Roller Sliding in Wind Turbine Gearbox High-SpeedShaft Bearings

4th Conference for Wind Power Drives 2019, Aachen, March 13, 2019

David Vaes, SKF

Jon Keller, National Renewable Energy Laboratory

Pietro Tesini, SKF

Yi Guo, National Renewable Energy Laboratory

Photo by Jonathan Keller, NREL 49037

NREL/PR-5000-73428
© SKF Group

• Background

• Measurement setup 

• Numerical model building and verification

• Transient load conditions

• Conclusions

Overview
Background

Photo from SKF
© SKF Group

Background: Premature Failures with White Etching Cracks

HS-S

HIS-S

LS-P

• During recent years several 

countermeasures have been 

taken

• Since introduction of blackoxidising, no serial failure 

case reported by gearbox 

original equipment 

manufacturer 

• Some failures still reported 

today by after market and 

end users:

– Proper statistics are 

missing.

Early cracks occur 

commonly within the 

first 1–3 years of 

operational time 

(<10% of the 

calculated rating life).

Photos from SKF

LS-P = Low-Speed Planet

HS-S = High-Speed Shaft

HIS-S = High Intermediate-Speed Shaft

Illustration from SKF 
© SKF Group

Premature Bearing Failures: Understanding the Drivers

‘Cyclic’ Stresses

and Loading

Rolling Contact 

Fatigue

‘Accelerated’ Fatigue (Premature Spalling)

Higher ‘Stresses’ Lower ‘Material Strength’

WEC in bearing test 

exposed to additional 

tensile stresses.

WEC in bearing test 

exposed to short heavy 

loads.

White Etching Cracks (WECs) Occurrence in Bearings

Structural Stresses

(Sub-Surface)

Frictional Stresses and Wear

(Surface, Near-Surface)

Hydrogen

(Surface and Sub-Surface)

WEC in larger bearing 

rolling contact fatigue.

WEC in bearing test 

exposed to water standstill corrosion.

WEC in bearing 

exposed to electrical 

currents.

WEC in bearing test 

running under mixed 

friction and slip.

WEC in bearing test 

that was hydrogen 

charged.

Short Heavy Loads

Standstill Corrosion Electrical Current

Role of lubricant and tribochemistry

Low Film and Slip

Photos from SKF 
© SKF Group

• The exact combination of drivers that explains the failures in wind gear 

units is not yet understood:

- Limits of current solutions are not fully understood

- A better understanding of critical operating conditions in wind gearboxes still 

required

• Simulations and measurements complete each other.

Critical Operating Conditions in Wind Gearboxes

Simulation

• Requires a detailed set of 

boundary conditions

• Requires tuning of model 

parameters

• Disturbances are negligible

• Possibility of in-depth 

analysis of roller kinematics.

Measurement

• Provides data in complex 

operating conditions

• Only requires to process the 

measured signals

• Limited number of output 

parameters

• Measurement disturbances and 

input uncertainties.

Photo from SKF 

Illustration from SKF 

Illustration from SKF 

Cons

Cons Pros

Pros
Measurement Setup

Photo by Jonathan Keller, NREL 49037
© SKF Group

• 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 iMX8 system.

Winergy PEAB 4410.4 Gearbox and SKF Cylindrical Roller Bearings

Gearbox Instrumentation

Photo by Mark McDade, NREL 49050

Sliding

Source: Keller, J. and S. Lambert, Gearbox Instrumentation for the Investigation of Bearing Axial Cracking, NREL/TP-5000-70639, 2018.
© SKF Group

GE 1.5 SLE turbine:

• Blade flap and edge bending

• Blade pitch angles

• Rotor azimuth and speed

• Main shaft torque and bending

• Active and reactive power

• Nacelle yaw

• Tower bending and torsion

• Wind vane offset

M5 met tower:

• Air temperatures and humidity

• Wind speed and direction

And more...GPS time stamped.

Turbine and Meteorological (Met) Tower 

Instrumentation

Photo by Dennis Schroeder, NREL 21884

GE 1.5 SLE

ESS Mk 6 controller

M5 

Met 

Tower

Photo by Dennis Schroeder, NREL 49409
© SKF Group

Roller and Cage-Speed Measurement

Cage-speed measurement:

• Pin passage detected by 

proximity sensor

• One speed measurement per 

cage revolution.

Roller-speed measurement:

• Magnetized roller

• Changing magnetic field detected 

by coil next to the bearing

• Position of magnetized roller 

determined by cage pin.

