Lubricating Oil Composition For Hydrodynamic Bearing And Hdd Motor Using The Same

Abstract

There is provided a lubricating oil composition for a hydrodynamic bearing including: 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil. According to the present invention, a hard disk drive (HDD) motor is manufactured using the lubricating oil composition for a hydrodynamic bearing having a low viscosity, a small evaporation loss, and improved oxidation stability at a room temperature, whereby the impact resistance and low-temperature operation stability of the hydrodynamic bearing motor may be improved.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the priority of Korean Patent Application No. 10-2013-0032942 filed on Mar. 27, 2013, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a lubricating oil composition for a hydrodynamic bearing capable of decreasing a lubricating oil evaporation rate in order to increase a lifespan of a hydrodynamic bearing motor, and a hard disk drive (HDD) motor using the same.

[0004] 2. Description of the Related Art

[0005] A hard disk drive (HDD), an information storage device, reads data stored on a disk or writes data to a disk using a read/write head.

[0006] The hard disk drive requires a disk driving device capable of driving the disk. In the disk driving device, a small-sized spindle motor is used.

[0007] The small-sized spindle motor uses a hydrodynamic bearing assembly. Lubricating oil is interposed between a shaft and a sleeve of the hydrodynamic bearing assembly, such that the shaft is supported by fluid pressure generated in the lubricating oil.

[0008] The lubricating oil has high viscosity at low temperatures at the time of rotation of the spindle motor, such that viscous resistance of the lubricating oil to a dynamic power generating groove generated at the time of the rotation of the motor increases, thereby increasing power loss of the motor.

[0009] On the other hand, the lubricating oil is thermally expanded and has reduced viscosity at high temperatures at the time of rotation of the spindle motor, such that it may not sufficiently perform a support role.

[0010] Due to the above-mentioned reason, the lubricating oil requires opposed viscosity behavior characteristics in which low viscosity is maintained in a low temperature region and viscosity is not reduced in a high temperature region.

[0011] In order to satisfy these viscosity characteristics, several technologies such as a technology of adding a material such as an anti-oxidant, an extreme-pressure additive, or the like, to base oil containing a specific ester compound as a main component, have been developed.

[0012] The lubricating oil to which the above-mentioned additives are added indicates an effect at the beginning. However, when it is used for a long period of time, the lubricant is evaporated and viscous characteristics are changed, such that it is difficult to continuously maintain this effect.

[0013] In addition, in accordance with the trend for miniaturization, high precision, high speed rotation, and low power consumption of the motor, characteristics such as heat resistance, oxidation stability, low degrees of evaporation, and abrasion prevention have been demanded in the lubricating oil.

[0014] Meanwhile, in order to increase a lifespan of a hydrodynamic bearing motor, it is important to maintain a uniform amount of the lubricating oil therein. Therefore, research into a technology of decreasing a lubricating oil evaporation rate has been demanded.

RELATED ART DOCUMENT

[0015] (Patent Document 1) Japanese Patent Laid-open Publication No. 2007-186710

SUMMARY OF THE INVENTION

[0016] An aspect of the present invention provide a lubricating oil composition for a hydrodynamic bearing capable of decreasing a lubricating oil evaporation rate in order to increase a lifespan of a hydrodynamic bearing motor, and a hard disk drive (HDD) motor using the same.

[0017] According to an aspect of the present invention, there is provided a lubricating oil composition for a hydrodynamic bearing including: 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

[0018] The ester-based base oil may be at least one selected from a group consisting of dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester.

[0019] The mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil may include at least one selected from a group consisting of dioctyl sebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA).

[0020] The lubricating oil composition may further include at least one additive selected from a group consisting of an antioxidant, an anti-abrasion agent, an anti-corrosion agent, an extreme-pressure additive, a viscosity index improver, an anti-static agent, and a deactivator.

[0021] The additive may include an anti-oxidant, 2,2'-methylene-bis(4-methyl-6-tert-butylphenol).

