Wave gear device and flexible externally toothed gear

A wave gear device comprising:

• an annular rigid internally toothed gear, a flexible externally toothed gear disposed on an inner side of the rigid internally toothed gear, and a wave generator disposed on an inner side of the flexible externally toothed gear;
• the flexible externally toothed gear having a cylindrical body part capable of flexing radially, a diaphragm expanding radially inward or outward as a continuation of one edge of the cylindrical body part, and external teeth formed in an external peripheral surface portion of the cylindrical body part on the other side of the cylindrical body part having an open edge;
• the wave generator having an external-teeth-pushing surface for pushing radially outward an internal peripheral surface portion of an external-teeth area of the cylindrical body part where the external teeth are formed;
• and
• the external-teeth area of the flexible externally toothed gear being made to flex into a non-circular shape by the external-teeth-pushing surface, the external teeth meshing with internal teeth of the rigid internally toothed gear in a plurality of circumferentially separated regions, and, when the wave generator is rotated, the meshing positions of the external teeth and the internal teeth also moving circumferentially and relative rotation corresponding to the difference in the number of the external teeth and internal teeth occurring between the flexible externally toothed gear and the rigid internally toothed gear;
• the wave gear device wherein:
• the external-teeth area in the cylindrical body part includes a pushed portion pushed radially outward by the external-teeth-pushing surface of an outer ring and wherein the pushed portion is in direct contact with the external-teeth-pushing surface of the outer ring, an adjacent portion adjacent to the pushed portion toward the diaphragm, and a groove formed in the adjacent portion;
• the groove extends along an internal peripheral surface of the external-teeth area through the entire periphery thereof;
• and
• a thickness of the adjacent portion where the groove is formed is reduced compared to that of the pushed portion.