Brake Drum and Shoe Inspection
Brake drums are cast iron cylinders that rotate with the wheel hub. Brake shoe linings press against the inside of the drum to create friction. Drum inspection criteria: cracks — any crack in a brake drum, especially cracks that penetrate through the drum wall, are cause for immediate out-of-service. Heat checks (surface cracks from thermal cycling) are more common and require evaluation: superficial, parallel heat checks (like dried mud pattern) under 0.025 inches deep may be acceptable; deeper or intersecting cracks approaching structural integrity are OOS. Drum-out-of-round: drums worn unevenly create pulsating brakes — check by measuring drum inside diameter at multiple points; out-of-round exceeding manufacturer specs requires replacement. Minimum thickness: drums are stamped with maximum allowable inside diameter — if the drum is worn to this diameter, it must be replaced. Shoe lining inspection: minimum thickness for air brake shoes varies but is typically 1/4 inch (6mm) for non-shoe-style and 1/8 inch (3mm) for shoe-style. Lining contaminated with oil or brake fluid is considered zero thickness (oil-soaked lining provides minimal friction). Cracks across the width of the lining indicate imminent failure. Uneven lining wear indicates misadjustment or hardware problems.