GB2059304A - Manufacture of mineral insulated electric cables - Google Patents

Abstract

In the manufacture of a mineral insulated electric cable by a continuous in-line process which comprises forming the sheath by passing metal strip through a tube-forming mill while simultaneously introducing one or more conductor wires and insulating powder into the formed sheath, and then passing the assembly through a series of reduction and annealing means, before entering the tube mill and thence the sheath each conductor wire is passed under tension through a wire drawing die, whereby the wires are located in the desired radial positions in the sheath and are prevented from being urged towards one another by compaction of the insulant powder during the first reduction step. <IMAGE>

Description

SPECIFICATION Manufacture of mineral insulated electric cables This invention relates to the manufacture of mineral insulated electric cables, that is to say cables of the type consisting of one or more electrical conductor wires enclosed within a tubular metal sheath and insulated from the sheath by compacted powdered insulating material, in which the wires are embedded. The term "mineral insulated cables" is to be understood to include, in addition to wiring cables for the conduction of electric current for general purposes, cables of the construction described above and employed for other purposes, for example heating cables, thermo-couple cables, and heating elements for electric cookers.

It has been proposed to manufacture mineral insulated cables by a continuous in-line process (hereinafter referred to as a process of the type specified) which comprises continuously forming the sheath from ductile metal strip, for example of copper or aluminium by causing the strip to travel through a tube-forming mill including means for bending the strip into tubular form and means for seam welding the abutting edges of the bent strip together, while simultaneously feeding one or more conductor wires through the tube-forming mill and into the formed sheath, and continuously introducing powdered insulating material, such as magnesium oxide, into the sheath, and then passing the resulting assembly through a series of reduction means to compact the insulant powder and to reduce the sheath to the desired overall diameter of the cable, each reduction step being followed by annealing and quenching. The cable components and the assembled cable are pulled through the tube-forming mill and the reduction, annealing and quenching system by suitable means, for example a rotating drum on which the completed cable is wound.

For carrying out the said process, the tube-forming mill may be disposed either vertically or horizontally; that is to say, the metal strip/tube and wire or wires may be arranged to travel either vertically downwards or horizontally during the procedure of forming the sheath and introducing the wire or wires and the insulant powder into the sheath: these arrangements will hereinafter be referred to as the "vertical process" and "horizontal process" respectively.In the vertical process, the first reduction and annealing steps are also carried out during the downward travel of the assembly, the first reduction means being situated immediately below the tubeforming mill, butthereafterthe assembly may be turned continuously through an angle of 90 , the remainder of the reduction, annealing and quenching steps conveniently being carried out during horizontal travel.

In the said process, the insulant powder may be delivered directly into the bent metal strip before it is closed, or, in the case of a vertical process, may be fed through a delivery tube inserted into the sheathforming strip/tube with the outlet end of the delivery tube located beyond the point at which the welding is effected; in the latter case the wire or wires may be introduced within the powder delivery tube.

One problem which arises in carrying out a process of the type specified is the control of the positioning of the conductor wire or wires in the completed cable. The wire or wires can be initially located in a desired radial position or positions within the sheath by being introduced into the sheath through a guide tube or tubes and/or one or more spacer members, but during the subsequent sheath reduction and powder compaction procedure an inward force is exerted upon the wire or wires which usually alters the position thereof, and in particular, where two or more wires are present, they are liable to be urged undesirably close together.

It is an object of the present invention to provide an improved method of, and means for, positioning the conductor wire or wires within the sheath of a mineral insulated cable, during the manufacture of the cable.

According to the invention, in a process of the type specified for the manufacture of mineral insulated electric cable each conductor wire, before entering the tu be-forming mill, is passed under tension through a wire drawing die having an aperture of diameter smaller than the initial diameter of the wire, the die being axially aligned with the direction of travel of the wire through the tube-forming mill and through at least the first reduction and annealing means, and being so positioned that the wire on passing therethrough is located in the desired final radial position in relation to the cable sheath and, when two or more wires are present, in relation to the other wire or wires.

When two or more wires are to be introduced into the cable sheath, the dies through which the respective wires are passed are preferably mounted in a single die block, appropriately spaced apart in the block for ensuring the correct location of the wires relative to one another.

Since the cross-section of each wire is reduced by drawing through the die, the force required to pull the wire through the die imparts tension to the wire, the magnitude of which depends upon the degree of reduction of the wire achieved by the die: usually only a small reduction will be required, for example from 1.0% to 1.5% of the initial diameter of the wire.

The wire will be under tension throughout the whole of its length from the die outlet, through the tube-forming mill and the series of cable reduction, annealing and quenching means, to the means for pulling the cable components and assembly through the system, and this tension enables the wire to be retained substantially in its initial position in relation to the cable sheath and any other wire orwires present, as determined by the position of the die or dies. Thus, where two or more wires are incorporated in the cable, the tension in the wires will prevent them from being urged towards one another by the compaction of the insulant powder occurring in the first reduction step. Furthermore, in a horizontal process, the tension in the wires will overcome their tendency to fall, under gravity, to the bottom of the sheath.

As a result of the initial location of the wire or wires by the die or dies, and the retention of such location by the effect of tension as aforesaid, little or no additional guide means will be necessary to ensure correct positioning of the wire or wires in the completed cable. If desired, however, guide tubes and/or spacers may be employed.

