CN118043518A

CN118043518A - Steel cord with adapted elongation properties

Info

Publication number: CN118043518A
Application number: CN202280065080.1A
Authority: CN
Inventors: B·弗肯斯; J·迪斯佩斯
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2021-09-28
Filing date: 2022-09-23
Publication date: 2024-05-14
Also published as: WO2023052257A1; KR20240060856A

Abstract

In tires, the strength of the steel cord and the resilience of the rubber are a successful combination. However, in some specific areas of the tire, it is desirable for the steel cord to have greater elongation while still desiring sufficient rigidity. A steel cord having these properties is provided. The steel cord comprises two or more steel elements twisted together. The steel element comprises one or more steel wires. The steel cord comprises a total of "N" filaments, each having a cross-sectional area "a". When the steel element is separated from the steel cord, the steel element exhibits a helical pitch length "L _o", while the individual pitch has a centerline length "S". The steel cord of the invention exhibits a "P" value of at least 50 newtons, whereinA method of manufacturing such a steel cord is also presented. The basic principle of this method is to plastically deform the steel element around a core wire, which is then detached from the steel cord without opening the steel cord.

Description

Steel cord with adapted elongation properties

Technical Field

The present invention relates to a steel cord for reinforcing rubber articles such as tires. The steel cord has adapted elongation properties to align with the rubber during construction of the tire.

Background

Steel cords remain one material choice for reinforcing the belt layers of vehicle tires. The excellent compression resistance, tensile strength, predictable fatigue properties, impact resistance, adhesion to rubber and adhesion retention of steel cords are advantageous compared to man-made organic fibers.

However, steel cords also have some drawbacks, namely their extensibility is less than that of organic fibers, which is important for zero degree applications (zero degree application) in tires. In zero degree applications, the steel cords are circumferentially aligned with the equatorial plane of the tire. The equatorial plane is a plane perpendicular to the axis of the tire and passing through the center of the belt region. In practice, during building of the tire, the green tire (GREEN TIRE) is blown in the tire curing mold, stretching the fibers in the equatorial plane.

In addition, efforts have been made to reduce the rolling noise of tires, which has become a more significant source of noise, particularly due to the introduction of electric vehicles. It is believed that this noise can be further reduced by selecting an appropriate vibration resistant material, such as a steel cord-rubber composition with high damping.

Accordingly, the inventors sought to design steel cords with sufficient elongation without compromising other properties (e.g., strength). In this regard, the following solutions exist:

(a) The open cord is of the 1 x n type made of "n" individual filaments having a larger helical preform than the filaments when in intimate contact at a given lay length. The publication of this base is patent document US 4258043. A problem with these cords is that there is a limit to the extent to which the individual filaments can be given a preform: see, for example, patent document WO2012/055677A2, which describes an open cord that has been preformed to a limit. Another problem is that the cord is pulled closed during the process, i.e. when the cord is pulled from the drum during calendering of the rubber. Other publications are patent document WO2020/021006A1 and patent document WO2020/021007A1;

(b) High elongation cords, for example 3 x 7 cords, i.e. steel cords comprising three strands, each comprising 7 filaments, each twisted with a relatively short lay length and in the same direction (so-called forward twist) (patent document WO2019/086929 A1). Other examples are 3×4 or 3×3 (patent document WO2015/014639 A1). These cords exhibit excellent impact resistance. However, these cords do not always achieve the desired elongation properties.

(C) Hybrid cords have also been proposed in which a steel wire (patent document WO2013/098738 A1) or a multi-strand steel wire (patent document US2005/0183808A1, patent document WO2005/014925A1, patent document JP 2007145125) is twisted around an organic material core. A core is added to keep the filaments or strands farther away from each other than can be achieved with conventional open cords (a). These cords may exhibit the disadvantage that fretting corrosion may occur between different materials due to the presence of water in the organic material core.

Thus, there remains a need to develop a cord made of a single material (without "mixing"), preferably steel, which can resist closure of the cord during calendering and still exhibit sufficient elongation during building of the tire and when incorporated into the tire.

Disclosure of Invention

It is therefore an object of the present invention to solve the problems of the prior art. According to a first aspect of the present invention, there is provided a steel cord having excellent elongation properties before breaking. At low tension, the steel cord exhibits a sufficiently high but not excessively high stiffness in tension, which rises sharply when the structural elongation is reached. More particularly, a steel cord is described having an unusually high structural elongation, for example higher than 3%. In a second aspect of the invention, a method of manufacturing such a steel cord is provided.

