JP2015205958A - Bearing oil composition and spindle motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bearing oil composition which is suppressed in rise of the viscosity at practical temperatures, reduced in evaporation loss and improved in oxidation stability and a spindle motor using it.SOLUTION: A bearing oil composition contains an ester base oil which consists of an ester of 2-ethyl-2-butyl-1,3-propanediol with an ester of a 6-12C carboxylic acid and has a total number of carbon atoms of the ester of 21-33 and one or more anti-wear agents selected from phosphorus-based anti-wear agents and polyoxyethylene sorbitol esters. A spindle motor is equipped with a fluid dynamic pressure bearing part using the bearing oil composition.

Description

  The present invention relates to a bearing oil composition and a spindle motor.

Conventionally, a sintered impregnated bearing obtained by impregnating a porous member obtained by sintering a metal powder with a lubricating oil or grease has been used in personal computers, other peripheral devices, AV equipment, automobile electrical systems, and the like. In recent years, fluid dynamic pressure bearings have been put to practical use in models that require high-speed and high-precision shaft rotation, such as hard disk and polygon mirror rotation motors.
As the performance required for lubricating oil for these sintered impregnated bearings and fluid dynamic bearings, as motors become smaller, more precise, and faster, heat resistance, oxidation stability, low evaporation, and wear resistance are improved. Many of them require long life because they are used without replenishment.

  Furthermore, fluid dynamic pressure bearings are also used in portable terminals such as personal computers, AV equipment, digital cameras, and mobile phones, but these are battery-driven, so it is necessary to reduce motor power consumption and battery consumption. It is done. As means for suppressing this power consumption, means such as a low-viscosity base oil is often used as the base oil of the lubricating oil, and a means such as reducing the friction with an additive is used. However, since the base oil generally has a tendency to increase the evaporation loss when the viscosity is lowered, a base oil having a low viscosity at a practical temperature and a reduced evaporation loss is required.

As such lubricating oils for bearings, conventionally, polyα-olefins having excellent fluidity at low temperatures, aliphatic monocarboxylic acids having a specific structure, and dibasic acid diesters represented by di-2-ethylhexyl sebacate , Neopentyl glycol, trimethylolpropane fatty acid ester, and the like as base oils, and various additives have been used (see, for example, Patent Documents 1 to 3).
Patent Document 4 discloses a bearing lubricant containing an ester compound composed of 2-ethyl-2-methyl-1,3-propanediol and a carboxylic acid having 5 to 12 carbon atoms.

JP 2000-336383 A JP 2002-146374 A Japanese Patent Application Laid-Open No. 2002-338979 JP 2010-138316 A

  Along with higher performance and smaller size of various machines, there is a demand for a bearing oil that is further excellent in viscosity characteristics and at the same time excellent in low evaporation.

  Therefore, the present invention has been made in recent years while bearing applications have been expanded and further increased in speed and accuracy, and high viscosity is suppressed at practical temperatures, evaporation loss is small, and oxidation is performed. It is an object of the present invention to provide a bearing oil composition having improved stability and a spindle motor using the same.

  As a result of intensive studies to solve the above problems, the present inventors have found that a bearing oil composition containing a specific ester as a base oil and further containing a specific antiwear agent solves the above problems. As a result, the present invention has been completed. That is, the present invention is as follows.

<1> An ester base oil which is an ester of 2-ethyl-2-butyl-1,3-propanediol and a carboxylic acid having 6 to 12 carbon atoms, wherein the ester has a total carbon number of 21 to 33 And at least one antiwear agent selected from the group consisting of phosphorus-based antiwear agents and polyoxyethylene sorbitol esters.
<2> The bearing oil composition according to <1>, further containing 0.05 to 2% by mass of at least one antioxidant selected from the group consisting of amine-based antioxidants and phosphites.
<3> The bearing according to <1> or <2>, containing 0.05 to 5% by mass of at least one type of antiwear agent selected from the group consisting of the phosphorus-based antiwear agent and polyoxyethylene sorbitol esters. Oil composition.
<4> A spindle motor comprising a stationary part including a stator, a rotating part including a rotor magnet, and a fluid dynamic pressure bearing part including the bearing oil composition according to any one of <1> to <3>.

  According to the present invention, it is possible to provide a bearing oil composition in which high viscosity is suppressed at a practical temperature, evaporation loss is small, and oxidation stability is improved, and a spindle motor using the same.

It is a schematic block diagram which shows an example of a structure of the spindle motor to which the bearing oil composition of this invention is applied.

  Hereinafter, the bearing oil composition of the present invention and the spindle motor to which it is applied will be described in detail. In addition, in this specification, "-" showing a numerical range represents the range containing the numerical value of the upper limit and the minimum.

  The bearing oil composition of the present invention is an ester of 2-ethyl-2-butyl-1,3-propanediol and a carboxylic acid having 6 to 12 carbon atoms, and the ester has a total carbon number of 21 to 21. 33 ester base oil, and at least one antiwear agent selected from the group consisting of phosphorus-based antiwear agents and polyoxyethylene sorbitol esters.

(A) Ester base oil The ester base oil used in the present invention is a total carbon number obtained by esterification of 2-ethyl-2-butyl-1,3-propanediol and a carboxylic acid having 6 to 12 carbon atoms. Is an ester oil of 21 to 33, and has a structure represented by the following general formula (1).

In the general formula (1), R ¹ and R ² each independently represent a linear or branched hydrocarbon group having 5 to 11 carbon atoms.

The diol component of the ester compound constituting the ester base oil used in the present invention is 2-ethyl-2-butyl-1,3-propanediol.
The diol component of this ester compound is one in which an ethyl group and a butyl group are bonded to the 2-position carbon of 1,3-propanediol. In this structure, the β-position carbon of the alcohol is a quaternary carbon. With this structure, hydrolysis stability and oxidation stability can be improved, and as a result, low evaporation is considered to be achieved.
At the same time, by aligning the number of carbons in the alcohol side chain with an even number of ethyl and butyl groups, the carbon located in the side chain is compared to methyl and pentyl groups, or propyl and propyl groups. It is considered that low evaporation can be achieved.

  On the other hand, the carboxylic acid component constituting the ester base oil used in the present invention has 6 to 12 carbon atoms, preferably 8 to 11 and more preferably 8 to 10 carbon atoms. If the number of carbon atoms is too small, it tends to evaporate, and if the number of carbon atoms is too large, the viscosity tends to be high.

  The carboxylic acid constituting the ester base oil used in the present invention may be any of a carboxylic acid having a plurality of carboxyl groups such as a dicarboxylic acid and a tricarboxylic acid in addition to a monocarboxylic acid. The carboxylic acid constituting the ester base oil of the present invention may be any of linear or branched, saturated or unsaturated, aliphatic, alicyclic, or aromatic.

The saturated aliphatic monocarboxylic acid may be linear or branched, and specific examples include hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, undecanoic acid, and dodecanoic acid. These carboxylic acids include all isomers.
Examples thereof include saturated alicyclic carboxylic acids such as cyclohexanecarboxylic acid.

The unsaturated aliphatic monocarboxylic acid may be linear or branched, and specific examples include hexenoic acid, heptenoic acid, octenoic acid, nonenic acid, decenoic acid, undecenoic acid, and dodecenoic acid. These carboxylic acids include all isomers.
Further, unsaturated alicyclic monocarboxylic acids such as 3-cyclohexenecarboxylic acid and aromatic monocarboxylic acids such as benzoic acid and naphthalenecarboxylic acid are also included.

The saturated aliphatic dicarboxylic acid may be linear or branched and includes hexanedioic acid, heptanedioic acid, octanedioic acid, nonanedioic acid, decanedioic acid, undecanedioic acid, and dodecanedioic acid. These saturated fatty acids include all isomers.
In addition, saturated alicyclic dicarboxylic acids such as 1,4-cyclohexanedicarboxylic acid and dimer acid are also included.

The unsaturated aliphatic dicarboxylic acid may be linear or branched and includes nonene diacid, decenedioic acid, undecenedioic acid, dodecenedioic acid, tridecenedioic acid, tetradecenedioic acid, and pentadecenedioic acid. These unsaturated fatty acids include all isomers.
Further, unsaturated alicyclic dicarboxylic acids such as cis-4-cyclohexene-1,2-dicarboxylic acid and aromatic dicarboxylic acids such as phthalic acid, isophthalic acid, and terephthalic acid are also included.

  Examples of the tricarboxylic acid include aliphatic tricarboxylic acids such as aconitic acid, cyclohexanetricarboxylic acid, citric acid, and isocitric acid, and aromatic tricarboxylic acids such as trimellitic acid and trimesic acid.

Of these carboxylic acids, monocarboxylic acids are preferred, aliphatic monocarboxylic acids are more preferred, and saturated aliphatic linear monocarboxylic acids are most preferred from the viewpoint of startability at low temperatures.
In addition, since there exists a tendency for a viscosity to become high when there are too many carbon numbers when there are too few carbon numbers and there exists a tendency for a viscosity to become high when there are too many carbon numbers, the thing whose carbon number is 6-12 is used. The number is preferably 8 to 11, and most preferably 8 to 10.

  The ester base oil used in the present invention is an ester oil obtained by esterifying the above-mentioned alcohol and carboxylic acid, and is esterified using two or more carboxylic acids having 6 to 12 carbon atoms. Also good.

  Note that the ester base oil used in the present invention tends to have poor evaporability when the carbon number is too small, and the viscosity tends to be high when the carbon number is too large. Therefore, the total number of carbon atoms of the ester base oil used in the present invention is 21 to 33, preferably 25 to 31 and most preferably 25 to 29.

  The bearing oil composition of the present invention comprises the above-described ester base oil. By satisfy | filling said structure, the ester base oil used by this invention can suppress high viscosity, and can also be used as a low evaporation base oil. For example, two or more ester base oils obtained by esterifying two or more carboxylic acids having 6 to 12 carbon atoms with 2-ethyl-2-butyl-1,3-propanediol separately are mixed. Alternatively, a base oil obtained by esterifying two or more carboxylic acids having 6 to 12 carbon atoms together with 2-ethyl-2-butyl-1,3-propanediol may be used.

  As long as the performance of the bearing oil composition of the present invention is not reduced, lubricating base oils other than the above esters, such as mineral lubricating base oils and synthetic lubricating base oils, may be used as the base oil. You may mix. Examples of the mineral oil base oil include base oil obtained by refining a lubricating oil fraction of crude oil by an appropriate combination such as solvent refining and hydrorefining. Examples of synthetic lubricant base oils include poly-α-olefins, monoesters, diesters, polyol esters, alkylbenzenes, polyglycols, and phenyl ethers. In order to obtain a bearing oil composition excellent in low viscosity and low evaporation, the ester base oil is 60 to 100 mass based on the total amount of the base oil, although it depends on the performance of the bearing oil required by the equipment used. %, Preferably 80 to 100% by mass.

(B) Antiwear agent The bearing oil composition of the present invention contains at least one antiwear agent selected from the group consisting of phosphorus antiwear agents and polyoxyethylene sorbitol esters in order to improve antiwear properties. contains.

Furthermore, in consideration of the influence on the hydrolysis of linear aliphatic monocarboxylic acid ester and the evaporability considered to be caused by this, it is preferable to select an antiwear agent that is unlikely to be a factor in promoting hydrolysis. .
From this viewpoint, as the antiwear agent, one or more selected from the group consisting of alkylaryl phosphates and polyoxyethylene sorbitol esters in which R ^{3 to} R ⁵ are alkylaryl groups in the general formula (2) described later are used. It is particularly preferable to use polyoxyethylene sorbitol esters.

The content of the antiwear agent is preferably 0.05 to 5% by mass, more preferably 0.1 to 3% by mass, further preferably 0.15 to 2% by mass, It is especially preferable that it is 0.15-1.5 mass%.
When the content of the antiwear agent in the bearing oil composition of the present invention is 0.05% by mass or more and 5% by mass or less, it is economical and a sufficient effect of improving the antiwear property can be obtained. In addition, when content of the said antiwear agent exceeds 5 mass%, the antiwear effect according to content may not be acquired.

  The said antiwear agent may be used individually by 1 type, and may combine 2 or more types. In the case of combining two or more, the total amount is preferably within the above range.

(I) Phosphorous antiwear agent Examples of the phosphorus antiwear agent include phosphates, acid phosphates, and amine salts of acid phosphates.
Examples of the phosphate include those having a structure represented by the following general formula (2).

In General formula (2), R < ³ > -R < ⁵ > represents a hydrogen atom or a C1-C22 alkyl group, an alkenyl group, an alkylaryl group, or an arylalkyl group, and may be same or different. When R ^{3 to} R ⁵ are an alkyl group, an alkenyl group, an alkylaryl group, or an arylalkyl group, a carbon number of 3 to 9 is preferable. If the alkyl group, alkenyl group, alkylaryl group or arylalkyl group represented by R ^{3 to} R ⁵ has more than 22 carbon atoms, the solubility in the base oil may be reduced.

  Examples of the phosphate include triaryl phosphate, trialkyl phosphate, and the like. Specifically, benzyl diphenyl phosphate, allyl diphenyl phosphate, triphenyl phosphate, tricresyl phosphate, tributyl phosphate, cresyl diphenyl phosphate, dicresyl phenyl Mention may be made of compounds such as phosphate, ethylphenyldiphenylphosphate, diethylphenylphenylphosphate, propylphenyldiphenylphosphate, dipropylphenylphenylphosphate, triethylphenylphosphate, tripropylphenylphosphate and the like.

  Examples of the acid phosphate include those having a structure represented by the following general formula (3) or the following general formula (4).

In General Formula (3) and General Formula (4), R ⁶ and R ⁷ represent a hydrocarbon group having 4 or more carbon atoms, and may be the same or different.
Specific examples of R ⁶ and R ⁷ include linear or branched saturated or unsaturated aliphatic hydrocarbon groups having 4 to 20 carbon atoms, that is, alkyl groups and alkenyl groups, and aromatic hydrocarbons having 4 to 20 carbon atoms. Group and a cycloalkyl group. When the carbon number is 4 to 20, the effect of improving the wear resistance is sufficiently exhibited. The hydrocarbon group represented by R ⁶ and R ⁷ preferably has 6 to 18 carbon atoms, and more preferably 8 to 12 carbon atoms.

  Specific examples of the acid phosphate include 2-ethylhexyl acid phosphate, isodecyl acid phosphate, di (2-ethylhexyl) phosphate, and the like.

  Preferred examples of the acid phosphate amine salt include those obtained by neutralizing the above acid phosphate with an alkylamine having a structure represented by the following general formula (5).

In the general formula (5), R ^{8 to} R ¹⁰ each independently represents a monovalent hydrocarbon group having 1 to 22 carbon atoms or a hydrogen atom, and at least one of R ^{8 to} R ¹⁰ is carbonized. It is a hydrogen group. R ^{8 to} R ¹⁰ are preferably a monovalent hydrocarbon group having 4 to 18 carbon atoms or a hydrogen atom.
Specific examples of the alkylamine include dibutylamine, octylamine, dioctylamine, laurylamine, dilaurylamine, oleylamine, coconut amine, and beef tallow amine.

  These phosphorus wear inhibitors may be used alone or in combination of two or more.

(Ii) Polyoxyethylene sorbitol esters The polyoxyethylene sorbitol esters are represented by the following general formula (6), and a polyoxyethylene group in which 3 to 100 moles of an oxyethylene group are polymerized to a hydroxyl group of sorbitol is bonded. It is a compound having a structure in which monocarboxylic acid is ester-bonded to oxyethylene sorbitol.

In General Formula (6), a, b, c, d, e, and f are integers in which a + b + c + d + e + f is 3 to 100, and R ^{11 to} R ¹⁶ are hydrogen or a C 5-25 monocarboxylic acid-derived moiety. It is.
The monocarboxylic acid is preferably an aliphatic monocarboxylic acid having 5 to 25 carbon atoms, more preferably an aliphatic monocarboxylic acid having 8 to 22 carbon atoms, and most preferably an aliphatic monocarboxylic acid having 10 to 20 carbon atoms. The ester of sorbitol and the above monocarboxylic acid may be any of monoester, diester, triester, tetraester, pentaester, and hexaester, and may be an ester with one fatty acid or a mixed ester with two or more fatty acids. .

