WO2018172691A1

WO2018172691A1 - Tyre crown reinforcement consisting of a working crown layer and a layer of circumferential elements

Info

Publication number: WO2018172691A1
Application number: PCT/FR2018/050673
Authority: WO
Inventors: Luc Bestgen; Orel FOURNIER; Patrick DAYET
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2017-03-22
Filing date: 2018-03-20
Publication date: 2018-09-27
Also published as: FR3064213B1; FR3064213A1

Abstract

The invention relates to a tyre (1), of which the reinforcing elements of the carcass reinforcement (2) are oriented in a radial direction in the sidewalls of the tyre, comprising a crown reinforcement (4) consisting of a working crown layer (42) of reinforcing elements forming, together with the circumferential direction, an angle (α2) of between 25° and 55° and a layer of circumferential reinforcing elements (41). According to the invention, in the crown region, the reinforcing elements of said carcass reinforcement layer (2) form, together with the circumferential direction, an angle (α1) such that the average angle a defined by the formula arctan((tan (|α1|)*tan (|α2|))1/2) is between 30° and 67°, and the utilisation ratio of the breaking potential F2/FR2 of the working layer is less than 1/10.

Description

REINFORCED PNEUMATIC TOP REINFORCEMENT

OF A WORK SUMMIT LAYER AND A

LAYER OF CIRCUMFERENTIAL ELEMENTS

The present invention relates to a tire, radial carcass reinforcement and more particularly a tire for fitting vehicles carrying heavy loads, such as, for example trucks, tractors, trailers or road buses.

[0002] In general, in heavy-vehicle tires, the carcass reinforcement is anchored on both sides in the bead zone and is radially surmounted by a crown reinforcement consisting of at least two layers, superimposed and formed of son or parallel cables in each layer and crossed from one layer to the next in making with the circumferential direction angles between 10 ° and 45 °. Said working layers, forming the working armature, can still be covered with at least one so-called protective layer and formed of advantageously metallic and extensible reinforcing elements, called elastic elements. It may also comprise a layer of metal wires or cables forming with the circumferential direction an angle of between 45 ° and 90 °, this so-called triangulation ply being radially located between the carcass reinforcement and the first crown ply of work, formed of parallel wires or cables having angles at most equal to 45 ° in absolute value. The triangulation ply forms with at least said working ply a triangulated reinforcement, which presents, under the different stresses it undergoes, few deformations, the triangulation ply having the essential role of taking up the transverse compression forces of which the object all the reinforcing elements in the area of the crown of the tire.

Cables are said to be inextensible when said cables have under a tensile force equal to 10% of the breaking force a relative elongation at most equal to 0.2%.

Cables are said elastic when said cables have under tensile force equal to the breaking load a relative elongation of at least 3% with a maximum tangent modulus of less than 150 GPa. [0005] Circumferential reinforcing elements are reinforcing elements which make angles with the circumferential direction in the range + 2.5 °, -2.5 ° around 0 °.

The circumferential direction of the tire, or longitudinal direction, is the direction corresponding to the periphery of the tire and defined by the rolling direction of the tire.

[0007] The transverse or axial direction of the tire is parallel to the axis of rotation of the tire.

The radial direction is a direction intersecting the axis of rotation of the tire and perpendicular thereto.

The axis of rotation of the tire is the axis around which it rotates in normal use.

A radial or meridian plane is a plane which contains the axis of rotation of the tire. The circumferential mid-plane, or equatorial plane, is a plane perpendicular to the axis of rotation of the tire and which divides the tire into two halves.

As regards the metal wires or cables, the measurements of force at break (maximum load in N), tensile strength (in MPa), elongation at break (total elongation in%) and of module (in GPa) are made in traction according to the ISO 6892 standard of 1984.

Some current tires, called "road", are intended to run at high average speeds and on longer and longer journeys, because of the improvement of the road network and the growth of the motorway network in the world. . The set of conditions under which such a tire is called to roll, undoubtedly allows an increase in the number of kilometers traveled, the wear of the tire being less. This increase in service life in terms of kilometers, combined with the fact that such conditions of use are likely to result, under heavy load, relatively high top temperatures, requires an at least proportional increase in the endurance potential of the crown reinforcement of the tires.

