Car Parts and Accessories

What the Auto Clutch has to do

A motor vehicle’s clutch takes the load off the gears when they have to be changed, by disconnecting the driveline between the engine and the driving wheels. The clutch also permits the engine to be revved up to develop sufficient power to move the vehicle before the drive is engaged.

Disengaging the clutch separates three parts of the clutch assembly the flywheel, the driven plate (also known as the centre plate or clutch plate) and the pressure plate. The flywheel is bolted to the end of the crankshaft and rotates with it; the driven plate is splined to the gearbox input shaft so that they rotate together; and the pressure plate clamps the driven plate to the flywheel.

When this pressure is released, by depressing the clutch pedal, the crankshaft and gearbox input shaft rotate independently; when the driver takes his foot off the pedal, they rotate as one.

Both sides of the driven plate, a thin disc of high-tensile steel, are faced with a friction material known as the lining. When the driven plate is clamped to the flywheel by the pressure plate, the clamping load must be great enough to prevent any slipping at the maximum torque delivered by the engine to the flywheel.

The terms coil-spring clutch, diaphragm clutch and centrifugal clutch refer to different methods of applying a load to the friction linings.

In a coil-spring clutch, the pressure plate is backed by a number of coil springs and housed with them in a pressed-steel cover bolted to the flywheel. The springs push against this cover.

Neither the driven plate nor the pressure plate is connected rigidly to the flywheel; both can move either towards it or away from it.

When the driver depresses the clutch pedal, a thrust pad, riding on a carbon or ball thrust-bearing to reduce wear, is forced towards the flywheel. Levers, pivoted so that they engage with the thrust pad at one end and the pressure plate at the other, pull the pressure plate back against its springs. This releases pressure on the driven plate, disconnecting the gearbox from the engine.

Auto Centrifugal Clutch

A few cars are equipped with centrifugal or centrifugally assisted clutches. These resemble the coil-spring clutch, but weights are fitted to the release levers.

As the clutch assembly rotates with the engine, these weights are flung outwards by centrifugal force and cause the levers to press more strongly against the pressure plate. The faster the engine runs, the greater is the force exerted.

This centrifugal clutch system can be used either in place of springs or as a supplement to spring loading.

Nowadays, centrifugal clutches are used only with some of the simpler automatic or semi-automatic transmissions, notably the DAF (now Volvo) Variomatic system.

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