Abstract:
A magnet held in a magnet holder is constrained against radial or axial instability, or against both, by a shape on either component in contact with complementary shape on the other component, the shaped components generally defining a tab-and-slot arrangement. The magnet component may be a bonded magnet and in one embodiment may be formed in place by injection molding. The magnet also will exhibit improved magnet properties when magnetized to have lines of polarity matching a path defined by the bulk of the magnetic material as governed by the location of tabs on the magnet. The invention is useful in magnet-sensor assemblies found in industrial applications and in automotive applications such as power steering systems.

Description:
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/330,394 filed May 2, 2010 by Stanley Byron Musselman et al and entitled “MAGNET AND HOLDER ASSEMBLY HAVING IMPROVED ROTATIONAL AND AXIAL STABILITY”. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     None. 
     REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISC APPENDIX 
     None. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to magnets and magnet holders and, more specifically, to magnet and holder combinations having improved axial and rotational stability. 
     2. Background 
     Magnets have many uses in modern devices. Magnets are often used with sensors to detect the position of machine elements or to determine the speed, torque or direction of the movement of parts relative to each other. Such applications may be found in computers, printers and other electronic devices. Magnets also work with sensors in many automotive applications such as automatic brake systems. Magnet-sensor combinations also are an important part of smart remote actuators in automotive applications such as those that control air intake vents in turbochargers for diesel engines. Recently magnet-sensor combinations have been used extensively in electrically assisted power steering applications. 
     In most such applications the magnet is a bonded magnet that has been magnetized to have a pre-selected polarity. In many applications the magnet is supported and positioned during use by a magnet holder. In electronically assisted power steering applications, for example, such a magnet is retained by a holder on the end of a variable-speed motor shaft. The shaft spins the magnet-magnet holder assembly both clockwise and counter clockwise at speeds that can range up to 3,000 rpm or higher depending, for example, on the direction and speed the driver turns a vehicle&#39;s steering wheel. Sensors detect the steering wheel movement by detecting the speed and the direction of rotation of the magnet. 
     Rotational and axial stability of the magnet relative to the magnet holder is of great importance, especially when sensing of the magnet&#39;s rotation is used to control or to assist in steering an automobile. Sensing the speed and direction of a magnet&#39;s rotation typically involves sensing movement of the pre-determined polarity of the magnet as it rotates. Rotational slipping of the magnet in the magnet holder would cause the sensor to send incorrect information to a power assisted brake or to a steering mechanism. Likewise, axial movement of the magnet in the magnet holder, or axial separation of the magnet from the magnet holder, during operation would at least cause a sensor to send incorrect information to the controlled device and could prevent operation of the device. 
     There is a need for a magnet-holder assembly that provides improved axial and rotational stability of the magnet in the holder. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of this invention to improve the axial and rotational stability of a magnet relative to a magnet holder in which the magnet is retained. 
     It also is an object of this invention to improve the safety and reliability of assemblies comprising a magnet and a holder. 
     These and other objects are accomplished by the present invention. In one aspect the present invention is a device comprising a shaped magnet, at least a portion of which is in contact with the shaped surface of a magnet holder. The shaped surface of the magnet holder is generally complementary to the portion of the shaped magnet with which it is in contact such that radial movement between the shaped magnet and the magnet holder is constrained. Additionally the shaped magnet and magnet holder may have complementary shapes that constrain axial movement between them. 
     The device may include an adhesive layer bonding at least a portion of the shaped magnet to the surface of the magnet holder whereby axial movement of the magnet and the magnet holder relative to each other is constrained. 
     The magnet holder component of the present invention may be made from any useful non-magnetic material. Aluminum is normally the non-magnet material of choice because of its strength, availability, workability and relative cost. Other useful materials include, for example, high strength plastics and non-magnetic composites, including composites containing carbon fibers. 
     A shaped magnet useful in the present invention may be made from a variety of magnetizable materials. Powdered magnetic materials are preferred because of the relative ease of forming powdered magnetic materials into desired shapes compared with other useful materials such as iron and iron-containing compounds. Magnets of various shapes may be formed from powdered magnetic material and may be isotropic or anisotropic in nature. Bonded magnets having pre-selected shapes are known. Their manufacture in shapes generally suitable for use as shaped magnets in the present invention also is known. Shaped magnets have been made by methods including sintering, extrusion, injection and compression molding of magnet powders. Shaped magnets useful in the present invention can be made by any such method; however, the making of such shaped magnets is illustrated here by a brief description of the compression molding method. 
