Patent Publication Number: US-2006002781-A1

Title: Captive fastener

Description:
The present invention relates to a captive fastener. Such fasteners are well known and are typically in the form of a threaded nut or stud. The present applicants market one form of captive fastener under the trade mark FLANGEFORM. Details of such a fastener and a manner of mounting the fastener in sheet metal are described in U.S. Pat. No. 3,938,239, for example.  
      Typically the fastener is inserted through a pre-formed circular hole in the metal sheet, and then the sheet and/or fastener are deformed to ‘capture’ the fastener. Formations, such as ribs, may be provided on the outside surface of the fastener to increase the torque resistance of the fastener: to resist it twisting in the metal sheet. Pierce fasteners are also known in which the fastener shank pierces the sheet material to form the hole, such as in U.S. Pat. No. 4,389,766.  
      We have realised that there is a need for a captive fastener for use in composite materials, particularly composite materials having a polymer matrix such as thermoplastics, and for use in plastics generally. Fasteners have been mounted in plastics material by using a force fit, when a sufficiently large depth of material is available, or by adhesive mounting, for example. Thus, in one aspect our invention provides a captive fastener mounted in a composite or plastics sheet material. The captive fastener is deformed during mounting to grip the sheet material and hold the fastener in position.  
      Composite materials and plastics may not have the same response to being worked as sheet metal. Thus in a preferred form of the invention we form an aperture in the composite material or plastics for receiving the fastener. The aperture may be formed during a moulding process, or by stamping or cutting. The aperture periphery is preferably non-circular and the fastener has a shank portion which fits in the aperture, the shank portion co-operating with the aperture periphery to prevent rotation of the fastener. In one form the aperture may be polygonal such as a triangle, square, hexagon or the like. A convex outline is preferred, and we presently prefer an outline without sharp points or apices. We presently prefer an aperture having a periphery which is rounded in parts. A completely rounded periphery such as an oval may also be used. The fastener shank is preferably a close fit in the aperture to prevent rotation of the fastener in the aperture.  
      A head is formed at one end of the shank. The head bears on the surface of composite or plastics material at or near the periphery of the aperture. At the other end of the shank, the fastener is deformed to bear on the composite or plastics material to prevent the fastener being pulled through the material from the direction of the head and so secure the fastener in the aperture. Tabs or a peripheral flange may extend from the fastener shank, and are deformed to secure the fastener in place. Thus, the composite or plastics material is sandwiched tightly between the fastener head and the tabs. During the deformation process, the main force is applied only to the fastener, avoiding damage to the composite or plastics material. By providing deformable tabs, the force required to deform the fastener is relatively low, and so avoids damage to the surrounding composite or plastics material.  
      Preferably an outer wall of a tab is contiguous with the periphery of the shank. An inner wall of the tab is angled to the fastener axis. Preferably it subtends an angle of about 55 to 65 degrees, more probably 58 to 62, and very preferably about 60 degrees. The inner wall is arcuate and curves about the fastener axis to maintain the slope relative to the fastener axis, forming an amphitheatre type shape.  
      In another embodiment the tab inner wall may subtend an angle of about 25 to 35 degrees, preferably about 28 to 32 degrees and more preferably about 30 degrees to the fastener axis. A flat may be provided at the upper surface of the tab.  
      Where the shank is elongate in the direction perpendicular to the fastener axis, the tabs may be formed on opposite ends, extending around the shorter width of the shank. The outer wall of the tabs may be parallel to the fastener axis along the mid-line of the shank, and chamfered at the sides of the shank.  
      Preferably the aperture in the composite or plastics material is formed with a chamfered lip in the vicinity of the tabs or flange on the fastener. The tabs are deformed over to bear against the chamfered lip, and preferably the shank end surface is flush with or slightly below the surface of the composite or plastics material.  
      Other aspects and preferred features of the invention will be apparent from the following description and the accompanying claims. 
    
    
      The invention will be further described by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a side view of a captive fastener forming an embodiment of the invention;  
       FIG. 2  is a plan view of the fastener of  FIG. 1 ;  
       FIG. 3  shows a cross-section through a tool for deforming the fastener of  FIG. 1  to mount it in a sheet of composite or plastics material;  
       FIG. 4  shows the fastener of  FIG. 1  mounted in the tool of  FIG. 3 , and placed in an aperture in a sheet of plastics material;  
       FIG. 5  shows the completion of the operation of deforming fastener to fix the fastener in the plastics sheet;  
       FIG. 6  is a perspective view of a captive fastener of the invention;  
       FIG. 7  is a plan view of another embodiment of a fastener in accordance with the invention;  
       FIG. 8  is a cross-section along the line VIII-VIII of  FIG. 7 , and  
       FIG. 9  is an end view of the fastener of  FIG. 7 . 
    
