Silk hat flexible engagement gear device

A silk-hat type flexible engagement device which comprises a circular rigid internal gear, a flexible external gear inside the rigid internal gear, and a wave generator arranged inside the flexible external gear for flexing the external gear radially to engage partially with the internal gear and for moving engagement portions of these gears along circumferential direction, wherein

• the flexible external gear has a silk-hat shape which comprises an annular body part formed on an outer circumferential surface at a side of one opening end thereof with external teeth, a circular diaphragm integrally connected at its inner circumferential end with the other opening end of the body part, and a circular boss integrally formed on an outer circumferential end portion of the diaphragm,
• the diaphragm has a root part connected to the boss, a plate-shaped part extending radially and inwardly from the root part, and a curved part for connecting the plate-shaped part smoothly to the body part, and
• the body part has a tooth part where the external teeth are formed, and a body main part extending from the tooth part to the curved part and connected smoothly to the curved part, characterized in that
• one side of a sectional shape of the plate-shaped part of the diaphragm, when cut along a plane including a device axial line, is defined by a first profile line which includes a straight line extending perpendicular to the device axial line,
• the other side of the sectional shape of the plate-shaped part is defined by a second profile line which includes a first convex circular arc smoothly connected to the root part, a concave circular arc smoothly connected to the first convex circular arc, and a second convex circular arc which is smoothly connected to the concave circular arc connected smoothly to the curved part, and
• a portion defined by the straight line and the concave circular arc is set to be the minimum thickness portion of the diaphragm.