JP2001139971A - Lubricant, hydrodynamic bearing, spindle motor and rotator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hydrodynamic bearing improved in starting characteristics at low temperature, capable of favorably keeping bearing characteristics in wide operating temperature range, having long life and suffi ciently exhibiting earthing function without providing a specific earthing mecha nism. SOLUTION: In this hydrodynamic bearing constituted of a shaft 1 having a flange and formed of a cylindrical member 2 and a ring member 3, a sleeve 4, a cyclic lid member 5 and a lubricant F packed in minute spaces R1 to R6 containing a bearing space formed among these constituent members, a lubricant comprising >=18C and <=24C phosphate ester or containing >=18C and <=24C phosphate ester as a base oil and having <=1.0×1011 Ωcm volume resistivity is adopted as the above lubricant F.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lubricating oil for a fluid dynamic bearing and a fluid dynamic bearing, and more particularly to an improvement in performance and a long life of a fluid dynamic bearing related to a lubricating oil.

[0002]

2. Description of the Related Art A fluid dynamic pressure bearing includes a shaft forming a bearing gap, a sleeve, and lubricating oil filled in the bearing gap, and a spiral dynamic pressure generating groove provided in the bearing gap. This is a sliding bearing that uses a dynamic pressure generated when a fluid is transported while being pushed by a screw pump action, and rotatably supports a shaft in a non-contact manner with a sleeve. Compared to rolling bearings that use ball bearings, they are smaller and lighter, have a longer life, have higher rotational accuracy during high-speed rotation, and can be driven to rotate at high speed even under conditions where a large unbalance load is applied. Fluid dynamic pressure bearings are polygon scanner motors for laser beam printers, spindle motors for rotating devices such as hard disk drives (HDD devices),
It is widely used as a bearing of a spindle motor for a rotating device such as an optical disk drive such as a DVD-ROM or a DVD-RAM or a magneto-optical disk drive.

As described above, since the bearing is used for the bearing of the spindle motor of various types of rotating devices, new requirements are imposed on the performance and life of the fluid dynamic bearing. That is,
Further, it is required to extend the life and to make the bearing characteristics constant over a wide temperature range. In particular, in a spindle motor for an HDD device, it is an important requirement to ensure measures against static electricity.

[0004] First of all, the life is extended. There are many factors which determine the life of the fluid dynamic bearing. Among them, the contact accident between the shaft and the sleeve and the evaporation of the lubricating oil are major factors. In the event of a shaft-sleeve contact, the shaft contacts the sleeve when starting or stopping, damaging the contact surface,
In some cases, it is an accident that burns in, and various means to prevent this have been developed, and the effect has been improved as such. However, this is closely related to the structure of the individual fluid dynamic bearings, and therefore, the degree of the effect differs depending on the type of the fluid dynamic bearing.

On the other hand, evaporation of the lubricating oil is a life determining factor common to all fluid dynamic bearings. Although the lubricating oil of the fluid dynamic pressure bearing has a low evaporation characteristic, the evaporation proceeds gradually though it is still. If the lubricating oil evaporates to such an extent that it is impossible to generate dynamic pressure due to long-term use, the fluid dynamic pressure bearing will not function at that point. In order to further extend the life, it is necessary to employ a lubricating oil having excellent low evaporation characteristics.

[0006] Next, to maintain the bearing characteristics constant over a wide temperature range, it is necessary to improve the starting characteristics at low temperatures and prevent the bearing rigidity from decreasing at high temperatures. Bearing stiffness decreases as the bearing gap widens,
It decreases as the viscosity (viscosity coefficient) of the lubricating oil decreases. Both the bearing gap and the viscosity coefficient of the lubricating oil change with temperature. Therefore, as shown in FIG. 5, when the temperature rises, the bearing rigidity decreases. Various measures have been developed to keep the bearing gap from expanding as much as possible as the temperature rises, but this is closely related to the structure of the individual fluid dynamic bearings and therefore the type of fluid dynamic bearing. The degree of the effect differs for each.

