Seal for a joint or juncture – Seal between relatively movable parts – Close proximity seal
Reexamination Certificate
1998-11-03
2001-03-13
Knight, Anthony (Department: 3626)
Seal for a joint or juncture
Seal between relatively movable parts
Close proximity seal
C310S090500
Reexamination Certificate
active
06199867
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention pertains to rotary motion feedthrough devices which are sealed by magnetic fluid (“ferrofluid”). Such devices commonly employ a magnetic pole piece assembly to provide suitable magnetic flux in a set of annular gaps disposed axially along a rotating shaft.
FIGS. 1A and 1B
taken from U.S. patent application Ser. No. 08/940,777 referenced above show a typical example of a rotary motion feedthrough structure
100
of the prior art. Five pole piece rings
20
are arranged in a stack with four ring magnets
18
to form a pole piece assembly
16
. The entire assembly is mounted within a housing
10
, which also supports a shaft
14
and a bearing
12
assembly.
Shaft
14
has an outside diameter slightly smaller than the inside diameter of pole rings
20
, so a small annular gap
22
exists between each pole ring and the shaft. This gap is typically 0.002 inch in radial dimension. Ferrofluid fills each gap, being held in place by magnetic forces.
A sealing material (not shown) fills empty spaces
21
(
FIG. 3
) between the magnets
18
and pole rings
20
, preventing leakage from the outer diameter of the pole rings radially inward to the ferrofluid sealing region. It is necessary to provide static sealing (as will be described below) at every one of the eight interfaces between pole rings
20
and ring magnets
18
. If this is not done, each of the seals made by the eight fluid rings would be bypassed by gas leaking across the pole ring/magnet interfaces. This would result in the full pressure differential (typically 1 atmosphere) appearing across the final fluid ring at the left end of the pole piece. Since it is not possible to support this much pressure difference across a single fluid ring, the seal would fail. If carefully formulated and applied, the sealing material also serves to provide mechanical retention of the magnets
18
in their proper locations. A single O-ring seal
30
provides static sealing between the pole piece assembly
16
and housing
10
at the vacuum side of the pole piece.
The five pole piece rings
20
must be precisely aligned (typically within 0.0005″) with each other and with the axis of the rotating shaft in order to produce eight annular gaps
22
which can be filled with ferrofluid. This alignment is accomplished during assembly of the pole piece by mounting the pole rings
20
on a fixturing shaft (not shown) having a diameter which matches the inner diameter (ID) of the pole rings very closely (typically within 0.0002″). The stack of pole rings and magnets is then held on the fixture and a static sealing material (typically epoxy resin and hardener) is applied and allowed to cure. Curing time is usually several hours.
FIG. 2
is an isometric view of a typical single pole ring
20
of the prior art with a circular array of short cylindrical magnets
18
A placed on one surface. Although a single ring magnet could be used, an array of small magnets is often used instead, because many different seal sizes can be made using only one or two types of standardized small magnets, thereby simplifying production planning and inventory control. Typical magnet dimensions are 4.5 mm or 9 mm diameter and 2 mm high. Enough magnets are placed in each layer to occupy substantially the entire space available. It is clear from
FIG. 2
that a lot of empty space must be filled with sealing material.
Close examination of
FIG. 2
also reveals that a small raised rim
15
exists at the outer diameter of the pole ring. Each magnet has been placed so that it abuts the inner diameter of this rim. The rim is required because the magnets exert mutually repulsive forces on each other, tending to push all magnets radially away from the axis of the pole ring. This force becomes particularly significant as the last magnet is placed on the ring. If there were no retaining rim, one or more magnets might move radially outward and protrude beyond the outer diameter of the pole ring.
FIG. 3
shows a stack of four pole rings
18
A and their associated magnet layers in a complete pole piece assembly. The topmost pole ring has been omitted for clarity.
SUMMARY OF THE INVENTION
In accordance with the invention a rotary motion feedthrough device is provided for coupling rotary motion from a high pressure (atmospheric) environment to a low pressure (vacuum) environment. The device is characterized by a unitary pole piece construction. The unitary pole piece is formed of a single cylindrical member having an inner and outer diameter and is made from a ferromagnetic metal, such as, stainless steel. Slots extending radially outward from the inner diameter of the member are filled with one or more magnets, the magnets in each slot having the same polarity, while magnets in alternate slots have opposite polarity. Magnetic pole tips are formed in the inner diameter laterally adjacent the slots. Ferrofluid (magnetic fluid) is contained in the space between the pole tips.
A rotatable shaft extends along the inner diameter of the pole piece in close proximity thereto and a stationary housing encircles the pole piece. The magnetic flux generated by the magnets is coupled to the fluid in the tip spaces and creates a non-rotating dynamic gas seal between the rotatable shaft and a housing which coaxially encircles the pole piece.
A groove for accepting an O-ring seal is formed on the outer diameter of the pole piece at an end of the pole piece disposed nearest the low pressure environment. Optional water cooling channels and O-ring sealing channels may be formed in the outer diameter of the pole piece to the extent water cooling of the device is desired.
Problems which are inherent to the prior art of
FIGS. 1A
,
1
B,
2
and
3
include cost, reliability, processing time, precision of alignment and uneven spacing of magnets. These problems are discussed in detail below.
Each pole ring must be produced to the required accuracy, and must be inspected to assure that it conforms to the requirement. The required assembly fixture must be produced to even tighter tolerance than the rings. The assembly process requires skilled labor. These are all costly aspects of the prior art. Because a pole piece constructed according to the prior art contains many individual pieces, the reliability of the assembly is reduced. If any portion (pole ring, magnet, sealing material) is defective, the reliability of the entire assembly is compromised.
Reducing the parts count increases the reliability of the whole. It is necessary to leave the entire assembly on the fixture during the curing cycle of the sealing material. Typically, overnight curing at room temperature is employed. This means that work in process is increased and that multiple fixtures may be required for pole pieces which are in large volume production. In addition to the obvious cost implications, these considerations result in a less flexible production environment.
Because there is some tolerance on the ID of pole rings, no two rings within a set will have exactly the same ID. Therefore, they cannot be perfectly aligned on a fixture. In most cases, alignment is good enough for practical purposes, but in extreme cases (e.g., extremely high speeds or minimum number of sealing stages) a closer approach to perfect alignment would be desirable. The multiple-piece nature of the prior art inherently limits how closely this art can approach perfection. Magnets should be evenly spaced when placed on the pole rings. If they are not, the overall magnetic field will be uneven and some deviation in seal properties (e.g., reduced pressure capacity) may be observed.
The present invention addresses and resolves four of the above referenced difficult aspects of the prior art: (1) precise axial alignment of sealing stages, (2) static sealing of interstage bypass leakage, (3) retention and radial distribution of magnets, and (4) additional size and cost incurred if water cooling of the seal is required.
As previously noted, the prior art pole piece assembly usually includes two to five pole rings and one or more mag
Helgeland Walter
Mahoney David G.
Hamilton Brook Smith & Reynolds P.C.
Knight Anthony
Peavey Enoch E
Rigaku/USA, Inc.
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