[0022] The additive may include a metal anti-oxidant, barium diphenylamine-4-sulfonate.

[0023] The additive may include an internal pressure preventing agent, tricresyl phosphate.

[0024] According to another aspect of the present invention, there is provided a hard disk drive (HDD) motor using a lubricating oil composition for a hydrodynamic bearing, the lubricating oil composition containing: 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

[0025] The ester-based base oil may be at least one selected from a group consisting of dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester.

[0026] The mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil may include at least one selected from a group consisting of dioctyl sebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA).

[0027] The lubricating oil composition may further include at least one additive selected from a group consisting of an antioxidant, an anti-abrasion agent, an anti-corrosion agent, an extreme-pressure additive, a viscosity index improver, an anti-static agent, and a deactivator.

[0028] The additive may include an anti-oxidant, 2,2'-methylene-bis(4-methyl-6-tert-butylphenol).

[0029] The additive may include a metal anti-oxidant, barium diphenylamine-4-sulfonate.

[0030] The additive may include an internal pressure preventing agent, tricresyl phosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and other aspects, features and other advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0032] FIG. 1 is a schematic cross-sectional view showing a hard disk drive (HDD) motor including a hydrodynamic bearing assembly according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawing. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawing, the shapes and dimensions of elements may be exaggerated for clarity, and the same reference numerals will be used throughout to designate the same or like elements.

[0034] A lubricating oil composition for a hydrodynamic bearing according to an embodiment of the present invention may contain 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

[0035] Hereinafter, the above configuration will be described in detail.

[0036] The lubricating oil composition for a hydrodynamic bearing may contain 95 to 99 wt % of an ester-based base oil.

[0037] The base oil is not particularly limited as long as it maybe generally used for the hydrodynamic bearing. For example, the base oil may be an ester-based compound.

[0038] More specifically, the ester-based base oil may be at least one selected from a group consisting of dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester, but is not limited thereto.

[0039] Meanwhile, according to the embodiment of the present invention, the lubricating oil composition for a hydrodynamic bearing may contain 1.0 to 5.0 wt % of the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

[0040] The mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil, ester-based mixed oil added to the ester-based base oil, may be different from the ester-based base oil.

[0041] Particularly, the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil may be considered as being added in order to decrease an evaporation rate in lubricating oil for the hydrodynamic bearing used in a hard disk drive (HDD) motor.

[0042] That is, the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil may have a low evaporation rate at a high temperature and a high viscosity due to its large molecular weight, as compared to the base oil.

[0043] According to the embodiment of the present invention, the lubricating oil composition for a hydrodynamic bearing may contain 1.0 to 5.0 wt % of the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil, thereby decreasing the evaporation amount while significantly decreasing a change in physical properties of the lubricating oil composition.

[0044] Therefore, the lubricating oil composition for a hydrodynamic bearing may have a low viscosity and a small evaporation loss, and oxidation stability thereof may be improved, at a room temperature.

[0045] The hard disk drive (HDD) motor is manufactured using the lubricating oil composition for a hydrodynamic bearing, such that impact resistance and low-temperature operation stability of the hydrodynamic bearing motor may be improved, and a lifespan thereof may be increased.

[0046] In the case in which the lubricating oil composition for a hydrodynamic bearing contains the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil in a content lower than 1.0 wt %, there may be no effect of decreasing an evaporation amount at a high temperature.

[0047] Meanwhile, in the case in which the lubricating oil composition for a hydrodynamic bearing contains the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil in a content higher than 5.0 wt %, the physical properties of the lubricating oil may be changed.

[0048] The mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil may include at least one selected from a group consisting of dioctyl sebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA), but is not limited thereto.

[0049] Meanwhile, the lubricating oil composition for a hydrodynamic bearing according to the embodiment of the present invention may further contain at least one additive selected from a group consisting of an antioxidant, an anti-abrasion agent, an anti-corrosion agent, an extreme-pressure additive, a viscosity index improver, an anti-static agent, and a deactivator, but is not limited thereto. That is, various additives may be added.

