Wind Turbine 

Tribology Seminar

A Recap

DRAFT

Authors:

R. Errichello, GEARTECH

S. Sheng and J. Keller, NREL

A. Greco, ANL

Sponsors:

National Renewable Energy Laboratory (NREL)

Argonne National Laboratory (ANL)

U.S. Department of Energy

Presented at:

Renaissance Boulder Flatiron Hotel

Broomfield, Colorado, USA

November 15-17, 2011

February 2011February 2011

Wind Turbine 

Tribology Seminar

A Recap

Authors:

R. Errichello, GEARTECH

S. Sheng and J. Keller, NREL

A. Greco, ANL

Sponsors:

National Renewable Energy Laboratory (NREL)

Argonne National Laboratory (ANL)

U.S. Department of Energy

Presented at:

Renaissance Boulder Flatiron Hotel

Broomfield, Colorado, USA

November 15-17, 2011

 

 

 
ii

NOTICE

This report was prepared as an account of work sponsored by an agency of the United States 

government. Neither the United States government nor any agency thereof, nor any of their 

employees, makes any warranty, express or implied, or assumes any legal liability or 

responsibility for the accuracy, completeness, or usefulness of any information, apparatus, 

product, or process disclosed, or represents that its use would not infringe privately owned rights. 

Reference herein to any specific commercial product, process, or service by trade name, 

trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views 

and opinions of authors expressed herein do not necessarily state or reflect those of the United 

States government or any agency thereof.

Printed on paper containing at least 50% wastepaper, including 10% post consumer waste.
iii

Table of Contents

Introduction................................................................................................................................... 1

Summary of Presentations........................................................................................................... 4

Introduction............................................................................................................................... 4

Wind Market and NREL Gearbox Reliability Research Overview.................................... 4

Argonne National Laboratory (ANL) Wind Tribology Overview ..................................... 4

Tribological Challenges in Wind Turbine Technology ...................................................... 4

Tribology Fundamentals........................................................................................................... 6

Elastohydrodynamic Lubrication (EHL) Fundamentals..................................................... 6

EHL Surface Interactions in Micropitting .......................................................................... 6

Effect of Lubricant Properties on EHL Film Thickness and Traction................................ 7

Surface Roughness and Micropitting.................................................................................. 7

Lubricant Fundamentals............................................................................................................ 8

Fundamentals of Lubrication Gear Oil Formulation .......................................................... 8

Selecting Synthetic Gear Oil............................................................................................... 9

Wind Turbine Grease Lubrication .................................................................................... 10

Wind Turbine Tribological Damage....................................................................................... 11

NREL Gearbox Reliability Collaborative Failure Database Project ................................ 11

Bearing and Gear Failure Modes Seen in Wind Turbines................................................ 12

Microstructural Alterations in Hertzian Fatigue ............................................................... 13

Microstructural Alterations in Rolling Contact ................................................................ 13

Classic Bearing Damage Modes....................................................................................... 14

Root Cause Hypotheses .......................................................................................................... 14

The Mechanism of White Structure Flaking in Rolling Bearings .................................... 14

Frictional Surface Crack Initiation and Corrosion Fatigue-Driven Crack Growth 

Hypothesis................................................................................................................... 15

Hammering Wear Impact Fatigue Hypothesis.................................................................. 16

Influence on Bearing Life by New Material Phenomena ................................................. 16

R&D Activities ....................................................................................................................... 17

Surface Treatment and Nano Lubricant Bearings............................................................. 17

Novel Macromolecular Nano Lubricant Oils and Greases............................................... 18

Update on the Development of a Full Life Wind Turbine Gearbox Lubricating 

Fluid ............................................................................................................................ 18

Modeling Tribological Contacts for Wind Turbine Gearbox Component Life 

Prediction .................................................................................................................... 19

Approaching Surface Fatigue Life by Integrated Contact and Lubrication 

Mechanics................................................................................................................... 20

Panel Discussion ..................................................................................................................... 20

Field Observations of Tribological Damage in Wind Turbines........................................ 20

A Look at Wind Turbine Gearbox Oil over Time ............................................................ 20

Damage as Seen from a Wind Turbine Gearbox Manufacturer’s Angle.......................... 21

Pitch and Yaw Bearing Damage ....................................................................................... 21

Summary and Conclusions......................................................................................................... 22

Research.................................................................................................................................. 22

Seminar Summary................................................................................................................... 22

Tribology and Lubrication Fundamentals............................................................................... 23

Factors Influencing Surface Failures...................................................................................... 23

Critical Surface Damage/Failure Modes................................................................................. 24
iv

Field Failure Tracking and Analysis....................................................................................... 26

Opportunities for Learning and Path Forward........................................................................ 27

Recommendations for Future R&D Activity ........................................................................... 28

Bibliography ................................................................................................................................ 31

Appendix A. Moderator, Speaker, and Panelist Biographies................................................. 41

List of Tables

Table 1. Seminar Agenda................................................................................................................ 2

Table 2. Comparison of Hypotheses for Axial Cracks................................................................. 26

Table 3. Recommended Failure Mode Nomenclature .................................................................. 26

Table 4. Research Needs in Wind Turbine Tribology: General R&D Approach......................... 29

Table 5. Research Needs in Wind Turbine Tribology: Specific Issues........................................ 30
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Foreword

Tribology is the science and engineering of interacting surfaces in relative motion. It 

includes the study and application of the principles of friction, lubrication, and wear that 

impact the design and operation of bearings and gears in wind turbine gearboxes, and

their subsequent maintenance requirements and overall reliability. The Wind Turbine 

Tribology Seminar was convened by the National Renewable Energy Laboratory 

(NREL), Argonne National Laboratory (ANL), and the U.S. Department of Energy 

(DOE) to explore the state-of-the-art in wind turbine tribology and lubricant technologies, 

raise industry awareness of this complex topic, present the science behind the

technologies, and identify possible R&D areas for improvements. The Wind Turbine 

Tribology Seminar was held at the Renaissance Boulder Flatiron Hotel in Broomfield, 

Colorado, on November 15-17, 2011. This report is a summary of the seminar and its 

conclusions. 

The presentations given at the meeting can be downloaded at:

http://www.nrel.gov/wind/pdfs/2011_wind_turbine_tribology_seminar.pdf

Interested readers who were not at the meeting may wish to consult the detailed 

publications listed in the bibliography section, obtain the cited articles in the public 

domain, or contact the authors directly.
vi

Acknowledgements

The Wind Turbine Tribology Seminar was organized by a small committee, but its value 

resulted from the contributions of many. The concept of the workshop is credited to

Brian McNiff (McNiff Light Industry). Brian and Bob Errichello (GEARTECH) used 

their long standing experience in the field of wind turbine gearbox failure analysis and 

contacts among world class experts to suggest both the format of the seminar and most of 

the speakers. We are deeply indebted to them for their contributions, not only for this 

seminar, but also their continuous contributions to wind turbine gearbox reliability.

Aaron Greco (ANL) provided additional perspective on the subject of tribology and 

suggested additional world class experts. Aaron also guided the seminar's organization, 

collaborated on the technical content, and contributed to this report.

Shawn Sheng and Jonathan Keller (NREL) provided the organizational and logistical 

planning aided by the previously mentioned contributor's technical insight.

Each of the speakers is a recognized expert who brought various perspectives from the

tribology field and how each affects wind turbine reliability. 

The organizers express their deepest thanks for the efforts of all the contributors.
1

Introduction

Surface damage and failure of contacting components (i.e., bearings, and gears) are among the 

more frequent and costly types of failures for a wind turbine and can be the root cause of system 

failure for the gearbox, main rotor bearing, generator, yaw system, and blade pitch systems. 

Understanding the fundamental tribological factors that influence contacting element 

performance is important to addressing these issues; disseminating this information to foster

collaboration on this topic is one of the main objectives of the 2011 Wind Turbine Tribology 

Seminar. The other objective is to identify the major tribological related issues impacting the 

wind energy industry leading to recommendations for future research and development strategies 

to improve turbine reliability and ultimately lower the cost of wind energy.

The Wind Turbine Tribology Seminar was conceived to: (1) present state-of-the art tribology 

fundamentals, lubricant formulation, selection of oils and greases, gear and bearing failure 

modes, R&D into advanced lubricants, and mathematical modeling for tribology, and field 

observations; (2) provide a forum for researchers, tribologists, lubricant engineers, wind turbine 

manufacturers, gearbox manufacturers, bearing manufacturers, owners, operators, and those in 

the supply chain to share their knowledge and learn from their colleagues; and (3) develop a list 

of R&D needs to guide future wind turbine tribology research.

The seminar consisted of six sessions with 29 moderators, speakers, and panelists. Three came 

from Germany, two from the UK, one from Belgium, and one from Japan. About 110 attendees 

attended the seminar and participated in the discussions. Table 1 details the seminar's agenda.

The next section summarizes each presentation, and final section presents conclusions. 

Appendix A contains the speakers’ biographies.
2

Table 1. Seminar Agenda

Day 1: November 15, 2011

Time Topic Speaker and Affiliation

8:00 AM Introduction Paul Veers,

NREL

Ali Erdemir,

ANL

8:30 AM Tribological Challenges in Wind Turbine Technology Gary Doll,

U of Akron

Session I: Tribology Fundamentals (Bob Errichello, GEARTECH)

10:00 AM Elasto Hydrodynamic Lubrication (EHL) Fundamentals Vern Wedeven,

Wedeven Associates

11:00 AM EHL Surface Interactions in Micropitting Pwt Evans,

Cardiff University

1:00 PM The Influence of Lubricant Properties on EHL Film Thickness 

and Traction

Andy Olver,

Imperial College

2:00 PM Surface Roughness and Micropitting Lane Winkelmann,

REM Surface Engr

Session II: Lubricant Fundamentals (Bill Herguth, Herguth Laboratories, Inc.)

3:00 PM Fundamentals of Lubrication Gear Oil Formulation Jon Leather,

Castrol

4:00 PM Selecting Synthetic Gear Oil Dennis A. Lauer,

Klüber Lubrication

5:00 PM Wind Turbine Grease Lubrication Henri Braun,

ExxonMobil

Day 2: November 16, 2011

Session III: Wind Turbine Tribological Damage (Shawn Sheng, NREL)

8:00 AM NREL Gearbox Reliability Collaborative Failure Database 

Project

Mark McDade,

NREL

8:30 AM Bearing and Gear Failure Modes Seen in Wind Turbines Bob Errichello,

GEARTECH

10:00 AM Microstructural Alterations in Hertzian Fatigue Bob Errichello,

GEARTECH

Andy Olver,

Imperial College

11:00 AM Classic Bearing Damage Modes Ryan Evans,

Timken

Session IV: Root Cause Hypotheses (Brian McNiff, McNiff Light Industry)

1:00 PM Introduction Bob Errichello,

GEARTECH

1:15 PM The Mechanism of White Structure Flaking in Rolling 

Bearings

Hideyuki Uyama,

NSK

2:15 PM The Bearing Axial Crack Root Cause Hypothesis of Frictional 

Surface Crack Initiation and Corrosion Fatigue-Driven Crack 

Growth

Jürgen Gegner,

SKF

3:45 PM Hammering Wear Impact Fatigue Hypothesis Johan Luyckx,

Hansen

5:00 PM Influence on Bearing Life by New Material Phenomena Walter Holweger,

Schaeffler
3

Day 3: November 17, 2011 

Session V: R&D Activities (Jim Johnson, NREL)

8:00 AM Surface Treatment and Nano Lubricant Ali Erdemir,

ANL

8:45 AM Novel Macromolecular Nano Lubricant Oils and Greases Ajay P. Malshe,

NanoMech Inc.

