Handbook of Lubrication part 8 ppsx

The four-ball machine, Figure la, is widely used for evaluating solid lubricant additives in oils; the pin/disc and pin/ring arrangements Figures 1b to d are used for wear testingself-lu

of the film is subjected.31Even in very carefully controlled conditions, repeat determinations

of wear life can show considerable scatter With the Timken apparatus, Figure 1e, scatter

in wear life determinations can exceed ±100% With Falex tests, Figure 1e, scatter isusually less than ±50% Falex tests are commonly incorporated into specification require-ments for thin film lubricants

The four-ball machine, Figure la, is widely used for evaluating solid lubricant additives

in oils; the pin/disc and pin/ring arrangements (Figures 1b to d) are used for wear testingself-lubricating composites as well as thin film lubricants; reciprocating line-contact arrange-ments (Figure 1d) show promise for wear testing thin, self-lubricating, bearing-liner ma-terials;32 the press-fit test (Figure 1h) is used for dry powders and rubbed films and thejournal and thrust-bearing configurations (Figures 1f and g) simulate bearing applicationsfor both thin films and self-lubricating composites

Behavior of rubbed MoS2films shows some general trends with operational parameters.Friction rises with increasing relative humidity,35 possibly as a result of increased hydrogenbonding between adsorbed water molecules Initial reduction in friction with increasingtemperature can be attributed to desorption of water vapor, but reduction in wear life astemperatures rise above 200°C is more probably a consequence of increasing oxidation ofthe MoS2 Effects of substrate roughness on wear life are consistent with the idea thatmechanical entrapment of particles plays a major role in film formation; if the topography

is very smooth, little lubricant is contained within the surface depressions, but if the surface

is very rough metal peaks may protrude through the lubricant film Relation of wear life tosubstrate hardness involves an uncertain trend.36,37

The possibility that MoS2might induce corrosion of ferrous substrates in humid ments has been the subject of much controversy Oxidation of MoS2 is accelerated bymoisture, and after prolonged storage of powder in air at room temperature, MoO3, adsorbed

environ-H2O, and H2SO4 can all be present as surface contaminants For this reason, pH limits ofaqueous extracts from MoS2 powder are required by most specifications,38 or a direct cor-rosion test.39 MoS2 powder is commonly protected against oxidation during storage either

by adsorption of long chain organic inhibitors or by enclosure in an inert gas atmosphere

Bonded Coatings

To overcome the dependence of burnished film thickness on relative humidity, and toobtain greater film thickness and wear lives, lamellar solids are often incorporated within asynthetic resin binder to produce a “bonded coating” An enormous number of coatingformulations has been developed40and some of the more widely used constituents are listed

in Table 6 MoS2 is by far the most common Relevant specifications are given in Table 7

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With the possible exception of polyimides, most binders have intrinsically poor frictionalproperties and the optimum lubricant to binder ratio usually ranges from 1:1 to 4:1 Highratios minimize friction while low ratios maximize wear life Other additives can also beincluded in the coating Sb2O3 generally increases the wear life of MoS2 coatings whenadded at a concentration of around 30% by weight, and is believed to function as a sacrificialantioxidant Inhibitors, such as dibasic lead phosphite, reduce substrate corrosion and othermetal sulfides can increase wear life Graphite additions increase wear life but are fallinginto disfavor because of possible electrochemical corrosion.

Bonded coatings are generally applied from dispersions in a volatile solvent by spraying,brushing, or dipping Spraying is usually the most consistent, but dipping is widely usedbecause of low cost Recommended thicknesses range from 5 to 25 µm, but even thickercoatings may be useful in low-stress applications Surface pretreatment is essential both toremove organic contamination and to provide a suitable topography for mechanical “key-ing” Optimum roughness depends on the finishing process used: abrasion 0.5 µm Ra, grit-blasting 0.75 µm Ra, grinding 1.0 µm and turning 1.25 µm Ra An alternative, or additional,pretreatment is phosphating for steels and analogous chemical conversion treatments forother metals

It is more difficult to generalize performance trends for bonded coatings than for rubbedfilms of lamellar solids because their properties depend on the type of binder and on thetest method, in low stress conditions wear life usually increases with film thickness but athigh stresses the reverse may occur.41Sliding speed usually has little effect on either friction

or wear until it becomes so high that frictional heating begins to soften or degrade organicresin binders The most important variable is temperature With organic binders, wear lifetends to decrease with increasing temperature but with inorganic binders the converse issometimes observed because of low-temperature brittleness Probably best all-round per-formance over the widest temperature range is given by formulations incorporating high-temperature resin binders such as polyimides Binder properties may also affect the way inwhich wear life depends on relative humidity

