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Better Cooling Needed for Braking Systems

16th December 1949
Page 51
Page 51, 16th December 1949 — Better Cooling Needed for Braking Systems
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Which of the following most accurately describes the problem?

Says I.M.Waller, B.A.,A.M.I.Mech.E., Technical Manager, Small and Parkes. Ltd. OUTSTANDING subjects requiring further research and

development in vehicle braking are: (1) Means for cooling; (2) facing and drum wear; (3) brake, squeal; (4) brake fading; (5) auto, matic adjustment; (6) the preven tion of lubricant reaching the faeings.

These were the points-mentioned

by 1. M. Waller, B.A., A.M.1.Mech.E., technical manager Of Small a,nd Parkes, Ltd., Manchester, in his paper, "Internal Expanding Shoe Brakes for Road Vehicles," which he read at the Institution of Mechanical Engineers (Automotive Division), on Tuesday.

The author, who dealt with the brake units themselves, as against the means for operating the shoes, said that a significant trend in brake design was the increasing use of floating shoes in place of the fixed-pivot type. The fundamental difference between them was that the floating shoe could centre itself in the drum when the brake was applied, whereas, in the fixed-pivot type, concentricity was difficult to obtain and easily upset.

Four Brake Types On the other hand, when the brakes were released, the fixed pivot provided. a positive location, whereas the floating shoe required some form of frictional device to centre it in the off position. There were, said Mr. Waller, four recognized types of brake in common use on road vehicles: these were the leadingand trailing-shoe brake with floating expander, leadingand trailingshoe br ake with fixed cam, the two-leading-shoe type, and the selfenergizing or duo-servo brake.

It was possible for any of them to be equipped with either pivoted or floating shoes, so that, theoretically, they could be compared on the basis of a common shoe, Of the two-leading-shoe brake, the author said that when separate .hydrau

lie cylinders were used, the two shoes were fully compensated for both load and movement, so that the drum reactions were balanced and produced no offset load on the wheel bearings. The two facings, too, would run at approximately the same temperature and wear would be equal. With the mechanical type, using bell-crank levers and a strut, frictional losses would prevent exact equalization.

The hydraulic two leading shoe brake could be made double-acting if the same factor were required for reverse as for forward rotation; otherwise the brake would be about three times as powerful going forwards as backwards. The fashion, however, was to use single-acting brakes for economy and to confine them to the front of the vehicle, leadingand trailing-shoe brakes with floating expanders being used at the Mar.

With this arrangement, the overall reverse braking was approximately three-fifths of normal, so that the pedal load which gave a degree of retardation of 12.88 ft. per sec. per sec. on the level when travelling forward, would be sufficient to hold the vehicle from rolling back on a 1-in-4 gradient.

One of the advantages of the mechanical two leading shoe brake using bell cranks and a strut, was that

it lent itself to dual operation by hand or foot, so that it could be employed at the rear as well as at the front.

Of the self-energizing, or duoservo brake, the author said that this had the highest factor of any of the internal-expanding types; it permitted the use of the lightest possible pedal load without need for power assistance.

To keep brakes cool a fan which would pump a substantial weight of air to the brake assembly was needed. The only way of doing this appeared to be to form a centrifugal impeller on the drum back plate, which would draw air through the centre of the road wheel and discharge it through orifices near the outer periphery of the brake carrier plate.

Silencing Squeal The most generally successful palliative for brake squeal was the anti-squeak band. Such a band would consist of asbestos tape, or other nonresonant material, wrapped round the brake drum and secured by a strip-steel clamp.

Successful results had been attained by cutting away the web of the shoe to introduce local flexibility, but the position and dimensions of the slot thus formed were critical.

A small amount of progressive fade with rapid recovery could be tolerated, and might even be desirable. Delayed fade, however, was intolerable, because a vehicle might descend a long gradient without difficulty, and then the brakes might fail to hold it at a cross-road half-a-mile farther on.

Of automatic adjustment, the speaker said that, to be fully effective, it must compensate for wear of the facings, but at the same time it must allow the shoes to follow changes in drum diameter.

It was, the author said, the study of surface phenomena and the search for better cooling which formed the fundamental basis. of progress in braking.


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