Technical Handbook O-rings 5. Designing With Rubber
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberIn designing an O-ring seal, it is important to determine the O-ring compoundearly, as the compound selected may have an influence on the gland design. Theapplication determines the rubber compound, the primary factor being the fluid to besealed. But the elastomer must also resist extrusion when exposed to the maximumanticipated pressure and be capable of maintaining good physical properties throughthe full temperature range expected.This chapter discusses the next criteria that must be considered like compressionset, hardness, tensile strength, chemical compatibility, thermal effects, pressure,and extrusion. Data and procedures enabling the designer to meet particularrequirements or obtain specific performance from the seal will be found in thischapter.Compression Set and SqueezeCompression set is the percentage of deflection that the elastomer fails to recoverafter a fixed period of time under a specific squeeze and temperature. Compressionset is a very important sealing factor, because it is a measure of the expected loss ofresiliency or "memory" of a compound. Compression set is generally determined inair and measured as a percentage of original deflection. Although it is desirable tohave a low compression set value, this is not so critical as it might appear becauseof actual service variables. For instance, an O-ring may continue to seal after takinga 100% compression set, provided the temperature and system pressure remainsteady and no motion or force causes a break in the line of seal contact. Also,swelling caused by contact with the service fluid, may compensate for compressionset. The condition most to be feared is the combination of high compression setand shrinkage. This will lead to seal failure unless exceptionally high squeeze isemployed. Compression set is calculated as follows:t0 - t1t0 - tsC x 100 %LoadSqueezeCompressionsetct0t1tsOriginalO-ring WUnderLoadAfter test and30 min. relaxationCompression set illustration22All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberLower compression set values indicate improved remaining seal capacity. Compressionset values generally increase with increased temperature and time.For O-rings the minimum squeeze should be about .007 inch. (0,175mm). The reason isthat with a very light squeeze almost all elastomers quickly take 100% compression set.A good compression set resistant compound can be distinguished from a poor one onlywhen the squeeze is more than .005 inch. (0,127mm)Most O-ring seal applications cannot tolerate the no squeeze condition, the exceptions arethe floating ring designs in special pneumatic and rotary applications.The most commonly used standards for the expression of compression set are ASTM D395 and DIN 53517 / ISO 815.Table 3A-1a gives compression set values for standard Eriks compounds, (Squeeze 25%).Note:It is important to notice that the compression set changes with time and depends oncross section diameter. This table shows these different values, measured on the samecompound.Table 3A-1aMaterialHardnessIRHD 5NBR 3662470NBR 4770290EPDM 5591470EPDM 55914 PC70Silicone 7147770Neoprene 329067070Viton black 5141470Viton green 5141490Viton black 514320Compression set22h/100 C, 25%,on O-ring 3.53 mm.max. 20%max. 30%max. 30%max. 25% (150 C)max. 40% (200 C)max. 25%max. 18% (200 C)max. 19% (200 C)max. 18% (200 C)Temp. Range C F-30 120-30 120-50 120-50 150-60 220-35 110-20 200-20 200-20 200-22 248-22 248-58 248-58 302-76 428-31 230-4 392-4 392-4 392NBR 36624 O-ringsCross section mmCompression set 22h/100 C (212 F)Compression set 70h/100 C (212 F)1,7814,823,93,5312,822,76,999,216,823All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberO-ring HardnessThe hardness of an O-ring is important for several reasons.The softer the elastomer, the better the seal material conforms to the surfaces to besealed and lower pressure is required to create a seal. This is particularly important inlow pressure seals that are not activated by fluid pressure.The softer the elastomer, the higher the coefficient of friction. In dynamic applicationshowever, the actual running and breakout friction values of a harder compound withlower coefficients of friction are higher because the load required to squeeze the hardermaterial into the O-ring groove is much greater.The softer the elastomer the more risk that at high operating pressure the elastomer ofthe O-ring will extrude into the clearance gap between the mating seal surfaces.The harder materials offer greater resistance to flow.With an increase in temperature, elastomers first become softer and then eventuallyharder as the rubber curing process continues with the application of heat.The hardness of most elastomers is indicated by a durometer rating on a gaugemanufactured by the Shore Instrument Company or equivalent. Most elastomers aremeasured on the Shore "A" scale. Shore A hardness of 35 is soft; 90 is hard. Shore"D" gauges are recommended where the Shore "A" rating is greater than 90. The mostcommon standards for measuring hardness are ASTM D2240, DIN 53505, BS 2719,and