GB1592593A

GB1592593A - Manufacture of electric cables

Info

Publication number: GB1592593A
Application number: GB465777A
Authority: GB
Original assignee: AEI
Current assignee: AEI
Priority date: 1978-05-03
Filing date: 1978-05-03
Publication date: 1981-07-08

Description

(54) IMPROVEMENTS IN OR RELATING TO THE MANUFACTURE OF ELECTRIC CABLES (71) We, ASSOCIATED ELECTRICAL IN DUSTRIES LIMITED, of 1 Stanhope Gate, London W1A IEH, a British Company, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a continuous process for the manufacture of electric cables of the type consisting of one or more conductors surrounded by a metal sheath and insulated from the sheath by compacted pulverulent insulating material, in which the conductors are embedded. The invention also relates to apparatus for use in carrying out the process, and to cable manufactured by the process.

It has been proposed to manufacture cable of the type referred to by forming the sheath from ductile metal strip, by bending the strip into tubular form and welding the edges together, while simultaneously introducing pulverulent insulating material and the conductor or conductors into the tube so formed.

A ram or tamper may be employed for compacting the powder in the tube, and the filled sheath may be subsequently passed through means, such as one or more dies, for reducing its diameter. A member with guide holes may be provided for locating the conductors in the desired positions within the sheath, but difficulties may arise as a result of the guide becoming clogged with powder, causing irregular packing of the powder in the sheath and possibly interfering with the freedom of travel of the conductors into the sheath.

It is an object of the present invention to provide an improved process and apparatus for the continuous production of cable of the type specified (hereinafter referred to as "mineral insulated cable"), whereby the above-mentioned difficulties can be avoided or reduced.

According to the first aspect of the invention, in a continuous process for the manufacture of mineral insulated cable, as hereinbefore defined, which process includes the steps of progressively forming a continuous strip of ductile metal into a tubular sheath and feeding pulverulent insulating material and one or more conductors into the sheath so formed while the said sheath is travelling substantially vertically downwards, the pulverulent material is delivered into the sheath at such a rate, in relation to the rate of travel of the sheath, that the said material fills the space around the conductor or conductors within the sheath and is partially compacted under gravity, each conductor is guided into a desired radial position within the sheath by passage through an individual shield tube in which the conductor fits loosely, each said shield tube being disposed substantially vertically and having its lower end so located that the conductor emerges therefrom into the sheath at or below the level at which the pulverulent material is delivered into the sheath, and each said shield tube being so arranged that the conductor guided therein does not come into contact with pulverulent material before so emerging into the sheath, and the filled sheath is subsequently subjected to one or more reduction steps for reducing the diameter of the sheath and further compacting the pulverulent material therein.

The sheath is suitably formed by progressively bending the metal strip, for example by means of forming rolls, so that the longitudinal edges of the strip abut, and then seam welding these edges together: both the pulverulent insulating material and the conductor or conductors are arranged to enter the sheath at a level below that at which the welding is effected. Preferably the powder is delivered into the sheath from a vessel located above the tube forming and welding means. through a substantially vertically disposed tube extending into the formed sheath; each shield tube through which a conductor is introduced into the sheath is then positioned within the powder delivery tube and extends upwards through the said vessel to a sufficient height to ensure that its upper open end is always above the surface of the powder in the vessel.

According to the second aspect of the invention, therefore, apparatus for carrying out a continuous process for thc manufacture of mineral insulated cable. as hereinbefore defined, includes means for progressively bending ductile metal strip into the form of a tube and means for welding the abutting longitudinal edges of the bent strip together while said tube is travelling substantially vertically downwards. to form a tubular sheath, a vessel for containing pulverulent insulating matcrial, which vessel is located above said tube forming and welding mcans.

a substantially vertically disposed powder delivery tube extending downwardly from said vessel to a level below that of the welding means and so arranged that. in opcration of the apparatus, the lower end of the delivery tube will lie within the formed sheath, at least one shield tube through which a single conductor is arranged to pass into the formed sheath, each said shield tube being disposed substantially vertically within the said vessel and powder delivery tube with the lower end of the shield tube at or below the level of the lower end of said delivery tube and the upper end of the shield tube above the level to which said vessel will be filled with said pulverulent material in operation, means for locating each said shield tube in a desired radial position within the powder delivery tube, and one or more means for reducing the external diameter of the sheath subsequently to the delivery of said pulverulent material and conductor or conductors into it.

