Grease composition and rolling bearing

Abstract

To provide a grease composition capable of suppressing an increase in torque even in a state where deterioration with time has progressed in urea-based grease, and to provide a rolling bearing lubricated with the grease composition.SOLUTION: The grease composition contains a base oil and a urea compound as a thickener, and contains an oil-soluble compound having one or more hydroxy groups or ester bonds in a molecule, wherein the oil-soluble compound has a kinematic viscosity at 40 °C. of 80mm2 / s or more, and the oil-soluble compound is contained in an amount of 0.1 mass% to 3.0 mass% based on the total amount of the grease composition.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a grease composition and a rolling bearing lubricated with the grease composition, and particularly to a rolling bearing used under high-speed rotation conditions. [Background technology]

[0002] Conventionally, bearings that support the spindles of machine tools such as machining centers and lathes are frequently used under high-speed rotation conditions, and the relationship between the dn value (bearing bore diameter d (mm) and rotation speed n (min -1 ) reaches over 1 million. Such bearings often use air-oil lubrication, which offers excellent lubrication properties, but because the use of compressed air consumes a lot of power, there is a growing trend to switch to grease lubrication, which does not use compressed air.

[0003] However, if grease-lubricated bearings are used under high-speed rotation conditions with a dn value of 1 million or more, there is a risk that problems such as abnormal bearing temperature rise and associated wear on the raceway surface may occur before the end of their service life.

[0004] As a grease-lubricated rolling bearing used under high-speed rotation conditions, a rolling bearing using grease containing a highly heat-resistant urea compound as a thickener is known. For example, Patent Document 1 describes a rolling bearing for machine tools that contains a grease composition containing at least one of an aromatic urea compound and an alicyclic urea compound as a thickener and having an NLGI consistency number of 3 to 4. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-35916 Summary of the Invention [Problem to be solved by the invention]

[0006] However, it was discovered that urea-based grease, which uses a urea compound as a thickener, has a high risk of abnormal temperature rise when used for extended periods.The mechanism behind this abnormal temperature rise is thought to be that the grease deteriorates over time (the base oil decreases) when used for extended periods, and the deteriorated grease becomes less fluid and gets caught between the bearing components, causing an increase in torque.

[0007] The present invention has been made in view of the above circumstances, and has an object to provide a grease composition that can suppress an increase in torque even when urea-based grease has undergone deterioration over time, and a rolling bearing lubricated with the grease composition. [Means for solving the problem]

[0008] The grease composition of the present invention is a grease composition containing a base oil and a urea compound as a thickener, wherein the grease composition contains an oil-soluble compound having one or more hydroxy groups or ester bonds in the molecule, and the kinematic viscosity of the oil-soluble compound at 40°C is 80 mm 2 / s or more.

[0009] The oil-soluble compound is contained in an amount of 0.1% by mass to 3.0% by mass relative to the total amount of the grease composition.

[0010] The oil-soluble compound is characterized in that it is a compound represented by the following formula (1): [ka] (In formula (1), X 1 , X 2 , X 3 each independently represents a hydrogen atom, an optionally substituted linear or branched alkyl group, an optionally substituted linear or branched alkenyl group, or an optionally substituted linear or branched acyl group.

[0011] The oil-soluble compound is characterized by having a tert-butyl-substituted hydroxyphenyl group in the molecule.

[0012] The base oil contains an ester oil and has a kinematic viscosity of 80 mm at 40°C. 2 / s.

[0013] The oil-soluble compound has an ester bond and a tert-butyl-substituted hydroxyphenyl group in the molecule and is represented by the following formula (1): The base oil contains an ester oil and has a kinematic viscosity of 80 mm at 40°C. 2 / s. [ka] (In formula (1), X 1 , X 2 , X 3 each independently represents a hydrogen atom or an optionally substituted linear or branched acyl group.

[0014] The rolling bearing of the present invention is a rolling bearing that has steel inner and outer rings and a plurality of rolling elements interposed between the inner and outer rings, and is lubricated with a grease composition, wherein the grease composition is the grease composition of the present invention.

