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POWER TRANSMISSION REFERENCE GUIDE
Power transmission components
— the mainstays of motion
M
otion designs continually evolve but will always rely on mechanical devices … particularly where the drive of an electric motor engages a load to execute machine tasks. In fact, as the technical features in this 2019 Power Transmission Reference Guide explain, applications for mechanical motion components continue to proliferate as innovations make them increasingly effective. There’s certainly increased demand for complete system solutions over components, which is changing the design of linear systems, actuators, and gearmotors … as well as subsystems such as conveyors and robotics. Consider the section on gearmotors in this Reference Guide. Here, manufacturers are predesigning more motors than ever with gearboxes. Such gearmotors are increasingly accurate as well … particularly those sporting planetary gearsets. That’s thanks in part to how manufacturers are making gearing with the latest approaches in design, machining, and assembly. Check out the sections in this Reference Guide covering gears — including custom gear design, strainwave gearing, and planetary and other shaftmount sets. These articles detail how leading gear designs work. Strain-wave gearing in particular continues to see expanded use in robotics applications. But even for other applications, software now helps engineers get design-specific gearing (and other power-transmission components) at lower cost than that of general-purpose offerings from just a decade ago. In fact, today’s moving designs rely on an increasingly diverse array of mechanical components to protect expensive subsystems and change motion-system dynamics to simplify programming. These actuators, ballscrews, bearings, brakes, chains, collars, couplings, and linear slides and guides transmit power in ways that get higher performance than ever. So use this Reference Guide as a review of basic component functions or as an update on what’s new in power-transmission designs … and to get instructions on how to make the
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most of proliferating features to meet evolving motion-system requirements. Also count on us Design World editors to bring you technology updates and help you specify and integrate the right components. We invite your feedback and requests for technical information. Email me at leitel@ wtwhmedia.com or tweet to @DW_LisaEitel, @ Linear_Motion and @Motion_Control. Connect with our Design World Network Facebook page at facebook.com/DesignWorldNetwork and let us know what designs you’re using or are looking to apply. By the way ... another brand-new option to get Design World’s technical information on power-transmission components organized in comprehensive libraries is to visit www.designworldonline.com/MC2. MC2 — short for Motion Control Classroom — is a new online reference series for design engineers needing information about motion components and systems. Curated by Design World’s motion editorial team, each installment is a digital content hub with comprehensive background information, trends, typical and emerging applications, and FAQs on one motion technology. The first May 2019 installment details linear guides and slide systems; check it out at the above URL and see what you think. Also look out for the 2019 Motion Systems Handbook and 2019 Motion Casebook coming to you in August and November for complete coverage of electronic and programming technologies for motion designs, as well as real-world application examples and illustrations to inform your next build. Otherwise, the latest motion-technology announcements (as well as technical archives) can be found on our motion tips sites — motioncontroltips.com, linearmotiontips.com, sensortips.com, bearingtips.com, and couplingtips.com.
Lisa Eitel | Motion editor
5 • 2019
5/15/19 1:04 PM
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POWER TRANSMISSION REFERENCE GUIDE
The importance
of the patent search There’s value in searches led by attorneys capable of analyzing results ... and guiding inventors before patent-application filings.
T
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By George Likourezos Patent attorney • Partner | Carter, DeLuca & Farrell LLP echnologists and thinkers can have eureka moments at any time: They see a problem and instead of just accepting that problem, they create a solution. Such solutions can be a new product idea or a redesign of an existing product. In other words, an invention is born. One of the next steps is for the inventor to meet with a patent attorney. Most inventors are indeed eager to get a patent application prepared and filed. However, especially now that all major patent jurisdictions (including the U.S.) award patents to the inventor who is first-to-file, we advise that a patent search be performed at the outset. Here are the five main reasons why it’s important to perform such a search.
The Home Depot Test is not realistic
It is not uncommon for an inventor to believe he was the first to think of his new product idea or improvement to an existing product. An inventor may accept anecdotal evidence that if the inventive product is not being sold by Home Depot (or any other brick-and-mortar or online store) — a.k.a. the “Home Depot Test” — he is the first person who ever thought of the new product idea or improvement. The Home Depot Test should not be relied upon, because only a small percentage of inventions described by issued patents and pending patent applications are manufactured and commercialized. Every week in the U.S., thousands of patents are issued and thousands more patent applications are published. Only a sliver of the inventions described by these patents and patent applications will see the light of day. Therefore, a patent search will more often than not reveal many patents and published patent applications (commonly referred to as prior art references) describing inventions that are similar (or even identical) to the inventor’s invention.
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Time and cost savings
Patent searches are much less expensive than preparing and filing a patent application only to trigger prosecuting the same in the U.S. Patent and Trademark Office (USPTO) and in foreign patent offices. Therefore, it is better to find out early on from your patent attorney that the patent search revealed one or more similar or identical inventions. Better avoided is learning that same information from a USPTO examiner two to four years later — after having spent time and money preparing and filing a patent application … and possibly having received money from investors … or having spent money manufacturing a product that has nothing proprietary.
Analysis of the patent search results
The information gleaned from a patent attorney’s analysis of the patent search results can provide guidance to the inventor for improving or redesigning his invention. In particular, if a patent is identified during the patent search that describes a similar or identical invention, the inventor can improve or redesign his invention in a manner that is non-obvious over the prior inventions. That is, the further improvement or redesign of the original invention may be patentable over the identified patent or patents, even though the original invention may not be patentable. For example, assume patents and published patent applications identified during the patent search describe the use of a mechanical gear mechanism to make an adjacent component move in a circular motion. If the invention is redesigned to include an electromagnetic mechanism to make the adjacent component move in a lateral motion, the USPTO may find the redesigned invention patentable over prior inventions.
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INDUSTRY INSIGHTS
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Gauging the scope of eventual patent claims
The patent search results also enable a patent attorney to gauge how broad the inventor’s patent claims are likely to be. The broader the patent claims, the more likelihood the patent will be found to cover a competing product that was designed to thwart the patent claims. Conversely, the narrower the patent claims, the less likelihood the patent can be used to stop a competitor or enter into a licensing agreement with the competitor. Therefore, if the latter, the inventor may decide to forgo the preparation and filing of a patent application and think of investing his resources in another invention where broader claims are more likely possible.
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The prompting of additional inventive features
If the patent search results indicate there are many patents and pending patent applications covering the overall inventive product idea or improvement, an inventor may be able to steer the patent prosecution process to his advantage if he thinks of additional inventive features regarding his invention. That is, if the patent search indicates the inventive product is known, the inventor can think of additional inventive features to increase the likelihood of being granted a patent. These additional inventive features can be included in the patent application before it is filed. Of course, once the patent application is filed, additional inventive features cannot be added to the patent application.
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5 • 2019
DESIGN WORLD — MOTION
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MDX Integrated Servo Motors Applied Motion Products is expanding our line of integrated motors with the addition of our first-ever integrated motors built on traditional 8-pole brushless servo motors. Using our innovative approach to integrated motors, developed over time with our integrated stepper and StepSERVO Integrated Motor products, we now introduce MDX Integrated Servo Motors for higher speed applications. Make it Move.
• High torque density, low inertia motors • Integrated drive/controller • 200 or 400 Watts continuous output power • Compact 60 mm frame size • IP65 or IP20 environmental rating • Built-in EtherNet/IP, CANopen, Modbus, or SCL over Ethernet or RS-485 • Dual-port communications for easy daisy-chain connections • Q Programmer for stored program execution and stand-alone operation
800-525-1609 www.applied-motion.com sales@applied-motion.com
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CONTENTS VO LU M E 5 N U M B E R 2
POWER TRANSMISSION REFERENCE GUIDE
Editorial........................................................ 2
Commentary on patent searches................. 4
Ballscrews..................................................... 10
Bearings........................................................ 12
Belts & pulleys.............................................. 15
Cam followers ............................................. 18
Clutches & brakes........................................ 20
Couplings..................................................... 22
Encoders....................................................... 26
Gearing......................................................... 28
Gearmotors.................................................. 33
Leadscrews................................................... 34
Linear slides, rails & guides.......................... 38
Motors.......................................................... 42
Sensors & transducers.................................. 46
Shock & vibration mitigation........................ 48
Wave springs................................................ 53
Cover image courtesy iStockphoto.com
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Power Transmission, Motion Control and Engine Braking Solutions
Creating A Premier Industrial Company Ameridrives
Deltran
Kilian
Portescap
Twiflex
Bauer Gear Motor
Formsprag Clutch
Kollmorgen
Stieber
Warner Electric
Bibby Turboflex
Guardian Couplings
Lamiflex Couplings
Stromag
Warner Linear
Boston Gear
Huco
Marland Clutch
Svendborg Brakes
Wichita Clutch
Delevan
Jacobs Vehicle Systems
Matrix
TB Wood’s
Nuttall Gear
Thomson
Delroyd Worm Gear
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POWER TRANSMISSION REFERENCE GUIDE
The straight story on
ball screw standards
B
all screws are a mainstay of many motion systems. They differ from other types of screws by using balls that roll between a nut, the screw, and helical grooves in the screw, with the balls in effect carrying the load. Ball screws commonly have higher load capacities than comparable lead screws. They’re also a better choice for uses requiring high accuracy. And because ball screws operate on point contact to transmit load, versus sliding elements that are line contact, they inherently have less friction and thus are more efficient, typically to 90% or better. There are several different standards that govern various aspects of ball screw design, from lead accuracy and load capacity to ball nut tolerances and rigidity. However, in many cases these standards are in harmony and provide the same (or virtually equivalent) specifications.
But in the areas where the standards diverge and the specifications differ, it can be difficult for engineers and designers to compare products and choose the ball screw that meets their design and application requirements. So until the industry comes together and adopts a single, international standard, here’s a guide to the similarities and differences between the most commonly used ball screw standards: DIN, ISO, and JIS. DIN (German Institute for Standardization) ISO (International Organization for Standardization) Originally two separate standards, DIN 69051 and ISO 3408, the DIN and ISO ball screw standards have been mostly combined and harmonized. The harmonized standard is referred to as “DIN ISO 3408,” which means the ISO standard has been adopted directly as the DIN standard. The DIN ISO 3408 standard consists of four sections: DIN ISO 3408-1: Ball screws – Part 1: Vocabulary and designation DIN ISO 3408-3: Ball screws – Part 3: Acceptance conditions and acceptance tests DIN ISO 3408-4: Ball screws – Part 4: Static axial rigidity DIN ISO 3408-5: Ball screws – Part 5: Static and dynamic axial load ratings and operational life
Ball screws with nut cooling, shown here from NSK, help machine tool systems run faster and more precise. Piping is attached to the outer periphery of the nut flange, which eliminates the need for sliding seals and rotary joints. This design achieves a cooling capacity greater or equal to existing hollow-shaft ball screws.
