Technical Report

NREL/TP-500-46572

February 2010 

Wind Turbine Micropitting 

Workshop: A Recap

Shuangwen Sheng, Editor
National Renewable Energy Laboratory

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Contract No. DE-AC36-08-GO28308

Technical Report

NREL/TP-500-46572

February 2010

Wind Turbine Micropitting 

Workshop: A Recap

Shuangwen Sheng, Editor

Prepared under Task No. WE101131
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iii

Foreword 

Micropitting is a Hertzian fatigue phenomenon that affects many wind turbine gearboxes, 

and it affects the reliability of the machines. With the major growth and increasing 

dependency on renewable energy, mechanical reliability is an extremely important issue. 

The U.S. Department of Energy has made a commitment to improving wind turbine 

reliability and the National Renewable Energy Laboratory (NREL) has started a gearbox 

reliability project. Micropitting as an issue that needed attention came to light through 

this effort. To understand the background of work that had already been accomplished, 

and to consolidate some level of collective understanding of the issue by acknowledged 

experts, NREL hosted a wind turbine micropitting workshop, which was held at the 

National Wind Technology Center in Boulder, Colorado, on April 15 and 16, 2009. 

Attendees agreed that it would be a small meeting of experts that allowed open 

discussion. To that end, it was also agreed that no formal proceedings would be 

published. The consensus of attendees, however, was that the workshop content was so 

valuable that a summary of the content and conclusions should be prepared. This report 

was developed for that purpose. The presentations given at the workshop can be 

downloaded at: 

http://wind.nrel.gov/public/Wind_Turbine_Gearbox_Micropitting_Workshop_2009.zip

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

publications listed in the bibliography, obtain the cited articles in the public domain, or 

contact the authors directly.
iv

Acknowledgments

This workshop was organized by a small committee, but its tremendous value was 

realized because of the contributions of many. The concept of the workshop came from 

Bob Errichello (GEARTECH). His longstanding experience in wind turbine gearbox 

failure analysis and his contact with many world-class experts enabled him to suggest 

both the format and most of the speakers. We are deeply indebted to him not only for his 

contributions for this workshop but also for his continuous contributions to wind turbine 

gearbox reliability. 

Shawn Sheng (NREL) provided all the organizational and logistical planning. He has the 

good technical insight into the technical topics needed to accomplish this task. He did an 

excellent job and clearly was the main reason for the success of the workshop.

Peter Blau (Oak Ridge National Laboratory [ORNL]) collaborated on determining the 

technical content, writing this report, and guiding the organization of the workshop. His 

contributions were a key to making the meeting a success. He brought a perspective of 

material science to the discussion and meeting organization.

Sandy Butterfield (NREL) gave the wind turbine context and related it to broad gearbox 

reliability issues. 

The speakers were by far the main attraction. Each one was a recognized expert who 

brought a new and different perspective of micropitting and how it could affect wind 

turbine reliability. The organizers want to express their deepest thanks to them for their 

efforts. 
v

Contents

1.0 Introduction................................................................................................................. 1

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

2.1 Micropitting Review................................................................................................. 4

2.1.1 State of the Art: Micropitting................................................................................. 4

2.1.2 Micropitting of Rolling Element Bearings ............................................................ 6

2.2 Lubricant Effects on Micropitting ............................................................................ 8

2.2.1 Fundamentals of Tribochemistry, Wear, and Surface Property Control ............... 8

2.2.2 Influence of Lubricant Chemistry on Micropitting.............................................. 10

2.2.3 Rolling Contact Fatigue (RCF): Lubricant Effects.............................................. 10

2.3 Characterization and Testing .................................................................................. 12

2.3.1 Helical versus Spur Gears.................................................................................... 12

2.3.2 Survey of Micropitting Research Work of Germany’s FVA and Status of Work 

within ISO-WG 15 “Micropitting” ............................................................................... 13

2.3.3 Systematic Tribology Test Methods for Micropitting ......................................... 14

2.3.4 Gear Testing to Aid the Understanding of Micropitting...................................... 16

2.3.5 Analytical and Experimental Methods for Micropitting Research...................... 18

2.4 Modeling................................................................................................................. 19

2.4.1 Micropitting, Micro-EHL, and Mixed Lubrication.............................................. 19

2.4.2 Modeling Micropitting: The CAT Model............................................................ 20

2.5 Prevention ............................................................................................................... 21

2.5.1 How to Prevent Micropitting ............................................................................... 21

3.0 Summary and Conclusions....................................................................................... 23

3.1 Possible Causes for Prevalence of Micropitting in Wind Turbines........................ 23

3.2 Test Rigs and Strategy for Wind Turbine Micropitting.......................................... 23

3.3 Standardized Test for Lubricant.............................................................................. 24

3.4 Run-in Procedure for Wind Turbine Gearboxes..................................................... 24

3.5 Needs for Further Research and Testing................................................................. 24

4.0 Bibliography .............................................................................................................. 30

Appendix A: Information on Attendees........................................................................ 38

A.1 Speaker Biographies .............................................................................................. 38

Robert Errichello........................................................................................................... 38

Stephen Hsu, Ph.D. ....................................................................................................... 39

Michael N. Kotzalas, Ph.D., P.E................................................................................... 39

Andy Olver, Ph.D. ........................................................................................................ 40

Brian Shaw, Ph.D.......................................................................................................... 41

Ray Snidle, Ph.D........................................................................................................... 42

Thomas Tobie, Ph.D. .................................................................................................... 42

Martin N. Webster, Ph.D. ............................................................................................. 43

Lavern (Vern) D. Wedeven, Ph.D. ............................................................................... 43

A.2 Attendee List.......................................................................................................... 44

A.3 Group Photo ........................................................................................................... 45
1

1.0 Introduction

Micropitting is a form of localized surface damage that is prevalent in wind turbine 

gearboxes, and it occurs on both gear teeth and bearings. It is especially detrimental to 

bearing function because it alters the geometry of rollers, raceways, or both. The altered 

geometry increases internal clearance and results in edge stresses that ultimately cause 

macropitting and bearing failure. It is not known why micropitting occurs frequently in 

wind turbine gearboxes, but the unique operation and environment may each contribute. 

Perhaps widely varying loads, rapid accelerations, and high vibration levels promote 

micropitting. Contamination by water is common in wind turbine gearboxes and could 

also be a contributing factor.

The National Renewable Energy Laboratory’s (NREL) micropitting workshop was 

conceived to (1) establish what is known about micropitting from prior research and (2) 

define what additional research may be necessary to prevent micropitting in wind turbine 

gearboxes.

Thirteen presentations by leading tribologists covered many aspects of micropitting in 

gears and bearings, including data from the field and laboratory and both analytical and 

experimental analyses. Thirty-two delegates representing 21 organizations from three 

different countries attended the workshop and participated in the discussions. Table 1-1 

gives the workshop agenda, and a summary of the concluding discussion follows the 

table. Section 2 summarizes each presentation, and Section 3 presents conclusions. 

Appendix A contains the speakers’ biographies, along with a list of attendees and a group 

photo.
2

Table 1-1. Workshop Agenda

Day 1: April 15, 2009

Time Topic Speaker and 

Affiliation

9:00 a.m. Introduction Sandy Butterfield, 

NREL

Session 1: Overview of Micropitting, Mechanistic Causes, and Modeling

9:30 a.m. State of the Art: Micropitting Robert Errichello, 

GEARTECH 

10:20 a.m. Fundamentals of Tribochemistry, Wear, and Surface 

Property Control

Stephen Hsu, George 

Washington 

University

11:00 a.m. Micropitting of Rolling Element Bearings Michael Kotzalas,

The Timken 

Company

1:30 p.m. Survey of Micropitting Research Work of Germany’s FVA 

and Status of Work within ISO-WG 15 “Micropitting”

Thomas Tobie, FZGTU-Munich

2:20 p.m. Micropitting, Micro-EHL, and Mixed Lubrication Ray Snidle, Cardiff 

University

3:20 p.m. Modeling Micropitting: The CAT Model Andy Olver, Imperial 

College

Session 2: Role of Lubrication in Micropitting and its Control

4:00 p.m. Rolling Contact Fatigue (RCF)—Lubricant Effects Martin Webster, 

ExxonMobil

Day 2: April 16, 2009

8:30 a.m. Influence of Lubricant Chemistry on Micropitting Andy Olver

10:20 a.m. Gear Testing to Aid the Understanding of Micropitting Brian Shaw, 

University of 

Newcastle

Session 3: Laboratory and Field Test Methods for Micropitting Studies

11:00 a.m. Analytical and Experimental Methods for Micropitting 

Research

Robert Errichello and 

Andy Olver

11:30 a.m. Systematic Tribology Test Methods for Micropitting Vern Wedeven, 

Wedeven Associates

Session 4: Concluding Discussion and R&D Roadmap Development

1:30 p.m. How to Prevent Micropitting Robert Errichello 

2:00 p.m. Concluding Discussion All Attendees

4:00 p.m. Tour of National Wind Technology Center Jim Johnson, NREL

During the concluding discussion in Session 4, the attendees explored the following 

topics, listed here by session:

• Session 1: 

o What is not known about micropitting? 

o Who other than the attendees is doing leading research on the subject? 

o Is there general agreement in the field on the knowns and the unknowns of 

micropitting?
3

• Session 2: 

o What scale of testing is most helpful in solving micropitting issues in wind 

turbine components? 

o Is a new type of test rig needed for full-sized bearings? 

o Who might design/build it?

