Many people know that "big rigs" are equipped with air brakes. However, one question that may be on the minds of those unfamiliar with trucking is, "How do air brakes function if air pressure is lost?" The concern is understandable, as the thought of an unstoppable tractor-trailer rolling along at high speeds is frightening.
However, the good news is that this outcome is highly unlikely to occur. Below is more information on how emergency braking is implemented in air brake-equipped vehicles.
Semi trucks and trailers use air-activated brakes for slowing and stopping, as opposed to the hydraulic systems found on light passenger vehicles. Instead of using liquid (ie, brake fluid) to compress brake shoes or pads against drums or rotors, air brakes rely upon the power of compressed air to function.
Whenever a truck driver presses on the service brake pedal in the cab, air is expelled into a brake chamber, and the brake shoe pushes against the drum. For this to function, there must be sufficient air pressure built up inside the system.
Thanks to the genius of an inventor named George Westinghouse, air brakes are all-but-guaranteed to work during an emergency situation when all air is lost. Inside the braking mechanism lies a powerful spring that is held back by pressurized air. If this spring is released, it will forcefully push the brake shoe against the drum and slow the truck until it stops.
This simple, but elegant system works as a fail-safe to prevent loss of control situations. Loss of air pressure due to a leak, compressor failure or other mechanical problem will cause the brakes to activate automatically. The spring brake mechanism also doubles as a parking brake for trucks, as it locks into position whenever the engine is shut down and the air is released from the tanks.