Rolling bearing

Abstract

Provided is a rolling bearing capable of suppressing ridge marks due to electrolytic corrosion. This rolling bearing 1 has an inner ring 2, an outer ring 3, and a plurality of balls 4 interposed between the inner ring 2 and the outer ring 3, and is lubricated with a grease composition 7, wherein: the oil film parameter Λ in a steady operation state is greater than 3; the grease composition 7 contains a base oil and a phosphorus-based additive; the phosphorus content is at least 0.2 mass% and less than 2.0 mass% with respect to the total amount of the grease composition 7; and the phosphorus-based additive is an aliphatic phosphoric acid ester having a linear or branched alkyl group.

Description

Rolling bearings

[0001] The present invention relates to a rolling bearing lubricated with a lubricant.

[0002] Rolling bearings are used in fan motors and servo motors used in industrial machinery, as well as in drive motors used in electric and hybrid vehicles. The rolling bearings used in these applications are filled with lubricant to provide lubrication, or are equipped with a mechanism that allows the lubricant to penetrate into the contact area.

[0003] In recent years, most industrial and traction motors are now inverter-controlled to achieve higher efficiency. Inverter control regulates the voltage and frequency input to the motor according to the motor's set rotation speed. As inverter switching frequencies increase, the frequency of shaft voltage increases. As a result, a potential difference can develop between the outer and inner rings and the rolling elements of rolling bearings installed in inverter-driven motors. If this potential difference grows large enough to exceed the breakdown voltage of the oil film formed between the bearing rings and rolling elements, discharges can occur between the bearing rings and rolling elements, causing damage known as electrolytic corrosion inside the bearing. Furthermore, increasing the power supply voltage allows for a smaller current for the same output, reducing copper loss in cables and inverter elements. However, the potential difference between the shaft potential and ground potential increases, making the oil film more susceptible to breakdown.

[0004] As damage to bearings due to this electrolytic corrosion progresses, striped irregularities called ridge marks form on the rolling surfaces of the outer and inner rings. These ridge marks can lead to bearing noise and vibration. For this reason, efforts are underway to develop bearings that are designed to suppress the formation of ridge marks on the rolling surfaces.

[0005] 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 also describes the provision of a conductive seal as a seal that seals off the end of the bearing space.

[0006] Patent No. 4599769 Patent No. 4177057

[0007] As described above, there are known techniques for imparting electrical conductivity to bearings. However, the conductive greases and conductive seals described in Patent Documents 1 and 2 generally have a volume resistivity of 10 ４ ~10 ８ Therefore, although it is effective in removing static electricity and preventing charging, when the shaft voltage of the motor caused by the inverter becomes a problem, it is difficult to suppress the generation of the shaft voltage due to the high resistance, and as a result, dielectric breakdown of the oil film caused by the lubricant in the rolling bearings can occur.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a rolling bearing that can suppress ridge marks caused by electrolytic corrosion.

[0009] The rolling bearing of the present invention has an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner ring and the outer ring, and is lubricated with a lubricant, wherein the rolling bearing has an oil film parameter Λ greater than 3 in a steady operating state, the lubricant contains a base oil and a phosphorus-based additive, and the phosphorus content of the total amount of the lubricant is 0.2 mass% or more but less than 2.0 mass%. Here, the steady operating state refers to a main operating state in which the rotational speed and load are generally stable.

[0010] The phosphorus-based additive is an aliphatic phosphate ester having a linear or branched alkyl group, and the aliphatic phosphate ester has three alkyl groups, each of which has 2 to 8 carbon atoms.

[0011] The base oil is at least one selected from synthetic hydrocarbon oils and ester oils, and the kinematic viscosity of the base oil at 40°C is 10 mm ２ / s~80mm ２ / s.

[0012] The phosphorus-based additive is an aliphatic phosphate ester having a linear or branched alkyl group, the aliphatic phosphate ester has three alkyl groups, each of which has 2 to 8 carbon atoms, the phosphorus content of the lubricant is 0.5 mass % or more and less than 2.0 mass % based on the total amount of the lubricant, and the lubricant does not contain a sulfur-based additive.

