U.S. patent application number 12/877341 was filed with the patent office on 2011-02-24 for sleeve for hydrodynamic bearing device, hydrodynamic bearing device and spindle motor using the same, and method for manufacturing sleeve.
This patent application is currently assigned to NIDEC CORPORATION. Invention is credited to Takafumi ASADA, Tsutomu HAMADA, Katsuo ISHIKAWA.
Application Number | 20110044837 12/877341 |
Document ID | / |
Family ID | 37804184 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110044837 |
Kind Code |
A1 |
HAMADA; Tsutomu ; et
al. |
February 24, 2011 |
SLEEVE FOR HYDRODYNAMIC BEARING DEVICE, HYDRODYNAMIC BEARING DEVICE
AND SPINDLE MOTOR USING THE SAME, AND METHOD FOR MANUFACTURING
SLEEVE
Abstract
A bearing stiffness of a sintered metal sleeve is prevented from
lowering. A sleeve includes an inner section formed of metal powder
for sintering and a resin for impregnation, and a surface
deformation section which covers a surface of the inner section and
is formed by shot blast process. Since the surface deformation
section is formed by the shot blast process, the number of pores
formed between the metal powder for sintering near the surface can
be reduced. In this way, a supporting pressure at a bearing portion
can be prevented from being released out through the pores, and the
bearing stiffness can be prevented from lowering.
Inventors: |
HAMADA; Tsutomu; (Osaka,
JP) ; ASADA; Takafumi; (Osaka, JP) ; ISHIKAWA;
Katsuo; (Ehime, JP) |
Correspondence
Address: |
NIDEC CORPORATION;c/o KEATING & BENNETT, LLP
1800 Alexander Bell Drive, SUITE 200
Reston
VA
20191
US
|
Assignee: |
NIDEC CORPORATION
Minami-ku
JP
|
Family ID: |
37804184 |
Appl. No.: |
12/877341 |
Filed: |
September 8, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11509770 |
Aug 25, 2006 |
|
|
|
12877341 |
|
|
|
|
Current U.S.
Class: |
419/27 ;
419/26 |
Current CPC
Class: |
F16C 2220/60 20130101;
F16C 2223/08 20130101; F16C 33/14 20130101; F16C 2223/04 20130101;
F16C 2220/20 20130101; F16C 17/107 20130101; F16C 2370/12
20130101 |
Class at
Publication: |
419/27 ;
419/26 |
International
Class: |
B22F 3/26 20060101
B22F003/26; B22F 3/12 20060101 B22F003/12; B22F 3/24 20060101
B22F003/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 26, 2005 |
JP |
2005-245936 |
Aug 31, 2005 |
JP |
2005-251177 |
Claims
1. A manufacturing method of a sleeve for a hydrodynamic bearing
device, the method comprising the steps of: forming a primary
compact form metal powder for sintering; sintering the primary
compact; sizing the sintered primary compact to form a secondary
compact; and contacting the secondary compact with a
high-temperature steam after the sizing step.
2. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, the method further comprising the step
of: finishing a surface of the secondary compact.
3. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, the method further comprising the step
of: removing at least a part of an iron oxide film formed on a
surface of the secondary compact.
4. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 2, wherein the primary compact or the
secondary compact is treated with nonelectrolytic nickel plating
process or DLC film coating process in the surface finishing.
5. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, wherein an average density of a
portion of the metal powder for sintering of the secondary compact
is 6.8 g/cm.sup.3 or higher.
6. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, wherein: the primary compact includes
a tubular sleeve main body and a tubular projection projecting from
the sleeve main body in an axial direction; and a rate of change in
a dimension of the tubular projection is larger than a rate of
change in a dimension of the sleeve main body in the sizing
process.
7. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, comprising: a sleeve; a shaft inserted
into a bearing hole of the sleeve so as to be relatively rotatable;
and at least one radial bearing having hydrodynamic grooves formed
on at least one of an outer peripheral surface of the shaft and an
inner peripheral surface of the sleeve, wherein a volume density of
a portion of the metal powder for sintering of the secondary
compact is 85% or higher.
8. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, wherein: the sleeve is brought into
contact with a high-temperature steam at an atmospheric temperature
within the range of 600.degree. C. to 700.degree. C. for 15 to 50
minutes in the steam process.
9. A manufacturing method of a sleeve for a hydrodynamic bearing
device according to claim 1, wherein: the sleeve is brought into
contact with a high-temperature steam at an atmospheric temperature
within the range of 400.degree. C. to 700.degree. C. for 25 to 80
minutes in the steam process.
10. A manufacturing method of a sleeve (for a hydrodynamic bearing
device) according to claim 1, wherein the metal powder for
sintering includes pure iron or an iron based powder having a
volume density of at least 80% iron.
11. A manufacturing method of a sleeve for hydrodynamic bearing
device according to claim 10, wherein the contacting step includes
arranging a layer including an oxide on a surface of the secondary
compact.
12. A manufacturing method of a sleeve for hydrodynamic bearing
device according to claim 1, wherein the secondary compact includes
thereon a Fe.sub.3O.sub.4 film including a thickness of
approximately 2 .mu.m to approximately 10 .mu.m after the
contacting step.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to Japanese Patent
Application Nos. JP2005-245936 and JP2005-251177. The entire
disclosures of Japanese Patent Application Nos. JP2005-245936 and
JP2005-251177 are hereby incorporated herein by reference.
BACKGROUND OF THE INVENTION
Field of the Invention
[0002] The present invention relates to a sleeve for a hydrodynamic
bearing device, particularly, a sleeve formed of sintered metal, a
hydrodynamic bearing device and a spindle motor using the same, and
a method for manufacturing the sleeve.
[0003] In recent years, recording and reproducing apparatuses and
the like using discs to be rotated experience an increase in a
memory capacity and an increase in a transfer rate for data. Thus,
bearings used for such recording and reproducing apparatuses are
required to have high performance and high reliability to
constantly rotate a disc load with a high accuracy. Accordingly,
hydrodynamic bearing devices suitable for high-speed rotation are
used for such rotary devices. The hydrodynamic bearing devices are
suitable for high-speed rotation since each of the hydrodynamic
bearing devices has oil which serves as a lubricant interposed
between a shaft and a sleeve, and generates a pumping pressure by
hydrodynamic grooves during rotation. Thus, the shaft rotates in a
non-contact state with respect to the sleeve, and no mechanical
friction is generated.
[0004] Hereinafter, an example of conventional hydrodynamic bearing
devices will be described with reference to FIGS. 32 through 34.
FIG. 32 is a cross-sectional view schematically showing a structure
of a conventional hydrodynamic bearing device. As shown in FIG. 32,
the hydrodynamic bearing device includes a shaft 911, a flange 912,
a sleeve 913, a thrust plate 914, oil 915, a rotor 916, and a base
917. The shaft 911 is formed integrally with a flange 912. The
shaft 911 is inserted into a bearing hole 913C of the sleeve 913 so
as to be rotatable. The flange 912 is accommodated within a
recessed portion of the sleeve 913. On at least one of an outer
peripheral surface of the shaft 911 and an inner peripheral surface
of the sleeve 913, hydrodynamic grooves 913A and 913B are formed.
On a surface of the flange 912 which opposes the sleeve 913 and on
a surface of the flange 912 which opposes the thrust plate 914,
hydrodynamic grooves 912A and 912B are formed. The thrust plate 914
is fixed to the sleeve 913. Bearing gaps near the hydrodynamic
grooves 913A, 913B, 912A, and 912B are filled with at least the oil
915. To the rotor 916, a disc 918 is fixed.
[0005] The sleeve 913 is fixed to the base 917. To the rotor, a
rotor magnet (not shown) is fixed. Furthermore, a motor stator (not
shown) is fixed to the base 917 at a position opposing the rotor
magnet.
[0006] An operation of the conventional fluid bearing type rotary
device having the above-described structure will be described. When
a rotational force is applied to the rotor magnet (not shown), the
rotor 916, the shaft 911, the flange 912, and the disc 918 start to
rotate. Due to the rotation, the hydrodynamic grooves 913A, 913B,
912A, and 912B gather the oil 915, and generate pumping pressures
between the shaft 911 and the sleeve 913, between the flange 912
and the sleeve 913, and between the flange 912 and the thrust plate
914. In this way, the shaft 914 can rotate in a non-contact state
with respect to the sleeve 913 and the thrust plate 914 and data on
the disc 918 can be recorded/reproduced by a magnetic head or an
optical head (not shown).
[0007] In general, a sleeve of a hydrodynamic bearing device is
made from metal materials by a cutting process and the like.
However, in order to further reduce the manufacturing cost, a
sleeve made of sintered metal has been proposed (see, for example,
Japanese Laid-Open Publication No. 2003-314536). Sintered metal
means a sintered body obtained by molding and sintering metal
powder of copper alloy or the like, for example. When a sleeve is
made from a metal rod by a cutting process, a large amount of swarf
is generated and the material is wasted. If a sleeve is made by
sintering, metal powder is molded and sintered. Thus, there is no
swarf and the materials are not wasted. Furthermore, for producing
hydrodynamic grooves on an inner peripheral surface of a sleeve, a
cutting process or an electrolytic machining is necessary in the
conventional art. On the other hand, if a sleeve is manufactured by
sintering, hydrodynamic grooves can be formed at the same time as
the sleeve is being formed by previously machining portions of a
mold which correspond to the hydrodynamic grooves.
[0008] As described above, the number of steps and a time period
required for manufacturing a sintered metal sleeve can be reduced a
few times from that for making the same sleeve by a cutting process
or the like. Manufacturing sleeves by sintering can significantly
reduce the manufacturing cost of the sleeves.
[0009] However, although the sintered metal sleeve can reduce the
manufacturing cost, it has problems in its properties.
Specifically, since sintered metal is an aggregate of metal powder,
it is porous and has a large number of pores (small spaces formed
between the metal powder) inside. The pores include pores inside
the sintered body, which are referred to as "structural pores", and
opened pores on a surface of the sintered body, which are referred
to as "surface pores". In normal sintered metal, surface pores and
structural pores communicate with each other. Thus, lubricating oil
can pass through the sintered body via the pores. When a sintered
metal sleeve is used for a hydrodynamic bearing device, lubricating
oil passes through the sleeve and a supporting pressure generated
at a radial bearing portion is released toward an outer periphery
of the sleeve. As a result, for example, the supporting pressure
generated at the radial bearing portion is reduced. A stiffness of
the radial bearing portion is decreased by about 30%.
[0010] In order to prevent the supporting pressure being released
toward the outer periphery of the sleeve, as described above, a
hydrodynamic bearing device having a member of a cup shape fitted
to the outer periphery of the sleeve has been proposed. However,
since the number of components forming the hydrodynamic bearing
device increases with such a structure, a benefit that the
manufacturing cost can be reduced by the sintered metal sleeve
becomes small. Therefore, in order to utilize the advantage of the
sintered metal sleeve of low cost, sintered metal sleeves which do
not reduce the bearing stiffness are desired.
[0011] In order to respond to such a demand, the present inventors
have proposed a technique of impregnating a sintered body bearing
with a resin to seal pores, and continue developing the
technique.
[0012] However, when a pressed-powder sintered body bearing is
impregnated with a resin, a resin impregnant tends to remain on a
surface of the bearing with a normal step. Thus, resin impregnation
tends to have an adverse influence on a precision of dimension.
Further, it is substantially impossible to completely fill the
pores on the surface and inside the pressed powder sintered body,
which is a porous material. Moreover, a remained resin attached on
a surface of the pressed powder sintered body, which is a porous
material, has to be removed from the surface. Thus, the resin
hardly remains on the surface. Under such circumstances, an effect
of impregnating a resin cannot be fully utilized.
[0013] As shown in FIG. 33, a porous sleeve 913 has holes (pores)
inside. Therefore, even oil 915 is first injected into an entire
bearing gap to a position indicated by the letter U in the figure,
oil 915A enters into holes inside the sleeve 913 after it is left
for about 500 hours. A level of a liquid surface of the oil 915 is
lowered to a position indicated by the letter V in the figure.
Thus, hydrodynamic grooves 913A rub a surface of the the shaft 911
and seizes.
[0014] As shown in FIG. 33, oil 915B oozes out on an external
surface of the porous sleeve 913. The oil 915B evaporates and the
oil in a gas form contaminates the surroundings.
[0015] Whether there is a problem of insufficient sealing of the
pores of the porous sleeve 913 can be checked as follows. First, a
sufficient amount of oil is put into a beaker (not shown). Then,
the sleeve 913 is dipped and left therein by itself, or with being
assembled with a shaft 911, a flange 912 and a thrust plate 914.
After about 500 hours, an increase in the total weight is measured
to obtain a weight of the oil soaked into the porous material. As
shown in FIG. 34, conventionally, a change of 2 mg or more (weight
change) has been recognized after dipping for 500 hours at
80.degree. C. The total amount of oil filled in the bearing
arrangement is about 10 milligrams. Thus, such a change largely
impairs reliability of the hydrodynamic bearing device.
[0016] Further, in general, a gap between the sleeve 913 and the
shaft 911 in a hydrodynamic bearing device is set to be about 5
.mu.m. Therefore, problems in accuracy in a surface treatment after
a pore sealing process, a difference in temperatures of use
circumstances in thermal expansion coefficient difference in use,
abrasion powder and the like are inevitable for the hydrodynamic
bearing device.
SUMMARY OF THE INVENTION
[0017] An object of the present invention is to prevent the bearing
stiffness of a sintered metal sleeve from decreasing.
