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Gearbox Reliability 

Collaborative Update: 

A Brief 

Shawn Sheng, Jon Keller, and Mark McDade

NREL/National Wind Technology Center 

AWEA Operations & Maintenance Working Group Meeting

January 10-11, 2012

NREL/PR-5000-53804
2 

Outline 

 Gearbox Reliability Collaborative (GRC) 

 Failure Database 

 GRC Test Gearbox 1 Damage

 Condition Monitoring

 GRC Status Update
3 

Gearbox Reliability Collaborative (GRC)

Improved Industry Practices & 

Design Standards

Test

Condition 

Monitoring

Modeling 

& Analysis

Teardown 

Inspections

Root Cause 

Analysis

 Disparity between expected and actual gearbox life

• Widespread, not due to

manufacturing issues

• Critical elements missed

in design process, and/or

• Analytical tools insufficient

 Gearbox largest contributor

to turbine downtime and

costliest to repair[1]

 How do I design a gearbox

for reliability?

Failure Database

Anecdotal FMECA

[1] WindStats Newsletter, Vol. 16 Issue 1 to Vol. 22 Issue 4, covering 2003 to 2009
4 

Failure Database

 Collect information on gearbox rebuilds

• In shop and on tower

• Existing data from papers, Excel spreadsheets

 Quantify magnitude and cost of gearbox 

problems

• Expensive uncertainty

 Focus research accurately

• Aid root cause analysis and correction

 Provide objective record of improvements

• Analyze and close loop when solutions found

 Sanitized data shared among GRC members
5 

Failure Database Software

 Structured data collection

• Navigation tree 

 Visually oriented

• Wireless image from camera to software fields

 Embedded failure codes
6 

Incidents Summary

 37 incidents: 36 involving bearings and 22 gear failures[2]

• Top modes highlighted in RED

Quantity Component Code Description 

1 bearing 5.4.3 Hertzian Fatigue, Micropitting, Edge of Raceway 

1 bearing 6.1.1 Wear, Adhesion, Mild 

3 bearing 6.1.3 Wear, Adhesion, Severe (Scuffing) 

4 bearing 6.2.1 Wear, Abrasion, Two-body 

10 bearing 6.2.2 Wear, Abrasion, Three-body 

1 bearing 6.2.3 Wear, Abrasion, Polishing 

14 bearing 8.1.1 Cracking: Roller and Ring Cracks, Hardening Cracks 

2 bearing 8.1.2 Cracking: Roller and Ring Cracks, Grinding Cracks 

1 gear 3.1 Overload, Fracture, Brittle 

1 gear 4.1 Bending Fatigue, Low Cycle 

2 gear 5.3 Hertzian Fatigue, Subcase Fatigue 

6 gear 6.4 Wear, Fretting-Corrosion 

2 gear 4.2.1 Bending Fatigue, High Cycle, Root Fillet Cracks 

3 gear 4.2.2 Bending Fatigue, High Cycle, Profile Cracks 

2 gear 6.1.1 Wear, Adhesion, Mild 

1 gear 6.2.2 Wear, Abrasion, Moderate 

4 gear not found 

[2] Wind Turbine Failure Modes – A Brief, ASME/STLE 2011 International Joint Tribology Conference, NREL PR-5000-53084. 
7 

One GRC Test Gearbox

 A 750-kW wind turbine 

gearbox

 One planet and two 

parallel stages, floating 

sun 

 Pressurized lubrication 

for all but the planet 

gears and the ring gear

Annulus

Planet

Sun

Gear

Gear Pinion

Pinion

Low-Speed

Stage

High-Speed 

Stage

Intermediate-Speed

Stage

Low-Speed

Shaft

Intermediate-Speed 

Shaft

High-Speed 

Shaft 

Planet 

Carrier
8 

Bearing Nomenclature 

PLC-A PLC-B

LSS-A

LSS-B LSS-C

PL-A PL-B

ISS-A ISS-B ISS-C

HSS-A HSS-B HSS-C Annulus

Planet

Sun

Gear

Gear Pinion

Pinion

Acronym Definition

PLC Planet Carrier

PL Planet

LSS Low-Speed Shaft

ISS Intermediate-Speed Shaft

HSS High-Speed Shaft 

Bearing PLC-A Outer Raceway (Severe Fretting 

Corrosion) Photo from GEARTECH, NREL/PIX 19748
9 

Real Damage [2] 

Failure # Component / Location Mode Severity 

1 HSS Gear Set Scuffing Severe 

 (HSS Gear and HSS Pinion) 

 

2 HSS Downwind Bearings Overheating Mild 

 (IR and Rollers) 

 

3 ISS Gear Set Fretting Corrosion Severe 

 (ISS Gear and Pinion) Scuffing 

 (All Teeth) Polishing Wear 

 (Sun Spline) 

 

4 ISS Upwind Bearing Assembly damage Moderate 

 (IR) Plastic deformation 

 Scuffing 

 False brinelling 

 Debris dents 

 Contact Corrosion 

 

5 ISS Downwind Bearings Assembly damage Severe 

 (OR Spacer for both Bearings) Plastic deformation 

 Dents 

 

6 Annulus/Ring Gear, or Sun Pinion Scuffing and polishing Moderate 

 Fretting Corrosion Severe 

 

7 Planet Carrier Upwind Bearing Fretting Corrosion Severe 

 (Container and OR) 

 Severe damage highlighted in RED
10

Real Damage (Cont.) 

