EP3310590A1

EP3310590A1 - Tyre tread for a heavy civil engineering vehicle

Info

Publication number: EP3310590A1
Application number: EP16728707.7A
Authority: EP
Inventors: Philippe Mansuy; Antoine PERRIOT
Original assignee: Compagnie Generale des Etablissements Michelin SCA
Current assignee: Compagnie Generale des Etablissements Michelin SCA
Priority date: 2015-06-17
Filing date: 2016-06-14
Publication date: 2018-04-25
Also published as: FR3037532B1; CN107743449A; US20180370287A1; WO2016202763A1; JP2018521893A; FR3037532A1; BR112017025642A2

Abstract

The present invention relates to the tread of a radial tyre for a heavy civil engineering vehicle, and aims to reduce the speed of wear thereof when used in mining. The tread is made up of a radial stack of a first portion (21) and a second portion (22) which is radially outside the first portion (21). The first portion (21) is made up of a radial stack of N layers C_1i, each layer C_1ihaving a substantially constant radial thickness E_1i and consisting of a polymer material M_1ihaving a dynamic shear modulus G_1i. The second portion (22) is made up of a single layer C₂, having a substantially constant radial thickness E₂and consisting of a polymer material M₂having a dynamic shear modulus G_2.According to the invention, the following relations are simultaneously verified: a. 1/(E₁/G₁+E₂/G₂) > G₀/(E₁+E₂), where E₁= Formula (I) where E_1i, E₁and E₂ are in mm, G_1i, G₁ and G₂ are in MPa, and where 1 MPa ≤ G₀ ≤ 1.8 MPa; b. G₁ < G₀; c. E₁ ≥ E₁ min. = 25 mm; d. G₂ > G₀ > G₁; e. E₂ ≤ E₂ max. = 70 mm; and f. Formula (II) for 1 ≤ j ≤ N-1.

Description

PNEUMATIC TIRE TREAD FOR HEAVY VEHICLE TYPE GENIE CIVIL

The present invention relates to a radial tire, intended to equip a heavy vehicle type civil engineering, and relates, more particularly, its tread.

According to the classification of the standard of the European Tire and Rim Technical Organization or ETRTO, a radial tire for heavy vehicle type civil engineering is intended to be mounted on a rim whose diameter is at least equal to 25 inches. Without being limited to this type of product, the invention is described for a large radial tire, intended to be mounted on a dumper type vehicle for transporting materials extracted from quarries or surface mines. Large radial tire means a tire intended to be mounted on a rim whose diameter is at least equal to 49 inches and can reach 57 inches or 63 inches.

At the extraction sites of materials, such as ores or coal, the use of a dumper type vehicle consists, in a simplified manner, in alternating cycles go charging and empty return cycles . During a load cycle, the loaded vehicle transports, mainly uphill, materials extracted from loading areas at the bottom of the mine, or bottom of the "pit", to unloading areas. During an empty return cycle, the unladen vehicle returns, mainly downhill, to the loading areas at the bottom of the mine.

Given the small dimensions of the loading and unloading areas, the vehicles are forced to perform maneuvers to load or unload, and in particular half-turns on trajectories of very small radii typically between 12 m and 15 m, which puts a lot of stress on the tires.

In addition the tracks on which the vehicles roll are made of materials generally from the mine, for example, crushed rocks, compacted and regularly watered to ensure the holding of the wear layer of the track during the passage cars. The load applied to the tire depends both on its position on the vehicle and the cycle of use of the vehicle. For example, for a slope of about 10%, during a load increase cycle, one third of the total load of the vehicle is applied to the front axle, generally equipped with two tires in single ride. , and two-thirds of the total vehicle load is applied to the rear axle, usually equipped with four twin-mounted tires. During the empty descent return cycle, for a slope of approximately 10%>, half of the total vehicle load is applied to the front axle and half of the total vehicle load is applied to the rear axle. . The tires fitted to the mining dumpers are, as a general rule, mounted as single tires on the front axle of the vehicle during the first third of their life, then swapped, and mounted, in twin tires, on the rear axle for the remaining two-thirds of life.

On an economic level, the transport of extracted materials can represent up to 50%) operating costs of the mine, and the contribution of tires in transport costs is significant. Therefore limiting the tire wear speed is a major axis of reduction of operating costs. From the point of view of the tire manufacturer, developing technical solutions to reduce the speed of wear is therefore an important strategic objective.

