What is an O-Ring.doc

8/14/2019 What is an O-Ring.doc

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hat is an O-ring?

O-ring is a simple and versatile ring-shaped packing and sealing device with a circular cross section.

ring functions as compact and reliable sealing devices by absorbing the tolerance stack-up between closely mated surfaces innamic and static applications. Although O-rings can be made from a variety of materials, they are most commonly molded in o

ce from an elastomeric material.

hat is an O-ring seal?

ring seal is used to prevent the loss of a fluid and gas between two closely spaced surfaces. The O-ring is generally installed achined groove in one of the surfaces to be sealed. As the two surfaces are brought together, forming a gland, they suee!e toss section of the O-ring. This suee!ing action results in a deformation of the O-ring cross section. With O-rings, the greater tuee!e, the larger the deformation.

ted below are some of the outstanding characteristics that make O-rings one of the most versatile, dependable, yet ine#pensials available, with sealing capability from hard vacuum to high pressure.

hey seal over a wide range of pressure, temperature and tolerancease of service, no smearing or retighteningo critical torue on tightening, therefore unlikely to cause structural damage

O-rings normally reuire very little room and are light in weightn many cases an O-ring can be reused, and advantage over non-elastic flat seals and crush-type gasketshe duration of life in the correct application corresponds to the normal aging period of the O-ring material

O-ring failure is normally gradual and easily identifiedhey are cost-effective

Where differing amounts of compression effects the seals function, an O-ring is not effected because metal to metal contact isnerally allowed for.

O-ring compounds can be selected to resist most environmental e#tremes

s a uniue characteristics of the elastomer material used in O-rings that makes O-ring such a good seal. The elastomer, a hig

cous, incompressible fluid with high surface tension, has a capacity for remembering its original shape for a long time.

ow-pressure applications 'that is, where the confined fluid e#erts little or no pressure on the O-ring(, the tendency of the elastmaintain its original shape creates the seal. As the O-ring is deformed when the mating surfaces are brought together, it e#ertce against the mating surfaces eual to the force it takes to suee!e it. The areas of contacts between the O-ring and the matrfaces 'contact bands( act as a barrier to block the passage of the fluid.

applications where higher pressure is e#erted by the confined fluid, the sealing action of the O-ring caused by the suee!e of oss-section is augmented by fluid pressure, transmitted through the elastomer. The O-ring is forced to the side of the gland, awm the pressure. As it is pressed against the side, the O-ring, cross section is deformed. The elastomer e#erts eual force in aections and is forced up to the gap between the mating surfaces.


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ROTARY APPLICATIONS

rotary O-ring applications, the O-ring continuously moves against tire same portions to the shaft. )eat due to friction is continunerated ion the same place, and elastomers are poor thermal conductors. &f heat is generated more uickly than it can besipated, temperature rise is rapid and seal failure uickly follows. Where surface speeds do not e#ceed *+ feetminute, or whation is brief and intermittent, this is rarely a problem and gland design criteria for reciprocating service are applicable. or

ntinuous rotation at surface speeds over *+ feetminute some developmental ad/ustments are often reuired to achieveceptable performance.

applications where rotating motions occur, the designer should consider the following0

1easures should be taken to reduce heat buildup0rovide absolute minimal suee!e, as little as .345 to minimi!e friction. This may permit some leakagerovide ample diametric clearance to increase fluid flow and facilitate better dissipation of heatelect O-ring with smallest cross section.aintain low system pressure 'not over 37psi(

8se a shaft of diameter no greater than that of the rela#ed O-ring &.9. This is important because when an O-ring is heated undess, it will tend to contract. :ontraction of the O-ring could cause it to sei!e the shaft and increase friction and heat resulting

pid failure.

The gland should be located as close as possible to the lubricating fluid and as far as possible from the shaft support bearings allows the O-ring to receive the ma#imum amount of cooling lubricant and minimi!es the effects of bearing-generated tre

;elative motion must occur e#clusively between the O-ring &9 and the rotating shaft. ;otation of the O-ring within the gland wd to rapid wear and leakage.

