Optimized tread and crown of the civil engineering tire

Abstract

Radial tire for construction vehicles with a tread pattern comprising two working layers (321, 322) and at least one protective layer. Each tread layer includes elastic metallic reinforcement elements, with the protective layer having reinforcement elements 30% more elastic than the working layers. The tread pattern is characterized by its thickness (ETE) at the shoulder and its thickness (ETM) at the center of the tread. The tread pattern described is optimal in terms of performance balance between durability and wear, with an ETE / ETM ratio of no more than 1.25. The Me2 point of the carcass layer, located at a distance of 0.85 times L4m (the distance to the furthest point from the median plane), is at a radial distance E85 from the point of intersection of the median plane and the carcass layer. The ratio between E85 and 0.85*L4m is between 0.10 and 0.15. (Shortcut figure: Figure 1)

Description

Title of the invention: Optimized tread and crown of an engineering tire

[0001] The present invention relates to a radial tire, intended to equip a heavy civil engineering vehicle, and relates more particularly to the crown reinforcement of such a tire and its tread.

[0002] Radial tires intended to equip a heavy civil engineering vehicle are designated in accordance with the standard of the European Tyre and Rim Technical Organisation (ETRTO).

[0003] A radial tire for heavy-duty construction vehicles, as defined by the European Tyre and Rim Technical Organisation (ETRTO) standard, is intended to be mounted on a rim with a diameter of at least 25 inches. Although not limited to this type of application, the invention is described for a large radial tire intended to be mounted on a dumper, particularly vehicles for transporting materials extracted from quarries or surface mines, by means of a rim with a diameter of at least 35 inches and up to 57 inches, or even 63 inches. Construction tires have a tread depth of at least 70 mm when new.

[0004] Since a tire has a geometry of revolution about an axis of rotation, the tire's geometry is generally described in a meridian plane containing the tire's axis of rotation. For a given meridian plane, the radial, axial, and circumferential directions respectively denote the directions perpendicular to the tire's axis of rotation, parallel to the tire's axis of rotation, and perpendicular to the meridian plane. The circumferential direction is tangent to the tire's circumference.

[0005] In what follows, the expressions "radially inward" and "radially outward," respectively, mean "closer" and "farthest," respectively, from the axis of rotation of the tire. "Axially inward" and "axially outward" mean "closer" and "farthest," respectively, from the equatorial plane of the tire, the equatorial plane of the tire being the plane passing through the midpoint of the tread surface and perpendicular to the axis of rotation. "An element A axially inward to an element B by an axial distance D" means that element A is closer to the equatorial plane than element B and that the axial distance between the two elements is equal to the distance D. This type of sentence can be generalized to the radial and circumferential directions and the outer position. versus internal, of one or the other of the elements.

[0006] Generally, a tire comprises a tread, designed to come into contact with the ground via a tread surface, the two axial ends of which are connected by means of two sidewalls to two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted. Once mounted on a rim, the tire and the rim define an internal volume designed to contain a pressurized gas. The surface and the rubber compound in contact with the gas are respectively called the inner surface and the inner rubber compound. This inner rubber compound has sealing properties designed to prevent the diffusion of oxygen into the tire in order to prevent the oxidation of the tire's components.

[0007] A radial tire further comprises a reinforcing reinforcement, consisting of a crown reinforcement, radially inside the tread, and a carcass reinforcement, radially inside the crown reinforcement.

[0008] The carcass reinforcement of a radial tire for heavy-duty construction vehicles typically comprises at least one carcass layer including metallic reinforcements coated with a polymeric material of the elastomeric or elastomeric type, obtained by mixing and called the coating compound. A carcass layer includes a main portion connecting the two beads and generally wrapping, within each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called a bead, to form a inversion. The metallic reinforcements of a carcass layer are substantially parallel to each other and form an angle of between 80° and 90° with the circumferential direction.

[0009] The crown reinforcement of a radial tire for construction vehicles comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer consists of reinforcements, generally metallic, parallel to each other and coated with a polymeric material of the elastomer type or coating mixture.

[0010] A metal reinforcement is mechanically characterized by a curve representing the tensile force (in N) applied to the metal reinforcement as a function of its relative elongation (in %), called the force-elongation curve. From this force-elongation curve, tensile mechanical characteristics of the metal reinforcement are deduced, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking force Fm (maximum load in N), and the tensile strength Rm (in MPa), these characteristics being measured according to ASTM D 2969-04:2014.

[0011] The total elongation At of the metallic reinforcement is, by definition, the sum of its al Structural, elastic, and plastic elongations (At = As + Ae + Ap), particularly at break, where each elongation is non-zero. The structural elongation As results from the relative positioning of the metal wires constituting the reinforcement under a small tensile force. The elastic elongation Ae results from the elasticity of the metal wires constituting the reinforcement, considered individually, the metal's behavior following Hooke's law. The plastic elongation Ap results from the plasticity, that is, the irreversible deformation, beyond the elastic limit, of the metal wires considered individually. These different elongations, as well as their respective meanings, well known to those skilled in the art, are described, for example, in documents US5843583, WO2005 / 014925, and WO2007 / 090603.

[0012] A tensile modulus, expressed in GPa, is also defined at every point on the stress-strain curve of a metallic reinforcement. This modulus represents the slope of the tangent line to the stress-strain curve at that point. In particular, the tensile modulus of the linear elastic portion of the force-strain curve is called the elastic tensile modulus or Young's modulus.

