Reliable sheath bonding connector and method of making

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.

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.

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' 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.