For other uses, see Clutch (disambiguation).
Rear side of a Ford V6 engine, looking at the clutch housing on the flywheel
Single, dry, clutch friction disc. The splined hub is attached to the disc with springs to damp chatter
A clutch is a mechanism for transmitting rotation, which can be engaged and disengaged.
In everyday use, the term clutch refers to a subcomponent of motor vehicle engine's transmission designed to allow engagement or disengagement of the engine to the gearbox or whatever apparatus is being driven. Invention of the clutch is attributed to Karl Benz.
There are many different vehicle clutch designs, but most are based on one or more friction discs, pressed tightly together or against a flywheel using springs. The friction material is very similar to the material used in brake shoes and pads and used to contain asbestos. Also, 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 and are unsuitable for road 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, and the friction plate is released and allowed to rotate freely. A wet clutch is immersed in a cooling lubricating fluid, which also keeps the surfaces clean and gives improved performance and longer life. A dry clutch, as the name implies, uses no fluid. Since the surfaces of a wet clutch can be slippery (as with a motorcycle clutch bathed in engine oil), stacking multiple clutch disks can compensate for slippage. Most Moto Guzzi and BMW motorcycles use a single plate clutch like a car.
In a car it is operated by the left-most pedal using hydraulics or a cable connection from the pedal to the clutch mechanism. No pressure on the pedal means that the clutch plates are engaged (driving), while depressing the pedal will disengage the clutch plates, allowing the driver to shift gears.
There are other clutches found in a car. For example, the radiator fan may have a clutch that is heat-activated. This is an electrorheological clutch, drive and driven members are selectively engaged by the application of a voltage to an electroheological fluid (ER). When the temperature is low, the fluid is thin and so the clutch slips. When the temperature is high, the fluid thickens, causing the fan to spin.
A manual transmission contains cogs for selecting gears. These cogs have matching teeth, called dog teeth, which means that the rotation speeds of the two parts have to match for engagement. This speed matching is achieved by a secondary clutch called a synchromesh, a device that uses frictional contact to bring the two parts to the same speed, and a locking mechanism called a blocker ring to prevent engagement of the teeth (full movement of the shift lever into gear) until the speeds are synchronized.
On most motorcycles, the clutch is operated by the clutch lever, located on the left handlebar. No pressure on the lever means that the clutch plates are engaged (driving), while pulling the lever back towards the rider will disengage the clutch plates, allowing the rider to shift gears. Some cars and mopeds have a centrifugal clutch, using centrifugal forces to engage the clutch above certain rpm, see Saxomat.
While engaging the clutch, the engine speed may need to be increased from idle, using the manual throttle, so that the engine does not stall. However, raising the engine speed too high will cause excessive clutch plate wear and cause a harsh, jerky start. This kind of start is desired in drag racing and other competitions, however.
A clutch may also be a device on a shaft that will "slip" when higher than normal resistance is encountered on a machine. An example of a clutch such as this may be mounted on the driving shaft of a large grass mower. The clutch will "slip" or "give" if the blades were to hit a rock, stump, or other immobile object.