According to a first aspect of the invention as defined in claim 1, a steel cord for reinforcing rubber products is proposed. The steel cord comprises two or more steel elements twisted together. The steel element may consist of one steel wire. Alternatively, the steel element may comprise more than one steel wire twisted around each other or bundled without twisting. A wire is a wire that cannot be unwound further into other wire-like objects. The total number of filaments in the steel cord is denoted by "N". Each wire has a cross-sectional area "a" expressed in square millimeters. It should be noted that different wires in the steel cord may have different cross-sectional areas due to production variations. If this is the case, the cross-sectional area "A" is the average cross-sectional area of all "N" filaments. If all wires are equal or about equal, the average becomes the area of the cross-section of the individual wires.

The steel element can be removed from the steel cord, unwound, i.e. the steel element is "individualized". The steel element has a centre line. When the steel element is a wire, the centre line is a line formed by connecting the centroids of the perpendicular cross-sections of the wires. When the steel element comprises two or more wires, the centre line is formed by connecting the centroids of the vertical cross-sections along the length of the steel element. The centroid is the average of all positions in the vertical cross section of the steel element.

After "singulation" the centre line of the steel element exhibits a spiral shape. The spiral shape is a three-dimensional curve with constant curvature and torsion. It is a locus of points rotating at a constant angular velocity about an axis at a constant distance from the axis and having a constant linear velocity along the axis (i.e. "spiral axis"). The spiral is fully defined by the spiral radius and the spiral pitch length. The helical pitch length is the axial distance along the helical axis, wherein one complete rotation about the helical axis is depicted on the centerline. This helical pitch length is denoted by "L _o" hereinafter. On one turn of the spiral, the centre line itself has a length "S" expressed in millimeters. It is clear to the person skilled in the art that "S.gtoreq.L _o" is true at all times, the equation being true when the wire is straight. It is noted that the helical pitch length "L _o" and the helical centre line length "S" must be obtained by measuring the steel elements which have been singulated. When these parameters of the steel elements are measured indirectly on the basis of the cross section of the steel cord by determining the spiral radius and the lay length, the possible influence of these steel elements on each other is abandoned.

The centre line of the steel element must be measured with each wire in said steel element being subjected to a tension of half newton. This is to ensure that the spiral shape is measured under normalized tension. Thus, if the steel element is a single wire, 0.5 newtons is applied axially to determine "S" and "L _o". If the steel element comprises two wires, a tension of 1 newton is applied, and so on. The inventors noted that since the measurements of "S" and "L _o" are not particularly sensitive to the type of wire used, some variation in the test force is allowed.

From the known amounts N, A and S of steel cords, the amount "P" can be calculated:

Where "pi" is the ratio of the circumference to the diameter of any circle in the plane and "E" is the tensile modulus of the material from which the wire is made, which in the case of steel and for the purposes of the present application is about 200000N/mm ². From this, it follows that "P" has the dimension of a force and can therefore be expressed in newtons.

The amount "P" is a measure of the tensile stiffness of the steel cord at low elongation. "tensile stiffness" is the proportionality constant between the applied elongation and the generated force (in newtons). "Low elongation" is an elongation that is much lower than the elongation of the "structure" (see below). Note that for the purposes of the present application, the "elongation" indicated hereinafter by "∈" refers to the ratio "Δl/l" of the amount of elongation to the initial length, which is not expressed in percent. If "elongation" is expressed in percent, it will be indicated.

The quantity "P" is not a random set of parameters. It combines the bending stiffness and torsional stiffness of the spiral geometry "S" and the "N" wires themselves through the cross-sectional area "a" with the material properties "E". It is not readily derived using the karst 'An Nuodi two Theorem (Castigliano's Second theory) under virtual work considerations for bending, twisting and stretching of helical wires.

When "P" is greater than 50 newtons, the steel cord of the present invention exhibits the desired properties of having a sufficiently large tensile stiffness at low elongation. Possibly, "P" is higher than 70, 85 or 90 newtons. If "P" is greater than 300 newtons, or even greater than 250, 200, 150 or 125 newtons, the initial tensile stiffness becomes too great and the steel cord becomes less useful for its purpose. Preferably, the "P" value is between 85 and 125. Such steel cords exhibit sufficient elongation capability for zero degree applications (zero-degree application) and at the same time create sufficient bending stiffness in the belt of the tire. The inventors speculate that the increased stiffness of the belt may have a positive effect on the noise generation of the tire.

