Grease composition for vehicular hub unit bearing

Abstract

Provided is a grease composition for a vehicular hub unit bearing, the grease composition having a mixture consistency of 220-270 and containing, with respect to the total mass of the composition, 0.1-5 mass% of a carboxylic acid-based rust-preventing agent, 0.1-5 mass% of a carboxylate-based rust-preventing agent, and 0.1-3 mass% of a fatty acid amine salt, wherein the base oil is a synthetic hydrocarbon oil having a kinematic viscosity at 40 °C of 25-50 mm2 / s, and the thickener is an alicyclic aliphatic diurea compound.

Description

Grease composition for vehicle hub unit bearings

[0001] The present invention relates to a grease composition for a vehicle hub unit bearing.

[0002] One method for improving the efficiency of automobiles is to reduce the torque of hub unit bearings used in automobile wheels. Similarly, it is important for the grease used to have low torque over a wide temperature range, from low to high. Methods for reducing grease torque include lowering the kinematic viscosity of the base oil as much as possible (see Non-Patent Document 1) or softening the grease to reduce the grease's stirring resistance. Lowering the kinematic viscosity of the base oil reduces the grease's stirring resistance, thereby reducing torque. However, this can lead to surface-initiated peeling due to insufficient oil film formation and reduced lubrication life due to evaporation, preventing the bearing from fulfilling its lifespan. Furthermore, softening the grease in particular can lead to increased grease leakage, resulting in reduced lubrication life and peeling due to insufficient oil film formation, resulting in an unsatisfactory peeling life.

[0003] Hiroaki Ishikawa: Technological Trends and Tribology of Hub Unit Bearings, Tribologist, Vol. 54, No. 9, 2009, pp. 580-585

[0004] Therefore, an object of the present invention is to provide a grease composition for vehicle hub unit bearings that has low torque and excellent durability.

[0005] The present invention provides the following grease composition: 1. A grease composition for a vehicle hub unit bearing, comprising a base oil, a thickener, and an additive, wherein the base oil has a kinematic viscosity at 40°C of 25 to 50 mm 2 / s, the thickener is an alicyclic aliphatic diurea compound, and the additives contain, based on the total mass of the composition, 0.1 to 5 mass% of a carboxylic acid-based rust inhibitor, 0.1 to 5 mass% of a carboxylate-based rust inhibitor, and 0.1 to 3 mass% of an amine salt of a fatty acid, and the grease composition has a worked penetration of 220 to 270. 2. The grease composition according to item 1 above, wherein the thickener is a mixture of a compound of the following formula (1), a compound of the following formula (2), and a compound of the following formula (3): R1-NHCONH-R2-NHCONH-R3 (1) R1-NHCONH-R2-NHCONH-R1 (2) R3-NHCONH-R2-NHCONH-R3 (3) (wherein R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, R1 represents a cyclohexyl group, and R3 represents a linear or branched alkyl group having 8 to 22 carbon atoms. The ratio of the number of moles of linear or branched alkyl groups having 8 to 22 carbon atoms to the total number of moles of cyclohexyl groups and linear or branched alkyl groups having 8 to 22 carbon atoms (R3 / (R1+R3))×100 is 10 to 20%).) 3. The grease composition according to 1 or 2 above, wherein the thickener is contained in an amount of 10 to 20 mass% based on the total mass of the composition. 4. 4. The grease composition according to any one of 1 to 3 above, further comprising triphenyl phosphorothioate as an additive. 5. The grease composition according to any one of 1 to 4 above, further comprising a phenolic antioxidant as an additive. 6. A vehicle hub unit bearing comprising a hub that constitutes an inner ring equivalent member together with an inner ring element, an outer ring, and a plurality of rolling elements disposed so as to be rollable between the hub and the outer ring, wherein the water-resistant grease composition according to any one of 1 to 5 above is sealed in a gap formed between the hub and the outer ring and in which the rolling elements are disposed.

