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FEATURE GEARS & GEARBOXES Selecting the right gear

With gearing systems, it’s not always clear which type of setup is best to use. Here, Graham Mackrell, managing director of Harmonic Drive, explores and critiques the four main gear types


ears play a critical role in the world we live in – from large

scale deep-sea oil and gas drilling and industrial manufacturing, to small scale applications such as the conveyor belt at the supermarket till or the tiny gearbox in your car’s windscreen wipers. 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. While the market is currently weighted

geographically towards the Asia-Pacific region, the slowdown of the Chinese economy and the increasing demand for high-precision gearing systems for broadcast and aerospace, is set to provide growth in the North American and European regions. 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. While it is possible to control the speed of a motor using VSDs, the drive cannot replace the gear’s other key benefits, torque multiplication and inertia matching, allowing a relatively small, low power, motor to move and accurately control a large load.

SPUR GEAR Cheap and easy to install, spur gears are used in everything from washing machines, cars and clocks, to industrial cutting machines and power plants. 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 have significant backlash and although they can be fitted with backlash compensation, this accuracy is not maintained throughout the gear’s life without adjustment. Spur gears can also be noisy at high

speed; helical gearing less so. And, while they have the ability to be variably configured, they can occupy a large footprint especially in high ratio’s, 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 where space is at a premium.

WORM GEAR 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. The larger ratios, however, suffer from low efficiency. The design of a worm gear means

The right choice of transmission

for an application can drastically alter operational efficiency, energy usage and ultimately the total cost of ownership

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. This, however, does increase wear on the teeth, reducing efficiency and meaning in-life adjustment is often necessary to maintain the accuracy.

PLANETARY GEARS Epicyclic gears, commonly known as planetary gears, 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. These provide a high power density, over 95% efficiency, are very compact, and accuracy can be high, with backlash achievable down to one 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,

Harmonic Drive has used its HPN and HPG range of planetary gears. These include a flexible annular ring that boosts accuracy to between one and three minutes of arc.

STRAINWAVE GEAR A strainwave gear, also known generically as a harmonic drive, offers accuracy and quality, and is essential for applications that require the highest power density and accuracy. 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.

Very high single stage ratios are possible with a strainwave gear – from 30:1 up to 320:1 – in the same space that a planetary gear can only achieve a 10:1 ratio. Of additional benefit, these are compact, very low weight, zero backlash, and have 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. The right choice of transmission for an application can drastically alter operational efficiency, energy usage and ultimately the total cost of ownership – and this is becoming an ever more important aspect of today’s decision making process.

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