Agricultural Vehicle Tire Comprising a Single-Layer Carcass Reinforcement

Abstract

A tire for an agricultural vehicle (1), and in particular the carcass reinforcement (4) thereof, for a tire running at low pressure on loose ground, is to reduce the compaction of the ground and to increase the traction capability by reducing the structural stiffness of the tire. According to the invention, the carcass reinforcement (4) is made up of a single carcass layer (41) having a mean thickness E at most equal to 2 mm and a breaking strength Fr, expressed in daN/cm, that satisfies the relationship:

Fr>=Fs=(Cs*Pmax*10.sup.−3)*((R.sup.2−((R+Rj)/2).sup.2)/2)/Rj, with Fs: reference threshold strength (in daN/cm) Cs: safety factor at least equal to 1, Pmax: recommended maximum inflation pressure (in kPa), R=D/2: outside radius of the tire (in mm), Rj=Dj/2: nominal radius of the rim (in mm).

Claims

1. A tire for an agricultural vehicle, intended to be mounted on a rim having a nominal diameter Dj and to be inflated to a recommended minimum pressure at most equal to 240 kPa, having an outside diameter D and comprising, radially from the outside to the inside, a tread, a crown reinforcement and a carcass reinforcement: the tread comprising tread pattern elements that are separated from one another by voids and having a radial height H at least equal to 20 mm and at most equal to 60 mm, measured between a tread surface and a bearing surface parallel to the tread surface and tangential to the bottoms of the deepest voids, the tread, which has an axial width L, comprising at least two axially outer voids, each having a mean line that forms an angle at most equal to 45° with a circumferential direction (XX′) of the tire, which are positioned on either side of an equatorial plane (XZ) passing through the middle of the tread and are spaced apart from one another by a mean axial distance L1 at least equal to 0.5*L, the tread having an overall volumetric void ratio TEV, defined as the ratio between the volume VC of voids and the total volume V of the tread assumed to be free of voids, comprised between the bearing surface and the tread surface, at least equal to 30% and at most equal to 60%, the crown reinforcement comprising at least two crown layers, each comprising metal reinforcers that are coated in an elastomer-based material, are mutually parallel, are crossed from one layer to the next and form an angle at least equal to 10° with the circumferential direction (XX′) of the tire, the carcass reinforcement comprising at least one carcass layer comprising textile reinforcers that are coated in an elastomer-based material, are mutually parallel and form an angle at least equal to 75° and at most equal to 105° with the circumferential direction (XX′) of the tire, wherein the carcass reinforcement is made up of a single carcass layer (41) having a mean thickness E at most equal to 2 mm and a breaking strength Fr, expressed in daN/cm, that satisfies the relationship:
Fr>=Fs=(Cs*Pmax*10.sup.−3)*((R.sup.2−((R+Rj)/2).sup.2)/Rj, with Fs: reference threshold strength Cs: safety factor at least equal to 1, Pmax: recommended maximum inflation pressure, R=D/2: outside radius of the tire, Rj=Dj/2: nominal radius of the rim.

2. The tire according to claim 1, wherein Cs*Pmax is at least equal to 1000 kPa.

3. The tire according to claim 1, wherein Cs*Pmax is at least equal to 1500 kPa.

4. The tire according to claim 1, wherein the mean thickness E of the carcass layer is at most equal to 1.2 mm.

5. The tire according to claim 1, wherein the textile reinforcers of the single carcass layer comprise an assembly made up of at least one multifilament strand made of aromatic polyamide or aromatic copolyamide and/or of aliphatic polyamide and/or of polyester and/or of cellulose.

6. The tire according to claim 1, wherein the textile reinforcers of the single carcass layer are hybrid textile reinforcers comprising an assembly made up of at least one multifilament strand made of aromatic polyamide or aromatic copolyamide, and of at least one multifilament strand made of aliphatic polyamide.

7. The tire according to claim 5, wherein the carcass layer, in its vulcanized state and when removed from the tire, has a law governing its behaviour under tension having a secant modulus M1 at the equivalent force developed by the carcass layer at 1% elongation such that the ratio M1/Fr of the secant modulus M1 at the equivalent force developed by the carcass layer at 1% elongation to the breaking strength Fr of the carcass layer is strictly less than 7.

8. The tire according to claim 7, wherein the ratio M1/Fr of the secant modulus M1 at the equivalent force developed by the carcass layer at 1% elongation to the breaking strength Fr of the carcass layers is greater than or equal to 0.5.

9. The tire according to claim 5, wherein the hybrid textile reinforcers of the carcass layer comprise an assembly made up of two multifilament strands made of aromatic polyamide or aromatic copolyamide, and of a single multifilament strand made of aliphatic polyamide, the strands being wound together in a helix.

