Patent Publication Number: US-2010130390-A1

Title: Multifunctional Driveline Fluid

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a lubricant for transmissions, having good lubricating properties and a relatively high traction coefficient. 
     There have been continuing efforts in the transmission industry to improve the fuel economy of vehicles in which the transmissions are installed. This trend first became apparent in passenger cars, as a result of fuel economy legislation, and is increasingly affecting other types of vehicles, including heavy trucks. 
     One approach to improving fuel economy, while meeting in emission and performance requirements, is to employ a combination of transmission technologies. For example, so-called dual-clutch transmissions incorporate a combination of wet clutches, typical of automatic transmissions, along with manual transmission componentry. Lubricating such a transmission is a challenge since manual transmission fluid technology is not necessarily compatible with the wet clutches. The present invention provides a fluid which is suitable for lubricating a wide variety of mechanical power transmission devices, including automatic transmissions, manual transmissions, automatic manual transmissions, dual clutch transmissions, traction drives such as toroidal traction drives, continuously variable transmissions such as push-belt transmissions and pullchain transmissions, infinitely variable transmissions, hybrid transmissions, and transmissions for hybrid-powered vehicles or for gasoline, diesel, or electric powered vehicles. 
     Traction fluids based on a variety of base fluids are known. For example, U.S. Pat. No. 6,372,696, Tipton, Apr. 16, 2002, discloses traction fluid formulations including a base fluid of polymers of at least one olefin containing 3 to 5 carbon atoms, hydrocarbon molecules containing non-aromatic cyclic moieties, or mixtures thereof; a low-temperature viscosity control agent (such as polymers or oligomers of linear α-olefins), and an additive package of dispersants and detergents. Examples contain phosphoric acid and dialkyl hydrogen phosphite and alkyl phosphite friction modifier. The same examples also include dialkyl dimercaptothiadiazole. 
     U.S. Pat. No. 5,043,497, Muraki et al., Aug. 27, 1991, discloses a lubricating oil for a traction drive, mainly composed of a naphthenic hydrocarbon having 19 carbon atoms comprising two substituted cyclohexane rings linked by a methylene group. Additives for ordinary lubricating oils such as antioxidants, agents for increasing the viscosity index, corrosion inhibitors, detergents, defoamers, and so forth are added as necessary. Calcium sulfonate is disclosed as a detergent. 
     U.S. Pat. No. 3,975,278, Wygant, Aug. 17, 1976, discloses hydrogenated dimers of α-alkyl styrene, which are useful as tractive fluids. Additives such as VI improvers, antioxidants, antiwear agents, corrosion inhibitors, dispersants, and dyes can be included. 
     U.S. Pat. No. 3,966,624, Duling et al., Jun. 29, 1976, discloses a blended traction fluid containing hydrogenated polyolefin and an adamantane ether. The lubricant described can contain other oils and additives, e.g., a sludge dispersant. An especially useful additive, combining detergency, corrosion inhibition and friction improvement at high speeds, is a Mg, Ca or Ba salt (especially a super-based salt) of certain weak acids. 
     A variety of additive formulation for use in transmission fluids and other functional fluids are generally known. For instance, U.S. Pat. No. 5,858,929, Sumiejski et al., Jan. 12, 1999, equivalent to European Patent publication 747,464, published Jun. 6, 1996, discloses a composition for use in lubricants and functional fluids to provide improved anti-shudder and shudder durability properties to an automatic transmission. The composition comprises alkoxylated fatty amines as well as a mixture of other friction modifiers. Preferred compositions include alkoxylated fatty amines, other friction modifiers, antioxidants, overbased metal organic acid, dispersants, viscosity index improver and/or dispersant-viscosity modifier, extreme pressure agent, seal swell agent, and 85% phosphoric acid. The base oils of lubricating viscosity include liquid petroleum oils and solvent treated or acid treated mineral lubricating, oils of the paraffinic, naphthenic or mixed naphthenic-paraffinic types. 
     U.S. Pat. No. 6,251,840, Ward et al., Jun. 26, 2001, discloses lubrication fluids for reduced air entrainment and improved gear protection. The improvement results from the use of 2,5-dimercapto-1,3,4-thiadiazole and derivatives thereof together with silicone and/or fluorosilicone antifoam agents. Phosphoric acid is also present. 
     U.S. Pat. No. 6,103,673, Sumiejski et al., Aug. 15, 2000, discloses compositions containing friction modifiers for continuously variable transmissions, including, an oil of lubricating viscosity, a shear-stable viscosity modifier, an overbased meta salt, a phosphorus compound, and at least two friction modifiers. 
     The present invention, therefore solves the problem of providing a single fluid that can satisfactorily lubricate one or more of a variety of transmissions, including automatic transmissions, manual transmissions, continuously variable transmissions, dual clutch transmissions, and other mentioned above. It is also suitable for use as a hydraulic fluid, final drive oil, hybrid vehicle fluid, and a fluid for other driveline and industrial applications. 
