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INTRODUCTION 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 energy into heat. 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. 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
use compressed air as the force used to push 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. It can apply the brake, release it or hold it "on"
after a partial application. The system is in widespread use throughout the world.
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 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.
The momentum of a moving body increases with weight and speed of the body as these
factors increase improvements in the brake become so important. The adhesion of
the wheels and speed of the train are the main factors that determines the total
retarding power. The maximum retarding force applied by the brake blocks at wheels
depends upon the coefficient of friction between the wheels and the rail and the
component of the weight of the wagon on the wheels. Mathematically the retarding
force F can be expressed as
F = ? * W
Where ? = the coefficient of friction
W = component of weight of wagon on the wheels
If the coefficient of
friction becomes equal to unity then the retarding force will be equal to the
weight of the wagon. Also the deceleration equals the acceleration due to gravity.
Then the braking efficiency is 100%. This is the theoretical limit for braking
efficiency. Highly efficient brakes giving a large deceleration might injure the
passengers due to sudden stopping of the train .More over this will cause the
brake shoes to wear rapidly and their is always the risk of derailment. The braking
efficiencies usually vary from 50% to 80%, which enables the train to stop safely
with in a reasonable distance. The equations used for the calculations of acceleration
can also be used for calculating the braking distance except to the accelerating
force becomes the braking force Fb The
brake force Fb = p* ? * ? Where
p = brake shoe pressure
? = co-efficient of fricton between brake shoe and
wheel
? efficiency of braking
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