Significant reductions in both wear life and load-carrying capacity of solid lubricant filmsoccur in the presence of conventional oils.42 In some cases the reduction in performance is

a consequence of the resin binder being attacked by certain fluids, e.g., acrylics by chlorinatedorganic solvents More generally, fluids tend to cause adhesion failures at the substrateinterface and also impede reaggregation of lubricant debris produced during wear Despitethese reductions in performance, some MoS2-bonded coatings persist sufficiently long in thepresence of oils to facilitate running-in,43and to reduce tool wear during machining operations.44

The most promising high-temperature coatings are those incorporating CaF2/BaF2eutectic.These may be applied by spraying from dispersions, followed by fusing at around 1000°C,

or bonded with metal salts such as monoaluminum phosphate.45 Thicker coatings, 0.1 mmupwards, can be produced by plasma-spraying mixtures of CaF2/BaF2 with metals, oxides,

or graphite, followed by machining and a final heat treatment to enrich the lubricant phase

in the surface.46Applications include seals for gas turbine regenerators and high-temperatureair-frame bearings Thin coatings of mixed fluorides have also been used on retainers ofball bearings for hostile environments.47 For cryogenic applications, bonded coatings con-taining either MoS2 or PTFE are generally satisfactory, although some resin binders canbecome rather brittle PTFE films tend to lose adhesion to metal substrates on cooling tolow temperatures as a result of their high thermal expansion coefficients; this may be offset

by low expansion fillers in the coatings, e.g., lithium aluminum silicate

Polymer Composites

Because low thermal conductivity inhibits dissipation of frictional heat, thermoplasticsundergo large increases in wear above critical loads and speeds as a consequence of surfacemelting Effects on thermosetting resins are less dramatic because oxidative degradation,leading to surface embrittlement, is a function of exposure time as well as temperature.Thermal conductivity of the counterface is also relevant and at high sliding speeds canbecome more important than the conductivity of the polymer composite itself Limitingspeeds for polymers sliding against themselves are, in general, several hundred times lowerthan those for polymers sliding against metals.48

Wear rates of polymer composites depend strongly on the surface roughness of metalcounterfaces In early stages of sliding, wear rate varies typically with initial Ra roughnessraised to a power of 2 to 4;49 for this reason smooth counterfaces are always recommendedfor applications such as dry bearings During running-in, however, the initial counterfaceroughness is frequently reduced, either by transfer of the polymer and/or fillers or bypolishing/abrasive action of fillers, leading to a reduction in wear rate Steady-state roughnessand steady-state rate of wear depend both on the composite composition and on relativehardness of the fillers and counterface.50 Relationships between steady-state rate of wearand initial counterface roughness thus become very variable and examples are shown inFigure 2 Although an optimum counterface roughness for minimum wear is sometimessuggested, experimental results are conflicting

For PTFE composites and other polymers incorporating solid lubricants which rely ontransfer film formation on the counterface to achieve low wear, wear behavior is stronglyinfluenced by environmental factors Relative humidity is particularly important and in-creasing humidity can either reduce or increase wear depending on the type of filler; thereare no systematic trends.51Liquid water, however, increases wear by inhibiting transfer filmformation and the aggregation of wear debris Other fluids, including conventional hydro-carbon lubricants, produce similar effects although to a smaller extent For polymer com-posites which do not rely on transfer film formation, e.g., nylons and acetals, hydrocarbonlubricants usually reduce wear52 and are often effective in extremely small amounts Smallpockets of fluid within the bulk structure can provide a continuous source of lubricant.53

Applications of polymer composites are extremely diverse For dry bearings, some of themost successful composites are of complex construction, e.g., a layer of sintered bronze ofgraded porosity on a steel backing and filled with PTFE/Pb,3or a fabric liner of interwovenPTFE and glass fibers impregnated with synthetic resin and adhesively bonded to a steelbacking.54Composites of the latter type are widely used in aerospace applications; a typicalmodern aircraft may contain several hundred For transfer lubrication of rolling-elementbearings, a particularly successful composite for retainers is PTFE/glass fiber/MoS2.55,56

