Tire for a heavy civil-engineering vehicle with a simplified crown reinforcement

Abstract

A tire for a heavy-duty vehicle of construction plant type comprises a crown reinforcement (35) radially on the inside of a tread (10) and radially on the outside of a carcass reinforcement (50). The crown reinforcement (35) comprises: at least one low-modulus layer (20) formed of elastic metal reinforcers having a structural elongation at least equal to 0.4%, and a total elongation at break at least equal to 3%, and a tensile elastic modulus of between 40 GPa and 130 GPa; at least one rigid layer (30) formed of rigid metal reinforcers, the structural elongation of which is less than or equal to 0.2% and the tensile elastic modulus of which is between 140 GPa and 200 GPa. The ratio of the breaking tension of the rigid layer to that of the low-modulus layer is greater than or equal to 1.2.

Claims

1. A tire for a heavy-duty vehicle, the tire comprising: a nominal section B within the meaning of the European Tyre and Rim Technical Organisation (ETRTO) standard; a tread; a carcass reinforcement; and a crown reinforcement radially on an inside of the tread and radially on an outside of the carcass reinforcement, the crown reinforcement including: at least one low-modulus layer formed of low-modulus elastic metal reinforcers, which are coated in an elastomeric material, the low-modulus reinforcers being mutually parallel and forming an angle Alpha1 with a circumferential direction tangential to a circumference of the tire, the low-modulus reinforcers having a structural elongation at least equal to 0.4% and a total elongation at break at least equal to 3%, and the tensile elastic modulus of the low-modulus reinforcers being between 40 GPa and 130 GPa; a rigid layer formed of rigid metal reinforcers, which are coated in an elastomeric material, the rigid metal reinforcers being mutually parallel and forming an angle Alpha2 with the circumferential direction tangential to the circumference of the tire, the structural elongation of the rigid metal reinforcers being less than or equal to 0.2%, and the tensile elastic modulus of the rigid reinforcers being between 140 GPa and 200 GPa; and a triangulation layer comprising metal reinforcers, which are coated in an elastomeric material, the metal reinforcers being mutually parallel and forming an angle Beta with the circumferential direction tangential to the circumference of the tire, wherein the angle Beta is greater than or equal to 50 in terms of absolute value, wherein a breaking tension of the rigid layer is greater than or equal to 120 daN/mm, wherein a ratio of the breaking tension of the rigid layer to that of the at least one low-modulus layer is greater than or equal to 1.2, wherein the breaking tension of the at least one low-modulus layer is greater than or equal to 80 daN/mm, wherein an axial width of the at least one low-modulus layer is greater than an axial width of the rigid layer by a distance DNT at least equal to 10 mm at each axial end, and wherein at least one low-modulus layer is in contact with the rigid layer over an axial width W at least equal to 20% of the nominal section B of the tire and the triangulation layer is positioned on an inside of the rigid layer and in contact therewith in the radial direction.

2. The tire according to claim 1, wherein the rigid layer comprises reinforcers provided with a wrapping thread of small diameter, between 0.10 mm and 0.3 mm, wound in a helix on an external surface of the reinforcers.

3. The tire according to claim 2, wherein the at least one low-modulus layer is positioned on an outside of the rigid layer in a radial direction.

4. The tire according to claim 1, wherein an axial width of the triangulation layer, measured in a meridian cross section of the tire, is less than that of the at least one low-modulus layer and greater than that of the rigid layer.

5. The tire according to claim 1, wherein an absolute value of the angle Alpha1 of the reinforcers of the at least one low-modulus layer is greater than or equal to an absolute value of the angle Alpha2 of the rigid layer.

6. The tire according to claim 1, wherein the crown reinforcement comprises at least one layer formed of elastic metal reinforcers, an angle of which with the circumferential direction of the tire is less than or equal to 10.

7. The tire according to claim 1, wherein a layer of elastomeric compound is positioned toward an inside in a radial direction beneath the at least one low-modulus layer and in contact with the at least one low-modulus layer vertically in line with the end of the rigid layer.

8. The tire according to claim 7, wherein the layer of elastomeric compound has a tensile stiffness modulus less than or equal to 4.5 MPa.

Description

DETAILED DESCRIPTION

(1) The main idea of the invention is to form a multilayer composite having a rigid layer and a low-modulus layer as a base element of the architecture of the crown. Depending on the intended use of the tyre, a triangulation layer, comprising reinforcers that form an angle at least equal to 50 with the circumferential direction, or a hoop reinforcement, made up of at least one hooping layer comprising reinforcers that form an angle at most equal to 10 with the circumferential direction, can be added to the base element.

