Rolling bearing

Abstract

[PROBLEM] The purpose of the present invention is to provide a rolling bearing in which electrolytic corrosion can be suppressed, and a motor incorporating the bearing. [SOLUTION] Provided are a rolling bearing and a motor including the rolling bearing. The rolling bearing comprises: an inner ring; an outer ring arranged coaxially with and on the outer peripheral side of the inner ring; a plurality of rolling elements arranged between the inner ring and the outer ring; a cage for holding the rolling elements; and a grease composition held between the inner ring and the outer ring. The grease composition contains base oils, including an aromatic ester-based base oil, a thickener, and an ionic liquid containing borate anions, and has an electrostatic capacity of at least 4.6 pF or more under conditions of an electrode diameter of 10 mm, an inter-electrode distance of 1 mm, and a frequency of 10 kHz, or a loss tangent (tanδ), expressed as the ratio between the storage modulus and loss modulus at 25°C measured under the conditions of a film thickness of 0.5 mm, a shear strain of 1%, and a frequency of 100 Hz in a dynamic viscoelasticity measurement using a rotary rheometer, of at least 0.7.

Description

Rolling bearings

[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.

[0002] Ionic liquids are salts in a liquid state composed only of ions (anions and cations). Ionic liquids possess characteristics such as low vapor pressure (non-volatility), high thermal stability, flame retardancy, low viscosity, and high ionic conductivity. Furthermore, various physical properties can be designed by combining cations and anions. Therefore, they are expected to be applied in a variety of technical fields, including electrolytes and solvents. Due to these characteristics, ionic liquids are also being considered for use in lubricants and greases. For example, a lubricant composition containing an ionic liquid has been proposed with the aim of maintaining low friction for extended periods under high load conditions (Patent Document 1). Another example of a grease composition with improved low friction is one that uses an ionic liquid as the grease base oil and combines it with a non-soap thickener (Patent Document 2).

[0003] JP 2019-065256 A JP 2019-123846 A

[0004] For example, in rolling bearings used in electric motors, 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 surfaces of the rolling elements and raceways of the rolling bearing. Such damage to the rolling elements and raceways can lead to deterioration of the rolling bearing and even to its shutdown, so measures to prevent this are required.

[0005] The present invention aims to provide a rolling bearing that is filled with a grease composition containing an ionic liquid and that can suppress electrolytic corrosion, and to provide a motor that incorporates such a bearing.

[0006] One aspect of the present invention relates to a rolling bearing comprising: an inner ring; an outer ring arranged coaxially with the inner ring and outer peripheral side of the inner ring; a plurality of rolling elements arranged between the inner ring and the outer ring; a cage for holding the rolling elements; and a grease composition held between the inner ring and the outer ring, the grease composition comprising a base oil containing an aromatic ester base oil, a thickener, and an ionic liquid containing borate anions, the grease composition having 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, and 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. The present invention also relates to a motor equipped with the rolling bearing.

[0007] The present invention will be described in detail below with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram illustrating an example of the structure of a rolling bearing according to the present invention; Fig. 2 is a schematic diagram illustrating an example of the structure of a motor according to the present invention;

[0008] As mentioned above, in rolling bearings used in motors and the like, electric current flows within the bearing, generating sparks on the rolling elements (balls) and raceways, causing damage (electrolytic corrosion) to these surfaces. In the early stages, damage caused by these sparks is limited to small spots (holes) on the surface, but as the damage progresses, corrugated irregularities (washboard-like electrolytic corrosion marks) appear, causing abnormal noise and vibration, and even bearing deterioration (stoppage). There are two approaches to dealing with this electrolytic corrosion: energizing the bearing and insulating it. The former involves the use of conductive grease, for example, while the latter involves the use of ceramic bearings, which do not conduct electricity. However, ceramic bearings have issues such as their high cost, limiting their application.

