Abstract:
An electric motor ( 20 ) has a stator ( 30 ) and a rotor ( 26 ). The latter is equipped with a cup-like rotor part ( 56 ) and with a ring magnet ( 60 ) adhesively bonded therein, which magnet has an outer circumference ( 61 ) on which are provided elevations ( 84 ) and depressions ( 86 ) that extend at least partly in the longitudinal direction of the ring magnet ( 60 ). The outer circumference ( 61 ) of the ring magnet ( 60 ), that faces toward the cup-like rotor part ( 56 ) after assembly, is formed with at least one opening or channel ( 68; 69, 88 ), extending in a circumferential direction, that is connected to at least a plurality of the flat depressions ( 86 ). Positive mechanical engagement between adhesive and ring magnet enhances durability, and discourages any tendency toward relative rotation between the magnet ( 60 ) and the cup-like rotor part ( 56 ).

Description:
CROSS-REFERENCES 
       [0001]    This application is a section 371 of PCT/EP09/05941, filed 21 Jul. 2008 and published 7 May 2009 as WO-2009-056180-A3, whose priority document is German application DE 10 2007 054 322.2, filed 31 Oct. 2007, the entire content of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to an electric motor having a stator and having a rotor, in which latter is mounted a cup-like yoke. A ring magnet of the rotor is adhesively bonded into said yoke. 
       BACKGROUND 
       [0003]    Such ring magnets can be manufactured in various ways, for example from a rubber-like material into which hard-ferromagnetic particles are embedded. This rubber-like material is then mounted in a yoke of this kind. Such ring magnets are also referred to as “rubber magnets.” 
         [0004]    There are also ring magnets that are manufactured by injection molding from a raw granulated plastic, for example from SmFeN with a polyamide “PA 12” matrix, and, in this case, a slip agent that contains silicone molecules is often added to the raw granulate. Such ring magnets are referred to hereinafter as “plastic-matrix ring magnets.” 
         [0005]    With this type of plastic-matrix ring magnet having added silicone, adhesive bonding is not reliable in processing terms, i.e. the adhesive compound cross-links with the yoke, but as a result of contaminants such as, for example
   silicone residues,   release agents,   oils,   fats, etc.
 
that are present on the side of the plastic-matrix ring magnet that is to be adhesively bonded, insufficient cross-linking, of the adhesive compound with the ring magnet at that location, is possible in some circumstances, and the risk therefore exists that the relative position of the ring magnet and rotor housing may change in the context of further process steps, e.g. under thermal or mechanical loads.
   
 
         [0010]    For this reason, laborious cleaning processes are necessary prior to adhesive bonding in order to increase process reliability. In addition, such cleaning processes clean the surface that is to be bonded only for a certain time. Subsequent diffusion of silicone molecules degrade adhesion capability not only directly prior to bonding, but also during the time when the adhesive compound is curing. 
         [0011]    Thus, whereas the adhesive compound reliably cross-links with the yoke, such cross-linking on the surface of a plastic-matrix ring magnet of this kind is not possible in every case to a sufficient extent, as a result of the aforementioned contaminants. 
       SUMMARY OF THE INVENTION 
       [0012]    An object of the invention is therefore to make available a novel electric motor. 
         [0013]    According to the invention, this object is achieved by providing a central stator, an external rotor constituted by a cuplike rotor part with an inner surface and a ring magnet adhesively bonded therein, the outer circumference of the ring magnet being formed with alternating longitudinal elevations and depressions, the rotor part and the ring magnet defining, between them, an annular void which interconnects at least a plurality of the flat depressions. The void helps distribute adhesive. Greater process reliability is obtained thereby. The low adhesion forces with respect to the plastic-matrix ring magnet that can occasionally occur, for the reasons mentioned, are compensated for by the mechanical positive engagements (gear-like engagements, undercuts, etc.). Entirely satisfactory process reliability is obtained, as well as a normal service life for the electric motor. Failure of the adhesive bond appears to be ruled out. 
         [0014]    Further details and advantageous refinements of the invention are evident from the exemplifying embodiments, in no way to be understood as a limitation of the invention, that are described below and depicted in the drawings, in which: 
     
    
     