[0050] The additive is added to the lubricating oil for a hydrodynamic bearing at a trace amount, such that the additive may play a specific role for improving long-term high-temperature reliability of the lubricating oil.

[0051] The additive may include an anti-oxidant, 2,2'-methylene-bis(4-methyl-6-tert-butylphenol), but is not limited thereto.

[0052] In addition, the additive may include a metal antioxidant, barium diphenylamine-4-sulfonate, but is not limited thereto.

[0053] Further, the additive may include an internal pressure preventing agent, for example, tricresyl phosphate, but is not particularly limited thereto.

[0054] A content of the additive may be 0.01 to 3.00 parts by weight based on 100 parts by weight of the base oil, such that the lubricating oil may maintain a low viscosity at a low temperature region, and the viscosity may not be decreased at a high temperature region.

[0055] In the case in which the additive is added in an amount lower than 0.01 part by weight based on 100 parts by weight of the base oil, an amount of the additive is small, such that an effect caused by the additive may not be sufficiently exhibited.

[0056] Further, in the case in which the additive is added in an amount higher than 3.00 parts by weight based on 100 parts by weight of the base oil, the physical properties of the lubricating oil for a hydrodynamic bearing may be deteriorated.

[0057] The lubricating oil composition for a hydrodynamic bearing is not particularly limited, but may be appropriate for being used as, for example, a fluid bearing of the HDD motor.

[0058] In the case of a small-sized hard disk drive, a power consumption amount needs to be low, and low-temperature operation stability and the impact resistance of the motor may be significant.

[0059] The lubricating oil composition according to the embodiment of the present invention may have a low frictional loss and also have low-temperature operation stability to thereby satisfy the above-mentioned conditions of the small-sized hard disk drive.

[0060] FIG. 1 is a cross-sectional view schematically showing a hard disk drive (HDD) motor including a fluid dynamic bearing assembly according to an embodiment of the present invention.

[0061] Referring to FIG. 1, a HDD motor according to another embodiment of the present invention may contain a lubricating oil composition for a hydrodynamic bearing containing 95 to 99 wt % of the ester-based base oil; and 1.0 to 5.0 wt % of the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

[0062] Hereinafter, the HDD motor according to another embodiment of the present invention will be described in detail. However, a portion overlapped with the description in the above-mentioned embodiment of the present invention will be omitted.

[0063] A motor 10 including a base assembly 100 for a motor (hereinafter, referred to as the base assembly) according to the embodiment of the present invention may include the base assembly 100 including a base 110 for a motor (hereinafter, referred to as a base), a sleeve 220 supporting rotation of a rotating member, and a core 240 having a coil 230 wound therearound.

[0064] Terms with respect to directions will be first defined. As viewed in FIG. 1, an axial direction refers to a vertical direction based on a shaft 210, and an outer diameter or inner diameter direction refers to a direction toward an outer edge of a hub 250 based on the shaft 210 or a direction toward the center of the shaft 210 based on the outer edge of the hub 250.

[0065] In addition, a circumferential direction refers to a rotation direction of the shaft 210, that is, a direction rotating around an outer peripheral surface of the shaft 210.

[0066] The base assembly 100 may include the base 110 and a pulling plate 120, and the core 240 having the coil 230 wound therearound may be coupled to the base 110.

[0067] In other words, the base 110 may be a fixed member supporting the rotating member including the hub 250 and be a fixed structure to which the coil 230 generating electromagnetic force having a predetermined magnitude at the time of the application of power and the core 240 having the coil 230 wound therearound are coupled.

[0068] Here, the base 110 may include a protrusion part 112 and a body part 114, wherein the protrusion part 112 may have an inner peripheral surface coupled to an outer peripheral surface of the sleeve 220 supporting the shaft 210 to thereby support the sleeve 220.

