Abstract
A tire for an agricultural vehicle with a metal crown reinforcement, with improved endurance of the crown reinforcement thereof through the choice of a suitable tread. For each tread portion (21), positioned axially, with respect to the equatorial plane (E) of the tire (1), at an axial distance DE at most equal to 0.36*L, and having an axial width LE equal to 0.08*L, the product TEVL*(H/B) of the local volumetric void ratio of the tread portion (21) and the circumferential slenderness H/B of each tread pattern element (22) of said tread portion (21) is at most equal to 0.35.
Claims
1. A tire for an agricultural vehicle, having a nominal section width L, within the meaning of the ETRTO standard, and comprising, radially from the outside to the inside, a tread and a crown reinforcement; the tread comprising tread pattern elements that are separated from one another by voids and extend radially towards the outside from a bearing surface to a tread surface, the tread having a volumetric void ratio TEV, defined as the ratio between the volume of voids VC and the total volume of the tread assumed to be free of voids V, comprised between the bearing surface and the tread surface, each tread pattern element having a circumferential slenderness H/B, H being the mean radial height between the bearing surface and the tread surface and B being the mean circumferential length of the tread pattern element, each tread portion, positioned axially, with respect to an equatorial plane (E) of the tire, at an axial distance DE, having an axial width LE and a local volumetric void ratio TEVL, defined as being the ratio between the volume VCL of the voids and the total volume VL of said tread portion, comprised between the bearing surface and the tread surface, the crown reinforcement comprising at least two crown layers, each comprising metal reinforcers that are coated in an elastomeric material, are mutually parallel and form an angle at least equal to 10° with a circumferential direction (XX′), wherein, for each tread portion positioned axially, with respect to the equatorial plane (E) of the tire, at an axial distance DE at most equal to 0.36*L, and having an axial width LE equal to 0.08*L, the product TEVL*(H/B) of the local volumetric void ratio of the tread portion and the circumferential slenderness H/B of each tread pattern element of said tread portion is at most equal to 0.35.
2. The tire according to claim 1, wherein the volumetric void ratio TEV of the tread is at least equal to 35%.
3. The tire according to claim 1, wherein the mean radial height H of each tread pattern element is at least equal to 20 mm.
4. The tire according to claim 1, wherein the mean radial height H of each tread pattern element is at most equal to 50 mm.
5. The tire according to claim 1, having, in a given circumferential plane (XZ), a circumferential void ratio TEC1 in the new state, measured along the curve (C1) of intersection between the circumferential plane (XZ) and the tread surface in the new state, TEC1 being defined as the ratio between the circumferential void length LC1 and the total circumferential length L1, and the tire having, in the circumferential plane (XZ), a circumferential void ratio TEC2 in the worn state, measured along the curve (C2) of intersection between the circumferential plane (XZ) and the tread surface in the worn state, the tread surface in the worn state being radially positioned on the outside of the bearing surface at a radial distance HR, TEC2 being defined as the ratio between the circumferential void length LC2 and the total circumferential length L2, wherein, in each circumferential plane (XZ) axially positioned at at most 0.4*L, the circumferential void ratio TEC1 in the new state is at least equal to 1.45 times the circumferential void ratio TEC2 in the worn state.
6. The tire according to claim 1, wherein the tread is made up of at least 5 circumferential rows of tread pattern elements that are separated from one another by substantially circumferential voids extending around the entire circumference of the tire, wherein the tread comprises transverse voids extending continuously from one axial edge of the tread to the other.
7. The tire according to claim 1, wherein the tread is made up of at least 5 circumferential rows of tread pattern elements that are separated from one another by substantially circumferential voids extending around the entire circumference of the tire, wherein the tread comprises transverse voids extending discontinuously from one axial edge of the tread to the other, such that the tread pattern elements of a given circumferential row have an angular offset in the circumferential direction (XX′) with respect to those of an adjacent row.
8. The tire according to claim 1, wherein the tread comprises a total number N of tread pattern elements, each tread pattern element comprising a contact face, a leading face and a trailing face, said leading face being inclined by an angle A towards the rear with respect to the radial direction (ZZ′) in the direction of running (R) of the tread, said tread comprising a number N1 of tread pattern elements for which the angle α is comprised between 50 degrees and 75 degrees, the number N1 being at least equal to 0.2×N.
9. The tire according to claim 1, 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 E0 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.
