WO2020115409A2

WO2020115409A2 - Tyre for a civil engineering vehicle, comprising a level-wound crown reinforcement with metal reinforcements

Info

Publication number: WO2020115409A2
Application number: PCT/FR2019/052876
Authority: WO
Inventors: François BARBARIN
Original assignee: Compagnie Generale Des Etablissements Michelin
Priority date: 2018-12-05
Filing date: 2019-12-02
Publication date: 2020-06-11
Also published as: WO2020115409A3

Abstract

The present invention relates to a tyre for a civil engineering vehicle, which comprises an at least partially level-wound crown reinforcement with metal reinforcements and is compatible with a use under high load and on aggressive soils. According to the invention, the reinforcements of the crown layers (211, 212) are metallic and are wound, in the form of a strip (5) of width W, in a zigzag circumferential trajectory according to a periodic curve (7) and extend over a number N of periods P distributed over a number T of circumferences 2ΠR and satisfying the two equations N*(W/sinA) = 2ΠR*t, wherein 0.6 ≦ t ≦ 1 and N*P=2ΠR*T, so as to constitute a bilayer (21). Furthermore, for at least 40% of the axially positioned strip crossovers (53) and with respect to the circumferential direction (XX') at an axial distance L1 being at most equal to 0.25 times the amplitude L of the periodic curve (7), the circular portion of the bilayer (213), which is centered on the strip crossover (53) and has a radius R1 equal to twice the width W of the strip (5), comprises Ne outer strip crossovers and Ni inner strip crossovers, such that |Ne-Ni|/(Ne+Ni)<= 0.3.

Description

Tire for civil engineering vehicle, comprising a crown reinforcement with metal reinforced reinforcements

The present invention relates to a tire intended to equip a civil engineering vehicle and relates more particularly to its crown reinforcement.

Typically a tire for a Civil Engineering vehicle, within the meaning of the European Tire and Rim Technical Organization or ETRTO standard, is intended to be mounted on a rim whose diameter is at least 25 inches and can reach 63 inches.

[0003] A tire having a geometry of revolution with respect to an axis of rotation, the geometry of the tire is generally described in a meridian plane containing the axis of rotation of the tire. For a given meridian plane, the radial, axial and circumferential directions respectively designate the directions perpendicular to the axis of rotation of the tire, parallel to the axis of rotation of the tire and perpendicular to the meridian plane. The circumferential direction is tangent to the circumference of the tire.

In what follows, the expressions “radially interior”, respectively “radially exterior” mean “closer”, respectively “further from the axis of rotation of the tire”. By “axially interior”, respectively “axially exterior”, is meant “closer”, respectively “further from the equatorial plane of the tire”, the equatorial plane of the tire being the plane passing through the middle of the rolling surface and perpendicular to the axis of rotation.

In general, a tire comprises a tread, intended to come into contact with a ground by means of a tread surface, the two axial ends of which are connected by means of two sidewalls with two beads. ensuring the mechanical connection between the tire and the rim on which it is intended to be fitted.

A tire also includes a reinforcing reinforcement, consisting of a crown reinforcement, radially internal to the tread, and a carcass reinforcement, radially internal to the crown reinforcement. The carcass reinforcement of a tire for a civil engineering vehicle usually comprises at least one carcass layer comprising generally metallic reinforcements, coated with a polymeric material of elastomeric or elastomeric type, obtained by mixing and called a mixture of coating. A carcass layer comprises a main part, connecting the two beads together and generally winding, in each bead, from the inside to the outside of the tire around a circumferential reinforcement element, most often metallic, called bead wire, to form a reversal. For a radial tire, the metal reinforcements of a carcass layer are substantially parallel to each other and form, with the circumferential direction, an angle between 85 ° and 95 °.

The crown reinforcement of a tire for a civil engineering vehicle comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each top layer consists of generally metallic reinforcements, parallel to each other and coated with a polymeric material of the elastomer type or elastomeric coating mixture.

