Patent ID: 7694607

Claim:
A wave gear drive, comprising: a circular rigid internal gear; a flexible external gear disposed within the rigid internal gear; a wave generator disposed within the flexible external gear; the flexible external gear having a flexible cylindrical body and an annular diaphragm that extends in a radial direction from a back end of the cylindrical body, a toothed portion formed on a front end opening portion 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 side thereof that is approximately proportional to distance from the diaphragm; wherein a cross-section of the flexible external gear perpendicular to the axis at a prescribed location in a tooth trace direction of the flexible external gear is set to be a main cross-section; the rigid internal gear and flexible external gear are gears with a module m; the flexible external gear has a number of teeth that is set to be smaller by 2 n, wherein n is a positive integer, than a number of teeth of the rigid internal gear; a radial flexing amount of a major axis of an elliptical rim neutral line of the flexible external gear is set at κmn, wherein κ>1; wherein a tooth of the flexible external gear follows a moving route (M) with respect to a tooth of the rigid internal gear that accompanies rotation of the wave generator when meshing of the teeth in the main cross-section is approximated by rack meshing; on an entering side of the meshing of the teeth to the major axis of the moving route (M), a first similar curve (AC) is obtained by similarity transformation of a pressure angle α defined as an angle of inclination in a direction orthogonal to the direction of the moving route (M) in a range that is from 90° at a first point (A) in an initial meshing phase to a minimum of 0° at a second point (B), or a range that is midway from the first point (A) to the second point (B), using a first ratio of similitude λ, wherein λ<1, with the first point (A) as center of similarity, the first similar curve (AC) is employed as a basic addendum tooth profile of the rigid internal gear, a second similar curve (CB) is obtained by rotating the first similar curve (AC) through an angle of 180° with an endpoint (C) thereof as center of similarity, and by similarity transformation in which the first similar curve (AC) is multiplied by a second ratio of similitude (1−λ)/λ, and the second similar curve (CB) is employed as a basic addendum tooth profile of the flexible external gear.