GB2106808A - The manufacture of mineral insulated cables - Google Patents

Abstract

In a process for the continuous manufacture of mineral insulated cable, comprising continuously forming a welded sheath (1) from a travelling strip (2) of ductile metal by bending the strip into tubular form and welding the abutting edges of the strip together, simultaneously introducing one or more conductor wires (8) into the sheath-forming strip/tube (2/1) and introducing refractory insulating powder via a delivery tube (7) at a point past an arc-welding head (5) and subsequently passing the resulting assembly through a series of reduction means (13, 17, 18), the said abutting edges of the strip are preheated as they approach the said welding head, preferably by inclining the welding head towards the approaching sheath. The said abutting edges are preferably forced together under constant pressure by one or more pairs of opposed hydraulically controlled rollers (26) and are preferably guided past the welding head by one or more pairs of weld-seam nipping rollers (23). <IMAGE>

Description

SPECIFICATION The manufacture of mineral insulated cables This invention relates to the manufacture of mineral insulated 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, thermocouple 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 a travelling strip of ductile metal, for example copper or aluminium, by bending the strip into tubular form and seam welding the abutting edges of the strip together, simultaneously introducing powdered insulating material, such as magnesium oxide, and one or more conductor wires into the sheath so formed, the powder being introduced through a powder delivery tube which is inserted into the sheath-forming strip/tube with the outlet end of said delivery tube located below the point at which the seam welding is effected, and 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. The wire or wires may be introduced into the sheath through a guide tube or tubes.

Such a process has been described in our copending U.K. patent applications 2,041,259A and 2,041,260A. The present invention provides a method of improving the reliability and consistency of the welding in the above process.

According to the present invention, in a process of the type specified, the said abutting edges of the strip are pre-heated as they approach the said point at which the seam welding is effected.

Preferably the said abutting edges are preheated by an arc-welding head inclined at an obtuse angle to the approaching said abutting edges. The said obtuse angle is preferably between 105 and 115 degrees, and is ideally approximately 110 degrees.

Preferably the said inclination of the welding head is adjustable.

This arrangement spreads out the arc and improves the quality of the weld, and has the advantage that the abutting edges of the strip may be pre-heated and seam-welded by a single inclined arc-welding head. The welding head may be arranged to be slidable parallel to the direction of travel of the developing sheath. This enables a new weld to overlap an old weld when the process is re-started after a stoppage, so that a continuous length of cable may be produced even if the process is only run intermittently. A return path for the welding current may be provided by two or more brushes spaced around the developing sheath and engaging a region of the sheath downstream of the welding head or heads.

The quality of the weld is affected not only by the degree of preheating of the abutting edges of the developing sheath, but also by the pressure acting on the edges to force them together. If the edges are held together by fixed rollers, small variations in the width of the strip and hence in the circumference of the sheath will cause large fluctuations in the forces acting on the abutting edges. Furthermore the resulting uneven compression of the sheath as it is forced through the fixed rollers will lead to uneven heating and tend to encourage the formation of localised hot spots in the welded region of the sheath. The benefits obtained by pre-heating the sheath may thus be seriously compromised.Therefore, according to a further aspect of the present invention, the compressive forces acting on the abutting edges of the developing strip are kept approximately constant by passing the sheath through a pair of opposed controlled-pressure rollers as it approaches the welding heads. Preferably the pressure of the rollers is controlled hydraulicaily.

The rollers are preferably free-running.

In the process described in our co-pending applications mentioned above, the abutting edges of the sheath are located by means of a narrow seam guide inserted between the said edges immediately upstream of the welding head. Although we have found that this method adequately locates the seam beneath the welding head, there can still be a slight tendency for the seam to twist away from the head, especially when a pair of pressure rollers are interposed between the seam guide and the welding head. Even though the edges are located sufficiently accurately to be welded, twists in the seam may cause it to be unevenly heated and may lead to an irregular weld.

According to a further aspect of the invention, this danger is avoided by gripping the bead formed by the welded joint between a pair of opposed rollers, the axes of which lie in a common plane perpendiculay to the direction of travel of the welded sheath.

The seam guide and the said pair of rollers guide the travelling sheath as it approaches and as it leaves the welding head and thereby align the seam.

The invention will now be described in more detail in conjunction with the accompanying drawings, of which Figure 1 is a side elevation schematically showing the plant layout for the manufacture of a mineral insulated cable by a process according to the invention; Figure2 is a front elevation showing in detail the arrangement of the rollers which carry the sheath past the welding head, the welding head being omitted from this Figure for the sake of clarity; and Figure 3 is a side elevation taken from Figure 2, showing the position and orientation of the welding head.

In the apparatus shown in Figure 1,thearrange- mentforforming the cable sheath 1 from a metal strip 2 consists of a tube forming machine comprising six opposed pairs of tube-forming rolls 3 (only one of each of the first and last pairs of rolls is shown in the drawing), a seam guide 4, and a vertically slidable inclined argon arc welding head 5. The tube slides over a brush 25 which provides a return-path for the welding current. The seam guide consists of a series of narrow metal rollers, which are located in the path of travel of the seam edges of the formed tube and are inserted between the edges before they reach the welding head, so as to keep the seam straight and prevent its mis-alignment with respect to the welding head. The bead 24 formed on the welded seam is gripped by a pair of opposed weld seam nipping rollers 23.The abutting edges ofthe seam are forced together under a constant pressure buy a pair of opposed pneumatically controlled rollers 26.

The arrangement for filling the sheath consists of a small internally heated hopper 6 to which insulant powder is supplied from a larger hopper (not shown) at a controlled rate, and a stainless steel powder delivery tube 7 into which the powder is fed from the hopper 6 and which extends into the sheath 1 for a considerable distance below the welding head 5.

