Definition
In Railway wagon as well as in
passenger cars the braking system plays a very important role to stop
the train, to maintain the speed of the train within specific limit.
Brakes are the devices on the trains to bring it to standstill.
A moving train contains energy, known as kinetic energy, which
needs to be removed from the train in order to cause it to stop. The
simplest way of doing this is to convert the kinetic energy into heat
energy. The conversion is usually done by applying a contact material to
the rotating wheels or to discs attached to the axles. The material
creates friction and converts the kinetic energy into heat energy. The
wheels slow down and eventually the train stops. The material used for
braking is normally in the form of a block or pad.
The vast majority of the world's trains are equipped with
braking systems which uses compressed air as the force to push the
blocks on to wheels or pads on to discs. These systems are known as "Air
Brakes" or "Pneumatic Brakes". The compressed air is transmitted along
the train through a "brake pipe". Changing the level of air pressure in
the pipe causes a change in the state of the brake on each vehicle. The
system is in widespread use throughout the world.
An alternative to air brake known as vacuum brake is also used
in railway wagon. Like the air brake the vacuum brake system is
contolled through a brake pipe conecting a brake valve in the driver’s
cab with braking equipment on each vehicle. The operation of the brake
equipment on each vehicle depends on the condition of vacuum created in
the pipe by an ejector or exhauster.
Another braking system used by electric train is Electric
Dynamic Braking System.The basic principle of operation is to convert
electric motor into a braking generator dissipating the kinetic energy
as heat energy. Regenerative braking is similar to Dynamic Braking. Only
difference is that, it transmits generated electricity to overhead
wires instead of dissipating it as heat, and is becoming more common due
to it’s ability to save energy.
To overcome the problems associated with air braking system, a
new braking system called “Electronically Controlled Pneumatic Brakes”
(E.C.P.) is generally used in Railway Wagons. Before E.C.P. Electro
Pneumatic Brakes are also in use.
This is all about braking systems used in railway wagon as
well as in passenger cars. To prevent accidents some Automatic Systems
like Automatic Warning Systems are used. They vary the speed or stop the
train according to the signal aspects.
Elctronically Controlled Pneumatic Brakes:
A new form of electronically control of air
braking is currently being tested by a number of railroads. It is known
as ECP and uses modern electronic techniques to overcome the problems of
air braking on long freight trains.
The pure air control brake system invented
by George Westinghouse in the 1860s and still used by almost all
freight trains in the US and in many other parts of the world suffers
from two main problems. It takes a long time for the air messages to
travel along the train and there is no graduated release. For example,
the delay for a reduction in train line pressure to travel from the
leading locomotive to the rear of a 150 car consists can be 150 seconds.
Also, you have to fully release the brake and wait for the supply
reservoirs to recharge before you can reapply. Electrical control can
overcome these difficulties
ECP refers to Electronically Controlled
Pneumatic brakes, key word being "Electronically" as opposed to
"electrically". Before E.C.P. Electro-pneumatic brakes were fitted on
trains. They use several train wires to operate individual valves or
variations in switching of the wires to control brakes. Most of these
systems use a second train line for main reservoir air supplies and they
do not have the built-in two-way communications that ECP systems have. A
car in an ECP brake train can do a self-diagnosis and report the
information to the driver and it only requires the standard train line
pipe
Operation
There is a control box on top of the
driver's console. When he wants to apply the brakes, he pushes the
button until the readout shows the amount of brake cylinder pressure (or
percentage of braking effort) he wants. He releases the button; the
control unit then codes and sends the signal to all cars. They in turn
receive and interpret the message. They then begin allowing compressed
air from their reservoirs to go to the brake cylinder until the desired
cylinder pressure is achieved. The microprocessors on the cars will
continuously monitor brake cylinder pressure against leakage and
maintain the desired pressure.
If the driver wants to reduce
brake cylinder pressure he simply pushes the release button until the
desired level is indicated, either partial or full release. Again a
signal is coded and transmitted to the cars. The cars in turn do as
commanded. If the driver asks for only a partial reduction of braking
effort, he can increase the effort again as needed without doing a full
release first. The processor on the car is constantly monitoring brake
pipe, reservoir tank and brake cylinder pressures.
When braking commands are not
being transmitted, the head end (control) unit is sending out status
messages. The last car in the train will respond to each status message
from the head end. All cars will monitor these messages, and if a car
fails to receive three status messages in a row from either the head end
or the rear end, it will assume that the train is broken in two or that
the electrical line is broken. It will then initiate an emergency stop,
while trying to tell the other cars and loco that it is doing so.
Each car has a rechargeable
battery to provide the high power requirements when solenoids need to be
activated. When the high power is not being used, the batteries will
trickle recharge from the communications/power cable.(If the train uses
radio communication the batteries will recharge while the car is in
motion via an onboard generator creating power from the motion of the
car, either an axle generator, or natural frequency vibration generator
or some other type of device.)
The hardwired system uses
roughly 25% of its signal capacity for brake commands and status
messages. Distributed power, controlled via the same cable uses another
10-15%, leaving 60-65% of the signal capacity for special monitors on
the car, such as bearing sensors, temperature sensors for reefers on
tankers, pressure sensors for tankers, etc.
Some of the benefits of ECP
braking are Instantaneous response to the engineer's commands on all
vehicles, graduated release of brakes and continuous replenishment of
reservoirs. With the new responsiveness of E.C.P. braking distances will
be reduced. This will allow shorter stopping distances and will in
turn, allow higher speeds.
Another benefits are in train handling improvements, cycle time
reduction energy saving, lower wagon maintenance cost
Vacuum Braking System
An alternative to the air brake,
known as 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 fully applied.
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 let’s air into the brake pipe to prevent further increase.
Driver’s Brake Valve
It is 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
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