[0013] The lubricant is characterized in that it is a lubricating oil composition.

[0014] In the above rolling bearing, the maximum surface pressure in a steady operating state is 0.3 GPa to 3 GPa, and the rotation speed is 500 min －１ ~20,000 min －１ It is characterized in that:

[0015] The rolling bearing of the present invention is lubricated with a lubricant, and has an oil film parameter Λ greater than 3 under steady-state operating conditions. The lubricant contains a base oil and a phosphorus-based additive, and the phosphorus content of the total amount of the lubricant is 0.2 mass % or more and less than 2.0 mass %, and the coating derived from the phosphorus-based additive acts as an insulating coating, making it possible to suppress ridge marks due to electrolytic corrosion.

[0016] The phosphorus-based additive is an aliphatic phosphate ester having a linear or branched alkyl group, and further, the three alkyl groups of the aliphatic phosphate ester all have 2 to 8 carbon atoms, so that the insulating properties of the coating are high and ridge marks can be suitably suppressed.

[0017] In the above rolling bearing, the maximum surface pressure in a steady operating state is 0.3 GPa to 3 GPa, and the rotation speed is 500 min －１ ~20,000 min －１ Therefore, it can be suitably used for motor bearings where electrolytic corrosion is a concern.

[0018] FIG. 1 is a cross-sectional view of a deep groove ball bearing, which is an example of a rolling bearing of the present invention. FIG. 2 is a schematic cross-sectional view of a motor. FIG. 3 is a diagram showing the structures of the phosphorus-based additive and the sulfur-based additive used in the present examples. FIG. 4 is an observation photograph of the inner ring raceway surface of Examples 1 to 5. FIG. 5 is an observation photograph of the inner ring raceway surface of Comparative Examples 1 and 2. FIG. 6 is a diagram showing an outline of an Anderon test. FIG. 7 is a graph showing the results of an Anderon test. FIG. 7 is a diagram showing an outline of an SRV test machine. FIG. 8 is a graph showing the changes in friction coefficient and electrical resistance value in an SRV test. FIG. 9 is a graph showing electrical resistance value when each additive is used at each load.

[0019] Conventionally, in rolling bearings used under boundary lubrication conditions (oil film parameter Λ<1), such as high load conditions, extreme pressure agents are often blended into the lubricants of the rolling bearings. Examples of extreme pressure agents include phosphorus-based (P-based) additives and sulfur-based (S-based) additives. Extreme pressure agents undergo chemical changes due to frictional heat generated, particularly when localized metal contact occurs, under boundary lubrication conditions, forming a reactive protective film on the metal surface and exhibiting wear resistance.

[0020] The present inventors have conducted extensive research into rolling bearings that can suppress ridge marks caused by electrolytic corrosion, and as a result have surprisingly found that when a phosphorus-based additive, which has conventionally been used as an extreme pressure agent, is used under hydrodynamic lubrication conditions (oil film parameter Λ<3), an insulating coating is formed, making it possible to suppress ridge marks caused by electrolytic corrosion. The present invention is based on this finding.

[0021] The rolling bearing 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 arranged concentrically, with a plurality of rolling elements 4 arranged 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 a grease composition 7 is enclosed 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 acts as a lubricant by being interposed between them and the rolling elements 4 to provide lubrication.

[0022] 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.

[0023] In the rolling bearing of the present invention, the oil film parameter Λ in a steady operating state is preferably greater than 3. This oil film parameter Λ is calculated using the following formula (1). However, the symbol in the formula is h ｍｉｎ : oil film thickness [μm], σ1, σ2: root mean square roughness [μm] of two contacting surfaces (rolling contact surfaces of rolling element and raceway ring).

[0024] The oil film thickness can be calculated by a theoretical formula. When using a theoretical formula, it can be calculated using the Cittenden calculation formula represented by the following formula (2) using the data of the base oil and the test conditions.