10. The tire according to claim 5, wherein the hybrid textile reinforcers of the carcass layer comprise an assembly made up of a single multifilament strand made of aromatic polyamide or aromatic copolyamide, and of a single multifilament strand made of aliphatic polyamide, the strands being wound together in a helix.

11. The tire according to claim 5, wherein the hybrid textile reinforcers of the carcass layer comprise an assembly made up of a core made up of a first multifilament strand made of aliphatic polyamide, and of a layer comprising at least two second multifilament strands made of aromatic polyamide or aromatic copolyamide, the second strands of the layer being wound together in a helix around the core.

12. The tire according to claim 1, each crown layer having a law governing its behaviour under tension characterized by a secant modulus at break M′=F′r/A′r (in daN/cm/%), F′r being the breaking strength of the crown layer and A′r its elongation at break (in %), wherein the crown reinforcement, comprising at least two crown layers that each have a secant modulus at break M′, has a resultant modulus at break M′s, defined as the sum of the secant moduli at break M′ of all of the crown layers, at least equal to 150 daN/cm/%, preferably at least equal to 300 daN/cm/%.

13. The tire according to claim 12, wherein the reinforcers of the at least two crown layers of the crown reinforcement are metallic.

14. The tire according to claim 13, wherein the metal reinforcers of the at least two crown layers of the crown reinforcement have a law governing their two-way elastic behaviour comprising a first portion having a first tensile modulus MG1 at most equal to 30 GPa, and a second portion having a second tensile modulus MG2 at least equal to 2 times the first tensile modulus MG1, said law governing the behaviour under tension being determined for a metal reinforcer coated in an elastomeric 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 elastomeric compound having a tensile elastic modulus at 10% elongation MA10 at least equal to 5 MPa and at most equal to 15 MPa.

15. The tire according to claim 14, wherein any metal reinforcer of a crown layer is a multi-strand rope of structure 1×N comprising a single layer of N strands, 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.

Description

[0087] The features of the invention are illustrated by the schematic FIGS. 1 to 8, which are not drawn to scale:

[0088] FIG. 1: Meridian half-section of a tire for an agricultural vehicle according to the invention.

[0089] FIG. 2: Perspective view of a tire for an agricultural vehicle according to the invention.

[0090] FIG. 3: View in cross section of a carcass layer portion.

[0091] FIG. 4: Model law governing behaviour under tension for a carcass layer comprising hybrid textile reinforcers according to a preferred embodiment of the invention.

[0092] FIG. 5: Section of a hybrid textile reinforcer for a carcass layer according to a first variant of the preferred embodiment of the invention.

[0093] FIG. 6: Section of a hybrid textile reinforcer for a carcass layer according to a second variant of the preferred embodiment of the invention.

[0094] FIG. 7: Section of a hybrid textile reinforcer for a carcass layer according to a third variant of the preferred embodiment of the invention.

[0095] FIG. 8: Respective laws governing behaviour under tension of a carcass layer comprising hybrid textile reinforcers according to two embodiment variants of the invention, and of a carcass layer comprising textile reinforcers of the prior art considered as reference.

[0096] FIG. 1 is a meridian half-section, in a meridian plane YZ, of a tire 1 for an agricultural vehicle according to the invention. The tire 1 for an agricultural vehicle is intended to be mounted on a rim 5 having a nominal diameter Dj and to be inflated to a recommended minimum pressure at most equal to 240 kPa. It has an outside diameter D and comprises, radially from the outside to the inside, a tread 2, a crown reinforcement 3 and a carcass reinforcement 4. The tread 2 comprises tread pattern elements 22 that are separated from one another by voids 23 and has a radial height H at least equal to 20 mm and at most equal to 60 mm, measured between a tread surface 25 and a bearing surface 24 parallel to the tread surface 25 and tangential to the bottoms of the deepest voids. The tread 2 has an axial width L and comprises two axially outer substantially longitudinal voids 231 which are positioned on either side of an equatorial plane XZ passing through the middle of the tread 2 and are spaced apart from one another by a mean axial distance L1 at least equal to 0.5*L. In the half-section in the figure, only the axial half-width L and the half-distance L½ between an axially outer longitudinal void 231 and the equatorial plane XZ are shown. Moreover, the tread 2 has an overall volumetric void ratio TEV, defined as the ratio between the volume VC of voids 23 and the total volume V of the tread 2 assumed to be free of voids, comprised between the bearing surface 24 and the tread surface 25, at least equal to 30% and at most equal to 60%. The crown reinforcement 3 comprises two crown layers (31, 32), each comprising metal reinforcers that are coated in an elastomer-based material, are mutually parallel, are crossed from one layer to the next and form an angle at least equal to 10° with the circumferential direction XX′ of the tire. The carcass reinforcement 4 comprises a single carcass layer 41 comprising textile reinforcers that are coated in an elastomer-based material, are mutually parallel, are crossed from one layer to the next and form an angle at least equal to 85° and at most equal to 95° with the circumferential direction XX′ of the tire.

