WO2020128236A1

WO2020128236A1 - Vehicle tyre comprising a stiffening structure

Info

Publication number: WO2020128236A1
Application number: PCT/FR2019/053022
Authority: WO
Inventors: Frédéric Perrin; Florian LACHAL; Gaël PATAUT; Richard CORNILLE; Olivier REIX
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2018-12-19
Filing date: 2019-12-11
Publication date: 2020-06-25

Abstract

The subject of the present invention is a tyre (1) for an agricultural vehicle, comprising a crown reinforcement (3), comprising at least two crown layers (31, 32) that each comprise metallic reinforcers coated in an elastomeric material. According to the invention, every metallic reinforcer of a crown layer (31, 32) has a law describing elastic tensile behaviour, known as bi-modulus, comprising a first portion having a first tensile modulus MG1 at most equal to 30 GPa, and a second portion having a second tensile modulus MG2 at least twice the first tensile modulus MG1, and every metallic reinforcer of a crown layer (31, 32) has a law describing compressive behaviour, characterized by a critical compressive buckling deformation E0 at least equal to 3%.

Description

Vehicle tire comprising a stiffening structure.

The present invention relates to a tire for an agricultural vehicle, such as an agricultural tractor or an agro-industrial vehicle, and more particularly relates to its crown reinforcement.

[0002] The dimensional specifications and the conditions of use (load, speed, pressure) of a tire for an agricultural vehicle are defined in standards, such as, for example, the ETRTO standard (European Tire and Rim Technical Organization). For example, a radial tire for a drive wheel of an agricultural tractor is intended to be mounted on a rim whose diameter is generally between 16 inches and 46 inches, or even 54 inches. It is intended to be driven on an agricultural tractor whose power is between 50 CV and more than 250 CV (up to 550 CV) and capable of traveling up to 65 km / h. For this type of tire, the minimum recommended inflation pressure corresponding to the indicated load capacity is most often at most equal to 400 kPa, but can drop to 240 kPa, for an IF (Improved Flexion) tire, or even 160 kPa, for a VF (Very High Flexion) tire.

Like any tire, a tire for an agricultural vehicle comprises a tread, intended to come into contact with a ground by means of a tread surface, the two axial ends of which are connected by means of two sidewalls with two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted.

In what follows, the circumferential, axial and radial directions designate respectively a direction tangent to the rolling surface and oriented according to the direction of rotation of the tire, a direction parallel to the axis of rotation of the tire and a perpendicular direction. to the axis of rotation of the tire.

A radial tire for an agricultural vehicle comprises a reinforcing reinforcement, consisting of a crown reinforcement, radially internal to the tread, and a carcass reinforcement, radially internal to the crown reinforcement.

[0006] The tread of a tire for an agricultural vehicle generally comprises a plurality of elements in relief, called tread elements, extending radially outward from a bearing surface to the surface of rolling, and most often separated from each other by hollows or grooves. These elements of sculpture are most often bars generally having an elongated shape generally parallelepiped, comprising at least one rectilinear or curvilinear portion.

The carcass reinforcement of a radial tire for an agricultural vehicle comprises at least one carcass layer connecting the two beads together. A carcass layer comprises reinforcements coated with a polymeric material comprising an elastomer, obtained by mixing, or elastomeric mixture. Carcass layer reinforcements most often consist of polymeric textile materials, such as polyester, for example polyethylene terephthalate (PET), an aliphatic polyamide, for example nylon, an aromatic polyamide, for example aramid , or even rayon. The reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle between 85 ° and 95 °.

The crown reinforcement of a radial tire for an agricultural vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each top layer consists of reinforcements coated with an elastomeric mixture and parallel to each other. When the crown layer reinforcements form, with the circumferential direction, an angle at most equal to 10 °, they are said to be circumferential, or substantially circumferential, and provide a hooping function limiting the radial deformations of the tire. When the top layer reinforcements form, with the circumferential direction, an angle at least equal to 10 ° and most often at most equal to 40 °, they are called angle reinforcements and have a function of taking up transverse, parallel forces. to the axial direction, applied to the tire. The top layer reinforcements can be made of polymeric textile materials, such as polyester, for example polyethylene terephthalate (PET), an aliphatic polyamide, for example nylon, an aromatic polyamide, for example aramid, or still rayon, or by metallic materials, such as steel.

