Abstract: On a rotatable shaft, having no lateral movement in a housing, a slanted collar fixedly received thereon has a given rotational speed being driven by an electric motor. The collar drives two adjacent cylinders slidably received on the shaft, having interlocking slants and kept in working engagement with the collar by two compression springs. When the rotational speed of one of the cylinders differs from the given rotational speed of the collar this cylinder is pushed by the collar on a switch that shuts the motor off or reverses its polarity.