A rotational speed control device includes a housing containing a viscous fluid and a shaft disposed in the housing and rotatable relative to the housing. A rotor may be movable axially on the shaft depending on the rotating speed of the shaft. The rotor is biased in a low torque direction. A braking torque between the rotor and the housing and/or between the rotor and a drive control ring is varied according to an axial position of the rotor on the shaft. The device may incorporate bypass channels to control a circulating pressure of the viscous fluid.