1inimi!e out-of-round shafts and eccentric rotation. 1a#imum eccentricity should not e#ceed .*45inish of the moving surface contacting the O-ring should not e#ceed *3 ;16 is recommended.8tili!e an O-ring of a hard, self-lubricating compound specifically developed for rotary service.

$;1$A&"&T@

ses diffuse into and through elastomaric compounds at various rates depending on the elastomer type and nature of the indivmpound. enerally, harder compounds which have more carbon black added have lower diffusion rates. Of the popular elastochlorohydrin and butyl have the lowest permeability, followed by fluorocarbons, polyurethanes, nitrites, heoprenes, polyacrylad 6;. The fluorosilicons and silicones have higher rates.

r any given compound, the permeability through the O-ring depends on the amount of its compression or suee!e, the area ofal, and the pressure, temperature and type of gas begin sealed. or the ma/ority of applications, the rate of gas permeation the O-ring is inconseuential and standard groove dimensions are applicable.

here gas pressure e#ceeds 7psi, and pressure is released alter a soak period, gas within the O-ring may e#ert considerableder the lower e#ternal pressure and may cause damage. The O-ring may blister or chunks of rubber may even be blown out.

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special consideration is usually warranted for pneumatic applications if they are static. With dynamic applications, the problemk of a system liuid to provide lubrication and cooling. &f reasonable life is to be achieved sortie lubricant must be provided.rticularly where operating temperature approaches the capabilities of the O-ring, an elastomer resistant to o#ygen should beosen 4B as temperature increase due to compression reuires consideration in determining system temperature.

nventional gland designs are applicable for pneumatic service. )owever, since slight leakage is usually not important, and fric


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reduced suee!e is desirable in reciprocating pneumatic applications.

A:881 6$A"6

cuum seals also warrant separate mention. 8nlike pneumatic seals, even slight leakage is often unacceptable in vacuum

plications. They have only one atmosphere differential pressure, so essentially all the sealing force must be provided bympression of the O-ring. The following factors should be considered0

namic vacuum seals reuire proper lubrication due to the absence of system liuids. 8se of vacuum grease is also desirable tic sealsespecially smooth finish in the gland is important to insure contact between the elastomer and the metal parts.

applications where absolute minimum leakage is a necessity, gland depth should be reduced to increase the amount of sueeminimi!e the possibility of gases begin trapped under the O-ring and escaping into the vacuum, reduced groove width and th

suitable vacuum grease to fill the e#cess void are recommended.rings may be used in series in vacuum applications, preferably with a separate vacuum between them.

;&B$ $"T6

rings provide e#cellent service in low power drive belt applications. Tire primary concern for O-rings used as drive belts is thempound from which they are made.

veral elastomers have been used successfully in drive belt applications. $thylenepropylene has provided superior performancts low stress rela#ation, high temperature resistance, and overall reliability silicon has also been used in high temperatureplications and lacking good wear and abrasion resistance, it provides reliable but somewhat limited service life. 2olyurethane en used successfully but it should not be used at temperatures above *7+o 'Co:(.

hen utili!ing O-rings as drive belts, the following factors should be considered0

, stretch should be app. *D-3E,ley grooves should be round, of depth and radius eual to the radius of the O-ring cross sections.

ley diameter 'at the bottom of groove( should be no less than four times the O-ring cross section diameter

mparison of :ommon 6eal Types

number of common seals types, T-seals, 8-cups, B-packing and other devices, have been, and are still used for both dynamictic seals. When compared with an O-ring seal, these seal types may show one or more design disadvantages which might be

ercome by use of an O-ring. As an aid in assessing the relative merits of and O-ring, below table lists several of the importanttors that must be considered in the selection of any effective seal geometry.