• Session 3: 

o What do applications engineers need to know about micropitting and how 

can that information be communicated? What are the deficiencies of 

current guidelines and standards? 

o What basic research issues remain in order to enable more informed 

solutions to micropitting problems?

o How should the research needs be prioritized: Improved mechanical 

design tools? Better materials or surface treatments? Improved lubricants 

or lubrication systems? Better test methods?

In addition, the participants discussed possible recommendations for research by the U.S. 

Department of Energy (DOE) on preventing micropitting in wind turbine gearboxes:

• Form advisory committee. 

• Recommend research for DOE.

• Suggest possible laboratory testing methods (disk machine?).

• Establish baseline (representative) micropitting conditions.

• Test coatings and lubricants.

• Design/build/test on full-scale laboratory bearing test rig.

• Repeat coatings/lubricants tests.

• Specify field tests.

Finally, the attendees toured the National Wind Technology Center (NWTC) before 

adjourning the workshop.

The synthesis and critical evaluation of the information presented here revealed a 

complex challenge for the wind power community and highlighted the need for additional 

research in multidisciplinary areas: mechanical engineering, contact mechanics, 

component design, lubrication, condition monitoring, basic tribology, and materials 

science.
4

2.0 Summary of Presentations

In this section, we group summaries of the workshop presentations under the following 

categories: (1) micropitting review; (2) lubricant effects on micropitting; (3) 

characterization and testing; (4) modeling; and (5) prevention. Note that, by design, these 

major groupings do not necessarily align with the workshop’s agenda. Instead, they are 

intended for ease of use and reference in this recap report. 

2.1 Micropitting Review

2.1.1 State of the Art: Micropitting

Robert Errichello, GEARTECH 

Morphology of Micropitting

Understanding the morphology of micropitting is important because it differs depending 

on the manufactured surface texture produced by different grinding machines.

Scanning electron microscopy (SEM) shows that the floor of a micropit crater slopes 

gently downward from its origin at the tooth surface. The floor has a rough surface 

typical of that caused by ductile fatigue crack propagation. A featheredge forms at the 

back of the crater because of plastic flow of material over the crater rim. The featheredge 

appears white in SEM when it becomes charged with electrons. Material surrounding a 

micropit generally appears smooth and featureless unless abraded.

Mechanism of Micropitting

Micropitting occurs under mixed-film elastohydrodynamic lubrication (EHL) where oil 

film thickness is of the same order as surface roughness average (Ra) and load is borne 

by surface asperities and lubricant. When asperities carry a significant portion of load, 

collisions between asperities on opposing surfaces cause elastic or plastic deformation, 

depending on the severity of local loads. Micropitting experiments show that specific film 

thickness is a useful parameter for assessing the severity of asperity contact, but Chiu 

(1997) has shown that surface roughness has a stronger influence on micropitting life 

than does EHL film thickness.

In addition to contact stress caused by normal loading, sliding between gear teeth causes 

tractive forces that subject asperities to shear stresses. Tokuda (1977), Spikes (1986), and 

Akamatsu (1989) have shown that the first 104 to 106 cycles of stress occurring during 

run-in are an incubation period during which damage consists primarily of plastic 

deformation at asperities. Cyclic contact and shear stresses accumulate plastic 

deformation on asperities and at shallow depths below asperities. Plastic flow produces 

tensile residual stresses (Olver 1984; Chiu 1997), and with sufficient cycles, fatigue 

cracks initiate.

After incubation, micropits rapidly nucleate, grow, and coalesce. Microscopy shows a 

continuously cracked surface. Periodic inspection of tooth profiles with a gear inspection 

machine discloses a steady rate of surface deterioration. Damage can be extensive after 
5

only 106 cycles. The length of the incubation period and the rate of surface deterioration 

depend on the relative hardness of the specimen and the mating components. Olver 

(1995) has shown that relative hardness and persistent roughness are the principal 

parameters that determine the severity of micropitting damage.

Significance of Micropitting

Micropitting degrades gear tooth accuracy when it causes nonuniform wear. It may 

preferentially attack pinion dedenda and cause disparity between base pitches of the 

pinion and gear. Consequently, spacing errors can cause noisy gears and create dynamic 

loads that can lead to gear tooth failure.

Micropitting can alter the manufactured profiles of rollers and raceways in a rolling 

element bearing and lead to a geometric stress concentration that results in macropitting 

at the ends of the rollers and edges of the raceways.

Failure analyses show that micropitting is frequently a primary failure mode responsible 

for initiating other secondary failure modes such as macropitting, scuffing, bending 

fatigue, and polishing.

Effects of Surface Coatings 

Sacrificial coatings such as manganese phosphate, copper, or silver help limit wear to 

mild adhesion during run-in and protect the surfaces during the critical run-in 

(incubation) period.

Effects of Lubricant Type

Polyalkyleneglycol (PAG) lubricants have significantly thicker EHL films than 

polyalphaolefin (PAO) and mineral (MIN) lubricants over most of the temperature range 

in which wind turbine gearboxes operate, and PAG lubricants have demonstrated very 

good micropitting resistance. PAO and MIN lubricants have essentially the same EHL 

film thickness for 70°–90°C, and both PAO and MIN lubricants can have good 

micropitting resistance. Differences in performance of PAO and MIN lubricants depend 

primarily on their additives.

Presentation Summary

• Low specific film thickness promotes micropitting.

• Asperities and oil film share load.

• Asperities deform and crack during run-in.

• Micropit incubation occurs early.

• After incubation, micropitting grows.

• Micropitting degrades accuracy.

• Micropitting can escalate into other failure modes.

• Coatings act as solid-film lubricant and protect surfaces during run-in.
6

• PAG lubricants have thick EHL films and very good micropitting resistance.

• PAO and MIN lubricants have similar EHL films and can have good micropitting 

resistance depending on their additives.

2.1.2 Micropitting of Rolling Element Bearings

Michael Kotzalas, Timken Company 

The morphology of micropitting in gears and bearings can be different, and crack 

propagation rate can be slower in bearings because there is less sliding in bearings. Under 

high sliding, cracks are oriented in the same direction and propagate opposite to the 

sliding direction. Under low sliding, the cracks are randomly oriented.

Background: Micropitting in Gears

• Surfaces

o Finishing process creates “lay” perpendicular to contact motion.

o Gear surfaces have larger tractive forces than bearings.

o Gear surfaces have larger surface roughness than bearings.

• Damage mechanism

o Cracks form quickly under asperities because of plastic deformation.

o Cracks quickly propagate owing to surface lay and sliding.

Background: Micropitting in Bearings

• Surfaces

o Finishing process creates “lay” parallel to contact motion.

o Bearings surfaces have smaller tractive forces than gears.

o Bearings surfaces have smaller surface roughness than gears.

• Damage mechanism

o Cracks form over time under asperities because of plastic deformation.

o Cracks slowly propagate because of surface lay and lack of sliding.

Application Issues

• Debris damage

o Debris dents cause raised shoulders.

o Raised shoulders act as stress concentrators and reduce oil film thickness.

o Micropits initiate at shoulders.

• Tractive forces—microslip
7

o Wind turbine bearings must be designed to accommodate high loads. This 

requires rollers with high end-relief that may lead to microslip under 

lighter loads, which may promote micropitting.

o Roller skewing increases microslip.

o Microslip has relatively small tractive forces.

• Tractive forces—gross slip

o Lighter loads and high speeds cause inertial forces that are large relative to 

the normal loads and smaller load zones that lead to sliding between 

rollers and inner raceway.

o Smaller load zones create higher roller-to-cage loads that promote slip.

o Full-complement bearings have low film thickness and high sliding 

velocity between rollers, which often leads to scuffing.

o Scuffing damage may initiate micropitting.

• Edge stresses

o Heavy concentrated load owing to misalignment, improper roller or 

raceway crowning, or extreme loading may promote micropitting.

• Lubricant effects

o Lubricant additives can be detrimental to fatigue life. Experiments have 

shown that additives can attack weak points on bearing surfaces.

o Specific film thickness calculations should use a modified lambda ratio as 

described by Moyer (1990). 

• Surface coatings

o Surface treatments such as black oxide and phosphate are sacrificial 

coatings that facilitate run-in and help prevent scuffing and micropitting.

Presentation Summary

• Gears have a surface lay perpendicular to the contact motion, large tractive forces, 

and relatively rough surfaces, whereas bearings have a surface lay parallel to 

contact motion, smaller tractive forces, and smoother surfaces. These differences 

may explain differences in micropitting morphology and crack propagation rate.

• In gears, cracks are oriented in the same direction and propagate opposite to the 

sliding direction, whereas in bearings cracks are randomly oriented.

• Debris dents cause raised shoulders that act as stress concentrators and reduce oil 

film thickness. Micropits initiate at shoulders.

• Wind turbine bearings must be designed to accommodate high loads. This 

requires rollers with high end relief that may lead to microslip under lighter loads, 

which may promote micropitting.
8

• High speed, light loads, centrifugal forces, and full-complement bearings may 

cause gross slip and promote micropitting.

• Heavy concentrated load resulting from misalignment, improper roller or raceway 

crowning, or extreme loading may promote micropitting.

• Certain lubricant additives can be detrimental to fatigue life. Experiments have 

shown that additives can attack weak points on bearing surfaces.

• Specific film thickness calculations should use a modified lambda ratio as 

described by Moyer (1990).

