The air brake system, or the compressed air brake, is a kind of friction brake for vehicles. Here, compressed air presses on a piston, applying pressure to the brake pad or brake shoe — to stop the vehicle.
Air brakes are usually used on heavy trucks and buses, and the normal operating pressure is approximately 100–120 psi (690–830 kPa or 6.9–8.3 bar).
But why do we need air brakes?
One of the primary reasons is safety.
Heavy load vehicles often deal with transporting tens of thousands of people and goods, making safety a top priority.
The air brake system uses compressed air pressure that’s plentiful to keep people safe (as opposed to hydraulic brakes that lose braking power if they leak.) That’s why heavy vehicles use this braking system to generate the necessary braking force.
This Article Contains:
- How Does A Compressed Air Brake System Work?
- 6 Key Air Brake Components You Should Know About
- 4 Key Advantages Of An Air Brake System
- 3 Key Disadvantages Of An Air Brake System
- How To Maintain An Air Brake System Properly?
How Does A Compressed Air Brake System Work?
Air brakes use compressed air instead of hydraulic fluid. This is unlike your car’s hydraulic brake system, which is prone to leaking because of the brake fluid. Brakes in an air brake system can be either drum brakes or disc brakes, or a combination of both.
Let’s go over the inner workings of the air brake:
- An engine-mounted compressor pressurizes air. The air compressor then pumps the air into the storage tanks, which store the compressed air until it’s needed.
- When a driver engages the brake pedal, the compressed air from the reservoir passes through the brake line.
- This compressed air in the brake lines gets transferred to the brake cylinder of the air system.
- The air pressure on the piston inside the brake cylinder helps apply the service brake and release the parking brake.
Note that the parking brake engages by spring force in the spring brake chamber when the air pressure is released. This allows the parking brake to also operate as the emergency brake system.
Now, let’s look at the components that help an air brake system function.
6 Key Air Brake Components You Should Know About
A compressed air brake system can be divided into supply and control systems.
The supply system helps compress, store, and supply high pressure air to the control system.
The control system consists of service brakes, parking brakes, a control pedal, and an air storage tank.
Here’s a closer look at each of these key components in the air system:
1. Air Compressor
The air compressor helps pump air into the air storage tanks or reservoirs. It’s connected to the vehicle’s engine through gears or a v-belt.
The air compressor may be air-cooled or cooled by the engine cooling system.
2. Air Compressor Governor
The governor manages when the air compressor pumps air into the storage tanks.
When air tank pressure rises to the “cut-out” level (that’s around 125 pounds per square inch or “psi”), the governor stops the compressor. And when the tank pressure falls to the “cut-in” pressure (around 100 psi), the governor allows the compressor to start pumping again.
3. Air Tank And Air Tank Drains
Air storage tanks (a supply reservoir or wet tank) hold compressed air.
Compressed air usually contains some moisture and oil traces, which are bad for the air brake system and can cause brake failure. So, each air tank has a drain valve or purge valve at the bottom to drain them out regularly.
A heavy vehicle or truck is also equipped with a relay valve. The relay valve is an air-operated brake valve that remotely controls the brakes at the rear of a heavy commercial vehicle.
4. Brake Pedal
The brakes are engaged by pushing down on the brake pedal (the treadle valve or foot valve.) Pushing the pedal harder will apply more air pressure.
5. Foundation Brakes
Foundation brakes are used at each wheel. All the brakes — the service brake, the parking brake, and the emergency brake — use the same foundation brake system in a motor vehicle.
Here are the key types:
A. S-Cam Drum Brakes
S-Cam brakes are a type of drum brakes that are located on each end of the vehicle’s axles. The wheels are bolted to the drums. To stop the vehicle, the brake shoe and lining are pushed against the inside of the drum.
B. Wedge Brakes
In this type of drum brake, the brake chamber push rod pushes a wedge between the ends of two brake shoes. This pulls them apart and against the inside of the brake drum.
C. Disc Brakes
Air-operated disc brakes see the air pressure acting on a brake chamber, like s-cam brakes.
Here, a power screw clamps the disc or rotor between the brake pads of a caliper.
6. Spring Brakes
A heavy vehicle must be equipped with parking brakes and emergency brakes. These vehicles are held on by mechanical force because air pressure can eventually leak away.
This is where spring brakes come in. When driving, these powerful springs are held back by air pressure. A parking brake control lets the driver remove the air pressure and release the springs.
By the same principle, a leak in the air brake system will also cause the springs to release onto the brakes — which is a rather ingenious design, as losing air means the emergency brakes engage.
So what are the positive aspects of an air brake system?
4 Key Advantages Of An Air Brake System
Here are four main advantages of an air brake system:
- Air supply is unlimited. This means that an air brake system can never run out of its operating fluid. This is unlike hydraulic brakes, which are prone to fluid leaks.
- Air line couplings or fittings that connect air tools to compressed air are easier to attach and detach when compared to hydraulic lines.
- Air not only serves as a fluid for the transmission of force but also stores potential energy as it is compressed. So it can control the force applied.
- An air braking system includes an air tank that holds sufficient energy to stop a heavy vehicle if the compressor fails. So, an air brake system is designed with enough fail-safe capacity to stop a truck safely, even when leaking.
Next, let’s look at the limitations of an air brake system.
3 Key Disadvantages Of An Air Brake System
Air brakes do have some drawbacks. Here’s a closer look:
- Air brakes generally cost more. Since air brake systems compress air, they produce moisture that needs to be removed with an air dryer. This increases the maintenance cost.
- A defective air dryer can lead to ice in the air brake system in cold locations.
- In the US, commercial drivers must obtain additional training to legally drive any vehicle using an air brake system.
Now that we’ve looked at the pros and cons of an air brake system, let’s go through how you can maintain the air brakes on your motor vehicle to avoid damage.
How To Maintain An Air Brake System Properly?
Regularly checking your air brake systems can help you avoid brake fade.
Ensure you’re checking for:
- Brake valve failure in the primary or secondary circuit
- Dirt and debris between the lining and braking surface
- Fractures and leaks
- Broken or weak springs
- Parking brake lines for leaks in each chamber
Understanding the working of an air brake system and its main components is key. This is helpful for preventive maintenance and the overall safety of your fleet.
And if you’re facing any trouble with your brakes, contact AutoNation Mobile Service!
AutoNation Mobile Service is a convenient mobile auto repair and maintenance solution that you can book online. We offer upfront pricing and a 12-Month, 12,000-Mile warranty on all our repairs.
Contact us, and our mechanics will drop by to fix any issue with your brake system right in your driveway!