ASIC O-RING ELASTOMERS

e following paragraphs briefly review the various elastomers currently available for use in O-rings and other elastomeric seals

Ar!l"nitrile- B#ta$iene %NBR&

rile ;ubber '%;( is the general term for acrylonitrile butadiene terpolymer. The acrylonitrile content of nitrile sealing compouries considerably '*+E to 7E( and influences the physical properties of the finished material.

e higher the acrylonitrile content, the better the resistance to oil and fuel. At the same time, elasticity and resistance to compreis adversely affected. &n view of these opposing realities, a compromise is often drawn, and a medium acrylonitrile content

ected. %; has good mechanical properties when compared with other elastomers and high wear resistance. %; is not res


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weathering and o!one.

at Resistaneto *F: '3*3F( with shorter life G *3*F: '37F(.

l$ (le)i*ilit!pending on individual compound, between ->HF: and -7CF:F and -CF(

e+ial Resistanephatic hydrocarbons 'propane, butane, petroleum oil, mineral oil and grease, diesel fuel, fuel oils( vegetable and mineral oils aeases.A, ) and ): fluidsute acids, alkali and salt solutions at low temperatures.ater 'special compounds up to *F:(

t "+,ati*le ithels of high aromatic content 'for fle# fuels a special compound must be used.(omatic hydrocarbons 'ben!ene(lorinated hydrocarbons 'trichloroethylene(lar solvents 'ketone, acetone, acetic acid, ethylenester(ong acids

ake fluid with glycol base.one, weather and atmospheric aging.

Car*")!late$ Nitrile %0NBR&

rbo#ylated %itrile 'I%;( is a special type of nitrile polymer that e#hibits enchanced tear and abrasion resistance. or this rea; based materials are often specified for dynamic applications such as rod seals and rod wipers.

at resistaneto *F: '3*3F( with shorter life G *3*F: '37F(.

l$ (el)i*ilit!pending on individual compound, between -*+F: and -H+F: and -77F(

e+ial resistanephatic hydrocarbons 'propane, butane, petroleum oil, mineral oil and grease, diesel fuel, fuel oils( vegetable and mineral oils aeases.A, ) and ): fluids

any diluted acids, alkali and salt solutions at low temperaturesater 'special compounds up to *F:(

t "+,ati*le ithels of high aromatic content 'for fle# fuels a special compound must be used(omatic hydrocarbons 'ben!ene(lorinated hydrocarbons 'trichloroethylene(.lar solvents 'ketone, acetone, acetic acid, ethyleneester(ong acids

ake fluid with glycol base.


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Eth!lene Ar!late %AEM&

hylene acrylate is a mi#ed polymer of ethylene and methyl acrylate with the addition of a small amount of carbo#ylated curingonomer. $thylene acrylate rubber is not to be confused with ethyl acrylate rubber 'A:1(.

at resistaneto *HJF: with shorter life up to *F:

l$ 2le)i*ilit!tween -3JF: and -HF:

e+ial resistaneoneidi!ing media

oderate resistance to mineral oils

t "+,ati*le ith

toneselsake fluids

Eth!lene Pr",!lene R#**er %EP4M5 EPM&

1 is a copolymer of ethylene and propylene. $thylene propylene-diene rubber '$291( is produced using a third monomer anrticularly useful when sealing phosphate-ester hydraulic fluids and in brake systems that use fluids having a glycol base.

at resistaneto *7F: 'ma# 3HF:( in water and or steam(

l$ (le)i*ilit!

wn to appro#imately -7CF:

e+ial resistanet water and steam up to *HJF: with special compounds up to 3HF:ycol based brake fluids up to *HJF:any organic and inorganic acidseaning agents, soda and potassium alkalis.osphate- ester based hydraulic fluids ')9-;(cone oil and grease

any polar solvents 'alcohols, ketones, esters(.one, aging and weather resistant.

t "+,ati*le ith0neral oil products 'oils, greases and fuels(

B#t!l R#**er %IIR&

tyl 'isobutylene, isoprene rubber, &&;( is produced by many companies in different types and varies widely in isoprene contentprene is necessary for proper vulcani!ation. utyl has a very low permeability rate and good electrical properties

at resistane


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to app. *3*F:

l$ 2le)i*ilit!wn to app. -7J F:

e+ial resistanet water and steam up to *3*F:ake fluids with glycol baseny acidst solutionsar solvents, eg. Alcohols, ketones and estery-glycol based hydraulics fluids '): fluids( and phosphate-ester bases ')9-; fluids(cone oil and greaseone, aging and weather resistant

t "+,ati*le ithneral oil and greaseelslorinated hydrocarbons

B#ta$iene R#**er %BR&

lybutadiene ';( is mostly used in combinations with other rubbers to improve cold fle#ibility and wear resistance. ; is primaed in the tire industry, for sure drive belts and conveyor belts and is not suitable as a sealing compound.

