Grounding Device For Shielded Electrical Cable

Abstract

A grounding device for a shielded electrical cable extending through a housing. A pair of relatively soft resilient O-rings, each of which is surrounded by a braided metal sheath, are disposed between the cable and housing. The rings are mounted adjacent oppositely facing bevel rings, with a straight sided washer disposed between them. A jam nut threadedly connected to the housing is tightened to compress and distort the O-rings and sheaths simultaneously. The sheaths are distorted into electrical grounding engagement with the shield of the cable, the bevel rings, washer and inner wall of the housing. A rigid metallic collar may be mounted between the cable and its shielding. The braiding of the O-ring sheaths and the cable shielding preferably interengage each other for better physical and electrical connection.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a grounding device for shielded electrical cables.

2. Description of the Prior Art

In wire cable harnesses, particularly air frame harnesses, metal braid shielding is used to prevent signals on the inside of the cable from being interfered with by signals on the outside of the cable bundle. It is becoming more difficult to accomplish effective shielding required by the delicacy of current day instrumentation. In addition to the normal types of interference, it is sometimes required that critical lines be shielded from a force called Electro-Magnetic Pulses (EMP). These pulses are generated from nuclear explosions. They precede the shock wave in great magnitude and have been known to reach in excess of several thousand volts. To shield from these effects, an extremely low resistive path is required between the aircraft ground and the shield of the cable run.

Present day means of shield grounding are inadequte for EMP. A current popular design conforms a metal ring around the braid in an electro-forming process, in which the two metals bond together. Functionally, the design is close to ideal, but does not permit any subsequent changes of modifications.

Various types of grounding devices have been used in the past, such as the coil spring of U.S. Pat. No. 3,739,076, which makes only a limited number of contacts with the shield of the cable over a relatively small area of engagement. The electrically conductive washers of U.S. Pat. No. 3,057,951 are predominantly formed of non-conductive resilient material and they accordingly have comparatively high electrical impedance, particularly at high frequencies. U.S. Pat. No. 3,154,632 shows braid which is folded with its free ends squashed against the shield of a cable for grounding. The braid is not supported by a resilient member to urge it continuously into engagement with the shield.

SUMMARY OF THE INVENTION

The present invention provides a grounding device for shielded electrical cables which is an improvement over the prior art because it provides better electrical contact with both the shield of the cable and the body of the connector than previous repairable devices. It accordingly provides an extremely low impedance path between the shield of the cable and the grounded bulkhead or connector which terminates the wiring. Its resistance is extremely low, and is almost as low as that provided by an electro-forming process.

While it is conventional to use braided wire to form the shield around the cable or wire bundle, it has not been known to use braided wire as part of the grounding device which engages the shield of the cable with an inner resilient member to maintain the braided wire in physical engagement and secure electrical contact with both the shield of the cable and the connector or bulkhead.

It is accordingly an object of the invention to provide an improved grounding device for shielded electrical cables which utilizes braided wire, the periphery of which engages both the cable shield and the housing to provide better electrical contact over a wider area than heretofore, resulting in an extremely low impedance path both between the cable shield and the housing.

Another object of the invention is to use such braided wire in combination engagement a resilient member or elastomer such as an O-ring disposed within the wire braid, with the resilient member providing the desired pressure to maintain the braid in direct physical engagment and electrical contact with both members.

A further object of the invention is to provide a grounding device in which better electrical contact is made with the braided cable shield through the use of braided wire in the grounding assembly which meshes and inter-engages with the braided wire of the cable shield.

The grounding device of the present invention is adapted to be used in or with a terminating connector or adapter and also in feedthrough applications as well, in which the cable is grounded to the bulkhead through which it passes.

It is accordingly among the objects of the invention to provide a new and improved grounding device which is economical to manufacture, as well as being simple and convenient to use, and which is capable of being disassembled and re-assembled for purposes of replacement or repair.

The invention also comprises such other objects, advantages and capabilities as will later more fully appear and which are inherently possessed by the invention.

While I have shown in the accompanying drawings a preferred embodiment of the invention, it should be understood that the same is susceptible of modification and change without departing from the spirit of the invention .