       BRIEF FIGURE DESCRIPTION 
         [0015]      FIG. 1  is an exploded view of an exemplifying embodiment of an electric motor; 
           [0016]      FIG. 2  is a greatly enlarged three-dimensional depiction of a first embodiment of a ring magnet  60  that is equipped on its external circumferential surface  61  with flat, relief-like elevations and depressions, and also with an annular groove  88  in the circumferential direction; 
           [0017]      FIG. 3  shows an enlarged detail of ring magnet  60  of  FIG. 2 ; 
           [0018]      FIG. 4  is a first drawing to explain the installation of ring magnet  60  in a yoke  56 ,  57  of the rotor; 
           [0019]      FIG. 5  is a second drawing, analogous to  FIG. 4 , in which ring magnet  60  is inserted completely into yoke  56 ,  57 ; 
           [0020]      FIG. 6  is a depiction, analogous to  FIG. 5 , in which rotor  26  is installed on stator  30 ; 
           [0021]      FIG. 7  shows a second embodiment of a ring magnet  60  that is equipped with relief-like elevations and depressions; 
           [0022]      FIG. 8  is an enlargement of a detail of  FIG. 7 ; 
           [0023]      FIG. 9  shows a third embodiment of a ring magnet  60  that is equipped with relief-like elevations and depressions; 
           [0024]      FIG. 10  is an enlargement of a detail of  FIG. 9 ; 
           [0025]      FIG. 11  shows a fourth embodiment of a ring magnet  60  that is equipped with relief-like elevations and depressions; 
           [0026]      FIG. 12  is an enlargement of a detail of  FIG. 11 ; 
           [0027]      FIG. 13  shows a fifth embodiment of a ring magnet  60  that is equipped with relief-like elevations and depressions; and 
           [0028]      FIG. 14  is an enlargement of a detail of  FIG. 13 . 
       
    
    
       [0029]    Identical or identically functioning parts are usually labeled hereinafter with the same reference characters, and are described only once. Terms such as “left,” “right,” “top,” “bottom” refer to the particular figure. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]      FIG. 1  is an exploded view of an external-rotor motor  20  that serves here, by way of example, to drive a fan wheel  22  that is depicted with five fan blades  24  that are mounted on an external rotor  26 . 
         [0031]    Located in the interior of external rotor  26  is an internal stator  30  of usual construction, here having four stator poles  32  that are wound with a stator winding arrangement  34 . Stator poles  32  are depicted, by way of example, as salient poles. The coil formers are labeled  36 . 
         [0032]    Stator poles  32  are part of a lamination stack  40  that is usually pressed onto a bearing tube  42  and is partly visible in  FIG. 6 . Located in it are bearings (not depicted), for example plain bearings or ball bearings, for journaling a rotor shaft  44 . In  FIG. 1 , the location of the rotation axis of external rotor  26  is labeled  46  and is indicated with dot-dash lines. 
         [0033]    Bearing tube  42  is mounted on a support flange  48  that is in turn connected, by means of struts  50  (which are merely indicated in  FIG. 1 ) to an air-guiding housing (not depicted) which surrounds fan blades  24  with a small spacing. Such fans are often used as equipment fans, e.g. in order to cool computers. 
         [0034]    External rotor  26  is implemented approximately in the manner of a can or a bell. The can is usually manufactured on its outer side from a suitable plastic  54 , and is immovably connected on its interior to a can or cup  56 , made of a magnetically conductive material, that has an open side  55  ( FIG. 1 ) on whose base  57  shaft  44  is mounted, and whose inner side is labeled  58 . Cup  56  serves as a magnetic yoke for a hollow-cylindrical permanent magnet  60 . 
         [0035]    Said permanent magnet  60  is adhesively bonded, with its outer side  61 , into said inner side  58  by means of a suitable adhesive compound  59  ( FIG. 4 ). Depending on requirements (e.g. environmental conditions, magnetic values, production technology, metering, curing method), a variety of adhesive types can be used:
   Anaerobic adhesives: advantages are, for example, a small adhesive gap, i.e. the volume of ring magnet  60  is maximized.   Epoxy resin adhesives having one or two components. The advantage is that high utilization temperatures are possible in this case.   Polyurethane adhesives. The advantage is that they are viscoplastic and can therefore compensate particularly effectively for differing thermal expansion values of the materials to be bonded.   
 
