Abstract

A pair of projection elements 42, 42 are formed on the opposite sides of an inner link 4. Each projection element 42 is formed with a cutout recess 42b. A fitting shaft part 72a, which is formed on each end of an abutment shaft 72, is inserted into the cutout recess 42b through an open part thereof and turnably fitted to a bottom part of the cutout recess 42b. A second abutment part 71c of a coiled spring 71 is abutted with an enlarged-diameter part 72b of the abutment shaft 72. The abutment shaft 72 is biased towards the bottom side of the cutout recess 43b from the open part side thereof by the coiled spring 71.