[0018] A sleeve for a hydrodynamic bearing device according to the
first invention comprises: an inner section formed of metal powder
for sintering and a resin for impregnation; and a surface
deformation section which covers a surface of the inner section and
is formed of metal powder for sintering. An average density of a
portion of the metal powder for sintering of the surface
deformation section is larger than an average density of a portion
of the metal powder for sintering of the inner section.
[0019] In such a sleeve, since the average density of the portion
of the metal powder for sintering of the surface deformation
section is larger than the average density of the portion of the
metal powder for sintering of the inner section, the inner section
is covered with a layer with fewer pores. Thus, a supporting
pressure of a bearing portion can be prevented from being released
out through the pores, and the bearing stiffness can be prevented
from lowering. The average density as used herein is obtained by
dividing the weight by volume. For example, the average density of
the sleeve is obtained by dividing the weight of the sleeve by the
volume calculated from an external shape of the sleeve.
[0020] A sleeve for a hydrodynamic bearing device according to the
second invention comprises: an inner section formed of metal powder
for sintering and a resin for impregnation; and a surface
deformation section which covers a surface of the inner section and
is formed of metal powder for sintering. A density of the portion
of the metal powder for sintering of the surface deformation
section becomes gradually larger from a side of the inner section
toward a surface.
[0021] In such a sleeve, since the density of the portion of the
metal powder for sintering of the surface deformation section
becomes gradually larger from the side of the inner section toward
the surface, the density of the surface of the surface deformation
section is the largest. Thus, a supporting pressure of a bearing
portion can be prevented from being released out through the pores
more securely particularly on the surface of the surface
deformation section, and the bearing stiffness can be prevented
from lowering.
[0022] A sleeve for a hydrodynamic bearing device according to the
third invention comprises: an inner section formed of metal powder
for sintering and a resin for impregnation; and a surface
deformation section which covers a surface of the inner section and
is formed by a shot blast process.
[0023] In such a sleeve, since the surface deformation section is
formed by the shot blast process, the number of the pores formed
between the metal powders for sintering near the surface can be
reduced. Thus, a supporting pressure of a bearing portion can be
prevented from being released out through the pores, and the
bearing stiffness can be prevented from lowering.
[0024] A sleeve for a hydrodynamic bearing device according to the
fourth invention is a sleeve of the first invention wherein an
average density of the portion of the metal powder for sintering of
the inner section is 6.5 g/cm.sup.3 or higher.
[0025] In such a sleeve, since the average density of the portion
of the metal powder for sintering of the inner section is 6.5
g/cm.sup.3 or higher, the average of the surface deformation
section is higher than that and there are less pores on the sleeve
surface. Thus, a supporting pressure of a bearing portion can be
prevented from being released out through the pores more securely,
and the bearing stiffness can be prevented from lowering.
[0026] A sleeve for a hydrodynamic bearing device according to the
fifth invention comprises: an inner section including metal powder
for sintering; a steam process layer which covers a surface of the
inner section and includes iron oxide; and a plating process layer
which covers a surface of the steam process layer.
[0027] In such a sleeve, a surface of the inner section is covered
by oxide generated by steam process, and pores near the surface
have their inner walls sealed by the oxide. Thus, a supporting
pressure of a bearing portion can be prevented from being released
out through the pores.
[0028] A sleeve for a hydrodynamic bearing device according to the
sixth invention is the sleeve of the fifth invention in which a
thickness of the steam process layer is 2 .mu.m or greater.
[0029] A sleeve for a hydrodynamic bearing device according to the
seventh invention is a sleeve of the fifth invention wherein an
average density of the portion of the metal powder for sintering of
the inner section is 6.8 g/cm.sup.3 or higher.
[0030] In such a sleeve, since the average density of the portion
of the metal powder for sintering of the inner section is 6.8
g/cm.sup.3 or higher, a supporting pressure of a bearing portion
can be prevented from being released out through the pores, and the
bearing stiffness can be prevented from lowering.
[0031] A sleeve for a hydrodynamic bearing device according to the
eighth invention is a sleeve of the fifth invention wherein the
iron oxide includes Fe.sub.3O.sub.4.
[0032] In such a sleeve, since the steam plating layer includes
Fe.sub.3O.sub.4 which has electric conductivity, a plating process
can be securely performed, and the strength of the plating process
layer can be improved.
[0033] A sleeve for a hydrodynamic bearing device according to the
ninth invention is a sleeve of the fifth invention further
comprising a plating process layer which covers a surface of the
steam process layer.
[0034] In such a sleeve, the inner section is covered by the steam
process layer and the plating process layer, and pores near the
surface have their inner walls sealed by the oxide formed by the
steam process or plating. Thus, a supporting pressure of a bearing
portion can be prevented from being released out through the pores.
Such a sleeve is employed for using a sintered material of
iron.
[0035] A sleeve for a hydrodynamic bearing device according to the
tenth invention is a sleeve according to the ninth invention
wherein: a thickness of the steam process layer is 2 .mu.m or
larger; and a thickness of the plating process layer is 2 .mu.m or
larger.
[0036] In such a sleeve, since the thickness of the steam process
layer and the plating process layer are both 2 .mu.m or larger, a
supporting pressure of a bearing portion can be prevented from
being released out through the pores more securely.
[0037] A sleeve for a hydrodynamic bearing device according to the
eleventh invention comprises: metal powder for sintering; and a
steam process section with iron oxide being formed between
particles of the metal powder for sintering.
[0038] A sleeve for a hydrodynamic bearing device according to the
twelfth invention is a sleeve for a hydrodynamic bearing device
into which a shaft of a hydrodynamic bearing device is inserted,
comprising: metal powder for sintering; and a steam process section
with iron oxide being formed between particles of the metal powder
for sintering. The steam process layer is removed at least from an
area which generates a dynamic pressure.
[0039] A sleeve for a hydrodynamic bearing device according to the
thirteenth invention is a sleeve for a hydrodynamic bearing device
into which a shaft of a hydrodynamic bearing device is inserted,
comprising: metal powder for sintering; a steam process section
with iron oxide being formed between particles of the metal powder
for sintering; and a steam process layer including iron oxide which
is formed to cover a surface of the steam process section. The
steam process layer is removed at least from an area which
generates a dynamic pressure.
[0040] A hydrodynamic bearing device according to the fourteenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the fifth invention which
is fixed to one of the stationary member and the rotating member; a
shaft which is fixed to the other of the stationary member and the
rotating member and is provided on an inner peripheral side of the
sleeve so as to be relatively rotatable; and a radial bearing
portion including a working fluid filled between the sleeve and the
shaft, and at least one hydrodynamic groove formed on either an
inner peripheral surface of the sleeve or an outer peripheral
surface of the shaft.
[0041] A hydrodynamic bearing device according to the fifteenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the eleventh invention
which is fixed to one of the stationary member and the rotating
member; a shaft which is fixed to the other of the stationary
member and the rotating member and is provided on an inner
peripheral side of the sleeve so as to be relatively rotatable; and
a radial bearing portion including a working fluid filled between
the sleeve and the shaft, and at least one hydrodynamic groove
formed on either an inner peripheral surface of the sleeve or an
outer peripheral surface of the shaft.
[0042] A hydrodynamic bearing device according to the sixteenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the twelfth invention
which is fixed to one of the stationary member and the rotating
member; a shaft which is fixed to the other of the stationary
member and the rotating member and is provided on an inner
peripheral side of the sleeve so as to be relatively rotatable; and
a radial bearing portion including a working fluid filled between
the sleeve and the shaft, and at least one hydrodynamic groove
formed on either an inner peripheral surface of the sleeve or an
outer peripheral surface of the shaft.
[0043] A hydrodynamic bearing device according to the seventeenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the thirteenth invention
which is fixed to one of the stationary member and the rotating
member; a shaft which is fixed to the other of the stationary
member and the rotating member and is provided on an inner
peripheral side of the sleeve so as to be relatively rotatable; and
a radial bearing portion including a working fluid filled between
the sleeve and the shaft, and at least one hydrodynamic groove
formed on either an inner peripheral surface of the sleeve or an
outer peripheral surface of the shaft.
[0044] A hydrodynamic bearing device according to the eighteenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the fifth invention which
is fixed to the stationary member; a shaft which is fixed to the
rotating member and is provided on an inner peripheral side of the
sleeve so as to be relatively rotatable; a radial bearing portion
including a working fluid filled between the sleeve and the shaft,
and at least one hydrodynamic groove formed on either an inner
peripheral surface of the sleeve or an outer peripheral surface of
the shaft; and a cover member of a tubular shape fitted to an outer
periphery of the sleeve.
[0045] Since such a hydrodynamic bearing device includes a sleeve
according to the fifth invention, a sleeve of a hydrodynamic
bearing device which has a circulating function can be manufactured
by sintering. Thus, the manufacturing cost can be reduced, and the
bearing stiffness can be prevented from lowering in such a
hydrodynamic bearing device.
[0046] A hydrodynamic bearing device according to the nineteenth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the eleventh invention
which is fixed to the stationary member; a shaft which is fixed to
the rotating member and is provided on an inner peripheral side of
the sleeve so as to be relatively rotatable; a radial bearing
portion including a working fluid filled between the sleeve and the
shaft, and at least one hydrodynamic groove formed on either an
inner peripheral surface of the sleeve or an outer peripheral
surface of the shaft; and a cover member of a tubular shape fitted
to an outer periphery of the sleeve.
[0047] A hydrodynamic bearing device according to the twentieth
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the twelfth invention
which is fixed to the stationary member; a shaft which is fixed to
the rotating member and is provided on an inner peripheral side of
the sleeve so as to be relatively rotatable; a radial bearing
portion including a working fluid filled between the sleeve and the
shaft, and at least one hydrodynamic groove formed on either an
inner peripheral surface of the sleeve or an outer peripheral
surface of the shaft; and a cover member of a tubular shape fitted
to an outer periphery of the sleeve.
[0048] A hydrodynamic bearing device according to the twenty-first
invention is a hydrodynamic bearing device for supporting a
rotating member so as to be rotatable with respect to a stationary
member, comprising: a sleeve according to the thirteenth invention
which is fixed to the stationary member; a shaft which is fixed to
the rotating member and is provided on an inner peripheral side of
the sleeve so as to be relatively rotatable; a radial bearing
portion including a working fluid filled between the sleeve and the
shaft, and at least one hydrodynamic groove formed on either an
inner peripheral surface of the sleeve or an outer peripheral
surface of the shaft; and a cover member of a tubular shape fitted
to an outer periphery of the sleeve.
[0049] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-second invention, comprises:
forming a primary compact from metal powder for sintering;
sintering the primary compact; sizing the sintered primary compact
with a sizing process to form a secondary compact; impregnating the
secondary compact with resin; and shot-blasting the secondary
compact.
[0050] In such a manufacturing method, since the secondary compact
is treated with the resin impregnation process, the resin can enter
the pores. Further, the secondary compact treated with the resin
impregnation process is treated with the shot blast process. Thus,
the pores formed near the surface of the secondary compact and
including the resin can be sealed, and a layer with less pores and
a high average density of the portion of the metal powder for
sintering can be formed on a surface of the primary compact. As a
result, a layer with a high average density of the portion of the
metal powder for sintering and the pores including the resin can be
formed on a surface of the sleeve. In this way, a sleeve which can
prevent a supporting pressure of a bearing portion from being
released out through the pores can be manufactured by the
manufacturing method. The bearing stiffness can be prevented from
lowering, and the manufacturing cost can be decreased compared to
the case of the conventional sintered metal sleeve.
[0051] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-third invention is a
manufacturing method of the twenty-second invention wherein an
average density of the portion of the metal powder for sintering of
the secondary compact is 6.5 g/cm.sup.3 or higher.
[0052] In such a manufacturing method, since average density of the
portion of the metal powder for sintering of the secondary compact
is 6.5 g/cm.sup.3 or higher, effects of the shot blast process and
the resin impregnation process can be enhanced.
[0053] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-fourth invention comprises:
forming a primary compact from metal powder for sintering;
sintering the primary compact; sizing the sintered primary compact
to form a secondary compact; and contacting the sintered primary
compact or secondary compact with a high-temperature steam;.
[0054] In such a manufacturing method, since the primary compact or
the secondary compact contact a high-temperature steam, steam
enters pores between particles of metal powder for metal powder for
sintering, and oxide is formed on the surface of the compact. Thus,
inner walls of the pores are sealed by the oxide. As a result, the
supporting pressure at the bearing portion can be prevented from
being released out.
[0055] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-fifth invention is a
manufacturing method according to the twenty-fourth invention,
further comprising: finishing a surface of the primary compact or
the secondary compact treated in the steam process.
[0056] The surface finishing may be plating process and DLC film
coating process and the like, for example.
[0057] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-sixth invention is a
manufacturing method according to the twenty-fourth invention,
further comprising: removing at least a part of an iron oxide film
formed on a surface of the primary compact or the secondary compact
at the steam process.
[0058] In this way, at least an oxide film on a surface of an inner
peripheral surface of the sleeve which opposes an outer peripheral
surface of the shaft is removed. Thus, problems such that a film
peels off due to a shock or the like and enters a bearing gap,
causing the shaft to wear out can be prevented.
[0059] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-seventh invention is a
manufacturing method of the twenty-fifth invention wherein the
primary compact or the secondary compact is treated with
nonelectrolytic nickel plating process or DLC film coating process
in the surface finishing.
