Summary
Air brake systems are essential for the safety of commercial vehicles, like trucks and buses.
These braking systems offer the stopping power needed for their massive size and weight.
While they share similar braking principles with passenger vehicles, the primary difference lies in their power source: compressed air for commercial vehicles versus hydraulic pressure for passenger cars.
Air brakes consist of three main functions:
1) service brakes (brake pedal) for everyday use,
2) parking brakes to hold vehicles stationary,
3) and emergency brakes that activate automatically during catastrophic air pressure loss.
Power sources for air brake equipped vehicles include compressed air stored in large tanks and large, powerful springs.
These large, powerful springs primarily engage the parking brakes.
In the event of catastrophic pressure loss, the large, powerful springs engage the emergency brakes.
This fail-safe ensures safety when air pressure is lost.
Compared to hydraulic brakes, air brakes provide greater force and reliability, meeting the demands of heavy-duty vehicles.
The integration of fail-safes and robust functionality makes them indispensable for safe commercial transportation.
Rick August, PhD
Working as as Expert Witness, Dr. August provides testimony in crash analysis, traffic safety, road user behavior, and driver education. His expertise is in defensive driving, commercial vehicles (semi-trucks & buses), duty of care, and air brakes.
Overview of Air Brake Systems
Air brake systems are a cornerstone of modern commercial vehicle safety, used in trucks, buses, and other heavy-duty vehicles.
The overall principle of air brake systems, however, isn’t all that different from the brakes found in passenger vehicles.
Owing to the increased weight and size of commercial vehicles, only the brake’s power sources and components’ size are different.
Most drivers learning air brake information to earn their endorsement are confused by the term “spring brakes.” For the novice, this ‘old‘ labelling leads them to erroneously believe there is another braking system.
Spring brakes is a term from the 1970s to designate the parking/emergency brakes.
In this blog post, we will explore how air brake systems work, the reasons for their application. Also, hydraulic systems used in passenger vehicles will be used as a starting point. Using hydraulic brakes are your car will illustrate that braking principles are more-or-less the same..
Unlike hydraulic braking systems, air brake system can transmit force over distance (ie trains), tolerate minor leaks, and have proven reliable for more than a century.
Brakes on Your Car (Hydraulic Brakes)
The easiest way to grasp the fundamentals of brakes is three, two, one.
There are three reasons for brakes to apply:
1) service brakes (the brake pedal),
2) parking,
3) and emergency.
There are two power sources:
1) hydraulic pressure,
2) You pulling a lever or pushing a pedal (newer cars have electric parking brakes).
And finally, there is only one braking system at each wheel.
Except the power sources, the fundamentals of a vehicle’s brakes are the same on both your car or on a big semi-truck.
Three Reasons Brakes are Applied on Any Vehicle
The application of brakes on a commercial vehicle is for one of three reasons, similar to the braking functions on passenger vehicles.
These are:
1. Service Brakes (brake pedal),
2. Parking Brakes, and
3. Emergency Brakes.
The underlying principle of the service brakes (brake pedal) in commercial vehicles is more-or-less the same as those in passenger vehicles: to slow or stop the vehicle under normal driving conditions.
1. Service Brakes
Service brakes are used for everyday braking needs.
These brakes are engaged when the driver presses the brake pedal.
In passenger vehicles, pressing the brake pedal activates hydraulic pressure to stop the vehicle.
In commercial vehicles, air brake systems use compressed air to apply pressure to the brake mechanisms at each wheel.
When the driver presses the brake pedal in a commercial vehicle, a valve is opened to allow compressed air from the air tank to flow into the brake chambers at the wheels.
This pressurized air pushes a diaphragm (drum brakes) or piston (disc brakes) that activates the brakes and slows the vehicle.
In addition to applying the service brakes, the air pressure in the system is maintained to keep the parking and emergency brakes disengaged.
In other words, for the service brakes to work, the large, powerful springs that activate the parking/emergency brakes must be off.
Like your car, if the parking brake is engaged, the vehicle shouldn’t move, and the service brakes (brake pedal) won’t work.
2. Parking Brakes
Parking brakes are used to keep the vehicle stationary indefinitely when it is parked.
In other words, the parking brake hold the vehicle in the same place, so after you come back to get your vehicle…it’s still in the place where you left it.
In passenger vehicles, parking brakes are usually mechanical and can be manually engaged by the driver, often via a lever or a pedal.
In modern cars, electric parking brakes are becoming more common, where the system uses electricity to activate the brake mechanism.
For commercial vehicles equipped with air brakes, the parking brake system works differently.
When the vehicle is parked, the driver will engage the parking brake, which releases air pressure from the system.
This reduction in air pressure causes the large, powerful springs in the brake system to expand, which applies the brakes.
The parking brakes will only release when air pressure is sent into the spring brake chambers compressing the large powerful springs.
