Explain construction and working of Air Brakes.
2 Answers

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  • Air Brakes (a.k.a. pneumatic brakes/harsh brake) are predominantly used on medium and heavy duty commercial vehicles having much higher braking torque requirements.
  • Air brakes are more effective and are actuated faster with shorter braking distances.
  • This is the reason why heavy commercial vehicles are fitted with sign boards reading “Caution Air Brakes” near the tail lights.


  • The simplest system consists of an air compressor, compressor cutout, pressure gauge, drain plug/safety valve and the reservoir constitute the compressing and control unit whereas the rest of them are termed as application units.
  • Figure shows the layout an air-brake system.
  • Air from the atmosphere is taken in and compressed in the compressor which is a centrifugal type driven by the engine.
  • The compressed air from the compressor is stored in the reservoir.
  • A cutoff valve is provided to control the pressure inside the reservoir and acts as a non-return valve.
  • The oil filter and air drier as the name suggests filters and dries the air coming out of the compressor.
  • The reservoir is connected to the brake chambers via a foot operated valve (a.k.a. brake valve).
  • The brake chamber is connected to the actuation mechanism of the drum brakes.


  • The Air-Brake system consists of a normal drum brake but the difference is in the actuation mechanism.
  • When the brakes pedal is pressed or the foot operated valve is pressed, the valve opens and air from the reservoir rushes into the brake chambers which actuates the drum brakes.
  • As the pressure in the air reservoir decreases, the cutoff valve opens and the air reservoir pressure is maintained.

An air or pneumatic braking system is used in medium & heavy duty vehicles. This system has following advantages:
i) The pressure on the pre-compressed air allow any force regulate for braking to developed very small effort applied by the driver to the brake pedal.
ii) The compressed air can be used for many purposes like to inflate the tyre , drive windshield wiper, actuate steering, gear etc.
iii) This system is most convenient for full trailers & semi-trailers.

The layout of air braking system shown in fig 9.1. It consists of air filter, air compressor, unloader valve, reservoir, brake valve & brake chamber with steel tubes & flexible bases. The compressor is operated by engine. The compressor sucks the air from atmosphere through air filter & compressed it to high pressure & discharge to the reservoir through oil separator. The pressure gauge is fitted to reservoir indicate the pressure in the tank. The reservoir contain enough compressed air for several braking operation. The air from reservoir tank is supplied to brake value & to other accessories. There is no braking effect until the brake pedal is not pressed as brake valve present the passage of air to the brake chamber.

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Working: When brake pedal is pressed, the brake valve changes its position & brake valve open. The compressed air from brake valve from to brake chamber acts on the flexible diaphragms in brake chamber. The diaphragm pushes the rods connected with the levers of brake gear cam. The cam turn & expand the brake shoe with make frictional contact with brake drum thus braking the wheels. When brake pedal is released the supply of compressed air is cut off from the brake chamber & they we connected to atmosphere. The pressure from the chamber drops & brake shoes are returned to their initial position & wheel run free. The brake valve is equipped with servo-mechanism which ensures that the braking force on the shoe proportion to the applied force on the pedal & also impart relative reaction to the movement of the pedal. So, driver can senior the degree of brake application.

The unloader valve is located between compressor reservoir air pressure line. The unloader valve is relieved the compressor of its pumping load once the uploader cut out pressure is obtained & seal the reservoir. When the compressor is built up a pressure depending upon the setting of the adjusting screw. The unloader then delivers the air discharge by the compressor to the atmosphere.Thus, allowing the compressor to run light while the reservoir contains sufficient supply of air. The air filter prevents the dust & foreign material entering the operating system. These are mounted on the chassis & have a drain plug to allowed the condensate to be easily removed.

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