Chl"r"*#t!l R#**er %CIIR&

lorobutyl ':&&;( is produced by chlorinating butyl polymer. &s chlorine content is appro#imately *.*E to *.>E. Apart from theoperties of butyl rubber '&&;(, chlorobutyl ':&&;( shows improved compression set properties and can be compounded with othaterials

Chl"r",rene R#**er %CR&

loroprene was the first synthetic rubber developed commercially and e#hibits generally good o!one, aging and chemical resisas good mechanical properties over a wide temperature range

at resistaneto app. *3*F:

l$ 2le)i*ilit!wn to app. -H F:

e+ial resistaneraffin base mineral oil with low 92&, eg0 A6T1 oil %o.*con oil and grease

ater and water solvents at low temperaturefrigerants

mmoniarbondio#ideproved o!one, weathering and aging resistance compared with %;


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+ite$ "+,ati*ilit!phthalene based mineral oil '&;1 J3 and &;1 J> oils(w molecular aliphatic hydrocarbons 'propane, butane, fuels(ycol based brake fluids

t "+,ati*le ithomatic hydrocarbons 'ben!ene(lorinated hydrocarbons 'trichloroethylene(lar solvents 'ketones, esters, ethers, acetones(.

Chl"r"s#l2"nate$ P"l!eth!lene %CSM&

e polyethylene polymer contains additional chlorine and sulfur groups. :hlorine gives the mineral resistance to flame and minand also improves the cold fle#ibility

at resistaneto *3*F:

l$ 2le)i*ilit!wn to app. -3JF:

e+ial resistaneany acidsany o#idi!ing mediacon oil and grease

ater and water solventsone, aging and weathering resistance

+ite$ "+,ati*ilit!w molecular aliphatic hydrocarbons 'propane, butane, fuel(

neral oil and greasemited swelling in aliphatic oil 'A6T1 oil %o.*(gh swelling in naphthene and aromatic base oils '&;1 J3 and &;1 J> oil(lar solvent 'acetone, methyl ether, ketone, ethyl acetate, diethyl ether, dio#ane(osphate-ester based fluids

t "+,ati*le ithomatic hydrocarbons 'be!ene(orinated hydrocarbons 'trichloroethylene(

E,ihl"r"h!$rin %CO5 ECO&

ichlorohydrin is available in 3 types0 the homopolymer ':O( and the copolymer '$:O(. oth :O and $:O have good resistan

neral oils, fuels and o!one. The high temperature resistance is good. :ompression set and the tendency to corrode metal seaes increase at *7F:. $:O has a good cold fle#ibility. :O has a high resistance to gas permeability

at resistaneto app. *>7F:

l$ 2le)i*ilit!wn to app. -HF:


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e+ial resistaneneral oil and greasephatic hydrocarbons 'propane, butane, fuel(cone oil and grease

ater at room temperature

one, aging and weather resistance

t "+,ati*ilit! ithomatic and chlorinated hydrocharbonstones and estersn-flammable hydraulic fluids in the groups )9-; and )9-6.ycol based brake fluids

(l#"r"ar*"n %( oilsn-flammable hydraulic fuels in the group )9.cone oil and grease

neral and vegetable oil and greasephatic hydrocarbons 'fuel, butane, propane, natural gas(omatic hydrocarbons 'ben!ene, toluene(orinated hydrocarbons 'trichloroethylene and carbon tetrachloride(

els, also fuels with methanol contentsh vacuumry good o!one, weather and aging resistance

t "+,ati*le ithycol based brake fluids

mmonia gas, amines, alkalisperheated steamw molecular organic acids 'formic and acetic acids(

