Technology explained

How do air brakes actually work?

(Bild: Knorr-Bremse)

Trucks have different brakes to cars. Everyone can hear this when a large truck stops next to them: The loud hissing sound is produced by the release of compressed air, which allows the brakes to apply forcefully instead of hydraulic fluid.

Air brakes are not mandatory for vehicles, but they are now the norm for trucks over 7.5 tons. The classic hydraulic brake would quickly reach its limits with such large and heavy vehicles - it would need extremely large brake boosters and vast amounts of hydraulic fluid. And the same again for every trailer.

The compressed air as the transmission medium for the pedal command, on the other hand, can be fed relatively easily from the towing vehicle to the trailer via a hose. Moreover, a small leak in the system does not immediately have catastrophic consequences: Compressed air is constantly regenerated, brake fluid is finite.

The functional principle
The basic mechanics of the pneumatic brake are initially largely similar to those of the hydraulic brake. When the trucker presses the brake pedal, the compressed air previously generated by an air compressor generator is fed to the wheel brakes via hoses and valves. There it presses the brake pads onto the discs via pistons or diaphragms. When the driver releases the pedal, this pressure escapes and causes the loud hissing sound that you hear, for example, when a truck brakes next to you at the end of a traffic jam. Or has braked.

The same, only in reverse
The hissing also occurs when the truck driver activates the parking brake. A second braking system is then used, which comes into play when the first one fails - the so-called spring-loaded brake. This brake works in reverse, so to speak, and is triggered when the pressure in the system drops. For example, if the engine, and therefore the air compressor, stops or a trailer comes loose.

The so-called spring accumulator then loses pressure, a spring is released and closes the brake. This system is also the reason why trucks cannot drive off immediately after a long period of downtime: First, the compressed air generator has to provide enough pressure in the system again so that the spring accumulator is filled and the brake is released. Fire trucks and other emergency vehicles are therefore always connected to a compressed air line in the depot.

The ingenious compressed air brake system not only acts on the towing vehicle, but also on the semi-trailers and trailers. This is ensured by the two spiral hoses, which can often be seen from the outside. The red hose conducts the compressed air for braking and for opening the spring accumulator, while the yellow hose transmits the control signals from the brake pedal.

The driver regulates the braking force via the pedal travel, not via foot force as in a car. The truck brakes are very powerful: a 40-ton truck has a braking distance that is only a good 50 percent longer than that of a two-ton car.