     A compression molded magnet is one type of bonded magnet. Typically, compression molded magnets are made from powdered magnetic materials. Generally speaking, the powdered magnetic material will be a compound that comprises rare earths, transition metals and boron. Magnetic materials include ferrites, samarium-cobalt, aluminum-nickel-cobalt, and neodymium-iron-boron type materials either alone or in combination. In recent years neodymium-iron-boron has been used for many bonded magnet applications. Preferably the compounds will be made from Nd, Pr, Fe, Co and B. Industrial use of powders as a component in the manufacture of powdered metal magnets has centered around Nd 2 Fe 14 B and its derivatives, such as Dy 2 Fe 14 B; Dy x Nd 2-x Fe 14 B; Pr 2 Fe 14 B, and Pr x Nd (2-x) Fe 14 B. As is well known in the art, cobalt may be substituted for all or part of the iron in the neodymium-iron-boron phase of the magnet. Other metals such as niobium, titanium, zirconium, vanadium, tungsten etc can be added to neodymium-iron-boron alloys to obtain desired magnetic properties. Other rare earth metals, such as, but not limited to, cerium, dysprosium, erbium, praseodymium and yttrium may be substituted for all or part of the neodymium. Part or all of the boron may be replaced by carbon, silicon or phosphorous. Other metals or nonmetals may be substituted for small portions of either the iron or the neodymium, and the relative proportions of the neodymium, iron, and boron may be varied slightly. Usually, Nd—Fe—B material for use in such pre-formed magnets is obtained by the rapid solidification process. Other methods, such as using hydrogen, also can be used to make these magnetic materials. 
     The particle size of useful powdered materials varies widely depending on particular applications. Typically, powdered metals useful in making compression molded bonded magnets have an average particle size of about 150 microns, although particle sizes ranging from about 20 to about 400 microns may be useful. Magnetic metal particles useful in the present invention are commercially available from, for example, Neo Materials Technologies (Magnequench), Toronto, Ontario, Canada. 
     Such powders typically are mixed with curable resins to form B-stage powders that are at least partially coated with an uncured resin system. The resin system may be a phenol novolac resin and a diamine crosslinker or hardener as described in the co-pending, commonly assigned U.S. application Ser. No. 12/152,383 filed May 13, 2008. As taught in that application other hardener systems can be used with the phenol novolac resin depending on the application and the curing method. Illustrative examples of other useful hardeners for phenol novolac resins are amines, polyamides, anhydrides, phenolic resins, polymercaptans, isocyanates and dicyandiamides which are selected depending on the application and the curing method. The resin system also may be, for example, bisphenol A-epichlorohydrin or a bisphenol F-epichlorohydrin epoxy resin system for example. The resin system also can be a thermoplastic material. 
     B-stage powders normally are used to form powdered metal shapes by a well-known compression process in which high pressure is applied to a pre-measured charge of the powder held in a die cavity. The pressure applied typically is about 60 tons/square inch. The resulting shape, referred to at this point as a B-stage item, is then cured by heating at atmosphere to a temperature sufficient to initiate crosslinking. Typically the curing temperature is about 170 degrees C. and is maintained for less than an hour. 
     As is taught in the co-pending, commonly-assigned U.S. patent application Ser. No. 61/308,941, filed Feb. 27, 2010, a shaped magnet also can be made by injection molding or extruding techniques and, optionally, can be sintered prior to assembly with the magnet holder. When a shaped magnet is made using injection molding techniques, the magnet material typically is applied in a viscous, fluid state to a cavity having a shape complementary to the desired shape of the shaped magnet. Upon curing, the magnet material forms a shape defined by the shape of the cavity. 
     In one manufacturing method the injection molded shaped magnet is removed from the cavity mold and assembled with a magnet holder having a shape complementary to the shape of the magnet. In a preferred manufacturing method the shaped magnet is formed in place in a cavity in the magnet holder. The injected magnet material, when cured, adheres to the walls of the cavity, forms a magnetizable shape that is complementary to the shape of the cavity and is thus constrained from radial separation from the magnet holder. 
     The magnets used in the present invention, however formed, are magnetized to have a pre-determined polarity by well-known means such as, for example, by the use of a U-Series Impulse Magnetizer available from Magnet-Physik Dr. Steingroever GmbH, Cologne, Germany. In a preferred embodiment the polarity is selected to coincide generally with the longest dimension of a tab extending from the shaped magnet because that dimension will normally will include the most magnetizable material. 
     The magnet and magnet holder may have any complementary shapes that, when in contact in the assembled device, will resist rotational movement relative to each other when the assembly is subjected to repeated, rapid reciprocal rotation by, for example, torque applied to the magnet holder. Any complementary shapes that will accomplish such resistance to relative rotation or slippage between the magnet and the magnet holder are useful in the present invention. Typically the complementary shapes will include some variety of a tab extending from one of the components that is generally complementary in shape to a slot or groove on the other component. Such a combination of shapes is often described as a “tongue-and-groove” or a “key-and-slot” arrangement. In a typical arrangement the tab is a portion of the magnet that extends into a complementary groove, slot or recess in the surrounding magnet holder. Alternatively the tab, tongue or key may extend from the wall of the magnet holder into a slot, groove or recess in the magnet that is held in the magnet holder. 
