Optimised tread and crown for a civil engineering tyre
Patent Information
- Application Number
- AU2025238999
- Authority / Receiving Office
- AU · AU
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2024-03-18
- Filing Date
- 2025-02-20
- Publication Date
- 2026-08-27
Smart Images
Figure 00000000_0000_ABST
Abstract
Description
Furthermore, if the carcass layer is manufactured with a shape far from its equilibrium curve, the pressurization will deform the carcass layer so as to bring it towards its equilibrium curve under pressure, putting certain parts of the crown, in particular the reinforcing elements of the hooping layers, in tension or compression depending on their positions. For good endurance, the reinforcing elements of the hooping layers should not be in absolute compression on any of the segments thereof on inflation, the compression of the reinforcing elements having the effect of very significantly worsening their fatigue strength. Designing a tyre therefore consists in finding a way to combine a tread and a crown, the optimum shape for wear of which is cylindrical with a carcass with a curved optimum shape. Rounding the tread profile results in a tyre that is less effective in terms of wear. Opting for a less round profile for the carcass is likely to produce a less durable tyre, since cracks in the rubber compounds close to the axial ends of the working layers are more likely to propagate as far as 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 away, the value of which is a linear variable of the tread pattern height multiplied by the axial width of the tread. Such a compromise is usually settled by combining the crown reinforcement with the carcass by way of rubbers, or rubber composition(s), known as crown filler rubbers at the shoulder, the shoulders representing the radially outermost 20% of the tread on either side of the mid-plane. The wider the crown, the thicker these crown filler rubbers are. At the centre of the tread, the thickness of these crown filler rubbers is at a minimum. For obvious reasons of symmetry, the carcass layer in the mid-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 at the centre, but in this case, this thickness does not comply with geometric constraints. In the widest part of the crown reinforcement, the filler rubbers are unnecessary, since the crown reinforcement stabilizes the position of the carcass layer. At the shoulder, the thicknesses of the filler rubbers are at a maximum. This solution causes a high level of heat at the crown, particularly at the shoulders, which tends to adversely affect endurance. If the thicknesses of these rubbers are reduced, the carcass layer, which, under the effect of the pressure, will reach the level of its equilibrium curve, will move the filler rubbers, the ends of the crown layers, the tread, to a lower radial position at the shoulders, and will place certain reinforcing elements in compression. When running, the shoulders are aligned with the centre in the contact patch by being squashed against the ground, and place the reinforcing elements in relative tension in the shoulder region. This repeated passage upon each wheel revolution from tension phase to compression phase can cause breakage of the reinforcing elements at the ends of the crown layers, and causes high levels of shear stress at the axial ends of the working layers, these being conducive to the creation of cracks which, depending on the case, open onto the sidewalls or toward the carcass layer. Maintaining a large volume of filler rubbers in the crown considerably increases the heat in this region, and this can result in thermal failure of the crown or premature ageing of the edge rubbers, causing a drop in the cohesion of the materials thereof, this drop promoting the creation of cracks. Reducing the thickness of the crown filler rubbers increases the maximum shear stresses to which they are subjected and normally decreases endurance. The edge rubbers are rubber compounds positioned between the axial ends of the working layers and each adjacent layer of metal reinforcing elements. In order to reduce cracking, tyre manufacturers seek above all to increase the stiffness of the crown by introducing hooping layers and to optimize their operation, for example as shown in US 2018 / 0056723 A1, in which the hooping layers are between the working layers, and the working layers are recoupled in order to improve their stiffness. Nevertheless, these solutions are complex in terms of manufacture, recoupling sometimes being difficult to carry out with metal reinforcing elements with large diameters for heavy-duty vehicles and the stiffening of the crown not being very favourable for resistance to puncturing by stones, this being a basic performance aspect for civil engineering tyres. The inventors set themselves the objective, for a civil engineering tyre, of improving 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 filler rubbers, while maintaining or improving resistance to attacks for the