Optimized structure of civil engineering tire

Abstract

The invention relates to a civil engineering tyre (1) comprising a first working ply having a width L1, said first working ply comprising a metal reinforcement at an angle A, and a second and a third working ply having widths L2 and L3, the second and third working plies comprise metal reinforcements at angles A12 and A13 on the widths L21 and L31 at the center, and second elastic metal reinforcements at angles A22 and A23 both less than 8 degrees on the axial outer sides of L21 and L23 on both sides of the median plane. The radially outermost ply of the tyre is two protective plies (31) with axial widths of LPN1 and LPN2, including elastic reinforcements at angles of APN1 and APN2. And the absolute values of the angles A1, A21, A31, APN1 and APN2 are greater than 20 degrees. A1 and A31 have the same sign, which is opposite to the sign of the angle A21. The width L1 is greater than the widths L2 and L3 and less than one of the widths LPN1 and LPN2.

Description

Technical Field

[0001] The subject of the invention is a radial tyre intended to be fitted to heavy vehicles of the construction site type and more particularly to a crown reinforcement for such tyres. Background Art

[0002] Radial tyres intended to be fitted to heavy vehicles of the construction site type are designated as radial tyres within the meaning of the European Tyre and Rim Technical Organisation (or ERTRO) standards.

[0003] For example, within the meaning of the European Tyre and Rim Technical Organization (or ERTRO) standards, radial tires for heavy vehicles of the construction site type are intended to be mounted on rims with a diameter at least equal to 25 inches. Although not limited to this type of application, the present invention is described for radial tires of large dimensions, intended to be mounted on dump trucks, in particular on vehicles used to transport materials extracted from quarries or open pits, with rims having a diameter at least equal to 35 inches, possibly up to 57 inches, or even 63 inches.

[0004] Since the geometry of the tire exhibits rotational symmetry about an axis of rotation, the geometry of the tire is usually described in a meridian plane including the axis of rotation of the tire. For a given meridian plane, the radial direction, the axial direction and the circumferential direction respectively denote the direction perpendicular to the axis of rotation of the tire, the direction parallel to the axis of rotation of the tire and the direction perpendicular to the meridian plane. The circumferential direction is tangential to the circumference of the tire.

[0005] In the following, the expressions "radially inner / located radially on the inside" and "radially outer / located radially on the outside" mean "closer to the axis of rotation of the tyre" and "further away from the axis of rotation of the tyre", respectively. "Axially inner / located axially on the inside" and "axially outer / located axially on the outside" mean "closer to the equatorial plane of the tyre" and "further away from the equatorial plane of the tyre", respectively, the equatorial plane of the tyre being the plane passing through the middle of the tread surface and perpendicular to the axis of rotation. "Element A is axially located on the inside of element B at an axial distance D" is understood to mean 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. Phrases of this type may be extended to radial and circumferential directions and to external and internal positions of one or the other of the elements.

[0006] Typically, a tire comprises a tread intended to come into contact with the ground via a tread surface, the two axial ends of which are connected by two sidewalls to two beads providing a mechanical connection between the tire and the rim on which it is intended to be mounted.

[0007] The radial tire also comprises a reinforcement constituted by a crown reinforcement radially on the inside of the tread and a carcass reinforcement radially on the inside of the crown reinforcement.

[0008] The carcass reinforcement of the radial tire for heavy vehicles of the construction site type generally includes at least one carcass layer, the carcass layer includes a metal reinforcement coated with an elastomer or an elastomer type polymer material, the polymer material is obtained by blending and is referred to as a coating compound. The carcass layer includes a main portion, the main portion connects two beads together and is generally wound around a circumferential reinforcement element (which is referred to as a bead wire) of a general metal from the inside to the outside of the tire in each bead to form a bead. The metal reinforcements of the carcass layer are substantially parallel to each other and form an angle between 85 ° and 95 ° with the circumferential direction.

[0009] The crown reinforcement of a radial tire for construction site type vehicles comprises superimposed crown layers extending circumferentially and radially on the outside of the carcass reinforcement. Each crown layer is generally composed of metal reinforcements parallel to each other and coated with a polymer material of the elastomeric type or of the coating compound type.

[0010] The mechanical characteristics of the metal reinforcement are represented by a curve representing the variation of the tensile force (in N) applied to the metal reinforcement with the relative elongation (in %) of the metal reinforcement, which is called the force-elongation curve. From this force-elongation curve, the mechanical tensile characteristics of the metal reinforcement are derived, such as the structural elongation As (in %), the total elongation at break At (in %), the breaking force Fm (maximum load in N) and the breaking strength Rm (in MPa), which are measured according to the 2014 standard ASTM D2969-04.

[0011] By definition, the total elongation At of a metal reinforcement is the sum of its elastic structural elongation and plastic structural elongation (At=As+Ae+Ap), in particular at break, each elongation being non-zero. The structural elongation As results from the relative position of the metal wires constituting the metal reinforcement under low tensile forces. The elastic elongation Ae results from the actual elasticity of the metal of the metal wires considered individually constituting the metal reinforcement, the behavior of the metal following Hooke's law. The plastic elongation Ap results from the plasticity of the metal of these metal wires considered individually (i.e., irreversible deformation beyond the yield point). These different elongations and their respective meanings are well known to those skilled in the art and are described, for example, in documents US5843583, WO2005 / 014925 and WO2007 / 090603.

