122spec Design

5/23/2018 122spec Design

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DesignHandbookEngineeringuideTo SpringDesign1987Edition

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section Table of Contents PageI Eor to Use the HandbookI Selccdng Spring Configurationshrsh. Pull. Twist or Energy StofageApplications: CommonAvailableConfigurations.3 Spring Materials

CommonSpecifications,Elastic Modulus, MagneticCharacteristics,Heat Treatment,Stress Relaxation,Corrosion,Coatingsand Finishes.SpringWire: TensileProperties,Cost and Availability.SprineStrip: Strength,Formability and Edge Condition.4 Reidual Stress, Fatigue and ReliabilityLoad-CarryingAbility, FatigueTerminology, Modified GoodmanDiagram, Weibull Analysis, Load Loss.

-i Eelical Compression SpringsGeneralDefinitions, Squareness, arallelism,Hysteresis,DesignEquations or SpringRate and Stress,BucklinlChoiceof Operating Stress or Static and Cyclic Applications,Dynamic Loading Impict and Resonance,RectangularWire, StrandedWire, Variable Diameter, VariablePitchand NestedSprings,CommercialToleranc,5 Hot-Wound SpringsDesignConsiderations,End Configurations,Materials,Choiceof OperatingStress,Tolerances.7 Eelirnl Extension SpringsInitial Tension, Types of Ends and Dimensions,DesignEquations,Choice of OperatingStress or Staticand Cyclic Applications,CommercialTolerances.t Garter SpringsJoint Design, DesignEquationsand Tolerances.9 Helical Torsion SpringsMean Diameter, Length, DesignEquations for Rateand Stress,End Configurations,Natural Frequency,Choiceof Operating Stress or Static and Cyclic Applications, Double Torsion and RectangularWire Springs, ioleranc,

l0 f,staining RingsExternal and Internal Types, Ends, DesignEquations,Choiceof StressLevel, Tolerances.

ll Belleville Spring WashersLoad-DeflectionCharacteristics,Mounting, DesignEquations.Choiceof StressLevel for StaticandCyclic Applications, Stackingand Tolerances.12 Flat Springs . .

DesignConsiderations nd Equations or Cantileverand SimpleBeams,Choice of StressLevel andTolerances.13 Specid Spring WashersDesignConsiderations nd Equations or Curved, Waveand Finger Washers,Choiceof StressLevel andTolerances.l{ Power SpringsGeneralDesignConsiderationsand Equations,OperatingStress or Power and hestressed PowerSprings.lS Constant Force SpringsExtension Type_, esign Equations, Mounting and Tolerances,Motor Type DesignEquations or "A"and "B" Type Motors, OperatingStress and Tolerances.f6 Spird SpringsDesignEquations or Hair Springsand Brush Springs.17 Volute SpringsDesignEquationsand Choiceof OperatingStressLevel.It Wire FormsGeneral nformation and How to Specify.19 IndexandReference ln fo rmat ion . . . . . . .1Glossaryof Spring Terminology, Bibliography, Trademarks,ConversionFactors, Abbreviations and Symbols,HardnessScaleConversions, ndex and Lists of Thblesand llustrations.

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Selecting pring ConfigurationsTabb2-1. Spring Configurations. Tv?u cor'if"IstJ&ATIoli ACTt0:ri

T\'PT. COTNCUNATION A.rTION Beam (Section12)Cantilever,Rectangular ection

SimpleBeam

Helical CompressionRound an dRecrangular\t-ire

Constant PitchV_*:

Conrcal : :I

i

Hourglass

(Section5)t

Barrel

Variable-Pitch

hrsh or pull-wide rangeof loads, low deflectionrange.Push-wide load and de-flection range-constant cantilever,rate' TrapezoidalSection

A.>,-:-=::=/:/^

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SpringMaterialsChemical and Physical CharacteristicsWhile certain materials have come to be regarded asry.i.ngmaterials, they are not speciallydesign-dalloys.Spring materials are high strength alloys wtrictr ofienexhibit the greatest strength in the alloy system. Forexample: in steels, medium and high-carbonsteelsareregardedas spring materials. Beryllium copper is fre-quently specifiedwhen a copper basealloy is required.For titanium, cold-workedand agedTi-l3v-llCr-3At isTahle 3-1 Typical Properties of Common Spring Materials.

used. The energy storage capacityof a spring is propotional to the square of the maximum operating strelevel divided by the modulus. An ideal spring materihas high strength,a high elastic imit anda low moduluBecausespringsare resilient structuresdesigned o undergo large deflections, spring materials must have aextensive elastic range. Other factors such as fatigustrength,cost, availability, formability, corrosion resitance, magnetic permeability and electrical conductivity

(l) Elastic moduli, density and electrical conductivity can vary withcold work, heat treatment and operating stress. These variations areusually minor but should be considered if one or more of theseproperties is critical.(2) Diameters or wire; thicknessesor strip.(3) Typicd surface quality ratings. (For most materials, special pro-cessescan be specified to upgrade typical values.)a. Maximum defect depth: 0 to 0.5Voof d or t.

b. Maximum defect depth: l.$Vo of d or t.c. Defect depth: less than 3.5Voof d or t.(4) Maximum service temperatures are guidelines and may vary duto operating stress and allowable relaxation.(5) Music and hard drawn are commercial terms for patented ancold-drawn carbon steel spring wire.INCONEL, MONEL and NI-SPAN-C are registered rademarksoInternational Nickel Company, Inc. BARTpi is a registered radmark of Theisof America. Inc.

Com*on lihmeY,o*qt*Modulrrs S {l}MPr | {ed}r t ' l i d]

il{odnkm ofRlddly G {l}lWPr | {Dd}r d l i d . Mtf {t}*1ffir5 tbl,tu';&lcr .leslCorr&rc-*iY,lty tl% f*c$

$ias lioruelly.tvrlhblc {2}Min. i Mrr.mm {hr.} i mm {h.}

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, i,,'i , ({} 3FtCarbonSteel Wires:Music (5)Hard Drawn (5)Oil TemperedValve Spring

207207207207(30)(30)(30)(30)

79.379.379.379.3

( .s)( .s)( 1.5)( l1 .5)7.86 (0.2E4)7.85 (0.2E4)7.E6 (0.2E4)7.86 (0.284)

0.10 0.004)0.13 0.00s)0.s0 0.020)1.3 (0.0s0)6.350.250)16 (0.625)16 (0.62s)6.350.250)

acca

r20150r501502525300300

Alloy SteelWires:Chrome VanadiumChrome Silicon 207207 (30)(30) 79.379.3 ( 1.5)( 1.5) (0.284)(0.2E4)7.867.86 75 0.50 0.020)0.50 0.020)l r (0.435)9.5 (0.375) ar barb 220 42245 47StainlessSteel Wires:Austenitic Type 302hecipitationHardening l7-7 PHNiCr A2E6

193203200

(28)(2e.s)(2e)

69.075.E7 r . 7

(10.)( l )(10.4)

(0.286)(0.2E2)(0.290)

7.927.E lE.03

222

0.13 0.00s)0.08 0.002)o.lm 0.016)

9.5 (0.375)r2.5 (0.500)5 (0.200)

bbb

2@ 500315 60510 95

CopperBaseAlloy Wires:PhosphorBronze (A)Silicon Bronze (A)Silicon Bronze (B)Beryllium CopperSpring Brass, CAz6/u.

10310 3t17128l r 0

(15)(15)(17)(1E.5)(16)

43.438.64 . 1+8.t42.0

(6.3)(5.6)(6.4)(7.0)(6.0)

8.86 (0.320)E.s3 (0.308)E.75 (0.316)8.26 (0.298)8.53 (0.30E)

l57t22rt70.10 0.004)0.10 0.004)0.r0 0.004)0.0E 0.003)0.10 0.004)

(0.500)(0.s00)(0.500)(0.500)(0.500)

12.512.5tz.512.512.5

bbbbb

9s 2095 2095 2m205 4095 20Nickel BaseAllovs:Inconelo Alloy 600

Inconel Allov X750Ni-Span-C@Monilo Allov 400Monel Alloy K500

2r42t4lE 6179r79

(3 )(3 )(27)(26)(26)

75.E79.362.96. 26. 2

( l t )( 1.5)(e.7)(e.6)(e.6)

8.43 (0.304)8.25 (0.298)8.14 (0.2%)8.83 (0.319)E.46 (0.306)

1 . 5Ir .53.53

0.10 0.004)0.10 0.004)0.10 0.004)0.05 0.002)0.05 0.002)

l2.s (0.500)12.5 0.500)r2.5 (0.500)9.s (0.375)9.5 (0.375)

bbbbb

320 700595 ll009s 200230 4502ffi 500Carbon Steel Strip:AISI 1050l06s1074, 1075r09sBartexo

2072W2W207207

(30)(30)(30)(30)(30)

79.379.379.379.379.3

( l . s )( l 1 . 5 )( l 1 . 5 )( r.5)( l 1 . 5 )

7.E6 (0.2E4)7.86 (0.284)7.E6 (0.2E4)7.E6 (0.2E4)7.86 (0.284)

77777

0.25 0.010)0.0B0.003)0.0E 0.003)0.08 0.003)0.10 0.004)

3 (0.125)3 (0.125)3 (0.125)3 (0.125)l (0.040)

bbbba

95 20095 2Wr20 250r20 2509s 200StainlessSteelStrip:Austenitic Types301,302Precipitation

Hardening l7-7 PH

193203

(28)(29.s)

69.075.8

(10)( 1 1 )

(0.2E6)(0.282)

7.927.81

)7

0.0E0.003)0.08 0.003)

1.5 (0.063)3 (0.125)

bb

315 60370 700

Copper aseAlloy Strip:Phosphor ronze A)BerylliumCopper 103r28 (15)(18.5) 4348 (6.3)(7.0) E.16 (0.320)8.26 (0.298) l52 l 0.08 0.003)0.08 0.003) 5 (0.18E)9.5 (0.375) bb 9s 200205 400

