NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

NREL Gearbox Reliability 

Collaborative: Comparing InField Gearbox Response to 

Different Dynamometer Test 

Conditions

Preprint

William LaCava, Jeroen van Dam, 

and Robb Wallen

National Renewable Energy Laboratory

Brian McNiff

McNiff Light Industry

Presented at WINDPOWER 2011

Anaheim, California

May 23-25, 2011

Conference Paper

NREL/CP-5000-51690

August 2011
NOTICE

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Table of Contents

List of Figures.............................................................................................................................1

List of Abbreviations..................................................................................................................2

1 Summary..........................................................................................................................................3

2 Background......................................................................................................................................3

3 GRC Test Program Description.......................................................................................................4

3.1 Overall Test Program................................................................................................................4

3.2 Field Testing .............................................................................................................................6

3.3 Dynamometer Testing ..............................................................................................................8

3.4 Measurement Setup ..................................................................................................................9

4 Data Analysis and Processing........................................................................................................11

4.1 Comparison of Field and Dynamometer Gearbox Response..................................................11

Main Shaft Torque....................................................................................................................11

Main Shaft Bending..................................................................................................................12

Planet Bearing Strains...............................................................................................................15

4.2 Simulating Field Loading Conditions in the Dynamometer...................................................18

Dynamic Torque Control..........................................................................................................18

Dynamic Non-Torque Loading Control....................................................................................19

5 Conclusions....................................................................................................................................21

6 Future Plans...................................................................................................................................22

7 Acknowledgments .........................................................................................................................22

8 References......................................................................................................................................22

Appendix A: Coordinate System......................................................................................................24

List of Figures

Figure 1. GRC drivetrain. ..................................................................................................................5

Figure 2. Wind speed histogram - field test data. ..............................................................................7

Figure 3. LSS torque vs. wind speed - field test data. .......................................................................7

Figure 4. NREL/PIX18952: Non-torque load mechanism in NREL dynamometer..........................8

Figure 5. Main shaft gauges - example..............................................................................................9

Figure 6. Gauges in machined slots in planet bearing inner rings...................................................10

Figure 7. Ring gear load distribution measurement.........................................................................10

Figure 8. Variations in LSS torque in field and dynamometer tests................................................11

Figure 9. FFT of LSS torque in field and dyno tests. ......................................................................12
2

Figure 10. Field and dyno test shaft bending vs rotor azimuth........................................................12

Figure 11. FFT of Y and Z components of shaft bending in field and dyno testing........................13

Figure 12. Effect of applied shaft moment on ring gear load distribution.......................................14

Figure 13. Load centroid sensitivity to Y bending. .........................................................................14

Figure 14. Similar planet bearing load magnitudes (at TDC)..........................................................15

Figure 15. FFT of peak-valley strain magnitudes............................................................................16

Figure 16. Bearing strain signal modulation with LSS/carrier rotation...........................................16

Figure 17. Planet load share derived from bearing loads.................................................................17

Figure 18. Planet A bearing loading vectors (at gauges).................................................................18

Figure 19. Time series of field torque data reproduced in dynamometer testing. ...........................19

Figure 20. FFT of field torque data reproduced in dynamometer testing........................................19

Figure 21. Dynamic NTL control diagram. .....................................................................................20

Figure 22. Time series of field shaft bending reproduced in dynamometer testing.........................20

Figure 23. FFT of field shaft bending reproduced in dynamometer testing. ...................................21

Figure 24. Coordinate system for gearbox and nacelle....................................................................24

Figure 25. Main shaft moment orientation and reference................................................................24

Figure 26. Negative gearbox pitch moment (-Y) reference.............................................................25

List of Abbreviations

DOE Department of Energy

NREL National Renewable Energy Laboratory

TDC Top Dead Center

NTL Non-Torque Load

FFT Fast Fourier Transform

GRC Gearbox Reliability Collaborative

IEC International Electrotechnical Commission

LDD Load Data Distribution

LSS Low Speed Shaft (Main Shaft)

