Vehicle Tire Comprising a Stiffening Structure

Abstract

A tire (1) for an agricultural vehicle, having a crown reinforcement (3), with at least two crown layers (31, 32), each having metal reinforcers which are coated in an elastomer material. Any metal reinforcer of a crown layer (31, 32) has a law, known as a bi-modulus law, governing its elastic behaviour under tension, comprising a first portion having a first extension modulus MG1 at most equal to 30 GPa, and a second portion having a second extension modulus MG2 at least equal to 2 times the first extension modulus MG1, and any metal reinforcer of a crown layer (31, 32) has a law governing its behaviour under compression that is characterized by a critical buckling strain EU at least equal to 3%.

Claims

1. A fire for an agricultural vehicle, comprising: a crown reinforcement, radially on the inside of a tread and radially on the outside of a carcass reinforcement, the crown reinforcement comprising at least two crown layers each comprising metal reinforcers which are coated in an elastomeric material, are mutually parallel and form an angle A at least equal to 10° with a circumferential direction (XX′), wherein any metal reinforcer of a crown layer has a law, known as a bi-modulus law, governing its elastic behaviour under tension, and comprising a first portion having a first extension modulus MG1 at most equal to 30 GPa, and a second portion having a second extension modulus MG2 at least equal to 2 times the first extension modulus MG1, said law governing the tensile behaviour being determined for a metal reinforcer coated in an elastomer compound having a tensile elastic modulus at 10% elongation, MA10, at least equal to 5 MPa and at most equal to 15 MPa, and wherein any metal reinforcer of a crown layer has a law governing its behaviour under compression that is characterized by a critical buckling strain EU at least equal to 3%, said law governing behaviour under compression being determined on a test specimen made up of a reinforcer placed at its centre and coated with a parallelepipedal volume of an elastomer compound having a tensile elastic modulus at 10% elongation, MA10, at least equal to 5 MPa and at most equal to 15 MPa.

2. The tire according to claim 1, wherein the crown reinforcement is made up of two crown layers and any metal reinforcer of a crown layer (31, 32) has a linear density, expressed in g/m, wherein the linear density of a metal reinforcer of a crown layer is at least equal to 6 g/m and at most equal to 13 g/m.

3. The tire according to claim 1, wherein any metal reinforcer of a crown layer is a multistrand rope of structure 1×N comprising a single layer of N strands of diameter DT wound in a helix at an angle AT and a radius of curvature RT, each strand comprising an internal layer of M internal threads wound in a helix and an external layer of P external threads wound in a helix around the internal layer.

4. The tire according to claim 3, wherein the helix angle AT of a strand is at least equal to 20° and at most equal to 30°.

5. The tire according to claim 3, wherein the crown reinforcement is made up of two crown layers and any metal reinforcer of a crown layer has a diameter D, wherein the diameter D of a metal reinforcer of a crown layer is at least equal to 1.4 mm and at most equal to 3 mm.

6. The tire according to claim 1, wherein the crown reinforcement is made up of two crown layers and a crown layer has a breaking strength R expressed in N/mm, wherein the breaking strength R of a crown layer is at least equal to 500 N/mm and at most equal to 1500 N/mm.

7. The tire according to claim 1, wherein the crown reinforcement comprises at least one hooping layer comprising reinforcers which are coated in an elastomeric material, are mutually parallel and form an angle B at most equal to 10° with the circumferential direction (XX′).

8. The tire according to claim 1, wherein the crown reinforcement comprises at least one additional crown layer comprising metal reinforcers which are coated in an elastomeric material, are mutually parallel and form an angle C at least equal to 60° and at most equal to 90° with the circumferential direction (XX′).

9. The tire according to claim 1, wherein the carcass reinforcement comprises at least one carcass layer comprising textile reinforcers which are coated in an elastomeric material, are mutually parallel and form an angle D at least equal to 85° and at most equal to 95° with the circumferential direction (XX′).

10. The tire according to claim 1, wherein the tread is made up of a first and a second row of lugs extending radially outwards from a bearing surface and disposed in a chevron pattern with respect to the equatorial plane (XZ) of the tire.