[0013] Among metallic reinforcements, a distinction is usually made between elastic metallic reinforcements, such as those used in protective layers, and non-stretchable or inextensible metallic reinforcements.

[0014] An elastic metal reinforcement, in its rubberized state from the tire, is characterized by a structural elongation As of at least 0.3% and a total elongation at break At of at least 3%. Furthermore, an elastic metal reinforcement has an elastic modulus in tension of at most 150 GPa, and is usually between 40 GPa and 120 GPa.

[0015] An inextensible metal reinforcement is characterized by a total elongation At, under a tensile force equal to 10% of the breaking force Fm, at most equal to 0.2%. Furthermore, a non-extensible metal reinforcement has an elastic modulus in tension usually between 150 GPa and 200 GPa.

[0016] Among the top layers, we usually distinguish the protective layers, which are part of the protective reinforcement and radially the outermost, comprising elastic reinforcement elements (or reinforcements), and the working layers, which are part of the working reinforcement and radially located between the protective reinforcement and the carcass reinforcement.

[0017] The protective reinforcement, comprising at least one protective layer, essentially protects the working layers from mechanical or physicochemical aggressions, which may propagate through the tread radially towards the inside of the tire.

[0018] The protective reinforcement for a civil engineering type tire often comprises two radially superimposed protective layers, formed of elastic metallic reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles of at least 15°.

[0019] The working reinforcement, comprising at least two working layers, serves to encircle the tire and provide it with rigidity and road holding. It withstands both the mechanical stresses of inflation, generated by the tire's inflation pressure and transmitted by the carcass reinforcement, and the mechanical stresses of rolling, generated by the tire rolling on a surface and transmitted by the tread. Furthermore, it must resist oxidation, impacts, and punctures, thanks to its intrinsic design and that of the protective reinforcement.

[0020] The working reinforcement usually comprises two radially superimposed working layers formed of non-extensible metallic reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming angles with the circumferential direction of at most 60°, and preferably at least 15° and at most 45°. To reduce the shear stresses on the rubber compounds, or rubber mixtures, at the axial ends of the working layers, it is common practice to axially offset the position of said ends relative to each other. The top reinforcement therefore usually comprises a working layer of greater axial width and a working layer of lesser axial width. The shear stresses on the rubber compounds are maximal at the end of the working layer with the smaller axial width.Indeed, these maximum shear stresses due to displacements at the end of the narrowest axially wide working layer are distributed across the radial thickness of the rubber compounds between the narrowest axially wide working layer and the widest axially wide working layer. These shear stresses are amplified by the deformations of the widest axially wide working layer. In fact, given the angle of the cross-bracing between the metal reinforcements and the metal reinforcements of the narrowest axially wide working layer, the widest axially wide working layer deforms in a different direction, which increases the deformations of the rubber compounds. These shear maxima are generally reduced by adding a decoupling rubber between the end of the narrowest axially wide working layer and the widest axially wide working layer.The end of the working layer with the greatest axial width is also subjected to strong shear, but generally of lesser amplitude, since at this end the thickness of the rubber compounds is greater and the deformations are no longer amplified by the presence of the other working layer.

[0021] To reduce the mechanical stresses of inflation and rolling transmitted to the working reinforcement and the shear stresses of the rubber compound covering it, it is known to place a shrink-fit reinforcement radially outside the carcass reinforcement. The shrink-fit reinforcement, whose function is to absorb at least some of the mechanical stresses of inflation, improves the durability of the top reinforcement by stiffening it. The shrink-fit reinforcement can be positioned radially inside the working reinforcement, between the two working layers of the working reinforcement, or radially outside the working reinforcement.

[0022] In civil engineering applications, the reinforcement cage may comprise two radially superimposed layers of reinforcement cages formed of parallel metallic reinforcements within each layer and intersecting from one layer to the next, forming angles of no more than 10° with the circumferential direction. For creating the reinforcement cage layers on large-diameter civil engineering tires, winding a layer of discontinuous reinforcement elements, the ends of which run from one axial edge of the layer to the other forming an angle of between 7° and 10° with the circumferential direction, may be preferred to winding a narrow strip of a few continuous reinforcement elements forming an angle of between 0° and 5° with the circumferential direction, for reasons of productivity. In both cases, the reinforcement cage layers have a smaller axial width than the working layer, which has the smallest axial width.Indeed, the stresses due to rolling at the ends of the working layers are very high in tension and compression and lead to the failure of the inextensible metal reinforcements arranged around the ends whose angles with the circumferential direction are less than 15°. Document WO2019 / 202239 discloses a common architecture of civil engineering tires comprising two layers of rigid reinforcement elements, radially inside two working layers of rigid reinforcement elements radially inside elastic protective layers, the two layers of reinforcement having an axial width significantly less than the axial width of the working layers.

[0023] These tires often have a large tread width compared to the overall transverse width of the tire, resulting in crown layers with a similarly large axial width. During inflation, for this type of tire, the crown deforms with a relative rise at the center and a relative drop at the shoulders, which, for a constant tread depth, leads to significant shoulder wear.