[0006] According to the present invention, it is possible to provide a grease composition that exhibits low torque, particularly low bearing torque and low starting torque at low temperatures, and excellent durability, particularly durability at high temperatures. Conventional greases have a tendency for torque to gradually decrease from the torque at the initial stage of bearing rotation (starting torque) as operating time passes. However, according to the present invention, torque is low from the start of bearing rotation and can be maintained at a stable low torque even after operating time has passed, compared to conventional greases. The composition of the present invention also has water resistance and rust prevention properties equivalent to or better than those of conventional techniques. The composition of the present invention also exhibits excellent fretting resistance, particularly at low temperatures.

[0007] <Base Oil> The base oil contained in the composition of the present invention has a kinematic viscosity at 40°C of 25 to 50 mm 2 It is a synthetic hydrocarbon oil having a kinematic viscosity of 25 to 50 mm / s at 40°C. 2 When the kinematic viscosity at 40°C is 25 to 40 mm / s, the grease has low viscosity resistance, which results in excellent low torque, and a sufficient oil film thickness can be ensured, which results in excellent durability such as peeling life. 2 / s, and 25 to 35 mm 2 / s. In this specification, the kinematic viscosity of the base oil at 40°C is a value measured in accordance with JIS K 2220 23. When the base oil is a synthetic hydrocarbon oil, the change in kinematic viscosity due to temperature change is small, and the fluidity at low temperatures is excellent, resulting in excellent low torque at low temperatures. Furthermore, the oil film thickness at high temperatures can be ensured, resulting in excellent durability at high temperatures. From the viewpoint of low traction coefficient and low temperature properties, poly-α-olefins are preferred. The base oil is preferably contained in an amount of 70 to 88% by mass, more preferably 75 to 85% by mass, based on the total mass of the composition. When the base oil content is within this range, a sufficient amount of lubricating oil can be ensured, and excellent durability can be exhibited.

[0008] <Thickener> The thickener contained in the composition of the present invention is an alicyclic aliphatic diurea compound. The alicyclic aliphatic diurea compound is obtained by reacting an alicyclic monoamine and an aliphatic monoamine with a diisocyanate. Therefore, in addition to the reaction products of the alicyclic monoamine and the aliphatic monoamine with a diisocyanate, the thickener includes a reaction product of the alicyclic monoamine and a diisocyanate (a so-called alicyclic diurea compound) and a reaction product of the aliphatic monoamine and a diisocyanate (a so-called aliphatic diurea compound). From the viewpoints of low torque and shear stability, the thickener of the present invention is preferably a mixture of a compound represented by the following formula (1), a compound represented by the following formula (2), and a urea compound represented by the following formula (3). R1-NHCONH-R2-NHCONH-R3 (1) R1-NHCONH-R2-NHCONH-R1 (2) R3-NHCONH-R2-NHCONH-R3 (3) In the formulae, R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms. R1 represents a cyclohexyl group. R3 represents a linear or branched alkyl group having 8 to 22 carbon atoms, preferably a linear alkyl group having 16 to 20 carbon atoms, and most preferably a linear alkyl group having 18 carbon atoms. The ratio of the number of moles of the linear or branched alkyl group having 8 to 22 carbon atoms to the total number of moles of cyclohexyl groups and the linear or branched alkyl group having 8 to 22 carbon atoms (R3 / (R1+R3))×100 is 10 to 20%. By using such a thickener, it is possible to adjust the consistency to the desired level with a small amount of thickener, thereby reducing the stirring resistance of the grease composition and achieving low torque, while at the same time providing a grease composition with excellent durability.The molar ratio of the alkyl group is more preferably 10 to 15%, and particularly preferably 11.5 to 13.5%.

[0009] In particular, the thickener of the present invention is most preferably a mixture of a compound represented by formula (1) in which R1 is a cyclohexyl group, R2 is a group derived from 4,4'-diphenylmethane diisocyanate, and R3 is a linear alkyl group having 18 carbon atoms, a compound represented by formula (2) in which R1 is a cyclohexyl group and R2 is a group derived from 4,4'-diphenylmethane diisocyanate, and a compound represented by formula (3) in which R2 is a group derived from 4,4'-diphenylmethane diisocyanate and R3 is a linear alkyl group having 18 carbon atoms, wherein the mole ratio of the mixture is 11.5 to 13.5%, from the standpoints of thickening ability (i.e., the ability to prepare a grease of the desired hardness with a small content) and shear stability, and ultimately low torque and durability. The thickener is preferably contained in an amount of 10 to 20 mass %, more preferably 13 to 17 mass %, based on the total mass of the composition. When the thickener content is within this range, the grease has low stirring resistance and excellent channeling properties, making it possible to achieve a satisfactory level of low torque.

