A torque limiter is a mechanical device that controls how much torque the drive shaft of a machine is subjected to at any given time. It is a protection mechanism and its objective is to prevent the machine from being damaged due to 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 alternately called the overload clutch. It can be found in anything from a ship’s propeller to a bottling plant assembly line and 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 disconnect torque limiter can come in many different designs. This includes shear pins, synchronous magnetic, lock and pawl and spring detent. Generally speaking, a disconnect type must be reset somehow after intervening during an overload. Depending on the type, this can be done automatically or manually.
Spindle pin stoppers work by inserting a small metal pin into the drive when it is overloaded, forcibly disconnecting it. In the process, the pin is destroyed and must be replaced before being used again. Shear pins are often compared 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, just like the name, a ball detention 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 movable arm section of a ratchet mechanism, is activated when needed, with the pawl dropping and catching a notch in the unit, forcing it to disconnect.
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 the sense that there is no need to restart the system after each use. Each type of torque limiting design can be engaged and disengaged in a timely manner, typically without damage to mechanical parts.