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Wind Turbine Gearbox Failure Modes – A Brief 

ASME/STLE 2011 International 

Joint Tribology Conference

October 24-26, 2011

Los Angeles, California

NREL/PR-5000-53084

Shuangwen (Shawn) Sheng

National Renewable 

Energy Laboratory

Mark McDade

National Renewable 

Energy Laboratory 

Robert Errichello

GEARTECH
2 

Outline 

 Gearbox Reliability 

Collaborative (GRC)

 Gearbox Failure Database

 Recorded Incidents 

Summary

 Detailed Damage of one 

GRC Test Gearbox

 Observations

DOE 1.5 MW Turbine. Photo by Lee Jay Fingersh, NREL/PIX 17245
3 

Gearbox Reliability Collaborative

 Wind turbine gearboxes are not always meeting 20-year 

design life

 Premature failure of gearboxes increases cost of energy

• Turbine downtime

• Unplanned maintenance 

• Gearbox replacement and rebuild

• Increased warranty reserves

3

Source: Wind Stats 2003-2009 aggregated downtime by turbine subsystem 

 The problem

• Is widespread

• Affects most Original Equipment 

Manufacturers 

• Not caused by manufacturing practices

 Need to improve gearbox reliability 

and reduce turbine downtime 
4 

GRC Technical Approach

 Technical Approach

• Modeling and analysis 

• Field test 

• Dynamometer test 

• Condition monitoring

• Failure database

 Goal

• To improve gearbox reliability 

and increase turbine uptime, 

which in turn will reduce the 

cost of energy 

Field Test

Dynamometer Test

Analysis

• Load cases

• System loads

• Internal loads

Test Turbine at NREL/PIX19022

NREL Dynamometer/PIX16913 
5 

Gearbox Failure Database 

 Gearbox problems = expensive 

uncertainty 

 Quantify magnitude and cost of 

the problem

 Focus research accurately

 Aid root cause analysis and 

correction

 Provide objective record of 

improvements
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Gearbox Failure Database (Cont.) 

 Collect information from gearbox rebuilds

• In shop and on tower

• Existing data from papers, Excel spreadsheets, etc.

• Share ‘sanitized data’ within the group

 Analyze and close loop when solutions are identified
7 

Gearbox Failure Database (Cont.) 

 Structured data collection: navigation tree 

 Visually oriented: wireless image transfer 

from camera to correct fields in software

 Embedded failure codes library
8 

Recorded Incidents Summary [1] 

 37 incidents: 36 with bearing and 22 with gear failures

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 
9 

Top Failure Modes for Bearings [2]

 Cracking: 8.1.1 roller and ring hardening cracks

• Generally inter-granular with the crack running from 

the surface of a roller or a ring toward its center of 

mass in a relatively straight line 

• Cracking in heat treatment usually occurs during or 

after quenching due to localized stresses caused by 

non-uniform cooling or unequal transformation of 

austenite to martensite. 
10

Top Failure Modes for Bearings [2]

 Abrasion: 6.2.1, 6.2.2 two-body and three-body 

• Two-body: embedded particles or asperities on one 

bearing surface abrade the opposing bearing surface

• Three-body: abrasion due to loose contaminants 

• Abrasion scratches or gouges on bearing surfaces are 

in the direction of sliding. Under magnification, 

scratches appear as parallel furrows that are smooth 

and clean

• Usually caused by contamination of lubricant by hard, 

sharp-edged particles. Common contaminations are 

sand, rust, machining chips, grinding dust, weld 

splatter, and wear debris. 
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Top Failure Modes for Bearings [2]

 Adhesion: 6.1.3 severe adhesion (scuffing)

• Severe adhesion or scuffing is transfer of material 

from one bearing surface to another due to welding 

and tearing

• Damage typically occurs in areas of slip in narrow or 

broad bands along the direction of sliding. It may 

occur in localized patches with load concentrations

• Scuffing areas appear to have a rough or matte 

texture, which under magnification, appears to be 

torn or plastically deformed. 
12

Top Failure Modes for Gears [3]

 Fretting Corrosion: 6.4

• Fretting corrosion is deterioration of contacting gear 

tooth surfaces caused by minute vibratory motion 

• It occurs between contacting surfaces that are pressed 

together and subject to cyclic, relative motion of 

extremely small amplitude. Under these conditions, 

lubricant is inadequate to replenish, permitting metalto-metal contact and causing adhesion of surface 

asperities 

• Fretting corrosion damages gear teeth by forming ruts 

along lines of contact. 
13

Top Failure Modes for Gears [3]

