Optimized tread and crown of civil engineering tire

Abstract

Radial tire for civil engineering vehicles, the crown of which comprises two working layers (321, 322) and at least one protective layer. Each crown layer comprises elastic metallic reinforcing elements, the protective layer having reinforcing elements 30% more elastic than the working layers. The crown is characterized by the thickness (ETE) at the shoulder and the thickness (ETM) at the center of the tread. The crown thus described is optimal in terms of balance of performance in endurance and wear with an ETE / ETM ratio at most equal to 1.25. The point Me2 of the carcass layer located at a distance of 0.85 times L4m, the distance to the point furthest 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. Abstract figure: figure 1

Description

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

[0001] The present invention relates to a radial tire, intended to equip a heavy civil engineering vehicle, and more particularly relates 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 European Tire and Rim Technical Organization (ETRTO) standard.

[0003] A radial tire for a heavy vehicle of the civil engineering type, within the meaning of the European Tyre and Rim Technical Organization (ETRTO) standard, is intended to be mounted on a rim whose diameter is at least equal to 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, in particular vehicles for transporting materials extracted from quarries or surface mines, by means of a rim whose diameter is at least equal to 35 inches and can reach 57 inches, or even 63 inches. Civil engineering type tires have tread heights when new at least equal to 70 mm.

[0004] A tire having a geometry of revolution relative to an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively designate the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane. The circumferential direction is tangent to the circumference of the tire.

[0005] In the following, the expressions "radially inward", respectively "radially outward" mean "closer", respectively "further from the axis of rotation of the tire". By "axially inward", respectively "axially outward", is meant "closer", respectively "further from the equatorial plane of the tire", the equatorial plane of the tire being the plane passing through the middle of the rolling surface and perpendicular to the axis of rotation. By "an element A axially inward to an element B by an axial distance D", is meant that the element A is closer to the equatorial plane than the element B and that the axial distance between the two elements is equal to the distance D. This type of sentence can be generalized with the radial and circumferential direction and the outer position versus interior, of one or the other of the elements.

[0006] Generally speaking, a tire comprises a tread, intended to come into contact with a ground via a rolling surface, the two axial ends of which are connected via two sidewalls with two beads ensuring the mechanical connection between the tire and the rim on which it is intended to be mounted. Once mounted on a mounting rim, the tire and the rim delimit an interior volume intended to contain a gas under pressure. The surface and the rubber mixture in contact with the gas are called respectively the interior surface and the interior rubber. This interior rubber has sealing properties suitable for preventing the diffusion of oxygen into the tire to prevent the oxidation of the tire's constituents.

[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 a heavy civil engineering vehicle usually comprises at least one carcass layer comprising metal reinforcements, coated with a polymeric material of the elastomer or elastomeric type, obtained by mixing and called a coating mixture. A carcass layer comprises a main part, connecting the two beads together and generally winding, in each bead, from the inside to the outside of the tire around a circumferential reinforcing element, most often metal, called a bead wire, to form a turn-up. The metal reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle of between 80° and 90°.

[0009] The crown reinforcement of a radial tire for a civil engineering vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of generally metallic reinforcements, 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 are deduced the mechanical characteristics in tension of the metal reinforcement, such as the structural elongation As (in %), the total elongation at break At (in %), the force at break Fm (maximum load in N) and the breaking strength Rm (in MPa), these characteristics being measured according to the standard ASTM D 2969-04 of 2014.

[0011] The total elongation At of the metal reinforcement is, by definition, the sum of its al structural, elastic and plastic elongations (At = As + Ae + Ap) and particularly at break where each of the elongations is non-zero. The structural elongation As results from the relative positioning of the metal wires constituting the metal reinforcement under a low tensile force. The elastic elongation Ae results from the very elasticity of the metal of the metal wires, constituting the metal reinforcement, taken individually, the behavior of the metal following a Hooke law. The plastic elongation Ap results from the plasticity, that is to say from the irreversible deformation, beyond the elastic limit, of the metal of these metal wires taken 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] We also define, at any point on the force-elongation curve of a metal reinforcement, an extension modulus, expressed in GPa, which represents the slope of the line tangent to the stress-elongation curve at this point. In particular, the elastic extension modulus or Young's modulus is the extension modulus of the linear elastic part of the force-elongation curve.