• Surface treatments such as black oxide and phosphate are sacrificial coatings that 

facilitate run-in and help prevent scuffing and micropitting.

2.2 Lubricant Effects on Micropitting 

2.2.1 Fundamentals of Tribochemistry, Wear, and Surface Property Control

Stephen Hsu, George Washington University

In the hydrodynamic lubrication (HL) regime, a continuous fluid film supports 100% of 

the load and separates opposing surfaces. Surface roughness has little or no influence. 

Lubricant film thickness depends on the EHL inlet conditions of speed, geometry, and 

fluid viscosity.

In EHL lubrication, the lubricant film and asperities share load. Flash temperatures from 

asperity contacts influence viscosity around asperities, and chemical reactions between 

the lubricant and the surfaces may occur.

Under boundary lubrication (BL), contacting asperities either fully or mostly support the 

load. Elastic and plastic deformation occurs at asperities, and chemical reactions between 

the lubricant and metal surfaces create surface films that protect the surfaces by 

preferentially wearing the surface films. With high loads, failure modes can include 

plastic deformation, scuffing, abrasion, and fatigue. To protect surfaces, chemical films 

must provide:

• Adhesion with surfaces.

• Cohesive strength to resist shear and abrasion.

• Load capacity to support normal load.

• Thickness and hardness sufficient to support a significant portion of the load.

Chemical Reactions 

The base oil and its polar constituents form the following:

• Oxidation products.

• Acids.
9

• Adsorption films.

• Hydrogen abstraction to form conjugated double bonds.

• Condensation to form high molecular-weight organometallic compounds (friction 

polymers).

Lubricant additives function by the following mechanisms:

• Thermal decomposition.

• Surface-active species.

• Preferential adsorption.

• Thermally induced bonding.

• Formation of an inorganic reinforcing layer.

Additives need soft, “movable” friction polymers from base oil reaction products to work 

properly.

A detailed collision by collision experiment using two 3 mm diameter steel spheres was 

done to examine the surface and subsurface damage mode under several conditions: 

unlubricated, lubricated with paraffins, and paraffins plus antiwear additives. A high 

speed video camera mounted on the side captured the shear motion and the resulting 

shape. The x, y, z forces were measured by force transducers. After each collision, the 

surface damage was photographed, and some experiments were repeated after the same 

number of collisions, but the sample was cross-sectioned to show the substrate 

phenomena. Results showed the metal subsurface grains deformed to align with the 

principal component shear stresses, forming slip lines. Each collision added some more 

shear strain and the strain accumulated over time, resulting in an increasing angle of the

slip lines toward the horizontal plane. Near the surface, the strain angle increased and 

permanent shear deformation and wear occurred. After 250 to 500 collisions, depending 

on the lubrication condition, micropits appeared on the surface.

The lubricant and additive tend to prolong the induction time by redistributing the 

stresses over a larger area, hence reducing the stress intensities and prolonging the 

induction time to micropitting. The additives do not change the basic process or the 

mechanism.

Presentation Summary

Micropitting depends on:

• Plastic deformation of surface asperities.

• Lubricant ability to form thick boundary layers.

• Chemical tribofilm’s ability to flow with deformation.
10

Micropitting in gears is related to cumulative shear strain in asperities under cyclic 

loading. Prevention methods include the following:

• Change materials to increase strength.

• Change design to lower stress intensity.

• Add large molecules with more branched chains to induce thicker fluid films.

• Introduce surface textures to promote thicker films.

2.2.2 Influence of Lubricant Chemistry on Micropitting 

Andy Olver, Imperial College, UK 

Experiments on a disk test rig show that zinc dialkyl-dithio-phosphate (ZDDP) antiwear 

(AW) additives can suppress run-in and promote micropitting. Other tests show that 

micropitting damage encountered with ZDDP additive can be greatly reduced when 

molybdenum bis-diethylhexyl dithio-carbamate (MoDTC) additive is also present. 

Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) 

show that ZDDP+MoDTC AW additive forms a tribofilm with an outer surface that is 

markedly smoother than the steel substrate.

Presentation Summary

• AW additives suppress mild wear.

• AW additives slow the run-in process.

• AW additives cause persistent roughness and persistently high local stresses.

• Persistently high local stresses cause micropitting.

• Different AW additives behave similarly. When a molybdenum friction modifier 

is added to AW oil, however, initial micropitting damage gradually disappeared 

with continued running. This combination of additive types seems to be a key to 

combining wear protection and micropitting resistance.

• It is postulated that improved micropitting resistance, obtained with 

ZDDP+MoDTC additive, is attributable to reduced asperity friction.

2.2.3 Rolling Contact Fatigue (RCF): Lubricant Effects 

Martin Webster, ExxonMobil 

Gear Oil Formulators

Gear oil formulators must achieve an overall best balance of competing properties:

• Antiwear/antiscuff.

• Macropitting/micropitting resistance.

• Oxidation resistance.

• Filterability.
11

• Demulsibility.

• Rust/corrosion resistance.

• Foam control.

• Deposit control.

• Seal/material compatibility.

Rolling Contact Fatigue

Properties that affect RCF include:

• Viscosity

o Affects EHL film thickness.

o Depends on molecular structure of lubricant base stock.

• EHL traction

o Affects friction transmitted across the lubricant film.

o Depends on molecular structure of lubricant base stock.

o Subjects surface cracks to tensile stress at the EHL inlet and promotes 

hydraulic propagation of fatigue cracks.

o The central portion of the EHL film determines the tractive forces. 

• EHL film thickness

o Increased specific film thickness reduces micropitting.

o Increased specific film thickness increases bearing life.

o EHL film thickness is determined by the conditions at the inlet, including 

geometry, speed, and temperature of the bodies.

• Chemical effects

o Mechanisms not well understood and no model exists.

o Experimental data are available for limited classes of lubricants.

o Industrial MIN gear oils give significantly shorter bearing lives than MIN 

base stock without additives.

o Industrial PAO gear oils have bearing L10 lives similar to industrial MIN 

oils, but longer L50 lives.

o Industrial PAO circulating oils with phosphorus-only additives give 

significantly longer bearing lives than industrial MIN and PAO gear oils.

o Automotive rear axle oils all give significantly shorter bearing lives than a 

nonadditized base stock.

o Water contamination significantly shortens bearing life. For example, 

increasing water concentration from 50 ppm to 500 ppm reduces L10 life 

by a factor of three.
12

Bearing Life Adjustment Factors for Common Lubricants

The ASME design guide (ASME 2003) titled “Life Ratings for Modern Rolling 

Bearings” gives an average life adjustment factor of 0.8 for hydraulic oils, AW roll 

bearing oils, turbine oils, AW circulating oils, MIN gear oils, and ATF automotive oils. 

The only oils with life adjustment factors greater than 1.0 are roll bearing oils without 

AW additives, circulating oils without AW additives, synthetic AW gear oils, and 

aviation turbine oils.

Presentation Summary

• Lubricant base stock affects EHL film thickness and surface tractive forces.

• Increased specific film thickness reduces micropitting.

• Increased specific film thickness increases bearing life.

• Lubricant additives affect bearing fatigue lives either negatively or positively, 

depending on additive composition.

• Water contamination significantly shortens bearing life.

2.3 Characterization and Testing 

2.3.1 Helical versus Spur Gears

Robert Errichello, GEARTECH 

Spur gears are not a good choice for achieving high repeatability in tribology tests 

because they have complex dynamics and are overly sensitive to changes in their tooth 

profiles. This makes it difficult to control test parameters and leads to wide scatter in test 

results. Helical gears operate much more smoothly, are less sensitive to profile changes, 

and have less scatter in test results.

Spur and Helical Hertzian Fatigue

• Hertzian fatigue is the same for both types of gears, but size-for-size helical gears 

have higher resistance to Hertzian fatigue.

• Spur gears suffer more because of loss of profile accuracy because micropitting 

destroys base pitch.

• Axial pitch may remain accurate on helical gears despite micropitting.

• Tip relief is critical on spur gears and much less important on helical gears.

Spur gear dynamic behavior is strange. Spur gear mesh stiffness is nonlinear and 

transmission error (TE) is strongly load dependent. Helical gear mesh stiffness has less 

variation and lower TE. Spur gears have more areas of resonance caused by 

superharmonics and instabilities.

Spur gears are poor test gears because:
13

• Wear degrades profiles, changes TE, and increases dynamic tooth loads.

• Nonlinear mesh stiffness promotes resonance.

• Test rig locked torque may vary 15% under slow roll.

• Debris is trapped between profiles.

• Bending fatigue is catastrophic.

• A dent at the Start-of-active-profile (SAP) promotes macropitting.

• Micropitting creates geometric stress concentration (GSC) macropits.

2.3.2 Survey of Micropitting Research Work of Germany’s FVA and Status 

of Work within ISO-WG 15 “Micropitting” 

Thomas Tobie, FZG-TU-Munich

The Forschungsvereinigung Antriebstechnik E.V. (FVA) research projects FVA 54/I+II 

(1977–1983) were performed to study micropitting and macropitting for different 

materials, lubricants, flank roughness, operating conditions, and other influences.

FVA 54/III+IV (1985–1992) tests were run at higher speed. Tests were conducted for 

different gear geometry, lubricants, flank roughness, operating conditions, and other 

influences such as burnishing, copper plating, and phosphate coatings.

