Brake Tests

226 427. The fact that the shock disappears entirely when the time element in the application of the successive brakes in the train is elimi- nated was shown by repeating the ten m.p.h. stop using the electro-pneu- matic equipment. Notwithstanding that the stop was made in a shorter distance than before (37 feet instead of 42 feet and 45 feet) the dif- ference in the action of the train was marked. There was no shock or violent slack action although a very high rate of retardation was produced. 428. As the only difference in the two cases was in the brake appli- cation being simultaneous on all cars with the electro-pneumatic and serial with the pneumatic equipment, it was clear that the per cent, braking power, by itself, could not possibly cause shocks due to the suddenness or the magnitude of the retardation produced by the brake shoes. The shocks disappeared with the elimination of the serial appli- cation of the brakes and the prevention of sudden changes in the amount of slack movement between the cars. CHARACTERISTIC SPEED, RESISTANCE, DECELERATION AND POWER CURVES. 429. During any stop the retardation at different instants is dependent upon the resultant normal pressure of the brake shoes on the wheels, the instantaneous value of the coefficient of brake shoe friction, and the effect of the air and internal resistances. The difference between the effects of the rotative energy of the wheels and the opposing air and internal resistances is relatively so small, compared with that due to the action of the brakes, that it does not here require consideration. The brake resistances increase as the brake cylinder pressure increases, during the time the brakes are being applied. After the maximum brake cylinder pressure is reached with the UC equipment, no further change in brake cylinder pressure takes place. Consequently, the resultant normal pressure is substantially constant from that point to the end of the stop. If the coefficient of brake shoe friction was constant throughout the stop, the resultant resistance and retardation would then be constant, and the speed-distance curve would be a true parabola, while the speed-time curve would be a straight line. It will be of interest to check these conclusions with the results obtained in several typical tests. 430. In order to study the relations of the factors above mentioned to the best advantage typical single car breakaway stops, and stops with the locomotive alone have been chosen. This eliminates the disturbing influences of slack action (which produce apparent changes in retar- dation which are not characteristic of the train as a whole) variations in the retardation on the different vehicles comprising the train (which would require the averaging of all data for all cars in order to arrive