G. L. Doll, M. N. Kotzalas, and Y. S. KangThe Timken Company

Canton, Ohio USA

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

Solutions

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Outline

Critical bearing locations in WTGs. The problems: micropitting, smearing, flaking; not fatigue. Causes

– Micropitting– Smearing– Flaking

Solutions– Micropitting– Smearing– Flaking

Conclusions

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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

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.

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The Problems

Micropitting Smearing

DESIRED

twear

ACTUAL

L10

BrittleFlaking

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Micropitting: Cumulative shear stresses from moderate roller/raceway sliding in low L leads to low cycle 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 Micropitting and Smearing – Roller/Raceway Sliding

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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.

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.

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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 fatigueC0 is a measure of the bearing’s ability to withstand the maximum applied load without function-reducing permanent deformations

3.10

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PCaaL ISOnm

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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 %

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Solutions to Micropitting & Smearing Wear

Tapered Roller Bearings Surface Treatments

Black OxideAdvanced

Engineered Surfaces

Preloaded TRBs, SRR~0%

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Black Oxide Surface Treatments (Rings & Rollers)

• 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.

Before Use After Use

Advanced Engineered Surfaces (ES) (Rollers Only)

ES TECHNOLOGY DESCRIPTION BENEFITSROLLER TEXTURING Low Roughness,

Low Asperity SlopeReduced Asperity Contact & Stress

ROLLER COATING MC/aC:H Coating1 mm thick

Increased Wear Resistant,Increased Fatigue Life,Increased Debris Resistance.

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Superfinishing NanocompositeCoating

ES322 Treated Rollers Eliminate Micropitting & Reduced Life from Debris

Reverse Sliding Zone

Rol

ler

Con

tact

A

rea

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MICROPITTING

Lact~0.6L10

Lact~1.2L10

Additional Benefits of ES Roller Treatments

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Increased Life of the Bearing in Thin Lube Film

A Debris Resistant Bearing for Gearboxes

Protection Against Loss of Lubrication

Reduction in the Friction Losses

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Possible Solutions to Brittle FlakingActions Material Benefit Performance BenefitImprove steel cleanliness

Reduced volume of Al2O3 inclusions

Reduction in sites for crack initiation

Increase fracture strength of steel

Fine grains, Small/dispersed carbides,~20% Retained Austenite

Inhibits crack propagation

Apply ES treatments to rolling elements

Reduces shear stresses on raceways surfaces

Drives Hertzian stresses deeper into raceway,L10-based fatigue life

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 L10 life. 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 L conditions.

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