     SUMMARY OF THE INVENTION 
     The present invention provides a lubricant composition suitable for lubricating a transmission, said composition comprising: (a) 1 to 20 weight percent of a compound having a traction coefficient of at least about 0.045 or at least 0.05, selected from the group consisting of: (i) hydrocarbons containing non-aromatic cyclic structures, (ii) polybutenes of number average molecular weight 180 to 600, (iii) esters having a branched or non-aromatic cyclic alkyl moiety (for instance, polyol esters having branched or non-aromatic cyclic alkyl moieties in the acid portion or in both the alcohol and acid portions thereof), and mixtures thereof; (b) at least 50 weight percent of an oil of lubricating viscosity, other than a material of component (a); (c) a phosphorus compound, for example, a phosphorus acid or phosphorus ester or salt thereof; and (d) 2,5-dimercapto-1,3,4-thiadiazole or a derivative thereof, said lubricant composition having a kinematic viscosity at 100° C. of up to 12 mm 2 /sec. 
     The invention also provides a method for lubricating a transmission, comprising, supplying thereto the composition as described above. The transmission may comprise a traction drive. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Various preferred features and embodiments will be described below by way of non-limiting illustration. 
     The first component of the lubricant composition of the present invention is a compound, that is, a fluid or oil, which may be characterized as a traction fluid. Such a fluid will exhibit a traction coefficient of at least 0.045 or at least 0.05. Certain such fluids will exhibit a traction coefficient of 0.053 or 0.06 up to 0.12, or 0.08 to 0.10. Traction coefficient is the ratio of force transmitted in a sliding/rolling contact, to the normal, or clamping force between rolling elements. 
       μ(Traction Coefficient)= F (Tangential)/ N (Normal) 
     Traction coefficient and various traction fluids are described in greater detail in SAE Technical Paper Series 2000-01-2906, Oct. 16-19, 2000. As used herein, the traction coefficient limitation of at least 0.05 and the like refers to measurement at 100° C. at a 10% slide/roll ratio (SRR), at a speed of 4 m/s, and at 1.25 GPa. 
     Certain types of fluids are particularly suited for use in traction fluids because of their inherently good (high) traction coefficients. The types of fluids which are particularly suitable include (1) hydrocarbon molecules containing non-aromatic cyclic structures, (2) polybutenes of number average molecular weight 180 to 600; and (3) esters having a branched or non-aromatic cyclic alkyl moiety, for instance, polyol esters having branched or non-aromatic cyclic alkyl moiety in the acid portion thereof or in both the alcohol and acid portions, that is to say, in the portion of the ester corresponding to the constituent alcohol or in the portion corresponding to the constituent acid. Mixtures of these types of materials can also be used. For suitable performance, the traction fluid component (base fluid) may have a viscosity of greater than 2.5 mm 2 /s (cSt) at 100° C. (ASTM D-445), such as at least 3.0 mm 2 /s (cSt) or 3.5 mm 2 /s (cSt), typically up to 12.0 mm 2 /s (12.0 cSt) or to 10.0 mm 2 /s (cSt) or to 8.0 mm 2 /s (cSt) or 6.0 mm 2 /s (cSt) at 100° C. This component may also contain at least 10 carbon atoms, or at least 12 or 15 or 18 carbon atoms and may, if desired contain up to 200 or to 100 or to 50 carbon atoms. 
     Suitable fluids of type (1) include a wide variety of cyclic-containing hydrocarbon molecules. Examples of these include di(cyclohexyl)alkanes, cyclohexyl hydrindans and adamantane compounds, as described in U.S. Pat. No. 3,966,624; esters of cyclohexanol and cylohexanecarboxylic acid, as described in U.S. Pat. No. 4,871,476; decahydronaphthalene (“Decalin”™), cyclohexyldecahydronaphthalene, alkyl-substituted decahydronaphthaline, alkyl-substituted cyclohexyldecahydronaphthalene, and mixtures thereof, as described in U.S. Pat. No. 3,803,037; various materials having two cyclohexane rings linked by a methylene group described in U.S. Pat. No. 5,043,497; various hydrocarbon compounds having a bicyclooctane skeleton described in U.S. Pat. No. 5,422,027; hydrogenated products of dimers, trimers, or tetramers of norbornanes and/or norbornenes described in U.S. Pat. No. 5,126,065; hydrogenated dimers, trimers, or polymers of cyclic monoterpenoid monomers described in U.S. Pat. No. 4,975,215; various ter-cyclohexyl compounds disclosed in U.S. Pat. No. 5,850,745; perhydrofluorene derivatives disclosed in U.S. Pat. No. 4,774,013; and preferably linear dimers of hydrogenated α-alkyl styrene, as described in U.S. Pat. No. 3,975,278. Any of the above materials may be used in a hydrogenated form, to assure the removal of carbon unsaturation; indeed, certain hydrogenated styrene derivatives (or cyclohexane derivatives) are inherently hydrogenated species. However; aromatic cyclic structures such as those derived from styrene may also be present in the base fluid, since aromatic cyclic structures are generally considered to be less deleterious than olefinic unsaturation. 
     Among the suitable materials for option (1) of the fluid are predominantly hydrogenated linear dimers of α-alkyl styrene. These dimers are said to be predominantly linear, in contrast to the cyclic dimers which represent another possible structure. Such materials can be represented by the general structure 
     
