August 2015 ∞ Volume 5
www.designworldonline.com
2015
Motion Systems Handbook
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Design with the power of small.
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1. Developing ideas 2. Drafting concepts 3. Implementing solutions 4. Manufacturing machines 5. Ensuring productivity
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motioncontroltips.com
MILES BUDIMIR Senior Editor
@DW_Motion
Happy Five-Year Anniversary,
MOTION HANDBOOK WELCOME
to the fifth edition of Design World’s annual motion control compendium. Observant readers will notice a slight change to this year’s handbook. In years past, it was called the Motion Control Handbook. This year we’ve changed the name to the Motion Systems Handbook. Why the change? For one, we think it more accurately reflects the state of the industry and the way real-world systems are built. The change also reflects the reality that a motion system is more than just a motor and controller; it comprises a host of individual components that are engineered together to form complete and functional designs. Motion systems encompass a broad range of components, both mechanical and electrical; from bearings, linear guide rails and ballscrews to the powerful software algorithms in motion controllers and the design software that aides engineers in developing moving machinery. Even beyond the obvious components, there are accessories such as limit switches, proximity switches and machine vision systems, as well as cabling and the associated harnesses and cable management hardware. In fact, according to some industry studies, these so-called ancillary components are one of the fastest growing areas in the industry.
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So whether you’re coming to this handbook as a seasoned veteran, looking for a refresher on a particular topic or component, or you’re new to the industry and figuring out what the difference is between a servo and stepper motor or just what exactly a gearbox does, we’ve got you covered. We especially appreciate all the feedback from years past and always welcome more. Receiving compliments on the handbook and requests for extra copies from engineering departments at colleges and universities across the country tells us we’re doing at least something right. So continue to let us know how we’re doing as we look for ways to improve going forward. Is there something we’ve missed? Do you want more detail or less? You’re welcome to send any feedback directly to me at mbudimir@wtwhmedia. com or to Senior Editor Lisa Eitel at leitel@wtwhmedia.com. Also follow us on twitter at @DW_Motion and @DW_Lisa_Eitel. Or connect with our linear motion editor Danielle Collins at @DW_Danielle. There, we deliver up-to-the-minute developments in motion control directly to you. And don’t forget to check in at motioncontroltips.com and linearmotiontips.com for the latest news, technical stories and industry trends.
www.designworldonline.com
8/18/15 3:27 PM
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INSIDE MOTION SYSTEMS HANDBOOK
the motion systems handbook 14
STATE OF TECHNOLOGY
in 2015 for motion systems design
22
ACTUATORS Electric, Pneumatic
48
98
146
54
102
150
56
106
153
CONTROLLERS CONVEYORS
30
COUPLINGS
34
DRIVES
BALLSCREWS BEARINGS
36
BELTS AND PULLEYS
40
CABLE MANAGEMENT
42
CABLING
46
COMPRESSION SPRINGS
66
HMI SOFTWARE LEADSCREWS LINEAR MOTION
Guide Rails, Slides & Ways
115
AC, DC
MACHINE VISION
ENCODERS
119 MOTORS
72 77
GEARING
88
RETAINING RINGS SHOCK & VIBRATION ABSORBERS TRANSDUCERS
154
WAVE SPRINGS
AC, DC, Integrated, Linear, Servo & Stepper
136
NETWORKS
GEARMOTORS
92
140
POSITIONING STAGES
GRIPPERS
94
144
RACK-AND-PINION SETS
HMI HARDWARE
WORLD A DESIGN WORLD RESOURCE
WTWH Media, LLC 6555 Carnegie Avenue, Suite 300 Cleveland, OH 44103 Ph: 888.543.2447 Fax: 888.543.2447
DESIGN WORLD does not pass judgment on subjects of controversy nor enter into dispute with or between any individuals or organizations. DESIGN WORLD is also an independent forum for the expression of opinions relevant to industry issues. Letters to the editor and by-lined articles express the views of the author and not necessarily of the publisher or the publication. Every effort is made to provide accurate information; however, publisher assumes no responsibility for accuracy of submitted advertising and editorial information. Non-commissioned articles and news releases cannot be acknowledged. Unsolicited materials cannot be returned nor will this organization assume responsibility for their care. DESIGN WORLD does not endorse any products, programs or services of advertisers or editorial contributors. Copyright© 2015 by WTWH Media, LLC. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, or by recording, or by any information storage or retrieval system, without written permission from the publisher. Subscription Rates: Free and controlled circulation to qualified subscribers. Non-qualified persons may subscribe at the following rates: U.S. and possessions: 1 year: $125; 2 years: $200; 3 years: $275; Canadian and foreign, 1 year: $195; only US funds are accepted. Single copies $15 each. Subscriptions are prepaid, and check or money orders only. Subscriber Services: To order a subscription or change your address, please email: designworld@halldata.com, or visit our web site at www.designworldonline.com POSTMASTER: Send address changes to: Design World, 6555 Carnegie Ave., Suite 300, Cleveland, OH 44103
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Senior Editor Miles Budimir mbudimir@wtwhmedia.com @dw_Motion Senior Editor Mary Gannon mgannon@wtwhmedia.com @dw_marygannon Senior Editor Lisa Eitel leitel@wtwhmedia.com @dw_LisaEitel Associate Editor Mike Santora msantora@wtwhmedia.com @DW_MikeSantora
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8 ∞ 2015
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8/18/15 11:19 AM
Precise motion drives automation productivity
Exactly
Move with a purpose. It’s not enough for linear motion technology to operate with precise endpoint accuracy. Cycle after cycle, day after day, your automation systems need linear technology with proven reliability, energy efficiency and minimal wear-and-tear. Bosch Rexroth delivers: our linear motion portfolio provides the industry’s broadest range of linear guide rails, linear modules and ball screw assembly options, all precision engineered for smooth, quiet, long-lasting performance. From individual components to complete pre-engineered linear actuators in single- or multi-axis configurations, count on Rexroth linear motion technology to boost the efficiency, throughput and productivity of your next automation solution. Free Linear Motion Handbook included in our Linear Motion and Mechatronics @ Work Resource Kit. Order yours today! Visit www.boschrexroth-us.com/mechatronicskit
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motioncontroltips.com
We would like to thank the following companies for their contributions to the editorial features: Ace Controls Inc. Altra Industrial Motion Corp. AMETEK Precision Motion Control AST Bearings LLC AutomationDirect Baldor, part of ABB Group Beckhoff Automation BEI Sensors Bimba Manufacturing Co. Bishop-Wisecarver Corp. Bosch Rexroth Corp. Clippard Instrument Laboratory, Inc. ContiTech North America Delts Tau Data Systems Inc. Dorner Mfg. Corp. Dunkermotor, part of AMETEK Exlar Actuation Solutions FESTO Groschopp Inc. H2W Technologies Inc. Harmonic Drive LLC Haydon Kerk, part of AMETEK HEIDENHAIN North America
www.designworldonline.com
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HELUKABEL USA IDEC Corp. igus, Inc. Intech Corp. Intercon Automation Parts Inc. ITT Enidine ITW Heartland Lampin Corp. Lapp Group Lee Spring Co. Lenze Americas Lin Engineering LinMot USA Inc. Maple Systems Inc. maxon precision motors, Inc. Mitsubishi Electric Automation Moog Inc., Components Group MS-Graessner GmbH & Co. KG Nexen Group Inc. NOOK Industries Inc. Opto 22 Oriental Motor U.S.A. Corp. Pacific Bearing Co.
PHD Inc. Pittman, part of AMETEK R+W America Rexnord Corp. Rotor Clip Co. Schaeffler Group USA Inc. SCHUNK SDP/SI (Stock Drive Products) Servometer/PMG Siemens Industry SKF Group Smalley Steel Ring Co. Steinmeyer USA TGW Logistics Group GmbH THK America Inc. Thomson Reuters Tolomatic Inc. U.S. Tsubaki Power Transmission US Digital Weiss North America Windjammer, part of AMETEK Yaskawa America Inc. Zero-Max Inc.
8 • 2015
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Motion Systems Design
State of TECHNOLOGY in 2015 for MOTION SYSTEMS DESIGN
[
Motion systems apply electromechanical or fluid power through machinery linkages to move loads. They work in myriad markets, including packaging,
food and beverage, plastics, paper, pharmaceuticals, agriculture, aerospace,
automotive, medical, mining, wood, machine tool and semiconductor.
NO
matter the scale or final objective, motion systems must satisfy design mechanics to overcome the inertia of external loads and machines; they must also overcome myriad sources of mechanical friction. As inertia defines how a setup resists acceleration, it dictates the force or (in the case of a rotary-motor-driven setup) torque to ultimately move the load. For real-world calculations to describe motor-driven motion designs, engineers calculate inertia and related parameters by analyzing the motion system using industry-standard calculations or (increasingly common) proprietary sizing and selection software. Often, the first step is to tentatively select one of the four most common motion-system setups, primarily defined by the inclusion of gear drives, tangential drives, rotary-to-linear drives or direct drives. These include an
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array of motors, controllers, drives, actuators, ballscrews, bearings, brakes, couplings, gearing and linear systems that generate and transmit power with better performance than ever. With all of these designs, the load-inertia portion reflected back to the electric motor is proportional to the speed ratio of intermediate mechanical linkages (such as gears) squared. Where applicable, engineers also account for a gear’s inertia (in addition to that of any pulleys, sprockets or pinion gears in the drivetrain) to get more accurate results. Where engineers don’t properly calculate inertia and friction, the motion design will be underpowered or overly costly. The easiest designs to evaluate are direct drives (such as linear motors) that eliminate mechanical linkages. www.designworldonline.com
8/18/15 11:59 AM
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Motion Systems Design
Here, a Windjammer variable-speed brushless blower sends a cool stream of air through a racecar driver’s crash helmet, allowing him to stay comfortable.
Therefore, inertia calculations are relatively simple, and load speed equals motor output speed. But engineers calculating values for designs that have tangential drives (including rack-and-pinion, belt-and-pulley and sprocket-and-chain sets) must calculate load reflected back to the motor shaft. After analysis of these design mechanics, the next step for specifying a rotary-motor-driven design is to calculate required output torque. Some caveats: Gearing efficiencies are always less than 100, so always increase overall system torque demand. Also, designs that use leadscrews need analysis that accounts for the effect on overall torque from (backlash-reducing) preload. Concurrent with these motion-design steps are the selection (where required) of machine subcomponents and the controller and drive. The latter must feed the motor enough power to meet duty-cycle demands; ramp through periods of acceleration; and overcome motor power-dissipation losses—a function of acceleration current during acceleration and rms current during normal running. Then design engineers finish any programming and motion-controller setup that wasn’t concurrent with the specification of the selection of motor, mechanical components, controller and drive. Today’s motion design types Of course, the most suitable motion system depends on the application. Many motion applications fall into the general category of speed control. These take the form of anything from basic setups (say, a soft-started motor paired with 16
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straightforward mechanical kinematics) to those with variable frequency drives paired with induction motors (or synchronous permanent-magnet-type setups). Typical applications are those for lift and hoist, grinding, fan, pump, compressor, conveyor, basic appliance and large machinery installations. Technologies common here are ac motors; traditional hydraulics; basic fieldoriented control for motors; linear speed control of stepper motors; and variable-speed pump and compressor motors. Case in point: Windjammer blower cools racecar-driver helmets Brushless variable-speed blowers in the Ametek Windjammer series have been used by NASCAR teams and independent racers for more than 15 years. The blowers keep the drivers cool during competitive racing events by supplying a controlled, cool airstream into their crash helmets. High aerodynamic performance, reliability and functional controllability in a lightweight package make the blowers suitable for racing. The Windjammer’s full-wave commutating electronics package enhances its versatility by providing functional performance options, protection features and increased customization potential. The blowers are fully programmable so end users do not need to develop their own hard control schemes, but can take advantage of a multi-wire input for specific operational functions, including closed-loop speed control, maximum speed set, electric braking, current limit, over temperature and locked rotor. Onboard signal level controllability allows end users to vary the speed (virtually 0 to 100% variation) without www.designworldonline.com
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WHEN SPEED MATTERS THE MOST. And More than 1,000 motion solutions to choose from, most with same-day shipping option. Design your own, today!
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8/15/15 1:52 PM
Motion Systems Design
any adjustments needed from the main input power supply. Windjammer blowers operate at the highest possible aerodynamic efficiencies, lowering cost of operation and making them better suited for energy efficient or battery operated systems. These units can deliver up to 50 in. (water column) of static pressure and output airflow to 26 CFM. Blower package sizes vary from 3.0 to 5.1 in., each providing aerodynamic operating characteristics allowing for a custom fit. All units are powered by Ametek’s 1.7-in. brushless 12 or 24 Vdc motor with tachometer output as a standard option. Another category: Precision motion In contrast with general speed control, precision motion machinery and those that use servomotor setups—for automation of tasks to sub-mm to µm and even nm scales—are more common in high-end positioning, fabricating and machining applications. These include semiconductor, precision machining and computer-assisted surgery applications. Technologies common here are micrometer-level positioning setups; modular designs; adaptive control; zero-backlash couplings; precision slides, stages and integrated actuators; and mechatronics. Common
motors are vector motors, linear motors, PMDC servomotors, and brushless servomotors. Controllers in these closed-loop systems use feedback signals to track load. They compare target and feedback position and then command the motor drive to minimize the difference. Most positioning applications use motion profiles in the controller software to define target speed and acceleration over time. To illustrate, a servomotor that gets a command from the controller (and full power-supply voltage and current from the drive) initially doesn’t move. Only after some instant does the system recognize a large and growing error that only diminishes after the motor overcomes inertia and friction. Trends in motion systems for 2015 New innovations in motion systems include the growing use of modular, customized and configurable motion components; digital and additive manufacturing of components for motion design; and an ever-increasing focus on efficient motion systems that leverage open-source technology.
Today’s motion designs, such as this aerospace electrical connector, include more customized setups than ever. Here, two Smalley-brand wave springs exert tailored force when compressed to maintain constant connection.
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At Catalyst Motion Group, Group we are able to configure motion systems by utilizing our own component level supply. What this means for our clients is full accountability for customized solutions while realizing the improved efficiencies of a reduced supplier count and receiving fully tested and delivered solutions. We can provide you with the ultimate solution for almost any application, while managing scheduling demands, technical requirements and financial considerations. We are able to accomplish this through coordinated utilization of our extensive engineering resources and vertically integrated manufacturing resources. We put it all together – customized motion systems that include stepper motors, brush and brushless DC motors, linear mechanics, drives, electronics and an the entire entire process, process, extensive array of peripheral system components. Our considerable and wide-ranging in-house manufacturing capabilities allow us full control of the from development of prototypes to delivery of fully tested production units. And, with our engineering capability, if we don't have it ...we can develop it! We have successfully developed and taken to production fully customized motion solutions for a wide variety of advanced technologies including: • Medical devices for diagnostics, surgical procedures, therapeutics and pharmaceuticals • Laboratory and analytical instrumentation and equipment • Industrial automation including robotics and production line processing operations MO M OTTI IOON NG RGORUOP U P We are innovative, flexible and most importantly a secure, reliable, and knowledgeable resource ready to respond to your most challenging motion control needs.
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Motion Systems Design
This year has also seen some mainstreaming of the industrial Internet of things (also called IIoT or Industry 4.0). Much of the progress has come from leveraging pre-existing Ethernet connectivity and distributed intelligence. Other progress has come from developments in controller and HMI technology to let plant managers access and manage their facilities from anywhere, anytime. For example, some such devices have built-in FTP servers to push production statistics online
in real time. These let management check operator screens and production statistics on the fly. Of course, central to the success of IIoT is networking. Here, the industries that make use of motion systems have led the charge to leverage the full capacity of networking today. There’s now a move toward an open Ethernet standard for IT; industrial applications could soon follow suit. No matter the ultimate outcome, myriad motion applications rely on factory networking—
and of enduring relevance here are traditional fieldbus, Ethernet and WiFi. Advances are also pushing bus standards, such as USB and open PC-based PCI eXtensions for Instrumentation (PXI), to new heights. Elsewhere, IIoT in the form of smart industrial devices, is helping to simplify implementation, mostly through better use of industrial Ethernet and communication standards such as IO-Link. Here, the concept of plug-and-play devices is a reality at last. One caveat to these IIoT developments is that industries only now are showing quantifiable progress to ensure network security for the 50 billion devices projected to have connectivity by 2020. In contrast, physical machine-safety components are the fastest spreading of automation, as workplace injuries cost U.S. businesses more than $171 billion a year, and standards are going global. Safety encompasses physical machinery and control architectures that assemble to meet increasingly normalized standards. Of
Pittman BI series motors from Ametek are suitable for home healthcare products. They’re small but deliver high torque with high slot fill; they’re also quiet and exhibit little or no vibration. Pairing with gearboxes is automatic.
This EC042B brushless dc motor from PITTMAN Motors of AMETEK has a maximum speed of 9,000 rpm and works with dc bus voltages up to 96 V. A four-pole rotor with rare earth magnets yields a high torque-to-inertia ratio.
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increasing relevance here are safety controls, networking and I/O, disconnects and power products, sensors, and safety feedback devices. Manufacturers are addressing these concerns with mechanical and electrical equipment, networking, sensors and standards to ensure safety in motion designs. Still another permutation of IIoT that’s actually long existed in motion design is predictive maintenance (or PM). This refers to the use of controllers with enough intelligence and sensor input to predict when an industrial process or machine is going to break down or degrade output. Here, it’s increasingly common to see motion systems with self-monitoring pneumatic systems; mechanical failure tracking; lifecycle calculators; electrical signature analysis; and ac motor testing and PM. www.designworldonline.com
8/18/15 11:59 AM
What’s 15 years to one of our blowers? A warm-up. Nothing moves air with more rock-solid reliability than AMETEK® Rotron regenerative blowers. Fifteen years’ service life is not unusual. These low-pressure, high-volume blowers feature rugged, compact construction and quiet operation. Their proven design makes them ideal in applications from chemicals, wastewater and furnaces to vapor recovery and more. Plus, they’re backed by the industry’s most knowledgeable engineering experts. AMETEK can customize your blower for harsh environments, high voltage and specialized applications, too. So make your next air-moving challenge a breeze. Call us at +1 330-673-3452 or visit our website to get started.
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Actuators–Electric
MOTION SYSTEMS HANDBOOK
This programmable actuator works in continuous-duty applications. This Nook Industries Series 500 has a direct or toothed belt drive for efficiency and quiet operation. Versions with a ballscrew come in parallel and in-line models with a servo or stepper motor.
Summary of today’s
ELECTRIC ACTUATORS MANY
applications call for converting rotary motion into motion that moves in a straight line. For these applications, linear electric or electromechanical actuators handle the task efficiently. In fact, today’s actuators are so efficient that the variety available for different design needs has proliferated. That means that actuators today are easier than ever to integrate into machinery; they’re also less costly. Electric actuators turn an electric motor’s power into linear motion in one of three ways: through a linear motor, belt or screw drive. Linear motors are the most technologically advanced and efficient method of directly transmitting the power of the motor into the motion of the actuator. Instead of the rotor rotating in the stator, the rotor travels in a linear, flat-array fashion along the stator. Belt drive actuators are less costly, but can still move loads at fairly high linear speeds. Because the motor is separate from the drive, the mechanical advantage can increase thrust speed. The disadvantage of belt drives is that they wear over time and require maintenance. Most screw drives take the form of either rod-style actuators or rodless cylinders. A motor transmits power through a coupler or pulley arrangement to rotate the screw and translate a nut along the screw axis. Attached to this nut is either the rod or saddle of the actuator. Screw drives can use roller, ball or leadscrews. Electric actuators have several benefits over hydraulic or pneumatic actuators. For one, the operation is cleaner because they operate without the need for fluids or
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ancillary equipment. They have the ability to integrate power, control and actuation mechanisms into one device. And they combine force, velocity and positioning in a single, compact motion control device. Another advantage is the ability to constantly monitor feedback directly from the motor and adjust performance accordingly. Though not necessary for every application, closed-loop operation has the ability to adjust and correct variances in the operation, resulting in repeatable and accurate motion with every move. Today, the prices for drives for electric actuators have come down, which has opened new application uses for the linear actuator. So, electric actuators are more viable for applications where hydraulic, pneumatic and manual operations once ruled. In many applications, servomotors are replacing induction motors because of their performance and energy efficiency. Direct drives are replacing traditional motor-gearbox combinations because of their high dynamic performance, high precision and long life. And electric actuators are replacing pneumatic cylinders in many applications for similar reasons. But the biggest improvements in the last five to ten years can be found in the control systems integrated with electric actuators. Faster bus systems, like industrial Ethernet and realtime communication, make the use of electric actuators simpler. Servo systems require fast communication and exchange of real-time data between the drive and the overlaid machine control. The bus was always the bottleneck in these systems. Now, with the much higher data rates and real-time capacity of industrial Ethernet, the integration and the use of electric www.designworldonline.com
8/14/15 2:32 PM
Save Time. Save Space. Save Money.
with Del-Tron’s DL Linear Actuators. These lead screw and ball screw actuators offer the benefits of a space-saving design, fast and simple assembly, long life, and a competitive price. The rigid enclosed aluminum box structure provides a compact envelope that incorporates the linear bearing and drive mechanism. Integrating all components into a single unit that includes the motor adaptor saves assembly time and eliminates the need to source additional parts. DL series linear actuators are offered in travel lengths up to 410 mm. The DL ball screw and lead screw actuators utilize recirculating guide technology to provide a low profile and compact design solution. Our DW series (double wide) is engineered to create a wider mounting platform while still maintaining the same low profile height as our standard width DL actuators. This double wide design is ideal for applications that need a greater carriage mounting area or where axial play must be minimized.
Precision Linear Motion Required. Del-Tron Preferred. • Compact space saving design • Available with ball and lead screw technology • Standard selection of stepper and servo motor mounts provided • Manufactured from corrosion resistant aluminum and stainless steel materials • Covers, limit switches and other options available
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Actuators–Electric
MOTION SYSTEMS HANDBOOK
Some actuators come with low-cost servo or stepper drivers and controllers. For example, these Tolomatic ACS actuators are ODVA conformant for Ethernet/IP, so naturally they work well with other ODVA conformant devices over Ethernet/ IP. The actuators deliver infinite positioning and can replace pneumatic cylinders and manual processes.
actuators is easier. Stepper and servo drive options with Ethernet protocols (Ethernet/IP, Modbus, TCP) turn singleaxis actuators into simple, low-cost motion devices with infinite positioning, precise control and longer life. Electric linear actuators are an alternative to pneumatic cylinders in several applications because of the flexibility they deliver in the design of production and monitoring systems. In conveying applications, for example, electric actuators now more frequently control diverting and sorting functions. Traditional setups use pneumatic actuators, but the manual adjustments they need introduce human error. Plus, pneumatic actuators can only handle minimal variability in product sizes. In contrast, electric actuators are flexible by design. For example, material handling applications have experienced an increase in the variety and variability of package sizes. In packaging machines, consumer products manufacturers are producing more package sizes with the same manufacturing lines, which require equipment to be adaptable enough to handle different product sizes and types. Electric actuators easily handle these variability requirements and, over the life of the motion system, can be less expensive.
These electric actuators come in versions for light-, general- or rugged-duty applications. Photo courtesy of Warner Linear.
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Selecting an Electric Actuator The process for selecting an electric actuator is similar to one for hydraulic or pneumatic actuators, with a few differences. Here are the essentials. Start with the motion profile. This establishes the demands for velocity and time as well as force (or torque) and the required travel distance. This is also the place to determine the maximum stroke needed as well as maximum and minimum speed requirements. Then calculate the load. This can have many different components including inertial load, friction load, the external applied load and the gravitational load. Load calculations also depend on the orientation of the actuator itself, whether it’s horizontal or vertical. Duty cycle is another important factor. This is the ratio of operating time to resting time; manufacturers usually express it as a percentage. The cycling rate may be in seconds, minutes, hours or even days; even knowing the operating hours per day may be necessary. Knowing the duty cycle helps the engineer estimate the system life requirements and can also eliminate problems such as overheating, faster wear and premature component failure due to an incorrectly sized actuator. Know the positional accuracy and precision demanded by the application. The actuator’s precision should meet or exceed the application’s requirements for accuracy, backlash, and straightness and flatness of linear motion. This directly impacts the cost of the system; if the application doesn’t demand high accuracy or precision, then there is no need to buy a more expensive actuator when a less expensive one will satisfy the demands of the application. Aside from the technical specifications mentioned above, select the proper configuration for the actuator in the final design. This includes mounting and any other external components, such as holding brakes and communication and power cables. Lastly, consider the operating environment for the actuator. What are the temperature requirements? Are there any contaminants such as water, oil or abrasive chemicals? Contaminants can affect seals and impact the working life of the actuator. In such cases, selecting the appropriate IP rating for an application can guard against the effects of contaminants. www.designworldonline.com
8/14/15 2:32 PM
ELECTRIC!