There are indeed constraints at the crown reinforcement and more particularly shear stresses between the crown layers which, in the case of too high a rise in the operating temperature at the ends of the crown. the axially shorter crown layer, result in the appearance and propagation of cracks in the rubber at said ends. The same problem exists in the case of edges of two layers of reinforcement elements, said other layer not necessarily being radially adjacent to the first. In order to improve the endurance of the tire crown reinforcement, the French application FR 2 728 510 proposes to have, on the one hand, between the carcass reinforcement and the reinforcing steel working ply. radially closest to the axis of rotation, an axially continuous sheet formed of non-extensible metal cables forming an angle of at least 60 ° with the circumferential direction, and whose axial width is at least equal to the axial width the shortest working crown ply, and secondly between the two working crown plies an additional ply formed of metal elements, oriented substantially parallel to the circumferential direction.

In addition, the French application WO 99/24269 proposes, on either side of the equatorial plane and in the immediate axial extension of the additional ply of reinforcing elements substantially parallel to the circumferential direction, to couple, over a certain axial distance, the two working crown plies formed of reinforcing elements crossed from one ply to the next ply and then decoupling them by rubber mixing profiles at least over the remainder of the width common to the two tablecloths. Furthermore, the use of tires on vehicles for trucks "type site" leads the tires to suffer shocks when driving on stony ground. These shocks are of course harmful in terms of endurance performance. It is still known to those skilled in the art to increase the number of webs constituting the crown reinforcement to improve the endurance of the tire with respect to such shocks.

Whatever one of these solutions as presented above, the presence of one or more layers of additional reinforcing elements leads to a larger mass of the tire and higher tire manufacturing costs.

An object of the invention is to provide tires for vehicles "Heavy-Duty", for example of the type "building site", whose endurance performance is preserved or improved especially in view of shocks suffered on the tape and whose overall mass is reduced.

This object is achieved according to the invention by a vehicle tire of heavy weight type, carcass reinforcement consisting of a layer of metal reinforcing elements oriented in a radial direction in the sidewalls of the tire, comprising a frame apex comprising a working crown layer of metal reinforcing elements forming with the circumferential direction an angle (a2) of between 25 ° and 55 °, said working crown layer being centered on the circumferential mid-plane, the reinforcement the crown being radially capped with a tread, said tread being joined to two beads by means of the two sidewalls, the crown reinforcement being completed by at least one layer of circumferential metal reinforcing elements, and on a portion centered on the circumferential median plane of axial width less than the width of said working crown layer, the reinforcing elements of said layer of the carcass reinforcement making with the circumferential direction an angle (a1) such that the average angle (a) defined by the Arctan relation ((tan (| al |) * tan (| a2 |)) ^1/2 ) is between 30 and 67 °, said angles a1 and a2 being oriented on either side of the circumferential direction, the utilization ratio of the fracture potential F 2 / FR 2 of the working layer being less than 1/10, wherein:

FR2 is the breaking force in uniaxial extension of the reinforcing elements of the working layer, expressed in N, F2 = 0.1 * P * Qs * p ₂ / [(sin (| a2 |) + tan (| al |) * cos (| a2 |)) * sin (| a2 |)], with P: inflation pressure, expressed in bar,

Qs = (Rs ² -RL ² ) / (2 * Rs),

p ₂ : the laying pitch of the reinforcing elements of the working crown layer, measured perpendicular to the reinforcing elements at the circumferential mid-plane,

Rs = Re - Es,

Re: outer radius of the tire measured at the radially outermost point on the surface of the tread of the tire, said surface being extrapolated to fill any cavities,

Es: radial distance between the radially outermost point of the tire and its orthogonal projection on the radially outer face of a reinforcing element of the working crown layer,

RL: average of the radii of the axially outermost points on each side of the tire, that is to say the average of the radii, measured in the radial direction, of the axially outermost points on each of the tire sidewalls.

The thickness Es and the pitch p ₂ are measured on a section of the tire and are expressed in millimeters.

The angles α1 and α2, expressed in degrees, are also measured on a section of the tire. The angle measurements are according to the invention made at the circumferential mid-plane.