       
         
         
             
             
         
       
     
     wherein each R is an alkyl group of 1 to 4 carbon atoms and C 6 H 11  represents a cyclohexyl group. Such materials and their preparation are described in detail in U.S. Pat. No. 3,975,278. 
     Suitable fluids of type (2) include polymers isobutylene, particularly those having a number average molecular weight of 180 to 600, or 200 to 500. The polymer may be hydrogenated to remove any residual unsaturation. Such materials and their preparation are well known and are described, for instance, in U.S. Pat. No. 3,966,624, as component A, described particularly in column 12 line 32 through column 16 line 1.1. 
     Suitable fluids of type (3) include esters of one or more polyols having 4 to 8 carbon atoms and 2 to 6 OH groups. The acid-derived moiety of the ester may have a branched or cyclic structure, and the polyol-derived portion may also have a branched or cyclic structure. Examples of suitable polyols include ethylene glycol, 1,2- and 1,3-propylene glycol, butylene glycol, glycerine, neopentyl glycol, pentaerythritol, 2-methylpropane-1,3-diol, 2-methylpropane-1,2-diol, 2-methylglycerol, 1,1,1-tris(hydroxymethyl)ethane, cyclohexane diol (1, 2 or 1, 3 or 1,4 isomers, cis or trans), cyclohexane triol, tetrols, pentols, and hexols such as inositol, and various carbohydrates such as fucose, allose, fructose, galactose, glucose, mannose, sorbose, tagatose, and talose. The alcohol functionality of such molecules may be fully esterified or there may be unreacted hydroxy groups. 
     The polyol is condensed with one or more acids, which may be cyclic or branched acids, and which are typically monocarboxylic acids. Suitable cyclic acids include cyclopentanecarboxylic acid, cyclopentylacetic acid, cyclohexylacetic acid, cyclohexanecarboxylic acid, substituted cyclic acids having alkyl groups with 1 to 8 carbon atoms, e.g., methylcylohexanecarboxylic acid and ethylcyclohexanecarboxylic acid. Also included are polycyclic carboxylic acids such as norbornanecarboxylic acid, norbornaneacetic acid, adamantanecarboxylic acid, adamantane acetic acid, decahydronaphthalenecarboxylic acid, and substituted versions of such materials. An example of a resulting ester could be neopentyl glycol cyclohexanecarboxylic acid diester. Suitable branched acids include those having one or multiple quaternary carbon atoms within the carbon chain. Examples include 2,2-dimethylbutyric acid and homologues thereof, such as 2,2,4,4-tetramethylpentanoic acid, as well as acids which contain both branching and cyclic groups, such as dicyclohexylacetic acid. Suitable linear acids include n-heptanoic acid, n-octanoic acid, n-nonanaoic acid, n-decanoic acid, and other acids of up to 30 or 24 or 1.8 carbon atoms. Mixtures of such acids may also be used. 
     In certain embodiments, the esters are those in which a cyclic group, such as one or more cyclohexyl groups or substituted cyclohexyl groups, is present in the acid-derived portion of the ester, or in both the acid- and the alcohol-derived portion of the ester. These esters and methods of their preparation are described in detail in one or more of U.S. Pat. Nos. 4,871,476; 4,886,613; 4,886,614; 4,889,650; 5,075,024; and 3,398,165. Suitable esters are commercially available as traction fluid materials. 
     The amount of the compound with the high traction coefficient will typically be 1 to 20 weight percent of the lubricant composition, alternatively 5 to 18 weight percent or 8 to 12 weight percent. 
     The lubricant composition will also contain at least 30 weight percent or at least 50 weight percent of an oil of lubricating viscosity, other than the traction fluid of component described above. The amount of the oil of lubricating viscosity (also referred to as a base oil) may also be 60 to 98 percent by weight or 75 to 95 percent by weight. 
     The base oil used in the inventive lubricating oil composition may be selected from any of the base oils in Groups I-V as specified in the American Petroleum Institute (API) Base Oil Interchangeability Guidelines. The five base oil groups are as follows: 
                                             Base Oil   Sulfur       Saturates   Viscosity       Category   (%)       (%)   Index                  Group I   &gt;0.03   and/or   &lt;90   80 to 120       Group II   &lt;0.03   and   &gt;90   80 to 120       Group III   &lt;0.03   and   &gt;90   &gt;120                         Group IV   All polyalphaolefins (PAOs)           Group V   All others not included in Groups I, II, III or IV                    
Groups I, II and III are mineral oil base stocks. The oil of lubricating viscosity, then, can include natural or synthetic lubricating oils and mixtures thereof. Mixture of mineral oil and synthetic oils, particularly polyalphaolefin oils and polyester oils, are often used.
 