The conductor or conductors, and the strip from which the cable sheath is formed, may each be composed of any suitable metal of high electrical conductivity, for example copper, or aluminium, or copper-clad aluminium, and the pulverulent material may consist of any insulating substance known for use for this purpose, for example magnesium oxide.

The tube forming means suitably consists of a series of opposed pairs of forming rolls, and seam welding of the edges of the formed tube may be effected by means of an argon arc welding head, in known manner. The edges of the metal strip are preferably shaped, for example by tapering or bevelling, in such a manner that the seam formed on welding is flat, with no ridge or other irregularity either inside or outside the tube.

The powder containing vessel and delivery tube arc preferably consituted by a single integral component, a suitable arrangement consisting of a funnel with an elongated stem forming the delivery tube, formed either of glass or a suitable metal such as stainless steel. The powder is supplied continuously to the said vessel, at an appropriate rate as aforesaid, from a hopper or other suitable reservoir.

Each of the shield tubes, which serves both to guide a conductor into the desired position within the sheath and to shield the conductor from premature contact with the powder, preferably extends below the lower end of the powder delivery tube; the shield tubes may also be formed of metal such as stainless steel.

One preferred means for locating the shield tube. or each shield tube, within the powder delivery tube, so as to ensure that the conductor passing through the shield tube is placed in the required position within the sheath. consists of one or more tongues of metal integral with the lower end of the powder delivery tube, which tongue or tongues can be bent to engage the shield tube and, if desired. can be attached thereto, for example by brazing. An additional tongue or tongues may be provided in or near the upper end of the powder delivery tube, in this case being brazed to the interior surface of the powder delivery tube if the latter is of metal. or otherwise sealed to the said surface if it is of glass.If required, further supports for the shield tube, or for each shield tube, may be provided at one or more intermediate points between the upper and lower ends of the powder delivery tube, for example screws may be inserted through the delivery tube wall and screwed into engagement with the shield tube or tubes, to ensure that the latter is or are held rigid. If two or more shield tubes are employed, for the introduction of two or more conductors, one through each tube. these tubes may if desired be brazed together along the whole of or parts of their lengths, to provide a substantially rigid structure, as well as being individually engaged with or attached to one or more locating tongues or other members.

Preferably at least a first step of reducing the external diameter of the filled sheath is carried out while it is still travelling vertically downwards, and is effected, for example, by rolling, ring rolling or drawing. Thus the apparatus of the invention preferably includes at least one ring roll or opposed pair of reducing rolls, located adjacent to the path of downward travel of the filled sheath and at a level below that of the lower end or ends of the shield tube or tubes. This reduction has the effect of reducing the diameter of the cable assembly without elongation, and of compacting the powder within the sheath sufficiently to ensure that no movement of the powder occurs when the assembly is subsequently passed round a drum or capstan, as will usually be necessary for changing the direction of travel of the assembly from vertical to horizontal. Further reduction steps may be carried out if required, suitably by drawing the assembly through dies while it is travelling horizontally, to effect further compaction of the powder insulation and to form a cable of a desired diameter. Preferably the assembly is an nealed between reduction stages.

Continuous travel of the metal strip and formed sheath and its contents, through the system of tube forming and welding means, reduction means and annealing ovens, can be effected by conventional pulling and transporting arrangements, the completed cable finally being wound on to a drum.

A specific cable manufacturing process in accordance with the invention, ad apparatus employed for carrying out the process, will now be described by way of example with reference to the diagrammatic drawings accompanying the Provisional specification, in which Figure 1 shows the apparatus in elevation, Figure 2 is a part-sectional elevation of part of the apparatus of Figure 1, in greater detail and on an enlarged scale, viewed in the direction of the arrow II in Figure 1, Figure 3 is a cross-section drawn on the line III III in Figure 2, and Figure 4 is a similar cross-section of a modified form of the apparatus.

Like parts in the different figures of the drawings are indicated by the same reference numerals.

In the apparatus shown in Figure 1 of the drawings, the arrangement for forming the cable sheath consists essentially of a drum 1 from which metal strip is supplied, a guide roll 2 for causing the strip to travel in a vertical downward direction, a series of opposed pairs of tube forming rolls 3, and an argon arc welding head 4. The arrangement for filling the sheath consists of a stainless steel funnel 5 with an integral elongated powder delivery tube 6 extending vertically downwards to a level below the welding head 4, and a stainless steel tube 7, constituting the shield tube for a conductor, supported within the funnel 5 and tube 6 and extending from above the top of the funnel to a level about 15 cm below the lower end 8 of the tube 6.

