1. A CONTROL DEVICE FOR PRODUCING CONTROLLABLE DISPLACEMENTS, THE CONTROL DEVICE COMPRISING FIRST AND SECOND PULLEY MEMBERS, FIRST AND SECOND SHAFTS SUPPORTING SAID PULLEY MEMBERS FOR ROTATION IN COAXIAL RELATION, MEANS OPERATIVELY COUPLING SAID SHAFTS TOGETHER TO ENABLE RELATIVE AXIAL DISPLACEMENT OF SAID SHAFT WITH THE PULLEY MEMBERS ROTATING AT DIFFERENT ANGULAR SPEEDS, SAID PULLEY MEMBERS INCLUDING PERIPHERAL SURFACES, AND FRICTION MEANS TEMPORARILY ENGAGEABLE WITH THE PERIPHERAL SURFACES OF THE PULLEY MEMBERS AND OPERATIVE THEREWITH TO CAUSE SAID PULLEY MEMBERS TO ROTATE ON SAID SHAFT AT A DIFFERENT ANGULAR SPEEDS TO OBTAIN SAID RELATIVE AXIS DISPLACEMENT.