Description
[0060] The features of the invention are illustrated by the schematic FIGS. 1 to 12, which are not drawn to scale:
[0061] FIG. 1: Meridian half-section of a tire for an agricultural vehicle according to the invention
[0062] FIG. 2: Perspective view of a tire for an agricultural vehicle according to a first embodiment of the invention
[0063] FIG. 3: Face-on view of a tire for an agricultural vehicle according to a first embodiment of the invention
[0064] FIG. 4: Detail of the tread of a tire for an agricultural vehicle according to a first embodiment of the invention
[0065] FIG. 5: Circumferential section through the tread of a tire for an agricultural vehicle according to a first embodiment of the invention
[0066] FIG. 6: Detail of the circumferential section through the tread of a tire for an agricultural vehicle according to a first embodiment of the invention
[0067] FIG. 7: Perspective view of a tire for an agricultural vehicle according to a second embodiment of the invention
[0068] FIG. 8: Face-on view of a tire for an agricultural vehicle according to a second embodiment of the invention
[0069] FIG. 9: Face-on view of a tire for an agricultural vehicle according to a third embodiment of the invention
[0070] FIG. 10: Circumferential section through the tread of a tire for an agricultural vehicle according to a third embodiment of the invention
[0071] FIG. 11: Typical example of a typical tensile force-elongation curve for an elastic metal reinforcer coated with an elastomeric material
[0072] FIG. 12: Typical example of a compressive force-compressive strain curve for an elastic metal reinforcer, obtained on a test specimen made of elastomeric material
[0073] FIG. 1 depicts a half-view in meridian section of a tire 1 for an agricultural vehicle, in a meridian plane YZ passing through the axis of rotation YY′ of the tire. The tire 1 has a nominal section width L, within the meaning of the ETRTO standard—only a half-width L/2 is depicted—and comprises 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 (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 (not depicted) at least equal to 85° and at most equal to 95° with the circumferential direction XX′. The tread 2 comprises tread pattern elements 22 that are separated from one another by voids 23 and extend radially towards the outside from a bearing surface 24 to a tread surface 25. Also depicted, with hatching, is a tread portion 21, positioned axially, with respect to the equatorial plane E of the tire, at an axial distance DE at most equal to 0.36*L, and having an axial width LE equal to 0.08*L. According to the invention, for such a tread portion 21, the product TEVL*(H/B) of the local volumetric void ratio of the tread portion 21 and the circumferential slenderness H/B of each tread pattern element 22 of said tread portion 21 is at most equal to 0.35. The local volumetric void ratio TEVL is defined as being the ratio between the volume VCL of the voids 23 and the total volume VL of said tread portion 21, comprised between the bearing surface 24 and the tread surface 25. The circumferential slenderness H/B is the ratio between the mean radial height H between the bearing surface 24 and the tread surface 25 and B being the mean circumferential length (not depicted) of the tread pattern element 22.
[0074] FIGS. 2 and 3 are, respectively, a perspective view and a face-on view of a tire 1 for an agricultural vehicle according to a first embodiment of the invention. According to this first embodiment, the tread 2 is made up of seven circumferential rows 20 of tread pattern elements 22 extending radially outward from a bearing surface 24 as far as the tread surface 25, and separated from one another by voids 23. The voids 23 are either circumferential voids 231 extending around the entire circumference of the tire, or transverse voids 232 extending continuously from one axial edge 27 of the tread to the other. In the case depicted, the tread pattern elements constitute chevron motifs. FIG. 3 depicts the detail C of the tread, which forms the subject of FIG. 4, and the circumferential plane XZ, according to the circumferential section A-A, that forms the subject of FIG. 5.
[0075] FIG. 4 is a detail of the tread of a tire 1 for an agricultural vehicle according to the first embodiment of the invention. This detail C depicts, in particular, in the form of hatching, a tread portion 21, positioned axially, with respect to the equatorial plane E of the tire, at an axial distance DE at most equal to 0.36*L, and having an axial width LE equal to 0.08*L, for which, according to the invention, the product TEVL*(H/B) of the local volumetric void ratio of the tread portion 21 and the circumferential slenderness H/B of each tread pattern element 22 of said tread portion 21 is at most equal to 0.35.
[0076] FIG. 5 is a circumferential section through the tread of a tire for an agricultural vehicle according to the first embodiment of the invention. Depicted on this section A-A are the mean radial height H between the bearing surface 24 and the tread surface 25, and the mean circumferential length B of the tread pattern element 22, extending radially towards the outside from a bearing surface 24 as far as a tread surface 25. The mean circumferential length B is the mean distance separating the leading face and the trailing face of the tread pattern element 22.
