Abstract:
In a snap-in electrical connector for attaching helical metal cable and conduit to a junction box or the like, the connector having the locking ring with flexible tabs engageable with the cable/conduit, the improvement wherein the tabs have legs angled inwardly of the ring at specified angles, tips that form specified angles with the legs, and tip ends that are biased relative to the legs in the same direction. The specified angles of the legs, tips and tip ends provide improved pull-out strength, electrical conductivity and stability for a wide range of sizes and types of cable and conduit, including metal clad cable, armored cable, and flexible metal conduit, made of aluminum and steel.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a Continuation-In-Part of U.S. patent application Ser. No. 11/422,097 filed Jun. 5, 2006, and is incorporated herein by reference in its entirety. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to electrical connectors, and more specifically to a new and improved snap-in connector for electrical junction boxes and the like. 
       BACKGROUND OF THE INVENTION 
       [0003]    The invention is particularly concerned with improvements in snap-in connectors for helical metal cable and conduit, such as metal-clad cable, armored cable and flexible metal conduit. A known connector of this type generally includes a cylindrical body having a leading end that snaps into a hole in the junction box, a trailing end that receives the helical cable/conduit, and inwardly extending, flexible fingers or tangs that are intended to seat in the helical grooves to lock the cable/conduit in the connector body and establish electrical contact. 
         [0004]    Underwriters Laboratories Standard 514B requires connectors of the type described to pass several stringent performance tests. One test requires the connector to resist a cable pull-out force of 75 pounds for 5 minutes. Another test measures the electrical resistance of the cable/connector assembly when installed in a junction box. In this test, the voltage drop from the outside of the cable/conduit to the junction box should not exceed 50 millivolts. 
         [0005]    Tests conducted on commercially available cable connectors as described above have not met one or more of the desired requirements. This is due in large part to the fact that the cable tangs or fingers do not seat properly in the helical grooves of the cable/conduit. The failure of the tangs to seat in the helical grooves results in poor pull-out strength and poor electrical contact so that the voltage drop across the cable/fitting interface exceeds 50 millivolts. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the invention is to provide a snap-in electrical connector for helical metal cable and conduit which meets the desired industry performance standards. 
         [0007]    A more particular object of the invention is to provide a snap-in electrical connector for helical metal cable and conduit characterized by improved cable gripping tabs that are designed to maximize engagement of the tabs in the helical cable/conduit groove, thereby providing improved electrical contact and higher and more consistent pull-out strength than prior art designs. 
         [0008]    The snap-in electrical connector of the invention generally comprises a body having a longitudinal axis, a leading end engageable in a hole of a junction box, a trailing end for receiving a helical cable or conduit, an inner cylindrical wall defining a locking ring chamber, and a spring steel locking ring in the chamber. The locking ring includes outwardly extending tabs engaged in holes in the cylindrical wall of the connector body, and inwardly extending cable/conduit engaging tabs including trailing and leading tabs that extend toward each other, and a middle tab between the trailing and leading tabs. 
         [0009]    The invention is based on the discovery that it is possible to attain consistently high pull-out strength by making a connector of the type described with cable engaging tabs having a combination of specific properties of thickness, hardness and angular directions. 
         [0010]    In accordance with one embodiment of the invention, the tabs are made from 0.020 inch gauge  1074  spring steel having a hardness in a range of about 45-50 Rockwell C. In order to maximize the engagement of the tabs in the cable/conduit groove, each tab has a leg portion forming an included angle with the ring of about 35°-55°, a tip forming an included angle with the leg of about 135°, and a tip end. The tabs are spaced along the longitudinal axis of the connector body so that the tip ends are engageable in the helical groove of the cable/conduit. 
         [0011]    One critical aspect of the invention is the form of the tip ends of the tabs. The tip ends are biased in the same direction in order to maximize engagement in the helical groove of the cable/conduit. In a specific example hereinafter described, the biased tips form angles in a range from about 9° to 20°. 
         [0012]    The middle tab is longer than the other tabs and preferably has a concave or scalloped end face. The concave end face presents two points of engagement with the bottom of the helical groove that is captured by all of the tabs. 
