JPH0737810B2 - Gas dynamic bearing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an aerodynamic bearing used in a spindle motor, and more particularly to an aerodynamic bearing that can be used in an environment where ordinary lubricants, which tend to scatter, cannot be used.

BACKGROUND ART In recent years, as the storage capacity of hard disk drives (HDDs) has increased, the bearings of the spindle motors used in them are required to have high-precision rotational performance and low power consumption, and the adoption of gas dynamic bearings has been proposed. In gas dynamic bearings, the bearing sliding surfaces are in contact during start-up and stop, and the material and
Depending on the machining conditions or assembly precision, durability may be an issue, raising questions about the necessity of lubricants.

Generally, the sliding parts of the bearing metal of the gas dynamic pressure bearing used in HDD spindle motors are lubricated by liquid lubricants such as oil, grease (hereinafter referred to as "oil"), or various coatings.

When the bearing material is a normal metal, a lubricant film thickness of 0.01 to several microns is formed, but the bonding strength between the metal substrate and the lubricant is weak, causing the lubricant to peel off and the oil to turn into a mist and splash outside, which can have a negative effect on the magnetic media of hard disks, etc.

Furthermore, if a 500 Å thick film of lubricant that is difficult to scatter is formed on a ceramic bearing surface with a surface roughness Ra of, say, 0.2 microns, the film thickness will be uneven, making it difficult to maintain uniformity. Furthermore, if the film thickness exceeds 100 Å, stiction occurs due to the meniscus effect of the lubricant, increasing static friction torque and preventing the spindle motor from starting. On the other hand, if the film thickness is too thin, the oil will peel off with repeated starts and stops, causing the ceramic substrates to come into contact with each other, accelerating wear and generating dust.

To solve these problems, the following methods have been proposed. One conventional method involves coating the sliding surfaces with a light, soft metal layer such as silver, lead, or white metal as a lubricating layer, and utilizing the plastic deformation, shear, and adhesion of this soft metal to prevent seizure. This method has the problem that wear particles from the soft metal form fairly large lumps, which escape to the outside or become trapped between the sliding surfaces or in the hydrodynamic grooves, preventing the bearing from functioning properly.

Another method that has been proposed in the past is to coat the sliding surface with a self-lubricating material, such as molybdenum disulfide or a lubricating polymer such as polytetrafluoroethylene, by means of sputtering (ion plating, vapor deposition) or the like.

However, methods using vacuum equipment such as sputtering require complex equipment and difficult conditions to set, making it difficult to coat the inner surface uniformly. Furthermore, the polymer can lose its lubricating properties, and as wear progresses, the lubricating layer can be rapidly damaged, resulting in an increase in the coefficient of friction, requiring a large torque at start-up.

Japanese Patent Laid-Open Publication No. 64-65322 discloses a hydrodynamic bearing in which at least one of the opposing rotating surfaces of the hydrodynamic bearing is coated with either a functional group-containing organopolysiloxane or a fluorine-containing polymer, or both. In the hydrodynamic bearing of this publication, the above substance reacts with the metal surface that constitutes the sliding surface to form a lubricant film. However, there are limitations to increasing the precision of metal processing, and the surface roughness of metal is relatively large, so in the hydrodynamic bearing of this publication, the lubricant becomes multi-layered.

DISCLOSURE OF THE INVENTION The present invention has been made in view of the above points, and resides in improving a hydrodynamic bearing for use in a highly clean environment, and has as its object to provide a hydrodynamic bearing that can improve the life and performance of a hydrodynamic bearing that slides between fixed parts during start-up and shutdown, and can reduce loss torque during start-up.

In the gas pressure bearing of the present invention, the sliding surface is made of a dense ceramic material with a roughness Ra of 0.3 microns or less, and is thinly coated with at least one lubricant selected from perfluoropolyethers having functional groups reactive with ceramics, to form a uniform thin film 20 to 100 Å thick on the sliding surface that provides protection and lubrication. Preferably, the lubricant contains an antistatic agent.

