Crown reinforcement for urban tires

Abstract

The present invention relates to a tire having a radial carcass reinforcement and a crown reinforcement comprising two working crown layers of crossed metallic reinforcing elements, which crown reinforcement also comprises at least one layer of metallic circumferential reinforcing elements radially outside the two working layers, and the distance D between the radially innermost point of the break axially closest to the equatorial plane and a point on the outer surface of the circumferential reinforcing elements of at least one layer is less than 8 mm.

Description

[Technical field]

[0001] The present invention relates to a tire having a radial carcass reinforcement, and more particularly to a tire intended to be fitted to mass transit guided vehicles capable of running on dedicated lanes, and more particularly to large or small, monorail, type rapid transit vehicles, or other vehicles of this type. [Background technology]

[0002] In tires with radial carcass reinforcement for heavy goods transport, the carcass reinforcement is generally fixed on both sides in the region of the beads, and radially surmounted by a crown reinforcement consisting of at least two layers formed of threads or cords, parallel in each layer, making an angle of between 10° and 45° with the circumferential direction and crossing from one layer to the next, called a working layer, which may also be covered by at least one layer called a protective layer, which may also include a layer of low-stretch metal threads or cords making an angle of between 45° and 90° with the circumferential direction, called a triangular lattice layer, located radially between the carcass reinforcement and the radially innermost crown layer, called the working layer.

[0003] A cord is said to be inextensible if it exhibits an elongation at most equal to 0.2% under a tensile force equal to 10% of the breaking force.

[0004] A cord is said to be elastic if it exhibits, under a tensile force equal to the breaking load, a maximum tangent modulus of elasticity of less than 150 GPa and an elongation at least equal to 3%.

[0005] Circumferential reinforcing elements are reinforcing elements that form an angle in the circumferential direction within the range of +2.5° to -2.5°.

[0006] The circumferential, or longitudinal, direction of the tire is the direction that corresponds to the outer periphery of the tire and is defined by the direction in which the tire travels.

[0007] The axis of rotation of a tire is the axis about which the tire rotates in normal use.

[0008] The radial or meridian plane is the plane containing the axis of rotation of the tire.

[0009] The circumferential median or equatorial plane is the plane perpendicular to the tire's axis of rotation and which bisects the tire.

[0010] The lateral or axial direction of the tire is parallel to the tire's axis of rotation. Axial distances are measured along the axial direction. The expression "axially inward of, or axially outward of, respectively" means "the axial distance measured from the equatorial plane is less than, or greater than, respectively".

[0011] A radial direction is a direction that intersects the tire's axis of rotation and is perpendicular thereto. A radial distance is measured along the radial direction. The phrase "radially inward of or radially outward of, respectively" means "a radial distance, measured from the tire's axis of rotation, that is less than or greater than, respectively."

[0012] For metal threads or cords, the breaking force (maximum load, N), breaking strength (MPa), elongation at break (total elongation, %), modulus of elasticity (GPa) are measured under tension according to standard ISO 6892 of 1984.

[0013] In combination with an internal tire structure, it is known to provide the tread, i.e. the part of the tire that comes into contact with the ground during running and is intended to be worn during running, with a tread pattern formed of raised elements bounded by grooves, in particular circumferentially oriented grooves.

[0014] It is also known to provide a plurality of laterally or diagonally oriented edge corners on the tread surface. One means of obtaining such edge corners is to provide a plurality of cuts in the tread, these cuts having the form of grooves or of sipes. In the present application, a distinction is made between sipes and grooves in that the sipes have a width suitable to allow, during running, at least partial contact between the opposing walls that define them, particularly in the contact surface that contacts the ground, which would not be the situation of a groove under normal conditions of use of the tire.

[0015] The lateral or diagonal edge corners of the tread ensure the grip of the tire, especially on the driving lanes on which high speed transport vehicles travel.

[0016] If the crown reinforcement consists of two working layers, each reinforcing element crossing each other to form an angle between 10° and 45°, stresses may arise in the crown reinforcement during use, in particular shear stresses between the crown layers, which may result in separation of the working layers at the shoulder of the tire under certain conditions of use, which may lead to tire deterioration.

[0017] The close proximity of the electrical conductors necessary to supply power to the vehicle poses a non-zero risk of electrical arcing between these conductors and metal pieces of the tire in the event of the above-mentioned deterioration.

[0018] In order to eliminate this risk and to avoid the presence of metal pieces, crown reinforcement solutions have been developed which only have reinforcing elements made of fibre material. WO 97 / 15463 describes a crown reinforcement which consists of one or more layers of circumferentially oriented fibre reinforcing elements.

