To keep the concentricity and parallelism of one and other side rims 20 and 30 with high accuracy over a long period of time while dispensing with inspection and adjusting works.

The mutual joining of one and other side rims 20 and 30 are performed by the surface contact of the other side surface 44a of an inside locking pawl 44 and one side surface 41a of an outside locking pawl 41 but, since these other side surface 44a and one side surface 41a are together constituted of a part of a conical surface having the same gradient, the position and posture of the freely displaceable other side rim 30 are corrected on the basis of the immovable one side rim 20 by the correction action of the conical surface. By this constitution, both rims 30 and 20 can keep highly accurate concentricity and parallelism over a long period of time.