Tire tread for a heavy vehicle with improved tensile strength

Description

[0001] The present invention relates to a tire tread for a heavy goods vehicle intended for mixed use, that is to say capable of running on tarred or stony ground. US 2018 / 0264889 A1 and EP 3 269 567 A1 describe tires with a tread comprising blocks delimited by hollows comprising a groove and an incision arranged in the bottom of the groove.

[0002] A tread, positioned at the circumference of the tire and made of at least one rubber-based material, is intended to be worn when it comes into contact with the ground via a rolling surface. It generally includes a sculpture which is a system of hollows separating raised elements and intended to ensure the grip of the tire, on rigid or soft ground, which may be covered with water, snow or mud.

[0003] A tread can be defined geometrically by three dimensions: a thickness, in a radial direction, a width, in a transverse direction, and a length, in a longitudinal direction.

[0004] These geometric characteristics of the tread are measured on the tire as follows: the thickness is the radial distance between the rolling surface and a bottom surface, substantially parallel to the rolling surface and passing through the bottom of the deepest depression, the width is the transverse distance between the end points of the rolling surface, when it is in contact with a rigid ground under the action of load and pressure conditions applied to the tire as specified, for example, by the standards of the "European Tyre and Rim Technical Organization" (ETRTO), the length is equal to the outer circumference of the tire.

[0005] The tread, when the tire rolls on the ground, is subjected to abrasion stresses due to the indentations in the ground, which cause its thickness to gradually reduce through wear. Furthermore, the grooves in the tread are likely to trap stones present on the ground, which can either be ejected during rolling or migrate to the bottom of the groove and generate cracks likely to propagate from the bottom of the groove towards the inside of the tread to a reinforcement reinforcement at the crown of the tire. These cracks can ultimately lead to degradation of the crown of the tire, reducing the endurance and service life of the tire and, consequently, requiring its premature removal.

[0006] Tread depressions can be of two types: grooves and incisions. Grooves delimit raised elements that can deform without coming into contact with each other, when the tread passes through the contact patch with the ground. Incisions, narrower than grooves, are of a width such that the walls of the opposing raised elements delimiting them come into contact at least partially with each other, when the tread passes through the contact patch. To determine the possibility or not of contact between the walls of a depression, the load and pressure conditions applied to the tire are those specified, for example, by ETRTO standards. Grooves, which are the widest depressions, have the highest probability of retaining the largest stones likely to migrate and attack the tread.

[0007] To reduce the impact of stone retention and migration in the tread depressions, and more specifically in the grooves, tire designers have implanted rubber protrusions in the depression bottoms, called bridges. A bridge, implanted in a depression, therefore connects the two raised elements delimiting the depression and thus provides local stiffening of the raised elements, in particular when they are subjected to shear stresses transverse to the mean direction of the depression. A bridge can be either continuous or discontinuous, depending on the mean direction of the depression in which it is implanted or in a direction perpendicular to this mean direction. It can also have a greater or lesser height compared to the depth of the depression.

[0008] The introduction of a discontinuity in a bridging, in the form of at least one incision extending along the average direction of the hollow in which the bridging is positioned, makes it possible to guarantee flexibility of the bridging and therefore a flexible connection between the raised elements, which contributes, in particular, to limiting the wear of said raised elements. A first type of known discontinuous bridging comprises a single incision opening into the hollow and positioned in the center of the bridging at a substantially equal distance from each raised element. In this first type of bridging, it has been found that the presence of an incision centered on the bridging is likely to reduce the protection performance of the hollow bottom by the bridging, since stones are likely to migrate into the incision.A second known type of discontinuous bridging comprises two incisions opening into the hollow, each incision being positioned at the junction between the bridging and one of the two raised elements. Due to the presence of an incision at each of its ends, this second type of bridging has the disadvantage of not connecting the two raised elements which border it and therefore of not effectively ensuring its bridging function.

[0009] The presence of an incision in a bridge implies that the hollow has a stepped structure, comprising a first radially outer portion of wide, groove-like hollow opening onto the rolling surface and a second radially inner portion of narrow, incision-like hollow opening into the groove.