     Optimally the magnet and magnet holder also will have complementary shapes that constrain relative axial movement between them during use. The complementary shapes that constrain relative axial movement between the components also will normally be some variety of key-and-slot or tongue-and-groove combination. In some embodiments the shapes that constrain axial separation of the magnet and magnet holder will include interlocking of grooves and ridges in the components. In other embodiments axial separation of the components is constrained by a shape or a pattern of shapes formed in the walls of the magnet holder and filled with a cured injection molded magnet material. In one such embodiment a screw thread shape is machined into the magnet holder. A curable magnet material is injected into the magnet holder and cured to form tabs that are complementary to the screw threads. 
     In yet another embodiment the end of the magnet holder that presents the magnet to a sensor is partially or completely covered by an end portion of the magnet holder. In such an embodiment the walls of a generally tubular magnet holder are narrowed or crimped at one end to prevent axial movement between the magnet holder and the contained magnet. Alternatively, the end of the magnet holder may be completely closed. A completely closed end also can facilitate manufacturing of the assembly when injection molding is used to form the magnetizable material in the magnet holder. 
     It will be clear to those of ordinary skill in the manufacturing arts that the most desirable complementary shapes in high volume production will be governed by the relative cost and speed of available manufacturing techniques. 
     Constraint against relative axial movement between the components of the present device also may be obtained or enhanced by an adhesive layer that bonds the magnet to the magnet holder. Ideally the adhesive layer, when present, is of uniform thickness and is present between all areas of contact between the components. However, uniform coverage is not a requirement for the adhesive layer to be effective in constraining relative axial movement between the magnet and the magnet holder over long periods in high frequency reciprocal applications at elevated temperatures. The ambient temperature range at which the device of the present invention is expected to operate can affect the glass transition temperature (Tg) of a cured adhesive, its tensile strength throughout the range of typical operating temperatures, its hardness properties and its thermal shock resistance. Any adhesive material that provides the desired constraint may be used. Typically, the adhesive is a chemically or thermally curable resin. The selection of a particular adhesive material or system also can depend on the specific magnet and magnet holder materials. 
     Many adhesives resins may be used to constrain relative axial movement between the magnet and the magnet holder in the device of the present invention. Useful resins include, for example, phenol novolac resins, Bisphenol A modified phenol novolac epoxy resins, Bis A epoxy resins, hydrogenated Bis A epoxy resins, Bis F epoxy resins and dimmer acid modified epoxys. These resins can be used in combination with a variety of hardeners including diamine, dicyandiamide, polyamide, dodecenyl succinic anhydride, NMA, HHPA, TETA and MeHHPA. Other adhesive systems such as polyurethane and acrylic also can be used to form a cured adhesive layer between the magnet and the magnet holder. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1   a  shows a plan view of a magnet and magnet holder assembly of the prior art. 
         FIG. 1   b  shows in schematic perspective view a magnet of the prior art. 
         FIG. 1   c  shows in cross section along line A-A of  FIG. 1   a  an elevation of the prior art device of  FIG. 1   a.    
         FIG. 2   a  shows in plan view a device according to the present invention. 
         FIG. 2   b  shows in schematic perspective a magnet useful in the present invention having a tab extending from a surface opposite its face. 
         FIG. 2   c  shows in cross section along line B-B of  FIG. 2   a  an elevation of the assembly of  FIG. 2   a.    
         FIG. 3   a  shows a plan view of an embodiment of the present invention in which relative axial movement between the magnet and the magnet holder also is constrained. 
         FIG. 3   b  is a cross sectional view of the device of the present invention along line A-A of  FIG. 3   a.    
         FIG. 3   c  is a cross sectional view of the device of the present invention along line B-B of  FIG. 3   a.    
         FIGS. 4   a  and  4   b  show a top and bottom view, respectively of a device according to the present invention in which the magnet holder covers the injection molded magnet. 
         FIG. 4   c  shows a cross sectional view of the device of  FIGS. 4   a  and  4   b.    
         FIG. 5   a  shows a plan view of the device of the present invention in which an adhesive layer adds axial stability to the device of the present invention. 
         FIG. 5   b  is a cross sectional view of the assembly of the present invention along line A-A of  FIG. 5   a.    
         FIG. 5   c  is a cross sectional view of the assembly of the present invention along line B-B of  FIG. 5   a.    
         FIGS. 6   a ,  6   b  and  6   c  show a magnet and magnet holder assembly according to the present invention in which an injection molded magnet is restrained from both axial and radial movement relative to the magnet holder by the complementary shapes of magnet and magnet holder. 
         FIGS. 7   a ,  7   b  and  7   c  show a magnet and magnet holder assembly according to the present invention in which a sintered or bonded magnet is restrained from both axial and radial movement relative to the magnet holder by the complementary shapes of the magnet and magnet holder with the assistance of a potting adhesive. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The call-out numbers used in the following detailed description are listed below for ease of reference: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 10 
                 Prior Art Magnet 
               
