Motion Systems Handbook 2019

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What ancient myths can teach us about

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As our editorial team was preparing this year’s Motion Systems Handbook, covering the basics of motion technologies as well as their ongoing evolution, I began to wonder about the origins of the technology we cover.

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This wonderment intersected with some of my summer reading, in particular Adrienne Mayor’s absorbing book Gods and Robots, subtitled “Myths, Machines, and Ancient Dreams of Technology.” One of the most revealing features of the book is that Mayor recounts, in great detail, vivid examples from the ancient Greek world of what we would recognize as some of the first automated systems. We discover that the ancient Greeks conceived of many of the technologies that we’re familiar with today – the desire for flight, animated statutes (proto-robots), artificial life forms, biological enhancements to human bodies, and more.

The stories and myths in which these technologies appear, in fact, continue to enthrall us today. In many ways, they functioned as some of the earliest forms of science fiction by taking the actual state of things and imagining how a small change here or there, or the addition of a new technology, brings about a new reality in some at first exciting but then quickly terrifying way.

The myths served many functions, one of which was as cautionary tales and lessons about our own humanity, reminding us of the price paid for acting on our desires. Take, for instance, the story of Prometheus, and his lesser-known brother Epimetheus. Prometheus (meaning “forethought”) is portrayed as the helper of humanity, giving them the gift of fire that enables human progress and civilization. His brother Epimetheus (meaning “afterthought”) in some stories is made responsible for all of humanity’s problems having thoughtlessly accepted the gifts of Pandora from the gods.

4

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However, even the story of Prometheus contains within it an ambivalent attitude towards human curiosity, that which propels science and knowledge, but also is not without often unintended, undesirable consequences.

Not surprisingly, a quick glance at science fiction writing over the years reveals a related concern with unintended outcomes of technological development. In the early 19th century, it was Mary Shelley’s Frankenstein (originally subtitled “The Modern Prometheus”), with its anxieties about the consequences of the industrial revolution. In the aftermath of World War II and the development and use of nuclear weapons, it was the Japanese figure of Godzilla that projected the fears of the new nuclear age.

Today, countless stories and movies portray our collective anxieties about the rise of robots and artificial intelligence. We’re even seeing some observers questioning the value of social media given the hatred and intimidation it aids in spreading across the internet and into the real world.

The ancient Greek myths remind us that the desire to automate processes, to create artificial life forms that would do our bidding and make our lives easier is as old as human civilization itself. And yet, the dark side of the myths were there to warn humans, reminding them of the things that they’d perhaps not want reminded of; the persistence of human shortsightedness, ignorance, and other darker, destructive motivations.

Today, as humans attempt to come to terms with the negative consequences of climate change including the ways in which our industrial activity has altered the planet, we need to think now more than ever about the consequences of our technological development. These ancient lessons are something humans in the 21st century should not forget as we continue to build versions of our own imagined worlds.

MILES BUDIMIR | SENIOR EDITOR 8 • 2019

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VO LU M E 5 N U M B ER 3

CONTENTS MOTION CONTROL HANDBOOK Actuators Electric......................................................10 Pneumatic.................................................18 Rigid chain................................................22

10

Ball & roller screws.........................................24 Bearings..........................................................26 Belts & pulleys • Chain drives .......................32 Brakes & clutches • Torque limiters...............38 Cables & connectivity.....................................44 Controls Motion......................................................50 PC & PAC.................................................54 Conveyors......................................................58

78

Couplings.......................................................63 Encoders.........................................................72 Gearing...........................................................78 HMIs...............................................................86 Linear guide rails • Slides & ways..................88 Motors DC............................................................96 Gearmotors........................................... 100 Linear..................................................... 104 Servo..................................................... 108 Stepper.................................................. 112 Positioning stages....................................... 116 Retaining rings............................................ 121 Seals............................................................ 122 Sensors & transducers................................. 125 Shocks • Damping & holding...................... 127

108

Springs: Compression & wave spring......... 134

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DESIGN WORLD — MOTION

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DESIGN WORLD EDITORIAL

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VP, Editorial Director Paul J. Heney pheney@wtwhmedia.com @dw_Editor

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Staff — Motion Control HB 08-19.indd 9

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DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

Linear actuators: Make versus buy

Parker Hannifin’s Hybrid Actuator Systems are high-force self-contained linear actuation. They offer te benefits of electrical actuators and the power density and failsafe characteristics of traditional hydraulics.

With the range of linear actuators on the market, it’s becoming easier for machine builders and end users to find a standard or “customized standard” product that meets even the most unique application requirements. But there are still times when it makes sense, from a financial or technical standpoint, to

LINEAR-ACTUATOR BUSINESS EVALUATION

design and build an actuator in-house.

The first question a company should ask before they embark on making their own linear actuator is: do we have the expertise to do this in-house? Keep in mind that this includes not only the mechanical design, but also sizing and selecting the motor and controls and integrating the system. In many cases, one department or functional area has experience in mechanical

However, the make vs. buy decision isn’t as clear-cut as simply adding up the costs of do-it-yourself components and comparing that to an off-the-shelf solution. Choosing between a homemade actuator and a pre-made one means considering many factors,

10

some of which are difficult to quantify in terms of time or dollars. In addition to the technical details of design and assembly, there’s the basic business evaluation of whether it can (and should) be done in-house, and the often-overlooked factor of after-sale support.

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Electric Actuators — Motion Control HB 08-19 V3.indd 10

8 • 2019

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MOTION SYSTEMS HANDBOOK

design, while another department has the electrical expertise. In these cases, capture both knowledge sets by setting up a crossfunctional team for the design and engineering phase.

If the answer to the expertise question is “yes” the next thing to consider is whether you have the capacity and resources to do it in-house. This is a question that is often glossed over, because managers assume that if a project is done in-house, the company has control over quality, cost, and timeline. But building an actuator means relying on multiple outside suppliers and being at the mercy of their delivery and quality. If the project is delayed, customers could be affected or profits could be impacted.

KEY QUESTIONS: Do we have the linear motion and controls expertise to build the actuator in-house? What are the consequences if the project is delayed?

MANUFACTURING AND ASSEMBLY REQUIREMENTS Building an actuator in-house means that you have the freedom

to design it for your specific needs. The main components of any actuator are the base, the guideway, and the drive unit. One of the simplest types of actuators to build in-house is a design based on an aluminum extrusion. The benefit of using an extrusion is that they are readily available in a broad range of cross-sections and lengths. They also require little or no modification for mounting the guides and the drive mechanism. The drawback to using an extrusion is that, when high travel accuracy is required, it’s difficult to obtain a precise mounting surface.

The next option is a machined base plate. Relatively inexpensive and easy to machine, aluminum plate is a good solution for many in-house built actuators. If very high travel accuracy and rigidity are needed, machined steel plate is typically the best choice. Whether aluminum or steel, if a machined base plate is used, be sure to assess whether the machining can be done in-house. This also applies to other machined parts, such as motor and gearbox mounts, carriage plates, and protective covers. These items can easily be overlooked when evaluating

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DESIGN WORLD — MOTION

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8/15/19 1:37 PM

ACTUATORS • ELECTRIC

the time and capacity needed for in-house production.

The type of guide to be used is often decided hand-in-hand with the decision regarding the actuator’s base structure.

When an aluminum extrusion is used as the base, the best choice is typically a single guide based on cam rollers, wheels, or round shaft and linear ball bushings. For higher rigidity, profiled rails and carriages can be used. But unless the extrusion can be machined for a precise mounting reference, the benefit of higher travel accuracy that profiled rails normally provide will be forfeited by inaccuracies in the extrusion.

Applications that have high moment loads typically require two guides mounted in parallel, which necessitates the use of either a very wide extrusion, or a base plate to accommodate the side-by-side guides.

The drive mechanism, regardless of whether belt, screw, rack & pinion, etc., should be mounted between the two guides in order to reduce the effects of binding. In dualguide designs, it’s important to align the two guides properly to avoid binding, which can cause additional forces on the bearings and reduce their service life.

Drive mechanisms are usually belts or screws. Belt drives are relatively easy to integrate because they don’t require precise alignment. However, they must be properly tensioned to remove “slack” from the belt and allow it to meet its force rating.

Tensioning during assembly is usually not an issue but think about how the end-user will check the belt tension and re-tension it if necessary. This should be a relatively simple maintenance process and not require significant disassembly and reassembly time for the user.

If a ball or lead screw is the drive mechanism, determine what type of end support is needed. The fixed-simple arrangement (an angular contact thrust bearing on the driven end and a single ball bearing on the non-driven end) is most common, but if significantly high speeds or buckling loads will occur, a fixed-fixed bearing arrangement may be appropriate. motioncontroltips.com | designworldonline.com

Electric Actuators — Motion Control HB 08-19 V3.indd 13

KEY QUESTIONS: Can the necessary

machining, alignment, and assembly be done in-house? Does manufacturing have the capacity to meet the required production?

CUSTOMER-SUPPORT NEEDS FOR ACTUATOR END USER Most actuators perform a critical function in the machine or process, and downtime can be catastrophic to production output.

This makes customer support an essential consideration in the make vs. buy decision.

Do you have staff who can support the actuator if troubleshooting or service are needed? Are you able to carry the spare parts required for quick repairs?

These considerations are key whether the actuator is part of a production machine sold to external customers, or part of an in-house machine — with one of your own departments as the customer.

Also remember that components with rolling or recirculating elements will likely need relubricating during the life of the machine. This is especially true for ball screws, linear ball bearing guides, and profiled rail guides. Even if components are “lubed for life” application conditions may warrant re-lubrication during the machine’s useful lifespan. Taking userfacilitated maintenance into account during the design phase will ease service requirements.

KEY QUESTIONS: Do we have the

structure and resources (staff and inventory) to support end users if troubleshooting and repairs are needed?

How easy is it for the user to perform basic maintenance?

Just as buying a pre-made actuator should include an assessment of all the costs involved, making an actuator in-house requires considering the engineering, manufacturing and service requirements, as well as the cost of components.

8 • 2019

Some actuators are application specific. Case in point: Tolomatic’s new RSX press model (RSX096P) electric linear actuator increases the force range of the existing RSX product family to 40,000 lbf (178 kN) and is for force in the extend direction. The RSX press-model electric actuator works for metal-forming and assembly pressing, punching, stamping, riveting, fastening, and joining.

DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

These PA programmable actuators from Nook Industries come as complete systems (including motor, drive, controller, and power supply) and are rated for continuous duty. Acme-screw actuators are for lower duty cycles where self-locking is needed; toothed belt drives are for parallel motor mounting. Strokes reach 24 in. with ±0.004 to ±0.001 in. repeatability; the actuators also have standard motor and gearhead flanges. There are IP54 or IP65 versions depending on the motor.

WHAT IS A 12-VOLT LINEAR ACTUATOR? Linear actuators are typically characterized by their drive mechanism — belt drive, ball or lead screw drive, or pneumatic drive. But it’s not unusual for rod style electric actuators to be classified by the input voltage — commonly 12 or 24 volts — of their integrated motors. These actuators provide thrust force, much like a pneumatic or hydraulic cylinder. In fact, rod style electric actuators are widely used to replace pneumatic or hydraulic cylinders, due to their simplicity and the potential cost savings that can be realized by switching from fluid power to electrically driven motion.

As the name implies, a 12-V linear actuator includes a 12-V dc motor, integrated into or tightly coupled with the actuator body. 12-V actuators are driven almost exclusively by one of two mechanisms – a ball screw or a lead screw. And most designs incorporate gearing or use a gear motor to optimize the thrust and speed characteristics of the actuator. The most basic design includes a limit switch at each end of the stroke, meaning that the actuator fully extends and retracts, with no intermediate positioning. But manufacturers offer programmable limit switches as an option for intermediate positioning capabilities.

Because these electric rod-style actuators are often used to replace hydraulic or pneumatic cylinders, some of their basic design features follow the precedents set by the other technologies. Mounting is a good example. A 12-V linear actuator is typically mounted in the same manner as a pneumatic or hydraulic cylinder, with most having both clevis and trunnion mounting options. In some 12-V linear actuator product lines, you’ll find body sizes and mounting options that meet ISO, NFPA, and other standards, which makes the conversion from a pneumatic or hydraulic actuator to an electrical actuator much simpler in existing applications.

One of the most crucial differences between rod style and slider type actuators is that rod style actuators provide only thrust force. Their primary use is for pushing or pulling a load, via a tube or rod that extends and retracts from the actuator. While a plain bushing guides the rod, there are no linear guides to support and

14

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Electric Actuators — Motion Control HB 08-19 V3.indd 14

8 • 2019

carry the load. In most applications, support and guiding for the load is provided by tracks or rails independent of the actuator.

This operating principle explains why these actuators have several different monikers, including electric cylinders, thrust type actuators, and rod style actuators.

Sizing and selection of a 12-V linear actuator is fairly straightforward, because the motor is preselected and integrated into the actuator. The first parameter to be considered is typically thrust, as it will often dictate the overall actuator size. Next is stroke length, since a small actuator may meet the thrust requirements, but may not deliver the necessary stroke length.

Note that like slider type linear actuators, rod-style actuators driven by a ball screw or lead screw can back drive. When the application requires vertical operation, remember to check that the vertical load doesn’t exceed the screw’s back driving torque.

With an initial actuator selection based on thrust force and stroke, the speed and duty cycle requirements can then be checked. The allowable force and speed combinations are typically provided by the manufacturer, in the form of a performance curve or chart. Once it’s confirmed that all other parameters are within the actuator’s capabilities, it’s important to check the required duty cycle, or “on time” because motor heating can be a limiting factor for the actuator’s performance.

SUITABLE APPLICATIONS FOR 12-V ACTUATORS Virtually any time a load needs to be pushed or pulled, without being guided or carried, a rod style actuator is a good choice.

This includes opening and closing sliding doors in applications such as rail cars and machining centers. In the medical industry, 12-V actuators are often used for ergonomic positioning of work tables or patient beds.

In conveying operations, these actuators can stop or divert product. Because they’re fully enclosed and available in IP-rated or hygienic designs, rod style actuators also excel in pharmaceutical and food and beverage designs needing purely thrust operations for inserting, labeling, or stamping. motioncontroltips.com | designworldonline.com

8/21/19 11:14 AM

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MOTION SYSTEMS HANDBOOK

HIGH-SPEED LINEAR ACTUATORS: WHAT QUALIFIES THEM AS SUCH? Like many terms used in the linear motion industry, such as heavy duty, miniature, and corrosion-resistant, to name a few, there is no industry standard that specifies what constitutes a high-speed linear actuator.

Nevertheless, there are some general guidelines that manufacturers follow when classifying and marketing their actuators as high speed. These guidelines are typically based on the drive mechanism, actuator type, and even primary use or industry.

Understanding these distinctions can help you make an informed decision when your application calls for a high-speed linear actuator.

SPEED IS PRIMARILY DEPENDENT ON THE DRIVE MECHANISM The limiting factor of a linear actuator’s speed capability is typically the drive mechanism.

Because lead screw designs are based on sliding contact and generate high heat due to friction, they often have lower maximum speeds than ball screws of a similar size. So of the screw technologies, actuators based on ball screw drives are more likely to be deemed high speed than those based on lead screw drives.

Actuators based on belt drives or rack and pinion assemblies are typically able to reach higher speeds than ball screws, provided they are properly tensioned (for belt drive versions) or preloaded (for rack and pinion versions). Actuators with steel reinforced belts can achieve speeds of 10 m/sec or higher, while rack and pinion driven actuators can commonly reach speeds of 5 m/sec.

ACTUATOR TYPE ALSO PLAYS A ROLE IN MAXIMUM SPEED Another factor comes into play when discussing high speed linear actuators: the type of actuator. The “high speed designation is most often applied to thrust-rod type actuators (also called electric cylinders) because their primary applications involve pushing-pulling and

16

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ACTUATORS • ELECTRIC

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Ballscrew and leadscrew critical speed depends on screw diameter, length, and end bearing arrangement. Information courtesy Haydon Kerk of AMETEK Advanced Motion Solutions (AMS)

inserting operations, which typically require very short extension and retraction times. These actuators can be either ball screw or lead screw driven, with speeds ranging from 0.1 m/sec to more than 1 m/sec.

A few manufacturers even offer belt-driven rod-style actuators that can reach speeds to 2.5 m/sec.

Slider or carriage-type actuators (also called rodless actuators) can achieve even higher speeds than rod-type actuators in many cases. But because their primary uses are for positioning and transport, typically with high loads, they are less often marketed as high speed. Rodless or slider-type actuators have a wide range of drive options, including lead screw, ball screw, rack and pinion, belt, and linear motor.

Linear motors inherently provide the highest speed capabilities, with no mechanical parts to limit speed or create friction and heat.

But when incorporated into a linear actuator, linear motor drives are limited by the speed of the guide mechanism. Similarly, steel reinforced belt drives can achieve speeds greater than 12 m/sec, but like linear motors, are limited by the maximum speed of the guide. The most common guide systems used with linear motors and belt drives are recirculating profiled rail bearings, whose maximum speeds typically reach up to 5 m/sec. limiting the overall speed of the actuator to 5 m/sec or less. motioncontroltips.com | designworldonline.com

Electric Actuators — Motion Control HB 08-19 V3.indd 17

However, higher speeds can be achieved when belt drives are paired with cam roller guides instead of recirculating profiled rail bearings. With preloaded cam roller guides and a properly tensioned, steel-reinforced belt drive, these high speed linear actuators win the race, with travel speeds up to 10 m/sec.

SPECIAL CASE OF MINIATURE LINEAR ACTUATION For micro and nano-positioning applications, actuators of choice are often based on voice coil or piezo technologies. Ultrasonic piezo actuators can reach speeds of 0.5 m/sec or greater, but typically have maximum strokes of 100 mm or less. Voice-coil actuators run to 0.30 m/sec with strokes to 150 mm.

While these specifications may not fit the general definition of high-speed linear actuators, considering the rapid acceleration that is required to reach these speeds in very short stroke lengths, piezo and voice coil designs can easily be classified as high-acceleration actuators.

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MOTION SYSTEMS HANDBOOK

Pneumatic

actuators

HOW ARE PNEUMATIC ACTUATORS USED IN AUTOMATED DESIGNS? Pneumatic actuators come in an array of permutations for automation.

Rod and rodless actuators are the two most common, though rodless are far more common in the U.S. Rodless actuators include bearings to address moment loads in X. Y, and Z. Tip: Avoid undersized pneumatic actuators for applications by accurately calculating application force; subtracting piston-rod area; accounting for the energy to overcome friction; and accounting for unusual application kinematics.

Note that most pneumatic-actuator applications are linear-stroke setups needing two or more position stops. Minimizing stop times in many pneumatic actuators are done by air cushions. Damping seals are another option. Choosing pneumatic actuators over electrical options is common in applications that need motion axes to work without electricity — where sparking poses a problem (as in applications that involve flammable materials for example).

Limitations include lack of total controllability and air consumption, but few actuators outperform pneumatic actuators in applications that need clean operation, low upfront cost, and high force-speed ratios. Even so, pneumatic actuators deliver moderate to high load at 120 in./sec or better. Plus advanced pneumatic actuators have wireless valve controls and sensors for IoT and predictivemaintenance functionality.

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8 • 2019

Festo DGST pneumatic mini slides are compact for precision handling, press fitting, pick and place, and light assembly applications. The DGST slide and yoke plate are machined from a single piece of aluminum for low torsion and accurate alignment. A backlash-free piston-rod-to-yoke connection contributes to the slide’s precision and extended service life. Maintenance free twin piston drives and high force capacity recirculating ball bearing guides provide the DGST with twice the power of other slides — from 7.6 to 132.6 lbf at 6 bar. Cushioning and proximity sensors do not extend beyond the slide’s footprint. Payload is 1.5 to 37.5 lb and stroke from 0.4 to 8 in.

ANOTHER VARIATION ON PNEUMATIC ACTUATION: BLADDERS Bladder actuators are another form of pneumatic actuation for short strokes and constant-force motion. Where appropriate they outperform other systems. In part positioning, footed bladder actuators output consistent lifting force for plates and other wide and flat loads and workpieces. Another application is automatic door seals to work as flood barriers and chamber closures. These inflatable actuators also work in material-handling applications. Such actuators can’t deliver on precision positioning, so only work where designs are forgiving on that parameter. They’re also unsuitable for point-source applications. That said, these actuators are capable of very accurate force control, and strokes can reach a couple inches.

THE BASICS OF ACTUATORS: PNEUMATICALLY OPERATED CYLINDERS Driving linear-motion applications with pneumatic actuators (or air cylinders) is a relatively easy and inexpensive approach. The motioncontroltips.com | designworldonline.com

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MOTION SYSTEMS HANDBOOK actuator technology has existed for more than 50 years, but better piston seals and rod wiper seals (of modern materials) materials make pneumatic actuators more resilient and efficient than ever. These 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 air-cushioning features make pneumatic actuators more useful than ever.

To review, pneumatics is the technology of compressed air. However, in some circles, it’s more fashionable to refer to it as a type of automation control. Pressurized gas— generally air that may be either of the dry or lubricated type—is used to actuate an end effector and do work. 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 is commonly used in industries that include medical, packaging, material handling, entertainment and even robotics.

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 hoses and is controlled by valves on the way to the actuator.

The air supply itself must be filtered and monitored constantly in order to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life.

New controller options including

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.

PNEUMATIC ACTUATOR OPERATION I MOTION DESIGNS 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, often referred to as an air cylinder. 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.

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ACTUATORS • PNEUMATIC Variations on how pneumatic actuators integrate into linear systems In some setups, load rides on a carriage.

Here the slide makes linear strokes.

Bearing block Guide shaft Tool bar

Air cylinder

In some setups, load rides on a moving frame.

Here, linear bearings support much of the load.

The slide makes linear strokes.

Guide shaft The pneumatic cylinder is stationary. 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, 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 subdivided 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 motioncontroltips.com | designworldonline.com

Pneumatic Actuators — Motion Control HB 08-19 V3.indd 21

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.

PNEUMATIC CYLINDER SELECTION When selecting any air cylinder, it’s important to properly match the cylinder to the application, particularly in terms of required force. The theoretical force available in the actuator is the piston surface area multiplied by the supplied air pressure. Spring force must be subtracted from this value for single acting cylinders. The actual force applied to the load will be 3% to 20% less due to pressure losses in the system.

When the required piston surface area (A) is known, the bore diameter (d) can be found by the formula:

The required travel of the machine element determines stroke length driven by the actuator.

The final selection criterion is the cylinder mounting arrangement, and the resulting configuration.

There are myriad pneumatic-actuator configurations available from various manufacturers. The more common ones include rigid nose or tail mount, trunnion mount, rear pivot mount and foot mount. Once the basic actuator size and configuration are known, other options such as end-of-stroke cushions or special seals should be considered. In some applications, position detection switches are required, typically accomplished with a magnetic piston and switches.

