Abstract:
A threaded fasteners combines two or more of (a) a self-drilling tip that is long enough and/or sufficiently distanced from the threads to assure that drilling action is complete before the first thread engages the surface of the work piece, (b) high-low threads, and (c) a factory pre-applied adhesive/sealant. Such fasteners are especially advantageous in applications where connections to fiberglass are needed, and especially in applications such as boats or other transportation vehicles where leaks resulting from drilled cavities are problematic.

Description:
[0001]    This application is a continuation-in-part of U.S. utility application Ser. No. 10/724,560, titled “Fastener for Fiberglass and Other Composite Structures” and filed Nov. 26, 2003, which claims the benefit of U.S. provisional application No. 60/429,870 tiled “Fastener for Fiberglass” filed on Nov. 27, 2002, and U.S. provisional application No. 60/508,143 titled “Fastener for Fiberglass and Other Composite Structures” filed on Oct. 1, 2003, each of which is incorporated herein by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The field of the invention is fasteners and fastening methods.  
         BACKGROUND OF THE INVENTION  
         [0003]    Threaded fasteners can readily damage the fiberglass reinforced plastic (FRP) or other composite material used in applications such as boat building. Among other things, many of the known fasteners can shatter and crack the un-reinforced outer gelcoat surface of the FRP, causing it to spall. Additional problems can occur when the rotating threads of a fastener, which is typically an externally threaded sheet metal screw, bore into the FRP, jamming the threads of the fastener into the bored cavity. The amount of force required to pull the jammed fastener out of the destructively fashioned FRP cavity is minimal, increasing the incidence of fastener “pull-out” and “strip-out”, which occurs when the fastener spins freely in the connection.  
           [0004]    It is known that these problems can be reduced by pre-drilling or pre-forming a cavity into FRP. In such instances it is recommended that the installer use a fastener with tapping threads having a major diameter, the measurement of the greatest outside diameter of the threads (the crest), which is slightly greater than the diameter of the cavity. In the boat building industry such fasteners commonly have type “A” or “AB” threads, but in other industries the fasteners are known to have type “25”, “B”, “Trilobular” “Thread-rolling” or “High-low” (Hi-Lo) threads. High-low threaded fasteners alternate one high and one low thread along the shank of the fastener.  
           [0005]    Pre-drilling or pre-forming a cavity are not, however, completely satisfactory solutions. In addition to the added manufacturing costs involved, pre-forming limits the amount of load that can be placed on the fastener before pull-out or spin-out. Additionally, the diameter of the threads can be only slightly larger than the diameter of the cavity to avoid shattering or cracking the FRP.  
           [0006]    The holding problem can be addressed by dispensing an adhesive/sealant (such as 3M™ 5200 adhesive/sealant) into the cavity. The adhesive aspect of the dispensed material bonds the fastener into the cavity, thereby reducing the chance of fastener spin-out and/or pull-out. The sealant aspect of the dispensed material helps to prevent moisture and water from migrating into the cavity. For brevity herein, the term adhesive/sealant is used to denote any material that comprises an adhesive and/or a sealant.  
           [0007]    Although advantageous in many ways, these prior art methods of using adhesive/sealants are also problematic. Among other things, strength of the attachment relies largely on the adhesive rather than the mechanical bond of the threads into the wall of the cavity. In addition there is no adequate method of determining if a seal has been formed around the entire circumference of the body of the fastener. This is especially important in the boat building industry, which often incorporates foam and balsa cores in its hulls and superstructures. Moisture or water migrating into the area between the outer laminate and the core material can compromise the bond between the two materials, and can cause them to delaminate. In addition, if water is allowed to saturate the core material of the assembly, it can seriously comprise the entire structure, sometimes rendering it unsalvageable. Applying an adhesive/sealant into a pre-formed cavity is also labor intensive, requiring multiple steps that can increase manufacturing costs.  
           [0008]    It is known to factory pre-apply a adhesive/sealant coating or “patch” onto the threads of some types of machine screws before they are sold to end-users. Factory pre-applied adhesives/sealants are “dry to the touch”, and ideally remain dormant until the shearing action of engaging the fastener into a nut or preformed cavity causes them to cure. There are numerous advantages to this approach, including improved resistance to pull-out, spin-out and vibration. Surprisingly, while this method is widely known in the automotive, aerospace and furniture industries, it has apparently never been applied to fasteners used in the assembly of FRP structures. The closest that the prior art comes to pre-coated screws that provide a water-tight, moisture and vapor-proof seal is U.S. Pat. No. 5,260,100 to Day (November 1993). But there, the threads are coated with a liquid or pasty sealant, which is then over coated with a dry, hard material.  
