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Rolling Element Bearing Dynamics in 

Wind Turbines 

Yi Guo and Jonathan Keller

National Renewable Energy Laboratory

NREL/PR-5000-72185

American Society of Mechanical Engineers International Design 

Engineering Technical Conferences Meeting

Quebec City, Canada

August 29, 2018
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• Predominant drivetrain failure modes are:

o Not accounted for in design standards

o Not attributable to material deficiencies or quality control

o Complex and independent of the component supplier

• Conduct testing and analysis to enable:

o Improvement of inherent reliability

o Increase of availability with less effort and drama

o Reduction in wind plant operation and maintenance costs.

Wind Turbine Drivetrain Reliability Challenges

Photo by Mark McDade, National Renewable 

Energy Laboratory (NREL) 40432

Photo by Dennis Schroeder, NREL 21864

Dynamometer

Experiment

Field Experiment

Bench 

Experiment

Photo from Argonne 

National Laboratory
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Most Frequent Failures

• High & intermediate speed stage 

bearings contribute 62% of total 

drivetrain failures.

• A planet bearing failure is more 

costly.

High-Speed and

Intermediate-Speed Stages

Source: S. Sheng, GRC Failure Database, 2018

Note: HSS = high-speed shaft; IMS = intermediate-speed shaft; LSS = 

low-speed shaft

Planet Bearings
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Gearbox Bearing Axial Cracking Main Bearing Failure

Uptower Experiment Objectives

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. Photo from Jurgen Gegner, SKF.

What turbine operations and grid conditions result in critical 

contact conditions for high-speed shaft and main bearings?

Load impacts on component reliability addressed properly?

Brake, D. “WTG SRB Main Bearing Failures.” Paper presented at the 2013 

UVIG Wind Turbine/Plant Operations & Maintenance Users Group 

Meeting.
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• Winergy PEAB 4410.4 gearbox and SKF cylindrical roller bearings

o Instrumentation focused on high-speed shaft, bearings, and lubricant

– Shaft speed

– Cage speed

– Roller speed

– Shaft torque

and bending

– Stray current

– Bearing temperatures

– Air temperature

and humidity

– Lubricant temperatures

and moisture content

– LogiLube and Poseidon

lubricant monitoring

and routine oil samples

– SKF IMx-8 system.

Gearbox Instrumentation

Photo by Mark McDade, NREL 49050

Sliding

[1]
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• Lumped-parameter dynamics model 

o Transmission error

o Bearing clearance

o Nontorque loads

o Gravity

• Simulate normal operation and transient events efficiently

• Failure modes, such as planet bearing fatigue, can be included

• Validation on loads will be performed during DRC 1.5 uptower testing.

Modeling of Bearing Loads & Stresses

Planet-Bearing-Stage Model

High-Speed-Shaft Model

Different modules used to 

reduce computation time

Nonlinear, Time-Dependent Equations of 

Motion

Illustration from [2]
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• Roller dynamics model (analytical) based on:

o Harris roller dynamics model [3,4]

• Lubricant hydrodynamics model based on:

o Bercea cage friction model [5]

o Dowson and Higginson lubricant model [6]

Model Development: Roller Dynamics

QQiiii − QQoo + FFcc = 0 (1)

FFiiii − FFoo + FFvv − QQcccc = 0 (2)

MMiiii − MMoo +

1

2 μμccccDDQQcccc = FFFFmm

dd rr

dddd (3)

�

=1

zz

QQiiii cccccc − FFrr = 0 (4)

ddmm�

=1

zz

QQcccc − DDcc FFcccc = 0 (5)

Forces and speeds of a roller Force balance of a single roller
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• Good agreement between model & experiments

• Outside the load zone, the roller speed is less than its 

theoretical value for pure rolling conditions.

Model Validation: Roller Speed Zone

Rated power

Load Zone
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Roller Speed Statistics

• Good correlation between model and experimental 

results

• Roller speed less than theoretical at low wind speed

o Indicates significant roller sliding

Note: GS-in = inboard generator side; rpm = revolutions 

per minute; m/s = meters per second
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• Significant sliding present between 110 & 220 seconds

o Related to controller settings

• Sliding occurred because of high speed with no load

o Roller/raceway wear could occur.

Roller Sliding During Startup

Operating Condition Cage Speed

Sliding Sliding

Note: kW = kilowatt; kNm = kilonewton-meter Note: GS-in = generator side inboard
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Roller Sliding During Emergency Stop

• No significant sliding occurred

o Limited roller sliding present only when braking started

• Strong impact loading initiated by the braking

• Maximum torque exceeded 169% of rated.

Operating Condition Cage Speed

Sliding

Sliding

Note: kW = kilowatt; kNm = kilonewton-meter Note: GS-in = generator side inboard
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• Sliding occurs when generator disconnected at

75 seconds

• High sliding risks under high-speed & low to zero load.

Roller Sliding During Normal Stop

Operating Condition Cage Speed

Sliding

Sliding

Note: kW = kilowatt; kNm = kilonewton-meter Note: GS-in = generator side inboard
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• Unique experimental results on bearing roller and cage 

speed presented

• Analytic model for calculating bearing speed described

o Model validated through uptower experiments

• Bearing speed affected by drivetrain load and speed

• Bearing sliding widely present during regular turbine 

operations

• Significant sliding occurs during transient events

o Can lead to bearing failures or shortened life

o Risks of sliding-induced failures to be quantified in the future.

Conclusions
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Photo by Dennis Schroeder, NREL 49418

This work was funded by the U.S. Department of Energy under Contract No. DE-AC36-

08GO28308 with the National Renewable Energy Laboratory and cooperative research 

and development agreement 17-694 with Flender Corporation, and 16-608 SKF GmbH. 

Funding for this work was provided by the DOE Office of Energy Efficiency and 

Renewable Energy, Wind Energy Technologies Office.

Contacts: 

yi.guo@nrel.gov

jonathan.keller@nrel.gov

Acknowledgments
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[1] J. Keller and S. Lambert. Gearbox Instrumentation for the Investigation of Bearing Axial

Cracking (Technical Report). NREL/TP-5000-70639. National Renewable Energy Laboratory,

Golden, CO (US). https://www.nrel.gov/docs/fy18osti/70639.pdf, 2018

[2] T. M. Ericson & R. G. Parker, Natural Frequency Clusters in Planetary Gear Vibration. J. Vib.

Acoust 135(6), 061002, 2013

[3] T. A. Harris, An analytical method to predict skidding in high speed roller bearing, ASLE

Transaction, 1966

[4] T. A. Harris and M. H. Mindel, Rolling element bearing dynamics, Wear, 1972

[5] I. Bercea, et al. Simulating roller – cage pocket friction in a tapered roller bearing, European 

J. Mech. Eng., 1997

[6] D. Dowson & G. R. Higginson, “The effect of material properties on the lubrication of elastic

rollers”, Journal of Mech. Eng. Sci., 1960 

References