               
                   
                 11 
                 Prior Art Magnet Holder 
               
               
                   
                 12 
                 Magnet Assembly 
               
               
                   
                 13 
                 Smooth-sided Depression 
               
               
                   
                 14 
                 Side of Magnet 10 
               
               
                   
                 20 
                 Magnet Assembly 
               
               
                   
                 21 
                 Shaped Magnet 
               
               
                   
                 22 
                 Tab 
               
               
                   
                 23 
                 Magnet Holder 
               
               
                   
                 24 
                 Slot 
               
               
                   
                 25 
                 Magnet Face 
               
               
                   
                 26 
                 Registration Flats 
               
               
                   
                 30 
                 Device 
               
               
                   
                 31 
                 Injection Molded Shaped Magnet 
               
               
                   
                 32 
                 Magnet Holder 
               
               
                   
                 33 
                 Slot 
               
               
                   
                 34 
                 Tab Portion 
               
               
                   
                 35 
                 Mounting Shape 
               
               
                   
                 40 
                 Assembly 
               
               
                   
                 41 
                 Injection Molded Magnet 
               
               
                   
                 42 
                 Magnet Holder 
               
               
                   
                 43 
                 Slots 
               
               
                   
                 44 
                 Radial Locking Tabs 
               
               
                   
                 45 
                 Axial Locking Threads 
               
               
                   
                 46 
                 Grooves 
               
               
                   
                 47 
                 Tongues 
               
               
                   
                 48 
                 End Portion 
               
               
                   
                 50 
                 Device 
               
               
                   
                 51 
                 Magnet 
               
               
                   
                 52 
                 Magnet Holder 
               
               
                   
                 53 
                 Tab 
               
               
                   
                 54 
                 Slot 
               
               
                   
                 55 
                 Adhesive Layer 
               
               
                   
                 60 
                 Assembly 
               
               
                   
                 61 
                 Magnet 
               
               
                   
                 62 
                 Magnet Holder 
               
               
                   
                 63 
                 shaped space 
               
               
                   
                 64 
                 Waist Portion 
               
               
                   
                 65 
                 Flange Area 
               
               
                   
                 66 
                 Shaped Void 
               
               
                   
                 70 
                 Device 
               
               
                   
                 71 
                 Magnet 
               
               
                   
                 72 
                 Magnet Holder 
               
               
                   
                 73 
                 Shaped Portion 
               
               
                   
                 74 
                 Shaped Portion 
               
               
                   
                 75 
                 Shaped Void 
               
               
                   
                 76 
                 Adhesive Potting Compound 
               
               
                   
                 77 
                 End Portion 
               
               
                   
                 78 
                 Shaped Void 
               
               
                   
                   
               
             
          
         
       
