Abstract:
Blind fasteners are used to connect two or more workpieces together, and are particularly useful in aerodynamic designs. In such uses, it is very desirable to have a substantially flush surface. The present fastener comprises a fastener body having an axial bore therethrough and an enlarged head, a corebolt passing through the axial bore, the corebolt having a first portion, a threaded second portion, and a localized weakened region therebetween, a drive nut configured to fit around and threadingly engage the first portion of the corebolt, and a stop member that substantially prevents further withdrawal of the corebolt relative to the drive nut after the corebolt has been driven a predetermined distance during installation to shear the corebolt at the weakened region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. Provisional Patent Application No. 60/534,261 filed on Jan. 5, 2004, which is hereby incorporated by reference. 
     
    
     FIELD OF INVENTION  
       [0002]     The present invention relates generally to blind fasteners for use in fastening workpieces together in overlapped outer and inner relation, and more particularly, to blind fasteners that provide positive mechanical engagement during installation of the fastener so that the corebolt break groove shears at a position at or near a fastener body flange head for a flush finish.  
       BACKGROUND  
       [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 usually is desired on the accessible side of the panels while access to the blind side may not be possible. In these cases, the use of a blind fastener is appropriate, since access to only one side of the panel is available to install the fastener.  
         [0004]     Typical blind fasteners comprise an internally threaded nut body and an externally threaded cylindrical corebolt or stem passing in threaded engagement through the nut body. The inserted end of the corebolt has an enlarged corebolt head while the other end of the corebolt has a wrench-engaging portion. Thus, upon insertion of the fastener into the aligned apertures of a pair of workpieces and upon turning motion of the corebolt relative to the nut body, the corebolt is moved in an axially outward direction through the nut body. This axially outward movement typically causes a deformable sleeve around the corebolt and intermediate the nut body and corebolt head to deform around the tapered nose of the nut body to provide a blind side head against the inner surface of the inner work piece. The corebolt further is provided with a localized weakened region or break groove adapted to sever the corebolt at a predetermined torque and location.  
         [0005]     Other types of fasteners utilize non-threaded fastener bodies and corebolts, such as rivets. These rivets include much of the same structure as identified above. However, where the stem has a threaded section and is pulled through the unthreaded fastener body by a drive nut system to cause the deformable sleeve located intermediate the stem head and the fastener body to deform on the blind side of the inner work piece. Again, the unthreaded to thread transition of the corebolt may be provided with a localized weakened region or break groove adapted to sever the corebolt at a predetermined tension stress.  
         [0006]     It is advantageous that the break groove, in either the threaded or non-threaded fasteners or any other fastener configuration, shears the corebolt in a substantially flush relation to the fastener body head after the fastener is fully set. Particularly, accurate corebolt 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.  
         [0007]     However, due to numerous factors including over-tightening, sometimes the break groove on the corebolt 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 corebolt may protrude beyond the fastener body head. It is often necessary to grind the protruding corebolt so that the corebolt 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 corebolt stem to provide a flush finish. Several designs have been offered to promote flush breaking of the corebolt stem, such as U.S. Pat. No. 4,752,169 issued on Jun. 21, 1988, hereby incorporated by reference herein, and having common ownership with the present invention.  
       SUMMARY OF THE INVENTION  
       [0008]     In accordance with the present invention there is provided a fastener comprising, a fastener body having an axial bore therethrough and an enlarged head, a corebolt passing through the axial bore of the fastener body, the corebolt having a first portion, a threaded second portion, and a localized weakened region between the first and second portions, a drive nut configured to fit around and threadingly engage the first portion of the corebolt, and a stop member that substantially prevents further withdrawal of the corebolt relative to the drive nut after the corebolt has been driven a predetermined distance during installation to shear the corebolt at the weakened region.  