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can also be important and must be considered n lightof cost,benefit. Consequently, areful selectionsmustte made to obtain the best compromise.Table3- I lists some commonlyusedalloys along withdau for material selectionpurposes.Data on mechanicalpropeniesare presented n the Spring Wire and SpringStripsubsectionsPages18 and 20 respectively).Speci-fications have been written by many national and inter-n a t i o n a lo rg a n i za t i o n s . h e se sp e c i f i ca t i o n s r ecross-referencedo AssociatedSpring specificationsnTable 3-2r However, correlationbetween he specifica-tions is only approximate.AssociatedSpring specifica-t ions were developedexclusively or high qual i tymaterialfor spring applicationsand are generallymoredetailedand stringent han other specifications.Surface quality has a major influence on fatiguestrength and is often not clearly delineated on nationalspecifications.t is important to useonly those materialsuith the best surface integrity for fatigue applications,particularly those in the high cycle region.In steel alloys, for which processingcosts are a largefraction of product cost, surface quality can vary overan appreciable range. Depth of surface imperfections,suchas seams,pits and die marks, can be up to3.5%of diameter for commercial spring wire grades (ASTMA-227 and A-229). Various intermediatequalities can beobtained. Highest levels are representedby music andvalve spring quality grades which are virtually free ofsurface mperfections. Decarburization, which can alsoadversely affect fatigue performance, follows a similarpattern. Surface quality of spring materials is a functionof the care exercised n their production and processesemployed.Materials producedwith a high level of sur-face integrity are more costly than commercial grades.Elastic ModulusThe modulus of elasticity in tension and shear s vitalto springdesign. Table 3-1 ists recommended alues orcommonly used spring alloys. For most steels and age-hardenablealloys, the modulus varies as a function ofchemical composition, cold work and degreeof agrng.Usually variations ar0 small and can be compensated orby adjustment of reference parametersof the spring de-sign, (e.g. number of active coils, and coil diameter).For most materials,moduli are emperature-dependentand vary inversely with temperature by approximatelyZVc er55'C (100"F).Sincenonambient emperature est-ing is costly, designcriteria should be specifiedat roomtemperature after having made appropriate compensa-tion for the application temperature. Certain nickel-chromium-iron alloys are designed to have a constantmodulusover the temperature ange from -5o to 65'C(-50" to 150'F) and are exceptions o the above rule.For true isotropic materials, heelasticmoduli in tension(E) and shear(G) are related hrough Poisson's atio by theexpression:

EP : 6 - rso hat, for commonspringmaterials,anyone of theparam-etersmay be approximated sing heother two.

SpringMaterialsMagnetic CharacteristicsFor most applications, he question of "magnetic ornot" is adequatelyansweredwith the useof a permanentmagnet. For some applications,even very low levels ofmagneticbehavior can be detrimental.Then, it is desir-able to know the magnetic permeability of candidatematerials and reach agreement between parties on amaximum allowable value. Table 3-3 lists approximatevalues for a number of low permeability materials alongwith other frequently used alloys.Sincepermeabilitycan be alteredby cold work, somevariation can be expected. n general, ow permeabilitymaterials are more expensiveso designersshould specifylow levels only when absolutelynecessary.Often, nitro-gen strpngthened manganesestainless steels are goodchoices because they have good strength at moderatecost.Heat Treatment of SpringsHeat-treating temperatures or springs can be dividedinto two ranges. Low temperature heat treatments n the175'to 510'C (347'to 950'F) rangeare applied o springsafter forming to reduce residual stresses and stabilizeparts dimensionally. For carbon steels, stainlesssteelsand some age-hardenablealloys, low temperature heattreatments are used to increaseor restore the set point.Electroplated carbon steel parts are heat-treatedat lowtemperaturesprior to plating, and baked afterward to re-duce the susceptibility o hydrogenembritflement. Mostlow temperature stress relieving and age-hardening ofsprings are done in air and a moderate amount of oxidemay be formed on the part. No detrimental effects of thisoxide have been noted.High temperature heat treatments are usedto strength-en annealed material after spring forming. High-carbonsteelsare strengthenedby austenittzing n the temperat-ure range760'to 900"C 1480" o 1652"F),quenching oform martensite and then tempering. Some nickel basealloys are strengthenedby high temperature aglng treat-ments. Becausesubstantialoxidation occursat theseel-evated emperatures, t is advisable o preventexcessiveoxidation by usingan appropriateprotective atmosphere.Heat treatmentssuitable or many commonly used ma-terials are listed in Table 3-4. Selection of a temperaturewithin a given rangecan only be made after consideringthe material, size, strength evel, applicationconditionsand desiredcharacteristics.For additionalguidance,As-sociatedSpring engineersshould be consulted. Unlessotherwise noted, 20 to 30 minutesexposureat tempera-ture is sufficient to obtain the bulk of the stress-relievingeffect.Many spring-likeparts involve forms which precludethe use of prehardenedmaterial. In these cases,soft orannealedmaterialmustbe usedand heat-treatedo attainspring propertiesafter forming. Thin high-carbonandalloy steel parts become distorted when hardenedbyquenching. Distortion may be reduced by fixture tem-pering; however, his process s costly and should beavoided f at all possibleby using pretemperedmat-erials.

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Spring MaterialsTable -2. RelatedSpringMaterial Specifications.

.twoeint*d$tri'n* CorrrnonTrldr F{ams saa ASTM AM,S Mfliarl' Ihderd Britlih fieffirlr':f,},[H, ,lltfsrye.tI$N 8SSpring WireAS-5 MusicWire 1085Jl78 1228 5tL2 s4@9 QQv/470(obsolete) 1408 r 5216520r 17223,Sheet1.1200 G3522,SWP-A,B, VAS-10 Oil TemperedCarbonSteel 1066J316 L229 QQw428 2803,grade 17223,Sheet ,1.1230 G3560,swo-A,BAS.2O Cold DrawnCarbonSteel 1066J r1 3 A227 49B 14085216NS or HS 17222, heet1.0500 G3521,sw-A,B,cAS-25 Oil TemperedCarbon Steel* 1070 42,30 5 1 1 5 2803,Grades & 2 L7223.Sheet G3561,swo-vAS-32 Oil TemperedChromeVanadium*

6150tr32 A232 &50 w-22826 QQw412 4750 17225,50CrV4 G3565,swocvryAS-33 Oil TemperedChrome ilicon* 9254n57 A40l QQw4l2 48A 17225,67SiC15 G3566,swosc-vAS-35 Stainless Steel 3030130302J230

A313;Type301,Type3025688 QQw423(obsolete) 58 A 2056 1.43001.4310t7224

G43T4,sus302AS.36 r7-7PH J2T7 A'313,I}pe 531 5678 t7224,1.4568 G4314,sus63IJlAS44 InconelX-750 5698.5699AS-45 CopperBeryllium cA-t72 Bt97 4725,Cond.A QQW-530,Cond.A 2873,cBl0l 1766,6,2.t247.55AS-55 SpringBrass cA-2@ BTY,n60 QQW-321,n@ 2786,czrw 17660,2.0265AS-60A. Phosphor ronze cA-510B159,#5rc 4720 QQw40l 2873,PBlO2 17662,2.1030.39AS-60C Phosphor rorue cA-52r#521AS-70 Chromium Steel s160H A'304A689 970,Part5Spring StripAS-100 1095 A682A684 5r2l5t22 s-7947AnnealedCold-Rolled

44D 1449,Part38,csl00 17232,| . t274 G3 3 1 1 ,SK4MAS- l0l r0741075 A682A684 5r20 42E. 14/19, arrt38,cs.cs80 t7222,l.t2r0 G3 3 1 1 ,s75CMAS-102 1050 A682A684 5085 l$g, Paft3B,cs50 G3 3 l l ,s50cMAS-103 1065 A682A684 5 1 1 5 42F 1440,Part38,cs60. s70 17222,l .1230 G3311,s65CMAS-105 Bartex 1085AS-135-AAS-135-B Stainless Steel 3030130302 At77 55175518, 519 s-5059 QQS-766 58A 14,y'l9, arrt4,302-S-25

t7224,1.43101.4300G43r3,sus-301-csPsus-302-csP

AS-136 t7-7PH 55285529SpringTempers-25043Cond. A t7224,1.4568 G4313,sus-631-csP

AS-144 InconelX-750 5542 N 7786AS-145 Copper cA-r728194,#r72 4s30(AT)4s32(LtzHT) QQC-533 2870,cB101 r7666,2.1247.55AS-155 SpringBrass cA-260 836,f260 4507. zH QQB-613,Comp.2TIzH

2870,czt08 17660,2.026sAS-1@A PhosphorBrorze cA-5r0 8103,#510 4510SpringTemper

QQB-750,Comp.A 2870,PB1O2 17662,2.1030.39AS-160C Phosphor ronze cA-5218r03,#521*Valve spring quality.

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T& 3-3. Magnetic Characteristicsof SomeMaterials.Spring Materials

Tempering is an effective stress-relieving treatment andresults n negligible evelsof residualstress.Somespringmaterials, such as beryllium copper and l7-7 PH, arestrengthened after forming by age hardening. This pro-vides a good stress elief, but may also cause distortionunless special techniquesare used.Environmental ConsiderationsFrequently, operatingenvironment s the single mostimportant consideration for proper spring material se-lection. For successfulapplication, material must becompatible with the environment and withstand effectsof temperature and corrosion without an excessive ossin spring performance. Corrosion and elevated tempera-tures decreasespring reliability. The effect of tempera-ture on spring materials is predictable and discussedbelow. Compatibility of springmaterials and spring coat-ing systemswith corrosiveenvironments s discussed ngeneral terms. For specific applications, the designer surged to rely upon previous experience or consult withAssociated Spring engineers.Stress RelaxationPrimary concern for elevated emperatureapplicationsof springs is stress rela:ration. Stress relaxation is theloss of load or available deflection that occurs when aspring is held or cycled under load. Temperature alsoaffects modulus, tensileand fatigue strength. For a givenspring, variables which affect stress reloration are:stress, time and temperature, with increases n any pa-rameter tending to increase the amount of rela;ration.Stress and temperature are related exponentially to re-laxation. Curves of relaxation versus these parametersare concave upward as is shown in Figures3-1 and3-2.Other controllable factors affecting relaxation include:1. Alloy Type - more highly alloyed materials are gen-

erally more resistantat a given temperature or can beused at higher temperatures.2. Residual Stress - residual stresses emaining fromforming operationsare detrimental to relaration resis-tance. Therefore, use of the highest practical stress-relief temperatures s beneficial. Shot peening s alsodetrimental to stress relaxation resistance.3. Heat Setting- various procedurescan be employedto exposesprings o stressand heat for varying timesto prepare for subsequentexposures. Depending onthe method used, the effect is to remove a usuallylarge first-stage relaxationand/or to establish a resid-

