Abstract:
A system for attaching layers of a material stack-up of at least three layers together using rivets alternatingly place on each side of the stack-up is disclosed. The system includes a material stack-up comprising an upper layer, an intermediate layer and a lower layer, a first rivet attaching the upper layer and the intermediate layer, and a second rivet attaching the lower layer and the intermediate layer. The first rivet attaches the upper layer to the intermediate layer through the upper layer and the second rivet attaches the lower layer to the intermediate layer through the lower layer. The first and second rivets are alternatingly positioned in a spaced apart relationship. The rivets are selected from the group consisting of self-piercing rivets, blind rivets and solid rivets. When self-piercing rivets are used, the first rivet includes rivet tail and the second rivet includes rivet tail.

Description:
TECHNICAL FIELD 
       [0001]    The disclosed inventive concept relates generally to the riveting of a workpiece. More particularly, the disclosed inventive concept relates to a system for riveting from opposite sides of a workpiece. 
       BACKGROUND OF THE INVENTION 
       [0002]    The automobile manufacturing industry is constantly faced with new challenges in a wide array of areas including vehicle safety, reliability, durability and cost. Perhaps the greatest challenge faced by the automobile industry today is the need to improve fuel mileage to both decrease carbon emissions and increase fuel economy for both environmental and cost reasons, all without compromising safety, power or durability. In 2011, new fuel economy requirements were imposed that establish a US vehicle fleet average of 54.5 miles per gallon by 2025. As the industry moves to that target year fuel annual economy requirements will be ramped up for different-sized vehicles. 
         [0003]    Efforts have been made to increase fuel economy for vehicles. These efforts can be divided into two approaches: the “supply” side and the “demand” side. 
         [0004]    On the supply side attention is drawn to improving energy conversion efficiency through use of, for example, electric or hybrid-electric drive trains. In addition, new vehicle drive trains, including smaller engines and more efficient transmission having multiple gears and transfer cases, are being developed and employed. Other technologies, including start-stop and engine cylinder deactivation strategies, are also proving effective at decreasing fuel consumption. Improved transmissions with multiple gears are also important elements to increased fuel consumption efficiencies. 
         [0005]    On the demand side weight reduction is key, though other aspects, such as improved aerodynamics and drag reduction, are also important. Conventional vehicles, particularly trucks, rely on steel components. For over 100 years the material of choice for most vehicles is steel. Today steel makes up about 60% of the average car by weight. 
         [0006]    Despite the improvement in steel composition the weight of steel regardless of type remains significant. It is also possible to reduce vehicle weight when steel is used by reducing component thickness. However, at a certain point it is no longer practical to reduce steel thickness regardless of the steel grade used. The use of high strength steel or advanced, high strength steel does not improve the realization that there are limits to how much vehicle weight can be reduced by steel thickness reduction without compromising vehicle performance. 
         [0007]    Thus as the automotive industry continues to focus on light weighting vehicles to meet customer expectations on fuel economy and CAFE requirements, interest in alternative materials including aluminum intensive vehicle applications has increased. This is because vehicle weight reduction is most directly accomplished through substituting lighter materials for currently used steel parts. However, a limited variety of materials are available as a substitute for automotive steel. One such material is carbon fiber which is both lightweight and strong. 
         [0008]    While carbon fiber offers certain performance advantages, replacement of the steel body-in-white with carbon fiber is expensive and brings with it a relatively slow production process. 
         [0009]    Accordingly, much attention is drawn to the use of aluminum which is about ⅓ the weight of steel. Aluminum is not a new material for automotive use and has been used as a material for castings for over 100 years. The use of aluminum not only provides weight reduction but also results in good crash performance. Research has shown that in collisions aluminum can perform as well as conventional steel and demonstrates the ability to absorb twice the crash energy per pound of mild steel, having good buckling and energy absorption characteristics. 
         [0010]    In body-in-white structures, joining methods have traditionally relied on resistance-spot welding (e.g., in steel structures). In the case of aluminum intensive vehicles and other mixed metal joining applications, self-piercing rivet (SPR) technology prevails. One advantage of self-piercing rivet technology is that it is a high production volume assembly process. Further, it is compatible with an adhesive, where both methods can be used in conjunction. 