The hydrodynamic bearing of the present invention differs substantially in dimensional accuracy from hydrodynamic bearings used in water-lubricated pumps and rotating parts of machine tools, etc., and is finished to flatness, cylindricity, and roundness within a few microns. It is also a hydrodynamic bearing used in a clean environment where no dust can enter from the outside.

In an environment where powder, oil, etc. are mixed in from the outside, the hydrodynamic bearing of the present invention will not exhibit its intended function and effect.

The surface roughness Ra of the ceramic sliding part must be finished to 0.3 microns or less, or the surface roughness Ra must be 0.3 microns or less using sputtering, CVD, ion plating, etc. If the surface roughness Ra is greater than 0.3 microns, the actual contact area will be smaller, resulting in increased surface pressure and making the thin film more likely to peel off, making it less effective.

Sliding surfaces have unevenness on their surfaces, and these convex portions actually come into contact with each other. Therefore, even if the apparent contact area is the same, if the surface roughness is large, the actual contact area will be smaller. If the actual contact area is small, the actual surface pressure will be large, causing the applied lubricant to peel off and wear to progress. If the surface roughness is small, the contact area will be larger, the effect of the lubricant will be greater, and the surface pressure will be lower, resulting in a lower coefficient of friction.

The perfluoropolyether lubricant having a functional group used as the lubricant is one or a mixture of two or more selected from hydroxyl group-containing perfluoropolyethers, carboxyl group-containing perfluoropolyethers, ester group-containing perfluoropolyethers, etc.

The application method involves thoroughly degreasing and drying the bearing member, dissolving a small amount of the lubricant in a solvent such as trichlorotrifluoroethane, applying it by spin coating, spraying, immersion, or the like, and then heating to form a thin film.

The perfluoropolyether lubricant having a functional group has a unit represented by the general formula _{-CxF2x -} O- (x is an integer of 1 to 4) as the main structural unit, and has an average molecular weight of
The polymer is a polymer of 1000-5000, and this portion controls the lubricating action. Furthermore, functional groups containing, for example, hydroxyl, glycidyl, amino, carboxyl, mercapto, isocyanate, sulfonic, or ester groups are preferred, as they contribute to the reaction with the ceramic surface. The lubricant to be used is determined by the type of ceramic.

The thickness of the thin film is controlled to a uniform thickness of 20 to 100 .ANG.. This thin film is composed of a monolayer or a multilayer.
The size of a monomolecular layer is about 20 Å, so a film thickness of 100 Å would consist of five molecular layers. It is not possible to apply a monomolecular layer less than 20 Å thick. Of course, the size of the molecule varies depending on the type of lubricant. The functional groups of the lubricant are strongly bonded to the ceramic surface mainly through chemical adsorption, preventing the lubricant from peeling off due to CSS and protecting the ceramic surface. If the film thickness is greater than 100 Å, stiction occurs due to the meniscus effect of the lubricant, increasing starting resistance.

The antistatic agent is selected from one or more surfactants or polyoxyethylene additives, preferably of the ester or amine type. The surfactants are fatty acid polyhydric alcohol esters having monoalkyl, dialkyl, glycerin, sorbitol, polyglycerin, or polyoxyethylene as the lipophilic group. The polyoxyethylene additives are polyoxyethylene additives having alkylamine, alkylamide, fatty alcohol, or alkylphenol as the lipophilic group.

The antistatic coating is carried out by adding a small amount of antistatic agent when the lubricant is applied.

By providing a protective and lubricating thin film of the above-mentioned material 20 to 100 Å thick on the sliding surfaces of the components of the hydrodynamic bearing, the gas pressure bearing of the present invention can reduce the static friction coefficient to 0.2 or less, as will be described in detail later, and has excellent durability, with the friction coefficient remaining unchanged even in a CSS (contact start stop) test of 40,000 revolutions or more.

In addition, in the present invention, a highly wear-resistant ceramic material is used on the sliding surface, so the sliding surface can withstand 80,000 CSS cycles.
No damage after the test.

