Grease Composition For Use In Constant Velocity Joints

Info

Publication number: 20080176776
Type: Application
Filed: Oct 8, 2007
Publication Date: Jul 24, 2008
Applicants: GKN Driveline International GmbH (Lohmar), Kyodo Yushi Co., Ltd. (Tokyo)
Inventors: Jisheng E. (Hennef), Shinya Kondo (Fujisawa), Frank Reher (Wenden), Akira Taniguchi (Fujisawa)
Application Number: 11/868,765

Abstract

In order to provide for a grease composition which has a good compatibility with boots made of rubber or thermoplastic elastomer, and which also gives low wear and low friction, a grease composition for use in constant velocity joints is suggested, comprising a) a base oil composition; b) at least one tri-nuclear molybdenum compound of the formula Mo3SkLnQz, wherein L are independently selected ligands having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 though 7, Q is selected from the group of neutral electron donating compounds such as amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes non-stoichiometric values; c) at least one urea derivative thickener.

Description

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of International Patent Application No. PCT/EP2006/009718, filed Oct. 7, 2006, and which is incorporated in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a lubricating grease which is intended primarily for use in constant velocity universal joints, especially ball joints or tripod joints, which are used in the drivelines of motor vehicles.

BACKGROUND OF THE INVENTION

The motions of components within constant velocity joints (CVJ) are complex with a combination of rolling, sliding and spinning. When the joints are under torque, the components are loaded together which can not only cause wear on the contact surfaces of the components, but also rolling contact fatigue and significant frictional forces between the surfaces. The wear can result in failure of the joints and the frictional forces can give rise to noise, vibration and harshness (NVH) in the driveline. NVH is normally “measured” by determining the axial forces generated in plunging type CVJ. Ideally the greases used in constant velocity joints need not only to reduce wear, but also have to have a low coefficient of friction to reduce the frictional forces and to reduce or prevent NVH.

Constant velocity joints also have sealing boots of elastomeric material which are usually of bellows shape, one end being connected to the outer part of the CVJ and the other end to the interconnecting or output shaft of the CVJ. The boot retains the grease in the joint and keeps out dirt and water.

Not only must the grease reduce wear and friction and prevent the premature initiation of rolling contact fatigue in a CVJ, it must also be compatible with the elastomeric material of which the boot is made. Otherwise there is a degradation of the boot material which causes premature failure of the boot, allowing the escape of the grease and ultimately failure of the CVJ. The two main types of material used for CVJ boots are polychloroprene rubber (CR) and thermoplastic elastomer (TPE), especially ether-ester block co-polymer thermoplastic elastomer (TPC-ET).

Typical CVJ greases have base oils which are blends of naphthenic (saturated rings) and paraffinic (straight and branched saturated chains) mineral oils. Synthetic oils may also be added. It is known that said base oils have a large influence on the deterioration (swelling or shrinking) of both boots made of CR and TPC-ET. Both mineral and synthetic base oils extract the plasticisers and other oil soluble protective agents from the boot materials, Paraffinic mineral oils and poly-α-olefin (PAO) synthetic base oils diffuse very little into especially boots made of rubber material causing shrinkage, but on the other hand naphthenic mineral oils and synthetic esters diffuse into boot materials and act as plasticisers and can cause swelling. The exchange of plasticiser or plasticiser compositions for the naphthenic mineral oil can significantly reduce the boot performance, especially at low temperatures, and may cause the boot to fail by cold cracking, ultimately resulting in failure of the CVJ. If significant swelling or softening occurs, the maximum high speed capability of the boot is reduced due to the poor stability at speed and/or excessive radial expansion.

In order to solve the aforesaid problems, U.S. Pat. No. 6,656,890 B1 suggests a special base oil combination comprising 10 to 35% by weight of one or more poly-α-olefins, 3 to 15% by weight of one or more synthetic organic esters, 20 to 30% by weight of one or more naphthenic oils, the remainder of the combination being one or more paraffinic oils, and, further, a lithium soap thickener, and a sulphur-free friction modifier, that may be a organo-molybdenum complex, and molybdenum dithiophosphate, and a zinc dialkyldithiophosphate and further additives such as corrosion inhibitors, anti-oxidants, extreme pressure additives, and tackiness agents. However, the friction coefficient and the wear of grease compositions according to U.S. Pat. No. 6,656,890 B1 as measured in SRV (abbreviation for the German words Schwingungen, Reibung, Verschleiβ) tests needs to be improved.

SUMMARY OF THE INVENTION

Thus, it is the object of the present invention to provide for a grease composition, primarily for use in constant velocity joints, which has a good compatibility with boots made of rubber or thermoplastic elastomer, and which also gives low wear and low friction in use in CVJ.