Metal-Lamellar Solid Composites

A wide variety of metal-solid lubricant mixtures have been developed and some examplesare listed in Table 8 With those containing lamellar solids, low friction is achieved viatransfer Since transfer film formation is an inefficient process, a high proportion of solidlubricant, 25% or more, is usually needed Since such composites are mechanically weak,low friction tends to be associated with high wear and vice versa, as shown in Figure 3.For any given materials, however, conditions which reduce friction, such as increasedtemperature with fluoride or oxide films, usually reduce wear rate also

A great deal of effort has been devoted to material combinations and/or composite rication to obtain both low friction and wear Incorporation of PTFE in lamellar solid-metalcomposites appears to facilitate transfer film formation, and carbides in Ta-Mo-MoS2improvestrength.57Fabrication techniques use conventional powder metallurgy, infiltration of porousmetals, electrochemical codeposition, plasma spraying, and machining of holes or recesses

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Table 9 CLASSIFICATION OF CARBONS AND GRAFITES

cermets over ceramics are greater toughness and ductility, but the metal content, usually Co

or Ni, reduces the maximum temperature

Few general guidelines are available to predict the wear behavior of ceramics, particularlycoatings where properties depend as much on method of deposition as on composition.Friction coefficients tend to be very variable but can be as low as 0.2 to 0.25 at hightemperatures, e.g., Cr2C3-Ni-Cr or Cr2O3sliding against themselves.5Attempts to incorporatesolid lubricants into bulk ceramics to reduce friction have met with little success, exceptwhen confining them to machined holes and recesses.67

Selection of Materials for Dry Sliding

Various attempts have been made to provide general guidelines for selection of materialsfor specific applications For dry bearings, one approach is to identify major applicationrequirements as listed down the left hand side of Table 11, and then select the group of

Volume II 285

Note: Key: 1 = unfilled thermoplastics, 2 = filled/reinforced

thermo-plastics, 3 = filled/reinforced PTFE, 4 = filled/reinforced thermosetting resins, 5 = PTFE impregnated porous metals,

6 = woven PTFE/glass fiber, 7 = carbons-graphites, 8 = metal-graphite mixtures, 9 = solid film lubricants, 10 = ceramics, cermets, hard metals, and 11 = rolling bearings with self-lubricating cages.

Table 10 SOME CERAMICS AND CERMETS FOR HIGH-TEMPERATURE USE

Table 11 SELECTION OF BEARING MATERIALS FOR

VARIOUS CONDITIONS

materials which offers the best compromise solution Published wear rates of the selectedmaterials obtained in low-duty sliding conditions where frictional heating is negligible arethen modified to take into account sliding conditions appropriate to the intended application.Figure 4 illustrates the range of wear rates typical of various groups of self-lubricatingcomposites, and approximate wear rate correction factors are listed in Table 12 A morecomplete account of this procedure, together with information about individual materials,

is given elsewhere.68Unfortunately, a similar approach is not yet available for self-lubricatingcomponents other than dry bearings, e.g., gears, seals, or thin-film solid lubricant coatings

Dispersions in Oils and Greases

Graphite and MoS are extensively used as additives in conventional oils and greases toreduce friction and wear when full-film hydrodynamic or elastohydrodynamic lubricationcannot be achieved The concentrations added vary widely, from 0.1 to 60% by weight, thehigher values producing pastes used primarily for component assembly purposes Relevantspecifications are listed in Table 13 Numerous rig tests have demonstrated that MoS2canprovide increases in load-carrying capacity, reductions in wear, and increased life of rollingbearings The optimum concentrations depend on the type of carrier fluid and the slidingconditions but are typically around 3% by weight in oils and 20% by weight in greases.Automotive experience has confirmed the beneficial effects of MoS2 additions to oils inreducing both wear and fuel consumption (friction).69Two cautionary comments are in order.First, detergent additives in automotive oils can inhibit the wear-reducing ability of MoS2and graphite, and some anti-wear additives can even increase wear rates slightly.70 Second,solid lubricant additions can affect the oxidation stability of oils and greases, and this mayinfluence the concentration of oxidation inhibitors required; smaller particles have a greatereffect on oxidation stability than larger ones

The influence of solid lubricant particle size on performance in oils and greases is fused.71 Particle shape can be important, and significant improvements in performance havebeen reported when using dispersions of “oleophilic” graphite and MoS2.72 These materialsare produced as very thin, plate-like particles by grinding in hydrocarbon media, and can

con-286 CRC Handbook of Lubrication

FIGURE 4 Order-of-magnitude wear rates of self-lubricating composites sliding against steel at room temperature, light loads, and low speeds.