         [0009]     In another embodiment of the present invention a fastener is disclosed. The fastener comprises a fastener body having an axial bore extending therethrough and an enlarged head, a corebolt passing through the axial bore of the fastener body, the corebolt having a first portion, a threaded second portion, and a break groove located between the first and second portions, a drive nut configured to threadingly engage the second portion of the corebolt and capable of engaging the enlarged head of the fastener body, and means for mechanically engaging the first portion of the corebolt with the drive nut to substantially prevent further withdrawal of the corebolt after the corebolt has been driven a predetermined distance during installation of the fastener to shear the corebolt at the break groove. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0010]     Objects and advantages together with the operation of the invention may be better understood by reference to the following detailed description taken in connection with the following illustrations, wherein:  
         [0011]      FIG. 1  is a cross-sectional view of a threaded fastener and threaded drive nut according to a first embodiment of the present invention;  
         [0012]      FIG. 2  is a cross-sectional view of the corebolt of the first embodiment showing first and second corebolt portions having different diameters prior to creation of threads;  
         [0013]      FIG. 3  is a modification of the corebolt of  FIG. 2  wherein the corebolt further includes a break groove and optional step;  
         [0014]      FIG. 4  is an enlarged view of segment A of  FIG. 1  showing the drive nut/fastener body/corebolt interface of the first embodiment;  
         [0015]      FIG. 5  is an enlarged segment view of the drive nut/fastener body/corebolt interface of a second embodiment of the invention utilizing a lock ring;  
         [0016]      FIG. 6  is an enlarged segment view of the drive nut/fastener body/corebolt interface of a third embodiment of the invention utilizing a deformable feature which engages the fastener body and drive nut and provides a screw stop;  
         [0017]      FIG. 7  is an enlarged segment view of the drive nut/fastener body/corebolt interface of a fourth embodiment of the invention utilizing a separate element to provide a screw action stop;  
         [0018]      FIG. 8  is a cross-sectional view of a drive nut according to the first embodiment of the invention;  
         [0019]      FIG. 9  is a cross-sectional view of a second embodiment of a deformable drive nut; and  
         [0020]      FIG. 10  is a rivet-type blind fastener using a thread stem section and a drive nut for mechanical installation with a flush break feature;  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]     The present invention is accomplished by providing a corebolt stem with at least two distinct diameter sections. Particularly, the corebolt section engaging the fastener body is of a larger diameter that the threaded corebolt stem section engaging the drive nut and separated by a break groove. Therefore, as the corebolt is rotated and driven radially outwardly the larger corebolt section engages the drive nut, and is prevented from being driving through the drive nut. Therefore, the torque as applied by the driving tool to the small threaded corebolt section causes the drive load to increase and the corebolt to shear in the predefined break groove at a predetermined position relative to the fastener body head. Further, it is anticipated that the present invention can be utilized with any fastener configuration to provide positive mechanical engagement between the corebolt and the drive nut to provide increased shear or tensile stress in the corebolt to accurately predict corebolt break off for a substantially flush finish.  
         [0022]     Referring now to  FIG. 1  of the drawings, a blind fastener  10  in accordance with the teachings of the invention is shown. Fastener  10  includes a fastener body  11  and a corebolt or stem  12 . The fastener  10  may further include a deformable sleeve  13 . The fastener body  11  has a body portion  14  with a threaded axial bore  15  therethrough. A nose  16  is provided at one end of the body portion  14  in the preferred form of a conically shaped chamfer having an angle of about 15 degrees to 30 degrees to the axis of fastener body  11 .  
         [0023]     An enlarged flange head  17  is provided at the other end of the body portion  14 . The flange head  17  is adapted to seat in a cavity in the access side of a pair of structural panels or the like being fastened together. The flange head  17  may also be of a protruding type, setting on the surface of the access side panel. The length of fastener body  11  is adapted to extend the external surface of the body portion  14  beyond the blind side of the parts being fastened even in the maximum grip situation by a distance sufficient to permit the thin wall of sleeve  13  to adapt to the external surface of body portion  14 , as will be discussed.  