[0012] At any point on the force-elongation curve of the metal reinforcement, the tensile modulus (expressed in GPa) is also defined, which represents the slope of the straight line tangent to the force-elongation curve at that point. In particular, the tensile modulus of the elastic linear portion of the force-elongation curve is called the tensile elastic modulus or Young's modulus.

[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] The elastic metal reinforcement in the rubberized state and extracted 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, the tensile elastic modulus of the elastic metal reinforcement is at most equal to 150 GPa and is generally between 40 GPa and 110 GPa.

[0015] The non-extensible metal reinforcement is characterized by a total elongation At of at most 2% at a tensile force equal to 10% of the breaking force Fm. Furthermore, the tensile elastic modulus of the non-extensible metal reinforcement is generally between 150 GPa and 200 GPa.

[0016] In the crown layer, a distinction is usually made between a protective layer constituting the protective reinforcement, radially outermost and comprising elastic reinforcing elements (or reinforcements), and a working layer comprising reinforcing elements constituting the working reinforcement and radially interposed between the protective reinforcement and the carcass reinforcement. The angle of the reinforcing elements of the protective layer or their elasticity (which is relatively greater than that of the reinforcing elements of the working layer) is such that, compared with the working layer, the protective layer absorbs only very small loads caused by running.

[0017] The protective reinforcement, which comprises at least one protective layer, essentially protects the working layers from mechanical or physicochemical attacks that could spread through the tread radially towards the inside of the tire.

[0018] The protective reinforcement generally comprises two radially superimposed protective layers formed by elastic metal reinforcements which are mutually parallel in each layer and cross from one layer to another, making an angle at least equal to 15° with the circumferential direction. The protective layers are radially on the outside of the other crown layers, protecting them from attack.

[0019] The working reinforcement, which consists of at least two working layers, serves to restrain the tire and to give it rigidity and grip. The working reinforcement absorbs both the mechanical inflation stresses, which are generated by the tire inflation pressure and transmitted through the carcass reinforcement, and the mechanical stresses caused by running, which are generated when the tire is running on the ground and transmitted through the tread. It also needs to withstand oxidation, impacts and punctures by virtue of its inherent design and protective reinforcements.

[0020] The working reinforcement usually comprises two radially superimposed working layers, formed by non-extensible metal reinforcements, which are mutually parallel in each layer and cross from one layer to another, forming an angle with the circumferential direction of at most 60°, preferably at least 15° and at most 45°. In order to reduce the shear stresses in the rubber compound or rubber mixture at the axial ends of the working layers, the positions of the ends are usually offset axially relative to each other. Therefore, the crown reinforcement usually has a working layer with a larger axial width and a working layer with a smaller axial width. The shear stresses in the rubber compound reach a maximum at the end of the working layer with a smaller axial width. This is because these maximum shear stresses caused by the displacement of the end of the working layer with a smaller axial width are distributed over the radial thickness of the rubber compound between the working layer with a smaller axial width and the working layer with a larger axial width. These shear stresses are amplified by the deformation of the working layer with a larger axial width. This is because, considering the angle at which the metal reinforcement crosses the metal reinforcement of the working layer with a smaller width, the working layer with a larger axial width will deform in the other direction, thereby increasing the deformation of the rubber compound. These maximum shear stresses are usually reduced by adding a disconnecting rubber between the ends of the axially smaller working layers and the axially larger working layers. The ends of the axially larger working layers are also subject to high shear stresses, but the amplitude of the shear stress is usually smaller, considering the greater thickness of the rubber compound at these ends and the fact that the deformation is no longer amplified by the presence of the other working layers.

[0021] In order to reduce the mechanical inflation stresses transmitted to the working reinforcement and the stresses caused by running and the shear stresses of the rubber compound covering it, it is known practice to arrange the hoop reinforcement radially on the outside of the carcass reinforcement. The role of the hoop reinforcement is to absorb the mechanical inflation stresses at least partially, which improves the durability of the crown reinforcement by stiffening the crown reinforcement. The hoop reinforcement can be arranged radially on the inside of the working reinforcement, between the two working layers of the working reinforcement, or radially on the outside of the working reinforcement.

[0022] In construction site type applications, the hoop reinforcement may comprise two radially superposed hoop layers formed by metal reinforcements which are mutually parallel in each layer and cross from one layer to another, making an angle with the circumferential direction of at most 10°. To produce the hoop layers, on construction site type tires with large diameters, winding a layer with discontinuous reinforcing elements, the ends of which extend from one axial edge of the layer to the other, making an angle of between 0° and 10° with the circumferential direction, may be preferred for reasons of productivity rather than winding a narrow strip with a plurality of continuous reinforcing elements, which make an angle of between 0° and 5° with the circumferential direction.

