Bearing Failure Modes – What They Look Like and What Actually Causes Them
20 Apr, 2026
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What They Look Like and What Actually Causes Them
A bearing comes off a failed machine and goes into the scrap bin. That’s a missed opportunity. Every failed bearing carries a physical record of what went wrong, damage patterns on the raceways, rolling elements, and cage that, read correctly, point directly at the root cause. A crack in one location and a spall in another are not the same failure. They don’t have the same cause, and they don’t get the same corrective action.
Most facilities treat bearing failure as a cost rather than a data source. The bearing fails, it gets replaced, and work continues. The next failure on the same equipment, often weeks or months later, gets the same response. Without understanding what the damage is telling you, the cycle repeats.
ISO 15243:2017 — the international standard for rolling bearing damage and failure classification, defines six primary failure modes based on the visible appearance of the damage. Each has characteristic appearance, and each points to a specific root cause.
Precision Maintenance, Bearings & Lubrication
Master bearing failure mode identification, lubrication selection, installation, and condition monitoring, hands-on training built for industrial maintenance professionals.
ISO 15243:2017 classifies failure modes occurring in service for rolling bearings made of standard bearing steels. For each failure mode, it defines the characteristics, appearance, and possible root causes. The standard is used by major bearing manufacturers including SKF, Schaeffler/FAG, and NSK as the basis for their bearing damage analysis documentation.
The classification is based on observed appearance rather than root cause alone, because in practice, damage and/or failure of a rolling bearing can often be the result of several mechanisms operating simultaneously. The visible damage pattern is the starting point. The root cause investigation follows from correctly identifying what type of damage is present.
The six primary ISO 15243 failure mode categories:
Rolling contact fatigue is the failure mode that bearing life calculations describe. It is the expected end-of-life mechanism: cyclic Hertzian contact stress between rolling elements and raceways creates a shear stress field below the surface. Over millions of load cycles, microcracks initiate at subsurface inclusions, propagate to the surface, and produce the characteristic spalling that defines bearing fatigue failure.
This is the only bearing failure mode that is inherent to bearing operation. Every other failure mode on this list is, in principle, preventable. Normal rolling contact fatigue is not, but it should only occur after the bearing has completed its rated service life.
Subsurface Fatigue — Classical Spalling
What it looks like: Irregular pits and craters on the raceway surface in the load zone. The spalls have rough, jagged edges initially. Over time, the damaged area widens as additional material breaks away from the edges of existing spalls. Vibration increases measurably as spalling progresses, this is what condition monitoring vibration analysis detects in the bearing defect frequency bands.
Root cause: Cyclic subsurface shear stress beginning 0.1–0.5 mm beneath the raceway. Premature occurrence indicates excessive load (doubling the load on a ball bearing reduces L10 life by a factor of 8), inadequate lubricant film, material quality issues, or contamination-induced surface damage.
What it is not: Spalling concentrated at one point on the outer ring indicates a stationary point load rather than rotating load fatigue. Indentations at exact rolling element spacing suggest true brinelling, not fatigue.
Surface-Initiated Fatigue — Peeling
What it looks like: Fine surface roughening progressing to superficial flaking. Early stages show a gray, matte, or frosty appearance on the raceway — similar in look to frosting from electrical erosion, but without the characteristic washboard pattern. Material removal is shallow and covers larger areas than classical subsurface spalling.
Root cause: Inadequate base oil viscosity for the operating speed and bearing size (kappa below 1.0). This is a viscosity selection problem, not a lubrication quantity problem. The bearing may be receiving the correct quantity of grease but with a base oil that cannot maintain an adequate film at operating temperature and speed.
Field Note — Reading the Raceway to Find the Real Cause
A critical pump experiences its third bearing failure in 18 months. Each time, the bearing is replaced and returned to service. On the third failure, a reliability engineer retains the bearing for inspection instead of scrapping it.
The inner ring shows spalling concentrated in a 90-degree arc rather than distributed around the circumference. The outer ring shows a linear spalling pattern across the full raceway width. The inner ring pattern indicates a high unidirectional load, consistent with belt tension. The outer ring pattern indicates angular misalignment, the contact line is tilted rather than uniform.
The fix wasn’t a better bearing, it was a belt tension check and a shaft alignment verification before returning to service.