HSS High Speed Shaft

CRB Cylindrical Roller Bearing

VFD Variable Frequency Drive

Dyno Dynamometer

UW Upwind

DW Downwind

Np N repetitions per revolution of the main shaft

Vin Cut-in wind speed

Vrated Maximum rated wind speed 
3

1 Summary

The NREL/ DOE Gearbox Reliability Collaborative has been reviewing the design process of wind 

turbine gearboxes in order to identify ways to improve the reliability of this fundamental 

component. As part of this effort, two 750-kilowatt (kW) gearboxes were removed from an 

operating population and redesigned and rebuilt to meet current megawatt (MW) power standards 

using state-of-the-art technology. To date, 300+ hours of operational data, including over 125 

signals, have been collected in the field and from NREL dynamometer testing on a 750-kW wind 

turbine platform. This data includes both internal and external loads, motions, deflections and 

other response during a broad range of operating modes and conditions.

This paper compares measured gearbox response in the NREL dynamometer to gearbox response

during field testing. It then describes the way in which dynamometer testing has been improved to 

better represent field loading conditions. Conclusions and recommendations are made for

augmented dynamometer testing.

2 Background

The National Renewable Energy Laboratory (NREL), a U.S. Department of Energy laboratory, has 

undertaken the difficult task of identifying the gaps and sensitivities in the design process that may 

contribute to the premature failure of wind turbine gearboxes. To this end, NREL has brought 

together the different parties involved in the wind turbine and component design, manufacture and 

operation through a program called the Gearbox Reliability Collaborative (GRC) that has been 

described previously (1,2). The GRC has utilized a comprehensive approach to reviewing the 

complete drivetrain design process using current design tools and validating them with operational 

test data.

There are four main GRC research activities: 

• testing with full-scale dynamometer and in the field

• building a database of observed failures

• performing drivetrain design, analysis and modeling

• investigating how condition monitoring can improve reliability.

This paper discusses selected parts of the testing and data analysis activities.

The testing utilized two identical, highly instrumented 750-kW gearboxes with over 125 signals 

each. The gearboxes were removed from the field and rebuilt to current state-of-the-art-design 

including active lubrication, different bearing configurations, modified gear micro-geometry, and a 

floating sun for improved load distribution. Measurements included bearing and gear load 

distributions, internal and external motions, deflections and accelerations, and temperatures.
4

Currently, hundreds of hours of experimental data have been obtained from testing the two 

gearboxes in the NREL 2.5-MW dynamometer facility and on a field test turbine. This data 

provides important information for comparing the behavior of the gearbox components in normal 

wind turbine operating modes with the behavior in dynamometer testing.

Historically, most dynamometer testing has been with steady state applied torque and limited 

(static) non-torque loading. Yet during field operation, wind turbine gearboxes are subject to 

continuously fluctuating loads caused by variations in the wind and control actions. These 

conditions must be considered in the design process (6) but are difficult to validate in a 

dynamometer. The GRC is working to improve the capability of the NREL dynamometer (and 

dynamometer testing in general) to more accurately reproduce in-field operation and response.

Additionally, the GRC data is being used to help improve analytical tools through the partnerships 

formed in the project. A wind turbine gearbox is comprised of many hundreds of moving and 

stationary parts interacting in a complex dynamic environment. Modeling this complex system 

efficiently and verifying that all the parts work together as designed has proven to be difficult (3). 

This is evidenced by problems occurring in the field with many gearbox designs (4,5). High 

confidence design verification through analysis and in a dynamometer could be critical in 

identifying potential problems in this key component at the pre-production prototype stage.

This paper describes the pertinent parts of the GRC field and dynamometer test programs and 

compares the measured gearbox response in the NREL dynamometer to operation in the field. The 

field test provided real wind turbine data which could be replicated in the dynamometer for 

controlled testing. Results are presented from the initial attempts to apply time history-based loads 

from the field to the gearbox in the dynamometer.

3 GRC Test Program Description

3.1 Overall Test Program

The main objective of the GRC tests was to build an understanding of how normal wind turbine 

loading conditions and transient events translate into gear and bearing response, including 

reactions, load distribution, displacements, temperature, stress, and slip. This information will help 

improve bearing selection and integration, gear and bearing design and provide input to relevant 

design and application standards (6,7,8). In the process, the acquired test data will be used to 

validate wind turbine drivetrain, gear, and bearing analytical tools, as well as assess a 

comprehensive matrix of design loads required for proper wind turbine gearbox design.

