Brake Tests

197 402. As an example of the allowance that should be made for brak- ing power, suppose that the curves of Fig. 115 are to be used in compar- ing the best performance of the two types of rigging tested, using plain brake shoes. It would be natural to assume that the distance between the curves for the best stops of the No. 2 and No. 3 clasp brakes at any given per cent, braking power would indicate the difference in stopping distance which resulted from the different characteristics of the two types of brake rigging. Thus, the difference in the best per- formance of the two types of rigging at 150 per cent, braking power might be taken from Fig. 115 to be 965 feet—875 feet or 90 feet. But this is not a fair comparison because when using the same brake pipe pressure (110 pounds) the same per cent, of braking power was not obtained on the two cars. Points “A” and “B” indicate the per cent, braking powers based on brake cylinder pressure actually obtained and the corresponding lengths of the best stops made. For test 0-99 with the No. 2 clasp brake and test 1557 for the No. 3 clasp brake, the difference in the lengths of these stops, no allowance being made, is 1,007 feet—873 feet or 134 feet. Evidently this difference is due not only to the different performance of the brake rigging but to a difference in the percentage of braking power obtained. For a fair comparison of the two types of rigging, proper allowance must be made for all inequalities in the two cases, except those strictly chargeable to the action of the brake rigging. The brake pipe pressure was the same for these two stops and the brake cylinder cards show that the air brake mechanism operated identically for the two cases. The brake rigging is, therefore, chargeable with any loss in cylinder pressure from a given brake pipe pressure if this loss is due to greater piston travel caused by the rigging design. 403. A fair comparison, however, would require that the nominal per cent, braking power per pound of brake cylinder pressure be the same in the two cases. This was not the case in the tests referred to, owing to a slight variation in the total weights of the cars when used, which were not allowed for in the design and application of the rigging. With the No. 2 clasp brake one pound of brake cylinder pressure was equivalent to 1.43 per cent, braking power; whereas, with the No. 3 rigging one pound of brake cylinder pressure was equivalent to 1.505 per cent, braking power. For any given stop obtained with the No. 2 clasp brake it is, therefore, proper to compensate for its relatively lower braking power by assuming that the distance of stop should be shortened in accordance with an increase in percentage of braking power in the ratio of 1.505 to 1.43, but not directly in this proportion because, as the curves of Fig. 115 indicate, the length of stop does not decrease in direct proportion to the increase in percentage of braking power, but at a much less rate than this. 404. Taking the example mentioned above, the stop indicated by point “A” was made in 1,007 feet with 140 per cent, braking power.