[0097] FIG. 2 is a perspective view of a tire 1 for an agricultural vehicle according to the invention. The tread 2 comprises tread pattern elements 22 that are separated from one another by voids 23 and has a radial height H at least equal to 20 mm and at most equal to 60 mm, measured between a tread surface 25 and a bearing surface 24 parallel to the tread surface 25 and tangential to the bottoms of the deepest voids. In the embodiment shown, the raised elements are grouped together into five circumferential rows of blocks. The blocks 22 are separated from one another by voids 23 that are either substantially longitudinal, in the circumferential direction, or substantially transverse, in the axial direction. The tread 2 comprises at least two circumferential distributions of axially outer substantially longitudinal voids 231 that act as hinges and make the flattening of the tread easier. Lastly, the tread 2 has an open tread pattern with an overall volumetric void ratio TEV, defined as the ratio between the volume VC of voids 23 and the total volume V of the tread 2 assumed to be free of voids, comprised between the bearing surface 24 and the tread surface 25, at least equal to 30% and at most equal to 60%.

[0098] FIG. 3 is a view in cross section of a portion of a carcass layer 41, the cross section being taken perpendicularly to the direction of the reinforcers 411. The carcass layer 41 comprises textile reinforcers 411 coated in an elastomer-based material 412. It has a total thickness Et and a trimmed thickness E corresponding to the diameter of the reinforcers. It is a mean value of this trimmed thickness E that, according to the invention, needs to be at least equal to 2 mm. This trimmed thickness E is measured on the cross section of the carcass layer portion. FIG. 2 also shows the spacing P, the distance between the mean lines of two consecutive reinforcers.

[0099] FIG. 4 shows a model law governing behaviour under tension for a carcass layer comprising hybrid textile reinforcers according to a preferred embodiment of the invention. This law governing behaviour represents the unitary tensile force F, expressed in daN/cm, applied to the carcass layer, as a function of its relative elongation A, expressed in %. In the example shown, the breaking strength Fr is equal to 775 daN/cm. In addition, the secant modulus M1 at the equivalent force developed by the carcass layer at 1% elongation is such that the ratio M1/Fr of the secant modulus M1 to the breaking strength Fr is equal to 3.5, and therefore less than 4.

[0100] FIG. 5 shows the section of a hybrid textile reinforcer for a carcass layer according to a first variant of the preferred embodiment of the invention. In this first variant, the hybrid textile reinforcers of the carcass layer comprise an assembly (A/A/N) made up of two multifilament strands made of aramid (A), and of a single multifilament strand made of nylon (N), the strands being wound together in a helix.

[0101] FIG. 6 shows the section of a hybrid textile reinforcer for a carcass layer according to a second variant of the preferred embodiment of the invention. In this second variant of the preferred embodiment of the invention, the hybrid textile reinforcers of the carcass layer comprise an assembly (A/N) made up of a single multifilament strand made of aramid (A), and of a single multifilament strand made of nylon (N), the strands being wound together in a helix.

[0102] FIG. 7 shows the section of a hybrid textile reinforcer for a carcass layer according to a third variant of the preferred embodiment of the invention. In this third variant, the hybrid textile reinforcers of the carcass layer comprise an assembly (N+A/A/A) made up of a core made up of a first multifilament strand made of nylon (N), and of a layer comprising three second multifilament strands made of aramid (A), the second strands of the layer being wound together in a helix around the core. This type of hybrid textile reinforcer is usually referred to as a “Core Insertion” reinforcer.

