Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
1999-11-22
2001-10-23
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C384S133000
Reexamination Certificate
active
06305694
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to improvement of a ferrofluid sealing device for use in, e.g., a spindle motor that drives a magnetic recording medium utilized in the information industry.
2. Description of the Prior Art
A conventional ferrofluid sealing device is constructed as shown in FIG.
7
. That is, a pair of pole pieces
1
,
2
made of magnetic bodies, each having a shape of a doughnut-like disk, sandwiched between their inner surfaces is a magnet
3
shaped into a doughnut-like disk and having an inner diameter larger than that of either of the pole pieces; the outer peripheral faces thereof are fixed to a housing
6
; a shaft
7
made of a magnetic body is inserted to the center holes of the doughnut-like pole pieces; and a ferrofluid (a magnetic fluid)
5
is retained, by a magnetic circuit formed by the magnet
3
, in a gap between this shaft
7
and a front end surface
4
of the outer pole piece
1
facing the shaft and located on the side where the ferrofluid is retained. Reference numeral
8
denotes a ball bearing.
In the ferrofluid sealing device described above, not all of the magnetic flux generated by the magnet
3
can flow from the pole pieces
1
,
2
to the shaft
7
. Some of the magnetic flux leaks when it flows from the vicinity of the inner diameters of the pole pieces
1
,
2
to the shaft
7
, causing the ferrofluid
5
, which is injected to and retained in the gap between the shaft and the pole pieces
1
,
2
, to have a shape bulging axially from the shaft toward the pole piece
1
. The bulging ferrofluid splashes outward upon high-speed rotation of the seal device, which brings about inconveniences such as shortened lifetime due to reduction in amount of the ferrofluid
5
, or stained surroundings due to the ferrofluid
5
flowing out onto the outer surface of the pole piece
1
.
SUMMARY OF THE INVENTION
The present invention solves such problems encountered in the prior art, and provides a high-speed rotation ferrofluid sealing device that can prevent, even when the ferrofluid seal is operated at a higher rotation speed, splashing of ferrofluid over the surroundings, thereby avoiding staining the surroundings as well as prolonging its lifetime.
A first aspect of the present invention is a ferrofluid sealing device retaining a ferrofluid between a shaft and inner and outer pole pieces rotating in opposite directions, wherein grooves are formed on a shaft surface that faces inner diameter surfaces of the pole pieces.
In the above sealing device, the grooves are preferably formed at an outer position shortly from a front end of the outer pole piece on the side where the ferrofluid is retained, and at an inner position shortly from the front end of the outer pole piece; and a projection portion wider than the thickness of an end surface of the outer pole piece is formed between the grooves. More specifically, the grooves formed on the shaft surface are effective when located at a position about 0.05 mm high from the upper surface of the outer pole piece and at a position about 0.05 mm below the lower surface of the outer pole piece. Providing these grooves can cut down the leakage of magnetic flux toward the outer surface side of the outer pole piece, as well as heighten the density of the magnetic flux in a gap between the shaft and the pole piece. Therefore, the ferrofluid does not splash even in high-speed rotation, showing enough sealing effect.
A second aspect of the present invention is a ferrofluid sealing device retaining a ferrofluid between a shaft and inner and outer pole pieces rotating in opposite directions, wherein the outer pole piece on the side where the ferrofluid is retained is 1.5 to 18.0 times as thick as the opposite inner pole piece.
The outer pole piece may be thicker than the opposite inner pole piece along its full length, or only at the inner circumferential portion beyond a stepped portion for supporting a magnet. The stepped portion may be formed on both the inside and outside the outer pole piece or only on the inside.
Thus, making the outer pole piece thicker can increase the magnetic flux and suppress the leakage of magnetic flux toward the outer surface side, presenting an effect equal to that of the formation of the grooves describedabove. The range between 1.5 times and 18.0 times is determined as a result of many tests. Also, the contact area between the ferrofluid and the pole piece is increased, so that the resistance value is lowered improving ground effect for grounding static electricity.
A third aspect of the present invention is a ferrofluid sealing device retaining a ferrofluid between a shaft and inner and outer pole pieces rotating in opposite directions, wherein grooves are formed on a shaft surface that faces inner diameter surfaces of the pole pieces, and the outer pole piece on the side where the ferrofluid is retained is 1.5 to 18.0 times as thick as the opposite inner pole piece.
Employing together the structures of the first and second aspects above makes it possible to further cut down the leakage of magnetic flux and, accordingly, show the above effect.
A fourth aspect of the present invention is a ferrofluid sealing device retaining a ferrofluid between a shaft and inner and outer pole pieces rotating in opposite directions, wherein the outer pole piece on the side where the ferrofluid is retained is 1.5 to 18.0 times as thick as the inner pole piece, and a projection portion with a width narrower than the thickness of the outer pole piece on the side where the ferrofluid side is retained is formed on a shaft surface that faces an inner diameter surface of the outer pole piece.
The magnetic flux density in a gap between the pole pieces and the shaft can be heightened also with such a structure. Thus, the ferrofluid does not splash even in high-speed rotation, showing enough sealing effect.
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patent: 4357024 (1982-11-01), Raj
patent: 4386784 (1983-06-01), Banks
patent: 4407518 (1983-10-01), Moskowitz et al.
patent: 4445696 (1984-05-01), Raj et al.
patent: 4630943 (1986-12-01), Stahl et al.
patent: 5108198 (1992-04-01), Nii et al.
patent: 5165701 (1992-11-01), Koba
patent: 5876037 (1999-03-01), Ishizaki et al.
patent: 2087988 (1982-06-01), None
patent: 1-220777 (1989-09-01), None
patent: 7-310746 (1995-11-01), None
patent: 10-89490 (1998-04-01), None
patent: 63-180774 (1998-07-01), None
Unozawa Heiichi
Yamazumi Noboru
Ferrotec Corporation
Flynn ,Thiel, Boutell & Tanis, P.C.
Knight Anthony
Pickard Alison K.
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