Any type of steel may be used for the wire as long as it can be obtained in the form of a wire. Typical steels are low carbon steels with carbon contents between 0.04 and 0.20 mass%. Alternatively, stainless steel may be used. The minimum chromium content of the stainless steel was 11%. More preferably, the steel of the wire is made of high carbon steel with the following typical composition: the minimum carbon content is 0.65%, the manganese content is 0.40% to 0.70%, the silicon content is 0.15% to 0.30%, the maximum sulfur content is 0.03%, the maximum phosphorus content is 0.30%, all percentages are weight percentages. Only trace amounts of copper, nickel and/or chromium. Typical steel tire cord compositions for high tensile steel cords have a minimum carbon content of about 0.80 weight percent, for example 0.78 to 0.82 weight percent.

The tensile strength of the filaments is at least 2000N/mm ², or even higher than 2500, 2700 or higher than 3000N/mm ². Since the filaments deform very severely, the ductility of the filaments is important. The wire should therefore be able to twist at least 200 times over a length of 1 meter before breaking. Even better the wire is able to withstand more than 250, 300, 350 or 400 twists over a length of one meter.

The steel wire may be coated with a metal or metal alloy that promotes adhesion to an elastomer such as rubber. Particularly preferred alloys for achieving 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. Zinc metal is also a viable substrate to which rubber can adhere.

In another preferred embodiment, the filaments have an equivalent diameter "d" such that "a = pi d ²/4", and wherein the dimensionless ratio "S/d" is less than 30, or even less than 25. If the ratio becomes higher than 30, then

(A) The wire becomes too thin to resist bending;

(b) In the case of a limited number of threads "N", the threads become too thin to reach a sufficient breaking load;

According to another preferred embodiment, the ratio "L _o/S" is less than 0.95, or even less than 0.945. The subscript "o" refers to the "open state" of the steel cord. By "open state" is meant that the steel elements are independent, loose from each other, movable relative to each other. This ratio is important because the inverse of this ratio represents how much "extra length" is available for the singulated steel elements before they are completely flat. The upper limit of 0.95 means that the singulated steel elements have at least 5.26% of the available "extra length" (S-L _p)/L_o) before being completely flat under a sufficiently large load.

When the steel cord is loaded under the boundary conditions of the limited end of rotation, the steel wires in the elements will come into contact with each other as the helical steel elements twist around each other. In this "closed state" the steel filaments in the steel cord are in contact with each other and in interference with each other. The wire cannot be further straightened. In the "closed state" the wire will exhibit a pitch length "L _c" in millimeters. The ratio "L _c/S" is an indicator of the degree to which the yarn straightens. The ratio should be as high as possible, i.e. as close to 1 as possible. "L _c/S" is preferably greater than 0.98, or even better greater than 0.985. By using a smaller number of wires of "N" and wires having a smaller diameter "d" relative to "S", a large "L _c/S" ratio is obtained.

In another preferred embodiment, "structural elongation" ∈ ₀ "is greater than 3.5% and less than 10%. For the purposes of the present application, "structural elongation" ∈ ₀ "is defined as (L _c-L₀)/L_o. Even more preferably, it is greater than 3.75, or 4.00, or 4.25, or 4.50%, or 4.75%, or even greater than 5.0%. The inventors believe that steel cords with structural elongation up to 10% can be achieved.

According to another preferred embodiment of the application, a steel cord is claimed having a predetermined force at the structural elongation "∈ ₀": the force required to extend, stretch, the steel cord to the structural elongation "∈ ₀" is greater than 50N but less than 100N. Within this relatively narrow window, the tensile stiffness at low elongation is well suited to the present application. If the force at "∈ ₀" is below 50N, the cord is initially too weak and does not provide sufficient reaction force in the low elongation region. If the force at "∈ ₀" becomes greater than 100N, the cord becomes too stiff and does not easily lengthen during building of the tire.

In another preferred embodiment the number of steel elements is two, three or four. More than four are not recommended because these steel elements block each other when they are drawn into alignment, i.e. the "L _c/S" value becomes too low, i.e. it is difficult to achieve the desired structural elongation.

Accordingly, the number of wires within one steel element is one of one, two or three. Namely: the steel element may have a different number of filaments within one steel cord. Preferably, the number of steel wires in each steel element is one, two or three, i.e.: all steel elements have the same number of steel wires.

Preferably, the total number of threads "N" is 2 to 8, including boundaries 2 and 8.

