Description

Technical Field

[0001 ] The invention relates to a steel cord for reinforcing a rubber article such as a tire. The steel cord has adapted elongation properties to align with the rubber during tire building.

Background Art

[0002] Steel cord is still the material of choice for reinforcing the belt of a vehicle tire. Its superior compression resistance, tensile strength, predictable fatigue behaviour, impact resistance, adhesion and adhesion retention to rubber are favoured when compared to manmade organic fibres.

[0003] However, steel cord also has some drawbacks in that it is less extensible than organic fibres which is important for zero degree applications in a tire. In a zero degree application the steel cord is circumferentially aligned with the equatorial plane of the tire. The equatorial plane is the plane that is perpendicular to the axis of the tire and goes through the centre of the belt area. Indeed, during tire building the green tire is blown up in the tire vulcanisation mould and thereby the fibres in the equatorial plane are stretched.

[0004] Further there is the constant strive to reduce the rolling noise of the tire, in particular as due to the introduction of electrical vehicles, this rolling noise has become the more prominent noise source. It is believed that by the selection of the right anti-vibration materials such as steel cord-rubber combinations with high damping this noise can further be abated.

[0005] Inventors have therefore sought to design steel cords that show sufficient elongation while not compromising other properties such as strength. In this respect the following solutions exist:

(a) Open cords are of the 1xn type made of ‘n’ single filaments that have been given a helical preformation larger than the filaments would have had when being in close contact at the lay given. A ground breaking patent publication in that respect is US 4 258 543. The problem with these cords is that there are limits to the degree of preforming that can be given to the individual filaments: see e.g. WO 2012/055677 A2 describing an open cord of which the preforming has been pushed to the limit. Another problem is that during processing i.e. when pulling the cords from the spool during calendering of the rubber, the cords are pulled closed. Other publications are WO 2020/021006 A1 and WO 2020/021007 A1 ;

(b) High elongation cords such as 3x7, that is a steel cord comprising three strands each comprising 7 filaments that are both twisted in relatively short lay lengths and in the same direction the so called Lang’s lay (WO 2019/086929 A1) . Other examples are 3x4 or 3x3 (WO 2015/014639 A1 ). These cords show excellent resistance against impact. However, these cords do not always reach the required elongation properties.

(c) Hybrid cords have also been suggested wherein steel filaments (WO 2013/098738A1 ) or strands of steel filaments (US 2005/0183808 A1 , WO 2005/014925 A1 , JP2007145125) are twisted around an organic material core. The core is added to keep the filaments or strands further away from one another than what can be achieved with the traditional open cords (a). These cords may show the drawback that fretting corrosion may occur between the dissimilar materials aggravated by the presence of water in the organic material core.

[0006] There remains therefore the desire to develop a cord made of a single material (no ‘hybrid’), preferably made of steel, that can resist the closing of the cord during calendering and still shows sufficient elongation during tire building and when incorporated in the tire.

Disclosure of Invention

[0007] It is therefore an object of the current invention to do away with the problems of the prior art. According a first aspect of the invention a steel cord is presented that has extreme elongation properties prior to break. At low tension the steel cord exhibits a sufficiently high - butt not too high - stiffness in tension that rises steeply when the structural elongation is reached. More specifically a steel cord is described that has an unusually high structural elongation for instance higher than 3%. In a second aspect of the invention a method to produce such steel cord is presented.

[0008] According a first aspect of the invention as defined in claim 1 a steel cord for the reinforcement of a rubber product is presented. The steel cord comprises two or more steel elements that are twisted together. The steel elements may consist of one steel filament. Alternatively the steel element may comprise more than one steel filaments that are twisted around each other or are bundled without being twisted. A filament is a wire that cannot further be disentangled in further filamentary objects. The total number of filaments in the steel cord is indicated with ‘N’. Each of the filaments has a cross sectional area ‘A’ expressed in square millimetres. It is to be noted that different filaments in the steel cord may have variant cross sectional areas as a result of production variations. If this is the case the cross sectional area ‘A’ is the average cross sectional area over all ‘N’ filaments. If all filaments are equal or about equal this average becomes the area of the cross section of one single filament.

