THE
VACUUM BRAKE
THE
FUNCTION OF THE BRAKE is
to absorb by motion the momentum of the train. Energy stored in the
moving train is converted to heat at the brake blocks when the brake
is applied.For passenger trains an Act of Parliament was passed
in 1889 that (1) the brake on the train should be continuous and capable
of being applied to every vehicle of the train (2) be instantaneous
in action and capable of being applied by the driver and/or guard (3)
be self applying in the event of a train becoming divided. The Automatic
Vacuum Brake fulfils these conditions.
Basic
vacuum brake system
THE
VACUUM BRAKE SYSTEM consists
of an exhausting device on the engine known as the vacuum ejector. This
has a large ejector to create the regulation vacuum quickly in the train
pipe reservoir and both sides of the vacuum cylinder,
together with a small ejector to maintain the vacuum. G. W. R. locomotives
are fitted with a large ejector and a vacuum pump, which fulfils the
same purpose as the small ejector.Atmospheric pressure is approximately 15lb p.s.i.
and vacuum is measured in inches of mercury. A perfect vacuum is equal
to 30 inches of mercury i.e. 0lb p.s.i. Therefore 2 inches of vacuum
represents approximately 1 p.s.i. of atmospheric pressure. The regulation
vacuum for working brakes is 21 inches of mercury (GWR 25 inches).To
apply the brakes air is admitted to the train pipe so that the vacuum
on the lower side of the brake piston is partially or completely destroyed.The vacuum on the reservoir and upper side of the
brake piston is retained, being sealed off by a ball valve. In a normal
application the driver admits air to the train pipe through the ports
in the drivers brake valve. The amount of air admitted regulating the
power of the application. Air can also be admitted to the train pipe
via the guards brake valve, by use of the communication cord or by the
train pipe connection being broken in the event of a divided train.THE VACUUM EJECTOR contains
two ejectors - one large and one small. The difference
being that the large uses more steam and therefore creates a vacuum more quickly
for use when standing or when a quick release of the brake is required.
The small ejector will maintain the vacuum throughout the train and
overcome any small leaks in the system. Each ejector is fitted with
a non-return valve to prevent smokebox gases being drawn back through
the ejector exhaust.Steam passing through the steam cone at great velocity
is discharged into the ejector air cone where it comes into frictional
contact with the air. The steam and air are exhausted via the ejector
exhaust pipe and out via the chimney thereby lifting the non-return
valves and creating a vacuum in the train pipe. Under each ejector ball
drain valves are fitted to allow condensation to drain away. These ball
valves are forced to their seat when the ejector is running to prevent
air from being admitted. A ball valve is provided so that in the event
of leakage taking place in either of the non-return valves the ball
valve will open to atmosphere and smokebox gases will not be drawn through
the ejector but vacuum will be maintained in the train pipe.The
drivers brake valve will admit air into the train pipe to reduce or
destroy the vacuum. VACUUM RELIEF VALVE is
provided to prevent the regulation vacuum being exceeded. It consists
of a spring-loaded valve, which lifts and allows air into the train
pipe if the ejector creates
more than 21 inches of vacuum.
COMBINATION BRAKE VALVE is
designed to admit steam to the steam brake cylinders when the train
pipe vacuum is reduced or destroyed and to release steam from the steam
brake cylinders when the train pipe vacuum is restored.With the brake released a vacuum is maintained in
the train pipe and on the inner side of the vacuum piston the opposite
side being held in by atmospheric pressure which, through the fulcrum
lever, maintains the steam brake plug in the closed position. A brake
application admits air into the train pipe and to the inner side or
the vacuum piston; the steam behind the steam plug exerts a pressure
forcing the steam plug out. The movement of the steam plug spindle closes
the exhaust port and at the same time via the fulcrum lever forces the
vacuum piston out. Steam from the plug is then admitted to the steam
brake cylinders. When the vacuum is restored in the train pipe atmospheric
pressure will force the vacuum piston in and via the fulcrum lever close
the steam plug and release the steam in the steam brake cylinders to
exhaust.VACUUM
PUMP as
fitted to G.W.R. locomotives consists of a piston, which reciprocates,
in the cylinder being directly driven from the locomotives crossheads.A chamber extends above the cylinder and clack valves
are fitted between the chamber and the cylinder. Similar clacks are
also fitted at each end of the cylinder, which provide access to the
atmosphere. The train pipe is also connected to a retaining valve. The
pump is double acting so that when air is being induced from the train
pipe on one side of the piston the air induced on the previous stroke
is expelled on the other side.
Vacuum brake cylinder -Brake shown released
BRAKE CYLINDERS When
the brake is applied air which enters the train pipe passes through
the flexible connecting pipe and enters the bottom of the brake cylinder
and at the same time forces the ball valve onto its seating on the passage
leading to the vacuum chamber, thereby retaining the vacuum on the top
side of the brake piston, the air having access to the bottom of the
piston only causing it to rise. When the brakes are released the action
of the ejector causes air to be drawn from the underside of the brake
piston.When the vacuum reaches. and commences to exceed
that in the top side, the ball valve lifts from its seat until vacuum
on both sides is equal causing the brake piston to drop to the bottom
of the cylinder by virtue of its own weight.
Vacuum brake cylinder -Brake shown applied
DIRECT ADMISSION VALVE is
fitted close to the brake cylinder in the running position the inlet
is closed and the interior of the valve is at 21 inches vacuum. When
a brake application is made air enters the train pipe and produces a
greater pressure on the underside of the diaphragm, which opens the
air inlet valve to atmosphere.Air let into the system does not have to pass through
the drivers brake valve. The train pipe is 'passing a message' to each
direct admission valve to allow uniform application of the valve more
rapidly.VACUUM
EXHAUSTERS (Diesel)The exhauster rotor carries six blades, which can
move in the rotor slots. The rotor revolves inside the cylindrical body,
but the axis of the rotor is not the same as that of the cylindrical
body. As the rotor revolves the vanes are kept in contact with the body
wall by the cam ring. Air is drawn from the train pipe between pairs
of blades and exhausted into the atmosphere. For the efficient operation
of the exhauster it is necessary for sufficient oil to be supplied to
the exhauster bearings and also into the body of the exhauster to provide
an effective seal between the blades and the bore of the body. The air
exhausted, therefore, contains a considerable proportion of oil and
it is necessary to force this through an oil separator before allowing
it to escape into atmosphere.QUICK
RELEASE BRAKE (Diesel Multiple Unit)The vacuum release chamber provided on each vehicle
is normally maintained at a high vacuum of 29 inches of mercury. This
chamber is connected to the exhauster through an automatic isolating
valve, which remains open until the vacuum in the vacuum release chamber
falls to 19 inches of mercury. A feed valve is fitted between the high
vacuum system and the drivers brake valve and the train pipe. This valve
ensures that the vacuum in the train pipe does not exceed 21 inches
of mercury. 
Twin
Pipe vacuum system
By
this arrangement when it is necessary to release the brakes not only
will the exhauster draw air out of the system but there is a reservoir
of high vacuum available which accelerates the withdrawal of air from
the train pipe and compensates for the fact that the belt driven exhauster
may be running at low speed for long periods.With the quick release brake it is necessary to have,
in addition to the normal vacuum pipe, a high vacuum pipe connecting
all the vacuum release chambers, and this pipe is maintained at 29 inches
of mercury in the running position.