       Examples of Adhesives  
       [0039]      
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Anaerobic adhesives: 
                 Henkel: 
                 Loctite 510, Loctite 128500 
               
               
                   
                   
                 Delo: 
                 Delo ML adhesives 
               
               
                   
                 Epoxy resin adhesives: 
                 Henkel: 
                 Loctite 95xx series 
               
               
                   
                   
                 Delo: 
                 Delo Monopox adhesives 
               
               
                   
                   
                   
                 Delo Duopox adhesives 
               
               
                   
                 Polyurethane adhesives: 
                 Henkel: 
                 Macroplast adhesives 
               
               
                   
                   
                 Delo: 
                 Delo PUR adhesives 
               
               
                   
                   
               
             
          
         
       
     
         [0040]    The hollow-cylindrical inner side of permanent magnet  60  delimits the magnetically effective air gap  109  ( FIG. 6 ) of motor  20  toward the outside, while stator poles  32  delimit air gap  109  toward the inside. 
         [0041]      FIG. 2  and  FIG. 3  show ring magnet  60  of  FIG. 1  in an enlarged, three-dimensional view. Its diameter D is in most cases between 20 and 40 mm, i.e. the drawing is greatly enlarged so that details can be depicted. 
         [0042]    Outer side  61  of ring magnet  60  is equipped with flat elevations (here in the form of longitudinal ribs  84 ) and with flat depressions (here in the form of longitudinal grooves  86 ) which, as depicted, extend in such a way that upon insertion of ring magnet  60  into yoke ring  56 , and upon metering of the adhesive onto the inner wall of cup  56 , adhesive compound  59  can flow into longitudinal grooves  86  (adhesive metered onto the inner wall of cup  56 ). 
         [0043]    Ring magnet  60  furthermore has a flat annular groove  88  that in this case is somewhat deeper than longitudinal grooves  86 . The adhesive fills annular groove  88  during insertion, and in the insertion direction it fills longitudinal grooves  86  located thereabove. The depth of longitudinal grooves  86  is on average less than 1 mm, and preferably less than 0.5 mm. To summarize, one might say that the outer circumference  61  of ring magnet  60  is provided with flat depressions  86 ,  88  in the manner of a gravure printing roller. 
         [0044]    Alternatively, the adhesive can also be metered into annular groove  88 . If the adhesive is metered onto ring magnet  60 , i.e. preferably into annular groove  88 , the adhesive is then distributed into longitudinal grooves  86  during the insertion of magnet  60  into cup  56 . 
         [0045]    In this example annular groove  88  forms, on both sides, shoulders  90  and  92 , respectively, that serve for positive connection with adhesive compound  59  after it has cured. In the same fashion, the steep transitions  85  between longitudinal ribs  84  and longitudinal grooves  86  serve as shoulders that prevent rotation between ring magnet  60  and the cup-like rotor part  56  ( FIG. 1 ) into which ring magnet  60  is adhesively bonded. 
         [0046]    On its right (in  FIG. 2 ) end face  94 , ring magnet  60  has three axially protruding pegs  96 ,  98 ,  100  that serve as spacers  57  during assembly and during the curing of adhesive compound  59 , and thereby establish the axial position of ring magnet  60  relative to rotor part  56 . As depicted in  FIGS. 1 to 3 , they can have oblique surfaces  96 ′,  98 ′,  100 ′ on their radial outer sides. 
         [0047]    Ring magnet  60  is also equipped, in the region of this end face  94 , with oblique surface  68 , which in this example has approximately the shape of a truncated cone and constitutes a transition from end face  94  to the flat longitudinal grooves  86 . This promotes the distribution of adhesive on ring magnet  60  during insertion. 
         [0048]    Upon assembly, ring magnet  60 , which can preferably be manufactured from a suitable raw granulate (plastic grains having embedded hard ferrite particles) by injection molding and whose outer side  61  is implemented in the manner described, is adhesively bonded by means of adhesive compound  59  into inner side or surface  58  of the cup-like rotor part  56 . Because the latter is usually manufactured from soft iron, adhesive bonding does not represent a problem in terms of rotor part  56 , since strong adhesive forces occur there as a result of the adhesion process at the interface between adhesive compound  59  and rotor part  56 . 
         [0049]    The situation is different for injection-molded plastic-matrix ring magnets  60 , since they can contain slip agents in the form of silicone molecules that can be removed, at best temporarily, by the usual cleaning methods. 
