How air brakes work ?

Air brake system has ability to do high force braking with small input force. It can be done because air brakes generating power from pressurized air to pull the brake shoe.

Then how air brake can do that? this article will show you how air brake system works.

Air brakes is selected to replace hydraulic brake that have less braking force on heavy duty vehicle. So, air brake is one of brake control system, or it is not the brake actuator system. It means, air brake system is a mechanism to activate the brake actuator on its vehicle.


Generally, the actuators that are combined with air brake control system, is drum brake system. The example, the truck. Here we have a set of drum brake system. At the tip of brake shoes, there is a cam. If we rotate this cam, it will push both brake shoes to the outside. So that the brake active.

And if we look more detail, the cam is connected to a push rod. So, to activate the drum brakes, we need to push this pushrod. This is the job of air brakes system. Air brake will high forcedly push the push rod. So, the result, both brake shoes will press to the drum with high force. It will create a high force braking.

For more detail, we start from the engine side. Here we can see an air compressor. Air compressor connected to the engine pulley via a fan belt. So, the compressor, works once engine run. It used to supply the air pressure to the air brake system.


Then from the compressor, there are three channels. An inlet channel and two output channels. Both output channel, use to distribute pressurized air to the air tanks. Inside the air tanks, the pressurized air is stored. Here the source of energy to operating the air brake.

But the air that are stored, certainly have contaminant. Seperti dust and water. We know that water, is uncompressible material. So that, if this water come into the system, it may break the air brake components.

To be done, before the air come into the tanks, pressurized air will be flow to the air dryer. This component will separate air from all contaminants. So that here, the air are free from water. and water will be thrown away from waste channel.

And then we back to compressor. the inlet hose has a connection to the valve mechanism. It called governor. It function to keep the pressure in working pressure and not overpressure. The working pressure of air brake system approximately 100 – 150 PSI.

So when the pressure inside the system over 150 PSI, the valve here will closed. And it will close the air path from outside, but it will open the air path from the air dryer. For more detail, governoor have four channel. An inlet channel coming from outside, an outlet channel go to compressor, a channel to the air tank, and another channel coming from air dryer.

A channel from tank, has a function to control the governoor valve. It located at the top. Inside of this room, there is a membrane wich is if the pressure inside this room get higher it will automatically close the inlet channel.

The maximum pressure is 150 PSI. so if it exceed the pressure, the membrane will move down. It will move the piston that are connected with it.

The result, inlet channel is closed and the channel from air dryer is opened. In this position, air compressor still operating but the air that are compressed is only circulating inside the system. So that it will not increase the pressure anymore.

When the pressure drop under 100 PSI, this membrane back to move up. So that it will slowly open the inlet channel. It will make compressor back to supply the air from outside to the tanks.

Then we move to the cabin area. Here we can see a brake valve. The valve is located under the brake pedal. So when we press the brake pedal, it will push the piston inside of this valve.

So what next ? if we see the opeating inside of brake valve, here the mechanism. There are two inlet hose from the tank, and outlet hose to the next parts. When the pedal is not step on, the piston surface will close all inlet channels.

When we step on the pedal, piston pushed down. And it will open the outlet channel. The interesting thing, the piston position influence the flow of air to the outlet channel. So when we hold the pedal on short range, the channel will open in tiny gap. It will limit the flow. But when we step on the pedal at full, the channel will open in maximum gap. So the flow will be maximum.

This mechanism aim to make the brake has variable of power. So that the driver can operate the brake according to the road conditions.

We go to the next part, we go to relay valve. It function to cut the air path so the air are arrived to the actuator faster. Because when the air has to flow to brake valve, it may occur the delay of braking. With this relay, the air from the tank can be distribute directly to the actuator without flow to the brake valve.

Then how it works ?

When the piston of brake valve is pressed, the air flow from outlet channel brake valve to the signal channel in relay valve. This air flow, will push the valve below of them. The pushed valve will open the main channel that connect air tank and brake chamber.

Brake chamber, is a component to transform air pressure to the mechanical movement. The air from relay valve will push the air chamber inside the brake chamber. So, it will expand the membrane. This membrane connected to pushrod.

So the expanded membrane, will push the push rod.

But before reach the brake chamber, the air from relay valve will flow throgh quick release valve. This valve aim to distribute the air on two side of brake chamber. Another function is to release the brake faster.

So when the braking is finished, the pressure at relay valve will drop. It will close the main channel so the air pressure on brake chamber side is thrown away from bottom of relay valve. But from brake chamber, it need more time to travel air. So, to make the release faster, the air at the brake chamber will be thrown away from quick release valve.

This componen works automatically once the pressure is drop.

So, on air brake system, the amount of air pressure is influence the power of braking. The pressure of this tank, should reach 100 PSI at minimum before the vehicle opeating. That’s why we always see the driver run the engine in approximately 30 minute before the they drive. It aims to full fill the pressure inside this tank to reach 100 PSI.

that’s the animation of air brakes system. Hope add our insight.