PURPOSE:To enable the stress applied to the ribbon spring in the caption to be exerted uniformely in the circumferential direction by performing a bending work at least on one end of a thin plate, and arranging a incorporated ribbon spring in an approximate circular shape. CONSTITUTION:Each folded portion 17 is set between two adjacent weavy bending portions 7e, 7e and set perticularly between adjacent bridge portions 7c, 7c and on a portion in which a tab 7b does not exist. And the bending direction of the bending work is set in the projecting direction of the weavy bending portion 7e. For this reason, the overlaid portion of both end portions 7' and 7'' approximately follows the circumferential direction in a state of incorporation and the ribbon spring 7 wholly approximates an exact circle. Accordingly, the stress applied to the ribbon spring 7 becomes approximately uniform along the longitudinal direction.
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