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

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

Causes of Micropitting and Smearing – Roller/Raceway Sliding 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 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.

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) 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 Tapered Roller Bearings Surface Treatments Advanced Engineered Surfaces Black Oxide Preloaded TRBs, SRR~0%

Black Oxide Surface Treatments (Rings & Rollers) Before Use After 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) ES TECHNOLOGY DESCRIPTION BENEFITS ROLLER TEXTURING Low Roughness, Low Asperity Slope Reduced Asperity Contact & Stress ROLLER COATING MC/aC:H Coating 1 mm thick Increased Wear Resistant, Increased Fatigue Life, Increased Debris Resistance. Nanocomposite Coating Superfinishing

ES322 Treated Rollers Eliminate Micropitting & Reduced Life from Debris Reverse Sliding Zone Roller Contact Area Lact~0.6L10 The pictures in this slide are of spherical roller bearing raceways that were deliberately damaged with steel debris and then life tested to fatigue in a low lambda (~0.5) condition. Debris damage is known to significantly reduce bearing life, especially for through-hardened material like 52100. The dark band in the upper image is called peeling or grey-staining and it is caused by micropitting. Micropitting of bearing raceways is caused by high cyclic shear stresses in low lambda conditions. The high shear stresses are associated with roller sliding, and rollers in SRBs always experience high amounts of sliding. The reverse sliding zone is indicated. There is also a large amount of debris damage on the raceway still observable at the end of the useful bearing life. In contrast, the image of the SPB raceway which ran against ES322 treated rollers is much smoother – the majority of the debris damage has been repaired. That is, the plastically deformed raised lips around the debris generated craters have been removed through the gentle lapping action of the coated rollers. In fact, whereas the SRB with the untreated rollers statistically experiences a 40% reduction in bearing life because of debris damage, SRBs with ES322-treated rollers statistically show a 20% improvement in bearing life over untreated, non-debris damaged bearings. Additionally, even thought the testing conditions of both bearing tests were identical, absolutely no micropitting has occurred in the SRBs with ES322-treated rollers. Lact~1.2L10

Additional Benefits of ES Roller Treatments Protection Against Loss of Lubrication Increased Life of the Bearing in Thin Lube Film Reduction in the Friction Losses A Debris Resistant Bearing for Gearboxes

Possible Solutions to Brittle Flaking Actions Material Benefit Performance Benefit Improve 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.