Rolling-element bearings fail. Even under ideal conditions, the cyclic contact stresses in a loaded bearing eventually cause subsurface fatigue cracks that propagate to the raceway surface and form spalls. But most bearings never reach their calculated fatigue life — they fail prematurely due to contamination, improper installation, inadequate lubrication, or operating conditions that exceed their design envelope. Understanding the common failure modes, their root causes, and their vibration signatures is essential for both failure prevention and accurate diagnosis when a bearing does deteriorate.
Subsurface Fatigue (Spalling)
Mechanism
Subsurface fatigue is the classical bearing failure mode — the one that the L10 life calculation describes. Repeated Hertzian contact stress between rolling elements and raceways creates a cyclic shear stress field below the surface (maximum shear stress occurs at a depth of approximately 0.5× the contact half-width, typically 0.1–0.5 mm below the surface for most industrial bearings). Over millions of stress cycles, micro-cracks nucleate at material inclusions or carbide particles and propagate parallel to the surface. When a crack network reaches the surface, a piece of material breaks away, forming a pit or spall.
The initial spall is typically small — a few hundred micrometers to a few millimeters across. It grows with continued operation as the exposed edges of the spall act as stress concentrators, accelerating crack propagation. Left unchecked, the spall expands around the raceway until the bearing produces severe vibration, elevated temperature, and eventual seizure.
Vibration Signature
Early-stage spalling produces sharp, periodic impulses as each rolling element crosses the spall. These impulses appear in the time waveform as brief, high-amplitude spikes at the bearing defect frequency (BPFO for outer race spalling, BPFI for inner race). In the frequency domain:
- Envelope spectrum shows clear peaks at BPFO (or BPFI) and harmonics (2×, 3×, 4×)
- Raw spectrum may show increased broadband energy in the 1–10 kHz range (the structural resonance band excited by the impulses)
- As the spall grows, the defect frequency harmonics increase in number and amplitude, and the time waveform shows a rising crest factor (peak/RMS ratio)
- Late-stage spalling produces a noisy, irregular waveform with elevated overall vibration velocity
Root Cause
Normal fatigue is inherent to bearing operation and is the expected end-of-life mechanism. Premature fatigue (well before L10 life) indicates excessive load, inadequate lubrication, or material quality issues. Overloading compresses the fatigue life exponentially — doubling the radial load reduces L10 life by approximately a factor of eight for ball bearings (the load-life exponent is 3).
Brinelling and False Brinelling
True Brinelling
Brinelling is permanent indentation of a raceway caused by static overload or shock loading. When a stationary bearing receives an impact force that exceeds the elastic limit of the raceway material, the rolling elements press permanent dents into the raceway. These dents are spaced at the rolling element pitch — one dent per ball position. The name comes from the similarity to a Brinell hardness test indent.
Common causes include rough handling during installation (dropping a bearing or driving it onto a shaft with hammer blows transmitted through the rolling elements), transportation vibration of heavy equipment (machinery shipped by truck or rail with the shaft locked in one position), and shock loads during operation (water hammer in pumps, sudden coupling engagement).
False Brinelling
False brinelling produces similar-looking raceway indentations but through a different mechanism: fretting corrosion from small oscillatory motion while the bearing is stationary or rotating very slowly. When a machine sits idle and is subjected to external vibration (from nearby operating equipment, for example), the rolling elements oscillate microscopically against the raceways. This micro-motion wears away the lubricant film and creates small, oxidized wear marks at each ball contact point.
False brinelling is common in standby equipment, spare pumps, transport vehicles, and any machinery that sits for extended periods in a vibrating environment.
Vibration Signature
Both true and false brinelling produce a series of evenly spaced surface irregularities on the raceway. As the shaft rotates and rolling elements traverse these dents, the vibration signature includes:
- Elevated vibration at BPFO (outer race) or BPFI (inner race) harmonics, similar to spalling but often with broader, less sharp spectral peaks
- Increased overall vibration level, particularly in the velocity range (10–1,000 Hz)
- In severe cases, audible rumbling or growling that is evident immediately upon startup
- The pattern is typically uniform around the raceway (multiple evenly spaced dents), which may produce a spectrum dominated by higher harmonics of the defect frequency rather than the fundamental
Contamination
Mechanism
Particle contamination — hard particles (sand, metal chips, scale) or soft particles (fibers, rubber fragments) entering the bearing — is the single most common cause of premature bearing failure. Industry studies consistently attribute 20–30% of all bearing failures to contamination. Particles enter through inadequate sealing, contaminated lubricant, or during installation (dirty work practices, contaminated grease in the supply chain).
Hard particles larger than the minimum oil film thickness (typically 0.2–2 μm for EHL contacts) are overrolled by the rolling elements and indent the raceways. Each indent acts as a stress concentrator that accelerates subsurface fatigue. The effect is cumulative: thousands of small dents distributed randomly across the raceways reduce the bearing fatigue life by factors of 2–10 or more, depending on contamination severity.