     
     Magnet  10  and magnet holder  11  of the prior art are shown in  FIG. 1   a . When assembled as shown in  FIG. 1   a  these components form a prior art magnet assembly generally designated  12 .  FIG. 1   b  shows prior art magnet  10  in schematic perspective view. Magnet  10  is seen to be a smooth, truncated cylinder that is held in an equally smooth-sided depression  13  in magnet holder  11 . Such prior art assemblies are used in magnet-sensor combinations such as those found in electrically assisted steering systems in vehicles. In such an application, magnet assembly  12  typically is placed on the shaft of a motor that spins clockwise and counter clockwise at speeds up to 3,000 RPM depending on the direction in which a vehicle&#39;s steering wheel is turned and on the speed at which it is turned. 
     The smooth, curved side  14  of prior art magnet  10  and the equally smooth sides of depression  13  contain no structures that might constrain radial movement between magnet  10  and magnet holder  11  during rapid, high speed reciprocation. Magnet  10  normally will be magnetized across its face as exemplified by the N-S line shown in  FIG. 1   b  so that the direction and speed of movement of the lines of flux resulting from the polarity of magnet  10  can be registered by a nearby sensor (not shown). Radial slippage of magnet  10  in depression  13  during rotation of assembly  12  will result in inaccurate readings of the speed and direction of movement of magnet  10 , as described above. 
     The magnet assembly generally designated  20  in  FIGS. 2   a  and  2   c  is made according to the present invention and resists radial slippage between shaped magnet  21  and magnet holder  23  during high speed reciprocal rotation of magnet assembly  20 . Magnet  21  is shaped to include tab  22  extending from the flat surface of magnet  21  opposite magnet face  25 . Magnet holder  23  includes slot  24  that is complementary to tab portion  22  of shaped magnet  21 . When tab  22  is positioned in slot  24  as shown in  FIGS. 2   a  and  2   c , the complementary shapes resist radial slippage relative to each other as assembly  20  spins at high speeds with sudden reversals of direction. 
     Registration flats  26  shown in  FIGS. 2   a  and  2   c  are used to align assembly  20  in a magnetizer when, after assembly, it is magnetized to have a pre-determined polarity. In preferred embodiments the polarity is of the magnet is aligned as much as possible with the length of a tab, such as tab  22 , so that a large amount of magnetizable material lies in a plane substantially parallel with the lines of polarity, resulting in gauss variability as shown in Example 2. 
       FIGS. 3   a ,  3   b  and  3   c  show a device  30  according to the present invention that is adapted also to provide axial stability. Device  30  includes an injection molded shaped magnet  31  assembled with shaped magnet holder  32 . During one exemplary manufacturing process, shaped magnet holder  32  is first formed as a closed end cup-like structure after which an oblong slot  33  is formed through the closed end. Slot  33  can be made by drilling overlapping holes through the closed end of holder  32 . 
     As is best seen in  FIGS. 3   a  and  3   c , slot  33  is longer on axis A-A than on axis B-B. Magnetic material is injected into holder  32  so that it fills slot  33  and is cured to form shaped magnet  31 . Shaped magnet  31  includes tab portion  34  that is complementary in shape to slot  33 . Tab portion  34  of shaped magnet  31  works with slot  33  to constrain relative radial movement between the components when device  30  is rotated at high speeds in reciprocal mode. 
     This embodiment of the present invention includes a mounting shape  35  at the end of assembly  30  that is illustrative of a variety of configurations that are useful in mounting assembly  30  on, for example, a motor shaft in industrial and automotive applications. 
     Another embodiment of a magnet assembly according the present invention that is constrained against both radial and axial slippage is shown in  FIGS. 4   a ,  4   b  and  4   c . The assembly, generally designated  40 , includes shaped magnet material  41  that has been injection molded and cured to form a shape complementary to the shape of the interior of magnet holder  42 . The interior of magnet holder  42  has been shaped by, for example, overlapping drilling to include slots  43  that are best seen in  FIGS. 4   a  and  4   c . The injection molded magnet material fills slots  43  and, when cured forms tabs  44  that are complementary in shape to the shape of slots  43 . Tabs  44  and slots  43  work together to constrain relative radial movement between magnet  41  and magnet holder  42  during high speed reciprocal rotation of assembly  40 . 
     The interior of magnet holder  42  also includes machined threads  45 . During injection molding, magnet material fills the depressions, or grooves  46 , that are a part of the thread structure. Upon curing, the material filling the grooves  46  between threads  45  forms complementary tabs or tongues  47  that work with grooves  46  to constrain axial relative movement between magnet  41  and magnet holder  42  during high speed reciprocal rotation of assembly  40 . 