[0025] h ｍｉｎ : Minimum oil film thickness U: Speed ​​parameter G: Material 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

[0026] In the above formula (2), 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 / Rｙ１ ) + (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 cylinder in the direction perpendicular to the flow. In addition, in the above equation (2), 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.

[0027] In the above formula (2), 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 １０It 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.

[0028] The rolling bearing of the present invention forms an oil film thicker than the surface roughness of the rolling elements and raceways under steady-state operating conditions. In the case of a metallic rolling bearing, under boundary lubrication conditions (oil film parameter Λ<1) or mixed lubrication conditions (1<oil film parameter Λ<3), contact between the rolling elements and raceways allows current to flow, making electrical discharge and electrolytic corrosion less likely to occur (see, for example, the bottom diagram in Figure 5). On the other hand, under hydrodynamic lubrication conditions (3<oil film parameter Λ), an oil film is present between the rolling elements and raceways. If the potential difference between the two becomes large enough to exceed the breakdown voltage of the oil film, electrical discharge may occur, potentially resulting in damage due to electrolytic corrosion (the occurrence of ridge marks).

[0029] In the present invention, the lubricant used to lubricate the rolling bearing contains a predetermined amount of phosphorus-based additive converted into phosphorus, so that an insulating coating is formed and the occurrence of ridge marks is suppressed.

[0030] The lubricant used to lubricate the rolling bearing of the present invention can be of two types: (A) a lubricating oil composition essentially containing a base oil and additives, and (B) a grease composition essentially containing a base oil, a thickener, and additives.

[0031] The phosphorus-based additives contained in the lubricant are additives containing phosphorus (P) in their molecular structure. Examples of phosphorus-based additives include phosphate esters, acid phosphate esters, phosphites, acid phosphites, thiophosphates, thiophosphites, zinc alkyldithiophosphates (ZnDTPs), and molybdenum alkyldithiophosphates (MoDTPs). These may be used alone or in combination.

[0032] Among the above, phosphate ester is preferred because it is easy to form an insulating coating. The phosphate ester is represented by the following formula (3): (R１ O) ｎ P(=O)H ３－ｎ In the above formula (3), n is 1 to 3, preferably 2 to 3, and more preferably 3. R １ are each independently an alkyl group or an aromatic group having 1 to 12 carbon atoms. For example, when n is 2 or 3, R １ may be the same or different. １ is, for example, a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, an octyl group, a phenyl group, a tolyl group, a tert-butylphenyl group, or a naphthyl group, and may be branched or may have a substituent containing an oxygen atom, a nitrogen atom, a fluorine atom, or the like.

[0033] R １ is preferably an aliphatic phosphate ester having a linear or branched alkyl group. Furthermore, since the electrical resistance of the insulating coating formed on the metal surface tends to be higher, it is more preferable that the aliphatic phosphate ester has three alkyl groups (n=3), and each of the alkyl groups has 2 to 8 carbon atoms. Examples of such phosphate esters include triethyl phosphate, tributyl phosphate, and trioctyl phosphate. The number of carbon atoms may be 2 to 6.

[0034] The lubricant has a phosphorus content of 0.2% by mass or more and less than 2.0% by mass based on the total amount of the lubricant. The phosphorus content may be 1.8% by mass or less, 1.5% by mass or less, 0.3% by mass or more, or 0.5% by mass or more. The phosphorus content is preferably derived solely from phosphorus-based additives contained in the lubricant.

[0035] Furthermore, it is preferable that the lubricant does not contain a sulfur-based additive (particularly a sulfur-based extreme pressure agent) as an additive. The sulfur-based additive is an additive containing sulfur (S) in the molecule, and examples of the sulfur-based extreme pressure agent include sulfide-based compounds.

[0036] The base oil used in the lubricant is not particularly limited as long as it is one that is typically used in rolling bearings. Examples include mineral oils such as paraffinic mineral oil and naphthenic mineral oil, synthetic hydrocarbon oils such as poly-α-olefin (PAO) oil and alkylbenzene oil, ester oil, ether oil, silicone oil, and fluorine oil. These oils may be used alone or in combination of two or more. Among these, at least one selected from synthetic hydrocarbon oils and ester oils is preferred, and it is more preferred to include at least a synthetic hydrocarbon oil.