The present invention has the excellent effect of providing a hydrodynamic bearing that has a small coefficient of static friction, is highly durable, does not use ordinary oil, and can be used in a clean environment without requiring any special measures to prevent dust generated by the bearing from leaking to the outside.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial cross-sectional view showing the structure of a hydrodynamic radial bearing to which the present invention is applicable, FIG. 2 is a plan view showing the spiral band of a hydrodynamic thrust bearing to which the present invention is applicable, FIG. 3 is a cross-sectional view showing the structure of a spindle motor using the hydrodynamic bearing of the present invention, and FIGS. 4, 5, 6 and 7
1 is a graph showing the results of a CSS test.

BEST MODE FOR CARRYING OUT THE INVENTION FIG. 1 is a partial cross-sectional view illustrating the structure of a hydrodynamic radial bearing to which the present invention can be applied, in which hydrodynamic pressure generating grooves, such as herringbone grooves _C1 , that generate fluid hydrodynamic pressure are formed on the surface of the movable or fixed member B of the bearing.

The sliding surface is made of ceramic material, has a surface roughness Ra of 0.3 microns or less, and is thinly coated with at least one lubricant selected from perfluoropolyethers having functional groups reactive with ceramics to form a thin film of lubricant that provides protection and lubrication.
20 to 100 Å. Preferably, the lubricant contains an antistatic agent.

The ceramic materials that make up the sliding surfaces are high-purity dense alumina, mullite, diamond, SiC, TiN, Si ₃ N ₄ , TiC, which are used as engineering ceramics.
The movable and fixed members of the bearing may be integrally formed from these ceramic materials, or the sliding surfaces may be coated with these ceramic materials. Also, any combination of these structures may be used.

In the hydrodynamic radial bearing of FIG. 1, when the movable member A and the fixed member B are rotated relative to each other, hydrodynamic pressure is generated in the gap between them.

FIG. 2 is a plan view diagrammatically showing spiral grooves of a hydrodynamic thrust bearing to which the present invention can be applied.

The surface of the fixed or movable member D of the thrust hydrodynamic bearing is formed with hydrodynamic grooves, such as spiral grooves _C2 , that generate hydrodynamic pressure. The sliding surface, like the hydrodynamic radial bearing of FIG. 1, is made of ceramic material with a surface roughness Ra of 0.3 microns or less. A thin layer of lubricant, provided with both protective and lubricating properties, is applied to the sliding surface. The thickness of the thin layer is 20 to 100 Å. Preferably, the lubricant contains an antistatic agent.

The ceramic material and structure of the sliding surface are the same as those of the hydrodynamic radial bearing shown in FIG.

In the hydrodynamic thrust bearing of FIG. 2, when the movable member A and the fixed member B are rotated relative to each other, hydrodynamic pressure is generated in the gap between them.

By applying the above lubricant to the sliding surface of the bearing and creating a thin film of lubricant, the static friction coefficient of the hydrodynamic bearing can be reduced to 0.
It was found that the coefficient of friction could be reduced to 2 or less, and durability was also improved by more than 80,000 CSS (contact start stop) tests, with no change in the coefficient of friction. It was also confirmed that there was no damage to the sliding surface.

Example 1 Next, a specific example of the hydrodynamic bearing of the present invention will be described.

Figure 3 is a side cross-sectional view of a spindle motor for an HDD (hard disk drive) that uses the hydrodynamic bearing of the present invention as its bearing. In Figure 3, a support shaft 2 is erected in the center of a mounting base 1, and a fixed member 4a of a concentric radial bearing 4 is fixed to the outer periphery of the support shaft 2. The motor rotating part 6 is cap-shaped and has a support shaft through-hole in the center through which the support shaft 2 passes, and a flange 6a that expands horizontally is formed at the lower end.

A concentric movable member 4b facing the fixed member 4a of the radial bearing 4 is fixed to the inner peripheral surface of the support through hole of the motor rotating part 6. The movable member 4b and the fixed member 4a constitute the radial bearing 4.

A movable member 3b of the thrust bearing 3 is fixed to the underside of the flange 6a of the motor rotating part 6, and a fixed member 3a facing the movable member 3b is fixed to the mounting base 1. The fixed member 3a and the movable member 3b constitute the thrust bearing 3.