     Natural oils include animal oils and vegetable oils (e.g. castor oil, lard oil and other vegetable acid esters) as well as mineral lubricating oils such as liquid petroleum oils and solvent-treated or acid treated mineral lubricating oils of the paraffinic, naphthenic or mixed paraffinic-naphthenic types. Hydrotreated or hydrocracked oils are included within the scope of useful oils of lubricating viscosity. 
     Oils of lubricating viscosity derived from coal or shale are also useful. Synthetic lubricating oils include hydrocarbon oils and halosubstituted hydrocarbon oils such as polymerized and interpolymerized olefins and mixtures thereof, alkylbenzenes, polyphenyl, (e.g., biphenyls, terphenyls, and alkylated polyphenyls), alkylated diphenyl ethers and alkylated diphenyl sulfides and their derivatives, analogs and homologues thereof. 
     Alkylene oxide polymers and interpolymers and derivatives thereof, and those where terminal hydroxyl groups have been modified by, for example, esterification or etherification, constitute other classes of known synthetic lubricating oils that can be used. 
     Another suitable class of synthetic lubricating oils that can be used comprises the esters of dicarboxylic acids and those made from C 5  to C 12 ) monocarboxylic acids and polyols or polyol ethers. 
     Other synthetic lubricating oils include liquid esters of phosphorus-containing acids, polymeric tetrahydrofurans, silicon-based oils such as the polyalkyl-, polyaryl-, polyalkoxy-, or polyaryloxy-siloxane oils, and silicate oils. 
     Hydrotreated naphthenic oils are also known and can be used, as well as oils prepared by a Fischer-Tropsch gas-to-liquid synthetic procedure. 
     Unrefined, refined and rerefined oils, either natural or synthetic (as well as mixtures of two or more of any of these) of the type disclosed hereinabove can used in the compositions of the present invention. Unrefined oils are those obtained directly from a natural or synthetic source without further purification treatment. Refined oils are similar to the unrefined oils except they have been further treated in one or more purification steps to improve one or more properties. Rerefined oils are obtained by processes similar to those used to obtain refined oils applied to refined oils which have been already used in service. Such rerefined oils often are additionally processed by techniques directed to removal of spent additives and oil breakdown products. 
     In certain embodiments of the present invention, the base oil is a synthetic oil such as a poly-alpha olefin (typically hydrogenated) such as a 4 centistoke polyalpha olefin (i.e., having a nominal viscosity of 4 mm 2 /sec at 100° C.). In certain embodiments, mixtures of synthetic and mineral base oils are used. In certain embodiments, at least 50, or at least 80, or at least 90 percent by weight of the oil of lubricating viscosity is a synthetic oil. 
     Another optional or typical component of the present lubricant composition is a phosphorus compound, typically a phosphorus acid or phosphorus ester or salt thereof (that is, a salt of a phosphorus acid or phosphorus ester), which can include a phosphorus acid, a phosphorus acid salt, a phosphorus ester, or mixtures thereof. The phosphorus acid or ester can be of the formula (R 1 X)(R 2 X)P(X) n X m R 3  or a salt thereof, where each X is independently an oxygen atom or a sulfur atom, n is 0 or l, m is 0 or 1, m+n is 1 or 2, and R 1 , R 2 , and R 3  are hydrogen or hydrocarbyl groups, and, in one embodiment, at least one of R 1 , R 2 , or R 3  is hydrogen. This component thus includes organic or inorganic phosphorous and phosphoric acids, thiophosphorous and thiophosphoric acids, as well as phosphite esters, phosphate esters, thiophosphite esters, and thiophosphate esters. Suitable salts include metal or amine salts of phosphorus esters and amine salts of phosphorus acids, such as the salt formed by reaction of a phosphorus acid with an amine-containing dispersant such as a succinimide dispersant. It is noted that certain of these phosphorus materials can exist in tautomeric forms, and that all such tautomers are intended to be encompassed by the above formula and included within the present invention. For example, phosphorous acid and certain phosphite esters can be written in at least two ways: 
     
       
         
         
             
             
         
       
     
     differing merely by the placement of the hydrogen. Each of these structures is intended to be encompassed by the present invention. 
     The phosphorus-containing acids can be at least one phosphate, phosphonate, phosphinate or phosphine oxide. These pentavalent phosphorus derivatives can be represented by the formula 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2  and R 3  are as defined above. The phosphorus-containing acid can be at least one phosphite, phosphonite, phosphinite or phosphine. An example of trivalent phosphorus derivatives can be represented by the formula 
     
       
         
         
             
             
         
       
     
