Harmonic Drive - A guide to choosing gears for robotics

The ongoing pressure on engineering companies to reduce costs, increase efficiency and yield a higher return on investment (ROI) is pushing many business leaders to consider alternatives to direct-drive motor systems in the form of various mechanical transmission solutions. Although gearing systems can offer simplicity, cost effectiveness and flexibility, it's not always clear which type of setup is best to use. Here, Graham Mackrell, managing director of Harmonic Drive UK, explores and critiques the four main gear types.

There is no doubt that gears play a critical role in the world we live in. From large scale deep-sea oil and gas drilling and industrial manufacturing the world over, to small scale applications such as the conveyor belt at the till in your local supermarket and even the tiny gearbox in your car's windscreen wipers, gears are invaluable.

It's no surprise then, that putting aside a short-lived decline during the 2009 financial crisis, the global market for gearboxes and geared motors has seen year-on-year growth over the last decade. Recent research by Frost & Sullivan has shown that the market earned revenues of $12.8 billion in 2013 and, due in part to continued innovation in wind power, is estimated to reach $15.67 billion by 2017.

Currently, the market is weighted geographically towards the Asia-Pacific region. However the slowdown of the Chinese economy due to overproduction in recent years, in addition to an increasing demand for high-precision gearing systems for broadcast and aerospace, is set to provide growth in the North American and European regions.

Gear system technology
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.

Spur Gear
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.

Worm gear
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.

Planetary gears
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.

For the more precise applications we have developed here at Harmonic Drive a range of Planetary gears. Our HPG range features a unique flexible ring gear allowing us to preload the mesh between planet and ring gearing, this increases the accuracy to one arc minute and testing has shown this preload system delivers excellent repeatability over time.

The improved HPGP range has 4 planet gears increasing torque capacity further size for size. Our HPN range is a more conventional gear with helical gearing to increase torque capacity and lower noise, this is available in accuracies of up to 5 arc minutes.

Strainwave gear
The ultimate in accuracy and quality is a strainwave gear, also known generically as a harmonic drive. For applications that require the highest power density and accuracy, a strainwave gear is essential. In demanding applications such as broadcast motion control, oil and gas extraction, robotics, aerospace, metrology and high precision industrial machine tools, strainwave gears are a necessity.

A strainwave gear consists of three parts. An outer Circular Spline, a fixed ring with gear teeth on the inside, is meshed to an inner Flexspline, a flexible ring with gear teeth on the outside, the Flexspline is smaller in diameter than the Circular Spline and has fewer teeth so does not mesh without the third component, an elliptical centrally mounted Wave Generator attached to an input shaft.

A strainwave gear is unique in that very high single stage ratios are possible, from 30:1 up to 320:1, in the same space that a planetary gear can only achieve a 10:1 ratio. This impressive feat is made even more impressive by maintaining a compact size, a very low weight, zero backlash, a low component count and very high torque levels.

The central shaft can even be bored to provide the largest hollowshaft possible on a concentric gear. It's these characteristics that resulted in Harmonic Drive being chosen for inclusion by NASA on the Mars Rover.

Gearing up
It's clear that the world of gears is more complex than first meets the eye. Making the right choice of transmission for your given application can drastically alter operational efficiency, energy usage and ultimately the total cost of ownership. This is becoming an ever more important aspect of the decision making process as we move towards cost saving oriented high-accuracy applications.

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