A clutch is that which connects a vehicle engine to the transmission. If there was no clutch the vehicle could not move from a standing start, could not come to a complete stop with the engine running nor would it be possible to change gears. The entire concept of a clutch is based on friction.
The components that make up a complete clutch assembly consist of the disc, pressure plate, throw out or release bearing and the clutch fork. Although it actually an integral part of the engine, being bolted to the crankshaft, the flywheel is also necessary for the clutch to function.
Clutch friction plate:
The key to the whole thing is the clutch friction plate; it is a circular metal plate with a splined hub and friction material bonded to both sides. The female splines of the clutch friction plate mesh with matching male splines on the transmission input shaft. When the clutch is not engaged the plate and transmission splines do not rotate, however, they do when the clutch is engaged.
The friction material on the face of the friction plate is made from heat resistant material that is bonded to the face of the metal core. For conventional vehicles the friction material is a mixture of fiberglass and copper or brass. Many manufacturers use exotic material such as ceramics, Kevlar or Fermalloy.
The clutch assembly, including the pressure plate, release bearing, fork and disc are attached to the flywheel. The pressure plate is bolted to the flywheel, the clutch friction plate or disc is held against the flywheel by spring pressure.
When the clutch pedal is depressed the fork moves the throw out bearing to the center of the bellville spring in the pressure plate, this allows the pressure plate to relax which releases the captive disc from the flywheel. The engine can continue to run while the vehicle is stationary. When the clutch pedal is released, the opposite action takes place and as the engine and transmission are now coupled together, the vehicle moves.