the best springs you haven’t tried yet
TRANSCRIPT
ROTORCLIPCOMPANY,INC.·187DavidsonAvenue·Somerset·NJ08873·USA
Evenifyouroutinelyusespringelementsinyourdesigns,chancesareyouhaven’ttriedwavespringsyet.You’renotalone.Mostengineershaveplentyofexperiencewithtraditionalcoilanddiscsprings,whilesingle-andmulti-turnwavespringsremainsomethingofamystery.Yetwavesprings,particularly
thosemadefromflatwire,shouldbebetterknowngiventheircompellingengineeringadvantages.
Chiefamongtheseadvantagesisanexcellentforce-to-workheightratio.Well-designedwavespringscan
producethesameorevengreaterforcesascoilspringswhoseworkheightsareuptotwiceaslarge.
Goodwavespringsalsoproduceamoreconsistentforceacrossawiderangeofdeflections.Theirdeflectioncurveshaveawider,flatterlinearforceregionthaneithercoilordiscsprings.Wavespringstransmitthosespringforcesonlyintheintendedaxialdirection.Theydon’tsufferfromthetorsionalloadingandtwistingthatcanmakeconventionalcoilspringslesseffective.
Inadditiontotheirfavorableforcecharacteristics,wavespringshaveassemblyadvantages.Manywavespringshavetheabilitytoself-locateinboredholesoronshafts.Andasinglemulti-turnwavespringcanfrequentlyreplaceanentirestackofdiscsprings,eliminatinganassemblystep.
Asdrop-inreplacementsfortraditionalspringelements,wavespringscanimprovetheperformanceofspring-loadedcomponents.Buttomaximizetheengineeringpotentialofwavesprings,youshoulddesignaroundtheiruniquequalities,whichinclude:
THEBESTSPRINGSYOUHAVEN’TTRIEDYETFlatwirewavespringsofferthebestbalanceofsizeandspringforce.Here’swhatyouneedtoknowtodesignwiththesehigh-performancealternativestotraditionalsprings.
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SINGLE-TURN
WAVE SPRING.Applicationsincludebearingpreload,connectorandfluidpowersealsaswellasnoiseandvibrationattenuation.Theytypicallytaketheplaceofstampedwavewashers.
MULTI-TURN
WAVE SPRING.Applicationsincludemechanicalsealsaswellasavarietyofpowertransmissionandfluidpowersystemsusedinautomotive,medicalandindustrialequipment.Thesetypicallyreplacecoiledspringsorstacksofdiscsprings.
Coil springs require as much as twice the working height of a wave spring.
ROTOR CLIP COMPANY, INC.·www.rotorclip.com·Phone:1-732-469-7333·Email:[email protected]
• Axial Space Savings.Themostimmediatelynoticeablequalityofwavespringsisjusthowmuchaxialspacetheycansave.Instaticapplications,awavespringwilltypicallyneedjust50%oftheworkheightofcoilspringtodeliveranequivalentforce.Indynamicapplications,theworkheightadvantageistypicallyabout30%,lessthanstaticapplicationsbutstillsubstantial.Thereasonforthedifferencecomesdowntothefundamentalsofmulti-turnwavespringdesign.Tominimizethebendingstressesindynamicapplications,thewavespringwilltypicallyrequiremoreturnsthaninastaticapplication.Andmoreturnsresultinadditionalworkheight.
• Consistent Spring Force.Springelementstypicallyexhibitbothlinearandnon-linearforcebehaviors,dependingontheirdeflection.Thislinearbehaviorcanbegraphicallyshownonthespring’sload-deflectioncurve.Ingeneral,thebroaderandflatterlinearregionofthecurve,theeasieritistohitspecificspringforcerequirements.Wavespringshaveaclearadvantageinthisdepartment.Theytypicallyhavealinearforcebetween30and70%deflections.Bothcoilanddiscspringhavemuchnarrowerlinearforceregions(SeeFigure1).Predictablespringforcescanbeabigbenefitinmanyapplications.Mechanicalseals,forexample,successfullyusewavespringstopreloadthesealingsurfaces.Havingapredictablespringforceallowsthesealdesignertostrikethebalancebetweenexcesswearfromtoohighapreloadandleaksfromtoolowapreload.
DESIGN TIP: Alwaysspecify
theloadatworkheight!