(C) Antioxidant The bearing oil composition of the present invention can be blended with an antioxidant to improve oxidation stability, in which case it is selected from the group consisting of amine-based antioxidants and phosphites. It is preferable to contain at least one antioxidant.
Furthermore, in order to further improve the oxidative stability, diphenylamines of the general formula (7) and alkylated phenyl-α-naphthylamines of the general formula (8), which are amine antioxidants described later, and phosphorus It is preferable to contain both phosphites of the general formula (9) which are antioxidants. Since the effect of suppressing the evaporation loss can be expected by improving the oxidation stability, it is preferable to contain the amine-based antioxidant and the phosphorus-based antioxidant from the viewpoint of suppressing the evaporation loss. As the oxidative deterioration progresses, the acid value may increase over a long period of use, but this may cause the evaporation loss due to the hydrolysis of the ester base oil.

The content of at least one antioxidant selected from the group consisting of the amine-based antioxidant and phosphite is preferably 0.05 to 2% by mass, and preferably 0.1 to 1% by mass. Is more preferable, and 0.1 to 0.5% by mass is even more preferable. When the content of the antioxidant is 0.05% by mass or more and 2% by mass or less, it is economical and a sufficient effect of the antioxidant can be obtained. In addition, when content of the said antioxidant exceeds 2 mass%, the effect corresponding to content may not be acquired.
The said antioxidant may be used individually by 1 type, and may combine 2 or more types. In the case of combining two or more, the total amount is preferably within the above range.

(I) Amine-based antioxidant In the bearing oil composition of the present invention, examples of the amine-based antioxidant preferably used as the antioxidant include diphenylamines and alkylated phenyl-α-naphthylamine.
Examples of the diphenylamines include those having a structure represented by the following general formula (7).

In the general formula (7), R ¹⁷ and R ¹⁸ represent a hydrogen atom or a linear or branched alkyl group having 1 to 16 carbon atoms. The linear or branched alkyl group is preferably a linear or branched alkyl group having 3 to 9 carbon atoms, and particularly preferably a linear or branched alkyl group having 4 to 8 carbon atoms. If the linear or branched alkyl group has more than 16 carbon atoms, the solubility in the base oil may be reduced.

The R ¹⁷ and R ¹⁸ may be the same or different. Specific examples of the diphenylamines include diphenylamine, butyldiphenylamine, octyldiphenylamine, dibutyldiphenylamine, octylbutyldiphenylamine, and dioctyldiphenylamine. Diphenylamines may be used alone or in combination of two or more.

  Examples of the alkylated phenyl-α-naphthylamine include those having a structure represented by the following general formula (8).

In the general formula (8), R ¹⁹ represents a linear or branched alkyl group having 1 to 16 carbon atoms, preferably a linear or branched alkyl group having 4 to 8 carbon atoms. Specific examples of the alkylated phenyl-α-naphthylamine include n-pentylated phenyl-α-naphthylamine, 2-methylbutylated phenyl-α-naphthylamine, 2-ethylhexylated phenyl-α-naphthylamine, and n-octylated phenyl. -Α-naphthylamine, 1-methyloctylated phenyl-α-naphthylamine and the like. The alkylated phenyl-α-naphthylamine may be used alone or in combination of two or more.

(Ii) Phosphite As a preferable antioxidant in the bearing oil composition of the present invention, phosphite which is a phosphorus-based antioxidant can be mentioned.
Examples of the phosphite include those having a structure represented by the following general formula (9).

In the general formula (9), R ²⁰ and R ²¹ represent a linear or branched alkyl group having 1 to 20 carbon atoms, preferably a linear or branched alkyl group having 2 to 6 carbon atoms. . These phosphites may be used alone or in combination of two or more.

(D) Other additives Various additives other than the above can be blended in the bearing oil composition of the present invention, if necessary. Examples include rust inhibitors such as benzotriazole and derivatives thereof, alkyl succinic acid derivatives, dispersants such as polyalkenyl succinimide and derivatives thereof, pour point depressants such as styrene-butadiene copolymers, polyisobutylene and polymethacrylate, Examples thereof include viscosity index improvers such as olefin polymers and conductivity imparting agents.

(E) Kinematic Viscosity / Viscosity Index of Composition The bearing oil composition of the present invention preferably has a kinematic viscosity at 40 ° C. of 9 to 16 mm ² / s, more preferably 11 to 15 mm ² / s. When the 40 ° C. kinematic viscosity is 9 mm ² / s or more, evaporation loss is suppressed and sufficient lubrication performance can be maintained. Further, when it is 16 mm ² / s or less, an increase in viscous torque when the shaft rotates and a decrease in low-temperature fluidity are suppressed. Therefore, it is considered that the kinematic viscosity is in the range of 9 to 16 mm ² / s from the viewpoint of low power consumption, evaporation loss, and lubrication performance.
In addition, the bearing oil composition of the present invention preferably has a viscosity index of 110 or more, more preferably 120 or more. When the viscosity index is 110 or more, a change in viscosity with respect to a change in temperature is suppressed, and an increase in viscosity torque at a low temperature is suppressed.

  The bearing oil composition of the present invention can be suitably used for fluid dynamic pressure bearings and sintered impregnated bearings used for bearing parts of rotating bodies such as spindle motors. For example, in a spindle motor comprising a stationary part including a stator, a rotating part including a rotor magnet, and a fluid dynamic pressure bearing part, the bearing oil composition of the present invention can be suitably applied to the fluid dynamic pressure bearing part. it can.

  FIG. 1 is a schematic configuration diagram showing an example of the configuration of a spindle motor to which the bearing oil composition of the present invention is applied. The spindle motor shown in FIG. 1 includes a stationary part 20 and a rotating part 30. The rotating part 30 is supported rotatably with respect to the stationary part 20 by a fluid dynamic pressure bearing which is a preferred embodiment. In the following description, when describing the positional relationship and direction of each member in the upper, lower, left and right directions, the positional relationship and direction in the drawings are only shown, and the positional relationship and direction when incorporated in an actual device are not shown. Absent.

  The base 10 includes a flat portion 11 and an annular boss portion 13 provided at the center of the flat portion 11. Between the annular boss part 13 and the annular step part 14 provided on the outer peripheral part of the flat part 11 is an annular recess. The stator 17 and a rotor magnet 34 attached to the hub 31 described later are disposed in an annular recess. The annular boss portion 13 has a cylindrical support wall 15 protruding upward, and a stator 17 is fixed to the cylindrical support wall 15.

  Inside the annular boss portion 13, a bearing stationary portion 20 constituting a part of the fluid dynamic pressure bearing portion is disposed. The bearing stationary portion 20 includes a substantially cylindrical sleeve 21 and a counter plate 22 that closes the lower opening of the sleeve 21. The inner peripheral surface of the sleeve 21 includes a small-diameter inner peripheral surface 21a, a medium-diameter inner peripheral surface 21b, and a large-diameter inner peripheral surface 21c. The small diameter inner peripheral surface 21a is a radial bearing surface. The medium diameter inner peripheral surface 21b is positioned below the sleeve 21 and has a larger outer diameter than the small diameter inner peripheral surface 21a. The large-diameter inner peripheral surface 21c is located at the lower end of the sleeve 21 and has an outer diameter larger than that of the medium-diameter inner peripheral surface 21b. The counter plate 22 is disposed on the large-diameter inner peripheral surface 21 c and is fixed to the sleeve 21. A tapered surface 23 described later is disposed on the upper outer peripheral surface of the sleeve 21.

The rotating unit 30 includes a rotor hub 31 and a shaft 32 fixed to the rotor hub 31.
The rotor hub 31 is formed from a ferromagnetic material such as iron or stainless steel. A cylindrical portion 31b is disposed on the outer peripheral portion of the disk portion 31a. A flange portion 31c extending radially outward from the cylindrical portion 31b is disposed at the lower portion of the cylindrical portion 31b. An annular wall 31d extending downward from the disk portion 31a is disposed inside the cylindrical portion 31b.
The outer peripheral surface 32a of the shaft 32 and the small-diameter inner peripheral surface 21a of the sleeve 21 are opposed to each other in the radial direction through a minute gap.

  A stopper 33 is disposed below the shaft 32. The outer diameter of the plate portion 33 a of the stopper 33 is larger than the outer diameter of the shaft 32 and smaller than the inner diameter of the medium-diameter inner peripheral surface 21 b. When the plate portion 33 a comes into contact with the sleeve 21, the shaft 32 is prevented from coming off the sleeve 21.

An annular rotor magnet 34 is disposed inside the cylindrical portion 31 b of the rotor hub 31. The rotor magnet 34 faces the stator 17 via a gap.
One or a plurality of recording disks are arranged on the flange portion 31 c of the rotor hub 31.

There are minute gaps between the small-diameter inner circumferential surface 21a of the sleeve 21 and the outer circumferential surface 32a of the shaft 32, and between the lower surface of the disk portion 31a of the rotor hub 31 and the upper end surface of the sleeve 21, respectively. Is filled with bearing oil 40.
The bearing oil 40 also fills a space surrounded by the inner diameter inner peripheral surface 21 b of the sleeve 21, the upper surface of the counter plate 22, and the circular plate portion 33 a of the stopper 33. A tapered seal portion 41 is formed between the inner peripheral surface 31 f of the annular wall 31 d of the rotor hub 31 and the tapered surface 23 on the upper outer periphery of the sleeve 21. The gap of the taper seal portion 41 is reduced as it goes upward. The bearing oil 40 exists in the taper seal portion 41, and the gas-liquid interface of the bearing oil 40 is located in the taper seal portion 41.

  On the small-diameter inner peripheral surface 21a of the sleeve 21, for example, a herringbone-shaped dynamic pressure generating groove array is disposed. A pair of radial dynamic pressure bearings 42 and 43 are formed in the minute gap between the small-diameter inner peripheral surface 21 a of the sleeve 21 and the outer peripheral surface 32 a of the shaft 32. The shaft 32 is supported in the radial direction by the dynamic pressure generated by the herringbone-shaped dynamic pressure generating groove array when the spindle motor rotates. Further, on the upper end surface of the sleeve 21, for example, a spiral-shaped dynamic pressure generating groove array is disposed. A thrust dynamic pressure bearing 44 is formed in a minute gap between the upper end surface of the sleeve 21 and the lower surface of the disk portion 31a. When the spindle motor rotates, the rotor hub 31 floats due to the dynamic pressure generated by the spiral-shaped dynamic pressure generating groove array.

Next, the present invention will be described more specifically with reference to examples. In addition, this invention is not restrict | limited at all by these examples.
The base oil and additive components used in the preparation of the compositions in each Example and Comparative Example are as follows.

(A) Base oil In the examples and comparative examples, the base oil components * 1 to * 4 shown below were evaluated.

* 1: Diester oil A
Obtained by a Fischer ester synthesis reaction of 2-ethyl-2-butyl-1,3-propanediol and decanoic acid (saturated aliphatic monocarboxylic acid having 10 carbon atoms) in the presence of an acid catalyst, represented by the following formula: The resulting ester compound was designated as diester oil A.

In the above formula, R ³¹ and R ³² each represent a straight-chain saturated hydrocarbon having 9 carbon atoms.

* 2: Diester oil B represented by the following formula

In the above formula, R ³³ and R ³⁴ each represent a straight-chain saturated hydrocarbon having 7 carbon atoms.

* 3: Diester oil C represented by the following formula

In the above formula, R ³⁵ and R ³⁶ each represent a straight-chain saturated hydrocarbon having 9 carbon atoms.

* 4: Diester oil D represented by the following formula

(B) Antiwear agent (Bi) Phosphate In general formula (2), R ^{3 to} R ⁵ are the same cresyl group.

(B-ii) Polyoxyethylene sorbitol ester In the general formula (6), a + b + c + d + e + f is 6, R ^{11 to} R ¹⁴ are oleic acid, and R ¹⁵ and R ¹⁶ are hydrogen.

(B-iii) Polyoxyethylene sorbitol ester In the general formula (6), a + b + c + d + e + f is 30, R ^{11 to} R ¹⁴ are oleic acid, and R ¹⁵ and R ¹⁶ are hydrogen.

(C) Antioxidant (Ci) Diphenylamines An alkylated diphenylamine in which R ¹⁷ and R ¹⁸ are linear or branched C ₈ H ₁₇ in the general formula (7).

(C-ii) Alkylated phenyl-α-naphthylamine In the general formula (8), alkylated phenyl-α-naphthylamine in which R ¹⁹ is linear or branched C ₈ H ₁₇ .

(C-iii) Phosphite Phosphite in which R ²⁰ and R ²¹ are t-butyl in the general formula (9).

(Evaluation method)
In this example, the following evaluation methods were used to evaluate the low viscosity, low evaporation, oxidation stability, and wear resistance required as lubricating oils for impregnated bearings.
[Kinematic viscosity / viscosity index]
The kinematic viscosity at 0 ° C., 40 ° C., and 100 ° C. was measured by the kinematic viscosity test method specified in JIS K 2283, and the kinematic viscosity and the viscosity index were evaluated.

[Evaporation]
A distillation curve was determined by a GC distillation test method for petroleum products and a test method based on ASTM D 2887. From the distillation curve, 50% distillation temperature was evaluated as an evaporating index. It can be evaluated that the higher the 50% distillation temperature, the better the evaporability.

[Rotating cylinder oxidation stability (RBOT test)]
This evaluation is called Rotating Bomb Oxidation Test, and is abbreviated as RBOT hereinafter. Based on JIS K 2514, the time until the specified pressure drop at 150 ° C. was evaluated as the RBOT life.

[Shell four-ball test method]
This was carried out in accordance with ASTM D 2783, which is one of the methods for evaluating the wear resistance of a lubricating oil, and the wear resistance was evaluated by the wear diameter under the following conditions.
Test conditions Rotation speed: 1200rpm
Load: 30kgf (294N)
Test time: 30min

<Examples 1-3, Comparative Examples 1-3>
As shown in Tables 1 and 2, in each base oil, various additives are blended in the amounts shown in Tables 1 and 2, and the bearing oil compositions of Examples 1 to 3 and Comparative Examples 1 to 3 are respectively prepared. The evaluation described in Tables 1 and 2 was performed. The results are shown in Tables 1 and 2.
In Tables 1 and 2, “◯” means that the corresponding base oil is contained, and the content of each base oil is such that the total amount with the additive is 100% by mass.

From Tables 1 and 2, Examples 1 to 3 using the diester base oil A which is the base oil of the bearing oil composition of the present invention are different in structure from the ester base oil which is the base oil of the bearing oil of the present invention. When compared with Comparative Examples 1 to 3 using a diester base oil, it can be seen that the viscosity characteristics are almost the same, but show low evaporability.
Further, in comparison with Comparative Examples 1 to 3, Examples 1 to 3 have almost the same wear diameter in the shell 4-ball test, but the RBOT life time is long, and the bearing oil composition of the present invention is oxidized. It turns out that it is excellent in stability.

DESCRIPTION OF SYMBOLS 10 Base 11 Flat part 13 Annular boss part 14 Annular step part 15 Cylindrical support wall 17 Stator 20 Bearing stationary part 21 Sleeve 21a Small diameter inner peripheral surface 21b Medium diameter inner peripheral surface 21c Large diameter inner peripheral surface 22 Counter plate 23 Tapered surface 30 Rotation Part 31 rotor hub 31a disk part 31b cylindrical part 31c flange part 31d annular wall 31f inner peripheral surface 32 shaft 32a outer peripheral surface 33 stopper 33a circular plate part 34 rotor magnet 40 bearing oil 41 taper seal part 42, 43 radial dynamic pressure bearing 44 thrust dynamics Pressure bearing

Claims (4)

An ester base oil of 2-ethyl-2-butyl-1,3-propanediol and a carboxylic acid having 6 to 12 carbon atoms, wherein the ester has a total carbon number of 21 to 33;
At least one antiwear agent selected from the group consisting of phosphorus-based antiwear agents and polyoxyethylene sorbitol esters;
Containing a bearing oil composition.   Furthermore, the bearing oil composition of Claim 1 which contains 0.05-2 mass% of at least 1 sort (s) of antioxidant chosen from the group which consists of an amine antioxidant and a phosphite.   The bearing oil composition according to claim 1 or 2, comprising 0.05 to 5% by mass of at least one antiwear agent selected from the group consisting of the phosphorus-based antiwear agent and polyoxyethylene sorbitol esters. .   A spindle motor comprising: a stationary part including a stator; a rotating part including a rotor magnet; and a fluid dynamic pressure bearing part having the bearing oil composition according to any one of claims 1 to 3.