9:45 AM Update on the Development of a Full Life Wind Turbine 

Gearbox Lubricating Fluid

Manfred Jungk,

Dow Corning

10:30 AM Modeling Tribological Contacts for Wind Turbine Gearbox 

Component Life Prediction

Nathan Bolander,

Sentient

11:15 AM Approaching Component Surface Fatigue Life by Integrated 

Contact and Lubrication Mechanics & Beyond

Jane Wang,

Northwestern Univ

Session VI: R&D Needs Development (Aaron Greco, ANL)

1:00 PM Panel Discussion:

Field Observations of Tribological Damage in Wind Turbines

A look at Wind Turbine Oil Over Time

Damage Seen From a Wind Turbine Manufacturer’s Angle

Pitch and Yaw Bearing Damage

Art Miller,

enXco

Bill Herguth,

Herguth Labs

Shawn Doner,

Winergy

Les Miller,

Kaydon

2:30 PM Future R&D Areas All Attendees

3:30 PM Tour of National Wind Technology Center Jim Johnson,

NREL

Critical evaluation of the seminar's information revealed the complex challenge of tribology for 

the wind industry and highlighted the need for multidisciplinary research in areas including

contact mechanics, tribology, materials science, lubrication, mechanical engineering, component 

design, condition monitoring, and modeling.
4

Summary of Presentations

Introduction

Wind Market and NREL Gearbox Reliability Research Overview

Paul Veers, NREL

The presentation began with an overview of the wind turbine assets at NREL and the wind 

energy market globally and in the United States and then segued into a discussion of wind 

turbine gearbox reliability challenges. The need for improved reliability in terms of failure 

frequency and resulting downtime in wind turbine drive trains, including the main shaft/bearings, 

gearbox, and generator, was demonstrated. An overview of the Gearbox Reliability 

Collaborative (GRC), including tests, modeling, analysis, overhaul database, and condition 

monitoring was presented along with drive train testing assets at the National Wind Technology 

Center (NWTC).

Argonne National Laboratory (ANL) Wind Tribology Overview

Ali Erdemir, ANL

An overview of ANL's tribological, surface treatment, and nano-lubricant research and its 

engineering staff was presented. Argonne’s tribology mission is to perform leading-edge R&D 

in the fields of materials, lubricants, surface engineering, and tribology to:

• Improve efficiency, durability, and reliability of machine components that operate under 

severe tribological conditions (including those in wind turbines)

• Understand fundamental tribological mechanisms through advanced surface/structure 

analytical methods and modeling/simulation.

Argonne tribology strengths, facilities, and recent success stories, such as super-hard nanocomposite coatings and carbide-derived carbon, were discussed. Current wind turbine R&D 

activity in ultra-fast boriding and nano-boron additives were presented. Future areas of R&D,

such as explaining the root causes of failure mechanisms, commercial implementation of 

advanced surface technologies, and investigation of hydrogen embrittlement were discussed.

Tribological Challenges in Wind Turbine Technology

Gary Doll, University of Akron 

Some wind turbine bearings are not achieving their desired operational lives because of life 

limiting wear modes. Tribological issues manifest themselves through different bearing failure 

modes in various systems of wind turbines. The primary mechanisms in pitch/yaw bearings, 

main shaft bearings, the gearbox, and the generator are false brinelling, micropitting, wear and 

cracking, and electrical arc damage. Micropitting and smearing are caused by large amounts of 

roller/raceway sliding in situations in which lambda (Λ), the ratio between the oil film thickness 

and the combined surface finishes of the parts, is low. Micropitting, smearing, and false 

brinelling problems can be solved with durable tungsten carbide-reinforced, amorphous,

hydrocarbon thin film (WC/aC:H) coatings on rollers. WC/aC:H coatings on rollers provide 

bearings with a high tolerance of debris damage. The solutions to micropitting and scuffing in 

gears are the same as in roller bearings. The root cause of radial cracking and wear from an 
5

Irregular White Etch Area (IrWEA) is controversial, but probably mechanical in nature. Cleaner 

steels, higher compressive stresses on raceways, increased Λ, and less roller skidding can reduce 

IrWEA wear and radial cracking, if the IrWEA wear is of mechanical origin. In generators, less 

electric arc damage is shown in oils than in greases. Examples of problems without current 

solutions are: 1) increasing seal life and 2) the development of a common nacelle lubricant.

Presentation Summary

• Wear problems in pitch and yaw bearings

o False brinelling because bearings and gears are not rotating, vibrations cause 

small motions termed dither. This leads to fretting.

o Fretting leads to false brinelling and fretting corrosion

o There is a critical dither angle

o False brinelling is avoided by regular rotation, along with adequate base oil 

viscosity and antiwear additives

• Wear problems in main shaft bearings

o Micropitting defined

 Surface initiated fatigue due to roller/raceway sliding and low Λ condition 

(oil film thickness)

 Uneven load distribution between upwind/downwind bearing rows

 Micropitting avoidance through reduction of roller/raceway sliding by 

using preloaded tapered roller bearings will reduce risk of micropitting 

and/or coatings and super-finishes on SRB rollers reduce shear stresses 

and increase Λ by polishing raceways in operation

• Wear problems in gearbox

o Scuffing wear

 Rollers skidding across raceway in low Λ condition generates local 

temperatures high enough to melt steel

 Caused by decreasing loads and transient conditions

 Avoid transients and reduce clearance, or use coated rollers

o Axial Cracking & Wear from IrWEA

 Caused by hydrogen embrittlement or mechanical causes such as scuffing

 Avoidance through black oxide on rings and rollers, usage of case 

carburized rings from ultra clean steel, and reduction in shear stress (preloaded TRBs and coatings)

o Debris Damage

o Gear scuffing

• Electric Arc Damage in Generator

o Loads generated during grid reversal

o Avoidance through usage of ceramic balls, electrical insulating coating on rings, 

use of oil instead of grease, and usage of dry oils with high dielectric strengths.
6

Tribology Fundamentals

Elastohydrodynamic Lubrication (EHL) Fundamentals 

Vern Wedeven, Wedeven Associates, Inc.

The formation of an elastohydrodynamic (EHD) film to provide elastohydrodynamic lubrication 

(EHL) is the key lubrication mechanism for long-life and robust operation of rolling element 

bearings and gears for wind turbines. Understanding the EHL mechanism, and how it uniquely 

links to theory, provides a foundation for engineering design, tribology technology development,

and problem solving. The performance of EHL films is linked to fundamental and inherent 

lubricant properties of viscosity, pressure-viscosity coefficient, and traction coefficient. Seven 

features characterize this mechanism and spell the acronym, MIRACLE:

M = Molecular attraction of adsorbed films, which drags lubricant along with the moving 

surfaces

I = In-flight refueling by fluid flow in the converging inlet region, in which the surfaces supply 

fluid and pump up the film

R = Radical increase of viscosity with pressure, in which the oil becomes a pseudo-solid

A = Accommodation of stress by elastic flattening of surfaces and by the pseudo-solid, which 

rides the Hertzian region

C = Cushioning of asperities due to asperity deformation

L = Limiting shear strength of pseudo-solid film, which limits traction forces

E = Exit without trauma, in which the pseudo-solid reverts to oil without damage

Wind turbine operational features, including start/stop, present unusual demands and limitations 

for EHL mechanisms to be operational. The linkage between EHL mechanisms and boundary 

lubrication mechanisms is essential for understanding bearing/gear performance limits and 

failure mechanisms. Five key tribology parameters (entraining velocity, film thickness-tosurface roughness ratio, sliding velocity, total contact temperature, and contact stress) are used to 

“manage” technology development for the lubricated contacts of bearings and gears. These 

parameters can be used for design, failure analysis, lubricant formulations, and evaluation of 

materials and surface engineering technologies. Specialized testing illustrates how controlling 

lubrication and failure mechanisms are expected to play out in service hardware.

EHL Surface Interactions in Micropitting

Pwt Evans, Cardiff University, UK

The presentation detailed the effect of surface roughness on Elastohydrodynamic Lubrication 

(EHL). The importance of surface roughness was discussed in terms of Λ. The importance of 

surface roughness was illustrated for cases with sub unity Λ in the extreme loading events that 

occur due to the interaction of surface asperity features. The application of suitable analysis 
7

methods to gear contacts operating in these conditions shows that their operation occurs in a 

mixed lubrication regime, with direct surface interaction of the asperity features occurring as 

transient high pressure events.

High pressure events, such as asperity interaction, can lead to scuffing failure when the asperity 

contact levels are high. They also lead to cyclic asperity loading within the EHL contact due to 

sliding effects. Cyclic loading is used as an input to fatigue calculations that identify the near 

surface zone beneath heavily loaded asperities and have high probabilities of fatigue failure. The 

manner by which these asperities are subjected to plastic deformation during the running-in 

process, and the resulting residual stress field, was considered.

Effect of Lubricant Properties on EHL Film Thickness and Traction

Andy Olver, Imperial College, UK

The presentation discussed the effects of various phenomena on film thickness and traction. 

Dimensionless speed, and material and load parameters were defined and basic regression 

equations to estimate the film thickness were presented. Methods to estimate the effect of 

temperature on lubricant viscosity were discussed. The phenomenon of “shear thinning," 

defined as the variation of viscosity with shear rate, was shown. The Ree-Eyring versus Carreau 

methods for estimation of the coefficient of friction were discussed, including accounting for the 

slide roll ratio and temperature droop as measured in simple bench tribometer tests. The 

following are proposed:

• A protocol for extracting a description of the EHL traction behavior of an oil from simple 

bench tribometer tests

• This can be used in conjunction with a coupled thermal EHL model to predict traction 

over a wide range of conditions for competing oils.

Surface Roughness and Micropitting

Lane Winkelmann, REM Surface Engineering

The presentation discussed typical wind turbine failure modes, and described the basics of 

micropitting on gears. The benefits of super-finishing gears were discussed. Super-finishing 

modifies the topography of the gears and can eliminate micropitting, increase lubricant life and 

cleanliness, and increase component life. It is relatively easy to implement. Isotropic superfinishing, in which directionally-oriented grinding asperity rows are eliminated, was discussed in 

detail. Isotropic super-finishing can reduce the mean roughness value by an order of magnitude 

while maintaining the overall component geometry. Supporting validation work and the results 

from micropitting tests through standard methods for Forschungsstelle für Zahnräder und 

Getriebebau (FZG) Brief Test of Grey Staining (BTGS) was shown. The current status of 

implementation of isotropic super-finishing was discussed.
8

Lubricant Fundamentals

Fundamentals of Lubrication Gear Oil Formulation

Jon Leather, Castrol Industrial

The presentation discussed a systematic approach to gear oil formulation and development, 

starting with the fundamentals of gear oils and their application in wind turbines. There are 

industrial and wind industry requirements for gear oils. The composition, effects and sideeffects of gear oils and their components is a balancing act when formulating the oils. An 

example project was discussed that demonstrated the basic process of building a new product. 

