Abstract:
A joining assembly for mounting a fastening element to an external surface of a structural member of deformable material. The structural member is provided at its external surface with one or a plurality of joining projections engaging into respective recesses of a joining flange of said fastening element. The joining projections are provided at their free ends with upset beads obtained by material deformation of the joining projections and positively fixing the associated joining projection to the joining flange.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a joining assembly for mounting a fastening element to an external surface of a structural member of deformable material. 
     Quite often it is necessary to have structural members of plastics such as plastic housings provided with fixing means enabling to mount other members to said structural members. To this end it has become known for example to press threaded inserts of a harder plastic material into respective openings of the structural members, for example by a hot melt operation, ultrasonic welding or the like. This method, however, cannot be used with thin-walled structural members such as housings of mobile telephones (cellular phones). One possibility would be to provide the structural member with an integral threaded projection by injection moulding. However this is possible only with structural members of very simple geometrical shape. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide an improved joining assembly for mounting a fastening element of a material of relatively higher strength to an external surface of a structural member of a material of relatively less strength. 
     It is a further object of the invention to provide a joining assembly for joining a fastening element and a structural member which allows to mount the fastening element to the separately manufactured structural member in a very simple manner, without the need of any bores or openings in the walls of the structural member. 
     In the joining assembly of the invention the fastening element comprises a main body provided with functional means and a joining flange integral with said main body, and the structural member is provided with at least one joining projection integral with the external surface thereof and engaging into a respective recess of a joining flange of the fastening element. The joining projection is provided at its free end with an upset bead provided by material deformation of the joining projection and positively retaining the joining projection to the joining flange. 
     This results in a positively secured surface-to-surface joint between the fastening element and the structural member which is of high resistance to twisting, withdrawal and shear stresses. Since it is not necessary to provide any bores or openings in the walls of the structural member, the walls of the structural member can be made of extremely small thickness. 
     The structural member is preferably made of thermoplastic material, but could be made also of light metal. In any case the structural member must be made of a material which can be deformed for making the upset bead at the end of the joining projection. 
     When the structural member is made of thermoplastic material, the upset bead can be made by a conventional hot melt operation such as ultrasonic welding, linear or rotation friction welding or another hot melt operation. When, however, the structural member is made of light metal, the upset bead is advantageously made by cold deformation (e.g. a riveting operation). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawing a preferred embodiment thereof, from an inspection of which, when considered in connection with the following description, the invention, its construction and many of its advantages should be readily understood and appreciated. 
     FIG. 1 is a perspective view of a fastening element and a structural member prior to a joining operation; 
     FIG. 2 is a view similar to FIG. 1 during an intermediate step of the joining operation; 
     FIG. 3 is a view similar to FIGS. 1 and 2 after the joining operation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A fastening element  2  shown in FIG. 1 is to be mounted to planar external surface  6  of a structural member  4 . The structural member  4  is a thin-walled member such as a housing of a mobile telephone (cellular phone). 
     The structural member  4  is made of a material of a relatively small strength such as thermoplastic material or light metal. The fastening element  2  is made of a material of greater strength, in particular metal or a harder plastic material. Therefore the fastening element  2  can perform a fastening function for the structural member  4  when the fastening element  2  has been integrally and rigidly connected to the structural member  4 . 
     The fastening element  2  comprises a main body  8  and a joining flange  10  integral with the main body  8  and provided with three recesses  12 ,  14  in the embodiment as shown. The external surface  6  of the structural member  4  has integral projections  16  and  18  matingly shaped with respect to the recesses  12  and, respectively,  14 . 
     The joining flange  10  is a thin annular flange which, in the embodiment as shown, is of circular shape; however, it could also be of another shape. As shown in the drawing, the recesses  12  are shaped as annular wall segments extending through the joining flange  10  while the recess  14  is a groove of generally semi-circular cross-section at the outer periphery of the joining flange  10 . The projections  16  and  18  at the external surface  6  of the structural member  4  are adapted to the recesses  12  and, respectively,  14  as to their cross-sectional shapes and relative positions, i.e. the projections  16  are shaped as annular wall segments, and the projection  18  is of cylindrical shape. 
