Abstract:
A blind bolt fastener is provided having a body, a deformable sleeve and a core bolt in threaded engagement with the deformable sleeve. The deformable sleeve has a groove formed therein. Rotation of the core bolt bulbs the deformable sleeve and moves the deformable sleeve along the body. The groove receives a tapered nose of the body and aids in completing the bulbing of the deformable sleeve. An optional drive nut is provided for preventing rotation of the body during installation of the fastener.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent Application No. 60/777,449, entitled “Blind Bolt Fastener” filed on Feb. 28, 2006, which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention is generally related to blind fasteners, and more particularly to blind bolt fasteners having a body and a deformable sleeve.  
       BACKGROUND OF THE INVENTION  
       [0003]     Blind fasteners are used in a variety of applications to connect two or more workpieces together. In the construction of aerodynamic designs, such as control surfaces on aircraft and the like, a substantially flush surface is typically desired on the accessible side of the panels. Often, however, access to the blind side of the workpiece is not possible. Such one-sided access complicates the installation process. In these cases, the use of a blind fastener is appropriate and simplifies installation.  
         [0004]     Typical blind fasteners comprise an internally threaded nut body and an externally threaded cylindrical core bolt in threaded engagement with the nut body. The inserted end of the core bolt has an enlarged core bolt head while the other end of the core bolt has a wrenchable portion. The fastener is inserted into aligned apertures of a pair of workpieces and the core bolt is rotated with respect to the nut body. The core bolt moves axially in an outward direction through the nut body. This axially outward movement typically causes a deformable sleeve around the core bolt and intermediate the nut body and core bolt head to deform about the nut body to provide a blind side head against the inner surface of the inner work piece. The core bolt further is provided with a localized weakened region or break groove adapted to sever the core bolt at a predetermined amount of torque and location.  
         [0005]     It is advantageous that the break groove shears the core bolt in a substantially flush relation to the fastener body head after the fastener is fully set. Particularly, an accurate core bolt break is sought for fasteners having countersunk body heads to provide a flush relationship between the set fastener and the outer panel, thus providing a smooth aerodynamic surface after the fastener is set.  
         [0006]     However, due to numerous factors including variations in combined panel thickness, sometimes the break groove on the core bolt extends beyond a flush position with the fastener body head. Therefore, when shear or breakage occurs at the break groove, a portion of the remaining core bolt may protrude beyond the fastener body head. Accordingly, it is often necessary to grind the protruding core bolt so that the core bolt is flush with the fastener body head. Prevention of such protrusion will provide a cost savings through the elimination of additional operations and manpower required in shaving, smoothing and trimming the protruding core bolt stem to provide a flush finish.  
         [0007]     Conversely, positioning the break groove to break below the head surface can result in cavities that must be filled. Again, eliminating the need to fill such cavities will provide a cost savings through the elimination of additional operations and manpower required to provide a flush finish. In addition, low (below flush) breaks may result in some loss of strength in the fastener head.  
         [0008]     Additionally, structural joints should have strengths at least equivalent to the panels in which they are installed. Otherwise, the fasteners will fail prior to panel failure in an overload situation. As most airframe joints are designed to carry shear loads, the joint shear strengths should be in line with the structure material bearing load strength. The shear load capability of a structural joint is usually measured using Metallic Materials Property Development and Standardization (FAA/DOD MMPDS) guidelines and testing in accordance with MIL-STD-1312 Test Method # 4 . A load versus elongation plot of a single fastener joint is shown in  FIG. 7 . Generally, the higher the yield strength and ultimate strength (i.e., higher curve), the more suitable the fastener is for structural applications.  
         [0009]     Having a relatively large residual clamp load in the joint enhances structural strength. This allows fasteners to close gaps between panels and keep them tightly clamped together as desired. High residual clamp reduces microscopic movement between metal panels during flight operations, thereby minimizing the likelihood that fretting and fatigue cracks will develop.  
         [0010]     Laminated carbon fiber composites are becoming increasingly prevalent in airframe structure because these composites provide lighter weight and accompanying fuel savings. Composites, however, cannot endure the high compressive stresses induced by the installation of conventional fasteners designed for metallic structure. It is, therefore, desired to spread the fastener clamp loads over a large region on the panels to minimize contact stresses while maintaining high clamp loads.  
