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 Reliability Collaborative -

Phase 1 and 2 Overview

Sandia Turbine 

Reliability Workshop

Aug 2-3, 2011

Hal Link

NREL/PR-5000-52463

NREL PIX # 19222 
NATIONAL RENEWABLE ENERGY LABORATORY

Why GRC?

• Wind turbine gearboxes do not last their 20 year design life

• Designs must meet design, manufacturer, and testing 

standards and certification requirements

• Problems have occurred throughout the wind industry

• Problems range across the turbine design cycle 

• load characterization 

• design methods 

• model fidelity

• testing/validation methods

• Competition inhibits communication and joint solutions

2
NATIONAL RENEWABLE ENERGY LABORATORY

Major Elements of GRC

• Collaboration - form a team of stakeholders including 

manufacturers, owners, researchers

• Generic drivetrain - Identify and investigate 

“representative” test gearboxes

• Testing - model validation, load characterization, 

detailed gearbox, and subcomponent response

• Modeling - improve design tools and understanding

• Failure Database - document and analyze 

failures/modes

• Condition Monitoring - investigate use of continuous 

inspection as design and O&M (operation and 

maintenance) tool to improve reliability

3
NATIONAL RENEWABLE ENERGY LABORATORY

Test Article Selection and Redesign

4

• 750kW - easy to test, 

representative, generic

• 2 gearboxes for field and 

dyno testing

• 3 point mount, 1 planet 

stage, 2 parallel

• Redesigned with expert 

input

• Upgraded using experience 

of design and rebuild 

experts

• active lubrication with kidney loop

• improved tooth profile and finish

• improved bearing arrangements
NATIONAL RENEWABLE ENERGY LABORATORY

Instrumentation (and lots of it)

5

• Planet load distribution (12 gauge pairs per planet)

• Planet race temperature (six sensors per planet)

• Planet rim movement (6 sensors on two planets) 

• Mainshaft bending and torque

• Ring gear load distribution

• Carrier deflection/movement

• Sun radial movement

• Gearbox movement relative to mainframe

• High speed shaft (HSS) bearing temperatures

• HSS torque

• HSS / Generator alignment
NATIONAL RENEWABLE ENERGY LABORATORY

Instrumentation – Ring Gear Tooth Load Distribution

6

Strain gauge installation 

in ring gear of GRC 

Gearbox . 

NREL PIX #19495 (top). 

NREL PIX #19494 (left).
NATIONAL RENEWABLE ENERGY LABORATORY

Testing

7

Early tests

Field test: torque and modal characterization

Planet bearing calibration

Trunnion stiffness characterization

Phase 1

Dyno Gearbox 1: run-in and instrumentation checkout

Field Gearbox 1: loads, responses, events

Dyno Gearbox 2: run-in, limited non-torque loading

Phase 2

Dyno Gearbox 2: extensive Non-Torque Loads (NTL), 

dynamics, HSS misalignment

Dyno Gearbox 1: NTL, condition monitoring
NATIONAL RENEWABLE ENERGY LABORATORY

Data Analysis

• Did we measure what we wanted accurately?

• Did we achieve the test conditions we wanted?

• What data are available and where?

• Can the data be released for model validation?

• Were unexpected loads observed?

8
NATIONAL RENEWABLE ENERGY LABORATORY

Gearbox Modeling

9

Source: Ansol (Calyx)

Gearing Analysis

• Gear tooth loading 

• Gear mesh stiffness 

• Gear tooth contact 

stress 

Shaft Analysis

• Torsional deflections

• Bending defection

and misalignment

Planet Carrier Analysis

• Torsional deformation of the

planet carrier 

• Misalignment the planet pin

• Planet carrier and pin interaction

Housing Analysis

• Deflections

• Misalignment 

• Tolerance stack up 

• Virtual modal testing 

Bearing Analysis

• Bearing stiffness 

• Roller contact stress

• Roller load distribution

• Bearing life
NATIONAL RENEWABLE ENERGY LABORATORY

Gearbox Failure Database

Why?

• Connects testing 

and simulations 

to actual failures

• Helps to identify 

root causes of 

failures

• Sanitized data 

can be shared 

among GRC 

members

Status

• Development of standardized 

collection software

• Signed partners: 17% of US 

generating capacity represented

10
NATIONAL RENEWABLE ENERGY LABORATORY

Findings

1. NTL (bending) affects planet/ring mesh

2. On-line particle counting for run-in

3. Controller adjustments to reduce torque spikes

4. NTL (thrust) affects carrier position

5. Planet bearing load share varies significantly

6. External gauges can indicate tooth contact pattern

0 50 100 150 200 250 300 350 0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

Main Shaft Azimuth [deg]

K

γ

Planet Load Share vs Main Shaft Rotation

A

B

C

0 20 40 60 80 100 120 50

100

150

200

250

300

Torque [kNm]

Time [sec]

Field Torque

Dyno Torque

11
NATIONAL RENEWABLE ENERGY LABORATORY

External Gauges Facilitate Tooth Load Measurements

• External gauges are MUCH easier to install

• Last longer and are repairable

• They accurately indicate the centroid of the 

contact area and, potentially, edge-loading

• They distort measurement of the important design 

parameter, Khß

0

0.2

0.4

0.6

0.8

1

1.2

0 50 100 150 200 250

Load Distribution

Width Across Ring Gear [mm]

NREL

OSU

Romax

NREL PIX # 19221

12
NATIONAL RENEWABLE ENERGY LABORATORY

Non-Torque Loads Affect Gearing

• Non-torque loads DO affect the gearbox

• Mainshaft bending loads up to 180 kNm in all 

azimuths simulating unbalanced rotor loads

• Caused significant increase in tooth edge loads

<-Upwind Downwind-> 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Gear Face Width

μStrain

Ring Gear Load Distribution at 120°

MY

 = -180 kNm

MY

 = 0 kNm

MY

 = 170 kNm

<-Upwind Downwind-> 0

200

400

600

800

1000

1200

1400

1600

1800

2000

Gear Face Width

μStrain

Ring Gear Load Distribution at 240°

MY

 = -180 kNm

MY

 = 0 kNm

MY

 = 170 kNm

13
NATIONAL RENEWABLE ENERGY LABORATORY

Next Steps in GRC

• Continue data validation and analysis

• Use data and experience to identify design 

sensitivities for next re-design

• Redesign and rebuild damaged Gearbox 1 

-> Gearbox 3

• Start Phase 3 testing in the dyno and field

• Convene a tribology / lubrication workshop 

in November 2011 with experts and shared 

WTG experiences from field

14
NATIONAL RENEWABLE ENERGY LABORATORY

Questions / Comments?

15