A rotor magnet 8 is fixed to the lower end of the motor rotating part 6 inside the movable member 3 b , and a stator coil 5 facing the rotor magnet 8 is fixed to the mounting base 1 .

A hard disk can be mounted on the outer periphery of the motor rotating part 6 .

The spindle motor is a so-called thrust gap type spindle motor, and the stator coil 5 and rotor magnet 8 constitute a drive section.

On the opposing surfaces of the fixed side member 4a and the movable side member 4b of the radial bearing 4, dynamic pressure generating grooves for generating dynamic pressure, such as herringbone grooves _C1 as shown in FIG.
On the opposing surfaces of the fixed-side member 3a and the movable-side member 3b, dynamic pressure generating grooves for generating dynamic pressure, such as spiral grooves _C1 as shown in FIG. 2, are formed.

In a spindle motor with the above structure, when current flows through the stator coil 5, the motor rotating part 6, to which the rotor magnet 8 is fixed, begins to rotate, and air dynamic pressure is generated between the upper surface of the fixed member 3a and the lower surface of the movable member 3b of the thrust bearing 3, forming an air dynamic pressure thrust bearing. Also, air dynamic pressure is generated between the outer peripheral surface of the fixed member 4a and the inner peripheral surface of the movable member 4b of the radial bearing 4, forming an air dynamic pressure radial bearing.

The material of the fixed member that constitutes the bearing is SiC, and the sliding portion between the fixed member 3a and the movable member 3b of the thrust bearing 3 has a flatness of 1 micron or less and a surface roughness Ra of 0.2 microns. The sliding surface between the fixed member 4a and the movable member 4b of the radial bearing 4 has a cylindricity or roundness of 1 micron and a surface roughness Ra of 0.1 micron.

As a lubricant, an isocyanate-modified fluoropolyether polymer (FOBLNZDISOC, manufactured by Nippon Montezon Co., Ltd.) represented by the following chemical formula was immersed in a 0.01 wt % solution using trichlorotrifluoroethane as a solvent, and then dried with hot air at 120°C for 1 hour.

(OCN- _C6H3 ( _CH3 )NHCO) _-CF3- (O- _{C2F4)-p-(O-CF2 ) q - OCF2 ( OCN} - _C6H3 ( _CH3 )NHC) At this time, a solid lubricant film of approximately 100 Å was formed. The results of the _CSS test were as shown in curve a in Figure 4, and no increase in the friction coefficient was observed up to 80,000 cycles.

Curve b in Figure 4 shows the CSS test results for Example 2.

The material of the bearing components was 99.7% alumina, and the other components were the same as in Example 1. The lubricant was a carboxyl group-containing perfluoroalkyl polyether (DUPON's KRYTOX 157
The coating method was to immerse the sample in a 0.005 wt% trichlorotrifluoroethane solvent and apply a 1
The heat treatment was carried out at 20°C for 30 minutes. At this time, a lubricant film of approximately 50 Å was formed. The CSS test results in this case were as shown by curve b in Figure 4, and no increase in the friction coefficient was observed up to 80,000 cycles.

Example 3 Curve c in Figure 4 shows the CSS test results for Example 3.

The test was carried out in the same manner as in Example 2, except that the members constituting the bearing were made of aluminum and TiN ion plating was applied to the sliding surface. The CSS test results are shown in curve c in Figure 4.
No increase in the coefficient of friction was observed up to 80,000 cycles.

Specific Example 4 Figure 5 shows the CSSS test results for Specific Example 4, which was set under the same conditions as Specific Example 1 except for the surface roughness.
i and j are the surface roughness Ra of the thrust bearing and radial bearing.
, respectively, 0.1 microns, 0.3 microns, and 0.4
This is the CSS test result when the microns are in contact with each other.
and the surface roughness Ra as shown by the curve i, respectively.
When the surface roughness Ra was 0.1 microns and 0.3 microns, the static friction coefficient hardly changed. However, when the surface roughness Ra was 0.4 microns, a sudden increase was observed after about 1000 strokes, and the coefficient continued to increase gradually thereafter. At this time, the lubricant film was formed to a thickness of 100 Å.

