Wheel Gauging: Tread Wear & Damage

Tread wear and damage gauging is the inspection of the wheel tread - the cylindrical running surface that rolls on top of the rail - for wear and for surface defects that can condemn the wheel. It is part of routine wheelset gauging during interchange inspection.

Conditions Inspected

• **Shelled tread** - sub-surface fatigue causes pieces of the tread metal to break away, leaving cavities; shelling tends to spread and roughen the running surface.
• **Spalling** - surface metal flakes off, often after wheel slide and localized heating.
• **Thermal cracks** - heat from heavy or sustained braking produces cracks in the tread that can propagate.
• **Built-up tread** - metal smeared and accumulated on the surface, raising it above the normal profile.
• **Flat spots** (slid-flat wheels) - a wheel that locked and slid grinds a flat onto the round tread, producing pounding impact loads on the rail and wheel.
• **Tread worn hollow** - the tread wears into a concave profile, changing how the wheel contacts the rail.

Why It Matters

These defects raise impact loads, weaken the wheel, and can lead to tread or rim failure. Some also degrade how the wheel steers: a tread worn hollow changes the wheel's contact with the rail and hurts its tracking through curves and switches, and built-up tread - metal accumulating on the tread surface - reduces the wheel's effective flange height, so the flange can no longer reliably guide the wheel and the risk of a flange-climb derailment rises at switches and curves. The inspector measures or templates the tread against the relevant gauge; a wheel beyond the limit is removed and replaced. Specific sizes and lengths that condemn each defect are defined in the AAR interchange rules and field manual.