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BALL SCREW STANDARDS
Notice that Part 2 of the ISO standard has not been adopted as a DIN standard, and each retains its own designation — i.e. DIN 69051 Part 2 and ISO 3408-2. In both standards, Part 2 defines nominal diameters and leads, so the variation between ISO and DIN means they allow for some differences in the available diameter and lead combinations (d0 x P). DIN 69051-2: Machine tools; ball screws; nominal diameters and nominal leads (1989) ISO 3408-2: Ball screws: Nominal diameters and nominal leads – Metric series (1991)
JIS (Japanese International Standard) Another common ball screw standard is JIS B1192-1997. (Note that JIS B1191 is sometimes referenced, but it was replaced by JIS B1192 in 1997.) JIS B1192-1997 is similar to DIN ISO 3408, but there are some noticeable differences. First, there are variations between the DIN ISO standard and JIS B1192-1997 regarding the travel deviation specifications ν300 and ν2π. In addition, JIS uses accuracy class designations of “C” for positioning screws and “Ct” for transport screws, whereas the DIN ISO standard uses the designations “P” for positioning screws and “T” for transport screws. The JIS B1192-1997 standard also includes several accuracy classes that DIN ISO 3408 doesn’t address. Other variations between JIS B1192-1997 and DIN ISO 3408 can be found in the dimensional tolerances and run-out specifications of the screw and nut, and in the permissible torque fluctuations of preloaded nuts. But these variations typically occur only in certain instances within a given specification — not across the entire range of products. For example, JIS B1192-1997 and DIN ISO 3408 specify different ν300 travel deviation limits for screws in accuracy classes 3 and 5, but they provide the same ν300 specification for class 7 screws. If you examine the specifications of a ball screw manufactured to the JIS B1192-1997 standard, you’ll likely notice that there are exceptions in the specifications, where the manufacturer has deviated from the JIS standard. In many cases, the manufacturer’s deviation provides a better tolerance or more stringent acceptance criteria. But it is a deviation from the standard, nonetheless. One ball screw expert explained it this way: “Where the DIN ISO 3408 standard is followed rigidly, manufacturers seem to take the JIS B1192-1997 standard as more of a suggestion.” The JIS B1192 standard discussed thus far is the 1997 publication (JIS B1192-1997), but it isn’t the most recent version. JIS B1192 was updated in 2013 (JIS B1192-2013) to conform with the ISO 3408-1, -2, and -3 specifications for ball screw definitions, nominal diameters and leads, and acceptance conditions. Another update was just published in August 2018 (JIS B1192-2018), putting the JIS standard in harmonization with the ISO 3408-4 and -5 specifications for axial rigidity and static and dynamic load ratings. Manufacturers of JIS ball screws still cite the 1997 version (JIS B1192-1997) in their specifications, but they will likely begin citing the more recent 2013 version (or even the 2018 version) in the nottoo-distant future, as they comb through the changes and make adjustments to specifications to be compliant with ISO 3408. 5 • 2019
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Because the variances between JIS B1192-1997 and ISO 3408 are generally quite small, the switch from the 1997 version to either of the updated JIS standards isn’t likely to be a major disruption to designers who use JIS-standard ball screws. The most noticeable difference will probably be in the form of dynamic load capacity changes. As one manufacturer of JIS-standard ball screws pointed out, the method for determining dynamic load capacity is the most significant difference between ISO 3408 and JIS B1192-1997. So it’s likely that the published dynamic load capacities of JIS-standard ball screws will undergo a change as manufacturers re-rate their products to meet the ISO specification. And while the changes in load ratings may catch some users off guard or cause a bit of confusion for a short time, as another ball screw manufacturer pointed out, “The ball screw standards were destined to come together eventually. Competition is hard enough, and the industry needs a single, international standard.”
Thanks to Michael Fuchsberger and Jason Winburn of Bosch Rexroth and to Dan Williams of NSK, for helping us understand the recent changes to the JIS B1192 standard and the potential implications for the ball screw industry.
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5/14/19 8:00 AM
POWER TRANSMISSION REFERENCE GUIDE
Rotary bearings
for power transmission
B
earings are internal machine components that are crucial to motion applications. They reduce friction between moving parts by giving a surface something on which to roll rather than slide. Rotary bearings consist of smooth rollers or metal balls and inner and outer surfaces (races) against which the rollers or balls travel. These rollers or balls carry load carrier and let axes spin freely. Bearings typically encounter radial and axial load. Radial loads are perpendicular to the shaft, and axial loads occur parallel to the shaft. Depending on the application, some bearings must withstand both loads simultaneously.
What’s the difference between ball and roller bearings? Because the contact area between balls and races is so small, ball bearings excel in light to moderate loads. The small areas of surface contact also minimize friction-generated heat, so 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 — so handle larger loads with-out deforming. Their shape also allows for a moderate amount of thrust load, as weight is distributed across cylinders instead of spheres.
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Dodge food-safe ball bearings withstand caustic and high-pressure washdowns with an IP69 rating without an end cover. That’s helpful in food and beverage applications requiring sanitary machinery. The bearings also include Hydro armor sealing with a stainless-steel flinger and four contact lip seals — to prevent water and contamination ingress.
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5/14/19 8:27 AM
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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Engineered Solutions for a World in Motion
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POWER TRANSMISSION REFERENCE GUIDE
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 extremely high radial loads. Usually a cage orients and contains the needle rollers. The outer race is sometimes machined 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).
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.
Tapered roller bearings Tapered roller bearings have tapered inner and outer ring raceways with tapered rollers between them — angled so the rollers’ surfaces converge at the bearing’s axis. These bearings are the only bearing type that can concurrently handle large amounts of axial and radial loads. Singlerow taper bearings only support high axial loads from one direction. However, installations that put one tapered roller bearing against a second counter-act 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 in relation to the outer ring — just a few minutes of
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arc at most. As with other roller bearings, tapered roller bearings must carry a minimum load, especially in high-speed applications where inertial forces and friction can damage rollers and raceways should they come out of contact.
How to pick the right lubricant for a rotary bearing? Lubrication of bearings with rolling elements takes the form of oil or grease; grease usually lasts longer, thanks to thickeners that sustain a layer between raceways and rolling elements. Grease with extreme-pressure additives also extends bearing life under higher forces. Even so, oil is more common for open bearings or those subject to low torque or high speeds. Oil’s lower viscosity imparts less drag than greases as rollers move through the lubricant. Mode of oil delivery, application rpm and temperature, and potential environmental contaminants dictate which oil is most suitable. Case in point: Operating temperature dictates which oil viscosity will work in a given application. Overly thick oil increases required torque to make the rotary bearing spin; overly thin oil won’t maintain the protective layer needed to prevent mental-on-metal contact.
Side note on plain bearings Plain bearings are cylindrical sleeves with an array of design elements to cater to specific applications. For more information, refer to the Motion Handbook section on plain bearings that precedes this one. Some plain bearings go into applications requiring slide plates for straight strokes. Other plain bearings do the same job as roller-based thrust bearings, but use pads arranged in a circle around the cylinder. The pads create wedgeshaped regions of oil to prevent hard contact with the rotating disc supporting the application thrust. Material innovations have made plastic plain bearings more useful than ever, though plain bearings of all types are lightweight and compact and can carry substantial load. Growing use of plain plastic bearings and increasingly stringent industry standards means these bearings must often meet FDA, RoHS, and other standards. Some even meet EU directive 10/2011/EC standards, which holds material manufacturing processes to certain criteria.
Applications for rotary bearings Bearings abound in industrial and consumer designs. For example, deep-groove ball bearings often go into in small to mediumsized electric motors because they can accommodate both high speeds and radial and axial loads. Self-aligning ball bearings, on the other hand, are work well in fans. These bearings have two rows of balls with a common raceway in the outer ring. The 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. 5 • 2019
These bearings are also common in gearboxes where they mount with a second bearing of the same type in a faceto-face or back-to-back orientation. They provide rigid shaft support to minimize deflection. This reduced shaft deflection minimizes gear backlash. Tapered bearings 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. motioncontroltips.com | designworldonline.com
5/14/19 8:27 AM
BELTS & PULLEYS
Belts and pulleys
for top efficiency
I
ndustrial belt drives consist of rubber belts that wrap around drive pulleys, in turn driven by electric motors. In a typical setup, the belt also wraps around one or more idler pulleys that keep the belt taut and on track. The main reasons that engineers pick belt drives over other options is that modern varieties require little if no maintenance; they’re less expensive than chain drives; and they’re quiet and efficient, even up to 95% or more. In addition, the tensile members of today’s belts — cords embedded into the belt rubber that carry most the belt load — are stronger than ever. Made of polyester, aramid, fiberglass or carbon fiber, these tensile cords make today’s belt drives thoroughly modern power-transmission devices.
motioncontroltips.com | designworldonline.com
Belts & Pulleys — Power Transmission HB 05.19 V3.indd 15
Manufacturers generally describe belts and pulleys with five main geometries. Pitch diameter is the drive-pulley diameter. Center distance is the distance between the two pulleys’ centers. Minimum wrap angle is a measure of how much the belt wraps around the smallest pulley. Belt length is how long the belt would be if cut and laid flat. Finally, in the case of toothed belts (also called synchronous belts) the pitch is the number of teeth per some length — so a 3-mm pitch means that the belt has one tooth every 3 mm.
5 • 2019
Pulleys and sprockets need tight tolerances to deliver top performance. SDP/SI Shaftloc shaft locks work on narrow pulleys for tight enclosures, as the former has engineered dimensions to transmit more torque than clamp or setscrew connections. It’s a mechanical component that fixes pulleys without marring the shaft— for easy repositioning and synchronizing rotating components.
DESIGN WORLD — MOTION
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POWER TRANSMISSION REFERENCE GUIDE V belt and synchronous belt geometry Synchronous belt uses positive engagement.
Timing belts (also known as synchronous belts or toothed belts) are most often used in transporting, indexing, and positioning applications where high torque or force transmission and high acceleration rates are required.
Curvilinear teeth
Unlike V-belts, which rely on friction between the belt and the pulley for power transmission, timing belts have positive engagement between the belt teeth and the pulley teeth, so the possibility of slip is eliminated.