(l#"r"sili"ne %(=M>&

1L contains trifluoropropyl groups ne#t to the methyl groups. The mechanical and physical properties are very similar to B1L1L offers improved fuel and mineral oil resistance but poor hot air resistance when compared with B1L.

at resistane


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to *CCF: ma#.

l$ 2le)i*ilit!wn to app. -C>F:

e+ial resistaneomatic mineral oils '&;1 J> oil(elsw molecular weight aromatic hydrocarbons 'ben!ene, toluene(

. '!$r"genate$ Nitrile %'NBR&

drogenated %itrile is a synthetic polymer that results from the hydrogenation of nitrile rubber '%;(. &n this process the molecuble bonds in the %; primiary polymer chain undergo a hydrogenation process and therefore the term hydrogenated nitrile%;(. The allow temperature range e#tends to *HJF: with short periods at higher temperature possible. y following designdelines effective sealing can be achieved at ->3F: for static applications. or dynamic applications however, operating

mperatures are limited to above - 3>F:. )%; compounds posses superior mechanical characteristics, particularly their highength. or sealing applications up to app *7JF:, this is an advantages as it prevents e#trusion and wear.

e+ial Resistanephatic hydrocarbonsgetable and animal fats and oilsA, ) and ): fluidsute acids, bases and salt solutions at moderate temperatureater and stream up to *HJF:one, aging and temperature

t "+,ati*le ithlorinates hydrocarbonslar solvents 'ketone and ester(ong acids

Per2l#"r"elast"+er %((F: depending on compound

l$ 2le)i*ilit!+F: or -3


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organic and organic acidsater and steamgh vacuum with minimal loss in weight

t "+,ati*le ith

roinated refrigerants ';**, *3, *>, **>, **H, etc(

. P"l!ar!late %ACM&

:1 or simply acrylate rubber consists of a polymeri!ed ester and a curing monomer. $thyl acrylate rubber has a good resistanat and mineral oil0 on the other hand butyl acrylate has a better cold fle#ibility. 2olyacrylate has a good resistance to mineral oygen and o!one even at high temperatures. The water compatibility and cold fle#ibility of A:1 are significantly worse than wit;.

at resistaneortened lifetime up to appro#imately *CCF:.

l$ 2le)i*ilit!wn to appro#imately -3*F:

e+ial Resistaneneral Oil 'engine, gear bo#, AT oil(one, weather and aging resistance

t "+,ati*le ithycol based brake fluidomatics and chlorinated hydrocarbonst water, steamds, alkalis, amines

P"l!#rethane %A5 E&

e must differentiate between polyester urethane 'A8( and polyether urethane '$8(. A8 type urethanes e#hibit better resistancdraulic fluids. 2olyurethane elastomers, as a class, have e#cellent wear resistance, high tensile strength and high elasticity inmparison with any other elastomers. 2ermeability is good and comparable with butyl.

at Resistaneto appro#imately +3F:

l$ 2le)i*ilit!wn to appro#imately -HF:

e+ial Resistanere aliphatic hydrocarbons 'propane, butane, fuel(neral oil and greasecone oil and grease

ater up to 7F: '$8 type(one and aging resistance

t "+,ati*le ithtones, esters, ethers, alcohols, glycols


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t water, steam, alkalis, amines, acids

Sili"ne R#**er %>5 M>5 =M>5 P=M>&

e term silicone covers a large group of materials in which vunyl-methyl-silicone 'B1L( is often the central ingredient. 6ilicone

stomers as a group have relatively low tensile strength, poor tear and wear resistance. )owever, they have many useful propwell. 6ilicones have good heat resistance up to 3>3F:, good cold fle#ibility down to -7JF: and good o!one and weather resiswell as good insulating and physiologically neutral properties.

