NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Gearbox Typical Failure 

Modes, Detection and 

Mitigation Methods

Shawn Sheng 

National Renewable Energy Laboratory/National Wind 

Technology Center 

AWEA Operations & Maintenance and Safety Seminar

January 15–16, 2014

NREL/PR-5000-60982
2

Outline

 Background

 Gearbox Typical Failure Modes

• Bearings

• Gears

• Lubricant 

 Detection Techniques 

• Real-time 

• Periodic 

 Mitigation Strategies

• Bearings

• Gears

• Lubricant 
Background
4

Gearbox Reliability Challenges

 Gearboxes do not always 

achieve their 20-year design 

life 

 Premature gearbox failures:

• Are widespread

• Affect most original equipment 

manufacturers 

• (Most) are not caused by 

manufacturing practices, except 

grind temper and nonmetallic 

inclusions 

 Gearbox largest contributor

to turbine downtime and

costliest to repair*

* Sheng, S. (2013). “Report on Wind Turbine Subsystem Reliability – A Survey of Various Databases.” June 2013. NREL/PR-5000-59111. 
5

Gearbox Damage

 Gearboxes can fail in drastically different ways

 Data: 257 gearbox damaged records 

 Bearings: 70%; gears: 26%; and others: 4% 

 Observations: 

• Both bearing and gear failures are concentrated in the parallel section

• Top failure mode is high-speed shaft (HSS) or intermediate-speed shaft (IMS) 

bearing axial cracks

Damage 

Records Bearings Gears Others

Planetary 44 23 21

IMS N/A 34 9

47

HSS N/A 123

Total 257 180 68 9
Typical Failure Modes
7

A Representative Configuration 

7

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

Typical Failure Modes: Bearings

 Axial Cracks

• Recent failure mode of significant proportions

• Most appear initiated with irregular white 

etching areas

• Manifests as axial cracks within the roller path

• Progresses to macropitting, split inner ring, or 

both

Scuffing

Photo Credits: Bob Errichello, GEARTECH, Johan Luyckx, Hansen Transmissions, Andy Milburn, Milburn Engineering, and Ryan Evans, 

Timken

Macropitting

Denting/Point-surfaceorigin (PSO) macropitting

Fretting corrosion 

 

False brinelling
9

Typical Failure Modes: Gears

Subsurface initiated bending fatigue 

(due to nonmetallic inclusion, 

intermediate-speed stage pinion)

Scuffing (high-speed stage pinion) 

Photo Credit: Bob Errichello, GEARTECH

Fretting corrosion (sun pinion) Micropitting/macropitting
10

Typical Failure Modes: Lubricant

Plot Credit: Bill Herguth, Herguth Laboratories

% Additives vs. Viscosity @ 40°C cSt D-445

R&O Mineral Oil

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

0 36 72 108 138

Days Aged in Turbine Oil Stability Test D-943

Percent of New Oil Additive

30

70

110

150

190

230

270

310

350

390

430

470

510

Viscosity @ 40

°C cSt

48.27 cSt

% Electrooxidizable Additives

Viscosity @ 40°C

New Oil Vis. = 46.70
11

Typical Failure Modes: Lubricant (Cont.)

 Contaminations include:

• Wear debris

• Dust

 Foam

• Breathing of 

housing

• Air trapped in oil 

• Not enough 

settling time

Photo Credits: Bill Herguth, Herguth Laboratories, and Art Miller, EDFR
Detection Techniques
13

Real-Time Continuous 

 Critical SCADA data trending: • Temperature of high-speed stage 

bearing, targeting surface damage 

modes: pitting, scuffing, and so 

on

 Vibration analysis: • Dedicated condition monitoring 

system with sensor bandwidth up 

to more than 10 kHz

• Effective for high-speed shaft, 

intermediate-speed shaft bearing, 

and gear surface damage modes: 

axial cracks, macropitting, 

scuffing, and so on 

• Earlier detection than oil debris 

analysis, but challenging to detect 

damage to planetary stage 

bearings and gears
14

Periodic 

 Oil sample analysis 

• Six months or annual oil samples sent to dedicated laboratories for typical physical 

and chemical property analysis: viscosity, additives depletion, and so on 

• Debris element analysis may pinpoint failed components: bearings, gears, and so on 

• Good for oil condition, not very good for conditions of bearings and gears 

 Filter element analysis 

• Old filter cartridge can be sent to dedicated laboratories for analysis when it is 

replaced with a new one 

• Reveals what is typically missed in conventional oil sample analysis

• Identifies composition of wear debris and pinpoints damaged components: bearings, 

gears, and so on

Reference Limits Analysis Results
15

Periodic (Cont.)