Cylinders and Linear Slides
Series ESFX Modular Electric Slides
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Actuators–Pneumatic
Update on MOTION SYSTEMS HANDBOOK
PNEUMATICALLY OPERATED ACTUATORS
DRIVING
linear-motion applications with pneumatic actuators (or air cylinders) is a relatively easy and inexpensive approach. The actuator technology has existed for more than 50 years, but better piston seals and rod wiper seals (of modern materials) make today’s pneumatic actuators more resilient and efficient than ever. The seals reduce leakage and withstand extreme temperatures to let engineers use the actuators in more environments. Likewise, surfaces with permanent lubrication, servo-pneumatic controls, improved corrosion resistance and aircushioning features make pneumatic actuators more useful than ever. To review, pneumatics technology uses compressed air. However, some refer to it as a type of automation control. Pressurized gas—generally air that may be either of the dry or lubricated type— works to actuate an end effector and move loads. End effectors can range from the common cylinder to more application-specific devices such as grippers or air springs. Vacuum systems, also in the pneumatic realm, use vacuum generators and cups to handle delicate operations, such as lifting and moving large sheets of glass or delicate objects such as eggs. Pneumatics
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are most common in medical, packaging, material handling, entertainment and robotics applications. By its nature, air is easily compressible, and so pneumatic systems tend to absorb excessive shock, a feature useful in some applications. Most pneumatic systems operate at a pressure of about 100 psi, a small fraction of the 3,000 to 5,000 psi that some hydraulic systems see. As such, pneumatics are generally used when much smaller loads are involved. A pneumatic system generally uses an air compressor to reduce the volume of the air, thereby increasing the pressure of the gas. The pressurized gas travels through pneumatic hose and is controlled by valves on the way to the actuator. The air supply itself must be filtered and monitored constantly to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life. In recent years, the control available within pneumatic systems (thanks to advanced electronics and componentry) has increased a great deal. Where once pneumatic systems could not compete with many comparable electronic automation systems, the technology today is seeing a renaissance of sorts. www.designworldonline.com
8/14/15 3:29 PM
The shortest distance to linear motion solutions Easiest, most affordable actuator control solution (ACS)
Single-axis servo or stepper drive and controller • Replace pneumatic cylinders, automate manual processes • Built-in configuration for all Tolomatic actuators • Infinite positioning, precise control • EtherNet/IP™ with ODVA™ conformance for worry-free setup and operation
Visit www.tolomatic.com/ACS
Tolomatic makes it easy to take your machine design from premise to production. Make your next machine everything you imagine it can be. Optimize cost and performance with our complete single-axis linear motion solutions—actuator, drive, motor and controls. We meet nearly any application requirement, and our online tools simplify specification. With over 60 years of product innovation and integrity, our technical and customer service support is unequaled. Great design ideas start on the back of a napkin. Contact us to help you get from point A to point B. Visit www.tolomatic.com or call 877-385-2234. Download our white paper comparing linear actuator solutions. www.tolomatic.com/Napkin
www.tolomatic.com
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Actuators–Pneumatic
MOTION SYSTEMS HANDBOOK
PNEUMATIC ACTUATOR OPERATION Many industrial applications require linear motion during their operating sequence. One of the simplest and most cost effective ways to accomplish this is with a pneumatic actuator. An actuator is a device that translates a source of static power into useful output motion. It can also be used to apply a force. Actuators are typically mechanical devices that take energy and convert it into some kind of motion. That motion can be in any form, such as blocking, clamping or ejecting. Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas. There are many styles of pneumatic actuators, including diaphragm cylinders, This is a pneumatic linear cylinder in an industrial application with a vacuum chamber that opens and closes. Image courtesy of Festo.
This application is a pneumatically operated penstock driven by linear actuators. It runs at a pumping station on the British island of Guernsey to direct rainwater into storage basins for treatment (to make drinking water) or pumping out to sea (to prevent flooding). Centralized controls automate the process.
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rodless cylinders, telescoping cylinders and through-rod cylinders. The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders use one air port to let compressed air enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed. Double-acting cylinders have an air port at each end and move the piston forward and back by alternating the port that receives the high pressure air. In a typical application, the actuator body is connected to a support frame and the end of the rod is connected to a machine element that is to be moved. An on/off control valve is used to direct compressed air into the extended port while opening the retract port to atmosphere. The difference in pressure on the two sides of the piston results in a force equal to the pressure differential multiplied by the surface area of the piston. If the load connected to the rod is less than the resultant force, the piston and rod will extend and move the machine element. Reversing the valving and the compressed air flow will cause the assembly to retract back to the “home” position. Pneumatic actuators are at the working end of a fluid power system. Upstream of these units, which produce the visible work of moving a load, are compressors, filters, pressure regulators, lubricators, on/off control valves and flow controls. Connecting all of these components together is a network of piping or tubing (either rigid or flexible) and fittings. Pressure and flow requirements of the actuators in a system must be taken into account when selecting these upstream system components to ensure desired performance. Undersized upstream components can cause a pneumatic actuator to perform poorly, or even make it unable to move its load at all. www.designworldonline.com
8/14/15 3:28 PM
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MOTION SYSTEMS HANDBOOK
Ballscrews
This cutaway of a caged ballscrew shows the main components of a typical ballscrew, including the screw itself, a nut, and balls that roll between the nut, screw and grooves. Photo courtesy of THK.
BALLSCREW basics
BALLSCREWS
are used to change the direction of motion—from either rotational motion to linear motion or linear to rotational. The advantage of ballscrews is that they accomplish this function with minimal friction. The basic components of a ballscrew are a nut, a screw with helical grooves, and balls (often made from steel, ceramic or hard plastic material) that roll between the nut, the screw and the grooves when either the screw or nut rotates. Balls are routed into a ball return system of the nut and travel in a continuous path to the ball nut’s opposite end. Seals are often used on either side of the nut to prevent debris from compromising smooth motion. Over the last few years, advances in manufacturing and materials have improved ballscrew performance so machine designers today can get better linear motion with them at lower cost. Some improvements include the fact that the latest generation of ballscrews has more load density than ever, giving designers higher capacity from a smaller package. There is also a trend toward more miniaturization, but also faster ballscrews with rolled and ground screw manufacturing methods.
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Ballscrews suit applications needing light, smooth motion, applications requiring precise positioning, and when heavy loads must be moved. Examples include machine tools, assembly devices, X-Y motion, Z motion and robots. Ballscrews are usually classified according to factors such as lead accuracy, axial play and preload, and life/load relationship. Lead accuracy refers to the degree to which the shaft’s rotational movements are translated into linear movement. With lead accuracy and axial play determined by the manufacturing method of the ballscrew shaft and the assembly of the nut, high lead accuracy and zero axial play is generally associated with relatively higher-cost, precision ground ballscrews, while lower lead accuracy and some axial play is associated with lower-cost, rolled ballscrews. Fabricated by rolling or other means, ballscrew shafts yield a less precise, but mechanically efficient and less expensive, ballscrew. Axial play is the degree to which a ball nut can be moved in the screw axis direction without any rotation of either nut or screw. Preload is applied to eliminate axial play. The process of preloading removes backlash and increases stiffness. Ball recirculation inside the ball nut can affect precision and repeatability. Thus, ball nuts are available with a range of preload options to reduce or remove the axial play as they rotate around the screw. Minimal axial play allows better accuracy, for example, because no motion is lost from the clearance in the balls as they re-engage. Common preloading methods are to oversize balls inside the nut housing; use a double-nut or tension-nut method; or use a manufactured offset in the raceway spiral to change the angle of ball engagement. Each method has its advantages www.designworldonline.com
8/18/15 3:37 PM
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Ballscrews
CUSTOM BALL SCREWS DESIGNED IN REAL TIME
Dynatect precision-ground ball screws can be quickly designed to your exact specifications with our automated CAD system.
• High precision, up to ANSI class 2/JIN Class 1 spec. • Virtually any length (longest to date: 54 feet) • Screw diameters from 1/2" to 6" • Designed to minimize backlash and eliminate deadband • We take into account: load, speed and accuracy, for optimal performance and long life
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and disadvantages, but all serve to minimize or eliminate backlash between the nut and screw. Perhaps the biggest overall benefit of ballscrews is that efficiencies exceed 90%. By contrast, Acme leadscrews average about 50% efficiency or less. There are also minimal thermal effects. Backlash can be eliminated through preloading. Ballscrews also offer smooth movement over the full travel range. The higher cost of ballscrews can be offset by decreased power requirements for similar net performance. One drawback to ballscrews is that they require high levels of lubrication. Ballscrews should always be properly lubricated, with the correct type of lubricant, to prevent corrosion, reduce friction, extend operating life and ensure efficient operation. Because ballscrews are a bearing system, they’ll need some type of lubrication to avoid metal-to-metal contact of the balls in the raceway. While the lubrication choice can be either oil or grease, it’s advisable to avoid solid additives (such as graphite) as they will clog the recirculation system. An NLGI No. 2 type grease is recommended, but it should also depend on the application, whether food-grade or another special type of lubrication is required. Ballscrews, especially those used in machine tools, generally require lubricants with EP additives to prevent excessive wear. The lube amount will be fixed, but the frequency of lubrication will vary depending on factors such as the move cycle characteristics or contamination in the environment. Contaminated lubrication can increase friction. In addition, ballscrews can fail if the balls travel over metal chips or dirt in the ball thread raceway. Using lubricants recommended by machine tool manufacturers can help prevent this effect. Using telescopic covers or bellows can help keep ballscrews clean when used in environments with many contaminants. Proper installation of ballscrews is also important; but care must be taken as ballscrews are prone to more damage during installation than leadscrews.
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Linear motion made easy, find a solution today! • • • • • •
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Bearings
BEARINGS MOTION SYSTEMS HANDBOOK
in motion systems
Needle roller bearings such as this one from Koyo offer high load carrying from a compact design.
BEARINGS
usually consist of smooth rollers or metal balls and smooth inner and outer surfaces known as races against which rollers or balls roll. These rollers or balls act as the load carrier for the device, allowing it to spin freely. The primary function of a bearing is to reduce friction between moving parts by giving a surface something to roll on, rather than slide over. The secondary function of a bearing is to transmit loads. Bearings carry two kinds of load: radial and axial. Radial loads are perpendicular to the shaft, while axial loads are parallel to the shaft. Depending on the application, some bearings experience both loads simultaneously. Ball bearings: One of the most common forms, ball bearings use balls to provide a low-friction means of motion between two bearing races. Ball bearings are usually inexpensive and, when selected properly, require little maintenance. Because of these characteristics, ball bearings are some of the most popular of all bearings. Because the contact area between the balls and races is so small, ball bearings can’t support as much load as other bearing types. However, their small surface contact also limits the heat generated by friction, which makes ball bearings useful in high-speed applications.
Ball bearings support radial loads and a bit of axial load. They come in open, sealed, shielded and ceramic-ball versions. Image courtesy of Koyo.
Roller bearings: Because of their shape, roller bearings have greater surface contact than ball bearings, so can handle larger loads without deforming, making them suitable for conveyor belt applications. Needle-roller bearings: These have rollers with lengths at least four times their diameter. Despite their low cross section, the large surface area of needle-roller bearings allows them to support extremely high radial loads. These bearings are often used in automotive applications such as rocker arm pivots, pumps, compressors and transmissions. The drive shaft of a rear-wheel drive vehicle typically has at least eight needle bearings (four in each U joint) and often more if it is particularly long, or operates on steep slopes. Thrust ball bearings: These are for applications with axial loads or shaft misalignment. These bearings are also useful in high-speed applications in aerospace and automotive industries. Thrust roller bearings: In these, load is transmitted from one raceway to the other (to accommodate radial loads). These bearings can also self-align. Tapered roller bearings: These have tapered inner and outer ring raceways with tapered rollers arranged between them, angled so the surface of the rollers converge at the axis of the bearing. They handle both axial and radial loads.
Tapered roller bearings consist of an inner ring (cone), an outer ring (cup), cage and rollers. The bearings carry high radial and axial (thrust) load capacities at moderate speeds. Left is a spherical roller bearing, a self-aligning option. Images courtesy of Koyo.
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One variation: Linear bearings Linear bearings include a straight array of rollers in a carriage to carry loads on a straight or curved slide or rail. Engineers usually pair linear bearings with a manual crank or motor. Note that linear bearings experience overturning moments of force instead of radial and axial loads.
www.designworldonline.com
8/18/15 3:42 PM
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Belts + Pulleys
Motion excels with MOTION SYSTEMS HANDBOOK
BELT DRIVES INDUSTRIAL
belt drives consist of rubber belts that wrap around drive pulleys, in turn driven by electric motors. In a typical setup, the belt also wraps around one or more idler pulleys that keep the belt taut and on track. The main reasons that engineers pick belt drives over other options is that modern varieties require little if no maintenance; they’re less expensive than chain drives; and they’re quiet and efficient, even up to 95% or more. In addition, the tensile members of today’s belts— cords embedded into the belt rubber that carry the majority of the belt load—are stronger than ever. Made of polyester, aramid, fiberglass or carbon fiber, these tensile cords make today’s belt drives thoroughly modern power-transmission devices. Manufacturers generally describe belts and pulleys with five main geometries. Pitch diameter is the drive pulley’s diameter. Center distance is the distance between the two pulleys’ centers. Minimum wrap angle is a measure of how much the belt wraps around the smallest pulley. Belt length is how long the belt would be if cut and laid flat. Finally, in the case of toothed belts (also called synchronous belts), the pitch is the number of teeth per some length—so a 3-mm pitch means that the belt has one tooth every 3 mm.
Applying synchronous belts Some general guidelines are applicable to all timing belts, including miniature and double-sided belts. First of all, engineers should always design these belt drives with a sufficient safety factor—in other words, with ample reserve horsepower capacity. Tip: Take note of overload service factors. Belt ratings are generally only 1⁄15 of the
Clockwise from top are ABB’s Baldor Maska classic sheaves for V-belt drives (also called friction drives for the way they operate), timing belts, split-taper sheaves, bushings, and synchronous sprockets and timing pulleys. The range shows how belt designs abound to suit myriad applications.
belt’s ultimate strength. These ratings are set so the belt will deliver at least 3,000 hours of useful life if the end user properly installs and maintains it. The pulley diameter should never be smaller than the width of the belt. As mentioned, belts are quieter than other powertransmission drive options … but they’re not silent. Noise frequency increases proportionally with belt speed, and noise amplitude increases with belt tension. Most belt noise arises from the way in which belt teeth entering the pulleys at high speed repeatedly compresses the trapped pockets of air. Other noise arises from belt rubbing against the flange; in some cases, this happens when the shafts aren’t parallel. Pulleys are metal or plastic; the most suitable option depends on required precision, price, inertia, color, magnetic properties and the engineer’s preference based on experience. Plastic pulleys with metal inserts or metal hubs are a good compromise. Tip: Make at least one pulley in the belt drive adjustable to allow for belt installation and tensioning. Also note that in a properly designed belt drive, there should be a minimum of six teeth in mesh and at least 60° of belt wrap around the drive pulley. Other tips: • Pretension belts with the proper recommended tension. This extends life and prevents belt ratcheting or tooth jumping. • Align shafts and pulleys to prevent belt-tracking forces and belt edge wear. Don’t crimp belts beyond the smallest recommended pulley radius for that belt section. This conveyor—a Dorner 2200—uses an integrated timing-belt drive to accurately move hundreds of pounds of product at hundreds of feet per minute. It leverages the main benefit of timing belts, which is precise movement.
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Belts + Pulleys
MOTION SYSTEMS HANDBOOK
• •
Select the appropriate belt for the design torque. Select the appropriate belt material for the environment (temperature, chemical, cleaning agents, oils and weather). Belt-and-pulley systems are suitable for myriad environments, but some applications need special consideration. Topping this list are environmental factors.
Dusty environments do not generally present serious problems as long as the particles are fine and dry. In contrast, particulate matter can act as an abrasive and accelerates belt and pulley wear. Debris should be prevented from falling into belt drives. Debris caught in the drive is generally either forced through the belt or makes the system stall. In either case, serious damage occurs to the belt and related drive hardware. Light and occasional contact with water—during occasional washdowns, for example—has little serious effect. However, prolonged contact with constant spray or submersion can significantly reduce tensile strength in fiberglass belts and make aramid belts break down and stretch out. In the same way, occasional contact with oils doesn’t damage synchronous belts. But prolonged contact with oil or lubricants, either directly or airborne, significantly reduces belt service life. Lubricants cause the rubber compound to swell, break down internal adhesion systems, and reduce felt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it’s best to prevent oil from contacting synchronous belts. The presence of ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials in much the same way as excessive temperatures. Although the bumper materials used in belts are compounded to resist the effects of ozone, eventually chemical breakdown occurs and they become hard and brittle and begin cracking. The amount of degradation depends on the ozone concentration and generation of exposure. Rubber belts aren’t suitable for cleanrooms, as they risk shedding particles. Instead, use urethane timing belts … keeping in mind that while urethane belts make significantly less debris, most can carry only light loads. Also, none have static conductive construction to dissipate electrical charges.
Shown here are timing belts, timing-belt pulleys and Fairloc hubs from Stock Drive Instruments/Sterling Instrument (SDP/SI). The Fairloc hub centers shafts and keeps mounted pulleys aligned.
These are Dura-Belt powered rollers that transmit power and convey loads. Their round polyurethane belts can carry more load than comparable rubber belts.
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Cable Management
The basics of MOTION SYSTEMS HANDBOOK
CABLE TRAYS & CARRIERS MOTION
systems vary from single-axis directdrive systems with little wiring to complex multi-axis robotics with a hornet’s nest of cables. This is usually where cabling, which was once an afterthought, now takes center stage. A simple form of cable management uses twist-tie type bundlers that tie together groups of wires and cables, which are low cost and easy to use. The problem is that with more and more cabling, they become impractical. Also, if the wiring and cabling is suspended, bundling them together can pose weight problems that cause sagging and strain. Cable trays: For largely stationary applications or where cable ties are insufficient, cable trays are an option. Cable trays safely and cleanly route cables to give them enough space to move and stay cool. Traditional trays have a U-shaped open channel into which cables safely lay. They can be open at the top or closed with a cover or lid. They’re available in a number of styles including solid, ventilated or perforated, with knockouts for cable exits or ladder style with rungs. They come in plastic, aluminum and fiberglass. When selecting a cable tray, consider the type of cable to be routed, including its diameter and weight; the span between supports; the distance of the cable run; environmental conditions; the need for complete enclosure or ventilation; and the ability to access cables along the run. Cable carriers: These house cables in enclosures of plastic, steel or a metal alloy. Cable carriers protect cables and hoses on moving machinery. They prevent tangling and increase safety by not having cables susceptible to getting caught in moving parts of a machine. Applications for cable carriers can range from machine tools and robotics to cleanroom applications and large industrial equipment like cranes and other construction machinery. Carriers can house a large volume of cables and wires and support the weight of them all without sagging or putting stress on the cabling. They also make managing and routing the cables through a machine or factory simpler and provide easy access for troubleshooting or maintenance. Selecting the right kind of cable carrier for an application starts with a few simple guidelines. The most important points to consider are the specifics of the application. These include the length of travel, number of cables or hoses, size and weight of the cables, required speed and acceleration
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and environmental factors such as exposure to any debris, excessive heat or chemicals. Knowing the weight of the cables ensures that the carrier won’t fail by snapping in two. Cable carriers are open or closed. Open varieties allow for easy access to the cables and visible access as well, whereas closed carriers shield cables from the environment. One importat carrier parameter is the bend radius— measured from the center of the curve loop to the center of the pivot pin on the side link. A larger bend radius means less stress on the cable and a longer service life. This radius (except with space restrictions) should be larger than the recommended minimum bend radii of cables in the carrier. All cable carriers have a predetermined radius stopping point on each link. When a number of links are assembled, these stopping points restrict the carrier from fully pivoting and form a curve loop, or minimum bend radius.
Photo courtesy of Helukabel
All cable carriers also have multiple bending radii to choose from, and every manufacturer suggests a minimum bend radius. The bend radius chosen for the cable carrier will depend on the cable or hose with the largest diameter. Selecting a considerably larger bend radius than required for the fill package will extend the lifespan of the cables and hose. www.designworldonline.com
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MOTION SYSTEMS HANDBOOK
Cabling
THE RIGHT CABLING
for motion control applications
A
lot of industrial automation equipment today operates continuously, with robots that execute motions repeatedly, sometimes thousands of times per day. These applications stress not only the moving parts of the machine, but also the electrical cabling. All too often, designers spend more time sizing components like motors, actuators and controllers than giving enough thought to the cabling needed. The result is that if standard cabling is used in these applications, the cables, not being designed to flex continuously, can’t handle the rigors of the application and can result in costly premature failures. Flexible cables are cables specially designed to cope with the tight bending radii and physical stress associated with motion control applications. These highly flexible cables were developed with unique characteristics to differentiate them from standard designs. These are sometimes called chain-suitable, high-flex or continuous-flex cables. A higher level of flexibility translates into an increase in service life for a cable inside a cable carrier. A regular cable typically manages 50,000 cycles, but a flexible cable can complete between one and three million cycles. Flexible cables can be divided into two types: those with conductors stranded in layers inside the cable, and those that have bundled or braided conductors. Cables with stranded layers are easier to produce, and therefore usually less expensive. The cable cores are stranded firmly and left relatively long in several layers around the center and are then enclosed in an extruded tube-shaped jacket. In the case of shielded cables, the cores are wrapped up with fleece or foils. However, this type of construction means that during the bending process, the inner radius compresses and the outer radius stretches as the cable core moves. This can work quite well because the elasticity of the material is still sufficient, but material fatigue can set in and cause permanent deformations. The cores move and begin to make their own compressing and stretching zones, which
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Photos courtesy of igus.
can lead to a “corkscrew” shape, and ultimately, core rupture. The other construction technique involves braiding conductors around a tension-proof center instead of layering them. Eliminating multiple layers guarantees a uniform bend radius across each conductor. At any point where the cable flexes, the path of any core moves quickly from the inside to the outside of the cable. The result is that no single core compresses near the inside of the bend or stretches near the outside of the bend, which reduces overall stress. An outer jacket is still required to prevent the cores from untwisting. A pressure-filled jacket fills all the gussets around the cores and ensures that the cores cannot untwist. The resulting flexible cable is often stiffer than a standard cable, but lasts longer in applications where it must constantly flex. An alternative to flexible cabling in some motion applications are flat cables. These cables can incorporate any variety of power, signal and video conductors in a single, compact cable. In addition to every type of electrical conductor, flat cables can also include tubing for air or liquids, even fiber optics. By incorporating all these elements into a single flat cable, motion equipment can be significantly smaller, quieter and more energy efficient. Flat cables are best for continuous flexing. Their wire conductors can individually flex in a single plane, which provides optimum flex life. Some motion control systems may encase separate wires, cables and tubes in a carrier track to contain and manage the separate elements and to constrain their motion. These tracks are usually made of plastic and have a rather large bend radius. These tracks do not add performance to the motion device or machine, as they are simply cable management devices. Cable tracks can add bulk, mass and inertia to the motion system, and moving this extra mass requires more energy. While certain motion systems such as robotic applications may require this type of cabling design, other designs may not and can use standard flat cabling instead to save weight and cost. Some flat-cable manufacturers offer cables with silicone jacketing. These types of flat cables are durable and need no www.designworldonline.com
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Cabling
When Size and Performance Matters Medical-Industrial Actual Micro-Coax cable bundle diameter compared to a penny.
Photos courtesy of igus.
7.7mm
external armor for protection. They resist abrasion and will even self-heal minor nicks. Silicone encapsulation also provides protection against oils, acids, ozone, steam and extreme temperatures. Selecting the right cable for an application starts with a few fundamental parameters. First, determine the application type. In a motion application, will the cable be stationary or will it be moving? If the latter, is the motion mainly flexing or is there torsional motion involved? Or does the application call for both flexing and torsion? Different applications have specifically designed cables for that application. If there is bending or motion involved, the bend radius must be specified. The bend radius ultimately depends on the gauge of the wire and the kind of conductors used in the cable. The cable size includes the gauge of the wire, which is dependent on the current requirements as well, and the number of conductors needed by the application. As a general rule, the finer the conductor gauge, the smaller the allowable bend radius. Flat cables with PTFE jackets can have a larger bend radius than cables with silicone jacketing, given that each cable contains the same conductors. For cabling used in flexing applications, the two key factors are the wire conductors and the cable jacket. With continuous flexing, conductors containing multiple strands of fine-gauge wire generally last the longest. Cable materials are also important. Specifically, this refers to the insulation and jacketing material. There is a wide array of selections depending, once again, on the application needs, from PVC and halogenfree to Neoprene, rubber, silicone and other materials. Consider whether the cables require electrical shielding. Also consider any approvals that the cables may need to meet such as UL, CSA, CE or RoHS. The environmental conditions in which the cable will operate are also important in the selection process. Chief environmental factors include exposure to harsh conditions such as temperature and humidity, and resistance to environmental contaminants such as any oil or corrosive materials. For instance, what is the operating temperature for the application; will the cables be in low-temperature (freezing and below freezing) or high-temperature environments? Also, will the cables need to be oil resistant? In this case, there are cables that provide minimal protection, which may be sufficient for low-level exposure, and cables that provide full immersion protection over a period of days. Lastly, consider flame resistance. Options can range from minimal protection to higher levels of protection as the application calls for them.
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Compression Springs
All about MOTION SYSTEMS HANDBOOK
COMPRESSION SPRINGS
This compression spring has reduced ends.
Surging is when a spring builds compressionwave motion when subject to vibrations close to its natural frequency. This coneshaped compression spring from Lee Spring resists surging. The larger outer coils collapse before the smaller inner coils, so forces on the spring also increase the spring rate for a natural damping effect.
ENGINEERS
incorporate compression springs in designs that need linear compressive forces and mechanical energy storage—designs such as pneumatic cylinders and push-button controls, for example. The most conventional compression spring is a round metallic wire coiled into a helical form. The most common compression spring, the straight metal coil spring, bends at the same diameter for its entire length, so has a cylindrical shape. Cone-shaped metal springs are distinct in that diameter changes gradually from a large end to a small end; in other words, they bend at a tighter radius at one end. Cone-shaped springs generally go into applications that need low solid height (the total height when compressed) and higher resistance to surging. Whether cylindrical or cone shaped, helical compression springs often go over a rod or fit inside a hole that controls the spring’s movement. Other configuration types include hourglass (concave), barrel (convex) and magazine (in which the wire coils into a rectangular helix). Most compression springs have squared and ground ends. Ground ends provide flat planes and stability under load travel. Squareness is a characteristic that influences how the axis force produced by the spring can be transferred to adjacent parts. Although open ends may be suitable in some applications, closed ends afford a greater degree of squareness. Squared and ground-end compression springs are useful for applications
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This concave (hourglass-shaped) compression spring can stay centered, even in largediameter bores.