[0008] The tires for mining use are subjected to strong mechanical stresses, both at the local level, when driving on tracks covered by indentors, consisting of pebbles whose average size is typically between 1 inch and 2.5. inches, and overall, when passing a large torque on slopes between 8.5% or 10%>, and during U-turns for loading and unloading maneuvers. These mechanical stresses lead to a relatively fast wear of the tires.

The technical solutions envisaged to date to reduce the speed of wear relate essentially to the design of the tread pattern, the choice of constituent materials of the tread, generally elastomeric mixtures, and on the optimization of the crown reinforcement radially inner to the tread. For example, in the field of tread sculpture, patent WO 2004085175 proposes the use of a tread whose sculpting elements have a tilt of the front and rear faces differentiated and variable in the width of the tread to generate coupling forces dependent on the applied load, and thus change the operating point of the tire in sliding and therefore limit the phenomena of wear. A tire having a geometry of revolution with respect to an axis of rotation, its geometry is usually 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. By convention, the terms "radially inner or radially outer" mean "closer or farther from the axis of rotation of the tire" respectively. By "axially inner, respectively axially outer" is meant "closer or more distant from the equatorial plane of the tire", the equatorial plane of the tire being the plane passing through the middle of the running surface of the tire and perpendicular to the tire. rotation axis of the tire.

The inventors have set themselves the objective of reducing the wear speed of the tread of a radial tire for a heavy vehicle of the civil engineering type subjected to heavy mechanical stresses induced by the mining use previously described.

This object has been achieved, according to the invention, by a tire for heavy vehicle type civil engineering comprising a tread, intended to come into contact with a ground,

the tread having an axial width L and constituted by a radial superposition of a first portion and a second portion radially external to the first portion;

the first portion consisting of a radial superposition of N layers Cn, i varying from 1 to N,

each layer Cn having a radial thickness, measured in an equatorial plane of the tire, substantially constant over at least 80% of the axial width L of the tread, and consisting of a polymeric material Mn having a dynamic shear modulus Gn, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C.,

the second portion consisting of a single layer C ₂ ,

the layer C ₂ having a radial thickness E ₂ , measured in the equatorial plane of the tire, substantially constant over at least 80% of the axial width L of the tread, and consisting of a polymeric material M ₂ having a module dynamic shear G ₂ , measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C,

the following relations being simultaneously checked:

at. 1 / (E1 / G1 + E2 / G2)> Go / (E1 + E2), with Ei = Σ _{= 1} Eu, Gi = Ej / (Σ ^ _{= 1} Eu / Gn) with E _H , Ei, E ₂ in mm, Gn, Gi, G ₂ in MPa and with MPa <G ₀ <1.8 MPa c. Ei> Eimin = 25 mm

e. E ₂ <E ₂ max = 70 mm

f- 1 / (Σ! _{= 1} En / Gn) <1 / (Σ¾ ₊₁ Εη / α for 1 <j <N-1

The tread of the tire of the invention is the wear portion of the tire and is intended to come into contact with a covered floor, in the context of the invention, by indentors, consisting of pebbles whose size maximum is at least 1 inch and not more than 2.5 inches. The passage of the tire on these indentors generates significant local deformations of the tread. The tread of the tire of the invention has an axial width L, measured parallel to the axis of rotation of the tire between the axial ends of the tread.

The tread is constituted by a radial superposition of a first portion and a second radially outer portion to the first portion. The first tread portion is constituted by a radial superposition of N layers Cn, i varying from 1 to N: it is therefore a multilayer portion, with N most often at most equal to 3. The first layer Cn, the radially inner of the first portion, is in contact, by a radially inner face, either directly with a crown reinforcement, or with an intermediate layer of polymeric material itself in contact with the crown reinforcement. The Nth layer C _IN , the radially outermost of the first portion, is in contact, by a radially outer face, with a radially inner face of the layer C ₂ of the second portion radially external to the first portion. Each layer Cn, for i varying from 1 to N, has a radial thickness, measured in an equatorial plane of the tire, substantially constant over at least 80% of the axial width L of the tread, and is constituted by a polymeric material Mn having a dynamic shear modulus Gn, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C. The polymeric materials are all different from each other and therefore have different dynamic Gn modules.