(2) The radially innermost rigid layer is reinforced in design terms in order to replace the two working layers of the usual working reinforcement. The radially outer low-modulus layer, closest to the tread, replaces the two protective layers of the usual protective reinforcement. Therefore, the invention represents a simplification of conventional crown architectures that generally comprise at least five crown layers necessary for achieving equivalent operation of the tyre.

(3) The same coating compound is used for the rigid layer and for the low-modulus layer. The thickness of the coating compound between the two layers, the angles of the reinforcers of each layer, and the Young's moduli of the reinforcers are chosen so as to ensure the mechanical coupling of the two layers.

(4) The mechanically coupled state of the multilayer is realized when the shear of the coating compound of the rigid layer and of the low-modulus layer in the meridian plane of the tyre is zero. The tension in the reinforcers of the two layers then reaches its maximum value. The tension in the reinforcers varies from a value of zero at the axial ends of the layers to the maximum possible value along the coupling distance.

(5) During the inflation of the tyre mounted on its rim, a uniform rise in the profile of the crown in the meridian plane is decisive for the performance aspects of the tyre, in particular for the wear pattern of the tread, which needs to be as uniform as possible. The adjustment of the radial stiffnesses in connection with the mechanical coupling of the rigid layer and low-modulus layer is therefore instrumental for ensuring the performance aspects of the tyre.

(6) According to the invention, the breaking tension of the rigid layer is greater than or equal to 120 daN/mm.

(7) A radial tyre subjected to its inflation pressure generates meridian tension that is taken up by the reinforcers of the radial carcass reinforcement, which, by shearing of the coating compound, transmits the tension to the reinforcers of the crown reinforcement.

(8) The radially innermost crown layer is reinforced such that the breaking tension of the layer is greater than or equal to 120 daN/mm. The design of the tyre according to the invention with a smaller number of crown layers allows it to resist the inflation pressure with a sufficient safety margin.

(9) Still according to the invention, the breaking tension of the low-modulus layer is greater than or equal to 80 daN/mm.

(10) The breaking tension of the low-modulus layer needs to be greater than a threshold which, according to the inventors, is equal to 80 daN/mm. This constraint is satisfied for elastic cables of 24.26 assembly, meaning cables made up of 24 threads each measuring 26 hundredths of a millimetre.

(11) These levels of tension determine the operating range of the invention.

(12) Also according to the invention, the ratio of the breaking tension of the rigid layer to that of the low-modulus layer is greater than or equal to 1.2, preferably greater than or equal to 1.4.

(13) The invention defines a crown reinforcement comprising a rigid layer facing a low-modulus layer. This design choice results from the compromise found in order to have sufficient cornering stiffness for the transverse operation of the tyre while having a relatively flexible elastic protective layer for protecting the crown. The low-modulus layer plays a dual role: firstly, this layer couples with the rigid layer to achieve the necessary stiffness level that contributes to guiding the vehicle, and secondly, said low-modulus layer protects the crown by being sufficiently elastic to shape itself when rolling over obstacles. The inventors have found that, in order to ensure these two functions under satisfactory conditions, the ratio of the tension in the rigid layer to that of the low-modulus layer needs to be equal to 1.2.

(14) According to the invention, at least one low-modulus layer is in contact with at least one rigid layer over an axial width W at least equal to 20% of the nominal section B of the tyre within the meaning of the ETRTO standard.

(15) The rigid layer and low-modulus layer are placed in contact to achieve mechanical coupling between the two layers which corresponds to the state of maximum stress of the reinforcers in the meridian plane. The stress in the reinforcers is related to the variation in shear stresses in the coating compound of the layer. The stress increases continuously from the axial end of the layer, where it is zero, to a maximum value that is reached starting at a certain axial position on the layers. The distance covered to reach the maximum value of the stress is the coupling distance. The width of the low-modulus layer needs to be sufficient to make it possible to achieve mechanical coupling.

(16) The coupling distance depends on several mechanical parameters such as the tensile elastic moduli of the reinforcers, the shear modulus of the coating compound. The inventors have established that if the rigid layer and low-modulus layer are in contact over a distance at least equal to 20% of the nominal width of the tyre, the coupling between the two layers can be achieved.

(17) It is advantageous that the rigid layer comprises reinforcers comprising a wrapping thread of small diameter, between 0.10 mm and 0.3 mm, wound in a helix on the external surface of the reinforcer.