[0023] In both cases, the hoop layer has a smaller axial width than the working layer with a smaller axial width. This is because the tensile stress generated when driving at the end of the hoop layer is very high, and increasing the width of these hoop layers will have an adverse effect on the durability of the tire. This avoids setting the reinforcing element that forms an angle between 0° and 10° with the circumferential direction at a position too far from the median plane in the axial direction. Document WO2019 / 202239 discloses a conventional structure for a tire of a construction site type, which includes two hoop layers, the hoop layer has a rigid reinforcing element, radially located on the inner side of two working layers with rigid reinforcing elements, the working layer is radially located on the inner side of the elastic protective layer, and the axial width of the two hoop layers is substantially less than the axial width of the working layer. The document discloses optimizing the protective layer to improve the resistance of the crown to attacks on tires of the construction site type. Specifically, tires of the construction site type, especially those used in open-pit mines, are usually attacked near the cutting surface where the ore protrudes more. As the tread wears, the adverse effect of the indentation stiffness of the crown block portion on the tread cut is felt. Despite the advantages of the optimization proposed in the prior art, the significant circumferential stiffness provided mainly by the hoop layer can lead to tread cuts and uneven wear in the center and sometimes complete perforation of the crown blocks. It is well known to those skilled in the art that increasing the average angle of the hoop layer above 15° can reduce the stiffness of the crown, but this correspondingly reduces durability by promoting cracks at the ends of the working layers. Summary of the invention

[0024] The goal that the inventors set themselves was to maintain good durability, especially at the ends of the crown layer, and to improve the wear caused by attacks and the resistance to punctures in the center.

[0025] According to the invention, this object is achieved by a radial tire for a construction site type vehicle, comprising:

[0026] a crown reinforcement radially on the inside of the tread and radially on the outside of the carcass reinforcement,

[0027] - a median plane, perpendicular to the axis of rotation of the tyre and passing through the middle of the tread,

[0028] a crown reinforcement comprising at least five layers having metal reinforcing elements, including at least 3 working layers and 2 protective layers, the protective layers being situated radially on the outside of the working layers,

[0029] a first radially innermost working layer having an axial width L1 and comprising mutually parallel metallic reinforcement elements forming, in the median plane, an angle A1 with a circumferential direction (XX') tangential to the circumference of the tire,

[0030] - the second working layer and the third working layer, which are located radially outside the first working layer, are called composite working layers, and have respective axial widths L2 and L3,

[0031] - the composite second working layer and the third working layer each comprise three parts: a central part and two axial parts located on either side of the central part,

[0032] - a central portion of the composite second and third working layers, which comprises, over respective axial widths L21 and L31 centred on the median plane, mutually parallel first metal reinforcing elements forming angles A21 and A31 respectively with the circumferential direction (XX') at the median plane,

[0033] - an axial portion of the composite second and third working layers comprising, on either side of the central portion, second elastic metal reinforcing elements parallel to each other, having a tensile modulus at most equal to 130 GPa and forming angles A22 and A23 respectively with the circumferential direction (XX'),

[0034] - two radially outermost protective layers, the radially innermost protective layer having an axial width LPN1 and the radially outermost protective layer having an axial width LPN2, said protective layers comprising mutually parallel elastic metal reinforcements having a tensile modulus of at least 40 GPa and at most 130 GPa, coated with an elastomeric material and forming angles APN1 and APN2 respectively with the circumferential direction (XX'), the absolute value of which is at least 15°,

[0035] the absolute value of the angle formed by the metal reinforcement elements of the working layer in the median plane with the circumferential direction (XX') is at least equal to 20°,

[0036] the angles A1 and A31 formed respectively by the reinforcing elements of the first and third layers with the circumferential direction (XX') in the median plane have the same sign and are opposite to the sign of the angle A21 formed by the reinforcing elements of the central part of the second working layer with the circumferential direction (XX') in the median plane,

[0037] the angles A22 and A32 formed by the elastic reinforcing elements of the axial portions of the second and third working layers with the circumferential direction (XX′) are at most equal to 10°,

[0038] The axial width L1 of the first working layer is at least equal to the axial widths L2 and L3 of the second and third working layers, and the axial width of at least one protective layer is at least equal to the axial width of the working layer of greater width.

[0039] The solution implemented makes it possible to better balance the circumferential stiffness in the axial direction, while still maintaining, or even increasing, the stiffness near the axial ends of the working layers and reducing the stiffness in the center. The inventors have recognized that the circumferential stiffness in the center is too high due to the presence of the hoop layers. On some tires, the hoop layers provide five sevenths of the circumferential stiffness in the center. Counterintuitively, it is necessary to eliminate the hoop layers in the center and replace them with working layers that may be classified as composite layers, the center of which is formed by a first metal reinforcement element, the stiffness of which can be adjusted by an angle at least equal to 20° and by the tensile modulus, and the axial part of which is formed by a second elastic metal reinforcement element, which has a tensile modulus at most equal to 130 GPa and a stiffness increased due to an angle less than 8°. A proper adjustment of the crown geometry requires placing the axial parts of the two composite working layers, whose angles are close to the circumferential direction, on a relatively rigid reinforcement element layer, such as, for example, a metal reinforcement element layer, which is inextensible or otherwise. If this were not the case, during the moulding process, the three parts of the composite working layer would tend to align in different ways and produce a geometry of low durability, since its central part would deform more easily than its two axial parts. The inventors' solution is to arrange a first composite working layer on a first working layer, which first working layer consists of metal reinforcement elements over its entire axial width, with the constraint that the axial width of this radially innermost working layer is at least equal to the width of the two composite working layers.

