What will cause an air brake to "cam over" during application?

To understand why worn drums and shoes can cause an air brake to "cam over" during application, it's essential to grasp the mechanics of how brake systems function. In an air brake system, the brakes engage through the pressure applied from the air supply, which pushes the brake shoes against the brake drums to create friction.

When the brake shoes and drums are worn, they lose their effective surface area and ability to generate adequate friction. Since there is less contact area, the brake shoes may not make sufficient contact with the drums during application. This scenario can result in the shoes being unable to maintain proper alignment and can lead to a condition known as "cam over," where the shoes rotate past their optimal position, causing the brakes to fail to apply effectively. Essentially, because of the excessive rotation or poor seating, the brake components cannot adequately decelerate the vehicle, leading to safety risks.

Therefore, the condition of worn drums and shoes is crucial for the effective operation of air brakes, as proper function relies on optimal contact and alignment between brake components.