[0037] 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.

[0038] Examples of the ester oil include polyol ester oil, phosphate ester oil, polymer ester oil, aromatic ester oil, carbonate ester oil, diester oil, and polyglycol oil.

[0039] The kinematic viscosity of the base oil at 40°C (in the case of a mixed oil, the kinematic viscosity of the mixed oil, the same applies hereinafter) is, for example, 10 mm ２ / s~80mm ２ / s, and 30 mm ２ / s~80mm ２ / s, and 50 mm ２ / s~80mm ２ The kinematic viscosity of the base oil at 100°C may be, for example, 5.0 mm ２ / s ~ 12 mm ２ / s.

[0040] When the above lubricant is used as a grease composition (form (B)), the lubricant further contains a thickener. There are no particular limitations on the thickener, and typically, a common thickener used in the field of rolling bearings 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, and examples of urea compounds and urea-urethane compounds include diurea compounds, triurea compounds, tetraurea compounds, other polyurea compounds, and diurethane compounds.

[0041] For example, a diurea compound can be 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.

[0042] In the above grease composition, the thickener is preferably contained in an amount of 10% by mass to 30% by mass, and more preferably 10% by mass to 20% by mass, relative to the total amount (100% by mass) of the base oil and thickener.

[0043] In the case of the above grease composition, the worked penetration (JIS K2220) is preferably in the range of 200 to 350. If the penetration is less than 200, oil separation is small and there is a risk of poor lubrication. On the other hand, if the penetration exceeds 350, the grease becomes soft and is likely to leak out of the bearing, which is undesirable.

[0044] The lubricant may further contain other additives within the scope of the present invention.

[0045] The rolling bearing of the present invention may have an oil film parameter Λ of 4 or more, or may be 5 or more, in a steady operating state. The larger the oil film parameter Λ, the thicker the oil film, making it more difficult for current to flow and making it easier to suppress the occurrence of ridge marks. On the other hand, if the oil film parameter Λ becomes too large, the bearing torque increases, which may increase, for example, the power consumption of the motor. The oil film parameter Λ is, for example, 10 or less, or may be 8 or less, or 6 or less.

[0046] In the rolling bearing of the present invention, the root mean square roughness of the raceway surfaces is preferably 0.01 μm, and the root mean square roughness of the surfaces of the rolling elements is preferably about 0.001 μm.

[0047] In the rolling bearing, the maximum surface pressure in a steady operating state is, for example, 0.3 GPa to 3 GPa, and may be 0.15 GPa to 4 GPa. In addition, the rotation speed in a steady operating state is, for example, 500 min －１ ~30,000 min －１ and 500 min －１ ~20,000 min －１ 1000 min －１ ~20,000 min －１ may be.

[0048] 1 shows a deep groove ball bearing as an example of the rolling bearing of the present invention, but the form of the rolling 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 self-aligning roller bearing, a needle roller bearing, a thrust cylindrical roller bearing, a thrust tapered roller bearing, a thrust needle roller bearing, or a thrust self-aligning roller bearing.

[0049] The rolling bearing of the present invention is suitable as a rolling bearing used in environments or 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), hybrid electric vehicles (HEVs), etc.

[0050] FIG. 2 shows an example of a rolling bearing supporting a motor rotor as one embodiment of the rolling bearing of the present invention. FIG. 2 is a schematic cross-sectional view of a motor using this bearing as an output motor bearing. As shown in FIG. 2, a motor 10 rotates a load by attaching a belt 18 to a pulley 17 that is linked to a main shaft 11. A rotor 13 is attached to the main shaft 11, and a pulley 17 is attached to one end of the rotor 13, to which a belt 18 that rotates an air conditioning fan or the like is attached. The main shaft 11 is rotatably supported on a flange 14 by a first radial ball bearing 15 and a second radial ball bearing 16 attached to both ends of the rotor 13. A stator 12 is fixed to the flange 14, facing the rotor 13. A wave spring washer is positioned between the flange 14 and the second radial ball bearing 16 to apply preload. The first radial ball bearing 15 and the second radial ball bearing 16 are rolling bearings of the present invention, and support the rotor 13 via the main shaft 11 .

