JP7642013B2 - Rolling bearings - Google Patents

Description

The present invention relates to a rolling bearing filled with a grease composition, and in particular to a rolling bearing for rotating electrical machines such as servo motors and encoders.

Servomotors are equipped with detectors for position and speed, and are capable of high-precision positioning and fine-grained control of rotational speed, so demand is increasing, mainly for use in machine tools and robots. Servomotors for robots are often used to drive joints, and so they must be small and have high output to increase the shaft speed and acceleration/deceleration of the joints. To reduce size, detectors and brakes are placed near the rolling bearings, but if some of the grease inside the bearings splashes onto the detectors or brakes, it will reduce detection accuracy and braking performance, so the grease must be low-dust generating.

Dust generation occurs when heat generated by the rotation of a rolling bearing and an increase in pressure inside the bearing cause some of the grease to evaporate or scatter and be released outside the bearing. In order to prevent dust generation, it is effective to increase the viscosity of the grease's base oil to increase the surface tension, or to reduce the amount of base oil in the grease. However, on the other hand, this can easily lead to insufficient supply (flow) of base oil to the rolling and sliding surfaces, which can cause excessive heating of the bearing and damage such as surface roughness and wear on the rolling and sliding surfaces.

Various grease compositions aimed at low dust generation have been studied in the past. For example, Patent Document 1 proposes a grease composition having a kinetic viscosity at 40°C of 30 ^mm2 /s to 180 ^mm2 /s, containing a base oil containing at least one oil selected from synthetic hydrocarbon oils and ether oils, a thickener containing a urea compound, and further containing an additive containing only nonmetallic elements in an amount of 0.1 to 1% by weight of the total amount of the grease.

Furthermore, Patent Document 2 describes that in a rolling bearing unit incorporated in a motor that rotates forward and backward, a rolling bearing disposed near an encoder is filled with a grease composition that contains at least one type of base oil selected from synthetic hydrocarbon oils and ether oils and has a kinetic viscosity at 40°C of 80 mm ² /s to 120 mm ² /s.

Japanese Patent Application Publication No. 11-166191 JP 2010-127344 A

In recent years, as motors have become smaller and more powerful, the environments in which their bearings are used are increasingly becoming hotter, and they are increasingly being used in environments that are unfavorable to dust generation. Even rolling bearings, which require low dust generation, are required to have durability, such as wear resistance and grease life. The grease composition described in Patent Document 1 is for rolling devices used in clean rooms in semiconductor manufacturing facilities, and dust generation and durability at high temperatures have not been considered, raising concerns about these points. In addition, the grease composition described in Patent Document 2 is for use in bearing units for motors that rotate forward and backward, and it is thought that simply specifying the base oil and its kinetic viscosity may be insufficient in terms of wear resistance and grease life.

The present invention was made in consideration of these circumstances, and aims to provide a rolling bearing that can achieve low dust generation while minimizing deterioration in wear resistance and lifespan.

The rolling bearing of the present invention has an inner ring, an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, sealing members provided at openings at both axial ends of the inner ring and the outer ring, and a grease composition sealed in the space within the bearing, the grease composition comprising a base oil and a thickener, the base oil being a mixed oil comprising a synthetic hydrocarbon oil and an ether oil and having a kinetic viscosity of 120 ^mm2 /s or more at 40°C, the thickener being an aliphatic diurea compound and an alicyclic diurea compound and being contained in an amount of 12 to 15 parts by mass per 100 parts by mass of the total amount of the base oil and the thickener, and the grease composition having a worked penetration of 200 to 240 as measured in accordance with JIS K 2220 and containing an organic acid metal salt that is solid at 20°C.

The organic acid metal salt is a sodium salt or a calcium salt. The organic acid metal salt is a metal salt of an organic carboxylic acid having 2 to 18 carbon atoms, and the grease composition does not contain any additives that contain sulfur and phosphorus in the molecular structure.

The grease composition further contains an amine-based antioxidant, and the amine-based antioxidant is N-phenyl-N'-isopropyl-p-phenylenediamine (PIPA).

The sealing member is characterized by being made of nitrile rubber containing a trimellitic ester plasticizer.

The rolling bearing is a bearing for a rotating electrical machine.