 Bending Fatigue: 4.2.2 High-Cycle 

• It consists of three distinctive stages: crack initiation, 

propagation and fracture

• During stage 1 no gross yielding of the gear teeth 

occurs. However, local plastic formation may occur in 

regions of stress concentrations or areas of 

discontinuities. The end of this stage is symbolized by 

the formation of microcracks inside grains. 
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Top Failure Modes for Gears [3]

 Stage 2 begins when the crack turns and grows across 

grain boundaries (transgranular) in a direction 

approximately perpendicular to the maximum tensile 

stress. Plastic deformation is confined to a small zone at 

the leading edge of the crack. As a result, the cracked 

surfaces usually appear smooth without signs of gross 

plastic deformation

 Sudden fracture occurs during stage 3. It may be ductile, 

brittle or mixed-mode depending upon material 

toughness and magnitude of applied stress

 High-cycle bending fatigue occurs when cyclic stress is less 

than the yield strength of the material and the number of 

cycles to failure is greater than 10,000.
15

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
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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 
17

Test History 

1. Completed dynamometer run-in test

2. Sent for field test: experienced two oil losses 

3. Stopped field test 

4. Retested in the dynamometer under controlled conditions.

NREL 2.5 MW Dynamometer. Photo by Lee Jay 

Fingersh, NREL/PIX 16913 
18

Real Damage [4] 

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) 
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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 Miaslignment Mild 

 
20

Gear Sets

High-Speed Stage Pinion (Severe Scuffing)

Photo from GEARTECH, NREL/PIX 19743

High-Speed Stage Gear (Severe Scuffing) 

Photo by Robert Errichello, NREL/PIX 19599

Sun Spline (Severe Fretting Corrosion)

Photo from GEARTECH, NREL/PIX 19751

Sun Pinion (Severe Fretting Corrosion)

Photo from GEARTECH, NREL/PIX 19750
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Gear Sets

Intermediate-Speed Stage Pinion Photo from GEARTECH, NREL/PIX 19745

 Failure mode 1: fretting corrosion, severe 

 Failure mode 2: polishing wear, severe 

(local) 

 Failure mode 3: scuffing, severe (local) 

 Description: Damage was imprinted on all 

teeth because the gear set had a hunting 

gear ratio.

Annulus Gear. Photo from GEARTECH, NREL/PIX 19749

 Failure mode 1: scuffing, moderate 

(local)

 Failure mode 2: polishing wear, 

severe

 Description: Two original, hand 

dressed macropits are shown. 

Polishing wear removed all traces 

of the grind marks and created a 

wear step at the end of contact.
22

Bearings 

Bearing ISS-B/ISS-C Outer Raceway Spacer 

(Severe Assembly Damage) Photo from 

GEARTECH, NREL/PIX 19747

Bearing PLC-A Outer Raceway (Severe 

Fretting Corrosion) Photo from GEARTECH, 

NREL/PIX 19748

Bearing HSS-C Inner Raceway (Mild 

Overheating) Photo from GEARTECH, NREL/PIX 19744

Bearing ISS-A Inner Raceway (Mild Contact 

Corrosion) Photo from GEARTECH, NREL/PIX 19746
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Observations 

 Gear failure database incidents:

• 37 among which 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 

 GRC damaged test gearbox: 

• 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.
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Summary 

 Motivation and technical approach for the GRC 

 Motivation, objectives, approach and features of the 

gearbox failure database

 Summary of incidents recorded in the database and 

detailed damage of one GRC test gearbox, along with 

some discussions

 Be careful with generalization of the results obtained 

based on the database, as population is still small 
25

References 

1. Link, H.; LaCava, W.; van Dam, J.; McNiff, B.; Sheng, S.; Wallen, R.; McDade, M.; 

Lambert, S.; Butterfield, S.; Oyague, F. (2011). Gearbox Reliability Collaborative 

Project Report: Findings from Phase 1 and Phase 2 Testing. 88 pp.; NREL Report 

No. TP-5000-51885. 

2. GEARTECH, Bearing Failure Analysis – A Textbook for the Bearing Failure Analyst. 

3. GEARTECH, Gear Failure Analysis – A Textbook for the Gear Failure Analyst.

4. Sheng, S. (2011). “Wind Turbine Gearbox Condition Monitoring Round Robin.” 

presented at the 2011 Wind Turbine Condition Monitoring Workshop, 

Broomfield, CO, September 19-21, 2011.

5. GEARTECH, GEARTECH Report No. 2443 – Failure Analysis of NREL Field Test 

Gearbox No. 1, January 21, 2011.
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Thank You!

NREL’s contributions to 

this presentation were 

funded by the Wind and 

Water Power Program, 

Office of Energy Efficiency 

and Renewable Energy of 

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/PIX 17855 

shuangwen.sheng@nrel.gov 

303-384-7106