[0051] FIG. 2 shows a state in which a method of applying preload using a spring is adopted, and a Belleville spring, a wave spring washer, etc. are positioned between the flange 14 and the second radial ball bearing 16 to apply preload.

[0052] The present invention will be specifically explained by way of examples and comparative examples, but is not limited to these examples in any way.

[0053] Lubricants were prepared with the compositions shown in Table 1. These lubricants were lubricating oil compositions consisting of base oils and additives. In Table 1, the numerical values ​​in the column for each additive indicate the mass % calculated as phosphorus (P) or sulfur (S) relative to the total amount of the lubricant.

[0054] The structural formulas of the phosphorus-based additive and sulfur-based additive used in this example are shown in Figure 3. For example, phosphate ester was used as the phosphorus-based additive, and more specifically, trioctyl phosphate was used as the branched alkyl additive, trimethyl phosphate and tributyl phosphate were used as the linear alkyl additive, and tricresyl phosphate was used as the aromatic additive.

[0055] [Electric corrosion test] Each lubricant was filled into a rolling bearing (inner ring, outer ring, and steel balls were made of bearing steel SUJ2) that supports the rotating shaft and simulates an industrial motor, and an operating test was carried out under the following conditions: combined load of 59 N, rotation speed of 1600 min －１ The operating conditions were set as above, and the test was carried out with a constant current of 5.0 A flowing through the test bearing (6206).

[0056] <Test conditions> Bearing: 6206T2X2CMLLU Rolling element: Steel ball (standard bearing) Power supply current: 5.0A Test machine rotation speed: 1600min －１ Radial load: 38.2N Axial load: 101N Oil film parameter Λ (35°C): 6.56 Test time: 7 hours Number of tests: 1 each

[0057] The oil film parameter Λ under the above test conditions was calculated using the above formula (1). The oil film thickness in the above formula (1) was calculated using the Cittenden formula, the base oil data, and the test conditions. In addition, the surface roughness (root mean square roughness) of the rolling element and the surface roughness (root mean square roughness) of the inner ring were measured using a stylus surface roughness tester for the rolling element and the inner ring before the test.

[0058] The surface quality of the inner ring raceway surface was then optically observed after 7 hours of testing. Photographs of the examples and comparative examples are shown in Figures 4 and 5. First, as shown in Comparative Example 1 in Figure 5, when no additive was added (PAO oil only), ridge marks (striped irregularities) were clearly formed on the inner ring raceway surface.

[0059] On the other hand, in the examples in which an additive was added (Comparative Example 2 and Examples 1 to 5), the suppression of ridge mark formation was evaluated in comparison with the ridge marks formed in Comparative Example 1. When the surface texture was observed, cases in which no ridge marks were formed were rated "A," cases in which ridge marks were formed but sufficient suppression of formation was confirmed were rated "B," and cases in which ridge marks were formed but little suppression of formation was confirmed were rated "C." The results are shown in Table 1.

[0060]

[0061] As shown in Figure 5, in Comparative Example 2, in which a sulfur-based additive was added, the ridge marks themselves were thinner than in Comparative Example 1, but their occurrence was not significantly suppressed. In contrast, as shown in Figure 4, in Examples 1 to 5, in which a phosphorus-based additive was added, ridge marks were not formed or their occurrence was sufficiently suppressed. It was found that the phosphorus-based additive is effective regardless of whether the additive is branched-chain alkyl, linear alkyl, or aromatic (Examples 1, 4, and 5). Furthermore, even when the base oil viscosity was increased and the oil film thickness was thickened, ridge marks were formed, but a sufficient suppression effect was observed (Example 3). Furthermore, a sufficient suppression effect was observed even when the concentration of the phosphorus-based additive was increased.