FVA research project 259/I (1994–1998) investigated the influence of gear geometry and 

gear size on micropitting.

FVA research projects 259/II+III (1998–2001) investigated the influence of gear tooth 

grinding process.

FVA research projects 286/I+II+III (1998–2009) investigated the influence of large 

gears.

FVA research projects 289/I+II (1995–2003) studied the influence of tribological layers.

FVA research project 459/I (2004–2006) studied the influence of micropitting on 

macropitting resistance.

FVA research project 482/I (2005–2007) investigated micropitting and macropitting 

behavior of low-speed external and internal gears.

FVA research project 488/I (2006–2008) studied the influence of water contamination on 

micropitting resistance.

Based on the theoretical and experimental results of these research projects, researchers 

developed an advanced calculation model to calculate the risk of micropitting of gears 

and to estimate the expected profile deviations caused by micropitting. 
14

Since 2005, the ISO TC 60/SC 2/WG 15 Committee has been developing a standardized 

calculation method for assessing the micropitting risk for gears.

Presentation Summary

FVA research has shown that micropitting depends on many variables, and it influences 

several characteristics of gears. For example, micropitting:

• Depends on flank roughness.

• Depends on surface hardness.

• Is initiated primarily in flank areas with negative specific sliding.

• Depends on lubricant and specific film thickness.

• Depends on lubricant additives.

• Depends on oil inlet temperature.

• Depends on gear size.

• Depends on grinding process.

• Influences dynamic forces and noise.

• May influence macropitting resistance.

• Depends on the tribological layer, which is a function of the lubricant, 

temperature, and operating conditions.

• Depends on low speed and whether external or internal gears.

• Depends on the level of water contamination.

2.3.3 Systematic Tribology Test Methods for Micropitting

Vern Wedeven, Wedeven Associates

In the present context, systematic tribology is defined as developing and testing tribology 

technology as a system with interactive and multiple failure modes. Tribology testing 

must be tuned to the service hardware and state of lubrication. Key tribology parameters 

for linking tests with service performance are:

• Specific film thickness.

• Entraining velocity.

• Sliding velocity.

• Contact temperature.

• Contact stress.

• Traction coefficient.

Micropitting in Gas Turbine Engines

Micropitting is a significant life-limiting failure mode for main shaft ball bearings. Low 

specific film thickness with near pure-rolling conditions produces micropitting.
15

Root Causes of Micropitting in Aeropropulsion

• Ultrathin films and high surface stresses (normal and tractive).

• Debris dents and surface defects as stress concentrators.

• Surface roughness (asperity peaks with high stress).

• Irregular surface geometry resulting from surface film wear protection and 

polishing wear.

Debris Dents and Surface Flaws

Micropitting initiates from raised shoulders associated with debris dents and other surface 

flaws such as abrasion scratches and scuffing furrows.

Influence of Specific Sliding (Ratio of Sliding Velocity to Rolling Velocity)

Micropitting decreases in severity as contact slip decreases, but it is also found at 0% slip 

(pure rolling). Exploratory tests show that specific film thickness and traction coefficient 

determine whether micropitting occurs. Therefore, all micropitting tests should include 

measurements of traction coefficient.

Polishing Wear and Micropitting

Micropitting can be initiated by unequal wear resulting from polishing in one area and 

wear protection by surface films in other areas. This phenomenon has been observed in 

ball bearings when micropitting formed at the crests of circumferential ridges that formed 

on the inner raceway. Polished valleys were found on either side of the ridges. Sulfide 

films were found on the ridges, whereas the highly polished valleys were free of sulfide 

films. It is postulated that the ridges form at points of true rolling where sulfide films 

protect the surfaces, and polishing wear removes the protective films and creates valleys 

on either side of the ridges in areas of Heathcote slip (slip that occurs because ball radii 

have varying velocity).

Presentation Summary

• Experience in aeropropulsion shows micropitting to be caused by low lambda 

ratio, surface defects, surface roughness, and irregular wear. Similar causes are 

expected in wind turbines.

• Initiation of micropitting and whether it propagates are critically linked to 

polishing wear at an asperity level and wear affecting the global geometry. 

Therefore, the material/oil chemical reaction and boundary lubrication are 

important.

• The mechanism of micropitting seems to be reasonably well understood, and 

micropitting resistance can be increased through improved design and 

manufacturing processes. Advanced lubrication and surface technologies for 

increased micropitting resistance, however, are needed. This needs to be an 

interdisciplinary and industry-wide team effort.

• Solutions to micropitting are likely to have negative effects on other performance 

attributes. Therefore, a systematic tribology approach is necessary.
16

2.3.4 Gear Testing to Aid the Understanding of Micropitting 

Brian Shaw, University of Newcastle

Experimental tests are necessary to study micropitting. Test rig development and testing 

methodologies must include mechanical design, metallurgical assessment, failure 

analysis, metrology, and condition monitoring.

Characteristics of Micropitting

• Can be found in all gear applications.

• Becoming more prevalent.

• Often regarded as secondary failure mode.

• Can increase noise and vibration.

• May escalate into macropitting, scuffing, and flank-initiated bending fatigue.

• Mechanism and root cause not well understood.

• Is a complex failure mode influenced by many variables.

Contributing Factors

• Material

o Composition.

o Quality (flaws such as grind temper).

o Heat treatment.

o Residual stress.

o Microstructure.

• Gear design

o Macrogeometry.

o Microgeometry.

o Specific sliding.

• Manufacturing process

o Grinding.

o Finishing.

o Surface roughness.

o Accuracy.

o Flaws.

• Surface treatments

o Shot peening.
17

o Superfinishing.

o Coatings.

• Operating conditions

o Torque.

o Speed.

o Lubrication (viscosity, additives).

o Temperature.

o Duty cycle.

Mechanism

• Gear running changes near surface material properties.

• Localized plastic deformation at asperities changes residual stresses and 

transforms retained austenite.

• Micropits initiate at high asperities.

• Micropits coalesce until entire original surface is removed.

Prevention

• Improve surface finish. Superfinished gears have greatest resistance to 

micropitting.

• Avoid shot-peened gears because they have low micropitting resistance because 

of increased surface roughness. Grinding after shot-peening, however, improves 

micropitting resistance.

• Optimize gear design to minimize specific sliding and avoid edge stresses.

• Increase gear accuracy.

• Use lubricants with high micropitting resistance.

Future Research Needs

• Identify ideal material properties such as retained austenite and residual stress and 

any benefits from alloy choice and heat treatment method.

• Modify surfaces to obtain optimum surface texture and minimum friction.

• Develop lubricant additives that give adequate scuffing resistance and maximum 

micropitting resistance.

• Develop coatings to maximize micropitting resistance.

• Develop analytical models for investigating mechanisms for crack initiation and 

propagation.

Presentation Summary
18

• With modern clean steels, Hertzian fatigue is dominated by micropitting rather 

than subsurface initiated macropitting.

• Asperity contact stresses are the root cause of micropitting.

• Micropitting is influenced by a complex interaction of many variables.

• Understanding micropitting is the key to improving performance.

• Research is needed to develop material properties, lubricant properties, and 

surface engineering to obtain gears and bearings with improved performance.

2.3.5 Analytical and Experimental Methods for Micropitting Research 

Both disc test rigs and gear test rigs have their place for experimental investigation into 

micropitting. The choice of the test rig depends on the objectives of the test and on the 

advantages and disadvantages of the different test rigs.

Andy Olver, Imperial College

Micropitting Test Objectives

• Create micropitting under controlled conditions.

• Obtain reproducible parameters to indicate severity or progression of 

micropitting.

• Study effects of variables such as materials, lubricants, load, temperature, and 

surface finish.

• Understand the relationship of the test to any relevant service components.

Typical Problems with Micropitting Tests

• Tests take too long (and speeding up machine eliminates failure).

• Too much fluid is required.

• Wear changes the macrogeometry and roughness.

• Initial micropitting “fizzles out.”

• Scuffing and macropitting occur instead of micropitting.

• Sliding causes uncertainty in film thickness because of thermal feedback.

• Wear affects dynamic loads.

• No micropitting occurs with viscous oils.

Test Rigs—A Comparison of Gear Testers with Disc-on-Disc Machines

• Advantages of gear testers

o Representative of some gears.

o Allows direct investigation of gear geometry effects.

o Representative failure progression.
19

o Wide range of contact conditions.

• Disadvantages of gear testers

o Results not convincing for other components.

o Rapidly varying contact conditions.

o Poor control of thermal and dynamic effects.

o Very expensive test specimens.

o Long test time.

o Large lubricant volume.

• Advantages of disc-type testers

o Close control of load, speed, and temperature.

o Precisely reproducible surface roughness.

o Much lower test time with multiple contact tester.

o Precise wear depth measurement.

• Disadvantages of disc-type testers

o Not convincing for gear engineers.

o Point contact fizzles out because of the decline in Hertzian stress.

o Test specimens still expensive and require careful finishing.

2.4 Modeling 

2.4.1 Micropitting, Micro-EHL, and Mixed Lubrication

Ray Snidle, Cardiff University

EHL theory explains the generally successful operation of most types of gears. EHL has 

been less successful, however, in explaining and predicting failures of gear tooth 

contacts. There is an urgent need to develop a detailed understanding of “mixed” 

lubrication because this is the mode of lubrication in highly loaded gears. Greater 

emphasis needs to be given to initial plastic deformation in real, lubricated contacts.

