Abstract:
Bonding geometries are disclosed between permanent magnets and steel portions of magnetic assemblies. When employed in permanent magnet motors, this particular bonding construction reduces or even eliminates the loosening of permanent magnets inside of electric motors. The bonding geometry consists of interlocking protrusions and voids at the bonding interface. A bonding agent is employed that may be filled with finely divided iron or other magnetic material. The result is a strong bond between the permanent magnet and steel assembly portions with virtually no loss in overall magnetic properties.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This non-provisional application claims benefit of the provisional application filed on Feb. 1, 2006 having application No. U.S. 60/764,085. This non-provisional application also claims benefit of provisional application filed on Jan. 25, 2007 having application No. 60/897,325. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to bonding permanent magnets in steel construction and more particularly to interlocking bonding geometries for bonding permanent magnets to steel portions of magnet assemblies. This bonding construction is particularly well suited for use in bonding permanent magnets to steel motor housings. 
   2. Description of the Related Art 
   Permanent magnets are pieces of material that retain a magnetic field after being exposed to a magnetizing field. Once magnetized they do not need energy input to maintain their field. As a result, they have found numerous uses. Of particular interest is their use in electric motors. Electric motors generate torque by the interaction of magnetic fields. Prior to the development of strong permanent magnets, electric motors employed both stationary and moving electromagnets to generate the interacting magnetic fields needed to generate torque. 
   Electricity is required to produce magnetic fields within electromagnets. The result is often a substantial amount of electrical loss within the electromagnets themselves. In addition, since power has to be supplied to moving electromagnet windings, some sort of brush system is also required. General electric developed the first permanent magnet material that was strong enough to replace some of the electromagnets in electric motors. This magnet material was called Alnico and shortly thereafter several grades became commercially available. Improved permanent magnets made possible permanent magnet motors. 
   A permanent magnet motor is an electric motor employing permanent magnets as the fixed magnetic field and electromagnets as the changing interacting magnetic field. The use of permanent magnets in electric motors improved their reliability and reduced manufacturing costs. There are numerous configurations possible with permanent magnet motors. One common configuration involves placing curved permanent magnets against a motor housing and having a rotatable electromagnet assembly located centrally within the field of the permanent magnets. The bonding of permanent magnets into motor housings may be accomplished using a balloon to press the permanent magnets against the housing and using epoxy resin to adhere the magnets firmly into place. When the epoxy finishes curing, the balloon may be deflated and removed. Another method that can be used for bonding permanent magnets in magnetic assemblies is disclosed in U.S. Pat. No. 4,011,120 titled “Method For Fastening Ceramic Magnets To A Flywheel Using Centrifugal Force”. U.S. Pat. No. 4,011,120 discloses the placement of permanent magnets having applied adhesive against the inner surface of a flywheel and spinning the flywheel in a special fixture having positioning pins. The centrifugal force created by the spinning action holds the magnets in place until the adhesive sets. 
   The torque generated in permanent magnet motors depends on the field strength of the permanent magnets and the field strength of the interacting field of the electromagnets. The stronger the field of the permanent magnets the greater will be the torque. The stronger the field of the electromagnets, the stronger will be the torque. 
   Permanent magnets can withstand a limited demagnetizing field. If too much current is applied to the electromagnets of a permanent magnet motor the permanent magnets will demagnetize, and the motor will cease to function properly. Because of this, the power supplied to permanent magnet motors needs to be limited. 
   As time progressed so did permanent magnet development. Today, there are strong permanent magnet materials that resist the forces of demagnetization and have inherently strong magnetic fields. Permanent magnets made from certain rare earth compositions such as neodymium iron boron can have very good properties for use in permanent magnet motors. 
   A bit needs to be said about the standard design of permanent magnet motors with respect to the permanent magnets themselves. In the standard configuration, curved permanent magnets are placed against a steel motor housing. The magnets face each other with opposite poles aligned. Magnetic flux travels through the steel motor housing from the back of one magnet to the other. The central portion where the rotating electromagnet assembly spins is open with the field of the permanent magnets traveling through. The pathway of magnetic flux forms closed loops between the permanent magnets, the steel motor housing, and the air space in the central portion of the motor. This pathway is sometimes referred to as the “magnetic circuit”. 
