Rolling bearings

Abstract

The object is to provide a rolling bearing capable of suppressing electrolytic corrosion, and to provide a motor incorporating such a bearing. [Solution] A rolling bearing and a motor equipped with said rolling bearing, comprising an inner ring, an outer ring arranged coaxially on the outer peripheral side of the inner ring, a plurality of rolling elements arranged between the inner ring and the outer ring, a retainer for holding the rolling elements, and a grease composition held between the inner ring and the outer ring, wherein the grease composition comprises a base oil including an aromatic ester base oil, a thickener, and an ionic liquid including a borate anion, and wherein the grease composition has a capacitance of 4.6 pF or more under conditions of an electrode diameter of 10 mm, an electrode distance of 1 mm, and a frequency of 10 kHz, or has a loss tangent (tan δ) of 0.7 or more, expressed as the ratio of storage modulus to loss modulus at 25°C, measured under conditions of a film thickness of 0.5 mm, a shear strain of 1%, and a frequency of 100 Hz, in dynamic viscoelasticity measurement using a rotational rheometer.

Description

[Technical field]

[0001] The present invention relates to a rolling bearing in which a grease composition containing an ionic liquid is sealed, and a motor equipped with the rolling bearing. [Background technology]

[0002] Ionic liquids are salts in a liquid state that are composed only of ions (anions and cations). Ionic liquids have characteristics such as low vapor pressure (non-volatility), high thermal stability, flame retardancy, low viscosity, and high ionic conductivity. In addition, various physical properties can be designed by combining cations and anions, so they are expected to be applied to various technical fields, including electrolytes and solvents. Due to these characteristics, ionic liquids are also being considered for use as lubricants and greases. For example, there has been a proposal for a lubricant composition to which an ionic liquid has been added in order to maintain low friction for a long period under high load conditions (Patent Document 1). Furthermore, for example, a grease composition that employs an ionic liquid as the base oil of the grease and combines it with a non-soap thickener has been proposed as a grease composition that aims to improve low friction properties (Patent Document 2). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-065256 A [Patent Document 2] JP 2019-123846 A Summary of the Invention [Problem to be solved by the invention]

[0004] For example, in rolling bearings used in electric motors, etc., a potential difference (shaft voltage) is likely to occur between the inner and outer rings, and this potential difference (shaft voltage) causes current to flow inside the bearing, resulting in electrolytic corrosion, which can damage the rolling elements and the surface of the raceway of the rolling bearing. Such damage to the rolling elements and raceway can lead to deterioration of the rolling bearing and even to its shutdown, so measures to prevent this are required.

[0005] An object of the present invention is to provide a rolling bearing packed with a grease composition containing an ionic liquid, which is capable of suppressing electrolytic corrosion, and to provide a motor incorporating such a bearing. [Means for solving the problem]

[0006] One aspect of the present invention is a rolling bearing, With the inner circle, an outer ring arranged coaxially with the inner ring on an outer circumferential side of the inner ring; A plurality of rolling elements disposed between the inner ring and the outer ring; A cage for holding the rolling elements; A grease composition is held between the inner ring and the outer ring, The grease composition includes a base oil including an aromatic ester-based base oil, a thickener, and an ionic liquid including a borate anion, The grease composition has a capacitance of 4.6 pF or more under the conditions of an electrode diameter of 10 mm, an electrode distance of 1 mm, and a frequency of 10 kHz, and In dynamic viscoelasticity measurements using a rotational rheometer, the loss tangent (tan δ), expressed as the ratio of storage modulus to loss modulus at 25°C, measured under conditions of a film thickness of 0.5 mm, a shear strain of 1%, and a frequency of 100 Hz, is 0.7 or more. Concerning rolling bearings. The present invention further relates to a motor equipped with the above-mentioned rolling bearing. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of the structure of a rolling bearing according to the present invention. [Diagram 2] 1 is a schematic diagram illustrating an example of a structure of a motor according to the present invention. [Diagram 3] FIG. 1 is a conceptual diagram of a test device used in an electrolytic corrosion test. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] As mentioned above, in rolling bearings used in motors, etc., electric current flows inside the bearing, generating sparks on the rolling elements (balls) and raceway surfaces, causing damage (electrolytic corrosion) to these surfaces. In the early stages, damage caused by this spark generation is limited to small spots (holes) on the surface, but if it progresses further, corrugated irregularities (washboard-like electrolytic corrosion marks) will appear, causing abnormal noise and vibration, and even causing the bearing to deteriorate (stop). There are two ideas for preventing electrolytic corrosion: bearing current and bearing insulation. The former involves the use of conductive grease, for example, while the latter involves the use of ceramic bearings that do not conduct electricity. However, ceramic bearings have problems such as being expensive, limiting their application.

