Conference Paper

NREL/CP-500-45325

May 2009

Gearbox Reliability Collaborative 

Analysis Round Robin

Preprint

F. Oyague, C.P. Butterfield, and S. Sheng 

To be presented at the American Wind Energy Association 

WINDPOWER 2009 Conference 

Chicago, Illinois

May 4 – 7, 2009
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iii

Table of Contents 

List of Figures ...............................................................................................................iv

List of Tables.................................................................................................................iv

Acronyms and Abbreviations ....................................................................................... v 

Introduction.................................................................................................................... 1

Gearbox Reliability Collaborative—Overview ............................................................. 1

Gearbox Redesign ......................................................................................................................1

Instrumentation ..........................................................................................................................2

Dynamometer and Field Testing ................................................................................................2

GRC Analysis Round Robin ......................................................................................................3

Analysis Description ..................................................................................................... 3

Preliminary Analysis ..................................................................................................................4

Gearing Analysis ........................................................................................................................4

Shaft Analysis ............................................................................................................................6

Planet Carrier Analysis ..............................................................................................................6

Housing Analysis .......................................................................................................................7

Bearing Analysis ........................................................................................................................8

Loading Analysis and Results Comparison ................................................................ 8

Load Cases Evaluated ................................................................................................................9

Calibration Load Cases ........................................................................................................9

Design Load Cases ...............................................................................................................9

Preliminary Field Transients ..............................................................................................11

Parameter Evaluation and Results ...........................................................................................11

Torque Distribution ............................................................................................................11

Gear Mesh Stiffness ...........................................................................................................12

Torsional Compliance ........................................................................................................12

Gear Tooth Stress ...............................................................................................................13

Gear Tooth Loading ...........................................................................................................14

Planet Carrier Torsional Misalignment ..............................................................................15

Planet Load Share ..............................................................................................................15

Sun Motion.........................................................................................................................15

Gear Tooth Contact Pattern ...............................................................................................15

Shaft Bending.....................................................................................................................15

Bearing Stiffness ................................................................................................................15

Bearing Roller Load Distribution ......................................................................................16

Housing Deflections...........................................................................................................16

Future Work..................................................................................................................16

References ...................................................................................................................17
iv

List of Figures

Figure 1. Exploded view of the GRC gearbox .................................................................... 4

Figure 2. Gearbox free-body diagram (GE Transportation) ............................................... 4

Figure 3. Detailed integrated model (Ansol) ...................................................................... 5

Figure 4. Shaft torsion FEA (NREL) .................................................................................. 6

Figure 5. Planet carrier pin misalignment (NREL) ............................................................. 7

Figure 6. Housing analysis (Ricardo) ................................................................................. 7

Figure 7. Shaft bearing configuration ................................................................................. 8

Figure 8. Bearing analysis illustration (Romax) ................................................................. 8

Figure 9. Fatigue load spectrum and simulated transient ................................................. 10

Figure 10. Torque time series from field testing ............................................................... 11

Figure 11. Gearbox torque distribution results ................................................................. 12

Figure 12. Planet ring gear mesh stiffness results............................................................. 12

Figure 13. Shaft torsional compliance results ................................................................... 13

Figure 14. Gear tooth contact stress .................................................................................. 13

Figure 15. Contact stress distribution and participant comparison ................................... 14

Figure 16. Planet ring gear loading result ......................................................................... 14

List of Tables

Table 1. Calibration Load Cases ......................................................................................... 9

Table 2. Gearbox Reliability Collaborative Turbine Class ............................................... 10
v 

Acronyms and Abbreviations 

AGMA American Gear Manufacturers Association

CLC Calibration load case 

FAST Fatigue Aerodynamics Structures and Turbulence

FEA Finite element analysis

GRC Gearbox Reliability Collaborative 

HS High speed 

IEC International Electrotechnical Commission

INP_SH Input shaft (main shaft) 

IMS_SH Intermediate speed shaft 

LOA Length of action 

LS_SH Low speed shaft

NREL National Renewable Energy Laboratory 

NWTC National Wind Technology Center 

PL Planet gear 

RING Annulus or ring gear 

SH shaft

ST Stage

Vave Annual average wind speed at hub height 

Vref Referenced wind Speed averaged over ten minutes
1 

Introduction

The National Renewable Energy Laboratory’s (NREL) National Wind Technology 

Center (NWTC) has embarked on the task of determining the causes and loading 

conditions that result in the premature failure of wind turbine gearboxes. The NWTC 

approaches this challenge by bringing together the parties involved in the gearbox-design 

process to achieve the common goal of the improving the lifetime of gearboxes. This is 

being accomplished through the Gearbox Reliability Collaborative (GRC).

