Grease composition and grease-filled bearing

Abstract

Provided are a grease composition and a grease-filled bearing that prevent electrolytic corrosion and have excellent noise reduction and wear resistance. A rolling bearing 1 is a grease-filled bearing in which a plurality of balls 4 are held between raceway surfaces 2a, 3a of an inner ring 2 and an outer ring 3, and in which a grease composition 7 is filled between the raceway surfaces 2a, 3a. The grease composition 7 includes a base oil, a thickening agent, a metal powder having a melting point of 800℃ or higher, and an ionic liquid. In particular, the metal powder is a copper powder, and the ionic liquid includes the elements phosphorus (P) and sulfur (S).

Description

Grease composition and grease-filled bearing

[0001] The present invention relates to a grease composition and a grease-filled bearing in which the grease composition is filled.

[0002] Motors for automotive accessories and industrial machinery are required to be smaller, more powerful, and have higher output every year, and the conditions for use are becoming more stringent. Rolling bearings are generally used for these applications.

[0003] For example, when current from the circuit flows into the rolling bearings installed in motors and inverters, sparks can occur through the oil film on the rolling contact surface between the rolling elements and the raceway. These sparks can cause electrolytic corrosion, which locally dissolves the raceway surface. The effects of electrolytic corrosion can damage the bearing raceway surface, leading to significant reductions in noise and durability (wear resistance and bearing life).

[0004] To prevent such problems, rolling bearings containing conductive grease are known. For example, Patent Document 1 describes a conductive grease containing a base oil made of at least one of perfluoropolyether and fluorosilicone, a thickener such as a fluorine compound, and carbon black. Patent Document 2 describes a conductive grease containing a lubricating base oil, a thickener, and a phosphonium-based ionic liquid.

[0005] Patent No. 4599769 Patent No. 7218242

[0006] Incidentally, the conductive grease containing a solid conductive additive, such as that described in Patent Document 1, generally has a volume resistivity of 10 ２ ~10 ４ Ωcm, which tends to be superior to the ionic liquids described in Patent Document 2. However, conductive greases using carbon black, such as those described in Patent Document 1, are not very suitable for long-term use because their conductivity decreases over time due to destruction of the chain structure of the carbon black particles, etc. On the other hand, conductive greases to which ionic liquids have been added, such as those described in Patent Document 2, are effective in preventing electrolytic corrosion, but there are concerns about wear due to the corrosive action specific to ionic liquids.

[0007] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a grease composition and a grease-sealed bearing that prevent electrolytic corrosion and have low noise and excellent wear resistance.

[0008] The grease composition of the present invention is characterized by comprising a base oil, a thickener, a metal powder having a melting point of 800° C. or higher, and an ionic liquid. The metal powder may be a metal powder containing an element of Groups 11 to 13.

[0009] The metal powder is characterized in that it is copper powder.

[0010] The ionic liquid is characterized by containing the elements phosphorus (P) and sulfur (S).

[0011] The grease composition is characterized by containing 1 to 10 parts by mass of the copper powder and 1 to 10 parts by mass of the ionic liquid relative to 100 parts by mass of a base grease comprising the base oil and the thickener.

[0012] The present invention is characterized by being a grease composition for preventing electrolytic corrosion that prevents electrolytic corrosion of rolling bearings.

[0013] The grease-sealed bearing of the present invention is a grease-sealed bearing in which a plurality of rolling elements are held between the raceway surfaces of an inner ring and an outer ring, and a grease composition is sealed between the raceway surfaces, and is characterized in that the grease composition is the grease composition of the present invention.

[0014] The rolling bearing is characterized in that the ratio of the average particle diameter (unit: μm) of the metal powder to the oil film thickness (unit: μm) of the grease composition when in use (average particle diameter of metal powder / oil film thickness) is greater than 1.

[0015] The grease-sealed bearing of the present invention is a grease-sealed bearing which holds a plurality of rolling elements between the raceway surfaces of an inner ring and an outer ring, and in which a grease composition is sealed between the raceway surfaces, and which is characterized in that the grease composition contains a base oil, a thickener, and a metal powder having a melting point of 800°C or higher, and in that the ratio of the average particle diameter (unit: μm) of the metal powder to the oil film thickness (unit: μm) of the grease composition when the rolling bearing is in use (average particle diameter of metal powder / oil film thickness) is greater than 1.

