D E S I G N W O R L DO N L I N E . CO M
M AY 2 0 1 8
M OT I O N CO N T R O L T I P S . CO M
POWER
TRANSMISSION RE FE RE N C E GUIDE
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9x10.875Drive HPN — Ad2.indd 1 5-18.indd 47 Harmonic PT Guide
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D E S I G N W O R L DO N L I N E . CO M
M AY 2 0 1 8
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Automation Direct — PT Guide 5-18.indd 1 1804-DesignWorldMotionControlSupp-GearboxesShaftMount-MAG.indd 1
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Applied Motion — PT Guide 5-18.indd 3
5/14/18 3:15 PM
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POWER TRANSMISSION R E F E R E N C E
G U I D E
from
2.25”
SEARCHING FOR THIS CENTURY’S
STEAM LOCOMOTIVE
THE TECHNOLOGIES WE COVER HERE in this special issue on mechanical power transmission in motion control are for the most part well aged; that is, not exactly new but tried and true. These are components like mechanical screws, electric motors and gears but also belts and pulleys and chains and sprockets, along with a supporting cast of components including bearings, clutches and brakes, and more. These echoes of Industrial Revolutionera technologies have no doubt been refined, improved and updated for contemporary uses. And this got me to thinking about one of the most revolutionary developments in motion and power in human history; namely, the steam locomotive. In the history of mechanical power transmission, you’d be hard-pressed to find an example of a technology that so completely demonstrated the revolutionary power of science and engineering. Steam locomotion is hardly used at all today largely due to the inefficiencies of steam compared with other methods of locomotion. Railroads long ago switched to electric and diesel locomotives as prime sources of motive power. A few steam locomotives are still used mostly in developing countries. In fact, the last bastion of sizable steam locomotives was in China, where they were manufactured up until the turn of the 21st century. For anyone who’s experienced a steam locomotive up close, it can most accurately be described as mesmerizing. It’s hard to find a more compelling example of a self-contained mechanical power transmission system in action that captures our attention on so many levels. A steam locomotive engages all of the senses; the sight of the driving rods powering the wheels on the rails, the puffs of steam escaping from valves and smokestack; the sound of building steam pressure in the cylinders and its release; the smell of burning coal heating the water in the boiler to steam; even the feeling of the vibrations from the tons
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4
Editorial — Power Transmission HB 05.18 V1.indd 4
DESIGN WORLD — MOTION
of hot metal on the track thundering past. It is an experience of the raw power of engineering and motion unlike any other that captures the imagination on so many different levels. Why am I bringing this up? Because of a sense that there may be a larger point here. Without overly romanticizing it, a steam locomotive was a succinct and accessible example of the intersection of science, cuttingedge engineering, art and craftsmanship, combined with the thrill and excitement of danger that touches at something primal within us. Looking at mechanical power transmission today, it’s hard to find something with quite the same appeal. There are many (good) reasons for this. Systems are quieter, smoother, and more efficient. Also, they’re mostly electromechanical systems, with an emphasis on the electrical side of things. Some of this includes the revolutionary presence of electronic computing and communication technology; specifically mobile electronic devices that are largely “black boxes” whose inner workings are hidden from view, and, I’d argue, from the understanding of most people as well. The magic, you might say, is no longer visible. Perhaps the question I’m asking is in our own age, is there a technology that is capable of engaging the imagination in the way that the steam locomotives of yore could do? Perhaps robots and artificial intelligence? The recent spate of videos showing robots doing amazing things may come close. The robotic revolution of today, which brings together mechanical elements with cutting edge computing power, may well be the 21st century example of something similar, though not quite equal to the enduring fascination of steampowered locomotives. Weigh in and let us know what you think.
MILES BUDIMIR SENIOR EDITOR
5 • 2018
5/15/18 11:13 AM
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2011 - 2017
5 • 2018
DESIGN WORLD — MOTION
5
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INSIDE
20
THE POWER TRANSMISSION REFERENCE GUIDE VOLUME 4 NUMBER 2
Actuators Electrical ....................................................... 08 Pneumatic ..................................................... 12 Rigid Chain .................................................... 18 Ball screws ........................................................... 20
FOR THIS CENTURY’S 02 SEARCHING STEAM LOCOMOTIVE BY: MILES BUDIMIR
Bearings .. .............................................................. 22 Belts & Pulleys . . ................................................... 25 Brakes, Clutches & Torque Limiters ................. 31 Chain ..................................................................... 36 Couplings .. ............................................................ 38 Drives .................................................................... 42 Encoders .............................................................. 44
44
Gearing ................................................................. 46 Gearmotors ........................................................... 51 Lead screws . . ....................................................... 55 Linear Guides, Slides & Ways . . ......................... 58 Retaining Rings .. .................................................. 64 Motors . . ................................................................. 66 Seals . . .................................................................... 67 Shocks & Vibration Damping ............................ 68
58
CO V E R P H O TO CO U R T E S Y O F i S TO C K
6
DESIGN WORLD — MOTION
Contents — PT Guide 05-18.indd 6
5 • 2018
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Baldor — PT Guide 5-18.indd 7
4/3/18 2:58 PM
5/14/18 3:17 PM
POWER TRANSMISSION R E F E R E N C E
G U I D E
Curtiss-Wright Sensors & Controls Division supplies next-generation Exlar FTX Series actuators with an electric rod-style design for high performance. A continuous force rating to 44 kN and speeds to 1,166 mm/ sec mean the FTX125 works in myriad linear motion applications.
HIGH SPEED
LINEAR ACTUATORS: WHAT QUALIFIES THEM AS HIGH SPEED? AS WITH MANY QUALITATIVE TERMS used in the linear motion industry — including 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 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 an actuator’s speed capability is typically the drive mechanism. Ballscrews and lead screws are limited in speed by their tendency to whip, which is a function of the screw diameter, length, and end bearing arrangement. Because lead screw designs are based on
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sliding contact and generate high heat due to friction, they often have lower maximum speeds than ballscrews of a similar size. So of the screw technologies, actuators based on ballscrew 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 ballscrews, provided they are properly tensioned (for belt drive versions) or preloaded (for rack and pinion versions). Actuators with steel-reinforced belts can reach speeds of 10 m/sec or higher, while rack and pinion driven actuators can commonly reach speeds of 5 m/sec. Actuator type also dictates maximum speed Another factor comes into play when discussing high speed linear actuators: the type of actuator. The highspeed designation is most often applied to thrust-rod type actuators — also referred to as electric cylinders –
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Del-Tron — PT Guide 5-18.indd 9
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5/14/18 3:16 PM
POWER TRANSMISSION R E F E R E N C E
G U I D E
SCREW WHIP IS LIMITING FACTOR
because their primary applications involve pushing/pulling and inserting operations, which typically require very short extension and retraction times. These actuators can be either ballscrew or lead screw driven, with speeds ranging from 0.1 m/sec to over 1 m/ sec. A few manufacturers even offer belt driven rod-style actuators, which can reach speeds up to 2.5 m/sec. Slider or carriage-type actuators (also referred to as rodless actuators) can reach 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 slidertype actuators have a wide range of drive options, including lead screw, ballscrew, 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 reach speeds greater than 12 m/sec, but like linear motors, are limited by the maximum speed of the guide. The most common guide
SIMPLE
RIGID
Ballscrews and leadscrews are limited in speed by their tendency to whip.
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. However, higher speeds can be reached 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. For micro and nano-positioning applications, the 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 they typically have maximum strokes of 100 mm or less. Voice coil actuators operate at maximum speeds of 0.25 to 0.30 m/sec, with strokes of 150 mm or less. While these specifications may not fit the general definition of highspeed 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.
LEADSCREW END SUPPORTS AFFECT CRITICAL SPEED
FREE
RIGID
SIMPLE
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LEADSCREW Ø 5/8 IN. (16 MM) Ø 1/4 IN. (6 MM)
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Ø 1/2 IN. (13 MM) Ø 7/16 IN. (11 MM) Ø 3/8 IN. (10 MM)
COLUMN LENGTH BETWEEN BEARINGS (IN.) ADJUSTED FOR BEARING SUPPORT
SIMPLE
SIMPLE
RIGID
RIGID
10
DESIGN WORLD — MOTION
electric actuators — Power Transmission HB 05.18 V2.LE.indd 10
Ballscrew and leadscrew critical speed depends on screw diameter, length, and end bearing arrangement. Image courtesy Haydon Kerk of AMETEK Advanced Motion Solutions (AMS)
RIGID
MOST SUPPORT
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POWER TRANSMISSION R E F E R E N C E
G U I D E
C I T A M U E N P TA ORS U T C A Fabc
PNEUMATIC CYLINDERS AND ACTUATORS are sometimes referred to as “bang-bang” devices, making quick moves from one end of their stroke to the other, with limited regulation of the force or move profile. On the other hand, electromechanical actuators with servo controls offer high levels of refinement in positioning, force/torque, and accuracy. Generally speaking, pneumatics offers a cost-effective solution for rather crude, point-to-point moves, while electromechanical actuators provide high precision, at a higher cost. However, there’s a spot between these two solutions where a relatively high level of control is needed, but without the complexity and cost of electromechanical servo driven systems. Bridging this gap are pneumatics that operate in a closed-loop system—in other words, servo pneumatics.
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A servo system is one that uses a feedback device and a controller to monitor and correct the system’s error (in position, speed, or torque/force). Hence, integrating a pneumatic cylinder or actuator with a feedback system and a controller that can issue commands based on that feedback, gives us a servo pneumatic device. Another key component of a servo pneumatic system is a proportional valve, which precisely regulates air delivery to ensure that the commanded position and/or force is achieved. Traditional pneumatics enable rapid, high-force, point-to-point motion. Servo pneumatics provide the same speed and force capabilities, with the advantage of higher accuracy positioning, not only at the ends of
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the stroke, but ADN compact also at intermediate cylinder image points along the travel. courtesy Festo In addition to obtaining feedback on position, servo pneumatics also monitor and regulate air pressure, which enables precise control of the force that’s produced. One drawback (real or perceived) to traditional pneumatics is air consumption. Air preparation and delivery costs money, and pneumatics can use a significant amount of air even when they’re not working. Servo pneumatics, on the other hand, control air flow based on the required position and force. This leads to less air consumption than standard pneumatics, by as much as 30%. It’s important to note that servo pneumatics require higher quality air than standard pneumatics. In addition to industrystandard filtration, a 5-micron filter is typically recommended for servo pneumatic systems. Why servo pneumatics instead of electromechanical actuators? Of course, electromechanical actuators can provide accurate position and force control, but servo pneumatics have a much higher power density—that is, force capability for a given size. A servo pneumatic cylinder or actuator typically provides many times the force capability of an electromechanical actuator of a similar body size, which is a significant advantage in pressing, inserting, and tightening applications. Servo pneumatics also operate with 24 Vdc power supplies, which allows them to be used in lowpower applications. Lower power also reduces heat generation and thermal build-up, so they perform well in continuous-duty applications and high-temperature environments. While electromechanical servo systems have been in use for decades, the adoption of servo pneumatics in industrial applications hinged on advancements in controls and software. Air is compressible, and this variable is much more
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14
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5/14/18 9:28 AM
P N E U M AT I C A C T U ATO R S
Bimba’s Stainless Steel Flat-1 Cylinders are for applications needing compact actuation and protection against corrosive washdown.