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

[0014] An elastic metal reinforcement, in its rubberized state from the tire, is characterized by a structural elongation As at least equal to 0.3% and a total elongation at break At at least equal to 3%. In addition, an elastic metal reinforcement has an elastic modulus in extension at most equal to 150 GPa, and 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 extension usually between 150 GPa and 200 GPa.

[0016] Among the crown layers, a distinction is usually made between the protective layers, constituting the protective reinforcement and radially the outermost, comprising elastic reinforcing elements (or reinforcements) and the working layers comprising inextensible reinforcing elements, constituting the working reinforcement and radially 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 attacks, likely to propagate through the tread radially towards the inside of the tire.

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

[0019] The working reinforcement, comprising at least two working layers, has the function of surrounding the tire and giving it rigidity and road holding. It absorbs both the mechanical stresses of inflation, generated by the inflation pressure of the tire and transmitted by the carcass reinforcement, and the mechanical stresses of rolling, generated by the rolling of the tire on the ground and transmitted by the tread. It must also be resistant to oxidation and to impacts and perforations, thanks to its intrinsic design and that of the protective reinforcement.

[0020] The working reinforcement usually comprises two working layers, radially superimposed, formed of non-extensible metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 60°, and, preferably, at least equal to 15° and at most equal to 45°. To reduce the shearing of the rubber compounds, or rubber mixtures, at the axial ends of the working layers, it is usual to axially offset the position of said ends relative to each other. The crown reinforcement therefore usually comprises a working layer of greater axial width and a working layer of smaller axial width. The shearing of the rubber compounds is maximum at the end of the working layer of smaller axial width.Indeed, these maximum shears due to the displacements of the end of the working layer of smaller axial width are distributed over the radial thickness of rubber compounds between the working layer of smaller axial width and the working layer of greater axial width. These shears are amplified by the deformations of the working layer of greater axial width. Indeed, given the angle of the crossed metal reinforcements with the metal reinforcements of the working layer of smaller width, the working layer of greater axial width deforms in another 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 working layer of smaller axial width and the working layer of greater axial width.The end of the working layer with greater axial width is also subjected to strong shears but generally of lesser amplitude given that for 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 shearing of the rubber mixture which covers them, it is known to arrange, radially outside the carcass reinforcement, a hoop reinforcement. The hoop reinforcement, the function of which is to take up at least part of the mechanical stresses of inflation, improves the endurance of the crown reinforcement by stiffening the crown reinforcement. The hoop 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 hoop reinforcement may comprise two radially superimposed hoop layers formed of metal reinforcements, parallel to each other in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 10°. To produce the hoop layers, on large diameter civil engineering tires, the winding of a layer of discontinuous reinforcing elements whose ends go from one axial edge of the layer to the other forming with the circumferential direction an angle of between 7 and 10°, may be preferred to the winding of a narrow strip of a few continuous reinforcing elements forming with the circumferential direction an angle of between 0° and 5° for productivity reasons. In both cases, the hoop layers are of smaller axial width than the working layer of smaller axial width.Indeed, the stresses due to rolling at the end of the working layers are very high in tension and compression and lead to the rupture of the inextensible metal reinforcements arranged around the ends whose angles formed with the circumferential direction are less than 15°. Document WO2019 / 202239 discloses a usual architecture of civil engineering tires comprising two hooping layers of rigid reinforcing elements, radially inside two working layers of rigid reinforcing elements radially inside elastic protection layers, the two hooping layers having an axial width substantially less than the axial width of the working layers.

[0023] These tires often have a large tread width compared to the total transverse width of the tire, involving crown layers with an equally large axial width. During inflation, for this type of tire, the crown deforms with a relative rise in the center and a relative fall at the shoulders, which gives, for a constant sculpture height, significant wear phenomena at the shoulders.

[0024] Furthermore, if the carcass layer is manufactured with a shape far from its equilibrium curve, the pressurization will deform the carcass layer for the bring closer to its pressure equilibrium curve, putting certain parts of the top, in particular the reinforcement elements of the hoop layers, according to their positions in tension or compression. For good endurance, it is advisable that the reinforcement elements of the hoop layers are not in absolute compression on any of its segments from the moment of inflation, the compression of reinforcement elements having the effect of very significantly deteriorating their fatigue resistance.

