Fluid seal and bearing with same

Details Fluid seal and bearing with same Fluid seal and bearing with same

2000-10-06

2003-12-30

Ometz, David L. (Department: 2653)

Dynamic magnetic information storage or retrieval

Record transport with head stationary during transducing

Disk record

C310S06700R

Reexamination Certificate

active

06671125

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a sealing structure for an electric motor, and more particularly, it pertains to a fluid seal which is disposed in a radial gap between a shaft and a hub of a motor for preventing contaminates such as oil, dusts and the like within the motor from leaking to the outside of the motor. In addition, the present invention relates to a spindle motor having such a fluid seal, and a disk drive using such a spindle motor.
2. Description of the Related Art
FIG. 16
schematically illustrates the internal construction of a typical disk drive
70
. The disk drive
70
comprises a housing
71
having a motor
72
mounted therein for driving a disk
73
capable of storing various kinds of information in digital format and at high density. The housing
71
also contains a head assembly
77
for reading and writing information to or from the disk plate
73
. The head assembly
77
comprises a head
76
for reading and writing information on the disk
73
, an arm
75
for supporting the head
76
, and an actuator portion
74
for moving the head
76
and arm
75
to a required position. In recent years, the information density capable of being stored on a disk has grown by leaps and bounds. A clean environment for installation and housing of a disk containing extremely little dust is required. Therefore, the inside of the housing
71
of the disk drive
70
has to form an extremely clean space and to this end is shut off from outside air. Also, the motor
72
used in a disk drive may not leak contaminates, such as oil mists and the like. In response to such a demand, fluid seals are used to prevent oil mists and the like that are caused by rotation of the shaft and ball bearing portions of the motor
72
.
In
FIG. 15
, an example of spindle motor with a fluid seal is shown as a cross-sectional view cut away by a plane passing through the shaft axis of the motor. In particular, shaft
1
is fixed and has a rotor hub
4
rotatably disposed thereon by upper and lower ball bearings
2
and
2
′. A stator core
10
and a coil
9
are disposed on a bracket
7
which constitutes a stationary portion, that is, the rotor hub
4
has the shape of a so-called outer rotor. A seal portion
3
is situated above the upper ball bearing, and is fitted in between the shaft
1
and the rotor hub
4
so as to fill the gap therebetween.
Hitherto, a magnetic fluid in schematic in
FIG. 13
has been used to form the seal portion
3
FIG.
15
. Since the magnetic fluid seal has a rotationally symmetrical shape with respect to the alternate long and short dash line exhibiting the shaft axis in the figure, only the right sides of the cross-sectional views cut away by a plane passing through the shaft axis is shown in the Figures below.
The magnetic fluid seal shown in
FIG. 13
comprises annular pole pieces
30
and
33
formed by of a pair of magnetic bodies, and an annular magnet
32
sandwiched therebetween. These pole pieces and magnet are disposed above the outer ring portion
21
of the upper bearing
2
via a holder
35
, and are fixed by an adhesive
34
. The inner peripheral end portions of the pole pieces
30
and
33
confront the outer sealing surface of the shaft with a gap located therebetween. Between the inner end portion of the upper pole piece and the outer surface of the shaft, a magnetic fluid
31
is held by the flux generated by the annular magnet
32
, and this forms an air barrier, which prevents dust generated by the rotation of the lower pole or the like from leaking upwardly along the shaft via the above-mentioned gap.
FIG. 14
is an enlarged view of only the fluid seal portion of FIG.
13
. Usually, the magnetic fluid
31
is stored between the upper and lower pole pieces and also in the annular concave portion formed on the inner peripheral side of the annular magnet
32
. At a standstill, the magnetic fluid
31
is held forming the meniscus indicated by the broken line in FIG.
14
.
In recent years, with the increase in the processing speed of personal computers and the like, storage devices used in them, such as magnetic disks, are increasing in storage capacity and the writing/reading speed for storing information is rising by leaps and bounds. In response to such a situation, the magnetic disk or the like is required to have a rotational speed, of for example, more than ten thousand rotations per minute. In such a case, the magnetic fluid held between the inner peripheral end portion of the upper pole piece
30
and the outer peripheral surface of the shaft is attracted by a rotational centrifugal force in the radial outward direction, and forms a meniscus, as indicated for example, by the solid line in FIG.
14
. That is, when the spindle motor is at a standstill, the magnetic fluid
31
forms the meniscus indicated by the broken line, but when attracted by the rotational centrifugal force, for example, a part of the upper portion of the meniscus
311
flows up to the magnetic fluid part
312
on the inner end portion of the upper pole piece
30
, while a part of the lower portion of the meniscus
313
is pushed back to the part
314
within the annular concave portion on the inner side of the upper and lower pole pieces. As a result, the sealing layer, formed by the magnetic fluid, held between the inner end portion of the upper pole piece
30
and the outer surface of the shaft becomes thin, and the sealing capability, especially the barrier pressure of the seal decreases. In addition, this location is when the barrier pressure of the seal is at the maximum equivalent pressure required to prevent oil mists and the like from flowing out through the fluid seal.
A magnetic fluid seal shown in
FIG. 13
has a thickness in the axial direction of several millimeters (e.g., 5 mm or 3 mm), that is, this magnetic fluid seal is a very thin precision component, although it is a complicated component comprising upper and lower pole pieces
30
and
33
, an annular magnet
32
, a holder
35
, and so on. Manufacturing thereof, therefore, requires numerous precision processes, a large number of man-hours, and a high cost.
Furthermore, in response to the recent reduction in thickness of note-book personal computers and the downsizing or weight-reduction thereof intended for an improvement in portability, the downsizing tendency of storage devices, such as magnetic disks and the like, is being increasingly promoted, and a corresponding decrease in thickness of the motors used in these storage devices also is being desired. With respect to the fluid seals, it is therefore also an essential condition to make these thinner than conventional ones.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a low-profile fluid seal capable of stably maintaining a sealing capability with respect to a high-speed rotation.
It is another object of the present invention to provide a fluid seal having a simple structure and being capable of meeting a high-speed rotation.
It is still another object of the present invention to provide a fluid seal capable of being manufactured with a small number of man-hours and at a low cost.
It is a further object of the present invention to provide a low-profile and low-cost spindle motor capable of meeting a high-speed rotation, and to provide a high-speed rotating disk drive using this motor.
In order to achieve these objects, a fluid seal in accordance with the present invention comprises an annular body having an annular sealing surface confronting the outer sealing surface of a motor shaft with a minute gap therebetween, and a fluid held in this minute gap. In this way, a fluid seal is constructed which has a much simpler structure than that of a conventional magnetic fluid seal.
In addition, in a fluid seal in accordance with a particular embodiment the present invention, the annular sealing surface of the annular body has an approximately concave shape in cross-section. In other words, the nearer to the upper or lower surface of the annular body in

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