ual stress system which will lessen relaxation influ-ences.In some cases, he latter approachcan be soeffective that in application, compressionspringsmay"grow" or exhibit negative relaxation. Increase infree length does not usually exceed I to ZVo.4. Grain Size - coarse grain size promotes relaxationresistance.This phenomenon s used only in veryhigh temperature applications.Becauseso many variablesare involved, it is impossibleto cite comprehensivedata in a publication of this type,but Table 3-l doesshowapproximatemaximum service

iffi-{.!rBrzsscs.BronzesCarbon SrcelsFlglol tlnconel -{,lloys:6m

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Spring MaterialsFig.3-1. Relaxationterials. versus nitial Stress or SpringMa-

lnitiolstress 103 si )7'5 100 125 150gECorbon Chrome 'steel silicon $Ploinspr ings -- -Shor-peened.- - flShot-peened- - .lond Heot set gflE tExposure f 100 hoursot l49oC (300"F) f IStresses olculoted ot room femPeroture o I.gt

f; r lg.u I

Iilr/ Il t

200 1000

temperatures for many commonly used materials. Itshould be remembered that, if a material is used at itsmaximum temperature, a substantial reduction must bemade in applied stress rom that used at room temper-ature.CorrosionThe effect of a corrosive environment on spring per-formance is difficult to predict with certainty. Generalcorrosion, galvaniccorrosion, stress corrosion and cor-rosion fatigue reduce life and load-carrying ability ofsprings. The two most common methods employed tocombat effects of corrosion are to specify materials thatare inert to the environment and to use protective coat-ings. Use of inert materials affords the most reliableprotection againstdeleterious effects of all types of cor-rosion; however, this is often costly and sometimes m-practical. Protective coatings are often the most cost-effective method to prolong spring life in corrosive en-vironments. In special situations, shot peeningcan beused o prevent stresscorrosionand cathodicprotectionsystems can be used to prevent general corrosion.

Fig.3-2. RelaxationversusTemperatureor Springterials.Exposureemperqture"F)250 300 350 400

Corbon Chromesteel siliconPloinsprings - - -Shot-peenedcrrr mShot-peened - - -ond Heot sei

_ffg,ff3aFg .* ' /f , t

120 r40 160 r80400 600 800Initiqlslress MPo) Exposure emperoture t)

3-5. Guide or Se ectingMinimum Thickne seZinc and Cadmium Coatings.Tahle

(l) Requirements or zinc coating (electrodeposited).(2) Requirements or cadmium plating (electrodeposited)'Finish Type:A. Without supplementary chromate or phosphate reatment.B. With supplementary chromate treatment.C. With supplementary phosphate treatment.

Zixon tranerd SrnelPart PerQ8rA325 t$Cedmirrron LuardStsclPrrrs perQQ'"I4I6 {Z}

ilfisirnur$3Th*nmm*n,till.l .*Ilri*hBrrS*li,$pqryTr*Sr''Lrn$,h:ftrr'.. ion:White :n$d,

,ill&turry!Thiekamrrnar,{lnJ fiilf,$l}pr,''Sl*.iffi,:iH$l*;1ltryC

0.025(0.0010)ABc

% r92r92

0.013(0.00050) B 96

0.013(0.000s0)ABc 96

0.008(0.00030) B 960.005(0.00020)

ABc %3636

0.005(0.00020) B 96

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Coatings may be classified as galvanically sacrificialor simple barrier coatings. Sacrificial coatings for highcarbon steel substrates nclude zinc, cadmium (and al-loys thereof) and, to a lesser degree,aluntinum. Due toits toxicity, cadmium coating should only be specifiedwhen absolutelynecessary.Becausesacrificial coatingsare chemically essnoble han steel, the substrates pro-tected in two ways. First, the coating acts as a barrierbetween substrate and environment. Second, galvanicaction between coating and substrate cathodically pro-tects the substrate. This characteristic allows sacrificialcoatings o continue their protective role even after thecoating s scratched,nicked or cracked. The amountofdamagea sacrificial coating can sustain and still protectthe substrate s a function of the sizeof the damagedareaand the effrciency of the electrolyte involved. The saltspray ife criteria for three thicknessesof sacrificial coat-ings are shown in Table 3-5. Use of conversion coat-ings,suchas chromates,engthens he time of protectionby protecting sacrificial coatings. SaIt spray (fog) is anaccelerated est and results may, or may not, correlatewith corrosive activity in the actual environment. Thetest is useful as a control to ensure the coating wasapplied properly.Metallic coatings are normally applied by electroplat-ing. Since most high hardnesssteelsare inherently verysusceptible o hydrogenembrittlement, plating must becarried out with great care to minimize embrittlementand subsequentdelayed racture. A baking operation af-ter plating s also essential.The designershould observethesepointsduring design and specification:l. Minimize sharp corners and similar stress- oncen-tration points rn design2. Keep hardnessas low as possible.3. Keep operatingstressdown, in accordancewith low-ered hardnessvalue.4. Specify plating thickness, depending upon require-ments.

Specify that parts be baked after plating.Consideruse of HEPrM strips to monitor the platingoperation.Residualstressfrom forming operations must be re-duced by stressrelief at the highest practical temper-ature. Otherwisethe combined effect of residual en-sion and hydrogen absorbed during plating can nducecracking even before plating is completed.

Similar cautions apply if acid cleaning procedures arecontemplated.

Spring MaterialsMechanicalplating providesan effectivemeansof zincor cadmium protection with minimum hydrogen embrit-tlement. It is particularly recommendedwhere partshave high residual stress, have been hardened aboveHRC48 and are used with high static oads. The processcan only be applied to parts that do not tangle and havea clean, fully accessiblesurface. Hydrogen embrittle-ment, although unlikely, is still possible if parts arecleanedby pickling. When appropriate,coatingsof zinc,tin, cadmium,or an alloy of cadmiumcan be applied by

mechanicalplating processes.Cadmium, zinc or more commonly alloys of the twocan be applied o steelspring wire during ts production,and under some circumstances this alternative is highlydesirable. It is best suited to small diameter wire and,in general, for the production of springs not requiringgrinding.Springs are almost always in contact with other metalparts. In a corrosiveenvironment, t is important that thespring material be more noble than components n con-tact with it. Table 3-6 shows a partial list of alloys inincreasingorder of nobility. When any two alloys areplaced in contact in the presence of an electrolyte, theless noble alloy (higher on the list) will be attacked. Theanack will be significantly more vigorous han that of theelectrolyteactingby itself.Table34. Order of Nobility.

G*lvnnic Serics$itb,,'gl$ te ,gneh s,,se*ry8t8r.MagnesiumZincAluminumCadmiumSteel or IronCast IronStainlessSteel, series300(active)Hastelloy@CNickel (active)Inconel (active)Hastelloy BBrasses,BronzesMonelNickel (passive)Inconel (passive)StainlessSteel, series300(passive)Titanium

Least noble(+), Anodic

Most noble (-), CathodicHASTELLOY is a registered rademarkof Cabot Corporation.

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10/49

Spring MaterialsThe list of coatingswhich protect the base materialbyacting as a barrier to the environment s extensive andincreasesas new finishesand techniquesare developed.Table 3-7 shows protection available from some of thecommonbarrier finishes. This information is not for se-lectionpurposes; t simply shows he rangeof protectionafforded. In fact, the hours of salt spray protection mayonly be valid for the specimenand test conditions em-ployed in this series of tests.The tests were conducted

on springs which had undergone 4 million cycles in afatigue test prior to salt spray exposure.While coatings frequenfly increase in effectiveness astheir thicknessesare incrbased,cautions are in order.Tendencies o crack increase as coating thickness in-creases,and the coating ncreases he size of the spring.For example, coatings ncrease he solid height and di-ametral clearances equired for compression springs.Brittle coatingssuch as epoxy can chip under impact,leavingunprotectedspots.Tough coatingssuch as vinylresist chipping, but bruises, tears or abrasionscan ex-pose the base material and trap corrosive agents. Thisallowscorrosion o continueafter exposure,and n thesecircumstances oatedspringsoccasionallyexhibit short-er lives than uncoated springs.Frequently oils, waxes or greasesprovide adequateprotection. Effectivenessof these coatings s often de-pendent on the nature of the surface to be protected. Ingeneral, ustrousor smoothpartswill not retain oils, andwaxes,paraffrn-based ils or greasesare recommended.Steelscan be phosphate oatedby a conversionprocess.Phosphate coatings have a high retention for oils,greasesor paints. The combination of a phosphate andoil coating becomesa colTosion nhibitor more effectivethan either of the components. A similar effect is ob-tained by retaining or deliberately forming oxides onmetal surfaces o hold corrosion inhibitors or lubricants.Oil tempered spring wire is a notable example of thistechnique.Spring WireTensile propertiesof springwire vary with size (Figure3-3). Common spring wires with the highest strengthare ASTM 228and ASTM 401materials.ASTM A313

Type 302, A232 and A230materials have slightly lotensile strength with surfacequalities suitable or fatapplications. Hard-drawn (ASTM 227) andoil temp(ASTM 229) are also supplied at lower strength leand are most suitable for static applications.Most spring wires can be wrapped on their ownmeter (bent around a pin with a diameter equal towire diameter). Exceptions include some copper-balloys and large diameter and/or high strength allBecausestress elieving ncreasesyield strengthofdrawn spring wire, all sharp bends of this gradematshould be made prior to stress relief.Tahles3J. SaIt Spray Resistanceof CommonBaFinishes.hotectiveMateriat Stsnderd SaIt SprayTest Resistancc,hours DescriptionPaints:Japan

LacquerEnamelPaint

15-20311005G4002s-300

Dark colored, usuallydipped, cured by bakingUsually applied by spraing. Air dried.Hard finish; appliedbyspray, brushor dip; curby air or baking.

Oils, waxes

Phosphateswithsupplementaloils, waxes,etc.Cadmium,zinc

l-3002440

24-100

Lubricating, rust-inhibiting, hard dryinganondrying oils.Chemical treatmentconing steel surface o ironphosphatecrystallinesurface. Affords a bondfor oils and paints.Electroplated or mechaplated.This information is basedon laboratory-controlledapplicationsandThe protective material selected,cleanlinessof parts, method of acation, subsequentoperationsand part usage affect performancechoice of a springfinish mustalso considershipping,assembly,enand total cost.


