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Less Facing Better Braking

26th October 1951
Page 49
Page 49, 26th October 1951 — Less Facing Better Braking
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Which of the following most accurately describes the problem?

46HE drum brake will be with its

for a long time to come, and future development will centre on employing shorter brake blocks in place of the conventional facings now used, so reducing drum temperature.." This forecast was made by Mr. Ivan M. Waller, B.A.. M.I.Mech.E., technical direetbr of Small and Parkes, Ltd., when he presented a paper entitled " Brakes and Clutches," before a meeting of the Institute of Road Transport Engineers, in London, last week.

He considered it a paradox that, in the past development of drum brakes. the tendency had been to increase the length of the facing for longer life, whereas this might have raised surface temperatures and so increased the rate of wear. The American practice of employing short lengths of friction material bolted to the shoes was an intermediate stage to using two short moulded brake blocks, with greater pressure operation at the shoes to offset the lower friction coefficient of the moulded material. .

Questioned on the potential life of such blocks, Mr. Waller expressed the opinion that replacement would probably be less frequent than* at present because, with a large air gap between the blocks, the drum-surface area was cooled more rapidly and the rate of wear on the drum and friction material reduced accordingly.

Improved Material He maintained it was the aiin of all brake manufacturers to produce longerlife facings at reduced cost, thereby lengthening the period between docking. Assuming that the vehicle manufacturer did all that was reasonably possible to provide adequate cooling and that the surface of the drums be. given a reasonably smooth finish, the problem of prolonging life could be met only by using a more durable facing material, He considered the, better. types:Of ,moulded brake blocks cOuld be regarded tis the answer..

Mr. Waller said it was not so well known that the moulded materials at present used here were a compromise to give a . friction value -as near as possible to the woven fabrie, and that by. employing a lower friction value an even longer life could be given. In th-,1. Meantime, driven, were able to discern advantages and disadvantages of brake facings which were not readily revealed

in: the:. conventional test run. Me. Waller, who returned last week from a six-weeks' tout of America, mentioned that the average brake life of the C.M: truck was 35,000-50,000 miles. .

The other approach to the wear problem, he considered, was ill 'the. better uSe or the materials employed. The ercscent-shaped moulded block used an many Timken axles showed a reduction in theamount of material pinehased: of approximately 10-12 per COL _ However; it was an unpopular itOdffet imvpropositicin because the cost tvg miglitTiot be commensurate with the saving in raw material.. He mentioned the crescent-Shaped block was at present -employed • on the heavier type Of Kirkstall 'bogie.

.Mr. Waller _thoughtthat a far more practical • proposition, which was now being examined, was the use of the Iwo short brake blocks, welt spaced on the shoe, and mounted at points where the wear pattern . was symmetrical, in addition to aiding cooling of the drum surface, it•could also be adopted without any change in the -brake design.

_Questioned on the possibility of fitting these blocks to existing shoes, he con's-idered that special brake shoes would be required to obtain the maximum benefit of cooling.

Concerning ease of maintenance. he considered that •whilst riveting Or bonding of friction material to the shoes were attractive methods in initial mane, facture, the three-quartet-inch blocks bolted to theshoes, as used onpractically every heavy vehicle in Atneriea. presented an advantage to the operatar little recognized in this country. The blocks could be hanged quicker and easier and without distortion to the shoes. Ile considered much wastage of shoes and damage was done when rivets were .being chippedoff fOr refacing.

During the discussion, Mr. Wal!:1mentioned that bonding had certain advantages. but replacement of friction material might be difficult. It was not always easy to remove the worn bonded facing from the shoe, and the alternative of "exchanging the shoe complete was, at Present made more difficult because of the steel . shortage •and high freight charges. •

Adjustment When Refuelling Mr. Waller mea tioned that' atitomatie adjusters should he an integral part of every ,brake design -; but unfortunately, they had not yetreached the state when they could be considered gencraliy aceeptable. -The present .trend was to concentrate -on .mbre. positive and easily operated manuaradjusters-. and he was. intetested in the practice of American bits companies in Which the brakes' were adjusted :every, time the bus vsias refuelled. The fuel pumps Were' situated alongside .a 'pit :over which -every bus passed' On its way -down the servicing line whichalso included facilities for washing and cleaning. Of the few types of automatic adjuster available, he considered the Wagner on the Studebaker, and those fitted to, the Laycock Neale air. brake showed most promise,

Outlining the defects of brake_dritms, he said there was little to learn beyond recogniting .. the . symptoms . they betrayed, such 'as black patches, blue

spotting . and thermal cracking, all giving indication of very rapid .heat rise such as heavy' braking from high speeds. This had the effect of transforming the surface of the cast-ironinto local patches of martensite which had a hard brittle structure with a higher 'coefficient of expansion than the parent mass.

Surface temperatures of 800-900 degrees C. were needed to produce this condition, although. the main mass of ihe drum Might he comparatively. cool. The chilling effect of the body of the metal played an essential part in forming he ma riensite.

Replying to a question regarding drum sleeving, he considered there was not much evidence of the heat path . being disturbed to any serious extent between the sleeve and the drum. Wear on a steel sleeve would probably be higher.

Mr. Wallet considered that the disc brake might,. eventually, be a replacement for the present arrangement, anti mentioned that most manufacturers were working on schemes of this kind; one was, in fad. being shown at the Motor Show on the Girling stand.

Disc Brake Tests

He mentioned that in tests in this country with disc brakes fitted to a car, it had been driven through the water splash on the M.I.R.A. track at Nuneaton, and the retardation had beet) little affected by immersion in water. He explained that with the disc brake it was open to derive maximum benefit from air cooling and that any water reaching the brakes was automatically thrown off by centrifugal action. In similar tests, with drum brakes, the results had been entirely different because it took some while to dry the drums.

Before outlining symptoms of clutch failure, Mr. Waller said that a survey had been made in Cincinnati to determine, the amount oftime the brakes were in, use. on a lboal bus' service, The

results had shown that in a area the brakes were applied for 50 pet cent. of the total running time, but this. of course, did not represent an equal pro portion of distance, _ The causes of clutch troubles, apart' from slip, were practically impossible to trace, once the unit was removed from the vehicle.

Judder on engagement was caused by a geometric quarrel between the pedal linkage and flexible engine mounting, and as usually marked more in one directiOn than the Other. The cauSe of spline wear coUld,. aometimes, be traced to out-of-line machining 'of the bell. housing, or other Misalignment between the gearbox and the engine, Hydraulic operation of the clutchwithdrawal mechanism had appeared. on the new Ford cars and was a line of development which Mr Wailer thought should be watched with interest because this could result in a reduction of clutch

troubles .


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