Abstract:
The subject invention relates to a new connector assembly design for making a low resistance, stable electrical connection to a plastic coated aluminum shield adhering to and underlying polyethylene jacket of telephone cables. When the connector in accordance with the invention is applied across the composite sheath, it eliminates the creep strain exhibited by the polyethylene jacket and aluminum shield and results in a stable low resistance electrical connection, which is essential for noise-free operation of the cable and good lightning protection of enclosed telephone circuits.

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The subject invention relates to a reliable connector for bonding shields of telephone cables to ground and in particular for the polyethylene coated aluminum shield adhering to and underlying a polyethylene jacket. 
     The most commonly used devices for providing shield bonding consist of a connector composed of two or more parts assembled together by stud and a nut. The lower sections of the connector, incorporating the stud, are placed in contact with the shield under the cable outer jacket. The other part of the connector rests on top of the cable jacket, whereupon the nut is threaded onto the stud, electrical contact is established between the shield through the lower sections then through the threaded stud when the nut is tightened. This technique is operative to provide shield bonding but it involves many problems. 
     The electrical contact resistance has been found to increase substantially with time and, as a result, the telephone operating companies have experienced noisy lines. 
     The high increase in contact resistance with commercially available connectors is attributed to the loss of contact between the lower section and the aluminum shield as a result of aluminum oxidation at the contact points. Aluminum, as well as the polyethylene cable jacket, which is normally low density type, have the tendency to cold flow or creep under sustained load, and, in addition, the dimensional stability of the jacket is very sensitive to temperature fluctuation. Therefore, with time, relocation of the initially applied pressure at the contact points takes place and aluminum oxide forms which is non-conductive and consequently results in increasing the electrical contact resistance. 
     The main problem, of course, is the bonded composite cable sheath. The coating on the aluminum shield was initially designed to eliminate corrosion problems. Further sheath modification consisted of bonding the shield to the polyethylene cable jacket. The latter improved the mechanical characteristics of the sheath and reduced substantially the moisture permeation into the cable core. The laminated composite sheath, however, created problems in bonding such sheath to ground, or in establishing electrical shield continuity at splice points, since high adhesion between the shield and the jacket made separation of the two difficult, if not practically impossible. This, in turn, created a necessity for electrical bonding of the composite sheath. 
     The invention described in this specification has built-in features which compensate for creep strain and, in addition, prevent oxidation of the aluminum at the contact points. A connector, according to the present invention, provides and maintains a stable low resistance electrical connection to a metallic shield when applied over the composite sheath of the plastic telephone cable. The present invention overcomes disadvantages of the prior art by providing means by which the contact resistance remains low and stable. 
     Other objects, features of the invention will appear or be pointed out as the description proceeds. 
    
    
     BRIEF DESCRIPTION OF DRAWING 
     In the drawing, forming a part hereof, in which like reference characters indicate corresponding parts in all the views: 
     FIG. 1 is an exploded isometric view showing the sheath bonding connector of this invention and illustrating the order of assembling its components at the end of a composite laminated cable sheath; 
     FIG. 2 is an isometric view of the clamping shoe used on the assembly shown in FIG. 1; 
     FIG. 3 is an enlarged, fragmentary view showing the sheath bonding connector of this invention when assembled with the cable shown in FIG. 1; and 
     FIG. 4 is a diagrammatic view showing the use of a plurality of sheath bonding connectors on the same cable. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENT 
     FIG. 1 shows a communication cable 10 having a core 12 surrounded by metal shield 14. A plastic outer jacket 16 is adhered to the metal shield 14 in accordance with conventional practice. 
     The shield 14 and jacket 16 are cut through along two lines 18 and 20 which are spaced from one another and which extend lengthwise of the cable. If the lines 18 and 20 do not extend to the end of the shield 14 and jacket 16, the shield and jacket are also cut along a line 22 which extends with a circumferential component and with the lines 18 and 20 forms a tab, indicated generally by the reference character 24. 
     The tab 24 can be hinged upward away from the core 12, at the end of the tab remote from the line 22, as shown in FIG. 1. This makes the metal shield 14 accessible for connecting it with the other parts of this invention. 
