How an air brake system works in trucks and buses?

  

      Air is a composition of various gases which float all around us. It exerts a tremendous pressure on everything. If this pressure is used in a well-versed manner it could be used for stopping the vehicles. Vehicles use various types of brakes to stop the rotating wheels. Each type of brake uses a different mechanism for stopping the vehicles. Particularly vehicles like cars use a liquid braking system to pull the brake system. But large vehicles such as buses and trucks cannot be easily stopped by the liquid braking system; due to the tremendous mass of the vehicle.

   So in such large vehicles, an air brake system is utilized. In an air brake system, the pressure exerted by the air is converted into frictional energy to stop the vehicles. The basic schematic system of an air brake system employed in the trucks and buses is shown below,

Air brake System in trucks and buses

   As seen in the above image there are several parts in the air brake system to complete the braking system. The main parts in an air brake system are,

• Compressor and the governor
• Control Valve
• Braking Chamber and Brake shoe assembly

Let’s see all of the parts one by one to understand the operation,

Compressor and the governor

    At first, in order to apply brakes to the wheels by applying air, the air must be sucked in from the atmosphere and delivered at a certain pressure to the wheels. To do that, a compressor is used.  A compressor is basically a mechanical part that is used for compressing a medium. An air compressor uses a motor or an engine, to suck the air from the atmosphere and deliver it at high pressure.

  This high-pressure air is then stored in the supply reservoir from which it is fed into the primary and secondary reservoirs. In these reservoirs, the air is maintained in the high pressure itself.

    A governor mechanism is also used with these reservoirs to control the amount of pressure of the air that should be present inside the tank. If the pressure inside the tank is above the nominal value, the governor cuts off the air supply from the compressor, thus safeguarding the air pressure tank from exploding. A safety valve is also provided to decrease the pressure inside the tank in case of an emergency or if the governor fails. Thus by this equipment, high-pressure air is attained to get to supply to the wheels.

Control valve

   The control valve is the most important part of the air brake system. It controls the amount of air to be applied into the drive wheels from the reservoirs at the right time. There are two types of valves used for stopping the vehicles. They are foot valve (also called as treadle valve) and the hand valve. The foot valve is normally used for the service brake and the control valve is used for the parking brake. The service brake is the type of brake which is used to temporarily stop the vehicle while the parking brake is the type of brake which is used to permanently stop the vehicle for a while. A control valve has input ports, output ports, and an exhaust port.

Control Valve

    The input ports of both the foot and hand valve are connected to the two reservoirs from which high-pressure air is delivered. For the foot valve, one output port is connected to the braking chamber of the front wheels and the other is connected to the braking chamber of the rear (back) wheels through the relay as a service line. The purpose of the relay in the rear-drive wheels is to quickly stop the wheel as the front wheel does; because the rear wheels are far away from the reservoir.

   So to stop the rear wheels as soon as the brakes are applied, relays are used. A relay usually needs two lines to operate namely the service line and the supply line. The supply line is the supply of high pressured air to stop the wheels. This supply line is directly fed by a reservoir because applying air from the foot valve to all the wheels takes a little delay to rear wheels. So a reservoir is used for the relay in the rear wheels. The service line is taken from the foot valve just to activate the relay, for supplying air to the wheels from the reservoir. For the hand lever (parking brake), the output ports are connected to the front wheels, and to the rear wheels as a supply line to the relay. The service line is given from the foot valve. The exhaust port is used to exhaust the excess air from the control valve when the operation is finished. So by using the control valve and a relay, a fixed value of air is ready to be applied to the braking chamber of the wheels.

Braking Chamber

   The braking chamber is another crucial part of the air braking system and it is present in all the wheels. The braking chamber usually contains two chambers. One is the service brake chamber and the other one is the parking brake chamber. The service brake chamber is used for the drive wheels during running, in case the vehicle has to stop and run again immediately. The parking brake chamber is used to completely halt a vehicle for a long time, in case the vehicle has to halt for a long time in someplace; so that the vehicle does not move anywhere even it was on a slippery surface. Let’s discuss each of these braking chambers in detail:

Braking Chamber

 

Service brake chamber

   The service brake chamber generally consists of a diaphragm placed in a normal position as shown in the diagram below. When the vehicle has to be stopped immediately, the brake pedal (foot valve) is pressed. When it does, high air pressure from the tank passes into the four chambers of the wheel, through the control valve and the relay. This high pressure air makes the diaphragm in the service brake chamber to compress. The compression of the diaphragm makes the brake assembly to engage.

Service brake operation

   The brake assembly consists of a rod attached to the braking chamber. When the braking chamber activates, a linear movement caused by the rod moves the brush shoe through the slack adjuster mechanism. Thus by this mechanism, the brake shoe comes in contact with the brake wheel, and the wheel comes to a halt position. When the driver releases the brake, the air present in the chamber will flow out through the exhaust port in the control valve and the diaphragm comes back to its original position and the brake shoes are released from the wheels.

  For front wheels, when the driver presses the foot valve, the service brake chamber gets operated directly without any relay function and stops the front wheels. But for the rear-wheel drive, the relay gets power from the supply line from a reservoir and the service line from the foot valve to actuate the braking chamber. So now the service brake chamber gets actuated on all the wheels and the vehicle stops temporarily.

Parking brake chamber

   The parking brake chamber which is attached to the service brake chamber is also the same as the service brake chamber. But unlike the service brake chamber, high pressure air is stored inside the parking brake chamber, thus making the diaphragm to be pulled out and making the slack adjuster to release the brake shoe from the wheel. When a vehicle has to be halted for some time at a place, the parking brake lever is pushed. When it does the high air pressure from the parking brake chamber is taken out and released out through the exhaust port in the control valve, which makes the held diaphragm by the air pressure to come back. This makes the slack adjuster move, thereby making the brake shoe to come in contact with the brake disc and making the frictional effect to stop the vehicle.

Parking brake operation

  So when the parking brake gets activated, the high pressurized air will flow from the reservoir to another relay and the relay gets activated and makes the parking brake chamber to get operated. Thus the vehicle can be stopped.

  The main thing to be considered in the service and the parking brake chamber is that; if the air is supplied from the control valve to the service brake chamber; the air is stored and the brakes are applied. But if the air is applied to the parking brake chamber the stored air is first removed and exhausted from the chamber and then brakes are applied. This arrangement is generally done for the safety of the vehicle, that even though sufficient air pressure is not available to stop the vehicle by foot valve, we can just release the stored air pressure from the parking brake chamber to stop the vehicle.

   Another system particularly used in towing vehicles is the trailer brake system. By this system, the vehicle part which is independent of the main vehicle can be controlled. The trailer brake system also uses the relay for its operation which uses the supply line and the service line from the main system.

Trailer Vehicles