ISO 7619. These standards define a gauge reading on a standard sample with athickness of 0,25 in. (6 mm.). Always use standard hardness discs 1.28 in. diam. by0,25 in. thick (ø32 x6 mm.), or 6 in.x 6 in.x 0.075 in. (150x150x2 mm.) sheets piled upto a minimum of 0,25 in. (6 mm.) to determine durometer hardness.It has been almost impossible to obtain reliable and reproducible hardness readings onseals with curved surfaces and variable cross sections such as O-rings. This problemhas plagued the industry for years and is acknowledged in some standard tests. LikeASTM Method D 2240-00, paragraph 6.2.1 states: "A suitable hardness determinationcannot be made on an uneven or rough point of contact with the indentor". Also MILP-5510B, paragraph 4.4.2. states : "Test specimens for the purpose of batch testingshall consist of one compression molded hardness specimen 0,25 in. thick and 1 in.diameter minimum ( 6 mm. thick and 25 mm. diameter)." The specification states in anote "Hardness shall not be determined from actual packings."However, for specimens that are too thin or provide too small an area for accurate Shoredurometer readings, the Wallace Micro Hardness Tester is the most recommendedmethod. Measurements in Micro-IRHD are more accurate for O-rings. This method ofmeasurement is recorded in the standards ASTM D1415 and DIN 53519. Differencesbetween IRHD and Shore "A" are negligible on the 6 mm thick sample.Normally, durometer hardness is referred to in increments of five or ten, as in 60durometer, 70 durometer, 75 durometer, etc. Not as 62, 66, or 72 durometer. Thispractice is based on the fact that hardness is generally called out in specifications witha tolerance of 5 and also on the inherent variance from batch to batch of a givenrubber compound due to slight differences in raw materials and processing techniquesand the variability encountered in reading durometers.24All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberDurometer Ranges PYLENEIRHD and Shore LL R2060RUBBERCAR TIRES1050400DUROMETER D30IHRD-MicroDIN 53 519 Teil 2Norm : 2 mm sheetTime : 30 sec.Shore A DIN 53 505Norm : 6 mm sheetTime : 3 sec.RUBBER BANDS20100DUROMETER A25All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberModulusModulus, as used by the rubber industry, refers to stress at a predeterminedelongation, usually 100%. It gives a comparison for good extrusionresistance. Modulus normally increases with increase in hardness and isprobably the best indicator of the strength of a compound, all other factorsbeing equal.IRHDHardness (IRHD) versus Young’s Modulus (M)Log 10M (M in psi)Stress versus Strain1 Speciality FKM2 Black FFKM3 Standard FKM4 White FKM5 Ethylene Propylene6 Nitrile7 Fluorsilicone8 Silicone1235476Stress (PSI)Stress (MPa)Tensile Strength and ElongationTensile strength is a measurement of theamount of force required to rupture an elastomeric specimen. Tensile strength is a fairproduction control measurement used toinsure uniformity of the compound, and alsouseful as an indication of deterioration of thecompound after it has been in contact with afluid for long periods of time. If a large reduction in the tensile strength occurs, the life of aseal may be relatively short. Exceptions to thisrule do occur.Elongation is an increase in length expressednumerically as a percentage of initial lengthat the point of rupture. This property primarilydetermines the stretch which can be toleratedduring the installation of a seal.An adverse change in the elongation of acompound after exposure to a fluid is adefinite sign of degradation of the material. Elongation, like tensile strength, is usedthroughout the industry as a check on production batches of compound.Tests are performed on dumb-bell shapedsamples on a machine pulling them apart axially at a constant speed of 500 mm per minute, during which the force and elongation ofthe sample are recorded.Standards tests for Tensile strength andElongation are ASTM D412, DIN 53505, andBS 903, Part A3.8Elongation (%)26All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberTensile Stress-StrainTensile strength is the maximum tensile stress reached instretching a test piece (either an O-ring or dumbbell).Elongation: the strain or ultimate elongtion is the amount ofstretch at the moment of break.Modulus: also called ‘Mod.100’; this is the stress required to produce a given elongation. In the case of Mod 100, the moduluswould be the stress required to elongate the sample 100%.In elastomers, the stress is not linear with strain. Therefore,the modulus is neither a ratio nor a constant slope - but ratherdenotes a specific point on the stress-strain curve.Tensile tests are used for controlling product quality and fordetermining the effect of chemical or thermal exposure on anelastomer. In the latter case, it is the retention of these physicalproperties, rather than the absolute values of the tensile stress,elongation or modulus, that is often significant.Tensile Strength (MPa)Tear strengthThe tear strength or tear resistance is relatively low for mostcompounds. This test measures the force to perpetuate a nickor cut. Seal compounds with poor tear resistance will fail