Once a prototype is developed, field trials occur through the final development stages.

The challenges facing wind turbine gear oil formulation are manifested by the requirements for 

wind turbine operation:

• Long oil life: +3 to 5 year minimum

• Use of anti-scuff/antiwear additives with high load carrying capacity

o Wear performance should remain constant as the oil ages

o Micropitting protection

• Oil cleanliness: 16/14/11 for new oil 18/16/13 used

• Wide temperature range

o Cold startup

o High operating temperatures

• Oxidation stability

o Resistance to sludging

o No effect on the service intervals of filters

• Stability with water and condensation: Rust and corrosion protection

Other requirements by gearbox and wind turbine manufacturers include: 

• Deutsches Institut für Normung (DIN)-minimum requirements

• Compatibility with elastomers and paints

o Static and dynamic tests

o Long-term tests with a duration of at least 1000 hrs

• Foam tests

o Mixed with anti-corrosion oil

o After filtration

• FZG-tests
9

o Micropitting tests

o Increased loads and/or tests without running in

• Tests of antifriction bearings:

o Corrosion protection, especially salt water

o Formation of residues under the influence of water and temperature

o Wear tests on an FE 8 test-rig

o Endurance tests on test benches for antifriction bearings

• Further requirements

o Filterability

o Good cleanliness class and automatic countability (ISO 4406 Particle Count)

Selecting Synthetic Gear Oil

Dennis A. Lauer, Klüber Lubrication

When selecting synthetic gear oil, the maximum performance level that the gear oil is able to 

meet should be compared to the actual performance of the gear oil. The maximum performance 

of gear oils, with advanced additive packages, and the impact of the base oil on performance 

parameters were summarized. Over the life cycle of the turbine, the highest performance oil 

proves to be the most cost effective, though it commands the highest price.

The requirements for wind turbine gear oil are higher than the industrial gear oil requirements 

specified by DIN 51517-3. Specifically, wind turbine gear oil is expected to meet the following 

requirements:

• High scuffing and micropitting load-carrying capacity

• Low friction behavior

• No negative influence on wear behavior and life time of rolling bearings

• High oxidation stability

• High upper operating temperature

• No residue formation

• No negative influence on radial shaft seals.

When selecting wind turbine gear oil, the following performance characteristics should be 

considered:

• Scuffing load-carrying capacity greater than LS 13

• Resistance to micropitting greater than, or equal to, LS 10

• Pitting load-carrying capacity
10

• Bearing load-carrying capacity

o Suitable for rolling bearing lubrication FAG FE8 test

o Maximum roller wear <= 10 mg, maximum cage wear <= 100 mg

• Foam test

• Elastomer compatibility

o Static elastomer compatibility, according to DIN ISO 1817

o Dynamic elastomer compatibility, according to DIN 3761

• Oil change intervals

• Viscosity-temperature behavior

• Efficiency, oil temperature, wear, and wear rate

• Friction behavior.

Wind Turbine Grease Lubrication

Henri Braun, ExxonMobil

Lubricating greases face a demanding environment in wind turbine applications. Wide operating 

temperature ranges, shifting wind forces and directions, high torques and loads, water 

contamination, and boundary lubrication conditions are the key performance challenges. These

are exacerbated by the operator's desire to minimize the number of greases used and to maximize 

re-greasing intervals. Thus, developing lubricating greases for wind turbines is a costly and 

complex undertaking that must consider numerous, often conflicting, targets. Synthetic-base oils 

can help meet the lubrication needs of main pitch and yaw bearings with one grease, and help 

reduce traction and extend grease life. Still, achieving optimum performance requires a carefully 

balanced formulation of base oil, thickener, and additives. Finally, extensive laboratory, bench,

and field testing is necessary to demonstrate overall performance.

The challenges facing wind turbine grease formulation are demonstrated by various and 

conflicting requirements across different turbine components as follows:

• Main bearing

o Fluctuating winds introduce thrust loads

o High load and slow speed make EHL conditions difficult to achieve; suggest the 

use of high viscosity base oils (ISO VG 460)

• Pitch bearing

o High torques and bending moments during rotation

o Bi-directional oscillation (partial rotation) results in boundary lubrication 

conditions

o Vibration leads to fretting wear and corrosion 
11

• Yaw bearing

o High thrust loads from weight of nacelle

o Bi-directional oscillation (partial rotation) results in boundary lubrication 

conditions

o Vibration leads to fretting wear and corrosion

• Yaw gears

o Vertical tooth flanks require excellent adhesion

o Exposure to environment necessitates excellent corrosion protection

• Generator bearing

o Moderate to high speed rotation

o Lower base oil viscosity than pitch, yaw, and main bearings (ISO VG 100)

o NLGI 2 or firmer, with good oil release control

To address these challenges, synthetic oil greases are advantageous over mineral oil products,

providing:

• Wider operating temperature range, lower viscosity at low temperatures and higher 

viscosity at high temperatures

• Lower traction, reducing heat generation and energy losses

• Longer grease life

As is the case for wind turbine gear oil development, the development of wind turbine grease 

also is a costly and complex undertaking that requires:

• Consideration of numerous, often conflicting, targets

• Extensive laboratory, bench and field testing to demonstrate performance

Two attempts to address these challenges include: 

• Main bearing protection through a balance of EHL and additive chemistry

• Pitch and yaw bearing protection through targeted additives and controlled bleed

Wind Turbine Tribological Damage

NREL Gearbox Reliability Collaborative Failure Database Project

Mark McDade, NREL

The four parts of the NREL Gearbox Reliability Collaborative were explained and include

modeling and analysis, testing, condition monitoring, and the failure database. The GRC Failure 
12

Database was presented in detail, including its value to industry and individual participants and 

demonstrating the actual software used to input information resulting from gearbox overhauls. 

The database houses information collected from gearbox rebuilds, such as:

• Background data from in-field operation

• Failure modes from tear-down inspection

• Existing data from records.

The goal of the database is to share “sanitized data” within the participating group and to identify 

the root cause of failures and prescribe remedies. The database software provides a structured 

data collection system with a navigation tree that is visually-oriented. It offers wireless image 

transfer from the camera to the software. To expedite data input, the database software includes 

embedded models of each gearbox, a failure atlas with classifications of failure modes, and 

interactive help. Reports are automatically generated that identify:

• The failure location

• Failure images, with comments from the inspector

• Failure mode identification and description.

Bearing and Gear Failure Modes Seen in Wind Turbines

Robert Errichello, GEARTECH

Wind turbine bearings and gears have a long history of failure modes such as Hertzian fatigue, 

consisting of macropitting, micropitting, or subcase fatigue, and by scuffing or bending fatigue. 

Current failures of rolling-element bearings in wind turbine gearboxes are manifested as axial 

cracks in the inner rings. Current failures in gears are manifested as bending fatigue that 

originate from non-metallic inclusions. Examples of bearing and gear failures were presented to 

demonstrate the morphology of the various failure modes. Current failure modes that are 

prevalent in wind turbine gearboxes include:

• Bending fatigue, originating from non-metallic inclusions

• Micropitting, due to rough surfaces or lubricants with inadequate micropitting resistance

• Subcase fatigue, due to grind temper or inadequate case depth

• Adhesion or abrasion, due to contaminated lubricants

• Fretting corrosion during parking

• Case/core separation, due to excessive case depth at tips of teeth

• Axial cracks in bearing inner rings.
13

Microstructural Alterations in Hertzian Fatigue

Robert Errichello, GEARTECH

Microstructural alterations in Hertzian fatigue have been studied for decades and are a key 

indicator for the fatigue mechanism in rolling-element bearings and gears for wind turbines. 

Several features were described that characterize microstructural alterations such as: butterflies, 

dark-etching areas (DEA), white-etching areas (WEA), flat white bands, and steep white bands. 

Microstructural alterations were shown to be linked to Hertzian stress and the number of load 

cycles.

Current failures of rolling-element bearings in wind turbine gearboxes are manifested as axial 

cracks in the inner rings. The white-etching areas associated with this failure mode are irregular 

and are termed irregular white-etching areas (IrWEAs) or white-etching cracks (WECs). Several 

theories have been proposed for the root cause of IrWEAs, WECs, and axial cracks. However, 

none of the theories have been proven, and it is an active field of research.

• It is probable that IrWEAs and WECs have identical driving mechanisms that manifest as 

different alterations in bearing microstructure depending on the specific metallurgy of the 

material's alloy and processing.

• How IrWEAs and WECs develop and progress is not understood yet. Both have been 

characterized as brittle fracture modes that generate cleavage fractures. It might be a 

single-step or a multiple-step process that generates cleavage cracks and white-etching 

bands.

• It is not understood how coatings, such as black oxide, help to prevent axial cracks. They 

might reduce traction stresses, damp vibrations, or prevent diffusion of hydrogen. On the

other hand, the temperatures used to coat the components might beneficially alter the 

bearing metallurgy.

Microstructural Alterations in Rolling Contact

Andy Olver, Imperial College, UK

Transformations, at inclusions and on crack faces, are indistinguishable from those occurring at 

the surface in simple rubbing experiments (e.g., fretting). Crack faces in rolling contact are 

subject to fretting displacements and high pressure. Surface cracks are typically associated with 

local plastic deformation due to reduced plastic constraint. The resultant residual stresses control 

the direction of small surface cracks. Microstructural alterations in Hertzian contacts have the 

following features:

• Transformations at inclusions and on crack faces are indistinguishable from those 

occurring at the surface in simple rubbing experiments (e.g., fretting)

• The alterations are nano-grained ferrite (e.g., cell ferrite)

• Crack faces in rolling contact are subject to fretting displacements and high pressure

• Surface cracks are typically associated with local plastic deformation, due to reduced 

plastic constraint
14

• Resultant residual stresses control the direction of small surface cracks.

Classic Bearing Damage Modes

Ryan Evans, The Timken Company

Rolling element bearing damage in the field is attributable to either material fatigue, or wear, in 

most applications. Bearing life prediction tools estimate the statistical likelihood of contact 

surface spalling due to material fatigue, given a set of assumptions about the bearing operating 

conditions. However, even if a bearing is designed, manufactured, and specified for an 

application, premature damage and wear may occur due to contamination, inadequate 

lubrication, and/or misuse. Classic examples of these damage modes and their causes were 

presented for steel rolling element bearings. In addition, transmission electron microscopy 

characterization of subsurface microstructural alterations, called “white etch areas,” from wind 

turbine gearbox bearings was discussed. Classic failure modes include:

• Macropitting (spalling)

o Inclusion origin

o Point Surface Origin (PSO)

o Geometric Stress Concentration (GSC)

• Micropitting (peeling)

• Wear or other damage

o Abrasive wear

o Debris denting

o Etching/corrosion

o False Brinelling/Fretting corrosion

o True Brinelling

o Heat discoloration

o Scuffing (smearing)

o Electric discharge

Investigations are underway to better understand WEA microstructural alterations and their 

causes.