The use of wire drawing dies in the method of the invention has additional advantages: thus, the wire is straightened, any curvature set therein as a result of previous coiling on a reel being removed as the cross-section of the wire is reduced on drawing through the die, and any surface damage on the wire is also removed.

In accordance with the normal practice in wire drawing procedure, a lubricant is employed in the die or dies: any lubricant adhering to the surface of a wire might affect the electrical properties of the cable and must therefore be removed before the wire enters the tube-forming mill. Each wire is therefore passed through cleaning means, for example a pair of felt pads impregnated with a suitable solvent for the lubricant, immediately after emerging from the die.

A specific method in accordance with the invention, for the manufacture of a mineral insulated cable incorporating two conductor wires, will now be described by way of example with reference to the accompanying diagrammatic drawings, in which Figure 1 shows, in elevation, part of the apparatus employed for the manufacture of the cable, together with the metal components of the cable, Figure 2 is a cross-section of the die arrangement included in the apparatus of Figure 1, and Figure 3 shows the die arrangement and wires in sectional elevation.

The apparatus shown in Figure 1 includes a vertically arranged tube-forming mill comprising a series oftube-forming rolls 1 and an argon arc welding head 2, a hopper 3 and delivery tube 4for introducing insulant powder into the sheath, and situated vertically below the tube mill, a series of reducing rolls 5 for carrying out the first reduction stage, and an annealing furnace 6. A die block 7, in which two wire drawing dies 8 are mounted, is situated vertically abvoe the tube mill, and a pair of solvent-impregnated felt pads 9 is provided immediately below the die block.The remainder of the apparatus, which is not shown in the drawing, comprises a quenching bath located below the annealing furnace 6, two or more further assemblies of reduction means, annealing furnace, and quenching bath, disposed horizontally, and a rotatable drum by means of which the cable components and assembly are pulled through the system, and on which the finished cable is wound.

The die block 7, as shown in detail in Figures 2 and 3, incorporates a pair of dies 8, mounted side by side in such relative positions that the wires 10, after passing therethrough with a reduction in crosssection, are the required distance apart, corresponding to their desired final spacing in the completed cable.

In operation of the arrangement shown in Figure 1, metal strip 11, suitably of copper, is fed through the tube mill to be formed into the tubular sheath 12, and at the same time the insulant powder is delivered into the sheath from the delivery tube 4, which extends within the sheath to a point below the level of the welding head 2, and the two conductor wires 10 (only one of which is seen in Figure 1, the second wire lying behind the one shown) are pulled vertically through the dies 8, between the felt pads 9, and into the sheath: the wires may be arranged to pass either inside or outside the powder delivery tube, if desired through individual guide tubes (not shown).The assembly then passes through the reduction rolls 5, by means of which the sheath diameter is reduced and the powder is compacted, the wires being retained in the relative positions in which they are located by the dies, and thence through the annealing furnace 6 and the remainder of the reduction system described above.

In a specific example of the process described above with reference to the drawings, copper wires initially 3.95 mm in diameter are drawn to 3.90 mm diameter by passing through the dies, the tension imparted to the wires by the pulling force required to effect this reduction being approximately 100 Kg force. The wires are spaced 1.90 mm apart on emerging fromthe dies. The felt pads are impregnated with tetrachloromethane, for removing a carbonaceous die lubricant from the surfaces of the wires. The insulant powder used is magnesium oxide. The sheath, initially 20 mm in diameter, is formed from copper strip 65 mm wide, and the final overall diameter of the cable, after completion of the reduction stages, is 5.7 mm. The rate of travel of the components and the cable assembly through the system is initially two metres per minute, being increased after each reduction, and the magnesium oxide powder is supplied to the hopper 3 at the rate of 0.99 Kg per minute.

Claims (4)

1. A process for the manufacture of mineral insulated electric cable which comprises continuousliy forming a sheath from ductile metal strip by causing the strip to travel through a tube-forming mill including means for bending the strip into tubularform and means for seam welding the abutting edges of the bent strip together, while simultaneously feeding one or more conductor wires through the tube-forming mill and into the formed sheath, and continuously introducing powdered insulating material into the sheath, and then passing the resulting assembly through a series of reduction means to compact the insulant powder and to reduce the diameter of the sheath, each reduction step being followed by annealing and quenching, wherein each conductor wire, before entering the tube-forming mill, is passed under tension through a wire drawing die having an aperture of diameter smaller than the initial diameter of the wire, the die being axially aligned with the direction oftravel of the wire through the tubeforming mill and through at least the first reduction and annealing means, and being so positioned that the wire on passing therethrough is located in the desired radial position in relation to the cable sheath and, when two or more wires are present, in relation to the other wire or wires.

2. A process according to Claim 1, wherein two or more wires are introduced into the cable sheath, and the dies through which the respective wires are passed are mounted in a single die block, appropriately spaced apart in the block for ensuring the correct location of the wires relative to one another.

3. A process according to Claim 1 or 2, wherein the diameter of the die aperture through which each wire is passed is such that the degree of reduction of the wire achieved by passage through the die is from 1.0% to 1.5% of the initial diameter of the wire.

4. A process for the manufacture of mineral insulated electric cable, substantially as hereinbefore described with reference to the accompanying drawings.