[0019] Although such tires effectively provide a solution to the risks associated with the presence of metal debris, driving reveals wear-related problems for the tires, more particularly irregular wear: the strong traction forces involved in the use of this type of vehicle, and in particular the very large load variations present, for example between busy periods with maximum load and less busy periods when the vehicle is traveling with few or no passengers (these load variations are associated with intensive and repetitive braking and acceleration phases), lead to very noticeable wear on the shoulders of the tires, necessitating frequent tire changes.

[0020] Furthermore, in the field of heavy vehicle tyres, it is known to introduce a layer of rubber compound between the ends of the working layers in order to form a decoupling between said ends in order to limit the shear stresses. However, such a decoupling layer must exhibit very good bonding. Such a layer of rubber compound is described, for example, in WO 2004 / 076204.

[0021] French Patent No. 2 222 232 teaches that in order to avoid separation between the crown reinforcement plies, the ends of the reinforcement are covered with a rubber cushion having a Shore A hardness different from that of the tread that rests on the reinforcement and greater than the Shore A hardness of a profiled element of rubber compound arranged between the crown reinforcement and the ends of the carcass reinforcing plies.

[0022] Furthermore, in order to produce tires with a very wide tread or to give a tire of given dimensions a high load-bearing capacity, it is known to introduce layers of circumferential reinforcing elements, the presence of which is described, for example, in WO 99 / 24269.

[0023] Furthermore, WO 99 / 00260 describes a tire for heavy vehicles in which a protective reinforcement consisting of a layer of circumferential reinforcing elements is arranged radially outside the working reinforcement. [Prior art documents] [Patent documents]

[0024] [Patent Document 1] WO 97 / 15463 [Patent Document 2] International Publication No. 2004 / 076204 [Patent Document 3] French Patent No. 2 222 232 [Patent Document 4] WO 99 / 24269 [Patent Document 5] International Publication No. 99 / 00260 Summary of the Invention [Problem to be solved by the invention]

[0025] The inventors therefore set themselves the task of providing a tyre for vehicles of the highway transport type which exhibits satisfactory performance in terms of wear without the risks associated with the proximity of electrical conductors. [Means for solving the problem]

[0026] According to the invention, this object is achieved with a tire for vehicles of high-speed transport type, comprising a radial carcass reinforcement, the tire comprising a crown reinforcement comprising two working crown layers with metal reinforcing elements crossing from one layer to the next and making an angle between 10° and 45° to the circumferential direction, the angle being oriented on both sides in the circumferential direction, the crown reinforcement being radially covered by a tread having at least one circumferential cut, the tread being joined to the two beads via two sidewalls, the crown reinforcement being radially slightly outwardly of the two working layers, the crown reinforcement has at least one layer of metallic circumferential reinforcing elements, the two working crown layers and the circumferential reinforcing elements of the at least one layer being the only layers present to constitute the crown reinforcement across the entire axial width of the crown reinforcement, and in the meridian plane, the distance D between the radially innermost point of the circumferential cut axially closest to the equatorial plane and a point on the outer surface of the circumferential reinforcing element of the at least one layer is less than 8 mm, the distance D being measured along the direction of an orthogonal projection of the radially innermost point of the circumferential cut axially closest to the equatorial plane onto the radially outer surface of the circumferential reinforcing element of the at least one layer.

[0027] Within the meaning of the present invention, the radially outer surface of a layer of circumferential reinforcing elements is defined by a surface extrapolated from the radially outermost points of the reinforcing elements that make up the layer of circumferential reinforcing elements.

[0028] According to the invention, the crown reinforcement may comprise one or more layers of metallic circumferential reinforcing elements. In case of more than one layer of metallic circumferential reinforcing elements, the two working crown layers and the layers of circumferential reinforcing elements are the only layers present to form the crown reinforcement over the entire axial width of the crown reinforcement.

[0029] Distance D is measured on a cross section of the tire, therefore with the tire in an uninflated state. In the case of several layers of metallic circumferential reinforcing elements radially outside the two working layers, distance D is measured between the radially innermost point of the cut axially closest to the equatorial plane and a point on the outer surface of the radially outermost layer of circumferential reinforcing elements.

[0030] The angles, expressed in degrees, are measured on the cross section of the tire. According to the invention, the angles are measured in the circumferential median plane.

[0031] Advantageously, according to the invention the tyre is intended to be inflated to an inflation pressure of 11.5 bar or greater.

[0032] In accordance with a preferred embodiment of the present invention, the discontinuity axially closest to the equatorial plane is a circumferential groove.

