Abstract:
A screw ( 10 ) having a screw head ( 14 ) equipped with an engagement portion ( 12 ), having a shaft ( 16 ) which defines a screw axis and which bears a thread ( 18 ), and having a thread-free shaft portion ( 20 ) arranged between the thread ( 18 ) and the screw head ( 14 ). A head-side end ( 22 ) of the thread ( 18 ) is truncated. Additionally, the thread ( 18 ) ends, adjacent to the thread-free shaft portion ( 20 ), in an end plane ( 32 ) which, with a constant spacing to the screw head ( 14 ), runs at least partially around the shank ( 16 ) and forms an angle α E  with the screw axis, and a head-side flank ( 34 ) of the thread ( 18 ) averted from the thread-free shank section ( 20 ) forms an angle α F  with the screw axis, and the angle α E  is smaller than the angle α F .

Description:
BACKGROUND 
       [0001]    The invention relates to a screw comprising a screw head that is provided with an engagement point, a shaft that defines a screw axis and has a thread, and a thread-free shaft portion that is arranged between the thread and the screw head. 
         [0002]    The invention further relates to a fastening arrangement and to a use of a screw according to the invention. 
         [0003]    The present screws, fastening arrangements and uses are mainly for fastening outer or inner metal sheets of sandwich panels. The sheet stacks used in this case are intended to be securely held together by screws which are, in turn, cost-effective and can be inserted simply and quickly during use. 
         [0004]    Fastening arrangements of the stated type are associated in particular with two problems. Firstly, a screw that has been screwed into the sheet stack should be prevented from being able to be removed again from the sheet stack by means of unintentional loosening. Secondly, it is desirable to ensure that the fit of the screw in the sheet stack is as free as possible from backlash. 
         [0005]    A fundamental requirement for the resistance to loosening is the mentioned thread-free shaft portion between the thread and the screw head, it still being entirely permissible, however, for contact between the final thread turn and the sheet stack to remain or to occur precisely in order to provide the mentioned zero backlash or as little backlash as possible of the screw in the fastening arrangement. 
       SUMMARY 
       [0006]    The object of the invention is therefore to specify a screw and thus a fastening arrangement and a use of the screw, in which the screw is resistant to loosening and is a component of the fastening arrangement that is as free as possible from backlash. It should be possible for large tightening forces to be generated and large shearing forces to be absorbed. 
         [0007]    This object is achieved by the independent claims. Advantageous embodiments of the invention are set out in the dependent claims. 
         [0008]    The invention further develops the generic screw in that a head end of the thread is cut off. The thread thus does not end in the manner of a conventional thread on the side of the screw head. Conventional threads gradually peter out, usually as a result of the thread production by cold forming methods, in particular thread rolling. This means that the material protruding through the thread above the shaft gradually reduces as it nears the screw head, until a thread is no longer discernible. The thread “peters out to zero”. This is different in the screw according to the invention. In this case, the head end of the thread is cut off, i.e. said thread ends abruptly and not gradually. There is thus still a significant amount of thread material immediately beside the transition between the thread-free shaft and the thread end. The thread does not peter out to zero, but rather ends abruptly and suddenly. The resistance of the screw to loosening is thereby improved. If the thread-free shaft portion has passed through the screw hole, the cut-off end of the thread makes loosening difficult. In a conventional screw, in which the thread gradually peters out to zero, loosening of the screw is promoted, since there is virtually no resistance to be overcome at the start of the engagement between the thread and the adjacent layer of the fastening arrangement, and there are no specific positions that would prevent the thread from re-engaging in the layers of the fastening arrangement. Minimal force in the return direction and loosening of the screw are sufficient to remove the screw again from the fastening arrangement, since the head thread end of said screw easily engages in the layers of the fastening arrangement. This is different in the screw according to the invention. In this case, the head end of the thread is cut off, with the result that it is far less likely, when the screw is withdrawn and loosened, that the thread end will engage in the stack of the fastening arrangement. Vibrations or other external influences will definitely not result in loss of the screw having the cut-off thread, but this can certainly occur in conventional screws and the “asymptotic” manner in which the thread peters out. 