[0016] The metal powder is copper powder, and is characterized in that its average particle size is 0.5 μm or more and less than 2.0 μm.

[0017] The grease composition is characterized in that it is a grease composition for preventing electrolytic corrosion that prevents electrolytic corrosion of rolling bearings.

[0018] One embodiment of the grease composition of the present invention is a grease composition containing an ionic liquid, which further contains a metal powder having a melting point of 800°C or higher. This prevents electrolytic corrosion, and the electrical conductivity of the metal powder reduces wear during electrical conduction due to the corrosive action of the ionic liquid. This metal powder can be said to function as a corrosion inhibitor. Furthermore, using a metal powder having a melting point of 800°C or higher as the metal powder makes it easier to maintain its function as a conductive additive and also reduces increases in noise and vibration caused by the melting of the metal powder. As a result, the grease composition prevents electrolytic corrosion and has excellent low noise and wear resistance.

[0019] The metal powder is copper powder, and copper is a soft metal, which can suppress aggressiveness against mating members and can also reduce wear due to conductive additives. In addition, copper has particularly high conductivity among soft metals, which reduces the amount of energy generated during insulation breakdown, which is thought to affect electrolytic corrosion, resulting in even lower noise and better wear resistance.

[0020] The grease composition contains 1 to 10 parts by mass of copper powder as a metal powder and 1 to 10 parts by mass of an ionic liquid relative to 100 parts by mass of a base grease consisting of a base oil and a thickener. This allows the amount of ionic liquid added to the entire grease composition to be reduced, making it easier to suppress the corrosive action of the ionic liquid.

[0021] One embodiment of the grease-sealed bearing of the present invention is filled with the grease composition of the present invention, and therefore is a bearing that is prevented from electrolytic corrosion and has low noise and excellent wear resistance.

[0022] Another embodiment of the grease-filled bearing of the present invention is filled with a grease composition containing a base oil, a thickener, and a metal powder having a melting point of 800°C or higher, and the ratio of the average particle size of the metal powder to the oil film thickness (average particle size of the metal powder / oil film thickness) is greater than 1. The metal powder imparts conductivity to the grease composition, while the metal powder is less likely to enter the sliding parts, making it easier to maintain its function as a conductive additive and also suppressing increases in noise and vibration caused by the melting of the metal powder. As a result, the bearing is protected from electrolytic corrosion and has low noise and excellent wear resistance.

[0023] Fig. 1 is a cross-sectional view showing an example of a grease-sealed bearing of the present invention. Fig. 2 is a view showing the surface properties of the rolling bearings of Examples 1 to 4. Fig. 3 is a view showing the surface properties of the rolling bearings of Comparative Examples 1 to 3. Fig. 4 is a view showing the surface properties of the rolling bearings of Comparative Examples 4 to 7. Fig. 5 is an element mapping image of Example 2 and Comparative Example 3. Fig. 6 is a schematic view of the conduction paths of Examples 1-2 and Examples 3-4.

[0024] As a result of extensive research into grease compositions that prevent galvanic corrosion, the present inventors have found that using a metal powder (preferably a soft metal powder) with a melting point of 800°C or higher as a conductive additive, more specifically, a metal powder with a melting point of 800°C or higher and containing an element from Groups 11 to 13, can impart appropriate conductivity to the grease composition, thereby preventing galvanic corrosion and effectively preventing damage caused by galvanic corrosion. Furthermore, they have found that the use of the metal powder in a grease composition containing an ionic liquid can also suppress the progression of corrosion caused by the ionic liquid. The present invention is based on these findings.

[0025] The grease composition of the present invention contains a base oil, a thickener, and a metal powder having a melting point of 800° C. or higher. In one embodiment, the grease composition further contains an ionic liquid.

[0026] The grease composition of the present invention contains a metal powder having a melting point of 800°C or higher. From the viewpoint of ensuring electrical conductivity, a larger number of free electrons is preferable, and the grease composition of the present invention preferably contains a metal powder containing an element of Groups 11 to 13 and having a melting point of 800°C or higher. The melting point of the metal powder may be 900°C or higher, 1000°C or higher, or 1200°C or lower. This metal powder contributes to imparting electrical conductivity to the grease composition. The type of metal powder is not particularly limited as long as it has a melting point of 800°C or higher (preferably containing an element of Groups 11 to 13). However, considering the attacking potential on the rolling contact surface and raceway surface, a soft metal powder is preferred. The soft metal powder has a Mohs hardness of, for example, 4 or lower, preferably 3.5 or lower.