This compression spring has a barrel shape for lateral stability.
that specify high-duty springs; unusually close tolerances on load or rate; minimized solid height; accurate seating and uniform bearing pressures; and minimized buckling. The key physical dimensions and operating characteristics of these springs include their outside diameter (OD), inside diameter, wire diameter, free length, solid height and spring rate or stiffness. • Free length is the overall length of a spring in the unloaded position. • Solid height is the length of a compression spring under sufficient load to bring all coils into contact with adjacent coils. • Spring rate is the change in load per unit deflection in pounds per inch (lb/in.) or Newtons per millimeter (N/mm). These dimensions, along with the load and deflection requirements, determine the mechanical stresses in the spring. When the design loads a compression spring, the coiled wire is stressed in torsion, and the stress is greatest at the wire surface. As the spring is deflected, the load varies, causing a range of operating stress. Stress and stress range affect the life of the spring. The higher the stress range, the lower the maximum stress must be to obtain comparable life. Relatively high stresses may be used when the stress range is low or if the spring is subjected to static loads only. The stress at solid height must be low enough to avoid permanent damage because springs are often compressed solid during installation. www.designworldonline.com
8/14/15 5:53 PM
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Controllers
Summary of MOTION SYSTEMS HANDBOOK
CONTROLLERS
MOTION
controllers act as the brains of moving machinery. In feedback-based designs, they take an input command, compare it with a feedback signal at the motor, and trigger corrective action to make the output (or actual position) and input (target position) match—ideally, with minimal error. Motion controllers also calculate trajectories for the machine axes’ motors to follow to meet the target commands. These trajectories form motion profiles, which are sequences of position commands (expressed as functions of time) that tell the motors where to position the load and how fast to do it. Common motion profiles are trapezoidal, ramp, triangular and complex polynomial profiles. Each of them satisfies certain motion conditions and tasks. For instance, a trapezoidal profile (with a velocity-versustime profile in the shape of a trapezoid) delivers constant acceleration, then velocity, then deceleration. Such actions need copious signal processing, so motion controllers typically use digital signal processors (DSPs) to perform the mathematical operations.
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This MP3300iec machine controller from Yaskawa America makes automated equipment more responsive by boosting processing speed and memory capacity. It works with the MECHATROLINK-III motion network to improve cycle time.
DSPs handle algorithmic processing better than standard microcontrollers incapable of heavy mathematical processing. To get the target motion profile out of the physical machine, the controller needs to track the profile and (in most cases) reject disturbances. Controllers track commands by commanding motor drives to follow positions, velocities, torques (or forces) and accelerations. Called feedforward control, this compensation relies on accurate machine and motor models. In contrast, disturbance-rejecting controls are more active, fixing output to correct for problems with sudden or unexpected loads on the machine or inaccurate feedforward models. The simplest of these is proportional (P) control for constant integer gain. Then programmers can add integral gain (I) and in some cases either derivative (D) gain or velocity (V) gain to make PID or PIV loops to actively reject errors. The integral value
integrates error over time and helps to drive it to zero. The derivative value helps to stabilize the P-I system. PID loops are particularly common and powerful algorithms to help machines track commanded trajectories. They work on the error signal—the difference between a commanded value and actual value of an output—and attempt to drive the error to zero while maintaining machine stability. Physical setup: Three main iterations On a physical level, most motion controllers are stand-alone controllers, PC-based controllers or microcontrollers. Stand-alone controllers are complete systems (including all electronics, power supplies and external connections) that mount in one physical enclosure. These controllers go into machines to command one application consisting of a singlemotion axis or multiple-motion axes. www.designworldonline.com
8/14/15 6:26 PM
Galil Motion Control
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• Process commands in as little as 31 microseconds • Handle all modes of motion, simple to complex • Handle ultra-high resolution systems • Support closed loop stepper motors • Up and running in minutes • Use virtually any encoder • EtherCAT master available
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Controllers
MOTION SYSTEMS HANDBOOK
PC-based controllers include a motherboard of a basic personal computer or a ruggedized industrial PC, as well as PC-type hardware components and a high-speed dedicated bus that transmits information to and from the processor. The latter exists because using a PC for control requires the same inputs and outputs as a basic PC … as well as interfaces to factory-floor devices. Here, typical I/O includes the electric motors and fluid-power devices for actuation, as well as discrete sensors, pushbuttons, signaling lights and mechanical switches for feedback. One key advantage of PCbased controllers is that they provide a ready-made graphical user interface for easier programming and tuning. PC-based controller software includes: • some operating system to manage internal processing and resources—OS instability issues are a thing of the past thanks to proliferating real-time operating systems, professional grades of Windows, and Linux • application software to trigger the target behavior in a machine or process Control programming languages include computer-science offerings (such as C+, Visual Basic, and so on) to more
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application-specific languages—including IEC-61131-recognized Ladder Diagram, Instruction List, Function Block Diagram, Structured Texas and Sequential Function Chart languages. Lastly, there are microcontrollers. These are individual integrated chips (ICs) that manufacturers usually design on printed circuit boards with I/O for feedback and driving connected motors. While these controllers are relatively inexpensive and have the advantage of giving designers chiplevel access to their systems, the drawback is that some require good programming skills to configure and implement.
PROGRAMMABLE
logic controllers (or PLCs) are specialized microprocessor-based controllers to command specific machine or process tasks. They work in automation and manufacturing to control assembly lines and factory-floor machinery as well as mechanical, electrical and electronic equipment in industrial applications. Design engineers have used PLCs for automation since the 1960s, when they began to replace physical relays setup through ladder logic. Basic PLC hardware includes the processor, I/O modules (to handle inputs to the processor and outputs to controlled devices), and a user interface. The latter can be anything from a simple keypad or a touchscreen to an Ethernet connection to a PC for more complex programming. No matter its form, users program the PLC through the user interface. I/O modules bring input signals to the PLC’s CPU and output control signals to devices such as electric motors, sensors, and fluid-power valves and actuators. One key metric of PLC performance is scan time—the time the PLC takes to run through the program to collect data and update outputs. Scan time usually takes only a few milliseconds, but can take much longer if the program is long or the processor is slow. Faster scan times accommodate processes with more real-time demands. In contrast, fast scans aren’t necessary for traditional applications that run more slowly. New breed of complete control: PACs PACs (programmable automation controllers) are similar to PLCs but include more hardware and software to satisfy a broader range of industrial functions. When manufacturers first introduced them in the early 2000s, their main strength was their ability to execute more complex automation. Today, their key advantage is that they help integrate other industrial tasks that surround motion setups in facilities. As a superset of PLCs, PACs excel where machines need multiple channels of communication, high data traffic and coordination with intelligent sub-systems. PACs work especially well here thanks to better real-time control of highend automation tasks. More specifically, PACs help integrate management monitoring, HMI inputs and outputs, process control and analog I/O, and business enterprise-level functions. In fact, as high-performance PAC microprocessors are ever more affordable, PACs are an increasingly common alternative for 50
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Remote monitor and control as close as your phone or tablet! You can’t always be on the factory floor or in the office, and with an IDEC HMI you don’t need to be. These 8.4” and 12.1” touchscreens not only offer easy, remote access, but also feature built-in multimedia capabilities with video and audio interfaces. Just connect a video camera and speakers to your IDEC HMI, then from anywhere in the world, you can use your PC, tablet or smart phone to remotely:
Troubleshoot, test or do maintenance View real-time video of machine conditions Capture and playback events leading to a fault Make changes to enhance safety and security Play an operation manual as a movie
Check out the family of HMIs at: http://HMI.IDEC.com www.IDEC.com/USA 800-262-IDEC
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Solve motion control problems with multiplechoice answers.
Controllers
complex control architectures. Though it’s true that most high-performance PLCs can host additional intelligent processors in their backplanes (such as Ethernet modules with multiple ports for expanded data and communications), such setups are usually more expensive that using PACs. That’s because vendors’ proprietary backplanes and internal operating systems are both additional overhead.
SERVO
Pick three. Whether you’re retrofitting an old machine or building a new one, Delta motion controllers give you precise closed-loop control for critical industrial applications. Our field-proven RMCTools software makes it easy to optimize hydraulic and electric servo operations for longer machine life, and interfacing to PCs and PLCs is a snap. So you see, it’s easy to choose the right box every time. Just call 1-360-254-8688 or choose your controller at deltamotion.com RMC75 and RMC150 Motion Controllers
1-2 Axis
Multi-axis
controllers are the core component of servo system—in turn made of controller, motor and feedback. Servo systems run closed-loop to outperform openloop systems. Namely, they improve transient response times and reduce steady state errors Manufacturers make more tools than ever to simplify motion- and system sensitivity to controller setup. Shown here is Elmo Motion Control’s different load parameters. Remote Practice System (RPS), online software that lets users Servo-controller configure, tune and run advanced motion control for Elmo circuitry usually includes servo controllers. a motion controller that generates motion profiles for the motor to follow, and an interface with a motor drive that supplies power to the motor based on controller commands. These perform two tasks: They make a machine’s axes track a commanded input, and they improve system disturbance rejection (often through a PID loop). Refer to the previous section on motion controllers for more on PID loops. There are a few important factors to consider when selecting a servo controller for an application. The first task it to determine which type of motor the system will control. Is the servomotor an ac or dc motor? If it’s dc, is it brushless or brushed? This will help determine the kind of commutation the motor needs and if the controller can accommodate it. How many axes of motion does the application have? Is it a single axis of control or are there multiple axes? Some servo controllers control simple single-axis applications as well as more complex motion, such as multi-axis robotic workcells. Next, how many channels of I/O does the machine need? Are special input types needed beyond inputs for feedback signals such as speed and position? Be sure that the controller can accommodate all necessary feedback devices, whether they’re signals from encoders, resolvers, SSIs or Hall sensors. One factor designers sometimes overlook is controller setup. Is the controller easy to setup and program? Is programming done with a keypad or does the controller let designers program it from a computer screen? Also consider the available communication links. Are there basic RS232 or RS485 links? Does the controller include bus interfaces for common networks such as CAN, DeviceNet, Sercos or Ethernet? Answering these questions helps identify servo controllers that are most suitable for a given application. 52
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Extraordinary Motion Control, Ordinary Hardware The semiconductor industry’s go-to controller, Power PMAC, now runs on standard industrial PCs. Motion Machine PC delivers the motion control capabilities you need on hardware you already use.
Key Features
This new controller consists of a Linux- or Windows-based PC running a motion kernel that offers all of the intelligence and capabilities of our latest Power PMAC controller.
• Supports CoE protocols for I/O and DS402 drives
With its built-in motion programs, software PLCs and I/O support, Motion Machine PC offers complete machine logic control over EtherCAT, MACRO or both networks together.
• Controls up to 256 axes of motion
• Runs on Linux or Windows • Eliminates costly specialized hardware • Allows programming in PMAC script or C • Seamlessly migrates existing software from Power PMAC controllers
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Conveyors
Basics of
CONVEYORS MOTION SYSTEMS HANDBOOK
AUTOMATED
tracks that move bulk material or discrete products from one area to another: Where would modern engineering be without conveyors? In fact, nearly all products move on a conveyor during manufacture. No wonder that conveyors come in all shapes and sizes, from belt widths of less than 2 in. (for moving extremely small parts) to several feet wide for transferring bulk substances.
Selecting a conveyor: Factors to consider To select a conveyor, first answer these questions about the application. 1. What types of product is the application moving? Conveyors for material handling of bulk product are more rugged than those for moving discrete product. In contrast, the latter often requires conveyors that can advance product with more precision. 2. How does surrounding equipment interact with the product riding on the conveyor? Conveyor Class 1 includes materialhandling uses in which the conveyor serves as an artery to transport bulk or discrete product in a steady stream (with little interaction along the way). Class 2 includes conveyors that act as bridges to take product from one location or machine to another. Class 3 includes conveyors that take materials into or out of machines or stations. Class 4 conveyors run right through machinery without break. The first two classes generally prioritize ruggedness or throughput. The last two classes need positioning and (in many cases) custom workpiece pucks to steady product while machines perform work on the product pieces. 3. What is the maximum weight of the product being moved? 4. Does the conveyor need to operate at a certain speed? 5. Does the application require the conveyor system to have inclines, declines or curves? Look for conveyor features that secure or enclose material or product onto the conveyor. 6. Will moisture be present in the application? Does the application need to be sanitary? Look for rugged or washdown-ready conveyors with open frames.
VersaMove non-synchronous workpiece-pallet conveyors from mk Technology Group boost process flexibility to improve manufacturing productivity and product quality.
Metal-free conveyors have Delrin bedplates (instead of the traditional steel bedplate) under sections where metal-scanning equipment checks product—usually food—for metal shavings. Pivot conveyors mount to a pivot base to swing out of the way when workers need to walk through the line. Servodrives accurately start and stop belt conveyors to provide precise part location. They also let engineers control acceleration and deceleration, so are most suitable for conveyors used in assembly operations. Single-drive, multi-belt conveyors serve two or more lanes of product for the sake of efficiency. Here, two or more conveyors run off a single gearmotor on a common drive shaft or coupled shafts. Timing-belt conveyors use toothed belts that engage synchronous drive pulleys while serving as the conveyor surface. These provide accurate part positioning. Vacuum conveyors work with a perforated belt that draws air through grooves in the conveyor bedplate to hold light or flimsy parts on inclines or during especially fast transport.
Photo courtesy of Dorner.
Conveyor choices for discrete-product transport Most conveyors in light- to medium-duty discrete-transport applications use belt that’s wrapped around two or more pulleys. A motor powers the pulleys that in turn engage the conveyor belt. Styles and materials abound to meet specific applications. Some belts are low friction, so product can slide a bit for accumulation. In contrast, high-friction belts have more grip to better hold products to the belt. Engineers can design such conveyors to meet exact application specifications. Here are some options:
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Couplings
MOTION SYSTEMS HANDBOOK
This is an EKM elastomer coupling from GAM. It’s suitable for motioncontrol applications that transmit torques from 2 to 1,000 Nm.
COUPLINGS
for motion systems
COUPLINGS
are simple but precisely engineered devices that connect two shafts together, usually on rotating equipment such as motors. This connection transmits torque, velocity and angular positioning. There are two basic types of couplings: torsionally rigid and flexible. Flexible couplings compensate for misalignments, while rigid designs handle drive components already in alignment. Myriad types of couplings fall under these two categories. Rigid couplings include sleeve-style and clamped, or compression style, and require precise alignment. Flexible couplings include bellows, jaw, Oldham, disc and beam styles.
Rigid couplings Rigid couplings are torsionally stiff and used when shafts are already in alignment. These couplings have two drawbacks: They fail if parallel shaft misalignment exceeds one thousandth of an inch, and they are susceptible to vibration and cannot run at high speeds. Sleeve-style rigid couplings are suitable for light- to medium-duty applications. The one-piece sleeve—a tube with an inner diameter the same as the shafts—connects to the shafts with set-screws. They are easy to use and offer high torque capacity, stiffness and zero backlash. Clamped, or compression style, couplings come in two parts that completely wrap around the shaft. Like most
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Bellows couplings (such as this one from R&W) connect drivetrains in applications that require precision control even where shaft misalignment is present. The couplings are flexible with zero backlash.
coupling designs, this protects the shaft from damage and provides high torsional strength. Because of their two-piece design, designers can easily remove these couplings for maintenance. Flexible couplings Flexible couplings are suitable for applications with slight shaft misalignment. They accommodate misalignment but still transmit torque. There are three types of misalignment: lateral, axial and angular; skewed misalignment is not a separate type, but a combination of lateral and angular misalignment. The greater the misalignment, the less efficient the motor in generating speed and torque. Misalignment also adds premature wear including broken shafts, failed bearings and excessive vibration. Some engineers see flexible couplings as the most compliant components in mechanical motion systems. This attribute makes torsional stiffness critical in maintaining positional control. Many designs need a shaft to start and stop multiple times per second, a dynamic requirement that necessitates a torsionally stiff coupling to reduce settling time between cycles. Regardless, flexible couplings frequently win out because of their torque capacity. Flexible couplings are naturally better for vibration damping, which is needed just as frequently in continuous motion applications as in cyclic duty applications.
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Simplify assembly Add value to your design by having the "best possible solution" Make your design standout by not using "off the shelf" parts Multitude of material choices Full staff of Application Engineers for design assistance
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Couplings
MOTION SYSTEMS HANDBOOK
Motion considerations for coupling selection Types of motion differ in applications as well. For instance, in manufacturing lines, motion may be either continuous or start and stop. With the latter, couplings can help dampen all-toocommon vibration, diminish the settling time of the system and improve throughput. In contrast, continuous-motion applications give greater weight to torsional strength over damping capabilities. Motion applications that require precise motion control, such as packaging, scanning and inspection, call for zero-backlash couplings. Bellows couplings Bellows couplings are common in motion control applications that require precision control and misalignment compensation. There are three components: a metal bellows and two hubs. One hub connects to the driving element. The other hub connects to the driven element. The bellows connects the two hubs. The bellows element of this coupling compensates for all three types of misalignment. Because of this design, metal bellows couplings are suitable for most applications. If your application requires precision, then it is important to understand the performance factors that are critical for selecting the best bellows coupling for the task. Designers most commonly choose couplings for their rated torque. Coupling rated torque should be at least 1.5 times the rated torque of the driving element. Tip: Account for acceleration torque when designing dynamic machinery. Moment of inertia has significant impacts on a coupling when the speed of the driven axis changes. Torsional rigidity and resonance frequency are other factors to consider when selecting a coupling. Some notes on backlash and stiffness To review, backlash is a mechanical clearance or (more informally) the empty space or play between the linkages of a coupling, meshing gear teeth, and a ballscrew nut and its screw. Most couplings are preloaded to eliminate backlash or are inherently backlash free, including bellows couplings. All bellows couplings have different levels of torsional stiffness, and it’s a value often sacrificed for greater lateral flexibility. Bellows couplings tend to have high torsional stiffness and handle less misalignment than others, but impose no heavy reaction loads onto the shafts and bearings as they flex. Typical bellows couplings are made from a stainless-steel tube hydroformed to create deep corrugations that increase flexibility. When joining shafts, bellows couplings absorb slight misalignments from perpendicularity and concentricity tolerances between the mounting surfaces of the two components.
Jaw couplings Jaw couplings have two metal hubs and a spider insert, usually made of elastomer, fitted together to absorb vibration and shock. The elastomer is available in a variety of hardness and temperature ratings, so engineers can choose spiders for specific applications. Because they are not as torsionally stiff as other couplings, they are better suited to constant motion applications. They are available in two types: straight jaw and curved jaw. Because accuracy of torque transmission can be an issue, straight jaw couplings are not used in most servo applications. Curved jaw couplings, on the other hand, reduce deformation on the spider and the effects of centrifugal forces when running at high speed (up to 40,000 rpm or more). Both types compensate for axial motion. If a spider breaks, the driving jaws can still contact the driven jaws directly, maintaining operation, making jaw couplings fails-safe designs.
KM bellows couplings from GAM transmit torques from 0.4 to 1,800 Nm with torsion resistance from 0.05 to 260.00 Nm/arc-min.
This is a flexible disc coupling from R&W. It accommodates misalignment between coupled shafts, which arises manufacturing tolerances, improper installation or system loads.
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Metal Bellows & Assemblies for Motion Control
Transfer Pressure or Temperature into Linear Movement Electrodeposited Bellows
Edge Welded Bellows
Features: • Zero backlash • Thinnest high strength walls • Seamless construction • Minimal side loading • Diameters as small as 0.020 in (5mm) • Highest cycle life • Design assistance for Customization
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MOTION SYSTEMS HANDBOOK
Couplings
This SKB-KP coupling from GAM is one example of a directdrive safety coupling. It has a bellows attachment for high stiffness and single-bolt radial clamping for easy installation. Bores range from 6 to 100 mm.
Oldham couplings Oldham couplings are a versatile and common drive component. Manufacturers often preload Oldham couplings to eliminate backlash. Depending on the disc material, they handle all sorts of misalignment. Engineers often use them as an alternative to straight jaw couplings on general industrial equipment such as pumps, valves, gearboxes and conveyors. They are versatile and offer long life, even in designs that exhibit significant misalignment. Their three-piece design—two hubs and a torquetransmitting center—makes them easy to install and disassemble. Oldham couplings are manufactured in a variety of materials for different applications, for example, a zero backlash versus vibration reduction application. They are best suited when parallel misalignment may be high. And because of their threepiece design, axial motion is limited.
Disc couplings Disc couplings are a logical choice for servomotor and demanding applications because they can transmit high torque, operate at high or changing speeds and handle misalignments. While a coupling’s torque, misalignment and speed capacities need to be evaluated against system requirements, the disc-pack is the most important aspect of the coupling’s construction; this piece affects all performance aspects of the coupling and the rest of the system. The most common type of disc-pack is made of metal and can be found in different shapes (straight-sided, scalloped edges, square, and so on). In metal disc couplings, double-flex designs compensate for parallel shaft misalignment. The single-flex variety of metal disc coupling is good for angular misalignment but not parallel. This design feature can be quite advantageous if a user needs to suspend a load between two single-flex couplings, because their lateral stiffness can support the weight of the intermediate component. Beam or helical couplings Beam or helical couplings are almost always made of aluminum, but stainless-steel versions are also available for use in corrosive environments and increased torque and stiffness. Two designs exist under this style—single and multiple beams. Single beams are best suited to low-torque applications where no parallel misalignment is present, while multiple-beam designs are stiffer, for higher maximum torque capabilities. No matter the material or subtype, the one-piece design of beam couplings makes them easy to maintain. The couplings are also zero backlash with spiral cuts to transmit torque over spaces with significant angular, parallel or axial misalignment. One caveat: Parallel motion can cause stress and possible failure of single-beam designs because such applications force couplings to bend in two directions.
Bellows couplings are torsionally stiff, operate with zero-backlash and have low inertia, making them ideal for precision systems that require accuracy and repeatability. Balanced designs reduce vibration to work at speeds to 10,000 rpm. Photo courtesy of Ruland
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THE PERfECTIONIsT FOR DYNAMIC PRECISION APPLICATIONS: OUR METAL BELLOWS COUPLING.
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THE COUPLING.
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MOTION SYSTEMS HANDBOOK
Couplings
Users can combine clamp or set screw hubs with inch, metric, keyed and un-keyed bores with zero-backlash jaw couplings from Ruland. Spiders are available in three durometers providing a range of dampening, torsional stiffness and misalignment capabilities. The dampening characteristics of these jaw couplings reduce settling time and shock loads.
Special couplings Most disc couplings use a metal disc-pack. However, some manufacturers sell composite disc couplings with shaped disc packs constructed of a composite material rather than metal. This composite material provides an alternative to metal disc couplings. Advantages include absorbing shock and vibration, misalignment capacity, electrical isolation and elimination of fatigue and fretting. Metal disc couplings may be less expensive initially, but overall cost of composite disc couplings is usually lower because they are maintenance free and rated for long life. One critical job of a flexible disc coupling is to accommodate misalignment from manufacturing tolerances, improper installation or imbalanced loads on the system. Engineers should regularly examine lateral, angular and axial misalignment between shafts to see if the coupling selected is up to the task. Tip: Know a coupling’s misalignment rating as well as the stiffness rating. Stiffer couplings tend to spur higher reaction loads to misalignment … and that in turn transmits to connected components. In this way, reaction loads shorten machine life. To limit these reaction loads, the Zero-Max composite disc couplings are less radially stiff than metal disc couplings. Therefore, they transmit lower reaction loads on the coupled equipment, increasing the life of connected components. 62
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How much misalignment a system can handle determines the selection between a single-flex (one flexible disc-pack) and a double-flex (two flexible discpack) coupling. While more compact in size than the double-flex variety, a single-flex coupling has lower misalignment capacity and higher reaction loads. A common misconception is that single-flex disc couplings cannot accommodate lateral misalignment. Although this is true for metal disc couplings, the design of the disc-pack used in certain couplings allow them to accommodate limited lateral misalignment. This permits designers to use a single-flex disc coupling in designs that may not have space for a double-flex coupling. Gear couplings Gear couplings are a type of mechanical device designed to transmit torque between two shafts that are not collinear. The coupling typically consists of two flexible joints, one fixed to each shaft. These joints are often connected by a third shaft called the spindle. Most joints consist of a 1:1 gear ratio from an internal-and-external gear pair. The tooth flanks and outer diameter of the external gear are crowned to get angular displacement between the two gears. Mechanically, the gears are equivalent to rotating splines with modified profiles. They are called gears because of www.designworldonline.com
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THE SMOOTH OPERATOR FOR ABSORBING VIBRATION AND SHOCK: OUR SERVOMAX速 ELASTOMER JAW COUPLINGS.
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THE COUPLING.
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Couplings
MOTION SYSTEMS HANDBOOK
the relatively large size of the teeth. Gear couplings are generally limited to angular misalignments of 4 to 5°. Gear couplings are either flanged-sleeve or continuous-sleeve. Flanged gear couplings consist of short sleeves surrounded by a perpendicular flange. A sleeve on each shaft helps the two flanges line up face to face. A series of screws or bolts in the flanges hold them together. Continuous-sleeve gear couplings use shaft ends coupled together and abutted against each other, which are then enveloped by a sleeve. Often made of metal, these sleeves can also be made of nylon. Single-joint gear couplings connect two nominally coaxial shafts. In this application, the device is called a gear-type flexible, or flexible coupling. The single joint allows for minor misalignments such as installation errors and changes in shaft alignment due to operating conditions. These types of gear couplings are generally limited to angular misalignments of 1⁄4 to 1⁄2°.
because they require no holes, so designs have a hermetically sealed feedthrough. Magnetic couplings don’t wear because none of the coupling parts come in contact with one another. What’s more, the use of permanent magnets eliminates the need for an external power source. Magnetic couplings also have a built-in safety feature to shift to the next position and keep going in the event of a coupling overload. Magnetic couplings have some limitations. Most only transmit light torque that starts gradually. Many only work in applications with low driven-side rotational inertia. Magnetic couplings are also rather large in diameter, considering their relatively light torque load. The couplings also have moderate radial loads on support bearings.
Magnetic couplings Magnetic couplings transfer torque through walls and thin barriers by way of a magnetic connection. This makes them suitable for fluid-pumping applications
Disc couplings are zero-backlash and are torsionally stiff, so are suitable for precision machinery that needs rapid movements over short increments. Single-disc couplings are best suited for compact installations, while doubledisc couplings (pictured) offer increased misalignment capabilities. Photo courtesy of Ruland
Ruland Oldham couplings are zero-backlash and accommodate parallel misalignment with low bearing loads. They are a three-piece coupling that can easily combine clamp or set screw hubs with inch, metric, keyed and un-keyed bores. The center disk is available in acetal for high torsional stiffness, nylon for shock absorption and PEEK for high temperature.
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Single Source
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DESIGN.
Widest range of motion control couplings including over 12,000 standard parts to simplify the design process and ensure the coupling you need is available from stock.
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coupling Hub ruland.com is your source for product specifications, cad models, technical articles, installation videos, live inventory, and application support.