[0077] FIG. 6 is a detail of the circumferential section through the tread of a tire for an agricultural vehicle according to the first embodiment of the invention. This detail D depicts a tread pattern element 22, separated from the adjacent tread pattern elements by voids 23. In a given circumferential plane XZ, the curve C1 of intersection between the circumferential plane XZ and the tread surface 25 when new can be used to define a circumferential void ratio TEC1 when new, this being defined as being the ratio between the circumferential void length LC1 and the total circumferential length L1, the tread surface 25 when new being positioned radially on the outside of the bearing surface 24 at a radial distance H. Similarly, the curve C2 of intersection between the circumferential plane XZ and the tread surface 26 when worn can be used to define a circumferential void ratio TEC2 when worn, this being defined as being the ratio between the circumferential void length LC2 and the total circumferential length L2, the tread surface 26 when worn being positioned radially on the outside of the bearing surface 24 at a radial distance HR. Advantageously, the circumferential void ratio TEC1 when new is at least equal to 1.45 times the circumferential void ratio TEC2 in the worn state.
[0078] FIGS. 7 and 8 are, respectively, a perspective view and a face-on view of a tire 1 for an agricultural vehicle according to a second embodiment of the invention. According to this second embodiment, the tread 2 is made up of seven circumferential rows 20 of tread pattern elements 22 separated from one another by voids 23. The voids 23 are either circumferential voids 231 extending over the entire circumference of the tire, or transverse voids 232 extending discontinuously from one axial edge 27 of the tread 2 to the other so that the tread pattern elements 22 of a given circumferential row 20 are angularly offset in the circumferential direction relative to those of an adjacent row.
[0079] FIG. 9 is a face-on view of a tire 1 for an agricultural vehicle according to a third embodiment of the invention. In this third embodiment, the tread 2 comprises a total number N of tread pattern elements 22, each tread pattern element 22 comprising a contact face 221, a leading face 222 and a trailing face 223, said leading face being inclined by an angle α towards the rear with respect to the radial direction ZZ′ in the direction of running R of the tread 2, said tread 2 comprising a number N1 of tread pattern elements 22 for which the angle α is comprised between 50 degrees and 75 degrees, the number N1 being at least equal to 0.2×N. Each tread pattern element 22 therefore comprises a contact face 221, a leading face 222 and a trailing face 223. The contact face is the face, at the crown, of the tread pattern element 22 that is intended to roll and bear the load on firm ground. On loose ground, the tread pattern elements 22 can sink into the ground. In the preferred direction of running of the tire, the leading face 222 is thus the face that is the first to enter the contact patch and can transmit a driving force, while the trailing face is the face that is the last to leave the contact patch. The trailing face 223 can only transmit force to the ground during a braking or reversing phase.
[0080] FIG. 10 depicts the section A-A from the face-on view of the tire shown in FIG. 9. This section makes it possible to clearly see the orientation of the leading faces of the tread pattern elements 22. The leading faces are inclined with respect to the radial direction Z in the opposite direction to the preferred direction of running R and form an angle α with this radial direction Z. In this example, the angle α is equal to 60° and therefore comprised between 50° and 70°.
[0081] FIG. 11 is a typical example of a tensile force-relative elongation curve for an elastic metal reinforcer according to one particular embodiment of elastic metal reinforcer, 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 this embodiment, 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.
[0082] FIG. 12 is a typical example of a compressive force-compressive strain curve for an elastic metal reinforcer according to the particular embodiment of elastic metal reinforcer described hereinabove, 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 approximately equal to 5% and therefore greater than 3%.
[0083] The invention was implemented on a tire for an agricultural vehicle of dimension 600/70 R 30, having a nominal section width L equal to 600 mm and comprising a tread having a volumetric void ratio TEV equal to 50% and a crown reinforcement comprising two crown layers of which the reinforcers are elastic metal reinforcers of formulae E18.23 or E24.26.
[0084] For a tread portion such as that depicted in FIG. 4, positioned axially, with respect to the equatorial plane E of the tire, at an axial distance DE equal to 79 mm, and therefore less than 0.36*L=216 mm, and having an axial width LE equal to 0.08*L=48 mm, the local volumetric void ratio TEVL is equal to 63% and the circumferential slenderness H/B of any tread pattern element is equal to 0.36, the mean radial height H being equal to 44 mm and the mean circumferential length B being equal to 124 mm Under such conditions, the product TEVL*(H/B) is equal to 0.22, and therefore less than 0.35, according to the invention.
[0085] In addition, in a circumferential plane positioned axially in the tread portion as depicted in FIG. 4, outside of the circumferential groove, the circumferential void ratio TEC1 when new is equal to 38% and the circumferential void ratio TEC2 when worn is equal to 17%, and therefore TEC1 is equal to 2.24 times TEC2, and therefore greater than 1.45 times TEC2, according to a preferred embodiment of the invention.
[0086] In comparison with an agricultural-vehicle tire of the prior art, with a lugged tread and a metal crown reinforcement, and operating at low pressure, such as an IF (Improved Flexion) tire or a VF (Very high Flexion) tire, the inventors have observed an improvement in the endurance of the crown reinforcement for a tire with a tread having low circumferential tilt as described in the invention.
