Abstract:
A rapid-release frangible fastener is provided that is operative in response to a force in excess of a predetermined value to quickly and reliably release and separate, e.g. for the rapid release of a pressure relief vent in switchgear for electrical power transmission and distribution. In a preferred embodiment, the frangible fastener utilizes a strain wire having a predetermined breaking strength. A resilient member is provided to ensure that the load on the frangible fastener is relatively constant, e.g. in response to vibration or distortion of the parts to which the fastener is connected.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This application claims the benefit of U.S. Provisional Application No. 60/340,395 filed on Dec. 18, 2001 in the names of R. T. Swanson and J. F. Fargo.  
           [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to the fields of fasteners and overpressure relief arrangements especially for the rapid release of a pressure relief vent in switchgear for electrical power transmission and distribution and more particularly to a rapid-release frangible fastener that is operative in response to a force in excess of a predetermined value to quickly and reliably release and separate.  
           [0004]    2. Description of the Related Art  
           [0005]    Various types of frangible fasteners and pressure relief arrangements are known in the prior art. For example, examples of pressure relief arrangements are shown in U.S. Pat. Nos. 4,583,750, 5,100,157 and 5,521,567 and copending application No. 10/017260 filed Dec. 18, 2001 in the names of J. Opfer et al.  
           [0006]    While the arrangements of the prior art may be generally suitable for their intended uses, it would be desirable to provide a rapid-release frangible fastener.  
         SUMMARY OF THE INVENTION  
         [0007]    Accordingly, it is a principal object of the present invention to provide a rapid-release frangible fastener that is operative in response to a force in excess of a predetermined value to quickly and reliably release and separate, e.g. for the rapid release of a pressure relief vent in switchgear for electrical power transmission and distribution.  
           [0008]    This and other objects of the present invention are efficiently achieved by a rapid-release frangible fastener that is operative in response to a force in excess of a predetermined value to quickly and reliably release and separate, e.g. for the rapid release of a pressure relief vent in switchgear for electrical power transmission and distribution. In a preferred embodiment, the frangible fastener utilizes a strain wire having a predetermined breaking strength. A resilient member is provided to ensure that the load on the frangible fastener is relatively constant, e.g. in response to vibration or distortion of the parts to which the fastener is connected. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0009]    The invention, both as to its organization and method of operation, together with further objects and advantages thereof, will best be understood by reference to the specification taken in conjunction with the accompanying drawing in which:  
         [0010]    [0010]FIG. 1 is a sectional view of a frangible fastener according to the present invention;  
         [0011]    [0011]FIG. 2 is an elevational view of a first assembly of the frangible fastener of FIG. 1;  
         [0012]    [0012]FIG. 3 is a sectional view taken generally along the line  3 - 3  of FIG. 2;  
         [0013]    [0013]FIG. 4 is an elevational view of a strain wire assembly of the frangible fastener of FIG. 1;  
         [0014]    [0014]FIG. 5 is a perspective view of the frangible fastener of FIG. 1 illustrating an application for the rapid release of a pressure relief vent in switchgear for electrical power transmission and distribution;  
         [0015]    [0015]FIG. 6. Is an elevational view of a first tool for adjusting the frangible fastener of FIG. 1 to provide a predetermined holding force;  
         [0016]    [0016]FIG. 7 is a sectional view taken generally along the line  7 - 7  of FIG,  6 ; and  
         [0017]    [0017]FIG. 8 is a sectional view of a second tool for adjusting the frangible fastener of FIG. 1. 
     
    
     DETAILED DESCRIPTION  
       [0018]    Referring now to FIGS.  1 - 4 , a frangible fastener  10  of the present invention is illustrated that is operative in response to a force in excess of a predetermined value, e.g. responsive to a force at  12  applied at a first point  14  of the frangible fastener  10  with respect to a second point at  16  of the frangible fastener. The frangible fastener  10  includes a first assembly  20  as shown in FIGS. 2 and 3 that is slidably disposed within a sleeve  30 . A spring  40  is disposed so as to act between the first assembly  20  and the sleeve  30 . The first assembly  20  includes two relatively movable members  22  and  24  with the member  22  being slidably received within a bore  23  of the member  24 . The first assembly  20  also includes a strain element  26  that is attached between the two members  22  and  24  as will be explained in more detail hereinafter to restrain their separation in a direction that would elongate the strain wire  26 , i.e. the strain element  26  acts as a control facility to prevent the two members  22  and  24  from separating until the predetermined force is exceeded in the direction of separation.  