[0025] Designing a tire therefore involves 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 tire that is less efficient in terms of wear. Opting for a less round 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 height multiplied by the axial width of the tread.

[0026] Such a compromise is usually settled by associating the crown reinforcement with the carcass by means of rubber compounds, or rubber composition(s), called crown fillers at the shoulder, the shoulders representing the 20% of the tread that are most radially outer on either side of the median plane. These crown filler compounds are all the thicker as the crown is wide. In the center of the tread, the thickness of these crown filler 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 composition between the carcass reinforcement and the crown reinforcement in the center, but in this case, this thickness does not meet geometric constraints.On the widest part of the crown reinforcement, the filler rubbers are useless, the crown reinforcement allowing to stabilize the position of the carcass layer. At the shoulder, the thicknesses of the filler rubbers are maximum. This solution brings a high thermal at the crown, particularly at the shoulders, which tends to degrade the endurance. If the thicknesses of these rubbers are reduced, the carcass layer which, under the effect of the pressure, will be at the level of its equilibrium curve, will bring the filler rubbers, the ends of the crown layers, the tread, to a lower radial position at the shoulders, and will put certain reinforcement elements in compression. When rolling, the shoulders align with the center in the contact patch by crushing on the ground, and puts in relative extension the reinforcement elements in the shoulder zone. This passage repeated. with each wheel revolution from the extension phase to the compression phase, can cause ruptures of the reinforcing elements at the ends of the crown layers, and leads to high shear rates at the axial ends of the working layers conducive to the creation of cracks which, depending on the case, open onto the sides or towards the carcass layer.

[0027] Maintaining a large volume of crown filler rubbers considerably increases the thermal stress in this area, which can lead to thermal failure of the crown or premature aging of the edge rubbers, leading to a decrease in the cohesion of their materials, a decrease favorable to the creation of cracks. Reducing the thickness of the crown filler rubbers increases the maximum shears they undergo and normally reduces endurance. The edge rubbers are rubber mixtures positioned between the axial ends of the working layers and each adjacent layer of metallic reinforcing elements.

[0028] To reduce cracking, tire manufacturers seek above all to increase the rigidity of the crown by introducing hoop layers and to optimize their operation, for example, as shown in document US 2018 / 0056723 A1 where the hoop layers are between the working layers, and the working layers are recoupled to improve their rigidity. However, these solutions are complex in terms of manufacturing, recoupling being sometimes difficult to carry out with large diameter metal reinforcement elements for heavy vehicles and the stiffening of the crown is not very favorable to resistance to perforation by rocks, a fundamental performance for civil engineering tires.

[0029] The inventors set themselves the objective for a civil engineering tire of improving endurance, particularly at the ends of the crown layers, both for the ends of the reinforcing elements of the crown layers and for the filling rubbers, while maintaining or improving resistance to attack for the same wear result.

