PURPOSE: To cause slowing-down revolution of a fan for a certain while after stopping of its turning shaft by the function of inertial force, by providing an inertial ring that is coupled with the turning shaft by means of helical splines and is designed to be capable of coming into contact with a tapered portion of the fan.

CONSTITUTION: A male tapered portion 7 and a female tapered portion 9 are held in tight contact with each other. A fan 3 begins to revolve together with its shaft 1 by the thrust of an inertial ring 8 and the frictional force acted at the faces where the above two tapered portions 7 and 9 are held in contact with each other. With such an arrangement, if revolution of the turning shaft 1 is stopped abruptly after the speed of revolution has reached a certain level, the inertial ring 8 produces a thrust in the opposite direction to the fan 3 by the function of helical splines since the ring 8 tends to keep up its revolution, so that the female tapered portion 9 is disengaged from the male tapered portion 7. Therefore, slippage is caused between the shaft 1 and a plain bearing 4 by the inertial force of the fan 3, so that it can cause slowing-down revolution for a certain while after stopping of the shaft 1.