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Sealing Elements

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All the information in this documentation has been compiled with the greatest of care.

Despite this we can bear no responsibility whatsoever for any errors present in the documentation. The recommendations are intended as guidelines.

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Technical Handbook O-rings

8. Qualifications

Functionalrequirementsshouldalwaysbegivenfirst.Afunctional

testisworthmorethanaphysicalandchemicalpropertytest.Thus

thefirststepistosettheoriginalphysicalpropertylimitswhichwill

assure that the mechanical properties desired in the O-ring are

present.Beawareofthefactthatthereisadifferencebetweenthe

originalphysicalpropertiesandtheagedphysicalproperties.

Original Physical Properties:

Hardness

DeterminetheIRHDhardnessbestsuitedfortheapplicationand

roundoffto40°,50°,60°,70°,80°,90°or95°-shoreAorIHRD.

A±5pointtoleranceisestablishedtoallowthemanufacturerarealisticworkingrangeandpermitnormalvariationsexperiencedin

measuring hardness.

Tensile Strength

Determinetheminimumtensilestrengthnecessaryforthe

application.Alwaystakeintoconsiderationtheinherentstrengthof

theelastomersmostlikelytobeusedtomeetthespecification.Most

siliconeshaveamuchlowertensilestrengththanotherelastomers.

Oncetheminimumtensilestrengthhasbeenset,multiplyitby

1.20.Thisisthelimitsetfortensilestrengthvariationof±15%

experiencedbetweenproductionbatchesofacompound.

Elongation

Investigateanddeterminethemaximumamountofstretchaseal

mustundergoforassemblyintheapplication.Multiplythisfigure

by1.25toallowasafetyfactorandtoprovidefornormalproduction

variationof±20%.

Modulus

Chooseaminimummoduluswhichwillassureagoodstateofcure,

goodextrusionresistance,andagoodrecoveryfrompeakloads.

Modulusisdirectlyandrelatedtotensileandelongation,andrefers

tothestressatapredeterminedelongation,usually100%.

Specific Gravity

Avalueforspecificgravityshouldnotbesetinthequalification

sectionofthespecificationbutthevaluemustbereported"as

determined".Therealisticfigurewillthenbeusedinthecontrol

section.

Age Physical Properties

DeterminetheresistanceoftheO-ringtotheanticipatedservice

environment.Thisisdonebymeasuringthechangeinvolume

andphysicalpropertiesoftestsamplesafterexposuretovarious

conditionsforaspecifiedtimeataspecifiedtemperature.

Recommendedtimes,temperatures,andtestfluidsforaccelerated

testscanbefoundinASTMD471.Itisusuallydesirabletousetheactualservicefluid.Becausethesefluidsarenotascontrolled

astestfluids,therecanbesomevariations.Thisfluidvariation

accountsfordifferencesintestresults.

Hardness Change

HardnessChangeisusuallycontrolledtoavoidexcessive

softening(causingextrusionfrompressure)orhardening(causing

cracking).

Tensile Strength Change

Areasonabletolerancelimitisusuallysetasinsuranceagainst

excessivedeteriorationandearlysealfailure.Eachindividualfluid

dictatesitsownspecificlimits.Experiencewillprobablydictate

theselimits.However,10% 

toleranceisnotrealisticsincemuchwidervarianceintensile

strengthcanbeexperiencedontwotestspecimenscutfromthesame sample.

Elongation Change

Experiencewilldictatethislimitasnotedundertensilechange.

Volume Change

Determinethemaximumamountofswellwhichcanbetolerated

intheO-ringapplication(usually15%to20%fordynamicand

50%forstaticapplications).Determinethemaximumamountof

shrinkagewhichcanbe 

toleratedintheO-ringapplication(usually3%forbothdynamic

andstatic).Includeadry-outtestaftertheimmersiontestto

provideacontrolfordry-outshrinkage.ShrinkageofO-rings

canbeacauseoffailure.Itisnecessarytostressthedifference

betweentestresultsondifferentsizeseals.AnO-ringwitha

thinnercrosssectionwillnothavethesamevolumeswellasan

O-ringwithathickercrosssectionwhentestedunderthesame

conditions.Thisdifferenceisatitspeak  

duringthefirst70hoursoftesting(mostacceleratedtesting

isspecifiedwithinthisperiod).Onlyafterfourtosixweeksthe

volumeswellofdifferentcrosssectionringsapproachesan

equilibriumvalue.

Compression Set

Arealisticvalueforcompressionsetisoftenallthatisnecessaryto

assureagoodstateofcureandresilienceofacompound.

Low Temperature Resistance 

DeterminethelowesttemperatureatwhichtheO-ringisexpected

tofunction.Mostlowtemperaturetestsaredesignedtoindicate

thebrittlepointofamaterialwhichonlytellsatwhattemperature

thecompoundismostlikelytobecompletelyuselessasaseal

inastandardO-ringdesign,butverylittleaboutthetemperature

atwhichitisuseful.OnlytheTR-10testgivesinformationabout

thelowesttemperatureatwhichthecompoundexhibitsrubber-

likepropertiesandthereforerelatestolowtemperaturesealing

capabilities.O-ringsindynamicapplicationswillsealattheTR-10value.O-ringsinstaticapplicationswillfunctionsatisfactorilyto

about10°C(ca.15°F)belowthisvalue.