David Clark explores the advances that belt drives present to users and designers of machine tools
When designing belt drives for machine tools, the user is presented with a range of options, thanks to the new materials and processes available to belt manufactures. For speeds of up to 60m/s, elastomer v-belts or multi-ribbed belts are sufficient. Beyond 60m/s, Polyflex and Polyflex JB polyurethane v-belts are the best option. The key properties of these drives are the low specific weight of the belts, their high power density compared to other types of belt and their quiet operation, even in highly dynamic applications. Alongside the construction of the belts and the materials used, Polyflex belts incorporate a special wedge angle of 60°, which enables the drive to reach circumferential speeds of up to 100m/s.
However, if the application in question involves a positioning drive that also runs at a high speed in the region of several thousands of revolutions per minute (rpm), the solution lies with synchronous belts as friction drives are not suitable for this purpose. Increasingly, synchronous belt drives are being used instead of simple transmissions and chains, which are not designed for higher speeds, and are loud and require frequent maintenance.
When selecting the right synchronous belt design for high-speed applications, the following principles should be considered:
Selecting a suitable tooth pitch and profile
Too small a tooth pitch will entail an increase in the construction space required as well as higher noise levels. Pitches that are too large, on the other hand, will necessitate larger pulley diameters, and will also result in high noise levels. The best solution is a belt drive combining the highest power density and the lowest possible tooth pitch. For high-speed drives and drives with power ratings up to 100kW, 8mm pitch belts with GT-profile involute tooth form are normally used.
For very compact drives, a 5mm pitch belt with an extremely rigid carbon tensile member is recommended. Belts with small pitches, such as 2mm and 3mm, are used for low-power applications that involve high-precision positioning, measurement, machining or engraving.
Selecting a minimal width to reduce noise level
The use of several narrow belts on one belt pulley, as opposed to one wide belt, has proven particularly effective in minimising transmission noise. The width of the individual belts used in these kinds of drives is typically between 12mm and 30mm and these are suitable for rotational speeds up to 10,000rpm, with an 8mm pitch belt system.
Setting the correct preload
This is important because, in the case of high-speed synchronous belt drives, the preload influences service life, heat generation, noise levels and the reliability of the components. The preload also has a significant impact on the stiffness of the system. As such, as early as during the planning stage of dimensioning a belt drive, it is necessary to bear in mind the stiffness and preload of the belts. The preload of the belts decreases at first due to post-installation run-in behaviour, but then remains constant after run-in for the rest of the service life of the machine, if the correct belts have been selected. Therefore, drives with synchronous belts can be expected to be maintenance free for the entire duration of their service life. This can be tens of thousands of operating hours depending on dimensioning and wear to the drive components.
For slow-running drives with a high torque, ranging from a few revolutions per minute to 1,000rpm, a belt pitch of 14mm provides the best solution. Spindles or tables can be rotated using endless belts, usually with carbon tensile members that deliver the correct level of stiffness within the minimum package size.
Systems for automatically changing tools and work pieces can utilise open-ended synchronous belts or flat belts made of elastomer or polyurethane materials, the end of which are clamped in place using profiled clamping plates. Depending on the mass of the object to be moved, belt drives can be used at acceleration and deceleration rates up to 60m/s2.
To ensure that belt drives reliably and sustainably perform their function, they must be resistant to ambient conditions. Belts in machine tools often come into contact with aggressive liquids, or at the very least oil vapours. For such applications, HSN/HNBR elastomer belts or polyurethane belts are most suitable. Although polyurethane v-belts are resistant to aggressive ambient conditions, if oil enters the grooves of the v-belt pulley, there is a risk of slippage.
The compact construction of machine tools results in poor heat dissipation. For temperatures of up to 100°C, polychloroprene belts are the best option, and for temperatures of up to 130°C, HSN/HNBR belts, special polyurethane belt models and standard Quad Power III belts made of an EPDM compound are most suitable.
A well designed and installed belt drive can last many thousands of hours on a variety of applications within a machine tool platform.
When deciding on the right solution for a specific project or application, it pays to get some expert advice. Gates is known globally as a leading manufacturer of power transmission belts and fluid power products. Its highly engineered products are critical components that are used in diverse industrial and automotive applications. The company offers a complete portfolio product and service solutions for both replacement and first-fit applications across its targeted end markets.
For more information visit www.engineerlive.com/ede
David Clark is a senior engineer at Gates PT, in Dumfries, Scotland.