difficult to define and model than the “compliance,” or backlash, in an electromechanical system. Before servo pneumatics could be commercialized, pneumatic control algorithms capable of taking into account this non-linearity had to be created. But the past ten years or so have seen the development and integration of high-response valves and digital signal processors (DSPs) that can perform highspeed computations, making servo pneumatic systems a reality, capable of providing accurate, highly responsive positioning and force control. What are the benefits of pneumatic actuators? Pneumatics is the technology of compressed air. Pressurized gas—generally air that is dry or lubricated— 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.
pneumatic actuators — Power Transmission HB 05.18 V3 FINAL.indd 15
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 common 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,
5/14/18 9:28 AM
POWER TRANSMISSION R E F E R E N C E
G U I D E
pneumatics is 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 to keep the system operating efficiently and the various components working properly. This also helps to ensure long system life. How do these fluid-power actuators work? 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. Pneumatic actuators are mechanical devices that use compressed air acting on a piston inside a cylinder to move a load along a linear path. Unlike their hydraulic alternatives, the operating fluid in a pneumatic actuator is simply air, so leakage doesn’t drip and contaminate surrounding areas. There are many styles of pneumatic actuators, including diaphragm cylinders, rodless cylinders, telescoping cylinders and through-rod cylinders. The most popular style of pneumatic actuator consists of a piston and rod moving inside a closed cylinder. This actuator style can be sub-divided into two types based on the operating principle: single acting and double acting. Single-acting cylinders use one air port to let compressed air enter the cylinder to move the piston to the desired position, as well as an internal spring to return the piston to the “home” position when the air pressure is removed. Doubleacting cylinders have an air
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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 connects 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 considered when selecting these upstream system components. Undersized upstream components can cause pneumatic actuators to perform poorly, or even make them incapable of Fabco moving their loads. Air F-series non-repairable air cylinders offer direct interchanges. Stainless piston rods are standard.
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HIGH PERFORMANCE. STANDARD.
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NSK — PT Guide 5-18.indd 17
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POWER TRANSMISSION R E F E R E N C E
G U I D E This is a push-pull die-moving machine based on SERAPID rigid-chain actuators. It works as an auxiliary machine to an industrial press to make quick die changes.
: N IO T O M R A E IN L R O F N ANOTHER OPTIO
S R O T A U T C A RIGID-CHAIN 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
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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 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
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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
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DW ha
R I G I D C H A I N A C T U ATO R S
actuator setups. Next, determine what type of actuator body and chainstorage magazine the application can accommodate. 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. Twinchain 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.
Common rigid-chain arrangements Chain link shoulders
Unguided chain with shoulders up coils downwards ...
... but guided chain is most stable. Actuator body 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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POWER TRANSMISSION R E F E R E N C E
G U I D E
BALL SCREW BASICS: FROM STANDARD TO MINIATURE ALL BALL SCREWS consist of the same basic components; a screw with helical grooves, a nut, and balls that roll between the nut, screw, and the grooves when either the screw or not rotates. The balls can be made from a number of materials including steel, ceramic, or hard plastic. They’re routed into a ball return system of the nut and travel in a continuous path to the ball nut’s opposite end. To prevent debris from compromising smooth motion, seals are often used on either side of the nut. The design of a ball screw minimizes friction and provides high efficiency. Ball screws are used in a wide variety of applications, but some of the most challenging are Based on Applied those on the extreme ends of the performance Motion Products’ spectrum – from large diameter, large lead StepSERVO Integrated screws for machine tools, to screws with Motor technology, the small diameters and extremely fine leads for TBSM11 Linear Actuator optical and medical applications. For small, features a ball screw shaft and nut assembly instead high-precision movements, designers and of a motor shaft, which engineers often turn to miniature ball screws. eliminates the need While there’s no industry for a coupling. standard for what classifies a screw as “miniature,” most manufacturers apply the designation to screws with a diameter smaller than 16 mm, while others include 16 mm screws in their “miniature” product line. To further segment the range of sizes, screws with a diameter smaller than 6 mm are sometimes referred to as “sub-miniature” or “ultra miniature.”
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BALL SCREWS
In some regards, miniature ball screws are the same as their larger, “standard” counterparts. For instance, miniature ball nuts are offered in many of the same styles as standard ball nuts, including flanged, cylindrical (also referred to as “compact” or “slim”), or with a threaded end for easy mounting into a carriage or table assembly. Ball recirculation can be done inside the ball nut or with external recirculation methods. Lead accuracies and preload classes follow the same designations regardless of the screw diameter. And sizing parameters, such as L10 life calculation, buckling load, and critical speed, are the same for both miniature and standard ball screws. Miniature ball screws, like standard versions, can be manufactured by either rolling or grinding the screw threads. Screw diameters below 8 mm are commonly produced by grinding, although some manufacturers offer screw diameters as small as 6 mm in rolled versions. In some miniature screw sizes, the journal diameter of the screw is too small to accommodate an appropriately sized end bearing. To address this, manufacturers can friction weld a larger journal onto the screw end, providing a sufficient journal for the support bearing. In addition to small screw diameters, miniature screws offer an advantage over standard versions with their option for extremely fine leads. The smallest miniature screws have a lead of just 0.5 mm, which means every rotation of the screw produces 0.5 mm of travel. This is a significant benefit in applications that require fine adjustments, such as positioning semiconductor wafers or optical equipment, or driving small medical pumps and dispensing equipment. Because miniature screws use small balls for load carrying, preload options are more limited, with only a light preload of 1 to 2 percent typically achievable, versus up to 5 percent with standard ball screws using the oversized ball preload method. However, miniature screws offer more customization options and more standard variations in some respects. For example, combination left- and right-hand screws (where one segment of the screw has left-handed threads and another segment has right-handed threads) are commonly available in miniature designs. Screws and nuts made from stainless steel are also readily available in miniature screw offerings, where they are rarely available for standard sizes. This can be attributed to the fact that miniature screws typically carry smaller loads, whereas stainless material would not be able to withstand the high loads that larger ball screws often transport. Due to their small leads, miniature ball screws can execute fine movements, and in vertical applications, these small leads make backdriving nearly impossible. Because ball screws operate with metal-to-metal contact, they’re not good for oscillating applications, even in miniature sizes where stroke lengths are typically small. For applications with oscillating-type motion, voice coil or piezo actuators may be a better choice.
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ballscrews — Power Transmission HB 05.18 V2 FINAL.indd 21
Designed for use in small spaces, miniature ball screw assemblies from Thomson Linear range from 4 to 14 mm in diameter with leads from 1 to 20 mm. Standard lead accuracies are ±52 µm/300 mm, making them suitable for laboratory, semiconductor, and medical applications.
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POWER TRANSMISSION R E F E R E N C E
G U I D E
ROLLING ELEMENT
BEARINGS: TECHNICAL REVIEW
BEARINGS REDUCE FRICTION in moving machine sections by giving a surface something on which to roll, rather than slide over. There are some fundamental elements that all bearings share, but specific application needs call for numerous variations of this universal motion component. Engineers incorporate different bearings in a motion system depending on the speeds, loads, and operating conditions a bearing will encounter. Here, we will specifically cover some of the different types of rolling element bearings. This bearing type, as opposed to plain bearings, uses multiple components in its design. The most important of these bearing components are cylinders or balls that roll inside a raceway to reduce friction. What’s the difference between ball and roller bearings? Because of the limited contact area between balls and races, ball bearings specifically excel in light to moderate load applications. The small areas of surface contact also minimize friction-generated heat, making ball bearings work well in high-speed applications.
In contrast, roller bearings have cylindrical rollers. They’re common in applications such as conveyor belt rollers because their rolling elements make more surface contact with their races — handling larger loads without deforming. Their shape also allows for a moderate amount of thrust load, as weight is distributed across cylinders instead of spheres. What kinds of applications use needle roller bearings? Needle-roller bearings operate in tight spaces — for example, in automotive applications such as rocker-arm pivots and transmissions. In short, these are roller bearings with rollers having a length at least four times the roller diameter. The large surface area of the needle roller bearing lets them support high radial loads. Usually a cage orients and contains the needle rollers. Occasionally, engineers machine the outer race into the housing interior. Needle-roller bearings come in two different arrangements — a radial arrangement (in which the rollers run parallel to the shaft) and a thrust arrangement (in which the rollers are flat in a radial pattern and run perpendicular to the shaft).
Because of the limited contact area between balls and races, ball bearings specifically excel in light to moderate load applications.
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Quality Bearings & Components The Bearing Marketplace
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Bearing Products
QBC is an authorized distributor for some of the world’s largest bearing and linear motion manufacturers, products include all types of bearings, rod ends and precision shafting.
Bearing Maintenance Equipment Proper bearing maintenance will maximize equipment life and lower operating costs. QBC offers a line of quality products, Bega Special Tools, to keep your business running smoothly.
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For better bearing performance, QBC provides relubing plus custom-made bearings for specific applications and operating environments. Call or e-mail for engineering and technical assistance.
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Get The Right Bearing 2018.indd 1
QBC — PT Guide 5-18.indd 23
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POWER TRANSMISSION R E F E R E N C E
G U I D E
What are thrust bearings? Thrust roller bearings transmit load from one raceway to the other to resolve radial loads; their self-aligning capability makes them immune to shaft deflection and alignment errors. Thrust ball bearings go in applications with primarily axial loads and handle shaft misalignment. These bearings also work on high-speed axes in the aerospace and automotive industries. However, installations that put one tapered roller bearing against a second counteract additional load; this allows the bearings to support high radial and axial loads from multiple directions. Other caveats: Tapered roller bearings can only accommodate slight angular misalignment of the inner ring about the outer ring — just a few minutes of arc at most. As with other roller bearings, tapered roller bearings must carry a minimum load, especially in highspeed applications where inertial forces and friction can damage rollers and raceways should they come out of contact. Applications for rotary bearings Bearings abound in industrial and consumer designs. For example, deep-groove ball bearings often go into in small to medium-sized electric motors because they can accommodate both high speeds and radial and axial loads. Selfaligning ball bearings, on the other hand, work well in fans. These bearings have two rows of balls with a common raceway in the outer ring. This design allows for angular misalignment while maintaining running accuracy. The only caveat is that they’re one of the most difficult bearings to install correctly. Tapered roller bearings go in needing support for axial and radial loads — as in a tire hub bearing vehicle weight and the axial loads associated with cornering. These bearings are also common in gearboxes where they mount with a second bearing of the same type in a face-to-face or back-to-back orientation. They provide rigid shaft support to minimize deflection. This reduced shaft deflection minimizes gear backlash. Tapered bearings also have the advantage of being lightweight but efficient, even while maintaining good overall speed capabilities. In applications where the bearings mount vertically, they typically mount in a face-to-face setup. In horizontal applications, they mount back-to-back.
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The igus xirodur B180 pillow block bearing uses built-in stainless-steel balls for smoothrunning and corrosion-resistance. The selflubricating and wear-resistant properties also eliminate potential contamination.
