Harmonic Drive

Harmonic Drive Applications

Harmonic Drive - A Guide to Choosing Gears for Robotics

Although there are now a variety of electrical configurations to choose from, this hasn't always been the case. Before widespread electrical innovation in induction motor technology and the advent of variable speed drives (VSDs), controlling the output speed of a system was achieved through the use of gears. This means that the final output speed of a typical squirrel-cage motor running at 1440rpm can be reduced as required by varying the gear reduction ratio. This increases flexibility by allowing the same motor to be used for different speed applications without a VSD. It is now of course possible to control the speed of a motor using VSDs however the drive cannot replace the gears other key benefits, torque multiplication and inertia matching, allowing a relatively small, low power, motor to move and accurately control a large load, hence reducing running costs and general machine weight and size.

Ask your child to draw a picture of a gear and you'll get a spur gear, a disk with teeth projecting radially. Used in everything from washing machines, cars and clocks, to industrial cutting machines and power plants, spur gears are cheap and easy to install. They offer a good power transmission efficiency and a constant velocity ratio, with the ability to transmit a large amount of power, up to 50,000kW. For anyone using this basic gear type and the closely related helical gear there are a few considerations. Generally these gears have significant backlash and although they can be fitted with backlash compensation, this accuracy is not maintained throughout the gear's life without adjustment. As well as this, spur gears can be noisy at high speed, helical gearing less so. Also, while they have the ability to be variably configured they can occupy a large footprint especially in high ratios, due in part to each individual gear shaft needing to be supported in its own bearings. A Bevel gear can be considered in the same family as a spur/helical gear and can also be straight or helical cut. Many of the considerations above apply, although the right angled nature of this gear can help in applications where space is at a premium.

So called, due to its earthworm like movement, the worm drive consists of two parts, the screw shaped worm gear and the larger spur-shaped worm wheel. Meshed perpendicular to the axis of rotation, the worm gear offers a compact solution and a large single-stage gear reduction can be achieved, however the larger ratios suffer from low efficiency. The design of a worm gear means that a large hollowshaft can be bored into the central cylinder of the worm wheel, making it convenient to pass through cables and services. With a few modifications, this gear type can also offer relatively good precision. By increasing the pressure on the surfaces in contact, backlash, the transverse movement apparent in a gearing system, can be reduced. However, this does increase wear on the teeth, reducing efficiency and meaning in-life adjustment is often necessary to maintain the accuracy of the gearbox.

Moving up to the next category we have epicyclic gears. More commonly known as planetary gears they are mounted in such a way that a numbers of gears, typically three to five, rotate like planets around a central sun gear, surrounded by an outer annular ring gear. Planetary gears provide a high power density, over 95% efficiency and, as a result of their design, are very compact. Accuracy can be high, with backlash achievable down to 1 arcmin. By combining several stages of gearing, high reduction ratios can be achieved with the maximum single stage ratio typically being 10:1. Planetary gears are generally more expensive than helical gears and can require more maintenance owing to a higher part count.