[0060] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-eighth invention is a
manufacturing method of the twenty-fourth invention wherein an
average density of a portion of the metal powder for sintering of
the secondary compact is 6.8 g/cm.sup.3 or higher.
[0061] In such a manufacturing method, since average density of the
portion of the metal powder for sintering of the secondary compact
is 6.8 g/cm.sup.3 or higher, effects of the steam process and the
surface finishing can be enhanced.
[0062] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the twenty-ninth invention is a
manufacturing method of the twenty-fourth invention wherein: the
primary compact includes a tubular sleeve main body and a tubular
projection projecting from the sleeve main body in an axial
direction; and a rate of change in a dimension of the tubular
projection is larger than a rate of change in a dimension of the
sleeve main body in the sizing process.
[0063] In such a manufacturing method, since the rate of change in
the dimension of the tubular projection is larger than the rate of
change in the dimension of the sleeve main body, a density at a
step such as joint between the sleeve main body and the projection,
for example, can be made high. In this way, by changing partially
the rate of change of the dimensions in the sizing process, the
density of the portion of the mold which is difficult to put the
metal powder for sintering can be made high by the sizing process.
The effects of the above-mentioned manufacturing method can be
enhanced.
[0064] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the thirtieth invention is a
manufacturing method of the twenty-fourth invention, comprising: a
sleeve; a shaft inserted into a bearing hole of the sleeve so as to
be relatively rotatable; and at least one radial bearing having
hydrodynamic grooves formed on at least one of an outer peripheral
surface of the shaft and an inner peripheral surface of the sleeve.
A volume density of a portion of the metal powder for sintering of
the secondary compact is 85% or higher.
[0065] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the thirty-first invention is a
manufacturing method according to the twenty-fourth invention,
wherein: the sleeve is brought into contact with a high-temperature
steam at an atmospheric temperature within the range of 600 to
700.degree. C. for 15 to 50 minutes in the steam process.
[0066] A manufacturing method of a sleeve for a hydrodynamic
bearing device according to the thirty-second invention is a
manufacturing method according to the twenty-fourth invention,
wherein: the sleeve is brought into contact with a high-temperature
steam at an atmospheric temperature within the range of 400 to
700.degree. C. for 25 to 80 minutes in the steam process.
[0067] A sleeve for a hydrodynamic bearing device according to the
thirty-third invention is a sleeve of the fifth invention
comprising at least one groove portion which is provided on an
outer peripheral side and extends in the axial direction.
[0068] A sleeve for a hydrodynamic bearing device according to the
thirty-fourth invention is a sleeve of the eleventh invention
comprising at least one groove portion which is provided on an
outer peripheral side and extends in the axial direction.
[0069] A sleeve for a hydrodynamic bearing device according to the
thirty-fifth invention is a sleeve of the twelfth invention
comprising at least one groove portion which is provided on an
outer peripheral side and extends in the axial direction.
[0070] A sleeve for a hydrodynamic bearing device according to the
thirty-sixth invention is a sleeve of the thirteenth invention
comprising at least one groove portion which is provided on an
outer peripheral side and extends in the axial direction.
[0071] A spindle motor according to thirty-seventh invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the fourteenth invention for supporting the rotor so
as to be rotatable with respect to the base plate.
[0072] Since the spindle motor includes a hydrodynamic bearing
device according to the fourteenth invention, the manufacturing
cost can be reduced.
[0073] A spindle motor according to thirty-eighth invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the fifteenth invention for supporting the rotor so as
to be rotatable with respect to the base plate.
[0074] A spindle motor according to thirty-ninth invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the sixteenth invention for supporting the rotor so as
to be rotatable with respect to the base plate.
[0075] A spindle motor according to fortieth invention, comprises:
a base plate as the stationary member; a stator of a circular shape
which is fixed to the base plate and to which a stator coil is
wound around; a hydrodynamic bearing device according to the
seventeenth invention for supporting the rotor so as to be
rotatable with respect to the base plate.
[0076] A spindle motor according to forty-first invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the eighteenth invention for supporting the rotor so
as to be rotatable with respect to the base plate.
[0077] A spindle motor according to forty-second invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the nineteenth invention for supporting the rotor so
as to be rotatable with respect to the base plate.
[0078] A spindle motor according to forty-third invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the twentieth invention for supporting the rotor so as
to be rotatable with respect to the base plate.
[0079] A spindle motor according to forty-fourth invention,
comprises: a base plate as the stationary member; a stator of a
circular shape which is fixed to the base plate and to which a
stator coil is wound around; a hydrodynamic bearing device
according to the twenty-first invention for supporting the rotor so
as to be rotatable with respect to the base plate.
[0080] A sleeve for a hydrodynamic bearing device according to the
present invention can prevent a supporting pressure of a bearing
portion from being released out through the pores, and the bearing
stiffness from lowering.
[0081] Further, a manufacturing method of a sleeve for a
hydrodynamic bearing device according to the present invention can
manufacture a sleeve which can prevent the bearing stiffness from
lowering, and the manufacturing cost for the sleeve can be
decreased without deteriorating bearing performance.
[0082] In a hydrodynamic bearing device according to the present
invention, as described above, a sleeve has the film of magnetite
(Fe.sub.3O.sub.4) formed on the porous material of pressed-powder
molded sintered metal body with a volume density of 85% or higher
(or 6.8 g/cm.sup.3 or higher). Further, the magnetite
(Fe.sub.3O.sub.4) film is formed by treating the porous material of
pressed-powder molded sintered metal body with a water vapor
process (steam process) at an atmospheric temperature within the
range of 400 to 700.degree. C. Thus, not only that the magnetite
(Fe.sub.3O.sub.4) film is formed even inside the sintered body
material through pores, the holes of the porous material are sealed
sufficiently, and the mechanical strength is increased, but also
the surface roughness of the sintered body bearing can be improved.
Particularly, the arrangement is useful as the hydrodynamic bearing
device, and is suitable for miniaturizing the spindle motor.
[0083] Moreover, in the hydrodynamic bearing device according to
the present invention, the surface of the sleeve is further treated
with a nonelectrolytic nickel plating of a component including
nickel or DLC film coating. In this way, abrasion of the surface,
and removal of the magnetite (Fe.sub.3O.sub.4) film can be
prevented. Thus, a bearing member with higher reliability can be
obtained. Further, if the sintered body is impregnated with a resin
or water glass, soaking of the oil into the holes can be prevented
even when there is a pinhole in the oxide film, the plating, or the
like.
BRIEF DESCRIPTION OF THE DRAWINGS
[0084] Referring now to the attached drawings which form a part of
this original disclosure:
[0085] FIG. 1 is a schematic diagram of a vertical cross-section of
a spindle motor 1 which includes a hydrodynamic bearing device
employing a sleeve according to Embodiment 1 of the present
invention.
[0086] FIG. 2 is a schematic diagram of a vertical cross-section of
the hydrodynamic bearing device 4.
[0087] FIG. 3 is a diagram showing a relationship between an
average density of sintered metal and an amount of soaked
lubricating oil.
[0088] FIG. 4 is a diagram showing a relationship between the
average density of sintered metal and the amount of the soaked
lubricating oil when a shot blast process is performed.
[0089] FIG. 5 is a diagram showing a relationship between the
average density of sintered metal and the amount of the soaked
lubricating oil when a resin impregnation process is performed.
[0090] FIG. 6 is a diagram showing a relationship between the
average density of sintered metal and the amount of the soaked
lubricating oil when the both the shot blast process and resin
impregnation process are used.
[0091] FIG. 7 is a schematic diagram of a vertical cross-section of
a sleeve 42 (left half).
[0092] FIG. 8 is a flow diagram of a method for manufacturing a
sleeve according to Embodiment 1 of the present invention
(including Modification 1).
[0093] FIG. 9 is a flow diagram of a method for manufacturing a
sleeve according to Modification 2 of Embodiment 1 of the present
invention.
[0094] FIG. 10 is a diagram showing a relationship between the
average density and the amount of the soaked lubricating oil when
the steam process is performed.
[0095] FIG. 11 is a diagram showing a relationship between the
average density of sintered metal and the amount of the soaked
lubricating oil when the plating process is performed.
[0096] FIG. 12 is a diagram showing a relationship between the
average density and the amount of the soaked lubricating oil when
both the steam process and the plating process are performed.
[0097] FIG. 13 is a schematic diagram showing a vertical
cross-section of a sleeve 142 according to Embodiment 2 of the
present invention (left half).
[0098] FIG. 14 is a flow diagram of a method for manufacturing a
sleeve according to Embodiment 2 of the present invention.
[0099] FIGS. 15A and 15B show a schematic view of the sizing
process of the manufacturing method according to Embodiment 3 of
the present invention.
[0100] FIG. 16 is a schematic view of a hydrodynamic bearing device
204 according to Embodiment 4 of the present invention.
[0101] FIG. 17 is a cross sectional view of a hydrodynamic bearing
device according to Embodiment 5 of the present invention.
[0102] FIG. 18 is a diagram illustrating a sleeve in the
hydrodynamic bearing device.
[0103] FIG. 19 is a diagram illustrating a rotor in the
hydrodynamic bearing device.
[0104] FIG. 20 is an enlarged view of a cross-section of the sleeve
in the hydrodynamic bearing device.
[0105] FIG. 21 is a diagram illustrating influence of volume
density in the hydrodynamic bearing device.
[0106] FIG. 22 is a diagram illustrating a change in a weight of
the sleeve in the hydrodynamic bearing device.
[0107] FIG. 23 is a diagram illustrating a steam process time in
the hydrodynamic bearing device.
[0108] FIG. 24 is a diagram illustrating a steam process time in
the hydrodynamic bearing device.
[0109] FIG. 25 is a schematic diagram showing a vertical
cross-section of a sleeve 442 according to Embodiment 7 of the
present invention (left half).
[0110] FIG. 26 is a flow diagram of a method for manufacturing a
sleeve according to Embodiment 7 of the present invention.
[0111] FIG. 27 shows a variation of Embodiment 7 of the present
invention.
[0112] FIG. 28 is a schematic diagram showing a vertical
cross-section of a sleeve 542 according to Embodiment 8 of the
present invention (left half).
[0113] FIG. 29 is a flow diagram of a method for manufacturing a
sleeve according to Embodiment 8 of the present invention.
[0114] FIG. 30 is a schematic diagram showing a vertical
cross-section of a sleeve 642 according to Embodiment 9 of the
present invention (left half).
[0115] FIG. 31 is a flow diagram of a method for manufacturing a
sleeve according to Embodiment 9 of the present invention.
[0116] FIG. 32 is a cross sectional view of a conventional
hydrodynamic bearing device.
[0117] FIG. 33 is a diagram illustrating a sleeve in the
conventional hydrodynamic bearing device.
[0118] FIG. 34 is a diagram illustrating a change in a weight of
the sleeve in the conventional hydrodynamic bearing device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0119] Selected embodiments of the present invention will now be
explained with reference to the drawings. It will be apparent to
those skilled in the art from this disclosure that the following
descriptions of the embodiments of the present invention are
provided for illustration only and not for the purpose of limiting
the invention as defined by the appended claims and their
equivalents.
First Embodiment
[0120] Hereinafter, embodiments of the present invention will be
described with reference to the drawings.
[0121] FIG. 1 is a schematic diagram of a vertical cross-section of
a spindle motor 1 which includes a hydrodynamic bearing device
employing a sleeve according to Embodiment 1 of the present
invention. Line O-O shown in FIG. 1 is a rotational shaft of the
spindle motor 1. In the description of the present embodiment, the
upper side and the lower side of the figure are respectively
referred to as "upper side in the axial direction" and "lower side
in the axial direction". However, these expressions are not
intended to limit how the spindle motor 1 is actually attached.
[0122] The spindle motor 1 mainly includes a base plate 2
(stationary member), a rotor 3 (rotating member), and a
hydrodynamic bearing device 4. Hereinafter, details of the
components will be described.
[0123] The base plate 2 forms a stationary portion of the spindle
motor 1 and is fixed to or forms, for example, a housing (not
shown) of a recording and reproducing apparatus. The base plate 2
includes a bracket portion 21, and a stator 22 is mounted. The
bracket portion 21 is a ring member which forms a main part of the
base plate 2. The bracket portion 21 includes a tubular portion 21a
extending upward in the axial direction on an inner peripheral
side. The stator 22 is a member forming a magnetic circuit, and is
fixed on an outer periphery of the tubular portion 21a. To an inner
periphery of the tubular portion 21a, the hydrodynamic bearing
device 4, which will be described below, is fixed.
[0124] The rotor 3 is a portion which is driven to rotate by a
rotational force generated at the magnetic circuit. The rotor 3
includes a rotor hub 31, a disc placement portion 32, a back yoke
33, and a rotor magnet 34. The rotor hub 31 is a portion having a
disc shape which forms a main part of the rotor 3, and is jointed
to the shaft 41 which will be described below. The disc placement
portion 32 is a portion for placing a recording disc (not shown),
and is located on an outer peripheral side of the rotor hub 31 in a
lower part in the axial direction. In the present embodiment, the
rotor hub 31 and the disc placement portion 32 are integrally
formed.
[0125] The back yoke 33 is a tubular member which is fixed to a
lower part of the rotor hub 31 in the axial direction and on an
inner peripheral side of the disc placement portion 32. The rotor
magnet 34 is fixed to an inner periphery of the back yoke 33, and
is located so as to oppose the stator 22 in a radial direction. The
rotor magnet 34 and the stator 22 together form the magnetic
circuit for driving the rotation of the rotor. Specifically, when
an electric current flows through a coil of the stator 22, a
rotational force is generated at the rotor magnet 34 and the rotor
3 is driven to rotate. The rotor 3 is supported by the hydrodynamic
bearing device 4 so as to be rotatable with respect to the base
plate 2.