The wind turbine drivetrain design is shown in Figure 1. The gearboxes feature a standard 

arrangement of first stage planetary and two parallel stages on a 3 point support (main bearing and 

elastomeric trunion supports). Both units were identically instrumented to allow for a broad array 

of measurements to meet the objectives. One of the gearboxes was tested in the NREL 2.5-MW 
5

Dynamometer Test Facility, and the other was installed and operated on an upwind, stallcontrolled, 3 bladed turbine at the Xcel Energy Ponnequin wind plant.

Figure 1. GRC drivetrain.

Achieving the test objectives was highly dependent on making measurements that correctly 

characterized the behavior of the critical drivetrain elements under the various loading scenarios. 

The original measurement goals are listed as follows:

• Relative displacement of planet carrier rim to gear housing

• LSS axial motion displacement relative to gear housing

• Planet load share and annulus gear face width load distribution

• Main shaft azimuth angle to sync with annulus gear stain gauges

• Planet bearing radial load distribution

• HSS axial displacement relative to gear housing

• HSS locating bearing axial load distribution

• Planet gear motion relative to carrier

• Sun pinion radial & axial motion

• Relative motion of gearbox to base frame

• Relative motion of LSS relative to base frame

• Relative motion of HSS relative to generator

• Planet bearing slip

A more complete description of the testing is included in a forthcoming NREL publication (9).
6

3.2 Field Testing

The field test was used to characterize motions and responses of the drivetrain elements during 

normal and unique loading situations. These include transient accelerations and decelerations due 

to rapid increase in wind speed at start-up or control actions such as shifting between generators 

and shutdown.

There were additional signals installed on the field test intended to:

• validate load assumptions for the analysis team to tune the aeroelastic models and modify 

the dynamic and multibody analysis codes

• provide means to verify that testing in the dynamometer can accurately reproduce in-field 

response

• provide direct connection between internal gearbox responses to commonly measured and 

simulated environmental conditions, primary rotor forces and moments and typical control 

situations.

The field test matrix is shown in Table 1 provides a summary of the number of events recorded in 

the field test program.

Table 1. Field Test Matrix – Events or Time

Wind Conditions

Situation Vin Vin<V<Vrated >Vrated

Start up 2 2 2

brake stop 2 2 2

E-stop 2 2 2

Gen shift down 2 2 2

Gen shift up 2 2 2

Accelerations

> 1 rpm/sec

2 2 2

Normal operation 3 x 10 min 3 x 10 min 3 x 10 min

Off yaw operation 3 x 10 min 3 x 10 min 3 x 10 min

Parked 3 x 10 min 3 x 10 min 3 x 10 min

Over 300 hours of data was collected in a broad range of wind speeds and torque levels. Figure 2

and Figure 3 show the collection statistics taken in 10 minute averages. The field test collected 

wind speed conditions up to 26 m/s and torque transients from 735 kNm (200% of rated torque) to

-273 kNm. The range of measurement values allowed for an appropriate selection of dynamometer 

loading conditions that formed the Phase 2 test program.
7

Figure 2. Wind speed histogram - field test data.

Figure 3. LSS torque vs. wind speed - field test data.

0 5 10 15 20 25 30 0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Wind Speed [m/s]

Bin Probability

Raw Data

0 5 10 15 20 25 30 -400

-200

0

200

400

600

800

Wind Speed [m/s]

Torque [kNm]

 q

Max

Mean

Min
8

3.3 Dynamometer Testing 

The dynamometer test was configured to match, as closely as possible, the response and situations 

measured in the field. To accomplish this, a complete turbine main frame was installed in the 

dynamometer facility. 

The 1st phase of dynamometer testing involved run-in, signal checks and static load response and is 

described in the NREL test plan (9). In Phase 2 of dynamometer testing, a rig was constructed to 

apply non-torque forces and moments to the test article (see Figure 4). This system could apply 

dynamic non-torque loading in order to help evaluate the effect on internal gearbox response.