A micro-EHL mathematical solver for Hertzian contacts must be capable of handling the 

following:

• Low lambda ratios (< unity). 

• Thin films (10 nm).

• Transient condition (moving roughness).

• Real loads and speeds (rolling/sliding).

• Thermal effects.
20

• Non-Newtonian lubricant properties.

Presentation Summary

• Transient micro-EHL analysis results are used to generate time histories of stress 

at each material point.

• Fatigue failure models of both critical plane (plane with most fatigue damage) and 

cumulative damage type give similar results for a particular rough surface.

• Calculated damage is greatest close to the surface.

• Calculated damage increases with sliding speed and decreases with increasing 

viscosity.

• Dry contact is significantly worse than lubricated contact from a calculated 

damage perspective.

A full, up-to-date list of papers published by the Cardiff group since 2001 is available, for 

example, at http://www.engin.cf.ac.uk/whoswho/profile.asp?RecordNo=69.

The Cardiff University’s tribology Web site is at http://tribology.engineering.cf.ac.uk/.

2.4.2 Modeling Micropitting: The CAT Model 

Andy Olver, Imperial College, on behalf of Caterpillar

The Caterpillar (CAT) theoretical model of rough-surface contact, developed by Ping 

Wang, Diann Hua, and Tom Beveridge (Caterpillar), and Emmanuel Laine, and Andy 

Olver (Imperial College), accounts for evolving roughness with time caused by mild wear 

and fatigue damage. The CAT model has the following features:

• Explicit three-dimensional description of both surfaces.

• Elastostatic model without lubricant mechanical properties.

• Model for gradual wear.

• Model for fatigue damage.

• Rules for material removal and shape change resulting from micropitting.

• Lubricant chemistry acts only by changing wear coefficient in the gradual wear 

model.

Flow Chart for the CAT Model

A. Input material properties, contact geometry, and operating conditions.

B. Calculate surface and subsurface stresses.

C. Calculate fatigue damage.

D. Calculate gradual wear.

E. Calculate modified surface.

F. Iterate A through E until surface change is small.
21

Presentation Summary

The CAT model has achieved the following:

• Accounts for competing mechanisms of gradual wear and micropitting.

• Discriminates behaviors of different oils.

• Predicted wear depth agrees with experiments.

• Predicted surface change on counterface agrees with experiments.

• Agrees with experimental results that show antiwear additives suppress run-in and 

promote micropitting.

2.5 Prevention 

2.5.1 How to Prevent Micropitting 

Robert Errichello, GEARTECH 

The following guidelines were developed from experiments and inspections of laboratory 

and industrial gears and from literature reviews. Not every measure will be achievable or 

applicable for a given application, but as many as possible should be implemented. The 

steps to prevent micropitting are as follows: maximize lambda, optimize gear geometry, 

optimize metallurgy, optimize lubricant properties, and protect surfaces during run-in.

Thick Oil Films Maximize Lambda

• Use highest practical viscosity.

• Use high speed.

• Use copious oil flow.

• Cool gear teeth.

• Use synthetic oil if tooth temperature > 80°C.

Smooth Surfaces Maximize Lambda

• Avoid shot-peened gear teeth. 

• Hone or polish teeth (or run-in against a hard, smooth master?).

• Make hardest gear as smooth as possible.

Optimize Gear Geometry

• Use aspect ratio (ratio of face width to pinion pitch diameter) ma ≤ 1.0.

• Use helical gears with face contact ratio (ratio of face width to axial pitch) mF ≥

2.0.

• Use at least 20 teeth in the pinion.

• Use a nonhunting gear ratio.

• Minimize contact stress.
22

• Use profile shift to minimize specific sliding.

• Use proper profile and helix modifications to obtain uniform load intensity 

without edge stresses.

• Avoid tip-to-root interference.

Optimize Metallurgy

• Maximize pinion hardness.

• Make pinion 2 Rockwell hardness (HRC) points harder than gear.

• Use ≈ 20% retained austenite.

Optimize Lubricant Properties

• Use oil with high micropitting resistance.

• Use oil with low traction coefficient.

• Use oil with high pressure-viscosity coefficient.

• Avoid oils with aggressive antiscuff additives.

• Avoid oils with viscosity index (VI) improvers.

• Keep oil cool, clean, and dry.

Protect Surfaces during Run-in

• Coat teeth with Mn-P, Cu, or Ag.

• Use special oil (avoid AW additives).

• Use series of increasing loads and appropriate speeds.

• Limit each load duration based on wear particle generation rate.

• Prefilter oil to ISO 16/14/11 cleanliness per cleanliness code ISO 4406.

• Use a fine filter (≤6 μm).

• Keep oil cool.

• Drain oil, flush, and change filter after run-in.

Presentation Summary 

Methods to prevent micropitting are as follows:

• Maximize lambda.

• Optimize gear geometry.

• Optimize metallurgy.

• Optimize lubricant properties.

• Protect surfaces during run-in.
23

3.0 Summary and Conclusions

At the open discussion session in Session 4, attendees discussed several topics that are 

also potential areas for future research and development and testing. They generally 

agreed on the need to conduct further research and tests of micropitting specific to wind 

turbines. The three main research components identified were gear, bearing, and 

lubricant. The long-term goal of these research and test efforts is to increase 

understanding of wind turbine micropitting phenomena and, subsequently, to standardize 

the relevant research and testing methods. 

This section summarizes the points made during the open discussion. 

3.1 Possible Causes for Prevalence of Micropitting in Wind 

Turbines 

Micropitting is much more prominent in the wind industry than in other industries. It is 

present in both bearings and gears. The possible reasons identified include 1) much 

longer life expectation: commercialized large-scale wind turbines are designed to 

continuously run for 20 years, which is much longer than the typical life expectation for 

cars in automobile industry, and for equipment in most other industries; 2) unsteady 

wind: changes in amplitude, direction, and gust; 3) contamination in lubricants such as 

water, dirt, and wear debris; 4) low rpm and high loads: typical operating condition for 

the planetary stage of a wind turbine gearbox, leading to slow moving gears and bearings 

with low pitch line velocities, low oil film thickness, and high traction; and 5) high duty 

cycle. Further research and tests are thus necessary to fully understand micropitting 

phenomena in wind turbines and pinpoint the exact causes.

3.2 Test Rigs and Strategy for Wind Turbine Micropitting

Before a test is designed, the test objectives must be identified. There are typically two 

test objectives: 1) understanding micropitting phenomena specific to the application, and 

2) finding solutions to solve the micropitting problems in wind turbine gearboxes. No 

matter which objective is targeted, an appropriate test rig is important for conducting 

meaningful and successful tests. Currently, there are three options: the Micropitting Rig 

(MPR) used by Imperial College, the Wedeven Associates Machine (WAM), and 

developing new test rigs. A specific test rig for wind turbines would be necessary because 

both MPR and WAM are not able to capture the simultaneous rotating and sliding actions 

occurring in gear sets. The rigs should be able to replicate the wind turbine field 

operational conditions, such as transients in loads, unsteady, high duty cycle, full 

lubrication system, typical contamination, and speeds, loads, and temperatures. When 

selecting test specimen, spur gears appear to be a bad choice, because the tooth profile 

characteristics change throughout the test. On the other hand, an accelerated life test 

needs to be designed and conducted to reflect the 20 years of wind turbine design life. 

Ultimately, wind turbine gearbox and bearing designers would use the test results. A 

proposal to the International Electrotechnical Commission IEC for a test standard for 

wind turbine micropitting is potentially achievable given the experience to be gained in 

these tests. 
24

3.3 Standardized Test for Lubricant

There is a large gap between lab and field tests for lubricant. FVA 54 has been used 

extensively as the standard micropitting test for wind turbine gearbox lubricant. FVA 54 

tests cannot, however, fully represent wind turbine field conditions, and they should be 

used as an initial guideline instead of a final step for a micropitting test. The FVA 54 test 

needs to be carefully planned so that a broad range of machine operating conditions can 

be covered. The test also needs to be conducted intelligently, to avoid deceptive results 

on lubricants with poor micropitting resistance performance. A progressive test approach 

for lubricant test is identified as the appropriate way to go. Proposing a standardized test 

approach for wind turbine lubricant would be one goal of the research and test efforts 

identified at this workshop. 

3.4 Run-in Procedure for Wind Turbine Gearboxes 

Present wind turbine gearbox manufacturers typically conduct gearbox run-in based on 

fixed time intervals. The attendees recommend conducting the run-in based on particle 

generation rates; some gearbox manufacturers have already taken this approach. There 

are, however, some remaining questions: What particles should be looked at, ferrous, 

nonferrous from gears, bearings, or other components, and how large? Also during the 

run-in, the level of antiwear additives in the lubricant is low (if the run-in oil has no 

antiwear additives), which could lead to scuffing if the run-in process is not performed 

carefully. As a result, some guidelines have to be specified for running-in a wind turbine 

gearbox. To reduce the chances of scuffing, the load has to be stepped up gradually 

before the full load is applied. How many load steps are appropriate? Should speed be 

different than the wind turbine operating speed? What oil temperatures should the run-in 

use and how should they be correlated with the load? The present IEC/ISO draft standard 

for wind turbine gearboxes requires the same oil to be used during the run-in and normal 

operation. This conflicts with the approach of running with low or no antiwear additives 

for the run-in process of wind turbine gearboxes. What practice does the wind industry 

need to follow? Recommending to IEC a run-in procedure for wind turbine gearboxes is 

another goal of the research and test efforts identified at this workshop. 

