Torque limiter
Encyclopedia
A torque limiter is an automatic device that protects mechanical equipment, or its work, from damage by mechanical overload. A torque limiter may limit the torque
by slipping (as in a friction plate slip-clutch), or uncouple the load entirely (as in a shear pin
). The action of a torque limiter is especially useful to limit any damage due to crash stops and jams.
Torque limiters may be packaged as a shaft coupling
or as a hub for sprocket
or sheave
. A torque limiting device is also known as an overload clutch.
Many cordless drills incorporate this type of torque limiter in a planetary gearset. It may be a part of an assembly of multiple gearsets used to primarily reduce speed and multiply torque as well as perform ratio changes. The torque limiter is typically the last gearset in the transmission. It uses the planet carrier as the input with the sun gear as the output, and the annulus normally locked. A series of ball detents act on the annulus to lock it, allowing power to be transmitted from the planet carrier to the sun gear. When the torque transmitted through the gearset reaches a determinate amount, the torque acting on the annulus causes it to unlock from its ball detents and freely rotate, causing power to be diverted from the load on the sun gear to the annulus and thereby stalling the output until torque is reduced to an amount where the ball detents can lock the annulus again. This system equally limits torque in both directions of rotation and also works with the sun gear as the input. The compression of the ball detents (and therefore the amount of torque at which the limiter is utilized) is typically adjusted by means of a rotating collar accessible to the user which is indexed and held in place with its own separate ball detents.
, where the slipping torque is set by means of a large hand nut in order that the reel will turn and allow more line to unwind before the line breaks under the pull of a fish.
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
by slipping (as in a friction plate slip-clutch), or uncouple the load entirely (as in a shear pin
Shear pin
A shear pin is a safety device designed to shear in the case of a mechanical overload, preventing other, more-expensive parts from being damaged...
). The action of a torque limiter is especially useful to limit any damage due to crash stops and jams.
Torque limiters may be packaged as a shaft coupling
Coupling
A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. Couplings do not normally allow disconnection of shafts during operation, however there are torque limiting couplings which can slip or disconnect when some torque limit is exceeded.The...
or as a hub for sprocket
Sprocket
A sprocket or sprocket-wheel is a profiled wheel with teeth, cogs, or even sprockets that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which are radial projections that engage a chain passing over it...
or sheave
Sheave
A sheave is a wheel or roller with a groove along its edge for holding a belt, rope or cable. When hung between two supports and equipped with a belt, rope or cable, one or more sheaves make up a pulley. The words sheave and pulley are sometimes used interchangeably.A sheave can also refer to a...
. A torque limiting device is also known as an overload clutch.
Disconnect types
Disconnect types will uncouple the drive, with little or no residual torque making its way to the load. They may reset automatically or manually.Shear pin
A shear pin type sacrifices a mechanical component, the pin, to disconnect the shafts.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. Because there is no mechanical contact between the two shafts, they are also used to transmit torque through a physical barrier like a thin plastic wall. On some models, the torque limit may be adjusted by changing the gap between the magnets.Ball detent
A ball detent type limiter transmits force through hardened balls which rest in detents on the shaft and are held in place with springs. An over-torque condition pushes the balls out of their detents, thereby decoupling the shaft. It can have single or multiple detent positions, or a snap acting spring which requires a manual reset. There may be a compression adjustment to adjust the torque limit.Many cordless drills incorporate this type of torque limiter in a planetary gearset. It may be a part of an assembly of multiple gearsets used to primarily reduce speed and multiply torque as well as perform ratio changes. The torque limiter is typically the last gearset in the transmission. It uses the planet carrier as the input with the sun gear as the output, and the annulus normally locked. A series of ball detents act on the annulus to lock it, allowing power to be transmitted from the planet carrier to the sun gear. When the torque transmitted through the gearset reaches a determinate amount, the torque acting on the annulus causes it to unlock from its ball detents and freely rotate, causing power to be diverted from the load on the sun gear to the annulus and thereby stalling the output until torque is reduced to an amount where the ball detents can lock the annulus again. This system equally limits torque in both directions of rotation and also works with the sun gear as the input. The compression of the ball detents (and therefore the amount of torque at which the limiter is utilized) is typically adjusted by means of a rotating collar accessible to the user which is indexed and held in place with its own separate ball detents.
Pawl and spring
This mechanical type uses a spring to hold a drive pawl against a notch in the rotor. It may feature automatic or manual reset. A compression adjustment on the spring determines the torque limit.Torque limiting types
Torque limiting types will limit the torque by slipping (ie. letting the drive shaft run faster than the driven shaft.) Excess power is dissipated as heat. They don't need to be reset.Friction plate
This type is similar to a friction plate clutch. Over-torque will cause the plates to slip. A simple example is found in a fixed-spool fishing reelFishing reel
A fishing reel is a "cylindrical device attached to a fishing rod used in winding the line". Modern fishing reels usually have fittings which make it easier to retrieve the line and deploy it for better accuracy or distance. Fishing reels are traditionally used in the recreational sport of angling...
, where the slipping torque is set by means of a large hand nut in order that the reel will turn and allow more line to unwind before the line breaks under the pull of a fish.