Root Cause Hypotheses

The Mechanism of White Structure Flaking in Rolling Bearings

Hideyuki Uyama, NSK, Japan

Microstructural change is observed on cross sections of the rolling bearings used for wind

turbine gearboxes. This microstructural change is the same phenomena, called a "white 

structure." NSK has experienced premature flaking, due to white structures in bearings, in

automotive electrical accessories, as an example. The main cause of white structures is due to 
15

hydrogen generated by decomposition of the lubricant. Formation mechanisms of white 

structures related to hydrogen embrittlement, and accelerating factors of hydrogen generation,

were reported. Our research shows the following:

• Bearing failures in wind turbine gearboxes may be classified as white structure flaking

• White structure flaking is induced by hydrogen that diffuses into the bearing's steel

• White structures are caused by localized, microstructural changes. The presence of white 

structures indicates that hydrogen-induced, localized plastic deformation is present in 

rolling contact fatigue

• The type of lubricant, slip, static electricity, and material influence white structure 

flaking.

It is hoped that our experience with automotive bearings will help solve wind turbine bearing 

failures.

The Bearing Axial Crack Root Cause Hypothesis of Frictional Surface Crack 

Initiation and Corrosion Fatigue-Driven Crack Growth

Jürgen Gegner, SKF, Germany

Results of failure analysis and research were presented. Some medium and large size bearings, 

such as those used in wind turbine gearboxes, suffer from premature failures due to axial 

raceway cracks. Root cause hypotheses from the literature were reviewed. Surface initiation,

and the subsequent chemically-assisted propagation of the cracks, occurs as brittle spontaneous 

fracture and corrosion fatigue, respectively. Local microstructural changes result from hydrogen 

impacts due to aging reactions of the lubricant at the tip and the rubbing faces of the advancing 

crack. Material response in the form of cleavage-like surface cracking suggests there are

causative tangential tensile stresses. Weaker areas with inhomogeneities and edge-zone 

embrittlement were considered. The tensile stresses are caused by sliding friction in the rolling 

contact, induced by vibrations, for example. A tribological model was presented. The tangential 

tensile stresses are estimated to be high enough for cracking. A combination of cold working, 

because it generates compressive residual stresses, together with black oxidizing and final lowtemperature reheating, is proposed as an effective countermeasure. Our conclusions are:

• Axial cracks initiate from the surface by brittle, spontaneous, cleavage-like fractures

• The root cause of the axial cracks is tangential tensile stress, due to high local sliding 

friction

• Surface cracks propagate by corrosion fatigue cracking (CFC)

• CFC is caused by lubricant decomposition at the crack tips and on the rubbing crack 

faces.
16

Hammering Wear Impact Fatigue Hypothesis

Johan Luyckx, Hansen Transmissions, Belgium

The main features and status of the WEC/IrWEA failure mode in roller bearings applied in wind 

gear units were described. The material observations in WEC/IrWEA were shown and a detailed 

interpretation concludes that different material damage patterns are generated by an impact load 

system. The updated Hansen wind experience regarding WEC/IrWEA failures was presented. It

detailed bearing variants that have improvement potential. The material research performed on 

the different drivers identified in the Hansen wind experience were discussed and interpreted. 

These resulted in the formulation of a root cause hypothesis. There is an observed correlation 

between WEC/IrWEA failed raceways and the hammered (flattened) appearance of the raceway. 

The hypothesis is that the impact load system is caused by a roller contact on a flattened wear 

particle from the raceway, henceforth called “hammering wear impact.” This impact load 

system generates subsurface material damage, which initiates a bearing fatigue failure process. 

Further research work is proposed and a summary of all potential solutions and parameters was 

presented with the following conclusions:

• Axial cracks are caused by the impact load system

• IrWEAs and WECs are consequences of the impact load system

• Countermeasures include:

o Raceway surface treatment, such as reduced roughness, black oxide, hot 

assembly, strain hardening, hard coatings

o Case carburizing 

Influence on Bearing Life by New Material Phenomena

Walter Holweger, Schaeffler

Classical failure modes in bearings appear as a subsurface dark etching area, followed by

strictly-oriented low and high angle white bands (LAB/HAB). The appearance of such features

is a function of pressure and is expressed in the Woehler line. In contrast to this classical

fatigue, irregular white etching bands indicate a new fatigue process not ruled by the classical

theorems (the Woehler Line). A majority of failures occur in wind mill applications, in the

high speed shaft at low nominal contact pressure.

Detailed forensic investigations show the degradation of the matrix and the matrix carbides in a

nanometer scale. Downsizing implies the presence of a severe plastic deformation mode, 

normally found in high pressure torsion experiments. Neither severe plastic deformation nor 

high pressure torsion is found in the real application.

Test rigs at Schaeffler technologies prove the occurrence of irregular white etching with 

respect to loading and materials. Those results show that IrWEAs occur due to multiple

influences, e.g., vibrations, straying currents, and the chemistry of lubricants and

preservatives. Recent results prove lubricants and preservatives to be of major importance,

while vibrations, straying currents, and mechanical loading are of minor importance.
17

A stringent model of irregular white etching area requires a multiple descriptor model. The 

hierarchy of root causes is:

• Nature of EHL lubrication (preservatives, additives)

• Nature of the surfaces (dark oxide, hot assembly, passivation)

• Nature of the material (type of heat treatment, microstructure, interstitials)

• Nature of the loading (vibration, slip, electric discharge)

R&D Activities

Surface Treatment and Nano Lubricant Bearings

Ali Erdemir, ANL

Leveraging over 25 years of tribology experience (specifically, surface engineering, advanced 

lubricants, materials and coatings, and surface analytical characterization), the Argonne tribology 

group has been active in the wind turbine area for several years. The objective of the current 

work has been to develop, test, and implement innovative surface engineering and nanolubrication technologies that can increase the reliability of wind turbine drivetrain components; 

primarily through an ultra-fast boriding process, lubricant- derived, diamond-like carbon (DLC) 

coatings, and nano-boron lubrication technology.

The ultra-fast boriding process was developed to enhance the hardness of gear and bearing 

surfaces, by more than a factor of two, compared to standard carburizing (1800 HK to 600 HK). 

This was achieved through an advanced electrochemical process that increases efficiency by 

80% compared to current boriding, making it economically practical for wind applications. The 

sliding wear performance of the borided surface is shown to be improved by an order of 

magnitude compared to that of standard carburized gear steels.

The lubricant-derived DLC technology introduces a paradigm shift in lubricant/material 

technologies. The concept is to design coatings that catalyze a reaction with base lubricants to 

form a protective boundary film. The advantage of this technique is that it can generate 

boundary films from oils that do not contain organometallic additives. This simplifies the 

lubricant formulation and removes additives, which in some cases can cause early failure in 

some wind applications. The results presented showed scuffing performance of these designer 

coatings that exceeded the limits of the test rig, Surface analysis showed evidence of diamondlike, sp3 bonding in the boundary film.

Finally, the nano-particle-based lubricant additives have been under development for several 

years to enhance performance compared to traditional additive packages. Nano-particle 

additives, mostly based on boron or carbon, are engineered to interact with the contacting surface 

to produce a low friction/protective boundary film. Results demonstrate the friction reduction 

qualities and surface analysis shows evidence of the tribofilm. Micro-pitting performance also 

was significantly enhanced with nano-boron additives compared to commercially-formulated 

oils. In the current test, micro-pitting was nearly eliminated. 
18

The conclusion is that ultra-fast surface boriding can enhance surface hardness and reduce wear

of certain components. More development is needed to optimize the processing for specific drive

train components:

• Lubricant-derived DLC coating technology is an area of development that can reduce the 

need for traditional lubricant additives

• Nano-particle-based additives demonstrate certain advantages on the bench-top scale that 

need verification at the system level.

Novel Macromolecular Nano Lubricant Oils and Greases

Ajay P. Malshe, NanoMech Inc.

Successful investigations of advanced nano-lubricant additives that favorably impact robust 

boundary tribofilm formation to reduce wear and friction in wind turbine components, such as 

gearboxes, was presented. These additives are designed as surface-stabilized nanoscale,

materials-based macromolecules that are “dropped-in” from off-the-shelf formulated oils and 

grease for the purpose of advancing effectiveness. They have been prepared for use in 

performance testing to closely simulate the conditions faced by oil lubricants and greases in wind 

turbines. Specimens of the most widely used materials in wind turbine gearboxes have been 

prepared for evaluation in several tribological tests to assess the performance of the developed 

nano-lubricants. These tests were used to demonstrate successful and consistent performance, 

meeting the demands for lubrication of wind turbine gearboxes.

Update on the Development of a Full Life Wind Turbine Gearbox Lubricating Fluid

Manfred Jungk, Dow Corning

The goal of this project is to prove and implement an alternative chemistry for the lubricating 

fluid used for wind turbine gearboxes. Due to the chemical robustness of the altered lubricating 

fluid, the expectation is that the need for condition monitoring and/or maintenance of the 

gearbox is reduced significantly and could potentially be eliminated. The fluid technology to be 

expanded into gearbox lubrication has been successfully used in "lube for life" specialty 

applications. In this study, bench testing like FZG gear and FE 8 bearing have been carried out 

to document the suitability of the fluid compared to existing gearbox oil specifications. The 

benefits, such as superior viscosity temperature profile with viscosity indices above 300, and 

material compatibility have been demonstrated. The target viscosity's superior viscosity 

temperature profile is lower than that currently used gearbox oils and results in friction reduction 

and reduced wear of the internal gears and components. Full-scale wind turbine gearbox trial 

results indicate improved power output efficiency. In addition to the increase of efficiency, the 

other potential impact is increased operating reliability through reduction in the downtime by 

eliminating planned and unplanned gear oil changes.
19

Modeling Tribological Contacts for Wind Turbine Gearbox Component Life 

Prediction

Nathan Bolander, Sentient Corporation

Modeling of lubrication and contacting surfaces can provide more accurate and flexible options 

for determining the life of wind turbine components. A few techniques that are being developed 

to improve the understanding of surface contact fatigue were presented. First, a brief 

introduction to EHL/mixed-EHL modeling was presented, including discussion of the governing 

equations and discretization techniques. The utility of a mixed-EHL approach, based on 

deterministic consideration of surface roughness profiles, was emphasized. In the next section, a 

microstructure-based approach for contact fatigue analysis was presented. The approach utilized

detailed mixed-EHL traction profiles as inputs. Users can model the effect of a wide range of 

parameters that influence surface fatigue, such as surface roughness, material properties, and 

load variation. The analysis resulted in a fatigue life distribution. The final section showed a 

method by which a prognostic model for the remaining life of wind turbine components can be 

assembled from a damage progression model, a source of diagnostic information, and an 

uncertainty management model to update the architecture. A prognostic model for bearing spall 

propagation was presented as an example application.

Inputs for traction modeling (EHL/mixed-EHL) assume:

• Real surfaces are rough

• Reynolds equation governing lubrication flow in the contact zone

• Model outputs are a time history of surface shear stress, due to lubricant flow and 

asperity interaction.

In terms of fatigue damage initiation, there are several points to remember:

• Fatigue cracks initiate in the microstructure

• To understand fatigue, an understanding of microstructure level stresses is needed

• Fatigue models must consider:

o Microstructure geometry/composition

o Residual stresses due to manufacturing

o Surface tractions

o Load history/non-linear damage accumulation.

