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Contract No. DE-AC36-08GO28308

High-Speed Shaft Bearing 

Loads Testing and Modeling in 

the NREL Gearbox Reliability 

Collaborative

Preprint

B. McNiff

McNiff Light Industry

Y. Guo, J. Keller, and L. Sethuraman

National Renewable Energy Laboratory

To be presented at the Conference for Wind Power Drives

Aachen, Delaware

March 3‒4, 2015

Conference Paper

NREL/CP-5000-63277

December 2014
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Acknowledgment

This work was supported by the U.S. Department of Energy under contract number DE-AC36-

08GO28308 with the National Renewable Energy Laboratory. Funding for the work was 

provided by the DOE Office of Energy Efficiency and Renewable Energy, Wind and Water 

Power Technologies Office.
iv

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Executive Summary

Bearing failures in the high-speed output stage of the gearbox are a leading cause of unscheduled 

maintenance in wind turbines. Accordingly, the National Renewable Energy Laboratory (NREL) 

Gearbox Reliability Collaborative (GRC) has performed an experimental and theoretical 

investigation of loads within these bearings. The purpose of this paper is to describe the 

instrumentation, calibrations, data post-processing, and initial results from this testing and 

modeling effort. Efforts to relate high-speed shaft (HSS) torque, bending, and bearing loads to 

model predictions are also discussed in this work. Of additional interest is examining whether the 

shaft measurements can be simply related to bearing load measurements, eliminating the need for 

making invasive modifications to the bearing races to accommodate such instrumentation.
v

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Table of Contents

Acknowledgment ....................................................................................................................................... iii

Executive Summary................................................................................................................................... iv

Table of Contents........................................................................................................................................ v

List of Figures ............................................................................................................................................. v

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

2 Test Summary....................................................................................................................................... 2

2.1 Instrumentation.............................................................................................................................. 2

2.2 Calibration..................................................................................................................................... 2

2.3 Dynamometer Testing ................................................................................................................... 2

3 Modeling and Analysis Approach....................................................................................................... 3

3.1 Bearing Forces From Analytical Model........................................................................................ 3

3.2 Bearing Forces From SIMPACK Model....................................................................................... 5

4 Results................................................................................................................................................... 6

4.1 Shaft Bending and Torque Correlation ......................................................................................... 6

4.2 Estimation of Global TRB Loads From Bearing Measurements .................................................. 7

5 Conclusions .......................................................................................................................................... 9

References................................................................................................................................................. 10

List of Figures

Figure 1. HSS free-body diagram .............................................................................................................. 3

Figure 2. Comparison of measured and SIMPACK-predicted HSS bending moments ....................... 6

Figure 3. Measured TRB load zone at rated torque................................................................................. 7

Figure 4. Comparison of directly measured, analytically inferred, and SIMPACK-predicted TRB..... 8
1

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1 Introduction

Since 2007, the NREL GRC has been investigating internal and external gearbox motion, loads, 

and deflections through modeling, analysis, and full-scale field and dynamometer testing. GRC 

Phases 1 and 2 focused primarily on acquiring measurements in the planetary sections of two 

750-kilowatt (kW) test gearboxes (Link et al. 2011).

Feedback from GRC partners highlighted the need for measurements of gear and bearing 

response on the high speed shaft (HSS) locating tapered roller bearing (TRB) pair, because of the 

high rate of failures and subsequent repair costs for these bearings in the industry (Sheng 2013, 

Scott et al. 2012). The reliability of this bearing configuration remains a weak link owing to a 

possible disconnect between the actual loading behavior, and the loading behavior assumed by 

the design. HSS bearings can be subject to low loads at high speeds, torque transients, and even 

torque reversals. When rollers enter and leave the shifting load zone during such events, bearings 

are prone to skidding, which leads to surface distress and damage (Jain and Hunt 2011).

In GRC Phase 3, instrumentation was added to measure the HSS bending, torque, pinion tooth 

load distribution, and radial load distribution and temperature of the locating TRBs. The current 

dynamometer test program measures these loads during normal operation with nontorque rotor 

loads, generator misalignment, and transient operation (Link et al. 2012). In this report, the 

experimental HSS measurements are compared to an analytical model and a SIMPACK 

multibody model.
2 

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2 Test Summary

The 2.5-megawatt dynamometer test facility at the National Wind Technology Center enabled 

researchers to apply loads to the GRC drivetrain under controlled, field-like conditions. The 

following sections summarize the instrumentation and testing of the GRC drivetrain and HSS.