   When magnetic flux travels through air, it is said to be traveling through an “air gap”. Generally speaking, the greater the air gap, the less will be the torque of the motor. Because of this, designers of electric motors often reduce this air gap to the bare minimum. It should be noted that the thin layer of epoxy bonding the permanent magnets to the housing can be considered to be an air gap due to the fact that like air, epoxy is a poor conductor of magnetic flux. Examples of this are numerous and thin non-magnetic materials are often used as part of magnet bonding and/or holding assemblies. An example of this can be found in U.S. Pat. No. 4,920,634 titled “Permanent Magnet Rotor With Magnet Retention Band” incorporated herein by reference. U.S. Pat. No. 4,920,630 discloses the use of a non-magnetic retention band wrapped around the rotor magnets of a permanent magnet electric motor. The opposite ends of the band are extended inwardly in the space between the magnets and firmly secured to the rotor core. In circumferentially spaced relation to the secured ends, a further securement of the band is established between the band and the rotor core. The apparatus and method disclosed in U.S. Pat. No. 4,920,634 has the advantage of allowing the band to be of minimal thickness thereby minimizing the air gap. 
   Strong rare earth permanent magnet motors can be made small in size, light in weight and very powerful. Because of this, it is often the case that permanent magnet motors employing rare earth magnets run hot. The result is that the epoxy holding the permanent magnets to the motor housing may be compromised resulting in bond failure. Once this happens, the permanent magnets slide around loosely within the motor and may jam the rotor. 
   In addition to thermal instabilities to the bond between permanent magnets and their motor housings, high torque values are often associated with rare earth permanent magnet motors. These high torque values add further strain to the bond between the motor housing and permanent magnets. 
   It is an object of this invention to provide good bonding of permanent magnets to steel in magnetic assemblies. 
   It is an object of this invention to provide good bonding of permanent magnets to motor housings. 
   It is a further object of this invention to provide bonds that are stable to high temperatures. 
   It is a further object of this invention to provide a strong bond between a permanent magnet and motor housing capable of withstanding large torque values. 
   Finally, it is an object of this invention to provide a bonding method between a permanent magnet and motor housing that minimizes magnetic path air gaps. 
   SUMMARY OF THE INVENTION 
   This invention therefore proposes permanent magnets having a pattern of protrusions and/or holes that minimizes the effective magnetic air gap to the steel portions of magnetic assemblies such as the inside surfaces of steel motor housings and bonding these permanent magnets to the steel portions of the magnetic assemblies using one or more bonding agents. The permanent magnets and steel portions of magnetic assemblies may be interposed with one another in order to minimize the effective magnetic air gap or alternatively, the permanent magnets and steel portions may employ ultra low profile bonding surfaces that interlock with bonding agents. The bonding agent may be selected from the group of liquid polymer resins such as epoxy. Furthermore, the bonding agent may have added ferromagnetic powdered material to effectively reduce the air gap of the overall magnetic circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a permanent magnet having surface protrusions facing a steel plate having matching holes. 
       FIG. 2  shows a curved permanent magnet having an outer beaded surface along with a matched hole steel construction. 
       FIG. 3  shows a cross sectional view of curved permanent magnets fixedly attached to a steel motor housing. 
       FIG. 4  shows a cross sectional view of curved permanent magnets having protrusions fixedly attached to the inside of a steel motor housing having matching holes. 
       FIG. 5  shows a curved permanent magnet having holes along with a matched steel construction having an outer beaded surface. 
       FIG. 6  shows a permanent magnet having holes facing a steel plate having matching protrusions. 
       FIG. 7  shows a permanent magnet having ultra low profile protrusions for bonding to another surface. 
       FIG. 8  shows a permanent magnet having ultra low profile bonding protrusions facing a steel plate having ultra low profile bonding protrusions. 
       FIG. 9  shows a cross sectional view of curved magnets having ultra low profile bonding protrusions fixedly attached to a steel motor housing having ultra low bonding protrusions. 
       FIG. 10  shows a cross sectional view of curved magnets having ultra low profile bonding protrusions fixedly attached to a steel motor housing having a matching interposed pattern of ultra low profile protrusions. 
   

   DESCRIPTION OF THE INVENTION 
     FIG. 1  shows a permanent magnet having surface protrusions facing a steel plate having matching holes. This particular configuration is suitable for the attachment of permanent magnets to numerous surfaces. Permanent magnet  2  is shown having a north pole face  4  and a south pole face  6 . Also shown are protrusions  8  extending outward from south pole face  6 . Steel portion  10  is shown having top surface portion  12  facing south pole face  6  of permanent magnet  2 . Holes  14  are blind holes drilled into top surface  12  of steel portion  10  and are shown in matching alignment with protrusions  8  of permanent magnet  2 . 