The rolling bearing of the present invention has a grease composition sealed in the space within the bearing, the base oil being a mixed oil containing a synthetic hydrocarbon oil and an ether oil and having a kinetic viscosity of 120 mm ² /s or more at 40° C., the thickener being an aliphatic diurea compound and an alicyclic diurea compound, being contained in an amount of 12 to 15 parts by mass per 100 parts by mass of the total amount of the base oil and the thickener, having a worked penetration of 200 to 240, and containing an organic acid metal salt that is solid at 20° C., so that low dust generation can be achieved while suppressing deterioration in wear resistance and life. Specifically, it is believed that by adding an organic acid metal salt that is solid at 20° C. to a combination of a specified base oil and a specified thickener, the surface tension of the base oil is not affected, and when the supply of base oil is insufficient and wear is a concern, the organic acid metal salt melts and can protect the steel surface as an extreme pressure agent.

The rolling bearing of the present invention is suitable for use in rotating electrical machines such as servo motors, and can contribute to the miniaturization and high output of such rotating electrical machines, leading to higher functionality.

In addition, the sealing member is made of nitrile rubber containing a trimellitate ester plasticizer, so by using a low-volatility trimellitate ester plasticizer as the plasticizer, evaporation of the plasticizer itself is suppressed, further improving the low dust generation properties of the bearing.

1 is a cross-sectional view of a deep groove ball bearing which is an example of a rolling bearing according to the present invention. 1 is a schematic diagram of a servomotor to which a rolling bearing according to the present invention is applied.

The grease composition used in the rolling bearing of the present invention is characterized in that it contains a base oil having a specified kinetic viscosity, a specified thickener, and an additive, and that the additive contains an organic acid metal salt that is solid at 20°C.

The base oil used in the grease composition is a mixed oil containing a synthetic hydrocarbon oil and an ether oil, and having a kinetic viscosity of 120 mm ² /s or more at 40° C. This kinetic viscosity is the kinetic viscosity of the mixed oil, and is preferably 120 mm ² /s to 160 mm ² /s at 40° C., and more preferably 125 mm ² /s to 140 mm ² /s.

As the synthetic hydrocarbon oil, poly-α-olefin oil (PAO oil) is more preferable. 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 of these are usually used.

Examples of ether oils include polyphenyl ether oil, alkyl diphenyl ether oil, alkyl triphenyl ether oil, and alkyl tetraphenyl ether oil. Among these, alkyl diphenyl ether oil is preferred from the viewpoint of durability at high temperatures, and it is more preferred that alkyl diphenyl ether oil accounts for 50 mass% or more of the entire base oil (mixed oil). Examples of alkyl diphenyl ether oils include monoalkyl diphenyl ether oil, dialkyl diphenyl ether oil, and polyalkyl diphenyl ether oil.

The base oil may be a mixture of synthetic hydrocarbon oil and ether oil alone, or may further contain other oils. As the other oil, any oil that is normally used in rolling bearings may be used without any particular restrictions. Examples include mineral oils such as paraffinic mineral oil and naphthenic mineral oil, ester oil, silicone oil, and fluorine oil.

The thickener used in the above grease composition is a mixture of an aliphatic diurea compound and an alicyclic diurea compound. These diurea compounds are obtained by reacting a diisocyanate component with a monoamine component. The aliphatic diurea compound is obtained from an aliphatic monoamine as the monoamine component and a diisocyanate component, and the alicyclic diurea compound is obtained from an alicyclic monoamine as the monoamine component and a diisocyanate component. Examples of the diisocyanate component include phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate, octadecane diisocyanate, decane diisocyanate, and hexane diisocyanate. Examples of the aliphatic monoamine include hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine, stearylamine, and oleylamine. Examples of the alicyclic monoamine include cyclohexylamine. The content of the aliphatic diurea compound in the thickener is preferably greater than the content of the alicyclic diurea compound.

A base grease is obtained by blending a diurea compound as a thickener with a base oil. A base grease using a diurea compound as a thickener is produced by reacting a diisocyanate component with a monoamine component in the base oil. The thickener is contained in an amount of 12 to 15 parts by mass per 100 parts by mass of the total amount of the base oil and thickener.

