What does the S-cam do in the air brake system?

The S-cam is a crucial component in the air brake system, specifically within drum brake assemblies. Its primary function is to force the brake shoes against the brake drum when air pressure is applied. When the driver activates the brakes, compressed air enters the brake chamber, which then pushes the diaphragm. This movement rotates the S-cam, causing its arms to spread and push the brake shoes outward against the interior surface of the brake drum.

This configuration enables effective braking by increasing the surface area in contact and generating friction, which ultimately slows down or stops the vehicle. The S-cam design is notable because its shape and motion provide a mechanical advantage, allowing for a powerful and efficient braking action. Understanding this process is essential for CDL candidates, as it underscores the importance of the S-cam in ensuring safe and responsive braking performance in commercial vehicles.