         [0024]     The corebolt  12  has a first threaded stem portion  25  extending through fastener body  11  and having an enlarged head  26  at one end thereof (on the blind side). The diameter of head  26  is generally related to the diameter of the body portion  14 . The corebolt  12  further comprises a second threaded stem portion  24  extending through the drive nut  23 . The thread diameter of the first threaded stem portion  25  is larger than the thread diameter of the second threaded stem portion  24  but has the same pitch, as shown in  FIG. 4 , so as to act as a screw stop as will be discussed in greater detail below. Corebolt  12  further includes a localized weakened region located between the first threaded stem portion  25  and the second threaded stem portion  24 . In particular, as shown in  FIG. 1 , the localized weakened region is a break groove  27 . The break groove  27  causes the corebolt  12  to fracture when a preselected stress is applied to it during installation of the blind fastener  10 . Such fracture is designed to occur when the break groove  27  is located flush with or slightly below the surface  28  on the access side of the part being fastened upon completion of the installation of blind fastener  10 .  
         [0025]     Drive nut  23  may be threaded on the second threaded stem portion  24 . Drive nut  23  may also include a deformable portion to engage the flange head  17  so as to help prevent relative rotation between the fastener body  11  and the drive nut  23  as well as to assist in aligning the thread leads of the fastener body  11  and drive nut  23  to prevent lock up.  
         [0026]     As particularly contemplated in the present invention, and clearly shown in  FIG. 2 , the corebolt  12  utilizes a stem having two threaded portions of different thread diameters. As shown in  FIG. 3 , the break groove  27  can be formed between the two threaded portions  25 ,  24  and could optionally include a step or unthreaded portion  30 .  
         [0027]     The operation of fastener  10  during installation thereof is shown in  FIG. 5  for the fastener of  FIG. 1 . When the fastener  10  is inserted into the aligned openings of a pair of workpieces, the drive nut  23  is held and a drive tool engages the wrenching flats  42  in order to the turn the corebolt  12  relative to the drive nut  23 . The drive nut  23  likewise is designed to engage the flange head  17  so as to prevent relative rotation therebetween, as through the use of a deformable feature  32 . Thus, as the corebolt  12  is rotated, the first threaded stem portion  25  is driven through the threaded fastener body  11  and the second threaded stem portion  24  is driven through the drive nut  23  so as to draw the corebolt head  26  toward nose  16  so as to deform the sleeve  13  against the inner surface of the inner workpiece to form a blind side head. Alternatively, however, the fastener  10  may not include a separate deformable sleeve. Instead the fastener body  11  includes a sleeve portion that is integral thereto or just the body portion  14  can be used. The operation of this alternative fastener is the same as previously discussed. As the corebolt  12  is rotated, the first threaded stem portion  25  is driven through the threaded fastener body  11  and the second threaded stem portion  24  is driven through the drive nut  23  so as to draw the corebolt head  26  toward nose  16  so as to deform the sleeve portion or the body portion  14  of the fastener body against the inner surface of the inner workpiece to form a blind side head.  
         [0028]     When the corebolt  12  has been driven a predetermined distance, a stop member, such as the step  30 , or in the alternative a shoulder  31  of the first stem portion  25 , engages the threads of the drive nut  23  so as to substantially prevent further withdrawal of the corebolt  12  relative to the drive nut  23  to shear the corebolt  12  at the break groove  27 . Therefore, with the first threaded stem portion  25  held by mechanical engagement with the drive nut  23 , the continued torque applied to the second threaded stem portion  24  of the corebolt  12  causes increased torque at the break groove  27  thereby forcing the corebolt  12  to shear a predetermined distance from the flange head  17 . Therefore, the present design forces a bolt break at a predetermined position with the head  17  so that the corebolt  12  stem breaks off in a flush relationship therewith.  