enhanced by additives Effects of additions of metal oxides and salts to graphite-oil pastesduring high-temperature extrusion have been surveyed by Cook.73

Solid lubricants other than graphite and MoS2 which have been used as additives toconventional fluid lubricants are various phosphates, oxides, and hydroxides such as Zn2P2O7and Ca(OH)2, and PTFE The former groups are of interest where the black color of MoS2

or graphite is a disadvantage, e.g., in textile machinery PTFE may also be used for thispurpose, but its special properties are more fully exploited in PTFE-thickened fluorocarbongreases, which can provide effective lubrication in oxidizing environments over a widetemperature range.74Typical applications are in rocket motors and space components

REFERENCES

1 Campbell, W E., Solid lubricants, in Boundary Lubrication: An Appraisal of World Literature, Ling, F.

F., Klaus, E E., and Fein, R S., Eds., American Society of Mechanical Engineers, New York, 1969, 197.

2 Lansdown, A R., Molybdenum disulphide: a survey of the present state of the art, Swansea Tribol Cent.

Rep., 74, 279, 1974.

3 Pratt, G C., Plastic-based bearings, in Lubrication and Lubricants, Braithewaite, E R., Ed., Elsevier,

Amsterdam, 1967, 377.

4 Claus, F J., Solid Lubricants and Self-Lubricating Solids, Academic Press, New York, 1972.

5 Lancaster, J K., Dry bearings: a survey of materials and factors affecting their performance, Tribology,

6, 219, 1973.

6 Lancaster, J K., Friction and wear (of polymers), in Polymer Science, Jenkins, A D., Ed., North Holland,

Amsterdam, 1972, 960.

7 Tabor, D., Friction, adhesion and boundary lubrication of polymers, in Advances in Polymer Friction and

Wear, Lee, L.-H., Ed., Plenum Press, New York, 1974, 1.

8 Roselman, I C and Tabor, D., The friction of carbon fibres, J Phys D., 9, 2517, 1976.

9 Peterson, M B and Johnson, R L., Friction Studies of Graphite and Mixtures of Graphite With Several

Metallic Oxides and Salts at Temperatures to 1000°F, TN-3657, National Aeronautics and Space istration, Washington, D.C., 1956.

Admin-10 Grattan, P A and Lancaster, J K., Abrasion by lamellar solid lubricants Wear, 10, 453, 1967.

11 Giltrow, J P and Lancaster, J K., The role of impurities in the abrasiveness of MoS2, Wear, 20, 137,

14 Play, D and Godet, M., Study of the Lubricating Properties of (CFx)n, Coll Int CNRS, 233, 441, 1975;

NASA Rep TM 75191, National Aeronautics and Space Administration, Washington, D.C., 1975.

15 McConnell, B D., Snyder, C E., and Strang, J R., Analytical evaluation of graphite fluoride and its

lubrication performance under heavy loads, paper 76-AM-5C-3, ASLE Trans., 1976 preprint.

16 Gisser, H., Petronic, M., and Shapiro, A., Graphite fluoride as a solid lubricant, Lubr Eng., 28, 161,

1972.

17 Martin, C., Sailleau, J., and Roussel, M., The ultra-high vacuum behavior of graphite-fluoride filled

self-lubricating materials, Wear, 34, 215, 1975.

18 Fusaro, R L., Effect of Fluorine Content, Atmosphere and Burnishing Technique on the Lubricating

Properties of Graphite Fluoride, TN-D-7574, National Aeronautics and Space Administration, Washington, D.C., 1974.

19 Bisson, E E., Non-conventional lubricants, in Advanced Bearing Technology, SP-38 Bisson, E E and

Anderson, W J., Eds., National Aeronautics and Space Administration, Washington, D.C., 1964, 203.

20 Olsen, K M and Sliney, H E., Additions to Fused Fluoride Lubricant Coatings for Reduction of Low

Temperature Friction, TN-D-3793, National Aeronautics and Space Administration, Washington, D.C., 1967.

21 Devine, M J., Cerini, J P., Chappell, W H., and Soulen, J R., New sulphide addition agents for

lubricant materials, ASLE Trans., 11, 283, 1968.