The second tread portion is constituted by a single layer C ₂ : it is therefore a monolayer portion. The layer C ₂ is in contact, by a radially inner face, with the radially outer face of the Nth layer C _IN , the most radially outer of the first portion, and is intended to come into contact with a ground, with a face radially exterior.

The layer C ₂ has a radial thickness E ₂ , measured in the equatorial plane of the tire, substantially constant over at least 80% of the axial width L of the tread, and consists of a polymeric material M ₂ having a dynamic shear modulus G ₂ , measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C.

A radial layer thickness is a measured distance, in the radial direction, between the respectively radially inner and radially outer faces of the layer. This thickness is measured in the equatorial plane of the tire, passing through the middle of the tread and perpendicular to the axis of rotation of the tire. pneumatic. This thickness is measured on a new tire, that is to say having not rolled and therefore not worn. By substantially constant radial thickness means a thickness in a range of + or -5% relative to an average thickness and at least 80% of the axial width L of the tread. A dynamic shear modulus is measured on a viscoanalyser Metravib type VA4000, according to ASTM D 5992-96. The response of a sample of vulcanized polymeric material, in the form of a cylindrical specimen 4 mm in thickness and 400 mm ² in section, subjected to a sinusoidal stress in alternating simple shear at the frequency of 10 Hz is recorded. , with a strain amplitude sweep from 0.1% to 45% (forward cycle), then from 45% to 0.1% (return cycle), and at a temperature of 60 ° C. The dynamic shear modulus is thus measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C.

According to the invention, six inequalities combining the radial thicknesses and / or the dynamic shear modules of the constituent layers of the first and second tread portions must be verified.

The first inequality l / (Ei / Gi + E ₂ / G ₂ )> G ₀ / (Ei + E ₂ ), with Ei = Σ _{= 1} Eu, Gi = Ei / (ΣiLi Eii / Gii) with In, E _h E ₂ in mm, G _H , d, G ₂ in MPa and with MPa <G ₀ <1.8 MPa, means that the rigidity of a tread according to the invention, constituted by a first portion, it -also constituted by the radial superposition of N layers Cii, having respective radial thicknesses E and being constituted by polymeric materials Mn having respective shear moduli Gn, and a second outer radial portion, constituted by a single layer C ₂ , having a radial thickness E ₂ and consisting of a polymeric material M ₂ having a respective shear modulus G ₂ , must be greater than the rigidity of a tread of the state of the art, constituted by a single equivalent layer, having a radial thickness equal to the sum of the radial thicknesses of all constituent layers res first and second portions, said equivalent layer consisting of a polymeric material having a dynamic shear modulus GB. The dynamic shear modulus of reference Go, in the field of tires for heavy vehicles of civil engineering type, is usually at least equal to 1 MPa and at most equal to 1.8 MPa.

To simplify the writing of the inequality, are introduced the equivalent radial thickness Ei and equivalent dynamic shear modulus Gi of the first portion assimilated to a single equivalent layer Ci. By definition, the equivalent radial thickness Ei of the first portion is equal to the sum of the respective radial thicknesses in layers CH. By definition also, the equivalent flexibility Ei / Gi of the first portion, which is the inverse of the equivalent stiffness Gi / Ei, is equal to the sum of the respective flexibilities En / Gn of the layers Cn, hence the expression of equivalent dynamic shear modulus Gi of the first portion.

This first inequality expresses that, on the new tire, that is to say at the beginning of its life, when it is mounted on the front axle of the vehicle, the multilayer tread of a tire according to the invention must be more rigid than the monolayer tread of a tire of the state of the art. Indeed the tread of a new tire, at the beginning of life on the front axle, uses predominantly under imposed force. However, locally, the force applied on the tread is the product of the rigidity of the tread by the local slip rate which is proportional wear. Therefore, imposed force, when the rigidity of the tread increases, the local slip rate, and therefore the wear, decrease. Thus, early in life, the multilayer tread of the invention, more rigid, will wear less quickly than the single-layer tread of the state of the art.