A tire for an agricultural vehicle is intended to roll on various types of soil such as the more or less compact earth of the fields, the unpaved paths for access to the fields and the paved surfaces of the roads. Given the diversity of use, in the field and on the road, a tire for an agricultural vehicle must present a compromise in performance between, in a non-exhaustive manner, traction in the field on loose ground, resistance to tearing, resistance to wear on the road, resistance to travel, vibrational comfort on the road. [0010] An essential problem for the use of a tire in the field is to limit the compaction of the soil by the tire as much as possible, liable to harm crops. This is the reason why, in the agricultural field, tires with low pressure, therefore with strong bending, have been developed. The ETRTO standard thus distinguishes IF (Improved Flexion) tires, with a minimum recommended inflation pressure generally equal to 240 kPa, and VF (Very high Flexion) tires, with a minimum recommended inflation pressure generally equal to 160 kPa. According to the standard, compared to a standard tire, an IF tire has a load capacity increased by 20%, and a VF tire has a load capacity increased by 40%, for an inflation pressure equal to 160 kPa.

However, the use of low pressure tires had a negative impact on behavior in the field. Thus, the drop in inflation pressure has resulted in a reduction in transverse stiffness and in tire drift, hence a reduction in the transverse thrust of the tire, and, consequently, in a deterioration of its behavior under transverse stresses. One solution to restore correct transverse thrust has been to transversely stiffen the crown reinforcement of the tire, by replacing the crown layers with textile reinforcements with crown layers with metal reinforcements. Thus, for example, a crown reinforcement comprising six crown layers with textile reinforcements of the rayon type has been replaced by a crown reinforcement comprising two crown layers with metallic steel reinforcements. Document EP 2934917 thus describes an IF tire comprising a crown reinforcement comprising at least two crown layers with metal reinforcements, combined with a carcass reinforcement comprising at least two carcass layers with textile reinforcements.

However, the use of crown layers with metal reinforcements in a tire for an agricultural vehicle can lead to a reduction in the endurance of the crown of the tire, due to premature rupture of the metal reinforcements.

The inventors then set themselves the objective of increasing the endurance of a crown reinforcement with metal reinforcements to a level at least equivalent to that of a crown reinforcement with textile reinforcements, in particular for a tire for agricultural vehicles operating at low pressure such as an IF tire (Improved Flexion) or a VF tire (Very High Flexion). This object was achieved according to the invention by a tire for an agricultural vehicle comprising a crown reinforcement, radially internal to a tread and radially external to a carcass reinforcement,

the crown reinforcement comprising at least two crown layers, each comprising metal reinforcements coated in an elastomeric material, mutually parallel and forming, with a circumferential direction, an angle A at least equal to 10 °,

-all metallic reinforcement of crown layer having an elastic behavior law in extension, called bi-module, comprising a first portion having a first module in extension MG1 at most equal to 30 GPa, and a second portion having a second module in extension MG2 at least equal to 2 times the first module in extension MG1, said behavior law in extension being determined for a metallic reinforcement coated in an elastomeric mixture having a modulus of elasticity in extension at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa,

-and any metallic reinforcement of crown layer having a law of behavior in

compression, characterized by a critical buckling deformation in compression E0 at least equal to 3%, said law of behavior in compression being determined on a test piece constituted by a reinforcement placed in its center and coated with a volume

parallelepiped of elastomeric mixture having a modulus of elasticity in extension at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa.

For a tire for an agricultural vehicle comprising a crown reinforcement with at least two crown layers with metallic reinforcements, the inventors propose to use elastic metallic reinforcements whose behavior laws have specific characteristics both in extension and in compression.

As regards its behavior in extension, a bare metal reinforcement, that is to say one not coated with an elastomeric material, is characterized mechanically by a curve representing the tensile force (in N), applied to the reinforcement metallic, as a function of its relative elongation (in%), called the force-elongation curve. From this force-elongation curve are deduced the mechanical characteristics in extension of the metal reinforcement, such as the structural elongation As (in%), the total elongation at break At (in%), the force at break Fm (load maximum in N) and the breaking strength Rm (in MPa), these characteristics being measured, for example, according to ISO 6892 of 1984 or ASTM D2969-04 of 2014. The total elongation at break At of a metal reinforcement is, by definition, the sum of its structural, elastic and plastic elongations (At = As + Ae + Ap). The structural elongation As results from the relative positioning of the metallic wires constituting the metallic reinforcement under a low tensile force. The elastic elongation Ae results from the very elasticity of the metal of the metallic wires, constituting the metallic reinforcement, taken individually, the behavior of the metal according to Hooke's law. The plastic elongation Ap results from the plasticity, that is to say from the irreversible deformation of the metal of these metallic wires taken individually, beyond the elastic limit.