[0009] The steel elements can be taken out, disentangled out of the steel cord, that is: the steel elements are ‘individualized’. The steel element has a centre line. When the steel element is one filament the centre line is the line formed by connecting the centroids of perpendicular cross sections of the filament. When the steel element comprises two or more filaments, the centre line is formed by connecting the centroids of the perpendicular cross sections along the length of the steel element. The centroid is the average of all positions in the perpendicular cross sections of the steel elements.

[0010] After ‘individualization’, the centre line of the steel elements show a helix shape. A helix shape is a three dimensional curve with constant curvature and torsion. It is the trajectory of a point rotating at constant angular speed around an axis at constant distance to said axis with a constant linear speed along said axis, the ‘helix axis’. A helix is completely defined by a helix radius and a helix pitch length. The helix pitch length is the axial distance along the helix axis where over the centre line describes one complete rotation around the helix axis. This helix pitch length will be indicated with ‘L _o ’ hereinafter. Over one turn of the helix the centre line itself has a length ‘S’ expressed in millimetre. It will be clear to the skilled person that ‘S > L _o ’ at all times, the equality applying when the wire is straight. It is to remarked that the helix pitch length ‘L _o ’ and the helix centre line length ‘S’ must be obtained by measurement on individualised steel elements. When these parameters of the steel elements are measured indirectly based on a cross section of the steel cord by means of determining the helix radius and the lay length, one foregoes the effect that these steel elements may have on one another.

[0011 ] The centre line of the steel elements must be measured under a tension of half a newton per filament in said steel element. This is to ensure that the helix shape is measured under a normalised tension. Hence, if a steel element is a single filament, 0.5 newton will be applied axially to determine ‘S’ and ‘L _o ’. If the steel element comprises two filaments, a tension of 1 newton will be applied, and so on. The inventors note that some variation on the test forces is allowed as the measurement of ‘S’ and ‘L _o ’ is not that sensitive for the type of filaments used.

[0012] With the known quantities of the steel cord, N, A, and S, a quantity ‘P’ can be calculated: wherein ’TT’ is the ratio of circumference to diameter of any circle in the plane, ’E’ is the tensile modulus of the material the wire is made off, in the case of steel, and for the purpose of this application, is about 200 000 N/mm ² . It follows that ‘P’ has the dimension of a force and can therefore be expressed in newton.

[0013] The quantity ‘P’ is a measure for the tensile stiffness of the steel cord at low elongation. ‘Tensile stiffness’ is the constant of proportionality between the elongation applied and the resulting force (in newton). A ‘low elongation’ is an elongation that is well below the ‘structural elongation’ (see further). Note that for the purpose of this application with ‘elongation’ - indicated with ‘e’ hereinafter - is meant the ratio of extension over initial length ‘AZ/Z’, not expressed in percentage. If ‘elongation’ is expressed in percent it will be indicated.

[0014] The quantity ‘P’ is not a random collection of parameters. It incorporates both the geometry of the helix ‘S', the bending and torsion stiffness of the ‘N’ filament themselves through the cross-sectional area ‘A’ in combination with the material property ‘E’. It derives non-trivially from virtual work considerations of bending, torqueing and stretching a helical wire thereby using Castigliano’s Second Theorem.

[0015] The steel cord of the invention shows the desired property of having a sufficiently large tensile stiffness at low elongation when ‘P’ is larger than 50 newton. Possibly ‘P’ is higher than 70, 85 or 90 newton. If ‘P’ becomes larger than 300 newton, or even larger than 250, 200, 150 or 125 newtonthe initial tensile stiffness becomes too large and the steel cord becomes less useful for its purpose. Best is if the ‘P’ value is between 85 and 125. Such a steel cord shows enough extensibility for zero-degree applications and at the same time produces sufficient bending stiffness in the belt of the tire. The inventors conjecture, that an increased stiffness of the belt may have a positive influence on the noise generation of the tire.