With regard to the metallic reinforcements, a metallic reinforcement is characterized mechanically by a curve representing the tensile force (in N), applied to the metallic reinforcement, as a function of its relative elongation (in%), called the force curve. elongation. From this force-elongation curve are deduced mechanical tensile characteristics of the metal reinforcement, such as the structural elongation As (in%), the total elongation at break At (in%), the force at break Fm (load maximum in N) and the breaking strength Rm (in MPa), these characteristics being measured according to ISO 6892 of 1984.

The total elongation at break At of the metal reinforcement is, by definition, the sum of its respectively structural, elastic and plastic elongations (At = As + Ae + Ap). The structural elongation As results from the relative positioning of the metal wires constituting the metal reinforcement under a low tensile force. The elastic elongation Ae results from the intrinsic elasticity of the metal of the metallic wires, constituting the metallic reinforcement, taken individually, the behavior of the metal according to Hooke's law. The plastic elongation Ap results from the plasticity, that is to say from the irreversible deformation, beyond the elastic limit, of the metal of these metallic wires taken individually. These different extensions as well as their respective meanings, well known to those skilled in the art, are described, for example, in documents US5843583, WO2005 / 014925 and WO2007 / 090603.

We also define, at any point of the force-elongation curve of a metal reinforcement, an extension module, expressed in GPa, which represents the slope of the straight line tangent to the force-elongation curve at this point. In particular, the elastic extension module or the Young modulus is called the extension module of the elastic linear part of the force-elongation curve.

Among the metallic reinforcements, a distinction is usually made between elastic metallic reinforcements and non-extensible or inextensible metallic reinforcements. In what follows, the characteristics relating to the metallic reinforcements are determined on metallic reinforcements, coated in a vulcanized elastomeric mixture. An elastic metallic reinforcement is characterized by a structural elongation As at least equal to 0.5%. In addition, an elastic metallic reinforcement has an elastic modulus in extension at most equal to 150 GPa, and usually comprised between 40 GPa and 150 GPa. A non-extensible metallic reinforcement is characterized by a total elongation At, under a tensile force equal to 10% of the breaking force Fm, at most equal to 0.2%. Furthermore, a non-extensible metal reinforcement has an elastic module in extension usually comprised between 150 GPa and 200 GPa.

Regarding the top layers of the top reinforcement, there are usually the protective layers, constituting the protective frame and radially outermost, and the working layers, constituting the frame and radially between the protective armature and the carcass reinforcement.

The protective reinforcement, comprising at least one protective layer, essentially protects the working layers from mechanical or physico-chemical attack, which may propagate through the tread radially towards the inside of the tire.

The protective frame often includes two protective layers, radially superimposed, formed of elastic metal reinforcements, mutually parallel in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at least equal to 10 °.

The working frame, comprising at least two working layers, has the function of encircling the tire and giving it rigidity and road holding. It takes up both mechanical inflation stresses generated by the inflation pressure of the tire and transmitted by the carcass reinforcement, and mechanical rolling stresses generated by the rolling of the tire on a ground and transmitted by the tread. . It must also resist oxidation and impact and puncture, thanks to its intrinsic design and that of the protective frame.

The working frame usually comprises two working layers, radially superimposed, formed of non-extensible metal reinforcements, mutually parallel in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 60 °, and preferably at least equal to 15 ° and at most equal to 45 °.

In addition, to reduce the mechanical inflation stresses transmitted to the working frame, it is known to have, radially outside the carcass frame, a hooping frame, having a rigidity in circumferential extension high. The hooping reinforcement, whose function is to at least partially absorb the mechanical inflation stresses, also improves the endurance of the crown reinforcement by stiffening the crown reinforcement when the tire is flattened under a radial load and, in particular, subject to a drift angle around the radial direction.

The hooping reinforcement usually comprises two hooping layers, radially superimposed, formed of metal reinforcements, mutually parallel in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at more equal to 10 °. The hooping frame can be positioned radially inside the working frame, between two working layers of the working frame, or radially outside the working frame.

Among the hooping layers, there are the hooping layers called closed angles, that is to say whose metal reinforcements form, with the direction circumferential, angles at least equal to 5 ° and at most equal to 10 °, and the circumferential hooping layers, more precisely substantially circumferential, that is to say whose metallic reinforcements form, with the circumferential direction, angles at most equal to 5 ° and which may be harmful. The closed-angle hooping layers include metallic reinforcements having free ends at the axial ends of the hooping layers. The circumferential hooping layers comprise metal reinforcements having no free ends at the axial ends of the hooping layers, because the circumferential hooping layers are most often obtained by the circumferential winding of a sheet of metal reinforcements or by the circumferential winding of a continuous metallic reinforcement.