[0033] Advantageously, according to the invention, the distance D is less than 5 mm.

[0034] Also advantageously, the reinforcing elements of the working crown layer form an angle with the circumferential direction of less than 30 degrees, preferably less than 25 degrees.

[0035] The results obtained with the tyre according to the invention effectively show that the performance is improved in terms of durability, so that the tyre does not present a risk with regard to electrical conductors present in the environment, and furthermore that its performance is satisfactory in terms of wear.

[0036] In particular, the inventors have been able to show that the combination of two working layers with metallic reinforcing elements associated with metallic circumferential reinforcing elements of at least one layer arranged radially outside the two working layers makes it possible to guarantee the durability of the crown reinforcement despite the shear stresses imposed on the working layers, and thus the tire according to the invention does not run the risk of generating metal chips in the vicinity of the electrical conductors.

[0037] Furthermore, the presence of circumferential reinforcing elements in at least one layer located at the radially outermost position of the crown reinforcement makes it possible to contain the entire tire, thus avoiding the generation of metal fragments in the event of separation of the working layer subjected to shear stresses.

[0038] Furthermore, the combination of the position of the circumferential reinforcing elements of at least one layer and the distance D, which corresponds to the thickness of the rubber material in the region radially inside the bottom of the tread pattern void, which is significantly reduced compared to conventional tire designs, has a favorable effect on the performance in terms of tire wear. In particular, the inventors have been able to show that the resulting proximity of the layers of circumferential reinforcing elements to the radially outer surface of the tire's tread in contact with the ground makes it possible to limit the shear experienced by the compounds constituting the tread when running and to make the forces on the tread more uniform, thereby contributing to a better performance in terms of wear, regardless of the loading conditions or loading variations of the high-speed transport vehicle.

[0039] According to one embodiment of the invention, the reinforcing elements of the working crown layer are inextensible metal cords.

[0040] According to an advantageous embodiment variant of the invention, the circumferential reinforcing elements of at least one layer have an axial width greater than 0.5xL.

[0041] L is the maximum axial width of the tire when mounted on its service rim and inflated to its recommended pressure.

[0042] The axial width of the layer of reinforcing elements is measured on a cross-section of the tire, the tire therefore being in an uninflated state.

[0043] According to a preferred embodiment of the invention, the two working crown layers have different axial widths, and the difference between the axial width of the axially widest working crown layer and the axial width of the axially narrowest working crown layer is between 10 and 30 mm.

[0044] According to an advantageous embodiment of the invention, the reinforcing elements of the circumferential reinforcing elements of at least one layer are metallic reinforcing elements having a secant modulus of elasticity at 0.7% elongation between 10 and 120 GPa and a maximum tangent modulus of elasticity of less than 150 GPa.

[0045] According to a preferred embodiment, the reinforcing elements have a secant modulus at 0.7% elongation of less than 100 GPa and greater than or equal to 20 GPa, preferably between 30 and 90 GPa, more preferably less than 80 GPa.

[0046] Also preferably, the maximum tangent modulus of elasticity of the reinforcing elements is less than 130 GPa, more preferably less than 120 GPa.

[0047] The above mentioned modulus is measured on a curve of tensile stress as a function of elongation determined at a preload of 20 MPa, which tensile stress corresponds to the measured tension corrected for the cross section of the metal of the reinforcing element. The measurement is carried out on a cord taken from the tire over a portion of the layer of the circumferential reinforcing element extending from the axial end of the layer towards the inside of the layer over an axial width of 50 mm.

[0048] The modulus for the same reinforcing element can be measured on a curve of tensile stress as a function of elongation determined at a preload of 10 MPa, the tensile stress corresponding to the measured tension corrected for the total cross section of the reinforcing element, the total cross section being the cross section of the composite element made up of metal and rubber, the latter being in a state in which it penetrates, in particular during the curing phase of the tire, this total cross section of the reinforcing element in question being approximately twice the cross section of the metal of the reinforcing element.

[0049] According to this formula for the entire cross section of the reinforcing element, the reinforcing elements of the axially outer and central parts of the circumferential reinforcing elements of at least one layer are metallic reinforcing elements having a secant modulus of elasticity at 0.7% elongation between 5 and 60 GPa and a maximum tangent modulus of elasticity of less than 75 GPa.

[0050] According to a preferred embodiment, the secant modulus of elasticity of the reinforcing elements at 0.7% elongation is less than 50 GPa and greater than 10 GPa, preferably between 15 and 45 GPa, more preferably less than 40 GPa.

[0051] Also preferably, the maximum tangent modulus of elasticity of the reinforcing elements is less than 65 GPa, more preferably less than 60 GPa.

