What Is Spalling? — Bearing Glossary

Spalling is a bearing surface fatigue failure in which metal fragments flake from the raceway or rolling elements, leaving irregular pitting — a signal that the bearing has reached end of fatigue life and requires immediate replacement.

Spalling (also called pitting or flaking) is the natural outcome of accumulated Hertz contact stress over millions of cycles. Fatigue cracks initiate below the surface (subsurface fatigue) at depths of 0.1–0.5 mm — where shear stress peaks. When cracks propagate to the surface, metal fragments break away, leaving the characteristic rough, cratered texture. Noise increases sharply, vibration escalates through ISO 13373 stages, and continued operation without stopping will lead to complete bearing seizure. See the full failure progression guide at bearing damage and failure modes and choose spherical roller bearings engineered for high-cycle fatigue resistance.

Development Mechanism and Accelerating Factors

Spalling progresses through four stages that can be tracked with vibration analysis. Stage 1: no visible signs, detectable only through high-frequency spectrum analysis (>10 kHz). Stage 2: characteristic bearing fault frequencies (BPFO, BPFI, BSF) appear clearly in the spectrum. Stage 3: sidebands and harmonics increase sharply, bearing temperature rises 5–10°C above baseline. Stage 4: overall vibration exceeds ISO 10816-3 limits — emergency shutdown required.

Three factors accelerate spalling: overload (every 25% load increase halves L₁₀ life), inadequate lubrication (thin oil film allowing metal-to-metal contact), and hard particle contamination (debris creates micro-indentations that seed cracks). According to SKF Rolling Bearings Catalogue, improving oil cleanliness from ISO 4406 class 20 to class 17 extends bearing life 2–3 times.

Distinguishing spalling from pitting matters for maintenance decisions. Pitting is small (<1 mm²), early-stage damage often caused by hard particle contamination pressing into the surface. Spalling is the progression — larger craters from subsurface fatigue cracks breaking through. Pitting can sometimes be slowed by improving lubrication and filtration; spalling cannot.

Practical Example: Bearing 22220 EK/C3

At a paper mill in Binh Duong, a drum roller bearing 22220 EK/C3 (d=100, D=180, B=46 mm, C=365 kN, C₀=490 kN) developed spalling on the outer raceway after 18,000 hours. Oil analysis detected a spike in dissolved Fe particles from 8 to 47 ppm before vibration increased — providing 6 weeks of advance warning. The maintenance team replaced the bearing during a scheduled shutdown, avoiding an unplanned stop estimated at 80 million VND in losses.

Inner Ring vs. Outer Ring Spalling Progression

Spalling location affects how quickly the failure develops. Inner ring spalling progresses faster because the inner ring rotates — the damaged zone passes through the load zone with every revolution, receiving a load impulse each time. Outer ring spalling progresses more slowly because the damage site only receives load when a rolling element passes over it.

This difference appears in vibration signatures. BPFI (inner ring fault frequency) shows characteristic sidebands modulated at shaft speed because the damage enters and exits the load zone. BPFO (outer ring fault frequency) produces a cleaner, steadier peak because the load is constant. Both require bearing replacement — there is no safe operating threshold once spalling has appeared.