(1) Field of the Invention
This invention relates to motion transmitting mechanisms, and more particularly to harmonic drive type transmissions.
(2) Prior Art
In U.S. Pat. No. 3,161,082 to C. W. Musser and assigned to the assignee of the current invention, there is shown a harmonic drive transmission having two wave generator discs each mounted on an eccentric ball bearing on a shaft. A flexspline is rotatively supported in a housing and on an output shaft. The housing has an inwardly directed array of teeth comprising a circular spline and the flexspline has thereon an outwardly directed array of matable teeth thereon, as shown in the aforementioned patent. The eccentric wave generators reduce the mass of the input elements which rotate at input speed, thereby increasing the response time.
As shown in the '082 patent to Musser, the wave generator discs are orthogonal to the axis of rotation of the wave generator shaft. No suggestion is made in these references, regarding the angle of the wave generator disc relative to the angle of the interengaging teeth of the flexspline and the circular spline. There is reference to the coning action of the flexspline when the distal end thereof is deflected from an essentially round condition into its ellipsoidal shape, which helps to reduce backlash. The outer surfaces of the wave generator discs described in this patent have "arcuate surfaces" in the longitudinal direction of their periphery.
U.S. Pat. No. 3,747,434 to P. Stahlhath describes a harmonic drive unit having a pair of wave generator discs, mounted parallel to one another and whose axes are coincident, both being mounted on a skewed bearing, so that the portion of tooth engagement generated by each respective disc lies in a common plane. The discs have arcuate outer surfaces in longitudinal cross-section.
The prior art does not minimize bearing wear, nor do any of the peripheral surfaces of the wave generator discs properly conform to the bore of the flexspline along the major axis when the optimum deflection shape is imposed upon the flexspline, to minimize its stress and therefore, maximize its life. The reference to Stahlhath mentions that asymmetrical loading and imbalance are reduced, but they do not say that the imbalance would be eliminated. Further, one of the wave generator discs tends to more nearly conform to the flexspline bore along one side of the major axis while the disc on the opposite side is skewed in a negative direction relative to the cone angle of the flexspline.
Thus it is an object of the present invention to provide a double eccentric wave generator arrangement for a harmonic drive type transmission which overcomes the disadvantages of the prior art.