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

1. 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

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

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 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.

4. The Problems Brittle Flaking Micropitting Smearing twear L10 DESIRED ACTUAL

5. 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

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 Al2O3/steel interface • Microcracks agglomerate & propagate → brittle flaking *A. Grabulov, R. Petrov, & H.W. Zandbergen, Intl. J. Fatigue, 32 (2010), pp. 576-583.

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, 5th Ed., (Taylor & Francis, Boca Raton, 2007), p. 132.

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 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

9. 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 %

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

11. 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.

12. Advanced Engineered Surfaces (ES) (Rollers Only) Nanocomposite Coating Superfinishing

13. ES322 Treated Rollers Eliminate Micropitting & Reduced Life from Debris MICROPITTING Reverse Sliding Zone Roller Contact Area Lact~0.6L10 Lact~1.2L10

14. 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

15. Possible Solutions to Brittle Flaking

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 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.