[0010] <Additives> Carboxylic acid-based rust inhibitors that can be used in the present invention include monocarboxylic acids such as lauric acid and stearic acid, and saturated carboxylic acids having a naphthenic nucleus. Dicarboxylic acids include succinic acid, alkylsuccinic acid, alkylsuccinic acid half esters, alkenylsuccinic acid, alkenylsuccinic acid half esters, succinic acid derivatives such as succinimides, tetrapropenylbutanedioic acid, hydroxy fatty acids, mercapto fatty acids, sarcosine derivatives, and oxidized waxes such as wax and petrolatum oxides. Among these, alkenylsuccinic acid half esters and tetrapropenylbutanedioic acid are preferred. Carboxylic acid salt-based rust inhibitors that can be used in the present invention include metal salts of naphthenic acid, abietic acid, lanolin fatty acid, alkenylsuccinic acid, and amino acid derivatives. Metal elements that constitute the salts include cobalt, manganese, zinc, aluminum, calcium, barium, lithium, magnesium, copper, and the like. Of these, zinc naphthenate and zinc abietic acid are preferred.

[0011] Regarding the fatty acid amine salts that can be used in the present invention, the fatty acids that constitute the fatty acid amine salts are preferably fatty acids having 4 to 22 carbon atoms, and more preferably fatty acids having 8 to 18 carbon atoms. The fatty acids may be saturated or unsaturated fatty acids, and may further be straight-chain fatty acids, branched fatty acids, cyclic fatty acids, or hydroxy fatty acids. Specific examples include stearic acid, palmitic acid, myristic acid, lauric acid, isostearic acid, octylic acid, undecylenic acid, oleic acid, and hydroxystearic acid. The amines that constitute the fatty acid amine salts are not particularly limited, but are preferably saturated or unsaturated amines having 1 to 42 carbon atoms, and more preferably saturated or unsaturated amines having 4 to 22 carbon atoms. Specific examples include octylamine, laurylamine, myristylamine, stearylamine, behenylamine, oleylamine, tallow alkylamines, hydrogenated tallow alkylamines, aniline, benzylamine, cyclohexylamine, diethylamine, dipropylamine, dibutylamine, diphenylamine, dibenzylamine, dicyclohexylamine, triethylamine, tributylamine, dimethyloctylamine, dimethyldecylamine, dimethylstearylamine, dimethyl tallow alkylamines, dimethyl hydrogenated tallow alkylamines, dimethyloleylamine, etc. As the fatty acid amine salt that can be used in the present invention, particularly preferred are two-kind mixtures (2:1 mass ratio) of salts of C8 fatty acids and C12 amines, and salts of C18 fatty acids and C12 to C20 (mixed) amines.

[0012] The additive of the present invention preferably contains zinc abietic acid as a carboxylate-based rust inhibitor for water resistance and rust prevention. The additive of the present invention particularly preferably contains alkenyl succinic acid half ester, zinc abietic acid, and a fatty acid amine salt. The additive of the present invention most preferably contains alkenyl succinic acid half ester, tetrapropenyl butanedioic acid, zinc naphthenate, zinc abietic acid, and a fatty acid amine salt.