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5/15/18 2:59 PM
BELTS & PULLEYS
WHICH TIMING BELT TOOTH PROFILE TO USE:
TRAPEZOIDAL, CURVILINEAR, OR MODIFIED CURVILINEAR? TIMING BELTS—also known as synchronous belts or may not be interchangeable. This is due to variations in tooth toothed belts—are most often used in transporting, indexing, dimensions between manufacturers' designs, and in some and positioning applications where high torque or force cases, even within a given manufacturer's product line. transmission and high acceleration rates are required. Unlike V-belts, which rely on friction between the belt and the pulley Trapezoidal tooth profiles for power transmission, timing belts have positive engagement Belts with a trapezoidal tooth profile are probably the most between the belt teeth and the pulley teeth, so the possibility widely used in timing belt applications, especially for linear of slip is eliminated. This makes timing belts very efficiency at positioning and conveying applications. They have good force transmitting power and gives them good positioning accuracy. transmitting capabilities and low backlash. But the trapezoidal Belt manufacturers typically offer their own exclusive tooth shape results in high stress concentrations at the beltor patented tooth designs, but even proprietary designs are pulley interface, which can lead to high wear rates when the based on one of three basic profiles—trapezoidal, curvilinear, transmitted torque or speed is high. or modified curvilinear. Each one offers benefits in power transmission, speed capability, or backlash. One caveat: Although SYNCHRONOUS (TIMING) BELT PROFILES timing belt tooth profiles are based on just three basic designs, belts with the same type of tooth profile and the same tooth pitch
TRAPEZOIDAL
Better load distribution and smooth meshing transitions mean curvilinear teeth are quieter than trapezoidal options. The catch is that curvilinear designs have higher backlash than trapezoidal profiles.
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belts & pulleys — Power Transmission HB 05.18 V3.indd 25
CURVILINEAR
MODIFIED CURVILINEAR
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CURVILINEAR BELT TEETH CONCENTRATE STRESS AT TOOTH CENTERS FOR MORE RELIABILITY.
DESIGN WORLD — MOTION
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POWER TRANSMISSION Manufacturers of Power Transmission and Motion Control Components
R E F E R E N C E
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Shown in the close-up is a Gates Carbon Drive CDN system—designed to be lower in cost for new bike applications. It leverages new materials and geometries, with nine carbon cords embedded within engineered polymer belt and a patented 11-millimeter tooth pitch profile for lower tension. Like many new belt applications, it replaces chain drives. Carbon belts are useful in industrial applications as well.
Curvilinear tooth profiles The curvilinear tooth profile was developed to alleviate the stress concentrations found in trapezoidal profiles and improve on torque and speed capabilities. Curvilinear profiles also have a larger tooth depth than trapezoidal designs, so belt ratcheting is less likely. And the smoother transition that curvilinear teeth provide during mesh means these profiles are quieter than their trapezoidal counterparts. The tradeoff, however, is that curvilinear designs have higher backlash than trapezoidal profiles. In fact, the curvilinear tooth profile was developed by Gates, and belts with this profile were referred to as high torque drives (HTD). Although various manufacturers use different names, the HTD designation is still quite common for belts with curvilinear tooth profiles. Modified curvilinear tooth profiles Further improvements to the curvilinear tooth profile resulted in the modified curvilinear design. This profile has a smaller tooth depth and a greater flank angle than the original curvilinear version, giving the modified curvilinear design the highest torque and force transmitting capabilities among the three tooth profiles. It also allows the areas of the belt between the teeth to share the load with the teeth that are engaged with the pulley. This means that belts with modified curvilinear teeth are the least likely to experience ratcheting, even under extreme loads, making them common in processing
American Engineering • American Made © 2017 Custom Machine & Tool Co., Inc.
belts & pulleys — Power Transmission HB 05.18 V3.indd 26
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SDP SI — PT Guide 5-18.indd 27 Design World-Motion Control Special Issue.indd 1
250 Duffy Avenue, Hicksville, NY 11801 Phone: (800) 819-8900 Fax: (516) 326-8827
5/14/18 3:24 PM 4/9/18 11:19 AM
POWER TRANSMISSION R E F E R E N C E
G U I D E
Shown here are timing belts, timing-belt pulleys, and Fairloc hubs from Stock Drive Instruments/ Sterling Instrument (SDP/ SI). The company specializes in these power-transmission components; its Fairloc hub is a particularly unique offering in that it centers shafts and keeps mounted pulleys perfectly aligned.
applications that require very high torque transmission at high speeds. One note here: Belts with a trapezoidal tooth profile are available in both imperial and metric dimension. However, belts with curvilinear and modified curvilinear tooth profiles are available only in metric dimensions.
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Where to apply synchronous belt and V-belt drives Power transmission in linear motion designs is often through rotaryto-linear devices, chain, or belt drives. The earliest belt iteration — and one that’s still economical today — is the friction-based V-belt design. These pair a belt with a pulley (often on an electric motor’s geared output shaft) to provide reliable operation in myriad enduser and industrial designs. Modern V belts are rubber, urethane synthetic, and neoprene designs with either a V or trapezoidal profile. The latter increases the amount of contact between V belts and pulleys to minimize tension needed to transmit torque. Even so, polyurethane outperforms rubber thanks to its higher resistance to chemicals and adaptability to specialized profiles. (Polyurethane also boosts the shear strength of the teeth on synchronous belts covered in this article’s next section). A V belt’s most important element — its tension-bearing top — includes fiber cords for strength to bear the actual traction load. Modern tension-member cords are often aramide, polyester, fiberglass, or even steel. Pre-stretched variations help minimize stretch. The cords embed into the main belt material that serves to hold the belt body together and shed heat. The part of most modern friction belts that engages the pulley is a compression section designed to actually wedge into pulley grooves as a way to boost engagement. In many instances, a rubberized fabric cover helps protect the belt and prevent slipping and overheating cords. Though they’re versatile and forgiving, improperly sized friction-based belt drives can slip (tangentially on the pulley — a form of lost motion) and creep axially. That can make for unreliable
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POWER TRANSMISSION R E F E R E N C E
G U I D E
Shown here are DuraBelt powered rollers, which are designed to transmit power and convey loads. Their round polyurethane belts can carry more load than comparable rubber belts.
speed output. Here are some things to remember if a V-belt drive makes the most sense for a motion axis: Output torque depends on belt resistance to tension and belt-pulley adherence. The latter is why oils and greases must be kept away from belt drives — or threaten drive failure due to slipping. Be prepared to specify V belts by cross section (including the belt’s top width, V angle, and depth) and overall pitch length (defined as a circumferential length along a belt’s pitch line). Then suitable V belts are narrowed further by which have sufficient power ratings (determined by rpm and sheave speed) to satisfy design demand of nominal horsepower (to be transmitted or output at the motor) with application of a service factor. Sound complicated? In fact, industry has simplified much of this with references that list fairly specific V-belt service factors that adjust for typical levels of
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special application demands and losses from variable loads and rpm, heat, environmental conditions, and shock and vibration. Where friction belts are insufficient for a motion design — as on positioning table, conveyor, and printing-machine axes needing true synchronous operation, for example — toothed synchronous belts excel. Such belt drives are also indispensable in compact designs that need power-dense linear drives in awkward or compact design envelopes. As with V belts, be prepared to specify synchronous belts by length and axis power demand. Here, additional factors include the teeth’s maximum shear strength (dictated by their cross section as well as pulley-engagement dynamics). On the topic of teeth engagement, remember that synchronous belt drives need tooth clearances at the engagement with pulley grooves (so teeth can enter and exit channels sans interference). That’s why most synchronous belts exhibit some backlash. In addition, a synchronous belt’s tooth shear strength must be high enough to withstand maximum application torque demand. As with V-belt selection, service factors can help engineers pick synchronous belts having shear strengths to withstand the application’s worst expected shocks and loading. Despite the extra considerations, synchronous belts are indispensable in precision motion designs. A mature technology is belting with teeth of a trapezoidal shape (not to be confused with V belts sporting trapezoidal cross sections) — although modified iterations are suitable for very precise positioning. More common in new designs are rounded profiles carry more load than belts with trapezoidal teeth. The belts do this in two ways: 1) They have inherently higher tooth shear strength and they 2) More evenly spread load over the belt’s tensile cords. Generic labels for synchronous belts with round-profile teeth are variations on the term high-torque drive or HTD for short — with the latter a trademark of belt and rubbercomponents manufacturer Gates Corp. In some cases, belts with round-profile teeth can triple horsepower ratings. Another design — belts with curvilinear teeth — help optimize pulleytooth engagement and pressure angles to boost overall power transmission. Many such belts go into automotive applications, which come with tensile cords and in sizes unsuitable for industrial designs.
pyramidbelts.com • 641.792.2405 • sales@pyramidbelts.com 30 201702_Pyramid4x475_lineshaftR2.indd 1 belts & pulleys — Power Transmission HB 05.18 V3.indd 30
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CLUTCHES & BRAKES
CAM CLUTCHES WORK, AND WHAT ARE OTHER OVERRUNNING CLUTCH OPTIONS?
HOW DO
MECHANICAL OVERRUNNING CLUTCHES, sometimes called one-way or backstop clutches depending on the context, transmit torque in one drive direction but let the output axis continue to turn (freewheel) when the driving motor slows or stops. Such clutch function is useful in large and small designs for material handling, packaging, conveying, medical, turbine auxiliary drives, and offhighway applications. Common mechanical overrunning clutches include those that use ratcheting, roller ramps, wrap-spring, and cam action for one-way axis engagement. Of these designs, cam clutches (sometimes called sprag clutches, especially in transportation industries) are relatively new and power dense. An array of bars profiled with asymmetrical cam-shaped geometry ring the circumference of the clutch. Cages at each end keep the bar cams evenly spaced. Upon input rotation, the cams act as rising wedges to lock between the races on the inner and outer clutch halves. More specifically, cams in some cam-clutch variations fill the space between the races to quickly act as a sprags and hold the clutch halves together via locking friction. Upon reversal, the cams quickly release (and tolerate high overrunning speeds). In other cam-clutch designs, centrifugal force makes the bar cams turn into liftoff to disengage the clutch halves. Then upon slowing or reversal, a
spring at each cam pushes them back into sprag-type engagement. No matter the variation, the geometry of cam clutches’ loadbearing cams tightly pack into the clutch body. That means it’s often two other factors that limit torque capacity: 1. 2.
Bore size (and connecting shaft size) The effects of tangential forces where the cams meet the races.
But some cam clutches exhibit comparably less ID and OD race deflection and contact stress than other designs thanks to distributed loading during engagement. That in turn extends life and makes for smoother engagement. Some cam clutches even use ground alloysteel races to maximize uniformity of load distribution. Because jamming under load can occur where wear is excessive, some cam clutches for demanding applications feature precision-formed cams to resist wear and prevent failure from fatigue. As mentioned, other overrunning clutches include those that use ratcheting, roller ramp, and wrap-spring action. Ratcheting clutches are basic two-element clutches that use a single-tooth arm that engages a toothed wheel in one direction and freewheels in the other. These are most common in powered consumer products and light industrial tools.