Trapezoidal teeth V belt uses friction
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19Pyramid_4x475_lineShafOring.indd 1 Belts & Pulleys — Power Transmission HB 05.19 V3.indd 16
Some general guidelines are applicable to all timing belts, including miniature and double-sided belts. First, engineers should always design these belt drives with a sufficient safety factor — in other words, with ample reserve horsepower capacity. Tip: Take note of overload service factors. Belt ratings are generally only 1/15 of the belt’s ultimate strength. These ratings are set so the belt will deliver at least 3,000 hours of useful life if the end user properly installs and maintains it. The pulley diameter should never be smaller than the width of the belt. As mentioned, belts are quieter than other powertransmission drive options … but they’re not silent. Noise frequency increases proportionally with belt speed, and noise amplitude increases with belt tension. Most belt noise arises from the way in which belt teeth entering the pulleys at high speed repeatedly compresses the trapped pockets of air. Other noise arises from belt rubbing against the flange; in some cases, this happens when the shafts aren’t parallel. Pulleys are metal or plastic, and the most suitable depends on required precision, price, inertia, color, magnetic properties and the engineer’s preference based on experience. Plastic pulleys with metal inserts or metal hubs are a good compromise. Tip: Make at least one pulley in the belt drive adjustable to allow for belt installation and tensioning. Also note that in a properly designed belt drive, there should be a minimum of six teeth in mesh and at least 60° of belt wrap around the drive pulley. Other tips: Pretension belts with the proper recommended tension. This extends life and prevents belt ratcheting or tooth jumping. Align shafts and pulleys to prevent belt-tracking forces and
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DESIGN WORLD — MOTION
5 • 2019
12/17/2018 4:17:22 PM 5/15/19 9:25 AM
BELTS & PULLEYS Manufacturers of Power Transmission and Motion Control Components On this R.A Jones Autoprod CF-400 cup-filling machine, an endless belt drive replaces the traditional chain drive. That boosts precision while reducing maintenance and the number of adjustments needed to stations.
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belt edge wear. Don’t crimp belts beyond the smallest recommended pulley radius for that belt section. Select the appropriate belt for the design torque. Select the appropriate belt material for the environment (temperature, chemical, cleaning agents, oils and weather). Beltand-pulley systems are suitable for myriad environments, but some applications need special consideration. Topping this list are environmental factors. Dusty environments don’t generally present serious problems as long as particles are fine and dry. But particulate matter can act as an abrasive and accelerates belt and pulley wear. Debris should be prevented from falling into belt drives. Debris caught in the drive is generally either forced through the belt or makes the system stall. In either case, serious damage occurs to the belt and related drive hardware. Light and occasional contact with water — during occasional washdowns, for example — has little serious effect. However, prolonged contact with constant spray or submersion can significantly reduce tensile strength in fiberglass belts and make aramid belts break down and stretch out. In the same way, occasional contact with oils doesn’t damage synchronous belts. But
5 • 2019
Belts & Pulleys — Power Transmission HB 05.19 V3.indd 17
prolonged contact with oil or lubricants, either directly or airborne, significantly reduces belt service life. Lubricants cause the rubber compound to swell, break down internal adhesion systems and reduce felt tensile strength. While alternate rubber compounds may provide some marginal improvement in durability, it’s best to prevent oil from contacting synchronous belts. Ozone can be detrimental to the compounds used in rubber synchronous belts. Ozone degrades belt materials in much the same way as excessive temperatures. Although the bumper materials used in belts are compounded to resist the effects of ozone, eventually chemical breakdown occurs, and they become hard and brittle and begin cracking. The amount of degradation depends on the ozone concentration and generation of exposure. Rubber belts aren’t suitable for cleanrooms, as they risk shedding particles. Instead, use urethane timing belts here … keeping in mind that while urethane belts make significantly less debris, most can carry only light loads. Also, none have static conductive construction to dissipate electrical charges.
Custom Synchronous Drives
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DESIGN WORLD — MOTION
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© 2017 Custom Machine & Tool Co., Inc.
5/14/19 8:46 AM
POWER TRANSMISSION REFERENCE GUIDE
Cam followers from Intech Corp. excel on high-load machines that don’t allow for wearing of the cams or traditional lubrication. Gravity cast nylon-12 bearing surfaces combine with metal hubs and roller bearings for quiet and lubrication-free designs.
Basics of cam followers
(including those for linear motion)
C
am followers are power-transmission devices with a rotary bearing core that bears load while serving as the interface between independently moving machine sections. Applications include those on rotary indexing tables and turntable conveyors, long-stroke robot transfer units (RTUs), and an array of highly customized machinery. The outer diameter (OD) of the cam-follower bearing assembly is its working face — typically made of steel, nylon, urethane, polyamide, or other engineered material. This OD mates with some machine surface … traditionally this was a mechanical cam of some type — such as the precision barrel of an indexing table. Such mechanically automated indexing tables have a motion profile cut into a cam drum that engages the followers, which in turn transmits the power to an output. Cam followers also find use in assemblies that pair them with linear tracks and other engineered paths on customized assemblies. Cam followers assemble onto machines in one of two ways.
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5 • 2019
Stud-type cam followers include a partially threaded shaft fixed to the follower inner diameter (ID) for assembly onto a machine frame with a nut or similar fastening device. Yoke cam-follower variations (identifiable by their open ID) often mate to machine frames via press fit at a hardened inner race usually held by the follower’s end plates. Because they’re not a cantilevered design, yoke followers exhibit minimal deflection. But stud cam followers are indispensable in an array of applications — including those that are subject to high loads. The most common cam-follower design employs needle rollers to carry high radial loads; where applications require the axis to run at high speeds, a cage can separate the rollers. Where loads are particularly high and the axis needs high dynamic load capacity, cam followers can include twin rows standard rollers. Though beyond the focus here, some light-load cam followers are even built around simple plain (sleeve) bearings. Note that cam followers differ from their roller-bearing cousins in a few ways. Because the latter are typically interference fit into motioncontroltips.com | designworldonline.com
5/14/19 10:07 AM
CAM FOLLOWERS
IKO Intl. CFKR series double hex-hole cam followers come in versions with outer-ring ODs of 22 to 90 mm. Their outer rings rotate with a small coefficient of friction and high load capacity. To illustrate, the CFKR 90 V model has a dynamic rating of more than 67,000 N compared to 40,500 N for comparable cam followers.
assemblies, they get circumferential reinforcement from the surrounding machine frame or housing. In contrast, the outer race of a cam follower must be thick to prevent deformation … especially under the localized line of loading. In addition, many cam followers include lubrication ports and more ruggedized surface finishes to withstand exposure to environments during operation — especially those that operate exposed on unprotected machine sections. Many cam followers have flat outer diameter (OD) profiles, while others (especially those for linear-motion applications) include crowned, edgeflanged, or vee-shaped ODs to engage tracks and rails that are engineered with mating geometry.
Crowned cam followers can compensate for ten times the misalignment that traditional flat-profile cam followers. Some cam followers serve as track followers by engaging rails to deliver linear motion. These designs are increasingly common in automated storage and retrieval systems (AS/RS) and seventh-axis RTUs mentioned earlier. That’s because cam-follower-based linear systems outperform the linear bearings known as profile guides where compactness and ultra-high accuracy are less important than ruggedness, quick and forgiving installation, high-speed reversals, and long life.
Linear cam-follower (track follwer) arrangement image courtesy Güdel US
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DESIGN WORLD — MOTION
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POWER TRANSMISSION REFERENCE GUIDE
Overview of
clutches and brakes P
ower-transmission and motion designs rely on stopping loads predictably, especially to ensure machinery and operator safety. To that end, torque limiters, clutches, and brakes stop, hold or index loads. Migration towards application-specific designs has quickened 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. The general steps to pick a given unit are to categorize axis orientation; determine total axis load and kinetic energy to stop it; and calculate 1. Allowable travel before stopping or slowing and time of engagement 2. Maximum load velocity and required clutch or brake force 3. Driving and backdriving torque and 4. Required brake or clutch geometry. Manufacturers provide quick-selection charts that list clutch and brake sizes for given motor horsepower ranges and shaft speed. Most of these charts are based on the dynamic torque capacity of the clutch or brake and the motor’s torque capacity plus an overload factor of some value. This presumes that the motor is appropriately sized to the application. Tip: Designs with aggressive cycle rates need manufacturer input to address heat-dissipation capacities. Harmonization of international safety standards is a factor in brake and clutch selection even as controls have come to integrate more safety features. Those include ANSI B11 Series and OSHA rules in the U.S. and EN ISO 13849 as a global standard on safety to sufficiently mitigate risk — with EN ISO 13849 and EN IEC 62061 possibly merging (as IEC/ISO 17305 should it go forward) requiring compliance of machines that go through the E.U. Many OEMs adhere to these standards for competitive advantage.
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Brakes Clutches & Limiters — Power Transmission HB 05.19 V3.indd 20
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This is a slip clutch from Mach III Clutch Inc. adapted to diameter restriction. In fact, brakes and clutches from the manufacturer come in torque capacities to 62,000 lb-in. and with mounting configurations for through-shaft, end-of-shaft, flange, NEMA frame, IEC frame, and custom motor-rame mounting.
Sizing to torque, speed, and cycles Expressed in lb/ft, N/m, or lb/in., static and dynamic torque values express clutch or brake output capability. Applications needing dynamic braking are those in which the brake controls rotating-axis motion by absorbing kinetic-energy changes. Dynamic clutching is that during which a clutch brings a stationary output to the input
motioncontroltips.com | designworldonline.com
5/15/19 3:19 PM
CLUTCHES & BRAKES
Shown here is a RSCI 20-130 series backstop clutch from Stieber of Altra Industrial Motion.
rpm by assuming the slower axis’ kineticenergy delta. Static-torque ratings (values that describe clutch and brake behavior when the units aren’t absorbing any kinetic energy) depend largely on torsional load. Reaching static torque operation with a clutch needs a clutch that must engage prior to rotating the input — so the clutch effectively functions as a coupling between in and output. In contrast, reaching static torque operation with a brake just takes holding the output element stationary. Static torque equals clutch holding torque when there’s no relative shaft rpm difference between input and output — or (in the case of a brake) when the shaft is stopped. The point at which a system exceeds a unit’s static torque is aptly called the breakaway torque. This is what a machine assembly must reach before relative motion arises between the shafts (in the case of a clutch) or before the shaft starts turning (in the case of brake). Designers sometimes boost dynamic torque rating (and shorten response time) by selecting larger brakes and clutches — though
5 • 2019
Brakes Clutches & Limiters — Power Transmission HB 05.19 V3.indd 21
keep in mind that this also increases torque and shear forces (due to torsion) on mounts to nonrotating machine-frame segments. Dynamic torque depends on the rpm delta between in and output (for a clutch) and operating and zero rpm for a brake. Dynamic torque is usually about 50 to 80% of static torque. In friction-based designs, this value depends on the contact surfacres’ friction coefficient. Because that changes slightly (with the portion of operation the brake or clutch is delivering) designers usually employ an average coefficient for design calculations. For axes where the brake must stop vertical loads, engineers must account for how motors can temporarily draw higher current to output more than rated torque. As always, refer to published performance curves to get dynamic torque ratings for operating speed ranges and match brakes and clutches to the output torque.