Several factors, such as system contamination, corrosion, minor leaks and wear, affect the available air pressure and flow used to drive the actuator. Engineers should size the actuator and fluid-power system so as not to waste energy, with a margin added to account for minor reductions in pressure and flow.

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MOTION SYSTEMS HANDBOOK

Chain actuation — rigid type

Linear actuators abound but some

Common rigid-chain arrangements

applications need telescoping actuators — mechanical linear-motion devices that extend

Chain link shoulders

into a volume beyond the machine base, drive housing, and guides. There are many variations, but the most common telescoping actuators are based on interlocking segments

Unguided chain with shoulders up coils downwards ...

of rigid chain; advancing spirals of helical band; segmented-cylinder spindle sets; and zipper-type mating belts. In one way or another, all extend the linkages or coils into

... but guided chain is most stable.

rigid columns that mechanically lock into a

Actuator body

column that advances, pulls, or lifts loads.

Rigid-chain actuators work by pairing a drive (usually an electric motor) with a length of chain sporting shoulders on each link. The motor output shaft—fitted with a specialty sprocket or pinion—applies tangential force to the chain. Then the chain comes out and straightens, and its links’ shoulders lock to form a rigid series. When the motor runs in the opposite direction, the chain shoulders disengage and allow for coiling.

Inside the actuator body, reaction plates and guides counter thrust resistance and keep the chain on track. Links travel around the pinion to exit the actuator body along the stroke path. Here, the motor’s torque comes to act as forward thrust via the link shoulder to the rest of the links’ shoulders. The last link in

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Chain Actuation — Motion Control HB 08-19 V3.indd 22

Pinion Input drive shaft Choose a rigid-chain actuator to satisfy the design geometry.

Unguided chain with shoulders up coils downwards, which is useful but not always stable enough for long strokes.

That with shoulders down (here, bottom) is slightly more stable. Use guided chain wherever space permits.

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DW half pa

ACTUATORS • RIGID CHAIN

the chain before the load has geometry that puts the thrust higher than the articulation axis. This makes a moment that effectively locks the link shoulders. In reverse, pulling force acts along the links’ cross axes.

Rigid-chain actuators have the mechanical benefits of conventional chain but can act in horizontal push setups or vertically as jacks. Plus they’re compact. In contrast, traditional chain drives can only pull, so need two drives for bidirectional motion.

Traditional screw jacks for vertical power transmission need space for retraction that’s as long as the working stroke itself.

Before specifying a rigid-chain actuator, determine the application’s total load, including the transported load, acceleration forces, external environmental forces, and that due to friction — with a coefficient between 0.05 and 0.5 for typical rigid-chain actuator setups. Next, determine what type of actuator body and chainstorage magazine the application can accommodate.

Next determine whether the chain will need to change direction on its way from the magazine to actuator body. Actuators usually feed chain around 90° or 180° turns.

Note that rigid-chain actuators can work alone or in tandem.

Twin-chain setups deliver high positioning accuracy and stability where loads are large or bulky. Here, a pushing bar acts as a yoke to

keep loads steady, with optional hooks for pulling as well. Optimized geometry has the force vector act on the load’s center for balance.

If twin-chain setups are impossible, consider adding framework to guide awkward loads.

Guides on the chain also help maintain stability — even over very long strokes — because they address side and buckling forces.

Such guides come in different shapes with different crampons and subcomponents to engage the chain. Where use of chain guides is impossible, most designs run the chain with link shoulders down for moderate stability.

Some last design notes: Standard chain is carbon steel to withstand heat to 200°C, but stainless, high-temperature, and coated chain for long life are other options.

The required length of chain is total design stroke plus a few links to engage the actuator pinions.

As with any power-transmission setup, consult the manufacturer for tips and guidance on determining necessary drive power and other details.

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MOTION SYSTEMS HANDBOOK

The basics of

ball & roller screws There are a number of

screw actuation methods providing

linear motion. These include basic lead screws to other types including ball screws and roller screws. Here’s how the latter two compare.

BALL SCREWS Ball screws consist of a screw with helical grooves, a nut, and balls that roll between the nut, screw and the grooves when either the screw or nut rotates. The balls, made of steel, ceramic, or a hard plastic material, are routed into a ball return system of the nut and travel in a continuous path to the ball nut’s opposite end.

Compared to other screw actuation methods, ball screws have a significant advantage with their high efficiency ratings that can be well over 90%. By way of comparison, a typical acme lead screw has efficiency ratings of 50% or less. Even though ball screws typically cost more than other screw types, this is usually offset by decreased power requirements for a similar net performance.

A number of factors play a role in assessing ball screw performance.

These include lead accuracy, axial play, preload, and life/load relationships. For starters, lead accuracy refers to the degree to which the shaft’s rotational movements are translated into linear movements.

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 typically applied to eliminate axial play, which also has the effect of removing backlash and increasing stiffness.

Lead accuracy and axial play are determined by the manufacturing method of the ball screw shaft and the assembly of the nut. For instance, high lead accuracy and zero axial play is usually associated with relatively higher-cost precision ground ball screws, while lower lead accuracy and some axial play is associated with lower cost rolled ball screws.

Other important performance parameters are precision and repeatability, which can be impacted by ball recirculation inside the ball nut. This is why 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 because no motion is lost from the clearance in the balls as they reengage.

mechanical advantage, meaning that less input torque is required for a given load.

Because their load-carrying rollers don’t contact each other, roller screws can typically travel at higher speeds than ball screws, which have to deal with the forces and heat generated by the balls colliding with each other and with the recirculation end caps.

Although their load carrying mechanism differs, many attributes of planetary roller screws are similar to ball screws. They can be preloaded to reduce or eliminate backlash, by using either a split nut (in which the nut is cut transversely and a precision spacer is inserted between the front and back halves), a double nut, or oversized rollers. Roller screws are also manufactured to the same precision classes as ball screws, so lead deviation and positioning accuracy are easy to determine. And they experience the same critical speed and buckling limitations that are found with ball screws.

Despite the load and speed advantages of roller screws over ball screws, where roller screw actuators really make sense are cases where an electromechanical actuator is replacing a hydraulic actuator. In many instances, the desire is to move away from hydraulics to eliminate the use of hydraulic fluid, or to simplify the overall system. Roller screw actuators are the best option because they provide load capacities and stiffness characteristics similar to hydraulics.

ROLLER SCREWS In contrast, a roller screw replaces the recirculating balls with threaded rollers. The ends of the rollers are toothed to mesh with geared rings at each end of the nut. The rollers both spin on their axes and orbit around the nut, in a planetary configuration. (This is why roller screws are also referred to as planetary roller screws.)

The geometry of a roller screw provides significantly more contact points than are possible with a ball screw. This means that roller screws typically have higher dynamic load capacities and rigidity than similarly sized ball screws. And the fine threads (pitch) provide a higher

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8 • 2019

For high-load applications, roller screws may be a better choice than a ball screw. In a roller screw, the load bearing mechanism is a set of recirculating rollers instead of balls.

The rollers have more total surface contact area than balls, which increases the load carrying capacity and can also increase service life. However, due to the precision machining and complex assembly required, roller screws are generally more expensive than ball screws. | courtesy of Thomson Linear motioncontroltips.com | designworldonline.com

8/19/19 3:47 PM

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MOTION SYSTEMS HANDBOOK

Rotary bearings for

precision motion applications

Rotary bearings are common components in motion systems. They’re used in gearboxes,

load — ball or roller. Within the ball and roller types, the

motors, pulleys, fans, pumps — virtually any time a shaft

that the bearing can withstand—radial or axial (aka

is rotating, rotary bearings are there to reduce friction and

thrust). Then, ball bearings are further classified by the

support radial or axial loads. The classification of rotary

configuration of the outer ring, while roller bearings are

bearings begins with the type of bearing that supports the

further categorized by the shape of the rollers.

next distinguishing feature is the primary load direction

While there are dozens of variations of rotary bearings — including designs made for specific environments, mounting configurations, and applications — the most common categories are covered here. Rotary bearings are categorized by whether they use balls or rollers … and then by the type of load they’re designed to support — radial or axial (thrust). Further classification for ball bearings is based on the configuration of the outer ring, whereas roller bearings are further classified by the shape of the rollers.

Ball bearings in the form of radial rotary bearings: Within the ball-bearing family, deepgroove ball bearings are the simplest type, with raceways that nearly match, or conform, to the diameter of the balls. This type is suitable for radial loads and for axial loads in either direction, although their load capacity is relatively small compared to other bearing types. For higher load capacity in a relatively small footprint, deep groove ball bearings are commonly available in a double-row design, with two rows of balls instead of one. | Rotary bearing image courtesy SIC Marking USA

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8 • 2019

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MOTION SYSTEMS HANDBOOK Angular contact ball bearings are designed so that the line that connects the points of contact of the inner ring, ball, and outer ring runs at an angle radially to the bearing. The contact angle increases load capacity, but it means the bearing can withstand an axial load only in one direction, so these bearings are typically used in pairs or in a double-row design (essentially two angular contact bearings mounted back-to-back) in order to withstand axial loads in both directions.

Four-point contact ball bearings are single angular contact bearings that are designed to withstand high axial loads in both directions. They can also withstand combined axial and radial loads, as long as the axial component is predominant.

Ball bearings for thrust loads: These are made of two relatively thin bearing rings (sometimes referred to as bearing plates or washers), with raceways designed to take axial loads, in either one direction or both directions. Radial loads are typically not permissible for thrust bearings, and speed capabilities are limited (as low as 20 to 30% of their radial bearing counterparts in some cases).

Roller bearings for radial loads: With rollers supporting the load, these have higher load capacities and higher rigidity than ball bearings of similar sizes. The type of load they can support depends primarily on the shape of the roller.

Cylindrical roller bearings can withstand high radial loads, while double-row cylindrical versions having extremely high radial load capacity and high rigidity in the radial direction. Although standard cylindrical roller bearings are not capable of taking axial loads, some designs include internal ribs or collars that allow them to handle relatively small axial loads in one or both directions.

Needle roller bearings use long, thin rollers, with a length that is anywhere between three and ten times the diameter. They have good radial load capacity, but the primary benefit of the needle design is that it has a thin cross-section, for applications where high radial capacity is required but space is limited.

Unlike cylindrical and needle roller bearings, tapered roller bearings (which use conical rollers to support the load) can withstand axial loads in one direction. Tapered roller bearings can be thought of as the roller version of the angular contact ball bearing, and like their ball bearing counterparts, tapered roller bearings are often used in pairs or in a double-row design, to counteract axial forces that are produced in the bearing when a radial load is applied. The tapered design also improves rolling properties and reduces friction within the bearing.

Spherical roller bearings use barrel-shaped rollers and have two inner raceways inclined at an angle to the bearing axis, and one spherical-shaped outer raceway. This gives them a self-alignment capability, very high radial load capacities, and the ability to withstand axial loads.

Roller bearings in the form of thrust bearings: Tapered thrust roller bearings have pure rolling motion, so they produce less heat and less wear than other options. Thrust versions of roller bearings generally follow the strengths of their radial counterparts but with thrust (axial) load capabilities. Cylindrical thrust roller bearings can accommodate high axial loads and can withstand impact loads, although radial loads are not permissible.

Similarly, needle thrust roller bearings can withstand high axial loads, but in a much smaller form factor due to the use of needle bearings.

Tapered thrust roller bearings can be single-direction or doubledirection type, indicating whether they can accommodate axial loads in one direction or in both directions. Like their radial counterparts, tapered thrust roller bearings have pure rolling motion, which means less heat generation and less wear than other bearing options.

Spherical thrust roller bearings can take very high axial loads and small to moderate radial loads, with the ability to withstand some misalignment.

REVIEW OF PLAIN BEARINGS Plain bearings are cylindrical sleeves that bear light to moderate radial loads. Plain bearings are made of many graphite, bronze, and plastics that include PTFE, nylon, and polyacetal. Material improvements have made plastic plain bearings increasingly common, even in demanding motion applications.

In short, plain bearings slide radially or axially over shafts to allow rotary motion or linear motion (or sometimes both) of these loads.

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BEARINGS • PLAIN Plain bearings have none of the moving parts that rolling-element bearings have, so minimize fail points; they’re also cost effective for even fairly rugged applications. Common variations are metallic sleeve bearings (which often ride loads on a hydrodynamic or full film of lubrication) and self-lubricating plastic bearings in an array of geometries for bushing, thrust bearing, and integral-slide applications.

Plain-bearing ratings are based in part on test results and its material modulus of elasticity, flexural strength, shore-D hardness, maximum surface pressure and running speed, rotating, and maximum load capacity — with the latter related to the plain bearing’s material compressive limit. (Here, recall that the compressive limit is the point at which 0.2% permanent deformation occurs.)

In addition, a pressure-speed (PV) value expresses plain-bearing load capacity — usually in in psi times the shaft rpm.

However, note that PV values are only one to help determine a plain bearing’s overall load capacity — especially where a PV expressions might mislead engineers into thinking that a plain bearing can bear excessively high loads if the speed is very low. In other words, use of PV values requires concurrent consideration of realworld speed and load limits.

plain-bearing performance and life. One common option is cold-rolled carbon steel. This shaft material makes for a suitable mating surface for plain bearings made of polymers. Ceramic shaft surfaces induce more wear, though are sometimes chosen for their ability to withstand harsh environmental conditions. Though aluminum shafts are lightweight and easy to machine, they also induce accelerated plain-bearing wear. Aluminum shaft made of anodized slightly improves the assembly performance.

In fact, shaft surfaces for mating with plain bearings shouldn’t be too smooth or rough. Overly smooth surfaces will cause stick-slip adhesion variations — in turn causing higher friction resistance to bearing movement. More of a disparity between dynamic and static friction will make for faster bearing wear and jerkier motion. In contrast, overly rough shaft surfaces quickly abrade plain bearings. In fact, the rates of wear induced by shaftbearing interfaces can vary a hundredfold.

Some manufacturers recommend shaftsurface finishes to 64 root mean square (rms) for precision applications needing low friction; a smoother shaft with roughness of 20 rms or so is more suitable where long plain-bearing life is a design objective.

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32

64

96

Roughness (RMS)

Morgan Hill, CA 95037 408-612-4970

www.dingsmotionusa.com 8 • 2019

Plain & Rolling Element — Motion Control HB 08-19 V3.indd 31

DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

Basics of

sprockets

and chain drives Roller-chain selection chart Chain strands 4

motion systems for more than a century. They are versatile and reliable components to drive machinery and convey products. Now, advances chains in more applications than ever. Remote installations benefit from long-life chain that requires no lubrication, for example.

Chain-based machinery abounds, but the most common industrial designs use roller chain. This type of chain consists of five basic components—pin, bushing, roller, pin link plate, and roller link plate. Manufacturers make and assemble each of these subcomponents to precise tolerances and heat treat them to optimize performance. More specifically, modern roller chains exhibit high wear resistance, fatigue strength, and tensile strength.

Roller-chain applications generally fall into two categories: Drives and conveyors.

Chain-drive applications: Most typical drive applications use an ASME/ANSI roller chain wrapped around a driver sprocket (connected directly to the motor or reducer) and the driven sprocket (often connected to a machine’s conveyor head-shaft). This portion of the drive lets the designer build a system that’s either faster or slower by simply changing the ratio of teeth between the drive and driven sprocket. The ratio of the teeth determines the reduction in rpm … so to reduce rpm, the driven

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Chain Drives — Motion Control HB 08-19 V4.indd 32

Roller-chain drive capacity (horsepower)

in precision and technology let designers use

3

2

900 1,000 700 1,000 800 500 800 600 400 600 400 300 400 300 200 300 200 200 100 100 80 60 40 30

100 80 60

30

30

20

20 10 8 6 4 3

1 0.8

500 400 300 200 100 80 60 40 30 20

10 8

10

6

8 6

4 3

4 3

2

2

1 0.8 0.6

2

2 1 0.8

40

40

20 10 8 6 5 4 3

80 60

1

1 0.8 0.6

0.6

0.4

0.6

0.4

0.3

0.4

0.3

0.2

19 T 19 22 T T 2 25 21T 5T 22 19 T T T 21 25 25 T 25 T 23 23 T T 2 17 T 21 25 T 1 20 40 T T T 25 1 9T 0 17 21 7T T 1 T 21 T 16 80 T 25 0 14 23 17 T 0 T T 12 2 15 25 1T 0 T 10 T 17 19 0 T T 19 80 T 23 60 T 15 50 T 40 35

Engineers have used chains in

0.4 0.3 0.2

10

30 20

50 80 200 500 1,000 3,000 7,000 40 60 100 300 700 2,000 5,000 10,000

Speed of roller chain’s small sprocket (rpm) 8 • 2019

motioncontroltips.com | designworldonline.com

8/15/19 2:31 PM

BELT DRIVES • CHAIN DRIVES

Morse inverted-tooth chain drives from Power Transmission Solutions of Regal-Beloit America comes in HV versions for high capacity at high speed. Silent chain is another option to make smooth, silent drives at slower speeds.

| courtesy Martin Sprocket & Gear Inc.

sprocket must be larger than the driver sprocket. For example, if the driver sprocket has 15 teeth and the driven sprocket has 30 teeth, the ratio is 2:1, so the rpm is halved at the driven sprocket.

The easiest way to select a roller chain is using horsepower charts. First, obtain the motor horsepower and rpm of the small driver sprocket. From this, determine the chain size and number of teeth for the driver sprocket. Where roller chain must drive applications that need long life without contamination, pick chain with self-lubricating subcomponents. Where roller chain must drive applications than need high precision, pick chain with precision roller bearings at each link connection.

Conveyor applications: Conveyor chains come in myriad versions to move product horizontally, vertically, or even around curved radii. The most common conveyor chains are ASME-style (ANSI-style) attachment chains. These chains include extended pins or plates with tabs onto which parts or product-holding shoes can bolt. Common versions are single-pitch attachment chain, doublepitch attachment chain, hollow-pin chain, curved-attachment chain, and plastic-sleeve chain. The attachments let engineers put special fixtures or blocks onto the chain to serve specific conveyor functions.

One subtype of conveyor chain is the accumulating conveyor.

These stop discrete products even while the chain is still moving, and they do so with minimal friction and wear. Accumulating conveyors are suitable for applications (such as assembly lines) that have products ride through several stations.

Tip: Select chain with top rollers or side rollers to let discrete products idle while the conveyor continues to run. Also pick custom attachments or work with manufacturers that make custom fixtures to handle specific parts. Many industries (including the automotive, food and beverage, and consumer-products industries) use custom attachments on their chain-based accumulator conveyors to economically and consistently move.

8 • 2019

DESIGN WORLD — MOTION

33

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12/17/2018 4:17:22 PM 8/15/19 2:31 PM

MOTION SYSTEMS HANDBOOK Manufacturers of Power Transmission and Motion Control Components

Concentric Maxi Torque

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CHAINS ENDURE SUBOPTIMAL ENVIRONMENTS The environments of many chain applications are less than ideal. Some require clean operation, without the lubrication that can contaminate products.

Others expose chain-driven machinery to weather, water, or chemicals. So, chain manufacturers offer several products to meet these challenges.

Consider roller chain: One critical area where roller chains need lubrication is the pin-bushing contact zone. Self-lubricating chains stay cleaner because the exterior of the chain is free of excess lube. These chains also attract less dust and particulates than regular chains. Such roller chains are useful where oil contamination is a concern, including paper-product or woodprocessing industries.

Specialty coatings and stainless steel can delay or prevent corrosion.

Nickel-plated chains offer another alternative for chain coatings, providing some protection for mildly corrosive

environments. Stainless steel chains offer superior corrosion resistance ... but designers must be aware that regular stainless steels cannot be hardened in the same manner as carbon steel. Therefore, the load carrying capacity of stainless steel is lower than carbon steel.

Proper chain maintenance requires periodic inspection. All chains must be checked for damage, wear, and chemical attack on a regular basis.

Another issue is elongation due to wear.

Eventually roller chains wear to so much that they necessitate replacement—typically at 1.5% (12.180 in./ ft) to 2% (12.240 in./ft) elongation. Chains may work until they reach 3% elongation but are at increased risk for suboptimal performance.

Power-transmission and conveyor chain attachment options K1 (B2 one-hole) and K2 (B2 two-hole) chains both have bent attachments on both sides.

Custom Synchronous Drives

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A1 chain (sometimes called B1 one-hole chain) has links with one hole and a bent attachment. A2 is similar but always double pitch with two attachment holes per link.

WSK1 (WCS2 one-hole or WM1) and WSK2 (WCS2 two holes or WM2) is wide-contour chain with straight attachments on both sides.

Single-pitch WA1 (WCB1 one hole) chain and wide-contour WA2 (WCB1 two holes) chain both have bent attachments on one side and one or two holes per link.

Timing Pulley Stock

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SK1 chain (sometimes called S2 one-hole or M1 chain) has straight attachments on both sides. SK2 (S2 two holes or M2) is the same but with two holes per link.

SA1 (S1 one-hole or M35) chain and SA2 (S1 two holes or M35-2) chain both have straight attachments on one side.

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Chain Drives — Motion Control HB 08-19 V4.indd 34

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DESIGN WORLD — MOTION

8 • 2019

8/21/19 2:03 PM

BELT DRIVES • CHAIN DRIVES This Festo ELGA-RF is accurate enough to function as the main axis on a machine that produces printed circuit boards. Its robust roller bearings let it move to 10 m/sec — faster than designs using ball-bearing guides. Two synchronized belt drives extend the length of travel.

Sizing and applying

belts and pulleys 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 powertransmission devices.

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Chain Drives — Motion Control HB 08-19 V4.indd 35

8 • 2019

DESIGN WORLD — MOTION

35

8/15/19 2:32 PM

MOTION SYSTEMS HANDBOOK SADDLE CAP TOP COVER

Linear-slide timing belt tensioning STROKE 50 TO 499 mm

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.

For toothed belts (also called synchronous belts) the pitch is the number of teeth per some length, so a 3-mm pitch means the belt has one tooth every 3 mm.

BAND CLAMP

STROKE 500 mm TO MAX

SIZE

DISTANCE L (mm)

55

250

3.5

9 [40]

500

7

9 [40]

56

250

3.5

16 [70]

500

7

16 [70]

58

250

3.5

30 [130]

500

7

30 [130]

DEFLECTION Y APPLIED FORCE (mm) lb [N]

DISTANCE L (mm)

DEFLECTION Y APPLIED FORCE (mm) lb [N]

L

L/2

Y

HOW TO INTEGRATE 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 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 power-transmission 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, and the most suitable 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.

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.

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Chain Drives — Motion Control HB 08-19 V4.indd 36

8 • 2019

| courtesy PHD Inc. 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. Some rubber compounds may provide some marginal improvement in durability, but it’s best to prevent oil from contacting synchronous belts.

