PURPOSE: To make a rotary member rotatable independently without receiving any effect by means of an engaging member when it rotates as far as the specified angle by forming the width of a second groove formed in the rotary member to be wide enough so as not to make an engaging pin of the engaging member contact with it.

CONSTITUTION: Width of a second groove 2II formed in a rotary member 2 is being formed wide enough so as not to make an engaging pin 3a of an engaging member 3 contact with it. Accordingly, when the rotary member 2 rotates as far as more than the specified angle and the pin 3a of the engaging member 3 is engaged with the second groove 2II of the rotary member, any load by the engaging member will not work on the rotary member at all. In consequence, when the rotary member 2 rotates as far as more than the specified angle, it is able to rotate separately without receiving any effect due to the engaging member.