RAILWAY BRAKES.
The Vacuum Automatic brake--The Westinghouse air-brake.
In the early days of the railway, the pulling up of a train necessitated
the shutting off of steam while the stopping-place was still a great
distance away. The train gradually lost its velocity, the process being
hastened to a comparatively small degree by the screw-down brakes on the
engine and guard's van. The goods train of to-day in many cases still
observes this practice, long obsolete in passenger traffic.
An advance was made when a chain, running along the entire length of the
train, was arranged so as to pull on subsidiary chains branching off
under each carriage and operating levers connected with brake blocks
pressing on every pair of wheels. The guard strained the main chain by
means of a wheel gear in his van. This system was, however, radically
defective, since, if any one branch chain was shorter than the rest, it
alone would get the strain. Furthermore, it is obvious that the snapping
of the main chain would render the whole arrangement powerless.
Accordingly, brakes operated by steam were tried. Under every carriage
was placed a cylinder, in connection with a main steam-pipe running
under the train. When the engineer wished to apply the brakes, he turned
high-pressure steam into the train pipe, and the steam, passing into the
brake cylinders, drove out in each a piston operating the brake gear.
Unfortunately, the steam, during its passage along the pipe, was
condensed, and in cold weather failed to reach the rear carriages. Water
formed in the pipes, and this was liable to freeze. If the train parted
accidentally, the apparatus of course broke down.
Hydraulic brakes have been tried; but these are open to several
objections; and railway engineers now make use of air-pressure as the
most suitable form of power. Whatever air system be adopted, experience
has shown that three features are essential:--(1.) The brakes must be
kept "off" artificially. (2.) In case of the train parting accidentally,
the brakes must be applied automatically, and quickly bring all the
vehicles of the train to a standstill. (3.) It must be possible to apply
the brakes with greater or less force, according to the needs of the
case.
At the present day one or other of two systems is used on practically
all automatically-braked cars and coaches. These are known as--(1) The
_vacuum automatic_, using the pressure of the atmosphere on a piston
from the other side of which air has been mechanically exhausted; and
(2) the _Westinghouse automatic_, using compressed air. The action of
these brakes will now be explained as simply as possible.
THE VACUUM AUTOMATIC BRAKE.
Under each carriage is a vacuum chamber (Fig. 85) riding on trunnions, E
E, so that it may swing a little when the brakes are applied. Inside the
chamber is a cylinder, the piston of which is rendered air-tight by a
rubber ring rolling between it and the cylinder walls. The piston rod
works through an air-tight stuffing-box in the bottom of the casing, and
when it rises operates the brake rods. It is obvious that if air is
exhausted from both sides of the piston at once, the piston will sink by
reason of its own weight and that of its attachments. If air is now
admitted below the piston, the latter will be pushed upwards with a
maximum pressure of 15 lbs. to the square inch. The ball-valve ensures
that while air can be sucked from _both_ sides of the piston, it can be
admitted to the lower side only.
[Illustration: FIG. 85.--Vacuum brake "off."]
[Illustration: FIG. 86.--Vacuum brake "on."]
Let us imagine that a train has been standing in a siding, and that air
has gradually filled the vacuum chamber by leakage. The engine is
coupled on, and the driver at once turns on the steam ejector,[21]
which sucks all the air out of the pipes and chambers throughout the
train. The air is sucked directly from the under side of the piston
through pipe D; and from the space A A and the cylinder (open at the
top) through the channel C, lifting the ball, which, as soon as
exhaustion is complete, or when the pressure on both sides of the piston
is equal, falls back on its seat. On air being admitted to the train
pipe, it rushes through D and into the space B (Fig. 86) below the
piston, but is unable to pass the ball, so that a strong upward pressure
is exerted on the piston, and the brakes go on. To throw them off, the
space below the piston must be exhausted. This is to be noted: If there
is a leak, as in the case of the train parting, _the brakes go on at
once_, since the vacuum below the piston is automatically broken.