[0016] Any type of steel can be used for the filaments provided it can be made available in wire rod form. Typical steels are low carbon steels with a carbon content ranging between 0.04 wt% and 0.20 wt%. Alternatively, stainless steels can be used. Stainless steels have a minimal chromium content of 11 %. More preferably the steel of the filaments is made of high carbon steel with a typical composition of having a minimum carbon content of 0.65%, a manganese content ranging from 0.40% to 0.70%, a silicon content ranging from 0.15% to 0.30%, a maximum sulphur content of 0.03%, a maximum phosphorus content of 0.30%, all percentages being percentages by weight. There are only traces of copper, nickel and I or chromium. A typical steel tire cord composition for high-tensile steel cord has a minimum carbon content of around 0.80 weight %, e.g. 0.78 - 0.82 weight %.

[0017] The tensile strength of the filaments is at least 2 000 N/mm ² , or even higher than 2 500, 2 700 or above 3 000 N/mm ² . As the filaments are quite heavily deformed, the ductility of the filaments is all important. Therefore the filaments should at least be able to take 200 twists over a length of one meter before breaking. Even better is if they can sustain more than 250, 300, 350, or 400 twists over a length of one meter.

[0018] The steel filaments may be coated with a metal or metal alloy that promotes the adherence to an elastomer such as rubber. Particularly preferred alloys to effectuate this are copper based alloys such as brass or bronze. Recently, ternary or quaternary alloys based on brass with one or two additional metals such as cobalt, nickel, manganese, or even iron have been considered. Also zinc metal is a possible substrate to which rubber can adhere.

[0019] In a further preferred embodiment the filaments have an equivalent diameter ‘d’ such that ‘A = πd ² 14 ‘ and wherein the dimensionless ratio ‘S jd' is smaller than 30, or even smaller than 25. If this ratio becomes higher than 30 the

(a) The filament becomes too thin to resist bending;

(b) The filament becomes too thin to reach sufficient breaking load with a limited number of filaments ‘/V’;

[0020] According a further preferred embodiment the ratio is smaller than 0.95, or even smaller than 0.945. The index ‘o’ refers to the ‘open condition’ of the steel cord. With ‘open condition’ is meant that the steel elements are independent, loose from one another, can move relative to one another. This ratio is important as the inverse of the ratio is indication how much ‘extra length’ the individualised steel element has available before being completely straight. The upper limit of 0.95 implies that the individualised steel element has an ‘extra length’ of at least 5.26% available before being completely straightened under a sufficiently large load.

[0021] When the steel cord is loaded under the boundary condition of rotationally restrained ends, the steel filaments in the elements will come to contact one another as the helical steel elements are twisted around each other. In this ‘closed condition’ the steel filaments in the steel cord contact one another, and hinder one another. The steel filaments cannot further straighten. In the ‘closed condition’ the steel filaments will show a pitch length of in millimetre. The ratio is an indicator how close the filaments get to being straight. This ratio should be as high as possible, that is as close as possible to one. By preference _c is larger than 0.98, or even better larger than 0.985. A large ratio is obtained by using a low number of filaments ‘N’ and filaments that have a small diameter ‘d’ relative to ‘S’.

[0022] In a further preferred embodiment, the ‘structural elongation ‘ε ₀ ” is larger than 3.5 percent and smaller than 10 percent. The ‘structural elongation - for the purpose of this application - ‘e ₀ ” is defined as . Even more preferred is that it is larger than 3.75, or 4.00, or 4.25 or 4.50 % or 4.75 % or even larger than 5.0%. The inventors believe that a steel cord with up to a 10% structural elongation can be achieved.

[0023] According another preferred embodiment of the invention, a steel cord that has a predetermined force at the structural elongation ‘ε ₀ ’ is claimed: the force needed to extend, stretch the steel cord to structural elongation ‘ε ₀ ’ is larger than 50 N, but smaller than 100 N. Within this relatively narrow window, the tensile stiffness at low elongation is just right for the application. If the force at ‘ε ₀ ’ would be below 50 N, the cord is too weak initially and will not give sufficient reactive force in the low elongation region. If the force at ‘ε ₀ ’ becomes larger than 100 N, the cord becomes too stiff, and is not easily extended during tire building.

[0024] In a further preferred embodiment the number of steel elements is two, three or four. More than four is not advised as then the steel elements hinder each other when they align when being pulled that is: the value ‘L _c /S’ becomes too low i.e. the required structural elongation is difficult to reach.

