Internal combustion engines must have a certain minimum speed (approx. 300-600 r.p.m.) before they can run on their own power and develop a sufficiently high torque to drive the vehicle. While the vehicle is stationary, with its engine running, it is necessary to disconnect the engine from the gearbox; this is done by means of the clutch.

When the vehicle is about to move off, the clutch must first gradually raise the speed of rotation of the gearbox input shaft to the speed of the engine shaft. In so doing, the clutch has to transmit a torque while the slip due to the difference in speed between the engine shaft and the gearbox input shaft steadily decreases. Short periods of disconnection of the engine from the gearbox by means of the clutch are also necessary for gear-changing - i.e., for changing the transmission ratio of the gearbox - so as to effect the temporary disengagement of intermeshing components in the gearbox.

In the commonest form of clutch the connection of the engine shaft to gearbox shaft is effected by friction between two or more discs. The slip which occurs during the period of equalisation of the speeds of the driving shaft and the driven shaft generates heat. In the normal clutch operations the amount of heat involved is negligible, but if the clutch is allowed to slip for a considerable time, the clutch lining will quickly be destroyed.

Nowadays dry single-plate clutches are mostly used. In this form of construction a drive plate, made of steel and faced on both sides with riveted-on segmental clutch linings, is so mounted on the clutch shaft that it cannot rotate but can be shifted axially in relation to the latter. The clutch shaft also forms the output shaft of the clutch to the gearbox. The drive plate is pressed between the flywheel and the clutch ring by thrust springs. This clutch ring is axially movable, but is so connected to the housing by drive elements (cogs) that it must always participate in the rotation of the housing. The clutch ring engages with the clutch release lever, the other end of which is moved by the movable clutch release sleeve.

When the clutch is disengaged, the clutch release sleeve moves towards the engine, with the result that the clutch release lever shifts the clutch ring against the pressure of the thrust springs: the connection between the engine and gearbox is severed.

The multi-plate clutch has been evolved from the single-plate clutch. Here again the clutch is integral with the flywheel of the engine. The housing contains the driving plates, which are non-rotatably connected to the housing, but can be shifted axially, and which are faced with clutch linings.

The driven plates are secured to the pressure plate, upon which the pressure spring acts. The pressure plate is mounted directly on the drive shaft, on which it is axially movable, in order to enable the clutch to be disengaged.