The measurements of the Re and RL spokes are performed on a tire mounted on its nominal rim and inflated to the nominal pressure.

Advantageously according to the invention, the working crown layer is radially external to said at least one layer of circumferential reinforcing elements.

The results obtained with tires according to the invention have indeed demonstrated that the performance in terms of endurance can be improved especially when driving on stony ground, the crown reinforcement of the tire being lightened. The lightening of the crown of the tire is accompanied by a simplification of manufacturing and a reduction in manufacturing costs.

Against all expectations, the results have indeed demonstrated that the tires according to the invention can be reduced by reducing the number of constituent layers of the crown reinforcement while maintaining or even improving the endurance properties of the crown of the tire. in particular against shocks appearing on the tread for example when driving on stony ground. It is known to those skilled in the art that in order to improve the endurance performance of the crown reinforcement of a tire with respect to this type of impact, it is customary to increase the number of layers of reinforcing elements.

The inventors think to interpret these results because of the angle formed with the circumferential direction by the reinforcing elements of the carcass reinforcement layer in the area of the crown of the tire, the latter functionally substituting elements a working top layer. It seems according to the inventors that it is thus possible for the carcass reinforcement layer to act as a working layer whose reinforcing elements would be crossed with the elements of the working crown layer in the zone of the crown. of the tire as provided by the tire according to the invention.

The inventors still find that it is possible to do without a protective layer. Such a layer is usually present to be sacrificed in the event of an attack of the tire-type cuts that can alter the integrity of metal reinforcing elements by corrosion phenomena associated with the fatigue of said reinforcing elements. The inventors actually observe that the reinforcing elements of the working crown layer, which is in a radially outermost position of a tire according to the invention, are less stressed when the tire is inflated or when it is use in normal rolling that the reinforcing elements of a radially outermost working crown layer of a tire more usual on the one hand because of the presence of the layer of circumferential reinforcing elements and secondly because of the relatively low utilization ratio of the F2 / FR2 rupture potential in comparison with a conventional tire. The elements reinforcing the working crown layer of a tire according to the invention thus have endurance properties much greater than those of a more usual tire; the inventors thus realize that the removal of the protective layer is made possible and contributes to the lightening of the tire.

In order to obtain a carcass reinforcement layer whose reinforcing elements form an angle with the circumferential direction between 80 and 90 ° in the sidewalls and whose reinforcing elements form an angle 1 with the circumferential direction on a part centered on the circumferential median plane, such that the average angle a is between 30 and 67 °, the inventors propose, for example, the use of a finishing drum having a membrane reinforced by reinforcing elements. In order to obtain the tire according to the invention, the reinforcing elements are only present on a portion of the membrane centered on a plane coming to merge with the equatorial plane position of the tire being made.

The presence of these reinforcing elements thus allows the reorientation of the reinforcing elements of the carcass reinforcement layer in the area of the crown of the tire. Indeed, the inventors have demonstrated that during the manufacture of the tire, the reinforcing elements of the carcass reinforcement layer which are initially oriented to form an angle of between 80 ° and 90 ° with the circumferential direction will have their advanced orientation due to the presence of reinforcing elements reinforcing the membrane of the finishing drum. After the laying of the carcass reinforcement layer and before the setting of the crown reinforcement, the tire blank thus formed is shaped to pass from a cylindrical structure to a toric structure. The inventors have shown that the presence of reinforcing elements allows the local reorientation of the reinforcing elements of the carcass reinforcement layer during this conformation phase.

According to a preferred embodiment of the invention, the reinforcing elements of the layer of reinforcing elements are distributed with not between 1.4 and 3.0 mm. Advantageously according to the invention, the utilization ratio of the fracture potential F2 / FR2 of the radially outermost working layer is less than 1/15. Such a ratio of utilization of the breaking potential F2 / FR2 further contributes to improving the endurance performance of the reinforcing elements of the radially outermost working layer during the use of the tire.