U.S. Tsubaki BS-F Series cam clutches excel in backstop applications. Bores are 2.360 to 18.310 in. and torques are 4,980 to 722,000 lb-ft.
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G U I D E
Highest Torque in the Smallest Space ... or the largest.
Maxitorq® clutches and brakes deliver power, reliability and are customized to meet your exact needs. Land, sea and air – CJM is everywhere. AS9100C:2009/Certified
Clutches, Brakes & Power Transmission Products • electrical, mechanical, pneumatic & hydraulic models • system design and integration • expert engineers working on every order
Roller-ramp clutches engage through ground steel rollers that run and lock on a wheel precision machined with incline ramps. Note that some roller-ramp clutch literature describes roller engagement with a cam. This isn’t to be confused with the cam arrays of cam clutches, as here the cam is singular and refers to the assembly’s wheel that’s machined with ramps. Operation relies on concentricity between inner and outer races, so most roller-ramp clutch iterations include compression springs to maintain roller-to-wheel contact. The
Note the array of sprags in this RSCI (20-130) Series centrifugal liftoff clutch. From Stieber of Altra Industrial Motion, the backstop clutch comes in sizes to accommodate operating speeds of 70 to about 1,300 rpm with torques to 250,000 Nm.
Left: BS-HS Series clutches from U.S. Tsubaki Power Transmission are high torque and rated for high maximum overrunning speed. Red-highlighted areas are where the cams backstop when the inner race turns clockwise. Right: BR-HT Series cam clutches use a different mode of engagement based on centrifugal force. When the inner race turns clockwise with sufficient speed, the cams pivot so their geometry clears both races. Otherwise, springs make the cams pivot back to default and lock the clutch halves together.
springs install at angles engineered to optimize initial actuation and holding grip. Because of the way the cylindrical rollers lock into flat steps on the inner wheel, these clutches transmit high torque and withstand high overrunning speeds. In addition, roller-ramp clutches are low cost and easy to repair even while offering high overrunning speed and torques.
Engineering Solutions for Clutches & Brakes
www.cjmco.com Phone: 860-643-1531 291 Boston Tpke, Bolton, CT 06043 32
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Power Transmission and Motion Control Solutions for Industrial Applications
Heavy Duty Clutches and Brakes
Electric Clutches and Brakes
Gear Motors
Overrunning Clutches
Gear Drives
Engineered Couplings & Universal Joints
Power Transmission Components Linear Actuators & Controls
Engineered Bearing Assemblies
Air Motors
Belted Drives & Sheaves
Geared Cam Limit Switches
The Power Brands in Power Transmission Ameridrives Couplings Bauer Gear Motor Bibby Turboflex Boston Gear Delroyd Worm Gear Formsprag Clutch
Guardian Couplings Huco Industrial Clutch Inertia Dynamics Kilian Lamiflex Couplings
Marland Clutch Matrix Nuttall Gear Stieber Clutch Stromag Svendborg Brakes
TB Woodâ&#x20AC;&#x2122;s Twiflex Warner Electric Warner Linear Wichita Clutch
www.altramotion.com
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In contrast with cam clutches (which excel at overrunning) jaw (tooth) clutches and brakes excel at holding. Shown here is a Carlyle Johnson JEB0325 low-backlash spring-set jaw holding brake. Widely used in medical-imaging applications, it eliminates backlash and can smoothly stop imaging-unit rotation to allow positioning sequences without vibration or shaking. This makes for clear imaging.
Wrap-spring clutches have springs that coil around a cylindrical output to tighten down when turned in the drive direction. This creates friction to lock the assembly and transmit torque. Wrap-spring clutches excel in applications of moderate speed — commonly to 2,000 rpm or so.
PRIMER ON BRAKES, CLUTCHES, AND TORQUE LIMITERS Torque limiters, clutches, and brakes stop, hold or index loads. Especially over the last five years, there have been more application-specific designs … as several industries are pushing the performance envelope of stock components. Brakes stop loads (typically rotating loads) and go in applications that need accurate stopping of the load with motors that stop as well. Clutches transfer torque and go in applications where the machinery must engage or disengage a load and motor while letting the motor continuously run. With a clutch, the design usually lets the load coast to a stop. Clutch and brake combinations go where a machine stops and starts a load while the motor continues to rotate. In fact, both clutches and clutch-brake combinations can mount to a motor shaft or mount to a base and engage the drive shaft with a belt drive, chain drive or coupling. A machine’s motor frame size and horsepower dictate brake and clutch types suitable for a given design. In the case of base-mounted units, the design engineer may need to define the rpm at that location. To this end, manufacturers provide quick-selection charts that list unit size (which engineers can find at the chart’s intersection of motor horsepower and speed at the clutch shaft). Manufacturers base most of these charts on the dynamic torque capacity for the product and the torque capacity for the motor … plus an overload factor of some value.
Using this method presumes that the design engineer has selected a motor that’s sized appropriately to the application. In applications where cycle rates are considered aggressive for the inertia of the load, it’s a good idea to consult with the application support staff of the manufacturer regarding the heat dissipation capacity. In the case of electric varieties, coil voltage is another clutch and brake consideration. The most common options are 6, 24 and 90 Vdc. 90 V is most common in North American markets. Versions that use 24 V are more common in Europe. In both cases, power supplies exist to convert ac to dc if the application needs it.
This is a stainless-steel tensioning brake from Mach III Clutch Inc. and a waterresistant clutch for shaft-to-output flange connection. These and other Mach III brakes and clutches come in torque capacities to 62,000 lb-in., bore sizes to 3.625 in., and in configurations for through-shaft, end-of-shaft, flange, NEMA frame, IEC frame, and custom-motor frame mounting.
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WOULDN’T IT BE NICE TO HAVE ONE LESS THING ON YOUR PLATE? Searching for components eats up valuable time. For more than 45 years machine designers have relied on us for made-to-order products that meet their exact requirements. We are easy to reach, quick to respond and deliver both catalog and custom products within reliable lead times. › Pneumatic and mechanical models › Torque capacities to 60,000 lb.in. › Experienced application assistance One call or email connects you with an engineer: 859-291-0849 engineering@machiii.com
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CHAIN DRIVES: POWER TRANSMISSION MAINSTAYS ONE OF THE OLDEST METHODS of transmitting mechanical power, chain drive are linear actuators used in a range of industrial machinery. A typical chain drive consists of two components; a chain and a sprocket. The chain itself is composed of several components including a pin, bushing, roller, pin link plate, and a roller link plate. The chain wraps around two sprockets; a driver sprocket and a driven sprocket. The driver sprocket is connected to the output of a motor or gear reducer and moves the chain, while the driven sprocket connects to either the driven load or some other part of the system that is moved.
Perhaps the most significant benefit of roller chain drives is that they are a relatively straightforward and simple method of transmission of mechanical power. They have been around for a long time and have a proven track record of functioning well in a range of applications and environmental conditions. Today’s roller chains feature high wear resistance, fatigue strength and tensile strength. Compared to other forms of drives such as belt drives, for instance, chain drives tend to have better efficiency due largely to less friction losses. There are plenty of applications where chains are still used, a common one being in conveying applications. These systems can use chain drives for either horizontal, vertical or curved conveyor sections.
Common rigid chain has two rows of link plates and shoulders; duplex chain has three; other options abound. (Image courtesy iwis Drive Systems)
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CHAIN
THE PERFECT BALANCE TSUBAKI CABLES, THE PERFECT COMPLEMENT TO YOUR CABLE CARRIER! Only Tsubaki Carriers matched with superior Tsubaki Cables can offer you the perfect balance of flexibility, endurance, and resistance in even the harshest production environments. That’s what makes this the ultimate package and the first choice of the most discriminating customers from around the globe. Both Carrier and Cable are designed and engineered with the same high standards of quality, reliability and performance you can depend on. So why compromise your mission-critical application with anything less than the perfect match: Tsubaki Carriers and Tsubaki Traxline Cables. Go with the single source provider of cables and carriers you can trust in any conditions – Tsubaki.
TRAXLINE CABLES
Operating environments can vary widely as well. There are uses in clean settings where the chain drives must operate lubrication free. Other applications expose chain drives to varying weather conditions, water, or harsh or corrosive chemicals. Examples include industries like wood or paper processing mills. A common design requirement is for anticorrosive chains. These can be either specially coated chains such as with special polymers, or materials such as stainless steel, titanium or nickel-plated chain. Other types include chains made from special lightweight plastic materials. Another common type of chain is a rigid chain actuator. These actuators work by pairing a drive (such as 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. 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. Rigid-chain actuators have the mechanical benefits of conventional chain but can act in horizontal push setups or vertically as jacks. They’re also 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.
ROLLER CLASS CHAINS • BACKSTOPS • SPROCKETS • CABLE & HOSE CARRIERS • POWER TRANSMISSION PRODUCTS ROLLERCHAINS CHAINS• •ENGINEERING ENGINEERING CLASS CHAINS • CAM CLUTCHES • SPROCKETS • CABLE & HOSE CARRIERS • PT COMPONENTS
®
Total Package
USTSUBAKI.COM ©2018 U.S. Tsubaki Power Transmission, LLC All Rights Reserved.
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G U I D E
COUPLINGS
WHAT ARE FOR POWER-TRANSMISSION DRIVE SHAFTS? AS OUTLINED in our coverage of couplings for motion control — online at couplingtips.com — couplings connect rotating shafts powered by a drive of some type — often an electric motor. All couplings serve to transmit drive torque and angular velocity. But applications for motion control (such as axes to position loads) usually use disc, slit or beam, curved-jaw, bellows, and other zerobacklash couplings capable of precise transmission of torque. In contrast, applications for power transmission (as in grinding machines, pumps, and material-handling machinery) commonly include disc, gear, chain, elastomer tire, grid, jaw, and Oldham couplings. Such PT couplings transmit more torque on average than couplings designed for motion control … even to millions of lb-in. Plus they’re more rugged to withstand challenging environments. Chain couplings — with typical maximum torques to 220,000 lb-in. at their largest — wrap lengths of chain around sprockets with clearances to impart flexibility. These power-transmission couplings excel in high-horsepower applications on axes needing correction of up to 2° and 0.01-in. angular and parallel misalignment. Diaphragm couplings — with typical maximum torques to 500,000 lb-in. at their largest — transmit power through a metal membrane (sometimes of varying thickness or ganged in arrays). Though often more costly than other options, diaphragm couplings mitigate and avoid problematic transmission of forces and moments t coupled equipment such as bearings. Profiles include straight-spoked diaphragms; tapered diaphragms; and convoluted diaphragms assembled in arrays. These correct up to 1° and 0.1-in. angular and parallel misalignment. Elastomeric tire couplings — with typical maximum torques to 550,000 lb-in. at their largest — transmit power through a tire-shaped rubber element that bridges the coupling’s two hubs. These correct up to 1° and 0.2-in. angular and parallel misalignment.
ABB’s Dodge Raptor elastomeric-tire couplings use a split natural-rubber element. This element transmits torque to 340,200 lb-in.