[0027] Examples of soft metal powders having a melting point of 800°C or higher and containing Group 11 to 13 elements include copper (melting point: approximately 1084°C), silver (melting point: approximately 961°C), gold (melting point: approximately 1064°C), and alloy powders thereof. These metal powders may be used alone or in combination. While aluminum (melting point: approximately 660°C) and zinc (approximately 419°C) are commonly known as soft metal powders, these do not fall under the category of metal powders with melting points of 800°C or higher. In other words, the grease composition of the present invention preferably does not contain metal powders with melting points below 800°C. For example, metal powders with melting points below 800°C have difficulty maintaining their particle shape under conditions where sliding parts are exposed to high temperatures, such as in rolling bearings, and tend to adhere to the sliding parts. This can result in increased noise and vibration.

[0028] The Mohs hardness scale is a measure of hardness in which, if a material is scratched by 10 standard materials in sequence, it is judged to be lower in hardness than the standard materials. The standard materials are, in order of decreasing hardness, 1: talc, 2: gypsum, 3: calcite, 4: fluorite, 5: apatite, 6: orthoclase, 7: quartz, 8: topaz, 9: corundum, and 10: diamond. For example, if a material is not scratched by calcite (Mohs hardness 3) but is scratched by fluorite (Mohs hardness 4), the hardness is expressed as 3.5. The Mohs hardness indicates the degree of hardness when a force is applied laterally and is suitable as a measure of wear on sliding surfaces. The Mohs hardness can be measured using a known Mohs hardness scale.

[0029] As the soft metal powder having a melting point of 800°C or higher, it is preferable to use a copper-based powder, which has excellent conductivity and is relatively inexpensive. Examples of copper-based powders that can be used include copper powder (single copper powder) and copper alloy powder containing copper as a main component. Examples of copper alloy powders that can be used include copper-zinc alloy (brass) powder and copper-tin alloy (bronze) powder. Note that various alloy powders may further contain a third compounding element. Furthermore, the metal powder may be one composed of Group 11 to 13 elements and having a melting point of 800°C or higher.

[0030] The metal powder may be in various shapes, such as spherical or scaly. From the viewpoint of dispersibility and flowability in the grease composition, it is preferable to use a spherical powder. For example, the metal powder may be prepared by an atomization method or may be a commercially available product.

[0031] The average particle diameter of the metal powder (D ５０ ) is not particularly limited, but is, for example, 0.1 μm to 10 μm, preferably 0.5 μm to 5.0 μm, and may be 0.5 μm or more and less than 2.0 μm. In terms of the relationship with the oil film, the average particle diameter is preferably larger than the oil film thickness. The average particle diameter (D ５０ ) is the particle size at which the cumulative value reaches 50% when the particle size distribution is expressed as a cumulative distribution on a volume basis, and can be measured using a particle size distribution measuring device that utilizes a laser light scattering method.

[0032] In a rolling bearing filled with the grease composition, the ratio of the average particle diameter (unit: μm) of the metal powder to the oil film thickness (unit: μm) of the grease composition during use of the bearing (average particle diameter of metal powder / oil film thickness) is preferably greater than 1. If this ratio is greater than 1, the metal powder is less likely to enter the sliding portion, which is considered to be disadvantageous for forming a conductive path. However, since the particle shape is more easily maintained (especially in the case of soft metal powder), the electrolytic corrosion prevention effect can be exerted over the long term. In this case, it is considered that the conductive path is mainly formed outside the contact ellipse. The ratio may be, for example, 1.2 to 10, or 1.5 to 5.0.

[0033] In the present invention, the oil film thickness can be calculated by a theoretical formula. When a theoretical formula is used, the oil film thickness can be calculated using the Cittenden formula shown in the following formula (1) using the data of the base oil and the test conditions.