288 CRC Handbook of Lubrication

22 Stott, F H., Lin, D S., Wood, G C., and Stevenson, C W., The tribological behavior of nickel and

nickel-chromium alloys at temperatures from 20° to 800°C, Wear, 36, 147, 1976.

23 Todd, M J and Bentall, R H., Lead film lubrication in vacuum, Proc ASLE 2nd Int Conf Solid Lubr.,

SP-6, American Society of Lubrication Engineers, Park Ridge, III., 1978, 1948.

24 Dearnaley, G and Hartley, N E W., Ion implantation of engineering materials, Proc Conf Ion Plating

and Allied Techniques, CEP Consultants Ltd., Edinburgh, 1977, 187.

25 Pooley, C M and Tabor, D., Friction and molecular structure: the behavior of some thermoplastics,

Proc R Soc London Ser A, 239, 251, 1972.

26 Spalvins, T., Sputtering technology in solid film lubrication, Proc, ASLE 2nd Int Conf on Solid Lubr.,

SP-6, American Society of Lubrication Engineers, Park Ridge, III., 1978, 109.

27 Fusaro, R L., Friction and Wear Life Properties of Polyimide Thin Films, TN-D-6914, National

Aero-nautics and Space Administration, Washington, D.C., 1972.

28 Brydson, J A., Plastic Materials, 3rd ed., Butterworths, London, 1975.

29 Theberge, J E., Properties of internally lubricated, glass-fortified thermoplastics for gears and bearings,

Proc ASLE Int Conf Solid Lubr., SP-3, American Society of Lubrication Engineers, Park Ridge, III.,

1971, 106.

30 Benzing, R J., Goldblatt, I., Hopkins, V., Jamison, W., Mecklenburg, K., and Peterson, M., Friction

and Wear Devices, 2nd ed., American Society of Lubrication Engineers, Park Ridge, III., 1976.

31 McCain, J W., A theory and tester measurement correlation about MoS2dry film lubricant wear, SAMPE

34 Stupian, G W., Feuerstein, S., Chase, A B., and Slade, R A., Adhesion of MoS2 powder burnished

on to metal substrates, J Vac Sci Technol., 13, 684, 1976.

35 Pritchard, C and Midgley, J W., The effect of humidity on the friction and life on unbonded molybdenum

disulphide films, Wear, 13, 39, 1969.

36 Tsuya, Y., Microstructure of wear, friction and solid lubrication, Tech Rep Mech Eng Lab Tokyo, 81,

1975.

37 Lancaster, J K., The influence of substrate hardness on the friction and endurance of molybdenum

disulphide films, Wear, 10, 103, 1967.

38 Military specifications, Molybdenum Disulphide Powder, Lubricating, U.K.; DEF-STAN 68-62/1; France: AIR 4223; W Germany: VTL - 6810-015; Canada: 3-GP-806a.

39 Military specifications, Molybdenum Disulphide, Technical, Lubrication Grade, U.S.: MIL-M-7866B.

40 Campbell, M E and Thompson, M B., Lubrication Handbook for Use in the Space Industry, Part A

— Solid Lubricants, CR-120490, National Aeronautics and Space Administration, Washington, D.C., 1972.

41 Hopkins, V and Campbell, M E., Film thickness effect on the wear life of a bonded solid lubricant

film, Lubr Eng., 25, 15, 1969.

42 Hopkins, V and Campbell, M E., Important considerations in the use of solid film lubricanis, Lubr.

Eng., 27, 396, 1971.

43 Kawamura, M., Hoshida, K., and Acki, I., Running-in effect of bonded solid film lubricants on

con-ventional oil lubrication, Proc ASLE 2nd Int Conf Solid Lubr., SP-6, American Society of Lubrication

Engineers, Park Ridge, III., 1978, 101.

44 Harley, D and Wainwright, P., Development of a dry film tool lubricant, Proc ASLE 2nd Int Conf on

Solid Lubr., SP-6, American Society of Lubrication Engineers, Park Ridge, III., 1978, 281.

45 Lavik, M T., McConnell, B D., and Moore, G D., The friction and wear of thin, sintered, fluoride

films, J Lubr Technol., Trans ASME, 95, 12, 1972.

46 Sliney, H E., Self-Lubricating Plasma-Sprayed Composites for Sliding-Contact Bearings to 900°C, TN

D-7556, National Aeronautics and Space Administration, Washington, D.C., 1974.