     wherein R 1 , R 2  and R 3  are defined as above. Generally, the total number of carbon atoms in R 1 , R 2  and R 3  is at least 8, and in one embodiment at least 12, and in one embodiment at least 16. Examples of useful R 1 , R 2  and R 3  groups include hydrogen, t-butyl, isobutyl, amyl, isooctyl, decyl, dodecyl, oleyl, C 1-8  alkyl, eicosyl, 2-pentenyl, dodecenyl, phenyl, naphthyl, alkylphenyl, alkylnaphthyl, phenylalkyl, naphthylalkyl, alkylphenylalkyl, and alkylnaphthylalkyl groups. 
     In another embodiment, the phosphorus acid or ester is characterized by at least one direct carbon-to-phosphorus linkage such as those prepared by the treatment of an olefin polymer, such as one or more of the above polyalkenes polyisobutene having a molecular weight of 1000) with a phosphorizing agent such as phosphorus trichloride, phosphorus heptasulfide, phosphorus pentasulfide, phosphorus trichloride and sulfur, white phosphorus and a sulfur halide, or phosphorothioic chloride. 
     In certain embodiments at least two of the X atoms in the above structure are oxygen, so that the structure will be (R 1 O)(R 2 O)P(X) n X m R 3  or (R 1 O)(R 2 O)P(X) n X m H. This structure can correspond, for example, to phosphoric acid when R 1 , R 2 , and R 3  are hydrogen. Phosphoric acid exists as the acid itself, H 3 PO 4  and other forms equivalent thereto such as pyrophosphoric acid and anhydrides of phosphoric acid, including 85% phosphoric acid (aqueous), which is the commonly available commercial grade material. The formula can also correspond to a mono- or dialkyl hydrogen phosphite such as dibutyl hydrogen phosphite (a phosphite ester) when one or both of R 1  and R 2  are alkyl, respectively and R 3  is hydrogen, or a trialkyl phosphite ester when each of R 1 , R 2 , and R 3  is alkyl; in each case where n is zero, m is 1, and the remaining X is O. The structure will correspond to phosphoric acid or a related material when n and m are each 1; for example, it can be a phosphate ester such as a mono-, di- or trialkyl monothiophosphate when one of the X atoms is sulfur and one, two, or three of the R groups are alkyl, respectively. 
     Phosphoric acid and phosphorous acid are well-known items of commerce. Thiophosphoric acids and thiophosphorous acids are likewise well known and are prepared by reaction of phosphorus compounds with elemental sulfur or other sulfur sources. Processes for preparing thiophosphorus acids are reported in detail in Organic Phosphorus Compounds, Vol. 5, pages 110-111, G. M. Kosolapoff et al., 1973. 
     Salts of the above phosphorus acids are well known. Salts include ammonium and amine salts as well as metal salts. Zinc salts, such as zinc dialkyldithiophosphates and zinc dialkylphosphates, are well known and are useful in certain applications. In certain embodiments, the salts may be metal or amine dihydrocarbyidithiophosphate salts or metal or amine mono- and dihydrocarbylphosphate salts. 
     Thus, the phosphorus compound can be any of the phosphorus acids, dialkyl hydrogen phosphites, metal dihydrocarbyldithiophosphates, metal dihydrocarbylphosphates, and mixtures thereof. The amount of the phosphorus compound may be a suitable amount to provide 0.03 to 0.1 weight percent phosphorus to the composition, or in other embodiments to provide 0.04 to 0.09 or 0.05 to 0.08 or to 0.06 weight percent phosphorus. The requisite amounts of the particular phosphorus compound of interest can be readily calculated by those skilled in the art. 
     The lubricant composition of the present invention will further comprise a 2,5-dimercapto-1,3,4-thiadiazole or a derivative thereof, which may function in part as a corrosion inhibitor. Examples of suitable dimercaptothiadiazoles include 2,5-dimercapto-1, 3-4-thiadiazole, hydrocarbyl-substituted 2,5-dimercapto-1,3,4-thiadiazole, and hydrocarbylthio substituted 2,5-dimercapto-1,3,4-thiadiazole. In several embodiments the number of carbon atoms on the hydrocarbyl-substituent group may be 1 to 30, 2 to 25, 4 to 20, or 6 to 16. Additional specific examples include 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-nonyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-decyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-undecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-dodecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tridecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-tetradecyklithio)-1,3,4-thiadiazole, 2,5-bis(tert-pentadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-hexadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-heptadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-octadecyldithio)-1,3,4-thiadiazole, 2,5-bis(tert-nonadecyldithio)-1,3,4-thiadiazole, and 2,5-bis(tert-eicosyldithio)-1,3,4-thiadiazole. Moreover the dimercaptothiadiazole or its derivatives may be provided by a combination or reaction product an of oil soluble dispersant, as described below, with dimercaptothiadiazole. Such treated dispersants (e.g., treated succinimide dispersants) and their preparation are known and are described in U.S. Pat. No. 4,136,043, see col. 9 lines 18-36, col. 10 line 47 through col. 11 line 25, and examples 26-35. 
     In one embodiment, the dimercaptothiadiazole of the present invention can be present in an amount of 0.05 to 4.0 or to 2.0 percent by weight, or 0.1 to 1.5 percent by weight, or about 0.15 to 1.0 percent by weight of the lubricant composition. In certain embodiments, the 2,5-dimercapto-1,3,4-thiadiazole or derivative thereof may be present in an amount to provide 0.005 to 1%, or 0.01% to 0.5%, by weight sulfur to the composition. 
     Each of the components or additives listed herein may be mixed as such into the final lubricant composition. Alternatively, any one or more of them may be supplied as a concentrate in oil. For example, the phosphorus compound and the thiadiazole compound, as well as any of the additional optional ingredients described below, may be prepared in a relatively small amount of a diluent, typically, oil. If desired, some or all of the component having the specified traction coefficient may be also included within the concentrate, and any such concentrates may be blended with the larger amounts of oil or oil plus traction component, as the case may be, to provide the final lubricant composition. The traction component itself may serve as the diluent for a concentrate. If an oil of lubricating viscosity is used as the diluent, the amount of oil in such a concentrate may be, for instance, 20 or 30 to 50 weight percent, and the amounts of the other components will be correspondingly increased. 
     The lubricant composition of the present invention may thus also include additional additives such as at least one dispersant, or at least one detergent, or mixtures thereof. Dispersants and detergents are extremely well-known and commonly used materials in the field of lubrication. 
     Detergents are typically overbased materials, otherwise referred to as overbased or superbased salts, are generally single phase, homogeneous Newtonian systems characterized by a metal content in excess of that which would be present for neutralization according to the stoichiometry of the metal and the particular acidic organic compound reacted with the metal. The overbased materials are prepared by reacting an acidic material (typically an inorganic acid or lower carboxylic acid, preferably carbon dioxide) with a mixture comprising an acidic organic compound, a reaction medium comprising at least one inert, organic solvent (mineral oil, naphtha, toluene, xylene, etc.) for said acidic organic material, a stoichiometric excess of a metal base, and a promoter such as a phenol or alcohol. The acidic organic material will normally have a sufficient number of carbon atoms to provide a degree of solubility in oil. The amount of excess metal is commonly expressed in terms of metal ratio. The term “metal ratio” is the ratio of the total equivalents of the metal to the equivalents of the acidic organic compound. A neutral metal salt has a metal ratio of one. A salt having 4.5 times as much metal as present in a normal salt will have metal excess of 3.5 equivalents, or a ratio of 4.5. 
     Such overbased materials are well known to those skilled in the art. Patents describing techniques for making basic salts of sulfonic acids, carboxylic acids, phenols, phosphonic acids, and mixtures of any two or more of these include U.S. Pat. Nos. 2,501,731; 2,616,905; 2,616,911; 2,616,925; 2,777,874; 3,256,186; 3,384,585; 3,365,396; 3,320,162; 3,318,809; 3,488,284; and 3,629,109. 
     Other overbased materials include salixarate detergents. These include overbased materials prepared from salicylic acid (which may be unsubstituted) with a hydrocarbyl-substituted phenol, such entities being linked through —CH 2 —or other alkylene bridges. It is believed that the salixarate derivatives have a predominantly linear, rather than macrocyclic, structure, although both structures are intended to be encompassed by the term “salixarate.” Salixarate derivatives and methods of their preparation are described in greater detail in U.S. Pat. No. 6,200,936 and PCT Publication WO 01/56968. 
     The amount of the detergent in the lubricant composition of the present invention, if it is present, may be 1 to 10 weight percent, or 1.5 to 7 weight percent, or 2 to 3 weight percent. 
     Dispersants are well known in the field of lubricants and include primarily what is known as ashless-type dispersants and polymeric dispersants. Ashless type dispersants are characterized by a polar group attached to a relatively high molecular weight hydrocarbon chain. Typical ashless dispersants include N-substituted long chain alkenyl succinimides, having a variety of chemical structures including typically 
     
