Ferrofluidic seal with controlled leakage to minimize...

Details Ferrofluidic seal with controlled leakage to minimize... Ferrofluidic seal with controlled leakage to minimize...

2002-05-22

2004-05-18

Knight, Anthony (Department: 3676)

Seal for a joint or juncture

Seal between relatively movable parts

Close proximity seal

C277S411000

Reexamination Certificate

active

06736402

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to ferrofluidic seals and, in particular, to apparatus that ameliorates the effect of seal stage bursting in high-vacuum environments.
BACKGROUND OF THE INVENTION
Ferrofluidic rotary seals have been widely used in vacuum applications over the past 20 years. The basic structure of the seal comprises one or more magnets, a rotary shaft, pole pieces, and a housing. Additional parts may also be present as is known in the art. The magnets, the pole pieces and the rotary shaft form magnetic circuits with air gaps that occur between the pole pieces and the shaft. A ferrofluid is placed in the air gaps and forms a liquid O-ring rotary seal between the pole pieces and the rotary shaft. As used herein, a ferrofluid comprises magnetic particles coated with a surfactant that are suspended in a carrier liquid that may be water or oil. The magnetic particles are sufficiently small (approximately 10 nanometers) that they are colloidally suspended in the carrier liquid. A rubber O-ring at the radial interface usually provides a seal between the stationary parts, such as that between a pole piece and the housing. Seals with the above structure have been effectively used in a wide variety of applications, such as semi-conductor manufacturing, optical coating, rotary gas unions etc.
Due to the fact that a liquid forms the seal, the pressure capacity of a single seal stage is limited and is dependent upon the magnetic circuit design and the magnetization of the ferrofluid. Typically, one stage can withstand a 2-4 psi pressure difference across the stage without failure. Consequently, in applications that require a seal to withstand a pressure differential more than can be supported by a single stage, multiple seal stages are used.
A typical seal and bearing unit
100
with multiple seal stages is shown in sectional view in FIG.
1
. The unit
100
comprises a non-magnetic housing
102
that surrounds a rotary shaft
104
fabricated from a magnetic material. The ferrofluid seal is comprised of magnetic pole pieces
106
and
108
, magnet
110
and the shaft
104
. The seal pieces form a magnetic circuit indicated schematically by dotted box
115
. The pole pieces
106
and
108
extend close to, but do not touch, the shaft
104
to form small gaps between the pole pieces
106
,
108
and shaft
104
. The magnetic circuit
115
extends across gaps between pole pieces
106
and
108
. A ferrofluid
112
, located in the gaps is held in position by the magnetic field in the gaps. Rubber O-rings
130
and
132
seal the stationary pole pieces,
106
and
108
, respectively, to the housing
102
to support the pressure differential between a low pressure (vacuum) area
101
and a high pressure (which may be atmospheric) area
103
.
The seal unit
100
may also include a bearing assembly
128
that has one or more bearings
124
and
126
. The unit is completed by a cover plate
118
, fastened to an end of the housing
102
by clamping screws, of which screws
120
and
122
are shown in FIG.
1
.
Although there are only two pole pieces
106
and
108
, slots
116
are cut into the shaft to form multiple seal stages. Alternatively, slots may be cut into the faces of the pole pieces that oppose the shaft
104
to form the seal stages. At each seal stage the magnetic fluid forms a liquid O-ring, which provides a hermetic seal between the rotary shaft
104
and the stationary pole piece
106
,
108
. Thus, multiple seal stages are formed each of which can support a pressure differential. This arrangement is shown in greater detail in FIG.
2
.
In
FIG. 2
, the shaft
104
has a plurality of slots
116
cut into its surface, leaving a plurality of ring-shaped teeth
250
,
252
,
256
, etc. The teeth extend close to, but do not touch the inner surface
105
of pole piece
106
. Because the magnetic field in concentrated in the gaps between the teeth
250
,
252
,
256
and inner surface
105
of pole piece
106
, ferrofluid
112
is attracted to the gaps and forms a plurality of seal stages. Each of these seal stages will be referred to below by the numeral designation of the tooth that forms it. Interseal areas
254
,
258
, etc. exist between each seal stage.
During a pump down process in which a pressure differential is applied across the seal, the differential pressure across the first stage
250
facing the vacuum side of the seal is increased due to the vacuum. Once the differential pressure exceeds the pressure capacity of the first stage
250
, the ferrofluid
112
at the first stage
250
is temporarily pushed out of the gap and the seal stage “bursts” to relieve some of the pressure differential across the stage
250
. When the seal stage
250
bursts, the stage
250
allows part of the gas stored in the interstage area
254
between the first stage
250
and the second stage
252
to leak into the vacuum area
101
, thereby reducing the gas pressure in the interstage area
254
. Thus, the differential pressure across the first stage
250
is reduced while the pressure differential the second stage
252
is increased.
Eventually, the pressure differential across the second seal stage
252
will exceed the capacity of the seal stage and it too will burst, thereby decreasing the pressure differential across it and increasing the pressure differential across the first seal stage
250
and the third seal stage
256
. Sometimes the increase in pressure differential caused by a seal stage bursting can increase the pressure differential across an adjacent stage causing it to burst also. A “cascade” effect results until a volume of gas is released into the low-pressure area. Such a process continues between the stages during the pump down process, until the differential pressure between vacuum area
101
and the atmospheric area
103
is approximately equally shared by a plurality of seal stages. Each time the first stage
250
bursts, the pressure in the vacuum area fluctuates and the gas in the interstage area is released into the vacuum area.
Typically, the aforementioned bursting phenomenon is not harmful during the pump down process because at this time, a processing job inside the vacuum chamber that requires a high vacuum has not been started. This processing job can be wafer processing in semiconductor industry, thin film coating in the optics component industry or some other conventional processing job that requires high vacuum. However, various factors, such as shaft rotation and pressure variations can cause a seal to burst after pump down. If seal stage bursting continues while the processing job proceeds, the resulting pressure fluctuations and release of gas into the vacuum chamber is not desirable. In particular, the larger the amplitude of the pressure fluctuation, the more deleterious the consequences to the processing job. A typical pump down profile is illustrated in
FIG. 5
that shows the processing chamber pressure on the vertical scale versus time on the horizontal scale. As shown, with a conventional multiple stage ferrofluid seal, seal stage bursting can cause the pressure in a vacuum chamber to fluctuate over three orders of magnitude, for example, from 10
−7
Torr to 10
−5
Torr, then back to 10
7
Torr in a period of a few seconds as shown by the pressure spike
500
.
Therefore, there is a need for a ferrofluid seal structure that minimizes the impact of seal stage bursting during the processing phase of a job.
SUMMARY OF THE INVENTION
In accordance with the principles of the invention, a reservoir is created between the first seal stage at the low-pressure side and its adjacent seal stage. The volume of the reservoir is relatively large compared to the volumes of the interstage areas between the other seal stages. In addition, a controlled leakage path bypasses the first stage from the reservoir to the low-pressure area. The leakage rate through the bypass path is controlled so that gas in the reservoir leaks to the low-pressure area relatively slowly; for example, the bypass path might equalize the pressure across the

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