         [0050]    The surface structure, as described and depicted, of ring magnet  60  eliminates the risk that the latter will detach over time from rotor part  56 , since adhesive compound  59 , which cures in the context of the adhesive bonding process, forms positively engaging connections in particular with depressions  86  of said relief-like surface structure  61 . Adhesive compound  59  flows around elevations  84 , which thereby produce additional positively engaging connections and prevent displacement and/or rotation of ring magnet  60 , even if the adhesive forces on its surface  61  sometimes happen to be very low. A favorable shape for the magnetic circuit in rotor  26  also results. 
         [0051]    This secure connection is therefore produced, on the one hand, by the adhesive forces of adhesive compound  59  at the interface with rotor part  56  and, on the other hand, by the cohesive forces between the molecules in adhesive layer  59 . The low adhesive forces at the interface with a plastic-matrix ring magnet  60  consequently cannot lead to failure of the adhesive bond. 
         [0052]    The above-described relief-like implementation of outer side  61  of ring magnet  60  thus yields, without additional cost, a substantial enhancement in the process reliability and service life of such motors, without thereby impairing their properties. 
         [0053]    As shown by  FIG. 4 ,  FIG. 5 , and  FIG. 6 , ring magnet  60  has, at its end region facing toward base  57  and on its radially outer side  61  there, its oblique surface  68  that forms, together with rotor part  56 , a void  69  ( FIG. 6 ) of approximately triangular cross section. In the embodiment according to  FIGS. 1 to 6 , this oblique surface  68  is formed by frustoconical surface segment  68  (see  FIGS. 2 to 6 ). This segment  68  has advantages, in the context of the adhesive bonding of ring magnet  60  into rotor part  56 , as will be explained below. As  FIGS. 4 to 6  show, the radial spacing, between oblique surface  68  and inner surface  58 , decreases in the direction away from base  57 . 
         [0054]    According to  FIGS. 2 and 3 , frustoconical segment  68  intersects the flat longitudinal grooves  86  and flat longitudinal ribs  84 , so that adhesive compound  59  can flow out of said segment into flat longitudinal grooves  86  and through them also into annular groove  88 . In this context, a film  59 F ( FIG. 4 ) of adhesive compound forms on the flat longitudinal ribs  84 ; this film should, however, be thin, since it acts in rotor  26  as a magnetic air gap and therefore might slightly attenuate the magnetic flux generated by ring magnet  60  in air gap  109  ( FIG. 6 ). 
       Installing Ring Magnet  60  In Rotor Part  58   
       [0055]    In one method, a bead  59 A of adhesive compound  59  is applied, prior to assembly, in the region of open end  55  of rotor can  56  on its inner side or surface  58 , as depicted in  FIG. 1 . 
         [0056]    An adhesive metering device such as the one usually utilized in industrial adhesive bonding for application of the adhesive, which device enables application of a defined quantity of adhesive  59  in the form of said bead  59   a,  is used for application. Said bead is preferably free of interruptions, i.e. is continuous. The necessary quantity of adhesive is ascertained beforehand, for example by means of experiments. 
         [0057]    Ring magnet  60  is then inserted from below (referring to  FIG. 1 ) into inner side  58  of rotor can  56 , such that frustoconical surface  68  abuts against bead  59 A and displaces it upward, as schematically depicted in  FIG. 4  and  FIG. 5 . 
         [0058]    As indicated schematically and by way of example with arrows  108  at the center of  FIG. 2 , adhesive compound  59  is distributed in this context by longitudinal ribs  84  in the manner of a spreader, and flows in largely identical quantities into the individual flat longitudinal channels  86  and from there to annular groove  88 . 
         [0059]    There the flat longitudinal ribs  84  once again produce a distribution of the adhesive compound, so that upon insertion of ring magnet  60  it flows—still uniformly distributed over outer side  61 —into the lower region of the flat channels  86  and likewise fills them up. 
         [0060]      FIG. 4  shows the process of introduction in the direction of an arrow  108 . Adhesive compound  59 , which initially is located at the top in the annular opening  68 , travels into the flat openings  86 ,  88  and from annular groove  88  further into the flat longitudinal grooves  86  in the lower part of ring magnet  60 . Longitudinal grooves  86  and annular groove  88  are, in this fashion, filled with adhesive compound  59  as a result of the longitudinal displacement of ring magnet  60  in the direction of arrow  108 . 
