Method of making a mineral-insulated, compacted, bendable cable

Abstract

A mineral-insulated cable has a non-circular cross-sectional shape that does not allow the internal wires to twist or change alignment during manufacturing of the cable.

Claims

1. A method of making a mineral-insulated, compacted, bendable cable having a shape that does not allow internal wires to twist or change alignment during production, the method comprising: inserting, into a metal tube having a non-circular cross-sectional shape with a major axis and a minor axis, a plurality of starting insulators of a crushable ceramic material having a non-circular cross-sectional shape that is similar to the non-circular cross-sectional shape of the metal tube and having a plurality of aligned spaced holes each containing an electrically conductive wire, the starting insulators shaped to have a non-circular outer surface that matches an inner surface of the metal tube to inhibit twisting and changes of alignment of the starting insulators and the wires; swaging a first end of the metal starting tube to form a pointed leading end for feeding the metal tube through a draw die and into a pulling mechanism; crimping a second end of the metal starting tube to contain the insulators within the metal tube; performing a series of drawing and annealing processes in which, during each successive drawing and annealing process, the metal tube, the insulators, and the wires are drawn through a draw die having a successively smaller draw opening shaped to match the non-circular cross-section of the metal tube and with a smaller major axis and a smaller minor axis, to reduce cross-sectional dimensions of the metal tube and to crush the crushable ceramic insulators to form a compacted ceramic powder that fills the metal tube and surrounds the wires, while maintaining the cross-sectional shape of the metal tube and without causing the wires to twist or change alignment.

2. The method of claim 1, wherein the plurality of aligned spaced holes are aligned in a plane and are parallel to one another.

3. The method of claim 2, wherein the plane is aligned with the minor axis of the starting insulators.

4. The method of claim 3, wherein the plurality of aligned spaced holes include three parallel holes.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a photograph of a bendable compacted MI cable having an ovular shaped cross section that prevents the internal wires from twisting or changing alignment during manufacturing of the MI cable.

(2) FIGS. 2A-2C show end views of the tubing, insulator, and wire components, respectively, used in the manufacture the MI cable of FIG. 1.

(3) FIG. 3 shows a cross-sectional view of the MI cable of FIG. 1.

(4) FIG. 4A is a front view of a draw die used in manufacturing of the MI cable of FIG. 1.

(5) FIG. 4B is a sectional view of the draw die of FIG. 4A along section 4B-4B.

(6) FIG. 4C is a sectional view of the draw die of FIG. 4A along section 4C-4C.

DETAILED DESCRIPTION

(7) FIG. 1 is a photograph showing MI cable 10, which includes sheath 12, insulation 14, and three wires 16. MI cable 10, in this embodiment, has an ovular cross-sectional shape with a major axis and a minor axis, which does not allow turning while being drawn. The objective of the design is to create a mineral-insulated, metal-sheathed, compacted, bendable cable whose shape does not allow the internal wires to twist or change alignment during production.

(8) FIGS. 2A-2C show the starting components used to produce MI cable 10, with their starting dimensions. FIG. 2A shows an end view of starting tube 12s. FIG. 2B shows an end view of starting insulator 14s, which includes wire holes 18s. FIG. 2C is an end view of starting wires 16s.

(9) Starting tube 12s must have a shape that does not allow turning while being drawn (any shape that is not perfectly circular). The tube material can be any material that will withstand the working conditions of the final product and be able to be drawn down through draw die reductions and anneals (i.e. various metals and metal alloys, including different grades of stainless steel (such as 304, 310, 316, etc.), high oxidation resistant alloys such as Inconel 600, copper alloys, nickel alloys, and refractory metals such as tantalum and platinum).

(10) Starting insulator 14s must have a shape that does not allow turning while being drawn (any shape that is not perfectly circular). The insulator should be made of a ceramic material (e.g. magnesium oxide) that is crushable and will electrically separate the wires from each other and the sheath in the final product at normal operating conditions. In the embodiment shown, the cross-sectional shape of starting insulator 14s matches the cross-sectional shape of starting tubing 12s, but is smaller to allow insertion of starting insulator 14s into starting tubing 12s.

(11) Any normal wire material and shape will work for starting wires 16s. The outer diameter of starting wires 16s is smaller than the diameter of the wire holes 18s in starting insulator 14s. The wire material used for starting wires 16s can be any material that will withstand the working conditions of the final product and be able to be drawn down through draw die reductions and anneals (i.e. various metals and metal alloys, including different grades of stainless steel (such as 304, 310, 316, etc.), high oxidation resistant alloys such as Inconel 600, copper alloys, nickel alloys, and refractory metals such as tantalum and platinum).

(12) FIG. 3 is a cross-sectional view of finished MI cable 10 after the drawing and annealing processes have been completed. The cable shown is in its compacted form where the major and minor O.D.'s have been reduced in size. The final cable has compacted the powder around the wires and provides the electrical insulation with the I.D. of the sheath in full contact with the compacted insulation.

(13) The draw dies used in manufacturing of MI cable 10 must have an equivalent shape to that of the tubing and ceramic insulation. The size of the die openings (bearing diameter) is progressively smaller to reduce the material.

(14) FIG. 4A is an end (or front) view of die 20, which is one of the draw dies used in the drawing process. FIGS. 4B and 4C are sectional views that show the minor bearing diameter and the major bearing diameter, respectively, of die 20.

(15) All tubing and wires are cleaned using standard procedures. The tubing has one end pointed for feeding the tubing through the draw die and into a pulling mechanism. The ceramic insulators are cleaned by firing at an elevated temperature using standard procedures. The wires have the special shaped ceramic insulators strung over them. The special shaped tubing is filled with the string of wires and ceramic insulators. The back end of the filled tubing is crimped to ensure the ceramic insulators do not fall out during the first draw procedure. The material is drawn and annealed to the desired size using a series of special shaped dies with successively smaller die openings that match the shape of the starting materials. Care must be taken during each drawing procedure. Operators must line up the material with the draw die opening profile to prevent twisting, tearing, and scratching of the cable. Because the special shaped ceramic insulators have a non-circular outer surface that matches the non-circular inner surface of the tubing, twisting and changes in alignment of the insulators and the wires within the insulators with respect to the tubing is inhibited.

(16) While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.