Tribology Of Elastomeric Seal Materials
DOC TOR A L T H E S I SDepartment of Applied Physics and Mechanical EngineeringDivision of Machine ElementsLuleå University of Technology 2009Mohammadreza MofidiISSN: 1402-1544 ISBN 978-91-86233-26-6Tribology of elastomeric seal materialsTribology of elastomeric seal materialsMohammadreza MofidiLuleå University of Technology
Tribology of elastomeric seal materialsMohammadreza MofidiLuleå University of TechnologyDepartment of Applied Physics and Mechanical EngineeringDivision of Machine Elements
Tryck: Universitetstryckeriet, LuleåISSN: 1402-1544ISBN 978-91-86233-26-6Luleå www.ltu.se
Tribology Of Elastomeric Seal MaterialsiiiPrefaceThe work presented in this thesis has been carried out at the Division of MachineElements, Department of Applied Physics and Mechanical Engineering at LuleåUniversity of Technology (LTU) in Luleå, Sweden.I would like to express my deep gratitude to my supervisors, Professor Braham Prakashand Professor Elisabet Kassfeldt, for their wholehearted support and guidancethroughout this work. I have learnt a lot from the various courses I attended at thisuniversity and would like to thank all my teachers, especially Professor Braham Prakashand Professor Roland Larsson.My sincere thanks also go to my previous teachers at BSc and MSc levels especially,Professor Mansour Nikkhah Bahrami at the University of Tehran, Dr. Mohammad RezaGhazavi and Professor Gholamhosein Liaghat at Tarbiat Modarres University.I wish to thank the “Ministry of Science, Research and Technology of IRAN” forawarding me the scholarship to pursue research at Luleå University of Technology. Thiswork would not have been possible without this support and I am really grateful to theGovernment of Iran for this.All my colleagues at the Division of Machine Elements, especially Dr. MarikaTorbacke, Gregory F. Simmons, Jens Hardell and Donald McCarthy have been veryhelpful whenever I had any difficulty and I sincerely acknowledge their support.A special thanks to my wife, Sedigheh and my son Aref for their support and patience.My sincere gratitude goes to my parents-in-law for their affection and kindness whopassed away during the last three years. I would like to extend my appreciation to all mybrothers and sisters for their profound kindness.Finally, I am deeply indebted to my parents, Parviz and Soghra, and feel a tremendoussense of appreciation for their genuine support, care, encouragement, patience andeternal dedication.Luleå, April 2009Mohammadreza Mofidi
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Tribology Of Elastomeric Seal MaterialsvAbstractElastomers are the most commonly used materials for various sealing applicationsowing to their low modulus of elasticity, large elongation-at-break, and high Poisson’sratio. Most seals operate in the presence of lubricants, therefore the sealing elastomeroil interaction plays an important role in determining the tribological performance ofelastomers. Furthermore, at times, such as during start-up periods, the seals may alsooperate under dry conditions and the seal material can be affected by high frictioncoefficient and wear.In this work, the tribological behaviour of different sealing elastomers has been studied.The influence of the ageing of a sealing elastomer in different lubricants on itstribological behaviour has been investigated. Further studies pertaining to the influenceof lubrication on the abrasive wear of a sealing elastomer have also been carried out.The results show that the friction coefficient of an elastomer in lubricated slidingagainst a hard counterface, at low contact pressure depends on the surface topography ofthe elastomer but at high contact pressure, it is mainly a result of the viscoelasticdeformation of the rubber by the counterface surface asperities. Even if the hard surfaceappears smooth to the naked eye, it may exhibit short-wavelength roughness, whichmay make the dominant contribution to rubber friction. Ageing of the nitrile rubber inester base fluids leads to a reduction of the friction coefficient.In unidirectional dry sliding of an elastomer against a counterface, the frictioncoefficient decreases during the running-in period. The longest running-in periods havebeen observed when the elastomers slide against relatively smooth surfaces.Depending on the elastomer-lubricant compatibility, abrasive coarseness, geometry ofsliding contact area and contact pressure, the two-body abrasive wear of elastomers mayincrease or decrease in the presence of lubricants.Ageing of nitrile rubber in lubricating fluids increases the abrasive wear both in dry andlubricated conditions. The abrasive wear of nitrile rubber in ester base fluids andrapeseed oil is higher than that in the mineral oils.
Tribology Of Elastomeric Seal MaterialsviAppended papersA. G. F. Simmons. M. Mofidi. B. Prakash. Friction evaluation of elastomers inlubricated contact: a comparison of different test methodologies. Submitted forpublication.B.M. Mofidi, B. Prakash, Influence of counterface topography on sliding frictionand wear of some elastomers under dry sliding conditions, Proc. Inst. Mech. Eng.Part J.-J. Eng. Tribol. 222(5) (2008) 667-673.C.M. Mofidi, E. Kassfeldt, B. Prakash, Tribological behaviour of an elastomeraged in different oils, Tribol. Int. 41 (2008) 860-866.D. M. Mofidi, B. Prakash, B. N. J. Persson, O. Albohr, Rubber friction on(apparently) smooth lubricated surfaces, J. Phys.-Condes. Matter 20 (2008)085223.E.M. Mofidi, B. Prakash, The influence of lubrication on two-body abrasive wearof sealing elastomers under reciprocating sliding conditions. Submitted forpublication (A part of this paper was presented at NordTrib 08, Tampere, June2008 and published in the conference proceedings).F.M. Mofidi, B. Prakash, Two body abrasive wear and frictional characteristics ofsealing elastomers under unidirectional lubricated sliding conditions. Submittedfor publication.