(18) As it runs, a construction plant tyre generates buckling stresses in the cables of the carcass ply, which are made to bend to a significant degree, such that they tend to buckle, leading to an increase in the curvature of the individual threads, causing thread breakages. These cables may then break prematurely, thus determining the running endurance limit of the tyre. Just as in the case of the carcass layer, the rigid layer uses the solution of wrapping with a thread of small diameter of between 0.10 mm and 0.3 mm in order to improve its endurance performance.

(19) According to one embodiment of the invention, the low-modulus layer is positioned on the outside of the rigid layer in the radial direction.

(20) The more flexible layer is positioned radially closest to the tread. Said layer contributes to obtaining a level of indentation stiffness that allows the crown to deform so as to absorb the obstacles on tracks while the tyre is running.

(21) Advantageously, the axial width of the low-modulus layer is greater than the axial width of the rigid layer by a distance DNT at least equal to 10 mm at each axial end.

(22) The ends of the reinforcers of the layers are mechanical singularities with zero-stress conditions and shear stresses of large amplitude. Said ends are thus staggered at spacings of at least 10 mm in the axial direction so as to avoid them coinciding in one and the same meridian plane.

(23) According to one embodiment of the invention, an additional, triangulation layer, comprising metal reinforcers, which are coated in an elastomeric material, are mutually parallel and form an angle Beta with a circumferential direction (XX) tangential to the circumference of the tyre, is positioned on the inside of the rigid layer and in contact therewith in the radial direction, said angle Beta being greater than or equal to 50 in terms of absolute value and in the same direction as the adjacent rigid layer.

(24) The triangulation layer is made up of rigid metal threads or cables of low extensibility that make an angle Beta of between 45 and 90 with the circumferential direction, and is positioned between the carcass reinforcement and the rigid layer, which likewise comprises rigid reinforcers that make an angle with a value of less than 45 in terms of absolute value with the circumferential direction. Said triangulation layer forms a triangulated reinforcement with the rigid layer, this reinforcement having low deformation under the various stresses which it undergoes, the triangulation layer essentially serving to absorb the transverse compressive forces that act on all the reinforcing elements in the crown region of the tyre.

(25) In this embodiment, in fine, the tyre comprises a carcass reinforcement on which the triangulation layer is positioned in the radially outward direction, followed by the stack of the rigid layer and the low-modulus layer, the whole being surmounted by the tread.

(26) Advantageously, the axial width of the triangulation layer, measured in a meridian cross section of the tyre, is less than that of the low-modulus layer and greater than that of the rigid layer.

(27) The low-modulus layer has an axial width greater than that of the rigid layer by at least 20 mm. As for the triangulation layer, its width is defined so as to bring about coupling with the low-modulus layer in order to limit the shear stresses at the axial ends so as to defend against problems of cleavage of the crown. This mechanical condition associated with the constraint of avoiding coincidence of the axial ends leads to the definition of the end of the triangulation layer between axial ends of the rigid layer and low-modulus layer.

(28) Preferably, the absolute value of the angle Alpha1 of the reinforcers of the low-modulus layer is greater than or equal to the absolute value of the angle Alpha2 of the reinforcers of the rigid layer.

(29) The absolute values of the angles of the low-modulus layer and of the rigid layer are both comprised in the range [15; 40], but the angle of the rigid layer is smaller than that of the low-modulus layer, meaning that it is closer to the 15 limit. In reality, the angle Alpha2 of the rigid layer closer to the circumferential direction is used to contribute to the stiffening of the rigid layer in the circumferential direction.

(30) By way of illustration, a combination of angles Alpha1 at 24 and Alpha2 at 20 allows good operation of the invention.

(31) According to one embodiment of the invention, the crown reinforcement comprises at least one hooping layer formed of elastic metal reinforcers, the angle of which with the circumferential direction of the tyre is less than or equal to 10.

(32) The simplified architecture of the tyre according to the invention is compatible with the use of a hoop reinforcement. For particular uses in which the tyres are highly stressed for example when cornering, a hoop reinforcement can be added to the crown proposed by the invention.

(33) The hoop reinforcement usually comprises two radially superposed hooping layers formed of metal reinforcers that are mutually parallel in each layer and are crossed from one layer to the next, forming angles at most equal to 10 with the circumferential direction.

(34) The hoop reinforcement, the function of which is to at least partially absorb the mechanical inflation stresses, improves the endurance of the crown reinforcement by stiffening the crown reinforcement. The hoop reinforcement can be positioned radially between the rigid layer and the low-modulus layer of the working reinforcement or radially on the outside of the working reinforcement.

