Grease composition

Abstract

To provide a grease composition for wheels having low torque, improving surface-initiated peeling caused by insufficient oil film formation, and further suppressing fretting wear under minute vibration at low temperature.SOLUTION: A grease composition for automotive wheel bearings comprises (a) a base oil, (b) a thickener, and (c) an additive, wherein (a) the base oil has a kinematic viscosity of 3 to 9 mm2 / s at 100°C, and (c) the additive contains (c1) a metal dialkyldithiophosphate, (c2) a metal organosulfonate, and (c3) a nitrogen- and sulfur-containing heterocyclic compound, and (c3) the nitrogen- and sulfur-containing heterocyclic compound contains at least one selected from the group consisting of 2(3H)-benzothiazolethione and 2,5-bis(alkyldithio)-1,3,4-thiadiazole.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a grease composition for automobile wheel bearings which has low torque properties, excellent resistance to flaking caused by surface-initiated flaking and excellent resistance to fretting at low temperatures, and to an automobile wheel bearing filled with the grease composition. [Background technology]

[0002] In recent years, from the perspective of reducing power consumption, there has been a demand for high efficiency in electrical equipment and machine parts used in various industries, including automobiles, and various studies are being conducted on weight reduction of parts, improvement of structure, etc. One method of increasing the efficiency of automobiles is to reduce the torque of wheel bearings. Furthermore, automobiles are often transported by rail or truck, and during this transportation, micro-vibrations caused by rail joints and rough roads may occur, causing fretting wear in lubricated parts coated with grease. In particular, in low-temperature environments, the base oil of the grease becomes less likely to flow, resulting in insufficient inflow of the grease into the lubricated parts, causing fretting wear. As a countermeasure against this fretting wear, efforts are being made to use grease with excellent fretting resistance. Conventional methods for reducing the torque of grease include using a base oil with a low kinetic viscosity (Non-Patent Document 1) and softening the grease to reduce the stirring resistance of the grease.

[0003] Lowering the kinetic viscosity of the base oil reduces the agitation resistance of the grease, which can reduce torque, but it can also cause surface-origin peeling due to insufficient oil film formation and reduced lubrication life due to evaporation, preventing the bearing from completing its lifespan. In addition, softening the grease makes it more likely to leak to the outside, which reduces the lubrication life and prevents the bearing from completing its lifespan. Furthermore, as a conventional countermeasure against surface-initiated flaking, in addition to the method of using a base oil with a high kinetic viscosity to form a sufficient oil film thickness, Patent Document 1, for example, proposes that a grease composition using a diurea-based thickener of the general formula shown below can effectively prevent direct contact between both surfaces to be lubricated and improve flaking resistance. R2-NHCONH-R1-NHCONH-R3 (In the formula, R1 represents a divalent hydrocarbon group having 6 to 15 carbon atoms, R2 and R3 represent a phenyl group and / or a cyclohexyl group, and the cyclohexyl group / (cyclohexyl group+phenyl group) ratio of R2 and R3 is 0.85 to 0.50 (molar ratio).) However, a base oil with a high kinetic viscosity cannot provide a satisfactory low torque property, and even the thickener described in Patent Document 1 cannot provide a satisfactory low torque property.

[0004] Also, measures against surface-initiated peeling have been taken using additives, and for example Patent Document 2 proposes the use of oxides and carbonates of typical metals such as zinc oxide, sodium nitrite, sodium molybdate, sodium benzoate, sodium sebacate, and calcium carbonate. However, all of the above additives are solid additives, and the amount of solid components in the grease increases, leading to increased stirring resistance, making it impossible to achieve satisfactory low torque properties. Fretting is surface damage that occurs under small amplitude, and in the atmosphere, it generates oxidized wear powder, which is said to often cause severe wear due to its abrasive action (Non-Patent Document 2). As measures to prevent fretting, (1) reducing the relative amount of slippage, (2) separating both sides and preventing direct contact, and (3) preventing adhesion between surfaces by covering the contact surfaces with a phosphate coating or supplying lubricating oil and grease have been proposed (Non-Patent Document 2). As a grease composition with excellent fretting resistance, a grease composition containing a urea-based thickener, a base oil, a phosphorothioate-based compound, and an amine-based compound has been disclosed (Patent Document 3). However, fretting wear suppression under conditions that are more severe than conventional conditions (low temperature: -30°C, small amplitude) is not sufficient, and further improvement in fretting wear is required compared to the above conventional techniques. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 4809793 [Patent Document 2] Patent No. 5738712 [Patent Document 3] Patent No. 5350597 [Non-patent literature]