[0126] FIG. 2 is a schematic diagram of a vertical cross-section of
the hydrodynamic bearing device 4. The hydrodynamic bearing device
4 is for supporting the rotor 3 so as to be rotatable with respect
to the base plate 2. The hydrodynamic bearing device 4 includes a
sleeve 42, a shaft 41, a thrust plate 44, and a thrust flange
43.
[0127] The sleeve 42 is a member of the stationary part of the
hydrodynamic bearing device 4, and is a sintered metal member of a
tubular shape which is fitted into an inner periphery of the
tubular portion 21a of the base plate 2. The sleeve 42 includes a
sleeve main body 42a, and at least one (in this embodiment, a
plurality of) first hydrodynamic grooves 71a and 71b, a tubular
projection 42b, a fixing portion 42d and a sealing portion 42e. The
sleeve main body 42a is a tubular portion which forms a main part
of the sleeve 42. The first hydrodynamic grooves 71a and 71b are
grooves formed on an inner peripheral surface of the sleeve main
body 42a and are located with an equal interval therebetween in a
circumferential direction. The first hydrodynamic grooves 71a and
71b have, for example, a herringbone pattern. The tubular
projection 42b is a circular portion protruding from an end of the
sleeve main body 42a in the axial direction. The fixing portion 42d
is a circular portion which is further protruding from an end of
the tubular projection 42b in the axial direction. The fixing
portion 42d is fixed to, for example an outer periphery of the
thrust plate 44, which will be described further below, by adhesion
or caulking The sealing portion 42e is a capillary sealing portion
which is formed on an inner peripheral side of the sleeve main body
42a in an upper end portion in the axial direction.
[0128] The shaft 41 is a member of the rotating part of the
hydrodynamic bearing device 4. The shaft 41 is a pillar member
located on an inner peripheral side of the sleeve 42. In a conical
bearing, the shaft 41 is a member having a conical shape. The shaft
41 has a recessed portion 41 a. The recessed portion 41 a is a
recessed portion having a circular shape formed on an outer
peripheral surface of the shaft 41. The recessed portion 41 a is
located at a position between the above-mentioned first
hydrodynamic grooves 71a and 71b.
[0129] The thrust flange 43 is a member of the rotating part of the
hydrodynamic bearing device 4. The thrust flange 43 is located on
an inner peripheral side of the tubular projection 42b.
Specifically, the thrust flange 43 is located in a space defined by
the sleeve 42 and the thrust plate 44 with a minute space
therefrom. The thrust flange 43 has second hydrodynamic grooves 72a
on a surface opposing the thrust plate 44 in the axial direction.
The thrust flange 43 also has third hydrodynamic grooves 73a on a
surface opposing the sleeve main body 42a in the axial direction.
Alternatively, the second hydrodynamic grooves 72a may be formed on
the thrust plate 44 and the third hydrodynamic grooves 73a may be
formed on an end of the sleeve 42.
[0130] As described above, in the hydrodynamic bearing device 4, a
radial bearing portion 71 for supporting the rotor 3 in the radial
direction is formed of the sleeve 42 having the first hydrodynamic
grooves 71a and 71b, the shaft 41, and the lubricating oil 46 as a
working fluid interposed therebetween. The working fluid may be
highly fluidic grease or ionic liquids beside the lubricating oil.
Further, a main thrust bearing portion 72 for supporting the rotor
3 in the axial direction is formed of the thrust flange 43 having
the second hydrodynamic grooves 72a, the thrust plate 44, and the
lubricating oil 46 interposed therebetween. Also, a sub thrust
bearing portion 73 is formed of the thrust flange 43 having the
third hydrodynamic grooves 73a, the sleeve 42, and the lubricating
oil 46 interposed therebeteween. When the members rotate with
respect to each other, supporting forces in the radial direction
and the axial direction of the shaft 41 are generated at the
bearings. Thus, the sleeve 42 is a significantly important member
in the hydrodynamic bearing device 4.
[0131] As mentioned above, the sleeve 42 according to the present
invention is made of sintered metal. The characteristics of
sintered metal will be described below in more details.
[0132] Sintered metal include a large number of pores (small spaces
formed between metal powder) inside. The pores can be divided into
two types: pores inside the sintered body which are referred to as
"structural pores"; and pores opening on a surface of the sintered
body which are referred to as "surface pores". In conventional
types of sintered metals, surface pores and structural pores
communicate with each other. Thus, the lubricating oil can pass
through the sintered body via the pores. When the sleeve of the
hydrodynamic bearing device is made of sintered metal, the
lubricating oil soaks into the sleeve. The lubricating oil passes
through the sleeve via the pores and oozes out from an outer
periphery of the sleeve. As a result, a supporting pressure
generated at the radial bearing portion decreases, for example, and
a stiffness of the radial bearing portion is reduced by about
30%.
[0133] In general, an amount of the lubricating oil soaks into the
sintered metal has a relationship with an average density of the
sintered metal. The average density as used herein is obtained by
dividing the weight of the sintered metal by the volume calculated
from its external shape. For example, the density is obtained based
on the weight of the sintered body and the volume calculated based
on Archimedes method with open pores on the surface of the sintered
body being sealed with wax or the like. FIG. 3 shows results of
experimentation in which the sintered metal is left in the
lubricating oil at 80.degree. C. for about 100 hours. A shown in
FIG. 3, when the average density of the metal powder of the
sintered metal is small, the amount of the soaked lubricating oil
increases since there are a large number of pores. In contrast,
when the average density of the metal powder of the sintered metal
is large, the amount of the soaked lubricating oil since there are
small numbers of pores.
[0134] Although the amount of the lubricating oil which soaks can
be reduced by increasing the average density, it is not realistic
to reduce the amount to a level which allows a sintered metal
sleeve to be used for a hydrodynamic bearing device only by
adjusting the average density. Thus, treating the sintered metal
with a pore-sealing process for sealing the pores is taken into
consideration in order to further reduce the amount of the
lubricating oil which soaks.
[0135] For example, pores may be sealed by treating the sintered
metal with a shot blast process to have steel balls strike against
the pores near the surface. FIG. 4 shows a relationship between the
average density and the amount of the soaked lubricating oil when
the shot blast process is performed. From comparison between FIGS.
3 and 4, it is shown that the amount of the soaked lubricating oil
is smaller when the sintered metal is treated with a shot blast
process than when the shot blast process is not performed. For
example, when the average density is 7.0 g/cm.sup.3 or higher,
there is no significant difference in the amounts of the soaked
lubricating oil due to whether the shot blast process is performed
or not. On the other hand, when the average density is 6.8
g/cm.sup.3 or smaller, the amount of the soaked lubricating oil
becomes small, and it is shown that there is an effect in the shot
blast process. However, even with the shot blast process, the
amount of the soaked lubricating oil through the sintered metal
cannot be reduced to a level which allows the sintered metal sleeve
to be used for a hydrodynamic bearing device.
[0136] Meanwhile the sintered body may be treated with a process
for impregnating a resin so that the pores are previously soaked
with the resin. The resin to be impregnated may be, for example,
acryl resins, epoxy resins, and the like. FIG. 5 shows a
relationship between the average density and the amount of the
soaked lubricating oil when a resin impregnation process is
performed. As shown in FIG. 5, the amount of the soaked lubricating
oil is smaller when the sintered metal is impregnated with a resin
than when the resin impregnation process is not performed (this is
clear from absolute values on vertical axis which indicates the
amount of the soaked lubricating oil after 100 hours).
Specifically, when the density is below 6.5 g/cm.sup.3, the
proportion of the pores in the sintered metal is so large. Thus,
the pores cannot be completely filled with the resin, and the
amount of the soaked lubricating oil cannot be reduced. However,
when the density is 6.8 g/cm.sup.3 or higher, the amount of the
soaked lubricating oil can be significantly reduced. Yet, although
the resin impregnation process has more effect than the shot blast
process, the amount of the lubricating oil soaked into the sintered
metal cannot be reduced to a level which allows the sintered metal
sleeve to be used for a hydrodynamic bearing device.
[0137] Now, a specific criterion of the amount of the lubricating
oil which soaks into the sintered body will be described. For the
sleeve 42 of the hydrodynamic bearing device 4 shown in FIG. 1, the
amount which can prevent a decrease in the bearing stiffness is
about 20 mg in 1000 hours. However, when 20 mg of the lubricating
oil actually soaked into the sleeve, the liquid surface of the
lubricating oil lowers. As a result, the lubricating oil is not
left in the radial bearing portion, and the hydrodynamic bearing
device may seize. Therefore, in view of the seizing of the
hydrodynamic bearing device, the amount of the lubricating oil
which soaks into the sintered metal which allows the sintered metal
sleeve to be used for a hydrodynamic bearing device is about 3.0 mg
in 1000 hours. It can be seen from FIG. 5 that even when the
average density becomes large and the resin impregnation process is
performed, the amount of the lubricating oil soaked into the
sintered metal cannot be reduced to a level which allows the
sintered metal sleeve to be used for a hydrodynamic bearing device.
The value of the criterion may vary depending upon the size of the
sleeve and the hydrodynamic bearing device, and is not limited to
such a numerical value.
[0138] In a manufacturing method according to the present
invention, the shot blast process and the resin impregnation
process are used together to achieve more effective pore-sealing
process than when they are separately used. FIG. 6 shows a
relationship between the average density and the amount of the
soaked lubricating oil when the sintered metal is treated with both
the shot blast process and resin impregnation process. As shown in
FIG. 6, when the sintered metal is treated with the shot blast
process and the resin impregnation process, the amount of the
soaked lubricating oil can be significantly reduced than when they
are separately used (FIGS. 4 and 5). When both of the processes are
performed, if the density is 6.5 g/cm.sup.3 or higher (more
preferably, 6.8 g/cm.sup.3 or higher), the amount of the soaked
lubricating oil can be significantly reduced to a level which
allows the sintered metal to be used for a sleeve. From comparison
between FIGS. 5 and 6, given the density of 6.5 g/cm.sup.3 or
higher, the amount of the soaked lubricating oil when the both
processes are used is reduced to about one eighth of the amount
when only the resin impregnation process is performed. In other
words, by treating the sintered metal with both the shot blast
process and the resin impregnation process, the amount of the
lubricating oil which soaks can be significantly reduced to a level
which allows the sintered metal sleeve to be used for a
hydrodynamic bearing device.
[0139] As described above, the sleeve 42 according to the present
invention is treated with the shot blast process and the resin
impregnation process in order to reduce the amount of the
lubricating oil which soaks into the sintered metal. In the above
description, the size of a bearing is about an outer diameter .phi.
12, an inner diameter .phi. 4, and length L15, and formed of
sintered metal of iron. However, one advantage of the present
invention is that iron material is not necessary in some cases, for
example, when a stem process which will be described later is used.
Similar effects can be obtained with different sizes and different
materials for sintered metal (for example, coppers). Hereinafter,
sleeve 42 according to the present invention and the manufacturing
method thereof will be described.
[0140] FIG. 7 shows a schematic view of a vertical cross-section of
the sleeve 42 (left half). The sleeve 42 is mainly formed of an
inner section 48a and a surface deformation section 48b. The inner
section 48a is a tubular portion formed of metal powder for
sintering and a resin for impregnation. The average density of the
portion of the metal powder for sintering of the inner section 48a
is 6.5 g/cm.sup.3 or higher (more preferably, 6.8 g/cm.sup.3 or
higher). The surface deformation section 48b covers a surface of
the inner section 48a. The surface deformation section 48b is
formed of metal powder for sintering and the resin for
impregnation. The surface deformation section 48b is a layer formed
by a shot blast process, as will be described below. In other
words, it is a section which has a shape deformed by the shot blast
process. The metal powder for sintering may be a material including
at least one of iron, iron alloys, copper, and copper alloys.
[0141] In the sleeve 42, the surface deformation section 48b is
formed by the shot blast process. Thus, at least a part of the
pores of the surface deformation section 48b are sealed. The
average density of the portion of the metal powder for sintering of
the surface deformation section 48b is higher than the average
density of the portion of the metal powder for sintering of the
inner section 48a. The inner section 48a is covered by the surface
deformation section 48b having less pores. Since the effect of the
shot blast process is higher in a surface portion, the density of
the portion of the metal powder of the surface deformation section
48b becomes gradually higher from an inner section 48a side toward
the surface portion. The density of the surface of the surface
deformation section is the highest. FIG. 7 clearly shows a boundary
between the inner section 48a and the surface deformation section
48b. However, actually, the density gradually changes in the
boundary between the inner section 48a and the surface deformation
section 48b. Additionally, since the surface deformation section
48b is formed of the metal powder for sintering and the resin for
impregnation, the resin enters into the pores inside the surface
deformation section 48b and the inner section 48a. In other words,
the sleeve 42 has a structure with fewer pores on the surface, and
a resin is in the pores. In this way, the supporting pressure of
the radial bearing portion 71 can be prevented from being released
out through the pores, and the bearing stiffness can be prevented
from decreasing.
[0142] Further, in the sleeve 42, the average density of the
portion of the metal powder for sintering of the inner section 48a
is 6.5 g/cm.sup.3 or higher. The surface deformation section 48b
has a density higher than that and has fewer pores around the
surface of the sleeve 42. Therefore, the supporting pressure of the
radial bearing portion 71 can be further prevented from being
released out through the pores, and the bearing stiffness can be
further prevented from decreasing.
