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Gearbox Reliability Collaborative Update

Sandia Reliability Workshop

Albuquerque, NM

August 13-14, 2013 

NREL/PR-5000-60141

Shawn Sheng and Jon Keller

National Wind Technology 

Center, NREL 

Chad Glinsky

US Technical Center 

Romax Technology
2

Outline

 Introduction 

• “Classic” Statistics 

• New Results 

• Gearbox Reliability Collaborative (GRC) 

 Status Update

• Key Failure Modes 

• Lessons Learned

• Design Changes in Gearbox #3

• Test Summary 

• Key Findings 

• Schedule

 Next Steps for the GRC

 Several Other Reliability Research and Development (R&D) Activities 
Introduction
4

“Classic” Statistics

 WMEP and LWK databases 

represent ~2,150 turbines [1]

 Data ranging from 1993 to 

2006, when both databases 

were closed [1] 

 Gearboxes do not fail often 

but cause the highest 

downtime [1]

 Wind Stats: aggregated downtime per turbine 

subsystem from 2003 to 2009 [2]

 Based on the data for the first quarter of 2010, it 

represents about 27,000 turbines, ranging from 

500 kilowatts (kW) to 5 megawatts (MW) [2]

 Highest downtime caused by gearboxes; reliability 

of generators and main bearings may also need to 

be considered because of crane costs [2]
5

New Results: Wind Stats [3]

 Aggregated downtime per turbine subsystems from 2003 to 2007 (left) and 

from 2008 to 2012 (right): 

• Both periods indicate the gearbox as the highest downtime driver

• The 2008−2012 period shows less downtime than the 2003−2007 period for most 

subsystems

• The top four drivers stay the same with a little variation in sequence: gearbox (1 = >1), 

generator (2 = >4), electric systems (3 = >2), and rotor (4 = >3)
6

New Results: Gearbox Failure/Reliability Database [3]

 Gearbox failure event data highlighting damaged components, failure modes, and possible 

root causes 

 About 20 partners involved, including turbine/gearbox manufacturers, owners/operators, 

gearbox rebuild shops, and operation and maintenance (O&M) service providers

 Assets owned by owner/operator partners represent ~31% of the United State’s end of 

2012 capacity

 The database contains 289 gearbox failure incidents with 257 confirmable damage records 

(Note: one incident may have multiple damage records and inconsistent data reporting)

 Observations: 

• Gearboxes fail in different ways

• Bearings: ~ 70%; Gears: ~ 26%; and 

Others: ~ 4% 

• Both bearing and gear faults are 

concentrated in the parallel section

• Top gearbox failure mode is highspeed shaft (HSS) or intermediatespeed shaft (IMS) bearing axial cracks
7

Gearbox Reliability Collaborative (GRC) 

 Gearboxes do not always achieve their 20-year design life

 Premature failure of gearboxes 

increases the cost of energy 

through:

• Extended turbine downtime

• Unplanned maintenance 

• Gearbox replacement and rebuild

• Increased warranty reserves

 The problem:

• Is widespread

• Affects most original equipment 

manufacturers (OEMs)

• Is not driven by manufacturing defects Illustration by NREL

Industry-wide collaboration is needed, but...
8

GRC (Continued) 

 Technical approach:

• Modeling and analysis 

• Field test 

• Dynamometer test 

• Failure database

• Condition monitoring 

 Goal:

• To improve gearbox 

reliability and decrease 

O&M costs, which will 

reduce the cost of energy 

Field Test Dynamometer Test

• Test plan

• Test article

• Test setup and 

execution

• Test plan

• Test turbine

• Test setup and execution

Analysis

• Load cases

• System loads

• Internal loads

NREL Dynamometer/Photo by Lee 

Jay Fingersh, NREL 16913

Test Turbine at NREL/Photo by Dennis Schroeder, NREL 19022

http://www.nrel.gov/wind/grc/ Illustration by NREL
9

GRC (Continued)

 Use smaller gearbox to control costs: 750 kW

 Lack of public models → redesign/rebuild gearbox

• First iteration brings to state-of-the-art circa 2007: gearbox 1 and 2

• Second iteration brings to state-of-the-art circa 2012: gearbox 3

 Significant internal and external instrumentation

• Main shaft, gearbox, coupling, and generator displacements

• Planetary section loads

• High-speed shaft, pinion, and bearing loads recently added

Photo by Lee Jay Fingersh, NREL 16913
Status Update
11

Segue Into GRC Updates

• Industry Experience – Key Failures and Observations

o Planetary bearings

o Manufacturing defects

o Gear teeth

o Bearing axial cracks

o Sun splines

• GRC Gearbox History

o Gearboxes 1 and 2 lessons learned

• GRC Gearbox 3

o Key design improvements
12

Key Failure Modes – Planetary Bearings

• Most costly repair

• Commonly due to rolling contact fatigue

• Bearings are designed close to the margins (safety factor close to 1)