Description

[0044] The features of the invention are illustrated by the schematic FIGS. 1 to 7, which are not drawn to scale:

[0045] FIG. 1: Meridian half-section of a tire for an agricultural vehicle according to the invention

[0046] FIG. 2: Typical example of a typical tensile force-elongation curve for an elastic metal reinforcer according to the invention, coated with an elastomeric material

[0047] FIG. 3: Tensile stress-elongation curves for two particular examples of elastic metal reinforcer according to the invention (E12.23 and E24.26) coated with an elastomeric material

[0048] FIG. 4: Typical example of a compressive force-compressive strain curve for an elastic metal reinforcer according to the invention, obtained on a test specimen made of elastomeric material

[0049] FIGS. 5 and 6: Assembly formulas for two particular examples of elastic metal reinforcer according to the invention (E18.23 and E24.26)

[0050] FIG. 7: Face-on view of a tire for an agricultural vehicle with lugged tread.

[0051] FIG. 1 shows a meridian half-section, on a meridian plane YZ, passing through the axis of rotation YY′ of the tire, of a tire 1 for an agricultural vehicle, comprising a crown reinforcement 3 radially on the inside of a tread 2 and radially on the outside of a carcass reinforcement 4. The crown reinforcement 3 comprises two crown layers (31, 32) each comprising metal reinforcers which are coated in an elastomeric material, are mutually parallel and form an angle A (not depicted) at least equal to 10° with a circumferential direction (XX′), The crown reinforcement 4 comprises three carcass layers comprising textile reinforcers that are coated in an elastomeric material, are mutually parallel and form an angle D (not depicted) at least equal to 85° and at most equal to 95° with the circumferential direction (XX′).

[0052] FIG. 2 is a typical example of a tensile force-relative elongation curve for an elastic metal reinforcer according to the invention, coated with an elastomeric material, showing its elastic behaviour under tension. The tensile force F is expressed in N and the elongation A is a relative elongation expressed as a %. According to the invention, the elastic and bi-modulus law governing the behaviour under tension comprises a first portion and a second portion. The first portion is delimited by two points of which the ordinate values correspond respectively to a zero tensile force and to a tensile force equal to 87 N, the respective abscissa values being the corresponding relative elongations (in %). A first tensile stiffness KG1 may be defined, this representing the gradient of the secant straight line passing through the origin of the frame of reference in which the behaviour law is represented, and the transition point marking the transition between the first and second portions. With the knowledge that, by definition, the tensile stiffness KG1 is equal to the product of the extension modulus MG1 times the cross-sectional area S of the reinforcer, the extension modulus MG1 can easily be deduced from it. The second portion is the collection of points corresponding to a tensile force greater than 87 N. Likewise, for this second portion, a second tensile stiffness KG2 may be defined, this representing the gradient of a straight line passing through two points positioned in a substantially linear part of the second portion. In the example depicted, the two points have the respective ordinate values F=285 N and F=385 N, these tensile force values corresponding to levels of mechanical loading indicative of the loadings applied to the metal reinforcers of the crown layers when the tire being studied is being driven on. As described previously, KG2=MG2*S, and so the extension modulus MG2 can be deduced therefrom.

[0053] FIG. 3 depicts two tensile stress-elongation curves, the tensile stress F/S, expressed in MPa, being equal to the ratio between the tensile force F, expressed in N, applied to the reinforcer, and the cross-sectional area S of the reinforcer, expressed in mm.sup.2, and the elongation A being the relative elongation of the reinforcer, expressed in %. The cross-sectional area S of the reinforcer is the cross section of metal equal to ML/ρ, ML being the linear density of the reinforcer, expressed in g/m and ρ being the volumetric density of the reinforcer, expressed in g/cm3 (for example, the volumetric density ρ of brass-coated steel is equal to 7.77 g/cm.sup.3). These curves are the laws governing the respective tensile behaviours of two examples of multistrand elastic reinforcers E18.23 and E24.26 coated with an elastomeric material. The first and second extension moduli MG1 and MG2 can be deduced directly from these curves. According to the invention, for each of the behaviour laws depicted, the first extension modulus MG1 is at most equal to 30 GPa, and the second extension modulus MG2 is at least equal to 2 times the first extension modulus MG1.