Consequently, a standard crown reinforcement of a tire for a civil engineering vehicle frequently comprises six crown layers, distributed in two protective layers, two working layers and two hoop layers. The complexity of this crown reinforcement results from the need to satisfy, in particular, a compromise between the capacity to carry heavy loads and the capacity to roll on aggressive grounds. At the same time, there is a need to simplify the architecture of the tire in order to reduce its manufacturing cost.

It is known, in another field of tires, that of aircraft tires, also intended to carry heavy loads, but for taxiing at high pressure and at much higher speed, on non-aggressive soils, use a so-called truncated crown reinforcement, and more specifically a tranched working reinforcement. Documents WO 2015059172, WO 2015063131, WO 2015071152, WO 2015124758 and WO2015150133 thus describe a crown reinforcement, comprising at least one crown bilayer formed at least in part by two radially overlapping crown layers and each constituted by reinforcements coated in a elastomeric material. Each top bilayer is formed by a circumferential zigzag winding of a strip of width W, in a circumferential direction of the tire and on a substantially cylindrical surface of revolution positioned at a radial distance R from the axis of rotation of the tire. The trajectory of the zigzag circumferential winding is a periodic curve having a period P and an amplitude L. The periodic curve forms, with the direction circumferential, an angle A measured at the points of the curve positioned in an equatorial plane passing through the middle of the tread and perpendicular to the axis of rotation. The periodic curve extends over a number N of periods P distributed over a number T of circumferences 2IIR. Such a crown reinforcement has the advantage of limiting the number of individual crown layers and of simplifying manufacture.

However, this cut crown reinforcement, for an airplane tire, is constituted by crown layers comprising textile reinforcements, preferably of the aliphatic polyamide type, such as nylon, or of the aromatic polyamide type, such as Taramide. , or any combination of the above types. However, in the field of Civil Engineering, given the particularly severe conditions of use, the use of textile reinforcements is hardly possible and that of metallic reinforcements is commonly accepted as a preferable solution.

The inventors have therefore set themselves the objective of proposing, for a Civil Engineering vehicle, a tire comprising a crown reinforcement with metal reinforcements at least partially trancannée, compatible with use under high load, at pressure and at speed moderate, and on aggressive soils.

This objective was achieved, according to the invention, by a tire for a civil engineering vehicle comprising:

a crown reinforcement radially internal to a tread and radially external to a carcass reinforcement,

the crown reinforcement comprising at least one crown bilayer constituted at least in part by two radially superposed crown layers and each constituted by reinforcements coated in an elastomeric material,

each bilayer at the top consisting of a circumferential zigzag winding of a strip of width W, in a circumferential direction of the tire and on a substantially cylindrical surface of revolution positioned at a radial distance R from the axis of rotation of the tire,

-the trajectory of the zigzag circumferential winding being a periodic curve having a period P and an amplitude L,

the periodic curve forming, with the circumferential direction, an angle A measured at level of the points of the curve positioned in an equatorial plane passing through the middle of the tread and perpendicular to the axis of rotation,

the periodic curve extending over a number N of periods P distributed over a number T of circumferences 2IIR,

-the trajectory of the zigzag circumferential winding generating strip crossings between a portion of radially outer strip and a portion of radially inner strip,

-a strip crossing being external, when the strip portion

radially outer forms, in an axial plane and with the circumferential direction, a positive angle B,

a strip crossing being internal, when the radially external strip portion forms in an axial plane and with the circumferential direction, a negative angle B,

the reinforcements of the top layers being metallic,

-the periodic curve of the trajectory of the zigzag circumferential winding satisfying the following relationships:

N * (W / sinA) = 2nR * t with 0.6 <= t <= l,

N * P = 2flR * T,

-and, for at least 40% of the strip crossings (53) axially positioned, with respect to the circumferential direction (XX '), at an axial distance L1 at most equal to 0.25 times the amplitude L of the periodic curve (7 ), the circular bilayer portion (213), centered on the strip crossing (53) and having a radius equal to twice the width W of strip (5), includes Ne cross strip crossings and Neither strip cross crossings, so that | Ne-Ni | / (Ne + Ni) <= 0.3.