[0009] In response to the issue of suppressing electrolytic corrosion in rolling bearings, the inventors investigated greases containing ionic liquids that function as electrical conductivity imparting agents, and discovered that combining the ionic liquid composition with other grease compositions, particularly base oils, is effective in suppressing electrolytic corrosion. As a result of further investigation, it was discovered for the first time that in a grease composition that employs an aromatic ester base oil and an ionic liquid containing borate anions, when the electrostatic capacitance of the grease composition is a certain value or above and the loss tangent (tan δ) of the grease composition determined by dynamic viscoelasticity measurement is a certain value or above, the occurrence of electrolytic corrosion in rolling bearings can be suppressed. Although capacitance represents the electrical properties of grease and can therefore be used as an indicator of the occurrence of electrolytic corrosion, there have been no reports to date on the threshold value of capacitance that leads to the occurrence of electrolytic corrosion. Furthermore, the dynamic viscoelasticity of grease is an index showing the flow characteristics and shape stability of the grease, and can be said to be an effective parameter for understanding, for example, the behavior of grease inside a rolling bearing when it rotates [ease of adhesion to rolling elements (balls), etc. (easily adheres: churning type, difficult to adhere: channeling type)]. However, there have been no reports to date that have examined the dynamic viscoelasticity of grease in conjunction with its composition and electrostatic capacitance from the perspective of the occurrence and suppression of electrolytic corrosion. As will be shown in the results of the examples described below, both the grease composition (base oil, ionic liquid) and the physical properties of the grease (capacitance, dynamic viscoelasticity) are important in suppressing electrolytic corrosion in rolling bearings. This point is demonstrated by the result that electrolytic corrosion occurred when the grease had a specific composition that deviated from the desired capacitance and dynamic viscoelasticity. The present invention will now be described in detail.

[0010] [Rolling bearings] First, preferred embodiments of the rolling bearing according to the present invention will be described in detail below with reference to the accompanying drawings. However, the present invention is not limited to the following embodiments.

[0011] 1 is a radial cross-sectional view of a rolling bearing 10 according to a preferred embodiment of the present invention. The rolling bearing 10 has a basic structure similar to that of a rolling bearing of the prior art, and includes an annular inner ring 11, an outer ring 12, a plurality of rolling elements 13, a cage 14, and a seal member 15. The inner ring 11 is a cylindrical structure installed coaxially with the center axis of a shaft (not shown) on the outer periphery side of the shaft. The outer ring 12 is a cylindrical structure arranged coaxially with the inner ring 11 on the outer periphery side of the inner ring 11. Each of the multiple rolling elements 13 is a ball arranged in a raceway in an annular bearing space 16 formed between the inner ring 11 and the outer ring 12. That is, the rolling bearing 10 in this embodiment is a ball bearing. The cage 14 is disposed within the raceway and holds the multiple rolling elements 13. The cage 14 is an annular body that is installed coaxially with the central axis of the shaft, and has a structure in which one side in the direction of the central axis is provided with multiple pockets for holding the rolling elements 13, with the rolling elements 13 accommodated in each pocket. The rolling elements 13 are held at predetermined intervals in the circumferential direction of the inner ring 11 and the outer ring 12 by the cage 14, and falling off of the rolling elements 13 and contact between adjacent rolling elements 13 are suppressed. The shape (crown-shaped, corrugated, etc.) and material (steel plate, resin, etc.) of the cage 14 are arbitrary and are not limited to a specific shape or material. The seal member 15 is fixed to the inner circumferential surface of the outer ring 12 and extends toward the inner ring 11, sealing the bearing space 16. A grease composition G is enclosed in the bearing space 16 sealed by the seal member 15. That is, the grease composition G is held between the inner ring 11 and the outer ring 12. A grease composition described below is used as the grease composition G. The amount of grease G enclosed inside the bearing space 16 can be, for example, 5 to 50% of its volume. The seal member 15 is made of, for example, steel plate or rubber, and may be a steel plate shield that does not come into contact with the outer periphery of the inner ring 11, or a non-contact rubber seal that does not come into contact with the outer periphery of the inner ring 11. Either of the seal members, the steel plate shield or the non-contact rubber seal, may be used in the present invention. Note that although the drawing shows an embodiment equipped with the seal member 15, the rolling bearing of the present invention also covers an embodiment of a rolling bearing that does not have a seal member. In the rolling bearing 10 having the above configuration, the grease composition G acts to reduce friction between the rolling elements 13 and the cage 14, and between the rolling elements 13 and the inner ring 11 or the outer ring 12. The reduction in friction reduces friction torque and suppresses the generation of frictional heat, promoting smooth rotation of the inner ring 11 and the outer ring 12. As can be seen from the configuration shown in Fig. 1, the grease composition G sealed in the rolling bearing 10 lubricates between the rolling elements 13 and the inner ring 11 or the outer ring 12 when the rolling bearing 10 rotates.

