How Air Brakes Work

Written by quinten plummer
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Storage tanks hold compressed air accumulated from the air brake system's air compressors. The amount of compressed air stored in the air storage tanks is regulated by the compressor's governor. The compressor's governor keeps the tank's accumulated pounds per square inch (PSI) from rising too high or dropping too low--turning the compressor on or off as needed. As compressed air will inevitably accumulate water, and oil which is used to lubricate the system, the air tanks are equipped with release valves to drain the liquid--moisture in the air tanks can freeze in cold weather, causing the air brake system to malfunction--it is essential that moisture inside the air tanks be kept to a minimum.

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Foundation Brakes

The foundation brakes--secured to each of the truck's axles and contained inside of a brake drum--use compressed air from the air storage tank to force the brake shoes inside the brake drum against the walls of the drum. Friction from the brake shoes, being pressed against the walls of the break drum, slows and eventually stops the revolution of the truck's axles--causing the wheels to stop turning and the truck to stop moving. Foundation breaks come in three different varieties."S-cam" breaks use an S-shaped mechanism that rotates and pushes the brake shoes against the walls of the drum when air pressure is applied, while "disc breaks" substitute a "power screw" for the S-cam--"wedge" brakes press the breaks shoes against the drum walls using a push rod as a wedge.

Safety Mechanisms

Although pressure in the air brake system's tanks are regulated by a governor, each tank is also fitted with pressure gauges that gauge each tank's PSI level--pressure gauges let the driver know how much pressure is left in the air tank(s) and gives him an idea about the truck's current braking abilities. When pressure in the air tank(s) is low, the driver receives a warning from the "low pressure indicator." When the air brakes are applied, air pressure not only engages the air brakes, it also triggers the electronic brake lights to alert other motorist of the driver's intentions. Spring breaks are another safety measure implemented in an air brake system. Springs coiled back by air pressure will engage the emergency spring breaks if the air tank's pressure drops too low--spring brakes are also released to park the truck.

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