Invented by George Westinghouse in the early 1870s, air brakes are an effective method of bringing large, heavily loaded vehicles to a safe stop. Smaller vehicles, such as the family automobile, ordinarily use hydraulically operated brakes to stop. The difference between these two braking systems is the force used to engage the braking mechanism.
Hydraulic brakes use compressed liquid to engage the brakes, while an air brakes use compressed air. Compressed liquid is fine for lighter vehicles, since compressed air would be too powerful, overwhelming the brake system. Pressurized liquid, however, would not be powerful enough to stop heavily laden vehicles, such as the large, 18-wheeled tractor-trailer rigs roaming the roads today. Some of these large trucks weigh close to 40 tons (36,288 kg) when fully loaded with cargo.
Essentially, the typical system operating air brakes in these large trucks consists of a supply system, and a control system. The supply system begins with the air compressor, which draws in air from the atmosphere, compresses the air and sends it to the air drier. The air drier removes moisture and impurities from the air, and sends the compressed air via a network of air valves and air lines to the air tanks, or reservoirs, where it is stored, ready for use. The control system is the process by which the compressed air is sent to all the wheel cylinders, thereby activating the braking mechanisms in each individual wheel.
When the driver steps on the brake pedal, the system of check valves throughout the brake system are activated, and the compressed air in the air tanks is sent through the brake lines to the wheels. The compressed air enters the brake cylinders where it is modulated according to how hard the driver is pressing the brake pedal. Air is increased in the brake cylinder as the driver presses the pedal harder. When the driver releases the air brakes, air bleeds from the brake cylinders, and the wheels can rotate freely.
Most air brakes are drum-type braking mechanisms, though disc-type air brakes are becoming more common with air brake systems. The drum brake mechanism consists of a hollow chamber, or drum, inside of which are two shoes. When the brakes are activated, air is forced into the brake cylinder to rotate a cam which, in turn, presses the two brake shoes against the walls of the brake drum. The friction of shoe against drum brings the vehicle to a stop. Disc brakes work on basically the same principle, with the friction of brake pads against brake rotors bringing the vehicle to a stop.