[0009] In response to the challenge of suppressing electrolytic corrosion in rolling bearings, the present inventors investigated greases formulated with ionic liquids that function as conductivity-imparting agents and found that combining the ionic liquid with other grease compositions, particularly base oils, is effective in suppressing electrolytic corrosion. Further investigations led to the first discovery that grease compositions incorporating aromatic ester-based base oils and ionic liquids containing borate anions can suppress electrolytic corrosion in rolling bearings when the grease composition has a capacitance equal to or greater than a certain value and a loss tangent (tan δ) of the grease composition determined by dynamic viscoelasticity measurement equal to or greater than a certain value. While capacitance represents the electrical properties of a grease and is therefore thought to be an indicator of the occurrence of electrolytic corrosion, there have been no reports to date on the threshold value of electrolytic capacitance that leads to the occurrence of electrolytic corrosion. Furthermore, the dynamic viscoelasticity of a grease is an index of the grease's flow characteristics and shape stability, and can be considered an effective parameter for understanding, for example, the behavior of the grease inside a rolling bearing during rotation (e.g., the ease of adhesion to rolling elements (balls) (easily adheres: churning type, difficult to adhere: channeling type)). However, no reports have been published to date that examine the dynamic viscoelasticity of grease in conjunction with its composition and capacitance from the perspective of the occurrence and suppression of electrolytic corrosion. As shown in the results of the Examples described below, both the grease composition (base oil, ionic liquid) and the grease's physical properties (capacitance, dynamic viscoelasticity) are important for suppressing electrolytic corrosion in rolling bearings. This point is demonstrated by the results showing that even greases with the desired capacitance and dynamic viscoelasticity caused electrolytic corrosion when the grease deviated from a specific composition. The present invention will now be described in detail.

[0010] [Rolling Bearing] 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] FIG. 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 the same basic structure as conventional rolling bearings, and includes an annular inner ring 11, an outer ring 12, multiple rolling elements 13, a cage 14, and a seal member 15. The inner ring 11 is a cylindrical structure that is installed coaxially with the center axis of a shaft (not shown) on the outer periphery of the shaft. The outer ring 12 is a cylindrical structure that is installed coaxially with the inner ring 11 on the outer periphery of the inner ring 11. Each of the multiple rolling elements 13 is a ball that is arranged in a raceway within an annular bearing space 16 formed between the inner ring 11 and the outer ring 12. In other words, the rolling bearing 10 according to this embodiment is a ball bearing. The cage 14 is arranged in the raceway and holds the multiple rolling elements 13. The cage 14 is an annular body installed coaxially with the central axis of the shaft. It has a structure in which it has multiple pockets on one side in the direction of the central axis for holding the rolling elements 13, with the rolling elements 13 housed in each pocket. The cage 14 holds the rolling elements 13 at predetermined intervals circumferentially around the inner ring 11 and the outer ring 12, thereby preventing the rolling elements 13 from falling off and preventing adjacent rolling elements 13 from coming into contact with each other. The shape (crown-shaped, corrugated, etc.) and material (steel plate, resin, etc.) of the cage 14 are optional 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. The grease composition described below is used as the grease composition G. The amount of grease G enclosed within the bearing space 16 can be, for example, 5 to 50% of its volume. The sealing member 15 is formed, for example, from steel plate or rubber, and examples include a steel plate shield that does not contact the outer periphery of the inner ring 11, and a non-contact rubber seal that does not contact the outer periphery of the inner ring 11. Either the steel plate shield or the non-contact rubber seal can be used in the present invention. Note that while the drawing shows an embodiment that includes the sealing member 15, the rolling bearing of the present invention also covers embodiments of a rolling bearing that does not include a sealing 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 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 Figure 1, the grease composition G sealed in the rolling bearing 10 lubricates the gap between the rolling elements 13 and the inner ring 11 or outer ring 12 when the rolling bearing 10 rotates.

[0012] The rolling bearing of the present invention is not particularly limited in size or conditions of use, 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, etc. The rolling bearing of the present invention can be used as a rolling bearing for motors (for example, fan motors and 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 one embodiment of the present invention taken along the shaft direction. Motor 20 has the same basic structure as conventional motors, and is composed of a housing 21, a stator 22, coils 23, a rotor magnet 24, a shaft 25, and a rolling bearing 26 that supports shaft 25. Motor 20 generates magnetic force by passing current supplied from a power source (not shown) via a drive circuit through coil 23 wound around stator 22, which rotates rotor magnet 24, and the rotation is transmitted to an external rotating body via 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 enclosed in the rolling bearing of the present invention will be described below.

[0016] <Base Oil> In the grease composition filled 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, ditridecyl phthalate; C-8 trialkyl esters of trimellitic acid such as trioctyl trimellitate and tri-2-ethylhexyl trimellitate; C-9 trialkyl esters of trimellitic acid such as trinonyl trimellitate; C-10 trialkyl esters of trimellitic acid such as tridecyl trimellitate; C-11 trialkyl esters of trimellitic acid; and C-16 trialkyl esters of trimellitic acid (the alkyl chains of the three alkyl ester groups in one compound may be the same or different, for example, "(C-atom number x, y, z) alkyl ester of trimellitic acid" refers to a combination of an alkyl ester of trimellitic acid having x carbon atoms and an alkyl ester of trimellitic acid having y carbon atoms). Examples of such an alkyl 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 an alkyl ester 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 carbon atoms having z carbon atoms (in which the 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 atoms having y carbon atoms and an alkyl of carbon atoms having z carbon atoms (in which the three alkyl ester groups in one compound are not the same); tetraoctyl pyromellitate, tetra-2-ethylhexyl pyromellitate, and mixtures of two or more thereof.