[0030] This objective has been achieved, according to the invention, by a radial tire for a civil engineering vehicle intended to be mounted on a rim with a nominal diameter greater than 25 inches comprising: • a carcass reinforcement extending between the two beads, each of the beads comprising a circumferential reinforcement or bead wire, and consisting of a carcass layer comprising metal reinforcements forming an angle, with a radial direction, of between -10° and 10°, the carcass layer comprising a main portion going from one bead wire to the other and two turns each going from one bead wire to one end of the carcass layer carcass, each bead comprising a protector made of a rubber mixture whose axially outermost points are intended to come into contact with a mounting rim and in each meridian a Mel point minimizing the axial distance from the bead to the axially outermost points of the protector, a crown reinforcement radially inside a tread and radially outside the carcass reinforcement, the tread having a rolling surface intended to come into contact with a rolling ground, the rolling surface 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 most radially inner point of the tire in the median plane (M) to the rolling surface, each point Mel being positioned at an axial distance from the median plane equal to half the width of the nominal rim plus 1.27 cm, the axially outermost point of the main portion of the carcass layer being at an axial distance L4m, the 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 thickness at the shoulder measured in a meridian plane, being the minimum distance from an axial end of the rolling surface and the inner surface of the inner rubber, the crown reinforcement comprising, at least one working reinforcement, and a protective reinforcement radially outside 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 reinforcing element of each of the layers of the crown 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 crown reinforcement layers having a structural elongation As at least equal to 0.3% and a total elongation At at break at least equal to 3% and a tangent modulus at (At+As) / 2 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 radially innermost 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 crown layers to act as a protective layer, it is appropriate that this layer of reinforcing elements only absorbs a small portion of the inflation and rolling forces. For this, a sufficient condition is that the reinforcing elements of said protective layer are significantly more elastic than the reinforcing elements of the working layers and therefore of the radially innermost working layer. According to the invention, the protective reinforcement being radially external to the working reinforcement and to the possible hoop reinforcement, there is at least one protective layer, a layer of metallic reinforcing elements, the radially outermost, the reinforcing elements of which have a structural elongation As at least equal to 1.3 times the structural elongation As of the reinforcing elements of the radially innermost working layer.The protective reinforcement possibly comprises two protective layers, each being radially external to the working reinforcement and the hoop reinforcement, the most radially inner protective layer will also be elastic and with a significantly higher structural elongation, of at least 30%, than the structural elongation of the most radially inner working layer. For reasons of productivity and standardization, the architecture of the reinforcing elements of the two protective layers, namely the diameters of the unit wires and the way in which these wires are arranged relative to each other, will preferably be identical. For better performance in crown rupture under aggression, it is advantageous to have two different protective layers. A preferred solution for puncture resistance is that the . reinforcing elements are crossed from one protective layer to the next, i.e. the angles formed by the reinforcing elements of the two protective layers and the circumferential direction are of opposite signs.

[0032] According to the invention, all of the reinforcing elements of the crown reinforcement are elastic. For an identical crown geometry, this specificity leads to greater fatigue failure performance under tensile and compressive stresses. Indeed, elastic reinforcing elements resist buckling much better than inextensible reinforcing elements provided that the structural elongation is at least equal to 0.3% and the total elongation At at break is at least equal to 3% and that the tangent modulus at (At+As) / 2 is at most equal to 120 GPa. Their lower rigidity surprisingly makes it possible to reduce the stresses in the packing rubbers whereas it should increase the deformation of the crown reinforcing elements and therefore the shearing of said rubbers. This reduction makes it possible to modify the geometry of the crown and to reduce the thicknesses of the packing rubbers at the crown, without degrading the endurance.One way to compare the thickness of the filling rubbers of one tire to another without taking into account the difference in the thickness of the crown layers is to compare the ratio of the thickness of the tire at the center of the crown, where it is the smallest, and the thickness of the tire at the shoulder where it is the greatest. However, to see if this change is due to an increase in the thickness of the tread volume, it is also necessary to consider the slope of the carcass layer under the crown. In fact, the crown layers are substantially axial, their role consisting, among other functions, of forming a cylindrical shell on which a tread is placed. The more cylindrical it is, the more it helps avoid differences in wear. The slope of the carcass is therefore essentially a function of the thickness of the crown filling rubbers between the carcass layer and the radially innermost crown layer.By using a crown whose reinforcing elements are all elastic, the inventors were able to reduce the thickness of these crown filling rubbers while maintaining the tire's endurance. Above a certain slope, namely 0.15, a tire with such characteristics of crown layer reinforcing elements is not optimized in terms of endurance and below a slope of 0.1, its endurance will be insufficient.

[0033] This slope measurement must be made in a situation close to the time 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 to distort the measurement of the slope of the carcass 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 of approximately 2.54 cm (one inch) on the total axial width, i.e. 1.27 cm more than the nominal half-rim in relation to the distance from the bead to the median plane. By mounting rim width, we mean the axial distance between the substantially radial portions of the rim intended to bear on the tire beads as defined in the ETRTO technical dictionary. This difference between the width of the curing mold and the nominal rim allows the beads to naturally bear on the rim during mounting and the tire to be naturally substantially airtight. This measurement is taken either on one or more cross-sections of the tire or on a dismounted tire with the beads correctly positioned as specified. The slope is measured at 0.85 times the axial width of the main portion of the carcass layer which gives a good estimate of the axial width of the crown layers. The ratio between the radial distance E85 and the axial length 0.85*L4m is a measure of this carcass layer slope. State-of-the-art tires, which primarily include non-elastic working layers, have crown slopes between 0.19 and 0.25, to ensure acceptable endurance.