The principle of the invention is to use, for at least part of the top layers, a cut-out structure comprising at least one bilayer, and, more precisely, to replace at least two top layers with a bilayer of crown obtained by zigzag circumferential winding of a strip, in a circumferential direction of the tire and on a surface of revolution.

This top bilayer firstly has the distinction of being obtained by cutting a strip comprising metal reinforcements, while the process is usually used for textile reinforcements, which are much easier to cut due to their greater deformability.

In addition, the periodic curve of the trajectory of the zigzag circumferential winding must satisfy two particular relationships, defined between the parameters of said trajectory and as specified below.

Regarding the first relationship, each of the two crown layers, making up the crown bilayer, is formed by the juxtaposition of N strip portions, the strip having a width W and forming an angle A, with the circumferential direction of the tire and in an equatorial plane passing through the middle of the tread and perpendicular to the axis of rotation. N is the number of periods P of the periodic curve, that is to say the number of times that one must repeat the trajectory of laying the strip to achieve the top bilayer. Consequently, the developed circumferential length of a vertex layer is equal to N * (W / sinA), where W / sinA is the width of the strip projected on the circumferential direction. The first relation N * (W / sinA) = 2IlR * t with 0.6 <= t <= l reflects the fact that the developed circumferential length of a top layer is less than or equal to the circumference 2IIR of the cylindrical laying surface of radius R, that is to say that the juxtaposition of strip portions is generally not carried out uniformly, except for t = 1. By non-uniform juxtaposition of strip portions, we mean a juxtaposition which may include a discontinuity or a hole between two adjacent strip portions. The coefficient t of the previous relationship is called the recovery rate, because it characterizes the greater or lesser amount of holes present in a top layer.

Concerning the second relation, the total length of strip, projected onto the circumferential direction, necessary for the realization of the top bilayer, is equal to N * P, where N is the number of periods P of the periodic curve and where P is the period of the periodic curve. The second condition N * P = 2I1R * T expresses the fact that the total projected length of strip is equal to a multiple T of the circumference 2IIR of the cylindrical laying surface of radius R. T represents the number of winding turns of the strip on the cylindrical installation surface of radius R, necessary for the realization of the top bilayer. Finally, given that the trajectory of the zigzag circumferential winding generates strip crossings between a radially outer strip portion and a radially inner strip portion, the inventors have classified these strip crossings as external crossings and crossings interiors. By definition, a strip crossing is external, when the radially external strip portion forms, in an axial plane and with the circumferential direction, a positive angle B. An axial plane is an oriented plane, defined by a first circumferential direction, tangent to the surface of revolution and oriented according to the direction of rolling of the tire, and a second axial direction, parallel to the axis of rotation of the tire: this is therefore a plane tangent to the surface of revolution. Still by definition, a strip crossing is internal, when the radially external strip portion forms in an axial plane and with the circumferential direction, a negative angle B.

The inventors have sought to obtain, in the vicinity of 40% of the strip crossings, positioned in a median zone of the axial width of the top bilayer, that is to say outside the strip overlap zones present at the axial ends of the top bilayer, a number of internal crossings close to the number of external crossings, in order to obtain a weaving homogeneity guaranteeing a homogeneous mechanical operation of the top bilayer. This object is achieved, according to the inventors, if, for at least 40% of the strip crossings axially positioned, relative to the circumferential direction, at an axial distance L1 at most equal 0.25 times the amplitude L of the periodic curve, c that is to say in the middle zone, the circular portion of bilayer, centered on the crossing of strip and having a radius equal to 2 times the width W of strip, that is to say a limited radius, includes Ne crossings of outer strip and Ni crosses of inner strip, such that | Ne-Ni | / (Ne + Ni) <= 0.3, which guarantees that the numbers of strip crossings respectively outer Ne and inner Ni are close.