When developing a prognostic system, it is necessary to include three primary components:

• A damage progression model

• A source of diagnostic information

• An architecture to update predictions and manage the model uncertainties.
20

Approaching Surface Fatigue Life by Integrated Contact and Lubrication 

Mechanics

Jane Wang, Northwestern University

Surface damage due to contact fatigue is a major threat to wind turbine transmission gears and 

bearings. Fatigue life prediction is based on interfacial mechanics, and therefore, is vital to 

design, operation monitoring, and performance/reliability improvements. Over the years, a 

comprehensive fatigue life prediction approach has been developed for components under cyclic 

motion, based on advanced contact mechanics and mixed elastohydrodynamic lubrication (EHL) 

models. The comprehensive approach is capable of simulating the entire transition from fullfilm and mixed EHL down to boundary lubrication, or even dry contact, of real machined rough 

surfaces under severe operating conditions. The presentation introduced the models and 

predicted gear pitting life results with comparisons to test data. In addition, issues like material 

inclusions, defects, reinforcements, and surface treatments, such as coatings and case hardening,

were discussed.

Panel Discussion

Field Observations of Tribological Damage in Wind Turbines

Art Miller, enXco

Four examples of wind turbine gearbox faults, discovered during end of warranty (EOW) 

inspections, were presented. The first case demonstrated micropitting and fretting on the planet 

gear, micropitting and scuffing on the ring gear, intermediate gear and pinion, and high speed 

gear and pinion. The second case was a gearbox with a failed non-drive end tapered roller 

bearing on one of the three planet gears. The gearbox in the third case demonstrated a pitted 

intermediate pinion and correlating oil debris sensor data. The gearbox in the fourth case 

showed pitting and smearing on the high speed shaft bearing. Conclusions were that oil 

cleanliness and quality does appear to be a contributing factor and flushing out the gearboxes and 

replacing clean gear oil appears to retard the progression of wear. Some of the gear oils seem to 

be better at controlling the foam. Testing brands that are outside of the OEM’s list of 

recommendations are worth looking into. Viscosity is the most important property when 

choosing a lubricant and working with higher viscosities and oil with a higher index (VI) has had

positive results

A Look at Wind Turbine Gearbox Oil over Time

Bill Herguth, Herguth Laboratories, Inc.

Examinations of seven different oils, from May 2004 to October 2011, totaling 3500 samples 

with run times from 1 to 72,649 hours used in a variety of gearbox makes, models and locations,

were presented. The results show that gearbox oils react in service in different ways and that 

trending the data helps in determining the end of life. There is enough data available to plan 

future lubricant change schedules. These should be based on new algorithms using oil type, inservice time, gearbox make and model and operating parameters. Additive concentration and 

type do not necessarily indicate the relative wear protection or useful life
21

Damage as Seen from a Wind Turbine Gearbox Manufacturer’s Angle

Shawn Doner, Winergy

The wind turbine industry has cost issues related to gearbox reliability. Solutions to the

reliability problems are paramount. The presentation began with an example of shallow spalling 

on the intermediate pinion that was widespread in a particular gearbox. It was determined that

the problem was a lubricant issue due to high water content in the oil. The particular oil was 

sensitive to moisture, which lowered its load carrying capacity. Water in oil is a continuing 

concern in the wind industry. Field data is extremely important to gearbox manufacturers to 

implement knowledge in future designs for continuous improvement. Winergy views the 

lubricant as a machinery element, an integral part of the overall design. Winergy has completed 

FZG tests to evaluate the effectiveness of oils with 500 parts per million (ppm) water, and also 

without water in the oil. One oil failed at load stage 5, others completed load stage 10.

Pitch and Yaw Bearing Damage

Les Miller, Kaydon

Dimensional challenges regarding the size and weight of pitch and yaw bearings in modern wind 

turbines were discussed, along with the magnitude of required preload to support the blade. Key 

issues with design and maintenance of pitch and yaw bearings were summarized. Smooth 

rotation is a continuing challenge in the presence of heavy preload and hub flexibility. Seal 

integrity and durability also are a continuing challenge and are important to increase resistance to 

false brinelling and fretting wear. There is currently no accurate means to predict friction torque 

under load. Life calculations such as L10 life are no longer required for certification, because the 

current designs are not failing due to fatigue. An example of a hub and bearing finite element 

analysis was presented, focusing on a large amount of deflection under a high preload. 

Examples of damaged pitch and yaw bearings also were shown. Adaptive pitch control was 

discussed with the possibility that it may resolve fretting, but it can result in fatigue issues.
22

Summary and Conclusions

Tribology is an interdisciplinary topic, involving interactions of fluid and surface chemistry to 

the physics of solid contact. In the wind energy community, tribological failures also are an 

inter-industry challenge because they include the bearing and gear OEM, lubricant formulator, 

turbine manufacturer, and operator. Therefore, a coordinated R&D effort is important to 

successfully identify, solve, and implement mitigation methods. DOE has the ability to approach 

the issues across industry boundaries and to provide information sharing that benefits the 

industry as a whole. DOE is also able to leverage the national laboratories and their unique 

capabilities and facilities, with the experience and application guidance of its industrial partners.

Research

There are several national laboratory research projects, in the past and ongoing, to address wind 

turbine tribological-related issues from varied approaches. Currently, the NREL Gearbox 

Reliability Collaborative (GRC) is working to understand the drive train system and account for 

load conditions that result in overstressed contacting elements. The GRC is utilizing advanced 

computer simulations, dynamometer testing, and field trials. In addition, its database effort is 

collecting information on field failures of gearboxes to elucidate the types of failures 

experienced by a range of turbine models at various locations. Argonne National Laboratory is 

engaged in the development of advanced surface treatments and lubricant additives that can 

benefit the wind industry, and has unique bench-top testing capabilities and lab-based coating 

and lubricant fabrication equipment. Oak Ridge National Laboratory also has unique tribology 

and metallurgy capabilities, and has been engaged in the forensic analysis of damaged wind 

turbine bearings using advanced microscopy techniques.

Seminar Summary

The reliability of a wind turbine is highly dependent on tribological issues associated with blade 

pitch systems, main shaft bearings, yaw systems, gearboxes, and generators. Many of the failure 

modes that occur in these systems such as Hertzian fatigue, adhesion, abrasion, corrosion, 

fretting corrosion, polishing, electric discharge, and scuffing are influenced by tribology. 

Lubricant base oil, additives, and cleanliness must be correctly specified for each of these 

systems to achieve their design life. Currently, bearings in blade pitch systems, main shafts, yaw 

systems, gearboxes, and generators suffer early failures despite well maintained systems, proper 

lubricant selection, and clean oil. Furthermore, micropitting continues to attack gear teeth and 

bearing components. False brinelling and fretting corrosion are the primary failure modes for 

blade pitch and yaw bearings, and electric discharge is often the root cause of failures of 

generator bearings. Lubricant contamination by solid particles is a principle mechanism that 

causes debris dents on bearing components. These eventually lead to micropitting and 

macropitting. A recent critical bearing problem manifesting as axial cracks is occurring

primarily in the inner rings of bearings, though also sometimes in outer rings. Currently, there 

are several controversial hypotheses for the root cause of this failure mode, but none of the 

hypotheses have been widely accepted across the entire tribology community.
23

Tribology and Lubrication Fundamentals

As previously stated, tribology is an interdisciplinary topic, and a review of these fundamentals 

is essential for an informative understanding on how the entire wind turbine system operates. 

For wind turbine applications, there are several factors that cause extreme tribological 

conditions, which lead to early failure and system down time. These conditions are listed in the 

following section. The fundamental tribological topics that are most relevant to wind 

applications, which were covered in this seminar, included elastohydrodynamic lubrication 

behavior and surface interaction, and lubricant fundamentals: formulation, synthetics, and 

greases.

Elastohydrodynamic lubrication (EHL) is one characteristic to counter formal contact, like 

rolling element bearings and gear tooth contact. In the EHL lubrication regime, the surfaces are 

separated by a thin lubricant film, which is influenced by the lubricant, surfaces and contacting 

conditions. If the surface roughness is on the same order or greater than the generated film 

thickness, contact occurs. This is characterized as mixed lubrication, increasing the risk of 

surface damage, such as asperity scuffing and micropitting. It can be effectively controlled by 

reducing the surface roughness through super finishing techniques. An important parameter for 

designing components that operate in the EHL regime is the traction or friction at the contact. A 

protocol for extracting this parameter was proposed.

Lubricants used in wind turbine applications are subjected to extreme conditions and are 

expected to maintain their performance throughout operation, and therefore, they are required to 

meet higher standards than similar lubricants for other industries. Synthetic lubricants typically 

offer the better cost benefit throughout the life cycle. Development of new lubricants typically 

involves a systemic approach of blending different concentrations of certain additives to balance 

the various performance characteristics: scuffing, micropitting, wear, oxidation stability, 

compatibility with elastomers and paints, foaming, and other techniques. In addition to gear oils, 

greases are used in many bearing components: generator, main shaft, pitch, and yaw; each with 

their own design requirements.

Factors Influencing Surface Failures

It is clear that wind turbines offer unique environments for mechanical systems due to factors 

such as:

• Vibration loading

• Rapid acceleration

• Frequent stops and starts

• Periods of standstill during parking

• Misalignment of gears and bearings, due to elastic deformations

• Exposure to contamination by water and dust

• Exposure to wide temperature variations

• Difficult access for maintenance.
24

Because of this unique operational environment, it is important to maintain the quality of the 

lubricant by:

• Monitoring the life cycle of the oil- like the onset of oxidation and organic species in the 

oil

• Monitoring viscometrics, additive chemistry, contamination species (wear) and 

degradation

• Filtering the oil. There is no filter too small that would filter out additives; however, 

lubricant additives may absorb on the filter medium.

Critical Surface Damage/Failure Modes

Several surface damage mechanisms were identified as significant for wind turbines throughout 

the seminar, including false brinelling and fretting corrosion, mainly in blade pitch and nacelle 

yaw components, micropitting, abrasive wear, scuffing (also called smearing in bearing 

literature), axial cracking of bearing rings associated with irregular microstructural changes (also 

known as IrWEAs), Hertzian fatigue, electrical discharge damage, mainly in generator bearings, 

and surface etching/corrosion. Without a comprehensive collection of failure data and analysis,

it is difficult to quantify the severity and frequency of each type of failure; however, anecdotal 

accounts during the seminar highlighted certain damage modes.

Some of the damage modes are summarized below:

• False brinelling and fretting corrosion, as it was pointed out by Doll and others, is a 

common issue for blade pitch and yaw bearing and gear components, which only 

experience a small range of motion or structural vibration. This condition causes 

lubricant to be squeezed out of the contact area and removes protective oxide layers. This 

results in an accelerated wear mode that forms indentations in the raceway, impeding the 

smooth operation of the positioning system. To alleviate these issues, certain control and 

design parameters to avoid critical dithering angles are recommended, in addition to the 

use of coatings and lubricating greases with proper antiwear additives.

