Will Friction Brakes and Clutches Survive?
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Interesting Facts on the Continued Use of Asbestos-base Materials for Brake and Clutch Facings
THE future of friction brakes and clutches was dealt with by Mr. E. W. Sisman, a director of British Belting and Asbestos, Ltd., Cleckheaton, in a recent lecture to the Bradford Centre of the Institute of the Motor Industry.
Mr. Sisman said that he was doubtful whether friction brakes would ever be dispensed with. The friction clutch had a rival in the fluid coupling, but he did not think it would ever be eclipsed by it. The range of use of the fluid coupling, he felt, would never include the lighter and cheaper types of vehicle.
Suggesting that friction facings, with an asbestos base, would continue to hold most of the field, Mr. Sisman said that the sintered product, as made in the United States, was about eight to ten times dearer than the asbestos-base material, and its scope was limited, particularly in respect of curved shoes.
Increased Stability
With regard to future development of asbestos-base facings, he thought they would generally tend towards the rigid types, moulded or partially moulded, with lower but more stable friction values. They would be called upon to operate under more severe temperatures, and the general refinement of design would call for increased stability of friction over the whole range.
Progress in the technique of spinning asbestos fibre into yarn, and particularly in bonding media, had, during the past few years, eliminated the necessity of adding to the, facings' mechanical strength by metallic reinforcement.
Under normal conditions, where the friction surfaces were clean and the ternoerature did not exceed 400 degrees F., the coefficient of friction for practically all types of brake facing, speaking generally, lay between 0.3 and 0.4. Appreciable variations in the frictional characteristics of the braking surfaces could, however, be caused not only by operating conditions—principally temperature—but by the presence of extraneous matter on the braking surfaces.
Some facings tended towards a large reduction in their coefficient of friction at temperatures in excess of 400 degrees F., whilst a limited number was inclined to a slight increase at the same point. Most asbestos-base materials showed a large and permanent increase if temperatures exceeding 700 degrees F. were encountered more than momentarily, but if such figures were experienced in the normal working of any brake, design and proportions obviously called for drastic revision.
Friction Plus "Sticktion"
Certain types of bonding media tended to soften and exude at relatively low temperatures, when the grip of the surfaces became the sum of friction plus " sticktion," and values of the order of 0.7 might obtain. This characteristic, however, was rarely manifested by modern products of the more reputable manufacturers.
As to the effect of water in reducing the coefficient of friction, Mr. Sisman's experience was that, driving on wet roads or in fog, with brakes having an average degree of protection, a drop ot 10-15 per cent. in friction resulted; on a freshly washed car, or after negotiating a water-splash, the drop might be as great as 60-80 per cent.
Effects of oil and grease could cause wide variations, but the result was always a reduction in friction value, except that a small amount of grease. mingled with abraded debris on lowtemperature applications, occasionally formed a glutinous paste and augmented the coefficient of friction.
Road grit, mostly water-borne, was a strong abrasive and its effects must tend to augment the friction. As its ingress was chiefly caused by water, a reasonable assumption was that its greatest accumulation would occur at the leading ends of the facings. A rise in friction at the leading ends of the facing was particulary prone to cause locking or harsh action.
Cement or Rivets
Remarking that the Americans had been talking a good deal about their development of the idea of using cement instead of rivets to fasten brake and clutch facings on to steel parts, Mr. Sisman said that, in this country, the idea had, for several years, been applied on a large scale to clutches for aircraf engines.
Striking a note of caution with regard to this technique, he commented; "We know how to put the facings on, but it seems to us, at this stage, and for a long way ahead, a process which will have to be confined to factory control; and even there I am very dubious whether the fatigue resistance in such cases is sufficient to enable any one to offer cemented brake facings with confidence to the public."