As shown in FIG. 6, the viscosity coefficient of the lubricating oil decreases as the temperature increases. In order to avoid this, the viscosity may be set to a value that allows for the decrease in the viscosity at high temperatures. However, in this case, the viscosity at low temperatures increases, and the starting characteristics at low temperatures deteriorate. Therefore, in order to maintain good bearing characteristics over a wide operating temperature range, it is necessary to employ a lubricating oil whose viscosity coefficient does not become as small as possible even when the temperature rises.

[0008] Further, as a countermeasure against static electricity, this is necessary when a fluid dynamic bearing is used as a bearing of a spindle motor for an HDD device. A magnetic head of an HDD device, particularly a high storage capacity type HDD device, has a magneto-resistance effect element (Magneto-Resistive element) whose electric resistance changes according to a change in a magnetic field.
MR heads utilizing Effective Element) are widely used. However, since the MR head is formed of a thin film, a countermeasure against static electricity, which is not a problem with the inductive magnetic head, is required for the HDD device using the MR head.

That is, an HD in which a fluid dynamic bearing is adopted as a bearing
In a spindle motor for a D device, a magnetic disk,
A hub for holding the magnetic disk, a rotor shaft to which the hub is coaxially fixed, a sleeve in which the rotor shaft is rotatably fitted, and a minute gap including a bearing gap between the rotor shaft and the sleeve. The lubricating oil and the motor frame to which the sleeve is fixed are all conductors, and these are mechanically fixed or in contact. However, if the volume resistivity of the lubricating oil is high, the fluid dynamic bearing Does not function well as a ground wire. In such a case, electric charges accumulate on the magnetic disk when the HDD device rotates at high speed, and in some cases, electrostatic breakdown occurs between the magnetic head and the magnetic disk. In particular, a fluid dynamic pressure bearing using a base oil lubricating oil having a volume resistivity of 1.0 × 10 ¹² Ωcm or more has a high risk of occurrence of this problem.

In order to prevent electrostatic breakdown, it is preferable that the magnetic disk is grounded with a resistance value of 1 Ωcm or less. However, for example, if the bearing gap is 1μ
m, the volume resistivity is 1.0 × 10
^When a lubricating oil having a resistance of ¹¹ Ωcm or less is used, the resistance value per 1 cm ² of the bearing area becomes 1 MΩ. Generally, the bearing gap of a fluid dynamic bearing is at least about 2 μm. For this reason, in the fluid dynamic pressure bearing, the bearing area is 1 cm ²
Lubricating oil resistance per 1MΩ or less, preferably 10
In order to reduce the volume resistivity to 0 kΩ or less, the volume resistivity of the lubricating oil must be 5.
0 × 10 ¹¹ Ωcm or less, preferably 1.0 × 10 ⁹ to
It is preferably 1.0 × 10 ¹⁰ Ωcm or less.

As described above, the lubricating oil is a major component directly related to the performance and life of the fluid dynamic bearing. Therefore, the characteristics required for the lubricating oil for fluid dynamic pressure bearings are low evaporation characteristics, low viscosity, low generation of sludge during use, wide use temperature range, and excellent lubricity. That is. Heretofore, mainly lubricating oils of ester base oils have been used as lubricating oils for fluid dynamic bearings.

For example, in the lubricating oil disclosed in Japanese Patent Application Laid-Open No. 1-188592, trimethylolpropane triester is used as a base oil, and 0.1 to 3.0 parts by weight based on 100 parts by weight of the base oil. Parts of a hindered phenolic antioxidant and 0.03 to 1.0 parts by weight of a methyl-benzotriazole corrosion inhibitor.

The lubricating oil disclosed in Japanese Patent Application Laid-Open No. 9-177766 uses diethyl ethylhexyl sebacate as a base oil and is added with 1 to 3% by weight of monoethyl phthalate. is there.

[0014] The lubricating oil of the ester base oil, when used as a lubricating oil for a fluid dynamic bearing, exhibits excellent characteristics such as low torque, good initial conformability, and good durability. is there. However, conventional lubricating oils based on ester base oils have high base oil viscosities of 11 to 31 cSt (40 ° C.), poor starting characteristics at low temperatures, and do not satisfy the required performance of the bearings in a wide operating temperature range. For this reason, there is a problem that it is not possible to find a conventional lubricating oil whose viscosity coefficient does not become as small as possible even when the temperature increases.