[0034] A crown dimension that also has a significant impact on endurance performance with respect to shoulder cracking is the distance from the axial end of the tread surface to the inner surface of the inner rubber, called the total shoulder thickness. There is a total shoulder thickness on each side of the meridian plane. They are usually substantially equal, apart from manufacturing variations. The measurement is carried out either on one or more meridian sections of the tire or on a disassembled tire. In the case of multiple measurements, the average of these will be considered. In the case of a cut in the tread at the shoulder, the measurement is carried out at the point on the tread surface that maximizes the total shoulder thickness. In general, 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 from the crown in the new condition to the median plane is called the median total thickness. It is measured on a new, disassembled tire or on a meridian section as for the total shoulder thickness. By new tire, we mean on a tire or piece of tire whose sculpture has not been modified compared to its exit from the curing mold. The median total thickness is the radial distance from the most radially inner point of the tire at the median plane to the rolling surface at the median plane. In the absence of a central groove, the median total thickness will be measured from a most radially outer point of the rolling surface in an obvious 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 rolling surface at the median plane as a function of the positions of the height blocks axially closest to the median plane by a simple linear approximation.

[0036] The tires according to the state of the art present on the market, comprising inextensible reinforcement elements, have a ratio of the total thickness at the shoulder to the total median thickness generally between 1.3 and 1.4. By using elastic reinforcement elements, it is possible to optimize the total thickness at the shoulder 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 that the crown reinforcement comprises a hoop reinforcement radially inside the protective reinforcement, comprising at least one hoop layer comprising elastic metal 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°. The preferred solution is to circumferentially wind a strip of one to several reinforcing elements with an angle of less than 2°. This solution allows better absorption of forces by the elastic hoop layers.

[0038] The use of elastic reinforcing elements for all the reinforcing elements of the crown reinforcement increases the elasticity of the crown and allows for homogenization of the stresses in all the crown layers and a reduction in the stresses of the hoop layers. However, the endurance of the reinforcing elements is strongly linked to their extension-compression cycle. It is therefore possible for the same level of endurance to increase the width of the hoop layers compared to the working layers. This makes it possible to lower the level of shear at the ends of the working layers and the hoop layers, and thus to increase the endurance in this zone, the objective of the invention.In market tires, the axial width of the largest axial width hoop layer, due to the stiffness of its reinforcing elements, is at most equal to 55% of the axial width of the largest axial width working layer. To improve crown endurance, a preferred solution is that the axial width of the largest axial width hoop layer is at least equal to 65% of the axial width of the largest axial width working layer. This is made possible by the use of elastic reinforcing elements for all crown layers.

[0039] Experiments have shown that the crown endurance is further improved if the tangent modulus at (At+As) / 2, of the reinforcing elements of the working layers, is at most equal to 80 GPa. Taking into account the increase in crown elasticity and the increases in width of certain crown layers, the rubber mixtures at the ends of the working layers can be more mechanically stressed and produce by hysteresis a local increase in temperatures. To overcome this problem, one solution is that rubber compositions with an axial width of at least 5 mm and a 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 equal to 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 equal to 0.06.

[0040] The elastic metal reinforcing elements of the crown layers are usually made up of several steel monofilaments or wires assembled together. To obtain an elastic reinforcing element, the geometry of the assembly of the monofilaments initially allows the reinforcing elements to deform by the displacement of the monofilaments in the reinforcing element by shearing the rubber between the monofilaments. In a second stage, the deformation of the reinforcing elements is mainly done by the deformations of the monofilaments.This property, allowing the first phase of structural deformation, implies that the diameters of the reinforcing elements are often large, which implies, for a number of crown layers and a certain thickness of the tread, very large total median and total shoulder thicknesses, implying a high temperature at the crown during rolling, limiting the possible gains in endurance brought about by the use of elastic reinforcing elements. To limit the 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 that the elastic reinforcing elements of at least two layers of the crown reinforcement are metal cables composed of several monofilaments, said monofilaments having a diameter at most equal to 0.32 mm and preferably at most equal to 0.29 mm, preferably from all top layers.