The organic acid metal salt used in the grease composition is a salt of a metal ion and an aliphatic or aromatic compound having a substituent, such as an organic carboxylic acid, an organic sulfonic acid such as alkylbenzenesulfonic acid, an organic sulfuric acid such as alkylsulfuric acid, an organic phosphonic acid such as alkylphosphonic acid, or an organic phosphinic acid such as alkylphosphinic acid. The organic acid metal salt is solid at 20°C, that is, it is sufficient that the melting point is above 20°C, and the organic acid and metal ion constituting the organic acid metal salt can be appropriately selected. The melting point of the organic acid metal salt is preferably 100°C or higher, and more preferably 150°C or higher.

The metal constituting the organic acid metal salt may be any metal capable of forming a salt with the organic acid, such as lithium, sodium, potassium, magnesium, calcium, barium, zinc, or aluminum. The metal salt is preferably a sodium salt or a calcium salt, and more preferably a sodium salt.

The organic acid constituting the organic acid metal salt may have a halogen group, a hydroxyl group, an amino group, an alkyl group, an alkoxy group, an epoxy group, or other substituent on the aliphatic or aromatic group in the molecular structure. The organic acid preferably does not contain a sulfur atom or a phosphorus atom in the molecular structure, and more preferably the constituent atoms are selected from carbon atoms, hydrogen atoms, nitrogen atoms, and oxygen atoms. Examples of such organic acids include organic carboxylic acids having 2 to 18 carbon atoms, such as acetic acid, propionic acid, butyric acid, hexanoic acid, lauric acid, myristic acid, palmitic acid, stearic acid, 12-hydroxystearic acid, oleic acid, and linoleic acid, among which organic carboxylic acids having 10 to 18 carbon atoms are preferred, and linear organic carboxylic acids having no substituent are more preferred.

Specific examples of organic acid metal salts that can be used include sodium acetate (melting point 324°C), calcium acetate (melting point 160°C), sodium stearate (melting point 220°C), and calcium stearate (melting point 179°C).

The amount of organic acid metal salt in the grease composition is preferably 0.5 parts by mass to 5 parts by mass, more preferably 0.5 parts by mass to 3 parts by mass, and even more preferably 1 part by mass to 3 parts by mass, per 100 parts by mass of the total amount of the base oil and thickener.

The grease composition preferably further contains an antioxidant as an additive. As the antioxidant, an amine-based antioxidant, a phenol-based antioxidant, a sulfur-based antioxidant, etc. can be used, and among these, it is preferable to use an amine-based antioxidant, such as phenyl-1-naphthylamine, diphenyl-p-phenylenediamine, dipyridylamine, phenothiazine, N,N'-diisopropyl-p-phenylenediamine, N-phenyl-N'-isopropyl-p-phenylenediamine, dialkyldiphenylamine (DDPA), etc.

In addition, other additives such as rust inhibitors and oily agents such as esters and alcohols can be added to the grease composition as necessary. For example, it is preferable to add a rust inhibitor to ensure the rust-preventing performance of the grease itself. The rust inhibitor is not particularly limited, but an ester-based rust inhibitor is preferable. Examples of ester-based rust inhibitors include partial esters of polyhydric alcohols such as sorbitan, sorbitol, pentaerythritol, sucrose, and glycerin with carboxylic acids such as oleic acid and lauric acid, and succinic acid half esters. It is more preferable that the amount of the ester-based rust inhibitor is 0.2 to 0.6 parts by mass per 100 parts by mass of the total amount of the base oil and thickener.

The grease composition preferably does not contain additives that contain phosphorus or sulfur atoms in the molecular structure. As shown in the examples below, the inclusion of these additives results in a decrease in the surface tension of the base oil, which tends to increase the amount of dust generated. Examples of such additives include phosphate esters such as tricresyl phosphate, phosphite esters such as tricresyl phosphite, thiophosphates, thiophosphites, zinc alkyldithiophosphates (ZnDTP), molybdenum alkyldithiophosphates (MoDTP), and zinc dithiocarbamate (ZnDTC) and other extreme pressure agents.

The worked penetration of the above grease composition (JIS K 2220) is in the range of 200 to 240. From the viewpoint of suppressing the amount of dust generation, the worked penetration is preferably in the range of 200 to 220.