       
         
         
             
             
         
       
     
     where each R 1  is independently an alkyl group, frequently a polyisobutylene group with a molecular weight of 500-5000, and R 2  are alkylene groups, commonly ethylene (C 2 H 4 ) groups. Such molecules are commonly derived from reaction of an alkenyl acylating agent with a polyatnine, and a wide variety of linkages between the two moieties is possible beside the simple imide structure shown above, including a variety of amides and quaternary ammonium salts. Also, a variety of modes of linkage of the R 1  groups onto the imide structure are possible, including various cyclic linkages. Succinimide dispersants are more fully described in U.S. Pat. Nos. 4,234,435 and 3,172,892 and in EP 0355895. 
     Another class of ashless dispersant is high molecular weight esters. These materials are similar to the above-described succinimides except that they may be seen as having been prepared by reaction of a hydrocarbyl acylating agent and a polyhydric aliphatic alcohol such as glycerol, pentaerythritol, or sorbitol. Such materials are described in more detail in U.S. Pat. No. 3,381,022. 
     Another class of ashless dispersant is Mannich bases. These are materials which are formed by the condensation of a higher molecular weight, alkyl substituted phenol, an alkylene polyamine, and an aldehyde such as formaldehyde. Such materials may have the general structure 
     
       
         
         
             
             
         
       
     