Ozone can also be detrimental to the compounds used in rubber synchronous belts. It degrades belt materials much like excessive temperatures do. 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 here … 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.

motioncontroltips.com | designworldonline.com

8/21/19 8:36 AM

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Stock Drive Products / Sterling Instrument 250 Duffy Avenue, Hicksville, NY 11801 Phone: (800) 819-8900 Fax: (516) 326-8827 email: sdp-sisupport@sdp-si.com www.sdp-si.com REQUEST A CATALOG

8/15/19 12:35 PM

MOTION SYSTEMS HANDBOOK

Brakes, clutches,

and torque limiters

There are three types of electrical braking for motors — regenerative braking, dynamic braking, and plugging. Of the three methods, plugging provides the fastest stop, but it can be harsh on both the electrical and mechanical components. Because of this, it’s the least commonly used method of braking … though appropriate for some applications.

Plugging (called reverse current braking in some contexts) is possible on both dc motors and ac induction motors. For dc motors, plugging is via reversal of armature-voltage polarity. When this happens, back EMF voltage no longer opposes supply voltage.

Instead, the back EMF and the supply voltage work in the same direction, opposing the motor’s rotation and causing it to come to a near-instant stop. The reverse current produced by the combined

supply voltage and back EMF is extremely high, so resistance is placed in the circuit to limit the current.

One potential problem with plugging for braking (especially when braking time is short) is that it can be difficult to brake the motor at exactly zero speed. Another drawback to plugging is that it can induce high mechanical shock loads on the motor and connected equipment, due to the abrupt stop that it causes.

Plugging is also very inefficient for stopping and generates significant heat.

So for most modern motion designs requiring high throughput — external brakes and clutches are required to get sufficiently quick stops or disengagement. This applies to conveyors, airport-baggage handlers, escalators, and elevators … as well as other axes that make frequent stops and starts — even as few as 10 cycles a minute in some cases. With stops and starts at much higher cycle rates,

Torque limiters versus overrunning clutches ROLLER CLUTCHES •

SOMETIMES CALLED ROLLER-RAMP CLUTCHES

Related components include those classified as mechanical overrunning clutches.

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8 • 2019

CAM CLUTCHES

EXHIBIT HIGH TORQUE DENSITY

RATCHE TING CLUTCHES ARE SIMPLE TO INTEGRATE

motioncontroltips.com | designworldonline.com

8/21/19 2:02 PM

BRAKES • CLUTCHES Torque limiters ...

Completely disconnect drive from driven load (or slip) Sprag engagement Pawl spring

Pawl engages notched wheel Upon overload, pawl rocks out of detent

Steel bean-shaped cams tilt to wedge between outer + inner races

Precision Control for Your Operations

Upon overload pawls in a pawl-spring torque limiter rock out of their detents to release the load until manual or automatic resetting. Spragengagement torque limiters use steel wedges that tilt during operation and wedge themselves between the assembly’s outer and inner races.

motor inertia may degrade the quickness with which starts and stops are possible. So here, clutch-brakes are often more suitable — as they disengage the driven load from the motor to allow the former to run even while the brake engages and stops the load. Of course, though we focus on responsiveness here, failsafe design features are another main driver of brake and clutch inclusion.

Mechanical, electric, fluidic, and selfactuated clutches and brakes are suitable for different applications. For example, springset brakes benefit motion designs that slow loads with the motor before the brake engages … and they’re suitable as holding mechanisms. Control of electric brakes is easy, and they can keep pace to a thousand cycles per minute. Most air-actuated brakes and clutches are cool-running and hold with minimal input. Friction brakes with drum, disc, and cone geometries deliver e-brake functionality with failsafe holding.

Brake or clutch size and type depend on whether the axis at hand will make emergency stops or softer stops that sacrifice the clutch or brake to protect systems and loads from shock. Or sometimes it’s more essential that the brake deliver soft stops to prevent shifting loads and misalignment.

After that, other criteria — cycle rates, thermal capacity, machine envelope, and MRO schedules — dictate final selection.

8 • 2019

Brakes & Clutches & Torque Limiters — Motion Control HB 08-19 V4.indd 39

Some tips: Size clutches and brakes to the machine axis’ motor torque. Where a brake must stop vertical loads, account for how motors can briefly draw current to output in excess of their rated torque.

Consult performance curves in manufacturer PDFs for dynamic torque ratings at set speeds to match the brake or clutch to peak motor-output torque.

CLUTCH AND BRAKE ENGAGEMENT Key parameters dictate which clutches and brakes are suitable for a given application: The first parameter is the mode of motion and whether the axis needs a brake or clutch. Clutches generally bridge moving machine sections to shape relative motion, while brakes’ main job is to stop and hold rotating elements. Several design variations exist, and a main differentiating factor is the way in which a brake or clutch engages.

Some options include mechanical, electric, fluidic, or self-actuation.

Brakes and clutches using fluidpower actuation usually include a piston to physically engage friction discs for torque transmission from an input to an output.

Hydraulically and pneumatically actuated clutches and brakes are quick-response units that act on fluid pressure and piston mass, so engagement speed follows control pressure.

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DESIGN WORLD — MOTION

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291 Boston Tpke, Bolton, CT 06043

8/20/19 3:53 PM

Power Transmission, Motion Control and Engine Braking Solutions

Creating A Premier Industrial Company Ameridrives

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Warner Linear

Boston Gear

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Wichita Clutch

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Altra Industrial Motion 08-19.indd 40

www.AltraMotion.com 8/15/19 12:36 PM

BRAKES • CLUTCHES Some air-actuated brake and clutch designs are specifically designed to run coolly … and maintain hold with minimal power.

Some caveats though: In many clutches, full piston force transmits through a bearing — effectively a wear point — to let the body rotate as the housing stays put. Here, the bearings and high loads also limit operating speed. Brakes using fluid-power actuation don’t need such bearings.

Brakes and clutches using mechanical actuation include a lever for mechanical advantage to trigger engagement and disengagement. This levering usually works to squeeze friction discs together for torque transmission. When the mechanism moves, the clutch or brake condition changes from engaged to disengaged or vice versa. Some such brakes and clutches have a locking mechanism to keep the unit as it is until repowering. Such clutches and brakes have no bearings to fail; are fairly speed tolerant; come in optional one-position setups; are unaffected by power failures; and offer automatic overload release during overtorqueing or locking. On the other hand, many mechanically actuated clutches and brakes don’t automatically disengage during power loss — and most need adjustments to compensate for wear.

Clutches and brakes that are electromagnetically engaged (or disengaged, depending on design) use a magnetic coil to generate a magnetic flux — either to move an armature from friction-disc contact or to move the armature for squeezing discs together. That’s because electromagnetic clutches and brakes are either electrically activated or spring-applied-electricallydeactivated setups. The latter does doubleduty as a failsafe during power loss.

Electromagnetic clutches and brakes are high-speed tolerant; exhibit long life; and come with self-adjustment mechanisms to compensate for friction-disc wear. But there are drawbacks with this technology too: They need bearings to support a stationary coil — and the bearings bear a light load from the radial flux-path use.

What’s more, electromagnetic clutch or brake engagement time exceeds that of other designs due to the time needed to generate the coil’s magnetic field.

Clutches and brakes called wet-running units contain fluid to work by the effects of a transmission-fluid boundary layer. More DESIGN WORLD — MOTION

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BRAKES • CLUTCHES • TORQUE LIMITERS

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MOTION SYSTEMS HANDBOOK

specifically, fluid shears between friction discs and drive plates. As the parts come together, the fluid in shear transmits torque. This eliminates direct friction-disc contact during high-speed slip. Just note that oil shear generates heat even in neutral; there’s oil shear until the input and output are almost to equal rpm.

But in applications that must rapidly stop, start, reverse, and change speed, clutch brakes that use oil-shear technology excel.

Heat from the friction dissipates as the fluid circulates to the housing.

That’s useful on axes with start-stop cycles or cycle rates to highcycle setups (to 300 cycles per minute, as a rule of thumb). Here, oil-shear brakes last five to ten times longer than dry clutch-brake offerings. (Just review the application before specification to prevent heat from becoming an issue.)

MORE ON SIZING BRAKES AND CLUTCHES TO TORQUE AND SPEED AND CYCLES The size and type of brake or clutch most suitable for a given design depends on whether the machine needs emergency stops or softer stops that sacrifice brakes to protect transported loads from shock.

Then other application criteria (including the machine envelope, thermal capacity, cycle rates, and inspection and repair schedules) dictate final selection. In other machines, it’s more important that brakes prevent shifting loads and misalignment.

Expressed in lb/ft, N/m, or lb/in., static and dynamic torque values express clutch or brake output capability. Applications needing dynamic braking are those in which the brake controls rotating-axis motion by absorbing kinetic-energy changes. Dynamic clutching is that during which a clutch brings a stationary output to the input rpm by assuming the slower axis’ kinetic-energy delta.

Static-torque ratings (values that describe clutch and brake behavior when the units aren’t absorbing any kinetic energy) depend largely on torsional load. Reaching static torque operation with a clutch needs a clutch that must engage prior to rotating the input — so the clutch effectively functions as a coupling between in and output.

In contrast, reaching static torque operation with a brake just takes holding the output element stationary.

Static torque equals clutch holding torque when there’s no relative shaft rpm difference between input and output — or (in the case of a brake) when the shaft is stopped. The point at which a system exceeds a unit’s static torque is aptly called the breakaway torque. This is what a machine assembly must reach before relative motion arises between the shafts (in the case of a clutch) or before the shaft starts turning (in the case of brake).

Common clutches come in pulley and shaft-to-shaft variations.

For the latter, the clutch output and input are inline.

Designers sometimes boost dynamic torque rating (and shorten response time) by selecting larger brakes and clutches — though keep in mind that this also increases torque and shear forces (due to torsion) on mounts to nonrotating machine-frame segments.

Dynamic torque depends on the rpm delta between in and output (for a clutch) and operating and zero rpm for a brake.

Dynamic torque is usually about 50 to 80% of static torque. In

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8 • 2019

friction-based designs, this value depends on the contact surfaces’ friction coefficient. Because that changes slightly (with the portion of operation the brake or clutch is delivering) designers usually employ an average coefficient for design calculations.

MORE ON DYNAMIC-TORQUE CALCULATIONS The specifications of the electric motor driving the load partially dictates clutch or brake size. Dynamic torque in lb-ft = (5,250· hp/ rpm) · safety factor. Here rpm = Rotational shaft speed the brake must stop (or the speed difference between a clutch output and input). The safety factor adjusts for the motor type and its typical torsional output during operation. Electric motors have a safety factor near unity, as their output is quite consistent.

Note how a major consideration is time needed to change the axis speed. Another parameter is the duty cycle: If there are more than a few incidences of engagement per hour, duty cycle becomes an influencing design factor. Note that high-cycle applications running to 300 cycles per minute (as in indexing with a clutch brake) each clutch engagement transfers a torque spike to the connection between the motor shaft and input shaft of the clutch brake.

High-cycle applications can cause severe hammering on the shaft connection of the motor to the clutch brake.

For convenience and ease of assembly, standard C-face connections are most common — but the loose fit can cause torque to transfer through the key and keyway, which hastens failure.

Here, clutches and brakes that connect with shrink-fit or clamping couplings for a 360° connection perform better.

The dynamic-torque calculation presented above is based on time to stop the load or bring it to speed — not accounting for time needed for clutch or brake actuation. Consider clutch brakes that are electromagnetically engaged: Here a coil must build an electromagnetic field for armature activation and friction-disc engagement — a process that can take 10 to 500 msec depending on unit size.

Cycle rate is how many times per minute a brake or clutch must engage — and the amount of energy a brake dissipates or a clutch absorbs before the next cycle. Except for regenerative setups, energy dissipates as heat — with heat-load maximums published by manufacturers. But applications that run fast or hot or are extreme in some other way need extra evaluation …

•

•

•

Machinery running beyond a few hundred cycles per minute may need clutches and brakes that are larger than performance curves indicate.

Spring-set brakes benefit motion designs that slow loads with the motor before the brake engages; they’re also suitable as holding mechanisms.

Brakes using permanent magnets automatically adjust for wear. Industrial designs operating in hot environments may benefit from friction brakes with specialty friction-surface geometries and phenolic resin or ceramic materials. Motion applications that won’t see a lot of maintenance also benefit from longer-life friction materials. motioncontroltips.com | designworldonline.com

8/21/19 11:18 AM

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The Bishop-Wisecarver QuickShip Program guarantees belt-driven LoPro® Linear Actuators with qualifying configurations to be on their way to you within 7 days of order placement.

LoPro® Linear Actuators excel in high speed, long length and dirty environments. Its low profile is accomplished by mounting two lengths of hardened steel track to milled anodized aluminum substrate.

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Bishop Wisecarver 8-19.indd 43

QUALIFYING CONFIGURATIONS •

Undriven or belt-driven

•

Sizes: 1 – 4 (1, 2S, 2L, 3, & 4)

•

Order qty: 1 – 5 systems

•

System travel: < 2.4 meters of travel

•

Unmounted or aluminum beam mounted for sizes 1–3

•

Unmounted steel beam for size 4

•

Corrosion resistant wheels available

•

Wiper or basic wheel plate options

•

May include multiple wheel plates on a single system

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MOTION SYSTEMS HANDBOOK

Flexible and controlled

cabling and connections

Electrical cabling is often overlooked in motion control system design. If you use something other than industrial cabling — which is designed to flex continuously — premature failures can result.

Continuous-flex cables are designed to cope with the tight bending radii and physical stress associated with motion control applications.

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 usually less expensive. The cable cores are stranded firmly and left relatively long in several layers around the center and are enclosed in an extruded tube-shaped jacket. However, this design can lead to core rupture.

Bundled designs feature braided conductors that surround a tension-proof center. A pressure filled outer jacket ensures the cores cannot twist, so these cables are often stiffer than standard cables, but last longer in constant flex applications.

Flat cables can incorporate power, signal, and video conductors as well as air or liquid tubing and fiber optics in a single compact cable suitable for continuous flexing.

One-cable technology, like igus’ encoder cable integrated into servo cable, combined with readychain energy chains from igus met the highspeed demands of wood processing machinenery.

Flat cables with silicone jacketing resist abrasion and other harsh environmental impacts and will even self-heal minor nicks.

To select the right cables, determine the application type, such as whether it will be stationary or moving, and what type of movement it is—flexing, torsion, or both?

If there is bending or motion involved, the bend radius must be specified. The bend radius depends on the wire gauge and kind of conductors used. With continuous flexing, conductors containing multiple strands of fine-gauge wire generally last the longest.

MAKING THE CONNECTION Most cable assemblies use some type of interconnect device to transmit the signals from the cable to the motor or drive. Commonly used connectors include M12 and M8 connectors, which are circular connectors with a 12- or 8-mm locking thread used for sensors,

Dynamic requirements — acceleration, speed, etc. – have increased and created more mechanical stress and electromagnetic interference (EMI) on cable designs.

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MOTION SYSTEMS HANDBOOK

Electrical power and signals through a slip ring: Where do they go?

Slip rings — also called electric swivels, collector rings, and rotary electrical joints — are electromagnetic devices of electromechanical, fiber-optic, or even non-contacting inductance and capacitance subtypes that transmit power and data signals between a rotating machine segment and a stationary segment.

Of course, power transmission is associated with higher voltage and higher current than that of data transmission via slip rings. That transmission of power can be for applications needing high voltage, dc, or ac power.

In many cases, the electrical power goes onward to drive electric motors, solenoids to trigger the action of hydraulics, or other electrically-driven actuators.

HOW DO THE SIGNALS THROUGH A SLIP RING GET USED?

| courtesy of Deublin If a machine assembly includes the rotation of one section with a fixed number of revolutions, it may be possible to use spools with sufficient cable length and rotating speed to allow for the required revolutions. But if a machine section continuously rotates, it’s not particularly practical or reliable to use cables to transmit signals between the rotating and stationary sections.

Not to say that slip rings don’t work in applications needing less than 360° of swiveling or oscillating. In fact, limitedstroke rotary axes are quite common.

One consideration here is that the slip ring include feature for clockwise and counterclockwise rotation.

Transmission of data signals can be that for controls, sensors and strain gauges of various types, audio and video, and digital information, often to support IoT and preventative-maintenance. Many manufacturers subcategorize these slipring functions for control and data transmission, as their requirements Transmissions between brush and metal ring often differ. The frequency of data signals can vary — from a straight Graphite or wire brushes ride the rings.

Bearing dc signal to GHz, with the highest frequencies and fastest speeds requiring sophisticated slip-ring Metal rings or traces designs.

Adding such functionality means slip rings enable unique applications that would otherwise be impossible — or at least Slip-ring brushes on stationary portion of assembly streamline mechanical designs while avoiding the use of wires at moving joints. They’re most common in wind turbine, medical, aerospace, Connections marine, and industrial applications. to rotating equipment

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actuators, Fieldbus and industrial Ethernet, primarily in factory automation applications.

Often with ratings of IP65, IP68 and IP69K, they are ideal for washdown and corrosive environments.

High-speed, miniature connectors are growing in use in industrial automation, where signal density, electrical performance, and mechanical dependability in smaller package sizes are critical for efficient, compact and accurate machines. Modular connectors, micro connectors, heavy-duty connectors with locking systems, and more are used to provide constant, reliable connectivity for high-vibration environments.

Where rotating motion is present, slip rings should be used. These electromechanical connectors transmit electrical signals, power and data from a stationary structure to a rotating machine.

ELIMINATE CABLE ENTANGLEMENT Cable management is also critical. A simple, low-cost form of cable management uses twist-tie type bundlers that tie together groups of wires and cables. However, bundling them together may pose weight problems that cause sagging and put undue strain on the cables.

For stationary applications, cable trays safely and cleanly route cables. They are made of an U-shaped open channel into which cables can safely lay. They can be open or closed for protection against contaminants. When selecting a cable tray, consider what type of cable is to be routed, including its diameter and weight, the span between supports, the distance of the cable run, and environmental conditions.

Cable carriers can house a large volume of cables and wires and support their weight all without sagging or putting stress on the cabling. They can be made of plastic, steel, or a metal alloy in open or closed designs.

Knowing length of travel, number of cables or hoses, size and weight of the cables, required speed and acceleration and environmental factors such as exposure to any debris, excessive heat or chemicals is critical to selection. A key specification factor is knowing the carrier’s bend radius,

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MOTION SYSTEMS HANDBOOK

factor is knowing the carrier’s bend radius, which is 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.

SEALED AND GROUNDED TERMINATIONS Cable glands are critical when terminating cables in harsh environments, where ingress protection, sealing and grounding is necessary to safely pass a cable, wire or tube through an enclosure. They provide strain relief and contain electrical sparks or flames that may occur in hazardous environments. Additionally, they act as a seal, preventing outside contaminants from causing damage to the cable and electrical system, such as dust, dirt, and fluids. They also prevent cables from twisting and being pulled out of the machine.

Cable glands are critical when terminating cables in hazardous or harsh environments, where ingress protection, sealing and grounding is necessary to safely pass a cable, wire or tube through an enclosure. They provide strain relief and contain electrical sparks or flames that may occur in hazardous environments. Lapp’s Hygienic and INOX series cable gland series feature stainless steel construction and smooth, edge-free surfaces that prevent fluids from accumulating which can otherwise cause microorganisms to gather.

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MOTION SYSTEMS HANDBOOK

Servo versus

closed-loop stepper

motion controls

Servo motors and stepper motors both include rotors with permanent magnets and stators with coiled windings … so application of voltage to those windings prompts rotor rotation. But this is where construction similarities between these motors end.

Stepper motors have 50 to 100 poles and common hybrid iterations are two-phase (bipolar) and four-phase (unipolar). In contrast, many common motors run under servo control have four to 12 poles and are three-phase units.

What’s more, drives commonly paired with stepper motors generate sine waves with constant amplitude and varied frequency to change motor speed — or drive the motor with pulse-width modulation (PWM). In contrast, servo-motor drives produce sine waves with variable frequency and amplitude for full control over speed and torque.

Regarding controls, traditional stepper motors move upon reception of some command to advance some pulses correlating to a distance but are openloop, so lack feedback to verify whether the target is reached. Servo motors also move on receipt of a controller command signal — but under closed-loop control with encoders tracking motion to inform motioncontroller corrections until target position or actuation is reached.

So with traditional stepper designs, inadequate motor torque for overcoming

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a load will cause skipped steps or stalling even while receiving current from the drive — for a discrepancy between commanded and actual position. One solution is to oversize the stepper motor to ensure sufficient margin between highest expected torque required (to move the load) and available motor torque. Another solution is to add an encoder for closedloop stepper operation.

Some closed-loop stepper systems include controls that compare expected position (based on the steps the motor should have taken) with actual position reached as reported by encoder feedback. Any difference between the two prompts corrective moves. Slightly more sophisticated closed-loop stepper systems continuously monitor any discrepancy between position steps and encoder feedback — and then command real-time corrections in the form of changes to drive pulse rate, current, or step angle.

Still more sophisticated closed-loop stepper systems use drives that deliver sinusoidal commutation. When rotor and stator magnetic fields are improperly aligned, digital signal processors (DSPs) leveraging encoder feedback command changes to the current into the motor — to prompt torque that’s sufficient for moving or holding the axis load. Because the controls use feedback to control torque with motorcurrent changes, this stepper control mode is sometimes called servo control. Basically, the stepper motor functions as a highpole-count servo motor — sans the noise 8 • 2019

of traditional stepper-motor operation and with smooth and precise motion. Dynamic rather than constant current input minimizes motor-heating issues … and these closedloop stepper designs also exhibit less velocity ripple than open-loop vaeriations.

Read Open-loop versus closed-loop stepmotor systems at motioncontroltips.com for more on the latter.

CHOOSING BETWEEN CLOSEDLOOP STEPPERS AND SERVOS Stepper motors under closed-loop control are a small but growing percentage of stepper applications — increasingly common where load on an axis may vary and even in machine designs involving advanced multi-axis coordination.

Where appropriate for the application, it’s not uncommon for closed-loop stepper systems to be 25% less expensive than comparable servo motor solutions … and with common NEMA dimensions for simple integration. Closed-loop stepper motors are typically most suitable for applications in which: • • •

Position loss is unacceptable The axis needs high torque at relatively low rpm The design has a simple architecture or particularly tight cost constraints

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MOTION SYSTEMS HANDBOOK

speed that can be twice as fast as that from comparable open-loop arrangements. Such capabilities can be useful in machines that must quickly index or pick workpieces for sufficient throughput.

But where are closed-loop stepper motors unsuitable — and true servo motors a better solution? Well, consider the speedtorque curves of these motors. Steppers typically generate higher continuous torque at low rpm than comparable servo motors.

