1. Field of the Invention
The present invention generally relates to electrical connectors and cable splicing, and more particularly to a heat-shrinkable shield terminator having a multilumen sealing ring.
2. Description of the Prior Art
There are presently available a wide variety of electrical couplers and splicing devices for connecting wires and cables in electrical systems. The subject invention relates to such a device primarily used in conjunction with coaxial cables.
A typical coaxial cable (also known as a coax, coaxial transmission line, or concentric cable) has a central wire surrounded by an insulating layer, which is further surrounded by a conductive sheath or shield, and an insulating covering or jacket (see FIG. 1). The conductive shield is usually connected to the electrical ground, whether it be earth ground, a chassis, or simply a common tie.
It is sometimes necessary to connect the ground shields of two or more coaxial cables together, often at a point along the coax where it is undesirable to completely cut through the cable. In such cases, a section of the outer jacket may be removed without cutting the remaining layers of the cable, thereby exposing the ground shield. One end of a ground lead wire may then be soldered to the ground shield and the other end connected to the second coax or other ground. Throughout the specification, the term "lead" in the phrase "ground lead wire" is meant in the verb sense, not as the metal lead (Pb). Similarly, the word "ground" is not to be taken in a limiting sense; as noted above, the lead wire may simply be connected to a common tie.
A problem arises, however, with respect to the exposed ground shield. If left bare, the ground wire may accidentally become detached, or environmental influences, particularly water, could corrode the shield, as well as affect the insulative ability of the layer surrounding the central wire. Also, when cables are so exposed in the presence of water (such as underground cables after a rain), the water may seep in between the outer jacket and the inner insulating layer, where it is then conveyed down the cable, sometimes for hundreds of feet. If openings in the inner insulating layer exist, even as small as a pinhole, the water will create a resistive fault in the cable. Even if there are no flaws in the insulating layer, the presence of water will still create a capacitive fault, and alter the impedance characteristics of the coax. Such defects can become critical in certain situations, such as in airborne equipment or combat vehicles.
It is, therefore, necessary to provide an environmental seal around the exposed area of cable. Devices for this purpose are known in the art, and are generally a protective sleeve. They are cylindrical in shape, and are usually constructed of a thermoelastic, or heat-shrinkable material. In this manner, the sleeve may be loosely fitted around the exposed section of cable and heat applied, resulting in a tight seal about the shield ground.
Improved versions of the protective sleeve include a solder band placed midway along the sleeve, and two rings of sealing material, or dams, placed at each end of the sleeve (see FIG. 2). This particular design, known generically as a shield terminator, is disclosed in U.S. Pat. No. 3,312,772 issued to Raychem Corporation on Apr. 4, 1967, which is hereby incorporated by reference. Other related U.S. Pat. Nos. are: No. 3,305,625 issued to R. Ellis; No. 3,316,343 issued to H. Sherlock; No. 3,396,460 issued to J. Wetmore; No. 3,525,799 issued to R. Ellis; No. 4,206,786 issued to J. Wetmore; and No. 4,341,921 issued to D. Simpson. Each of these patents is hereby incorporated by reference.
The shield terminator is slipped around the exposed portion of the cable shield, an the ground lead wire is inserted between the sleeve and dam material (FIG. 3). When heat is applied, the solder melts, eliminating the separate step of soldering the ground lead wire to the ground sheath. At the same time, the sealing material becomes somewhat fluid and, as the thermoelastic material begins to shrink, the sealant should fill any gaps between the ground lead wire and the outer jacket of the cable. Notably, the abovereferenced patent does not disclose use of the sealing material to protect against external influences, but rather contemplates use of the sealant only to keep the solder from flowing out of the protective sleeve. A detailed discussion of desirable performance characteristics and use of these shield terminators may be found in U.S. Department of Defense military specifications MIL-S-83519A, MIL-S-83519/lC and MIL-S-83519/2A, which are hereby incorporated by reference.
Unfortunately, however, the sealing material does not always completely fill the space between the ground lead wire and the outer jacket. Failures sometimes occur at interstices between the sleeve and the ground lead wire, above the dam, as illustrated in FIG. 4A. Alternatively, if the ground lead wire is inserted between the dam material and the cable jacket, failures may occur adjacent to the jacket as shown in FIG. 4B. Such failures completely nullify the effectiveness of the shield terminator, and may lead to catastrophic results under certain conditions. It would, therefore, be desirable and advantageous to devise a shield terminator having improved sealing means which overcomes this deficiency.
Accordingly, the primary object of the present invention is to provide a shield terminator for electrical wires and cables.
Another object of the invention is to provide such a device which has improved sealing means for protection against environmental influences.
Still another object of the invention is to provide a dam for a shield terminator which may be used to interconnect several wires or cables.
USPTO Backgrounds
USPTO Backgrounds