[0062] In the above examples, the above-mentioned inhibitory effect was confirmed using a lubricant based on a PAO-based synthetic oil, but a similar effect can also be obtained with mineral oil or ester oil.

[0063] When the same test was performed using the lubricant of Comparative Example 1 with an oil film parameter Λ of 1.6, no ridge marks were formed. This is thought to be because electricity was passed through the metal-to-metal contact between the rolling element and the inner ring (see the lower diagram in Figure 5).

[0064] [Anderon Test] The inner ring after the electrolytic corrosion test, a new outer ring, a crown-type cage, and ceramic rolling elements were prepared. They were ultrasonically cleaned using petroleum benzene and assembled so that the outer ring marking, inner ring marking, and the openings of the retaining claws of the crown-type cage were flush with each other (see Figure 6). 10 μL of PAO6 was dropped onto the raceway surface, and the inner ring 22 was gently rotated. The inner ring 22 was placed on the shaft of the Anderon measuring machine with the opening of the crown-type cage 25 facing forward. An axial load of 100 N was applied to the outer ring 23 of the rolling bearing 21, and the bearing was rotated at a speed of 1,800 rpm to measure the Anderon value. The frosted area of ​​the inner ring 22 is indicated by cross-hatching. One data point was obtained every 10 seconds of rotation, and the average of five samples was calculated for each band. The results are shown in Figure 7.

[0065] Since the Anderon H band (frequency 1800 Hz to 10000 Hz) is particularly useful for evaluating electrolytic corrosion, the calculated Anderon value was rated as "A" if it was less than 100, "B" if it was 100 or more but less than 200, and "C" if it was 200 or more.

[0066] As shown in Figure 7(a), it was found that the sulfur-based additives and phosphorus-based additives suppressed the increase in the anderon H-band. Furthermore, Example 3, which had a higher base oil viscosity, suppressed the increase in the anderon H-band compared to Example 1. Furthermore, among the phosphate esters, trioctyl phosphate (branched alkyl ester) suppressed the increase the most, followed by trimethyl phosphate (linear alkyl ester) and tricresyl phosphate (aromatic ester).

[0067] In the following, in order to consider the difference in the results between the phosphorus-based additives and the sulfur-based additives, the electrical resistance (insulation properties) of each coating formed under wear test conditions was evaluated.

[0068] The wear test was performed using an SRV tester. An outline of the SRV tester is shown in FIG. 8 . Using the SRV tester 31, an arm 33 holding a ball 32 was horizontally vibrated back and forth while a load F was applied to a disk 34 coated with a lubricant 35. The lubricant 35 used was a lubricating oil composition containing only PAO10 or PAO10 with various sulfur-based or phosphorus-based additives. The sulfur-based additive was added so that the total amount of the lubricant was 0.5% by mass in terms of sulfur, and the phosphorus-based additive was added so that the total amount of the lubricant was 0.2% by mass in terms of phosphorus. The test conditions were as follows:

[0069] <Test conditions> Temperature: 40°C Load: 50N / 5 min, up to 200N Initial surface pressure: 1.7GPa, 2.2GPa, 2.5GPa, 2.8GPa Frequency: 50Hz Amplitude: 1mm Test time: 20 minutes Ball: SUJ2 (φ9.525mm) Disk: SUJ2 (φ24mm)

[0070] Figure 9(a) shows the change in the coefficient of friction when tert-butyl disulfide is used as an additive. As shown in Figure 9(a), the coefficient of friction decreases as the load increases, indicating that tert-butyl disulfide forms a coating and exerts extreme pressure effects.

[0071] Figures 9(b) and (c) show the change in electrical resistance when each additive was used. It can be seen that the electrical resistance value changed at a higher value when using the phosphorus-based additive than when using the sulfur-based additive. Figure 10 shows a bar graph showing the average values ​​for the last minute at each load.

[0072] As shown in Figure 10, when only PAO oil was used, the electrical resistance decreased as the load increased. On the other hand, when a sulfur-based additive was added, the electrical resistance was low regardless of the load.