         [0011]    The challenge often faced when using the self-piercing rivet to fasten together multiple layers is that the substrate material must have sufficient thickness to enable a satisfactory mechanical interlock between the rivet and the bottom layer while simultaneously avoiding a rivet break-through out of the lower layer. Production downtime due to rivet break-through can be exacerbated for applications which contain adhesive as the glue can contaminate the rivet equipment. Additionally, material stack-ups that are three layers (3T) and greater can be especially challenging to rivet as the bottom layer thickness relative to the total stack is too small to provide adequate interlock. 
         [0012]    In cases where riveting a 3T application is not possible due to no interlock, the situation may be remedied occasionally by using a rivet having a greater length. In some instances, however, a solution is not found prior to causing break-through when increasing the length of the rivet. Conversely, when break-through occurs there are applications where using a shorter rivet will result in no interlock, resulting in a joint having no mechanical strength. 
         [0013]    As in so many areas of vehicle technology there is always room for improvement related to the mechanical fastening of the materials through self-pierce riveting. 
       SUMMARY OF THE INVENTION 
       [0014]    The disclosed inventive concept overcomes the problems associated with known systems for riveting a material stack-up of at least three layers. The system includes a material stack-up comprising an upper layer, an intermediate layer and a lower layer, a first rivet attaching the upper layer and the intermediate layer, and a second rivet attaching the lower layer and the intermediate layer. The first rivet attaches the upper layer to the intermediate layer through the upper layer and the second rivet attaches the lower layer to the intermediate layer through the lower layer. 
         [0015]    The first and second rivets are alternatingly positioned in a spaced apart relationship. The rivets are selected from the group consisting of self-piercing rivets, blind rivets and solid rivets. When self-piercing rivets are used, the first rivet includes rivet tail and the second rivet includes rivet tail. The rivets are alternatingly positioned such that the tail of the first rivet are adjacent the tail of the second rivet when the layers of material are attached by the rivets. 
         [0016]    Optionally, an adhesive may be included between the upper layer and the intermediate layer. Alternatively or additionally, an adhesive may also be included between the intermediate layer and the lower layer. 
         [0017]    By alternating rivets according to the disclosed inventive concept, only two layers are joined from each side, thereby avoiding difficulties such as “break-through” of the bottom layer where adhesive is exposed and can contaminate the rivet die and associated equipment and the “no interlock” result between layers where the rivet has not splayed sufficiently to lock the layers together. Accordingly, the disclosed inventive concept enables greater application of rivet joining, particularly self-piercing rivet joining, and more particularly in difficult stacks, such as where thin layers are on the bottom of the sheet metal stack-up. 
         [0018]    The above advantages and other advantages and features will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    For a more complete understanding of this invention, reference should now be made to the embodiments illustrated in greater detail in the accompanying drawings and described below by way of examples of the invention wherein: 
           [0020]      FIG. 1A  is a schematic illustration of the first step of a self-piercing rivet process according to the prior art in which the blankholder and the punch are in position above the rivet prior to pressure being applied to the punch; 
           [0021]      FIG. 1B  is a schematic illustration of the second step of the self-piercing rivet process according to the prior art in which initial pressure has been applied to the punch; 
           [0022]      FIG. 1C  is a schematic illustration of the third step of the self-piercing rivet process according to the prior art in which the rivet has pierced the upper layer and is interlocked into the lower layer; 
           [0023]      FIG. 1D  is a schematic illustration of the fourth step of the self-piercing rivet process according to the prior art in which the rivet process has been completed and the punch and blankholder have been removed; 
           [0024]      FIG. 2A  is a cross-section view of a self-piercing rivet joint illustrating a break-through on the lower layer according to the prior art; 
           [0025]      FIG. 2B  is a plan view of the lower layer where the self-piercing rivet joint has broken through according to the prior art; 
           [0026]      FIG. 3A  is a cross-section view of a self-piercing rivet joint illustrating a rivet that has not splayed sufficiently to lock layers of sheet-metal together according to the prior art; 
           [0027]      FIG. 3B  is a cross-section view of a self-piercing rivet joint illustrating a rivet that has splayed sufficiently to lock layers of sheet-metal together according to the prior art; and 
           [0028]      FIG. 4  is a cross-section view of a rivet arrangement according to the disclosed inventive concept in which the rivets are inserted in alternating directions. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0029]    In the following figures, the same reference numerals will be used to refer to the same components. In the following description, various operating parameters and components are described for different constructed embodiments. These specific parameters and components are included as examples and are not meant to be limiting. 
