MULTI-STRAND CABLE WITH TWO MULTI-STRAND LAYERS

Description

The invention relates to cords and to a tyre comprising these cords.

Cords of structure 1×N are known from the prior art, as described in document WO2016/131862. These cords comprise a single layer of N=4 strands wound in a helix at a pitch 20 mm. Each strand comprises, for the one part, an internal layer of 3 internal threads wound in a helix at a pitch p1=6.7 mm and an external layer of 8 external threads wound in a helix around the internal layer at a pitch p3=10 mm. The structural elongation of the cord is 2.8% and the diameter of the cord is 3.8 mm, the linear mass being 36.4 g/m, and the endurance criterion equal to 3635 N×m/g.

These cords have the advantage of having a relatively high structural elongation but there is scope for improvement of the endurance criterion in order to increase the endurance performance of the reinforcers while at the same time reducing the shear in the polymer matrix.

Nowadays, there is an emerging need to develop new cords for application in crown plies, particularly zero-degree plies such as hooping crown plies. The purpose of these plies is to hoop the tyre in order both to reduce shear at the edge of the ply and to reduce the stiffness of the crown block at the centre with respect to attack.

The object of the invention is a cord that offers sufficient flexibility and structural elongation to allow the building of the tyre and reduce the stiffness of the crown block, with an improved endurance criterion to withstand cyclic tensile stress loadings.

To this end, one subject of the invention is a multi-strand cord with two layers of multi-strand elements, wherein the cord comprises:

• • a cord internal layer made up of X=1 multi-strand element comprising K>1 strands wound in a helix about a main axis, each strand having at least two layers comprising:
  • an internal layer made up of Q1 internal metal thread(s) of diameter d1, and
  • an external layer made up of Q3 external metal threads of diameter d3 wound around the internal layer, and
  • a cord external layer made up of Y>1 multi-strand elements wound around the internal layer of the cord, each multi-strand element comprising L>1 strands wound in a helix about an axis, each strand having at least two layers comprising:
  • an internal layer made up of Q1′ internal metal thread(s) of diameter d1′, and
  • an external layer made up of Q3′ external metal threads of diameter d3′ wound around the internal layer,
    the multi-strand elements being wound in a helix about the main axis,
    the cord having an endurance criterion V1=Δσ_bending/(M/D)<3000 N×m/g; where

Δσ bending = Msteel × Max ⁡ ( di ; di ′ ) 2

in MPa·mm is the maximum bending stress per unit curvature experienced by the internal and external threads of the internal and external strands, where di and di′ are the diameter of the metal threads and i and i′ range from 1 to 3 and where Msteel=200 000 MPa;

• • M is the linear mass of the cord in g/m, M being the sum of the cross sections of metal of all the metal threads in the cord multiplied by the density of steel, Rho, where Rho=7.79 g/cm3;
  • D is the diameter of the cord (50) in mm; and
    the cord (50) has a structural elongation As such that As ≥1.0%, the structural elongation As being determined by applying the standard ASTM D2969-04 of 2014 to the cord so as to obtain a force-elongation curve, the structural elongation As being equal to the elongation, in %, corresponding to the intersection between the tangent to the elastic portion of the force-elongation curve at some point along the elastic portion thereof and the elongation axis of the force-elongation curve.

Thanks to this multi-strand cord configuration with two layers of multi-strand elements, the cord according to the invention makes it possible to obtain a cord with a sufficient mass of metal while at the same time keeping the threads slender to make it possible to achieve improved endurance performance and thus improve the trade-off between shear in the polymer matrix and endurance performance of the cord and improve resistance to splitting.