Advantageously, any metal reinforcement having a circular section of diameter D, the width W of the strip is at least equal to D. The strip, from which the top bilayer is made, is at least made up of a reinforcement single metallic, coated in an elastomeric mixture. The diameter D of the reinforcement is its overall diameter, not its compacted diameter.

Still advantageously, the width W of the strip is at most equal to 0.2 times the amplitude L of the periodic curve. The strip, from which the top bilayer is made, must have a sufficient width, that is to say must be composed of a sufficient number of reinforcements to allow a realization of the top bilayer sufficiently quickly. However, this width must remain limited to allow the winding of the strip without twisting thereof at the extremes of the periodic curve, that is to say at the level of changes in direction of the trajectory. The phenomenon of twisting, outside the plane of the strip, is in fact particularly noticeable with metallic reinforcements.

Advantageously, the periodic curve having extrema and having a radius of curvature R ’at its extrema, the radius of curvature R’ of the periodic curve is at least equal to the amplitude L of the periodic curve. The radius of curvature of the periodic curve, minimum at its extrema, must have a minimum value to allow the winding of the strip with metal reinforcements without twisting thereof at the extrema, at which the trajectory of the strip changes direction.

Again advantageously, the periodic curve having extrema and having a radius of curvature R 'at its extrema, the radius of curvature R' of the periodic curve is at most equal to 7 times the amplitude L of the curve periodic. The radius of curvature of the periodic curve, minimum at its extremes, must however remain limited to allow obtaining an angle A, relative to the circumferential direction, not too small, conventionally at least equal to 15 °, that is to say sufficient to obtain a transverse rigidity of the crown bilayer making it possible to guarantee correct transverse behavior of the tire.

Advantageously, the T / N ratio is at least equal to 0.15 and at most equal to 1.3. A zigzag circumferential winding defined by a number N of periods P distributed over a number T of circumferences 2I1R * T, such that the T / N ratio is included in the preceding interval, makes it possible to obtain the trajectory characteristics targeted, as previously defined, namely a sufficient angle A and minimum radii of curvature R 'at the extremes in the required ranges.

Preferably, N and T are prime integers between them. The fact that N and T are integers prime to each other makes it possible to have a circumferential offset of the trajectory of the strip at each winding revolution so that there can be no radial superposition of the trajectory of the strip.

According to a preferred embodiment, the metal reinforcements of the crown layers are elastic metallic reinforcements having a structural elongation As at least equal to 0.5% and a modulus of elasticity in extension at most equal to 150 GPa. The previous characteristics are determined on an elastic metallic reinforcement, coated in a vulcanized elastomeric mixture. The elastic behavior of the metal reinforcements, as previously defined, with in particular a structural elongation As at least equal to 0.5%, facilitates the carrying out of the trancanning, that is to say the zigzag winding of the strip.

According to a variant of the preferred embodiment, the elastic metal reinforcements of the strip are multi-strand cables of lxl structure comprising a single layer of I strands wound in a helix, each strand comprising an internal layer of J internal wires wound helically and an outer layer of K outer wires wound helically around the inner layer. Multi-strand cables are conventional embodiments of elastic metallic reinforcements, in the field of tires.

A crown reinforcement preferably comprises at least two crown bilayers. Knowing that a crown reinforcement of a tire for a civil engineering vehicle usually comprises six crown layers, distributed in two protective layers, two working layers and two hoop layers, the inventors have imagined replacing the two layers. working by a working bilayer and / or the two hooping layers by a hooping bilayer and / or the two protective layers by a protective bilayer. Thus, various configurations of crown reinforcement can be envisaged: a crown bilayer combined with four individual crown layers, two crown bilayers combined with two individual crown layers, three crown bilayers. It should be noted that a frame of Simplified top, with only two top bilayers, was also considered by the inventors.

The features of the invention are illustrated in Figures 1 to 9

schematic and not shown to scale:

-Figure 1: Half meridian section of a tire crown for a Civil Engineering vehicle according to the invention

-Figure 2: Perspective view of a zigzag circumferential winding of a strip, according to a periodic curve, on a cylindrical laying surface

-Figure 3: Plan view of the trajectory of the strip at the end of T = 1 turn

strip winding

-Figure 4: Plan view of the trajectory of the strip at the end of T = 2 turns of winding of the strip

-Figure 5: Plan view of the trajectory of the strip at the end of T = 28 turns of winding of the strip, corresponding to the top bilayer in its final state

-Figure 6: Plan view of the trajectory of the strip at the end of T = 28 turns of winding of the strip with location of the strip crossings in a middle part

-Figure 7: Diagram of an outer strip crossing

-Figure 8: Diagram of an interior strip crossing

-Figure 9: Principle of counting crossings of internal and external strips in the vicinity of a strip crossing.