[0013] It is preferable that the content of the carboxylic acid-based rust inhibitor is 0.1 to 5 mass%, the content of the carboxylate-based rust inhibitor is 0.1 to 5 mass%, and the content of the amine salt of a fatty acid is 0.1 to 3 mass%, as these provide excellent water resistance and rust prevention. It is even more preferable that the content of the carboxylic acid-based rust inhibitor is 0.2 to 3 mass%, the content of the carboxylate-based rust inhibitor is 0.3 to 3 mass%, and the content of the amine salt of a fatty acid is 0.3 to 2.5 mass%, as these provide even better water resistance and rust prevention. It is even more preferable that the content of the carboxylic acid-based rust inhibitor is 0.2 to 1 mass%, the content of the carboxylate-based rust inhibitor is 0.3 to 1 mass%, and the content of the amine salt of a fatty acid is 0.5 to 2 mass%, as these provide even better water resistance and rust prevention. Among these, a carboxylic acid-based rust inhibitor containing an alkenyl succinic acid half ester having a content of 0.2 to 0.4 mass%, a carboxylate-based rust inhibitor containing zinc abietic acid having a content of 0.3 to 0.4 mass%, and an amine salt of a fatty acid having a content of 0.9 to 1.1 mass% are preferred, as these provide even more excellent water resistance and rust prevention. In particular, a carboxylic acid-based rust inhibitor containing an alkenyl succinic acid half ester and tetrapropenyl butanedioic acid having a total content of 0.2 to 0.4 mass%, a zinc naphthenate and zinc abietic acid having a total content of 0.3 to 0.4 mass%, and an amine salt of a fatty acid having a total content of 0.9 to 1.1 mass% are preferred, as these provide the most excellent water resistance and rust prevention.

[0014] The worked penetration of the grease composition of the present invention is 220 to 270. A worked penetration within this range indicates excellent channeling properties (sometimes referred to as "channeling ability"). Generally, grease inside a rolling bearing flows due to rotation. As a result, a small amount of grease remains in the area where the rolling elements run. The remaining majority of grease does not flow and remains on the cage or is pushed out to the side seal. This behavior of grease being expelled from the rolling elements and forming tunnels is called channeling. Excellent channeling properties mean that less grease adheres to the rolling element surfaces and raceway surfaces, thereby reducing running torque. The worked penetration of the composition of the present invention is preferably 230 to 265, more preferably 235 to 265. The worked penetration in this invention refers to the 60-stroke worked penetration, measured in accordance with JIS K 2220 7.

[0015] The composition of the present invention is a base oil having a kinematic viscosity of 25 to 50 mm at 40°C. 2 By using a synthetic hydrocarbon oil with a viscosity of 1 / s, rolling viscous resistance and traction coefficient can be reduced, and by using an alicyclic aliphatic diurea compound as a thickener, stirring resistance can be reduced, and by using the base oil and thickener to achieve a consistency of 220 to 270, excellent channeling properties can be achieved. These features are organically integrated to reduce bearing torque and starting torque at low temperatures, and further improve fretting resistance at low temperatures. By using the above combination as additives, the composition of the present invention can form a strong surface film of the additives. As a result, a grease composition with excellent water resistance and durability can be obtained.

[0016] <Optional Components> The composition of the present invention may further contain antiwear agents, antioxidants, extreme pressure agents, oiliness improvers, metal deactivators, etc., either alone or in combination of two or more. However, from the viewpoint of water resistance, it is preferable that the composition does not contain barium sulfonate. The content of the additives is typically 0.01 to 15 mass%, preferably 0.05 to 12 mass%, and more preferably 0.1 to 10 mass%, based on the total mass of the lubricant composition. Examples of antiwear agents include triphenyl phosphorothioate, triphenyl phosphite, triethyl phosphite phosphite esters, glycerol monocaprate, and glycerol monostearate fatty acid esters. Examples of antioxidants include amine-based antioxidants such as N-n-butyl-p-aminophenol, 4,4'-tetramethyl-di-aminodiphenylmethane, α-naphthylamine, N-phenyl-α-naphthylamine, phenothiazine, and alkyldiphenylamines (e.g., p,p'-dioctyldiphenylamine); 2,6-di-tert-butyl-p-cresol (BHT), 2,2'-methylenebis(4-methyl-6-tert-butylphenol), and 4,4'-butylidenebis(3-methyl-6-tert-butylphenol); Examples of suitable antioxidants include phenolic antioxidants such as 2,6-di-tert-butylphenol, 2,4-dimethyl-6-tert-butylphenol, tert-butylhydroxyanisole (BHA), 4,4'-butylidenebis(3-methyl-6-tert-butylphenol), 4,4'-methylenebis(2,3-di-tert-butylphenol), 4,4'-thiobis(3-methyl-6-tert-butylphenol), and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. Phenol-based antioxidants are preferred as antioxidants. Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate is particularly preferred.