In the context of the invention, the constitutive law in extension of a metallic reinforcement is determined for a metallic reinforcement coated in a cooked elastomeric material, corresponding to a metallic reinforcement extracted from the tire, on the basis of the standard ISO 6892 of 1984 as for a bare metal reinforcement. By way of example, and in a non-exhaustive manner, a cooked elastomeric coating material is a rubber-based composition having a modulus of elasticity in secant extension at 10% elongation MA10 at least equal to 5 MPa and at more equal to 15 MPa, for example equal to 6 MPa. This modulus of elasticity in extension is determined from tensile tests carried out in accordance with French standard NF T 46-002 of September 1988.

From the force-elongation curve, for a two-module elastic law of behavior comprising a first portion and a second portion, it is possible to define a first rigidity in extension KG1, representing the slope of the secant line passing through l origin of the reference frame, in which the constitutive law is represented, and the transition point between the first and second portions. Likewise, a second rigidity in extension KG2 can be defined, representing the slope of a straight line passing through two points positioned in a substantially linear part of the second portion.

From the force-elongation curve, characterizing the behavior in extension of a reinforcement, one can also define a stress-deformation curve, the stress being equal to the ratio between the tensile force applied to the reinforcement and the surface of the section of the reinforcement, and the deformation being the relative elongation of the reinforcement. For a two-module elastic behavior law comprising a first portion and a second portion, a first module can be defined in extension MG1, representing the slope of the secant line passing through the origin of the reference frame, in which the law of behavior, and the transition point between the first and second portions. Similarly, we can define a second module in extension MG2, representing the slope of a straight line passing through two points positioned in a substantially linear part of the second portion. The rigidities in extension KG1 and KG2 are respectively equal to MG1 * S and MG2 * S, S being the area of the section of the reinforcement. It should be noted that the bi-module behavior laws falling within the scope of the invention include a first portion with low module and a second portion with high module.

According to the invention, concerning the behavior in extension of the metallic reinforcements, any metallic reinforcement of the top layer has a law of elastic behavior in extension, called bi-module, comprising a first portion having a first module in extension MG1 at more equal to 30 GPa, and a second portion having a second module in MG2 extension at least equal to twice the first module in MG1 extension.

With regard to the compression behavior, a metal reinforcement is characterized mechanically by a curve representing the compression force (in N), applied to the metal reinforcement, as a function of its compression deformation (in%). Such a compression curve is in particular characterized by a limit point, defined by a critical buckling force Fc and a critical buckling deformation E0, beyond which the reinforcement is subjected to compression buckling, corresponding to a state of mechanical instability characterized by large deformations of the reinforcement with a reduction in the compressive force.

The compression behavior law is determined, using a Zwick or Instron type test machine, on a test piece of dimensions 12 mm x 21 mm x 8 mm (width x height x thickness). The test piece is constituted by a reinforcement, placed in its center and coated with a parallelepiped volume of elastomeric mixture defining the volume of the test piece, the axis of the reinforcement being placed according to the height of the test piece. In the context of the invention, the elastomeric mixture of the test piece has a modulus of elasticity in secant extension at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa, for example equal to 6 MPa. The test piece is compressed in the height direction at a speed of 3 mm / min until a deformation in compression, that is to say a crushing of the test piece equal to 10% of its initial height, ambient temperature. The critical buckling force Fc and the corresponding critical buckling deformation E0 are reached when the applied force decreases while the deformation continues to increase. In other words, the critical buckling force Fc corresponds to the maximum compression force Fmax. According to the invention, concerning the compression behavior of the metallic reinforcements, any metallic reinforcement of the top layer has a compression behavior law, characterized by a critical buckling deformation in compression E0 at least equal to 3%.

The inventors have shown that so-called elastic metal reinforcements, characterized by behavior laws in extension and in compression as previously described, have a limit of fatigue endurance, during cycles repeated alternately in extension and in compression, higher than that of conventional metal reinforcements.

