LIMITED BEND CRUSH-RESISTANT CABLE

FIELD OF THE INVENTION

The invention pertains to cables which are internally crush- resistant and also have limited bend features.

BACKGROUND OF THE INVENTION

In the various types of signal transmitting cables, such as coaxial electrical signal cables, fiber optic cables, and microwave waveguides, it is desirable to make the cables resistant to harsh environments or abusive handling to protect the integrity of the signals being carried by the cables. This is often done by ruggedization processes such as enlarging or hardening the jacketing of the cables at the expense of the electrical and mechanical properties. There is usually an increase in the dielectric constant of the. cable insulation, as well as generally adding weight, size, and stiffness to the cable.

In a coaxial electrical cable, there is a need to maintain or reduce the dielectric constant of a given insulation to provide the preferred electrical properties of lower capacitance and higher velocity of signal propagation or smaller size and weight of construction for a given impedance and wire size, for example. This has often been achieved by either expanding the insulation, such as the case of expanded polytetrafluoroethylene (ePTFE), or foaming of polyethylene or fluorinated ethylene-propylene copolymer insulation. Use has also been made of convoluted or corregated insulation, such as that described in U.S. Patent 4,866,212.

Ruggedization of coaxial electrical cables has been provided by an external armor sheath, such as that disclosed in U.S. Patent 4,731,502, or internal spiralled rigid metal wire in the case of U.S. Patent 5,061,823. Such structures also provide a high degree of bend radius limitation as well as crush-resistance.

Crush-resistance and some limitation to bending of a cable may also be supplied by a helically slit spiral of hard polymer tubing housed within the structure of the cable, such as shown in U.S. Patent 5,138,684, for protecting and isolating signal cables, optical fibers, hydraulic lines, and pneumatic gas lines.

Both crush-resistance and bend limitation are important in the case of optical fiber-containing cables. Minimum bend characteristics are needed in such a cable to prevent optical fiber damage which will radically affect signal transmission of the fiber and long-term mechanical reliability. The usual solution to these problems, armoring, is at the expense of adding size and weight to the cable as is the case for electrical cables.

Similar considerations also apply to the protection and maintenance of the signal transmitting properties of microwave transmitting dielectric waveguides of the types disclosed in U.S. Patents 4,463,329, 4,525,693, and 4,603,942.

The cable of the invention can provide desirable properties of crush-resistance and limited bend characteristics to cables of all the above types of structure along with lightweight and overall low dielectric constant.

SUMMARY OF THE INVENTION

The invention comprises a limited bend crush-resistant cable in which a signal carrying elongated core is surrounded by a spiralled layer of dielectric insulation which has indentations spaced along the outer surface so that the spaced apart edges of the spirals of insulation are configured to contact adjacent bands of insulation when the core of the cable is bent. The edges of the spirals of insulation are spaced apart specified distances so as to limit the bend of the cable by their contact with each other. The edges are configured by milling an insulation which surrounds a core to the desired shape or by extruding an insulation of the desired configuration around the core in a spiral pattern.

The core of the cable may be an electrically conductive wire, optionally having already formed on it a layer of insulation, a fiber optic cable, including buffering and other protective layers

surrounding an optical fiber, or a dielectric microwave waveguide, which may have protective layers already applied to it.

The edges of the spiralled insulation of the cable may be undercut in some cases in order to provide for the maximum content of air within the cable to provide as low a dielectric constant for the cable as possible.

Any solid, porous, or foamed plastic dielectric material may be used as the spiralled insulation of the cable which can be milled or extruded to the configuration desired. Otherwise unusable polymers of high dielectric constant may therefore be usable in the invention since the overall dielectric constant can be lowered b the air included between the spirals of polymer.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a side view with cross-section of an electrical wire form of cable core surrounded by spiralled insulation.

Figure 2A is a cross-sectional view of a signal carrying core surrounded by spiralled insulation.