           [0009]    Thus, there is still a need for new types of fasteners in boat building and other industries that use fiberglass and other composites. There is also a need to adapt some of the methods known in other fields to the boat building and other industries that use composites.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention is directed to threaded fasteners that combine two or more of (a) a self-drilling portion that is long enough and/or sufficiently distanced from the threads to assure that drilling action is effectively complete before the first thread engages the surface of the FRP work piece, (b) external high-low threads, and (c) a factory pre-applied adhesive/sealant. Thus, the various fastener features disclosed herein may be combined in any suitable manner. For example, some of the novel fasteners contemplated herein may comprise high-low threads coated with adhesive/sealant, whereas others may comprise high-low threads and a self-drilling tip or point (with the terms “tip” or “point” being used interchangeably herein), with or without an adhesive/sealant.  
           [0011]    The self-drilling tip or point is preferably chosen to be appropriate for the material of the work piece. For example, a fastener inserted into a gelcoated fiberglass panel would preferably have a forged or milled drill tip that minimizes cracking and spalling of the gelcoat. Milled self-drilling points are generally more expensive to produce, but are considered to be advantageous in that regard. Regardless of the type of drill-tip chosen, the distance between the tip of the fastener and the thread is preferably about ⅛ th -⅝ th  inches, (or about 3-15 mm) when working with FRPs&#39;. In other embodiments that distance can be as small as 0.25 mm, or greater than 15 mm. All ranges set forth herein include the endpoints.  
           [0012]    Even though it has apparently not been previously appreciated, course high-low threads are well adapted for engagement into FRPs&#39;. The high, threads provide deep thread engagement and high shear values, while the inclusion of low threads reduces the tendency of the FRP to crack as the fastener is being driven. All suitable hi-lo threads are contemplated, including those where there is only minimal differences between the crests of the high and low threads, to those where there is a very significant difference between the crests of the high and low threads.  
           [0013]    Pre-coated adhesives and/or sealants preferably lie dormant on the fastener until the shearing action of the threads engaging the FRP releases, mixes, activates the substance(s), or in some other manner triggers the curing process. After curing, the adhesive/sealant reduces the incidence of fastener pull-out and spin-out, and provides a seal around the body of the fastener. In some instances fasteners comprising a factory pre-applied adhesive/sealant can even be reused, since some of the adhesive/sealant may remain unactivated when the fastener is first used.  
           [0014]    It should be noted that although the use of a pre-applied adhesive/sealant is described primarily in relation to threaded fasteners in the form of screws, it is applicable to other fasteners as well, with nail type fasteners being but one example. The concept is also applicable to different types of threaded fasteners, and, more particularly, to different types of screws. As such, the style, shape and dimensions of the fastener will likely vary between embodiments. For example, when attaching a thick bracket to a fiberglass wall, a screw may be used that has a non-threaded portion near the head that is approximately the length of the hole in the bracket through which the screw is inserted. As another example, inventive fasteners may have a flat, pan, oval, truss or other head design, and can have a hex, Phillips, slotted, square, Torx™ or other drive design.  
           [0015]    Contemplated fasteners can be used advantageously in many different applications. Of particular interest are boat building and other construction where connections to FRPs&#39; are needed, and where leaks around fasteners, resulting from bored or drilled cavities, are problematic.  
           [0016]    Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a perspective view of a screw.  
         [0018]    [0018]FIG. 2 is a perspective view of an alternative screw.  
         [0019]    [0019]FIG. 3A is an end view of another alternative screw.  
         [0020]    [0020]FIG. 3B is an side view of the screw of FIG. 3A.  
         [0021]    [0021]FIG. 3C is a enlarged view of a portion of the screw of FIG. 3B.  
         [0022]    [0022]FIG. 3D is a side view of another alternative screw.  
         [0023]    [0023]FIG. 4 is a cutaway view of a thread of a screw.  
         [0024]    FIGS.  5 A- 5 E are cutaway views of portions of alternative screws.  
         [0025]    FIGS.  6 A- 6 B are cutaway views of the heads of screws.  
