U.S. patent application number 12/304595 was filed with the patent office on 2009-10-01 for method of preventing lubricant from deteriorating, lubricant, and dynamic-pressure bearing device.
This patent application is currently assigned to NIDEC CORPORATION. Invention is credited to Toshimasa Kobayashi, Chikara Sagae, Sayuri Tsubata.
Application Number | 20090247433 12/304595 |
Document ID | / |
Family ID | 38831818 |
Filed Date | 2009-10-01 |
United States Patent
Application |
20090247433 |
Kind Code |
A1 |
Tsubata; Sayuri ; et
al. |
October 1, 2009 |
METHOD OF PREVENTING LUBRICANT FROM DETERIORATING, LUBRICANT, AND
DYNAMIC-PRESSURE BEARING DEVICE
Abstract
Disclosed are a lubricant whose oxidation degradation is
inhibited, and a dynamic pressure bearing device which has a long
lifespan by prolonging the period of time of depression of a
lubricating function of the lubricant occurring as a result of
oxidation degradation. Ester-based lubricating oil serving as the
lubricant is brought into continuous or intermittent contact with
an ionic compound or an ionic compound solution obtained by
dissolving the ionic compound in a solvent. In the dynamic pressure
bearing device, the ionic compound is provided in a portion of a
bearing or a shaft member and is thus brought into contact with the
lubricating oil, in which the ionic compound or ionic compound
solution is substantially insoluble in the ester-based lubricating
oil.
Inventors: |
Tsubata; Sayuri; (Kyoto,
JP) ; Sagae; Chikara; (Kyoto, JP) ; Kobayashi;
Toshimasa; (Kyoto, JP) |
Correspondence
Address: |
WESTERMAN, HATTORI, DANIELS & ADRIAN, LLP
1250 CONNECTICUT AVENUE, NW, SUITE 700
WASHINGTON
DC
20036
US
|
Assignee: |
NIDEC CORPORATION
Minami-ku, Kyoto
JP
|
Family ID: |
38831818 |
Appl. No.: |
12/304595 |
Filed: |
June 15, 2007 |
PCT Filed: |
June 15, 2007 |
PCT NO: |
PCT/JP2007/062074 |
371 Date: |
June 10, 2009 |
Current U.S.
Class: |
508/100 ;
508/180; 508/463 |
Current CPC
Class: |
F16C 41/008 20130101;
C10M 2207/141 20130101; F16C 2370/12 20130101; C10M 2207/126
20130101; C10M 2207/123 20130101; C10N 2010/04 20130101; C10M
169/04 20130101; C10M 2207/283 20130101; C10M 2215/04 20130101;
C10N 2010/02 20130101; G11B 19/2036 20130101; F16C 33/109 20130101;
C10M 2207/122 20130101; C10M 2207/127 20130101; C10M 2201/062
20130101; C10M 2207/281 20130101; C10N 2040/02 20130101; C10N
2030/10 20130101; C10M 2207/285 20130101; F16C 33/10 20130101; C10M
2207/284 20130101; C10M 2207/282 20130101; C10M 105/32
20130101 |
Class at
Publication: |
508/100 ;
508/463; 508/180 |
International
Class: |
C10M 125/10 20060101
C10M125/10; C10M 105/32 20060101 C10M105/32 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 15, 2006 |
JP |
2006-165426 |
Claims
1. A method of preventing degradation of a lubricant, comprising:
bringing an ester-based lubricating oil serving as the lubricant
into contact with an ionic compound continuously or intermittently,
in which the ester-based lubricating oil contains mainly an ester,
and the ionic compound includes a cation and an anion bonded to
each other mainly through an ionic bond thus forming a molecule or
crystal, wherein the ionic compound is substantially insoluble in
the ester-based lubricating oil.
2. A method of preventing degradation of a lubricant, comprising:
bringing an ester-based lubricating oil serving as the lubricant
into contact with an ionic compound solution continuously or
intermittently, in which the ester-based lubricating oil contains
mainly an ester, and the ionic compound solution is obtained by
dissolving an ionic compound, including a cation and an anion
bonded to each other mainly through an ionic bond thus forming a
molecule or crystal, in a solvent, wherein the ionic compound and
the solvent are substantially insoluble in the ester-based
lubricating oil.
3. The method as set forth in claim 1 or 2, wherein the ionic
compound is a salt in which a hydrogen atom of an acid is
substituted with a metal ion.
4. The method as set forth in claim 3, wherein the salt has an acid
dissociation constant (pKa) ranging from 9 to 11.
5. The method as set forth in claim 3, wherein the salt is a
bicarbonate of an alkali metal other than lithium.
6. The method as set forth in claim 3, wherein the salt is a
carbonate of an alkali metal other than lithium.
7. The method as set forth in claim 3, wherein the salt is a
carboxylate of an alkali metal other than lithium.
8. A lubricant comprising: an ester-based lubricating oil
containing mainly an ester; and an ionic compound including a
cation and an anion bonded to each other through an ionic bond thus
forming a molecule or crystal, wherein the ionic compound is
substantially insoluble in the ester-based lubricating oil and an
interface is formed between the ester-based lubricating oil and the
ionic compound.
9. A lubricant comprising: an ester-based lubricating oil
containing mainly an ester; and an ionic compound solution obtained
by dissolving an ionic compound, including a cation and an anion
bonded to each other through an ionic bond thus forming a molecule
or crystal, in a solvent, wherein the ionic compound is insoluble
in the ester-based lubricating oil and an interface is formed
between the ester-based lubricating oil and the ionic compound
solution.
10. The lubricant as set forth in claim 8 or 9, wherein the ionic
compound is a salt in which a hydrogen atom of an acid is
substituted with a metal ion.
11. The lubricant as set forth in claim 10, wherein the salt has an
acid dissociation constant (pKa) ranging from 9 to 11.
12. The lubricant as set forth in claim 10, wherein the salt is a
bicarbonate of an alkali metal other than lithium.