On the one hand, by virtue of its relatively low endurance criterion, the cord according to the invention makes it possible to reduce the levels of stress in the cord subjected to tensile stress loading and therefore to extend the life of the tyre. Specifically, the inventors behind the invention have discovered that the first determining criterion for improving the endurance performance of a cord in a corrosive environment was not only the force at break, as is widely taught in the prior art, but also the endurance criterion, which is represented in the present application by an indicator equal to a combination of bending stress, cord diameter and mass of metal in the cord:

• • if the bending stress per unit curvature experienced by the internal and external threads of the strands: Δσ_bending is the maximum bending stress per unit curvature experienced by the metal threads, in this type of cord, the inventors behind the invention have discovered that the tensile stress-loading of the cord generated both tensile stresses and bending stresses in the individual threads at the same time; hence reducing this bending stress criterion therefore has a positive impact on the tensile endurance performance of the cord by relieving it of the share of the loading caused by the bending;
  • the mass of metal in the cord divided by the diameter of the cord which, if increased, chiefly enables the cord to be relieved of the tensile stresses:
    where M is the linear mass of the cord in g/m, which can be defined in a simplified manner as being the sum of the cross sections of metal of all the individual metal threads in the cord multiplied by the density of steel, Rho=7.79 g/cm3, where the sum of the cross sections is determined by image processing on a cross section of the cord; and where the diameter of the cord D is measured on the cord in accordance with standard ASTM D2969-04.

By definition, the diameter of the cord is the diameter of the smallest circle inside which the cord without the wrapper can be circumscribed.

The structural elongation As, which is a parameter well known to a person skilled in the art, is determined for example by applying the standard ASTM D2969-04 of 2014 to a cord tested so as to obtain a force-elongation curve. As is derived from the curve obtained as being the elongation, in %, corresponding to the point of intersection between the tangent to the elastic portion of the force-elongation curve and the elongation axis of the force-elongation curve. It will be recalled that a force-elongation curve comprises, in the direction of increasing elongations, a structural portion, an elastic portion and a plastic portion. The structural portion corresponds to a structural elongation of the cord that results from the moving-together of the different strands and metal threads that make up the cord. The elastic portion corresponds to an elastic elongation that results from the construction of the cord, in particular of the angles of the various layers and of the diameters of the metal threads. The plastic portion corresponds to the plastic elongation that results from the plasticity (irreversible deformation beyond the elastic limit) of the metal threads.

In the invention, the cord comprises two layers of multi-strand elements, which means to say that it comprises an assembly made up of a layer of Y>1 multi-strand elements, wound around a single layer of multi-strand elements, neither more nor less, which means to say that the assembly has two layers of multi-strand elements, not one, not three, but only two.

In the invention, the multi-strand element has one layer of strands, meaning that it comprises an assembly made up of one layer of strands, neither more nor less, meaning that the assembly has one layer of strands, not zero, not two, but only one.

In one embodiment, the internal multi-strand element of the cord is surrounded by a polymer composition and then by the external layer.

Advantageously, each strand has cylindrical layers.

Advantageously, each strand in the multi-strand element has two layers, meaning that it comprises an assembly made up of two layers of metal threads, neither more nor less, meaning that the assembly has two layers of metal threads, not one, not three, but only two. The external layer of each strand is wound around the internal layer of this strand in contact with the internal layer of this strand.

Highly advantageously, each strand of the internal layer and each strand of the external layer have cylindrical layers. It will be recalled that such cylindrical layers are obtained when the various layers of strands are wound at different pitches and/or when the directions of winding of these layers differ from one layer to the other. A strand with cylindrical layers is very highly penetrable, unlike a strand with compact layers in which the pitches of all the layers are the same and the directions of winding of all the layers are the same, and which exhibits far lower penetrability.

Advantageously, each strand of the internal layer and each strand of the external layer are desaturated, which means that there is enough space between the threads of the external layer to allow an elastomer compound to impregnate each strand.

For preference, the strands do not undergo pre-shaping.

Advantageously, the cord as defined above is bare, meaning that it does not have any polymer composition; in particular the cord does not have any elastomer composition.

A metal thread is understood to be a metal monofilament comprising a core made up predominantly (that is to say more than 50% of its weight) or entirely (100% of its weight) of a metal material, for example a carbon steel. The metal thread may advantageously comprise a layer of a metal coating covering the core, the metal coating being chosen from zinc, copper, tin and alloys of these metals, for example brass. Each thread is preferably made of pearlitic or ferritic-pearlitic carbon steel.

