Why Semi-Trucks Use Air Brakes

Posted by Jarrett on Jul 27, 2021 10:30:00 AM



A fully loaded tractor trailer can weigh tens of thousands of pounds, and basic physics teaches us that as the mass (weight) of a moving object increases, the force required to stop it also increases. That’s why the braking system on a semi-truck and trailer are one of its most important components. The braking system used on semi-trucks is the air brake system.


How air brakes on a truck work

First, a short explanation of how hydraulic brakes work. When an automobile driver presses their foot on the brake pedal, a lever pushes a piston into the master cylinder, which is filled with hydraulic fluid. The hydraulic brake fluid is forced through the brake lines, and pressure is transmitted to all 4 brakes. The hydraulic braking system multiplies the force of your foot on the brake pedal so that it applies enough force to apply the brakes and make the car slow and stop.


Air brakes on a semi-truck work using compressed air instead of hydraulic brake fluid. Since semi-trucks are carrying so much weight, they rely on air because compressed air can be constantly produced, unlike hydraulic fluid, which requires refills and can leak, causing the brakes to fail.


Who invented air brakes?

George Westinghouse, who was an engineer at the time (and went on to found Westinghouse Electric and introduced the standard of AC electric still used today), invented the triple-valve air brake system for use in the railroad industry.


Before air brakes, trains required that a brakeman in each car apply a hand brake to slow or stop a train. Soon came the direct air brake, which used compressed air to apply the brakes. The Westinghouse system works the opposite of a direct air brake system and uses air pressure to release the brakes. So, the brakes remain engaged until air is pumped throughout the system, which means that if the system lost air pressure, the brakes would engage and stop the train.


The triple-valve system is the basic concept at work in today’s truck air brake systems.


Because they’re a critical component of semi-trucks, truck technicians need to know how to troubleshoot, service, and maintain air brake systems.


Trained diesel techs will check for water in the air brake system, which is a byproduct of the condensed air. Water is not good for air brake lines, especially in cold climates where the water can freeze and prevent air from reaching the brake mechanism and cause the brakes to lock up.


Air couplers can also be trouble spots. Old and worn rubber seals cause air to escape, and even though a compressor can compensate for a small leak, overtaxing a compressor can lead to failure. If the air leak is large, you’ll be stuck because the brakes will lock.


Another issue with air brakes is brake sensitivity, which can lead to accidents. Have you ever seen dual skids marks on a highway? Since air brake systems are designed to work on trucks with a heavy load, chances are good that an empty trailer locked its wheels because of brake sensitivity.


When you need truck air brake service and are near I-71, I-76, I-77, I-80, or I-271, Jarrett Fleet Services can help keep you on the road and your brakes functional and safe. We’re at 8860 Wooster Pike in Seville, OH, or if you need 24/7 roadside repair, give us a call at 330-925-5339.


Topics: Fleet


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