[0034] H ｍｉｎ : Minimum oil film thickness U: Speed ​​parameter W: Load parameter R ｘ : Equivalent radius of curvature in the flow direction R ｙ : Equivalent radius of curvature in the direction perpendicular to the flow

[0035] In the above formula (1), R ｘ is the equivalent radius of curvature in the flow direction, and R ｙ is the equivalent radius of curvature in the direction perpendicular to the flow. ｘ is 1 / R ｘ = (1 / R ｘ１ ) + (1 / R ｘ２ ) and R ｙ is 1 / R ｙ = (1 / R ｙ１ ) + (1 / R ｙ２ ) is calculated by R ｘ１ is the radius of curvature of one cylinder in the flow direction, and R ｘ２ is the radius of curvature of the other cylinder in the flow direction, and R ｙ１ is the radius of curvature of one of the cylinders in the direction perpendicular to the flow, and R ｙ２ is the radius of curvature of the other cylindrical body in the direction perpendicular to the flow. In addition, in the above formula (1), U is a velocity parameter, and (η ０ ×u) / (E′×R ｘ ) is calculated as follows: η ０ is the normal pressure viscosity. ０ is calculated by ρ×ν, where ρ is the density of the lubricating oil and ν is the kinematic viscosity of the lubricating oil. u is the average value of the peripheral speed of one cylinder and the peripheral speed of the other cylinder. E' is the equivalent Young's modulus. E' is calculated by 2 / E'={(1-ν １ ２ ) / E １}+{(1-ν ２ ２ ) / E ２} is calculated by E １ is the Young's modulus of one cylinder, and E ２ is the Young's modulus of the other cylinder. １ is the Poisson's ratio of one cylinder, and ν ２ is the Poisson's ratio of the other cylinder.

[0036] In the above formula (1), G is a material parameter and is calculated by α×E′. α is a viscosity pressure coefficient. α is calculated by the Wu-Klaus-Duda formula. More specifically, α is (0.1657+0.2332×log １０ v) × m × 10－８ where ν is the kinematic viscosity of the lubricating oil. m is a constant determined by the lubricating oil and is calculated by the Walther-ASTM formula. More specifically, m is log １０ {log １０ (ν+0.7)}=-m×log １０ It is calculated by T + K, where T is the temperature and K is a constant determined by the lubricating oil. By substituting the two temperatures and the kinematic viscosities at those two temperatures into the Walther-ASTM formula and solving the simultaneous equations, the values ​​of m and K can be calculated. W is a load parameter, and is expressed as w / (E' x R ｘ ２ ) where w is the load.

[0037] In the grease composition of the present invention, the content of the metal powder is, for example, 0.1 to 20 parts by mass, preferably 1 to 10 parts by mass, and may be 5 to 10 parts by mass, relative to 100 parts by mass of the base grease consisting of the base oil and the thickener. Furthermore, when used in combination with an ionic liquid, the ionic liquid can also be endowed with a conductive function, and the content of the metal powder may be, for example, 1 to 5 parts by mass, relative to 100 parts by mass of the base grease. By ensuring this range, it becomes easier to ensure the conductivity necessary to prevent electrolytic corrosion.

[0038] The grease composition of the present invention may contain the above-mentioned metal powder in combination with an ionic liquid. An ionic liquid is an ionically bonded compound consisting of a cationic component and an anionic component, but is liquid at around room temperature (e.g., 25°C).

[0039] In the grease composition of the present invention, the content of the ionic liquid is, for example, 0.1 to 20 parts by mass per 100 parts by mass of the base grease. From the viewpoint of corrosion resistance, the content is preferably 0.1 to 15 parts by mass, and may be 1 to 10 parts by mass. Furthermore, by adding the above-mentioned metal powder to the grease composition, the content of the ionic liquid that is normally used can be reduced. The content of the ionic liquid may be less than the content of the above-mentioned metal powder.

[0040] Examples of the cationic component of the ionic liquid that can be used in the present invention include aliphatic amine cations, alicyclic amine cations, imidazolium cations, pyridine cations, and quaternary phosphonium cations. － ) as Br － , Cl － Halide ions such as SCN － , B.F. ４ － , ClO ４ － , P.F. ６ － , (CF ３ SO ２ ) ２ N － , (CF ３ CF ２ SO ２ ) ２ N － , C.F. ３ SO ３ － , C.F. ３ COO － , Ph ４ B － , (CF ３ SO ２ ) ３ C － , P.F. ３ (C ２ F ５ ) ３ － The ionic liquid may be a commercially available product.