In addition to this collaborative effort, the GRC seeks to achieve its goal by exploring 

three avenues of research: drivetrain numerical analysis and modeling, full-scale 

dynamometer testing, and field testing. These different avenues are correlated and 

iterated to obtain the closest representation of actual load behavior in a controlled 

environment. 

The numerical analysis plays a key role in the design process of the gearbox; thus, the 

“Gearbox Reliability Collaborative, Analysis Round Robin” presented here seeks to 

examine various analytical approaches to gain insight in the validity and level of fidelity 

required to satisfy the design practice. The analysis is based on modeling techniques that 

range from simple torsional models to models that account for gear tooth contact and gear 

tooth load distribution, as well as bearing compliances and roller load distribution. The 

results from the GRC Analysis Round Robin will be validated with experimental data, and 

eventually will be a valuable data set that can be used for the validation of analysis tools.

Industry designers typically have used different design approaches, but there is some 

consensus based on AGMA 6006 [1]. It would be useful to establish a definition of this 

consensus as a baseline and then compare it against a broad range of analytical 

approaches. The GRC Round Robin could provide an objective database of analytical 

results as well as experimental validation to aid the evaluation of the consensus. 

The main focus of this document is the analytical approaches overseen by the Round 

Robin. It also gives a brief overview of the GRC and describes the gearbox redesigns, 

instrumentation, and testing, as well as the GRC Round Robin. Next is a description of 

the different analytical approaches presented by the participants, and discussion of the 

levels of fidelity implemented. The load cases and the parameters provided to the 

participants are presented along with the results obtained.

Gearbox Reliability Collaborative—Overview 

This section presents a brief overview of the GRC effort. It describes the steps followed 

to bring the gearbox up to today’s design practices and manufacturing standards. It also 

discusses the instrumentation utilized and experimental effort. Lastly, it gives a 

description of the GRC Analysis Round Robin. 

Gearbox Redesign

The GRC gearbox was redesigned so that its configuration and characteristics are 

consistent with the current industry standards. Additionally, the redesign reflects the 
2 

configuration of a 1.5-MW gearbox, which is an integral part of the currently installed 

fleet. The redesign of the gearbox also enables NREL to provide the GRC participants 

with the design information needed to perform the analysis. This is an important aspect of 

the GRC, because it is quite difficult to obtain manufacturing drawings, material 

properties, and solid models due to the proprietary nature of the information. Further, one 

of the goals of the GRC is to produce a reference data set that is substantiated by reliable 

data and can be used for the validation of new analytical tools. 

The redesigned gearbox included the addition of a floating sun configuration. This 

configuration allows the sun to move freely between the planets, which improves the 

planet load-share characteristics of the gearbox. Bearing configuration was changed in 

the planets, abandoning the old standard of spherical roller bearings. The bearing 

configuration of the parallel shaft stages also was updated to use tapered roller bearings.

The internal lubrication system was upgraded from a splash-lubricated system to a jetlubricated system adding lubricant distributions to each individual planet. The filtration 

system also was updated to reflect the highest standard in the industry by the addition of a 

self-standing kidney loop and desiccant filters. The gearing microgeometry was updated 

using up-to-date analytical tools to compensate for deflections and misalignments. 

Instrumentation

The redesigned and rebuilt process noted above was performed for two independent 

gearboxes. The first is the test subject for the field and the other is used in the 

dynamometer. Both gearboxes are heavily instrumented, and include more than 150 

channels to capture the internal behavior of the gearbox. Among the parameters to be 

evaluated are the bearing roller load distribution in the planet bearings as well as the

planet bearing load share. Planet bearings are a common source of failure in wind turbine 

gearboxes [2]. These measurements will help validate the analytical models and shed 

some light on the uncertainty associated with planet bearing behavior. 

