When used in ordinary service stops it must be capable of gradual and uniform application (followed, if necessary, by a full emergency application at any part of the service application) and of prompt release under all conditions of application. (8) It must be simple in operation and construction, not liable to derangement, and inexpensive in maintenance.
The Westinghouse non-automatic or “straight” air-brake,
patented in 1869, consists in its simplest form of a direct-acting,
steam-driven air-pump, carried on the locomotive, which
forces compressed air into a reservoir, usually placed
under the foot-plate of the locomotive. From this reservoir
Simple
air-brake.
a pipe is led through the engine cab, where it is fitted with a
three-way cock, to the rear of the locomotive tender, where it terminates
in a flexible hose, on the end of which is a coupling. The
coaches are furnished with a similar pipe, having hose and coupling
at each end, which communicates with one end of a cylinder containing
a piston, to the rod of which the brake-rods and levers are
connected. The application of the brakes is effected by the engine-driver
turning the three-way cock, so that compressed air flows
through the pipe and, acting against one side of the brake-cylinder
piston, applies the brake-shoes to the wheels by the movement of
this piston and the rods and levers connected to it. To release the
brakes the three-way cock is turned to cut off communication
between the main reservoir and the train-pipe, and to open a port
permitting the escape of the compressed air in the train-pipe and
brake-cylinders. This brake was soon found defective and inadequate
in many ways. An appreciable time was required for the air
to flow through the pipes from the locomotive to the car-cylinders,
and this time increased quickly with the length of the trains. Still
more objectionable, however, was the fact that on detached coaches
the air-brakes could not be applied, the result being sometimes
serious collisions between the front and rear portions of the train.
 |
| Fig. 1.—Westinghouse Air-Brake.
Section through Triple-Valve and Brake-Cylinder. |
In the Westinghouse “ordinary” automatic air-brake a main air reservoir on the engine is kept charged with compressed air at 80 ℔ per sq. in. by means of the steam-pump, which may be controlled by an automatic governor. On electric railways a pump, driven by an electric motor, is generally Automatic air-brake. employed; but occasionally, on trains which run short distances, no pump is carried, the main reservoir being charged at the terminal points with sufficient compressed air for the journey. Conveniently placed to the driver’s hand is the driver’s valve, by means of which he controls the flow of air from the main reservoir to the train-pipe, or from the train-pipe to the atmosphere. A reducing-valve is attached to the driver’s valve, and in the normal or running position of the latter reduces the pressure of the air flowing from the main reservoir to the train-pipe by 10 or 15 ℔ per sq. in. From the engine a train-pipe runs the whole length of the train, being rendered continuous between each vehicle and between the engine and the rest of the train by flexible hose couplings. Each vehicle is provided with a brake-cylinder H (fig. 1), containing a piston, the movement of which applies the brake blocks to the wheels, an “auxiliary air-reservoir” G, and an automatic “triple-valve” F. The auxiliary reservoir receives compressed air from the train-pipe and stores it for use in the brake-cylinder of its own vehicle, and both the auxiliary reservoir and the triple-valve are connected directly or indirectly with the train-pipe through the pipe E. The automatic action of the brake is due to the construction of the triple-valve, the principal parts of which are a piston and slide-valve, so arranged that the air in the auxiliary reservoir acts at all times on the side of the piston to which the slide-valve is attached, while the air in the train-pipe exerts its pressure on the opposite side. So long as the brakes are not in operation, the pressures in the train-pipe, triple-valve and auxiliary reservoir are all equal, and there is no compressed air in the brake-cylinder. But when, in order to apply the brake, the driver discharges air from the train-pipe, this equilibrium is destroyed, and the greater pressure in the auxiliary reservoir forces the triple-valve to a position which allows air from the auxiliary reservoir to pass directly into the brake-cylinder. This air forces out the piston of the brake-cylinder and applies the brakes, connexion being made with the brake-rigging at R. The purpose of the small groove n which establishes communication between the two sides of the piston when the brakes are off, is to prevent their unintended application through slight leakage from the train-pipe. To release the brakes, the driver, by moving the handle of his valve to the release position, admits air from the main reservoir to the train-pipe, the pressure in which thus becomes greater than that in the auxiliary reservoir; the piston and slide-valve of the triple-valve are thereby forced back to their normal position, the compressed air in the brake-cylinder is discharged, and the piston is brought back by the coiled spring, thus releasing the brakes. At the same time the auxiliary reservoir is recharged.
With this “ordinary” brake, since an appreciable time is required for the reduction of pressure to travel along the train-pipe from the engine, the brakes are applied sensibly sooner at the front than at the end of the train, and with long trains this difference in the time of application becomes a matter of Quick-acting air-brake. importance. The “quick-acting” brake was introduced to remedy this defect. For it the triple valve is provided with a supplementary mechanism, which, when the air pressure in the train-pipe is suddenly or violently reduced, opens a passage whereby air from the train-pipe is permitted to enter the brake-cylinder directly. The result is twofold: not only is the pressure from the auxiliary reservoir acting in the brake-cylinder reinforced by the pressure in the train-pipe, but the pressure in the train-pipe is reduced locally in every vehicle in extremely rapid succession instead of at the engine only, and in consequence all the brakes are applied almost simultaneously throughout the train. The same effect is produced should the train break in two, or a hose or any part of the train-pipe burst; but during ordinary or “service” stops the triple-valve acts exactly as in the ordinary brake, the quick-acting portion, that is, the vertical piston and valve seen in fig. 1, not coming into operation. When the handle Z is turned to the position X the quick-acting mechanism is rendered inoperative, and when it is at Y the brake on the vehicle concerned is wholly cut out of action.
A further improvement introduced in the Westinghouse brake in 1906 was designed to give quick action for service as well as emergency stops. In this the triple-valve is substantially the same as in the ordinary brake. The additional mechanism of the quick-acting portion is dispensed with, but instead, a small chamber, normally containing air at atmospheric pressure, is provided on each vehicle, and is so arranged that it is put into communication with the train-pipe by the first movement of the triple-valve. As soon, therefore, as the driver, by lowering the pressure in the train-pipe, causes the triple-valve in the foremost vehicle of the train to operate, a certain quantity of air rushes out of the train-pipe into the small chamber; a further local reduction in the pressure of the train-pipe in that vehicle is thereby effected, and this almost instantaneously actuates the triple-valve of the succeeding vehicle, and so on throughout the train. In this way, on a train 1800 ft. long, consisting of sixty 30-ft. vehicles, the brake-blocks may be applied, with equal force, on the last vehicle about 212 seconds later than on the first.
Brake-blocks can be applied, without skidding the wheels, with greater pressure at high speeds than at low. Advantage is taken of this fact in the design of the Westinghouse “high-speed” brake, invented in 1894, which consists of attachments enabling the pressure in the train-pipe and High-speed air-brake. reservoirs to be increased at the will of the driver. The increased pressure acting in the brake-cylinder increases in the same proportion the pressure of the brake-shoes against the wheels. Attached to the brake cylinder is a valve for automatically reducing