at Resistaneto appro#imately 3HF: 'special compound up to 3>3F: (

l$ (le)i*ilit!wn to appro#imately -7JF: to -7HF: with special compounds down to -**7F:

e+ial resistanegine and transmission oil 'eg0 A6T1 oil %o.*(imal and vegetable oil and greaseake fluid 'non-petroleum base(e-resistance hydraulic fluid, )9-; and )9-6gh molecular weight chlorinated aromatic hydrocarbonsoderate water resistanceuted salt solutionsone, aging and weather resistance

t "+,ati*le ith@perheated water steam over *3*F:ds and alkalis

w molecular weight chlorinated hydrocarbonsomatic mineral oildrocarbon based fuelsomatic hydrocarbons 'ben!ene, toluene(

St!rene-B#ta$iene %SBR&

; probably is better known under its old names una 6 and ;6. 6; was first produced under government control betwee> and *J7 as a replacement for natural rubber. The basic monomers are butadiene and styrene, with styrene contentpro#imately 3>.7E. About one third of the world output of 6; is used in tire production. 6; is mostly used in seals and nonneral oil based brake fluid applications

at Resistaneto appro#imately *CF:

l$ 2le)i*ilit!wn to appro#imately -7CF:

+,ati*le ithater, alcohol, glycol and certain ketones 'acetone(n-mineral oil based brake fluid.con oil and greaseuted water solutions, weak acids


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t "+,ati*le ithneral oilstroleum greases and fuelsphatic hydrocarbons like ben!ene, toluene, #ylol.lorinated hydrocarbons- such as chloroform, trichloroethylene, carbon tetrachloride

idi!ing, media like nitric acids, chromic acid, hydrogen pero#ide, chlorine, bromine.

Tetra2l#"r"eth!lene-Pr",!lene %A(LASR&

s elastomer is a copolymer of tetrafluoroethylene 'T$( and propylene. &ts chemical resistance is e#cellent across a wide rangressive media.

at Resistaneto appro#imately 3>3F:

l$ (le)i*ilit!wn to appro#imately -HF:

+,ati*le ithsesosphate esters

minesgine Oilseamlp and paper liuors

t "+,ati*le ithomatic fuelstonesrbon Tetrachloride

election of ase 2olymer

stem operating temperatures and compatibility with the media to be sealed are the most important parameters which must bensidered when selecting a base polymer. Only when these two factors are identified 'including any lubricants and potential cleds(, can a reliable recommendation be given concerning selection of the proper elastomer base. or the seal designed, ampromise often has to be made between specifying high uality, sealing grade materials and cheaper commercial products 'wually contain less polymer and more ine#pensive fillers(

e application temperatures given as shown below chart refer to long-term e#posure to non-aggressive media. At highermperatures, new crosslink sites may be formed between the polymer chains and lead to a loss of seal fle#ibility. The stiffness ymer chains may be observed as e#cessive compression set in highly filled compounds. This condition prevents an O-ring cro

ction from re-turning to its original, pre-compressed shape after deformation forces are removed. 9uring compression, a seal

anges its original shape to effect a seal and over time, and with e#cessive temperature, elastic memory loss in the elastomer sment can cause leakage. $#ceeding the normal ma#imum temperature limit for a given compound always result in reduced s.


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actically all elastomers undergo a physical or chemical change when in contact with a sealed medium. The degree of changepends on the chemistry of the medium and on the system temperature. An aggressive medium becomes more active withreasing temperature. 2hysical changes are caused by two mechanisms which can work concurrently when0

The elastomer absorbs a medium2lastici!ers and other components of the compound are dissolved and e#tracted or leached out by the media.

e result is volume changes, i.e, swelling or shrinkage of the elastomer seal. The degree of volume change depends on the typedium, molecular structure of the rubber compound, system temperature, geometrical seal shape, and the stressed conditionsbber part. When deformed and e#posed to a medium, rubber, when confined in a gland, swells significantly less than in free stp to 7E( due to a number of factors including lessened surface area in contact with the medium.

e limit of permissible volume change varies with the application. or static seals, a volume change of 37E to >E can be tolewelling leads to some deterioration of the mechanical properties, and in particular, those properties which improve e#trusion

sistance.

dynamic applications, swelling leads to increased friction and a higher wear rate. Therefore, a ma#imum swell of *E shouldnerally not be e#ceeded. 6hrinkage should also be avoided because the resulting loss of compressive force will increase the re leakage.