[0006] [Non-Patent Document 1] Hiroaki Ishikawa: Technological Trends and Tribology of Hub Unit Bearings, Tribologist, Vol. 54, No. 9, 2009, pp. 580-585 [Non-Patent Document 2] Yuji Yamamoto et al.: Tribology, Rikogakusha, February 28, 1998, pp. 201-203 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a grease composition for wheels which has low torque properties, improves surface-initiated flaking caused by insufficient oil film formation, and can suppress fretting wear under micro-vibrations at low temperatures. [Means for solving the problem]

[0008] The present invention provides a grease composition as follows: 1. A grease composition for automobile wheel bearings, comprising: (a) a base oil; (b) a thickener; and (c) an additive, (a) The kinetic viscosity of the base oil at 100°C is 3 to 9 mm 2 / s, (c) the additive comprises (c1) a dialkyldithiophosphate metal salt, (c2) an organic sulfonate metal salt, and (c3) a nitrogen- and sulfur-containing heterocyclic compound, and the nitrogen- and sulfur-containing heterocyclic compound comprises at least one selected from the group consisting of 2(3H)-benzothiazolethione and 2,5-bis(alkyldithio)-1,3,4-thiadiazole; The grease composition for use in wheel bearings of automobiles. 2. (c1) The grease composition according to 1 above, wherein the dialkyldithiophosphate metal salt is a zinc salt or a molybdenum salt. 3. (c2) A grease composition according to 1 or 2 above, wherein the metal salt of an organic sulfonic acid is a calcium salt or a zinc salt. 4. The grease composition according to any one of 1 to 3 above, wherein (a) the base oil is a synthetic oil or a mixed oil of a synthetic oil and a mineral oil. 5. The grease composition according to any one of 1 to 4 above, wherein (b) the thickener is a diurea compound represented by the following formula (1): R2-NHCONH-R1-NHCONH-R3 (1) (In the formula, R1 is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R2 and R3 may be the same or different and are an alkyl group having 6 to 30 carbon atoms, an aryl group having 6 or 7 carbon atoms, or a cyclohexyl group.) 6. (b) The grease composition according to 5 above, wherein the thickener is a diurea compound in which the ratio of the number of moles of alkyl groups to the total number of moles of alkyl groups and cyclohexyl groups [{(number of alkyl groups) / (number of cyclohexyl groups+number of alkyl groups)}×100] in formula (1) is 10 to 100 mol %. 7. A bearing for an automobile wheel, in which the grease composition as defined in any one of 1 to 6 above is packed. Effect of the Invention

[0009] The grease composition of the present invention has low torque properties, and is excellent in peeling life against surface-initiated peeling and in fretting resistance at low temperatures. [Brief description of the drawings]

[0010] [Figure 1] Schematic diagram of the four-ball rolling test DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] (a) Base oil The base oil used in the present invention has a kinematic viscosity at 100°C of 3 to 9 mm 2 This allows the desired low torque to be achieved. The preferred range is 3.5 to 7.5 mm / s. 2 / s. The base oil that can be used in the present invention may be a mineral oil or a synthetic oil, but is preferably a synthetic oil or a mixture of a synthetic oil and a mineral oil. As the synthetic oil, synthetic hydrocarbon oils such as poly-α-olefin or polybutene, ester-based synthetic oils such as diester or polyol ester, and ether-based synthetic oils such as alkyl diphenyl ether or polypropylene glycol can be used. From the viewpoint of low traction, it is preferable to include poly-α-olefin. The synthetic oil may be a so-called biomass oil produced from biological resources derived from animals and plants. For example, biomass ester oils synthesized from various fatty acids and alcohols using vegetable oils as raw materials, and biomass hydrocarbon oils using vegetable oils such as palm oil, corn oil, and soybean oil can also be used. The base oil may be used alone or in a mixture of two or more types. When the base oil of the present invention is a mixed oil of synthetic oil and mineral oil, the mixing ratio is not particularly limited. The mixing ratio can be selected as necessary. When the base oil of the present invention is a mixed oil, the ratio of the mineral oil is preferably 10 to 60 mass% based on the total mass of the base oil from the viewpoint of low temperature fluidity. The synthetic oils and mineral oils may each be used in combination of two or more kinds. For example, a high- or medium-viscosity synthetic oil and a low-viscosity synthetic oil may be used in combination, a high- or medium-viscosity mineral oil and a low-viscosity synthetic oil may be used in combination, a high- or medium-viscosity synthetic oil and a low-viscosity mineral oil may be used in combination, or a low-viscosity synthetic oil and a low-viscosity mineral oil may be used in combination. As the base oil of the present invention, it is particularly preferable to use poly-α-olefin alone from the viewpoint of low traction. 2 It is most preferred to use polyalphaolefins of the formula (I) / s alone. The content of the base oil is preferably 60 to 90 mass %, more preferably 70 to 88 mass %, based on the total mass of the composition. By including the base oil in such a ratio, a sufficient amount of oil for lubrication can be secured, resulting in good durability.