         [0009]    Rather more specially, the invention is particularly advantageously developed in that, remote from the thread-free shaft portion, the thread has a defined thread depth, and in that the head end of the thread forms a shoulder to the shaft that has a maximum radial height at a starting point that is greater than 10% of the thread depth, preferably greater than 20% of the thread depth, preferably greater than 40% of the thread depth, preferably greater than 60% of the thread depth, preferably greater than 80% of the thread depth, and preferably 100% of the thread depth. Simply a thread shoulder to the shaft having a small radial height of approximately 10% can be sufficient for significantly improving the resistance of the screw to loosening. Depending on the type of fastening arrangement in which the screw is used, larger or smaller radial heights in the region of the shoulder may be useful or necessary. In an extreme case, the thread may be cut off at its full height. In this case, the shoulder has a maximum radial height that is 100% of the thread depth of the screw remote from the thread-free shaft portion, i.e. in the “normal region” of the thread. 
         [0010]    It can be advantageous for the shoulder to be defined by a radial cut having an axial cutting plane. The cutting plane thus produced is therefore perpendicular on the shaft and is parallel to the screw axis. Accordingly, there is no petering-out region of the thread proceeding from the shoulder towards the screw head, which region would have a circumferential component that could, rather, promote engagement of the thread in the stack of the fastening arrangement. 
         [0011]    It can, however, also be provided for the thread, proceeding from the starting point of the shoulder, to end in at least one non-radial end face that tapers towards the shaft. It is thus not necessary to cut off the head end of the thread in a sharp and radial manner. Rather, it is also within the scope of the invention for the thread to end having a non-radial end face that is likewise designed such that there is a larger force for screwing the screw into the fastening arrangement upon loosening and/or less likelihood thereof than in a conventional screw having a thread that peters out in an asymptotic manner. 
         [0012]    The invention further develops the generic prior art in that the thread ends adjacently to the thread-free shaft portion in an end plane that surrounds the shaft at least in part at a constant spacing from the screw head and forms an angle α E  with the screw axis, in that, remote from the thread-free shaft portion, a head flank of the thread forms an angle α F  with the screw axis, and in that the angle α E  is smaller than the angle α F . An end plane of this kind comes into contact with the adjacent layer of the fastening arrangement at a plurality of points on the circumference of the screw, either when there is sufficient tension in the finished state of the fastening arrangement, or when the screw is withdrawn for any reason. In each case, the “flattened” end plane improves the contact between the thread and the rest of the fastening arrangement, with the result that the backlash of the screw in the fastening arrangement is reduced. 
         [0013]    In this context, it can be particularly preferable for the end plane to extend radially. 
         [0014]    According to another embodiment of the invention, it is provided for the thread to have a constant pitch when tapering towards the screw head across a plurality of thread turns, and to end at a pitch of 0 after a kink. The relationships between the angles α E  and α F  result almost automatically from the “kinking” of the thread from a true thread pitch to a thread pitch of 0. The threaded part having a thread pitch of 0 can then again be formed advantageously having a cut-off head end. 
         [0015]    The screw according to the invention is particularly advantageously developed in that the thread-free shaft portion has, at least in part, a diameter that is greater than the core diameter of the thread and smaller than the outer diameter of the thread. Since at least the outermost layer of the layer stack in which the screw is inserted is generally pre-drilled such that the screw, together with the thread thereof, can be guided through the hole without contact, the hole has a significantly greater diameter than the core of the thread. Since the thread-free shaft portion is now selected having, at least in part, a greater diameter than the core of the thread, the backlash in the hole of the outer layer is reduced. 
         [0016]    It can also be provided for the thread-free shaft portion to have, at least in part, a diameter that is smaller than or equal to the core diameter of the thread. As a result, the thread-free shaft portion penetrates the layer arrangement in a frictionless manner. 
         [0017]    It can be provided for the thread-free shaft portion to comprise a step, adjacent to the screw head, having a shaft diameter that is greater than the core diameter of the thread. 
         [0018]    According to a particular embodiment of the present invention, the screw is designed such that the screw is a drilling screw having a shaft that peters out into a boring tip. If the screw is formed as a drilling screw, the layer stack can optionally be provided with pre-drilled holes in one or more layers, or even without any pre-drilled holes. 