 Two main root causes 

• Oil starvation 

• Assembly damage

 Simple triggers may lead to 

complex damage

9 Oil Transfer Ring for Planet Carrier Polishing Mild 

 (Bore) 

 

10 LSS Scuffing Severe 

 (Shaft and O-ring Seal Plate) 

 

11 LSS Downwind Bearings Abrasion Severe 

 (Locknut) 

 

12 HSS Shaft Misalignment Mild 

 

High-Speed Stage Pinion (Severe Scuffing)

Photo from GEARTECH, NREL/PIX 19743
11

Observations 

 Gear failure database incidents

• 37 incidents: 36 related to bearings and 22 related to gears 

 may need to pay attention to both bearings and gears

• Top bearing failure modes: hardening cracks, abrasion 

(scratching of surfaces), adhesion (scuffing, welding and tearing 

of materials)

• Top gear failure modes: fretting corrosion, high-cycle bending 

fatigue 

• Be careful with generalization of the database results 

 the population is still small 

 GRC test gearbox damage 

• Simple causes may lead to complex damage

• Operation and maintenance improvements are necessary to 

minimize the possibility of oil loss

• Additional attention to assembly is needed to avoid damage
12

Condition Monitoring (CM)

 Multiple CM systems demonstrated in Phases 1 and 2

 Various vendors and techniques (27 partners)

• Vibration

• Stress Wave

• Electrical Signature

• Oil Condition

Particle count

Moisture/Quality

 Compared capability

to detect gearbox fault

in dynamometer test
13

CM Key Findings[3]

 ISO cleanliness measurements could be used to monitor 

and control the run-in of wind turbine gearboxes 

 Recommend an integrated approach when conducting 

wind turbine condition monitoring 

• At a minimum, a combination of vibration or acoustic emission, 

with oil debris monitoring techniques, is recommended. 

 If the sensor mounting location is appropriate, similar 

trends in wear debris counts can be obtained between 

the offline filter loop and the inline filter loop

[3] Wind Turbine Drivetrain Condition Monitoring During GRC Phase 1 and Phase 2 Testing, NREL TP-5000-52748. 
14

GRC Accomplishments

 Failure database software deployed

• Partners see business value for themselves and customers

 Designed, built, instrumented, and tested two 

gearboxes

• Generated unprecedented public domain test data from 

both field testing and dynamometer testing

• Different responses from ‘identical’ gearboxes

• Demonstrated importance of non-torque loading and 

modeling approach

 Active collaborative, with wide industry support, 

leveraging DOE funding

• Modeling round robin

• Condition Monitoring round robin
15

GRC Recent Activities

 Deployed Failure Database software

• Users represent 17% of installed U.S. capacity

 Continued Modeling and Analysis

• Model-to-model-to-test comparisons for dynamic data

• Effect of carrier and planet bearing clearances/gravity on 

loads

 Planning Phase 3 Testing

• Field representative loading tests of Gearbox 2

• Design and eventual tests on Gearbox 3

 Meetings

• Hosted Condition Monitoring Workshop

46 speakers, 11 sessions, and about 150 attendees

• Hosted Tribology Seminar

21 speakers and about 110 attendees
16

GRC Recent Publications and Presentations 

 Failure Database

• Wind Turbine Failure Modes – A Brief, ASME/STLE 2011 International Joint 

Tribology Conference, PR-5000-53084

 GRC General

• Gearbox Reliability Collaborative Project Report: Findings from Phase 1 and Phase 2 

Testing, 2011 Sandia Wind Turbine Reliability Workshop, TP-5000-51885 & PR5000-52463

• Wind Turbine Tribology Seminar presentations & TP-5000-53754 

 Testing, Modeling and Analysis

• NREL Gearbox Reliability Collaborative: Comparing In-Field Gearbox Response to 

Different Dynamometer Test Conditions, WINDPOWER 2011, CP-5000-51690

 Condition Monitoring

• Investigation of Various Condition Monitoring Techniques Based on a Damaged 

Wind Turbine Gearbox, 8th International Workshop on Structural Health 

Monitoring, CP-5000-51753

• Wind Turbine Drivetrain Condition Monitoring During GRC Phase 1 and Phase 2 

Testing, 2011 Sandia Wind Turbine Reliability Workshop, TP-5000-52748 & PR5000-52352

• Condition Monitoring Workshop presentations
17

Thank You!

NREL’s contributions to 

this presentation were 

funded by the Wind and 

Water Power Program, 

Office of Energy Efficiency 

and Renewable Energy, 

the U.S. Department of 

Energy under contract No. 

DE-AC02-05CH11231. 

The authors are solely 

responsible for any 

omissions or errors 

contained herein. 

Photo from HC Sorensen, Middelgrunden Wind Turbine Cooperative, NREL/PIX17855

shuangwen.sheng@nrel.gov 

303-384-7106