Agricultural Vehicle Tire with Lightweight Crown

Abstract

A radial tire (1) for an agricultural vehicle, which tire has a speed index at least equal to D for a maximum speed equal to 65 km/h, has a load index at least equal to 140 and at most equal to 170, and has a tread pattern comprising lugs with a radial height at least equal to 45 mm, having a crown reinforcement (3), consisting of 3 crown layers (31, 32, 33), each having reinforcing elements made of PET having an ultimate tensile strength at least equal to 32 daN, and the angles ANT1 and ANT3 of the reinforcing elements of the radially inner and outer layers (31, 33) with the circumferential direction have the same orientation and are less than 20°, and the angle ANT2 of the reinforcing elements of the median layer (32) with the circumferential direction has an opposite orientation to ANT1 and is greater than 10° plus the absolute value of the average of ANT1 and ANT3.

Claims

1. A radial tire for a vehicle of the agricultural and NF (“Normal Flexion”) type, which, within the meaning of the 2020 ETRTO (“European Tire and Rim Technical Organization”) standard, has a speed index at least equal to D for a maximum speed equal to 65 km/h, which tire is intended to be mounted on a rim with a diameter at least equal to 28 inches, and has a load index at least equal to 140 and at most equal to 170, comprising: a tread intended to come into contact with the ground, which tread has an axial width LT and comprises a central portion with an axial width LT/2 and two axially outer portions, each with an axial width LT/4, the two axially outer portions comprising tread pattern blocks with a radial height at least equal to 45 mm, these blocks covering at least 80% of the axial width LT/4 of the axially outer portions of the tread and having a circumferential thickness ep at least equal to 42 mm, a crown reinforcement radially on the inside of the tread, comprising 3 working layers a radially innermost working layer a radially outermost working layer and an intermediate working layer, each working layer comprising mutually parallel textile reinforcing elements made of PET (polyethylene terephthalate), which are coated in a rubber material and form angles at least equal to 12° and at most equal to 45° with the circumferential direction of the tire, the reinforcing elements of the working layers having an ultimate tensile strength at least equal to 32 daN, wherein the angles (ANT1, ANT3) respectively formed by the reinforcing elements of the radially innermost working layer and the radially outermost working layer with the circumferential direction are at most equal to 20° and have the same orientation, wherein the angle ANT2 formed by the reinforcing elements of the intermediate working layer with the circumferential direction has an opposite sign to the angles ANT1 and ANT3, wherein the absolute value of the angle ANT2 is greater than 10° plus the absolute value of the average of the angles ANT1 and ANT3.

2. The tire according to claim 1, wherein the reinforcing elements of the working layers consist of two PET strands, one strand being an assembly of elementary fibres.

3. The tire according to claim 2, wherein each PET strand of the reinforcing elements of the working layers has a linear density at least equal to 320 g per km.

4. The tire according to claim 3, wherein the reinforcing elements of the working layers are cords which consist of two PET strands with a linear density of between 320 and 360 g per km and are spaced apart at a pitch of between 1.2 mm and 1.5 mm.

5. The tire according to claim 4, wherein the absolute value of the angles ANT1 and ANT3 is between 13° and 17° and the absolute value of the angle ANT2 is between 27° and 33°.

6. The tire according to claim 5, wherein the absolute value of the angles ANT1 and ANT3 is between 14.5° and 15.5° and the absolute value of the angle ANT2 is between 29.5° and 30.5°.

Description

[0035] FIG. 1: shows a diagram of a meridian half-section through the tire according to the invention,

[0036] FIG. 2: shows a segment of the tread pattern of the tire according to the invention.

[0037] FIG. 1 [FIG. 1] shows a segment of the crown of the tire according to the invention in a meridian plane YZ passing through the axis of rotation YY of the tire. The tire 1 for an agricultural vehicle for a medium-power tractor 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 3 crown layers (31, 32, 33), each comprising mutually parallel textile reinforcing elements which are composed of two PET strands coated in an elastomeric material and form an angle A (not shown) at least equal to 15° and at most equal to 45° with a circumferential direction (XX′). The carcass reinforcement 4 comprises one or more carcass layers comprising mutually parallel textile reinforcing elements that are coated in an elastomeric material and form an angle (not shown) at least equal to 85° and at most equal to 95° with the circumferential direction (XX′). The tread 2 comprises tread pattern blocks 21, in this instance continuous blocks usually referred to as lugs with a radial height h at least equal to 45 mm. The tread has an axial width LT and comprises a central portion 23 having an axial width LT/2 and two axially outer portions 24, each having an axial width LT/4. The lugs or tread pattern blocks have an axial width Lba greater than 0.8*LT/4.

[0038] FIG. 2 [FIG. 2] shows a segment of the tread 2 of a tire for an agricultural vehicle. The tread 2 has an axial width LT and comprises tread pattern blocks 21, in the present case lugs, and voids 22 that enable good grip on loose ground. The tread pattern blocks 21 on the axially outer portions of the tire have an axial width Lba greater than 80% of LT/4. When the tire is running, since the thickness of the lugs increases the thermal stresses, this type of tread generates cycles of compressive/tensile loading in the reinforcers of the crown layers, for which it is necessary to balance the angles particularly well so as to be able to reduce the weight of the crown reinforcement to only 3 crown layers. To resist chunking, the lugs have a circumferential thickness ep, measured halfway up the tread pattern in the circumferential plane passing through the centre of each axially outer portion of the tread, at least equal to 42 mm. This thickness also has an impact on the thermal stresses in this area.

[0039] The invention was implemented more particularly for an agricultural tire of size 650/65R38 NF, specifically one that is not classified as IF or VF according to the 2020 ETRTO standard. The tire according to the prior art comprises 4 crown layers with reinforcing elements consisting of two PET strands having a linear density of 220 g per km, the reinforcing elements being disposed at a pitch of 1.15 mm. The angles formed by the reinforcing elements of the working layers in the median circumferential plane with the circumferential direction, from the radially innermost layer to the radially outermost layer, are −25°, 25°, −25°, 25°. The tread pattern is composed of lugs with a radial height of 60 mm, a circumferential thickness ep measured in the middle of each axially outer portion that is equal to 74 mm, and an axial width representing 51% of the total width of the tread.

[0040] The tire according to the invention is identical to the control except that it comprises 3 crown layers, the reinforcing elements of which are cords consisting of two PET strands with a linear density equal to 344 g per km, the reinforcing elements of the working layers being disposed in the working layers at a pitch equal to 1.35 mm. The PET strand has a force at break equal to 20 daN, and the reinforcing elements have a force at break equal to 35 daN. The angles formed by the reinforcing elements of the working layers in the median circumferential plane with the circumferential direction, from the radially innermost to the radially outermost, are equal to −15°, 30°, −15°.

[0041] The tire according to the invention makes it possible to decrease the weight of the crown layers by 5.2 kg, that is to say 27%, and has been tested in terms of endurance. The inventors tested the invention by comparing the service life, as regards the endurance of the crown reinforcement, of the two tires for medium-power tractors. Each tire was run on a steel rolling road with a diameter of 1.7 m at a speed V equal to 15 km/h, each tire being inflated to a pressure P equal to 260 kPa, specifically the nominal pressure increased by 1 bar, and subjected to load levels equal to 4047 daN for 24 h, then 4856 daN for 24 h, then 6475 daN for 24 hours, and lastly 8094 daN for 328 hours. The tire according to the invention, with a lower weight, travelled an equivalent number of kilometres as the tire according to the prior art, without exhibiting any failure of the reinforcing elements of the working layers, demonstrating the advantage of the invention in realizing a more lightweight tire with equivalent endurance.