CONTINUOUS TRAIN BRAKES.
QUESTION 441. _What are meant by automatic or continuous train brakes?_
_Answer._ Continuous train brakes are brakes which can be applied to
all the cars of a train by the locomotive runner on the locomotive. In
some cases such brakes are arranged in such a way that they can also be
applied from any car in the train, or are made self-acting in case of
an accident, such as a car getting off the track or a train breaking in
two.
QUESTION 442. _What are the principal systems of brakes of this kind in
use?_
_Answer._ What is called, after its inventor, the Westinghouse
atmospheric brake is now used more than any other. Next to this,
Smith’s vacuum brake is used most. Besides these two, Creamer’s,
Ward’s, Loughridge’s and Henderson’s systems of brakes are used to a
limited extent. The two first are, however, the only ones which have
come into sufficiently extensive use as yet to justify us in describing
them here.
QUESTION 443. _How does the Westinghouse brake act and how is it
constructed?_
_Answer._ As its name indicates, the medium employed for transmitting
the power for operating the brakes is atmospheric air.
[Illustration: Fig. 223. Scale ¹⁄₂ in. = 1 foot.]
This is compressed to any required density by a steam pump which is
located between the driving-wheels, or in any other convenient place on
the locomotive. This pump is shown in section in fig. 223 and consists
of two cylinders, the upper one, _A_, the steam cylinder, the piston of
which is connected by its rod with the piston in the lower cylinder,
_B_. This latter is operated by the steam piston, and at each stroke a
quantity of air, equal to the space swept through by the lower piston,
is compressed and thus forced into a cylindrical reservoir, which is
usually placed under the foot-board of the locomotive, in which it is
stored for use at any time when the brakes are to be applied. The air
and steam cylinders are supplied with suitable valves for admitting
and releasing the air and steam. From this reservoir it is conducted
back under the tender and cars by pipes, which are connected together
between the engine and tender and between the cars by India rubber
hose. Two pieces of hose are attached to the engine and also to each
end of the tender and cars, so that in case one piece should break
the others will act. Each of these pieces is united or coupled to the
corresponding piece opposite to it by a peculiar coupling made for the
purpose, so that they can be quickly disconnected if the cars, engine
or tender are uncoupled.
Under the tender and also under each car is a cylinder and piston. The
compressed air is conducted to this cylinder in front of the piston
when the brakes are to be applied. As the piston-rod is connected by
a bell crank to the brake levers when the piston is forced out by the
pressure of the air, the brakes are at once applied to the wheels. As
the reservoir under the foot-board is connected by the pipes which have
been described with the cylinders under each car and the tender, by
simply opening communication between the reservoir and the pipes, the
air at once rushes from the reservoir back through the whole length of
the train, and so rapid is its motion and quick its action that only a
second or two intervenes between the opening of communication and the
application of the brakes. To relieve or “let off” the brakes it is
only necessary to close the reservoir cock and open communication from
the air-pipes to the external atmosphere, when the compressed air in
the brake cylinders will escape, and the springs ordinarily used on car
brakes will cause the pistons to resume their former positions.
For the purpose of opening the connection from the reservoir to the
brake cylinder, and closing this connection and opening one from the
latter to the external air, a single three-way cock is commonly used.
This is arranged at such a point as to be under the control of the
engineer, so that he can at pleasure turn on the compressed air with
any degree of force, instantaneously, or slowly, or with a varying
power, or by another turn of the cock let it off as freely, still
keeping it under the same complete control.
[Illustration: Fig. 224. Scale 1¹⁄₂ inch = 1 foot.]
QUESTION 444. _How does the vacuum brake act and how is it constructed?_
_Answer._ The power is applied to the brakes of the cars in this system
by exhausting instead of compressing the air. This is done by means
of an ejector, of which fig. 224 is a section. This operates somewhat
like an injector. Steam is admitted into the pipe _B_, and escapes
through the annular or circular opening _a a_. The effect of this is
to create what is called an “induced current,” or to draw the air from
the pipe _C′ C_, which, with the steam, escapes at _A_. This produces
a partial vacuum in the pipe _C_, which extends back under the cars.
The pipes under the cars are connected together by rubber hose, which
are prevented from collapsing by coils of wire inside. Under the tender
and under each car are India rubber cylinders with cast iron ends, one
fastened to the car and the other movable. The rubber cylinders can be
extended or compressed somewhat like the bellows of an accordeon. The
rubber is supported by iron rings inside, placed from 4 to 6 inches
apart, so as to prevent them from collapsing when the air is exhausted
from them. When this is done the pressure on the movable cast iron
end draws it towards the fixed one, and by attaching the former to
the brake levers by a rod, the force of the pressure on the head is
communicated to the brakes.
The ejector is placed on top of the boiler, and when the brakes are
to be applied the locomotive runner opens a valve, which admits steam
into the ejector, which instantly begins to produce a partial vacuum
and thus apply the brakes. When the pressure of the brakes is to be
released, the release valve, _D_, is opened, which admits air into
the pipe, _C_, through which it is conducted back to each of the India
rubber cylinders, and thus counteracts the pressure on the ends and
releases the brakes.
Both the atmospheric and the vacuum brakes have recently been applied
to the driving-wheels of locomotives with very excellent results.