2 — Wear: When the Lubricant Film Breaks Down
Wear in bearings is defined by the progressive removal of material from raceway and rolling element surfaces. ISO 15243 distinguishes two sub-modes: abrasive wear from hard particles in the lubricant, and adhesive wear (smearing) from metal-to-metal contact. Research attributes 20 to 30 percent of all bearing failures to contamination.
Abrasive Wear
What it looks like: Dull, matte, or rough finish on the raceway rather than the mirror-like surface of a properly lubricated bearing. In early stages, fine scoring lines parallel to the rolling direction. In advanced stages, the raceway surface is visibly rough with material removal across the full contact band. Bearing clearance increases as material is removed.
Root cause: Hard contaminant particles in the lubricant passing through the contact zone. Contaminated lubricant (poor storage, open containers, unclean dispensing equipment), installation contamination, inadequate sealing, and wear particles from adjacent components.
Secondary evidence: Hard contaminants produce indentations when over-rolled at low speed or high load. These indentations then act as stress concentration sites for subsequent surface fatigue, a bearing showing both contamination indentations and adjacent spalling has almost certainly failed from contamination-induced surface fatigue.
Adhesive Wear — Smearing
What it looks like: Torn, rough surface with visible material smearing in the direction of sliding. Localized heat discoloration (brown, blue, or dark) from the friction generated during smearing. The damage is irregular rather than following the systematic pattern of abrasive wear or fatigue.
Root cause: Insufficient load or excessive speed causes rolling elements to slip against the raceway rather than roll. Cylindrical roller bearings have minimum load requirements (SKF specifies 0.02C). Smearing is common at startup when lubricant film has not yet been established, and in applications where operating load drops below the minimum.
3 — Corrosion: Three Different Mechanisms, Three Different Appearances
ISO 15243 classifies corrosion into three distinct sub-modes: moisture corrosion, fretting corrosion (fit corrosion), and false brinelling. They look different from each other, they have different root causes, and they require different corrective actions.
Moisture Corrosion
What it looks like: Rust spots or dark oxidation on raceway and rolling element surfaces. In early stages, small pits where the rust has been removed by rolling contact. In severe cases, the rust pitting follows the contact pattern of the rolling elements when the bearing was stationary, a characteristic pattern that helps distinguish moisture corrosion from other damage types.
Root cause: Seal failure allowing process fluid or wash-down water ingress; condensation in housings experiencing temperature cycling; water contamination in grease from improper storage; long standstill periods during which condensation accumulates.
Fretting Corrosion — Fit Corrosion
What it looks like: Reddish-brown metallic oxide debris (often mistaken for rust) on the bore surface of the inner ring or the outside diameter of the outer ring. The mating shaft or housing bore shows matching wear and discoloration. In advanced cases, the bore or OD is visibly undersized or out-of-round from material loss.
Root cause: Incorrect fit (too loose for the load condition) causes the ring to microslip relative to the shaft or housing under load. The micromotion destroys the lubricant film at the interface, causes metal oxidation, and generates reddish-brown metallic oxide debris.
Why it matters beyond the obvious: Fretting corrosion at the inner ring bore concentrates stress at the ring, accelerating fatigue crack initiation from the bore surface. It is a primary pathway to ring cracking, a bearing showing fretting corrosion internally may fail catastrophically from ring fracture rather than gradual raceway wear.
False Brinelling — Standstill Vibration
What it looks like: Shallow depressions at rolling element pitch spacing, the same spacing as true brinelling, but with reddish-brown fretting corrosion debris at each depression. The combination of depression shape and debris color distinguishes false brinelling from true brinelling, which produces clean metallic indentations without the rust-colored debris.
Root cause: Stationary bearing subjected to vibration, from nearby machinery, transport, or process vibration, that causes small oscillatory movements of the loaded rolling elements against the raceway. Common in standby equipment, during transport, and in idle sections of conveyor systems with structural vibration.
Field Note — False Brinelling on Standby Pumps
A standby pump shows bearing noise and elevated vibration within weeks of startup following an extended standby period. The bearing is pulled. The outer ring shows evenly spaced shallow depressions at rolling element pitch, with reddish-brown fretting debris at each one, classic false brinelling.
The pump sat on standby for eight months adjacent to a running process pump that transmitted structural vibration through the common baseplate. The standby pump’s shaft was not rotating, but the loaded bearing was oscillating slightly with the vibration.
The corrective action: schedule periodic slow shaft rotation for all standby equipment. A different bearing grade won’t fix the problem, stopping the mechanism is the only effective corrective action.
