Stopping a battleship - Correct Brake Adjustment is Critical
Correct brake adjustment is critical in keeping the brake system operating in a fail-safe manner.
The brake chamber strokes, and through the linkage, mechanically moves the brake shoes against the brake drum. Typically, the maximum stroke is two to three inches, depending on the size of the brake chamber. As the brake shoes wear, the shoe/drum clearance continues to increase. Excessive clearance (brakes out of adjustment) will cause the chamber(s) to stroke near their maximum before the shoes contact the drum. With the brake chambers near maximum stroke, as the brake drum heats and expands, the shoes follow the expansion. Due to the mechanical advantage (ratio) between the brake chamber rod and shoe movement, very little drum growth will cause the chamber rod to move sufficiently to become fully stroked! Braking action on that wheel will be greatly reduced!
Once this concept is understood, it is easy to grasp that descending a grade with brake chambers at near maximum stroke spells pending disaster.
Air Brake Basics 101, by Dave Galey & Dick Stafford, Bus Conversions Magazine, January, 2006.
