GB1579470A

GB1579470A - Applying a layer of wires to linearly extended material

Info

Publication number: GB1579470A
Application number: GB47907/77A
Authority: GB
Original assignee: KM Kabelmetal AG
Current assignee: KM Kabelmetal AG
Priority date: 1977-02-11
Filing date: 1977-11-17
Publication date: 1980-11-19
Also published as: NO148834B; FI69530C; DK380577A; JPS53100485A; YU277677A; SE443679B; IT1090701B; JPH0113168B2; FI772703A; CH632792A5; NL172386B; DK147720C; FR2380370B1; NL172386C; HU180818B; DD131865A5; FI69530B; NO780459L; NL7801513A; NO148834C

Abstract

At least one ply consisting of a plurality of metal wires (9) is applied via guide grooves (7) to the surface of a strand (1), for example a cable core, by means of the hollow nipple (6) rotating in an alternating direction of rotation. In the run-on region (13) of the wires (9), a holding band (10) is wound by means of a band winder (11) on to the strand (1), into the windings of which the wires (9) are successively inserted. This holding band (10) remains on the strand above the wire ply. The wires (9) are consequently held securely after winding. So that the stranding cannot become twisted, there is arranged after the run-on region (13) a holding device (12) which is fixed or is displaceable axially relative to the strand (1) and which can consist of rollers radially adjustable relative to the strand. In order to improve the transverse conductivity of the stranding in an electrical cable, a conductive band (4), for example a copper band, is applied under the wires (9) by means of a band spinner (2). <IMAGE>

Description

(54) APPLYING A LAYER OF WIRES TO LINEARLY EXTENDED MATERIAL (71) We, KABEL- UND METALLWERKE GUTEHOFFNUNGSHUTTE AKTIENGESELLSCHAFT, a body corporate organised under the laws of Germany of 271 Vahrenwalder Strasse, Hannover, Germany, 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 method of and an apparatus for applying, with twist, one or more layers comprising a plurality of metallic wires, whose direction of twist is alternated during their application, to the surface of linearly extended material, more particularly of electric cables (or cable cores) or lines.

It has already been proposed (Gebrauchsmuster Specification No. 1875570) that a concentric screen or neutral conductor should be formed from a plurality of individual wires which are applied with reversing twist around a core of an electric cable. For this purpose the individual wires are guided by an annular disc, disposed coaxially with respect to the cable core, around whose circumference there are apertures through which the wires pass. This disc is oscillated, so that the disc in co-operation with a stranding nipple constrains the individual wires to be undulatingly deformed and embedded on to the cable core, or into a composition which has already been applied around the core. The concentric conductor wires are held by a guide nipple designed to hold them until a copper spiral is applied around them.

It is a disadvantage of this method that it necessitates the use of an adhesive composition as a support and also a sufficiently long guide means to prevent the concentric conductor wires from falling away from the core. This is necessary because the conductor wires are not stranded around the core. The use of an adhesive composition is frequently undesirable, however, e.g. in the production of cables for high tension and very high tension service, and also in the production of reinforcements for various forms of linearly extended material.

In view of these difficulties, it has also previously been proposed that, in the production of cables with a concentric screen or neutral conductor, an endless band should be fed over rollers to the cable core, meeting it at a predetermined stranding point, this band being caused to wrap itself one or more times around the core and then removed again, likewise over rollers, and fed back to the starting point (Federal German Patent Specification No. 151097).

The mechanical equipment requirements necessary for this purpose have hitherto prevented this proposal from being adopted in practice.

It is an object of the present invention to make it possible, with minimal mechanical equipment requirements, to apply, with twist, one or more layers comprising a plurality of metallic individual wires, with reversing stranding, to the surface of any desired linearly extended material.

