Abstract:
The present invention is a self-piercing rivet wherein the hollow shell has an external axial asymmetry wherein the asymmetry is with respect to the axis of rotation of the hollow shell. Once installed in a stack of sheets, the asymmetric self-piercing rivet prevents relative sheet rotation of the sheets relative to the axis because of rotational interference caused by the external axial asymmetry of the hollow shell.

Description:
TECHNICAL FIELD 
     The present invention relates to self-piercing rivets used to join together metallic sheets, and more particularly to a self-piercing rivet having features which inhibit rotation of the rivet with respect to the metallic sheets joined thereby. 
     BACKGROUND OF THE INVENTION 
     The joining of metallic sheets can be accomplished by various mechanical modalities, as for example, threaded fasteners, rivets and welding. The modality used depends upon the application. One type of rivet having an excellent ability to join together a stack of metallic sheets is a self-piercing rivet. 
     Various aspects of a prior art self-piercing rivet are shown at FIGS. 1 through 4. A self-piercing rivet  10  has a hollow shell  12  of cylindrical shape which is closed at one end by an overhanging head  14  and which has at the opposite end a point  16 . A ram  18  and opposing die  20  are used to drive the self-piercing rivet  10 , point  16  first, into two or more sheets  22 , typically a metal, such as for example aluminum. The ram  18  has a convex contour  24  and the die has a concave contour  26 , such that after stroking of the ram, the hollow shell  12  pierces the sheets  22  with a deformation D defined by the concave contour  26  and wherein the head  14  is countersunk by the convex contour  24 . The deformation D involves a bending of the hollow shell  12  outwardly so as to lock the self-piercing rivet  10  in position with respect to the sheets  22  such as to prevent its removal from the sheets and, as a result, any possible disjoinder of the sheets. 
     It will be noticed that the deformation D, while preventing the self-piercing rivet and the sheets  22  from releasing from one another, involves a circular symmetry about the shell axis A of the hollow shell  12 . Because of this a circular symmetry, there is the possibility that, in spite of rivet clamping and the resultant high frictional forces which are present, the sheets  22  may, over time, rotatively wander relative to each other and one or more with respect to the self-piercing rivet. The possibility of rotational wandering of the sheets relative to each other can be undesirable in a number of self-piercing rivet applications. 
     Accordingly, what is needed in the art is some way to prevent rotational wandering of sheets joined by a self-piercing rivet. 
     SUMMARY OF THE INVENTION 
     The present invention is a self-piercing rivet wherein the hollow shell has an external axial asymmetry wherein the asymmetry is with respect to the centerline axis of the hollow shell (or simply put, the shell axis). Once installed in a stack of sheets, the asymmetric self-piercing rivet prevents relative sheet rotation of the sheets relative to the shell axis because of rotational interference caused by the external axial asymmetry of the hollow shell. 
     A preferred external axial asymmetry of the hollow shell is provided by the exterior surface of the hollow shell having irregular surface features which are oriented generally parallel to the shell axis. The irregular surface features may be either protruding surface features, such as for example ribs, or intruding surface structures, such as for example flutes (i.e., grooves or slots). In operation, as the asymmetric self-piercing rivet is driven into a plurality of sheets (composed of for example metal or metal composite) to be joined, the irregular surface features interact with the sheets such that after joinder, the sheets are prevented from relative rotation because of an interference fit between the sheets and the irregular surface features. 
     The external axial asymmetry can be provided alternatively, or in addition to the aforementioned surface irregularity features, by any geometrical configuration of the hollow shell which is non-circular with respect to the shell axis, as for example an oval or square geometry with respect to the shell axis of rotation of the hollow shell. 
     Accordingly, it is an object of the present invention to provide a self-piercing rivet having an external axial asymmetry which serves to prevent relative rotation of sheets joined thereby. 
     This and additional objects, features and advantages of the present invention will become clearer from the following specification of a preferred embodiment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional side view of a prior art self-piercing rivet. 
     FIG. 2 is a top end view of the self-piercing rivet, seen along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a schematic side view of a prior art ram and die mechanism, shown operative with respect to a prior art self-piercing rivet and a stack of sheets to be riveted. 
     FIG. 4 is a sectional side view of the stack of sheets and prior art self-piercing rivet of FIG. 3, showing the rivet joinder created after the ram has stroked. 
     FIG. 5 is a side view of an axially asymmetrical self-piercing rivet according to the present invention; wherein the axial asymmetry is provided by a plurality of external surface irregularities in the form of a plurality of axially oriented flutes. 
     FIG. 6 is a top end view of the axially asymmetric self-piercing rivet, seen along line  6 — 6  of FIG.  5 . 
     FIG. 7 is a sectional view of the axially asymmetric self-piercing rivet, seen along line  7 — 7  of FIG.  5 . 
     FIG. 8 is a sectional side view of a stack of sheets and the axially asymmetric self piercing rivet of FIG. 5, showing the rivet joinder thereby created. 
     FIG. 9 is a side view of a second preferred axially asymmetrical self-piercing rivet according to the present invention, wherein the axial asymmetry is provided by a plurality of external surface irregularities in the form of a plurality of axially oriented ribs. 
     FIG. 10 is a top end view of the second preferred axially asymmetric self-piercing rivet, seen along line  10 — 10  of FIG.  9 . 
