Unit 2: Air Brakes in Commercial Vehicles

Navigation: ETAP 623 Fall 2017 Section 7619 | Akshar Shastri Mini-Course
The red and blue hoses between the tractor and the trailer carry air for the trailer brakes. The green wire carries electricity for the trailer lights ("Air Brake Hose," n.d.). Image from Raven Transport.


Air brake assemblies are “one of the most complex braking systems found on any type of vehicle” (“Truck,” 2017). These braking systems are utilized in tractor-trailers, school buses, trains, and fire trucks (“Lessons in School Bus,” 2008; Peters, 2015, pp. 4-5; “Truck,” 2017). Air brakes are the preferred method for “stopping heavier vehicles, but they are more complex than the simple hydraulic brake systems used on lighter vehicles (Peters, 2015, pp. 4-5). Therefore, it is important to understand how they operate and their limitations since people interact with vehicles that use air brake technology every day.

Learning Objectives

1. Students will be able to identify the emergency brake used in all commercial vehicles.
2. Students will be able to explain why air-brakes are used in commercial vehicles as opposed to hydraulic brakes.

Why Are Air Brakes Used In Commercial Vehicles?

Air brakes were invented in 1872 by George Westinghouse for trains. After they were successful in railroad applications, truck manufacturers adopted the systems for roadway use. Air brakes are preferable to hydraulic brake systems for the following reasons for commercial vehicle applications:

1. The supply of air is unlimited, so an air brake system cannot be depleted of the medium it relies on.
2. Compressed air is stored in tanks, so if the compressor fails the brakes can still bring the vehicle to a stop.
3. If there is a significant leak in an air brake system, the fail-safe spring brake will activate and bring the vehicle to a halt. Therefore, if the system is properly maintained it can bring the vehicle to a stop even if there is an unsatisfactory amount of air in the system (Peters, 2015, p. 6). If a hydraulic brake system is depleted of brake fluid, the brakes will not operate.

How Air Brakes Function on School Buses

Please view the video below illustrating the use of air brakes in school buses. https://youtu.be/-R7J9BIjNEw


The air brake chamber (Peters, 2015, p. 10).

The two main components in an air brake system are the compressor and the spring brake chamber. When the air governor detects that the pressure in the system is below 90 pounds per square inch (PSI), it sends a signal to the compressor to fill the storage tanks with the appropriate amount of air. The governor stops the compressor when the pressure in the system “reaches approximately 125 pounds per square inch.” The governor ensures that the pressure in the system is always at a safe level (Peters, 2015, p. 9).

The air is routed to brake chambers located near the wheels through air lines. The video outlines this process in a detailed manner at the four minute and 33 second mark (link). Essentially, the air and the springs in the brake chamber provide an equal force against each other when the system in a neutral position and the vehicle is rolling freely. However, the brakes are applied if the operator uses the foot pedal to introduce air into the brake chamber. The air pressure is greater than the force exerted by the service brake return spring, and thus the brakes are applied to the rear wheels when the pushrod is moved out of the chamber. Similarly, if the air pressure is not sufficient enough to act against the force exerted by the emergency spring brake, the spring brake will be engaged. This ensures that the vehicle will stop if the system loses air pressure (Peters, 2015, p. 9; “Lessons in School Bus,” 2008).

Interacting With Air Brakes

The air brake parking button located on the dashboard of most commercial vehicles. Image from marketplacepulse.com

Given that heavy-duty commercial vehicles are less prevalent than smaller passenger cars, most people will never need to operate air brakes on the vehicles they drive. However, most students will be passengers on a school bus during their educational careers, and most people will share the road with large vehicles such as tractor-trailers and buses that utilize air brakes. Therefore, it is important to know how to operate air brakes in an emergency and be cognizant of the limitations air brakes.

If the driver of a school bus has become incapacitated, the quickest way to stop a school bus is to pull the yellow parking brake button located on the dashboard of the bus. This is similar to using the emergency hand brake in a car. As the video illustrated, the air will rush out of the spring brake chambers, and the bus will grind to a halt. The vehicle will stop suddenly, so it is important that all the occupants are prepared for the force that will be exerted on their bodies. If your state mandates bus safety drills, be sure to ask about this important safety device during the next session!

The other important fact about air brakes is that they have a delay. Hydraulic brakes work instantly, but air brakes “can take up to a half-second for the air to flow through the lines and act against the diaphragms to activate the brakes” (Peters, 2015, p. 14). As noted in the introduction, air brakes are usually utilized on heavy vehicles. The momentum that these vehicles have along with the delay that air brakes inherently possess means that commercial vehicles take longer to stop. Therefore, when driving around these vehicles give the commercial driver a break and do not cut them off on the road!

Review Question

Question: Give one reason why air brakes are used on commercial vehicles.

Answer: As the name implies, air brakes rely on air. Since there is virtually an unlimited supply of air on earth, the brake system will never be depleted of the fluid that it relies on. If the system contains an unsatisfactory amount of air, the spring brakes will automatically deploy. Therefore, if the vehicle is properly maintained, the automobile will be able to stop even if a catastrophic amount of air is lost from the air brake system.

Next Unit

In the next unit, different types of trailers will be examined.
Unit 3: Different Trailer Types & Configurations


Air Brake Hose at Ryder Fleet Products. (n.d.). Retrieved December 17, 2017, from https://www.ryderfleetproducts.com/brake-hose-air-c-9284
Lessons in School Bus Air brakes. (2008, December 18). Retrieved December 17, 2017, from https://www.youtube.com/watch?v=-R7J9BIjNEw&feature=youtu.be
Peters, W. C. (2015). Understanding Air Brakes on Fire Apparatus. Fire Engineering, 168(6), 4.
Truck. (2017). In Encyclopædia Britannica. Retrieved from http://academic.eb.com/levels/collegiate/article/truck/110737#64090.toc