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Me tal Bel low s Exp ert s
Delivering precision and performance, on time and on budget...our 50 years of experience comes free! Features: • Zero backlash • Thinnest high strength walls • Seamless construction • Premium alloy FlexNIckel® • Diameters as small as 0.020 in (5mm) • Highest cycle life • Design assistance for customization
For more than 50 years, Servometer has specialized in the manufacture of precision electrodeposited bellows, assemblies, and electroforms. Our engineering team focuses on your application and quickly responds with a custom design and high quality prototype or part. That’s why OEM manufacturers call us. ISO9001:2008 Certified.
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Now some balancing machines incorporate CD couplings from Zero-Max to get reliable performance for better onmachine trim balancing during assembly into the machine. For this application, the Composite Disc pack has high torsional stiffness. The flex element ensures zero backlash, high torsional stiffness, and low bearing loads. Specially sized and located holes are drilled and positioned on the CD coupling’s hubs to allow addition of weight during final trim balancing of the drive system on the machine … although Zero-Max pre-balances the couplings at 4,000 rpm so that final assembly only requires fine-tune balancing.
Jaw couplings — with typical maximum torques to 550,000 lb-in. at their largest — include both straight and curved variations. Much like disc couplings, the design lends itself to adaptation to both power transmission and backlash-free motion control. The coupling hubs have jaws that lock into a spider made of bronze, elastomer, or other material. Power transmission is reliable even through 1° and 0.01 in. angular and parallel misalignment. Oldham couplings — with typical maximum torques to 550,000 in. at their largest — include a metal or polymer disc with slots on each face 90° offset. Usually hub fins or tenons engage a slotted disc that’s free to slide even while transmitting torque. Oldham couplings for accommodation of angular misalignment might transmit through 6° and 0.05 in. Oldham couplings to primarily address parallel misalignment might address 0.15 in. or more and 0.5° or so. Disc couplings — with typical maximum torques to 5,000,000 lb-in. at their largest — are one of a few coupling types that come in variations to satisfy motion-control or powertransmission applications. Single thin discs or multi-disc packs (made of metal or engineered composite) bridge the hubs. In representative designs, the discs impart flexibility to transmit torque even while addressing up to 2° and 0.05 in. angular and parallel misalignment. Grid couplings — with typical maximum torques to 5,000,000 lb-in. at their largest — include a heavy spring that weaves between slots on the coupling hubs. Compliant connection damps torsional vibration and shock loading — typically even through 0.3° and 0.30 in. angular and parallel misalignment. Gear couplings — with typical maximum torques to 55,000,000 lb-in. at their largest — include a flexible joint on each hub. In most variations, a spindle joins the two. Each joint includes gearset that mates with a 1:1 ratio. The tooth flanks and external gearing’s outer diameter are crowned to allow rotating-spline action and accommodate misalignment of 3° and 0.04 to 0.4 in. on average.
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COUPLINGS COUPLINGS FOR POWER TRANSMISSION CHAIN COUPLINGS
COURTESY IWIS DRIVE SYSTEMS
DIAPHRAGM COUPLINGS
ELASTOMERIC TIRE COUPLINGS TB WOOD’S DURA-FLEX COUPLING COURTESY ALTRA INDUSTRIAL MOTION
THE COUPLING. ABSOLUTE PRECISION 0.1-100,000 NM.
JAW COUPLINGS
JAW-IN-SHEAR (JIS) COUPLING LOVEJOY INC.
OLDHAM COUPLINGS
RULAND MFG. CO. INC.
TYPICAL TORQUE CAPACITY
FLEXIBLE JAW COUPLING MARTIN SPROCKET & GEAR INC.
DISC-ELEMENT COUPLINGS ROBA-DS DISC-PACK COUPLINGS MAYR POWER TRANSMISSION
BIBBY TURBOFLEX • ALTRA INDUSTRIAL MOTION
MORE
GRID COUPLINGS
GEAR COUPLINGS FALK GEAR COUPLING REXNORD CORP.
These are just a few coupling types used for power transmission between shafts. Others include uni-lat, finger, K-flex, and fluid (hydraulic or hydrodynamic) couplings as well as flexible shafts and Hooke's joints, also called Cardan or universal or U joints.
WWW.RW-AMERICA.COM
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WHAT IS A
G U I D E
DRIVE?
THERE ARE A NUMBER of ways that the term “drive” is used. Roughly speaking, the word can refer to either mechanical uses or electrical ones. Mechanical uses focus on the mechanical transmission of power and can include screw drives (like ball or lead screws) or other types such as chain or belt drives. Here, the focus is on the electrical side of things. A drive takes an input signal from a controller and amplifies that signal which is then used to power a motor. An amplified signal is necessary because control signals are low current and can’t be used to power the windings of a motor, which require higher current levels. Internally, drive components include power electronic devices such as SCRs (silicon controlled rectifiers), transistors and thyristors. Drives are designed to power the many different types of motors used in motion control applications. These can include drives for powering
ac induction or synchronous motors, dc motors, servo and stepper motors. To take ac drives as an example, they convert ac to dc, then, using a range of different switching techniques, generate variable voltage and frequency outputs to drive a motor. More specifically, variable frequency drives (VFD) operate by switching their output devices—which can be transistors, IGBTs (insulated gate bipolar transistors), or thyristors—on and off. VFDs can be either constant voltage or constant current. Constant voltage types are the most common type of VFD. They use pulse width modulation (PWM) to control both the frequency and the voltage applied to the motor. Functionally speaking, signal amplification is what is going on inside of a drive. But this can create confusion because some manufacturers refer to drives as amplifiers but also as inverters. Focusing on the electronics helps to clear things up. An electronic inverter converts dc power to ac. Drives contain inverters in order to generate the ac signals needed to drive a motor. So labeling something as an inverter really only refers to one of the electronic systems in a drive, even though engineers may use it interchangeably with the word “drive” to refer to the same thing. However, adding to the complexity is that modern drives integrate other functions as well. For instance, a drive in addition to performing standard drive functions may also be generating the control signals as well. Or a controller may have drive functionality within it, for example. Either way, knowing what a drive functionally does (as well as having access to product specs) can help determine what is being referred to when any of these terms is used.
Manufacturers such as Siemens offer a complete system of drives, motors and controllers designed to simplify motion control for machine builders. For example, the company's Sinamics S210 drive comes with integrated safety functions and connects via Profinet to controllers and servo motors.
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+
=
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Makes no cents! Adding an inefficient single-stage worm gearbox to a premium efficient motor doesnâ&#x20AC;&#x2122;t make sense if you are trying to save money. Why gain 2-3% energy savings with a more efficient motor and then lose 50% or more through the worm gear? Solution: Use a helical-bevel gearmotor from SEW-EURODRIVE and obtain 96% efficiency. Now that makes a lot of cents!
seweurodrive.com | 864-439-7537
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Magnetic sensing technology in these SIKO LEC series magnetic linear sensors ensures that measurements are resistant to environmental contaminants such as dust, oils, grease and moisture.
OVERVIEW OF
ENCODERS
ENCODERS ARE CRUCIAL components in motion control systems that provide information on a number of parameters including position, distance, and speed. Typically classified as rotary or linear, absolute or incremental, encoders can also be categorized according to their internal sensing architecture such as optical, magnetic, or capacitive. A key performance parameter for encoders is resolution. For incremental encoders, resolution is typically specified in pulses per revolution (PPR), or, in the case of linear encoders, pulses per inch (PPI) or pulses per millimeter (PPM). These square-wave pulses are precisely spaced, and the encoder determines its position by counting the number of pulses generated during a movement. Incremental encoders generally supply squarewave signals in two channels, A 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. In order for the encoder to provide any useful position information, the position of the encoder has to be referenced in some way, traditionally using an index pulse. So the incremental encoder sends incremental position changes to electronic circuits that perform the counting function.
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Absolute encoders have a unique code for each shaft position. The encoder interprets a system of coded tracks to create position information where no two positions are identical. Another feature is that absolute encoders do not lose position when power is switched off. Because each position is distinctive, the verification of true position is available as soon as power is switched on without the need for a homing routine. Linear encoders typically are composed of a scale such as a coded strip, and a sensing head. Reading the space between the scale coding determines position. The resolution of linear encoders is measured in pulses per inch or millimeter. The scale typically has a fixed resolution with embedded markings that is read by the sensing head. Rotary encoders measure position in pulses per revolution. Similar to linear encoders, a typical rotary encoder contains an internal coded disk and a sensing head. Think of a linear encoder as a type of tape measure while a rotary encoder is more like a measuring wheel. One area where encoder use is seeing growth is in robotic applications, with one of the most important considerations being functional safety. Designers of machines and industrial equipment are taking safety seriously. The basis for functional safety is the need to reduce the risk of injury in industrial machinery and systems, which functional safety does by addressing machines and systems as a whole.
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ENCODERS
The DFS60S incremental encoder from SICK simplifies the integration of safe motion into machine safety architecture by reducing the amount of work required for verification, validation and safety engineering.
The two main standards that govern industrial machine safety are the IEC 62061 and the EN/ISO 13849-1 standard. Two measures, SIL (Safety Integrity Level) and PL (Performance Level), are defined within the IEC 62061 and the EN/ISO 13849-1 standard as probabilities of events occurring, with lower probability numbers reflecting safer systems. An encoder can be designed so that it fits readily within a larger machine or system that can be certified functionally safe. Typical features of safety rated or enabled encoders may be diagnostic features, safe mechanical interfaces such as oversized or redundant features, or safety rated communication protocols. For robotic systems, using encoders in joints can provide critical feedback on position, motion and velocity of robotic arms. This data can then be used by a controller to evaluate system performance as a whole. So the encoder, along with the controller and other components, together contribute to making the entire robotic system functionally safe.
NEW Programmable Incremental
Encoders Give You Flexibility Program your CPR, waveform, and output type The new Model 25SP and Model 58TP Programmable Accu-CoderPro™ encoders are so configurable, they can go into almost any application. With the easy-to-use interface, you can program these specs:
CPR – any resolution from 1 to 65,536 Waveform – choose from 32 options Output Type – 6 different outputs
Highly Mechanically Configurable: • Available in Size 25 Shaft, or 58 mm Thru or Hollow Bore • Flexible Mounting Options
• Variety of Connector Types • Operating Temperature Range of -40º to 100º C • Sealing up to IP67
Call EPC today to learn how an Accu-CoderPro™ programmable encoder can fit your application. 1-800-366-5412 • www.encoder.com
WATCH ONLINE!
