The silk-hat-shaped flexible external gear (13)of a silk-hat flexible meshing gear device (11) comprisesa body (22), a diaphragm (23) and a boss (25). Between themain portion of the body (22) and a curved portion (222)is formed a thinned portion (221) whose thickness is about80% that of the main portion. The thickness of thediaphragm (23) is defined to be largest at the inner edge(point A), smallest at the middle portion (point B) andslightly smaller than the largest thickness at the outeredge (point C). To make the change in thickness smooththroughout, the contour is defined by use of a group ofcurved lines (223, 224). Providing the thinned portion(221) and defining the thickness of the diaphragm (23) inthe foregoing manner relieves stress concentration at theinner and outer edges and smooths the stress distribution.As a result, the axial length and the outer diameter can bereduced to realize a compact silk-hat flexible meshing geardevice.
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