[0040] The absence of a hoop layer or other crown layer with metal reinforcements (forming an angle with the circumferential direction of less than 15° in absolute value) in the center of the tire makes the center of the tire significantly more flexible, thereby significantly improving its wear resistance and puncture resistance. However, this elimination increases the shear stresses at the ends of the working layers and has a negative impact on the cracking durability. This problem is solved by adding reinforcing elements forming an angle with the circumferential direction of less than 10° in absolute value, which are made of metal but have elasticity to avoid that these reinforcements are overstressed by the tension / compression cycles increased at this location. Elasticity is understood here to mean that the metal reinforcing elements have a tensile modulus of at most 130 GPa. This elasticity also makes it possible to mold the crown correctly.

[0041] However, in order to achieve the performance of the tire in terms of durability by limiting the shear stresses in particular at the ends of the first working layer (i.e. the radially innermost crown layer), it is necessary to maintain a specific balance between the angles of these second elastic metal reinforcement elements with the reinforcement elements of the first working layer and the angles of the reinforcement elements of the central part of the second and third working layers (the latter two layers are called composite layers). Thus, the absolute value of the angle formed by the metal reinforcement elements of the working layers in the median plane is at least equal to 20°. Below this value, these working layers would make the crown too stiff and the object of the invention would not be achieved.

[0042] Furthermore, it is necessary to connect the centers of the three first working layers, taking into account that the center of the tire is no longer hooped. To this end, the angles A1 and A31 formed respectively by the reinforcing elements of the first and third layers with the circumferential direction (XX') at the median plane must have the same sign, opposite to the sign of the angle A21 formed by the reinforcing elements of the second layer with reinforcing elements with the circumferential direction (XX') at the median plane. These relative orientations make it possible to provide sufficient rigidity to the center of the tire.

[0043] In view of the presence of hoop reinforcement elements at the axial ends of the second and third working layers, it is necessary to protect them from attacks on the crown, taking into account the wear of construction site type tires, and therefore at least two protective layers are provided, namely the radially outermost crown layer. Their radially innermost protective layer has an axial width LPN1 and their radially outermost protective layer has an axial width LPN2. These protective layers include elastic metal reinforcements for giving the crown good resistance to punctures and not absorbing any driving tensions, this function being performed by the working layers. These protective layers therefore have elastic reinforcement elements with a tensile modulus at least equal to 40 GPa and at most equal to 130 GPa. Like the other crown layers, their reinforcement elements are coated with an elastomeric material, are parallel to each other and the radially innermost protective layer and the radially outermost protective layer respectively form angles APN1 and APN2 with the circumferential direction (XX'), the absolute value of the angles APN1 and APN2 being at least equal to 15°, preferably at least equal to 20°. For such angles, the protective layer with elastic reinforcing elements will not absorb the circumferential loads, but above 20° the balance is better, especially with regard to the portion of the working layer around the median plane.

[0044] The presence in the composite layer towards its axial ends of metal reinforcement elements which form angles with the circumferential direction of less than 8° in absolute value and which are more susceptible to breaking than other parts of the crown layer in the event of significant circumferential deformations, such as an attack on the crown when crossing an obstacle, means that these parts must be protected by a protective layer as well as by the radially innermost working layer. Therefore, the axial width of at least one protective layer must be at least equal to that of the working layer having the greater width.

[0045] The preferred solution to achieve standardization of the tire component products is that the reinforcing elements of the central portion of the second and third working layers are identical, and so are the reinforcing elements of the axial portions of the second and third working layers. However, such standardization is not necessary for the invention to work properly.

[0046] In order to better protect the crown at the end of the working layer, the wider protective layer must be closest to the working layer to limit the bending stress generated during impact. Therefore, the preferred solution is that the axial width LPN1 of the radially innermost protective layer is at least equal to the axial width LPN2 of the radially outermost protective layer.

[0047] A solution for the protective layer to absorb little of the loads caused by driving and actually protect the working layers is that the absolute values ​​of the angles APN1 and APN2 are at least equal to the maximum absolute value of the angles of the reinforcing elements of the working layers (ie all working layers) in the median plane.

[0048] Advantageously, the angle APN1 formed by the reinforcing elements of the radially innermost protective layer with the circumferential direction (XX') has the same sign as the angle formed by the reinforcing elements of the radially outermost working layer with the circumferential direction (XX') at the mid-plane. This is because any attack will cause cracking of the compound of the crown (from the rubber compound of the tread to the coating compound of the protective layer), resulting in the entry of water from the contact surface into the radially innermost protective layer. If the reinforcing elements of the radially innermost protective layer intersect the reinforcing elements of the radially outermost working layer (for example at the mid-plane of the composite or non-composite working layer), the water flowing from the contact surface to the reinforcing elements of the protective layer and following this reinforcing element is likely to cause corrosion to many connected reinforcing elements of the working layers through which it passes. The corrosion of several reinforcing elements close to the working layers can greatly weaken the crown. This risk can be reduced by orienting the reinforcing elements of the radially outermost working layer with the same sign at the mid-plane as the reinforcing elements of the radially innermost protective layer. In addition, this arrangement makes it possible to avoid shear stresses in the rubber composition between the radially outermost working layer and the radially innermost protective layer.

[0049] It is also preferred that the angle APN1 formed by the reinforcing elements of the radially innermost protective layer and the circumferential direction (XX') and the angle APN2 formed by the reinforcing elements of the radially outermost protective layer and the circumferential direction (XX') have opposite signs, so that the attack resistance of the crown is consistent in all directions, thereby improving the impact resistance of the crown.