[0041] The ionic liquid preferably contains phosphorus (P) and sulfur (S) as contained elements. Specifically, the cation component is a quaternary phosphonium cation, and the anion component is SCN － , (CF ３ SO ２ ) ２ N － , (CF ３ CF ２ SO ２ ) ２ N － , or (CF ３ SO ２ ) ３ C －In addition, the ionic liquid may have a sulfur content greater than a phosphorus content on a mass basis.

[0042] In the chemical formula of the quaternary phosphonium cation, the four substituents on the phosphorus (P) are hydrogen, a hydrocarbon-containing group, or an alkyl group, and these substituents may be the same or some or all of them may be different.

[0043] For example, when an ionic liquid containing phosphorus (P) and sulfur (S) is included, the phosphorus content is 0.1 to 2.0 mass%, or alternatively 0.1 to 1.0 mass%, or alternatively 0.1 to 0.5 mass%, relative to the total amount (100 mass%) of the grease composition. The sulfur content is 0.1 to 2.0 mass%, or alternatively 0.1 to 1.0 mass%, relative to the total amount (100 mass%) of the grease composition.

[0044] The total amount of the phosphorus content and the sulfur content may be 0.1 to 1.0 mass % or 0.1 to 0.5 mass % relative to the total amount (100 mass %) of the grease composition.

[0045] The base oil used in the grease composition of the present invention can be any oil normally used in rolling bearings, without any particular limitations. Examples include mineral oils such as paraffinic mineral oil and naphthenic mineral oil, synthetic hydrocarbon oils such as PAO oil and alkylbenzene oil, ester oil, ether oil, silicone oil, and fluorine oil. These base oils may be used alone or in combination of two or more. Among the above, synthetic hydrocarbon oils, ester oils, and mixed oils thereof are preferred.

[0046] The synthetic hydrocarbon oil is more preferably PAO oil. PAO oil is a mixture of α-olefins or isomerized α-olefin oligomers or polymers. Specific examples of α-olefins include 1-octene, 1-nonene, 1-decene, 1-dodecene, 1-tridecene, 1-tetradecene, 1-pentadecene, 1-hexadecene, 1-heptadecene, 1-octadecene, 1-nonadecene, 1-eicosene, 1-docosene, and 1-tetradocosene, and mixtures thereof are usually used.

[0047] Examples of ester oils include polyol ester oils, phosphate ester oils, polymer ester oils, aromatic ester oils, carbonate ester oils, diester oils, polyglycol oils, etc. Among these, aromatic ester oils are preferred, and trimellitic acid esters, for example, can be used.

[0048] The kinematic viscosity of the base oil (in the case of a mixed oil, the kinematic viscosity of the mixed oil) is 20 to 100 mm at 40°C. ２ / s is preferable, and more preferably 40 to 100 mm ２ / s, 40 to 80 mm ２ / s may also be used.

[0049] The thickener used in the grease composition of the present invention is not particularly limited, and any of the conventional thickeners typically used in the field of greases can be used. For example, soap-based thickeners such as metal soaps and composite metal soaps, and non-soap-based thickeners such as bentone, silica gel, urea compounds, and urea-urethane compounds can be used. Examples of metal soaps include sodium soaps, calcium soaps, aluminum soaps, and lithium soaps, while examples of urea compounds and urea-urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, and diurethane compounds. Among these, it is preferable to use a diurea compound as the thickener.

[0050] The diurea compound is obtained by reacting a diisocyanate component with a monoamine component. Examples of the diisocyanate component include phenylene diisocyanate and diphenylmethane diisocyanate (MDI). Examples of the monoamine component include alicyclic monoamines such as cyclohexylamine, aromatic monoamines such as p-toluidine, and aliphatic monoamines such as octylamine.

[0051] As the diurea compound, an aliphatic / alicyclic diurea compound prepared using an aliphatic monoamine and an alicyclic monoamine as the monoamine components, and an aliphatic / aromatic diurea compound prepared using an aliphatic monoamine and an aromatic monoamine as the monoamine components are preferred. For example, the molar ratio of the aliphatic monoamine to the alicyclic monoamine used in preparing the aliphatic / alicyclic diurea compound is aliphatic monoamine:alicyclic monoamine = (4:1) to (2:1). For example, the molar ratio of the aliphatic monoamine to the aromatic monoamine used in preparing the aliphatic / aromatic diurea compound is aliphatic monoamine:aromatic monoamine = (4:1) to (2:1).

