What is Spalling in Rolling Element Bearings? • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors What is Spalling in Rolling Element Bearings? • Portable balancer, vibration analyzer "Balanset" for dynamic balancing crushers, fans, mulchers, augers on combines, shafts, centrifuges, turbines, and many others rotors

What is Spalling in Rolling Element Bearings?

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Understanding Spalling in Rolling Element Bearings

Definition: What is Spalling?

Spalling (also called spall, flaking, or pitting when small) is the localized flaking, chipping, or fracture of material from the surface of bearing races or rolling elements due to rolling contact fatigue. A spall appears as a crater, pit, or chip where a piece of material has broken away from the surface, leaving a rough, damaged area. When rolling elements pass over a spall, they generate impact forces that create characteristic vibration at specific bearing fault frequencies.

Spalling is the most common and normal bearing failure mode, representing the end of a bearing’s fatigue life. It is distinct from wear (gradual material removal) or pitting (corrosion-induced surface damage). Spalling is detectable through vibration analysis months before the bearing fails completely, making it a key target for predictive maintenance programs.

Physical Mechanism of Spalling

Rolling Contact Fatigue Process

Spalling develops through a progressive fatigue process:

  1. Cyclic Loading: Every time a rolling element passes a point on the race, it creates a Hertzian contact stress (typically 1000-3000 MPa)
  2. Subsurface Shear Stress: Maximum shear stress occurs slightly below the surface (typically 0.2-0.5 mm depth)
  3. Crack Initiation: After millions or billions of cycles, microscopic crack initiates at subsurface stress concentration
  4. Crack Propagation: Crack grows parallel to surface, then branches toward surface and deeper into material
  5. Material Separation: Crack network isolates a piece of material
  6. Spall Formation: Isolated material breaks free, leaving crater or pit

Typical Spall Characteristics

  • Size: Initially 1-5 mm diameter, can grow to 10-20 mm or more
  • Depth: 0.2-2 mm deep into hardened case
  • Shape: Irregular crater with rough bottom and edges
  • Location: Most often on outer race in load zone
  • Appearance: Metallic, bright surface with sharp edges initially; darkens with continued operation

Causes and Contributing Factors

Normal Fatigue Life

  • All bearings have finite fatigue life (L10 life – 90% survive to this point)
  • Spalling is the expected end-of-life failure mode
  • Proper bearing selection ensures adequate life for application
  • Not a defect if occurs at or beyond calculated L10 life

Premature Spalling Causes

  • Overloading: Loads exceeding bearing rating drastically reduce life (Life ∝ 1/Load³)
  • Poor Lubrication: Inadequate film thickness increases surface stress
  • Contamination: Particles creating stress risers that initiate cracks
  • Misalignment: Edge loading creating high local stresses
  • Incorrect Installation: Damage during mounting initiating early failures
  • Corrosion: Surface pits acting as crack initiation sites
  • Material Defects: Inclusions in bearing steel

Vibration Detection of Spalling

Early Stage (Micro-Spall)

  • Spall < 1-2 mm diameter
  • Small peaks at bearing fault frequencies in envelope spectrum
  • May not be visible in standard FFT spectrum
  • Amplitude in envelope: 0.5-2 g
  • Remaining life: 6-18 months typically

Moderate Stage

  • Spall 2-10 mm diameter
  • Clear fault frequency peaks in both FFT and envelope spectra
  • 2-3 harmonics visible
  • Beginning of sideband formation
  • Amplitude: 2-10 g
  • Remaining life: 2-6 months

Advanced Stage

  • Spall > 10 mm, may be multiple spalls
  • Very high amplitude fault frequency peaks
  • Numerous harmonics (4-8 or more)
  • Complex sideband structure
  • Elevated noise floor
  • Amplitude: > 10 g
  • Remaining life: Days to weeks

Severe/Critical Stage

  • Extensive spalling, multiple defects
  • Broadband noise dominant
  • Individual fault frequencies may become obscured
  • Very high overall vibration
  • Audible noise from bearing
  • Elevated temperature
  • Imminent failure – immediate replacement required

Progression and Secondary Damage

Spall Growth

Once initiated, spalls grow progressively:

  • Impact loading at spall edges creates high stress
  • Adjacent material fatigues more rapidly
  • Spall grows outward and deeper
  • Exponential growth rate – small spall can become large in weeks

Secondary Damage

Spalling creates debris that causes cascading damage:

  • Debris Generation: Metal particles from spall circulate in bearing
  • Three-Body Abrasion: Debris acts as lapping compound
  • Secondary Spalls: Debris particles initiate new spalls in other areas
  • Rapid Deterioration: Once multiple spalls present, failure accelerates
  • Complete Failure: Eventually bearing loses all load-carrying capacity

Response and Corrective Actions

Upon Detection

  1. Confirm Diagnosis: Verify fault frequency matches bearing geometry
  2. Assess Severity: Determine stage based on amplitude and harmonics
  3. Increase Monitoring: Change from monthly to weekly or daily based on severity
  4. Schedule Replacement: Plan bearing change-out during appropriate downtime
  5. Procure Bearing: Order replacement (verify correct model and specifications)

Emergency Indicators

Immediate shutdown recommended if:

  • Vibration amplitude doubling in less than one week
  • Bearing temperature rising rapidly (> 5°C in one shift)
  • Audible grinding, squealing, or roughness from bearing
  • Multiple bearing frequencies present (multiple defects)
  • Loss of lubricant or visible contamination

Prevention Through Design and Maintenance

Design Phase

  • Select bearings with adequate life rating (L10 > required service life)
  • Provide proper lubrication system
  • Design effective sealing
  • Ensure adequate cooling for operating conditions

Installation Phase

  • Clean installation practices
  • Proper mounting tools (prevent installation damage)
  • Verify correct bearing clearance
  • Precise alignment

Operation Phase

  • Vibration monitoring program with envelope analysis
  • Lubrication program (intervals, quantities, quality)
  • Temperature monitoring
  • Good balance quality to minimize dynamic loads

Spalling is the inevitable end point of bearing fatigue life, but through proper bearing selection, installation, lubrication, and condition monitoring, bearing life can be maximized and failures can be detected early enough to prevent secondary damage and allow planned, cost-effective maintenance.

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