An intelligent alternative

Published:  17 April, 2007

Graham Mackrell, UK sales manager of Harmonic Drive UK, discusses how harmonic drive gear technology can offer an effective replacement for conventional high ratio or two stage planetary gearboxes in plant applications.

The conventional planetary gearbox has offered a combination of torque, stiffness and reliability to plant engineers for many years, providing an alternative to other gear technology. Unlike competing gearing mechanisms, the design of planetary gearboxes, with an arrangement of several planetary gears around an input pinion or sun gear enables engineers to achieve high reduction ratios and levels of torque from a relatively compact unit, as the torque being transmitted at any time is shared between multiple sets of teeth on the primary drive pinion. Compared with a parallel shaft system, for example, a planetary gearbox can often achieve the same ratio with a large saving in weight and volume of typically up to 60% as well as offering a coaxial footprint.

Most planetary gearboxes incorporate spur gears, allowing them to deliver high levels of efficiency.  Normally, the output support shaft is independent of the gear output, the planet carrier, which means that large external radial loads do not effect the gear meshing; this in is direct contrast to most helical gearboxes, which have the output gear located on the shaft between the output load support bearings. This offers a number of benefits to the planetary system, including the option of specialist output bearing arrangements for items such as mixer and slew drives, and can often mean that the driven load can be mounted directly on the output shaft, eliminating the need for complex, and often costly, couplings and alignment mechanisms.

Planetary gearboxes have proved useful in applications where both forward and reverse motion is required, and for high torque/low speed applications, such as turbo systems for compressors and pumps, mobile plant equipment, mixers and agitators, low speed conveyors and winches.

Although planetary gearboxes are always worth considering, they do have a number of limitations.  In particular, they can become heavy and lengthy when using larger ratios (>30:1) that require two or three stages. Additionally, the lubrication in such gearboxes has to work particularly hard due both to the number of gears in mesh and the relatively small volume of oil available; consequently, the condition of the oil must be carefully monitored especially in arduous or critical duty applications, as its quality will directly affect the performance of the roller bearings supporting the planet gears.  Also, the sun gear which typically meshes simultaneously with three planet gears must be sized correctly to avoid premature fretting corrosion and subsequent failure.  Perhaps the most important feature in any precision application, however, is that these gearboxes suffer from an inherent level of backlash, which can cause significant inaccuracies and inefficiencies in the driven system.

This backlash, or the degree of play between gear teeth, can be reduced by carefully controlled manufacturing techniques, often to less than five minutes of arc in a single stage reduction unit, or eight minutes of arc in a double stage unit, but this can add considerably to production costs. Additionally, as it is impossible to eliminate backlash completely from the gear assembly, it therefore becomes necessary to incorporate mechanisms to compensate for the effect; again adding to cost and complexity, and increasing the overall size of the gear unit.

The presence of backlash in planetary gear systems can also cause a rise in hysteresis effects and a reduction in torsional stiffness. In each case, the impact on the performance of downstream production equipment can be significant, with repeatability and accuracy affected, and a potential decrease in long term reliability, with a corresponding rise in maintenance and operating costs.

Although planetary gearboxes are appropriate for a number of less critical applications, the growth in market demand for specialised and often highly demanding applications is increasing the requirement for smaller, lighter but nonetheless powerful gear sets that offer improved levels of performance, reliability and repeatability.

Providing one solution to this requirement is harmonic drive gear systems, which were originally developed in the nineteen fifties as part of the American space and military programme, but have since evolved to become mainstream industrial products. The technology now features a largely standardised design, making it suitable for use in a wide range of applications, with gears constructed from three basic components: a wave generator, a flexspline and a circular spline, which fit into one another to form an integrated component set. 

The wave generator is essentially a 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; as the circular spline is slightly larger in diameter that the flexspline it also has two more gear teeth.  In fitting the circular spline over the flexspline a pre-load is introduced between the two thereby eliminating clearance backlash.

When the gear operates, 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 180 effectively regresses the position of the flexspline by one gear tooth, relative to the circular spline; thus a complete revolution will change the relative positions of the two components by two teeth.

This simple yet robust construction enables each component to be used as an input or output drive, or as a fixed part; as a result, harmonic drive gear set can be used for reduction gearing, gearing for increasing speed, or differential gearing. This technology offers a number of important advantages in a wide range of high precision applications. As power is transmitted through multiple teeth engagement, with typically up to 30% of the gear teeth being in contact under load, the system provides high output torque, with zero backlash, while the use of a simple three component assembly enhances reliability. A wide range of reduction ratios can be achieved, from 50:1 to 320:1, with high operating efficiencies. Additionally, as the gear teeth come into contact with an almost perfect radial motion, the sliding and frictional forces associated with standard gears are removed, even at high operating speeds, limiting wear on the gears and giving greater repeatability of over time.

The ability to eliminate backlash enables a harmonic drive gear set to offer high levels of precision, allowing production systems to operate at high speeds with positional accuracies that are under one minute of arc, with a repeatability within a few seconds of arc.

The technology has a further important advantage over planetary and other types of gear systems in that it is possible to build harmonic drive gear sets with hollow central shafts. This means that cabling, piping and even laser beams for measuring or guidance devices can easily be routed through the gear set, saving both operating space and machine assembly costs.

Harmonic drive gears can also be easily incorporated into a base machine design, enabling a high degree of system integration to be achieved, or be supplied as standard gear units and complete motor actuators, incorporating large capacity output bearings that can be used as load bearing members.

In conclusion, the benefits of harmonic drive gears over planetary gearboxes can be summarised as a 300% reduction torque/volume and torque/weight, with harmonic drive gear units being considerably shorter; zero backlash, as opposed to 3 arc minutes or greater; and less wear and degradation in performance over time.

Harmonic drive gear technology offers greater choice in the design and build of production line systems. Increasingly, plant managers and system designers are recognising the potential in implementing harmonic drive gears, which are not only capable of offering significant improvements in performance, but can also result in long term cost savings.

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