Brake Tests

62 122. The effect of the lower equalization pressure of the UC equip- ment in connection with a higher per cent, braking power per pound of cylinder pressure than for the PM equipment, is to provide a greater flexibility for service brake operation. For example: a 24-pound brake pipe reduction is required to obtain a maximum (90 per cent.) service braking power with the UC equipment, whereas, with the PM equip- ment the maximum service braking power (80 per cent.) is obtained with the brake pipe reduction of approximately 19 pounds. 123. To obtain the same per cent, of braking power with each equipment requires a greater reduction with the UC equipment than with the PM equipment, 75 per cent, braking power requiring a brake pipe reduction of about 173 pounds with the PM equipment and 20 pounds with the UC equipment. Thus, with the smaller reservoirs, a longer time is available in which the engineman can exercise his judgment as to what rate retardation is being obtained and how best to control the speed of the train. 124. The valve mechanism is designed to require a drop in brake pipe pressure of approximately four pounds before it is possible to obtain an application of the brakes. The equalizing piston moves on a dif- ferential much lower than this, however, so as to close the feed groove and thus prevent back leakage from the auxiliary reservoir. Thus a service application of the brakes is positively insured when the required four pounds brake pipe reduction is reached. From this point the rise in brake cylinder préssure corresponds to the reduction in brake pipe pressure in the proper relation to produce a full service brake application (90 per cent, braking power) for a brake pipe reduction of 24 pounds. 125. The rate of build-up of brake cylinder pressure during a service application is controlled in the same manner as already explained in connection with the PM equipment. (Par. 81.) 126. The maximum brake cylinder pressure obtainable in a service application is limited by the setting of a quick blow-down and positive acting safety valve which is connected to the brake cylinder through the emergency portion of the universal valve at all times except when an emergency application of the brakes is made. When making an emergency application, the safety valve is automatically cut off from communication with the rest of the equipment. This safety valve is adjusted to limit the maximum obtainable service brake cylinder pres- sure to 60 pounds per square inch. Releasing and Recharging. 127. When the brake pipe pressure is increased above that of the auxiliary reservoir after an application, the universal valve operates so as to:—