According to a second aspect of the invention, a method of manufacturing a steel cord is described, the method comprising the steps of:

(a) A plurality of steel elements having a diameter d _e are unwound from a reel. These steel elements can be prepared by twisting the steel wire and winding it onto a reel. Alternatively, the steel elements are one or more steel wires wound on a reel without twisting;

(b) Next, a "heart wire" is provided. The core is a means, tool, to provide the correct preforming of the steel element. The core wire has a diameter "D";

(c) The steel element is then twisted around the core wire in the cord twisting direction, with a number of turns of "N _c" times per unit length. Thereby obtaining an intermediate cord. In this intermediate cord, the steel element has obtained a certain degree of plastic deformation, thus obtaining a helix;

(d) Thereafter, the core wire is removed from the intermediate cord by unscrewing and twisting the core wire from the intermediate cord;

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

In a preferred variant of the method, the resulting steel cord comprises in total "N" steel wires, each of which has a cross-sectional area "a" expressed in square millimeters, the steel element having a centre line after singulation and under tension of half newtons, said centre line having a spiral shape, wherein the centre line of the steel element has a length of "S" millimeters over one pitch, such that the amount "P" expressed in newtons:

Greater than 50 newtons, and wherein "E" is the modulus of the steel.

Conventionally, in the present application: the sign of the number of twists indicates the direction of twisting: if "N _c" is a negative number, this corresponds to twisting "N _c" times in the "S" direction. Conversely, if "N _c" is a positive number, this corresponds to twisting "N _c" times in the "Z" direction.

The presence of the heart wire is important to the process. The core wire is a moving forming pin, moving thorn, mandrel around which the steel element is plastically formed. In prior art methods for manufacturing open cords, each filament is preformed by guiding the filament over a pin. This technique has limitations in the ability to shape the wire because the wire may be overstrained during preforming, resulting in transverse cracking and lower fatigue life. On the other hand, when manufacturing an elongated cord made of different strands preformed by virtual twisting, there is a limit to the degree of preforming that can be given. The presence of the heart wire greatly expands the limits of the degree of preforming that can be given.

Preferably, in step (c), the steel element is wound around the core, i.e.: the core wire has a central position relative to the intermediate cord.

Preferably, in step (c), the intermediate cord has been treated to obtain zero or near zero residual twist. This may be accomplished by means of one or more straighteners or one or more virtual twisters. This is a procedure known to those skilled in the art.

The method may be performed by first winding the intermediate wire onto an intermediate wire reel after step (c) and unwinding from the intermediate reel in a next step to another device performing step (d).

The method can also be performed without intermediate winding, i.e.: the intermediate cord is directed from the device performing step (c) directly to the device performing step (d) without the use of an intermediate spool.

In another embodiment detailing step (d), the intermediate cord is moved linearly, for example by unwinding from an intermediate reel or by moving directly from the device performing step (c) to the device performing step (d). The core wire is "turned out" of the intermediate cord by a freewheel ring which rotates relatively around the intermediate cord, leaving the steel element as a steel cord. In this way, the intermediate wire is separated into the final steel cord and the core wire. The steel cord and the core are wound on separate driven drums.

It is noted that in step (d) the twisting of the cord is not changed. That is, the cord is not twisted open and then closed again to the final step in order to easily remove the core wire. The inventors' experience is that this procedure does not work properly because of the loss of elongation of the steel cord due to repeated twisting of the filaments.

The flywheel ring rotates about the intermediate cord relative thereto. There are two extreme cases: (i) The intermediate cord is forced to rotate about its axis and pulls the core wire on a driven drum with a stationary axis by a stationary flyer ring, or (ii) the intermediate cord does not rotate and turns out the core wire at the same lay length of the steel cord by a flyer ring rotating around the cord. (i) Mixing of (i) and (ii) is also possible, but more complex and therefore less preferred.

A preferred embodiment of the method is that both the mandrel reel and the take-up reel have a stationary axis. This can be achieved by placing the driven take-up reel on a support of a wire harness machine having a flywheel ring rotating around the support. Namely: the driven take-up reel is positioned in the flywheel ring. The core wire is led out of the intermediate cord at the entrance of the wire-tying machine through a flyer ring and wound on a driven outer mandrel drum. The steel cord runs straight at the entrance of the wire harness machine and is wound on a take-up reel. Since no plastic torsion is introduced during operation, the preforming of the steel element is maintained and thus the elongation properties remain optimal.

This situation can also be reversed, although this is somewhat less preferred. In this case, the driven mandrel drum is located on a support about which the flywheel ring rotates, and the take-up drum is located outside the support. Since steel cords are generally somewhat more difficult to handle, it may be somewhat difficult to guide the steel cords out of the intermediate cords.

The diameter D of the core wire is important relative to the diameter D _e of the element, as this determines the degree of preforming that can be achieved. The degree of preforming determines the radius of the spiral. Preferably, the ratio D/D _e is between 0.5 and 2. When the steel element is a wire, it is preferred that the ratio is at the higher side, e.g. between 0.8 and 2, or even between 1 and 2, including the boundary. On the other hand, when the steel element is made of a plurality of steel wires, a ratio towards the lower side is preferred, for example between 0.5 and 1.2, or even between 0.5 and 1.0, including boundaries.