11/49

Spring MaterialsTabk 34. PreferuedDiameters for Spring Steel Wire. Spring StripMost flat springsare made rom AISI grades1050,11074and 1095steel strip. Thesecompositions are lisin ASTM specifications A.682and A684.Tensile strength and formability characteristicsshown in Figure 3-4. The vertical inclined bands deeatethree strength levels as functions of stock thicknand hardness. Horizontal curves indicate minimum bding radii required for the strength evels they inters

Interpolations can be made between any two bandlines for intermediate levels. Formability criteria areen for relatively smooth bends made at reasonablebding rates. Operations which apply forming forces othan smooth bending, or have impact characterismay require larger radii to prevent fracture. Four-spart manufacture, progressive die work and seconforming are examples of operations that often prodless than ideal bending.

Table34. Ranking of RelativeCosts of CommonSpWires.

Most spring wires can be purchased o tighter tolerances. Musicand most nonferrous materials are regularly made to closer tolera

M*trk Sillri {ftm,ftrril Sr*cnd ThirdHrrtnct PrEfrnmr Prcfcrcns0.10 0 . 10. 2 0. 40 . 1 6 0. 80.20 0.220.25 0.2E0.30 0.350.40 0.450.50 0.550.60 0.650.700.80 0.901 . 0 I . lt . 2

1 . 41 . 8') ,

2 . 83 .5

4 .55 .56 .57 .0

9.0r 1 . 01 3 . 01 5 . 0

EnglicbSirrs {la.)First $cmrdhrcfcrrnc hefttwmcc0.0040.0050.0060.0080.0100.0120.0140.0160.01E0.0200.0220.0240.0260.0280.0300.0350.0380.u20.0450.04E0.0510.0550.0590.0630.0570.0720.0760.08r0.0850.0920.0980. 050 . 1 r 20.1250.1350.1480.t620.t770.1920.2070.2250.2500.2E10 . 3 1 20.3430.3620.3750.4060.4370.4590.500

0.0090 . 0 1 l0.0130.0150.0r70.0190.021

0.0310.0330.0400.u7

t . 62.0

2.53.0

2 .12.42.63. 23 .84.24.85 . 0

6.00 . 1 0 20.1200.1300.1400.1560 . 1 7 00.2000.2180.2620.306

7.5E.59. 510.0

12.014.016.0

Wir! Snt tffii;$:::,,.,,::::, , , , , , i l1, , , , , , :i , t

ffi,,#.2.|1}...'1 -Wrl l&wc Patentedand Cold DrawnOil Tempered

ASTM A227ASTM A229

1.01 .3 11MusicCarbon Valve Spring

ASTM A,228ASTM A23O

2.63 . 1 1 .1 .Chrome Silicon ValveStainlessSteel (Type 302)

ASTM A4OIASTM A3l3 (302)

4.07.6

3.4.

PhosphorBronzeStainlessSteel (Type 631)(17-7 PII)ASTMASTM A 313 631) 8.0l l 6.8.

Beryllium CopperInconel Alloy X-750 ASTM BI97 2744 t73 1

Table3-10. Standard Tolerancesor Spring Wire.Dlrrmttr: rnar ln,} tohrw: ulur,{lil;} tlm:Ost,d"Mnrrr:,,il;il,,,{*n.}0.514.71 0.020-0.028)0.71-2.000.0284.078) 0.oloo.ooo4)0.0150.0006) 0.0100.00.0150.02.00-3.00 0.0784. tE)3.00{.00 (0.118J.240) 0.0200.000E)0.0300.0011E)

+f

0.0200.00.0300.06.0G9.00 (0.24G4.354)9.50-r6.00 0.37s4.62s) r 0.050 0.00197)+ 0.070 0.00276) 0.0500.00.0700.0


12/49

N t n N r f f i---t,.Tronsverse Bend

Also known os ocross he groin,perpendicu lor o the ro l l ingdirection. Eosyor good woy.

Longitudinol BendAlso known os with the groin,porollel to rolling direction.Hord or bod woy.

SpringMaterialsHardness evels above HRC 50 result in high strengthbut are not generallyrecommendeddue to notch sensitivity. Surfaceand edge smoothnessbecomecritical andplated parts become highly susceptible to static fracturedue to trapped hydrogen.Parts which cannot be made within formability limitsof pretempered strip are made from annealedstrip andhardenedand tempered after forming. To maintain crit-ical dimensions, t is often necessary o fixture temperthese parts. Sharp bends are not only diffrcult to fabri-cate but arealso undesirable n servicebecause f stressconcentration.The formability limits of annealed pringsteels are presented n Table 3-1 1.In flat spring designswhere the edgeof the strip be-comes an edgeof the part, the type of edge s important,particularly for cyclic applications. Common types ofedges availableare shown in Figure 3-7. Slit edge(No.3) and deburred (No. 5) are preferred for blanked partsand static applications. No. I round edge is recom-mended or cyclic applications o reducethe stresscon-centration and eliminate the edge flaws due to slitting.Configurations shown in Figure 3:7 are approximate,and it is advisable o use both the numericaldesignationand a description when specifying edge condition.Commercial thickness tolerances for spring steel stripEIre resented n Table 3-12. Many flat springsand springwasher designscan tolerate this variation. Since he loadvaries as the cube of the thickness, critical designsmayrequire closer tolerances.Fig. 3-6. TensileStrength versusHardnessof Quenchedand Tempered pring Steel.

l0 k9. DPH or Vickers (VHN)350 450

16 0

Direction of bending with respect to rolling directionis an important consideration.Formability of strip isgreater n transverse han in longitudinal directions(Fi-gure3-5). If a part is designedwith two identicalbendsat 90" to each other, it is commonpractice to orient thepart so that both bendsare madeat 45o o rolling direc-tion. Dmensionless parameter2rlt, often referredto asbend actor, is frequently usedas a measureof formabil-it1'. Materials with low values are more formable thanmaterialswith high values.This measure s only a guidesince it does not allow for tooling considerationsandcomplex strains associatedwith forming operations.Springsteelsare nonnally produced to specifiedhard-ness evels which are relatedto tensile strength(Figure3{). Composition is not shown in Figure 34 becausethe lowest carbon level (AISI 1050)can be usedat highstrength evels and the highestcarbon level (AISI 1095)canbe tempered o the oweststrength evels.In general,higher carbon levels are used when strength s criticaland lower carbon levels when formabilitv is critical.Fig. 34. Minimum TransverseBending Radii or Var-

ious Tempersand Thicknessesof TemperedSpring Steel.

44 46 48 50 52RockwelHordncss HRC)

oJ

F

E 1 . 0EoS o.7s.c

F

Fig. 3-5. orientation of Bend Axis to Rollins Directionfor Transverseand Longitudinal B-ends.-) lndicotesDirectionOf Rolling

N 1 : 2 r

24o E32zo *,g62oo+, ogr8o E5

alo

. o

t4 0

1 20

28 32 36 40 444648 50 52 54Rockwell Hordness (HRC)


13/49

Spring MaterialsOther Spring MaterialsA variety of materials other than carbon steel strip isused or flat springs Table3-13). When high conductiv-ity is required, copperbase alloys are usually specified.Stainlesssteelsare used n applications equiringheat orcorrosion resistance.Typical tensile strength evels'forthese alloys after heat treatment are shown in Table3-13. Bend factors and tensileelongationsare for alloysin "as received" conditionprior to final heat treatment.SpecifyrngHardnessHardness tests are used extensively to inspect stripand flat springsand t is necessary o specify he correctscale. Recommendedhardness scales for steels arepresented in Table 3-14. To obtain accurate readingsfree from the effect of the anvil, it is important to limitthe thickness of the material for each hardnessscale asshown in Figure 3-8 for hard materials and Figure 3-9for soft materials.

Fig.3-7. EdgesAvailable on Steel Strip.No. I Edge

SQUAREStondordmoximumornerrodius:0.08 m(0.003')

ROUNDStondord

BLUNT OUNDSpeciol

OVALSpeciol

BROKENCORNERSSpeciol

No.3 Edge

I

NOR'VIALAS SIIT

'No.5 Edge

_lNo.3DEBURRED

_ll:l)

_l

Formability s determined y slowly bendinga sample ver l80ountil itsends are parallel. The measureddistance between he ends s Nt.For example,f N1 = 4 and :2, then Nr/1= 2*Availableas Barco-Form@rom WallaceBarnesSteelsubsidianv fTheisof America, nc.

Precision olled high-carbonsteelstrip is availablecommerciallyattolerances onsiderablv ess han hevaluesstatedabove.

Table3-11. Formability of AnnealedSpring Steels.

Ttulcknmr(}mrn(in,l Di:recti*nnf:Ssn6

*Iffi r0sNtll , AI$I T065Hrlt *IS,*qid. :, :,:il,{i: tfrs '::il$#$.i..'t,,.,...[-{i t,..',.,Ann*n|ed{*ta rdtpr*iffitrnnx.)

*nntr$st.lmregt ,,t.!l{x"}i

Annwkd($tardflrdlsw{stmnx.)

Annerlsd{sp l. ;'lwnst ,filflX,l*

Aaar*bd{ffiidbnffit'.' , .m&X;}, , '

A W,,.{W.,,..,.l{illffit::,,.: ,.,'1114.1g;)it,, i#ffiffi'.,: ,,r,s**:l} ,,

,,,i{ffi::..,,i,[Uft,,,.ifm.i|

1.9mm(0.076)-over Il l 24 03 24 0 24 03 3) I40.9-1.89mm(0.036{.075") Il l I2 0I I2 0I I2 0I 23 020.374.89mm(0.0154.035) Iil 0I 00 0I U 2 0I II U 2 0I 12 0I0.2-O.36 m(0.008{.014j il 00 00 00 00 1I 00 II 0U2

Table -12. TyoicalHigh-CarbonStripThickness oleraThhkns*: mrn{in,}

'#*tqgry,,Pffi;1iffi. rl;,,Srrip t#idth: rnm.{tor.}t?.7*?6.1 t 76,3*]S4.fl(0.50*?.99) | f3 fffl*12.00)0. 0-0.25 0.004-0.010) 0.005 0.00020) 0.9060.000250.25-0.5(0.010-0.020) 0.0060.00025) 0.0090.0m350.5-0.76 0.020-0.030) 0.0090.00035) 0.0130.000500.7Gt.020.030-0.erc) 0.0100.00040) 0.0r3 0.000s0t .02- .s2 0.040-0.60) 0.0130.00050) 0.0190.00075| .52-2.030.060-0.80) 0.025 0.00100) 0.0380.001s02.03-2.s40.0E0-0.00) 0.038 0.001s0) 0.0510.002002.54-3.r80. 00-0.25 ) 0.0510.00200) 0.0630.002s0


14/49

DPH ks)500 600

FA. J-d. Ilinimum Safe Thicknessesor Hardness Test-ing Hard Materials.800 900.r_----t0.040

0.020

0.0r

035 40 45 50 55 60 65 70HordnessN.umber

Spring MaterialsFig. 3-9. Minimum SafeThicknessesor HardnessTest-ing Soft Materials.