[0103] FIG. 8 shows the respective laws governing behaviour under tension of a carcass layer comprising hybrid textile reinforcers according to the embodiment variants shown in FIGS. 5 and 7, and of a carcass layer comprising textile reinforcers of the prior art considered as reference. Each law governing behaviour represents the unitary tensile force F, expressed in daN/cm, applied to the carcass layer, as a function of its relative elongation A, expressed in %. The law governing behaviour I1 relates to a carcass layer comprising hybrid textile reinforcers of the carcass layer comprising an assembly (A/A/N) made up of two multifilament strands made of aramid (A), and of a single multifilament strand made of nylon (N), the strands being wound together in a helix. The law governing behaviour I2 relates to a carcass layer comprising hybrid textile reinforcers comprising an assembly (N+A/A/A) made up of a core made up of a first multifilament strand made of nylon (N), and of a layer comprising three second multifilament strands made of aramid (A), the second strands of the layer being wound together in a helix around the core. The law governing behaviour E relates to a carcass layer comprising textile reinforcers comprising an assembly (PET/PET) of two multifilament strands made of polyester of the PET (polyethylene terephthalate) type of the reference prior art. For the respective laws governing behaviour I1 and I2, the secant moduli M1 at the equivalent force developed by the carcass layer at 1% elongation are such that the ratios M1/Fr of the secant modulus M1 to the breaking strength Fr of the carcass layer are respectively equal to 3.5 and 0.8, and therefore less than 4. The breaking strengths Fr are respectively equal to 775 daN/cm for I1 and 641 daN/cm for 12. The above values are to be compared with the reference prior art, for which the ratio M1/Fr is equal to 7.7, for a breaking strength Fr equal to 213 daN/cm. Thus, the carcass layers according to the invention have a secant modulus M1 at 1% elongation that is lower than the reference, and a higher breaking strength Fr, the latter being achieved for a relative elongation at break Ar at least 50% less than that of the reference.

[0104] The invention was studied more particularly in the case of a tire for an agricultural vehicle of size 710/70R42 VF, i.e. having, according to the definitions in the ETRTO standard, a section width S=710 mm, a ratio of the section height to the section width H/S=70% and a mounting rim having a diameter Dj=42 inches=1066.8 mm. This tire is also a VF (Very high Flexion) tire. From the above features, it is possible to deduce the outside radius of the tire R=Rj+H, with Rj, the rim radius, being equal to Dj/2=533.4 mm and H=0.7*S=497 mm With these conditions, R is equal to 533.4+497=1030.4 mm Taking Cs*Pmax to be equal to 1000 kPa, the reference threshold strength Fs is equal to Fs=(Cs*Pmax*10.sup.−3)*((R.sup.2−((R+Rj)/2).sup.2)/2)/Rj=10.sup.3*10.sup.−3*(1030.4.sup.2−((1030.4+533.4)/2).sup.2)/2)533.4=422 daN/cm. Consequently, the carcass reinforcement, made up of a single carcass layer, must have a mean thickness E at most equal to 2 mm and a breaking strength Fr at least equal to Fs=422 daN/cm.

[0105] Table 1 below presents the characteristics of the respective textile reinforcers of the compared carcass layers, corresponding to those of which the laws governing behaviour are presented in FIG. 8, namely the reference carcass layer with textile reinforcers comprising an assembly of 2 strands made of PET, a carcass layer according to a first embodiment I1 with hybrid textile reinforcers comprising an assembly made up of two multifilament strands made of aramid, and of a single multifilament strand made of nylon, and a carcass layer according to a second embodiment 12 with hybrid textile reinforcers comprising an assembly made up of a core made up of a first multifilament strand made of nylon, and of a layer comprising three second multifilament strands made of aramid.

TABLE-US-00001 TABLE 1 Carcass layer E I1 I2 Nature of the PET/PET Aramid/ Nylon/1 + strands Aramid/Nylon Aramid/3 Counts of the 144/144 330/330/188 47 + 167/167/167 strands (tex) Twists T of the 420/420 270/270 340 + 315/315 strands (turns/m) Twist factor K 192 212 196 Density (number of 120 67 81 reinforcers/dm) Reinforcer diameter 0.62 mm 1.11 mm 0.92 mm d (mm) Layer thickness Et 1.41 mm 1.90 mm 1.80 mm (mm) Ratio d/Et 0.44 0.58 0.51