[0012] The rolling bearing in the present invention is not particularly limited in size, conditions of use, etc. and may be, for example, a rolling bearing with an outer diameter of 10 mm or less, or a rolling bearing with an outer diameter of more than 50 mm. The rolling bearing of the present invention can be used as a rolling bearing for motors (for example, fan motors, cleaner motors) used in automobiles, home appliances, information devices, etc.

[0013] [Motor] As an example, an embodiment of a motor equipped with the rolling bearing of this embodiment will be described in detail with reference to FIG. 2, but the present invention is not limited to the following embodiment.

[0014] 2 is a cross-sectional view of a motor according to an embodiment of the present invention taken along the shaft direction. The motor 20 has a basic structure similar to that of a conventional motor, and is composed of a housing 21, a stator 22, a coil 23, a rotor magnet 24, a shaft 25, and a rolling bearing 26 that supports the shaft 25. In the motor 20, magnetic force is generated by passing current supplied from a power source (not shown) via a drive circuit through a coil 23 wound around a stator 22, which causes a rotor magnet 24 to rotate, and the rotation is transmitted to an external rotating body via a shaft 25.

[0015] [Grease composition] The grease composition used in the rolling bearing of the present invention comprises a base oil containing an aromatic ester-based base oil, a thickener, and an ionic liquid containing a borate anion. The grease composition packed in the rolling bearing of the present invention will now be described.

[0016] <Base oil> In the grease composition packed in the rolling bearing according to this embodiment, an aromatic ester-based base oil is used as the base oil. Examples of aromatic ester-based base oils include aromatic esters having an ester group as a cyclic substituent, such as esters of aromatic polycarboxylic acids such as phthalic acid, trimellitic acid, and pyromellitic acid with aliphatic monoalcohols having 4 to 16 carbon atoms. Specifically, there are trialkyl esters of trimellitic acid having 8 carbon atoms, such as ditridecyl phthalate, trioctyl trimellitate, and tri-2-ethylhexyl trimellitate, trialkyl esters of trimellitic acid having 9 carbon atoms, such as trinonyl trimellitate, trialkyl esters of trimellitic acid having 10 carbon atoms, such as tridecyl trimellitate, trialkyl esters of trimellitic acid having 11 carbon atoms, and trialkyl esters of trimellitic acid having 4 to 16 carbon atoms (the alkyl chains of the three alkyl ester groups in one compound may be the same or different, and for example, "an alkyl ester of trimellitic acid (having x, y, z carbon atoms)" refers to an alkyl ester of trimellitic acid having x carbon atoms, an alkyl ester of trimellitic acid having y carbon atoms, and a trialkyl ester of trimellitic acid having 4 to 16 carbon atoms). Examples of the ester include, but are not limited to, a mixture of alkyl esters of trimellitic acid having z carbon atoms (a mixture of esters in which all three alkyl ester groups in one compound are the same), a mixture of alkyl esters of trimellitic acid having x carbon atoms (in which all three alkyl ester groups in one compound are the same) and an ester of an alkyl of trimellitic acid having y carbon atoms and an alkyl of trimellitic acid having z carbon atoms (in which all three alkyl ester groups in one compound are not the same), or an ester of an alkyl of trimellitic acid having x carbon atoms, an alkyl of carbon atom, and an alkyl of carbon atom (in which all three alkyl ester groups in one compound are not the same); tetraoctyl pyromellitate, tetra-2-ethylhexyl pyromellitate, and mixtures of two or more of these.

[0017] The aromatic ester-based base oil used in the present invention has a kinematic viscosity at 40° C. of, for example, 53 to 130 mm 2 / s range, 55-130mm 2 / s range, e.g. 70-130 mm 2 / s range, or for example 70-100 mm 2 An aromatic ester-based base oil in the range of 0.1 to 1.0 wt. / s can be used. The aromatic ester base oil having a kinematic viscosity within the above-mentioned range is not particularly limited, and examples thereof include aromatic ester compounds having an ester group as a substituent on the ring as described above, for example, esters of aromatic polycarboxylic acids such as phthalic acid, trimellitic acid, and pyromellitic acid with aliphatic monoalcohols having 4 to 16 carbon atoms.

[0018] The above-mentioned base oil may be contained in a proportion of, for example, 70 mass % or more based on the total mass of the grease composition used in the present invention, and for example, the above-mentioned base oil may be contained in a proportion of 70 mass % to 98 mass %, 70 mass % to 90 mass %, or 80 mass % to 98 mass % based on the total mass of the grease composition.

[0019] <Thickener> In the grease composition used in the present invention, the type of thickener is not particularly limited, but among them, urea-based thickeners are preferably used. Urea compounds have excellent heat resistance and water resistance, and are particularly stable at high temperatures, and therefore are suitably used as thickeners in applications where the application is in a high-temperature environment. As the urea-based thickener, a urea compound such as a diurea compound, a triurea compound, or a polyurea compound can be used. As these urea-based thickeners, conventionally known urea compounds can be used.