In conventional lubricating oils using an ester base oil, although the base oil itself generally has low evaporation characteristics, it prevents oxidation, corrosion, and improves the wettability of bearing components to metals. Various additives mixed with the base oil for the purpose of evaporating more easily than the base oil. For this reason, there is a problem that a lubricating oil having further low evaporation characteristics cannot be found among lubricating oils using an ester base oil which have been developed or proposed so far.

What is more problematic is that in conventional lubricating oils,
It means that the one having a volume resistivity of 1.0 × 10 ¹¹ Ωcm or less is not found. Incidentally, the volume resistivity of the above-mentioned di-2-ethylhexyl sebacate is 1.0 × 10 ¹² Ω.
cm or more.

[0017]

The first problem to be solved by the present invention is that, compared to conventional lubricating oils, it has excellent low evaporation characteristics and low viscosity, and other lubricating oil characteristics are comparable or equal. An object of the present invention is to provide a lubricating oil for a fluid dynamic bearing.

A second problem to be solved is to provide a lubricating oil for a fluid dynamic pressure bearing which is excellent in low volume resistivity and has the same or higher lubricating oil characteristics as compared with the conventional lubricating oil. To provide.

A third problem to be solved is to provide a fluid dynamic pressure bearing which has improved starting characteristics at low temperatures, good bearing characteristics over a wide operating temperature range, and a long life. To provide.

A fourth object of the present invention is to provide a fluid dynamic pressure bearing having a sufficient grounding function without providing a special grounding mechanism.

A fifth object to be solved is to improve the performance and extend the life of a spindle motor provided with a fluid dynamic pressure bearing or a rotating device using a spindle motor provided with a fluid dynamic pressure bearing as a driving source of the rotating body. It is to make it.

[0022]

In order to solve the above first and second problems, a lubricating oil for a fluid dynamic bearing has a carbon number of one.
It was composed of 8 to 24 phosphate esters alone.

In order to solve the first and second problems, the lubricating oil for a fluid dynamic pressure bearing is constituted by using a phosphate ester having 18 to 24 carbon atoms as a base oil.

In order to solve the third problem, a lubricating oil for a fluid dynamic pressure bearing is composed of a single phosphoric acid ester having 18 to 24 carbon atoms or a lubricating oil having 18 to 24 carbon atoms.
A lubricating oil composed of the following phosphate ester as a base oil was employed.

In order to solve the fourth problem, an HDD
Lubricating oil composed of a simple substance of phosphate ester having 18 to 24 carbon atoms or lubricating oil composed of phosphate ester having 18 to 24 carbon atoms as a base oil for lubricating oil of a fluid dynamic pressure bearing of a spindle motor for a device. A lubricating oil which was oil and had a volume resistivity of 1.0 × 10 ¹¹ Ωcm or less was employed.

In order to solve the fifth problem, a spindle motor having a fluid dynamic bearing or a rotating device in which a rotating body is driven by a spindle motor having a fluid dynamic bearing is provided. As the lubricating oil for the bearing, a lubricating oil composed of a phosphate alone having 18 to 24 carbon atoms or a lubricating oil composed of a phosphate ester having 18 to 24 carbon atoms as a base oil was employed.

[0027]

FIG. 1 is a sectional view of an embodiment of a fluid dynamic bearing according to the present invention, in which a minute gap is exaggerated, and FIG.
Is a perspective view of a flanged shaft used in the fluid dynamic bearing of FIG. 1, FIG. 3 is a longitudinal sectional view of one embodiment of the spindle motor according to the present invention, and FIG. 4 is an application of the spindle motor according to the present invention. FIG. 1 is a perspective view of a main part of a hard disk drive as an example of a rotating body device. In FIG. 4, SM is a spindle motor, and MD is a magnetic disk.

As shown in FIGS. 1 and 2, the fluid dynamic bearing according to the present invention comprises a flanged shaft 1 comprising a ring member 3 and a cylindrical member 2, a sleeve 4 for receiving the flanged shaft 1, and a thrust holding member. And an annular lid member 5 which also functions as As shown in FIG. 2, a spiral thrust dynamic pressure groove G2 such as a herringbone groove is formed on the upper and lower surfaces of the ring member 3, and a radial dynamic pressure groove G1 such as a herringbone groove is formed on its outer peripheral surface. ing.