1 shows a meridian half-section, in a meridian plane YZ passing through the axis of rotation YY 'of the tire, of a tire 1 for a civil engineering vehicle comprising a crown reinforcement 2 radially inner to a tread 3 and radially external to a carcass reinforcement 4. The crown reinforcement 2 comprises a crown bilayer 21, constituted at least in part by two crown layers (211, 212) radially superposed, and obtained by circumferential winding in a zigzag of a strip of width W, on a cylindrical laying surface of radius R (not shown), having as axis of revolution the axis of rotation YY 'of the tire. In the embodiment shown, the top bilayer 21 is a working bilayer, radially inside a protective frame 22 made up of two protective layers. In the meridian plane YZ, each vertex layer (211, 212) is formed by an axial juxtaposition of strip portions 5 of width W / cosA, where W is the width (not shown) of strip 5, measured perpendicular to the mean line of the strip 5, and A is the angle (not shown) formed by the mean line of the strip 5, with the circumferential direction XX ', in the equatorial plane XZ.

Figure 2 is a perspective view of a circumferential zigzag winding of a strip 5 of width W, according to a periodic curve 7, on a cylindrical laying surface 6, of revolution about the axis of rotation YY ′ of the tire and having a radius R. This circumferential zigzag winding of a strip 5 constitutes a top bilayer.

Figure 3 is a plan view of the trajectory of the strip at the end of T = 1 turn of the strip winding or, more precisely, of the average fiber of the strip, the width of the strip n ' not being shown for better readability of the figure. This plan view, in an XY plane, corresponds to the developed surface of the cylindrical laying surface, having a length equal to the circumference C = 2L1R, where R is the radius of the cylindrical laying surface, and a width equal to l amplitude L of the periodic curve 7. The trajectory of the zigzag circumferential winding is a periodic curve 7 having a period P and an amplitude L. The periodic curve 7 forms, with the circumferential direction XX ', an angle A measured at level of the points of the curve positioned in an equatorial plane XZ passing through the middle of the tread and perpendicular to the axis of rotation YY '. The periodic curve 7 has extrema (71, 72) and having a radius of curvature R ’at its extrema (71, 72). In the embodiment shown, with the choices of angle A and radius of curvature R ’made, the period of the periodic curve 7 extends over 2.2 periods P, on a winding turn.

Figure 4 is a plan view of the trajectory of the strip at the end of T = 2 turns of winding of the strip. In the embodiment shown, the periodic curve 7, in the second winding turn, crosses the trajectory of the ^1st winding turn in 5 points, thus creating 5 strip crossings 53. Figure 5 is a plan view of the trajectory of the strip at the end of T = 28 turns of winding of the strip, corresponding to the top bilayer in its final state. In the embodiment shown, the top bilayer is in its final state when the curve 7 extends over a number N = 61 of periods P distributed over a number T = 28 of circumferences 2IIR.

Figure 6 is a plan view of the trajectory of the strip at the end of T = 28 turns of winding of the strip, corresponding to the top bilayer in its final state, on which is shown an example of circular portion of bilayer 213, centered on a crossover of strip 53, positioned at the axial distance L1, and having a radius RI equal to twice the width W of strip. This circular bilayer portion 213 is a reference element on which the number Ne crosses of external strip and the number Ni crosses of internal strip are determined, in the vicinity of the cross strip 53.

Figure 7 is a diagram of an outer strip crossing 53, between a radially outer strip 51 and a radially inner strip 52. In the case of an outer strip crossing, the strip portion 51 radially outer forms , in an axial plane XY and with the circumferential direction XX ', a positive angle B. An axial plane XY is an oriented plane, defined by a first circumferential direction XX ', tangent to the surface of revolution and oriented according to the direction of travel of the tire, and a second axial direction YY', parallel to the axis of rotation of the tire. pneumatic: it is therefore a plane tangent to the surface of revolution.