Precision Control for Your Operations
Clutches, Brakes and Motion Control Products Clutches and brakes for complete motion control, designed to meet exact specifications. From ultra-precise medical procedures to heavy equipment across a variety of industries – CJM is everywhere. • electrical, mechanical, pneumatic and hydraulic models • system design and integration • expert engineers working on every order
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www.cjmco.com Phone: 860-643-1531 DESIGN WORLD — MOTION
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5/14/19 9:31 AM
POWER TRANSMISSION REFERENCE GUIDE
What are couplings for
power-transmission drive shafts?
A
s 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 zero-backlash 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 powertransmission couplings excel in high-horsepower applications on axes needing correction of up to 2° and 0.01-in. angular and parallel misalignment.
ABB’s Dodge Raptor elastomeric-tire couplings use a split naturalrubber element. This element transmits torque to 340,200 lb-in.
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Couplings — Power Transmission HB 05.19.v4.indd 22
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This specialty double-disc coupling from R+W America is for machine-tool OEMs building designs with long spindle drives. It can send coolant directly to cutting heads via an integrated tube with interfaces at each end. These couplings operate in dynamic highspeed applications with lightweight CFRP or aluminum spacer tubes to boost performance; symmetric conical clamping ring hubs and fine balancing guarantee smooth operation. Torisonally rigid disc packs compensate for misalignment between the drive and driven shafts.
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 to coupled equipment such as bearings. Profiles include straightspoked 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. 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
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5/14/19 12:48 PM
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POWER TRANSMISSION REFERENCE GUIDE
Chain Couplings
Courtesy IWIS Drive Systems
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.
Diaphragm Couplings
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 power-transmission 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.
Elastomeric Tire Couplings TB Wood’s Dura-Flex Coupling courtesy Altra Industrial Motion
Jaw Couplings
Jaw-in-shear (JIS) coupling lovejoy inc.
Oldham Couplings
Typical Torque Capacity
Flexible jaw coupling martin sprocket & gear inc.
Ruland MFG. Co. Inc.
Disc-Element Couplings Roba-DS disc-pack couplings mayr power tramission
Bibby turboflex • Altra Industrial Motion
More
Grid Couplings
Gear Couplings Falk gear coupling Rexnord Corp.
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DESIGN WORLD — MOTION
5 • 2019
5/14/19 12:48 PM
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POWER TRANSMISSION REFERENCE GUIDE
Encoders — the
I
n motion systems, encoders provide information on a number of parameters including position, distance, and speed. They can be classified a number of ways including as rotary or linear, incremental or absolute, or according to their operating principle as optical, magnetic, or capacitive. The most important 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 square-wave signals in two channels, A and B, which are offset (or out-of-phase) by 90
basics
degrees and help determine direction of rotation. The output signals of an incremental encoder only have information on relative position not absolute position. 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. In contrast, 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. Encoders for industrial uses typically are either optical or magnetic. While optical encoders were, in the past, the primary choice for highresolution applications, improvements in magnetic encoder technology now
allow them to achieve resolutions down to one micron, competing with optical technology in many applications. Magnetic technology is also, in many ways, more robust than optical technology, making magnetic encoders a common choice in industrial environments. Then there are capacitive encoders, a relatively new introduction. They offer resolution comparable to optical devices, with the ruggedness of magnetic encoders. Currently, there are only a handful of vendors for capacitive encoders, but their suitability for applications requiring high precision and durability make them a good choice for the semiconductor, electronics, medical, and defense industries. Magnetic rotary encoders rely on three main components: a disk, sensors, and a conditioning circuit. The disk is magnetized, with a number of poles around its circumference. Sensors detect the change in magnetic field as the disk rotates and convert this information to a sine wave. The sensors can be Hall effect devices, which sense a change in voltage, or magnetoresistive devices, which sense a change in magnetic field. The conditioning circuit multiplies, divides, or interpolates the signal to produce the desired output.
HEIDENHAIN’s RCN 6000 series absolute sealed angle encoders use the company’s METALLUR process in which the graduation is applied directly to the bearing ring and uses a reflected light scanning method which gives it compact dimensions. The graduations consist of lines and gaps at defined intervals with minimal deviation, forming structures with high edge definition and making them resistant to mechanical and chemical influences as well as to vibration and shock.
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EP • • • • • •
Ca
W
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1
ENCODERS The resolution of a magnetic rotary encoder is determined by the number of magnetic poles around the disk and by the number of sensors. Incremental encoders (whether magnetic or optical) use quadrature output and can employ X1, X2, or X4 encoding to further increase resolution. The primary difference between incremental and absolute encoders, regardless of sensing technology, is that absolute versions assign a unique binary code, or word, to each measuring position. This allows them to track the encoder’s exact position, even if power is discontinued. The operation of linear magnetic encoders is analogous to their rotary counterparts, except that they use a linear scale (also referred to as a tape, since they typically have an adhesive backing) and a read head. The read head can employ either a Hall effect or a magnetoresistive sensor, and
detects signals generated by the magnetic code on the scale to provide position information. For absolute linear magnetic encoders, each position on the scale represents a unique binary word, indicating the exact linear position of the read head. For incremental versions, one or more reference marks are included on the scale, to enable homing after a power-off situation. Linear magnetic scales can be provided in long lengths—up to 100 meters from some manufacturers. The most significant advantage of magnetic encoders may be their robustness. Unlike optical encoders, magnetic versions are insensitive to contaminants such as dust, dirt, liquids, and grease, as well as to shocks and vibrations. Similar to optical encoders, magnetic encoders do require an air gap between the magnetic disk and the sensor. However, the air gap in a magnetic encoder does not need to be clean and transparent, as it does for an optical encoder. As long as no ferrous material is present between the disc and the sensor, the magnetic pulses will be detected. Two important specifications for proper operation of magnetic encoders are the radial placement of the sensor in respect to the disk (or tape), and the gap distance between the sensor and the magnet. This magnetic absolute linear encoder, the MSA213C from SIKO, features absolute resolution of 1 μm and supplies absolute position without referencing and without requiring a backup battery.
For Absolute Feedback, we’ve got your solution EPC’s Absolute Encoders offer: • 16 bits resolution for single turn, and 43 bits resolution for multi-turn • SSI, CANopen, or EtherCAT communication protocols • Electronic cam switches on CANopen and EtherCAT models • Maintenance-free and environmentally-friendly magnetic design • Energy harvesting magnetic multi-turn technology • No gears or batteries
Call us today.
We’ll help you find the motion feedback solution you need.
1-800-366-5412 • encoder.com Encoders— Power Transmission HB 05.19_v2.indd 27
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POWER TRANSMISSION REFERENCE GUIDE
Review of
G
general gear operation
ears mesh with other gears to transmit altered torque and rotation. In fact, gearing can change the speed, torque and direction of motion from a drive source. When two gears with an unequal number of teeth engage, the mechanical advantage makes their rotational speeds and torques different. In simple setups, gears are flat with spur teeth (with edges parallel to the shaft) and the input gear’s shaft is parallel to that of the output. Spur gears mostly roll through meshing, so can be 98% or more efficient per reduction stage. However, there is some sliding between tooth surfaces, and initial tooth-to-tooth contact occurs along the whole tooth width at once, causing small shock loads that induce noise and wear. Sometimes lubrication helps mitigate these issues.
In more complex setups, parallel-axis gearsets have helical gears that engage at an angle between 90° and 180° for more tooth contact and higher torque capacity. Helical reducers are suitable for higher-horsepower applications where long-term operational efficiency is more important than initial cost. Helical gear teeth engage gradually over the tooth faces for quieter and smoother operation than spur gearsets. They also tend to have higher load capacities. One caveat: Angled tooth contact generates thrust that the machine frame must resolve. No matter the subtype, most parallelaxis gearsets have gear teeth with tailored involute profiles — customized versions of the rolled trace off a circle with an imaginary string. Here, mating gears have tangent pitch circles for smooth rolling engagement that minimizes slipping. A related value, the pitch
point, is where one gear initially contacts its mate’s pitch point. Involute gearsets also have an action path that passes through the pitch point tangent to a base circle. Besides parallel-axis gearsets, there are non-parallel and right-angle gearsets. These have input and output shafts that protrude in different directions to give engineers more mounting and design options. The gear teeth of such gearsets are either bevel (straight, spiral or zerol), worm, hypoid, skew or crossed-axis helical gears. The most common are bevel gearsets with teeth cut on an angular or conical shape. Hypoid gears are much like spiral-bevel gearsets, but the input and output shaft axes don’t intersect, so it’s easier to integrate supports. In contrast, zerol gearsets have curved teeth that align with the shaft to minimize thrust loads.
Gear-geometry variations S Sp ur gea Spur gearsets are simple ...
Zerol bevel gearsets are a special veriation of straight right-angle bevel sets.
Pitch circle Pit
Reaction force ... but helical gearsets are more efficient. Cross-axis sets are another option.
Planetary gearsets are compact and run to 10,000 rpm. Here, a lightweight Schaeffler differential for a hybrid vehicle has an axial spline to boost efficiency.
The ratio of a helical or bevel gearset is simply the number of teeth in the larger gear divided by the number of teeth in the smaller gear. Other gear types such as planetary gears have more complex ratio relationships.
Worm gearsets are rugged and don’t let designs backdrive ... which can eliminate the need for brakes. Note there’s some overlap between bevel and worm applications. Case in point: The MS-Graessner DynaGear here is a single-stage bevel gear with a 30:1 ratio.