2.1 Instrumentation

The GRC HSS is supported by a cylindrical roller bearing (CRB) upwind, and a close-coupled 

32222 J2 TRB pair mounted in an O-configuration downwind of the gear mesh. As shown in 

Figure 1, orthogonal shaft bending moments were measured using full-bridge strain gauge 

arrangements mounted downwind (location A) and upwind (location B) of the gear mesh, and 

also downwind of the TRB pair (location C) where torque was also measured. Bearing loads 

were measured using Poisson half-bridge strain gauge arrangements installed in machined slots 

on the outer race of each TRB at two different axial locations in four circumferential positions

(Link et al. 2012, Keller and McNiff 2014). 

2.2 Calibration

The shaft torque, bending, and TRB signals were calibrated to obtain the conversion coefficients 

between measured voltages and engineering units of loads for direct comparison against 

predictions from modeling tools. The calibration coefficients for shaft bending moments and 

torque were obtained and verified by static in-situ calibration (Keller and McNiff 2014), and a 

special test rig was used for calibration of the TRB gauges (Keller et al. 2013). 

2.3 Dynamometer Testing

The current test program includes a broad matrix of operations to identify those that result in 

anomalous HSS responses; however, for this work, the drivetrain was operated at full speed 

(1800 revolutions per minute on the HSS) and at power levels up to 100% (750 kW) in torqueonly conditions. HSS data were acquired at 2 kilohertz for 1 second (30 shaft revolutions). 
3 

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3 Modeling and Analysis Approach

This section presents the analytic model used to estimate TRB loads from measured shaft 

bending moments and torque, along with a description of the SIMPACK model. 

3.1 Bearing Forces From Analytical Model

This section presents the analytical model developed to derive unknown CRB and TRB loads 

from measured shaft bending moments and torque. If validated, this analytical model would 

preclude the need for invasive instrumentation on the TRBs. In this model, the HSS is supported 

by the generator coupling and three bearings, which are treated as unknowns and are determined 

by solving the force and moment equations satisfying static equilibrium. The free-body diagram 

of the HSS is shown in Figure 1. 

Figure 1. HSS free-body diagram

The following assumptions were made in developing the free-body diagram: 

• The shaft is rigid and its weight is negligible

• The CRB does not carry a bending moment or react to axial force 

• The upwind TRB does not carry axial load 

• The TRBs do not carry bending moments, because their radial stiffnesses are much 

greater than their tilting stiffnesses

• The generator coupling does not transmit moments or axial force. 
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These assumptions are anticipated to be valid during normal operating conditions, but may be 

inappropriate when significant shaft misalignment is present. Writing the force balance equations 

in three directions, moment balance in two directions at three locations, and torque in 

equilibrium yields ten equations to solve for the ten unknowns in terms of HSS geometric 

properties and measured shaft bending and torque yields:

Fm = T

Rcosβ (1)

Fy

g = − Mz

C

LC

(2)

Fz

g = +

My

C

LC

(3)

Fy

CC = +

Mz

L − LCC

(4)

Fz

CC = − My

L − LCC

(5)

Fy

UU = 1

LSS �− Mz

C

LC

(L − LSS − LUU) + Mz

A − T

R sinα(LSS + LUU − LA)

− Mz

L − LCC

(LSS + LUU − LA)�

(6)

Fz

UU = 1

LSS �+

My

C

LC

(L − LSS − LUU) − My

A − T

R cosα(LSS + LUU − LA)

+

My

L − LCC

(LSS + LUU − LA)�

(7)

Fx

D = T

R tanβ (8)

Fy

D = 1

LSS �+

Mz

C

LC

(L − LUU) − Mz

A +

T

R sinα(LUU − LA)

+

Mz

L − LCC

(LUU − LA)�

(9)

Fz

D = 1

LSS �− My

C

LC

(L − LUU) + My

A +

T

R cosα(LUU − LA)z

− My

L − LCC

(LUU − LA)�

(10)
5 

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In this formulation, R, α, and β are the pitch diameter, the angle between the line of action and 

the z axis, and helix angle. The properties of the GRC HSS are: L = 775 mm, R = 57 mm, α = 

10o, and β = 14o. The bearing locations are LCB = 33.5 millimeters (mm), LUW = 333.5 mm, and 

LSP = 79 mm. The instrumentation locations are LA = 260.5 mm, LB = 88.5 mm, LC = 298 mm 

(Keller and McNiff 2014). 

