Brake Tests

69 TESTS MADE AND RESULTS. FULL SERVICE Brake Application. Standard PM Brake EQUIPMENT. 153. The characteristic action of the standard PM equipment during a service application of the brakes, is illustrated by the curves (Fig. 41). These curves are plotted to show the brake cylinder pressure and corresponding percentage of braking power on the train as a whole, developed during the time required to make a full service brake pipe reduction. 154. The cars at the head end of the train begin to apply and reach their maximum pressure before those at the rear of the train, which is true of any form of pneumatically controlled brake. The time of commencing to apply on different cars varies through a range of about four seconds, which is an indication of the relatively slow serial re- sponse of the brake mechanism to a gradual fall in brake pipe pressure controlled by pneumatic means alone. From these cards and similar cards for the UC pneumatic equipment (Fig. 42), it is easily seen that there is a considerable time element involved in starting the service application of all the brakes in the train when operating pneumatically. It follows from this that the rate of “build-up” of brake cylinder pres- sure in a service application must be relatively slow in order to avoid shocks which result from the brakes applying with a slow serial action combined with too rapid " build-up " of pressure on the individual cars. 155. The total time required to reach maximum full service brake cyl- inder pressure is nearly 12 seconds which shows clearly the effects of the long train (large brake pipe volume) in extending the time required to make a pneumatic full service brake application beyond a minimum which is fixed by the design of the equalizing discharge feature of the brake valve which requires that about six seconds at least be occupied in making a full service brake pipe reduction. 156. The rate of rise of brake cylinder pressure is more rapid than it would otherwise be, however, because of the use of the larger size auxiliary reservoirs with the PM equipments (Par. 80). For a given brake pipe reduction this results in a higher brake cylinder pressure than is obtained with a smaller size reservoir as used with the UC pneumatic equipment for the purpose of insuring flexibility of service operation of the brakes. 157. The maximum full service brake cylinder pressure is a trifle over 60 pounds, which is equivalent to a nominal braking power of 80 per cent. The variations in the setting and the individual action of the different high-speed reducing valves is the cause of the varying degrees of maximum braking power obtained on the different cars.