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Drives–AC
Basics of MOTION SYSTEMS HANDBOOK
AC DRIVES
Safety functions are key features on newer servo drives. For example, ACOPOSmulti servo drives from B&R Industrial Automation include a Safely Limited Acceleration (SLA) function that monitors axis acceleration or deceleration. If those exceed system limits, the SafeMOTION module transmits an error code.
AT
This U1000 Industrial Matrix drive from Yaskawa has nine bi-directional switches in a matrix to convert a threephase ac input voltage directly into three-phase ac output.
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its most basic level, a motor drive controls the speed of a motor. Some manufacturers refer to a controller and motor together as a drive system. However, from the electrical side of things, the drive is often specifically the electrical components that make up the variable frequency inverter itself. So drives are the interface between the control signals and the motor and include power electronic devices such as SCRs (silicon controlled rectifiers), transistors and thyristors. Matching the correct drive to the type of motor in an application is critical for getting the best fit. There are a wide range of drives available depending on the needs of the specific application and motor type. In general though, drive types typically fall into two categories: dc and ac. A basic dc drive is similar in operation to an ac drive in that the drive controls the speed of the motor. For dc motor control, a common method is a thyristor-based control circuit. These circuits consist of a thyristor bridge that rectifies ac into dc for the motor armature. Varying the voltage to the armature controls the motor’s speed. 8 • 2015
Ac drives An ac drive controls an ac motor, such as induction motors and synchronous motors. The ac drive converts ac to dc, then uses a range of different switching techniques to generate variable voltage and frequency outputs to drive the motor. An adjustable speed drive is a general term used sometimes interchangeably with variable speed drive or variable frequency drive (VFD). It does so by varying the frequency of the output power. Again, from an electrical perspective, all of these ultimately refer to the frequency converter circuitry. An ac motor’s speed is determined by the number of poles and the frequency. Thus, as frequency is adjusted, the motor’s speed can be controlled as well. A common way to control frequency is by the use of pulse width modulation (PWM). A PWM drive outputs a train of dc pulses to a motor that modulates the pulse width, making it either narrower or wider, developing an ac current waveform to the motor. Another drive feature is regenerative (or regen) braking. A way of stopping a motor’s rotation by www.designworldonline.com
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Nobody enjoys a larger installed base of inverters, servos and robots around the world than Yaskawa. Our experienced engineers, proven technology and unsurpassed quality can all be combined to give proven solutions in which you can have total confidence. In a recent internal study of 100,000 servo motors shipped, Yaskawa found that only 7 were returned for warranty repair. To put that in perspective, a typical out-of-box failure rate goal for manufacturers of brushless servo motors is 0.5% (or 500 failures per 100,000 motors shipped). No matter how you add it up, nobody equals Yaskawa. Call us today.
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Drives–AC
MOTION SYSTEMS HANDBOOK
using the same solid-state components that control the motor’s voltage, this channels energy back into the ac mains or a braking resistor. Advantages of regen drives include higher efficiency and the ability to stop a motor faster than it would normally coast down. VFDs VFDs operate by switching their output devices, which can be transistors, IGBTs (insulated gate bipolar transistors), or thyristors, on and off. VFDs can be either constant voltage or constant current. Constant voltage types are the most common type of VFD. It uses PWM to control both the frequency and the voltage applied to the motor. Why use VFDs? They are a powerful way to control the speed of ac induction motors and are fairly simple and easy to use. Among the benefits of using a VFD for motor speed control is the actual energy savings. Controlling the amount of current drawn by the motor can decrease energy costs
because the motor will not run at full load all of the time. Motor efficiency has become a top design priority. For instance, single-phase induction machines (specifically, permanent split-capacitor motors) and universal motors, widely used in industrial washers, are managed with simple voltagecontrol techniques. Contrast this with high-end, highperformance machines where three-phase motors are more common and which use VFDs. In the same way, an OEM using a universal motor with simple triac control may now find that a three-phase VFD control will provide better energy efficiency, while OEMs using three-phase/VFD configurations may make the move to technologies like brushless dc motors. Another advantage of VFDs is seen on motor start-up. Without a VFD, an induction motor on startup has to handle a high initial in-rush current. As the motor speeds up and approaches a constant speed, the current levels off from the peak in-rush values.
This decentralized 100X drive from Vacon has a power range to 37 kW (or 50 hp) and is approved for public networks.
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motioncontroltips.com
So with a VFD, the motor’s input starts off with low voltage and a low frequency, avoiding the problem of high in-rush currents. Another benefit of using a VFD for motor speed control is the reduction of mechanical wear. Eliminating in-rush currents upon startup gets rid of excessive torque. Plus, mechanical controls such as throttles, valves, dampers and louvers can be completley removed, which reduces maintenance costs. One caveat: VFDs can introduce harmonic distortion, which degrades power quality and machine performance. However, VFD manufacturers have been developing solutions that mostly eliminate this problem. Selecting an ac drive There are several key factors to consider when selecting an ac drive. Consider the elements of the power supply: input voltage; number of phases (three or one); grounded or not grounded; and input frequency (60 or 50 Hz). Also consider the drive enclosure (NEMA 1, NEMA 12 ventilated); motor cable length and shielding; and motor cable type (fixed or flexing in machine operation). Next, look at the machine’s motor. What are the motor’s voltage, current rating and horsepower? These can be found on the motor’s nameplate. Also determine what speed control the application
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needs—servo, closed-loop vector, open-loop vector or volts/Hz. Another consideration is feedback. For closed-loop systems, it’s often HTL/TTL, resolver, sin/cos 1 Vpp or multiturn absolutevalue EnDat feedback. Next, look at the interface needs of the application. Almost all drives require digital control, analog control or communications. These dictate the specification of operator-machine interactions and the drives that can support such functions.
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Some ac drives are easy to setup and operate. With a power rating ranging from 1 ⁄6 to 40 hp at 480 Vac, Sinamics V20 drives come in five frame sizes and suit material handling, conveyor, pump, fan and compressor applications. Photo courtesy of Siemens.
Key factors include: number of digital inputs and outputs; I/O signals (and types including 0 to 10 Vdc, 4 to 20 mA) communications (PROFIBUS-DP, PROFINET, Ethernet/IP) interface ports (RS232, RS485), PLC and HMI
Next, consider any safety features that the drive may need. These can range from basic features, such as brake control and stops, to more advanced capabilities, such as acceleration monitoring, speed monitoring and speed limits. Lastly, don’t overlook environmental factors such as temperature and humidity, as well as dust and pollutants.
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MOTION SYSTEMS HANDBOOK
Drives–DC
Modern dc motor drives can range from simple to complex. This brushless dc motor and drive package from Oriental Motor, for example, can incorporate a built-in controller and is capable of handling speed, position and torque control with a speed control range of 2 to 4,000 rev/min.
Basics of
DC DRIVES A
dc drive controls a dc motor. A basic dc drive is similar in operation to a typical ac drive in that it controls the speed of the motor. A drive supplies voltage to the motor to operate at a desired speed. The motor draws current from the power source in proportion to the torque (or load) applied to the motor shaft. A controller lets operators start, stop and change the direction and speed of a motor. A typical drive converts a three-phase ac voltage to an adjustable dc voltage, which is then applied to a dc motor armature. The dc motor converts power from the adjustable dc voltage source into rotating mechanical force. The rotation and direction of the motor shaft are proportional to the magnitude and polarity of the dc voltage applied to the motor. For dc motor control, one of the most common methods is a thyristor-based control circuit. These circuits consist of a thyristor bridge that rectifies ac into dc for the motor armature. And varying the voltage to the armature controls the motor’s speed. There are two basic types of dc motors; brushed and brushless (or BLDC). Each type of motor has different drive techniques. For instance, to drive a brushed dc motor you can apply a voltage or pulse width modulated (PWM) voltage and the motor will start running and increasing in speed (while reducing torque) until the torque and speed match the load. BLDC motors are a bit more complicated to drive compared with brushed dc motors. For example, a typical brushless motor will have three sets of windings connected in a star or “Y” or “Delta” configuration. The motor controller energizes each of the windings to turn the motor. To control the windings properly you need to know where the motor is so that you can energize the correct winding or windings. The controller can be a sensorless type, meaning that it relies on detecting the back EMF (electromotive force) of the windings to detect position and provide the sequence information for the controller. A sensorless drive allows the use of a motor without Hall effect sensors on the motor, making the motor less expensive and requiring fewer connections.
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For most BLDC motor controllers, the control signal is a speed control signal, such as a PWM input signal rather than an analog signal. As for PWM drives, two common types are sinusoidal (or sine) and trapezoidal. A sine PWM drive increases and decreases the current to each winding to follow a sinusoidal curve to smooth the drive power and produce a smoother motor torque. Simple on/off control of the windings will tend to produce an uneven torque through the rotation of the motor and also tends to generate more audible noise due to the uneven torque. A trapezoidal PWM drive is similar to a sine PWM drive and will increase the current to each winding in a straight line based on the motor position and then decrease it in a straight line while increasing the current to the next winding. To use a trapezoidal or sinusoidal PWM drive, motor controllers need to know where the motor rotor is to a higher degree of accuracy than a simple Hall effect switch position provides. They do this by monitoring the motor velocity and predicting the position with time. While this won’t be perfectly accurate, it is considerably better than simple on/off drive. Also, a trapezoidal drive will be quieter and smoother than a simple on/off drive, but not as smooth or quiet as a sinusoidal drive. In actual practice, dc motors and drives are still among the most common types of motors and drives in many industrial and consumer applications, including automotive designs and consumer appliances. The fact is that tried and true motors, like brushed dc motors, are capable of high peak torques. Also, the fact that they have a linear torque-speed relationship makes control easier, meaning they can be controlled using simple speed controllers and often cost less than other motor and control options. Manufacturers are also finding that using existing dc motors and upgrading the dc drives is often a good option. This is because dc motors are usually well built and can offer many years of reliable service.
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Encoders
An overview of
ENCODERS MOTION SYSTEMS HANDBOOK
IN
motion control applications, an encoder is a type of transducer that measures position. This position information provides a point of orientation for controlling a system. There are several common ways to classify all encoders. One is the method of sensing (capacitive, optical or magnetic sensing) and another is whether the position output from the encoder is absolute or incremental. Another parameter is how much force the encoder can handle on the shaft, so there are light-, medium- and heavy-duty encoders. But perhaps the most common way encoders are classified is as either rotary or linear, depending on the type of position they measure, with rotary encoders measuring rotary position and linear encoders measuring linear position. What are linear encoders? Linear encoders typically are made up of a scale of some sort, such as a coded strip, and some type of sensing head. Reading the space between the scale coding determines position. The resolution of linear encoders is measured in pulses per inch or millimeter. The scale typically has a fixed resolution with embedded markings, which are read by the sensing head. For example, a linear encoder with a 100 points per inch resolution would read 100 marks for every inch of movement. In contrast, a rotary encoder measures resolution in pulses per revolution. Similar to linear encoders, a typical rotary encoder contains an internal coded disk and a sensing head. Think of a linear encoder as a type of tape measure, while a rotary encoder is more like a measuring wheel. For instance, a rotary encoder with a 100-pointper-revolution resolution would have 100 marks on its coded disk. Scale-based linear encoders can use different types of sensing technology. The most common type are optical encoders, but they can also be magnetic, capacitive and even inductive.
These absolute encoders (KĂźbler Sendix EtherCAT encoders) have the shortest bus cycle time for EtherCAT currently available, with a position update every 62.5 Îźsec.
Absolute encoders Encoders can also be either absolute or incremental. Absolute encoders have a unique code for each shaft position. In other words, every position of an absolute encoder is distinctive. The absolute encoder interprets a system of coded tracks to create position information where no two positions are identical. Another feature is that absolute encoders do not lose position whenever power is switched off. Since each position is distinctive, the verification of true position is available as soon as power is switched on. It is not important to initialize the system by going back to a home base for a reference. Furthermore, absolute encoders can be either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion control applications where position verification is needed within a single turn of the encoder shaft. Multi-turn encoders, on the other hand, are better for applications that involve complex or lengthy positioning requirements. Absolute encoders have a number of advantages. First is the non-volatility of memory. True position is not lost if power is lost or the system moves while power is switched off. A continuous reading of position is not needed. This is specifically useful in applications where position verification is key, such as satellitetracking antennas.
This Three Point Flex (SE) configuration of an Encoder Products Company Model 25T has the largest thru-bore available in a 2.5-in. encoder, mounting directly on shafts to 1.125 in. Resolutions to 10,000 CPR let the encoder work on fast motors.
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Encoders
MOTION SYSTEMS HANDBOOK
Safety is another benefit. In some applications where a loss of position could lead to operator injury or machine damage, an absolute encoder automatically provides position verification when the power is switched on. Absolute encoders also have good immunity to electrical noise. The device determines position by frequently reading a coded signal. Stray pulses from electrical noise will not build up; accurate position is presented again on the next reading. Incremental encoders Incremental encoders, like absolute encoders, are used to track motion as well as to determine speed and position. Incremental encoders generally supply square-wave signals in two channels, A and B, which are offset, or out-of-phase, by 90°. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position. For the encoder to provide any useful position information, the position of the encoder has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function. On start-up, the encoder will home in on a known, fixed position, which serves as a fixed reference point going forward. This fixed position can be either fixed by a magnetic point or strip, or mechanically by a limit switch. One traditional limitation of an incremental encoder is that because the number of pulses counted is stored in an external or buffer counter, the data can be lost if there is an interruption of power. For instance, if a machine with an encoder is turned off, the encoder will not know its position when switched on again. The encoder has to perform a homing routine to know its exact position, forcing the motor to move until a home limit switch is activated. Then, a counter or buffer will be zeroed and the system will determine where it is relative to fixed positional points. One way around this issue is to use a battery backup system. Such a solution ensures that the memory is backed up and can store the count information and provide an absolute count once power is restored.
Encoder performance As position measuring devices, the most critical performance parameter for encoders is their ability to accurately measure position. One of the main factors is that the encoder should have the necessary level of resolution required by the application. However, resolution and accuracy are not the same. For instance, an encoder may have a high resolution but low accuracy, or vice versa. The ideal encoder has both the necessary resolution and a high level of accuracy. Accuracy depends on a number of factors. For starters, there are differences between optical and magnetic technologies. Generally speaking, magnetic encoders are more resistant to shock and vibration than optical encoders. They’re also better able to withstand environmental contaminants such as dust, grease and moisture. As far as electrical interference goes, optical encoders are fairly immune to it since position measurement is not electrical-based but optical. Also, magnetic technologies may be subject to strong magnetic fields that could impact readings. Other components apart from the encoder itself can impact encoder performance. For instance, encoder bandwidth, with respect to command response and control reliability, can be limited by the rigidity of the coupling between the motor shaft and encoder shaft as well as by the natural frequency of the coupling. Encoders are qualified to operate within a specified acceleration range. However, if the application itself or poor mounting cause long lasting resonant vibration, this can limit performance and possibly damage the encoder. Encoder communication protocols The advent of more sophisticated communication protocols means that users have a lot more control over encoder operation. For instance, encoder resolution can be programmed through a USB interface on a computer. Other parameters can be changed on the fly through simple software commands. Data communication options at their most basic include parallel and serial formats. Older parallel formats could be fast, but require more bulky cables. Today, serial protocols are more common and offer more flexibility for users. Some of the most common encoder communication protocols include SSI (Synchronous Serial Interface), BiSS (bidirectional serial/synchronous), ProfiBus, DeviceNet and Ethernet/IP. These protocols are “open,” meaning they are non-proprietary and not tied to a specific manufacturer’s products. Closed or proprietary protocols include Hiperface (High Performance Interface) and EnDat (Encoder Data). Absolute encoders such as the ones here from HEIDENHAIN come in a wide variety of shapes and sizes.
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Encoder solutions for a world in motion Advanced Opto-ASIC circuitry and intelligently designed, robust mechanical packaging combine to ĚĞůŝǀĞƌ ƌĞůŝĂďůĞ͕ ŚŝŐŚͲƉĞƌĨŽƌŵĂŶĐĞ ƌŽƚĂƌLJ ŵŽƟŽŶ deliver reliable, high-performance rotary motion feedback. Made in the USA, and backed by an industry-best three year warranty, fast delivery and global support, Encoder Products Company rotary encoders work when it counts ͶǁŚŝĐŚ ŝƐ Ăůů ƚŚĞ ƟŵĞ͘ —which is all the time.
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Technical gear review: GEOMETRY & GENERAL GEAR DESIGN THE primary function of a gear is to mesh with other gears to transmit altered torque and rotation. In fact, gearing can change the speed, torque and direction of motion from a drive source. When two gears with an unequal number of teeth engage, the mechanical advantage makes their rotational speeds and torques different. In the simplest setups, gears are flat with spur teeth (with edges parallel to the shaft) and the input gear’s shaft is parallel to that of the output.
Worm gear
Spiroid® gear
Spiroid® or Helicon® gear Helicon® gear Hypoid gear
Spiral bevel gear
Spiroid and Helicon brand gearing operate on non-intersecting and non-parallel axes. Compared to traditional right-angle bevel and worm gearing, the gear-centerline offset of Spiroid and Helicon branded gearing allows for more tooth-surface contact and higher contact ratios. This boosts torque capacity and smooths motion transmission. Spiroid gearsets deliver ratios from 3:1 to 300:1 and beyond.
Spur gears mostly roll through meshing, so can be 98% or more efficient per reduction stage. However, there is some sliding between tooth surfaces, and initial tooth-to-tooth contact occurs along the whole tooth width at once, causing small shock loads that induce noise and wear. Sometimes lubrication helps mitigate these issues. In slightly more complex setups, parallel-axis gearsets have helical gears that engage at an angle between 90 and 180° for more tooth contact and higher torque capacity. Helical reducers are suitable for higher-horsepower applications where long-term operational efficiency is more important than initial cost. Helical gear teeth engage gradually over the tooth faces for quieter and smoother operation than spur gearsets. They also tend to have higher load capacities. One caveat: Angled tooth contact generates thrust that the machine frame must resolve. No matter the subtype, most parallel-axis gearsets have gear teeth with tailored involute profiles—customized versions of the rolled trace off a circle with an imaginary string. Here, mating gears have tangent pitch circles for smooth rolling engagement that minimizes slipping. A related value, the pitch point, is where one gear initially contacts its mate’s pitch point. Involute gearsets also have an action path that passes through the pitch point tangent to a base circle. Besides parallel-axis gearsets, there are non-parallel and right-angle gearsets. These have input and output shafts that protrude in different directions to give engineers more mounting and design options. The gear teeth of such gearsets are either bevel (straight, spiral or zerol), worm, hypoid, skew or crossed-axis helical gears. The most common are bevel gearsets with teeth cut on an angular or conical shape. Hypoid gears are similar to spiral-bevel gearsets, but the input and output shaft axes don’t intersect, so it’s easier to integrate supports. In contrast, zerol gearsets have curved teeth that align with the shaft to minimize thrust loads. www.designworldonline.com
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Gears
Technical gear review: MOTION SYSTEMS HANDBOOK
CONSULTATION, CUSTOM GEAR DESIGNS & GEAR ANALYSIS
CUSTOM
gearboxes are increasingly common, mainly because they’re easier than ever to manufacture to specification. That’s not to say that the design work isn’t challenging. However, modern manufacturing lets some suppliers make gearboxes and components to meet specific application requirements. New supplier approaches to giving engineering support, as well as new machine tools, automation and design software, now let OEMs and end users get reasonably priced gearing even in modest volumes. When enlisting help from a consultant or manufacturer, an engineer is more likely to get gearing that mounts properly and performs to specification after reviewing the following and answering as many of these questions as possible: • What’s the input speed and horsepower? • What’s the gearbox target output speed or output torque? This partially defines the required gear ratio. • What are the characteristics of use? How many hours per day will the gearbox run? Will it need to withstand shock and vibration? • How overhung is the load? Is there internal overhung load? Remember that bevel gears usually can’t accommodate multiple supports, as their shafts intersect … so one or more gears often overhang. This load can deflect the shaft which misaligns the gears, in turn degrading tooth contact and life. One potential fix here is straddle bearings on each side of the gear. • Does the machine need a shaft or hollow-bore input ... or a shaft or hollow-bore output? • How will the gearing be oriented? For instance, if specifying a
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Lampin MITRPAK right-angle gearboxes help reduce downtime and parts inventory for higher asset availability.
right-angle worm gearbox, does the machine need the worm over or under the wheel? Will the shafts protrude from the machine horizontally or vertically? • Does the environment necessitate corrosion-resistant paints or stainless-steel housing and shafts? Service factor: The starting point for most gearbox manufacturers is to define a service factor. This adjusts for such concerns as type of input, hours of use per day, and any shock or vibration associated with the application. An application with an irregular shock (a grinding application, for example) needs a higher service factor than one that’s uniformly loaded. Likewise, a gearbox that runs intermittently needs a lower factor than one used 24 hours a day. Class of service: Once the engineer determines the service factor, the next step is to define a class of service. A gearbox paired to a plain ac motor driving an evenly loaded, constant-speed conveyor 20 hours per day may have a service class 2, for example. This information comes from charts from gearbox manufacturers that list classes of service. To use these charts, the design engineer must know input horsepower, application type and target ratio. For instance, suppose that an application needs a 2-hp motor with a 15:1 ratio. To use the chart, find the point where 2 hp and 15:1 ratio intersect. In this case, that indicates a size 726 gearbox. According to one manufacturer’s product-number system, size 726 defines a gearbox that has a 2.62 center distance. Such charts also work in reverse to www.designworldonline.com
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MOTION SYSTEMS HANDBOOK
let engineers confirm the torque or speed of a given gearbox size. Overhung load: After the designer picks a size, the gearbox manufacturer’s catalog or website lists values for the maximum overhung load that is permissible for that sized unit. Tip: If the load in an application exceeds the allowed value, increase the gearbox size to withstand the overhung load. Mounting: At this point, the designer or manufacturer has defined the gearbox size and capability. So, the next step is to pick the mounting. Common mounting configurations abound, and gearbox manufacturers offer myriad options for each unit size. A flanged input with hollow bore for a C-frame motor combined with an output shaft projecting to the left may be the most common mounting, but there are many other choices. Options such as mounting feet for either above or below the body of the gearbox, hollow outputs, and input and output configuration are all possible. All gearbox manufacturers list their mounting options as well as dimensional information in catalogs and websites. Lubricant, seals and motor integration: Once unit size and configuration is complete, a few specifications remain. Most manufacturers can ship gearboxes filled with lubrication. However, most default to shipping units empty to let users fill them on-site. For applications where there is a vertical shaft down, some manufacturers
Pre-engineered gearmotors reduce design time and project risk because the manufacturer ensures that the motor-gearbox combination works well.
recommend a second set of seals. Finally, because many gearboxes eventually mount to a C-frame motor, many manufacturers also offer to integrate a motor onto the gearbox and ship the assembly as a single unit. It’s best to work with consultants and even use custom gear designs if the application needs a unique motorgearbox combination. Some combinations are more efficient. In fact, working with manufacturers to get a pre-engineered gearmotor ensures that the motor-gearbox combination will work and deliver the specifications from calculations and testing performed by the manufacturer. Review the manufacturer’s performance calculations to determine if the chosen gearmotor will cause any issues within the application. Remember that today’s custom and standard gearing aren’t mutually exclusive. Where fully-custom gearboxes aren’t feasible (if quantities aren’t high enough, for example) consider working with manufacturers that sell gearboxes built to order from standard modular subcomponents. Otherwise, for small quantities of true custom gearboxes, look for manufacturers that leverage the latest CAD software, CAM software and machine tools to streamline post-processing work and reduce the cost of one-offs. One final tip: Once the gearmotor has been chosen and installed in the application, perform several test runs in sample environments that replicate typical operating scenarios. If the design exhibits unusually high heat, noise or stress, repeat the gear-selection process or contact the manufacturer.
WEISS North America makes a direct-drive TO220C rotary indexing table that goes into this test bench. The machine inspects precisionturned automotive parts. Thanks to the WEISS gear drive, it has a switching time of just 0.3 sec … for a cycle time that’s 1.9 sec (compared to 2.6 sec from previous designs).
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Gears Common gear options
Technical gear review:
Spur gearsets are simple ...
MOTION SYSTEMS HANDBOOK
GENERAL SPEED REDUCERS, SHAFT-MOUNT SETS & WORM DRIVES GEAR
reducers, called speed reducers, are a component of many mechanical, electrical and hydraulic motors. Essentially, it is a gear or series of gears combined in such a manner as to alter the torque of a motor. Typically, the torque increases in direct proportion to the reduction of rotations per unit of time. Speed reducers come in two varieties: base mounted and shaft mounted. Shaftmounted types come in two versions. One is truly shaft mounted in that the input shaft of the drive motor supports it … with a special coupling to address torque reactions. The other mounts to the machine housing so the input shaft doesn’t support the reducer’s weight or address torque reactions. By the American Gear Manufacturers Association (AGMA) definition, engineers apply the term “speed reducer” to units operating at pinion speeds below 3,600 rpm or pitch-line velocities below 5,000 fpm. (The AGMA is an international group of gear manufacturers, consultants, academics, users and suppliers.) Reducers operating at speeds higher than these are called high-speed units. Manufacturers base catalog ratings and engineering specifications for speed reducers on these AGMA standards. There are as many types of speed reducers as there are gear types. Consider reducers in which the input and output shafts are at different angles. The most common of these are worm-gear reducers. Worm-gear reducers are used in low- to moderate-horsepower applications. They offer low initial cost, high ratios and high output
torque in a small package, along with a higher tolerance for shock loading than helical gear reducers. In a traditional setup, a cylindrical toothed worm engages a disk-shaped wheel gear with teeth on its circumference or face. Most worm gears are cylindrical with teeth of consistent size. Some worm-gear reducers use a double-enveloping tooth geometry— with a pitch diameter that goes from deep into short and back to deep—so more teeth engage. No matter the version, most wheel gears in worm-based reducers sport cupped teeth edges that wrap around the worm shaft during engagement. In many cases, the sliding engagement lowers efficiency but extends life, as worm-gear mating holds a film of lubricant during operation. The ratio of a worm-gear is the number of wheel teeth to the number of threads (starts or leads) on the worm. A few words on gearheads A gearhead is similar to a gear reducer, but a gearhead doesn’t just reduce speed. Engineers use them wherever an application calls for high torque at low speed. It reduces a load’s reflected mass inertia, which makes accelerating heavy loads easier, enabling designs to run off smaller motors. Gearheads come in a variety of styles from basic spur gearheads to more complex planetary gearheads and harmonic type gearheads, each with their own characteristics and suitable applications. One caveat: In some applications, gearhead backlash is an issue. In this case, consider using a gearhead with low or zero backlash.