4 — Electrical Erosion: The Failure Mode That Wasn’t Common Until VFDs
Electrical erosion is the fastest-growing category of industrial bearing failure in the past decade, driven directly by the proliferation of variable frequency drives (VFDs) in motor applications. It is also one of the most misidentified failure modes, the damage pattern can be confused with lubrication-related surface wear by inspectors who don’t know what to look for.
The mechanism: the PWM switching action of a VFD creates common-mode voltage on the motor shaft. This voltage builds up and discharges through the bearing, the lowest impedance path to ground, when the voltage exceeds the dielectric strength of the lubricant film. Each discharge is a microscopic electric arc, similar to electrical discharge machining (EDM).
What Electrical Erosion Looks Like
Early stage — Frosting
Gray or matte bands on the otherwise polished raceway surface. The pattern may be localized to the load zone or distributed around the raceway circumference. At this stage, the bearing still functions but noise and vibration are beginning to increase.
Advanced stage — Fluting
Evenly spaced ridges or grooves running transverse to the rolling direction, the “washboard” pattern that is pathognomonic for electrical erosion. Running a fingernail across the race, you can feel the ridges. By the time fluting is visible, bearing failure is near.
Grease condition indicator
Electrically damaged bearings consistently show gray or black grease — a metallic gray color from the fine ferrous particles eroded from the raceways. Normal grease degradation produces brown or darkened grease. Gray or black grease with metallic particles is a distinctive indicator of electrical damage.
Who Is at Risk
Any motor controlled by a VFD is at risk for bearing currents, particularly motors above 100 hp where high-frequency circulating currents are a concern in addition to capacitive discharge. Facilities that have converted existing motors to VFD control without evaluating shaft grounding are especially vulnerable. VFD-induced bearing damage typically produces failure at 25 to 50 percent of normal bearing L10 life.
Prevention — Addressing the Electrical Mechanism Directly
Shaft grounding rings or brushes that provide a low-impedance path for shaft current to ground, diverting the discharge away from the bearing
Insulated bearings (with ceramic-coated rings) or hybrid bearings (with ceramic rolling elements) that break the current path through the bearing
Both measures combined for motors above 100 hp where both capacitive discharge and circulating current are present
Correct VFD grounding and shielded motor cable installation per IEC TS 60034-25
Critical Point
Changing the grease type or increasing the regreasing frequency will not prevent electrical erosion. The electrical mechanism does not respond to lubrication changes. If fluting is being observed on VFD-driven motors that have correct lubrication practices, the root cause is electrical, and the corrective action must address the electrical circuit.
5 & 6 — Plastic Deformation and Fracture
True Brinelling — Plastic Deformation
What it looks like: Clean, shiny metallic indentations at rolling element pitch spacing on the raceway. No debris, no discoloration. The indentations are smooth and geometrically defined by the rolling element contact geometry. Easily confused with false brinelling, the key distinguishing feature is the absence of reddish-brown fretting debris.
Root cause: Permanent indentation caused by a force that exceeds the elastic limit of the bearing steel at the rolling element contact. True brinelling is immediate, it happens in a single event. The C0 static load rating is the parameter that determines whether true brinelling will occur.
When it happens: Shock loading during transport, installation impact (hammer blows transmitted through the shaft or housing), and startup impact on equipment that experiences sudden loading.
Cage Fracture
What it looks like: Fractured cage; scattered rolling elements; catastrophic raceway damage from multiple uncontrolled contacts. Cage fracture is one of the few bearing failures that can cause immediate catastrophic machine damage rather than gradual degradation.
Root cause: Almost always a secondary failure. Cages fail because they are driven beyond their design load through: misalignment that loads the cage rather than the raceway, severe contamination that jams rolling elements, inadequate lubrication, vibration-induced fatigue, or over-speeding.
Key investigation principle: The root cause investigation for a fractured cage must look past the cage itself to what caused the unusual loading. The cage did not fail on its own, it was driven to failure by something that should be identified and corrected.