The bearing raceway temperature differential also will be monitored, which will help in 

calculating bearing operating clearances—an important component in the bearing-life 

calculations. Sun motion with respect to the planet carrier shows the behavior of the sun 

as it adjusts for the changing planet load share among the planets. Planet carrier 

misalignment will be measured and its effects on the planet load share will be evaluated 

through measuring the root stresses on the planet ring mesh. These parameters represent a 

portion of what is evaluated in the experimental side of the GRC, and will be used to 

validate the analytical branch of the GRC. 

Dynamometer and Field Testing

The GRC testing will operate in the field and in the dynamometer in parallel, allowing an 

important interaction and cross feeding between both tests. This interaction will help 

determine how the field loading can be simulated in the dynamometer. The control 

environment of the dynamometer will serve as an important validation tool for the 

analysis, and the parallel field test will ensure that the validation is representative of the 

loading experienced on the field. Entire load cases generated in the field will be fed to the 

validated models, so that real turbine behavior can be simulated.
3 

GRC Analysis Round Robin

The GRC Analysis Round Robin is intended to evaluate the different analytical tools and 

approaches utilized in the development and design of horizontal axis wind turbine 

drivetrains. The advantage of the round-robin approach is that it brings together many 

experts. This pool of expertise foments progressive discussion surrounding the 

assumptions and analytical discrepancies produced by the round robin. Additionally, the 

round-robin configuration allows the evaluation of different analytical approaches as well 

as different levels of complexity, exploring a broad gamut of tools used in the industry. 

Lastly, the participation of industry partners’ consulting companies as well as software 

development companies ensures that the feedback of the knowledge gained by the GRC 

will be rapidly absorbed by the industry, thus inherently improving drivetrain reliability. 

The GRC Round Robin utilizes the GRC drivetrain to provide its participants with a 

comprehensive characterization of the gearbox and other drivetrain components. The 

participants perform their analyses using this information. All analysis results are

compared, based on a platform provided by GRC. The platform includes parameters with 

a narrow definition and provides assumptions to avoid discrepancies in the results as well 

as specific load conditions. 

The parameter comparison (noted above) initially evaluates parameters of low 

complexity which are independent of other parameters. Subsequently, the parameter 

complexity is increased to evaluate the capabilities of the higher-fidelity models. The 

progressive comparison approach, as well as the different levels of model fidelity, result

in a comprehensive evaluation of the different analytical tools. 

The parameters are obtained under certain loading conditions which follow a progressive 

load spectrum. Simple load cases—such as steady state or static loading—are initially 

evaluated. Subsequently, load cases of more complexity—such as dynamic and transient 

events—are evaluated. The combination of the progressive modeling and loading 

conditions prevents rootless discrepancies on highly compounded models with complex 

dynamic transient loading. 

GRC Round robin Participants 

The GRC participants that have provided results for the round robin are the following 

CENER, DONG Energy, GE Transportation, KiSSsoft, Ricardo Inc., and Romax

Technology. The number of participants involved in the round robin is more extensive 

than the previously mentioned. However participants that are currently building the 

models or tuning already existing models to provide agreeable results have not been 

included in the list. Nevertheless we feel confident that the participants involved in the 

project are representative of the analytical tools currently used in the industry. 

Analysis Description

This section describes the different analytical approaches applied to the different areas of 

the gearbox. A brief description of the level of fidelity implemented by the different 
4 

participants also is presented. Figure 1 shows an exploded view of the different 

components of the GRC gearbox. 

Figure 1. Exploded view of the GRC gearbox

Preliminary Analysis

As an initial step, some participants generate a free-body diagram. This approach gives a 

first glimpse of the load path traveling through the gearbox. This initial calculation is 

fairly simple; however, it provides a good guideline of the loading and the expected results 

throughout the analysis. As the analysis increases in complexity a number of assumptions 

must be made. The initial free-body calculation becomes a reference to validate these 

assumptions. Figure 2 shows the layout of the free-body diagram for the gearbox. 