[0069] That is, the protrusion part 112 may have a hollow and protrude upwardly in the axial direction, and the sleeve 220 supporting the shaft 210 may be inserted into the hollow and coupled thereto by a method such as a welding method, a bonding method, a press-fitting method, or the like.

[0070] In addition, the protrusion part 112 may have the core 240 coupled to an outer peripheral surface thereof, wherein the core 240 has the coil 230 wound therearound. Rigidity needs to be secured in order to secure rotational stability of the motor 10 according to the embodiment of the present invention.

[0071] Here, the pulling plate 120 may be coupled to the body part 114 of the base 110, and excessive floating of the rotating member including the shaft 210 and the hub 250 may be prevented by the pulling plate 120.

[0072] In detail, the pulling plate 120 may be coupled to the body part 114 corresponding to a bottom surface of a magnet 260 coupled to the hub 250 by a coupling method such as a bonding method, and may have magnetism so that attractive magnetic force acts between the pulling plate 120 and the magnet 260.

[0073] The shaft 210 and the hub 250, rotating members of the motor 10 according to the embodiment of the present invention, need to be floated at a predetermined height so as to be stably rotated. However, in the case in which the shaft 210 and the hub 250 are floated at a height higher than a pre-designed floating height, it may have a negative effect on performance.

[0074] In this case, in order to prevent excessive floating of the shaft 210 and the hub 250, the rotating members, the pulling plate 120 may be coupled to the base 110. Therefore, the excessive floating of the rotating member may be prevented by the magnetic attractive force acting between the pulling plate 120 and the magnet 260.

[0075] The shaft 210, the rotating member coupled to hub 250 to thereby rotate together with the hub 250, may be supported by the sleeve 220.

[0076] The sleeve 220, a component supporting the rotation of the shaft 210 and the hub 250, the rotating member 250, may support the shaft 210 so that an upper end of the shaft 210 protrudes upwardly in the axial direction, and may be formed by forging Cu or Al or sintering a Cu--Fe-based alloy powder or an SUS-based power.

[0077] In addition, the sleeve 220 may include a shaft hole having the shaft 210 inserted thereinto so as to have a micro clearance therewith, wherein the micro clearance is filled with oil O, such that the shaft 210 may be stably supported by radial dynamic pressure through the oil O.

[0078] The hub 250 may be a rotating structure rotatably provided with respect to the fixed member including the base 110, and the above-mentioned annular ring shaped magnet 260 may be provided so as to correspond to the core 240, having a predetermined interval therebetween.

[0079] Here, the magnet 260 may interact with the coil 230 wound around the core 240, whereby the motor 10 according to the embodiment of the present invention may obtain rotational driving force.

[0080] The HDD motor according to another embodiment of the present invention is manufactured by using the lubricating oil composition 170 for a hydrodynamic bearing, whereby frictional loss of a device may be more effectively reduced while a viscosity becomes low, and low-temperature operation stability may be significantly excellent and the impact resistance may be excellent.

[0081] In addition, the HDD motor is manufactured by using the lubricating oil composition for a hydrodynamic bearing having low viscosity, low evaporation loss, and improved oxidation stability at a room temperature, whereby quality reliability according to the use of the motor for a long period of time may be improved.

[0082] A manufacturing method of the HDD motor 10 may be the same as a general manufacturing method except that lubricating oil composition 170 for a hydrodynamic bearing is used.

[0083] Hereafter, although the present invention will be described in detail with reference to Examples, the present invention is not limited thereto.

EXAMPLES 1 TO 5

[0084] In Examples 1 to 5, in order to measure a high-temperature evaporation amount, dioctyl adipate (DOA) was used as ester-based base oil, and the content thereof was 97 wt % or 98.5 wt %.

[0085] Dioctyl adipate (DOA) may be represented by the following Chemical Formula 1.

##STR00001##

[0086] In addition, three kinds of ester-based oils were mixed as the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil and then added to the base oil, thereby preparing lubricating oil for a hydrodynamic bearing.