[0020] An example of the diurea compound used in the urea-based thickener is a diurea compound represented by the following formula (1). R 1 -NHCONH-R 2 -NHCONH-R 3 ...Equation (1) In the above formula (1), R 1 and R 3each independently represents a monovalent aliphatic hydrocarbon group, a monovalent alicyclic hydrocarbon group, or a monovalent aromatic hydrocarbon group, and R 1 and R 3 At least one of R represents a monovalent aliphatic hydrocarbon group or a monovalent alicyclic hydrocarbon group. For example, R 1 and R 3 one of R may represent an aliphatic hydrocarbon group and the other may represent a monovalent alicyclic hydrocarbon group or a monovalent aromatic hydrocarbon group, or alternatively R 1 and R 3 One of these may represent an aliphatic hydrocarbon group and the other may represent a monovalent aromatic hydrocarbon group. Also R 2 represents a divalent aromatic hydrocarbon group. The diurea compound represented by formula (1) may be a mixture of a plurality of types. For example, R 1 and R 3 and R 1 and R 3 and R 1 and R 3 It is also possible to use a mixture of compounds in which one of the above represents an aliphatic hydrocarbon group and the other represents a monovalent aromatic hydrocarbon group.

[0021] The monovalent aliphatic hydrocarbon group may, for example, be a straight-chain or branched, saturated or unsaturated alkyl group having 6 to 26 carbon atoms. The monovalent alicyclic hydrocarbon group may, for example, be a cycloalkyl group having 5 to 12 carbon atoms. Examples of the aromatic hydrocarbon group include monovalent or divalent aromatic hydrocarbon groups having 6 to 20 carbon atoms.

[0022] The urea compound used as the urea-based thickener can be synthesized using an amine compound and an isocyanate compound. Examples of the amine compound include aliphatic amines such as hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine (stearylamine), behenylamine, oleylamine, etc., alicyclic amines such as cyclohexylamine, etc., and aromatic amines such as aniline, p-toluidine, ethoxyphenylamine, etc. Two or more of these amine compounds can be used in combination when synthesizing the urea compound. As the isocyanate compound, aromatic diisocyanates such as phenylene diisocyanate, tolylene diisocyanate (TDI), diphenyl diisocyanate, diphenylmethane diisocyanate (MDI), dimethylbiphenyl diisocyanate (TODI), etc., and aliphatic diisocyanates such as octadecane diisocyanate, decane diisocyanate, hexane diisocyanate, etc. can be used. In addition, when an aromatic diurea compound obtained by using an aromatic monoamine and an aromatic diisocyanate as the amine raw material is used as a urea-based thickener, there is a risk of abnormal noise being generated, so its use must be considered.

[0023] The urea-based thickener (urea compound) can be blended in an amount of, for example, 10 to 20% by mass based on the total amount of the grease composition used in the present invention.

[0024] <Ionic liquid> The grease composition applied to the rolling bearing according to this embodiment contains an ionic liquid containing a borate anion. Conventionally, grease compositions and lubricants have been made conductive as necessary to dissipate static electricity that is generated between parts due to rotational friction, and the addition of ionic liquids has been considered as one method for achieving this. In the present invention, the use of a specific ionic liquid containing a borate anion can contribute to suppressing the occurrence of electrolytic corrosion in rolling bearings.

[0025] The ionic liquid is not particularly limited as long as it contains a borate anion. As an example, the ionic liquid may have, as a cationic component, at least one type of cation selected from the group consisting of a tetraalkylphosphonium cation represented by formula (A) and a tetraalkylammonium cation represented by formula (B) described below, and, as an anionic component, at least one type of anion selected from the group consisting of a borate anion represented by formula (C-1), a borate anion represented by formula (C-2), and a borate anion represented by formula (C-3) described below.

[0026] <Cation> The tetraalkylphosphonium cation used in the ionic liquid according to the present invention is represented by formula (A). [ka] In the above formula (A), R 4 , R 5 , R 6 , and R 7 each independently represents a linear or branched alkyl group having 1 to 18 carbon atoms. Preferably, R 4 , R 5 , R 6 , and R 7 each independently represents a linear or branched alkyl group having 4 to 18 carbon atoms.

[0027] Examples of the alkyl group having 1 to 18 carbon atoms in the above formula (A) include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, and an octadecyl group.

[0028] R in the above formula (A) 4 , R 5 , R 6 , and R 7 For example, the combination of R 4is a linear or branched alkyl group having 11 to 18 carbon atoms; R 5 ~R 7 are each independently a linear or branched alkyl group having 4 to 10 carbon atoms, and R 4 is a linear or branched alkyl group having 12 to 16 carbon atoms; R 5 ~R 7 are each independently a linear or branched alkyl group having 4 to 8 carbon atoms, or 4 ~R 7 are each independently a linear or branched alkyl group having 6 to 12 carbon atoms. In addition, R in the formula (A) 4 , R 5 , R 6 , and R 7 The total number of carbon atoms may be, for example, 32. The tetraalkylphosphonium cation represented by formula (A) is, for example, R 1 is a tetradecyl group and R 2 ~R 4 is a hexyl group, and the like.