The values of the features described in the present application for the bare cord are measured on or determined from cords directly after they have been manufactured, that is to say before any step of embedding in a polymer matrix, in particular an elastomer matrix.

In the present application, any range of values denoted by the expression “between a and b” represents the range of values from more than a to less than b (that is to say excluding the end points a and b), whereas any range of values denoted by the expression “from a to b” means the range of values from the end point “a” as far as the end point “b”, namely including the strict end points “a” and “b”.

Advantageously, As≥1.5% and preferably As≥2.0%.

Another subject of the invention is a cord extracted from a polymer matrix, wherein the extracted cord comprises:

• • a cord internal layer made up of X=1 multi-strand element comprising K>1 strands wound in a helix about a main axis, each strand having at least two layers comprising:
  • an internal layer made up of Q1 internal metal thread(s) of diameter d1, and
  • an external layer made up of Q3 external metal threads of diameter d3 wound around the internal layer, and
  • a cord external layer made up of Y>1 multi-strand elements wound around the internal layer of the cord, each multi-strand element comprising L>1 strands wound in a helix about an axis, each strand having at least two layers comprising:
  • an internal layer made up of Q1′ internal metal thread(s) of diameter d1′, and
  • an external layer made up of Q3′ external metal threads of diameter d3′ wound around the internal layer,
    the multi-strand elements being wound in a helix about the main axis,
    the extracted cord (50′) having an endurance criterion V1=Δσ_bending/(M/D)<3000 N×m/g; where

Δσ bending = Msteel × Max ⁡ ( di ; di ′ ) 2

in MPa·mm is the maximum bending stress per unit curvature experienced by the internal and external threads of the internal and external strands, where di and di′ are the diameter of the metal threads and i and i′ range from 1 to 3 and where Msteel=200 000 MPa;

• • M is the linear mass of the cord in g/m, M being the sum of the cross sections of metal of all the metal threads in the cord multiplied by the density of steel, Rho, where Rho=7.79 g/cm3;
  • D is the diameter of the cord (50′) in mm; and
    the cord (50′) has a structural elongation As' such that As' 0.3%, the structural elongation As' being determined by applying the standard ASTM D2969-04 of 2014 to the cord so as to obtain a force-elongation curve, the structural elongation As' being equal to the elongation, in %, corresponding to the intersection between the tangent to the elastic portion of the force-elongation curve at some point along the elastic portion thereof and the elongation axis of the force-elongation curve.

Preferably, the polymer matrix is an elastomer matrix.

The polymer matrix, preferably elastomer matrix, is based on a polymer, preferably elastomer, composition.

A polymer matrix is understood to be a matrix comprising at least one polymer. The polymer matrix is thus based on a polymer composition.

What is meant by an elastomer matrix is a matrix containing at least one elastomer. The preferred elastomer matrix is thus based on the elastomer composition.

The expression “based on” should be understood as meaning that the composition comprises the compound and/or the product of the in situ reaction of the various constituents used, some of these constituents being able to react and/or being intended to react with one another, at least partially, during the various phases of manufacture of the composition; the composition thus being able to be in the fully or partially crosslinked state or in the non-crosslinked state.

A polymer composition is understood as meaning that the composition comprises at least one polymer. Preferably, such a polymer may be a thermoplastic, for example a polyester or a polyamide, a thermosetting polymer, an elastomer, for example natural rubber, a thermoplastic elastomer or a combination of these polymers.

An elastomer composition is understood as meaning that the composition comprises at least one elastomer and at least one other component. Preferably, the composition comprising at least one elastomer and at least one other component comprises an elastomer, a crosslinking system and a filler. The compositions that can be used for these plies are conventional compositions for the skim coating of filamentary reinforcing elements and comprise a diene elastomer, for example natural rubber, a reinforcing filler, for example carbon black and/or silica, a crosslinking system, for example a vulcanizing system, preferably comprising sulfur, stearic acid and zinc oxide, and optionally a vulcanization accelerant and/or retarder and/or various additives. The adhesion between the metal threads and the matrix in which they are embedded is afforded for example by a metal coating, for example a layer of brass.