         [0013]    The spacing of the tabs is optimized so that the tabs can engage in all types of cable/conduits, i.e. metal-clad cable, armored cable and flexible metal conduit made of steel and aluminum. The angle directions of the ends of the tabs, including the bias angles, are such as to capture and firmly engage in the helical grooves of wide range cable/conduit sizes. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIG. 1  is a side elevational view of the new snap-in electrical connector of the invention with a helical cable inserted into the connector from its trailing end. 
           [0015]      FIG. 2  is an end elevational view taken in the plane of the line  2 - 2  of  FIG. 1 . 
           [0016]      FIG. 3  is a cross-sectional view taken in the plane of the line  3 - 3  of  FIG. 2 . 
           [0017]      FIG. 4  is a cross-sectional view taken in the plane of the line  4 - 4  of  FIG. 2 . 
           [0018]      FIG. 5  is a perspective view of a spring steel locking ring made according to the invention. 
           [0019]      FIG. 6  is an end elevational view of the locking ring shown in  FIG. 5 . 
           [0020]      FIG. 7  is an edge elevational view of a partially formed blank used to form the locking ring of  FIGS. 5 and 6 . 
           [0021]      FIG. 8  is plan view taken in the plane of the line  8 - 8  of  FIG. 7 . 
           [0022]      FIG. 9  is a cross-sectional view taken in the plane of the line  9 - 9  of  FIG. 8 . 
           [0023]      FIG. 10  is a cross-sectional view taken in the plane of the line  10 - 10  of  FIG. 8 . 
           [0024]      FIG. 11  is an end elevational view taken in the plane of the line  11 - 11  of  FIG. 8 . 
           [0025]      FIG. 12  is a plan view of a blank prior to being formed to the shape of  FIG. 8   
           [0026]      FIG. 13  is an elevational view of a duplex connector embodying two locking rings of the construction shown in  FIGS. 5 and 6 . 
           [0027]      FIG. 14  is a plan view of a blank used in forming a modified duplex locking ring shown in  FIG. 16 . 
           [0028]      FIG. 15  is an edge elevational view of a partially formed blank used to make a duplex locking ring shown in  FIG. 16 . 
           [0029]      FIG. 16  is an end elevational view of a modified duplex locking ring made with the blank shown in  FIGS. 14 and 15 . 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    Referring now to the drawings and to  FIGS. 1-12  in particular, the new snap-in connector embodying the present invention is generally indicated by reference numeral  15  in  FIG. 1 . The connector  15  includes a cylindrical body  16  having a longitudinal axis  17 , a leading end  18  and a trailing end  19 . 
         [0031]    The leading end or nose  18 , which does not form a part of the present invention, may be constructed as disclosed in U.S. Pat. No. 6,827,604, the disclosure of which is incorporated by reference, to provide a quick-connect snap-in connection when pushed through an opening of a junction box. The end opening of the nose  18  is shown provided with an insulated bushing  25 . In  FIGS. 1-4 , reference numeral  48  designates a helical metal cable or conduit that is engaged in the connector  15  in a manner to be described. The cable/conduit wires (not shown) pass through the connector and the bushing  25 . 
         [0032]    The connector body  16  has a cylindrical wall  20  that defines a locking ring chamber  21 . A spring steel locking ring  30  is engaged in the chamber  21 . As shown in  FIGS. 2-4 , the locking ring  30  includes outwardly extending tabs  31  that are engaged in holes in the cylindrical wall  20  of the connector body  16 . In the illustrated construction, the side edges of the tabs  31  are bent toward each other to form a concavo-convex shape in order to enhance their columnar strength. 
         [0033]    The locking ring  30  has flexible, inwardly extending tabs that engage and lock a helical cable/conduit in the connector  15 . These tabs include a leading tab  32  and a trailing tab  33  on either side of the longitudinal axis  17 . A middle tab  34  is located between the tabs  32 ,  34 . The tabs  32 - 34  are spaced along the axis  17  by a distance determined by the helical angle of the cable/conduit so that all three tabs will seat in the helical groove. 