Said object of the present invention is solved by a grease composition for use in constant velocity joints comprising

a) a base oil composition; and

b) at least one tri-nuclear molybdenum compound, preferable 0.25% by weight to 5% by weight, more preferable 0.3% by weight to 3% by weight, referred to the total amount of the grease composition, of the formula

Mo₃S_kL_nQ_Z, (I)

wherein L are independently selected ligands having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 though 7, Q is selected from the group of neutral electron donating compounds such as amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes non-stoichiometric values;

c) at least one urea derivative thickener;

The number of carbon atoms present in the tri-nuclear molybdenum compound among all the ligands, organo groups is at least 21 carbon atoms, preferably at least 25, more preferably at least 30, and most preferably at least 35. Tri-nuclear molybdenum compounds usable in the present invention are disclosed in U.S. Pat. No. 6,172,013 B1, the disclosure of which is incorporated in the present invention insofar by reference. The presence of at least 0.25% by weight of the tri-nuclear molybdenum compound according to claim 1 is preferred and significantly lowers the friction coefficient as well as the wear when used in CVJ.

As a base oil composition according to the present invention, a base oil composition as disclosed in U.S. Pat. No. 6,656,890 B1, the disclosure of which is incorporated insofar herein by reference, may preferably be used. However, any further kind of base oil composition, especially a blend of mineral oils, a blend of synthetic oils or a blend of a mixture of mineral and synthetic oils may be used. The base oil composition should preferably have a kinematic viscosity of between about 32 and about 250 mm²/s at 40° C. and between about 5 and about 25 mm²/s at 100° C. The mineral oils preferably are selected from the group comprising at least one naphthenic oil and/or at least one paraffinic oil. The synthetic oils usable in the present invention are selected from a group comprising at least one poly-α-olefin (PAO) and/or at least one synthetic organic ester. The organic synthetic ester is preferably a di-carboxylic acid derivative having subgroups based on aliphatic alcohols. Preferably, the aliphatic alcohols have primary, straight or branched carbon chains with 2 to 20 carbon atoms. Preferably, the organic synthetic ester is selected from a group comprising sebacic acid-bis(2-ethylhexylester) (“dioctyl sebacate” (DOS)), adipic acid-bis-(2-ethylhexylester) (“dioctyl adipate” (DOA)), and/or azelaic acid-bis(2-ethylhexylester) (“dioctyl azelate” (DOZ)).

If poly-α-olefin is present in the base oil composition, preferably poly-α-olefins are selected having a viscosity in a range from about 2 to about 40 centistokes at 100° C. The naphthenic oils selected for the base oil compositions have preferably a viscosity in a range between about 20 to about 150 mm²/s at 40° C., whereas if paraffinic oils were present in the base oil composition, preferably the paraffinic oils have a viscosity in a range between about 25 to about 170 mm²/s at 40° C.

According to the present invention, the grease composition comprises at least one urea derivative thickener. The urea derivative thickener used in accordance with the present invention may also be a urea complex thickener, that is defined as a mixture of at least one urea derivative thickener with at least one other thickener not being a urea derivative thickener. Especially preferred urea complex thickeners in accordance with the present invention are mixtures of at least one urea derivative thickener with at least one calcium and/or lithium-based thickener and/or complex thickener.

As a urea-derivative type thickener in the present invention, especially a urea thickener manufactured by the company Kyodo Yushi Co., Ltd., Tokyo, Japan, is used as defined in U.S. Pat. No. 5,589,444. The urea-derivative thickener is preferably a reaction product of at least one organic aliphatic amine with at least one organic phenyl isocyanate. However, the urea-derivative thickener is not restricted to specific ones and may be, for instance, also a diurea compound and/or a polyurea compound.

Examples of diurea compounds include those obtained from a reaction of a monoamine with a diisocyanate compound. Examples of useful diisocyanates include phenylendiisocyanate, dephenyldiisocyanate, phenyldiisocyanate, dephenylmethandiisocyanate, octadecanediisocyanate, decanediisocyanate, and hexanediisocyanate. Examples of useful monoamines include octylamine, dodecylamine, hexadecylamine, octadecylamine, oliylamine, aniline, t-toluidine, and cyclohexylamine. Especially preferred diurea compounds are compounds obtained by reaction of (4,4′-methylenediphenyldisocyanate) (MDI) with octadecylamine.

Examples of useful polyurea compounds include those obtained from a reaction of a diamine with a diisocyanate compound. Examples of useful diisocyanate include those used for the formation of the diurea compounds as mentioned above, whereas examples of useful diamines include ethylendiamine, propanediamine, butanediamine, hexanediamine, octanediamine, phenylenediamine, tolylenediamine, and xylenediamine. Most preferred examples of urea type derivative thickeners include those obtained through a reaction of arylamine such as aniline or p-toluidine, cyclohexylamine or a mixture thereof with a diisocyanate. The aryl group in the diurea compound has preferably 6 or 7 carbon atoms.