The rolling bearing filled with the above grease composition will be described with reference to FIG. 1. FIG. 1 is a cross-sectional view of a deep groove ball bearing. In the rolling bearing 1, 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 are concentrically arranged, and a plurality of rolling elements 4 are arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3a. The rolling elements 4 are held by a cage 5. In addition, the axial end openings 8a, 8b of the inner and outer rings are sealed by a seal member 6, and the above grease composition 7 is sealed at least around the rolling elements 4. The seal member 6 in FIG. 1 is a contact type seal member in which the inner diameter lip comes into contact inside the seal groove. The inner ring 2, the outer ring 3, and the rolling elements 4 are made of an iron-based metal material, and the grease composition 7 is interposed between the raceway surface and the rolling elements 4 to lubricate them.

In the rolling bearing 1, the iron-based metal material constituting the bearing components such as the inner ring 2, outer ring 3, rolling elements 4, and cage 5 is any material commonly used as a bearing material, such as high carbon chromium bearing steel (SUJ1, SUJ2, SUJ3, SUJ4, SUJ5, etc.; JIS G 4805), carburized steel (SCr420, SCM420, etc.; JIS G 4053), stainless steel (SUS440C, etc.; JIS G 4303), high speed steel (M50, etc.), cold rolled steel, etc.

The sealing member 6 may be made of metal or rubber alone, or may be a composite of a rubber molded body and a metal plate, a plastic plate, or a ceramic plate. From the viewpoint of durability and ease of attachment, a composite of a rubber molded body and a metal plate, as shown in FIG. 1, is preferred. Materials used for the rubber molded body include nitrile rubber (NBR), acrylic rubber, silicone rubber, and fluororubber, and the rubber molded body contains a plasticizer.

式（１）において、Ｒ^１、Ｒ^２、Ｒ^３は同一であっても異なってもよい。好ましくは同一である。また、Ｒ^１、Ｒ^２、Ｒ^３は炭素数が７～９の脂肪族１価アルコール残基であることが好ましい。この脂肪族１価アルコール残基は、直鎖状アルキル基であっても分岐状アルキル基であってもよい。具体的には、トリメリット酸トリ－ｎ－オクチル、トリメリット酸トリ－２－エチルヘキシルなどが挙げられる。トリメリット酸エステル系可塑剤としては、レオルーブＬＴＭ、レオルーブＴＭ１０（ＦＭＣ製）などを用いることができる。 The plasticizer can be appropriately selected from known plasticizers, but it is preferable to use an ester-based plasticizer. Examples of the ester-based plasticizer include phthalate ester-based plasticizers such as di-n-octyl phthalate, di-2-ethylhexyl phthalate, and diisononyl phthalate, and ester compounds such as adipate ester-based plasticizers, trimellitate ester-based plasticizers, and pyromellitate ester-based plasticizers. Among these, trimellitate ester-based plasticizers are more preferable from the viewpoint of suppressing the amount of dust generation. The trimellitate ester-based plasticizer is represented by the following formula (1).

In formula (1), R ¹ , R ² and R ³ may be the same or different. They are preferably the same. Furthermore, R ¹ , R ² and R ³ are preferably an aliphatic monohydric alcohol residue having 7 to 9 carbon atoms. This aliphatic monohydric alcohol residue may be a linear alkyl group or a branched alkyl group. Specific examples include tri-n-octyl trimellitate and tri-2-ethylhexyl trimellitate. As the trimellitic acid ester plasticizer, Rheolub LTM, Rheolub TM10 (manufactured by FMC), etc. can be used.

In addition to the plasticizers described above, the rubber moldings may also contain various known additives, such as flame retardants, lubricants, stabilizers, fillers, foaming agents, antioxidants, processing aids, neutralizing agents, UV absorbers, pigments, antistatic agents, dispersants, thickeners, metal deterioration inhibitors, antifungal agents, and flow adjusters, as necessary.

Although FIG. 1 shows a ball bearing as an example of the bearing, the rolling bearing of the present invention can also be used as a cylindrical roller bearing, a tapered roller bearing, a spherical roller bearing, a needle roller bearing, a thrust cylindrical roller bearing, a thrust tapered roller bearing, a thrust needle roller bearing, a thrust spherical roller bearing, etc.