The values of the features described in the present application for the extracted cord are measured on or determined from cords extracted from a polymer matrix, in particular an elastomer matrix, for example of a tyre. Thus, for example on a tyre, the strip of material radially on the outside of the cord that is to be extracted is removed in order to be able to see the cord that is to be extracted radially flush with the polymer matrix. This removal can be done by stripping using cutters and grippers, or else by planing. Next, the end of the cord that is to be extracted is disengaged using a knife. The cord is then pulled so as to extract it from the matrix, applying a relatively shallow angle in order not to plasticize the cord that is to be extracted. The extracted cords are then carefully cleaned, for example using a knife, so as to detach any remains of polymer matrix locally adhering to the cord, while taking care not to damage the surface of the metal threads.

In order to determine the linear mass of the extracted cord, a cross section of the cord in the elastomer matrix is taken and the surface area of steel is determined using image processing and multiplied by the density of the steel.

In order to measure the linear mass of the extracted cord it is also possible, after the operation described hereinabove, to weigh one metre of cleaned cord in order to determine, over 10 measurements, the mean linear mass of cleaned cord.

The advantageous features described below apply both to the bare cord and to the cord extracted from a polymer matrix.

Advantageously, the criterion V1 is greater than or equal to 1000 N×m/g and preferably greater than or equal to 1500 N×m/g.

Advantageously, the criterion V1 is less than or equal to 2500 N×m/g.

Advantageously, M ranges from 30 to 150 g/m, and preferably from 40 to 120 g/m.

As a preference, the cord has a cord diameter such that D ranges from 3 mm to 9 mm, preferably from 4 mm to 7 mm.

By definition, the diameter of a strand is the diameter of the smallest circle inside which the strand can be circumscribed.

For preference, the diameters of the metal threads range, independently of one another, from 0.15 mm to 0.50 mm, preferably from 0.18 mm to 0.35 mm and more preferably from 0.20 mm to 0.30 mm.

For preference, the threads of the one same layer of a predetermined strand all have substantially the same diameter. Advantageously, the external strands all have substantially the same diameter. What is meant by “substantially the same diameter” is that the threads or the strands have the same diameter to within the industrial tolerances.

Advantageously, Y is equal to 6, 7, 8, 9 or 10, preferably Y=6, 7 or 8 and more preferentially Y=6.

Advantageously, K=2, 3 or 4, preferably K=3 or 4.

Advantageously, L=2, 3 or 4 and preferably L=3 or 4.

In a first embodiment, each strand of the internal layer has two layers.

Advantageously, each strand of the external layer has two layers.

Advantageously, in this first embodiment, in a preferred variant, each strand of the internal and external layers has two layers.

In a second embodiment, each strand of the internal layer has three layers and comprises:

• • an intermediate layer made up of Q2 intermediate metal threads wound around the internal layer, and
  • an external layer made up of Q3 external metal threads wound around the intermediate layer.

Advantageously, each strand of the external layer has three layers and comprises: an intermediate layer made up of Q2′ intermediate metal threads wound around the internal layer, and

• • an external layer made up of Q3′ external metal threads wound around the intermediate layer.

Advantageously, in this second embodiment, in a preferred variant, each strand of the internal and external layers has three layers.

Advantageously, each strand is of the type not rubberized in situ. Not rubberized in situ means that, before the strands are assembled with one another, each strand is made up of the threads of the various layers and does not exhibit any polymer composition, in particular elastomer composition.

Strands of the Internal Multi-Strand Elements of the Cord According to the Invention

Advantageously, Q1=1, 2, 3 or 4, preferably Q1=1, 2 or 3, and more preferably Q1=1 or 3.

Advantageously, Q3=5, 6, 7, 8, 9 or 10, preferably Q3=6, 7, 8 or 9, and more preferably Q3=6 or 9.

In one embodiment, Q1=1.

Advantageously, Q3=5, 6 or 7, and preferably Q3=6.

