Quantification of Roller Sliding Energy in 

Wind Turbine Gearbox High-Speed Shaft 

Bearings

Yi Guo, Jonathan Keller, and Shawn Sheng

Drivetrain Reliability Collaborative Meeting

National Renewable Energy Laboratory

February 19, 2019

NREL/PR-5000-73320
NREL | 2

1

2

3

4

5

Background

Roller Sliding Measurement and Modeling

Bearing and Roller Loads Measurement and Modeling

Energy Accumulation and Reliability Assessment

Summary and Ongoing Research
NREL | 3

Gearbox Bearing Axial 

Cracking—Dominant 

Drivetrain Failure Mode

What turbine operational conditions 

result in critical contact conditions?

Errichello, R., S. Sheng, J. Keller, and A. Greco. 2012. Wind Turbine 

Tribology Seminar- A Recap. U.S. Department of Energy Wind and 

Water Power Program (image provided by Jurgen Gegner of SKF).

High-Speed 

Shaft Bearings

IntermediateSpeed Shaft 

Bearings

Planet 

Bearings
NREL | 4

Cumulative frictional energy is considered a 

potential failure metric for axial cracking.

E N Vt = ∆ μ

Cumulative Frictional

Energy 

Gould, B., and A. Greco. The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks. doi:10.1007/s11249-015-0602-6.

MPR Test Bench Results

MPR=Micro Pitting Rig

WEC=White Etching Cracks
NREL | 5

Measure 

Model

Measure

Model

E Quantification

∆V

∆V,μ

N

N

Frictional 

Energy E

This Photo by Unknown 

Author is licensed 

under CC BY-SA

Wind Plant

E

Model validated by experiments for a 1.5-MW turbine Apply models for wind plants
NREL | 6

1

2

3

4

5

Background

Roller Sliding Measurement and Modeling

Bearing and Roller Loads Measurement and Modeling

Energy Accumulation and Reliability Assessment

Summary and Ongoing Research
NREL | 7

Roller dynamics model (analytical):

– Harris roller dynamics model

Lubricant hydrodynamics model based on:

– Bercea cage friction model

– Dowson and Higginson lubricant model

Gould, B., and A. Greco, The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks. doi:10.1007/s11249-015-0602-6.

0 QQ F i oc −+=

0 FF FQ i o v cg +− − =

1

2

r

i o cg cg c

d M M DQ J dω

μ ω φ

−+ =

Tangential

Radial

Torsional

Analytical Model 

Predicts Roller and 

Cage Sliding

Cage

Primary Governing Equations

Frictional Energy Loss

Guo, Y., and J. Keller. Forthcoming. “Analytic Formulations of Rolling Element Bearing 

Sliding in Wind Turbine Gearboxes.” Mechanism and Machine Theory.
NREL | 8

High-Speed Shaft 

Bearings and Load 

Zone

Load Zone Entry

Load Zone

Load Zone Exit

Unloaded Zone

NU 232 (Bearing B) NU 2326 (Bearing A)
NREL | 9

• Rollers slide even at rated torque

• Sliding affected by lubricant temperature and clearance

• Low torque = Severe sliding

Rotor Sliding: 

Power Production

Vaes, D., Y. Guo, P. Tesini, and J. Keller. 2019. Investigation of Roller Sliding in Wind Turbine Gearbox High-Speed Shaft Bearings. NREL/TP-5000-73286. 

Golden, CO: National Renewable Energy Laboratory. 

Pure rolling

Pure rolling

Model Validation Effects of Clearance Effects of Temperature

Load Zone Entry

Load Zone Center

Load Zone Exit
NREL | 10

1

2

3

4

5

Background

Roller Sliding Measurement and Modeling

Bearing and Roller Loads Measurement and Modeling

Energy Accumulation and Reliability Assessment

Summary and Ongoing Research
NREL | 11

• High-speed shaft bearing loads derived 

from the measured shaft-bending moments 

and torque through force and moment 

balance

• Bearing loads distributed among rollers 

using the Harris approach.

