Patent Publication Number: US-6910841-B2

Title: Thread-forming screw

Description:
TECHNICAL FIELD 
   The invention refers to a thread-forming screw. 
   BACKGROUND ART 
   Thread-forming screws typically are screws having a thread adapted to produce a counter or female thread in a bore of a workpiece. Usually, one differentiates between a tapping and forming thread of a screw. In case of a tapping thread a counter thread is produced in a preformed hole by a chip-generating forming process. In case of a self-forming thread the counter thread is generated substantially without chips through a plastic deformation of the workpiece. In this connection it is known to provide self-forming screws with a thread-generating zone by which particularly effectively a counter thread can be produced. If the workpieces are made of plastic material, the preformed hole or the preformed bore normally has not the core or root diameter of the thread-forming screw, rather, is provided with a greater diameter, preferably with the thread pitch diameter of the thread-forming screw. 
   For the improved insertion of the thread-forming screw in the preformed bore, the thread-forming screw normally is provided with a first cut or a conical portion by which a centering in the bore is achieved and an easy moving of the threads of a thread-forming portion into the wall of the bore, as well. 
   Threaded connections with thread-forming screws suffer under undesired release, for example by vibrations. Frequently, such thread-forming screws are pre-assembled with corresponding workpieces, e.g. of synthetic material. In this case, the thread-forming screw is partially turned into the bore of the workpiece. During transportation of such pre-assembled units vibration may occur which may lead to a loosening of the screws from the workpieces. It is known to undertake respective provisions, e.g. by applying micro capsules on the thread. The micro capsules effect an increased friction and thus may avoid an undesired loosening of the screw in the bore. However, the application of such micro capsules is relatively expensive. 
   SUMMARY OF THE INVENTION 
   With the invention, a thread-forming screw is to be provided which in the pre-assembled and also in the finally assembled state guarantees a maximum safety against undesired release. 
   In the thread-forming screw according to the invention the shaft at the end opposite to the head has a spherical portion with a maximum diameter in a plane perpendicular to the axis of the shank which is approximate to the pitch thread diameter of the thread on the shank. The shape of the spherical portion may be different. Essential is that the portion which comes into contact with the wall of the bore is spherical. 
   According to an embodiment of the invention, the spherical portion may be a section of a ball. According to a further embodiment of the invention, the diameter of the ball section adjacent to the thread-forming zone has the same diameter as the thread-forming zone at this location. The whole spherical portion adjacent to the thread-forming zone may be formed as ball portion. 
   In case of a pre-assembly with a workpiece consisting of a material suited for a thread-forming process, e.g. plastic material according to an embodiment of the invention the diameter of the bore is approximately equal to the pitch diameter of the thread portion. 
   Upon insertion of a thread-forming screw according to the invention into the work-piece the spherical portion serves for the centering of the screw shank. Sub-sequently, the thread-forming portion forms a thread into the wall of the bore. In case the maximum diameter of the spherical portion is approximately equal to the diameter of the bore, the spherical portion encounters no considerable resistance during the forming of the thread. Independent from the depth the shank is turned into the bore, the thread-forming screw at any time is sufficiently secured against release. The safety in turning-in direction takes place through the bore portion not yet provided with a thread. The safety in the opposite direction is taken over by the spheri-cal portion. If it is made an attempt to withdraw the thread-forming screw from the bore, this can take place only if the already formed thread is destroyed. This can be only done by applying a considerable force which normally is not generated by vibrations. 
   If the screw is completely threaded-in so that the spherical portion protrudes beyond the bore in the workpiece, the screw is also protected against release. A safety also against a minimum loosening is achieved only if no substantial gap is between the associated surface of the workpiece and the spherical portion. 
   The manufacture of a screw according to the invention can take place similar to that for conventional screws. First a blank without thread is produced in a screw press, i.e. with a spherical or ball-shaped extension. Thereafter, a thread is rolled and the spherical portion is rolled into the desired shape. The rolling of the spherical or ball-shaped portion can take place contemporarily with the rolling of the thread in case a specific rolling tool is available. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment example is subsequently described in connection with accompanying drawings wherein 
       FIG. 1  shows a thread-forming screw according to the invention outside a workpiece, 
       FIG. 2  shows a thread-forming screw of  FIG. 1  partially threaded into a workpiece, 
       FIG. 3  shows a thread-forming screw according to the invention completely threaded into a workpiece, and 
       FIGS. 4   a  to  d  show the manufacture of a screw blank. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIGS. 1  to  3 , a thread-forming screw  10  is depicted. It includes a head  12  with a hexagon  14  and a flange  16  adjacent thereto. The screw  10  includes further a shank  18  provided with a thread. In the end portion of thread  20  a thread-forming zone is provided having a conical portion  22 . A spherical portion  24  joins to conical portion  22 . The diameter of the spherical portion  24  immediately adjacent the conical portion  22  corresponds to the diameter of the conical portion at this location. The maximum diameter of the spherical portion shown at  26  corresponds to the pitch thread diameter of thread  20 . 
   In  FIGS. 1  to  3  there is shown a workpiece  28  of plastic material. It includes a throughbore  30  having a diameter which corresponds to the pitch diameter of thread  20 . 
   In the illustration of  FIG. 2  it is shown that the thread-forming screw is partially threaded into bore  30 . By means of the thread-forming zone (not shown in detail), a thread is formed in the wall of bore  30 , i.e. a so-called internal or female screw thread. Correspondingly, the geometry of bore  30  changes. As the ball-shaped portion  24  corresponds to the diameter of bore  30 , portion  24  can substantially freely pass bore  30 . If upon vibrations a retraction force is exerted onto screw  10  by a back-turning of the screw, this is prevented by the ball-shaped portion  24 . In this case, the ball-shaped portion  24  would be pressed against the already formed thread and deform or destroy it. For this, a considerable force is necessary. 
   In the finally reached position the screw head  12  clamps a plate  32  against work-piece  28  through its flange  16 . The ball-shaped portion  24  protrudes out of bore  30 , i.e. through its complete extension in axial direction. In other words, the chamfer start  22  terminates at the lower surface of the, workpiece  28 . As already explained, by this a safety for screw  10  is achieved. 
   In  FIGS. 4   a  to  4   d  a partial manufacture of screw  10  is illustrated. A rod-like blank  40  is pressed and rolled to a pre-form  42  having a head  44  and a shank with a first shank portion  46  and a second shank portion  48  having a reduced diameter. In the next step head  44  is formed to head  44   c  which has a flange  50  and countersunk wrench surfaces as shown in  FIG. 4   d . By a further deformation, shank portion  46  is formed to shank portion  46   d  and shank portion  48  is formed to a ball-shaped extension  54  with a narrowed portion  56  of smaller diameter being formed between shank portion  46   d  and ball-shaped portion  54 . By a subsequent thread-forming, the thread-forming screw is produced as shown in  FIGS. 1  to  3 . It can be seen that in  FIG. 4   d  the head of the screw differs from that of the screw of  FIGS. 1  to  3 . In view of shank  18  and the ball-shaped portion, there is conformity with the screw of  FIGS. 1  to  3 . 
   As can be seen in  FIGS. 1-3 , the maximum diameter  26  of portion  24  is greater than the minimum diameter of thread  20  and smaller than the maximum diameter of thread  20 .