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Understanding How Air Brakes Work On A Commercial Motor Vehicle (CMV)

July 25, 2023

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Originally published February 15, 2017.

Air brakes are the most important component of modern commercial motor vehicles (CMVs). The air brake system on a semi-truck uses compressed air to push the brake shoe lining onto the brake drum creating friction to slow the wheel down.

That may sound complicated--because it is. When slowing down a vehicle that can weigh 80,000 pounds, society is okay with complicated, so long as the air brakes work effectively, the driver knows how to use them, and the company that owns and services the truck knows what to look for to adequately maintain the air brake system.

The 4 Steps of Air Brake Functionality

To ensure air brakes are operating safely, you must first understand how the air brake operates. The following is a basic explanation of how the air brake system works on a commercial motor vehicle.

The air is pumped through the air brake lines where it eventually reaches the air brake. Notice the gold rod on backside of brake which the air pressure pushes out.
The air is pumped through the air brake lines where it eventually reaches the air brake. The black brake shoe lining creates the friction and slow-down effect depending on how much air is forced into the brake drum.
  1. The air compressor pumps air into the air storage tanks, or reservoirs, that are stored beneath the truck.
  2. That air is then pumped through the air brake lines where it eventually reaches the air brake (pictured here). On most typical “S-Cam” brake systems, when you push the brake pedal on a commercial motor vehicle, air pressure pushes a rod out (see the gold rod on the back side of the brake in the photograph), moving the slack adjuster. The slack adjuster is used to essentially calibrate the brake system and ensure that the internal spring mechanism is working appropriately (aka the spring is not fully extended, which would not create the appropriate friction needed to slow the vehicle).
  3. Air flows through the nozzle to the air brake chamber (the black/metal cylinder with gold rings around it), which then causes the spring to move the “S-cam.”
  4. The “S-cam” forces the brake shoe linings away from one another and presses them inside the brake drum. This creates the friction and pressure needed to slow the wheel down. Similarly, when you release your foot from depressing the brake pedal, the “S-cam” rotates back, causing the spring to pull those same brake shoes away from the brake drum eliminating the friction and slow-down effect.

The brake shoes, like a brake pad, wear over time, especially with added pressure from depressing the brake pedal too often or too heavily. Safe practices require the commercial driver and company that operates a CMV to maintain their brake shoes at a 1/4-inch minimum thickness.

Since both the brake shoe and the brake drum deteriorate over time and with excess heat/wear, it is essential that truck drivers and the companies they drive for are consistently and regularly checking the air brake system to ensure that there are no unsafe or malfunctioning components.

When trucking companies fail to maintain any critical part of the vehicles in their fleet, especially the brake system, they can be held liable for crashes. Victims of truck crashes can file a claim against the trucking company for the cost of their vehicle repairs, medical expenses, lost wages, and more.

Contact Our Ohio Truck Accident Lawyers After a Crash

As powerful and impressive as air brakes are, they aren’t infallible. When air brakes aren’t regularly serviced or replaced, they can fail, putting truck drivers and the people near them on the road at risk of serious injuries.

In addition, air brakes are only as effective as the drivers using them. When drivers are distracted, fatigued, or impaired and they brake too late, air brakes aren’t enough to slow down their trucks and prevent accidents. This is an example of negligence, and negligent drivers can be held liable for crashes they caused via injury lawsuits.

If you or someone you love was injured in a truck accident, the Ohio truck accident attorneys at Nurenberg, Paris, Heller & McCarthy want to help. We have decades of experience building successful truck accident claims for victims like you, and we know what it takes to win. Contact our legal team anytime for a free case review.

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