47 Sliney, H E., A Calcium Fluoride-Lithium Fluoride Solid Lubricant Coating for Cages of Ball-Bearings

to be Used in Liquid Fluorine TMX-2033, National Aeronautics and Space Administration, Washington, D.C., 1970.

48 Evans, D C and Lancaster, J K., The wear of polymers, in Treatise on Materials Science and

Tech-nology, Vol 13, Scott, D., Ed., Academic Press, New York, 1979, 85.

49 Lancaster, J K., Relationships between the wear of polymers and their mechanical properties, Proc Inst.

Mech Eng., 183 (3P)), 98, 1969.

50 Lancaster, J K., Polymer-based bearing materials: the role of fillers and fibre reinforcement, Tribology,

5, 249, 1972.

51 Arkles, B C, Gerakaris, S., and Goodhue, R., Wear characteristics of fluoropolymer composites.

Advances in Polymer Friction and Wear, Plenum Press, New York, 1974, 663.

Volume II 289

52 Evans, D C., Fluid-polymer interactions in relation to wear, Proc 3rd Leeds-Lyon Symp Wear of

Non-Metallic Materials, Mechanical Engineering Publication, London 1978, 47.

53 Ikeda, H., Piastic-Based Anti-Friction Materials, Japanese Patent, 75101441, 1975.

54 Williams, F, J., Teflon airframe bearings — their advantages and limitations, SAMPE Quart., 8, 30, 1977.

55 Sitch, D., Self-lubricating rolling element bearings with PTFE-composite cages, Tribology, 6, 262, 1973.

56 Anon., Self-Lubricating Bearings — A Performance Guide, U.K Natl Center of Tribology, Risley,

War-rington, 1977.

57 McConnell, B D and Mecklenburg, K R., Solid lubricant compacts — an approach to long-term

lubrication in space, 76-AM-2E-1, ASLE Trans., 1976, preprint.

58 Gardos, M N., Some Topographical and Tribological Characteristics of a CaF2/BaF2, Eutectic-Containing

Porous Nichrome Alloy Self-Lubricating Composite, 74LC-2C-2, ASLE Trans., 1974, preprint.

59 Sliney, H E., Wide-Temperature-Spectrum Self-Lubricating Coatings Prepared by Plasma Spraying,

TM-79113, National Aeronautics and Space Administration, Washington, D.C., 1979.

60 Paxton, R R., Carbon and graphite materials for seals, bearings, and brushes, Electrochem Tech., 5,

1974, 1967.

61 Strugala, E W., The nature and cause of seal carbon blistering, Lubr Eng., 28, 333, 1972.

62 McKee, D W., Savage, R H., and Gunnoe, G., Chemical factors in carbon brush wear, Wear, 22, 193,

1972.

63 Giltrow, J P., The influence of temperature on the wear of carbon fibre-reinforced resins, ASLE Trans.,

16, 83, 1973.

64 Lancaster, J K., The wear of carbons and graphites, in Treatise on Materials Science and Technology,

Vol 13, Scott, D., Ed., Academic Press, New York, 1979, 141.

65 Shobert, E I., Carbon Brushes: The Physics and Chemistry of Sliding Contacts, Chemical Publishing Co.,

New York, 1965.

66 Mayer, E., Mechanical Seals, 2nd ed., Illiffe, London, 1972.

67 Van Wyk, J W., Ceramic Airframe Bearings, 75-AM-7A-3, ASLE Trans., 1975, preprint.

68 Anon., A Guide on the Design and Selection of Dry Rubbing Bearings, Item 76029, Engineering Sciences

Data Unit, London, 1976.

69 Braithewaite, E R and Greene, A B., A critical analysis of the performance of molybdenum compounds

in motor vehicles, Wear, 46, 405, 1978.

70 Bartz, W J and Oppelt, J., Lubricating effectiveness of oil soluble additions and graphite dispersed in

mineral oil, Proc 2nd ASLE Int Conf on Solid Lubr., SP-6, American Society of Lubrication Engineers,

Park Ridge, III., 1978, 51.

71 Barlz, W J., Solid lubricant additives — effect of concentration and other additives on anti-wear

per-formance, Wear, 17, 421, 1971.

72 Groszek, A J and Witheredge, R E., Surface properties and lubricating action of graphite and MoS2,

Proc ASLE Conf Solid Lubr., SP-3, American Society of Lubrication Engineers, Park Ridge, III., 971,

371.

