Gearbox 2 High-Speed Shaft Loads Analysis

Gearbox 2 High-Speed ShaftLoads AnalysisLatha Sethuraman1, Jon Keller1 , Yi Guo1, and Brian McNiff21NationalRenewable Energy Laboratory2McNiffLight IndustryGearbox Reliability Collaborative All-Members MeetingBoulder, ColoradoFebruary 17‒18, 2015NREL is a national laboratory of the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Gearbox Reliability Collaborative (GRC) Research Motivation High failure rates and repair costs of high-speedshaft (HSS) bearings1%o7% 1%HSS bearings represent nearly 28%of the total cost over the differentfailure modes for a wind gearbox(Horenbeek et al. 2014) Common bearing failure modes(Stadler and Baum 2014)o White etch crackingo SkiddingHSS BEARINGSINTER-MEDIATE SHAFTBEARINGSLOW SPEED SHAFT BEARINGS2% 1%18%48%PLANET CARRIER BEARING13%2%7%PLANET BEARINGHELICAL GEARPLANET GEAR0%Failure % - Data based on 257 gearbox damagerecords in GRC database (Sheng 2013)RING GEARINTERNAL SHAFTHSS COUPLING ReasonsCombinations of speed and loading beyond the “safearea”o Impact loads, transient events or torque reversals canoccur up to 15,000 times per year (Gearsolutions.com2010)o HSS shaft/coupling misalignmento Mismatch between actual loads and design loads.oHSS bearingsSafe area of bearing operation, reproducedfrom (Stadler and Baum 2014)2

Objectives GRC Phase 3 focused on identifying any abnormal loading on the gear, shaft,and bearings on the HSS stage. HSS section of the GRC gearbox 2 was instrumented and tested underdifferent loading conditions. The measurements were used to:o Validate models against dynamometer test data– First shaft torque and bending, then bearing loads– First SIMPACK, then Transmission3D Establish and validate the relationships between external measurementsof shaft loads and bearing loadso INDIRECT APPROACH: Can invasive instrumentation be avoided in future?3

GRC HSS InstrumentationPhoto by Jon Keller,NREL 27895Encoder for azimuthand speedShaft bending strainBearing strain for relative loading(two axes, three axial locations) (four axial slots, two Poisson gauges per slot)and shaft torsion for torqueand temperature (both bearings)Pinion tooth strain forGear KhβSlip ringsPhoto by Scott Naucler,NREL 30252Photo by Scott Naucler,NREL 30251Photo by ScottNaucler, NREL 26259Photo by Scott Naucler,NREL 302504

Strain Gauge Calibration Convert measured bending and torque signals from millivolt per volt (mV/V) tokiloNewton-meter (kNm) for comparison against model predictionsoPinion gauges not calibrated; load distribution is based on relative differences Bending and torque strain gauges were calibrated in situ in the high bay of theNational Wind Technology Center’s (NWTC)’s 2.5-megawatt (MW) dynamometer(Keller and McNiff 2014)oCrane applied torque, positive and negative moments to downwind end of HSSoGauge coefficients derived from moments and shear stress for shaft.HSS calibrationFcalTapered roller bearing(TRB) pairCylindrical roller bearing(CRB)5

Dynamometer Testing The NWTC 2.5-MW dynamometer was used in Phase 3 testing Load cases particularly of interest with respect to HSS stage(Link et al. 2013)oooNontorque loadsRadial misalignment of high-speed shaftEmergency shutdown Model/test validation with torque and nontorque loads (NTL) Dynamometer operated in torque control modeooTorque commanded and applied by the dynamometer at fixed speedOne second of 2,000 Hertz (Hz) data were recorded on the HSS.6

Multibody Model in SIMPACKExisting multibody model (Guo 2014)103GRC gearbox model in SIMPACK (Guo 2014)15SIMBEAM models for HSS in SIMPACK(Image courtesy of Latha Sethuraman)o HSS modeled as flexible three-dimensional(3D) beam structure using node-basednonlinear finite difference approachTo HSS-gearFFE-225o Each beam element modeled as a crosssection connected by nodes with sixdegrees of freedom (DOF)o Bearings modeled as visco-elastic springs.Pinion modeled using force elementFE-225o Nodes 3, 10, and 15 provide orthogonalmoments and torque.CRBTRB-UWForce element(FE)TRB-DWUpwind (UW)Downwind (DW)SIMPACK model for HSS(Image courtesy of Latha Sethuraman)7

Effect of Torque and Nontorque Loads on HSS Loads HSS loading behavior tested at five different power levelsoGenerator offline (zero torque), 25%, 50%, 75%, and 100% rated torque. Combinations of torque and NTL (up to 300 kNm) were alsoexamined to:oInvestigate the impact of thrust, pitch, and yaw on HSS loads.oExamine behavior of generator coupling and how it affects the HSS loads.oIsolate gearbox motion at different power levels and identify potentialcontributions to HSS misalignment.8

HSS Loads at Different Power Levels HSS torqueTorque pulsationsobserved at all powerlevels–10%/revProf. Don Houser (OhioState University) hasbeen investigating thisphenomenon Tooth spacing errors inthe HSS pinion could be apossible contributor tothis variation.9

HSS Loads at Different Power Levels Bending momentsUpwind gaugemeasurementscompare very well.10

HSS Loads at Different Power Levels Bending momentsSIMPACK: twice perrotor revolutionfrequency contentbecomes moreprominent at higherpower.11

HSS Loads at Different Power Levels Bending momentsSIMPACK: relativelyconstant at 0.35 kNm.Experimental results:demonstrate variations.12