3.5 Needs for Further Research and Testing

Presentations at this workshop and the discussions they stimulated support the need for an 

integrated, interdisciplinary approach to micropitting avoidance. Both experimental work 

and modeling work are needed, and the latter requires better information on wind turbine 

gearbox forces, stresses, and transients. Clearly, not all of the types of loads and stresses 

imposed on gearbox components will result in micropitting, but under some 

circumstances, they could contribute to its onset. 

Because the micropitting on gears differs in certain respects from that seen on bearings, 

different approaches to defining the testing requirements and research needs are indicated 

for gears and bearings. This finding is consistent with the systematic tribology approach 

presented by Wedeven. The interdisciplinary nature of micropitting, component design, 

and lubricant selection also supports the need to establish standardized methods to define 

the magnitude of micropitting problems, and to use consistent terminology to better 

communicate results across the disparate fields of failure root-cause analysis, component 
25

performance modeling, materials development, lubrication, and simulative 

experimentation. 

Basic mechanisms responsible for micropitting have been proposed by several 

investigators, and the degree to which they are fully understood remains a subject of 

controversy in the tribology and mechanical engineering communities. If these 

mechanisms were indeed well understood, as some have asserted, then a proper research 

focus would be on selecting materials, surface treatments, lubricants, manufacturing 

methods, and bearing designs that reduce or eliminate the operation of those identified 

mechanisms. It may also be true that the causes of micropitting are better understood in 

some kinds of mechanical systems than in others. Increasing complexity of such 

interdisciplinary approaches results from the highly variable operating characteristics of 

wind turbines. Likewise, the similarities and differences between micropitting and other 

forms of surface damage must be better understood.

Scale effects in tribology enter the discussion because certain approaches to bearing and 

gear analysis treat mechanical components from a macroscopic point of view; that is, 

they look at geometrical arguments, stresses, and strains that are on the scale of 

dimensional drawings of production parts. Microcontact issues enter because of the size 

scale of the operative mechanisms that researchers associate with micropitting. These 

include attributes like surface roughness parameters, asperity and micropit sizes, and the 

dimensions of grains and inclusions within the gear and bearing alloys. As the 

presentations in the workshop indicated, the process of micropitting has also been 

associated with the state of the lubricating films that separate, or fail to separate, bearing 

surfaces. Lubricant formulations affect the composition and properties of such films and 

the surfaces on which they reside.

Several of the presentations alluded to the so-called lambda ratio or film parameter (i.e., 

the ratio of the fluid film thickness to composite surface roughness) to define the 

lubrication conditions between mating surfaces. Using this parameter for mechanistic 

studies has shortcomings because micropitting tends to reflect individual asperity 

interactions rather than the statistical average of the conditions within the larger nominal 

contact zone. Micropits may initiate and grow on a scale of a few micrometers but 

coalesce and spread to produce surface damage on a scale of millimeters or centimeters. 

Micro-EHL, mentioned by Snidle, is one approach to address this, but it is also important 

to recognize that only the arithmetic average roughness of many such features, not the 

shapes of the surface features, are directly reflected in the lambda ratio. In other words, 

two surfaces could have different asperity slopes, shapes, and lay but still have the same 

average roughness parameters that are used in the lambda ratio calculation. That subtlety 

may become important in determining which microfeatures could produce micropits and 

which might be more benign. Nevertheless, lambda does reflect the general tendency for 

solid contact to occur under a given set of liquid-lubricated operating conditions. Film 

thickness calculations require information such as pressure-viscosity coefficients of 

lubricants, which are not straightforward to measure and require specialized apparatus.

A basic understanding of mixed-lubrication and boundary lubrication regimes, in which 

lubricant film thickness and surface features have similar dimensions, becomes 
26

particularly challenging, but it is critical if micropitting is to be better understood. 

Presentations on the pros and cons of antiwear films (Olver) and the role of 

tribochemistry (Hsu) exemplify a need to develop lubricating approaches that facilitate 

run-in without truncating long-term bearing surface reliability.

Another recurrent topic of discussion was that of running-in (a.k.a. break-in, wear-in). 

Tribologists and bearing designers recognize its importance in avoiding “infant mortality 

failures” and in extending the useful service life of bearings, seals, and gears. Despite this 

recognition, there has been relatively little systematic research on the nature of running-in 

as a function of lubricant type, surface roughness, material combination, heat treatment, 

coating type, and so on. Although research in the former Soviet Union produced quite a 

number of papers on running-in (including at least one standard for run-in ability of 

materials), much of the research in the western literature seems to reside in proprietary 

reports or in the undocumented “art” of engineering. Technicians or manufacturers may 

apply a certain running-in method to new or rebuilt mechanical systems, but fail to 

document them fully. Even when running-in procedures are routinely applied to certain 

types of machinery, these may have been developed by trial and error, without a firm 

scientific rationale. Therefore, systematic studies of running-in and its effects on 

micropitting are indicated.

From a broader perspective, running-in is only a part of the challenging area of 

tribological transitions. These include wear-out and the response of mechanical systems 

to transient events. Translated to wind turbine gearboxes, such events include rapid 

accelerations or decelerations caused by gusting or by side loads from changes in the 

wind direction. Bearing manufacturers sometimes complain that their customers specify 

bearings using only steady-state operating conditions, but not the transients that are more 

typical of the actual turbine gearbox applications. In fact, transients may affect wear life 

more than steady-state operation. From that point of view, a better method to simulate the 

actual spectrum of loads and directions experienced by gearbox bearings is needed. There 

is a need to develop a multiaxial gearbox rolling element bearing test system that can 

apply standardized operational spectra typical of various wind farm locations (e.g., 

Midwest, offshore, desert). Designing such a testing system requires detailed operational 

data from instrumented turbine components.

Another issue was only briefly mentioned at the workshop, but it may have an effect on 

micropitting and wear of bearing components. That is the issue of fretting from 

superimposed vibrations on the contact area, either when the rotors are turning or when 

the system is at rest. High-frequency, low-amplitude slip between contacting surfaces can 

generate fine, powdery wear debris, and depending on conditions, the debris can clump or 

accumulate in surface pockets. Perhaps a study of the role of fretting on the initiation 

phase of micropitting is needed, but this type of research was not explicitly advocated at 

the workshop.

In formulating recommendations for future work, we must distinguish between basic and 

applied research in light of the need to move wind turbine technology forward. The 

history of technology has many examples of engineering and empirical approaches 

leading fundamental understanding of the reasons how and why certain strategies and 
27

designs work. Some of the issues discussed in the workshop are fundamental: How do 

lubricants work on a nano-scale level? How do dislocation pile-ups and lattice defects 

created at the subasperity level coalesce into microcracks that eventually lead to 

micropits? Some issues are highly applied: What kind of finishing process is required for 

certain types of bearings? What running-in method should be applied to condition a new 

gearbox? What is the minimum inspection interval? What kinds of sensors should be used 

to monitor bearing health? 

Given that there are still some unknowns in the technical community’s understanding of 

micropitting, Table 3-1 presents a summary of research needs. It has been reviewed by 

the organizers and speakers for content, but does not necessarily represent a 

unanimous agreement on what needs to be done. No attempt was made to list these 

items in order of priority. Several involve standardization. Because a number of 

contributors have expressed these ideas, they have not been ascribed to a particular 

person or organization. Furthermore, considering the limited attendance at the workshop, 

it is possible that additional micropitting research needs have been overlooked. 

Table 3-1. Research Needs in Micropitting

Item Subject Description

A Quantification of 

micropitting damage

Need a standardized method to quantify the degree of 

micropitting damage observed on contact surfaces. This will 

help diagnose the severity of field failures, assess the 

evolution of surface damage, and help in the correlation of 

laboratory test data with field observations.

B Employment of a 

standardized 

‘systematic tribology’ 

approach to 

characterize bearings 

and gear assemblies 

that are prone to 

micropitting

Define the gearbox components characteristics in ways that 

allow their tribological characteristics to be specified. This 

includes obtaining a better understanding of load histories 

and transients that can affect gear and bearing life.

C Identification of 

standard test methods 

that can used to 

generate micropitting 

damage that simulates 

field observations

Allow standardization of laboratory-scale tests to be used to 

cost-effectively and accurately screen lubricants, surface 

treatments, and coatings for resistance to micropitting.

D Improved gearbox 

damage sensors

Identify condition monitoring strategies that can sensitively 

detect the presence of micropitting in operating gearboxes. 

Alternatively develop methods to detect the symptoms of

micropitting, even if surfaces cannot be observed directly.

E Hydrogen 

embrittlement studies 

of gear and bearing 

alloys

Determine whether or not hydrogen embrittlement is a factor 

in micropitting initiation.

F Run-in-ability studies Understand the asperity-level changes that occur during 

run-in that could lead to micropitting using various lubricants 

and surface treatment approaches. Determine the optimum 

run-in lubricant, test parameters, and run-in procedure to 

maximize resistance to micropitting. Determine condition 

monitoring techniques necessary for controlling a proper 
28

run-in.

G Material alloy, heat 

treatment, and surface 

treatment selection 

strategies

Using established micropitting test methods, establish a set 

of material (alloy, heat treatment, and surface treatment) 

selection rules to help designers avoid micropitting. This 

may need to be part of a suite of tests to ensure that the 

approaches used to reduce micropitting do not induce some 

other form of surface damage, like abrasion, macropitting, 

or scuffing. Determine the differences of through-hardened 

bearing steels and case hardened gear and bearing steels 

on micropitting resistance.