13. The lubricant as set forth in claim 10, wherein the salt is a
carbonate of an alkali metal other than lithium.
14. The lubricant as set forth in claim 10, wherein the salt is a
carboxylate of an alkali metal other than lithium.
15. A dynamic pressure bearing device comprising: an ester-based
lubricating oil containing mainly an ester; a first member having a
first bearing surface; and a second member rotatably disposed with
respect to the first member and having a second bearing surface
facing the first bearing surface through a fine gap in which the
lubricating oil is retained, wherein a salt in which a hydrogen
atom of the acid is substituted with a metal ion is disposed in at
least one of a portion of a surface of the first member coming into
contact with the lubricating oil and a portion of a surface of the
second member coming into contact with the lubricating oil, and the
salt is substantially insoluble in the ester-based lubricating
oil.
16. A dynamic pressure bearing device comprising: an ester-based
lubricating oil containing mainly an ester; a first member having a
first bearing surface; and a second member rotatably disposed with
respect to the first member and having a second bearing surface
facing the first bearing surface through a fine gap in which the
lubricating oil is retained, wherein a salt solution obtained by
dissolving a salt in which a hydrogen atom of the acid is
substituted with a metal ion in a solvent is retained in a portion
of a surface of the first member coming into contact with the
lubricating oil or a portion of a surface of the second member
coming into contact with the lubricating oil, and the salt is
substantially insoluble in the ester-based lubricating oil.
17. The device as set forth in claim 15, wherein at least a portion
of the first member or the second member is made of a porous
material, and pores of the porous material are filled with the
salt.
18. The device as set forth in claim 16, wherein at least a portion
of the first member or the second member is made of a porous
material, and pores of the porous material are filled with the salt
solution.
19. The device as set forth in any one of claims 15 to 18, wherein
the salt has an acid dissociation constant (pKa) ranging from 9 to
11.
20. The device as set forth in any one of claims 15 to 18, wherein
the salt is a bicarbonate of an alkali metal other than
lithium.
21. The device as set forth in any one of claims 15 to 18, wherein
the salt is a carbonate of an alkali metal other than lithium.
22. The device as set forth in any one of claims 15 to 18, wherein
the salt is a carboxylate of an alkali metal other than lithium.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a method of preventing
degradation of a lubricant which is mainly used to lubricate a
bearing device, to a lubricant, and to a dynamic pressure bearing
device using the lubricant.
BACKGROUND OF THE INVENTION
[0002] A bearing mechanism is typically lubricated using a
lubricant. The lubricant functions to lubricate components of the
bearing mechanism but is gradually degraded due to oxidation or the
like. In particular, because a dynamic pressure bearing mechanism
used for a spindle motor for a hard disk drive is heated to about
60.degree. C. upon use and also is used for a long period of time,
the degradation of the lubricant is considered to be a major
problem.
[0003] As a method of preventing the degradation of the lubricant,
in particular, the oxidation degradation thereof, there have been
proposed, for example, a lubricant (Japanese Patent Laid-open
Publication No. Hei. 1-188592) using trimethylolpropane fatty acid
triester as base oil and containing a hindered phenol-based
oxidation inhibitor and a benzotriazole derivative, a lubricant
(Japanese Patent Laid-open Publication No. Hei. 1-225697)
containing a hindered phenol-based oxidation inhibitor and an
aromatic amine-based oxidation inhibitor at a specific ratio, a
lubricant (Japanese Patent Laid-open Publication No. Hei. 8-34987)
using carbonate ester as base oil and containing a
sulfur-containing phenol-based oxidation inhibitor and a zinc-based
extreme pressure additive, and a lubricant (Japanese Patent
Laid-open Publication No. Hei. 10-183159) containing base oil
composed mainly of carbonate ester and a phenol-based oxidation
inhibitor.
[0004] A lubricant containing a conventionally known amine-based
oxidation inhibitor or phenol-based oxidation inhibitor is
inhibited from degradation compared to lubricants to which neither
of them is added. However, in the case where such a lubricant is
used under high-temperature conditions for a long period of time,
oxidation degradation is not sufficiently inhibited.
[0005] Although the amount of inhibitor added may be increased to
prolong the degradation inhibition period, it is difficult to
drastically improve the inhibitory effect. The degradation
inhibition period may be prolonged to some degree by an increase in
the amount of the inhibitor. However, in the case where the amount
thereof exceeds a critical level, even when the amount is further
increased, the inhibition period is not prolonged any further.
SUMMARY OF THE INVENTION
[0006] In view of the above, the present invention provides a
lubricant which is used to lubricate a bearing device or the like,
thus enabling the inhibition of degradation thereof, in particular,
oxidation degradation thereof, for a long period of time.
[0007] In the present invention, a degradation inhibitor selected
from among ionic compounds substantially insoluble in ester-based
lubricating oil is prepared, and is thus brought into contact with
the ester-based lubricating oil. Using the ester-based lubricating
oil in this state, a target is lubricated.
[0008] In this case, there is no need to bring the lubricating oil
into continuous contact with the degradation inhibitor. Typically,
lubricating oil for lubricating the bearing device or the like
circulates inside the bearing device through sliding of the
components of the device, and, during the circulation thereof, it
may come into contact with the degradation inhibitor.
[0009] In the case where the device to be lubricated includes a
vessel for reserving the lubricating oil, the granular degradation
inhibitor may be introduced into the vessel. In the present
invention, because the degradation inhibitor substantially
insoluble in the lubricating oil is used, even though such a
degradation inhibitor is added, separation or precipitation occurs.
Nevertheless, because the lubricating oil is brought into contact
with the degradation inhibitor, the degradation of the lubricating
oil may be prevented.
[0010] Also in the present invention, the ionic compound is a
material made up of a cation and an anion bonded to each other
mainly through an ionic bond to thus form a molecule or crystal.
This material is not typically dissolvable in oil.
