Bearing Failure Analysis Guide

1. Fatigue failure (spalling and flaking)

What you see: Pitting and spalling of the raceway surfaces — small craters where material has broken away in flakes. Starts as sub-surface micro-cracks propagating to the surface. In early stages, detectable by vibration analysis (BPFO/BPFI frequencies) weeks before visible damage.

Key characteristics:

• Spalling progresses in the direction of rotation (follows rolling contact)
• Inner race spalling: evenly spaced damage around the circumference if stationary inner race, localised if rotating inner race
• Outer race spalling: damage localised in the load zone (bottom of housing)
• Grey, rough, flaked surface texture — not smooth pitting

Root causes:

• Normal end-of-life fatigue (L10 life reached) — no change needed, bearing was correctly selected
• Overloading — bearing misapplied, dynamic load exceeded C/P rating
• Misalignment — creates uneven load distribution, accelerates fatigue in one zone
• False brinelling from vibration during storage/transit

Corrective action: If premature (below 80% of calculated L10 life), investigate load conditions, alignment, and mounting practices. If at end-of-life, bearing was correctly sized — continue with same type.

2. Wear

What you see: Gradual removal of metal from raceway and rolling element surfaces. Raceways appear dull, polished, or grooved. Rolling elements may show flat spots.

Abrasive wear

Hard particles (metal swarf, dirt, sand) in the lubricant act as abrasives, cutting fine grooves into the bearing surfaces. Under a magnifying glass you can see parallel scratch marks (striations) on the raceway.

Root cause: Contaminated lubricant, failed seal, inadequate contamination control. Corrective action: Improve sealing, flush and re-lubricate with clean grease, implement contamination control (ISO 4406 cleanliness targets).

Adhesive wear (smearing)

Occurs when rolling elements slide rather than roll — caused by sudden acceleration, deceleration, or grossly inadequate lubrication. Surface appears as torn, rough patches with material transfer between surfaces.

Root cause: Slip during starting (excessive axial preload on angular contact bearings), lubricant film collapse due to extreme temperature or incorrect viscosity grade.

3. Corrosion

Rust (moisture corrosion)

What you see: Red-brown pitting on raceway and rolling element surfaces, often with a pattern following the rolling element spacing. Corrosion pits then act as fatigue crack initiation points.

Root cause: Water ingress past seals, condensation during temperature cycling (especially common in intermittent-duty motors), inadequate corrosion inhibitors in the grease, or improper storage.

Corrective action: Improve shaft seal type (lip seal → labyrinth → positive air purge), specify corrosion-inhibited grease, store replacement bearings in a controlled environment. Electric motors standing still for months should be manually rotated monthly and re-greased.

Fretting corrosion

What you see: Red-brown powder (iron oxide) between bearing outer ring and housing bore, or inner ring and shaft. The mating surfaces show orange-red staining and micro-pitting.

Root cause: Relative micro-motion between the bearing ring and its seat — typically caused by incorrect fit (too loose). The correct fit for most rotating inner ring applications is an interference fit (k5 or m5 on the shaft).

4. Electrical erosion

What you see: Two distinct patterns depending on current magnitude:

• Frosting: Dull, matte, grey surface texture on both raceway and rolling elements — caused by small parasitic currents (mA range). Common in VFD-driven motors.
• Fluting: Parallel corrugations (channels) across the raceway, spaced evenly, resembling a washboard. Caused by higher currents (A range). The channels run perpendicular to the rolling direction.

Root cause: Shaft voltage induced by variable frequency drives (VFDs) finding a path to ground through the bearings. Induced currents arc across the bearing lubricant film, creating micro-craters that coalesce into fluting.

Corrective action: Install shaft grounding rings (e.g., AEGIS SGR) on the drive end of all VFD-driven motors above 75 kW. Use insulated bearing housings on the non-drive end to prevent circulating currents in larger motors. Specify bearings with hybrid ceramic rolling elements (ceramic does not conduct) for high-risk applications.

5. Plastic deformation

True brinelling

What you see: Permanent indentations in the raceway at rolling element spacing — sharp-edged craters matching the rolling element diameter and spacing exactly. Occurs when a stationary bearing is overloaded beyond the material's yield stress.

Root cause: Shock loading during transport (forklift drops), hammer blows during mounting, or external overload while stationary (hydraulic press loads on a bearing in a press machine).

False brinelling

What you see: Indentations similar to true brinelling but with red-brown iron oxide debris — caused by fretting, not yielding. Often found in stationary bearings subjected to vibration.

Root cause: Equipment transported by road or rail without bearing supports, motors mounted close to vibrating machinery while idle, or standby equipment sitting on vibrating foundations.

Corrective action: Use shipping restraints for transport, install isolation mounts or rotate standby equipment regularly to redistribute contact stress.

6. Fracture and cracking

What you see: Visible cracks or complete fracture of the inner ring, outer ring, or rolling elements. Usually catastrophic — the bearing often disintegrates in service.

Root cause:

• Incorrect mounting — driving force applied through rolling elements (inner ring mounting with hammer on outer ring)
• Excessive interference fit on hollow shaft
• Thermal cracking from overheating
• Pre-existing material defect

Corrective action: Always use induction heaters or bearing fitting kits to mount bearings — never a hammer on the outer ring. Check fit tolerances match shaft/housing dimensions exactly.

Failure analysis process

1. Collect information: Operating hours, operating conditions, lubrication history, previous failures on same equipment
2. Clean the bearing: Wash in solvent and dry — do not blast clean with compressed air (drives debris further in)
3. Visual inspection: Use a strong light and 10× magnifier — identify the failure mode from the characteristics above
4. Check the lubricant: Grease colour, consistency, any contamination visible
5. Check the seat: Shaft and housing bore for correct dimensions and surface finish
6. Confirm root cause and implement corrective action
7. Document in CMMS: Failure mode, root cause, corrective action — build your failure database

Prevention summary

Failure mode	Primary preventive action
Fatigue (spalling)	Correct bearing selection (C/P ratio ≥3), precision alignment, correct fit
Abrasive wear	Contamination control, correct seal selection, clean re-lubrication
Adhesive wear (smearing)	Correct grease viscosity, avoid axial preload on angular contact bearings at start-up
Rust corrosion	Corrosion-inhibited grease, improved sealing, proper storage
Fretting corrosion	Correct interference fits, anti-fretting compounds during assembly
Electrical erosion	Shaft grounding rings on all VFD-driven motors, insulated non-drive end housings
True brinelling	Shipping supports, no hammer mounting, avoid shock loads
False brinelling	Rotate standby equipment, vibration isolation, transport restraints
Fracture	Induction heater mounting, correct fits, avoid thermal shock