• Dimensional Tolerances Improvements.Asidebenefitofawavespring’spredictableforcesinvolvesdimensionaltolerancesforthecavitiesandshaftsthatholdspringelements.Thesetolerancesareoftendrivenbytheneedtoaccommodatethespring’sforce-to-workheightrequirements.Byspecifyingwavespringswhoseforceisrelativelyconsistentacrossarangeofdeflections,it’softenpossibletoloosendimensionaltolerancesonspringcavitiesandshafts.Thisabilityrepresentsamajor,butoftenoverlooked,costfactorinfavorofwavesprings.
• No Torsional Loads.Wheneveryoucompressacoilspringtoitsworkheight,loadsarenotjustintheaxisofcompressionbutalsotorsional.Thesetorsionalloadscancausethepre-loadedcomponenttorotateinuse,potentiallyresultinginexcesswear.Torsionalloadscanalsodecreasethespring’sworkingload.Manyapplications,includingseals,cansufferfromthisrotationalwearproblem.Wavespringsdon’thavethisissue.Theirwaveformscanonlycompressaxially,insomecaseschangingtheirdiameterslightly.
• Increased Travel.Whencomparedtoatraditionaldiscspring,multi-turnwavespringsofferfarmoretravel.Asinglemulti-turnwavespringcaneasilyreplacetheassembliestheusemultiplediscspringstoachievethenecessarytravel.Mostapplicationsrequirerelativelyshorttraveldistances,usuallylessthan1mm.Buttogiveyouanideaofwhat’spossible,we’veengineeredmulti-turnwavespringsthatofferupto50-mmoftravel.Replacingastackeddiscspringassemblywithonewavespringcanresultinbothcostandqualitybenefits.Notonlywillthesinglespringcostlesstoinstallthanthestackeddiscsprings,butitalsoreducesthechancethatthethewrongnumberofdiscspringswillmakeitintoafinishedassembly.
• Dial In Required Stiffness.Wavespringstiffnessisdeterminedbythicknessandtypeofspringmaterialaswellasbythenumberofwavesperturnofspring.Youcaneasilydialinarequiredstiffnessbyoptimizingthenumberofwavesperturnusingwell-knowndesign
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Mechanical seal application
ROTOR CLIP COMPANY, INC.·www.rotorclip.com·Phone:1-732-469-7333·Email:[email protected]
Comparison Load-Deflection-Curves
Deflection, s[mm]
Load
, F[N
]
Disc Spring
Coil Spring
Multiple Turn Wave Spring
formulas(seeSidebar).Increasingthenumberofwavesperturndoeshaveaninfluenceonfreeheightandcompresseddiameterofthewavespring.Moreturnsalsoincreasesthehysteresissinceeachwaveshassomefrictionassociatedwithit.Alltheseeffectsareeasilyoffset.Youcanaccountforfree-heightanddiametereffectsduringdesign.Andhysteresiscanbeeliminatedbypresettingthewavespring—thatis,compressingtoitsworkheightoverseveralcycles.
• Fast Customization.Unlikestampedproduces,whichrequiretooling,wavespringscanbecustomizedonthefly—bychangingtheparametersofthecoilingequipment.Thisabilityallowsyoutospecifycustomwavespringswithoutworryingaboutcostordelaysassociatedwithcustomtooling.Thiscapabilityisyetanothercostfactorweighinginfavorofwavesprings.
WAVE SPRINGS ARE NOT CREATED EQUAL
Alltheadvantageswe’velaidoutforwavespringsassumethatthespringsaremadefromhigh-qualitymaterials,manufacturedinacontrolledenvironmentandproperlydesignedfortheapplicationathand.AtRotorClip,forexample,weproducethewavespringsinaverticallyintegratedmanufacturingenvironment—inwhichwerollourownflatwireandformitinourownmachines.Verticalintegrationallowstoadjusttobatchtobatchvariationsinourrawmaterials,whichincludeavarietyofsteelsandotheralloys.Italsoallowsustokeeptightstatisticalcontrolonalltherollingandcoilingprocesses.
Whenitcomestowavesprings,theimportanceofthismanufacturingexpertisecan’tbeunderstated.Evensmallvariationsinwirethickness,wavegeometryormetallurgycanallhavedrasticeffectsonawavespring’sperformanceinthefield.Forcecharacteristicsinparticularcansufferifthespringhasn’tbeenmadeproperly.Soitpaystospecifyhigh-qualitysprings.Ifyoudo,you’llbeabletotakeadvantageofallthedesignbenefitswavesspringscanoffer.