         [0019]    It is also possible for the screw to be a drive-out screw having a thread that extends into the tip of the drive-out screw. A drive-out screw removes essentially no material, but rather displaces the material of layers that have not been pre-drilled or have been insufficiently pre-drilled, such that funnel-like structures form when the screw is screwed in. For example, if the lowest layer of a layer arrangement has not been pre-drilled, the funnel-like structure thus forms in this lowest layer. If the length of the thread-free shaft portion, the thickness of the stack and the thickness of the screw are well matched to one another, the funnel-like structure rests, in the finished state, with the edge thereof on the final thread, i.e. in particular on the end plane of the thread, loosening of the screw nonetheless being impossible or unlikely due to, in particular, of the cut-off end of the thread. 
         [0020]    According to a preferred embodiment, it is provided for the thread to be a tapping-screw thread. 
         [0021]    The screw can also be selected such that the thread is a single-start or multi-start thread. 
         [0022]    The invention further consists in a fastening arrangement comprising a layer stack having at least one thin metal sheet and a screw according to the invention, wherein the thickness of the layer stack corresponds to the maximum length of the thread-free shaft portion. In this case, the focus should in particular be the thickness of the layer stack in the end state and in the immediate vicinity of the screw, since this thickness changes when the screw is screwed in, in particular if said screw is a drive-out screw. 
         [0023]    It is preferred for the thin metal sheet to be the layer of the layer stack that is furthest from the screw head. This thin metal sheet is suitable in particular for shaping a funnel edge by the drive-out screw being screwed in, and thus for resting the funnel edge on the thread end in an advantageous manner. 
         [0024]    A particular advantage is that the layer that is furthest from the screw head has a maximum thickness of 1 mm. In particular the thickness of the layer that is furthest from the screw head has a significant influence on the displacement mechanism described, as a result of which specific layer thicknesses have particularly advantageous effects, in particular thicknesses of less than 1 mm or even of 0.7 mm or even of less than 0.5 mm. The method is suitable, on this basis, for attaching profiles and other superstructures that require high tightening values to facades having very thin outer layers. 
         [0025]    Thus, the invention also consists in a use of a screw according to the invention externally or internally on a building. 
         [0026]    In particular, the use is designed for fastening outer or inner metal sheets of sandwich panels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will now be explained on the basis of particularly preferred embodiments, with reference to the accompanying drawings in which: 
           [0028]      FIG. 1  is a partially sectional partial side view of a first embodiment of a screw according to the invention; 
           [0029]      FIG. 2  is a first detail from  FIG. 1 ; 
           [0030]      FIG. 3  is a second detail from  FIG. 1 ; 
           [0031]      FIG. 4  is a side view of a second embodiment of a screw according to the invention; 
           [0032]      FIG. 5  is a first detail from  FIG. 4 ; 
           [0033]      FIG. 6  is a second detail from  FIG. 4 ; 
           [0034]      FIG. 7  is a side view of a third embodiment of a screw according to the invention; 
           [0035]      FIG. 8  is a first detail from  FIG. 7 ; 
           [0036]      FIG. 9  is a second detail from  FIG. 7 ; 
           [0037]      FIG. 10  is a side view of a fourth embodiment of a screw according to the invention; 
           [0038]      FIG. 11  is a detail from  FIG. 10 ; 
           [0039]      FIG. 12  is a side view of a fifth embodiment of a screw according to the invention; 
           [0040]      FIG. 13  is a partially sectional view of a fastening arrangement according to the invention; 
           [0041]      FIG. 14  is a side view of a sixth embodiment of a screw according to the invention; and 
           [0042]      FIG. 15  is a detail from  FIG. 14 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0043]    In the following description of the figures, like reference numerals denote like or similar components. 
         [0044]      FIG. 1  is a partially sectional partial side view of a first embodiment of a screw  10  according to the invention.  FIG. 2  is a first detail from  FIG. 1 .  FIG. 3  is a second detail from  FIG. 1 . The screw  10  has a screw head  14  that provides an engagement point  12  for a tool. A shaft  16  is connected to the screw head  14  and, just like the screw head  14 , defines an axis. The shaft  16  has a thread  18  in portions. The shaft  16  also comprises a thread-free portion  20 . 