[0041] The characteristics of the invention are illustrated by [Fig.l] schematically and not shown to scale, with reference to a civil engineering tire.

[0042] [Fig.l] represents a half-meridian 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 tori substantially symmetrical apart from manufacturing variations, and an axially outer point 211, - a carcass reinforcement, comprising a carcass layer 4 of which the metal reinforcing elements forming an angle, with a radial direction, between -10° and 10°, the carcass layer, extending between the two beads 7, each of the beads 7 comprising a circumferential reinforcement or bead wire 6; the carcass layer 4 comprising a main portion 41 going from one bead wire 6 to the other and two turns 42 each going from a bead wire 6 to one end of the carcass layer 4, - Each bead 7 comprising a protector 71 and for each meridian a Mel point minimizing the axial distance from the rod 6 to the most axially outer points of the protector 7, - When the points Mel are at a distance from the median plane M equal to half the width of the mounting rim plus 1.27 cm Ljc, the most axially outer point of the main portion 41 of the carcass layer 4 is at a distance L4m, which makes it possible to construct the point Me2 of the carcass layer at an axial distance of 0.85*L4m, this point Me2 being radially outer to the most axially outer 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 radially innermost point 512 on the median plane M, - a crown reinforcement 3, comprising a working reinforcement 32, a hoop reinforcement 31 and a protective reinforcement 33 - the hooping reinforcement 31, comprising two hooping layers 311 and 312, - the working reinforcement 32, comprising two working layers 321 and 322, - a protective frame 33, radially external to the hoop frame and the working reinforcement here comprising two protective layers 331, 332, the most radially inner protective layer being the crown layer of greatest axial width to protect all of the other crown reinforcement elements from crown attacks, - the median total thickness ETM measured on the new tire, is the radial distance from the most radially inner point 512 of the tire at the median plane M to the rolling surface 212 at the median plane M, - the total shoulder thickness (TST) measured in the meridian plane, on a new tire, is the minimum distance from the axial end 211 of the rolling surface 21 and the inner surface 51 of the inner rubber 5, - the axial width LFM of the wider hooping layer 311 is at least equal to 65% of the axial width LTM of the wider working layer 321, here 85%.

[0043] The invention is compared to a control tire on the market (“Michelin XTRA- LO AD PROTECT B ») of the same dimension, namely 24.00R35, the top of which is composed of two radially inner hoop layers to two radially inner working layers to two protective layers. The reinforcement elements of the hoop layers form angles of 8 and -8°. They are inextensible in 26.30, i.e. 26 steel wires of 30 hundredths of a millimeter in diameter, having a structural elongation on the cables taken from a tire of 0% and a total elongation At at break equal to 2.4% and a tangent modulus at (At+As) / 2 equal to 185 GPa. They are arranged in a pitch of 3.4 mm. 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 hoop layers, namely 26.30. They are arranged in 3.4 mm increments. The reinforcement elements of the protective layers are extensible in 24.26 or 24 steel wires of 26 hundredths of a millimeter in diameter and forming angles with the circumferential direction of -24 and 24°, arranged in 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 at (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 with two radially inner hooping layers with two protective layers. The reinforcing elements of the hooping layers form angles of 1 and -1°. They are extensible in 21.28, i.e. 21 steel wires or monofilaments of 28 hundredths of a millimeter in diameter, having a structural elongation on the cables taken from a tire of 0.5% and a total elongation At at break equal to 3.3% and a tangent modulus at (At+As) / 2 at most equal to 95 GPa. They are arranged at a pitch of 2.3 mm. The reinforcing elements of the working layers form angles of -33 and +33°. They are extensible, with the same characteristics as the reinforcement elements of the hoop layers, namely 21.28. They are arranged in a pitch of 2.3 mm.The reinforcing elements of the protective layers are extensible in 5.35 or 5 steel wires of 35 hundredths of a millimeter in diameter and form angles of -33 and 33°, having a structural elongation on cables taken from a tire of 5% and a total elongation At at break equal to 8% and a tangent modulus at (At+As) / 2 equal to 45 GPa arranged in a pitch of 2.2 mm. The structural elongation As of the reinforcing elements of the protective layers is greater than 1.3 times the As of the reinforcing elements of the working layers. The tire according to the invention has a value L4m identical to the control 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 wider working layer, in the control tire, to 73% in the tire according to the invention and to reduce the slope of the carcass layer under the crown by almost 50% while maintaining maximum shear levels of the same order of magnitude for the tire according to the invention.

