PURPOSE: To prevent the removal of a loading ring supporting member from a mounting shaft and to decrease the number of parts for the purpose of mounting.

CONSTITUTION: The loading ring supporting member 1 is mounted on the mounting shaft. A loading ring 3a is first mounted thereon. The loading ring supporting member 1 is rotated at a prescribed angle when the loading ring 3a is mounted. The loading ring 3a is supported by a collar 5b and is no more removable. A loading ring 3b is then mounted. The loading ring supporting member 1 is further rotated at a prescribed angle when the loading ring 3b is mounted. The loading ring 3b is then supported by a collar 5a and is no longer removed. A spring type member 4 intrudes into a hole part 8 at the point of this time and, therefore, the member is hooked by the part d and is no longer removed in a thrust direction. The spring type member 4 is hooked in the rotating direction as well and the rotation thereof is prevented.