How Can Brake Fade be Cured?
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CAUSES of brake fade and its possiblesible cure formed one of the topics at a discussion evening held in Leeds by the North-Eastern Centre of the Institution of Mechanical Engineers (Automobile Division).
Mr. L. W. Clegg, of British Belting and Asbestos, Ltd., Cleckheaton, who opened discussion on this topic, remarked that the main cause of reduction in the coefficient of friction of brake facings was excessive brake temperature. The heat, he explained, drove off volatiles in the bonding medium, and thus created a lubricant on the rubbing surfaces.
Time and Temperature The fall in frictional efficiency was not due entirely to temperature, but rather to a combination of time and temperature. Therefore, the betterquality facings were those that would maintain friction when working at high temperature over long periods.
A sudden reduction in frictional efficiency also sometimes occurred when brakes which had been run for some time at high temperatures were allowed to cool, Here, again, the trouble was caused by sweating of the bonding medium in the brake facing. Recently, however, high-friction facings had been produced which did not suffer from this defect.
A30 High-quality non-me.tallic woven asbestos facings had a coefficient of friction of about 0.4 to 0.42 when cold, and this was well maintained at all temperatures up to about 400 degrees F. Thereafter, a slight reduction took place up to 500 degrees F.
It was generally recommended -that this type of facing should not be run at more than 500 degrees F., because in that case not only did frictional efficiency tend to drop sharply but excessive wear took place.
In the case of heavy-duty woven facings with zinc or brass wire bonding, again the maximum working temperature was 500 degrees F.; so it would seem that temperatures above that figure could be considered excessive for the normal type of brake facing. There were certain materials that would hold their coefficient of friction at temperatures up to 1.400 degrees F., but they had only a moderate coefficient of friction of 0.28 to 0.30.
Mr. Clegg added that he had found an increase in the mass of the drum was by far the most successful method of keeping down brake temperature.
Mr. T. Anthonies commented that brake-facing manufacturers were often asked to take responsibility for fade troubles when the question was fundamentally one of design. In the case of public service vehicles, the greater weight of the double-deck bus and the higher overall speed were factors which tended to raise brake temperatures. Thousands of foot-pounds of kinetic energy were converted into heat in a matter of seconds every few minutes. The real cause of the trouble was that the heat was not dissipated rapidly enough.
Mr. P. M. Baker, of Leeds Passenger Transport Department, agreed that fundamentally the problems arising from excessive heat were not for the brake-facing manufacturer, but he testified to benefits obtained from manufacturers' co-operation in the selection of facings which were better able to withstand heat.
Plated Aluminium Drums One heat-dissipation idea mentioned in the discussion was the use of aluminium brake drums, plated with chromium. It was stated that encouraging results had been obtained from aluminium drums with very heavy ribs. Aluminium drums of earlier design were found to be too light.
A plea for research into the heat problem in the design of vehicles, and for consultation between vehicle and brake-facing manufacturers, prompted Mr. Clegg to remark that there had been a good deal of improvement in the degree of co-operation of that kind.