The spiral pitch length "L _o" together with the radius of the spiral determines the length "S" of the centerline. In order for the quantity "P" to be large enough, the length "S" of the centerline must be small enough. Therefore, in order to obtain the desired characteristics of the steel cord, the number of twists introduced in the steel cord must be sufficiently large. The inventors claim that the number of twists per unit length N _c in the intermediate cord must be at least 150 twists per meter. I.e. a lay length of less than 6.666.

Remarks: the helical pitch length "L _o" in the steel cord is greater than, preferably equal to, the lay length given to the steel element in the intermediate cord. In fact, the plastically deformed steel element will rebound when untwisted from the intermediate steel cord. The amount of spring back will depend on material properties such as the yield strength of the steel and the curvature given by the core. Thus, the number of twists "N _c" and the spiral radius (D _e +d)/2 applied in the intermediate cord do not allow deriving "S" and "L _o" on the final steel cord: these amounts have to be measured on the final steel cord.

The wires in the steel element are twisted together at an element twist number N _e. This number applies to the steel elements present in the intermediate cord. Hereinafter, the number of twists given to the steel element before assembly into the intermediate cord will be designated as "n _e". When assembling the intermediate cord, various techniques can be used:

The steel element together with the core material can be twisted together by means of a wire harness. In this technique, when the twisting directions of both the steel element and the cord are the same, the number of twists "N _e" in the steel element is added to the number of twists "N _c" during formation of the cord. Thus, N _e is equal to "N _e+N_c". This results in a first preferred embodiment wherein the steel element is twisted into a steel element having a number of turns N _e in the cord twisting direction, said number of turns being greater than said number of cord turns N _c.

In a preferred embodiment, the total number of twists "N _e" obtained from the wire is greater than 200, or greater than 250, 300 or even greater than 350 twists/meter.

When the steel element is provided in the form of a bundle, the number of twists N _e eventually present in the steel element is equal to N _c. Within the scope of the present application, a "bundle" refers to a group of parallel wires that are not twisted together (i.e. "n _e" is zero).

In another preferred embodiment, the filaments in the steel element are first twisted to a twist number "n _e" in a direction opposite to the direction of cord twist and then twisted together with the core. The number of twists per meter N _e obtained in the steel element is then N _e+N_c, which means that the filaments in the steel element of the steel cord are twisted to an absolute number of element twists |n _e | which is smaller than the absolute number of cord twists |n _c |.

In the extreme case where the number of twists "N _e" in the steel element is equal to the number of cord twists "N _c" but opposite thereto, the resulting "N _e" can be made arbitrarily small, for example less than 10 twists per meter or even zero twists per meter.

The latter two embodiments have the additional advantage that they have obtained some plastic deformation into a spiral shape, since the steel elements are disassembled when they enter the intermediate cord. This results in a very loose structure, further contributing to increased structural elongation, without giving longitudinal stiffness at low elongation.

In an alternative method, the steel element and the core wire may be twisted together by means of cabling. In this technique, the number of turns "N _e" of the filament prior to assembly is substantially unchanged, i.e. "N _e" remains equal to "N _e", when twisted into an intermediate cord. When the steel elements are bundled, i.e. "N _e" is zero, the last described embodiment is obtained, i.e. "N _e" is less than 10 twists per meter or zero. For this embodiment, the steel elements appear as parallel wires on the drum before being twisted into the intermediate cord. For example, three reels with two wires each thereon may be twisted in a middle cord with three steel elements and a core wire therein.

As is clear from the above, the core wire has only a temporary function and is removed from the intermediate cord. The core wire may be one of the group comprising or consisting of metal wires, steel cords, organic yarns, organic cords, organic filaments. Since the number of cord twists given to the steel element may be very high, the core wire is strongly axially compressed, especially when using a wire-binding process. Thus, organic yarns, organic cords or organic threads may be advantageous. When wire is used, compression can be overcome by applying a large tension to the wire prior to entering the wire bonding machine.

Drawings

Figure 1 shows the geometric elements of the helix that are important for understanding the invention;

FIG. 2 shows a typical load elongation diagram of a steel cord and an individualized steel element indicating its differences;

fig. 3a and 3b show a first method of manufacturing the steel cord of the present invention;

FIG. 4 shows a second method of manufacturing the steel cord of the present invention;

Fig. 5 shows a load elongation diagram of the samples in table 1.

The figures have reference numerals, with units and tens digits referring to like features in the drawings and hundreds digits referring to the numbers of the drawings.