According to a preferred embodiment of the invention, the average angle α formed by the reinforcing elements of the carcass reinforcement layer is greater than 75 ° on either side of the circumferential mid-plane on two sides. parts axially inside the ends of the working layer and each of said two parts having an axial width of between 10 mm and 35 mm. More preferably, the average angle α1 is greater than 82 ° on these two parts. The usual inflation pressures of the truck tires impose an orientation in the radial direction to the reinforcing elements of the carcass reinforcement layer at the level of the tire shoulders and especially since the working layer is no longer radially superposed on the tire. the carcass reinforcement layer. It is therefore preferable to provide these two parts, which are assimilated to transition zones between the portion of the carcass reinforcement layer centered on the equatorial plane and the parts of the carcass reinforcement layer present in the flanks. of the tire.

After shaping, when the working layer is put in place, the reinforcing elements are thus advantageously present on the membrane of the finishing drum over an axial width narrower than the axial width of the working layer, a axial width between 14 and 80 mm. Advantageously, the reinforcing elements are present on a narrower width than the layer of circumferential reinforcing elements. According to an advantageous embodiment of the invention, said at least one layer of circumferential reinforcing elements has an axial width greater than 0.5xL.

L is the maximum axial width of the tire, when the latter is mounted on its service rim and inflated to its recommended pressure. The axial widths of the reinforcing element layers are measured on a cross section of a tire, the tire is therefore in a non-inflated state.

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

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

Also preferably, the maximum tangent modulus of the reinforcing elements is less than 130 GPa and more preferably less than 120 GPa.

The modules expressed above are measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa reduced to the metal section of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element.

The modules of the same reinforcing elements can be measured on a tensile stress curve as a function of the elongation determined with a prestress of 10 MPa reduced to the overall section of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the overall section of the reinforcing element. The overall section of the reinforcing element is the section of a composite element made of metal and rubber, the latter having in particular penetrated the reinforcing element during the baking phase of the tire.

According to this formulation relating to the overall section of the reinforcing element, the reinforcing elements of the axially outer portions and the central portion of at least one layer of circumferential reinforcing elements are metal reinforcing elements having secant modulus at 0.7% elongation between 5 and 60 GPa and a maximum tangent modulus of less than 75 GPa.

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

Also preferably, the maximum tangent modulus of the reinforcing elements is less than 65 GPa and more preferably less than 60 GPa.

According to a preferred embodiment, the reinforcing elements of said at least one layer of circumferential reinforcing elements are metal reinforcing elements having a tensile stress curve as a function of the relative elongation having slight slopes for the low elongations and a substantially constant and strong slope for the higher elongations. Such reinforcing elements of the additional ply are usually referred to as "bi-module" elements.

According to a preferred embodiment of the invention, the substantially constant and strong slope appears from a relative elongation of between 0.1% and 0.5%.

The various characteristics of the reinforcement elements mentioned above are measured on reinforcing elements taken from tires.

Reinforcement elements more particularly adapted to the production of at least one layer of circumferential reinforcing elements according to the invention are, for example, assemblies of formula 21.23, the construction of which is 3x (0.26 + 6x0.23). 4.4 / 6.6 SS; this strand cable consists of 21 elementary wires of formula 3 x (1 + 6), with 3 twisted strands each consisting of 7 wires, a wire forming a central core of diameter equal to 26/100 mm and 6 coiled wires of diameter equal to 23/100 mm. Such a cable has a secant module at 0.7% equal to 45 GPa and a maximum tangent modulus equal to 98 GPa, measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa brought back to the section. of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element. On a tensile stress curve in according to the elongation determined with a preload of 10 MPa brought back to the overall section of the reinforcement element, the tensile stress corresponding to a measured tension brought back to the overall section of the reinforcing element, this cable of formula 21.23 has a secant modulus at 0.7% equal to 23 GPa and a maximum tangent modulus equal to 49 GPa.

In the same way, another example of reinforcement elements is an assembly of formula 21.28, whose construction is 3x (0.32 + 6x0.28) 6.2 / 9.3 SS. This cable has a secant module at 0.7% equal to 56 GPa and a maximum tangent modulus equal to 102 GPa, measured on a tensile stress curve as a function of the elongation determined with a preload of 20 MPa brought to the cross section. metal of the reinforcing element, the tensile stress corresponding to a measured voltage brought back to the metal section of the reinforcing element. On a tensile stress curve as a function of the elongation determined with a preload of 10 MPa brought back to the overall section of the reinforcing element, the tensile stress corresponding to a measured tension brought back to the overall section of the element of reinforcement, this cable of formula 21.28 has a secant module at 0.7% equal to 27 GPa and a maximum tangent modulus equal to 49 GPa.