73 Cook, C R., Lubricants for high temperature extrusion, Proc ASLE Conf Solid Lubr., SP-3, American

Society of Lubrication Engineers, Park Ridge, III., 1971, 13.

74 Messina, J., Rust-inhibited, non-reactive perfluorinated polymer greases, Proc ASLE Conf Solid Lubr.,

SP-3, American Society of Lubrication Engineers Park Ridge, III., 1971, 326.

290 CRC Handbook of Lubrication

NATURE OF A GAS

In the gaseous state of matter, individual atoms or molecules are in constant motion andare separated from each other by distances of several times their diameter The gas particlescollide with each other frequently and travel in straight lines between collisions The averagevelocity of the particles is an expression of the gas temperature, increasing with temperature.When a gas particle hits a solid surface and bounces off, the change in momentum of theparticle exerts a force on the surface The sum of the countless surface collisions is thepressure the gas exerts on the surface If one of a pair of parallel surfaces is moving, it willimpart an additional component of velocity to each gas particle hitting it This additionalvelocity is transmitted to other particles in the course of collisions and eventually to theother surface The result is a force on the other surface expressed as the product of the area

of the surface, the rate of shear, and the viscosity The rate of shear is defined as the velocitydifference between the surfaces divided by the distance between them

If a volume of gas is compressed, more particles must hit each unit of surface, and thepressure increases If the temperature is increased, average particle velocity is increased,momentum change in each surface collision increases, and again the pressure is increased.This behavior is expressed in the “perfect gas” law PV= nRT where R is the “gas constant”and n the mass (moles) of the volume of gas involved

Almost all gases are “perfect” at low pressures, usually one atmosphere or less Deviationoccurs at very low pressures when not enough particles are present to provide many collisionsbetween impacts with the surface When the pressure is high, the gas particles are forcedmore closely together, molecular attractions between particles begin to exert an influence,and deviations from the perfect gas law are observed

Mixtures of gases behave as if each were alone in the total volume Each exerts a partialpressure equal to the pressure it would exert if it were alone in the volume The total pressure

is then the sum of the partial pressures of the gases that are mixed in the volume

PROPERTIES OF A GAS

In designing gas bearings, viscosity is usually the property of prime interest A number

of other physical properties may also be required, however, and are described in this section.Chemical properties of any particular gas may influence mixing of the gas with fluids inthe system, reactions with other gases, or reaction with bearings or other surfaces Thedesigner should, therefore, ascertain from other sources the chemical reactivity of the gas

Boiling point — TB, is the absolute temperature in degrees Kelvin at which a gas willcondense into a liquid Boiling point increases with pressure

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Density—ρ, also termed mass density, is the mass of gas in kilograms in a volume ofone cubic meter.

Absolute viscosity—μ, is used in determining flow of a gas in both hydrostatic andhydrodynamic designs It is the force in Newtons on an area of one square meter that isexerted by the gas when that area is moved at a velocity of one meter per second parallel

to a second surface one meter distant The units are Newtons per square meter × seconds,

or Pascal-seconds (Pa-sec)

Kinematic viscosity—ν, is the absolute viscosity divided by the mass density Units areNewton-meter-seconds per kilogram Since the Newton is the force required to accelerateone kilogram by one meter per second squared, kinematic viscosity has units of m2/sec

Temperature — The relation between absolute temperature T in degrees Kelvin and

relative temperature in degrees Celsius C is C = T – 273.1

Specific heat—Cpand Cv, is the energy required to raise the temperature of a unit quantity

of gas by one degree When the process is carried out at constant pressure, the quantity is

Cp At constant volume, the quantity is Cv The units are kilo-Joules per kilogram per degree

Sonic velocity—Φ, is the speed with which a pressure wave is transmitted through a gas.Since an increase in pressure can be transmitted only through particle collisions, the speed

of transmission will be related to the particle velocities, and hence to gas temperature

Mean free path—λ, is the average distance traveled by a gas particle between collisions.This quantity is of interest in bearings operating with very close clearances or at very lowpressures where the mean free path approaches the surface separation distance Its calculation

is treated in a following section

Equation of state — Relates the physical properties of a perfect gas to each other and to

the quantity of gas present It is PV = nRT, where V is the volume in m3 occupied by nkg-mol of gas at an absolute temperature T Gas constant R = 8.3143 kJ/kg-mol·K