quicklyunder further flexing or stress, once a crack is started. Lowtear strength of a compound is also indicative of poor abrasionresistance which may lead to early failure of an O-ring used as adynamic seal.Volume changeVolume change is the increase or decrease of the volume of anelastomer after it has been in contact with a medium. It is measured as a percentage (%). Increase by swell or decrease byshrinkage in volume is almost always accompanied by a changein hardness.Volume swell is caused by absorption of gaseous or liquid medium by the O-ring. In static applications, even extreme volumeswell can sometimes be tolerated. Actually an O-ring can swellonly until 100% gland fill and further increase of volume is notpossible, regardless of how much volume swell is observed in afull immersion test. If the free state swell exceeds 50 percent;however, a radially squeeze assembly may be almost impossibleto take apart because of the friction generated.In dynamic applications, volume swell up to 15 or 20 percent isusually acceptable, but higher values are likely to increase friction and reduce toughness and abrasion resistance to the pointthat use of a particular compound is no longer feasible.Volume shrinkage is often caused by fluids which extract theplasticizers from the compound. Decrease in volume is usually accompanied by an increase in hardness. Also, as swellcompensates for compression set, shrinkage will intensify thecompression set effect, causing the O-ring to pull away fromsealing surfaces - providing a leakage path. It is apparent then,that shrinkage is far more critical than swell. More than 3 or 4%shrinkage can be a serious problem for dynamic O-ring seals.Hardness (Shore A)27All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberChemical CompatibilityThe chemical guide is intended to assistthe user in determining the suitabilityof various elastomers in many differentchemical environments. The ratings arebased on a combination of published literature, laboratory tests, actual field experience, and informed judgments. ERIKSuses the DuPont Performance Elastomersguide.Note: Volume swell is only one indicator ofelastomer fluid compatibility and may bebased on the solubility parameter alone.Fluid attack on the backbone of the polymer may show up as a change in physicalproperties such as tensile strength, elongation at break, and hardness.Chemical Compatibility Guide Rating SystemRatingDescriptionALittle or noeffectBPossible lossof physicalpropertiesCNoticeablechangeUExcessive changeVolumeChange 10%10-30%30-50% 50%CommentsElastomer may exhibit slight swelling and/or lossof physical properties under severe conditions.Elastomer may exhibit swelling in addition to achange in physical properties.May be suitable for static applications.Elastomer exhibits a noticeable change in swellingand physical properties.Questionable performance in most applications.Elastomer not suitable for service.Elevated temperatures and extendedexposure times may create more aggressive conditions.In some cases, specific elastomer compounds within a material family mayprovide improved compatibility. Pleasecontact the Application EngineeringDepartment for assistance or consult theDupont Performance internet chemicalresistance guide - where you can find thelatest information.Elastomers can swell and/or degrade inchemical environments through reactionswith the polymer backbone and crosslink system, or by reactions with the fillersystem. In the semiconductor industry,this degradation can be seen in increasedcontamination and reduced seal life.visit our website: www.o-ring.info28All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberAttack mechanisms: Chemical Compatibility The process of chemical degradation or chemical incompatibility is very complex. In general, degradation of the polymerbackbone and cross-link may occur by means of: nucleophilic attack - nucleophiles are ions or molecules thatcan donate electrons. This is the main cross-linking mechanism. In certain chemical media, nucleophilic attack canresult in increased cross-linking and embrittlement. dehydrofluorination - in fluorocarbon elastomers the attack ofaliphatic amines can result in the formation of unsaturatedbonds in the polymer backbone. polar attack - swelling caused by electrostatic interactionsbetween the dipole and polymer chainNucleophilic Attack(Unsaturated)C CPolar AttackH2OOxidationO-Chemical Attack MechanismsDegradation may also occur due to interactions of the chemicalenvironment and elastomer filler systems. This type of degradation may be caused by oxidation of fillers, or by chemical attackof certain fillers or process aids.In many applications special considerations should be made forcontamination or vacuum performance. Contamination is critical in semiconductor fabrication and medical applications. Thismay take the form of particle generation, extractable ions or otherresidual gas contamination.Test methods:ISO 1817 (Liquids)ASTM D471, D1460, D3137 (Liquids)Volume Swell:The most common measure of chemical compatibility is volumeswell. The following formula is used in reporting volume swellmeasurements. This takes into account dimensional changes inall three dimensions, and is more precise than specific dimensional change readings for most sealingapplications.Volume Swell:VS (%) (Weight in Air-Wt. in Water) final - (Wt. in Air - Wt. in Water) initialx100(Weight in Air - Weight in Water) initialNote:The "Weight in Water" measurement is performed by suspendinga sample in a container of water and recording it's weight. Thistakes into consideration that the density of a solid is equal to it'sweight in air divided by the difference of it's weight in air and it'sweight in water.29All 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.www.eriks.info