[0011] The degradation inhibitor which is not dissolvable in the
lubricating oil or is very slightly soluble therein is used in a
state of being brought into contact with the lubricating oil, and
thereby the following effects are obtained. During use, surrounding
impurities dissoluble in the lubricating oil are absorbed on the
degradation inhibitor, thereby preventing the change in properties
of the lubricating oil. Further, because the degradation inhibitor
which is dissolvable in only a very small amount in the lubricating
oil may always be supplied, a condition in which the lubricating
oil contains a very small amount of the degradation inhibitor may
be maintained for a long period of time. Furthermore, the
degradation inhibitor is barely dissolvable in the lubricating oil,
and thus the properties of the lubricating oil, including
viscosity, are not changed even in the presence of the degradation
inhibitor.
[0012] The useful degradation inhibitor may be selected from among
salts in which the atom of an acid molecule, in particular, the
hydrogen atom emitted as a hydrogen cation upon electrolytic
dissociation, is substituted with a metal ion. This material mainly
has a low solubility in the ester-based lubricating oil.
[0013] In the case where the salt, more particularly, an alkali
metal carbonate, alkali metal bicarbonate or alkali metal
carboxylate, all of which are alkaline when provided in the form of
an aqueous solution, is used, a material exhibiting superior
degradation inhibitory effects is found.
[0014] Also, the degradation inhibitor need not be a solid. The
ionic compound may be used in the form of a solution by dissolving
it in a solvent such as water or the like. In either case, an
interface is present between the degradation inhibitor and the
lubricating oil which are in contact with each other. When two
materials, for example, water and oil, which are not mixed with
each other, come into contact, the interface is a boundary
therebetween. These materials are not limited to liquids, and the
boundary between a solid and a liquid is also referred to as an
interface.
[0015] In the case where the lubricating oil of the present
invention is applied to the dynamic pressure bearing device, the
degradation inhibitor is retained in a portion of any one of a
shaft and a bearing, and the surface thereof is brought into
contact with the lubricating fluid. An example of the portion in
which the degradation inhibitor is retained includes a dent, a
groove and a hole. Alternatively, the hole of a sintered member
such as a metal sintered body or the like may be filled with the
degradation inhibitor.
[0016] A conventional degradation inhibitor which is dissolved in
the lubricating oil may be used along with the degradation
inhibitor which is not dissolved in the lubricating oil, thereby
further inhibiting the degradation.
[0017] In accordance with the present invention, lubricating oil
having good properties and which is usable for a long period of
time can be provided. Also, by the use of the lubricating oil, a
dynamic pressure bearing device exhibiting stable performance and
high reliability for a long period of time can be provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a longitudinal cross-sectional view showing a
storage disk drive apparatus.
[0019] FIG. 2 is a longitudinal cross-sectional view showing a
spindle motor including a dynamic pressure bearing device in
accordance with the present invention.
[0020] FIG. 3 is a graph showing the oxidation degradation rates of
base oil to which each alkali metal carbonate was added.
[0021] FIG. 4 is a graph showing the relationship between the
amount of sodium carbonate added to base oil and an oxidation
degradation rate thereof.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(1) Lubricating Oil Containing Solid Ionic Compound
(1-1) Base Oil
[0022] Base oil used for the lubricating oil of the present
invention is an ester-based oil, and specific examples thereof
include monoester, diester, polyolester (trimethylol propane,
pentaerythritol, dipentaerythritol, neopentyldiol ester, complex
ester), polyglycol ester, glycerin ester, and aromatic ester.
[0023] Further, the above ester-based lubricating oil may be added
with ether oil, such as alkylated diphenyl ether, alkylated
triphenyl ether, alkylated tetraphenyl ether and alkylated
polyphenyl ether, various poly-.alpha.-olefins, various silicone
oil species, and various fluorinated oil species.
[0024] Also, examples of the monoester include monoesters composed
of any one organic acid selected from among caprylic acid, capric
acid, lauric acid, myristic acid, palmitic acid, palmitoleic acid,
stearic acid, oleic acid, ricinoleic acid, linoleic acid, linolenic
acid, arachidonic acid, eicosa pentaenoic acid, erucic acid, docosa
hexaenoic acid and lignoceric acid, and any one monovalent alcohol
selected from among methanol, ethanol, propanol, butanol, pentanol,
hexanol, heptanol, octanol, nonanol, decanol, undecanol, dodecanol,
tridecanol, tetradecanol and pentadecanol.
[0025] Examples of the diester include diesters composed of any one
organic acid having two carboxylic groups selected from among
malonic acid, methyl malonic acid, succinic acid, methyl succinic
acid, dimethyl malonic acid, ethyl malonic acid, glutaric acid,
adipic acid, dimethyl succinic acid, pimelic acid, tetramethyl
succinic acid, suberic acid, azelaic acid, sebacic acid and
brassylic acid, and a same type of or different types of two
monovalent alcohol molecules selected from among methanol, ethanol,
propanol, butanol, pentanol, hexanol, heptanol, octanol, nonanol,
decanol, undecanol, dodecanol, tridecanol, tetradecanol and
pentadecanol.
[0026] Examples of the polyolester include polyolesters composed of
any one selected from among trimethylol ethane, trimethylol propane
and pentaerythritol, and any one selected from among caprylic acid,
capric acid, lauric acid, myristic acid, palmitic acid, palmitoleic
acid, stearic acid, oleic acid, ricinoleic acid, linoleic acid,
linolenic acid, arachidonic acid, eicosa pentaenoic acid, erucic
acid, docosa hexaenoic acid and lignoceric acid.
[0027] Examples of the polyglycol ester include glycol esters
composed of polyglycol and any one selected from among caprylic
acid, capric acid, lauric acid, myristic acid, palmitic acid,
palmitoleic acid, stearic acid, oleic acid, ricinoleic acid,
linoleic acid, linolenic acid, arachidonic acid, eicosa pentaenoic
acid, erucic acid, docosa hexaenoic acid and lignoceric acid.