[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 to 130 mm ２ / s range, e.g., 70 to 130 mm ２ / s range, or for example 70-100 mm ２ The aromatic ester base oil having a kinematic viscosity in the above-mentioned range is not particularly limited, but examples thereof include the above-mentioned aromatic ester compounds having an ester group as a substituent on the ring, 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 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 may be contained in a proportion of, for example, 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 urea-based thickeners can be preferably used. Urea compounds have excellent heat resistance and water resistance, and are particularly stable at high temperatures, so they are preferably used as thickeners in applications under high-temperature environments. Urea compounds such as diurea compounds, triurea compounds, and polyurea compounds can be used as urea-based thickeners. Conventionally known urea compounds can be used as these urea-based thickeners.

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

[0021] Examples of the monovalent aliphatic hydrocarbon groups include linear or branched, saturated or unsaturated alkyl groups having 6 to 26 carbon atoms. Examples of the monovalent alicyclic hydrocarbon groups include cycloalkyl groups having 5 to 12 carbon atoms. Examples of the aromatic hydrocarbon groups include monovalent or divalent aromatic hydrocarbon groups having 6 to 20 carbon atoms.

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

[0023] The urea-based thickener (urea compound) can be blended in an amount of, for example, 10 to 20 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 borate anions. Conventionally, grease compositions and lubricants have been imparted with electrical conductivity as needed to dissipate static electricity generated between parts due to rotational friction, and the addition of an ionic liquid has been considered as one method for achieving this. In the present invention, the use of a specific ionic liquid containing borate anions 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). In the above formula (A), R ４ , R ５ , R ６ , and R ７ are each independently a linear or branched alkyl group having 1 to 18 carbon atoms. 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 As a combination of, for example, R ４ is a linear or branched alkyl group having 11 to 18 carbon atoms, and R ５ ~R ７are 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, and 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. ４ , R ５ , R ６ , and R ７ The total number of carbon atoms in the formula (A) can be, for example, 32. The tetraalkylphosphonium cation represented by formula (A) can be, 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). In the above formula (B), R ８ , R ９ , R １０ , and R １１ are each independently a linear or branched alkyl group having 1 to 18 carbon atoms. 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 As a combination of, for example, R ８ is a linear or branched alkyl group having 1 to 4 carbon atoms, and 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, and 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. ８ , R ９ , R １０ , and R １１ The total number of carbon atoms in the formula (B) can be, for example, 24 to 40. The tetraalkylammonium cation represented by formula (B) can be, for example, R ８ ~R １１ is a hexyl group, a tetrahexylammonium cation, R ８ is a methyl group and R ９ ~R １１ is an octyl group, methyltri(octyl)ammonium cation, R ８ is a tetradecyl group and R ９ ~R １１ is a hexyl group, (tetradecyl)tri(hexyl)ammonium cation, R ８ ~R １１ is an octyl group, a tetraoctylammonium cation, R 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). 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] The above R １２ , R １３ , R １４ and R １５ Examples of the alkyl group having 1 to 22 carbon atoms in the above R 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. １２ , R １３ , R １４ and R １５ Examples of the aryl group having 6 to 10 carbon atoms in the formula (C-1) include a phenyl group and a naphthyl group. １２ , R １３ , R １４ and R １５ As a combination of, for example, 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 １２ and R １４are 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, a combination of cations and anions shown in the following (i) to (v).

[0035] The ionic liquid may 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 can contain additives commonly used in grease compositions as needed, as long as the effects of the present invention are not impaired. 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, thickeners, etc. When these other additives are contained, the amount added (total amount) is usually 0.1 to 10 mass% of 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-thiazolinone, 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], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl) hindered phenol-based antioxidants such as N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxyphenyl)propionate and N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamic acid; 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, and alkylated phenothiazine.

[0038] Examples of extreme pressure agents include phosphorus compounds such as phosphate esters, phosphites, and phosphate amine salts; sulfur compounds such as sulfides and disulfides; chlorine compounds such as chlorinated paraffin and chlorinated diphenyl; and metal salts of sulfur compounds such as zinc dialkyldithiophosphate and molybdenum dialkyldithiocarbamate.