According to the present invention, we provide a method of applying, with twist, one or more layers comprising a plurality of metallic wires, whose direction of twist is alternated during their application, to the surface of linearly extended material, wherein the individual metallic wires are introduced, in the region of their arrival on the said surface, into a stranding nipple in the form of an encircling containing formation which is formed in the said arrival region during stranding, and which is left as a lost nipple (as herein defined) on the wire layer or layers applied.

By a "lost" nipple, we mean, in the present description and claims, one which encircles an assembly comprising a linearly extended material and metallic wires applied thereto not only at the time at which this assembly is formed, but also in the respective finished product. The material of which the lost nipple is composed is thus treated as an expendable material, in contrast with the endless band of the above-mentioned prior proposal.

The present measure, whereby the lost nipple is left on the wire layer(s) applied, permits the application of a so-called Ceander layer to any desired substrates, including more particularly substrates which have little or no adherence. This is the case when it is required (e.g.) to apply a metallic screen in making a medium or high-tension cable, and when it is desired to omit the compsoition used for embedding the wires in the relevant prior process.

Thus a complete production stage is saved compared to the prior process. Furthermore, due to the wires being stranded into a nipple which is in the process of formation, but which directly grips the individual wires, a uniform laying of the individual wires is possible even in the region of the reversal of the direction of twist.

In the method of the invention, the lost nipple is preferably constituted by a band wound on to the layer or layers of wire being formed in the arrival region. It is important that the wires should be gripped and held actually in the arrival region, and in fact around the entire circumference of the linearly extended material. For this reason it has also been found particularly advantageous that the band should be wound on with a length per turn which is not greater than the band width.

Any desired material may be used as the band material, provided that it exhibits the necessary tensile strength; it is convenient to use a band of insulating material, or a band backed with a conductive layer, which may additionally be provided with friction resistance increasing means on its surface facing the wire layer(s), e.g. an adhesive layer or a grooved surface. However, metal bands with a smooth or grooved surface, may also be used advantageously, depending upon the other materials used.

In making electrical cables, more particularly those for medium or high tensions, a copper band is in many cases wound spirally with long twist on to a screen comprising stranded wires, to improve transverse conductivity. Now an improvement in transverse conductivity can be achieved in a simple manner by the method of the invention, in that the copper band can be wound on to the cable before the application of the wire layer(s). The subsequent applicationof the wire layer(s) is not in any way affected by the application of the copper band, so that the equipment requirements for applying the wires can be kept to a minimum.

The invention includes an apparatus suitable for use in performing a method according to the invention as described above, this apparatus incorporating a hollow nipple provided with substantially conically converging wire guiding means on its circumference, whereby (in use) the wires are guided on to the linearly extended material, the hollow of this nipple comprising a bore through which this linearly extended material is passed, and this nipple being (in use) rotated in alternate senses of rotation; the apparatus also incorporating means for rotating the hollow nipple in alternate senses of rotation, and additionally incorporating a band winder whereby (in use) band material is wound on to the wires to form the said lost nipple.

As in known arrangements, the wire guiding means ont he circumference of the abovementioned hollow nipple may take the form of grooves extending in the longitudinal direction to guide the individual wires, and the hollow nipple advantageously terminates close to the surface of the linearly extended material to be covered.

In order to ensure a uniform covering of this surface with the wires serving as a screen, even in the case of flexible material, e.g. with single-core electrical cables, it is advisable to provide a separate holding or gripping means for the material passing through. It is important in this respect that no undesirable effects should be produced by the unavoidable torsional forces which are transmitted to the material in passing through and which could possibly lead to a loosening of the stranding band, if no steps were taken to prevent this. In this connection, use may be made of any desired holding or gripping elements, e.g. counter-rotated guide nipples, or band haul-off means.

The present aspect and also certain additional aspects of the invention will be explained with reference to the accompanying drawings.