60 65 70 7s 80 85Hordnessumber

2; E2 ;P 3r i

F iC oi ;S EE ?7 .=E . :

(l ) Beforeheat treatment.Tsblc3-14. RecommendedHardness Scalesfor Hardand Soft Spring Alloys.

Thickness:mm {in.}0.890.035)ndover0.64J.860.025-0.034)0.35{.610.01-0.024)0.204.360.0084.014)Under .20 0.008)

"tnncrlcd Stcdand Finnfcrmnr AlbysCA30Nr 5 NDPH

B457307r5TDPH

DPH kg)r00 r20 r40

in =o

, tat5Eo . E=Tabb 3-13. Typical Properties of Spring TemperAlloy Strip.

ldfrtrr,*$l Tendlc@rySrntra{rtr rd} in t:l:t:.,fst.m|,.,, ,,,,,,:8fsd ,:, .';F l'{.1},;,',,,., .,rif.1;tl:':.,,rr,t;,,,;,,;,;:.h,qns., b) b: l , : t ; i 1 , , : : ,

Steel, spring temperStainless 01Stainless 02

r70o246)1300r89)1300 189)c50c40c40

285534

20.7 30)19.3 28)19.3 28)0.300.310.31

Monel 400Monel K500Inconel 600

6e0100)rz0or74)1040151)895c34c30

2402552

17.9 26)17.9 26)2r.4 3r)0.320.290.29

Inconel X-750Copper-BerylliumNi- Span-C

1050 152)1300 189)1400 203)

c35c40c42

2026

352

2r.4 3r)12.8 18.5)r8.6 27)

0.290.33BrassCA 260PhosphorBronzeI1J PH RH95Ol7J PH Condition C

620 (e0)6e0 100)1450210)1650239)

B90890c4c463J61

J2.5flat2.5

l l (16)10.31s)20.3 2e.s)20.3 29.s)

0.330.200.340.34

Assogfrlfi8&tr*ffiwsfi


15/49

HelicalCompressionpri gs

ffq*

1*


16/49

HelicalCompression pringsDiameter increases when a spring is compressed.Although the increase in diameter is usually small,it must be consideredwhen minimum clearancesareestablished.The increase n diameter is a function ofinitial spring pitch and can be estimated from thefollowing equation where p : pitch.

O.D.4*1;6 = + d (s-tIf the spring ends are allowed to unwind, the O.D. atsolid may be greater than calculated by this equation.Long springs buckle (seeFigure 54, page 35) and mayrequire lateral support and larger diametral clearances.Spring IndexSpring index is the ratio of mean diameter to wirediameter or radial dimensionof the cross section (Figure5-15, page 40). The preferred index range is 4 to lZ.Springs with high indexes tangle and may require indi-vidual packagrng,especially f the ends are noi squared.Springs with indexes lower than 4 are diffrcult to form.Free Length

Free length is overall spring length in the free or un-loadedposition (Figure5-1). If loadsare not critical, freelength should be specified. When definite loads are re-quired, free length shouldbe a referencedimension hatcan be varied to meet load requirements.Pitch is thedistancebetweencentersof adjacentcoils and s relatedto free length and number of coils.Type of EndsTypes of ends availableare: plain ends, plain ends -ground, squaredendsandsquaredends ground (Figure5-2).To improve squareness nd reducebucklingduringoperation, a bearing surface of at least 270" s required.Sguaredand ground springs are normally supplied witha bearing surface of 270 o 330". Additional grinding re-sults n thin sections. squared endsonly" arepreferredon springs with small wire diameters (less than 0.5 mmor 0.020), a large index (gtreaterhan 12)or low springrates. Squared ends cost less to manufacture thansquaredand ground ends.

IntroductionHelical compressionspringsare usedto resistappliedcompressionorces or to storeenergy n the push mode.They have the most commonspringconfigurationand arefound in many applications such as auiomotive, aero-space and consumer goods. While the most prevalentform o-fcompression spring s a straightcylindri-d springmade rom round wire, many other forms are produced.Conical, barrel, hourglassor cylindrical forms Ere avail-able, with or without variable spacingbetween coils.Suchconfigurationsareused o reducesolid height,buck-ling and surging, or to produce nonlinear oad deflectioncharacteristics. Energy storage capacity is greater forround wire compressionsprings han for rectangularwirecompressionsprings and can be increasedby nesting.Rectangularwire is sometimesemployed to reduce solidheightor increase he space ffrciencyof the design.Mostdie springs are made from rectangularwire for this rea-son.The SPEC ine of springscontainshundredsof com-pressionspringdesignsusingwire sizes rom 0.15 o 5.26mm (0.006"1o 0.207') diametermusic or stainlesssteelwire.SpecifyingSPECspringssaves esign ime, reducescost for low volume applications and offers improveddelivery.Helical Compression Spring TerminologrSpecial erminology hasevolved in the spring industryto describe eaturesof helicalcompression prings.Theseterms are defined and the relationship between terms isreviewed n Figure 5-1. Communicationbetweendesign-er and springmaker is improved if thesecommon termsareused.Spring DiameterOutsidediameter, insidediameterand meandiameterare all used to describe helical compressionspring di-mensions.Mean diameter is equal to the sum of O.p.and I.D. divided by two, and is employed n spring de-sign calculations for stressand deflection. The O.D. isspecifiedfor springs that operate in a cavity, while theI.D. is specified or springs hat operateover a rod, seator shaft. Minimum diametral clearance between thespring and cavity or rod is:

0.05D- when D. is greater han 13 mm (0.512')0.10D when D. is less han 13 mm (0.512,)D. is the diameter of the rod or cavity.Fig. 5-1. Dimensional Terminology or Helical Com-

+ I f",.r ld L rI-T

Porollels m ( e p )I, lan.h1 |A

P 2 - d :*2tl

nssgpffi,&gmp$


17/49

Helical Compression pringsNumber of CoilsTotal number of coils should be specified as a refer-ence number. For springswith squaredends, the totalnumber of coils minus two is the number of active coils.There s some activity in end coils, but duringdeflectionsomeactive material comes n contactwith the end coilsand becomes inactive. Experience indicates that thisequation s a good approximation.The number of activecoils in springswith plain ends s greater than those withsquaredends and dependsupon the seatingmethod em-ployed. Someuseful guidelines or estimating he num-ber of active coils are presented n Table 5-1.Solid HeightSolid height is the length of a spring with all coilsclosed. For ground springs, solid height is the numberof coils multiplied by wire diameter. For ungroundsprings, solid height is the number of coils plus one,Fig. 5-2. Typesof Endsfor Helical CompressionSprings

Squored nd GroundEndsCoiled eft-hond

PloinEndsGroundCoiled Lefi-hondSquoredor ClosedEndsNot Ground, CoiledRighrhond

ffi@ffiffi@loinEndsCoiled Right-hondTable5-1. Guidelinesor DimensionalCharacteristicsofCompression Springs.

,Di 'cTtxtqd':SS$

Ollftior,,,fhfu,,:,,{tr{t:,,$K[{l:,Oif,;1. , f i , . ,{G $gurd Ouly r.s c*d' , ' , , , ,G ,"

Solid Height(Lr) (Nr + l)d Nt d (Nt + l )d Ntd*Pitch(p) L r - dN. LrNt L r - 3 dNa L r - 2 dNaActive Coils(NJ L r - dp

L r rp

L r - 3 dp

L r - 2 dp

Total Coils(Nt) Na N a + l N " * 2 N " + 2Free Length(Lr) p N t + d P N t p N a + 3 d p N 6 + 2 d

*For small index springs ower solid heightsare possible.

multiplied by wire diameter(Table5-1). If critical, soheight should be specified as a maximum dimensiAfter allowances are madefor plating or other coatinit is good practice to add one-half of the wire diameto determine maximum solid height. With larger wsizesand fewer coils, this allowancecan be decreasSolid height is often measuredby applying a force eqto 110 o l5Vo of the calculated oad at solid. If soheigtrt s not critical, this dimensionshould be omitteDirection of CoilingA helical compression springcanbe either left or righand coiled. If the index finger of the right hand canbent to simulate direction of coil; so that the fingernand coil tip are approximately at the same angular poition, the spring is right-hand wound (Figure 5-3). If tindex finger of the left hand simulates he coil directiothe spring is left-hand wound. If direction of coilingnot specified, springs may be coiled in either directioNested springs with small diametral clearancesshouldcoiled in opposite directions.Squarenessand Parallelism

Squareness f helical compression pringscan be msuredby standing a sample spring on end on a horizonflat plate and bringing the spring against a straighteat right angles to the plate. The spring is rotated to pduce a maximum out-of-square dimension e, (Fig5-l). Normally squared and ground springs are squwithin 3owhen measuredn the freeposition.Squarenshould be checked at both ends. Specifying squarenor parallelism in the free position does not assuresquaness or parallelism under load.Parallelism (Figure 5-1) refers to the relationshipthe ground ends, and is determined by placing a spron a flat plate and measuringthe maximum differenin free length around the spring circumference ep.HysteresisHysteresis s the loss of mechanicalenergyunderclic loading and unloading of a spring. It results frfrictional losses in the spring support system due ttendency of the ends to rotate as the spring is copressed.Hysteresis for compressionsprings s lowFig.5-3. Direction of Coiling Helical CompresSprings.o (:-f-*--)fL---( -----l- CoileLeft-ho----- CoiledRight-hond

WA"SS;$ &ffi&ffiffi$


18/49

the contribution due to internal friction in the springmaterial itself is generally negligible.Spring RateSpringrate for helical compressionsprings s definedas the change n loadper unit deflection and s expressedas shown:

HelicalCompressionpringsStressWire in a helicalcompressionpring s stressedntorsion.Torsionalstresss expressed s:

This equation is valid when the pitch angle s less thanl5o or deflectionper turn is less thanD/4. For largedeflections per turn, a deflection correction factor (Re-ference3, page 102) should be employed.The load deflection curve for helical compressionsprings s essentiallya straight line up to the elastic imit,provided that the amount of active material is constant.The initial spring rate and the rate as the spring ap-proachessolid often deviate from the averagecalculatedrate. When it is necessaryo specify a rate, it shouldbespecified between two test heights which lie within 15ro 85Voof the full deflection range (Figure 54).When compression prings are used in parallel, thecomposite ate is the sum of the rates for individualsprings. For compressionsprings in series,the rate iscalculated rom:k - (s-3)

This relationship is often used to calculate the rate forspringswith variable diameters. The technique nvolvesdividing the spring into many small incrementsand cal-culating the rate for each increment. The rate for thewhole spring is computed rom the rate of the incrementsaccording o the equationabove.Fig.54. Curve for Helical

(s4)BendingstressesEuepresentbut can be ignored exceptwhen the pitch angle is greater than 15oand deflectionof each coil greater thanD/4 (Reference3, page 102).Under elastic conditions,torsionalstress s not uniformaround the wire cross section due to coil curvature anda direct shear load. Murimum stressoccurs at the innersurfaces of the spring and is computed using a stresscorrection factor. The most widely used stress correc-tion factor Kwr is attributed to Wahl. It is shown belowand in Figure 5-5.

t :H*-

4C - I 0.615r ( w r : 4 c - 4 - c

I I -. , 4 C - 1 0 . 6 1 5A w r = 4 c - 4 - cFor 2e/oset pointor fot igue

n qK w 2 = l * tFor springs withset removedI

{*,* '

I

, P G d 4r : - : -f gD3N. (s1)

l l l l- J - - I -k r kz k r " ' kn

(s-s)In some circumstancesafter yielding occurs, resultantstressesare distributed more uniformly around the crosssection. Then, a stresscorrection factor Ks,2which ac-counts only for the direct shearcomponent is used.

K w z : 1 + 0.5C (s4)In other circumstances, uch asstatic oadingat elevatedtemperatures, stress distribution tends to become uni-form around the cross sectionand can best be estimatedby using no correction factor. Use of different stresscorrection factors can lead to confirsion. In publisheddata, t is essential o know which stresscorrection fac-tors were used. (The stresscorrectionfactor used by adesigner must be the same as that used to develop thedata.) Methods to calculate stress or different applica-tions and the use of stress correction factors will beFig. 5-5. Wahl SrressCorrectionFactors or Round WtreHelical Compression nd ExtensionSprings.2 . 2

2 .0

J r . 8o;Y ' t . 6oI

t r . 43

o

6C = D / d

Typical Load DeflectionCompressionSprings.

Assosl$lfi8&ffi*ffiff


19/49

HelicalCompressionSpringsdiscussedn the following paragraphson choice of oper-ating stresses.loadsWhen deflection is known, loads are determined bymultiplying deflection by the spring rate (Equation 5-Z).When the stress s known or assumed, oads are deter-mined from Equation 5-4. The procedure used to deter-mine loads of variable rate springs is complex. In thiscase, the load deflection curve is approximated by aseriesof short chords. The spring rate is calculated foreach chord and multiplied by deflection to obtain theload. The load is then added to that calculated for thenext chord. The process s repeateduntil load has beencalculated for the desired value of deflection (Figures4).Loads shouldbe specifiedat a test height. Because heload deflectioncurve is often not linear at very low loadsor at loads near solid, loads should be specifiedat testheightsbetween 15 and 85% of the full deflection range(Figure 54).Loads are classified as static, cyclic or dynamic. Instatic loading applications, the spring is expected tooperatebetween specified oads only a few times. Fre-quently, springs in static applications remain loaded forlong periods of time. In typical cyclic applications,springs are required to cycle between specified loadsfrom 10,000 o more thana billion cycles. During dynam-ic loading, the rate of load application is high and causesa surgewave in the springwhich will induce stresses hatexceed the value calculated from Equation 5-4.Buckling of Compression SpringsCompression springs that have lengths greater thanfour times the spring diameter can buckle. If properlyguided, either in a tube or over a rod, buckling can beminimized. However, friction between the spring andtube or rod will affect the loads, especially when theaspect ratio (I4lD) is high.Fig. 54. Load Deflection Curve for a Variable RateSpring.

f1 t2 f3 f4Defleaion --1.ps = krfr + kz(fr fr)...ks(fs fr )

Critical buckling conditions are shown in Figure 5for axially loaded springs with squaredand groundendCurve A is for springswith one end on a flat plate athe other end free to tip (Figure 5-8). It indicates thbuckling will occur when the spring design s above ato the right of the curve. A tendency for bucklingconsiderably ess or springs compressedbetweenparlel platesas shown n curve B. For applications equirisprings with a high aspect ratio and large deflectionseveral springs can be used in series n a tube or ova rod, with guides between the springs to prevent bining.Choice of Operating Stress - Static ConditionsFor static applications, the yield strengthor stressrlaxation resistance of the material limits the loacarrying ability of a spring. The spring is requiredoperate for a limited number of cycles, and the velocof the end coils is low to preclude high stressesdue surgtng or impact conditions. Maximum allowable tosional stress or helical compression springsused n stic applications is presented n Table 5-2 as a percentaof the tensile strength for common spring materials. Fsprings that do not contain beneficial residual stressinduced by set removal, maximum allowable torsionstressvalues are from 35 to SVo of the tensile strengTo calculate the stressbefore set removal, it is necessto use the Ks,1correction factor. If the calculatedstreat solid is greater than the indicated percentageof tensstrength, the spring will take a permanent set when dflected to solid. Amount of set is a function of tamount that calculated stress at solid exceeds he incated percent of tensile strength.Fig. 5-7. Critical Buckling Condition Curves.

CD0,o

.9uq,ooiiod,


20/49

To increase the load-carrying ability of springs n stat-ic applications, t is commonpractice to makethe springlonger han its required ree length and to compress hespring o solid. This causes he spring to set to the de-sired inal lengthand nduces avorable residualstresses.Thisprocess s calledremovingset or presettingandcanbe conducted at either room or elevated temperatures.The loss of deflection from the free position to solid bycold setremovalshouldbe at least Vo.If the set s ess,it is diffrcult to control the spring's free length. Ratiosof stressgreater than 1.3 lead to distortion and do notappreciably increasethe load-carrying ability. This is il-lustrated schematically n Figure 5-9.Allowable torsion stressesn springswith setremoved(Table 5-2) are significantly hiefier than for springs hathave not had set removed. It is important to note thatbecause yielding has occurred during presetting, thestress s relatively unifonn around the cross section andit is calculatedusing he Kwzstresscorrection actor. Setremoval is an added springmaking operation which in-creases he manufacturing cost but gfeatly increases heenergy storage capacity of the spring. Set removal iscommonfor critical springs made from premium materi-als. In some instances,springs have the set removedduring an assemblyoperation.Fig.54. End Conditions Used to Determine CriticalBuckling.

BEndFixedAgoinst ippingP++ffi- ' /ArA_--

- V AH\ \ \ \ \ \ \ \ \ \ \FixedEn dTable 5-2. Maximum Allowable Torsional StressesorHeIicaI CompressionSprings in Static Appli-

cations. Bendingor bucklingstresses ot included.Maximun EaoI Tcmile

Mitsrirk r $ctErurgrcd fi[#r]

HelicalCompressionSpringsIf the calculatedstressusing he Ks,2stresscorrectionfactor exceeds he percentageof tensile stength indicat-ed in Table 5-2, the spring cannotbe made. n this case,it is necessary o either lower the stress by alteringspring design or selecting a higher strength material.In some applications, maximum operating stress islimited by material stress relaxation resistance andamount of load loss that the design can tolerate. Whenload is constant, these designs are limited by material

creep resistance.When the spring is compressedat afixed test height, stress relaxation resistanceof the ma-terial is limiting. Designs imited by stressrelaxation re-sistanceaxemore common than designs imited by creepresistance. t is suggestedhat creep-limiteddesignsbereviewed by Associated Spring engineers.Stress relaxation is defined as percentcording to the following relationship: loadoss c-(sJ)

StressesAreColculotedAt Solid.

vokelaxationT x tooP" is load at test height before testing.Pr is load at test height after testing.

AEnd Free o Tip

ffi@ryR

Typical stress relaxation data (Figure 5-10) indicate thatat high stresses, some spring materialssuch as musicwire exhibit appreciable stress relaxation after only 100hours at temperaturesas low as 100"C zn"q. Thesedata are only representative of the conditions indicated.Stress relaxation is affected by material, spring pro-cessingvariables, ime, temperatureand stress. Associ-ated Spring engineers should be contacted for criticalapplications involving stress rela,ration resistance.When set is removed at an elevated temperature, theprocess is called heat setting. It significantly improvesthe stress relaxation resistance of springs (Figure 3-2,page 16) at moderate temperatures and is frequently amore cost-effective method for achieving low levels ofstress relaxation than specifying a more costly springmaterial.Fig. 5-9. Spring -oad-Carrying Ability versusAmount of

EoEodo3noooo

.t1

s.Dco

tnottg

0.6

0.20.4atentedand colddrawn carbon steelHardenedand temperedcarbonand low allovsteelAustenitic stainlesssteels\onferrous allovs