         [0011]     Additional information will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention relates to a blind bolt fastener having a core bolt in threaded engagement with a deformable sleeve. The fastener has a body with an enlarged head positioned between the deformable sleeve and wrenching flats of the core bolt. The bore of the deformable sleeve is tapered and has a leading edge that may abut the end of the body. An optional drive nut having protrusions may be provided for engagement with the head of the body. Bulbing of the sleeve causes the leading edge of the sleeve to move along the length of the body. The leading edge of the sleeve engages the blunt end of the body to flatten the deformable sleeve in a completely bulbed position. The core bolt has a break groove that fractures when installation of the fastener is complete. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0013]     Operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:  
         [0014]      FIG. 1  is a diagrammatical cross sectional view of an embodiment of a blind bolt fastener in an embodiment of the present invention;  
         [0015]      FIG. 2  is another cross-sectional view of the fastener of  FIG. 1 .  
         [0016]      FIG. 3  is a perspective view of a blind bolt fastener having a deformable sleeve and a drive nut in an embodiment of the present invention;  
         [0017]      FIG. 4  is a side and top perspective view of a blind bolt fastener in an embodiment of the present invention;  
         [0018]      FIG. 5  is a generally exploded view of a blind bolt fastener having a drive nut in an embodiment of the present invention;  
         [0019]      FIG. 6  is a generally exploded view of a blind bolt fastener in a side perspective view in an embodiment of the present invention; and  
         [0020]      FIG. 7  is a lap joint shear load versus elongation plot of a single fastener joint in an embodiment of the blind bolt fastener.  
         [0021]      FIG. 8  illustrates a blind bolt fastener having a drive nut in another embodiment of the present invention.  
         [0022]      FIG. 9A  illustrates a cross sectional view of the blind bolt fastener of  FIG. 1  installed into a pair of panels.  
         [0023]      FIG. 9B  illustrates a top view of the blind bolt fastener of  FIG. 9A  having a flush core bolt break at the head of the body of the fastener.  
     
    
     DETAILED DESCRIPTION  
       [0024]     While the present invention is described with reference to the embodiments described herein, it should be clear that the present invention should not be limited to such embodiments. Therefore, the description of the embodiments herein is illustrative of the present invention and should not limit the scope of the invention as claimed.  
         [0025]     Reference will now be made in detail to the embodiments of the invention as illustrated in the accompanying figures. Embodiments of a blind bolt fastener  10  are shown in  FIGS. 1 through 6 . The blind bolt fastener  10  has a core bolt  20  at least partially engaged with a body  30  and a deformable sleeve  40 . The core bolt  20  is threadingly engaged with the deformable sleeve  40  and rotatively engaged with the body  30 . In one embodiment, the sleeve  40  abuts an end of the body  30  when assembled.  
         [0026]     The core bolt  20  has a core bolt head  21  with a wrench engaging portion  22  on one end, as shown in  FIGS. 1-6 . The wrench engaging portion  22  may be enlarged wrenching flats. In an embodiment, the core bolt  20  has a non-threaded portion  23  proximate the core bolt head  21 . The core bolt  20  has a threaded portion  24  that may be adjacent the non-threaded portion  23  and distal the core bolt head  21 . In one embodiment, the threaded portion  24  of the core bolt  20  has buttress threads.  
         [0027]     Further, the core bolt  20  has a core bolt break groove  26 . The core bolt break groove  26  is a weakened region in the core bolt  20  that causes the core bolt  20  to fracture at a predetermined amount of torque or stress. For example, the break groove  26  may break or fracture the core bolt  20  when a preselected amount of torque is applied to wrenching flats  22  during installation of the blind bolt fastener  10 . Such fracture is designed to occur upon completion of the installation of the blind bolt fastener  10 , such as, when the torque required to deform the sleeve  40  exceeds the torsional strength of the break groove  26 .  
         [0028]     The core bolt  20  is insertable through the body  30 . The body  30  may be sized to allow the core bolt  20  to rotate within the body  30 . The body  30  has an enlarged head  32  (hereinafter “the body head  32 ”) at one end, such as, the end adjacent the wrenching flats  22  of the core bolt  20  when assembled. The body head  32  is capable of seating in a cavity in the access side of a pair of structural panels being fastened together. The body head  32  may be of a protruding type, setting on the surface of the access side panel.  
         [0029]     The body  30  has a tapered nose  36 . In a preferred embodiment, the tapered nose  36  is located at an opposing end of the body head  32 . The outer diameter of the body  30  may taper or otherwise decrease toward the end of the body  30  adjacent the deformable sleeve  40 . In an embodiment, the tapered nose  36  may abut the deformable sleeve  40 .  