     Grooves  46  are formed in the embodiment of  FIGS. 4   a ,  4   b  and  4   c  by producing threads on the inner surface of the magnet holder. Threads were chosen because the technology for producing threads on the interior of cylindrical surfaces already is well established. Other shapes will be useful in practicing the present invention. For example grooves such as those intended to hold O-rings also are useful. A plurality of methods for creating slots on the inner surface of the magnet holder will be apparent to those of ordinary technical skill, and such slots are intended to be within the scope of the appended claims. Axial instability also can be constrained by tapering the interior of magnet holder  42  so that the injection molded magnet material forms a truncated cone with its wide end in contact with end portion  48 , causing the complementary shapes of magnet  41  and magnet holder  42  to resist relative movement. 
     The face of assembly  40  that holds the rotating magnet  41  functionally close to a sensor (not shown) is covered by a relatively thin end portion  48  of magnet holder  42 . This optional configuration facilitates control of the magnetic slurry during the injection molding process and may protect the surface of magnet  41  against environmental damage during use in some applications. 
       FIGS. 5   a ,  5   b  and  5   c  show a device according to the present invention, generally designated as  50 , in which constraint against radial instability between magnet  51  and magnet holder  52  is provided by tab  53  and complementary slot  54  as illustrated in  FIGS. 2   a ,  2   b  and  2   c , above. In the embodiment of  FIGS. 5   a ,  5   b  and  5   c , however, constraint against axial instability is provided by adhesive layer  55  that bonds magnet  51  to magnet holder  52 . As described in greater detail above, layer  55  typically is a thermally or chemically curable adhesive. 
       FIGS. 6   a ,  6   b  and  6   c  show another embodiment the present invention in which assembly  60  is constrained against both radial and axial relative movement between magnet  61  and magnet holder  62 . Magnet holder  62  includes shaped space  63 , illustrated in  FIG. 6   a  by dashed lines, which is a generally cylindrical void having a narrowed waist portion  64 . Magnet  61  is injection molded so as to fill shaped space  63  such that the complementary shapes of magnet  61  and magnet holder  62  in flange areas  65 , which generally define the ends of waist portion  64 , prevent lateral movement between injection molded magnet  61  and magnet holder  62 . 
     As is best seen in  FIG. 6   b , the complementary cross sectional shapes of magnet holder  62  and injection molded magnet  61 , both of which are generally circular in this embodiment, also constrain relative axial movement between them. Shaped void  66 , shown in  FIG. 6   b , enables mounting of assembly  60  on, for example, the shaft of an electric motor (not shown) for high speed reciprocation during use. 
       FIGS. 7   a ,  7   b  and  7   c  show an embodiment of the present invention in which device  70  includes magnet  71  held in magnet holder  72 . Relative movement between magnet  71  and holder  72  is constrained by the complementary shapes of sintered or bonded magnet  71  and shaped portions  73  and  74  of shaped void  75  acting in combination with adhesive potting compound  76 , which fills portions of shaped void  75  not occupied by magnet  71 . End portion  77  of magnet holder  72  covers magnet  71 , preventing exposure to harsh elements during operation. Shaped void  78 , shown in  FIG. 7   b , enables mounting of device  70  on, for example, the shaft of an electric motor (not shown) for high speed reciprocation during use. 
     EXAMPLE 1 
     A prior art magnet and magnet holder assembly as shown in  FIG. 1  was made using a sintered NdFeB magnet and an aluminum magnet holder. The assembly was tested for torque to failure. The assembly failed at force of 1 Nm. For comparison, a magnet and magnet holder assembly as shown in  FIG. 2  was made using the same materials and tested for torque to failure. The assembly of  FIG. 2  failed at 4.3 Nm, indicating that the construction according to the present invention can withstand much higher torque than the assembly of the prior art and resists radial slippage to a greater degree. 
     EXAMPLE 2 
     A prior art assembly was made according to  FIG. 1  using a sintered ferrite magnet. The magnet was magnetized across its diameter. The gauss value of the magnet was measured at a height of 10 mm above the magnet surface in the Z plane. The gauss value varied within +/−5.0%. 
     A similar design was made using the tabbed magnet as shown in  FIG. 2  and was magnetized with polarity lines parallel to the tab. The gauss value was measured in the Z plane as with the assembly of  FIG. 1 . The variation was +/−2%, indicating the tabbed design provides improved magnetic properties. 
     With the above description it is to be understood that the MAGNET AND HOLDER ASSEMBLY HAVING IMPROVED ROTATIONAL AND AXIAL STABILITY is not to be limited to only the disclosed embodiment. The features of the MAGNET AND HOLDER ASSEMBLY HAVING IMPROVED ROTATIONAL AND AXIAL STABILITY are intended to cover various modifications and equivalent arrangements included within the spirit and scope of the description.