Pitch circle
Reaction force ... but helical gearsets are more efficient. Cross-axis sets are another option.
Planetary gearsets are compact and run to 10,000 rpm. Here, a lightweight Schaeffler differential for a hybrid vehicle has an axial spline to boost efficiency. Zerol bevel gearsets are a special veriation of straight right-angle bevel sets.
Worm gearsets are rugged and don’t let designs backdrive ... which can eliminate the need for brakes. Note there’s some overlap between bevel and worm applications. Case in point: The MS-Graessner DynaGear below is a single-stage bevel gear with a 30:1 ratio.
The ratio of a helical or bevel gearset is simply the number of teeth in the larger gear divided by the number of teeth in the smaller gear. Other gear types, such as planetary gears, have more complex ratio relationships.
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Your Custom Gearbox Solutions are CGI Standard Products.
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Gears
Technical gear review: MOTION SYSTEMS HANDBOOK
STRAIN-WAVE GEARING STRAIN-WAVE
gearing is a special gear design for speed reduction. It uses the metal elasticity (deflection) of a gear to reduce speed. (Strain-wave gearing sets are also known as Harmonic Drives, a registered trademark term of Harmonic Drive Systems Inc.) Benefits of using strain-wave gearing include zero backlash, high torque, compact size and positional accuracy. A strain-wave gearset consists of three components: wave generator, flexspline and circular spline. The wave generator is an assembly of a bearing and a steel disk called a wave generator plug. The outer surface of the wave generator plug has an elliptical shape machined to a precise specification. A specially designed ball bearing is pressed around this bearing plug causing the bearing to conform to the same elliptical shape of the wave generator plug. Designers typically use the wave generator as the input member, usually attached to a servomotor. The flexspline is a thin-walled steel cup. Its geometry lets the walls of the cup be radially compliant but remain torsionally stiff (because the cup has a large diameter). Manufacturers machine the gear teeth into the outer surface near the open end of the cup (the brim). The flexspline is usually the output member of the mechanism. The cup has a rigid boss at one end to provide a rugged mounting surface. The wave generator is inserted inside the flexspline so the bearing is at the same axial location as the flexspline teeth. The flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. Circular spline This conforms the teeth on the outer surface of the flexspline to the elliptical shape. That way, the flexspline effectively has an elliptical gear-pitch diameter on its outer surface. The circular spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the flexspline. The tooth pattern of the Flexspline
This is a progression of flex-spline tooth engagement with circular-spline teeth. The profile of Harmonic Drive gear teeth lets up to 30% of the teeth engage for higher stiffness and torque than gearsets with involute teeth.
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flexspline engages the tooth profile of the circular spline along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse contacts a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement. Roughly 30% of the teeth are always engaged. The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the wavegenerator bearing. The teeth of the flexspline and circular spline engage near the ellipse’s major axis and disengage at the ellipse’s minor axis. Note that the flexspline has two less teeth than the circular spline, so every time the wave generator rotates one revolution, the flexspline and circular spline shift by two teeth: number of flexspline teeth ÷ (number of flexspline teeth number of circular spline teeth)
The tooth engagement motion (kinematics) of the strain wave gear is different than that of planetary or spur gearing. The teeth engage in a manner that lets up to 30% of the teeth (60 for a 100:1 gear ratio) engage at all times. This contrasts with maybe six teeth for a planetary gear, and one or two teeth for a spur gear. In addition, the kinematics enable the gear teeth to engage on both sides of the tooth flank. Backlash is the difference between the tooth space and tooth width, and this difference is zero in strain-wave gearing. As part of the design, the manufacturer preloads the gear teeth of the flexspline against those of the circular spline at the major axis of the ellipse. The preload is such that the stresses are well below the material’s endurance limit. As the gear teeth wear, this elastic radial deformation acts like a stiff spring to compensate for space between the teeth that would otherwise cause an increase in backlash. Strain-wave gearing offers high torque-to-weight and torqueto-volume ratios. The lightweight construction and single-stage gear ratios (of up to 160:1) let engineers use the gears in applications requiring minimum weight or volume. Small motors can exploit the large mechanical advantage of a 160:1 gear ratio to create a compact and low-cost package. Another tooth profile for strain-wave gearing is the S tooth design. This design lets more gear teeth engage to double torsional stiffness, double peak torque ratings and lengthen operational life. The S tooth form uses a series of pure convex and concave circular arcs that match the loci of engagement points dictated by theoretical and CAD analysis. The increased root filet radius makes the S tooth much stronger than an involute curve gear tooth. It resists higher bending (tension) loads while maintaining a safe stress margin.
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New Small Hollow Shaft Servo Actuator Surgical Robots • Medical Imaging • Exoskeleton • Lab Automation
Compact, Hollow Shaft Design The new SHA-20A servo actuator is the smallest in the SHA series. • High Accuracy: Zero-backlash Harmonic Drive® Gear Reducer • High Torque Density: ~ 60 Nm/kg (for 161:1 ratio) • Maximum Torque: 73~120 Nm
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High-Precision Gears
Ideally suited for robotic and machine tool applications, the new CSG-LW and CSF-LW gearheads are 30% lighter than previous
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Gears
Technical gear review: MOTION SYSTEMS HANDBOOK
GEARBOXES, SPECIALTY GEARHEADS & SERVO GEARSETS SERVO
systems are precision-motion setups with feedback and (in most cases) fairly stringent accuracy demands. So for these designs, engineers should pick servogear reducers with good torsional stiffness, reliable output torque and minimal backlash. OEMs tasked with integrating servo systems should look for quiet reducers that easily mount to the motor and require little or (if possible) no maintenance. In fact, a lot of advanced machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have specific labels. Here is a look at some of the most widespread. Gearmotor: This complete motion component is a gear reducer integrated with an ac or dc electric motor. Usually the motor includes the gears on its output (typically in the form of an assembled gearbox) to reduce speed and boost available output torque. Engineers use gearmotors in machines that must move heavy objects. Speed specifications for gearmotors are normal speed and stall-speed torque. Gearbox: This is a contained gear train … a mechanical unit or component consisting of a series of integrated gears. Planetary gears are common in integrated gearboxes. Planetary gears: Particularly common in servo systems, these gearsets consist of one or more outer planet gears that revolve about a central, or sun, gear. Typically, the planet gears mount on a movable arm or carrier that rotates relative to the sun gear. The sets often use an outer ring gear, or annulus, that meshes with the planet gears. The gear ratio of a planetary set requires calculation, because there are several ways they can convert an input rotation to an output rotation. Typically, one of these three gear wheels stays stationary; another is an input that provides power to the system, and the last acts as an output that receives power from the driving motor. The ratio of input rotation to output rotation depends on the number of teeth in each gear and on which component is held stationary. Planetary gearsets offer several advantages over other gearsets. These include high power density, the ability to get large reductions from a small volume, multiple kinematic combinations, pure torsional reactions and coaxial shafting. Another advantage to planetary gearbox arrangements is
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Shown here is a high-speed bevel gearbox for dynamic servo drivetrains. Efficiency reaches 98% and torque reaches 45 to 360 Nm (with emergency-stop torques of 90 to 720 Nm) depending on the version. Photo courtesy of MS-Graessner
power-transmission efficiency. Losses are typically less than 3% per stage, so rather than waste energy on mechanical losses inside the gearbox, these gearboxes transmit a high proportion of the energy for productive motion output. Planetary gearbox arrangements distribute load efficiently, too. Multiple planets share transmitted load between them, which greatly increases torque density. More planets in a system mean greater load ability and higher torque density. This arrangement is also stable due to the even distribution of mass and increased rotational stiffness. Disadvantages include high bearing loads, inaccessibility and design complexity. In servo systems, besides boosting output torque, gearboxes impart another benefit—reducing settling time. Settling time is a problem when motor inertia is low compared to load inertia … an issue that’s the source of constant debate (and regular improvement) in the industry. Gearboxes reduce the reflected inertia at the controls by a factor equal to the gear reduction squared.
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Features & Benefits:
Features & Benefits:
Features & Benefits:
• • • • • •
• • • • • •
• • • • • •
Economical price Sizes from 40 to 155 mm Self-centering design Maintenance-free Versatile mounting Integral steel ring gear
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Low backlash Right angle or inline Shaft, flange, or foot mounting High efficiency Cast iron housings Wide range of ratios
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Features & Benefits:
Features & Benefits:
• • • • • •
• • • • • •
• • • • • •
1 arc minute of backlash 5 output shaft options 10 sizes available Aluminum housing High radial capacity Maintenance-free
NEMA 23 & 34 motor interface Metric 40 & 60 motor interface Standard or hollow shafts Ground worm gears Flexible coupling connection Light weight aluminum
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Zero Backlash Precise Positioning High Repeatability High Stiffness Supports Tilting 10 Sizes
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MOTION SYSTEMS HANDBOOK
Gearmotors
Technical review of
GEARMOTORS GEARMOTORS
—what are they? A gearmotor is a single component that integrates a gear reducer and either an ac or dc electric motor. Thanks to its gearset, a gearmotor can deliver high torque at low horsepower or low speed. Gearmotors are most common in applications that need a lot of force to move heavy objects. Most industrial gearmotors incorporate fixed-speed ac motors. However, some gearmotors use dc motors. These dc gearmotors are common in automotive applications—to electronically adjust side-view mirrors and make automatic seat adjustments, for example. OEM engineers can mix and match motors and gears as needed to best fit application requirements. However, housing design, assembly gearings, gear lubrication, and specific mode of integration of pinion gear and motoroutput shaft all affect gearmotor performance. Motor and gear-reducer combinations abound: For example, right-angle wormgear, planetary and parallel shaft gears can combine with permanent-magnet dc, ac induction or brushless dc motors to form a gearmotor unit. Though it’s possible to combine many different motors and gearsets, not just any one will work for every application, because certain combinations are more efficient and cost-effective than others. Knowing the application and getting an accurate estimation of its required torque and operating speeds is the foundation for successfully integrating a gearmotor into a system.
Common gearmotor types and variations As gearmotors can be built on either ac or dc motors, there are a number of choices for the gear reducer. There are five basic types of gears that can be paired with a motor to form a complete gearmotor: bevel, helical, hypoid, spur and worm.
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Gearmotors are available in a range of configurations, such as these in-line, right angle and hollow shaft models for the food and beverage industry from Brother Gearmotors.
Another way to classify gearmotors is by the physical arrangement of the final complete unit. For instance, there are so-called in-line gearmotors where the gear shaft is parallel with the motor shaft, also called a parallel shaft. These can either be offset from the output shaft or completely in line with it. The other configuration is the right-angle gearmotor, where the output shaft is at a 90° angle to the motor shaft. Benefits Gearmotors give better performance than other motorgear combinations. More importantly, gearmotors simplify design implementation because they save engineers from integrating motors with gears, which in turn reduces engineering costs. If the application requirements are known, engineers can order the right gearmotor from a supplier directly. What’s more, if a gearmotor is sized properly, having the right combination of motor and gearing can prolong operating life and boost overall design efficiency. Another benefit of gearmotors is that they eliminate the need for couplings and potential alignment problems that come with those components. Such problems are common when a design includes the connection of a separate motor and gear reducer—which in turn increases the potential for misalignment and bearing failure, and ultimately reduces useful life.
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Gearmotors
MOTION SYSTEMS HANDBOOK
Gearmotor accessories Gearmotors, like non-geared motors, come in versions with myriad options to satisfy specific application and environmental conditions. For instance, common options include shaft type, bearing choices, seals and lubrication, and mounting and housing options. Housing types also abound. Housing materials include stainless steel or aluminum. Some common options for aluminum housings are billet aluminum, sand-cast aluminum, permanent mold aluminum casting and die-cast aluminum. Other options are die-cast zinc and magnesium injection molding. Despite being a fairly well-established technology, advances in gearmotor technology continue. Some of these include the use of new specialty materials, coatings and bearings, and improved gear-tooth designs that reduce noise, increase strength and extend life, all of which boost performance and reduce overall design size. Gearmotor performance curve One tool to simplify selection of a gearmotor is the performance curve. These plots relate torque and speed and sometimes efficiency. To help designers select a pre-engineered gearmotor, manufacturers do much of the heavy lifting to ensure that their motor-gearing combinations will work together. Because manufacturers make performance calculations and do testing in advance, gearmotor failures from miscalculations or improper component matching are rare. The speed and torque that a given application needs are critical factors in gearmotor selection. Use speed and torque measurements to identify manufacturer’s performance curves that match the application needs. Gearmotor curves unify information (including that about speed, torque and efficiency) to summarize the performance of the motor-gearset combination. If an OEM or end user buys a complete gearmotor assembly from a manufacturer, the latter supplies its performance curve. Note: After identifying gearmotors with performance curves that appear to meet the application needs, review all calculations and use the values to
determine which of the chosen gearmotors will cause problems once installed. Remember to consider thermal characteristics, full-load gearbox torque, gearbox input speed, gearbox yield strength and intermittentduty effects. Once the speed and torque requirements are identified, that may not be the end of the story. That is, proper selection of a gearmotor is not all science. The fact is that this is merely a beginning, a starting point. This is because often times the manufacturer’s data may not be derived from empirical testing, so there will be some variation between calculated requirements and the actual application. This is why it’s important to test a sample load under the actual operating conditions of the application. In summary, follow these steps to selecting the right gearmotor: • Start by determing the torque. • Also adjust for duty cycle. Determine if the cycle is intermittent or continuous and adjust accordingly. • Factor in the type of load (for example, overhung loads on the output shaft) and the type of drive mechanism (chain and sprocket, gear, belt, and so on). • Consider any shock loads and their potential impact on the output. • Based on application and output requirements, determine the best type of motor to use.
Newer gearmotor designs, such as this 730 Series PowerSTAR right-angle Hypoid Gearmotor from Bison Gear, are equipped with a super-reduction hypoid (SRH) gearset, allowing for greater energy efficiency, cooler operation and increased torque.
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Grippers The gripper on this arm is from Robotiq. It’s a universal adaptive gripper with two fingers.
MOTION SYSTEMS HANDBOOK
Summary of
GRIPPERS & END EFFECTORS GRIPPERS
and end effectors are the devices on robot peripheries that interact with materials or discrete objects. Usually they connect to the end of a robot to execute material handling, welding, cutting or scanning tasks. The robot can take the form of an SCARA arm, delta robot, or Cartesian robot or gantry. Manufacturers often sell these with hardware that makes it easy to attach end effectors of various types. Recently, robotic grippers have made headlines thanks to breathtaking new prosthetics, smarter materialhandling machines and fairly brisk robotic innovation. That said, the most common use of grippers is still in industrial applications—on pick-and-place robotic setups to service assembly lines, conveyors or other automated machines. Grippers use pneumatic, electromechanic, and (least common) hydraulics for power. Traditional pneumatic grippers still dominate industrial-assembly and robotic applications. That’s because pneumatic grippers are reliable, come in myriad sizes and grip quickly. In fact, pneumatic grippers were the first gripper design and still offer the advantage of simplicity. The majority are two-finger types; versions with angled fingers swing on a pivot and close like a lobster claw. Such parallel jaws are simple because they exert constant force that can reach 12,000 lbf or better. In contrast, many newer three-finger grippers work off brushless motors and embedded servocontrollers. Sometimes the fingers have multiple degrees of freedom, so (unlike two-finger types) aren’t limited to handling only one kind of part or geometry. In fact, electromechanical grippers use ballscrews, belts, or racks and pinions to connect to the jaw or finger that is touching or gripping the part. They are driven by a motor for tasks like loading
and unloading machines to assembly lines. They can grip small parts, like an IC chip, up to very large pieces, such as a railroad wheel assembly greater than 3,000 lb. They are also suitable for designs that need tight precision or those that lift larger, heavier items. Most electromechanical grippers are aluminum and steel, though some are made of composites like carbon fiber and plastic for faster dynamics. When selecting a gripper, consider: • size, weight and material of the part that the gripper or end effector will handle • clearance issues around the part that could potentially interfere with the gripping robot’s reach • environment in which the gripper will operate—including proximity to corrosive materials or food and beverages • motion path of the robot or linear device moving the gripper and available power supply • potential need for anti-collision sensors, brushes, cameras, cutting tools, drills, grippers, magnets, sanders, screw drivers, spray guns, vacuum cups, or welding guns Also, will the gripper pick parts off a conveyor? Will it take the form of a three-jaw gripper to grab round items? Would the setup work better with a long-law gripper capable of grasping different workpieces? Other grippers that don’t fall into neat categories include single-jaw grippers, magnetic grippers and bladder grippers that inflate against part ODs and IDs. Such adaptive grippers are blind to shape or capable of dexterous manipulation; they wrap around and adapt to parts of varied sizes. In fact, pneumatic muscles, elastomertube fingers and mechatronic setups with feedback are finding use in research and specialty motion applications.
This ROTA NCS from SCHUNK is a power lathe chuck with active jaw pull-down. The chuck is suitable for external and internal clamping applications. In a six-jaw version, the pendulum mechanism is integrated, which means it centers workpieces between six contact points.
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HMI Hardware
MOTION SYSTEMS HANDBOOK
Pro-Face GP-4000M Series HMIs use GP-Pro EX development software that supports dedicated and PC-based HMI setups for open functionality and connectivity. Main HMI hardware benefits include extra-sharp display, efficiency and remote monitoring.
Summary of
HUMAN MACHINE INTERFACE HARDWARE HUMAN
machine interfaces, or HMIs, are control terminals added to machinery to give operators a way to monitor and adjust controls and functions. This interaction is through a graphical user interface that facilitates information exchange and communication between supervisory and machine-level HMIs. HMI hardware consists of compact controllers with embedded functions, usually in the form of ruggedized touchscreens—for example, LCDs with tempered glass and cast-aluminum frames or even sealed enclosures for outdoor applications or indoor plant applications with a lot of oil, dirt and machining
byproducts. Hardware standardization is increasingly common for HMIs on open-source and proprietary setups alike. In addition, HMI hardware options can make the displays satisfy rather specific application requirements. Depending on the application’s complexity, myriad I/O options exist— including digital and analog. Advances in HMI technology have quickened with increasingly affordable touchscreens and replacement of resistive displays with capacitive. In fact, these capacitive displays are particularly helpful in medical and food-and-beverage applications that need bezel-free designs, as they’re sleeker and let personnel clean and even sterilize machinery more easily. Solid-glass capacitive touchscreens also last longer than HMI hardware based on resistive technologies, because the screens don’t use pressure points to form circuits, so don’t wear or lose sensitivity over time. What’s more, many HMIs with capacitive displays have the multitouch capabilities of smartphones, which lets OEMs leverage user familiarity to offer intuitive interfaces. In fact, electronics innovation for consumer products—with ever-improving wireless communications, displays and portable processing power—is letting users connect to HMIs for industrial applications with tablets and smartphones for real controls. This is an NA series HMI from Omron Automation and Safety. Its wide screen gives users control over more objects—so operators can visualize whole machines at a glance. NA series HMIs are suitable for Sysmac NJ/ NX machine-automation projects, as well as new machines with CS rack PLC, CP micro and CJ modular controls.
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HMI Hardware
These Automation Panel displays from B&R work as remote controls (with a Smart Display Link 3 or SDL3 receiver) and combine with modules to make panel PCs. The HMIs can also display legacy programs and visualization applications in a 4:3 format without forcing users to modify software. There’s a 19-in. panel with SXGA resolution; other panels are 12.1 and 15 in. with XGA resolution for control-cabinet installs. HMI hardware networking Communication protocols range from simple RS-232 links to more advanced protocols such as CANOpen, SERCOS, and Ethernet-based communications. Depending on the HMI setup, multinetwork communications usually go through ControlNet and DeviceNet. ControlNet is a control-level network for high-speed transmission of time-critical messaging data and I/O data. In contrast, DeviceNet handles industrial devices like drives, limit switches, motor starters, operator displays, photoelectric cells and valve manifolds—as well as PCs and PLCs that may connect to the machine. Designs that use both communications facilitate HMI data management between machines and operators. More sophisticated HMIs also accept input from mobile devices and platforms such as Microsoft Windows CE—a scalable version of Windows for handheld devices. Such HMIs can often save designers money, as the operating systems are distributed on machine-level embedded HMIs, solid-state open HMI machines and distributed HMI servers, as well as relatively low-cost portable devices acting as HMI interfaces.
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INNOVATIVE AUTOMATION SOLUTIONS
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HMI Software
Summary of MOTION SYSTEMS HANDBOOK
HUMAN MACHINE INTERFACE SOFTWARE This Samba PLC with HMI and onboard I/O is from Unitronics. It gives OEMs on a tight budget a compact controller for simple machine control (but with a color-touch HMI panel). The HMI uses VisiLogic software—a single environment for hardware and communications configuration, Ladder application development, HMI design, remote PC access and data export.
HUMAN
machine interface (HMI) software is programming that gives machine operators a way to interact with and manage machine command panels. Interaction is through a graphical user interface (GUI) that facilitates information exchange and communication between two types of HMI—supervisory and machine level. Generally, programmers write HMI software for either one or the other, with applications suitable for both types. Such software has high upfront cost, but is inexpensive long-term thanks to the way it reduces redundancies. Case in point: Even lower-tech applications (in which most machines interact through switches and pushbuttons) entry-level HMI offerings are making inroads—as they often reduce interface-part count and simplify controls. More sophisticated applications benefit in a different way: Pharmaceutical and medical machinery use the latest HMI features to differentiate from competitive offerings.
But no matter the performance grade, selecting HMI software starts with an analysis of product specifications and features. What kind of GUI will the machine operator need? Will operators need to view diagrams, digital photos and detailed system schematics? Other considerations include system architecture, performance requirements, integration, cost of procurement, and operations. HMI software editors HMI software editors let designers add touchscreen functions and configure control functions for industrial automation. Usually, programming is through Windows-based software or screen-editor software. That lets designers quickly edit schematics and set the right communication protocols in a familiar programming environment. HMIs now link to Siemens Simotion 4.4 motion-control hardware and software to let end users setup and network motion applications through a graphical user interface. Simotion on Simatics HMI panels also let engineers eliminate programming and the risk of configuration errors, while leveraging a Totally Integrated Automation (TIA) framework for system compatibility even to the operations level.
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HMI Software
MOTION SYSTEMS HANDBOOK
Standard HMI software satisfies the needs of simpler machinery that’s not task-intensive. High-performance HMIs run software that lets users customize the interface to meet specific operator requirements. Sometimes, HMI software lets users program advanced control functions as well—for editing servomotor parameters and issuing global commands to other control axes on a machine, for example. (For motion-control applications, the visual GUI can range from simple 4 line x 20 column text displays to color monitors with touchscreen controls.) Such motion functions go well beyond basic HMI tasks, observing processes or making simple changes to some individual variables, parameters or setpoints. Remote HMI access One iteration of HMI software that’s increasingly common are programs that let users remotely monitor and control HMIs from smartphones, tablets or off-site PCs. Traditional setups only let users get to the HMI on the factory floor, but this new cloud-based HMI software gives operators remote access, so they can check machines from anywhere. Sometimes called web-based visualization, this is particularly helpful where machines run in hard-to-reach places. Related innovations in HMI software even let remote users make on-the-fly changes to machine functions (for variable production output).
This Maple Systems Graphic HMI is a high-performance operator panel with a high-speed processor.
SoftPAC software from Opto 22 lets Microsoft Windows PCs work like a stand-alone or rack-mounted PAC. The software controller lets users build a tag database when programming control logic; then it’s ready to use in the HMI. In fact, HMI functions include operator security, trending, and alarming. Users can import graphics or use a built-in graphics library.
This Opto 22 groov software is on a phone— and the groov Box hardware appliance GROOV-AR1. The web-based software brings data from sensors, drives, meters and other industrial devices to smartphones, tablets and PCs. It uses OPC Unified Architecture (OPC UA) and Modbus/TCP protocols to communicate with myriad plant-floor devices.
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Leadscrews
LEADSCREWS: MOTION SYSTEMS HANDBOOK
Converting rotary to linear
THERE
are a number of ways to convert the rotary motion of a motor to linear motion. Often times, the battle between the different methods and technologies can get quite contentious. However, as a rule, it’s best to remember that specific applications are better for some technologies than others, and as always, cost and product lifetime are also critical factors in determining which technology to use in a given application. One of the most common technologies is the leadscrew. A leadscrew, also known as a power screw, is a threaded rod or bar that translates rotational motion into linear motion. Leadscrews generate sliding rather than rolling friction between a nut and the screw. Consequently, higher friction means a lower overall efficiency. And efficiency, when talking about leadscrews, is simply the ability to convert torque to thrust while minimizing mechanical losses. Leadscrews are a staple of motion designs, driving axes on machines big and small. They usually sport higher ratings than comparable ballscrews thanks to more contact between the nut and screw load surfaces. Now, innovations
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Shown here are planetary screw assemblies from Bosch Rexroth. As an alternative to traditional rotary-to-linear devices, the drive elements have screw diameters to 75 mm and leads to 20 mm to transmit up to 1,496 kN. The design principle of planetary screw assemblies allows a high load capacity in relation to the installation dimensions. The planets rotate with a defined contact profile in a nut around a screw in parallel to the axis for smooth and quiet running.
in materials and helix geometry address old issues associated with leadscrew friction, bringing it down to better than 0.10 in some cases—good for fast and dynamic applications. In fact, there’s also been an uptick in leadscrew use because of proliferating machines for 3D printing, manufacturing and medical applications. Industries across the board are adopting new leadscrew components and linear systems. Designers of kiosk and automated retail applications, for instance, are looking for ways to simplify machines, reduce design weight and www.designworldonline.com
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On the outside, this looks like an ordinary DC motor. In fact, this particular motor is not a standard off-the-shelf part, but designed exactly to a customer’s specific technical requirements. PITTMAN provided an experienced team of engineers to work directly with the customer to design the perfect motor to meet a demanding motion application…
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Leadscrews
MOTION SYSTEMS HANDBOOK
The pressure velocity (PV) factor is the product of the pressure and velocity between the nut and screw. It defines the load, speed and duty cycle that the nut can handle. Plastic materials have an intrinsic PV rating, the point at which frictional heat causes permanent deformation of the plastic.
simplify assembly and maintenance. In a similar way, both additive manufacturing (3D printing) and traditional subtractive processes—plasma cutter, laser and waterjet manufacturing—are driving new leadscrew uses. The same holds true for factory automation. Leadscrew manufacturing processes can determine the performance and cost of the leadscrew. For instance, there are three ways leadscrews can be manufactured: machining, rolling or grinding. Ground leadscrews are the most expensive and are generally considered to be the highest performing as well. Another determinant of efficiency is the thread type. Acme threads are the simplest to produce and the most inexpensive, but also among the least efficient. Other types include buttress threads and square threads, which generally have the least amount of friction and higher efficiencies. Leadscrews have a number of advantages including a relatively high load carrying capacity. They are also compact and simple to design into a system with a minimal number of parts. The motion is also generally smooth and quiet and requires little maintenance. Leadscrews also work well in washdown environments because the materials used and the lubricant-free operation allows total immersion in water or other fluids. On the other hand, leadscrews do not have high efficiencies. Because of lower efficiency ratings, they’re not used in applications requiring continuous power
transmission. There’s also a high degree of friction on the threads, meaning that the threads can wear quickly. Because the nut and screw mate with rubbing surfaces, they have relatively higher friction and stiction compared to mechanical parts that mate with rolling surfaces and bearings. There are several parameters that help determine leadscrew performance. These include thrust, speed, accuracy and repeatability. The two most important factors in determining the performance of a leadscrew are the screw pitch and lead. The pitch is the linear distance between the threads, while the lead is the linear distance the nut travels. Speed is another critical parameter. Leadscrews have a critical velocity, which is the rotational velocity limit of the screw. Reaching this limit induces vibrations in the leadscrew. Accuracy and repeatability are also important factors. The accuracy of a leadscrew is a measure of how close to a desired end point the assembly can move a load to within a given tolerance. The accuracy of the leadscrew will mostly determine the system’s accuracy. On the other hand, repeatability is a measure of how well a leadscrew assembly can repeatedly move a load to the same position.