The second inequality Gi <GB means that the equivalent dynamic shear modulus Gi of the first portion must be smaller than the dynamic shear modulus GB of the single polymeric material constituting the tread of a tire of the state of the technique, measured under the same conditions. If we call E _r the residual radial thickness of tread, at the end of life of the tire on the rear axle, measured from the crown reinforcement, the second inequality can also be written Gi / E _r <GB / E _r . For the tire of the invention, E _r corresponds to the residual radial thickness of the first radially inner portion of the partially worn tread, a portion of the C n most radially outer having been completely worn. This new relationship expresses that the rigidity of the multilayer tread of the invention at end of life Gi / E _r must be lower than that of the tread of the state of the art Go / E _r . The tread of a worn tire, at the end of its life on the rear axle, uses predominantly under imposed deformation. However, the local slip rate is the ratio of the local force applied on the tread, the rigidity of the tread. Thus, when the rigidity of the tread decreases, the local effort decreases. As wear is an increasing function of the local effort, as the rigidity of the tread decreases, the wear, varying in the same direction as the local effort, decreases. Therefore the tread of the invention, less rigid, will wear less quickly than the tread of the state of the art.

Thus the first two inequalities express that the wear of a tread of a tire according to the invention is slower than that of a tire of the state of the art, at the beginning of life as in end of life, that is to say throughout the life of the tire.

The third inequality Ei> Eimin = 25 mm means that the equivalent radial thickness Ei of the first radially inner portion must be at least equal to a minimum value Eimin equal to 25 mm and corresponding to the depth of influence of the indentors usually covering the running tracks. In other words, the first radially inner portion must be thick enough to be able to have sufficient flexibility to ensure a cushion effect to envelop the indenter.

The fourth inequality G ₂ >GB> G ₁ means that the dynamic shear modulus G ₂ of the second portion must be both greater than the reference dynamic shear module GB and the equivalent dynamic shear modulus Gi of the first portion, that is, there must be a decreasing gradient of dynamic shear modules when moving from the second portion to the first portion.

The fifth inequality E ₂ <E ₂ max = 70 mm means that the radial thickness E ₂ of the single layer C ₂ of the second radially outer portion must at most be equal to a maximum value E ₂ max, equal to 70 mm and corresponding to the thickness radial limit beyond which the rolling of the tire on the indentors no longer impacts the deformations of the radially inner layers of the first portion. In other words, the second radially outer portion should not be too thick to allow the cushioning effect of the first radially inner portion and to ensure sufficient rigidity of the second radially outer portion intended to come into contact with the indentors.

The sixth inequality 1 / (Σ { _{= 1} E _H / G _H ) <1 / (Σ¾ ₊₁ Επ / Gn), for 1 <j <N-1, means that within the first portion , the rigidity of the assembly constituted by the most radially inner layers Ci _j must be less than the rigidity of the assembly constituted by the (Nj-1) most radially outer layers. There is thus a decreasing gradient of stiffness, for the layers of the first portion, when passing from the most radially outer layers to the most radially inner layers. Thus the radially most radially inner layers, the least rigid and therefore the most flexible, provide a cushioning role vis-à-vis the most radially outer layers.

The invention makes it possible to act simultaneously at the local level on the stresses imposed on the tread and at the overall level on the operating range of the tire during its life on the vehicle, mounted successively on the front axle. then on the rear axle, to improve the wear performance of the tire.

[0033] Advantageously, the relationship Gi> 0.5 * GB is verified. Thus the equivalent dynamic shear modulus Gi of the first radially inner portion must be greater than 0.5 times the dynamic shear modulus GB of the single polymeric material constituting the tread of a tire of the state of the art, measured under the same conditions. This relationship indicates that equivalent dynamic shear modulus Gi should not be too low, to ensure compliance with the first inequality defined above and sufficient overall tread stiffness.

Still advantageously the relationship G ₂ <3 * Gi is verified. The ratio between the dynamic shear modulus G ₂ of the second portion and the equivalent dynamic shear modulus Gi of the first portion should not be too high, in practice less than 3, to ensure a significant cushion effect of the radially inner layers of the first portion.

It is also advantageous for the relationship E ₂ > E ₂ min = 25 mm to be verified. In other words, the second radially outer portion should be sufficiently thick, with, in practice, a radial thickness E ₂ at least equal to 25 mm, to ensure sufficient rigidity of this second radially outer portion, at the beginning of life, when the tire is mounted on the front axle of the vehicle.