[0028] Examples of the glycerin ester include monofatty acid
glycerin ester, difatty acid glycerin ester, and trifatty acid
glycerin ester. The fatty acid linked to glycerin includes one or
more selected from among caprylic acid, capric acid, lauric acid,
myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic
acid, ricinoleic acid, linoleic acid, linolenic acid, arachidonic
acid, eicosa pentaenoic acid, erucic acid, docosa hexaenoic acid
and lignoceric acid.
[0029] The polyphenyl ether may have no alkyl group, or may have a
linear or branched alkyl group. Specific examples of the alkyl
group include methyl, ethyl, n-propyl, isopropyl, n-butyl,
isobutyl, tert-butyl, n-pentyl, isopentyl, neopentyl, tert-pentyl,
2-methylbutyl, n-hexyl, isohexyl, 3-methylpentyl, ethylbutyl,
n-heptyl, 2-methylhexyl, n-octyl, 2-ethylhexyl, 3-methylheptyl,
n-nonyl, methyloctyl, ethylpentyl, n-decyl, n-undecyl, n-dodecyl
and n-tetradecyl.
[0030] Although a diester-based oil is used as the base oil of the
lubricating oil in the present embodiment, the aforementioned base
oil species may be used in various mixture forms. The mixing of two
or more oil species may be carried out through a known mixing
process.
(1-2) Degradation Inhibitor
[0031] In the present invention, to the base oil of the lubricating
oil, a degradation inhibitor selected from among ionic compounds is
added. In this regard, the ionic compound indicates a material made
up of a cation and an anion bonded to each other mainly through an
ionic bond to thus constitute a molecule or crystal. The ionic
compound typically has a low solubility in oil. In the present
invention, a material which is barely dissolvable in the base oil
is used as the degradation inhibitor.
[0032] In order to prevent oxidation degradation, particularly
useful is an alkali metal carbonate, an alkali metal bicarboante or
an alkali metal carboxylate. Among them, however, lithium salts
exhibit insignificant oxidation degradation inhibitory effects. The
metal carbonates may be used alone or in combinations of two or
more. In the case where an alkali metal carbonate, an alkali metal
bicarboante or an alkali metal carboxylate is used in an aqueous
solution form, the solution thereof is alkaline. The acid
dissociation constants pKa of these materials fall in the range of
about 9 to 11.
[0033] The carboxylic acid used for the metal carboxylate may vary,
and examples thereof include aliphatic saturated monocarboxylic
acid, aliphatic unsaturated carboxylic acid, aliphatic dicarboxylic
acid, and aromatic carboxylic acid. Examples of the aliphatic
saturated monocarboxylic acid include linear saturated acids such
as formic acid, acetic acid, propionic acid, butyric acid, valeric
acid, caproic acid, caprylic acid, capric acid, laurylic acid,
myristic acid, palmitic acid, stearic acid, arachic acid, cerotic
acid and laccelic acid, and branched fatty acids such as
isopropionic acid, isobutanoic acid, isopentanoic acid, 2-methyl
pentanoic acid, 2-methyl butanoic acid, 2,2-dimethyl butanoic acid,
2-methyl hexanoic acid, 5-methyl hexanoic acid, 2,2-dimethyl
heptanoic acid, 2-ethyl-2-methyl butanoic acid, 2-ethyl hexanoic
acid, dimethyl hexanoic acid, 2-n-propyl pentanoic acid,
3,5,5-trimethyl hexanoic acid, dimethyl octanoic acid,
isotridecanoic acid, isomyristic acid, isostearic acid, isoarachic
acid and isohexanoic acid. Examples of an unsaturated carboxylic
acid include palmitoleic acid, oleic acid, elaidic acid, linoleic
acid and linolenic acid, and unsaturated hydroxylic acids such as
ricinoleic acid. Examples of the aliphatic dicarboxylic acid
include adipic acid, azelaic acid and sebacic acid, and examples of
the aromatic carboxylic acid include benzoic acid, phthalic acid,
trimelitic acid and pyromellitic acid. Also, an alicyclic fatty
acid such as naphthenic acid may be used. The carboxylic acids may
be used in combinations of two or more.
[0034] The metal element associated per carboxylic acid may be not
only one type but also may be of two or more types. Also, metal
carbonates and metal carboxylates each may be used alone or in
combinations of two or more.
[0035] In addition to the ionic compound, a conventional inhibitor
such as a phenol-based oxidation inhibitor or an amine-based
oxidation inhibitor may be used together, thereby more effectively
preventing oxidation degradation.
[0036] Examples of the phenol-based oxidation inhibitor include
4,4'-methylenebis(2,6-di-tert-butylphenol),
4,4'-bis(2,6-di-tert-butylphenol),
4,4'-bis(2-methyl-6-tert-butylphenol),
2,2'-methylenebis(4-ethyl-6-tert-butylphenol),
2,2'-methylenebis(4-methyl-6-tert-butylphenol),
4,4'-butylidenebis(3-methyl-6-tert-butylphenol),
4,4'-isopropylidenebis(2,6-di-tert-butylphenol),
2,2'-methylenebis(4-methyl-6-nonylphenol), 2,2'-isobutylidenebis
(4,6-dimethylphenol),
2,2'-methylenebis(4-methyl-6-cyclohexylphenol),
2,6-di-tert-butyl-4-methylphenol, 2,6-di-tert-butyl-4-ethylphenol,
2,4-dimethyl-6-tert-butylphenol,
2,6-di-tert-.alpha.-dimethylamino-p-cresol,
2,6-di-tert-butyl-4(N,N'-dimethylaminomethylphenol),
4,4'-thiobis(2-methyl-6-tert-butylphenol),
4,4'-thiobis(3-methyl-6-tert-butylphenol),
2,2'-thiobis(4-methyl-6-tert-butylphenol),
bis(3-methyl-4-hydroxy-5-tert-butylbenzyl)sulfide,
bis(3,5-di-tert-butyl-4-hydroxybenzyl)sulfide,
2,2'-thio-diethylenebis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate],
tridecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate,
pentaerythrityl-tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]-
, octadecyl-3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate, and
3-methyl-5-tert-butyl-4-hydroxyphenyl-substituted fatty acid
esters. Mixtures of two or more thereof may be used as the
degradation inhibitor.