     (including a variety of isomers and the like) and are described in more detail in U.S. Pat. No. 3,634,515. 
     Other dispersants include polymeric dispersant additives, which are generally hydrocarbon-based polymers which contain polar functionality to impart dispersancy characteristics to the polymer. 
     Dispersants can also be post-treated by reaction with any of a variety of agents. Among these are urea, thiourea, dimercaptothiadiazoles, carbon disulfide, aldehydes, ketones, carboxylic acids, hydrocarbon-substituted succinic anhydrides, nitriles, epoxides, boron compounds, and phosphorus compounds. References detailing such treatment are listed in U.S. Pat. No. 4,654,403. 
     The amount of the dispersant in the lubricant composition of the present invention, if it is present, may be 1 to 10 weight percent, or 1.5 to 7 weight percent, or 2 to 3 weight percent. 
     The compositions of the present invention may also contain a viscosity index modifier, for example, in limited amounts, that is, up to 10 percent by weight of the composition. In certain embodiments the amount of this component is 0 to 1 percent by weight, and in one embodiment the traction fluids are substantially free from (that is, less than 1 percent or less than 0.1 percent) polymeric viscosity index modifiers. 
     Polymeric viscosity index modifiers (VMs) are extremely well known in the art and most are commercially available. Hydrocarbon VMs include polybutenes, poly(ethylene/propylene) copolymers, and hydrogenated polymers of styrene with butadiene or isoprene. Ester VMs include esters of styrene/maleic anhydride polymers, esters of styrene/maleic anhydride/acrylate terpolymers, and polymethacrylates. The acrylates are available from RohMax and from The Lubrizol Corporation; polybutenes from Afton Corporation and Lubrizol; ethylene/propylene copolymers from ExxonMobil and Afton; hydrogenated polystyrene/isoprene polymers from Shell; styrene/maleic esters from Lubrizol, and hydrogenated styrene/butadiene polymers from BASF. 
     Suitable VMs include acrylate- or methacrylate-containing copolymers or copolymers of styrene and an ester of an unsaturated carboxylic acid such as styrene/maleic ester (typically prepared by esterification of a styrene/maleic anhydride copolymer). Preferably the viscosity modifier is a polymethacrylate viscosity modifier. Polymethacrylate viscosity modifiers are prepared from mixtures of methacrylate monomers having different alkyl groups. The alkyl groups may be either straight chain or branched chain groups containing from 1 to 18 carbon atoms. When a small amount of a nitrogen-containing monomer is copolymerized with alkyl methacrylates, dispersancy properties are also incorporated into the product. Thus, such a product has the multiple function of viscosity modification, pour point depressancy and dispersancy. Such products have been referred to in the art as dispersant-type viscosity modifiers or simply dispersant-viscosity modifiers. Vinyl pyridine, N-vinyl pyrrolidone and N,N′-dimethylaminoethyl methacrylate are examples of nitrogen-containing monomers. Polyacrylates obtained from the polymerization or copolymerization of one or more alkyl acrylates also are useful as viscosity modifiers. It is preferred that the viscosity modifier of the present invention is a dispersant viscosity modifier. 
     When the viscosity modifier component includes a polyisobutene, this component is distinguished from the material indicated as having a traction coefficient of at least 0.045 or 0.05, on the basis of its higher molecular weight. The traction fluid component has a relatively low molecular weight, e.g., 180 to 600, while the polybutene viscosity modifier component, if present, will have a higher molecular weight, as described below, or alternatively a molecular weight such as 800 to 6000 or 1000 to 3600 or 1200 to 2400. The polybutene viscosity modifier component may, nonetheless, impart some improved traction properties to the composition. 
     The polymers described above may commonly have a weight average molecular weight (  M   w ) of 1,000 or 2,000 or 10,000 up to 500,000, such as 30,000 to 250,000, or alternatively 20,000 to 100,000, and polydispersity values (  M   w /  M   n ) of 1.2 to 5. 
     Another optional material that may be present are present invention is one or more friction modifiers. Friction modifiers include alkoxylated fatty amines, borated fatty epoxides, fatty phosphites, fatty epoxides, fatty amines, borated alkoxylated fatty amines, metal salts of fatty acids, fatty acid amides, glycerol esters, borated glycerol esters, and fatty imidazolines. The amount of friction modifier or modifiers, if present, may be 0.01 to 2 percent by weight of the fluid composition, or 0.05 to 1.2 percent, or 0.1 to 1 percent by weight. 
     Other conventional components such as antioxidants, seal swell agents, corrosion inhibitors, anti-foam agents, and dyes may be present in conventional amounts. 
     In certain embodiments, molybdenum-containing additives such as molybdenum dithiocarbamates and titanium-containing additives may also be present to impart desirable properties such as antiwear performance, anti-oxidancy, and friction modification. 
     The lubricant composition thus prepared should have a kinematic viscosity at 100° C. of up to about 12 mm 2 /sec, for example, 2 to 10 or 6 to 8 mm 2 /sec. Obtaining a lubricant with such viscosity will be within the skills of the person skilled in the art, by means of selection of a base stock and other components with suitable viscosity. 
     It is known that some of the materials described above may interact in the final formulation, so that the components of the final formulation may be different from those that are initially added. For instance, metal ions (of, e.g., a detergent) can migrate to other acidic or anionic sites of other molecules. The products formed thereby, including the products formed upon employing the composition of the present invention in its intended use, may not be susceptible of easy description. Nevertheless, all such modifications and reaction products are included within the scope of the present invention; the present invention encompasses the composition prepared by admixing the components described above. 
     As used herein, the term “hydrocarbyl substituent” or “hydrocarbyl group” is used in its ordinary sense, which is well-known to those skilled in the art. Specifically, it refers to a group having a carbon atom directly attached to the remainder of the molecule and having predominantly hydrocarbon character. Examples of hydrocarbyl groups include: 
     hydrocarbon substituents, that is, aliphatic (e.g., alkyl or alkenyl), alicyclic (e.g., cycloalkyl, cycloalkenyl) substituents, and aromatic-, aliphatic-, and alicyclic-substituted aromatic substituents, as well as cyclic substituents wherein the ring is completed through another portion of the molecule (e.g., two substituents together form a ring); 
     substituted hydrocarbon substituents, that is, substituents containing non-hydrocarbon groups which, in the context of this invention, do not alter the predominantly hydrocarbon nature of the substituent (e.g., halo (especially chloro and fluoro), hydroxy, alkoxy, mercapto, alkylmercapto, nitro, nitroso, and sulfoxy); 
     hetero substituents, that is, substituents which, while having a predominantly hydrocarbon character, in the context of this invention, contain other than carbon in a ring or chain otherwise composed of carbon atoms and encompass substituents as pyridyl, furyl, thienyl and imidazolyl. Heteroatoms include sulfur, oxygen, and nitrogen. In general, no more than two, preferably no more than one, non-hydrocarbon substituent will be present for every ten carbon atoms in the hydrocarbyl group; typically, there will be no non-hydrocarbon substituents in the hydrocarbyl group. 
     EXAMPLES 
     Examples 1-3 
     Lubricant compositions are prepared as shown in Table I, and have the 100° C. kinematic viscosities as shown. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Example 
                 1 
                 2 
                 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 PAO 4 mm 2 /s (cSt) 
                 70.5 
                 70.5 
                 70.5 
               