[0015] The rolling bearing is characterized in that it is a rolling bearing that supports a main spindle of a machine tool. [Effects of the Invention]

[0016] The grease composition of the present invention is a urea-based grease and contains an oil-soluble compound (e.g., a glycerol derivative represented by the above formula (1)) having one or more hydroxyl groups or ester bonds in the molecule. The oil-soluble compound exhibits an interaction (orientation force) with the polar group of the urea compound, thereby reducing the coefficient of friction between the urea compound and steel, and particularly suppressing an increase in the coefficient of friction of the grease as it ages. Furthermore, the interaction between the polar group of the oil-soluble compound and the polar group of the urea compound reduces the intermolecular force between urea molecules during ageing (when the base oil is reduced), thereby suppressing an increase in the viscosity of the thickener and, ultimately, the viscosity of the grease. As a result, torque increases due to factors such as the entrapment of degraded grease can be suppressed, and abnormal temperature increases can also be expected.

[0017] Furthermore, the oil-soluble compound has a kinematic viscosity of 80 mm at 40°C. 2 / s or more, which at first glance seems to have a detrimental effect on suppressing torque increase, but it is thought to have a favorable effect on the interposition and maintenance (retention) of oil-soluble compounds between urea molecules and the expression of interactions with the polar groups of urea compounds during continued deterioration (when the base oil decreases).

[0018] The oil-soluble compound has a tert-butyl-substituted hydroxyphenyl group in the molecule, and therefore, in addition to the interaction with the urea compound as described above, the hindered phenol moiety can also contribute to the thermal oxidation stability of the base oil.

[0019] The base oil contains an ester oil and has a kinematic viscosity of 80 mm at 40°C. 2 / s, it is excellent in solubility of oil-soluble compounds having ester bonds, for example, and also reduces the viscosity of the grease as a whole, making it easy to suppress an increase in torque. [Brief explanation of the drawings]

[0020] [Figure 1] 1 is a cross-sectional view of an angular contact ball bearing, which is an example of a rolling bearing according to the present invention. [Figure 2] FIG. 10 is a diagram showing a speculated mechanism of abnormal temperature rise in a bearing. [Figure 3] FIG. 1 is a diagram showing a spindle device using a rolling bearing according to the present invention. [Figure 4] 10 is a graph showing changes in torque in Comparative Example 1 and Comparative Example 2. [Figure 5] 1 is a graph showing the changes in torque in Comparative Example 1 and Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0021] The present inventors have considered the mechanism of abnormal temperature rise in grease-lubricated bearings, and as a result of conducting research assuming degraded grease, have found that adding a specific oil-soluble compound to urea-based grease can reduce torque and is therefore effective in suppressing abnormal temperature rise. The present invention is based on this finding.

[0022] The rolling bearing of the present invention will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view of an angular contact ball bearing. This angular contact ball bearing is a grease-lubricated rolling bearing in which the grease composition of the present invention is packed as a lubricant.

[0023] As shown in Figure 1, angular contact ball bearing 1 comprises steel inner ring 2 and outer ring 3, with multiple rolling elements 4 placed between them. Inner ring 2 has an inner ring raceway on its outer peripheral surface for rolling contact with rolling elements 4, and outer ring 3 has an outer ring raceway on its inner peripheral surface for rolling contact with rolling elements 4. Rolling elements 4 are held in place by cage 5 and roll between the inner ring raceway and the outer ring raceway. Openings at both axial ends of the inner and outer rings are sealed by sealing members 6, and a grease composition is enclosed in the bearing space, at least around the rolling elements 4. A line connecting the contact points between rolling elements 4 and the inner ring 2 and outer ring 3 has a contact angle α with respect to the radial direction, allowing the bearing to withstand radial loads and unidirectional axial loads.

[0024] A circumferential groove-like grease pocket is formed on the inner peripheral surface of the outer ring 3, which serves to further prevent leakage of the grease composition.

[0025] In angular contact ball bearing 1, the iron-based metallic material constituting the bearing components of inner ring 2, outer ring 3, and balls 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 G 4805), carburized steel (SCr420, SCM420, etc.; JIS G 4053), stainless steel (SUS440C, etc.; JIS G 4303), high-speed steel (M50, etc.), and cold-rolled steel.

[0026] Furthermore, the balls 4 can be made of not only iron-based metal materials but also lightweight ceramic materials. Using ceramic materials is preferable because it can suppress the increase in preload caused by centrifugal force during high-speed rotation, for example. Examples of ceramic materials include silicon nitride, silicon carbide, aluminum oxide (alumina), zirconium oxide (zirconia), sialon, and glass. Among these, silicon nitride is more preferable because of its excellent mechanical properties and heat resistance.