Servo motors on the other hand can output exceptionally high intermittent peak torques at low rpm — as well as peak and continuous torques over wider ranges of rpm. Here are some questions to ask about these and other parameters: What are the speed requirements?

Typical steppers excel on axes that run slower than about 800 rpm. Above that, core losses and the effect of time constants cause stepper-motor torque to wane. (For more information on this, search Why is the electrical time constant important for stepper motors? on motioncontroltips.com.) Torqueproducing current through a stepper motor has a certain rise time, and a given voltage may only get limited speed increase.

But in many cases from about 30 rpm to stall or when holding loads, step motors output more torque than servo motors of comparable size.

Of course, servo motor torque doesn’t fall off until several thousand rpm. That helps servo motors excel where repeatability and resolution requirements are especially stringent … to outperform certain closed-loop stepper systems on high-speed axes, machine designs needing high torque at high speed, and (as we’ll detail) systems that handle dramatically variable loads. One footnote: Servo motors are known for their ability to run at high speeds, but certain specialty controllers allow some motors (especially those with direct-drive construction) to work at speeds down to one revolution per second.

Note that some axes to run between about 800 and 2,700 rpm or so may be able to use either motor technology. Here, continuous and peak torque requirements often indicate which may be more suitable.

What are the torque requirements?

Servo motor systems output high torque on demand, even over wide speed ranges.

So servo motors may be more suitable where closed-loop stepper designs are incapable of peaking torque enough to get sufficiently quick acceleration … or on particularly dynamic axes. Plus servo motors come in wider torque ranges than most comparable stepper-motor options … and some servo drives today even include software to simplify or

Servo motor

Torque (Nm)

τpeak

Closed-loop stepper

8

τcontinuous

Servo motor

6

τcontinuous

4 2 0

Open-loop step motor 5

10

300

52

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Closed-loop step motor

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15 900

τcontinuous

20 25 30 Speed (rev/sec) 1,500 Speed (rpm) 8 • 2019

35 2,100

40

45 2,700

What is the required accuracy? Some manufacturers differentiate applications suitable for closed-loop steppers from those requiring full servo motor control with measures of rigidity requirements. A linearmotion axis with cantilevered (overhung) load or employing a belt drive is likely to accept closed-loop stepper operation. An axis that employs bearings, linear slides, and a ballscrew for maximum stiffness and minimal deflection may be one particularly reliant on rigidity for accuracy to machine, scan, or assemble delicate workpieces … and one requiring a servo motor.

Does the axis require a lot of position holding? Closed-loop stepper motors excel on point-to-point positioning and on axes that must hold in place. That’s because a stepper motor at rest can hold load absolutely still, with none of the hunting some servo-motor systems exhibit. Where efficiency is a key concern, sometimes axes are even setup to use stepper-motor detent torque to hold loads sans power input.

Example servo and stepper speed-torque curves 12

automatically execute system tuning and compensation tasks.

That said, many of today’s stepper motors have power ratings surpassing those of legacy variations thanks to advances in construction featuring stronger magnets and smaller airgaps. High torque lets stepper systems deliver accurate and stiff low-speed motion sans gearing. Larger rotors on otherwise equivalent geometries and frame sizes also make for more torque density. One caveat is that rotor inertia affects acceleration … though some stepper designs can see 20:1 rotor-to-load inertia ratios ... and more modest acceleration requirements, advanced closed-loop controls, and microstepping allow ratios to 100:1 or greater.

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Will the axis see a lot of disturbances and load variability? Servo motors tend to be most suitable where high throughput or bandwidth for disturbance correction are top design objectives. Such responsiveness depends in part on the system’s inertial arrangement. Many stepper motors require

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CONTROLS • MOTION

a load-to-motor inertia ratio below 30:1.

(Read Why use a gearbox with a stepper motor? at motioncontroltips.com for more on stepper-motor inertia.) Traditional geared servo motors often have loadto-motor inertia ratios below 10:1 for sufficient responsiveness and acceleration capabilities. But advanced servo systems with high-resolution encoder feedback and stiffness (with no sources of lost motion) allow higher ratios for top efficiency as well as responsiveness.

Visit motioncontroltips.com/steppermotor-servomotor for more on this topic.

Motion Controls with 35+ years of Experience and Outstanding Support

What resolution does the axis require?

We cover the relationship between stepper-motor mode and resolution at the motioncontroltips.com stepper-motor library. One rule of thumb is that when a design needs positioning resolution better than a couple hundred steps per revolution, stepper motor systems (especially five-phase variations and those that microstep) can work if employed with encoders — which have resolutions to a thousand steps per revolution or better.

In contrast, servo motor resolution is (at least in theory) infinite … though encoder resolution is a factor. Of course, encoders 28-bit and beyond allow millions of counts per motor revolution.

Might direct-drive servo motors be suitable? Further blurring the lines between motor-construction types are direct-drive servo motors with high pole counts that work without gearing or intermediate mechanical components. These are suitable for axes (often with high inertia) needing slower operation — generally though not always below about 1,200 rpm. Systems with these motors can see 300:1 and higher rotor-to-load inertia ratios for maximum responsiveness and performance … with high-resolution feedback as well as system stiffness maintained by specialty materials in the drivetrain, upsized bearings, and (in the case of frameless direct-drive servo motors) integration right into the machine frame.

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MOTION SYSTEMS HANDBOOK

Programmable automation controllers (PACs) and industrial PCs

A programmable automation controller (PAC) is an industrial controller that combines the functionality of a PLC with the processing capability of a PC. The term programmable automation controller is generally accepted as having been coined by the ARC Advisory Group, which specified five characteristics that define a PAC: • • • • •

Multi-domain functionality A single, multi-discipline development platform Flexible software tools that maximize process flow across machines or processes An open, modular architecture Compatibility with enterprise networks

But with no industry-standard definition of a PAC, the distinction between PACs and PLCs is blurry. Higher-end PLCs now incorporate some of the characteristics described above and are encroaching on what was once considered PAC territory. In fact, many PLCs now include standard programming languages, the ability to expand functionality through add-on modules, and connectivity to various bus systems.

PACs still differentiate themselves from PLCs by employing a more open architecture and modular design. They’re also more capable than PLCs at monitoring and controlling a large number of I/O, such as in a large processing plant or a complex automation system. They do this because data can be exchanged between devices and applications in different domains, such as motion and process control.

In addition, a programmable automation controller can send and receive data to and from other PACs, creating a distributed control system of PACs. Programmable automation controllers (PACs) excel in commanding complex automation setups that involve PC-based and HMI functions as well as process control (largely because of the way they handle I/O).

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Programmable automation controllers integrate multiple components MAIN CONTROL UNIT (PROCESSOR) KEYPAD and CONTROL LED STATUS INDICATORS MODULES

POWER SUPPLY (on main unit or a module)

COMMUNICATIONS (on main unit or a module)

I/O MODULES

Programmable automation controllers (PACs) integrate multiple subcomponents. All include a base unit with CPU and power supply; this base unit or complementary modules can also include analog, pulse, and other I/O; positioning, high-speed counter, motion, energy and power functions; and safety communications, controls, and relays.

PACs are also increasingly common for motion applications for machining or handling discrete product thanks to the flexibility and interoperability they offer machine designs.

Today’s PACs evolved as an option for complex control when microprocessors with significantly more performance became affordable and commonly available. PACs differ from the still-dominant form of control for motion — the programmable logic controller (PLC) — in that all PACs can perform as PLCs but not vice versa. That’s because PACs serve multiple channels of communication; high-data traffic; and coordination with intelligent subsystems. Most performance PLCs can host intelligent processors in their backplanes — such as Ethernet modules with multiple ports for expanded data and communications, for example. But such setups can be expensive where vendors’ proprietary backplanes and operating systems are costly.

Consider how PACs emulate the behavior of electric-relay controls. Relay logic executes sequentially with repeatability and reliability — on hardware rugged enough 8 • 2019

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CONTROLS • PACs AND PCs

Programmable automation controllers (PACs) compare most closely to what exactly?

According to some, today’s most demanding motion designs either incorporate a PAC or industrial PC — and these are engineers’ two leading options for machine control right now.

Recall that until recently, industrial automation (including motion control) was extremely segmented. So supervisory control and data acquisition (SCADA) systems were at the system top layer … separate from machine-level HMIs … in turn separate from PLCs on the machinery … with those oftentimes separate from devices reaching down to the motion to survive hard industrial settings. All logic rungs are defined by inputs and outputs that are logically connected to trigger actions after satisfying precise circuit-logic conditions. This means one logic rung can’t execute without preceding conditions met first. Extra left-hand power rails for loops and jumps extend relay-system capabilities to build complex logic ... but larger and more complex permutations of these setups can be costly to build and maintain.

In fact, PLCs themselves replaced relay-based controls and were the first standard as electricity became the dominant power source for manufacturing. As control requirements became increasingly complex, hard-wired relay control became impractical because manufacturing needed more reliable and reconfigurable (programmable) systems.

Given the primitive hardware and rigid task execution, early PLCs were difficult to network. Today’s PLCs are quite easy to implement.

The first PLC in the late 1960s used available electronics that duplicated the behavior of relays — plus were programmable instead of hardwired. This necessitated proof of repeatability and reliability on the plant floor. So custom memory boards, logic-controller boards, backplane interfaces to I/O modules, and heavy-duty circuitry helped these PLCs to run in industrial environments. It was a new concept to let users write application code using the symbols and logic of relays.

Today’s PLCs are still more appropriate than PACs in standalone applications such as machine axes that run preset sequences. The rule of thumb is that anywhere PAC functions would otherwise go unused, it still makes sense to use the a more economical PLC. Pressure from plant personnel and the enduring value of ladder logic also make PLCs the first choice in many applications.

control level — or at the end reaches of the machine executing other complex processes beyond the capabilities of a PLC. In contrast, many of today’s automation systems blend motion control with robotics, vision systems, I/O, HMIs, at a single PAC. Some even connect to or serve as IoT gateways to allow communications with enterprise-level systems and the Cloud.

Of course, non-PAC solutions (employing PLCs and motion controls) remain suitable for myriad applications.

defending designs in product-liability lawsuits. What’s more, it’s not just product-data logging that leverages the data-tracking functions of PACs; running predictive maintenance and operations monitoring uses data from controls, too. That necessitates more data and complex network interactions — which means PACs will only become increasingly common.

Just as PLCs, the controls known as distributed control systems (DCSs), PCs, and remote terminal units (RTUs) include hardware and programming to satisfy specific applications. PAC hardware can run functions as software to replicate legacy forms of these and other pieces of motion-system hardware. Here’s the catch: Early-generation versions of all these controls were engineered with features to serve specific

WHERE PLC SETUPS YIELD TO PACS — AND THE DRIVE OF DCS, RTU, AND PC TASKS As various industries came to accept PLCs, they came to be used in myriad applications. Increased PLC use has also followed the continuous improvements in their speed, ability to run complex math functions, and communication networks.

But once PLC behavior was proven reliable on a computer, the PAC was born. The aerospace and medical industries are two driving industries. The FAA and FDA mandate that day, date, and timetagged data about manufacturing processes are stored for extended periods of time — particularly well run on PACs. Even manufacturers of simple consumer products are finding such information necessary for 8 • 2019

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SAFETY

PROCESS CONTROL

markets. So for today’s engineers working in siloed industries served by these legacy controls, the way in which PACs replicate several control schemes MOTOR boosts convenience and familiarity during CONTROLS CNC implementation.

DIGITAL I/O Note that computer processors’ DRIVES increasing capabilities and declining cost FEEDBACK have blurred distinctions between various control types. Case in point: The PAC itself MOTION and SEQUENTIAL is the extension of the PLC to incorporate CONTROL greater data-processing and communications capabilities incomprehensible when the PLC was invented.

Originally, DCSs were a collection of RTUs operating on normal phoneline networks, and their communications were simple alarm states from remote equipment. RTUs were small standalone HMIs controllers to execute modest logic tasks — usually ENTERPRISE off simple information such as elapsed runtime or and IIoT CONNECTIVITY total units of counted, for example. Early DCSs didn’t transmit data, but one could rent a line from the phone company and create an alarm to indicate SCADA that a process value had been exceeded or the RTU needed reading, for example.

Industrial PCs have advanced with processor capabilities following Moore’s law. Early industrial PCs were merely programming terminals and storage devices, because their operating systems weren’t robust enough for industrial controls. Now there’s no such limitation, because of vastly improved hardware and widely available real-time operating systems such as Linux for IPCs. Some PACs have multi-processor architecture to support multiple programming environments — and run multiple DCS, RTU, PLC and PC functions. PACs

DCS

ANALOG I/O

PROGRAMMABLE LOGIC CONTROLLER (PAC)

COLLISION DETECTION INTEGRATED VISION ROBOTICS

PACs can operate in multiple domains simultaneously — including motion and sequential control, process control, logic, data management, and communications on a single platform. PACs excel in Cartesian, SCARA, and six-axis robot applications needing coordinated motion across multiple axes as well as integration with other motion and data systems.

Two controls for motion automation — PLCs and PACs

56

Typical programmable automation controller (PAC)

Typical programmable logic controller (PAC)

Processor modules

Multiple processors per rack

One per rack

Processor chips per module

Two or more high-performance chips allow multitasking

One microprocessor (288)

Communication

Open networking • Multiple options • Ethernet/IP

Typically one option (though more are increasingly common)

Programming

Structured text • Function block diagrams • Ladder logic • Most support for IEC 61131

Ladder logic — but with new PLCs supporting IEC 61131

Functionality

Dual scan of logic • Motion control • Data acquisition • Process control

Sequential scan of logic

Memory

Up to 32,000K

Up to 64K

I/O

128,000 (with thousands of analog I/Os possible)

3,000

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CONTROLS • PACs AND PCs also have high-bandwidth internal architectures that allow multiple processors and multiple tasks to simultaneously execute ... so designers can create controls to satisfy complex and concurrent requirements.

PACs AND CONNECTIVITY PACs are generally sold by companies with mature I/O offerings, but the fact that PACs support Ethernet means there’s more I/O independence than ever for system designers. Design engineers can choose the I/O product that best suits a given application — or one that’s compatible with an existing installed base of products.

Integration of a PAC into data-reporting schemes is usually easy thanks to builtin communications options in the basic hardware. There’s also support for modems and wireless-network layers — for built-in remote communications to data systems outside the plant environment. There are also protocols to facilitate interfaces with data systems such as Oracle and SAP — so tools abound for using PACs in datacentric applications.

Video interfaces are more common (and growing) in discrete part manufacturing. More PACs than ever work with smart videos to verify dimensional accuracy — which in turn boosts product quality. Common video subsystems can interface directly to PACs. In the past, videoto-PLC interfaces required a great deal of extra programming.

Motion control integration into discrete part-manufacturing operations often works well on PLCs if the motion consists mostly of independent single axes. Here, a smart-axis controller built into the motor drive electronics can operate with digital handshakes to the PLC. Runs, stop inputs, and busy signals from the motor control can often suffice.

However, applications that need more coordination or speedier movements don’t run well off PLCs. That’s because PLCs limit system performance — and that can jeopardize whole automation projects.

In contrast, PACs use of high-density memory and solid-state disks, so can process much more data on the fly. Should there be a machine problem, PACS can access extensive documentation or serve it under software control to facilitate repairs. That improves machine availability and productivity. In fact, these features can sometimes integrate into HMIs when there is sufficient memory. However, PACs integrate such function directly into application programming.

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Conveyors

for simple to complex

transport

Conveyors move bulk material or discrete products from one area to another and serve as main material-handling arteries to improve efficiency and throughput. Advances in materials, controls and modular subcomponents have spurred new large conveyors for bulk material transport, miniature conveyors for discrete sorting, and everything in between.

During manufacture, myriad products move on conveyors. So conveyors come in an array of shapes and widths of less than 2 in. (for moving extremely small parts) to several feet wide. Once viewed as an afterthought, conveyors have become an integral component in nearly all automated facilities and applications.

Select a conveyor by first asking: 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.

How does surrounding equipment interact with the product riding on the conveyor? Conveyor Class 1 includes material-handling 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

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that take materials into or out of machines or stations. Class 4 includes conveyors that 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.

What is the maximum weight of the product being moved?

Does the conveyor need to operate at a certain speed? 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. Will moisture be present in the application? Does the application need to be sanitary? Look for rugged or washdown-rated conveyors with open frames.

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.

Magnetic conveyors are built with ceramic magnets for applications that need parts to adhere to the belt during processing, or for jobs that require elevation changes. The designer can specify motioncontroltips.com | designworldonline.com

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MOTION SYSTEMS HANDBOOK

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higher magnet strength for use in inverted applications. In contrast, 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. (Delrin is an inflexible polymer that works as a tough, heat-resistant metal substitute.)

This lets a device check passing product without getting false readings.

Pivot conveyors mount to a pivot base to swing out of the way when workers need to walk through the line. Interlock switches and a timer let the conveyor clear before the gate opens. Some controls can automatically resume product flow after the conveyor returns to the inline position.

Servo drives 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. Manufacturers mount encoders to a conveyor’s drive shaft to sense shaft rotation or count pulley revolutions for accurate control of the belt in feeding or indexing applications.

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. In some arrangements, the belts even mount to a single conveyor frame.

Timing-belt conveyors use toothed belts that engage synchronous drive pulleys while serving as the conveyor surface as well. These provide excellent belt-movement control for accurate part or fixture 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.

POWER AND FREE CONVEYORS: WHERE DO THEY EXCEL? Power and free conveyors are designed for manufacturing environments where products need to be transported in a non-linear fashion — that is, where materials aren’t necessarily delivered in the order they were loaded or at the same pace. Traditional linear conveyors on the other hand lack the flexibility to handle manufacturing environments where different production processes run at different cadences or where various materials have different flow paths.

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TITLE GOES HERE

Conveyors that provide basic machine input and output can be fairly simple.

Conveyors that move product through a standalone machine can also use one-way conveyors with the most basic encoder feedback.

Conveyors that move product from one machine to another in a continuous manufacturing line must be precision setups — often with a servodrive and motor running a toothed pulley to (positively) engage a belt.

When a conveyor is a manufacturing line’s main artery, it needs a precision setup as well. Software setup helps facilitate drive sizing to accomodate distance moved, load, and startup inertia.

Many conveyor manufacturers (including Dorner Mfg Corp.) classifies conveyors by how they interface with surrounding environments.

Note that because of their flexibility in handling and delivering products, power and free conveyors are also referred to as asynchronous or non-linear conveyors.

The defining feature of a power and free conveyor is that it consists of two tracks — an upper track and a lower track. The upper track is powered by a chain, and the lower track is unpowered. Trolleys, which carry the load, run on the lower track, supported by rolling wheels. Mechanical devices — often referred to as pusher dogs — on the powered track engage with the trolleys to move them and disengage with the trolleys to stop them. The pusher dogs are engaged and disengaged by cam action caused by a trolley in front or by a stop blade positioned along the powered chain. Air-activated stops, triggered

by switches, can also be used to control the movement of trolleys.

Much like cars on a road, conveyor traffic can also be merged or diverted among multiple conveyor lines, and both sharp turns and elevation changes can be executed. And unlike traditional linear conveyors, power and free versions can allow products to accumulate, or “stack up” by holding trolleys stationary while other trolleys “catch up” and join them. The trolleys can then be released in the required sequence. All of this is accomplished through mechanical linkages via the pusher dogs and trolleys, without requiring complex automation.

To maximize space, especially during accumulation, trolleys can be connected via rigid load bars that set the spacing between trolleys at as little as 6 inches (depending on

This is a stainless-steel micro-pitch plastic modular belt conveyor from mk North America with a diverter for bottles.

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the size and shape of the load). And storage density can be maximized with trolleys that hold the load diagonally relative to the conveyor path.

Circuits of conveyors up to 300 feet can be controlled with just one motor, and complex “traffic” patterns and timing sequences can be controlled via sensors triggered by a PLC or PC-based controller. In addition, bar codes or RFID tags on the trolleys can be used to identify individual loads and carry instructions for sequencing.

While most power and free conveyors are mounted overhead, above the working area, and carry products below the tracks, floormounted designs are available that carry the product above the tracks. This is especially useful when overhead space is a concern, or when potential contamination from the conveyor or product is an issue, as is often the case in cleanroom environments. And when overhead space is limited but floor-mounting is not an option, some manufacturers offer overhead versions with powered and nonpowered tracks positioned side-by-side, rather than in a top-and-bottom configuration.

Power and free conveyors are customdesigned for each application and can carry loads from just a few pounds to several hundred pounds. They’re used extensively in the automotive industry, where robot cells are sometimes used to load/unload products from the trolleys or to perform work on the parts being conveyed as they’re held stationary.

Power and free designs are available in heat and corrosion-resistant versions, so they can transport loads through processes such as washing and painting or through ovens.

DESIGN WORLD — MOTION

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COUPLINGS

Flexible couplings for motion design

Couplings connect rotating shafts in equipment powered by electric motors and other drives. All transmit torque and angular velocity. Flexible variations compensate for misalignment. Many of the latter even address vibration and improve system dynamics.

Design considerations include machine or installation construction and backlash, torsional stiffness, damping, inertia, torque ratings, maximum rpm, size, misalignments, ease of installation, robustness, and cost. For power transmission (as in motors for pumps and large material-handling setups) common choices are gear, disc, elastomeric tire, grid, jaw, and Oldham couplings because of their ruggedness and ability to transmit very large torques. Motion-control applications (as for axes employed in precise positioning of loads, for example) typically employ couplings capable of oft-more modest but far more precise torque transmission. These include curved-jaw, beam (slit), bellows, disc, and other zero-backlash couplings.

As we’ll explore, any misalignment that couplings accommodate should be what’s otherwise unavoidable even after proper machine-axis squaring and installation adjustments. That’s because misalignment — manifest as parallel, axial, and angular misalignment— degrades efficiency, induces bearing wear, and excites machine natural frequencies.

To review, the maximum amount of angular misalignment for which a coupling can compensate is expressed in degrees;

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Wafer holder

Servo motor Ball screw SERVO FLEX SFC couplings from Miki Pulley Co. complete the assembly of this semiconductor wafer stage.

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COUPLINGS HYBRID STEPPER MOTOR LEAD SCREW LINEAR ACTUATORS parallel misalignment between the shafts a coupling connects is expressed in inches or millimeters. Axial misalignment is also a length value; it’s the maximum permissible spread between coupled shafts — and in fact, a misalignment permutation often most affected by thermal effects. Flexible couplings for motion control are often less forgiving of misalignment than those for more straightforward power transmission and resolve it with specialty design features.

A related phenomenon and a coupling consideration specific to motion-control installations is backlash. In applications for strict power transmission, backlash is far less of a concern than that of efficient torque transmission — and actually a characteristic that (in normal moderate quantities) helps make some couplings in these settings more efficient and forgiving of misalignment. In contrast, couplings on the outputs of steppers and servomotors are designed to prevent the lost motion that can degrade output-product quality or overall machine throughput.

