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(11) | EP 0 735 546 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Laying wire around core |
| (57) A method of laying a layer of at least one steel wire (18) around a core (12), said
at least one steel wire consisting of several lengths, each of said lengths having
a leading end and a trailing end ; said method comprising following steps : (a) connecting the trailing end of the lengths to the leading end of a subsequent length resulting in at least one steel wire with affected zones in the neighbourhood of the connections and in non-affected zones elsewhere, the non-affected zones having a yield strength which is substantially greater than the yield strength in the affected zones ; (b) subjecting said at least one steel wire (18) to a first torsion in a first direction and to a first extent necessary to plastically deform said at least one steel wire into a helically twisted configuration about its longitudinal axis ; (c) subjecting said at least one steel wire (18) to a second torsion in a second direction opposite to said first direction and to a second elastical extent necessary to avoid that said helically twisted configuration would concentrate mainly on said affected zones ; (d) wrapping said at least one steel wire (18) around said core (12). |
Field of the invention.
[0001] The present invention relates to a method of laying a layer of at least one steel wire around a core of a cable wherein the steel wire consists of several lengths, each length having a leading end and a trailing end, the trailing end of the length being connected to the leading end of a subsequent length.
Background of the invention.
[0002] The core of such a cable may comprise a plurality of copper or aluminium wires which serve as conductors of electrical energy and which may be embedded in a matrix of synthetic material. Alternatively, or in addition thereto, such a core may comprise a plurality of fibres such as glass fibres for the transfer of communication.
A layer of at least one steel wire around the core serves to give the necessary strength to the entire cable, gives protection to the cable and builds a Faraday screen against magnetic fields. The layer may itself be embedded in or enveloped by a matrix of another synthetic material.
The steel wire or steel wires in the layer are subjected to a plastical deformation, e.g. a plastical torsion, prior to the cable formation or just alter the cable formation so that the steel wires take their proper position and form in the layer.
[0003] In order to bridge over greater distances or in order to be used at greater depths in the sea without having to use more than one layer of steel wires, it is a necessity to use steel wires with higher tensile strengths.
The use of steel wires with higher tensile strengths, however, has led to the problem that during the cable formation substantial discontinuities occur resulting in an unacceptable quality and even in an increase of the frequency of fractures of the outer wires to such an extent that economic cable formation is no longer possible.
[0004] It has been discovered that, with existing techniques and if the above-cited plastical deformation is given prior to the cable formation, this plastical deformation concentrates mainly in affected zones near the ends of the wire lengths where the wire lengths have been connected to each other. This concentration results in the mentioned occurrence of discontinuities.
[0005] Applying the plastical deformation just after the cable formation could avoid the fractures but necessitates large forces and would increase the risk of harming the core of the cable.
Summary of the invention.
[0006] The present invention aims at avoiding the drawbacks of the prior art.
It is an object of the present invention to provide for a method of laying at least one steel wire around a core where the steel wire or steel wires with a higher tensile strength can be used and/or where the quality of the finished product is better and/or where fractures of the steel wires are reduced.
[0007] According to the present invention, there is provided for a method of laying a layer of at least one steel wire around a core. The steel wire consists each of several lengths where each of the lengths have a leading end and a trailing end.
The method comprises following steps :
(a) connecting the trailing end of the lengths to the leading end of a subsequent length resulting in a continuous wire with affected zones in the neighbourhood of the connections and in non-affected zones elsewhere, the yield strength in the non-affected zones being substantially greater than the yield strength in the heat-affected zones ;
(b) subjecting the steel wire or steel wires to a first torsion in a first direction and to a first extent necessary to plastically deform the steel wires into a helically twisted configuration about their longitudinal axes ;
(c) subjecting the thus plastically deformed steel wires to a second torsion in a second direction opposite to said first direction and to a second elastical extent necessary to avoid that the helically twisted configuration would concentrate mainly on the affected zones ;
(d) wrapping the steel wire or the steel wires around the core of the cable.
[0008] Connecting the trailing end of the lengths to the leading end of a subsequent length can be done by welding which results in heat-affected zones in the neighbourhood of the welds and in non-affected zones elsewhere.
[0009] The yield strength is hereby defined as the strength at 0.2 % permanent elongation and is in the litterature often referred to as σ0.2. With commonly deformed industrial steel wires, the yield strength increases as the tensile strength increases and vice versa.
[0010] Each steel steel wire can be considered as a series connection of strong parts with a higher tensile strength or yield strength, i.e. the non-affected zones, and weak parts with a lower tensile strength or yield strength, i.e. the heat-affected zones. The stresses introduced in the outer steel wires during their plastical deformation in step (b) seem to concentrate mainly in the weak parts, which results in the discontinuities and even in fractures of the steel wires.
[0011] Equalizing the yield strengths in both the non-affected zones and the heat-affected zones could be considered but does not give adequate results for the following reasons.
Reducing the yield strength in the non-affected zones to become substantially equal to the yield strength in the heat-affected zones avoids the alternation of weak and strong parts and, as a consequence, avoids that the helical twisted configuration mainly concentrates in the heat-affected zones. The helical twisted configuration, on the contrary, is spread over a much larger distance along the steel wires. This reduces the occurrence of discontinuites and, as a consequence, the frequency of wire fractures, on the one hand, but, on the other hand, lowers considerably the overall yield strength of the outer wires.
Increasing the yield strength in the heat-affected zones in order to obtain substantially the same yield strength in the non-affected zones by reinforcing the heat-affected zones mechanically, e.g. by applying clamps in the heat-affected zones, could be envisaged but should be avoided since this would slow down or substantially change the cable formation and would lead to increased cross-sectional dimensions.