The tapered minute gap R1 opened to the atmosphere.
Is formed between the outer peripheral surface of the upper part of the cylindrical member 2 and the inner peripheral surface of the annular lid member 5. The annular minute gap R2 is formed between the lower surface of the annular lid member 5 and the upper surface of the ring member 3. The ring-shaped minute gap R3 is formed between the inner peripheral surface of the large diameter portion of the sleeve 4 and the outer peripheral surface of the ring member 3. The annular minute gap R4 is formed between the bottom surface of the large diameter portion of the sleeve 4 and the lower surface of the ring member 3. The ring-shaped minute gap R5 is formed between the inner peripheral surface of the small diameter portion of the sleeve 4 and the outer peripheral surface of the lower part of the cylindrical member 2. Further, the disc-shaped minute gap R6 is formed between the bottom surface of the small diameter portion of the sleeve 4 and the lower end surface of the columnar member 2. These minute gaps R1, R2, R3, R4, R5 and R6 are filled with lubricating oil F.

Each of the annular minute gaps R2 and R4 is a thrust bearing gap, and the ring-shaped minute gap R3 is a radial bearing gap. The clearance interval of the bearing gap depends on the size of the fluid dynamic pressure bearing, the number of revolutions, and the viscosity coefficient of the lubricating oil.
m to several hundred μm. The gap between the ring-shaped minute gap R4 and the disk-shaped minute gap R6 functioning as a lubricating oil reservoir is larger than this. The tapered minute gap R1 functions as a capillary seal.

The lubricating oil F is a lubricating oil represented by the following chemical formula, wherein the lubricating oil is composed of a single phosphoric acid ester having 18 to 24 carbon atoms, or a lubricating oil having 18 to 24 carbon atoms.
A lubricating oil composed of the following phosphate ester as a base oil and having a volume resistivity of 1.0 × 10 ¹¹ Ωcm or less is employed.

[0032]

Embedded image

As shown in FIG. 3, the spindle motor according to the present invention has a rotor rotatably supported on a stator by a fluid dynamic pressure bearing as shown in FIG. The rotor includes a cup-shaped hub 6 on which a magnetic disk is mounted, and a rotor magnet 7, and the stator includes a stator coil 8 disposed close to and opposed to the rotor magnet 7. The magnetic disk is mounted on an extension that extends horizontally from the lower end of the skirt of the cup-shaped hub 6. The rotor magnet 7 is a multi-pole magnetized ring-shaped permanent magnet attached to the inner peripheral surface of the cup-shaped hub 6. When an exciting current is supplied to the stator coil 8 from an exciting current source (not shown), the spindle motor rotates due to the interaction between the exciting current and the magnetic field of the rotor magnet 7.

Hereinafter, Examples 1 to 4 of the present invention will be described.
The characteristics will be described in comparison with Comparative Examples 1 and 2. (Example 1) A phosphoric acid ester having 100% of an n-octyl group as a saturated alkyl group R was used alone as a lubricating oil.

(Example 2) As the saturated alkyl group R, n-
A phosphoric acid ester having 100% heptyl groups was used alone as a lubricating oil.

(Example 3) As the saturated alkyl group R, n-
A phosphoric acid ester having 100% of hexyl groups was used alone as a lubricating oil.

(Example 4) As the saturated alkyl group R, n-
A phosphoric ester mixture containing 86.7% of octyl groups and 13.3% of total of 3-methyl-1-hexyl group and 5-methyl-1-hexyl group was used as lubricating oil.

Comparative Example 1 Dioctyl sebacate alone was used as a lubricating oil.

Comparative Example 2 Dioctyl adipate was used alone as a lubricating oil.

