Drivetrain Reliability from

Inherent to Operational

Shawn Sheng and Jonathan Keller

National Renewable Energy Laboratory

Special Reliability Workshop, October 23, 2018

Albuquerque, New Mexico
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Outline

• Reliability from Two Angles

• Drivetrain Reliability Efforts 

at NREL

– Inherent

– Operational

• A Wishful List 

Photo by Lee Jay Fingersh, NREL 17245
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Product Development: Inherent Reliability As the 

Focus

Design

Development

Deployment Testing

Analyze the 

Problem

Requirements 

Constraints

Source: Valvano, Jonathan, and Ramesh Yerraballi. 2014. Embedded Systems: Introduction to ARM Cortex-M Microcontrollers.

Austin: The University of Texas. http://users.ece.utexas.edu/~valvano/Volume1/E-Book/C7_DesignDevelopment.htm.

New requirements 

New constraints

• Hardware

• Software

• Block diagrams

• Data flow graphs

• Specifications

• Constraints

Done
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Wind Turbine Operation and Maintenance: Operational

Reliability As the Focus
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Inherent Reliability

Illustration from Jon Leather, Castrol

Illustration by Cisco Oyague, NREL

• Gearbox loads:

– Planet bearings 

• Operational conditions:

– High-speed shaft and 

main bearings

• Dynamic modeling 

approaches and 

recommended practice.
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Gearbox Loads Investigation: Planet Bearings

• Compare planet bearing load-sharing 

characteristics

– Preloaded tapered roller bearings 

versus cylindrical roller bearings in 

clearance.

Project Summary Report

Industry partners: Romax Technology, Powertrain 

Engineers, Timken, and Brad Foote Gearing

Photo by Mark McDade, NREL 40432

Photo by Jon Keller, NREL 36521
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Operational Conditions: High-Speed Shaft Bearings

• Determine turbine operations conducive 

to crack formation

– Torque, loads, bearing roller sliding

– Tribological conditions: temp, 

moisture, current.

Instrumentation Details

Photo by Jonathan Keller, NREL 40981

Photo by Jonathan Keller, NREL 40979

Photo by Jonathan Keller, NREL 49044

Industry partners: Argonne, SKF GmbH, and Winergy
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Operational Conditions: Main Shaft Bearings

• Instrument and install a SKF SRB-Wind main bearing

– Bearing strain (load), temperature, acoustic emission, vibration

– Electric current across bearing, axial movement, misalignment

– Auto-lube system with routine grease sampling.

Photo from SKF Photo by Jonathan Keller, NREL 49379

Industry Partners: SKF USA Uptower Drivetrain Testing
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Dynamic Modeling Approaches

Gear Contact Analysis

• Gear tooth loading 

• Time-varying stiffness 

• Tooth contact stress 

Structure Finite Element 

(FE) Analysis

• Torsional windup

• Pin misalignment

• Carrier/pin fit

Bearing Analysis

• Nonlinear stiffness 

• Load distribution

• Bearing life

In-House Program

• Gravity/transmission error

• Clearance

• Tooth wedging

Gearbox Analysis

• Deflections

• Misalignment 

• Clearance

• Vibration

• Dynamics 

Full Turbine Analysis

• International Electrotechnical 

Commission (IEC) load cases

• Turbulent wind

• Aerodynamics

• Drivetrain dynamics

• Hydrodynamics

• Structural dynamics
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Recommended Modeling Practice

Major Drivetrain 

Components Recommended Modeling Approach Requirements for Degrees 

of Freedom (DOF)

Rotor/Hub Rigid body with lumped weight N/A

Main shaft Flexible, FE beams Six DOF

Main bearing Stiffness matrices Five DOF

Gearbox housing Flexible, condensed FE N/A

Planetary carrier Flexible, condensed FE N/A

Gearbox shafts Rigid shaft with correct bearing locations N/A

Gearbox support Stiffness matrices Six DOF

Gears Rigid body with contact stiffness Six DOF

Gearbox bearings Stiffness matrices Five DOF (except rotation)

Spline Stiffness matrices Two DOF (tilting)

Bedplate Rigid body or condensed FE N/A

Generator coupling Stiffness matrices Five DOF (except rotation)