e e#traction of plastici!er from a seal material is sometimes compensated for by partial absorption of the contact medium. Thiuation however, can still lead to une#pected shrinkage and resultant leakage when an elastomer dries out and the absorbed fl


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aporate.

chemical reaction between sealed or e#cluded medium and the elastomer can bring about structural changes in the form of fuosslinking or degrading. The smallest chemical change in an elastomer can lead to significant changes in physical properties, embrittlement.

e suitability of an elastomer for a specific applications can be established only when the properties of both the medium and thstomer are known under typical working conditions. &f a particular seal material suits a medium, it is referred to as begin comph the medium.

++"n Seal (ail#res

*rasi"n 4esri,ti"n The seal or parts of the seal e#hibit a flat surface parallel to the direction ormotion. "oose particles and scrapes may be found on the seal surface.

C"ntri*#ting (at"rs;ough sealing surfaces. $#cessive temperature. 2rocess environcontaining abrasive particles. 9ynamic motion. 2oor elastomer surface finish.

S#ggeste$ S"l#ti"ns8se recommended gland surface finishes. :onsider internally luelastomers. $liminate abrasive components.

"+,ressi"n Set 4esri,ti"nThe seal e#hibits a flat-sided cross-section, the flat sides correspoding to thmating seal surfaces.

C"ntri*#ting (at"rs$#cessive compression. $#cessive temperature. &ncompletely cuelastomer. $lastomer with high compression set. $#cessive volume swell in chemical.

S#ggeste$ S"l#ti"ns"ow compression set elastomer. 2roper gland design for the speelastomer. :onfirm material compatibility.

he+ial 4egra$ati"n 4esri,ti"nThe seal may e#hibit many signs of degradation including blisters, cracks,

or discoloration. &n some cases, the degradation is observable only by measurement ofphysical properties.

C"ntri*#ting (at"rs:ontributing actors0 &ncompatibility with the chemical andor theenvironment.

S#ggeste$ S"l#ti"ns6election of more chemically resistant elastomer.

),l"sie 4e"+,ressi"n 4esri,ti"nThe seal e#hibits blisters, pits or pocks on its surface. Absorption of gas at pressure and the subseuent rapid decrease in pressure. The absorbed gas blisters andruptures the elastomer surface as the pressure is rapidly removed.

C"ntri*#ting (at"rs;apid pressure changes. "ow-modulushardness elastomer.

S#ggeste$ S"l#ti"ns)igher-modulushardness elastomer. 6lower decompression 'relof pressure(.

)tr#si"n 4esri,ti"nThe seal develops ragged edges 'generally on the low-pressure side( whicappear tattered.

C"ntri*#ting (at"rs$#cessive clearances. $#cessive pressure. "ow-modulushardne


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elastomer. $#cessive gland fill. &rregular clearance gaps. 6harp gland edges. &mproper s

S#ggeste$ S"l#ti"ns9ecrease clearances. )igher-modulushard-ness elastomer. 2rogland design. 8se of polymer backup rings.

stallati"n 4a+age4esri,ti"nThe seal or parts of the seal may e#hibit small cuts, nicks or gashes.

C"ntri*#ting (at"rs6harp edges on glands or components. &mproper si!ing of elasto"ow-modulushardness elastomer. $lastomer surface contamination.

S#ggeste$ S"l#ti"ns;emove all sharp edges. 2roper gland design. 2roper elastomersi!ing. )igher-modulushardness elastomer.

#tgassing E)trati"n 4esri,ti"nThis failure is often very difficult to detect from e#amination of the seal. Themay e#hibit a decrease in cross-sectional si!e.

C"ntri*#ting (at"rs&mproper or improperly cured elastomer. )igh vacuum levels. "ohardnessplastici!ed elastomer.

S#ggeste$ S"l#ti"nsAvoid plastici!ed elastomers. $nsure all seals are properly post-to minimi!e outgassing.

er"+,ressi"n 4esri,ti"nThe seal e#hibits parallel flat surfaces 'corresponding to the contact areas(may develop circumferential splits within the flattened surfaces.

