Clutches are useful in devices that have two rotating shafts. In these devices, one of the shafts is typically driven by a motor or pulley, and the other shaft drives another device.

In a car, you need a clutch because the engine spins all the time, but the car’s wheels do not. In order for­ a car to stop without killing the engine, the wheels need to be disconnected from the engine somehow. The clutch allows us to smoothly engage a spinning engine to a non-spinning transmission by controlling the slippage between them.

It transmits engine power to the gearbox, and allows transmission to be interrupted while a gear is selected to move off from a stationary position, or when gears are changed while the car is moving.

Most cars use a friction clutch operated either by fluid (hydraulic) or, more commonly, by a cable.

When a car is moving under power, the clutch is engaged. A pressure plate bolted to the flywheel exerts constant force, by means of a diaphragm spring, on the driven plate.

Earlier cars have a series of coil springs at the back of the pressure plate, instead of a diaphragm spring.

Parts of the clutch

The modern clutch has four main components: the cover plate (which incorporates a diaphragm spring), the pressure plate, the driven plate, and the release bearing.

The cover plate is bolted to the flywheel, and the pressure plate exerts pressure on the driven plate through the diaphragm spring or through coil springs on earlier cars.

The driven plate runs on a splined shaft between the pressure plate and flywheel.

It is faced on each side with a friction material which grips the pressure plate and flywheel when fully engaged, and can slip by a controlled amount when the clutch pedal is partially depressed, allowing the drive to be taken up smoothly.

The release bearing is pushed hard against the diaphragm spring, either hydraulically or by a cable and lever, and releases spring load to interrupt power transmission.

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