To reduce costs and to facilitate assembly operations by providing a protection member consisting of front and rear extended parts extended inward from both ends of an annular part along a fitting and connecting direction, in order to cover at least a part of the outer peripheral surface of each of the front and rear end faces of a receiving member that are perpendicular to the fitting and connecting direction.

In order to cover a part of the outer peripheral surface 15a of the two front and rear end faces 14b, 14b of a receiving member 14 which are perpendicular to a fitting and connecting direction, a protective tire 19, made from synthetic rubber to serve as a protection member consisting of front and rear extended parts 19B, 19B extended inward from both ends of the receiving member 14 that extend in the fitting and connecting direction, is fitted into place. Therefore, the outer peripheral surface 14a of the receiving member 143 and the front and rear end faces 14b, 14b at portions located at corners continuing to the outer peripheral surface 14a can be protected by the protective tire 19, and the need for performing secondary working such as cutting on the outer peripheral surface 14a and the front and rear end faces 14b, 14b of the receiving member 14 can be eliminated.