Detailed Description

A first manufacturing method is shown in fig. 3a and 3 b. In a first step, as shown in fig. 3a, the intermediate cord 304 is made by means of an external wire-bonding machine, which is a wire-bonding machine with a take-up reel outside the machine and a pay-off reel on a stationary support 326. A plurality of steel elements (four in this example) are unwound from reels 330 mounted on brackets 326. The heart wire 312 is fed from the spool 314', from the flywheel ring inlet, through the first flywheel ring 320, to the flywheel ring outlet. The core material may be, for example, a metal steel wire having a diameter of 0.30 to 0.40 mm. On the brackets 326, steel elements are engaged with the core 312. After entering the second flywheel ring of the wire bonding machine 322, the steel elements are twisted together at a twist number "N _c/2" that is half the cord twist number. At the outlet of the second flywheel ring 322, the steel element attains its final number of turns "N _c".N_c, typically between 50 and 250.

To remove residual twist from the steel elements twisted about the core, the assembly is fed through a false twister 324 to plastically over-twist the steel elements. The resulting intermediate cord 304 has little or no residual twist and is wound onto the intermediate spool 302.

The steel element comprises two or three steel wires, which are twisted together in the twisting direction with a lay length such that each meter is twisted "n _e" times. For steel elements, typical values are 25 to 150 twists per meter. The heart wire is used as a moving deformation pin, thorn, mandrel or the like around which the steel element is plastically formed. The core allows a higher degree of plastic deformation of the steel element than in conventional preforming systems. Furthermore, by using a core wire, it is also possible to deform steel elements in the form of strands. The strands cannot be deformed with, for example, preformed pins.

Thus, the final number of plastic torsions "N _e" present in the steel element after the wire-bonding step is "N _e+N_c". Typically, this number will be less than 300 twists per meter. If "n _e＝-N_c", i.e. the steel element is made with the same number of turns as the cord but in the opposite direction, this number will be close to zero.

In a further step of the method, as shown in fig. 3b, the intermediate steel cord 304 is unwound from an intermediate reel 302 on a support 309 of an external wire harness machine. At the entrance of the flyer ring 306, the core wire 312 is separated from the intermediate cord 304 by rotating it out of the cord. The core wire unwinding speed (twist/second) is equal to the linear speed (meters/second) times "N _c" (twist/meter). The core wire is guided over the flyer ring 306 and wound onto the driven take-up reel 314. The steel cord 308 released from the core is wound on a take-up reel 310 mounted on the support. Precautions must be taken to bring the amount of rotation of the heart wire into line with the exit velocity.

In an alternative embodiment of the method shown in fig. 4, the use of an intermediate spool is made superfluous by feeding the intermediate cord 404 made on an external cord breaker for forming the intermediate cord directly to an internal cord breaker for winding up the steel cord 410. The intermediate cord 404 is formed by unwinding the core wire 412' from the spool 414', passing through the first flywheel ring 420, adding four steel elements from the spool 430 to the core wire 412', thereby forming the intermediate cord after guiding it through the second flywheel ring 422 and the virtual twister 424. The intermediate cord is fed into an internal wire bonding machine, wherein the centerline 412 is wound from the intermediate cord 404 at the entrance of the freewheel ring 406. The core is then wound onto a mandrel take-up reel 414. The heart wire can be reused in the next production cycle.

Turning now to product performance, fig. 1 shows the basic geometrical elements of the main product claim. Fig. 1 shows a singulated steel element 100 having a helical shape with a Z-axis as the helical axis. The helix has a helix pitch length denoted by "L ₀". The steel element has a cross-sectional area indicated by "a". The steel element has a centerline (indicated by dashed lines) with a centerline length "S".

The cross-sectional area of the wire can be easily obtained by measuring the diameter of the wire and calculating the surface area. The number "N" of threads can be obtained by counting them.

The length "S" of the centre line can be determined by means of an axial scanning device as described in patent document WO95/16816 or a similar device IM6000 as obtained from KEYENCE. The apparatus comprises two axially aligned chucks spaced 100 to 500mm apart for holding the singulated steel elements at their ends during testing. By weight, a controlled tension is applied to the steel element such that each wire in the steel element is subjected to a tension of half newtons. A linear scanning device (e.g. a KEYENCE LS 3034 laser scanning system combined with KEYENCE LS 3100 processing unit) is made to travel parallel to the axis of the steel element by means of a coded high precision linear drive (precision better than ± 10 μm at a step size of 50 μm). The measurement plane of the laser scanning system is perpendicular to the Z-axis. The laser scanning system can scan the outer edge of the steel element with the accuracy of + -0.5 μm.