[0143] Next, a method for manufacturing the sleeve 42 will be
described. FIG. 8 shows a flow diagram of a method for
manufacturing a sleeve according to Embodiment 1 of the present
invention. As shown in FIG. 8, the manufacturing method includes
filling step S1, forming step S2, sintering step S3, shot blast
process step S4, sizing process step S5, and resin impregnation
process step S6.
[0144] In the filling step S1, metal powder including for example,
iron, copper or the like is filled in a mold for primary formation.
In the forming step S2, the metal powder material filled in the
filling step S1 is compressed by using an upper mold and a lower
mold for primary formation, and a primary compact is formed. Then,
the primary compact is sintered at a high temperature in the
sintering step S3.
[0145] Next, in the shot blast process step S4, the sintered
primary compact is treated with the shot blast process. For
performing the shot blast process, steel balls strike against the
surface of the primary compact. As a result, pores formed near the
surface of the primary compact are sealed, and a layer with no pore
or with less pores compared to the inside (the inner section) is
formed on a surface of the primary compact. In other words, the
surface deformation section having fewer pores compared to the
inside with the higher average density of the portion of the metal
powder for sintering is formed on the primary compact by the shot
blast process. Conditions for the shot blast process may be as
follows. The average particle size of the steel balls is 0.3 mm.
The amount of blasting the steel balls is 60 kg/min. The rate of
blasting the steel balls is 50 m/s. This set of conditions provides
a better result in reducing the amount of the lubricating oil which
soaks compared to other conditions.
[0146] In the sizing process step S5, the dimension of the primary
compact is adjusted. Specifically, in the sizing process step S5,
the primary compact treated with the shot blast process is set in a
metal mold for secondary formation, which is formed of an inner
mold and an outer mold in which the primary compact is placed at a
predetermined position, and an upper mold and a lower mold which
can be moved up and down freely. The primary compact is compressed
by these molds. As a result, the dimension accuracy of the inner
and outer peripheral surfaces and both end surfaces of the primary
compact is improved, and the secondary compact is formed. By
performing the sizing process, the dimension of the primary compact
is adjusted, and also, the average density of the metal powder
portion of the primary compact can be further increased. For
example, the process can increase the average density of the
portion of the metal powder of the secondary compact to 6.5
g/cm.sup.3 or higher.
[0147] After the secondary compact is formed in the sizing process
step S5, the secondary body is treated with the resin impregnation
process in the resin impregnation process step S6, and the
secondary compact is now the sleeve 42. The resin for impregnation
may be, for example, acrylic resins, epoxy resins, and the like. By
performing the resin impregnation process, the resin enters into
the pores formed on the surface of and inside the secondary
compact. As a result, the pores formed on the surface of and inside
the secondary compact can be sealed. More specifically, the surface
deformation section 48b having the density higher than that of the
inside and the pores sealed with the resin can be formed on a
surface of the sleeve 42. Further, as described above, when the
average density of the sintered metal is at a certain level or
higher, the lubricating oil can be prevented from passing through
the sintered body by performing resin impregnation. Therefore, it
is ensured that the sleeve 42 which is manufactured by the above
process can prevent the lubricating oil from passing inside through
the pores with the surface deformation section 48b formed on the
surface. In this way, the manufacturing method can prevent the
supporting pressure of the radial bearing portion 71 from being
released out through the pores, and the bearing stiffness from
decreasing. Also, the manufacturing cost can be reduced.
[0148] The above manufacturing method can provide the sintered
metal sleeve 42 which can prevent the bearing stiffness from
decreasing. Thus, there is no need to provide a covering member for
preventing the supporting pressure from being released to the outer
periphery of the sleeve 42, and it is ensured that the
manufacturing cost can be reduced.
[0149] As Modification 1 of Embodiment 1, a manufacturing method
which also includes finishing process step S7 after the above
mentioned steps may be considered. More specifically, as shown in
FIG. 8, in the finishing process step S7, the secondary compact
treated with the resin impregnation process is treated with at
least one of the shot blast process and the sizing process. In the
finishing process step S7, both the shot blast process and the
sizing process may be performed. In this modification, since at
least one of the shot blast process and the sizing process is
performed after the resin impregnation process, the surface
roughness of the sleeve 42 after the resin impregnation process can
be improved.
[0150] Further, as Modification 2 of Embodiment 1, a manufacturing
method with an order of performing steps being changed from that of
Embodiment 1 may be considered. FIG. 9 shows a flow diagram of a
method for manufacturing a sleeve according to Modification 2 of
Embodiment 1 of the present invention. As shown in FIG. 9, the
manufacturing method includes filling step S11, forming step S12,
sintering step S13, sizing process step S14, resin impregnation
process step S15, and shot blast process step S16. In this
manufacturing method, the dimension of the primary compact is
adjusted by the sizing process step S14 after the sintering step
S13. The secondary compact is treated with the resin impregnation
process in the resin impregnation process step S15 before
performing the shot blast process. Then, after the resin
impregnation process is performed, the secondary compact is treated
with the shot blast process in the shot blast process step S16.
[0151] This manufacturing method also results in the surface
deformation section 48b having the less pores and the pores filled
with the resin. Therefore, the same or close effects as those of
the above-described manufacturing method of Embodiment 1 can be
achieved. Further, since the shot blast process is performed after
the resin impregnation process, the surface roughness of the
secondary formation after the resin impregnation process can be
improved. In short, this manufacturing method can provide the same
or close effects as those of steps S1 through S7 of the
above-described manufacturing method with fewer steps. The bearing
stiffness can be prevented from lowering, and the manufacturing
cost can be further reduced.
Alternate Embodiments
[0152] Alternate embodiments will now be explained. In view of the
similarity between the first and alternate embodiments, the parts
of the alternate embodiment that are identical to the parts of the
first embodiment will be given the same reference numerals as the
parts of the first embodiment. Moreover, the descriptions of the
parts of the alternate embodiment that are identical to the parts
of the first embodiment may be omitted for the sake of brevity.
Second Embodiment
[0153] In Embodiment 1, the sleeve with a high pore-sealing effect
is formed by together using the shot blast process and the resin
impregnation process. However, another pore-sealing process and
manufacturing method is possible. The pore-sealing process of
sleeve according to Embodiment 2 and the manufacturing method
thereof will be described below. The present embodiment is applied
mainly when sintered materials of iron are used.
[0154] Besides the shot blast process and the resin impregnation
process described above, a pore-sealing process by having the
sintered metal contact steam of a high temperature and subjecting a
surface to high-temperature oxidization. FIG. 10 shows a
relationship between the average density and the amount of the
soaked lubricating oil when the steam process is performed. FIG. 10
shows the example in which the film formed by the steam process has
a thickness of 2 .mu.m. From comparison between FIGS. 3 and 10, it
is shown that the amount of the lubricating oil soaked into the
sintered metal treated with the steam process is smaller than the
amount when the steam process is not performed. For example, when
the figures obtained at 100 hours are compared, the amount of the
soaked lubricating oil becomes small when the average density is
6.8 g/cm.sup.3 or higher. It is shown that the steam process is
effective. However, with only the stem process, the amount of the
soaked lubricating oil cannot be reduced to a level which allows
the sintered metal sleeve to be used for the hydrodynamic bearing
device. This is because the thickness of the film formed by the
steam process is thin and is about 2 to 5 .mu.m.
[0155] Meanwhile, in order to reduce the amount of the lubricating
oil which soaks, it is possible to treat the sintered metal with a
plating process to previously form a plating process layer on a
surface. FIG. 11 shows a relationship between the average density
and the amount of the soaked lubricating oil when the plating
process is performed. The results shown in FIG. 11 are when the
thickness of the plating process layer is 100 .mu.m. As shown in
FIG. 11, when the sintered metal is treated with the plating
process, the amount of the soaked lubricating oil of the sintered
metal treated with the plating process is smaller than that of the
sintered metal which is not treated with the plating process.
However, with only the plating process, the amount of the
lubricating oil which soaks cannot be reduced to a level which
allows the sintered metal sleeve to be used for a hydrodynamic
bearing device. This is because, for performing the plating
process, grease and rust have to be removed beforehand, and thus, a
liquid used for removing grease and rust enters into pores of the
sintered metal which prohibits the plating liquid from entering
into the pores.
[0156] Therefore, in the manufacturing method according to the
present invention, the steam process and the plating process are
used together allowing the pore-sealing process which is more
effective than in when the each of the processes is performed
separately. FIG. 12 shows a relationship between the average
density and the amount of the soaked lubricating oil when both the
steam process and the plating process are performed. As shown in
FIG. 12, when the sintered metal is treated with the steam process
and the plating process, the amount of the soaked lubricating oil
can be significantly reduced compared to that when they are
performed separately (FIGS. 10 and 11). When both of the processes
are used, if the density is 6.8 g/cm.sup.3 or higher (more
preferably, 7.0 g/cm.sup.3 or higher), the amount of the soaked
lubricating oil is reduced to 3.0 mg or lower in 1000 hours. The
amount of the soaked lubricating oil can be reduced to a level
which allows the sintered metal sleeve to be used for a
hydrodynamic bearing device. From comparison between FIGS. 11 and
12, when the density is 6.8 g/cm.sup.3, the amount of the soaked
lubricating oil of the sintered metal treated with both of the
process is shown to be smaller than the amount when only the
plating process is performed. In other words, by treating the
sintered body with both of the steam process and the plating
process, the amount of the lubricating oil which soaks can be
reduced to a level which allows the sintered metal sleeve to be
used for a hydrodynamic bearing device.
[0157] As described above, in order to reduce the amount of the
lubricating oil which soaks, the sleeve 42 of the present invention
is treated with the steam process and the plating process during a
manufacturing process. Hereinafter, the sleeve 42 and details of
the manufacturing method will be described.
[0158] FIG. 13 is a schematic view of a vertical cross-section of
the sleeve 142 according to Embodiment 2 of the present invention
(left half). As shown in FIG. 13, the sleeve 142 is formed of an
inner section 148a, a steam process layer 148b, and a plating
process layer 148c. The inner section 148a is a tubular portion
formed of metal powder for sintering and a resin for impregnation.
The steam process layer 148b is a layer including iron oxide, which
covers a surface of the inner section 148a. The iron oxide may be,
for example, Fe.sub.3O.sub.4 and the like. The plating process
layer 148c is a layer covering the surface of the steam process
layer 148b. The plating process may be, for example,
nonelectrolytic nickel plating or the like. Further, it is
preferable that the thickness of the steam process layer 148b is 2
.mu.m or larger and the thickness of the plating process layer 148c
is 2 .mu.m or larger.
[0159] In such a sleeve 142, when the iron oxide of the steam
process layer 148b has electric conductivity, the plating process
can be performed. As a result, the strength of the plating process
layer 148c can be increased from that when only the plating process
layer 148c is provided. Particularly, when the steam process layer
148b includes Fe.sub.3O.sub.4 having electric conductivity, one
electrode is formed electrochemically. Thus, the plating process
can be performed securely, and the strength of the plating process
layer 148c can be increased. In this way, in the sleeve 142, the
supporting pressure of the radial bearing portion 71 can be
prevented from being released out through the pores, and the
bearing stiffness can be prevented from lowering.
[0160] Further, in the sleeve 142, the average density of the
portion of the metal powder for sintering of the inner section 148a
is 6.8 g/cm.sup.3 or higher (more preferably, 7.0 g/cm.sup.3 or
higher). Thus, the effect of the stem process and the plating
process can be enhanced, and the strength of the plating process
layer 148c can be further improved.
[0161] Moreover, in the sleeve 142, the thickness of the steam
process layer 148b and the plating process layer 148c are both 2
.mu.m or larger. Thus, the effect of the stem process and the
plating process can be enhanced, and the supporting pressure of the
radial bearing portion 71 can be prevented from being released out
through the pores more securely.
[0162] Next, a method for manufacturing the sleeve 42 will be
described. FIG. 14 shows a flow diagram of a method for
manufacturing a sleeve according to Embodiment 2 of the present
invention. As shown in FIG. 14, the manufacturing method includes
filling step S21, forming step S22, sintering step S23, steam
process step S24, sizing process step S25, and plating process step
S26.
[0163] In the filling step S21, metal powder including for example,
iron, copper or the like is filled in a mold for primary formation.
In the forming step S22, the metal powder material filled in the
filling step S21 is compressed by using an upper mold and a lower
mold for primary formation, and a primary compact is formed. Then,
the primary compact is sintered at a high temperature in the
sintering step S23.
[0164] Next, in the steam process step S24, the sintered primary
compact is treated with the steam process. More specifically, the
primary compact is exposed to high-temperature steam to have a
surface of the primary compact high-temperature oxidized. As a
result, the steam process layer 148b including iron oxide is formed
on a surface of the primary compact. The iron oxide included in the
steam process layer 148b may be, for example, Fe.sub.3O.sub.4 and
the like.
[0165] In the sizing process step S25, the dimension of the primary
compact is adjusted. Specifically, in the sizing process step S25,
the primary compact treated with the shot steam process is set in a
metal mold for secondary formation, which is formed of an inner
mold and an outer mold in which the primary compact is placed at a
predetermined position, and an upper mold and a lower mold which
can be moved up and down freely. The primary compact is compressed
by these molds. As a result, the dimension accuracy of the inner
and outer peripheral surfaces and both end surfaces of the primary
compact is improved, and the secondary compact is formed. By
performing the sizing process, the dimension of the primary compact
is adjusted, and also, the average density of the metal powder
portion of the primary compact can be further increased. For
example, the process can increase the average density of the
portion of the metal powder of the secondary compact to 6.8
g/cm.sup.3 or higher.