[0017] Examples of extreme pressure agents include sulfurized oils and fats, sulfurized olefins, polysulfides, ashless dithiocarbamates, trioctyl phosphate, tricresyl phosphate, zinc dialkyldithiocarbamates, dialkyldithiocarbamates of molybdenum dialkyldithiocarbamates, zinc dialkyldithiophosphates, and dialkyldithiophosphates of molybdenum dialkyldithiophosphates. Examples of oiliness improvers include lanolin, methyl oleate, monoesters of polyethylene glycol monooleate, bis(2-ethylhexyl) adipate, and diesters of bis(2-ethylhexyl) sebacate. Examples of metal deactivators include benzotriazole. The inclusion of an antiwear agent as an additive is preferred because it can further improve the durability of the grease composition. Triphenyl phosphorothioate is particularly preferred as an antiwear agent. When the composition of the present invention contains triphenyl phosphorothioate, its content is preferably 0.1 to 5 mass%.

[0018] It is preferable to further include a phenolic antioxidant as an additive, as this can further improve the durability of the grease composition. Octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate is particularly preferable as the phenolic antioxidant. When the composition of the present invention includes octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, its content is preferably 0.1 to 3 mass%. It is preferable to further include an antiwear agent and a phenolic antioxidant as additives, and more preferably, triphenyl phosphorothioate and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate. It is particularly preferred to contain 0.1 to 5% by weight of triphenyl phosphorothioate and 0.1 to 3% by weight of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, based on the total weight of the composition.

[0019] In particular, a grease composition for a vehicle hub unit bearing, comprising a base oil, a thickener and an additive, wherein the base oil has a kinematic viscosity at 40°C of 25 to 40 mm 2 / s, the thickener is a mixture of a compound of the above formula (1), a compound of the above formula (2), and a compound of the above formula (3) (in formulas (1) to (3), R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, R1 represents a cyclohexyl group, and R3 represents a linear alkyl group having 16 to 20 carbon atoms. The ratio of the number of moles of linear alkyl groups having 16 to 20 carbon atoms to the total number of moles of cyclohexyl groups and linear alkyl groups having 16 to 20 carbon atoms (R3 / (R1+R3))×100 is 10 to 20%), the content of the thickener is 10 to 20% by mass based on the total mass of the composition, the content of the carboxylic acid-based rust inhibitor is 0.2 to 1% by mass, the content of the carboxylate-based rust inhibitor is 0.3 to 1% by mass, and the content of the amine salt of a fatty acid is 0.5 to 2% by mass, A grease composition which further contains triphenyl phosphorothioate as an additive and has a worked penetration of 220 to 270 is preferred because it has excellent water resistance and rust prevention properties.

[0020] More particularly, a grease composition for a vehicle hub unit bearing, comprising a base oil, a thickener and an additive, wherein the base oil has a kinematic viscosity at 40°C of 25 to 40 mm 2 / s, the thickener is a mixture of a compound of the above formula (1), a compound of the above formula (2), and a compound of the above formula (3) (in formulas (1) to (3), R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, R1 represents a cyclohexyl group, and R3 represents a linear alkyl group having 18 carbon atoms. The ratio of the number of moles of linear alkyl groups having 18 carbon atoms to the total number of moles of cyclohexyl groups and linear alkyl groups having 18 carbon atoms (R3 / (R1+R3))×100 is 10 to 15%), the content of the thickener is 10 to 20% by mass based on the total mass of the composition, A grease composition containing 0.2 to 0.4 mass% of a carboxylic acid-based rust inhibitor containing an alkenyl succinic acid half ester, 0.3 to 0.4 mass% of a carboxylate-based rust inhibitor containing zinc abietate, 0.9 to 1.1 mass% of an amine salt of a fatty acid, further containing triphenyl phosphorothioate and a phenol-based antioxidant as additives, and having a worked penetration of 220 to 270 is preferred because it has excellent water resistance and rust prevention properties.