         [0045]    The head end  22  of the thread  18  has a particular design. Unlike in conventional screws, said end does not gradually peter out in the shaft  20  in an almost asymptotic manner or “to zero”, but instead is cut off. In this sense, the head end  22  of the thread  18  has a shoulder  24 , wherein the thread, proceeding from a starting point  26  of the shoulder  24 , tapers in a surface  30  towards the shaft  16 . In the present case, the surface  30  has a curvature. Due to the particular design of the head end  22  of the thread  18 , the screw  10  is provided with particular resistance to loosening, since a radial height of the thread that exists virtually to the end of the thread makes it more difficult for the thread to engage in the other components of the fastening arrangement when the screw  10  is withdrawn and loosened. 
         [0046]    A further particular feature of the screw  10  can be identified in the angles α E  and α F  that the end plane  32  of the thread  18  and the normal flanks  34  of the thread  18  respectively form with the screw axis. In this case, the end plane of the thread  32  encloses a smaller angle α E  with the screw axis than the flank  34  does in its angle α F . This ensures better contact of the screw  10  with other components of the fastening arrangement, as a result of which the backlash of the screw  10  in the fastening arrangement is reduced and which also makes a positive contribution to improved resistance to loosening. In the embodiment of  FIGS. 1 to 3 , the angle α E  is slightly larger than a right angle. 
         [0047]      FIG. 4  is a side view of a second embodiment of a screw  10  according to the invention.  FIG. 5  is a first detail from  FIG. 4 .  FIG. 6  is a second detail from  FIG. 4 . The screw  10  shown here corresponds, in most of its parts, to that of  FIG. 1 , although a further particular feature of the screw can be identified here in that said screw has a blunt end. Further differences in the screw according to  FIG. 4  compared with that according to  FIG. 1  relate to the head end  22  of the thread  18  and to the end plane  32  of the thread  18 . 
         [0048]    In this case, the head end  22  of the thread is defined by a radial cut having an axial cutting plane  28 . This provides the main advantages in terms of resistance to loosening. 
         [0049]    In this case, the end plane  32  of the thread encloses a right angle with the screw axis. In any case, here, too, the angle α E  that the end plane  32  encloses with the screw axis is greater than the angle α F  that an otherwise arbitrary flank  34  of the thread  18  forms with the screw axis. 
         [0050]      FIG. 7  is a side view of a third embodiment of a screw  10  according to the invention.  FIG. 8  is a first detail from  FIG. 7 .  FIG. 9  is a second detail from  FIG. 7 . Unlike the screw according to  FIG. 4 , the screw  10  shown here does not have a blunt end but ends in a boring tip  38 . Otherwise, most of the features of the screw  10  are again identical to the screws according to  FIGS. 1 and 4 . In contrast to the screws described above, however, the screw  10  according to  FIG. 7  has further particular properties with respect to the head end  22  of the thread  18  thereof, and to the end plane  32  of the thread. 
         [0051]    In this case, the head end  22  of the thread  18  ends, proceeding from a starting point  26 , in a plane, i.e. a surface without a curvature, that tapers towards the shaft  16 . 
         [0052]    In contrast to the screws according to  FIGS. 1 and 4 , the screw  10  according to  FIG. 7  has an end plane  32  of the thread  18  that encloses an acute angle α E  with the screw axis. Once again, the angle α E  between the end plane and the screw axis is smaller than the angle α F  of an otherwise arbitrary flank  34  of the thread  18 . 
         [0053]      FIG. 10  is a side view of a fourth embodiment of a screw  10  according to the invention.  FIG. 11  is a detail from  FIG. 10 . The screw  10  that can be seen here is a drive-out screw comprising a thread  18  that extends into the tip  40  of the screw  10 . Otherwise, the screw  10  again has particular features with respect to the head end  22  of the thread  18  and the end plane  32  of the thread  18 . 
         [0054]    The head end  22  of the thread is again cut off, in a manner comparable to  FIG. 1 , the end surface  30  of the thread being drawn slightly longer here than in  FIG. 1 . 