This bevel-gear screw jack from Nook Industries has an acme leadscrew. They’re suitable in machines that have high duty cycles or high speed.
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Need some help getting your project moving?
Have questions about linear slide systems? Not sure what product can handle the load? Our experienced team will answer your questions, brainstorm with you and help solve your system challenges. It’s what we’ve been doing for more than 45 years—and why we’re the go-to partner for linear motion systems in aerospace, medical, automotive and other industries. Just pick up the phone and speak with a seasoned application engineer and get your motion moving.
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MOTION SYSTEMS HANDBOOK
Linear Motion
This linear plain bearing is a PBC Linear Uni-Guide with a Frelon self-lubricating liner to lower the coefficient of friction, reduce wear and boost load capacity.
Technical Summary of
LINEAR MOTION: GUIDE RAILS, SLIDES & WAYS LINEAR-MOTION
systems are essential in all sorts of applications, including everything from manually operated industrial drawers to advanced Cartesian robots. Mechanisms that include the former operate without power, using inertia or manual power to move loads. Components to complete the latter include readyto-install drive and guidance designs in the form of self-contained actuators or linear-motion machinery subsections. Some designs simply rely on the rotary-to-linear mechanism or actuator structure for total load support. However, most industrial linear designs have pneumatics, linear motors or motor-driven, rotary-to-linear mechanisms to advance attached loads, as well as rails that guide and support the loads. Here, linear rails, rotary rails, guide rails, linear slides and linear ways are just a few options to facilitate singleaxis motion. Their main function is to support and guide load with minimal friction along the way. Typical linearmotion arrangements consist of rails or shafts, carriages
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and runner blocks, and some type of moving element. Engineers differentiate these systems by the type of surface interaction (sliding or rolling), the type of contact points, and (if applicable) how the design’s rolling-element recirculation works. In fact, slides and rails are more advanced than ever, with advances in materials and lubrication setups (to help designs last longer in harsh applications), innovative rail geometries (to help designs withstand more misalignment and load than ever), and modular guide mounts (to boost load capacity and minimize deflection). No matter the ultimate installation, linear-motion rails, guides and ways enable motion along an axis or rail either through sliding or rolling contact. Myriad moving elements can produce either sliding or rolling support: ball bearings, cam roller sliders, dovetail bearings, linear roller bearings, magnetic bearings, fluid bearings, X-Y tables, linear stages and machine slides. One classic rail with sliding contact is a dovetail slide; a classic rail with rolling contact is a ball rail with a recirculating system. www.designworldonline.com
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CUSTOM ENGINEERED SPINDLES & SLIDES DESIGNED & MANUFACTURED TO YOUR SPECIFICATIONS
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1-800-543-0470
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Linear Motion
This Rexroth CKL Compact Module incorporates a linear motor to deliver highforce density with a compact package … for travel velocities up to 5 m/sec. A ball rail with central relubrication helps the module deliver precise positioning and zero backlash.
Sliding-contact bearings are the more straightforward type of linearmotion component. These consist of a carriage or slide that rides over a surface. Sliding contact occurs when the moving part directly contacts the rail section. Older versions of these sliding-contact rails generated considerable friction during movement, so were only suitable for basic applications. However, newer versions have self-lubricating sleeves and other features to boost positioning accuracy and repeatability. In contrast, rolling-element linear-motion systems are either recirculating or non-recirculating. Non-recirculating types use rolling elements such as bearing balls, rollers and cam followers for movement. Recirculating types use some type of moving platform that houses a bearing block. This bearing block contains raceways with rolling elements that let the platform move along the rail with little friction. Recirculating types include linear guides and ball-bushing bearings. More specifically, rolling-element linear guides come in two basic versions—those with circular arc grooves and those with Gothic arc grooves. These groove choices are a result of industry evolution that’s enabled new geometries for better load handling. Circular arc grooves contact bearing balls at two points. The Gothic arch contacts the balls at four points for bidirectional load capacity. Another option for rolling-element linear motion is ball bushings that have a bushing nut lined with recirculating bearing balls. This nut rides along a round shaft to allow axial movement. History lesson: In 1946, the manufacturer Thomson introduced ball bushings, and the technology established the basic mechanism of rolling-element linear-motion bearings. In today’s designs, the bushings may also have integral flanges to support axial loads. Sliding-contact rail geometries A distinguishing feature of sliding carriage-and-rail setups is that manufacturers typically incorporate a ground groove in a rectangular track’s geometry (to serve as a working surface). Manufacturers typically build these rails in one of three shapes: • Rails with a boxway or square shape are simplest. Square rails excel at carrying large loads without a lot of deflection. Manufacturers often preload square rails, and most linear systems based on square rails do not self-align. Square rails often have a smaller envelope size; the boxway rails handle the highest loads in all directions. 108 DESIGN WORLD
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Linear Motion • Rails with a dovetail shape (or twin rail) have male geometry that securely engages female saddle geometry. That boosts stability and load capacity, even in unusual orientations or applications with unsteady loads. • Round rails deflect less under load. In addition, systems based on round rails are inherently self-aligning, so are easier to install than the other options. Rolling-contact functions and options Rolling-element linear systems need little force to initiate motion. In addition, friction-force variations due to speed are minimal, so these systems can position loads with small and precise steps. The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help machinery maintain a level accuracy for much of the linear system’s operating life. Manufacturers produce rolling-contact guides in several variations. The differences are in rolling element shape (ball or roller); rolling element size; whether the rolling contact is two or four-point; conformity of ball contact; whether the design has two, four, six or some other number of rolling-element rows; contact angle; and how the rolling-element rows are arranged—in an X or O configuration. All these design factors determine load capacity, rigidity and friction. For example, O-shaped arrangements can withstand higher torque than X arrangements. In general, the number of load-bearing, rolling-element rows influences the load capacity … so more rail rows means more load capacity and rigidity. However, more rows makes systems more complex and costly. Here are more details on these rolling-contact options: • Rolling elements are either linear rollers or balls. Because the rolling elements recirculate in recirculating rolling-element guides, they have a nearly infinite stroke length. They are available on flat guide ways and guide way rails. Flat guide ways are available in single or double row rolling elements. Guide way rails are often square rails. • Non-recirculating roller type units have limited stroke length. Flat guide ways are dominant here and have either a grooved race compatible with crossed rollers, or non-grooved race, which uses cage and rollertype rolling elements. • Recirculating elements (ball or roller bearings) between the rail and the bearing block enable precise linear motion. The coefficient of friction with roller-element-based systems is much less than with slide based linear motion guides … about 1⁄50 that of non-recirculating systems. • Ball-type rolling element units are also subdivided into recirculating and non-recirculating types. The flat guide ways here typically use double row recirculating rolling elements. The guide way rail can be either round or square. If the raceway is not grooved, the rolling element is typically a linear ball bushing. If the raceway is grooved, the unit usually uses a ball spline. For square rails, the raceway is usually grooved. For ball-type rolling element units that are non-recirculating, the flat guide ways are grooved and use linear ball guides. The guide ways are round rail, without a grooved raceway, and use stroke bearings. Quick note on fluid-floated bearings Less common types of linear systems include hydrostatic or aerostatic linear-motion bearings. Because these systems have no mechanical contact, they are suitable for applications that need extremely accurate 110
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HEAVY DUTY SLIDES DESIGNED FOR DEMANDING NEEDS
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9308
up to 500 lbs.
Heaviest load-carrying slides, for the most demanding conditions such as mobile storage in commercial and emergency vehicles.
7957
up to 350 lbs.
Good for wider drawer applications such as toolboxes and equipment storage. Mid-range load capacity in a .75” cross section.
AL4140
up to 1323 lbs.
Super heavy-duty slides for your heaviest loads. Made of aluminum and stainless steel components for corrosion resistance.
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accuride.com
© 2015 Accuride International Inc.
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Linear Motion
or quiet operation. Here’s how they work: A pressure regulator sends pressurized fluid between the rail and carriage. That lifts the carriage off the guideway by about 0.01 mm or so. Aerostatic versions use air as the fluid; hydrostatic linear bearings use specially formulated hydraulic oil. This type of guide is difficult to manufacture and expensive, but damps vibrations and allows for moves to 120 m/min. and 10 g—useful for ultraprecision machines. Linear-rail lubrication Some linear-motion systems need periodic application of lubricant, but many are available pre-lubricated. In addition, a number of systems use self-lubricated moving elements, eliminating the need for lubrication during the useful life. Note that the rails, ways and guides of linear motion systems tend to pick up dirt and debris from their application environment. For this reason, use carriages and slides with some kind of wiper system to keep the systems clean. When selecting linear systems, engineers should consider space limitations, accuracy needs, stiffness, travel length, magnitude and direction of loads, moving speed and acceleration, duty cycle, and the application’s environment. Note that an excessively large load or an impact load can permanently deform the raceway surface whether the linear guideway is at rest or in motion. Most manufacturers offer tables on the basic dynamic load rating, which can help engineers determine the proper load ratings for a system. Another caveat about friction: Friction measurements are carried out on all profiled rails. The friction values are given in tables in the manufacturers’ respective product catalogs. The level of friction depends on load, preload and sealing, travel speed, lubricant and runner block temperature. Total friction from a runner block includes associated rolling or sliding friction, lubricant friction and the friction of any seals.
This Schaeffler INA Linear Technology is a linear recirculating-ball bearing and guideway assembly. Called the KUVE-B-HS, it has conventional steel rolling elements for speeds to 10 m/sec. Plastic in the recirculation mechanism prevents rolling-element tilting and pulse loads. The guides run on standard guideways.
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Mechatronics Enabled
™
Bring mechanical design & electronic motion control together with an optimized solution from PBC Linear®.
PBC Linear’s full range of linear slides and actuators are easily integrated for a complete mechatronics solution.
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1-800-962-8979 • www.pbclinear.com • 6402 Rockton Road, Roscoe, IL 61073 USA
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FOR MAINTENANCE-FREE LINEAR WAY PERFORMANCE, HEAD STRAIGHT FOR IKO TECHNOLOGY.
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Your Global Automation Partner
An introduction to
MACHINE VISION SYSTEMS
WARNING
Not suitable for repairing crummy sensors
Integrated smart cameras, such as the BOA M1280 from Dalsa, are complete machine vision systems featuring resolutions of 1280 x 960 and operating at up to 24 frames per second.
(or your reputation).
MACHINE
Rugged, reliable industrial
vision refers to the technology and methods used to provide imaging-based automatic inspection and analysis. Machine vision systems are becoming more common in applications such as medicine, security and traffic, aside from their traditional use in manufacturing lines and robotics. In these applications and many others, vision systems replace manual inspection and offer higher speeds and greater accuracy, improving product quality and reducing production costs. Vision systems can range from extremely simple sensors (photoelectric sensors, for example) that detect the presence of an object or part to advanced cameras that capture an image and pass the data to a computer and software algorithms for further analysis. These can include edge detection, color comparisons, 3D imaging and analysis of height, volume or tilt.
automation products from TURCK are built to perform in the toughest conditions, and our engineered solutions are customized to meet your application challenges. Cheap knockoffs can’t compare. TURCK works!
Some key areas of advancement in vision systems include: • Higher-resolution image sensors, which means higher accuracy for better defect detection • The rise of 3D vision systems capable of measuring height and volume as well as tilt of objects, including using laser triangulation methods • Software and algorithm advances such as self-learning pattern-matching tools
uprox®3 Inductive Proximity Sensors Developed to combine compact sensor design with the longest switching distance to all metals of all inductive proximity sensors on the market.
Machine vision systems are integral to many industrial systems that involve motion control. For instance, vision systems are key to many types of robots, providing Call 1-800-544-7769 or visit info.turck.us/sensors www.designworldonline.com
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Your Global Automation Partner
visual data for pick-and-place applications or material handling. The image produced from the vision system is used as a form of feedback to let the robot know where an object is in physical space. Basic format The basic machine vision system can be understood in two different ways: one, as a system of components, and the other as a process. For example, the basic parts of any machine vision system include the lighting, the camera (which includes the lenses and sensors) and software. Hand in hand with a component approach is a functional understanding of vision systems as a process. So, the process of machine vision involves several steps including capturing an image (the camera and lighting), image analysis (software) and using the analysis to take an action of some kind, such as a Pass/Fail decision of a part on a manufacturing or assembly line. Lighting In many ways, the lighting is the most crucial piece of a vision system. You can have the best camera and imaging sensor and powerful analysis software, but if the lighting is wrong or off (too bright and reflective off of a metallic surface, or too soft, which doesn’t offer enough contrast), the image produced will be worthless. The right type of lighting needs to be chosen to match environmental conditions and the object to be imaged. So choose a lighting source that’s appropriate for the size, shape, texture, color, transparency, reflectivity and heat tolerance of the target object. Common types of lighting include backlights, ring lights, directional lighting, on-axis lighting and diffused lighting.
WARNING
Not suitable for repairing crummy sensors (or your reputation).
Sensors Every machine vision system is essentially based on an image sensor, one which generates a photographic image. Image sensors are key to camera performance. There are basically two types of imaging sensors: a CCD (charge-coupled device) and CMOS (complementary metal oxide semiconductor). Both devices convert light into electrical signals, with CCDs using analog signals while CMOS sensors are completely digital. CCD sensors are older, better established technologies, while CMOS
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QR24 Rotary Inductive Sensors An expanded line of noncontact inductive rotary position sensors with optional stainless steel housing. Wide variety of outputs includes HTL Incremental, SSI, CANopen and
Some companies, such as National Instruments with its CompactRIO platform, offer the option of integrating vision capabilities with precise control and monitoring functions on one system.
analog in voltage and current.
Call 1-800-544-7769 or visit info.turck.us/sensors www.designworldonline.com
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Machine Vision
MOTION SYSTEMS HANDBOOK
TYPICAL MACHINE VISION SYSTEM Lens
Sensor
Software, Image Processing
Camera
Object
Output
Signal
Light Source
sensors have faster readout rates and generally use less power. Image quality from both sensors is comparable, so there is not a big difference between the two sensors in that regard. Acquisition Once an image is acquired by the camera, it is processed to extract the information needed. The image itself can be either monochromatic (black and white) or color, depending on the application needs. For instance, if an application calls for simple presence detection, a monochromatic image may be enough. But if distinguishing between different colored parts is called for, then a color image would be needed. Optics Another important consideration is the camera optics. Much of this will be familiar to people with some knowledge of photography. One key factor is the focal length of the lens. This determines the field of vision, so a longer focal length means a smaller field of vision or a more magnified image, whereas a shorter focal length means a wider field of vision, comparable to the operation of a wideangle lens. The aperture, or f-stop, determines the 118
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quantity of light entering the lens. A smaller number means a larger opening and more light entering the lens and hitting the image sensor, while a larger number means a smaller opening and less light entering the lens. The aperture also determines the depth of field, which is the amount of the image that is in focus. So a smaller opening means a greater depth of field (hence more of the image is in focus), while a larger opening produces a more shallow depth of field, with less of the image in focus.
The diagram shows a typical machine vision setup, including a lighting source, camera and image-processing block.
Software/Processing Most of the hard work of a vision system is in the image processing. For instance, image processing can include a range of functions such as edge detection, color analysis and pattern recognition, as well as measuring part dimensions. Edge detection is a common function. For it to work properly, the pixels of the desired object and the background should be high enough in contrast so the algorithm can distinguish the difference. Of course, this depends on proper lighting that will produce a sufficiently high-contrast image. And this is also where users can specify how much contrast is necessary to count as identifying an object and discerning the object from the background. www.designworldonline.com
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The GVM series of specialty ac motors from Parker are permanent magnet synchronous ac motors (PMAC) for electric and hybrid vehicle applications. They feature continuous power up to 167 kW with max torque to 710 Nm.
An introduction to
AC MOTORS AC
(alternating current) motors take an ac current as their input, but differ from dc (direct current) motors in that there is no mechanical commutation involved, and they can be single or multiphase. An ac motor, like any electric motor, converts electrical energy to mechanical energy. A typical ac motor has two basic parts: a stationary stator with coils energized with alternating current that produce a rotating magnetic field, and a rotor attached to an output shaft that produces a second rotating magnetic field. The magnetic field in the rotor can be generated in several different ways including with dc or ac electrical windings, permanent magnets or through reluctance. Because ac motors have no commutators or brushes, they require less maintenance than brushed dc motors. With dc motors, control is done by varying voltage and current, while on ac motors, the voltage and frequency (along with the number of magnetic poles) are used to control the motor. A common way to break down ac motors is based on the magnetic principle that produces rotation. There are two fundamental types of ac motors: induction motors and synchronous motors. Induction motors In induction motors, the key idea is the rotating magnetic field. The rotor turns in response to the induction of a rotating magnetic field within the stator. The most common source of this in ac motors is the squirrel cage configuration. The setup is essentially two rings, one at each end of the motor, with bars of aluminum or copper connecting the two ends. Induction motors have properties that make them especially suited to a number of industrial and home appliance applications. For
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starters, they are simple and rugged motors that are easy to maintain. They also run at constant speed across a range of load settings, from zero to full-load. The only drawback is that induction motors are generally not amenable to speed control, although the availability of sophisticated variable-frequency drives means that even induction motors, usually three-phase induction motors, can now be speed controlled as well. Synchronous motors Synchronous motors are so named because they run synchronously with whatever the frequency of the source is. The motor speed is fixed and doesn’t change with changes to the load or voltage. These motors are primarily used where precision and constant speed are required. Most synchronous motors are used in heavy industrial applications, with horsepower ratings ranging from the low hundreds up to the thousands. Because of the rotor size, synchronous motors typically experience a poor response in incrementing applications. Also, because acceleration of inertial loads may not be as high as other motor types, these motors may operate at irregular speeds and produce undesirable noise. And generally, synchronous motors are larger and more costly than other motors with the same horsepower rating. Sometimes terms used to describe motors can be a little confusing. For instance, it’s not uncommon to hear brushless dc motors referred to as ac motors because of the similarity of the moving magnetic field. However, the important point to remember is that the designation “ac” and “dc” refer to the type of current driving the motor. Comparing ac and dc brushed and brushless motors, all three have power losses in the form of I-R losses. Because dc motors use permanent magnets, none of their energy needs to be used to generate the magnetic field as in ac motors. The energy used by ac motors to create the magnetic field decreases the efficiency of an ac motor in comparison to dc motors. As for brushless dc motors, for the same mechanical work output, they will usually be smaller than a brushed dc motor, and always smaller than an ac induction motor. The brushed dc motor is smaller because its body has less heat to dissipate. Also, brushed and brushless dc systems provide flat torque over a wide speed range, while ac motors typically lose torque as speed increases. 8 • 2015
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MOTION SYSTEMS HANDBOOK
Motors–DC
DC MOTORS:
brushed and brushless
A
direct current (dc) motor is an electric machine that converts electrical energy to mechanical energy. All dc motors generate a magnetic field, either with electromagnetic windings or permanent magnets. An armature, which is often a coil of wires, is placed between the north and south poles of a magnet. When current flows through the armature, the field produced by the armature interacts with the magnetic field from the magnets and eventually generates a torque and thereby motion. According to common industry naming conventions, the most common dc motor types are basic brushed dc motors and brushless dc motors. Brushed dc motors Some engineers call brushed dc motors wound-field motors because they have wound and lacquered coils of copper wire that make the electromagnetic field. No matter the term, there are permanent magnet (PM), shunt, series and compound-wound brushed motors. In a brushed dc motor, the magnet acts as the stator. The armature is integrated onto the rotor and a commutator switches the current flow. Brushed dc motors use commutators and brushes to pass current to the rotating rotor’s copper-wire windings. Designers can control speed by changing rotor voltage (and current with it) or by changing the magnetic flux between rotor and stator through adjustments of the field-winding current. In fact, the way dc brush motors let designers control field and rotor windings means they’re suitable for applications that need simple and cost-effective torque and speed control. That said, increased functionality from electronics for PM motors means that this advantage is less pronounced than it once was. Brushed dc motors have the advantage of generally low initial cost and simple control of the motor speed. However, there are some drawbacks. At certain periods during the dc motor rotation, the commutator must
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Frameless kit motors, such this one from Parker, are designed to be directly integrated with a mechanical transmission device, eliminating parts that add size and complexity, as well as providing higher dynamic stiffness by eliminating the compliance of shaft attachments.
reverse the current, causing arcing and friction wear on the brushes. Because of this spark hazard, brushed dc motors aren’t suitable for explosive settings. Brushed dc motors also require more maintenance in the form of replacement of springs and brushes that carry the electric current, as well as replacement or cleaning of the commutator. Brush particles also mean that the motors can’t be used in cleanroom applications. The same goes for applications that need high precision, as friction from brush-commutator engagement makes for long positionsettling times. Brushless dc motors A brushless dc (BLDC) motor is essentially a dc motor without the mechanical commutation of the brushed motor. BLDC motors are powered by direct current and have electronic commutation systems instead of the mechanical brushes and commutators used in brushed dc motors, eliminating mechanical wear issues. In BLDC motors, the permanent magnet is housed in the rotor and the coils are www.designworldonline.com
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Motion made easy.
System Solutions from one single source
Great ideas move people, and maxon moves great ideas. Our precision brushed and brushless DC motors, gearheads, encoders and control electronics have made us the leading single source of motion solutions in a world of innovation. Scan QR code to visit our web site.
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Motors–DC
MOTION SYSTEMS HANDBOOK
placed in the stator. The coil windings produce a rotating magnetic field because they’re separated from each other electrically, which enables them to be turned on and off. The BLDC’s commutator does not bring the current to the rotor. Instead, the rotor’s PM field trails the rotating stator field, producing the rotor field. For successful commutation, it’s important to have precise rotor position data, which is often achieved through magnetic sensing with a Hall effect sensor, which also allows for tracking of speed and torque. BLDC motors have quite a few advantages over their brushed counterparts. Compared to brushed motors, BLDC motors are typically more efficient, mainy due to the elimination of the friction from the brushes. They’re also more reliable and typically have longer life spans. Getting rid of the brushes also means a decrease in EMI (electromagnetic interference) noise and no sparking from the brushes making contact with the commutator. Selecting the right motor Selecting the right motor for an application requires having a full understanding of the application parameters, including power, speed, torque, physical size, efficiency and lifetime expectations. Key criteria include the following: • Voltage availability is a critical element in motor selection. Remote applications or portable devices, for example, are battery operated, while many rackmounted devices and tools operate from a 24-V power supply. Typical dc motors are available for use at voltages from 1.5 to 48 V and higher depending on required power. • Physical size is often one of the limiting factors in motor selection because more applications have smaller footprints, like desktop 3D printers, portable medical devices and hand tools. Often a compromise needs to be made between which motor to use and the available space it needs to fit into. Efficiency becomes a primary concern when you need to worry about power consumption to maximize battery life in a surgical tool or unmanned security drone. • Torque and speed also have an effect on motor frame size. High-torque motors are often larger in size than their low-torque counterparts, which means that larger mounting hardware and larger housings may be an important machine requirement. For example, it takes a larger motor to rotate the magnets in an 122
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•
MRI than it does to run the windows in the doors of an automobile. Although speed and torque are independent requirements in many applications, typically speaking, when the torque increases the speed will decrease—if the voltage stays the same. This connection is based on the slope of the speed/torque curve. Motor duty cycle could be one of the most telling aspects of many semiconductor production machines. Intermittent operation not only reduces the wear and tear on the motor and increases the life of the motor, but it also means that a smaller motor size can be used without depleting the positive characteristics of the machine itself.
Brushed or brushless? Brushless motors typically last much longer than brushed motors, which rely on a mechanical connection for operation. Brushless motors run much faster as well. If you’re using a brushless motor for reliability, you won’t want to add a gearhead to the mix, though. The mechanical nature of a gearhead automatically means that it’ll have a shorter life cycle. Using a gearhead with a brushless motor will only negate the longevity of the combined system, thereby reducing the longevity of the machine it was designed into. On the other hand, there are times when using a gearhead on a brushless motor is advisable. For example, if the environment is such that noise is a concern or that a higher torque is needed, a gearhead will do the job. Don’t use gearheads to increase the speed of brush motors. Using a gearhead with a brushed motor won’t change the life cycle to any great extent. Both are mechanical components that are subject to wear and tear. A real issue in selecting between a brushed and brushless motor is the expertise of the machine builder. Brushless motors either come with built-in or external electronics to operate the motor. It takes some experience to provide the custom electronics many machine builders choose to provide. But for high sales volumes, the costs are easily regained. Brushed motors, on the other hand, don’t need electronics to run the motor, offering a plug-and-play option to the designer. This means that if the machines are expected to sell in low quantities, a brushed motor will save on the overall cost of the system. Overall, many machine builders are electing to use brushless motors whenever possible. Long life and high speeds make these motors applicable to a broader array of applications.