same result in terms of wear. This objective has been achieved, according to the invention, by a radial tyre for a civil engineering vehicle, which is intended to be mounted on a nominal rim with a nominal diameter greater than 25 inches and comprises: - a carcass reinforcement, which extends between two beads of the tyre, each of the beads comprising a circumferential reinforcement or bead wire, and is made up of a carcass layer comprising metal reinforcers that form an angle of between -10° and 10° with a radial direction, the carcass layer comprising a main portion, extending from one bead wire to the other, and two turn-ups, each extending from a bead wire to one end of the carcass layer, - a crown reinforcement, radially on the inside of a tread of the tyre and radially on the outside of the carcass reinforcement, the tread having a tread surface intended to come into contact with underlying ground, the tread surface having, for each meridian plane, two axial ends, and - an inner liner having an inner surface intended to be in contact with an inflation gas, wherein the tyre has a mid-plane, which is perpendicular to an axis of rotation of the tyre and which passes through the middle of the tread, wherein a total median thickness is the radial distance from the radially innermost point of the tyre in the mid-plane to the tread surface, wherein a total shoulder thickness, measured in a meridian plane, is the minimum distance from the axial end of the tread surface to the inner surface of the inner liner, wherein the ratio of the total shoulder thickness to the total median thickness is at most equal to 1.25, wherein each bead comprises a protector made up of a rubber compound having axially outermost points which are intended to come into contact with the nominal rim, each bead including, in each meridian, a point Me1 which minimizes the axial distance from the bead wire to the axially outermost points of the protector and which is positioned at an axial distance from the mid-plane equal to half the width of the nominal rim plus 1.27 cm, wherein the main portion of the carcass layer includes an axially outermost point, which is at an axial distance L4m from the mid-plane, and a point Me2, which is radially on the outside of said axially outermost point of the main portion of the carcass layer and which is at an axial distance from the mid-plane equal to 0.85*L4m, the point Me2 being situated at a radial distance E85 from the intersection point between the midplane and the carcass layer, wherein the ratio between E85 and 0.85*L4m is between 0.10 and 0.15, wherein the crown reinforcement comprises crown layers that each comprise metal reinforcing elements, each reinforcing element of each of said crown layers being characterized by a structural elongation As and a total elongation at break At, which are measured according to the standard ASTM D 2969-04 of 2014, wherein the crown reinforcement comprises a working reinforcement, which comprises at least two working layers among said crown layers, and a protective reinforcement, which is radially on the outside of the working reinforcement and which comprises at least one protective layer among said crown layers, wherein the reinforcing elements of each working layer are parallel to one another, forming oriented angles with an absolute value at least equal to 15° and at most equal to 45° with the circumferential direction, the angles of two working layers among the at least two working layers having opposite signs from one working layer to the next, wherein the reinforcing elements of the or each protective layer are parallel to one another, forming oriented angles with an absolute value at least equal to 15° and at most equal to 45° with the circumferential direction, wherein the reinforcing elements of each of the crown layers have a structural elongation As at least equal to 0.3%, a total elongation at break At at least equal to 3%, and a tangent modulus at (At+As) / 2 at most equal to 120 GPa, and wherein the reinforcing elements of the or each protective layer 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. In order for at least one of the crown layers to be able to act as a protective layer, this layer of reinforcing elements should absorb only a small part of the inflation and rolling forces. For this purpose, a sufficient condition is that the reinforcing elements of said protective layer be significantly more elastic than the reinforcing elements of the working layers and therefore of the radially innermost working layer. According to the invention, since the protective reinforcement is radially on the outside of the working reinforcement and of the possible hoop reinforcement, there is at least one protective layer, the radially outermost layer of metal reinforcing elements, 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 optionally comprises two protective layers, each of which is radially on the outside of the working reinforcement and of the hoop reinforcement, and the radially