BELTS, PULLEYS & CHAINS FEATURE


Belt, leadscrew or chain drive? Putting the choice in motion


With the rise in ‘desktop manufacturing’ and 3D printing, people are now physically closer to mechanical components than ever before and, as a result, precise positional control is needed to ensure safety. So what components should you specify to achieve this goal? Patrick Faulkner, application engineer at Accu, explores the options


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sed in applications where power must be transmitted, often through a complex path,


or as a speed reduction, belt drives are specified for a wide range of products – from consumer electronics to automotive applications. As a generalisation, belts are used to transmit rotary force from one location to another, in the speed reduction of a drill press, for example. They are often used to replace gear trains in applications where the force must be transmitted over either a long distance, or through a complex path such as the aam belt in an engine. Replacing a geartrain or chain drive with a belt is more cost effective and significantly lighter in weight. Belts have a range of advantages over chain


drive or gear drive mechanisms. A belt is a low maintenance device and, providing the correct tension is kept, requires little to no attention until the end of its service life. V-belts are designed to transmit greater loads than flat belts, however they still have a tendency to slip under high loads. This led to the development of synchronous or timing belts. These are found in hobby robotics, the cam belts in an engine, and even the drive mechanisms of some vehicles. Of benefit, not only do the teeth of the


timing belt allow it to replace a more expensive gear train, but they produce less noise. This enables belt drives to be used as a speed reduction device for acoustically critical applications, such as a record turntable, and in automotive applications where they help reduce engine and transmission noise. With regards to motion control as opposed to


rotational applications, belts boast a higher motion speed due to their direct-drive nature, although this comes at the sacrifice of lower precision. This lends them to being suited to low-torque, high speed movements. The low backlash present in such a system has led to their use in desktop 3D printers, laser cutters and other low-load motion applications.


LEADSCREWS Leadscrews are motion control devices that convert the rotational motion of either the nut or screw into a linear motion. These are commonly used in robotics, hobby machinery and other applications which either require accurate motion or high loads to be transferred. Leadscrews use a helix design, similar to a standard nut and bolt, albeit with a thread form


designed to transmit force, rather than fasten parts together. They are available in a wide variety of sizes and, due to different lead values, can provide varying levels of motion for one revolution. The lead is defined by the pitch of the screw, which is the distance between the same point on adjacent threads, multiplied by the number of thread starts. This range of values allows leadscrews to be


specified for both high torque, low speed applications, and high-speed, high-precision applications. A comparative example would be comparing the leadscrew in a bench vice to the leadscrew in a DVD Drive Reader Head. The disadvantages to a leadscrew system are both cost and backlash. Backlash is lost motion, often observed as play.


In an application where force is only to be applied in one direction, such as a vice, this is not an issue. On a product where repeatable motion is required, however, this leads to the distance to be moved being less than expected, as the play in the mechanism is taken back up in the opposite direction with each change of heading. This unexpected loss – if not compensated for – would result in the difference between the expected location of an effector and its actual location increasing every time the force changes direction. In the case of a CNC router, for example, this would result in parts that are incorrectly shaped and sized, despite the program being correct. Backlash is inherent in leadscrews due to the


tolerances required for free movement: they have a built in ‘slop’ which is necessary to provide the clearances required for motion. In applications such as CNC machines, 3D printers and other devices where positional accuracy is paramount, this needs to be compensated by either an anti- backlash nut or by software. With regards to cost, for a medium precision application a belt drive system would typically be a fraction of the cost, so leadscrews are typically reserved for the most critical parts of an assembly.


CHAIN DRIVE A chain drive uses metal links, riveted together to transmit power. These are commonly found in bicycles, timing applications and industrial machinery. Chains are similar to belts in that they offer a ‘flexible’ power transmission method, but they have a few key disadvantages


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to belts, as well as some advantages. Chains are heavier than a comparably sized


belt, and they require regular lubrication and cleaning to ensure they last for their expected lifetime. They are also noisier and cannot be used in an application where slippage of the drive is required to reduce shock loads on the drivetrain. The advantages of the chain drive, however,


include its ease of installation. In the case of a chain with a master link, its resistance to wear and degradation from UV rays, oil and solvents, and its longer expected lifespan in harsh or abrasive conditions. Its inability to slip is also an advantage over belts, but it means that in the case of machine failure there is a higher risk of the chain breaking. These advantages allow its use in applications


such as a motorcycle’s cam chain, where it is submerged in oil, or for the drive on a mining excavator where constant exposure to dust and debris is expected.


SO WHY CHOOSE? Take 3D printers as an example, where sales are driven by both performance and price. A typical Cartesian style printer will utilise both leadscrews and belt drives for motion. The most critical aspect of a printer is its accurate Z motion, often the figure most printers are advertised by. To achieve high figures, leadscrews are used to move the Z axis, whereas belts and pulleys are used to move the higher speed X and Y axes. Anti-backlash measures are not typically used, due to the bottom-up printing method resulting in a constant load and motion direction of the leadscrew. This single direction removes the need for any expensive anti-backlash measures to be implemented. As always, there are a wide range of factors


that can affect the choice of drive, and this article outlines just a few of the most common choices.


Accu www.accu.co.uk DESIGN SOLUTIONS | OCTOBER 2020 35


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