Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
2002-01-08
2004-01-06
Miller, William L. (Department: 3677)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C277S418000
Reexamination Certificate
active
06672592
ABSTRACT:
This is a nationalization of PCT/JP00/04600 filed Jul. 10, 2000 and published in Japanese.
TECHNICAL FIELD
The present invention relates to a magnetic fluid seal that seals various fluids using a magnetic fluid. For instance, it relates to a magnetic fluid seal that is applied as a shaft sealing apparatus used for a semiconductor manufacturing process, a vacuum process, various pneumatic devices or the like.
BACKGROUND ART
As a reciprocating seal for sealing a circular space between a shaft and a housing that reciprocate relatively, there has conventionally been used an elastic seal that is made of an elastic material such as rubber.
In the case where such an elastic seal is used, the elastic seal seals a target member that should be sealed while contacting the target member, and thus there occur problems concerning slide resistance and friction due to sliding motion.
On the other hand, there has been known a magnetic fluid seal as a seal that achieves a sealing effect in a non-contact manner, which does not cause the stated problems.
Magnetic fluid seals according to conventional techniques will be described with reference to
FIGS. 5 and 6
.
First, a magnetic fluid seal
100
shown in
FIG. 5
is provided to seal a circular space between a shaft
300
and a housing
400
.
This magnetic fluid seal
100
substantially includes: a circular permanent magnet
101
that is arranged so that one end thereof corresponds to a north pole and the other end thereof corresponds to a south pole in a shaft extending direction; and a pair of pole pieces
102
and
103
that are respectively provided on both sides so that the permanent magnet
101
is sandwiched therebetween.
Here, the housing
400
shown in
FIG. 5
is made of a non-magnetic material and a space between the magnetic fluid seal
100
and the interior surface of the housing
400
is sealed with O rings D
10
and D
20
.
On the other hand, the shaft
300
is made of a magnetic material and a space between the magnetic fluid seal
100
and the surface of the shaft
300
is sealed with a magnetic fluid J.
That is, there are formed a plurality of projections
102
a
,
102
b
, and
102
c
on the pole piece
102
and there are also formed a plurality of projections
103
a
,
103
b
, and
103
c
on the pole piece
103
. The magnetic fluid J is held by a magnetic force in a space between the tips of these projections and the surface of the shaft
300
, thereby achieving a sealing effect.
Also, in the same manner, a magnetic fluid seal
200
shown in
FIG. 6
is provided to seal a circular space between a shaft
301
and a housing
401
.
Like the magnetic fluid seal
100
shown in
FIG. 5
described above, this magnetic fluid seal
200
also includes a permanent magnet
201
and a pair of pole pieces
202
and
203
.
Here, the shaft
301
shown in
FIG. 6
is made of a non-magnetic material and a space between the magnetic fluid seal
200
and the surface of the shaft
301
is sealed with O rings D
30
and D
40
.
On the other hand, the housing
401
is made of a magnetic material and a space between the magnetic fluid seal
200
and the interior surface of the housing
401
is sealed with a magnetic fluid J.
That is, there are formed a plurality of projections
202
a
,
202
b
, and
202
c
on the pole piece
202
and there are also formed a plurality of projections
203
a
,
203
b
, and
203
c
on the pole piece
203
. The magnetic fluid J is held by a magnetic force in a space between the tips of these projections and the interior surface of the housing
401
, thereby achieving a sealing effect.
For instance, Japanese Utility Model Application Laid-Open No. Sho 55-20746 discloses a technique with which, between a rotary shaft and a fixed casing, a pair of circular ball blocks is fixed to bearings for holding the rotary shaft, a magnet is also provided between these paired circular ball blocks, and a magnetic fluid is held by this magnetic force between the rotary shaft and projections formed at tips of the circular ball blocks. In this manner, there is achieved a construction in which the rotary shaft is sealed with this magnetic fluid.
Also, Japanese Utility Model Examined Publication No. Sho 63-11427 discloses a technique with which, in a space between a casing and a rotary shaft supported by bearings within the casing, a magnetic fluid is held between the rotary shaft and projections formed on the surfaces of bushes that are fixed to the rotary shaft so that a magnet is sandwiched between these bushes and the rotary shaft. In this manner, there is achieved a construction in which a space between the rotary shaft and the projections formed on the bush surfaces is sealed with the magnetic fluid.
In the cases of the aforementioned conventional techniques, however, there occurs a problem to be described below.
As described above, in the case where a magnetic fluid seal is used, there occur no problem concerning slide resistance and friction due to sliding motion.
However, in the case of a magnetic fluid seal, there occurs a problem that a held magnetic fluid leaks, losing its sealing property.
This problem in which a magnetic fluid leaks rarely becomes serious in the case where a magnetic fluid is applied as a rotary seal for sealing a space between two members that rotate relatively to each other. However, in the aforementioned case where a magnetic fluid is applied as a reciprocating seal for sealing a circular space between a shaft and a housing that reciprocate relatively, it is difficult to prevent leakage of the magnetic fluid. This makes it difficult to apply a magnetic fluid to a reciprocating seal.
This point will be described in more detail by taking, as an example, the magnetic fluid seal shown in
FIG. 5
or
6
described above.
The length in the shaft extending direction of each of the permanent magnets
101
and
201
that are respectively shown in
FIGS. 5 and 6
is generally set to about 5 mm or longer so as to secure a magnetic force.
Accordingly, in the case of the magnetic fluid seal
100
shown in
FIG. 5
, there is maintained a distance having a substantially similar length between the projections
102
a
and
103
a
of the respective pole pieces
102
and
103
that are provided so that the permanent magnet
101
is sandwiched therebetween. As a result, there is formed a space indicated by reference symbol P in the drawing.
Also, in the same manner, in the case of the magnetic fluid seal
200
shown in
FIG. 6
, there is maintained a distance having a substantially similar length between the projections
202
a
and
203
a
of the respective pole pieces
202
and
203
that are provided so that the permanent magnet
201
is sandwiched therebetween. As a result, there is formed a space indicated by reference symbol Q in the drawing.
Meanwhile, the surfaces of magnetic materials forming these P space and Q space are areas having the strongest magnetic fields.
Accordingly, there are cases where a magnetic fluid that should be held by the projections
102
a
and
103
a
or by the projections
202
a
and
203
a
leaks to the surface of the magnetic material forming the P space or the Q space. As a result, it becomes impossible to secure a sealing property.
Also, in the case where the moving speeds of a shaft and a housing that reciprocate relatively are high, there may be a case where a magnetic fluid leaks outside of a magnetic fluid seal.
The present invention has been made to solve the aforementioned problem of the conventional techniques, and the object of the present invention is to provide a magnetic fluid seal, which reduces leakage of a magnetic fluid with an excellent quality.
DISCLOSURE OF THE INVENTION
To achieve the above-mentioned object, according to the present invention, a magnetic fluid seal for sealing a circular space between a shaft and a housing, comprises: a magnetic force generating member that is arranged so that one end thereof corresponds to a north pole and the other end thereof corresponds to a south pole in a shaft extending direction; and a pair of magnetic pole members that are respectivel
Jacobson & Holman PLLC
Miller William L.
NOK Corporation
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