B&B precision: how do torque limiters work

When a force is used to begin to spin an object, or stop an object from spinning, a torque is created. The most prevalent example of torque would be inside an engine. A torque limiter protects mechanical equipment or loads from excessive torque. They shut down a machine and dissipate any rotational energy, protecting the machine from any damage. Torque Limiters fall into two main categories; disconnect and slip.

Disconnect types work by physically disconnecting a drive from the load, and there are several methods of achieving this. In a shear pin design the pin itself acts as a sacrificial component, breaking in the case of mechanical overload but in doing so preventing more expensive parts from being damaged. A Ball Detent limiter has a number of metallic balls on the annulus of the shaft which lock it in place, allowing power to be transmitted. When excess torque is used these balls ride up out of their seats causing power to be diverted and stalling the output. A magnetic torque limiter has a series of permanent magnets mounted to the shaft, when the torque limit is exceeds these magnetic forces break off and the clutch slips. All of these Torque Limiter function through permanently separated the drive and drive shaft and would require manual re-engagement.

Slip type torque limiters work by letting the drive shaft run faster the driven shaft (this is called slipping) and then will automatically re-engage when the torque falls below a set limit. Friction Plates use the friction between contact surfaces to transmit power, over-torque will cause the plates to slip. Friction torque limiters are intended for momentary overload only and will continue to slip until the source of the torque overload is cleared or the machine is shut down. Another variety of Slip Torque Limiter would be a magnetic particle clutch where instead of using friction plates a fine powder of magnetically susceptible material. When voltage is run through this powder is bound together and builds a magnetic field, transferring torque. When over-torque occurs this current is removed from the clutch.

It’s important that torque limiters are installed as physically close to the point of impact or overload as possible, and determining the torque rating of the device should help to determine which torque limiter should be used.

B&B Precision are sub-contract machinists that have many years experience in producing torque limiting components friction discs, plates, sprockets, shafts, sleeves and more.

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