Product demos and more at youtube.com/user/EncoderProducts
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GEARBOX
WHAT FEATURES MAKE A SUITABLE FOR WASHDOWN ENVIRONMENTS? WASHDOWN PROCESSES are common in the food and beverage and pharmaceutical industries, where cleanliness of the manufacturing and handling equipment plays an important role in the quality and safety of the product. In fact, the US Food and Drug Administration (FDA) and the European Hygienic Engineering and Design Group (EHEDG), along with other government and industryspecific agencies, provide regulations and standards for the safe and hygienic processing, handling, and packaging of food, beverage, and pharmaceutical products. However, these standards often address overall machine or process design and leave
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the choice of individual components, such as bearings, motors, and gearboxes, up to the equipment designer and manufacturer. Washdown applications introduce two significant hazards to the performance and life of a gearbox. The first being that the washdown solution could make its way inside the gearbox and damage the internal gears and bearings, and the second being the possibility of corrosion due to exposure to water and chemicals – specifically alkalines, chlorine, and acids. Fortunately, gearbox manufacturers have found ways to address these hazards, and it’s now common to see washdown and “hygienic” designs among gearbox manufacturers’ standard offerings. But what features make a gearbox suitable for washdown or hygienic applications? Gearbox materials for washdown First and foremost, the materials used for the housing and other external components, such as the output shaft and hardware, are critical to making a gearbox suitable for washdown duty. While some equipment manufacturers
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Servo Mount Gearheads Low Backlash For Life!
HPN Harmonic Planetary® value series provides a low cost solution without the need to compromise on quality or performance. This new value series of planetary gears carry the reputation for quality and reliability for which Harmonic Drive® products are known throughout the world. • • • •
Helical Gearing Available in 5 frame sizes Peak Torque: 9Nm to 752Nm Ratios: Single Stage: 3:1 to 10:1, Two Stage: 15:1 to 50:1 • Quick Connect® Mounting System
• High Efficiency • Backlash: <5 arc-min (single stage), <7 arc-min (two stage) • Fast Delivery • Shaft output available with key and centertapped hole or with center-tapped hole
Harmonic Drive LLC | 247 Lynnfield Street | Peabody, MA 01960 | 800.921.3332 | www.HarmonicDrive.net Harmonic Drive®, Harmonic Planetary® and Quick Connect® are registered trademarks of Harmonic Drive LLC.
9x10.875Drive HPN — Ad2.indd 1 5-18.indd 47 Harmonic PT Guide
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choose to use gearboxes with epoxy painted housings for washdown applications, these can chip and need to be repainted periodically. In most cases, stainless steel is a better alternative than painted options. Both 304 and 316 series stainless are suitable for washdown and hygienic applications, although 316 has better resistance against pitting, especially when exposed to the caustic chemicals found in many washdown solutions. However, the use of stainless steel presents a higher up-front cost than painted designs, and 316 series stainless even more so than 304. To mitigate the cost, some manufacturers use 316 stainless for the housing and 304 stainless for other components with less exposure (such as shafts and some hardware).
NORD DRIVESYSTEMS two-stage helical bevel gearboxes come in 3.58 to 61.88 ratios and transmit torque from 50 to several hundred Nm even while maintaining efficiency. Unicase gear housings and large bearings let the gearbox withstand high radial and axial loading on the output shaft. Some variations include NORD’s nsd tupH corrosion-resistant treatment of aluminum housings to withstand washdown.
Geometry to maximize gearbox cleanability Another important design aspect of washdown or hygienic gearboxes is their shape. The purpose of washdown is to clean the equipment and prevent bacteria from growing, but if the gearbox has crevices, seams, or grooves where particles can be trapped, thorough cleaning will be difficult or even ineffective. This is why many gearboxes designed for these applications have a round housing with smooth surfaces and no external seams. And if corners are necessary in the design, they’re executed with a radius rather than sharp angles. Gearbox lubrication and seals In the ideal world, lubrication would never make its way out of the gearbox and into the process, but in the real world, it can (and does) happen. To prepare for
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Your Custom Gearbox Solutions are CGI Standard Products.
Advanced Products for Robotics and Automation CGI Motion standard products are designed with customization in mind. We understand most off-the-shelf products or a complete in-house design may not fit your application, so our standard products are designed for functional flexibility. Our team of experts will work with you on selecting the optimal base product and craft a unique solution to help differentiate your product or application. So when you think customization, think standard CGI gearbox assemblies. Connect with us today to explore what CGI Motion can do for you.
copyrightŠ2015 cgi inc. all rights reserved. 0804stdp
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In this PT Reference Guide feature we focus on closed gearbox designs for washdown, but open gearing is particularly easy to sanitize. Here, KLEENLine retractable conveyors use precision-machined PowerCore gears from Intech Corp. to replace stainless-steel pinions in their rack and pinion drives. That eliminates the need for lubrication after washdown.
this possibility, applications that pose the risk of incidental contact with the product should be lubricated with NSF H1 food-grade grease or oil. The primary task of gearbox seals is to keep lubrication inside the gearbox (see above) and keep liquids and contamination out. Washdown conditions make this especially difficult, since the seals must withstand the high pressure of the washing operation and be impervious to harsh chemicals. This is why it’s common to find gaskets or O-rings on the input side of the gearbox and two-part seals on the output shaft. Most gearboxes for washdown and hygienic applications use seals made of Viton due to its broad chemical compatibility, ability to withstand high temperatures, and excellent mechanical properties. Note that the most common system used to classify a product’s washdown compatibility is the IP rating, which is published by the International Electrotechnical Commission (IEC). IP stands for International Protection — but is sometimes referred to as Ingress Protection. IP ratings that convey suitability for washdown are:
IP65: protection against water jets IP66: protection against powerful water jets IP67: protection against immersion, up to 1 meter IP69k: protection against powerful, high-temperature water jets
The National Electrical Manufacturers Association (NEMA) has also developed and published protection ratings, but NEMA ratings are used almost exclusively in the US, while IP ratings are used internationally. It’s important to note that there is not a direct correlation between NEMA and IP ratings.
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G E A R M O TO R S
COMMON DESIGN CONSIDERATIONS FOR
GEARMOTORS
A GEARMOTOR is an electromechanical device that integrates a gear reducer and an electric motor into one physical unit. The motors can be any variety of ac or dc motor, depending on many factors such as required power, torque, and application environment, among others. Different motors and gear types can be mixed and matched to suit unique application requirements. Common types of gearmotors include integral horsepower gearmotors, which are composed of a 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. However, there are a few other considerations that are just as important: Motor type – ac or dc, brushed or brushless, permanent magnet. Often times the application will determine which motor is best suited for that particular application. 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. A self-powered hub-wheel drive from ABM Drives Suzhou Co. Ltd. features a wheel with tire, ac motor, gearbox and brake with a power rating of 0.6 kW.
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Gear type – 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. Other factors – these can include environmental factors such as temperature, IP rating for washdown or debris or spray, as well as mounting configurations, lubrication needs, and the type of output shaft needed.
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Often tossed in with the “other” factors are gearmotor accessories. Although they may not be the first thing most people look to when selecting a motor for an application, they are important considerations. And inevitably, accessories will come up at some point during gearmotor installation, and that’s when some tiny seemingly trivial detail can become central to whether or not a gearmotor works or doesn’t in a given setup. To avoid this fate, here’s a look at three of the most common types of accessories for gearmotors. Shaft kits: A shaft kit can contain a number of devices to achieve various aims. For example, shaft kits can sometimes contain seals for sealing the shaft from outside contaminants, depending on what type of environment it is used in. So for instance, seals may protect the gearmotor from fluids like oil or water, or particulate pollution like dust or other fine particles. Another common accessory is an adapter sleeve. This is used to increase the shaft diameter, if that’s what’s needed for a particular application and fit. A common alteration may have an adapter sleeve turn a 1/2-in. shaft into a 5/8-in. shaft.
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BOD designWorld 2018_03-drives Bodine — PT Guide 5-18.indd 53 innovation-6.indd 1
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Another common accessory is a shaft grounding kit. NEMA recommends shaft grounding as a way to protect motor bearings for electric motors that run on inverter power. Face adapter: Face plates refer to fixed mechanical mounting standards for motors used in America. The most common type of mount is the C-face. The mounting holes in C-face mounts are threaded and can accept bolts. Usually these are used to attach something like a gear reducer to the motor. Contrast this with a D-face mounting plate, which does not have threaded holes. Mounting bases: Gearmotor mounting bases are typically correlated to motor frame sizes, where different mounting bases are available for different motor frame sizes. The variety here is fairly extensive and there are a number of different possibilities depending on a number of factors such as how and where the motor must be mounted.
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Did you mean: DieQua Corporation?
The search is over! The Tandler high precision right angle servo gearheads provide unmatched performance, durability, and design flexibility. Look no further than DieQua. • Backlash to ≤ 3 arcmin • Torque capacities up to 1250 Nm • Single or DUAL output options include solid shaft, hollow bore, or shrink disk • Peak torque up to 2800 Nm To find what you’re looking for, check us out on the web, or give us a call.
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LEAD SCREWS
LEAD SCREWS; BASICS AND SIZING TIPS offers n Kerk Haydo d screw r e r u t a fac klash r its le manu ices fo nd anti-bac screw e Serv a d p Lead a y m t o le o t t s d u Pro achine e ding c Rapid s inclu igns, fully m fts and guid e li b m s a asse s. nut de line sh p m w s e e t l r s e c e sy lead s less st uided plex g s, stain e com screw r o m nd rails, a
A LEAD SCREW is a threaded rod or bar that translates rotational motion into linear motion. Lead screws generate sliding rather than rolling friction between a nut and the screw. This generates higher friction and means a lower overall efficiency, defined as the ability to convert torque to thrust while minimizing mechanical losses. A lot of motion designs incorporate lead screws to drive axes on a range of machines. They usually sport higher ratings than comparable ball screws thanks to more contact between the nut and screw load surfaces. And recent innovations in lead screw design have reduced friction, in some cases to better than 0.10, making them an attractive option in some applications. When it comes to sizing a lead screw, that’s a bit of a hybrid exercise. There are similarities to the process of ball screw sizing, but incorporating some of the considerations that come with sizing a plain bearing. While many factors are involved in the selection of a specific lead screw assembly, lead screw sizing — choosing the right diameter and lead for the application — can be done in four simple steps.
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leadscrews — Power Transmission HB 05.18 V2 FINAL.indd 55
1. Determine the axial load Lead screws are often used in linear actuators and positioning equipment to provide thrust (axial) force to drive a load. The amount of axial force that a lead screw assembly can withstand is determined by the diameter and lead of the screw and the material of the nut — plastic or bronze. Plastic lead screw nuts are often self-lubricating and are less sensitive to chemicals and other contaminants, but they have significantly less load capacity than bronze versions. If the application requires high axial loads (thrust forces), it may be necessary to use a lead screw assembly with a bronze nut. The application’s maximum axial load must also be checked against the load that can be supported by the screw shaft, commonly referred to as the buckling strength or column strength. In horizontal applications, where the load is supported by a low-friction linear guide, the weight of the moved load contributes only a small amount to the axial load on the screw. But in vertical applications, where gravity causes the screw
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shaft to see the full force of the load, buckling can become a significant factor in lead screw sizing. Calculate the maximum buckling load using this equation:
Fb = fb
dr4 Lc
2
x 104 (N)
P =
Fb = maximum compressive (buckling) load (N) fb = factor based on end support bearings dr = root diameter of screw (mm) Lc = unsupported length of screw (mm) 2. Check the critical speed Like ball screws, lead screws have an inherent maximum rotational speed, which is referred to as the critical speed. The critical speed is based on the natural frequency of the screw, and if exceeded, severe vibrations can develop and damage the screw assembly. The determining factors for the critical speed of a screw shaft are its length, diameter, and end fixity. (Note that mounting orientation — vertical or horizontal — does not affect critical speed.) While the length of the screw is typically set by the application requirements, the critical speed for a given screw length can be increased by choosing a larger diameter screw or by using a more rigid end bearing arrangement. The critical speed equation is:
nc = f c
dr Lc2
x 107(min-1)
nc = critical speed (rpm) fc = factor based on end support bearings dr = root diameter of screw (mm) Lc = unsupported length of screw (mm) 3. Calculate the PV value Lead screw assemblies are the drive versions of plain bearing guides, with heat generation due to sliding friction being a critical factor in their operation. For lead screw assemblies with plastic nuts, in addition to checking the axial load and critical speed, the pressure-velocity (PV) value of the application needs to be compared to the PV rating of the nut. If the PV rating is exceeded, excessive heat generation can cause premature wear and failure. In the PV equation, pressure equals the axial load on the nut divided by the contact area between the nut and the screw. Velocity is the relative velocity between the nut and
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the screw, which depends on the helix length of the screw (making it slightly different from the linear velocity based on the screw lead). High loads — which result in more pressure — can limit the screw’s permissible speed (velocity), and high speeds can limit the screw’s available load capacity.