Figure 2. Gearbox free-body diagram (GE Transportation)

Gearing Analysis

The simplest approach to the gearing behavior used by the participants included the basic

calculation of the tooth interaction forces. These values are dependent on the helix angle 

and the pressure angle, which is assumed to be constant. This type of approach is used to 

obtain the loading conditions in the previously mentioned free-body diagram. 
5 

Gear mesh stiffness also has been evaluated using a number of techniques. The simplest 

assumes that the gear mesh behaves as a cantilever beam. This approach assumes a 

constant mesh stiffness throughout the mesh cycle [3]. Additional modeling uses twodimensional analytical modules to estimate the gear mesh stiffness. The gear behavior is 

dependent on the gears’ center distances and the gear tooth characteristics. This type of 

modeling approximates the influence of microgeometry by including the tip relief 

characteristics of the profile. This approach approximates the gear mesh stiffness through

the mesh cycle with a parabolic function [4]. More comprehensive models also have been 

used; they generally were finite element analysis (FEA) based, however they specifically 

were tailored and designed for the gear analysis. This type of modeling encompasses all 

the corresponding tooth geometries, including lead modification as well as crowning and 

tip relief. This type of analysis generates a series of stiffness values throughout the mesh 

cycle, giving an accurate representation of the gearing behavior. 

Gear contact stress was evaluated by some of the participants; however, there was no 

simplified modeling to obtain the stresses. Generally, FEA-based and highly specific 

codes were used for this task. The difference in the analytical approach is in the 

integrated nature of the modeling. Some of the participants presented decoupled models 

and others presented a fully coupled modeling approach. Gear contact stress is highly 

dependent on the misalignments generated by the compliance of the gearbox components.

The decoupled approach brings together different tools that calculate the misalignment 

generated by the various components of the gearbox. The contributions of the 

components are added to obtain the effective misalignment experienced by the gear 

mesh. The information is input into the stress-specific codes to calculate the contact 

stresses. The downfall of this approach is that different codes use different sign 

conventions and coordinate systems. Consequently, the addition of the misalignment 

should be evaluated carefully to ensure that the final misalignment used for the 

calculation is correct. Some coupled codes also were used in this approach. The 

integration of a number of tools into a single code eliminates the need for the manual 

addition of deflections. Generally, to decrease computational time this approach reduces 

the fidelity of some of its components. Nevertheless, some of the participants have 

generated fully coupled models with very high fidelity. Figure 3 shows a fully coupled 

model capturing the entire gearbox behavior and able to calculate gear tooth contact 

stresses.

Figure 3. Detailed integrated model (Ansol)
6 

Shaft Analysis

Shaft modeling has taken several approaches. The simplest approach has ignored the

shaft deflection, assuming that the shafts are infinitely stiff. A simplified version of the 

shaft behavior also has been included by considering only the torsional contribution of 

the shafts. Generally, the assumption of a linearly elastic body has been used and the 

shaft geometry simplified such that the torsion of circular bars mechanics of solids can be 

used. The more comprehensive analysis used the finite element analysis of the shaft to 

predict the torsional and bending behavior. Figure 4 shows the combined torsional 

compliance of the low speed shaft components. 

Figure 4. Shaft torsion FEA (NREL)

Planet Carrier Analysis

Due to its complicated geometry, the planet carrier was not simplified so that a basic 

analytical calculation could be used. Most of the analysis used FEA to account for the 

deflections generated at the planet carrier. These deflections are very influential in the 

evaluation of the microgeometry and the tooth stresses. Due to the dependence (noted 

above) the misalignment is measured at the planet pin so that its translation to planet 

misalignment easily could be taken into account.

The main difference in the analytical approach was in the assumptions of the models. For 

instance, some analyses modeled the planet carrier pin interactions by the use of contact 

elements. This approach resulted in a more compliant planet carrier with greater misalignments. Others assumed an infinitely stiff joint increasing the stiffness of the components. 

Figure 5 shows the reference planes used to measure the planet pin misalignment (on the 

left) and a linear approximation of the pin misalignment (on the right). 
7 

Figure 5. Planet carrier pin misalignment (NREL)

Housing Analysis

The main approach taken to simulate the housing behavior has been the FEA method. 

Housing deflections also are of great importance because they produce misalignment that 

affects the gearing performance. Additionally, the interaction between the housing and 

the bearings was modeled by some participants. This interaction can have a significant 

effect on the internal clearance of the bearings, which is a determinant factor in bearing 

life. Furthermore, misalignment (discussed above) also has a significant impact on the 

bearing life. For the dynamical behavior of the gearbox housing, the participants 

performed virtual modal simulations which are exported to multibody dynamics codes to 

capture the dynamic behavior of the gearbox. Figure 6 shows housing stress 

characteristics for the housing underrated torque. 

