Application Notes

The diagram shows the three basic Harmonic Drive® strain wave gearing elements CIRCULAR SPLINE - FLEXSPLINE - WAVE GENERATOR assembled in a normal configuration. Ordinarily, the Circular Spline is held stationary or fixed and input is through the Wave Generator, while output is via the Flexspline. Under these circumstances, operation is as follows.

As the Wave Generator is rotated by the primary power source, it imparts a continuously moving elliptical form or wave-like motion to the Flexspline. This causes the meshing of the external teeth of the Flexspline with the internal teeth of the Circular Spline at their two equidistant points of engagement. This meshing progresses in a continuous rolling fashion. It also allows for a full tooth disengagement at the two points along the minor axis of the Wave Generator. Since the Flexspline has two less teeth than the Circular Spline and because full tooth disengagement is made possible by the elliptical shape of the Wave Generator, each complete revolution of the Wave Generator causes a two tooth displacement of the Flexspline in relation to the Circular Spline. This displacement is always in the opposite direction of the rotation of the Wave Generator (see diagram). For example, if the Wave Generator is rotating in a clockwise direction, the two-tooth per revolution displacement of the Flexspline will be in a counterclockwise direction and vice versa. In this way, a basic three elements are capable of functioning as a speed reducer. Input from a main power source through the Wave Generator is at a high speed, but the two-tooth per revolution displacement causes the Flexspline, which is considerably slower speed than, the Wave Generator. The reduction ratio which results can be calculated by dividing the number of teeth on the Flexspline by two (the difference between the number of teeth on the Circular Spline and the Flexspline). If a fixed Circular Spline had 202 teeth and an output Flexspline has 200 teeth, the ratio would be ...

*The negative sign indicates that the input and output are turning in opposite directions.

Advantages

The advantages of strain wave gear over other, more conventional gear trains are apparent. A simple three element construction combined with the unique principle puts extremely high reduction ratio capabilities into a very compact and lightweight package. This remarkable union of simplicity of construction with an operating principle that is unique, also enables backlash to be held to an absolute minimum, (zero backlash for cup-type components) an exceedingly important.