In their simplest form, the
locomotive is braked by cast iron shoes that act on the wheels. The brakes
are connected together by a compensating linkage so that when they are
applied, an equal pressure is exerted on all wheels. A hand screw, usually on
a pillar allows the brakes to be applied manually and in addition, a steam
cylinder can be actuated to apply the brake.
compensating brake gear on a
Trains also, usually have a
system that brakes the wheels of the train. This is either operated by vacuum
The vacuum brake, was
introduced around the early 1870s, the same time as the air brake. Like the
air brake, the vacuum brake system is controlled through a brake pipe
connecting a brake valve in the driver's cab with braking equipment on every
vehicle. The operation of the brake equipment on each vehicle depends on the
condition of a vacuum created in the pipe by an ejector or exhauster. The
ejector, using steam on a steam locomotive, or an exhauster, using electric
power on other types of train, removes atmospheric pressure from the brake
pipe to create the vacuum. With a full vacuum, the brake is released. With no
vacuum, i.e. normal atmospheric pressure in the brake pipe, the brake is
The pressure in the atmosphere is defined as 1 bar or about 14.5 lbs. per
square inch. Reducing atmospheric pressure to 0 lbs. per square inch, creates
a near perfect vacuum which is measured as 30 inches of mercury, written as
30 Hg. Each 2 inches of vacuum therefore represents about 1 lb. per square
inch of atmospheric pressure.
In the UK, vacuum brakes operated with the brake pipe at 21 Hg, except on the
Great Western Railway which operated at 25 Hg.
The vacuum in the brake pipe is created and maintained by a motor-driven
exhauster. The exhauster has two speeds, high speed and low speed. The high
speed is switched in to create a vacuum and thus release the brakes. The slow
speed is used to keep the vacuum at the required level to maintain brake
release. It maintains the vacuum against small leaks in the brake pipe. The
vacuum in the brake pipe is prevented from exceeding its nominated level
(normally 21 Hg) by a relief valve, which opens at the setting and lets air
into the brake pipe to prevent further increase.
Principal Parts of the Vacuum Brake System
This diagram shows the principal parts of
the vacuum brake system as applied to an electric or diesel train.
The means by which the driver controls the brake. The brake valve will have
(at least) the following positions: "Release", "Running", "Lap" and "Brake
On". There may also be a "Neutral" or "Shut Down" position, which locks the
valve out of use. The "Release" position connects the exhauster to the brake
pipe and switches the exhauster to full speed. This raises the vacuum in the
brake pipe as quickly as possible to get a release.
In the "Running" position, the exhauster keeps running but at its slow speed.
This ensures that the vacuum is maintained against any small leaks or losses
in the brake pipe, connections and hoses.
"Lap" is used to shut off the connection between the exhauster and the brake
pipe to close off the connection to atmosphere after a brake application has
been made. It can be used to provide a partial release as well as a partial
application, something not possible with the original forms of air brake.
"Brake On" closes off the connection to the exhauster and opens the brake
pipe to atmosphere. The vacuum is reduced as air rushes in.
Some brake valves were fitted with an "Emergency" position. Its operation was
the same as the "Brake On" position, except that the opening to atmosphere
was larger to give a quicker application.
For some reason, exhausters
are called ejectors on steam locomotives. They are, of course, steam
operated. The ejector consists of a series of cones inside a tube. Steam is
allowed to pass through the cones so that a vacuum is created in the tube and
thus in the brake pipe to which it is connected. There are always two
ejectors, large and small, which provide the brake release and vacuum
maintenance functions respectively. The large ejector provides the rapid
build-up of vacuum required for brake release and the small ejector provides
the constant vacuum needed to keep the brake pipe and cylinder vacuum at the
correct level to maintain brake release.
On some locomotives, ejectors
were combined with the driver's brake valve. Most had only "Brake On",
"Running" and "Brake Off" positions and many were combined with a steam brake
fitted to the locomotive and tender. The more sophisticated allowed a single
brake application to apply the brakes on the train before they were applied
on the locomotive. This gave a smooth and even stop and prevented "bunching"
of coaches behind the locomotive. Drivers were taught that it was best to
slow the train down and then gently ease off the brakes as the train came to
a stop. This required them to restore much of the vacuum by the time the
train was brought to a stand, allowing a quick release without having to run
the large ejector for very long, thereby saving steam.
The vacuum-carrying pipe running the length of the train, which transmits the
variations in pressure required to control the brake. It is connected between
vehicles by flexible hoses, which can be uncoupled to allow vehicles to be
separated. The use of the vacuum system makes the brake "fail safe", i.e. the
loss of vacuum in the brake pipe will cause the brake to apply.
