Torque is the tendency of a body to rotate because of a force being applied at a distance from the center of rotation. A torque consists of a force and a distance(arm). A rotating operation can develop a lot of torque, and if there is any malfunction, the force can affect other parts, including the couplings, the drive or motor shafts, and the gears. It is a mechanical overload because the system encounters torques more than it is designed to handle. A torque limiter is essential to avoid these cases.
A torque limiter is a device that protects mechanical equipment from damage by excessive torque loads and works by slipping during torque overload. In the case of a torque overload, the transmitted torque will exceed the set torque point of the torque limiter. The frictional force will not be strong enough to transmit the torque from the driving shaft to the driven member, and the driven member will slip between the friction disks. The torque limiter resets and resumes transmitting torque after the torque overload is eliminated. Installing a torque limiter in the drive system provides protection and reduces downtime and loss of production. Torque reducers are used in gearboxes, motors, conveyors, and generators.
The main types of torque limiters are friction, ball-detent, magnetic particle, and shear pin.
A friction torque limiter is similar to an automobile clutch and consists of spring-actuated frictional discs interacting with each other. Adjusting the amount of spring force preload on the friction disks produces the set torque slip threshold. When the set torque threshold is exceeded, the friction discs will slip relative to each and limit power transmission, thus protecting the machine from damage. The friction limiter automatically resumes the torque transmission when excess torque is removed.
Friction Torque Limiter
A ball-detent torque limiter consists of a series of balls or rollers positioned in mating detents to connect the drive and driven elements. A spring keeps the rollers/balls engaged with the detents. When the threshold torque is exceeded, the balls/rollers overcome the spring pressure and slide out of their respective detents. A ball-detent limiter can be manually or automatically reset depending on the design.
Ball-Detent Torque Limiter
A shear pin is the simplest form of a torque limiter and is manually reset after a torque overload. A shear pin torque limiter consists of metal pins connecting the two rotating bodies together. A constant shear force is applied to the pins when torque is transmitted. But the shear pins will break when the torque exceeds the threshold. The main problem with shear pin torque limiters is that shear pins must be replaced after torque overload. Shear pins have been replaced with ball-detent torque limiters but are still used in applications requiring extremely high torque.
Shear Pin Torque Limiter
A magnetic particle clutch can be used effectively as a torque limiter. The torque setting reasonably approximates a linear relationship with the current passing through the windings, which can be statically or dynamically set depending on the needs.
Another type of magnetic torque limiter is synchronous magnetic. A synchronous magnetic torque limiter uses permanent magnets mounted to each shaft, with an air gap between. They are very fast-acting but may have more backlash than mechanical types.
Magnetic Torque Limiter