One of the objectives of the Phase 2 dynamometer test was to investigate the effects of increased 

complexity of loading (static and dynamic non-torque loading, torque feedback control, etc) to 

more accurately reproduce in-field response. This included developing a repeatable sequence of 

drivetrain speed and load changes to facilitate the comparison of test setup configuration changes.

Figure 4. NREL/PIX18952: Non-torque load mechanism in NREL dynamometer.

The basic test matrix for the Phase 2 dynamometer testing is shown in Table 2. The load cases 

selected increase in complexity from steady torque to in-field, data driven dynamics. The 

progressive approach of the test plan provided good insight on the loading conditions that generate 

uncharacteristic gearbox behavior that would not be observed in typical dynamometer testing.
9

Table 2. General Test Matrix for Phase 2 Dynamometer Tests

Load case Objective

1 Steady torque Baseline dynamometer testing

2 Steady NTL and steady torque Testing to account for rotor overhang moment

3 Dynamic torque without NTL Tuning of torque control loop at several frequencies

4 Dynamic torque with steady NTL Identify effect of NTL on torque control loop

5 Steady torque with dynamic NTL Tune NTL system, compensation for any 1P effects

6 Dynamic torque and dynamic NTL Simple case to prove concept

7 Simulation of field data with 

dynamic torque and NTL

Demonstrate difference in dynamometer loading and 

field loading

3.4 Measurement Setup

Torque and two orthogonal bending moments (see Figure 5) were measured on the main shaft 

between the main bearing and the gearbox using strain gauges in full bridge arrangements.

Figure 5. Main shaft gauges - example.

For the planet bearings, strain gauges were applied to 3 axial slots machined into the inner surface

of the inner ring of all 6 planet CRB bearings as shown on the left graphic in Figure 6. The slots 

were located at different locations in the bearing load zone for each planet, but they all had slots at 

the orthogonal to the sun-planet axis (referred to here as TDC). Two gauge sets in each slot and 

two bearings on each planet provided an axial distribution of radial loads at 4 locations along each 

planet pin (right graphic in Figure 6). These gauges were calibrated to loads applied to the fully 

assembled planet pins and bearing pairs in a bench-top test setup (10).
10

 

Figure 6. Gauges in machined slots in planet bearing inner rings.

Ring gear load distribution was measured using a cascade of strain gauges placed in the tooth roots 

at 8 axial locations (oriented circumferentially), as shown in Figure 7. Three tooth load 

distributions at 0°, 120°, and 240° were measured for each rotation of the low speed shaft from a 

total of 24 strain gauges. The planets were labeled A, B, and C in counterclockwise fashion with 

Planet A at TDC when the main shaft azimuth angle equaled 0 degrees.

Figure 7. Ring gear load distribution measurement.

A

B C

Positive

Rotation
11

4 Data Analysis and Processing

4.1 Comparison of Field and Dynamometer Gearbox Response

In this clause, gearbox measurements taken while running at 100% load in the field are compared 

to a 100% steady state load case tested in the dynamometer. Signals are compared in the time and 

frequency domains. Additional static dynamometer test cases are identified and used for 

comparison to field variations where indicated.

Main Shaft Torque

Figure 8. Variations in LSS torque in field and dynamometer tests

The gearbox main shaft torque response in dynamometer testing matches the average magnitude of 

the shaft torque in field testing, but there is much less torque variance in the dynamometer (see 

Figure 8). The constant torque applied in the dynamometer excites the shaft at its rotation 

frequency alone (see Figure 8), while the dominating excitation frequency in the field is 3p, 

corresponding to the blade pass frequency. The field torque also shows energy at the drivetrain 

natural frequency and multiples of the blade pass frequency.This is a significant gearbox loading 

difference not captured with a standard time at level dynamometer test.

0 10 20 30 40 50 60 70 280

300

320

340

360

380

400

420

Time [sec]

Torque [kNm]

Field Torque

Dyno Torque
12

Figure 9. FFT of LSS torque in field and dyno tests.