In fact, during the rolling of a tire for an agricultural vehicle comprising a tread with bars, the tilting of the bars under torque (motor or brake) causes the crown layers to rock, positioned radially inside the bars. This tilting causes alternately positive and negative curvatures of the crown layers, and correspondingly cycles alternately in compression / extension of the metal reinforcements of the crown layers.

It should also be noted that the crown layers of a tire for an agricultural vehicle often have initial curvatures, both in the circumferential direction and in the axial direction, resulting from the movements of the various elastomeric constituents and reinforcements at the during manufacturing, during molding and curing of the tire. These initial deformations are added to the deformations resulting from the tilting of the bars and therefore also contribute to the compression / extension cycles of the metal reinforcements of the crown layers, when the tire is rolling.

Thus elastic metal reinforcements of crown layers according to the invention are able to better withstand the compression / extension cycles described above, which leads to an improvement in the endurance of the crown reinforcement of the tire, and therefore an increase in the life of the tire.

Advantageously, in the case where the crown reinforcement consists of two crown layers, the linear mass of a metallic reinforcement of crown crown is at least equal to 6 g / m and at most equal to 13 g / m. The linear mass of a metal reinforcement is the metal mass of a reinforcement portion having a unit length of 1 m. The linear mass is correlated to the module in extension of the reinforcement, therefore to its rigidity. Consequently, this range of linear mass values has been considered optimal with respect to the target stiffness of reinforcement. More generally, for a crown reinforcement constituted by 2n layers crown, the linear mass of the reinforcements constituting each crown layer is advantageously at least equal to 6 / ng / m and at most equal to 13 / ng / m.

According to a preferred embodiment of the metallic reinforcements, any metallic reinforcement of the top layer is a multi-strand cable of lxN structure comprising a single layer of N strands of diameter DT wound in a helix having an angle AT and a radius of RT curvature, each strand comprising an internal layer of M internal wires wound in a helix and an external layer of P external wires wound in a helix around the internal layer. It is a type of metallic reinforcement commonly used in the pneumatic field.

Most often all the strands have the same diameter DT. Each strand is wound in a helix around the axis of the cable, this helix being characterized by a helix pitch PT, a helix angle AT and a radius of curvature RT. The propeller pitch PT is the distance at the end of which the strand has made a full propeller turn. The radius of curvature RT is calculated according to the relation RT = PT / (p * Sin (2 * AT)).

In the particular case where the metallic reinforcements of the top layer are multi-strand cables, the helix angle AT of a strand is advantageously at least equal to 20 ° and at most equal to 30 °. This range of values of helix angle AT of a strand conditions the geometry of the cable and, in particular, the curvature of the strands which impacts the level of critical buckling deformation in compression E0 and contributes to obtaining a value at least equal to 3%.

Still in the particular case where the metal reinforcements of the top layer are multi-strand cables, the ratio RT / DT between the helix radius of curvature of a strand RT and the diameter of a strand DT is also advantageously at most equal to 5. This maximum value of the RT / DT ratio is a criterion which also contributes to a level of critical buckling deformation in compression E0 at least equal to 3%.

In the case where the crown reinforcement is constituted by two crown layers and where the metallic reinforcements of crown crown are multi-strand cables, the diameter D of a metallic reinforcement of crown crown is still advantageously at least equal to 1.4 mm and at most equal to 3 mm. This range of values of diameter D is compatible with the range of values targeted for the linear reinforcement mass. Such reinforcements are obtained from an assembly of steel wires generally having a diameter at most equal to 0.35 mm, or even at most equal to 0.28 mm. Still in the case where the crown reinforcement is constituted by two crown layers, the breaking strength R of a crown layer is at least equal to 500 N / mm and at most equal to 1500 N / mm . The breaking strength R of a crown layer is equal to the unit breaking strength, in N, of a metal reinforcement divided by the pitch, in mm, that is to say the distance between two consecutive reinforcements. The breaking strength R conditions, in particular, the bursting resistance under pressure of the tire, with a given safety factor.