[0050] Advantageously, the absolute value of the angle A1 formed by the reinforcing element of the first working layer and the circumferential direction (XX') is at most equal to the absolute value of the angle A21 and A31 formed by the first reinforcing element of the central part of the second working layer and the third working layer at the median plane respectively with the circumferential direction (XX'). This configuration makes it possible to avoid that too much proportion of the circumferential load is absorbed by the elastic reinforcing element of the axial part of the second working layer and the third working layer. In this way, a good balance can be achieved between the decline in stiffness at the center (which requires the angle of the reinforcing element of the central working layer to be at least equal to 20°, if possible, greater) and the acceptable value of the tension in the axial part in the elastic reinforcing element of the second working layer and the third working layer. The reinforcing element of the first working layer absorbs more circumferential tension at a smaller angle. Advantageously, the difference in absolute value between the average value of the absolute value of the angle of the reinforcing element of the central part of the second working layer and the third working layer and the absolute value of the angle of the reinforcing element of the radial innermost working layer is at least equal to 2°, preferably at least equal to 4°.

[0051] An advantageous solution is that the reinforcing elements of the first working layer and of the central part of the composite second and third working layers are inextensible, their tensile modulus being at least equal to 150 GPa. These parts thus absorb the majority of the running loads and give good durability to the crown and in particular to the axial parts of the second and third working layers.

[0052] Preferably, the axial width L21 of the central part of the second working layer is at least equal to the axial width L31 of the central part of the third working layer. Specifically, the closer the axial area of ​​the composite layer is to the tread, the greater the bending load absorbed by its reinforcing elements, so it is necessary to reduce the load in the reinforcing elements of the axial part of the third working layer, because it is axially farther from the inside than the axial part of the second working layer.

[0053] Furthermore, an advantageous solution is that the difference between the axial widths L21 and L31 of the central parts of the second and third working layers, comprising the first inextensible metal reinforcing elements, is at least equal to four times the diameter of the inextensible reinforcing elements of the second working layer. This is because the transition from the central area to the axial area of ​​the working layer generates considerable differences in stiffness, especially in bending. It is risky to concentrate this difference in a single axial position. Concentrating this difference in a single axial position of the two composite working layers will contribute to concentrating the bending deformations caused by tire extrusion in this area, with the risk of fatigue fractures in the reinforcing elements of the other crown layers, which fractures may cause defects caused by the circumferential cuts in the crown. Taking into account the dimensions of the reinforcing elements used for such applications, the proposed axial offset of the axial ends of the central parts of the second and third working layers is sufficient to avoid this problem.

[0054] A preferred solution is that the elastic reinforcing elements of the axial part of the second and third working layers are formed by winding a continuous strip of the second layer circumferentially around the third working layer, the strip comprising at least 1 and at most 5 elastic reinforcing elements. The strip comprises circumferential ends which are optimally located at the axial ends of the central parts of the second and third working layers. The advantage of the strip is to increase the laying speed of the axial part of the composite working layer. It is still possible to lay bare cords, which is a feasible solution, but it is preferred to lay a strip consisting of cords (or reinforcing elements) surrounded by a rubber composition, thereby avoiding the creation of circumferential grooves (which may become filled with water and cause corrosion of the reinforcing elements of the various working layers). A strip consisting of a plurality of reinforcing elements is also advantageous because it reduces the time required for laying the axial part in proportion to the number of reinforcing elements. However, this possibility is limited because the ends of such a strip, even if beveled, will create a weak area in the crown, the wider the strip, the more obvious this weakness is. A very advantageous solution is to form one of the two axial portions of the second working layer with the same strip and then, without interruption, the axial portion of the third working layer radially outside it. This makes it possible to avoid the presence of cord ends at the axial ends of the second and third working layers, which are weak areas where the elastic reinforcing element is subjected to the greatest stress. To increase productivity, such a strip can be laid simultaneously on both sides of the central portion.

[0055] The best balance in terms of properties is obtained when the elastic reinforcing elements of the axial portions of the second and third working layers have a tensile modulus at least equal to 80 GPa and a structural elongation at least equal to 0.3%.

[0056] A preferred solution comprises 4 working layers, the axially outermost fourth working layer having an axial width L4 comprising mutually parallel inextensible metal reinforcement elements having a tensile modulus at least equal to 150 GPa and forming an angle A4 with a circumferential direction (XX') tangential to the circumference of the tire in the median plane, said axial width L4 being at least equal to the axial widths L2 and L3 of the second and third working layers and less than the axial width L1 of the first working layer, said angle A4 having an opposite sign to the angle A1 formed by the reinforcement elements of the first working layer with the circumferential direction (XX') and having an absolute value at least equal to 20°, preferably at least equal to the absolute values ​​of the angles A21 and A31 formed by the inextensible first reinforcement elements of the central part of the second and third working layers, respectively, with the circumferential direction (XX') in the median plane. This version is the most optimized version in terms of crown performance of the best performing construction site type tires available on the market, comprising two central hoop layers, two working layers and two protective layers. They are of equal quality.