[0012] (b) Thickener The thickener that can be used in the present invention is not particularly limited, but is preferably a diurea compound represented by the following formula (1). R2-NHCONH-R1-NHCONH-R3 (1) (In the formula, R1 is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R2 and R3 may be the same or different and are an alkyl group having 6 to 30 carbon atoms, an aryl group having 6 or 7 carbon atoms, or a cyclohexyl group.) R1 in formula (1) is preferably a diphenylmethane group. R2 and R3 may be the same or different and are an alkyl group having 6 to 30 carbon atoms, an aryl group having 6 or 7 carbon atoms, or a cyclohexyl group. R2 and R3 may be the same or different and are preferably an alkyl group having 6 to 30 carbon atoms, or a cyclohexyl group.

[0013] When either R2 or R3 is a cyclohexyl group, the other is preferably an alkyl group having 6 to 30 carbon atoms, more preferably an alkyl group having 8 or 18 carbon atoms. Such diurea compounds are mixtures of so-called aliphatic diurea compounds (diurea compounds in which R2 and R3 in formula (1) are both alkyl groups having 6 to 30 carbon atoms), alicyclic diurea compounds (diurea compounds in formula (1) where R2 and R3 are both cyclohexyl groups), and alicyclic aliphatic diurea compounds (diurea compounds in formula (1) where one of R2 and R3 in formula (1) is an alkyl group having 6 to 30 carbon atoms, and the other is a cyclohexyl group)) formed during the reaction of diisocyanate with monoamine. The ratio of the number of moles of alkyl groups to the total number of moles of alkyl groups and cyclohexyl groups [{(number of alkyl groups) / (number of cyclohexyl groups+number of alkyl groups)}×100] is preferably 10 to 100 mol %, more preferably 10 to 80 mol %.

[0014] As the thickener of the present invention, alicyclic aliphatic diurea compounds and aliphatic diurea compounds are preferred. Among them, alicyclic aliphatic diurea compounds are preferred. In particular, diurea compounds in which, in formula (1), one of R2 and R3 is an alkyl group having 18 carbon atoms, the other is a cyclohexyl group, and the ratio of the molar number of the alkyl group to the total molar number of the alkyl group and the cyclohexyl group is 10 to 30 mol % are preferred. The content of the thickener in the grease composition of the present invention varies depending on the type of thickener. The consistency of the grease composition of the present invention is preferably 200 to 350, more preferably 235 to 325. The content of the thickener is the amount required to obtain this consistency. The content of the thickener in the grease composition of the present invention is usually 5 to 25 mass%, preferably 8 to 20 mass%.

[0015] (c) Additives (c1) Dialkyldithiophosphate metal salt The dialkyldithiophosphate metal salt used in the present invention includes those represented by the following formula (2). [(RO)(RO)P(=S)-S]2-M1 x O m S n ...Equation (2) (In the formula, R4 and R5 are an alkyl group having 1 to 24 carbon atoms or an aryl group having 6 to 30 carbon atoms, and the alkyl group may be linear or branched and may be primary, secondary or tertiary. M1 represents a metal element such as Zn, Mo, Cu, Fe, Ni, or Te, x is 1 or 2, and n and m are 0 to 4.) The metal dialkyldithiophosphate salt used in the present invention is a zinc salt or a molybdenum salt, preferably a zinc salt. The content of the dialkyldithiophosphate metal salt is usually 0.1 to 5.0 mass %, and preferably 0.5 to 3.0 mass %, based on the total mass of the grease composition.