         [0034]    The tabs  32 - 34 , respectively, have leg portions  32   a ,  33   a ,  34   a  and angularly extending tips  32   b ,  33   b ,  34   b  that terminate in tip ends  32   c ,  33   c ,  34   c . As most clearly shown in  FIGS. 7 and 8 , the tip ends  32   c ,  33   c ,  34   c  are biased in the same angular direction. This assures that the tip ends will engage the bottom of the helical groove of cable/conduit of different sizes. The corners of the tip ends  32   c ,  33   c ,  34   c  may be radiused in order to prevent the corners from damaging the insulation of the conductor wires of the cable/conduit  48  that are pulled through the connector  15  and the bushing  25 . 
         [0035]    The spacing of the tabs  32 ,  33 ,  34 , the angularity of the tabs relative to the ring  30 , the angle of the tips  32   b ,  33   b ,  34   b , and the bias angle of the tip ends  32   c ,  33   c ,  34   c  are important in order that the tab ends can be captured in the helical grooves of a full range of cable/conduit sizes, for example, cable diameters ranging from 0.405-0.612 in diameter, and will engage the bottoms of the helical grooves to provide good electrical contact and high pull-out resistance. 
         [0036]    The high pull-out strength of the connector  15  is further enhanced by the formation of the middle tab  34 . As shown, it is concavo-convex lengthwise and is longer than the tabs  32 ,  33  which results in the cable/conduit being forced toward the wall  20  of the connector body. The concavo-convex shape of the middle tab adds to its columnar strength so that it firmly holds the cable/conduit in place in the connector body. The tip end  34   c  is concave. The concave tip end provides two-point contact with the bottom of the helical cable/conduit groove. This two-point contact is important in achieving good pull-out strength and stability, as will better electrical conductivity. 
         [0037]    Specific examples of the invention are as follows: 
         [0038]    In the following examples, the ring  30  is made of spring steel, preferably, 1074 grade steel, having a thickness and hardness of 0.020 inch gauge and 45-50 Rockwell C. The physical properties of the ring, in combination with the angle directions of the tabs  31 - 34 , are responsible for the high pull-out strength of the connector  15 . 
         [0039]    The specific angularity of the tabs  32 - 34  relative to the ring  30  is a compromise between high pull-out strength and electrical conductivity on the one hand and ease of inserting the cable or conduit  48  on the other hand. A steep angle of the tabs results in optimum pull-out strength and electrical conductivity, but makes it more difficult to engage the cable/conduit between the tabs. Shallower tab angles make it easier to insert the cable/conduit, but decrease the pull-out strength and conductivity. In the example of the invention, the tabs and ring form included angles in a range from about 35°-55°, and, more particularly, from 37°-52°. 
         [0040]    Referring to  FIGS. 9 and 10 , the tab  34  is shown as forming an included angle  40 , and the tab  33  is shown to form an included angle  41 . The included angle formed by the tab  32  may be the same as that formed by the tab  33 . In one example of the invention exhibiting high pull-out strength and conductivity, the angle  40  is about 52° and the angle  41  is about 42°. According to another example characterized by easier insertion of the cable/conduit, the angle  40  may be about 45° and the angle  41  about 37°. 
         [0041]    Each of the tips  32   b ,  33   b ,  34   b  is bent inwardly of the leg portions  32   a ,  33   a ,  34   a  to form included angles of about 135°. The bias angles of the tip ends  32   c ,  33   c ,  34   c  are shown in  FIG. 7  and range from about 9° to 20°. A specific angle  45  of the tip end  32   c  is about 20°, the bias angle  46  of the tip  33  is about 10°, and the bias angle  47  of the middle tab tip end  34   c  is about 9°. 
         [0042]    A partially formed, spring steel blank is shown in  FIGS. 7-11 . These drawings show the angularity of the tabs  31 - 34 , the angularity of the tips  32   b ,  33   b ,  34   b , and the bias angles of the tip ends  32   c ,  33   c ,  34   c . Because the tip ends are biased in the same angular direction, they face the same helical direction when the blank is bent into a ring. 