         [0019]    With the first assembly  20  and the sleeve  30  arranged in an operative position as shown in FIG. 5 so as to compress the spring  40 , a predetermined restraining force is applied to restrain the relative movement between the components  20  and  30 . For example, as shown in FIG. 5, the frangible fastener  10  is installed in operative position between a vent  50  and a housing portion  60  of switchgear housing  70 , the vent  50  being hinged at  52  with respect to the switchgear housing  70 . The first point  14  of the frangible fastener  10  is retained by the vent  50  and the second point  16  of the frangible fastener  10  is retained by the housing portion  60 . Accordingly, when the pressure at  80  within the switchgear housing  70  exceeds a predetermined value, the forces on the vent  50  overcome the spring  40  and apply force to separate the two end points  14 ,  16  sufficient to break the strain element  26  whereupon the members  22 ,  24  move apart from each other releasing the vent  50  for opening in direction  54  to relieve the overpressure and vent the interior  80  of the switchgear housing  70 .  
         [0020]    Referring now to FIGS.  1 - 4  and  8 , the strain element  26  is retained at first end  27  within the member  24  via a pin  28  (FIG. 1) that fits within a slotted or recessed portion  32  (FIG. 3) of the member  24 . Further, the strain element  26  is retained at a second end  29  within the member  22  via a pin  34  (FIG. 1) that fits within a bore  36  (FIG. 2) formed through the member  22 . As shown in FIG. 4, the strain element  26  in the illustrative embodiment is fabricated as a looped wire  38  retained within a central ferrule element  39 . The member  22  is retained within the member  24  against relative movement in both the radial and axial directions via two pins  42 ,  44  (FIG. 1) that are inserted through respective passages  46 ,  48  formed through the member  24  and extending into open slotted portions  47 ,  49  of the member  22 .  
         [0021]    As shown in FIG. 5, in the illustrative application, the frangible fastener  10  is installed between an attachment member  51  extending from the inner side of the vent  50  and the housing portion  60 , e.g. the member  22  is installed through a hole  61  in the attachment member  60  and a hole  53  of the attachment member  51 , a nut  55  is threaded unto a threaded portion  25  of the member  22  that extends through the hole  53  of the attachment member  51 , and the collar  30  acts against the attachment member  60  to retain the other end of the frangible fastener  10 .  
         [0022]    Referring now additionally to FIGS. 6 and 7, a tool  100  is provided to tighten and apply a desirable pretension to the frangible fastener  10  after the installation of the frangible fastener  10 . This pretension ensures that the vent  50  remains closed until the predetermined pressure is exceeded, i.e. pressures lower than the predetermined pressure do not partially open the vent  50  even on a temporary or transient basis either due to internal pressure or wind outside the switchgear housing  70 . The tool  100  functions as a wrench and includes a pointer  102  to facilitate counting the number of turns the frangible fastener  10  is rotated relative to the nut  55  and a locating pin  104  that that cooperates with a receiving socket  106  (FIG. 8) in the member  24  to accomplish rotation of the member  24  via rotation of the tool  100 .  
         [0023]    In FIG. 8, there is illustrated another tool  110  that includes provisions and facilities to grasp the collar  30  so as to establish the relative position of the member  24  to the collar  30  when the spring  40  is contacting both the member  24  and the collar  30  at base shoulder  31 . The tool  110  is clamped to the frangible fastener  10  via the rotation of a threaded handle portion  111  within a mating threaded portion  113  of a body portion  115  of the tool  110 , the handle portion  111  acting against a clamp bar  117  of the tool  110 . The tool  110  includes the wrench pin  104  as in the tool  100  and a flush pin gauge assembly generally referred to at  112  to accomplish the desired function. Specifically, the flush pin gauge assembly  112  includes a movable pin member  114  that extends from the flush pin gauge assembly  112 . As the tool  110  is rotated, the pin member  114  moves downward in FIG. 8. The geometry of the pin member  114  is arranged so that the pin member  114  is flush or even with the top surface  116  of the flush pin gauge assembly  112  when the appropriate amount of predetermined pretension has been achieved by compressing the spring  40 .  
         [0024]    The frangible fastener  10  is provided with tamper resistant features via the provision of a self locking nut  55  and the recessed access to the member  24  via the collar  30  extending over the member  24 , i.e. such that there is limited access to the collar  30  and the member  24  can not be rotated with pliers or ordinary wrenches etc.  
         [0025]    While there has been illustrated and described a preferred embodiment of the present invention, it will be apparent that various changes and modifications will occur to those skilled in the art. Accordingly, it is intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of the present invention.