The use of such reinforcing elements in at least one layer of circumferential reinforcing elements makes it possible in particular to maintain rigidities of the satisfactory layer including after the shaping and baking steps in conventional manufacturing processes.

According to a second embodiment of the invention, the circumferential reinforcing elements may be formed of inextensible metal elements and cut so as to form sections of length much shorter than the circumference of the least long layer, but preferably greater than 0.1 times said circumference, the cuts between sections being axially offset with respect to each other. More preferably, the tensile modulus of elasticity per unit width of the additional layer is less than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer. Such an embodiment makes it possible to confer, in a simple manner, on the layer of reinforcing elements circumferential a module that can easily be adjusted (by the choice of intervals between sections of the same row), but in all cases lower than the module of the layer consisting of the same metallic elements but continuous, the module of the additional layer being measured on a vulcanized layer of cut elements, taken from the tire.

According to a third embodiment of the invention, the circumferential reinforcing elements are corrugated metal elements, the ratio a / λ of the amplitude of waviness over the wavelength being at most equal to 0, 09. Preferably, the tensile modulus of elasticity per unit width of the additional layer is smaller than the tensile modulus of elasticity, measured under the same conditions, of the most extensible working crown layer.

According to one embodiment of the invention, the reinforcing elements of the working crown layers are inextensible metal cables.

According to an advantageous embodiment of the invention, the working crown layer and said layer of circumferential reinforcing elements are the only layers present to constitute the crown reinforcement over at least 80% of the axial width. of the crown frame. This advantageous embodiment of the invention goes in the direction of an even greater relief of the tire.

According to other embodiments of the invention offsetting the compromise of performance of the tire in a less favorable manner with regard to lightening, the crown reinforcement comprises an additional layer, called protective, radially external to the working crown layers, preferably centered on the circumferential mid-plane. The reinforcing elements of such a protective layer are preferably so-called elastic reinforcing elements, oriented with respect to the circumferential direction with an angle of between 25 ° and 55 ° and in the same direction as the angle formed by the elements. reinforcement of the working layer. More preferably, the reinforcing elements of such a protective layer are parallel to the reinforcing elements of the working layer. Other variants may also provide that the crown reinforcement can be completed between the carcass reinforcement and the radially inner working layer closest to said carcass reinforcement, by a triangulation layer of elements of the carcass reinforcement. Inextensible steel metal reinforcement making, with the circumferential direction, an angle greater than 45 ° and in the same direction as that of the angle formed by the elements of the carcass reinforcement at the equatorial plane. According to a preferred embodiment according to these embodiments, at the equatorial plane, the angle formed by the reinforcing elements of the triangulation layer with the circumferential direction the angle is greater than that formed by the reinforcing elements of the the carcass reinforcement layer with the circumferential direction. Advantageously, said triangulation layer consists of two half-layers positioned axially on either side of the circumferential mid-plane.

The tire according to the invention may also comprise one or more layers of additional circumferential reinforcing elements, advantageously consisting of two half-layers positioned axially on either side of the circumferential mid-plane.

Other details and advantageous features of the invention will emerge below from the description of an exemplary embodiment of the invention with reference to the figure which shows a meridian view of a diagram of a tire according to an embodiment of the invention.

The figure is not shown in scale to simplify understanding. The figure represents only a half-view of a tire which extends symmetrically with respect to the axis XX 'which represents the circumferential median plane, or equatorial plane, of a tire. In the figure, the tire 1, of size 12 R 22.5 has a form ratio H / L equal to 0.90, H being the height of the tire 1 on its mounting rim and L its maximum axial width. Said tire 1 comprises a radial carcass reinforcement 2 anchored in two beads, not shown in the figure. The carcass reinforcement 2 is formed of a single layer of metal cables. They still include a tread 5. In the figure, the carcass reinforcement 2 is shrunk according to the invention by a crown reinforcement 4, formed radially from the inside to the outside: of a layer of circumferential reinforcing elements 41 formed 21x28 steel wire ropes, of "bi-module" type, a working layer 42 formed of metal cables oriented at an angle equal to 45 °.