[0021] Among these, a grease composition for a vehicle hub unit bearing, which comprises a base oil, a thickener, and an additive, wherein the base oil has a kinematic viscosity at 40°C of 25 to 35 mm 2 / s, the content of the base oil is 75 to 85 mass% based on the total mass of the composition, the thickener is a mixture of a compound of the above formula (1), a compound of the above formula (2), and a compound of the above formula (3) (in formulas (1) to (3), R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, R1 represents a cyclohexyl group, and R3 represents a linear alkyl group having 18 carbon atoms. The ratio of the number of moles of linear alkyl groups having 18 carbon atoms to the total number of moles of cyclohexyl groups and linear alkyl groups having 18 carbon atoms (R3 / (R1+R3))×100 is 10 to 15%), the content of the thickener is 10 to 20 mass% based on the total mass of the composition, A grease composition containing 0.2 to 0.4 mass% of a carboxylic acid-based rust inhibitor containing an alkenyl succinic acid half ester, 0.3 to 0.4 mass% of a carboxylate-based rust inhibitor containing zinc abietate, 0.9 to 1.1 mass% of an amine salt of a fatty acid, and further containing triphenyl phosphorothioate and octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate as additives, and having a worked penetration of 230 to 265 is preferred because it has excellent low torque properties, water resistance, and rust prevention properties.

[0022] In particular, a grease composition for a vehicle hub unit bearing, comprising a base oil, a thickener and an additive, wherein the base oil has a kinematic viscosity at 40°C of 25 to 35 mm 2 / s, the content of the base oil is 75 to 85 mass% based on the total mass of the composition, the thickener is a mixture of a compound in which, in formula (1), R1 is a cyclohexyl group, R2 is a group derived from 4,4'-diphenylmethane diisocyanate, and R3 is a linear alkyl group having 18 carbon atoms, a compound in formula (2) in which R1 is a cyclohexyl group, R2 is a group derived from 4,4'-diphenylmethane diisocyanate, and a compound in formula (3) in which R2 is a group derived from 4,4'-diphenylmethane diisocyanate, and R3 is a linear alkyl group having 18 carbon atoms, the proportion of the number of moles being 11.5 to 13.5%, the content of the thickener is 13 to 17 mass% based on the total mass of the composition, A grease composition having a total amount of alkenyl succinic acid half ester and tetrapropenyl butanedioic acid of 0.2 to 0.4 mass%, a total amount of zinc naphthenate and zinc abietate of 0.3 to 0.4 mass%, a total amount of amine salts of fatty acids of 0.9 to 1.1 mass%, further containing, as additives, 0.1 to 5 mass% of triphenyl phosphorothioate and 0.1 to 3 mass% of octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and having a worked penetration of 235 to 265 is preferred because it has excellent low torque properties, water resistance, and rust prevention properties.

[0023] <Applications> The grease composition of the present invention is used for vehicle hub unit bearings, particularly for vehicle hub unit bearings for electric vehicles.

[0024] Grease compositions containing an alicyclic aliphatic diurea thickener were prepared by reacting a predetermined amount of raw amine (cyclohexylamine and stearylamine, molar ratio: cyclohexylamine:stearylamine = 7:1) with 1 mole of 4.4'-diphenylmethane diisocyanate in a base oil at a ratio of 2 moles, followed by heating and cooling to obtain a base grease. A predetermined amount of additive was mixed with the base oil, added to the base grease, mixed well, and kneaded on a three-roll mill to produce a grease composition with a predetermined worked penetration. Grease compositions containing an aromatic diurea thickener were prepared by reacting a predetermined amount of raw amine (p-toluidine) with 1 mole of 4.4'-diphenylmethane diisocyanate in a base oil at a ratio of 2 moles, followed by heating and cooling to obtain a base grease. A predetermined amount of additive was mixed with a base oil, added to the base grease, mixed thoroughly, and kneaded in a three-roll mill to produce a grease composition with a predetermined worked consistency.