According to an advantageous embodiment of the crown reinforcement, the crown reinforcement comprises at least one hooping layer comprising reinforcements coated in an elastomeric material, parallel to each other and forming, with the circumferential direction (XX ' ), an angle B at most equal to 10 °. The function of a hooping layer is to participate in the resumption of mechanical inflation stresses and, also, in improving the endurance of the crown reinforcement by stiffening it when the tire is crushed under a radial load and, in particular, subject to a drift angle around the radial direction. Among the hooping layers, a distinction is made between hooping layers known as closed angles, that is to say whose reinforcements form, with the circumferential direction, angles at least equal to 5 ° and at most equal to 10 °, and the circumferential hooping layers, more precisely substantially circumferential, that is to say whose reinforcements form, with the circumferential direction, angles at most equal to 5 ° and which may be harmful. The closed angle hoop layers include reinforcements having free ends at the axial ends of the hoop layers. The circumferential hooping layers comprise reinforcements having no free ends at the axial ends of the hooping layers, because the circumferential hooping layers are most often obtained by the circumferential winding of a reinforcement ply or by the circumferential winding of a reinforcement. The hoop layer reinforcements can be either continuous or split. The hoop layer reinforcements can be either metallic or textile.

According to another advantageous embodiment of the crown reinforcement, the crown reinforcement comprises at least one additional crown layer comprising metal reinforcements coated in an elastomeric material, parallel to each other and forming, with the circumferential direction , an angle C at least equal to 60 ° and at most equal to 90 °. This additional crown layer includes metallic reinforcements, which are not necessarily elastic and not necessarily of the type of those of the invention and forming, with respect to the circumferential direction, angles between 60 ° and 90 °. These angles are higher than those formed by the elastic reinforcements of the crown layers according to the invention, generally between 10 ° and 40 °. This additional crown layer, radially positioned either inside or outside of the crown layers according to the invention, and most often being decoupled from said layers, that is to say separated from them by a layer in elastomeric mixture, contributes to the stiffening of the crown reinforcement by a hooping effect by triangulation with the other crown layers.

Most often the carcass reinforcement comprises at least one carcass layer comprising textile reinforcements coated in an elastomeric material, mutually parallel and forming, with the circumferential direction, an angle D at least equal to 85 ° and at more equal to 95 °. But a lower angle D, typically at least equal to 65 °, is also conceivable.

According to a usual embodiment of the tread, the tread consists of first and second rows of bars extending radially outward from a bearing surface and arranged in chevrons relative to the equatorial plane of the tire.

The invention is applicable in particular to a radial tire for the drive wheel of an agricultural tractor, and even more particularly to an IF (Improved Flexion) tire, with a minimum recommended inflation pressure generally equal to 240 kPa, and a VF (Very High Flexion) tire, with a minimum recommended inflation pressure generally equal to 160 kPa. It can even be extended to a tire inflated to a low pressure, as recommended for a VF tire, but having a load capacity greater than that of a VF tire.

The characteristics of the invention are illustrated by Figures 1 to 7 schematic and not shown to scale:

-Figure 1: Half-cut meridian of tire for agricultural vehicle according to the invention

-Figure 2: Typical example of a typical tensile force-elongation curve of an elastic metal reinforcement according to the invention, coated with an elastomeric material

-Figure 3: Tensile stress-elongation curves for two particular examples of elastic metallic reinforcement according to the invention (E12.23 and E24.26), coated with an elastomeric material

-Figure 4: Typical example of compression force-compression deformation curve of a elastic metal reinforcement according to the invention, obtained on a test piece of elastomeric material

-Figures 5 and 6: Formulas for assembling two particular examples of elastic metallic reinforcement according to the invention (E18.23 and E24.26)

-Figure 7: Front view of a tire for an agricultural vehicle with bar tread.

FIG. 1 represents a meridian half-section, in a meridian plane YZ passing through the axis of rotation YY ′ of the tire, of a tire 1 for an agricultural vehicle comprising a crown reinforcement 3 radially internal to a strip of bearing 2 and radially external to a carcass reinforcement 4. The crown reinforcement 3 comprises two crown layers (31, 32), each comprising metal reinforcements coated in an elastomeric material, mutually parallel and forming, with a circumferential direction (XX '), an angle A at least equal to 10 ° (not shown). The crown reinforcement 4 comprises three carcass layers comprising textile reinforcements coated in an elastomeric material, mutually parallel and forming, with the circumferential direction (XX '), an angle D at least equal to 85 ° and at most equal to 95 ° (not shown).