[0087] As the three kinds of ester-based oils, dioctylsebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA) represented by the following Chemical Formula 2 to 4, respectively, were used.

##STR00002##

COMPARATIVE EXAMPLES 1 AND 2

[0088] In Comparative Example 1, for comparison with Examples, as the ester-based base oil, dioctyl adipate was used, and the content thereof was 100 wt %.

[0089] In Comparative Example 2, for comparison with Examples, 94 wt % of dioctyl adipate was used as the ester-based base oil, and the three kinds of ester-based oils were used in a content of 2 wt %, respectively.

[0090] An experiment of measuring the evaporation amounts was performed by putting each of 5 g of lubricating oil for a hydrodynamic bearing containing each component on an evaporation dish formed of an SUS material and then introducing it into a thermostat of 100.degree. C.

[0091] The experiment was performed for 144 hours (six days), and an initial weight of the lubricating oil for a hydrodynamic bearing put in the evaporation dish and an weight of the lubricating oil after hardening for 144 hours at 100.degree. C. were measured, thereby comparing the evaporation amounts of the lubricating oil for a hydrodynamic bearing in Examples and Comparative Examples with each other.

[0092] The following [Table 1] shows the results obtained by measuring the high-temperature evaporation amount according to the Examples and Comparative Examples and to compare the measured values with each other.

TABLE-US-00001 TABLE 1 Comp- Comp- arative Ex- Ex- Ex- Ex- Ex- arative Example ample ample ample ample ample Example 1 1 2 3 4 5 2 Base oil DOA 100 97 97 97 98.5 97 94 (wt %) Mixed oil DOS -- 3 -- -- 0.5 1 2 (wt %) DOZ -- -- 3 -- 0.5 1 2 DIDA -- -- -- 3 0.5 1 2 High-temperature 13.22 12.89 12.97 12.73 8.14 6.96 6.01 evaporation amount (%)

[0093] Referring to [Table 1], it may be appreciated that the lubricating oil composition according to the present invention (Examples 1 to 5) had an effect of decreasing the high-temperature evaporation amount.

[0094] Particularly, in Examples 1 to 3, 3 wt % of DOS, DOZ, or DIDA was used alone, and in Examples 4 and 5, the three kinds of oils were mixed at a predetermined ratio. It may be appreciated that the effect of reducing the high-temperature evaporation amount was increased in Examples 4 and 5 as compared to Examples 1 to 3.

[0095] Therefore, according to the embodiment of the present invention, it may be appreciated that in the case in which at least two kinds, particularly, at least three kinds were mixed and added at a predetermined ratio as the mixed oil containing at least two kinds of ester-based oils different from the base oil, the effect of decreasing the high-temperature evaporation amount was further increased.

[0096] On the other hand, it may be appreciated that in the case of Comparative Example 1 in which 100 wt % of the base oil was used, there was no effect of decreasing the high-temperature evaporation amount.

[0097] In addition, in the case of Comparative Example 2 in which the three kinds of ester-based oils were mixed in a content of 2 wt %, respectively, so as to be outside of the numerical ranges of the present invention, respectively, there was the effect of decreasing the high-temperature evaporation amount, but the physical properties of the lubricating oil were changed.

[0098] Therefore, according to the embodiment of the present invention, the HDD motor was manufactured using the lubricating oil composition for a hydrodynamic bearing having a small high-temperature evaporation loss, whereby the lifespan of the motor may be increased.

[0099] The following Table 2 shows the results obtained by variously applying a kind of base oil while using the mixed oil composition in Example 5 to measure the high-temperature evaporation amounts of the lubricating oil compositions and compare the measured values with each other.

[0100] As the base oil, dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester were used, respectively.

[0101] Dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester were represented by the following Chemical Formulas 5 to 8, respectively.