In a preferred embodiment of the present invention the urea derivative thickener is selected from the group comprising urea complex thickeners. Urea complex thickeners are defined as a mixture of at least one urea derivative thickener with any further kind of thickener, especially calcium-based thickeners. Especially preferred as a urea complex thickener in accordance with the present invention is a mixture of a urea derivative thickener as defined above with at least one calcium complex thickener and/or calcium thickener (calcium-based thickeners).

In the sense of the present invention, a calcium thickener (soap) is a reaction product of at least one fatty acid with calcium hydroxide. Preferably, the thickener may be a simple calcium soap formed from 12-hydroxy stearic acid or from other similar fatty acids or mixtures thereof or methylesters of such acids. Alternatively, a calcium complex thickener (soap) may be used formed for example from a mixture of long-chained fatty acids together with a mixture of short and/or medium chained carboxylic acids. However, mixtures of all of the aforesaid thickeners may also be used.

The urea derivative thickener may be present in the grease composition claimed in an amount of about 1% by weight to about 25% by weight, referred to the total amount of the grease composition, most preferred in an amount of about 3% by weight to about 11% by weight, referred to the total amount of the grease composition.

In a further embodiment of the present invention, the grease composition claimed further comprising at least one zinc dithiophosphate, molybdenum dithiocarbamate and/or molybdenum dithiophosphate as an additive package. Preferably, the amount of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is in a range of between about 0.1% by weight to about 7% by weight, preferably to about 5% by weight, more preferably about 0.3% by weight to about 2% by weight, in each case referred to the total amount of the grease composition. Most preferably, the weight percent added, referred to the total amount of the grease composition, of each of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is essentially identical. In such an embodiment of the present invention, preferably the amount of the zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is about 0.4% by weight, 0.5% by weight, 0.6% by weight, and/or 0.7% by weight, in each case referred to the total amount of the grease composition.

In a preferred embodiment of the present invention, the further molybdenum containing compound is selected from the group comprising molybdenum dithiocarbamates and/or molybdenum dithiophosphates. The at least one molybdenum dithiophosphate (MoDTP) and/or molybdenum dithiocarbamate (MoDTC) is preferably present in the grease composition according to the present invention in an amount in a range between about 0.3% by weight, more preferred about 0.5% by weight, most preferred about 1.5% by weight, to about 3.5% by weight, most preferred about 3% by weight, in each case referred to the total amount of the grease composition. However, also any further molybdenum containing compound may be present in the grease composition according to the present invention as component c), of which organic molybdenum compounds are preferred. The grease composition according to the present invention may contain one or more MoDTC and/or MoDTP, and especially mixtures thereof. The MoDTP according to the present invention is of the following general formula:

wherein X or Y represents S or O and each of R¹to R⁴inclusive may be the same or different and each represents a primary (straight chain) or secondary (branched chain) alkyl group having between 6 and 30 carbon atoms.

The MoDTC according to the present invention is of the following general formula:

[(R⁵)(R⁶)N—CS—S]₂—Mo₂O_mS_n (III)

wherein R⁵and R⁶each independently represents an alkyl group having 1 to 24, preferably 3 to 18 carbon atoms; m ranges from 0 to 3 and n ranges from 4 to 1, provided that m+n=4.

The grease composition comprises in the additive package at least one zinc compound additive, more preferably a zinc compound additive in an amount of about 0.1% by weight to about 3.5% by weight, preferably to about 2.5% by weight, more preferably to about 0.5% by weight to about 2.0% by weight, referred to the total amount of the grease composition. Most preferred the zinc compound additive is selected from the group comprising at least one of zinc dithiophosphates (ZnDTP) and/or zinc dithiocarbamates (ZnDTC), and ZnDTPs are most preferred. The zinc dithiophosphate is preferably selected from the group of zinc dialkyldithiophosphate of the following general formula:

(R⁷O)(R⁸O)SP—S—Zn—S—PS(OR⁹)(OR¹⁰) (IV)

wherein each of R⁷to R¹⁰inclusive may be the same or different and each represents a primary or secondary alkyl group of which primary alkyl groups are most preferred having 1 to 24, preferably 3 to 20, most preferably 3 to 5 carbon atoms. In particular, excellent effects can be expected if the substituents R⁷, R⁸, R⁹and R¹⁰represent a combination of primary and secondary alkyl groups, each having 3 to 8 carbon atoms.

The zinc dithiocarbamate may be preferably selected from zinc dialkyldithiocarbamate of the following general formula:

wherein R, R¹¹, R¹², R¹³, and R¹⁴may be same or different and each represents an alkyl group having 1 to 24 carbon atoms or an aryl group having 6 to 30 carbon atoms.