[0025] Accordingly the number of filaments within one steel element is one out of one, two or three. That is: the steel elements may have different number of filaments within one steel cord. Preferably the number of steel filaments in each one of the steel elements is one, two or three, that is: all steel elements have the same number of steel filaments. [0026] Preferably the total number of filaments ‘N’ is two to eight wherein the limits two and eight are included.

[0027] According a second aspect of the invention, a method to produce the steel cord is described comprising the following steps:

(a) Unwinding a number of steel elements with diameter d _e from spools. The steel elements may have been prepared by twisting steel filaments and winding them on a spool. Alternatively the steel elements are one or more steel filaments wound on a spool without being twisted;

(b) Next to that a ‘mandrel wire’ is provided. This mandrel wire is a means, a tool to provide the correct preformation to the steel elements. The mandrel wire has a diameter ‘D’;

(c) The steel elements are then twisted around the mandrel wire with a number of twists ‘N _c ’ over a unit length in a cord twist direction. This results in an intermediate cord. In this intermediate cord, the steel elements have obtained a degree of plastic deformation, resulting in a helix;

(d) Thereafter the mandrel wire is removed again from the intermediate cord by turning, twisting the mandrel cord out of the intermediate cord;

(e) The resulting steel cord is wound on a final spool.

[0028] In a preferred version of the method the resulting steel cord comprises in total ‘N’ steel filaments, each of said steel filaments having a cross sectional area ‘A' expressed in square millimetres, the steel elements have, after individualisation and under a tension of half a newton per filament in said steel element, a center line, said center line having a helix shape, wherein the length of the center line of the steel element over one pitch is ‘S' millimeter, such that the quantity ‘P'expressed in newton: is larger than 50 newton and wherein ‘E’ is the modulus of steel. [0029] By convention in this application: the sign of the twist number indicates the twisting direction: if ‘N _c ’ is a negative number, this corresponds to ‘N _c ’ twists in the ‘S’ direction. Conversely, when ‘N _c ’ is a positive number, this corresponds to ‘N _c ’ twists in the Z’ direction.

[0030] The presence of the mandrel wire is all important to the method. The mandrel wire is the moving forming pin, the moving thorn, the mandrel around which the steel elements are plastically formed. In prior art methods for producing open cords, individual filaments are preformed by guiding the filaments over pins. This technique has its limits in the ability to shape the filaments as the filaments can be overstrained during preforming, leading to transversal cracks and lower fatigue life. On the other hand, when making elongation cords made of different strands that are preformed by false twisting them, there is a limit to the degree of preforming that can be given. The presence of a mandrel wire extends the limits that can be given to the degree of preforming greatly.

[0031] Preferable in step (c) the steel elements are wound around the mandrel wire that is: the mandrel wire has a central position to the intermediate cord.

[0032] Preferably in step (c) the intermediate cord has been treated to obtain zero, or close to zero residual torsions. This can be done by means of one or more straighteners or one or more false twisters. This is a procedure known to the skilled.

[0033] The method can be performed by first winding the intermediate wire on an intermediate wire spool after step (c) and unwinding from said intermediate spool in a next step, on another apparatus to perform step (d).

[0034] The method can be performed also without intermediate winding that is: the intermediate cord is directly lead from an apparatus performing step (c) to an apparatus performing step (d), without using an intermediate spool.

[0035] In a further embodiment detailing step (d), the intermediate cord is made to move linearly e.g. by unwinding from the intermediate spool or by direct moving from the apparatus performing step (c) to the apparatus performing step (d). The mandrel wire is ‘turned out’ of the intermediate cord through a flyer relatively rotating around the intermediate cord and thereby leaving the steel elements as a steel cord. In this manner the intermediate wire is split into the final steel cord and the mandrel wire. The steel cord and mandrel wire are wound onto separate driven spools.

[0036] Important to note is that in step (d) the lay of the cord is not changed. That is the cord is not twisted open in order to easily remove the mandrel wire and subsequently closed again to final step. It is the inventors’ experience that such a procedure does not work properly as there is a loss in elongation of the steel cord due to the repeated torqueing of the filaments.

[0037] The flyer rotates around the intermediate cord relatively thereto. Two extremes exist: (i) the intermediate cord is made to turn around its axis, and the mandrel wire is drawn on a driven spool with stationary axis through a stationary flyer or (ii) the intermediate cord is not rotating and the mandrel wire is turned out at the same lay of the steel cord through a flyer turning around the cord. Mixtures of (i) and (ii) are also possible but more complicated and thus less preferred.