The metal cables constituting the reinforcing elements of the layer of circumferential reinforcing elements are spaced from each other by 2 mm, depending on the normal direction of the average line of the cables.

The metal cables constituting the reinforcing elements of the working layer are cables of formula 9.35. They are distributed in the working layer with a distance between the reinforcing elements, measured according to the normal to the direction of the average line of the cable equal to 2 mm and form an angle a2 with the circumferential direction equal to 45 °.

The tire is inflated to a pressure of 9 bar.

The axial width L of the layer of circumferential reinforcing elements 41 is equal to 158 mm.

The axial width L ₄₂ of the working layer 42 is equal to 202 mm.

The axial width of the tread L ₅ is equal to 215 mm. The axial width L is equal to 302 mm.

The cumulative mass of the working layer 42 and the layer of circumferential reinforcing elements 41, including the mass of the cables and calendering mixtures, thus amounts to 6.5 Kg.

The angle α1 formed by the reinforcing elements of the carcass reinforcement layer with the circumferential direction at the equatorial plane is equal to 70 °. The reinforcing elements of the carcass reinforcement layer 2 form this angle α1 on a width D, substantially equal to 138 mm, defined by the axial width on which are present the reinforcing elements on the finishing drum, after shaping.

The average angle a is equal to 58.9 ° and is well between 30 ° and 67 °.

On a width 1, measured according to the curvilinear abscissa of the carcass reinforcement layer equal to 29 mm, advantageously between 10 and 35 mm, substantially corresponding to the distance between the end of the working layer 42 and the end of the reinforcing elements on the finishing drum, the mean angle α1 formed by the reinforcing elements of the carcass reinforcement layer 2 with the circumferential direction has a value equal to 83 °.

Beyond the end of the working layer 42, the average angle α formed by the reinforcement elements of the carcass reinforcement layer 2 with the circumferential direction has a value greater than 88 °.

The measured value of Re is equal to 541.7 mm.

The measured value of Es is equal to 22.3 mm.

The average value RL of the measured rays is equal to 410 mm.

The calculated value of Qs is equal to 97.9 mm.

The value of F2 is equal to 94 N.

The breaking force of the reinforcing elements of the working crown layer FR2 is equal to 2600 N.

The utilization ratio of the rupture potential F2 / FR2 is equal to 3.6%.

The tire according to the invention is compared to a reference tire of the same size which differs from the tire according to the invention by its crown reinforcement formed radially from the inside to the outside: of a triangulation layer, consisting of two half-plies, formed of non-stranded inextensible metal cables 9.28, oriented at an angle equal to 65 °, a first working layer formed of metal cables oriented at an angle equal to 26 °, a second working layer formed of metal cables oriented at an angle of 18 ° and crossed to the metal cables of the first layer working, the cables of each of the working layers being oriented on either side of the circumferential direction, a protective layer formed of elastic metal cables 6.35.

The metal cables of the two working layers are cables of formula 9.35. They are distributed in each of the working layers with a distance between the reinforcing elements, measured according to the normal to the direction of the average line of the cable equal to 2.5 mm.

The reference tire is inflated to a pressure of 9 bar.

The overall axial width of the triangulation layer is equal to 180 mm, each of the half-plies having a width equal to 60 mm.

The axial width of the first working layer is equal to 220 mm.

The axial width of the second working layer is equal to 200 mm.

The axial width of the protective layer is equal to 136 mm.

The cumulative mass of the working layers of the reference tire, the protective layer and the triangulation layer, comprising the mass of the metal cables and calendering mixtures, amounts to 10.0 Kg.

Tests were carried out with tires made according to the invention in accordance with Figure 1 and with the reference tire.