By adding at least one zinc compound additive to the grease composition according to the invention, the friction coefficient as well as the wear in CVJ are diminished further significantly.

According to a further embodiment of the present invention, the grease composition may further comprise an agent comprising at least one anti-oxidation agent, corrosion inhibitor, anti-wear agent, wax, friction modifier, and/or extreme pressure agent (EP agent), that may also be part of the additive package. The additive package, thus, may not only comprise zinc dithiophosphates, molybdenum dithiocarbonates and/or molybdenum dithiophosphates, but also the aforesaid agents.

The EP agent is preferably a metal-free polysulfide or a mixture thereof, e.g. sulphurised fatty acid methyl ester agents, with preferably a viscosity of about 25 mm²/s at 40° C., being present preferably in an amount between about 0.1 to about 3% by weight, preferably 0.3 to about 2% by weight, referred to the total amount of the grease composition. The total inactive sulphur amount of the EP agent at room temperature preferably ranges from about 8 to about 50% by weight, preferably to about 45% by weight. The active sulphur amount as measured in accordance with ASTM D1662 may be about up to 11% by weight, preferably up to about 8% by weight at 100° C., and preferably up to about 20% by weight at 140° C., the weight percent being referred to the amount of the EP agent itself. Such EP agents exhibit excellent effects with respect to the prevention of scuffing of contacting CVJ internal components. If the sulphur content exceeds the upper limit defined above, it may promote the initiation of rolling contact fatigue and wear of the contacting metal components and may lead to degradation of the CVJ boot material.

As an anti-oxidation agent, the grease composition of the present invention may comprise an amine, preferably an aromatic amine, more preferably phenyl-α-naphthylamine or di-phenylamine or derivatives thereof. The anti-oxidation agent is used to prevent deterioration of the grease composition associated with oxidation. The grease composition according to the present invention may range between about 0.1 to about 2% by weight, referred to the total amount to the grease composition, of an anti-oxidant agent in order to inhibit the oxidation degradation of the base oil composition, as well as to lengthen the life of the grease composition, thus prolonging the life of the CVJ.

Typically, the last operation before the assembly of CVJ is a wash to remove machining debris, and it is therefore necessary for the grease to absorb any traces of remaining water and to prevent the water from causing corrosion and adversely effecting the performance of the CVJ, thus a corrosion inhibitor is required. As a corrosion inhibitor, the grease composition according to the present invention may comprise at least one metal or dimetal salt selected from the group consisting of metal salts of oxidised waxes, metal salts of petroleum sulphonates, especially prepared by sulphonating aromatic hydrocarbon components present in fractions of lubricating oils, and/or metal salts of alkyl aromatic sulphonates, such as dinonylnaphthalene sulphonic acids, alkylbenzene sulphonic acids, or overbased alkylbenzene sulphonic acids. Examples of the metal salts include sodium salts, potassium salts, calcium salts, magnesium salts, zinc salts, quaternary ammonium salts, the calcium salts being most preferred. Calcium salts of oxidised waxes also ensure an excellent effect. Especially preferred is disodium sebacate as corrosion inhibitor.

Anti-wear agents according to the present invention prevent a metal-to-metal contact by adding film-forming compounds to protect the surface either by physical absorption or chemical reaction. ZnDTP-compounds may also be used as anti-wear agents. As anti-corrosion agents according to the present invention preferably calcium sulphonate salts are used, preferably an amount between about 0.5 to about 3% by weight, referred to the total amount of the grease composition.

As a wax compound, the grease composition of the present invention may comprise any kind of waxes, preferably oiliness waxes, known in the state of the art to be used in grease composition or mixtures thereof, of which montan waxes, especially ester montan waxes being a reaction product of at least one acid montan wax with an ester, and polyolefin waxes including micronized montan and/or polyolefin waxes, or mixtures thereof are most preferred. Montan waxes in the sense of the present invention preferably comprise esters of C₂₂-C₃₄-fatty acids and probably wax alcohols having 24 to 28 carbon atoms. Esters may be present in the montan wax in accordance with the present invention in an amount in a range of about 35% by weight to about 70% by weight. Further, free fatty acids as well as free wax alcohols as well as montan resins may be present. Useful montan waxes are offered for example by the company Clariant GmbH, 86005 Augsburg, Germany, especially montan waxes offered and sold under the trade name “Licowax”. Usable polefin waxes in the sense of the present invention are especially polypropylene and/or polyethylene waxes or mixtures thereof, also including modified polyolefin waxes, obtained especially by copolymerization of ethylene with useful comonomers like vinyl esters or acrylic acid. The wax has preferably a viscosity of at least about 50 mPa·s at 100° C., more preferred of at least about 100 mPa·s at 100° C., and most preferred of at least about 200 mPa·s at 100° C., measured in accordance with DIN 53 018. The wax used in the grease composition may be supplied as a powder or flakes, and is added to the grease composition with a long period of stirring, preferably at elevated temperatures, especially at temperatures about 80° C. to about 100° C.