The configuration in which the rolling bearing of the present invention is applied to a servo motor is described with reference to Figure 2. As shown in Figure 2, the servo motor 11 has a rotor 12, a stator 13, a shaft 14, and an encoder 15 that reads the angle of the rotor 12 on the anti-load side of the shaft 14. The shaft 14 is supported by a rolling bearing 16 arranged on the load side and the rolling bearing 1 of the present invention arranged on the anti-load side. Although the rolling bearing 1 of the present invention is arranged near the encoder 15, the amount of dust generated from the bearing is suppressed, so that a decrease in the detection accuracy by the encoder 15 can be prevented.

The rolling bearing of the present invention can be used not only in servomotors, but also in other rotating electrical machines that require low dust generation. Examples include generators and electric motors other than servomotors, such as spindle motors, stepping motors, and fan motors.

Grease compositions were prepared according to the compositions shown in Tables 1 and 2. In Tables 1 and 2, the contents of base oil, thickener, and additives are shown as the content (mass%) relative to the base grease (base oil + thickener). The grease compositions of Examples 1 to 8 in Table 1 contain only organic acid metal salts and amine-based antioxidants as additives. Dioctyldiphenylamine was used as the DDPA in Tables 1 and 2. Items 1) to 9) below in Table 1 are also the same in Table 2.

<Dust generation test>
Each grease composition was filled into a deep groove ball bearing 6900 (inner diameter 10 mm × outer diameter 22 mm × width 6 mm) equipped with a contact type seal member to prepare a test bearing. For the seal members other than Example 8, NBR containing a phthalate ester plasticizer (di-2-ethylhexyl phthalate) was used as a plasticizer, and for the seal member of Example 8, NBR containing a trimellitate ester plasticizer (Reolube LTM, manufactured by FMC, kinetic viscosity at 40°C 53 mm ² /s) was used as a plasticizer. The inner ring of the obtained test bearing was rotated at an ambient temperature of 120°C, 6000 min ^-1 , and 200 hours. Glass plates were placed on both axial ends of the test bearing, and the grease composition scattered from the bearing was attached to them. The amount of dust was calculated by subtracting the weight of the glass plate before the test from the weight of the glass plate after the test. The amount of dust generated is evaluated in Tables 1 and 2 as follows: 3 mg or less is marked with a ◎, 3 mg to 4 mg is marked with a ○, 4 mg to 6 mg is marked with a △, and more than 6 mg is marked with an ×.

<Fretting wear test>
A wear test was performed for each grease composition using a ball-on-disk type reciprocating tester. Each grease composition was applied to a steel plate (20 mm square, 5 mm thick) test piece, and a steel ball (7/16 inch) made of SUJ2 was pressed against it with a load of 98 N, and a small reciprocating movement was performed for 8 hours at an amplitude of 0.47 mm and a frequency of 30 Hz. The wear depth (μm) of the steel plate after this test was measured. The evaluation of the wear depth is shown in Tables 1 and 2, with 3 μm or less marked with ◎, more than 3 μm and less than 5 μm marked with ○, more than 5 μm and less than 10 μm marked with △, and more than 10 μm marked with ×.

<Grease life test>
A test bearing was prepared by filling 1.8 g of each grease composition into a 6204 deep groove ball bearing (inner diameter 20 mm, outer diameter 47 mm, width 14 mm). The test bearing was rotated at a speed of 10,000 min ^-1 under the conditions of an outer diameter temperature of the outer ring of the bearing of 180°C, an axial load of 67 N, and a radial load of 67 N, and the time until seizure occurred was measured. The evaluation of the grease life time is shown in Tables 1 and 2 with a ◎ mark for 300 hours or more, a ○ mark for 230 hours or more but less than 300 hours, a △ mark for 150 hours or more but less than 230 hours, and an × mark for less than 150 hours.

Regarding the dust generation evaluation in Table 1, Example 8 generated less than 2 mg of dust, which was superior to the other test examples marked with ◎ in terms of dust generation.

