23 Clutch friction materials

Clutch friction linings or buttons are subjected to severe rubbing and generation of heat for relatively short periods. Therefore it is desirable that they have a combination of these properties:

a) Relatively high coefficient of friction under operating conditions, b) capability of maintaining friction properties over its working life, c) relatively high energy absorption capacity for short periods, d) capability of withstanding high pressure plate compressive loads, e) capability of withstanding bursts of centrifugal force when gear changing, f) adequate shear strength to transmit engine torque, g) high level of cyclic working endurance without the deterioration in friction properties, h) good compatibility with cast iron facings over the normal operating temperature range,

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Axial Weer (mm!

Fig. 2.8 Characteristics of driven plate torsional damper thrust spring i) a high degree of interface contamination tolerance without affecting its friction take-up and grip characteristics.

2.3.1 Asbestos-based linings (Figs 2.2 and 2.3) Generally, clutch driven plate asbestos-based linings are of the woven variety. These woven linings are made from asbestos fibre spun around lengths of brass or zinc wire to make lengths of threads which are both heat resistant and strong. The woven cloth can be processed in one of two ways:

a) The fibre wire thread is woven into a cloth and pressed out into discs of the required diameter, followed by stitching several of these discs together to obtain the desired thickness. The resultant disc is then dipped into resin to bond the woven asbestos threads together.

b) The asbestos fibre wire is woven in three dimensions in the form of a disc to obtain in a single stage the desired thickness. It is then pressed into shape and bonded together by again dipping it into a resin solution. Finally, the rigid lining is machined and drilled ready for riveting to the driven plate.

Development in weaving techniques has, in certain cases, eliminated the use of wire coring so that asbestos woven lining may be offered as either non- or semi-metallic to match a variety of working conditions.

Asbestos is a condensate produced by the solidification of rock masses which cool at differential rates. When the moisture content of one layer is transferred to another, fibres are produced on solidification from which, as a result of high compression, these brittle, practically straight and exceptionally fine needle-like threads are made. During processing, these break down further with a diameter of less than 0.003 mm. They exhibit a length/thickness ratio of at least three to one. It is these fine fibres which can readily be inhaled into the lungs which are so dangerous to health.

The normal highest working temperature below which these asbestos linings will operate satisfactorily giving uniform coefficient of friction between 0.32 and 0.38 and a reasonable life span is about 260 °C. Most manufacturers of asbestos-based linings quote a maximum temperature (something like 360 °C) beyond which the lining, if operated continuously or very frequently, will suffer damage, with consequent alteration to its friction characteristics and deterioration in wear resistance.

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