60-7065-75

55-6555-65

old,

1 0 r . l r . 2 1 . 3 1 . 4 1 . 5Stress Before Set Removol c" t : f f i : i ,

::..,B rc.r5tl,{Ktvi}

Assog&ifi8f&ffi#ffiffisfi


21/49

HelicalCompressionSpringsChoice of Operating Stress - Cyclic ApplicationsIn cyclic applications, the load-carrying ability of aspring s limited by material fatigue strength. Velocity ofend coils is low compared o the natural frequency. Toselect he optimum stress evel, t is necessaryo balancespring cost versusreliability. Reducingoperatingstress-es ncreases pringreliability as well as cost. A completeknowledge of operating environment, expected life,stress ange, requency of operation,speedof operationand permissible evels of stress elaxation are requiredin order to make the best choicebetweencost and reli-ability.Becausemaximum stress s at the wire surface, anysurfacedefects such as pits or seamsseverely reducefatigue life. Shot peening improves fatigue life and min-imizes the harmful effect of surface defects, but it doesnot totally remove them.Maximum allowable design stresses or fatigue appli-cations should be calculatedusing the Kwr correctionfactor and are shown for common spring materials inTable5-3. Thesevaluesare for a stressratio of 0 in anambient environment with no surging. Note that shotpeening ncreases he fatigue strengthby asmudr asZVoat lives of 10 million cycles.Values in Table 5-3 are guidelines for designers andshouldonly be used n the absence f specificdata. Mostspringsdesigned o recommendedstress evels will ex-ceed he indicated ives; however, n the absenceof de-tailed information on material, manufacturing methodand operatingconditions, it is not possible to quantifythe reliability level.Fatigue Life Estimation ExampleFatigue life at other stress ratios can be determinedfrom Table 5-3 according to the procedures outlined inSection 4. A short example illustrates the procedure:

Estimate the fatigue life of a not-shot-peened helicalcompressionspring loaded sinusoidally at a rate of onecycle per second.The spring s flooded with oil and oper-ates at a maximum temperatureof 40'C (104"F).The ma-terial is ASTM A228 wire and ends are squared andground.The design s given here:d = 1.00mm (0.039')C = 8I+ = 20.5 mm (ref) (0.807')Lr : 17.5mm (0.689?Lz: l0 mm (0.394')L , : 8 m m ( 0 . 3 1 5 ' )

N t : 8Springrate is determined from equation:

, G d o .t= Etr =3'2NimmLoads are calculated from the deflections and found tobe :P, : (20.5 17.5) 3.2 : 9 .6NP2 (20.5 10.0) 3.2 : 33.6NP,= (20 .5 8) x 3 .2 :40 N

Stressesare calculatedusing Equation 5-4 and are:Sr : 232 MPaSz 810MPaS, : 955MPa

Tensilestrength of the wire is 2180MPa (Figure 3-3, pa19). The stress at solid is 44Voof the tensile strengReferring to Table 5-2, the maximum stress allowabefore sit removal for ASTM A228 is 45% of tensstrength.Therefore, the spring can be madeand doesnrequire set removal.To estimate the fatigue life, it is necessary o:l. Plot an S-N curve on a modified Goodman diagr(Figure 5-11) using the data from Table 5-3 for nshol-peened springs and a tensile strength of 21MPa.2. Plot point A on the 45" line at 67Voof the tenstrength.3. Plot the stress angecoordinates,point B.4. Estimate the life by drawing a line through AB. At intersection of this line with the vertical axis, point' draw a horizontal line to intersect a S-N curve. Tpoint of, interse'Jon, D, is the estimated life 2,500,000 ycles.Dynamic Loading - ImpactWhen a spring is loaded or unloaded, a surgewavestablished which transmits torsional stress frompoint of loading along the spring length to the poiniestraint. The surg wave travels at a velocity apprmately I / l0 of a normal torsional stresswave. Velocitythe torsional stresswave (V1) is given by:

i: I=Vr = 10.1./9 m/sec (or) Vr : ./9 in./sec- Y p Y pVelocity of the surge wave V. varies with materialspring design, but is usually in the range of 50 tom/sec. The surgewave limits the rate at which a spcan absorb or releaseenergy by limiting impact veloV. Impact velocity is the spring velocity parallel toTabte 5-3. Maximum Allowable Tbrsional StessesRound Wire Helical CompressionSpring

Cyclic Applications.f*tiglrrLlfr {*yrhr}

Pcrccat af Ttuih S&eagth*$rll{ A?p8',*s$teinllrs $e?cltrdl{mfrrrous A$TM AEN d Al3tlot ShotF*ilrrd

'fu.',f*md llld;$hot-,f,,e,nd shdPoGn

I$ 5t0 6r$?36''J3{1

4?393Sat*0t?,

494?4SThis nformation s basedon the followingconditions:no surging,temperatureand noncorrosiveenvironment-Stressatio n fatigue Pi,lil+ = gs maxlmum

s=#r*,

Jwnsso5ff1ffiffi m*ffir s


22/49

Helical Compression SpringsFig. 5-10. Spring RelaxationDataforVarious Materials. Springs ere reset t room emperaturend estedM hoursindicatedemperatures.he nitialstresss Kwtcorrected.

MusicWire,ASTM4228, 1.57mm 0.062")rl40 14 0

r30r20

r30

l-roo3o

v,

5 r 0 1 5 2 0Reloxolion, Lmd Loss (o/o)

Chrome-Sil icon ire,ASTMA401, 1.57 o3.76 mm(0.062,'to .l4S"). Chromium onodiumWire, ASTMA232, 1 57 o 3.76 mm (0.062" o 0.140")r000

? 8oo3! 7ooar,=s 600

CLoo

a.no

o-=o

tn

s00400

14 0r30

CorbonVolveSpringWire,ASTMA230, l. 57 to 3.76 mm (0.062" o0.148,,)

r20i l o ?'100=

*9l-140I

ii-r 0l l 0 .i-roo

04=rt=

al=rtl

s 1 0Reloxotion, oodLoss "/o)

Inconel l loyX 750 Wire, 1.93mm 0.075')i l It ,

tt v,

706050

Stoinless reel 02 Wire,ASTMA313, 1.57mm .062")

Reloxolion, Lood Loss (o/o)

l-80 s

l l o i .roo3

o9 0 ;E8 0 =

7060

g*" 1,.

i ^oi-----


23/49

HelicalComPression Pringsspring axis and is a function of stressand material con-stantsshown as:

ITv = 10.tt{ft m/secor) = tV# in./sec s4)This is a surprisingresult because mpact velocity andstressare independentof the spring configuration. Forsteels, mpact velocitY reducesto:

V : * m / s e c ( o r ) v : * i n . / s e c 6 - t o tIf a spring is compressed o a given stress level andreleased istantanebusly, the maximum spRqg velocityil;6;;rseo as tti stressdivided by 35.5. Similarly, ifu tpting is loaded at a known velocity, instantaneousstresscin be calculated.At very high oadingvelocities,instantaneous stress will exceed the stress calculatedfrom the conventional static formula (Equation 5-4) and*ifi fitnit design performance. Thesg equatigns for im-pu.t u.focity are bttt' concernedwith the_primary surge*uu". Frequently, this wave will reflect from the otherend of the-spring, in.t."ring stress. Springs loaded at

high velocities are frequently subject to resonancephnomena (page 39).'Wh;; itreiatio'of the weight to be accelerated o tweijtrt of the spring s lesslhan 1, surge-wave heo;";;;"iy frea-icls-design performance (Figure 5-1Ai frigtt iveight ratios and lower velocities, an enet"d; is ujed to predict velocity of a weight projecil;- th;- rprng "ia o, deflection of the spring whi.pu.t"o uv u ."rr. velocity and deflection arelated as:For horizontal loading:

r: 3l.ovfY.-(or) : v,ffii"'For vertical loading:

- / w w - m wr: 31.6v#i+ u mm (or) : v{*[ +1-mw/g is the mass that is being acceleratedor deceler"nO-V is the axial velocity of the spring'

(s

(s

Fig. 5-11.Modified GoodmanDiagram for Estimating Fatigue Ltfe'

StressCycles

oA=o

tnEExo=

Minimum Stress 103Psi)


24/49

Theseequations assume hat the spring is masslessand shouldonly be used when the spring mass s lessthen 114 of the mass to be acceierated.When the ratio of spring load to weight is less than4. the energy required to accelerate he spring tself be-comes appreciable. By assuming hat all mass of thespring s concentratedat the movingend, Equations5-10and l-l I can be corrected by substituting (W + W,/3)for W where W, is the springweight.Qnamic Loading - ResonanceResonanceoccurs in a springwhen the frequency ofthe cyclic loading is near natural spring frequency or amultiple of it. Resonancecan increase individual coildeflectionand stress evelswell above amountspredict-ed by staticor equilibrium analysis.Resonance an alsocausespringbounce,which results n loadsconsiderablylower than calculated at the minimum spring deflection.To avoid resonance,naturalspringfrequency shouldbe

Helical Compression pringsFor a vibration isolation system, the essentialchar-acteristic is that the natural frequency of the spring-mass system be as far as possible from the disturbingfrequency. Filtering of disturbing forces may be calculated as:

% offorce ransmitted +, x 100 6-14(na /n ) ' - Iwhere no s the frequency of the disturbing orce and nthe natural frequencyof the spring-mass ystem(Figure5-13).If rq/n is less han l, the denominatorn Equation5-14should be changed to I - (no/n)2.Note that the fre-quency n in this equation s the frequencyof the springmass system and not the natural springfrequency. Infact, the most commonly used equation neglecti thesplng weightand s only deflectiondependenr. he gen-eral equation is:

n : 15'8.E metricor)n: j,p ensrirn6-t7 r Y PSpecial SpringsPreviously in this section, design considerations orround wire helical compression springsof uniform diam-eter were discussed.These designtechniquesare mod-ified below andapplied to many specialspringconfigura-tions. Specialspringsare chosen to fulfill a unique setof design criteria. Springs from rectangular wire andstranded wire as well as variable diameter springs withconical, hourglassand barrel shapes,zlrdiscussedbe-low. Helpful guidelines or nested springsare also re-viewed.RectangUlarWire

In applications where space s limited and particularlywhere solid height is restricted, springsdesigned romrectangularor keystonedwire are oftenselected.Associ-ated Spring manufactures hundreds of rectangular wirespring designs.These springsare commonlyreferred toas die springs and are available for immediate delivery.Fig. 5-13. Transmissibilityof Spring Mounting.

at least i3 times the operating requency.The natural frequencyof a compressionspringversely proportional to the time required for awave o traverse he spring. For a compressionwithoutdampingand with both endsfixed:

ls ln -surgespring(s-r3)" : %#,ry' ror teel $fi!} metric

,./qg: for steel l4oood F\ p r: ffiF English= gD2N"n is in hertz.If a springcannotbe designed o the natural requencyis more than 13 times operating frequency, or if thespring s to serve as a vibration dampingdevice, t mustutilize one of several methods of energy absorption.Generally, heseare friction devices n which the springrubs againstanother element such as an internal damp-er coil, arbor, housingor anotherportion of the spring.Variablepitch springs and springs in combinationaiealso occasionallyused to avoid or minimize resonantfrequencyeffects.Fig.5-12. Velocity of an Object Propelled by a Com-pression Spring.