[0027] The seal member 6 may be made of metal or rubber molded body alone, or may be a composite of a rubber molded body and a metal plate, plastic plate, or ceramic plate. From the viewpoint of durability and ease of attachment, a composite of a rubber molded body and a metal plate is preferred, as shown in Figure 1. In Figure 1, the seal member 6 is a contact seal in which the seal lip portion on the inner diameter side contacts the seal groove, but it may also be a non-contact type seal.

[0028] The grease composition of the present invention is a urea-based grease containing a base oil, a urea compound as a thickener, and an oil-soluble compound having one or more hydroxy groups or ester bonds in the molecule. It has been found that urea-based greases are prone to abnormal temperature rise when used under high-speed rotation conditions, for example, and the addition of the oil-soluble compound is intended to suppress this.

[0029] Specific examples of oil-soluble compounds include glycerol derivatives, glycol derivatives, sugar esters such as sucrose fatty acid esters, sugar alcohols, monohydric alcohol esters of fatty acids, monohydric alcohol esters of polybasic acids, etc. These may be used alone or in combination of two or more.

[0030] As the glycerol derivative, one represented by the following formula (1) can be used. [ka] (In the above formula (1), X 1 , X 2 , X 3 each independently represents a hydrogen atom, an optionally substituted linear or branched alkyl group, an optionally substituted linear or branched alkenyl group, or an optionally substituted linear or branched acyl group.

[0031] The alkyl group, alkenyl group, and acyl group may have, for example, 1 to 22 carbon atoms. Furthermore, in the alkyl group, alkenyl group, and acyl group, the hydrogen atoms bonded to the carbon atoms on the chain may be substituted with one or more of various substituents. Specifically, the substituents may be halogen atoms; functional groups such as hydroxyl groups, amino groups, carboxyl groups, ester groups, and amide groups; aromatic groups which may be substituted with halogen atoms, functional groups, alkyl groups, alkenyl groups, acyl groups, aromatic groups, and alicyclic groups; and alicyclic groups which may be substituted with halogen atoms, functional groups, alkyl groups, alkenyl groups, acyl groups, aromatic groups, and alicyclic groups.

[0032] In the above formula (1), more specifically, X 1 , X 2 , X 3 each independently represents a hydrogen atom, a group represented by the following formula (2), or a group represented by the following formula (3), in which m is 6 to 20 (preferably 8 to 12), and in which n is 1 to 8 (preferably 2 to 6), R 1 , R 2are each independently an alkyl group having 1 to 8 carbon atoms. For example, R 1 , R 2 Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a tert-butyl group, and a pentyl group, and among these, a tert-butyl group is preferred. [ka]

[0033] The oil-soluble compound preferably has a tert-butyl-substituted hydroxyphenyl group in the molecule. This hindered phenol moiety can also exert an antioxidant effect, making it possible to omit the addition of a separate antioxidant, for example. The tert-butyl-substituted hydroxyphenyl group is preferably a 3,5-di-tert-butyl-4-hydroxyphenyl group.

[0034] Furthermore, as the monohydric alcohol ester of a polybasic acid, a diester represented by the following formula (4), a trimellitic acid ester, or the like can be used. R 4 OOC-R 3 -COOR 4 ···(4) (In the above formula (4), R 3 represents a linear, branched or cyclic hydrocarbon group, and R 4 represents a linear, branched, or cyclic hydrocarbon group.

[0035] The oil-soluble compound preferably contains two or more hydroxy groups or ester bonds in the molecule, and more preferably three or more. The polar group in the molecule may contain a polar group other than a hydroxy group or an ester bond, but the oil-soluble compound preferably does not contain sulfur (S) or phosphorus (P) as a constituent element, and may not contain nitrogen (N).

[0036] The oil-soluble compound has a kinematic viscosity of 80mm at 40°C. 2It is preferable that the kinematic viscosity is 120 mm / s or more. It is thought that the use of a high-viscosity oil-soluble compound makes it easier to exhibit an interaction with the urea compound. 2 / s or more, 200 mm 2 The kinematic viscosity may be, for example, 800 mm 2 / s or less.

[0037] The oil-soluble compound is contained in an amount of, for example, 0.1 to 10% by mass, preferably 0.1 to 5.0% by mass, and may be contained in an amount of 0.1 to 3.0% by mass, relative to the total amount of the grease composition.

