Bead for a tire for a civil-engineering heavy vehicle

Abstract

Improving the endurance of the beads (1) of a radial tire for a civil-engineering heavy vehicle by proposing a solution which blocks the propagation of the cracks initiated in the coating elastomer of the bead reinforcing layer (5), by inserting a cushion rubber (6) interposed between the coating elastomer of the carcass layer turn-up (312) and the coating elastomer of the bead reinforcing layer (5). The elastic modulus in extension of the cushion rubber (6) measured at 100% deformation must be less than the elastic modulus of the coating compound of the carcass layer. Still according to a disclosed embodiment, the thickness of the cushion rubber (6) is at least equal to the thickness of the bead reinforcing layer (5).

Claims

1. A tire for a heavy vehicle, comprising: two beads each intended to come into contact with a rim by way of a contact face, said contact face comprising a first axial portion substantially parallel to an axial direction parallel to the axis of rotation of the tire, connected by a second circular portion, having a centre, to a third radial portion, parallel to a radial direction, said tire comprising a carcass reinforcement connecting the two beads to one another and comprising at least one carcass layer made up of reinforcers coated in an elastomeric coating compound M1 having an elastic modulus in extension at 100% elongation E1, said carcass layer comprising a main part wrapping in each bead, from the inside to the outside of the tire, around a bead wire core inscribed in a circle having a centre G and a radius R, to form a turn-up, a bead reinforcing layer having a thickness and made up of reinforcers coated in an elastomeric coating compound M2 having an elastic modulus in extension at 100% elongation E2 and forming, with the radial direction, an angle A at most equal to 45°, wherein the bead reinforcing layer comprises a first leg extending along the first axial portion of the contact face and a second leg extending radially and a curved portion between the first and second legs, wherein an axial inner end of the first leg is radially inside the bead; a radially outer portion of the second leg of said bead reinforcing layer being at least partially in contact with the carcass reinforcement and extending on the outside of the carcass reinforcement from a first end radially on the outside of an axial straight line (DY) passing through the centre G of the circle circumscribed on the bead wire core as far as a second end axially on the inside of a radial straight line (DZ) passing through the centre G of the bead wire core, wherein each bead comprises a cushion rubber arranged directly between an axially inner side of the bead reinforcing layer and axially outer side of the carcass layer turn-up in an area of the bead and is made up of an elastomeric compound M3 having an elastic modulus in extension at 100% elongation E3, said cushion rubber being interposed between the carcass reinforcement and the bead reinforcing layer and extending over an angular sector defined by the angle formed by the straight lines passing respectively through the centre G of the bead wire core and the radially outer end of the cushion rubber and through the centre G of the bead wire core and the radially inner end of the cushion rubber; wherein said cushion rubber has a maximum thickness Emax, at least equal to the thickness Er of the bead reinforcing layer, measured on the straight line passing through the centre of the second circular portion of the contact face and perpendicular to the carcass reinforcement, wherein the angular sector of the cushion rubber is greater than or equal to 45° and less than or equal to 90°, and in that the elastic modulus at 100% elongation E3 of the elastomeric compound M3 making up the cushion rubber is less than the elastic modulus at 100% elongation E1 of the elastomeric coating compound M1 of the carcass layer.

2. The tire according to claim 1, wherein the radially inner end of the cushion rubber is positioned, with respect to the radial straight line passing through the centre G of the bead wire, at an axial distance greater than or equal to 10 mm.

3. The tire according to claim 1, wherein the elastic modulus in extension at 100% elongation E3 of the elastomeric compound M3 making up the cushion rubber is at most equal to 60% of the elastic modulus at 100% elongation E1 of the elastomeric coating compound M1 of the carcass layer.

4. The tire according to claim 1, wherein the elastic modulus in extension at 100% elongation E3 of the elastomeric compound M3 making up the cushion rubber is less than or equal to the elastic modulus in extension at 100% elongation E2 of the elastomeric coating compound M2 of the bead reinforcing layer.

5. The tire according to claim 1, wherein the elastic modulus in extension at 100% elongation E3 of the elastomeric compound M3 making up the cushion rubber is equal to 1.6 MPa.

6. The tire according to claim 1, wherein the elastic modulus in extension at 100% elongation E1 of the elastomeric coating compound M1 of the carcass layer is equal to 2.6 MPa.

7. The tire according to claim 1, wherein the reinforcers of the bead reinforcing layer form, with the radial direction of the tire, an angle at least equal to 22° and at most equal to 28°.