Gould, B., and A. Greco. The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks. doi:10.1007/s11249-015-0602-6.

Semi-Experimental 

Approach—Strain Gaging 

Bearing Ring Not Needed

Bearing loads approximated as

1 - - sin( )( - ) (- )

CB B

y z a p

a cb b

T F M LL

LL R γ

  =    

1 - - cos( )( - ) (- )

CB B

z y a p

a cb b

T F M L L

LL R γ

  =    

- - sin( ) UW CB

y y

b

T F F

R = γ

- - cos( ) UW CB

z z b

b

T FF W

R = γ +

Guo, Y., and J. Keller. Investigation of High-Speed Shaft Bearing Loads in Wind Turbine Gearboxes Through Dynamometer Testing. doi:10.1002/we.2150.
NREL | 12

• Three degrees of freedom lumpedparameter model calculates bearing loads

• Bearing loads distributed among rollers 

using the Harris approach.

Gould, B., and A. Greco. The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks. doi:10.1007/s11249-015-0602-6.

Simple Analytical 

Model Calculates 

Bearing Loads

Governing Equation

.. .

Mq Cq K q t q f q ++ = ( ,) ( ,)t

C

M

K

q

f

Mass

Damping

Stiffness

Displacement

Applied loads
NREL | 13

1

2

3

4

5

Background

Roller Sliding Measurement and Modeling

Bearing and Roller Loads Measurement and Modeling

Energy Accumulation and Reliability Assessment

Summary and Ongoing Research
NREL | 14

• Accumulates most sliding energy at the load zone entry

• Rollers slide most outside the load zone

– No frictional energy generated

Frictional Energy: Power Production

∆V μ N = E t /

Pure rolling

Pure rolling

Load Zone Entry

Load Zone Center

Load Zone Exit

Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and Frictional Energy in a Wind Turbine Gearbox. NREL/TP-5000-72415. 

Golden, CO: National Renewable Energy Laboratory.
NREL | 15

Frictional Energy 

Varies with Power 

and Temperature

Cold operations at low power

generate more frictional energy than warm 

operations at high power.

Effects of Temperature on E

Temperature Drop

Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and 

Frictional Energy in a Wind Turbine Gearbox. NREL/TP-5000-72415. Golden, CO: 

National Renewable Energy Laboratory.
NREL | 16

Frictional Energy: Startup Event

• Normal startup

• Energy accumulates during the runup once grid is connected

∆V μ N t = E

Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and Frictional Energy in a Wind Turbine Gearbox. NREL/TP-5000-72415. 

Golden, CO: National Renewable Energy Laboratory.
NREL | 17

Frictional Energy: Emergency Stop Event

• Emergency stop—induced from tower base

• Limited energy accumulation

– But many torque oscillations and reversals contact stress up to 2 GPa

∆V μ N t = E

Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and Frictional Energy in a Wind Turbine Gearbox. NREL/TP-5000-72415. 

Golden, CO: National Renewable Energy Laboratory.
NREL | 18

Energy Accumulation Comparison

• Compare total sliding energy between turbine operations

• Transients vs. 10-minute projections of normal power

– Normal power contributes more energy

– RS (A) has more energy than generator-side inboard (GS-in or B)

Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and Frictional Energy in a Wind Turbine Gearbox. NREL/TP-5000-72415. 

Golden, CO: National Renewable Energy Laboratory.
NREL | 19

• Nearly 200 wind turbines with multiple 

gearbox suppliers

• Investigate high-speed and intermediatespeed stage bearings

• Correlate energy accumulation with failure 

records

Gould, B., and A. Greco. The Influence of Sliding and Contact Severity on the Generation of White Etching Cracks. doi:10.1007/s11249-015-0602-6.

Reliability Assessment 

and Remaining Life 

Prediction

Turbine E vs. Time

GG = ee∗ − (EEnnnn + EEtttt)

Compute Probability of Failure

Preliminary Results

What are other contributors to WECs? Higher-resolution SCADA? 

