232 Asbestos substitute friction material

The DuPont Company has developed a friction material derived from aromatic polyamide fibres belonging to the nylon family of polymers and it has been given the trade name Kevlar aramid.

The operating properties relative to asbestos based linings are as follows:

1 High endurance performance over its normal working pressure and temperature range.

2 It is lighter in weight than asbestos material therefore a reduction in driven plate spin shortens the time required for gear changing.

3 Good take-up characteristics, particularly with vehicles which were in the past prone to grab.

4 Weight for weight Kevlar has five times the tensile strength of steel.

5 Good centrifugal strength to withstand lining disintegration as a result of sudden acceleration which may occur during the changing of gears.

6 Stable rubbing properties at high operating temperatures. It is not until a temperature of 425 °C is reached that it begins to break down and then it does not simply become soft and melt, but steadily changes to carbon, the disintegration process being completed at about 500 °C.

Kevlar exists in two states; firstly as a 0.12 mm thick endless longitudinal fibre, which has a cut length varying between 6 and 100 mm, and secondly in the form of an amorphous structure of crushed and ground fibre known as pulp. In either form these fibres are difficult to inhale because of their shape and size.

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