Generally the performance of
hydraulic brake is based on the amount force applied on the brake pedal by the driver. Hence for obtaining high braking force the driver has to press the brake pedal at high force. This causes the driver
more tiresome. To eliminate this difficulty and obtaining maximum braking force by less pressing, other brake systems such as air type, vacuum type and electric type brakes are used. The air brake is discussed
in the following section. Absorbing the ambient air, compressing it to a very high pressure and operating the braking system is called as air brake which is used mostly in all vehicles.
The construction of the normal air brake is shown in Figure. The
brake cam ‘S’ placed on the brake carrier plate is actuated by the pressurized air obtained from the air compressor operated by the engine. Hence the brake shoes are expanded and the wheels are stopped from rotation. Brake chambers are located
nearer to each wheel of the vehicle.
Each chamber is connected to the
reservoir tank through pipelines. When the brake pedal is pressed the brake valve is started to operate. Through the brake valve the pressurized air from the reservoir tank is sent to the brake chamber.
In the brake chamber a slack adjuster is operated with help of diaphragm lever which is connected to it. By pressing the top end of the slack adjuster the bottom end turns the “S” cam and rotates the lever. Hence the brake shoes which are connected with the S cam get expanded and the brake drum is stopped from rotation.
While releasing brake pedal
While releasing the brake pedal,
exhaust valve in the brake valve is opened. Hence the pressurized air in the brake chamber is forced through exhaust valve from the same way as the air enters to the chamber. At that time, the diaphragm inside the brake chamber is pushed in backward direction with the help of the spring force. At the same time slack adjuster and “S” cam are moved back to their original position. Thus brake shoes are pulled inward and released from the brake drum.
Parts of air brake
1. Air cleaner
2. Air compressor
3. Unloader valve
4. Air tank
5. Pressure gauge
6. Relief valve
7. Brake valve and brake pedal
8. Brake light switch
9. Brake chamber and
10. Slack adjuster
The device used to produce compressed air which is required for operating the air brake is the air compressor. It is a reciprocating type air compressor. It works on the principle that when the piston moves downwards inside the cylinder, the suction port is opened and through this way the atmospheric air enteres in to the cylinder. When thenpiston moves upward, exhaust valve is
opened and through the exhaust port the compressed air is sent to the reservoirntank.
Governor or unloader valve
This valve controls the compressed
air being stored in excess pressure in the air storage tank. Normally the air pressure should be maintained at 7.5 kg/cm2 in the reservoir tank. When the pressure exceeds above this limit, the unloader valve opens automatically and releases thecompressed air to the atmosphere. When the pressure of the air in the reservoir reaches to 6.5 kg/cm2, the governor valve automatically closes.
Reservoir tank is used for storing amount of air required for operating the brake system. It is made up of steel plate. There are provision given for air inlet and draining the impurities from the tank. In addition to these arrangement a safety valve is also located at the top of the tank for maintaining the air pressure.
This valve is fitted below the brake
pedal. When the driver presses the brake pedal, the piston in the brake valve is moved downward. It connects the compressed air with the brake chambers. Therefore the compressed air is sent to the brake chamber located at the four wheels.
Near to the each wheels, one brake
chamber is located and connected with the wheels. The brake chamber is made up of steel plate. The brake chambers are separated by the diaphragm made up of rubber. One chamber is connected to the pressurized air coming from the
brake valve and the another chamber is attached to the brake cam that operates the brake through the diaphragm. The diaphragm element is compressed by a spring and the spring is located in the brake chamber. While pressing the brake
pedal, the compressed air against the
spring force moves the diaphragm and tends to expand the brake shoes.
The slack adjuster is attached at the end of the diaphragm lever coming out from the brake chamber. The bottom edge of the slake adjuster is attached to the S cam which is used to operate the brake system. Therefore due to the compressed air diaphragm is moved forward and allowed to rotate the S cam that expands
the brake shoes which actuate the brake.
Advantages of air brake system
1. As compared to mechanical and
hydraulic brakes it has more power
and efficiency. Hence it is mostly used
in heavy vehicles.
2. The parts of the brake system can be fitted to anywhere in the vehicle. So the chassis structure becomes simpler.
3. Compressed air in the reservoir tank can be also used to operate air horn,screen wiper and to fill air for tyres.
4. The driver does not feel tiresome by
operating the air brake because of its
5. It has low maintenance cost.
6. The air brake system is widely used
in heavy vehicle because it response
is faster than any other systems and it