EP-4403615-B1 - GREASE COMPOSITION FOR VEHICULAR HUB UNIT BEARINGS

Inventors

• ISA KAZUKI
• TAKEYAMA YUKI
• IJIMA Masatoshi
• SUGAWARA SUGURU
• ISHIKAWA HIROAKI

Dates

Publication Date

20260506

Application Date

20220913

Claims (8)

1. A grease composition for vehicular hub unit bearings comprising: a base oil; a thickener; and an additive, wherein the base oil is a synthetic hydrocarbon oil having a kinematic viscosity at 40°C of 25-50 mm 2 /s, the thickener is an alicyclic aliphatic diurea compound, the additive includes, based on a total mass of the composition, 0.1-5% by mass of a carboxylic acid corrosion inhibitor, 0.1-5% by mass of a carboxylic acid salt corrosion inhibitor selected from the group consisting of: metal salts of naphthenic acid, of abietic acid, of lanolin fatty acids, or of alkenyl succinic acids, and 0.1-3% by mass of an amine salt of a fatty acid, and a 60-stroke worked penetration measured in accordance with JIS K 2220 7 is 220-270.
2. The grease composition according to claim 1, wherein the thickener is a mixture of a compound represented by formula (1) below, a compound represented by formula (2) below, and a compound represented by formula (3) below: 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; R3 represents a linear or branched alkyl group having 8 to 22 carbon atoms; and a ratio (R3/(R1+R3))×100 of a molar amount of the linear or branched alkyl group having 8 to 22 carbon atoms to a total molar amount of the cyclohexyl group and the linear or branched alkyl group having 8 to 22 carbon atoms is 10-20%.
3. The grease composition according to claim 1 or 2, wherein the thickener is contained in an amount of 10-20% by mass based on the total mass of the composition.
4. The grease composition according to any one of claims 1 to 3, further comprising: triphenyl phosphorothioate as an additive.
5. The grease composition according to any one of claims 1 to 4, further comprising: a phenolic antioxidant as an additive.
6. The grease composition according to any one of claims 1 to 5, wherein the carboxylic acid corrosion inhibitor is selected from the group consisting of: succinic acid, alkyl succinic acids, alkyl succinic acid half esters, alkenyl succinic acids, alkenyl succinic acid half esters, succinic acid imides, (tetrapropenyl)butanedioic acid, hydroxy fatty acids, and mercapto fatty acids.
7. The grease composition according to any one of claims 1 to 5, wherein the carboxylic acid corrosion inhibitor is an alkenyl succinic acid half ester and/or (tetrapropenyl)butanedioic acid.
8. A vehicular hub unit bearing comprising: a hub constituting an inner ring-equivalent member together with an inner ring element; an outer ring; and a plurality of ball elements rollably arranged between the hub and the outer ring, wherein the water-resistant grease composition according to any one of claims 1 to 7 is sealed in a gap formed between the hub and the outer ring where the ball elements are arranged.

Description

Technical Field The present invention relates to a grease composition for vehicular hub unit bearings. Background Art One method for improving the efficiency of automobiles is reducing the torque of hub unit bearings used in automobile wheels, and similarly, it is important that the grease used has low torque over a wide temperature range from low to high temperatures. Methods for reducing grease torque include lowering the kinematic viscosity of the base oil as much as possible (Non Patent Literature 1) and softening the grease to reduce grease agitation resistance. Patent Literature 1 discloses a grease composition comprising a base oil having a kinetic viscosity of 20 to 50 mm2/s at 40°C, a urea compound as a thickener in an amount of 8 to 30 mass% of the total amount of the grease, and at least one rust inhibitor selected from carboxylic acids, carboxylates, and ester-based rust inhibitors, each added in an amount of 0.1 to 10 mass% of the total amount of the grease and 0.1 to 15 mass% in total of the total amount of the grease. The rust-preventive additive may be at least one selected from the group consisting of naphthenates and succinic acid derivatives. Lowering the kinematic viscosity of the base oil reduces torque by decreasing grease agitation resistance, but can cause insufficient oil film formation leading to surface-originated flaking and reduced lubrication life due to evaporation, resulting in inability of the bearing to achieve its full service life. In addition, especially when the grease is made softer, the grease tends to leak out more easily, leading to reduced lubrication life and insufficient oil film formation causing flaking, with unsatisfactory flaking life. Citation List Patent Literature 1: JP 2004-224823 Non Patent Literatures Non Patent Literature 1: Hiroaki Ishikawa, Technical trends and tribology of hub unit bearing, Journal of Japanese Society of Tribologists, Vol. 54, No. 9, 2009, pp. 580-585 Summary of Invention Problems to be solved by the invention Therefore, an object of the present invention is to provide a grease composition for vehicular hub unit bearings that has low torque and excellent durability. Means for solution of the problems The present invention provides grease compositions and a vehicular hub unit bearing as defined in the appended claims. Advantageous Effects of Invention According to the present invention, it is possible to provide a grease composition having low torque, specifically low bearing torque and low starting torque at low temperatures, as well as excellent durability, specifically high-temperature durability. With conventional greases, the torque tends to gradually decrease over operating time from the initial bearing rotation torque (starting torque), but with the present invention, the torque is low from startup and remains stably low compared to conventional greases even after passage of operating time. The composition of the present invention also has water resistance and corrosion prevention equal to or better than the related art. The composition of the present invention also has excellent fretting resistance, specifically low-temperature fretting resistance. Description of Embodiments <Base Oil> The base oil contained in the composition of the present invention is a synthetic hydrocarbon oil having a kinematic viscosity at 40°C of 25-50 mm2/s. Having a kinematic viscosity at 40°C of 25-50 mm2/s provides excellent low torque due to low viscous drag of the grease and also excellent durability such as flaking life by securing sufficient oil film thickness. Preferably, the kinematic viscosity at 40°C is 25-40 mm2/s, and more preferably 25-35 mm2/s. Note that in the present specification, the kinematic viscosity of the base oil at 40°C is a value measured in accordance with JIS K 2220 23. By the base oil being a synthetic hydrocarbon oil, the change in kinematic viscosity due to temperature change is small, fluidity at low temperatures is excellent thus providing excellent low torque at low temperatures, and oil film thickness at high temperatures can also be secured thus providing excellent high temperature durability. From the viewpoint of low traction coefficient and low temperature properties, polyalphaolefin is preferable. Preferably, the base oil is contained in an amount of 70-88% by mass based on the total mass of the composition, and more preferably 75-85% by mass. When the content of the base oil is within this range, a sufficient amount of lubricating oil can be secured and excellent durability can be exhibited. <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 product of the alicyclic monoamine and aliphatic monoamine with the diisocyanate, it includes a reaction product of the alicycli