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G. L. Doll, M. N. Kotzalas, and Y. S. Kang The Timken Company Canton, Ohio USA Life-Limiting Wear of Wind Turbine Gearbox Bearings: Origins and Solutions.

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Presentation on theme: "G. L. Doll, M. N. Kotzalas, and Y. S. Kang The Timken Company Canton, Ohio USA Life-Limiting Wear of Wind Turbine Gearbox Bearings: Origins and Solutions."— Presentation transcript:

1 G. L. Doll, M. N. Kotzalas, and Y. S. Kang The Timken Company Canton, Ohio USA Life-Limiting Wear of Wind Turbine Gearbox Bearings: Origins and Solutions

2 Outline  Critical bearing locations in WTGs.  The problems: micropitting, smearing, flaking; not fatigue.  Causes –Micropitting –Smearing –Flaking  Solutions –Micropitting –Smearing –Flaking  Conclusions 2

3 Critical Bearing Locations in WTGs 3  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

4 The Problems 4 Micropitting Smearing DESIRED t wear ACTUAL L 10 Brittle Flaking

5  Micropitting: Cumulative shear stresses from moderate roller/raceway sliding in low  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 5

6 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 Al 2 O 3 /steel interface –Microcracks agglomerate & propagate → brittle flaking 6 *A. Grabulov, R. Petrov, & H.W. Zandbergen, Intl. J. Fatigue, 32 (2010), pp

7 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, 5 th Ed., (Taylor & Francis, Boca Raton, 2007), p

8 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 C 1 → highly crowned raceways.  Low P → small traction forces.  Gearbox torque reversals rapidly change load zone to sliding/misaligned rollers Load Distribution in Loaded Zone C 1 is a measure of the bearing’s ability to withstand rolling contact fatigue C 0 is a measure of the bearing’s ability to withstand the maximum applied load without function-reducing permanent deformations 8

9 Results of DBM Calculations (Intermediate Shaft CRB for 1.5mW WTG)  When C 1 /P  1, –SRR=5 ~ 10 %  When C 1 /P  1000, –SRR=20 ~ 100 %  Tighter clearances, –SRR decreases but > 0  Acceleration/Deceleration –SRR  50 % 9

10 Solutions to Micropitting & Smearing Wear Tapered Roller BearingsSurface Treatments 10 Black Oxide Advanced Engineered Surfaces Preloaded TRBs, SRR~0%

11 Black Oxide Surface Treatments (Rings & Rollers) 11 Chemical conversion of the surfaces of the rollers and raceways from steel to magnetite (Fe 3 O 4 ). 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

12 Advanced Engineered Surfaces (ES) (Rollers Only) ES TECHNOLOGYDESCRIPTIONBENEFITS ROLLER TEXTURINGLow Roughness, Low Asperity Slope Reduced Asperity Contact & Stress ROLLER COATINGMC/aC:H Coating 1  m thick Increased Wear Resistant, Increased Fatigue Life, Increased Debris Resistance. 12 Superfinishing Nanocomposite Coating

13 ES322 Treated Rollers Eliminate Micropitting & Reduced Life from Debris Reverse Sliding Zone Roller Contact Area 13 MICROPITTING L act ~0.6L 10 L act ~1.2L 10

14 Additional Benefits of ES Roller Treatments 14 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

15 Possible Solutions to Brittle Flaking ActionsMaterial BenefitPerformance Benefit Improve steel cleanliness Reduced volume of Al 2 O 3 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, L 10 -based fatigue life 15

16 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 . 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 L 10 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  conditions. 16


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