• Excellent filtration can save the rest of the gearbox

rollers

severe pitting raceway

gearbox filter

debris from failing gears/bearings

Photos by Romax Technology
13

Key Failure Modes – Planetary Bearings

• Poor load sharing reduces bearing life

• Clearances, tolerances, deflections, and bearing selection affect load sharing

• GRC redesign to preloaded taper roller bearings (TRB) to improve load sharing

Upwind (UW)

Downwind (DW)

row row row row

Photos from Romax teardown

Photos by Romax Technology
14

Key Failure Modes – Manufacturing Defects

• Manufacturing defects

o Nonmetallic inclusions

o Microstructural banding

o Inconsistent induction hardening depth

o Gear-grinding temper

• Defects alone are not a failure mode, 

but serve as the initiation of failure

• More common in gears than bearings

poor deoxidation process

nonmetallic 

(oxide) inclusions

nucleation point 

of failure

0.025 mm

crack along inclusions

1.0 mm

indents from hardness tests

banding in case

gear surface

poor heat treat

banding in 

case-hardened 

layer

reduced local 

hardness

Photos by Romax Technology
15

Key Failure Modes – Gear Teeth

• Gear rating standards assume that the designer has good 

quality steel and a well-controlled heat treat process

Tooth fatigue crack

0.025 mm Crack along inclusions

Missing tooth

Housing joint failure

Liberated gear teeth

Liberated tooth jam

1 2

3

4

5

6

Photos and illustration by Romax Technology
16

Key Failure Modes – Bearing Axial Cracks

• Widespread problem with unknown root cause

o Topic of research

o Some OEMs are moving to case carburized bearings

• Occurrences typically on inner race of cylindrical roller bearings 

(CRBs)

o Current rating standards do not cover this failure

o White etching areas (local hardening) seem to serve as nucleation points

Inner ring axial crack

Boroscope of axial crack

Photos by Romax Technology
17

Note About Sun Splines

• Commonly show signs of fretting and pitting

o Difficult to lubricate

• Not a common failure mode

o Debris generated is ideally flushed away with minimal 

consequential damage to gears and bearings

Sun spline flank wear

Photos by Romax Technology
18

Brief History Lesson

• GRC Gearbox 1 and 2

o Gearbox 1: Dynamometer and in-field testing 

o Gearbox 2: Dynamometer testing

• Key Items:

1. Misalignment between carrier and ring gear

– Application: rotor-bending moments

– Design: operating radial internal clearance in planet carriers (PLCs)

2. Poor load share between upwind and downwind planetary bearings

– Application: torque load with carrier windup and planetary pin deflection

– Design: operating radial clearance in the two single-row CRBs

3. Oil feed into rotating frame (planetary stage)

– Application: off-axis and transient loads

– Design: distribution ring sensitivity to carrier misalignment; jams with poor 

sealing

Illustration by Romax Technology
19

Carrier Bearings

Planet Bearings

Ring/Torque 

Arm/Housing 

Joint

Ring Gear

Hollow Shaft 

Bearings Improved 

Reliability

Lubrication

Illustration by Romax Technology

Key Design Improvements (GRC Gearbox 3)
20

TRBs used to 

improve planetary 

alignments with 

load variation

Robust bolt system 

to accommodate 

increased housing 

load from TRBs

Nitrided to 

improve fatigue life

Integral TRBs; 

improve load share 

and eliminate 

outer race fretting

X-arrangement; 

tighter inner rings 

to avoid slip

Carrier radial feed;

three feeds center 

plate;

Spray all gears;

high-speed TRBs axial 

feed Illustration by Romax Technology

Key Design Improvements (GRC Gearbox 3)
21

Test Summary

 Phase 1 (300+ hours of data) included:

• Gearbox 1 dynamometer test - 125+ signals

• Gearbox 1 field test

• Oil loss event led to gearbox damage

 Phase 2 (700+ hours of data) included:

• Gearbox 2 dynamometer test - 150+ signals

• Dynamic torque and some dynamic non-torque 

loads

• Gearbox 1 dynamometer test

• Condition monitoring evaluation and gearbox 

teardown

 Phase 3 (underway) includes:

• Gearbox 2 retest with high-speed section instrumentation

 Field loads (normal power production and transient); generator misalignment

• Gearbox 3 test

 Replace planetary CRBs with preloaded TRBs

April

2009

Oct

2009

July 

2010

Nov

2010

Jan 

2011

July 

2013

July 

2014

Photo by Jeroen van Dam, NREL 

19257
22

Key Findings

 Three-point drivetrain design sensitive to nontorque loads 

• Nontorque load (bending) disturbs load sharing among planetary 

gears and between planet-bearing rows

• Main shaft misalignment can cause pitting on the ring gear

• Non-torque load (thrust) affects planet carrier position 

 Controller adjustments to reduce torque spikes

 External gauges can indicate tooth contact pattern

 High bearing skidding risks at low torque

 On-line particle counting can be used for run-in

 Reliability improvement needs a comprehensive approach 

that can include:

• Design and testing 

• Metallurgy and material

• Operation and maintenance 

http://www.nrel.gov/wind/grc/publications.html
23

Schedule 

 Main housing, ring gear, shafts and gears, and so on

• Contract award August 2013

• Instrumentation installation April 2014

• Gearbox acceptance/completion May 2014

 Planet gear/bearings

• Planet inners manufactured September 2013

• Planet inner strain gages calibrated October 2013

• Planet gears manufactured December 2013

• Delivery to gearbox manufacturer January 2014

 Dynamometer testing 

• Gearbox 2 with added HSS instrumentation, September 2013

• Gearbox 3, June 2014 
Next Steps
25

GRC – FY14 and Beyond

 Draw down current GRC 750-kW project

• Complete build and test of gearbox 3

• Validate design tools used for design improvements → increase in L10 life 

for gearbox bearings

 Launch new GRC 1.5 project

• Field test 1.5-MW turbine (highest interest to GRC members)

 Offers of in-kind cost share from industry

• Gather main shaft (input) and high-speed shaft (reaction) loads

 Focused measurement and modeling → NREL reference load distribution

 Needed by industry for design improvements

• Use 2.5-MW dynamometer, U.S. Department of Energy (DOE) 1.5-MW 

turbine, and NWTC controllable grid interface (CGI) as stepping stones to 

conduct field testing in wind farm(s)

Meet Industry Desires with Applied Testing, Analysis, and Modeling 
Several Other Reliability R&D 

Activities
27

Testing 

Siemens

2.3 MW

Alstom

3 MW

DOE/GE

1.5 MW

Gamesa

2 MW

Control Advanced 

Research Turbines

2 x 600 kW 

PV Array

1.1 MW

2.5-MW

dynamometer

 Field testing of commercial megawatt scale and small (under 100 kW) wind 

turbines

 Dynamometer testing facilities: 250 kW, 2.5 MW, and 5 MW 

 Static and fatigue tests of blades 

 Grid compliance testing: a 7-MW controllable grid interface (CGI)

 Multimegawatt energy storage testing capability under development 

5-MW

dynamometer,

7-MW CGI

Photo by Vahan Gevorgian, NREL 
28

Data Collection and Modeling 

 Gearbox reliability database => drivetrain major 

subsystems/components reliability database => O&M research 

[3,4]

• Highlight failure locations, failure modes, and possible root causes 

• Opportunities: data sharing and solution packages (from fault 

detection to maintenance recommendation) R&D, and so on 

 Historical operational and expense cost data and major 

component replacements data collection [5]

• Collected by DNV KEMA and GL GH for NREL and the combined data 

represents 10 GW of U.S. capacity 

• Opportunities: results update and validation, and so on 

 Offshore O&M cost modeling [6]: 

• Energy Research Center of the Netherlands (ECN) tool purchased by 

NREL and baseline studies for U.S. offshore project conducted 

• Opportunities: data sharing and model validation, and so on 
29

References 

1. Tavner, P. “How Are We Going to Make Offshore Wind Farms More Reliable?” 

Presented at the 2011 SUPERGEN Wind General Assembly on March 20, 2011 

at Durham University, UK. 

2. Sheng, S. “Investigation of Various Wind Turbine Drivetrain Condition 

Monitoring Techniques.” Presented at the 2011 Wind Turbine Reliability 

Workshop, August 2−3, 2012 Albuquerque, NM. 

3. Sheng, S. “Report on Wind Turbine Subsystem Reliability - A Survey of Various 

Databases.” NREL/PR-5000-59111.

4. Sheng, S. “Wind Turbine O&M and Condition Monitoring Research.” 

Presented at the 2013 GRC All Member Meeting, February 4−5, 2013, 

Golden, CO. 

5. Lantz, E. “Operations Expenditures: Historical Trends and Continuing 

Challenges.” Presented at AWEA Wind Power Conference, May 5−8, 2013, 

Chicago, IL. NREL/PR-6A20-58606. 

6. Meadows, R. Offshore O&M Cost Drivers - A U.S. Case Study, NREL Report No. 

TP-5000-58908 (forthcoming).
30

Thanks for your attention! 

HC Sorensen, Middelgrunden Wind Turbine 

Cooperative, Photo by HC Sorensen, NREL 17855 

shuangwen.sheng@nrel.gov, 303-384-7106

jonathan.keller@nrel.gov, 303-384-7011 

chad.glinsky@romaxtech.com, 303-351-5418

Special thanks to the U.S. Department of Energy and the 

GRC project partners.