Main Shaft Bending

Figure 10. Field and dyno test shaft bending vs rotor azimuth.

Figure 10 shows the measured shaft bending magnitude for field and dynamometer test cases. The 

gearbox in this 3-point drivetrain support is not completely uncoupled from the rotor moments and 

forces. The main shaft experiences a larger amount of static bending in the field since the overhung 

weight of the rotor is larger than the combined weight of the dynamometer jack shaft, the couplings 

and the non-torque loading equipment. Wind variation in the field imparts shifting asymmetric 

rotor loads as well, and these greatly affect the shaft bending. 

An FFT of the stationary Y and Z components of main shaft bending is shown in Figure 11. It can 

be seen that the main shaft experiences significantly higher bending variation in the field than it is 

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0

2

4

6

8

10

12

Single-Sided Amplitude Spectrum of Data

Frequency [HZ]

Magnitude 

Field Torque

Dyno Torque

3p

9p

ωn

Drivetrain

6p

1p

0 10 20 30 40 50 60 70 50

100

150

200

250

Moment [kNm]

Time [sec]

Field Bend Magnitude

Dyno Bend Magnitude
13

subjected to in standard, torque-only dynamometer testing for the reasons mentioned above. The 

coordinate system used in this document is shown in Appendix A.

Figure 11. FFT of Y and Z components of shaft bending in field and dyno testing.

In order to examine the importance of variations in main shaft bending on the gearbox response, 

seven distinct magnitudes of vertical non-torque loads were applied in the dynamometer while 

operating at 100% power. The ring gear load distribution was then evaluated as a function of the 

resultant main shaft bending moment.

Figure 12 shows the change in ring gear load distribution caused by the bending condition in the 

main shaft. As the Y direction main shaft moment increases, upwind edge loading of the ring gear 

teeth at the 120° location increases. Conversely, downwind edge loading increases at the 240° 

degree location. At both ring gear locations, the increased edge loading at one end of the teeth is 

coupled with a decrease of the load at the other edge.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0

1

2

3

4

5

6

7

8

Single-Sided Amplitude Spectrum of Data

Frequency [HZ]

Magnitude

Field Y Bending

Field Z Bending

Dyno Y Bending

Dyno Z Bending

3p

2p

1p
14

Figure 12. Effect of applied shaft moment on ring gear load distribution.

In order to quantify this edge loading effect, a third order polynomial was fit to the strain 

distributions and the centroid of the area under each load distribution curve is calculated using the 

formula:

Equation 1

where is the fraction of gear face width (0 to1), and is the strain value at that location. By 

plotting these centroid values versus main shaft bending, a strong correlation is observed between 

main shaft Y bending moment and the load centroid on the ring gear at 120° and 240°, as shown in 

Figure 13.

Figure 13. Load centroid sensitivity to Y bending.

Ring gear tooth load misalignment of this type has been shown to have an adverse effect on 

gearbox life (11). Gear tooth edge loading or poor load distribution is a deterrent to gear life that is 

<-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

x dx

x x dx

Centroid ∫

∫ ⋅ = ( )

( )

ε

ε

x ε (x)

R2 = 0.9357

R2 = 0.9613

0.3

0.4

0.5

0.6

-200 -100 0 100 200 Load Centroid 

Main Shaft Y Bending [kNm]

120

240
15

generally controlled by micro geometry and crowning of the gear tooth. These corrections can 

compensate for small deflections and misalignments of the gearing system. However the 

introduction of the bending loads as shown above can potentially undermine the microgemoetry 

and result in undesired loading condition for the gears. Further testing and understanding of the

effects of the bending loads is necessary, however the potential negative effects of non-torque 

loading is evident.

Planet Bearing Strains

The planet bearing strain signals show generally good agreement in magnitude for the field and 

dynamometer test cases with the exception of planet C, as shown in 3 of the signals presented in

Figure 14. By processing the signals into peak-valley ranges and calculating the FFT, a 3p 

magnitude change can be observed in the field data which does not occur in dynamometer testing, 

as shown in Figure 15. However, the main modulation is once per revolution for both cases, and is 

shown in shorter period time series of a raw bearing signal in Figure 16.