         [0029]     A second embodiment is shown at  FIG. 6  where the stop member comprises a deformable lock ring  38  that prevents relative movement between the fastener body portion  14  and the corebolt  12 , acting as a self locking feature after the fastener  10  is installed. Drive nut  23  forces and deforms the locking ring  38  into the cavity between the remaining first threaded stem portion  25  and the inner walls of the fastener body portion  14  in the region  38 A. A third embodiment is shown at  FIG. 7  wherein the drive nut  23  includes the stop member, which is a deformable feature  40  that engages the first threaded portion  25  so as to prevent further withdrawal of portion  25 . It is contemplated that this deformable feature  40  for engaging the first threaded portion  25  could either be integral with the drive nut  23  or be a separate member. A fourth embodiment is shown at  FIG. 8  wherein the stop member comprises a separate member  42  that is located between the first threaded portion  25  and the drive nut  23  so as to provide a corebolt screw stop. As shown in  FIG. 8 , the corebolt screw stop member  42  preferably a washer, stops the withdraw of the first threaded portion  25  relative to the drive nut  23  so as to break the corebolt stem at a predetermined position relative to the head  17 . This fourth embodiment may also optionally utilize a deformable portion  43  on the drive nut  23  for engaging the head  17 .  
         [0030]     An internally threaded blind fastener with a corebolt that breaks off flush with the top of the nut head in a grip condition is shown in the accompanying drawings. Other fastener designs attempt to achieve break-off requirements of 0.000/0.103″ when measured from the top of the installed fastener nut head. The break-off distances can vary within the required limits and still be considered a satisfactory installation, but requires a subsequent milling operation to mill the protruding corebolt flush with the surface of the workpiece. The present design is targeted to provide break-off from approximately 0.000/0.010″ when measured from the top of the installed fastener nut head to eliminate subsequent corebolt milling operations after installation. This improvement will reduce break off distance variation and eliminate the need for subsequent milling operations. Therefore, when installation is complete, the unthreaded portion of the corebolt or step and the smaller thread size of the drivenut prevents further thread engagement, resisting the applied torque resulting in the corebolt breaking off at the break-off-groove with no need for any secondary grinding operation.  
         [0031]     Alternatively, as shown in  FIG. 11  where it should be understood that for simplicity, like elements are identified by like numbers, the proposed fastener could also be applied to a rivet-type fastener  110 . The fastener  110  comprises a corebolt  112  having a first portion  125  and a second portion  124 . The first portion  125  is not threaded, whereas the second portion  124  is threaded. Additionally, the first portion  125  has a diameter that is greater than that of the second portion  124 . The fastener  110  further comprises a fastener body  111  with a non-threaded axial bore  115  therethrough similar to that of the previous embodiment. Finally, the fastener  110  comprises a drive nut  23  identical to that previously described.  
         [0032]     During installation of the fastener  110 , the drive nut  23  pulls the second portion  124  which causes the first portion  125  to be pulled through the unthreaded axial bore  115  of the fastener body  112  to cause the deformable sleeve  13  located intermediate the corebolt head  26  and the fastener body  114  to deform on the blind side of the inner work piece. Again, the unthreaded to thread transition of the corebolt  112  may be provided with a localized weakened region or break groove  27  adapted to sever the corebolt  112  at a predetermined tension stress.  
         [0033]     Finally, when the corebolt  112  has been driven a predetermined distance, the stop member, or more particularly a step  30 , or in the alternative the first portion  125  because of its greater diameter, engages the threads of the drive nut  23  so as to prevent further withdrawal of the corebolt  112 . Therefore, with the first portion  125  held by mechanical engagement with the drive nut  23 , the continued torque applied to the second portion  124  of the corebolt  112  causes increased torque at the break groove  27  thereby forcing the corebolt  112  to shear a predetermined distance from the flange head  17 . Therefore, the present design forces a break at a predetermined position with the flange head  17  so that the corebolt  112  breaks off in a flush relationship therewith.  
         [0034]     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.