Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
2001-04-11
2003-04-08
Knight, Anthony (Department: 3676)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C277S411000
Reexamination Certificate
active
06543782
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to magnetic fluid rotary seals designed to provide a pressure barrier in a variable pressure environment, and, more particularly, to magnetic fluid seals designed to prevent bursting during operation.
BACKGROUND OF THE INVENTION
Magnetic fluid rotary seals have been used for many years in different environments. The prototypical seal device of this type is described in U.S. Pat. No. 3,620,584. A comprehensive review describing numerous applications and modifications of the seal is presented in Chapter 5 authored by Kuldip Raj in
Magnetic Fluids and Applications Handbook
, B. Berkovski, Ed., Begell House, Inc., New York (1996). A treatment describing the synthesis, make-up, fluid and magnetic properties, and flow of magnetic fluids is given in the monograph
Ferrohydrodynamics
, R. E. Rosensweig, Cambridge University Press, New York (1985), reprinted by Dover Publications, Inc., Mineola, N.Y. (1997).
As stated in the aforementioned publications, and as used herein, a magnetic fluid is an ultra-stable colloidal dispersion of approximately 10 nm size magnetic particles in a liquid carrier. Such colloidal magnetic fluids are also known as “ferrofluids.” The particles are sufficiently small that they are prevented from settling in gravitational or magnetic fields by thermal motion. A surface coating of adsorbed surfactant(s) or electric charges prevents agglomeration of particles to one another so that the colloids are stable over a long period of time.
Such a fluid may be used to create a gas tight seal. In prior art seal devices, magnetic fluid is retained as a ring in a gap, for example, a gap surrounding a cylindrical rotating shaft, by a magnetic field in the gap. The magnetic fluid serves as a barrier to the passage of gas along the shaft while permitting rapid rotation of the shaft, if desired. For a given magnetic field, the amount of pressure differential across the fluid ring that the seal can support is primarily determined by the magnetic field intensity in the gap.
FIG. 1
illustrates components and magnetic field lines of such a conventional magnetic fluid seal having permanent magnet
1
with annular, permeable pole pieces
2
and
8
defining gaps, such as gaps
3
and
7
, between pole pieces
2
and
8
and permeable rotating shaft
4
. Magnetic lines of flux
5
circulate through the magnetic circuit formed by magnet
1
, pole piece
2
, gap
3
, shaft
4
, gap
7
and pole piece
8
. The magnetic flux lines
5
concentrate to a high intensity in the gaps defined by a tooth, such as tooth
6
. Magnetic fluid is magnetically retained in discrete rings such as rings
9
,
11
and
13
circling the shaft
4
and bridging the gaps
3
and
7
between teeth and a pole piece.
The rings
9
,
11
and
13
of magnetic fluid prevent the flow of gas under pressure from a region
14
to a lower pressure region
15
. A ring of magnetic fluid is referred to as a seal “stage.” The seal stages are separated by gas filled interstage regions, such as regions
10
and
12
. In variations of this conventional seal, the teeth may be recessed in the shaft rather than extending from the surface, teeth may be located on a pole block opposite the smooth surface of a shaft, teeth may be present at the gaps of both pole blocks, teeth may be tapered or otherwise shaped, and multiple magnets may be employed. For simplicity,
FIG. 1
omits bearings supporting the shaft, housing, static seals, retaining rings, etc. that are part of a total seal package, as these elements are well known to one skilled in the art.
In magnetic fluid seals, the magnetic field intensity in the gaps is, in turn, determined by the configuration of the magnetic circuit that generates the field. The intensity of magnetic field established in the gap depends on the magnetomotive force of the magnet and the magnetic reluctance of the magnetic circuit elements and is analogous to the flow of current in a resistive electrical circuit containing a source of electromotive force, as is well known in the design of magnetic and electrical systems. In conventional seals, the magnetic circuit insures that the seals operate with a constant magnetic field in the seal gaps.
A single fluid ring, which constitutes one stage, can withstand only a limited pressure differential, and when this differential is exceeded, the ring “bursts.” When a burst occurs, a leakage path develops through the fluid ring and allows gas to pass by the seal. This process is illustrated in
FIGS. 2A-C
. As shown in
FIG. 2A
, with no pressure differential across the stage, the cross-section of a ring of magnetic fluid is symmetrically positioned, bridging the gap between a pole piece and the shaft. When a pressure differential is applied across the seal stage, the ring of fluid is displaced toward the low-pressure side as shown in FIG.
2
B. Due to fringing, magnetic field intensity is weakest at the shaft surface. When the pressure difference is excessive, a leakage path opens up adjacent to the shaft surface with the magnetic fluid lifted away from the surface. The bursting condition is illustrated in FIG.
2
C.
Accordingly, a seal designed to support a high-pressure differential is typically equipped with multiple stages of rings arranged in series longitudinally on the shaft. Upon initial exposure to the pressure differential, the outer seal rings are exposed to large pressure differences and a seal ring may burst temporarily and distribute excess pressure to the next stage. When pressure-holding capacity of that next stage is exceeded, the ring associated with that stage bursts and permits transfer of gas to the subsequent stage. This process continues until the seal stages reach equilibrium.
After a seal stage bursts, the pressure difference across the seal stage is reduced and the integrity of the fluid ring in that stage is restored as the fluid in the gap reseals itself. Thus, after the initial application of pressure across the seal, the seal stages will reach equilibrium and reseal them selves. However, each seal stage that has burst will subsequently operate near its burst condition. Later, if a pressure fluctuation occurs that increases the pressure difference across a stage, or if a condition develops that decreases the pressure holding capacity of a stage (for mechanical, thermal, magnetic or other reasons), the seal stage may burst during operation. This second burst may then release a volume of gas trapped in the interstage region into a process chamber, e.g. a vacuum chamber in which integrated circuits are fabricated and the gas can be detrimental to the processing operation.
FIGS. 3A-3D
illustrate the interstage pressurization conditions in a conventional magnetic fluid seal used in a material processing system. The figures show the seal during sequential processing steps in an environment where the pressure varies between vacuum and atmospheric pressure.
FIG. 3A
depicts the seal prior to establishing vacuum at one end. The interstage regions all hold air at one atmosphere pressure. The distribution of interstage pressurization after pumpdown is illustrated in FIG.
3
B. Typically, about 3 psi of pressure differential is established across each seal stage that previously has burst and resealed as discussed in relation to
FIG. 2. A
number of stages remain in the unburst condition with no pressure difference across them. They furnish a reserve or margin of safety and ensure long life of the seal should magnetic fluid evaporate or otherwise be removed from the working stages. During the course of processing, the vacuum vessel may be backfilled, or re-pressurized, in order to remove processed material.
FIG. 3C
illustrates the distribution of interstage pressures following a backfill. Following a subsequent pumpdown, the interstage pressurizations return to their previous pumped down values as shown in FIG.
3
D. It will be understood that the pressure changes presented are notional and intended for illustration only, and may not coincide with conditions in an actual seal device.
Prior-ar
Black, Jr. Thomas J.
Raj Kuldip
Rosensweig Ronald E.
Ferrotec (USA) Corporation
Peavey E
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