[0016] (c2) Metal salts of organic sulfonic acids The organic sulfonic acid metal salt used in the present invention is preferably a compound represented by the following formula (3). [R6-SO3]M2...Formula (3) In the formula, R6 represents an alkyl group, an alkenyl group, an alkylnaphthyl group, a dialkylnaphthyl group, an alkylphenyl group, or a residue of a high-boiling petroleum fraction. The alkyl or alkenyl is linear or branched and has a carbon number of 2 to 22. M2 represents an alkaline earth metal or zinc, preferably calcium or zinc. The organic sulfonic acid metal salt used in the present invention includes calcium salt or zinc salt of alkylbenzenesulfonic acid, calcium salt or zinc salt of dinonylnaphthalenesulfonic acid, or highly basic salts thereof, etc. Among these, calcium salt of dinonylnaphthalenesulfonic acid and calcium salt of alkylbenzenesulfonic acid are preferred. As the organic sulfonate metal salt of the present invention, calcium sulfonate or zinc sulfonate is preferred from the viewpoint of abrasion resistance and rust prevention properties because a strong coating film is formed when combined with zinc dialkyldithiophosphate. In particular, a calcium salt or zinc salt of dinonylnaphthalenesulfonic acid having a base number of 5 or less is preferred, and calcium dinonylnaphthalenesulfonate is more preferred. From the viewpoint of improving heat resistance, the organic sulfonate metal salt of the present invention is preferably a highly basic calcium sulfonate having a base number of 50 or more, and more preferably a calcium salt of an alkylbenzenesulfonic acid having a base number of 300 or more. In this specification, the base number is a value measured in accordance with JIS K 2501. The content of the organic sulfonic acid metal salt is usually 0.1 to 5.0 mass %, and preferably 0.5 to 4.0 mass %, based on the total mass of the grease composition.

[0017] (c3) Nitrogen- and sulfur-containing heterocyclic compounds Nitrogen- and sulfur-containing heterocyclic compounds are heterocyclic compounds with a five-membered ring structure that contains one or two nitrogen atoms and one sulfur atom and has one or two double bonds in the backbone, such as thiazole and thiadiazole. Specific examples of thiazoles include 4,5-dimethylthiazole, 2-isopropyl-4-methylthiazole, 1,2-benzisothiazole, 2-methyl-β-naphthothiazole, 2-methoxythiazole, thiazole-2-carbaldehyde, 2-acetylthiazole, benzothiazole-2-acetonitrile, 4-thiazoleamine, 2-amino-4-methylthiazole, benzothiazole-6-amine, 2-aminobenzothiazole, 2-amino-6-methylbenzothiazole, 4-(2-methyl-4-thiazolyl)aniline, 2-amino-5-phenylthiazole, 2-amino-6-methoxybenzothiazole, 5-methoxybenzothiazole-2 -amine, 2-aminobenzothiazol-6-ol, 2-amino-α-(methoxyimino)-4-thiazole ethyl acetate, 3-isothiazole carboxylic acid, 4-thiazole carboxylic acid, 3-methyl-4-isothiazole carboxylic acid, 4-methyl-5-thiazole carboxylic acid, 2-methylthiazole-5-carboxylic acid, benzothiazole-2-carboxylic acid, 2-benzothiazole acetic acid, 4-methyl-5-thiazole ethanol, 2(3H)-benzothiazole thione, 4-methyl-2-mercaptobenzothiazole, bis(benzothiazol-2-yl) persulfide, 2-(methylthio)benzothiazole, 2-methylbenzothiazole, and the like. The thiazole compound used in the present invention is 2(3H)-benzothiazolethione.

[0018] The thiadiazole is specifically 2,5-dimercapto-1,3,4-thiadiazole or a derivative thereof, specifically 2,5-dithioacetic acid-1,3,4-thiadiazole, 2-amino-1,3,4-thiadiazole, 2-thioacetic acid-5-mercapto-1,3,4-thiadiazole, 5-amino-1,3,4-thiadiazole-2-thiol, 2,2'-dithiobis(1,3,4-thiadiazole-5-thiol), 2,5-bis(n-dodecyldithio)thiadiazole, 2,5-bis(tert-decyldithio)-1,3,4-thiadiazole, 2,5-bis(n-octyldithio)thiadiazole, 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole, 2,5-bis-(tert-nonyldithio)-1,3,4-thiadiazole, and 2,5-bis(diethyldithiocarbamic acid)thiadiazole can be mentioned.

