Method for evaluating adhesion of base oil of grease composition to magnetic disk

Abstract

To provide: a grease composition for a pivot assembly; a pivot assembly bearing in which the grease composition is sealed; a bearing device which, with the use of the grease composition and the bearing, can suppress an adhesion of a volatile component to a magnetic disk or the like even when the grease composition is volatilized, and thus can suppress occurrence of hard disk drive read / write errors; and a driving device comprising the bearing device.SOLUTION: There are provided: a grease composition for a pivot assembly bearing, which is a grease composition comprising an aromatic ester base oil and a thickening agent, where the aromatic ester base oil has, as a substituent on a ring thereof, an ester group +-(CO)O- (in which + represents a bonding site with the aromatic ring), and includes an aromatic ester compound in which an alkyl group having eight or more total carbon atoms binds to an oxygen atom of the ester group; a pivot assembly bearing in which the grease composition for pivot assembly bearings is sealed; a bearing device that comprises the pivot assembly bearing; and a disk driving device that is equipped with the bearing device.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a grease composition for a pivot assembly, a pivot assembly bearing filled with the grease composition, and a bearing device including the bearing.The present invention also relates to a disk drive device including the bearing device. [Background technology]

[0002] Various lubricants such as grease and oil are used in the pivot assemblies used in the fulcrum of the actuator of a disk drive (HDD) and in the bearings built into the spindle motor to facilitate the operation of these parts and the drive of the device. For example, in the case of a rolling bearing incorporated in an actuator of a disk drive device, there has been a proposal for a rolling bearing filled with grease made by blending a diurea compound having at least one type of alicyclic hydrocarbon group and aliphatic hydrocarbon group in the skeleton as a thickening agent with a base oil containing aromatic ester oil (Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2006-236410 A Summary of the Invention [Problem to be solved by the invention]

[0004] One of the causes of read / write errors in HDDs is the volatilization of base oil, which is one component of the lubricant sealed in the bearings built into the actuator and spindle motor. When the volatilized base oil cools and condenses on the surface of the magnetic disk or the magnetic head, and adheres to these as a liquid or solid, the magnetic disk and the magnetic head may stick to each other, making normal reading and writing impossible, which is thought to be one of the causes of read / write errors. The volatilization of the lubricant base oil that accompanies the rise in temperature while the HDD is in operation can be controlled, for example by selecting a low-volatility base oil, but it is difficult to completely prevent the volatilization of the components.

[0005] The present invention aims to provide a grease composition for pivot assemblies and a pivot assembly bearing containing the same, as well as to provide a bearing device and a disk drive device equipped with the same, in which, by applying the grease composition and bearing, adhesion of volatile components to magnetic disks, etc. is suppressed even if the grease composition volatilizes, thereby suppressing the occurrence of read / write errors in HDDs. [Means for solving the problem]

[0006] One aspect of the present invention relates to a grease composition for pivot assembly bearings, which contains an aromatic ester base oil and a thickener, wherein the aromatic ester base oil contains an aromatic ester compound having an ester group *-C(=O)O- (* indicates the bonding site to the aromatic ring) as a substituent on a ring, and an alkyl group having a total of 8 or more carbon atoms bonded to the oxygen atom of the ester group. The present invention also relates to a pivot assembly bearing filled with the grease composition for pivot assembly bearings. The present invention further relates to a bearing device including the pivot assembly bearing. The present invention also relates to a disk drive device equipped with the bearing device. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a schematic diagram illustrating an example of the structure of a pivot assembly bearing (rolling bearing) of the present invention. [Diagram 2] FIG. 1 is a schematic diagram illustrating an example of the structure of a drive device (disk drive device) according to the present invention. [Diagram 3] 1 is a schematic diagram illustrating an example of the structure of a bearing device (pivot assembly bearing device) of the present invention. FIG. [Figure 4]FIG. 13 is a diagram showing a heater temperature program used in a read / write error occurrence test. [Diagram 5] 1 is a diagram showing a crown cage used in the pivot assembly bearing (rolling bearing) of the present invention. FIG. [Figure 6] 1 is a photograph of a ball after a high-speed Grade 4 test for evaluating sludge generation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] As mentioned above, for the lubricants used in HDD actuators and spindle motors, proposals have been made to suppress the volatilization of lubricant components (e.g., outgassing), which is thought to be one of the causes of read / write errors in HDDs. Even if the volatilization of commonly used lubricating components is suppressed, the volatilization itself cannot be eliminated. In conventional disk drive devices, the fly height (the distance between the magnetic head and the disk) was sufficiently large. Therefore, if the volatile components could be suppressed, it was possible to avoid read / write errors. However, with the improvement of recording density, the fly height has been reduced to about a few nm. In this case, it is considered that a negative pressure state is created between the magnetic head and the disk. This causes the surrounding gas to move toward the gap between the magnetic head and the disk and compress. This causes the gas to condense, and even traces of volatile components may liquefy. In recent years, with the increase in recording capacity per HDD, the number of disks in the device has increased, and disk drive devices with nine or more 3.5-inch disks have been released. In such devices, the spatial volume within the device has become even smaller. In an environment where the spatial volume is small and the fly height is on the order of a few nm, even a small amount of contamination may lead to read / write errors. In addition, disk drive devices whose internal space is filled with a gas (e.g., helium, etc.) that is less dense than air are also beginning to become popular. In such disk drive devices, the air pressure inside the device may be less than 1 atmosphere. In that case, it becomes more difficult to suppress the volatilization of the lubricant components. Furthermore, in the case of HDDs that adopt the next-generation recording technology, the heat-assisted magnetic recording (HAMR) method, the temperature of the head part of the actuator may locally reach a high temperature of 400°C. This causes the internal temperature of the HDD to rise, and even if a low-volatility base oil is used, it may not be possible to reduce the amount of volatilization of the lubricant components. As described above, while the volatilization of the lubricant components is becoming more and more of a problem, the present inventors have further advanced the conventional challenge of making the constituent components of the lubricant low-volatile. Based on the new idea that even if volatilization occurs, the volatile components are unlikely to adhere to the disk, etc. (even if they do adhere, they do not remain), the present inventors have proceeded with the study of the constituent components. Then, by adopting an aromatic ester compound having an alkyl chain length of a certain length or more as the base oil, it was found for the first time that a grease composition that realizes the above idea can be obtained. Furthermore, they found a correlation between the adhesion of the components of the grease composition to the disk (adhesion suppression) and the occurrence of read / write errors (error suppression) in an actual HDD.

