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Friction is the clutch's best friend

By unknown | Aug 08, 2007 | COMMENTS [ 0 ]

The clutch is the mediator between the engine's torque and the drive- train's traction.

The clutch is the mediator between the engine's torque and the drive- train's traction.

As such, it has a hard life.

The principle on which it works can be summarised in one word: friction. A disc, perhaps the size of a dinner plate, and lined on both sides with friction material similar to brake linings, is the heart of a clutch. This is the so-called clutch plate, which is splined to the gearbox input shaft, and which can be sandwiched tightly between the face of the flywheel and a movable pressure plate.

When you depress the clutch pedal to disengage the clutch, the pressure plate backs off slightly to release its pressure on the clutch plate.

The clutch plate is now free to do its own thing - stop spinning or spin faster - independently of the flywheel; there is no connection between the flywheel, bolted to the crankshaft, and the gearbox input shaft, splined to the clutch plate.

When you let out the clutch pedal to engage the clutch, the pressure plate moves towards the flywheel, clamping the clutch plate between itself and the flywheel with increasing pressure.

As the pressure increases, so does the friction between the linings on the clutch plate and metal surfaces between which it is clamped, until the point is reached where the clutch plate is forced to spin at the same speed as the flywheel.

The clutch is then fully engaged and there is essentially a solid connection between the flywheel and the gearbox input shaft.

When the clutch is only partially engaged, the friction linings on the clutch plate rub across the metal faces of the flywheel and pressure plate, and each time this happens, a tiny amount of friction material is rubbed off the linings in the form of fine dust. At the same time heat is generated by the sliding friction.

This occurs perhaps 30 times on a typical trip to work in peak-hour traffic, and though it is only for a split second each time as the clutch is let out in stop-start conditions, it eventually takes its toll on the clutch plate. In a worst-case scenario, where you are caught in stop-start traffic on a long uphill stretch while towing a heavy trailer, the build-up of heat as you are forced to slip the clutch repeatedly can do more damage in a single day than 40000km on the open road.

Small wonder there's a litany of clutch complaints. The main ones can be described as a slipping clutch, a dragging clutch, and a juddering clutch. A slipping clutch is one that never engages solidly, even when you let out the pedal completely.

A dragging clutch is exactly the opposite - the clutch never disengages completely, even when the pedal is fully depressed. A juddering clutch is unmistakable.

A good driver can do a lot to preserve a clutch. Never use the clutch pedal as a footrest, and never slip the clutch to keep the vehicle stationary on an incline.

Remember: a clutch should be either fully in or fully out.


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