5 • 2018
FA A
P = pressure between screw and nut (MPa) FA = axial load on nut (N) A = area of contact (m2)
V = lhr · rpm V= linear velocity between screw and nut (m/s) lhr = helix length per screw revolution (m) rpm = required rotational speed of screw Note that both the pressure and velocity values must be within the maximum allowable limits, even if, when combined, they provide an acceptable PV value. 4. Consider back driving Although back driving is only applicable in vertical applications, it’s sometimes a key factor in the decision of whether to use a lead screw or a ball screw. This is because a lead screw’s relative inefficiency, when compared with a ball screw, makes it much more difficult for a load to back drive. This inherent inefficiency can help prevent the load from falling and “crashing” if power to the motor is lost. While the lead screw’s resistance to back driving should not be relied on as the only safety measure, using a screw that is difficult to back drive provides a safeguard that can supplement a proper safety mechanism, such as a failsafe brake. The equation to calculate back driving torque is:
Tb =
(F · P · ɳ2) 2π
Tb = back driving torque (Nm) F = axial load (N) P = screw lead (m) η2 = reverse efficiency* *Efficiency when back driving is typically less than the efficiency for normal operation. Be sure to check the manufacturer’s specification for back driving efficiency.
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HOW TO CHOOSE BETWEEN
This doubleacting RTC-SB rodless cylinder from Aventics includes an oval piston shape and lubrication-free slide bearing for speeds to 21.3 ft/sec over strokes to 21.6 ft.
ROUND SHAFT OR PROFILED RAIL
WHEN DESIGNING A LINEAR MOTION SYSTEM, engineers have two primary choices in recirculating linear guides — round shaft or profiled rail. Choosing the wrong linear motion system can be a costly mistake, resulting in design or structural changes, poor machine performance, or oversized components with higher costs. Although both round shafts and profiled rails may appear to be suitable options for most applications, there are typically a few criteria that dictate which technology is more appropriate. Round shaft guides were invented in the 1940s and have been applied successfully in virtually every industry since their inception. But their limitations in rigidity forced manufacturers of precision equipment, such as machine tools, to use machined ways or cross rollers in order to achieve the required load capacities and accuracies. The introduction of profiled rail guides in the 1970’s offered a less-costly and less timeconsuming alternative, providing high load capacity |and high rigidity in a fairly compact envelope.
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But for designers, the decision regarding when to use round shafts became less clear, since profiled rail guides were suitable for many of the applications that had been the province of round shafts. As a result, decisions were, and sometimes still are, based on past successes (or failures) with a given technology. Fortunately, there are some key performance criteria that can guide the choice between round shafts and profiled rails and take the guesswork out of the initial selection. Load capacity — advantages of profiled rail With conformity between the balls and raceways, profiled rail systems have a larger contact area, and thus a higher load carrying capacity for a given size, than round shaft systems. Profiled rails are also better suited for moment loads than round shafts are, and typically have equal load capacities in all four directions. Conversely, the load capacity for round shafts depends on the direction of loading, which is the orientation of the load to the ball bushing.
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GAM — PT Guide 5-18.indd 59
© 2018 GAM. ALL RIGHTS RESERVED
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Rigidity — advantages of profiled rail The larger contact area between the balls and raceways yields less deflection for a profiled rail system than for a round shaft. And while round shaft guides can be lightly preloaded, profiled rail systems are often supplied with preload ranging from 2 to 8% which provides further rigidity to the guide system. Accuracy — advantages of profiled rail With ground raceways and reference edges, profiled rails commonly achieve travel accuracies that are an order of magnitude better than round shaft guides. In these criteria, round shafts are more commonly valued for their ability to handle inaccuracies (self-aligning) than for their travel accuracy. Speed — advantages of profiled rail Round shaft guides can generally achieve a maximum speed of 2 m/sec being limited by the ability to control the balls as they move in and out of the load zone. Profiled rail bearings, with a more sophisticated recirculation method, can reach speeds of 5 m/sec. Mounting — advantages of round shaft Where profiled rail guides must be fully supported and mounted along their length, round shaft guides can be supported only on their ends, for lengths up to 20 times the shaft diameter. Round shafts also do not require machined surfaces for mounting, since ball bushings inherently compensate for some misalignment, reducing cost and time for designing and preparing mounting surfaces. drylin W turn-tofit adjustable carriage (image courtesy igus)
60
Harsh environments — advantages of round shaft Round shafts are generally less sensitive to debris than profiled rails and are available in a variety of materials, coatings, and sealing options to withstand caustic or abrasive contamination. These range from all steel ball bushings to assemblies composed of stainless steel shafts with plastic bushings.
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Profiled rails often offer higher load capacity, rigidity, and accuracy. Round shafts as the ones shown here are often easier to integrate, more rugged, more forgiving of installation challenges, and lower maintenance. Image courtesy Lee Linear
Maintenance — advantages of round shaft Due to the smaller contact area between the load carrying balls and the running tracks, round shafts have less demanding lubrication requirements than profiled rails. Round shafts and ball bushings are also one of the few linear motion components that are, for the most part, interchangeable between manufacturers, which means that replacements are more readily available. More details on sliding-contact rail geometries One feature of sliding carriage-and-rail setups is that manufacturers typically incorporate a ground groove in a rectangular track’s geometry (to serve as a working surface). Manufacturers typically build these rails in one of three shapes. Rails with a boxway shape or square shape are simplest. Square rails excel at carrying large loads without a lot of deflection. Manufacturers often preload square rails, and most linear systems based on square rails do not self-align. Square rails often have a smaller envelope size; the boxway rails handle the highest loads in all directions.
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Rails with a dovetail shape (or twin rail) have male geometry that securely engages female saddle geometry. That boosts stability and load capacity, even in unusual orientations or applications with unsteady loads. Round rails deflect less under load. In addition, systems based on round rails are inherently self-aligning, so are easier to install than other options. No matter the type, rails come in myriad sizes and lengths. More details on rolling-contact options Rolling-element linear systems need little force to initiate motion. In addition, friction-force variations due to speed are minimal, so these systems can position loads with small and precise steps. The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help machinery maintain a level accuracy for much of the linear system’s operating life. Manufacturers produce rolling-contact guides in several variations. The differences are in rolling element shape (ball or roller); rolling element size; whether the rolling contact is two or four-point; conformity of ball contact; whether the design has two, four, six or some other number of rolling-element rows; contact angle; and how the rolling-element rows are arranged—in an X or O configuration. All these design factors determine load capacity, rigidity and friction. For example, O-shaped arrangements can withstand higher torque than X arrangements. In general, the number of load-bearing rollingelement rows influences load capacity … so more rail rows means more load capacity and rigidity. However, more rows make systems more complex and costly. Here are more details on these rolling-contact options: Rolling elements are either linear rollers or balls. Because the rolling elements recirculate in recirculating rolling-element guides, they have a nearly infinite stroke length. They are
available on flat guide ways and guide way rails. Flat guide ways are available in single or double row rolling elements. Guide way rails are often square rails. Non-recirculating roller type units have limited stroke length. Flat guide ways are dominant here and have either a grooved race compatible with crossed rollers, or non-grooved race, which uses cage and roller-type rolling elements. Recirculating elements (ball or roller bearings) between the rail and the bearing block enable precise linear motion. The coefficient of friction with roller-element-based systems is much less than with slide based linear motion guides … about 1/50th that of non-recirculating systems. Ball-type rolling element units are also subdivided into recirculating and non-recirculating types. The flat guide ways here typically use double row recirculating rolling elements. The guide way rail can be either round or square. If the raceway is not grooved, the rolling element is typically a linear ball bushing. If the raceway is grooved, the unit usually uses a ball spline. For square rails, the raceway is usually grooved. For ball-type rolling element units that are non-recirculating, the flat guide ways are grooved and use linear ball guides. The guide ways are round rail, without a grooved raceway, and use stroke bearings.
Long-stroke LoPro linear actuators from Bishop-Wisecarver Corp. are typically 3 to 8 m long, but units to 15 m are possible. The actuators are designed around DualVee guide wheel technology for smooth and quiet motion even over long lengths.
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POWER TRANSMISSION R E F E R E N C E
G U I D E
RETAINING RINGS: THE BASICS Smalley is able to manufacture a wide range of retaining rings from a variety of materials, including carbon steel, titanium, and a number of alloys.
AS WITH OTHER JOINING HARDWARE (such as cotter pins, screws, and bolts) retaining rings prevent mating components from excessive moving. In short, they create a removable shoulder preventing components from migrating out of proper position during operation. Retaining rings are thin, circular, metal components that can be either stamped from a sheet or coiled from wire. Retaining rings are designed to fit into a machined groove either on the inside of a bore or on the outside of a shaft. These components reduce vibration, retain two parts of an assembly, and can withstand axial loading. They are used in assemblies as a cost-saving solution and to avoid machining a shoulder onto a mating component. There are many forms of retaining rings including stamped, spiral, constant section, hoop, e-clip, beveled, and wave-ring forms. They’re also available in a diverse range of materials with the type of material dictated by the application environment. When the ring is not exposed to corrosive elements, carbon steel is typically the standard material used. Retaining rings are available in a range of stainless steels if the ring is destined for a corrosive environment. With high-temperature environments, an exotic alloy may be required. These
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alloys include Elgiloy, Hastiloy, and Inconel. Retaining rings can also undergo different types of plating or receive different coatings to withstand harsh environments. Common finishes include black oxide, cadmium plating, oil dip, passivation, zinc phosphate, vapor degrease (cleaning), and vibrator or hand deburr. Retaining rings can be customized to application requirements. Traditionally, rings produced from coiling are easier to customize. Often only a simple setup is required (with no tooling) to produce a specific ring. If the application requires a specific end type or configuration, this is easier to produce from a coiled ring than from a stamped ring. Custom rings can be manufactured in sizes from 0.157 in. (or 4 mm) to 120 in. or 3,000 mm in diameter. A custom stamped ring would require an additional set up and die. When choosing a retaining ring, consider the kind of assembly that it will be in; is it a housing or a shaft? Also, what are the critical dimensions such as the diameter of the shaft or housing as well as the grooves where the ring will fit? What is the speed and thrust force of the assembly? Lastly, consider environmental factors such as temperature variations and any corrosive materials present, which will help to determine the proper retaining ring material for the application.