[0024] Furthermore, if the carcass layer is manufactured with a shape far from its equilibrium curve, the pressurization will deform the carcass layer to the To bring the system closer to its equilibrium curve under pressure, certain parts of the apex, particularly the reinforcement elements of the compression layers, are placed in tension or compression depending on their positions. For good endurance, the reinforcement elements of the compression layers must not be under absolute compression on any of their segments immediately upon inflation, as compressing these reinforcement elements significantly reduces their fatigue resistance.

[0025] Designing a tire therefore consists of finding a way to combine a tread and a crown whose optimal shape for wear is cylindrical with a carcass whose optimal shape is curved. Rounding the tread profile results in a less wear-efficient tire. Opting for a less rounded profile for the carcass is likely to produce a less durable tire, as cracks in the rubber compounds near the axial ends of the working layers are more likely to propagate to the carcass layer. It is also possible to reduce the axial width of the tread, but this also has a direct impact on the volume of rubber to be worn, the value of which is a linear variable of the tread depth multiplied by the axial width of the tread.

[0026] Such a compromise is usually achieved by attaching the crown reinforcement to the carcass via crown filling compounds, or rubber compounds, at the shoulder, the shoulders representing the outermost 20% of the tread on either side of the median plane. These crown filling compounds are thicker the wider the crown. At the center of the tread, the thickness of these crown filling compounds is minimal. For obvious reasons of symmetry, the carcass layer at the median plane is parallel to the transverse direction. It is possible to have a thickness of rubber compound between the carcass reinforcement and the crown reinforcement at the center, but in this case, this thickness does not meet geometric constraints.Over most of the crown reinforcement, the tread blocks are unnecessary, as the crown reinforcement stabilizes the position of the carcass layer. At the shoulder, the tread blocks are thickest. This design results in high temperatures at the crown, particularly at the shoulders, which tends to reduce durability. If the thickness of these blocks is reduced, the carcass layer, under pressure, will move to its equilibrium curve, causing the tread blocks, the ends of the crown layers, and the tread itself to a lower radial position at the shoulders, and putting some reinforcement elements under compression. During rolling, the shoulders align with the center in the contact patch due to the impact on the ground, and put the reinforcement elements in the shoulder area under relative extension. This repeated action... With each wheel rotation from extension phase to compression phase, it can cause ruptures of the reinforcing elements at the ends of the top layers, and brings high shear rates to the axial ends of the working layers, conducive to the creation of cracks which, depending on the case, lead to the flanks or to the carcass layer.

[0027] Maintaining a large volume of filler rubber at the top significantly increases the temperature in this area, which can lead to thermal failure of the top or premature aging of the edge rubber, resulting in a decrease in the cohesion of their materials, a decrease conducive to crack formation. Reducing the thickness of the top filler rubber increases the maximum shear stresses they are subjected to and normally decreases their endurance. Edge rubbers are rubbery compounds positioned between the axial ends of the working layers and each adjacent layer of metallic reinforcing elements.

[0028] To reduce cracking, tire manufacturers primarily seek to increase crown stiffness by introducing compression layers and optimizing their function, for example, as shown in US patent 2018 / 0056723 A1, where compression layers are placed between the working layers, and the working layers are re-bonded to improve their stiffness. However, these solutions are complex in terms of manufacturing, as re-bonding is sometimes difficult to perform with large-diameter metallic reinforcement elements for heavy vehicles, and crown stiffening is not conducive to puncture resistance by rocks, a fundamental performance characteristic for earthmoving tires.

[0029] The inventors have set themselves the objective for a civil engineering tire, to improve the endurance in particular at the ends of the top layers both for the ends of the reinforcement elements of the top layers and for the filling rubbers while maintaining or improving the resistance to aggressions for the same result in wear.

[0030] This objective has been achieved, according to the invention, by a radial tire for construction vehicles intended to be mounted on a rim with a nominal diameter greater than 25 inches, comprising: • a carcass reinforcement extending between the two ribs, each rib comprising a circumferential reinforcement or rod, and consisting of a carcass layer comprising metal reinforcements forming an angle, with a radial direction, between -10° and 10°, the carcass layer comprising a main portion running from one rod to the other and two turns each running from one rod to one end of the layer carcass, each bead comprising a protector made of a rubber compound, the outermost axial points of which are intended to come into contact with a mounting rim and in each meridian a Mel point minimizing the axial distance of the bead to the outermost axial points of the protector, a crown reinforcement radially internal to a tread and radially external to the carcass reinforcement, the tread having a tread surface intended to come into contact with a running surface, the tread having, for each meridian plane, two axial ends, an inner rubber having an inner surface intended to be in contact with an inflation gas, a median plane perpendicular to the axis of rotation (YY') of the tire and passing through the middle of the tread, a median total thickness being the radial distance from the innermost radial point of the tire in the median plane (M) to the tread surface, each point Mel being positioned at an axial distance from the median plane equal to half the nominal rim width plus 1.27 cm, the outermost axial point of the main portion of the carcass layer being at an axial distance L4m, point Me2 being the point of the carcass layer at an axial distance of 0.85*L4m and at a radial distance E85 from the point of intersection between the median plane M and the carcass layer, a total shoulder thickness measured in a meridian plane, being the minimum distance from an axial end of the tread surface and the inner surface of the inner rubber, the crown reinforcement comprising at least one working reinforcement, and a protective reinforcement radially external to the working reinforcement, the working reinforcement comprising at least two working layers and the protective reinforcement comprising at least one protective layer, each working layer comprising metallic reinforcing elements,parallel to each other, forming, with the circumferential direction, oriented angles whose absolute value is at least equal to 15° and at most equal to 45°, the angles of two working layers being of opposite sign from one working layer to the next, each reinforcement element of each of the layers of the top reinforcement being characterized by a structural elongation As, a total elongation at break At, these characteristics being measured according to the ASTM standard, D 2969-04 of 2014, • each protective layer comprising reinforcing elements, parallel to each other, forming, with the circumferential direction, oriented angles whose absolute value is at least equal to 15° and at most equal to 45°, • the reinforcing elements of the top reinforcement layers having a structural elongation As of at least equal to 0.3% and a total elongation At at break of at least equal to 3% and a modulus tangent to (At+As) / 2 of at most equal to 120 GPa, • the reinforcing elements of each protective layer having a structural elongation As at least equal to 1.3 times the structural elongation As of the reinforcing elements of the innermost radially positioned working layer, • the ratio of the total thickness at the shoulder to the total median thickness being at most equal to 1.25, • the ratio between the radial distance E85 and the axial length 0.85*L4m being between 0.10 and 0.15.