In another preferred embodiment, Q1>1, and preferably Q1=2, 3 or 4.

Advantageously, Q3=7, 8, 9 or 10, and preferably Q3=7, 8 or 9.

In a first variant, Q1=2 and Q3=7 or 8, and preferably Q1=2, Q3=7.

In a second variant, Q1=3 and Q3=7, 8 or 9, and preferably Q1=3, Q3=8.

in a third variant, Q1=4 and Q3=7, 8, 9 or 10, and preferably Q1=4, Q3=9.

Strands of the External Multi-Strand Elements of the Cord According to the Invention

Advantageously, Q1′=1, 2, 3 or 4, preferably Q1′=1, 2 or 3, and more preferably Q1′=1 or 3.

Advantageously, Q3′=5, 6, 7, 8, 9 or 10, preferably Q3′=6, 7, 8 or 9, and more preferably Q3′=6 or 9.

In one embodiment, Q1′=1.

Advantageously, Q3′=5, 6 or 7, and preferably Q3′=6.

In another preferred embodiment, Q1′>1, and preferably Q1′=2, 3 or 4.

Advantageously, Q3′=7, 8, 9 or 10, and preferably Q3′=7, 8 or 9.

In a first variant, Q1′=2 and Q3′=7 or 8, and preferably Q1′=2, Q3′=7.

In a second variant, Q1′=3 and Q3′=7, 8 or 9, and preferably Q1′=3, Q3′=8.

In a third variant, Q1′=4 and Q3′=7, 8, 9 or 10, and preferably Q1′=4, Q3′=9.

Advantageously Q1=1 and Q3=6, Q1′=1 and Q3′=6.

Reinforced Product According to the Invention

Another subject of the invention is a reinforced product comprising a polymer matrix and at least one cord or extracted cord as defined above.

Advantageously, the reinforced product comprises one or several cords according to the invention embedded in the polymer matrix and, in the case of several cords, the cords are arranged side-by-side in a main direction.

Tyre According to the Invention

Another subject of the invention is a tyre comprising at least one extracted cord or a reinforced product as defined above.

What is meant by a tyre comprising an extracted cord is a tyre comprising a cord of which the properties, measured prior to extraction from the tyre, are those of the extracted cord, this cord being, prior to being incorporated into the tyre, a cord such as the cord described hereinabove.

For preference, the tyre has a carcass reinforcement anchored in two beads and surmounted radially by a crown reinforcement which is itself surmounted by a tread, the crown reinforcement being joined to said beads by two sidewalls, and comprising at least one cord as defined above.

In one preferred embodiment, the crown reinforcement comprises a protective reinforcement, a working reinforcement and a hoop reinforcement comprising at least one cord as defined hereinabove, the protective reinforcement being interposed radially between the tread and the working reinforcement, and the hoop reinforcement preferably being interposed between the two plies of working reinforcement.

The cord is most particularly intended for industrial vehicles selected from heavy vehicles such as “heavy-duty vehicles”—i.e. underground trains, buses, road haulage vehicles (lorries, tractors, trailers), off-road vehicles—agricultural vehicles or construction plant vehicles, or other transport or handling vehicles.

As a preference, the tyre is for a vehicle of the construction plant type. Thus, the tyre has a size in which the diameter, in inches, of the seat of the rim on which the tyre is intended to be mounted is greater than or equal to 40 inches.

The invention also relates to a rubber item comprising an assembly according to the invention, or an impregnated assembly according to the invention. What is meant by a rubber item is any type of item made of rubber, such as a ball, a non-pneumatic object such as a non-pneumatic tyre casing, a conveyor belt or a caterpillar track.