Failed

Not Failed
NREL | 20

Summary and Ongoing Research

• Up-tower testing campaign investigated major contributors to WECs

– Roller sliding and frictional energy accumulation

• Newly developed analytical tools calculate roller loads and sliding

– Can simulate a variety of turbines and plants

– Validated by experiments

• Frictional energy accumulated the most during power production

– Transient events contribute less energy

• Lubricant temperature greatly affects energy generation

– Lubricant heater/cooler function improvement?

• Relate frictional energy with plant failure records (ongoing)

• Reliability assessment during early design phase (ongoing)

• Prediction of remaining useful life (ongoing)
NREL | 21

Recent References

• Bercea, I. et al. Simulating roller – cage pocket friction in a tapered roller bearing, European J. Mech. Eng., 

1997

• D. Dowson & G. R. Higginson, The effect of material properties on the lubrication of elastic rollers, Journal of 

Mech. Eng. Sci., 1960

• Guo, Y., and J. Keller. 2018. Investigation of High-Speed Shaft Bearing Loads in Wind Turbine Gearboxes 

through Dynamometer Testing. Wind Energy. doi: 10.1002/we.2150.

• Guo, Y., and J. Keller. Forthcoming. Analytic Formulations of Rolling Element Bearing Sliding in Wind Turbine 

Gearboxes. Mechanism and Machine Theory.

• Keller, J., and S. Lambert. 2018. Gearbox Instrumentation for the Investigation of Bearing Axial Cracking. 

NREL/TP-5000-70639. Golden, CO: National Renewable Energy Laboratory.

• Keller, J. 2018. Investigating Main and High-Speed Shaft Bearing Reliability Through Uptower Testing. 

NREL/PR-5000-70958. Golden, CO: National Renewable Energy Laboratory.

• Keller, J., Y. Guo, and L. Sethuraman. 2018. Main and High-Speed Shaft Bearing Reliability Uptower Test 

Report. NREL/TP-5000-71529. Golden, CO: National Renewable Energy Laboratory.

• Keller, J., and Y. Guo. 2019. Analysis of High-Speed-Shaft Bearing Roller Sliding and Frictional Energy in a 

Wind Turbine Gearbox. NREL/TP-5000-72415. Golden, CO: National Renewable Energy Laboratory.

• Keller, J., Y. Guo, Z. Zhang, and D. Lucas. 2018. “Comparison of Planetary Bearing Load-Sharing 

Characteristics in Wind Turbine Gearboxes.” Wind Energy Science. doi: 10.5194/wes-2018-36.

• Sheng, S., and Y. Guo. 2019. A Prognostics and Health Management Framework for Wind Energy. ASME 

Turbo Expo GT2019-91533.

• T. A. Harris, An analytical method to predict skidding in high speed roller bearing, ASLE Transaction, 1966

• T. A. Harris and M. H. Mindel, Rolling element bearing dynamics, Wear, 1972

• Vaes, D., Y. Guo, P. Tesini, and J. Keller. 2019. Investigation of Roller Sliding in Wind Turbine Gearbox HighSpeed Shaft Bearings. NREL/TP-5000-73286. Golden, CO: National Renewable Energy Laboratory.
NREL | 22

This work was authored by the National Renewable Energy Laboratory, operated by 

Alliance for Sustainable Energy, LLC, for the U.S. Department of Energy (DOE) under 

Contract No. DE-AC36-08GO28308. Funding provided by the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy Wind Energy Technologies Office and 

CRADAs 17-694 with Flender Corporation and 16-608 with SKF GmbH. The views 

expressed in the article do not necessarily represent the views of the DOE or the U.S. 

Government. The U.S. Government retains and the publisher, by accepting the article for 

publication, acknowledges that the U.S. Government retains a nonexclusive, paid-up, 

irrevocable, worldwide license to publish or reproduce the published form of this work, 

or allow others to do so, for U.S. Government purposes.

Thank You!

Flender Corporation,

SKF GmbH, and SKF USA

Acknowledgments