Every failed bearing tells a story. The pattern of damage on the raceways, rolling elements, and cage reveals the cause of failure as clearly as fingerprints at a crime scene. The problem is that most maintenance teams replace the bearing and throw the old one away without examining it. They lose the evidence and the failure repeats.
This guide covers the most common bearing failure modes, what the damage looks like, and what corrective actions prevent recurrence. Learning to read a failed bearing is the single most valuable diagnostic skill in plant maintenance.
Fatigue Spalling: The Natural End of Life
Fatigue spalling is the only failure mode predicted by the L10 life formula. A crack initiates below the raceway surface (10 to 20 micrometers deep) due to cyclic shear stress from the rolling elements passing over that point billions of times. The crack propagates to the surface and a flake of hardened steel pops out, leaving a pit.
Spalling from normal fatigue looks like this: it starts at one point on the inner ring raceway and spreads circumferentially as the pit edges create stress concentrations. The surface inside the spall is rough and granular. The rest of the raceway may still be smooth and mirror-like. The rolling elements may show minor pitting but are not severely damaged.
If the spalling is localized to one spot on the inner ring (at the same angular position), the cause is likely a static overload, a dent from installation, or a hard particle that passed through the contact zone. If the spalling extends all the way around the raceway, the cause is normal rolling contact fatigue and the bearing reached or approached its design life.
• Inner ring, full circumference → Normal fatigue or overload
• Inner ring, one spot → Static dent, installation damage, or point load
• Outer ring, one zone → Housing bore out-of-round or point load on outer ring
• Both rings → Severe overload or contamination-accelerated fatigue
Bearing Failure Symptom Triage
Diagnose bearing problems from observed symptoms, noise, vibration data, and visual inspection. Rule-based diagnostic tool that identifies likely causes and recommends corrective actions.
Lubrication Failure: The Number One Killer
Lubrication failure causes roughly half of all premature bearing replacements. The damage pattern is distinctive: the raceways show a polished or glazed appearance (from metal-to-metal rubbing under inadequate film), progressing to micro-pitting and then adhesive wear. The rolling elements have a dull, roughened surface instead of the original mirror finish. The grease, if any remains, is dark, dry, or hardened.
Discoloration is the most obvious sign. Bearing steel turns straw-colored at about 200°C, blue at 300°C, and black above 400°C. Any discoloration on the raceways or rolling elements means the bearing experienced temperatures well above normal, almost certainly from lubrication starvation.
Causes of lubrication failure include: wrong grease type, insufficient grease quantity, exceeded regreasing interval, grease incompatibility from mixing, and blocked grease channels. In many plants, the root cause is not that maintenance forgot to grease the bearing but that the grease fitting is painted over, the grease channel inside the housing is blocked with hardened old grease, or the bearing seal prevents fresh grease from reaching the contact zone.
Contamination: Death by a Thousand Dents
Hard particles (dirt, sand, grinding dust, metal chips) that enter the bearing create dents in the raceway surface. Each dent is a stress raiser that accelerates fatigue. A bearing running in contaminated grease can fail at one-tenth of its calculated L10 life.
The damage pattern from contamination is distinctive: the raceways show a uniformly rough, matte finish rather than the localized pitting of fatigue or the glazed appearance of lubrication failure. The dents are small, numerous, and distributed across the entire loaded zone. Under magnification, each dent has a raised rim (material displaced by the particle) that further damages the surface.
Water contamination is equally destructive. Water in the grease causes hydrogen embrittlement of the bearing steel, etching and corrosion of the raceways, and breakdown of the lubricant film. As little as 0.1 percent water in the lubricant can reduce bearing life by 50 percent. Water contamination shows as dark staining or black spots on the raceways and may include visible rust.
Bearing Failure Symptom Triage
Diagnose bearing problems from observed symptoms, noise, vibration data, and visual inspection. Rule-based diagnostic tool that identifies likely causes and recommends corrective actions.
Electrical Damage: The VFD Problem
Variable frequency drives (VFDs) generate common-mode voltage on the motor shaft. This voltage discharges through the bearings as electrical arcing, creating tiny craters in the raceway surface. Over time, the craters connect to form visible parallel lines called fluting, which looks like a fine washboard pattern on the raceway.
Electrical damage causes the bearing to run rough and noisy long before its mechanical fatigue life is reached. The fluting pattern creates periodic vibration at the ball pass frequency, which may be detected by vibration monitoring. The grease in an electrically damaged bearing often turns gray or black from the metal particles generated by arcing.
The fix is shaft grounding: a conductive brush or ring that provides a low-impedance path for the shaft voltage to discharge to ground instead of through the bearing. Insulated bearings (ceramic-coated outer ring) are an alternative but more expensive. In many plants, the cheapest and most effective solution is a shaft grounding ring on the non-drive end of the motor.