As engine power is transmitted through the gearbox, the gears themselves will have a tendency to stay in mesh making gear changing very difficult. Even more difficult, once a gear has been disengaged, is the selection of another. The difficulty arises from the fact that the speed of the two gears being introduced to each other is very different. To make this gear changing process smoother and more gradual a device is required that will interrupt the engine output before it reaches the gearbox internals. This device is the the most commonly used clutch today is the single plate diaphragm spring type. Put simply, a circular plate (driven plate) with a splined centre boss and a ring of friction material fixed to either side is slid onto the input shaft of the gearbox, one side of this plate will press against the working surface of the engine flywheel which is, in turn, bolted to the engine crankshaft, the other friction lined side of the driven plate will be pressed against another working surface mounted onto an assembly called the pressure plate. With the clutch in the at rest position this friction sandwich is clamped firmly between the flywheel and the pressure plate and will, therefore, rotate at the same speed as the engine. As the driven plate is splined it can move back and forth a small amount when required. At the centre of the pressure plate is a circular spring steel diaphragm which is slightly conical in the at rest position. Pushing on the centre of the diaphragm, flattening it out, will have the effect of reducing the clamp effect produced. The last part of the clutch assembly is the release bearing which is mounted around a tube over the input shaft being able to slide quite freely and in turn indirectly attached to the clutch pedal via a cable or a system of hydraulics.
When the clutch pedal is depressed, the release bearing moves forward pressing against the diaphragm spring unclamping the driven plate and interrupting the power supplied to the gearbox, with the drive gone, selecting or changing gear is easily achieved. The gradual release of the clutch pedal by the driver once a gear is selected will reinstate the clamp force on the driven plate and power will once again pass to the gearbox and on to the driven wheels. Even with the engine running and the clutch not being operated, no vehicle movement will occur until a gear is selected.
Although various mechanisms are incorporated in the driven plate to facilitate smooth power take up, the main advantage of the friction material on the driven plate is that it will slip for a very short time greatly assisting this power transfer.
Various other permutations on this basic theme exist e.g.; paddle clutches, multi-plate clutches, electro-mechanical clutches, damped flywheel clutches, centrifugal clutch, etc; but they all exist to interrupt engine power to the gearbox so that gears can be selected and the vehicle moved away smoothly.