innermost protective layer will also be elastic, with a structural elongation significantly higher, by at least 30%, than the structural elongation of the radially innermost working layer. For reasons of productivity and standardization, the architecture of the reinforcing elements of the two protective layers, namely the diameters of the individual threads and the way in which these threads are arranged in relation to each other, will preferably be identical. For a better crown breaking performance under attack, 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, meaning that the angles formed by the reinforcing elements of the two protective layers and the circumferential direction have opposite signs. According to the invention, all the reinforcing elements of the crown reinforcement are elastic. For an identical crown geometry, this specificity leads to a better performance in terms of fatigue failure under tensile and compressive stresses. Specifically, the elastic reinforcing elements resist buckling much better than the inextensible reinforcing elements provided that the structural elongation is at least equal to 0.3% and the total elongation at break At is at least equal to 3% and that the tangent modulus at (At+As) / 2 is at most equal to 120 GPa. Their lower stiffness surprisingly makes it possible to reduce the stresses in the filler rubbers when it should increase the deformation of the reinforcing elements of the crown and therefore the shear stresses in said rubbers. This reduction makes it possible to modify the geometry of the crown and to reduce the thicknesses of the crown filler rubber, without adversely affecting endurance. One way of comparing the thickness of the filler rubbers from one tyre to another without involving the difference in the thickness of the crown layers is to compare the ratio of the thickness of the tyre at the centre of the crown, where it is smallest, and the thickness of the tyre at the shoulder, where it is at a maximum. Nevertheless, in order to see whether this modification is not caused by an increase in the thickness of the volume of the tread, the gradient of the carcass layer below the crown should also be taken into consideration. Specifically, the crown layers are substantially axial, with their role consisting, among other functions, in forming a cylindrical shell ring on which a tread is laid. The more cylindrical it is, the more it makes it possible to avoid differences in wear. The gradient of the carcass is therefore essentially a function of the thickness of the crown filler rubber between the carcass layer and the radially innermost crown layer. By using a crown having reinforcing elements that are all elastic, the inventors were able to reduce the thickness of these crown filler rubbers while preserving the endurance of the tyre. Above a certain gradient, namely 0.15, a tyre with such characteristics of crown layer reinforcing elements is not optimized in terms of endurance, and below a gradient of 0.1, its endurance will be insufficient. This gradient measurement should be taken in a situation close to the time when the tyre is in the curing mould. Any measurement under pressure creates a deformation of the tyre that tends to modify the shape of the crown by rounding it and therefore tends to distort the measurement of the gradient of the carcass below the crown. A tyre is cured in a mould in which the beads are at a distance slightly greater than their position in the mounting rim by approximately 2.54 cm (one inch) over the total axial width, or 1.27 cm more than the nominal half-rim with respect to the distance from the bead to the midplane. The width of the mounting rim is understood to mean the axial distance between the substantially radial portions of the rim that are intended to bear on the beads of the tyre as defined in the ETRTO technical dictionary. This difference between the width of the curing mould and the nominal rim makes it possible, on mounting, for the beads to bear naturally on the rim and for the tyre to be naturally substantially gas-tight. This measurement is taken either on one or more cross sections of the tyre or on a removed tyre with the beads properly positioned as specified. The gradient 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 gradient of the carcass layer. The prior art tyres comprising mainly non-elastic working layers have gradients below the crown of between 0.19 and 0.25, in order to ensure acceptable endurance. A dimension of the crown that also has a significant impact on the endurance performance with regard to shoulder cracking is the distance from the axial end of the tread surface to the inner surface of the inner liner, known as the total shoulder thickness. There is a total shoulder thickness on each side of the meridian plane. They are usually substantially equal give or take manufacturing variations. The measurement is carried out either on one or more meridian sections of the tyre or on a removed tyre. In the case of multiple