What Bearing Damage Patterns Actually Tell You
When a bearing fails, the raceway is a diagnostic record of how it was loaded, lubricated, and installed. Simply pressing a new bearing onto the shaft without reading the old one guarantees the replacement will fail on the exact same schedule. Understanding the physical mechanisms behind the damage patterns allows you to identify and eliminate the root cause, whether it is a housing distortion issue, a VFD-induced shaft current, or a lubrication breakdown.
True Brinelling vs. False Brinelling
Both conditions leave indentations on the raceway, but their mechanisms and root causes are entirely different. True brinelling occurs when the rolling elements exceed the elastic limit of the bearing steel, permanently deforming the raceway. You will see evenly spaced dents matching the ball or roller pitch. The cause is almost always static overload—either a severe impact during installation, dropping the assembly, or leaving a machine subjected to heavy vibration while stationary. The steel yields under point contact stress.
False brinelling looks similar but happens under dynamic conditions with no rotation. When a machine is idle but subjected to external vibration (like a standby pump next to a running unit), the rolling elements vibrate microscopically in their tracks. This motion breaks down the elastohydrodynamic lubricant film, and the resulting metal-to-metal contact causes fretting wear at the contact points. The key visual difference is that false brinelling wear marks often have a rusty or oxidized appearance and lack the sharp, polished edges of true brinelling. The fix for false brinelling is isolating idle equipment from vibration or rotating the shaft periodically.
Spalling and Subsurface Fatigue
Spalling is the macroscopic result of subsurface fatigue. Under normal Hertzian stress, the highest shear stress occurs slightly below the surface of the raceway, not at the surface. Over time, micro-cracks initiate at this subsurface depth, typically at non-metallic inclusions in the steel, and propagate upward. Eventually, a flake of metal detaches, leaving a rough, cratered surface. This is classic fatigue life—the L10 life calculation predicts when 10% of a population will reach this state.
However, if spalling appears prematurely, you need to determine what accelerated the fatigue. Heavy external shock loads, continuous overloading, or poor shaft/housing geometry that concentrates stress will force the subsurface shear stress to exceed the material's endurance limit. When inspecting rolling bearings for spalling, trace the spall location: a spall on one specific area of the inner ring points to a stationary inner ring with a rotating load zone, while a circumferential spall suggests the ring was rotating in its housing.
Brinelling vs. Spalling: How to Tell the Difference
These two failure modes are the most commonly confused on a raceway because both leave visible pitting or indentation — but the mechanism, timing, and root cause are opposite. Brinelling is an instantaneous, static-overload event; spalling is a slow, cyclic-fatigue event that only shows up after extended running time.
| Signal | Brinelling | Spalling |
|---|---|---|
| Cause | Static overload or impact — installation shock, a dropped assembly, or stationary vibration | Subsurface rolling-contact fatigue — normal wear-out, or accelerated by shock loads and misalignment |
| When it appears | Immediately, often before the bearing ever rotates in service | After extended running time, once fatigue life is reached — or sooner if overloaded |
| Pattern | Evenly spaced dents matching the ball or roller pitch | Irregular, cratered patches where flakes of metal have detached |
| Depth of origin | Surface — the steel yields under point contact stress | Subsurface — micro-cracks start below the surface at non-metallic inclusions and propagate upward |
| Fix | Handle and install correctly; isolate idle equipment from vibration | Address the overload, shock, or misalignment accelerating the fatigue — not just replace the bearing |
Electrical Damage and Fluting
Variable frequency drives (VFDs) induce common-mode currents on motor shafts. When the voltage builds up enough to overcome the dielectric strength of the lubricant film, it discharges through the bearing. The arc is incredibly hot and localized, melting a microscopic pit in the raceway and the rolling element. Over time, these pits accumulate, creating a frosted or dull appearance known as electric erosion.