    
       FIG. 1  shows a fastener  2  in accordance with the invention having a shank  4  with a head  6  at one end  8  of the shank  4 . Head  6  overhangs the periphery  10  of the shank  4 , the exposed undersurface  12  of the head bearing, in use, on a surface of the material in which the fastener  2  is mounted. The other end  14  of the shank  2  carries a fastener configuration, namely a threaded shaft  16  forming a screw or stud. In place of shaft  16 , fastener  2  may have a fastener configuration in the form of a threaded bore through the shank  2  and/or head  6  to form a nut.  
      Fastener  2  is of metal, typically of Boron Steel. The material used may depend on the particular requirements, but needs to be deformable as described hereinafter. The fastener  2  may be cold formed.  
      Shank  4  is generally rectangular in plan view ( FIG. 2 ) with flat sides  20  and arcuate ends  22 . Ends  22  merge with sides  20  at rounded comers  24 .  
      Projecting upwards (as viewed in  FIG. 1 ) from ends  22  are tabs  26 . The outer surface  28  of tabs  26  is a continuation of the outer surface  30  of the ends  22 ,  24 . The inner surface  32  of tabs  26  is formed by a plane which slopes at a predetermined angle to the axis A-A of the fastener  2 .  
      The height H of the shank  4  is matched to the thickness of the material in which the fastener is to be mounted. The height H will preferably be equal to or slightly less than the thickness of the material, typically up to 0.3 mm less than the thickness of the sheet material.  
      As will be described hereinafter, the fastener is mounted in a sheet of composite or plastics material by placing the shank  4  in a closely fitting aperture in the material, the head  6  abutting one surface of the material, and deforming the tabs  26  to sandwich the material between the head  6  and tabs  26 . Preferably, the aperture is equal to the size of the shank +/−0.2 mm. The fit will depend on the material and the application.  
      Referring to  FIG. 3 , a tool for mounting the fasteners  2  in a sheet of material comprises a hardened steel lower mandrel  40  having a recess  42  for receiving the head  6 . Recess  42  is of substantially the same depth as the thickness of head  6 . A hardened steel punch  44  is positioned above mandrel  40  and has a bore or recess  46  to straddle the shaft  16 .  
      Referring to  FIG. 4 , a flat sheet  46  of nylon resin material has been moulded with an aperture  48  which corresponds in outline to the shank  4  of fastener  2 . The fastener  2  is positioned in the aperture  48 . Shank  4  is a close fit in aperture  48  in order to hold the fastener in place during handling of the sheet. It will be appreciated that in the production process other possibilities exist. For example: the fastener may be positioned in the recess  42 , and the sheet then placed over the fastener; the assembly may be inverted, the fastener projecting downwards through the sheet, and the mandrel  40  positioned above the sheet.  
      The lower surface  50  of sheet  46 , as viewed in  FIG. 4 , rests on the mandrel  40 , the fastener head  6  sitting in recess  42  in the mandrel. The depth of recess  42  is equal to the height of the head  6 . The upper edge  52  of aperture  48  has a chamfer  54  at the junction with the upper surface  56  in a region corresponding to the tabs  26 .  
      Referring to  FIG. 5 , the punch  44  is brought down onto the tabs  26 . The lower surface  60  of punch  44  deforms the tabs outwardly and comes to rest at the end surface  14  of the shank  4 . The press setting or a dead stop limits the downward movement of the punch. The tabs  26  are deformed so as to lie substantially flush with the surface  56 . The force applied to the tabs depends on the material used and the design of the tabs, but a force in the region of 5000 lbs may typically be used.  
      Certain dimensions of the fastener are important to the most effective operation of the fastener and its installation. Thus, the depth of the shank  4  should preferably be close to, or slightly less than, the thickness of the sheet  46 . The height of the tabs  26  and the angle of the sloping surface  32  is also important to ensure that the tabs  26  are adequately deformed over by the punch  44 . This can be determined by trial.  
      The deformed tabs  26  cooperate with head  6  to hold the fastener in place in the sheet  46 , and the shape of the shank  4 , corresponding to the aperture  48 , prevents rotation of the fastener in the sheet  46 .  
       FIG. 6  shows a perspective view of a fastener in accordance with the invention. The outer wall  28  of the tabs  26  is chamfered at the junction  40  with the sidewalls  20  of the shank  4 . The inner wall  32  subtends an angle of θ of 30 degrees with the plane perpendicular to the fastener axis A-A.  
      In another embodiment, the wall  32  of the tab subtends an angle of between 25 and 35 degrees with the fastener axis, preferably 28 to 32 degrees, more preferably about 30 degrees. In yet another embodiment the angle is about 40 to 50 degrees, preferably about 43 to 47 degrees, most preferably about 45 degrees.  
      The captive fastener of FIGS.  7  to  9  is similar to the fasteners of  FIGS. 1 and 2  and  FIG. 6 . Head  6 ′ of the fastener is oval in plan view as seen in  FIG. 7 . In the embodiment of FIGS.  7  to  9 , the main difference is in the shape of the tabs  26 ′. Tabs  26 ′ are thicker in the radial direction of the fastener, to form a truncated wedge shape as seen in the cross-sectional view of  FIG. 8 . Thus the tabs  26 ′ have a flat  62  at their upper surface. The inner walls  32 ′ of the tabs  26 ′ in this embodiment subtend an angle of 30 degrees with the fastener axis. Thus the tabs  26 ′ are of substantially the same height as the tabs  26  of the previous embodiment but provide a greater volume of material to be deformed or peened over when installing the fastener in the sheet material and so enhancing the push through resistance of the fastener in the direction towards the head  6 .  
      Although the fastener is particularly applicable to plastics and other rigid or semi-rigid materials and components, it may also be used in metals, particularly softer metals such as magnesium. The performance of the fastener will depend in part on the nature of the sheet material.