10.2 Bearing Defect Analysis: Spalling, Brinelling & Overheating

10.2 Bearing Defect Analysis: Spalling, Brinelling & Overheating

Approved-Data Control

The figures and hardware examples in this section illustrate principles. For an actual aircraft or component, current approved maintenance data, product instructions, organisation procedures, and applicable law control the material, limit, interval, sequence, tooling, PPE, and acceptance decision.

Rolling-element bearings in aircraft systems are manufactured from ultra-clean, vacuum-degassed alloy steels (such as AISI 52100 high-carbon chromium steel or AISI M50 tool steel for high-temperature turbine sumps) heat-treated to a surface hardness of 58 to 64 Rockwell C (HRC). Despite this metallurgical excellence, bearings operate under concentrated dynamic contact stresses (Hertzian contact pressures often exceeding 1,500 to 2,500 MPa / 200,000 to 350,000 psi). Under these intense cyclic stress fields, minor handling errors, lubricant contamination, static vibration, or electrical leakage currents manifest as distinct physical damage modes. An EASA Part-66 maintenance engineer must possess the diagnostic expertise to identify these defects, pinpoint their root mechanical causes, and enforce uncompromising workshop safety standards.

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Diagnostic Matrix: True Brinelling vs. False Brinelling

A classic diagnostic distinction frequently examined in aviation maintenance certification is the difference between True Brinelling and False Brinelling.

+-------------------------------------------------------------------------+
|                   TRUE BRINELLING VS FALSE BRINELLING                   |
|                                                                         |
|   TRUE BRINELLING (Static Overload)    FALSE BRINELLING (Fretting Wear) |
|                                                                         |
|            Smooth Indentation                  Micro-Scuffed Pocket     |
|            No metal removed                    Metal torn and oxidized  |
|                                                                         |
|              |   Ball   |                           |   Ball   |        |
|              v  (STATIC)|                           <->(VIBRATION)      |
|             ---.     .---                          ---.     .---        |
|                 \___/                                  \xxx/            |
|       =========================             =========================   |
|             Plastic Flow                           Fretting Wear        |
|       Original grind marks PRESERVED        Original grind marks WIPED  |
|       Raised metallic rim at edges          Reddish-brown Fe2O3 debris  |
|                                                                         |
|   Causes: Hard landing shock, or        Causes: Ground vibration in     |
|   pressing inner ring via outer ring.   parked aircraft (wind/APU run). |
+-------------------------------------------------------------------------+

1. True Brinelling (Permanent Plastic Indentation)

• Physical Mechanics: Occurs when static or shock loads exceed the elastic limit (yield point) of the bearing raceway steel, resulting in permanent, localized plastic deformation without loss of metal.
• Visual & Microscopic Characteristics:
  • Smooth, cup-shaped indentations in the raceways matching the exact contour and pitch spacing of the balls or rollers.
  • Critical Diagnostic Criterion: Under magnification, the original manufacturing grinding lines and micro-honing marks remain intact and clearly visible at the bottom of the indentation depressions.
  • A slight raised metallic lip or displaced ridge is typically visible around the perimeter of each dent where the steel was forced upward by plastic displacement.
• Root Causes:
  • Extreme static shock loads (e.g., hard aircraft landings, gust wind slams against locked control surfaces, ground handling towbar collisions).
  • Improper Installation Practice: Pressing a bearing onto a shaft by applying force to the outer ring, or pressing a bearing into a housing by applying force to the inner ring. Forcing the press load through the rolling elements causes instant Brinelling.

