Brake Tests

147 317. For full service applications the UC equipment was ar- ranged to give 90 per cent, braking power based on 60 pounds brake cylinder pressure. While this is 10 per cent, higher than the maximum service braking power which is standard with PM equipment, the smaller size of reservoirs used with the UC equipment in service appli- cations, gives a slower rate of build-up of braking power than with the PM equipment on P-70 cars, so that a greater flexibility in service operation is thereby secured, but at the same time a higher maximum of service braking power is available. 318. For emergency applications the UC equipment, both pneu- matic and electro-pneumatic, was used with braking powers of 90 per cent., 125 per cent, and 180 per cent, in addition to the normal emer- gency braking power of 150 per cent. The object in making tests throughout such a wide range of emergency braking powers was to obtain data from which conclusions might be drawn with respect to the effect of different percentages of braking power upon the length of the stop from different initial speeds and upon the tendency to slide wheels. 319. For the purposes of the tests the different braking powers were obtained either by a change in the brake cylinder levers or by a change in the brake pipe pressure or both. The various cylinder levers used are referred to below as 80 per cent, and 90 per cent, levers. The 80 per cent, levers are used with the present arrangement of PM equip- ment. The 90 per cent, levers were required when using the UC equip- ment at its standard percentage of braking power. 320. To obtain the various percentages of emergency braking power different from the normal, for the various equipments used, the brake pipe pressure was changed as required. The table on page 148 shows the various brake pipe pressures corresponding to the different percentages of emergency braking power used in the tests. 321. For the emergency tests, made at 80 per cent, braking power (PM equipment) special safety valves were used on the brake cylinders, and for those at 90 per cent. (UC equipment) limiting valves were placed between the universal valve and the brake cylinder. The object of these devices was to limit the brake cylinder pressure to that re- quired for the low percentage of braking power desired, in as con- venient and simple a manner as possible. 322. The percentage braking power on the locomotive was changed slightly by the change in the brake pipe pressure, made to obtain the different percentages of braking power used on the cars as mentioned above. But as this change was slight, the locomotive braking power was considered substantially the same throughout all tests, except when using the by-pass valve (Par. 147), or when a change in tender brake rigging was made (Par. 301).