Vibration Signature
Contamination damage produces a distinctive vibration signature that differs from localized spalling:
- Elevated broadband vibration floor (raised noise floor across the spectrum) rather than discrete peaks at defect frequencies
- Increased high-frequency energy (acceleration domain, above 1 kHz), reflecting the many small surface irregularities
- Kurtosis of the time waveform increases as contamination worsens (more frequent impulsive events from overrolling particles and dents)
- In early stages, overall velocity may remain near baseline while high-frequency metrics (HFD, acceleration envelope RMS) rise — this is because the individual dents are too small to produce significant low-frequency vibration but collectively increase high-frequency impulsiveness
- As contamination damage progresses, the multiple dents eventually merge into larger spalls, and the signature transitions to resemble classical fatigue spalling
Distinguishing contamination-induced distributed damage from early-stage localized spalling is important for root cause analysis. Contamination suggests a sealing or lubrication supply problem; localized spalling suggests overloading, misalignment, or normal end-of-life fatigue. Vibration monitoring systems designed for forensic root cause determination, such as Fault Ledger, capture high-resolution waveform data that preserves the statistical character of the vibration (kurtosis, crest factor, and impulsive event distribution), enabling analysts to distinguish distributed contamination damage from localized defects during post-failure investigation.
Misalignment
Mechanism
Bearing misalignment occurs when the shaft axis and the bearing bore axis are not concentric (radial misalignment) or not parallel (angular misalignment). This forces the rolling elements to traverse a non-ideal load distribution, generating axial forces in bearings designed for radial loads, uneven contact stresses, and cage loading.
Common causes include imprecise shaft machining or housing boring, thermal growth that changes alignment as the machine warms from ambient to operating temperature, soft foot (uneven mounting surfaces that distort the housing when bolts are tightened), and improper shimming or coupling alignment during installation.
Vibration Signature
Misalignment produces a distinctive vibration pattern:
- Axial vibration dominance: Misaligned bearings generate significantly elevated axial (parallel to shaft) vibration relative to radial vibration. A 2:1 or greater ratio of axial to radial vibration amplitude at shaft frequency is a strong misalignment indicator.
- 2× shaft speed: Angular misalignment produces a strong 2× RPM component, often dominant over the 1× component. The once-per-revolution variation in contact conditions creates a twice-per-revolution vibration response.
- Harmonic series at shaft speed: Severe misalignment generates a series of shaft-speed harmonics (1×, 2×, 3×, 4× RPM and higher), with 2× typically dominant.
- Bearing defect frequency modulation: Misalignment alters the load distribution around the raceway, causing amplitude modulation of any existing defect signatures. This appears as sidebands around bearing defect frequencies spaced at shaft speed.
Consequences for Bearing Life
Misalignment does not cause immediate catastrophic failure, but it accelerates fatigue by creating non-uniform contact stress distribution. The most heavily loaded region of the raceway sees stresses that exceed the design basis, while other regions are under-loaded. The net effect is a reduction in fatigue life proportional to the severity of misalignment. Industry data suggests that 0.001 inch/inch of angular misalignment can reduce bearing life by 30–50%.
Other Failure Modes
Lubrication Failure
Inadequate lubrication — wrong viscosity, insufficient quantity, degraded grease, or excessive relubrication interval — is a contributing factor in an estimated 40–50% of bearing failures. Without an adequate elastohydrodynamic (EHL) lubricant film, metal-to-metal contact between rolling elements and raceways causes adhesive wear, surface distress, and accelerated fatigue. The vibration signature of lubrication-related distress includes elevated high-frequency energy (often detected with ultrasonic techniques in the 25–50 kHz range), increased bearing temperature, and eventually the onset of spalling signatures as the surface degrades.
Electrical Erosion (Fluting)
Stray electrical currents passing through bearing rolling contacts — common in variable-frequency drive (VFD) applications — cause electrical discharge machining (EDM) of the raceways. The damage appears as a pattern of closely spaced pits arranged in circumferential bands (fluting). The vibration signature includes elevated broadband energy and, as fluting progresses, peaks at BPFO and BPFI with a characteristic washboard pattern on the raceways.
Corrosion
Moisture ingress (from process leaks, washdown, condensation) causes rust and oxidation on raceway surfaces. Corroded surfaces create surface roughness that accelerates fatigue and increases vibration. The signature is similar to contamination — elevated broadband noise floor with increased high-frequency content.
Matching Failure Mode to Vibration Signature: Why It Matters
Detecting that a bearing is deteriorating is only the first step. Effective root cause analysis requires identifying which failure mode is responsible so that corrective action addresses the underlying problem, not just the symptom. Replacing a bearing that failed from contamination without fixing the seal that allowed particle ingress guarantees a repeat failure. Replacing a bearing that failed from misalignment without correcting the shaft alignment wastes the new bearing.
This is where the depth of the vibration data matters. Simple overall vibration level (a single RMS velocity number) can detect that something is wrong, but it cannot distinguish contamination from fatigue from misalignment. Frequency-domain analysis with defect frequency identification can detect and classify bearing defects. Full waveform capture with statistical analysis (kurtosis, crest factor, impulse event characterization) provides the deepest diagnostic insight.
Fault Ledger approaches this as a forensic evidence problem: by capturing and preserving high-resolution vibration waveforms throughout the bearing deterioration process, the system creates a time-stamped record of how the fault developed. This forensic record allows reliability engineers to trace a failure back to its root cause — distinguishing, for example, a contamination-initiated fatigue failure from a misalignment-initiated one — by examining the temporal progression of the vibration signature from first detection to final failure.
Conclusion
Bearing failures are not random events — they follow characteristic patterns determined by the specific failure mechanism. Fatigue produces localized spalls with periodic impulses at defect frequencies. Brinelling produces evenly spaced raceway dents from static overload or fretting. Contamination produces distributed surface damage with elevated broadband noise. Misalignment produces axial vibration dominance and strong 2× shaft-speed harmonics. Recognizing these patterns in vibration data enables both timely detection and accurate root cause identification, closing the loop between condition monitoring and reliability improvement.