2. False Brinelling (Fretting Corrosion / Vibration Wear)

• Physical Mechanics: False Brinelling is not plastic deformation at all; it is a fretting wear and surface oxidation phenomenon caused by minute, high-frequency oscillatory vibrations (micro-motion) between rolling elements and raceways in a stationary (non-rotating) bearing.
• Visual & Microscopic Characteristics:
  • Elongated, shallow depressions or hollows in the raceway at ball/roller pitch intervals.
  • Critical Diagnostic Criterion: The original manufacturing grinding marks are completely obliterated and scrubbed away within the contact pockets.
  • The surface of the pocket appears dull, scored, or rough, accompanied by the accumulation of a fine, reddish-brown iron oxide ($Fe_2O_3$) debris known in the workshop as "cocoa" or "bleeding rust".
• Root Causes:
  • Aircraft parked on aprons exposed to high ambient ground vibrations (e.g., adjacent aircraft taxiing, high surface winds oscillating control surfaces, ground power units).
  • Running an Auxiliary Power Unit (APU) or engine ground runs while rotorcraft main rotor bearings or accessory gearboxes remain stationary.
  • Transporting unmounted dynamic components or spare gearboxes in trucks or aircraft cargo holds without securing shafts with locking jigs to prevent micro-fretting.

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Spalling (Flaking / Fatigue Breakdown)

Spalling—often referred to in aerospace maintenance manuals as flaking or fatigue pitting—is the classic end-of-life fatigue failure of rolling-element bearings.

The Subsurface Shear Mechanism

Unlike surface abrasion, spalling is a subsurface-initiated fatigue phenomenon:

1. As rolling elements pass over the raceway under heavy load, the maximum shear stress (Hertzian shear stress, $\tau_{max}$) does not occur at the outer surface, but at a depth of 0.1 mm to 0.5 mm below the raceway contact surface.
2. Over millions of cyclic stress reversals, microscopic shear cracks initiate at subsurface metallurgical anomalies, such as non-metallic inclusions, carbide clusters, or micro-voids.
3. These subsurface micro-cracks propagate parallel to the raceway surface until they deflect upward at a 45° angle, breaking through the outer surface layer.
4. Triangular or jagged metallic flakes separate from the raceway or ball, leaving behind an irregular, rough-bottomed crater with sharp, stepped edges.

Operational Hazards & Progressive Nature

Spalling is progressive and irreversible. Once a single spall crater forms:

• The sharp crater edges create severe geometric stress risers, accelerating crack growth.
• Liberated hardened steel flakes are rolled over by subsequent elements, indenting the raceways (debris denting).
• The bearing emits loud, low-frequency rumbling noise, accompanied by intense high-frequency vibration detectable on engine health monitoring (EHM) accelerometers.
• Left unaddressed, spalling rapidly degenerates into total structural fragmentation of the raceways and catastrophic bearing seizure. Any bearing exhibiting spalling must be rejected and scrapped immediately.

+-------------------------------------------------------------------------+
|                   SUBSURFACE SPALLING / FLAKING MECHANISM               |
|                                                                         |
|         Rolling Element (Ball / Roller)                                 |
|                     ( O )                                               |
|                      v  Heavy Contact Pressure                          |
|       ============== . =================  Raceway Surface               |
|       |   .  .  .  . . .  .  .  .  .   |                                |
|       |  [Subsurface Max Shear Stress] |  <-- Depth: 0.1 to 0.5 mm      |
|       |        *---* Micro-Crack       |      initiates at inclusions   |
|       ==================================                                |
|                                                                         |
|       PROGRESSION:                                                      |
|       1. Cyclic shear stress generates subsurface micro-cracks.         |
|       2. Cracks propagate parallel to surface, then turn upward.        |
|       3. Metallic flake breaks away -> Sharp-edged spall crater forms.   |
|       4. Debris contaminates sump; progressive, rapid destruction.       |
+-------------------------------------------------------------------------+

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Overheating & Metallurgical Temper Breakdown

Bearing steels undergo specialized heat treatment (quenching and tempering) to lock in their ultra-hard martensitic crystalline microstructure. If a bearing exceeds its design operating temperature ceiling (typically 120°C to 150°C for standard 52100 steel, or 200°C to 300°C for M50 / Cronidur 30 steels), the steel begins to over-temper, permanently losing its hardness.

Visual Discoloration Progression (Temper Colors)

As steel heats in the presence of oxygen, a thin surface iron oxide film forms whose optical interference thickness reflects specific temperatures:

1. Faint Straw Yellow (approx. 180°C to 200°C): Initial thermal distress; lubricant oxidation commences.
2. Straw / Bronze Yellow (approx. 210°C to 230°C): Significant oil breakdown; light loss of temper.
3. Reddish Purple / Violet (approx. 240°C to 260°C): Advanced thermal distress; irreversible loss of hardness.
4. Deep Blue / Cobalt Blue (approx. 270°C to 300°C): Severe metallurgical over-tempering; surface hardness drops below serviceable limits (HRC < 55).
5. Dull Slate Grey / Black (exceeding 320°C): Complete structural anneal, cage distortion, charred lubricant, imminent total lockup.