         [0055]    The same applies to the end plane  32  of the thread  18  as has been set out above relating to  FIG. 1 . 
         [0056]      FIG. 12  is a side view of a fifth embodiment of a screw  10  according to the invention. The screw  10  according to  FIG. 12  is also a drive-out screw. A particular feature to be mentioned here is that the step  36  of the shaft  16  that is adjacent to the head  14  of the screw  10  is significantly longer here than in the case of the screws described above. 
         [0057]      FIG. 13  is a partially sectional view of a fastening arrangement  52  according to the invention. The fastening arrangement  52  comprises a thin metal sheet  42  and a further metal sheet  44  arranged thereon. The metal sheets  42 ,  44  form a sheet stack or layer stack  42 ,  44 . The fastening arrangement  52  also comprises a screw  10 , the screw head  14  of which rests on the outer metal sheet  44  of the sheet stack  42 ,  44 . A shaft of the screw is guided through holes in the metal sheets  42 ,  44 . Insulating material  46  is arranged on the thin metal sheet  42 . The screw  10  is designed as a drive-out screw. When the screw  10  is screwed into the sheet stack  42 ,  44 , the outer metal sheet  44  is pre-drilled while the thin metal sheet  42  is intact. The tip penetrates into the thin metal sheet  42  and, when the screw  10  is screwed into the sheet stack  42 ,  44 , the thin metal sheet  42  assumes a funnel shape  48  in the vicinity of the screw  10 . When the screw  10  is completely screwed into the sheet stack  42 ,  44 , the edge of the funnel is preferably supported on the end plane of the thread, i.e. on the part of the thread that, in the context of the further figures above, is defined by its angle α E  with the screw axis compared with the angle α F  of an otherwise arbitrary flank of the thread with the screw axis. The supported situation can either always be present, or can arise only when the screw  10  is withdrawn, specifically when said screw again has a certain backlash in the fastening arrangement  52 . The screw  10  obtains resistance to loosening in that the head end of the thread  18  is designed as described in conjunction with the further figures above. As a result, it is made more difficult or less likely for the screw  10  to penetrate into the funnel edge  48  when the screw  10  is loosened. 
         [0058]      FIG. 14  is a side view of a sixth embodiment of a screw according to the invention.  FIG. 15  is a detail from  FIG. 14 . The screw  12  shown here comprising its screw head  14  that provides an engagement point  12 , its shaft  16  that has a thread  18 , its thread-free shaft portion  20 , its tip  40  and its step  36  between the shaft  16  and the screw head  14  in turn has a particular design with regard to the head end  22  of the thread  18  and to the end plane  32  of the thread  18 . The thread  18  tapers towards the screw head  14  at a constant or almost constant pitch as far as a kink  50 . After the kink  50 , the thread has a smaller pitch as far as its cut-off end  22 . In the present case, the thread end has a pitch of 0 after the kink  50 . If the thread  19  otherwise has a comparable shape before the kink  50  and after the kink  50 , the angle between the screw axis and the end plane  32  is again smaller than the angle between the screw axis and the thread flank  34  at other points. The shoulder  26  on the end  22  of the thread in turn forms the transition to an end face  30 , by means of which the thread tapers towards the shaft  16 . 
         [0059]    Up to now, screws have been described of which the threads have symmetrical thread flanks remote from the thread-free region. However, the invention can also be achieved using screws that have asymmetrical thread flanks 
         [0060]    The features of the invention disclosed in the above description, the drawings and in the claims can be essential to the implementation of the invention both individually and in any combination. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           10  screw 
           12  engagement point 
           14  screw head 
           16  shaft 
           18  thread 
           20  thread-free shaft portion 
           22  head end 
           24  shoulder 
           26  starting point 
           28  cutting plane 
           30  surface 
           32  end plane 
           34  thread flank 
           36  step 
           38  boring tip 
           40  tip 
           42  thin metal sheet 
           44  outer metal sheet 
           42  sheet stack/layer stack 
           44  sheet stack/layer stack 
           46  insulating material 
           48  funnel shape 
           50  kink 
           52  fastening arrangement