Air Brake System – Parts, Working, Diagram, Principle, Advantages

Construction and working of Pneumatic ( Air) Brake System used in Automobile

Introduction To Air Brake :

An air brake or, more formally, 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. Air brakes are used in large heavy vehicles, particularly those having multiple trailers that must be linked into the brake system, such as trucks, buses, trailers, and semi-trailers in addition to their use in railroad trains.

Construction Of Air Brakes :

The air brake system consists of a two-stage air-compressor driven by the crankshaft or gearbox shaft. It takes air from the atmosphere, compresses it, and delivers to the air reservoir through an un-loader valve. Where the pressure of the reservoir reaches the maximum degree, the unloader valve opens to the atmosphere. Then the compressed air is directed into the atmosphere directly.
Each of the four wheels fitted with brake chambers consists of a diaphragm, and which the air pressure is applied and pushes it. This force operates the cam actuating lever and applies the brake. Each of the brake chambers is connected to the brake pedal, and the air filter is also fitted between the brake valve and reservoir.

Air Brake Components and their Functions :

Following are the main parts of an air brake:
1. Air compressor
  • It is used to build-up and maintain air pressure.
  • The function of the air compressor is to build up and maintain air pressure required to operate air brakes and air-powered accessories.
  • A compressor is designed to pump air into a reservoir which results in pressurized air.
  • The compressor is driven by the vehicle’s engine, either by belts and pulleys or shafts and gears.
  • The compressor is in constant drive with the engine. Whenever the engine is running, so is the compressor.
2) Reservoir
  • The reservoir is used to store the compressed air. Reservoirs are pressure-rated tanks, which hold a supply of compressed air until required for braking or operating auxiliary air systems.
  • They must store a sufficient volume of air to allow several brake applications if the engine stops or the compressor fails.
  • The number and size of the reservoirs on a vehicle will depend on the number of brake chambers and their size, along with the parking brake configuration.

3. Air Dryer 

  • An air dryer may be installed between the compressor and the wet reservoir to help remove moisture from the compressed air.
  • It may be partially filled with a high moisture–absorbent desiccant and an oil filter, or it may be hollow with baffles designed to assist in separating the moisture from the air.

4. Safety Valve

  • A safety valve protects reservoirs from becoming over-pressurized and bursting if the governor malfunctioned and did not place the compressor in the unloading stage.
  • The valve consists of a spring-loaded ball that will allow air to exhaust from the reservoir into the atmosphere. The valve’s pressure setting is determined by the force of the spring
5. Foot valve.
  • Foot Valve is used to draw compressed air from reservoirs when it is needed for braking.
  • This foot-operated valve applies air to operate the brakes.
  • The amount of air delivered to the brakes is regulated by the driver according to the distance the treadle or brake pedal is depressed. Releasing it exhausts air in the service brakes through its exhaust port.
  • The distance the treadle of the foot valve is depressed by the driver determines the air pressure that will be applied, but the maximum application will not exceed the pressure in the reservoir. Releasing the foot valve treadle releases the brakes.
air foot valve diagram
air foot valve diagram
  • When the driver applies the brakes, depressing the treadle partway, the foot valve will automatically maintain the application air pressure without the driver having to adjust the pressure of his foot on the treadle.
  • Releasing the treadle allows the application air to be released through the exhaust ports into the atmosphere. Air treadles are spring-loaded, producing a different “feel” from hydraulic brake applications.

(v) Brake chamber. 

  • Brake chamber is used to transfer the force of compressed air to mechanical linkages.
  • Service-brake chambers convert compressed air pressure energy into mechanical force and movement, which apply the vehicle’s brakes.
  • A brake chamber is a circular container divided in the middle by a flexible diaphragm.
  • Air pressure pushing against the diaphragm causes it to move away from the pressure, forcing the push rod outward against the slack adjuster.
Brake chamber assembly
Brake chamber assembly
  • The force exerted by this motion depends on air pressure and diaphragm size. If a leak occurs in the diaphragm, air is allowed to escape, reducing the effectiveness of the brake chamber.
  • A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking.
(vi) Brake Assembly
  • Brake assembly includes brake chamber and slack adjuster mounted on the backing–plate because of the steering action.
  • A brake chamber is usually mounted on the axle, near the wheel that is to be equipped for braking.
  • Air pressure is fed through an inlet port. The air pushes against the diaphragm and the pushrod.
air brake assembly
air brake assembly
  • The pushrod is connected by a clevis and pin to a crank arm–type lever called a “slack adjuster”.
  • This converts the pushing motion of the pushrod from the brake chamber to a twisting motion of the brake camshaft and S–cams.
  • When the air is exhausted, the return spring in the brake chamber returns the diaphragm and pushrod to the released position.
air brake used for automobile vehicles
air brake used for automobile vehicles

Working of Air Braking System
When the brake pedal is pushed the brake valve opens and compressed air is allowed into the brake chamber. 
The brake valve consists of three passages.
1. Air intake 
2. Exhaust 
3. Brake chamber
When the brake pedal is pressed the exhaust passage will be closed and Air intake passage open and compressed air goes back to the chamber. During return stroke the exhaust passage opens while intake closes and used air goes to the atmosphere. This system fitted with an emergency mechanical brake, which can be used when air supply fails the air brake system, which is called an air-assisted hydraulic braking system.