\ \ I For moss rotios ofI t Over 4 - use cose i )l -4 - use cose (2 )| | Under 'l - use cose (3 )t l t

Id Moss Theory (2) |Concenirole

\Mortt"ss Spr ing Theory l )t lVo = Veloci tyVm = Moximum Veloci ty

0.15f :7.3, there ismore spaceavailable. Try a larger preferred wire size(Table 3{, page 20) of 4.8 mm.TS = 1400MPa,D : 38.0 4.8 : 33.2mm, C = 6.9N " : (7.93 l0a)(4.8y: 6.48(33.2)3z.s)

- 5 06 0 -Gd48m

4.

C.

L, : 8.4 x 4.8 : 40.3mmL z - L . : 5 0 - 4 0 . 3 : 9 . 7 m m

f, = 72.2 40.3 31.9mm(Lz - L,) : 9.7 > 0.15 , = 4.8mm

P, : (31.9)(22.5) 718N( + X 6 . e ) _ l * # : r . z zr a w t = ( 4 x 6 . 9 ) - 4 - e t =s , - @ : 6 7 t M p a(4.8)'S. 671MPaot ffi x loo

- 48vo f rS

WircIXa.,E(iD.)

Tolemmffr: t:mm:,(lui)$pring lrdn {D/d}

4 6 t IB L2 l{ t60.38(0.01t 0.05(0.002) 0.05(0.002) 0.08(0.003) 0 .10(0.004) 0 .13(0.00s) 0 . 1 5(0.006) 0 . lE(0.007)0.58r0.023) 0.05(0.002) 0.08(0.003) 0.10(0.004) 0.15(0.006) 0.18(0.007) 0.20(0.008) 0.25(0.010)0.E9r0.035) 0.05(0.m2) 0 .10(0.004) 0.15(0.006) 0. lE(0.007) 0.23(0.00e) 0.28(0.01l) 0.33(0.013)1.30(0.05) 0.08(0.003) 0.13(0.005) 0 .18(0.007) 0.25(0.010) 0.30(0.012) 0.38(0.0r5) 0.43(0.017)1.93(0.076) 0 .10(0.004) 0 . lE(0.007) 0.25(0.010) 0.33(0.013) 0.41(0.016) 0.48(0.019) 0.53(0.021):.90r0 .114 )0 . 1 5(0.006) 0.23(0.00e) 0.33(0.013)0.46(0.01E) 0.53(0.021) 0.9(0.025) 0.74(0.029)1.y(0 .171)0.20(0.008) 0.30(0.012)0.43(0.017)0.58(0.023) 0.71(0.028) 0.84(0.033) 0.97(0.038)6.3,((0.250) 0.28(0 .01 l ) 0.38(0.01s) 0.53(0.021) 0.71(0.028) 0.90(0.035) 1.07(0.042) 1.24(0.049)9.53(0.37t 0.41(0.016) 0 .51(0.020) 0.66(0.026) 0.94(0.037) t . l 7(0.046) 1 .37(0.054) 1.63(0.064)

11.70r0.500) 0.53(0.021)0.76(0.030) r.02(0.040)r.57(0.062)2.03(0.0E0) 2.54(0.100) 3 . 1 8(0.12s)

Assos$tf;g&ffi*ffir sB


29/49

Helical CompressionSpringsCOMPR,ESSIONPRING PECIFICATIONHECKLIST(Fill n requireddoto only)

Frequencyof loodingRequired life cYcles'

Moteriol: Required reliobility (see Section 4)Wo*ing Conditions: Speciol Informotion:To work in m(in.) diometer hole

m(in. dismeter shoft Squoreness Porol lel ismTo work over FinishLood (1b0, tl (lb0 Moximum operoting fem Peroture :c("F)Operoting environmentx (tbO Electricol/mognetic

Rote -N/mm ( lbf / in . ) , t -N/mm ( lbf / in . )between -rnm(in.) qnd -mm(in.)

Moximum solid height mm(in.)Direction of coil (right-hond, left-hond or opfionol) -Type of endsAlfowobfe reloxotion -o/o Hours/doys -

lmpoct looding mm/sec ( in./sec)

Design Dofo (Reference):Wire diometerOutside diomlnside diometerFree length mm(in.)Totol number of coils

Tablc 54. Load Tolerances f Helical CompressionSprings.

First oad estat not less han 5Vo f availableeflectron.Final oad est at not more han85%of available eflection.Fis.5-22. Tansled Helical CompressionSprings (Left)and-Spring Flow Packaging.

Again referring to Table 5-2, page35, it is clear thatspring can be made without presetting. TolerancesobtaineO rom Tables 54, 5-5 and 5-5. The final desbecomes:Final Design Specifications:Material: ASTM A229Wire Diameter d: 4.8 mm (0.189) ReferenceO.D.: 38.0 r 0.4 mm (1.500 0.050)Free Length I-r: 72.2 mm (2.843')ReferenceTest Height Lr: 60 mm (2.362')Test Height Lz: 50 mm (1 96Y)Pr Load at Lr: 275 N (61.8 bO = ll.0%Pz Load at L2: 500N (112 bO =7VoFinal DesignStressS,: 671MPa (97,300 si) or 48%N,: 8.4

.

X.*ugthTskrarr*::mm {in.}

r"*,l""*' qb."r1yrd..y{,*tl.e.rypry|,]:i , .Y'H.bl,f:, .:I fien f,rom re*,,I ,,to',.[ , mm {in.)'l;3;7'(s.s3o)?.54{0,ltr} 3,*r.{0,1$6} 5"S0{0"m,} ,,f,$${0,2sJ "V,67,t0.300) 10.2(0.400,1 1:,7{ 0 } '19*,1,'{s-.?50}"28*,{r;tr} t*.1(r fO;il,.,(2'ffi1 'l?6 ,,.1,{,s,; .i,ltrEl{*$1 ,,:1 ,:{6iffi}

0.130.00s)0.250.010)0.510.020),: 7.t2.22.

6.8 .515.55 .7 .t2 . 6 .510. l.s8.s 5 .7 . 6.

0.76 0.030)1.0 (0.040)r.3 (0.050) ? 17 .22. T4r8)) t2.15 .519. 9.512.14.5 8l012 6.7 .59. 567 s5.51 .51 . 82.0

(0.060)(0.070)(0.080) ! 22.25 . 17 .19.522. l4l6l8 10.1 1 .12.5 8910 6.6.57.5 5.) . )6.2.3 (0.0e0)2.s (0.100)5.1 (0.200) :

20.? 14.15.5 1 lt222 8.8.515.5 67t2 5 .5 .58.5 7.7.6 (0.300)10.2 (0.400)12.7 (0.500)

? 172l25 t2.15 .18.5 9.5t2.14.5 7 .8.510.5


30/49

Helical xtensionSpri gs I

o"tF'!fi349*nrrs


31/49

!tHelicalExtensionSpringsIntroductionHelical extension springsstore energy and exert a pul-ling force. Usually, they are made from round wire andareclose-woundwith initial tension. Typical applicationsinclude apecassette layers,balancescales, oys, garagedoors, auiomatic waJhingmachinesand various types ofspringtensioning devicesHelical extension springsare stressed n torsion in thebody. Designprocedures or the body are similar to thosediscussed leviously for compressionsprings Section5)with the following major exceptions. Most helical exten-sion springs are coiled with initial tension, equal to theminimum force required to separateadjacentcoils. Heli-cal extension springsdo not normally have set removed.Furthermore, untit


32/49

beendevelopedand used successfully or many years -for example, hreaded nserts, swivelhooks, twisi loops,side loops, cross-center loops and extended hooks.l-oops are attachment ends that have small gaps(Figure-{). while hooks.are loops with a largegap. In fact, the'ariet-v-of ends is almost unlimited. The most commonconfigurations are those that can be formed during thespringmaking operation. Typical types include twist,crosscenter,side oopsand extendedhooks(Figure74).Man), of these configurations are made by bending thelast coils of an extension spring to form loops. Mostspecial hooks are formed from straight sectioni of wireon the so-called"tangent ends" of an extensionspringbody.Guidelines for the lengths of common loops arepresented n Figure 74. Alrhough other configuiationsand lengths are available, common loops of preferredlengthsare generally the most eccnomical. f possible,a spring should be designedwith one or both loops atthe prefered length. For example, if a designrequiresa-total oop lengthequal o five timesthe I.D., a popularchoice s one twist loop with a length equalto the I.D.and one extended loop with length equal to four timesthe I.D. Wheneverpossible or extended oops, he de-signer should allow for a straight section approximatelythree wire diameters long at the end of the wire (A,Figure 74). Loops at each end can be madewith a con-trolled angular relationship. Specifying an angular rela-tionship may add to the cost; therefore, whenever anapplication permits, a random angular relationshipshould be allowed. Production of special end configurir-tions may involve tool chargesand generallyresufts inincreased osts.

HelicalExtensionSpringsStresses n loops are often higher than in the springbody. This limits spring performance,particularly in cy-clic applications. Generousbend radii in loops and re-duced end coil diameters are two methods frequentlyemployed o reduce stresses. n a full twist loop, stressreachesa maximum at point A in bending and a maxi-mum in torsion at point B (Figure7-5). Stress at theselocations s complex, but can be estimatedwith reason-able accuracy by:

se S*, - #bending t-rtw h e r e ? - 4 C t 2 - C t - t l R rK t : + t f f i a n d c r : ?

A 8 D P / 4 C , - l \ 2 R ,Ss A \4ffi) andCz ? torsion e4tRecommended ractice s to make C2geater than four.

Fig.7-5. r,ocation of Maximum Bending and TorsionStressesn TwistLoops.Prl

\ l t l,-r;lF-( : f - ---=r- )Torsion Slresso t B

Fig. 74. common End configurationsfor Helical ExtensionSprings.?yprTwistLoop orHook

::'--::i"T_- m - AWJ \UZ # @hceCImm*ndd,,Lrn#h*

Min"-t\{sx,0 .5-1 .7.D.

CrossCenterLooporHookt lrmA /ATrnF - 7r -Tr-rr-7r-ullv/ \Jlz I .D.

SideLoop orHookI I-uNzrz v p- 0.9-1.0.D.

ExtendedHook l . l I .D .andupas requiredby design

SpecialEnds A Avvvvvvv - f f i'

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