In a first scan at spaced equidistant discrete measurement locations "Z _j", "Δz", the lower edge and upper edge of the steel element are determined and the average of the two is used as a location along the centerline of the axis perpendicular to the Z-axis (i.e., the X-axis). In this way, the position "x (z _j)" is measured and stored in the computer. The subscript "j" is the sequence number of the sample.

The chuck is then rotated 90 deg. and the scan repeated. Now, the value "Y (Z _j)", along the Y-axis, which is perpendicular to the X-axis and the Z-axis, is measured and stored. In this way, a triplet "(x (z _j),y(z_j),z_j)", which determines the shape of the centre line of the steel element, is obtained, the curves (z _j,x(z_j) and (z _j,y(z_j)) being similar to the cosine and sine functions of z _j, since the shape is essentially a spiral. The beginning of the first turn and the end of the last turn can be determined and this is the axial length "l" over which there are "n" helical pitches. The axial length covers "m" measurement points.

Now, by summing the "m-1" measured segments, the total length "s" of the centerline over the axial length "l" can be calculated:

Thus, the length "S" of the centerline of the steel element over one pitch is equal to "S/n". In the same manner, the helical pitch length "L _o" is equal to "L/n". Since the number of turns "n" measured is easily greater than 50 or even 100, the values "S" and "L _o" are averages over a large number of turns.

The relationship between the geometric parameters "L _o,L_c, S" and the load elongation diagram is shown in FIG. 2. The load elongation curve of the steel cord according to the invention is shown as 202. Parallel to the elongation axis 206, the axial length of one helical pitch "L" as a function of applied load F (ordinate axis, 210) is shown on axis 208. When the steel cord is at very low measured tension, the axial length of the single helical turn is L _o and the elongation e is 0."∈" is associated with "L" by:

∈＝(L-L_o)/L_o

When a tangent line (shown in phantom) is drawn to the straight portion of the curve, the tangent line may extend toward the elongation axis 206. The intersection corresponds to the structural elongation e ₀, since at this point the steel element closes and reaches the corresponding helical pitch "L _c". It can be demonstrated that this point does correspond to the closure of the steel element by assuming that the steel element has an increased modulus: when the slope of the tangent rises to vertical, all the corresponding tangent curves will pass this point (0, ∈ ₀). When the steel cord is further stretched beyond ∈ ₀, the ratio (L/S) remains constant, while both "L" and "S" increase further due to elongation of the steel.

For the purposes of the present application, the ratio (L _c/S) is calculated as:

where "d" is the equivalent diameter of the steel wire.

When now considering the steel element after singulation, a curve similar to 204 is obtained. But here corresponds to a fully stretched helix, i.e. a helix having a length "S", intersecting the "L" axis. The elongation was then (S-L _o)/L₀).

In a series of experiments, samples were prepared according to the methods described in fig. 3a and 3 b. The results for these samples are summarized in table 1.

(A) The first column is the reference number of the curve in fig. 5 showing the load elongation curve of the mentioned structure;

(b) The "structural" column is an indication of "intermediate cord", wherein the first digit indicates the diameter of the core wire, followed by the arrangement of the steel elements. If for intermediate cords detached from strands, this is indicated with brackets (..). For example, 0.4+2× (2×0.225) represents 2 steel elements, each comprising two wires of diameter 0.225 which have been twisted around a 0.4mm diameter wire.

(C) The column "N _e" indicates the number of twists (t/m) per meter of steel wire obtained in the intermediate cord.

(D) The column "N _c" indicates the number of twists (t/m) per meter of steel element obtained in the intermediate cord.

(E) The ratio "D/D _e" is the ratio between the diameter of the core wire and the diameter of the steel element;

Note that: the twisting direction of the filaments in the steel element is the same as the twisting direction of the steel element in the steel cord and in the "S" direction.

In the final steel cord, i.e. the steel cord from which the core wire has been removed, the following different geometrical and mechanical properties are obtained:

(a) "N" is simply the number of steel filaments in the steel cord;

(b) "A x 1000" is the cross-sectional area in mm ² times 1000 for a single wire;

(c) "S" is the length of the centerline in one helical pitch according to the described measurement procedure;

(d) "L _o" is the axial length of one helical pitch according to the described measurement procedure;

(e) "P" is an amount calculated according to the definition of the claims;

(f) "S/d" is the ratio of "S" divided by the equivalent diameter of the wire;

(g) "L _o/S" and "L _c/S" are ratios of the indicated amounts;

(h) "∈ ₀" is the structure elongation determined by the flow of FIG. 2.

(I) "F (∈ ₀)" is the force at the structural elongation from the load elongation plot.