Figure 6. Housing analysis (Ricardo) 
8 

Bearing Analysis

Modeling the bearing behavior is quite complex. Bearing response is dependent on 

factors including the loading conditions, temperature gradients, misalignment, rotational 

velocity, preload and operating clearance, off-roundness, and defects of the housing. For 

the bearing models to account for the previously noted parameters, the entire gearbox 

generally is modeled. Simplified gearing gives estimated loading to the shaft and 

housing, which interacts with the bearings. The bearing portion of the models generally is 

of very high resolution with sophisticated contact elements that account for the 

interaction of all the bearing components. These models may be used to calculate a 

linearized bearing stiffness, which may then be applied to simpler dynamic models 

(examples of this type of modeling can be found in reference [5]), this simplification is 

appropriate for transient time domain simulations. The detailed non-linear models are 

required for calculating accurate bearing life and roller contact stresses. Figure 7 and 

Figure 8 show the shaft bearing configuration, roller stress characteristics and roller 

contact stress for all rollers. Here the calculation accounts for bearing misalignment, 

variation in contact angle with misaligment, increase in contact area and number of 

rollers contacting with load and the microgeometry of the bearing, such as roller 

crowning. When modeling the whole gearbox the contact and deflection calculation 

incorporates the elasticity of the surrounding structure, this is important to correctly 

predict misalignment, which directly effects the calculation for stress and component life 

[6]

Figure 7. Shaft bearing configuration

Figure 8. Bearing analysis illustration (Romax)

Loading Analysis and Results Comparison

This section briefly describes the load cases that are used in the GRC Round Robin. The 

load cases range from simple static load cases to complex and dynamic load cases. The 
9 

simple static-load cases are used as a calibration tool for the analysis. More complex load 

cases follow the guidance of current standards used in the traditional design process. Also 

discussed are load cases including time series from field measurements which are 

representative of loading conditions experienced by the turbine under normal operation. 

A brief description of the parameters evaluated in the GRC Round Robin effort also is 

presented. The parameters increase in complexity as the comparison process progresses.

The simple independent parameters are evaluated first and subsequently are increased to 

parameters that are dependent on previously evaluated parameters. As the project 

progresses, the new parameters are added to better correlate the analytical effort to the 

experimental data. The results presented in this section are the result of the calibration 

load cases.

Load Cases Evaluated

Calibration Load Cases

The calibration load cases (CLC) are the simplest load cases evaluated by the GRC 

Round Robin. These CLC cases include static-load cases as well as constant-speed load 

cases. This subdivision was created to accommodate the analysis, which only can 

implement static, semi-static, or dynamic-load cases.

The calibration load cases range from 25% to 100% rated power in magnitude. The 100% 

load case gives good insight on the deflections and general behavior that is seen by the 

drivetrain under normal operation. The 25% rated load case serves as another point of

comparison for the study. The operation of the drivetrain at 25% also utilizes the fourpole configuration of the generator operating at a lower constant speed. This enables 

testing of the models under different conditions. A 39.5% rated power case also is 

evaluated. This case is representative of a 100% rated load for the four-pole generator. 

This load precedes dynamic load cases such as generator shifting events, therefore an 

understanding of the behavior of the gearbox prior to these events could prove valuable. 

The table below shows a summary of the load cases noted above. 

Table 1. Calibration Load Cases

Classification Load Case Description RPM Torque Units

Static

CLC 1.1 25% rated N/A 120.77 kNm

CLC 1.2 100% rated N/A 322.61 kNm

Dynamic

CLC 2.1 25% rated 14.824 120.77 kNm

CLC 2.2 39.5% rated 14.824 128.83 kNm

CLC 2.3 100%rated 22.199 322.61 kNm

Design Load Cases

The design load cases are more complex than the CLC cases included here, and they 

follow the guidance of the IEC 61400-1 3rd ed. standard [8]. These design load cases are 

representative of the conditions used in the current industry design process. To generate 

the loading conditions the entire turbine must be modeled. In this case, the simulation 
10

code FAST was used. FAST is an aeroelastic simulation code capable of simulating 

two- and three-bladed horizontal axis turbines. The code can simulate the transient 

dynamics of the entire system from which the fatigue and extreme events can be 

obtained. FAST uses a multibody and modal approach, incorporating rigid and flexible 

bodies to represent the turbine [7]. The entire drivetrain, however, is represented by a 

two-degrees of freedom system connected by a linear stiffness torsional spring; hence, 

there exists a need for further exploration of the drivetrain. 