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ELECTRIC MOTOR OVERVIEW ELECTRIC MOTORS are one of the prime sources of motive power in motion systems. All electric motors convert electrical energy into mechanical energy used to power machines and systems. Motors are usually divided into ac and dc, and rotary and linear as well. They can also be classified according to their commutation method; either self commutated or external, and further by mechanical means (brushes in dc motors) or electronically commutated (as in brushless dc motors.) Electric motors are usually divided into either ac – alternating current, or dc – direct current. The two main types of ac motors are synchronous and asynchronous or induction motors. As for dc motors, the most common industry naming conventions recognize three subtypes: brush motors, permanent-magnet (PM) motors, and universal motors. In motion control applications, one of the most common types is the servomotor. The hallmark of any servomotor is the presence of feedback for closed-loop control. Closed-loop control gives servomotors precise positioning ability by greatly reducing error. That means they can accommodate complex motion patterns and profiles more readily. They offer precise control of torque and speed and they can also operate at zero speed while maintaining enough torque to maintain a load in a given position. Another common motor for motion control is the stepper motor. They’re used mostly in positioning applications and have the advantage of being able to be accurately controlled for the most precise positioning applications, down to fractions of a degree without the use of feedback devices such as encoders or resolvers. They operate open loop without the need for tuning parameters as in closed-loop servo systems. In many linear motion applications, linear motors are a suitable option. One way to think of a linear motor is essentially a rotary motor that has been unrolled, so that
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instead of producing torque via rotation it generates a straight-line force along its length. Like many rotary motors, linear motors consist of a coil (primary part or forcer) and magnets (secondary part). Although there are many types of linear motors, brushless ironcore and ironless designs prevail in automation and positioning applications. Linear motors offer several advantages over belts, screws, and other drive mechanisms, including almost unlimited lengths, low maintenance, and higher accuracy and repeatability. There are no mechanical-transmission components – such as pulleys, couplings, or gearboxes – to introduce elasticity and backlash. The system’s accuracy and The EC 60 Flat repeatability are determined Frameless Brushless by the controls and do not Servo Motor Kit from maxon degrade over time. The consists of only a rotor and stator, with no bearings or motor lack of rotating or sliding shaft. The motors are electronically components also means commutated, feature a 60-mm linear motors are almost diameter, and the hollow-shaft maintenance-free, with construction makes them only the support bearings, suitable for direct integration into a variety of machines or linear guides, requiring and robots. periodic maintenance.
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5/15/18 4:06 PM
SEALS
BASICS OF
and
SEALS
SEALS PERFORM A VITAL JOB in any power transmission system—they keep dirt and other ingress materials from entering and damaging critical, internal components. They also have the equally important job of preventing leakage of necessary lubricants, such as oil, grease or hydraulic fluid. 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 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 that is used in 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.
SEAL DELIVER
While many seals are designed primarily to keep 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. 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 noncontact 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.
Don’t Get Caught In a Labyrinth of Bad Seals Our Patented Centrifugal Pressure Seals: ✓ Keep lubricants in and contaminants out ✓ Reduce down time ✓ Support horizontal and vertical applications ✓ Prevent friction and overheating with a non-contact design ✓ Create a dynamic pressurized barrier ✓ Support extremely low-viscosity fluids
Centritec Seals are centrifugal designs that use a rotating chamber within the seal to create a pressurized barrier as the lubricating fluid exits the chamber. This pressurized barrier keeps the lubricant in, and when used as a double seal, keeps the contaminants out.
A Carlyle Johnson Company
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POWER TRANSMISSION R E F E R E N C E
G U I D E
Several components exist to damp vibration and reduce shocks in motion systems. Here is a look at some of ACE Controls’ offerings, including its tow bar snubber, industrial gas spring, and hydraulic damper products.
SHOCK ABSORBERS & VIBRATION DAMPING WHERE THERE IS MOVEMENT, there will be shock and vibration. In any given industrial automation system, damage and fatigue can be caused by stopping a motion system or changing its direction. Preventing these damaging motions is done via shock and vibration attenuating components that are used to decelerate a load smoothly and precisely. The most common types of shock and vibration attenuating components are shock absorbers, linear dampers, wire rope or spring isolators, elastomeric isolators, air springs, and structural damping materials. These devices help manufacturers reduce equipment downtime and costly cycle time limitations. Shock absorbers and dampers provide smooth deceleration of a given payload. A pneumatic or hydraulic shock absorber will use fluid or gas power for deceleration, and a spring to return the piston to its initial position. Linear dampers (also known as a velocity controller)—whether hydraulic or pneumatic—are used when a load is in constant
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contact with the damper and the operator wants a smooth deceleration in either the compression or tension direction. Here, the load is in contact with the damper when the deceleration starts. That is, there is no impact of the damper by the load. That is the key difference between a damper and a shock absorber. Most shock absorbers achieve their damping characteristics through the use of hydraulic fluids. The fluid is pushed by a piston and rod through small orifice holes to create damping, 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 high-strength 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.
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5/15/18 3:16 PM
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· Online sizing tool & calculator · One-on-one support from an application expert
800-521-3320 | www.acecontrols.com ACE Controls -- Motion Systems Handbook 8.17.indd 69 DesignWorld_ACEControls_find_a_solution_fast_FullPage_040417.indd 1
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POWER TRANSMISSION R E F E R E N C E
G U I D E
Vibration isolation products rely generally on mechanical designs to achieve their isolation characteristics. A spring function provides support for the mounted equipment, while decoupling it from the vibration source. Friction and elastomeric material properties give the isolators their damping characteristics. Isolators can be made from a variety of materials. Wire rope and spring isolators can be made from carbon steel, stainless steel or aluminum. Elastomeric isolators generally have metallic components that function as mounting brackets, separated by an elastomeric material that provides the stiffness and damping desired. Common elastomeric compounds include natural rubber, neoprene and silicone; however, a vast selection of compounds and compound blends can be used to achieve different characteristics specific to the application. Air springs are comprised of metallic end fittings coupled by a composite elastomeric-based bladder that contains the compressed air used to provide isolation. These single-acting designs are comprised of a pressurized bladder and two end plates. As air is directed into the air bladders, they are expanded linearly. Energy or power dissipation is key when selecting a damper or shock-absorbing device. The size and characteristics of the device are based on these inputs, so it is generally the first consideration to make. Dynamic spring rate and damping are the two biggest considerations when selecting an isolator. These characteristics will define the natural frequency (sometimes
Wire rope isolators, like these from ITT Enidine, help to reduce the fatigue and damage caused by system vibration.
IMPACT CAN DESTROY AN OBJECT OR A SINGLE ELEMENT OF THAT OBJECT
A PRIMER ON DA M P I N G
INNOVATING SHOCK & VIBRATION SOLUTIONS Visit sorbothane.com for Design Guide and Technical Data
Damping—whether done by hydraulic shock absorbers, wire rope isolators, or damping materials—is simply the reduction or dissipation of vibration and motion in a system. However, the type of damping involved is defined by the device used to restrain the vibrations. Viscous damping is achieved with the hydraulic fluids in shock absorbers. Here, the piston moves, forcing oil through the device to control resistance. Instead of the energy resulting in damaging shock, it is converted into heat. Coulumb damping is achieved with wire rope isolators, which provide stiffer damping through thicker or larger wire diameters.
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Hysteretic damping is achieved through the use of elastomers, which reduce the effects of vibrations and impacts.
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S H O C K A B S O R B E R S & V I B R AT I O N DA M P I N G
referred to as resonant frequency) of the isolation system and are important in achieving the desired performance. Elastomer and other synthetic and rubber pads can also damp vibration and isolate shock loads. They are available in a number of shapes, including tubes, bushings, blocks, pads and washers. These components are commonly used in heavy-duty applications to create strong cushioning plates or foundations in heavy machinery such as cranes, presses, and also for vibration reduction in lab and testing equipment, aerospace, and for pipelines and bridges. The rubber-like materials with which they are designed allow these padding materials to meet specific requirements, such as natural frequency, load, and area. And because they are soft, they are forgiving in most environments. 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 (69 N/mm2) depending on plate form and size. Another custom product called Sorbothane (from a company with the same name) is a thermoset that attenuates shock with near-faultless memory. That means its 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 damping, acoustic damping and isolation. Sorbothane works by turning 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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Like all its elastomeric materials, these water-resistant Sorbothane shock absorbing and vibration isolating materials attenuate shock and control vibration while being impervious to moisture.
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AD INDEX
POWER TRANSMISSION REFERENCE GUIDE
ACE Controls Inc. ..................................................... 69 All Motion ......................................................................4 Altra Industrial Motion Corp. . ................................ 33 Applied Motion Products . .........................................3 AutomationDirect ........................................................1 Baldor .................................................................... 7, BC Bison Gear & Engineering Corp. .........................IBC Bodine Electric Company ...................................... 53 C-Flex Bearing Co, Inc. ........................................... 40 Carlyle Johnson ....................................................... 32 Centritec Seals ......................................................... 67 CGI Motion . ................................................................ 49 CMT ............................................................................. 26 ContiTech .................................................................. 28 Del-tron Precision, Inc. ..............................................9 DIEQUA Corporation ....................................... 52, 54 Encoder Products Company . ................................ 45 Festo. ............................................................................ 13 Fluid Lines Products ................................................. 15 GAM ............................................................................ 59 Harmonic Drive . ........................................................ 47 igus . ............................................................................. 63
IKO International, Inc. .. ............................................. 61 Intech .. ........................................................................ 50 ITT Enidine .. ................................................................ 71 Lin Engineering .. ......................................................... 11 Mach III Clutch Inc. ................................................... 35 NSK Precision ............................................................ 17 PBC Linear .. ............................................................... 57 PHD Inc. .. ..................................................................... 14 Pyramid Inc ............................................................... 30 QBC Quality Bearings & Components ................ 23 R+W America .. ........................................................... 41 Rotor Clip Company, Inc. .. ...................................... 65 Ruland Manufacturing .. ........................................... 29 SDP/SI-Stock Drive Product ................................. 27 Serapid Inc. ................................................................ 19 Servometer ............................................................... 39 SEW Eurodrive ......................................................... 43 Sorbothane ............................................................... 70 THK America, Inc. ................................................... IFC US Tsubaki ................................................................ 37 Zero-Max, Inc. .. ............................................................2
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— Baldor-Reliance® motors Local manufacturing Global support
For more than 100 years, we’ve set out to do things better. And that’s still our goal today. Every day we produce the AC, DC and variable speed motors you trust and prefer from Fort Smith, Ozark and Clarksville, Arkansas; Westville, Oklahoma; Columbus, Mississippi; Athens and Gainesville, Georgia; and Kings Mountain, North Carolina. We are proud to continue to offer the same products and service you prefer with the global ABB technologies and innovation you deserve.
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