Traditional friction modifiers used in the present invention such as fatty acid amides and fatty amine phosphates have been used in greases and other lubricants for many years (see, e.g., the modifiers disclosed in Klamann, Dieter—“Lubricants”, Verlag Chemie GmbH 1983, 1st edition, chapter 9.6). Their role is to give the lubricant stable but not necessarily low friction over a wide range of operating conditions.

In a preferred embodiment of the present invention, the grease composition claimed comprises about 50% by weight to about 98.9% by weight of the base oil composition, about 0.1% by weight to about 5% by weight of at least one tri-nuclear molybdenum compound, about 1% by weight to about 25% by weight of at least one urea derivative thickener. Most preferred, the grease composition according to the present invention comprises about 55% by weight to about 98.1% by weight, preferably to about 97.8% by weight, more preferred to about to 92.8% by weight, most preferred to about 92.5% by weight of the base oil composition, about 0.1% by weight to about 5% by weight, preferably about 0.3% by weight to about 2% by weight, of the tri-nuclear molybdenum compound, about 1% by weight to about 25% by weight of the urea derivative thickener, about 0.5% by weight to about 15% by weight of at least one calcium complex thickener, about 0.1% by weight to about 5% by weight of at least one ZnDTPs, about 0.1% by weight to about 5% by weight of at least one MoDTPs, and about 0.1% by weight to about 5% by weight of at least one MoDTCs. A urea derivative thickener may be present in a range between about 5 to about 20% by weight. Most preferred is a grease composition comprising about 70% by weight, preferably about 80% by weight, to about 92% by weight, preferably to about 92.4% by weight, more preferred to about 92.7% by weight, of the base oil composition, about 0.3% by weight to about 2% by weight, preferably to about 1.2% by weight of the least one tri-nuclear molybdenum compound, about 4.5% by weight to about 20% by weight, preferably to about 11% by weight, of the least one urea derivative thickener, about 1.5% by weight to about 3.5% by weight of at least one molybdenum dithiocarbamate, about 0.5% by weight to about 3% by weight of the least one zinc dithiophosphate, and about 0.5% by weight to about 2% by weight of at least one wax. Further, 0.3% by weight to about 2% by weight of an EP additive are preferably added.

Preferably the grease composition in accordance with the present invention is characterized in that the weight percent added, referred to the total amount of a grease composition, of tri-nuclear molybdenum compounds is essentially identical with the weight per cent of each one of zinc dithiophosphate, molybdenum dithiophosphate and/or molybdenum dithiocarbamate added. The compounds mentioned before may also be composed of different zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates, and, thus, they may present a mixture of different zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates. For clarification purposes it is noted that the essential identity of the weight percent of the tri-nuclear molybdenum compound or mixtures of such compounds added refers to each one of the compounds added, and not to mixtures of the different compounds mentioned. In a further referred embodiment of the present invention, the grease composition is characterized in that the weight percent added, referred to the total amount of the grease composition, of tri-nuclear molybdenum compound or mixtures of different tri-nuclear molybdenum compounds is 4 to 10 times lower than the weight percent of all of zinc dithiophosphate, molybdenum dithiophosphate and/or molybdenum dithiocarbamate added. If, for example, in the grease composition 2 weight % dithiophosphate(s) and 1 weight % molybdenum dithiocarbamate(s) are present as well as 0.5 weight % of a tri-nuclear molybdenum compound, thus, the weight percent of the tri-nuclear molybdenum compound added is 6 times lower than the weight percent of zinc dithiophosphate(s) and molybdenum dithiocarbamate(s) added. It has to be noted that also the zinc dithiophosphate, molybdenum dithiophosphate and/or molybdenum dithiocarbamate may be present as mixtures of said compounds having different structural formulas.

Further, the grease composition according to the present invention has a sliding friction coefficient of not more than 0.08, as measured with a SRV test.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates the friction coefficient as a function of grease sample;

FIG. 1B illustrates the welding load as a function of grease sample;

FIG. 2A illustrates the friction coefficient as a function of grease sample;

FIG. 2B illustrates the welding load as a function of grease sample;

FIG. 3A illustrates the friction coefficient as a function of grease sample;

FIG. 3B illustrates the welding load as a function of grease samples;