A better understanding of the invention will be obtained on reading the examples which will follow, given solely by way of non-limiting examples and made with reference to the drawings, in which:

FIG. 1 is a view in cross section perpendicular to the circumferential direction of a tyre according to the invention;

FIG. 2 is a view of details of the region II of FIG. 1;

FIG. 3 is a view in cross section of a reinforced product according to the invention;

FIG. 4 is a schematic view in cross section perpendicular to the axis of the cord (which is assumed to be straight and at rest) of a cord (50) according to a first embodiment of the invention;

FIG. 5 is a schematic view in cross section perpendicular to the axis of the cord (which is assumed to be straight and at rest) of an extracted cord (50′) according to a first embodiment of the invention;

FIG. 6 is a view similar to that of FIG. 4 of a cord (60) according to a second embodiment of the invention; and

FIG. 7 is a photograph of a cord (50) according to a first embodiment of the invention.

EXAMPLE OF A TYRE ACCORDING TO THE INVENTION

A frame of reference X, Y, Z corresponding to the usual respectively axial (X), radial (Y) and circumferential (Z) orientations of a tyre has been depicted in FIGS. 1 and 2.

The “median circumferential plane” M of the tyre is the plane that is normal to the axis of rotation of the tyre and is situated equidistantly from the annular reinforcing structures of each bead.

FIGS. 1 and 2 depict a tyre according to the invention and denoted by the general reference 10.

The tyre 10 is for a heavy vehicle of construction plant type, for example of “dumper” type. Thus, the tyre 10 has a dimension of the type 53/80R63.

The tyre 10 has a crown 12 reinforced by a crown reinforcement 14, two sidewalls 16 and two beads 18, each of these beads 18 being reinforced with an annular structure, in this instance a bead wire 20. The crown reinforcement 14 is surmounted radially by a tread 22 and connected to the beads 18 by the sidewalls 16. A carcass reinforcement 24 is anchored in the two beads 18 and is in this instance wound around the two bead wires 20 and comprises a turnup 26 positioned towards the outside of the tyre 20, which is shown here fitted onto a wheel rim 28. The carcass reinforcement 24 is surmounted radially by the crown reinforcement 14.

The carcass reinforcement 24 comprises at least one carcass ply 30 reinforced by radial carcass cords (not depicted). The carcass cords are positioned substantially parallel to one another and extend from one bead 18 to the other so as to form an angle of between 80° and 90° with the median circumferential plane M (plane perpendicular to the axis of rotation of the tyre which is situated midway between the two beads 18 and passes through the middle of the crown reinforcement 14).

The tyre 10 also comprises an airtight sealing ply 32 made up of an elastomer (commonly known as “inner liner”) which defines the radially internal face 34 of the tyre 10 and which is intended to protect the carcass ply 30 from the diffusion of air coming from the space inside the tyre 10.

The crown reinforcement 14 comprises, radially from the outside to the inside of the tyre 10, a protective reinforcement 36 arranged radially on the inside of the tread 22, a working reinforcement 38 arranged radially on the inside of the protective reinforcement 36 and a hoop reinforcement 40 interposed radially between the two plies 48, 46 of the working reinforcement 38. The protective reinforcement 36 is thus interposed radially between the tread 22 and the working reinforcement 38.

The protective reinforcement 36 comprises first and second protective plies 42, 44 comprising protective metal cords, the first ply 42 being arranged radially on the inside of the second ply 44. Optionally, the protective metal cords make an angle at least equal to 10°, preferably in the range from 10° to 35° and more preferably from 15° to 35°, with the circumferential direction Z of the tyre.

The working reinforcement 38 comprises first and second working plies 46, 48, the first ply 46 being arranged radially on the inside of the second ply 48.

The hoop reinforcement 40, also referred to as a limiter unit, comprises at least one cord 50 that makes an angle at most equal to 10°, preferably ranging from 0° to 5°, with the circumferential direction Z of the tyre 10.

Example of a Reinforced Product According to the Invention

FIG. 3 depicts a reinforced product according to the invention and denoted by the general reference 100. The reinforced product 100 comprises at least one cord 50, in this instance a plurality of cords 50, embedded in the polymer matrix 102.

FIG. 3 depicts the polymer matrix 102, the cords 50 in a frame of reference X, Y, Z, in which the direction Y is the radial direction and the directions X and Z are the axial and circumferential directions. In FIG. 3, the reinforced product 100 comprises a plurality of cords 50 arranged side-by-side in the main direction X and extending parallel to one another within the reinforced product 100 and collectively embedded in the polymer matrix 102.