The IEC standard provides guidelines for the inflow characteristics that should be applied 

to the models, such as turbulence intensity, wind speed, and wind-shear conditions, 

depending on the turbine classification. The standard also gives guidelines for the fault 

conditions that would generate extreme loading conditions and which should be included 

in the design. The table below shows the characteristics of the GRC turbine class [8]. 

Table 2. Gearbox Reliability Collaborative Turbine Class

GRC Turbine Characteristics 

Wind Turbine Class II

Vref (m/s) 42.5

Vave (m/s) 8.5

Turbulence Class B

Turbulence Intensity at 15 m/s 0.14

The guidelines in combination with the aeroelastic model were used to generate the 

fatigue load spectrum and ultimate strength time series. The fatigue load spectrum is used 

to calculate the predicted life of the different gearbox components. The ultimate strength

time series along with the fatigue time series is used to correlate the simulated prediction 

of the design process to the behavior observed in the field. These time series prove 

valuable because they can be fed into the models without the inherent noise existent in

experimental data. Figure 9 shows the time at level load spectrum (left) for torque and a 

time series for a torsional extreme event (right). 

Figure 9. Fatigue load spectrum and simulated transient

Time at Level Load Spectrum 

0

10000

20000

30000

40000

50000

60000

70000

80000

90000

100000

-20 30 80 130 180 230 280 330 380

Torque [Kn.m]

# of Cycles 

DLC 1.3 Wind 20 m/sec seed 02

2.50E+00

3.00E+00

3.50E+00

4.00E+00

4.50E+00

5.00E+00

50 100 150 200 250 300 350 400 450 500 550

Time [sec]

 GeneratorTorque [kN·m] 
11

Preliminary Field Transients

An initial set of experimental data also has been included in the GRC load envelope. The 

purpose of this data is to validate the response of the entire drivetrain. Characteristics 

such as bulk stiffness and damping can be extrapolated from this information. The 

purpose of the data set is the validation of the models, therefore it is not very extensive; 

however, it is comprised of specific events that aid the validations. Among these are 

braking events, start-up, and generator-shifting events. Figure 10 shows torque at the 

main shaft of the turbine for an emergency braking event at low angular velocity and a 

startup event to rated power. 

Figure 10. Torque time series from field testing

Parameter Evaluation and Results

Torque Distribution

Torque distribution can be defined as the torque entering each of the stages of the 

gearbox. It mainly was used for the initial comparison of the calibration load cases. 

Although it is a simple calculation it serves as a check for the models generated. 

Differences in gear ratios at any of the stages yield different results. Calculation of the 

torque distribution also ensures that the different participants are using the same sign 

convention and units. The result for the torsional distribution also is used as an input for a 

more-detailed analysis in models which are not fully coupled. Figure 11 shows the 

comparison of the different participants through the different stages of the gearbox. 

-200

-100

0

100

200

300

400

500

600

700

0 5000 10000 15000 20000 25000 30000

Time [sec]

Torque [kN.m]
12

Torque Distribution

0

50

100

150

200

250

300

350

INP_SH PL LS_SH IMS_SH

Location

Torque [kN.m]

A

B

C

D

E

F

Figure 11. Gearbox torque distribution results

Gear Mesh Stiffness

The stiffness along the line of action through the mesh cycle is evaluated. The average 

throughout the mesh cycle is used as the means of comparison. The gear mesh stiffness is 

dependent on the gearing material properties, geometry, and the center distance. For the 

purpose of the CLC comparison, the center distance is assumed to be nominal; therefore,

no housing compliance or misalignment is included. Figure 12 below shows the result for 

the ring planet mesh stiffness.

Ring Planet Mesh Stiffness

0.00E+00

1.00E+09

2.00E+09

3.00E+09

4.00E+09

5.00E+09

6.00E+09

Participant 

Mesh Stiffness [kN.m]

A

B

C

D

E

F

Figure 12. Planet ring gear mesh stiffness results 

Torsional Compliance

The shaft torsional deflection is quantified throughout this parameter. The previously 

described torsional distribution is used as an input parameter to calculate the deflection 

on each shaft of the gearbox. This approach can be used to calculate the torsional 

stiffness needed for the generation of torsional models (Figure 13). This model approach 

is required by some certification agencies such as Germanischer Lloyd WindEnergie [9]. 
13