FIG. 4 illustrates the friction coefficient as a function of grease sample; and

FIG. 5 illustrates the wear as a function of grease sample.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to determine the effect of the lowering of the friction coefficient as well as the wear by the grease composition according to the invention, SRV tests are carried out using an Optimol Instruments SRV tester. Flat disc lower specimen made of the 100Cr6 standard bearing steel from Optimol Instruments Prüftechnik GmbH, Westendstrasse 125, Munich, properly cleaned using a solvent are prepared and contacted with the grease composition to be examined. The SRV test is an industry standard test and is especially relevant for the testing of greases for CVJ. The test consists of an upper ball specimen with a diameter of 10 mm made from 100Cr6 bearing steel reciprocating under load on the flat disc lower specimen indicated above. In tests for mimicking tripod joints a frequency of 7 Hz (for examples D1 and D2 only) and 40 Hz, respectively, with an applied load of 200 N were applied for 60 minutes (including running-in) or 3 hours (for examples D1 and D2 only) at 80° C., or 40° C. (examples D1 and D2). The stroke was 0.5 mm (for examples D2 and D2 only), 1.5 mm and 3.0 mm, respectively. The friction coefficients obtained were recorded on computer. For each grease, the reported value is an average of four data (two data for examples D1 and D2) at the end of tests in four runs or two runs, respectively (two runs at 1.5 mm stroke and two runs with 3.0 mm stroke with the exception of examples D1 and D2 with two runs with 0.5 mm stroke). Wear is measured using a profilometer and a digital planimeter. By using the profilometer, a profile of the cross section in the middle of the worn surfaces can be obtained. The area (S) of this cross section can be measured by using the digital planimeter. The wear quantity is assessed by V=Sl, where V is the volume of the wear and l is the stroke. The wear rate (W_r) is obtained from W_r=V/L [μm³/m], where L is the total sliding distance in the tests. For the running-in, it is started with an applied load of 50 N for 1 minute under the above-specified conditions. Afterwards, the applied load is increased for 30 seconds by 50 N up to 200 N.

Further, the welding load exerted on CVJs with a different grease composition is measured in accordance with a bear 4 ball EP test according to standard IP-239 (Energy Institute, London, UK).

The following substances are used in the examined grease compositions:

Base Oil Composition (Oil Blend)

The base oil compositions used have a kinematic viscosity of between about 32 and about 250 mm²/s at 40° C. and between about 5 and about 25 mm²/s at 100° C. Two base oil blends are used in this invention. The base oil blend A is a mixture of one or more naphthenic oils in a range between about 10 to about 60% by weight, one or more paraffinic oils in a range between about 30 to about 80% by weight and one or more poly-alpha-olefins (PAO) in a range between about 5 to about 40% by weight, referred to the total amount of the oil mixture. Oil blend A does not contain an organic synthetic ester, whereas oil blend B contains DOS in a range between about 2 to about 10% by weight referred to a total amount of the oil mixture.

The naphthenic oils are selected with a range of viscosity between about 20 to about 180 mm²/s at 40° C., paraffinic oils between about 25 to about 400 mm²/s at 40° C., and PAO between about 6 and about 40 mm²/s at 100° C.

Tri-Molecular Molybdenum Compound (TNMoS)

The tri-molecular molybdenum compound used in the grease compositions according to the present invention is a sulphur-containing tri-nuclear molybdenum compound obtainable under the trade name C9455B by Infineum International Ltd., UK. Its structure is defined in U.S. Pat. No. 6,172,013 B1.

Further Molybdenum Compounds

A molybdenum dithiophosphate (MoDTP) sold under the commercial name Sakuralube 300 (S-300) by Asahi Denka Co. Ltd., Japan, with the chemical formula 2-Ethylhexyl molybdenum dithiophosphate, diluted with mineral oil, is used. Further, a molybdenum dithiocarbamate (MoDTC) sold under the trade name Sakuralube 600 (S-600) in the solid state, produced by Asahi Denka Co. Limited, Japan, is used.

Zinc Compound Additive

As zinc compound additives, ZnDTP, sold by Infineum International Ltd., Oxfordshire, UK, under the trade name C9425, is used, being a zinc dialkyldithiophosphate with primary and/or secondary alkyl groups, especially having 3 to 8 C-atoms, preferably having 4 to 5 C-atoms, diluted with mineral oil.

Thickener

The urea thickener (“Thickener” in the examples) manufactured by the company Kyodo Yushi Co., Ltd., Tokyo, Japan, is used as defined in U.S. Pat. No. 5,589,444 (hereinafter referred to as Thickener), and is a reaction product of (4,4′-Methylenediphenyl diisocyanate) with octadecylamine.

Further, a calcium complex thickener (Calcium complex thickener) being a reaction product of calcium hydroxide with two carboxylic acids, one with a short carbon chain length of 2 to 5 carbon atoms and one with a long carbon chain length of 16 to 20 carbon atoms, in which the short to long chain ratio is between 1:2 and 1:5 is used. Examples having mixtures containing a urea thickener as well as a calcium complex thickener, thus, comprising a urea complex thickener in accordance with the definition in the present invention.