         [0061]      FIG. 5  shows how, at the end of the longitudinal displacement, ring magnet  60  abuts with its pegs  96 ,  98 ,  100  against base  57  of can  56 , thereby completing assembly. Annular groove  88  is thereby largely filled with adhesive compound  59 , likewise the longitudinal grooves  86  (not depicted), while annular channel  68  contains either no further adhesive compound  59  at all, or only small residues. This reduces imbalances and facilitates further processing, since adhesive compound  59  can cure quickly and is distributed very uniformly on outer side  61  of ring magnet  60 . The latter is thus also, during assembly, simultaneously a tool that produces a uniform distribution of adhesive compound  59  on its own outer circumference  61 ; as a result of the engagement of the cured adhesive compound  59  into the flat longitudinal grooves  86  and into annular groove  88 , a secure, positively engaged attachment of ring magnet  60  is produced, even if its outer side  61  happens to be contaminated with traces of silicone or the like. 
         [0062]    Once adhesive compound  59  has cured, rotor  26  is mated to stator  30  (see  FIG. 6 ), in which context shaft  44  ( FIG. 1 ) mounted on base  57 , which shaft is not depicted in  FIG. 6 , is inserted e.g. into plain bearings or rolling bearings (not depicted) in bearing tube  42 . A magnetically effective air gap  109  (see  FIG. 6 ) is produced in this context between outer circumference  107  of stator lamination stack  40  and inner side  50  of ring magnet  60 . Ring magnet  60  can be radially magnetized, which is not depicted. 
         [0063]    As an alternative, with the version of ring magnet  60  according to  FIGS. 1 to 6 , adhesive  59  can also be metered in the requisite quantity into circumferential groove  88  before ring magnet  60  is inserted into cup  56 . 
         [0064]    In this case, the excess adhesive  59  is pressed in both directions into the flat longitudinal grooves  86 . Adhesive that emerges at the top from longitudinal grooves  86  travels first to oblique surface  68  (see  FIGS. 4 and 5 ) and is then transported back down into the flat longitudinal grooves  86  by the continuing motion of ring magnet  60  in the direction of arrow  108  of  FIG. 4 . The advantage is that it is particularly easy to monitor whether groove  88  has been completely filled with adhesive  59  in the context of metering. This is especially important because laborious balancing operations might otherwise be necessary. 
         [0065]    Many variations, such as those explained below with reference to  FIGS. 7 to 14 , are of course possible within the scope of the invention. In this context, the same reference characters as those in  FIGS. 1 to 6  are used for identical or identically functioning parts. The motor ( FIG. 1 ) is not depicted again, since it can be unchanged except for the external shape of ring magnet  60 . 
         [0066]      FIG. 7  shows a variant in which, just as with ring magnet  60  of  FIG. 2 , the right end of ring magnet  60  is implemented, up to a boundary  66 , in the form of a truncated cone  68 , while the left part has a substantially cylindrical profile. Provided on truncated cone  68  are flow dividers  118  which, when ring magnet  60  is inserted in the direction of arrow  108  into yoke  56 , serve to deflect adhesive compound  59  (from bead  59 A) in the manner indicated schematically in  FIG. 7  by flow lines  120 , and thereby to optimize the distribution of adhesive compound  59  in the adhesive gap (between ring magnet  60  and yoke  56 ). In this context, adhesive compound  59  is deflected into flat channels  122  that extend in a longitudinal direction between flat elevations  124 , as shown by the drawings. 
         [0067]    Flat elevations  124  are shaped approximately like traffic arrows, i.e. they extend from a narrow left end  126  toward the right to a widest point  128  and from there taper toward the right to a narrow tip  130 , which in this case is located at boundary  66  with truncated cone  68 . As indicated symbolically by flow lines  120 , adhesive compound  59  is deflected by the relevant flow divider  118  and by elevations  124 , and directed to one of the flat depressions  122  between two flat elevations  124  in order to fill up said depressions. The steep edge  132  between elevations  124  and depressions  122  results in a corresponding positively engaged connection in both the longitudinal and the circumferential directions. 
         [0068]    This kind of shape of ring magnet  60  is often easier to manufacture by plastic injection molding than the shape according to  FIGS. 2 and 3 . Its advantage is maximum magnet volume, i.e. good utilization of permanent ring magnet  60 . 
         [0069]      FIG. 8  shows an enlarged detail of  FIG. 7 . 
         [0070]      FIG. 9  and  FIG. 10  show a variant of the embodiment according to  FIGS. 7 and 8 . Ring magnet  60  according to  FIGS. 9 and 10  once again has on the right a frustoconical segment  68  that, however, unlike in  FIGS. 7 and 8 , is not equipped with flow dividers. Flat elevations  124  have the same shape as in  FIGS. 7 and 8 , and the reader is therefore referred to that description. 
