How It Starts, How It Spreads, and the Warning Signs Your Team Can Catch Early
Bearing spalling does not happen without warning. The warning is just often invisible by the time the maintenance team finds it, buried in a vibration spectrum that nobody analyzed, or bypassed because the bearing looked fine on the last visual walk-around. By the time spalling is audible from across the room, the bearing has consumed most of its remaining service life.
Spalling, the pitting or flaking of material from bearing raceways and rolling elements under cyclic contact stress, is classified in ISO 15243:2017 as a fatigue failure mode and is one of the most common end-stage bearing failure presentations in industrial equipment. For the full ISO 15243 framework and how spalling fits alongside the other five primary bearing failure categories, see our article on bearing failure modes: what they look like and what actually causes them. What matters for maintenance teams is not the end-stage presentation. It is the weeks and months of detectable progression that precede it.
What Spalling Is: The Physical Mechanism
A rolling element bearing operates on a thin elastohydrodynamic lubricant film at the contact zone between rolling elements and raceways. Under load, the contact pressure at that zone is extreme, Hertzian contact stresses in bearing raceways can exceed 500,000 psi. Even with proper lubrication, the material below the surface is subject to cyclic shear stresses with every load cycle. Over millions of revolutions, those cyclic stresses cause material structural changes.
Microcracks initiate, either at subsurface inclusions in the bearing steel under normal fatigue, or at surface stress concentrations created by contamination dents, lubrication failure, or misalignment-induced edge loading. These cracks propagate under continued cyclic loading. When the crack network reaches the surface, a fragment of material detaches. That is a spall.
ISO 15243:2017 classifies spalling under two sub-modes of rolling contact fatigue: subsurface-initiated fatigue and surface-initiated fatigue. The distinction is more than taxonomic, it tells the investigator where the failure started, which directly determines the root cause and the corrective action.
In practice, most industrial bearing spalling is surface-initiated and triggered by preventable conditions. The most important diagnostic question when a spalled bearing is removed is not what does the spall look like? — but where is the spall located and what does that location tell us about the stress that initiated it?
The Three Spalling Types: PSO, GSC, and Inclusion Origin
Within the fatigue failure category, the industry recognizes three specific spalling types based on origin location, appearance, and root cause. Each has distinct visual characteristics, distinct root causes, and distinct corrective actions. Identifying the type correctly is what makes failure analysis actionable rather than generic.
| Spalling Type | Prevalence | Appearance | Root Cause |
|---|---|---|---|
| Point Surface Origin (PSO) | Most common in industrial environments | Arrowhead-shaped spalls propagating in direction of rotation; multiple origin points where debris dents exist | Hard-particle contamination creating stress-concentration dents. Also nicks, etching, and handling damage. Contamination control is the primary preventive measure. |
| Geometric Stress Concentration (GSC) | Misalignment, shaft deflection, edge loading, poor housing geometry | Spalling concentrated at edges of raceways rather than distributed across the contact zone | Shaft misalignment, housing bore out-of-round, machining errors, or inadequate clearance under load. GSC is a reliable indicator of an installation or alignment problem, not bearing material failure. |
| Inclusion Origin (IO) — Subsurface Fatigue | Inherent material characteristic; rare in modern clean steels | Elliptically shaped spalls at the center of the raceway; irregular in size; associated with the bearing’s expected fatigue life | Cyclic Hertzian contact stress over millions of load cycles. Microcrack initiates at subsurface inclusion, propagates to surface. The only inherent failure mode — not preventable but predictable from bearing life calculations. |
Sources: ISO 15243:2017; SKF Evolution bearing damage analysis; Pit & Quarry bearing damage guide; Tribonet; Timken bearing damage analysis reference.
Distinguishing the Types in the Field
How Spalling Progresses: The Three-Stage Research Data
Spalling is progressive, once initiated, it expands continuously during operation. Understanding the rate of progression is what allows maintenance teams to plan replacements rather than react to failures.
No visible surface damage. The stress concentration from the dent is cycling below the surface, developing the microcrack network. A contamination dent can sit in a bearing for a very long time before surface damage appears.
Exponential growth in spalled area following the first visible surface crack. Corresponds to Stage 2–3 in the vibration progression model — detectable by envelope analysis, then by standard FFT.
Growth rate more than doubles. Corresponds to Stage 4 vibration — broadband noise, rapidly increasing temperature, imminent failure.