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Motors–Integrated
An introduction to INTEGRATED MOTORS AN
MOTION SYSTEMS HANDBOOK
increasingly common motor type or style is what is known as the integrated motor, or smart motor. The hallmark of an integrated motor design is the combination of the motor with one or more other motion system components. The motor part of the integrated motor can be any number of types including brushless dc motors, servomotors or stepper motors. A typical motion system has a number of common components including the motor, controller, a drive and power for drive and control electronics, cabling, and feedback devices such as encoders. As a result, what constitutes an integrated motor can vary. For instance, the most basic type may simply consist of a motor and drive or a motor and controller. Additional add-ons can include an encoder as well as communication ports. Integrated motors are said to offer greater reliability mainly because there are fewer parts to connect. Also, fewer external connections means less cabling and wiring. Less cabling and wiring reduces costs, as does the fact that the components that one would usually purchase separately, such as the motion controller and the drive, are integrated into one package. These integrated motors are also designed to be programmed easily and quickly, which can help reduce development times. Communication options range from simple serial communication links such, as RS232 or RS485, to more advanced network topologies suited to complex motion control tasks, such as CANopen, DeviceNet or Ethernet protocols. For machine builders, designing with integrated motors has a number of benefits, including helping to reduce machine size, cost and complexity. Integrated motors can also eliminate external controllers such as PLCs. Such integrated systems can significantly reduce the amount of space required for a machine by consolidating components and eliminating cabling, as well as possibly the need for entire enclosures. Integrated motors became more prevalent with the advent of de-centralized motion
control architectures. An alternative to centralized motion control, a de-centralized architecture distributes motion control to a number of individual motion axes (in this case, to individual integrated motors), eliminating the need for a central controller. This means that individual motors can execute the control closer to the actual axis of motion or load, thereby taking the computational burden off a central controller and distributing it to individual integrated motors. Selecting a motor As with any motor, when selecting an integrated motor for an application, the most important step is determining the characteristics of the load. This is why properly calculating the load torque is such an important part of selecting the right motor and designing it into the application. A good rule of thumb is to try and keep the actual operating conditions below the published limits of the motor to ensure reliable and long-life operation. Motor sizing parameters are usually based on the torque curve and moment of inertia of the load. These two factors can help determine the motor’s operating bandwidth. Sets of torque curves depict limits of both continuous and peak torque for the given motor over their full range speed. There are different types of torque curves, dealing with peak torque and continuous torque as well. Peak torque curves can be derived from dyno testing and represent the point at which peak current limit hardware settings of the drive prevent further torque in an effort to protect drive stage components. For any mechanical system, if the motor is operating in its optimum range, then the system will be performing at its best. Beyond the motor itself, depending on the specifics of the application, it may be necessary to adjust mechanical components such as gear reducers, belts, leadscrew pitch or pinion gears in order to achieve optimal system performance.
Integrated motors, such as these Lexium MDrive units from Schneider Electric Motion, combine a motor with any number of components including drives, controllers and encoders into one package. Options can include communication links for Ethernet TCP/IP or CANopen or different connector styles such as M12.
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MOTION SYSTEMS HANDBOOK
Motors–Linear
Basics of
One classic analogy is that linear motors are conceptually similar to a rotary motor that someone’s cut open and flattened out. Instead of a rotating shaft for torque, load connects to a flat moving forcer for linear movement and force. Depending on size, Tecnotion UXA series linear motors (shown here) output 120 to 846 N continuous force and 615 to 4,200 N peak force.
LINEAR MOTORS LINEAR
motors are a relatively new addition to the arsenal of motion components for designers building machinery. They consist of two main components: a stationary platform that manufacturers call a platen or secondary (with electromagnetic windings), and a moving forcer or primary that sometimes includes permanent magnets. Linear motors are fast and precise for positioning, but can move slowly and steadily for material processing. Depending on the type, linear motor speeds range from a few inches to thousands of inches per second. They’re capable of unlimited strokes and (with an encoder) accuracy to ±1 μm/100 mm. Myriad inspection, medical and material-handling applications use linear motors to boost throughput. As is the case with their rotary counterparts, linear motors use common drives and motion controllers. Linear-motor accessories also include cable carriers, feedback encoders, limit switches and stages for multiaxis movement. But unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive. So, they avoid the gradual wear of traditional rack-and-pinion sets. Linear motors
also avoid the drawbacks of a rotary motor with a belt and pulley for translation. These downsides are limited thrust because of tensile-strength limits; lengthy settling times; belt stretching, backlash and mechanical windup; and typical speed limits of 15 ft/sec or so. In the same way, linear motors avoid lead- and ballscrew efficiencies (of 50 and 90%, respectively), as well as whip and vibration. They don’t force engineers to sacrifice speed (with higher pitches) for lower resolution, either. Multi-axis stages that use linear motors on each axis are more compact than traditional setups, and so fit into smaller spaces. Their lower component count also boosts reliability. Here, the motors connect to regular drives, and (in servo operation) a motion controller closes the position loop. How linear motors work Linear motors use electromagnetic flux for their operation. Flux in this context is the rate of electromagnetic energy flow through the airgap, and flux density is the magnetic flux through the airgap area. In linear motors, the latter is proportional to magnetic and electrical loading—the vector quantity of flux lines between platen and forcer. Engineers express this value in Tesla or Gauss. Typical airgap flux densities range from fractions to a few Tesla. Linear stepper motors—An older design, this type of linear motor has a toothed forcer consisting of laminated steel cores wound with coils. The platen also has teeth cut into a steel bar. Linear stepper-motor platens mount end-to-end for unlimited travel. Thrust originates from reluctance force.
This SR stage from H2W Technologies has a linear Z axis running off a brushless linear motor under closed-loop position control. The theta axis is a closed-loop, three-phase, brushless servomotor and encoder.
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motioncontroltips.com Linear stepper motors deliver speeds to 70 in./sec, suitable for relatively quick-acting pick-and-place and inspection machines. Other applications include parttransfer stations. Some manufacturers sell twin linear steppers with a common forcer to form X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few Οm for every hundred mm to output accurate movement. Hybrid linear motors—Most of these have low-cost ferromagnetic platens. (Those with solid steel platens move to 3 m/sec; those with laminated platens move faster.) Much like linear stepper motors, they vary magnetic saturation from the platen to shape opposition to magnetic flow; in other words, thrust originates from reluctance force. Feedback plus a PID loop with positioning control helps the motor output servo-grade performance. Key to hybrid linear-motor performance is a yoke on the platen that makes paths through which flux travels and closes flux loops between platen teeth and forcer. Hybrid-motor drawbacks are limited output and cogging from reluctance coupling between the forcer and platen. Two solutions here are phase-teeth offset or driving to get partial saturation of platen teeth and sections of F guide 7-13-15.pdf
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This LinMot PR01 linear-rotary motor comes in myriad sizes for different force and torque outputs. The motor integrates a linear motor with an attached rotary torque motor. Controls independently command the two.
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Introducing LinMot’s new
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YOUR DESIGN · OUR MOTOR
Motors–Linear
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This Electric Thrust Tubular (ETT) linear motor (from Parker’s Electromechanical Automation Div.) houses a motor stator with magnets in the moving rod to form a direct-drive, rodstyle thrust actuator. Acceleration reaches 200 m/sec2 and maximum speed is 4 m/sec.
forcer teeth. Here, the drive only magnetically saturates working teeth sections. Some hybrid-core motors also use external cooling to get more output during continuous operation. Linear ac induction motors—Linear ac induction motors that run to 2,000 in./sec work for people movers, rollercoasters and large aerospace applications. Generalpurpose types can move a few inches to 150 ft/sec or faster. Linear ac synchronous motors—These are either iron-core or ironless-core motors. Ironless-core (or air-core) linear motors—An epoxy forcer plate holds copper coils. It slides inside a U-shaped magnetic platen to output up to 3,000 N and speeds to 230 in./sec or better. Sometimes called brushless cog-free linear motors, these are lighter motors with potentially unlimited travel and quick acceleration. However, their main benefit (particularly in semiconductor applications) is smooth output. Their speed is helpful in flying-shear applications and longstroke pick-and place machines. Other applications exist for waterjet and laser cutting and robotics tasks. Iron-core motors—Slotted steel lamination stacks (insulated to reduce Eddy currents) can output 7,000 N or more. The forcer coil assemblies include steel laminations and windings in a single or three-phase configuration. This allows for control directly from a line or through an inverter or vector drive. Some such linear ac motors use water-cooling to boost force output—enough to let the motors drive large baggage handling and amusement-ride axes. Iron-core motors are suitable for certain machine-tool applications as well. Cylindrical linear motors—These have steel rods and a moving coil or rods filled with stacked magnets. With the same footprint as a traditional linear actuator, these work in myriad machines that need quick and accurate strokes. 128
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StepSERVOTM combines the best features of stepper and servo systems.
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Motors–Servomotors
MOTION SYSTEMS HANDBOOK
An ongoing trend in servomotors is more power in smaller packages. The MPP series of brushless servomotors from Parker exemplify this with lower inertia and higher power all in a smaller motor package. The motors feature segmented core technology, which can yield up to 40% higher torque per unit size than conventionally wound servomotors.
SERVOMOTOR basics and selection tips SERVOMOTORS
can provide precise control of torque, speed or position using closed-loop feedback. They also have the ability to operate at zero speed while maintaining enough torque to maintain a load in a given position. Servomotors have several distinct advantages over other types of motors. For starters, they offer more precise control of motion. This means they can accommodate complex motion patterns and profiles more readily. Also, because the level of precision is high, the position error is greatly reduced. All servo systems have essentially three components: an electric motor, a feedback device and some type of electronic control. The electric motor can be either ac or dc, rotary or linear. Under the dc heading, brushed dc servomotors are generally less expensive than brushless servos, but do require more maintenance due to the brushes needed for motor commutation. Brushless dc servomotors are highly reliable and virtually maintenance free. However, the drives for brushless dc servomotors are more complex because
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the commutation is done electronically. Servomotors also require a form of feedback, often with the feedback device, such as an encoder, built into the motor frame. The feedback signal is needed by the control circuitry to close the control loop. Lastly, the control circuitry typically involves a motion controller, which generates the motion profile for the motor, and a motor drive (or servo amplifier), which supplies power to the motor based on the commands from the motion controller. Sizing a servo motor Load: Correctly sizing a servomotor begins with knowing the load, which is also referred to in terms of inertia (or the load’s resistance to change in speed). Generally speaking, the important figure is the inertia ratio, which is the ratio of the load inertia to the motor inertia, or Inertia Ratio = JL / JM where JL is the moment of inertia of the load and JM is the moment of inertia of the motor.
The motor’s moment of inertia can be found from manufacturer data sheets. However, the moment of inertia of the load is a bit more complex. Basically, each component that is moved by the motor contributes to the total load inertia. This includes not only the load itself, but any other mechanical components of the transmission system such as couplings, leadscrews, rails and so forth. Speed: Another important factor is the speed or velocity. This involves knowing how far and how fast the load must travel. Knowing the inertia ratio can help with this as well as knowing the motion profile of the system. Figuring out what the motion profile is and knowing the system inertia helps determine the required speed, acceleration and torque. Torque: Once the load and speed are known, calculate the required torque values. This can be determined from the motor’s torque-speed curve. Calculations need to be made to determine the required continuous torque, peak torque and maximum motor speed. This means the peak torque measurements
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must be calculated, usually during acceleration or deceleration, along with the running/normal torque. A motor’s continuous torque is its ability to produce the rated torque and speed without overheating. Intermittent torque indicates how much torque a motor can produce in a short period of time based on current limits of the drive and motor. The required amount of continuous torque must fall inside the continuous operating region of the system torque-speed curve. The required amount of peak torque must also fall within the servo system’s intermittent operating region of the system torque-speed curve. Sizing software There are many different software programs available to make the servomotor selection process easier. These programs calculate the torque, speed and inertia requirements according to application specifications. Sizing software will ask for the system hardware that will be used (drive, motor, gearing, load), and will ask for a definition of the type of move the system will perform (the motion profile). Providing the distance and time, or providing the distance and speed of the required move, will allow the software to determine the continuous torque and the peak torque required by the motor. Sizing software can also help calculate the inertia mismatch of the load to the motor. A general rule of thumb is to keep the inertia mismatch to less than 10:1 (inertia of the load:inertia of the motor). While many servo systems can handle mismatches of much higher than 10:1, better system control and response will result with an inertia mismatch of 10:1 or less. Gearing The servomotor size directly affects other servo system components, so right-sizing the motor is critical. If a servomotor is oversized, it will need a larger amplifier than that required for a smaller motor. This means higher hardware costs as well as increased energy requirements. Servomotors generally run at speeds in the 3,000 to 5,000 rpm range, and in many applications, the motor is paired with some type of gearing to increase output torque. Gearing increases the available torque by the amount of the gear ratio. Gearing will also lower the inertia mismatch ratio by the square of the gear ratio, so a 10:1 gearbox will reduce the reflected load inertia by a factor of 100.
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Motors–Servomotors
Multi-Axis Communications Just Got Simpler.
In many instances, gearing allows smaller motors to be used successfully, more than offsetting the cost of the gearing system.
1 READY. Use our free SMI™ software to auto detect motors. 2 SET. Set CAN addresses and baud rate. 3 GO. Send commands; start talking.
Same for 2 or 120 motors. That’s all there is. Simple!
Special classes of servomotors, like the Kollmorgen AKMH series, can withstand washdown conditions typical of applications in food processing. These motors are built to IP69K standards and use corrosion-resistant materials.
More about inertia ratios Servomotor inertia ratios impact overall machine efficiency. The latest digital servo systems can get higher inertia ratios while maintaining stable control to target velocities and positions. What is a servo’s inertia ratio or mismatch? Simply put, inertia ratio or mismatch helps express the overall controllability and risk of system instability. The two terms of the moment-of-inertia ratio or mismatch for a rotary servo system are: •
•
The load’s total moment of inertia, JL. Here, the inertial load is that from all the axis’ components (reflected through mechanisms when applicable) and summed at the motor’s shaft. The motor’s moment of inertia, designated here as JM. Inertia mismatch is not a definite number or even a definite range for every application. That said, there are some ratio ranges that are generally applicable to specific applications and machine designs. The usual rule of thumb is that an ideal inertia mismatch is 1:1. This is the ideal mismatch to maximize power transfer and minimize potential control issues, while the acceleration and deceleration energy is evenly split between JL and JM (where JL = JM and JTOTAL = 2 · JL. However, the most efficient dynamic applications maximize acceleration of the load’s inertia (within the confines of axis stability, controllability, accuracy and repeatability). So for a fixed JL, the most efficient version of a machine gets maximum acceleration with the lowest possible JM, not a minimal matched JL.
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Motors–Steppers
The Quietest Brush Motor on the Market
STEPPER MOTOR basics
A cut-away view of a motor from LIN Engineering shows the internal construction of a hybrid stepper.
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motors are one of the most common motors used in motion control applications. They’re used in positioning applications and can be accurately controlled down to fractions of a degree without the use of feedback devices such as encoders or resolvers. They are operated in open-loop mode as opposed to closed loop like many other motor types. Stepper motors are typically classified by the number of allowable steps they can be commanded to move. For instance, a 1.8° step motor is capable of 200 steps/revolution (1.8 x 200 = 360°, or one full revolution) in full-step mode. If operated in half-step mode, each step becomes 0.9° and the motor can then turn 400 steps/revolution. Another mode called microstepping subdivides the degrees per step even further, allowing for extremely precise movements. Several different stepper motor technologies exist including permanent magnet (PM), variable reluctance (VR) and hybrid types. The principle of operation for stepper motors is fairly straightforward. Traditional VR stepper motors have a large number of electromagnets arranged around a central gear-shaped piece of iron. When any individual electromagnet is energized, the geared iron tooth closest to that electromagnet will align with it. This makes them slightly offset from the next electromagnet, so when it is turned on and the other switched off, the gear moves slightly to realign. This continues with the energizing and de-energizing of individual electromagnets, thus creating the individual steps of motion. Hybrid steppers combine the best features of both the PM and VR type motors. The rotor is multi-toothed, like the variable reluctance motor, and contains an axially magnetized concentric magnet around its shaft. The teeth on the rotor provide a path to help guide the magnetic flux to preferred locations in the airgap. This further increases the detent, holding and dynamic torque characteristics of the motor when compared with both the VR and PM motors. Hybrid stepper motors are usually more expensive than PM stepper motors, but can provide better performance with respect to step resolution, torque and speed. Control techniques, such as half-stepping and microstepping, let designers get even finer movements of rotation, which make for more exact output than that from VR stepper motors (which can’t usually be microstepped). Hybrids also have higher torque-to-size ratios and higher output speeds than other stepper-motor types, and are quieter than VR models. When the current changes, the rotor can turn a small amount—an improvement over basic PM motors and VR motors.
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Because a stepper motor’s speed is proportional to the frequency of the input pulses, a wide range of speeds is attainable. However, while stepper motors are capable of producing high torque at low speeds, they generally are well suited for lower power applications, not for applications requiring lots of torque to move heavier loads. A few other drawbacks include the possibility that not properly controlling the motor can produce undesired resonance in the system. Also, stepper motors are generally not easy to operate at extremely high speeds, and as the motor speed increases, torque decreases. For two-phase stepper motors, there are two basic kinds of winding structures: unipolor and bipolar. A unipolar arrangement uses six wires, but current can only flow in one direction. These types of motors also require a unipolar driver. A bipolar winding uses four wires; current can flow in two directions, and it requires a bipolar driver. Bipolar motors are generally more efficient and can provide more torque than unipolar models, although they can heat up faster than unipolar motors. A stepper motor’s low-speed torque varies directly with current. How quickly the torque falls off at higher speeds depends on a number of factors, such as the winding inductance and drive circuitry including the drive voltage. Steppers are generally sized according to torque curves, which are typically specified by the manufacturer. One of the most critical parameters for stepper motor performance is pullout torque, which is the highest torque a stepper motor can output at a given speed without losing steps. Manufacturers find a stepper motor’s pullout torque by accelerating the motor up to the target speed and then increasing the torque load until the motor starts missing steps or stalling. Performing this test over a range of speeds and torques lets the manufacturer plot the data in a complete torque or pullout curve, which designers use when evaluating different motor options. To put it another way, the pullout-torque plot (also called slew rate) for a stepper motor shows the maximum torque at various speeds that a stepper motor can generate. If the motor runs outside of this curve, it will stall. The drive must decelerate or accelerate out and into the stepper motor’s pullout curve. A related value is stepper motor pull-in curve—the maximum frequency at which a loaded stepper can start and stop without losing steps. The torque-speed curve changes with inertial mismatch, so designers should aim for a 25 to 50% safety margin when sizing stepper motors. If this is impossible for the application at hand—not unusual for precision applications—other means of compensation may be in order.
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A typical stepper motor, such as the SC3518 series motor from Nanotec pictured here, is a two-phase motor with a NEMA 14 frame size and a 1.8° step angle. Depending on the motor length, torque can range from 0.18 to 0.32 Nm.
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SMART
Download a 20-page brochure with specifications, drawings, and performance curves at
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Networks
MOTION SYSTEMS HANDBOOK
Motion Control
NETWORKS Motion networks, such as EtherCAT, are usually fast and deterministic, processeing data using dedicated hardware and software. EtherCAT is a full duplex, master-slave protocol that accommodates any topology. It can process 1,000 I/O points in 30 μsec and communicate with 100 servo axes in 100 μsec. Photo courtesy of Beckhoff Automation
WHAT’S
the best network for a motion system application? The answer, of course, depends on the specifics of the application. Since most multi-axis motion control uses eventbased synchronization, a limited number of network protocols are suitable. The Internet of Things, even the Industrial Internet of Things, will not affect most motion systems much. Nearly every motion system already makes use of sensor data to measure performance, and nearly all data transmits on some form of Ethernet, the basic backbone of the Internet, including wireless versions. The basic topology of a motion network will begin with a form of real-time version of Ethernet connected to the motion components (motors, drives, sensors, controllers). These components send the data to a higher level network, usually an IT version of Ethernet, which may then be sent through the Internet to dashboards, various storage media or software. Assuring real-time control Event-based synchronization is defined as scheduled, absolute hard delivery of time-critical cyclic data. Data must be delivered usually in less than 1 μsec; slower delivery rates tend to result in jitter, which increases the chances of uncontrolled motion in a machine tool or other motion system.
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Thus, it’s important to define the time delivery needs for the given application. Synchronized multi-axis motion will have different timing needs than a divert actuator on a conveyor system. EtherCAT Ethernet for Control Automation Technology (EtherCAT) was developed by the Beckhoff company. It’s fast and deterministic, and processes data using dedicated hardware and software. It uses a full duplex, master-slave configuration, and accommodates any topology. It can process 1,000 I/O points in 30 μsec and communicate with 100 servo axes in 100 μsec. Axes receive set values and control data and report actual position and status. A distributed clock technique that’s a simple version of IEEE 1588 synchronizes the axes with less than 1 μsec of jitter. This protocol can deliver fast throughput because messages are processed in hardware before they’re forwarded to the next slave. Slaves read data relevant to them as the data frame passes and they insert new data into that same data stream on the fly. This procedure does not depend on the run-time of the protocol stack, so processing delays are typically just a few nanoseconds. Ethernet/IP (EIP) Standard Ethernet cannot guarantee data delivery of less than 1 μsec because of the data layer’s use of Carrier Sense Multiple www.designworldonline.com
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Servomotor
50 V, 4 A
EtherCAT Terminals
Bus Terminals
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(IP 20)
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MOTION SYSTEMS HANDBOOK
Networks
Access/Collision Detection (CSMA/CD) techniques to control packet transmission. To overcome this issue, ODVA developed Ethernet/IP, and it did so without changing any of the four lower layers of Ethernet. Ethernet/IP is an industrial application layer protocol operating over the Ethernet medium that is used for communication between industrial control systems and their components, such as programmable automation controllers, programmable logic controllers or I/O systems. The “IP” stands for “Industrial Protocol,” referring to Rockwell Automation’s adoption of Common Industrial Protocol (CIPTM) standards as Ethernet/IP was developed. Ethernet/IP with CIP Motion as the application layer removes the requirement for strict determinism from the network infrastructure and entrusts the end devices with the timing information necessary to handle the real-time control needs of the application. CIP is also used by DeviceNet and ControlNet, so these networks are interoperable. CIP Motion accomplishes real-time data transmission through the application profiles that define the technology. Those profiles allow position, speed and torque loops to be set within a drive. This protocol makes use of CIP Sync technology, the IEEE-1588 compliant Precision Clock Synchronization, which is also mapped into the CIP object model, to coordinate precise, synchronized motion control. Sercos Sercos is a digital bus that interconnects motion controllers, drives, I/O, sensors and actuators for numerically controlled machines and systems. It’s designed for high-speed serial communication of standard closed-loop, real-time data over a noise-immune, fiber optic ring (Sercos I & II) or Industrial Ethernet cable (Sercos III). In a Sercos interface system, all servo loops are normally closed in the drive to reduce the computational load on the motion controller and synchronize more motion axes than it otherwise could. In addition, closing all the servo loops in the drive reduces the effect of the transport delay between the motion control and drive. Sercos III is the open, IEC-standard third-generation version that transmits data over Industrial Ethernet cabling and protocol for real-time control. It combines the best of both Ethernet and previous Sercos designs for deterministic bi-directional, real-time motion and I/O control. It delivers rich I/O communication capabilities while enabling all conventional protocols to be transmitted over the same Ethernet network efficiently in parallel with Sercos real-time communication. 138
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Sercos III achieves cycle times as low as 31.25 μsec. It supports up to 511 slave devices in one network, with multiple networks possible in a system.
How to select the right motion network
With the vast range of motion control applications, no one-size-fits-all approach to selecting a motion control networks exists. However, a few key factors will help narrow your choices. 1. Reaction time/precision. When selecting a motion network, closely evaluate the scan rate, how the scan is done and how fast the system can react. At a minimum, and critical for a motion network, is the ability to close motion loops back at the controller to each drive. This function is paramount if the application has any level of coordinated motion, as each axis needs to receive its own unique commands. Some network protocol promoters will insist that slave-to-slave communication is a substitute for closed-loop control, but this is only true in the simplest of applications, such as conveyor drive applications where there is only one master command. 2. Determinism. Determinism is not related to network speed or raw scan rate. It is the feature of predictable data transmission; regardless of how fast or slow, you will know exactly when data comes through. Many industrial Ethernet fieldbus systems available today actually have a poor scan rate, but the vendors behind each will promote their system as deterministic, which may be true. But determinism will not guarantee quick reactions to external events. 3. Specific controller requirements. Check to see if the chosen controller already has an embedded PC hardware platform specified, or requirements around a particular Real Time OS (RTOS). 4. Third-party device support. Consider whether the desired motion network has a number of “third party” master controllers and slave devices available. 5. Diagnostics. Modern industrial Ethernet protocols provide a wealth of built-in diagnostics to help you know more about a network’s status and where maintenance may be required. 6. Cost of system. Ethernet-based motion networks have some advantages through the use of standard Ethernet media and hardware.
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MOTION SYSTEMS HANDBOOK
Positioning Stages
An overview of
POSITIONING STAGES POSITIONING
stages and tables are used in motion control applications to hold down a part or component and position it for some operation. A positioning stage or table is most often a complete motion sub-system. That is, they’re composed of a system of motion control components such as linear motion assemblies made up of linear guides or carriages and some type of drive mechanism, like a motor or actuator, but also encoders, sensors and controllers. Stages and tables are used in a range of positioning applications, such as in fiber optic and photonic manufacturing and assembly, vision systems, machine tools, semiconductor manufacturing equipment, micromachining and electronic manufacturing. Stages can provide one of two basic An exploded view of a positioning types of motion: linear or rotary. Linear stage, such as this indexing table stages can be configured to provide motion from Weiss North America, shows in one direction only (single axis x, y, or z), the various components including motion in two dimensions (x-y positioning, the direct-drive motor.