         [0030]    The disclosed inventive concept may find use in any number of applications where plural layers of the same or dissimilar materials are being attached. Accordingly, the disclosed inventive concept may be used in the production of automotive vehicles and trucks. 
         [0031]    The use of self-piercing rivets in the assembly of plural components is a known technique as illustrated in  FIGS. 1A through 1D . These figures schematically show steps involved in the self-piercing rivet process. As the rivet is inserted into the stack. the material deforms into the die and the resultant form is called a “button.” 
         [0032]    As illustrated in  FIG. 1A , the first step of a self-piercing rivet process according to the prior art is illustrated. A first layer is shown in position over a second layer  12 . A rivet  14  is illustrated in position over the first layer  10 . A punch  16  and a blankholder  18  are illustrated in position with the rivet  14  prior to pressure being applied to the punch  16 . A die  20  is in position beneath the second layer  12 . 
         [0033]    In  FIG. 1B , the second step of the self-piercing rivet process according to the prior art is illustrated. In this step, initial pressure has been applied to the punch  16  and the rivet  14  is shown beginning to deform the first layer  10  and the second layer  12 . 
         [0034]    In  FIG. 1C , the third step of the self-piercing rivet process according to the prior art is illustrated. In this step, the punch  16  has been fully inserted through the blankholder  18  such that the rivet  14  pierced the first layer  10  and forms the second layer  12 . 
         [0035]    In  FIG. 1D , the fourth step of the self-piercing rivet process according to the prior art is illustrated. In this step, the rivet  14  is shown fully inserted through the first layer  10  and a button is formed in the second layer  12 . The punch  16  and the blankholder  18  have been moved out of contact with the first layer  10 . 
         [0036]    While a valuable mechanical fastener in many automotive and other assembly applications, use of the self-piercing rivet is occasionally challenged by the fact that the substrate material must have sufficient thickness to enable mechanical interlock between the rivet and the bottom layer while simultaneously avoiding a condition known in the industry as “rivet break-through.” This condition is illustrated in  FIGS. 2A and 2B . 
         [0037]    Referring to  FIG. 2A , a cross-section view of a self-piercing rivet joint is shown and is generally illustrated as  30 . The self-piercing rivet joint  30  includes a material stack-up  32  and a self-piercing rivet  34 . The material stack-up  32  includes a first or upper layer  36 , a second or middle layer  38 , and a third or lower layer  40 . Thus the second or middle layer  38  is sandwiched between the first or upper layer  36  and the third or lower layer  40 . An upper adhesive layer  42  is formed between the first or upper layer  36  and the second or middle layer  38 . A lower adhesive layer  44  is formed between the second or middle layer  38  and the third or lower layer  40 . 
         [0038]    When riveted, the rivet joint  30  may experience “break-through” where some of the lower adhesive layer  44  is exposed at a breach  46  formed through the third or lower layer  40 . The breach  46  is shown more fully in  FIG. 2B  which is a plan view of the third or lower layer  40 . In the event that the breach  46  is formed, a portion of the adhesive of the lower adhesive layer  44  is exposed and may contaminate the self-piercing rivet die and installation equipment, thus exacerbating production downtime due to rivet break-through in material stack-ups where adhesive is used as glue between layers. 
         [0039]    Self-piercing rivets suffer from shortcomings in other applications as well. Referring to  FIG. 3A , a cross-section view of a self-piercing rivet joint, generally illustrated as  50 , is shown. The self-piercing rivet joint  50  includes a material stack-up  52  and a self-piercing rivet  54 . The material stack-up  52  includes a first or upper layer  56 , a second or middle layer  58 , and a third or lower layer  60 . Thus the second or middle layer  58  is sandwiched between the first or upper layer  56  and the third or lower layer  60 . The self-piercing rivet  54  includes a pair of spaced-apart and opposed tails  62  and  62 ′. 
         [0040]    When riveted, the rivet joint  50  may suffer from a “no interlock” condition in which tails  62  and  62 ′ do not splay sufficiently as illustrated. Under such a circumstance, the self-piercing rivet  54  fails to lock together the first or upper layer  56 , the second or middle layer  58 , and the third or lower layer  60 . 