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motioncontroltips.com | designworldonline.com
5/15/19 9:48 AM
GENERAL GEAR OPERATION
Basics of strain-wave gearing
T
ooth-engagement kinematics of strain-wave gears is different than that of planetary or spur gearing. Strain-wave gearing uses the metal elasticity (deflection) of a gear to reduce speed. Benefits of strain-wave gearing include high torque, compact size, positional accuracy, and zero backlash. Recall that backlash is the difference between the tooth space and tooth width ... and this difference is zero in strain-wave gearing. A strain-wave gearset consists of a wave generator, flexspline, and circular spline. The wave generator is an assembly of a bearing and a steel disc called a wave-generator plug. The outer surface of the wave generator plug has an elliptical shape machined to precise specification. A specialty ball bearing is pressed around this plug to cause conformity to the same elliptical shape of the wave-generator plug. Designers typically use the wave generator as the input member, usually attached to a servomotor. The flexspline is a thin-walled steel cup. Its geometry lets the walls of the cup be radially compliant but remain torsionally stiff (because the cup has a large diameter). Manufacturers machine the
Wave generator Circular spline
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Gearing — Power Transmission HB 05.19 V3.indd 29
gear teeth into the outer surface near the open end of the cup (near the brim). The flexspline is usually the output member of the mechanism. The cup has a rigid boss at one end to provide a rugged mounting surface. The wave generator is inserted inside the flexspline so the bearing is at the same axial location as the flexspline teeth. The flexspline wall near the brim of the cup conforms to the same elliptical shape of the bearing. This conforms the teeth on the outer surface of the flexspline to the elliptical shape. That way, the flexspline effectively has an elliptical gearpitch diameter on its outer surface. The circular spline is a rigid circular steel ring with teeth on the inside diameter. It is usually attached to the housing and does not rotate. Its teeth mesh with those of the flexspline. The tooth pattern of the flexspline engages the tooth profile of the circular spline along the major axis of the ellipse. This engagement is like an ellipse inscribed concentrically within a circle. Mathematically, an inscribed ellipse contacts a circle at two points. However, the gear teeth have a finite height. So there are actually two regions (instead of two points) of tooth engagement. Roughly 30% of the teeth are always engaged ... 60 teeth for a 100:1 gear ratio. This contrasts with maybe six teeth for a planetary
This example of strain-wave gearing is the HIWIN DATORKER — a gearing set specifically for robotic applications. gear and one or two teeth for a spur gear. The pressure angle of the gear teeth transforms the output torque’s tangential force into a radial force acting on the wave-generator bearing. The teeth of the flexspline and circular spline engage near the ellipse’s major axis and disengage at the ellipse’s minor axis. Note that the flexspline has two less teeth than the circular spline, so every time the wave generator rotates one revolution, the flexspline and circular spline shift by two teeth. That means the gear ratio = number of flexspline teeth ÷ (number of flexspline teeth - number of circular spline teeth).
Illustration courtesy Harmonic Drive Systems Inc. Note that strain-wave gearing sets are also known as Harmonic Drive gears, a registered trademark of this manufacturer.
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DESIGN WORLD — MOTION
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POWER TRANSMISSION REFERENCE GUIDE
Gearboxes, specialty gearheads, and servogear sets
S
ervo systems are precision-motion setups that demand servogear reducers with good torsional stiffness, reliable output torque and minimal backlash. OEMs tasked with integrating servo systems should look for quiet reducers that easily mount to the motor and require little or (if possible) no maintenance. In fact, a lot of advanced machinery integrates servogears into application-specific electromechanical arrangements, and several of these arrangements are common enough to have specific labels. Here is a look at some of the most widespread. Gearmotor: This complete motion component is a gear reducer integrated with an ac or dc electric motor. Usually the motor includes the gears on its output (typically in the form of an assembled gearbox) to reduce speed and boost available output torque. Engineers use gearmotors in machines that must move heavy objects. Speed specifications for gearmotors are normal speed and stall-speed torque. Gearbox: This is a contained gear train … a mechanical component consisting of a series of integrated gears. Planetary gears
are common in integrated gearboxes. Planetary gears: Particularly common in servo systems, these gearsets consist of one or more outer planet gears that revolve about a central, or sun, gear. Typically, the planet gears mount on a movable arm or carrier that rotates relative to the sun gear. The sets often use an outer ring gear, or annulus, that meshes with the planet gears. The gear ratio of a planetary set requires calculation, because there are several ways they can convert an input rotation to an output rotation. Typically, one of these three gear wheels stays stationary; another is an input that provides power to the system, and the last acts as an output that receives power from the driving motor. The ratio of input rotation to output rotation depends on the number of teeth in each gear and on which component is held stationary. Planetary gearsets offer several advantages over other gearsets. These include high power density, the ability to get large reductions from a small volume, multiple kinematic combinations, pure torsional reactions and coaxial shafting. Another advantage of planetary gearbox
arrangements is efficiency. Losses are typically less than 3% per stage, so rather than waste energy on mechanical losses inside the gearbox, these gearboxes transmit a high proportion of the energy for productive motion output. Planetary gearbox arrangements distribute load efficiently, too. Multiple planets share transmitted load between them, which greatly increases torque density. The more planets in the system, the greater load ability and the higher the torque density. This arrangement is also very stable due to the even distribution of mass and increased rotational stiffness. Disadvantages include high bearing loads, inaccessibility and design complexity. In servo systems, besides boosting output torque, gearboxes impart another benefit — reducing settling time. Settling time is a problem when motor inertia is low compared to load inertia … an issue that’s the source of constant debate (and regular improvement) in the industry. Gearboxes reduce reflected inertia by a factor equal to the gear reduction squared.
Internal ring gear Sun gear Planet gear
GAM Enterprises now offers SSP Series washdown servo gearboxes for food, medical or sanitary applications. The precision inline planetary gearboxes include a 300-series stainless steel housing. Shown here is the SSP-W gearbox with a stainless KG-VA coupling.
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5/10/19 9:36 AM
POWER TRANSMISSION REFERENCE GUIDE
Sorting through
A
gearmotor types
gearmotor is a combination of an electric motor and a gear reducer that delivers high torque at low speed. Common motor types include ac permanent magnet or induction and brushless dc, and gears can range from straight and right angle to parallel shaft versions. Gearmotors can be classified in several different ways, but one common method is to divide them into standard and inverter duty. Standard gearmotors are designed to operate over a wide range of conditions producing a range of output speed and torque. However, there are special motor designs aimed at specific applications, such as washdown motors used in food and beverage processing or explosion-proof motors used in hazardous locations. Inverter-duty gearmotors are a bit like this. As the name implies, inverter-duty refers to a gearmotor whose speed is controlled by an inverter, or VFD (variable frequency drive). The difference between an inverter-duty gearmotor and a standard gearmotor is in the construction. These motors are specifically designed to operate at low speeds and not overheat. Because of the special way the windings are insulated, they are better able to withstand the voltage spikes of the fast-switching PWM signals generated by VFDs. The insulation will not break down and cause motor failure. Inverter-duty gearmotors can thus produce a wider constant-torque speed range than a standard gearmotor. Inverter-duty gearmotors are commonly used in applications such as material handling equipment, packaging equipment and conveyors. In short, any application requiring adjustable speed, hence the use of a VFD. A different way of classifying gearmotors is by function – as either controlling position or speed. A good example of position control is in packaging or material handling where a load needs to be picked up or placed at a specified location, or applications with more precise requirements such as micro-positioning in high-precision manufacturing or dispensing. Application requirements will dictate the level of accuracy needed from the gearmotor. For instance, low-accuracy applications are ones where the position can be “good enough,” such as a robotic arm placing boxes on pallets. On the other hand, high-accuracy positioning may require accuracy in thousands of an inch or even at the micron level. In reality, just about any type of gearmotor can be used for position control. The real question is how accurate does the positioning have to be, in which case the focus shifts to the type of gears in the gearmotor. One of the main culprits for inaccuracy in
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These H Series gearmotors with inline helical speed reducers from DieQua offer high torque capacity to 14,000 Nm (10,325 ft lbs). The company’s cat4CAD software supports simplified selection and configuration of gearbox and gearmotor systems. gearmotor positioning is backlash in the gears. So, if positioning accuracy is a key variable in the application, make sure that the choice of gearmotor minimizes backlash in the gearing. Manufacturers have several methods for minimizing backlash in gears including preloading and using gears that are designed to minimize or eliminate backlash from the start. Aside from their use in position control applications, gearmotors are used in speed-control applications as well. As with any design problem, the requirements of the unique application will dictate what type of motor is best suited for the job. A good question to ask right off the bat is what are the speed demands. For instance, does the application call for high speed, low speed, or variable speed operation? Another important factor is the type of motor and the associated control. For instance, is a dc or ac gearmotor the right fit? This could depend on the type of power source available as well as the required torque at the load. As for the type of gearing, this too depends on the application needs. Other design considerations may include different frame sizes as well as gear reduction ratios. In the end, the gearmotor selected for the job will ultimately have to satisfy the application’s speed and torque requirements. motioncontroltips.com | designworldonline.com
5/14/19 8:06 AM
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5/14/19 8:04 AM 5/1/19 8:52
POWER TRANSMISSION REFERENCE GUIDE
Key lead screw
selection criteria
M
any motion systems incorporate lead screws to drive machine axes. Lead screws are often used in linear actuators and positioning equipment to provide thrust or axial force to drive a load. A lead screw is basically a threaded rod that uses rolling friction between the screw and a nut to translate rotational motion into linear motion. The sliding action generates high friction, meaning a lower efficiency rating. However, lead screws generally sport higher power ratings than comparable ball screws, mainly because of the greater area of surface contact between the screw and nut. Selecting the right lead screw involves knowing the specific application parameters as well as a bit of manual calculation aided by online selection tools offered by most manufacturers. The most
important is choosing the right diameter lead screw and the lead. There are four steps to getting this right; determining the axial load, checking the critical speed of the screw, calculating the PV value, and considering back driving. Once you’ve gone through the process of sizing a lead screw, you know what diameter and lead will provide the required load capacity, speed, and back driving torque for the application. But there are hundreds — maybe thousands — of lead screw designs on the market. Beyond the basics of diameter and lead, how do you choose the specific lead screw assembly that will work best in your application? The final lead screw selection often comes down to three factors: backlash, material, and customization.
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Lead Screws — Power Transmission HB 05.19_v2.indd 34
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A
•
LEAD SCREWS Backlash Caused by clearance, or play, between the screw and nut, backlash affects the repeatability of the screw assembly, preventing it from returning to the same position over repeated movements. Although backlash was once taken as an unavoidable trait that prevented lead screws from being used in precision applications, manufacturers now offer anti-backlash versions of most lead screw nuts, greatly improving their repeatability. A common method to eliminate backlash in a lead screw nut is to incorporate a spring or other compliant part (such as a washer) between two nut halves. Other methods use a stiff spacer that can expand or move relative to the nut halves,
Thomson stepper motor linear actuators, like the MLA and MLS series, combine a hybrid stepper motor and a precision lead screw in one compact envelope. They can be incorporated into pipetting applications that require tiny, precise, and repeatable vertical motion.
