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Pneumatic or air braking system in automobile | Construction and working of pneumatic braking system

Rishabh Pandey



  • A brake which uses air as a working fluid is known as pneumatic brake. The system actuated to apply this phenomenon is know as pneumatic brake system.
  • An pneumatic brake system 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 the vehicle.

Construction of pneumatic braking system



  • The simplest air brake system consists of
    • An air compressor
    • A brake valve
    • series of brake chambers at the wheels
    • unloader valve
    • A pressure gauge and a safety valve
    • and an air reservoir.
    • These are all connected by tubes.
  • Some air braking systems may have additional components such as
    • stop light switch
    • low pressure indicator
    • An air supply valve to supply air for tyre inflation
    • A quick release air quickly from the front brake chambers when the brake pedal is released
    • A limiting valve for limiting the maximum pressure in the front brake chambers and a relay valve to help in quick admission and release of air from the rear brake chambers.
  • 599d2254d0fa7_pneumaticbrakingsystem.jpg.8d78c61033c02891a256a25e7bc3a43b.jpg

Working of pneumatic braking system

  • The air compressor operated by the engine forces air at a pressure of 9-10 kscm (kilo standard cubic meters) through the water and oil separator to the air reservoir.
  • The air pressure in the reservoir is indicated by a pressure gauge.
  • The reservoir contains enough compressed air for several braking operations. From the reservoir the air is supplied to the brake valve.
  • As long as brake pedal is not depressed, brake valves stop the passage of air to brake chambers and there is no braking effect.



  • When the brake pedal is depressed, the brake valves varies its position and compressed air is admitted into the wheel brake chambers.
  • In the chambers the air acts upon flexible diaphragms, moves them the pushes out the rods connected with the levers of the brake gear cams.
  • The cams turn and separate the shoes thus braking the wheels.
  • When the brake pedal is released, the supply of compressed air is cut off from the brake chambers and they are connected to the atmosphere.
  • The pressure in the chambers drops, the brake shoes are returned to their initial position and the wheels run free.
  • The brake valve is equipped with a servo mechanism which ensures that the braking force on the shoes is proportional to the force applied to the pedal.
  • Besides the valve imparts a relative reaction to the movement of the pedal so that the driver can sense the degree of brake application.

IMAGE SOURCE :- google



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