PURPOSE: To reduce wind loss by providing a weight part with a chamfered part for generating thrust in the direction of the rotary shaft of a motor while breaking the wind.

CONSTITUTION: A weight part 2 is provided with a chamfered part 4 for breaking the wind to generate thrust in the direction of the rotary shaft of a motor. When the weight part 2 rotates, the chamfered part 4 acts as a fan for breaking the wind to generate thrust in the direction of the rotary shaft of the motor. Consequently, lateral shift (thrust) acting on the rotary shaft of motor is reduced thus reducing friction caused by contact of the rotary shaft with the end face of bearing in the motor. This constitution allows reduction of wind loss at parts other than the thrust generating part, i.e., the chamfered part 4.