Cage Pin

Magnetized

Roller

Photos by Jonathan Keller, NREL 40979 and 40981

Illustration from SKF 
Numerical Model Building and 

Verification

Illustration from SKF 

Illustration by NREL 
© SKF Group

Bearing B

Bearing A

• Mean cage speed during each revolution is available

• Instantaneous roller speed is available but highly disturbed

• Operating bearing clearance is unknown (bearing inner ring temperature often not available)

Measurement Limitations and Processing

Processed Rolling Element (RE) Speed vs. Averaged 

at Stationary, Fully Loaded Operating Condition

Speed [rpm]

Time [s]

Postprocessing of the measurement is necessary:

1. Select time intervals where the cage speed is 

constant

2. Use several cage revolutions to filter the 

disturbance of the roller speed

3. Select best measured intervals.
© SKF Group

Measurement Screening

1. Systematic detection of all cage-speed plateaus: Speed [rpm] 

Time [s] 

2. Least-squares fit of a piece-wise approximation of the roller speed:

Azimuth [deg] 

Speed [rpm] 

3. Systematic selection based on error: Speed [rpm] 

Azimuth [deg] 

Final selection, based on 

most interesting and diverse 

operating conditions to 

increase the validity of the 

semiempirical model.

rpm = revolutions per minute | s = seconds | deg = degree
© SKF Group

• At lower operating temperature, the rollers decelerate significantly more in unloaded zone

• Higher temperature  lower viscosity  less drag losses on rollers in unloaded zone 

slower deceleration

Effect of Temperature on Roller Speed: “Down Slope” 

0 100 200 300 400

Cage position [deg]

1500

2000

2500

3000

3500

4000

Roller speed [rpm]

RS

RE speed using averaging

Temp = 36°C

0 100 200 300 400

Cage position [deg]

1500

2000

2500

3000

3500

4000

RS

RE speed using averaging

Temp = 57°C

0 100 200 300 400

Cage position [deg]

1500

2000

2500

3000

3500

4000

RS

RE speed using averaging

Temp = 67°C
© SKF Group

• A significantly larger temperature difference is measured on bearing B than on bearing A:

– Bearing B has much smaller radial clearance and a larger loaded zone than bearing A

• Slower deceleration of the rollers in unloaded zone at increasing temperature (lower oil 

viscosity)

• Drag losses increase with the size of the roller (larger projected surface of the rollers).

Effect of Temperature and Oil Viscosity – Overview Temperature Difference [C] 

Time [min]

min = minute | μm = micrometers

Temperature Difference of Bearing Rings

Clearance as Function of

Temperature Difference of Bearing Rings

Bearing A

Bearing B
© SKF Group

The model is designed by two SKF proprietary software.

SKF Numerical Modelling

SKF BEAST SKF SimProExpert

QJ bearing replaced by 

nonlinear stiffness.

• Transient multibody 

dynamic solver

• Detail contact calculation, 

elastohydrodynamic layer 

lubrication

• Cage-roller interaction

• Drag losses not 

automatically modeled.

Linear rotational damping torque is applied 

to both the cage and the rollers.

Illustrations from SKF 
© SKF Group

Analytical Model Predicts Roller and Cage Sliding

Roller dynamics model (analytical):

• Harris roller dynamics model

Lubricant hydrodynamics model based on:

• Bercea cage friction model

• Dowson and Higginson lubricant model

0 QQ F i oc −+=

0 F FFQ ij o v cg −+− =

1

2

r

i o cg cg c

d M M DQ J d

ω

μ ω φ

−+ =

Tangential

Radial

Torsional

Primary Governing Equations

Cage

Source: Guo, Y. and J. Keller. Forthcoming. Analytic Formulations of Rolling Element 

Bearing Sliding in Wind Turbine Gearboxes, Mechanism and Machine Theory.

Frictional Energy Loss

Roller Free Body Diagram

Illustration by NREL 
© SKF Group

Parametric Studies To Verify Model Parameters

Example in BEAST: Influence 

of Rotational Damping

Speed [rpm] 

Azimuth [rad] 

Example in Analytical Model: 

Influence of Temperature

Speed [rpm] 

Azimuth [rad] 

Nms = newtonmeter-second | rad = radian
© SKF Group

Verification of Simulation Results

Speed [rpm] 

Azimuth [rad] 

Torque = 7,930 Nm, 

TOR ~ 41o

C, Toil = 45oC

δB = 5μμm, δA = 95μμm 

Azimuth [rad] 

Torque = 7,930 Nm, 

TOR ~ 63oC, Toil = 63o

C

δB = 30μμm, δA = 145μμm

Azimuth [rad] 

Torque = 9,520 Nm, 

TOR ~ 57o

C, Toil = 41oC

δB = 20μμm, δA = 145μμm 

Input

Nm = newtonmeters | TOR = Outer ring temperature | Toil = Oil supply temperature | δ = Clearance

Roller Speeds Roller Speeds Roller Speeds
Measurements at Transient 

Conditions

kW = kilowatt | LSS = Low-speed shaft | HSS = High-speed shaft | kNm = kilonewtonmeter Power, Torque and Speed

Drivetrain Conditions during Emergency Stop
© SKF Group

Transient Conditions – Emergency Stop

21:49:00 21:49:10 21:49:20

Mar 04, 2018 

-500

0

500

1000

1500

2000

Power [kW]

Torque LSS [kNm]

HSS speed [rpm]

21:49:00 21:49:10 21:49:20

Mar 04, 2018 

-1000

0

1000

2000

3000

4000

5000

Roller speed [rpm]

Roller speed at emergence stop: RS

Measured

Theoretical

21:49:00 21:49:10 21:49:20

Mar 04, 2018 

-1000

0

1000

2000

3000

4000

5000

6000

Roller speed [rpm]

Roller speed at emergence stop: GS

Measured

Theoretical

• Torque oscillations at drivetrain 1st

eigenfrequency

• Oscillations result in cage and roller dynamics

• Rotor side more sensitive to torque oscillations 

than generator side

• Roller speed measurements unreliable at low 

speed conditions.