[0019] The thiadiazole compound used in the present invention is 2,5-bis(alkyldithio)-1,3,4-thiadiazole, and the alkyl group may be linear or branched with 6 to 18 carbon atoms, and may be primary, secondary, or tertiary. Examples of the thiadiazole compound include 2,5-bis(n-dodecyldithio)thiadiazole, 2,5-bis(tert-decyldithio)-1,3,4-thiadiazole, 2,5-bis(n-octyldithio)thiadiazole, 2,5-bis(tert-octyldithio)-1,3,4-thiadiazole, and 2,5-bis-(tert-nonyldithio)-1,3,4-thiadiazole. Of these, 2,5-bis-(tert-nonyldithio)-1,3,4-thiadiazole is preferred.

[0020] The component (c3) of the present invention contains at least one selected from the group consisting of 2(3H)-benzothiazolethione and 2,5-bis(alkyldithio)-1,3,4-thiadiazole, and preferably contains 2(3H)-benzothiazolethione. The composition of the present invention may contain other nitrogen- and sulfur-containing heterocyclic compounds, but preferably does not contain other nitrogen- and sulfur-containing heterocyclic compounds. The content of the nitrogen- and sulfur-containing heterocyclic compound is usually 0.1 to 3.0 mass% and preferably 0.1 to 2.0 mass% based on the total mass of the grease composition. When the nitrogen- and sulfur-containing heterocyclic compound contains another nitrogen- and sulfur-containing heterocyclic compound, the above content refers to the content of 2(3H)-benzothiazolethione and / or 2,5-bis(alkyldithio)-1,3,4-thiadiazole, and the ratio of the other nitrogen- and sulfur-containing heterocyclic compound to 2(3H)-benzothiazolethione and / or 2,5-bis(alkyldithio)-1,3,4-thiadiazole is preferably 0.1 to 1.8, more preferably 0.1 to 1.0.

[0021] The component (c) of the present invention preferably contains as (c1) a zinc dialkyldithiophosphate or a molybdenum dialkyldithiophosphate, as (c2) a neutral Ca sulfonate, a highly basic Ca sulfonate or a Zn sulfonate, and as (c3) 2(3H)-benzothiazole thione. It is more preferable that the component (c) of the present invention contains zinc dialkyldithiophosphate as (c1), neutral Ca sulfonate (e.g., base number 0.1 to 3.0) or highly basic Ca sulfonate (e.g., base number 300 to 500) as (c2), and 2(3H)-benzothiazolethione as (c3).

[0022] Other additives The grease composition of the present invention can contain any additives that are typically used in greases, as necessary, such as rust inhibitors other than organometallic sulfonic acids, antioxidants, metal corrosion inhibitors, oiliness agents, and anti-wear agents other than dialkyldithiophosphate metal salts. Rust inhibitor Carboxylic acids and their derivatives: Alkenylsuccinic anhydrides, alkenylsuccinic acid esters, alkenylsuccinic acid half esters Carboxylate: Metal salts (Ca, Ba, Mg, Al, Zn, Na, etc.) or amine salts of fatty acids, dibasic acids, naphthenic acids, lanolin fatty acids, alkenyl succinic acids, etc. Antioxidants Amine antioxidants: Phenyl alpha naphthylamine, alkylated phenyl alpha naphthylamine, alkylated diphenylamine, etc. Phenolic antioxidants: Hindered phenols such as 2,6-di-tert-butyl-p-cresol, pentaerythrityl tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate], and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. Metal corrosion inhibitors: benzotriazole or its derivatives, zinc oxide, etc. Oiliness agents: sorbitan trioleate, sorbitan monooleate, etc. Anti-wear agent Phosphorus compounds: amine phosphate, tricresyl phosphate, etc. Sulfur compounds: dibenzyl disulfide, various polysulfides, etc. Sulfur-phosphorus compounds: Triphenylphosphorothionate, etc. Of these, it is preferred to include a rust inhibitor (particularly a mixture of a fatty acid amine salt and zinc naphthenate), an antioxidant (particularly an amine antioxidant, more particularly an alkylated diphenylamine), and / or an anti-wear agent (particularly an amine phosphate). The content of these other additives is usually 1.0 to 12 mass %, preferably 1.0 to 6.0 mass %, based on the total mass of the composition.

