Changing gears: decades of development

Graham Mackrell, UK Sales Director of Harmonic Drive UK, takes a look at some of the latest developments in gear technology.

Gears and gear mechanisms are probably one of the oldest mechanical technologies known to mankind.

Early examples include simple wooden gears used in potters' wheels, while wooden, stone and metal gears were used by early Greek and Roman civilisations.

Today, the humble gear has evolved into a high precision component used throughout industry and is based on advanced metals and composites to provide a tough, reliable and often exceptionally accurate method of controlling the rate of rotation of shafts, direction of rotation or of converting rotary to linear motion.

In many respects, the development of gear systems mirrors that of industry as a whole, with an ever growing need for greater machine and component performance at an increasingly competitive cost.

This changing demand has led to the evolution of a number of new gear technologies, of which harmonic drive gear technology is the one that offers perhaps the greatest potential, especially in the growing range of applications where high precision and repeatability, low or zero backlash, or high output torque are required from small, lightweight gear units.

Harmonic drive gear technology was originally developed in the late 1950s as part of the American space and military programme.

The first satellite equipped with a harmonic drive unit was launched as early as 1962 and by 1971 the gear systems had been hermetically sealed to be used on the Lunar Rover vehicles used in the Apollo 15, 16 and 17 missions, converting the drive from electric motors mounted on each Rover wheel.

Since then, this gear technology has been used in machine tools, automation equipment, jet aircraft and offshore oil and gas platforms.

Harmonic drive gear systems are constructed from three simple but precision-engineered components: a wave generator, flexspline and circular spline, which are designed to fit one within the other to form an integrated set of parts.

In essence, the wave generator is a thin-raced ball bearing surrounding an elliptical plug, which acts as torque convertor.

The flexspline is constructed from a flexible metal cylinder, with external teeth and a flanged mounting ring, and is mounted over the wave generator, holding it in an ellipse.

Finally, the circular spline is produced from a solid steel ring with internal teeth and is fitted outside the flexspline, so that the gear teeth mesh; the circular spline is larger in diameter than the flexspline and has two additional gear teeth.

In operation, the outer and inner teeth of the flexspline and circular spline engage across the major axis of the ellipse, so that as the Wave Generator turns the zone of engagement follows the rotation of the ellipse.

Rotating the Wave Generator by 180 degrees effectively regresses the position of the flexspline by one gear tooth, relative to the circular spline, with a complete revolution changing the alignment of the two components by two teeth.

This construction offers a number of important benefits over traditional types of gear; in particular, as power is transmitted through the simultaneous engagement of a large number of gear teeth it is possible to produce levels of output torque which, size for size, are twice that of conventional mechanisms, while overall unit size can be reduced by up to two thirds.

Similarly, there is an inherent gear preloading, so that backlash between mating teeth can be almost eliminated, with the effects of tooth pitch errors and accumulated pitch errors, commonly found with traditional gear mechanisms, being exceptionally low to give high levels of operating efficiency of up to 85% when measured between the input and output shafts.

Perhaps as importantly, as the gear teeth come into contact in an almost true radial motion with virtually zero sliding velocity, even at high input speeds, it is possible to reduce frictional losses between gear teeth to an absolute minimum, so that operating life can be significantly increased.

This inherent efficiency, combined with the ability to eliminate the effects of backlash, also produces exceptionally high levels of positional accuracy, typically less than one minute of arc, with repeatability being to within just a few seconds of arc.

By comparison, some of the best planetary gear systems are typically only accurate to three minutes of arc and to achieve this require additional components, which add to the weight and size of the overall gear assembly.

Harmonic drive gear systems also offer high levels of torsional stiffness, across their operating range, and with only three component parts can easily be manufactured in a range of single-stage reduction ratios from 50:1 and 320:1.

In addition, each component can become the input or output drive, or be a fixed part, enabling the system to be used for reduction gearing, gearing for increasing speed, or differential gearing.

One final advantage of this technology is that, unlike most traditional gear systems, harmonic drive gears can be constructed with a hollow central shaft, which can be used for routing cables, pipes and even laser beams.

This helps to simplify the overall system design and, as importantly, can make a significant reduction in unit size, weight and cost - all of which are increasingly important factors to OEMs and end users alike in today's highly competitive global marketplace.

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