Every Failed Part Is Evidence
When a bearing, gear, or seal fails, maintenance teams focus on getting the equipment back in service — which is the right priority. But in the rush to rebuild, critical evidence gets tossed into a scrap bin. That failed bearing carries information about why it failed, and without examining it, you’re likely to repeat the same conditions and get the same result.
Component failure analysis doesn’t require a metallurgical lab for most industrial applications. A visual examination with a flashlight and magnifying glass, combined with knowledge of common failure patterns, identifies the root cause in the majority of cases. It takes 5-10 minutes. The return is preventing the next failure of the same type.
Bearing Failure Patterns
Fatigue Spalling
Subsurface cracks propagate to the raceway surface, causing material to flake off in small pieces. The spalled area starts small and grows over time. The surface inside the spall appears rough and granular, with clean edges where material has broken away.
What it means: This is the “normal” bearing failure mode — the bearing reached its fatigue life. If the bearing ran for many years before spalling appeared, the application and maintenance were probably appropriate. If spalling occurred prematurely (within the first 1-2 years on a bearing calculated for 10+ years of L10 life), investigate overloading, misalignment, or improper fit.
Location matters: Spalling on the outer race only (in the load zone) suggests static overloading or insufficient internal clearance. Spalling on the inner race with a pattern that moves around the circumference suggests rotating overload — often from misalignment or imbalance.
Brinelling
Permanent dents in the raceway that correspond to the spacing of the rolling elements. Each dent is the shape of the rolling element contact area — elliptical for ball bearings, rectangular for roller bearings.
What it means: Impact loading — the bearing was subjected to shock forces that exceeded the elastic limit of the raceway steel. Common sources include equipment dropping during installation, impact loads during operation (pressing, stamping, crushing operations), and transportation damage.
False brinelling looks similar but is caused by vibration when the bearing is stationary. The rolling elements wear shallow depressions in the raceway through micro-fretting motion. This occurs in standby equipment, equipment in storage, and equipment transported without shaft locking.
Electrical Discharge Damage (Fluting)
Parallel grooves across the raceway surface, oriented perpendicular to the direction of rolling element travel. The pattern resembles a washboard. Under magnification, the groove bottoms show a frosted or cratered appearance from electrical discharge machining (EDM) of the surface.
What it means: Electrical current passed through the bearing, causing micro-arcs between rolling elements and raceways that erode the surface. The primary source in modern plants is VFD-induced shaft voltage. Other sources include welding ground current flowing through the bearing (always clamp the welding ground close to the weld point — never let current path through bearings) and static discharge from belts or processes.
Contamination Damage
Denting of raceways by hard particles (dirt, metal chips, grinding debris) that entered the bearing. The dents are randomly spaced and shaped. Surrounding each dent is a raised ring of displaced metal that acts as a stress riser, accelerating fatigue. Bearings with contamination damage often show premature spalling originating at the dent sites.
What it means: The sealing system failed to keep contaminants out, or contamination was introduced during installation or lubrication. Examine the seals, the breather, and the lubrication practices. Also examine whether contamination was introduced during the bearing installation itself — mounting bearings in a dirty environment, using contaminated lubricant, or leaving the bearing unwrapped on a workbench.
Lubrication-Related Damage
Discoloration (heat tinting) of raceways and rolling elements ranging from straw yellow to blue-black indicates the bearing operated at excessive temperatures. Smearing — areas where surface metal has been plastically deformed by sliding contact — indicates lubricant film failure. Micro-pitting (a dull, matte surface finish) indicates marginal lubrication — the film was thin enough for asperity contact but not thin enough for gross smearing.
What it means: Insufficient lubricant quantity, wrong lubricant viscosity, excessive temperature reducing viscosity below operating requirements, or too-long intervals between relubrication. Check the lubrication type, quantity, interval, and delivery method. A grease-lubricated bearing that shows heat discoloration near the seal on one side may indicate over-greasing — excess grease trapped by the seal generates heat from churning.
Gear Failure Patterns
Pitting
Small surface craters on gear teeth, typically starting at the pitch line where contact stress is highest. Initial pitting (micro-pitting) gives the tooth surface a dull, frosted appearance. Progressive pitting produces visible craters 1-3 mm in diameter.
What it means: Surface fatigue from contact stress exceeding the material’s endurance limit. Contributing factors include overloading, improper tooth contact pattern (misalignment concentrating load on a portion of the tooth face), inadequate lubricant viscosity, and contaminated lubricant. Minor pitting that stabilizes and doesn’t progress (corrective pitting) is acceptable — the surfaces are self-correcting their contact pattern.
Scoring and Scuffing
Linear scratches or smeared metal on the tooth flanks running in the direction of sliding. Scoring indicates abrasive particles in the lubricant scratching the surface. Scuffing indicates metal-to-metal adhesion from lubricant film breakdown — the surface metal welds momentarily and tears apart, leaving a rough, torn appearance.
What it means: Scoring — contamination in the oil. Check filtration, seals, and breathers. Scuffing — lubricant film failure. The oil viscosity may be too low for the load and temperature, the EP additive package may be depleted, or the gear may be overloaded beyond the lubricant’s capacity. Scuffing often occurs during initial break-in if the gear set wasn’t run at reduced load per the manufacturer’s break-in procedure.
Tooth Breakage
Complete fracture of one or more teeth. Examine the fracture surface. A fatigue fracture shows a smooth, relatively flat surface with beach marks (concentric arc patterns) radiating from the crack origin. The crack origin is usually at the tooth root fillet on the tension side. An overload fracture shows a rough, granular surface without beach marks — the tooth broke in a single event rather than from progressive cracking.
What it means: Fatigue fracture — the tooth root stress exceeded the material’s endurance limit through overloading, stress concentration (nicks or grinding marks at the root fillet), or case/core problems in case-hardened gears. Overload fracture — a single extreme load event such as a jam, foreign object, or sudden torque spike exceeded the tooth’s ultimate strength.
Seal Failure Patterns
Mechanical Seal Face Damage
Heat checking — a network of fine cracks on the seal face from thermal shock or excessive heat generation. Indicates dry running, insufficient flush, or excessive face loading. Chipping at the outer edge of a hard face (silicon carbide, tungsten carbide) — thermal shock from intermittent dry running, or a damaged O-ring allowing the seal to cock. Grooving — circumferential grooves worn into the seal face by abrasive particles. Indicates inadequate seal flush cleanliness.
Lip Seal Damage
Hardened, cracked lip — heat aging from excessive operating temperature. The rubber has lost its elasticity and no longer conforms to the shaft. Worn lip (groove worn in the contact band) — normal wear if it occurred over an extended period. Premature wear indicates shaft surface roughness, misalignment, or contamination at the sealing surface. Inverted lip — the seal was installed backward, or differential pressure pushed the lip in the wrong direction.
Making Failure Analysis a Habit
Incorporate failed component examination into your standard maintenance workflow. When a bearing, gear, or seal is removed, the technician should spend 5 minutes examining it before discarding it. A simple checklist works:
- What’s the primary damage pattern? (Spalling, contamination, heat damage, etc.)
- Where is the damage concentrated? (Load zone, everywhere, one side only?)
- Does the pattern indicate a specific root cause? (Misalignment, lubrication, contamination, overload?)
- Has this same pattern been seen before on this equipment?
Photograph significant findings with a phone camera. Attach the photo to the work order in the CMMS. Over time, you build a visual library of failure patterns that supports training, root cause analysis, and design improvement decisions. The equipment is telling you why it failed — all you have to do is look at the evidence before throwing it away.