Abstract:
A threaded blind fastener intended for installation in aerodynamically smooth or aesthetically critical panels, further adapted to minimize discontinuities on paintable surfaces of the head resulting from clearances between male and female threads that may adversely affect the quality of paint coverage.

Description:
This application is a continuation of International Application No. PCT/US2009/039083, filed Apr. 1, 2009 which claims the benefit of U.S. Provisional Application No. 61/041,739, filed Apr. 2, 2008. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a threaded, flush-head, blind fastener constructed and arranged to produce an aerodynamic, paintable exposed head surface on the exterior surface of an aircraft. 
     BACKGROUND 
     Preferably, exterior aircraft surfaces should be aerodynamically smooth and aesthetically pleasing. Current (conventional) assembly processes typically utilize two-sided fasteners such as solid rivets, lock bolts with collars, and threaded pins with nuts. The manufactured heads of these installed fasteners are often positioned on the airflow surfaces of the aircraft. Their construction means that the exposed surface(s) can be easily painted with acceptable results because of the absence of head surface discontinuities. 
     Newer generation aircraft are to be assembled robotically, and it is not practical to install conventional two-sided fasteners such as solid rivets, lock bolts and threaded pins with this type of equipment. The primary issue with robotic assembly is the equipment cost and programming coordination. The requisite equipment is essentially doubled due to requiring equipment on each side so as to have access to each end of the fastener. Further, the timing and synchronization of the equipment on one side has to be coordinated with the timing and synchronization of the equipment on the opposite side. As a result, a need exists for blind (one-sided) fasteners that are easily installed with a robot, and optionally shaved, but also leave a flaw free head surface that may be painted with a good quality finish. 
     There are many types of blind fasteners presently available for assembly of aircraft structures and components, but these fasteners have drawbacks that make their use in many of the painted aerodynamic applications that are emerging less than optimal. Mechanically-locked pull-type blind rivets generally fail to provide sufficient joint strength. Mechanically-locked pull-type blind bolts, as disclosed in U.S. Pat. No. 4,127,345, issued Nov. 28, 1978 to Angelosanto et al., and rivets, as disclosed in U.S. Pat. No. 5,378,098, issued Jan. 3, 1995 to Andrews et al., typically incorporate a locking ring in the manufactured head that must not be shaved smooth, lest the structural integrity of the fastener would be adversely affected. Threaded blind bolts, as disclosed in U.S. Pat. No. 4,457,652, issued Jul. 3, 1984 to Pratt, may have sufficient strength but leave a small discontinuity on the head surface after shaving, resulting from gaps between male and females threads. The concern over having a discontinuity of this type is that the negative aesthetics are exaggerated by the application of paint. This discontinuity arises from small, but necessary, clearances between the female and male threads of the fastener. 
     SUMMARY 
     The present disclosure relates to a threaded blind fastener comprising a body, a core bolt, and a sleeve. A disposable drive nut may be used to facilitate installation and a plastic insert may be incorporated to assist in formation of the blind head. 
     One object of the present disclosure is to describe an improved flush-mount, blind fastener. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a front elevational view of a blind fastener, as initially supplied, according to the present disclosure. 
         FIG. 2  is a front elevational view, in full section, of the  FIG. 1  blind fastener, as viewed along line  2 - 2  in  FIG. 1 . 
         FIG. 3  is a front elevational view of the  FIG. 1  blind fastener, as installed, prior to finishing. 
         FIG. 4  is a front elevational view, in full section, of the  FIG. 3  installed blind fastener, as viewed along line  4 - 4  in  FIG. 3 . 
         FIG. 5  is a front elevational view of the  FIG. 3  installed blind fastener, as finished. 
         FIG. 6  is a front elevational view, in full section, of the  FIG. 5  finished installation of the blind fastener, as viewed along line  6 - 6  in  FIG. 5 . 