Sealing ElementsTechnical Handbook O-rings5. Designing with RubberThermal EffectsAll rubber is subjected to deterioration at high temperature.Volume change and compression set are both influenced byheat. Hardness is influenced in a complex way. The first effectof high temperature is to soften the compound. This is a physical change, and will reverse when the temperature drops. Inhigh pressure applications the O-ring may begin to flow throughthe clearance gap as the temperature rises, due to this softening effect. With increasing time at high temperature, chemicalchanges occur. These generally cause an increase in hardness,along with volume and compression set changes. Changes intensile strength and elongation are also involved. Being chemicalin nature, these changes are not reversible.The changes induced by low temperature are primarily physicaland reversible. An elastomer will almost completely regain itsoriginal properties when warmed.Thermal ExpansionCoefficient of linear thermal expansion is the ratio of the changein length per F or C to the original length at 0 F or 0 C.Coefficient of volumetric expansion for solids is approximately3 times the linear coefficient. As a rough approximation, elastomers have a coefficient of thermal expansion 10-times that ofsteel. With Fluoroelastomers and Perfluoroelastomers the coefficient of thermal expansion is even greater. This can be a criticalfactor at high temperature if the gland is nearly filled with theO-ring or at low temperature if the squeeze is marginal. Leakingcan be the result of seal failure at low temperature if the squeezeis small.There are certain reactions which in some circumstances causean
0,25 in. thick (ø32 x6 mm.), or 6 in.x 6 in.x 0.075 in. (150x150x2 mm.) sheets piled up to a minimum of 0,25 in. (6 mm.) to determine durometer hardness. It has been almost impossible to obtain reliable and reproducible hardness readings on seals with curved surfaces and variable cross sections such as O-rings. This problem
2.4 Homomorphism and quotient rings 28 2.5 Special rings 30 2.6 Modules 34 2.7 Rings with chain conditions 35 3. Smarandache rings and its properties 3.1 Definition of Smarandache ring with examples 38 3.2 Smarandache units in rings 41 3.3 Smarandache zero divisors in rings 46
Heavy Duty Back-Up Rings Heavy Duty back-up rings are configured with a greater axial thickness (T) dimension. The extra thickness provides more extrusion protec-tion for O-Rings. These rings are interchange-able with MS27595, MS28774, MS28782 and MS28783 back-up rings used in MIL-G-5514 packing glands. Heavy duty rings are available
seminear-rings using loops and groupoids which we call as groupoid near-rings, near loop rings, groupoid seminear-rings and loop seminear-ring. For all these concepts a Smarandache analogue is defined and several Smarandache properties are introduced and studied. The ninth chapter deals with fuzzy concepts in near-rings and gives 5
Missing rings & false rings (to identify these, need a "master chronology") Species limitations (some trees have no rings, non-annual rings, or poorly defined rings) Trees must crossdate! (can't develop a chronology or link to climate without this) Top of p 52 Today's class activity
Seal Rings The sets are comprised of double-turn rings in either 2 rings per set or 3 rings per set. The double-turn rings fit tighter in the bore or on the shaft, provide complete 360 surface contact and resist higher axial/radial forces than single-turn ring sets, for medium to heavy duty applications.
Technical Handbook O-rings The sealing principle of the Quad-Ring /X-Ring is nearly the same as the O-ring sealing. The initial sealing is achieved by the diametrical squeeze in a right angled groove. The system pres-sure itself creates a positive sealing force. Following are some advantages of Quad-Rings /X-Rings:
A hybrid design is possible: rings can be constructed in a hier-archy such that groups of nodes share a simple ring interconnect, and these “local” rings are joined by one or more “global” rings. Figure 1 shows an example of such a hierarchical ring design. Past works [43, 51, 21, 44, 19] proposed hierarchical rings as a scalable network.
Saturn’s most prominent rings are the A, B, and C The age and fate of Saturn’s rings Jonathan Henry When Saturn was the only known ringed planet, the rings were believed to be as old as the solar system—4.6 billion years in the conventional chronology. The existence of the rings to the present day was taken as evidence of this chronology.
