Air Brakes or Pneumatic Brakes

1.9. Air Brakes (Pneumatic Brakes)

Air brakes or more formally named pneumatic brakes is a type of compressed air brake system for heavy duty trucks. In which compressed air used to apply the pressure on the brake pad needed to stop the vehicle.

In air brakes, the brake shoe operating cam is operated by means of air pressure which is developed by an air compressor driven by the engine. There are separate brake chambers for the separate brake shoe operating cams.

The brake chambers are connected with the air reservoir by means of pipe line. A brake valve operated by the foot pedal controls the pressure of air which affects the brake chambers.

1.9.1. Construction of air brake:

Fig.1.21 shows the schematic layout of an air brake system for bus or truck.

Fig 1.20: Schematic layout of air brake (Trucks or Bus)

Following are the main parts of an air brake:

i)                   Air compressor.

ii)                Un-loader valve

iii)              Reservoir.

iv)              Brake valve.

v)                Brake chamber.

vi)              Quick release valve.

vii)           Relay valve.

viii)         Warning signal.

(i) Air compressor:

It builds up air pressure in reservoir. It is a heart of air brake system.

A piston type air compressor is commonly employed in the brake system. When piston moves downwards air is drawn into the cylinder through intake valve.

When the piston moves upward, the intake valve is closed and outlet valve is opened by the air pressure and the air is forced out into the reservoir.

 (ii) Un-loader valve:

This valve is mounted in the air pressure system between the compressor and the reservoir to control the pressure of air in the reservoir. It relieves the compressor of its pumping load once the unloader cut-out pressure is obtained.

The un-loader then diverts the air delivered by the compressor to the atmosphere thus allowing the compressor to run right whilst the reservoir contains an adequate supply of air.

It consists mainly of a governor valve, an unloader plunger and a non -return valve.

(iii) Reservoir:

It stores compressed air at the specified pressure for brake application.

It is made of sheet steel and a safety valve is provided at the top of the reservoir to regulate the air pressure. 

A drain plug is also provided at the bottom for periodic draining of the reservoir, without which the lubricating oil from the compressor and moisture in the air would form emulsion which would damage the other brake units (e.g., brake valve, brake chambers etc.).

(iv) Brake valve:

It is the control valve which is operated by the brake pedal. It controls the intensity of braking in an air pressure system.

It is located between the reservoir and air lines leading to individual brake chambers.

(v) Brake chamber:

The brake chambers convert the energy of the compressed air into mechanical force and motion necessary to operate the vehicle brakes.

A brake chamber consists of a housing which encloses a movable diaphragm connected by a rod linked to the brake shoe operating camshaft. The chamber is divided into two parts by the diaphragm, the side opposite to the rod being air tight. 

Air pressure acts in the air tight portion of the chamber which causes deflection of diaphragm in and application of brakes.

(vi) Quick release valve:

It is employed in the front brake lines to accelerate the release of air from the brake chambers.

It directly releases pressure to the atmosphere rather than through the brake valve.

(vii) Relay valve:

The relay valve speeds up the application and release of air from the brake chambers.

It supplies air to the brake chambers directly from the reservoir for quick application of the brake. 

It also exhausts compressed air from the rear brake chambers directly to the atmosphere rather than through the brake valve.

(viii) Warning signal:

It is a warning light or buzzer which warns low pressure in the circuit.

A compressed air brake system is divided into a supply system and a control system.

The supply system compresses, stores and supplies high-pressure air to the control system as well as to additional air operated auxiliary truck systems (gearbox shift control, clutch pedal air assistance servo, etc.).

1.9.2. Supply system:

The air compressor is driven by the engine either by crankshaft pulley via a belt or directly from the engine timing gears.

It is lubricated and cooled by the engine lubrication and cooling systems.

1.9.3. Control system:

The control system is further divided into two service brake circuits: the parking brake circuit and the trailer brake circuit.

1.9.4. Working of air brake:

Fig. 1.21 illustrates line diagram for general layout of an air brake system. While in Fig 1.20 illustrates schematic layout with complete parts. When the foot pedal is pressed down, air pressure acts on diaphragm of brake chamber. 

The diaphragm is linked with the brake shoe operating camshaft. The diaphragm is pushed outward in the brake chamber causing movement of brake shoe operating cam. 

The brake shoe expands outwards and holds the moving brake drums as they come into its contact. The brakes are thus applied.

Fig 1.21: Line diagram / layout of Air brake system

As soon as the pressure is released from the brake pedal, it comes back with the help of return spring.

This results in closing of brake valve and release of pressure inside the brake chamber. 

The brake shoe operating cam moves in the reverse direction as a result of pressure release on the brake chamber.

The brake shoe contracts inward with the help of retracting spring, releasing the brake drum of the binding effect. The brakes are thus released.

1.9.5. Advantages, limitations and applications of air brakes:

1.9.5.1. Advantages:

1. The supply of air is unlimited, so the brake system can never run out of its operating fluid, as hydraulic brakes can. Minor leaks do not result in brake failures.

2. Air-line couplings are easier to attach and detach than hydraulic lines; there is no danger of letting air into hydraulic fluid. So air brake circuits of trailers can be attached and removed easily by operators with little training.

3. Air not only serves as a fluid for transmission of force, but also stores potential energy. So it can serve to control the force applied. Air brake systems include an air tank that stores sufficient energy to stop the vehicle if the compressor fails.

4. 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.

5. Air brakes are much more powerful than the ordinary mechanical or hydraulic brakes and that is the reason they are excessively used on heavy vehicles.

6. Apart from braking, the compressed air from the reservoir can be used for tyre inflation, windscreen wipers, horns and many other accessories.

7. Much more powerful than the ordinary mechanical or hydraulic brakes (that is why these are exclusively used in heavy vehicles).

8. Simplified chassis design.

9. The compressed air from reservoir, apart from braking, can be used for tyre inflation, windscreen wipers, horns etc.

1.9.5.2. Limitations:

2. However, air brakes involve relatively more parts. Besides,

3. The air compressor uses a certain amount of the engine power.

4. Also thus decreases fuel economy or mileage of the vehicles.

1.9.5.3. Applications:

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

George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. 

In the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy road vehicles.

Fig 1.22: Typical air brake system - A Layout