         FIG. 7  is a partial, perspective view of the  FIG. 5  finished installation. 
         FIG. 8  is a prior art structure that includes a discontinuity ring. 
         FIG. 9  is a front elevational view of a blind fastener, as initially supplied, according to the present disclosure. 
         FIG. 10  is a side elevational view, in full section, of the  FIG. 9  blind fastener, as viewed along line  10 - 10  in  FIG. 9 . 
         FIG. 11  is a front elevational view, in full section, of the  FIG. 9  blind fastener. 
         FIG. 12  is a front elevational view, in full section, of an alternative embodiment of blind fastener, as initially supplied. 
         FIG. 13  is a front elevational view, in full section, of another alternative embodiment of blind fastener, as initially supplied. 
         FIG. 14  is a front elevational view, in full section, of the  FIG. 12  blind fastener in an intermediate installed position. 
     
    
    
     DETAILED DESCRIPTION 
     For the purposes of promoting an understanding of the disclosure, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended, such alterations and further modifications in the illustrated device and its use, and such further applications of the principles of the disclosure as illustrated therein being contemplated as would normally occur to one skilled in the art to which the disclosure relates. In several figures, where there are the same or similar elements, those elements are designated with the same or similar reference numerals. 
     Referring to  FIGS. 1 and 2 , there is illustrated a flush-mount, blind fastener  10  that is constructed and arranged according to the following disclosure. Blind fastener  10  includes body  20 , deformable sleeve  40 , core bolt  50 , and a drive nut  60 . The body  20  includes an enlarged, frustoconical head  21  at one end, a conical nose  22  at the opposite end, a generally cylindrical shank  23 , a through bore  24 , and a threaded bore  25 . The deformable sleeve  40  defines a through bore  41 . The core bolt  50  includes an enlarged head  55  at one end, a wrenching region  54  at the opposite end, primary threads  51 , secondary threads  53 , and a generally cylindrical unthreaded shank  52 . An optional plastic insert  30  is included as a way to assist in formation of the blind head end during installation. The functioning of such an insert  30  is described in U.S. Pat. No. 4,457,652. 
     The enlarged head  21  has a frustoconical shape that is constructed and arranged to correspond to the countersink shape in the outer workpiece  70   a . The upper surface  26  is slightly domed (convex), or raised, the function of which will be described hereinafter. The interior bore  24  is a cylindrical form that is sized for a close fit around the (unthreaded) cylindrical shank  52 . 
     The enlarged head  55  of the core bolt  50  includes a horizontal shoulder  56  that seats (i.e., abuts against) the lower end  42  of deformable sleeve  40 . Accordingly, as bolt  50  is drawn upwardly, the sleeve  40  is pulled up the outer surface  27  of body  20  starting with the conical nose  22 . The shank  52  is positioned between the upper and lower sets of threads, the secondary threads  53  and primary threads  51 , respectively. 
     Formed in shank  52  is an annular groove  57  that creates a weakened location for separation of the core bolt  50 , as will be described. The drive nut  60  is assembled into threaded engagement with the (upper) secondary threads  53 . This is essentially what is illustrated in  FIGS. 1 and 2 . 
     Referring to  FIGS. 3 and 4 , the fastener  10  is installed through a plurality of workpieces  70   a ,  70   b , and  70   c . While three workpiece layers (laminations) are illustrated, it will be understood that a lesser number (2) or a greater number can be secured together by fastener  10  as described and illustrated herein. The referenced workpieces  70   a - 70   c  are composite laminations consistent with what would be expected or anticipated for the aircraft industry, but could also be metal panels or workpieces. The form of fastener  10  in  FIGS. 3 and 4  is after the initial installation steps as further described hereinafter. 
