The Working of the Westinghouse Automatic Air-
CONTROL MECHANISM (in the driver’s cab) by means of which the Westinghouse brake is applied and released. The two indicators on the gauge register the pressure in the main and equalizing air reservoirs respectively.
THE earliest form of railway brake was an adaptation of the old horse-
Because it was found deficient in power for locomotive purposes, the lever was replaced by a screw and nut, usually arranged in a vertical position at the front of the tender on the fireman's side of the locomotive. A cross handle, with one end bent up crank fashion, was attached to the top of the screw shaft, so that it could be operated quickly, and yet allow plenty of pressure to be obtained at the brake blocks. The nut was connected by a rod to the brake rigging, and the turning of the screw pulled up the nut and applied the brakes.
This type of hand brake is still fitted to all locomotives, though it is not now used for service stops. It is useful in the rare instance of failure of the power brake, and can also be applied when the engine is left unattended. A similar arrangement was installed in the end carriages of each train in the old days (hence the term “brake van”) and was operated by the guard on hearing a whistle signal from the engine.
It was soon found that more powerful brakes were essential. The driver of an express train consisting of, say, a small “single-
The remedy for this was to apply the brakes by power. Steam was the only power available and small cylinders were installed under the engine and tender, the piston-
It was obvious that brake blocks should be applied to every wheel on the train. But it was difficult to find a means of doing this. Extension of the steam brake to the carriages was suggested, but was not practicable, as the steam would probably have condensed in the cold pipes before reaching the brake cylinder under each carriage.
Inventors prepared schemes for continuous braking. Systems operated by chains, liquids, and compressed air were tried out on the different railways. After many experiments, a series of trials was arranged at Newark in 1875, in which nine engines and trains took part.
Use of Compressed Air
George Westinghouse was quick to see that if steam would condense on entering cold pipes and cylinders, compressed air would not, and would operate the steam brake apparatus as well as steam itself.
His first brake equipment consisted of the usual steam brake-
This brake was a great improvement on the old hand and steam brakes, but it had three drawbacks. It took a long time to apply and release, even on a train of moderate length; it was “uneven” in action, as the engine and the carriages nearest to it had their brakes hard on before air had travelled to the last carriage; if a train broke in two, and tore the hose couplings of the train pipe apart, all the air was let out and the brake was useless.
To rectify these defects, George Westinghouse invented the “triple-
There is thus no pressure at all in the brake cylinder, and the release springs operate to keep the brake blocks clear of the wheels.
When the triple-
To apply the brake, the driver, by means of his valve, lets some of the air out of the train pipe, and so reduces the pressure on the underside of the piston in each triple-
The latter first shuts the port communicating with the atmosphere, and then opens the port leading to the brake cylinder, allowing air to flow from the auxiliary reservoir into the brake cylinder, and applying the brakes.
A small additional valve, called a graduating valve, is also connected to the piston in such a way that it closes the brake cylinder port while the piston is stationary. This enables the driver to control the braking power by regulating the air pressure in the train pipe. When the driver wishes to release the brakes after application he admits air, by means of his control valve, to the train pipe from the main reservoir on the engine. This pushes up the triple-
WESTINGHOUSE BRAKE EQUIPMENT, as fitted to a locomotive. On an electric train the steam-
All the time the train is in service the pressure in the train pipe must be maintained to keep the brakes in “release” position; and it will thus be seen that, should a train break in two, and the train pipe hose couplings be torn apart, the sudden release of air from the train pipe would operate all the triple-
Another advantage is that the guard of the train has an emergency control, as a cock is provided in his van, by which he can release air from the train pipe, and stop the train in the event of emergency. In most trains the passengers' alarm signal chain is also connected to an air valve, so that any passenger who “pulls the communication cord” not only gives the alarm, but also begins the application of the brakes.
Trains still increased in length and weight Once again it was found that the brake was becoming too slow in action, all the air for operating it having to pass through the driver’s valve. The brake was thus applied with greater force at the front end of the train, to the discomfort of the passengers and the detriment of the couplings and draw-
There is yet another kind of triple valve which is the same in principle as the foregoing types, but has certain improvements that tend to produce simultaneous action of the whole of the brakes in the train, and is so arranged that the local reductions of pressure in the forward end of the train pipe produce a correspondingly quicker action of the triples at the rear end. Other improvements in the valves, ports and passageways tend to produce the smooth, swift action of the modern brake as used to-
TENDER AND COACH FITTINGS used for the Westinghouse air brake. The brake cylinder on the tender is similar to that on the locomotive. On guards' vans additional fittings are provided so that the guard can apply the brakes if necessary. The carriage brake-
There are several varieties of steam-
The “singles” have a steam cylinder at the top, coupled to a single, two, or three-
The pump in the single-
WESTINGHOUSE PUMP, as fitted to locomotives. The piston in the upper steam cylinder drives another piston in the lower cylinder to supply compressed air to the reservoirs.
Similarly, air is compressed to moderate pressure in the larger air cylinder, and from there it goes to the smaller one, where it is increased to full pressure and discharged into the main reservoir. These reservoirs, also the auxiliary reservoirs under the coaches, are merely cylindrical steel tanks with either riveted or welded seams, built to carry a working pressure of 90 lb per sq in. The actual brake-
ELECTRIC AIR COMPRESSORS are used for the Westinghouse brakes on the trains of the Southern Electrified system and on London’s Underground Railways. The valves of the air cylinders are seen at the right of the photograph.
Everybody is acquainted with the small cast-
THE INTERIOR of an electric compressor with cover removed. The motor is at the left of the picture and works two air pumps through gearing.
The Driver’s Valve
Among the interesting “gadgets” used in the working of the brakes are the compressor governor and the driver’s brake valve. The former is an ingenious device mounted on the steam-
The driver’s valve consists of a slotted and holed disk valve working over a port-
Passengers on some of the suburban lines are under the impression that the driver uses the fifth position every time he pulls up. When a train has two engines, the leading driver controls the brakes, the brake valve on the second engine being cut out of action by a cock on the air pipe, called an isolating cock. Isolating cocks are also used on the coaches to cut out defective triple valves and brake cylinders, so that the train may be worked with the rest of the apparatus in action. Duplex pressure gauges are fitted on the engine, indicating to the driver the pressure in main reservoir and train pipe; and single needle gauges, showing train pipe pressure, are provided in the guards' brake vans.
The Westinghouse brake, as fitted to modern electric trains, has much in common with that on steam trains, the principal difference being that the compressor is operated by an electric motor, and usually has two pump cylinders. The motor is geared to a shaft like an automobile engine’s crankshaft, and the connecting rods and pistons bear a similar resemblance, ordinary types of suction and discharge valves being used. The simplest type of governor is similar to a pressure gauge, the needle making a contact at its low-
A NEW ZEALAND LOCOMOTIVE with a brake air-
In common with every other invention of great utility, the Westinghouse brake stimulated the work of other inventors, who devised various alterations and more or less useful variations; and the principle of the complete apparatus has been made the basis of other forms of air brakes. These include the “New York” air brake, used in USA, one form of which employs a direct-
In USA and other countries where grades fifteen miles or more in length have to be negotiated with heavy trains, retaining valves are fitted, which enable the auxiliary reservoirs to be recharged without releasing the brakes.
In contrast to the Westinghouse brake, using air under pressure, another type is the automatic vacuum brake, which is used on steam trains. With this brake air is exhausted from one side of a piston in a cylinder, so that the air pressure of the other applies brake-
[From part 8, published 22 March 1935]
“The Vacuum Automatic Brake” on this website.