[0029] The tetraalkylammonium cation is represented by formula (B). [ka] In the above formula (B), R 8 , R 9 , R 10 , and R 11 each independently represents a linear or branched alkyl group having 1 to 18 carbon atoms. Preferably, R 8 , R 9 , R 10 , and R 11 each independently represents a linear or branched alkyl group having 5 to 18 carbon atoms.

[0030] Examples of the alkyl group having 1 to 18 carbon atoms in the above formula (B) include a methyl group, an ethyl group, an n-propyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, and an octadecyl group.

[0031] R in the above formula (B) 8 , R 9 , R 10 , and R 11 For example, the combination of R 8 is a linear or branched alkyl group having 1 to 4 carbon atoms; R 9 ~R 11 are each independently a linear or branched alkyl group having 6 to 14 carbon atoms; 8 is a linear or branched alkyl group having 11 to 16 carbon atoms; R 9 ~R 11 are each independently a linear or branched alkyl group having 6 to 10 carbon atoms; or 8 ~R 11 are each independently a linear or branched alkyl group having 6 to 12 carbon atoms. R in the above formula (B) 8 , R 9 , R 10 , and R 11 The total number of carbon atoms may be, for example, 24 to 40. The tetraalkylammonium cation represented by formula (B) is, for example, R 8 ~R 11 R is a tetrahexylammonium cation, where R is a hexyl group. 8 is a methyl group and R 9 ~R 11 is the methyltri(octyl)ammonium cation, R 8 is a tetradecyl group and R 9 ~R 11is a hexyl group, (tetradecyl)tri(hexyl)ammonium cation, R 8 ~R 11 is the tetraoctylammonium cation, R is the octyl group 8 ~R 11 is a decyl group, and the like.

[0032] <Anion> The anion used in the ionic liquid according to the present invention can be, for example, a borate anion selected from the group consisting of a borate anion represented by formula (C-1), a borate anion represented by formula (C-2), and a borate anion represented by formula (C-3). [ka] In the above formula (C-1), R 12 and R 14 each independently represents a linear or branched alkyl group having 1 to 22 carbon atoms or an aryl group having 6 to 10 carbon atoms; R 13 and R 15 each independently represents a hydrogen atom, a linear or branched alkyl group having 1 to 22 carbon atoms, or an aryl group having 6 to 10 carbon atoms.

[0033] Above R 12 , R 13 , R 14 and R 15 Examples of the alkyl group having 1 to 22 carbon atoms in the above formula (I) include a methyl group, an ethyl group, an n-propyl group, an isopropyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, an undecyl group, a dodecyl group, a tridecyl group, a tetradecyl group, a pentadecyl group, a hexadecyl group, a heptadecyl group, an octadecyl group, a nonadecyl group, an eicosyl group, a heneicosyl group, and a docosyl group. Above R 12 , R 13 , R 14 and R 15Examples of the aryl group having 6 to 10 carbon atoms in the above formula include a phenyl group, a naphthyl group, and the like. R in the above formula (C-1) 12 , R 13 , R 14 and R 15 For example, the combination of R 12 ~R 15 are each independently a linear or branched alkyl group having 1 to 6 carbon atoms; or 12 ~R 15 are all methyl groups, or R 12 and R 14 are each independently a linear or branched alkyl group having 1 to 6 carbon atoms; R 13 and R 15 is a hydrogen atom, or R 12 and R 14 are each independently an aryl group having 6 to 10 carbon atoms; R 13 and R 15 is a hydrogen atom, etc.

[0034] The ionic liquid used in the present invention may be, for example, any of the following combinations of cations and anions (i) to (v). [ka]

[0035] The ionic liquid can be blended in an amount of, for example, 0.1 to 10 mass % relative to the total amount of the grease composition used in the present invention.

[0036] <Other additives> The grease composition used in the present invention may contain additives that are usually used in grease compositions, if necessary, within the range that does not impair the effects of the present invention. Examples of such additives include antioxidants, extreme pressure agents (extreme pressure additives), metal deactivators, antifriction agents (antiwear agents), rust inhibitors, oiliness improvers, viscosity index improvers, and thickeners. When these other additives are contained, the amount (total amount) of them added is usually 0.1 to 10 mass % based on the total amount of the grease composition.

[0037] Examples of the antioxidant include octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,2-thiazine, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] ... hindered phenol-based antioxidants such as N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamic acid, etc.; other phenol-based antioxidants such as 2,6-di-t-butyl-4-methylphenol and 4,4-methylenebis(2,6-di-t-butylphenol); and amine-based antioxidants such as diphenylamine, alkylated diphenylamine, triphenylamine, hindered amine, phenyl-α-naphthylamine, alkylated phenyl-α-naphthylamine, phenothiazine, alkylated phenothiazine, etc.