These large, powerful springs apply the brakes to keep the vehicle stationary, even if the vehicle is on a slope.
The system is designed this way to prevent the vehicle from rolling away, even if there is air pressure loss in the air brake system itself.
3. Emergency Brakes
Emergency brakes are activated in situations where the air brake system experiences a catastrophic loss of air pressure.
This is a critical safety feature that ensures the vehicle will be able to stop if there is a radical drop in air pressure.
In most systems, the emergency brakes will apply automatically when system pressure is between 20 and 45 pounds per square inch (psi).
In the event of a sudden loss of air pressure, such as a major leak in the system, the air pressure that normally keeps the parking brake off becomes insufficient.
When this happens, the large, powerful springs in the brake system will automatically engage the parking brakes, which then act as emergency brakes.
This automatic application of the parking brakes ensures that the vehicle will stop and remain stationary, even if the service brakes are no longer functional due to air loss.
The ability of the parking brake to automatically engage in an emergency situation provides a backup.
This fail-safe ensures that commercial vehicles are not left in dangerous situations without a means to stop.
On a passenger vehicle, the parking brakes too can be used as emergency brakes.
In the event that the brake pedal stops working, the parking brake can be used to bring the vehicle to a stop.
The difference between a passenger and a CDL vehicle, is that the emergency brakes are applied automatically in an air brake equipped unit.
The basic principle of all braking systems are the same: service brakes (brake pedal), the parking brake, and the parking brake being used to bring the vehicle to a stop in the event of an emergency. Thus, emergency brake.
The Role of Compressed Air and Large, Powerful Springs in Air Brake Systems
The operation of air brake systems depends on two primary power sources:
- compressed air
- and large, powerful springs.
Compressed Air
The most essential component of an air brake system is the compressed air itself.
Compressed air is stored in large tanks, and when the driver applies the brake pedal, air pressure is released into the brake chambers at each wheel.
The pressurized air is used to apply the brake shoes to the drums or the brake pads to the discs, depending on the vehicle’s brake configuration.
The pressure in the system is maintained to ensure that the brakes function when needed and that the parking brakes remain off unless manually engaged.
Large, Powerful Springs
These springs are crucial for both parking and emergency braking.
These large, powerful springs can exert more than 1,800 psi.
In the event of air pressure loss, the springs automatically expand and apply the parking brake.
This mechanical action ensures the vehicle comes to a stop, even if the supply of compressed air is lost.
The springs are designed to hold the vehicle in place indefinitely, preventing it from rolling away when parked.
The combination of air and springs allows air brake systems to be incredibly reliable.
These are capable of providing large amounts of braking force over distance.
Something that hydraulic systems can’t accomplish.
Differences Between Air Brakes and Passenger Vehicle Brakes
The fundamental difference between the braking systems in passenger vehicles and those in commercial vehicles lies in their power source.
1. Power Source:
As mentioned earlier, passenger vehicles rely on hydraulic pressure to apply brakes, whereas commercial vehicles use compressed air.
The hydraulic brake systems in passenger vehicles are generally simpler.
Hydraulic systems have fewer moving parts.
Using brake fluid to create hydraulic pressure and apply pressure to the brakes.
On the other hand, air brake systems in commercial vehicles rely on compressed air that is stored in large tanks.
This compressed air in combination with levers allows for greater braking force.
2. Braking Force:
The sheer size and weight of both trains and commercial vehicles necessitates a braking system that can generate significantly more force than that required for a passenger vehicle.
Air brakes are capable of handling the greater demands of heavy vehicles, ensuring that these can stop safely and effectively even when fully loaded.
3. Emergency Features:
One of the key differences is the emergency brake system.
In passenger vehicles, emergency brakes—vis-à-vis the parking brake—are manually engaged by the driver, usually as a backup system if the service brakes fail.
In commercial vehicles, however, the parking brake also serves as an emergency brake, automatically engaging in the event of a overwhelming loss of air pressure.
This ensures that the vehicle can still stop even if the primary brake system fails.
4. Placement of Brakes:
In a commercial vehicle, parking and emergency brakes are always located on the rear axles of the truck and trailer(s).
This placement ensures that the brakes do not interfere with the vehicle’s steering capabilities during emergency braking situations.
The power of air brakes is essential for commercial vehicles because of their size and weight.
These vehicles require more stopping power than a typical passenger car, and air brakes are specifically designed to provide that force consistently and safely.
Conclusion
Air brake systems are a vital component in ensuring the safety and control of commercial vehicles.
With three distinct brake functions—service brakes, parking brakes, and emergency brakes—air brake systems provide the necessary stopping power and backup mechanisms to keep heavy-duty vehicles safe on the road.
The use of compressed air and large, powerful springs offers both reliability and efficiency, making air brakes the preferred choice for trucks, buses, and other large vehicles.
Understanding how air brakes function not only highlights the complexity of these systems but also underscores their importance in the world of commercial transportation.