342
BRAKES
atmosphere. Attached to it is a reducing valve, which, in the normal or running position of the former, reduces the pressure of the air flowing from the main reservoir to the train-pipe to 70 K) per square inch, or 20 lb per square inch less than the pressure in the main reservoir. A train-pipe runs the whole length of the train. In addition to the brake-cylinder each vehicle is supplied with an air reservoir, called the auxiliary reservoir, and an automatic valve called a triple valve. The auxiliary reservoir receives air from the train-pipe and stores it for use in the brake-cylinder of its own vehicle, and both the auxiliary reservoir and triple valve are connected directly or indirectly with the train-pipe. The general arrange-
reservoir about 20 per cent. This causes practically instantaneous action of the brakes to their highest efficiency throughout the entire train. The same effect is produced should the train break in two or a hose or any part of the train-pipe burst. Sometimes the passengers are provided with the means of thus arresting the train on an emergency, a cord running along the inside or outside of the coaches, which on being pulled opens a valve connected to the train-pipe; this arrangement is applicable both to the vacuum and compressed - air brakes. When the pressure in the brake-pipe is again restored to an amount in excess of that remaining in the auxiliary reservoir, the piston and slide valve are forced in the opposite direction ♦to their normal position, communication is opened from the train-pipe to the auxiliary reservoir, and the air in the brakecylinder escapes to the atmosphere, thus releasing GUARDS VALVE the brakes. The quick - action triple valve is shown in section in Fig. 4. It consists essentially of two mechanisms: first, a piston and slide valve, 5 and 3, which, properly arranged, constitute the automatic feature ; and second, the piston, 8, puppet valve, 10, and check valve, 15, by means of which quick action is obtained. The latter feature is only essential -when stops are to be made in the shortest distance possible, and therefore this part of the mechanism does not come into action during ordinary or “service” stops. The train-pipe is connected at 18, admitting compressed air to chamber 13, thence to chamber 19, underneath piston 5, which is forced to the position shown in the 1 diagram, there admitting air taO= through feed ports I and k, above the slide valve 3 to the auxiliary reservoir at B, in which pressure soon equals that in the train-pipe and the flow stops. In this position port G, which communicates with a conduit leading to the brake - cylinder, is in Fig. 3.—Diagram of the principal parts of an Air-Brake on one car. communication with the atmosphere through ports N ment of the brake apparatus on a passenger coach is shown and p and the brakes are released. If a slight or gradual reducin Fig. 3. The brake-cylinder is in section disclosing the tion of pressure is made in the train-pipe, piston 5 moves to the left until it is arrested by graduating stem 21. Its first piston and release spring. movement opens graduating valve 7 to port z. When in the The automatic action of the brake is due to the con- position against graduating stem 21 communication between ports struction of the triple valve, the principal parts of which G and p is cut off, and port z is open to g, causing a flow are a piston and slide valve so arranged that the air in the of air from the auxiliary reservoir to the brake - cylinder, which until the pressure on the reservoir side of piston 5 is auxiliary reservoir acts at all times on the side of the continues slightly less than that on the train-pipe side, when the piston 5 is piston to which the slide valve is attached, while the air moved far enough to the right to close port z, by graduating valve 7. in the train-pipe exerts its pressure on the opposite side. The resistance of the slide valve 3 is too great to be overcome by A moderate reduction of air pressure in the train-pipe this slight diflerence in pressure, so its position is unchanged and brakes are held applied. A further reduction in the train-pipe causes the greater pressure remaining stored in the the pressure will admit more air through port z, by moving the piston auxiliary reservoir to force the piston of the triple valve again to the left, increasing the force of the application, which, if with, its slide valve to a position which will allow the air allowed to continue, will cause an equalization of pressure between in the auxiliary reservoir to pass directly into the brake- the auxiliary reservoir and the brake-cylinder when the brakes are fully applied. To release, the full pressure in the train-pipe and cylinder and apply the brake. A sudden or violent re- consequently underneath piston 5 is restored, which restores duction of the air in the train-pipe produces the same all parts to position shown in the diagram, permitting the air in effect, and in addition causes supplemental valves in the the brake-cylinder to escape to the atmosphere through ports r, N, triple valve to be opened, permitting the pressure and p. When the shortest stop is desired, a quick and considerable reduction in pressure is made in the train-pipe. The result is in the train-pipe also to enter the brake-cylinder, that the piston 5 moves its full traverse, compresses spring 22, augmenting the pressure derived from the auxiliary bringing port s in register with port r, and allowing the auxiliary