 Portable vibration sweep

• Sensors are similar to what are used by permanently installed 

vibration analysis systems

• Typically conducted during end-of-warranty (EOW) inspections 

targeting the same damage modes as permanently installed vibration 

systems

• Difficult to track down failure history 

 Borescope inspection 

• Dedicated scope to inspect all accessible stages of gearboxes

• Normally triggered by abnormal symptoms on the gearboxes 

detected by other methods and used to confirm the damage, 

evaluate its severity level, and inform maintenance planning: damage 

modes observable on component surfaces, pitting, scuffing, and 

problems with the lubricant such as foaming 

• Another scenario for conducting borescope inspections is during an 

EOW sweep 

• Depending on gearbox configuration, the inspection may not be 

thorough because of the difficulty in accessing and photographing 

certain components
Mitigation Strategies
17

Bearing Axial Cracks 

 Carburized instead of through hardened bearings1

 For carburized bearings, the level of retained 

austenite is recommended to be at least around 

20%1

 Assembly temperature of bearing inner rings 

around 120 degrees Celsius2

 Reduce transient event frequencies and their 

magnitudes

 Keep lubricant cool, clean, and dry

1. Errichello, R.; Budny, R.; Eckert, R. (2013) “Investigations of Bearings Failures Associated with White Etching Areas (WEAs) in Wind 

Turbine Gearboxes.” Tribology Transactions, Vol. 56, No. 6, pp. 1069-1076, DOI:10.1080/10402004.2013.823531

2. Luyckx, J. (2011) “Hammering Wear Impact Fatigue Hypothesis WEC/irWEA Failure Mode on Roller Bearings.” Wind Turbine 

Tribology Seminar, Broomfield, CO. 
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Bearing/Gear Micropitting 

 Grind/hone/polish gear teeth

 Superfinish, coat, or smooth bearing rings or elements

 Avoid shot-peened flanks

 Make hardest gear/bearing element as smooth as 

possible

 Make pinion 2 HRC points harder than gear

 Use oil with high micropitting resistance

 Keep lubricant cool, clean, and dry

 Use high-viscosity lubricant

 Operate at high speeds

 Coat teeth with phosphate, Cu, or Ag

 Run-in with special lubricant [w/o ZDDP (Zinc Dialkyl 

Dithio Phosphate)] at controlled loads

Errichello, R. (2011) “Wind Turbine Gearbox Failures.” Wind Turbine Tribology Seminar, Broomfield, CO.
19

Gear Scuffing

 Optimize gear geometry/accuracy

 Use nitrided steel

 Grind/hone/superfinish gear teeth 

 Coat gear teeth with phosphate, Cu, Ag

 Use high-viscosity antiscuff lubricant 

 Cool gear teeth with lubricant

 Run-in new gear-sets at reduced loads

Errichello, R. (2011) “Wind Turbine Gearbox Failures.” Wind Turbine Tribology Seminar, Broomfield, CO.
20

Lubricant Contamination 

 Pre-filter oil

 Run-in oil flushing 

 Main loop filtration 

 Offline loop filtration 

 Breather 

 Heat exchanger 

 Periodic seal examinations 

Photo Credit: Don Roberts, DA Roberts LLC. 
Summary
22

Summary 

 Wind turbine gearboxes can fail in dramatically different ways.

 Improvements in reliability and availability have to take a 

holistic approach involving design, manufacturing, testing, 

packaging/shipping/handling, installation, operation, and 

maintenance. It also involves almost all stakeholders, gearbox 

OEMs, turbine OEMs, owners/operators, research institutes, 

governmental agencies, and so on. 

 Each detection technique has its own advantages and 

limitations. The same is true for oil sample/filter element 

analysis, and end users need to come up with a solution that is 

the most economical and effective for their assets. Note that 

one solution for one plant may not apply to a different site. 

 Tracking and knowing your fleet condition through various 

instrumentation and data mining are critical. 
23

Gearbox Reliability Improved 

 Both 2007 and 2012 data indicate the gearbox as the highest downtime driver, 

but this was reduced by 67.7% in 2012 with respect to 2007

 Most subsystems show improved reliability and total downtime caused by all 

subsystems per turbine; in 2012 shows 47.1% reduction with respect to 2007

 Wind Stats data from Germany: about 5,000 turbines, majorities megawattscale
24

Thank You!

NREL’s contributions to 

this presentation were 

funded by the Wind and 

Water Power Program, 

Office of Energy Efficiency 

and Renewable Energy, 

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 17855 

shuangwen.sheng@nrel.gov

303-384-7106

Valuable reviews and comments from Bob Errichello are greatly appreciated!