In the drawings, Figure 1 is a diagrammatic side view showing apparatus employed in accordance with the invention for the application of metallic wires to the surface of a cable core, Figure 2 is a diagrammatic cross-section showing a three-cored low-tension cable in accordance with the invention, Figure 3 is a diagrammatic cross-section showing a single-cored high-tension cable in accordance with the invention, Figure 4 is a diagrammatic side view showing another apparatus employed in accordance with the invention for the application of metallic wires to the surface of a cable core, Figure 5 is a fragmentary sectional view, on an enlarged scale, of a gripping device forming a component of the apparatus of Figure 4, and Figure 6 is a graph wherein rate of rotation is plotted against time, showing a preferred procedure for alternating the twist of wires applied in a method in accordance with the invention.

In the apparatus of Figure 1, a cable core 1 comprising a conductor, an insulation and an outer conductive layer is fed from an extruder (not shown) to a band spinner 2, by means of which a copper band 4 drawn off from a supply reel 3 is applied as a spiral band winding around the cbre 1. By means of a guide nipple 5, the band winding is pressed on to the conductive layer of the cable core 1 located within it.

The cable core with its copper band winding passes through a hollow nipple 6, which is rotated in alternate senses of rotation, and which is provided with conically converging guide grooves 7 which are distributed uniformly around its surface and serve to receive individual wires 9 drawn off from supply reels 8, which wires are thus applied to the cable core 1 with alternating twist. The application of these wires is effected by arranging for the wires 9 to be introduced, following stranding, into a nipple which is formed by a holding band 10 in a region 13, this being the region of the arrival of the wires 9 on the surface of the cable core 1.

The wrapping of the cable core 1 provided by the holding band 10, which acts as a nipple, is allowed to remain on the cable core 1, as a lost nipple, so that the wires 9 are securely held after application to the cable core 1. The holding band 10 is applied by a band spinner 11, which is a central spinner in the case illustrated, but which may instead be a tangential spinner.

It may sometimes occur, more particularly with cable cores with a small conductor cross-section and correspondingly small insulating wall thickness, that the stranded structure held within the holding band 10 tends to become partially slackened as a result of torsional forces acting upon the core. To prevent this, a holding or gripping device 12 for the said stranded structure, which prevents torsion thereof, is provided adjacently to the abovementioned arrival region 13.

The method and apparatus of the present invention can be employed to produce inter alia a single or multiple-cored electric cable, more particularly a medium or high-tension cable, incorporating a screen disposed around an insulation, or around an outer conductive layer, the screen comprising a plurality of individual wires applied with alternating stranding as already described.

In the above-mentioned Ceander cables (cf. Gebrauchsmuster Specification No.

1875570), a soft packing composition is provided around the cable core, and into this composition are introduced the individual wires of a concentric screening or neutral conductor applied with reversing stranding, their mutual position thus being determined. The transverse conductivity required in this concentric conductor by typical regulations, i.e. its conductivity in the peripheral direction, can be obtained by means of a copper band wound helicoidally around the wires concerned, which connects these wires electrically with one another. This cable construction was proposed solely for low-tension distributor cables, for which it has indeed been used for a long time, and is generally unsuitable for high-tension cables, i.e. those for use above 10 KV, owing to the packing composition present.Moreover, increasing voltage also necessitates greater wall thicknesses, which entail an increased expansion in the radial direction in the heating cycles which occur during service. It is then possible for the band providing the transverse conductivity to lift off the screen wires, thereby ending the usefulness of this band.

It is therefore an object of the invention, in a second aspect thereof, to provide, independently of the cable type under consideration, a means of securing transverse conductivity between screen wires applied to a cable core, in all situations.