PHYSICAL DATA

Data in Table 1 are abstracted from an extensive listing of thermophysical properties ofliquids and gases.1The first three columns give the common name of the gas, its chemicalformula, and its molecular weight Column four gives the boiling point in K at a pressure

of 760 mmHg or 1.01 bar Also, given are specific volume in m3/kg, heat capacity Cp inkJ/kg·K, speed of sound in m/sec, viscosity in Pa·sec, and the viscosity-temperature exponent

in Equation 1

Viscosity

The viscosity of a gas is nearly independent of pressure over a wide range of lowerpressures, but at higher pressures it will increase significantly Figure 1 illustrates this pointfor nitrogen, the principal component of air: the viscosity is 18 × 10–6Pa·sec up to 40 atmpressure, 20 × 10–6 at 100 bar, and 53 × 10–6 at 1000 bar The viscosities of airat severalpressures from 1 to 100 bar are shown in Figure 2 as a function of absolute temperature.This shows that the effect of pressure increases at lower temperatures

Viscosities of a number of common gases at 1 bar are shown in Figure 3 to increaserapidly with absolute temperature, contrary to the behavior of liquids The low viscosity ofhydrogen is striking, as is the deviation of water vapor from the general trend The watervapor curve terminates at its boiling point of 373 K Data for air at a number of temperaturesand pressures are shown in Table 2

In determining viscosity as a function of temperature, two equations are often used Ascan be seen from Figure 2, log (gas viscosity) is nearly linear with log (temperature) andcan be represented by:

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Copyright © 1983 CRC Press LLC

If no viscosities are known, the critical viscosity may be estimated from the values of Pcr,

Tcr, and molecular weight M, as follows:

(6)Table 4 illustrates the application of the two methods to the calculation of the viscosity

of nitrogen For each temperature, Tris calculated from Tcr = 126.3 K Next are listed the

Volume II 297

FIGURE 4 Generalized reduced viscosity of gases (From Hougen, O A., Watson, K M., and

Ragatz, R A., C.P.P Charts, 2nd ed., John Wiley & Sons, New York, 1960 With permission.)

Table 4 COMPARISON OF ESTIMATED NITROGEN VISCOSITIES AT 1 BAR

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values of μrestimated from Figure 4 from the low density limit curve Assuming we knowthe viscosity is 17.9 × 10–6Pa·sec at 300 K where μr is estimated to be 1.00, the value of

μcris the same and the viscosities at the other temperatures are directly calculated as shown

If no viscosities are known, Equation 6 is used:

μcr= 7.70 × 10–7(28.02)0.5(34.0)0.667(126.3)–0.167= 19.1 × 10–6

Values for the other temperatures are calculated directly from the estimated values of μr.The final column shows the actual viscosities, indicating a reasonable check

For convenience, the following are conversions to the SI system: 1 reyn = 1.45 × 10–10

Pa·sec and 1 P= 0.1 Pa·sec

Specific Volume

The specific volumes listed in Table 1 indicate the degree of “perfection” of a gas At273.1 K and I bar pressure, 1 g-mol of a perfect gas occupies a volume of 22.4  Adjusting

this to 300 K gives 24.6  If the specific volumes in Table 1 are multiplied by the molecular

weight for each gas, the result is liters per gram mole (/g-mol) and also cubic meters perkilogram-mole (m3/kg-mol) Values for air, nitrogen, and oxygen are 24.9 Freon 21, Freon

11, and sulfur dioxide are below the perfect gas figure, indicating some degree of associationbetween molecules

Pressure is given here in bars or atmospheres For use in the SI system, 1 bar is equivalent

to 101,300 Pa

APPLICATION OF DATA

Hydrodynamic bearings principally require knowledge of the viscosity of the gas at thetemperature and pressure involved When the pressures are very low or the spacing betweensurfaces is very small, consideration must be given to the mean free path Hydrostaticbearings involve feeding of gas at an elevated pressure into the bearing film area Viscosity

is required in calculating the flow through the film; thermal properties are required incalculating flow through feed orifices or ports Sonic velocity sets a limit on flow rate inthese situations

Hydrodynamic and Hydrostatic Designs

The viscosity enters directly in hydrodynamic calculations through the principal terms inReynolds equation which are of the form:

Because of the usually good heat transfer to the surfaces, hydrodynamic films are treated

as isothermal and at the bearing surface temperature Ambient temperature and pressureconditions are adequate for the determination of operating viscosity from the data in theprevious section

In hydrostatic design computations, one is concerned with mass flow through thin slotsobeying the equation: