Fluid dynamic pressure bearing oil, spindle motor, and disk drive device

Abstract

[Problem] To provide a fluid dynamic pressure bearing oil and a fluid dynamic pressure bearing having the same sealed therein. Further, to provide: a spindle motor which, by having the fluid dynamic pressure bearing oil and bearing applied therein, is capable of suppressing adhesion of a volatile component to a magnetic disk, etc., even when the fluid dynamic pressure bearing oil is volatilized, and which can also suppress HDD reading and writing error occurrence; and a disk drive device comprising the same. [Solution] Provided is a fluid dynamic pressure bearing oil which comprises at least one compound selected from the group consisting of a monoester compound represented by formula (1) and a diester compound represented by formula (2). (1): R1-C(=O)O-R2 (In formula (1), R1 is an alkyl group having a total of ten or more carbon atoms, and R2 is an alkyl group having a total of nine or more carbon atoms.) (2): R3-E1-R4-E2-R5 (In formula (2), R3 and R5 are each independently an alkyl group having a total of eight or more carbon atoms; R4 is an alkylene group having a total of four or more carbon atoms; and E1 and E2 each independently represent -C(=O)O- or -OC(=O)-.)

Description

Fluid dynamic bearing oil, spindle motor and disk drive device

[0001] The present invention relates to a fluid dynamic bearing oil, a fluid dynamic bearing filled with the fluid dynamic bearing oil, and a spindle motor.

[0002] Pivot assemblies used at the fulcrum of actuators in hard disk drives (HDDs) and bearings built into spindle motors use various lubricants such as greases and oils to ensure smooth operation of these components and smooth driving of the devices. For example, a rolling bearing incorporated into a hard disk drive actuator has been proposed, which is filled with grease made by blending a diurea compound having at least one of an alicyclic hydrocarbon group and an aliphatic hydrocarbon group in the skeleton as a thickener with a base oil containing an aromatic ester oil (Patent Document 1).

[0003] Japanese Patent Application Laid-Open No. 2006-236410

[0004] One cause of read / write errors in HDDs is the volatilization of lubricant components, such as base oil, enclosed in bearings built into the actuator or spindle motor. When the volatilized base oil cools and condenses on the surface of the magnetic disk or the magnetic head, adhering to these as a liquid or solid, it is believed that the magnetic disk and the magnetic head may adhere to each other, preventing normal reading and writing, which is one of the causes of read / write errors. Even if attempts are made to suppress the volatilization of lubricant components due to temperature increases during HDD operation, for example by selecting a low-volatility base oil, it is difficult to completely eliminate the volatilization of the components.

[0005] The present invention aims to provide a fluid dynamic bearing oil and a fluid dynamic bearing containing the same, and to provide a spindle motor and a disk drive device equipped with the same, which, by applying the fluid dynamic bearing oil and bearing, can prevent volatile components from adhering to magnetic disks, etc., even if the fluid dynamic bearing oil volatilizes, thereby preventing HDD read / write errors.

[0006] One aspect of the present invention relates to a fluid dynamic bearing oil for use in a disk drive device, the fluid dynamic bearing oil comprising at least one compound selected from the group consisting of a monoester compound represented by the following formula (1) and a diester compound represented by the following formula (2): １ -C(=O)O-R ２ (1) (In formula (1), R １ is a linear or branched alkyl group having a total of 10 or more carbon atoms, and R １ When R is a branched alkyl group, the branched chain has 10 or more carbon atoms; ２ is a linear or branched alkyl group having a total of 9 or more carbon atoms, and R ２ When R is a branched alkyl group, the branched chain has 7 or more carbon atoms. ３ -E １ -R ４ -E ２ -R ５ (2) (In formula (2), R ３ and R ５ are each independently a linear or branched alkyl group having a total of 8 or more carbon atoms, ３ and R ５ When E is a branched alkyl group, １ or E ２ The carbon atom number of the longest chain, counting from the carbon atom bonded to R, is 9 or more, ４ is a linear or branched alkylene group having a total of 4 or more carbon atoms; １ and E ２ each independently represents -C(=O)O- or -OC(=O)-. The present invention also relates to a fluid dynamic bearing filled with fluid dynamic bearing oil. The present invention further relates to a spindle motor equipped with a fluid dynamic bearing. And the present invention also relates to a disk drive device equipped with a spindle motor.

[0007] 1 is a conceptual diagram illustrating an example of the structure of a main part of a spindle motor according to the present invention, and is a schematic diagram illustrating an example of the structure of a drive device (disk drive device) according to the present invention.

[0008] As mentioned above, proposals have been made to suppress the volatilization (e.g., outgassing) of lubricant components used in HDD actuators and spindle motors, which is thought to be one of the causes of HDD read / write errors. Even if volatilization of commonly used lubricant components is suppressed, volatilization itself cannot be eliminated. Conventional disk drive devices have a sufficiently large fly height (the distance between the magnetic head and the disk). Therefore, suppressing volatile components could prevent read / write errors. However, with increasing recording density, the fly height has decreased to a few nanometers. In this case, a negative pressure 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, potentially liquefying even traces of volatile components. Furthermore, with the recent increase in recording capacity per HDD, the number of disks in the device has increased, and disk drive devices equipped with nine or more 3.5-inch disks have become available. These devices further reduce the internal volume. In environments with such small spatial volumes and fly heights on the order of a few nanometers, even trace amounts of contamination can lead to read / write errors. Disk drive devices whose internal space is filled with a gas (e.g., helium) that is less dense than air are also becoming more common. In such disk drive devices, the internal pressure can be less than 1 atmosphere. In such cases, suppressing the volatilization of lubricant components becomes more difficult. Furthermore, in HDDs employing the next-generation recording technology, heat-assisted magnetic recording (HAMR), the temperature of the actuator head can locally reach as high as 400°C. This increases the internal temperature of the HDD, potentially making it impossible to reduce the amount of volatilization of lubricant components even when a low-volatility base oil is used. As described above, as the volatilization of lubricant components becomes increasingly problematic, the inventors have taken a step further in addressing the conventional challenge of making lubricant components less volatile. The present inventors have investigated the constituent components based on the new idea that even if volatilization occurs, the volatile components are unlikely to adhere to the disk or the like (and even if they do adhere, they will not remain).The inventors then discovered for the first time that by using an aliphatic monoester compound or diester compound having an alkyl chain length of at least a certain length as one component of a lubricant, it was possible to obtain a fluid dynamic bearing oil that realizes the above-mentioned concept. The fluid dynamic bearing oil of the present invention will be described in detail below.

[0009] [Fluid Dynamic Bearing Oil] The fluid dynamic bearing oil used in the fluid dynamic bearing and spindle motor of the present invention described below contains at least one compound selected from the group consisting of aliphatic monoester compounds and diester compounds having a specific alkyl chain length.

[0010] <Monoester Compound> The monoester compound is represented by formula (1): １ -C(=O)O-R ２ (1) In the above formula (1), R １ R is a linear or branched alkyl group having a total of 10 or more carbon atoms, preferably 23 or less carbon atoms. １ When R is a branched alkyl group, the number of carbon atoms in the branched chain can be 10 or more, preferably 15 or less. ２ R is a linear or branched alkyl group having a total of 9 or more carbon atoms, preferably 20 or less carbon atoms. ２ When R is a branched alkyl group, the number of carbon atoms in the branched chain is 7 or more, and preferably 8 or less. In this specification, the number of carbon atoms in the branched chain means the number of carbon atoms in the branched portion of the branched alkyl group, and does not mean the number of carbon atoms counted from the carbon atom bonded to the carbonyl group (-C(=O)-) or the oxygen atom (-O-). In a preferred embodiment, R １ and R ２ One of the groups may be a straight chain alkyl group and the other may be a branched alkyl group.

[0011] <Diester Compound> The diester compound is represented by formula (2). ３ -E １ -R ４ -E ２ -R ５ (2) In formula (2), R ３ and R ５R each independently represents a linear or branched alkyl group having a total of 8 or more carbon atoms, preferably 10 or less carbon atoms. ３ and R ５ When E is a branched alkyl group, １ or E ２ Counting from the carbon atom bonded to R, the longest carbon chain may have 9 or more carbon atoms, preferably 9 carbon atoms. ４ is a linear or branched alkylene group having a total of 4 or more carbon atoms, preferably 6 or less carbon atoms. ３ and R ５ are both linear alkyl groups, and R ４ is a branched alkyl group, or R ３ and R ５ are both branched alkyl groups, and R ４ is preferably a linear alkyl group. １ and E ２ each independently represents —C(═O)O— or —OC(═O)—.

[0012] In a preferred embodiment, R ３ and R ５ can be the same group. １ and E ２ Is E １ represents —C(═O)O— and E ２ represents —OC(═O)—, or E １ represents —OC(═O)— and E ２ may represent -C(=O)O-. For example, R ３ and R ５ represent the same linear alkyl group, and R ４ represent the same branched alkyl group, E １ represents —C(═O)O— and E ２ Alternatively, R ３ and R ５ represent the same branched alkyl group, R ４ represent the same linear alkyl group, E １ represents —OC(═O)— and E２ may represent -C(=O)O-.

[0013] <Additives> The fluid dynamic bearing oil of the present invention may contain additives commonly used in fluid dynamic bearing oils as needed, within the range that does not impair the effects of the present invention. Examples of the additives include extreme pressure additives, mineral oils, and base oils used in combination, such as poly-α-olefins, antioxidants, metal detergents, oiliness agents, antiwear agents, metal deactivators, corrosion inhibitors, rust inhibitors, viscosity index improvers, pour point depressants, conductivity imparting agents, dispersants, antifoaming agents, and hydrolysis inhibitors.

[0014] As the extreme pressure additive, conventionally known additives containing sulfur, chlorine, phosphorus, etc. can be used, and examples thereof include phosphorus-based compounds such as phosphate esters, phosphites, and phosphate amine salts; sulfur-based compounds such as sulfides and disulfides; chlorine-based compounds such as chlorinated paraffin and chlorinated diphenyl; and metal salts of sulfur-based compounds such as zinc dialkyldithiophosphate and molybdenum dialkyldithiocarbamate.

[0015] Examples of antioxidants include phenolic antioxidants, diphenylamines, phosphorus-based antioxidants, and sulfur-based compounds such as phenothiazine. These antioxidants may be used alone or in combination. Among these, from the viewpoint of disc adhesion, phenolic antioxidants, particularly hindered phenolic antioxidants selected from the group consisting of 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-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], and octyl-3,5-di-tert-butyl-4-hydroxyhydrocinnamic acid, are preferred. In addition, from the viewpoint of disc adhesion, it is desirable to avoid the use of alkylated phenyl-α-naphthylamine.

[0016] Examples of anti-wear agents include phosphate, phosphite, acid phosphate, etc. However, from the viewpoint of disk adhesion, it is desirable to avoid the use of amine salts of acid phosphates, which are commonly used as anti-wear agents.

[0017] Examples of rust inhibitors include dodecenyl succinic acid half ester. Examples of metal deactivators include benzotriazole-based compounds and thiadiazole-based compounds. Examples of viscosity index improvers include polyalkyl methacrylate, polyalkylstyrene, polybutene, etc. Examples of pour point depressants include the viscosity index improvers polyalkyl methacrylate, polyalkylstyrene, polybutene, etc. Examples of conductivity imparting agents include nonionic surfactants, ionic liquids, phenylsulfonic acid, etc. Examples of dispersants include polyalkenyl succinimides, polyalkenyl succinic acid amides, polyalkenyl benzylamines, polyalkenyl succinic acid esters, etc. Examples of hydrolysis inhibitors include alkyl glycidyl ether-type epoxy compounds, glycidyl ester-type epoxy compounds, alicyclic epoxy compounds, carbodiimides, etc.