     The preparation of the workpieces  70   a - 70   c  for receipt of blind fastener  10  includes first drilling a cylindrical bore  71  through the three workpieces and then finishing the outer surface  72  of the outer workpiece  70   a  with a concentric countersink  73 . Into this prepared aperture, the fastener  10  is installed such that head  55  extends beyond the blind side surface  74  of workpiece  70   c  with the enlarged, frustoconical head  21  seated into the countersink  73 . 
     The installation of  FIGS. 3 and 4  is accomplished by rotationally threading the core bolt  50  into drive nut  60 , with the enlarged head  55  of core bolt  50  forcing the deformable sleeve  40  up the conical nose  22  of body  20 . If not initially in threaded engagement, the primary core bolt threads  51  and body threads  25  come into engagement during the early stages of the installation cycle. The workpieces  70   a - 70   c  become tightly clamped as the deformable sleeve  40  comes to bear against blind side surface  73 , whereupon the wrenching region  54  separates from the core bolt at groove  57  as the torsion strength of the core bolt material is exceeded. The movement of sleeve  40  into this bulbed configuration may be facilitated by plastic insert  30 . These actions and interactions relative to sleeve  40  and insert  30  are further explained in part in U.S. Pat. No. 4,457,652. 
     Referring now to  FIGS. 5 and 6 , it is noted that, after installation, the exposed upper surface  26  on head  21  of the body  20  is shaved substantially flush with the outer surface  72  of workpiece  70   a  to provide an aerodynamically-smooth surface  81 . Raised portions of core bolt  50  are also removed during this shaving step to provide flush head  80 . The enlarged head  21  of the body  20  is provided with a dome of excess material defined by surface  26  for shaving to avoid having to weaken the fastener  10  by excessive material removal. Further, the fastener body may be colored or dyed such that, after shaving flush the enlarged head  21 , a band  83  of color on the periphery of the exposed head is left (see  FIG. 7 ). This annular ring of color serves as an indicator as to how much safely removable material remains. 
     One of the considerations in shaving off material to create a flush finish is that the fastener not be weakened by removing too much material. When an annular ring of color or surface finish is left, this can be visually inspected as a way to confirm that the fastener  10  has not been weakened, due to excess material removal. Further, since the shank  52  is sized to be a close fit, ideally line-to-line, with bore  24 , there is only a negligible discontinuity  82  at most between the core bolt  50  and the shaved head surface  21 . Accordingly, in those applications where the fastener is to be painted, this negligible discontinuity  82  is not exaggerated by the application of paint. The result is an aesthetically pleasing, flush-mounted, paintable blind fastener that is suitable for aircraft applications and robotic installation. 
     In comparison, the prior art construction of  FIG. 8  illustrates the use of a screw  90  and a fastener body  91  that are threaded together. This specific assembly style results in a discontinuity gap  92  between the mating threads that is of a size or magnitude that results in an aesthetically unattractive combination. Due to thread geometry factors such as thread angle and pitch, and due to the realities of threaded engagement, the discontinuity gap  92  gradually becomes smaller as it extends from its maximum to its minimum 180 degrees apart. Even when this gap or discontinuity is relatively small, the addition of paint visually exaggerates the magnitude of the discontinuity. The result is an unacceptable visual appearance that is the result of using this prior art style of blind fastener. The need to use blind fasteners has been explained relative to the realities of robotic installation. Consequently, the fastener disclosed in  FIGS. 1-7  provides an improvement over the prior art structures. 
     Referring to  FIGS. 9-11 , blind fastener  110  is illustrated as constructed and arranged as follows. Blind fastener  110  includes body  120 , deformable sleeve  40 , core bolt  150  and drive nut  160 . Body  120  includes enlarged frustoconical head  121  at one end and conical nose  122  at the opposite end, a generally cylindrical shank  123  and through bore  124  and threaded bore  125 . Threaded bore  125  is an internal thread with crest truncation to the pitch diameter. Upper surface  126  defines the top of head  121  and body  120  includes outer surface  127 . 