[0010] The inventors set themselves the objective of increasing the endurance of the crown of a heavy goods vehicle tire, comprising a tread with incised bridged hollows, by reducing the risk of migration of stones trapped in these hollows, when the tire is rolling.

[0011] This objective was achieved by a tire tread for a heavy goods vehicle, intended to come into contact with a ground via a rolling surface and comprising hollows, delimited by elements in relief relative to a base surface and extending over a given length, at least one hollow comprising, over at least part of its length, a first radially outer hollow portion, opening into the rolling surface, and a second radially inner hollow portion, opening into the first hollow portion, the first hollow portion being a groove having a width W11, measured at the rolling surface perpendicular to a mean surface of the groove, and having a depth H1, measured perpendicularly from the rolling surface to a groove bottom having a width W12, the second hollow portion being an incision, having a width W2, measured perpendicular to a mean surface of the incision, and having a depth H2, measured perpendicularly from the groove bottom to an incision bottom, the incision opening into the groove bottom at a distance E measured between the average surface of the groove and the average surface of the incision, the distance E, at which the incision opens into the groove bottom, being at least equal to the width W12 of the groove bottom divided by 6.

[0012] Essentially, according to the invention, when a tire tread for a heavy goods vehicle comprises hollows whose bottom is provided with an incised bridging, that is to say a bridging comprising a single incision extending in the mean direction of the hollow, this incision opens at a radially outer face of the bridging, eccentrically relative to a mean surface of the hollow. This eccentricity is defined by a distance E between the mean surface of the hollow and the intersection of the mean surface of the incision with the radially outer face of the bridging, said face corresponding to the bottom of the first portion of hollow, of groove type, radially outside the bridging. According to the invention, the distance E is at least equal to the width W12 of the groove bottom divided by 6, which means that the incision opens outside the central third of the bridging.

[0013] In other words, the recess with incised bridging can be described as a stepped recess comprising a first groove-type recess portion, delimited by two raised elements, radially external, opening into the rolling surface, and a second incision-type recess portion, at least partially crossing the bridging, radially internal, opening into the first recess portion at the radially external face of the bridging. The total depth of the stepped recess is the radial distance between the rolling surface and the bottom of the recess.

[0014] The first radially outer recess portion is a groove whose walls cannot come into contact with each other when the tread passes through the ground contact surface. It extends along a mean surface substantially perpendicular to the rolling surface. The first recess portion has, at the rolling surface, a width W11, and, at its bottom, a width W12, respectively measured perpendicular to the mean surface and between the walls opposite the raised elements delimiting the groove. The first recess portion has a depth H1, measured perpendicularly from the rolling surface to the bottom of the groove. In other words, the depth H1 is the maximum radial distance between the rolling surface and the bottom of the groove.

[0015] The second radially inner recess portion is an incision whose walls come into contact at least partially with each other, when the tread passes through the contact surface. It extends along a mean surface which may be inclined, and therefore not necessarily perpendicular, relative to the bottom of the groove. The second recess portion has a width W2, measured between the walls of the incision perpendicular to a mean surface of the incision, and a depth H2, measured perpendicularly from the groove bottom to an incision bottom. In other words, the depth H2 is the radial distance between the groove bottom and the incision bottom. The width W2 of the incision is generally substantially constant over the entire depth H2 and can therefore be measured at any radial level between the groove bottom and the incision bottom.

[0016] The distance E, at which the incision opens into the groove bottom, is therefore the distance between the intersection of the mean surface of the groove with the groove bottom and the intersection of the mean surface of the incision with the groove bottom.

[0017] The advantage of such an eccentric incision in the bridging is to reduce the probability of migration of a stone trapped in the groove, because the trajectory of the stone initially in the axis of the groove is deflected by the eccentricity of the incision. Therefore the probability of migration of the stone in the incision is reduced and the stone is potentially blocked at the level of the groove, or even ejected from the hollow. Consequently, the risk of attack on the crown reinforcement, radially inside the tread, is reduced and its endurance is increased, therefore the service life of the tire is increased.

[0018] Advantageously, the distance E is at most equal to half the width W12 of the groove bottom. The greater the distance E, the more the trajectory of the stone is deflected, which implies that the risk of migration of the stone in the incision decreases. When the distance E is equal to half the width W12 of the groove bottom, the incision opens at the foot of one of the two raised elements delimiting the groove, which is the limit position of the opening end of the incision.

