The unique principle of operation of gears, from which the name Strain Wave Gearing is derived, was patented in 1955, by its inventor, C.Walton Musser. Initially this new gear concept was applied in aerospace and other highly specific applications.
All strain wave gearing products employ the same three basic elements: a Circular Spline, a Flexspline and a Wave Generator. Although these elements may take alternate forms depending on their application, the types described here are typical and representative
WAVE GENERATOR (WG)
The Wave Generator is an elliptical cam enclosed in an antifriction ballbearing assembly. It normally functions as the rotating input element. When inserted into the bore of the Flexspline, it imparts its elliptical shape to the Flexspline, causing the external teeth of the Flexspline to engage with the internal teeth of the Circular Spline at two equally spaced areas 180 degrees apart on their respective circumferences, thus forming a positive gear mesh at these points of engagement.
The externally toothed Flexspline is a nonrigid or flexible, thin-walled, cylindrical cup which is smaller in circumference and has two less teeth than the Circular Spline. It is normally the rotating output element but can be utilized as the fixed, non rotating member when output is through the Circular Spline.
CIRCULAR SPLINE (CS)
The Circular Spline is a thick-walled, rigid ring with internal spline teeth. It normally functions as the fixed or non rotating member but can, in certain applications, be utilized as a rotating output element as well.
The Principles of Strain Wave Gearing
The diagram shows the three basic strain wave gearing elements (Circular Spline, Flexspline and 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 of the strain wave gearing unit 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 to progress in a continuous rolling fashion. It also allows for full tooth disengagement at the two points opposite the minor axis of the Wave Generator. Since the Flexspline has two less teeth than the Circular Spline and because full teeth 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 counter-clockwise direction and vice versa. In this way, a basic three element strain wave gearing component set is 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 the output element, to rotate in the opposite direction of, and at a 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 200/(202-200)=100:1
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