Torque limiters and slip clutches automatically slip or disengage the clutch at a predetermined torque. A variety of torque limiters and slip clutches are available. A unidirectional clutch often used as a safety mechanism.  With an overrunning clutch, the drive element can stop, and the load is allowed to wind down freely. Backstop clutches are freewheeling clutches that transmit torque in one direction only; they are specifically used for preventing drives from "backing up" or driven systems such as inclined conveyors from going down. Freewheeling clutches are unidirectional torque transmitters that spin freely in the direction opposite the intended drive rotation.