Figure 8 is a diagram of an inner strip crossing 53, between a radially outer strip 51 and a radially inner strip 52. In the case of an inner strip crossing, the strip portion 51 radially outside forms , in an axial plane XY and with the circumferential direction XX ', a negative angle B.

Figure 9 shows schematically the principle of counting crossings of interior and exterior strip in the vicinity of a strip crossing, the latter being interior in the case shown. For each strip crossing 53 axially positioned, with respect to the circumferential direction XX ', at an axial distance L1 at most equal to 0.25 times the amplitude L of the periodic curve 7 (see the FIG. 6), a circular portion of bilayer 213 is defined, centered on the crossover of strip 53 and having a radius equal to 2 times the width W of strip 5. Then we count the respective numbers Ne of crossings of external strip and Ni of interior strip crossings contained in said circular portion. It should be noted that the crossing of strip 53, being interior in the case shown, is taken into account in Ni. The parameters of the periodic zigzag winding are then optimized so that, for at least 40% of the selected strip crossings, whether they are themselves interior or exterior, the following condition is satisfied: | Ne-Ni | / (Ne + Ni) <= 0.3. This condition guarantees that the numbers of outer strip Ne and inner Ni crosses are close, this criterion being, according to the inventors, characteristic of a homogeneous distribution of the inner and outer strip crossings, resulting in a homogeneous mechanical operation of the top bilayer .

The inventors defined a top bilayer with optimized trenching, as defined in the invention, for a tire for a Civil Engineering vehicle of dimension 24.00R35.

The tire studied comprises a crown reinforcement constituted by two crown bilayers, for which each of the crown layers is constituted by elastic metallic reinforcements of the multi-strand cable type.

The structure of these multi-strand cables is of the lxl type, comprising a single layer of I strands wound in a helix, each strand comprising an internal layer of J internal wires wound in a helix and an external layer of K external wires wound in helix around the inner layer. In the example studied, the multi-strand cables used have a structure 52.26 = 4 * (5 + 8) * 26, that is to say consist of 1 = 4 strands, each strand comprising an internal layer of J = 5 internal wires and an external layer of K = 8 external wires wound in a helix around the internal layer, the wires having a section of diameter d = 0.26 mm. These reinforcements have a diameter D equal to 3.1 mm, an elastic modulus in extension E equal to 70 GPa, a structural elongation As equal to 0.7%, and are distributed axially according to an axial pitch equal to 5 mm. Table 1 below summarizes the properties of the elastic multi-strand cables tested:

Table 1

The top bilayer consists of a zigzag circumferential winding of a strip of width W equal to 35 mm, in a circumferential direction XX 'of the tire and on a substantially cylindrical surface of revolution positioned at a radial distance R equal 985 mm from the rotation tax YY 'of the tire, this radial distance R being measured in an equatorial plane XZ passing through the middle of the tread and perpendicular to the rotation tax YY'. The trajectory of the circumferential zigzag winding is a periodic curve having a period P equal to 2841 mm and an amplitude L equal to 400 mm. The periodic curve forms, with the circumferential direction XX ’, an angle A equal to 24 °, measured at the points of the curve positioned in the equatorial plane XZ. The periodic curve extends over a number N, equal to 61, of periods P distributed over a number T, equal to 28, of circumferences 2L1R. The periodic curve of the trajectory of the circumferential zigzag winding satisfies the following relationships N * (W / sinA) = 2LlR * t = 5.3 m with t = 0.85, and N * P = 2L1R * T = 173.3 m.

Table 2 below summarizes the properties of the periodic curve of the circumferential zigzag winding of the strip, constituting the top bilayer:

Table 2

The inventors have verified that, for 58% of the strip crossings axially positioned, relative to the circumferential direction XX ', at an axial distance L1 at most equal to 100 mm (= 0.25 * L), the circular portion of bilayer, centered on the strip crossing and having a radius RI equal to 70 mm (= 2 * W) includes Ne external strip crossings and Ni internal strip crossings, so that | Ne-Ni | / (Ne + Ni ) <= 0.3.

