A torque limiter is a mechanical device that controls how much torque a machine's driveshaft is subject to at a given time. It is a protective mechanism and its goal is to prevent the machine from suffering damage due to what is known as mechanical overload. This is a situation wherein too much torque is imposed on the drive. For this reason, the limiter is sometimes alternately referred to as an overload clutch. It can be found on anything from a ship's propeller to a bottling plant assembly line to a fishing reel.
There are several different ways in which a torque limiter can function. Some uncouple the load completely when an overload is detected. These kind are known as disconnect types. Others simply allow the load to slip during overload, similar to how a clutch in a car's manual transmission works. These are known as torque limiting types.
A disconnect torque limiter can come in several different designs. These include shear pins, synchronous magnetic, bawl detent, and pawl-and-spring. Generally speaking, a disconnect type must be reset in some way after it intervenes during an overload. Depending on the type, this may be done automatically or manually.
Shaft pin limiters work by inserting a small metal pin into the drive when it is overloading, forcibly disconnecting it. In the process, the pin is destroyed and must be replaced before it can be used again. Shear pins are often likened to electrical fuses, in the sense that they are sacrificed to protect more expensive parts.
A synchronous magnet system, as its name implies, uses a pair of strong magnets to quickly disconnect the shaft with a magnetic impulse. Again, much like its name, a ball detent system works by having a number of spring-loaded metal balls installed in the drive, which pop out to disconnect the drive when needed. A pawl-and-spring torque limiter, which basically uses the moving arm section of a ratchet mechanism, activates when needed, with the pawl dropping down and catching a notch in the drive, forcing it to disconnect.
Torque limiting types, which work like clutches, include friction plate, magnetic particle, and magnetic hysteresis designs. As opposed to disconnect types, clutch-based torque limiters are not as irrevocable in their use, and can be modulated while the machinery is running. They are also less catastrophic in the sense that a reset of the system is not required after each use. Each type of torque limiting design can be engaged and disengaged in a timely fashion, typically with no damage to mechanical parts.