[0038] The base oil used in the grease composition of the present invention can be any oil normally used in rolling bearings, without any particular limitations. Examples include mineral oils such as paraffinic mineral oil and naphthenic mineral oil, synthetic hydrocarbon oils such as 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.

[0039] Among the above, the base oil is preferably selected from the group consisting of synthetic hydrocarbon oils, ester oils, ether oils, and mixtures thereof. Furthermore, from the viewpoint of the solubility of oil-soluble compounds having ester bonds, the base oil more preferably contains an ester oil. In this case, the ester oil alone or a mixture of the ester oil and the synthetic hydrocarbon oil can be used.

[0040] Ester oils are compounds that have an ester group in the molecule and are liquid at room temperature, and examples thereof include diester oils such as dibutyl sebacate, di-2-ethylhexyl sebacate, and dioctyl adipate, aromatic ester oils such as trioctyl trimellitate, tridecyl trimellitate, and tetraoctyl pyromellitate, polyol ester oils such as trimethylolpropane caprylate, trimethylolpropane bellargonate, and pentaerythritol ester oil, carbonate ester oil, phosphate ester oil, and polymer ester oil. When an ester oil is used as the base oil, the oil-soluble compound used is different from the ester oil.

[0041] The synthetic hydrocarbon oil is 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.

[0042] The kinematic viscosity of the base oil at 40°C (in the case of a mixed oil, the kinematic viscosity of the mixed oil) is 80mm 2 / s, and the specific range is 10 mm 2 / s~50mm 2 / s is preferred, and 10 mm 2 / s~40mm 2 In contrast to oil-soluble compounds, using a low-viscosity base oil makes it easier to suppress an increase in torque as a grease.

[0043] The content of the base oil in the grease composition is, for example, 60% by mass or more, preferably 70% by mass or more, or may be 80% by mass or more, or may be 85% by mass or more, based on the total amount of the grease composition.

[0044] The thickener used in the grease composition is a urea compound obtained by reacting a polyisocyanate component with a monoamine component. Examples of the polyisocyanate component include phenylene diisocyanate, tolylene diisocyanate, diphenyl diisocyanate, diphenylmethane diisocyanate (MDI), octadecane diisocyanate, decane diisocyanate, and hexane diisocyanate. Among these, aromatic diisocyanates are more preferred. Polyisocyanates obtained by reacting a diamine (such as ethylene diamine or propane diamine) with a molar excess of a diisocyanate relative to the diamine can also be used. Examples of the monoamine component include alicyclic monoamines such as cyclohexylamine, aromatic monoamines such as p-toluidine, and aliphatic monoamines such as octylamine and stearylamine.

[0045] For example, a base grease is obtained by blending a diurea compound as a thickener with a base oil. The 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 content of the thickener is, for example, 5% to 30% by mass, preferably 10% to 20% by mass, based on the total amount (100% by mass) of the base oil and thickener.

[0046] The worked penetration (JIS K2220) of the grease composition is preferably in the range of 200 to 350, and more preferably in the range of 220 to 295. If the penetration is less than 200, oil separation may be small, resulting in 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.

[0047] The grease composition may also contain known additives as needed, such as antioxidants such as amine-based, phenol-based, and sulfur-based compounds; other oil-soluble compounds; extreme pressure agents such as phosphate esters such as tricresyl phosphate, phosphite esters such as tricresyl phosphite, thiophosphates, thiophosphites, zinc alkyldithiophosphate (ZnDTP), molybdenum alkyldithiophosphate (MoDTP), and zinc dithiocarbamate (ZnDTC); and rust inhibitors such as calcium sulfonate and barium sulfonate.

[0048] 1 shows an angular contact 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. For example, it can also be used as a deep groove ball bearing, cylindrical roller bearing, tapered roller bearing, spherical roller bearing, needle roller bearing, thrust cylindrical roller bearing, thrust tapered roller bearing, thrust needle roller bearing, thrust spherical roller bearing, etc.

[0049] The rolling bearing of the present invention can suppress an increase in torque even when the grease composition is in a deteriorated state, and therefore can suppress the occurrence of abnormal temperature rise, and therefore can be used even under high-speed rotation conditions where abnormal temperature rise can be a problem. Specifically, the high-speed rotation conditions are those under which the dn value is 60×10 4 That's 100 x 10 4 The upper limit of the dn value is not particularly limited, but for example, it is 200×10 4 is.