Figure 2B is a cross-sectional view of the core of Figure 2A after bending. Figure 3 is a cross-sectional view of a cable of the invention in which the core is a wire surrounded by porous insulation housed within the spiralled insulation and the spiralled insulation surrounded in turn by conductive shielding and a protective jacket. Figure 4 depicts an alternate configuration of the spirals of the insulation.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described with reference to the drawings to give in more detail the features of the invention and the materials useful in its manufacture. Figure 1 depicts a side view of a cable of the invention along with a cross-sectional view of the same cable. Cable core \ is surrounded by a spiralled layer 3 of insulation which is divided into strips or bands of insulation 3 separated by grooves 2. In

Figure 1, cable core I is shown as an electrically conductive metal wire. This arrangement of core \ and insulation 3 may be applied as well where core 1 comprises a bare, buffered, or insulated optical fiber, a microwave waveguide, or a coaxial electrical signal cable.

A suitable optical fiber is often coated with a cladding and a soft buffering layer and/or a hard polymer protective layer, such as disclosed in U.S. Patents 4,072,400, 3,980,390, 4,768,860, 4,681,400, 4,798,445, 5,002,359, 4,307,938, and 4,465,336, for example. Dielectric waveguides such as the type listed above may also be used as cable core 1.

Almost any of the types of coaxial electrical cables customarily used in that art may be substituted for the wire shown as cable core I, preferably without a protective jacket. Useful polymers for forming the spiralled, grooved insulation 3 may include those polymers commonly thought of as useful in the insulation art, including polyethylene, polypropylene, polytetrafluoroethylene (PTFE), polyvinyl chloride, urethane, rubber, and fluorinated hydrocarbon polymers and copolymers, for example. Particularly useful are foamed thermoplastic polymers and porous expanded PTFE, such as that disclosed in U.S. Patents 3,953,566, 3,962,153, 4,096,227, 4,187,390, 4,902,423, and 4,478,665, assigned to W. L. Gore & Associates, Inc. Also useful are strong polymers of high durometer hardness not normally used as insulation since they have high dielectric constants. Use of the air gaps spiralled into the insulation of the invention may lower the overall dielectric constant sufficiently so that advantage may be taken of their strength properties. An example of this type of polymer is Hytrel* polyester (DuPont). Figures 2A and 2B display cross-sectional views of slices of a cable of the invention which show how the edges 4. of the spiralled bands of insulation 3_ come into contact with each other when the cable of Figure 2A is bent to the configuration of Figure 2B. This contact, with the bend limitation resulting therefrom, would occur for any cable of the invention at some angle of bend θ 5 regardless of what sort of core 1 might be used in the cable. Angle θ 5 can be predetermined by proper spacing and considering the insulation properties of the spiralled bands of insulation 3.

Bend limitation will occur from this source regardless of any shielding or jacketing layers covering the insulation in alternative forms of the cable of the invention.

An alternative form of cable of the invention is shown in cross-section in Figure 3 wherein Ms a metal wire, 2 is the space between spirals of insulation 3 and undercut edges 4 of that insulation, 6 is a porous insulation to aid in control of the overall dielectric constant of the cable, 7 is a metallic shielding to render the cable coaxial, and 8 is a protective jacket surrounding the cable to protect it against the outside environment, such as weather. Expanded PTFE may serve as an excellent example of a suitable insulation for that purpose. Shielding 7 may be metal foil, metal coated polymer tape, or metal wire or tape braiding, for example. Jacket 8 may be polyvinyl chloride, rubber, urethane rubber, a thermoplastic fluorocarbon, or polyolefin, for example. Such a cable as shown in Figure 3 is adaptable to use with other electrical cables, optical fibers, and microwave transmitting dielectric waveguides as well as the insulated wire shown as 1 with insulation 6. Figure 4 shows a cross-sectional example of differently contoured spirals 3 of insulation which are undercut to maximize the air space 2 between sp ^' irals 3 and which have firm smoothly contoured edges 4 to the spirals for effective contact with adjacent spirals of the insulation to provide strong bend limitation to the cable.

Figures 5 and 5A describe another alternative configuration of spiralled insulationj, in a cross-sectional example wherein spirals _3_of insulation are straight-sided and closely spaced.

The cables of the invention thus advantageously greatly reduce the amount of dielectric insulation and metal needed for both bend- limitation and crush-resistance to provide very small very lightweight cables of superior properties for their size and weight.