To prevent the abrasion caused by the contact of an outer periphery of a coil spring with an inner peripheral face of a blind hole, by coating the coil spring with a cover made of an abrasion resisting material to be mounted in the blind hole, and locking a point of the coil spring with an inner face of the blind hole.

A coil spring 41b is coated with a cover 50 made of an abrasion resisting material, and is mounted in a blind hole 45b. In the operation, an outer periphery of the coil spring 41b is not directly kept into contact with an inner peripheral face of the blind hole 45b even when the coil spring 41b is expanded and contracted, so that the abrasion of the inner peripheral face of the blind hole 45b can be prevented. Further a point 51 of the coil spring 41b is bent and projected in the expanding direction to lock the point 51 in a small hole 52 formed on a bottom of the blind hole 45b. In the operation, the coil spring 41b is expanded and contracted, but is not rotated in the blind hole 45b, that is, they are relatively rotated in a state that an outer periphery of the coil, spring 41b is kept into contact with an inner peripheral face of the blind hole 45b, so that the abrasion of the inner peripheral face of the blind hole 45b can be prevented.