         [0026]    [0026]FIG. 7 is an exploded view of an object being attached to a section of FRP using three screws. 
     
    
     DETAILED DESCRIPTION  
       [0027]    General Aspects Of Fasteners  
         [0028]    In FIG. 1, a fastener in the form of a screw  100  has a head  110 , a body  120 , and a self-drilling tip  130 . Head  110  includes bearing surface  111 . Body  120  includes cylindrical shank  121  and high and low threads  123  and  122 , with threads  111  comprising surface  122 A. Threads  122  and  123  are at least partially coated with, and thereby “hold” a self-curing adhesive/sealant (not shown in FIG. 1 that is dry to the touch under usual circumstances, but discussed in relation to FIGS.  4 - 6 B).  
         [0029]    Similarly in FIG. 2, a fastener in the form of a screw  200  has a head  210 , a body  220 , and a self-drilling tip  230 . Head  210  includes bearing surface  11 . Body  220  includes cylindrical shank  221  and high and low threads  223  and  222 , with threads  211  comprising surface  222 A. Threads  222  and  223  hold a substantially dry self-curing adhesive/sealant (not shown in FIG. 2, but discussed in relation to FIGS.  4 - 6 B). Screw  200  differs from screw  100  both in regard to the type of head ( 110 ,  210 ), and how much of the shank ( 121 ,  221 ) is covered by threads ( 122 ,  123 ,  222 ,  223 ).  
         [0030]    FIGS.  3 A- 3 D illustrate various lengths and diameters of screws  100  and  200  where L1 is the overall length of the screw, L2 is the length/thickness of the head ( 110 ,  210 ), L3 is the length of the threaded portion of the body ( 120 ,  220 ), L4 is the length of the self-drilling tip ( 130 ,  230 ), L5 is the depth of the recess, and for screws which do not have threads extending the entire length of the body between the head ( 110 ,  210 ) and self-drilling tip ( 130 ,  230 ), L6 is the length of the non-threaded portion of body ( 120 ,  220 ), D1 is the diameter of the shank ( 121 ,  221 ) and is sometimes referred to as the minor diameter of the fastener, D2 is the diameter (crest to crest) of the high threads ( 123 ,  223 ) and is sometimes referred to as the major diameter of the fastener, D3 is the diameter (crest to crest) of the low threads ( 122 ,  222 ), D4 is the head outer diameter, and D5 is the diameter of the recess (not shown in FIGS. 1 and 2). Dimensions of corresponding screws in the prior art are already known.  
         [0031]    Although the various dimensions will vary between embodiments, it is contemplated that it may be advantageous for an embodiment having threads extending the length of the body to have L1 approximately 2.4 mm, L2 approximately 1.4 mm, L3 approximately 16.65 mm, L4  6 . 35  mm, L5 approximately 1.4 mm, D1 approximately 3.85 to 3.95 mm, D2 approximately 4.5 to 4.7 mm, D3 approximately 3.8 to 4.0 mm, D4 approximately 7.43 to 8.43 mm, and D5 approximately 3.94 mm.  
         [0032]    The tip, shank, threads, and head are preferably made of a material that is corrosion resistant. The presently preferred material is a series  300  stainless steel, sometimes written as 18-8, and especially  316  or  316 L stainless steel. It should be appreciated that alternatives may utilize any other suitable materials or combinations of materials, including but not limited to other forms of stainless steel, zinc plated steel, brass, bronze, and plastic.  
         [0033]    Although contemplated fasteners can have any type of head, as partially illustrated by the pan head  110  of fastener  100  and the flat head  210  of fastener  200 , preferred head styles are flat, oval, pan and truss. The preferred drive style is hex. The choice of head will depend mostly on the application for which the fastener is intended. As such, other embodiments may include, but are not necessarily limited, to flat heads, oval heads, pan heads, round heads, fillister heads, binding heads, (holt?) heads, truss heads, hexagon heads, and acorn heads with Phillips, slotted, square, Torx™ or other drive design. It is also contemplated that in applications where a seal is to be formed about the fastener to preserve the integrity of any barrier pierced by the fastener (such as a fiberglass boat hull), the use of a head having a bearing surface that conforms to the shape of the juxtaposed surface will assist in the formation of such a seal.  
