Brake Tests

249 mean coefficient of friction from the data of speed and length of stop from the point of equivalent instantaneous application, as measured on the card, instead of detërmining the mean coefficient of friction by in- tegrating the entire indicator card. The mean coefficient of friction as recorded in the log was computed from the speed and this stop distance, using the following equation:— .0334 V2 1 P’S, where f = mean coefficient of friction V = initial speed in miles per hour, P1 = the braking power in per cent, used on the machine and is equal to the corresponding nominal per cent, braking power (P) for the train tests, multiplied by the assumed car rigging efficiency (e) and S2= distance from point of equivalent instantaneous application to the stop A < 1: 1 0.0334V2 in feet. The formula can also be written f= —o P eS2 481. For example let speed be 60 miles per hour and the total stopping distance 707 feet (S) for a nominal braking power of 150 per cent, and 80 per cent, car rigging efficiency. The distance from the trip to the point of equivalent instantaneous application (S1) is 66 feet, then S2 =S-St = 641 feet. Then the mean coefficient of friction f = 0.0334 (60)2 149 ------------ —2— — = 15.63 per cent. 1.50 X 0.80 X 641 P TEMPERATURES. 482. While testing the plain solid and slotted shoes, temperature measurements were made of the brake shoes throughout the tests and the temperature of both the wheel and the shoe, before the test, was noted. The initial temperature of the shoes was usually taken as that of the room, the shoes having plenty of time to cool back to room tem- perature between tests. In a few cases the shoes did not have time to cool and then the initial temperature was noted by means of an electric pyrometer or a mercury thermometer. The temperatures during the tests were observed by means of an electric pyrometer, the thermo- couple of which was inserted in a hole drilled from the back of the shoe diagonally through to the face. The elliptical shaped opening marked “For Pyrometer” (Figs. 151 to 154) illustrates the position of the thermo-couple in the shoe. The thermo-couple was adjusted so that it was close to or touching the wheel during the stop. The object being to obtain if possible the actual temperature of the working metal at the surface of the shoe. The diagram of the arrangement of the thermo- couple in the drilled opening through the shoe is shown in Fig. 150.