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A torque limiter is a mechanical device that controls how much torque a machine’s driveshaft is subjected to at any given time. It is a protection mechanism and its objective is to prevent the machine from being damaged by what is called mechanical overload. This is a situation where too much torque is imposed on the drive. For this reason, the limiter is sometimes alternatively called an overload clutch. It can be found in anything from a ship’s propeller to a bottling plant assembly line to a fishing reel.
There are several different ways a torque limiter can work. Some decouple the load completely when an overload is detected. These types are known as disconnect types. Others simply allow the load to slip during overload, similar to how a clutch works in a car’s manual transmission. They are known as torque limiting types.
A tripping torque limiter can come in many different designs. This includes shear pins, synchronous magnetic, lock and pawl, and spring retainer. Generally speaking, a trip type must be reset in some way after intervening during an overload. Depending on the type, this can be done automatically or manually.
Spindle pin stops work by inserting a small metal pin into the drive when it is overloaded, forcibly disengaging it. In the process, the pin is destroyed and must be replaced before being used again. Safety pins are often compared to electrical fuses in that they are sacrificed to protect more expensive parts.
A synchronous magnet system, as the name implies, uses a pair of powerful magnets to rapidly disengage the shaft with a magnetic pulse. Again, just like the name, a ball arrest system works by having several spring-loaded metal balls installed in the unit, which pop out to disengage the unit when needed. A spring and pawl torque limiter, which basically uses the moving arm section of a ratchet mechanism, engages when needed, with the pawl dropping and catching a notch in the unit, forcing it to disengage.
Torque limiting types, which function like clutches, include friction plate, magnetic particle, and magnetic hysteresis designs. Unlike disconnect types, clutch-based torque limiters are not irrevocable in their use and can be modulated while the machinery is running. They are also less catastrophic in that there is no need to reboot the system after each use. Each type of torque limiting design can be turned on and off at the right time, usually without damaging mechanical parts.