• Micropitting is a wear mode that affects both gears and bearings, and is typically 

associated with tangential shear stress caused by rolling-sliding contact. In bearings, this 

is typically caused by sliding or skidding during unsteady operation. Micropitting is 

commonly a precursor to larger surface failures such as Hertzian fatigue (also caused by 

overstressed contact, material inclusions, and other factors). Micropitting has been and 

continues to be an issue that the wind industry has dealt with over the years and was the 

topic of a similar seminar titled “Wind Turbine Micropitting Workshop.” The outcome 

of this report included recommendations for run-in procedures, discussion of root causes, 

and recommendations for testing and future research. In general, the major factors 

influencing micropitting include inadequate EHL film thickness, surface roughness, 

unsteady operating conditions, and antiwear lubricant additives. The fundamental 

mechanisms that cause micropitting are still a matter of discussion and more work is 

needed in this area to fully characterize this damage mode. The standardized testing 

method for micropitting performance is reported to be inaccurate and misleading to 
25

designers, which has perpetuated this problem. There is a critical need to reevaluate the 

standardized test method and consider different approaches.

• Scuffing is a surface damage mode of sliding contact typically characterized by the 

severe adhesion and rapid plastic deformation. It is caused by local frictional heating at 

the surface where lubrication film thickness is inadequate. This is generally understood 

as a lubrication short-fall, caused by inadequate design, lubricant supply, and additive 

formulation. Mitigation methods are fairly well established in the form of EP lubricant 

additives or surface coatings. However, this type of damage is commonly observed in 

field failures and a better understanding of the occurrences is needed. This would 

involve an analysis of specific failures including operating conditions leading to failure 

and bearing/gear location and design.

• Electric discharge damage can occur when faulty insulation, induction effects, or 

improper grounding allows the electric current to pass through the bearing, thereby

damaging the bearing surfaces. Electric discharge damage is caused by electric arc 

discharge across the oil film, between the rollers and raceways. Electric current might 

originate from electric motors, electric clutches, instrumentation, or it might be due to the 

accumulation of static charge and subsequent discharge. Damage might occur during 

electric welding on, or near, the gearbox, if the path to ground is not properly made 

around the bearings rather than through them. Wind turbine bearings might be damaged 

by lightning strikes. When an electrical arc occurs, it produces temperatures high enough 

to melt bearing surfaces. Microscopically, the damage appears as small, hemispherical 

craters. Edges of the craters are smooth and they might be surrounded by burned or fused 

metal in the form of rounded particles that were once molten. A metallurgical section,

taken transversely through the craters and acid etched area, might reveal austenitized and 

rehardened areas in white, bordered by tempered areas in black. Sometimes microcracks 

are found near the craters. Overall, damage to bearings is proportional to the number and 

size of the arcing points. Depending on its extent, electric discharge damage might be 

destructive to bearings. Associated microcracking might lead to subsequent Hertzian 

fatigue or bending fatigue. If arc burns are found on bearings, all associated gears should 

be examined for similar damage. Electric discharge damage can be prevented by 

providing adequate electrical insulation or grounding and by ensuring that proper welding 

procedures are enforced.

o Arcing - Electric discharge damage might consist of randomly spaced discharge 

craters; this damage is designated as arcing

o Fluting - Electric discharge damage is often periodically spaced around the raceway 

giving it a “fluted” or “washboard” appearance. The flutes are depressions in the 

raceway transverse in the rolling direction and are separated from each other by 

lands of undamaged surface. Fluting generally leads to high vibration and noisy 

operation.

• Microstructural alteration (i.e., white etching area cracks) is a class of damage mode that 

can lead to axial cracking and macropitting at 1% to 20% of the L10 bearing design life 

(Gegner). As discussed previously in this report, this is one of the more critical, and least 

understood, failure modes experienced in wind turbines. While not unique to the wind 

industry, it is found to be much more prevalent than in other applications. As reported,
26

there are several theories about the cause of WEA cracks including hydrogen induced 

embrittlement from lubricant decomposition (Uyama); mechanically induced, from high 

stress and slip conditions (Evans); mechanical impact loading (Luyckx); or multiple 

influencing factors, without one root cause (Holweger). No one root cause, or 

combination of multiple root causes, has been fully established and more fundamental 

research effort is needed to address this critical issue. Furthermore, the influence of 

materials, lubricants, and loading conditions on IrWEA cracking occurrence needs to be 

established through carefully designed test procedures, which are not yet established. 

Mitigation methods need to then be validated and optimized. Table 2 compares the 

current hypotheses for axial cracks.

Table 2. Comparison of Hypotheses for Axial Cracks

Hypothesis Crack origin Failure mechanism Root cause

NSK Subsurface Hydrogen enhanced 

localized plasticity (HELP)

Hydrogen embrittlement, due to 

lubricant decomposition

SKF Surface

Brittle fracture followed by 

crack propagation due to 

corrosion fatigue cracking

Tensile stress, due to high 

surface traction

Hansen Subsurface Adiabatic shear bands Elastic stress waves, due to 

impact on surface asperities

Schaeffler Subsurface Severe plastic deformation

Complex interaction between 

lubricant, surfaces, materials, 

and loads

To facilitate communication between failure investigators, the nomenclature of failure modes for 

gears and bearings should be harmonized. Table Error! No text of specified style in document.

recommends failure mode nomenclature and includes commonly used, but non-preferred names.

Table 3. Recommended Failure Mode Nomenclature

Failure mode Preferred name Non-preferred names

Hertzian fatigue Macropitting Spalling, pitting

Hertzian fatigue Micropitting Peeling, superficial spalling

Scuffing Scuffing Smearing, scoring

Field Failure Tracking and Analysis

Without statistical tracking of the field failures and root cause analysis, it is difficult to have a 

comprehensive account of the critical failure modes and to quantify the impact of each failure 

type. The NREL Gearbox Reliability Collaborative database effort is focused on facilitating this 

activity by working with wind farm owners and operators to develop the strategy to collect and 

analyze failure data. This helps to focus research efforts, and also aids in the development of 

appropriate mitigation methods through detailed forensic analysis. The database currently has 

collected 36 failure instances to date and expected to expand significantly in the near future.
27

Opportunities for Learning and Path Forward

The following list of discussion points was made by presenters and seminar participants on the 

topic of lessons learned within this and other industries.

• Comparisons with the aerospace industry should be considered for approaches to 

overcoming tribological challenges.

• Rolling element bearings in the aerospace industry generally have much longer, lives, ten 

years or more, even though they are subjected to highly loaded conditions.

• WEA issues are of interest to the aerospace industry as bearings reach their design life.

• The aerospace industry has experience of solving one issue at the expense of another 

issue. Therefore, system testing is critical for wind turbine design.

• Moving technology through technology readiness levels (TRL) 5, 6 ,7, and 8 

(prototyping, system testing, demonstration, and application) and then to market requires 

costly testing. Coordinated and focused R&D programs are critical, including 

collaboration where applicable. 

• TRL 4 (component level lab testing) is the critical point when technologies merge 

together. In tribology, synergies between lubricants, materials, and system design are 

identified at this point.

• Some tests are not appropriate and may mislead the design and, thus, the choice of 

solutions. Some testing methodologies are in house and not publicly available. 

Improvement of test methods is recommended versus the addition of more tests. 

Identification of tests that appropriately replicate the conditions leading to a certain 

failure or performance metric are necessary where there is no well excepted standardized 

test method.

• With many MW size turbines reaching the end of warranty, operators now can test new 

oils and may be willing to share data to spur development.

• Verification of new lubricant and surface/material technologies is critical to 

advancement. Typical verification period out-of-laboratory is 2 years of field testing for 

lubricants by some OEMs.

• Scaling issues seem to be a root cause for many tribology related failures, along with 

rapid market development, which requires time to tease-out issues experienced in the 

field. 

• Material, coating, and lubricant development will take an industry-wide collaboration.
28

Recommendations for Future R&D Activity

The final session of the seminar was dedicated to a discussion of R&D needs moving forward to 

address the critical tribological issues discussed throughout the seminar. Given the breadth and 

depth of the seminar participants, the input to this process was informative.

It is noted that the surface failures observed in wind turbines are not fundamentally different 

from failures observed in other applications; however, the severity, frequency, and uncertainty of 

root causes for many of these failures has led to a reexamination of fundamental understandings

and testing methodologies. Throughout this seminar, specific tribological topics clearly highlight 

needed R&D efforts. General R&D approaches are listed in 4 and recommendations for 

addressing specific issues are in 5.
29

Table 4. Research Needs in Wind Turbine Tribology: General R&D Approach

Item Subject Description

A Failure analysis of 

components

Collection of in-field failed components and conducting 

detailed analysis and characterization using advanced 

methods to determine the morphology of failures across 

a large population and across component types and 

varied operating conditions. Collection of operational 

load and environmental spectra and the examination of 

components before failure also provides a better 

understanding of the factors that cause certain failures 

and how to control them.

B Quantification and 

classification of failures

Utilize field failure data to quantify the criticality of 

different failure modes and focus efforts. Also requires 

an effort to standardize the description and classification 

of failure modes, especially for WEA and micropitting.

C Test methodology 

development to mimic field 

failures

In conjunction with item ‘A,’ develop bench-top test 

methodology that accurately recreates the surface 

damage observed in the field. Reexamine standardized 

tests and make recommendations to improve test 

standards. Publicize test methodology.

D Identify drivers: 

operational, 

environmental, materials, 

and others

Isolate the individual drivers for each failure mode and 

test the acceptable limits for each driver; this can 

include water composition of lubricant, particle size of 

contamination, operation limits at the contact, and more

E Bench-top testing of 

mitigation methods

Utilizing test methodologies established in item ‘C;’ 

conduct screening of specific mitigation methods; such 

as coatings, lubricants, materials, and others.

F System validation Further validate mitigation methods proven in bench-top 

testing at the system level, in dynamometer and infield 

environments.

G Modeling and analysis Develop models used for simulating contact conditions 

and failure analysis. Validate with experimental data 

and utilization for understanding the fundamental 

tribological interactions.
30

Table 5. Research Needs in Wind Turbine Tribology: Specific Issues

Recommendations for Addressing Specific Issues

H Analysis of lubricant 

additive interaction

Typical additives may have an adverse effect in certain 

conditions. For example, antiwear additives might 

inhibit run-in and promote micropitting; decomposition of 

additives might cause adverse effects on the bearing 

and gear material.

I Establish a strategic 

approach to investigating 

WEA

A standardized approach to examining WEA formation 

is needed to understand root causes and test different 

theories. Some key items include the role of material 

microstructure, i.e. level of retained austenite in 

carburized layer; identification of mechanical drivers; 

development of test methodology to mimic mechanical 

conditions, i.e., impact loading; understanding the 

connections between mechanical drivers and material 

science; computer modeling to isolate certain 

mechanisms; role of lubricant chemistry

J Lubricant aging studies Changes in lubricity, antiwear characteristics, and 

corrosiveness of lubricants subjected to aging in the 

field need to be better characterized. At what point 

these changes can lead to surface damage needs to be 

better established.

K Black oxide influence Black oxide is a common treatment method; however, it 

is uncertain how it is influencing certain failure modes 

(adversely or beneficially). Comprehensive testing is 

needed to examine its role and recommendations for 

future application.

L Lubricant filtering study Filtering lubricants to remove particle and water 

contamination is important to keep oil clean and 

functioning properly. There is some question as to 

whether lubricant additives react with the filtering 

medium removing it from the oil. Testing and 

consensus is needed.