According to a second aspect of the invention, we provide a single or multiple-cored electric cable, more particularly a medium- or high-tension cable, incorporating a screen disposed around an insulation, or around an outer conductive layer, the screen comprising a plurality of individual wires applied with alternating stranding by a method according to the invention as described above, and incorporating also a conductive band serving to ensure adequate transverse conductivity for these wires, wherein the conductive band is disposed beneath the said individual wires.Now if the cable core(s) expand(s), e.g. owing to heat generated in service. then the conductive band, e.g. in the form of a spiral, is urged firmly against the individual wires of the screen so that considerably better contact can be obtained than in the known arranement with the spiral applied around the concentric wire layer. This also applied more particularly to all cases where the individual wires are embedded in a soft packing composition. Thus in these latter cases, it is possible, in the course of production or owing to heating during service, for the packing composition to pass between the individual wires or totally surround them, thus lifting off the spiral intended to provide transverse conductivity.However, if, according to the present aspect of the invention, the conductive band is disposed beneath the individual screen wires, then satisfactory contact is always maintained at least in the region occupied by the conductive band.

The present conductive band may be applied helicoidally or spirally with a short or long twist. However a particular advantage is obtained if the twist is so short that the adjacent band edges overlap one another, so that a closed layer is obtained; this layer can simultaneously perform the function of a moisture barrier.

A similar effect can be obtained if the band is one which has been applied longitudinally to, and folded around, the layer underlying it. Both in this case and in the previous one, the adjacent edges of the band may advantageously be secured to one another, e.g. by adhesive bonding, soldering or welding.

If, as described above, the conductive band simultaneously performs the function of a moisture barrier, then the further advantage is obtained that this is protected mechanically by the wire layer applied around it. The screen wires additionally perform the function of an armouring, which is particularly efficacious in the case of minor damage to an outermost sheath of the cable e.g. damage due to scraping tools, pressing stones or rodent attack.

The conductive band employed can usually be a copper band, but other conductive band materials are also suitable, and thus metallised bands, or synthetic-resin-backed metal foils, may also be used, being wound around the core(s) of the single or multiple-cored cable as an open or closed spiral, as desired.

This second aspect of the invention will be explained more fully with reference to Figures 2 and 3 of the accompanying drawings.

The low-tension cable of Figure 2 comprises three cores 21, 22 and 23, which have a generally sector-shaped cross-section. A packing material 24 overlies these cores. A conductive band which is actually a copper band 25 is wound in a spiral around the packing material 24, and is covered by a wire layer 26 comprising a plurality of individual wires applied with reversing twist.

The individual wires here are not embeded in a soft packing material, and so it is appropriate for them to be applied by a method in accordance with the invention as already described; more specifically, the wires for layer 26 are introduced by means of a hollow nipple (cf. 6 in Figure 1) rotated in alternate senses of rotation, into a nipple which is formed by a synthetic resin holding band 27 (cf. 10 in Figure 1), the wrapping formed by the band 27 being left on the wire layer 26 as a lost nipple. An outer sheath, of (e.g.) a polyvinyl chloride or polyethylene composition, is applied around the band 27, as shown at 28.

The single-cored high-tension cable of Figure 3 incorporates a conductor 29 composed of a multiplicity of individual wires, and an inner conductive layer 30.

Around the layer 30 there is a high-tension insulation 31 which is in turn covered by an outer conductive layer 32. Around this, a copper band 33 is wound in a spiral (optionally with the interposition of a buffer layer), and around this is a screen 34, comprising a plurality of individual wires applied with reversing twist by a method in accordance with the invention as already described. A synthetic resin foil 35, wound on so as to form a complete covering, covers the wires of the screen 34, and is left as a lost nipple on the layer which these wires form. The foil covering 35 is surrounded by an outer sheath 36.

It will be appreciated that this second aspect of the invention is not restricted to the types of cables actually shown in Figures 2 and 3. Low-tension distributor cables with and without a packing material may be similarly produced so as to achieve the advantages of the invention.

The method and apparatus of the invention first described herein make it possible to apply a so-called Ceander layer to any desired substrates, including more particularly substrates which have little or no adherence. However, there is sometimes a danger, more particularly with flexible material or cables of small diameter, that the layer(s) of wire applied may slip, so that the desired uniform covering effect is prejudiced.

It is consequently an object of the present invention, in a third aspect thereof, to provide a method and an apparatus which will ensure uniform covering by the layer(s) of wire applied, even with unpromising materials such as those just mentioned.

