Worm gearbox offers near-zero backlash

A range of right angle precision gearheads has been developed that offers less than 1 arcmin backlash during service through a unique design. Dean Palmer reports A range of precision right angle gearheads have been developed which promise no increase in backlash during operation, by ensuring there is 90 to 95% contact between the worm and wheel, and by using a specially developed bronze alloy gear wheel that results in less wear and pitting. The Servoplan SWG gearhead, developed by ZF Great Britain, also has an extra wide worm wheel and, unlike other gearheads on the market, does not require adjustment due to preload. This results in a range of gearheads that can offer three levels of backlash, a less than 10 arcmin standard version, then precision units from 3 arcmin, which should satisfy most applications, down to a very high precision unit that offers less than 1 arcmin backlash, an impressive level of precision for a worm gearbox. The gearheads also benefit from large diameter taper bearings on the output shaft that maximise radial load capability, making them ideal for demanding rack-and-pinion applications. The high quality bearings also ensure a minimum of 25,000 hours service life, 5,000 hours more than most competing gearheads. Most precision right angle gearheads on the market use a key or 'cast over' method to attach the worm wheel to the shaft, which can lead to increased backlash or shearing of the key. But with the SWG, the worm wheel is lightly pressed onto the shaft to eliminate any run-out. Then, the worm wheel is pinned to the shaft with up to 10 pins. The torque is transmitted evenly through the pins to eliminate any chance of shearing or movement. The gearbox is also very quiet, with a less than 55dB rating. Other clever design features include a double taper bearing which gives the gearbox higher stiffness to the worm shaft. This not only reduces any vibration but also increases the life of the unit. On the other side, an axial-free, 'floating' ball bearing allows the worm shaft to expand as the temperature increases. This eliminates side load between the wheel and the shaft, improving service life and maintaining the stated backlash. Rob Pearson, group manager industrial products at ZF Great Britain, told Eureka: "The one arcmin version has a hardened, ground worm shaft and a dual lead shaft gear form, which enables adjustment down to very precise levels without pre-load. Each gearbox is supplied motor-ready and flanges are available for all types of servo motor. The motor can be connected to the gearbox in a matter of minutes." The body of the gearhead is manufactured from one-piece in cast and heat treated aluminium-magnesium alloy, which offers excellent rigidity and low weight. There's also a large number of fixing holes which gives maximum flexibility for mounting options, and all the fixing faces are machined to the same height. The compact gearbox case lends itself well to weight-critical automated assembly and packaging applications and the range of mounting options, including a robot flange, make the boxes ideal for conveyors and other materials handling systems. Ratios are from 5.2:1 to 90:1 and output torques are available up to 6,900Nm. Input speeds go up to true servo speeds of 6,000rpm and the gearbox comes in 10 different sizes and four output options. ZF has also recently launched a selection CD for its range of precision planetary gearheads, which covers both single and twin stage versions with ratios between 3:1 and 100:1. Ideal for engineers and designers in printing, handling, robotics, winding and machine tool industries, the CD allows users to specify a gearhead by entering the fields for smallest ratio, highest ratio, lowest nominal torque, required output torque and choosing from continuous duty (S1) or cyclic S4/S5 designs. Other options include selecting compatibility according to the top 50 motor brands, fitting requirements or model number. Pointers * The gearhead offers a backlash of less than 1 arcmin, very impressive for this type of gearbox * Near-zero backlash is achieved during operation by lightly pressing the worm wheel onto the shaft to eliminate any run-out. The wheel is then pinned to the shaft with up to 10 pins. The torque is transmitted evenly through these pins to eliminate any chance of shearing or movement * Applications include automated assembly tasks, materials handling, conveyors and robotics