[0038] A preferred embodiment of the method is that both the mandrel spool and the take-up spool have a stationary axis. This can be implemented by putting the driven take-up spool on the cradle of a buncher with a flyer turning around the cradle. That is: the driven take-up spool is inside the flyer. The mandrel wire is lead out of the intermediate cord at the entry of the buncher, through the flyer and is wound on a driven outside mandrel spool. The steel cord goes straight at the entry of the buncher and is wound on the take-up spool. As no plastic torsions are introduced during that operation, the preforming of the steel elements is preserved and consequently the elongation properties remain optimal.

[0039] The situation can also be reversed, although this is slightly less preferred. In that case the driven mandrel spool is situated on the cradle, around which the flyer turns and the take-up spool is outside the cradle. As the steel cord is generally a bit more difficult to handle, guiding the steel cord outside the intermediate cord maybe somewhat more difficult.

[0040] The diameter of the mandrel wire D in relation to the diameter of the element d _e is important as this determines the degree of preforming that can be obtained. The degree of preforming determines the radius of the helix. Preferably the ratio of D/d _e is between 0.5 and 2. When the steel element is a filament, it is preferred that the ratio is on the higher side for example between 0.8 to 2, or even 1 to 2, the limits included. On the other hand when the steel element is made of a plurality of steel filaments a ratio towards the lower side for example between 0.5 to 1 .2, or even between 0.5 to 1.0, the limits included, is preferred.

[0041 ] Together with the radius of the helix, the helix pitch length ‘L _o ’ determines the length of the centerline ‘S’. In order to make the quantity ‘P’ sufficiently large, the length of the center line ‘S’ must be sufficiently small. Therefore the number of twists introduced in the steel cord must thus be sufficiently large in order to obtain the desired features of the steel cord. The inventors assert that the numbers of twists N _c per unit length in said intermediate cord must be at least 150 twists per meter. That is a lay length of less than 6.666.. mm.

[0042] Remark: the helix pitch length ‘L _o ’ found in the steel cord is larger and - at best - equal to the lay length given to the steel elements in the intermediate cord. Indeed, the plastically deformed steel elements will spring back when being disentangled from the intermediate steel cord. The amount of springback will depend on the material properties such as the yield strength of the steel and the curvature given by the mandrel wire. So the applied number of twists ‘N _c ’ and the helix radius (d _e +D)/2 in the intermediate cord do not allow to derive ‘S’ and ‘L _o ’ on the final steel cord : these quantities have to be measured on the final steel cord.

[0043] The steel filaments in the steel elements are twisted together with an element twist number N _e . This number applies to the steel elements as present in the intermediate cord. In what follows the number of twists given to the steel element prior to assembly in the intermediate cord will be denominated by ‘n _e ’. When assembling the intermediate cord, a number of techniques may be used:

[0044] The steel elements, together with the mandrel wire, can be twisted together by means of bunching. In this technique the number of twists in the steel elements ‘n _e ’ are added to the number of twists during the forming of the cord ‘N _c ’ when the twist directions of both steel element and cord are the same. N _e is then equal to ‘n _e +N _c ’. This results in a first preferred embodiment wherein the steel elements are twisted into the steel element with a twist number N _e in the cord twist direction, said element twist number being larger said cord twist number N _c .

[0045] In a preferred embodiment, the total number of twists ‘N _e ’ obtained by a steel filament is larger than 200, or larger than 250, 300 or even larger than 350 twists per meter.

[0046] When the steel elements are provided as bundles the number of twists finally present in the steel elements N _e is equal to N _c . Within the scope of this application, ‘bundle’ means a set of parallel steel filaments that have not been twisted together, that is ‘n _e ’ is zero.

[0047] In a further preferred embodiment, the filaments in the steel element are first twisted in a direction opposite to cord twisting direction to a number of twists ‘n _e ’ and then twisted together with the mandrel wire. The resulting number of twists per meter in the steel element N _e is then n _e +N _c meaning the filaments in said steel elements of said steel cord are twisted to an absolute element twist number |N _e | that is smaller than the absolute cord twist number | N _c | .

