TECHNICAL SEMINAR ON
PNEUMATIC BRAKING SYSTEM
Department of Mechanical Engineering
S.J.C institute of Technology
A brake is a device by means of which frictional resistance is
applied to a moving machine member, in order to retard or stop the
motion of a machine.
A brake is a mechanical device which inhibits motion, slowing or
stopping a moving object or preventing its motion.
Most commonly brakes use friction between two surfaces pressed
together to convert the kinetic energy of the moving object into heat
to slow down the motion of vehicle.
A brake which uses air as working fluid is a pneumatic brake.The
system actuated to apply this phenomenon is known as Pneumatic
An pneumatic brake or a compressed air brake system, is a type of
friction brake for vehicles in which compressed air pressing on a
piston is used to apply the pressure to the brake pad needed to stop
George Westinghouse first developed pneumatic brakes for use in
railway service. He patented a safer air brake on March 5, 1872.
Westinghouse made numerous alterations to improve his air
pressured brake invention, which led to various forms of the
automatic brake used in modern vehicles for more safer and
advance brakeing system.
In the early 20th century, after its advantages were proven in railway
use, it was adopted by manufacturers of bus ,trucks and heavy road
Brake system that is based upon a design Air patented by George
Westinghouse on March 5 1872. The Westinghouse Air Brake
Company (WABCO) was subsequently organized to manufacture
and sell Westinghouse invention which has been universally
Compressed air from compressor passes through the valve and
maintains its pressure. This air is stored in the reservoir. And from
the reservoir it goes to the Brake Chambers through many brake
valves. In the brake chamber this pneumatic force is converted into
the mechanical force and then it is converted into the rotational
torque by the slack adjuster, which is connected to S-cam. This
torque applies air brakes which is used to slow down the motion of
During this stage, brake pipe is charged to
5kg/cm2 pressure which in turn charges
control reservoir and auxiliary reservoir to
5 kg/cm2 pressure via distributor valve.
At this stage brake cylinder gets vented to
atmosphere through passage in Distributor
valve and there is no application of brake
in the stage.
For application of brakes, the pressure in brake pipe has
to be dropped. This is done by venting air from driver’s
Reduction in brake pipe pressure positions the distributor
valve in such a way that the control reservoir gets
disconnected from brake pipe and auxiliary reservoir
gets connected to brake cylinder.
The magnitude of braking force is proportional to
reduction in brake pipe pressure which is done by the
application of force on the brake pedal.
For releasing brakes, the brake pipe is again charged to 5 kg/cm2 pressure
by compressor through driver’s brake valve.
This action positions distributor valve in such a away that auxiliary reservoir
gets isolated from brake cylinder.
Brake cylinder is vented to atmosphere through distributor valve and thus
brakes are released.
The air brake system is actually three separate
This system applies and releases the brakes when the
driver pushes/releases the brake pedal. Pushing the
brake pedal, opens a valve to let air flow from the air
tank through the airlines to a brake chamber.
PARKING BRAKE :
Parking brake releases air from the brake chamber
which allows the brake spring to expand and forces the
pushrod out which pushes a slack adjuster that turns the
camshaft, twisting the S-Cam and forcing the brake
linings against the brake drum.
When there is insufficient air in the system to keep the spring in
the chamber restrained, the emergency brakes automatically
engage. When air pressure falls below 420 kpa, a low pressure
warning light will come on along with an audible buzzer. If air
pressure continues to fall, the emergency brake will
automatically apply when air pressure drops to 140-310 kpa.
The fail safe engineering of air brakes will not allow you to
control activation of the emergency brake so look for a safe
place to pull off the roadway as soon as the low pressure
Air is available everywhere in atmosphere so the brake system can
never run out of its operating fluid.
Air doesnot corrode the metals so the life of pneumatic brake is
Air line couplings are easier to attach and detach than hydraulic
lines there is no danger of letting air into a pneumatic circuit.
Pneumatic brake has got emergency brake which gets activated
when the service brake failes, so it is very safe.
Air brakes are effective even with considerable leakage, so an air
brake system can be designed with sufficient fail-safe capacity to
stop the vehicle safely even when leaking.
If the brakes are applied before recharging has been
completed, a larger brake pipe reduction will be required
in order to achieve the desired amount of braking effort.
If the brake pipe pressure is too low due to an excessive
no. of brake applications, an emergency application will
not produce a large enough volume of air flow to trip the
triple valves, leaving the engineer with no means to stop
This system is complicated and needs trained person to
The pneumatic brake is mainly used in large vehicles such as
trucks, buses, trailers, and semi-trailers in addition to their use in
Operation of pneumatic brakeis
different from more common hydraulic
systems,so most people require
additional training and licensing in
order to legally drive any vehicle using
an air brake system.
Driving a vehicle with air brakes
requires basic knowledge of
proper maintenance as well. A
driver should have knowledge to
inspect the air pressurization
system prior to driving and make
sure all tanks are in working order.
Brakes are the parts of the vehicle that absorb the energy
generated by a vehicle in motion in order to stop it. Pneumatic
brakes became popular because of their tremendous stopping
power and long life as compared to hydraulic system. In order
to do this, a system with several components with prpoer
functioning is required. Air provides a simple outlet to
accomplish this task. Thought the pneumatic brake is
complicated to use and training has to be provided for its
efficient use because of its tremendous power and its
effiectiveness it is very usedful in large vehicles such as trucks,
buses, trailers, and semi-trailers in addition to their use in
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