[0057] In an improved version, the absolute value of the angle formed by the reinforcing elements of the fourth working layer with the circumferential direction (XX') is less than the absolute value of the angles A21 and A31 formed by the inextensible first reinforcing elements of the central part of the second and third working layers with the circumferential direction (XX') at the median plane, respectively, and is preferably equal to the absolute value of the angle A1 formed by the inextensible reinforcing elements of the first working layer at the median plane (give or take manufacturing variations). As with the angle of the first working layer, the purpose of this is to avoid that the elastic reinforcing elements of the axial part of the second and third working layers absorb too much circumferential tension. In the same way, the difference in absolute value between the average value of the absolute value of the angle of the inextensible reinforcing elements of the central part of the second and third working layers with the circumferential direction and the absolute value of the angle of the reinforcing elements of the radially outermost fourth working layer is at least equal to 2°, preferably at least equal to 4°. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Refer to the tire size 24.00R35. Figure 1 The features of the invention are shown schematically and not to scale: Figure 1 A meridian section through the crown of a tire according to the invention is shown. DETAILED DESCRIPTION

[0059] Figure 1A meridian section through a tyre 1 for a heavy vehicle of the construction site type is shown, said tyre 1 comprising a crown reinforcement 3 radially on the inside of the tread 2 and radially on the outside of the carcass reinforcement 4. The crown reinforcement 3 comprises, radially from the outside to the inside, a protection reinforcement 32 and a working reinforcement 31. The protection reinforcement 32 comprises two protective layers (321, 322) of respective axial width LPN1 and LPN2, comprising elastic metal reinforcements coated with an elastomeric material, said elastic metal reinforcements being parallel to each other and forming respective angles APN1 and APN2 with a circumferential direction XX' tangential to the circumference of the tyre in a median plane, the respective metal reinforcements of each protective layer crossing from one protective layer to the next. In the present case, the working reinforcement 32 comprises four working layers 311, 312, 313, 314, including two composite working layers 312, 313 located between two non-composite working layers 311 and 314, whose respective metal reinforcements are coated with an elastic material, are parallel to each other and form corresponding angles A1 and A4 measured at the median plane with the circumferential direction XX', and the reinforcing elements are continuous in their respective axial widths L1 and L4. A version without a fourth working layer is possible, but is not shown here. The version with four working layers is the solution that exhibits the best performance. The two composite working layers, namely the second working layer and the third working layer, have respective axial widths L2 and L3, and each comprises three parts: a central part 3121, 3131, and two axial parts located on both sides of the central part, of which only the left axial parts 3122, 3123 are shown in the figure. The central portion 3121, 3131 of the composite second and third working layers (312, 313) comprises, over respective axial widths L21 and L31 centered on the median plane (M), first metal reinforcement elements forming angles A21 and A31 with the circumferential direction (XX') at the median plane, respectively. The axial portions 3122, 3132 of the composite second and third working layers 312, 313 located on either side of the central portion 3121, 3131 comprise elastic metal reinforcement elements forming angles A22 and A23 with the circumferential direction (XX'), respectively.

[0060] The angles APN1, APN2, A1, A21, A22, A31, A32, A4 are not shown in the figures. Those skilled in the art will know how to measure these angles on a tire using non-destructive testing devices or by cutting the tire open and accessing the individual crown layers. The same applies to the measurement of the axial widths of the different crown layers, which are usually measured in a meridian section. Preferably, the angles are measured at the center of the crown layer in question (usually at the mid-plane) or at the center of the axial portion in question to provide a reference value, the measured angles possibly varying slightly with the axial position at which the measurement is taken. The axial width L1 of the first working layer 311 is at least equal to the axial widths L2 and L3 of the second working layer 312 and the third working layer 313, and the axial width LPN1 of at least one protective layer (in this case the radially innermost protective layer 321) is at least equal to the axial width of the working layer with the larger width (in this case the radially innermost working layer 311). Figure 1 In the embodiment, the axial width L21 of the central portion 3121 of the second working layer 312 is at least equal to the axial width L31 of the central portion 3131 of the third working layer 313 .

[0061] The invention has been tested on a 24.00R35 tire, the crown of which has 4 working layers, two of which (the second and third layers) are composite layers. The first and fourth layers consist of inextensible reinforcing elements of 26×30 construction, including twenty-six steel wires with a diameter of thirty percent of a millimeter, with a tensile modulus (equal to 160 GPa) measured on the cords removed from the tire, the reinforcing elements are arranged at a spacing of 3.4 mm and form angles A1 and A4 with the circumferential direction equal to 24° and -24° respectively in the median plane. The central parts 3121 and 3131 of the composite working layers also include inextensible reinforcing elements of 26×30 construction, including twenty-six steel wires with a diameter of thirty percent of a millimeter. , having the same tensile modulus and arranged at a pitch of 3.4 mm, the angles A21 and A31 formed by the reinforcing elements with the circumferential direction in the median plane being equal to -28° and 28° respectively. The axial portion of the second and third working layers as a composite working layer, comprising elastic reinforcing elements in a 44×35 configuration, with a tensile modulus measured on the cord removed from the tire equal to 90 GPa, a structural elongation equal to 0.6%, the reinforcing elements being arranged at a pitch of 4.4 mm and forming angles A21 and A31 formed by the reinforcing elements with the circumferential direction in the median plane being equal to 2° and -2° respectively. 22 and A32. The axial portion is prepared by winding a continuous strip of 1 cord from the axial end of the central portion 3121 to the axial end of the axial portion of the second layer and then back to the axial end of the central portion of the third working layer. The crown layer also includes two protective layers 321 and 322, which are composed of elastic reinforcing elements in a 24×26 configuration, that is, twenty-four steel wires with a diameter of 2600 millimeters, a tensile modulus measured on the cord removed from the tire equal to 60 GPa, a structural elongation equal to 0.6%, and reinforcing elements with a thickness of 4 .4 mm spacing and form angles APN1 and APN2 equal to 33° and -33° respectively with the circumferential direction at the median plane. The axial width of the first working layer is 444 mm. The axial width of the second working layer is 360 mm, and the axial width of its central part is 250 mm. The axial width of the third working layer is 340 mm, and the axial width of its central part is 200 mm. The axial width of the fourth working layer is 402 mm. The axial width of the radially innermost protective layer is 524 mm. The axial width of the radially outermost protective layer is 360 mm.