         [0041]    The failed results of the rivet joint  50  may be compared with an acceptable interlock illustrated in  FIG. 3B  in which a cross-section view of a self-piercing rivet joint, generally illustrated as  70 , is shown. The self-piercing rivet joint  70  includes a material stack-up  72  and a self-piercing rivet  74 . The material stack-up  52  includes a first or upper layer  76  and a second or middle layer  78 . The self-piercing rivet  74  includes a pair of spaced-apart and opposed tails  80  and  80 . As illustrated, because the material stack-up  72  includes only two layers as opposed to three layers of the material stack-up  52  of  FIG. 3A , the self-piercing rivet  74  has less material to pierce and thus the tails  80  and  80 ′ can more easily splay to their proper position in which an acceptable interlock can be achieved as shown in  FIG. 3B . Accordingly, material stack-ups which are three layers (3T) and greater can be especially challenging to rivet as the bottom layer thickness relative to the total stack is too small to provide adequate interlock. 
         [0042]    The disclosed inventive concept combines the relative effectiveness of riveting two layers of material with the advantage of using rivets to attach material stack-ups having three or more layers while preventing lower layer “break-through” and consequent adhesive exposure and while also preventing a “no interlock” condition that is often found when three layers of material are riveted using a single rivet. 
         [0043]    Particularly, and referring to  FIG. 4 , a cross-section view of a self-piercing rivet joint according to the disclosed inventive concept is shown and is generally illustrated as  90 . The self-piercing rivet joint  90  includes a material stack-up  92  and self-piercing rivets  94 ,  94 ′,  94 ″ and  94 ′″ that are alternatingly positioned on the self-piercing rivet joint  90  such that the self-piercing rivets  94  and  94 ″ enter the self-piercing rivet joint  90  through one side while the self-piercing rivets  94 ′ and  94 ′″ enter the self-piercing rivet joint  90  through the opposite side. Any number of self-piercing rivets may thus be used provided that some degree of alternating directions between one side and the other is employed. Furthermore, while self-piercing rivets are illustrated, it is to be understood that other types of rivets, such as blind rivets and solid rivets, may be employed as well, 
         [0044]    The material stack-up  92  includes a first or upper layer  96 , a second or middle layer  98 , and a third or lower layer  100 . Thus the second or middle layer  98  is sandwiched between the first or upper layer  96  and the third layer or lower layer  100 . However, it is to be understood that more than three layers of material may be included in the material stack-up. The first or upper layer  96 , the second or middle layer  98  and the third or lower layer  10  may be any of a variety of materials including metals such as steel or, more particularly, carbon steel grade (DP800) or carbon-fiber composites. An adhesive may be applied between said first or upper layer  96  and said second or middle layer  98  prior to assembly. In addition or alternatively, an adhesive may be applied between said third or lower layer  100  and said second or middle layer  98  prior to assembly, 
         [0045]    Thus arranged, each of rivets  94 ,  94 ′,  94 ″ and  94 ′″ join only two layers. Specifically, and as illustrated, the self-piercing rivets  94  and  94 ″ join the first or upper layer  96  and the second or middle layer  98  while the self-piercing rivets  94  and  94 ′″ join the third or lower layer  100  and the second or middle layer  98 . When all of the rivets  94 ,  94 ,  94 ″ and  94 ′″ are considered in combination, the resulting monolith represented as the self-piercing rivet joint  90  is robustly joined. The disclosed inventive concept may be extended to material stack-ups having more than three layers. 
         [0046]    The disclosed inventive concept enables greater application of rivet joining. particularly self-piercing rivet joining, and more particularly in difficult stacks, such as where thin layers are on the bottom of the sheet metal stack-up. The disclosed inventive concept also avoids the weakening of such joints via other solutions such as “scalloping,” or separating the application into two two-layer stack-ups, thus potentially eliminating adhesive applicability, saving both material and labor costs. 
         [0047]    For at least the above reasons the disclosed invention as set forth above overcomes the challenges faced by known methods for riveting multiple layers of material by rivets inserted with alternating directions. However, one skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the true spirit and fair scope of the invention as defined by the following cairns.