[0038] Examples of the extreme pressure agent include phosphorus-based compounds such as phosphates, phosphites, and phosphate amine salts; sulfur-based compounds such as sulfides and disulfides; chlorine-based compounds such as chlorinated paraffin and chlorinated diphenyl; and metal salts of sulfur-based compounds such as zinc dialkyldithiophosphates and molybdenum dialkyldithiocarbamates.

[0039] Examples of the metal deactivator include benzotriazole-based compounds such as benzotriazole, 1-[N,N-bis(2-ethylhexyl)aminomethyl]-benzotriazole, and 1-[N,N-bis(2-ethylhexyl)aminomethyl]-4-methylbenzotriazole; thiadiazole-based compounds such as thiadiazole, 2-mercaptothiadiazole, and 2,5-bis(alkyldithio)-1,3,4-thiadiazole; benzimidazole-based compounds such as benzimidazole, 2-mercaptobenzimidazole, and 2-(decyldithio)-benzimidazole; and sodium nitrite.

[0040] Examples of the antifriction agent (antiwear agent) include tricresyl phosphate and polymer ester. Examples of the polymer ester include esters of aliphatic monovalent carboxylic acids and divalent carboxylic acids with polyhydric alcohols. Specific examples of the polymer ester include, but are not limited to, the PRIOLUBE (registered trademark) series manufactured by Croda Japan.

[0041] The grease composition used in the present invention can be obtained by blending the above-mentioned aromatic ester base oil, a thickener, an ionic liquid containing a borate anion, and, if desired, other additives. In addition, for example, a urea-based grease (base grease) consisting of the aromatic ester-based base oil and the urea-based thickener can be blended with an ionic liquid containing a borate anion and, if desired, other additives to obtain a grease composition. Typically, the content of the thickener in the base grease is about 10 to 30 mass %, and for example, the content of the diurea compound (urea-based thickener) in the above-mentioned urea-based grease can be, for example, about 10 to 25 mass %, or about 10 to 20 mass %.

[0042] <Capacitance and loss tangent (tan δ) in dynamic viscoelasticity measurement> In addition to the above-mentioned composition, the grease composition used in the present invention has a capacitance and a loss tangent (tan δ) determined by dynamic viscoelasticity measurement that are within appropriate ranges, thereby making it possible to suppress the occurrence of electrolytic corrosion in rolling bearings. Specifically, the capacitance of the grease composition is 4.6 pF or more under the conditions of an electrode diameter of 10 mm, an interelectrode distance of 1 mm, and a frequency of 10 kHz, and can be, for example, 5.0 pF or more, 5.5 pF or more, or 6.0 pF or more. The upper limit of the capacitance is not particularly limited, and can be, for example, 4.6 pF to 15 pF, 4.6 pF to 10 pF, 4.6 pF to 8 pF, or 4.6 pF to 7 pF. Furthermore, in the grease composition used in the present invention, the loss tangent (tan δ), expressed as the ratio (G" / G') of the storage modulus (G') to the loss modulus (G") at 25°C measured under conditions of a film thickness of 0.5 mm, a shear strain of 1%, and a frequency of 100 Hz, is 0.7 or more, and can be, for example, 0.75 or more. There is no particular upper limit to the loss tangent (tan δ), and it can be, for example, 0.7 or more and 1.5 or less, or can be, for example, 0.75 or more and 1.3 or less.

[0043] The present invention is not limited to the embodiments and specific examples described in this specification, and various changes and modifications are possible within the scope of the technical ideas described in the claims. EXAMPLES

[0044] The present invention will be described in more detail below with reference to examples, although the present invention is not limited thereto.