8. The tire according to claim 1, wherein the reinforcers of the bead reinforcing layer form, with the radial direction of the tire, a variable angle at least equal to 25° on one part of the layer and at most equal to 45° on the remaining part.

9. The tire according to claim 1, wherein the thickness Er of the bead reinforcing layer is less than or equal to 2.5 mm.

10. The tire according to claim 1, wherein the cushion rubber is arranged radially outside the bead wire and extends radially from a radially inner portion of the bead wire core inscribed in the circle to at least the axial straight line (DY) passing through the centre G of the circle circumscribed on the bead wire core.

11. The tire according to claim 1, wherein the tire for a heavy vehicle configured to bear a load of 104 tonnes at a use pressure of 6.8 bar.

Description

DETAILED DESCRIPTION OF THE EMBODIMENTS

(1) The features of the invention will be better understood with the aid of the description of the attached FIGS. 1, 2 and 3:

(2) FIG. 1 represents a sectional view in a meridian plane of the bead of a tire for a civil-engineering heavy vehicle of the prior art.

(3) FIG. 2 represents a sectional view in a meridian plane of the bead of a tire for a civil-engineering heavy vehicle, according to the invention.

(4) FIG. 3 represents a sectional view in a meridian plane of the bead of a tire for a civil-engineering heavy vehicle, according to the invention, defining the angular positioning of the cushion rubber.

(5) In order to make them easier to understand, FIGS. 1, 2 and 3 are not drawn to scale.

(6) FIG. 1 shows a bead 1 of a tire for a civil-engineering heavy vehicle of the prior art, mounted on a rim 2, comprising: a carcass reinforcement 3 comprising a single carcass layer 31 made up of metallic reinforcing elements coated with an elastomeric coating material, with a main carcass layer part 311 wrapping, from the inside towards the outside of the tire, around a bead wire core 4, to form a carcass layer turn-up 312; a filler element 91 radially outwardly extending the bead wire core 4, having, in every meridian plane, a substantially triangular cross section and being made up of an elastomeric filler material; a protection element 7 axially inwardly extending a sidewall 8 and made up of an elastomeric protection material; a filling element 92 axially on the inside of the protection element 7 and of the sidewall 8 and axially on the outside of the carcass reinforcement turn-up 312 and made up of an elastomeric filling material; a bead reinforcing layer 5 extending between a radially outer end 51 and a radially inner end 52, axially on the outside of and in contact with the carcass layer turn-up 312.

(7) FIG. 2 shows a bead of a tire for a civil-engineering heavy vehicle, according to the invention, comprising: a carcass reinforcement 3 comprising a single carcass layer 31 made up of metallic reinforcing elements coated with an elastomeric coating material, with a main carcass layer part 311 wrapping, from the inside towards the outside of the tire, around a bead wire core 4, to form a carcass layer turn-up 312; a filler element 91 radially outwardly extending the bead wire core 4, having, in every meridian plane, a substantially rectangular cross section and being made up of an elastomeric filler material; a protection element 7 axially inwardly extending a sidewall 8 and made up of an elastomeric protection material; a filling element 92 axially on the inside of the protection element 7 and of the sidewall 8 and axially on the outside of the carcass reinforcement turn-up 312 and made up of an elastomeric filling material; a bead reinforcing layer 5 extending between a radially outer end 51 and a radially inner end 52, axially on the outside of and in contact with the carcass layer turn-up 312; a cushion rubber 6 in contact, via an axially inner face, with the elastomeric coating material of the axially outer face of the carcass layer turn-up 312 and, via an axially outer face, with the elastomeric coating material of the bead reinforcing layer 5.

(8) FIG. 3 shows a zoom on the bead to demonstrate the positioning of the cushion rubber: the respective radially outer and radially inner ends 61 and 62 which delimit the cushion rubber 6; the angular sector 100 of the cushion rubber 6, defined by the angle formed by the straight lines passing respectively through the centre G of the bead wire core 4 and the radially outer end 61 of the cushion rubber 6 and through the centre G of the bead wire core 4 and the radially inner end 62 of the cushion rubber 6; the bead wire core 4 of centre G, of hexagonal shape.

(9) The invention was more particularly studied in the case of a tire for a dumper-type heavy vehicle of dimension 59/80R63, designed to bear a load of 104 tonnes at a use pressure of 6.8 bar.

