GB2205263A - Manufacture of mineral insulated cables - Google Patents

Abstract

A metallic material 10 is fed continuously downwards through an annular die 6 by a "Conform" machine 1 to form a cylindrical sheath 12. Conductors 15 and powdered insulating material 13 are fed into the sheath through guide tubes 14, 16, and the filled sheath is passed through at least one reduction stage to compact the powdered insulating material and reduce the diameter of the sheath. The reduction of the filled sheath may be carried out by feeding it through a series of rolls. The sheath 12 may be of copper. <IMAGE>

Description

Manufacture of Mineral Insulated Cables This invention relates to a method of manufacturing cables and, in particular, to a method of manufacturing mineral insulated cables, that is to say cables of the type consisting of one or more electrical conductors enclosed within a tubular metal sheath and insulated from the sheath by compacted powdered insulating material.

Usually the sheath is formed by bending a strip of ductile metal, for example copper or aluminium into tubular form, and seam welding the abutting edges of the strip together. However the welding of the strip generates an appreciable amount of heat which needs to be dissipated.

An object of the invention is to provide a method of manufacturing mineral insulated cables in which this welding step is avoided.

Now U.K. Patent Specification No. 1370894 describes a form of extrusion process commonly referred to as the conform" process.

Apparatus used for carrying out this process comprises movable and fixed members defining a passageway- therebetween, and an abutment member projecting from the fixed member to block the passageway. The movable member is arranged to rotate relative to the fixed member so that, if material is fed into the end of the passageway remote from the abutment member, the material is drawn along the passageway towards the abutment member. A die orifice is formed in the abutment member so that the material is then extruded through the die orifice.

According to the invention in its broadest aspect, a method of manufacturing a mineral insulated cable comprises feeding a metallic material continuously downwards through an annular die orifice by means of a UConformU process to form a cylindrical sheath, introducing one or more electrical conductors and powdered mineral insulating material into the sheath so formed, and passing the filled sheath through at least one reduction stage so as to compact the powdered insulating material and reduce the diameter of the sheath.

Such a process avoids the need for seam welding as is necessary when the sheath is formed by bending a travelling metal strip into tubular form.

Moreover the initial sheath, prior to its passage through the reduction stages, can be produced with a diameter which is nearer to that of the finished cable than those formed by conventional techniques, and requires a diameter reduction typically of between 10% and 20X to compact the powder filling and produce the finished cable. Consequently the number of reduction stages that are required can be reduced, compared with the number commonly employed for the manufacture of mineral insulated cables at the present time.

The process, which is fully continuous, can be used for the manufacture of cables having different numbers of conductors, typically 1-, 2-, 3-, 4-, 7-, 12- or 19-core cables.

A method in accordance with the invention conveniently comprises the steps of feeding metallic-material into one end of a passageway formed between first and second members with the second member having a greater surface area for engaging the material than the first member, said passageway having a blocked end remote from said one end and having said annular die orifice associated with said blocked end; moving the passageway-defining surface of the second member relative to the passageway-defining surface of the first member in a direction towards the die orifice from said one end to said blocked end such that the frictional drag of the passageway-defining surface of the second member draws the metallic material substantially in its entirety through the passageway and extrudes it through the die orifice to form said sheath.

The or each reduction stage is conveniently followed by annealing and quenching stages as is customary in the manufacture of mineral insulated cables.

The metallic material is preferably copper.

The powdered insulating material, which is conveniently powdered magnesium oxide as used in conventional mineral insulated cables, may be fed into the formed sheath through a tube, for example of stainless steel, from a hopper which can be vibrated during the process to assist the flow of powder. The powder can Incorporate any suitable moisture repellent such as a silicone, which can be introduced into the powder either in liquid or solid form. Where the moisture repellent is in the form of powder it can be mixed with the powdered insulating material prior to its introduction into the sheath, but if it is in liquid form it is conveniently Introduced into the sheath through a separate tube.

The conductor or conductors can also be fed into the sheath through respective guide tubes, which can be located within the tube feeding the powdered insulating material, if this is of sufficiently large diameter.

One embodiment of the invention will now be described by way of example with reference to the accompanying diagrammatic drawing in which Figure 1 is a part-sectional longitudinal elevation (not to scale) of extrusion apparatus in accordance with the invention.

The apparatus comprises a wheel 1 which is rotatably mounted on a shaft 2. The wheel 1 has a square cross-section circumferential groove 3 machined around Sts outer edge. A shoe member 4 fits against the edge of the wheel 1. An abutment member 5 formed on the underside of the shoe member 4 projects into and blocks the groove 3. An extrusion orifice 6 is formed in the abutment member 5. A chamber 7 integrally formed with the shoe member 4 has a box 8 connecting with the groove 3. A sealing block 9 formed on the underside of the shoe member 4 at the opposite end to the abutment member 5 projects into the groove 3.