According to another advantageous embodiment of the invention, the relation 0.3 <Ei / (Ei + E ₂ ) <0.7 is verified. This relationship characterizes the positioning of the geometrical interface of contact between the first radially inner portion and the second radially outer portion in a range of values, making it possible to have the desired overall rigidity evolution of the tread of the tire during of his life on the vehicle, mounted successively on the front axle and on the rear axle. This condition ensures a relatively rigid tread in the first third of the life of the tire mounted on the front axle and a relatively smooth tread in the last two thirds of the life of the tire mounted on the rear axle.

According to a particular embodiment, the relation Go = 1.3 MPa is verified. The dynamic shear modulus GB of the single polymeric material constituting the tread of a tire of the state of the art, referred to in the invention, is equal to 1.3 MPa. This value is a usual dynamic shear value of a single-layer elastomeric tread mixture of the state of the art.

According to a preferred embodiment of the invention, each polymeric material Mn constituting each layer Cn of the first portion is an elastomeric mixture, that is to say a polymeric material comprising a diene elastomer of natural rubber type. or synthetic, obtained by mixing the various components of the material. This is the type of material most often used in the tire field. Also preferably, the polymeric material M2 constituting the layer C ₂ of the second portion is an elastomeric mixture. Most often the different polymeric materials of the various constituent layers of the tread, that is to say both of the first portion and the second portion, are all elastomeric mixtures.

Generally, the first portion consists of a radial superposition of N layers Cn, with N at most equal to 3, preferably at most equal to 2. In other words, preferably, the tread is constituted by a superposition. radial of not more than 3 layers.

Even more preferentially, the first portion consists of a single layer Cn. In other words, the tread is constituted by a radial superposition of 2 layers, which is the most usual configuration of the state of the art. the technique.

The features of the invention are illustrated in Figures 1, 2, 3A, 3B, 4A, 4B, 5 and 6 schematic and not shown in scale.

In Figure 1, there is shown a meridian section of the top of a tire 1 for a heavy vehicle type civil engineering according to the invention, comprising a tread 2, intended to come into contact with a floor. The directions XX ', YY' and ZZ 'are respectively the circumferential, axial and radial directions of the tire. The plane XZ is the equatorial plane of the tire. The tread, having an axial width L, is constituted by a radial superposition of a first portion 21 and a second portion 22 radially external to the first portion 21.

The first portion 21 is constituted by a radial superposition of N layers Cn, i varying from 1 to N, each layer Cn having a radial thickness En, measured in an equatorial plane XZ of the tire, substantially constant over at least 80% of the axial width L of the tread 2, and consisting of a polymeric material Mn having a dynamic shear modulus Gn, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak deformation amplitude and a temperature of 60 ° C. The first multilayer portion 21 can be likened to a monolayer portion whose equivalent radial thickness Ei is equal to the sum of the respective radial thicknesses in layers Cn and whose equivalent flexibility E ₁ / G ₁ of the first portion is equal to the sum of the respective flexibility Επ / Gn CH layers.

The second portion 22 is constituted by a single layer C ₂ , the layer C ₂ having a radial thickness E ₂ , measured in the equatorial plane XZ of the tire, substantially constant over at least 80% of the axial width L of the tread 2, and being constituted by a polymeric material M ₂ having a dynamic shear modulus G ₂ , measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C.

Radially inside the first radially inner portion 21 is shown the crown reinforcement 3, comprising two crown layers comprising metal reinforcements. Radially inside the crown reinforcement 3 is shown the carcass reinforcement 4 comprising a carcass layer comprising metal reinforcements.

In Figure 2, there is shown a meridian section of the top of a tire 1 for heavy vehicle type civil engineering according to a preferred embodiment of the invention, comprising a tread 2, intended to come into contact with a ground. According to this preferred embodiment, the first portion 21 is constituted by a single layer Ci. In this particular case, the tread is constituted by the radial superposition of two layers, the first and second portions being monolayer: the tread is called bilayer.