measurements, the average of these measurements will be considered. If there is a cut in the tread at the shoulder, the measurement is taken at the point on the tread surface which 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. The reference thickness from the crown when new to the mid-plane is known as the median total thickness. It is measured on a new tyre, a removed tyre, or on a meridian cut as for the total shoulder thickness. New tyre is understood to mean on a tyre or piece of tyre having a tread pattern that has not been modified compared to when it was removed from the curing mould. The total median thickness is the radial distance from the radially innermost point of the tyre in the mid-plane to the tread surface in the mid-plane. In the absence of a central furrow, the total median thickness will be measured in an obvious manner from a radially outermost point of the tread surface. When there is a central furrow, for a meridian cut, a person skilled in the art will easily be able to determine a theoretical tread surface in the mid-plane depending on the positions of the height of the blocks axially closest to the mid-plane by a simple linear approximation. Commercially available prior art tyres that comprise inextensible reinforcing 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 reinforcing elements, it is possible to optimize the total shoulder thickness by reducing its value so that said ratio is at most equal to 1.25, preferably at most equal to 1.20. A preferred solution is that the crown reinforcement comprises a hoop reinforcement, which is radially on the inside of the protective reinforcement and which comprises at least one hooping layer among said crown layers, the reinforcing elements of the or each hooping layer forming oriented angles with an absolute value at least equal to 0° and at most equal to 10°, preferably at most equal to 2°, with the circumferential direction. The preferred solution is to circumferentially wind a strip of one or more reinforcing elements with an angle of less than 2°. This solution allows better absorption of the forces by the elastic hooping layers. The use of elastic reinforcing elements for all the reinforcing elements of the crown reinforcement increases the elasticity of the crown and makes it possible to make the stresses in all the crown layers uniform and to reduce the stresses in the hoop layers. However, the endurance of the reinforcing elements is strongly linked to their extensioncompression cycle. It is therefore possible, for one and the same endurance level, to increase the width of the hooping layers compared with the working layers. This makes it possible to lower the level of shear stress at the ends of the working layers and of the hooping layers, and thus to increase the endurance in this zone, which is the objective of the invention. In commercially available tyres, the axial width of the hooping layer of greatest axial width, on account of the stiffness of its reinforcing elements, is at most equal to 55% of the axial width of the working layer of greatest axial width. In order to improve the crown endurance, a preferred solution is that the axial width of the hooping layer of greatest axial width is at least equal to 65% of the axial width of the working layer of greatest axial width. This is made possible by the use of elastic reinforcing elements for all the crown layers. Experiments have shown that the crown endurance is improved further if the tangent modulus at (At+As) / 2 of the reinforcing elements of the at least two working layers is at most equal to 80 GPa. Given the increase in the elasticity of the crown and the increases in the width of certain crown layers, the rubber compounds at the ends of the working layers may be subjected to more mechanical stress and produce a local increase in temperature by hysteresis. To remedy this problem, one solution is that rubber compositions having an axial width at least equal to 5 mm and a radial thickness at least equal to 3 mm are positioned between the axial ends of the at least two working layers and each adjacent crown layer, said rubber compositions having an elongation at break at 100°C according to the standard NF T 46-002 which is at least equal to 500%, and a maximum dynamic loss tan6, measured according to the standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz which is at most equal to 0.06. The elastic metal reinforcing elements of the crown layers are usually made up of a plurality of steel monofilaments or assembled threads. In order to obtain an elastic reinforcing element, the geometry of the assembly of monofilaments firstly allows the reinforcing elements to deform via the movement of the monofilaments in the reinforcing element by shearing the rubber between the monofilaments. Secondly, the deformation of the reinforcing elements is mainly brought about by the deformations of the monofilaments. This property allowing the first phase of structural deformation means that the diameters of the reinforcing