[0018] [Fluid Dynamic Bearing] Preferred embodiments of a fluid dynamic bearing will now be described in detail with reference to the accompanying drawings. Figure 1 is a schematic diagram illustrating a fluid dynamic bearing according to one embodiment of the present invention and a spindle motor equipped with the fluid dynamic bearing. Note that the embodiments described below are exemplary embodiments of the present invention, and the present invention is not limited to these.

[0019] As shown in Figure 1, spindle motor 1 is used as a motor for driving data storage devices equipped with magnetic disks, optical disks, etc. used in computers. Overall, it is composed of a stator assembly 2 and a rotor assembly 3. Although the spindle motor 1 in Figure 1 is a rotating shaft type motor, the present invention is also applicable to fixed shaft type motors.

[0020] The stator assembly 2 is fixed to a cylindrical portion 5 that protrudes upward from a housing 4 (base plate) that constitutes the enclosure of the data storage device. A stator core 8 around which a stator coil 9 is wound is fitted and attached to the outer periphery of the cylindrical portion 5.

[0021] The rotor assembly 3 has a rotor hub 10, which is fixed to the upper end of a shaft portion 11 and rotates together with the shaft portion 11. The shaft portion 11 is inserted into a sleeve 7, which is a bearing member, and is rotatably supported by the sleeve 7. The sleeve 7 is fitted into and fixed to the inside of the cylindrical portion 5. A lower cylindrical portion 10a of the rotor hub 10 rotates inside the housing 4, and a back yoke 13 is attached to the inner circumferential surface of the lower cylindrical portion 10a. A rotor magnet 14 is fitted and fixed inside the back yoke 13 and is magnetized to multiple poles, namely, north and south poles.

[0022] When current is applied to the stator coil 9, a magnetic field is generated by the stator core 8, and this magnetic field acts on the rotor magnet 14 disposed within the magnetic field, causing the rotor assembly 3 to rotate. A recording disk, such as a magnetic disk (not shown), which forms the storage section of a data storage device, is attached to the outer peripheral surface of the intermediate cylindrical section 15 of the rotor hub 10 of the rotor assembly 3. The disk rotates or stops depending on the operation of the spindle motor 1, and information is written and data is processed by a recording head (not shown).

[0023] In the spindle motor 1 of this embodiment, a fluid dynamic bearing 6 is provided at the portion of the sleeve 7 that rotatably supports the shaft portion 11. A large-diameter first recess 16 that opens downward is formed at the lower end of the sleeve 7, and a small-diameter second recess 17 is formed on the top surface of this first recess 16. A counter plate (thrust receiving plate) 18 is fitted into the large-diameter first recess 16 and fixed thereto by means of welding, adhesive, or the like, so that the inside of the sleeve 7 is airtight.

[0024] A thrust washer 19 is fitted and pressed into the lower end of the shaft portion 11 and fixed thereto, and this thrust washer 19 is positioned within the second recess 17 of the sleeve 7, facing the counter plate 18 and the top surface of the second recess 17, so as to rotate together with the shaft portion 11.

[0025] The gap between the sleeve 7 and the shaft 11, the gap between the thrust washer 19 and the second recess 17, and the gaps between the thrust washer 19, the shaft 11 and the counter plate 18 are all interconnected, and the fluid dynamic bearing oil 12 according to the present invention is sealed in these interconnected gaps. The fluid dynamic bearing oil 12 is injected from between the sleeve 7 and the shaft 11.