[0012] In great contrast herewith, the solution given by the inventors in step (c) keeps the overall yield strength of the steel wires unaffected. Furthermore, the present invention can be introduced without necessitating amendments to the other cable formation steps. Moreover, the present invention does not slow down the cable formation or wrapping process.
[0013] The steel wires have preferably a non-round transversal cross-section, such as a flat, rectangular, trapezoidal, triangular or Z-like cross-section in order to increase the filling factor of the cable, i.e. the percentage of steel cross-section with respect to the global cross-section of the cable.
[0014] The steel wires preferably have a carbon content ranging between 0.10 and 0.70 %, most preferably between 0.30 and 0.50 %. The lower limits are imposed by reasons of minimum tensile strength, the upper limits are imposed by reasons of sufficient ductility.
[0015] The yield strength of the steel wires in the heat-affected zones is usually at least ten per cent, e.g. at least thirty per cent, lower than the yield strength of the steel wires in the non-affected zones.
[0016] The tensile strength of the steel wires in the non-affected zones is preferably greater than 500 MPa, e.g. greater than 600 MPa, e.g. 800 MPa or 1050 MPa.
Brief description of the drawings.
[0017] The invention will now be described into more detail with reference to the accompanying drawings wherein
- FIGURE 1 shows the cross-section of an electrical cable;
- FIGURE 2 illustrates the way of wrapping steel wires around a cable core;
- FIGURE 3 illustrates into more detail how the steel wires are wrapped around a steel core;
- FIGURE 4 shows schematically a steel wire comprising at least two lengths welded together.
Description of the preferred embodiments of the invention.
[0018] FIGURE 1 shows, by way of example, a cross-section of an electrical cable 10. The electrical cable comprises a core 12 having electrical conductors 14 embedded in a matrix of synthetic material 16. A plurality of flat steel wires 18 is wrapped around the core 12 and forms a layer around the core 12. This layer may, in its turn, be enveloped by an outer layer of synthetic material 20.
[0019] A transversal cross-section of the flat steel wires 18 can have a width (this is the greatest dimension in the cross-section) ranging from 4.0 to 10.0 mm and a thickness (this is the smallest dimension in the cross-section) ranging from 1.0 to 4.0 mm, the width-to-thickness ratio being preferably greater than 2.
Some examples are 7.5x2.5 mm, 7.5x3.0 mm, 8.0x3.0 mm, 9.0x3.0 mm wires. The carbon content of these flat steel wires 18 can range between 0.10 and 0.70 %, and can be about 0.35 %.
The tensile strength of the flat steel wires 18 is preferably greater than 500 MPa and is e.g. about 1000 MPa.
The flat steel wires can be coated with a corrosion-resistant coating such as zinc or a zinc alloy, e.g. a zinc aluminium alloy comprising about 95 % zinc and about 5 % aluminium. This latter alloy of zinc-aluminium is particularly interesting, since it allows the wrapping of the thus coated flat or non-round steel wires around a core without chipping or flaking of the coating layer.
[0020] FIGURE 2 gives a global view of how the flat wires 18 are wrapped around the core 12. Flat steel wires 18 are drawn from spools 22. The spools 22 may have their axes fixed during the rotation around the core 12 or may have their axes rotated during the rotation around the core 12. Alter being drawn from spools 22 the flat steel wires 18 pass through holes 24 of a disc 26 and between a pair of rollers 28. This pair of rollers 28 is fixed by an angular plate 30 to the disc 26. Thereafter the flat wires pass through holes 32 of a second disc 34 and between a pair of rollers 36, which are fixed by an angular plate 38 to the second disc 34. Finally the flat wires pass through holes 40 of a third disc 42 and between a pair of rollers 44, which are fixed by an angular plate 46 to the third disc 42.
[0021] FIGURE 3 gives a more detailed view of this process. A welding apparatus 50 may be positioned between the spools 22 and the first disc 26 in order to weld the trailing end of a length, e.g. at the end of a spool, to the leading end of a subsequent length of wire, which is e.g. at the beginning of a spool.
[0022] Referring more particularly to FIGURE 4, this welding operation results in a weld 52 and in a heat-affected zone 54 around the weld. The rest of the wire lengths 56 are non-affected zones, i.e. zones where the tensile strength and the yield strength have remained substantially unchanged during the welding operation.
[0023] Coming back to FIGURE 3, the flat wires are forced and subjected to a plastical torsion in a first direction between the first pair of rollers 28 and the second pair of rollers 36. This particular angle of a plastical torsion is determined by the position the steel wire must take around the core of the cable.
Thereafter, the flat wires are forced and subjected to an elastical torsion in a second direction, opposite to the first direction, between the second pair of rollers 36 and the third pair of rollers 44. This particular angle of an elastical torsion is determined empirically. Its value must be such that the helically twisted configuration given by the plastical torsion does no longer concentrate on the heat-affected zones, which reduces the frequency of fractures in the steel wires.
The thus deformed flat steel wires are brought toghether at a die 48 which eventually fixes the layer of flat wires 18 around the core 12.
said method comprising following steps :
(a) connecting the trailing end of the lengths to the leading end of a subsequent length resulting in at least one steel wire with affected zones in the neighbourhood of the connections and in non-affected zones elsewhere, the non-affected zones having a yield strength which is substantially greater than the yield strength in the affected zones ;
(b) subjecting said at least one steel wire to a first torsion in a first direction and to a first extent necessary to plastically deform said at least one steel wire into a helically twisted configuration about its longitudinal axis ;
(c) subjecting said at least one steel wire to a second torsion in a second direction opposite to said first direction and to a second elastical extent necessary to avoid that said helically twisted configuration would concentrate mainly on said affected zones ;
(d) wrapping said at least one steel wire around said core.