Tribology Of Elastomeric Seal MaterialsviiList of symbolsVariableMeaningUnits ofMeasurementE(Ȧ)Complex dynamic modulusPaE1The real part of E(Ȧ)PaE2The imaginary part of E(Ȧ)PaȦOscillation frequencys-1ȣPoisson’s ratio-Ȝwavelength of the surface roughnessmqThe wavevector corresponding to the wavelength Ȝ 2ʌ/qm-1q0The low-wavevector cut-off to the wavelength Ȝ0m-1q1The large-wavevector cut-off to the wavelength Ȝ1m-1C(q)The power spectrum of the surface roughness1/mAThe contact area observed at the highest magnificationM2A0The nominal or apparent contact areaM2pMean normal pressurePavSliding velocityms-1ȘDynamic viscosityPa s
Tribology Of Elastomeric Seal Materialsviii
Tribology Of Elastomeric Seal MaterialsixTable of contentsPreface . iiiAbstract .vAppended papers . viList of symbols . viiTable of contents . ix1. Introduction. 12. Seals and sealing. 22.1. Seal Classification. 22.1.1. Static & semi-static seals . 22.1.2. Rotary Seals. 32.1.3. Reciprocating seals . 42.2. Elastomeric seal materials . 42.3. Failure of elastomeric seals. 73. Tribology of elastomers. 83.1. Oil- elastomer interaction . 83.2. Friction . 103.3. Wear 124. Objectives and Limitations. 174.1. Objectives of the research. 174.2. Limitations . 175. Experiments. 185.1. Lubricated sliding friction . 185.1.1. High frequency short stroke reciprocating machine (Optimol SRV). 185.1.2. Low frequency, long-stroke reciprocating test rig (Cameron-Plint) . 185.1.3. Low frequency, long stroke with O-rings (Cameron-Plint) . 195.2. Dry sliding friction . 205.3. Two body abrasive wear . 215.3.1. Two body abrasive wear in reciprocating sliding . 215.3.2. Two body abrasive wear in unidirectional sliding . 226. Summary of important results . 236.1. Friction evaluation of elastomers in lubricated contact (Paper A). 236.2. Friction and wear behaviour of selected sealing elastomers under dry sliding conditions(Paper B) . 246.3. Influence on tribological behaviour from ageing of an elastomer in different oils (Paper C) 256.4. Rubber friction on (apparently) smooth lubricated surfaces (Paper D). 266.5. The influence of lubrication on two-body abrasive wear of selected sealing elastomers inreciprocating sliding (Paper E). 296.6. The influence of lubrication on two-body abrasive wear of selected sealing elastomers inunidirectional sliding (Paper F). 31
Tribology Of Elastomeric Seal Materialsx7. Conclusions . 348. Suggestions for future work . 359. References . 36Paper A . 41Paper B . 57Paper C . 67Paper D . 77Paper E . 87Paper F . 101
Tribology Of Elastomeric Seal Materials11. IntroductionA seal is a component which prevents the leakage of fluid or gas from a machine andprevents contamination from entering the machine. Elastomers have quite uniqueproperties which enable them to function reliably as seal materials. Many seals(dynamic seals) slide against a sealing surface during their operation and have to beoptimized to reduce leakage, friction and wear. Friction of a seal sliding against asealing surface has to be minimised to increase the overall efficiency of machines andsimultaneously the thickness of lubricant film has to be reduced to minimise theleakage. Leakage can be avoided by surface patterning on the elastomeric seal (whichmay be formed during the production or operation) but in some seals, such asreciprocating seals, the leakage can be prevented by increasing the contact pressure anddecreasing the lubrication film thickness. In such situations there exists a high risk ofinsufficient lubrication and direct contact between the elastomeric seal and sealingsurface. This risk may be further aggravated by several other factors such as side loads,vibrations and inadequate surface finish.Seals may fail through different mechanisms resulting in leakage or contaminationentering the lubricant. The most important types of seal failure are abrasion, thermaldegradation, chemical degradation, compression set, plasma degradation, overcompression, extrusion and extraction. Further, high seal friction impairs the efficiencyof machines. High friction also results in an increase in temperature and accelerates thefailure of a seal through different mechanisms such as thermal degradation, chemicaldegradation, abrasion and so on. Most seals operate in lubricated conditions but mayalso occasionally operate in dry conditions or with insufficient lubrication such asduring running-in periods. Thus, understanding the frictional behaviour and wearmechanisms of sealing elastomers is important in determining their performance andservice life. While the frictional behaviour of elastomers has been investigatedextensively, most of the previous studies pertain to their behaviour under either dryconditions or in the full film and elastohydrodynamic lubrication regimes. Thetribological behaviour of sealing elastomers in boundary or mixed lubrication has beeninvestigated scantily. An enhanced understanding of the tribological behaviours ofelastomers can potentially help a designer to choose the best material with suitableproperties for a specified application.
