The clutch is a mechanism for disconnecting the engine's power from the manual gearbox. In order for this operation to provide a smooth take-up of the drive and allow for quick gear selections, modern vehicles use a single plate, diaphragm spring clutch. The clutch assembly is made up from a pressure plate, a friction plate and a release bearing. When the clutch pedal is depressed, either mechanically or hydraulically, a pivoted lever located in the gearbox 'bell' housing, (the clutch release fork), exerts pressure onto the release bearing which in turn forces the pressure plate's diaphragm springs inward, releasing the friction plate from contact with the fly wheel and the pressure plate, thus disconnecting the drive.
When the clutch pedal is released the fork and bearing are now idle allowing the pressure plate diaphragm springs to return to the engaged position, clamping the friction plate between the pressure plate and flywheel's drive surfaces.
Clutch assemblies are robust in design and construction, as they need to transmit high torque loads at very high revolutions for long periods covering thousands of road miles. However, at some time in a vehicle's life the clutch will need to be replaced. The replacement intervals vary greatly due to driver style, vehicle loads and road conditions. The most common cause of failure is wear, causing clutch slip, which is identified by the vehicles engine revs increasing yet the vehicles speed remaining the same or reducing and in extreme cases, difficulty selecting gears. Clutch slip is also usually evident by a strong burning smell from the engine bay. Clutches can fail due to contamination from oil that has leaked from the engine, gearbox or hydraulic cylinder, which gives the same symptoms as a worn clutch. On occasion pressure plate diaphragm springs can fail preventing full clearance on the friction plate when the clutch pedal is depressed. When this fault has developed, as full disengagement has not occurred and the transmission has not slowed sufficiently, the driver will have difficulty selecting gears and the gears will crunch during selection.