[0037] Examples of the amine-based oxidation inhibitor include
monoalkyl diphenylamines such as monooctyl diphenylamine and
monononyl diphenylamine, dialkyl diphenylamines such as
4,4'-dibutyl diphenylamine, 4,4'-dipentyl diphenylamine,
4,4'-dihexyl diphenylamine, 4,4'-diheptyl diphenylamine,
4,4'-dioctyl diphenylamine and 4,4'-dinonyl diphenylamine,
polyalkyl diphenylamines such as tetrabutyl diphenylamine,
tetrahexyl diphenylamine, tetraoctyl diphenylamine and tetranonyl
diphenylamine, and naphthylamines such as .alpha.-naphthylamine,
phenyl-.alpha.-naphthylamine, butylphenyl-.alpha.-naphthylamine,
pentylphenyl-.alpha.-naphthylamine,
hexylphenyl-.alpha.-naphthylamine,
heptylphenyl-.alpha.-naphthylamine,
octylphenyl-.alpha.-naphthylamine and
nonylphenyl-.alpha.-naphthylamine. Mixtures of two or more of the
amino acid-based oxidation inhibitors may be used as the
degradation inhibitor.
[0038] A combination of the phenol-based oxidation inhibitor and
the amine-based oxidation inhibitor may also be used.
[0039] In the case where the phenol-based oxidation inhibitor or
the amine-based oxidation inhibitor is contained in the lubricating
oil for use in the dynamic pressure bearing device of the present
invention, the amount thereof should be set to 5.0 wt % or less,
preferably 3.0 wt % or less, and more preferably 1.0 wt % or less,
based on the total amount of the lubricant. When the amount thereof
exceeds 5.0 wt %, an adequate oxidation inhibitory effect in
comparison to the amount added is not obtained. In order to attain
a desired oxidation degradation inhibitory effect, the oxidation
inhibitor should be used in an amount of at least 0.1 wt % based on
the total amount of the lubricant.
[0040] If necessary, various additives which are conventionally
known, such as a viscosity enhancer, a pour point depressant, a
metal inactivator, a surfactant, a rust-proof agent, and an
anticorrosive agent may be added while exhibiting the effects of
the present invention.
(1-3) Description of Lubricant
[0041] Below, the compositions of 12 types of lubricants as
examples of the present invention and 7 types of lubricants of the
comparative examples will be described. The base oil used for the
following examples is diester.
(1-3-1) Compositions of Exemplary Lubricants of the Present
Invention
Example 1
[0042] 1 wt % of sodium carbonate was added to 100 parts by weight
of the base oil.
Example 2
[0043] 1 wt % of sodium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 3
[0044] 1 wt % of sodium bicarbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 4
[0045] 1 wt % of lithium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 5
[0046] 1 wt % of potassium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 6
[0047] 1 wt % of rubidium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 7
[0048] 1 wt % of cesium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 8
[0049] 1 wt % of sodium formate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 9
[0050] 1 wt % of sodium acetate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 10
[0051] 1 wt % of ethylenediamine tetraacetate-tetrasodium
(EDTA-4Na) and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 11
[0052] 0.5 wt % of sodium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Example 12
[0053] 0.25 wt % of sodium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
[0054] The lubricants of Examples 1 to 12 were formulated by adding
the additives to the base oil and then performing a stirring
process. Because of the stirring process, the additives except for
the salt were dissolved in the base oil, but most of the added salt
did not dissolve but was precipitated.
(1-3-2) Comparative Examples
Comparative Example 1
[0055] The base oil was used alone.
Comparative Example 2
[0056] 0.2 wt % of an oxidation inhibitor composed of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine was
added to 100 parts by weight of the base oil.
Comparative Example 3
[0057] 1 wt % of calcium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Comparative Example 4
[0058] 1 wt % of barium carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Comparative Example 5
[0059] 1 wt % of diethyl carbonate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Comparative Example 6
[0060] 1 wt % of sodium sulfate and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
Comparative Example 7
[0061] 0.3 wt % of sodium hydroxide and 0.2 wt % of a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine were
added to 100 parts by weight of the base oil.
[0062] The lubricants of Comparative Examples 1 to 7 were
formulated by adding the additives to the base oil and then
performing a stirring process. Because of the stirring process, the
additives except for the salt and sodium hydroxide were dissolved
in the base oil.
(1-4) High-Pressure Oxygen Test
[0063] For the lubricants of Examples 1 to 10 and Comparative
Examples 1 to 7, the following high-pressure oxygen test was
conducted, and the oxidation degradation of the lubricating oil was
evaluated.
(1-4-1) Test Conditions
[0064] Each of the lubricants was sealed with oxygen (0.9 MPa) and
allowed to stand in a thermostatic bath at 150.degree. C. for 60
hours (but 8 hours in Example 1 and Comparative Example 1), after
which the degradation rate of the lubricating oil was measured.
[0065] The degradation rate was determined using liquid
chromatography. The peak of the base oil and the peak of the
polymer or monomer subjected to oxidation degradation were
detected. The proportion (%) of the peak area of each of the
polymer and monomer subjected to oxidation degradation per the
total peak area was calculated, and thus determined to be a polymer
degradation rate and a monomer degradation rate. The results are
shown in Table 1 below.