H Wear-corrosion 

synergy

Determine if the presence of corrosive agents such as water 

or acidic species in lubricants initiates or accelerates 

micropitting beyond that which occurs by mechanical factors 

alone. Resolve whether antiwear and antiscuff additives are 

beneficial or detrimental to micropitting resistance.

I Micropitting 

morphology studies

Determine the effects of percent slip in the characteristics of 

micropits. This would differentiate the morphology of 

micropits in bearings from that in gears, where slip tends to 

be greater. Determine the influence of load magnitude and 

load transients on slip in bearings and influence on 

micropitting resistance.

J Manufacturing studies Determine the influence that various grinding machines 

have on surface topography of gear teeth and rolling 

element bearing surfaces and establish the degree (cost) of 

surface finishing required to reduce the incidence of 

micropitting in materials that vary in composition, hardness, 

and heat treatment. Determine the influence of grind temper 

on micropitting.

K Gearbox lubricant data Film thickness calculations require lubricant property data, 

some of which is not always readily available to modelers. 

Data such as pressure-viscosity coefficients for gearbox 

lubricants need to be obtained and made more widely 

available.

L 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 micropitting needs to be better 

established.

M Solid contaminant 

influence

Determine the influence of solid contaminants on 

micropitting. Establish required oil cleanliness for wind 

turbine gearboxes.

N Hunting vs. nonhunting 

gear ratio

Determine whether wind turbine gears should have hunting 

or nonhunting gear ratios to maximize micropitting 

resistance.

O Gear and bearing 

rating standard for 

micropitting risk

Develop a rating standard for calculating the risk of 

micropitting for gears and rolling element bearings.

P Bearing rating 

standard

Modify DIN ISO 281 to include the influence of micropitting 

on calculated bearing life.
29

Footnote to Table 3-1: the letters in the first column are intended to facilitate 

communications regarding items in the table.
30

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31

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32

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33

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34

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35

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38

Appendix A: Information on Attendees 

A.1 Speaker Biographies

The following biographies give information on the speakers. See Section A.2 for contact 

information.

Robert Errichello

President, GEARTECH, Townsend, MT, USA

Robert Errichello heads his own gear consulting firm, 

GEARTECH, and is the founder of GEARTECH Software, 

Inc. He has more than 40 years of industrial experience and 

has worked for several gear companies. For the past 31 

years, he has worked as a consultant to the gear industry, as 

well as to more than 40 wind turbine manufacturers, 

purchasers, operators, and researchers. 

Errichello is a graduate of the University of California at 

Berkeley and holds his B.S. and M.S. in mechanical 

engineering, along with a master’s of engineering in 

structural dynamics. Errichello 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 has also 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.

Errichello is a registered professional engineer in the state of California, and a member of 

several committees, including the American Gear Manufacturers Association (AGMA) 

Gear Rating Committee, the AGMA/American Wind Energy Association (AWEA) Wind 

Turbine Committee, and the American Society of Mechanical Engineers (ASME) Power 

Transmission and Gearing Committee. He is a member of AGMA, ASM International, 

ASME, the Society of Tribologist and Lubrication Engineers (STLE), NREL’s Gearbox 

Reliability Collaborative (GRC), and the Montana Society of Engineers.

Errichello has published more than 60 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 a technical editor for Gear Technology and STLE’s Tribology 

Transactions.

Errichello is recipient of the AGMA Technical Division Executive Committee (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.
39

Stephen Hsu, Ph.D. 

Professor, Department of Mechanical & Aerospace Engineering, George Washington 

University, Washington, D.C., USA

Stephen Hsu obtained his B.S. at Virginia Tech and M.S. and 

Ph.D. degrees in chemical engineering from The Pennsylvania 

State University. After graduation, he joined the Amoco 

Chemicals Research Lab in Naperville, Illinois. In 1978, he 

joined the National Institute of Standards and Technology 

(NIST) to lead a U.S. congressionally mandated program on 

recycled oils. After successfully concluding the program in

1984, he established his own research group on tribochemistry. 

After a stint as the founding division chief of the Ceramic 

Division, he went back to basic research and began working 

closely with various industries on a variety of technical issues 

challenging the economy: machining of advanced materials, 

magnetic hard disk interface, lubrication of new materials, wear 

maps to guide materials selection, nanofriction, and nanomechanical properties of 

nanomaterials. He has published some 250 papers, received 7 U.S. patents, and given 

about 40 plenary lectures in international conferences. He was a visiting professor at 

Pennsylvania State, Esback Fellow at Northwestern University, and adjunct professor at 

University of Maryland, where he supervised M.S. and Ph.D. theses. 

He is a Fellow of ASME, STLE, and the recipient of STLE International Award, Captain 

Alfred Hunt Memorial Medal, Al Sontag Award, and Bronze and Silver Medals from 

NIST. He served on the editorial boards of Tribology Letters and Tribology International, 

and has been a foreign expert reviewer for European Science Foundation, Australia, 

Canada, Singapore, Hong Kong, and Sweden, among others.

In January 2007, he joined George Washington University (GWU) as a professor of 

engineering and applied science, and in 2009, he cofounded the GWU Energy Institute. 

His current research focuses on wind energy, gear reliability, advanced materials for 

blades, self-repairing technology, and advanced coating development. 

Michael N. Kotzalas, Ph.D., P.E.

Chief Engineer, Product Design, Global TRB, The Timken Company, Canton, OH, USA

Michael Kotzalas attended The Pennsylvania State 

University, graduating with a B.S. in 1994, an M.S. in 1997, 

and a Ph.D. in 1999. All his degrees are in mechanical 

engineering. The focus of his university study and research 

was the analysis of rolling bearing technology, including 

quasi-dynamic modeling of ball and cylindrical roller 

bearings for high acceleration applications and spall 

progression testing and modeling for use in condition-based 

maintenance algorithms.

Since graduation, Kotzalas has been employed by The 

Timken Company within technology unit and most recently 
40

the bearings and power transmission business unit. His current responsibility is to 

globally coordinate and lead the design of tapered roller bearings for the Timken 

Company. Other responsibilities included leading the needle roller bearing design group 

for off-highway customers, new product development, and analysis algorithm research 

and development. From this work, Kotzalas has received two U.S. patents for cylindrical 

roller bearing designs.

Outside of work, Kotzalas is also active in industrial societies. A member of ASME, he 

serves as the secretary and treasurer of the Tribology Division, and previously served as 

chair of the Publications Committee and webmaster of the Tribology Division. With 

STLE, he previously served as chair of the Awards Committee. Also, Kotzalas has 

published 13 peer-reviewed papers in journals and conference proceedings, and 

coauthored the fifth edition of the two-volume book titled Rolling Bearing Analysis. 

Working with the American Bearing Manufacturer’s Association (ABMA), Kotzalas is 

one of many instructors for short courses on “Advanced Concepts of Bearing 

Technology” and “Essential Concepts of Bearing Technology.”

Kotzalas received the ASME Tribology Division’s Best Paper Award in 2001 and 

STLE’s Hodson Award in 2003 and 2006. In 2007, He was awarded the Outstanding 

Young Engineer for the Off-Highway Industry by the Society of Automotive Engineers 

(SAE) and the Association of Equipment Manufacturers (AEM).

Andy Olver, Ph.D.

Reader, Department of Mechanical Engineering, Imperial College, London, UK

Andy Olver is a reader, the rank between senior lecturer 

and professor, in the Department of Mechanical 

Engineering at Imperial College, UK. He worked as a 

materials scientist, tribologist, and transmission engineer 

in the helicopter industry before joining the staff of 

Imperial College in 1992. His research interests include 

lubrication, fretting, rolling contact fatigue, and 

tribological materials. 

Olver is part of the tribology group at the Imperial College. His motivation springs from 

the desire to solve practical engineering problems by making advances in fundamental 

understanding of tribology. Andy has contributed to the tribology group's research in 

experiments on rolling contact fatigue and fretting, theoretical modeling of spline 

couplings, microstructural studies of contact damage and gear, and rolling bearing 

tribology. His current projects include running-in and damage accumulation in rolling 

contact, and texture and roughness effects in hydrodynamic and elastohydrodynamic 

lubrication and diesel engine friction.

Olver’s work has been published in peer-reviewed journals, conference proceedings, and 

book chapters. He was awarded the William Sweet Smith Prize under the Aerospace 

Industries Division of IMechE in 1992. He also won the PE publishing award for the 

Journal of Engineering Tribology under IMechE in 2005. 
41

Olver is a member of the Tribology Group Committee and Technical Activity Committee 

on Aircraft Transmissions, an academic member of the National Advisory Committee on 

Transmissions and Mechanical Systems, and a member of the editorial board for 

Proceedings of International Mechanical Engineers Part J. He was also a member of the 

tribology steering group for Engineering Sciences Data Unit. 

More details on Olver can be found at: http://www3.imperial.ac.uk/people/a.v.olver

Brian Shaw, Ph.D. 