Fig. 5 shows the different load elongations of the prepared samples. Samples 522, 523, 503 show the most preferred characteristics in the use of steel cords. Less preferred, but still very useful, load elongation curves are shown by samples 507, 521. Other samples 524, 514 and 509 are not preferred.

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Claims

1. A steel cord for reinforcing rubber products, said steel cord comprising two or more steel elements twisted together, said steel element comprising one or more steel filaments, said steel cord comprising a total of "N" steel filaments, each of said steel filaments having a cross-sectional area "a" expressed in square millimeters, said steel element having a centre line after singulation and under tension of half newtons in said steel element, said centre line having a helical shape with a helical pitch length "L _o" in millimeters, wherein the length of the centre line of said steel element on one pitch is "S" millimeters,

The amount "P" expressed in newtons:

Greater than 50 newtons, and wherein "E" is the modulus of the steel.

2. A steel cord according to claim 1, wherein the steel wire has an equivalent diameter "d" defined by a = pi d ²/4, where "S/d" is less than 30.

3. A steel cord according to claim 1 or 2, wherein the ratio "L _o/S" is less than 0.95.

4. A steel cord according to any one of claims 1 to 3, wherein the steel filaments of the steel cord have a pitch length "L _c" in millimeters when in a closed state, wherein the ratio "L _c/S" is greater than 0.98.

5. The steel cord according to claim 4, wherein the structural elongation "∈ ₀" defined as (Lc-L0)/Lo is greater than 3.5% and less than 10%.

6. The steel cord according to any one of claims 1 to 5, wherein the force at the structural elongation "∈ ₀" is greater than 50N and less than 120N.

7. The steel cord according to any one of claims 1 to 6, wherein the number of steel elements is two, three or four, and wherein the number of steel wires within one steel element is one, two or three.

8. The steel cord according to any one of claims 1 to 7, wherein the number "N" of steel filaments is 2 to 8 and comprises 2 and 8.

9. A method for manufacturing a steel cord, the method comprising the steps of:

(a) Unwinding a plurality of steel elements of diameter d _e from a reel;

(b) Providing a core wire having a diameter D;

(c) Twisting the steel element around the core wire in a cord twisting direction at a cord twisting number of N _c times per unit length, thereby forming an intermediate cord;

(d) Removing the mandrel wire from the intermediate cord by unscrewing the mandrel wire from the intermediate cord, thereby producing a steel cord;

(e) The steel cord is wound on a take-up reel.

10. The method of claim 9, wherein the intermediate cord is wound on an intermediate reel after step (c) and unwound from the intermediate reel to perform step (d).

11. The method of claim 9, wherein the intermediate cord is directed from step (c) to step (d).

12. The method according to any one of claims 9 to 11, wherein the step of removing the core wire from the intermediate cord by unscrewing the core wire from the intermediate cord is performed by the steps of:

-linearly moving the intermediate cord;

-unscrewing the core wire from the intermediate cord by means of a freewheel ring rotating relatively around the intermediate cord, leaving the steel element as a steel cord;

-winding the core on a driven mandrel reel;

-winding said steel cord on a driven take-up reel.

13. The method of claim 12, wherein the driven take-up reel is within the flywheel ring or the driven mandrel reel is within the flywheel ring.

14. The method according to any one of claims 9 to 13, wherein the steel element is a steel wire and the ratio D/D _e is greater than or equal to 0.8 and less than or equal to 2.

15. The method of any one of claims 9 to 14, wherein the steel element is a plurality of steel wires and the ratio D/D _e is greater than or equal to 0.5 and less than or equal to 1.2.

16. The method according to any one of claims 9 to 15, wherein the steel wires in the steel elements of the intermediate cord are twisted into steel elements having a number of twists N _e in the cord twisting direction, the number of element twists being greater than or equal to the cord twisting number N _c, and wherein the cord twisting number N _c is greater than 150 twists/meter.

17. The method according to claims 9 to 16, wherein the steel wires in the elements of the steel cord are twisted to an element twist number N _e in the cord direction, the element twist number being smaller than the cord twist number.

18. The method of any one of claims 9 to 17, wherein the heart wire is one of the group comprising: metal wire, steel cord, organic yarn, organic cord, organic thread.

19. The method according to any one of claims 9 to 18, wherein the total number of steel wires "N" is a combination, wherein each of the steel wires has a cross-sectional area "a" expressed in square millimeters, the steel element has a centre line after singulation and under tension of half newtons, the centre line having a spiral shape, wherein the centre line of the steel element has a length of "S" millimeters on one pitch, such that the amount "P" expressed in newtons "

Greater than 50 newtons, and wherein "E" is the modulus of the steel.