         [0030]     The head  32  may have body-wrenching members  33 , which are shown in  FIG. 5 . The wrenching members  33  may be crevices, grooves, slots or the like that are capable of engagement with a tool or drive nut that may prevent rotation of the body  30 . For example, the wrenching members  33  may engage an end of a non-rotating tool that is in rotational engagement with another rotating tool used to drive the core bolt  20 , and more particularly, to a tool that engages the wrenching flats  22  of the core bolt  20 .  
         [0031]     The deformable sleeve  40  may be positioned at the threaded portion  24  of the core bolt  20 . The sleeve  40  may be made of a malleable material that has the ability to bulb or expand a predetermined amount without fracturing. For example, polished and annealed AISI 304 stainless steel is able to undergo a strain of approximately 100% without fracturing. Alternatively, Commercially-Pure Titanium, 300-Series Stainless steel, and A-286 Corrosion and Heat Resisting Steel can be used. The deformable sleeve  40  should not be deemed as limited to any specific material. One of ordinary skill in the art will appreciate the use of various materials for the deformable sleeve  40 .  
         [0032]     The deformable sleeve  40  has a tapered or stepped bore  42 . The bore  42  may have threads  43  capable of threaded engagement with the core bolt  20 . The threads  43  of the bore  42  may be buttress threads that matingly engage the threaded portion  24  of the core bolt  40 . The buttress threads incorporate a steep pressure flank and a shallow non-pressure flank. For example, the pressure flank may be approximately between 75 and 90 degrees off the axis of the core bolt  20 . In an embodiment, the non-pressure flank may be approximately 45 degrees. The pitch of the thread or threads per inch may be, for example, similar to that used for the 60 degree thread used on existing threaded blind bolts, such as, threads similar to MIL-S-8879 and MIL-S-7742. The buttress profile results in a lower radial component of force so that the female threads  43  of the sleeve  40  is not forced to expand radially as much as the sleeve  40  would be required to expand if a conventional thread form were used instead. A large radial component of force can cause the female threads  43  of the sleeve  40  to become disengaged with the male threads  24  of the core bolt  20  resulting in a weaker structural connection. Use of conventional threads would require more threads in engagement than the buttress threads to provide the same strain capability; however, additional threads requires additional length and, as a result, increased weight. The buttress threads minimize undesired radial expansion of the threaded portion  43  of the sleeve  40  with a minimal amount of thread engagement. The length of engagement is driven by the shear strength of the threads, rather than by concerns over radial expansion.  
         [0033]     The sleeve  40  may have a counter bore or groove  44 . The inner diameter of the groove  44  may be greater than the inner diameter of the threaded portion  43  of the sleeve  40 . The sleeve  40  may be crimped at or around the groove  44  such that an edge  46  contacts or abuts the body  30 . During installation, the edge  46  engages the outer surface of the tapered nose  36  of the body  30 . To this end, the edge  46  and the groove  40  aid in allowing the sleeve  40  to slide or otherwise move on the body  30  toward the body head  32 . As the sleeve  40  bulbs, the groove  40  is capable of causing the sleeve  40  to completely flatten against, for example, the blind side of a panel.  
         [0034]     The drive nut  50  is positioned between the wrenching flats  22  of the core bolt  20  and the head  32  of the body  30 . For example, the drive nut  50  is positioned at the break groove  26 . The drive nut  50  is trapped axially between the body head  32  and the core bolt head  21  so that the drive nut  50  remains engaged with the body head  32  throughout the installation sequence.  
         [0035]     The drive nut  50  may be provided with protrusions  52  that extend from an underside of the drive nut  50 . The protrusions  52  extend toward the body  30 . In an embodiment, the protrusions  52  correspond in size and shape to engage and fit into the wrenching members  33  of the body head  32 . The drive nut  50  can be positioned such that the protrusions  52  engage the body head  32  upon assembly. Failure to preposition the protrusions into the recesses may prevent the core bolt head  21  from properly seating in the body head  32  throughout the installation process.  
         [0036]     In an another embodiment, the protrusions  52  of the drive nut  50  may be a raised deformable portion adjacent the body head  32 . The deformable portion may be capable of deforming into the body head  32  during assembly of the fastener components or during installation of the fastener  10 .  