[0050] Here, we will explain the mechanism of abnormal temperature rise that occurs in bearings lubricated with urea-based grease using Figure 2. It is believed that this mechanism is caused by (1) the grease hardening due to degradation over time during long-term operation, making it less fluid than new grease, and (2) the grease's friction coefficient increasing as the grease deteriorates. Both (1) and (2) above lead to an increase in the torque of the rolling bearing, causing a temperature rise due to heat generation. For example, if degraded grease is accidentally caught in the sliding area between the bearing's cage pocket and the rolling elements, the bearing's torque increases due to the poor fluidity of the degraded grease, causing a temperature rise. Furthermore, this temperature rise triggers several other phenomena that lead to a temperature rise in the bearing, as shown in Figure 2. Therefore, it is believed that once degraded grease is caught in the bearing, this triggers an accelerated increase in heat generation within the bearing, causing a large temperature rise (abnormal temperature rise) in a short period of time.

[0051] The abnormal temperature rise is defined as a phenomenon in which the temperature of the inner or outer ring of a bearing rises by about 10°C from a steady state, and the rate of this rise is about 0.15°C / min.

[0052] In contrast, the grease composition of the present invention contains the above-mentioned oil-soluble compound as an additive component in a urea-based grease. This oil-soluble compound has a polar group, such as a hydroxyl group (-OH) or an ester bond (-COO), within the molecule. The presence of such a polar group is believed to facilitate adsorption of the oil-soluble compound to a urea compound, which also has a polar group. Urea compounds as simple substances are known to have a high coefficient of friction with steel (e.g., SUJ2) (see Non-Patent Document 1; Tribologist, Vol. 64, No. 1 (2019), pp. 33-41). However, when an oil-soluble compound is adsorbed on the surface of a urea compound, the coefficient of friction with the steel primarily depends on the properties of the oil-soluble compound adsorbed on the surface. Furthermore, the coefficient of friction between the thickener and steel is believed to be lower than when the urea compound is present alone. This suppresses the increase in the grease's friction coefficient as the grease deteriorates over time, which is expected to reduce the occurrence of abnormal temperature increases in rolling bearings due to the aforementioned reason (2).

[0053] Furthermore, because urea molecules share multiple polar groups and the same molecular structure, they are believed to be bound together by strong intermolecular forces. In viscous materials such as grease, the stronger the intermolecular forces, the greater the viscosity. Therefore, in urea-based greases, as the base oil decreases with age and the urea compound content in the grease increases, the viscosity may increase rapidly. In contrast, when the oil-soluble compounds mentioned above (e.g., glycerol derivatives) are included, the oil-soluble compounds are believed to be able to intervene between urea molecules even after the base oil is reduced. This is because the oil-soluble compounds have multiple polar groups and are likely to exhibit interactions (orientation forces) with the polar groups in the urea molecules. This effect is particularly pronounced for high-viscosity oil-soluble compounds. Therefore, when oil-soluble compounds are included in a grease, the intermolecular forces between urea molecules are reduced as the base oil decreases, suppressing the increase in the viscosity of the thickener and, ultimately, the viscosity of the grease. As a result, the abnormal temperature rise in rolling bearings due to reason (1) above is also believed to be suppressed.

[0054] In the examples described below, in order to suppress the occurrence of abnormal temperature rise due to such a mechanism, deteriorated grease is assumed and its effect on increased torque is confirmed using the deteriorated grease.

[0055] Figure 3 shows an example of a spindle unit of a machine tool that uses the rolling bearing of the present invention. This spindle unit 13 is a built-in motor type, with a spindle 15 supported by a front rolling bearing unit 11 and a rear rolling bearing unit 12, and a motor 16 installed between the two bearing units 11, 12. The spindle 15 has a spindle head at its front end that is equipped with a chuck (not shown) that grips a tool or workpiece. The motor 16 is composed of a stator 18 mounted on the inner periphery of the spindle accommodating space in the housing 14, and a rotor 17 mounted on the outer periphery of the spindle 15. A coolant flow path is provided in the housing 14, positioned around the motor 16.