[0062] The present invention is compared with a commercially available control tire ("Michelin XTRA Load Protect E4B***") of the same size, the crown of which consists of two hoop layers, which are radially located inside the two working layers, which are radially located inside the two protective layers. The reinforcing elements of the hoop layers form angles of 8° and -8°. They are inextensible and have a 26×30 construction, that is, 26 steel wires with a diameter of 300ths of a millimeter, a tensile modulus equal to 160GPa measured on the cord removed from the tire, and are arranged at a spacing of 3.4 millimeters. The reinforcing elements of the working layers form angles of 33° and -19°. They are inextensible and identical to the reinforcing elements of the first (and fourth) working layer of the present invention. The reinforcing elements of the protective layer form angles of 24° and -24°. They are elastic and identical to the reinforcing elements of the protective layer of the present invention. The axial width of the radial innermost layer of the hoop layer is 240 millimeters, and the axial width of the radial outermost layer is 200 millimeters, which cannot be increased without the risk of these reinforcing elements breaking. The axial width of the first working layer is 450 millimeters. The axial width of the second working layer is 380 millimeters. The axial width of the radially innermost protective layer is 520 millimeters. The axial width of the radially outermost protective layer is 400 millimeters. More specifically, the inextensible metal reinforcement of 26×30 construction, that is to say 26 cords of 30-thousandth millimeters in diameter, is arranged in three layers, wherein the middle layer comprises 3 threads, the second layer comprises 9 threads and the outer layer comprises 14 threads. The elastic metal reinforcement of the protective reinforcement of the control tire is a cord of 24×26 construction, that is to say 4 cords, said cords having 6 threads of 26-thousandth millimeters in diameter.

[0063] The control tire and the tire according to the invention are identical except for the crown reinforcement. They have the same tread pattern, the same carcass layer reinforcement and the same rubber compound in each tire part.

[0064] The invention has been simulated on a computational tool and tested on a tire of size 24.00R35. The simulations make it possible to evaluate, using the finite element method, the mechanical and thermal stresses imposed on the tire at large displacements and strains, taking into account the mechanical and hysteresis properties of the material.

[0065] The purpose of the invention is to reduce the circumferential rigidity in the center in order to improve the wear resistance and the resistance to attacks and perforations. The reduction in rigidity is estimated from the deformation of the crown blocks under the effect of increased pressure. The rigidity in the center of the crown is reduced by a factor of four, while the rigidity at the ends of the crown layers increases by a factor of two to four. The invention shows a very uniform rigidity of the crown over an axial width of about 360 mm, with a variation of about 25% over this area, while the control tire shows a variation of about 20 times between the minimum and maximum values. This reduction in rigidity in the center allows the invention to improve the resistance to perforations by about 20% and solves the problem of uneven wear caused by the excessive rigidity in the center of the control tire.

[0066] As regards the durability of the crown, the safety factor of the reinforcing elements of all the crown layers (breaking strength of the cords when subjected to maximum stress) is at least equal to the minimum safety factor of the control tire. Similarly, the stress in the rubber composition at the ends of the working layers is improved by the axial portion of the composite working layers. From this point of view, the inventors expect a performance in terms of durability that is at least the same as that of the crown, if not better. The quality of the control tire and the tire according to the invention is equivalent.

[0067] All these performance aspects demonstrate the advantages of the present invention.