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POWER TRANSMISSION REFERENCE GUIDE Actuator lead screw geometries LENGTH CALCULATIONS NON-CAPTIVE SHAFT PRODUCTS
SCREW DIMENSIONS FOR NEMA 14 AND 17 MOTORS 3.0 TO 18.0 (77.5 TO 455.0)
MOUNTING SURFACE PLATE THICKNESS + DESIRED STROKE LENGTH
0.25 (6.35)
+ (A/B/C)
0.25 (6.35)
SCREW LENGTH
14 — ADD 1.4" OR 35.6 MM 17 — ADD 1.4" OR 35.6 MM
23 — ADD 1.8" OR 45.7 MM
EXTERNAL SHAFT PRODUCTS
SCREW END OPTIONS
STRAIGHTNESS 0.002 PER INCH
SCREW DIMENSIONS FOR NEMA 23 MOTOR 3.0 TO 24.0 (77.5 TO 610.0)
THREADED END
MOUNTING SURFACE PLATE THICKNESS – SCREW LENGTH
SMOOTH END
0.375 (9.52)
– NUT LENGTH AVAILABLE STROKE LENGTH
NONE
Lead screws are often paired with stepper motors for linear actuation. Case in point: Schneider Electric Motion actuators combine lead screws with single-stack stepper motors in NEMA sizes 14, 17, and 23. Variations having an external shaft include a rotating screw that is integral to the motor’s rotor and a mating nut that travels axially along the threaded screw shaft. In contrast, versions with a non-captive lead screw have the threaded shaft pierce through the motor to move axially with motor-rotor output. | courtesy Schneider Electric Motion
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0.50 (12.7)
STRAIGHTNESS 0.002 PER INCH (0.05 PER MM)
continually compensating for clearance that develops as the nut wears. For applications that require high repeatability, look for an anti-backlash nut, keeping in mind how wear will affect its performance over time. Also note that drag torque may be higher due to the preload that’s placed on the nut, and that additional space may be required to accommodate the anti-backlash design.
Material Lead screws are able to operate in very challenging environments, including some conditions that would be impossible for ball screws to withstand. One reason for this is that lead screw nuts don’t have recirculating elements, which are highly susceptible to debris and contamination. But another reason is that lead screw nuts can be made from a wide variety of materials, including bronze and various plastics, to suit the environmental conditions — whether they include caustic liquids, abrasive particles, or temperature extremes. Lead screw shafts can be made from carbon steel, stainless steel, or
5 • 2019
aluminum, and they’re commonly supplied with a surface coating for additional protection against contamination. In their standard offerings, many lead screw manufacturers supply PTFE-based coatings, which provide durability in harsh environments and supply lubrication to the nut, eliminating the need for grease or oil.
Customization In addition to the standard customization options for screw end machining — to accommodate fixed or floating end bearings, or to add keyways for couplings, for example — most lead screw manufacturers offer truly customized designs. These can include special leads, thread forms, or nut bodies. Going one step farther, some manufacturers offer integration of the lead screw with a linear guide to provide a custom slide or actuator. This level of customization is especially helpful when an OEM or end user is replacing legacy parts that are no longer offered, or changing an existing design that has fit, form, or function constraints. motioncontroltips.com | designworldonline.com
5/14/19 8:12 AM
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POWER TRANSMISSION REFERENCE GUIDE
Technical summary of
linear-motion guides, rails, and systems
L
inear-motion systems are essential in all sorts of applications, including everything from manually operated industrial drawers to advanced Cartesian robots. Mechanisms that include the former operate without power, using inertia or manual power to move loads. Components to complete the latter include ready-to-install drive and guidance designs … in the form of self-contained actuators or linear-motion machinery subsections. Some designs simply rely on the rotary-to-linear mechanism or actuator structure for total load support. However, most industrial linear designs have pneumatics, linear motors or motor-driven, rotary-to-linear mechanisms to advance attached loads, as well as rails that guide and support the loads. Here, linear rails, rotary rails, guide rails, linear slides and linear ways are just a few options to facilitate single-axis motion. Their main function is to support and guide load with minimal friction along the way. Typical linear-motion arrangements consist of rails or shafts, carriages and runner blocks, and some type of moving element. Engineers differentiate these systems by the type of surface interaction (sliding or rolling), the type of contact points, and (if applicable) how the design’s rolling-element recirculation works. In fact, slides and rails are more advanced than ever, with advances in materials and lubrication setups (to help designs last longer in harsh applications), innovative rail geometries (to help designs withstand more misalignment and load than ever), and modular guide mounts (to boost load capacity and minimize deflection). No matter the ultimate installation, linear-motion rails, guides, and ways enable motion along an axis or rail either through sliding or rolling contact. Myriad moving elements can produce either sliding or rolling support: ball bearings, cam roller sliders, dovetail bearings, linear roller bearings, magnetic bearings, fluid bearings, X-Y tables, linear stages and machine slides. One classic rail with sliding contact is a dovetail slide, and one classic rail with rolling contact is a ball rail with a recirculating system. Sliding-contact bearings are the more straightforward type of linear-motion component. These consist of a carriage or slide that rides over a surface known as a rail, way or guide. Sliding contact occurs when the moving part directly contacts the rail section. Newer versions have self-lubricating sleeves and other features to boost positioning accuracy and repeatability.
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In contrast, rolling-element linear-motion systems are either recirculating or non-recirculating. Non-recirculating types use rolling elements such as bearing balls, rollers and cam followers for movement. Recirculating types use some type of moving platform that houses a bearing block. This bearing block contains raceways with rolling elements that let the platform move along the rail with little friction. Recirculating types include linear guides and ballbushing bearings. More specifically, rolling-element linear guides come in two basic versions — those with circular arc grooves and those with Gothic arc grooves. These groove choices are a result of industry evolution that’s enabled new geometries for better load handling. Circular arc grooves contact bearing balls at two points. The Gothic arch contacts the balls at four points for bidirectional load capacity. Another option for rolling-element linear motion is ball bushings that have a bushing nut lined with recirculating bearing balls. This nut rides along a round shaft to allow axial movement.
Sliding-contact rail geometries A distinguishing feature of sliding carriage-and-rail setups is that manufacturers typically incorporate a ground groove in a rectangular track’s geometry (to serve as a working surface). Manufacturers typically build these rails in one of three shapes: Rails with a boxway 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. Round rails deflect less under load. In addition, systems based on round rails are inherently self-aligning, so are easier to install than the other options.
Rolling-contact functions and options Rolling-element linear systems need little force to initiate motion. In addition, friction-force variations due to speed are minimal, so these systems can position loads with small and precise steps. The low friction also lets these systems move at high speeds without generating too much heat. That minimizes wear to help
motioncontroltips.com | designworldonline.com
5/14/19 1:39 PM
LINEAR SLIDES, RAILS & GUIDES
Heavy-duty slides from HepcoMotion work for longlength applications such as pick-and-place and robottranslation stages. V-slide rails are of bearing-grade steel in sections to four meters long. The slides typically bolt to aluminum extrusions or supporting back plates. A guide wheel bearing with matching V geometry rides the raceway. | Courtesy Bishop-Wisecarver
This linear plain bearing is PBC Linear Uni-Guide with a Frelon self-lubricating liner. The liner serves to lower the coefficient of friction, reduce wear, and boost load capacity.
Shown here are ball bushing bearings, 60 Case shafting, square and round rail, and other linear-motion components from Thomson Industries Inc.
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DESIGN WORLD — MOTION
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POWER TRANSMISSION REFERENCE GUIDE
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Linear Guide Rails Slides & Ways — Power Transmission HB 05.19 V3.indd 40
machinery maintain a level accuracy for much of the linear system’s operating life. O-shaped arrangements can withstand higher torque than X arrangements. In general, the number of load-bearing rolling-element rows influences the load capacity … so more rail rows means more load capacity and rigidity. However, more rows 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 rollingelement 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-elementbased 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 nonrecirculating 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.
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DESIGN WORLD — MOTION
5/14/19 1:39 PM
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POWER TRANSMISSION REFERENCE GUIDE
Using torque limiters
to protect servo-driven machinery
S
ervomotor-driven machinery abounds in today’s automated applications. These designs typically require high speeds for top throughput, dynamic operation for nimble positioning, and (in many cases) quick reversals for indexing. So it’s key to have some means to: Protect the servomotor from damage by sudden load changes, over-torque conditions, or shock due to machine issues. The latter can include errors in programming, mechanical or feedback malfunctions, faulty braking, and machine-operator mistakes. Protect the power-transmission assembly (including gearing and couplings as well as ballscrew, belt, or other drive components) from the behavior of its own inertia, motor-rotor inertia, and (reflected) load inertia — and even leverage system inertia — in atypical situations.
Torque-limiter selection requires calculation of torques to be experienced by the axis on which the limiter will operate — from the axis end back to the servomotor. Electronic protection sometimes fails to protect mechanical components in between. Shown here are Series T ball and detent torque limiters from DieQua Corp. Their low inertia makes them suitable for servomotor-driven designs (and those incorporating ballscrews) needing quick acceleration.
That’s because the processes that servo-driven machinery execute usually involve either expensive workpieces or high volumes … mission-critical operations with equipment that is itself costly. Such processes are intolerant of unexpected downtime from shock and jam-induced failures of subcomponents. Overload protection includes often-complementary electronic and mechanical means. Consider how servosystems can include electronic fusing, current limiting, and other modes of electronic overload protection to address crashes and sudden axis stops. Such protection uses controls (such as PLCs or servo drives) and feedback to track the axis state — monitoring and controlling motor-winding temperatures, current and voltage, as well as output force or torque and position in some cases. Where applicable, these modes of electronic overload protection also complement the use of powerful low-inertia servomotors to boost overall performance and efficiency. Then when there are issues with any monitored parameters (especially those that indicate total axis jams) motion controls trigger preset corrections with motor-drive shutdowns, brake engagement, or other commands to trigger a motor-based response.
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But one limitation of electronic torque limiting is that it’s not always quick enough to prevent machine or workpiece (or process) damage. That’s especially true for addressing inertial effects on dynamic motion axes; the masses of the mechanical components exert behavior that electronic corrections generally don’t resolve. In other words, the reliance of electronically programmed torque limiting on motor operation to correct problems leaves no immediate mode of addressing the action of components’ rotating masses (inertia) downstream. In contrast, mechanical torque limiters provide nearinstantaneous response (breaking free in just 1 msec in some cases) to issues. When installed at the right location in the drivetrain, these disconnect system inertia from the locked portion of the axis and let the drivetrain coast to a stop. That helps prevent damage in the first dozen or so milliseconds of machine-axis crashes or over-torqued conditions, when the most damage typically occurs. Torque limiters used this way are also called torque-overload motioncontroltips.com | designworldonline.com
5/14/19 1:20 PM
MOTORS & TORQUE LIMITERS
devices, safety couplings, overload clutches, and slip clutches. Note: Though this may be just a matter of semantics, torque limiters are technically not clutches because they are not built for continuous slipping. Related (though also nonequivalent) technologies are mechanical overrunning clutches that use wrap-spring, roller ramp, ratcheting, and cam action for one-way engagement.