[0019] Preferably, the width W11 of the groove is at least equal to the width W12 of the groove bottom. This condition implies that the walls of the groove are not radial and that the groove has, in a radial plane perpendicular to the mean direction of the groove, a trapezoidal section whose large base is the part opening into the rolling surface and the small base is the groove bottom. This trapezoidal section facilitates the ejection of stones during rolling, by creating an ejection cone, and also facilitates the manufacture of the tire, more precisely the demolding of the hollow at the end of curing of the tire.

[0020] Preferably, the width W11 of the groove is at least 4 mm. Below this value, the groove no longer functions as a groove without self-contact and there is no longer any risk of stone retention and therefore stone migration.

[0021] In addition, the W11 / W12 ratio must remain limited, typically at most equal to 3. This condition limits the inclination of the groove walls, and therefore limits the volume of the groove, which guarantees, with respect to the raised elements delimiting the groove, a volume of material to be worn sufficient for the wear performance of the tread.

[0022] The depth H1 of the groove is advantageously at least equal to 0.5 times the depth H2 of the incision. In other words, the depth H1 of the groove is at least equal to one third of the total depth H of the hollow, defined as the sum of the depth H1 of the groove and the depth H2 of the incision. This is a usual design domain guaranteeing a sufficient volume of grooves with regard to adhesion.

[0023] The depth H1 of the groove is still advantageously at most equal to 2 times the depth H2 of the incision. In other words, the depth H1 of the groove is at most equal to two thirds of the total depth H of the hollow, defined as the sum of the depth H1 of the groove and the depth H2 of the incision. This is a usual design domain guaranteeing a sufficient bridging height with respect to the hollow bottom protection.

[0024] Preferably the width W2 of the incision is at most equal to 2 mm. Above this value, the incision no longer functions as an incision, because its walls no longer come into contact with each other.

[0025] Preferably, the width W2 of the incision is at least 0.4 mm. The value of 0.4 mm is a technological minimum for manufacturing, below which the production of a molding strip of the incision becomes problematic.

[0026] It is advantageous for the incision to have a mean surface forming, with respect to a radial direction perpendicular to the rolling surface, an angle D at most equal to 15°. An inclination of the mean surface of the incision makes it possible to slow down the migration of a stone inside it, knowing that the preferred direction of migration of a stone is a priori radial. In addition, a greater limitation of this inclination to 15° facilitates substantially radial demolding, at the end of curing of the tire.

[0027] The incision bottom advantageously has a rounded contour with a minimum radius R at least equal to 1.5 times the width W2 of the incision. This rounding at the bottom of the groove, usually called "water drop" by designers, is an anti-cracking device which delays the initiation of cracking at the bottom of the incision by reducing local stresses.

[0028] According to a particular embodiment, the average surface of the groove forms, with the longitudinal direction, an angle at most equal to 45°. In other words, the groove, and therefore the stepped hollow of which it is the first radially outer portion, is substantially longitudinal, that is to say longitudinal or oblique with an inclination of its average surface relative to the longitudinal direction less than or equal to 45°. Indeed, such a longitudinal or oblique hollow traps a stone more easily, due to its orientation substantially parallel to the rolling direction of the tire: hence the interest of a bridge according to the invention in this type of hollow.

[0029] According to a preferred variant of a substantially longitudinal hollow as defined above, the hollow is continuous over the entire length of the tread. A substantially longitudinal hollow comprising a bridge according to the invention over the entire length of the tread is thus effectively protected against stone retention, over the entire circumference of the tire.

[0030] According to another particular embodiment, the average surface of the groove forms, with the transverse direction, an angle at most equal to 45°. In other words, the groove, and therefore the stepped hollow of which it is the first radially external portion, is substantially transverse, that is to say transverse or oblique with an inclination of its average surface relative to the transverse direction less than or equal to 45°.