[Illustration: FIG. 87.--Guard's valve for applying the Vacuum brake.]
For ordinary stops the vacuum is only partially broken--that is, an
air-pressure of but from 5 to 10 lbs. per square inch is admitted. For
emergency stops full atmospheric pressure is used. In this case it is
advisable that air should enter at _both_ ends of the train; so in the
guard's van there is installed an ingenious automatic valve, which can
at any time be opened by the guard pressing down a lever, but which
opens of itself when the train-pipe vacuum is rapidly destroyed. Fig. 87
shows this device in section. Seated on the top of an upright pipe is a
valve, _A_, connected by a bolt, B, to an elastic diaphragm, C, sealing
the bottom of the chamber D. The bolt B has a very small hole bored
through it from end to end. When the vacuum is broken slowly, the
pressure falls in D as fast as in the pipe; but a sudden inrush of air
causes the valve A to be pulled off its seat by the diaphragm C, as the
vacuum in D has not been broken to any appreciable extent. Air then
rushes into the train pipe through the valve. It is thus evident that
the driver controls this valve as effectively as if it were on the
engine. These "emergency" valves are sometimes fitted to every vehicle
of a train.
When a carriage is slipped, taps on each side of the coupling joint of
the train pipe are turned off by the guard in the "slip;" and when he
wishes to stop he merely depresses the lever E, gradually opening the
valve. Under the van is an auxiliary vacuum chamber, from which the air
is exhausted by the train pipe. If the guard, after the slip has parted
from the train, finds that he has applied his brakes too hard, he can
put this chamber into communication with the brake cylinder, and restore
the vacuum sufficiently to pull the brakes off again.
When a train has come to rest, the brakes must be sucked off by the
ejector. Until this has been done the train cannot be moved, so that it
is impossible for it to leave the station unprepared to make a sudden
stop if necessary.
THE WESTINGHOUSE AIR-BRAKE.
This system is somewhat more complicated than the vacuum, though equally
reliable and powerful. Owing to the complexity of certain parts, such as
the steam air-pump and the triple-valve, it is impossible to explain the
system in detail; we therefore have recourse to simple diagrammatic
sketches, which will help to make clear the general principles employed.
The air-brake, as first evolved by Mr. George Westinghouse, was a very
simple affair--an air-pump and reservoir on the engine; a long pipe
running along the train; and a cylinder under every vehicle to work the
brakes. To stop the train, the high-pressure air collected in the
reservoir was turned into the train pipe to force out the pistons in the
coach cylinders, connected to it by short branch pipes. One defect of
this "straight" system was that the brakes at the rear of a long train
did not come into action until a considerable time after the driver
turned on the air; and since, when danger is imminent, a very few
seconds are of great importance, this slowness of operation was a
serious fault. Also, it was found that the brakes on coaches near the
engine went on long before those more distant, so that during a quick
stop there was a danger of the forward coaches being bumped by those
behind. It goes without saying that any coaches which might break loose
were uncontrollable. Mr. Westinghouse therefore patented his _automatic_
brake, now so largely used all over the world. The brake ensures
practically instantaneous and simultaneous action on all the vehicles of
_a train of any length_.
[Illustration: FIG. 88.--Diagrammatic sketch of the details of the
Westinghouse air-brake. Brake "off."]
The principle of the brake will be gathered from Figs. 88 and 89. P is a
steam-driven air-pump on the engine, which compresses air into a
reservoir, A, situated below the engine or tender, and maintains a
pressure of from 80 to 90 lbs. per square inch. A three-way cock, C,
puts the train pipe into communication with A or the open air at the
wish of the driver. Under each coach is a triple-valve, T, an auxiliary
reservoir, B, and a brake cylinder, D. The triple-valve is the most
noteworthy feature of the whole system. The reader must remember that
the valve shown in the section is _only diagrammatic_.
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.
[Illustration: 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.
[Illustration: 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.