[0045] Grease compositions of Examples 1 to 16 were prepared in the amounts shown in Table 1 below, and various evaluations were carried out according to the procedures described below. In the following description, the example numbers of the grease compositions will also be treated as the example numbers of the evaluations of each test. The details and abbreviations of the components used in the preparation of the grease composition are as follows: <Base oil> Aromatic ester oil A: Mixture of trioctyl trimellitate (TOTM) and tetraoctyl pyromellitate (TOPM) [Kinematic viscosity at 40°C: 100mm 2 / s] Aromatic ester oil B: Alkyl ester of trimellitic acid (with 9, 10, 11 carbon atoms) [Kinematic viscosity at 40°C: 80 mm 2 / s] Aromatic ester oil C: Alkyl ester of trimellitic acid with 10 carbon atoms [Kinematic viscosity at 40°C: 55 mm 2 / s] Aromatic ester oil D: Trimellitic acid alkyl ester with 10 carbon atoms [Kinematic viscosity at 40°C: 52 mm 2 / s] Aromatic ester oil E: Alkyl ester of trimellitic acid (with 8, 9, 10 carbon atoms) [Kinematic viscosity at 40°C: 39 mm 2 / s] Polyalphaolefin (PAO) oil A [Kinematic viscosity at 40°C: 95mm 2 / s] Polyalphaolefin (PAO) oil B [Kinematic viscosity at 40°C: 48mm 2 / s] <Thickener> Diurea compound 1: Aliphatic-aromatic diurea compound (a diurea compound obtained by reacting a mixture of an aliphatic amine and an aromatic amine with a diisocyanate compound) Diurea compound 2: Alicyclic-aliphatic diurea compound (diurea compound obtained by reacting a mixture of alicyclic amine and aliphatic amine with a diisocyanate compound) <Additives> Ionic liquid: an ionic liquid consisting of a combination of anions and cations represented by the following formulas (i) to (v), (X), (Y), (Z-1), and (Z-2) [ka] [ka] Other additives: Extreme pressure additive: TPPT (triphenyl phosphorothioate) IRGALUBE TPPT manufactured by BASF Japan Ltd. Metal deactivator: Benzotriazole compound BT-LX manufactured by Johoku Chemical Co., Ltd. Antioxidant: Diarylamine antioxidant IRGANOX L57 manufactured by BASF Japan Ltd. Other additives were added such that the total amount of the above extreme pressure additives, metal deactivators, and antioxidants was 3 mass % in each grease composition (total mass: 100 mass %).

[0046] <(1)Electrical corrosion evaluation> An electrolytic corrosion test was carried out using the device shown in FIG. FIG. 3 is a schematic diagram of an accelerated electrolytic corrosion test device 50, and shows a test motor 51, a lower housing 52, a support bearing unit 53, a test bearing unit 54 (a ball bearing 54a to be tested, and a ball bearing 54b not to be tested), a metal shaft 55, a coupler 56, a resin coupler 57, and a pulse oscillator 58. In this device 50, to ensure that current does not flow anywhere other than the test ball bearing 54a, an insulating material (resin material) is used for the peripheral components of the test ball bearing 54a, and a ceramic ball bearing is used for the non-test ball bearing 54b, and the non-test ball bearing 54b is insulated from the test ball bearing 54a. Each grease composition was filled into a steel-loaded ball bearing 54a (inner diameter 3 mm, outer diameter 8 mm, width 3 mm) to be tested, at 25% to 35% of the bearing volume. This ball bearing 54a was set in a housing to form a test bearing unit 54, and a preload of 20 N was applied to the outer ring in the axial direction, after which a metal shaft 55 was inserted into the inner diameter of the bearing, and the metal shaft 55 was connected to the rotating shaft of a test motor 51 via a resin coupler 57 and a coupler 56, so that the ball bearing rotated around the inner ring. A low resistance slip ring (not shown) was brought into contact with the tip of the metal shaft 55, and a pulse generator 58 was connected between the slip ring and the outer ring housing of the test bearing unit 54, and a voltage of 10 V was applied. The test temperature was 25° C. and the rotation speed was 1,000 rpm for 100 hours. After 100 hours of rotation, the test ball bearing 54a was removed from the housing, disassembled, and visually inspected for the presence or absence of electrical pitting marks on the inner ring, outer ring, and balls. The results are shown in Table 1. <Judgment criteria> A: No electrical corrosion marks N: Electrical corrosion marks present

[0047] <(2) Capacitance [pF] Evaluation> The capacitance was measured using a TOYO Corporation SH2-Z type 4-terminal sample holder equipped with a guard electrode around the lower electrode, with each grease composition of the example sandwiched between an upper electrode of φ25 mm and a lower electrode of φ10 mm to a thickness of 1 mm, and the capacitance of each grease composition was measured using an impedance meter (NF Corporation, LCR meter ZW2371) at a frequency of 10 kHz, a voltage of 5 V, and room temperature. The results obtained are shown in Table 1.

[0048] <(3) Dynamic viscoelasticity: Loss tangent (tan δ) evaluation> Frequency dispersion measurements were performed using a stress-controlled rotational viscometer (rheometer, product name MCR302) manufactured by Anton Paar to measure the storage modulus (G') and loss modulus (G") of each grease composition and determine the loss tangent (tan δ). A test sample was sandwiched between the upper and lower plates of a jig, and the upper plate was set to a constant shear strain (1%). The frequency was gradually changed to evaluate the responsiveness through frequency-dependent measurements, and the mechanical relaxation time was evaluated from the temporal response of the network structure of the test sample. In detail, the grease composition to be measured was sandwiched between the plates under the test conditions of a jig: parallel plate φ25 mm (PP25), plate gap 0.5 mm, frequency 100 Hz, and temperature 25°C, and the storage modulus (G') and loss modulus (G") were measured. From the obtained storage modulus (G') and loss modulus (G"), the loss tangent tan δ was calculated using the following formula. Loss tangent tanδ=G” / G' A large loss tangent tan δ value can be considered to reflect the behavior of the substance (grease composition) becoming viscous and easily flowing into the lubrication area to form a sufficient oil film (churning), whereas a small value can be considered to reflect the behavior of the substance (grease composition) becoming elastic and being removed from the lubrication area over time (channeling).