Note there’s a difference between backlash (which is true mechanical clearance) and the torsional deflection or windup that all loaded rotary components exhibit. Most couplings for motion applications are inherently backlash free or preloaded to eliminate backlash — but they all have different torsional stiffnesses, which is sometimes a tradeoff for lateral flexibility.

AVOID PITFALLS IN SELECTION OF COUPLINGS FOR MOTION Design engineers often run into trouble when they neglect to account for environmental effects on couplings — particularly flexible couplings installed in gritty or caustic areas, vacuum environments, or places that are extremely hot or cold. Beyond that and the common design considerations already listed, designers must account for dynamic forces to which a coupling will be subject. Steer clear of using published an axis’ gearset or motor peak-torque values for setting its coupling’s nominal torque rating. That’s because this approach usually makes for an assembly with an oversized coupling and an unnecessary inertial increase.

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Designers should also avoid the application of a coupling type simply because it’s a familiar technology. For example, beam couplings are extremely well known in industry, and they excel on axes transmitting moderate to light torque — as on leadscrew-driven motorized axes or where there’s a need for attachment of a precision encoder, for example. However, some particularly demanding designs may necessitate a flexible coupling type that maintains higher torsional stiffness.

On the other hand, it’s also unadvisable to simply pick a coupling based on high torsional stiffness. Many flexible couplings have an inherent stiffness that exceeds application requirements for servo tuning and motion accuracy. Even in motion designs requiring high stiffness for the shortest possible response time (as in equipment for electronics manufacturing, for example) couplings with good damping characteristics often offer more effective optimization than more torsional stiffness. (Refer to the section, “When motion designs need torsional stiffness — as well as damping” at the end of this section for information on this topic.)

That’s because overly stiff couplings of many designs pose an unnecessary risk of fatigue.

IP54 STEPPER LINEAR ACTUATORS

CAN STACK LINEAR ACTUATORS

TORSIONAL RIGIDITY Torsional rigidity is an object’s resistance to torsion or twisting under applied torque.

Torsional rigidity in couplings is torque per value of angular displacement, and it’s a value that affects overall machine design.

Even slight variations degrade positioning accuracy and limit cycle speeds.

It’s usually unnecessary and impractical to test all components in design, which is why engineers use theoretical systemstiffness values. One caveat here related to couplings is that different manufacturers’ coupling-stiffness ratings vary with measurement methods. There can also be differences between published and measured values. So use caution when designing a motion machinery relying heavily on overall stiffness for good design performance. Also, look for evaluations that faithfully model performance characteristics to ensure stiffness and machine-assembly performance.

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MOTION SYSTEMS HANDBOOK Couplings accommodate various misalignment types Axial

WHAT ARE REACTION FORCES?

All flexible couplings compensating for misalignment cause reaction forces, and their effect is significant if misalignment is Parallel (lateral) excessive. These often-overlooked reaction forces transmit to connected shafts and support bearings and can cause damage to Angular motion axes — especially on precision designs with delicate bearings and slender shafts.

Though couplings get their compliance from elastomeric deflection, sliding contact, and flexing coupling members, here we focus on the types most common for motion designs employing stepper or servomotors.

In fact, the most suitable coupling choice ultimately Ultimately, reaction-force magnitudes depend on the level depends on application requirements and machine throughput. of misalignment and the coupling type in use. Bellows couplings, If improving axis positioning and cycle time is priority, focus so-called membrane couplings such as disc couplings, and on boosting powertrain stiffness. Selecting couplings with high beam couplings have thin sections of various designs capable torsional rigidity (among other things) can minimize lost motion of radial flexing. Resistance to misalignment — a spring-rate from torsional windup. reaction defined as a force per unit of deflection — increases Shorter couplings or those with reinforced bellows can proportionally with shaft deflection. Because these couplings bend boost torsional rigidity values ... But keep in mind that while a to accommodate misalignment, reaction force depends on the shorter coupling has higher rigidity (to 60 to 70%) misalignment thickness of the flexible element. compensation capabilities also decrease with length.

But bellows and beam couplings have multiple coils or convolutions, so work as flexible shafts that sweep through ALIGNMENT: complementary bows as the mode of misalignment compensation.

ITS IMPORTANCE CANNOT BE OVERSTATED Torque transmission is through members in shear, so the No coupling — no matter how engineered — can correct for convolutions can be thin and keep radial forces low while shafts that are excessively misaligned. The nature of flexible maximizing torsional stiffness. couplings occasionally misleads design engineers and assembly In contrast, membrane-coupling variations transmit torque via personnel (or more often, end users) into believing that they’re bending members, so need thick members to get high torsional a fix-all for compromised or less exacting machine builds. But stiffness. Such couplings’ bending (through complementary flexible couplings put into designs with excessive misalignment directions) also compensates for shaft misalignment. The catch is exhibit material stresses and fatigue and premature failure. that these torsionally stiff couplings can induce significant radialThough coupling failures do occasionally originate from reaction forces if excessive misalignment is present. couplings themselves, it’s far more common that coupling issues Because radial-force magnitude depends on bend severity, arise as a symptom of other design problems. If a motion design minimizing bending angles reduces the detrimental forces on does exhibit coupling problems, avoid the temptation to simply support bearings (though can reduce misalignment capacity, upsize or upgrade that coupling. Such upgrades are often too). Some membrane couplings address more misalignment unnecessarily expensive and short-lived solutions that actually put with a central member between the flexible members; the added system bearings as well as gearing and connected motors at risk distance imparts an ability to turn while making shallower bends of collateral damage. (and lower radial forces) for a radial shaft offset.

Instead, make a holistic analysis of the design and consult the In fact, aforementioned beam and bellows couplings with coupling manufacturer for assistance. divided flex-element arrays also sometimes leverage more Note that when motion systems exhibit coupling issues long distance between flexure points to get shallower bends for a after a proper installation and run of service, it’s sometimes a result given radial shaft offset. Short models sometimes connect via an of some other change in the drive assembly. Even small changes intermediate shaft. to the motor, drive, or programming can be to blame — especially In contrast, elastomeric couplings have myriad torsionalif a new motion sequence demands higher transmission of motor damping properties and transmit torque in shear, bending, and torque or the elimination of a previously held electronic limitation. compression. Coupling class is key with this design: Some versions

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MOTION SYSTEMS HANDBOOK FLEXIBLE COUPLINGS FOR MOTION

Shown here are some zero-backlash coupling options.

SOME JAW COUPLINGS ARE FOR SIMPLER POWER-TRANSMISSION DESIGNS; LOOK FOR CURVED-JAW AND ZERO-BACKLASH DESIGNATIONS FOR MOTION.

STANDARD

JAW (SPIDER)

τ MAX

TO 7,000 LB-IN.

ALLOWABLE MISALIGNMENT

MORE

ELASTOMERIC INSERT

BEAM STANDARD TO 260 LB-IN.

τ MAX

BEAM COUPLINGS AS THESE FROM HELICAL PRODUCTS CO. ARE SINGLE-PIECE DESIGNS WITH NO ASSEMBLY POINTS TO LOOSEN. NOTE THE ATTACHMENTS AND OTHER CUSTOM VARIATIONS.

RUBBER INSERT

THIS COUPLING DELIVERS TORSIONAL STIFFNESS PLUS DAMPS VIBRATION FOR EASIER SERVO TUNING. THE CENTER MOLDED PORTION IS HNBR. IMAGE COURTESY NBK AMERICA

WHEN MOTION DESIGNS NEED TORSIONAL STIFFNESS — AS WELL AS DAMPING

BELLOWS τ MAX

STANDARD TO 900,000 LB-IN.

APPLICATION EXAMPLE COURTESY GAM ENTERPRISES

ROBA-DS DISC-PACK COUPLINGS HAVE SPLIT CLAMPING HUBS FOR EASY ASSEMBLY. MADE BY MAYR POWER TRANSMISSION, THE COUPLINGS DELIVER HIGH TORQUE.

MORE

LESS

STANDARD TO 1,600,000 LB-IN.

TYPICAL TORSIONAL STIFFNESS

DISC ELEMENT τ MAX

RIGID

RIGID COUPLINGS ONLY WORK ON EXCEPTIONALLY WELL-ALIGNED AXES.

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exhibit zero backlash while others rated for use in power-transmission applications can exhibit minute inter-hub rotation. Most variations transmit torque (and address misalignment) through compressible elastomeric-insert spiders trapped between jawed halves ... and induce reaction forces when connecting shafts with excessive radial shaft deflection. Jaw designs can accommodate more misalignment (and minimize detrimental forces on the shafts’ support bearings) with softer elastomer spiders, though that sometimes reduces torsional stiffness.

Remember that if excessive misalignment is a concern, consult with coupling manufacturers on the design. Their engineers may suggest design improvements; offer coupling types to resolve the misalignment without inducing unacceptable reaction forces; and supply charts of reaction forces that a given coupling is projected to induce under a given set of conditions.

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New technological improvements in servomotors have spurred dramatic improvements in response frequencies. The catch is that vibration (and hunting) tend to arise when designers apply high gain settings to servo systems and use advanced couplings with high torsional stiffness — such as disc or bellows-type couplings, for example.

One way to resolve hunting in setups with high gain settings is to use couplings with vibration-damping capabilities. Here, couplings with hydrogenated nitrile butyl rubber (HNBR) center elements are one option to make servo systems for precision automation tasks (as those in semiconductor manufacturing) more responsive.

Sometimes called high-gain rubber couplings, these have an integrated structure that includes aluminum hubs on both ends molded with vibration-reducing HNBR to prevent backlash but stay flexible. The rubber-lined claw structure optimizes torsional rigidity as well as damping.

Bode plots show how high-gain rubber couplings increase servomotor gain beyond the capacity of comparable couplings with high torsional stiffness.

Gain width between 0 dB and the point at which there’s a phase delay in the Bode plot is -180° — and this is called the gain margin. General guidelines for servo systems recommend gain margins between 10 and 20 dB. As servomotor gain rises, gain margin decreases. When the gain margin falls below 10 dB, hunting tends to occur.

Consider the limit gain (the servo gain at which hunting occurs) of assemblies using high-gain rubber-type couplings. The value of 17.40 dB surpasses that of other coupling types. Plus because the gain margin is above 10 dB, the servomotor gain of the rubber-insert couplings effectively shortens stabilization time and increasing throughput. More specifically, a system with a 12-msec limit gain might see improvement to a 3-msec limit gain simply by switching to a rubber-type coupling. This suppresses hunting and minimizes time for stabilization.

Visit Design World’s own couplingtips.com for more on servomotor couplings and related stiffness, hunting, damping, and stabilization considerations. motioncontroltips.com | designworldonline.com

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How the Right Flexible Coupling Can Reduce Stabilization Time and Increase Productivity Q: How can one to bring out their servo system’s performance?

A: There are lots of ways to improve productivity in the Servo System. For example, there is the possibility that you can shorten the production cycle time if you use a high performance servo motor and an actuator. But, actually there are some points customers are overlooking. Mainly, the evolution of coupling technology that utilizes the linking of a motor and an actuator.

In using standard coupling technology, you may not be getting optimal performance from your servo system.

Q: What is the history of coupling’s performance improvement?

A: Generally, couplings used for a Servo System require flexibility and torsional rigidity. To fulfill these technical specifications, the Slit coupling was utilized during the 1990s and then updated to Disc couplings in the 2000s. Around 2005, servo motors with a higher speed response frequency appeared. There was a technological limit in regards to the shortening of the production cycle time even if we increase the coupling’s torsional rigidity.

Q: What is the limitation of “Torsional Rigidity”?

A: The cause is co-vibration. The increase of a coupling’s rigidity is insufficient to increase the rigidity throughout the whole servo system.

Due to the high-speed response frequencies, the co-vibration: “hunting” of the servo system cannot be avoided. So what can be done?

NBK America LLC 08-19 1.indd 70

8/19/19 1:00 PM

Q: How can we prevent the vibration: “hunting”?

A: The solution is to provide higher capacity for absorbing the vibrations that are a result of the higher speed response frequencies.

We propose NBK’s Anti-Vibration Couplings: “XG-series”.

We succeeded in realizing both the coupling’s rigidity and vibration reduction by using an innovative molding technique within the internal structure. An additional benefit is that the couplings have Zero-Backlash.

In 2007, NBK took the lead in the industry launching the Anti-Vibration Coupling.

In 2013, we launched a new coupling, the “XG2” with a capacity to achieve much higher torsional rigidity and damping performance.

With the XG2, you can increase your productivity and control of your Servo System through the high gain it offers compared to traditional couplings.

Anti-vibration couplings, the XG series, shorten manufacturing times by reducing vibration even in high gain environments.

NBK America LLC 307 East Church Road, Suite 7, King of Prussia, PA 19406 Phone: 484-685-7500 Fax: 484-685-7600 E-mail: info.us@nbk1560.com Web: www.nbk1560.com

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MOTION SYSTEMS HANDBOOK

NUMERIK JENA’s LIKgo linear encoders are designed for the demands of production and inspection machines in the semiconductor industry. A new two-field scanning principle ensures that signals are produced free of offset errors and phase errors. The measuring standard is a SINGLEFLEX steel scale tape 8 mm wide with a grating period of 20 µm, making possible measuring steps down to 78.125 nm.

Encoders — the basics In automation and control systems using closed-loop control, speed and position are two critical variables. And the most common way to acquire position information is using an encoder.

Encoders can be classified in a number of different ways – whether they are rotary (the more common) or linear, by the sensing technique, whether the output is absolute or incremental, as well as by some common performance parameters.

SENSING TECHNIQUES Common sensing techniques include optical, magnetic, and capacitive sensing. While optical encoders have historically been the only option for resolutions below 5 microns, improvements in magnetic scale technology now allow them to achieve resolutions down to 1 micron. Optical encoders use a light source and a photo-detector to determine position, but their use of light makes them sensitive to dirt and debris, which can disrupt the signal.

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The performance of optical encoders is influenced by the gap between the sensor and the scale, which must be properly set and maintained to ensure that signal integrity isn’t compromised.

This means that mounting must be done carefully, and shocks and vibrations should be avoided.

Magnetic encoders use a magnetic reader head and a magnetic scale to determine position. Unlike optical encoders, magnetic encoders are mostly unaffected by dirt, debris, or liquid contamination. Shock and vibration are also less likely to affect magnetic encoders. They are, however, sensitive to magnetic chips, such as steel or iron, as they may interfere with the magnetic field.

Capacitive encoders, a relatively new technology, offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. The basic principle behind capacitive encoders is that they detect changes in capacitance using a high-frequency reference signal. This is accomplished with the three main parts—a stationary transmitter, a rotor, and a stationary receiver. (Capacitive encoders can also be provided in a “two-part” configuration, with a

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MOTION SYSTEMS HANDBOOK NEW HOLLOW-SHAFT KIT ENCODERS

rotor and a combined transmitter/receiver.)

The rotor is etched with a sinusoidal pattern, and as it rotates, this pattern modulates the high-frequency signal of the transmitter in a predictable way. The receiver disk reads the modulations, and on-board electronics translate them into increments of rotary motion.

The primary concern when using capacitive encoders is their susceptibility to noise and electrical interference. To combat this, the ASIC circuitry must be carefully designed and the algorithms for de-modulation must be fine-tuned.

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Encoder output can be either absolute or incremental. Absolute encoders have a unique code for each shaft position, so that every position is distinct. The absolute encoder interprets a system of coded tracks to create position information where no two positions are identical. Absolute encoders also don’t lose position if there is power loss.

Because each position is distinct, true position is verified as soon as power is restored and so does not require a homing routine.

Absolute encoders can be either single-turn or multi-turn. Single-turn encoders are well suited to short-travel motion applications where position verification is needed within a single turn of the encoder shaft. On the other hand, multi-turn encoders are better for applications that involve complex or lengthy positioning requirements.

Absolute encoders have a number of advantages. Safety is one.

In some applications where a loss of position could lead to operator injury or machine damage, an absolute encoder automatically provides position verification when power is restored.

Absolute encoders also have good immunity to electrical noise because they determine position by reading a coded signal. Stray pulses from electrical noise will not build up and accurate position is presented again on the next reading.

Incremental encoders generally supply square-wave signals in two channels, A

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and B, which are offset (or out-of-phase) by 90 degrees. This helps in determining the direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position like an absolute encoder.

To provide useful position information, encoder position 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.

A traditional limitation of an incremental encoder comes with a loss 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 in order to know its exact position.

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 of loss of power 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.

PERFORMANCE PARAMETERS Encoder resolution and accuracy are essential to the proper operation of a closedloop system. The ideal encoder has both the necessary resolution and is highly accurate.

Resolution is the distance over which a single encoder count takes place – it’s the smallest distance the encoder can measure.

For rotary encoders, resolution is typically specified in terms of measuring units, or pulses, per revolution (PPR). Linear encoder resolution is most commonly specified as the distance over which the count takes place and is given in terms of microns (μm) or nanometers (nm). The resolution of an absolute encoder is specified in bits, since absolute encoders output binary “words” based on the encoder’s position.

Accuracy is the difference between the true position (or speed) of the device being measured and the position (or speed) reported by the encoder. For rotary encoders, it is specified in arcseconds or arcminutes, and for linear encoders accuracy is typically given in microns.

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ENCODERS

Note that higher resolution does not mean higher accuracy. Consider two encoders â&#x20AC;&#x201C; one with 100 PPR resolution and one with 10,000 PPR resolution, but both with the same accuracy specification. The lower resolution (100 PPR) encoder can report a movement of 90 degrees just as accurately as the higher resolution (10,000 PPR) model. The higher resolution encoder just has the ability to break up that 90 degree movement into much smaller increments.

An encoderâ&#x20AC;&#x2122;s resolution is based on the number of lines (for an incremental encoder) or the pattern (for an absolute encoder) on the encoder disk or scale. Physically, resolution is fixed. Once an encoder is manufactured, there is no option to add more lines or patterns to the code disk. But the resolution of an incremental encoder can be increased through signal decoding. Incremental encoders output square wave signals, and by counting both the leading and trailing edges of one signal (signal

These kit encoders for stepper motors from POSITAL are modular devices designed to be integrated into a motor housing, measuring the rotary position directly from the drive shaft. The magnetic measurement module is compact measuring 37 mm in diameter and 23 mm deep, and highly resistant to dust, moisture and shock and vibration loading.

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MOTION SYSTEMS HANDBOOK

These QUANTiC rotary (angle) encoders from Renishaw use a 40 µm-pitch ring scale, available in a range of sizes from 52 to 550 mm. The encoder system addresses applications with wide through-bore requirements and can operate at rotational axis speeds of up to 8,800 rpm with angular resolutions to 0.04 arc seconds.

A), the resolution of the encoder is doubled. When the leading and trailing edges of both signals (A and B) are counted – referred to as quadrature decoding – resolution is increased by a factor of four.

While the number of lines or measuring units determines resolution, accuracy is affected by the width and spacing of these lines or units. Inconsistent width and/or spacing will cause errors in the timing of the pulses. For absolute encoders, accuracy is influenced by the precision with which the pattern is placed on the code disk.

Factors external to the encoder can also affect its accuracy. These include the rigidity of the assembly and mounting errors, such as a lack of concentricity between the encoder disc and the shaft to which it’s mounted. For linear encoders, thermal expansion of the scale and of the mounting surface can also degrade accuracy.

For Absolute Feedback, we’ve got your solution EPC’s Absolute Encoders offer: • 16 bits resolution for single turn, and 43 bits resolution for multi-turn • SSI, CANopen, or EtherCAT communication protocols • Electronic cam switches on CANopen and EtherCAT models • Maintenance-free and environmentally-friendly magnetic design • Energy harvesting magnetic multi-turn technology • No gears or batteries

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MOTION SYSTEMS HANDBOOK

Gearbox

service factor and service class

explained

Sizing a gearbox (or gearmotor) for an industrial application typically begins with determining the appropriate service factor. In simple terms, the service factor is the ratio of the gearbox rated horsepower (or torque) to the application’s required horsepower (or torque). Service factors are defined by the American Gear Manufacturers Association (AGMA), based on the type of gearbox, the expected service duty, and the type of application.

While service factors may seem to be very specific, with thousands of combinations of gearbox types and applications each assigned its own numerical value, the criteria used to determine these values are based not on testing and empirical data, but rather on extensive review and analysis of gearbox manufacturers’ experience.

In general, the horsepower (or torque) rating of a gear tooth is based on the durability of the gear surface — its resistance to pitting — or on its bending fatigue. As the service factor of a

Shown here is an animatronics design featuring a robolink from igus. The worm-gear assembly is complemented by strain-wave gearing elsewhere.

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MOTION SYSTEMS HANDBOOK

AFX high-precision planetary gearboxes from Apex Dynamics USA include a lowfriction output bearing and sealing. They deliver high torque and precision; long service life; and low-noise operation. The planetary gearboxes also exhibit limited temperature rise and work in continuous-running applications. The AFXR rightangle version has a 90° input that transmits through a helical bevel gear. The planetary gearboxes feature an extremely short and light (yet rigid) housing and full compatibility with standard motor adapters.

gearbox is increased, the relationship between the gear teeth life (based on durability of the gear surface) and load is proportional to the increase in service factor, raised to the 8.78 power. In other words, if the service factor is increased by 30 percent (from 1.0 to 1.30, for example), the gear tooth life will increase 10 times (1.308.78 = 10.01).

To determine the gearbox service factor, start by consulting a set of tables or charts provided by the manufacturer, based on the type of gearing — worm, spiral bevel, helical, and so on. These tables list a wide range of applications ... conveyors, cranes, winders, saws, blowers, and so on, Each is listed with (typically) three levels of service duty the gearbox is expected to see: zero to 3 hours per day; 3 to 10 hours per day; or greater than 10 hours per day.

Each of these application-service duty combinations is assigned a recommended service factor.

Remember the gearbox service factor is much like a safety factor to ensure the gearbox meets the application requirements, taking into account typical operating conditions known to exist for various types of applications.

Once the AGMA-recommended service factor is determined, consider other, non-typical, working conditions that can cause additional stress and wear on the gear teeth, bearings, or lubrication.

If any of these conditions exist, increase the service factor accordingly to ensure a sufficient safety margin and life of the gearbox.

the application, and the result is the output horsepower (or torque) required by the gearbox.

Some conditions that may require an increase in the service factor are: • Elevated temperatures • Extreme shock loads or vibrations • Non-uniform loads (cutting versus conveying, for example) • Cyclic loads (frequent starts and stops) • High peak versus continuous loads

Inertia or more specifically the inertia ratio is one of the most important factors in sizing a servo system. Inertia is defined an object’s resistance to change in velocity and in servo-driven systems, it can be used as a measure of how well the motor is able to control the acceleration and deceleration of the load.