Wax

As wax compound, an oiliness montan wax sold by Clariant GmbH, Augsburg, Germany, under the trademark “Licowax OP” being an ester montan wax, partially saponified, with a drop point about 100° C. (DIN 51 801/1 or ASTM D 127) and a viscosity of about 300 mPa*s at 120° C. (DIN 53 018) is used (“Montan wax” in the examples).

Corrosion Inhibitor

As a corrosion inhibitor, disodium sebacate is used.

Anti-Oxidation Agent

As an anti-oxidation agent (Anti-Oxidant), a diphenylamine with butyl- and/or octyl groups is used, supplied by Ciba Speciality Chemicals, Switzerland, under the trade name “L57” (Irganox L57).

EP Additive

As an EP-agent, a sulphurized organic compound (Di-t-butyl Polysulfide) sold under the trade name C9002 by Infineum International Ltd., Oxfordshire, UK, with an inactive sulphur amount of about 45% (“EP additive” in the examples)) at room temperature (20° C. or 25° C.) and an active sulphur amount at 100° C. of about 5% by weight, and at 140° C. of about 15% by weight, the weight percent referred to the amount of the EP agent itself, is used.

First, the advantages of the grease composition according to the present invention were examined by measuring the friction coefficient and the welding load. Six different grease compositions were produced, as listed in Table 1:

TABLE 1 Grease Composition Example Example Example Example Example Example [wt %] A1 A2 A3 A4 A5 A6 TNMoS 0.5 0.5 0.5 0.5 0.5 0.5 ZnDTP 0.5 0.5 0.5 — 0.5 0.5 MoDTP 0.5 0.5 — 0.5 — 0.5 MoDTC 0.5 0.5 — 0.5 0.5 — Calcium complex 3.0 — 3.0 3.0 3.0 3.0 thickener oil blend 87 90 88 85 85 85 Thickener 8 8 8 8 8 8

The results from the SRV-measurements of the friction coefficient as well as the welding load measurements of examples A1 to A6 may be derived from FIG. 1. Example A2 does not contain any calcium complex thickener and/or calcium thickener, and, thus, does not comprise a urea complex thickener, whereas the other examples comprise an urea complex thickener. Further, the amounts of the additive package as well as the composition of the same are amended in examples A1 to A6. The friction coefficient of example A1 is below 0.06, and is the lowest friction coefficient measured in said test series. The friction coefficient of example A2 is above 0.08, and is the highest friction coefficient measured. Further, also the friction coefficients of examples A4 and A5 are slightly higher than the friction coefficients of examples A1, A3 and A6. One may derive from the friction coefficient measurements that the addition of an additive package containing at least one ZnDTP, at least one MoDTP, and at least MoDTC gives the lowest values for the friction coefficient. Further, the addition of at least one ZnDTP as well as at least one MoDTP, preferably in combination with each other (see example A6), is preferred.

From the measurements of the welding load in FIG. 1b) one may derive that the welding load of example A1 as well as example A5 is higher than the welding load measured for the other examples. Thus, the grease composition according to example A1 shows the best values not only for the friction coefficient, but also with respect to the welding load, and, thus, exhibits a good extreme pressure performance.

In a further series of tests, the amount of the TNMoS as well as the additive package composition is amended. Three grease compositions were prepared in accordance with Table 2.

TABLE 2 Grease Composition Example Example Example [wt %] B1 = A1 B2 B3 TNMoS 0.5 1.0 0.1 ZnDTP 0.5 1.0 0.1 MoDTP 0.5 1.0 0.1 MoDTC 0.5 1.0 0.1 Calcium complex 3.0 3.0 3.0 thickener oil blend 87 85 88.6 Thickener 8 8 8

In all of the examples B1 to B3, the amount of the thickener remains unamended, whereas the amount of the TNMoS compound as well as the components of the additive package were amended to 0.1% by weight, 0.5% by weight and 1.0% by weight, respectively, in each case referred to the total amount of the grease composition. The results from the SRV measurements with respect to the friction coefficient as well as the welding load may be derived from FIG. 2.

Example B1 (=A1) shows the lowest friction coefficient and highest welding load, and, thus, exhibits a very good extreme pressure performance when compared to examples B2 and B3. Further, the lowering of the amount of the TNMoS compound as well as the components of the additive package at values around 0.1% by weight clearly results in an increase of the friction coefficient and a decrease in the welding load. Thus, at least about 0.25% by weight of the TNMoS compound as well as at lest one of ZnDTPs, MoDTPs and MoDTCs should preferably be present in the grease composition.

In a third test series, the effect of the addition of a calcium complex thickener added to four grease compositions C1 to C4 in accordance with Table 3 is studied.

