Wikipedia
S-cam
An S-cam 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 and lever 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.