   The resulting fit between permanent magnet  2  and steel portion  10  forms a good bond with a bonding agent (not shown). In addition, the interlocking geometry helps to maintain good magnetic conductivity at the magnet to steel interface with the bonding of permanent magnet  2  to steel portion  10 . 
   Protrusions  8  extending outward from south pole  6  of permanent magnet  2  may be formed by compressing a water based slurry of a suitable magnetic material such as strontium ferrite into the voids of a flexible rubber mold followed by careful removal and subsequent sintering at elevated temperatures. Depending on the permanent magnet material, it may be desirable to carry out the sintering process in the presence of a magnetic field. 
     FIG. 2  shows a permanent magnet having an outer beaded surface along with a matched hole steel construction. Permanent magnet  12  is shown having a north pole face  14  and a south pole face  16 . Bead protrusions  18  are located on outer south pole face  16 . Also shown is steel section  20 . Steel section  20  has holes  22 . Holes  22  in steel section  20  are aligned with beads  18  in permanent magnet  12 . 
   This particular construction is suitable for bonding permanent magnets into steel motor housings. It should be noted that the opposite configuration may be employed with the permanent magnet having the holes and the inner surface of the motor housing having beads or bead like protrusions. This configuration may provide enhanced bonding properties that are desirable in permanent magnet electric motors. Tight spacing of permanent magnet material and the motor housing is still preserved and therefore the integrity of the magnetic circuit is preserved as well. 
     FIG. 3  shows a cross sectional view of two curved permanent magnets fixedly attached to a steel motor housing. Motor housing assembly  24  is shown having permanent magnets  26  and  28  fixedly attached to steel motor housing  30  with epoxy bonding agent  32 . Permanent magnet  26  is shown having its north pole face against steel motor housing  30  and its south pole face pointing inward toward the central portion of steel motor housing  30 . Permanent magnet  28  is shown having its south pole face against steel motor housing  30  and its north pole face pointing inward toward the central portion of steel motor housing  30 . 
   This is a commonly employed permanent magnet assemblies used with brush timed D.C. permanent magnet motors. Unfortunately while being rather effective in design, this particular construction may not be best suited for rare earth permanent magnet motors run at high temperatures and under the conditions of high torque. An improved construction is shown in  FIG. 4 . 
     FIG. 4  shows a cross sectional view of curved permanent magnets having protrusions fixedly attached to the inside of a steel motor housing having matching holes. Motor housing assembly  34  is shown having permanent magnets  36 , and  38 . Permanent magnets  36  and  38  have protruding portions  40  extending from pole faces  42  and  48 . Also shown are blind holes  44  in steel motor housing  46 . Blind holes  44  in steel motor housing  46  are matched to protrusions  40  of permanent magnets  36  and  38 . Permanent magnets  36 , and  38  are shown attached to the inside portion of steel motor housing  46  with bonding agent  50 . 
   Bonding agent  50  may be selected from the epoxy group of liquid bonding agents and may furthermore contain powdered ferromagnetic materials to reduce overall air gap effects. The overall result is a bonded magnet construction that is highly resistant to magnet loosening resulting from high temperature and high torque conditions. 
     FIG. 5  shows a curved permanent magnet having holes along with a matched steel construction having an outer beaded surface. Permanent magnet  52  is shown having a north pole face  54  and a south pole face  56 . Holes  58  are located on outer south pole face  56 . Also shown is steel section  60 . Steel section  60  has protrusions  62 . Protrusions  62  in steel section  60  are aligned with holes  58  in permanent magnet  52 . 
     FIG. 6  shows a permanent magnet having holes facing a steel plate having matching surface protrusions. This particular configuration is suitable for the attachment of permanent magnets to numerous surfaces. Permanent magnet  64  is shown having a north pole face  66  and a south pole face  68 . Also shown are blind holes  70  extending inward from north pole face  66 . Steel portion  72  is shown having top surface portion  74  facing north pole face  66  of permanent magnet  64 . Protrusions  76  from top surface  74  of steel portion  72  are shown in matching alignment with holes  70  of permanent magnet  64 . 
   It should be noted that for  FIGS. 1 ,  2 ,  4 ,  5 , and  6  that the holes may be modified from straight wall geometry to a geometry that may represent a hollow cavity having more of a spherical shape than the standard cylindrical shape of traditional holes. The spherically modified holes may be produced in a variety of ways including angled machining, chemical etching and EDM (electrode discharge milling). Holes modified in this manner may provide improved anchorage for the finished part when employing bonding agents. 