[0046] The control tires and those according to the invention are identical except for the crown layers and the geometry of the crown, namely the ETE / ETM ratio and the slope of the carcass layer under the crown. 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 rolling 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 was simulated on calculation tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tires using the finite element technique in large displacements and large deformations, taking into account the mechanical and hysteretic characteristics of the materials. The estimated gain in temperature 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] The crown crack endurance performance, also called crown straight line cleavage resistance, is measured by machine tests in which two tires of the same type (reference over reference, invention over invention) roll over each other at a speed of 28 km / h, the tires being inflated to 7.25 b for a crushing force of 20 t. The test is stopped at the moment of loss of pressure in one of the tires. The result considered is the number of kilometers traveled before the tire fails.

[0051] In this test, the control tires were driven for 470 hours and had to be stopped due to a cracking pocket at the ends of the crown layers. The tire according to the invention was driven for 970 hours without any damage, nor cracking of the crown filling rubbers nor rupture of the reinforcing elements.

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

[0053] Furthermore, the increase in the flexibility of the crown is known by those skilled in the art to be favorable to endurance under stress and to the reduction of wear linked to these stresses.

[0054] By this series of tests, the invention has clearly demonstrated its capacity to improve endurance, in particular at the ends of the crown layers, both for the ends of the reinforcing elements of the crown layers and for the filling rubbers, while maintaining or improving resistance to attack for the same expected wear result.

[0055] A second dimension of larger diameter was also tested. The invention is compared to a control tire on the market ("Michelin XDR3 MC4 NL") of the same dimension, namely 53.80R63, the crown of which is composed of two radially inner hoop layers to two radially inner working layers to two protective layers. The reinforcement elements of the hoop layers form angles of 8 and -8°. They are inextensible in 77.35, i.e. 77 steel wires of 35 hundredths of a millimeter in diameter, having a structural elongation on the cables taken from a tire of 0% and a total elongation At at break equal to 1.7% and a tangent modulus at (At+As) / 2 equal to 165 GPa. They are arranged at a pitch of 5.5 mm. The reinforcement elements of the working layers form angles of -33 and 19°. They are inextensible, with the same characteristics as the reinforcing elements of the hoop layers, namely 77.35.They are arranged in a pitch of 5.5 mm. The reinforcing elements of the protective layers are extensible in 52.26 or 52 steel wires of 26 hundredths of a millimeter in diameter and form angles with the circumferential direction of -24 and 24°, arranged in a pitch of 3.7 mm, 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 at (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 size, namely 53.80R63, the crown of which is composed of two radially inner working layers with two radially inner hooping layers with two protective layers. The reinforcing elements of the hooping layers form angles of 1 and -1°. They are extensible in 147.28, i.e. 147 steel wires or monofilaments of 28 hundredths of a millimeter in diameter, having a structural elongation on the cables taken from a tire of 0.3% and a total elongation At at break equal to 2.5% and a tangent modulus at (At+As) / 2 at most equal to 103 GPa. They are arranged at a pitch of 7.4 mm. The reinforcing elements of the working layers form angles of -26 and +26°. They are extensible, with the same characteristics as the reinforcement elements of the hoop layers, namely 147.28. They are arranged in a pitch of 7.4mm. The reinforcing elements of the protective layers are extensible in 52.26 or 52 steel wires of 26 hundredths of a millimeter in diameter and form angles of -26 and 26°, having a structural elongation on cables taken from a tire of 1.5% and a total elongation At at break equal to 4.2% and a tangent modulus at (At+As) / 2 equal to 70 GPa arranged in a pitch of 3.7 mm. The structural elongation As of the reinforcing elements of the protective layers is greater than 1.3 times the As of the reinforcing elements of the working layers. The tire according to the invention has a value L4m identical to the control to within 0.26% but the ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 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 wider working layer, in the control tire, to 72% in the tire according to the invention and to reduce the slope of the carcass layer under the crown by almost 35% while maintaining maximum shear levels of the same order of magnitude.