[0026] First and second radial dynamic pressure grooves 20 and 21 for generating dynamic pressure are formed axially apart on the inner peripheral surface of the sleeve 7 facing the shaft portion 11. These radial dynamic pressure grooves 20 and 21 generate dynamic pressure that keeps the shaft portion 11 and the sleeve 7 out of contact in the radial direction when the shaft portion 11 rotates. First and second thrust dynamic pressure grooves 22 and 23 are formed on the top surface of the second recess 17 facing the upper end surface of the thrust washer 19 and on the upper end surface of the counter plate 18 facing the lower end surface of the thrust washer 19, respectively. These thrust dynamic pressure grooves 22 and 23 generate dynamic pressure that stably lifts the shaft portion 11 in the thrust direction when the shaft portion 11 rotates. The action of these dynamic pressure grooves allows the shaft portion 11 to rotate stably at high speed without contacting the sleeve 7. Known patterns of dynamic pressure grooves, such as herringbone grooves and spiral grooves, can be used as the dynamic pressure grooves.

[0027] [Disk Drive Device] Fig. 2 is a perspective view showing the overall configuration of a disk drive device 30 using a spindle motor according to this embodiment. As shown in Fig. 2, the disk drive device 30 according to this embodiment includes a substantially rectangular box-shaped base plate 31, a spindle motor 1 mounted on the base plate 31, a magnetic disk 32 rotated by the spindle motor 1, a swing arm 33 having a magnetic head 34 that writes information to a predetermined position on the magnetic disk 32 and reads information from any position on the magnetic disk 32, a pivot assembly bearing device 35 that supports the swing arm 33 so that it can swing, an actuator 36 that drives the swing arm 33, and a control unit 37 that controls these devices.

[0028] The disk drive device of the present invention may be, for example, a disk drive device equipped with nine or more 3.5-inch diameter magnetic disks. In devices with such a large number of disks, the spatial volume within the device is further reduced. The disk drive device may have its internal space filled with a gas having a density lower than that of 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 lower than 1 atmosphere. The disk drive device may also employ a thermally assisted magnetic recording (HAMR) method as its recording method. In a disk drive device employing a thermally assisted magnetic recording (HAMR) method, the temperature of the actuator head may locally reach a temperature as high as 400°C.

[0029] As described above, by applying a fluid dynamic bearing oil containing an aliphatic monoester compound or diester compound having an alkyl chain length of at least a certain length to a fluid dynamic bearing and a spindle motor, the present invention makes it possible to reduce adhesion of the volatilized components to magnetic disks, etc., even in the event that volatilization of the bearing oil components occurs during operation at high temperatures, thereby making it possible to suppress disk read / write errors in disk drive devices.

[0030] 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 idea described in the claims.

[0031] The present invention will be described in more detail below with reference to examples, although the present invention is not limited thereto.

[0032] [Evaluation of Various Ester Compounds Used in Fluid Dynamic Bearing Oils] A disk adhesion test was carried out using the monoester compounds shown in Table 1 and the diester compounds shown in Table 2 according to the following procedure. In addition, a read / write error occurrence test was carried out using the diester compounds of Examples 4 to 6 according to the following procedure.

[0033] <Test Method> (1) Disk Adhesion Test An electroless nickel-plated aluminum magnetic disk was washed twice with n-hexane and isopropyl alcohol of 99% or higher purity, and then completely dried. 5 μL of a monoester compound / diester compound (sample oil) diluted to 10% by volume with hexane was dropped onto the disk and allowed to stand for 1 hour. Images of the droplets after dropping were taken with a camera fixed above the disk. The total area of ​​the droplets immediately after dropping (approximately 5 seconds later) and after leaving the droplets for 1 hour was calculated using image analysis software. The percentage (%) of the area value after leaving the droplets for 1 hour relative to the area value immediately after dropping [area value after 1 hour (final area) / area value immediately after dropping (initial area)] was taken as the "disk adhesion" (if the area value before and after leaving the droplets did not change at all, the disk adhesion was 100%). This test was carried out at a temperature of 20-30°C and a humidity of 30-70% RH, with each sample being repeated multiple times, and the average value of the values ​​where reproducibility was achieved (area value results within ±5%, N=4 or more) was adopted as the test result. Based on the results obtained, disc adhesion was evaluated using the following criteria. The results obtained are shown in Tables 1 and 2. <Criteria> A: Disc adhesion is less than 50% N: Disc adhesion is 50% or more