[0031] According to a preferred variant of a substantially transverse hollow as defined above, the hollow is positioned in a median portion of the tread having a width preferably equal to the width of the tread divided by 3. A transverse hollow being, by convention, a hollow whose average surface forms, with the transverse direction, an angle at most equal to 45°, therefore closer to the transverse direction than to the longitudinal rolling direction, the ejection of stones from the transverse hollows is in fact more difficult.Consequently, when a tread comprises transverse hollows in its middle portion, usually defined as the central third of the tread, it is particularly advantageous for these hollows to be protected by an incised bridging according to the invention, because this portion of the tread is subjected to high contact pressures, which promotes the possible capture of stones present in the contact surface of the tire with the ground.

[0032] The invention also relates to a tire for a heavy goods vehicle comprising a tread according to any one of the embodiments previously described.

[0033] The features of the invention are illustrated by the figures 1 to 3 schematic and not shown to scale: Figure 1 : Radial sectional view of a stepped hollow according to the invention. Figure 2: Perspective view of an example of a portion of tread according to the invention. Figure 3 : Radial sectional view of a stepped hollow of the example of a portion of tread according to the invention, presented on the figure 2 .

[0034] There figure 1is a radial sectional view of a stepped hollow according to the invention. The radial section is made in a plane defined by the radial direction zz' and a direction yy', perpendicular to a mean direction of the hollow xx'. The hollow 3 of the tire tread 1 for a heavy goods vehicle, intended to come into contact with a ground via a rolling surface 2, is delimited by elements 4 in relief relative to a bottom surface 5 and extends over a given length. The hollow 3 comprises, over at least part of its length, a first radially outer hollow portion 31, opening into the rolling surface 2, and a second radially inner hollow portion 32, opening into the first hollow portion 31.The first hollow portion 31 is a groove having a width W11, measured at the rolling surface 2 perpendicular to a mean surface S1 of the groove 31, and having a depth H1, measured perpendicularly from the rolling surface 2 to a groove bottom 311 having a width W12. The second hollow portion 32 is an incision, having a width W2, measured perpendicular to a mean surface S2 of the incision 32, and having a depth H2, measured perpendicularly from the groove bottom 311 to an incision bottom 321. The incision 32 opens into the groove bottom 311 at a distance E measured between the mean surface S1 of the groove 31 and the mean surface S2 of the incision 32. According to the invention, the distance E, at which the incision 32 opens into the groove bottom 311, is at least equal to the width W12 of the groove bottom 311 divided by 6. In the embodiment shown in the . figure 1, the incision 32 has a mean surface S2 forming, with respect to a radial direction zz' perpendicular to the rolling surface 2, an angle D at most equal to 15°. In addition, the incision bottom 321 has a rounded contour having a minimum radius R at least equal to 1.5 times the width W2 of the incision 32.

[0035] There figure 2 is a perspective view of an example of a tread portion 1 according to the invention. The directions shown in the figure are respectively the circumferential direction XX', the transverse direction YY' and a radial direction ZZ' of the tread. The figure 2represents a portion of a tire tread 1 for a heavy goods vehicle, intended to come into contact with a ground via a tread surface 2 and comprising hollows 3, according to the invention, delimited by elements 4 in relief relative to a bottom surface 5. At least a portion of the hollows 3, in particular the substantially longitudinal hollows 3, comprise a first radially outer hollow portion 31, opening into the tread surface 2, and a second radially inner hollow portion 32, opening into the first hollow portion 31. The tread 2 also comprises substantially transverse hollows 3, positioned in a median portion and in two lateral portions, said substantially transverse hollows opening into the substantially longitudinal hollows.The sculpture of such a tread is therefore a block sculpture, particularly suitable for use on a heavy goods vehicle drive axle.

[0036] There figure 3 is a radial sectional view of a stepped hollow of the example tread portion according to the invention as shown in the figure 2 . It is carried out in a plane defined by the radial direction zz' and a direction yy', perpendicular to an average direction of the hollow xx'. The directions xx', yy' and zz' define a local reference point relative to the hollow considered, the local radial direction zz' corresponding to the radial direction ZZ' of the tread as represented on the figure 2 . The characteristics represented on the figure 2 are identical to those of the figure 1. In the particular case shown, the incision 32 opens into the groove bottom 311 at a distance E from the average surface S1 of the groove 31 equal to half the width W12 of the groove bottom 311, that is to say at the foot of the wall with maximum eccentricity. Furthermore, the incision 32 is perpendicular to the groove bottom 311, which implies that its average surface S2 is parallel to the average surface S1 of the groove 31.