         [0034]    For certain flat head and other fasteners adapted to be countersunk into the outer surface of FRPs&#39;, it is desirable to provide serrations (nibs) that extend radially on the underside, bearing surface of the head. Nips provide for a self-countersinking head that reduces friction when the underside of the head first engages the surface of the FRP minimizing surface spalling. Nibs also aid in locking the fastener in the material providing protection from vibration. At least four such nibs are preferable, and six nibs are considered to be optimal. Other embodiments may include 4-8 nibs or some other quantity of nibs.  
         [0035]    As known in the prior art, fastener heads may be shaped, or may include recesses, protrusions, or combinations thereof to facilitate mating the fastener with a screwdriver or other driving tool. Drives may include Phillips, slotted, square, Torx™ or other design.  
         [0036]    Driving tools include Philips and flat blade screwdrivers, Torx drivers®, and Allen wrenches.  
         [0037]    Threads  
         [0038]    The specifications of the pitch for the preferred high-low course threads are 6-15, 8-15, 10-12, ¼-10. The first number is the body diameter and the second shows the combined number of high and low threads per inch. The preferred threads have a 70˜73 degree included angle thread profile, which is steeper than the standard pitch specifications (6-20, 8-18, 10-16, 12-14) for course self-drilling screw threads The steeper profile provides faster insertion into the FRP, reducing friction which causes cracking. Fasteners for masonry, which are not self-drilling, have sharp points and thread specifications with an included angle of 76 degrees which is comparable to the threads on the inventive fasteners. However, the crest of the “hi” thread on a masonry fastener, as compared to the differential in the major diameter of the “hi” thread and the diameter of the self-drilling tip on the inventive fasteners, combined with the lower included angle would cause unacceptable cracking and spalling of the gelcoat or outer surface of a FRP structure. As used herein “included angle” is the angle between the flanks, measured in an axial plane section where the flanks of a thread are the straight sides that connect the crest and the root, the crest of a thread is the prominent part of a thread, and the root is the bottom of the groove between the two flanking surfaces of the thread. As used herein the “pitch” of a thread is the distance, measured parallel to its axis, between corresponding points on adjacent surfaces, in the same axial plane. However, pitch may be approximated herein by specifying the number of threads per inch (TPI).  
         [0039]    Contemplated fasteners include those where all of the threads are of the high-low design, as well as those having a section of high-low threads and another section of a different type of threads. The threads may extend along the entire length of the body of the fastener, as shown by threads  122 ,  123  of body  120  of fastener  100 , or may extend only along a portion of the length as shown by threads  222 ,  223  of body  220  of fastener  200 . Having the threads extend only part way along the body of a fastener helps prevent pull-through during the drilling stage, and jacking, where a top piece of material pulls away from a base material because the fastener threads in the top material force the top material up the body of the fastener.  
         [0040]    Drilling Tip  
         [0041]    Drilling tip  130  is preferably a self-drilling, fluted point, adapted to effectively drill completely through the FRP or other composite before the threads ( 122 ,  123 ,  222 ,  223 ) engage the outer surface. This arrangement is intended to prevent pull-through, which causes cracking and spalling and prevents a strong mechanical connection between the fastener and the FRP. To that end it is advantageous to have the drill tip distanced from the threads by between ⅛″ and ⅝″, and more preferably between ¼″ and ⅜″. In metric terms the preferred tip to thread distance is preferably between about 3 mm and 1.5 mm, although it is contemplated that the distance can be as low as 0.25 mm, or greater than 1.5 mm. Depending on the application, preferred tip to thread distances are at least 3 mm, 5 mm, 10 mm, or 15 mm. Drilling tip  130  can comprises a forged or pinch point or any other suitable configuration, but most preferably is milled for precise drilling to reduce the likelihood of cracking and spalling the outer surface of the FRP.  
         [0042]    The length of the drill flute advantageously corresponds to the thickness of the FRP that can be drilled. The flute provides a channel for chip removal during drilling, and will increase the friction and slow down the cutting action if it becomes impeded in the material. For these reasons the length of the flute on the point of the inventive fastener should approximate the practical overall length of the tip.  
