Brake Tests

39 of what is desirable and obtainable, when due attention is given the elements entering into the design, the curve shown in Fig. 24 has been drawn to express the characteristic relation between brake cylinder pressure and piston travel for an ideal brake installation. The perform- ance of existing types of brake rigging will approach more or less closely to that illustrated in Fig. 24 according to the degree in which sources of loss resulting in so-called “false” piston travel are eliminated. 80. Assuming the piston travel to vary with the brake cylinder pressure, as shown in Fig. 24, the characteristic relation between brake pipe reduction and resulting brake cylinder pressure, when using the PM equipment with the size of auxiliary reservoirs standard on our P-70 cars, is shown by the dotted line Fig. 25. This curve is to be under- stood as characteristic and not necessarily exactly representative of the performance of any particular car brake installation in service. The effect of leakage, slight variations in piston travel, reservoir volume or piping and the effect of temperature changes, during and immediately after the brake pipe reduction is made, can vary the resulting cylinder pressure two or three pounds at least from that indicated in Fig. 25. The curve, therefore, is to be considered characteristic and true for the particular conditions assumed. For comparison, a scale of per cent, braking power is also shown on Fig. 25, based on the same relation between cylinder pressure and percentage of braking power, as in the case of the P-70 car, namely 80 per cent, braking power with 60 pounds brake cylinder pressure. A comparison of the dotted curve (Fig. 25), with the full line in the same diagram (for smaller size auxiliary reser- voirs as explained, Par. 121), illustrates the effect of an auxiliary reservoir volume larger than required to give a brake cylinder pressure of 50 pounds with 8-inch piston travel when the brake pipe reduction of 20 pounds is made. With the larger size reservoir the brake cylinder pressure and consequently the percentage of braking power is built up at a more rapid rate as the brake pipe reduction progresses, althoügh the lower percentage of braking power (80 per cent, on 60 pounds instead of 90 per cent, on 60 pounds), partially compensates for the quicker rise in brake cylinder pressure. This fact should be borne in mind whenever a comparison is made between the service applications of the PM and the UC brake equipments. The difference in reservoir volumes used necessarily results in a faster rate of building up of brake cylinder pressure with the PM equipment and large reservoirs than for the UC equipment with smaller size reservoirs for the same rate of brake pipe reduction. The effect of this was to provide a high minimum braking force and a low degree of flexibility. 81. The rate of building up brake cylinder pressure in service applications with the PM equipment is dependent upon the rate of fall of brake pipe pressure. This is limited by:—