[0106] Definition of the characteristics in Table 1: [0107] Count of the strands (tex): mass in g of a multifilament strand of length equal to 10 000 m (1 tex=1 g/10 000 m). [0108] Twist T of the strands (turns/m): number of twist turns applied. The 1.sup.st value is the twist applied to a multifilament strand in a first direction of twisting S, and the 2.sup.nd value is the twist applied to the assembly of elementary multifilament strands in a second direction of twisting Z, opposite to the first. [0109] Twist factor K: The twist factor K is defined by the relationship K=T×[(Count/(1000.ρ)].sup.1/2, in which the twist T of the assembly of elementary multifilament strands, that is to say of the reinforcer, is expressed in the number of turns per metre, the count of the reinforcer is expressed in tex (mass in g of a reinforcing element having a length equal to 10 000 m), and lastly p is the density or mass per unit volume (in g/cm3) of the material (for example: around 1.50 g/cm3 for cellulose, 1.44 g/cm3 for aramid, 1.38 g/cm3 for a polyester such as PET, 1.14 g/cm3 for nylon). In the case of a hybrid reinforcer, p is a mean of the densities weighted by the respective counts of the materials of the reinforcing element. [0110] Density (number of reinforcers/dm): Number of reinforcers per dm of carcass layer. It is the reverse of the spacing P separating the mean lines of two consecutive reinforcers. [0111] Diameter d of the reinforcer (mm) Diameter of the circle circumscribed on the assembly of multifilament strands making up the reinforcer. [0112] Layer thickness Et (mm): Total thickness of the carcass layer, equal to the sum of the diameter of the reinforcer d and the thicknesses of the elastomeric compound, known as the skim compound, coating the reinforcers on either side. The trimmed thickness E of the carcass layer is equal to the diameter d of the reinforcer.

[0113] Table 2 below presents, for a carcass layer according to the reference E, and for the respective carcass layers according to the embodiment variants I1 and I2 of the invention, the following mechanical characteristics:

TABLE-US-00002 TABLE 2 Carcass layer E I1 I2 Breaking strength Fr 213 daN/cm 775 daN/cm 641 daN/cm of a carcass layer (daN/cm) Ratio M1/Fr of the 7.70 3.53 0.82 secant modulus M1 at 1% elongation to the breaking strength Fr of the carcass layer

[0114] The reference tire, comprising a carcass reinforcement having three carcass layers, each carcass layer having the characteristics of the reference E in Tables 1 and 2, was compared with two tires comprising a carcass reinforcement having a single carcass layer, having the characteristics of the variants I1 and I2 in Tables 1 and 2. Table 3 below presents this comparison:

TABLE-US-00003 TABLE 3 Tire E I1 I2 Number of carcass 3 1 1 layers Total thickness of the 4.23 mm 1.90 mm 1.80 mm carcass reinforcement (mm) Trimmed thickness E N/A 1.11 mm 0.92 mm of the single-layer carcass reinforcement (mm) Total breaking 638 daN/cm 775 daN/cm 641 daN/cm strength of the carcass reinforcement (daN/cm)

[0115] Table 3 shows that the carcass reinforcements of the variants I1 and I2 clearly comply with the two essential characteristics of the invention: a mean thickness E less than 2 mm, and even less than 1.2 mm, and a breaking strength Fr greater than the reference threshold strength Fs=422 daN/cm.

[0116] Table 4 below presents the characteristics of the tread pattern and crown reinforcement combined with the characteristics of the carcass reinforcement, for the reference E and the embodiment variant I1.

TABLE-US-00004 TABLE 4 Reference E (710/70R42 IF 179D Invention I1 Characteristics Michelin AXIOBIB) (710/70R42 VF 179D) Number of carcass 3 1 layers Nature of the PET/PET Aramid/Aramid/Nylon reinforcers of the carcass layer Number of crown 6 2 layers Nature of the Assembly of 3 rayon Elastic metal reinforcer reinforcers of threads, having a 24.26, made up of 24 the crown layer diameter equal to steel threads, having a 0.94 mm unit diameter equal to 26/100 mm, distributed in 4 strands of 6 threads Overall volumetric >65%  <50%  void ratio TEV of the tread Mean radial height 65 mm 45 mm H of the tread Type of tread Lugged tread pattern Tread pattern having pattern of the blocks comprising 4 tread longitudinal voids, axially positioned on either side of the median equatorial plane of the tire, the two intermediate longitudinal voids being spaced apart from one another by an axial distance equal to 370 mm and the two axially outer longitudinal voids being spaced apart from one another by an axial distance equal to 568 mm.

[0117] The reduction in structural stiffness of the tire, targeted by the inventors and obtained by virtue of a single-layer carcass reinforcement as proposed by the invention, can also be enhanced by the combination of said single-layer carcass reinforcement with a crown reinforcement and a tread that are less thick than those of the reference tire. Thus, the crown reinforcement advantageously comprises two crown layers having elastic metal reinforcers, rather than six layers of textile reinforcers made of rayon. Furthermore, the traditional tread with a lugged tread pattern also advantageously comprises a tread pattern with blocks of smaller radial height (45 mm rather than 65 mm), and therefore thinner, but with a volumetric void ratio that is lower (less than 50% rather than greater than 60%), and therefore more closed. This tread pattern having blocks lastly comprises two axially outer longitudinal voids that are sufficiently far apart from one another (568 mm) to provide a hinge function, ensuring that the flattening of the tread is easier.