[0031] In order for at least one of the top layers to act as a protective layer, this layer of reinforcing elements must only bear a small portion of the inflation and rolling forces. A sufficient condition for this is that the reinforcing elements of said protective layer be significantly more elastic than the reinforcing elements of the working layers, and therefore of the innermost radially working layer. According to the invention, since the protective reinforcement is radially external to the working reinforcement and any possible reinforcing reinforcement, there is at least one outermost protective layer, a layer of metallic reinforcing elements, whose reinforcing elements have a structural elongation As at least equal to 1.3 times the structural elongation As of the reinforcing elements of the innermost radially working layer.The protective reinforcement may include two protective layers, each radially external to the working reinforcement and the reinforcing reinforcement. The innermost radially protective layer will also be elastic and have a significantly higher structural elongation, at least 30% higher, than the structural elongation of the innermost radially working layer. For reasons of productivity and standardization, the architecture of the reinforcing elements in both protective layers, namely the diameters of the individual wires and the way these wires are arranged relative to each other, will preferably be identical. For better peak failure performance under stress, it is advantageous to have two different protective layers. A preferred solution for puncture resistance is that the... reinforcement elements are crossed from one protective layer to the next, that is to say that the angles formed by the reinforcement elements of the two protective layers and the circumferential direction are of opposite signs.

[0032] According to the invention, all the reinforcement elements of the apex reinforcement are elastic. For an identical apex geometry, this characteristic leads to greater fatigue failure performance under tensile and compressive stresses. Indeed, elastic reinforcement elements resist buckling much better than inextensible reinforcement elements, provided that the structural elongation is at least 0.3%, the total elongation At at break is at least 3%, and the tangent modulus to (At+As) / 2 is at most 120 GPa. Their lower stiffness surprisingly reduces the stresses in the rubber pads, whereas it should increase the deformation of the apex reinforcement elements and therefore the shear stresses in said pads. This reduction makes it possible to modify the apex geometry and decrease the thickness of the rubber pads at the apex, without degrading durability.One way to compare the thickness of the tread blocks of one tire to another, without considering the difference in the thickness of the crown layers, is to compare the ratio of the tire's thickness at the center of the crown, where it is thinnest, to the tire's thickness at the shoulder, where it is greatest. However, to determine whether this variation is due to an increase in the tread volume thickness, it is also necessary to consider the slope of the carcass layer under the crown. Indeed, the crown layers are essentially axial, their role being, among other functions, to form a cylindrical ring onto which the tread is placed. The more cylindrical this ring is, the better it helps to prevent uneven wear. The carcass slope is therefore primarily a function of the thickness of the crown tread blocks between the carcass layer and the innermost radially positioned crown layer.By using a tread pattern with elastic reinforcement elements, the inventors were able to reduce the thickness of these tread blocks while maintaining tire durability. Above a certain gradient, namely 0.15, a tire with such tread reinforcement characteristics is not optimized for durability, and below a gradient of 0.1, its durability will be insufficient.

[0033] This slope measurement must be taken in a situation close to the moment when the tire is in the curing mold. Any measurement under pressure creates a deformation of the tire tending to modify the shape of the crown by rounding it and therefore distorting the measurement of the carcass slope under the crown. A tire is cured in a mold in which the beads are at a distance slightly greater than their position in the mounting rim by approximately 2.54 cm (one inch) on the The total axial width is 1.27 cm greater than the nominal half-rim width, measured from the bead to the median plane. The mounting rim width refers to the axial distance between the substantially radial portions of the rim intended to bear against the tire beads, as defined in the ETRTO technical dictionary. This difference between the curing mold width and the nominal rim width ensures that, during mounting, the beads naturally rest on the rim and that the tire is naturally airtight. This measurement is taken either on one or more cross-sections of the tire or on a dismounted tire, ensuring the beads are positioned correctly as specified. The slope is measured at 0.85 times the axial width of the main portion of the carcass layer, which provides a good estimate of the axial width of the crown layers. The ratio between the radial distance is E85 and the axial length is 0.85*L4m is a measurement of this carcass layer slope. State-of-the-art tires, mainly composed of non-elastic working layers, have slopes under the crown between 0.19 and 0.25, in order to guarantee acceptable durability.

