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
– Micropitting
– Smearing
– Flaking
 Conclusions
2
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.
3
The Problems
Micropitting
Smearing
twear
DESIRED
ACTUAL
4
Brittle
Flaking
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
5
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.
6
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.
7
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
10.3
Lnm
8
 C1 
 a1aISO  
P 
 
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 %
9
Solutions to Micropitting & Smearing Wear
Tapered Roller Bearings
Preloaded TRBs, SRR~0%
10
Surface Treatments
Black Oxide
Advanced
Engineered
Surfaces
Black Oxide Surface Treatments
(Rings & Rollers)
Before Use
•
•
•
•
11
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.
Superfinishing
12
Nanocomposite
Coating
MICROPITTING
Lact~0.6L10
Lact~1.2L10
13
Reverse
Sliding Zone
Roller
Contact
Area
ES322 Treated Rollers Eliminate Micropitting
& Reduced Life from Debris
Additional Benefits of ES Roller Treatments
Protection Against Loss of Lubrication
Reduction in the Friction Losses
14
Increased Life of the Bearing in Thin Lube Film
A Debris Resistant Bearing for Gearboxes
Possible Solutions to Brittle Flaking
15
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
•
•
•
•
•
•
16
because bearings in three critical positions suffer field failures due to lowcycle 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.