     FIG. 7  shows a cross sectional view of a permanent magnet having ultra low profile protrusions for bonding to another surface. Permanent magnet construction  76  is shown comprised of hour glass shaped protrusions  78  along with permanent magnet portion  80 . Also shown is top surface portion  82 . Hour glass shaped protrusions  78  are shown extending in an outward direction from top surface portion  82  of permanent magnet portion  80 . Also shown are cavities  84  having a suitable geometry for interlocking with liquid bonding agents (not shown). Also shown are exposed top surface portions  86  of hour glass shaped protrusions  78 . Exposed top surface portions  86  of hour glass shaped protrusions  78  are shown having significant surface area thereby establishing significant pathways for the efficient transference of magnetic flux with contacting ferro-magnetic surfaces such as steel. Also shown are bottom portions  88  of hour glass shaped protrusions  78 . Bottom portions  88  of hour glass shaped protrusions  78  are shown having significant contacting surface area with top surface portion  82  of permanent magnet portion  80  thereby establishing significant pathways for the efficient transference of magnetic flux from top surface portion  82  of permanent magnet portion  80  and bottom portions  88  of hour glass shaped protrusions  78 . 
   Hour glass shaped protrusions  78  extending in an outward direction from top surface portion  82  of permanent magnet portion  80  may be formed by casting an aqueous slurry of a suitable magnetic material (such as strontium ferrite) into a mold made from a burnable material and subsequently sintering. The sintering process may therefore be used to simultaneously form the magnet and attached protrusions while and burn away the mold. 
     FIG. 8  shows a permanent magnet having ultra low profile bonding protrusions facing a steel plate having ultra low profile bonding protrusions. Permanent magnet  90  is shown having hour glass shaped protrusions  92  extending in an outward direction from surface  94 . Hourglass shaped protrusions  92  are shown facing hourglass shaped protrusions  96  extending in an outward direction from surface portion  98  of high permeability steel plate  100 . 
     FIG. 9  shows a cross sectional view of curved magnets having ultra low profile bonding protrusions fixedly attached to a steel motor housing having ultra low bonding protrusions. Motor housing assembly  102  is shown having permanent magnets  104 , and  106 . Permanent magnets  104  and  106  have protruding portions  108  having an ultra low profile geometry as shown in detail in  FIG. 7  extending from pole faces  110  and  116 . Also shown are ultra low profile protrusions  112  extending in an outward direction from steel motor housing  114 . Ultra low profile protrusions  112  extending in an outward direction from steel motor housing  114  are shown aligned to protrusions  108  extending in an outward direction from pole faces  110  and  116  of permanent magnets  104  and  106 . Permanent magnets  104 , and  106  are shown attached to the inside portion of steel motor housing  114  with bonding agent  118 . 
   Bonding agent  118  may be selected from the epoxy group of liquid bonding agents and may furthermore contain powdered ferromagnetic materials to reduce overall air gap effects. The overall result is a bonded magnet construction that is highly resistant to magnet loosening resulting from high temperature and high torque conditions. 
     FIG. 10  shows a cross sectional view of curved magnets having ultra low profile bonding protrusions fixedly attached to a steel motor housing having a matching interposed pattern of ultra low profile protrusions. Motor housing assembly  120  is shown having permanent magnets  122 , and  124 . Permanent magnets  122  and  124  have protruding portions  126  having an ultra low profile geometry as shown in detail in  FIG. 7  extending from pole faces  128  and  134 . Also shown are ultra low profile protrusions  130  extending in an outward direction from steel motor housing  132 . Ultra low profile protrusions  130  extending in an outward direction from steel motor housing  132  are shown aligned with and extending into spaces  138  between protrusions  126  extending in an outward direction from pole faces  128  and  134  of permanent magnets  122  and  124 . Permanent magnets  122 , and  124  are shown attached to the inside portion of steel motor housing  132  with bonding agent  136 . 
   Bonding agent  136  may be selected from the epoxy group of liquid bonding agents and may furthermore contain powdered ferromagnetic materials to reduce overall air gap effects. The overall result is a bonded magnet construction that is highly resistant to magnet loosening resulting from high temperature and high torque conditions. 
   Those skilled in the art will understand that the preceding exemplary embodiments of the present invention provide foundation for numerous alternatives and modifications. These other modifications are also within the scope of the limiting technology of the present invention. Accordingly, the present invention is not limited to that precisely shown and described herein but only to that outlined in the appended claims.