Claims

1. A radial tire (1) for a construction site type vehicle, comprising: a crown reinforcement (3) situated radially on the inside of the tread (2) and radially on the outside of the carcass reinforcement (4), a median plane (M) perpendicular to the axis of rotation of the tyre and passing through the middle of the tread (2), a crown reinforcement (3) comprising at least five layers with metal reinforcing elements, including at least three working layers (31) and two protective layers (32), the protective layers (32) being radially outside the working layers (31), a first radially innermost working layer (311) having an axial width L1 and comprising mutually parallel metal reinforcement elements which, in the midplane, form an angle A1 with a circumferential direction (XX') tangential to the circumference of the tire, - a second working layer and a third working layer (312, 313), which are located radially outside the first working layer (311), and are called composite working layers, and have respective axial widths of L2 and L3, - the composite second working layer (312) and the third working layer (313) each comprise three parts: a central part (3121, 3131) and two axial parts (3122, 3123) located on both sides of the central part, - the central portion (3121, 3131) of the composite second and third working layers (312, 313) comprises, over respective axial widths L21 and L31 centered on the median plane (M), mutually parallel first metal reinforcement elements forming angles A21 and A31 respectively with the circumferential direction (XX') at the median plane, - the axial portion (3122, 3132) of the composite second and third working layers (312, 313) comprises, on either side of the central portion (3121, 3131), second elastic metal reinforcing elements parallel to one another, having a tensile modulus at most equal to 130 GPa and forming angles A22 and A23 respectively with the circumferential direction (XX'), - two radially outermost protective layers (321, 322), the radially innermost protective layer having an axial width LPN1 and the radially outermost protective layer having an axial width LPN2, the protective layers (321, 322) comprising mutually parallel elastic metal reinforcements having a tensile modulus of at least 40 GPa and at most 130 GPa, coated with an elastic material and the radially innermost protective layer and the radially outermost protective layer forming angles APN1 and APN2 respectively with the circumferential direction (XX'), the absolute values ​​of the angles APN1 and APN2 being at least 15°, Features: the absolute value of the angles A1, A21, A31 formed by the metal reinforcement elements of the working layers (311, 312, 313) with the circumferential direction (XX') in the median plane is at least equal to 20°, the angles A1 and A31 formed respectively by the reinforcing elements of the first layer (311) and the third layer (313) with the circumferential direction (XX') in the median plane have the same sign and are opposite to the sign of the angle A21 formed by the reinforcing elements of the central portion (3121) of the second working layer (312) with the circumferential direction (XX') in the median plane, the angles A22 and A32 formed by the elastic reinforcing elements of the axial portions (3122, 3132) of the second and third working layers and the circumferential direction (XX') are at most equal to 8°, - The axial width L1 of the first working layer (311) is at least equal to the axial widths L2 and L3 of the second and third working layers (312, 313), and the axial width LPN1, LPN2 of at least one protective layer (321, 322) is at least equal to the axial width of the working layer (311, 312, 313) with a larger width.

2. The tire (1) according to claim 1, wherein: The axial width LPN1 of the radially innermost protective layer (321) is at least equal to the axial width LPN2 of the radially outermost protective layer (322).

3. Tyre (1) according to any one of the preceding claims, wherein: The absolute values ​​of the angles APN1 and APN2 are at least equal to the maximum absolute value of the angles A1, A21, and A31 of the reinforcing elements of the working layer at the mid-plane, and the angle APN1 formed by the reinforcing elements of the radially innermost protective layer (321) and the circumferential direction (XX') has the same sign as the angle formed by the reinforcing elements of the radially outermost working layer at the mid-plane and the circumferential direction (XX'), but has the opposite sign as the angle APN2 formed by the reinforcing elements of the radially outermost protective layer (322) and the circumferential direction (XX').

4. Tyre (1) according to any one of the preceding claims, wherein: The absolute value of the angle A1 formed by the reinforcing elements of the first working layer (311) and the circumferential direction (XX') is at most equal to the absolute values ​​of the angles A21 and A31 formed by the first reinforcing elements of the central parts (3121, 3131) of the second working layer (312) and the third working layer (313) respectively with the circumferential direction (XX') at the midplane.

5. Tyre (1) according to any one of the preceding claims, wherein: The reinforcing elements of the first working layer and of the central portion of the composite second and third working layers are inextensible and have a tensile modulus at least equal to 150 GPa.

6. Tyre (1) according to any one of the preceding claims, wherein: The axial width L21 of the central portion (3121) of the second working layer (312) is at least equal to the axial width L31 of the central portion (3131) of the third working layer (313).

7. Tyre (1) according to any one of the preceding claims, wherein: The difference between the axial widths L21 and L31 of the central portions (3121, 3131) of the second and third working layers (312, 313) comprising the first metal reinforcing elements is at least equal to four times the diameter of the inextensible reinforcing elements of the second working layer (312).

8. Tyre (1) according to any one of the preceding claims, wherein: The second elastic reinforcing element of the axial part (3121, 3131) of the second working layer and the third working layer (312, 313) is formed by winding a continuous strip of the second layer circumferentially around the third working layer, the strip comprising at least 1 and at most 5 elastic reinforcing elements, the strip comprising a circumferential end portion located at the axial end portion of the central part (3121, 3131) of the second working layer and the third working layer (312, 313).

9. Tyre (1) according to any one of the preceding claims, wherein: The second elastic reinforcing element of the axial portion (3121, 3131) of the second and third working layers (312, 313) has a tensile modulus at least equal to 80 GPa and a structural elongation at least equal to 0.3%.

10. Tyre (1) according to any one of the preceding claims, comprising four working layers (311, 312, 313, 314), wherein: The axially outermost fourth working layer (314) with an axial width of L4 includes mutually parallel inextensible metal reinforcement elements, wherein the metal reinforcement elements have a tensile modulus of at least 150 GPa and form an angle A4 with a circumferential direction (XX') tangential to the tire circumference at a mid-plane, wherein the axial width L4 is greater than the axial widths L2 and L3 of the second working layer and the third working layer and is less than the axial width L1 of the first working layer, wherein the angle A4 has an opposite sign to the angle A1 formed by the reinforcement elements of the first working layer (311) at the mid-plane with the circumferential direction (XX') and has an absolute value of at least 20°.

11. Tyre (1) according to the preceding claim, wherein: The absolute value of the angle formed by the reinforcing elements of the fourth working layer (314) and the circumferential direction (XX') is smaller than the absolute values ​​of the angles A21 and A31 formed by the inextensible first reinforcing elements of the central parts (3121, 3131) of the second working layer (312) and the third working layer (313) and the circumferential direction (XX') at the midplane, respectively, and is preferably equal to the absolute value of the angle A1 formed by the inextensible reinforcing elements of the first working layer (311) at the midplane.

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