Go Deeper — Precision Maintenance: Bearings & Lubrication
The following table summarizes all major bearing failure modes, their root causes, appearance, detection approach, and corrective actions for field reference:
Failure Mode
Root Cause
What It Looks Like
Detection
Corrective Action
Subsurface Fatigue — Spalling
Cyclic Hertzian contact stress — normal end-of-life or overload/inadequate film
Pits and craters in load zone; flaked particles; progressive widening
Vibration trending; L10 life calculation; bearing temp monitoring
Verify load vs. rated capacity; check kappa; inspect for contamination
Surface Fatigue — Peeling
Boundary lubrication from inadequate film (kappa < 1)
Superficial surface roughening; matte/frosty appearance; fine flaking
Verify kappa ratio; check base oil ISO VG vs operating conditions
Select correct base oil viscosity for nDm and operating temperature
True Brinelling
Static overload or shock impact on stationary bearing exceeding elastic limit
Evenly spaced clean metallic indentations at rolling element pitch; no debris
Check S0 static safety factor
Review handling and transport practices; protect during installation and storage
False Brinelling
Micro-oscillation of loaded stationary bearing; lubricant film breakdown
Reddish-brown fretting debris at rolling element spacing; shallow depressions
Inspect standby equipment bearings periodically
Schedule slow shaft rotation of standby equipment; improve C0 margin
Shaft grounding ring; insulated or hybrid bearing; verify VFD grounding
Cage Fracture
Secondary failure — caused by misalignment, contamination, inadequate lubrication, over-speeding
Fractured cage; scattered rolling elements; catastrophic raceway damage
Often sudden/catastrophic — condition monitoring trend
Root-cause the cage fracture — always a secondary failure; find the primary cause
Misalignment Damage
Angular misalignment: bent shaft, poor installation, deflection under load
Linear spalling across full raceway width; tilted contact pattern on outer race
Vibration analysis; shaft alignment measurement
Verify shaft alignment; check housing bore parallelism; use self-aligning bearing if needed
Sources: ISO 15243:2017; SKF bearing damage analysis; IoT Bearings; American Roller Bearing; AEGIS; NFM Consulting.
How to Use Failure Evidence: The Inspection Approach
A failed bearing provides useful information only if you inspect it before scrapping it. The inspection takes five minutes and can prevent a repeat failure that might cost days of downtime.
What to Look For
Damage location on the outer ring
Spalling concentrated in one area (~90° of circumference) indicates a stationary point load, belt tension, weight, press fit load. Spalling distributed around the full circumference indicates rotating load or general fatigue.
Damage location on the inner ring
Spalling in a 360° band indicates the inner ring rotates under the load. Spalling at one point indicates the inner ring is stationary and the outer ring rotates, check the application architecture.
Damage pattern across the raceway width
Spalling concentrated in the center is consistent with pure radial load and correct alignment. Spalling at one edge indicates edge loading from misalignment, axial displacement, or overcrowning.
Grease condition
Brown/darkened = oxidation and heat. Milky/emulsified = water contamination. Gray/black metallic = electrical erosion. Dry/crusty = lubrication failure or over-temperature degradation.
Bore and OD surfaces
Reddish-brown debris = fretting corrosion from loose fit. Polished, worn bore = creep from loose fit. Correct fit: no debris, uniform contact marking.
What Good Looks Like — The Retained Bearing Program
A reliability engineer at a paper mill implements a retained bearing program: every bearing removed from a critical or important asset is inspected, photographed (damage zone, grease condition, bore/OD surfaces), and the findings are logged against the equipment record with a damage classification per ISO 15243.
Within six months, a pattern emerges: three specific bearing positions on the press section show consistent fretting corrosion on the inner ring bore. All three are on horizontal rolls where the inner ring fit was borderline transition, occasionally losing interference under thermal expansion. The corrective action is a modified fit specification for those positions during the next planned rebuild.
The cost of the retained bearing program is negligible. The insight it produces prevents a class of failures that had been causing unplanned press section stoppages at a cost of roughly 45 minutes of production per event.
The Bottom Line
Bearing failure modes are not mysterious. Each one has a characteristic appearance, a specific root cause mechanism, and a corrective action that addresses that mechanism, not just a replacement schedule. ISO 15243:2017 provides the standardized classification framework that connects the visible damage to the appropriate investigation pathway.
The failures that repeat — the motor bearing that fails every six months, the conveyor head pulley that goes through bearings every season, the VFD pump that consumes bearings at twice the rated life, are almost never failures of bearing quality. They are failures of application: wrong fit, wrong lubricant, inadequate sealing, electrical current with no discharge path, or loading conditions that exceed the bearing’s design envelope.
Reading the bearing before it goes in the scrap bin is the lowest-cost reliability improvement available in most industrial facilities. Five minutes of inspection, correctly interpreted, tells you whether the next bearing is going to last its rated service life or repeat the failure in a few months.
