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.

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