(35) According to another embodiment of the invention, a layer of elastomeric compound is positioned towards the inside in the radial direction, beneath the low-modulus layer and in contact with the latter vertically in line with the end of the rigid layer.

(36) Preferably, the layer of elastomeric compound positioned radially beneath the low-modulus layer and in contact with the latter vertically in line with the end of the rigid layer has a tensile stiffness modulus less than or equal to 4.5 MPa.

(37) The low-modulus layer is the one with the greatest axial width, and at its axial ends, the deformations in the meridian and circumferential planes have large amplitudes. The addition of an edging layer at the end of the low-modulus layer on the inside in the radial direction has the advantage of localizing the shear deformations there. This solution makes it possible to improve the endurance of the tyre with respect to cleavage.

(38) Located in the environment of the layer of compound added to the low-modulus layer on the inside in the radial direction are the coating compound of the low-modulus layer and the edging compound, which borders the axial end. The tensile elastic modulus of the compound attached to the low-modulus layer needs to be lower than that of the surrounding compounds. The inventors have found that a value of the tensile elastic modulus less than or equal to 4.5 MPa is sufficient for proper operation of the invention.

BRIEF DESCRIPTION OF THE FIGURES

(39) The features of the invention are illustrated in the schematic FIGS. 1 and 2, which are not to scale, with reference to a tyre of size 29.5R25:

(40) FIG. 1: meridian cross section through the crown of the tyre according to the invention;

(41) FIG. 2: portion of a meridian cross section through said crown;

(42) FIG. 3: illustration of the assembly of a cable as used in the invention.

(43) FIG. 1 shows a tyre 1 for a heavy-duty vehicle of construction plant type, comprising a crown reinforcement 35 radially on the inside of a tread 10 and radially on the outside of a carcass reinforcement 50. The crown reinforcement 35 comprises at least one layer 20, known as the low-modulus layer, and at least one layer 30, known as the rigid layer. A layer of elastomeric compound 25 is positioned radially on the interior side of the low-modulus layer. The radially innermost crown reinforcement layer 40, in contact with the carcass reinforcement 50, is known as the triangulation layer. The ends of the layers 20, 30 and 40 are embedded in an elastomeric edging compound 110. The rigid crown layer 30 and low-modulus crown layer 20 are separated at their axial ends by a decoupling compound 90.

(44) FIG. 2 shows the crown reinforcement 35 with its characteristic dimensions. The rigid layer has an axial width LNT1 and the low-modulus layer has a width LNT2. The width LNT2 is greater than LNT1 such that the low-modulus layer extends beyond the ends of the rigid layer on both sides. The axial ends of the triangulation layer of width LNTS are interposed between the axial ends of the low-modulus layer and rigid layer. In order to avoid coincidence of the axial ends of the crown layers, an offset of at least 10 mm is provided between two consecutive layers. LCOUP is the coupling distance for achieving the maximum level of tension in the reinforcers of the crown layers.

(45) FIG. 3 shows an example of low-modulus reinforcers used for the low-modulus layer. The cable comprises 4 strands 200 wound in a helix, each strand being made up of an internal layer 220 of one thread, surrounded by an external layer 230 formed of 4 threads 210 wound around the internal layer.

(46) The inventors compared a tyre I according to the invention against a reference tyre R of the same size 29.5R25. The reference tyre is a conventional prior art solution, the description of which is summarized in Table 1 below:

(47) TABLE-US-00001 TABLE 1 Crown reinforcement of the reference tyre Reinforcers Assembly Angle Protective Layer 1 E18.23 3 (1 + 5) 0.23 24 reinforcement Layer 2 E18.23 3 (1 + 5) 0.23 24 Working Layer 3 13.30 FR (4 + 9) 0.30 24 reinforcement Layer 4 13.30 FR (4 + 9) 0.30 24 Triangulation layer Layer 5 13.30 FR (4 + 9) 0.30 65

(48) Table 1 describes the layers that make up the crown of the reference tyre. The reinforcers of the layers are cables, the assembly of which is mentioned for each layer. The angles that the reinforcers make with the circumferential direction of the tyre are also mentioned.

(49) The formulations of the assemblies of the cables that end in FR mean that the cables are wrapped. The cables are provided with a wrapping thread of small diameter, between 0.10 mm and 0.3 mm, wound in a helix on the external surface of the cable. For example, the cable 13.30 FR has a wrap with a diameter of 0.18 mm.

