torque limiter
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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 In physics and mechanics, torque is the rotational equivalent of linear force. It is also referred to as the moment of force (also abbreviated to moment). It represents the capability of a force to produce change in the rotational motion of th ...
by slipping (as in a friction plate slip-
clutch A clutch is a mechanical device that engages and disengages power transmission, especially from a drive shaft to a driven shaft. In the simplest application, clutches connect and disconnect two rotating shafts (drive shafts or line shafts). ...
), or uncouple the load entirely (as in a
shear pin {{unreferenced, date=September 2018 A shear pin is a mechanical detail designed to allow a specific outcome to occur once a predetermined force is applied. It can either function as a safeguard designed to break to protect other parts, or as a con ...
). 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 A coupling is a device used to connect two shafts together at their ends for the purpose of transmitting power. The primary purpose of couplings is to join two pieces of rotating equipment while permitting some degree of misalignment or end mov ...
or as a hub for
sprocket A sprocket, sprocket-wheel or chainwheel is a profiled wheel with teeth that mesh with a chain, track or other perforated or indented material. The name 'sprocket' applies generally to any wheel upon which radial projections engage a chain passi ...
or
sheave A sheave () or pulley wheel is a grooved wheel often used for holding a belt, wire rope, or rope and incorporated into a pulley. The sheave spins on an axle or bearing inside the frame of the pulley. This allows the wire or rope to move fre ...
. 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

{{main, Shear pin A shear pin type sacrifices a mechanical component, the pin, to disconnect the shafts. The use of shear pins as torque limiters has been well known since at least the early 20th century.A. L. De Leeuw, Methods of machine tool design, American Machinist Vol. 57, No. 11 (Sept. 14, 1922)
page 402


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 An epicyclic gear train (also known as a planetary gearset) consists of two gears mounted so that the center of one gear revolves around the center of the other. A carrier connects the centers of the two gears and rotates the planet and sun gea ...
. 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 Transmission may refer to: Medicine, science and technology * Power transmission ** Electric power transmission ** Propulsion transmission, technology allowing controlled application of power *** Automatic transmission *** Manual 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 (i.e. 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 reel A fishing reel is a hand- cranked reel used in angling to wind and stow fishing line, typical mounted onto a fishing rod, but may also be used to retrieve a tethered arrow when bowfishing. Modern recreational fishing reels usually have fittings ...
, 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.


Magnetic particle

A magnetic particle clutch can be used effectively as a torque limiter. The torque setting fairly approximates a linear relationship with the current passing through the windings, which can be statically or dynamically set depending on needs.


Magnetic hysteresis

This type is non-synchronous in normal operation, so there is always some slippage.


See also

*
Torque converter A torque converter is a type of fluid coupling that transfers rotating power from a prime mover, like an internal combustion engine, to a rotating driven load. In a vehicle with an automatic transmission, the torque converter connects the power ...


References

Mechanisms (engineering) Torque Safety equipment