Brake Tests

73 163. The maximum full service brake cylinder pressure averages 60 pounds which is equivalent to a nominal braking power of 90 per cent. This maximum pressure is limited to approximately 60 pounds by the safety valve used with the equipment. 164. When proper allowance is made for the difference in reservoir volumes used with the UC and PM equipments, it will be seen that the rate cf development of brake cylinder pressure in service is sub- stantially the same in each case, the rate of brake pipe reduction being the controlling factor. 165. This shows that there can be no undesirable difference in the action of the PM and UC equipments when operating together in the same trains in ordinary service. The results of tests made with trains of mixed UC and PM equipment, for the purpose of bringing out this particular point, confirm this statement. (Par. 200.) Electro-Pneumatic Equipment. 166. The advantages of the electro-pneumatic control of the service brake are apparent from a comparison of Fig. 43 with Fig. 42. With the electro-pneumatic brake (Fig. 43), the application started almost simultaneously on all cars and built up to maximum brake cylinder pressure at a uniform rate. Furthermore, the rate of build-up of brake cylinder pressure is not dependent upon the length of train as it is in the case of any pneumatically controlled service application The brake pipe reduction is accomplished locally on each car by the operation of the service magnet valves. Consequently, it can be made as rapid as would be permissible on a single car operating pneumatically, the limit being that established by experience as the fastest rate at which a service brake application can be made, with assurance of convenience in manipulation, comfort to the passengers and an ample margin against quick action. 167. The maximum service brake cylinder pressure is obtained in eight seconds instead of sixteen seconds, required by the same brake equipment operating pneumatically. As would be expected this more prompt and uniform action of the brakes produces a shorter stop. 168. The important advantages of the electro-pneumatic control of the service brake application are clearly brought out by a study of the curves of Fig. 43. They show the almost absolutely uniform and simultaneous action of all the cars in the train. In fact, when making an electro-pneumatic service application of the brakes, the application on different cars in the train is much more uniform than in the case of a pneumatic emergency application. This diagram also shows the great advantage of eliminating the time element in starting the appli- cation and the ability to quicken the rate of brake application on the entire train, and at the same time retain the necessary flexibility which enables the engineman to readily control the brake application as desired.