Harmonic drive

A harmonic drive is an input/output gearing mechanism. It is typically used for gearing reduction, but may also be used to increase rotational speed or for differential gearing. Very high gear reduction ratios are possible in a small volume (100:1 is possible in the space that Planetary gears typically only produce 10:1).

History

The basic concept of a harmonic drive was introduced by C.W. Musser in 1957. It was first used successfully in 1964 by Hasegawa Gear Works, Ltd. and USM Co., Ltd.Later, Hasegawa Gear Works, Ltd. became Harmonic Drive Systems Inc. located in Japan and USM Co., Ltd. Harmonic Drive division became Harmonic Drive Technologies Inc.

On January 1, 2006, Harmonic Drive Technologies/Nabtesco of Peabody, MA and HD Systems of Hauppauge, NY, merged to form a new joint venture, Harmonic Drive LLC ("Gear Product News", April 2006, page 36). HD Systems, Inc. was a subsidiary company of Harmonic Drive System, Inc. Offices are maintained in both Peabody and Hauppauge.

Mechanics

The harmonic drive theory is based on elastic dynamics and utilizes the flexibility of metal. The mechanism has three basic components: a wave generator, a flex spline, and a circular spline. More complex versions will have a fourth component normally used to shorten the overall length or to increase the gear reduction within a smaller diameter, but still follows the same basic principles.

The wave generator is made up of two separate parts: an elliptical disk called a "wave generator plug" and an outer ball bearing. The gear plug is inserted into the bearing, giving the bearing an elliptical shape as well.

The flex spline is like a shallow cup. The sides of the "cup" are very thin, but the bottom is thick and rigid. This results in significant flexibility of the walls at the open end due to the thin wall, but in the closed side being quite rigid and able to be tightly secured (to a shaft, for example). Teeth are positioned radially around the outside of the flex spline. The flex spline fits tightly over the wave generator, such that when the wave generator plug is rotated, the flex spline deforms to the shape of a rotating ellipse but does not rotate with the wave generator.

The circular spline is a rigid circular ring with teeth on the inside. The flex spline and wave generator are placed inside the circular spline, meshing the teeth of the flex spline and the circular spline. Because the flex spline has an elliptical shape, its teeth only actually mesh with the teeth of the circular spline in two regions on opposite sides of the flex spline, along the major axis of the ellipse.

Assume that the wave generator is the input rotation. As the wave generator plug rotates, the flex spline teeth which are meshed with those of the circular spline change. The major axis of the flex spline actually rotates with wave generator, so the points where the teeth mesh revolve around the center point at the same rate as the wave generator. The key to the design of the harmonic drive is that there are fewer teeth (for example two fewer) on the flex spline than there are on the circular spline. This means that for every full rotation of the wave generator, the flex spline would be required to rotate a slight amount (two teeth, for example) backward relative to the circular spline. Thus the rotation action of the wave generator results in a much slower rotation of the flex spline "in the opposite direction".

For a harmonic drive mechanism, the gearing reduction ratio can be calculated from the number of teeth on each gear:

Reduction ratio = (flex spline teeth - circular spline teeth) / flex spline teeth.

For example, if there were 202 teeth on the circular spline and 200 on the flex spline, the reduction ratio is: (200 - 202) / 200 = -0.01

Thus the flex spline would spin at 1/100th the speed of the wave generator plug and in the opposite direction. This allows different reduction ratios to be set without changing the mechanism's shape, increasing its weight, or adding stages. The range of possible gear ratios is limited by teeth size limits for a given configuration.

Harmonic Drive provides unique advantages such as:

* Zero Backlash
* High Reduction Ratio with Single Stage
* Compact & Light weight
* High Torque Capability
* Coaxial input & output shaft

Lauletta, Anthony. "The Basics of Harmonic Drive Gearing", [http://www.gearproductnews.com/issues/0406/gpn.pdf"Gear Product News", April 2006] . pp 32-36.

ee also

External links

* [http://www.waltmusser.org/HD.htm Waltmusser.org]
* [http://www.harmonicdrive.net/reference/operatingprinciples/ Harmonicdrive.net]
* [http://www.chrisdesign.com/projects/hdinfonet/firstpage.htm Chrisdesign.com]