         [0043]    Adhesive/Sealant  
         [0044]    Where the threads are at least partially factory pre-coated, the coating is preferably made from just above the drill tip along a distance corresponding to the thickness of the FRP structure. Such coatings can be made in a vertical or angled strip, in a patch, or full coated around the circumference of the area. As discussed elsewhere herein, the coating preferably contains an adhesive/sealant that is dry to the touch. In any event, the coating advantageously remains dormant until the shearing action of engaging the fastener into a nut or cavity triggers a curing process. Curing can then occur in any suitable manner, but generally occurs as a result of a chemical reaction between two different components in the adhesive. The presently preferred material is Scotch-Grip™ Fastener Adhesive 2353, marketed by the 3M Company. The adhesive/sealant can have any color, but is preferably white for use in boats and other FRP structures.  
         [0045]    The adhesive/sealant can be advantageously encapsulated in microcapsules. As used herein, the term “microencapsulate” means to enclose in microcapsules, where a “microcapsule” is a small, sometimes microscopic capsule designed to release its contents when broken by pressure, dissolved, or melted. FIG. 4 illustrates the surface of a fastener comprising a shank  421 , a thread  422 , a thread surface  422 A and a plurality of microcapsules  470 . FIG. 4 and FIGS. 5A and 5B depict microcapsules deposited on the threads as distinct units. FIGS.  5 C- 5 E depict microcapsules that are included as part of a layer of material that fully or partially covers surface  422 A.  
         [0046]    Microcapsules (in individual or layered form) may consist essentially of only an adhesive/sealant as illustrated by FIGS.  5 B- 5 D, or may comprise an adhesive or sealant encapsulated by another material. This is illustrated in FIGS. 5A and 5E. In FIG. 5A, adhesive/sealant  572  is encapsulated by material  571  in microcapsules  570  on the surface  522 A of a thread  522 . In FIG. 5B, adhesive/sealant  573  forms  570  on surface  522 A of a thread  522 . In FIG. 5C, layer  573  comprises adhesive/sealant microcapsules on surface  522 A of thread  522 . In FIG. 5D, the layer  573  of FIG. 5C is non-continuous as it contains or more exposed regions  580 . In FIG. 5E adhesive/sealant  572  is partially encapsulated by material  571  but is non-continuous so leaves exposed areas  580  of surface  522 A.  
         [0047]    Although it is preferred that the threads be coated with adhesive/sealant as illustrated by FIG. 4, it is contemplated that other portions of the fastener may be coated in addition to or in place of the coating on the thread surfaces. As such the shank may be coated and/or the bearing surface of the head may be coated as illustrated by FIGS. 6A and 6B. In FIGS. 6A and 6B, bearing surface  611  of head  610  attached to shank  620  is coated with microcapsules that are either distinct units  670  as shown in  6 A, or in a layer  673  as shown in FIG. 6B.  
         [0048]    Methods and Applications  
         [0049]    [0049]FIG. 7 generally depicts a handle  810  being attached to a section of composite  820  using two screws  830 . The handle  810  is employed euphemistically in the figure to represent any object that can be attached to the composite section using fasteners. Other contemplated objects, for example, include brackets, hinges, wall hangings, furniture, rails, and electronics such as telephones and radios. Handle  810  thus also represents a second piece of composite that is being screwed into the composite section  820 .  
         [0050]    The composite section  820  is likewise intended to be generically representative of any composite material, including a fiberglass wall, beam, or other structure. Thus, although the portion of the composite section  820  in FIG. 7 is shown as having three layers, a gelcoat  822 , a hard plastic section  824  and a soft core  826 , alternative contemplated composite sections may have different types and/or number of layers. The composite section  820  shown should therefore be interpreted as any composite portion of a boat, plane, car or other transportation vehicle, or in any other application.  
         [0051]    The screws  830  should once again be viewed euphemistically, to include any of the fasteners contemplated herein that have drill-tip points. Thus, although both of the screws  830  have a flat head design, with high-low threads to which a self-curing adhesive/sealant may have been applied, the screws  830  shown are representative of other fasteners described herein, with different head or thread features, with or without a self-curing adhesive/sealant. Of particular interest is the tip to thread distance  832 . That distance should be long enough to prevent significant pull through as the screw is being inserted. In most circumstances this means that the tip to thread distance  832  would be at least as long as the thickness  825  of the FRP  824 .  
         [0052]    Although specific examples of novel fasteners and methods using same have been disclosed, those skilled in the art will appreciate that many more modifications besides those already described are possible without departing from the inventive concepts described herein. Moreover, in interpreting the disclosure, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.