Other Factors Effects Priority

Manufacturing tolerance Affects component motions but has limited effects 

on loads Medium

Bearing clearance or 

preload

Affects component motion and loads. Operational 

values w/operating temperature are recommended High

Gear tooth microgeometryAffects frequency spectrum of component motions 

and gear tooth load distribution Low

Bedplate tilting angle Causes gearbox axial loads due to gravity Medium

Gravity Affects component motion and loads High

Nontorque loads Affects component motion and loads High

Gear mesh stiffness or 

transmission error Affects frequency spectrum of component motions Medium

Recommended Minimum Model Fidelity

Recommended Factors to Consider

Source: Guo, Y. et al. 2014. “Gearbox System Modeling Practice, 

Preprint.” Golden, CO: National Renewable Energy Laboratory. 

NREL/CP-5000-63444. https://www.nrel.gov/docs/fy15osti/63444.pdf.
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Operational Reliability

• Benchmarking 

• Performance monitoring

Photo by Gary Norton/DOE, NREL 27358

• Condition monitoring 

• Remaining useful life prediction
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Benchmarking To Identify Gaps 

• Performance and reliability based on industry standardized key 

performance indicators 

• Gearbox reliability database as an example: partner-owned data compared 

with global data.
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Performance Monitoring Using Supervisory Control 

and Data Acquisition Data

• High-speed shaft (HSS) ratio: 

HSS torque to HSS rpm 

• Model developed based on 

normal operation

• Thresholds established 

based on a certain allowable 

false alarm rate

• Two angles: response and 

residual

• Abnormal: outside of the 

established thresholds.

Source: N. Yampikulsakul, E. Byon, S. Huang, S. Sheng, M. You. 

"Condition Monitoring of Wind Power System With 

Nonparametric Regression Analysis." IEEE Transactions on Energy 

Conversion, Vol. 29 (2014): pp. 288-299. 

https://dx.doi.org/10.1109/TEC.2013.2295301. 
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Condition Monitoring via Dedicated Instrumentation

• Stress wave analysis

• Vibration analysis 

• Main filter loop 

particle counting 

• Offline filter loop 

particle counting 

and oil condition 

monitoring 

• Electric signature 

monitoring 

• Periodic oil sample 

analysis.

Round robin testing of wind turbine drivetrain condition 

monitoring techniques.*

*Source: Sheng, S. et al. 2011. Wind Turbine Drivetrain Condition Monitoring During GRC Phase 1 and Phase 2 Testing. 

Golden, CO: National Renewable Energy Laboratory. NREL/TP-5000-52748. https://www.nrel.gov/docs/fy12osti/52748.pdf.
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Frictional Energy Loss-Based Component Remaining 

Useful Life Prediction

• High-speed-stage-bearing axial cracking 

• Frictional energy loss under both normal and transient operations: operation EEnnnn = ff μμ, NN, FF, ∆vv

• μμ: friction coefficient

• NN: number of cycles (roller)

• FF: load

• ∆vv: sliding velocity between roller/raceway

Transient

EEtttt = ff μμ,tt, FF, ∆vv

• μμ: friction coefficient

• tt: time

• FF: load

• ∆vv: sliding velocity between roller/raceway

Operation Transient

EE = EEnnnn + EEtttt

• First- and second-order reliability methods:

GG = ee∗ − (EEnnnn + EEtttt) PPff = PP GG UU < 0

Source: Guo, Yi and Shawn Sheng. 2018. Probability of Failure for Gearbox High-Speed-Stage Bearings in Wind Turbines. 

Golden CO: National Renewable Energy Laboratory. NREL/PR-5000-70972. https://www.nrel.gov/docs/fy18osti/70972.pdf.
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Reliability Improvements: A Wishful List 
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A Recommended Maintenance Practice

Source: Kühn, P. et. al. “Standardization of Data Collection for Wind Turbine Reliability 

and Maintenance Analyses” (workshop, Dublin, Ireland, November 17, 2011).
www.nrel.gov

NREL/PR-5000-72757

Thank You

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. 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.

Special thanks go to the U.S. Department of Energy, drivetrain reliability 

collaborative research partners! 

The Block Island Wind Farm—the first offshore wind farm in 

the United States. Photo by Dennis Schroeder, NREL 40389