Brake Tests

148 BRAKE PIPE PRESSURE, Pounds. Nominal Per Cent. Braking Power on Cars 80 Per Cent. Levers 90 Per Cent. Levers Air Brake Equipment 80 _ 110 PM 90 110 UC 113..... _ 110 98 PM 125 92 UC 144 140 PM 150 125 110 UC 180 132 UC EFFECT OF VARIOUS PERCENTAGES OF BRAKING POWER. 323. The leading considerations in determining the braking force are, the flexibility of the brake and the smoothness of action in service applications, and the length of stop and tendency to slide wheels in emergency applications. LENGTH of Stop. 324. The effect of various percentages of emergency braking power on the length of stop can best be studied by a consideration of the curves shown in Fig. 84. These curves are plotted to show the relation between the percentage of braking power and the length of stop from 60 m.p.h. for single car breakaway stops with the No. 3 clasp brake, electro-pneumatic air brake equipment, plain brake shoes well worn in and cracked. Throughout the range of braking power used the conditions were substantially the same. In this way many dis- turbing elements are eliminated which would have more or less effect when making a train stop such as locomotive effect, time element in applying the brakes on different cars and variation in the action of different units of the train. It thus becomes possible to determine how the length of stop is affected by different percentages of braking power, other factors being substantially constant. 325. Taking from these curves the stop at 90 per cent, as the basis, the stop at 125 per cent, is 17.5 per cent, shorter, that at 150 per cent, braking power 25.5 per cent, shorter and that at 180 per cent., 33 per cent, shorter. An interesting development from this is that for a given increase in braking power anywhere throughout the range, a constant decrease in length of stop will result, this constant decrease, however, not being equal to the corresponding increase in braking power. For example, an increase of 25 per cent, in braking power (from 90 per cent, up to 112.5 per cent, braking power) results in a decrease of about 12 per cent, in the length of step (from 1,177 feet to 1,033 feet).