Bearing Failure Diagnosis: How to Read Damage Patterns

Reading the Evidence

Every failed bearing tells a story. The damage patterns on the races, rolling elements, and cage reveal exactly what went wrong — and more importantly, what to fix so the replacement doesn't fail the same way.

Before discarding a failed bearing, examine it carefully. Note the location and appearance of damage on:

Inner race — the surface the rolling elements contact on the shaft-mounted ring

Outer race — the surface in the housing-mounted ring

Rolling elements — balls or rollers

Cage — the retainer that separates the rolling elements

Seal or shield — if damaged, contamination may be the root cause

Take photographs before cleaning. The lubricant condition (color, consistency, debris) is itself diagnostic evidence.

Fatigue Spalling

Appearance: Flaking, pitting, or crater-like removal of material from the raceway surface. Starts as small pits that grow and connect into larger flaked areas. Surface has a rough, granular texture where material has broken away.

Location: Typically in the loaded zone of the race. On the inner race, spalling in a circumferential band indicates normal rotating load fatigue. On the outer race, spalling concentrated in one area indicates a stationary point load.

Root Causes:

Normal fatigue — Every bearing has a finite life. If the bearing ran for its calculated L10 life or longer, this is expected end-of-life failure.

Overloading — Loads exceeding the bearing's rated capacity accelerate fatigue. For ball bearings, doubling the load reduces life by a factor of 8.

Insufficient internal clearance — Thermal expansion of the shaft and housing can eliminate clearance, creating excessive preload and concentrated stress.

Poor lubrication — Inadequate film thickness increases metal-to-metal contact stress.

Corrective Actions:

Verify actual loads against bearing rated capacity

Check that the bearing selection provides adequate L10 life for the application (target at least 20,000 hours for general industrial use)

Review fit and clearance — ensure the bearing has appropriate internal clearance for the operating temperature range

Verify lubrication type and quantity

True Brinelling

Appearance: Permanent dents or indentations in the raceway at rolling element spacing. Each dent corresponds exactly to the position of a ball or roller. The surface within the dent is smooth and shiny, with clearly defined edges.

Location: On both inner and outer races, spaced evenly to match the number of rolling elements.

Root Causes:

Static overload — Excessive force applied while the bearing is stationary, such as pressing the bearing onto a shaft through the wrong ring

Shock loads — Impact loading from being dropped, hammered during installation, or subjected to machinery collisions

Transportation damage — Vibration during shipping of assembled equipment with bearings under preload

Corrective Actions:

Never apply installation force through the rolling elements — press only on the ring being fitted

Use proper installation tools (induction heaters, hydraulic press, installation sleeves)

Block shafts during transport to prevent bearing load

Verify static safety factor S0 is adequate (minimum 1.0 for normal, 1.5 for vibration, 2.0 for precision applications)

False Brinelling

Appearance: Shallow depressions or wear marks in the raceway at rolling element positions, similar to true brinelling but with a dull, matte, or oxidized appearance. The worn areas may have a reddish-brown color from fretting corrosion.

Location: On the loaded zone of the non-rotating race. Common in bearings that vibrate while stationary.

Root Causes:

Vibration while stationary — Equipment that vibrates during transport or when shut down (e.g., electric motors on standby, railway axle bearings in parked trains)

Small oscillations — Pivoting or rocking motion that doesn't allow the rolling elements to complete a full revolution, preventing lubricant replenishment

Corrective Actions:

Isolate non-running equipment from vibration sources

Rotate shafts periodically during extended shutdown

Use lubricants with anti-wear (EP) additives

Consider bearing types designed for oscillating loads

Smearing and Skidding

Appearance: Streaked, smeared, or dragged areas on the raceway surface. The damaged areas have a torn or welded appearance with material displacement in the direction of sliding. Surface may appear rough and discolored.

Location: Commonly on the inner race of cylindrical roller bearings or on the roller ends and guide flanges.

Root Causes:

Insufficient load — When a bearing is too lightly loaded for its speed, rolling elements can skid instead of roll. The momentary sliding causes instantaneous welding and tearing of surface material.

Sudden acceleration or deceleration — Rapid speed changes cause rollers to slide before achieving rolling contact.

Inadequate lubrication — Insufficient lubricant film allows metal-to-metal sliding contact.

Corrective Actions:

Ensure minimum load is met (especially for cylindrical roller and angular contact bearings — SKF recommends minimum load of 0.02C for roller bearings)

Apply preload to angular contact or tapered roller bearings

Use a lighter lubricant to reduce viscous drag

Apply load gradually during startup

Corrosion and Moisture Damage

Appearance: Rust or dark oxidation on raceway surfaces. In advanced cases, pitting that follows the rolling element contact pattern. Etching or dark stains on polished surfaces. Lubricant may appear milky or emulsified.

Location: Often uniform across the raceway in static bearings, or in the loaded zone pattern in rotating bearings where moisture enters past seals.

