Consists of a driving hub having one or more weighted sections fitted with friction lining on the outer radial surfaces that contact a driven hub having a flange covering that portion of the driving hub containing the radial elements. Upon starting, the radial elements of the driving hub have no appreciable drag, but upon accelerating to the operating speed the force produced by the centrifugal action increases rapidly as the square of the speed and the elements grip the driven element, thereby causing it to speed up to the required speed of the driving hub.