[0054] FIG. 4 is a typical example of a compressive force-compressive strain curve for an elastic metal reinforcer according to the invention, showing its elastic behaviour under compression. The compressive force F is expressed in N and the compressive strain is a relative compression, expressed as a %. This compression-behaviour law, determined on a test specimen made of elastomeric compound having a secant extension elastic modulus at 10% elongation, MA10, equal to 6 MPa, exhibits a maximum corresponding to the onset of buckling of the reinforcer. This maximum is reached for a maximum compression force Fmax, or critical buckling force, corresponding to a critical buckling strain E0. Beyond the point of buckling, the compressive force applied decreases while the strain continues to increase. According to the invention, the critical buckling strain E0 is at least equal to 3%.

[0055] FIGS. 5 and 6 show two examples of structures of multistrand elastic reinforcer assemblies, which are particular embodiments of the invention. FIG. 5 depicts a multistrand rope of E18.23 type, having a 3*(1+5)*0.23 structure, namely comprising a single layer of 3 strands, each strand comprising an internal layer of 1 internal thread wound in a helix and an external layer of 5 external threads wound in a helix around the internal layer. Each thread is made of steel and has an individual diameter equal to 0.23 mm FIG. 6 depicts a multistrand rope of E24.26 type, having a 4*(1+5)*0.26 structure, namely comprising a single layer of 4 strands, each strand comprising an internal layer of 1 internal thread wound in a helix and an external layer of 5 external threads wound in a helix around the internal layer. Each thread is made of steel and has an individual diameter equal to 0.26 mm These cords are obtained by twisting.

[0056] FIG. 7 depicts a face-on view of a tire for an agricultural vehicle with lugged tread. The tire 1 comprises a tread 2 made up of a first and a second row of lugs 21 extending radially outwards from a bearing surface 22 and disposed in a chevron pattern with respect to the equatorial plane of the tire. As described previously, when driven on, this type of tread generates cyclic compressive/tensile loadings of the metal reinforcers of the crown layers, which elastic reinforcers according to the invention, which have a large elongation under tension with a low modulus and a high critical buckling strain, are better able to withstand.

[0057] The invention has been implemented more particularly for an agricultural tire of size 600/70R30 comprising a crown reinforcement with two crown layers with elastic metal reinforcers of formula E18.23 or E24.26.

[0058] The geometric and mechanical characteristics of the two examples of elastic metal reinforcers studied are summarized in Table 1 below:

TABLE-US-00001 TABLE 1 Type of metal Multistrand rope Multistrand rope reinforcer E18.23 E24.26 First extension 21 GPa 17 GPa modulus MG1 Second extension 67 GPa 50 GPa modulus MG2 Ratio MG2/MG1 3.2 2.9 Critical buckling 4.5% 4.4% strain E0 (%) Linear density of the 6.4 g/m 10.7 g/m reinforcer (g/m) Reinforcer diameter D 1.46 mm 1.92 mm (mm) Strand diameter DT 0.70 mm 0.80 mm (mm) Strand helix angle AT 24° 25.5° (°) Strand helix pitch PT 8 mm 6 mm (°) Crown layer breaking 616 N/mm (P = 2.5 mm) 781 N/mm (P = 3 mm) strength R (N/mm) for a reinforcer pitch spacing P in mm

[0059] The inventors tested the invention by comparing the life, from a crown reinforcement endurance viewpoint, of a tire of size 600/70R30, comprising two crown layers with elastic metal reinforcers according to the invention, with that of a reference tire, comprising six crown layers with textile reinforcers. Each tire, inflated to a pressure P equal to 50 kPa and subjected to a load Z equal to 2600 daN was run, on an asphalted surface, under torque, with an applied circumferential loading F.sub.X equal to 520 daN and at a speed V equal to 27 km/h.