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Loaded wave spring
Bearing preload
Load-Deflection Curves Compared
Piloted by Shaft Piloted by Bore
ROTOR CLIP COMPANY, INC.·www.rotorclip.com·Phone:1-732-469-7333·Email:[email protected]
DIAMETER EXPANSIONThediameterofaflatwirewavespringincreaseswhencompressedintheaxialdirection.Thefollowingformulaisanexampleofitsmaximumachievableoutsidediameterwhencompressedattheblocklevel.
R=WaveRadius*1[mm] Z=NumberofwavesperTurn
SW=RadialWidthofMaterial[mm] θ=Angle,degrees*2[Grad]
X=1/2WaveFrequency*3 Y=MeanFreeHeight*4[mm]
H=FreeHeight[mm] T=ThicknessofMaterial[mm]
*1WaveRadiusR=[(4Y2+X2):8Y]
*2AngleÐ=[ArcSin(X:2R)]
*31/2WaveFrequencyX=[π·Dm:2Z]
*4MeanFreeHeightY=[(H-T):2]
Max.outsidediameterat100%deflection=0.02222·R·Z·θ+SW
FATIGUE STRESS RATIOCalculatingtheratiobetweenWH12andWH2candeterminetherequirednumberofloadcycles.Theresultsarethencomparedtotheguidelinetablevaluestofigureoutfinalnumber.
δ=FatigueStressRatio σ=Calculatedoperatingstressatlowerworkheight[N/mm2]
σ=Materialtensilestrength[N/mmÐ]
σ2=Calculatedoperatingstressatupperworkheight[N/mm2]
FatigueStressRatioδ=(σ
Mat-σ1)
(σMat
-σ2)
FATIGUE GUIDELINES
FatigueStressRatioδ
EstimatedCycleLife
<0.40 <30.000
0.40–0.49 30.000–50.000
0.50–0.55 50.000–75.000
0.56–0.60 75.000–100.000
0.61–0.67 100.000–200.000
0.68–0.70 200.000–1.000.000
>0.70 >1.000.000
NOMENCLATURE
T=ThicknessofMaterial[mm] OD=OutsideDiameter,[mm]
SW=RadialWidthofMaterial,[mm] ID=InsideDiameter,[mm]
F=Load,[N] Dm=MeanDiameter,*[mm]
W.H.=WorkHeight,[mm] B=Deflection[mm]
H=Freeheight,[mm] σ=OperatingStress[N/mm2]
N=NumberofTurns E=ModulusofElasticity,[N/mm2]
Z=NumberofWavesperturn K=WaveFactor**
*MeanDiameterDm=[(OD+ID):2]***WaveFactorK:
NumberofWavesperturn[Z]: 2.0-4.0 4.5-6.5 7.0-9.5 ≥10.0
WaveFactor[K]: 3.88 2.90 2.30 2.13
SINGLE TURN WAVE SPRING WITH GAP OR OVERLAP Theoperatingstressofasingleturnwavespringshouldneverexceedtheminimumtensilestrengthoftheflatwirematerial.Keepdeflectionbetween30and70%.
DeflectionB=F·K·Dm3
·ID
E·SW·T3·Z4 OD
OperatingStressσ =3·π·F·D
m
4·SW·t2·Z2
MULTI-TURN WAVE SPRINGSTheoperatingstressofasingleturnwavespringshouldneverexceedtheminimumtensilestrengthoftheflatwirematerial.Keepdeflectionbetween20and80%.
DeflectionB=F·K·Dm3
·ID
E·SW·T3·Z4 OD
OperatingStressσ =3·π·F·D
m
4·SW·t2·Z2
NESTED MULTI-TURN WAVE SPRINGSTheoperatingstressofasingleturnwavespringshouldneverexceedtheminimumtensilestrengthoftheflatwirematerial.Keepdeflectionbetween30and70%.
DeflectionB=F·K·Dm3
·ID
E·SW·T3·Z4·N OD
OperatingStressσ =3·π·F·D
m
4·SW·t2·Z2·N
FORMULAS FOR SPIRAL WOUND WAVE SPRINGS