Accompanying Overheating Symptoms

• Lubricant Carbonisation: Grease or oil cooks into a hard, black, gritty carbon crust, resembling baked lacquer.
• Loss of Clearance & Ball Smearing: Differential thermal expansion between inner ring and outer ring completely eliminates internal radial clearance. Rolling elements bind, skid against raceways, and melt surface micro-welds, leaving frosted, smeared tracks.
• Disposal Mandate: Any aircraft bearing displaying temper discoloration past straw yellow, or showing signs of hardened carbon residue, has lost its structural hardness and fatigue resistance. Re-hardening in the field is impossible; the bearing must be permanently scrapped.

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Electrical Arcing & Fluting (Electric Current Damage)

When electrical current passes through a bearing—caused by static electricity discharge from unbonded airframe structures, starter-generator ground faults, or lightning strike currents seeking ground through engine shafts—it arcs across the ultra-thin hydrodynamic oil film between the rolling elements and raceways.

Characteristics of Electrical Damage

• Microscopic Arc Pits: Each electrical spark discharge acts as a localized electric arc furnace, melting tiny points of steel (reaching >1,500°C) and vaporizing the surrounding oil film. The molten steel droplets quench rapidly, leaving tiny microscopic craters.
• Fluting / Washboarding: Under continuous shaft rotation and AC current passage, these craters align into a distinct, closely spaced series of parallel transverse ridges and grooves across the raceways, resembling a corrugated washboard. The rolling elements take on a dull, frosted, or satin finish.
• Consequences: Fluting causes intense acoustic hum and severe high-frequency vibration. The rough ridges destroy the rolling elements within hours. Prevention mandates verifying airframe ground bonding straps and shaft grounding brush assemblies.

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Bearing Cleaning, Handling & Inspection Protocols

During routine maintenance and scheduled overhaul, unsealed bearings must be cleaned, dried, and inspected according to rigid technical procedures.

Approved Solvents & Cleaning Procedure

1. Approved Solvents: Technicians must use approved petroleum-base non-chlorinated solvents, such as Stoddard solvent (MIL-PRF-680 Type II) or Varsol. Prohibited: chlorinated solvents (e.g., trichloroethylene, which induces stress corrosion cracking in high-strength steels), harsh caustic tank cleaners, and water-based detergents that cause flash rusting.
2. Soaking & Agitation: Immerse bearings in a clean solvent bath. Swirl gently by hand to dissolve grease. For caked-on lubricant, use an ultrasonic cleaning tank with approved solvent.
3. Brushing: Use only natural-bristle or nylon brushes. Wire brushes, steel wool, and metallic scrapers are strictly prohibited, as they scratch precision-honed raceways and leave magnetic metallic debris behind.
4. Lint-Free Wiping: Bearings must be wiped dry using certified aerospace lint-free cloths. Ordinary shop rags shed fibers that contaminate lubricant and induce bearing skid.

The Absolute Safety Prohibition: Spinning Dry Bearings with Compressed Air

A fatal violation of workshop safety and engineering discipline is using shop compressed air to spin an unlubricated, dry bearing. Aircraft maintenance regulations strictly forbid this practice.

+-------------------------------------------------------------------------+
|          DANGER: ABSOLUTE PROHIBITION ON COMPRESSED AIR SPINNING        |
|                                                                         |
|          [Air Nozzle] =======>  High-Velocity Air Blast                 |
|                                       |                                 |
|                                       v                                 |
|                           (((((( [ Bearing ] ))))))                     |
|                           INSTANT OVERSPEED >40,000 RPM                 |
|                                                                         |
|   THE THREE CATASTROPHIC HAZARDS:                                       |
|   1. Instantaneous Over-speed: Lack of hydrodynamic film causes dry     |
|      ball skidding, friction welding, and scoring of raceways.         |
|   2. Centrifugal Cage Explosion: Ball separators disintegrate, expelling|
|      balls and steel shrapnel at ballistic speeds -> SEVERE INJURY!    |
|   3. Particulate Bombardment: Shop air moisture and grit blast into    |
|      delicate races, causing microscopic impact indentation.            |
|                                                                         |
|   CORRECT PROCEDURE: If using air to blow out solvent, HOLD BOTH RINGS  |
|   FIRMLY BY HAND TO PREVENT ANY ROTATION.                               |
+-------------------------------------------------------------------------+