[0166] After the secondary compact is formed in the sizing process
step S25, the secondary body is treated with the plating process,
which is surface finishing, in the plating process step S26, and
the secondary compact is now the sleeve 142. The plating process
may be, for example, nonelectrolytic nickel plating or the like. If
the steam process layer 148b includes Fe.sub.3O.sub.4, the plating
metal more easily enter the pores since Fe.sub.3O.sub.4 has
electric conductivity. Thus, stronger plating layer can be formed.
By performing the steam process S24 and the plating process 148c, a
strong plating process layer 148c with the pores sealed with the
plating metal can be formed on a surface of the sleeve 142.
Therefore, the sleeve 142 which is manufactured by the above
process can ensure to prevent the lubricating oil for passing
inside through the pores with the steam process layer 148b and the
plating process layer 148c formed on the surface. In this way, the
manufacturing method can prevent the supporting pressure of the
radial bearing portion 71 from being released out through the
pores, and the bearing stiffness from lowering. Also, the
manufacturing cost can be reduced.
[0167] The above manufacturing method can provide the sintered
metal sleeve 142 which can prevent the bearing stiffness from
lowering. Thus, there is no need to provide a covering member for
preventing the supporting pressure from being released to the outer
periphery of the sleeve 142, and the manufacturing cost can be
further reduced.
[0168] In the manufacturing method, the average density of the
secondary compact after the sizing process is 6.8 g/cm.sup.3 or
higher, the supporting pressure of the radial bearing portion 71
can be prevented from being released out through the pores with the
steam process and the plate process, and the manufacturing cost can
be further reduced more securely.
Third Embodiment
[0169] In the sleeves according to Embodiments 1 and 2 as described
above can prevent the supporting pressure of the bearing from being
released out through the pores. However, when the shaft has a
thrust flange, the shape of an end of the sleeve is complicated as
shown in FIG. 2. More specifically, a joint of the sleeve main body
and the tubular projection has an intricate shape. The shape of the
molds for molding is also complicated. As a result, it becomes
difficult to fill the metal powder in such a portion in the filling
process. Thus, the density of the metal powder cannot be increased.
As Embodiment 3, a manufacturing method of the sleeve will be
described with reference to the manufacturing method of Embodiment
1.
[0170] The manufacturing method has its feature in the sizing
process step. FIGS. 15A and 15B show a schematic view of the sizing
process of the manufacturing method according to Embodiment 3 of
the present invention. FIG. 15A shows a secondary compact 42'
before the sizing process, while FIG. 15B shows a secondary compact
42'' after the sizing process. As shown in FIGS. 15A and 15B, in
the sizing process step, the rate of change in a dimension of a
portion corresponding to the tubular projection 42b is set to be
larger than the rate of change in a dimension of a portion
corresponding to a sleeve main body 42a. For example, When it is
assumed that the dimension in the axial direction of the portion
corresponding to the tubular projection 62b and the fixing portion
42d before the sizing process is L1, L2, and L3, and a dimension in
the axial direction after the sizing process are L4, L5, and L6,
the dimensions of the primary formation mold and the secondary
formation mold are determined to satisfy L1/L4<L2/L5, and
L1/L4<L3/L6. Instead of satisfying the L1/L4<L3/L6, the
dimension may be determined.
[0171] In such a manufacturing method, the density of the metal
powder at step portions such as the joint between the sleeve main
body 42a and the tubular projection 42b, and the joint between the
tubular projection 42b and the fixing portion 42d. As a result, by
partially changing the rate of the change in the dimension at the
sizing process, the density of the portion where it is difficult to
fill the metal powder for sintering with the sizing process can be
increase. The effect of the manufacturing method of Embodiments 1
and 2 can be further enhanced.
Fourth Embodiment
[0172] In the above Embodiments 1 and 2, a hydrodynamic bearing
device which does not have a circulating function has been
described. Thus, a hydrodynamic bearing device with a circulating
function will be described as Embodiment 4.
[0173] FIG. 16 is a schematic view of a hydrodynamic bearing device
204 according to Embodiment 4 of the present invention. As shown in
FIG. 16, the hydrodynamic bearing device 204 supports a rotor 203
so as to be rotatable with respect to a base plate 202, and
includes a sleeve 242, a shaft 241, and a thrust plate 244 and a
sleeve cover 245.
[0174] The sleeve 242 is a member of a stationary part of the
hydrodynamic bearing device 204. The hydrodynamic bearing device
204 is a tubular member fitted to an inner periphery of a tubular
portion 221a of the base plate 202. The sleeve 242 is a sintered
metal sleeve manufactured by using one of the manufacturing methods
of the above embodiments. The sleeve 242 further includes a sleeve
main body 242a, a plurality of first hydrodynamic grooves 271a and
271b, a first recessed portion 242c, a second recessed portion
242f, and a circulating groove 242g. The sleeve main body 242a is a
tubular portion which forms a main part of the sleeve 242. The
first hydrodynamic grooves 271a and 271b are grooves formed on an
inner peripheral surface of the sleeve main body 242a and are
located with equal intervals therebetween in a circumferential
direction. The first hydrodynamic grooves 271a and 271b have, for
example, a herringbone pattern. The first recessed portion 242c is
a circular recessed portion formed on an upper end of the sleeve
main body 242a in the axial direction. The second recessed portion
242f is a circular recessed portion formed on a lower end of the
sleeve main body 242a in the axial direction. The circulating
grooves 242g is a portion for circulating the lubricating oil and
is a groove formed on an outer periphery and the ends of the sleeve
main body 242a in the axial direction. The circulating grooves 242g
will be described in more detail below. The shaft 241 is a member
of a rotating part of the hydrodynamic bearing device 204, and is a
pillar member located on an inner peripheral side of the sleeve
242.
[0175] The thrust plate 244 is a circular plate located on an end
of the sleeve 242, and has a second hydrodynamic groove 272a. The
hydrodynamic groove 272a has, for example, a spiral pattern or a
herringbone pattern, and is formed at a position opposing a lower
end of the shaft 241 in the axial direction.
[0176] The sleeve cover 245 is a circular member located on an
outer peripheral side of the sleeve 242. Specifically, the sleeve
cover 245 has a cover main body 245a, a circular plate portion
245b, and a fixing portion 245c. The cover main body 245a is a
tubular portion extending along the axial direction. The sleeve 242
is fitted on an inner periphery side thereof. The circular plate
portion 245b is a circular portion provided on an upper end of the
cover main body 245a in the axial direction. The circular plate
portion 245b extends from the cover main body 245a toward the inner
periphery. The fixing portion 245c is a circular portion protruding
downward in the axial direction from an end of the cover main body
245a. The fixing portion 245c sandwiches, for example, the outer
periphery of the thrust plate 244 with the cover main body
245a.
[0177] The circular plate portion 245b abuts the outer periphery of
the sleeve main body 242a in the axial direction. The circular
plate portion 245b forms a first oil chamber 261 having a circular
shape with the first recessed portion 242c. The thrust plate 244
abuts the outer peripheral portion of the sleeve main body 242a.
The thrust plate 244 forms a second oil chamber 262 having a
circular shape with the second recessed portion 242f. The
lubricating oil 246 is filled between the shaft 241, the sleeve
242, the thrust plate 244, and the sleeve cover 245 as a working
fluid.
[0178] As described above, in the hydrodynamic bearing device 204,
a radial bearing portion 271 for supporting the rotor 203 in the
radial direction is formed of the sleeve 242 having the first
hydrodynamic grooves 271a and 271b, the shaft 241, and the
lubricating oil 246 interposed therebetween. Further, a thrust
bearing portion 272 for supporting the rotor 203 in the axial
direction is formed of the thrust plate 244 having the second
hydrodynamic groove 272a, the shaft 241, and the lubricating oil
246 interposed therebetween.
[0179] Next, the circulating groove 242g will be described in
detail. The circulating groove 242g is formed of at least one (in
this embodiment, a plurality of) first groove portions 242h, at
least one (in this embodiment, a plurality of) second groove
portions 242i, and at least one (in this embodiment, a plurality
of) third groove portions 242j. The first groove portions 242h are
groove portions extending in the axial direction which are formed
in the sleeve main body 242a on the outer peripheral side. The
second groove portions 242i are groove portions extending in the
radial direction in an upper end in the axial direction. The second
groove portions 242i extend from the first groove portions 242h
toward the inside in the radial direction, and connect the first
groove portions 242h and the first recessed portion 242c. The third
groove portions 242j are groove portions extending in the radial
direction which are formed in the lower end in the axial direction.
The third groove portions 242j extend from the first groove
portions 242h toward the inside in the radial direction, and
connect the first groove portions 242h and the second recessed
portions 242f.
[0180] To summarize, a circulating fluid channel 270 is formed of
the circulating groove 242g between the sleeve 242, the sleeve
cover 245, and the thrust plate 244. The circulating fluid channel
270 connects the first oil chamber 261 and the second oil chamber
262 as described above. The first oil chamber 261 and the second
oil chamber 262 communicates with each other via a gap between the
outer periphery of the shaft 241 and the inner periphery of the
sleeve 242. This means that, in the hydrodynamic bearing device
204, the lubricating oil 246 between the shaft 241 and the sleeve
cover 245 can circulate through the second oil chamber 262, the
circulating fluid channel 270, and the first oil chamber 261.
[0181] The sleeve 242 has the circulating groove 242g instead of a
circulating hole which penetrates in the axial direction. Thus, the
circulating fluid channel 270 can be secured without forming the
circulating hole in the sleeve 242. As a result, it is no longer
necessary to form a circulating hole penetrating the sleeve in the
axial direction as in conventional art. It becomes possible to
manufacture a sleeve for a hydrodynamic bearing device with a
circulating function by sintering. Thus, a manufacturing cost for a
sleeve for a hydrodynamic bearing device with a circulating
function can be reduced.
[0182] Further, since the sleeve 242 is formed by a manufacturing
method according to the above described embodiments, it has a
surface deformation section or a steam process layer and a plating
process layer formed on its surface. Thus, a supporting pressure of
the bearing can be prevented from being released out through the
pores, and the bearing stiffness can be prevented from lowering.
Further, it becomes also possible to prevent the lubricating oil
flowing through the circulating fluid channel 270 from passing
through the inside of the sleeve 242, and the circulating function
can be prevented from deteriorating. The sleeve described in
Japanese Laid-Open Publication No. 2003-314536 as mentioned above
is formed of conventional porous sintered metal, and thus suffers
from lowering of the bearing stiffness and the circulating
function. However, the sleeve of the present embodiment can prevent
lowering of the bearing stiffness and the circulating function.
Thus, sleeves can be manufactured by sintering, and the
manufacturing cost of the sleeve can be reduced securely.
Fifth Embodiment
[0183] FIGS. 17 through 19 are cross sectional views showing
structure of a hydrodynamic bearing device according to the present
invention. The hydrodynamic bearing device is mainly formed of a
shaft 301, a flange 302, a sleeve 303, lubricating oil which is a
lubricating fluid (or oil) 304, an upper cover 305, a lower cover
306, a rotor 307, and a base 308. The shaft 301 is integrally
formed with the flange 302. The shaft 301 is inserted into a
bearing hole 303A of the sleeve 303 so as to be relatively
rotatable. The flange 302 opposes a lower surface of the sleeve
303. Hydrodynamic grooves 303B are provided on at least one of an
outer peripheral surface of the shaft 301 and an inner peripheral
surface of the sleeve 303. Hydrodynamic grooves 302A are provided
on at least one of an a lower surface of the sleeve 303, and a
surface of the flange 302 which opposes the lower surface 303C of
the sleeve 303. The upper cover 305 and the lower cover 306 are
fixed to the sleeve 303 or the rotor 307. The bearing gaps near the
hydrodynamic grooves 302A and 303B are filled with at least the oil
304. To the rotor 307, a disc 309 is fixed. To the base 308, a
shaft 301 is fixed. A rotor magnet (not shown) is attached to the
rotor 307. A motor stator (not shown) is fixed to the base 308 at a
position opposing the rotor magnet. The rotor magnet generates an
attracting force in the shaft direction, and presses the sleeve 303
toward the flange 302 with a force of a magnitude of about 10 to 50
g.
[0184] The sleeve 303 as shown in FIG. 18 is a pressed-powder
molded metal sintered body containing an iron component by 80% by
weight or higher. It is formed of porous material with a volume
density of 85% or higher (when the material is pure iron, the
specific gravity when the volume density is 100% is about 7.86).
The volume density as used herein is also referred to as sintering
density. The volume density equals a density of the sintered body
divided by a trued density of normal component of the sintered
body. The density of the sintered body equals the weight of the
sintered body divided by the volume of the sintered body being
sealed open pores on the surface with wax or the like based on
Archimedes method. A film of magnetite (Fe.sub.3O.sub.4) having a
thickness of 2 to 10 .mu.m is formed on a surface of the sleeve
303. The magnetite (Fe.sub.3O.sub.4) film is filling the pores
which have remained on a surface of the pressed-powder molded metal
sintered body. The bearing hole 303A of the sleeve 303 has to be
machined with a high machining precision of 1 .mu.m or lower. Thus,
a sizing process is performed at least before or after the
magnetite (Fe.sub.3O.sub.4) film is formed. Specifically, the
sleeve 303 is inserted into the mold of the bearing hole 303A, and
the sizing process is performed with a press. The sizing process
can be performed more readily before the magnetite
(Fe.sub.3O.sub.4) film is formed because a load is light. When the
process is performed after the magnetite (Fe.sub.3O.sub.4) film has
been formed, the variance in the thickness of the magnetite
(Fe.sub.3O.sub.4) film can be adjusted in the sizing process, and
thus, the precision of finishing becomes better.