Tribology Of Elastomeric Seal Materials22. Seals and sealingSealing is the control of fluid interchange between two regions sharing a commonboundary. Some structural or design issues or tolerance considerations may necessitatea relatively large gap between two surfaces which cannot therefore perform the sealingfunction autonomously. Such gaps can be reduced to small dimensions by introducing“seals” as additional components [1].2.1. Seal ClassificationSeals can be subdivided into static and dynamic seals. Static seals provide sealingfunction between surfaces which do not move relative to each other while dynamic sealsprovide sealing function between surfaces in relative motion. Dynamic seals can besubdivided into rotary and reciprocating seals [2, 3]. A general classification of seals isshown in Figure 1. Tribological aspects are significant in dynamic seals owing to theirsliding against sealing surfaces.Figure 1: Seals classification [3]2.1.1. Static & semi-static sealsStatic seals are used where there is no relative motion between the mating surfacesbeing sealed. In semi-static seals some motion is possible through the elastic deflectionsof the seal. Figure 2 shows an O-ring as a static seal. As pressure is applied to the seal,the flexibility of the elastomer material and its resistance to volume change, transfersthe pressure to the sealing surface. O-rings are also used in reciprocating seals andoccasionally in rotary seals. The same mechanism is prevalent when an O-ring is usedin reciprocating seal applications [4].
Tribology Of Elastomeric Seal Materials3Figure 2: The sealing action of an elastomeric O-ring: (a) with no applied pressure; (b) whenpressure is applied [4].Given that in static seals and semi-static seals there exists no relative motion betweenthe sealing surfaces, the tribological issues are not critical.2.1.2. Rotary SealsA rotary seal provides sealing between a rotating shaft and an outer surface, such as agroove or housing bore.Rotary lip seals are the most commonly used dynamic seals. The sealing surface islubricated by a very thin layer of the sealed fluid. At very low speed and during therunning-in period, mixed lubrication occurs [3].Figure 3: Schematic of a rotary lip seal (left) and the mechanism of reverse pumping in rotarylip seals (right) [3].Figure 3 shows the region in the vicinity of the sealing zone. A thin liquid film separatesthe lip from the shaft surface. This film is of the order of 1 μm in thickness and 0.05 to1 mm in length [3]. A mechanism which is called “reverse pumping” prevents leakagefrom the oil side to the air side. Several reasons have been identified to explain thereverse pumping. The most important reason is the effect of shear deformation. Asshown in Figure 3, when the shaft rotates, shear stresses in the film cause the asperitiesto deform into vane-like shapes which pump the fluid from the air side of the seal to theoil side [3-7]. Extensive modelling and numerical calculations have been done by Salantand his co-workers on the lubrication of rotary lip seals [e. g. 8-15].Other types of rotary seals that are available include labyrinth seals, viscoseals,mechanical face seals and grooved seals.
Tribology Of Elastomeric Seal Materials42.1.3. Reciprocating sealsA reciprocating seal provides sealing in relative reciprocating motion along the shaftaxis between the inner and outer elements (Figure 4). Reciprocating seals are subdivided into rod seals, piston seals and wipers.Figure 4: Schematic figure of typical reciprocating sealsThe behaviour of rod seals and piston seals are similar to that presented in Figure 2 forstatic seals. Significant effort has been made to develop the applied materials,technology, shape and geometry, accuracy and reliability of reciprocating seals toimprove their operating characteristics, such as reducing leakage and frictional losses,reducing the required housing volume, increasing the seal life and so on. Since O-ringsand other elastomeric reciprocating seals with symmetrical cross-sections usually havepoor performance, V-rings, U-rings or a combination of several seals are used in manyapplications. Some reciprocating seals are reinforced with harder materials such asmetals or hard polymers to improv
Tribology Of Elastomeric Seal Materials _ 1 1. Introduction A seal is a component which prevents the leakage of fluid or gas from a machine and prevents contamination from entering the machine. Elastomers have quite unique properties which enable them to function reliably as seal materials. Many seals
RS seal RS OMD seal RS PPS/S single-side seal RS PPS back-to-back seal Sleev-it Fire penetration seal Sleev-it Watertight penetration seal Sleev-it Transition collar R X seal RS X seal C RS T seal RG M63 seal CRL seal Wedge Stayplate C Wedge C Stayplate Lubricant Assembly Gel SE PPS extension frame GE extension frame Preholed entry plates
Mechanical shaft seal types and sealing systems 25 1. Mechanical shaft seal types 26 2. Sealing systems 31 3. Selecting a mechanical shaft seal 42 Chapter 3. Materials 45 1. Seal face materials 46 2. Seal face material pairings 51 3. Testing of shaft seals 55 4. Secondary seals 59 5. Materials of other shaft seal parts 61 Chapter 4. Tribology 63 1.
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