Understanding Pitting in Bearings and Gears
Definition: What is Pitting?
Pitting is the formation of small cavities, craters, or depressions on the surface of bearing races, rolling elements, or gear teeth. Pitting can result from two distinct mechanisms: (1) fatigue pitting from rolling contact fatigue, representing early-stage surface fatigue damage, and (2) corrosion pitting from electrochemical attack, creating surface cavities through material removal. In bearings, pitting is often considered an early or mild form of spalling, while in gears, pitting is recognized as a specific failure mode distinct from tooth breakage or excessive wear.
Pitting creates surface roughness and stress concentrations that generate vibration and noise, and if left uncorrected, pit initiation can progress to larger spalls and eventual component failure.
Types of Pitting
1. Fatigue Pitting (Mechanical)
Results from rolling contact fatigue:
Micropitting
- Size: Very small pits (10-50 micrometers diameter)
- Appearance: Matte gray surface texture, barely visible to naked eye
- Location: Concentrated in high-stress areas
- Progression: Can progress to macropitting or stabilize depending on conditions
- Common In: Gears with thin lubricant films, high-speed bearings
Macropitting (Initial Pitting)
- Size: 1-5 mm diameter, 0.1-0.5 mm deep
- Appearance: Clearly visible craters or pits
- Process: Subsurface fatigue crack propagates to surface
- Progression: Usually grows into larger spalls
- Time Frame: May take months to years to progress to failure
2. Corrosion Pitting (Chemical)
Results from electrochemical processes:
- Cause: Moisture, acids, or chemicals in contact with bearing surfaces
- Appearance: Rough, rust-colored pits often with corrosion products
- Distribution: May be widespread across surfaces rather than localized to load zones
- Depth: Typically shallower than fatigue pits initially
- Stress Risers: Corrosion pits act as initiation sites for fatigue cracks
3. Electrical Pitting
- Cause: Electric current passing through bearing creating arcing
- Appearance: Small, closely-spaced craters in regular patterns (fluting)
- Common In: Motors with VFD drives, inadequate shaft grounding
- Characteristic: Washboard or corrugated appearance
- Detection: Distinctive pattern in visual inspection
Detection Methods
Vibration Analysis
Pitting generates characteristic vibration signatures:
- Early Stage: Small increase in high-frequency vibration
- Fault Frequencies: Appearance of BPFO, BPFI, or BSF depending on location
- Envelope Analysis: Most effective for detecting early-stage pitting
- Progression: Amplitude and harmonic count increase as pits grow
Visual Inspection
- Requires bearing disassembly or borescope access
- Look for gray matte areas (micropitting) or visible craters (macropitting)
- Count and size pits to assess severity
- Photograph for documentation and trending
Oil Analysis
- Metal particles in oil samples indicate active material removal
- Ferrography shows particle morphology characteristic of pitting vs. wear
- Increasing particle concentration indicates progressive damage
Ultrasonic Testing
- Surface roughness from pitting increases ultrasonic emissions
- Portable ultrasound detectors for non-invasive monitoring
- Effective for early detection before vibration symptoms clear
Causes and Prevention
Fatigue Pitting Prevention
- Adequate Bearing Rating: Select bearing with L10 life exceeding required service
- Proper Lubrication: Ensure adequate film thickness (λ ratio > 3)
- Cleanliness: Minimize contamination that creates stress risers
- Alignment: Prevent edge loading from misalignment
- Load Control: Avoid overloading (bearing life ∝ 1/Load³)
Corrosion Pitting Prevention
- Effective Sealing: Prevent moisture ingress
- Proper Lubricant: Use lubricants with corrosion inhibitors
- Corrosion-Resistant Materials: Stainless steel bearings for wet environments
- Storage Protection: Protect spare bearings from humidity
- Condensation Prevention: Avoid thermal cycling causing condensation
Electrical Pitting Prevention
- Proper shaft grounding in VFD motor applications
- Insulated bearings for one end of motor
- Ceramic rolling elements (non-conductive)
- Eliminate or minimize bearing currents
Pitting in Gears
In gear applications, pitting has specific characteristics:
Gear Tooth Pitting
- Occurs on tooth surfaces in contact zone
- Concentrated near pitch line where sliding and rolling combine
- Creates characteristic noise and vibration at gear mesh frequency
- Can be acceptable in moderate amounts for some applications
- Excessive pitting leads to tooth breakage
Detection
- Increased gear mesh frequency vibration
- Sidebands at shaft frequencies
- Audible noise changes
- Visual inspection of tooth flanks
Severity Assessment
Bearing Pitting Severity Criteria
- Incipient: Few scattered micropits, < 5% of load zone affected
- Light: Visible pitting, 5-25% of load zone area
- Moderate: Extensive pitting, 25-50% area, beginning to coalesce
- Severe: > 50% area, pits joining to form spalls, immediate replacement required
Decision Criteria
- Continue Operation: Incipient to light pitting with monitoring
- Plan Replacement: Moderate pitting, schedule change-out within 1-3 months
- Urgent Replacement: Severe pitting, replace at first opportunity
- Emergency Shutdown: Rapid progression, high vibration or temperature
Pitting represents an important intermediate stage in bearing degradation—more serious than general surface wear but potentially manageable if detected early. Through vibration monitoring with envelope analysis and proper maintenance practices, pitting can be identified in its early stages, allowing planned maintenance before progression to catastrophic spalling failures.
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