According to a third aspect of the invention, the material obtained downstream of the said arrival region in the method of the invention first described herein is held against rotation following the applicaiton and fixing of the wire layer(s) at a predetermined distance from the said arrival region. The distance just mentioned can be settled according to the diameter and/or length of the free path and/or flexibility of the material concerned. In this way one can obviate complications in the method first described herein, which might arise from two force components being exerted against the material at right angles as it travels through.

More particularly, there are, on the one hand, a constant pull exerted upon the linearly extended material by the band which is preferably employed to form the nipple, and, on the other hand, the torsional forces which the individual wires with their changing direction of twist continually apply to the material.

In performing the method of this aspect of the invention, it has been found advantageous to use an apparatus as broadly described in connection with the first aspect of the invention, provided with two or more rollers with a yielding resilient lining on their operative surfaces, these rollers being mounted so as to be adjustable radially with respect to the material obtained downstream of the said arrival region and so as to hold the latter against rotation at the above-mentioned predetermined distance from the arrival region. Such rollers ensure a reliable holding of the material, and can have a long service life. The material can still be held if fluctuations in thickness appear in the material due to production tolerances.For this reason, however, it is particularly advantageous if the lining of the rollers is of rubber or synthetic rosin and has a Shore hardness of 50 to 70 kp/mm2, e.g. 60 + 3 kp/mm2. It can then be ensured that, even though the material is effectively held against rotation, it is not damaged by the holding rollers. In this context it has been found that roller linings of the above-mentioned Shore hardnesses do give particularly satisfactory results.

It is also advantageous that the rollers should be exchangeably mounted. Furthermore it is advisable that they should be sufficiently small to ensure that they cannot in any case obstruct the winding on of the band forming the stranding nipple. Here again, the above-mentioned lining on the operative surfaces of the rollers can be designed to allow for (e.g.) the irregularities in a cable core dictated by production tolerances.

If the rollers are exchangeably mounted, the apparatus can be adapted to any desired diameters. However, it is also advantageous that the rollers should be mounted so as to be axially adjustable. This has the substantial advantage of making it possible to employ an optimal distance between the arrival region and the holding point. The torsional rigidity of the material (e.g. an electric cable) which is to be given a concentric wire layer is a function, not only of the conditions of production, but also, and to a considerable degree, of the proportion between the material's diameter and the length of the section of it which is to be held against rotation. The angle of twist can in all cases be reduced to a minimum by axial adjustment of the rollers, e.g. manually when they are at rest, and by means of gear-wheels during production.

The optimal distance between the arrival region and the holding point can be determined for any given linearly extended material under such test conditions as may be desired. For this purpose, either the rollers can be displaced towards the nipple with which they are employed, or conversely the said nipple can be displaced towards the rollers in a case in which the rollers are stationary, i.e. non-displaceable.

This third aspect of the invention will be explained more fully with reference to Figures 4 and 5 of the accompanying drawings.

In the apparatus of Figure 4, a cable core 45 which is fed from an extruder, not shown, and which in the case of a medium-tension cable comprises a conductor, an inner conductive layer, an insulation and an outer conductive layer, is provided with a band covering 46 with a long twist. A plurality of individual wires 47 which are drawn off from supply reels, not shown, are applied to the cable core 45 with the aid of a guiding and twisting cone 48 rotated in alternate senses of rotation; thus the wires 47 follow the rotary movements direction of the cone 48, and are accordingly applied to the core 45 in an "SZ" stranding pattern.

To enable the wires 47 to be applied so as to give a smooth substrate, a holding band 55 is applied by a spinner 49; this holding band 55 immediately wraps the wires 47 in the arrival region 54, and anchors them upon the core. The holding band 55 remains on the core as a lost nipple.