(50) The tyre according to the invention comprises the components described in Table 2 below:

(51) TABLE-US-00002 TABLE 2 Crown reinforcement of the tyre according to the invention Elastic Reinforcers Assembly Angle ER TR moduli Spacing Low-modulus Layer 1 E24.26 4 (1 + 5) 0.26 24 2600N 1060 N/mm 42 GPa 2.5 mm layer Rigid layer Layer 2 26.30 FR 3 0.35 + (4 + 14) 0.30 20 6200N 1800 N/mm 144 GPa 2 mm Triangulation Layer 3 13.30 FR (4 + 9) 0.30 65 3000N 1500 N/mm 174 GPa 3.4 mm layer

(52) Table 2 describes the layers that make up the simplified crown of the tyre according to the invention. Indicated are the cables used as reinforcers, their assembly, and the angles that they make with the circumferential direction of the tyre. Mechanical properties of the layers are mentioned: the breaking force of the cables, the breaking tension of the layers and the axial spacing of the reinforcers for each layer.

(53) The tyre according to the invention, in the configuration tested here, differs from the reference tyre by the elimination of layers of the crown reinforcement so as to be limited to a radially innermost triangulation layer, followed by a rigid layer formed of reinforcers wrapped with 26 metal threads each measuring 30 hundredths of a millimetre, and a low-modulus layer formed of 24 metal threads each having a diameter of 26 hundredths of a millimetre.

(54) The assembly of the reinforcers in the low-modulus layer has the formulation 4(1+5)0.26, meaning that the reinforcers are multistrand ropes made up of 4 strands, each strand comprising an internal layer of 1 internal thread and an external layer of 5 external threads wound in a helix around the internal layer, the threads having a section with a diameter of 0.26 mm.

(55) The reinforcers of the low-modulus layer have a tensile elastic modulus equal to 42 GPa, a force at break equal to 260 daN, are axially distributed at a spacing equal to 2.5 mm, and form an angle Alpha1 of 24 with the circumferential direction XX. The breaking tension of the low-modulus layer is 106 daN/mm.

(56) As regards the rigid layer, the reinforcers are cables having three layers of threads, assembled according to the formulation 30.35+(4+14)0.30. The first layer comprises 3 threads with a diameter of 0.35 mm, followed by the second with 4 threads with a diameter of 0.30 mm, and the third layer comprises 14 threads with a diameter of 0.30 mm. The reinforcers thus defined have a force at break equal to 620 daN, are axially distributed at a spacing equal to 3.4 mm, and form an angle Alpha1 of 20 with the circumferential direction XX. The breaking tension of the rigid layer is 176 daN/mm.

(57) The triangulation layer comprises reinforcers which are cables having two layers of threads, assembled according to the formulation (4+9)0.30. The first layer comprises 4 threads with a diameter of 0.30 mm, followed by the second layer with 9 threads with a diameter of 0.30 mm. The reinforcers thus defined have a force at break equal to 300 daN, are axially distributed at a spacing equal to 2.0 mm, and form an angle Alpha1 of 65 with the circumferential direction XX. The breaking tension of the triangulation layer is 140 daN/mm.

(58) The above-described tyre according to the invention clearly meets the objective of the claimed patent. The intended objective of a saving in the industrial manufacturing cost is achieved with a weight of the tyre according to the invention that is lower than that of the reference tyre by 7%. The saving of weight results from the elimination, in the reference tyre, of a protective layer and a working layer.

(59) The strength of the crown of the tyre according to the invention when passing over obstacles on the road is at a level comparable to that of the reference tyre. The low-modulus layer, which is formed of elastic E24.26 multistrand ropes, also has a protective function with a performance level sufficient to replace the two layers of the E18.23 protective reinforcement of the crown of the reference tyre.

(60) The endurance was evaluated by way of finite-element calculations on the reference tyre and the tyre according to the invention, making it possible to calculate the tensions in the carcass layer and the shear stresses at the ends of the layers of the crown reinforcement.

(61) To evaluate these performance aspects, the tyre was subjected to calculations with a vertical load of 14 000 kg, a drift thrust of 6000 daN, and an inflation pressure of 450 kPa.

(62) In the version of the tyre with the triangulation layer, the carcass layer is still under relative tension, thereby obviating the risk of buckling. The shear stresses at the ends of the layers of the crown are sufficiently low as to not have a negative effect on endurance.

(63) The invention makes it possible to lighten the tyre while making it possible to transmit the forces for guiding the vehicle without impairing the integrity of the compounds at the axial ends of the layers.