The apparatus shown further includes reducing rolls 9, situated between guide rolls 10 and 11, a guide drum 12 and an annealing oven 13.

The powder and conductor delivery arrangement is shown in more detail in Figure 2, in which the lowermost pair of tube forming rolls 3 and the welding head 4 are also included. As shown in Figure 2, the shield tube 7 is supported and located within the powder delivery tube 6 by a first pair of stainless steel tongues 14 extending from the lower end 8 of the tube 6, a second pair of stainless steel tongues 15 brazed to the upper end of the tube 6, and two or more screws 16 inserted through the wall of the tube 6; the tongues 14 and 15 are bent inwards from the delivery tube wall and are brazed to the shield tube. The lower tongues 14 are also shown in Figure 3, which is a cross-section of the lower end portions of the powder delivery tube 6 and shield tube 7, with the surrounding sheath 18.The screws 16 might be located higher up the tube 6, rather than in the position shown, or one or more additional sets of screws might be inserted at intervals along the tube 6 which, as indicated by the break, is of considerably greater length than that shown in Figure 2.

In operation of the apparatus shown in Figures 1, 2 and 3, metal (for example copper) strip 17, with edges tapered and sheared off to provide clean surfaces for welding, is passed continuously from the drum 1, around the guide roll 2, to the forming rolls 3 by means of which it is bent into tubular form, and pass the welding head 4, by which the tube is closed by seam welding to form the sheath 18; at the same time a copper (or other suitable metal) wire 19 is fed through the shield tube 7 into the sheath, and pulverulent insulating material 20 continuously supplied from a hopper (not shown) to the funnel 5 flows down the delivery tube 6 to fill the sheath around the wire 19.The filled sheath is subjected to an initial reduction by means of the rolls 9, and then passes round the drum 12 to travel horizontally through the annealing oven 13, which is maintained at a temperature of 400or to 600"C and contains a non-oxidising atmosphere such as nitrogen or forming gas.

The cable assembly so formed continues travelling in the direction indicated by the arrow A (Figure 1), and is further reduced in diameter and elongated by being passed through six or seven drawing dies, with intermediate anneals. The dies, annealing ovens, and means for pulling and transporting the cable assembly may all be of conventional form and are not shown in the drawling. In addition, further guide rolls may be included in the system where required.

Only one shield tube, for the introduction of a single conductor into the sheath, is shown in Figures 1, 2 and 3, for the sake ofa simplicity and clarity of the drawings. Usually, however, a cable manufactured by the process of the invention will be required to include at least two conductors within the sheath: Figure 4 shows, in cross-section, an arrangement for introducing two wires into the sheath. this arrangement consisting of two shield tubes 21, 22, brazed together along their whole lengths, at 23, and attached to the lower end of the powder delivery tube 6 by respective tongues 24, 25, and to the upper end of the tube 6 by similar tongues (not shown), and further supported at one or more intermediate points by screws as described above with reference to Figure 2.In other respects the apparatus employed for manufacturing a cable incorporating two (or more) inner conductors is as described above with reference to Figure I.

In a specific example of a process in accordance with the invention. carried out by means of the apparatus described above with reference to the drawings, for the manufhc- ture of a cable consisting of a copper sheath with two inner copper wires and magnesium oxide powder as the insulant. copper strip 65 mm wide is formed into a tubular sheath initially 20 mm in external diameter. For the introduction of copper wires of diameter 4. 1 mm shield tubes of internal diameter 5.5 mn are used; the lengths of the power deliver) tube 6 and the shield tubes 7 are respectively 3.6 metres and 4. 1 metres.The rate of travel of the strip/sheath and the wires through the system is 5 metres per minute. and magnesium oxide powder is supplied to the funnel 5 at the rate of 2 kg per minute. The initial density of the powder in the sheath, before reduction of the latter is approximately 2 g/cc, and after the initial reduction step this is increased to 2.6 g/cc, the external diameter of the sheath being reduced to 17 mm. After further reduction by passage through seven drawing dies, the diameter of the final cable is 5.7 mm.