[0048] In the extreme case that the number of twists in the steel element is ‘n _e ’ is equal but opposite to the cord twist number N _c the resulting ‘N _e ’ can be made arbitrary small for example less than 10 twists per meter or even zero twists per meter.

[0049] The last two embodiments have the additional advantage that because the steel elements are being untwisted when entering the intermediate cord, they have already obtained some plastic deformation into a helix shape. This results in a very lose structure further contributing to an increased structural elongation, without giving in on longitudinal stiffness at low elongation.

[0050] In an alternative method, the steel elements, together with the mandrel wire, can be twisted together by means of cabling. In this technique the number of twists of the filaments prior to assembly ‘n _e ’ are not substantially changed when twisted into the intermediate cord, that is ‘N _e ’ remains equal to ‘n _e ’. When the steel elements are bundled - that is ‘n _e ’ is zero - the last described embodiment is obtained that is N _e is smaller than 10 twists per meter or zero. For this embodiment the steel elements are presented as parallel wires on a spool prior to being cabled into the intermediate cord. For example three spools with each two wires on it can be cabled into an intermediate cord with three steel elements in it plus the mandrel wire.

[0051 ] As will be clear from the above, the mandrel wire has only a temporary function and is removed from the intermediate cord. The mandrel wire can be one out of the group comprising, alternatively consisting of, a metal wire, a steel wire, a steel cord, an organic yam, an organic cord, an organic filament. As the number of cord twists given to the steel elements may be very high the mandrel wire is axially strongly compressed in particular when the bunching process is used. Therefore organic yarns, organic cords or organic filaments may have an advantage. When metal wires are used, the compressing can be overcome by giving a large tension to the mandrel wire prior to entering the buncher.

Brief Description of Figures in the Drawings

[0052] FIGURE 1 shows the geometrical elements of a helix of importance for understanding the invention;

[0053] FIGURE 2 shows a typical load - elongation diagram of a steel cord and the individualized steel element indicating the different thereof;

[0054] FIGURE 3a and FIGURE 3b illustrate a first method of manufacturing the inventive steel cord;

[0055] FIGURE 4 shows a second method of manufacturing the inventive steel cord;

[0056] FIGURE 5 shows the load-elongation diagrams of the samples in TABLE I. [0057] Figures are provided with reference signs of which the unit and tens digit refers to similar items across figures and the hundred digit refers to the number of the figure. Mode(s) for Carrying Out the Invention

[0058] A first method of manufacturing is presented in FIGURE 3a and 3b. In a first step, illustrated in FIGURE 3a, the intermediate cord 304 is produced by means of an external buncher that is a buncher that has the take-up spool outside the buncher and the pay-off spools on the stationary cradle 326. A number of steel elements - in this case four - are unwound from spools 330 mounted on the cradle 326. The mandrel wire 312 is fed from spool 314’, from the flyer entrance, over a first flyer 320, to the flyer exit. The mandrel wire may e.g. be a metallic steel wire of diameter 0.30 to 0.40 mm. On the cradle 326 the steel elements are joined with the mandrel wire 312. After entering the second flyer of the buncher 322, the steel elements are twisted together with half the cord number of twists ‘Nc/2’. At the exit of the second flyer 322, the steel elements obtain their final number of twists ‘N _c ’. Typical values for N _c are between 50 and 250.

[0059] In order to remove the residual torsions from the steel elements twisted around the mandrel wire, the assembly is fed through a false twister 324 for plastically overtwisting the steel elements. The resulting intermediate cord 304 is free or practically free of residual torsions and is wound on an intermediate spool 302.

[0060] The steel elements comprise two, or three steel filaments that are twisted together with a lay length in a lay direction resulting in ‘n _e ’ twists per meter. A typical value is from 25 to 150 twists per meter for the steel elements. The mandrel wire acts as a moving deformation pin, thorn, mandrel... around which the steel elements are plastically formed. It allows to give much higher degrees of plastic deformation of the steel elements than would be possible with conventional preforming systems. Moreover, by the use of a mandrel wire it becomes possible to deform also steel elements that are in the form of a strands. Strands cannot be deformed with e.g. preforming pins.