[0034] (2) Read / Write Error Occurrence Test: The cover of an unused disk drive was removed, and 20 mg of base oil (sample oil) was applied to the upper area of ​​the control unit (control unit 37 in Figure 2) on the back side of the cover (the surface facing the inside of the housing, not shown in Figure 2). The cover with the sample oil applied was then 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 attached to the cover surface (the surface facing the outside of the housing, not shown in Figure 2) around the oil application area. The heater temperature was maintained at 120°C for 48 hours, and then the disk drive was left at room temperature for 48 hours. During this time, the disk drive continued to operate by repeatedly measuring the speed using the disk drive's speed measurement software (e.g., CrystalDiskInfo). The disk drive was monitored for read / write errors during operation using a connected computer. The time when the software determined that the disk drive had failed due to a read / write error and stopped monitoring was recorded as the test stop time. The test was deemed to have passed if monitoring did not stop during the 96-hour test period. The results are shown in Table 2. <Judgment Criteria> A: Software monitoring did not stop during the 96-hour test period. N: Software monitoring stopped during the 96-hour test period.

[0035] If base oil evaporates due to an increase in ambient temperature, some of the evaporated base oil will condense as the temperature drops. This condensed base oil may adhere to the disk or head of a disk drive, causing an error. This means that errors are more likely to occur when the temperature drops. However, if no errors occur during this temperature drop, the temperature rise level before the temperature drop can be deemed a pass. Furthermore, this test, particularly the process of removing and reinserting the disk drive cover, must be performed in a clean room to avoid external contamination. This test was conducted without applying sample oil, and it was confirmed that monitoring did not stop even after the 96-hour test cutoff time.

[0036]

[0037]

[0038] As shown in Tables 1 and 2, it was confirmed that the monoester compounds of Examples 1 to 3 and the diester compounds of Examples 4 to 7 were compounds that were less likely to adhere to disks than the compounds of the comparative examples. Furthermore, as shown in Table 2, the results of the read / write error occurrence test using the diester compounds of Examples 4 to 6 confirmed that compounds that were less likely to adhere to disks were less likely to cause read / write errors in an actual machine. This confirmed that there is a correlation between disk adhesion and the occurrence of read / write errors.

[0039] The best mode for carrying out the invention has been described in detail above, but the invention is not limited to the above mode for carrying out the invention, and modifications and improvements within the scope of achieving the object of the invention are included in the invention.

[0040] REFERENCE SIGNS LIST 1...spindle motor, 2...stator assembly, 3...rotor assembly, 4...housing, 5...cylindrical portion, 6...fluid dynamic bearing, 7...sleeve, 8...stator core, 9...stator coil, 10...rotor hub, 10a...lower cylindrical portion, 11...shaft portion, 12...fluid dynamic bearing oil, 13...back yoke, 14...rotor magnet, 15...intermediate cylindrical portion, 16...first recess, 17...second recess, 18...counter plate, 19...thrust washer, 20...first radial dynamic pressure groove, 21...second radial dynamic pressure groove, 22...first thrust dynamic pressure groove, 23...second thrust dynamic pressure groove, 30...disk drive device, 31...base (base plate), 32...magnetic disk, 33...swing arm, 34...magnetic head, 35...pivot assembly bearing device, 36...actuator, 37...control unit

