6 months ago

MDA Technologies 5/2014

MDA Technologies 5/2014

Belts as alternative

Belts as alternative drives for machine tools Sasha Kolouch, Hans Offermanns For a long time, it was believed that linear motors and direct drives provided the answer to the need for higher machine tool productivity, and that belt drives were on the decline. However, thanks to new materials, belt drives may once again be the number one choice. W hen using belt drives for machine tools, the user is presented with a range of options, depending on the application. For speeds of up to approximately 60 m/s, elastomer v-belts or multi-ribbed belts are sufficient. Beyond 60 m/s, Polyflex polyurethane v-belts are the best option, especially in the form of banded belts. The key properties of these belt 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 high-dynamic applications. Author: Sasha Kolouch works in the Eastern Europe Regional Sales Department at Gates. Hans Offermanns is an Applications Engineer at Gates. Alongside the construction of the belts and the materials used, this is achieved using a special wedge angle of 60°, which enables the drive to reach circumferential speeds of up to 100 m/s. This special wedge angle requires the use of customized pulleys. Standard v-belt pulleys have an angle of 34° to 38°, depending on their diameter. However, producing belt pulleys with a special flank angle is usually not a problem for machine tool manufacturers. Synchronous belts If the application in question involves a positioning drive which runs at high speed, several thousands of revolutions per minute and since friction drives are not suitable for this purpose, the solution lies with synchronous belts. Increasingly, synchronous belt drives are being used instead of simple transmissions and chains, which are not designed for higher speeds, are loud and require frequent maintenance. When selecting the right synchronous belt design for high-speed applications, the following three principles should be taken into account: n Select a suitable tooth pitch and a suitable tooth profile in line with the power to be transmitted and the speed produced. Too small a tooth pitch will entail an increase in the construction space required as well as higher noise levels. Pitches that are too high, on the other hand, will necessitate a larger belt pulley diameter, and will also result in high noise levels. The best solution is a belt drive with the highest power density and the lowest possible tooth pitch at the same time. For high-speed drives and drive powers up to 100 kW, belts with 8mm pitch and GT-profile are normally used. For very compact belt drives, a 5 mm pitch with an extremely rigid carbon tensile member is recommended. Small 32 MDA Technologies 5/2014

MAchine Elements 01 Composite belts in a grinding machine drive 02 Oil-resistant polyurethane synchronous belts pitches of 2 and 3 mm are used for lowpower applications which involve highprecision positioning, measurement and machining or engraving. nSelect a belt drive with a minimal width in order to reduce noise levels as much as possible. The use of several split belts on one belt pulley with separating discs has proven particularly effective. The width of the 8 mm pitch belts used in drives with speeds of approximately 10,000 min -1 fall between 12 and 30 mm. nSet 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. It is important to consider the effect of preload on drive dimensions early in the planning of a belt drive. The preload of the belts decreases at first due to post-installation runin behavior, but then remains constant after runin 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 require no maintenance for the entire duration of their service life, which can last tens of thousands of operating hours depending on dimensioning and wear to the belt pulleys. The operating mode of machine tools is usually complex. Roughing operations are performed at low speeds, e.g. 200 min -1 , and high-speed processing at 10,000 min -1 . Under such complicated conditions, it is advisable to design the belt drive with the help of suitable calculation software. Programs such as Gates Design Flex Pro can be downloaded online free of charge. Polyurethane synchronous belts For slow-running drives with speeds, ranging from a few revolutions per minute to 1,000 min -1 and the high torques resulting from this, belt pitches of 14 mm provide the best solution. Spindles or tables can be rotated using endless belts, usually with a carbon tensile member which provides the correct level of stiffness. For the purpose of automatically changing tools and work pieces, open-ended synchronous belts or flat belts made of elastomer or polyurethane are used, and are clamped in place using clamping plates. Depending on the mass of the object to be moved, belt drives are also used where acceleration and deceleration rates are up to 60 m/s². In the case of high-speed belt drives, proper balancing of the belt pulleys must be ensured. Unbalanced belt pulleys cause vibrations, and in the case of cast iron pulleys this entails a risk of collapse at high speeds. Dynamic balancing of belt pulleys should only be performed on the shaft onto which they will ultimately be installed. Users have often encountered problems in the past due to ex warehouse belt pulley balancing. To ensure that the 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 vapors. 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. About Company name: Gates Europe Headquarters: Erembodegem, Belgium Employees: 14,000 worldwide Products: industrial belts, sprockets, sheaves, tensioners, industrial hose, couplings, industrial hydraulic equipment MDA Technologies 5/2014 33


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