[0058] The control tires and those according to the invention are identical except for the crown layers and the geometry of the crown, namely the ETE / ETM ratio, and except for the slope of the carcass layer under the crown. 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 rolling 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 was simulated on calculation tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tires using the finite element technique in large displacements and large deformations, taking into account the mechanical and hysteretic characteristics of the materials. The estimated gain in temperature at the shoulder is 4°.

[0061] Increasing the flexibility of the crown will also be beneficial to endurance under stress and to reducing wear and tear associated with these stresses.

[0062] These calculation elements show the ability of the invention to improve endurance, particularly at the ends of the crown layers, both for the ends of the reinforcing elements of the crown layers and for the filling rubbers, while maintaining or improving resistance to attack for the same expected wear result over a wide spectrum of civil engineering tire dimensions.

Claims

1. Claims Radial tire (1) for civil engineering vehicles intended to be mounted on a rim with a nominal diameter greater than 25 inches comprising: • a carcass reinforcement extending between the two beads (7), each of the beads (7) comprising a circumferential reinforcement or bead wire (6), and consisting of a carcass layer (4) comprising metal reinforcements forming an angle, with a radial direction, of between -10° and 10°, the carcass layer (4) comprising a main portion (41) going from one bead wire (6) to the other and two upturns (42) each going from a bead wire (6) to one end of the carcass layer (4), • each bead (7) comprising a protector (71) made of a rubber mixture whose axially outermost points are intended to come into contact with a mounting rim and in each meridian a Mel point minimizing the axial distance from the bead wire (6) to the axially outermost points of the protector (7), • a crown reinforcement (3), radially inside a tread (2) and radially outside the carcass reinforcement (4), • the tread (2) having a rolling surface (21) intended to come into contact with a rolling ground, the rolling surface having, for each meridian plane, two axial ends (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 most radially inner point (512) of the tire in the median plane (M) to the rolling surface (212), • each Mel point being positioned at an axial distance from the median plane equal to half the width of the nominal rim plus 1.27 cm, the most axially outer point of the main portion (41) of the carcass layer being at a distance axial 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 (ETE) measured in a meridian plane, being the minimum distance from an axial end (211) of the rolling 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 outside 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 reinforcing element of each of the layers of the crown reinforcement (3) being characterized by a structural elongation As, a total elongation at break At, these characteristics being measured according to the standard ASTM D 2969-04 of 2014, each protective layer (331) 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°, characterized in that the reinforcing elements of the layers (311, 312, 321, 322, 331) of the crown reinforcement (3) have a structural elongation As at least equal to 0.3% and a total elongation At at break at least equal to 3% and a tangent modulus at (At+As) / 2 at most equal to 120 GPa, in that the reinforcing elements of each protective layer have an elongation structural As at least equal to 1.3 times the structural elongation As of the reinforcing elements of the radially innermost 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. A tire (1) according to claim 1, wherein the crown reinforcement (3) comprises a hoop reinforcement (31), radially inside the protective reinforcement, comprising at least one hoop layer (311, 312) comprising elastic metal 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. A tire (1) according to any one of claims 1 or 2, wherein the axial width (LFM) of the hooping layer of greater axial width is at least equal to 65% of the axial width (LTM) of the working layer of greater axial width.

4. Tire (1) according to any one of the preceding claims in which the ratio of the total thickness at the shoulder (ETE) to the total median thickness (ETM) is at most equal to 1.

20.

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

6. Tire (1) according to any one of the preceding claims in which rubber compositions of axial width at least equal to 5 mm and radial thickness at least equal to 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 equal to 500%, and the maximum dynamic loss tanô of said crown decoupling rubbers, measured according to standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, being at most equal to 0.

06.

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

Images