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What Is A Clutch

Updated on March 24, 2013


The question has been asked countless times, what is a clutch? And what is its purpose? Simply put the purpose clutch is to transmit the turning force or torque from any engine to the transmission. Because of the design this is accomplish easily as it engage and disengage the engine from the transmission.

When the drive is disengaged the clutch allows the gears to be changed smoothly and it also provides a neutral position. The transmission is now able to selected or disengage gears whilst the engine is still running.

The clutch assembly is housed in the bell housing sandwich between the engine and gearbox. The major components of the clutch are the pressure plate, clutch disc and the flywheel. To engage and disengage the clutch a foot pedal operated mechanism is employed.

There are two types of pressure plates, namely the diaphragm and coil spring pressure plates. From the outside both looks similar but the coil spring pressure plates is deeper then the diaphragm spring pressure plate, when inspected closely the different components can be seen.

The strength of the springs and the diameter of the clutch disc determine the transmission of the torque from the engine. Greater torque can be transmitted if the springs are stronger and the diameter of the clutch disc is larger.

Coil spring clutch

The clutch assembly rotates with the engine because it is bolted onto the flywheel. The clutch disc is held against the flywheel by the force of the coil springs, which acts on the pressing plate which is fitted inside the pressure plate. The pressing plate is connected to the cover of the pressure plate with either straps or protrusions into eyes, which cause the pressing plate and the springs to rotate with the engine.

The center of the clutch disc is splined along with the input shaft from the gearbox is slid into th splines in the clutch disc so that the input shaft turns with the clutch disc. Because of the arrangement, when the engine rotates the gearbox input shaft also rotates

In order to disengage the clutch, the release bearing or thrust race is pushed forward toward the inner end of the fingers.

Diaphragm spring clutch

The following is the advantage the diaphragm spring clutch has over the coil spring clutch.

  1. Because it has a single flat spring it is smaller.
  2. Having less metal makes it lighter
  3. There is less wear because of fewer moving parts.

When it is engaged the diaphragm takes the shape of a saucer. The force applied by the outer ring forces the pressing plate against the clutch disc. The construction of the diaphragm spring pressure plate is similar to the coil spring pressure plate.

It is also bolted to the flywheel and the pressing plate is connected to the cover with flexible metal straps which causes both of them to rotate with the flywheel. The diaphragm spring pivots on the cover with shouldered rivets. It is against these rivets that the spring forces itself to transmit pressure to the pressing plate and the flywheel to transmit the drive.

The release bearing presses the center of the diaphragm spring to disengage the clutch. This action causes it to turn on the rivet and lifts its outer rim. Because of this the outer rim pulls the pressing plate away from the clutch disc, which allows the clutch to rotate freely, disengaging the drive between the engine and the gearbox.

The clutch disc assembly consists of two discs which is the friction material and a steel hub which fits onto the splines of the gearbox input shaft. The disc material consists of a mixture of asbestos and resin. This material has a very high coefficient of friction, along with the ability to resist wear and a natural resistance to burning.

This material is riveted onto the disc clutch plate. Today two types of clutch disc is common. The solid center type and the spring center type. The purpose of the springs is to absorb shock loads when the clutch is engage suddenly and to absorb small fluctuations in the engine speed and vibrations, giving a smooth transmission of power to the road.

Operating mechanisms

Depressing the left hand pedal engage or disengages the clutch. The pedal can be connected to the release bearing assembly by either a rod, cable or hydraulic.

Faults and adjustments

Grabbing and slipping are the two main faults that occur in a clutch. Slipping occurs when drive is not transmitted and can be brought about by:

  1. Oil on the friction lining.
  2. Fatigued springs.
  3. Worn friction lining.
  4. Wrong adjustment.

Grabbing occurs when the clutch cannot be engaged smoothly. It grabs and takes up the drive with a thud. Grabbing can be brought about by:

  1. Uneven spring wear.
  2. Damaged components.
  3. Wrong adjustment.

Adjustment is usually made by turning the adjustment nuts on the rod or cable. Most hydraulic systems are self adjusting. When any adjustment is made, there should be at least 2mm of free play at the end of the operating arm and this can be checked by the pedal. At the pedal there should be at least 25mm of free play.


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