How Do Air Brakes Work?

The air brakes are one of the most important pieces of safety equipment on a commercial truck. This device minimizes the chances of a runaway truck that could cause a tragic accident. Unlike traditional vehicles that rely on hydraulic fluid to control the brake system, air brakes use simple air. Because there is a lower chance of failure if there is no fluid to leak, air brakes are considered more reliable. As an added precaution, air brakes simply lock up and immediately stop the truck during a failure unlike hydraulic breaks that don’t work at all.

Large trucks have air brakes that operate on the triple-valve principle. The air pressure must build up inside the pipes in order to release the brakes. The vast majority of systems include a graduated release system in which a partial build up results in a proportionate release of the brakes. The weird squeaky and hissing noises that are heard coming from a parked truck are a result of the air brakes. As air is released from the brakes, the squeaking noise is made. The hissing occurs as the bypass safety valves maintain the air pressure at the correct level. Air brakes are the reason why trucks must sit for a few minutes when they are first started; the brake pressure must build to the right level.

In 1869, George Westinghouse invented air brakes for the railroad industry, and the basic operation remains the same today. Westinghouse’s triple-valve air brake system includes three basic functions. To begin, the system must be charged. Before the brakes can be released, the system must be brought up to pressure. Next, is the brake application. During this step, air pressure drops and air flows back through the valve into the reservoir tanks. Finally, during the final release step, the pressure is increased to the correct level. If this sounds completely backwards, you’re not alone. With a typical braking system, you stop as you apply more pressure. However, with air brakes, the vehicle stops as pressure is reduced.

Although there is a lot of confusion about air brakes, this system rarely has a complete failure. In addition to the primary set of air brakes, most trucks have another set of emergency air brakes in case of a failure. Some trucks even have an exhaust brake system that is powered by the engine independently of the air system. However, even with multiple backup systems, a complete failure is possible, and maintenance should never be neglected.
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