Patent ID: 7249536

Claim:
A wave gear device comprising: an annular rigid internal gear; a cup-shaped external gear provided with a cylindrical body having flexibility in a radial direction, an annular diaphragm extending towards inside in the radial direction from one end in a central axis direction of the cylindrical body, a discoid rigid boss integrally formed in a center portion of the diaphragm, and external teeth formed on an external peripheral surface portion of the other end in the central axis direction of the cylindrical body; and a wave generator for elliptically flexing a portion of the cylindrical body where the external teeth of the external gear are formed to mesh partially with the rigid internal gear, and causing the meshing positions of both gears to move in a circumferential direction; wherein a thickness t is a value that satisfies the following conditional expression (1) when t is the thickness of the diaphragm in a position of a radius r about the central axis thereof, and A is a constant: t=A/r 2 (1).