The incubation period, during which nothing is visible and standard vibration analysis cannot detect the developing fault, is very long. The period from first detectable signal to catastrophic failure is comparatively short. Early detection requires monitoring technology that catches Stage 1 or early Stage 2. Waiting for a visual indication or an alarm on overall vibration level means the bearing has already consumed most of its remaining service life.
The Four-Stage Vibration Progression
As spall size increases, the impact generated when rolling elements traverse the damaged area increases in energy, appearing at progressively lower and more easily detected frequencies.
| Stage | What Is Happening | How It Shows in Vibration | Recommended Action | Time Horizon |
|---|---|---|---|---|
| 1 | Subsurface microcrack initiation. No surface damage visible. Bearing functions normally. | Only detectable at ultrasonic frequencies (20–60 kHz). Shock Pulse Method (SPM), high-frequency envelope analysis. Standard FFT shows nothing unusual. | Lubrication condition and monitoring only. No maintenance action on the bearing itself. | Months |
| 2 | Cracks propagate toward surface. Bearing natural frequencies begin to ring at higher bands (2–6 kHz). Sidebands appear around resonances. | Detectable with envelope analysis in 2–6 kHz range. Bearing defect frequencies (BPFO, BPFI) not yet prominent. | Critical machines: plan replacement at next available window. Non-critical: increase monitoring to weekly. | Weeks to months |
| 3 | Surface spalling begins. BPFO, BPFI, BSF, and FTF appear in standard FFT spectrum with harmonics and sidebands. | Clear bearing defect frequencies visible in standard velocity spectrum. Audible with ultrasonic probe. | Replace at earliest planned opportunity. Bearing has 1–5% of useful fatigue life remaining. | Days to weeks |
| 4 | Multiple spalls, possible cage damage. Bearing frequencies disappear as damage becomes widespread. | High overall vibration; discrete frequencies replaced by broadband noise. Temperature rising rapidly. Audible from distance. | Remove from service immediately. Catastrophic failure imminent. | Hours |
Sources: MRO Magazine vibration guide; Reliabilityweb; AllTestPro motor bearing vibration stages; NCD bearing fault detection vibration analysis.
Stage 1 can last months. Stage 4 can last hours. A condition monitoring program that only detects Stage 3 — which is what a standard monthly vibration route with overall velocity measurement reliably catches, is detecting bearings that have 1 to 5% of their fatigue life remaining.
Stage 2 detection provides weeks to months of lead time. Stage 2 detection requires envelope analysis capability, not just overall velocity trending.
A plant’s vibration program collects monthly route data on pump bearings using overall velocity measurement. A 100 hp cooling water pump shows stable overall velocity of 2.1 mm/s, within normal range, for six consecutive months. A reliability engineer runs envelope analysis on the same data. The envelope spectrum shows a clear BPFO pattern at 2.3× and 3.5× harmonics in the 3–5 kHz range, classic Stage 2 bearing behavior. The standard velocity spectrum shows nothing.
The bearing is scheduled for replacement during a planned maintenance window five weeks out. At removal, the outer raceway shows early PSO spalling with four identifiable arrowhead origin points. Outcome: planned replacement, zero unplanned downtime. Without envelope analysis, the first indication would have been Stage 3 or Stage 4 — at which point the five-week planning window no longer exists.
Reading the Frequency Signatures: BPFO, BPFI, BSF, and FTF
When vibration analysis detects a bearing fault at Stage 3, the spectrum contains characteristic frequencies tied to the physical geometry of the specific bearing and shaft rotational speed.
| Freq. | Full Name | Physical Meaning | Diagnostic Significance |
|---|---|---|---|
| BPFO | Ball Pass Frequency Outer Race | Rate at which rolling elements strike a defect on the fixed outer raceway. Stable, load-independent frequency pattern. | Outer race spalling. Clean harmonic series. BPFO ≈ 0.4 × N × shaft speed. |
| BPFI | Ball Pass Frequency Inner Race | Rate at which rolling elements pass a defect on the rotating inner race. Harder to detect, defect moves in and out of the load zone with shaft rotation. | Inner race spalling. Spectrum shows BPFI with sidebands at shaft frequency. BPFI ≈ 0.6 × N × shaft speed. |
| BSF | Ball Spin Frequency | Rate at which a rolling element rotates about its own axis. Frequency smearing from slip makes detection difficult. | Rolling element surface damage. Most difficult of the four to detect reliably. |
| FTF | Fundamental Train Frequency | Rotational frequency of the cage. Cage defects and lubricant starvation produce energy at FTF and its harmonics. | Cage damage. Also appears as sideband spacing in BPFI spectra. FTF ≈ 0.4 × shaft speed. |
The most significant diagnostic observation in spalling analysis: the presence of bearing defect frequencies in a standard spectrum means the bearing has entered Stage 3. The bearing has lost over 95% of its remaining useful life by the time BPFO or BPFI appears clearly in the velocity spectrum.