[0185] Further, as shown in FIG. 20, nonelectrolytic plating film
303H of nickel or a DLC film (for example, a three dimensional DLC
coating of a plasma ion injection type for a three dimensional
object, which is available from KURITA Seisakusho Co., Ltd., or the
like may be used) is used as an overcoat of the magnetite
(Fe.sub.3O.sub.4) film as necessary to improve anti-abrasion effect
of the sleeve 303 and to achieve a complete surface sealing-pore
effect. Further, to achieve a perfect result in sealing the pores
of the sleeve 303 when there is a pinhole or a surface defect on
the overcoat 303H or the magnetite (Fe.sub.3O.sub.4) film 303G,
inner pores which are probable to remain inside the sleeve 303 as a
sintered body material 303F is impregnated with a resin 303J under
a low pressure as necessary.
[0186] FIG. 19 shows the rotor 307. In this example, not only
sleeve, but also rotor or hub sis formed of a sintered metal. The
rotor is a pressed-powder molded sintered body which includes
metals such as stainless metals, copper metals, hard resins, or
iron or copper with a volume density of 85% or higher. When the
rotor 307 is the pressed-powder molded sintered body, the magnetite
(Fe.sub.3O.sub.4) film is formed on a surface thereof to a
thickness of 2 to 10 .mu.m. The magnetite (Fe.sub.3O.sub.4) film is
filling the pores which have remained on a surface of the
pressed-powder molded metal sintered body. The rotor 307 is
integrally formed with the sleeve 303. A vertical circulating flow
path 302 which allows the lubricating fluid to circulate is
provided on at least one of an outer peripheral surface of the
sleeve 303 and an inner periphery of the rotor 307. The rotor 307
has a step portion 307A so that the disc or the like can be readily
fixed.
[0187] When the sleeve 303 and the rotor 307 are the pressed-powder
molded metal sintered body including iron content by 80% by weight
or more and are porous material with the density by weight of 85%
or higher, the rotor 307 is press-fitted after two components are
separately sintered, and then, the magnetite (Fe.sub.3O.sub.4) film
having a thickness of 2 to 10 .mu.m may be formed. In this way, the
manufacturing cost can be low. Furthermore, the thermal
coefficients of the materials of the sleeve 303 and the rotor 307
are the same. Thus, the members do not have distortion or do not
deform even under a temperature change. The performance of the
hydrodynamic bearing device becomes good.
[0188] Moreover, the sleeve 303 and the rotor 307 may also be
processed integrally as the pressed-powder metal sintered body, and
one or four pit(s) may be formed by a drilling process.
[0189] Hereinafter, an operation of a conventional fluid bearing
type rotary device having a structure as described above will be
described.
[0190] When a rotational force is applied to the rotor magnet (not
shown), the rotor 307, the sleeve 303, the upper cover 305, the
lower cover 306, and the disc 309 as shown in FIG. 17 start to
rotate. As the sleeve 303 rotates, the hydrodynamic grooves 303B
and 302A gather the oil 304. A pumping pressure is generated
between the shaft 301 and the sleeve 303 and between the flange 302
and the sleeve 303. In this way, the shaft 301 rotates without
contacting the sleeve 303 and the flange 302. Recording and
reproduction of data on the disc 309 is performed by a magnetic
head or an optical head (not shown). The oil can circulate because
of a vertical groove 303E of the sleeve 303 as a circulation
channel. Thus, necessary oil can be more easily supplied to a place
where oil is insufficient, and insufficiency of oil can be
prevented.
[0191] The oil 304 is held in the gap between the flange 302 and
the lower cover 306 by a surface tension. When the bearing is
rotating, a centrifugal force is applied to the oil 304 and the oil
leakage can be prevented further. The oil 304 is also held in the
gap between the upper cover 305 and an inclined surface 303D
provided on an upper portion of the sleeve 303. When the bearing is
rotating, a centrifugal force is applied to the oil 304 and the oil
leakage can be prevented further. An inner circumference of the
upper cover 305 opposes a small diameter portion 301A of the shaft
301 and the diameter is smaller than the outer diameter of the
shaft 301. Thus, the centrifugal force to be applied to the oil 304
can be fully applied. The oil 304 in the gap tends to move toward
an external peripheral portion where the gap is smaller because of
the inclined surface by the surface tension. Thus, the oil 304
easily flows into the vertical groove 303E, and readily moves
within the bearing. Therefore, when there is insufficiency in oil
in the bearing, the oil 304 can move through the vertical groove
and is supplied to the portion where it is required. As shown in
FIG. 17, the lower end 301B of the shaft 301 is also formed to be
have a diameter smaller than the outer diameter of the shaft 301.
The internal diameter of the lower cover 306 is also formed to be
smaller than the outer diameter of the shaft 301. Thus, the
centrifugal force to be applied to the oil 304 can be fully applied
and a strong oil sealing effect can be obtained.
[0192] The sleeve 303 as shown in FIG. 17 includes the magnetite
(Fe.sub.3O.sub.4) film formed on a porous surface of the
pressed-powder molded metal sintered body material. The metal
powder used for pressed-powder molding can be kinds of copper such
as brass. However, in order to reduce the difference in the thermal
coefficients with the rotary shaft of the motor, iron powder
containing iron content by 80% by weight of the total or pure iron
is preferable. In this case, iron powders are pressed-powder
molded, and then sintered to obtain sintered body material for the
bearings.
[0193] As shown in FIG. 20, the remaining holes on the surface of
the sleeve 303 are closed by the magnetite (Fe.sub.3O.sub.4) film.
Thus, the oil 304 in the bearing gap does not enter the holes and
become insufficient. Further, a problem that the oil 304 passes
through the remaining hole inside the sleeve 303 and leaks out the
sleeve 303 does not occur.
[0194] FIG. 21 shows data of the amount of oil soaked into the
porous material obtained by impregnating the sleeve 303 with a
sufficient amount of oil put into a beaker (not shown) by itself,
leaving it at a temperature of 80.degree. C., and then measuring an
amount of change in the total weight after 1000 hours. The sleeve
303 is a porous material of the pressed-powder molded metal
sintered material. The present inventors found that the pores on
the surface cannot be sufficiently sealed even when the magnetite
(Fe.sub.3O.sub.4) film is formed if the volume density is less than
85%. The weight of the soaked oil increases as indicated in FIG.
21. When the volume density is 85% or higher, the pores remaining
on the surface of the sleeve 303 are sealed by providing the
magnetite (Fe.sub.3O.sub.4) film and the sleeve 303 does not absorb
the oil 315. Thus, there is no weight change after 1000 hours and
the good result can be obtained.
[0195] The present inventors also found that the thickness of the
magnetite (Fe.sub.3O.sub.4) films of the sleeve 303 which provides
the good results is within the range of 2 to 10 .mu.m. When the
thickness is 2 .mu.m or smaller, effects of sealing the porous
surface are insufficient. When the thickness is 10 .mu.m or
greater, defects such that the magnetite (Fe.sub.3O.sub.4) film is
peeled off, or broken occurs. It is found that when the thickness
of the magnetite (Fe.sub.3O.sub.4) film is 2 to 10 .mu.m, the
surface is sealed and the oil does not soak in a combination with
the condition that the volume density of the porous material is 85%
or higher. With the sleeve 303 formed based on these conditions,
the surface of the porous material is completely sealed and the
effect of generating a pressure in the hydrodynamic bearing device
can be improved.
[0196] FIG. 22 shows data of the weight change measured after the
sleeve 303 formed by forming the magnetite (Fe.sub.3O.sub.4) film
on the porous surface of the pressed-powder molded metal sintered
body having the volume density of 85% or higher according to the
present invention is impregnated into the oil of 80.degree. C. for
300 hours. FIG. 22 indicates that the there is no weight change in
the sleeve 303 after it has been left for 3000 hours, and the oil
is not absorbed inside. Since such a sleeve 303 with the surface
being sealed completely is used, the hydrodynamic bearing device
shown in FIG. 17 does not experience lowering of the pressure
during rotation, and thus, it has high performance and high
stiffness. A problem such that the oil soaks inside the sleeve 303
and the oil 304 of the bearing gap becomes insufficient to result
in the bearing to seize, or the oil leaks out the sleeve 303 and
the surroundings of the bearing arrangement is contaminated with
gas of the oil does not occur.
[0197] In the present embodiment, the materials used for the shaft
301 and the flange 302 are a stainless steel, a high manganese
chrome steel, or a carbon steel. A material finished to have a
surface roughness within a range of 0.01 to 0.8 .mu.m by machining
is used for a radial bearing surface of the shaft 301.
[0198] In a fluid bearing type rotary device of the present
embodiment described above, both ends of the shaft 301 can be fixed
and the sleeve 303 rotates. However, as shown in FIG. 25, the
present invention can be applied to a hydrodynamic bearing device
which has a structure that the shaft rotates without providing an
adhesion groove even when the and the sleeve is directly adhered
and fixed to the base without interposing other member
therebetween, since an adhesion strength to fix the sleeve to the
rotor by adhesion is strong because the sleeve has more coarse
surface roughness than that of the sleeve formed by metal
cutting.
[0199] In a fluid bearing type rotary device of the present
embodiment described above, both ends of the shaft 301 can be fixed
and the sleeve 303 rotates. However, the present invention can also
be applied to a hydrodynamic bearing device shown in FIG. 18 of
Japanese Patent Gazette No. 3155529 (Motor including hydrodynamic
bearing device and recording and reproducing apparatus including
the same). More specifically, the hydrodynamic bearing device has a
rotor fixed to an upper side of a shaft, and a member of a ring
shape attached to a lower side of the shaft. Around the ring-shaped
member, an oil sump is provided adjacent to the radial bearing
surface, and a thrust bearing surface is formed with the lower
surface of the rotor and the upper surface of the sleeve opposing
each other.
Sixth Embodiment
[0200] Since the sleeve 303 shown in FIG. 17 is a porous sintered
body material, a pressure may be lowered due to leakage of a
dynamic pressure when it is used for a hydrodynamic bearing device.
Thus, a hydrodynamic bearing device may have a magnetite
(Fe.sub.3O.sub.4) film formed on at least a portion of an inner
peripheral surface of the sintered metal bearing material which has
been treated with a sizing process, which slides with respect to a
motor shaft. More specifically, in a hydrodynamic bearing device
according to Embodiment 6, a magnetite (Fe.sub.3O.sub.4) film is
formed on a porous surface by treating the sleeve 303 formed of the
pressed-powder molded metal sintered body material with a water
vapor process. Further, the pressed-powder molded metal sintered
body material is treated with the water vapor process at an
atmospheric temperature of 400 to 700.degree. C. to form the
magnetite (Fe.sub.3O.sub.4) film on the porous surface.
[0201] The water vapor process is known to be a method for
separating contents or fixing the deformation of wood, or
stabilizing dimensions of wood, or a stabilizing method for food
used in a relatively low atmospheric temperature of 230.degree. C.
at most. However, the present inventors have succeeded to apply the
process to the sintered metal bearing material by changing a
process temperature and a process time.
[0202] A heat treatment furnace with a pressure resistant structure
is preferable for the water vapor process. A sintered metal bearing
material and water are put inside and the inside is sealed by
putting a cap thereon. Then, the furnace is heated to a high
temperature of 400 to 700.degree. C. The water inside is evaporated
by heating. As the pressure inside the chamber rises, the heating
process of the sintered metal bearing material is started. After
around 25 to 80 minutes of the water vapor process, depending upon
the temperature inside the furnace, a dense and stable oxidized
film of magnetite (Fe.sub.3O.sub.4), which is a spinel phase oxide,
is formed on a surface of the sintered metal bearing material. The
film thickness is 2 to 10 .mu.m in this embodiment, and there is
substantially no influence on a dimension accuracy of the
bearing.
[0203] When the atmospheric temperature for the water vapor process
is less than 400.degree. C., sufficient film of magnetite
(Fe.sub.3O.sub.4) cannot be formed on the bearing material. On the
other hand, even when the atmospheric temperature is more than
700.degree. C., there is no further change in generation of
magnetite (Fe.sub.3O.sub.4). Also, the heat treatment furnace
becomes expensive. Thus, in view of the productivity and the
density of the film to be generated, it is preferable to perform
heating at a temperature in the range of 400 to 700.degree. C., as
described above, and more preferably, the range of 600 to
700.degree. C.
[0204] Further, a time period required for water vapor process when
the atmospheric temperature is within the range of 400 to
700.degree. C. as described above is about 25 minutes at the
atmospheric temperature of 600.degree. C., about 40 minutes at
550.degree. C., about 65 minutes at 450.degree. C., and about 80
minutes at 400.degree. C. for obtaining a film of magnetite
(Fe.sub.3O.sub.4) having a thickness of about 5 .mu.m. Thus, a time
period for process is preferably within the range of 25 to 80
minutes.
[0205] The sintered metal bearing material treated with the above
water vapor process may be treated again with the sizing process as
necessary to further improve the precision. The sleeve 303 treated
with the water vapor process not only has improved corrosion
resistance, anti-abrasion property, and mechanical strength, but
also has its surface covered by metal. Thus, the water vapor
process is good as an surface preparation for plating.