(1-4-2) Test Results
TABLE-US-00001 [0066] TABLE 1 Addition of Polymer Monomer (Polymer
+ Added Oxidation Oxidation Oxidation Monomer) Oxidation Sample
Inhibitor Degradation (%) Degradation (%) Degradation (%) Ex. 1
Na.sub.2CO.sub.3 No 3.69 13.32 17.25 Ex. 2 Na.sub.2CO.sub.3 Yes
1.41 5.56 6.97 Ex. 3 NaHCO.sub.3 Yes 3.30 7.67 10.97 Ex. 4
Li.sub.2CO.sub.3 Yes 11 12.11 23.46 Ex. 5 K.sub.2CO.sub.3 Yes 0.00
2.97 2.97 Ex. 6 Rb.sub.2CO.sub.3 Yes 0.75 5.93 6.68 Ex. 7
Cs.sub.2CO.sub.3 Yes 1.82 8.55 10.37 Ex. 8 HCOONa Yes 4.28 7.99
12.27 Ex. 9 CH.sub.3COONa Yes 8.05 9.75 17.80 Ex. 10 EDTA-4Na Yes
6.90 7.71 14.61 C. Ex. 1 None No 13.62 15.07 28.69 C. Ex. 2 None
Yes 13.73 10.14 23.87 C. Ex. 3 Ca.sub.2CO.sub.3 Yes 10.28 14.19
24.47 C. Ex. 4 Ba.sub.2CO.sub.3 Yes 14.19 13.61 27.80 C. Ex. 5
Et.sub.2CO.sub.3 Yes 13.52 9.29 22.81 C. Ex. 6 Na.sub.2SO.sub.4 Yes
23.02 15.60 38.62 C. Ex. 7 NaOH Yes 12.06 10.17 22.23 * C. Ex.
stands for Comparative Example.
[0067] As is apparent from the results of Example 1 and Comparative
Example 1 in Table 1, the oxidation degradation rate of polymer in
Example 1 is considerably low. That is, sodium carbonate can be
seen to remarkably inhibit oxidation degradation of polymer.
[0068] As is apparent from the results of Example 2 and Comparative
Example 2, the oxidation degradation rate polymer in Example 2 is
further improved compared to that of Example 1, and also the
oxidation degradation rate of monomer is drastically improved. That
is, sodium carbonate can be seen to further inhibit both the
polymer oxidation degradation and the monomer oxidation degradation
in the presence of the conventional oxidation degradation
inhibitor.
[0069] Table 2 below shows the results of Examples 4, 2, 5, 6 and 7
among the examples of Table 1, in order to exhibit the effects of
carbonates of alkali metals from lithium to cesium in the periodic
table. These results are graphed in FIG. 3.
TABLE-US-00002 TABLE 2 Polymer Monomer {Polymer + Oxidation
Oxidation Monomer} Alkali Degradation Degradation Oxidation Metal
(%) (%) Degradation (%) Ex. 4 Li 11.35 12.11 23.46 Ex. 2 Na 1.41
5.56 6.97 Ex. 5 K 0.00 2.97 2.97 Ex. 6 Rb 0.75 5.93 6.68 Ex. 7 Cs
1.82 8.55 10.37
[0070] As is apparent from the results of Table 2 and FIG. 3,
carbonates of alkali metals from sodium to cesium play a role in
drastically improving both the polymer degradation rate and the
monomer degradation rate. The greatest improvement effect is
exhibited in the case of potassium. Sodium and rubidium exhibit
improvement effects slightly inferior to those of potassium, and
effects based on cesium become a lot more inferior. However, it is
obvious that a carbonate of any metal selected from among these
metals can inhibit the degradation.
[0071] Among alkali metal carbonates, lithium carbonate has an
insignificant oxidation degradation inhibitory effect. Among alkali
metals, lithium is a slightly special element, and a carbonate
thereof has properties different from those of the other alkali
metal carbonates, which may result in a different improvement
effect.
[0072] Table 3 below shows the results of a high-temperature oxygen
test of Examples 2, 11 and 12 and Comparative Example 2. These
results are graphed in FIG. 4.
TABLE-US-00003 TABLE 3 Polymer Monomer {Polymer + Sodium Oxidation
Oxidation Monomer} Carbonate Degradation Degradation Oxidation (wt
%) (%) (%) Degradation (%) Ex. 2 1.00 1.41 5.56 6.97 Ex. 11 0.50
2.66 6.62 9.28 Ex. 12 0.25 8.41 9.31 17.72 C. Ex. 2 0 13.73 10.14
23.87
[0073] As is apparent from Table 3 and FIG. 4, as the amount of
sodium carbonate added increases, the polymer oxidation degradation
rate and the monomer oxidation degradation rate are improved. Even
when 0.25 wt % of sodium carbonate is added, the inhibitory effect
is exhibited but is not largely different from that of the
comparative example. As depicted in FIG. 4, when the sum of the
polymer oxidation degradation rate and the monomer oxidation
degradation rate is determined to be 20% or less as the minimum
level for improvement, sodium carbonate should be added in an
amount of at least 0.1 wt %.
(2) Lubricating Oil Added with Aqueous Solution
(2-1) Composition of Additives
[0074] 50 g of base oil added with the mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine was
added with 1.2 ml of an aqueous metal carbonate solution, thus
preparing a lubricant (Example 13). In addition, 50 g of base oil
added with the mixture of 2,6-di-tert-butyl-4-ethylphenol and
4,4'-dibutyl diphenylamine was added with 1.2 ml of an aqueous
EDTA-4Na solution, thus preparing a lubricant (Example 14).
Thereafter, an oxidation stability test was conducted.
(2-2) Oxidation Stability Test (RBOT Test)
[0075] The oxidation lifespan of the lubricating oil was measured
using a RBOT method according to a JIS standard test (JIS K2514),
and the RBOT value was calculated. That is, in a sealable vessel,
water, a copper coil and the lubricating oil were introduced into
the above aqueous solution and then pressurized to 620 kPa by
oxygen, after which the sealed vessel was placed in a thermostatic
bath at 150.degree. C. and was then continuously rotated at 100 rpm
while being kept inclined at an angle of 30.degree.. When the inner
pressure reached the maximum level, the period of time required to
drop the pressure to 175 kPa was measured. The lubricating oil of
Comparative Example 2 was subjected to the same test without the
use of the above aqueous solution. In this case, the lubricating
oil used for the test was in a state of being in contact with water
in lieu of the aqueous solution. The results are shown in Table 4
below.