Shaft Torsional Contribution LS-SH

0.00E+00

5.00E-04

1.00E-03

1.50E-03

2.00E-03

2.50E-03

3.00E-03

3.50E-03

CLC 1.1 CLC 1.2

Deflection [rad]

A

B

C

D

E

F

G

H

Figure 13. Shaft torsional compliance results

Gear Tooth Stress

The gear tooth contact stress is evaluated throughout the mesh cycle. Microgeometries 

such as tip relief crown and lead modifications are include in the calculation. For the 

initial comparison phase, misalignment has been ignored; hence, housing deflections, 

bearing compliance, and clearance are ignored. As the comparison process progresses 

these values will be validated and included in the stress calculations. Figure 14 shows the 

ring planet stress. Figure 15 shows a “slice” comparing the results for all the participants. 

Figure 14. Gear tooth contact stress 
14

Figure 15. Contact stress distribution and participant comparison

Gear Tooth Loading

In this case, gear tooth loading refers to the load generated by the gear mesh. Wind 

turbine gearboxes generally have helical teeth producing a radial, tangential, and axial 

component as reaction forces from the gear mesh. These forces impact the gearbox 

shafts, bearings, and housing, producing deflections which result in misalignments—

which emphasizes the importance of the gear tooth loading. Figure 16 shows the forces 

generated by the planet annulus mesh.

Gear Load PL1 Ring

0.00E+00

2.00E+04

4.00E+04

6.00E+04

8.00E+04

1.00E+05

1.20E+05

1.40E+05

1.60E+05

1.80E+05

2.00E+05

Circumferential Radial Axial

CLC 1.2

Load [N]

A

B

C

D

E

F

Figure 16. Planet ring gear loading result
15

The following parameters currently are being evaluated; therefore, the results are not 

presented in this document. 

Planet Carrier Torsional Misalignment

As the planet carrier undergoes torsional loading it twists slightly; this twist is one 

contributor to planet misalignment. This parameter quantifies the angular misalignment 

of the pin responsible for holding the planet. These values will be included in contact 

stress calculations and the evaluation of the microgeometry. 

Planet Load Share

Planet load share can be defined as the load carried by each planet in the planetary stage. 

Theoretically, it should be equal for all planets; however, there is debate with respect to

the load equalizing capabilities of the floating sun configuration under dynamic 

conditions. This parameter is intended to clarify some of the existing doubts regarding the 

planet load share with validation by means of experimental data. To date, the main focus 

of comparison has been static cases, so there have not been any major deviations from the 

theoretical approach. 

Sun Motion

This parameter is closely correlated to the previously noted load share. It specifically 

examines the motion of the sun with respect to the planet carrier. The correlation of the 

motion and the load share might give better insight into the behavior of the internal 

components of the gearbox. 

Gear Tooth Contact Pattern 

The gear tooth contact pattern is closely related to the tooth contact stress and 

microgeometry. The contact patterns will be monitored in the initial stages of the 

dynamometer testing as the run-in procedure is followed. This possibly will provide an 

approximate dimension and location of the contact patch for several loading conditions. 

This information will be used to validate the global deflections of the gearbox, especially 

in the planetary stage. 

Shaft Bending

Shaft bending, which is evaluated under the loading conditions generated by the gears, is 

of great importance to the misalignment of the internal components. For the purpose of 

the GRC Round Robin, specific locations were chosen along the shaft so that they can be 

compared among the different participants. 

Bearing Stiffness

The highly dependent nature of the bearing stiffness on misalignment and loading 

requires modeling that captures the integrated behavior of the gearbox. For the purpose of 

the GRC Round Robin, specific load cases which exemplify the nonlinear behavior of the 

bearings have been chosen for the comparison. The calculated stiffness from the highfidelity models will be simplified and used for the dynamic models. 
16

Bearing Roller Load Distribution

This parameter refers to the number of rollers in contact as well as the load carried by 

each roller. The parameter is highly dependent on the preload of the bearing, which

changes the operating clearances. It is of particular relevance in the planetary stage, 

where the planet generally has a very thin wall so as to reduce overall weight. The thin 

wall of the planet deforms under the load, leading to an oval shape and thus changing the 

loading characteristics of the load zone. Roller contact stress also will be evaluated. 

Housing Deflections

The housing behavior will be evaluated by monitoring its influence on other parameters. 

