Clutch

A clutch is a mechanical device that provides for the transmission of power from one component to another when engaged, but can be disengaged.

Friction clutches

In a pull type clutch, the action of pressing the pedal pulls the release bearing, pulling on the diaphragm spring and disengaging the vehicle drive.
Multiple plate clutch This type of clutch has several driving members interleaved or "stacked" with several driven members.
Since the surfaces of a wet clutch can be slippery , stacking multiple clutch discs can compensate for the lower coefficient of friction and so eliminate slippage under power when fully engaged.
The Hele-Shaw clutch was a wet clutch that relied entirely on viscous effects, rather than on friction.
A centrifugal clutch is used in some vehicles and also in other applications where the speed of the engine defines the state of the clutch, for example, in a chainsaw . This clutch system employs centrifugal force to automatically engage the clutch when the engine rpm rises above a threshold and to automatically disengage the clutch when the engine rpm falls low enough.

The system involves a clutch shoe or shoes attached to the driven shaft, rotating inside a clutch bell attached to the output shaft.

In the case of a chainsaw this allows the chain to remain stationary whilst the engine is idling; once the throttle is pressed and the engine speed rises, the centrifugal clutch engages and the cutting chain moves.
Torque limiter Also known as a slip clutch or safety clutch, this device allows a rotating shaft to slip when higher than normal resistance is encountered on a machine.

application

There are different designs of vehicle clutch but most are based on one or more friction discs pressed tightly together or against a flywheel using springs .
Clutches found in heavy duty applications such as trucks and competition cars use ceramic clutches that have a greatly increased friction coefficient. However, these have a "grabby" action generally considered unsuitable for passenger cars.

The spring pressure is released when the clutch pedal is depressed thus either pushing or pulling the diaphragm of the pressure plate, depending on type. However, raising the engine speed too high while engaging the clutch causes excessive clutch plate wear.
Engaging the clutch abruptly when the engine is turning at high speed causes a harsh, jerky start. This kind of start is necessary and desirable in drag racing and other competitions, where speed is more important than comfort. This plastic pilot shaft guide tool is used to align the clutch disk as the spring-loaded pressure plate is installed.

In a modern car with a manual transmission the clutch is operated by the left-most pedal using a hydraulic or cable connection from the pedal to the clutch mechanism.
If the engine is running with clutch engaged and the transmission in neutral, the engine spins the input shaft of the transmission, but no power is transmitted to the wheels.
The clutch is located between the engine and the gearbox, as disengaging it is required to change gear. Although the gearbox does not stop rotating during a gear change, there is no torque transmitted through it, thus less friction between gears and their engagement dogs.
Clutches in typical cars are mounted directly to the face of the engine's flywheel , as this already provides a convenient large diameter steel disk that can act as one driving plate of the clutch.
The propeller shaft between front and rear rotates continuously as long as the engine is running, even if the clutch is disengaged or the transmission is in neutral.
Motorcycles typically employ a wet clutch with the clutch riding in the same oil as the transmission.
A set of coil springs or a diaphragm spring plate force the plates together when the clutch is engaged.
No pressure on the lever means that the clutch plates are engaged , while pulling the lever back towards the rider disengages the clutch plates through cable or hydraulic actuation, allowing the rider to shift gears or coast.

Other clutches—such as for an air conditioning compressor—electronically engage clutches using magnetic force to couple the driving member to the driven member.
Electromagnetic clutch are, typically, engaged by an electromagnet that is an integral part of the clutch assembly.

Another type, magnetic particle clutches, contain magnetically influenced particles in a chamber between driving and driven members—application of direct current makes the particles clump together and adhere to the operating surfaces.
Overrunning clutch or freewheel : If some external force makes the driven member rotate faster than the driver, the clutch effectively disengages.

An oscillating member where this clutch can then convert the oscillations into intermittent linear or rotational motion of the complimentary member; others use ratchets with the pawl mounted on a moving member
The winding knob of a camera employs a wrap-spring type as a clutch in winding and as a brake in preventing it from being turned backwards.
The rotor drive train in helicopters uses a freewheeling clutch to disengage the rotors from the engine in the event of engine failure, allowing the craft to safely descend by autorotation.
Rotating the driving member the other way makes the spring wrap itself tightly around the driving surface and the clutch locks up.

Specialty clutches and applications

Single-revolution clutch

If the trip mechanism is operated when the clutch would otherwise disengage the clutch remains engaged.
When the sleeve's tooth contacted the pawl the sleeve and the load's inertia unwrapped the spring to disengage the clutch.

When the clutch locked up the driven mechanism coasted and its inertia rotated the disc until a tooth on it engaged a pawl that kept it from reversing.

Cascaded-pawl single-revolution clutches

These superseded wrap-spring single-revolution clutches in page printers, such as teleprinters , including the Teletype Model 28 and its successors, using the same design principles.
Inside the hollow disc-shaped housing were two or three freely floating pawls arranged so that when the clutch was tripped, the load torque on the first pawl to engage created force to keep the second pawl engaged, which in turn kept the third one engaged.
As the clutch rotated it would stay locked up if the trip lever were out of the way, but if the trip lever engaged the clutch would quickly unlock.