     There is an inner shoe comprising a plate 26 and a shim 28 which rests on the plate 26 but is shown spaced from it in FIG. 1 for clearer illustration. A fastening element 30, shown as a threaded screw, connects with the plate 26 and passes through an opening in the shim 28. 
     The fastening element 30 extends through an opening 32 in the tab 24, and when the structure is assembled, the fastening element 30 holds the plate 26 and shim 28 in firm contact with one another and holds the shim 28 in firm contact with the metal shield 14 which is preferably adhered to the outer jacket 16. The fastening element 30 extends through a leaf spring 36 and then through an opening 38 in a clamping shoe 40. A nut 42 screws over the end of the fastening element 30 and clamps all of the parts together which are between the plate 26 and the nut 42. 
     The shim 28 is preferably of substantially the same size as the plate 26 but is much thinner than the plate 26. The shim is preferably made of a thin resilient metal plate with a hole that fits loosely over the fastening element 30. The shim is formed with dimple-like openings having sharp burrs 43 projecting outwardly from the side which faces the cable shield 14. The side of the plate having the sharp points formed by the burrs 43 is preferably plated with indium metal. The sharp contacts on burrs 43 increase the contact pressure between the shim and the metal shield 14. Indium has the characteristic of being a good bearing metal. The shield 14 may be made of aluminum, which becomes non-conductive as a result of oxidation, but the sharp contacts of the shim 28 cut through such oxide as may form. 
     The spring 36 is an energy-storing device. When the parts are brought together, and the spring is compressed against the outer jacket 16 by the clamping shoe 40, the spring 36 is stressed and presses the outer jacket 16 and the metal shield 14 toward the shim 28 and maintains the contact pressure between the contacts of the shim 28 and the inside surface of the shield 14 in spite of changes in dimensions and cold flow of any of the parts. 
     The sharp burrs on the shim plate will cut through any thin plastic coating on the inner face of the metal shield 14 and make electrical connection with the metal shield. The indium plating on the burrs flows and forms a gas-tight seal at the contact points. Further tightening of the nut 42 stores compressive energy in the spring 36, and this compressive energy is automatically released to compensate for creep strain that tends to reduce the pressure on the electric contact points. 
     The clamping shoe 40 has walls 44 on opposite sides and another wall 46 which extends between the walls 44 at one end, and only one end, of the clamping shoe 40; with sharp edges at the bottoms of the walls 44. The sharp edges can be continuous straight line edges at the lower ends of the walls; but in the preferred construction, the sharp edges are saw tooth edges with triangular teeth as shown in FIGS. 1 and 2. The saw tooth edges make the clamping shoe 40 have less tendency to cut completely through the cable sheath tab 24. There is preferably no wall extending downward at the end of the clamping shoe opposite the end wall 46, because if there were there would be danger that such a cutting edge, away from the end of the cable, would cut off the tab 24 from the cable. 
     As the nut 42 is tightened during assembly of the structure, the knife edges 48 of the clamping shoe penetrate the plastic jacket 16 and confine part of it into an enclosed area within the walls 44 and 46 so as to prevent lateral movement of the tab 24 from creep or temperature fluctuation. 
     The height of the cutting edges 48 on the clamping shoe 40 can be varied for cables that employ very thick jackets; that is, different shoes can be provided. Complete cutting by the clamping shoe 40 of the outer jacket will not affect significantly the reliability of the connections. Cutting edges of the clamping shoe 40 can also be made in a tooth form for easier penetration into the sheath. 
     In order to insure high current-carrying capacity for the grounding connection, more than one tab in the composite cable sheath should be cut, and each should be fitted with a connector such as shown in FIG. 1. FIG. 4 shows diagrammatically a cable 10&#39; equipped with three grounding connections 54, each of which has a ground wire 56 clamped between the nut 52 and a lock nut 58 screwed over the threaded fastening element 30 shown in FIGS. 1, 2, and 3, already described. 
     The preferred embodiment of the invention has been illustrated and described, but changes and modifications can be made, and some features can be used in different combinations without departing from the invention as defined in the claims.