Harmonic drive

A harmonic gear is a strain wave gear which can improve certain characteristics compared to traditional gearing systems. Harmonic Drive is trademarked by the Harmonic Drive company. The strain wave gear was invented in 1957 by C.W. Musser.

It is very commonly implemented in robotics today and used in aerospace as well, for gear reduction but may also be used to increase rotational speed, or for differential gearing.

The basic concept of strain wave gearing (SWG) was introduced by C.W. Musser in his 1957 patent. It was first used successfully in 1960 by USM Co. and later by Hasegawa Gear Works, Ltd. under license of USM. Later, Hasegawa Gear Works, Ltd. became Harmonic Drive Systems Inc. located in Japan and USM Co. Harmonic Drive division became Harmonic Drive Technologies Inc.

The electrically driven wheels of the Apollo Lunar Rover included strain wave gears in their gearing. Also, the winches used on Skylab to deploy the solar panels were powered using strain wave gears. Both of these system were developed by The Harmonic Drive Division of United Shoe Machinery Corp.

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. HD Systems, Inc. was a subsidiary company of Harmonic Drive System, Inc. Offices are maintained in Peabody, MA, Hauppauge, NY, San Jose, CA and Oak Park, IL. Harmonic Drive Systems, Inc., Japan is headquartered in Tokyo with its primary manufacturing location in Hotaka, Japan. Harmonic Drive AG has its European headquarters and manufacturing in Limburg/Lahn,Germany.

The strain wave gearing theory is based on elastic dynamics and utilizes the flexibility of metal. The mechanism has three basic components: a wave generator (2 / green), a flex spline (3 / red), and a circular spline (4 / blue). More complex versions have a fourth component normally used to shorten the overall length or to increase the gear reduction within a smaller diameter, but still follow 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 shaped like a shallow cup. The sides of the spline are very thin, but the bottom is relatively rigid. This results in significant flexibility of the walls at the open end due to the thin wall, and 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, so that when the wave generator plug is rotated, the flex spline deforms to the shape of a rotating ellipse and does not slip over the outer elliptical ring of the ball bearing. The ball bearing lets the flex spine rotate independently to the wave generator's shaft.

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