How Air Brakes Work

George Westinghouse and Air-brake History

Air is everywhere. Hydraulic fluid isn't. Trains, buses and tractor-trailers use air-brake systems so they don’t have to rely on the hydraulic fluid in car braking systems, which can run out in the event of a leak. All of these types of transportation are weighed down by heavy passenger or cargo loads, so safety is of the utmost importance. A speeding locomotive that relied on hydraulic brakes would turn into a deadly steel bullet if the brake system suddenly busted a leak.

Before air brakes, trains used a primitive brake system that required an operator, or brakeman, in each car to apply a hand brake at the signal of the train director or engineer. This inefficient manual system was replaced by direct air-brake systems, which used an air compressor to feed air through a brake pipe into air tanks on each car. When the engineer applied these brakes, the pipe filled with air and squeezed the brakes.

In 1869, an engineer named George Westinghouse realized the importance of safety in the relatively new railroad industry and invented the first triple-valve air-brake system for railcar use. Westinghouse’s system worked the opposite way of a direct air-brake system. The triple-valve system performed three functions, thus its name. Let’s take a look at those functions.

  1. Charging: The system must be pressurized with air before the brakes will release. At rest, the brakes remain engaged. Once the system reaches its operating pressure, the brakes are freed and ready to use.
  2. Applying: As the brakes are applied, air pressure decreases. As the amount of air decreases, the valve allows air back into the reservoir tanks, while the brakes move to the applied position.
  3. Releasing: Once the brakes are applied and the air escapes after braking, the increased pressure releases the brakes.

Instead of using force or directed air to apply the brakes much like hydraulic fluid in our cars, the triple-valve system fills a supply tank and uses air pressure to release the brakes. In other words, the brakes in a triple-valve system remain fully engaged until air is pumped throughout the system. Pretty ingenious, considering if this type of system had a complete loss of air, the brakes would engage and stop the train. Think about that when you are zooming down the freeway and you hit the brake pedal. If your car’s brake fluid leaked out, your brakes wouldn’t work.

The triple-valve system is the basic concept at work in today’s air-brake systems in trains, buses and tractor-trailers. Let’s switch gears and learn how air brakes in roadgoing vehicles work in the next section