PURPOSE: To improve the direct welding workability between the upper end wall of a hub 10 and an upper flange.

CONSTITUTION: By mean of making the minimum dimension of a spring receiving protrusion 17 larger than the maximum dimension of a hole 22 of the upper flange 12, the hole 22 and the spring receiving protrusion 17 are engaged somewhat tightly by providing an intefference between them. By this arrangement, the upper flange 12 is stably placed on the hub 10 at the time of automatic transporting operation in the welding without playing or falling off due to the vibration, etc., and the welding workability is improved. A working hole 21 for removing a clamp piece 20 fitted to a clamping groove 19 of the hub 10 is arranged not on the upper flange 12 but on a lower flange 9. Thus, the positioning work with the hub 10 at the time of welding the upper flange 12 is unrequired, thereby the assembling easiness and working rate are improved much.