PURPOSE: To simplify a nut fastening work and improve productivity by providing an engagement means preventing a power shaft from rotating along its circumferential direction relative to a center plate and preventing any twist due to rotation of the power shaft during a nut fastening work.

CONSTITUTION: By means of a nut 13 engaged with a male thread portion 12 at an end of a power shaft 3, a center plate 11 is fixed on the power shaft 3. In order to prevent the power shaft 3 from rotating along its circumferential direction relative to the center plate 11, an engagement means 14 is provided. Consequently, when the power shaft 3 is going to twist during its rotation when a nut 13 is fastened, the center plate 11 fixed to a diaphragm 8 prevents the shaft 3 from twisting. In this way, any twisting force is not applied to a coil spring 10 fixed to the power shaft 3 and any damage is caused neither on the coil spring 10 nor on the diaphragm 8. Accordingly, fastening work of the nut 13 can be simplified and productivity can be improved.