[0073] When phosphorus-based additives were added, the electrical resistance values ​​of all phosphate esters were higher than when sulfur-based additives were added. In particular, the three phosphate esters except trimethyl phosphate had high electrical resistance values, and the change in resistance due to load was small. Furthermore, the linear alkyl and branched alkyl phosphate esters had higher electrical resistance values ​​than the aromatic phosphate esters.

[0074] As described above, the addition of additives shows a different tendency from that observed when PAO oil is used alone, suggesting that these additives contribute to the early formation of a coating. Furthermore, it is believed that the difference in electrical resistance between the sulfur-based additives and the phosphorus-based additives affected the results of the ridge mark suppression test described above. In other words, it can be said that a coating based on phosphorus-based additives has high insulating properties (non-conductive), while a coating based on sulfur-based additives has low insulating properties (conductive).

[0075] From the results of the SRV test, it is believed that the film formed by the phosphorus-based extreme pressure agent is an insulating film. Furthermore, when the oil film parameter is greater than 3, extreme pressure agents generally do not work, but because the discharge at the time of insulation breakdown generates high heat of several thousand joules, the heat generated during discharge raises the rolling surface to the temperature at which the extreme pressure agent acts, forming an insulating film, which is thought to have suppressed the occurrence of ridge marks.

[0076] The rolling bearing of the present invention is capable of suppressing ridge marks caused by electrolytic corrosion and is therefore suitable for use in environments where electrolytic corrosion is likely to occur, such as rolling bearings used in motors for automotive accessories and industrial machinery, and rolling bearings used in motor drive devices for electric and hybrid vehicles.

[0077] REFERENCE SIGNS LIST 1 Rolling bearing 2 Inner ring 3 Outer ring 4 Ball (rolling element) 5 Cage 6 Seal member 7 Grease composition (lubricant) 8a Opening 8b Opening 10 Motor 11 Main shaft 12 Stator 13 Rotor 14 Flange 15 First radial ball bearing 16 Second radial ball bearing 17 Pulley 18 Belt 21 Rolling bearing 22 Inner ring 23 Outer ring 24 Rolling element 25 Crown cage 31 SRV tester 32 Ball 33 Arm 34 Disk 35 Lubricant

Claims

1. A rolling bearing having an inner ring, an outer ring, and a plurality of rolling elements interposed between the inner ring and the outer ring, and lubricated with a lubricant, wherein the rolling bearing has an oil film parameter Λ greater than 3 under steady-state operating conditions, the lubricant contains a base oil and a phosphorus-based additive, and the phosphorus content of the total amount of the lubricant is 0.2 mass% or more but less than 2.0 mass%.

2. The rolling bearing according to claim 1, wherein the phosphorus-based additive is an aliphatic phosphate ester having a linear or branched alkyl group.

3. A rolling bearing according to claim 2, wherein said aliphatic phosphate ester has three alkyl groups, each of which has 2 to 8 carbon atoms.

4. The base oil is at least one selected from synthetic hydrocarbon oils and ester oils, and the kinematic viscosity of the base oil at 40°C is 10 mm ２ / s~80mm ２ 2. The rolling bearing according to claim 1, wherein the rolling bearing has a rolling resistance of 1 / s.

5. A rolling bearing according to claim 1, characterized in that the phosphorus-based additive is an aliphatic phosphate ester having a linear or branched alkyl group, the aliphatic phosphate ester having three alkyl groups, each of which has 2 to 8 carbon atoms, the phosphorus content of the total amount of the lubricant being 0.5 mass % or more but less than 2.0 mass %, and the lubricant does not contain any sulfur-based additives.

6. The rolling bearing according to claim 1, wherein the lubricant is a lubricating oil composition.

7. In the rolling bearing, the maximum surface pressure in a steady operating state is 0.3 GPa to 3 GPa, and the bearing rotation speed is 500 min －１ ~20,000 min －１ 2. The rolling bearing according to claim 1, wherein:

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