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view of a grounding device for electrical cables embodying my invention, with the upper portion broken away and shown in section, the parts being shown assembled prior to final tightening;

FIG. 2 is an enlarged partial sectional view corresponding to the upper portion of FIG. 1, showing the parts after final tightening;

FIG. 3 is a top plan view of a contact ring, with the sheath partly broken away and shown in section.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment which has been selected to illustrate my invention comprises a housing 10, which is sometimes referred to in the trade as an adapter of "backshell." The housing 10 may have means at the end thereof not shown in the drawings for terminating a cable or wire bundle 11 which extends into the housing 10. The cable 11 is conventionally covered with a braided wire shield 12.

One end of the housing 10 is provided with an enlargement 13, the outer surface of which is knurled to facilitate holding it stationary while a connection is made to it. The outer end of the enlargement 13 is provided with internal screw threading 14, which is adapted to engage the external screw threading 15 on the inner end of a jam nut 16. The jam nut 16 is also externally knurled and is removably connected to the enlargement 13 when a connection is made.

The interior of the enlargement 13 is slightly enlarged diametrically and has a bevelled inner wall against which a first metallic bevel ring 17 is removably mounted. Next to the bevel ring 17 is a removably mounted first contact ring 18. The ring 18 comprises any suitable elastomer such as an O-ring 19 or the like formed of rubber or other resilient material surrounded by a sheath 20 preferably formed of fine drawn tinned copper wire braided in tubular form of the type commonly used as shielding over electrical conductors. A typical braiding would be 34-36 AWG in a continuous knit around the diameter and circumference of the ring 19.

A flat metallic washer 21 is disposed on the opposite side of the first contact ring 18. On the opposite side of the washer 21 is a second contact ring 22 having an inner O-ring 23 and an outer braided wire sheath 24. Outwardly from the second contact ring 22 is a second metallic bevel ring 25 which is directed inwardly as opposed to the outward direction of the first bevel ring 17. The inner end 26 of the jam nut 16 engages the outer surface of the second bevel ring 25.

In order to provide a firm surface for the contact rings 18 and 22 to engage, a rigid metallic tubular collar 27 may be inserted around the exterior of the cable or wire bundle 11, directly beneath its ground shield 12.

The strucutre of the contact rings 18 and 22 is shown in FIG. 3 of the drawings. The ends of the O-ring and sheath may be secured together by adhesive or otherwise, or they may optionally be left unattached, since the rings in use are securely held in place.

In use, the elements of the grounding assembly are brought together in the configuration shown in FIG. 1 of the drawings. The jam nut 16 is then tightened to compress the contact rings 18 and 22 between the bevel rings 17 and 25 respectively and the washer 21.

When the contact rings 18 and 22 are compressed, their inner resilient O-rings 19 and 23 are somewhat compressed to a smaller diameter and are deformed into a substantially elliptical configuration, as shown in FIG. 2 of the drawings. The braided wire sheaths 20 and 24 cannot be compressed, but are deformed outwardly to fill all of the available free space around the outer periphery of the O-rings 19 and 23.

The sheaths 20 and 24 accordingly engage and make electrical contact with the entire adjacent side wall of the washer 21, with the adjacent portion of the inner wall of the enlargement 13 between the washer 21 and the adjacent bevel ring 17 and 25 and with the major portion of the inner side wall of the adjacent bevel ring 17 or 25.

At the same time, the inner portions of the sheaths 20 and 24 are compressed tightly and flatly against the ground shield 12 of the cable 11, making secure electrical contact therewith over a comparatively wide area.

The numerous metal-to-metal points of contact provided by the braided sheaths 20 and 24 create an ideal low electrical DC resistance path.

The material used to form the O-rings 19 and 23 should preferably be comparatively soft and of a relatively low durometer, so that it is capable fo being somewhat compressed in use. Material in the range of approximately 40-60 Shore hardness may be used, with approximately 50 Shore being considered preferable. The material should also preferably be capable of operating in environmental extremes, particularly when used on aircraft. Silicone and buna are among the materials which may used.

