is part of a braking
system used in heavy vehicles such as trucks
and wheeled machinery. It consists of a shaft, usually around 4 to 25 inches long, turned at one end by means of an air-powered brake booster with an 'S' shaped cam at the wheel end. Turning the shaft pushes the brake shoes against the drum
, producing friction
The design allows bulky air cylinders to be located outside of the wheel.