M Surface finish It is generally accepted that smoother surfaces are 

better for avoiding micropitting, scuffing, and wear. 

Super finishing techniques are available; however, there 

is some debate on what level of smoothness is required 

and if there is a level of “too smooth.”

N Surface engineering Coating and other surface engineering is an ongoing 

area of research in many industries. In wind, where cost 

is critical, an evaluation of coating performance and 

benefit is needed. This may lead to development of 

new, better performing, and cost effective coatings that 

meet wind requirements.

O Advanced base lubricants Synthetic lubricants are becoming common in the wind 

industry. New classes of synthetic lubricants need to be 

evaluated with respect to cost and safety.

P Advanced lubricant 

additives

Traditional organo-metallic EP additives have certain 

performance limitations and questionable interaction 

with certain materials, in addition to environmental 

concerns. New additives,need to be evaluated and 

optimized for wind application.
31

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41

Appendix A. Moderator, Speaker, and Panelist Biographies

Nathan Bolander

Nathan Bolander is the Chief Scientist at Sentient Corporation in Idaho Falls, Idaho. He 

received a BSME degree from the University of Wyoming, and MSME and PhD degrees from 

Purdue University with an emphasis in Tribology. Nathan's expertise is in modeling of mixedelastohydrodynamic lubrication and surface fatigue for bearings and gears.

H. R. (Henri) Braun

Henri Braun was born in 1957 in Cologne, Germany. He received his PhD in Inorganic 

Chemistry from the University of Cologne in 1988. In 1987, he began his career with Esso in its

Research & Technical Support Laboratory in Hamburg. Since then, he has worked through 

various positions and jobs with Exxon, Exxon Chemical, and ExxonMobil in Germany, the UK,

and Belgium. He is currently the Global Grease Technology Manager working from 

ExxonMobil Headquarters in Fairfax, VA, USA.

Shawn Doner

Shawn has been employed by Winergy Drive Systems Corporation since 2005 and has 25 years 

of mechanical systems experience. He has worked in operations and commercial roles, including

gearbox remanufacturing, remanufacturing operations management, field service, and 

aftermarket sales. Shawn is currently the Technical Sales Support Manager at Winergy. He is 

currently pursuing his Bachelor's degree in Business Administration with a concentration in 

Green Sustainable Enterprises.

Gary Doll

Gary Doll is the Timken Professor of Surface Engineering and the Director of the Timken 

Engineered Surfaces Laboratories. His research interests include engineering surfaces to address 

friction, wear, and corrosion; the mechanical properties of superelastic materials and

nanocomposites; and lubrication strategies for challenging environments. He received his PhD 

in Condensed Matter Physics from the University of Kentucky, where he studied the optical and 

structural properties of layered materials. As a postdoctoral fellow in Physics at the 

Massachusetts Institute of Technology, he conducted research on a wide range of materials 

including copper oxide superconductors, composites, carbon fibers, and polyimides. After MIT, 

he joined the General Motors Research Laboratories, where he began his research in thin film 

coatings, surface engineering, and tribology. Later, he became the Chief Technologist of 

Tribology at the Timken Company, where he was responsible for global research and 

development activities in bearing tribology, lubrication, surface engineering, and non-ferrous 

materials. Dr. Doll was elected as an ASM Fellow in 2009 for his contributions to the field of 

Surface Engineering. He currently serves as chair of the STLE/ASME Wind Energy Tribology 

Committee and is an assistant editor for Tribology Transactions. Throughout his career, Dr. Doll 

has published more than 150 articles and book chapters, edited four proceedings, and received 

more than 25 U.S. Patents.

Ali Erdemir

Ali Erdemir is a pioneering researcher, with international recognition and significant 

accomplishments, in the fields of materials science, surface engineering, and tribology. Dr. 
42

Erdemir's discoveries of nearly frictionless carbon and superhard nanocomposite coatings, as 

well as boron-based solid lubricants and lubrication additives, have been hailed as major 

breakthroughs in his field. He has received numerous awards, including four R&D-100 Awards; 

holds 15 U.S. patents; and has published more than 260 papers,16 invited book/handbook 

chapters, and two edited books. His archival publications have generated more than 4,000 

citations. His current research is directed toward nano-scale design and large-scale 

manufacturing of more advanced nanolubricants for even higher fuel economy in engines. His

latest ground-breaking development has the capacity to reshape the conventional heat-treatment 

and thermal diffusion processes like nitriding, carburizing, nitrocarburizing and conventional 

boriding, which are used extensively to combat friction and wear in all kinds of machine 

elements. 

Robert Errichello

Robert Errichello heads his own gear consulting firm, GEARTECH, and is founder of 

GEARTECH Software, Inc. He has more than 40 years of industrial experience. He has been a 

consultant to the gear industry for the past 33 years to over 40 wind turbine manufacturers, 

purchasers, operators, and researchers. He has taught courses in material science, fracture 

mechanics, vibration, and machine design at San Francisco State University and the University 

of California at Berkeley. He presented numerous seminars on design, analysis, lubrication, and 

failure analysis of gears and bearings to professional societies, technical schools, and the gear, 

bearing, and lubrication industries. He is a graduate of the University of California at Berkeley 

and holds BS and MS degrees in Mechanical Engineering and a Master of Engineering degree in 

structural dynamics. Bob is a member of several AGMA Committees including the AGMA Gear 

Rating Committee, AGMA/AWEA Wind Turbine Committee, ASM International, ASME Power 

Transmission and Gearing Committee, STLE, NREL GRC, and the Montana Society of 

Engineers. Bob has published more than 65 articles on design, analysis, and application of gears, 

and is the author of three widely-used computer programs for design and analysis of gears. He is 

technical editor for GEAR TECHNOLOGY and STLE Tribology Transactions. He is recipient 

of the AGMA TDEC Award, the AGMA E.P. Connell Award, the AGMA Lifetime 

Achievement Award, the STLE Wilbur Deutch Memorial Award, and the AWEA Technical 

Achievement Award.

Professor H.P. (Pwt) Evans

H.P. (Pwt) Evans is a Deputy Director of the Cardiff School of Engineering, Cardiff University, 

UK. He has been an active researcher in Tribology since 1977 when he embarked on post 

doctoral research in numerical modeling of Elstohydrodynamic Lubrication. His main research 

interests have been in aspects of EHL in gearing applications and he has published 120 papers in 

the field. He holds BSc, PhD and DSc degrees from the University of Exeter and was awarded 

the Tribology Trust Silver Medal in 2009 for achievement in Tribology.

Ryan Evans

Ryan D. Evans is the Manager of Bearing Fundamentals & Tribology within the Product 

Technology department at The Timken Company. He earned his PhD in Chemical Engineering 

from Case Western Reserve University, and has been a full-time R&D staff member at Timken 

in various roles for 10 years. Currently, he manages a team of engineers that apply expertise in 

bearing fundamentals and tribology to enhance Timken’s analytical methods and products. 
43

Ryan’s personal contributions have been in the areas of wear, surface engineering, thin film 

coatings, lubrication, and advanced surface characterization. He is a co-inventor on five U.S. 

patents, has published more than 20 refereed technical papers, and has given numerous 

presentations at technical conferences in the fields of tribology and thin film coatings. Ryan 

actively participates on organizing committees within the Society of Tribologists and Lubrication 

Engineers (STLE) and was the recipient of the Walter D. Hodson award in 2007.

Jürgen Gegner

Jürgen Gegner has been employed by SKF since 2000 and is currently the Head of the Material 

Physics Department in Schweinfurt, Germany. He holds a Habilitation degree with venia 

legendi in Material Engineering from the University of Siegen, Germany, where he has been an 

associate professor since 2005. He defended his doctoral thesis at the Max Planck Institute for 

Metal Research in Stuttgart, Germany, and received a PhD degree in Materials Science from the 

University of Stuttgart in 1995. The Friedrich-Alexander University of Erlangen-Nuremberg, 

Germany, awarded him a Diploma in Physics in 1989. He is the author of two textbooks and one 

book chapter, the editor of a proceedings volume, and has published more than 100 papers in 

journals and proceedings.

Aaron Greco

Dr. Aaron Greco is a researcher in the Tribology group at Argonne National Laboratory, where 

he is involved in the development of lubricants and surface engineering solutions for advanced 

wind turbine and vehicle drive trains. Currently, he is serving as a technical advisor to the U.S. 

Department of Energy, Wind Power Program office, assisting with R&D strategies for advanced 

wind turbine drive train and reliability programs. Aaron received his PhD from Northwestern 

University and a BS from Iowa State University, both in Mechanical Engineering, and has 

received several awards for presentations of research and publications. He is an active member 

of the Society of Tribologists and Lubrication Engineers, where he has served on several 

technical committees including Wind Energy Tribology.

Bill Herguth

Bill Herguth has been analyzing and managing laboratories for testing lubricants, fuels and 

machine failures for almost 40 years. He is the co-founder of Herguth Laboratories, Inc. 

Incorporated in 1980, Herguth Labs have two state-of the-art testing laboratories in the Chicago 

and San Francisco areas. He has been married to his wife Kathy for 40 years and has two sons 

and five grandchildren.

Walter Holweger

Walther Holweger studied at University of Tübingen from 1974-1982, when he completed his 

doctoral thesis. From 1982-2001, he worked in Product Development and Product Management 

Lubrication Technologies group. From 2001-2006, he was a Technical Consultant at SKF ERC 

Nieuwegein. Since 2006, he has been employed at Schaeffler Technologies, Central Materials, 

Herzogenaurach.

Jim Johnson

Jim Johnson is a Senior Mechanical Engineer at the NREL, Golden, Colorado. Jim’s 

responsibilities include external business development with mid-size wind turbine component 
44

manufacturers and OEMs. He is a Department of Energy and ARPA-E FOA Prime and sub-tier 

proposal co-author, and a point-of-contact and team member of the Gearbox Reliability 

Collaborative. He also performs database engineering support and is part of the DOE Drive 

Train FOA research effort implementation team. In 1995, he received an NREL Staff Award for 

outstanding individual leadership and technical engineering support for utility-class wind turbine 

blade destructive and nondestructive testing. He received an NREL Staff Team Award in 1999 

for designing and building the National Wind Technology Center’s 2.5 MW wind turbine drive 

train dynamometer. In 2009, he received the NREL Staff Team Award for participation in the 

Gearbox Reliability Collaborative (GRC), changing the way the wind industry designs and 

builds wind turbine drive trains to increase reliability.

Manfred Jungk

Manfred received his PhD in Chemistry from the University of Cologne, Germany, in 1986. 

From 1987-1990, he was a Development Chemist and carried out lubricant formulation work for 

Dow Corning. From 1990-1994, he was the Technical Service Group Leader in Plymouth, 

Michigan, and established an application engineering laboratory for automotive and industrial 

lubrication. From 1995-2003, he held various global roles in management, technology and 

marketing of lubricants. From 2004-2011, he led Research and Development for the Lubricants 

product line utilizing open Innovation Concept, cooperating with universities, institutions and 

suppliers. He is presently the Associate Industry Scientist, Wind Energy Solutions, for Dow 

Corning in Wiesbaden, Germany.