WHAT WE CLAIM IS:- I. A continuous process for the manufacture of mineral insulated cable, as hereinbefore defined, which process includes the steps of progressively forming a continuous strip of ductile metal into a tubular sheath and feeding pulverulent insulating material and one or more conductors into the sheath so formed while the said sheath is travelling substantially vertically downwards. wherein the pulverulent material is delivered into the sheath at such a rate, in rclation to the rate of travel of the sheath, that the said material fills the space around the conductor or conductors within the sheath and is partially compacted under gravity, each conductor is guided into a desired radial position within the sheath by passage through an individual shield tube in which the conductor fits loosely, each said shield tube being disposed substantially vertically and having its lower end so located that the conductor emerges therefrom into the sheath at or below the level at which the pulverulent material is delivered into the sheath, and each said shield tube being so arranged that the conductor guided therein does not come into contact with pulverulent material before so emerging into the sheath, and the filled sheath is subsequently subjected to one or more reduction steps for reducing the diameter of the sheath and further compacting the pulverulent material therein.

2. A process according to Claim 1, wherein the said sheath is formed by progressively bending a metal strip so that the longitudinal edges of the strip abut, and seam welding the said edges together, and wherein the pulverulent insulating material and the conductor or conductors are arranged to enter the sheath at a level below that at which said welding is effected.

3. A process according to Claim I or 2, wherein at least a first reduction step for reducing the diameter of the filled sheath is carried out while the filled sheath is still travelling substantially vertically downwards.

4. Apparatus for carrying out a process according to Claim 2, which includes means for progressively bending ductile metal strip into the form of a tube and means for welding the abutting longitudinal edges of the bent strip together while the said tube is travelling substantially vertically downwards. to form a tubular sheath, a vessel for containing pulverulent insulating material, which vessel is located above said tube forming and welding means, a substantially vertically disposed powder delivery tube extending downwardly from said vessel to a level below that of the welding means and so arranged that, in operation of the apparatus, the lower end of the said delivery tube will lie within the formed sheath, at least one shield tube through which a single conductor is arranged to pass into the formed sheath, each said shield tube being disposed substantially vertically within the said vessel and powder delivery tube with the lower end of the shield tube at or below the level of the lower end of said delivery tube and the upper end of the shield tube above the level to which said vessel will be filled with said pulverulent material in operation, means for locating each said shield tube in a desired radial position within the powder delivery tube, and one or more means for reducing the external diameter of the sheath subsequently to the delivery of said pulverulent material and conductor or conductors into it.

5. Apparatus according to Claim 4, wherein the tube forming means consists of a series of opposed pairs of forming rolls.

6. Apparatus according to Claim 4 or 5, wherein the said powder containing vessel and powder delivery tube are constituted by a single integral component consisting of a funnel with an elongated stem.

7. Apparatus according to Claim 4, 5 or 6, wherein the means for locating the shield tube, or each shield tube, in a desired radial position within the powder delivery tube consists of one or more tongues of metal integral with the lower end of the powder delivery tube, which tongue or tongues can be bent to engage the shield tube.

8. Apparatus according to Claim 7, wherein an additional tongue or tongues is or are provided in or near the upper end of the powder delivery tube, being attached to the interior surface of the said tube.

9. Apparatus according to Claim 7 or 8, wherein further supports for the shield tube,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **.

accordance with the invention. carried out by means of the apparatus described above with reference to the drawings, for the manufhc- ture of a cable consisting of a copper sheath with two inner copper wires and magnesium oxide powder as the insulant. copper strip 65 mm wide is formed into a tubular sheath initially 20 mm in external diameter. For the introduction of copper wires of diameter 4. 1 mm shield tubes of internal diameter 5.5 mn are used; the lengths of the power deliver) tube 6 and the shield tubes 7 are respectively 3.6 metres and 4. 1 metres.The rate of travel of the strip/sheath and the wires through the system is 5 metres per minute. and magnesium oxide powder is supplied to the funnel 5 at the rate of 2 kg per minute. The initial density of the powder in the sheath, before reduction of the latter is approximately 2 g/cc, and after the initial reduction step this is increased to 2.6 g/cc, the external diameter of the sheath being reduced to 17 mm. After further reduction by passage through seven drawing dies, the diameter of the final cable is 5.7 mm.