FIGS. 3A and 3B show the local deformation of the tread during the passage over an indenter, respectively for a tire of the state of the art with a monolayer tread and a tire according to the invention comprising a bilayer tread. For the tire of the state of the art, the monolayer tread consists of an elastomeric mixture having a dynamic shear modulus GB, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the amplitude of crest-peak deformation and a temperature equal to 60 ° C and its local deformation has a projected length on the ground equal to Ao. For the tire according to the invention, the bilayer tread is constituted by a first radially inner layer constituted by a first elastomer mixture having a dynamic shear modulus Gi, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C, and a second radially inner layer constituted by a second elastomer mixture having a module dynamic shear G2, measured under the same conditions. In this case, the local deformation of the tread has a projected length to the ground A greater than Ao. The bilayer tread of the invention more envelopes the inventor than the monolayer tread, because of the cushioning effect of the first radially less rigid inner layer than the second radially outer layer. FIGS. 4A and 4B respectively represent a go-up cycle in load and a return cycle in empty descent of a dumper, as well as a turning maneuver of a dumper.

For up and down operation, illustrated in Figure 4A, the slope is, for example, between 8.5% and 10%>. For a 400 ton dump truck loaded, uphill, the load applied to a front-mounted or rear-mounted tire is 67 t, and the F _{x force} applied to a rear-mounted tire is equal to 10000 daN. For a dumper of 400 tons empty, downhill, the load on a front-mounted tire is 60 t, and the load on a rear-mounted tire is 30 t. In this use up and down, the tread of a tire has a mechanical operation force imposed.

For the curve operation, during the loading / unloading maneuvers, illustrated in Figure 4B, the gyration radius during operation is, for example, between 7 m and 12 m. In this use curve, the tread of a tire has a mechanical operation to deformation imposed.

FIG. 5 shows an example of comparative evolution of the relative stiffness K, expressed in%, of the tread, between a tire of the state of the art R and a tire according to the invention I, in according to the mileage traveled d, expressed in km, as a first step, on the front axle in the "front" position (F), then, in a second step, on the rear axle in the "drive" position (D). The base 100 relative rigidity of the tread is the rigidity of the tread of the tire of the state of the art R in the new state, that is to say 0 km traveled. In the example presented, for use in the "front" position, up to 35000 km, the relative stiffness K of the tread of the tire according to the invention remains higher than that of the tread of the tire of the tire. The state of the art R. The tire operating preferentially effort imposed in this area of low mileage, early life, an increase in the relative stiffness K of the tread can limit slip rates and drift levels , thus to limit the loss of mass by wear of the tread. Then, for use in the "drive" position, the relative positioning is reversed: the relative stiffness K of the tread of the tire according to the invention I becomes lower than that of the tire tread of the state R. At the end of its life, because of the very high loads experienced during maneuvers with a small radius of gyration, the tire operates essentially with imposed deformation, and a lower relative rigidity K of the tread makes it possible to reduce the applied stresses on the electrostatic mixture in contact with the ground, and thus reduce the loss of mass due to wear of the tread.

Figure 6 shows the evolution of the height H of the sculpture of the tread, in mm, depending on the distance traveled d, in km. The sculpture of the tread is constituted by a set of elements in reliefs or loaves, separated by recesses or grooves and constituting the wearing part of the tread. The height H, which reflects the state of the wear of the tread, decreases with the distance traveled d. FIG. 6 shows two wear-type curves respectively for a tire according to the invention I and for a tire of the state of the art R. Each curve comprises two substantially linear portions. The first portion, of lower slope, has the wear of the tire mounted at the front of the vehicle, at low mileage. The second portion, of steeper slope, has the tire wear, mounted at the rear of the vehicle, the high mileage. The break in the slope of each curve corresponds to the distance at which the rotation of the tire takes place between the "front" or "front" position and the "rear" or "drive" position. Thus the distances dp (R) and dp (I), abscissa of the points of rupture of slope, represent the distances traveled on front axle in position "front" respectively for a tire of the state of the art R and for a tire according to the invention I. Similarly, the distances do (R) and do (I), corresponding to the total wear of the tire, represent the distances traveled. on the rear axle in "drive" position respectively for a tire of the state of the art R and for a tire according to the invention I. It should be noted that the height H of the sculpture decreases less rapidly, that is to say that is to say that the speed of wear is lower, both in the "front" position and in the "drive" position for a tire according to the invention I. In other words, the distances traveled respectively on front axle, before permutation on rear axle, and on rear axle, before removal of the tire with total wear, are higher for the tire according to the invention I.

The invention has been more particularly studied in the case of a tire size 40.00R57, equipping a rigid dumper 400 tons of total load.