To ensure that the layer of wires 47 remains on the core in the desired manner, the assembly leaving the spinner 49 is held by a gripping device 50. This may be of any applicable construction, but advantageously, as shown on a larger scale in Figure 5, it comprises two rollers 51 and 52 which cooperate to grip the assembly leaving the spinner 49, and render torsional forces virtually harmless. A yielding resilient lining 53 on the operative surfaces of the rollers prevents permanent deformation of the cable in the nature of fluctuations in thickness.

The gripping device 50, which may also function as a haul-off device, or at least the rollers 51 and 52, may be adjustable in the axial direction, as indicated by the double-headed arrow in Figure 4, so that according to the prevailing conditions, and the diameter and flexibility of the material to be gripped, the length of the portion which is to be protected from being twisted, between the arrival region 54 and the gripping device 50, can be optimised. It is important in this connection that the angle of twist of the material concerned should be kept as small as possible without adversely affecting the twisting of the wires 47 and the spinning operation performed by the spinner 49.

To conclude the present description, some observations may be made on the manner in which the alternation of twist in a method in accordance with the present invention may be carried out.

According to the particular configuration required in the metallic wires applied to the surface of the linearly extended material, use may be made of a method in accordance with the present invention wherein, when the direction of twist is to be reversed, the rate of rotation of the nipple employed (the hollow nipple 6 in the case of Figure 1) is increased immediately before the reversal point, and the reversal is effected at the increased rate of rotation, the latter being decreased again to the normal rate of rotation when the reversal has been effected. In this connection, it is possible to effect the requisite changes in the rate and direction of rotation by the use of contactless electronic control means.

More specifically, as shown in Figure 6, which is a graph wherein rate of rotation is plotted against time, one can start with a certain rate, shown as +V1, determined by the desired length of lay in stranding. Shortly before a time T1 has elapsed, at which reversal from positive to negative rotation is to take place, the rate of rotation is increased from + V1to +V2. At this increased rate of rotation +V2, the reversal of the sense of rotation takes place, to give the value -V2, and thereafter this increased rate of rotation on the negative side is brought back to the basic rate of rotation on the negative side, namely -V1. As operation continues, the rate of rotation is increased again to -V2 and a reversal of the sense of rotation takes place to give the corresponding value +V2 on the positive side.The absolute values of the basic rates of rotation should be equal to one another, and the absolute values of the increased rates of rotation should be equal to one another. If the number of lays which the rotating hollow nipple 6 gives the wires is B, then in the example to which the graph refers the ratio B1: B2: B3 is 10:1:1.

The control of the time interval is obtained by making it dependent on the number of revolutions of the hollow nipple 6, i.e. the revolutions or parts of a revolution are counted by a pre-set counter. A second pre-set counter is coupled with the first one, and determines the extent and timing of the increase in the rate of rotation before and after the reversal of rotation. The two pre-set counters control (e.g.) relays of known type, whose contacts close at the appropriate desired values, the amount of the desired values (iV1 and tV2) being obtained from a variable resistance or potentiometer.The first pre-set counter counts the revolutions on the positive side starting from zero, reverses at a predetermined value, and then counts downwards, through zero, in the negative direction, e.g. as in the following sequence: 0, 1,2,3,4,3,2, 1,0, -1, -2, -3, -4, -3, -2, -1,0, 1,2 ... etc.

WHAT WE CLAIM IS: 1. A method of applying, with twist, one or more layers comprising a plurality of metallic wires, whose direction of twist is alternated during their application, to the surface of linearly extended material, wherein the individual metallic wires are introduced, in the region of their arrival on the said surface, into a stranding nipple in the form of an encircling containing formation which is formed in the said arrival region during stranding, and which is left as a lost nipple (as herein defined) on the wire layer or layers applied.

2. A method according to claim 1, wherein the said lost nipple is constituted by a band wound on to the layer or layers of wire being formed in the said arrival region.

3. A method according to claim 2, wherein the band is wound on with a length per turn which is not greater than the band width.

4. A method according to claim 1,2 or 3, wherein the said material and the wire layer or layers applied thereto are held together immediately following the fixing of the said layer or layers upon the said surface.