Figure 14. Similar planet bearing load magnitudes (at TDC).

0 100 200 300

140

160

180

200

220

240

260

280

300

320

340

Main Shaft Azimuth [deg]

Load [kN]

Planet A UW 0° 25%

0 100 200 300

140

160

180

200

220

240

260

280

300

320

340

Main Shaft Azimuth [deg]

Planet B UW 0°

 25%

0 100 200 300

140

160

180

200

220

240

260

280

300

320

340

Main Shaft Azimuth [deg]

Planet C UW 0°

 25%

Field

Dyno
16

Figure 15. FFT of peak-valley strain magnitudes.

Figure 16. Bearing strain signal modulation with LSS/carrier rotation.

The load share of the planets can be calculated for each time step using these bearing inner ring 

radial load measurements taken at four axial locations in the TDC slot on each planet along the pins

(see Figure 17). The mesh load factor Kγ (12, 13) is calculated for each planet, i, using the equation

∑∫

∫

=

= 3

1

( ) 3

1

( )

i

i

i

i

x dx

x dx

K

ε

ε

γ

Equation 2.

0 0.5 1 1.5 2 2.5 3 0

5

10

15

20

25

30

35

40

Single-Sided Amplitude Spectrum of Data

Frequency [HZ]

Magnitude

Field Planet A UW 0° 25%

Field Planet B UW 0° 25%

Field Planet C UW 0° 25%

Dyno Planet A UW 0° 25%

Dyno Planet B UW 0° 25%

Dyno Planet C UW 0° 25%

2p 3p

1p
17

The Kγ results for the field data are plotted versus the rotation of the main shaft and carrier in 

Figure 17. A value of 1 would indicate that all 3 planets are equally loaded. The time varying 

planet load share is related to planet/sun gear misalignment, and this has been reproduced in 

modeling by Crowther et al (11). Planet load share in this sense is not heavily influenced by 

dynamic loading above 1p.

Figure 17. Planet load share derived from bearing loads.

The dynamic loading conditions experienced in the field may influence planet bearing load zones, 

which experience a dynamic upwind/downwind shift. The helical planet-ring and planet-sun gear 

mesh create a force couple on the planet gear about the axis tangent to the path of its center of 

gravity. This helical moment induces a shift in the upwind and downwind bearing load zones, as 

shown in the bearing load zone schematic of Figure 18(left side). These opposing load zone shifts 

are observed in the strain measurements for planet A upwind and downwind bearings in the field 

(Figure 18, right side).

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
18

Figure 18. Planet A bearing loading vectors (at gauges).

These upwind and downwind bearing load zone shifts, the axial shuttling forces caused by varying 

centroid of the gear mesh forces, and friction forces caused by gear sliding operate at higher 

frequencies than 1p. These behaviors must be studied more in depth to determine whether higher 

order torque and bending loads have a significant effect.

4.2 Simulating Field Loading Conditions in the Dynamometer

Dynamic Torque Control

An attempt to reproduce a main shaft torque time history from the field was conducted. In the 

existing 2.5-MW dynamometer, the control problem proved challenging due to system properties. 

Firstly, the coupled response of the test article and dynamometer exhibited torsional resonances 

within the frequency band of interest. Secondly, the current source VFD driving the dynamometer 

responds slowly to torque command changes, resulting in poor system response and closed loop 

instability in the operating band. The solution was to apply offline, open loop control techniques to 

achieve the best response.

The system was first run with band limited pseudo-random noise in order to obtain the transfer 

function. The input command was then compensated with a calculated error (the difference 

between command torque and output torque) and iterated upon to minimize the error.

A comparison of the field and dynamometer torque measurements is shown in Figure 19. As can be 

seen in Figure 20, the system resonances introduced extra torque dynamics into the testing. The 
19

response was hampered by the slow dynamometer response, but the concept was successfully 

demonstrated nontheless. The lessons learned are being applied to the new 5 MW dynamometer to 

be constructed at NREL in the very near future.

Figure 19. Time series of field torque data reproduced in dynamometer testing.

Figure 20. FFT of field torque data reproduced in dynamometer testing.