Servo-system inertia in context of torque limiters Recall that according to Newton’s second law for rotating systems, applying torque τ (in N∙m) to a mass induces angular velocity ω in rad/sec — with acceleration inversely proportional to the moment of inertia J (in kg∙m2) of that mass. Power P = τ ω = τ 2π∙n — with 1 rad = 360° ÷ 2π∙n and n = revolutions per sec. Also: ω τ= — J t Where t = Time to accelerate or decelerate (sec) so that stopping time and torque τ are inversely proportional. Often, the torque τ of a sudden jam or braking (inducing an axis stop within just a few milliseconds) is quite high … to 100,000 N∙m/ sec is not atypical ... and in fact, overload via reflected inertia (from the mass of all the load and power-transmission components on the
motioncontroltips.com | designworldonline.com
Motors — Power Transmission HB 05.19_v3.indd 43
axis) can easily exceed the servomotor’s peak-torque rating. Though machine axes can be built with torque ratings that allow for some momentary exceedances, a torque limiter in such a design can disengage at torques dangerously higher than continuous motor torque. These slip during shock loading of significant inertia-induced torque and not during startup or otherwise normal operation. Or the torque limiter (with the help of a speed sensor) can disengage the axis at some torque below peak motor torque and concurrently trigger a motor shutdown. When paired with electronic overload protection and installed where jams may occur (typically at the furthest reaches of axis actuation) still other mechanical torque limiters include a ring or other subcomponent to allow the mounting of proximity switches that in turn communicate overload conditions to a controller for electronic follow-up remediation. Where axis cycle rates are aggressive for the application’s load inertia, the torque limiter may need to have expanded range and capacity. That’s certainly true where torque limiters protect against damaging levels of shock loading but allow system inertia to temporarily increase torque and power the axis through certain conditions, if that’s part of the design. As mentioned, some servomotors are specifically engineered
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POWER TRANSMISSION REFERENCE GUIDE
to have low (rotor) inertia; these often find use in high-performance applications having high inertia ratios of (reflected) load inertia to motor inertia — breaking the somewhat arbitrary rule of thumb that this ratio not exceed 5 to 10:1 or so. Visit motioncontroltips.com and search “servomotor inertia ratio” for more information on this. Also read the FAQ: Why does servomotor inertia matter? Such motors exhibit quick deceleration (to stop from top speed in 20 msec, for example) but the inclusion of mechanical torque limiters is often required for wholly safe execution of dynamic tasks.
Torque-limiter variations for adding to servomotor-driven systems
TOO MUCH ON YOUR PLATE?
Don’t eat up valuable time searching for friction brakes, clutches and torque limiters. 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: USA 859-291-0849 engineering@machiii.com
Torque limiters are either releasable or continuously engaged. Releasable designs include ball-detent torque limiters (which are particularly common in servomotor applications), shear-pin limiters, permanent-magnet and loaded pawl-spring torque limiters, and spragengagement torque limiters. Slipping torque limiters requiring no reset upon load correction include magnetic-hysteresis and magneticparticle torque limiters as well as frictiondisc torque limiters. During a machineaxis jam, these slip just enough to absorb inertia shock load and (as mentioned) trigger controls to let the motor stop. When specifying torque limiters, look for availability of torque-specific models or those with some means of adjustment; backlash-free operation; and quantification of accuracy as a percent — ±4% of the trip torque, for example. Limiters billed as precision-tripping models will not exhibit lost motion (play). Torque limiters that integrate couplings also include misalignment-compensation values in degrees.
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#gearheads
Our team has only one objective: to design and manufacture the right gearmotor, motor or control for your application. So whether you’re buying one gearmotor or hundreds, you’ll have the expertise of the entire Bodine team working for you. Visit our new website at bodine-electric com. info@bodine-electric.com
Bodine Electric Company 05-19.indd 45
| 773.478.3515 (USA)
5/15/19 2:51 PM
POWER TRANSMISSION REFERENCE GUIDE
Sensors measure distance,
detect objects
P
46
ower transmission systems use a number of different types of sensors for various purposes. For instance, rotary and linear encoders for position feedback are perhaps the most common. But others include tachometers for speed sensing and proximity sensors for a variety of functions such as object detection and distance measurement. Many industrial and manufacturing applications use proximity sensors to sense the presence of objects or materials and then either initiate some action or simply flag their presence or absence. Key to their operation is that they don’t require physical contact with the target or object being sensed, and why they’re often called non-contact sensors. There are a number of common sensing techniques employed in proximity sensors that serve to categorize sensor types. The most common are photoelectric, ultrasonic, capacitive and inductive.
photoelectric sensor; reflective, and throughbeam. Reflective sensors work by emitting a beam of light that strikes the object and is reflected back to the detector, usually in the same physical housing as the emitter beam. Through-beam sensors, on the other hand, have two separate units, an emitter or source of light and a separate receiver or detector. When an object breaks the light beam, the detector registers this as a break.
Photoelectric – these sensors operate on the basis of light, dependent on a change in the amount of light available to a detector in the sensor. There are two basic types of
Capacitive – as the name indicates, these sensors operate by noting a change in the capacitance, capacitance being a function of both electrical charge and voltage between
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Ultrasonic – these sensors use sound waves to detect objects. They emit a high frequency sound wave (higher than human ears can detect) and when it strikes an object it’s reflected back to the sensor where the distance of the object can be calculated based on the time required for it to return. They’re used in applications to measure distance of objects, such as in automotive park-assist functions, and in bottling and filling applications to detect fluid levels.
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two surfaces with either an air gap or some other material between them, which is the dielectric constant. When an object to be detected enters the field of the sensor, it effects the dielectric and thus changes the capacitance. Inductive – these types of sensors are based on changing inductance, which is a measure of the ability of inducing a voltage in a conductor as a result of a changing current in a different conductor. Inductive sensors work with metallic objects because these have inductive properties, so can’t be used to detect plastic, for instance. Also, the type of material will influence the sensing distance. For example, ferromagnetic materials like steel generally have the longest sensing distances, whereas other metals such as aluminum or copper have much shorter sensing distances. The inductive type of proximity sensor is one of the more common types in motion and automation systems, partly because it’s one of the oldest types but also because of its long useful life and versatility and durability.
motioncontroltips.com | designworldonline.com
5/14/19 2:11 PM
SENSORS & TRANSDUCERS
Inductive sensors operate on the basis of Faraday’s Law that states that a change in magnetic flux in a coil of wire will induce a voltage in a nearby coil. This is applied in inductive proximity sensors in the following way: The sensor itself contains an oscillator circuit and a coil from which an electromagnetic field radiates out and induces eddy currents in any nearby metallic objects. The eddy currents have the effect of attenuating the oscillations from the amplifier. This reduction in oscillations is registered as the presence of a metallic object. Because only metallic objects have inductive properties, inductive sensors can’t be used to detect plastic or cardboard or other non-metallic objects. However, different metals have different inductive properties and the type of metal being sensed will influence the sensing distance. For instance, ferromagnetic materials like steel generally have the longest sensing distance, while non-ferrous metals such as aluminum or copper have much shorter sensing distances. In general, inductive proximity sensors are well suited to shorter-range applications as the inductive effect wears off with growing distance between the sensor and object to be detected. Inductive proximity sensors hold up well in dirty environments where contaminants don’t interfere with the sensor’s ability to detect metallic objects. For example, they’re resistant to dirt, dust, and smoke in the environment between the sensor and the object to be detected. As for build-up of contaminants on the sensor face such as dirt and dust, oil, grease or soot, these don’t effect the inductive sensing. However, metallic contaminants such as metal chips in machining applications will impact sensor operation. The key is to be sure to understand what type of contaminants an application contains in order to select the correct type of sensor that can handle them and operate effectively.
DESIGN WORLD — MOTION
Sensors & Transducers — Power Transmission HB 05.19_v3.indd 47
This new family of safety sensors from Pepperl+Fuchs are TÜV-certified in accordance with the Machinery Directive (EN 13849) Performance Level PLd, Category 2, and SIL 2. They are used to safeguard machines and plant components as well as for reliable position detection within these environments.
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POWER TRANSMISSION REFERENCE GUIDE
Mechanical, fluid-power, and elastomeric
vibration dampers
V
ibration in industrial machinery often originates from imbalances inherent to motors, gearboxes, and other turning component that excites natural frequencies. Dampers in this context are components that counteract and often isolate the vibration source from the rest of the design or machine — to prevent its propagation of mechanical oscillation. Passive vibration mitigation includes passive isolators — such as closed-cell foam slabs, metal coil or wave springs, wire rope isolators (common in military applications) and rubber machine mounts. These reduce system natural frequency to less than that of excitation frequency… though to be clear, traditional springs and rubber have near-zero damping capabilities. Precision passive isolators include negativestiffness isolators, which decrease system natural frequency upon loading via a kinematic linkage of beam columns and springs … typically stacked in series for tilt, horizontal, and vertical motion. In fact, many isolators do impart a small amount of damping. Some (including wire-rope isolators) also attenuate shock impact at sudden move ends, hard stops, and collisions without recoil — on conveyor e-brakes or axis stroke ends, for example. In contrast with isolators, passive dampers (such as material slabs and mechanical linkages) change the kinetic energy of vibration into heat. Passive damping usually employs viscous fluids, viscoelastic materials, piezo elements, or simple magnetics. Active dampers include an array of electronically controlled force cancellers that employ a power source, sensors, and actuation to counteract vibration with disruptive interference … out of phase with the source vibration. These include: •
•
Shocks containing magnetorheological fluid that stiffen to solid with controllable yield strength under an applied magnetic field in milliseconds Powered piezoelectric elements affixed to the design frame at critical points to execute active countermotions and address vibration (usually detected by acceleration sensors) and under the control of a DSP.