     FIG. 11 is a side view of a third preferred axially asymmetrical self-piercing rivet according to the present invention, wherein the axial asymmetry is provided by an asymmetrical geometry of the hollow shell. 
     FIG. 12 is a top end view of the third preferred axially asymmetric self-piercing rivet, seen along line  12 — 12  of FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Drawing, FIGS. 5 through 11 depict various aspects and examples of an axially asymmetric self-piercing rivet according to the present invention. 
     Referring firstly to FIGS. 5 through 8, a first preferred axially asymmetric self-piercing rivet  100  is depicted. The hollow cylinder  102  has an axis of rotation (shell axis) A′ and terminates at one end in a point  104  and at the opposite end is connected to a head  106  which is oriented transversely with respect to the shell axis A′. The head  106  has a larger diameter than that of the hollow cylinder such that it overhangs the hollow cylinder. 
     External axial asymmetry of the hollow shell  102  is provided by the exterior surface  108  of the hollow shell having a plurality of irregular surface features  110  which are oriented parallel to the shell axis A′ and radially intrude into the exterior surface, such as for example flutes  112 . The flutes  112  may be of any shape, preferably circularly concave. By way of example, there may be eight flutes equally spaced apart circumferentially around the exterior surface  108 . 
     In operation, the asymmetric self-piercing rivet  100  is placed at the ram of a ram and die mechanism as generally depicted at FIG. 3, and sheets of metal or metal composite (i.e., aluminum sheets)  114 ,  116  are stacked at the die and the ram is then stroked. FIG. 8 depicts an example of the deformation D′ resulting from the ram and die mechanism driving the asymmetric self-piercing rivet into the sheets. As can be discerned by this exemplification, each sheet  114 ,  116  has a respective portion  114   p ,  116   p  which has intruded radially inward with respect to the exterior surface  108  into the flutes  112 . As a result, an interference fit between the sheets and the asymmetric self-piercing rivet is established which prevents relative rotation of the sheets. 
     Referring next to FIGS. 9 and 10, a second preferred axially asymmetric self-piercing rivet  100 ′ is depicted. As in FIG. 5, the hollow cylinder  102 ′ has an axis of rotation (shell axis) A″ and terminates at one end in a point  104 ′ and is connected to a head  106 ′ which is transversely oriented relative to the shell axis A″. The head  106 ′ has a larger diameter than that of the hollow cylinder such that it overhangs the hollow cylinder. 
     External axial asymmetry of the hollow shell  102 ′ is now provided by the exterior surface  108 ′ of the hollow shell having a plurality of irregular surface features  110 ′ which are oriented parallel to the shell axis A″ and radially protrude from the exterior surface, such as for example ribs  118 . The ribs  118  may be of any shape, preferably circularly convex. By way of example, there may be eight ribs equally spaced apart circumferentially around the exterior surface  108 ′. 
     In operation, the asymmetric self-piercing rivet  100 ′ is placed at the ram of a ram and die mechanism as generally depicted at FIG. 3, and sheets of metal or metal composite (i.e., aluminum sheets) are stacked at the die and the ram is then stroked. The deformation resulting from the ram and die mechanism driving the asymmetric self-piercing rivet into the sheets is similar to that shown at FIG. 8 except that now each sheet has respective portions which are invaded by the ribs  118  in a direction which is radially outward with respect to the exterior surface  108 ′. As a result, an interference fit between the sheets and the asymmetric self-piercing rivet is established which prevents relative rotation of the sheets. 
     Referring lastly to FIGS. 11 and 12, a third preferred axially asymmetric self-piercing rivet  100 ″ is depicted. A hollow cylinder  102 ″ has a shell axis A′″ and terminates at one end in a point  104 ″ connected to a head  106 ″ which is transversely oriented relative to the shell axis A′″. 
     The external axial asymmetry of the hollow shell  102 ″ is provided by the exterior surface  108 ″ of the hollow shell having any axially asymmetric geometrical configuration of the hollow shell which is not circular with respect to the shell axis A′″. While an oval shape of the hollow shell, which serves also to improve mechanical strength at the joinder, is shown at FIG. 12, wherein L 1 &gt;L 2 , other shapes are possible, such as triangles, squares, hexagons, etc. The head  106 ″ is connected in transverse orientation to the hollow shell  102 ″ opposite the point  104 ″, and overhangs the hollow shell. The shape of the head may be similar to the non-circular shape of the hollow shell (as shown at FIG. 12) or may be circular irrespective of the shape of the hollow shell. 
     In operation, the asymmetric self-piercing rivet  100 ″ is placed at the ram of a ram and die mechanism as generally depicted at FIG. 3, and sheets of metal or metal composite (i.e., aluminum sheets) are stacked at the die and the ram is then stroked. The deformation D′ resulting from the ram and die mechanism driving the asymmetric self-piercing rivet into the sheets may resemble that shown at FIG. 4, except that now the non-circular shape of the hollow shell is imparted to the deformation such that an interference between the sheets and the self-piercing rivet is established which prevents relative rotation of the sheets. 
     The first and second preferred axially asymmetric self piercing rivets  100 ,  100 ′ have an advantage over the third preferred axially asymmetric self-piercing rivet  100 ″ in that no modification of the aforementioned ram and die mechanism would be needed with respect to interchangeability with conventional self-piercing rivets  10 . 
     To those skilled in the art to which this invention appertains, the above described preferred embodiment may be subject to change or modification. Such change or modification can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.