The yardstick by which this is judged is the inertia ratio, which is defined as the inertia of the driven component(s) divided by the inertia of the motor.

Once the appropriate gearbox service factor is determined, multiply the service factor by the horsepower (or torque) required for

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HOW DOES SERVICE CLASS DIFFER FROM SERVICE FACTOR? In some cases, manufacturers cite gearbox “service classes” rather than service factors. Service classes are designated as I, II, or III, and are generally translated to numerical service factors of 1.0, 1.4, and 2.0, respectively, to be used in gearbox sizing calculations. It’s common that even if a manufacturer publishes service classes for general application types, they also publish the more specific service factors for specific applications as well.

WHY DON’T SOME CATALOGS LIST GEARBOX SERVICE FACTORS? Using service factor to guide the selection of a gearbox is appropriate for applications driven by traditional AC induction motors. But because gearbox output torque, speed, and inertia are much more critical for the proper operation of a servo system, sizing a so-called “servo-rated” gearbox requires a more detailed and exact method.

For gearboxes that are used in servo systems, the primary emphasis in the sizing process is on required torque and inertia match.

CALCULATING THE INERTIA OF A SERVO-DRIVEN SYSTEM

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g reliable products.

ustom Gears MOTION SYSTEMS HANDBOOK JL = Inertia of load reflected to motor JD = Inertia of drive (ball screw, belt, rack & pinion) JE = Inertia of external (moved) load JC = Inertia of coupling

JL = inertia of load reflected to motor JM = inertia of motor

An inertia ratio that is too low means the motor is likely oversized, leading to higher than necessary cost and energy usage. An inertia ratio that is too high means the motor will have a difficult time controlling the load, which results in resonance and causes the system to overshoot its target parameter (position, velocity or torque).

While it seems logical that an inertia ratio of 1:1 would be the goal, it’s not always achievable or cost-effective. Most servo motor manufacturers recommend that the inertia ratio be kept to 10:1 or of Gear precision grade less, although there are many applications that operate successfully rd Gear precision Gradehigher ratios. The best inertia ratio for an application comes at much 0 1 2 3 4 5 976 down to the dynamics of the move and the accuracy required.

N4 N5 N6 N7 N8 N9 2:1998 How to calculate inertia of a drive system: Load inertia includes 4 5 6 7 8 9 the inertia of all rotating parts, including the drive (such as a belt and 3(1973) screw, or rack and pinion), the load being moved, and 13 12 10 pulley 9 system, 8 7 ce 14 12 11 the 10 9 8 coupling between the load and the motor.

nd "Gear precision grade"

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aterial table : g corresponding materials of SCr415) is defined as rial of SCM415. izing (case hardening), SCM415 is normally used. ements for harder material, SNCM220 and/or SNCM420 . ess is determined by producti hardness value is inf

Manufacturers typically provide the inertia value (or a simple equation to calculate the inertia value) of drive systems, such as ball screws. But if the inertia value isn’t provided, it must be calculated manually. One of the most common ways to do this is to model the drive system as a shape for which the inertia equation is easily defined.

Some common examples are solid cylinders and hollow cylinders.

Inertia of solid cylinder (screw or pinion) is:

Inertia of hollow cylinder (pulley) is:

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Mechanical Drive Solutions For Motion Control Applications

GCR Robotic Cycloidal Gearbox with integral Dyna Series right angle Pre-Stage

SPH High Performance Helical Gearbox 2-week lead time, drops in for competitor gearboxes

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Helical Rack & Pinion System Pairs with GAM Gearboxes for optimized system performance

From gearboxes to couplings to rack & pinion, GAM has the flexibility and broad product range for all your motion control applications With one of the largest product offerings in the motion control industry as well as the engineering expertise and manufacturing capabilities to develop customized solutions, GAM can help with your application. GAM Can.

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MOTION SYSTEMS HANDBOOK

Helical rack and pinion systems from GAM pair with GAM gearboxes for optimized system performance. A helical rack provides smooth and precise operation. The rack comes in 1-meter lengths in module 2, 3 or 4 and ISO quality 6 or 10. Eight gearbox and pinion combinations include inline and right-angle gearboxes in multiple sizes.

Where m = Mass of cylinder; r = Radius of solid cylinder; ro = Outer radius of hollow cylinder; and ri = Inner radius of hollow cylinder How to calculate inertia of a load: To determine the inertia of a screwdriven load, the effect of the screw’s lead must be taken into account.

Inertia of load driven by screw is:

m = Mass of load ls = Lead of screw Note that this equation is based on the screw’s lead (expressed as inches or mm per revolution), not on the pitch (expressed as revolutions per inch or per millimeter).

When a load is driven by a belt and pulley system, the mass of both the load and the belt must be considered ... because both of these components are being driven by the motor.

Rack and pinion systems are treated similarly, with the mass of the pinion being added to the mass of the load.

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Inertia of load driven by belt (or rack and pinion):

Where m = mass of load + mass of belt (or pinion) Also, the radius in this case is the outer radius of the pulley, since this is the axis around which the belt and load are being rotated.

How to reduce inertia ratio: If the inertia ratio is too high, one way to reduce it is to add a gearbox to the system. In this case, the inertia of the load is divided by the square of the gear ratio. Inertia of load with gear reduction:

Where JG = inertia of gearbox and i = Gear ratio Note that the inertia of the gearbox is added to the system, but its addition is small compared to the reduction provided by the gear ratio, which has an inverse squared effect on the load inertia.

8 • 2019

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MOTION SYSTEMS HANDBOOK

Human-machine interfaces (HMIs) in evolution from operator terminals

HMIs and other operator interfaces and terminals function as software and hardware access points between machine and personnel. In some cases, components called operator interface terminals or OITs (consisting of pushbuttons, LEDs, switches, and hard keypads and small and moderately customizable displays) have yielded to HMIs. The latter in the context of factory and machine automation today usually refers to full ruggedized touchscreen electronics and memory loaded with operational recipes; advanced connectivity options; signal and data processing capabilities; and the ability to display relevant information and menus (even across multiple screens) to human operators.

In fact, HMIs initially evolved from man-machine interfaces (MMIs) as well as graphical user interfaces (GUIs) into iterations accepting input beyond simple text — with the touchscreens just mentioned for image-based interfaces the world now takes for granted.

Even HMIs from a few years ago are being superseded by new HMI hardware and software systems capable of supporting IoT functionality — usually by letting operations make more use of plant and machine data (data handling) in increasingly automated operations as well as remote operations. Such HMI functions also include system real-time supervision, event logging and triggering, diagnostics, and enterprise-

level monitoring. HMI integration with any existing supervisory control and data acquisition (SCADA) and manufacturing execution systems (MES) is key here.

Where HMIs serve as a machine’s centralized controls, there is either connection to or integration of a PLC or motion controller.

These controls serve to process axis feedback and I/O data for higher-level uses. Many HMIs today run real-time operating systems (OS); a few even include electronics with what’s called an asymmetric multi-processing (AMP) architecture. In contrast with common symmetric multiprocessing (SMP) architecture that runs a single OS across all CPU cores, AMP architecture lets the HMI run different OSs — usually as general-purpose logic on one CPU core and realtime controls on the other. That lets the control logic maintains real-time operations while HMI logic executes data collection, batch processing, and display tasks.

IDEC Corp. HG2G-V5 5.7-in., HG3G-V8 8.4-in., HG3G-VA 10.4-in., and HG4G-VC 12.1-in. touchscreen HMIs are suitable for both new and retrofit applications.

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HMIs Maple Systems Inc. in a strategic alliance with Weintek Labs Inc. serves the North American HMI market. Shown here is a Weintek cMT3090 HMI that extends accessibility to beyond that of typical products — because it supports remote access via smartphone and tablet applications. A built-in server lets the cMT3090 serve as a gateway between the plant and outside world.

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Today’s HMIs also aim to minimize the impact of operator mistakes and other inefficiencies. One approach is what HMI manufacturers call situational awareness — a machine-based capability to recognize problems or situations that are typical and atypical for the automated design.

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HMIS SUPPORT PERSONNEL IN CHARGE OF MANY TASKS Today’s machine operators handle an ever-increasing amount of data — and always more than any one person can process alone. Here, well-designed HMIs distill data to let machine operators respond quickly and efficiently to situations … and help keep them safe from harm. (In contrast, poorly designed HMI notifications can sometimes distract uninvolved plant personnel and slowly render involved personnel insensitive to the alarms — especially if the HMI throws an excessive number of warnings or irrelevant signals.) Where plant personnel isn’t parked next to a given piece of machinery all day, HMIs can also help communicate what would have once been observed by sound, feel, or sight.

In contrast with typical HMI applications of the past, which merely communicated machine status, today’s HMIs assist operators in understanding what’s normal for a given machine axis or sensor and what’s not. That lets even inexperienced personnel understand and effectively act upon system parameter values to address issues where needed.

Where it’s appropriate that machine operators be fully informed and empowered, that can include communication through the HMI of potential consequences of ignoring the issue or addressing the issue with set actions.

DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

Calculating

linear bearing life When selecting linear guides, have you ever seen a disclaimer in the catalog or on the manufacturer’s website regarding dynamic load capacities? It typically reads something like this: Note that dynamic load capacities are based on 100,000 m of travel. When comparing to products whose load capacities are based on 50,000 m of travel, multiply the 100,000 m rating by 1.26.

If you saw this, you probably wondered where the 1.26 comes from and why it’s important. To find out, we’ll look at the bearing life equations for each rating to derive the conversion factor. But first, let’s review the difference between static and dynamic load capacity.

Static load capacity according to ISO 14728 Part 2 is the force that causes permanent deformation of the rolling element (ball) and raceway, which equals 0.0001 times the rolling element diameter.

This is an important specification because if it is exceeded, the running characteristics of the bearing will be compromised.

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However, the static load capacity is only valid when the bearing is in a non-moving state. So while it’s critical to consider the static load capacity in your linear bearing selection, it is the dynamic load capacity which is used for calculating bearing life.

Dynamic load capacity is the load at which a linear guide will, with 90 percent certainty, achieve the targeted service life before fatigue (flaking) occurs on the rolling elements or raceways. The ISO 14728 Part 1 specification allows either 50,000 m or 100,000 m to be used as the targeted service life for dynamic load capacity.

Therefore, in order to make a comparison between different manufacturer’s linear guides, you need to know whether their dynamic load ratings are based on 50,000 m or 100,000 m. This is where the 1.26 conversion factor becomes important.

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MOTION SYSTEMS HANDBOOK

To begin, we’ll set up the bearing life equations for 50,000 m service life and 100,000 m service life:

L = service life C = dynamic load capacity F = applied force Setting the life equations equal to each other allows us to begin deriving the ratio of C50 to C100:

Moving the variables to one side of the equation gives us:

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The applied force is the same for both life calculations, so F can removed from the numerator and the denominator, leaving us with:

And conversely, the load capacity of a bearing based on 50,000 m should be divided by 1.26 in order to make an accurate comparison with a bearing whose load capacity is based on 100,000 m:

To get rid of the third power on the left side, we’ll take the cube root of both sides:

This gives us the ratio we’re looking for:

In a more practical form, the load capacity of a bearing based on 100,000 m should be multiplied by 1.26 in order to make an accurate comparison with a bearing whose load capacity is based on 50,000 m:

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As an analogy, think of jogging with a backpack full of weights. If you’re jogging 10 miles, you might only be able to carry 20 lb of weights without becoming tired. But if you’re only jogging 5 miles, you can easily carry 25 lb of weights without fatigue. This same principle underlies the conversion between load capacity based on 100,000 m of life and 50,000 m of life.

By understanding the theory behind the 1.26 conversion factor and applying it when necessary, you can avoid the upfront cost of oversizing the linear bearings for your application or the long-term cost of downtime and repair due to undersizing.

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MOTION SYSTEMS HANDBOOK

Hertz contact stresses:

How they affect linear bearings Linear bearings that use balls or rollers to carry a load are subjected to Hertz contact stresses — a type of material stress that plays a significant role in determining the bearing’s load capacity and fatigue life.

When two surfaces of different radii are in contact and a load is applied (even an extremely small load) a small contact area is formed, and the surfaces experience very high stresses. These stresses are known as Hertz (or Hertizian) contact stresses. In rolling element linear bearings, Hertz contact stresses occur on the balls (or cylinders) and the raceways.

In theory, the contact between two spheres occurs at a point, and the contact between two cylinders occurs as a line. In either case — point or line contact — the resulting pressure between the two surfaces would be infinite and the surfaces would experience immediate yielding. But in real-world applications, when two surfaces are pressed together with a force, some elastic deformation occurs at each surface, and a contact area is formed. The stresses that occur on the two surfaces may still be very high — sufficient to initiate spalling or other forms of failure — but they are no longer infinite.

The analysis of Hertzian contact stresses relies on four primary assumptions: • • • •

The surfaces are smooth and frictionless The bodies are isotropic and elastic The contact area is small relative to the sizes of the bodies in contact The strains on the bodies are small and within the elastic limit

Hertzian stresses are present when any two surfaces with different radii are in contact — even if one surface is flat or if one surface is convex and the other is concave, which is the case for rolling element bearings: the ball or roller is convex, and the raceway is concave. In the analysis of Hertz contact stresses, a convex surface (the ball or roller) has a positive radius and the concave surface (the raceway) has a negative radius. Flat surfaces have an infinite radius.

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Because the surfaces have different radii, the contact area between a spherical ball and a bearing raceway has an elliptical shape. Under these conditions, the maximum pressure between the two surfaces for a spherical ball and raceway is given as:

Where a and b are the width and length of the contact area and F = Normal force on the assembly.

For a cylindrical roller and raceway the contact is rectangular with:

Where (2)b = Width of the contact area and L = the length of contact.

In fact, Hertz contact stresses have a significant effect on bearing dynamic load capacity and L10 life. Shear stresses, which cause fatigue — a primary mode of failure of rolling elements — are proportional to the maximum Hertz pressure between the two bodies.

Hertz contact, and the resulting deformation of surfaces, is also what causes bearings to skid rather than roll. This is because the Hertz contact areas have different diameters than the rolling elements themselves, so the rolling elements slip. Hertz contact has implications for bearing preload as well. Preloading the rolling elements gives them a larger — and finite — Hertz contact area, which increases stiffness. But the increased contact results in high heat generation.

This is why a preload amount of just 8% is considered high preload for linear bearings, with preload greater than 10 to 15% being extremely rare. Also, because Hertz contact is nonlinear, a small amount of preload can provide significant stiffness boost without resulting in unacceptable slip, friction, and heat.

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8/21/19 12:50 PM

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MOTION SYSTEMS HANDBOOK

The Schneeberger Monorail AMS offers both guidance and measurement, combining a high-precision linear encoder with a monorail guide. The result is an integrated measuring system that saves space and is simple to install without the need for any separate assembly or adjustment work, providing cost savings in the design, manufacture, and maintenance of equipment.

When should you use

dual guide rails?

Linear guide rails and bearings offer high stiffness and good travel accuracy. And they can support not only downward, upward, and side loads, they can also withstand overhung, or moment loads. Of course, the larger the linear rail and bearing system, the more moment capacity it has, but the arrangement of the bearing raceways — face-to-face or back-to-back — also influences the amount of overhung load that it can support.

While the face-to-face design (also known as the “X” arrangement) provides equal load capacities in all directions, it results in a shorter moment arm along which overhung loads are applied, which reduces moment load capacity. The back-to-back arrangement (also known as the “O” arrangement) provides a larger moment arm and gives higher moment load capacities.

Even with the back-to-back arrangement, however, linear guides have a relatively short distance between the raceways (essentially equal to the width of the rail) which limits their ability to handle roll moments ... which are caused by loads overhung in the Y direction. To counter this limitation, using two rails in parallel — with either one

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or two bearings on each rail — allows the roll moment to be resolved into forces on each bearing block. Because linear bearings have much higher capacity for forces than for moments (especially roll moments) bearing life can be significantly increased.

Another benefit of using dual guide rails and allowing moments to be resolved into forces is that linear bearings generally deflect less under pure forces than under moment loads.

Many linear actuator designs include two rails in parallel, with the drive mechanism (belt, screw, or linear motor) incorporated between the rails. While it’s not imperative that the drive be centered between the guide rails, doing so helps ensure even loading on all of the bearings, and reduces cogging, or uneven drive forces on each rail and bearing set. This arrangement also reduces the height of the actuator, making it relatively compact given the high load and moment capacity provided by the dual guide rails.

Recirculating bearings (bushings) that ride on round shafts are unable to handle roll moments. This is why most linear bushing applications require that two shafts be used in parallel.

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8/21/19 12:50 PM

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MOTION SYSTEMS HANDBOOK

Fundamentals of

dc motors

Motors can be classified in a number of different ways, but one of the most common is whether they are ac or dc motors. This division refers to the type of current that drives the motor with ac meaning alternating current and dc being direct current.

The EnduraMax 75i Series from Allied Motion Technologies is a 75-mm diameter brushless dc motor with an all-digital integrated drive. Command inputs to the motor can be ±10 Vdc, 4 to 20 mA, or via an optional CANopen or Modbus port.

Specifically, dc motors operate using magnetic fields produced in the motor, either via electromagnetic windings or permanent magnets. There is also some type of mechanism that commutates or switches the current flow, using either electromechanical means or electronically.

Common dc motor types include brush, brushless, and permanent magnet (PM) motors. Brushed dc motors are sometimes referred to as wound-field motors, because a wound and lacquered coil of copper wire produces the electromagnetic field. There are permanent magnet, shunt, series, and compound-wound brushed dc motors.

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MOTION SYSTEMS HANDBOOK DC Motor Torque-Speed Curve

Stall Torque

Torque

Brush dc motors have commutators and brush contacts 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. Brush orientation to the rotor’s commutator bar segments mechanically controls the No Load Speed phase commutation.

The fact that dc brush motors let designers control field and rotor windings means they’re suitable for applications that need simple and cost-effective torque Rotational Speed and speed control. However, increased functionality from electronics for PM motors means that this advantage is This dc motor speed-torque curve highlights two key less pronounced than it once was. Moreover, current on parameters; the motor’s stall torque and the no-load speed. both rotor and stator generate heat that limits the motors’ | National Instruments Corporation continuous-current ratings. The motors also present a spark hazard, so can’t go in explosive settings. At certain periods during the dc motor rotation, the commutator must reverse the current, reducing motor life with arcing and No-load speed: The speed at which the motor turns when running friction. So, brushed dc motors require more maintenance in at nominal voltage and without a load (zero torque production). the form of replacement of springs and brushes that carry the electrical current, and replacement or cleaning of the commutator.

Stall torque: The maximum torque produced by the motor when And because brush wear produces particles, designers shouldn’t running at nominal voltage. use dc brush motors in cleanrooms. Same goes for applications that need high precision, as friction from brush-commutator Other parameters that are given at nominal voltage include: engagement make for long position-settling times.

Nominal speed: The speed achieved when the motor operates at nominal voltage and nominal torque at ambient temperature VOLTAGE AND SPEED RELATIONSHIPS (typically 20 to 25°C).

In a dc motor, the relationships between voltage and speed and between current and torque are straightforward.

When the load (torque) on the motor is constant, speed is proportional to the supply voltage. And when the supply voltage is constant, speed is inversely proportional to the load (torque) on the motor.

Specifications for dc motors include two voltage parameters: nominal voltage and a range of operating voltages. Nominal is the recommended voltage at which the motor should be operated, although the motor can be run with a voltage that falls anywhere within the specified range.

Nominal voltage is a key parameter for dc motors. In fact, a motor’s torque-speed curve is based on its nominal voltage. The slope of the torque-speed curve is determined by dividing the no-load speed of the motor by the stall torque. No-load speed is the speed at which the motor turns when running at nominal voltage and without a load. Stall torque is the maximum torque produced by the motor when running at nominal voltage. (The term “stall torque” is used because it occurs when the motor has zero velocity—i.e., when it is stalled.)

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No-load current: The current the motor draws when operating at nominal voltage and with no load (zero torque production).

Operating a motor at a voltage below nominal generally has no detrimental effect on performance. In fact, running a motor at lower than nominal voltage (and, therefore, slower than nominal speed) can result in less brush and commutator wear (for brushed motors), lower current consumption, and longer motor life. On the other hand, running a motor at a voltage higher than nominal increases current draw and can cause the motor coils to overheat, decreasing motor life.

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8/20/19 2:12 PM

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MOTION SYSTEMS HANDBOOK

Fundamentals of

gearmotors

Gearmotors are combinations of an electric motor (either ac or dc) with a gear reducer integrated together into one unit. They can be bought direct from manufacturers as integrated units, saving designers the effort of integrating separately purchased motors with gear reducers.

These A and F Series helical parallel shaft gearmotors and speed reducers from DieQua feature torque capacities up to 2,800 Nm (2,065 ft lb) for Series A, and up to 14,000 Nm (10,325 ft lb) for series F.

There are as many gearmotor types as there are combinations of electric motors and gears.

Motor type – these can be ac or dc, brushed or brushless, or permanent magnet. Often times the application will determine which motor is best suited for that particular task. In general, brushless dc gearmotors have really good speed regulation properties, so they’re usually preferred in applications requiring tight variable speed control.

However, ac gearmotors have gotten better at speed control and they are now competitive with many dc gearmotors.

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MOTION SYSTEMS HANDBOOK

Gear type – here again, the application demands can help determine the best gear type.

Is efficiency the most important parameter?

Torque? For instance, planetary gears have high power density and are more compact than other arrangements. They also tend to cost more than other gear types. On the other hand, worm gears are prevalent in many gearmotors, especially right-angle gearmotors, but their efficiency ratings are lower than other gear types.

Another way to classify gearmotors is according to their horsepower rating; as either fractional or integral. An integral horsepower gearmotor is any motor with a horsepower of 1 and above, as opposed to fractional horsepower motors that are less than 1 hp. The design considerations for integral horsepower motors are fairly similar to fractional horsepower motors because the same types of parameters are important. So for instance, torque and speed matter in all cases.

Gearmotors are also differentiated by the output shaft orientation, with two standard types being parallel and right-angle arrangements. Beyond shaft configuration itself, other factors such as the type of gears used in the gearmotor determine performance.

Generally speaking, a right-angle gearmotor is less efficient than a parallel configuration. A typical right-angle gearmotor arrangement includes a worm gear assembly, which serves to translate the rotary motion of the motor by 90 degrees. In parallel-shaft gearmotors, the gear setup can take any number of forms, from helical and spur gears to complex planetary gear systems. In terms of gear efficiency, spiral bevel gears tend to have the greatest efficiency, followed closely by helical and spur gear types, all of which are in the range of 93 to 99%.

More demand for compact machine designs means that space considerations are an important design priority. So if the application calls for a right-angle gearmotor as the only one that will fit the design requirements without having to redesign the entire machine, then that’s the one to go with. Even if it sacrifices some efficiency, compactness and fit may be the more relevant design factors.

