PURPOSE: To constantly control the motion of a bowl, even when the suspension condition is impaired, by providing an intermediate member fixed to driver shafts with a straight groove part at a fitted section, coupling it to a groove part on the surface of a driving shaft in contact therewith and providing the intermediate member with a threaded time rod.

CONSTITUTION: Coupling is ensured by the intermediate member 6 fitting into the driving shaft 1, the fitted section is provided with the straight groove part 8 and the groove part 8 transmits the rotation of the driving shaft 1 as far as the intermediate member 6 in cooperation which the same groove part provided on the inner surface of the shaft 1. When the coupling is disengaged due to damage to the tie rod 12, the driver shafts 2 and 11 and the intermediate member 6 go down to be abutted on the surface 5 of a support. Thus, continuity of a mechanical coupling between the driver shafts 2, 11, the intermediate member 6 and the driving shaft 1 is maintained, thereby permitting the sliding in the vertical direction and the maintaining of rotary driving. In this case, the suspension of the bowl is similarly maintained, too, and further, coupling of the shaft 1 to its driving power is maintained. Consequently, the motion of the bowl is kept under control.