1. 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

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

3. 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

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

5. 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

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

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)
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.

12. 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

13. 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 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

14. 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

15. 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

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 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.