[0037] The tread pattern example shown on the Figures 2 and 3 is an example studied by the inventors in the tire size 13R22.5, for a mixed-use tire more particularly intended to equip a drive axle. This tire is intended to carry a load equal to 4000 kg, for an inflation pressure equal to 8.6 bars, according to the “Standards Manual 2019” of the ETRTO standard.

[0038] Table 1 below shows the characteristics of the tested tread: [Table 1] Features Values Width W11 of groove 31 10.4 mm Width W12 of groove 31 4 mm Height H1 of groove 31 10 mm Width W2 of the incision 32 0.8 mm Height H2 of the incision 32 11.2 mm Inclination D of incision 32 0° Distance E 2 mm

[0039] The tread according to the invention, developed for a standard tire subjected to inflation pressure, can also be used for a non-pneumatic tire, but also for any non-pneumatic rolling assembly intended to equip a vehicle.

Claims

1. Tread (1) for a tyre for a heavy-duty vehicle, which tread is intended to come into contact with the ground via a tread surface (2) and comprises voids (3) that are delimited by elements (4) in relief with respect to a bottom surface (5) and extend over a specific length, - at least one void (3) comprising, over at least some of its length, a first, radially outer void portion (31) leading into the tread surface (2), and a second, radially inner void portion (32) leading into the first void portion (31), - the first void portion (31) being a groove having a width W11, measured on the tread surface (2) perpendicularly to a mean surface (S1) of the groove (31), and having a depth H1, measured perpendicularly from the tread surface (2) to a groove bottom (311) having a width W12, - the second void portion (32) being a sipe having a width W2, measured perpendicularly to a mean surface (S2) of the sipe (32), and having a depth H2, measured perpendicularly from the groove bottom (311) to a sipe bottom (321), - the depth H1 of the groove (31) being at least equal to 0.5 times the depth H2 of the sipe (32) and at most equal to twice the depth H2 of the sipe (32), - the sipe (32) leading into the groove bottom (311) at a distance E, measured between the mean surface (S1) of the groove (31) and the mean surface (S2) of the sipe (32), characterized in that the distance E at which the sipe (32) leads into the groove bottom (311) is at least equal to the width W12 of the groove bottom (311) divided by 6.

2. Tread (1) according to Claim 1, wherein the distance E is at most equal to half of the width W12 of the groove bottom (311).

3. Tread (1) according to either of Claims 1 and 2, wherein the width W11 of the groove (31) is at least equal to the width W12 of the groove bottom (311).

4. Tread (1) according to any one of Claims 1 to 3, wherein the width W11 of the groove (31) is at least equal to 4 mm.

5. Tread (1) according to any one of Claims 1 to 4, wherein the width W2 of the sipe (32) is at most equal to 2 mm.

6. Tread (1) according to any one of Claims 1 to 5, wherein the width W2 of the sipe (32) is at least equal to 0.4 mm.

7. Tread (1) according to any one of Claims 1 to 6, wherein the sipe (32) has a mean surface (S2) forming an angle D at most equal to 15° with respect to a radial direction (ZZ') perpendicular to the tread surface (2).

8. Tread (1) according to any one of Claims 1 to 7, wherein the sipe bottom (321) has a rounded contour having a minimum radius R at least equal to 1.5 times the width W2 of the sipe (32).

9. Tread (1) according to any one of Claims 1 to 8, wherein the mean surface (S1) of the groove (31) forms, with the longitudinal direction (XX') which is the circumferential direction of the tread, an angle at most equal to 45°.

10. Tread (1) according to Claim 9, wherein the void (3) is continuous over the entire length of the tread.

11. Tread (1) according to any one of Claims 1 to 8, wherein the mean surface (S1) of the groove (31) forms, with the transverse direction (YY') of the tread, an angle at most equal to 45°.

12. Tread (1) according to Claim 11, wherein the void (3) is positioned in a median portion of the tread having a width preferably equal to the width of the tread divided by 3.

13. Tyre for a heavy-duty vehicle, comprising a tread (1) according to any one of Claims 1 to 12.