[0034] A dimension of the tread that also has a significant impact on endurance performance with regard to shoulder cracking is the distance from the axial edge of the tread surface to the inner surface of the inner rubber compound, called the total shoulder thickness. There is a total shoulder thickness on each side of the meridian plane. These are usually approximately equal, within manufacturing variations. The measurement is taken either on one or more meridian cross-sections of the tire or on a disassembled tire. In the case of multiple measurements, the average of these will be considered. If there is a cut in the tread at the shoulder, the measurement is taken at the point on the tread surface that maximizes the total shoulder thickness. Generally, the points of the cuts do not belong to the tread surface. On the carcass layer, the measurement points are taken on its neutral line of the reinforcing elements.

[0035] The reference thickness of the crown in its new condition at the median plane is called the median total thickness. It is measured on a new tire, either disassembled or on a meridian section, as with the total shoulder thickness. By new tire, we mean a tire or section of tire whose tread pattern has not been modified since it came out of the mold. The median total thickness is the radial distance from the innermost radial point of the tire at the median plane to the tread surface at the median plane. In the absence of a central groove, the median total thickness will be measured from the outermost radial point of the tread surface in a straightforward manner. In the presence of a central groove, for a meridian section, a person skilled in the art will easily be able to determine a theoretical tread surface at the median plane based on the height positions. blocks axially closest to the median plane by a simple linear approximation.

[0036] State-of-the-art tires on the market, comprising inextensible reinforcement elements, generally have a ratio of the total shoulder thickness to the median total thickness between 1.3 and 1.4. By using elastic reinforcement elements, it is possible to optimize the total shoulder thickness by reducing its value so that said ratio is at most equal to 1.25, preferably at most equal to 1.20.

[0037] A preferred solution is for the top reinforcement to include a radially internal bracing reinforcement within the protective reinforcement, comprising at least one bracing layer with elastic metallic reinforcing elements forming, with the circumferential direction, oriented angles whose absolute value is at least 0° and at most 10°, preferably at most 2°. The preferred solution is to circumferentially wrap a strip of one or more reinforcing elements at an angle of less than 2°. This solution allows for better load transfer by the elastic bracing layers.

[0038] The use of elastic reinforcing elements for all the reinforcement elements of the top reinforcement increases the elasticity of the top and allows for a homogenization of stresses in all the top layers and a reduction in stresses on the reinforcement layers. The endurance of the reinforcing elements is strongly linked to their tension-compression cycle. It is therefore possible, for the same level of endurance, to increase the width of the reinforcement layers compared to the working layers. This makes it possible to lower the shear stress at the ends of the working and reinforcement layers, and thus increase the endurance in this area, which is the objective of the invention.In commercially available tires, the axial width of the widest axial reinforcement layer, due to the rigidity of its reinforcing elements, is at most 55% of the axial width of the widest axial working layer. To improve crown durability, a preferred solution is for the axial width of the widest axial reinforcement layer to be at least 65% of the axial width of the widest axial working layer. This is made possible by using elastic reinforcing elements for all crown layers.

[0039] Experiments have shown that the peak endurance is further improved if the tangent modulus to (At+As) / 2 of the reinforcing elements of the working layers is at most 80 GPa. Given the increased elasticity of the peak and the increased width of certain peak layers, the rubber compounds at the ends of the working layers can be subjected to greater mechanical stress and to produce by hysteresis a local increase in temperatures. To remedy this problem, one solution is to position rubber compositions with an axial width of at least 5 mm and a radial thickness of at least 3 mm between the axial ends of the working layers and each adjacent layer of metallic reinforcing elements, the elongation of said rubber compositions at break at 100°C according to standard NF T 46-002 being at least 500%, and the maximum dynamic loss tanô of said rubber compositions, measured according to standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, being at most 0.06.

[0040] The elastic metallic reinforcing elements of the top layers are usually made up of several steel monofilaments or wires joined together. To obtain an elastic reinforcing element, the geometry of the monofilament assembly initially allows the reinforcing elements to deform through the movement of the monofilaments within the reinforcing element, shearing the rubber between the monofilaments. Subsequently, the deformation of the reinforcing elements occurs primarily through the deformation of the monofilaments.This property, which enables the first phase of structural deformation, implies that the diameters of the reinforcing elements are often large. This, in turn, results in very large total median and total shoulder thicknesses for a given number of crown layers and a certain tread thickness, leading to high crown temperatures during rolling and limiting the potential endurance gains achieved through the use of elastic reinforcing elements. To limit crown thicknesses, one solution is to limit the diameters of the reinforcing elements by using monofilaments of a limited diameter. Thus, an advantageous solution is for the elastic reinforcing elements of at least two crown reinforcement layers to be steel cables composed of several monofilaments, these monofilaments having a diameter of no more than 0.32 mm and preferably no more than 0.29 mm, preferably from all the top layers.

[0041] The features of the invention are illustrated by the schematic [Fig.1] not shown to scale, with reference to a civil engineering tire.