Here, the polymer matrix 102 is an elastomer matrix based on an elastomer composition.

Cord According to a First Embodiment of the Invention

FIG. 4 depicts the cord 50 according to a first embodiment of the invention.

With reference to FIG. 5, each marginal hoop reinforcing element is formed, after extraction from the tyre 10, by an extracted cord 50′ as described below. The cord 50 is obtained by embedding in a polymer matrix, in this instance in a polymer matrix respectively forming each polymer matrix of each working ply.

FIG. 7 depicts a photograph of the cord 50 in the polymer matrix.

The cord 50 and the extracted cord 50′ are made of metal and are of the multi-strand type with two multi-strand cylindrical layers. Thus, it will be understood that there are two layers, not more, not less, of multi-strand elements of which the cord 50 or 50′ is made.

At least 50% of the metallic threads, preferably at least 60%, more preferably at least 70% of the metallic threads, and highly preferably each metallic thread of the cord comprises a steel core having a composition in accordance with standard NF EN 10020 of September 2000, and a carbon content C>0.80% and preferably C≥0.82% and at least 50% of the metallic threads, preferably at least 60%, more preferably at least 70% of the metallic threads, and highly preferably each metallic thread of the cord comprises a steel core having a composition in accordance with standard NF EN 10020 of September 2000, and a carbon content C s 1.20% and preferably C s 1.10%. Here, each metallic thread comprises a steel core having a composition in accordance with standard NF EN 10020 of September 2000, and a carbon content C=0.86%.

Each thread has a breaking strength, denoted Rm, such that 2500≤Rm≤3100 MPa. The steel of these threads is said to be of SHT (“Super High Tensile”) grade. Other threads may be used, for example threads of an inferior grade, for example of NT (“Normal Tensile”) or HT (“High Tensile”) grade, just as may threads of a superior grade, for example of UT (“Ultra Tensile”) or MT (“Mega Tensile”) grade.

Method for Manufacturing the Cord According to the Invention

One example of a method for manufacturing the multi-strand cord 50 will now be described.

Each aforementioned internal strand T1 is manufactured according to known methods involving the following steps, preferably performed in line and continuously:

• • first of all, a first step of assembling, by cabling or by twisting, the 6 external threads F3 around the internal thread F1 of the internal layer C1 at the pitch p3 and in the S-direction to form the external layer C3 at a first assembling point;
  • preferably a final twist-balancing step.

Each aforementioned external strand T2 is manufactured according to known methods involving the following steps, preferably performed in line and continuously:

• • first of all, a first step of assembling, by cabling or by twisting, the 6 external threads F3′ around the internal thread F1 of the internal layer C1 at the pitch p3′ and in the S-direction to form the external layer C3′ at a first assembling point;
  • preferably a final twist-balancing step.

What is meant here by “twist balancing” is, as is well known to those skilled in the art, the cancellation of the residual torque (or the elastic return of the twist) applied to each thread of the strand, in the external layer.

After this final twist-balancing step, the manufacture of the strand is complete. Each strand is wound onto one or more receiving reels, for storage, prior to the later operation of assembling the elementary strands by cabling in order to obtain the multi-strand cord.

In order to manufacture the multi-strand cord of the invention, the method, as is well known to those skilled in the art, is to cable together the strands previously obtained, using cabling machines rated for assembling strands.

In a step of manufacturing the multi-strand element M1 of the internal layer C1, the K=3 internal strands T1 are assembled by cabling at the pitch P1 and in the S-direction to form the multi-strand element M1 of the internal layer C1 at a first assembling point.

In a step of manufacturing multi-strand elements M2 of the external layer CE, the L=3 external strands T2 are assembled by twisting at the pitch P2 and in the S-direction to form the multi-strand elements M2 of the external layer CE at a first assembling point.