Conductor wires 8, of which two are shown, are fed into the sheath through guide means 9, 10 attached to the exterior of the powder delivery tube 7. The wire guide means may be of any convenient form, and in Figure 1 one of the guide means, 10, is shown, by way of example only, as a tube provided with an inlet for rare gas at 11, and having a ring seal 12 fitted around the top to prevent back flow of the gas.

The apparatus shown in Figure 1 further includes means for reducing the diameter of the cable in three stages, consisting of a reduction machine 13 and an annealing furnace 14, both situated vertically below the sheath forming and filling arrangements, a water quenching tank 15 in which the cable 16 is turned in a catenary curve to continue travelling horizontally through two further reduction machines 17, 18, followed respectively by annealing furnaces 19, 20 and water quenching tanks 21, 22. Each reduction machine comprises a number of opposed pairs of reducing rolls, of which only two pairs of the machine 13 and one pair of each of machines 17 and 18, are shown in the drawing.

Before the metal strip 2 is fed through the sheath-forming apparatus described above, its edges are sheared to provide clean, tapered surfaces suitable for welding, and the strip is checked for correct width and edgewise bow, and is thoroughly degreased. The wires 9 are passed through means (not shown) for straightening, locating and tensioning them, and their surfaces are also cleaned and thoroughly degreased, before the wires are introduced into the guides 9, 10. These operations are carried out continuously as the strip and wires travel towards the tube forming and wire guiding apparatus.

Continuous travel of the metal strip and formed sheath and its contents through the system of sheath forming and filling means and reduction, annealing and quenching arrangements, at the desired speed and tension, is effected by conventional pulling and transporting arrangements (not shown), including pinch rolls which follow the final reduction stage, for maintaining continuous tension through the system, and a rotating drum on which the completed cable is finally wound. The rate of delivery of the insulant powder into the sheath is controlled, by weight, in synchronism with the rate of travel of the strip/ sheath and wires, to achieve the desired density of powder within the sheath.

In Figures 2 and 3 the weld seam nipping rollers 23 and pressure-controlled rollers 26 are shown together with two pairs of opposed mechanically set rollers 27 and 28, these being omitted from Figure 1 for simplicity. The rollers 27 and 28 may be independently set by means of knurled screw rods 29 in order to clamp together the edges of the downwardlytravelling sheath 1 and guide the sheath smoothly between the pressure-controlled rollers 26. The sheath is welded together at the point 31, midway between the centres of the rollers 26. The pressurecontrolled rollers are controlled by hydraulic pistons such as 32 pressurised by a hydraulic pump 33. The pressure is indicated by pressure gauges 34 and the hydraulic system is connected to a reservoir 35. The pistons 32 may be isolated by valves 36.Further control over the position of the seam is provided by a conventional seam guide 4, which is best seen in Figure 3.

Figure 3 shows the argon arc welding head 5 directed towards the vertically travelling sheath 1.

The upward inclination of the welding head towards the sheath results in a degree of preheating of the abutting edges of the strip before they are brought to the actual welding position, which has been found to give an improved weld. The vertical inclination of the welding head may be varied at 38; to give the most satisfactory weld, and will generally be of the order of 20 to the horizontal with a vertically moving strip.

The spacing of the welding head from the sheath may be achieved by adjusting the control knob 39, and the height of the welding head may be altered by adjusting the control knob 41.

The height adjustment enables the welding head to be lowered if the process has to be stopped for any reason. On restarting therefore the new weld overlaps the old so as to ensure a continuous weld.

The welding torch can then be gradually raised to its original position during the restarted process an indicator or stop being provided to ensure accurate re-location of the welding head.

Claims (13)

1. A process forthe manufacture of mineral insulated cable in which a sheath is continuously formed from a travelling strip of ductile metal by bending the strip into tubular form and arc welding the abutting edges of the strip together at a welding head to form a seam weld, powdered insulating material and one or more conducting wires are simultaneously introduced into the sheath so formed, and the resulting assembly is subsequently passed through reduction means, wherein said abutting edges are pre-heated as they approach the point at which seam welding is effected.

2. A process according to Claim 1 wherein the abutting edges are pre-heated by inclining said arc-welding head at an obtuse angle to the approaching said abutting edges.

3. A process according to Claim 2 wherein said angle is between 105 and 115 degrees.

4. A process according to Claim 2 wherein said angle is approximately 110 degrees.

5. A process according to any preceding claim wherein said arc-welding head is arranged to be slidable parallel to the direction of travel of the developing sheath.

6. A process according to any preceding claim wherein the abutting edges of the developing strip are continuously forced together under a substantially constant force as they approach said welding head.

7. A process according to Claim 6 wherein said force is applied by a pair of opposed controlledpressure rollers.

8. A process according to any preceding claim wherein the bead formed by the welded joint is constrained to travel in straight path immediately down-line of the welding head.

9. A process according to Claim 8 wherein said head is gripped by a pair of opposed rollers, the axes of which lie in a common plane perpendicular to the direction of travel of the welded sheath.

10. A process substantially as described hereinabove with reference to Figure 1 of the accompanying drawings.

11. A process substantially as described hereinabove with reference to Figures 2 and 3 of the accompanying drawings.

12. Apparatus for manufacturing mineral insulated cable by a process of the type specified incorporating an arc welding head for seam welding the abutting edges of the strip together, wherein the arc welding head is inclined at an obtuse angle to the path of said abutting edges approaching the welding head, so that, in use, the welding head preheats the abutting edges prior to effecting the welding of the edges together.

13. Apparatus for manufacturing mineral insulated cables substantially as shown in and as hereinbefore described with reference to Figure 1 to 3 of the accompanying drawings.