[0056] The bearing device 11 has four angular contact ball bearings 1a, 1b, 1c, and 1d arranged in the axial direction, and the rolling bearing of the present invention can be used as each angular contact ball bearing. In FIG. 3, the four angular contact ball bearings 1a to 1d are positioned by inner ring spacers and outer ring spacers located on the inner and outer ring sides, respectively. In FIG. 3, the four angular contact ball bearings are arranged back-to-back with two rows of angular contact ball bearings 1a and 1b at the front (left side of the figure) and two rows of angular contact ball bearings 1c and 1d at the rear (right side of the figure). The front angular contact ball bearings 1a and 1b and the rear angular contact ball bearings 1c and 1d may also be arranged face-to-face (with contact angles in an inverted V-shape). Furthermore, a single-row angular contact ball bearing is used for the bearing device 12. The number and types of bearings in each of the bearing devices 11 and 12 are not limited.

[0057] The spindle device can also be applied to various machine tools such as machining centers, lathes, milling machines, and grinding machines.

[0058] The rolling bearing of the present invention is not limited to a rolling bearing that supports the main spindle of a machine tool, but can also be suitably used as a rolling bearing used under high-speed rotation conditions. [Example]

[0059] The present invention will be specifically explained by way of examples and comparative examples, but is not limited to these examples in any way.

[0060] The torque test conditions are shown in Table 1 below. The test bearings were 6204 deep groove ball bearings, and a resin crown-type cage was used. Before the test, grease was applied to the sliding parts of the cage pockets (the parts that slide against the rolling elements during rotation), and the bearing torque was measured during the test.

[0061] [Table 1]

[0062] Table 2 below shows the composition of the grease used in the test and the presence or absence of additional base oil components. At all levels, a commercially available urea-based grease (base oil: poly-α-olefin oil + ester oil, kinematic viscosity at 40°C 22 mm 2 In Comparative Example 1, a new grease was used. In contrast, Comparative Example 2 and Example 1 used a "reduced base oil product" in which the base oil content of the new grease was artificially reduced. The "reduced base oil product" was obtained by stirring new grease in petroleum benzine, centrifuging it, and then drying the remaining material at 60°C for one month after removing the supernatant. In other words, by artificially removing the base oil, a grease that reproduced the deterioration in an actual machine was prepared, and the test was conducted by applying it to the sliding parts of the pocket.

[0063] When testing was conducted using only the "reduced base oil" grease, excessive torque caused the bearing to lock and not rotate. Therefore, to ensure a minimum level of lubrication, a small amount of additional base oil was added and the test was conducted. The "reduced base oil" grease also had significantly lower consistency than new grease.

[0064] In Example 1, a glycerol derivative was added as an oil-soluble compound together with the additional base oil. This glycerol derivative is represented by the formula (1) above, X 1 , X 2 , X 3 are each independently a hydrogen atom, a group represented by the above formula (2) (m is 10), a group represented by the above formula (3) (n is 2, R 1 and R 2 is a tert-butyl group), and the kinematic viscosity at 40°C is about 400 mm 2 / s.

[0065] [Table 2]

[0066] The results of the torque test are shown in Figures 4 and 5, respectively. First, Figure 4 shows the results of the torque test for Comparative Example 1 and Comparative Example 2. Comparative Example 1 exhibited a stable torque of approximately 10 N / mm or less immediately after the start of the test. This is thought to be because the grease was new and contained a large amount of base oil, resulting in high fluidity and a low coefficient of friction. Comparative Example 2 exhibited a higher torque than Comparative Example 1 up to approximately 600 seconds after the start of the test.

[0067] The reason for the high torque up to 600 seconds in Comparative Example 2 is believed to be that the grease (reduced base oil content) applied to the sliding portions of the cage pockets before the test had poor fluidity and a high friction coefficient, which hindered smooth sliding in the sliding portions of the pockets. On the other hand, the reason for the decrease in torque over time in Comparative Example 2 is believed to be that the "reduced base oil content" grease initially applied to the sliding portions of the pockets was removed by shearing in the sliding portions during operation, moved within the bearing, and then deposited in locations that would not interfere with rotation (such as beside the rolling contact areas of the raceway surfaces or non-sliding areas of the cage). Note that once the grease has deposited in a location that does not interfere with rotation, the subsequent torque is primarily dependent on the stirring resistance and rolling viscous resistance of the base oil. Because the base oil content in the bearing of Comparative Example 2 is lower than that of Comparative Example 1, the torque due to stirring resistance and rolling viscous resistance is smaller than that of Comparative Example 1.