Metallic material 10 is fed under pressure through the bore 8 of the chamber 7 and fills that part of the groove 3 underneath the shoe member 4.

The wheel 1 is rotated clockwise as shown by arrow 11. The metallic material 10 in the groove 3 is carried forward by the frictional drag of the groove walls. Thus pressure is generated so that the material 10 is extruded downwards through an annular die so as to produce a continuous tubular sheath 12.

A hopper (not shown) feeds powdered magnesium oxide 13 into the formed sheath 12 through a stainless steel feed tube 14, and a pair of conductor wires 15 are introduced into the sheath 12 through two stainless steel guide tubes 16, a spacer 17 serving to locate the lower ends of the tubes 14, 16 in the correct positions relative to the sheath 12.

The sheath assembly is then fed through a series of reducing rolls (not shown) to reduce the diameter of the sheath and to compact the magnesium oxide filling within it, and following each reduction stage the sheath may be annealed and quenched In accordance with known manufacturing techniques.

Although in the method above described the sheath is formed of copper, it will be appreciated that the technique can be used for forming mineral insulated cables with sheaths of other metals or alloys capable of being extruded by the "Conform" process.

However such a process, in addition to its use in the manufacture of mineral insulated cables, can be used to fabricate other forms of cable requiring a continuous copper or other suitably ductile metal sheath, for example coaxial or submarine cables.

Moreover the process can be used to manufacture cables having one or more tubular conductors within the metal sheath, for example so-called composite mineral insulated cables incorporating one or more optical fibres, for example as a preformed optical fibre cable, within such a tubular conductor. The fibre or fibres of such a cable can be fed into the conductor at a suitably higher rate than the rate at which the sheath is formed, so that they are not subjected to any strain during the subsequent reduction stages. The tubular conductor or another tubular conductor could be used to carry water or other fluid, for example Freon (Registered Trade Mark), for cooling the composite cable in high temperature applications or in case of fire.

The metallic material from which the sheath is formed can be in any suitable form, for example rod, stranded wire, strip, chopped cathode or chopped scrap. Thus cleaned, reclaimed copper from mineral insulated cable scrap could conveniently be utilised for this purpose.

In some cases it may be possible to form, not only the sheath, but also one or more internal conductors, either solid or tubular, by the conform" extrusion process.

Claims (13)

1. A method of manufacturing a mineral insulated cable comprising the steps of feeding a metallic material continuously downwards through an annular die orifice by means of a "Conform" process, as hereinbefore described to form a cylindrical sheath; introducing one or more electrical conductors and powdered mineral insulating material into the sheath so formed; and passing the filled sheath through at least one reduction stage so as to compact the powdered insulating material and reduce the diameter of the sheath.

2. A method of manufacturing a mineral insulated cable as claimed in Claim 1 comprising the steps of feeding the metallic material into one end of a passageway formed between first and second members, the second member having a greater surface area for engaging the material than the first member, said passageway having a blocked end remote from said one end and having said annular die orifice associated with said blocked end; mdving the passageway-defining surface of the second member relative to the passageway-defining surface of the first member in a direction towards the orifice from said one end to said blocked end, such that the frictional drag of the passageway-defining surface of the second member draws the metallic material substantially in its entirety through the passageway and extrudes it through the die orifice to form said sheath.

3. A method of manufacturing a cable as claimed in Claim 1 or 2 in which the metallic material is copper.

4. A method of manufacturing a cable as claimed in Claim 1,2 or 3 in which the metallic material is fed through the orifice in the form of rods, stranded wire, strip or chopped scrap.

5. A method of manufacturing a cable as claimed in any preceding claim including the step of feeding the powdered insulating material into the sheath through a tube.

6. A method of manufacturing a cable as claimed in any preceding claim in which the insulated powder incorporates a moisture repellent.

7. A method of manufacturing a cable as claimed in Claim 5 in which the moisture repellent, in liquid form, is incorporated into the powder by introducing it into the sheath through a further tube.

8. A method of manufacturing a cable as claimed in any preceding claim including the step of feeding the conductor or conductors into the sheath through respective guide tubes.

9. A method of manufacturing a cable as claimed in Claim 8 in which said respective guide tubes are located within the tube through which the powdered insulating material is fed.

10. A method of manufacturing a cable as claimed in any preceding claim in which an optical fibre or fibres are fed into the cable.

11. A method of manufacturing a cable as claimed in Claim 10 in which said optical fibre or fibres are each located within a tubular conductor.

12. A method of manufacturing a cable as claimed in any preceding claim in which the conductor or conductors are formed by the 'Conform' process hereinbefore described.

13. A method of manufacturing a cable as hereinbefore described and illustrated in the accompanying drawing.