TABLE 3 Grease Composition Example Example Example Example [wt %] C1 = A1 C2 = A2 C3 C4 TNMoS 0.5 0.5 0.5 0.5 ZnDTP 0.5 0.5 0.5 0.5 MoDTP 0.5 0.5 0.5 0.5 MoDTC 0.5 0.5 0.5 0.5 Calcium complex 3.0 1.5 15 thickener oil blend 87 90 88.5 75 Thickener 8 8 8 8

Example C2 is identical to example A2. One may derive from the SRV measurements of the friction coefficient as well as the measurement of the welding load (see FIG. 3) that the addition of 3% by weight calcium complex thickener resulted in the lowest friction coefficient values and a welding load above 3000 N. The welding load is especially increased by adding 15% by weight calcium complex thickener in accordance with example C4, however, also the friction coefficient is increased to values about 0.08. This third test series indicates that the amount of calcium complex thickener used in the grease composition may be in a range of about 0.5% by weight to about 20% by weight, preferably to about 15% by weight, thus forming a urea complex thickener with the Thickener.

Further preferred grease compositions are grease compositions as listed in Table 4.

TABLE 4 Grease Composition Example Example [wt %] D1 D2 TNMoS 0.5 0.5 ZnDTP 1.0 2.0 MoDTC 2.5 2.5 Montan Wax 1.0 1.0 Corrosion inhibitor 0.2 0.2 Anti-oxidant 0.5 0.5 EP additive 0.5 Oil blend 88.3 86.8 Thickener 6 6

As may be derived from FIG. 4, the friction coefficient of example D2 is below 0.05, and even lower than the friction coefficient of example C1. Further, the wear of example D2 is not detectable. Thus, the adding of an EP additive as well as the increase in ZnDTP amount lead to a grease composition with highly preferred properties, when comprising D1 and D2.

In summary, the grease composition according to the present invention has an advantageous significant influence on the friction coefficient and wear, leading to a good extreme pressure performance as well as a good NVH performance in CVJs.

Claims

1. A grease composition for use in constant velocity joints comprising:

a) a base oil composition;

b) at least one tri-nuclear molybdenum compound of the formula Mo3SkLnQz,

wherein L are independently selected ligands having organo groups with a sufficient number of carbon atoms to render the compound soluble or dispersible in the oil, n is from 1 to 4, k varies from 4 though 7, Q is selected from the group of neutral electron donating compounds such as amines, alcohols, phosphines, and ethers, and z ranges from 0 to 5 and includes non-stoichiometric values;

c) at least one urea derivative thickener.

2. A grease composition according to claim 1, characterised in that the urea derivative thickener is selected from the group comprising di-urea and/or polyurea compounds and mixtures of said compounds with calcium-based thickeners.

3. A grease composition according to claim 1, further comprising at least one zinc dithiophosphates, molybdenum dithiocarbamate and/or molybdenum dithiophosphates.

4. A grease composition according to claim 3, characterised in that the amount of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is in a range of between 0.1% by weight to 5% by weight, referred to the total amount of the grease composition.

5. A grease composition according to claim 4, characterised in that the weight percent added, referred to the total amount of the grease composition, of each of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates is essentially identical.

6. A grease composition according to claim 1, further comprising an agent having at least one anti-oxidation agent, corrosion inhibitor, anti-wear-agent wax, friction modifier and/or extreme pressure agent (EP agent).

7. A grease composition according to claim 1, comprising 50% by weight to 98.9% by weight of the base oil composition, 0.1% by weight to 5% by weight of at least one tri-nuclear molybdenum compound, 1% by weight to 25% by weight of at least one urea derivative thickener, in each case referred to the total amount of the grease composition.

8. A grease composition according to claim 1, comprising 70% by weight to 92% by weight of the base oil composition, 0.3% by weight to 2% by weight of the least one tri-nuclear molybdenum compound, 4.5% by weight to 20% by weight of the least one urea derivative thickener, 1.5% by weight to 3.5% by weight of at least one molybdenum dithiocarbamate, 0.5% by weight to 3% by weight of the least one zinc dithiophosphate, and 0.5% by weight to 2% by weight of at least one wax.

9. A grease composition according to claim 3, characterised in that the weight percent added, referred to the total amount of the grease composition, of tri-nuclear molybdenum compounds is essentially identical with the weight percent of each one of zinc dithiophosphates, molybdenum dithiophosphates and/or molybdenum dithiocarbamates added.

10. A grease composition according to claim 3, characterised in that the weight percent added, referred to the total amount of the grease composition, of tri-nuclear molybdenum compounds is four to ten times lower than the weight percent of the total amount of zinc dithiophosphates, molybdenum dithiophosphate and/or molybdenum dithiocarbamate added.

11. A grease composition according to claim 1, characterised in that the sliding friction coefficient is at most 0.08.