A surface failure mode of rolling‑element or journal bearings in which fragments or flakes separate from the raceway or rolling element because of subsurface fatigue cracks or localized overloads, producing pits, roughness and progressive loss of load‑bearing area.

Principle

Spalling arises when cyclic contact or tensile/shear stresses beneath the contact surface initiate subsurface cracks that grow and coalesce until they intersect the surface, causing discrete fragments to detach and locally redistribute load.

Demonstration

Illustrative scenario — Situation: A wheel bearing in repeated service experiences high rolling contact cycles. Recognition: Microscopic subsurface cracks are detected near the raceway beneath contact patches. Action: Continued operation allows cracks to propagate and break out to the surface. Consequence: Visible flakes and pits form, vibration and load redistribution accelerate further spalling and eventual bearing seizure.

Misapplication

Interpreting any surface roughness or pitting as spalling. Why it appears plausible: both produce surface defects. Semantic error: spalling specifically implies subsurface fatigue crack initiation and material detachment, not only surface wear, corrosion pitting, or adhesive transfer.

Consequence

Loss of effective contact area increases local stresses, raises vibration and temperature, and shortens bearing life; left uncorrected, spalling commonly leads to accelerated failure modes (severe pitting, cage damage, seizure) through progressive load redistribution and secondary cracking.

Reversal

When material is removed by a single high‑energy impact or by abrasive cutting (e.g., foreign object damage or scouring), the resulting surface loss is not classical spalling because it lacks the subsurface fatigue crack growth mechanism that defines spalling.

Boundary

Clearly within: rolling bearing raceway exhibiting subsurface‑initiated flakes and pits. Boundary case: surface‑initiated material loss where corrosion or fretting begins at the surface and then propagates inward—diagnosis depends on microstructural evidence. Clearly outside: uniform abrasive wear, scuffing, or lubricant starvation without subsurface crack growth.

Semantic Tension

Material hardness and surface treatments increase contact fatigue resistance but may reduce toughness; designers must balance hardness (to reduce contact stresses) against toughness (to resist crack growth) and operational constraints (load, lubrication, contamination).

Synthesis

Spalling is fundamentally a subsurface fatigue phenomenon whose recognition depends on linking surface fragments to prior subsurface crack growth; prevention focuses on reducing contact stress, improving lubrication/contamination control, and selecting microstructures that limit subsurface crack initiation.