elements are often large, meaning, for a number of crown layers and a certain thickness of the tread, total median and total shoulder thicknesses that are very large, causing a high temperature at the crown when running, limiting the possible improvements 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 with a limited diameter. Thus, an advantageous solution is that the reinforcing elements of at least two of the crown layers, preferably of all the crown layers, are metal cords made up of a plurality of monofilaments, said monofilaments having a diameter at most equal to 0.32 mm and preferably at most equal to 0.29 mm. The features of the invention are illustrated schematically and not to scale in Figure 1, with reference to a civil engineering tyre. Figure 1 shows a meridian half-section through a civil engineering tyre 1 comprising: a tread 2, a tread surface 21, having a point 212 in the mid-plane M that divides the tyre into two torus shapes that are substantially symmetrical give or take manufacturing variations, and an axially outer point 211, a carcass reinforcement, comprising a carcass layer 4 having metal reinforcing elements that form an angle of between -10° and 10° with a radial direction, 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 extending from one bead wire 6 to the other and two turn-ups 42 that each extend 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 point Me1 that minimizes the axial distance from the bead wire 6 to the axially outermost points of the protector 7, when the points Me1 are at a distance Ljc from the mid-plane M equal to half the width of the mounting rim plus 1.27 cm, the axially outermost 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 on the outside of the axially outermost point of the carcass layer, and at a radial distance E85 from the intersection point between the mid-plane M and the carcass layer 41, an inner liner 5 having an inner surface 51 and a radially innermost point 512 in the mid-plane M, a crown reinforcement 3 comprising a working reinforcement 32, a hoop reinforcement 31 and a protective reinforcement 33, the hoop reinforcement 31 comprising two hooping layers 311 and 312, the working reinforcement 32 comprising two working layers 321 and 322, a protective reinforcement 33, radially on the outside of the hoop reinforcement and the working reinforcement, comprising in this case two protective layers 331, 332, the radially innermost protective layer being the crown layer of greatest axial width in order to protect all the other crown reinforcing elements from crown attacks, the total median thickness ETM measured on the new tyre is the radial distance from the radially innermost point 512 of the tyre in the mid-plane M to the tread surface 212 in the mid-plane M, the total shoulder thickness ETE measured in the meridian plane, on a new tyre, is the minimum distance from the axial end 211 of the tread surface 21 to the inner surface 51 of the inner liner 5, the axial width LFM of the hooping layer of greatest axial width 311 is at least equal to 65% of the axial width LTM of the working layer of greatest axial width 321, in this case 85%. The invention was compared with a commercially available control tyre (“Michelin XTRA-LOAD PROTECT B”) of the same size, namely 24.00R35, the crown of which is made up of two hooping layers radially on the inside of two working layers radially on the inside of two protective layers. The reinforcing elements of the hooping layers form angles of 8 and -8°. They are inextensible with a 26.30 configuration, i.e. 26 steel wires 30 hundredths of a millimetre in diameter, having a structural elongation on the cables taken from a tyre of 0% and a total elongation at break At equal to 2.4% and a tangent modulus at (At+As) / 2 equal to 185 GPa. They are disposed at a spacing of 3.4 mm. The reinforcing elements of the working layers form angles of -33 and 19°. They are inextensible, having the same characteristic as the reinforcing elements of the hooping layers, namely a 26.30 configuration. They are disposed at a spacing of 3.4 mm. The reinforcing elements of the protective layers are extensible with a 24.26 configuration, i.e. 24 steel wires 26 hundredths of a millimetre in diameter, and form angles of -24 and 24° with the circumferential direction, disposed at a spacing of 2.5 mm, having a structural elongation on the cables taken from a tyre of 0.6% and a total elongation at break At 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. The invention was tested on a tyre of the same size, namely 24.00R35, the crown of which is made up of two working layers radially on the inside of two hooping layers radially on the inside of two protective layers. The reinforcing elements of the hooping layers form angles of 1 and -1°. They are extensible with a 21.28 configuration, i.e. 21 steel wires or monofilaments 28 hundredths of a millimetre in diameter, having a structural elongation on the cables taken from a tyre of 0.5% and a total elongation at