Now for the operation of the brake. When the engine is coupled to the train, the compressed air in the main reservoir is turned into the train pipe, from which it passes through the triple-valve into the auxiliary reservoir, and fills it till it has a pressure of, say, 80 lbs. per square inch. Until the brakes are required, the pressure in the train pipe must be maintained. If accidentally, or purposely (by turning the cock C to the position shown in Fig. 89), the train-pipe pressure is reduced, the triple-valve at once shifts, putting B in connection with the brake cylinder D, and cutting off the connection between D and the air, and the brakes go on. To get them off, the pressure in the train pipe must be made equal to that in B, when the valve will assume its original position, allowing the air in D to escape.
The force with which the brake is applied depends upon the reduction of pressure in the train pipe. A slight reduction would admit air very slowly from B to D, whereas a full escape from the train pipe would open the valve to its utmost. We have not represented the means whereby the valve is rendered sensitive to these changes, for the reason given above.
Fig. 89.—Brake "on."
The latest form of triple-valve includes a device which, when air is rapidly discharged from the train pipe, as in an emergency application of the brake, opens a port through which compressed air is also admitted from the train pipe directly into D. It will easily be understood that a double advantage is hereby gained—first, in utilizing a considerable portion of the air in the train pipe to increase the available brake force in cases of emergency; and, secondly, in producing a quick reduction of pressure in the whole length of the pipe, which accelerates the action of the brakes with extraordinary rapidity.
It may be added that this secondary communication is kept open only until the pressure in D is equal to that in the train pipe. Then it is cut off, to prevent a return of air from B to the pipe.
An interesting detail of the system is the automatic regulation of air-pressure in the main reservoir by the air-pump governor (Fig. 90). The governor is attached to the steam-pipe leading from the locomotive boiler to the air-pump. Steam from the boiler, entering at F, flows through valve 14 and passes by D into the pump, which is thus brought into operation, and continues to work until the pressure in the main reservoir, acting on the under side of the diaphragm 9, exceeds the tension to which the regulating spring 7 is set. Any excess of pressure forces the diaphragm upwards, lifting valve 11, and allowing compressed air from the main reservoir to flow into the chamber C. The air-pressure forces piston 12 downwards and closes steam-valve 14, thus cutting off the supply of steam to the pump. As soon as the pressure in the reservoir is reduced (by leakage or use) below the normal, spring 7 returns diaphragm 9 to the position shown in Fig. 90, and pin-valve 11 closes. The compressed air previously admitted to the chamber C escapes through the small port a to the atmosphere. The steam, acting on the lower surface of valve 14, lifts it and its piston to the position shown, and again flows to the pump, which works until the required air-pressure is again obtained in the reservoir.
Fig. 90.—Air-pump of Westinghouse brake.
[21] This resembles the upper part of the rudimentary water injector shown in Fig. 15. The reader need only imagine pipe B to be connected with the train pipe. A rush of steam through pipe A creates a partial vacuum in the cone E, causing air from the train pipe to rush into it and be expelled by the steam blast.