[0023] The grease composition of the present invention is used for wheel bearings of automobiles, specifically for wheel bearings of ordinary automobiles, medium-sized automobiles and large automobiles. EXAMPLES

[0024] <Preparation of test grease> Grease compositions of Examples and Comparative Examples were prepared using the thickeners and base oils shown below. Specifically, diphenylmethane diisocyanate (1 mol) was reacted with a specific amine (2 mol) in the base oil, the temperature was raised, the mixture was cooled, and then a three-roll mill was used to obtain the grease compositions of Examples 1 to 11 and Comparative Examples 1 to 7 so that the worked penetration (test method JIS K2220 7.) was 300. The kinetic viscosity of the base oil at 100°C was measured in accordance with JIS K2220 23., and the base number was measured in accordance with JIS K 2501. Unless otherwise specified, the units of values ​​in Tables 1 and 2 indicate mass % relative to the total mass of the grease composition.

[0025] (a) Base oil Polyalphaolefin A: 100℃ kinematic viscosity = 39.2~40.7mm 2 / s Polyalphaolefin B: 100℃ kinematic viscosity = 5.7~6.3mm 2 / s Polyalphaolefin C: 100℃ kinematic viscosity = 3.8~4.0mm 2 / s ·Mineral oil A 100℃ kinematic viscosity: 6.0~6.2mm 2 / s Ester oil A: 100℃ kinematic viscosity = 5.4~5.8mm 2 / s Ether oil A: 100℃ kinematic viscosity = 9.1~9.3mm 2 / s Ether oil B: 100℃ kinematic viscosity = 3.3~3.5mm 2 / s

[0026] (b) Thickener Thickener A: Alicyclic aliphatic diurea (the molar ratio of cyclohexylamine to stearylamine is 7:1) Thickener B: Alicyclic aliphatic diurea (the molar ratio of cyclohexylamine to stearylamine is cyclohexylamine:stearylamine=3:7) · Thickener C: Aliphatic diurea (octylamine) · Thickener D: Aliphatic diurea (stearylamine)

[0027] (c) Additives Additive A: (c2) Calcium salt of dinonylnaphthalenesulfonic acid (base number: 0.26) Additive B: (c2) Highly basic calcium salt of alkylbenzene sulfonic acid (base number: 405) Additive C: (c2) Zinc salt of dinonylnaphthalenesulfonic acid (base number: 0.50) Additive D: (c1) Zinc dialkyldithiophosphate (c) Additive E: (c1) Molybdenum dialkyldithiophosphate Additive F: (c3) 2(3H)-benzothiazolethiol Additive G: (c3) 2,5-bis(tert-nonyldithio)-1,3,4-thiadiazole Rust inhibitor: A mixture of fatty acid amine salt and zinc naphthenate Amine antioxidants: alkylated diphenylamines Anti-wear agent: tertiary alkylamine-dimethyl phosphate

[0028] <Test method and judgment> (1) Resistance to surface-originating flaking: 4-ball rolling test Test Overview Three φ15mm steel bearing balls are prepared and placed in a cylindrical container with an inner diameter of 40mm and a height of 14mm, and the container is filled with approximately 20g of test grease. As shown in Figure 1, a rotating ball, a φ5 / 8inch steel bearing ball, is placed on top of the three steel balls and set into the testing machine. The test is started by applying a load in the W direction in Figure 1 and rotating the ball. The bottom three balls revolve while rotating on their own axes. This is continued to rotate until flaking occurs. *Delamination occurs between the balls, where the surface pressure is the highest. *The service life is the total number of contacts of the upper ball at the time when flaking occurs. This is repeated 3 to 5 times to determine the L50 service life (the number of times at which 50% of the service life is reached). The results are shown in Tables 1 and 2.

