How Stuff Works

Fuzz separates the jive from the drive

Clutch release bearing

’Early engines had a limited rev range – often in the hundreds of rpm’

The internal combustion engine was liberating, in both automotive and stationary forms, but it came with restrictions. The first of these limitations was rev range.

Early engines had a very limited rev range indeed, often in the hundreds of revolutions per minute (rpm) rather than the thousands available with more advanced designs. This meant that once maximum revs had been reached, no greater output speeds could be reached without the use of gearing, which led to limited power.

The need for greater output speeds was affected by engine power. Developing enough torsional force to turn an output at speed against whatever resistances were imposed from a standing start would be problematic and, in many cases, have required an engine of great power. Larger engines tended to operate within a lower rev range and so the circle returned to the first problem.

One solution was to use a system of multiple gear ratios to enable output rpm to be increased while enabling a given engine to work within its

design-limited rev range. This system became known as a gearbox.

This solution, however, brought with it another problem – how to smoothly transition between gear ratios. In an automotive environment the additional problem of how to take up drive smoothly from a standing start also came to bear.

Several methods were established, including petrol-electric drive and fluid and centrifugal clutches, but the dry-friction plate clutch eventually became the standard in motor cars.

The way in which this assembly worked was by use of strong springs holding a circular metal pressure plate, located within a pressed or cast clutch cover, against a friction material-lined clutch centre plate, itself sandwiched between the engine flywheel and the pressure plate.

Releasing this spring pressure required an amount of ingenuity on the designers’ parts and so the method of fulcrum levers came to be used.

In unreleased state, the friction plate remained sandwiched, giving drive between engine and gearbox. However, with the clutch operated, a release bearing pressed against the fulcrum levers, overcoming the spring pressure, thus disengaging drive. This allowed gear selection to be made and for drive to be taken up from the engine in a gradual, smooth fashion.

This release bearing came in numerous formats, but the most common became the enclosed roller type, or the bonded graphite type. Located around and sliding over the gearbox input shaft, it was held securely in place onto a clutch control arm, most often using spring clips.

When actuated, the bearing quickly came up to speed against either a central ring connected to the clutch fulcrum levers or, on diaphragm spring clutch types, a ring located centrally amid the spring fingers, which themselves acted as fulcrum levers.

This allowed for relatively noiseless clutch operation and thus the harmonious matching of engine power to output to useful driving speeds.

Remarkably, it remains much the same today, in the twilight years of the internal combustion enginepowered motor car.