Then, in a later manufacturing step, the Y=6 external multi-strand elements M2 are assembled by cabling around the internal layer C1 at the pitch pe and in the Z-direction to form the assembly of the layers C1 and CE. Possibly, in a last assembly step, the wrapper F is wound, at the pitch pf and in the S-direction, around the assembly previously obtained.

The cord 50 is then incorporated by calendering into composite fabrics formed from a known composition based on natural rubber and carbon black as reinforcing filler, conventionally used for manufacturing crown reinforcements of radial tyres. This composition essentially contains, in addition to the elastomer and the reinforcing filler (carbon black), an antioxidant, stearic acid, an extender oil, cobalt naphthenate as adhesion promoter, and finally a vulcanization system (sulfur, accelerator and ZnO).

The composite fabrics reinforced by these cords have an elastomer composition matrix formed from two thin skim layers of elastomer composition which are superposed on either side of the cords and which have a thickness ranging between 1 and 4 mm, respectively. The calendering pitch (spacing at which the cords are laid in the elastomer composition fabric) ranges from 4 mm to 8 mm.

These composite fabrics are then used as working ply in the crown reinforcement during the method of manufacturing the tyre, the steps of which are otherwise known to a person skilled in the art.

Cord According to a Second Embodiment of the Invention

FIG. 6 depicts a cord 60 according to a second embodiment of the invention.

Unlike in the first embodiment described hereinabove, the cord 60 according to the second embodiment is such that Q1=Q1=1; Q2=Q2=5 and Q3=Q3=10.

Table 1 below summarizes the characteristics of the various cords 50, 50′ and 60.

TABLE 1
Cords	50	50′	60

M1	X = 1	K	3	3	3
	T1	Q1/Q2/Q3	1/—/6	1/—/6	1/5/10
		d1/d2/d3	0.26/—/0.23	0.26/—/0.23	0.12/0.12/0.12
		direction for	S/inf	S/inf	S/inf
		C1/pitch p1
		(mm)
		direction for	—	—	S/8.1
		C2/pitch p2
		(mm)
		direction for	S/13.7	S/13.7	S/11.5
		C3/pitch p3
		(mm)

	direction for T1/P1	S/7.5	S/7.5	S/10
	(mm)

M2	Y = 6	L	3	3	3
	T2	Q1′/Q2′/Q3′	1/—/6	1/—/6	1/5/10
		d1′/d2′/d3′	0.26/—/0.23	0.26/—/0.23	0.12/0.12/0.12
		direction for	S/inf	S/inf	S/inf
		C1′/pitch p1′
		(mm)
		direction for	—	—	S/8.1
		C2′/pitch p2′
		(mm)
		direction for	S/13.7	S/13.7	S/11.5
		C3′/pitch p3′
		(mm)

	direction for T2/P2	S/7.5	S/7.5	S/10
	(mm)

Direction of cord/pi/pe	Z/inf/70	Z/inf/70	Z/inf/50
As (%)	2	—	2
As′ (%)	—	0.5	—
At (%)	3.6	—	4
D (mm)	4.9	4.9	4.2
M (g/m)	53	53	32
M/D (kg · m2)	11	11	7.75
Δσbending (MPa · mm)	26000	26000	12000
V1 (N × m/g)	2375	2375	1550

And Table 2 below summarizes the characteristics of the cord of the prior art described in document WO2016/131862.

	TABLE 2
	Cord	PA
	K/direction of cord	4/S
	direction of strands	S
	Q1	3
	Q3	8
	p1 (mm)	6.7
	p2 (mm)	10
	Pe (mm)	20
	At %	6.0
	As %	2.8
	D (mm)	3.80
	M (g/m)	36.4
	M/D (kg · m2)	9.5
	Δσbending (MPa · mm)	34800
	V1 (N × m/g)	3635

It is found that the cords 50, 50′ and 60 according to the invention make it possible to obtain a cord exhibiting sufficient flexibility and structural elongation to allow the building of the tyre and reduce the stiffness of the crown block with an improved endurance criterion to account for the cyclic tensile stress loadings compared with the cord of the prior art.

The invention is not limited to the above-described embodiments.