Other factors to consider include environmental factors such as temperature, IP rating for washdown or debris or spray etc., mounting configurations, as well as lubrication needs, among others.

CONTINUOUS VS. INTERMITTENT DUTY The classification of a motor as either continuous or intermittent duty revolves around the idea of duty cycle. That is, how long the motor is on and off. So the basic decision for determining continuous vs. intermittent duty is fairly straightforward. If the motor is switched on and runs continuously then the motor should be rated for continuous duty. If the motor is switched on for a short time or has a cyclic

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Gearmotors can be designed specifically for particular applications. For instance, the Conveyor Series from Brother Gearmotors are double-sealed with an O-ring and feature high-grade grease for lifetime lubrication and a durable e-coat paint to withstand tough environments. operation, then the motor can be rated intermittent operation and the motor has no efficiency requirement. As a rule of thumb, if the motor is running more than one hour continuously with no switching on or off during that time, then, in general, the application is continuous. If the motor runs for less than an hour continuously, then the application can be considered intermittent.

One reason this is so important is that if the wrong motor is selected (intermittent instead of continuous, for instance), the risk is that the motor could overheat, damaging the motor and/or other devices.

Some of the most common examples of intermittent applications include packaging equipment, metering pumps, palletizers and winders and coilers. Typical continuous applications range from bulk material handling conveyors to continuous flow pumps and continuous mixers.

A properly sized gearmotor with the right combination of motor and gearing can prolong operating life and boost overall design efficiency. Gearmotors also 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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8/20/19 2:09 PM

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MOTION SYSTEMS HANDBOOK

LinMot PR01 linear-rotary motors come in myriad sizes for different force and torque outputs. They integrate a linear motor with a rotary torque motor. Controls independently command the two.

Summary of

direct-drive motors A direct drive motor is any motor — rotary or linear — in which the load is connected directly to the motor, without mechanical transmission elements such as gearboxes or belt and pulley systems. In other words, the motor directly drives the load.

Regardless of its design, a direct-drive motor has the benefit of eliminating mechanical components that can introduce backlash or compliance and degrade positioning accuracy and repeatability. The elimination of mechanical connections also reduces load inertia and allows more dynamic moves (higher acceleration and deceleration rates with heavier loads) with less overshoot and oscillation. Direct drive motors also have lower noise production than conventional motors, which is important for noise-sensitive applications, like those in the medical and laboratory industries.

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Without additional transmission elements, direct drive motors tend to be more compact than traditional motors, making them easier to integrate into machines and systems with tight spaces. And with fewer mechanical components (often, the only wear components are linear guides), maintenance is reduced and mean time between failures (MTBF) is increased.

Rotary direct drive motors are used to drive goniometers, gimbals, rotary tables, and SCARA and 6-axis robot arms. Many designs have a center bore, which allows electrical cables and pneumatic lines to be routed through the center of the motor.

Linear versions are used in numerous automation applications, including packaging machines that require rapid strokes on a continuous basis, machine tools thatWebsite: requirewww.linmotusa.com extreme positioning accuracy and high load carrying capability, and Phone: 262-743-2555 semiconductor manufacturing equipmentEmail: that requires ultrausasales@linmot.com smooth and precise motion. motioncontroltips.com | designworldonline.com

8/19/19 11:51 AM

LINEAR MOTORS LINEAR MOTOR SUBTYPES Linear motors include a stationary platform that industry calls a platen or secondary (with electromagnetic windings) and a moving forcer or primary that sometimes includes permanent magnets. Linear motors can make fast and precise moves for positioning or move slowly and steadily for material processing. Visit linearmotiontips.com/linear-motors for more on this.

Linear-motor speeds range from a few inches to thousands of inches per second. The motors deliver 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.

Unlike rotary motors (which need mechanical rotary-to-linear devices to get straight strokes) linear motors are direct drive.

So linear motors avoid the drawbacks of a rotary motor with a belt and pulley for translation — lower thrust because of tensile-strength

Shown here are iron-core linear-motor options from Chieftek Precision USA.

limits; longer settling times; belt stretching, backlash and mechanical windup; and typical speeds to 15 ft/sec or slower. They also avoid the gradual wear of traditional rack-and-pinion sets ... lead and ballscrew efficiencies (usually around 50 and 90%) and whip and vibration. Plus they don’t force engineers to sacrifice speed (with higher pitches) for lower resolution.

PRO2

Linear Rotary Motor The new PR02 motor series is characterized by a new design where both motors are integrated in a slim housing. In addition to the linear motor and the rotary motor, further options such as a magnetic spring «MagSpring», a torque sensor and a force sensor can be provided.

The MagSpring ensures that the weight force of the moving load is passively compensated and also prevents the axis from lowering in the current-less state. The torque sensor and force sensor enable precise, reproducible and recordable sealing processes as required in the pharmaceutical or medical industries.

The user also benefits from the shorter installation length of the entire unit and the hygienic design with easy-to-clean surfaces.

Website: www.linmot-usa.com

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Phone: 262-743-2555

Email: usasales@linmot.com

8/21/19 2:27 PM

MOTION SYSTEMS HANDBOOK What is a linear motor? A direct drive Linear motors use electromagnetic flux — the rate of electromagnetic energy flow through an airgap.

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.

COIL

IN THIS ARRANGEMENT, A MAGNETIC FORCERMOVES THE LOAD

N S

S N

CORE

In linear motors, magnetic flux through the airgap area 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 are an established design with a toothed forcer of laminated steel cores wound with coils. Toothed platens mount end-to-end for unlimited travel. Thrust originates from reluctance force. Linear steppers deliver speeds to 70 in./sec — useful in inspection machines or parts transfer. Some manufacturers pair twin linear steppers off a common forcer to work as X-Y stages. These stages mount in any orientation and have high stiffness and flatness to a few μm for every hundred mm to output accurate movement.

Now consider hybrid linear motors which usually have ferromagnetic platens. Those with steel platens move to 3 m/sec; those with laminated platens move faster. Much like linear steppers, they vary magnetic saturation to shape opposition to magnetic flow ... so thrust originates from reluctance force. Feedback and controls make for servo 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 setups are phase-teeth offset or

Linear motor — stepper-motor variation

driving to get partial saturation of platen teeth and sections of 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 that run to 2,000 in./sec work for people movers, rollercoasters and large aerospace applications.

General-purpose types can move a few inches to 150 ft/sec or faster.

Linear ac synchronous motors are either iron-core or ironless-core motors. Ironless-core linear motors have an epoxy forcer plate holding copper coils. This forcer moves in a U-shaped magnetic platen to output up to 3,000 N and speeds exceeding 230 in./sec. These cogfree linear motors are lighter motors with potentially unlimited travel and quick acceleration — though their main benefit is smooth output.

Their speed is helpful in flying-shear applications and long-stroke pickand place machines in semiconductor fabrication and elsewhere. Other applications exist for waterjet and laser cutting and robotics tasks.

Iron-core motors have slotted steel lamination stacks (insulated to reduce Eddy currents) to output 7,000 N or more. The forcer coil setup includes these steel laminations and windings in a single or three-phase configuration. This allows for control directly from a line or through a drive. Some such linear motors use water cooling to boost force output — for heavy uses as on baggage handling and amusement-ride axes. Iron-core motors are suitable for some machine-tool applications as well.

Cylindrical linear motors are sometimes called tubular linear motors. These have steel rods and a moving coil or rods filled with stacked magnets. With the same footprint as a lot of linear actuators, these offer high stiffness and other advantages over other linear-motor designs … and work in myriad machines that need quick and accurate strokes.

HOW DO IRON-CORE LINEAR MOTORS WORK? Linear motors are often classified as either ironless or iron core, referring to how their primary parts are constructed. Ironless linear motors have a primary of windings embedded in epoxy resin. Iron-core linear motors have windings mounted in an iron lamination stack.

For both motor types, the number and length of the windings determine how much force the motor can produce. Iron-core motors have a force density (force per working area) up to twice that of ironless motors. So to produce a given continuous force,

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LINEAR MOTORS

an ironless motor must be twice as large as a comparable iron-core design. Force here is the product of current through the windings and the flux density in the primary: F = I x B. Ironless linear motors have a primary that’s embedded in resin, so there’s no magnetic attraction between the primary and secondary ... so forces these motors can produce are smaller than those produced by iron-core designs. For iron-core linear motors, magnetic attraction between the primary’s iron and secondary’s permanent magnets allows high force output — though this attractive force also creates cogging.

Cogging is detent force that the motor experiences when the steel laminations of the coil cross the magnets of the secondary. This force degrades the smoothness of movement and can be significant ... making iron-core motors less desirable for applications

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that need extremely smooth motion. That said, some manufacturers have developed methods to reduce the effects of cogging — by skewing the magnets of the secondary part (which eases the change in attractive forces as the primary moves across the magnets) or by using feedback and controls to compensate for the effects of cogging.

In addition to their ability to produce very high thrust forces, iron-core motors also dissipate heat thanks in part to their relatively open design. But this design also leaves them susceptible contamination — particularly metallic chips or flakes that attract to the permanent magnets and cause damage.

Another factor that contributes to the ironcore linear motor’s good heat dissipation is the fact that their primary is mostly metal (as opposed to the epoxy enclosure of an ironless design) which acts as a heat sink.

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When designing a linear system using iron-core motors, account for the attractive force between the primary and secondary parts, as that’s essential to properly sizing the support bearings. Linear profiled rails are the most common guide systems used with linear motors, although air bearing systems are sometimes used. Because air bearings need a preload, the attractive force between the windings and the magnets is beneficial for an iron-core motor using air-bearing guides.

With high continuous forces and good heat dissipation, iron-core linear motors are suitable for pressing, molding, and machining applications. They also excel at high-speed testing that requires the application of high forces or pressures.

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MOTION SYSTEMS HANDBOOK

Servomotors: the basics Servomotors are used on machine axes that need to make complex moves or position loads with really high precision. Servomotors can

The Kollmorgen AKMH hygienic servomotor includes FDA-approved food-grade materials and meets IP69K, EHEDG, and 3A standards. The motor helps food processors reduce food-recall risks and decrease cleaning time. The motor can withstand 1,450 psi spray and pH 2 to pH 12 chemicals.

also run at zero rpm while holding torque to keep a load at a set position.

A servomotor (or servo system) is characterized mainly by the use of closed-loop control.

The system takes an error in position, speed, or torque and corrects it through the use of a feedback device.

A controller compares the system’s actual performance with its commanded performance and takes corrective action to eliminate the error.

A servomotor can be built around one of several types of motors. Common types include dc brushed and brushless motors as well as ac induction motors. The advantage of brushed dc servomotors is their linear and predictable performance that makes them easy to apply. Brushless motors usually run applications needing more torque; the only catch here is that their drives are more complex because commutation is done electronically and not mechanically as in brushed dc motors. What’s more, industry categorizes motors in part by their number of electrical phases. Brushed dc servomotors as well as voice coil motors are in fact single-phase motors, whereas brushless servomotors most commonly have three phases.

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8/19/19 2:36 PM

MOTION SYSTEMS HANDBOOK

The IDEA motor from Haydon Kerk Pittman integrates a brushless servomotor with a drive and feedback connections, reducing motion system component count and simplifying machine troubleshooting. The integrated motors are designed for real-time embedded motion control and are suitable for autonomous precise execution of complex single-axis motion.

There are also instances of classifying induction-motor-based designs running off vector controls as servomotor setups where the design incorporates feedback (usually from an encoder) to track and control speed and sometimes even position. These induction motors typically adhere to NEMA or metric standards, whereas other servomotor offerings are less uniform.

Manufacturers classify motors for constant-speed tasks by horsepower or torque at base speed. In contrast, servomotors operate over varying speed ranges and aren’t rated in this way.

Instead they have speed-torque curves that express continuous torque capabilities (that won’t threaten to overheat the motor) and intermittent or peak torque for acceleration.

Many non-direct-drive servomotors have top speeds up to thousands of rpm. To better leverage their full capabilities, designers will often combine such motors with gearing to trade an increase in output torque with lower output speed. Much of the time, this gearing takes the form of planetary or harmonic gearheads, precision arrangements with high accuracy and efficiency. In many instances, gearing even lets machine builders use smaller motors on some axes. This equates to cost savings that may even offset the price of the additional gearing.

One common factor that can impact servomotor performance (and thus accuracy and precision) is the presence of electrical noise in a system. The sources of electrical noise are varied, but one of the most common is from the high-frequency

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signals generated by variable frequency drives (VFDs.) This noise, called electromagnetic interference (EMI), can be picked up through cabling.

Cables themselves can be manufactured with special shielding to guard against EMI issues. The purpose of cable shielding is two-fold. One, it prevents electrical signals emitting from the cable, and two, it prevents other signals or electrical noise (EMI) from entering through the cable and disrupting other electrical equipment down the line. Shielded cable options include copper braiding, steel braiding, or combinations of these and other materials.

Another common way to minimize cable noise issues is to use twisted pair wires. This also reduces the amount of electromagnetic radiation of signals that could effect other equipment or unshielded cables nearby.

Optimizing the cable will go a long way toward eliminating noise issues. For instance, one simple fix (and what should always be a key design consideration) is shortening the length of cable used to connect equipment together. The longer a cable is, the more it acts as an antenna, making electrical noise problems worse.

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8/20/19 1:59 PM

RELIABLE MOTION SOLUTIONS. QUIET, EFFICIENT, BRUSHLESS MOTORS DESIGNED FOR MEDICAL APPLICATIONS. Medical equipment requires high performance motors. Moog Silencer® series brushless DC motors offer unique designs that deliver results. Ultra-quiet functionality, smooth operation at various speeds and the advantage of high torque at a low cost.

Learn more about Moog’s solutions for ventilators, centrifuges and other medical devices. Standard and custom motor models are available with options. Contact us to discuss your requirements, email mcg@moog.com.

How can we improve your machines performance?

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MOTION SYSTEMS HANDBOOK

Stepper motors designed to withstand harsh washdown conditions, like the IP65-rated SureStep line from AutomationDirect, are completely protected from solids and dust ingress and from low pressure water jets from any direction. Waterproof white epoxy coating on the motor laminations and special bearings and seals help keep moisture out of the motor.

Stepper motors — an overview

Stepper motors are one of the many motor options used in motion control systems. They’re known for their accurate

positioning capabilities and delivering high torque at low speeds. A distinguishing feature is that a typical stepper motor operates on open-loop control as opposed to closed loop like many other motor types, including servomotors.

One way to classify stepper motors is according to the number of steps they can be commanded to move. So a 1.8° step motor is capable of 200 steps per

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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.

The two most common types of stepper motors are the permanent magnet (PM) and the variable reluctance (VR) type. Stepper motors operate in a fairly straightforward way. Traditional VR stepper motors have a large number of electromagnets arranged around a central gear-shaped piece of iron.

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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.

Newer hybrid stepper motors combine the best features of both PM and VR types.

Here, the rotor is multi-toothed like the variable reluctance motor and contains an axially magnetized concentric magnet motioncontroltips.com | designworldonline.com

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MOTION SYSTEMS HANDBOOK

A-

AB+

B-

+ B

1.8°

A+

A+

B-

Stepper motors use current through their stator coils to advance magnetized rotors.

Stator poles and rotor are both toothed for finer electromagnetic control than otherwise possible.

B+

Aaround 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 variable reluctance and permanent magnet motor. Hybrid steppers 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, which make for more exact output than that from VR stepper motors, which usually can’t be microstepped. Hybrid steppers also have higher torque-to-size ratios and higher output speeds than other stepper-motor types, and are also quieter than VR stepper motors.

The low-speed torque of a stepper motor 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 specified by the manufacturer.

Sufficient documentation of a stepper motor includes a torque curve that shows both pull-in and pullout torque, two critical stepper motor parameters.

USING A GEARBOX WITH A STEPPER MOTOR Stepper motors require careful sizing to ensure the motor matches the load and application parameters, to minimize the possibility of lost steps or motor stalling. Adding a gearbox to a stepper motor system can improve the motor’s performance by decreasing the load-to-motor inertia ratio, increasing torque to the load, and reducing motor oscillations.

One cause of missed steps in stepper motor applications is inertia. The ratio of the load inertia to the motor inertia determines how well the motor can drive, or control, the load — especially during acceleration and deceleration portions of the move profile.

If the load inertia is significantly higher than the motor inertia, the motor will have a difficult time controlling the load, causing overshoot (advancing more steps than commanded) or undershoot

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(missing steps). A high load-to-motor inertia ratio can also cause the motor to draw excessive current and stall.

Inertia ratio = JL / JM JL = inertia of load JM = inertia of motor One way to reduce the inertia ratio is to use a larger motor with higher inertia. But that means higher cost, more weight, and trickle-down effects on other parts of the system such as couplings, cables, and drive components. Instead, adding a gearbox to the system reduces the load-to-motor inertia ratio by the square of the gear ratio.

Inertia ratio = JL / (JM x i2 ) i = gear reduction Another reason to use a gearbox with a stepper motor is to increase the torque available to drive the load. When the load is driven by a motor-gearbox combination, the gearbox multiplies the torque from the motor by an amount proportional to the gear ratio and the efficiency of the gearbox.

To = Tm x i x η To = torque output at gearbox shaft Tm = torque output at motor shaft η = gearbox efficiency But while gearboxes multiply torque, they reduce speed. (This is why they’re sometimes referred to as “gear reducers” or “speed reducers.”) In other words, when a gearbox is attached to a motor, the motor must turn faster — by a factor equal to the gear ratio — to deliver the target speed to the load. motioncontroltips.com | designworldonline.com

8/21/19 11:33 AM

STEPPER MOTORS

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A cutaway of a hybrid stepper motor shows two teeth wheels that are slightly offset. Such motors are called hybrid because the rotor includes the soft iron teeth wheels as well a permanent magnet with its N-S axis set axially in the assembly.

fully customizable rotary and linear stepper motors for precision positioning, dispensing and diluting ultra-precise Linear Shaft Motor servos

No = Nm / i

linear stages, driven by linear steppers or servos and configured for simultaneous multi-axis movement

No = speed output at gearbox shaft Nm = speed output at motor shaft And stepper motor torque generally decreases rapidly as speed increases, due to detent torque and other losses. This inverse relationship between speed and torque means it’s only practical to increase speed by a certain amount before the motor is unable to deliver the required torque (even when multiplied by the gear ratio).

But speeding up the motor does have a benefit. The additional speed required by the motor when a gearbox is installed means the motor operates outside its resonant frequency range, where oscillations and vibrations can cause the motor to lose steps or even stall.

In addition to ensuring the gearbox has the correct torque, speed, and inertia values, it’s important to choose a high-precision, low-backlash gearbox — especially when connecting the gearbox to a stepper motor.

Recall that stepper motors operate in an open-loop system, and backlash in the gearbox degrades the system’s positioning accuracy, with no feedback to monitor or correct for positioning errors. This is why stepper applications often use high-precision planetary gearboxes, with backlash as low as 2 to 3 arcminutes. And some manufacturers offer stepper motors with harmonic gears that can exhibit zero backlash under most application conditions.

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MOTION SYSTEMS HANDBOOK

Positioning systems: An overview Positioning stages and tables are integrated systems consisting of motors and mechanical power transmission devices, or linear motors and actuators complete with encoders, sensors, and controllers.

Better mechanical components and feedback and control options are enabling stages capable of motion that’s more accurate than ever. So, positioning stages today can execute tasks with tighter synchronization than in the past, useful for complicated axis commands.

Stages and tables are used in a range of highperformance applications, such as industrial robots, fiber optics and photonics, vision systems, machine tools, semiconductor equipment, medical component laser machining, micromachining and electronic manufacturing.

Stages can provide one of several different types of motion.

They can be linear, rotary or even lift types (Z-axis positioning stages). Among these, they can be configured in many different ways including movement in one direction (or axis) only, in multiple directions (X-Y positioning), or for extremely small and precise movements, as in nanopositioning applications where moves are in the micro- or nanometer range.

The drive mechanisms for positioning stages and tables can also vary significantly depending on a number of factors including cost and desired accuracy. For instance, stages can be direct-drive types driven by linear servomotors or by a combination of motors and gearing and couplings, and can be linear or rotary actuator driven (either using electric actuators, or even pneumatic or hydraulic actuation). Other methods can include belt and pulley systems, ball screws or lead screws.

Precision and accuracy requirements can also dictate design decisions such as what components to use in assembling a positioning stage.

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One kind of component used in stages requiring reliability and high accuracy are air bearings. 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. For instance, so-called planar stages are typically constructed of air bearing guides and linear motor drives.

Unlike ordinary bearings, the surfaces of an air bearing do not make mechanical contact, so these systems don’t need lubrication. 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 failure for many years.

X-Y TABLES X-Y tables are similar to X-Y Cartesian systems, in that they have two axes (X and Y, as their name implies) mounted on top of each other, and typically have strokes of one meter or less. But the key difference between X-Y Cartesian systems and X-Y tables lies in how the load is positioned. Instead of being cantilevered, as in a Cartesian system, the load on an X-Y table is almost always centered on the Y axis, with no significant moment created on the Y axis by the load.

The new long travel linear motor stage from PI (Physik Instrumente) features travel length of 32 in. (813 mm). The direct-drive stage achieves high velocities to 79 in./sec (2 m/sec) based on a direct drive ironless linear servo motor. High accuracy, repeatability, and functionality are guaranteed by an integrated absolute-measuring linear position encoder featuring 1-nm sensor resolution.

motioncontroltips.com | designworldonline.com

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HIGH-SPEED FIBER-OPTIC POSITIONING WITH NANOMETER STEP SIZES Aerotech’s FiberAlign® series is designed to meet the demands of fiber-to-fiber, fiber-to-laser, and fiberto-waveguide alignments. FiberAlign® productioncapable systems provide 1 nm step sizes and high speeds. Standard alignment routines including spiral scan, hill climb, and virtual pivot-point reduce system programming, setup, and qualification time. Multiaxis direct-drive rotary and goniometer solutions are available for complex alignments.

Aerotech award-winning multi-axis control systems complete your motion solution.

Visit aerotech.com or Call 412-963-7470 Since 1970, Aerotech has designed and manufactured the highest performance motion control, positioning tables/stages, and positioning systems for our customers in industry, government, science, and research institutions around the world.

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MOTION SYSTEMS HANDBOOK

What are hexapod robots — also called Stewart platforms?

Multi-axis systems can be designed and built with either serial kinematics or parallel kinematics. In systems built on the principle of serial kinematics, one axis sits on top of another axis, and each axis is driven individually, independent of the others. Cartesian robots are a good example of serial kinematic design.

In parallel kinematic systems, multiple axes work together to move a common platform or end effector.

The movement of one axis is constrained by the other axes, forming a type of closed-loop kinematic chain.