TABLE-US-00004 TABLE 4 Ex. 13 Ex. 14 C. Ex. 2 RBOT value [min]
10394 15202 4828
[0076] As is apparent from Table 4, in Example 13 using the aqueous
metal carbonate solution, the RBOT value is at least two times that
of Comparative Example 2 without the use of the aqueous solution.
Also, in Example 14 using the aqueous EDTA-4Na solution, the RBOT
value is at least three times that of Comparative Example 2. In
both cases, oxidation stability is remarkably improved. The reason
why the oxidation stability is increased in the RBOT test under a
condition in which water is added to the lubricating fluid is
because hydrolysis of the base oil is inhibited.
(3) Dynamic Pressure Bearing Device, Spindle Motor & Disk Drive
Apparatus
(3-1) Disk Drive Apparatus
[0077] FIG. 1 shows the internal construction of a disk drive
apparatus 60 (in the present embodiment, a hard disk drive
apparatus). A housing 61 of the disk drive apparatus 60 has a clean
space in which an amount of dust or impurities is very small. The
housing 61 include therein a spindle motor 1 which drives a disk
and is provided with a disk-shaped storage medium 62 for storing
information, and an access unit 63 for writing/reading information
to/from the storage medium 62.
(3-2) Spindle Motor
[0078] FIG. 2 is a longitudinal cross-sectional view showing the
construction of the spindle motor 1. The spindle motor 1 includes a
stationary member and a rotational member. By the dynamic pressure
bearing device in accordance with the embodiment of the present
invention, the rotational member is rotatably supported with
respect to the stationary member via a rotary shaft 32.
(3-2-1) Stationary Member of Spindle Motor
[0079] A base 10 has a flat portion 11 provided at the center
thereof and an annular boss portion 13 provided on the central
region of the flat portion 11. An annular recess is defined between
the annular boss portion 13 and an annular stepped portion 14
provided on the outer periphery of the flat portion 11. A stator 17
fixed to the flat portion 11 and a rotor magnet 34 attached to a
hub 31 which will be described later are disposed in the above
recess. The annular boss portion 13 is positioned near the outer
periphery of a cylindrical support wall 15 protruding upwards, and
the stator 17 is fixed to the above outer periphery. The stator 17
includes an annular stator core 17a formed by laminating a
plurality of electromagnetic steel plates, and multi-phase (e.g.,
three-phase) coils 17b wound on respective teeth of the stator core
17a. The stator core 17a of the stator 17 is fitted onto the
cylindrical support wall 15 and is fixed through press fitting,
adhesion or the like. Thus, the stator 17 is fixed to the
cylindrical support wall 15. The fixing process includes press
fitting, adhesion or the like.
[0080] A bearing stationary portion 20 made of stainless steel,
constituting a part of the dynamic pressure bearing device, is
fitted into the annular boss portion 13 and is fixed thereto. The
bearing stationary portion 20 includes a substantially cylindrical
sleeve 21, and a counter plate 22 closing the lower-end opening of
the sleeve 21. The inner peripheral surface of the through hole of
the sleeve 21 is divided into a small-diameter inner peripheral
surface 21a extending over substantially the entire length of the
sleeve 21 where a radial bearing portion is located, an
intermediate-diameter inner peripheral surface 21b located at the
lower portion of the sleeve 21 and formed to have a greater
diameter than that of the small-diameter inner peripheral surface
21a, and a large-diameter inner peripheral surface 21c located at
the lowest end of the sleeve 21 and formed to have a greater
diameter than that of the intermediate-diameter inner peripheral
surface 21b. The counter plate 22 is disposed in the space inside
the large-diameter inner peripheral surface 21c and is fixed to the
sleeve 21 through press fitting, caulking, welding, adhesion or the
like. The lower half portion of the outer peripheral surface of the
sleeve 21 is fixed to the inner peripheral surface of the annular
boss portion 13 through press fitting, adhesion, welding or the
like. The upper outer peripheral surface of the sleeve 21 is formed
with a tapered surface 23 which forms an inner peripheral surface
of a tapered sealing portion which will be mentioned later. As the
tapered surface 23 extends upward in the drawing, it becomes more
distant from the central axis of the bearing.
(3-2-2) Rotational Member & Dynamic Pressure Bearing Device
[0081] A rotor 30 includes an inverted cup-shaped hub 31 and the
rotary shaft 32 disposed in the rotational center of the hub 31.
Because the rotary shaft 32 is supported by the bearing stationary
portion 20, the rotor 30 is rotatable with respect to the flat
portion 11.
[0082] The hub 31 is made of a ferromagnetic material such as iron
or stainless steel. Connected to the outer periphery of a
disk-shaped portion 31a constituting a top plate is a cylindrical
portion 31b extending downward in the drawing. Provided at the
lower end of the cylindrical portion 31b is a flange 31c protruding
radially outwardly. Inside the cylindrical portion 31b, an annular
wall 31d extending downward from the disk-shaped portion 31a is
disposed. The annular wall 31d is disposed between the sleeve 21
and the cylindrical support wall 15 to surround the upper outer
periphery of the sleeve 21. Also, there is formed a labyrinth gap
for defining a labyrinth seal between the annular wall 31d and the
cylindrical support wall 15.
[0083] An attachment hole 31e is formed in the center of the
disk-shaped portion 31a, and the upper end of the rotary shaft 32
having a slightly smaller diameter is press fitted into the hole.
Accordingly, the hub 31 and the rotary shaft 32 are integrated with
each other. The rotary shaft 32 is hollow, and a female threaded
portion 32b is formed over substantially the entire length of the
inner peripheral surface thereof. The outer peripheral surface 32a
of the rotary shaft 32 and the small-diameter inner peripheral
surface 21a of the sleeve 21 are radially arranged with a slight
gap therebetween.
