Air Brakes

Various ways have been found to utilize the air we breathe for our work. Pneumatic drills use air to drill and break the concrete. Heavy loads are supported on air filled rubber tires. Air is also used to provide smooth and comfortable rides in the form of suspension. One such application of air is in the field of brakes, where compressed air is used to arrest the motion of the tires.

Who Invented Air Brake?

George Westinghouse patented a safer air brake on March 5, 1872. These air brakes were first used in the railways. Later, in the early 20^th century, vehicle manufacturers started using air brakes in trucks and other heavy load carrying vehicles. In modern days, you will find air brakes in most of the buses, trucks, trailers and semi-trailers.

Air Brakes Components:

Air brake system uses driver’s control to transmit air pressure to the service brakes. The components of the air brake system are as follows:

·         Air Compressor: It pumps the air to the storage tank or reservoir. The compressor is run by the engine crankshaft through gears or belt drive. The compressor is either air cooled or oil cooled.

·         Governor: The governor controls the pumping of air from the compressor by monitoring the pressure in the air tank. If the air tank reaches the cut-in pressure level (5.85 bar), the governor allows the compressor to pump the air. On the other hand, if the air tank reaches the cut-out pressure (9.3 bar), the governor stops the compressor from pumping air.

·         Air Dryer: Air dryers are installed before the reservoir tank to absorb the moisture from the air. Moisture build up in the tank can lead to brake failures.

·         One-Way Check Valve: It allows the air to flow in one direction only. It is located between the air compressor and the 1^st tank. It prevents the air from going out if the compressor develops a leak.

·         Air Storage Tank: It stores the compressed air pumped by the compressor. The tanks store enough air for braking even if the compressor has stopped working. There is a possibility of moisture and oil entering the tank. Water can cause brake failures, especially in winter where water gets converted into ice. Usually they provide two tanks. First the air is supplied to Wet tank, where the moisture and oil is drained. The moisture free air is then supplied to the Dry tank.

·         Drain Valve: To overcome the problem of moisture and oil getting accumulated in the tank, drain valves are provided at the bottom to drain the water and oil. It usually drains automatically.

·         Safety Valve: Safety valves are imperative in order to make sure that pressure doesn’t build up beyond a point of danger. The valve usually opens if the pressure crosses 10 bar. You would have often heard a loud hissing noise coming out of a truck. The release of pressure through the safety valves causes such noise.

·         Brake Pedal (Foot Valve): Brake pedal or foot valve allows the air pressure from the tank to be applied to the brake chambers. When we press on the brake pedal, the foot valve opens in the storage tank. Higher the force applied on the brake pedal, greater will be the air pressure applied to the brake chambers.

·         Drum Brake: Let’s consider a drum brake in use. When the brake pedal is pressed, the compressed air is supplied to the brake chamber. It is a cylindrical container that pushes the push rod out when pressure is applied. The push rod then moves the slack adjuster, thereby twisting the S-cam. The twisting of S-cam forces the brake shoes to move away from each other and press against the inside of the brake drum.

Alcohol Evaporators:

Some vehicles equipped with air brakes have alcohol evaporators that release alcohol in the storage tank to prevent the formation of ice in the tank.

Emergency Brakes and Parking Brakes:

All vehicles equipped with air brakes must mandatorily have emergency brake and parking brake. Usually spring brakes are used for this purpose. When you are driving, the spring brakes are held back by air pressure. If the air pressure is removed, spring brakes will apply the brakes. When using the parking brake, air pressure is removed off the spring brakes so that spring brakes can apply brake.

The spring brakes will also fully come on if the air pressure falls down due to leakage. Spring brakes will come on if the pressure falls in the range of 1.5 bar to 3 bar.  

Anti-lock Braking System (ABS):

All truck tractors manufactured after March 1, 1997 in US with air brakes are equipped with ABS and also other heavy load vehicles such as trucks, buses have been equipped with ABS after March 1, 1998. ABS is a safety device that prevents the wheels from locking up when heavy brakes are applied.

Related Topics:

• Disc Brakes

• Drum Brakes

• Anti-lock Braking System (ABS)

• Traction Control System (TCS)

• Electronic Brake force Distribution (EBD)