[0052] According to a preferred embodiment, the reinforcing elements of the circumferential reinforcing elements of at least one layer are metallic reinforcing elements, which have a curve of tensile stress as a function of elongation which exhibits a shallow slope for small elongations and a substantially constant and steep slope for large elongations.

[0053] The various above-mentioned properties of the reinforcing element are measured on the reinforcing element removed from the tire.

[0054] A reinforcing element more particularly suitable for the production of at least one layer of circumferential reinforcing elements according to the invention is, for example, an assembly of formula 21.23, the structure of which is 3x(0.26+6x0.23) 4.8 / 7. 5 SS, the stranded cord being made up of 21 elementary threads of formula 3x(1+6) twisted together from 3 elementary threads, each strand being made up of 7 threads, one thread of diameter equal to 26 / 100 mm forming the central core, and 6 winding threads of diameter equal to 23 / 100 mm. Such a cord has a secant modulus of elasticity at 0.7% equal to 45 GPa and a maximum tangent modulus of elasticity equal to 98 GPa, measured on a curve of tensile stress as a function of elongation determined with a preload of 20 MPa corrected for the cross section of the metal of the reinforcing element, the tensile stress corresponding to the measured tension corrected for the cross section of the metal of the reinforcing element. This code of formula 21.23 has, on the curve of tensile stress corresponding to the measured tension corrected to the entire cross section of the reinforcing element as a function of the elongation determined by a preload of 10 MPa corrected to the entire cross section of the reinforcing element, a secant modulus of elasticity at 0.7% equal to 23 GPa and a maximum tangent modulus of elasticity equal to 49 GPa.

[0055] Similarly, another example of a reinforcing element is the assembly of formula 21.28, the structure of which has a composition of 3x(0.32+6x0.28) 5.6 / 9.3 SS. This code has a secant modulus of elasticity of 0.7% equal to 56 GPa and a maximum tangent modulus of elasticity equal to 102 GPa, measured on a curve of tensile stress corresponding to the measured tension corrected to the cross section of the metal of the reinforcing element as a function of the elongation determined with a preload of 20 MPa corrected to the cross section of the metal of the reinforcing element. This code of formula 21.28 has a secant modulus of elasticity of 0.7% equal to 27 GPa and a maximum tangent modulus of elasticity equal to 49 GPa on a curve of tensile stress corresponding to the measured tension corrected to the cross section of the metal of the reinforcing element as a function of the elongation determined with a preload of 10 MPa corrected to the full cross section of the reinforcing element.

[0056] The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible to maintain a satisfactory layer stiffness, especially after the molding and curing steps of conventional manufacturing methods.

[0057] According to a second embodiment of the invention, the circumferential reinforcing elements can be formed of metal elements that are inextensible and cut to form sections of length very smaller than the perimeter of the shortest layer, but preferably greater than 0.1 times the perimeter, the cuts between each section being axially offset from each other. In this case too, preferably the tensile modulus per unit width of the additional layer is smaller than the tensile modulus of the most extensible working crown layer, measured under the same conditions. Such an embodiment allows in a simple way to impart to the layer of circumferential reinforcing elements a modulus of elasticity that can be easily adjusted (by the choice of the spacing between the sections of one and the same row), but which is in all cases smaller than the modulus of elasticity of a layer of the same metal elements but in which the latter is continuous, the modulus of elasticity of the additional layer being measured on a vulcanized layer of cut elements removed from the tire.

[0058] According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metal elements, the ratio of the wave amplitude to the wavelength a / λ being at most equal to 0.09. Preferably, the tensile modulus per unit width of the additional layer is smaller than the tensile modulus of the most extensible working crown layer, measured under the same conditions.

[0059] Further details and advantageous features of the invention will become apparent from the description of exemplary embodiments of the invention, in particular with reference to the figures showing meridian diagrams of tire designs according to the invention. [Brief description of the drawings]

[0060] [Figure 1] FIG. 2 is a meridian diagram of a tire design according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0061] The drawings are not drawn to scale for ease of understanding.

[0062] In the figure, the tire 1 has a size of 305 / 70 R 22.5. The tire 1 comprises a radial carcass reinforcement 2 fixed to two beads, not shown. The carcass reinforcement is made of a single layer of metal cords. This carcass reinforcement 2 is surmounted by a crown reinforcement 5 formed radially from the inside to the outside, its first working layer 51 is made of uncoated 11.35 inelastic metal cords, continuous over the entire width of the layer and oriented at an angle equal to 18°; its second working layer 52 is formed of uncoated 11.35 inelastic metal cords, continuous over the entire width of the layer, oriented at an angle equal to 18° and intersecting the metal cords of layer 51; Its circumferential reinforcing element layer 53 consists of 21x28 steel metal cords.