[0042] Figure 1 represents a meridional half-section of a civil engineering tire 1 comprising: - a tread 2, a tread surface 21, having a point 212 in the median plane (M) cutting the tire into two substantially symmetrical tori, subject to manufacturing variations, and an axially external point 211, - a carcass frame, comprising a carcass layer 4 of which the metallic reinforcing elements forming an angle, with a radial direction, between -10° and 10°, the carcass layer, extending between the two ridges 7, each of the ridges 7 comprising a circumferential reinforcement or rod 6; the carcass layer 4 comprising a main portion 41 going from one rod 6 to the other and two turns 42 each going from one rod 6 to one end of the carcass layer 4, - Each bead 7 comprising a protector 71 and for each meridian a point Mel minimizing the axial distance of the rod 6 to the most axially external points of the protector 7, - When the Mel points are at a distance from the median plane M equal to half the width of the mounting rim plus 1.27 cm Ljc, the outermost axial point of the main portion 41 of the carcass layer 4 is at a distance L4m, which allows the Me2 point of the carcass layer to be constructed at an axial distance of 0.85*L4m, this Me2 point being radially external to the outermost axial point of the carcass layer, and at a radial distance E85 from the point of intersection between the median plane M and the carcass layer 41, - an inner rubber 5 having an inner surface 51 and a most radially inner point 512 to the median plane M, - a top reinforcement 3, comprising a working reinforcement 32, a restraining reinforcement 31 and a protective reinforcement 33 - the shrink-fit frame 31, comprising two shrink-fit layers 311 and 312, - the working frame 32, comprising two working layers 321 and 322, - a protective reinforcement 33, radially external to the shrink-fit reinforcement and the working reinforcement comprising here two protective layers 331, 332, the innermost radially protective layer being the top layer with the greatest axial width to protect all the other top reinforcement elements from apex aggressions, - the total median thickness ETM measured on the new tire, is the radial distance from the innermost radial point 512 of the tire to the median plane M to the tread surface 212 to the median plane M, - the total thickness at the shoulder (TTY) measured in the meridian plane, on a new tire, is the minimum distance from the axial end 211 of the tread surface 21 and the inner surface 51 of the inner rubber 5, - the axial width LFM of the widest shrinkage layer 311 is at least equal to 65% of the axial width LTM of the widest working layer 321, here 85%.

[0043] The invention is compared to a benchmark tire on the market (“Michelin XTRA- LO AD PROTECT B") of the same dimensions, namely 24.00R35, the top of which is composed of two radially inner reinforcement layers, two radially inner working layers, and two protective layers. The reinforcement elements of the reinforcement layers form angles of 8° and -8°. They are inextensible, made of 26.30 steel wires, each 0.30 mm in diameter, with a structural elongation of 0% on the cables taken from a tire, a total elongation at break of 2.4%, and a modulus tangent to (At+As) / 2 of 185 GPa. They are arranged at a 3.4 mm pitch. The reinforcement elements of the working layers form angles of -33° and 19°. They are inextensible, with the same characteristics as the reinforcement elements of the reinforcement layers, namely 26.30 steel wires. They are arranged at a 3.4 mm interval. The reinforcing elements of the protective layers are expandable in 24.26, i.e., 24 steel wires of 26 hundredths of a millimeter in diameter, forming angles with the circumferential direction of -24 and 24°, arranged at a pitch of 2.5 mm, having a structural elongation on the cables taken from a tire of 0.6% and a total elongation At at break equal to 3.9% and a tangent modulus to (At+As) / 2 at most equal to 75 GPa. The ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 0.212.

[0044] The invention is tested on a tire of the same size, namely 24.00R35, the crown of which is composed of two radially inner working layers, two radially inner reinforcement layers, and two protective layers. The reinforcement elements of the reinforcement layers form angles of 1° and -1°. They are 21.28, i.e., 21 steel wires or monofilaments with a diameter of 28 hundredths of a millimeter, having a structural elongation of 0.5% on the cables taken from a tire, a total elongation At at break of 3.3%, and a tangent modulus to (At+As) / 2 of at most 95 GPa. They are arranged at a pitch of 2.3 mm. The reinforcement elements of the working layers form angles of -33° and +33°. They are extensible, with the same characteristics as the reinforcement elements of the shrink-fit layers, namely 21.28. They are arranged at a pitch of 2.3 mm.The reinforcement elements of the protective layers are 5.35, consisting of 5 steel wires with a diameter of 0.35 mm, forming angles of -33° and 33°. They have a structural elongation of 5% on cables taken from a tire, a total elongation At at break of 8%, and a tangent modulus of (At+As) / 2 equal to 45 GPa, arranged at a 2.2 mm pitch. The structural elongation As of the reinforcement elements of the protective layers is more than 1.3 times the As of the reinforcement elements of the working layers. The tire according to the invention has an L4m value identical to the control tire, but the ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 0.117.

[0045] The use of elastic reinforcing elements made it possible to have a width axial width of the frets increased from 53% of the axial width of the widest working layer, in the control tire, to 73% in the tire according to the invention and to decrease the slope of the carcass layer under the top by almost 50% while keeping maximum shear levels of the same order of magnitude for the tire according to the invention.

[0046] The control tires and the tires according to the invention are identical except for the tread layers and the tread geometry, namely the ETE / ETM ratio and the slope of the carcass layer under the tread. They have the same tread pattern and the same reinforcements for the carcass layer, and the same rubber compounds for the different parts of the tires. The radius of the tread surface has been adapted so that the geometry of the contact patch and the contact pressures are reproduced in order to maintain the same wear performance. The quality and thickness of the inner rubber are unchanged from the control tire to the tire according to the invention.

[0047] The ETE / ETM ratio was reduced by more than 5%, from 1.3 to 1.24, for the same value of the median total thickness (ETM).