As the bearing rotates, the rolling elements pass over these pits, and the resulting micro-vibration causes the arcs to strike at regular intervals. This creates a distinct washboard pattern of dark and light bands called fluting. Fluting is a dead giveaway of shaft current discharge. Replacing the bearing will not stop the electrical discharge; you must address the root cause by installing shaft grounding rings, using insulated bearings, or applying VFD output filtering.
Smearing, Skidding, and Scoring
Rolling elements are designed to roll, not slide. If the bearing operates with insufficient load, the rolling element can skid rather than roll, causing smearing. This happens frequently on the unloaded side of the bearing or in lightly loaded vertical shaft applications. Smearing appears as a smeared, streaky metal transfer on the raceway or roller ends, caused by the lubricant film breaking down under sliding friction. The fix is increasing the bearing preload or switching to a bearing geometry better suited for light loads.
Scoring and abrasive wear, on the other hand, look like deep circumferential scratches or a general dulling of the raceway surface. This is caused by hard particles circulating through the bearing. The rolling elements crush the particles into the raceway, plowing microscopic furrows. If you find abrasive wear, the lubricant is contaminated. Check your seals, verify that the breather is functioning, and ensure lubrication procedures are not introducing debris into the housing.
Discoloration and Overheating
Heat changes the appearance of bearing steel and lubricant. A blue or brown tint on the raceway or rolling elements indicates the steel reached temperatures where oxidation or tempering occurs—typically above 150°C to 200°C. This heat usually comes from lubricant starvation, excessive speed, or an overly heavy preload. The heat not only discolors the steel but softens it, reducing hardness and accelerating subsequent fatigue.
Lubricant breakdown is closely tied to overheating. The general rule of thumb is that for every 10°C increase in operating temperature above 70°C, the oxidation life of mineral oil is halved. When the lubricant oxidizes, it forms varnish and carbonaceous sludge. If you open a bearing and find a dark, baked-on residue or a crusty, lacquered film on the cage and raceways, the lubricant has thermally failed. You must evaluate the viscosity and base oil type, not just change the bearing.
Fretting Corrosion at the Fit
Fretting corrosion happens at the interface between the bearing inner ring and the shaft, or the outer ring and the housing. It requires two conditions: a loose fit and micro-motion. If the bearing is under-loaded or pressed onto an undersized shaft, the ring can shift microscopically relative to the shaft during operation. This oscillation wears away the protective oxide layer, exposing fresh metal which oxidizes again, creating a reddish-brown iron oxide dust.
This red dust is often mistaken for rust from moisture, but it is actually wear debris. Fretting corrosion destroys the bearing seat, meaning a simple bearing replacement will fail quickly because the new bearing will also fret. The repair requires building up the shaft or housing bore to restore the correct interference fit specified by the manufacturer. For high-precision applications, sourcing from established manufacturers like SKF or Timken ensures you have access to the exact engineering tolerance data needed to machine the seat correctly.
Reading the Load Zone
The load zone pattern tells you how the bearing was loaded in its housing. In a properly aligned radial bearing with a stationary outer ring and a rotating inner ring, the load zone is a single, narrow, heavily worn track at the bottom of the outer ring raceway, opposite the direction of the applied radial load. The rest of the raceway should look relatively untouched.
If the load zone track is skewed or wider than the rolling element width, the bearing was misaligned. Misalignment forces the rolling elements to bridge the gap between the inner and outer rings at an angle, scrubbing the raceway edges. If you see a load zone that is diametrically opposed—two heavy wear tracks 180 degrees apart—you likely have a rotating outer ring load or a severe housing resonance. If the wear track on the inner ring is wide and continuous while the outer ring track is narrow, the inner ring was stationary relative to the load. Matching the wear tracks on deep groove ball bearings or roller bearings to the housing geometry is the fastest way to confirm if the shaft was bending or the housing was distorting under load.
If you are diagnosing a bearing failure and need to source replacements or cross-reference an existing part, our team can help you find the exact fit and specification for your application.
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