[0040]

[Table 1]

(Rotation Torque) When the lubricating oil according to the present invention was used for a fluid dynamic pressure bearing, the rotation torque was generally low, and the driving time could be greatly increased. That is, Table 1
As shown in the figure, the rotational torque of the embodiment of the present invention is close to that of the comparative example 2 of the low rotational torque, and that of the comparative example 1 of the high rotational torque.
Although it is various to those close to the above, it generally shows low rotational torque characteristics. However, the driving time is 400
Above 0 hours, this is a significant increase over the comparative example. The values shown in Table 1 are obtained by using the lubricating oils of the examples and comparative examples as the lubricating oil of the spindle motor of FIG.
The driving time was obtained under the conditions of ° C and a rotation speed of 5000 rpm, and the rotation torque obtained under the conditions of 20 ° C and a rotation speed of 5000 rpm.

(Viscosity and Viscosity Change) The lubricating oil according to the present invention has a low viscosity at low temperatures and a substantially constant viscosity coefficient with temperature changes. That is, as shown in Table 2, the viscosities and changes in viscosity in the examples of the present invention are substantially the same as those in the comparative examples, although differences are seen in the individual examples. The viscosity and the change in viscosity of Example 1 are very close to those of Comparative Example 1 having high viscosity. On the other hand, Examples 2, 3 and 4 all show viscosities close to those of Comparative Example 2 having a low viscosity, and the change in viscosity is equivalent to that of Comparative Examples 1 and 2. Shows a low numerical value. In Table 2, the viscosity is a numerical value at 20 ° C., and the change in viscosity is −5.
It is a numerical value of the difference between the viscosities at 40 ° C. and 40 ° C.

[0043]

[Table 2]

(Evaporation Loss) The lubricating oil according to the present invention has excellent low evaporation characteristics. That is, as shown in Table 2, the minimum evaporative loss of the example of the present invention was 0.39% by weight of Example 4 and the maximum was 1.25% by weight of Example 3; It is an order of magnitude less than 4% by weight. But,
Although slightly less than 0.38% by weight of Comparative Example 1 having excellent low evaporation characteristics, each of the examples shows a value close to this.

(Volume Resistivity) The lubricating oil according to the present invention has a very low volume resistivity. That is, as shown in Table 2, the minimum volume resistivity of the embodiment of the present invention is 4.8 × 10 ⁷ Ωcm of the third embodiment, and the maximum is 5.0 × 10 ⁷ Ωcm of the first embodiment.
This is a very low value of 10 ⁸ Ωcm. This shows a very large decrease of four to five orders as compared with 2.3 × 10 ¹² Ωcm of Comparative Example 1. Therefore, according to the present invention, when a fluid dynamic bearing is used as a bearing of a spindle motor for an HDD device, a low volume resistivity lubricating oil required to prevent electrostatic breakdown, that is, a volume resistivity of 1.0 × 10 ¹¹ Ωcm or less, desirably 1.0 × 10
Lubricating oil of ^{9 to} 1.0 × 10 ¹⁰ Ωcm or less was realized. In addition, the numerical values of the evaporation loss and the volume resistivity in Table 2 were obtained by injecting the lubricating oil to be measured into a glass container,
This is a result measured 72 hours after storing in a thermostat under the conditions described in (1).

As described above, the lubricating oil for a fluid dynamic pressure bearing composed of a simple substance of a phosphate ester having 18 to 24 carbon atoms is used in the three embodiments, and a phosphate ester having 18 to 24 carbon atoms is used as a base oil. Although one embodiment has been disclosed as the lubricating oil for a fluid dynamic bearing, other additives may be added as necessary.

[0047]

The lubricating oil for fluid dynamic pressure according to the present invention comprises a lubricating oil composed solely of a phosphate ester having 18 to 24 carbon atoms or a phosphate ester having 18 to 24 carbon atoms as a base oil. Because of this lubricating oil, its low-evaporation characteristics and low viscosity were able to be improved as compared with the conventional lubricating oil. Moreover, four to five orders of magnitude compared to conventional lubricating oils
It was possible to provide a fluid dynamic pressure lubricating oil having a low volume resistivity and a significantly reduced girder.

The hydrodynamic bearing according to the present invention is characterized in that a lubricating oil composed of a simple substance of a phosphate ester having 18 to 24 carbon atoms or a lubricating oil composed of a phosphate ester having 18 to 24 carbon atoms as a base oil is used. Because it is used, it has better evaporation characteristics than conventional lubricating oils, has low viscosity at low temperatures, and has a viscosity coefficient that is almost constant with temperature changes. As a result, the starting characteristics at low temperatures and the bearing stiffness at high temperatures have been improved, and the bearing performance can be maintained well over a wide operating temperature range.

