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Interesting Brake Tests.

17th December 1908
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Page 4, 17th December 1908 — Interesting Brake Tests.
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A Few Useful Extracts from the Report of the Brake Committee of the Tramways and Light Railways Association.

The disastrous tramway accident, which took place at Highgate Archway, on the 23rd June, :two, will still be fresh in the minds of many of our readers, It will be remembered that a heavy double-deck bogie tramcar got out of control while it was coming down the long gradient from Highgate Archway to the terminus at the Archway Tavern; the car eventually ran off the line, and crashed into a shop, with fatal resulls to three of the passengers. The Board of Trade inspector, Colonel Yorke, in his report, suggested that it would be desirable for the Tramways Associations carefully to consider the whole question of braking and sanding in connection with tramcars.

The Council of the Tramways and Light Railways Association, with commendable promptitude, appointed a special committee to investigate the whole subject of brakes and braking, with, of course, special reference to their application to tramway work. So extensive did the field of en• 4.p.firy appear to be to the newly-constituted committee, that it resolved to divide itself info two sub-committees : the first was charged to discuss and recapitulate "the general objects to be aimed at to obtain braking effects, and the physical laws which govern them "; the second sub-committee was requested " to examine and report upon the various typos of brakes, brake blocks, and sanding gears." The committee soon arrived at the opinion that " the available records and book figures for coefficients of friction stood in need of experimental revision," and, therefore, with the cord'al co-operation of Mr. A. L. Coventry Fell, the

tnterprising Chief Officer of the L.C.C. Tramway, a number of very interesting tests was carried out, and the results were carefully classified. It is with these practical attempts to ascertain definitely the coefficient of friction for various pairs of braking surfaces that we are now primarily concerned, and we do not hesitate to state that much of the information on this subject, which has been so carefully obtained for the Tramway Associatidn, can be applied very usefully to some of the problems, such as braking, clutching, and skidding, which are engaging those who have anything to do with the commercial road motor.

"f.; those of our readers who are sufficiently interested in the committee's researches on such questions as the most us■.:u1 form of tramcar brake, the limits of safety gradients, the limits of speed of deceleration and acceleration, the employment of sanding gear, and the proportions of available weight to be used for the braketi, we cannot do better than ro.ornmend a careful study of the committee's full report; IL is published 'by the Tramways and Light Railways Associ.dion (as part of the official circular), at 35, Parliament Street, Westminster.

Tne committee found very little reliable data available, and it was, therefore, decided to institute a new series of tests which should definitely settle the relative values of a number of different braking materials under varying conditions of speed and pressure. The first series of tests was made by the pressing of blocks of different materials on to steel-tired wheels which had been turned smooth on the fare, and these wheels were run at varying speeds as occasion demanded. The results of this series are directly appl'cable to similar problems in connection with the braking

of heavy motor vehicles. Broadly speaking, the results obtained showed that " the coefficient of friction of wood blocks on a wrought-iron wheel is considerably higher than that of cast iron or wrought iron, and that the lengihening of the block from 12 inches to 18 inches has no appreciable effect. The curves obtained from the wood-block experiments are consistent, from low speeds to speeds of to to

Miles per hour. At the higher speeds, a rapid rise is found to take place. 'This is probably due to the exudation of .gum from the wood under the influence of heating, and is most marked in the experiments with heavily-weighted blocks. The effect of the weight is also noticeable in an increase of the coefficient of friction of wood blocks, due, no doubt, to an increase in the density of the material caused by the pressure."

We reproduce a number of the most striking series of curves, and on all these the vertical ordinates divide the charts for speed, and the horizontal lines, or abscissa, in-., diewe the graded coefficient of friction. The curves, ex except those shown on Test 6, show, until the effect of hewing is apparent, a drop in the coefficient as the speed increases, and this, of course, con firms, in principle, the results that have been obtained by other experiments. In

TVs'. No. 6 it will be seen that the coefficient rose throughout with the speed. The results of the tests of wood blocks, when dry, show that elm has the highest coefficient of friction on a steel wheel, followed by ash, maple, and oak, in the order named. The chart published as Test No. 6 snows the remarkably straight lines plotted owing to the coefficient's rising directly with the speed. Test No. 3, which gives the results obtained with dry oak blocks, shows considerable variation of friction with the load. The sharp drop after the " heating " rise is probably due to the lessening of viscosity of exuded gummy matter at h'gh temperatures. The curves shown on Test 9 agree with the generally-accepted performances of cast-iron blocks, in that they show a steady fall with the increase of speed. A slight rise is noticeable in all the curves at the higher speeds, and this is probably due to the production of iron dust by the grinding action between the surfaces. At low speeds, the curves for wrought iron show the coefficient to be very high, particularly in the case of the 2,o3o1b. load, e.g., .55 at 4 miles per hour ; but this, as is usual, rapidly falls as the speed increases, It will be noticed that the 2,0301b. load shows a very heavy fall, and this is probably due to the rolling action of small pieces torn from the surface of the block. On the last chart, Frood's patent lining shows remarkably constant results, in the neighbourhood of .375 for all speeds, except in -one case. We dealt very fully with this cotton-fabric material in our issue of the 7th May last.

A second and third series of tests were carried out, with the object of determining the variations in coefficients of friction between rails and various materials for use as track-brake blocks; but, for our present purpose, sufficient information may be gleaned from the results of the first series of tests.


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