Dennis A. Lauer

After attending the United States Air Force Academy, Dennis Lauer received his Bachelor of 

Science in Engineering degree from Penn State University. He also received his Master’s in 

Business Administration from the same university. Mr. Lauer was the Director of Technical 

Marketing for Hilti Incorporated in the United States. Hilti is a construction tool and fastener 

manufacturer with headquarters in Liechtenstein. After that position, he became the Head of 

Engineering for Lubrication Engineers, an independent industrial lubricant manufacturer located 

in Fort Worth, Texas. Currently, Mr. Lauer is the Vice President of Engineering for Kluber 

Lubrication, North America L.P. Mr. Lauer is a Registered Professional Engineer in the states of 

Pennsylvania, Oklahoma, Texas, and Missouri and is the author of many articles for trade 

journals and co-author of several books on lubrication technology. He is a member of the NSPE, 

STLE, ASTM, AGMA, and AIST.

Jon Leather

Jon Leather is the Wind Energy Technical Manager for Castrol Industrial. He received his BS in 

Mechanical Engineering from Worcester Polytechnic Institute. He is an STLE Certified 

Lubrication Specialist with 32 years of field experience in Industrial Lubrication (with Tribol, 

Castrol, and BP) and more than six field years of experience in wind industry lubrication.

Johan Luyckx

Johan Luyckx is a Mechanical Engineer with 25 years experience in the domain of transmission 

engineering. He has been working in the wind sector at Hansen Transmissions for six years. He 

is responsible for root cause analysis and performance improvement plans of failures in wind 

gearbox transmissions. He is actively involved with the WEC/irWEA failure mode analysis via 
45

several bearing failure cases observed in wind gear units, the first observed occurring 22 years 

ago.

Ajay P. Malshe

Ajay P. Malshe (PhD 1992): He is the Founder, Executive VP and Chief Technology Officer 

(CTO) of NanoMech (www.nanomech.biz), founded in 2002. NanoMech Inc. is a 

nanotechnology platform innovations global corporation. NanoMech associates ThinkSmall 

delivers breakthrough product innovations to improve quality of life. NanoMech has delivers 

breakthrough nanotech innovations, introducing nanomanufactured product platforms to market, 

in machining (TuffTek) and lubrication (nGlide), sustainable and biocide additives (nGuardTM) 

and functional nano metal powders (ElementXTM). As an inventor, Malshe has received 32 

awards/recognitions, including a Fellowship of the American Society of Mechanical Engineering 

(ASME); voted in NanoBusiness Alliances as one of the Most Influential Nanotechnology 

Leaders of 2010; a nomination in Marquis Who's Who in America (2009; 63rd Edition); and a 

2005 Frost and Sullivan Technology Excellence Award. He is also a Fellow of the Institute of 

Physics, London, UK. He has authored more than 200 plus peer reviewed publications, 13 books 

and book chapters, and holds 10 patents. He has an extensive track record of global 

collaborations with academic institutions and companies from Australia, Japan, India, Germany, 

and Ireland. He is a member of professional societies such as ASME, CIRP, SME, IEEE, MRS, 

ASEE, ASM, and IMAPS. At NanoMech, he is directing a team of world-class scientists and 

engineers. He is a Distinguished Professor of Mechanical Engineering and the 21st Century 

Endowed Chair Professor of Materials, Manufacturing Processes and Integrated Systems at the 

Department of Mechanical Engineering, and an adjunct-faculty member of the Microelectronics 

and Photonics Program at the University of Arkansas. He also is the Director of the Materials 

and Manufacturing Research Laboratories (MMRL; a cluster of five laboratories). Malshe leads 

multidisciplinary research programs in the fields of nanomanufacturing, IC, MEMS, and micro 

and nano device packaging and integration, and surface engineering for advanced machining. 

He has graduated more than 45 graduate students (PhD/MS), trained numerous post-doctoral 

fellows, and provided research experience to several undergraduate and high school students. 

His graduates work at leading organizations, such as IBM-Almaden, University of Florida, 

Qualcomm Technology, and Texas Instruments.

Mark McDade

Mark McDade is a project manager with NREL's National Wind Technology Center. He is 

responsible for the Gearbox Reliability Collaborative Database and the NREL Grid Simulator 

programs and is part of the 5 MW dynamometer upgrade team. Mr. McDade's background is in 

information systems management. He has been with NREL and its predecessor, SERI for more 

than 10 years.

Brian McNiff

Brian McNiff provides wind turbine testing, failure analysis, and related consulting though his 

company, McNiff Light Industry, based in Harborside, Maine. He holds a BS and MS in 

Mechanical Engineering from the University of Massachusetts Alternative Energy program. He 

has been involved in the wind industry as an engineer since 1979.
46

Art Miller

Arthur Miller is a CBM Specialist for enXco Service Corp (eSC) and has been with the company 

since 1998. He worked as a field technician/supervisor until 2005 when he took a position in the 

Quality Control department. He managed the condition monitoring of hundreds of gearboxes 

through analyzing oil sample results and performing internal gearbox inspections. In 2011, he 

transitioned into eSC’s Center for Technical Excellence engineering department, specializing in 

the area of condition monitoring.

Les Miller

Les Miller is currently the VP of Engineering at Kaydon Bearings and has responsibility for the 

design, development, and application of all products. Kaydon is specifically involved in 

supplying pitch and yaw bearings for the Wind Turbine industry. Kaydon is North America’s 

largest manufacturer of such bearings. Les has been active in the bearing industry for 34 years 

and received his BSME from Tri-State College. He has been active in the ABMA Engineering 

Education Committee and Bearing Technical Committee for the past 15 years. He also has been 

a member of the ASME Tribology Division and is a long time member of STLE. His work 

history includes 26 years with NSK and a total of 8 years with Kaydon in ever increasing levels 

of responsibility for Product Design and Application Engineering.

Andy Olver

Andy Olver worked as a materials scientist, tribologist, and transmission engineer in the 

helicopter industry for nearly twenty years before taking up an academic career in 1992. His 

interests include lubrication, fretting, rolling contact fatigue, failure analysis, gearing, rolling 

bearings, and tribological materials. He has published in excess of 100 papers in the field and is 

currently head of the Tribology Research Group at Imperial College London.

Shuangwen (Shawn) Sheng

Shawn Sheng is a senior engineer at the National Renewable Energy Laboratory (NREL). He 

has BS and MS degrees in electrical engineering and a PhD in mechanical engineering. Shawn 

is currently leading the wind turbine condition monitoring work at NREL. Under his leadership, 

NREL started an active investigation of various wind turbine condition monitoring techniques. 

Shawn also has experience in mechanical and electrical system modeling and analysis, soft 

computing techniques, and automatic control. He has published his work in various journals, 

conference proceedings, and book chapters.

Hideyuki Uyama

Hideyuki Uyama has a Doctorate of engineering and has held positions as assistant manager and 

research engineer with the material laboratory in the basic technology research center at NSK in 

Japan. He joined NSK in 1999 and has been involved in estimation of rolling contact fatigue life 

and development of long-life material for rolling bearings.

Paul Veers

Paul Veers is the Chief Engineer at NREL’s National Wind Technology Center and was 

previously a Distinguished Member of the Technical Staff at Sandia National Laboratories. He 

has worked in the area of Wind Energy Technology for 30 years conducting research on various 

aspects of wind systems including atmospheric turbulence simulation, fatigue analysis, 
47

reliability, structural dynamics, aeroelastic tailoring of blades, and the evaluation of design 

requirements. Paul is currently the Chief Editor for Wind Energy, an international journal for 

progress and applications in wind power. He has a MS in Engineering Mechanics from the 

University of Wisconsin and a PhD in Mechanical Engineering from Stanford University.

Q. Jane Wang

Q. Jane Wang received her PhD from Northwestern University in 1993. She taught for about 

five years at Florida International University. She is now a Professor in the Mechanical 

Engineering Department at Northwestern University. She served the Northwestern Mechanical 

Engineering department as the Director of the Graduate Studies Committee during 2003-2007,

and as the Adviser of the ASME Student Chapter during 1998-2003. She was elected a Fellow 

of the American Society of Mechanical Engineers (ASME) in 2009 and a Fellow of the Society 

of Tribologists and Lubrication Engineers (STLE) in 2007. She is one of the awardees of the 

STLE 2010 Edmond E. Bisson Best Written Contribution Award and the 1997 STLE Captain 

Alfred E. Hunt Best Paper Award. She received a 1997 NSF CAREER Award. Her professional 

society work includes service as a Board of Director of STLE during 2008-2009, Chair of the 

2011 ASME/STLE International Joint Tribology Conference, Chair of the STLE Annual 

Meeting Program Committee during 2007-2008, the Secretary of the 2005 Word Tribology 

Congress Technical Program Committee, and a Member of the Organization Committee of the 

2007 International Symposium of Computational Mechanics.

Lavern (Vern) D. Wedeven

Vern D. Wedeven holds a PhD in Mechanical Engineering from Imperial College, London in 

1970; an MS in Mechanical Engineering from the University of Michigan in 1967; a second B.S. 

in Mechanical Engineering from the University of Michigan in 1966; and a B.S. from Calvin 

College in Grand Rapids, Michigan, in 1963. He is currently the President of Wedeven 

Associates, Inc., established in 1988. He has held positions as the Manager, Advanced 

Engineering & Analysis for SKF-America from 1983–1987; and was a Materials Engineer at 

NASA Lewis & NASA Headquarters from 1970-1983. His honors and accomplishments include 

the Walter D. Hodson Award for the best paper published by author under the age of 35 in ASLE 

in 1975, the Captain Alfred E. Hunt Award for best paper published in ASLE (STLE) in 1979, 

the Arch T. Colwell Cooperative Engineering medal for contributions to the SAE Propulsion 

Lubricants Committee, and the STLE International Award for outstanding contributions in 

tribology in 2008. He has published more than 60 technical papers in the area of Tribology, 

more than 100 Contract Reports in the area of Tribology, and is the author of three book 

chapters, and holds three patents. He is a member of ASME, ASM, ASTM, STLE (Fellow), 

SAE; Chairman of the Gordon Research Conference on Tribology 1984; Member SAE 

Committee E-34 Propulsion Lubricants Committee, and chairperson of E-34C Lubricating 

Characteristics.

Lane Winkelmann

Lane Winkelmann joined REM Surface Engineering, Inc., in 1996 where he was a part of the 

Research and Development Group as a Senior Research Associate. Since leaving the R&D 

group, he served for four years as the Products Manager and has been in his current position as 

the Director of Services for the last three years. He manages three ISF® Service Centers serving 

the United States and European markets. During his career, he has developed numerous products 
48

and processes for superfinishing a wide variety of alloys for both decorative and engineered 

surfaces. Mr. Winkelmann has co-authored several patents and has written and/or presented 

numerous papers on the use of superfinishing to improve gear performance. As a result of his 

accomplishments, this technology has been widely adopted by industries such as wind turbines, 

heavy equipment, motorsports, and aerospace. He received a BS from Texas A&M University in 

1991 and his MBA from Tulane University in 2005.
1 

DOE/GO-102012-3496 ▪ February 2012

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.

DOE/GO-102012-3496 ▪ February 2012

Printed with a renewable-source ink on paper containing at 

least 50% wastepaper, including 10% post-consumer waste.