Particularly, surface roughness is smoothed out by filling the
holes. This is suitable for a hydrodynamic bearing device.
[0206] More specifically, by treating the sleeve 303 formed of a
porous material of the pressed-powder molded metal sintered body
with the water vapor process at the atmospheric temperature of 400
to 700.degree. C., the size of the pores can be reduced. Further,
the water vapor process can alleviate difficulty in attachment of
plating to a resin surface, and enhance effects of the following
plating process. Further, depending on process conditions, the
air-permeability can be substantially zero. Thus, there is no
lowering in the pressure due to dynamic pressure leakage, and the
stiffness and rotation accuracy of the bearing can be improved.
Additionally, a plating liquid can be prevented from entering and
the corrosion resistance can be improved.
[0207] The thickness of the magnetite (Fe.sub.3O.sub.4) film to be
produced can be adjusted as desired. A standard thickness is about
5 .mu.m. Thus, dimension can be adjusted by re-compressing using a
mold.
[0208] For performing the water vapor process, process can be
proceeded without colliding the bearing members against each other.
Thus, there is no dent left in the products. Further, the process
oil remained inside can be removed by a high-temperature process
before treating. Thus, no extra cleaning process is required. The
produced magnetite (Fe.sub.3O.sub.4) film has good durability.
[0209] FIG. 23 shows data obtained by measuring weight change after
the sleeve 303 is treated with the water vapor process for each
period of time in the temperature condition of 400 to 700.degree.
C., and each of the sleeves is left in the oil of 80.degree. C. for
500 hours. The temperature range of 400 to 700.degree. C. is wide,
and there were variances. Thus, FIG. 23 shows average values. The
result of the experimentation shows that a time period of 25 to 80
minutes is good for water vapor process. When the time period is
not more than 25 minutes, sealing of the surface is insufficient.
On the other hand, when the time period is more than 80 minutes,
the pores can be sealed, but there are problems such that the
magnetite (Fe.sub.3O.sub.4) film can be easily removed in the
following sizing process or the like, and the process is not
cost-effective.
[0210] FIG. 24 shows data obtained by measuring weight change after
the sleeve 303 is treated with the water vapor process for each
period of time in the temperature condition of 600 to 700.degree.
C., and each of the sleeves is left in the oil of 80.degree. C. for
500 hours. The result of the experimentation at temperature range
of 600 to 700 .degree. C. shows that a time period of 15 to 50
minutes is good for water vapor process. When the time period is
not more than 15 minutes, sealing of the surface is insufficient.
When the time period is 50 minutes or more, sealing of the pores
can be performed sufficiently. On the other hand, when the time
period is more than 50 minutes, the pores can be sealed, it is not
cost-effective, and problems such that the magnetite
(Fe.sub.3O.sub.4) film can be easily removed in the following
sizing process or the like occur in some cases.
[0211] Compared to performing the water vapor process at a
temperature of 600.degree. C. or below, performing the water vapor
process at a temperature between 600 and 700.degree. C. can reduce
the processing time, and thus, the productivity is high. Further,
since the surface temperature of the porous material rises and the
activity is increased, adhesion property between the porous
material layer and the magnetite (Fe.sub.3O.sub.4) film is high.
FeO which is instable oxidized iron does not remain on the surface
of the magnetite (Fe.sub.3O.sub.4) film, and a uniform magnetite
(Fe.sub.3O.sub.4) film with a high purity can be produced. This is
suitable for the sleeve 303 of the hydrodynamic bearing device
which generates a high pressure during rotation.
[0212] Further, for further forming a nonelectrolytic plating film
including nickel or DLC film on a surface of the magnetite
(Fe.sub.3O.sub.4) film, it is found that the adhesion property is
improved, and the strength against the removal of the films is
increased by about 20%. Particularly, the film thickness of about 5
.mu.m same as that of the magnetite (Fe.sub.3O.sub.4) film is
desirable. The gap between the sleeve 303 and the shaft 301 is set
to be about 5 .mu.m. However, by performing the sizing process, an
appropriate gap can be secured. By performing a plating surface,
abrasion powder can be suppressed from flowing out. An iron
sintered body which has a nonelectrolytic plating film including
nickel or DLC film which has small difference in thermal expansion
with the rotary shaft of the motor is suitable. By combining the
water vapor process and the plating process, the amount of soaked
lubricating fluid shown in FIG. 22 can be further improved.
[0213] The sleeve 303 is obtained by: pressing iron powder;
sintering the pressed powder; performing the sizing process to
obtain three types of bearing materials; putting the materials into
a heat treatment furnace having pressure resistant structure (a
homogenous treatment furnace of a batch type available from Tokyo
Netsushori Kogyo KK.); heating to 550.degree. C.; and maintaining
for 55 minutes to perform the water vapor process. As a result, a
magnetite (Fe.sub.3O.sub.4) film having an average thickness of 5
.mu.m is formed on the bearing material surface. The heat furnace
as used herein is not limited to the above example, and an
industrial superheated water vapor process furnace (ST furnace)
available from Sunray Reinetsu Co., Ltd., a combination of a bit
furnace and a steam producing device and the like may be used.
[0214] The range of 400 to 700.degree. C. of the atmospheric
temperature is set as a condition of the water vapor process for
the porous material of the pressed-powder molded metal sintered
body in the present invention. However, by combining an superheat
water vapor process allows the heating process to be performed
readily under non-oxidization (inactive) condition with
far-infrared ray heating, it is also possible to form a similar
magnetite (Fe.sub.3O.sub.4) film by using an superheat water vapor
process device with low energy load compared to the above
atmospheric temperature range. A speed of transfer is fast in the
superheat water vapor process, but it has a disadvantage that
thermal efficiency is low. Thus, non-oxygen heating processing
method as mentioned above adds a high thermal efficiency to the
advantages of the superheat water vapor process. In this way, the
quality of the bearing can be improved, and the reduction of the
processing time and the cost can be achieved.
Seventh Embodiment
[0215] In the above Embodiment 2, the plating process is performed
after the steam process so that the amount of soaking into the
sleeve becomes equal to or lower than a predetermined amount.
However, even when only the steam process is performed, the amount
of soaking may not cause any practical problem in some cases. That
is when there is a structure that the lubricating oil can circulate
within the bearing (see FIG. 16). Such a bearing with the
lubricating oil circulating structure has a lubricating oil sump
(the first oil chamber 261 in FIG. 16). Even when the amount soaked
into the sleeve is large, just the lubricating oil in the
lubricating oils sum decreases, and the lubricating oil is always
provided to the dynamic pressure grooves. Thus, problems such as
seizing are less likely to occur. More specifically, as shown in
FIG. 25, the sleeve 442 according to Embodiment 7 is a sleeve
mainly used for a small fluid bearing device, and is formed of a
steam process section 448d, and a steam process layer (oxide film
layer) 448b covering the steam process section 448d. The steam
process layer 448b is a layer including the oxide formed on a
surface of the steam process section 448d by the steam process as
in Embodiment 2 described above. The steam process section 448d is
a section having the oxide formed on a surface of each particle of
the metal powder for sintering with a high temperature steam
entering inside the steam process layer 448b. Thus, there is oxide
inside the pores between the particles of the steam process section
448d.
[0216] As shown in FIG. 26, a flow of the manufacturing method
includes filling step S421, forming step S422, sintering step S423,
sizing step S424, and steam process step S425. The differences from
the flow of Embodiment 2 are that a step corresponding to the
plating process step S26 for the secondary compact is omitted, and
the steam process step S425 is performed after the sizing step
S424.
[0217] The supporting pressure of the radial bearing portion can be
prevented from being released out through the pores, and the
bearing stiffness can be securely prevented from lowering. Further,
since the steam process layer 448b is relatively hard layer,
anti-abrasion property of the same level as in Embodiment 2 can be
achieved.
[0218] As shown in FIG. 27, an inner section 448a (a section which
cannot be treated by the steam process) formed of sintered metal,
which corresponds to the inner section 148a described in Embodiment
2, sometimes remains inside the steam process section 448d
depending upon the size and the shape of the sleeve 442. Even in
such cases, effects similar as those of the sleeve 442 shown in
FIG. 25 can be achieved.
Eighth Embodiment
[0219] In Embodiment 7 described above, the sleeve 442 is covered
by the steam process layer 448b, and pore sealing effect and
anti-abrasion property of the level same as that when plating
process is performed can be obtained. However, the steam process
layer 448b is hard, but vulnerable to shock. If a crack opens due
to shock, it may peel off while the bearing is being used. When the
steam process layer 448b peels off, a peeled piece undesirably
accelerates abrasion of the shaft. Thus, an embodiment in which at
least a part of the steam process layer 448b is removed after the
steam process step S425 is possible.
[0220] Specifically, as shown in FIG. 28, a sleeve 542 according to
Embodiment 8 is similar to the sleeve 442 of Embodiment 7, but a
steam process layer (oxide film layer) 448b on an inner peripheral
side is removed and is formed of only a steam process section
548d.
[0221] As shown in FIG. 29, a method of manufacturing includes
filling step S521, forming step S522, sintering step S523, steam
process step S524, film removing step S525, and sizing step S526.
The difference from the flow of Embodiment 7 is that film removing
step S525 is added after the steam process step S524. A method for
removing the steam process layer may be, for example, shot
blasting, barreling, cutting or the like. For removing the steam
process layer (oxide film layer) on the entire surface as shown in
FIG. 28, shot blasting is preferable. For removing only portions
corresponding to radial bearing or thrust bearing, cutting such as
reaming, lathing, or the like is used.
[0222] Even in this embodiment, the soaked amount may not cause a
problem in a practical use sometimes depending upon the steam
process layer 548b. Since the steam process layer does not peel
off, the acceleration of abrasion of the shaft caused by a peeled
off piece can be prevented. If the bearing has a structure which
does not cause a problem in a practical use even when the soaked
amount increases, cause of defects due to peeling off is removed,
and there is a significant effect that the reliability
improves.
[0223] The steam process section 548d has lower anti-abrasion
property compared to the steam process layer 448b. However, since
there is oxide, the anti-abrasion property of a level which does
not cause a problem as a bearing can be secured. Moreover, the
sizing step S526 after the film removing step S525 is provided for
improving dimension accuracy, and surface accuracy. Thus, the step
can be omitted in terms of the pore-sealing effect. Further, an
inner section (a section which cannot be treated by the steam
process) formed of sintered metal may remain inside the steam
process section 548d as in Embodiment 7.
Ninth Embodiment
[0224] In Embodiment 2 described above, the plating process
performed from above the steam process layer 148b. However, the
steam process layer 148b may be removed first as described in
Embodiment 8, and then the plating process may be performed as
described in Embodiment 2.
[0225] Specifically, as shown in FIG. 30, a sleeve 642 is formed of
a steam process section 648d, and a plating process layer 648c
which covers the steam process section 648d. Similarly to
Embodiment 8, in the steam process section 648d, steam enters
inside and the oxide enter the pores between particles of the
sintered metal. Similarly to Embodiment 2, a plating process layer
648c is a layer formed by nonelectrolytic nickel plating process,
and covers the steam process section 648d. In this way, the
anti-abrasion property and the pore sealing effect can be improved
in Embodiment 8.
[0226] As shown in FIG. 31, a method of manufacturing includes
filling step S621, forming step S622, sintering step S623, steam
process step S624, film removing step S625, sizing step S626, and
plating process step S627. The difference from the flow of
Embodiment 8 is that the plating process step S627 is added after
the sizing step S626.
[0227] This embodiment can provide a pore sealing effect similar to
that of Embodiment 2. In addition, since the steam process layer is
removed, even a variance in the plating process due to steam
processing layer can be suppressed.
Other Embodiments
[0228] In First through Third Embodiments as described above, the
thrust flange is provided on the end of the shaft. However, the
present invention can also be applied to the hydrodynamic bearing
device which does not include a thrust flange.
[0229] In the above-described embodiments, the working fluid is
lubricating oil. However, it may be highly fluidic grease, ionic
liquids and the like.
General Interpretation of Terms
[0230] In understanding the scope of the present invention, the
term "configured" as used herein to describe a component, section
or part of a device includes hardware and/or software that is
constructed and/or programmed to carry out the desired function. In
understanding the scope of the present invention, the term
"comprising" and its derivatives, as used herein, are intended to
be open ended terms that specify the presence of the stated
features, elements, components, groups, integers, and/or steps, but
do not exclude the presence of other unstated features, elements,
components, groups, integers and/or steps. The foregoing also
applies to words having similar meanings such as the terms,
"including", "having" and their derivatives. Also, the terms
"part," "section," "portion," "member" or "element" when used in
the singular can have the dual meaning of a single part or a
plurality of parts. Terms that are expressed as "means-plus
function" in the claims should include any structure that can be
utilized to carry out the function of that part of the present
invention. Finally, terms of degree such as "substantially",
"about" and "approximately" as used herein mean a reasonable amount
of deviation of the modified term such that the end result is not
significantly changed. For example, these terms can be construed as
including a deviation of at least .+-.5% of the modified term if
this deviation would not negate the meaning of the word it
modifies.
[0231] While only selected embodiments have been chosen to
illustrate the present invention, it will be apparent to those
skilled in the art from this disclosure that various changes and
modifications can be made herein without departing from the scope
of the invention as defined in the appended claims. Furthermore,
the foregoing descriptions of the embodiments according to the
present invention are provided for illustration only, and not for
the purpose of limiting the invention as defined by the appended
claims and their equivalents.
* * * * *