Patent ID: 7735396

Claim:
A wave gear drive having a circular rigid internal gear, a flexible external gear disposed therein, and a wave generator inserted therein, the flexible external gear is equipped with a flexible cylindrical body and an annular diaphragm that extends in a radial direction from a back end of the cylindrical body, a tooth portion formed on a front end opening portion side of the cylindrical body that is flexed into an elliptical shape by a flexing amount generated by the wave generator from a back end portion on the diaphragm side to a front end portion on the opening end side thereof that is approximately proportional to distance from the diaphragm, wherein: a basic rigid internal gear and flexible external gear are spur gears with a module m, a number of teeth of the flexible external gear is smaller by 2n, wherein n is a positive integer, than a number of teeth of the rigid internal gear, a flexing amount of a major axis of an elliptical rim neutral line of the flexible external gear at a cross-section of the flexible external gear perpendicular to the axis at an arbitrarily selected location in the direction of a tooth trace thereof (hereinafter referred to as main cross-section) is set to be 2κmn, wherein κ<1, meshing of the teeth of the rigid internal gear and flexible external gear in the main cross-section is approximated by rack meshing and a movement locus M of a tooth of the flexible external gear with respect to a tooth of the rigid internal gear that accompanies rotation of the wave generator is obtained, on an entering side of meshing of the teeth towards a major axis of the movement locus M, a curved portion AB extending from a position of an inflection point of the movement locus M to a deepest meshing position B of the two gears is selected, the curved portion AB is enlarged by similarity transformation using a first ratio of similitude λ to obtain a first similar curve GF that is employed as a tooth profile of the flexible external gear, the curved portion AB is enlarged by similarity transformation using a second ratio of similitude λ+1 to obtain a second similar curve that is employed as a tooth profile of the rigid internal gear, and the tooth profile of the rigid internal gear has a range whereby a tooth addendum thereof touches the movement locus M of a tooth addendum of the flexible external gear.