5. A method according to any preceding claim, wherein the stranding nipple which remains as a lost nipple is formed from a band of insulating material.

6. A method according to claim 5, wherein the band of insulating material is provided with friction resistance increasing means on its surface facing the wire layer or layers.

7. A method according to claim 6, wherein the band of insulating material is provided with an adhesive layer.

8. A method according to any preceding claim wherein the linearly extended material to which the one or more wire layers are applied comprises an electric cable, or cable core, or line.

9. A method according to any preceding claim in which an electric medium or hightension cable is produced, this cable being provided with a copper band increasing the transverse conductivity of the screen constituted by the wire layer or layers applied, wherein the copper band is wound on to the cable core before the application of the wire layer or layers.

10. A method according to claim 1, substantially as described with reference to Figure 1 of the accompanying drawings.

11. An electric cable, or cable core, or line, or other linearly extended material produced by a method according to any preceding claim.

12. Apparatus suitable for use in performing a method according to any of claims 1 to 9, incorporating a hollow nipple provided with substantially conically converging wire guiding

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. surface of the linearly extended material, use may be made of a method in accordance with the present invention wherein, when the direction of twist is to be reversed, the rate of rotation of the nipple employed (the hollow nipple 6 in the case of Figure 1) is increased immediately before the reversal point, and the reversal is effected at the increased rate of rotation, the latter being decreased again to the normal rate of rotation when the reversal has been effected. In this connection, it is possible to effect the requisite changes in the rate and direction of rotation by the use of contactless electronic control means. More specifically, as shown in Figure 6, which is a graph wherein rate of rotation is plotted against time, one can start with a certain rate, shown as +V1, determined by the desired length of lay in stranding. Shortly before a time T1 has elapsed, at which reversal from positive to negative rotation is to take place, the rate of rotation is increased from + V1to +V2. At this increased rate of rotation +V2, the reversal of the sense of rotation takes place, to give the value -V2, and thereafter this increased rate of rotation on the negative side is brought back to the basic rate of rotation on the negative side, namely -V1. As operation continues, the rate of rotation is increased again to -V2 and a reversal of the sense of rotation takes place to give the corresponding value +V2 on the positive side.The absolute values of the basic rates of rotation should be equal to one another, and the absolute values of the increased rates of rotation should be equal to one another. If the number of lays which the rotating hollow nipple 6 gives the wires is B, then in the example to which the graph refers the ratio B1: B2: B3 is 10:1:1. The control of the time interval is obtained by making it dependent on the number of revolutions of the hollow nipple 6, i.e. the revolutions or parts of a revolution are counted by a pre-set counter. A second pre-set counter is coupled with the first one, and determines the extent and timing of the increase in the rate of rotation before and after the reversal of rotation. The two pre-set counters control (e.g.) relays of known type, whose contacts close at the appropriate desired values, the amount of the desired values (iV1 and tV2) being obtained from a variable resistance or potentiometer.The first pre-set counter counts the revolutions on the positive side starting from zero, reverses at a predetermined value, and then counts downwards, through zero, in the negative direction, e.g. as in the following sequence: 0, 1,2,3,4,3,2, 1,0, -1, -2, -3, -4, -3, -2, -1,0, 1,2 ... etc. WHAT WE CLAIM IS:

1. A method of applying, with twist, one or more layers comprising a plurality of metallic wires, whose direction of twist is alternated during their application, to the surface of linearly extended material, wherein the individual metallic wires are introduced, in the region of their arrival on the said surface, into a stranding nipple in the form of an encircling containing formation which is formed in the said arrival region during stranding, and which is left as a lost nipple (as herein defined) on the wire layer or layers applied.

means on its circumference, whereby (in use) the wires are guided on to the linearly extended material, the hollow of this nipple comprising a bore through which this linearly extended material is passed, and this nipple being (in use) rotated in alternate senses of rotation; the apparatus also incorporating means for rotating the hollow nipple in alternate senses of rotation, and additionally incorporating a band winder whereby (in use) band material is wound on to the wires to form the said lost nipple.