[0052] The thickener is preferably contained in an amount of 5 to 30 mass %, more preferably 10 to 20 mass %, and may be contained in an amount of 10 to 15 mass %, based on the base grease consisting of the base oil and the thickener.

[0053] The grease composition of the present invention may further contain other additives within the scope of the present invention, such as antioxidants such as amine-based, phenol-based, and sulfur-based compounds, rust inhibitors such as sulfonates, and oiliness agents such as esters and alcohols.

[0054] The worked penetration (JIS K 2220) of the grease composition is preferably in the range of 200 to 350. If the penetration is less than 200, oil separation is small and lubrication may be poor. On the other hand, if the penetration exceeds 350, the grease becomes soft and tends to leak out of the bearing, which is undesirable.

[0055] A grease-filled bearing filled with the grease composition of the present invention will be described with reference to Figure 1. Figure 1 is a cross-sectional view of a deep groove ball bearing. Rolling bearing 1 comprises an inner ring 2 having an inner ring raceway surface 2a on its outer peripheral surface and an outer ring 3 having an outer ring raceway surface 3a on its inner peripheral surface, which are concentrically arranged, with a plurality of rolling elements 4 disposed between the inner ring raceway surface 2a and the outer ring raceway surface 3a. These rolling elements 4 are held in place by a cage 5. Openings 8a, 8b at both axial ends of the inner and outer rings are sealed by sealing members 6, and the above-described grease composition 7 is filled in the bearing space at least around the rolling elements 4. The inner ring 2, outer ring 3, and rolling elements 4 are made of steel, and grease composition 7 is interposed between them and the rolling elements 4 to provide lubrication.

[0056] In the rolling bearing 1, the steel materials constituting the bearing components, such as the inner ring 2, outer ring 3, and rolling elements 4, may be any material commonly used as a bearing material, such as high-carbon chromium bearing steel (SUJ1, SUJ2, SUJ3, SUJ4, SUJ5, etc.; JIS G4805), carburized steel (SCr420, SCM420, etc.; JIS G4053), stainless steel (SUS440C, etc.; JIS G4303), high-speed steel (M50, etc.), and cold-rolled steel. The sealing member 6 may be made of metal or a rubber molded body alone, or may be a composite of a rubber molded body and a metal plate, a plastic plate, or a ceramic plate. A composite of a rubber molded body and a metal plate is preferred for its durability and ease of attachment.

[0057] In the grease-sealed bearing of the present invention, the grease composition is sealed within the bearing space, and the grease composition is interposed between the raceway surfaces of the inner and outer rings and the rolling surfaces of the rolling elements, thereby preventing the occurrence of electrolytic corrosion due to its electrical conductivity.

[0058] 1 shows a deep groove ball bearing as an example of the bearing, but the grease-sealed bearing of the present invention is not limited to this and can also be used as, for example, an angular contact ball bearing, a cylindrical roller bearing, a tapered roller bearing, a spherical roller bearing, a needle roller bearing, a thrust cylindrical roller bearing, a thrust tapered roller bearing, a thrust needle roller bearing, or a thrust spherical roller bearing.

[0059] The grease-sealed bearing of the present invention is suitable as a rolling bearing used in environments and applications where electrolytic corrosion is likely to occur. For example, it is suitable as a bearing that rotatably supports the rotating shaft of a motor or refrigerant compressor, a bearing for an inverter, etc. It can also be used as a motor bearing for electric vehicles (EVs) and hybrid electric vehicles (HEVs).

[0060] The present invention will be specifically explained by way of examples and comparative examples, but is not limited to these examples in any way.

[0061] Grease compositions were prepared according to the compositions shown in Table 1. In Table 1, the contents of the base oil and thickener are shown in mass % relative to 100 mass % of the base grease (base oil + thickener). The sulfur and phosphorus contents of the ionic liquid are shown in mass % relative to the total amount of the grease composition. The contents of the copper powder and aluminum powder are shown in parts by mass relative to 100 parts by mass of the base grease.