• At brake engagement roller speed reduces to 

about 80% slip and accelerates back in about 

1.5 seconds.

Rotor Side Generator Side

Power, Torque and Speed

Drivetrain Conditions

21:49:00 21:49:10 21:49:20

Mar 04, 2018 

0

100

200

300

400

500

600

700

Cage speed [rpm]

Cage speed at emergence stop: RS

Measured

Theoretical

21:49:00 21:49:10 21:49:20

Mar 04, 2018 

0

100

200

300

400

500

600

700

Cage speed [rpm]

Cage speed at emergence stop: GS

Measured

Theoretical

Rotor Side Generator Side
© SKF Group

Transient Conditions – LVRT (50% Drop for 300 milliseconds)

21:46:00 21:46:04 21:46:08 21:46:12 21:46:16 21:46:20

Mar 05, 2018 

0

200

400

600

800

1000

1200

1400

1600

1800

Power [kW]

Torque LSS [kNm]

HSS speed [rpm]

21:46:00 21:46:04 21:46:08 21:46:12 21:46:16 21:46:20

Mar 05, 2018 

2500

3000

3500

4000

4500

5000

5500

6000

Roller speed [rpm]

Roller speed at LVRT: GS

Measured

Theoretical

21:46:00 21:46:04 21:46:08 21:46:12 21:46:16 21:46:20

Mar 05, 2018 

1000

1500

2000

2500

3000

3500

4000

4500

Roller speed [rpm]

Roller speed at LVRT: RS

Measured

Theoretical • Torque oscillations at drivetrain 1st

eigenfrequency after Low-Voltage 

Ride Through (LVRT)

• Load oscillations resulting in cage 

and roller dynamics

• Roller speed reduces to about 50% 

slip and accelerates back in about 

0.5 seconds.

Rotor Side Generator Side

21:46:00 21:46:04 21:46:08 21:46:12 21:46:16 21:46:20

Mar 05, 2018 

460

480

500

520

540

560

580

600

620

Cage speed [rpm]

Cage speed at LVRT: RS

Measured

Theoretical

Rotor Side

21:46:00 21:46:04 21:46:08 21:46:12 21:46:16 21:46:20

Mar 05, 2018 

500

520

540

560

580

600

620

640

660

Cage speed [rpm]

Cage speed at LVRT: GS

Measured

Theoretical

Generator Side

Power, Torque and Speed

Drivetrain Conditions
© SKF Group

• Operating at low 

load results in much 

higher slip levels.

Roller Speed at “Curtailment”

1500 kW 250 kW

Rotor Side

Generator Side

Inboard
Future Steps and Conclusions

Photo by Dennis Schroeder, NREL 49418
© SKF Group

• Repeat the procedure for the cage speed at low load. The proximity sensor is not affected by the 

same disturbance as the induction coil.

• Simulation of transient conditions (i.e., when measurement cannot be efficiently filtered from noise).

• Use simulation results to evaluate critical conditions for the bearings (e.g., by power slip density or 

cumulative frictional energy).

Ongoing Steps

Power Slip Density [W/mm2] 

Azimuth [rad] 

Speed [rpm] 

Azimuth [rad] 

Roller Speed Power Slip Density Frictional Energy

W/mm2 = watts per millimeters squared | J/s = joules per second

Azimuth [rad] 

Frictional Energy [J/s] 
© SKF Group

• Measurement of roller and cage speed gives useful insight in the bearing kinematics at 

different operating conditions:

– Low load/curtailment

– Emergency stop

– LVRT

• High roller slip and accelerations have been measured at these events

• BEAST model has been built and shown to be able to accurately predict roller and cage 

behaviour at different loads and temperature

• Next steps:

– Apply and validate the models at special events

– Evaluate the roller slip losses at special events.

Conclusions
© SKF Group

Thank you for your attention!

Vaes, D., Y. Guo, P. Tesini and J. Keller. 2019. Investigation of Roller Sliding in Wind Turbine 

Gearbox High-Speed Shaft Bearings (Technical Report). NREL/TP-5000-73286. National 

Renewable Energy Laboratory (NREL), Golden, CO (US). 

http://www.nrel.gov/docs/fy19osti/73286.pdf

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 16-608 with SKF GmbH and 17-

694 with Flender Corporation. 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.