[0029] Test conditions Test steel balls: 15mm grade 40 and 5 / 8inch grade 200 bearing steel balls Test load (W): 250kgf (5.6GPa) Rotational speed (n): 1500 rpm Test repetition rate: 3 to 5 times

[0030] judgement Total contact count: 300 x 10 5 More than 1000 rotations: 〇 (pass) 300×10 5 Less than rotation: × (fail)

[0031] (2) Fretting resistance at low temperatures: Fafnir test The Fafnir test was performed according to ASTM D 4170. The test grease was applied to two sets of test thrust bearings described below, and the specified oscillating operation was carried out to determine the amount of wear (weight loss due to fretting wear). The results are shown in Tables 1 and 2.

[0032] Bearing: 51204 Thrust bearing Load: 4000N (surface pressure: 1.9GPa) Oscillation angle: ±3° Oscillation cycle: 12Hz Time: 2h Temperature: -30℃ Grease quantity: 1.0g per bearing Amount of wear: Loss in race mass per bearing set (total loss in test bearing race mass / 2)

[0033] judgement Abrasion amount: Less than 2.0 mg: ○ (pass) 2.0mg or more...×(Fail)

[0034] (3) Low torque: Rheometer test (shear stress) The test was performed using a viscoelasticity measuring device, Physica MCR 301, manufactured by Anton Paar Japan Co., Ltd. Test grease was applied between the specified plates, and after setting the specified plate distance, excess grease was removed. The shear rate was 0.1 S -1 From 2500S -1 Increase speed logarithmically up to 2500S -1 The shear stress at was calculated and used as the measurement result. The results are shown in Tables 1 and 2.

[0035] Shear rate: 0.1~2500S -1 Test temperature: 25℃ Plate: Cone type Cone angle: 2° Distance between plates: 0.05mm Plate diameter: 25mm

[0036] judgement Shear stress: 2000 Pa or less: ○ (pass) Over 2000Pa...×(Fail)

[0037] <Overall rating> Passed all the tests: 〇(Pass) If any one of the tests is failed, it is marked as × (failed).

[0038] [Table 1]

[0039] [Table 2]

[0040] The kinematic viscosity of the base oil at 100°C is 3 to 9 mm 2 The grease compositions of Examples 1 to 13, which contained a thickener and predetermined additives, passed all of the four-ball rolling test, the Fafnir test, and the rheometer test. On the other hand, the kinetic viscosity at 100°C is 14.8mm 2 The grease compositions of Comparative Example 1, which had a high viscosity index of 0.1 to 0.25 / s, and Comparative Examples 2 to 5, which did not contain any of the additives, failed at least one of the rolling four-ball test, the Fafnir test, or the rheometer test.

Claims

1. A grease composition for automobile wheel bearings, comprising: (a) a base oil; (b) a thickener; and (c) an additive, (a) The kinematic viscosity of the base oil at 100°C is 3 to 9 mm 2 / s, (c) the additive comprises (c1) a dialkyldithiophosphate metal salt, (c2) an organic sulfonate metal salt, and (c3) a nitrogen- and sulfur-containing heterocyclic compound, and (c3) the nitrogen- and sulfur-containing heterocyclic compound comprises at least one selected from the group consisting of 2(3H)-benzothiazolethione and 2,5-bis(alkyldithio)-1,3,4-thiadiazole; The grease composition for use in wheel bearings of automobiles.

2. 2. The grease composition according to claim 1, wherein the dialkyldithiophosphate metal salt (c1) is a zinc salt or a molybdenum salt.

3. The grease composition according to claim 1, wherein the metal salt of an organic sulfonic acid (c2) is a calcium salt or a zinc salt.

4. 2. The grease composition according to claim 1, wherein the base oil (a) is a synthetic oil or a mixture of a synthetic oil and a mineral oil.

5. 2. The grease composition according to claim 1, wherein the thickener (b) is a diurea compound represented by the following formula (1): R2-NHCONH-R1-NHCONH-R3 (1) (In the formula, R1 is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R2 and R3 may be the same or different and are an alkyl group having 6 to 30 carbon atoms, an aryl group having 6 or 7 carbon atoms, or a cyclohexyl group.)

6. The grease composition according to claim 5, wherein the thickener (b) is a diurea compound in which the ratio of the number of moles of alkyl groups to the total number of moles of alkyl groups and cyclohexyl groups [{(number of alkyl groups) / (number of cyclohexyl groups+number of alkyl groups)}×100] in formula (1) is 10 to 100 mol %.

7. A bearing for an automobile wheel, which contains the grease composition according to any one of claims 1 to 6.

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