         [0043]      FIG. 12  shows a blank prior to being partially formed to the shape of  FIG. 8 . The tab forming portions are designated by reference characters  32   d ,  33   d , and  34   d , and the portions forming the locking tabs  31  are designated by reference character  31   d . As described previously, the locking ring  30  is inserted in the body  16  from its trailing end  19  so that the tabs  32 - 34  are directed inwardly toward the leading end or nose  18  of the connector. An electrical cable or conduit  48  is then inserted through the trailing end of the connector with the cable wires (not shown) extending out through the insulating bushing  25 . 
         [0044]      FIG. 13  shows a duplex snap-in connector  50  which receives two helical metal conduits or cables. The connector  50  has a leading end  51  that is similar to the leading end of the connector  15  described above, and a trailing end. The leading end  51  may be provided with an insulated bushing  52  similar to the bushing  25 . 
         [0045]    The trailing end of the connector  50  has two barrels or locking ring chambers  53 ,  54 . Each barrel or locking ring chamber  53 ,  54  has a locking ring  30  that may be identical to the locking ring  30  described above in connection with the embodiment of  FIGS. 1-12 . 
         [0046]    In use, the cable/conduits are inserted into the locking rings  30  and the cable wires are pulled or pushed through the bushing  30 . 
         [0047]      FIG. 16  illustrates a modified locking ring  60  for a duplex snap-in connector. The body of the duplex connector is not shown, but it may be same as shown in  FIG. 13 . 
         [0048]    The duplex locking ring  60  has two cable receiving sections  61 ,  62 . The section  61  has leading and trailing locking tabs  63 ,  64 , respectively, and a middle tab  65 . The locking ring section  62  has corresponding tabs  66 - 68 . The tabs  63 - 68  are formed and located identically to the tabs  32 - 34  described above in connection with the embodiment of  FIGS. 1-12 . 
         [0049]    Each locking ring section  61 ,  62  has outwardly extending locking tabs  69  that correspond in structure and function to the locking tabs  31  described in connection with the embodiment of  FIGS. 1-12 . 
         [0050]      FIG. 14  shows a spring steel blank  80  for the duplex ring  60 . The blank  80  has tab forming portions  82 - 87  that are bent into the tabs  63 - 68 , respectively, and locking tab forming portions  89  that correspond to the finally formed tabs  69 . 
         [0051]    The middle of the blank  80  has slots  95 ,  96  between the tab portions  84 ,  85 . Locking fingers  98  extend from one end of the blank  80  and locking fingers  99  extend from the other end. When the blank  80  is fully formed and bent into a ring, the locking fingers  98  are inserted into the slot  96  and the locking fingers  99  are inserted into the slot  95  as shown in  FIG. 15 .  FIG. 15  illustrates the blank  80  after being partially formed before being bent into the duplex shape of  FIG. 16 . In  FIG. 15 , the blank portions are given the same reference numerals as in  FIG. 14  followed by the letter “a”. 
         [0052]    As previously described, the invention provides an improved snap-in electrical connector having better electrical conductivity and pull-out strength than previous designs. This objective is achieved by a combination of important features. The locking tabs are spaced apart axially of the connector so that they engage in a full range of helical metal cable and conduit of different diameters and types. The angle direction at the ends of the tabs, i.e. the angle between the tab and its leg portion, also is optimized to capture the helical groove over a range of cable diameters and types. 
         [0053]    The angles of the tip ends with respect to the sides of the leg portions is optimized to engage the bottoms of the helical grooves in a range of cable/conduit sizes. The tip ends of the locking and trailing tabs will almost fully engage the groove bottoms of larger cables. The concave or scalloped tip of the middle tab provides two points of contact with the groove bottom of all sizes of cable/conduit, thereby enhancing electrical conductivity, strength and stability. 
         [0054]    Further advantages will be apparent to those skilled in the art in light of the foregoing disclosure. Therefore, it is to be understood that, within the scope of the appended claims, the invention can be practiced otherwise that as specifically shown and described.