[0062] [Electrical corrosion test] Each grease composition was filled into an inner ring rotating rolling bearing (inner ring, outer ring, and steel balls made of bearing steel SUJ2) that supports a rotating shaft, simulating an industrial motor, and an operating test was carried out. The operating conditions were set at a combined load of 59 N and a rotation speed of 1600 rpm, and the test was carried out with a current of 5.0 A flowing through the test bearing (6206).

[0063] The oil film thickness of each grease composition under the above test conditions was calculated using the Cittenden equation, based on the base oil data and test conditions. The results are shown in Table 1.

[0064] The surface properties of the inner ring raceway surface were observed 30 hours after the electrolytic corrosion test. The results of the example and comparative example are shown in Figures 2 to 4. Each figure shows, from top to bottom, the observed image, the roughness curve along the circumferential direction of the inner ring raceway surface (dashed line 1 in the figure) and its maximum roughness height Rz, the cross-sectional curve along the radial direction of the inner ring raceway surface (dotted line 2 in the figure) and its protrusion valley depth Pvk, and the roughness curve along the radial direction.

[0065] The maximum roughness height Rz of the roughness curve, the protrusion valley depth Pvk of the profile curve, and the protrusion peak height Ppk, which will be described later, were measured using a stylus surface roughness tester. Rz, Pvk, and Ppk are defined in JIS B 0601:2013. In this test, a protrusion valley depth Pvk of the profile curve of less than 0.5 was rated "A," a value of 0.5 or more but less than 1.0 was rated "B," and a value of 1.0 or more was rated "C." Measurements were taken at five locations, and an evaluation of A to C was performed based on the average value. The results are shown in Table 1.

[0066] All roughness profiles, including the circumferential roughness profile and the axial cross-sectional profile, were obtained under the following conditions: Measurement length: 7.0 mm Measurement pitch: 0.0005 mm Roughness analysis measurement pitch: 0.0005 mm Measurement speed: 0.2 mm / sec Reference length: 0.8 mm Number of sections: 5 λs: 0.0025 mm Correction: R correction (automatic) was performed using the non-sliding surface as the reference.

[0067] [Anderon Test] To evaluate acoustics, the inner ring after the electrolytic corrosion test was used, and a bearing with all components except the inner ring being brand new was operated to measure the Anderon value in the H band (frequency 1,800 Hz to 10,000 Hz). In the above test, the radial vibration component transmitted to the outer ring was measured. The rotation speed was 1,800 rpm, the axial load was 100 N, and sampling was performed to obtain one data point after 5 seconds of rotation, with the average value of a total of seven data points calculated. A calculated Anderon value of less than 50 was assigned an "A," a value between 50 and 100 was assigned a "B," and a value of 100 or greater was assigned a "C." The results are shown in Table 1.

[0068]

[0069] As shown in Table 1, in Examples 1 to 4, which used only copper powder or copper powder and ionic liquid as the conductive additive, the profile valley depth Pvk was less than 1.0, and wear of the raceway surface was suppressed. On the other hand, when no conductive additive was added (Comparative Example 1) or when only ionic liquid was added (Comparative Examples 4 to 7), the profile valley depth Pvk was 1.0 or more.

[0070] In Examples 1 to 4, the ratio of "average particle size of metal powder / oil film thickness" was greater than 1, which suggests that metal powder is less likely to get into the sliding portion, but compared to Comparative Example 1, the adverse effects of electrolytic corrosion were prevented by the formation of a conductive path. Furthermore, Examples 3 and 4, which also used an ionic liquid, had lower anderon values ​​than Examples 1 and 2, and were more effective at preventing electrolytic corrosion.

[0071] Although aluminum powder is a soft metal and electrically conductive, it exhibited a high anderon value. To consider these results, Figure 5 shows elemental mapping images of the center and edge portions of the frosting on the inner ring raceway surface of Example 2 and Comparative Example 3. In Example 2, slight copper adhesion was observed on the frosting. In contrast, in Comparative Example 3, a large amount of aluminum was observed on the frosting. Furthermore, to evaluate the degree of this adhesion, the peak height Ppk was measured (see Table 2 below). It was found that Comparative Examples 2 and 3 had higher Ppk values ​​than the other examples, indicating significant aluminum adhesion (i.e., aluminum deposition). Thus, the high anderon values ​​in Comparative Examples 2 and 3 are thought to be due to aluminum adhesion, rather than electrolytic corrosion. In contrast, copper has a higher melting point than aluminum, and no significant adhesion to the raceway surface was observed. It is believed that the favorable results were obtained because the particle shape was maintained.