3.2 Bearing Forces From SIMPACK Model

A multibody model of the GRC drivetrain was created in SIMPACK to simulate the 

dynamometer test (Guo et al. 2014). The HSS bearings were modeled as six-axial 

elastokinematic force elements, which follow a force-displacement relationship based on known 

stiffnesses and clearances. The bearing stiffnesses were calculated from RomaxWIND at rated 

torque, accounting for a 5-mm HSS axial offset (Keller and McNiff 2014) and assuming zero 

TRB preload. The axial stiffness of the upwind TRB was then assumed to be zero. SIMPACK 

solved the equations of motion by time integration and provided a time history of bearing forces. 

The bearing internal load distributions cannot be obtained from SIMPACK; instead, they will be 

the subject of future study using other tools. 
6 

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4 Results

The following sections compare the tests first to the SIMPACK model, and then to the analytic 

model. All cases were at rated torque and zero nontorque load.

4.1 Shaft Bending and Torque Correlation

Figure 2 compares the measured shaft torque and bending moment measurements to those 

predicted by SIMPACK. Generally, the results compared very favorably with the largest 

disparity at the location downwind of the TRB pair (location C). However, the moments at this 

location were very low in magnitude compared to those at either side of the gear mesh (locations 

A and B). Although the generator was aligned prior to testing, some generator misalignment 

could have resulted in the disparity at location C. 

Figure 2. Comparison of measured and SIMPACK-predicted HSS bending moments
7 

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4.2 Estimation of Global TRB Loads From Bearing Measurements

Figure 3 shows the measured TRB loads and estimated load zones. Each TRB slot has two 

gauges in the axial direction, the measurements of which were averaged into one value. The 

resulting four average circumferential points were then used to describe the load zone, which 

was assumed to be circular. Other load zone fits will be the subject of future study. The resultant 

global bearing force vector was then obtained by integrating the load zone. 

The upwind bearing has only a few rollers in contact whereas all downwind bearing rollers carry 

load. The load sharing between these rows was clearly unequal. One possible reason is that the 

TRB pair was designed with essentially zero preload and mistakenly manufactured with no axial 

upwind restraint. In this situation, the HSS gear mesh force unloaded the upwind TRB (Keller 

and McNiff 2014). 

Figure 3. Measured TRB load zone at rated torque

Figure 4 compares the direct TRB load measurements, derived as described above, to the 

SIMPACK model predictions. Additionally, the measured shaft bending and torque were used to 

estimate the TRB loads using the analytic model described in equations 6 to 10. The downwind 

TRB axial load was plotted directly in Figure 4, and the y and z axis loads for both bearings were 

combined into a single radial load, calculated as Fr

D = ��Fy

D �

2

+ (Fz

D )2 and Fr

UU =

��Fy

UU�

2

+ (Fz

UU)2. Both SIMPACK and the analytical method assumed that the upwind TRB 

axial load was zero.

There was very good agreement between the SIMPACK model and the analytical model for the 

downwind TRB axial load, each having a value of approximately 18 kN. The direct 

measurement, however, was only approximately half of this value. 
8 

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The upwind TRB radial load had the largest disparity between the direct measurement and the 

models. The direct measurement was approximately 35 kN on average, and the analytic 

estimation was approximately 55 kN with a variation of ±20 kN. This variation was driven by 

the variation in the measured torque shown in Figure 2. Although the torque only varied by 

±10% over each shaft revolution, the upwind TRB radial load was very sensitive to torque. The 

source of this consistent variation in torque is still being investigated. The SIMPACK model 

predicted that the upwind TRB was very lightly loaded, averaging approximately 15 kNm. 