Root Causes:

Water ingress — Washdown environments, condensation from temperature cycling, leaking seals

Chemical exposure — Acids, alkalis, or process fluids contaminating the bearing

Improper storage — Bearings stored in humid environments without protective coating

Galvanic corrosion — Dissimilar metals in contact in the presence of moisture

Corrective Actions:

Upgrade to sealed bearings (2RS) in wet environments

Use stainless steel bearings (AISI 440C or X65Cr13) for washdown applications

Store bearings in original packaging until installation

Apply preservation coating to spare bearings

Use moisture-resistant lubricants (calcium sulfonate greases)

Verify seal condition during maintenance intervals

Electrical Erosion (Fluting)

Appearance: A pattern of parallel grooves or ridges across the raceway, spaced at regular intervals. Under magnification, the surface shows pitting from electrical discharge craters. In early stages, the raceway has a frosted or gray appearance.

Location: On both races, typically across the full width of the raceway. The pattern is remarkably uniform and evenly spaced — this regularity distinguishes electrical damage from mechanical causes.

Root Causes:

Stray currents — Current passing through the bearing from the motor shaft to ground via the housing. Common with variable frequency drives (VFDs) that generate common-mode voltage.

Static discharge — Belt-driven equipment or processes generating static electricity.

Welding current — Ground current from welding operations passing through nearby bearings.

Corrective Actions:

Install shaft grounding brushes or grounding rings on VFD-driven motors

Use insulated bearings (ceramic-coated outer ring or hybrid bearings with ceramic rolling elements)

Ensure proper motor grounding

Route welding ground cables to avoid current paths through bearings

Use conductive grease if shaft grounding is not practical

Cage Failure

Appearance: Cracked, broken, or deformed cage. Cage pockets may be worn or enlarged. Metal cages may show fatigue cracks. Polymer cages may show melting, discoloration, or embrittlement.

Location: Examine the cage for cracks at the bridges between pockets and for wear on the guiding surfaces (bore or OD depending on cage design).

Root Causes:

Vibration and shock — Excessive vibration causes cage fatigue, especially in pressed steel cages

High acceleration — Rapid speed changes stress the cage as rolling elements shift in their pockets

Contamination — Particles jamming between cage and rolling elements

Inadequate lubrication — The cage is the last component to receive lubricant and the first to suffer from starvation

Excessive temperature — Polymer cages (nylon/polyamide) degrade above 120 degrees C; brass cages are more temperature-resistant

Corrective Actions:

Use brass or machined cages for high-vibration or high-temperature applications

Ensure adequate lubrication reaches the cage

Review bearing clearance and preload settings

Consider a bearing with a different cage design for the application

Misalignment Damage

Appearance: Wear path on the raceway that is not parallel to the race edges. On ball bearings, the ball track appears tilted or wider on one side. On roller bearings, one end of the roller shows heavier wear than the other.

Location: Inner and outer races show opposing tilt patterns. Rollers may show tapered wear or edge loading marks.

Root Causes:

Shaft deflection — Undersized shaft bending under load

Housing bore misalignment — Housing bores not concentric or not perpendicular to the mounting surface

Mounting errors — Cocked bearing on the shaft or in the housing

Foundation settling — Equipment base shifting over time

Corrective Actions:

Check and correct shaft and housing alignment (laser alignment for coupled shafts)

Verify mounting surface flatness and perpendicularity

Use self-aligning bearings (spherical roller or self-aligning ball) where alignment cannot be guaranteed

Review shaft stiffness and add support if deflection is excessive

SKF recommends that misalignment not exceed 2 to 4 minutes of arc for cylindrical roller bearings, or 0.5 degrees for self-aligning types

Diagnostic Flowchart

Use this systematic approach when examining a failed bearing:

1. Examine the lubricant first.

Dark, burnt, or dry lubricant points to lubrication failure or overheating.

Milky or emulsified lubricant indicates water contamination.

Metal particles in the lubricant suggest advanced wear was occurring before failure.

Clean, adequate lubricant with sudden failure suggests overload or installation damage.

2. Check the damage location on the race.

Circumferential band on inner race with localized patch on outer race is normal rotating inner ring fatigue pattern.

Localized damage on inner race with circumferential band on outer race indicates rotating outer ring (less common).

Damage evenly spaced at ball pitch suggests brinelling (static overload or vibration).

Uniform grooves across the full raceway suggest electrical erosion.

3. Examine the rolling elements.

Spalled or pitted balls/rollers confirm fatigue damage.

Flat spots or skid marks suggest insufficient load or sudden speed changes.

Discoloration indicates overheating.

Uniform wear suggests contamination.

4. Check the cage.

Broken cage with otherwise clean races suggests vibration or acceleration damage.

Worn cage pockets with contaminated lubricant suggests particle ingestion.

Melted or discolored polymer cage indicates excessive temperature.

For complex failure analysis or recurring failures, professional bearing forensics can identify root causes that visual inspection may miss. Bearing Consultants (bearingconsultants.com) provides detailed failure analysis services.