[0084] The leading end of the rotary shaft 32 passed through the
sleeve 21 slightly protrudes downward from the small-diameter inner
peripheral surface 21a. A removal prevention member 33 has a male
threaded portion 33a threadedly engaged with the female threaded
portion 32b of the rotary shaft 32 and a circular plate 33b. The
circular plate 33b has an outer diameter larger than the outer
diameter of the rotary shaft 32 and smaller than the inner diameter
of the intermediate-diameter inner peripheral surface 21b. A gap is
defined between the circular plate 33b and the sleeve, and the
rotary shaft 32 including the removal prevention member 33 is
rotatable with respect to the sleeve 21. In the case where force is
applied in the direction in which the rotary shaft 32 is removed
from the sleeve, the circular plate 32b comes into contact with the
sleeve 21, thereby preventing the removal of the rotary shaft
32.
[0085] Provided inside the cylindrical portion 31b of the hub 31 is
an annular rotor magnet 34 including a plurality of magnetic poles
arranged in a circumferential direction. The rotor magnet 34 is
disposed to surround the outer periphery of the stator 17. The hub
31 made of a ferromagnetic material also functions as a back yoke
of the magnet 34.
[0086] Mounted on the flange 31c of the hub 31 is a single or a
plurality of storage disks (hard disks) (not shown). The hard disk
has a hole at the center thereof, and the periphery of the hole is
in contact with the outer peripheral surface of the cylindrical
wall 31b. A clamp member is attached to the hub. The clamp member
is brought into contact with the upper surface near the hole of the
disk to hold the disk together with the flange 31c
therebetween.
[0087] The clamp member is fixed to the rotary shaft by means of a
screw threadedly engaged with the female threaded portion 32b of
the rotary shaft 32 from above.
[0088] A fine gap is formed between the small-diameter inner
peripheral surface 21a of the sleeve 21 and the outer peripheral
surface 32a of the rotary shaft 32 and between the lower surface of
the disk-shaped portion 31a of the hub 31 and the upper end surface
of the sleeve 21, and is filled with the lubricating oil 40. The
lubricating oil 40 contains a mixture of
2,6-di-tert-butyl-4-ethylphenol and 4,4'-dibutyl diphenylamine.
[0089] The lubricating oil 40 is also filled in the space defined
by the intermediate-diameter inner peripheral surface 21b of the
sleeve 21, the surface of the counter plate 22, and the surface of
the circular plate 33b of the removal prevention member 33. The
lubricating oil 40 is in contact with the air in the tapered
sealing portion 41 defined by the inner peripheral surface 31f of
the annular wall 31d of the hub 31 and the tapered surface 23 of
the upper outer periphery of the sleeve 21, and the level surface
thereof has an arcuate section shape. The tapered sealing portion
41 has a tapered shape in which the gap is gradually reduced as it
goes upward.
[0090] In the small-diameter inner peripheral surface 21 of the
sleeve 21, herringbone-shaped dynamic pressure generating grooves
are respectively formed at two positions separated from each other
in the axial direction, corresponding to reference numerals 42 and
43 in the drawing. The dynamic pressure generating grooves create a
bearing force for holding the rotary shaft 32 in a radius direction
when the spindle motor is rotated in a specific direction. That is,
a pair of radial dynamic pressure bearings is disposed in the
positions 42 and 43. Also, a spiral-shaped dynamic pressure
generating groove is formed in the upper end surface of the sleeve
21 to constitute a thrust dynamic pressure bearing 44. The
spiral-shaped groove functions to increase the pressure of the
lubricating oil inward compared to the region where the dynamic
pressure generating groove is formed when the spindle motor is
rotated in the specific direction, and also to create a force for
lifting the hub 31 upward in the axial direction.
[0091] The sleeve 21 has a communication hole 45, which extends in
the axial direction thereof and is filled with the lubricating oil
40. The lower end of the communication hole 45 is opened toward the
intermediate-diameter inner peripheral surface 21b and the upper
end thereof is opened at an inside area of the thrust dynamic
pressure bearing 44 in the thrust gap. The communication hole 45 is
formed such that both ends of two radial dynamic pressure bearings
42, 43 communicate with each other, and enables the circulation of
the lubricating oil 40 in the bearing device.
[0092] A recessed portion 70 is provided in the outer periphery of
the intermediate-diameter inner peripheral surface 21b. Potassium
carbonate is applied inside the recessed portion 70 and is always
in contact with the lubricating oil 40. Instead of potassium
carbonate, an aqueous solution of potassium carbonate may be
used.
[0093] The sleeve 21 may be made of a porous sintering metal
instead of stainless steel. In this case, pores of a portion of the
sleeve may be filled with potassium carbonate or an aqueous
solution thereof and then sealed, whereas pores of the other
portion of the sleeve are filled with the lubricating oil. In this
way, the lubricating oil and the potassium carbonate may be in
contact with each other in the sintered body.
[0094] Alternatively, there may be provided a construction in which
potassium carbonate is disposed in a lower portion 71 of the wall
surface of the tapered sealing portion 41, so that the lubricating
oil 40 comes in contact with potassium carbonate only when the
lubricating oil 40 expands with an increased temperature and the
interface thereof is moved downward. In this case, only at high
temperatures at which degradation rapidly progresses, does the
lubricating oil 40 come into contact with the potassium carbonate
serving as the degradation inhibitor. While the contact between the
potassium carbonate and the lubricating oil is held at a minimum,
the degradation of the lubricating oil may be effectively
prevented.
[0095] As described hereinbefore, the lubricant, the method of
preventing the degradation of the lubricating oil, and the dynamic
pressure bearing device in accordance with the present invention
are illustrated, but the present invention is not limited thereto
and various modifications can be made without departing from the
scope of the present invention.
[0096] For example, in the embodiment of the present invention, the
dynamic pressure bearing device includes two radial dynamic
pressure bearings and one thrust dynamic pressure bearing, but the
structure thereof is not limited thereto. Also, the positions of
the dynamic pressure generating grooves are not limited to those in
the above embodiment.
[0097] Also, examples of the ionic compound which is brought into
contact with the lubricating oil are not limited to those
generating oxidation degradation inhibitory effects. For example, a
material such as silica gel having hygroscopicity may be disposed
in the recessed portion 70 of FIG. 2.
* * * * *