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Are these the world’s smallest precision, backlash-free gear sets?

23 February 2007

Harmonic Drive has introduced what it believes are the world’s smallest backlash-free precision gear sets. Called the Micro Harmonic Drive, the new gear components typically measure less than 8.0mm in diameter, with an axial length of under 1.0mm and a tooth module of just 34 microns, yet when supplied as part of a miniaturised gearbox or actuator can provide high reduction ratios, plus exceptional acceleration, repeatability and torque with low moments of inertia.

The latest micro gears sets are based on proven Harmonic Drive technology, with a conventional assembly of a Wave Generator, Flexspline and Circular Spline. In the Micro Harmonic Drive gear sets, however, the Wave Generator comprises a sun gear, which is normally connected to a motor shaft, with two elastically deformable planetary gears being used to transmit motion from the motor shaft to the Flexspline and thus the Circular Spline. Additionally, it is also possible to incorporate a Dynamic Spline as an output element when the gear set is used as a speed reducer.

Although conventional manufacturing techniques enable gear teeth modules down to 60 microns to be produced, depending on the geometry, the profile of each gear tooth is invariably compromised, affecting factors such as stiffness and torque capacity. To overcome these problems, Harmonic Drive uses a specially modified LIGA process, called Direct-LIG, which combines X-ray lithography and electro-forming to create tough nickel-iron or stainless steel components with dimensional tolerances that are typically better than one micron.

The manufacturing process, combined with the unusual construction and characteristics of the Micro Harmonic Drive components, delivers a number of important benefits. In particular, it is possible to create large reduction ratios in a small envelope; for a given size of gear, these ratios can be from 160 up to 1000 in a single stage. Perhaps as importantly, as the planetary gear wheels are hollow and elastically deformable, it is possible to eliminate backlash completely by pre-loading the gears in the planetary gear stage.

In addition, when mounted in a Micro Gearbox and used with a miniature motor, the high torque capacity and low moment of inertia enable extremely fast acceleration of up to 550,000 rad/s² to be achieved at the input shaft. This corresponds to an acceleration of the motor shaft from 0 to 100,000 rpm in just 25 milliseconds, and in the correct configuration also enables extremely fast angular movements; for example, rotation of 180° in less than 80 milliseconds.

The latest Micro Harmonic Drive gear sets can be supplied with hollow output shafts and as gearboxes and actuators, with miniature motors, making them ideal for use in applications ranging from electronics and semiconductor manufacturing, including high vacuum, to medical and surgical equipment, aerospace systems and Micro Scara robots.

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