Director, Design Unit, Newcastle University, UK

Brian Shaw is director of the Design Unit at Newcastle 

University (www.ncl.ac.uk/gears); he has more than 15 years 

of experience researching gear metallurgy and failure 

processes and carrying out industry-related failure 

investigations, including work on bearings. This has led 

to more than 90 scientific publications and more than 200 

technical reports to industry. Shaw is the primary 

metallurgical gearing consultant to the UK Navy and also 

leads gear failure training for the British Gear Association, of 

which he is a member of Executive Board and Technical and 

Research Committee. Shaw is also a member of the UK BSI 

committee contributing to the review of ISO6336-5 on gear 

material specification and the development of micropitting 

gearing standards through ISO WG15. Shaw's research interests include improving 

gearing performance through material and processing developments assessed by testing 

gears on sophisticated gear test rigs developed at Newcastle University. Particular current 

interests include understanding methods to prevent micropitting, strength enhancements 

through control of residual stresses with novel methods (e.g., laser peening), improving 

performance through design optimization of microgeometry, and the use of coatings to 

enhance contact fatigue performance. Shaw leads a group of 22 staff in the Design Unit 

working across the board on gearing and transmissions research and development 

covering gear design, transmission testing, general mechanical design, bespoke test rigs, 

in-service measurement and condition monitoring, materials analysis, failure 

investigations, metrology, and seminars and training. 
42

Ray Snidle, Ph.D.

Professor, Department of Mechanical Engineering, Cardiff University, UK

Ray Snidle is a professor of mechanical engineering in the 

School of Engineering at Cardiff University, where he leads 

the Tribology and Contact Mechanics Group. His research is 

mainly concerned with fundamental aspects of surface 

durability in machine elements such as gears, bearings, and 

other concentrated rolling/sliding contacts where the 

mechanism of lubrication is described as 

elastohydrodynamic or micro-elastohydrodynamic. He is 

currently leading a major EPSRC-sponsored project on 

“Mixed lubrication, wear and contact fatigue of rough 

surfaces.” He recently completed an experimental 

investigation of the effects of hard coatings on the scuffing 

resistance of gears. Ray holds his Ph.D. and D.Sc. from 

Leicester University in the United Kingdom and is a fellow of ASME. His Web site can 

be found at http://tribology.engineering.cf.ac.uk/.

Thomas Tobie, Ph.D.

Gear Research Centre, Technical University of Munich, Germany

Thomas Tobie works at the Gear Research Centre (FZG) at 

Munich as associate to Prof. Dr.-Ing. B.-R. Höhn. He has been 

a chief engineer at FZG since 1999. In 2001 he graduated with 

the title of Dr.-Ing. He specializes in gear material, heat 

treatment, and gear load capacity research on tooth root 

bending fatigue, pitting, and micropitting.
43

Martin N. Webster, Ph.D. 

ExxonMobil Corporate Strategic Research, Annadale, NJ, USA

Martin Webster currently holds the position of senior 

research associate and program leader at ExxonMobil’s 

Corporate Strategic Research laboratory in Annandale, New 

Jersey. He is responsible for a number of the longer range 

lubrication and tribology research programs.

Webster gained his Ph.D. in tribology from Imperial College 

London, where his work on rough surface contact mechanics 

resulted in being awarded the I Mech. E Tribology Bronze 

Medal in 1986. Following a postdoctoral assignment with 

Shell Research in the UK, he spent 2 years working on the 

design and analysis of wind turbine systems with the UKbased Wind Energy Group (WEG). He has since accumulated 

more than 20 years of experience working with ExxonMobil 

in both research and product development positions. His 

research focus has been on the fundamentals of lubricated contacts, including the 

measurement and characterization of EHL performance, modeling EHL contacts, rolling 

contact fatigue phenomenon and the interactions of lubricant components with 

engineering surfaces. He has published numerous papers and patents in each of these 

areas. He has been active in various societies and technical committees, including the 

Gear Research Institute, the ASME Rolling Element Bearing Committee, and the STLE 

Gears and Gear Lubrication technical committee. He currently serves as a director of the 

STLE.

Lavern (Vern) D. Wedeven, Ph.D. 

President, Wedeven Associates, Inc., Edgemont, PA, USA

Vern Wedeven is president of Wedeven Associates, Inc., a 

small business devoted to tribology research and development. 

Since 1987 he and his associates have developed specialized 

tribology test machines and test methods that link fundamental 

principles of tribology to service applications. Wedeven 

Associates Machine (WAM) technology is providing tribology 

research capability for major bearing companies, military 

laboratories, and research institutes. He is involved in major 

research projects and consulting that serve aerospace and other industries.

Wedeven is a graduate of Calvin College (B.S.), University of Michigan (B.S.ME) and 

Imperial College, London (Ph.D.). Previous employment includes NASA Glenn Research 

Center, NASA Headquarters, and SKF-America. He is the author of more than60 

technical papers in the area of tribology. He has written chapters in three books and holds 

three patents. He is the recipient of several awards: Walter D. Hodson Award (1975) for 

best published paper in STLE by an author under the age of 35 years; Captain Alfred E. 

Hunt Award (1979) for best paper published in STLE; Arch T. Colwell Cooperative 

Engineering Medal for contributions of SAE Propulsion Lubricants Committee; and the 

STLE International Award (2008) for outstanding contributions in tribology. 
44

A.2 Attendee List

# Name Affiliation Email Address

1 Peter J. Blau ORNL

2

blaupj@ornl.gov

Sandy Butterfield NREL

3

Sandy.Butterfield@nrel.gov

Jim Carey ExxonMobil

4

jim.t.carey@exxonmobil.com

Ekkehard Daschner ExxonMobil

5

ekkehard.daschner@exxonmobil.com 

Kevin Dinwiddie Amsoil kdinwiddie@amsoil.com 

6 Gary Doll Timken

7

gary.doll@timken.com

Ali Erdemir Argonne National Lab.

8

erdemir@anl.gov

Bob Errichello GEARTECH geartech@mt.net

9 Ed Hahlbeck PowerTrain ENG ehahlbeck@wi.rr.com

10 Ted Harris Harris Consulting

11

tah10@psu.edu

Bill Herguth Herguth Lab wherguth@herguth.com 

12 Donald R. Houser OSU Gear Lab

13

houser.4@osu.edu

Stephen Hsu

George Washington 

University stephen.hsu@erols.com 

14 Peter Jacobs ExxonMobil

15

peter.w.jacobs@exxonmobil.com

James A Johnson NREL

16

James.A.Johnson@nrel.gov

Michael N. Kotzalas Timken

17

michael.kotzalas@timken.com 

Jon Leather Castrol

18

jon.leather@castrol.com. 

Hal Link NREL

19

Hal.Link@nrel.gov

Matthew Malkin DNV GEC

20

Matthew.Malkin@dnv.com

David Mikalonis SKF david.r.mikalonis@skf.com

21 Andy Milburn Milburn Engineering andy@milburnengineering.com

22 Jane Muller GEARTECH

23

jrmgears@mt.net

Andy Olver Imperial College a.v.olver@ic.ac.uk

24 Francisco Oyague NREL

25

Francisco.Oyague@nrel.gov

David Pallister SKF

26

Dave.M.Pallister@skf.com

Don Roberts DNV GEC Donald.Roberts@dnv.com

27 Brian Shaw University of Newcastle

28

b.a.shaw@newcastle.ac.uk

Shawn Sheng NREL

29

Shuangwen.Sheng@nrel.gov

Ray Snidle Cardiff University snidler@cf.ac.uk

30 Thomas Tobie FZG-TU-Munich tobie@fzg.mw.tum.de

31 Martin Webster ExxonMobil

32

martin.n.webster@exxonmobil.com

Vern Wedeven Wedeven Associates, Inc. wedeven@aol.com
45

A.3 Group Photo

Left to Right: Ekkehard Daschner, Gary Doll, Ray Snidle, Peter Jacobs, Michael N. Kotzalas, Sandy Butterfield, Don Roberts, Andy Olver, Ted Harris, Bob Errichello, 

Vern Wedeven, Andy Milburn, Matthew Martin, Jane Muller, Peter Blau, Stephen Hsu, Jim Carey, David Mikalonis, Ed Hahlbeck, David Pallister, Don Houser, Martin 

Webster, Thomas Tobie, Ali Erdemir, Brian Shaw, Bill Herguth, Francisco Oyague, Shawn Sheng
F1147-E(10/2008)

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1. REPORT DATE (DD-MM-YYYY)

February 2010 

2. REPORT TYPE

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3. DATES COVERED (From - To)

April 15-16, 2009

4. TITLE AND SUBTITLE

Wind Turbine Micropitting Workshop: A Recap

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DE-AC36-08-GO28308

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6. AUTHOR(S)

S. Sheng, Editor

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13. SUPPLEMENTARY NOTES

14. ABSTRACT (Maximum 200 Words)

Micropitting is a Hertzian fatigue phenomenon that affects many wind turbine gearboxes, and it affects the reliability 

of the machines. With the major growth and increasing dependency on renewable energy, mechanical reliability is an 

extremely important issue. The U.S. Department of Energy has made a commitment to improving wind turbine 

reliability and the National Renewable Energy Laboratory (NREL) has started a gearbox reliability project. 

Micropitting as an issue that needed attention came to light through this effort. To understand the background of work 

that had already been accomplished, and to consolidate some level of collective understanding of the issue by 

acknowledged experts, NREL hosted a wind turbine micropitting workshop, which was held at the National Wind 

Technology Center in Boulder, Colorado, on April 15 and 16, 2009. 

15. SUBJECT TERMS

micropitting; Hertzian fatigue phenomenon; wind turbine gearboxes; wind turbine reliability; turbine micropitting 

workshop

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