Prevention: Addressing the Root Causes
Spalling prevention is not a bearing selection problem. For PSO and GSC, which together account for the majority of premature bearing spalling, prevention is a maintenance program problem. The bearing’s material is not the failure mechanism. Contamination and misalignment are.
Contamination Control — PSO Prevention
PSO spalling is initiated by surface dents from hard particles in the bearing contact zone. Those particles enter through failed or inadequate seals, through unclean lubrication practices, or through contamination introduced during installation or handling.
- Seal selection and inspection: verify seal type is appropriate for the contamination level; inspect seals for wear and damage at every PM event; replace seals that have lost their exclusion capability before the bearing, not after it.
- Lubrication cleanliness: use closed-system dispensing; keep grease fittings clean before and after use. One frequently overlooked contamination pathway is over-greasing itself, when excess grease pressure ruptures the lip seal from the inside. See our article on over-lubrication in bearings and why too much grease causes heat, seal damage, and premature failure.
- Handling and installation: never handle bearing raceways with bare hands; work on clean surfaces; use bearing installation tools that apply force to the correct ring.
Alignment and Installation — GSC Prevention
- Shaft alignment verification using precision laser alignment tools before commissioning any new installation or after any coupling or drive train work.
- Housing and shaft bore tolerance verification: out-of-round housing bores produce edge loading from the moment of first rotation. For the ISO 286 interference-fit framework, see our guide on bearing fits and tolerances.
- Deflection assessment for overhung loads. Our article on bearing load ratings (C, C₀, and the C/P ratio) walks through how dynamic and static load ratings translate into L10 life.
Lubrication Adequacy — Surface-Initiated Fatigue Prevention
Surface-initiated fatigue from inadequate lubrication requires correct viscosity selection (kappa ≥ 1.0 at operating temperature), calculated regreasing intervals based on operating conditions rather than calendar defaults, and verified grease delivery.
The Inspection Framework: Reading a Spalled Bearing
When a spalled bearing is removed, five minutes of systematic observation before the bearing is discarded produces the failure analysis data that prevents the next failure. This is a specific application of the broader retained-bearing inspection approach described in our bearing failure modes article.
The answers to these observations define the corrective action. A bearing showing PSO spalling with contaminated grease and a damaged seal requires a different response than one showing GSC spalling in a clean, well-lubricated housing. Replacing both with the same bearing and resuming operation without a root cause determination produces the next failure.
Spalling vs. Pitting: A Practical Distinction
ISO 15243 uses spalling (and its synonym flaking) to describe material removal from fatigue. Pitting in the bearing context refers to corrosion pits, small, discrete, chemically-formed surface depressions classified separately under ISO 15243 as a corrosion failure mode, not a fatigue mode.
In practice, corrosion pits often become PSO spalling origins. The pit creates a stress concentration; cyclic loading initiates a microcrack at the pit edge; the crack propagates and the material fragments. Inspection that identifies the original corrosion pits alongside the spall origins helps reconstruct this sequence.
Bearing spalling is not a single failure mode. It is three distinct failure presentations, PSO from contamination, GSC from misalignment and edge loading, and inclusion-origin from normal material fatigue, each with different root causes, different preventions, and different investigative responses. Treating all three as bearing failure, replace bearing leaves the root cause in place and produces the next failure.
The vibration progression from Stage 1 to Stage 4 provides weeks to months of advance warning, but only if the monitoring program can detect Stage 1 or 2. Standard overall vibration measurement reliably detects Stage 3, when the bearing has 1 to 5% of its remaining useful life. Envelope analysis detects Stage 2, when the replacement can be planned rather than reactive.
PSO: keep contamination out. GSC: align shafts and verify fits before commissioning. Lubrication-initiated surface fatigue: calculate intervals and verify viscosity. Three root causes, three clear countermeasures.
