Abstract:
A permanent magnet assembly, for engaging a generally planar surface of a ferromagnetic object to affix items of interest thereto, includes a shell of a magnetic material. The shell has a portion of either a magic sphere or a magic cylinder and a cavity and the shell terminates in a surface that is generally planar for engaging a ferromagnetic object. The permanent magnet assembly also has an insert located in the cavity. A method of fabricating the permanent magnet assembly is also presented.

Description:
GOVERNMENT INTEREST 
   The invention described herein may be manufactured, used, imported, sold, and licensed by or for the Government of the United States of America without the payment of any royalty thereon or there for. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to simple, permanent magnets and, more particularly, to simple, permanent magnets having an increased magnetic force. 
   2. Related Art 
   There is considerable use for simple, permanent magnets of the horseshoe and bar magnet varieties. Common uses include simple paper anchors on refrigerators, field modifiers for more complex magnet systems, the retrieval of iron and other magnetic objects, location indicators for surgical implements, and machine shop anchors and dogs. Generally, these magnets are very limited in the strength of the magnetic field that they can generate. 
   Examples of permanent magnets having a relatively high field strength but that extends within an interior cavity of the magnet include permanent magnets configured in the form of a magic sphere or a magic cylinder. U.S. Pat. No. 5,216,401 describes each of these permanent magnet structures. 
   One way to further increase the field strength located within an interior cavity of a magic sphere or magic cylinder type of a permanent magnet, is to employ a magnetic insert. U.S. Pat. Nos. 5,382,936, 5,428,334 and 5,428,335, each of which are incorporated by reference herein, describe a permanent magnet assembly comprising a cylindrical or spherical shell of magnetic material having a hollow central cavity. The shell is permanently magnetized to produce a magnetic field within the central cavity and a magnetic insert is disposed in the central cavity to enhance the magnetic field therein. 
   SUMMARY OF THE INVENTION 
   In accordance with an embodiment of the present invention, a permanent magnet assembly, for engaging a generally planar surface of a ferromagnetic object to affix items of interest thereto, comprises a shell of a magnetic material. The shell comprises a portion of either a magic sphere or a magic cylinder and a cavity and the shell terminates in a surface that is generally planar for engaging a ferromagnetic object. The permanent magnet assembly also comprises an insert located in the cavity. 
   In accordance with another aspect of the present invention, a method of fabricating a permanent magnet assembly, for engaging a generally planar surface of a ferromagnetic object to affix items of interest thereto, comprises providing a shell of a magnetic material that comprises a portion of either a magic sphere or a magic cylinder and wherein the shell also comprises a cavity and terminates in a surface that is generally planar for engaging a ferromagnetic object; and locating an insert in the cavity. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description is made with reference to the accompanying drawings, in which: 
       FIG. 1  is a diagrammatical, cross-sectional view of a prior art magic sphere having a cavity through which a uniform magnetic field extends; 
       FIG. 2  is a diagrammatical, cross-sectional view of a prior art magic sphere having a cavity and an access bore and an iron insert located within the cavity; 
       FIG. 3  is a diagrammatical, cross-sectional view of a permanent magnet assembly comprising a permanent magnet shell, an insert and a plate in accordance with an embodiment of the present invention; and 
       FIG. 4  is a diagram showing a prior art, solid permanent magnet hemisphere that is mounted to a plate. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The present invention concerns a permanent magnet assembly that has a significant increase in magnetic field strength over that of prior art horseshoe and bar magnets. In one embodiment, the permanent magnet assembly may comprise a modified magic sphere or magic cylinder which provides for a relatively large magnetic field that extends away from the magnet rather than internal to it. In this way, the permanent magnet assembly may function as a powerful lifting or securing magnet that is many times stronger than a prior art horseshoe or bar magnet while maintaining a relatively small structural mass and bulk. 
   Referring now to  FIG. 3 , a permanent magnet assembly in accordance with one embodiment of the present invention is illustrated generally at  10 . In this embodiment, the permanent magnet assembly  10  comprises a shell  12  and an insert  14  which together may be attracted to an item of interest such as a plate  16 . The shell  12  may be composed of a magnetic material such as a cobalt-rare earth or a rare earth-iron and may comprise an arcuate portion  17  (diagrammatically shown in cross section) of, e.g., a magic sphere or a magic cylinder as described in U.S. Pat. No. 5,216,401 and incorporated herein by reference. Briefly and in accordance with a magic sphere or cylinder, the shell  12  is composed of a magnetic material that is permanently magnetized in a direction such that the remanence varies continuously about a polar angle of the shell. The shell  12  is shown for illustrational purposes as that of a portion of a magic sphere, although, it will be appreciated that a magic ring or cylinder may instead be employed to form a horseshoe-like outer configuration. 
   The shell  12  may also comprise a generally flat portion  18  and a cavity  20 . The insert  14  may be configured to fit within the cavity  20  and may also include a generally flat portion  21 . The insert  14  may comprise a ferromagnetic material such as iron. The plate  16  is shown for illustrational purposes and may represent a portion of a refrigerator door or other such surface to which it is desired to releasably affix items (not shown), such as exemplary homework assignments or other items that would heretofore require a fastener. 
   The magnetization of the shell  12  is represented in direction by arrows  22  that may rotate in direction about the periphery of the shell and a uniform magnetic field represented by arrow  24  is disposed within the cavity  20 . 
   For a better understanding of the present invention and the magnetic field extending from and about the shell  12 , it may be first useful to describe the magnetization of a magic sphere, as shown in  FIG. 1 , and thereafter describe the combination of a magic sphere and a magnetic insert as shown in  FIG. 2 . It will be appreciated that a magic cylinder, is similarly combinable with a magnetic insert to form a horseshoe-like magnet, although, for brevity this is not described in detail. 
   Referring to  FIG. 1 , a cross section of a magic sphere  28  having a shell  30  including a cavity  32  is shown. A magnetization of the shell  30  is represented by arrows  34  and disposed within the cavity  32  is a uniform field (H), represented by arrow  36 , which may be found by equation (1).
 