Claims

1. A fluid dynamic bearing oil for use in a disk drive device, The fluid dynamic bearing oil contains at least one compound selected from the group consisting of a monoester compound represented by the following formula (1) and a diester compound represented by the following formula (2): Fluid dynamic bearing oil. R １ -C(=O)O-R ２ (1) (In formula (1), R １ is a linear or branched alkyl group having a total of 10 or more carbon atoms, and R １ is a branched alkyl group, the branched chain has 10 or more carbon atoms; R ２ is a linear or branched alkyl group having a total of 9 or more carbon atoms, and R ２ When is a branched alkyl group, the branched chain has 7 or more carbon atoms. R ３ -E １ -R ４ -E ２ -R ５ (2) (In formula (2), R ３ and R ５ are each independently a linear or branched alkyl group having a total of 8 or more carbon atoms, R ３ and R ５ is a branched alkyl group, E １ or E ２ The carbon chain that is the longest chain, counting from the carbon atom bonded to R ４ is a linear or branched alkylene group having a total of 4 or more carbon atoms, E １ and E ２ each independently represents —C(═O)O— or —OC(═O)—.

2. In formula (1), R １ is a linear or branched alkyl group having a total of 23 or less carbon atoms, and R １ is a branched alkyl group, the branched chain has 15 or fewer carbon atoms; R ２ is a linear or branched alkyl group having a total of 20 or less carbon atoms, and R ２ When is a branched alkyl group, the branched chain has 8 or fewer carbon atoms; The fluid dynamic bearing oil according to claim 1.

3. In formula (1), R １ and R ２ one of which is a linear alkyl group and the other is a branched alkyl group; The fluid dynamic bearing oil according to claim 1.

4. In formula (2), R ３ and R ５ are each independently a linear or branched alkyl group having a total of 10 or less carbon atoms, R ４ is a linear or branched alkylene group having a total of 6 or less carbon atoms; The fluid dynamic bearing oil according to claim 1.

5. In formula (2), R ３ and R ５ are both linear alkyl groups, and R ４ is a branched alkyl group, or R ３ and R ５ are both branched alkyl groups, and R ４ is a linear alkyl group; The fluid dynamic bearing oil according to claim 4.

6. In formula (2), R ３ and R ５ represent the same group, E １ represents —C(═O)O— and E ２ represents —OC(═O)—, or E １ represents —OC(═O)— and E ２ represents —C(═O)O—; The fluid dynamic bearing oil according to claim 5.

7. In formula (2), R ３ and R ５ represent the same linear alkyl group, and R ４ represent the same branched alkyl group, E １ represents —C(═O)O— and E ２ represents —OC(═O)—, or R ３ and R ５ represent the same branched alkyl group, R ４ represent the same linear alkyl group, E １ represents —OC(═O)— and E ２ represents -C(=O)O-; The fluid dynamic bearing oil according to claim 6.

8. Does not contain alkylated phenyl-α-naphthylamine as an antioxidant. The fluid dynamic bearing oil according to claim 1.

9. Further comprising a phenolic antioxidant. The fluid dynamic bearing oil according to claim 1.

10. The phenolic antioxidant is a hindered phenolic antioxidant. The fluid dynamic bearing oil according to claim 9.

11. the hindered phenol antioxidant is at least one selected from the group consisting of 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-diethylenebis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], and octyl-3,5-di-tert-butyl-4-hydroxy-hydrocinnamic acid; The fluid dynamic bearing oil according to claim 10.

12. Does not contain amine salts of acid phosphates as anti-wear agents, The fluid dynamic bearing oil according to claim 1.

13. A fluid dynamic bearing containing the fluid dynamic bearing oil according to any one of claims 1 to 12. Bearing.

14. A spindle motor comprising the fluid dynamic bearing according to claim 13.

15. A disk drive device equipped with the spindle motor according to claim 14.

16. 16. The disk drive device according to claim 15, comprising nine or more disks each having a diameter of 3.5 inches.

17. 16. The disk drive device according to claim 15, wherein the internal space is filled with a gas having a density less than that of air.

18. 16. The disk drive device according to claim 15, wherein a heat-assisted magnetic recording method is adopted.