               
                   
                 Hydrogenated dimer (see below) 
                   
                 10.0 
               
               
                   
                 Isobutene oligomer 
                   
                   
                 10.0 
               
               
                   
                 Synthetic ester 
                 10.0 
               
               
                   
                 PMA VM 
                 7.0 
                 7.0 
                 7.0 
               
               
                   
                 Polybutene 2000 M n   
                 6.0 
                 6.0 
                 6.0 
               
               
                   
                 Additive package 
                 6.5 
                 6.5 
                 6.5 
               
               
                   
                 Kinematic viscosity, mm 2 /s 
                 7.6 
                 7.1 
                 7.3 
               
               
                   
                   
               
            
           
         
       
     
     The materials designated PAOs are commercial poly-alpha olefin based fluids (oils of lubricating viscosity) having nominal kinematic viscosity at 100° C. of 4 mm 2 /s. The synthetic ester is a pentaerythritol (tech. grade) ester prepared with 3,5,5-trimethylhexanoic acid (70%), n-heptanoic acid (15%) and n-C8-10 acids (15%), available under the trade names Solest 68 NA™ or Hatcol 3368™. The hydrogenated dimer is a hydrogenated linear α-methyl styrene dimer, available under the trade name Santotrac 20™. The isobutene oligomer is a hydrogenated oligomer of isobutene, M n  300-350, available under the trade name Panalane™. The PMA VM is a commercially available polymethacrylate viscosity modifier, and the polybutene is an additional viscosity modifying additive having approximately the indicated molecular weight (significantly greater than that of the isobutene oligomer). The additive package comprises a dialkyl hydrogen phosphite antiwear agent/friction modifier and 2,5-bis(t-nonyldithio)-1,3,4-thiadiazole (a dimercaptothiadiazole), as well as a small amount (0.5%, including 49% diluent oil) of a succinimide dispersant that is treated with about 6% dimercaptothiadiazole. The additive package also contains a borated succinimide dispersant, an aromatic amine antioxidant, overbased calcium detergents, a seal swell agent, corrosion inhibitor, anti-foam and additional diluent oil. 
     Materials of certain of the above examples are tested for traction coefficient (results reported at 100° C., 10% SRR) and wear testing. The wear test is the FZG step load test A10/16.6R/120, which uses a 10 mm wide “A” profile test gear operated in a reverse direction (wheel driving) at 16.6 m/s pitchline velocity and a starting temperature of 120° C. The test measures the scuffing load capacity of lubricants. The lubricant is tested at increasing torque levels (load stages) until the failure criteria is met. This is known as the fail load stage; the immediately preceding passing stage is reported as the pass stage. A higher number represents better performance. (Reproducibility is typically ±1 stage.) The results of these tests are shown in Table II. Each of the fluids exhibits good performance. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Example 
                 1 
                 2 
                 3 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Traction Coefficient 
                 0.018 
                 0.021 
                 0.019 
               
               
                   
                 FZG Reverse (stages) 
                 8 
                 8 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Each of the documents referred to above is incorporated herein by reference. Except in the Examples, or where otherwise explicitly indicated, all numerical quantities in this description specifying amounts of materials, reaction conditions, molecular weights, number of carbon atoms, and the like, are to be understood as modified by the word “about.” Unless otherwise indicated, each chemical or composition referred to herein should be interpreted as being a commercial grade material which may contain the isomers, by-products, derivatives, and other such materials which are normally understood to be present in the commercial grade. However, the amount of each chemical component is presented exclusive of any solvent or diluent oil, which may be customarily present in the commercial material, unless otherwise indicated. It is to be understood that the upper and lower amount, range, and ratio limits set forth herein may be independently combined. Similarly, the ranges and amounts for each element of the invention can be used together with ranges or amounts for any of the other elements. As used herein, the expression “consisting essentially of” permits the inclusion of substances that do not materially affect the basic and novel characteristics of the composition under consideration.