As shown in Tables 1 and 2, Examples 1 to 8, which combine a base oil containing PAO oil and ether oil and having a predetermined kinetic viscosity, a thickener containing an aliphatic diurea compound and an alicyclic diurea compound, and an organic acid metal salt, showed good results in all tests. Compared with Comparative Examples 2 and 6, it can be seen that the organic acid metal salt contributes particularly to improving wear resistance and grease life. It is believed that by adding an organic acid metal salt that is solid at 20°C, the surface tension of the base oil is not affected, and when the supply of base oil is insufficient and wear is a concern, the organic acid metal salt melts and protects the steel surface as an extreme pressure agent. As an organic acid metal salt, the metal stearate showed better results in terms of dust generation (Examples 1, 4, 6, and 7), and the sodium salt showed better results in terms of wear resistance (Examples 4 and 5).

In Comparative Example 1, Comparative Example 4, and Comparative Example 10, in which ZnDTP was added as an additive, the wear resistance and grease life were relatively good, but the amount of dust generation increased. Separately, a surface tension test was conducted in accordance with JIS K2241, and the surface tension of the PAO oil alone with a kinetic viscosity of 47 ^mm2 /s at 40°C was 29.8 mN/m, while the surface tension of the test oil in which 2 mass% of ZnDTP was added to the PAO oil was 26 mN/m. From this result, it is considered that the increase in the amount of dust generation due to the addition of ZnDTP is due to a decrease in surface tension. In the case of TCP, it is considered that the amount of dust generation increased as a result of the decrease in surface tension (Comparative Example 3).

In addition, when NBR containing a trimellitate ester plasticizer was used as the sealing member (Example 8), the amount of dust generation was reduced compared to when NBR containing a phthalate ester plasticizer was used (Example 3). Trimellitate ester plasticizers have lower volatility (higher boiling point) than phthalate ester plasticizers, and as a result, it is believed that the evaporation of the plasticizer itself is suppressed, reducing the amount of dust generation. In this way, the low dust generation properties are improved by using a heat-resistant NBR sealing member.

In view of the above, in the rolling bearing of the present invention, the grease composition is a combination of a specified base oil and a specified thickener, specifically, a combination of a base oil with excellent heat resistance and high surface tension, a specified thickener, and low volatility/oil separation, to which an organic acid metal salt that is solid at 20°C is added, so that the surface tension of the base oil is not affected, and when the supply of base oil is insufficient and wear is a concern, the organic acid metal salt melts and can protect the steel surface as an extreme pressure agent.

The rolling bearing of the present invention achieves low dust generation while suppressing deterioration in wear resistance and life span, making it particularly suitable for use as a rolling bearing for rotating electrical machines such as servo motors and encoders, which require low dust generation.

REFERENCE SIGNS LIST 1 Rolling bearing 2 Inner ring 3 Outer ring 4 Rolling element 5 Cage 6 Sealing member 7 Grease 8a, 8b Opening 11 Servo motor 12 Rotor 13 Stator 14 Shaft 15 Encoder 16 Rolling bearing

Claims (3)

A rolling bearing comprising an inner ring, an outer ring, a plurality of rolling elements interposed between the inner ring and the outer ring, seal members provided at openings at both axial ends of the inner ring and the outer ring, and a grease composition sealed in a space within the bearing,
The grease composition includes a base oil and a thickener,
the base oil is a mixed oil containing a synthetic hydrocarbon oil and an ether oil and having a kinematic viscosity at 40°C of 120 ^mm2 /s or more;
the thickener is a diurea compound obtained by reacting a diisocyanate component with a monoamine component, the monoamine component being an aliphatic monoamine and an alicyclic monoamine, and is contained in an amount of 12 parts by mass to 15 parts by mass per 100 parts by mass of the total amount of the base oil and the thickener;
The grease composition has a worked penetration of 200 to 240 as measured in accordance with JIS K 2220 and contains an organic acid metal salt that is solid at 20°C,
the organic acid metal salt is a sodium salt or a calcium salt of an organic carboxylic acid having 2 to 18 carbon atoms ;
The rolling bearing is characterized in that the grease composition does not contain any additives containing sulfur and phosphorus in the molecular structure . The rolling bearing according to claim 1, characterized in that the grease composition further contains an amine-based antioxidant, the amine-based antioxidant being N-phenyl-N'-isopropyl-p-phenylenediamine. 3. The rolling bearing according to claim 1, wherein the rolling bearing is a bearing for a rotating electrical machine.

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