[0048] The invention has been simulated using computer tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tires using the finite element technique under large displacements and large deformations, taking into account the mechanical and hysteretic characteristics of the materials. The estimated temperature gain at the shoulder is 7°.

[0049] The tires are also compared by rolling tests. The same test protocol is applied to the tires according to the invention and to the reference tires according to the state of the art.

[0050] Endurance performance against crown cracking, also called straight-line crown cleavage resistance, is measured by machine tests in which two tires of the same type (reference on reference, invention on invention) are driven over each other at a speed of 28 km / h, the tires being inflated to 7.25 bar for a crushing force of 20 t. The test is stopped when one of the tires loses pressure. The result considered is the number of kilometers traveled before the tire fails.

[0051] In this test, the control tires ran for 470 hours and had to be stopped due to a cracking pocket at the ends of the tread layers. The tire according to the invention completed 970 hours of driving without any damage, neither cracking of the tread blocks nor breakage of reinforcing elements.

[0052] Shoulder temperature measurements taken by pricking during taxiing confirmed thermal gains compared to the control, ranging between -4°C and -10°C depending on the position of the pricking point.

[0053] Moreover, the increase in the flexibility of the apex is known to those skilled in the art to be favorable to endurance under aggression and to the reduction of wear related to these aggressions.

[0054] Through this series of tests, the invention has clearly demonstrated its ability to improve endurance, particularly at the ends of the top layers, both for the ends of the top layer reinforcement elements and for the filling rubbers, while maintaining or improving resistance to aggressions for the same expected result in wear.

[0055] A second, larger diameter dimension was also tested. The invention is compared to a standard commercial tire (“Michelin XDR3 MC4 NL”) of the same size, namely 53.80R63, the crown of which is composed of two radially inner reinforcement layers, two radially inner working layers, and two protective layers. The reinforcement elements of the reinforcement layers form angles of 8° and -8°. They are inextensible in 77.35, i.e., 77 steel wires with a diameter of 0.35 mm, having a structural elongation of 0% on the cords taken from a tire, a total elongation at break of 1.7%, and a tangent modulus of (At+As) / 2 of 165 GPa. They are arranged at a 5.5 mm pitch. The reinforcement elements of the working layers form angles of -33° and 19°. They are inextensible, with the same characteristic as the reinforcement elements of the shrink-fit layers, namely 77.35.They are arranged at a 5.5 mm pitch. The reinforcing elements of the protective layers are 52.26, i.e., 52 steel wires of 0.26 mm diameter, and form angles with the circumferential direction of -24° and 24°, arranged at a 3.7 mm pitch, having a structural elongation on the cables taken from a tire of 1.5% and a total elongation At at break equal to 4.2% and a tangent modulus to (At+As) / 2 at most equal to 70 GPa. The ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 0.201.

[0056] The invention is tested on a tire of the same dimensions, namely 53.80R63, the crown of which is composed of two radially inner working layers, two radially inner reinforcement layers, and two protective layers. The reinforcement elements of the reinforcement layers form angles of 1° and -1°. They are 147.28, i.e., 147 steel wires or monofilaments with a diameter of 0.28 mm, having a structural elongation of 0.3% on the cords taken from a tire, a total elongation At at break of 2.5%, and a tangent modulus to (At+As) / 2 of at most 103 GPa. They are arranged at a pitch of 7.4 mm. The reinforcement elements of the working layers form angles of -26° and +26°. They are extensible, with the same characteristics as the reinforcement elements of the shrink-fit layers, namely 147.28. They are arranged at a pitch of 7.4mm. The reinforcement elements of the protective layers are 52.26, consisting of 52 steel wires, each 0.26 mm in diameter, forming angles of -26° and 26°. They have a structural elongation of 1.5% on cables taken from a tire, a total elongation At at break of 4.2%, and a tangent modulus (At+As) / 2 of 70 GPa, arranged at a 3.7 mm pitch. The structural elongation As of the reinforcement elements of the protective layers is more than 1.3 times the As of the reinforcement elements of the working layers. The tire according to the invention has an L4m value identical to the control tire to within 0.26%, but the ratio between the radial distance E85 and the axial length 0.85*L4m is 0.133.

[0057] The use of elastic reinforcing elements made it possible to have an axial width of the hoops increased from 58% of the axial width of the widest working layer, in the control tire, to 72% in the tire according to the invention and to decrease the slope of the carcass layer under the top by almost 35% while keeping maximum shear levels of the same order of magnitude.

[0058] The control tire and the tire according to the invention are identical except for the tread layers and the tread geometry, namely the ETE / ETM ratio, and except for the slope of the carcass layer under the tread. They have the same tread pattern and the same reinforcements for the carcass layer, and the same rubber compounds for the different parts of the tires. The radius of the tread surface has been adapted so that the geometry of the contact patch and the contact pressures are reproduced in order to maintain the same wear performance. The quality and thickness of the inner rubber are unchanged from the control tire to the tire according to the invention.

[0059] The ETE / ETM ratio was reduced by more than 12%, from 1.26 to 1.09, for the same value of the median total thickness (ETM).

[0060] The invention has been simulated using computer tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tires using the finite element technique under large displacements and large deformations, taking into account the mechanical and hysteretic characteristics of the materials. The estimated temperature gain at the shoulder is 4°.