This invention, studied in the field of tires for civil engineering vehicles, can be applied to any tire comprising a crown reinforcement comprising at least two metallic crown layers, such as for example a tire for a heavy vehicle.

Claims

1 - Tires (1) for Civil Engineering vehicles including:

a crown reinforcement (2) radially internal to a tread (3) and radially external to a carcass reinforcement (4),

the crown reinforcement (2) comprising at least one crown bilayer (21) constituted at least in part by two crown layers (211, 212) radially superimposed and each constituted by reinforcements coated in an elastomeric material,

-each two-layer crown (21) consisting of a circumferential zigzag winding of a strip (5) of width W, in a circumferential direction (XX ') of the tire and on a substantially cylindrical surface of revolution (6) positioned at a radial distance R of rotation tax (YY ') of the tire,

-the trajectory of the zigzag circumferential winding being a periodic curve (7) having a period P and an amplitude L,

-the periodic curve (7) forming, with the circumferential direction (XX '), an angle A measured at the points of the curve positioned in an equatorial plane (XZ) passing through the middle of the tread and perpendicular to Tax rotation (YY '),

-the periodic curve (7) extending over a number N of periods P distributed over a number T of circumferences 2IIR,

-the trajectory of the zigzag circumferential winding generating crossings of strip (53) between a portion of radially outer strip (51) and a portion of radially inner strip (52),

a strip crossing (53) being external, when the radially external strip portion (51) forms, in an axial plane (XY) and with the circumferential direction (XX ’), a positive angle B,

-a strip crossing (53) being inside, when the radially outer strip portion (51) forms in an axial plane (XY) and with the circumferential direction (XX ’), a negative angle B,

characterized in that the reinforcements of the crown layers (211, 212) are metallic, in that the periodic curve (7) of the trajectory of the zigzag circumferential winding satisfies the following relationships:

N * (W / sinA) = 2TIR * t with 0.6 <= t <= l, N * P = 2I1R * T,

and in that, for at least 40% of the strip crossings (53) axially positioned, with respect to the circumferential direction (XX '), at an axial distance L1 at most equal to 0.25 times the amplitude L of the periodic curve (7), the circular bilayer portion (213), centered on the crossover of strip (53) and having a radius RI equal to twice the width W of strip (5), includes Ne crossings of outer strip and Ni crosses of inner strip, so that | Ne- Ni | / (Ne + Ni) <= 0.3.

2 - A tire (1) according to claim 1, any metal reinforcement having a circular section of diameter D in which the width W of the strip (5) is at least equal to D.

3 - A tire (1) according to any one of claims 1 or 2, in which the width W of the strip (5) is at most equal to 0.2 times the amplitude L of the periodic curve (7).

4 - A tire (1) according to any one of claims 1 to 3, the periodic curve (7) having extremes (71, 72) and having a radius of curvature R 'at its extremes (71, 72), in which the radius of curvature R 'of the periodic curve (7) is at least equal to the amplitude L of the periodic curve (7).

5 - A tire (1) according to any one of claims 1 to 3, the periodic curve (7) having extremes (71, 72) and having a radius of curvature R 'at its extremes (71, 72), in which the radius of curvature R 'of the periodic curve (7) is at most equal to 7 times the amplitude L of the periodic curve (7).

6 - A tire (1) according to any one of claims 1 to 5, in which the T / N ratio is at least equal to 0.15 and at most equal to 1.3.

7 - A tire (1) according to any one of claims 1 to 6, in which N and

T are prime numbers between them.

8 - A tire (1) according to any one of claims 1 to 7, in which the metal reinforcements of the crown layers (211, 212) are elastic metallic reinforcements having a structural elongation As at least equal to 0.5% and a module elasticity in extension at most equal to 150 GPa. 9 - A tire (1) according to claim 8, in which the elastic metallic reinforcements of the strip (5) are multi-strand cables of lxl structure comprising a single layer of I strands wound in a helix, each strand comprising an internal layer of J internal wires wound in a helix and an external layer of K external wires wound in a helix around the internal layer.

10 - A tire (1) according to any one of claims 1 to 9, in which the crown reinforcement (2) comprises at least two crown bilayers (21).