break At equal to 3.3% and a tangent modulus at (At+As) / 2 at most equal to 95 GPa. They are disposed at a spacing of 2.3 mm. The reinforcing elements of the working layers form angles of -33 and +33°. They are extensible, having the same characteristic as the reinforcing elements of the hooping layers, namely a 21.28 configuration. They are disposed at a spacing of 2.3 mm. The reinforcing elements of the protective layers are extensible with a 5.35 configuration, i.e. 5 steel wires 35 hundredths of a millimetre in diameter, and form angles of -33 and 33°, having a structural elongation on cables taken from a tyre of 5% and a total elongation at break At equal to 8% and a tangent modulus at (At+As) / 2 equal to 45 GPa disposed at a spacing 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 tyre according to the invention has an L4m value identical to the control but the ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 0.117. The use of elastic reinforcing elements has made it possible to have an axial width of the hoops increased from 53% of the axial width of the working layer of greatest width, in the control tyre, to 73% in the tyre according to the invention, and to reduce the gradient of the carcass layer under the crown by almost 50% while keeping maximum shear stress levels of the same order of magnitude for the tyre according to the invention. The control tyre and tyre according to the invention are identical except for the crown layers and the geometry of the crown, namely the ETE / ETM ratio and the gradient of the carcass layer under the crown. They have the same tread pattern and the same reinforcers for the carcass layer and the same rubber compounds for the different parts of the tyres. The radius of the tread surface has been adapted such 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 liner are unchanged from the control tyre to the tyre according to the invention. The ETE / TME ratio has been reduced by more than 5% from 1.3 to 1.24, for the same value of the median total thickness (TME). The invention was simulated on calculation tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tyres using the large-movement and high-strain finite element technique, taking into account the mechanical and hysteretic characteristics of the materials. The estimated improvement in shoulder temperature is 7°. The tyres were also compared using running tests. The same test protocol was applied to the tyres according to the invention and to the reference tyres according to the prior art. The performance in terms of endurance with respect to cracking of the crown, also known as straight-line split resistance of the crown, was measured using tests on a machine in which two tyres of the same type (reference tyre on reference tyre, tyre according to the invention on tyre according to the invention) run on one another at a speed of 28 km / h, with the tyres inflated to 7.25 bar for a compressive force of 20 t. The test was stopped when one of the tyres experiences a drop in pressure. The result considered is the number of kilometres travelled before failure of the tyre. In this test, the control tyres ran for 470 hours and had to be stopped because of a pocket of cracking at the ends of the crown layers. The tyre according to the invention ran for 970 hours without having any damage, or cracking of the crown filler rubbers or breakage of reinforcing elements. The shoulder temperature measurements taken by tapping during running confirmed thermal improvements compared with the control, of between -4°C and -10°C depending on the position of the tapping point. Furthermore, the increase in flexibility of the crown is known to those skilled in the art to be favourable for endurance under attack and for the reduction in the wear associated with these attacks. Via this test series, the invention has clearly demonstrated its ability 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 filler rubbers, while maintaining or improving resistance to attack for the same expected result in terms of wear. A second size of larger diameter was also tested. The invention was compared with a commercially available control tyre (“Michelin XDR3 MC4 NL”) of the same size, namely 53.80R63, the crown of which is made up of two hooping layers radially on the inside of two working layers radially on the inside of two protective layers. The reinforcing elements of the hooping layers form angles of 8 and -8°. They are inextensible with a 77.35 configuration, i.e. 77 steel wires 35 hundredths of a millimetre in diameter, having a structural elongation on the cables taken from a tyre of 0% and a total elongation at break At equal to 1.7% and a tangent modulus at (At+As) / 2 equal to 165 GPa. They are disposed at a spacing of 5.5 mm. The reinforcing elements of the working layers form angles of -33 and 19°. They are inextensible, having the same characteristic as the reinforcing elements of the hooping layers, namely a 77.35 configuration. They are disposed at a spacing of 5.5 