[0061] The increase in the flexibility of the apex will also be favorable to endurance under stress and to the reduction of wear related to these stresses.

[0062] These calculation elements show the ability of the invention to improve endurance, particularly at the ends of the top layers, both for the ends of the top layer reinforcement elements and for the filling rubbers, while maintaining or improving resistance to aggression for the same expected result in wear over a wide range of civil engineering tire sizes.

Claims

1. Demands Radial tire (1) for construction vehicles intended to be mounted on a rim with a nominal diameter greater than 25 inches, comprising: • a carcass reinforcement extending between the two ribs (7), each of the ribs (7) comprising a circumferential reinforcement or rod (6), and consisting of a carcass layer (4) comprising metallic reinforcements forming an angle, with a radial direction, between -10° and 10°, the carcass layer (4) comprising a main portion (41) going from one rod (6) to the other and two turns (42) each going from one rod (6) to one end of the carcass layer (4), • each bead (7) comprising a protector (71) made of a rubbery mixture, the outermost axial points of which are intended to come into contact with a mounting rim and in each meridian a point Mel minimizing the axial distance of the rod (6) to the outermost axial points of the protector (7), • a crown reinforcement (3), radially internal to a tread (2) and radially external to the carcass reinforcement (4), • the tread (2) having a running surface (21) intended to come into contact with a running surface, the running surface having, for each meridian plane, two axial extremities (211), • an inner rubber (5) having an inner surface (51) intended to be in contact with an inflation gas, • a median plane (M) perpendicular to the axis of rotation (YY') of the tire and passing through the middle of the tread (2), a median total thickness (ETM) being the radial distance from the innermost radial point (512) of the tire in the median plane (M) to the tread surface (212), • each point Mel being positioned at an axial distance from the median plane equal to half the nominal rim width plus 1.27 cm, the outermost axial point of the main portion (41) of the carcass layer being at a distance axial L4m, point Me2 being the point in the carcass layer at an axial distance of 0.85*L4m and at a radial distance E85 from the point of intersection between the median plane M and the carcass layer, a total shoulder thickness (TST) measured in a meridian plane, being the minimum distance from an axial end (211) of the tread surface (21) and the inner surface (51) of the inner rubber (5), the crown reinforcement (3) comprising, at least one working reinforcement (32), and a protective reinforcement (33) radially external to the working reinforcement, the working reinforcement (32) comprising at least two working layers (321, 322) and the protective reinforcement (33) comprising at least one protective layer (331), each working layer (321, 322) comprising metallic reinforcing elements, parallel to each other, forming, with the circumferential direction, oriented angles whose absolute value is at least equal to 15° and at most equal to 45°, the angles of two working layers being of opposite sign from one working layer to the next, Each reinforcement element in each layer of the top reinforcement (3) is characterized by a structural elongation As and a total elongation at break At, these characteristics being measured according to ASTM D 2969-04:2014. Each protective layer (331) comprises reinforcement elements parallel to each other, forming oriented angles with the circumferential direction whose absolute value is at least 15° and at most 45°. The reinforcement elements in the layers (311, 312, 321, 322, 331) of the top reinforcement (3) have a structural elongation As of at least 0.3% and a total elongation At at break of at least 3% and a modulus tangent to (At+As) / 2 of at most 120 GPa. The reinforcement elements in each protective layer have a structural elongation As. at least equal to 1.3 times the structural elongation As of the reinforcement elements of the innermost radially working layer (321), • in that the ratio of the total thickness at the shoulder (ETE) to the total median thickness (ETM) is at most equal to 1.25, • and in that the ratio between the radial distance E85 and the axial length 0.85*L4m is between 0.10 and 0.

15.

2. Pneumatic (1) according to claim 1, wherein the top reinforcement (3) comprises a shrink-fit reinforcement (31), radially internal to the protective reinforcement, comprising at least one shrink-fit layer (311, 312) comprising elastic metallic reinforcing elements forming, with the circumferential direction, oriented angles whose absolute value is at least equal to 0° and at most equal to 10°, preferably at most equal to 2°.

3. Pneumatic (1) according to any one of claims 1 or 2, wherein the axial width (LFM) of the greatest axial width shrinkage layer is at least equal to 65% of the axial width (LTM) of the greatest axial width working layer.

4. Pneumatic (1) according to any one of the preceding claims wherein the ratio of the total thickness at the shoulder (TST) to the total mid-thickness (MST) is at most equal to 1.

20.

5. Pneumatic (1) according to any one of the preceding claims wherein the tangent modulus to (At+As) / 2 of the reinforcing elements of the working layers is at most equal to 80 GPa.

6. Pneumatic (1) according to any one of the preceding claims wherein rubber compositions of axial width of at least 5 mm and radial thickness of at least 3 mm are positioned between the axial ends of the working layers and each adjacent layer of metallic reinforcing elements, the elongation of said rubber compositions at break at 100°C according to standard NF T 46-002 being at least 500%, and the maximum dynamic loss tanô of said apex decoupling rubbers, measured according to standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, being at most 0.

06.

7. Pneumatic (1) according to any one of the preceding claims wherein the elastic reinforcing elements of at least two layers (311, 312, 321, 322, 331) of the top reinforcement are metal cables composed of several monofilaments, said monofilaments having a diameter of at most equal to 0.32 mm and preferably at most equal to 0.29 mm.

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