[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 esters. Examples of the polymer esters include esters of aliphatic monocarboxylic acids and dicarboxylic acids with polyhydric alcohols. Specific examples of the polymer esters 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 aromatic ester-based base oil described above with a thickener, an ionic liquid containing borate anions, and, if desired, other additives. Alternatively, a grease composition can be obtained by blending a urea-based grease (base grease) consisting of the aromatic ester-based base oil and the urea-based thickener with an ionic liquid containing borate anions and, if desired, other additives. Typically, the content of the thickener relative to the base grease is about 10 to 30% by mass. For example, the content of the diurea compound (urea-based thickener) relative to the above-mentioned urea-based grease can be, for example, about 10 to 25% by mass, or about 10 to 20% by mass.

[0042] <Capacitance and Loss Tangent (tan δ) in Dynamic Viscoelasticity Measurement> In addition to the above-described composition, the grease composition used in the present invention has an appropriate range for the capacitance and loss tangent (tan δ) determined by dynamic viscoelasticity measurement, thereby suppressing 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 electrode 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 (tanδ), expressed as the ratio (G" / G') of the storage modulus (G') to the loss modulus (G") at 25°C measured in dynamic viscoelasticity measurement using a rotational rheometer 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. The upper limit of the loss tangent (tanδ) is not particularly limited, but 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 idea described in the claims.

[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 using the blending amounts shown in Table 1 below, and various evaluations were carried out using the procedures described below. In the following explanation, the example numbers of the grease compositions will also be used as the example numbers for the evaluation of each test. Details of the components used in preparing the grease compositions and their abbreviations are as follows: <Base oil> Aromatic ester oil A: Mixed oil of trioctyl trimellitate (TOTM) and tetraoctyl pyromellitate (TOPM) [kinematic viscosity at 40°C: 100 mm ２ / s] Aromatic ester oil B: alkyl ester of trimellitic acid (having 9, 10, or 11 carbon atoms) [kinematic viscosity at 40°C: 80 mm ２ / s] Aromatic ester oil C: C10 alkyl ester of trimellitic acid [kinematic viscosity at 40 ° C: 55 mm ２ / s] Aromatic ester oil D: C10 alkyl ester of trimellitic acid [kinematic viscosity at 40 ° C: 52 mm ２ / s] Aromatic ester oil E: alkyl ester of trimellitic acid (having 8, 9, or 10 carbon atoms) [kinematic viscosity at 40°C: 39 mm ２ / s] Polyalphaolefin (PAO) oil A [kinematic viscosity at 40 ° C: 95 mm ２ / s] Polyalphaolefin (PAO) oil B [kinematic viscosity at 40 ° C: 48 mm 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 (a diurea compound obtained by reacting a mixture of an alicyclic amine and an aliphatic amine with a diisocyanate compound) <Additive> Ionic liquid: an ionic liquid consisting of a combination of an anion and a cation shown in the following formulas (i) to (v), (X), (Y), (Z-1), and (Z-2) 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-based antioxidant IRGANOX L57 manufactured by BASF Japan Ltd. Other additives were added so that the total amount of the extreme pressure additive, metal deactivator, and antioxidant was 3 mass % in each grease composition (total mass: 100 mass %).

[0046] <(1) Evaluation of Electrolytic Corrosion> An electrolytic corrosion test was conducted using the device shown in Figure 3. Figure 3 is a schematic diagram of an electrolytic corrosion acceleration test device 50, showing a test motor 51, a lower housing 52, a support bearing unit 53, a test bearing unit 54 (a test ball bearing 54a and a non-test ball bearing 54b), a metal shaft 55, a coupler 56, a resin coupler 57, and a pulse oscillator 58. In this device 50, to prevent current from flowing anywhere other than the test ball bearing 54a, insulating materials (resin materials) were used for the peripheral components of the test ball bearing 54a, and ceramic ball bearings were used for the non-test ball bearings 54b, which were insulated from the test ball bearing 54a. Each grease composition was filled into the test steel-tipped ball bearing 54a (inner diameter 3 mm, outer diameter 8 mm, width 3 mm) at 25% to 35% of the bearing volume. This ball bearing 54a was set in a housing to form a test bearing unit 54. An axial preload of 20 N was applied to the outer ring, and then a metal shaft 55 was inserted into the bearing bore. The metal shaft 55 was connected to the rotating shaft of the test motor 51 via a resin coupler 57 and a coupler 56, allowing the ball bearing to rotate around the inner ring. A low-resistance slip ring (not shown) was attached to 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, applying a voltage of 10 V. The test was run for 100 hours at a test temperature of 25°C and a rotational speed of 1,000 rpm. After 100 hours of rotation, the test ball bearing 54a was removed from the housing and disassembled. The inner and outer rings and balls were visually inspected for signs of electrolytic corrosion. The results are shown in Table 1. Evaluation Criteria: A: No signs of electrolytic corrosion; N: Signs of electrolytic corrosion.