This includes the change in gear center distance due to housing compliance as well as its 

influence in the bearing and shaft behavior. 

Future Work

The GRC Round Robin is a work in progress. At this point, the main focus has been the 

calibration load cases and the less-involved parameters. Regardless of the simplicity of 

the load cases, a great deal of progress has been made in the comparison, initially 

presenting large discrepancies in the results. As the project progresses, the importance of 

the assumptions made for the different analytical approaches and their impact on the 

results will be very informative. Additionally, the evaluation of the strengths and 

weaknesses of different tools and their partial overlap will prove valuable for their 

implementation in the different stages of the design process.

The GRC Analysis Round Robin will continue the comparison of the analytical

parameters, advancing to the more complex parameters with a higher dependence on the 

global behavior of the gearbox. As the analysis and the experimental testing progress, the 

parameters evaluated will be adapted to better reflect and evaluate the internal behavior 

of the gearbox. Finally, the parameters evaluated will expand to include the behavior of 

the entire drivetrain, thereby truly capturing its dynamics.

As the data from the field and dynamometer test is generated, the validation of the 

analysis will take place. This process will include the reevaluation of the analytical 

assumptions and the final tuning of the models. The GRC Round Robin also will provide 

feedback to the dynamometer testing so that specific load cases that are tailored to the 

validation of the models can be input into the GRC test subject.

It is hoped that the GRC Analysis Round Robin as an integrated part of the GRC will 

shed light on the root causes of the premature failure of gearboxes. This should provide 

guidance in the use of analytical tools for the design process, and point out the impact of 

the assumptions inherent in the implementation of these tools.
17

References 

[1] American Gear Manufacturing Association. Standard for Design and Specification of 

Gearboxes for Wind Turbines. ANSI/AGMA/AWEA6006-A03 (2003). 

http://www.agma.org/. Available from http://webstore.ansi.org/. 

[2] Musial, W., Butterfield, S. and McNiff, B. Improving Wind Turbine Gearbox 

Reliability. NREL/CP-500-41548. May 2007. http://www.nrel.gov/wind/pdfs/41548.pdf. 

Accessed Mar. 3, 2009. 

[3] Oyague, F. Gearbox Modeling and Load Simulation of a Baseline 750-kW Wind 

Turbine Using State-of-the-Art Simulation Code. NREL/TP-500-41160. February 2009. 

http://www.nrel.gov/docs/fy09osti/41160.pdf. Accessed Mar. 3, 2009. 

[4] Mauer, L. The New, Powerful Gearwheel Module. SIMPACK User meeting 2006 

INTEC GmbH Wessling. http://www.simpack.com/ 

uploads/media/um06_intec_mauer.pdf. Accessed Mar. 3, 2009.

[5] Oyague, F. Progressive Dynamical Drive Train Modeling as Part of NREL Gearbox 

Reliability Collaborative.NREL/CP-500-43473. http://www.nrel.gov/

docs/fy08osti/43473.pdf. Accessed Mar. 3, 2009.

[6] Coultate J., Zhang Z., Halse C., Crowther A.R., The Impact of Gearbox Housing and 

Planet Carrier Flexibility on Wind Turbine Gearbox Durability, European Wind Energy 

Conference (EWEC), Marseille, March 2009

[7] Jonkman, J. & Buhl, M. Fast Users Guide. NREL/EL-500-38230. 

http://wind.nrel.gov/designcodes/simulators/fast/FAST.pdf. Accessed Mar. 3, 2009. 

[8] International Electrotechnical Commission. Wind Turbines Part 1: Design 

Requirements. IEC 61400-1, 3rd ed. (2005). 

[9] Germanischer Lloyd WindEnergie GmbH. Rules and Guidelines, Industrial Services, 

“Guidelines for Certification of Wind Turbines.” November 2003. http://www.glgroup.com/en/ren/6454.php. Accessed Mar. 3, 2009. 
F1147-E(10/2008) 

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Gearbox Realiability Collaborative Analysis Round Robin: Preprint

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F. Oyague, C.P. Butterfield, and S. Sheng

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The Gearbox Reliability Collaborative is working to determine the causes and loading conditions that result in the 

premature failure of wind turbine gearboxes. 

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wind turbine gearboxes; wind turbine drivetrains; gearbox reliability collaborative; gearbox failures

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