HSS Loads at Different Power LevelsUncertainties inmodeling:o Limited information onstiffness for gearboxbushing and generatorcoupling (Zaidi andCrowther 2009,DNV GL 2013).o Gearbox motion notobserved.o Experimental datashowed couplinginduced harmoniccontent.o Coupling assumed tobehave as a linearspring.o Validity of thisassumption remains tobe investigated.13

Effect of NTL on HSS loadso Downwind portionalso relativelyinsensitive to NTL.o Loads upwind ofpinion scale withpower.o External gauge lessaffected by power.o Pitch/yaw momentshave no effect onbending loads.-100o Bending loads arelowest at positive NTL.14

Effect of NTL on HSS loadsInferences:o CRB and upwind TRBare less likely to beinfluenced by NTL.o Downwind TRBexpected to beinfluenced by couplingbehavior.-10015

Effect of Misalignment on HSS Loads Misalignment is a common problem in rotating machineryo Computational studies by Whittle et al. 2011 have demonstrated the influence onbearing lifeIn wind turbine gearboxes, misalignment is typically causedby:oInaccurate assemblyoThe relative position of components shifting after assemblyoGearbox tilting or rocking, or bushing deflectionoLarge torque or transient conditionsoRubber that is sensitive to environmental conditions, creep, or fatigue Important: Can be pre-existing/dynamically induced (gearboxmotion) IMPACT: Possible increase/decrease in shaft bending loadsand therefore TRB loads.16

Misalignment and Gearbox MotionTypes of misalignmentParallel Misalignment(Piotrowski 2006)Angular Misalignment (Piotrowski 2006)“Mixed” Misalignment (Piotrowski 2006)17

Misalignment and Gearbox MotionMisalignment can be initiated by gearbox motionGearbox roll motion measurement(Image courtesy of Latha Sethuraman)Trunnion Z stbdRolling from low bushing stiffness in torsionPitching from low bushing stiffness about YTrunnion Z portSensors for gearbox motion--Trunnion Z Stbd, port: Displacements in Z--My Trunnion bottom: Gearbox tiltMy Trunnion bottomGearbox Tilt measurement ( Image courtesy: Latha Sethuraman)18

Gearbox Motion and MisalignmentLGENGRC gearbox and generator (GEN) inaligned conditionGRC gearbox19

Gearbox Motion and MisalignmentφGENLPhoto by Jon Keller, NREL 32491Misaligning the generator using shims20

Gearbox Motion and MisalignmentGENφβ θ φ β Upwindβ - Gearbox tilt angleDetermined fromproximity sensor xβ x 3" β tan 1 Gearbox tilting upwind21

Gearbox Motion and MisalignmentGENφ θ φ-β Downwindβ - Gearbox tilt angleDetermined fromproximity sensor xβ x 3" β tan 1 Gearbox tilting downwind22

Gearbox Motion and MisalignmentMyy SIMPACKGENφLFz SIMPACK xφ z (millimeters)000.5o5.51o112o223033Fz SIMPACK Kz L φMyy SIMPACK Kyy φβMisalignment test conditions andSIMPACK modeling23

Effect of Misalignment on HSS Loadso Bending loads upwind of pinionrelatively insensitive to misalignment.24

Effect of Misalignment on HSS Loadso Bending loads downwind ofpinion: dips observed at 1.35oand 2o at 50% power.25

Effect of Misalignment on HSS Loadso SIMPACK simulationspredicted lowest loads at1.35oo Only five experimentaldata points: Difficult tolocate the minima.Could be the actualminima?26

Effect of Misalignment on HSS LoadsInterpreting gearbox roll motiono Gearbox bushings extremely stiffin radial directiono Roll motion 0o No influence on external gauge.27

Effect of Misalignment on HSS LoadsInterpreting gearbox tilt motiono Average gearbox tilting,β up to 0.36oo Influences loads onexternal gauge28

Effect of Misalignment on HSS LoadsGearbox tilting caused thevalleys to shift to the leftAverage gearbox tilting, βup to 0.36o29

Conclusions Examined HSS torque and bending loadsoStrain gauge measurements used to validate SIMPACK modeloShaft torque and bending compare well with SIMPACK model Few major observations:– Measured shaft torque displayed 10% variation per revolutionHSS loads insensitive to NTLo Loads upwind and downwind of the HSS pinion linear with torqueo Loads downwind of the TRB insensitive to torque, more sensitiveto generator alignmento Shaft misalignment reduced loads on downwind portion of HSSo– Relieved weight of brake disc– Reduction in downwind TRB loadsoGearbox motion contributed to misalignment.30

Further Work Conduct TRB load analysis from measurements Compare measurements with analytical tools and SIMPACKmodels for predicting bearing loads Examine additional test cases, such as impact events,including emergency shutdown Use INDIRECT approach as real-time bearing life assessmenttool Assess bearing load zone distribution and contact stress Evaluate source of torque variation, generator couplingdynamics, and their influence on bearing load zone.31

AcknowledgmentsThis work was funded by the U.S. Department of Energy under Contract No. DEAC36-08GO28308 with the National Renewable Energy Laboratory. Funding forthe work was provided by the DOE Office of Energy Efficiency and RenewableEnergy, Wind and Water Power Technologies Office.Photo by Mark McDade, NREL 32734Contact informationNWTC 2.5MW dynamometerLatha SethuramanEmail: Latha.Sethuraman@nrel.govPhone: 303-384-7481Photo by HC Sorensen, Middelgrunden Wind Turbine Cooperative, NREL 1785532