Figure 2 is a typical example of a tensile force-relative elongation curve of an elastic metal reinforcement according to the invention, coated with an elastomeric material, representing its elastic behavior in extension. The tensile force F is expressed in N and the elongation A is a relative elongation, expressed in%. According to the invention, the law of behavior in extension, elastic and bi-module, comprises a first portion and a second portion. The first portion is delimited by two points, the ordinates of which correspond respectively to a zero tensile force and a tensile force equal to 87 N, the respective abscissae being the corresponding relative elongations (in%). One can define a first rigidity in extension KG1, representing the slope of the secant line passing by the origin of the reference mark, in which the constitutive law is represented, and the point of transition between the first and second portions. Knowing that by definition, the rigidity in extension KG1 is equal to the product of the module in extension MG1 by the surface S of the section of the reinforcement, we can easily deduce the module in extension MG1. The second portion is the set of points corresponding to a tensile force greater than 87 N. Similarly, for this second portion, we can define a second rigidity in extension KG2, representing the slope of a straight line passing through two positioned points in a substantially linear part of the second portion. In the example shown, the two points have for respective ordinates F = 285 N and F = 385 N, these tensile force values corresponding to levels of mechanical stress representative of those which are applied to the metal reinforcements of the crown layers, during the rolling of the tire under study. As described above KG2 = MG2 * S, so we can deduce the extension module MG2 from it.

FIG. 3 represents two tensile stress-elongation curves, the tensile stress F / S, expressed in MPa, being equal to the ratio between the tensile force F, expressed in N, applied to the reinforcement, and the surface S of the section of the reinforcement, expressed in mm ² , and the elongation A being the relative elongation of the reinforcement, expressed in%. The area S of the reinforcement section is the metal section equal to ML / p, ML being the linear mass of the reinforcement, expressed in g / m, and p being the density of the reinforcement, expressed in g / cm3 (by example the density p of the brass-coated steel is equal to 7.77 g / cm ³ ). These curves are the respective laws of behavior in extension of two examples of elastic multi-strand reinforcements E18.23 and E24.26 coated with an elastomeric material. From these curves, we can directly deduce the first and second modules in extension MG1 and MG2. According to the invention, for each of the behavior laws represented, the first module in extension MG1 is at most equal to 30 GPa, and the second module in extension MG2 is at least equal to 2 times the first module in extension MG1.

FIG. 4 is a typical example of a compression force-compression deformation curve of an elastic metal reinforcement according to the invention, representing its elastic behavior in compression. The compression force F is expressed in N and the compression strain is a relative crushing, expressed in%. This compression behavior law, determined on an elastomeric mixture test piece having a modulus of elasticity in secant extension at 10% elongation MA10 equal to 6 MPa, has a maximum corresponding to the appearance of buckling in compression of the reinforcement. This maximum is reached for a maximum compression force Fmax, or critical buckling force, corresponding to a critical buckling deformation E0. Beyond the buckling point, the compressive force applied decreases while the deformation continues to increase. According to the invention, the critical buckling deformation in compression E0 is at least equal to 3%.

Figures 5 and 6 show two examples of assembly structures of elastic multi-strand reinforcements, particular embodiments of the invention. FIG. 5 represents a multi-strand cable of type E18.23 having a structure 3 * (l + 5) * 0.23, that is to say comprising a single layer of 3 strands, each strand comprising an internal layer of 1 internal wire wound in a helix and an external layer of 5 external wires wound in a helix around the internal layer. Each wire is made of steel and has a unit diameter equal to 0.23 mm. FIG. 6 represents a multi-strand cable of type E24.26 having a structure 4 * (l + 5) * 0.26, that is to say comprising a single layer of 4 strands, each strand comprising an internal layer of 1 inner wire wound in a helix and an outer layer of 5 outer wires wound in a helix around the inner layer. Each wire is made of steel and has a unit diameter equal to 0.26 mm. These cables are obtained by twisting.

FIG. 7 represents a front view of a tire for an agricultural vehicle comprising a tread with bars. The tire 1 comprises a tread 2, consisting of first and second rows of bars 21 extending radially outward from a bearing surface 22 and arranged in chevrons relative to the equatorial plane of the pneumatic. As described above, when running, this type of tread generates compression / extension cycles of the metal reinforcements of the crown layers, which elastic reinforcements according to the invention better resist, with a large elongation in extension with low modulus and a high critical buckling deformation.

The invention was more particularly implemented for an agricultural tire of size 600 / 70R30 comprising a crown reinforcement with two crown layers with elastic metallic reinforcements of formulas E18.23 or E24.26.