[0068] Next, Figure 5 shows the torque test results for Comparative Example 1 and Example 1. In Example 1, the period during which torque was higher than in Comparative Example 1 was approximately 300 seconds after the start of the test. This indicates that torque reduction occurred earlier in Example 1 than in Comparative Example 2. In Example 1, the additional base oil contained a glycerol derivative, which is thought to have adsorbed onto the urea compounds in the "reduced base oil" grease applied to the sliding surfaces of the cage pockets, thereby weakening the intermolecular force between the urea molecules. It is also thought that this weakened the adhesive force of the "reduced base oil" grease to the sliding surfaces of the cage pockets. These effects improved the fluidity of the grease, which led to the "reduced base oil" grease being removed from the sliding surfaces of the pockets more quickly, resulting in an earlier torque reduction. Furthermore, it is thought that adsorption of the glycerol derivative reduced the friction coefficient between the "reduced base oil" grease and the steel raceway ring compared to the case without the glycerol derivative. These effects are also thought to have contributed to the earlier torque reduction in Example 1.

[0069] Based on the above results, it is believed that if an oil-soluble compound (e.g., a glycerol derivative) is contained in a urea-based grease from the beginning, the torque increase caused by the deterioration of the grease fluidity and the increase in the friction coefficient when the grease is in an advanced state of deterioration (when the base oil has decreased due to grease loss, etc.) can be suppressed, which in turn leads to the suppression of abnormal temperature rise in the rolling bearing. In this way, the present invention can be expected to suppress abnormal temperature rise in the bearing caused by the continued deterioration of the urea-based grease. [Industrial Applicability]

[0070] The grease composition of the present invention can suppress an increase in torque even when urea-based grease has deteriorated over time, and is therefore widely applicable to rolling bearings that use urea-based grease, and is particularly suitable for rolling bearings used in high-speed rotation applications (e.g., dn values ​​of 1 million or more), such as rolling bearings that support the main spindles of machine tools. [Explanation of symbols]

[0071] 1, 1a, 1b, 1c, 1d Angular contact ball bearings (rolling bearings) 2. Inner circle 3 outer ring 4 rolling elements 5 Cage 6 Sealing material 11 Bearing device 12 Bearing device 13 Spindle device 14 Housing 15 Spindle 16 Built-in motor 17 rotor 18 Stator

Claims

1. A grease composition comprising a base oil and a urea compound as a thickener, The grease composition contains an oil-soluble compound having one or more hydroxy groups or ester bonds in the molecule, and the kinematic viscosity of the oil-soluble compound at 40°C is 80 mm ２ / s or more.

2. 2. The grease composition according to claim 1, wherein the oil-soluble compound is contained in an amount of 0.1% by mass to 3.0% by mass relative to the total amount of the grease composition.

3. 3. The grease composition according to claim 1, wherein the oil-soluble compound is a compound represented by the following formula (1): 【Chemistry 1】 (In formula (1), X １ , X ２ , X ３ each independently represents a hydrogen atom, an optionally substituted linear or branched alkyl group, an optionally substituted linear or branched alkenyl group, or an optionally substituted linear or branched acyl group.

4. 4. The grease composition according to claim 3, wherein the oil-soluble compound has a tert-butyl-substituted hydroxyphenyl group in the molecule.

5. The base oil contains an ester oil and has a kinematic viscosity of 80 mm at 40°C. ２ 3. The grease composition according to claim 1, wherein the viscosity of the grease composition is less than 1 / s.

6. The oil-soluble compound has an ester bond and a tert-butyl-substituted hydroxyphenyl group in the molecule and is a compound represented by the following formula (1): The base oil contains an ester oil and has a kinematic viscosity of 80 mm at 40°C. ２ 3. The grease composition according to claim 1, wherein the viscosity of the grease composition is less than 1 / s. 【Chemistry 2】 (In formula (1), X １ , X ２ , X ３ each independently represents a hydrogen atom or an optionally substituted linear or branched acyl group.

7. A rolling bearing having steel inner and outer rings and a plurality of rolling elements interposed between the inner and outer rings, and lubricated with a grease composition, 3. A rolling bearing, wherein the grease composition is the grease composition according to claim 1 or 2.

8. 8. A rolling bearing according to claim 7, wherein said rolling bearing is a rolling bearing for supporting a main shaft of a machine tool.

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