Regarding passive vibration damping ... those in mechanical formats include tuned mass dampers that affix to machinery or structures to damp narrow frequency bands of vibration. Other options for
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Shock and vibration-damping technologies safely decelerate loads in motion systems. Pictured left to right are a heavy industrial shock absorber, TUBUS profile damper, PET shock absorber, MAGNUM industrial shock absorber, and FYN series rotary damper — all from ACE Controls. passive mechanical damping are friction and piezoelectric action prompted by simple resistive shunt. Passive vibration mitigation in fluid-power formats include hydraulic mounts, air springs, and bladders to isolate and damp industrial machines and other moving equipment. Passive air tables are another iteration, though increasingly displaced by other technologies. Gas springs (of the piston-type plunger design) act as kinematic holds. Many pneumatic dampers have a similar structure — with a cylindrical chamber containing a piston and compressed air behind it — though work on moving axes. Oil dashpots employ a contained volume of fluid to resist motion with viscous friction to damp (though not isolate) vibration. Now consider passive vibration mitigation in elastomeric material formats — which offer simplicity in format and application. Foam slabs isolate well though lack durability. Rubber and neoprene excel at isolation but not damping. In contrast, another option is viscoelastic material from Sorbothane Inc. that excels at vibration and shock control — absorbing up to 94.7% of the latter. The material is classified as viscoelastic, as it exhibits both viscous and elastic characteristics when subject to deformation. A high delta tangent — also called the loss factor or damping coefficient — means an out-of-phase time relationship between shock impact or vibration motioncontroltips.com | designworldonline.com
5/14/19 2:02 PM
SHOCK & VIBRATION MITIGATION Passive isolators reduce natural frequency to less than excitation frequency • Closed-cell foam slabs • Metal coil • (Most) wave springs • Wire rope isolators • Rubber machine mounts
and force transmission. The viscoelastic material absorbs more than half the energies at 1 to 30,000 Hertz for powerful vibration damping … shedding the energy as heat (through hysteresis) and directing remaining energy perpendicularly — 90° out of phase from the vibration or shock source.
Special case of gas springs Gas springs, also called gas dampers, tension springs, or gas-pressure springs depending on the setup and context, are compressed-air or oil cylinders that install in motion designs to damp forces and return kinematic linkages (and more complicated assemblies) to default positions. Gas springs work through a piston on the end of a rod that protrudes from a steel cylinder body; usually compressed gas (often nitrogen) within the cylinder exerts force on this piston to reassume and maintain set positions. Nitrogen is common here because it’s inert and nonflammable. In such designs, oil or grease between the piston and other contacting parts minimize friction. In fact, the small amount of oil in these gas springs serves another function — to
further damp and gently decelerate gas springs during full extension or compression. Some setups even include a fine hole in the piston for damping that’s still slower than with other designs; such slow-damper springs are common on safety gates and doors. In contrast, extended-reach gas springs usually leverage telescoping mechanisms pairing multiple cylinders on one rod; then the smaller cylinder extends from within the larger cylinder. Consider one particularly long-stroke application: Passive heave compensators — systems on ships or offshore oil-rig systems that reduce the effect of waves on engineered structures — use gas springs with strokes to many meters long. Still other gas-spring applications include those for medical beds and hoists; industrial equipment such as machine-tool presses; off-highway and automotive equipment for hatches, hoods, and covers; office equipment and furniture; and general strut and support applications. Fast-acting gas springs find use in weaponry and aerospace design. Specific variations include gas springs with standard or fixed-height cylinders; spindle-only designs;
The Sentinel 1 onboard monitoring system for shock absorbers imparts IIoT functionality to energy-absorption technologies. From ITT Enidine, the new microelectronic solution lets plant operators get real-time energy management and performance monitoring on critical machine sections. The Sentinel 1 uses energy harvesting for power to allow wireless operation … and has a reach of 2,000 ft to communicate with PLCs and other automation hardware in the facility. motioncontroltips.com | designworldonline.com
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Common isolating and damping models Vibration isolation Vibration damping Amplitude
Mass
Mass
Vibration source
Undamped axis overshoots
Step input
Mass
Damped Vibration source
IMPACT CAN DESTROY AN OBJECT OR A SINGLE ELEMENT OF THAT OBJECT
INNOVATING SHOCK & VIBRATION SOLUTIONS Visit sorbothane.com for Design Guide and Technical Data
MADE IN THE U.S.A.
800.838.3906
sorbothane.com
and cable, return, adjustable autoreturn, nonrotating, stage, and multi-mode cylinders. No matter the iteration, gas-spring extension force — a value that usually ranges from 1 to 5,000 N — depends on piston-rod cross-section multiplied by fill pressure. Manufacturers commonly express extension force with two values — for rod extension and rod retraction — at normal ambient temperature and with the piston rod pointing downward. (Note that typical ranges are only those most common; some gas-spring applications in heavy industries use gas springs delivering several hundred-thousand Newtons cases.) Other gas-spring definitions include two pull-in forces — at rod extension and rod retraction — and overall friction force. These values depend on the gas spring’s gas and damping-oil volumes. Various nozzle orifices and oil quantity allow control of push-out and push-in speed. If design parameters are unknown, look for manufacturers capable of prototyping — especially for designs requiring an exact force that’s hard to pre-estimate — as in lifting a frame in a set time, for example. Here, some manufacturers sell prefilled cylinders sporting bleed valves. Then installers can bleed gas from the cylinders after system setup to get the correct forceacceleration actuation profile. The only caveat here is that if too much gas is bled, the assembly will need a new spring. That’s why OEM-level quantities of gas springs justify pre-engineered cylinders with preset pressurization. Or gas springs can offer full in-design adjustability via bleed valves and movable-endstop pressurization mechanisms, Bowden cables, knobs, and more. Some emergency-use gas springs also employ gas-generator cartridges that resemble those in airbags.
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Slow down to speed up
...with ACE industrial shock absorbers. · Decelerate moving loads · Protect against wear and tear on machines · Increase throughput & reduce downtime 800-521-3320 | www.acecontrols.com 23435 Industrial Park Drive, Farmington Hills, MI 43885
ace controls — MC Handbook 8-18.indd1 51 WTWH_ACE_fullpg_shocks_073118.indd
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POWER TRANSMISSION REFERENCE GUIDE
Wave-spring specification
S
prings are flexible mechanical components to store and release energy or apply and release forces on machine axes. Wave springs combine flat (non-coiled) bow springs (as their waves) with traditional compressionspring coil geometry. For the design’s compactness (as a high force-to-work height ratio) and other benefits, some motion systems have migrated from traditional helical or coil springs to flat-wire wave springs. Design engineers typically work with manufacturers to customize wave springs to specific operating conditions by material, thickness, number of turns, and other geometric features. That helps address or avoid the following springinstallation challenges. Column buckling in springs occurs with long free lengths and spring ends that can’t evenly distribute load around the spring circumference. Buckling mainly depends on geometry and not spring material properties. Traditional springs tend to buckle when deflection (for a set free length) is excessive.
Such buckling is preventable by keeping the design to below critical deflection and length values. The former is the ratio of deflection to spring free length; critical length is the ratio of that length to the spring diameter. One rule of thumb for avoiding buckling in traditional springs is to keep free length to less than quadruple the spring diameter — and load to less than the product of spring rate, length, and buckling factor. Wave-spring buckling factors vary greatly, but the value is nearly always higher than those of other springs. That means they readily hold their centered cylindrical shape and often self-locate into assembly bores — even operating reliably in machined assembly features held to relatively loose tolerances. Spring surge is a potential concern in assemblies with a compression spring having one free end. Depending on the motion input, such springs can exhibit resonance that’s large enough to cause temporary loss of contact with the assembly housing — and damage surrounding machine elements. That’s an issue of highest concern if the spring material provides
Formulas for specifying a wave spring Here are basic formulas to narrow down the wave spring type, shape and size that best suit an application. Note that wave springs are helping today’s push toward design miniaturization. Smaller designs need shorter and smaller diameter wave springs. That’s spurred some developers to make wave springs less than 0.250 in. in diameter.
Operating stress S = (3πPDm)/4bt2N2
Fatigue stress ratio x = (s - s1/(s - s2)
Where: S = Operating stress P = Load, lb Dm = Mean diameter, in. b = Radial width of material, in. t = Thickness of material, in. N = Number of waves per turn
Where: s = Material tensile strength s1 = Calculated operating stress at lower working height (not to exceed s) s2 = Calculated operating stress at upper working height
Deflection f = ((PKZDm3)/(Ebt3N4)) x (ID/OD)
: Where: R = Spring rate, lb/in. P = load, lb f = Deflection, in. b = Radial width of material, in. t = Thickness of material, in. N = Number of waves per turn K = Multiple wave factor Dm = Mean diameter, in. Z = Number of turns
Where: f = Deflection, in. P = Load, lb K = Multiple wave factor Dm = Mean diameter, in. Z = Number of turns E = Modulus of elasticity b = Radial width of material, in. t = Thickness of material, in. N = Number of waves per turn
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Spring rate R = (P/f) = (Ebt3N4)/(KZDm3) x (ID/OD)
5 • 2019
motioncontroltips.com | designworldonline.com
5/15/19 8:46 AM
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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POWER TRANSMISSION REFERENCE GUIDE
little damping and the spring operates on an axis that must make fast reciprocating strokes. Well-chosen wave-spring geometry and material can help avoid issues of spring surge and resonance excitation. Mean, cycling, and localized stresses on the spring each load cycle (and the load cycle itself) dictate when spring fatigue failure could occur. For any spring design, more turns make for a longer MTBF … while longer springs generally have shorter MTBF. But no matter the variation, wave springs fatigue more slowly and have longer MTBF than traditional springs. Springs under normal operation exhibit no permanent spring-rate or dimensional changes — called relaxation or set. But deflection under full load with stresses exceeding the spring material’s yield strength will induce permanent deformation that compromises the spring ability to deliver full design force or energy. This is often a concern in designs that must operate in extremely hot ambient conditions. Wave-spring deflection is about 25% lower than that exhibited by traditional springs. Wave springs are also more resistant to relaxation for comparable diameters, free lengths, and turns. One caveat: In designs necessitating fewer turns and shorter free lengths, sometimes traditional compression springs outperform wave springs by exhibiting less deformation. Any spring under a twisting moment load exhibits shear stress and stores strain energy. Wave springs exhibit less strain energy than traditional spring variations (and lower equivalent stress values) so better withstand such loading.
| courtesy Smalley Steel Ring Co.
Deformation and strain energy for wave springs
Strain energy (MJ)
60
Traditional spring
Wave spring
50 40
35 30 25 20
30
15
20
10
10
5 0
400
800
1,200
1,600
2,000
2,400
Deformation (mm)
} }
70
2,800
Load (N) Wave springs outperform other springs under the application of normal (non-twisting) forces. Data courtesy P. Ravinder Reddy and V. Mukesh Reddy
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Rotor Clip — MC Handbook 8-18.indd 55
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AD INDEX
POWER TRANSMISSION REFERENCE GUIDE
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motioncontroltips.com | designworldonline.com
5/15/19 3:11 PM
A BELT FOR ANY APPLICATION WE HAVE AN ANSWER FOR THAT Continental has the products and know-how to solve any drive system issue — from synchronous belts and banded V-belts to specialty belts for variable transmissions. We’ll even create customer-specific drive solutions for industrial equipment. To see our full line of power transmission belts and sprockets, visit continental-industry.com.
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