Understanding Bearing Wear
Definition: What is Bearing Wear?
Bearing wear is the progressive loss of material from bearing surfaces (races, rolling elements, and cage) through mechanical processes such as abrasion, adhesion, corrosion, or surface fatigue. Unlike sudden failures from fatigue spalling, bearing wear is a gradual degradation process that increases bearing clearances, reduces precision, and ultimately leads to functional failure when clearances become excessive or surface damage becomes severe.
Bearing wear is detectable through vibration monitoring (increasing high-frequency content and overall levels), temperature monitoring (changes in friction), and physical inspection (visible wear patterns, increased play). Understanding wear mechanisms enables proper bearing selection, lubrication practices, and maintenance strategies.
Mechanisms of Bearing Wear
1. Abrasive Wear
The most common wear mechanism in industrial bearings:
- Cause: Hard particles (dirt, metal chips, wear debris) entering bearing
- Process: Particles trapped between rolling elements and races act like grinding compound
- Result: Material removed from softer surfaces (usually races), creating grooves or polished wear tracks
- Rate: Proportional to contamination level and particle hardness
- Prevention: Effective sealing, filtration, clean assembly practices
2. Adhesive Wear (Scuffing)
Occurs under boundary lubrication or dry contact conditions:
- Cause: Inadequate lubrication allowing metal-to-metal contact
- Process: Microscopic welding and tearing at contact points
- Result: Rough, discolored surfaces; material transfer between races and rolling elements
- Progression: Can rapidly escalate once initiated
- Prevention: Adequate lubrication quantity and quality
3. Fretting Wear (False Brinelling)
Occurs in stationary or oscillating bearings:
- Cause: Small-amplitude oscillatory motion while bearing not rotating (vibration during transport or storage)
- Process: Micro-slip between rolling elements and races creates oxide debris
- Result: Reddish-brown deposits in contact areas, shallow depressions
- Visual: Appearance similar to true brinelling but without permanent deformation
- Prevention: Vibration isolation during storage/transport, slight bearing rotation, or adequate preload
4. Corrosive Wear
- Cause: Moisture, chemicals, or aggressive environments
- Process: Chemical attack creating pitting and surface roughness
- Result: Rust-colored deposits, rough surfaces, material loss
- Common In: Food processing, marine environments, chemical plants
- Prevention: Corrosion-resistant bearings, effective sealing, proper lubricant selection
5. Erosive Wear
- Cause: High-velocity fluid flow carrying particles
- Common In: Contaminated lubricants with circulation systems
- Result: Smoothly eroded surfaces, material removal
- Prevention: Filtration, clean lubricants, proper seal design
Vibration Symptoms of Bearing Wear
Gradual Changes
Wear produces characteristic progressive vibration changes:
- Increasing Overall Level: Total RMS vibration gradually increases
- High-Frequency Content: More energy in high-frequency range (> 1000 Hz)
- Broadband Noise: Elevated noise floor across spectrum
- Multiple Small Peaks: Rather than single dominant defect frequency
- Loss of Tracking: 1× peak may become less prominent relative to higher frequencies
Distinguishing Wear from Defects
| Characteristic | Localized Defect (Spall) | General Wear |
|---|---|---|
| Fault Frequencies | Clear BPFO, BPFI, BSF peaks | No clear defect frequencies |
| Spectrum Appearance | Discrete peaks with harmonics | Broad elevated noise floor |
| Progression | Exponential amplitude growth | Gradual linear increase |
| Envelope Analysis | Strong response, clear peaks | Moderate broadband increase |
| Time to Failure | Weeks to months once detected | Months to years of gradual degradation |
Detection Methods
Vibration Monitoring
- Trend overall RMS levels over time
- Monitor high-frequency acceleration (HFD – High Frequency Defect indicator)
- Crest factor may remain relatively normal (unlike spalling where it increases)
- Kurtosis doesn’t show dramatic changes (distributed wear vs. discrete impacts)
Temperature Monitoring
- Bearing temperature trending
- Wear often causes temperature increase from higher friction
- Gradual rise (2-5°C/year) indicates progressive wear
- Sudden jumps suggest transition to more severe damage
Ultrasound Monitoring
- Ultrasonic emissions increase with surface roughness
- Effective for detecting early-stage wear
- Portable ultrasound instruments for route-based inspections
Oil Analysis
- Wear debris in oil samples
- Particle counting and analysis
- Ferrography showing wear particle characteristics
- Increasing particle concentration indicates progressive wear
Causes and Contributing Factors
Lubrication-Related
- Inadequate lubricant quantity (starvation)
- Wrong lubricant viscosity for operating conditions
- Contaminated lubricant (particles, water, chemicals)
- Degraded lubricant (oxidation, loss of additives)
- Improper re-lubrication intervals
Operating Conditions
- Excessive bearing loads (static or dynamic)
- High operating temperatures
- Contaminated environment
- Inadequate sealing allowing particle ingress
- Vibration from external sources (nearby equipment)
Installation and Maintenance
- Improper installation causing misalignment
- Incorrect bearing clearance selection
- Contamination during installation
- Damaged seals allowing contamination entry
Prevention and Life Extension
Lubrication Best Practices
- Use correct lubricant type and grade for application
- Maintain proper lubricant level (not too much or too little)
- Establish appropriate re-lubrication intervals
- Monitor lubricant condition, replace when degraded
- Use clean practices during lubrication
Contamination Control
- Effective sealing to prevent particle ingress
- Clean installation practices
- Filtered lubrication systems where applicable
- Environmental controls (enclosures, positive pressure)
- Regular inspection and seal replacement
Operating Condition Management
- Operate within bearing design limits (load, speed, temperature)
- Maintain good balance to minimize dynamic loads
- Ensure precision alignment to prevent edge loading
- Control operating temperatures through cooling if needed
Bearing wear, while gradual and less dramatic than sudden spalling failures, represents a significant portion of bearing deterioration in industrial service. Proper lubrication, contamination control, and condition monitoring enable early detection and allow planned bearing replacement before wear progresses to functional failure, optimizing both equipment reliability and maintenance costs.
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