13. Apparatus according to claim 12, having holding or gripping means for the assembled material, the holding or gripping means being disposed immediately downstream of the point of formation of the lost nipple.

14. Apparatus according to claim 12, substantially as described with reference to Figure 1 of the accompanying drawings.

15. An electric cable, or cable core, or line, or other linearly extended material, produced by means of an apparatus according to claim 12, 13, or 14.

16. Single or multiple-cored electric cable incorporating a screen disposed around an insulation, or around an outer conductive layer, the screen comprising a plurality of individual wires applied with alternating stranding by a method according to claim 1, and incorporating also a conductive band serving to ensure adequate transverse conductivity for these wires, wherein the conductive band is disposed beneath the said individual wires.

17. Medium- or high-tension cable having the features specified in claim 16.

18. Cable according to claim 16 or 17, wherein the band is applied helicoidally around the layer underlying it.

19. Cable according to claim 16 or 17, wherein the band is one which has been applied longitudinally to, and folded around, the layer underlying it.

20. Cable according to claim 18, wherein the band is so wound as to form a closed layer.

21. Cable according to any of claims 16 to 20, wherein adjacent edges of the band are secured to one another.

22. Cable according to claim 21, wherein the adjacent edges of the band are adhesively bonded to one another.

23. Cable according to any claims 16 to 22, wherein the said conductive band is a metallised band.

24. Cable according to any of claims 16 to 23, wherein the lost nipple left around the screen wires in accordance with claim 1 comprises a foil or a band applied around the said wires.

25. Cable according to claim 24, wherein the foil or band applied around the screen wires is wound around these wires.

26. Cable according to claim 16, substantially as described with reference to Figure 2 or 3 of the accompanying drawings.

27. A method according to any of claims 1 to 9, wherein the material obtained downstream of the said arrival region is held against rotation following the application and fixing of the wire layer(s) at a predetermined distance from the said arrival region.

28. A method according to claim 27, substantially as described with reference to Figures 4 and 5 of the accompanying drawings.

29. An electric cable, or cable core, or line, or other linearly extended material, produced by a method according to claim 27 or 28.

30. Apparatus suitable for use in performing a method according to claim 27, comprising an apparatus according to claim 12 or 13 provided with two or more rollers with a yielding resilient lining on their operative surfaces, these rollers being mounted so as to be adjustable radially with respect to the material obtained downstream of the said arrival region and so as to hold the latter against rotation at the said predetermined distance from the said arrival region.

31. Apparatus according to claim 30, wherein the said lining is of rubber or synthetic resin and has a Shore hardness of 50 to 70 kp/mm2.

32. Apparatus according to claim 31 wherein the said Shore hardness is 6013 kp/mm2.

33. Apparatus according to claim 30, 31 or 32. wherein the rollers are exchangeably mounted.

34. Apparatus according to claim 30, 31, 32 or 33, wherein the rollers are mounted so as to be axially adjustable.

35. Apparatus according to any of claims 30 to 34, wherein the hollow nipple comprises a guiding and twisting cone which is axially adjustable.

36. Apparatus according to claim 30. substantially as described with reference to Figures 4 and 5 of the accompanying drawings.

37. An electric cable, or cable core, or line, or other linearly extended material, produced by means of an apparatus according to any of claims 30 to 36.

38. A method according to any of claims I to 10, or claim 27 or 28, wherein, when the direction of twist is to be reversed, the rate of rotation of the nipple employed is increased immediately before the reversal point, and the reversal is effected at the increased rate of rotation, the latter being decreased again to the normal rate of rotation when the reversal has been effected.

39. A method according to claim 38, substantially as described with reference to Figure 6 of the accompanying drawings.

40. An electric cable, or cable core, or line, or other linearly extended material, produced by a method according to claim 38 or 39.