##STR00003##

TABLE-US-00002 TABLE 2 High-temperature evaporation amount (%) Base oil Base oil (97 wt %) + Decrease Base oil (100 wt %) mixed oil (3 wt %) rate (%) Dioctyl 13.22 6.96 47.4 Adipate Heptylnonyl 8.45 4.79 43.3 Adipate 3-Methyl-1,5- 4.78 2.56 46.4 Pentandiol, dinonanoate Neopentylglycol 12.89 7.12 44.8 ester

[0102] Referring to FIG. 2, it may be appreciated that in the case in which 97 wt % of the base oil and 3 wt % of the mixed oil, which was the lubricating oil composition in Example 5, were used, the effect of decreasing the high-temperature evaporation amount was increased by 40% or more as compared to the case of using 100 wt % of each base oil.

[0103] That is, according to the embodiment of the present invention, since the mixed oil was added in an amount of 5 wt % or less, the effect of decreasing the high-temperature evaporation amount may be excellent without changing the physical properties of the lubricating oil, and the lifespan of the motor to which the lubricating oil is applied may be increased.

[0104] As set forth above, according to the present invention, the hard disk drive (HDD) motor is manufactured using the lubricating oil composition for a hydrodynamic bearing having a low viscosity, a small evaporation loss, and improved oxidation stability at a room temperature, whereby the impact resistance and low-temperature operation stability of the hydrodynamic bearing motor may be improved, and the lifespan thereof may be increased.

[0105] While the present invention has been shown and described in connection with the embodiments, it will be apparent to those skilled in the art that modifications and variations can be made without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A lubricating oil composition for a hydrodynamic bearing comprising: 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

2. The lubricating oil composition of claim 1, wherein the ester-based base oil is at least one selected from a group consisting of dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester.

3. The lubricating oil composition of claim 1, wherein the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil includes at least one selected from a group consisting of dioctyl sebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA).

4. The lubricating oil composition of claim 1, further comprising at least one additive selected from a group consisting of an antioxidant, an anti-abrasion agent, an anti-corrosion agent, an extreme-pressure additive, a viscosity index improver, an anti-static agent, and a deactivator.

5. The lubricating oil composition of claim 4, wherein the additive includes an anti-oxidant, 2,2'-methylene-bis(4-methyl-6-tert-butylphenol).

6. The lubricating oil composition of claim 4, wherein the additive includes a metal anti-oxidant, barium diphenylamine-4-sulfonate.

7. The lubricating oil composition of claim 4, wherein the additive includes an internal pressure preventing agent, tricresyl phosphate.

8. A hard disk drive (HDD) motor using a lubricating oil composition for a hydrodynamic bearing, the lubricating oil composition containing: 95 to 99 wt % of an ester-based base oil; and 1.0 to 5.0 wt % of the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil.

9. The hard disk drive (HDD) motor of claim 8, wherein the ester-based base oil is at least one selected from a group consisting of dioctyl adipate (DOA), heptylnonyl adipate, 3-methyl-1,5-pentandiol(dinonanoate), and neopentylglycol ester.

10. The hard disk drive (HDD) motor of claim 8, wherein the mixed oil containing at least two kinds of ester-based oils different from the ester-based base oil includes at least one selected from a group consisting of dioctyl sebacate (DOS), dioctyl azelate (DOZ), and diisodecyl adipate (DIDA).

11. The hard disk drive (HDD) motor of claim 8, wherein the lubricating oil composition further includes at least one additive selected from a group consisting of an antioxidant, an anti-abrasion agent, an anti-corrosion agent, an extreme-pressure additive, a viscosity index improver, an anti-static agent, and a deactivator.

12. The hard disk drive (HDD) motor of claim 11, wherein the additive includes an anti-oxidant, 2,2'-methylene-bis(4-methyl-6-tert-butylphenol).

13. The hard disk drive (HDD) motor of claim 11, wherein the additive includes a metal anti-oxidant, barium diphenylamine-4-sulfonate.

14. The hard disk drive (HDD) motor of claim 11, wherein the additive includes an internal pressure preventing agent, tricresyl phosphate.