 H= 4/3( B   r   In (r o /r i ))  (1)
 
where:
         B r  is the magnetic remanence; and   r o  and r i  are an inner and outer radii, respectively.       

     FIG. 2  illustrates a cross section of a magic sphere  38  that includes a shell  40  and an insert  42  that is disposed within a cavity  44 . The shell  40  may comprise a magnetic material similar to that of the shell  12  and the insert  42  may comprise a ferromagnetic material such as iron. An arrow  46  represents the magnetization of the insert  42  and arrows  48  represent a magnetization of the shell  40 . Access ports  50  communicate with a bore or a gap  52  that extends through the insert  42 . Arrow  54  represents a uniform magnetic field extending through the gap  52 . Further details of a magic sphere similar to that of magic sphere  38  may be found in U.S. Pat. Nos. 5,382,936, 5,428,334 and 5,428,335 previously incorporated herein by reference. 
   The total force (F) of attraction to a passive material (such as plate  16 ) may be found, under a known principle of virtual work, by taking a derivative of energy stored in the gap  42 . Accordingly, the force (F) then may be represented by the energy density in the gap  42  multiplied by a cross-sectional area (A) of the gap as reflected in equation (2).
 
 F= ( B   2 /8π)(A)  (2)
 
where:
         B is a flux density (in Gaussian units);   (B 2 /8π) is an energy density; and   A is a cross-sectional area.       

   In view of the foregoing and referring again to  FIG. 3 , the force (F) of attraction for the insert  14 , using, for example, an r o /r i =2, r i =1 cm for the flat portions  18  and  21  and an average field (B) that may be approximately 25,000 Gauss and or 2.5 Teslas for the permanent magnet assembly  10 , may be found by equation (3):
 
 F   (insert) =(25000) 2 /8π)(π)(1) 2    (3)
 
 F   (insert) =176 Pounds.
 
   Next, a force (F) of attraction exerted by the shell  12  may be found given that an area of the flat portion  18  may be found by π(r o   2 −r i   2 )=π(2 2 −1 2 )=3π. Thereafter the force (F) by the shell may be determined by equation (4).
 
 F   (shell) =((25000) 2 /8π)(3π)  (4)
 
 F   (shell) =528 Pounds
 
   In sum, the total force (F) of attraction for the example permanent magnet assembly  10  is 528+176 or 704 pounds. It will be appreciated that the dimensions of the example permanent magnet may be reduced to provide for a suitable total force of attraction depending on the application. 
   Referring now to  FIG. 4 , a solid, conventional, permanent magnet  56  comprising a hemispherical shape and being attracted to a plate  58  is shown. The permanent magnet  56  has a direction of magnetization represented by arrows  60  and produces an anti-image  62  and magnetic field in the direction of arrows  64 . The permanent magnet  56  may comprise the same material as that of the shell  12 , described above and r=2 cm. 
   The total force (F) of attraction for the example permanent magnet assembly  10  may compared to the force (F) of attraction of the solid conventional permanent magnet  56  that is of the same dimension and material magnetized (arrows  60 ) perpendicular to the base which found by equations (5) and (6).
 
 B=B   r (2/3)  (5)
         B=9.33 kiloGaus
 
 F= (9,333) 2 /8π)(4π)  (6)
   F=97.9 Pounds
 
Accordingly, the example permanent magnet assembly  10  is approximately 704/97.9 or 7.2 times as strong as the conventional permanent magnet  56 .
       

   While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to these herein disclosed embodiments. Rather, the present invention is intended to cover all of the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.