Abstract:
The present invention is a self-piercing rivet wherein the hollow shell has an external spiraling, wherein the spiraling is with respect to the axis of rotation of the hollow shell. Once installed in a stack of sheets, the spiraling prevents the riveted sheets from mutually separating and the rivet loosening with respect to the sheets.

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 spiraling which resists pull-out of the rivet with respect to 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 prevents the self-piercing rivet  10  and the sheets  22  from releasing from one another under normal loading conditions. However, there is yet the possibility that under certain loading conditions the self-piercing rivet could be loosened from the sheets, in which case the sheets may then become mutually spaced apart or even separated from each other. 
     Accordingly, what is needed in the art is some way to prevent loosening of sheets mutually attached by a self-piercing rivet. 
     SUMMARY OF THE INVENTION 
     The present invention is a self-piercing rivet wherein the hollow shell thereof has an external spiral feature, wherein the spirals thereof constitute external surface features (intrusive or protrusive) which twist (or turn) about the axis of rotation of the hollow shell (or simply put, the shell axis) from generally between the head and the point. Since forces which will tend to loosen the rivet and separate the sheets must have a component parallel to the shell axis of the rivet, once the spiraled self-piercing rivet has been installed in a stack of sheets, the rivet is prevented from being loosened from the sheets because the spirals of the spiral feature resist interferingly (with respect to the sheets) any component of force which is parallel to the shell axis. 
     A preferred external spiral feature is provided by the exterior surface of the hollow shell having a cornered geometry (for example, a square, a hexagon, an octagon, etc.), wherein the corners are twisted about the shell axis from the head to the point. The pitch of the twist is preferably coarse; for example, between about one-quarter turn to about one or two turns over the length of the hollow shell. 
     In another preferred external spiral feature, irregular surface features extend twistingly generally between the head and the point. The irregular surface features may be either protruding surface features, such as for example ribs, or intruding surface strutures, such as for example flutes (i.e., grooves or slots). 
     In operation, as the spiraled self-piercing rivet is driven into a plurality of sheets (composed of for example, metal or metal composite) to be joined, the spiral features interact with the sheets such that after joinder, the sheets are prevented from mutual separation because the component of applied forces which is parallel to the shell axis is resisted by an interference relationship between the spiral features and the stack of sheets. 
     Accordingly, it is an object of the present invention to provide a self-piercing rivet having a spiral feature which serves to resist loosening of the rivet with respect to the 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 a spiraled self-piercing rivet according to the present invention, wherein the spiraling is provided by a plurality of twisted corners on the external surface of the hollow shell. 
     FIG. 6 is a sectional view of the axially asymmetric self-piercing rivet, seen along line  6 — 6  of FIG.  5 . 
     FIG. 7 is a sectional side view of a stack of sheets and the spiraled self-piercing rivet of FIG. 5, showing the rivet joinder thereby created. 
     FIG. 8 is a schematic side view of a ram operatively interfaced with the spiraled self-piercing rivet of FIG.  5 . 
     FIG. 9 is a side view of a second preferred spiraled self-piercing rivet according to the present invention, wherein the spiral feature is provided by a plurality of twisted flutes. 
     FIG. 10 is a partly sectional view of the spiraled self-piercing rivet, seen along line  10 — 10  of FIG.  9 . 
     FIG. 11 is a sectional side view of a stack of sheets and the spiraled self-piercing rivet of FIG. 9, showing the rivet joinder thereby created. 
     FIG. 12 is a side view of a third preferred spiraled self-piercing rivet according to the present invention, wherein the spiral feature is provided by a plurality, of twisted ribs. 
     FIG. 13 is a partly sectional view of the spiraled self-piercing rivet, seen along line  13 — 13  of FIG.  12 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to the Drawing, FIGS. 5 through 13 depict various aspects and examples of a spiraled self-piercing rivet according to the present invention. 
     Referring firstly to FIGS. 5 through 8, a first preferred spiraled self-piercing rivet  100  is depicted. The hollow cylinder  102  has an axis of rotation 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. 