[0061 ] The final number of plastic twists ‘N _e ’ that after the bunching steps are present in the steel elements is thus ‘n _e +N _c ’. Typically this number will be smaller than 300 twists per meter. It will be nearing zero if ‘n _e = -N _c ’ that is the steel elements is made with the same number of twists as that cord number twists but are twisted in the opposite direction.

[0062] In a further step of the method, as illustrated in FIGURE 3b, the intermediate steel cord 304 is unwound from intermediate spool 302 on cradle 309 of an external bunching machine. At the entrance of the flyer 306, the mandrel wire 312 is separated from the intermediate cord 304 by turning it out of the cord. The mandrel wire unwinding speed (twists per second) is equal to the linear speed (meter per second) times ‘N _c ’ (twists per meter) The mandrel wire is guided over the flyer 306 and wound on the driven take-up spool 314. The steel cord 308 that is freed from the mandrel wire is wound on the take-up spool 310 mounted on the cradle. Precautions have to be taken to align the turning out of the mandrel with the speed of exit.

[0063] In an alternative embodiment of the method depicted in FIGURE 4, the use of an intermediate spool is made superfluous by feeding the intermediate cord 404 made on an external buncher for forming the intermediate cord directly to the internal buncher for taking up the steel cord 410. The intermediate cord 404 is formed by unwinding the mandrel wire 412’ from spool 414’, through first flyer 420, adding the four steel elements from spools 430 to the mandrel wire 412’ thereby forming the intermediate cord after guiding it through second flyer 422 and false twister 424. The intermediate cord is fed into the internal buncher whereby the mandrel wire 412, is wound out of the intermediate cord 404 at the entrance of the flyer 406. The mandrel wire is subsequently wound onto mandrel take-up spool 414. The mandrel wire can be reused in a next production cycle.

[0064] Turning now to the product properties, FIGURE 1 illustrates the basic geometrical elements of the main product claim. FIGURE 1 shows an individualised steel element 100 that has a helix shape with the Z-axis as the helix axis. The helix has a helix pitch length indicated ‘L _o ’. The steel element has a cross sectional area indicated ‘A’. The steel element has a centreline (indicated dashed) that has a centreline length ‘S’ [0065] The sectional area of the steel filament can be easily established by measuring the diameter of the steel filaments and calculating the surface. The number of filaments ‘N’ can be found by counting them.

[0066] The length of the centreline ‘S’ can be determined by an axial scanning apparatus as described in WO 95/16816 or similar apparatus IM6000 obtainable from KEYENCE. The apparatus comprises two axially aligned chucks, 100 to 500 mm apart, for holding the individualised steel element at its ends during the test. A controlled tension is applied to the steel element of half a newton per filament in the steel element by means of weight. A linear scanning apparatus such as a KEYENCE LS 3034 laser scan system in combination with a KEYENCE LS 3100 processing unit is made to travel parallel to the axis of the steel element by means of an encoding high precision linear drive (accuracy is better than ±10 pm at a step size of 50pm). The measurement plane of the laser scan system is perpendicular to the Z-axis. The laser scan system can scan the outer edges of the steel element up to a precision of ±0.5 pm.

[0067] In a first scan at the equidistant discrete measuring positions ‘z ₇ ’, ‘Δz’ apart, the lower and upper edges of the steel element are determined and the average of both is used as the position of the centreline along the axis perpendicular to the Z-axis, i.e. the X-axis. In this way the positions ‘x(z ₇ )’ are measured and stored in a computer. The index ‘j’ is the sequential number of the sampled.

[0068] Then the chucks are turned 90° and the scan is repeated. Now, the values ‘y(z ₇ )’ along the Y-axis, perpendicular to X and Z-axis are measured and stored. In this way the triplets are obtained that determine the shape of the centreline of the steel element. As this shape is substantially a helix the curves are similar to a cosine and sine as a function of z ₇ . The start of the first turn and the end of the last turn can be determined and this is the axial length ‘I’ over which ‘ri helix pitches are counted. This axial length covers ‘m’ measuring points [0069] Now the total length ‘s’of the centreline over the axial length ‘I’ can be calculated by adding the ‘m - 1’ measured sections:

[0070] The length of the center line of the steel element over one pitch ‘S’ is thus equal to ‘s/ri. In the same manner the helix pitch length ‘L _o ’ is equal to ‘l/ri. As the number ‘ of helix turns measured is readily larger than 50 or even 100, the numbers ‘S’ and ‘L _o ’ are averages over a large number of helix turns.

