The secret of the strain wave gear

The invention of the strain wave gear dates back more than 60 years. Even today, the principle and aesthetics of the gear inspire developers, designers and technology enthusiasts around the globe. Responsible for this: the creativity of one man – Clarence Walton Musser.

Clarence Walton Musser was a contemporary man who incorporated both the past and the future into his thinking and inventive ways. His genius and technical understanding resulted in well over 200 patents that were not concentrated or even limited to Musser's special field, mechanical engineering. Whether in the areas of physics, chemistry or biology - the US-American distinguished himself through his wide-ranging interests, with his completely restless mind and analytical abilities.

Equal to Musser's patriotism were ethics and honesty. His intellectual abilities were as legendary as his sense of humour - and so a day with Walt, as friends and acquaintances called him, was always like an adventure. To recognise his genius, it was enough to listen to him. Equally strong in character was his choice of friends. He avoided aloof people and rather had close contact with those he truly liked, regardless of their position in society.

After Musser had devoted himself to aviation in the late 1940s and early 1950s, which resulted in the invention of the ejection seat and the canopy for jet aircraft, among other things, his attention fell on elastic bodies. So he then began his study and research in this field and, in doing so, he sometimes relied on controlled bending and deflection. Later, he applied these to new mechanical designs, structures and concepts - and finally developed the functional principle of the strain wave gear.

Even today, the features of Harmonic Drive® Gears presented above are largely based on the interaction of their simple design, but above all on the unique principle of elastic deformation of gears and the Flexspline respectively. The latter is a high strength, torsionally stiff, yet flexible component that reliably transmits high loads.

Whether in a housing, as an actuator with motor and encoder or as part of customised configurations, adaptability of the Harmonic Drive® Gear enables a wide range of applications. However, the functional principle itself has remained constant over all these years.

The Harmonic Drive® Gear is based on three components. The basis is embodied by the Wave Generator, an elliptical steel disc at the heart of the gear. It has a centric hub and a thin, elliptically deformable special ball bearing. The Wave Generator uses the ball bearing to deform the Flexspline, a deformable cylindrical steel sleeve with teeth arranged radially around the outside. As a rigid and cylindrical outer ring, it is the Circular Spline, which is the third component surrounding both Wave Generator and Flexspline and is equipped with internal teeth.

As the Wave Generator rotates, the large elliptical axis shifts and with it the gear meshing area. The number of teeth themselves is of great importance - the external teeth of the Flexspline have exactly 2 teeth less than the internal teeth of the Circular Spline. Accordingly, during a half rotation of the elliptical Wave Generator, there is a relative movement of one tooth between the Flexspline and the Circular Spline; during a full rotation, there are already two teeth.

The fact that Harmonic Drive® Gears are used in such a diverse range of sectors, industries and applications is due to their wide-ranging performance characteristics. These include the following: