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Life-Limiting Wear of Wind Turbine Gearbox Bearings: Origins and Solutions. G. L. Doll, M. N. Kotzalas, and Y. S. Kang The Timken Company Canton, Ohio USA. Outline. Critical bearing locations in WTGs. The problems: micropitting, smearing, flaking; not fatigue. Causes Micropitting

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life limiting wear of wind turbine gearbox bearings origins and solutions

Life-Limiting Wear of Wind Turbine Gearbox Bearings: Origins and Solutions

G. L. Doll, M. N. Kotzalas, and Y. S. Kang

The Timken Company

Canton, Ohio USA

outline
Outline
  • Critical bearing locations in WTGs.
  • The problems: micropitting, smearing, flaking; not fatigue.
  • Causes
    • Micropitting
    • Smearing
    • Flaking
  • Solutions
    • Micropitting
    • Smearing
    • Flaking
  • Conclusions
critical bearing locations in wtgs
Critical bearing locations are defined as places that have exhibited a high percentage of application failures in spite of the use of best current design practices.

1. Planet bearings

2. Intermediate shaft bearings

3. High-speed shaft bearings

Critical Bearing Locations in WTGs

W. Musial, S. Butterfield, and B. McNiff, “Improving wind turbine gearbox reliability”, Proceedings of the 2007 EWEC Conference, Milan, Italy, Paper DW2.1, pp. 1-10.

the problems
The Problems

Brittle

Flaking

Micropitting

Smearing

twear

L10

DESIRED

ACTUAL

causes of micropitting and smearing roller raceway sliding
Causes of Micropitting and Smearing – Roller/Raceway Sliding
  • Micropitting: Cumulative shear stresses from moderate roller/raceway sliding in low L leads to lowcycle micropitting (< predicted life)…Moderate Pv
  • Smearing: High roller/raceway sliding with higher local surface temperature causes micro-welding of the surface leading to smearing…High Pv
causes of brittle flaking
Causes of Brittle Flaking*
  • Transient loads & high stress/high cycle loading
    • Butterflies at nonmetallic inclusions → ferrite in wing
  • Continued cyclic strain:
    • Voids/microcrack initiate at Al2O3/steel interface
    • Microcracks agglomerate & propagate → brittle flaking

*A. Grabulov, R. Petrov, & H.W. Zandbergen, Intl. J. Fatigue, 32 (2010), pp. 576-583.

slide7

Mechanics of Micropitting and Smearing in Spherical Roller Bearings (SRBs)

  • SRB geometry dictates that there are at most two points of rolling contact (A).
  • Differential or Heathcoat Slip exists (pure sliding) at Q.
  • Low lambda conditions prevail in WTGs.
  • Cannot effectively load SRBs in WTGs to reduce sliding at Q.
  • Micropitting in moderate Pv situations
  • Smearing in high Pv situations.

T. A. Harris and M. N. Kotzalas, Advanced Concepts of Bearing Technology, Rolling Bearing Analysis, 5th Ed., (Taylor & Francis, Boca Raton, 2007), p. 132.

slide8

Mechanics of Micropitting and Smearing in Cylindrical Roller Bearings (CRBs)

  • When traction forces at roller/raceway cannot overcome roller inertia, sliding increases.
  • Rollers slide/misaligned outside load zone.
  • WTG bearings designed for very large & rarely experienced static & dynamic loads.
    • Large C1 → highly crowned raceways.
    • Low P → small traction forces.
  • Gearbox torque reversals rapidly change load zone to sliding/misaligned rollers

Load Distribution in Loaded Zone

C1 is a measure of the bearing’s ability to withstand rolling contact fatigue

C0 is a measure of the bearing’s ability to withstand the maximum applied load without function-reducing permanent deformations

results of dbm calculations intermediate shaft crb for 1 5mw wtg
Results of DBM Calculations(Intermediate Shaft CRB for 1.5mW WTG)
  • When C1/P  1,
    • SRR=5 ~ 10 %
  • When C1/P  1000,
    • SRR=20 ~ 100 %
  • Tighter clearances,
    • SRR decreases but > 0
  • Acceleration/Deceleration
    • SRR  50 %
solutions to micropitting smearing wear
Solutions to Micropitting & Smearing Wear

Tapered Roller Bearings

Surface Treatments

Advanced

Engineered Surfaces

Black Oxide

Preloaded TRBs, SRR~0%

black oxide surface treatments rings rollers
Black Oxide Surface Treatments (Rings & Rollers)

After Use

Before Use

  • Chemical conversion of the surfaces of the rollers and raceways from steel to magnetite (Fe3O4).
  • Black oxide employed on bearings for many years to inhibit corrosion and in some cases to facilitate a break-in.
  • Black oxide surface treatments are sacrificial and can wear rapidly in operation (depends upon Pv).
  • After black oxide wears, smoother raceways. Benefit to micropitting, not beneficial to smearing.
advanced engineered surfaces es rollers only
Advanced Engineered Surfaces (ES) (Rollers Only)

Nanocomposite

Coating

Superfinishing

es322 treated rollers eliminate micropitting reduced life from debris
ES322 Treated Rollers Eliminate Micropitting & Reduced Life from Debris

MICROPITTING

Reverse Sliding Zone

Roller Contact Area

Lact~0.6L10

Lact~1.2L10

additional benefits of es roller treatments
Additional Benefits of ES Roller Treatments

Increased Life of the Bearing in Thin Lube Film

Protection Against Loss of Lubrication

Reduction in the Friction Losses

A Debris Resistant Bearing for Gearboxes

summary
Summary
  • Some wind turbine gearboxes are not achieving their desired operational lives because bearings in three critical positions suffer field failures due to low-cycle micropitting/smearing and/or brittle flaking, not fatigue.
  • Micropitting & smearing are caused by excessive amounts of roller/raceway sliding in low L.
  • Rollers slide when C1/P is large.
  • High cyclic stresses cause WEAs which cause brittle flaking in low fracture strength material.
  • Black oxide surface treatments applied to raceways and rollers should delay the onset of micropitting, but will probably not allow the bearings to attain their predicted L10life. Black oxide surface treatments are ineffective against smearing.
  • Advanced ES surface treatments applied to rollers eliminate micropitting and provide durable barrier against smearing. Low shear forces move peak stresses away from raceway surfaces.
  • In addition bearings with ES-treated rollers are also
    • resistant to damage due to lubricant interruption,
    • immune to damage from gearbox debris,
    • operate with about 15% less frictional torque,
    • have about 3.5 times greater fatigue life in low Lconditions.