C"ntri*#ting (at"rs&mproper design4Ifailure to account for thermal or chemical volumchanges, or e#cessive compression.

S#ggeste$ S"l#ti"nsland design should take into account material responses to che

and thermal environments.

as+a 4egra$ati"n 4esri,ti"nThe seal often e#hibits discoloration, as well as powdered residue on thesurface and possible erosion of elastomer in the e#posed areas.

C"ntri*#ting (at"rs:hemical reactivity of the plasma. &on bombardment 'sputtering($lectron bombardment 'heating(. &mproper gland design. &ncompatible seal material.

S#ggeste$ S"l#ti"ns2lasma-compatible elastomer and compound. 1inimi!e e#posed$#amine gland design.

,iral (ail#re 4esri,ti"nThe seal e#hibits cuts or marks which spiral around its circumference.

C"ntri*#ting (at"rs9ifficult or tight installation 'static(. 6low reciprocating speed. "ow

modulushardness elastomer. &rregular O-ring surface finish 'including e#cessive parting$#cessive gland width. &rregular or rough gland surface finish. &

S#ggeste$ S"l#ti"ns:orrect installation procedures. )igher-modulus elastomer. &nterlubed elastomers. 2roper gland design. land surface finish of +4B*< microinch ;16.2ossible use of polymer backup rings.

er+al 4egra$ati"n 4esri,ti"nThe seal may e#hibit radial cracks located on the highest temperature surfa&n addition, certain elastomers may e#hibit signs of softening4Ia shiny surface as a resul


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e#cessive temperatures.

C"ntri*#ting (at"rs$lastomer thermal properties. $#cessive temperature e#cursionscycling.

S#ggeste$ S"l#ti"ns6election of an elastomer with improved thermal stability. $valuaof the possibility of cooling sealing surfaces.

RING (AILRE ANALYSIS

seal failure can cost customer time and money as well as possiblydangering personnel. The analysis of premature or une#pected sealure includes many factors, including the environment, the sealsign and the elastomer itself. The appearance of t

he semiconductor industry, the failure of a single seal can result in millionsdollars in damaged production, downtime and maintenance costs. &n manyvironments, a seal failure can result in the complete evacuation of a facility-worse, the e#p

evention of seal failures through proper design, material selection andintenance certainly minimi!es the risk of failure. Attention to the condition of

placed seals, as well as the euipment performance over time, will result inproved process re

ring seals often fail prematurely in applications because of improper designcompound selection. This section is designed to provide the viewer withamples of common failure modes. y correctly identifying the failure mode,anges in the design or

om the end-userMs point of view, a seal can fail in three '>( general ways0

eakingontamination

hange in Appearance

ese three effects are demonstrated with special emphasis on theowing three analysis areas.

$nvironment6eal 9esign$lastomer

ir"n+ent Anal!sis

e ma/or factor in possible o-ring or seal failure is the e#treme and harsh environment in which o-rings and seals are e#pected to perform. &n& semiconductor industry, the sealing environment can consist of virtually anything from inert gases at

ntributing factors to seal failure in the sealing environment include0

emical 0 - type of chemical's( in serviceermal 0 - operating ranges of the seal 'also any thermal cycling(essureBacuum 0 - range of pressures or vacuum levels in the process

eal 4esign Anal!sis

alysis of the seal application is crucial to the understanding of possible failure. 1ost seal design is performed by component suppliers and


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uipment manufacturers. The designs are refined as e#perience is gained. As uickly as process technology chang

e seal design and application can provide information about the cause of failure0

tic 6eals 0 - a#ial and radial, confined or unconfined

namic 6eals 0 - a#ial 'open-close( or radial 'reciprocating or rotary(

aling land 9imensions 0 - shape 'suare, trape!oidal, etc.(ompressionand

retch

tallation 2rocedures 0 - stretch

ast"+er Anal!sisalytical techniues are used to identify the specific polymer type and compound. They can also be used to identify contamination sources oface, or surface properties which may have contributed to the failure. Traditional elastomer test methods


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Documents

What is an O-Ring.doc