[0047] <(2) Evaluation of Capacitance [pF]> The capacitance was measured using an SH2-Z type four-terminal sample holder manufactured by Toyo Corporation, which is equipped with a guard electrode around the lower electrode, and the grease composition of each example was sandwiched between an upper electrode φ25 mm and a lower electrode φ10 mm to a thickness of 1 mm, and the capacitance of each grease composition was measured using an impedance meter (LCR meter ZW2371 manufactured by NF Corporation) 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 measurement was 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 subjected to a constant shear strain (1%). The frequency was gradually changed to evaluate the responsiveness through frequency-dependent measurement, 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 test conditions of a jig: parallel plates 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. The loss tangent tan δ was calculated from the obtained storage modulus (G') and loss modulus (G") using the following formula: Loss tangent tan δ = G" / G' A large value of loss tangent tan δ 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), while a small value can be considered to reflect the behavior of the substance (grease composition) becoming elastic and being excluded from the lubrication area over time (channeling).

[0049]

[0050] As shown in Table 1, it was confirmed that the grease compositions of Examples 1 to 7, which contained a base oil containing an aromatic ester-based base oil, a thickener, and an ionic liquid containing borate anions, had a capacitance of 4.6 pF or more, and a dynamic viscoelasticity: loss tangent (tanδ) 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 using the same ionic liquid (iii) as in Examples 3 to 5, 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 electrolytic corrosion was observed. The grease compositions of Examples 10 and 11 had a dynamic viscoelasticity: loss tangent (tanδ) of 0.7 or more, but had a capacitance of less than 4.6 pF, and electrolytic corrosion was observed. The grease compositions of Examples 12 and 13, which used ionic liquids (Z-1) or (Z-2) containing anions other than borate anions, had a capacitance of 4.6 pF or more, and in Example 13, the dynamic viscoelasticity: loss tangent (tanδ) was 0.7 or more, but electrolytic corrosion was observed in both cases. Furthermore, the grease compositions of Examples 14 and 15 had a capacitance of 4.6 pF or more, but had a dynamic viscoelasticity: loss tangent (tanδ) of less than 0.7, and electrolytic corrosion was observed. The grease composition of Example 16, which did not contain an ionic liquid, had a capacitance of less than 4.6 pF, and electrolytic corrosion was confirmed.

[0052] The best mode for carrying out the invention has been described in detail above, but the invention is not limited to the above mode for carrying out the invention, and modifications and improvements within the scope of achieving the object of the invention are included in the invention.

[0053] DESCRIPTION OF SYMBOLS 10... rolling bearing, 11... inner ring, 12... outer ring, 13... rolling element, 14... cage, 15... seal member, 16... bearing space, 20... motor, 21... housing, 22... stator, 23... coil, 24... rotor magnet, 25... shaft, 26... bearing 50... electrolytic 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 comprising: an inner ring; an outer ring arranged coaxially with the inner ring on the outer peripheral side of the inner ring; a plurality of rolling elements arranged between the inner ring and the outer ring; a cage 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, 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, and wherein 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.

2. The base oil has a kinetic viscosity of 53 to 130 mm at 40°C. ２ 2. The rolling bearing according to claim 1 , comprising an aromatic ester-based base oil having a molecular weight of 1:1 or more; 3. The rolling bearing according to claim 1, wherein the thickener is a urea-based thickener, and the urea-based thickener contains a diurea compound represented by the following general formula (1): １ -NHCONH-R ２ -NHCONH-R ３ ...(1) (wherein, R １ and R ３ each 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; ２ represents a divalent aromatic hydrocarbon group.

4. The above R １ and R ３ 4. The rolling bearing according to claim 3, wherein one of said groups represents a monovalent aliphatic hydrocarbon group and the other represents a monovalent aromatic hydrogen group.

5. The rolling bearing according to claim 1, wherein the ionic liquid is at least one selected from the five combinations of anions and cations shown in (i) to (v) below.

6. A motor equipped with a rolling bearing according to any one of claims 1 to 5.

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