(10) The cushion rubber is placed on the axially outer face of the carcass layer turn-up with a curvilinear length of 60 mm between a radially outer end 61 and a radially inner end 62. Its angular sector measured with respect to the centre G of the bead wire between the segments joining this centre G and, on the one hand, the radially outer end of the cushion rubber and, on the other hand, its radially inner end is 80°. In this embodiment of the invention, the maximum thickness of the cushion rubber is equal to 8 mm, measured on the axis orthogonal to the carcass reinforcement that passes through the centre of the fillet 12 connecting the axial part 13 and radial part 11 of the rim flange.

(11) The carcass reinforcement is made up of a carcass layer formed by metallic reinforcers coated in an elastomeric compound. The metallic reinforcers are cords made up of 189 threads of 23 hundredths of a millimetre in diameter each.

(12) The elastomeric coating compound of the carcass layer is obtained according to the following composition, expressed in parts per hundred elastomer (phr):

(13) TABLE-US-00001 TABLE NO. 1 Elastomer NR Carbon OPF (Natural black Stearic Epoxy Cobalt Composition Rubber) N326 Antioxidant ZnO acid Sulfur DCBS resin salt CTP Elastomeric 100 47 1.5 7.5 0.9 5.63 0.8 0.5 1.5 0.15 coating compound of the carcass

(14) With the above composition, the elastomeric coating compound of the carcass layer has an elastic modulus in extension measured at 100% deformation with a value of 2.6 MPa, and at 10% deformation the elastic modulus in extension has a value of 5.2 MPa.

(15) In a simplified embodiment of the invention, the elastomeric coating compound of the reinforcing layer is made up of the same elastomeric coating compound as that of the carcass layer.

(16) The cushion rubber has an elastic modulus in extension measured at 100% deformation with a value of 1.6 MPa, and at 10% deformation the elastic modulus in extension has a value of 3.2 MPa.

(17) The cushion rubber is significantly softer than the elastomeric coating compound of the carcass layer: at 100% deformation, the elastic modulus in extension of the cushion rubber represents 60% of that of the reinforcing layer. This offset in the elastic modulus in extension is found more or less for all the levels of deformation.

(18) The 59/80R63 tire was designed according to the invention, as depicted in FIG. 2.

(19) Simulations of finite-element calculations were carried out respectively on a reference tire, as depicted in FIG. 1, and on a tire according to the invention, as depicted in FIG. 2.

(20) The inventors established that the deformation energy density is the relevant physical quantity for analysing the propagation of the cracks. They considered the peak-to-peak evolution of the deformation energy density over one wheel revolution, calculated in the elastomeric coating compounds of the carcass and reinforcing layers of the bead for the reference tire and the tire of the invention. For the tire of the invention, the deformation energy density was also calculated in the cushion rubber.

(21) The inventors also performed parametric sensitivity calculations in relation to the angle formed between the reinforcers of the reinforcing layer of the bead and the direction ZZ′ for the purpose of determining the optimal angle for maximizing the circumferential rigidities of the bead.

(22) The results of the calculations are compared between the reference tire and the tire of the invention:

(23) TABLE-US-00002 TABLE NO. 2 Deformation energy density calculated in the cracking region Coating Coating compound of compound Degree of the of the participation reinforcing carcass of the cushion Region layer layer rubber Reference tire 100 100 NA Tire of the invention 110 109 82 provided with a reinforcing layer with reinforcers at 25° Tire of the invention 77 121 66 provided with a reinforcing layer with reinforcers at variable 25°/45° angles

(24) On the reference tire (base 100), the crack is initiated in the elastomeric coating compound of the reinforcing layer and then propagates in the direction of the protection rubber in contact with the rim at the seat.

(25) On the tire of the invention, in the fourth line of Table No. 2, the peak-to-peak variation in the deformation energy density is reduced by approximately 10% in the coating compound of the reinforcing layer. The gain is approximately 9% in the coating elastomer of the carcass layer. The cushion rubber participates to the amount of 82% in this improvement, which is sufficient to prevent the propagation of the crack.

(26) In one embodiment of the invention with a reinforcing layer provided with reinforcers at variable angles, in this instance 25° and 45°, the gain is clearly perceptible in the coating elastomer of the carcass layer (121%), but the peak-to-peak variations in the deformation energy density continue to be high in the coating elastomer of the reinforcing layer. Here, too, the invention operates, with the crack not propagating.

(27) The results of optimizing the angle of the reinforcing layer converged towards an angle value of 25°.

(28) The invention should not be interpreted as being restricted to the example illustrated in FIG. 2, but may be extended to other variant embodiments, such as, for example, and in a non-exhaustive manner, relating to the number of elastomeric transition materials comprised between the elastomeric coating material and the elastomeric filling material.

(29) The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.