[0049] [Table 1]

[0050] As shown in Table 1, it was confirmed that the grease compositions of Examples 1 to 7, which contain a base oil containing an aromatic ester-based base oil, a thickener, and an ionic liquid containing a borate anion, have a capacitance of 4.6 pF or more, and a dynamic viscoelasticity: loss tangent (tan δ) of 0.7 or more, suppressed the occurrence of electrolytic corrosion in the rolling bearings in which they were packed.

[0051] On the other hand, even when the same ionic liquid (iii) as in Examples 3 to 5 was used, the grease compositions of Examples 8 and 9, which used polyalphaolefin oil A or B as the base oil, had a capacitance of less than 4.6 pF, and the occurrence of electrolytic corrosion was confirmed. The grease compositions of Examples 10 and 11 had a dynamic viscoelasticity: loss tangent (tan δ) of 0.7 or more, but had a capacitance of less than 4.6 pF, and the occurrence of electrolytic corrosion was confirmed. The grease compositions of Examples 12 and 13, which used ionic liquid (Z-1) or (Z-2) containing anions other than borate anions, had a capacitance of 4.6 pF or more, and in the case of Example 13, the dynamic viscoelasticity: loss tangent (tan δ) was 0.7 or more, but the occurrence of electrolytic corrosion was confirmed in all cases. In addition, although the grease compositions of Examples 14 and 15 had a capacitance of 4.6 pF or more, the dynamic viscoelasticity: loss tangent (tan δ) was less than 0.7, and the occurrence of electrolytic corrosion was confirmed. In the case of the grease composition of Example 16, which did not contain an ionic liquid, the electrostatic capacitance was less than 4.6 pF, and the occurrence of electrolytic corrosion was confirmed.

[0052] Although the best embodiment has been described in detail above, the present invention is not limited to the above embodiment, and modifications and improvements within the scope of the present invention that can achieve the object of the present invention are included in the present invention. [Explanation of symbols]

[0053] 10... rolling bearing, 11... inner ring, 12... outer ring, 13... rolling element, 14... cage, 15... sealing member, 16... bearing space, 20...motor, 21...housing, 22...stator, 23...coil, 24...rotor magnet, 25...shaft, 26...bearing 50...electrical corrosion acceleration test device, 51...test motor, 52...lower housing, 53...support bearing unit, 54...test bearing unit (54a...ball bearing to be tested, 54b...ball bearing not to be tested), 55...metal shaft, 56...coupler, 57...resin coupler, 58...pulse generator

Claims

1. A rolling bearing, With the inner circle, an outer ring arranged coaxially with the inner ring on an outer circumferential side of the inner ring; A plurality of rolling elements disposed between the inner ring and the outer ring; A cage for holding the rolling elements; A grease composition is held between the inner ring and the outer ring, The grease composition comprises a base oil containing an aromatic ester-based base oil having a kinetic viscosity at 40° C. of 53 to 130 mm 2 / s, a urea-based thickener, and an ionic liquid containing a borate anion; The grease composition comprises: The capacitance is 4.6 pF or more under the conditions of an electrode diameter of 10 mm, an inter-electrode distance of 1 mm, and a frequency of 10 kHz, and In dynamic viscoelasticity measurement using a rotational rheometer, the loss tangent (tan δ) expressed as the ratio of storage modulus to loss modulus at 25° C. measured under conditions of a film thickness of 0.5 mm, a shear strain of 1%, and a frequency of 100 Hz is 0.7 or more, and The urea-based thickener contains a diurea compound represented by the following general formula (1): R 1 -NHCONH-R 2 -NHCONH-R 3 ... (1) (In the formula, R 1 and R 3 each independently represent a monovalent aliphatic hydrocarbon group, a monovalent alicyclic hydrocarbon group, or a monovalent aromatic hydrocarbon group, and at least one of R 1 and R 3 represents a monovalent aliphatic hydrocarbon group or a monovalent alicyclic hydrocarbon group; R 2 represents a divalent aromatic hydrocarbon group. The ionic liquid is at least one selected from the five combinations of anion and cation shown in the following (i) to (v), and the ionic liquid is contained in an amount of 0.1 to 10 mass % based on the total amount of the grease composition. Rolling bearing. 【Chemistry 1】

2. The R １ and R ３ one of which represents a monovalent aliphatic hydrocarbon group and the other represents a monovalent aromatic hydrocarbon group; 2. The rolling bearing according to claim 1.

3. A motor comprising the rolling bearing according to claim 1 or 2.

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