[0072]

[0073] Furthermore, the ionic liquid contents of Comparative Examples 4 to 7 differ from one another. Comparative Examples 4 to 6 had good Anderon values, but relatively large amounts of wear. This is thought to be due to corrosion by the ionic liquid. Furthermore, the grease compositions of Comparative Examples 4 to 7 had relatively high volume resistivities, resulting in high energy discharges. However, because the ionic liquid was a liquid, it is thought that a uniform conductive path was formed both inside and outside the contact ellipse.

[0074] Figure 6 shows a schematic diagram of the conduction paths in Examples 1-2 and Examples 3-4. In Examples 1-2, due to the relationship between the oil film thickness and the particle size of the copper powder, it was difficult for copper particles to enter the contact ellipse, making it difficult to form a conduction path within the contact ellipse, and as a result, ridge marks were formed in some areas. As shown in Table 2, the surface roughness Rz within the contact ellipse was relatively rough (relatively large wear amount), and the surface roughness Rz within the contact ellipse was better when an ionic liquid (liquid) was used as the conductive additive. It is believed that the conduction paths in Examples 1-2 were formed by wrapping around outside the contact ellipse.

[0075] On the other hand, in Examples 3 and 4, by adding copper powder to the ionic liquid, the conductivity within the contact ellipse was mainly covered by the ionic liquid, and the conductivity outside the contact ellipse was mainly covered by the copper powder, which is thought to have resulted in reduced wear within the contact ellipse and suppressed corrosion, resulting in a better Anderon value. In this way, by combining ionic liquid and copper powder, the disadvantages of each were seemingly canceled out.

[0076] The grease-sealed bearing of the present invention prevents electrolytic corrosion and has low noise and excellent wear resistance, making it suitable for use in environments where electrolytic corrosion is likely to occur. For example, it is suitable for rolling bearings used in motors for automotive accessories and industrial machinery, and for rolling bearings used in motor drive devices equipped in electric vehicles and hybrid vehicles.

[0077] REFERENCE SIGNS LIST 1 Rolling bearing (greased bearing) 2 Inner ring 3 Outer ring 4 Ball (rolling element) 5 Cage 6 Seal member 7 Grease composition 8a Opening 8b Opening

Claims

1. A grease composition comprising a base oil, a thickener, a metal powder having a melting point of 800°C or higher, and an ionic liquid.

2. The grease composition according to claim 1, wherein said metal powder is copper powder.

3. The grease composition according to claim 2, characterized in that the ionic liquid contains the elements phosphorus (P) and sulfur (S).

4. The grease composition according to claim 2, characterized in that the grease composition contains 1 to 10 parts by mass of the copper powder and 1 to 10 parts by mass of the ionic liquid per 100 parts by mass of base grease consisting of the base oil and the thickener.

5. The grease composition according to claim 1, which is a grease composition for preventing electrolytic corrosion of rolling bearings.

6. A grease-sealed bearing comprising a plurality of rolling elements held between the raceway surfaces of an inner ring and an outer ring and a grease composition sealed between said raceway surfaces, said grease composition being the grease composition defined in claim 1.

7. A grease-sealed bearing as described in claim 6, characterized in that the ratio of the average particle diameter [unit: μm] of the metal powder to the oil film thickness [unit: μm] of the grease composition when the rolling bearing is in use (average particle diameter of the metal powder / oil film thickness) is greater than 1.

8. A grease-sealed bearing which holds a plurality of rolling elements between the raceway surfaces of an inner ring and an outer ring and has a grease composition sealed between the raceway surfaces, wherein the grease composition contains a base oil, a thickener, and a metal powder having a melting point of 800°C or higher, and wherein the ratio of the average particle diameter [unit: μm] of the metal powder to the oil film thickness [unit: μm] of the grease composition when the rolling bearing is in use (average particle diameter of metal powder / oil film thickness) is greater than 1.

9. A grease-sealed bearing according to claim 8, characterized in that the metal powder is copper powder, the average particle size of which is 0.5 μm or more and less than 2.0 μm.

10. A grease-sealed bearing according to claim 8, characterized in that the grease composition is an anti-electrolytic corrosion grease composition for preventing electrolytic corrosion of rolling bearings.

Images