6 shows the results of the CSS test for Example 5, which was conducted under the same conditions as Example 1 except for the thickness of the thin film of the lubricant.
Curves m, n, and s are for lubricant thin film thicknesses of 50
These are the results of CSS tests when the thickness of the lubricant film was 50 Å, 100 Å, and 120 Å. As shown by curves m and n, when the thickness of the lubricant film was 50 Å and 100 Å, respectively, the static friction coefficient hardly changed. However, when the thickness of the lubricant film was 150 Å, stiction occurred after only 100 strokes, as shown by curve s.
It became impossible to measure the static friction coefficient.

Comparative Example 1 Curve d in Figure 7 shows the CSS test results for Comparative Example 1. In Comparative Example 1, the conditions were the same as those for Specific Example 1, except that no lubricant was applied. In this case, an increase in frictional resistance was observed after about 50 CSS tests.

Comparative Example 2 The curve e in Fig. 7 shows the CSS test results for Comparative Example 2. In Comparative Example 2, the members constituting the bearing were made of stainless steel SU
The CSS test was carried out on S303 without any anti-wear treatment under the same conditions as in Example 1. In this case, an increase in the friction coefficient was observed after about 500 cycles. Numerous sliding marks were also observed on the sliding surface of the bearing.

Comparative Example 3: Curves f, g, and h in Figure 7 show the CSS test results for Comparative Example 3. In Comparative Example 3, the lubricants were boronite, molybdenum disulfide, and graphite fluoride, respectively, under the same conditions as in Example 1. The lubricants were applied by sprinkling powder of each lubricant on the sliding surfaces of the bearing components, and also on the polishing cloth, and then rubbing them together under pressure. Curves f, g, and h in Figure 7 show the results for boronite, molybdenum disulfide, and graphite fluoride, respectively. As shown in the figure, the friction coefficient increased with the number of CSS tests, reaching 0.8 or higher after approximately 3,000 cycles.

──────────────────────────────────────────────────── Continued from the front page (56) References: JP 63-57914 (JP, A) JP 63-251618 (JP, A) Utility Patent Publication No. 2-21324 (JP, U)

Claims (6)

[Claims] Claim 1: The sliding surface is made of dense ceramics,
The bearing surface is finished to a surface roughness Ra of 0.3 microns or less, and is coated with at least one lubricant selected from pearl fluoropolyethers having functional groups reactive with ceramics, thereby forming a uniform thin film with a thickness of 20 to 100 Å that provides protective and lubricating properties on the sliding surface. 2. The gas dynamic pressure bearing according to claim 1,
1. A gas pressure bearing, wherein the lubricant contains an antistatic agent. 3. The gas dynamic pressure bearing according to claim 1, wherein the lubricant is selected from the group consisting of hydroxyl group-containing perfluoropolyethers, carboxyl group-containing perfluoropolyethers, and ester group-containing perfluoropolyethers. 4. The gas dynamic pressure bearing according to claim 1,
A gas dynamic pressure bearing characterized in that the application of lubricant is carried out by degreasing and drying the bearing member, immersing it in the lubricant dissolved in a solvent, or by applying the lubricant to the bearing surface by spin coating or spraying, and then heating it. 5. The gas dynamic pressure bearing according to claim 1,
The perfluoropolyether lubricant having functional groups is -
The main structural unit is a unit represented by the general formula _{CxF2x -} O- (x is an integer of 1 to 4), and the average molecular weight is 1000 to 50
00的聚合物的气体压力轴承。 00 polymer gas dynamic pressure bearing. 6. The gas dynamic pressure bearing according to claim 2,
Antistatic agent, monoalkyl, dialkyl as lipophilic group,
1. A gas dynamic pressure bearing comprising one or more surfactants selected from glycerin, sorbitol, polyglycerin, and fatty acid polyhydric alcohol esters having polyoxyethylene, or a polyoxyethylene additive having an alkylamine, alkylamide, fatty alcohol, or alkylphenol as a lipophilic group.