[0009] The grease composition for pivot assembly bearings (hereinafter also simply referred to as the grease composition) of the present invention will be described in detail below.

[0010] <Base oil> The grease composition of the present invention uses an aromatic ester base oil as the base oil. The aromatic ester base oil used in the present invention is an aromatic ester compound having an ester group as a substituent on the ring and an alkyl group having a total of 8 carbon atoms bonded to the oxygen atom of the ester group. It is characterized by the use of The present inventors have found for the first time that by using an aromatic ester compound having the above structure as a base oil, it is possible to obtain a grease composition having the property that even when the base oil is exposed to high temperatures and volatilizes, the volatilized base oil is less likely to adhere to the surface of a magnetic disk or the like.

[0011] The aromatic ester compounds mentioned above are compounds in which an alkyl group having a total of 8 or more carbon atoms is bonded to an aromatic ring via an ester group *-(CO)O- (* is the bond site with the aromatic ring). In other words, they are compounds in which the hydrogen atoms on the aromatic ring are substituted with alkyl ester groups having 8 or more carbon atoms (the number of carbon atoms here refers to the number of carbon atoms in the alkyl group portion). Examples of the aromatic ring include a benzene ring and a naphthalene ring, and particularly, a benzene ring. The number of alkyl ester groups substituted on the aromatic ring is not particularly limited, and examples thereof include compounds substituted with 1 to 3 alkyl ester groups. When the aromatic ester compound is a compound substituted with two or more alkyl ester groups, the alkyl ester groups may be the same or different. When the aromatic ester compound is a compound substituted with two or more alkyl ester groups, it is preferable that at least one of the alkyl ester groups is an alkyl ester group having 8 or more carbon atoms, and all of the alkyl ester groups are alkyl ester groups having 8 or more carbon atoms (the number of carbon atoms here refers to the number of carbon atoms in the alkyl group portion) (the same applies to the examples of linear and branched alkyl groups described below, and when the aromatic ester compound has two or more alkyl ester groups, at least one of the alkyl ester groups has the exemplified group, and preferably all of the alkyl ester groups have the exemplified group).

[0012] The alkyl group having a total of 8 or more carbon atoms may be linear or branched. A branched alkyl group may have a plurality of branched chains, and the branching positions are not particularly limited.

[0013] In the aromatic ester compound, the linear alkyl group having a total of 8 or more carbon atoms may be, for example, a linear alkyl group having 8 to 11 carbon atoms, or alternatively, a linear alkyl group having 9 to 11 carbon atoms. Additionally, a branched alkyl group having a total of 8 or more carbon atoms can have a total of 9 or more and 16 or less carbon atoms, for example, a total of 11 or more and 16 or less carbon atoms. The branched alkyl group having a total of 8 or more carbon atoms may be, for example, a branched alkyl group formed by bonding a branched chain to a linear alkyl group having 8 to 11 carbon atoms. The branched alkyl group is an alkyl group having a longest carbon chain with 8 to 11 carbon atoms, counting from the carbon atom bonded to the oxygen atom of the ester group. The branched alkyl group may be, for example, a branched alkyl group formed by bonding a plurality of branched chains to a linear alkyl group having 6 to 11 carbon atoms, so long as the total number of carbon atoms is 8 or more. In the above aromatic ester compound, the embodiment in which the alkyl group having a total of 8 or more carbon atoms is a linear alkyl group having the above-mentioned specific total number of carbon atoms, or the embodiment in which the alkyl group having a total of 8 or more carbon atoms is a branched alkyl group having the above-mentioned specific total number of carbon atoms means that the aromatic ester compound essentially contains the linear alkyl group or branched alkyl group as the alkyl group bonded to the oxygen atom of the ester group *-(CO)O-. In other words, for example, when an aromatic ester compound is substituted with two or more alkyl ester groups, the alkyl group of at least one of the alkyl ester groups may be the above-mentioned specific linear alkyl group or specific branched alkyl group, and it is not excluded that the alkyl group of the remaining alkyl ester group may be another alkyl group, and it is not intended to particularly exclude the use of aromatic ester compounds having other alkyl groups having a total of 8 or more carbon atoms as a base oil. In short, in the above aromatic ester compound, for example, the embodiment in which "the branched alkyl group having a total number of 8 or more carbon atoms is a branched alkyl group having a total number of 9 or more and 16 or less carbon atoms" can include any of the following embodiments in which the aromatic ester base oil consists only of aromatic ester compounds containing only the branched alkyl group as the alkyl group bonded to the oxygen atom of the ester group; the aromatic ester base oil consists only of aromatic ester compounds containing the branched alkyl group and another alkyl group (for example, a linear alkyl group having a total number of 8 or more and 11 or less carbon atoms) as the alkyl group bonded to the oxygen atom of the ester group; and the aromatic ester base oil contains one or both of these aromatic ester compounds and an aromatic ester compound containing an alkyl group other than the branched alkyl group as the alkyl group bonded to the oxygen atom of the ester group (for example, an aromatic ester compound containing a linear alkyl group having a total number of 8 or more and 11 or less carbon atoms). In one embodiment of the present invention, the aromatic ester base oil comprises at least an aromatic ester compound in which a branched alkyl group having a total number of 9 to 16 carbon atoms is bonded to an oxygen atom of the ester group, and in another embodiment, the aromatic ester base oil comprises at least an aromatic ester compound in which a branched alkyl group having a total number of 11 to 16 carbon atoms is bonded to an oxygen atom of the ester group.

[0014] An example of the aromatic ester compound is a triester of trimellitic acid (1,2,4-benzenetricarboxylic acid).

[0015] A preferred aromatic ester compound is a triester compound of trimellitic acid represented by the following formula: [ka] In the formula, R is each independently a linear or branched alkyl group having a total of 8 or more carbon atoms, for example, a linear alkyl group having from 8 to 11 carbon atoms, a branched alkyl group having from 8 to 11 carbon atoms to which a branched chain is bonded, or a branched alkyl group having from 6 to 11 carbon atoms to which two or more branched chains are bonded, and the total number of carbon atoms in the branched alkyl group can be, for example, 9 to 16.

[0016] The base oil has a kinetic viscosity at 40° C. of, for example, 40 to 150 mm 2 Anything in the range / s can be used.

[0017] The above-mentioned base oil can be contained in a proportion of, for example, 80 mass % or more based on the total mass of the grease composition of the present invention, and for example, the above-mentioned base oil is contained in a proportion of 80 mass % to 98 mass % based on the total mass of the grease composition.

[0018] <Thickener> The grease composition of the present invention can preferably use a urea compound as a thickener. Urea compounds have excellent heat resistance and water resistance, and are particularly stable at high temperatures, so they are preferably used as thickeners in applications under high temperature environments.

[0019] In the present invention, as the urea-based thickener, for example, an alicyclic aliphatic diurea compound can be used. A specific example of the diurea compound is represented by the following formula (1): . R1-NHCONH-R2-NHCONH-R3...Formula (1) (In formula (1), R1 and R3 each represent a monovalent aliphatic hydrocarbon group or a monovalent alicyclic hydrocarbon group, and the molar ratio of the alicyclic hydrocarbon group to the aliphatic hydrocarbon group in the total amount of the diurea compound is 6:4 to 8:2; R2 represents a divalent aromatic hydrocarbon group.

[0020] The above R1 and R3 may be the same, i.e., both may be monovalent aliphatic hydrocarbon groups or monovalent alicyclic hydrocarbon groups, or one may be a monovalent alicyclic hydrocarbon group and the other may be a monovalent aliphatic hydrocarbon group. However, the molar ratio of the alicyclic hydrocarbon group to the aliphatic hydrocarbon group in the total amount of the diurea compound represented by formula (1) is preferably in the range of 6:4 to 8:2. By setting the molar ratio of the alicyclic hydrocarbon group to the aliphatic hydrocarbon group in the above range, the storage modulus and the amount of oil separation of the grease composition containing the diurea compound can be set within a predetermined range. As described below, by employing a diurea compound in consideration of the storage modulus and amount of oil separation of the grease composition, when the grease composition is enclosed in a rolling bearing and the rolling bearing is operated, the shape of the enclosed grease composition is maintained and an appropriate amount of oil (base oil) is supplied to the rolling elements, resulting in a grease composition that has appropriate lubricating performance while suppressing dust generation.

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

[0022] The alicyclic aliphatic diurea compound used in the present invention can be synthesized using an amine compound and an isocyanate compound. For example, an alicyclic amine and an aliphatic amine are used as amine raw materials, and the compound is synthesized and obtained using the amine raw materials and an aromatic diisocyanate. The alicyclic amine and aliphatic amine, which are amine raw materials, are charged in an amount of, for example, alicyclic amine:aliphatic amine=6:4 to 8:2, and reacted with an aromatic diisocyanate to obtain a compound in which the molar ratio of alicyclic hydrocarbon group:aliphatic hydrocarbon group in the total amount of the diurea compound is 6:4 to 8:2. Examples of the amine compound include aliphatic amines such as hexylamine, octylamine, dodecylamine, hexadecylamine, octadecylamine (stearylamine), behenylamine, and oleylamine, as well as alicyclic amines such as cyclohexylamine. As the isocyanate compound, aromatic diisocyanates such as phenylene diisocyanate, tolylene diisocyanate (TDI), diphenyl diisocyanate, diphenylmethane diisocyanate (MDI), dimethylbiphenyl diisocyanate (TODI) and the like are used.

[0023] The thickener is contained in a proportion of, for example, 10% by mass to 15% by mass based on the total mass of the grease composition of the present invention. If the thickener is used in an amount exceeding 15% by mass, the amount of oil separation from the grease composition is too small, which may cause poor lubrication. On the other hand, if the thickener is used in an amount less than 10% by mass, the amount of oil separation is too large, which may cause contamination of the device, and may cause the grease to flow out of the grease pockets of the cage and become caught between the rolling elements and races of the bearing, resulting in an increase in rotational torque. In particular, from the viewpoint of obtaining a grease composition having an appropriate amount of oil separation and particularly excellent flow characteristics and life characteristics, it is preferable that the grease contains a thickener in a ratio of, for example, 10% by mass to 13% by mass. is preferred.

[0024] <Other additives> In addition to the above essential components, the grease composition may contain additives that are usually used in grease compositions, if necessary, within the range that does not impair the effects of the present invention. Examples of such additives include antioxidants, rust inhibitors, extreme pressure additives (extreme pressure agents), metal deactivators, antifriction agents (antiwear agents), oiliness improvers, viscosity index improvers, and thickeners. When these other additives are contained, the amount (total amount) of them added is usually 0.1 to 10 mass % based on the total amount of the grease composition.

[0025] Examples of the antioxidant include octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, pentaerythritol tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, 1,3,5-trimethyl-2,4,6-tris(3,5-di-t-butyl-4-hydroxybenzyl)benzene, triethylene glycol-bis[3- hindered phenolic antioxidants such as N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamate), octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid, 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 1,6-hexanediol-bis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], 2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], N,N'-hexamethylenebis(3,5-di-t-butyl-4-hydroxy-hydrocinnamate), and octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid; Other phenol-based antioxidants such as 2,6-di-t-butyl-4-methylphenol and 4,4-methylenebis(2,6-di-t-butylphenol); and amine-based antioxidants such as diphenylamine, alkylated diphenylamine, triphenylamine, hindered amine, phenyl-α-naphthylamine, alkylated phenyl-α-naphthylamine, phenothiazine, and alkylated phenothiazine. Among these, from the viewpoint of disk adhesion, phenol-based antioxidants, particularly octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], 2,2-thio-diethylene bis[3-(3,5-di-t-butyl Preferred are hindered phenol-based antioxidants selected from the group consisting of octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid, diphenylamine, alkylated diphenylamine, phenyl-α-naphthylamine, alkylated phenyl-α-naphthylamine, and other diarylamine compounds, and further preferred are hindered phenol-based antioxidants from the viewpoint of sludge suppression.

[0026] Examples of the extreme pressure additive include phosphorus-based compounds, chlorine-based compounds, and polymer esters. Among these, phosphate compounds such as phosphate esters, phosphites, and amine salts of phosphate esters, that is, phosphorus compounds, can be preferably used. Suitable phosphoric acid ester compounds include, for example, phosphoric acid triesters such as tricresyl phosphate (CAS No. 1330-78-5), triphenyl phosphate, tributyl phosphate, trioctyl phosphate, and trioleyl phosphate; dilauryl hydrogen phosphite (CAS No. 21302-09-0), tricresyl phosphite (CAS No. 25586-42-9), tris(2-ethylhexyl) phosphite (CAS No. 301-13-3), triisodecyl phosphite (CAS No. 301-13-3), and the like. S No. 25448-25-3), trilauryl phosphite (CAS No. 3076-63-9), tris(triisodecyl)phosphite (CAS No. 77745-66-5), trioleyl phosphite (CAS No. 13023-13-7), and other phosphite diesters and / or phosphite triesters; 2-ethylhexyl acid phosphate (CAS No. 12645-31-7), alkyl (C12, C14, C16, C18) acid phosphates, isotridecyl acid phosphate (CAS No. 52933-07-0), oleyl acid phosphate (CAS No. 37310-83-1), and other phosphoric acid monoesters and / or phosphoric acid diesters (acidic phosphoric acid esters); which are also available as commercial products. Among these, from the viewpoint of sludge suppression, the phosphoric acid triester, the phosphoric acid monoester and / or the phosphoric acid diester are preferred, and among these, the phosphoric acid triester is preferred.Specific examples include at least one selected from the group consisting of tricresyl phosphate (CAS No. 1330-78-5), triphenyl phosphate, tributyl phosphate, trioctyl phosphate, trioleyl phosphate, 2-ethylhexyl acid phosphate (CAS No. 12645-31-7), alkyl (C12, C14, C16, C18) acid phosphate, isotridecyl acid phosphate (CAS No. 52933-07-0), and oleyl acid phosphate (CAS No. 37310-83-1). Particularly from the viewpoint of corrosion inhibition, one selected from the group consisting of tricresyl phosphate (CAS No. 1330-78-5), triphenyl phosphate, tributyl phosphate, trioctyl phosphate, and trioleyl phosphate is preferred, and among these, tricresyl phosphate is preferred. From the viewpoint of suppressing sludge, it is desirable to avoid the use of sulfur-containing additives that have conventionally been used as extreme pressure additives, such as metal salts of sulfur-based compounds (e.g., calcium sulfonate) and thiophosphate triesters such as triphenoxyphosphine sulfide (TPPS), which can also be classified as phosphorus-based compounds.

[0027] Examples of the metal deactivator include benzotriazole and sodium nitrite.

[0028] Anti-wear agents include tricresyl phosphate and polymer esters. Examples of the polymer ester include esters of aliphatic monovalent carboxylic acids and divalent carboxylic acids with polyhydric alcohols. Specific examples of the polymer ester include, but are not limited to, the PRIOLUBE (registered trademark) series manufactured by Croda Japan.

[0029] The grease composition of the present invention can be obtained by blending the aromatic ester base oil, the thickener, and, if desired, other additives. Furthermore, for example, a urea-based grease (base grease) consisting of the aromatic ester-based base oil and the urea-based thickener may be blended with other additives as desired to obtain a grease composition. Typically, the content of the thickener in the base grease is about 10 to 30 mass %, and for example, the content of the diurea compound (urea-based thickener) in the above-mentioned urea-based grease can be, for example, about 10 to 25 mass %, or about 10 to 20 mass %.

[0030] <Storage modulus> The grease composition of the present invention preferably has a storage modulus in an appropriate range, for example, a storage modulus of 1,200 to 3,000 Pa at 25° C. measured under conditions of a film thickness of 1 mm and a shear strain of 1%.

[0031] The storage modulus is a value that indicates the shape stability of the grease, and is an effective parameter for understanding the shape stability of the grease immediately after it is packed into the bearing device and when the bearing device is oscillating. It is. For example, in a pivot assembly bearing device, grease is filled only in the grease pockets of the crown retainer, so if the shape of the grease changes from the shape at the time of filling, the grease will get tangled with the balls (rolling elements), etc., which not only leads to increased torque and torque fluctuations in the rolling bearing, but can also be a cause of dust generation. Therefore, the shape retention ability (shape stability) of the grease is an important factor in preventing initial and long-term torque stability and dust generation. From the viewpoint of the shape stability of the grease, it is preferable that the storage modulus of the grease composition of the present invention at 25°C under the above measurement conditions (film thickness 1 mm, shear strain 1%) is 1,200 Pa or more. However, if the storage modulus is too high, there is a possibility that the grease composition will fall out of the grease pocket of the crown-shaped retainer while maintaining its shape. In this case, since the grease is located on the orbital path of the rolling elements, resistance increases when the balls pass over the grease, and there is a concern of an increase in torque. Therefore, it is preferable that the storage modulus does not exceed 3,000 Pa.

[0032] <About oil separation amount> The grease composition of the present invention has an oil separation amount in an appropriate range, for example, an oil separation amount at 80°C of 200 to 270 mm 2 / mg is preferred.

[0033] Conventionally, there has been an oil separation measurement test as a method for evaluating the amount of oil (base oil and additives) that seeps out from grease. Since the life of the grease changes depending on the amount of oil separation, understanding the amount of oil separation is not only important for understanding the life characteristics of the grease, but also for obtaining appropriate lubrication performance. For example, in a pivot assembly bearing in which grease is applied to the grease pockets between the ball pockets of a crown-shaped retainer, if the amount of oil separation is too small, the lubricant components (base oil and additives) supplied to the balls (rolling elements) will be insufficient over time, which may lead to torque roughness and seizure. On the other hand, if the amount of oil separation is too large, there is also the problem that contamination due to oil leakage is more likely to occur.

[0034] Greases containing the urea-based thickener used in the present invention generally have a low amount of oil separation, so when the amount of oil separation is measured using an oil separation measurement method according to a publicly known standard such as JIS K2220, which specifies the method for measuring the degree of oil separation, it may be difficult to see a clear difference in the measurement results. For this reason, in the present invention, a unique method was adopted that makes the difference in the amount of oil separation clearer. Specifically, 9 mg of the grease composition was placed in a cylindrical shape with a diameter of 3 mm on the side of the medicine wrapping paper on which the medicine is placed, and after leaving it in an environment of 80°C for 24 hours, the area of ​​the oil-stained part (stained base oil) that occurred on the medicine wrapping paper was measured. The area of ​​the oil-stained part per mass of grease was then calculated as the amount of oil separation (mm 2 In this test, the drug packaging paper used was "Pure White Imitation (Medium)" (size: 105 mm × 105 mm, thickness: 42 μm, basis weight: 30 g / m 2 ) and the grease composition was allowed to stand on the side (glossy side) on which the medicine was to be placed, as described above. Based on the above definition, when conventional greases that did not experience poor lubrication were evaluated using this unique method, the amount of oil separation was approximately 230 to 280 mm 2 It was confirmed that the amount of oil separation was about 200 mm 2 With conventional greases with a grease content of less than 1 / mg, seizure due to poor lubrication was confirmed. Also, considering that too much oil separation can cause oil leakage, the upper limit was set at 300 mm 2 / mg.

[0035] Based on the above results, for the grease composition of the present invention, 9 mg of the grease composition was placed in a cylindrical shape of φ3 mm on a medicine wrapping paper and left in an environment of 80°C for 24 hours. At that point, the area of ​​the oil-stained part on the medicine wrapping paper was measured, and the amount of oil separation, which is the area of ​​the oil-stained part per mass of the grease composition, was determined to be 200 mm 2 / mg~270mm 2 / mg is evaluated as being suitable.

[0036] [Pivot assembly bearing (rolling bearing)] The pivot assembly bearing according to the present invention is, in other words, a rolling bearing. Preferred embodiments of the rolling bearing will be described in detail below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the following embodiments.

[0037] 1 is a radial cross-sectional view of a rolling bearing 10 according to a preferred embodiment of the present invention. The rolling bearing 10 has a basic structure similar to that of a rolling bearing of the prior art, and includes an annular inner ring 11, an outer ring 12, a plurality of rolling elements 13, a cage 14, and a seal member 15. The inner ring 11 is a cylindrical structure installed coaxially with the center axis of a shaft (not shown) on the outer periphery side of the shaft. The outer ring 12 is a cylindrical structure arranged coaxially with the inner ring 11 on the outer periphery side of the inner ring 11. Each of the multiple rolling elements 13 is a ball arranged in a raceway in an annular bearing space 16 formed between the inner ring 11 and the outer ring 12. That is, the rolling bearing 10 in this embodiment is a ball bearing. The cage 14 is disposed in the raceway and holds the rolling elements 13. The cage 14 is an annular body that is installed coaxially with the central axis of the shaft, and has a structure in which a plurality of pockets for holding the rolling elements 13 are provided on one side in the direction of the central axis, and the rolling elements 13 are accommodated in each pocket. The rolling elements 13 are held at predetermined intervals in the circumferential direction of the inner ring 11 and the outer ring 12 by the cage 14, and the falling off of the rolling elements 13 and the contact between adjacent rolling elements 13 are suppressed. The shape (crown shape, corrugated shape, etc.) and material (steel plate, resin, etc.) of the cage 14 generally used in rolling bearings are arbitrary, but a crown-shaped cage (see FIG. 5) is preferably used in the pivot assembly bearing according to the present invention. As shown in FIG. 5, the crown-shaped cage 60 has a cylindrical annular member 61 centered on the central axis (rotation axis) of the rolling bearing 10 (not shown). The annular member 61 has an outer peripheral surface, an inner peripheral surface, and two end faces 61a connecting the outer peripheral surface and the outer peripheral surface. On one end face 61a of the annular member 61, a plurality of ball pockets (recesses) 62 for rotatably accommodating balls (rolling elements 13, not shown) are formed at predetermined intervals along the circumferential direction. Furthermore, the annular member 60 has a pair of claws 63 (63a, 63b) extending from the one end face 61a at both ends of each ball pocket 62. The pair of claws 63 are curved to approach each other so as to follow the curved surface of the ball accommodated in each ball pocket 62, thereby preventing the balls accommodated in each ball pocket 62 from falling out. In addition, a grease pocket 64 is formed between the two ball pockets 62 due to the presence of the claws 63. A grease composition G (not shown) described later is accommodated in the grease pocket 64 and contributes to lubrication between the ball pocket 62 and the balls (rolling elements 13) accommodated therein. The seal member 15 is fixed to the inner peripheral surface of the outer ring 12 and extends toward the inner ring 11 to seal the bearing space 16. A grease composition G is enclosed in the bearing space 16 sealed by the seal member 15. The grease composition G is the grease composition for pivot assembly bearings of the present invention described above. The amount of the grease composition G enclosed in the bearing space 16 is, for example, 2% to 30% of the volume. In particular, in a pivot assembly bearing device described below that requires low torque, 3% to 10% is more preferable. By setting the amount of the grease composition G within this range, the grease composition G can sufficiently lubricate the rolling elements 13, the inner ring 11, and the outer ring 12 in the bearing space 16 of the rolling bearing 10, reducing frictional resistance and reducing frictional torque. The seal member 15 is formed, for example, from a steel plate or rubber, and examples thereof include a steel plate shield that does not contact the outer periphery of the inner ring 11, and a non-contact rubber seal that does not contact the outer periphery of the inner ring 11. Either of the above-mentioned steel plate shields or non-contact rubber seals can be used in the present invention. From the viewpoint of suppressing outgassing, it is preferable to use a steel plate shield. Note that while this drawing shows an embodiment equipped with the seal member 15, the rolling bearing of the present invention also covers an embodiment of a rolling bearing that does not have a seal member. In the rolling bearing 10 having the above-mentioned configuration, the grease composition G acts to reduce friction between the rolling elements 13 and the cage 14, and between the rolling elements 13 and the inner ring 11 or the outer ring 12. The reduction in friction reduces the friction torque and suppresses the generation of frictional heat, promoting smooth rotation of the inner ring 11 and the outer ring 12. As can be seen from the configuration shown in FIG. In this way, the grease composition G enclosed in the rolling bearing 10 lubricates the spaces between the rolling elements 13 and the inner ring 11 or the outer ring 12 when the rolling bearing 10 rotates.

[0038] The rolling bearing 10 of this embodiment is used as a rolling bearing provided in a pivot assembly bearing device, i.e., as a pivot assembly bearing, as described above. The rolling bearing 10 of this embodiment has the advantage that, by using the specific grease composition described above, even if volatile components are generated during operation, the volatile components are less likely to adhere to the disk, thereby making it possible to suppress the occurrence of magnetic disk read / write errors, which are partly caused by the adhesion of the volatile components.

[0039] [Bearings and drive units] The bearing device according to the present invention is a pivot assembly bearing device, and the drive device is a disk drive device. A pivot assembly bearing device including the pivot assembly bearing (rolling bearing) of the above-described embodiment, and a disk drive device equipped with the bearing device will be described below with reference to the accompanying drawings. It should be noted that the present invention is not limited to the following embodiments.

[0040] FIG. 2 is a perspective view showing the overall configuration of a disk drive device 20 according to a preferred embodiment of the present invention. As shown in FIG. 2, the disk drive device 20 of this embodiment includes a substantially rectangular box-shaped base (base plate) 21, a spindle motor 22 mounted on the base 21, a magnetic disk 23 rotated by the spindle motor 22, a swing arm 24 having a magnetic head 25 that writes information to a predetermined position on the magnetic disk 23 and reads information from any position, a pivot assembly bearing device 30 that supports the swing arm 24 so that it can swing, an actuator 26 that drives the swing arm 24, and a control unit 27 that controls these devices.

[0041] The disk drive device of the present invention may be, for example, a disk drive device equipped with nine or more magnetic disks having a diameter of 3.5 inches. In a device having such a large number of disks, the spatial volume within the device is further reduced. The disk drive device may have an internal space filled with a gas having a density lower than air. In a disk drive device whose internal space is filled with such a low-density gas, the air pressure within the device may be less than 1 atmosphere. The disk drive device may also employ a thermally assisted magnetic recording (HAMR) method as a recording method. In a disk drive device employing a thermally assisted magnetic recording (HAMR) method, the temperature of the head portion of the actuator may locally reach a high temperature of 400°C.

[0042] FIG. 3 is a cross-sectional view of a pivot assembly bearing device 30 according to a preferred embodiment of the present invention. The pivot assembly bearing device 30 of this embodiment is mainly composed of a shaft (axis) 31, a first bearing 40 and a second bearing 50 which are two rolling bearings fitted onto the shaft 31 with a predetermined length of space S between them, and a sleeve 32 (outer peripheral member) which exteriorly covers the two rolling bearings 40, 50. The sleeve 32 has a spacer portion 32a provided for arranging the two rolling bearings 40, 50 with a predetermined length of space S in the axial direction. In this manner, the shaft 31 is supported in a freely rotatable state by the first bearing 40 and the second bearing 50 . The spacer portion 32a is not limited to being integrally formed with the sleeve 32 as in the embodiment shown in FIG. 3, and the sleeve and the spacer may be formed as separate parts.

[0043] For the first bearing 40 and the second bearing 50, the rolling bearing 10 according to the embodiment of the present invention described above is used. The first bearing 40 is mainly composed of a first inner ring 41, a first outer ring 42, balls 43 which are multiple rolling elements arranged in a raceway formed between the first inner ring 41 and the first outer ring 42, a retainer 44 arranged in the raceway and holding the balls 43, a sealing member 45 which isolates the raceway from the outside world, and the grease composition of the present invention (not shown) sealed in the raceway. Similarly, the second bearing 50 is mainly composed of a second inner ring 51, a second outer ring 52, balls 53 which are multiple rolling elements arranged in a raceway formed between the second inner ring 51 and the second outer ring 52, a retainer 54 arranged in the raceway and holding the balls 53, a sealing member 55 which isolates the raceway from the outside world, and the grease composition of the present invention (not shown) sealed in the raceway. The shaft 31 has a cylindrical shaft body 31a and a flange portion 31b formed on one end side of the shaft body 31a, and is attached to the base 21 with the flange portion 31b positioned on the base 21 (see FIG. 2) side of the disk drive device 20. One end of the second inner ring 51 of the second bearing is in contact with the flange portion 31b of the shaft.

[0044] The pivot assembly bearing device 30 of this embodiment uses first and second bearings 40, 50 which are rolling bearings (pivot assembly bearings) filled with the above-mentioned grease composition for pivot assembly bearings of the present invention. A typical rolling bearing rotates continuously in one direction, but the pivot assembly bearing device 30 performs a high-speed oscillating motion that repeats forward and reverse rotations at a small angle in order to move the magnetic head 25 of the disk drive device 20 above the magnetic disk 23. It is necessary to move the magnetic head 25 to an accurate position with high response speed.

[0045] The grease composition used in this embodiment is such that even if the base oil volatilizes at high temperatures, the volatilized base oil is less likely to adhere to magnetic disks, etc., making it possible to suppress disk read / write errors in disk drive devices. Furthermore, the grease composition used in this embodiment can achieve an appropriate range of oil separation amount and exhibits excellent grease shape stability. This makes it possible to prevent insufficient supply of lubricant and oil leakage. As a result, the disk drive device 20 of this embodiment can stably drive the rolling bearings (first and second bearings 40, 50) for a long period of time. This leads to the suppression of disk read / write errors in the disk drive device and enables the life of the pivot assembly bearing device and the disk drive device to be extended.

[0046] The present invention is not limited to the embodiments and specific examples described in this specification, and various changes and modifications are possible within the scope of the technical ideas described in the claims. EXAMPLES

[0047] The present invention will be described in more detail below with reference to examples, although the present invention is not limited thereto.

[0048] [Evaluation of base oils used in grease compositions for pivot assembly bearings] Using various base oils (Examples 1 to 15) shown in Table 1, (1) a disk adhesion test and (2) a read / write error occurrence test were carried out according to the following procedures. The aromatic ester compound having a branched alkyl group with a total of 11 carbon atoms used in Example 12 is a compound represented by the following formula (K): Examples 9, 10, 13, and 15 are esters of trimellitic acid and a mixture of two or three alcohols in which a hydroxy group is bonded to an alkyl group bonded to a benzene ring shown in each example, and the branched (1) alkyl group in Example 13 and the branched (1) alkyl group in Example 15 are alkyl groups corresponding to R in the following formula (K). [ka]

[0049] <Test Method> (1)Disc adhesion test(1) An electroless nickel-plated aluminum magnetic disk was washed twice with n-hexane and isopropyl alcohol of 99% purity or higher, and then completely dried. 5 μL of base oil (sample oil) diluted to 10 vol% with hexane was dropped onto the disk and left to stand for 1 hour. The state of the droplet after dropping was photographed with a camera fixed above the disk. The total area of ​​the droplet immediately after dropping (about 5 seconds later) and after leaving it for 1 hour was calculated using image analysis software, and the percentage (%) of the area value after leaving it for 1 hour to the area value immediately after dropping [area value 1 hour after dropping (final area) / area value immediately after dropping (initial area)] was defined as "disc adhesion" (when the area value before and after leaving it still did not change at all, the disc adhesion was evaluated as 100%). This test was performed multiple times for each sample at a temperature of 20-30°C and a humidity of 30-70% RH, and the average value of the values ​​where reproducibility was obtained (area value results: within ±5%, N=4 or more) was adopted as the test result. The obtained results are also shown in Table 1.

[0050] (2) Read / write error occurrence test The cover of an unused disk drive was removed, and 2 mg of base oil (sample oil) was applied to the upper periphery of the control unit (control unit 27 in FIG. 2) on the back surface of the cover (the surface on the inner side of the housing, not shown in FIG. 2), and then the cover with the sample oil applied was attached to the disk drive. The same type of disk drive was used for all samples. Five units (N=5) were tested for each test condition. A heater was placed in contact with the cover surface (the surface facing the outside of the housing, not shown in FIG. 2) around the oil application area, and the temperature of the heater was changed from a reference temperature of 40°C to 80°C, 110°C, 140°C, 175°C, 190°C, or 200°C in sequence based on a predetermined program (Table 2 and FIG. 4) described later. The disk drive continued to operate by repeatedly measuring the speed of the disk drive using speed measurement software (e.g., CrystalDiskMark). The disk drive was monitored for read / write errors during operation using a connected computer. During the operation and monitoring of the disk drive, the time when at least one sector on the disk became unreadable (confirmed as "Reallocated Sectors Count" in the disk drive status monitoring software) was recorded as the time when a read / write error occurred. The results are shown in Table 1. As a reference test, each base oil was left at 80°C for one week, and the amount of evaporation [%] of the base oil after leaving it was calculated (N=2). The results are also shown in Table 1.

[0051] If the base oil volatilizes due to an increase in the ambient temperature, some of the volatilized base oil will condense when the temperature drops, and this condensed base oil may adhere to the disk or head of a disk drive device, causing an error in the device. In other words, it can be said that errors are likely to occur when the temperature drops, but if no errors occur during this time, the temperature rise level before the temperature drop can be judged to be acceptable. In this test, if a read / write error occurred after 408 hours, the test was evaluated as passed. The criterion of 408 hours is the time elapsed until heating began with the heater temperature set to 140°C, as shown in Table 2. Generally, HDDs are not designed to be used in environments exceeding 70°C. The temperature around the pivot assembly bearing inside the HDD is slightly higher than the environmental temperature. Therefore, the criterion for passing the test was that no read / write errors (no replacement sectors) occurred even after a cycle (241 to 408 hours) of heater temperature: 110°C, at which the temperature of the sample oil applied inside the HDD becomes about 80°C. In addition, it is essential that this test, especially the process of removing the cover of the disk drive device and then reinstalling it, be carried out in a clean room to prevent contamination from the outside.In addition, this test was carried out without applying sample oil, and it was confirmed that no errors occurred even after 1080 hours had passed, which was the test cutoff time.

[0052] [Table 1]

[0053] [Table 2]

[0054] As shown in Table 1, the area ratio (%) of the base oils of Examples 7 to 15 after one hour of dropping to the area value immediately after dropping was less than 15%, and it was confirmed that the base oils were less likely to adhere to the disk compared to the base oils of Examples 1 to 6. Furthermore, it was confirmed that the base oils of Examples 7 to 15 did not produce any replaced sectors after 408 hours had passed, and thus cleared the criterion for passing the read / write error test. Furthermore, as shown in Table 1, it was confirmed that there is a correlation between disk adhesion and the time when a replaced sector occurs.

[0055] [Various additives] <(3) Disc adhesion test (2)> Using the same procedures and test procedures as those in the above <(1) Disc Adhesion Test (1)>, the disc adhesion of the antioxidants shown in Table 3 used in the grease compositions for pivot assembly bearings was evaluated. The abbreviations in Table 3 are as follows. <Hindered phenol-based antioxidant> Irganox L115: 2,2-thio-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], BASF Japan Ltd. Irganox L135: Octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamate Irganox 1076FD: Octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate, BASF Japan Ltd. Irganox 245: Triethylene glycol-bis[3-(3-t-butyl-5-methyl-4-hydroxyphenyl)propionate], BASF Japan Ltd. Irganox 565: 2,4-bis-(n-octylthio)-6-(4-hydroxy-3,5-di-t-butylanilino)-1,3,5-triazine, BASF Japan ( KK) <Amine-based antioxidants> <Diarylamine antioxidant> Irganox L57: Diphenylamine represented by the following formula [B] Irganox L67: Diphenylamine represented by the following formula [B] [ka] (In the formula, R' and R" each independently represent an octyl group, a hydrogen atom, or a tert-butyl group.) Irganox L06: Octylated phenyl-α-naphthylamine, BASF Japan Ltd. <Hindered amine antioxidants> Adeka STAB LA-72: Bis(1,2,2,6,6-pentamethyl-4-piperidyl) sebacate, ADEKA Corporation · Irgalube Base 10: Dodecanoic acid (2,2,6,6-tetramethyl-4-piperidyl), BASF Japan Ltd.

[0056] An electroless nickel-plated aluminum magnetic disk was washed twice with n-hexane and isopropyl alcohol, each having a purity of 99% or higher, and then completely dried. Each antioxidant shown in Table 3 was diluted to 10 vol % with an alkyl ester of trimellitic acid having 11 carbon atoms (compound represented by the above formula (K)), and then further diluted to 10 vol % with hexane to prepare an antioxidant sample. 5 μL of this antioxidant sample was dropped onto the above-mentioned washed and dried disk, and the disk was allowed to stand for 1 hour. The state of the droplet after dropping was observed in the same manner as in <(1) Disc Adhesion Test (1)> above, and the disc adhesion [%] was calculated from the area of ​​the droplet before and after leaving it for 1 hour after dropping. The results were evaluated according to the following criteria. <Disc adhesion criteria> A: Disc adhesion is less than 30% N: Disc adhesion is 30% or more

[0057] [Table 3]

[0058] As shown in Table 3, it was confirmed that the hindered phenol-based antioxidants and diarylamine-based antioxidants are antioxidants that are less likely to adhere to the disk. On the other hand, it has been evaluated that hindered amine-based antioxidants tend to adhere to disks, and it has been confirmed that they are not suitable for addition to the grease composition for pivot assembly bearings which is the subject of the present invention.

[0059] <(4) Sludge generation evaluation> The extreme pressure additives shown in Table 4 used in the grease composition were evaluated for sludge formation. The extreme pressure additives shown in Table 4 were each diluted with an alkyl ester of trimellitic acid having 11 carbon atoms (the compound represented by the above formula (K)) to a concentration of 1 to 2 vol %. Each of the extreme pressure additive samples was subjected to a Shell high speed class 4 test in accordance with ASTM D 4172 at a rotation speed of 1,200 rpm, a load of 392 N, a temperature of 75° C., and a time of 5 minutes.

[0060] Images of the balls after the high-speed four-ball test were taken with an optical microscope (magnification: 200x). As reference images, Fig. 6 shows the images of balls that were given an E, A, or N rating based on the judging criteria described below [Fig. 6(a): Rating E, Fig. 6(b): Rating A, Fig. 6(c): Rating N] (image analysis, described below, was performed based on the images shown in Fig. 6). The following analysis of the images was performed using the image analysis software ImageJ 1.53f. The captured image was converted to 16-bit grayscale (65,536 gradations), and then converted to monochrome two-gradation, with the areas with color tones of 0 to 100 treated as black. These black areas correspond to areas where sludge has occurred. After conversion, the left and right ends of the image, where the amount of light is unstable, were each removed by 15% of the image width. The monochrome two-tone image after excluding both the left and right ends was used as the image to be analyzed, and the sum of the area of ​​the black parts of the image to be analyzed was calculated using the Analyze Particles function of the image analysis software ImageJ 1.53f. The ratio of the sum of the areas of the black parts to the total area of ​​the image to be analyzed [sum of the areas of the black parts / total area of ​​the image to be analyzed] (percentage (%)) was taken as the area rate, and was evaluated according to the following criteria. <Judgment criteria> E (Very good): Area ratio is less than 0.1% A (Good): Area ratio is 0.1% or more and less than 10% N(Not Good): Area ratio is 10% or more

[0061] [Table 4]

[0062] As shown in Table 4, the sludge rating for the phosphate triester, phosphate monoester and / or phosphate diester was E (very good), and the sludge rating for the phosphite diester and / or phosphite triester was A (good), confirming that the phosphate ester-based extreme pressure additives suppress sludge. On the other hand, the sulfur-containing additive was rated as N (not suitable) for sludge, which resulted in it being deemed unsuitable for grease compositions.

[0063] Although the best embodiment has been described in detail above, the present invention is not limited to the above embodiment, and modifications and improvements within the scope of the present invention that can achieve the object of the present invention are included in the present invention. [Explanation of symbols]

[0064] 10... rolling bearing, 11... inner ring, 12... outer ring, 13... rolling element, 14... cage, 15... sealing member, 16... bearing space, 20... disk drive device, 21... base (base plate), 22... spindle motor, 23... magnetic disk, 24... swing arm, 25... magnetic head, 26... actuator, 27...Control unit, 30... pivot assembly bearing device, 31... shaft (axis), 31a... shaft main body, 31b... flange portion, 32... sleeve (outer peripheral member), 32a... spacer portion, 40... first bearing, 41... first inner race, 42... first outer race, 43... ball (rolling element), 44... retainer, 45... seal member, 50... second bearing, 51... second inner race, 52... second outer race, 53... ball (rolling element), 54... retainer, 55... seal member, 60: crown-shaped cage; 61: annular member; 61a: end face; 62: ball pocket (recess); 63 (63a, 63b): claw; 64: grease pocket

Claims

[Claim 1] A method for evaluating the adhesion of a base oil to a magnetic disk, comprising: a disk preparation process in which electroless nickel-plated aluminum magnetic disks are washed and then dried; a dropping step of dropping a base oil diluted with an organic solvent onto the washed and dried disk and leaving the disk to stand; A photographing step of photographing the state of the droplets immediately after dropping and after leaving them to stand with a camera; a calculation step of calculating the total area of ​​the droplets using image analysis software, and calculating the percentage (%) of the area value of the droplets after standing to the area value of the droplets immediately after dropping [area value after standing (final area) / area value immediately after dropping (initial area)], which is the disc adhesion (%); and a method for evaluating disc adhesion.

Images