In use, the O-rings 19 and 23 are slightly compressed diametrically due to the force exerted against them by the bevel rings 17 and 25. The sheaths 20 and 24 may tend to enlarge or distort diametrically, opening their weave pattern, which tends to move them into the free areas surrounding them and thereby into tighter engagement with the surrounding members.

The sheaths 20 and 24 are preferably woven in the same pattern and at the same angle as the ground shield 12 of the cable 11. When they are brought into engagement with each other in use, the highs of one braid tend to move into the lows of the other. There is accordingly an intermeshing of the braided materials which results in a larger amount of metallic contact being made, to provide high electrical conductivity. The individual wires forming the two braids will in many cases tend to be in contact with each other over relatively long areas.

The inter-engagement between the sheaths 20 and 24 and the ground shield 12 also provides strain relief by resisting forces tending to pull the cable 11 longitudinally out of the connector housing 10.

The material forming the sheaths 20 and 24 tends to "flow" with age and might by itself move away from contact with the ground shield 12. This discrepancy is resisted by the resilient action of the O-rings 19 and 23, which act to constantly urge the sheaths 20 and 24 inwardly against the ground shield 12. The O-rings accordingly compensate for any tendency of the sheaths to move outwardly and act to maintain complete electrical contact and a low impedance path at all times.

The structure shown and described comprises a preferred embodiment of the invention, but it is quite feasible and may in some instances be preferable to use one contact ring instead of two. The operation of the device remains exactly the same. It is also possible, of course, to use more than two rings. The essential feature is the provision of an O-ring or other resilient member surrounded by a sheath of metallic braid or the like which provides the desired electrical contact and low impedance path between the ground shield of the cable and the body of the connector.

The structure is easily disassembled for repair or replacement purposes in which case the rings 19 and 23 resiliently urge the sheaths 20 and 24 back to their normal configurations. The device can be assembled and disassembled rapidly without the rings or sheaths being rendered inoperative.

Claims

I claim:

1. A grounding assembly in association with a shielded electrical cable extending through a housing, said assembly comprising a resilient O-ring extending around said cable, said housing having an inner electrically conductive wall extending substantially parallel to said cable, said O-ring being disposed between said inner wall and said cable, a braided metallic sheath extending around said O-ring for its entire length, a bevel ring mounted on one side of said O-ring and sheath and extending between said inner wall and said cable, a member disposed on the opposite side of said O-ring and sheath, stop means within said housing for limiting longitudinal movement of said member said member having a straight surface extending between said inner wall and said cable, said bevel ring being movable into engagement with said O-ring and sheath to compress and distort said O-ring and sheath into a multi-sided configuration in which said sheath is held in resiliently supported electrical grounding engagement with a substantial portion of said bevel ring, a substantial portion of said straight surface of said member, a substantial portion of said inner wall between the outer ends of said bevel ring and said member, and a substantial portion of the shield of said cable disposed between the inner ends of said member and said bevel ring.

2. The structure described in claim 1, said cable having a braided shield, said sheath being woven in substantially the same braid pattern as said cable shield, whereby said braided sheath forms a locking interengagement with the braided shield of said cable.

3. The structure described in claim 2, said O-ring being formed of material having a relatively low durometer in the range of approximately 40 to 60 Shore hardness.

4. The structure described in claim 2, said assembly including a substantially rigid metallic collar disposed around said cable between the cable and its shield beneath the O-ring to provide a firm surface for electrical grounding engagement between said sheath and the braid of said cable.

5. The structure described in claim 1, said member being a straight sided washer and said assembly including a pair of said O-rings and sheaths disposed on opposite sides of said straight sided washer, with a pair of oppositely facing bevel rings disposed on the opposite sides of said O-rings and sheaths remote from said washer, and means coupled to said housing for compressing and distorting both of said O-rings and sheaths simultaneously, each of said O-rings and sheaths when compressed being distorted into a multi-sided configuration in which said sheaths are held in resiliently supported electrical grounding engagement with a substantial portion of one side of said washer, said inner wall, said bevel ring and the shield of said cable.