Finally, the highest correlation between the direct measurement and the models was found for 

the downwind TRB radial load. Here, all three methods averaged about 20 kN, with a variation 

of ±12 kN. 

Figure 4. Comparison of directly measured, analytically inferred, and SIMPACK-predicted TRB
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5 Conclusions

This work examined the loads on the HSS and TRBs of the GRC drivetrain at full-speed and full 

power. Experimental measurements of shaft bending and torque were compared to a SIMPACK 

multibody model. The global TRB loads were also calculated from direct experimental 

measurements of the load zone from instrumented slots on the TRB outer races, assuming a 

circular load zone distribution. Measured loads were compared to the global TRB loads predicted 

by the SIMPACK model and also to an estimation of the global TRB loads predicted by a 

simple, analytic model of the HSS that used measured shaft bending and torque as inputs. The 

analytic model was of interest because it could preclude the need for invasive instrumentation on 

the TRBs themselves and special calibration testing.

The measured shaft torque and bending moments compared favorably with the SIMPACK 

model, with the exception of the portion of the shaft downwind of the TRB pair. This section of 

the shaft was the most sensitive to assumptions regarding generator misalignment and generator 

coupling stiffness. In addition, the measured shaft torque displayed a variation of ±10% in 

magnitude very consistently over each revolution of the shaft. This variation was a primary 

contributor to nonconstant bearing loads; its source is still under investigation.

The directly measured global TRB loads compare favorably to SIMPACK model predictions and 

analytical model predictions for the downwind TRB axial and radial force, but disparities exist 

for the upwind TRB radial force.

Future work required to validate the analytical model will examine additional test cases for other 

steady state torque levels and intentional generator misalignment conditions. The source of the 

variation in torque signal is still being investigated as it plays a primary role in determining the 

bearing load zone. Additionally, load zone distributions other than circular will be investigated.
10 

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References

Link, H.; LaCava, W.; van Dam, J.; McNiff, B.; Sheng, S.; Wallen, R.; McDade, M.; 

Lambert, S.; Butterfield, S.; Oyague, F. (2011). “Gearbox Reliability Collaborative 

Project Report: Findings from Phase 1 and Phase 2 Testing.” NREL/TP-5000-51885. 

Golden, CO: National Renewable Energy Laboratory, 88 pp. 

Sheng, S. (2013). “Report on Wind Turbine Subsystem Reliability - A Survey of Various 

Databases.” NREL/PR-5000-59111. Golden, CO: National Renewable Energy 

Laboratory, 43 pp. 

Scott, K. G.; Infield, D.; Barltrop, N.; Coultate, J. K.; Shahaj, A. (2012). “Effects of 

extreme and transient loads on wind turbine drive trains.” 50th AIAA Aerospace Sciences 

Meeting Including the New Horizons Forum and Aerospace Exposition. AIAA; pp. 

2012-1293. 

Jain, S.; Hunt, H.E.M. (2011). “A dynamic model to predict the occurrence of skidding 

in wind-turbine bearings.” Journal of Physics: Conference Series 305, IOP Publishing; 

doi:10.1088/1742-6596/305/1/012027. 

Link, H.; Keller, J.; Guo, Y.; McNiff, B. (2012). “Gearbox Reliability Collaborative 

Phase 3 Gearbox 2 Test Plan.” NREL/TP-5000-58190. Golden, CO: National Renewable 

Energy Laboratory, 105 pp. 

Keller, J.; McNiff, B. (2014). “Gearbox Reliability Collaborative High-Speed Shaft 

Calibration.” NREL/TP-5000-62373. Golden, CO: National Renewable Energy 

Laboratory, 31 pp. 

Keller, J.; Guo, Y.; McNiff, B. (2013).“Gearbox Reliability Collaborative High Speed 

Shaft Tapered Roller Bearing Calibration.” NREL/TP-5000-60319. Golden, CO: 

National Renewable Energy Laboratory, 34 pp. 

Guo, Y.; Keller, J.; LaCava, W. (2014). “Planetary Gear Load Sharing of Wind Turbine 

Drivetrains Subjected to Non-Torque Loads.” Wind Energy, doi: 10.1002/we.1731.