Abstract:
A torque-limiting fastener assembly, comprising: a fastener body having an externally threaded shaft and a drivable head, said head being provided with elements adapted for frictional contact with a driver capable of delivering torque to said threaded shaft, wherein said elements comprise flexible flanges/toggles that deflect at a predetermined torque to prevent further frictional contact between said driver and said head elements that is sufficient to drive said fastener.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of priority of U.S. provisional application No. 62/054,075, filed Sep. 23, 2014, the contents of which are herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to fasteners and, more particularly, to fasteners having an externally threaded shaft and a drivable head, the head being provided with elements adapted for frictional contact with a driver capable of delivering torque to the threaded shaft. 
         [0003]    As can be seen, there is a need for such fasteners that are not subject to over-torqueing. 
       SUMMARY OF THE INVENTION 
       [0004]    In one aspect of the present invention, there is provided a torque-limiting fastener assembly, comprising: a fastener body having an externally threaded shaft and a drivable head, the head being provided with elements adapted for frictional contact with a driver capable of delivering torque to the threaded shaft, wherein the elements comprise flexible flanges/toggles that deflect at a predetermined torque to prevent further frictional contact between the driver and the head elements that is sufficient to drive said fastener. 
         [0005]    These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a perspective view of an embodiment of the invention shown being tightened by a driving tool. 
           [0007]      FIG. 2  is an elevational view of an embodiment of the invention. 
           [0008]      FIG. 3  is a section view along line  3 - 3  in  FIG. 1  shown in non-maximum developed torque state. 
           [0009]      FIG. 4  is a section view line  3 - 3  in  FIG. 1  shown in maximum developed torque state. 
           [0010]      FIG. 5  is an exploded view of an alternate embodiment of the invention. 
           [0011]      FIG. 6  is a section detail view of the alternate embodiment of  FIG. 5  shown in assembled and in non-maximum-developed-torque state. 
           [0012]      FIG. 7  is a section detail view of the alternate embodiment of  FIG. 5  shown in assembled and in maximum-developed-torque state. 
           [0013]      FIG. 8  is an exploded view of an alternate embodiment of the invention. 
           [0014]      FIG. 9  is an exploded view of an alternate embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The following detailed description is of the best currently contemplated modes of carrying out exemplary embodiments of the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention, since the scope of the invention is best defined by the appended claims. 
         [0016]    Over and under tightening of fasteners lead to machine failure. Over torqueing a fastener can lead to screw/bolt breakage by torsional shear of the body of the fastener as well as drive head stripping rendering further tightening or loosening of the fastener impossible. Under tightening of fasteners can lead to failure of adequately joining members, machine failure due to vibrations, reduced performance and early failure of components. 
         [0017]    Furthermore the user has no way of knowing if the screw has been properly tightened without looking up a tightening torque value and purchasing a torque limiting wrench or driver. Torque limiting of fasteners is conventionally accomplished using a torque wrench which requires the user to set a specific value. These wrenches are expensive and require calibration. Furthermore their ability to properly limit the torque depends on components which can degrade over time or when exposed to severe ambient conditions such as corrosion and extreme heat. 
         [0018]    Existing visual torque indicating bolts do not limit the torque and rely on the user having a visible view of the bolt&#39;s head in order to assess if the indicating feature has changed color. In essence these devices are not torque limiting but torque indicating devices. Furthermore their ability to indicate torque relies on a color changing indicator installed on the head of the screw which can produce highly variable values. Torque indicating bolts are expensive and only allow the user to see if a certain torque range has been set. These devices are not true torque limiting in nature and rely on the user having a direct line of sight on the head of the screw. Over or under tightening can still occur using these devices. 
         [0019]    A fastener with a built in torque limiting feature greatly simplifies the user experience of tightening a screw. The user does not have to buy additional specialized equipment or be burdened by properly setting the equipment or any calibration. 
         [0020]      FIGS. 1-4  of the drawings illustrate one embodiment  10  of the torque limiting fastener of the invention which consists of externally threaded shaft  30  and drivable head  16 . The latter is depicted here as hexagonal; however, those skilled in the art will be aware that the invention is not limited to a drivable head having any particular geometry. The hexagonal head  16  comprises external faces  28  adapted to mate frictionally with internal faces (not shown) of driver tool  24 . Slots  14  in head  10  extend internally from the exterior of faces  28  to optional stress relief notches  22  on a line parallel to the clockwise adjacent face. The slots  14  define torque limiting flanges/toggles  12  having surfaces  18  which, when a predetermined torque is delivered to said fastener  10  by driving tool  24 , deflect toward and contact adjacent surface  20 . This deflection of the flanges/toggles  12  changes the geometry of the faces  28  such that there is insufficient frictional contact thereof with the internal faces of tool  24  to deliver the torque required to drive the head  16  and threaded shaft  30 . 
         [0021]    It will be understood by those skilled in the art that the fastener of the invention is insertion torque-limiting where the screw thread is left handed and removal torque-limiting where the screw thread is right-handed. 
         [0022]      FIGS. 5-7  illustrate a second embodiment  34  of the invention. The torque limiting fastener which consists of externally threaded shaft  38 , drivable head  36  and torque limiting insert  44 , adapted for insertion into and fixation within bore  40  in head  36 . Insert  44  is provided with grip protrusions  46  which mate with slots  42  in bore  40  to achieve secure fixation of insert  44  within head  36 . The insert  44  is provided with internal bore  45 , depicted here as hexagonal; however, those skilled in the art will be aware that, like drivable head  16  of the embodiment of  FIGS. 1-4 , the invention is not limited to any particular geometry of bore  45 . Insert  44  comprises internal faces  49  adapted to mate frictionally with external faces  51 / 26  of driver tool  24 . Insert  44  is provided with slots  54  which extend internally from the exterior of faces  49  on a line parallel to the clockwise adjacent face. The slots  54  define torque limiting flanges/toggles  48  having surfaces  50  which, when a predetermined torque is delivered to the fastener  34  by driving tool  24 , deflect toward and contact adjacent surfaces  52 . This deflection of the flanges/toggles  48  changes the geometry of the faces  49  such that there is insufficient frictional contact thereof with external faces  51 / 26  of tool  24  to deliver the torque required to drive the head  34  and threaded shaft  38 . 
         [0023]      FIGS. 8-9  illustrate a third embodiment  56  of the invention comprising externally threaded shaft  58  provided with internal bore  60 , torque limiting insert  64  provided with internal bore  63  and drivable head  70 . The insert  64  is provided with grip protrusions  66  which mate with slots  62  in bore  60  to achieve secure fixation of insert  64  within bore  60 . Similar to the insert  44  of  FIGS. 5-7 , insert  64  is provided with flange/toggles  68  which, when a predetermined torque is delivered to fastener  56  by driving tool  24 , deflect sufficiently to changes their geometry such that there is insufficient frictional contact thereof with external faces  51 / 26  of tool  24  to deliver the torque required to drive the fastener  56 . 
         [0024]    It should be understood, of course, that the foregoing relates to exemplary embodiments of the invention and that modifications may be made without departing from the spirit and scope of the invention as set forth in the following claims.