• Hazard 1: Rotational Overspeed & Dry Boundary Scoring: High-pressure shop air (90–100 psi / 6–7 bar) impinged tangentially onto dry rolling elements accelerates the bearing to over 30,000 to 50,000 RPM in seconds. Because the bearing is completely dry, there is zero hydrodynamic fluid film. The rolling elements skid and skid-mark across the dry steel, generating intense local frictional flash temperatures that instantly score, gall, and ruin the raceways.
• Hazard 2: Centrifugal Cage Burst (Lethal Shrapnel Hazard): Thin pressed steel or phenolic cages are not engineered to withstand the extreme centrifugal bursting forces generated at 50,000 RPM without oil damping. The cage fragments explosively, firing steel balls and razor-sharp metal shards outward at ballistic velocities. Technicians have suffered severed arteries, blindness, and fatal facial trauma from exploding bearing cages.
• Correct Procedure: If compressed air is used to blow solvent from a bearing, the technician must hold both inner and outer rings firmly with fingers or a holding mandrel to prevent all rotation.

Tactile & Visual Inspection

• Tactile Feel Check: Lightly lubricate the cleaned bearing with clean system oil. While applying a firm axial hand force to the inner ring, slowly rotate the outer ring. The rotation must feel smooth, velvet-like, and silent. Any click, hitch, catch, gritty drag, or roughness indicates raceway spalling, Brinelling, or contamination requiring rejection.
• Optical Magnification: Inspect all raceways, balls/rollers, and cage pockets under a 10x optical magnifier with oblique lighting to detect hairline cracks, early spalling, or Brinelling dents.

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Realistic Maintenance Scenario & Common Exam Traps

Realistic Maintenance Scenario

During an annual scheduled C-check on a regional turboprop aircraft, an engineer inspects the primary flight control aileron push-pull rod bellcrank bearings. The aircraft had been parked outdoors on an active ramp in gusty wind conditions for six months prior to the check.

1. Tactile Check: Rotating the bellcrank by hand, the technician detects a pronounced, repetitive "notched" or "indexed" detent feel as the bearing rotates through its neutral travel.
2. Disassembly & Cleaning: The deep-groove ball bearing is extracted using a mechanical puller bearing squarely against the outer ring. The bearing is degreased in Stoddard solvent without spinning.
3. Defect Diagnosis: Under 10x magnification, the technician observes multiple shallow depressions spaced exactly at ball pitch across the inner raceway. Examining the bottom of the indentations, the technician notes that the original grinding marks are completely scrubbed away, and a faint reddish-brown iron oxide dust is visible. The technician correctly identifies the defect as False Brinelling caused by wind-induced control surface fluttering while parked stationary on the ramp. The bearing is rejected, scrapped, and replaced with a new serviceable unit.

Common Exam Traps

• Trap 1: Confusing True Brinelling with False Brinelling on grind marks. True Brinelling preserves grind marks (plastic displacement); False Brinelling wipes grind marks away (fretting wear).
• Trap 2: Believing compressed air spinning is acceptable if done briefly. Spinning a dry bearing with compressed air is never acceptable. It destroys the raceways within two seconds and presents an explosive shrapnel hazard.
• Trap 3: Misdiagnosing electrical fluting as mechanical chatter. Electrical fluting produces a distinctive, regular washboard pattern of transverse ridges across the entire raceway accompanied by frosted balls, caused by arcing electrical currents.
• Trap 4: Attempting to polish out light raceway spalling. Spalling is a subsurface fatigue breakdown. Polishing the surface cannot restore the fatigued subsurface crystalline lattice; spalling will rapidly recur and propagate.