Ratchet torque limiters (symmetrical)

The ratchet torque limiter is able to interrupt the transmission of power in the event of a torque peak or overload. Symmetrival units transmit torque in both directions of rotation

Ratchet torque limiters (one-way)

A ratchet torque limiter is a device able to interrupt the transmission of power int the event of a torque peak or overload that exceeds the setting. The torque limiter is automatically re-engaged after the cause of the overload is removed. Ratchet torque limiters are generally employed to protect implements subject to constant or alternating torques from overloads. The settings is normally two to three the median torque to be transmitted.
During operation of the device, the spring loaded ratchets try to reengage the grooves in the housing. This generates small torque peaks and noise. This noise warns the operator that the device is engaged. Once the device is slipping, the user should promptly stop the PTO to avoid excessive wear. 

Symmetrical ratchet torque limiters transmits the same torque values in both directions of rotation. Symmetrical limiters does not permit quite as high torque settings as standard one-way torque limiters with an equal amount of ratchets.