     A spiral feature  110  of the hollow shell  102  is provided by the exterior surface  108  thereof having a plurality of corners  112  which twist (or turn) to form spirals about the shell axis A′ between the head  106  and the point  104 . The number of corners is preferably related to that of a symmetrically balanced geometrical object having corners, such as a square (having 4 corners) or a hexagon (having 6 corners), wherein the number of corners may be other than those exemplified. The corners  112  may be radiused to “soften” the right-angularity of the corners so as to inhibit nonuniform stretch of the rivet during inserting and risk of rivet splitting. 
     In operation, the spiraled 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 (i.e., aluminum, another metal, or metal composite)  114 ,  116  are stacked at the die and the ram is then stroked. FIG. 7 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, the corners  112  now provide an interference location with respect to at least one of the sheets which resists any force component applied to the sheets in a direction parallel to the shell axis A′ which would tend to loosen the rivet with respect to the sheets. 
     In order to ensure a tight, clamping fit between the spiral feature (spirals)  110  of the spiraled self-piercing rivet and the sheets where the sheets are non-resilient, as in the case of aluminum sheet, it is desirable to include a drive interface  120  between the rivet and the ram so that the ram turns the rivet at a rate of rotation consonant with the penetration speed of the rivet into the sheets and the pitch of the spiral features. For example, the drive interface  120  may be in the form of the ram having a driver  122  which interfaces with a slot  124  in the head  106  to thereby rotate the rivet. A load cell, other sensor, or the mechanical function of the movement of the ram relative to the die can be used to synchronize the rotation rate of the rivet with respect to the speed of insertion of the rivet. 
     Alternative to corners  112 , the spiral feature may be in the form of irregular surface features  110 ′ which extend spirally (twistingly) generally between the head and the point. The irregular surface features  110 ′, as shown at FIGS. 9 through 13, may be either intruding surface features  112 ′, such as for example a plurality of flutes, or protruding surface structures  112 ″, such as for example a plurality of ribs. 
     As shown firstly at FIGS. 9 through 11, the second preferred spiraled self-piercing rivet  100 ′ has intruding surface features (i.e., flutes, slots or grooves)  112 ′, which may be of any shape, preferably circularly concave. By way of example, there may be four flutes equally spaced apart circumferentially around the exterior surface  108 ′, having for example, between one-quarter and two turns over the length of the hollow shell  102 ′. 
     In operation, the spiraled 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 (i.e., aluminum, another metal, or metal to composite)  114 ′,  116 ′ are stacked at the die and the ram is then stroked. FIG. 11 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, at least one of the sheets intrudes radially inward with respect to the exterior surface  108 ′ into the flutes  112 ′. As a result, an interference fit between the sheets and the spiraled self-piercing rivet is established which prevents forces acting parallel to the shell axis from loosening the sheets relative to the rivet, as well as relative separation of the sheets. 
     Referring finally to FIGS. 12 and 13, a third preferred axially asymmetric self-piercing rivet  100 ″ is depicted. As in FIG. 9, the hollow cylinder  102 ″ has an axis of rotation A′″ and terminates at one end in a point  104 ″ and is connected to a head  106 ″ which is transversely oriented relative to the axis A′″. The head  106 ″ has a larger diameter than that of the hollow cylinder such that it overhangs the hollow cylinder. 
     The second preferred spiraled self-piercing rivet  100 ″ has protruding surface features (i.e., ribs)  112 ″, which may be of any shape, preferably circularly convex. By way of non-limiting example, there may be four ribs equally spaced apart circumferentially around the exterior surface  108 ″, having for example, between one-quarter and two turns over the length of the hollow shell  102 ″. 
     In operation, the spiraled 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 (i.e., aluminum, another metal, or metal to composite) 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. 11 except that now each sheet has respective portions which are invaded by the ribs  112 ″ 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 spiraled self-piercing rivet is established which prevents loosening of the rivet and relative separation of the sheets. 
     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.