         [0037]     The drive nut  50  can be used on other fasteners as will be appreciated by one of ordinary skill in the art, including, without limitation, other known fasteners. In an embodiment, a drive nut  150  is used with a blind bolt fastener  100  as shown in  FIG. 8 . The blind bolt fastener  100  includes a core bolt  120 , a body  130 , an insert  135 , a deformable sleeve  140 , a nut  148  with a thread locking feature  149  and a drive nut  150 . In this embodiment, the drive nut  150  is positioned between a body head  132  and a core bolt head  121 . The drive nut  150  may have a raised deformable portion adjacent the body head  132 . In an embodiment, the protrusions  52  of the drive nut  150  may deform into the body head  132  during assembly of the fastener components or alternatively during the installation.  
         [0038]     The drive nut  150  may have features similar to the drive nut  50  as illustrated in  FIGS. 1-6 . For example, the drive nut  150  may have protrusions, such as, the protrusions  52  as best illustrated in  FIG. 6 . The protrusions  52  of the drive nut  150  may fit or otherwise engage into mating devices (not shown), such as the wrenching members  33  as illustrated in  FIG. 5 . of the body head  132 .  FIG. 8  illustrates that the drive nut  150  may be positioned such that the protrusions  52  engage the body head  132  upon assembly. The protrusions  52  may engage the wrenching members  33  of the head  132  to prevent rotation of the body  130  with respect to the sleeve  140 . Failure to preposition the protrusions  52  into the wrenching members  33  may prevent the core bolt head  121  from properly seating in the body head  132  throughout the installation process.  
         [0039]     Turning to the fastener  10 , an example of how to use the fastener  10  as illustrated in  FIGS. 1-6  is set forth below. The drive nut  50  may be pre-positioned between the wrenching flats  21  and the body head  32 . The core bolt  20  is inserted into the body  30  and into the bore  42  of the deformable sleeve  40 . Torque is applied to the wrenching flats  21  of the core bolt  20  to thread the core bolt  20  into the deformable sleeve  40 . The drive nut  50  may be engaged to prevent rotation of the body  32  with respect to the deformable sleeve  40 . A tool assembly having a rotating wrench and non-rotating housing may engage the wrenching flats  21  and the drive nut  50 , respectively. To this end, the tool may rotate the core bolt  20  and prevent rotation of the body  30  by engaging the drive nut  50 . For example, the protrusions  52  of the drive nut  50  may engage the wrenching member  33  of the body  30 .  
         [0040]     The core bolt  20  rotates and is threaded into the deformable sleeve  40  causing the sleeve  40  to bulb. The tapered or stepped bore  42  controls the blind side formation of the sleeve  40 . As the core bolt  20  is threaded into the sleeve  40 , the sleeve  40  is driven against the tapered nose  36  of the body  30 . In an embodiment, the sleeve  40  bulbs prior to moving up the body  30  toward the head  32 . For example, the sleeve  40  buckles against the body  30 , and then moves against the tapered nose  36  and along the body  30 . In a preferred embodiment, the rotational friction force at the interface between the body  30  and sleeve  40  is greater than the rotational friction force between the threaded interface of the core bolt  20  and sleeve  40 . Knurls on the body  30  may be used to enhance or increase frictional force.  
         [0041]     The edge  46  of the sleeve  40  reaches the outer surface of the tail-side panel and as a result, the sleeve  40  flattens completely against the blind side surface and causes the strain on the core bolt  20  to increase such that the core bolt  20  fractures at the break groove  26 . The residual clamp load is near maximum during formation of the blind side upset because there is no axial recoil upon torsional fracture at the break groove  26 . The frangible portion of the core bolt  20  and the drive nut  50  are discarded upon completion of the installation.  
         [0042]      FIG. 9B  illustrates an embodiment of the fastener  10  as installed into a pair of panels  90   a ,  90   b . As illustrated, the deformable sleeve  40  bulbs against the blind side of the panel  90   b . The deformable sleeve  40  moves along the body  30  to engage the tapered nose of the  36 . In this embodiment, the tapered nose  36  engages the groove  44  to prevent further movement of the deformable sleeve  40  and to aid in flattening or bulbing of the deformable sleeve  40 . The remaining portion of the core bolt  20  is flush with the head  32  and/or the access side of the panel  90   a  as illustrated in  FIG. 9A .  
         [0043]     The invention has been described above and, obviously, modifications and alternations will occur to others upon a reading and understanding of this specification. The claims as follows are intended to include all modifications and alterations insofar as they come within the scope of the claims or the equivalent thereof.