A bilayer tread, according to the invention, consisting of a first, radially inner, monolayer portion 21 having a radial thickness Ei equal to 30 mm and an elastomeric material Mi whose dynamic shear modulus Gi, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C., is equal to 1.16 MPa, and a second radially outer, single-layer portion 22 having a radial thickness E ₂ equal to 10 mm and elastomeric material M ₂ whose dynamic shear modulus, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and an equal temperature at 60 ° C, is G ₂ equal to 1.85 MPa, was evaluated in wear, on a mining-type soil in an imposed-force use, and compared to a monolayer tread, consisting of a single layer having a e radial thickness Eo equal to 40 mm and elastomeric material M ₂ .

Although the bilayer tread has a stiffness equal to 75% of the rigidity of the monolayer tread, which would suggest a significant degradation in wear performance, of the order of 20 to 30%, by a increasing the sliding rate, the modification of the local operating point of the radially outer surface layer 22, thanks to the cushioning effect of the layer radially Inner 21, finally provides a performance in wear equal to or even greater than that of the tread monolayer reference.

The invention is however not limited to the features described above and can be extended to other types of treads, for example, to different multilayer structures according to the axial portions of the tread.

Claims

1 - Pneumatic tire (1) for a heavy vehicle of the civil engineering type comprising a tread (2) intended to come into contact with a ground,

the first portion (21) consisting of a radial superposition of N layers Ci i, i varying from 1 to N,

each layer Cn having a radial thickness, measured in an equatorial plane (XZ) of the tire, substantially constant over at least 80% of the axial width L of the tread (2), and consisting of a polymeric material Mn having a dynamic shear modulus Gn, measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C,

the second portion (22) consisting of a single layer C ₂ ,

the layer C ₂ having a radial thickness E ₂ , measured in the equatorial plane (XZ) of the tire, substantially constant over at least 80% of the axial width L of the tread (2), and consisting of a material polymeric M ₂ having a dynamic shear modulus G ₂ , measured for a frequency equal to 10 Hz, a deformation equal to 50% of the peak-peak deformation amplitude and a temperature equal to 60 ° C,

characterized in that the following relationships are simultaneously verified:

at. l / (Ei / Gi + E2 / G ₂ )> Go / (Ei + E ₂ ), with Ei = Σ _{= 1} En, Gi = Ει / β ^ En / Gn) with E _H , Ei, E ₂ in mm , Gn, Gi, G ₂ in MPa and with MPa <G ₀ <1.8 MPa c. Ei> Eimin = 25 mm

e. E ₂ <E ₂ max = 70 mm

f Eii / Gii) <1 / (Σi _{j + 1} Eii / Gii) for 1 <j <N-1 2 - Pneumatic tire (1) for civil engineering heavy vehicle according to claim 1 wherein the relationship Gi> 0.5 * GB is verified.

3 - Pneumatic tire (1) for civil engineering heavy vehicle according to one of claims 1 or 2 wherein the relationship G ₂ <3 * Gi is verified. 4 - Pneumatic tire (1) for heavy vehicle type civil engineering according to any one of claims 1 to 3 wherein the relation E ₂ > E ₂ min = 25 mm is verified.

5 - Pneumatic tire (1) for heavy vehicle type civil engineering according to any one of claims 1 to 4 wherein the relation 0.3 <Ei / (Ei + E ₂ ) <0.7 is verified.

6 - Pneumatic tire (1) for heavy vehicle type civil engineering according to any one of claims 1 to 5 wherein GB = 1.3 MPa.

7- pneumatic tire (1) for civil engineering heavy vehicle according to any one of claims 1 to 6 wherein each polymeric material Mn constituting each layer Cn of the first portion (21) is an elastomeric mixture.

8- pneumatic tire (1) for civil engineering heavy vehicle according to any one of claims 1 to 7 wherein the polymeric material M ₂ constituting the layer C ₂ of the second portion (22) is an elastomeric mixture.

9- tire (1) for heavy vehicle type civil engineering according to any one of claims 1 to 8 wherein the first portion (21) is constituted by a radial superposition of N layers Cn, with N at most equal 3, preferably at most 2.

10- tire (1) for heavy vehicle type civil engineering according to any one of claims 1 to 9 wherein the first portion (21) is constituted by a single layer Ci.