     Core bolt  150  includes enlarged head  155  at one end, wrenching portion  154  at the opposite end, first threaded portion  151 , second threaded portion  153  and a generally cylindrical smooth wall shank  152 . First threaded portion  151  is an externally threaded portion with full threads illustrated. However, in alternative embodiments, the root of the thread could be truncated to the pitch diameter. The second threaded portion  153  is externally threaded with crest truncation to the pitch diameter. 
     Similar to the embodiment illustrated in  FIGS. 1-2 , plastic insert  30  is optionally included in thin walled recess  43  as a way to assist in the formation of the blind head end during installation. The characteristics and function of insert  30  is described in U.S. Pat. No. 4,457,652. 
     Drive nut  160  includes internally threaded portion  161 , unthreaded recess  162 , wrenching flats  163  and nose  164 . Internally threaded portion  161  is illustrated with full threads; however, alternative embodiments could utilize the threads with root truncation to the pitch diameter. Unthreaded recess  162  provides a space in which annular groove  157  can fracture after installation separating a top portion of core bolt  150  and drive nut  160  to be removed from the installed blind fastener  110 . Nose  164  engages upper surface  126  in a frictional engagement that rotationally constrains drive nut  160  with body  120  during installation. Nose  164  and/or upper surface  126  may optionally include friction enhancing features such as knurling or small projections to better engage drive nut  160  with body  120 . 
     Body  120  includes enlarged head  121  having a frustoconical shape that is constructed and arranged to correspond to a counter sunk recess in a work piece. Upper surface  126  is slightly domed (convex) or raised above machining line  185  that represents a location where body  120  can be machined flush with the outer surface of the work pieces. Threaded bore  125  is an internal thread with crest truncation to the pitch diameter. Threaded bore  125  and second threaded portion  153  are configured and arranged to not engage each another when body  120  is positioned over second threaded portion  153 . Similarly, threaded bore  125  is configured and arranged to permit the passage of unthreaded shank  152  there through while threaded bore  125  is constructed and arranged to threadingly engage first threaded portion  151  as illustrated in  FIG. 14 . These fit characteristics can be obtained by using the same pitch diameter for first and second threaded portions  151  and  153  and threaded bore  125  with the illustrated root and crest truncations to the pitch diameter. In addition, the diameters of both unthreaded shank  152  and through bore  124  closely approximate the pitch diameter of the threaded portions. 
     Core bolt  150  includes horizontal shoulder  156  that is sloped at an angle, which in the illustrated embodiment is approximately 100° and in other embodiments the angle can be between 90° and 120°. In alternative embodiments (not illustrated), lower end  42  can be beveled to match the inclination of horizontal shoulder  156 . 
     Referring now to  FIG. 12 , an alternate embodiment is illustrated as blind fastener  210  that is constructed and arranged as follows. Blind fastener  210  includes body  220 , deformable sleeve  40 , core bolt  150  and drive nut  260 . Body  220  includes enlarged head  221 , nose  222 , a generally cylindrical shank  223 , smooth wall bore  224 , internally threaded bore  225 , upper surface  226 , extension  228  extending therefrom and extension threads  229  on extension  228 . Extension threads  229  include root truncation to the pitch diameter while internally threaded portion  225  has internal threads with crest truncation to the pitch diameter. As such, internally threaded portion  225  is constructed and arranged to not threadingly engage second threaded portion  153  while internally threaded portion  229  is constructed and arranged to threadingly engage second threaded portion  153 . Smooth wall bore  224  extends through a portion of extension  228  with extension threads  229  positioned distally from upper surface  226 . This configuration provides space for unthreaded smooth wall shank  152  to extend to or through upper surface  226  without interference from extension threads  229 . Extension threads  229  keep threaded bore  225  rotationally aligned with first threaded portion  151  preventing cross threading when body  220  reaches first threaded portion  151  to become threadingly engaged with first threaded portion  151 . 