         [0071]    Located in each flat depression  122  between two elevations  124  is a respective flat elevation  138 . Proceeding from a short segment  140  at its left end, elevation  138  widens to a point  142  of maximum width. From point  142 , its width decreases sharply toward the right down to a narrow, finger-like segment  144  that acts as a flow divider for adhesive  59 , as symbolically indicated by flow lines  146 . 
         [0072]      FIGS. 11 and 12  show another variant that is put together from elements of  FIGS. 7 and 8  and elements of  FIGS. 9 and 10 . 
         [0073]    Ring magnet  60  according to  FIGS. 11 and 12  corresponds in its basic structure to the preceding variants, i.e. it has a cylindrical opening  50  on the inside. On the left if has on its outer side  61  firstly a short frustoconical segment  150 , then a segment  152  that is substantially cylindrical and extends approximately to a boundary  66 , and to the right of that a frustoconical segment  68  that tapers to the right and ends at end face  94 . Also adjacent to this, to the right, are spacing members  96 ,  98 ,  100 , whose function has already been described. 
         [0074]    Located on frustoconical segment  68  are flow dividers  118  that, unlike in  FIG. 8 , are shorter than segment  68  but otherwise have the same configuration as flow dividers  118  of  FIGS. 7 and 8 . Here as well, their function is to distribute adhesive compound  59  from bead  59 A uniformly on ring magnet  60 . The structure of flow dividers  118  is clearly evident to the skilled artisan from the drawings, and they extend radially outward approximately as far as enveloping cylinder  106  that is depicted in  FIG. 2 , so that adhesive compound  59  is forced to flow past a flow divider  118  on both of its sides, thereby producing effective flow division. 
         [0075]    Located on cylindrical segment  152  are elongated flat elevations  154  that correspond approximately, in terms of their shape and function, to elevations  138  according to  FIGS. 9 and 10 . 
         [0076]    Elevations  154  begin at the left at a narrow region  156  and widen from there toward the right to a widest point  158 . Their width then decreases to the right very sharply, down to a narrow segment  157 . All the way on the right they have a substantially sharp point  160  that coincides approximately with boundary  66 . 
         [0077]      FIG. 11  schematically shows flow lines  162  of adhesive  59 . These are divided once by flow dividers  118 , and on the other hand by segments  157  and their right-hand tips  160 , so that approximately the same amount of adhesive compound  59  flows into all the flat depressions  164  (between flat elevations  154 ). The result is that a secure join is obtained even if silicone residues or the like remain on outer side  61  of ring magnet  60 . 
         [0078]      FIG. 13  and  FIG. 14  show a fifth exemplifying embodiment. In this, the relief-like outer side  61  of ring magnet  60  has a very simple configuration that also largely simplifies production by means of plastic injection molding. Frustoconical surface  68  extends to the left as far as boundary  66 , adjoining which to the left is the substantially cylindrical segment  152  with its relief-like surface structure. 
         [0079]    No volumetric flow dividers are arranged on oblique surface  68  in the case of  FIGS. 13 and 14 . Instead, two types of elongated flat elevations, which act as flow dividers, are provided on cylindrical segment  152 . These are on the one hand longer flat elevations  168  that have approximately the same shape as elevations  124  according to  FIGS. 9 and 10 . They extend from a narrow point  170  at the left end ( FIG. 13 ) to an even narrower point  172  at the right end, which here is located on boundary  66 . Proceeding from point  170 , they widen over approximately 80 to 95% of their longitudinal extension up to a widest point  174 , and then decrease over the remainder up to point  172 . 
         [0080]    Located between longer elevations  168  are shorter and narrower elevations  176  that extend from a narrow point  178  (left) via a wide point  180  to a tip  182 . Tip  182  is at a distance a from boundary  66 , and this distance a is equal to approximately 5-15% of the length of flat elevations  168 . 
         [0081]      FIG. 13  also schematically shows the profile of flow lines  184 , and it is evident that here as well, the slightly deeper zones  186  between elevations  168  and  176  are uniformly filled with adhesive compound  59 , so that no overflow occurs at the right end of flat zones  186 , and the adhesive gap (between magnetic yoke  56  of  FIG. 1  and ring magnet  60  of  FIGS. 13 and 14 ) is uniformly filled with adhesive compound  59  during assembly in order to achieve an optimum mechanical bond. 
         [0082]    As the examples show, numerous possible variations exist within the scope of the invention.