     The length of the projections  16  is greater than the thickness of the joining flange  10  so that the projections  16  extend beyond the upper surface of the joining flange  10  for a predetermined amount when the structural member  2  has been positioned so as to be engaged by the projections  16  and  18  (see FIG.  2 ). The free ends of the projections  16  will be deformed by upsetting so as to provide upset beads  20  (see FIG. 3) which positively retain the fastening element  2  to the structural member  4  as will be explained in more detail below. 
     The cylindrical projection  18 , which is not absolutely necessary and could be dispensed with, is of a length corresponding to the thickness of the joining flange  20  so that it does not extend beyond the upper surface of the joining flange  20  and will not be provided with an upsetting bead. Its function is to increase the joining assembly&#39;s resistance to relative rotational movements. 
     It should be understood that the number, shape and position of the recesses  12 ,  14  and the projections  16 ,  18  can be selected to be different from those in the shown embodiment. The annular segmental shape of the recesses  12  and projections  16  provides for substantial shear resistance at minimal space. Depending on the particular application other geometrical shapes may be appropriate. As shown the walls of the recesses  2  and projections  16  extend perpendicularly to the external surface  6  of the structural member  4 . However, it would be possible to provide for small tapers of the recesses  12  in order to facilitate insertion of the projections into the structural member  2 . As an alternative the recesses  12  could be tapered in the opposite direction so that there will be additional space for receiving material when material is deformed for making the upsetting beads  20 . This would increase the joining assembly&#39;s resistance to withdrawal. Preferably the projections  16  should be dimensioned somewhat smaller than the recesses  12  in order to facilitate insertion of the projections  16  into the structural member  2 . 
     Of course the projection  18  which could be dispensed with may also be of a shape different from the shape as shown. For example it could be of angular, profiled or toothed shape. Furthermore, the projection  18 , similar to projections  16 , could be of an axial length sufficient to provide for an upsetting bead  20  so that it would also assist in axially securing the fastening element  2 . 
     The main body  8  of the fastening element  2  which is a hollow cylinder in the embodiment as shown is provided with functional means  22 . In the embodiment shown the functional means  22  are comprised of a smooth bore; alternatively they could be comprised of a tapped bore, a portion of a closure or snapping means, a smooth or threaded bolt or similar fastening means. 
     As may be seen from the figures, the bottom surface of the joining flange  10  may be ribbed or fluted in order to increase the resistance to relative rotational movements between the fastening element  2  and the structural member  4 . 
     The joining assembly comprising the fastening element  2  and the structural member  4  will be manufactured as follows: 
     Initially the fastening element  2  and the structural member  4  are manufactured separately from each other, for example by injection moulding or in any other suitable manner. 
     Thereafter the fastening element  2  is positioned upon the external surface  6  of the structural member  4  such that the projections  16  and  18  are received from the recesses  12  and, respectively,  14  as shown in FIG.  2 . The dimensions of the respective components may be selected such that e.g. the projections  16  are received in the recesses  12  with play while the projection  18  is received in the recess  14  by a press-fit. This allows precisely to position the fastening element  2  relative to the structural member  4  if required. 
     Thereafter the projecting free ends of the projections  12  are deformed into upsetting beads  20  by plastifying and deforming the material by means of a welding tool  24  (schematically indicated in FIG.  2 )—similar to a riveting or grimping operation—to provide for a positive joint between the fastening element  2  and the structural member  4 . The dimensions and shape of the upsetting beads  20  will be chosen in dependence of the specific application. 
     When the structural member  4  is made of light metal, the projections  16  and  18  are preferably formed as hollow rivets so that the upsetting beads  20  can be made by a conventional riveting operation (cold deformation).