Abstract:
A rotary motion feedthrough is described for coupling rotary motion from a rotatable shaft between an atmospheric side to a vacuum side by providing a dynamic magnetic seal of ferrofluid about the shaft using a non-rotating magnetic system formed of a unitary pole piece with magnets contained in radial slots formed in an inner diameter of the pole piece opposite the shaft.

Description:
RELATED APPLICATION(S) 
     This application is a Continuation-in-Part of application Ser. No. 08/940,777 filed Sep. 30, 1997, now U.S. Pat. No. 5,975,536 the contents of which are incorporated herein by reference in its entirety. 
    
    
     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 magnets. All of these elements must be assembled in a manner which establishes and maintains critical mechanical alignment among the elements. One method of establishing this alignment is to build up the pole piece as a subassembly using special fixtures. Another method is to use the feedthrough housing to provide the alignment at the time of final assembly. In both methods, sealing means must be provided to prevent interstage bypass leakage along the outside of the pole piece. If multiple small magnets are used, as is commonly the case, some means must also be employed to position the magnets correctly, and to retain them in that position during and after assembly. 
     This invention provides all pole rings as geometric features within a single machined part. The critical alignment within the pole piece is easily achieved by conventional machining operations, and is automatically built into the pole piece. No assembly fixture is needed to achieve this alignment. Accuracy requirements for the housing are also less critical. This invention also provides a continuous exterior wall with no breaks or openings separating sealing stages. Interstage bypass leaks cannot exist, so it is not necessary to provide sealing means for such leaks. Finally, this invention provides a simple, built-in means of correctly positioning and retaining the magnets during and after assembly. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
     FIG. 1A is a longitudinal partial schematic section of a prior art rotary feedthrough. 
     FIG. 1B is an exploded view of a portion of FIG.  1 A. 
     FIG. 2 is a perspective view of a single prior art pole piece ring with five circular magnets. 
     FIG. 3 is a view as in FIG. 2 of four separate pole piece rings of the prior art construction. 
     FIG. 4 is a longitudinal partial schematic section of a rotary feedthrough of the present invention. 
     FIG. 5 is an enlarged longitudinal partial schematic section of a unitary pole piece for the feedthrough of FIG. 4 of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Note, since the device is radially symmetrical, only the top half is shown in some of the drawings for simplicity. 
     Referring now to FIGS. 4 and 5 of the drawings a preferred embodiment of the invention is shown in which the entire set of pole rings and bypass seals is machined as a single piece. For example, a single piece  40  of ferromagnetic stainless steel, e.g., 17-4 PH alloy or 400-series stainless steel alloy is machined into a ring with an O-ring sealing groove  42  formed on the OD and magnetic pole tips  60  on the ID. The pole tips  60  at the ID of the unitary pole ring  40  are machined as a series of small V-grooves  63  in the ID of the single machined part  40 . The single machined part  40  is first made with a smooth bore at a carefully controlled diameter. Then large slots (for magnets) are machined into the ID. Then the series of V-grooves  63  are machined to a depth which leaves a small portion of the original ID intact between each pair of adjacent V-grooves. FIG. 5 shows two magnet slots  44  and a plurality of V-grooves  63  in an arrangement which results in small regions  60  which are left over from the original ID bore. These regions  60  are the pole tips. It is in the gap between these pole tips and the shaft that the most intense magnetic field develops, and it is here that the magnetic fluid (represented by “dots”  65  in FIG. 5) is retained by magnetic forces. Also machined into the ID of the pole piece ring  40  are a pair of slots  44  on either side of the central pole tips. The slots are large enough to accept magnets  46 . The slot width is slightly larger than the magnet thickness (e.g., 2.05 mm slot width for 2.00 mm magnet thickness). This permits easy insertion of magnets  46  and allows the magnets to move radially and longitudinally within thc slots. As more magnets are inserted, the mutually repulsive force serves to position each magnet equidistant from its neighbors, thereby automatically providing even spacing throughout the magnet layer. Magnets are added to each slot until the slot cannot accept any more magnets. 
     Typically the magnets are short cylinders, although they could also be quadrants, sextants, or octants. Rare earth magnets, such as SmCo or Nd B Fe with high energy products (20 to 35 MGO) are preferred to overcome the losses arising from the inherent shunting effect discussed below. Magnets are polarized through their thickness (parallel to the shaft axis). Within each magnet slot  44  the polarity is the same. From one slot to the next, the polarity alternates, so that alternate layers of magnets oppose each other. Any number of magnet layers can be used, but an even number is preferred (for cancellation of fringe fields). One layer is sufficient for all vacuum applications, although two are normally employed. For applications with larger pressure differentials, a greater number of layers can be used. Note that the outer surface of the pole piece  40  is continuous from the atmosphere side to the vacuum side. While it does contain grooves (three grooves are illustrated), it must not contain breaks which would connect from any interior region (e.g., magnet slots) to the OD. This precludes bypass leaks and also insures that all pole tip sealing stages will be very well aligned because they all will be made in the same final machining operation. 
     If water cooling is desired, an optional cooling water channel  48  on the OD of the pole piece may be provided, as illustrated, along with an O-ring seal channel  62 . Very simple water supply connections (not shown) via the housing. Only one water cooling channel is needed, and it is provided as a simple machined groove without increasing either the length or diameter of the pole piece  40 . In the prior art (FIG. 1) it is customary to increase the length of the outermost pole rings in order to provide water channels. In other competitive products two separate grooves are required because two separate pole pieces are employed. 
     The continuous outer surface of the pole piece  40  provides a magnetic shunt around each magnet. This dissipates some of the magnetic energy which would otherwise be available to the magnetic circuits which contain the sealing gaps. The situation is the mirror image of that described in the previously referenced parent application, application Ser. No. 08/940,777. In that invention, ferrofluid sealing is accomplished on the OD of a rotating shaft containing magnets in slots, with the interior of the shaft serving as a magnetic shunt. In the current invention, sealing is accomplished on the ID of a stationary pole piece containing magnets in slots, with the exterior of the pole piece serving as a magnetic shunt. In both cases, there is sufficient magnetic energy in the permanent magnets in the slots to provide high flux density in the sealing gaps, despite the shunting effect. 
     A ferrofluid  65  is provided in the tips  60  and the pole piece  40  is affixed to the housing  50  and the housing affixed to a flange (not shown) as described in the parent Helgeland reference U.S. Pat. No. 5,826,885 incorporated herein it its entirety by reference. In turn, the flange can be affixed to a suitable fixture disposed between the two atmospheres with the shaft  80  extending therebetween. 
     Equivalents 
     While this invention has been particularly shown and described with references to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described specifically herein. Such equivalents are intended to be encompassed in the scope of the claims.