First endurance tests were performed on a test machine imposing on each tire a straight line running at a speed equal to the maximum speed index prescribed for said tire (speed index) under an initial load of 3550 Kg gradually increased to reduce the duration of the test. Other endurance tests were performed on a test machine imposing cyclically a transverse force and a dynamic overload to the tires. The tests were carried out for the tires according to the invention with conditions identical to those applied to the reference tires.

The tests thus carried out showed that the distances traveled during each of these tests are substantially identical for the tires according to the invention and the reference tires. It therefore appears that the tires according to the invention have substantially equivalent performances in terms of endurance to those of the reference tires when driving on bituminous floors.

Tests to characterize the breaking strength of a tire crown reinforcement subjected to shocks were also carried out. These tests consist in rolling a tire, inflated to a recommended pressure and subjected to a recommended load, on an obstacle or cylindrical indenter of diameter equal to 1.5 inches, that is to say 38.1 mm, and of a determined height. The breaking strength is characterized by the critical height of the indenter, that is to say the maximum height of the indenter resulting in a total rupture of the crown reinforcement, that is to say of the breaking of all vertex layers. The values express the energy necessary to obtain the break of the vertex block. The values are expressed from a base 100 corresponding to the value measured for the reference tire.

These results show that despite lightening of the tire by a reduction in the mass of its crown reinforcement, the energy at rupture during an impact on the surface of the tread is significantly higher.

Claims

A tire for a heavy vehicle, carcass-reinforced vehicle consisting of a layer of metal reinforcing elements oriented in a radial direction in the sidewalls of the tire, comprising a crown reinforcement comprising a working crown layer of metal reinforcing elements forming with the circumferential direction an angle (a2) of between 25 ° and 55 °, said working crown layer being centered on the circumferential mid-plane, the crown reinforcement being radially capped with a tread , said tread being joined to two beads by two sidewalls, characterized in that the crown reinforcement is completed by a layer of metal circumferential reinforcing elements, in that, on a portion centered on the median circumferential plane of axial width less than the width of said working crown layer, the reinforcing elements said layer of the carcass reinforcement makes with the circumferential direction an angle (a1) such that the average angle a defined by the Arctan relation ((tan (| al |) * tan (| a2 |)) ^{1 / 2} ) is between 30 ° and 67 °, in that said angles α1 and α2 are oriented on either side of the circumferential direction, in that the utilization ratio of the fracture potential F 2 / FR 2 of the work is less than 1/10, in which:

FR2 is the breaking force in uniaxial extension of the reinforcing elements of the working layer, expressed in N, F2 = 0.1 * P * Qs * p ₂ / [(sin (| a2 |) + tan (| al |) * cos (| <x2)) * sin (| a2 |)], with

P: inflation pressure, expressed in bar,

Qs = (Rs ² -RL ² ) / (2 * Rs),

p ₂ : the laying pitch of the reinforcing elements of the working crown layer, measured perpendicularly to the reinforcing elements at the circumferential mid-plane, Rs = Re-Es,

Es: radial distance between the radially outermost point of the tire and its orthogonal projection on the radially outer face of a reinforcing element of the working crown layer, RL: average of the radii of the axially outermost points on each side of the tire.

2 - A tire according to claim 1, characterized in that the working crown layer is radially external to the layer of circumferential reinforcing elements.

3 - tire according to one of claims 1 or 2, characterized in that the working crown layer and said layer of circumferential reinforcing elements are the only layers present to form the crown reinforcement on at least 80% of the axial width of the crown reinforcement.

4 - tire according to one of the preceding claims, characterized in that the utilization ratio of the fracture potential F2 / FR2 of the working layer is less than 1/15.

5 - tire according to one of the preceding claims, characterized in that the angle α formed by the reinforcing elements of the carcass reinforcement layer is greater than 75 ° on either side of the circumferential mid-plane on two parts axially inside the ends of the working layer and in that each of said two parts has an axial width of between 10 mm and 35 mm.

6 - tire according to one of the preceding claims, characterized in that the reinforcing elements of the working crown layer are inextensible metal cables.

7 - A tire according to one of the preceding claims, characterized in that the reinforcing elements of said at least one layer of circumferential reinforcing elements are metal reinforcing elements having a secant modulus at 0.7% elongation included between 10 and 120 GPa and a maximum tangent modulus less than 150 GPa.