[0063] The crown reinforcement is itself covered by the tread 6 .

[0064] A carcass reinforcement 2 formed by a single layer of metal cords is wound around a bead wire 4 in each of the beads 3 and forms a turn-up 7 in each of the beads 3 .

[0065] The tire is inflated to a pressure of 11.5 bar.

[0066] Axial width L of the first working layer 51 51 is equal to 200 mm.

[0067] Axial width L of the second working layer 52 52 is equal to 180 mm.

[0068] Axial width L of layer 53 of circumferential reinforcing elements 53 is equal to 136 mm.

[0069] The axial width L6 of the tread is equal to 221 mm.

[0070] Distance D is measured between the radially innermost point 8 of the break 10 that is axially closest to the equatorial plane XX' and point 9 on the outer surface of the layer 53 of the circumferential reinforcing element. Distance D is measured along the orthogonal projection of point 8 onto the outer peripheral surface of layer 53 of the circumferential reinforcing element. Distance D is equal to 2.1 mm and is therefore less than 8 mm.

[0071] The tests were carried out on a tire I manufactured according to the invention and on a reference tire T.

[0072] The reference tire T differs from the tire according to the invention in that it has a crown reinforcement consisting of a stack of three-layered circumferential reinforcing elements made of aromatic polyamide.

[0073] The durability tests were carried out by running the tyres, inflated to a pressure of 13.5 bar and loaded with 7000 daN, on a roller test road at a speed of 40 km / h. The same type of tests were carried out with various pressure and load cycles, also at 40 km / h.

[0074] During the tests, the tire I according to the invention showed better results than the reference tire T. The tire I according to the invention covered nearly twice the distance achieved by the reference tire T.

[0075] Another type of test was performed to test different tires from a wear standpoint.

[0076] These tests were carried out on flat ground and involved measuring the geometry and stresses of the contact area. These tests, complemented by direct comparisons on vehicles, made it possible to evaluate the performance of tyres in terms of their lifespan.

[0077] For the reference tire T, irregular wear occurs and the tread edges are highly worn, which may be the result of uneven flattening between the center and shoulder.

[0078] In the tire I according to the invention, the deformation of the contact surface during flattening is more uniform, which ensures regular wear. This tire can therefore travel longer distances without exhibiting excessively pronounced irregular wear that would require tire replacement. The tire I according to the invention can therefore travel 40% longer distances than the distance achieved by the reference tire T.

Claims

1. A tire (1) for vehicles of urban type, comprising a radial carcass reinforcement (2), said tire comprising a crown reinforcement (5) including two working crown layers (51, 52) with metal reinforcing elements crossing from one layer to the next and making an angle between 10° and 45° with respect to the circumferential direction, said angle being oriented on both sides in the circumferential direction, said crown reinforcement (5) being radially covered by a tread (6) having at least one circumferential cut (10), said tread being joined to two beads (3) via two sidewalls, said crown reinforcement comprises at least one layer of metallic circumferential reinforcing elements (53) radially outside the two working layers; the two working crown layers and the circumferential reinforcing elements of the at least one layer are the only layers present to constitute the crown reinforcement over the entire axial width of the crown reinforcement, a distance D, in a meridian plane, between the radially innermost point of the circumferential cut axially closest to the equatorial plane and a point on the outer surface of the circumferential reinforcing element of said at least one layer is less than 8 mm, said distance D being measured along the direction of an orthogonal projection (9) of said radially innermost point (8) of said circumferential cut (10) axially closest to said equatorial plane (XX') onto said radially outer surface of said circumferential reinforcing element (53) of said at least one layer; A tire (1).

2. The cut (10) axially closest to the equatorial plane is a circumferential groove. A tire (1) according to claim 1.

3. The distance D is less than 5 mm. A tire (1) according to claim 1 or 2.

4. the reinforcing elements of the working crown layers (51, 52) are inextensible; A tire (1) according to any one of claims 1 to 3.

5. the reinforcing elements of the circumferential reinforcing elements (53) of said at least one layer are metallic reinforcing elements having a secant modulus of elasticity at 0.7% elongation between 10 and 120 GPa and a maximum tangent modulus of elasticity of less than 150 GPa; A tire (1) according to any one of the preceding claims.

6. The angle that the reinforcing elements of the working crown layers (51, 52) make with the circumferential direction is less than 30 degrees. Tyre (1) according to claim 5.

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