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for instance), or in three dimensions (x-y-z positioning). A typical multi-axis positioning stage has 6 degrees of freedom. These can be thought of as three linear axes (horizontal x and y axes and a vertical z axis) and three rotational axes, one about each linear axis (usually designated with a θ as θx θy and θz). There are some common positioning errors that crop up for both linear and rotary positioning stages. For instance, in linear stages, the critical parameters include flatness and straightness, where sizable deviations can cause errors in motion. Sometimes there can also be a tilt along the axis, a slight angular deviation which can cause errors in positioning. Accuracy and precision are key parameters for positioning stages. Depending on what is being positioned, the accuracy demands can vary greatly, from millimeter precision down to the nanometer range. While more conventional positioning stages can handle lower accuracy positioning demands, high-precision applications requiring extremely small and precise positioning may often use nanopositioning systems built on piezo motors or actuators that are capable of resolutions in the micro- or nanometer range. The drive mechanisms for positioning stages and tables can also vary significantly,
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TW Series: An integrated torque motor combined with high-accuracy gear reduction: The TW combines the dynamics, precision, flexibility, and ease of use of the direct drive with a high power density and the precise and robust WEISS mechanics. TW 150
TW 200
TW 300
SW / ST Series: The SW/ST rotating modules with absolute rotary encoder are just what you need when fast, precise, and highly dynamic rotating, swivelling, and gripping movements are called for. Compact profile, light weight, with various mounting-possibilities.
ST 75 SW 140
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Technology that inspires
Servo Index Tables – NR / CR / NC NR Series: Freely programmable rotary indexing ring with very large central opening, extremely flat design and a high level of parts accuracy. The ring-shaped design allows extra free design space. The rotating aluminium ring can be adjusted to your specifications in terms of diameter and thickness.
NC Series: The NC combines robustness and durability with the advantages of a freely programmable rotary table offering a high level of torque. The NC differentiates itself from the TC range through its use of a brushless AC servo motor drive. In addition, the drive curve has a constant rise.
CR Series: Flat heavy duty ring with large central opening. The extremely flat design frees up space and offers flexibility for creating ergonomically sound workplaces. Using the WAS control system, the ring is completely freely programmable.
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MOTION SYSTEMS HANDBOOK
Positioning Stages
For extremely high-precision applications, piezo nanopositioning stages such as the QNP-XY Piezo Stages from Aerotech, feature travel distances from 100 to 600 μm with naonmeter resolution.
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depending on a number of factors including cost and desired accuracy. For instance, linear or rotary stages can be direct-drive types driven by brushless dc motors and ballscrews or linear servomotors, or by a combination of motors and gearing and couplings. Other methods can include electric, pneumatic or hydraulic actuation combined with belt and pulley systems, ballscrews or leadscrews. Precision and accuracy requirements can also dictate design decisions such as the components used in assembling a positioning stage. For example, air bearings are used in stages where reliability and high accuracy are desired. Air bearings support a load with a thin film of pressurized air between the fixed and moving elements. They are typically referred to as aerostatic bearings, because a source of pressure rather than relative motion supplies the film of air. Unlike ordinary bearings, the surfaces of an air bearing do not make mechanical contact, so these systems do not need to be lubricated. Because the surfaces do not wear, the systems don’t generate particulates, which makes them suitable for clean-room applications. When supplied with clean, filtered air, the bearings can operate without a failure for many years. Some important parameters for selecting the proper positioning stage include things like the needed resolution of the application (or the smallest increment to move or measure), the required repeatability and accuracy, and the acceptable levels of backlash or hysteresis. Other factors involved in selecting a positioning stage include knowing the load specifications. How much does it weigh? Loads vary from a few grams up to hundreds of kilograms. Also, how far and how fast must it travel? Travel distance will help determine the length of the stage, which can be a few millimeters (as in nanopositioning stages) to several meters.
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Rack-and-pinion Sets
MOTION SYSTEMS HANDBOOK
This roller-pinion system from Nexen Group acts as a cam and follower. Its cycloidal contact curves means there’s no slipping. Rack-tooth geometry minimizes backlash by loading pairs of pinion rollers in opposition.
RACK-AND-PINION SETS: How they work
OPTIONS
for linear motion abound, but today’s rack-andpinion sets are precision mechanical devices that, in some cases, deliver performance rivaling that of electromechanical alternatives. First modernized 150 years ago to drive railway trains up steep passages, rack-and-pinion sets today are useful in everything from small consumer devices that move a few ounces to large industrial machinery that moves tons of load. That includes off-highway machinery, food-processing, packaging lines and other applications that involve reciprocating motion. Here’s how rack and pinions work: A parallelaxis gearset converts rotary motion to linear motion through a motor-driven pinion (essentially a specialty spur gear or engineered roller) that engages the rack (which is essentially a gear of infinite diameter). In rare cases, an input moves the rack to get pinion rotation. Usually, rack-andpinion sets get paired with servomotors or (less commonly) with step motors. One rack-and-pinion variation is based on a roller pinion. These ultra-quiet rack-andpinion sets have a pinion of bearing-supported
rollers instead of a spur gear. Rollers ride the rack-tooth surfaces with repeatability to about 2.5 μm from one direction (or better than 5.8 μm from both). Manufacturers also make the rollers with meshing geometry to trace paths tangent to tooth faces. So, the rollers glide into engagement with the rack teeth, which eliminates the wear and inaccuracy from sliding friction and tooth slap of some rack-and-pinion sets. No matter the version, the benefits of rack-and-pinion sets are that they can operate without enclosures or protective covers; they are efficient to 98% or better; and many exhibit backlash of 1 arc-min or less. Another strength is that they’re often less expensive than comparable linear motors when stroke lengths are long … so that a rack-and-pinion set may cost half of what a linear motor costs … especially for many-meter strokes. Rack-and-pinion sets sometimes perform better than ballscrew actuators because they’re not affected by adjacent bearings, couplings or bores; they’re also immune to stiffness degradation, even over long lengths. What’s more, as with any gear-based power transmission, rack-andpinion sets come in several gear versions to satisfy various application requirements. For example, some helical-toothed racks sport helix angles engineered for quiet operation (with a high tooth-contact ratio), even under high loads at high speeds. Rack-and-pinion installation is straightforward. The racks mount on flat surfaces, and many versions sport forgiving designs and mounting features to maintain performance without perfect assembly. Even so, misaligned or improper mounting can damage sets and the attached gearmotor’s bearings. One common mistake is to put the motordriven pinion too far from the rack. Here, design engineers should ensure that the pinion-to-rack distance is set to the manufacturer’s recommendation and that the rack and motor-driven pinion is perpendicular to the rack within tolerances.
This dynamic rack-and-pinion system from Wittenstein can drive linear motion to unlimited travel lengths. Some preassembled versions run true to 10 µm; those with actuators and servogears exhibit less backlash than standard setups.
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HIGH SPEEDS. 11 m/s (36 ft./sec.)
INCREDIBLE ACCURACY. ± 30 µm (± 0.00118 in.)
ZERO BACKLASH. UNLIMITED LENGTH.
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Introducing the patented Precision Roller Pinion System (RPS)
Make your machines run faster and more accurately with less maintenance. The new, patented RPS engages two teeth simultaneously to achieve unmatched accuracy with zero backlash—in any desired length! This amazing system is also extremely quiet, 99% efficient and can run without lubrication. Overcome the speed, accuracy, load and noise limitations of rack and pinion, ball screw and belt drive technologies with a lower maintenance, longer lasting solution.
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OVERCOMING COMMON PROBLEMS Found in Traditional Drive Systems
INDUSTRY PROBLEMS
Ball Screws
Traditional Rack/Gear & Pinion Systems
Low Accuracy Backlash/Vibrations
4
4
High Cost
4
4
Dirty Operation
4
4
High Maintenance
4
4
Low Load Capacity 4
4
Low Speed
4
4
4
4
High Positional Accuracy
4
4
Near-Zero Backlash 4
4
4
4
4
Little to No Maintenance
4
High Load Capacity Quiet: Pinion Rollers Glide Smoothly Along Teeth
4
High Speeds (Up to 11 m/sec) 4
4
Limited System Length/Size
4
4
Economical, Efficient Components No Dust Emissions
4
Magnetic Field High Wear/Low Life
NEXEN ROLLER PINION SYSTEMS
Chain Drives
4
Noisy
Linear Motors Direct Rotary Stages Direct Drive Motors
Belt Drives
No Magnetic Field
4
4
Long Life (Up to 36 million meters)
4
4
Custom Rack Sizes & Modular Components
Discover the Nexen Advantage by going to www.nexengroup.com
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Retaining Rings
MOTION SYSTEMS HANDBOOK
Spiral retaining rings do not need special tools for removal. The removal notches on the rings make them easy to extract from a groove. Image courtesy of Smalley Steel Ring Company.
RETAINING RINGS:
Holding parts together RETAINING
rings are fasteners that hold together components on a shaft or in a housing when installed in a groove. Three main types of retaining rings include tapered section, constant section and spiral. Tapered section rings typically show a decreasing thickness from the center of the ring out to the ends. They can be mounted either axially or radially. The taper ensures full contact with the groove when installed. Constant-section retaining rings have a constant width around the circumference of the ring. When installed, these rings don’t maintain uniform contact with the entire component. They take on an elliptical shape and contact the groove at three points. Spiral retaining rings install into a housing or onto a shaft, making full contact with the groove and component. Their grooves are relatively
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shallow, so their load bearing capability is reduced. They are often selected when full contact with the retained component or a lower axial profile is required. Spiral rings have no protruding ears to interfere with mating components in an assembly. The ring has a uniform cross-section and no gap or lugs for a functional and aesthetically pleasing ring. Unlike traditional fasteners, retaining rings eliminate machining and threading, reducing costs and weight. Spiral retaining rings do not require special tools for removal and are supplied standard with removal notches for easy extraction from a groove.
Brief history of retaining rings
In 1930, inventor Hugo Heiermann patented a spring ring to prevent axial component displacement. As in modern setups, the spring installed in a shaft groove but projected beyond the shaft diameter to form a secure shoulder for better assemblies. Originally manufactured to DIN metric standards, early retaining rings were thicker rings that locked into shallow grooves. Sudden overload on such an assembly dislodges the ring from the groove without damaging the housing or shaft. Later designs from U.S. manufacturers were made to standard English dimensions and with thinner formats to
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THE ENGINEER’S CHOICE™
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Retaining Rings This setup shows a single-turn wave spring and endcap retaining a bearing on a shaft. The ring can be a replacement that fits into a preexisting groove. This is an assembly shown without an endcap. Image courtesy of Rotor Clip.
MOTION SYSTEMS HANDBOOK
engage deeper and narrower shaft grooves. Sudden overload on such an assembly makes the shaft or housing fail first and minimizes damage to held components. As U.S. retaining-ring manufacturers moved to making more metric-approximate versions in the 1970s, a line of retaining rings to meet metric applications arose. Called the ANSI Metric line, these MHO, ME and MC series rings have a thinner ring that seats in a deeper groove to conform to metric specifications. However, global industries have increased the use of DIN metric rings in addition to ANSI metric and inch standards.
Selecting retaining rings
When selecting a retaining ring for an application, several factors dictate which retaining ring is most suitable. First ask: What kind of assembly does the application require? Is it a housing assembly or shaft assembly? Next, determine the main critical dimensions. These include the housing or shaft diameter, groove diameter and the groove width. Also, what is the rotational speed (usually in rpm) of the assembly? Next, determine the maximum thrust applied to the ring. Generally speaking, designers define this thrust as either a light, medium or heavy-duty load. It’s important that the design engineer define the maximum thrust because its value also helps determine if groove deformation or ring shear could be a problem. Basically, groove deformation occurs because the groove material is soft, which in turn limits maximum capacity. Ring shear, on the other hand, occurs when the groove material is hardened but the load exceeds the ring’s maximum capacity. Other factors, such as the temperature as well as
the presence of any corrosive media, also dictate the most suitable choice for ring material. Engineers can use modern retaining devices (including spiral rings, wave springs and retaining rings we cover here) to replace traditional fasteners and reduce overall design cost. Retaining rings also reduce overall design size and machining complexity. Case in point: Retaining rings work in bearing-retention setups as traditional alternatives, but they add rigidity and end-play takeup. They also work like hard shoulders to trap wave springs and help them apply preloads to assemblies that need it. Consider a machine setup with a bearing that must stay on a shaft. Here, the retaining ring must withstand axial loads and rotational speed. Traditionally, a combination of an endcap and wave spring holds bearings in place. But a multi-turn wave spring can make 360° (gap-free) contact with the groove periphery of groove. This geometry also eliminates lugs, which equates to designs that are radially more compact. Retaining rings can have stainless, carbon and alloy construction with finishes that include black oxide, oil dip, zinc phosphate and cadmium plating. Engineers can also replace end caps with multi-turn rings as a way to accommodate axial bearing loads and high speed but still keep the assembly compact.
Spirolox retaining rings hold a range of components in diverse products ranging from small electric appliances to jet engines. The design speeds product assembly. Image courtesy Smalley Steel Ring Company.
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Shock & Vibration Absorbers
The basics of MOTION SYSTEMS HANDBOOK
SHOCK & VIBRATION ABSORBERS
MOTION
is present in almost all industrial automation systems. Stopping or changing the direction of that motion releases kinetic energy, which can cause shock and vibration to occur. Any sudden shock in a system can cause immediate damage to the overall machine and the components it may be manufacturing or processing. And consistent vibration inputs can cause damaging fatigue over time. This is why it’s necessary to decelerate a system smoothly through the use of shock and vibration attenuation components. Based on the type of inputs present in the application, vibration and shock attenuation components can be comprised of shock absorbers, linear dampers, wire rope or spring isolators, elastomeric isolators, air springs or structural damping treatments. These devices help manufacturers reduce equipment downtime and costly cycle time limitations.
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These products can be used in a broad range of applications, from the rate control mechanisms that slow the motion of the overhead luggage bin or seat recline on commercial aircraft, to the isolators that keep GPS systems from losing signal or becoming damaged on farm and construction equipment as they harvest crops or pave roadways. Most shock absorbers achieve their damping characteristics through the use of hydraulic fluids. The fluid is pushed by a piston and rod through small orifice holes to create damping; this action compresses some type of gas. This in turn creates a spring force to return the rod back to its starting position when the load is removed. Shock absorbers and dampers are generally made of high-strength steel to handle the pressures from the internal hydraulic forces. Elastomeric seals prevent the fluid from leaking out of the cylinder, and
Vibration and shock attenuation components, such as these wire rope isolators, help reduce vibration in industrial applications. Photo courtesy of ITT Enidine special plating and coatings keep the units protected from harsh operating environments. Recent and ongoing developments in sealing technologies and in the internal designs of shock absorbers and dampers have allowed for longer service life and more compact designs. Ongoing research in the field of noise attenuation (high-frequency, low-amplitude vibration) has led to an increased effectiveness in noise reduction technologies. www.designworldonline.com
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Shock & Vibration Absorbers A unique application for these types of hydraulic damping devices has come with the increased awareness for seismic and environmental protection of our infrastructure. By adding damping to these critical structures, energy is absorbed by the hydraulic devices instead of damaging the structure. Vibration isolation products rely generally on mechanical designs to achieve their isolation characteristics. A spring function provides support for the mounted equipment, while decoupling it from the vibration source. Friction and elastomeric material properties give the isolators their damping characteristics. Isolators can be made from a variety of materials. Wire rope and spring isolators can be made from carbon steel, stainless steel or aluminum. Elastomeric isolators generally have metallic components that function as mounting brackets, separated by an elastomeric material that provides the stiffness and damping desired. Common elastomeric compounds include natural rubber, neoprene and silicone; however, a vast selection of compounds and compound blends can be used to achieve different characteristics specific to the application.
Air springs are comprised of metallic end fittings coupled by a composite elastomeric-based bladder that contains the compressed air used to provide isolation. These single-acting designs are comprised of a pressurized bladder and two end plates. As air is directed into the air bladders, they are expanded linearly. All of these reusable designs are self-contained, offering a number of advantages over any other technology that may require outside componentry. For example, hydraulic systems may require plumbing, while electrical systems may require wiring and power. Energy or power dissipation is key when selecting a damper or shock-absorbing device. The size and characteristics of the device are based on these inputs, so it is generally the first consideration to make. Dynamic spring rate and damping are the two biggest considerations when selecting an isolator. These characteristics will define the natural frequency (sometimes referred to as resonant frequency) of the isolation system and are important in achieving the desired performance.
solution, the ACE Application Engineers can save you time and help you size the right gas spring for your application.
Shock absorbers use hydraulic fluids slowly and smoothly to stop machine operations. Photo courtesy of Ace Controls
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motioncontroltips.com
Basics of
TRANSDUCERS IN One of the most common transducer types is the inductive proximity sensor, such as this one from Balluff, which uses a non-contact principle to detect metal objects.
general, a transducer is a device that converts one form of energy into another. Common types used in industrial applications include sensors to measure temperature, pressure, force, strain, liquid levels and flow rates. These physical quantities are converted to electrical signals in either analog or digital form and can be used to gain information about or control some process. In motion control applications, transducers can refer to any one of a number of sensors, such as rotary or linear encoders, or resolvers for position feedback, such as tachometers for speed sensing, and even proximity switches to initiate or halt some machine action. They also come in various types and the choices available depend on the quantity being measured or controlled. For motion control applications, the measured variable is typically position or speed. Depending on a number of factors, the right transducer may be an encoder or a resolver or a simple potentiometer. A common approach to measuring position is a magnetostrictive linear displacement transducer (MLDT), which is typically mounted inside the cylinder. MLDTs are best because they use moving magnets that don’t come in contact with the sensor tube, avoiding mechanical wear. They also provide an absolute position readout, requiring no homing step before beginning to work with the position information from the MLDT. Advances in MLDT technology have led to resolutions down to 1 μm, with fast signal processing of up to 1.5 MHz.
There are a few key factors to consider when selecting any transducer, including the desired variable to be measured, the accuracy or resolution needed and the type of output, as well as any size or space restrictions, environmental factors, and product lifetime and cost. For starters, what kind of motion is involved, rotary or linear? Specific transducer types exist for each kind of motion. Determine the accuracy needed for the application. This includes factors such as linearity, resolution and repeatability. What range must the transducer measure? For linear applications, is the range on the order of nanometers, a few millimeters or several feet? For rotary applications, if measured in degrees, is the angular distance more or less than 360°? Is the type of encoder needed a single-turn or multiturn device? Also, if selecting an encoder, does the output need to be absolute or incremental? Consider the type of output. Is it voltage or current? Digital or analog? Many transducers are programmable with a data connection, such as a PC-to-USB link. Other interface options can include encoder-specific communication links like SSI (synchronous serial interface), BiSS (bi-directional serial/synchronous) or PROFINET.
Linear position sensors, like this Micropulse linear position transducer from Balluff, use a non-contact magnetostrictive technology and produce absolute output position signals.
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Wave Springs
Storing mechanical energy: MOTION SYSTEMS HANDBOOK
WAVE SPRINGS THERE
As compressed air goes through this valve, a crest-to-crest wave spring from Smalley Steel Ring compresses to maintain force that precisely regulates flow.
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are almost as many variations of wave springs as there are applications. Although often out of sight, these springs are essential in many motion-control applications—including gear assemblies, actuators and different kinds of clutches—and consumer applications. At its most basic level, a spring is a device that stores mechanical energy. There are three types of springs: tension, compression and torsion. Tension springs operate in tension with a load attached, so as the application applies load, this spring stretches out. Compression springs, as the name implies, operate under compression, so as the application applies load, this spring shortens. Torsion springs operate under twisting loads, so the application applies torque to the spring. Compression springs are the most common wave spring type. Manufacturers make these out of coiled flat wire with waves added along the coils for more spring effect. Such wave springs can replace conventional round-wire compression springs in applications that have minimal space but need tight load deflection. As an alternative to other coil springs, wave springs occupy 30 to 50% of the compressed height space of comparable round-wire springs with the same deflection and load specifications. Manufacturers typically make wave springs from a single filament of flat wire formed in continuous precise coils with uniform diameters and waves—most commonly out of 17-7pH hardened stainless steel. Manufacturers make the springs with either plain ends (for wavy) or squared-flat ends (with shim ends). 8 • 2015
Wave springs operate as load bearing devices. They take up play and compensate for dimensional variations within assemblies. They can produce a range of forces whereby loads build either gradually or abruptly to reach a predetermined working height. This establishes a precise spring rate in which load is proportional to deflection. Some manufacturers offer single, nested and multi-turn wave springs. Single-turn wave springs with overlapping ends save axial space so there’s more space for travel; overlapping ends also prevent radial jamming by allowing circumferential movement. The spring clings to the bore, which saves radial space. Manufacturers classify these springs according to the specification known as spring load at work height. Nested wave springs suit applications requiring higher forces to meet safety regulations, such as those in government or military applications. A nested wave spring provides a higher load than single-turn wave springs (a stamped wave washer), but uses the same radial space. Multiple-turn wave springs do not cling to the bore, because radial jamming affects the specified load at work height. If the design of the multi-turn wave spring results in peripheral movement of the turns against each other, this can render the spring unstable. Compared to a single-turn design, longer travel or deflection is possible because the deflection in total is split. Every turn must tolerate less deflection than a single-turn design. Use of a multi-turn wave spring can also save 50% in axial space compared to a traditional coil spring. What’s more, these springs eliminate the risk of torsional movements during compression to work height … a real problem with coil springs. In contrast, a mutliturn wave spring always applies its load in an axial direction. They also apply consistent loads within a small tolerance range at different work heights. Here, a wave spring from Smalley Steel Ring keeps a needle bearing secure inside an assembly.
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HOW DO I KNOW IF I’M TALKING TO AN ENGINEER OR A SALESMAN? Ask Smalley. We have nothing against sales people. But when it comes to differentiating Inconel from Elgiloy or overcoming dimensional variations within a complex assembly, wouldn’t you rather work with an engineer? Our customers would. That’s why they collaborate directly with our world-class team of Smalley engineers—experienced professionals whose only focus is helping you specify or design the ideal wave spring, Spirolox® retaining ring or constant section ring for your precision application.
Smalley wave springs reduce spring operating height by 50%, saving space and weight, fitting tight radial and axial spaces. We offer more than 4,000 stock sizes in carbon and stainless steel.
Visit smalley.com for your no-charge test samples.
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Ad Index Encoder Products Co................................................ 75 FESTO..................................................................... 116 Galil Motion Control.................................................. 49 GAM............................................................................ 5 Groschopp, Inc.......................................................... 91 Harmonic Drive LLC.................................................. 85 Helical Products Co................................................... 57 HELUKABEL USA....................................................... 45 Hitachi Cable America Inc......................................... 44 HIWIN Corp............................................................... 33 IDEC Corp................................................................. 51 igus, Inc............................................................... 35, 43 IKO International, Inc.............................................. 114 Intech Corp. .............................................................. 77 ITT Enidine.............................................................. 151 KB Electronics, Inc................................................... 131 Keubler Inc................................................................ 73 Lee Spring Co. .......................................................... 47 Lenze Americas........................................................... 3 Lin Engineering....................................................... 133 LinMot USA, Inc. ..................................................... 127 Maple Systems, Inc. .................................................. 96 Martin Sprocket & Gear, Inc...................................... 39 maxon precision motor, Inc..................................... 121 MICROMO................................................... Cover, 123 Mitsubishi International Corp.................................... 97 Moog Inc., Animatics............................................... 132 Moog Inc., Components Group.................................. 1 NB Corp. of America................................................. 29 Neugart USA Corp.................................................... 79 Nexen Group, Inc.................................................... 145
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Nippon Pulse America, Inc...................................... 128 NOOK Industries, Inc.............................................. 105 NSK Precision America.............................................. 31 Omron Corp............................................................ 139 Opto 22..................................................................... 95 Patrick Plastics Corp.................................................... 8 PBC Linear............................................................... 113 PHD, Inc.................................................................... 25 PITTMAN................................................................. 103 Pro-face America....................................................... 99 R+W America...................................................... 61, 63 Renishaw Inc................................................................ 2 Rotor Clip Co., Inc................................................... 149 Ruland Manufacturing Co., Inc.................................. 65 Schneider Electric Motion USA............................... 125 SCHUNK, INTEC....................................................... 93 Servometer................................................................ 59 Setco Inc. ................................................................ 107 SEW Eurodrive........................................................... 89 Siemens Industry....................................................... 71 Smalley Steel Ring Co..................................... 147, 155 Stock Drive Products/Sterling Instrument................. 37 THK America, Inc...................................................... IFC Tolomatic, Inc............................................................ 27 TURCK, Inc...................................................... 115, 117 US Digital................................................................... 76 Vena Engineering Corp............................................. 50 Weiss North America............................................... 141 Yaskawa America, Inc................................................ 67
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Accuride International Inc....................................... 111 ACE Controls Inc..................................................... 152 Aerotech Inc............................................................ 143 All Motion.................................................................... 4 Altech Corp............................................................... 12 AMETEK PMC/Dynamic Fluid Solutions.................... 15 AMETEK Catalyst Motion Group............................... 19 AMETEK PMC/Dynamic Fluid Solutions.................... 21 Applied Motion Products, Inc. ................................ 129 AutomationDirect........................................................ 7 Beckhoff Automation............................................... 137 BEI Sensors................................................................ 13 BellowsTech, LLC....................................................... 59 Bimba Manufacturing Co.......................................... 10 Bishop-Wisecarver Corp. ........................................ 109 Bison Gear & Engineering Corp. .............................IBC Bodine Electric Co. ................................................... 81 Bosch Rexroth Corp. .................................................. 9 CC-Link.................................................................... 101 CGI, Inc. .................................................................... 83 Chieftek Precision Co., LTD..................... 108, 110, 112 Cicoil Corp. .............................................................. 41 Clippard Instrument Laboratory, Inc..........................BC Crouzet Motors................................................ 134, 135 Del-Tron Precision, Inc............................................... 23 Delta Computer Systems, Inc.................................... 52 Delta Tau Data Systems, Inc...................................... 53 DieQua Corp............................................................. 87 Dorner Mfg. Corp. .................................................... 55 Dunkermotor, part of AMETEK.................................. 17 Dynatect Manufacturing Inc...................................... 32
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