[0071 ] The relation between the geometrical parameters ‘L _O ,L _C ,S’ and the loadelongation diagram is illustrated in FIGURE 2. There the load-elongation curve of a steel cord according the invention is depicted as 202. Parallel to the elongation axis 206, the axial length of one helix pitch ‘L’ is shown on the axis 208 as it varies with the load F applied (ordinate axis, 210). When the steel cord is at the very low measuring tension, the axial length of a single helix turn is L _o and the elongation e is zero, ‘e’ relates to ‘L’ through:

[0072] When a tangent line (shown dashed) is drawn to the straight part of the curve, this can be extended towards the elongation axis 206. The crossing corresponds to the structural elongation e ₀ as at this point the steel elements are closed and reach the corresponding helix pitch ‘L _c ’. That this point indeed corresponds to the closing of the steel elements can be demonstrated by imagining that the steel elements have an increasing modulus: all the corresponding tangent curves will go through the point (0, e ₀ ) as the slope of the tangent line rises to vertical. When the steel cord is further stretched over e ₀ , the ratio (L/S) remains constant while both ‘L’ and ‘S’ further increase due the elongation of the steel.

[0073] For the purpose of this application, the ratio (L _c /S) is calculated as: wherein ‘d’ is the equivalent diameter of the steel filament.

[0074] When now considering an individualized steel element, a curve akin to 204 is obtained. But here the crossing with the ‘L’ axis corresponds to a fully stretched helix that is a helix with length ‘S’. The elongation is then

[0075] In a series of experiments, samples were prepared according the method depicted in FIGURES 3a and 3b. The results on these samples are summarised in TABLE I.

(a) The first column is a reference number to the curves in FIGURE 5 showing the load-elongation curve of the mentioned constructions;

(b) The ‘Construction’ column is a representation of the ‘intermediate cord’, wherein the first number indicates the diameter of the mandrel wire, followed by the arrangement of the steel elements. If for the intermediate cord one departed from a strand this is indicated by the parenthesis (..). E.g. 0.4+2x(2*0.225) indicates 2 steel elements each comprising two filaments of diameter 0.225 that have been twisted around a mandrel wire of diameter 0.4 mm.

(c) The ‘N _e ’ column indicates the number of twists per meter (t/m) the steel filaments have obtained in the intermediate cord.

(d) The ‘N _c ’ column indicates the number of twists per meter (t/m) the steel elements have obtained in the intermediate cord.

(e) The ratio ‘D/d _e ’ is the ratio between the diameter of the mandrel wire and the steel element diameter;

Note: the lay direction of filaments in the steel element and the lay direction of the steel element in the steel cord where identical and in the ‘S’ direction.

[0076] The different geometrical and mechanical properties following have all been obtained on the final steel cord, that is the steel cord wherefrom the mandrel wire has been removed:

(a) ‘N’ is simply the number of steel filaments in the steel cord; (b) ‘A x 1000’ is the cross sectional area of a single steel filament expressed in mm ² times 1000.

(c) ‘S’ is the length of the centre line in one helix pitch according the described measurement procedure; (d) ‘L _o ’ is the axial length of one helix pitch according the described measurement procedure;

(e) ‘P’ is the quantity calculated according the definition of the claim;

(f) ‘S/d’ is the ratio of ‘S’ divided by the equivalent diameter of the steel filament; (g) ‘L _o /S’ and ‘L _c /S’ are the ratios of the indicated quantities;

(h) ‘ε ₀ ’ is the structural elongation as determined by the procedure of FIGURE 2.

(i) ‘F(e ₀ )’ is the force at the structural elongation as derived from the load elongation diagram. [0077] FIGURE 5 shows the different load elongation of the samples made. In the use of the steel cord, the samples 522, 523, 503 show the most preferred behaviour. Less preferred, but still very useful load elongation curves are exhibited by 507, 521 . The other samples 524, 514 and 509 are not preferred.