mm. The reinforcing elements of the protective layers are extensible with a 52.26 configuration, i.e. 52 steel wires 26 hundredths of a millimetre in diameter, and form angles of -24 and 24° with the circumferential direction, disposed at a spacing of 3.7 mm, having a structural elongation on the cables taken from a tyre of 1.5% and a total elongation at break At 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. The invention was tested on a tyre of the same size, namely 53.80R63, the crown of which is made up of two working layers radially on the inside of two hooping layers radially on the inside of two protective layers. The reinforcing elements of the hooping layers form angles of 1 and -1°. They are extensible with a 147.28 configuration, i.e. 147 steel wires or monofilaments 28 hundredths of a millimetre in diameter, having a structural elongation on the cables taken from a tyre of 0.3% and a total elongation at break At equal to 2.5% and a tangent modulus at (At+As) / 2 at most equal to 103 GPa. They are disposed at a spacing of 7.4 mm. The reinforcing elements of the working layers form angles of -26 and +26°. They are extensible, having the same characteristic as the reinforcing elements of the hooping layers, namely a 147.28 configuration. They are disposed at a spacing of 7.4 mm. The reinforcing elements of the protective layers are extensible with a 52.26 configuration, i.e. 52 steel wires 26 hundredths of a millimetre in diameter, and form angles of -26 and 26°, having a structural elongation on cables taken from a tyre of 1.5% and a total elongation at break At equal to 4.2% and a tangent modulus at (At+As) / 2 equal to 70 GPa disposed at a spacing 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 tyre according to the invention has an L4m value identical to the control give or take 0.26% but the ratio between the radial distance E85 and the axial length 0.85*L4m is equal to 0.133. The use of elastic reinforcing elements has made it possible to have an axial width of the hoops increased from 58% of the axial width of the working layer of greatest width, in the control tyre, to 72% in the tyre according to the invention, and to reduce the gradient of the carcass layer under the crown by almost 35% while keeping maximum shear stress levels of the same order of magnitude. The control tyre and tyre 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 gradient of the carcass layer under the crown. They have the same tread pattern and the same reinforcers for the carcass layer and the same rubber compounds for the different parts of the tyres. The radius of the tread surface has been adapted such 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 liner are unchanged from the control tyre to the tyre according to the invention. The ETE / TME ratio has been reduced by more than 12% from 1.26 to 1.09, for the same value of the median total thickness (TME). The invention was simulated on calculation tools. The simulations make it possible to evaluate the mechanical and thermal stresses on tyres using the large-movement and high-strain finite element technique, taking into account the mechanical and hysteretic characteristics of the materials. The estimated improvement in shoulder temperature is 4°. Here too, the increase in flexibility of the crown will be favourable for endurance under attack and for the reduction in the wear associated with these attacks. These calculation elements demonstrate 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 filler rubbers, while maintaining or improving resistance to attack for the same expected result in terms of wear across a wide range of civil engineering tyre sizes.
Claims
1. Radial tyre (1) for a civil engineering vehicle, which is intended to be mounted on a nominal rim with a nominal diameter greater than 25 inches and comprises:- a carcass reinforcement, which extends between two beads (7) of the tyre, each of the beads (7) comprising a circumferential reinforcement or bead wire (6), and is made up of a carcass layer (4) comprising metal reinforcers that form an angle of between -10° and 10° with a radial direction, the carcass layer (4) comprising a main portion (41), extending from one bead wire (6) to the other, and two turn-ups (42), each extending from a bead wire (6) to one end of the carcass layer (4),- a crown reinforcement (3), radially on the inside of a tread (2) of the tyre and radially on the outside of the carcass reinforcement, the tread (2) having a tread surface (21) intended to come into contact with underlying ground, the tread surface having, for each meridian plane, two axial ends (211), and- an inner liner (5) having an inner surface (51) intended to be in contact with an inflation gas,wherein the tyre has a mid-plane (M), which is perpendicular to an axis of rotation (YY’) of the tyre and which passes through the middle of the tread (2),wherein a total median thickness (ETM) is the radial distance from the radially innermost point (512) of the tyre (1) in the