At the ends of each vehicle, a dummy coupling point is provided to allow the
ends of the brake pipe hoses to be sealed when the vehicle is uncoupled. The
sealed dummy couplings prevent the vacuum being lost from the brake pipe.
The brake pipe is carried between adjacent vehicles through flexible hoses.
The hoses can be sealed at the outer ends of the train by connecting them to
Cylinder (shown in blue)
Each vehicle has at least one brake cylinder. Sometimes two or more are
provided. The movement of the piston contained inside the cylinder operates
the brakes through links called "rigging". The rigging applies the blocks to
the wheels. I do not know of a vacuum brake system which uses disc brakes.
The piston inside the brake cylinder moves in accordance with the change in
vacuum pressure in the brake pipe. Loss of vacuum applies the brakes,
restoration of the vacuum releases the brakes.
The operation of the vacuum brake relies on the difference in pressure
between one side of the brake cylinder piston and the other. In order to
ensure there is always a source of vacuum available to operate the brake, a
vacuum reservoir is provided on, or connected to the upper side of the
piston. In the simplest version of the brake, shown above, the brake cylinder
is integral with the vacuum reservoir. Some versions of the brake have a
separate reservoir and a piped connection to the upper side of the piston.
This is the friction material which is pressed against the surface of the
wheel tread by the upward movement of the brake cylinder piston. Often made
of cast iron or some composition material, brake blocks are the main source
of wear in the brake system and require regular inspection to see that they
are changed when required.
This is the system by which the movement of the brake cylinder piston
transmits pressure to the brake blocks on each wheel. Rigging can often be
complex, especially under a passenger car with two blocks to each wheel,
making a total of sixteen. Rigging requires careful adjustment to ensure all
the blocks operated from one cylinder provide an even rate of application to
each wheel. If you change one block, you have to check and adjust all the
blocks on that axle.
The ball valve is needed to ensure that the vacuum in the vacuum reservoir is
maintained at the required level, i.e. the same as the brake pipe, during
brake release but that the connection to the brake pipe is closed during a
brake application. It is necessary to close the connection as soon as the
brake pipe vacuum is reduced so that a difference in pressure is created
between the upper and lower sides of the brake cylinder piston.
Operation on Each Vehicle
This diagram shows the
condition of the brake cylinder, ball valve and vacuum reservoir in the
release position. The piston is at the bottom of the brake cylinder. Note
how the brake cylinder is open at the top so that it is in direct connection
with the vacuum reservoir.
A vacuum has been created in the brake pipe, the vacuum reservoir and
underneath the piston in the brake cylinder. The removal of atmospheric
pressure from the system has caused the ball valve to open the connection
between the vacuum reservoir and the brake pipe. The fall of the piston to
the bottom of the brake cylinder causes the brake blocks to be released from
This diagram shows the
condition of the brake cylinder, ball valve and vacuum reservoir in the
application position. The vacuum has been reduced by the admission of
atmospheric pressure into the brake pipe. This has forced the piston upwards
in the brake cylinder. By way of the connection to the brake rigging, the
upward movement of the piston has caused the brake blocks to be applied to
The movement of the piston in the brake cylinder relies on the fact that
there is a pressure difference between the underside of the piston and the
upper side. During the brake application, the vacuum in the brake pipe is
reduced by admitting air from the atmosphere. As the air enters the ball
valve, it forces the ball (in red in the diagram above) upwards to close the
connection to the vacuum reservoir. This ensures that the vacuum in the
reservoir will not be reduced. At the same time, the air entering the
underside of the brake cylinder creates an imbalance in the pressure compared
with the pressure above the piston. This forces the piston upwards to apply
Westinghouse brake system
click on image to enlarge
The Westinghouse system uses
air pressure to charge air reservoirs (tanks) on each car. Full air pressure
signals each car to release the brakes. A reduction or loss of air pressure
signals each car to apply its brakes, using the compressed air in its
The pressurized air comes
from an air compressor in the locomotive and is sent from car to car by a
train line made up of pipes beneath each car and hoses between cars. The
principal problem with the straight air braking system is that any separation
between hoses and pipes causes loss of air pressure and hence the loss of the
force applying the brakes. This deficiency could easily cause a runaway
train. Straight air brakes are still used on locomotives, although as a dual
circuit system, usually with each bogie (truck) having its own circuit.