The geometric and mechanical characteristics of the two examples of elastic metallic reinforcements studied are summarized in Table 1 below:

Table 1

The inventors tested the invention by comparing the life, from the point of view of the endurance of the crown reinforcement, of a tire of size 600 / 70R30, comprising two crown layers with reinforcements elastic metal according to the invention, to that of a reference tire, comprising six crown layers with textile reinforcements. Rolling on paved ground, under torque, with a circumferential force F _x applied equal to 520 daN, and at a speed V equal to 27 km / h was carried out for each tire, inflated to a pressure P equal to 50 kPa and subjected to a load Z equal to 2600 daN.

Claims

1. Tire (1) for agricultural vehicle, comprising:

a crown reinforcement (3), radially internal to a tread (2) and radially external to a carcass reinforcement (4),

the crown reinforcement (3) comprising at least two crown layers (31,

32), each comprising metal reinforcements coated in an elastomeric material, mutually parallel and forming, with a circumferential direction (XX ’), an angle A at least equal to 10 °,

characterized in that any metallic reinforcement of the crown layer (31, 32) has a law of elastic behavior in extension, called bi-module, comprising a first portion having a first module in extension MG1 at most equal to 30 GPa, and a second portion having a second module in MG2 extension at least equal to 2 times the first module in MG1 extension, said law of

behavior in extension being determined for a metallic reinforcement coated in an elastomeric mixture having a modulus of elasticity in extension at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa, and in that any metallic reinforcement vertex layer (31, 32) has a law of

behavior in compression, characterized by a critical buckling deformation in compression E0 at least equal to 3%, said law of behavior in compression being determined on a test piece constituted by a reinforcement placed in its center and coated by a parallelepipedic volume of elastomeric mixture having a modulus of elasticity in extension at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa.

2. A tire (1) according to claim 1, the crown reinforcement (3) being

consisting of two top layers (31, 32) and any metallic top layer reinforcement (31, 32) having a linear mass expressed in g / m, in which the linear mass of a metallic top layer reinforcement (31 , 32) is at least equal to 6 g / m and at most equal to 13 g / m.

3. A tire (1) according to one of claims 1 or 2, in which any metallic reinforcement of the crown layer (31, 32) is a multi-strand cable of lxN structure comprising a single layer of N strands of DT diameter wound. helically having an angle AT and a radius of curvature RT, each strand comprising a inner layer of M inner wires wound in a helix and an outer layer of P outer wires wound in a helix around the inner layer.

4. A tire (1) according to claim 3, in which the helix angle AT of a strand is at least equal to 20 ° and at most equal to 30 °.

5. A tire (1) according to any one of claims 3 or 4, the crown reinforcement (3) being constituted by two crown layers (31, 32) and any metallic crown layer reinforcement (31, 32) having a diameter D, in which the diameter D of a metallic reinforcement of the top layer (31, 32) is at least equal to 1.4 mm and at most equal to 3 mm.

6. A tire (1) according to any one of claims 1 to 5, the crown reinforcement (3) being constituted by two crown layers (31, 32) and a crown layer (31, 32) having a breaking strength R expressed in N / mm, in which the breaking strength R of a crown layer (31, 32) is at least equal to 500 N / mm and at most equal to 1500 N / mm.

7. A tire (1) according to any one of claims 1 to 6, in which the crown reinforcement (3) comprises at least one hooping layer (33) comprising reinforcements coated in an elastomeric material, parallel to each other and forming, with the circumferential direction (XX '), an angle B at most equal to 10 °.

8. A tire (1) according to any one of claims 1 to 7, in which the crown reinforcement (3) comprises at least one additional crown layer comprising metallic reinforcements coated in an elastomeric material, mutually parallel and forming, with the circumferential direction (XX '), an angle C at least equal to 60 ° and at most equal to 90 °.

9. A tire (1) according to any one of claims 1 to 8, in which the carcass reinforcement (4) comprises at least one carcass layer (41) comprising textile reinforcements coated in an elastomeric material, mutually parallel and forming , with the circumferential direction (XX '), an angle D at least equal to 85 ° and at most equal to 95 °.

10. A tire (1) according to any one of claims 1 to 9, in which the tread (2) consists of first and second rows of bars (21) extending radially outwardly. from a surface bearing (22) and arranged in chevrons relative to the equatorial plane (XZ) of the tire.