mid-plane (M) to the tread surface (212),wherein a total shoulder thickness (ETE), measured in a meridian plane, is the minimum distance from the axial end (211) of the tread surface (21) to the inner surface (51) of the inner liner (5),wherein the ratio of the total shoulder thickness (ETE) to the total median thickness (ETM) is at most equal to 1.25,wherein each bead (7) comprises a protector (71) made up of a rubber compound having axially outermost points which are intended to come into contact with the nominal rim, each bead including, in each meridian, a point Me1 which minimizes the axial distance from the bead wire (6) to the axially outermost points of the protector (7) and which is positioned at an axial distance (Ljc) from the mid-plane (M) equal to half the width of the nominal rim plus 1.27 cm,wherein the main portion (41) of the carcass layer (4) includes an axially outermost point, which is at an axial distance L4m from the mid-plane (M), and a point Me2, which is radially on the outside of said axially outermost point of the main portionof the carcass layer and which is at an axial distance from the mid-plane (M) equal to 0.85*L4m, the point Me2 being situated at a radial distance E85 from the intersection point between the mid-plane (M) and the carcass layer (4),wherein the ratio between E85 and 0.85*L4m is between 0.10 and 0.15,wherein the crown reinforcement (3) comprises crown layers (311, 312, 321, 322, 331) that each comprise metal reinforcing elements, each reinforcing element of each of said crown layers being characterized by a structural elongation As and a total elongation at break At, which are measured according to the standard ASTM D 2969-04 of 2014,wherein the crown reinforcement (3) comprises a working reinforcement (32), which comprises at least two working layers (321, 322) among said crown layers (311, 312, 321, 322, 331), and a protective reinforcement (33), which is radially on the outside of the working reinforcement (32) and which comprises at least one protective layer (331) among said crown layers (311, 312, 321, 322, 331),wherein the reinforcing elements of each working layer (321, 322) are parallel to one another, forming oriented angles with an absolute value at least equal to 15° and at most equal to 45° with the circumferential direction, the angles of two working layers among the at least two working layers (321, 322) having opposite signs from one working layer to the next,wherein the reinforcing elements of the or each protective layer (331) are parallel to one another, forming oriented angles with an absolute value at least equal to 15° and at most equal to 45° with the circumferential direction,wherein the reinforcing elements of each of the crown layers (311, 312, 321, 322, 331) have a structural elongation As at least equal to 0.3%, a total elongation at break At at least equal to 3%, and a tangent modulus at (At+As) / 2 at most equal to 120 GPa,and wherein the reinforcing elements of the or each protective layer (331) 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 (321).
2. Tyre (1) according to Claim 1, wherein the crown reinforcement (3) comprises a hoop reinforcement (31), which is radially on the inside of the protective reinforcement and which comprises at least one hooping layer (311, 312) among said crown layers (311, 312, 321, 322, 331), the reinforcing elements of the or each hoopinglayer forming oriented angles with an absolute value at least equal to 0° and at most equal to 10°, preferably at most equal to 2°, with the circumferential direction.
3. Tyre (1) according to Claim 2, wherein the axial width (LFM) of the hooping layer of greatest axial width is at least equal to 65% of the axial width (LTM) of the working layer of greatest axial width.
4. Tyre (1) according to any one of the preceding claims, wherein the ratio of the total shoulder thickness (ETE) to the total median thickness (ETM) is at most 1.20.
5. Tyre (1) according to any one of the preceding claims, wherein the tangent modulus at (At+As) / 2 of the reinforcing elements of the at least two working layers (321, 322) is at most equal to 80 GPa.
6. Tyre (1) according to any one of the preceding claims, wherein rubber compositions having an axial width at least equal to 5 mm and a radial thickness at least equal to 3 mm are positioned between the axial ends of the at least two working layers (321, 322) and each adjacent crown layer, said rubber compositions having an elongation at break at 100°C according to the standard NF T 46-002 which is at least equal to 500%, and a maximum dynamic loss tan6, measured according to the standard ASTM D 5992 - 96, at a temperature of 100°C and at 10 Hz, which is at most equal to 0.06.
7. Tyre (1) according to any one of the preceding claims, wherein the reinforcing elements of at least two of the crown layers (311, 312, 321, 322, 331) are metal cords made up of a plurality of monofilaments, said monofilaments having a diameter at most equal to 0.32 mm and preferably at most equal to 0.29 mm.