     Drive nut  260  includes internally threaded portion  261 , unthreaded recess  262  and nose  264 . Unthreaded recess  262  is constructed and arranged to closely encapsulate extension  228  permitting extension  228  to have a relatively thin walled cross section that is supported by drive nut  260 . Optionally, unthreaded recess  262  may have a near interference fit over extension  228 . 
     Referring now to  FIG. 13 , an alternate embodiment is illustrated as blind fastener  310  that is constructed and arranged as follows. Blind fastener  310  includes body  320 , deformable sleeve  40 , core bolt  350  and drive nut  360 . 
     Core bolt  350  is similar to core bolt  150  except for first threaded portion  351  being longer thereby moving smooth wall shank portion  352  further away from enlarged head  355  and similarly moving annular groove  357  further away from enlarged head  355 . 
     Internally threaded portion  361  includes internal threads with root truncation to the pitch diameter as opposed to the full threads illustrated in other embodiments of the drive nut. 
     Blind fasteners  210  and  310  illustrate different embodiments for use in different ranges of work pieces stack height for the work pieces being clamped together. In practice, individual blind fastener configurations are utilized for limited thickness range with a total thickness that extends from a minimum to a maximum total work piece thickness. For example, in one embodiment the range between minimum and maximum thicknesses may be 0.050 of an inch (approximately 1.3 mm). Blind fasteners  210  and  310  illustrate configurations for different stack up heights. For example, blind fastener  210  illustrates configuration for a comparatively thinner work piece thickness. By way of comparison blind fastener  310  illustrates configuration for a comparatively thicker total stack up heights. In blind fastener  310  threaded bore  325  is within one revolution of beginning threading engagement with first threaded portion  351 . In this embodiment, extension threads  329  still provide rotational guidance for body  20  as it engages first threaded portion  351  to prevent cross threading of threaded bore  325  on first threaded portion  351 . 
     For yet other embodiments with thicker work pieces, threaded bore  325  could begin in engagement with first threaded portion  351  thereby assuring cross threaded situation does not exist permitting extension  328  and extension thread  329  to be optionally omitted. 
     Extensions  228  and  328  are removed with the top portion of the upper surface  226  and  326  during machining or “shaving” to provide a flush surface as described in other embodiments. 
     Referring now to  FIG. 14 , blind fastener  210 ′ is illustrated in an intermediate installation configuration clamping work pieces  270   a  and  270   b  together with enlarged head  221  positioned in counter sunk recess  273 . Blind fastener  210 ′ corresponds to partially installed blind fastener  210  after clamping work pieces  270   a  and  270   b  together but before fracturing annealed annular groove  157 . As shown, threaded bore  225  is threadingly engaged with first threaded portion  151  with deformable sleeve deformed forming a blind side head against blind surface  274  defined by deformed sleeve  40 ′, in particular, thin wall recess  43 ′ and plastic insert  30 ′ as described in U.S. Pat. No. 4,457,652. In the illustrated configuration, smooth wall through bore  224  and smooth unthreaded shank  152  are aligned and smooth wall shank  152  is positioned in the region of machining line  185  with annular groove  157  positioned above machining line  185 . 
       FIG. 14  illustrates blind fastener  210 ′ clamping work pieces  270   a  and  270   b  together. Further tightening of core bolt  150  with respect to drive nut  260  will fracture annular groove  157 , thereby separating removable portion  159  from the rest of core bolt  150  and fastener  210 ′. After fracturing annular groove  157 , installation of fastener  210 ′ is completed by the removal of removable portion  159  and drive nut and the removal of extension  228  and upper surface  126  to machine line  185  to create a flush finish as described above. The material removal to machine line  185  can be done by any material removal methods known, including, but not limited to, conventional milling, cutting, grinding. Such material removal methods are referred to as “shaving” above. 
     While the preferred embodiment of the invention has been illustrated and described in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that all changes and modifications that come within the spirit of the invention are desired to be protected.