Brake Tests

245 brake (single shoe) and clasp brake (double shoe). The conditions of the test in each case were planned to be as nearly as possible equivalent to those under which the standard and clasp brakes were tested on the road. 465. The wheel load provided on the test machine for the stand- ard brake conditions was 15 900 pounds. It would have been desirable to make this wheel load slightly less, say, 15 000 lb., in order to more nearly approximate the actual average wheel load of the test train, but owing to the fact that it was only possible to change the wheel load set- ting of the machine by certain predetermined steps, 15 900 pounds was the nearest possible approach to the actual road test figures. 466. The nearest obtainable wheel load to represent clasp brake conditions was 7220 pounds. 467. The nominal value of the braking power used throughout the tests was computed on the basis of the wheel load allowing 80 per cent, for brake rigging efficiency. Thus, the shoe load to be used in order to provide 90 per cent, braking power under clasp brake conditions was computed as follows: The product of wheel load, per cent, brak- ing power and rigging efficiency, or 7 220 X 0.90 X 0.80=5198 pounds. The object of introducing the rigging efficiency factor of 80 per cent, was to make the conditions of the machine tests approximate as nearly as possible those of the road tests, it being decided that 80 per cent, was a fair figure for the average value of the efficiency of the brake rigging. In order to make the data generally applicable the stops and the mean coefficient of friction were plotted against the actual per cent, braking power independent of any allowance for rigging efficiency. For ex- ample, all stops made at a nominal braking power of 90 per cent, have been plotted at 90 X 0.80 or 72 per cent.: likewise at 120 per cent, and 144 per cent, for the nominal braking powers of 150 per cent, and 180 per cent., respectively. 468. The shoe was held away from the wheel by air pressure, the releasing of this pressure applied the shoe to the wheel, the weight then acting through a system of levers (Figs. 146 and 148). These were arranged so as to make the time of application of the brake shoe to the wheel as nearly as possible equivalent to the rate of development of the braking power on the test train during an electro-pneumatic emergency application. Tlie time from the instant the shoe first began to touch the wheel to the instant that the full pressure came upon the shoe was approximately 2.5 seconds. 469. The method of starting and conducting the test was as fol- lows: The surface of the shoe was chalked to assist in the observation of the bearing area after the test. The wheel was accelerated to a speed slightly above that desired for the test and allowed to drift until its velocity had decreased to that desired for the test when the shoe was applied to the wheel.