Delta robots, for example, use parallel kinematics.

Hexapod robots are a type of parallel kinematic mechanism with six degrees of freedom: X, Y, Z, roll, pitch, and yaw.

The term hexapod is derived from Latin and means “six feet,” and the hexapod robot, fittingly, consists of six actuators (also referred to as “legs”) that connect a stationary platform (typically the base) with a moving platform (typically the top). The legs are extending-rod style actuators — also referred to as “prismatic” actuators — which can be driven by ball screws, roller screws, linear motors, or even piezo devices. Ball or universal joints connect the ends of the actuators to the stationary and moving platforms.

The first practical applications for hexapod mechanisms were in tire testing and flight simulator equipment, both of which still use hexapod robots today. But hexapod mechanisms are also widely used in photonics and optical alignment, positioning for machine tool equipment, and inspection systems.

Hexapod robots are unique among multi-axis systems because they allow a user-defined center of rotation, or pivot point, for all six axes of motion.

They also have much higher stiffness and load-carrying capability than serial kinematic robots (Cartesian, SCARA, or 6-axis) because their actuators, or “legs,” work together to support the load. The parallel design also means that the errors of each actuator are averaged, rather than accumulating as they do in serial robots.

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Because only the top platform moves — motors, gearboxes, and other transmission components are stationary — hexapod robots have much lower inertia than serial designs, making them well-suited for highly dynamic moves with large payloads, or for submicron precision with light loads. From an integration standpoint, hexapod robots are easier and faster to commission. Controllers typically allow a simple input for the pivot point, and hexapod robots often forego servo tuning, since there is only one moving platform and system stiffness is inherently high. And if tuning is required, the parameters for all six axes are identical, making the tuning process much easier and faster than with serial robots.

Hexapod robots are sometimes referred to as Stewart platforms, but Stewart platforms were originally defined as having actuators connected in pairs at either the stationary or moving platform (or both).

Hexapod devices are analyzed using reverse kinematics. In other words, rather than the displacement of the moving platform being determined by the actuator movements, the actuator movements are determined based on the displacement of the platform. Reverse kinematic analysis is necessary due to the fact that a given set of actuator displacements can result in 40 different platform positions, but each unique platform position corresponds to only one set of actuator displacements.

The kinematics of hexapod devices also give rise to singularities, or positions where the platform becomes uncontrollable and its position cannot be uniquely determined by fixing the displacements of the actuators. (In extreme cases, a singularity may cause the entire system to effectively collapse.) But users of hexapod robots rarely have to deal with the issue of singularities, since manufacturers define the robot’s working area to a space that does not include any points of singularity.

motioncontroltips.com | designworldonline.com

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POSITIONING SYSTEMS Z-Axis stages from Optimal Engineering Systems, Inc. (OES) are driven by an 8-mm diameter ball screw with a 1-mm lead, and guided by cross roller bearings. They feature vertical travel distances up to 20 mm with resolutions of 0.1 micron or 0.05 microns with 10/20 micro-steps per step motor drive, and repeatability of +/- 0.5 microns.

This is where the principle of “how the system is used” helps distinguish between the various types of multi-axis systems. X-Y tables generally work only within their own footprint, meaning the load does not extend beyond the Y axis. This makes them best suited for applications where a load needs to be positioned in the horizontal plane (X-Y). A typical example is a semiconductor wafer being positioned for inspection, or a part being positioned for a machining operation to take place. Designs referred to as “openframe” or “open aperture” have a clear opening through the center of the table. This allows them to be used in applications where light or objects need to pass through, such as back-lit inspection applications and insertion processes.

Because X-Y tables are primarily used for high-precision applications, the guideway of choice is crossed roller slides, which provide extremely smooth and flat travel. Drive mechanisms are typically ball screw or linear motor, although fine pitch lead screws are also common.

Hexapods for Multi-Axis Precision Motion PI Hexapod 6-axis parallel positioning systems with userprogrammable pivot point speed up alignment, complex positioning, and motion simulation tasks. With load capacities from 1kg to 2,000kg and precision from nanometers to microns, Hexapod success stories include photonics, medical, and automotive applications since 1993.

Hexapod Alignment Applications Radio Telescope Reflectors

Automotive Assembly

Silicon Photonics Wafer Test

Image: FormFactor

Physik Instrumente www.pi-usa.us 508-832-3456 (East) 949-679-9191 (West)

PI designs and manufactures precision motion systems at locations in the USA, Europe, and Asia. With over 40 years of experience developing standard and custom products based on piezoceramic and electromagnetic drives and more than 1,300 employees in 13 countries, PI can quickly provide a solution for your positioning and automation projects in industry and research.

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RETAINING RINGS

The basics of retaining rings Bowed preloading ring

•

Radial retaining ring

Retaining rings are fasteners that hold components together on a shaft or in a housing when engaging a groove. Three main types of retaining rings include tapered section, constant section and spiral.

Tapered-section rings typically have decreasing thickness from the center of the ring out to the ends. They mount either axially or radially.

The taper is there to ensure 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 do not maintain uniform contact with the entire component. They take on an elliptical shape and make contact with the groove at three points.

Spiral retaining rings are installed into the housing or onto the shaft, making full contact with the groove and component. Their grooves are relatively shallow, so their load bearing capability is reduced.

Spiral rings 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.

When selecting a retaining ring for an application, several factors dictate which is most suitable. The first question to 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 motioncontroltips.com | designworldonline.com

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•

Internally mounted axial ring

Bowed retaining rings eliminate play in assemblies. Radial rings widen during installation for an interference fit once in place. They only wrap a portion of the groove circumference so withstand lower forces than alternatives, but are easy to install. Axial rings slide into internally machined grooves. 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.

What is edge margin?

This is the distance from the groove (for the retaining ring) to the end of a shaft or housing.

Edge margin depends partially on the groove’s depth. Rule of thumb: When edge margin is about triple (or more) the groove depth, the groove can withstand the same level of thrust load as the mating ring.

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MOTION SYSTEMS HANDBOOK

Update on

sealing technologies

Non-contact seals, like these from Centritec Seals, form a reliable seal even when subjected to severe vibration and shaft motion. They do not require tight axial or radial alignment between the inner and outer ring.

In any motion control system, the risk from dirt and other ingress materials is great. Contaminants like these can damage bearings and other rotating equipment, destroy whole machinery systems and cause unwanted downtime and costs. To prevent these catastrophic events, all motion systems require some type of sealing system. In addition to stopping the ingress of contamintants, seals also prevent leakage of necessary lubricants, such as oil, grease or hydraulic fluid.

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Molded seals and v-shaped seals are two of the most common seals found in power transmission applications. V-shaped seals, such as wipers, are used most commonly in fluid power systems to prevent contaminants from entering a system while allowing lubricating oils to return to a system on inward stroke of the hydraulic piston.

Molded seals, which are more common in power transmission applications, can be further divided into O-rings, radial lip seals and shaft seals. O-rings are one of the most common types of seals because of their simple and inexpensive construction. They are designed to create a seal between the interfaces of two or more components. They generally consist of an elastomer ring with a circular cross section and are usually placed in a groove. They are used frequently in hydraulic components, particularly on cylinder pistons and rotating pump shafts.

Mechanical face seals, or heavy-duty seals, are used in extreme applications, such as bearings, gearboxes, turbines and machinery used in 8 • 2019

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SEALING TECH

SEAL and

DELIVER

Radial shaft seals are used between rotating and stationary machine components or between two components in relative motion. A cylindrical outer covering of sheet steel (case) or an elastomer has the requisite interference fit to seal statically against the housing bore.

Additionally, a sealing lip made of an elastomeric or thermoplastic material seals dynamically and statically against the shaft. | courtesy of SKF

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extremely tough and dirty environments, such as mining and agriculture. They feature two metal seal rings identical in nature that mount separately on a lapped face seal. A flexible, elastomer element centers the metal rings, allowing one half to rotate while the other remains still.

While many seals are designed primarily to prevent debris from entering a machine, radial lip seals are designed to keep lubricants within a machine that has rotating or oscillating parts. These seals are available as one of two types — spring loaded and non-spring loaded. Each is suited to a particular type of lubricant, grease or oil. Non-spring loaded seals are suited for applications that use a highly viscous lubricant and operate at slower shaft speeds. Spring-loaded seals are best paired with lubricants with low viscosity and higher speeds. The spring helps the seal lip maintain its contact with the shaft even as the seal material itself breaks down.

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In addition to keeping contaminants out and fluids in, rotary and shaft seals have the extra benefit of providing low friction and resistance to wear, thus extending component life.

To reduce efficiency and power loss, bearing users can turn to non-contact seal designs. These seals eliminate efficiency and frictional power loss, and also reduce maintenance and contamination problems associated with contact or rubber seals.

Several different styles of non-contact seals exist, including labyrinth and centrifugal seals. Designed to eliminate the physical contact between a machine’s stationary and rotating elements, they don’t suffer parasitic drag or wear.

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SENSORS

Sensors for motion systems Motion systems employ sensors for a variety of functions, the most common of which are measuring position or speed. Encoders are chiefly used for position measurement (both rotary and linear), while tachometers are common for speed measurement. Proximity sensors are used for both object detection as well as distance measurement.

In fact, many industrial and manufacturing applications use proximity sensors to sense the presence of objects or materials, then either initiate some action or simply flag their presence or absence. Key to their operation is that they don’t require physical contact with the target or object being sensed, and why they’re often called non-contact sensors.

Proximity sensors employ one of several sensing techniques including photoelectric, ultrasonic, capacitive and inductive.

Photoelectric sensors operate on the basis of light, detecting light levels present to the detector in the sensor. Ultrasonic sensors work by emitting a high-frequency sound wave. The wave strikes an object and then reflects back to the sensor where the distance of the object can be calculated based on the time it takes for the emitted signal to return.

Capacitive sensors operate by detecting a change in capacitance or what is really the dielectric constant between the sensor and the detected object. Inductive sensors are based on the principle of electromagnetic induction and are used to sense metallic objects.

Regardless of sensor type, technological developments across industries are impacting new generations of sensor designs, leading manufacturers to add features to accommodate the changes. For example, the ongoing expansion of IoT and IIoT demands the use of more sensors as data collecting tools and to monitor system operations for both predictive maintenance and condition monitoring. As a result, new sensors have added functions, are easier to program, and include better connectivity.

Manufacturers are also seeing interest in wireless sensors, particularly for smart factory applications. This interest is driven mainly by the cost savings that wireless sensors offer over hard wiring sensors to machines. With better wireless connectivity prevailing, wireless sensors are also beginning to find more use in some applications.

SENSOR SELECTION TIPS The most important factors for selecting a sensor include the desired variable to be measured, the required accuracy or resolution, the type of output, as well as any size or space restrictions, environmental factors, and product lifetime and cost. motioncontroltips.com | designworldonline.com

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A new portfolio of sensors from Pepperl+Fuchs includes reduction factor 1 capability, simplifying setup and configuration in changing application conditions. The inductive sensors also feature an IO-Link connection for added diagnostic and maintenance capabilities.

For motion applications, is the motion rotary or linear? Encoders or resolvers can measure rotary position. For linear motion, there are linear encoders using a variety of sensing technologies including optical, capacitive, inductive, and magnetic.

Also, consider the needed precision for the application including factors such as linearity, resolution, and repeatability. Generally speaking, higher precision sensors are more expensive. So knowing the application’s required precision and accuracy helps with not paying for more precision than the application demands.

What is the required measuring range? For linear measurements, 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 degrees? Is the type of encoder needed a single-turn or multi-turn device?

What type of output is needed? Is it voltage or current? Digital or analog? Many transducers are programmable through a simple 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. There may be other considerations such as any physical size or weight restrictions or special installation or mounting requirements.

Environmental conditions are another factor. The sensor should be able to withstand the environmental conditions of the application. Some of the most common conditions to consider are EMI/RFI noise, shock and vibration disturbances, extreme heat or cold, and environmental contaminants such as dirt, dust, moisture, and corrosive chemicals.

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Slow down to speed up

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SHOCK & VIBRATION DAMPING

Update on shock & vibration

technologies

When high-speed, heavy-duty industrial automation systems must decelerate and stop, damage to the load and the machine itself can result. The cause of this damage is the release of kinetic energy, which induces longterm vibration and fatigue and sudden shock in a system.

Smooth deceleration of a system should be done with shock and vibration attenuation components.

Bansbach Easylift’s industrial shock absorbers are designed for energy absorption and linear deceleration. They absorb the impact with no bounce back. This steady, gradual deceleration allows faster cycle rates, increased equipment life, and higher production rates while reducing maintenance expenses.

Based on the type of inputs present in the application, vibration and shock attenuation components can include 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.

motioncontroltips.com | designworldonline.com

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SHOCK & VIBRATION DAMPING

These products work in a broad range of applications — from the rate control mechanisms that slow the motion of overhead luggage bins or seat recline on commercial aircraft, to the isolators that keep GPS from losing signal or becoming damaged on farm and construction equipment as they harvest crops or pave roadways.

INTEGRATED DRIVER SOLUTIONS FOR NEMA 17 AND 23 MOTION SYSTEMS

SHOCK AND VIBRATION DAMPING Most shock absorbers deliver 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, and 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 highstrength steel to handle the pressures from the internal hydraulic forces. Elastomeric seals prevent the fluid from leaking out of the cylinder, and 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.

Miniaturization is a growing trend in these devices, as systems require tighter tolerances and smaller machine footprints. In machine automation and robotics, motion stabilization requires the use of hydraulic dampers, particularly micro-hydraulic designs.

STEPPER BALL SCREW ACTUATORS

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VIBRATION ISOLATION 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 but many compounds and blends deliver various characteristics specific to satisfy different applications.

Air springs include metallic end fittings coupled by a composite elastomeric-based bladder that contains the compressed air to provide isolation. These single-acting designs have a pressurized bladder and two end plates. As air is directed into the air bladders, they expand linearly.

ACE Controls’ new HB50 hydraulic damper is designed for use in heavy-duty safety applications. Featuring a steel body, it offers individual adjustment for push and pull motion and reduces downtime and component wear. These maintenance free, ready-to-install machine elements are adjustable and provide a constant feed rate.

CUSTOM SOLUTIONS AVAILABLE

DINGS’ MOTION USA 335 Cochrane Circle Morgan Hill, CA 95037 408-612-4970

www.dingsmotionusa.com 8 • 2019

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MOTION SYSTEMS HANDBOOK

How do you size an industrial shock absorber?

Sizing a shock absorber is relatively straightforward.

Here are a few guidelines to quickly come up with suitable products for a given task.

Before sizing a shock absorber, users first need to determine the relevant operating conditions, including weight and velocity of the moving mass and how frequently the shock is loaded. For simplicity, let’s look at a linear-motion application and use Imperial units for the calculations.

Determine kinetic energy in the system from: Ek = W/(722)(V2) where Ek = kinetic energy, lb-in.; W = weight of moving mass, lb; and V = velocity of moving mass, in./sec. This equation represents the amount of kinetic energy that the shock absorber will convert to thermal energy on each impact.

Next calculate the work energy in the application, defined as the amount of energy an external device generates to move the load: Ew = Fd(S) where Ew = work or drive energy, lb-in.; Fd = drive force, lb; and S = stroke of the shock absorber, in.

Note that Fd should not exceed the unit’s maximum rated propelling force. If it does, select a larger size and recalculate the work energy.

The next step is to calculate the total energy, Et (lb-in.) per cycle, shown as: Et = Ek + Ew Again, if this exceeds the model’s energy-absorbing capacity, select a larger unit and recalculate the work energy. Otherwise, the shock’s temperature may rise beyond rated limits and critical internal components like hydraulic seals could fail.

If the application uses more than one shock absorber, divide the total energy Et by the number of shocks to determine the total energy per shock.

Then determine the total energy a unit must convert in one hour. That’s because even though a shock might absorb an acceptable amount of energy

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in a single impact, it might not be able to dissipate the generated heat if the cycle rate is too fast. Here, multiply Et by C, the total number of cycles per hour.

The device’s hourly capacity must exceed this calculated amount. If not, choose a larger absorber (and recalculate Ew if the stroke changes) or, possibly, add an external oil tank or a cooling device to help dissipate the heat.

Finally, consider the shock force, Fp (lb) in the application. Shock force, in essence, is the resistive force required by the shock absorber to stop the moving load: Fp = Et/(Sη) where S = the stroke of the shock absorber and η is the unit’s damping efficiency. While the efficiency can vary with the type and model, 85% efficiency is a good baseline for typical industrial shocks.

This is important when selecting a suitable shock absorber because the machine structure and mounting must have the necessary strength and rigidity to withstand the transmitted force. The efficiency of various units is measured by evaluating how much of the shock’s stroke is used for actual damping of the motion. Shock absorber efficiency increases as more energy dissipates over the stroke, and more-efficient products typically yield the lowest shock forces for a given stroke.

Considerations such as the machine’s structural integrity and the payload’s ability to withstand forces without damage are also key to successful damping configurations. And some applications or payloads may have specified g-load rating limits. For example, an operator housed in a large overhead crane must be protected from excessive g forces.

Calculate this g-load from: g = (Fp – Fd)/W Make certain that the selected model matches or exceeds requirements for energy absorbed per cycle and per hour, as well as the shock force. Otherwise, it will likely cause damage or fail prematurely.

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SHOCK & VIBRATION DAMPING System of interlocking Sorbothane layers can be customized to meet individual energy absorbing requirements. These specialized 4 X 4 in.

Sorbothane pads can be layered and “locked” together to assemble a specific shock attenuating and vibration isolating assembly. The steel base plate provides column integrity and helps maintain the correct shape factor. A steel top plate can also be utilized. Layers of interlocking Sorbothane pads are added — or removed — to meet the desired size and isolation requirements.

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 shockabsorbing 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 define system natural frequency (resonant frequency) and the most suitable isolation system.

ELASTOMER, RUBBER PADS FOR VIBRATION AND SHOCK REDUCING Elastomer and other synthetic and rubber pads can also damp vibration and isolate shock loads. They come in tubes, bushings, blocks, pads and washers.

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MOTION SYSTEMS HANDBOOK Wire rope isolators from ITT Enidine have stainless steel cable and aluminum retaining bars to provide high multi-axis isolation. The isolators are completely unaffected by oil, chemicals, abrasives, ozone, and temperature extremes. The compact designs provide simultaneous shock and vibration attenuation where sway and package space are a premium. Their symmetrical design provides consistent, multi-axis isolation and their small size permits isolation of individual system components, rather than the entire system.

SORBOTHANE

These components work in heavy-duty applications to deliver strong cushioning for cranes, presses, and pipelines and bridges; they also excel in vibration reduction for lab and testing equipment and designs for aerospace.

Rubber-like materials let these padding elements satisfy specific requirements related to natural frequency, load, and area. Because they are soft, they are also forgiving.

Predicting the natural frequency of an application lets material manufacturers target known disturbance frequencies to dissipate energy. The lower the ratio of natural system frequency to disturbance frequency, the more it’s possible to isolate problem vibrations.

These cushioning plates can protect machinery subsystems against impacts and isolate vibration and structure-borne noise. For example, PAD plates from ACE Controls withstand compressive loads to 10,000 psi or 69 N/mm2 depending on plate form and size.

Another product called Sorbothane (from a company with the same name) is a thermoset that attenuates shock with near-faultless memory. Deformation is elastic and not plastic, so pads of the material reliably return to their original shape. Custom pieces of the material work for vibration and acoustic damping and isolation. Sorbothane turns mechanical energy into heat as the material is deformed. Molecular friction generates heat energy that translates perpendicularly away from the axis of incidence.

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DESIGN WORLD — MOTION

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MOTION SYSTEMS HANDBOOK

The basics of

compression springs Engineers incorporate compression springs in designs that need linear compressive forces and mechanical energy storage—designs such as pneumatic cylinders and pushbutton 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

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and ground end compression springs are useful for applications 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).

The 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.

8 • 2019

This concave (hourglass-shaped) compression spring can stay centered, even in large-diameter bores.

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8/19/19 2:30 PM

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AD INDEX

MOTION CONTROL HANDBOOK ACE Controls...........................................126 Aerotech...................................................117 AllMotion......................................................4 Altra Industrial Motion...............................40 Apex Dynamics, USA.................................79 Applied Motion Products, Inc....................51 AutomationDirect........................................2 Bansbach Easylift.....................................128 Bay Associates Wire Technologies, Inc......48 Beckhoff Automation.................................62 Bishop-Wisecarver.....................................43 Bodine Electric Company........................103 Carlyle Johnson..........................................39 Centritec Seals.........................................123 CGI Motion................................................85 Chieftek Precision....................................107 Clippard .....................................................19 CMT ...........................................................34 Continental Contacts.................................35 Control Techniques......................................1 Del-tron......................................................93 Deublin.......................................................45 Diequa......................................................101 Dings Motion USA.........................31,65,129 Dorner........................................................60 Dynatect Manufacturing, Inc.....................16 Emerson’s Aventics......................................5

Encoder Products Company......................76 FAULHABER MICROMO................. Cover,99 Festo........................................................113 Fluid Line Products, Inc..............................57 GAM...........................................................83 HEIDENHAIN CORPORATION..................73 HELUKABEL USA.......................................47 HIWIN.........................................................25 igus.............................................................27 IKO International........................................29 Intech.........................................................84 ITT Enidine...............................................131 KHK USA....................................................82 LAPP USA...................................................49 LeadLift....................................................124 Lee Spring Company...............................135 Lenze Americas............................................6 Lin Engineering..........................................11 LinMot USA Inc........................................105 Mach III Clutch Inc.....................................41 Maple Systems...........................................87 Master Bond...............................................55 maxon precision motor..............................97 Mitsubishi Electric Automation................133 mk North America, Inc...............................59 MOOG, Inc...............................................111 MW Industries - Helical..............................64

SALES

NBK America LLC.................................70,71 Neugart......................................................81 Nippon Pulse Americas, Inc.....................115 NSK Precision.............................................30 OMS Motion, Inc........................................53 PBC Linear.............................................88,89 Pepperl+Fuchs...........................................75 Physik Instrumente L.P.............................119 POSITAL-FRABA Inc...................................74 Pyramid Incorporated................................33 R+W America......................................67, BC Renishaw....................................................77 Rollon.........................................................95 Rotor Clip Company, Inc..........................120 Ruland Manufacturing Co., Inc..................69 Schneeberger ............................................91 Schneider Electric Motion USA..................15 SDP/SI-Stock Drive Product.......................37 Serapid Inc.................................................23 Smalley Steel Ring........................................8 Sorbothane...............................................132 THK America, Inc..................................... IFC Tolomatic....................................................12 Traco Power.............................................IBC Ultra Motion...............................................20 Yaskawa Electric America........................109 Zero-Max, Inc...............................................3

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8/20/19 4:02 PM

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