Harmonic approach to gear technology01 December 2006

Much of today's plant machinery uses high-ratio planetary gearboxes to provide a wide range of operating speeds and output torques for use in applications such as turbo systems for compressors and pumps, mobile equipment and machine tools. Due to their design, with several smaller planetary gears around an input pinion or sun gear, these systems enable high reduction ratios to be achieved via a single stage.

The popularity of these systems is due to a number of benefits they offer, including high levels of efficiency and the ability to cope with greater radial loads. Planetary gear boxes also allow a driven load to be mounted directly to a shaft, thereby eliminating the need for complex and often costly couplings and alignment mechanisms.

Yet high-ratio planetary gearboxes can, however, often be relatively large and heavy, effectively requiring two separate units and typically suffering from problems such as backlash, leading to inaccuracy or inefficiency. Although this degree of play between the gear teeth can be reduced by carefully controlled manufacturing techniques, this can greatly increase production costs. Also, it is impossible to eliminate backlash completely from the gear assembly and therefore it becomes necessary to incorporate mechanisms to compensate for the effect. Again, this adds to cost and complexity, and can increase the overall size of the gear unit.

While planetary gearboxes continue to be appropriate for a number of applications, there are many instances where smaller, lighter, but nonetheless powerful, harmonic drive gear sets can be used as effective replacements, eliminating the problem of backlash, and offering greater accuracy and similar reduction ratios and levels of torque. Moreover, these systems are more energy efficient and offer the advantage of hollow shafts through which cabling, pipes or laser beams can be routed.

Harmonic drive technology
Harmonic drive gear systems were originally developed 50 years ago as part of the American space and military programme. However, the technology has since evolved to become incorporated in to many industrial products. Today, the systems are largely standardised, delivering benefits in a diverse range of applications.

Harmonic drive gears are constructed from three basic components - a wave generator, flexspline and circular spline - which fit one within the other to form an integrated component set.

The wave generator is a type of thin-raced ball bearing, set around a precision-machined elliptical plug that acts as a highly efficient torque converter. The flexspline fits over the wave generator to hold it in an ellipse and is constructed from a flexible metal cylinder, with external teeth and a flanged mounting ring. In turn, the circular spline fits over the flexspline and is formed from a solid steel ring with internal teeth; the circular spline is slightly larger in diameter than the flexspline, with two more 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 starts to turn, the zone of engagement follows the rotation of the ellipse. Rotating the wave generator by 180º effectively regresses the position of the flexspline by one gear tooth, relative to the circular spline; a complete revolution will change the relative positions of the two components by two teeth.

Engineering benefits
Due to this revolutionary design, each component can be used as either the input or output drive or as a fixed part. This makes harmonic drive gear sets ideal for reduction gearing, gearing for increasing speed or differential gearing, while offering a number of important advantages in a wide range of high-precision applications.

In particular, these systems can provide high output torque with zero backlash, as power is transmitted through multiple teeth engagement. This ability to eliminate backlash means that a harmonic drive gear set can offer exceptionally high levels of precision, allowing systems to operate at high speeds with positional accuracies that are less than one minute of arc, with a repeatability within a few seconds of arc.

Also, a wide range of reduction ratios can be achieved with the technology, from 30:1 to 320:1, with high operating efficiencies, thanks to the use of the simple three component assembly. Also, the sliding and frictional forces associated with standard gears are eliminated, even at high operating speeds, as the gear teeth come into contact with an almost perfect radial motion.

Harmonic drive technology offers a further significant advantage over planetary and other types of gear systems, as it is possible to build these gear sets with hollow central shafts. As a result, cabling, piping and even laser beams for measuring or guidance devices can be quite easily routed through the gear set, saving operating space and, perhaps more importantly, machine assembly time and costs.

Although harmonic drive gears can at first appear to be more expensive than conventional devices, this technology becomes a far more cost-effective choice in many plant engineering applications when the costs resulting from backlash compensation or cable-routing mechanisms are taken into account. The development of harmonic drive gear technology offers manufacturers another option when designing and building machine and production line systems, enabling them to find the right components for any system and achieve significant improvements in performance in the process.

Graham Mackrell is UK sales manager of Harmonic Drive UK. For further information, visit: www.harmonicdrive.co.uk

SOE

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