In order to design a system without the shortcomings of the straight air
system, Westinghouse invented a system wherein each piece of railway rolling
stock was equipped with an air reservoir and a triple valve, also known as a
The triple valve is often
described as being so named because it performs three functions, but this is
a widespread myth, as the triple valve simply performs two functions: it
applies the brakes and releases them. In so doing, it supports certain other
actions (i.e. it 'holds' or maintains the application and it permits the
exhaust of brake cylinder pressure and the recharging of the reservoir during
the release). In his patent application, Westinghouse refers to his
'triple-valve device' because of the three component valvular parts
comprising it: the diaphragm-operated poppet valve feeding reservoir air to
the brake cylinder, the reservoir charging valve, and the brake cylinder
release valve. When he soon improved the device by removing the poppet valve
action, these three components became the piston valve, the slide valve, and
the graduating valve.
If the pressure in the train line is lower than that of the reservoir, the
brake cylinder exhaust portal is closed and air from the car's reservoir is
fed into the brake cylinder to apply the brakes. This action continues until
equilibrium between the brake pipe pressure and reservoir pressure is
achieved. At that point, the airflow from the reservoir to the brake cylinder
is lapped off and the cylinder is maintained at a constant pressure.
If the pressure in the train line is higher than that of the reservoir, the
triple valve connects the train line to the reservoir feed, causing the air
pressure in the reservoir to increase. The triple valve also causes the brake
cylinder to be exhausted to atmosphere, releasing the brakes.
As the pressure in the train line and that of the reservoir equalize, the
triple valve closes, causing the air pressure in the reservoir and brake
cylinder to be maintained at the current level.
Unlike the straight air system, the Westinghouse system uses a reduction in
air pressure in the train line to apply the brakes. When the engineer
(driver) applies the brake by operating the locomotive brake valve, the train
line vents to atmosphere at a controlled rate, reducing the train line
pressure and in turn triggering the triple valve on each car to feed air into
its brake cylinder. When the engineer releases the brake, the locomotive
brake valve portal to atmosphere is closed, allowing the train line to be
recharged by the compressor of the locomotive. The subsequent increase of
train line pressure causes the triple valves on each car to discharge the
contents of the brake cylinder to atmosphere, releasing the brakes and
recharging the reservoirs.
Under the Westinghouse system, therefore, brakes are applied by reducing
train line pressure and released by increasing train line pressure. The
Westinghouse system is thus fail safe—any failure in the train line,
including a separation ("break-in-two") of the train, will cause a loss of
train line pressure, causing the brakes to be applied and bringing the train
to a stop.
Modern air brake systems are in effect two braking systems combined:
The service brake system, which applies and releases the brakes during normal
The emergency brake system, which applies the brakes rapidly in the event of
a brake pipe failure or an emergency application by the engineer.
When the train brakes are applied during normal operations, the engineer
makes a "service application" or a "service rate reduction”, which means that
the train line pressure reduces at a controlled rate. It takes several
seconds for the train line pressure to reduce and consequently takes several
seconds for the brakes to apply throughout the train. In the event the train
needs to make an emergency stop, the engineer can make an "emergency
application," which immediately and rapidly vents all of the train line
pressure to atmosphere, resulting in a rapid application of the train's
brakes. An emergency application also results when the train line comes apart
or otherwise fails, as all air will also be immediately vented to atmosphere.
In addition, an emergency application brings in an additional component of
each car's air brake system: the emergency portion. The triple valve is
divided into two portions: the service portion, which contains the mechanism
used during brake applications made during service reductions, and the
emergency portion, which senses the immediate, rapid release of train line
pressure. In addition, each car's air brake reservoir is divided into two
portions--the service portion and the emergency portion--and is known as the
"dual-compartment reservoir”. Normal service applications transfer air
pressure from the service portion to the brake cylinder, while emergency
applications cause the triple valve to direct all air in both the service
portion and the emergency portion of the dual-compartment reservoir to the
brake cylinder, resulting in a 20-30% stronger application.
The emergency portion of each triple valve is activated by the extremely
rapid rate of reduction of train line pressure. Due to the length of trains
and the small diameter of the train line, the rate of reduction is high near
the front of the train (in the case of an engineer-initiated emergency
application) or near the break in the train line (in the case of the train
line coming apart). Farther away from the source of the emergency
application, the rate of reduction can be reduced to the point where triple
valves will not detect the application as an emergency reduction. To prevent
this, each triple valve's emergency portion contains an auxiliary vent port,
which, when activated by an emergency application, also locally vents the
train line's pressure directly to atmosphere. This serves to propagate the
emergency application rapidly along the entire length of the train.
The most visible item is the pump, operated by steam pressure. These can be
one or two cylinder.
single cylinder pump on a small steam engine