A cylindrical drum is driven for rotation about its axis by a drive mechanism. A braking mechanism selectively applies braking forces to the drum to impede or brake its rotational movement. A first movement sensor senses movement of the drive mechanism while a second movement sensor senses movement of the drum. An electronic comparator circuit compares these sensed movements and generates a control signal when the drive mechanism movement and drum movement are not in synchronism. In response to such loss of synchronism, a control mechanism causes the braking mechanism to apply braking forces.