A spindle motor according to the present invention and a rotator device using the spindle motor as a drive source of a rotator include:
Providing the above-mentioned hydrodynamic bearing has increased the life and improved the performance. In addition, a lubricating oil composed of a phosphate alone having 18 to 24 carbon atoms or a lubricating oil composed of a phosphate having 18 to 24 carbon atoms as a base oil having a volume resistivity of 1.0 to 1.0. When a dynamic pressure bearing employing a lubricating oil of × 10 ¹¹ Ωcm or less is provided, it is not necessary to provide a special grounding mechanism, and a spindle motor for an HDD device can be provided with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional view of one embodiment of a fluid dynamic bearing according to the present invention, in which a minute gap is exaggerated.

FIG. 2 is a perspective view of a flanged shaft having a ring member in which a thrust dynamic pressure groove and a radial dynamic pressure groove are respectively formed.

FIG. 3 is a longitudinal sectional view of one embodiment of a spindle motor according to the present invention.

FIG. 4 is a perspective view of a main part of the hard disk drive.

FIG. 5 is a diagram showing a change in bearing stiffness of a fluid dynamic bearing with respect to temperature.

FIG. 6 is a diagram showing a change in a viscosity coefficient of a lubricating oil with respect to a temperature.

[Explanation of symbols]

 Reference Signs List 1 shaft with flange 2 cylindrical member 3 ring member 4 sleeve 5 annular lid member 6 cup-shaped hub 7 rotor magnet 8 stator coil 9 motor frame F lubricating oil G1 radial dynamic pressure groove G2 thrust dynamic pressure groove R1, R2, R3, R4, R5, R6 Micro gap MD Magnetic disk SM Spindle motor

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tadao Iwaki 1-8-8 Nakase, Mihama-ku, Chiba-shi, Chiba Seiko Instruments Inc. (72) Inventor Shigeo Mori 35-1 Katsura-Chiyoharacho, Nishikyo-ku, Kyoto, Kyoto (72) Inventor Yoshihisa Okamoto 1-18-2 Nishiikebukuro, Toshima-ku, Tokyo F-term (reference) 3J011 AA04 BA09 CA02 JA02 KA04 MA22 4H104 BH03A LA01 LA04 PA01 PA04 5H607 AA00 BB01 BB14 BB17 BB25 CC01 CC05 DD01 DD02 DD03 FF12 GG01 GG02 GG12 GG15 KK10

Claims (8)

[Claims] 1. A lubricating oil for a fluid dynamic bearing comprising a simple substance of a phosphate ester having 18 to 24 carbon atoms. 2. A lubricating oil for a fluid dynamic bearing, comprising a phosphate having 18 to 24 carbon atoms as a base oil. 3. A shaft comprising a shaft forming a bearing gap, a sleeve and a lubricating oil filled in the bearing gap, wherein the shaft is rotatable in a non-contact manner with the sleeve by dynamic pressure generated in the bearing gap during rotation. 3. The fluid dynamic pressure bearing according to claim 1, wherein said lubricant for fluid dynamic pressure is employed as said lubricating oil. 4. A shaft which forms a bearing gap, a sleeve and a lubricating oil filled in the bearing gap, and the shaft is rotatable in a non-contact manner with the sleeve by dynamic pressure generated in the bearing gap during rotation. Wherein the lubricating oil is the lubricating oil according to claim 1 or 2, and has a volume resistivity of 1.0 × 10 ¹¹ Ωcm or less. Fluid dynamic pressure bearing. 5. A spindle motor in which a rotor is rotatably supported on a stator by the fluid dynamic bearing of claim 3. 6. A spindle motor in which a rotor is rotatably supported on a stator by the fluid dynamic pressure bearing according to claim 4. 7. A rotator device in which a rotator is driven to rotate by the spindle motor according to claim 5. 8. A hard disk drive device wherein a hub on which a magnetic disk is mounted is rotationally driven by the spindle motor according to claim 6.