Dynamic Non-Torque Loading Control

Non-torque loads were applied in the dynamometer using three closed loop servo-hydraulic 

actuators with displacement and force feedback. The control software translated magnitude and 

direction commands loaded from a time history file into separate actuator forces. The force control 

loops are closed separately for the two actuators. All commands were generated at 100 Hz in open 

0 20 40 60 80 100 120 50

100

150

200

250

300

Torque [kNm]

Time [sec]

Field Torque

Dyno Torque

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0

2

4

6

8

10

12

Single-Sided Amplitude Spectrum of Data

Frequency [HZ]

Magnitude

Field Torque

Introduced Dyno Torque Dynamics

3p

ωn

Drivetrain

1p
20

loop without feedback from the main shaft bending measurements. The control block diagram is 

shown in Figure 21.

Figure 21. Dynamic NTL control diagram.

Main shaft bending magnitude and direction data from the field testing were fed into the NTL 

system to demonstrate the concept. The data was low pass filtered to 2 Hz and a comparison of 

main shaft bending magnitude for the field and dynamometer cases is shown in Figure 22. The 

system calibration contributed to the magnitude error. The time varying 3p excitation was 

reproduced successfully as seen in Figure 23.

Figure 22. Time series of field shaft bending reproduced in dynamometer testing.

65 70 75 80 85 90 95

40

60

80

100

120

140

160

Time [sec]

Shaft Bending [kNm]

Field Magnitude

Dyno Magnitude
21

Figure 23. FFT of field shaft bending reproduced in dynamometer testing.

This new capability in dynamic non-torque loading allows the NREL 2.5-MW dynamometer to 

capture non-torque field loading conditions without the costs associated with in-field test 

operations. Further advancement and analysis of this tool will allow the GRC team to analyze the 

effect of other field and dynamometer variables, such as tower structural vibrations, while forcing 

the system with the equivalent rotor loads it will experience in the field.

5 Conclusions

Gearboxes designed for wind turbines do not always achieve the 20 year design life as required in 

the IEC 61400-1 standard (7). Commercial and intellectual property considerations have hindered

the solution of the problems with the gear and bearing elements, but it is clear that something 

fundamental is missing in the overall process of specifying, designing and validating gearboxes for 

this unusual application. As part of an effort to address these shortcomings, the GRC is evaluating 

dynamometer test approaches that reproduce the loading conditions a gearbox will experience in 

the field.

Two identically rebuilt and instrumented gearboxes were tested on a nacelle in the NREL 

dynamometer and on a turbine in the field. In comparing GRC field and dynamometer test data, we 

have reached the following conclusions:

• Dynamic non-torque load testing is able to reproduce the bending moment variations 

measured in the field.

• The non-torque loads have an appreciable effect on ring gear face width load distribution.

This illustrates the importance of accurately reproducing these loads in dynamometer 

testing.

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0

5

10

15

20

Single-Sided Amplitude Spectrum of Data

Frequency [HZ]

Magnitude

Field Bending Magnitude

Dyno Bending Magnitude 3p
22

• Torque variations in the field are not reproduced in typical dynamometer testing. These 

variations can be reproduced in the dynamometer but limitations in the test equipment must 

be considered because they can introduce unwanted dynamics. The lessons learned in our 

attempts to reproduce field torque time histories will be applied to the control of NREL’s 5 

MW dynamometer.

• The planets experience a 1p cyclical load share variation. Further modeling and analysis 

should reveal whether dynamic loads above 1p have a significant effect on the bearing load 

distribution.

The data sets will eventually become publicly available to those willing to share their analysis. 

Currently, the NREL GRC is working with our analysis partners to use test data to advance the 

state of the art of drivetrain design tools.

6 Future Plans

The knowledge that is gained by analyzing the first phases of testing and comparing that data to the 

modeling efforts will be used in the next phase of the GRC. This phase will consist of a review of 

the design process that was executed as part of the GRC for the first two test articles. In addition 

this next phase will consist of a redesign of one of the gearboxes to make small design changes. 

The redesign will allow for additional instrumentation in those areas where the modeling has 

shown the need for validation. An enhanced field test is also envisioned for the coming year.