[0025] [Bearing Torque Test] The grease composition was packed into a rolling bearing, the inner ring was rotated under the following conditions, and the torque value applied to the outer ring of the bearing after 60 minutes of rotation was measured. Bearing type: deep groove ball bearing 6204 Test temperature: room temperature Rotation speed: 3000 rpm Test load: axial load 500 N: radial load 250 N Amount of grease packed: approximately 1.4 g Evaluation: ○: bearing torque less than 70 mN m ×: bearing torque 70 mN m or more

[0026] [Low temperature torque test] The test was carried out in accordance with JIS K 2220 18. Low temperature torque test method. Test temperature: -40°C Evaluation: ○: Starting torque less than 500 mN·m ×: Starting torque 500 mN·m or more

[0027] [High-Temperature Durability Test] This is an inner ring rotation test in which a grease composition is filled into a rolling bearing and the bearing lubrication life at high temperatures is evaluated. The rolling bearing was operated under the following conditions, and the lubrication life was determined as the time until the motor generated an overcurrent, and durability was evaluated based on this. Bearing type: Angular contact ball bearing 7206 Test temperature: 150°C Rotation speed: 6000 rpm Test load: Axial load 1500 N Amount of grease filled: Approximately 10 g Evaluation: ○: Lifetime of 150 hours or more ×: Lifetime of less than 150 hours

[0028] [Water Resistance Test] The water resistance of the test grease was evaluated by a rolling four-ball test. Three 15 mm diameter steel bearing balls were prepared and placed in an equilateral triangular shape in a cylindrical container with a bottom inner diameter of 36.0 mm, an upper inner diameter of 31.63 mm, and a depth of 10.98 mm. 20 g of test grease mixed with 20% water was applied. Furthermore, one 5 / 8 inch diameter steel bearing ball (with six evenly spaced indentations of approximately 200 μm at the contact point with the three steel balls) was placed in the depression formed by the three steel balls. The 5 / 8 inch diameter steel bearing ball was rotated at 1200 rpm at room temperature while applying a surface pressure of 4.1 GPa. The three 15 mm diameter steel bearing balls also rotated and revolved around their own axes, and were continuously rotated until peeling occurred. The total number of rotations at the time of peeling was defined as the lifespan. Evaluation: ○; total number of rotations was 1000 x 10 3 More than 1000 rotations ×; total number of rotations is 1000 × 10 3 Less than rotation

[0029] [Rust prevention test - durability test in the presence of water] The test was carried out in accordance with ASTM D1743 bearing rust prevention test. However, while the standard requires that the bearings be immersed in distilled water, this test used 1% salt water, under more severe conditions. Evaluation was carried out based on the presence or absence of rust on the outer ring rolling surface. Evaluation: ○: No rust ×: Rust present

[0030]

Claims

1. A grease composition for a vehicle hub unit bearing, comprising a base oil, a thickener, and an additive, The base oil has a kinematic viscosity at 40°C of 25 to 50 mm 2 / s synthetic hydrocarbon oil, the thickener is an alicyclic aliphatic diurea compound, The additives include, based on the total mass of the composition, 0.1 to 5 mass% of a carboxylic acid-based rust inhibitor, 0.1 to 5 mass% of a carboxylate-based rust inhibitor, and 0.1 to 3 mass% of an amine salt of a fatty acid, The grease composition has a worked penetration of 220 to 270.

2. 2. The grease composition according to claim 1, wherein the thickener is a mixture of a compound of the following formula (1), a compound of the following formula (2), and a compound of the following formula (3): R1-NHCONH-R2-NHCONH-R3 (1) R1-NHCONH-R2-NHCONH-R1 (2) R3-NHCONH-R2-NHCONH-R3 (3) (In the formula, R2 represents a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms. R1 represents a cyclohexyl group. R3 represents a linear or branched alkyl group having 8 to 22 carbon atoms. The ratio of the number of moles of linear or branched alkyl groups having 8 to 22 carbon atoms to the total number of moles of cyclohexyl groups and linear or branched alkyl groups having 8 to 22 carbon atoms (R3 / (R1+R3))×100 is 10 to 20%.)

3. 3. The grease composition according to claim 1, wherein the thickener is contained in an amount of 10 to 20 mass % based on the total mass of the composition.

4. 3. The grease composition according to claim 1, further comprising triphenyl phosphorothioate as an additive.

5. 3. The grease composition according to claim 1, further comprising a phenolic antioxidant as an additive.

6. A vehicle hub unit bearing comprising a hub which, together with an inner ring element, constitutes an inner ring equivalent member, an outer ring, and a plurality of rolling elements arranged so as to be able to roll freely between the hub and the outer ring, wherein the water-resistant grease composition according to claim 1 or 2 is sealed in a gap formed between the hub and the outer ring and in which the rolling elements are arranged.