Page:1902 Encyclopædia Britannica - Volume 26 - AUS-CHI.pdf/383

 BRAKES reservoir air to flow into the brake-cylinder. At the same time air admitted through port t moves piston 8, which unseats valve 10, and allows train-pipe air to lift check valve 15 and charge the brake-cylinder with air from the train-pipe, in addition to that from the auxiliary reservoir. In this way not only is a very sudden application of the brakes brought about, but also an increased force of application of about 20 per cent, over the greatest force of an ordinary application. This sudden venting of the train-pipe into each brake-cylinder gives an extremely rapid serial action of the brakes on a train of vehicles by means of which the various brakes throughout the train are successively applied almost with the rapidity of sound. It is well known that brake-shoes can be applied,

Pig. 4.—Air-Brake Triple Valve.

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The object of the former series was to determine the co-efficient of friction between the brake-shoe and the wheel, and between the wheel and rail at different velocities when the wheels were revolving and when skidded, i.e., stopped in their rotation and caused to slide. These experiments were the first of their kind ever undertaken, and for many years their results furnished most of the trustworthy data obtainable on the friction of motion. It was found that the co-efficient of friction between cast-iron shoes and steel-tired wheels increased as the speed of the train decreased, varying from 0T11 at 55 miles an hour to 0'33 just moving. It also decreased with the time during which the brakes were applied; thus at 20 miles an hour the co-efficient was at the beginning 0T82, after ten seconds 0T33, after twenty seconds 0'099. Generally speaking, especially at moderate speeds, the decrease in the coefficient of friction due to time is less than the increase due to decrease of speed, although when the time is long the reverse may be true. When the wheels are skidded the retardation of the train is always reduced ; therefore, for the greatest braking effect, the pressures on the brakeshoes should never be sufficient to cause the wheels to slide on the rails. The Burlington brake tests were undertaken to determine the practicability of using powerbrakes on long and heavy freight trains. In the 1886 tests there were five competitors— three buffer-brakes, one compressed-air-brake, and one vacuum-brake. The tests comprised stops with trains of twenty-five and fifty vehicles, at 20 and 40 miles an hour, on the level and on gradients of 1 in 100. They demonstrated that the buffer-brakes were inadequate for long trains, and that considerable improvements in the continuous brakes, both compressed-air and vacuum, Avoukl be needed to make them act quickly enough to avoid excessive shocks in the rear vehicles. In 1887 the trials of the year before were repeated by the same committee, and at the same place. Trains of fifty vehicles fitted with each brake were again provided, and there were again five competitors, but they all entered continuous brakes — three compressed-air-brakes, one vacuum, and one electric. The results of the first day’s test of the train equipped with Westinghouse brakes are shown in Table I., the distances in which are the feet run by the train after the brakes were set, and the times the seconds that elapsed from the application of the brakes to full stop.

without skidding the wheels, with greater pressure at high speeds than at low. Advantage is air-brake.*1 of this fact in the design of the highspeed brake, invented in 1894, which consists of attachments, by means of which the pressure in the train-pipe and reservoirs can be increased at the will of the driver. The increased pressure acting in the brake-cylinder increases in the same proportion the pressure of the brake-shoes against the wheels. Attached to the brake-cylinder is a valve for automatically Table L- -Stops of a Train of Fifty Empty Gars, 1887— Automatic Air-Brakes. reducing the pressure therein proportionately to the reduction in speed, until the maximum pressure under which Equivalent the brakes are operated in making ordinary stops is Speed in Distance Time in Distance at Miles per 20 Miles and Seconds. in Feet. reached, when this valve closes, and the maximum safe Hour. 40 Miles. pressure for operating the brakes at ordinary speeds is 196 91 186 retained until a stop is made. m 233 11 215 m Famous tests of railway brakes were those made by the 693 588 17 36£ late Sir Douglas Galton, K.C.B. (then Capt. Galton, R.E.), Brake tests.

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^ Westinghouse on the London, Brighton, The remarkable shortness of these stops is the more and South Coast railway, in England, in 1878, and by a committee of the Master Car Builders’ evident when they are compared with the best results Association, near Burlington, Iowa, in 1886 and 1887. obtained in 1886, as shown in Table II.