7 Acknowledgments

The National Renewable Energy Laboratory’s GRC work is 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 would like to acknowledge the 

contributions of several individuals who have helped formulate the ideas and actions expressed in 

this paper including Sandy Butterfield, Walt Musial, Don McVittie, Bob Errichello, Ed Hahlbeck, 

Francisco Oyague, and Hal Link. In addition, the U.S. Department of Energy is recognized for its 

continued support of this project.

8 References

1. Musial, W. D., Butterfield, C. P. and McNiff, B. “Improving Wind Turbine Gearbox 

Reliability,” Proceedings of EWEC 2007, Milan, Italy, April 2007. 

2. Oyague, F., Gorman, D., and Sheng, S., “NREL Gearbox Reliability Collaborative 

Experiemental Data Overview and Analysis,” NREL/CP-500-48232, May 2010. 

3. Peeters, J., Vandepitte, D., and Sas, P., “Analysis of Internal Drivetrain Dynamics in a 

Wind Turbine,” Wind Energy. DOI: 10.1002/we.173, John Wiley and Sons, July 2005.
23

4. Windpower Monthly, “Facing up to the Gearbox Challenge: A survey of gearbox failure 

and collected industry knowledge,” Volume 21 (11), November 2005.

5. Rasmussen, F., Thomsen, K., and Larsen, T. J., “The Gearbox Problem Revisited,”Risø 

Fact Sheet AED-RB-17(EN), Risø National Laboratory, Roskilde, DK, 2004.

6. International Electrotechnical Commission. “Wind Turbines – Part 4: Standard for Design 

and Specification of Gearboxes,” ISO/IEC 61400-4 DIS, ISO Geneva, Switzerland.

7. International Electrotechnical Commission. “Wind Turbine Generator Systems - Part 1: 

Safety Requirements,” IEC 61400-1, Ed. 3. IEC, Geneva, 2004. 

8. International Organization for Standardization: ISO 281:2010. “Rolling bearings -

Dynamic load ratings and rating life,” ISO Geneva CH, 2010.

9. McNiff, B, van Dam, J., et al. “Gearbox Reliability Collaborative: Phase 1 Test Plan,”

National Renewable Energy Laboratory, Draft Publication December 2009, to be 

published.

10. van Dam, J., “Gearbox Reliability Collaborative Bearing Calibration” ; NREL Report No. 

TP-500--47852.

11. Crowther, A. R., Ramakrishnan, V., Zaidi, N. A., and Halse, C. “Sources of time-varying 

contact stress and misalignment in wind turbine planetary sets,” Wind Energy. DOI: 

10.0069, John Wiley and Sons, 19 May 2010. 

12. International Organization for Standardization: ISO 6336-1:2010 “Calculation of load 

capacity of spur and helical gears,” ISO Geneva CH, 2010. 

13. American Gear Manufacturers Association, ANSI/AGMA 6123-BXX, “Design Manual for 

Enclosed Epicyclic Gear Drives,” AGMA, Virginia USA, 2006. 
24

Appendix A: Coordinate System

Figure 24. Coordinate system for gearbox and nacelle.

The coordinate system implemented in this document follows the IEC standard (6). It is a right 

hand coordinate system where the X axis is aligned with the drive shaft and therefore has a 5 

degree tilt with respect to the inertial frame. The coordinates describing the orientation of the load 

or bending are polar coordinates with 0 degrees at top dead center of the main bearing. The angle 

increases in the clockwise direction while pointing down wind. Figure 24 and Figure 25 show a 

visual representation of the coordinate system used in this document.

 

Figure 25. Main shaft moment orientation and reference.

5 deg 

0 deg 

90 deg 
25

In the case of the moments, the angle in Figure 25 describes the direction of the vector in which a 

positive moment is applied in accordance with the right hand rule. Figure 26 shows a moment with 

a direction of 90 degrees. With respect to the axes, this is a negative moment in the Y direction.

Figure 26. Negative gearbox pitch moment (-Y) reference.

Port refers to the negative Y direction and Starboard refers to the positive Y direction.