WHAT WE CLAIM IS:- I. A continuous process for the manufacture of mineral insulated cable, as hereinbefore defined, which process includes the steps of progressively forming a continuous strip of ductile metal into a tubular sheath and feeding pulverulent insulating material and one or more conductors into the sheath so formed while the said sheath is travelling substantially vertically downwards. wherein the pulverulent material is delivered into the sheath at such a rate, in rclation to the rate of travel of the sheath, that the said material fills the space around the conductor or conductors within the sheath and is partially compacted under gravity, each conductor is guided into a desired radial position within the sheath by passage through an individual shield tube in which the conductor fits loosely, each said shield tube being disposed substantially vertically and having its lower end so located that the conductor emerges therefrom into the sheath at or below the level at which the pulverulent material is delivered into the sheath, and each said shield tube being so arranged that the conductor guided therein does not come into contact with pulverulent material before so emerging into the sheath, and the filled sheath is subsequently subjected to one or more reduction steps for reducing the diameter of the sheath and further compacting the pulverulent material therein.
2. A process according to Claim 1, wherein the said sheath is formed by progressively bending a metal strip so that the longitudinal edges of the strip abut, and seam welding the said edges together, and wherein the pulverulent insulating material and the conductor or conductors are arranged to enter the sheath at a level below that at which said welding is effected.
3. A process according to Claim I or 2, wherein at least a first reduction step for reducing the diameter of the filled sheath is carried out while the filled sheath is still travelling substantially vertically downwards.
4. Apparatus for carrying out a process according to Claim 2, which includes means for progressively bending ductile metal strip into the form of a tube and means for welding the abutting longitudinal edges of the bent strip together while the said tube is travelling substantially vertically downwards. to form a tubular sheath, a vessel for containing pulverulent insulating material, which vessel is located above said tube forming and welding means, a substantially vertically disposed powder delivery tube extending downwardly from said vessel to a level below that of the welding means and so arranged that, in operation of the apparatus, the lower end of the said delivery tube will lie within the formed sheath, at least one shield tube through which a single conductor is arranged to pass into the formed sheath, each said shield tube being disposed substantially vertically within the said vessel and powder delivery tube with the lower end of the shield tube at or below the level of the lower end of said delivery tube and the upper end of the shield tube above the level to which said vessel will be filled with said pulverulent material in operation, means for locating each said shield tube in a desired radial position within the powder delivery tube, and one or more means for reducing the external diameter of the sheath subsequently to the delivery of said pulverulent material and conductor or conductors into it.
5. Apparatus according to Claim 4, wherein the tube forming means consists of a series of opposed pairs of forming rolls.
6. Apparatus according to Claim 4 or 5, wherein the said powder containing vessel and powder delivery tube are constituted by a single integral component consisting of a funnel with an elongated stem.
7. Apparatus according to Claim 4, 5 or 6, wherein the means for locating the shield tube, or each shield tube, in a desired radial position within the powder delivery tube consists of one or more tongues of metal integral with the lower end of the powder delivery tube, which tongue or tongues can be bent to engage the shield tube.
8. Apparatus according to Claim 7, wherein an additional tongue or tongues is or are provided in or near the upper end of the powder delivery tube, being attached to the interior surface of the said tube.
9. Apparatus according to Claim 7 or 8, wherein further supports for the shield tube,

or for each shield tube, are provided at one or more intermediate points between the upper and lower ends of the powder delivery tube.
10. Apparatus according to any of the preceding Claims 4 to 9, wherein two or more shield tubes are included, each for the introduction of a single conductor into the sheath, and these tubes are brazed together along the whole or parts of their lengths.
11. Apparatus according to any of the preceding Claims 4 to 10, which includes at last one ring roll or opposed pair of reducing rolls, located adjacent to the path of downward travel of the filled sheath and at a level below that of the lower end or ends of the said shield tube or tubes.
12. Apparatus for carrying out the process according to Claim 1 for the manufacture of mineral insulated cable, substantially as shown in, and as hereinbefore described with reference to, the drawings accompanying the Provisional Specification.
13. A continuous process for the manufacture of mineral insulated cable, according to Claim 1, carried out substantially as hereinbefore described by way of example and with reference to the drawings accompanying the Provisional Specification.
14. Mineral insulated cable, as hereinbefore defined, manufactured by a process according to any of the preceding Claims 1, 2, 3 and 13.