Air Braking System Diagram :

air braking system diagram
air braking system diagram

Working principle:

As shown in the figure, in the air brakes the compressed air (around 700 kPa) is used to actuate the brake mechanism. The figure shows the complete layout of the Air Brake System. It consists of Air filter, unloading valve, Air compressor, Air reservoir, Brake valve, and 4 numbers brake chamber. The compressor takes atmospheric air through the air filter and compresses the air. This air is stored under pressure in the air reservoir. From this reservoir air goes to various accessories of the vehicle which operates on compressed air. Part of the air goes to the brake valve. The control of the brake valve is done by a driver who controls the intensity of braking according to an emergency.
Pedal Depressed: When the brake pedal is depressed, compressed air from the reservoir is transmitted through pipes equally in all directions to the brake chambers through brake valve which further applies the brake.
Pedal Released: When the driver releases the brake pedal, the master cylinder piston returns to its original position due to return spring, and the pressure is dropped. It releases brake shoes from brake drum to their: original position and brakes are released

Difference between air brakes and hydraulic brakes :

Air brakesHydraulic brakes
1. Compressed air is used as a working substance.1. Hydraulic oil is used as a working substance.
2. Air brake has more powerful than a hydraulic brake.2. Hydraulic brake has less powerful than air brake.
3. Components: Air compressor, unloader valve, brake valve, brake chamber.3. Components: Master cylinder, wheel cylinder, oil reservoir.
4. The air brake system is used in trucks, buses, trains, etc.4. Hydraulic oil brake system is used for light vehicles such as cars, light-duty trucks, etc.
5. Air compressor uses a certain amount of engine power.5. No engine power is used.
6. It is not self lubricating.6. Hydraulic brakes are self lubricating.

Probable causes and remedies of the air brake system

Sr no.Cuases / Problems in air brake systemRemedies / Solution
1.Insufficient air in the system.Identify the source of cause and rectify it.
2.Brake shoes worn outRepair or replace
3.No air pressure in brake systemIdentify the source of cause and rectify it
4.Restricted or broken pipe or hoseClean or replace
5.Leaky, defective brake valveReplace
6.Broken return spring of brake shoe, brake pedalReplace spring
7.Leaky drain plug of reservoirRepair or replace
8.Defective air compressorRepair or replace
9.Loose compressor beltTight it properly
10.Leakage in pipes or jointsRepair or replace
11.Brake linkages jammedLubricate
12.Damaged air boostersReplace

Advantages of Air Braking System :

1. It is more effective when compared to other brakes.
2. The air brake parts are easily located where the chassis design making is simple.
3. The compressed air can be used for tyre inflation wipers, horn, and other accessories.
4. It employs only air as the working medium which is easily available.
5. It is easy to store air at high pressure.
6. It provides a heavy braking effect used in heavy vehicles and trucks.
7. It provides better control.
8. It reduces the stopping distance.
9. It mainly allows less wear and tear of parts.
10. It has a flexible hose connection.

Disadvantage Of Air Braking System :

1. If there is any leakage in the passage the entire system will fail. Therefore sealing of air is very difficult. 

Applications of Air Braking System:

(1) Trucks.
(2) Buses.
(3) Trailers
(4) Semi Trailers
(5) Railway Train.
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Sachin Thorat

Sachin is a B-TECH graduate in Mechanical Engineering from a reputed Engineering college. Currently, he is working in the sheet metal industry as a designer. Additionally, he has interested in Product Design, Animation, and Project design. He also likes to write articles related to the mechanical engineering field and tries to motivate other mechanical engineering students by his innovative project ideas, design, models and videos.

6 thoughts on “Air Brake System – Parts, Working, Diagram, Principle, Advantages

  1. Hello,
    This is a very nice post. This is very informative and helpful blog. Thank you for sharing this post. Thank you for sharing about the working details. Hope your next post will published soon.

  2. I think you have operation of air brakes all wrong. Any CDL driver can tell you it is a big deal to have sufficient air pressure in order to move the vehicle. When the air pressure is lowered, or completely lost due to leakage, the brake springs force the brake lining against the drum or rotors (which ever system is used). This stops the vehicle. The compressed air is used to release the brakes, not apply them. The treadle valve is how the operator controls the amount of air released thereby allowing the spring brake to apply a controlled force of the lining against the drum or rotors. The parking brake is when a hand operated valve releases all air pressure thus allowing full spring pressure to activate the brakes.

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