TIRE TRAFFIC SURFACE FOR A HEAVY CONSTRUCTION VEHICLE WITH IMPROVED COMPROMISE BETWEEN STRENGTH AND THERMAL WEAR

Description

[0001] The present invention relates to a tire tread for a heavy vehicle intended to carry heavy loads and to drive on uneven, stony and / or muddy ground such as, for example, a dumper-type civil engineering vehicle intended for use in mines or quarries.

[0002] A tread comprises at least one rubber-based material and is intended to form the peripheral part of a tire and to be worn down when its tread surface comes into contact with the ground.

[0003] A tread can be geometrically defined by three dimensions: a smaller dimension or thickness, perpendicular to the tread surface; an intermediate dimension or width, transversely; and a larger dimension or length, longitudinally. When the tread is integrated into the tire, the direction perpendicular to the tread surface is also called the radial direction, as it is defined by a radius of the tire; the transverse direction is also called the axial direction, as it is parallel to the tire's axis of rotation; and the longitudinal direction is also called the circumferential direction, as it is tangent to the tire's circumference along the tire's direction of travel. In what follows, "radially" means "perpendicular to the tread surface" or "along a radial direction."

[0004] To ensure satisfactory performance in longitudinal grip, under engine torque and under braking torque, and in transverse grip, it is necessary to form, in the tread, a combination of cutouts separating raised elements, called a sculpture.

[0005] A cutout is geometrically characterized by its thickness, which is the distance between the material walls delimiting it, and by its depth, which is the distance between the running surface and the bottom of the cutout, that is, its innermost radial point. The thickness can vary depending on the radial position of the measurement point between the running surface and the bottom of the cutout.

[0006] Tread cuts can be of two types: grooves and incisions. Grooves are wide cuts, primarily used for the storage and evacuation of water or mud from the ground. A cut is considered wide when its thickness is such that the opposing material walls delimiting it do not come into contact with each other when the tread passes over the contact patch, provided the tire is subjected to recommended inflation and load conditions, as defined, for example, by ISO 4250 and the Tire and Rim Association (TRA) standard. Incisions are narrow cuts whose intersections with the tread, or edges, contribute to grip on wet surfaces by creating an edge effect in the contact patch that breaks up the film of water on the ground.A cut is said to be narrow when it has a width such that the opposing material walls delimiting it come into contact at least partially with each other, when the tread passes through the contact surface, under the load and tire pressure conditions specified by the TRA standard as seen previously.

[0007] A cut is often characterized by a mid-surface, equidistant from the sides of the cut and intersecting the tread. The intersection of this mid-surface and the tread is called the mid-cut line. The mid-cut line is not necessarily straight and can, for example, have a wavy or zigzag shape. A cut is said to be longitudinal when its mid-cut line has a longitudinal mean direction, meaning it forms a mean angle of no more than 45° with the longitudinal direction. A longitudinal cut generally runs around the entire tire. A cut is said to be transverse when its mid-cut line has a transverse mean direction, meaning it forms a mean angle of no more than 45° with the transverse direction.A transverse cut connects two longitudinal cuts together or a longitudinal cut to an edge of the tread.

[0008] In the case of a tire tread for a heavy construction vehicle, the raised elements are generally blocks. A block is a volume of material delimited by a contact patch, contained within the tread surface, by a base surface, and by lateral faces connecting the contact patch to the base surface. These blocks can be arranged to form longitudinal rows, these rows being separated in pairs by longitudinal cuts such as grooves or incisions, also called longitudinal furrows. Furthermore, within the same longitudinal row of blocks, the blocks are most often separated in pairs by transverse cuts such as grooves or incisions.

[0009] The tread, an integral part of the tire, is most often geometrically characterized by a width L, measured transversely, and a height H, measured perpendicular to the tread surface. The width L is defined as the transverse width of the contact patch of the new tire's tread with a smooth surface, such as asphalt, when the tire is subjected to the recommended nominal pressure and load conditions, for example, those defined by the TRA standard. The height H is conventionally defined as the maximum radial depth measured in the tread cutouts, corresponding to the maximum radial block height when new. For example, in the case of a tire for a dumper-type construction vehicle, the width L is at least 600 mm and the height H is at least 60 mm, or even 70 mm.

[0010] The typical operating conditions for tires on construction vehicles are particularly harsh. Such vehicles are generally designed to travel on rough, rocky tracks, requiring good resistance to wear and tear on the tire treads to ensure a satisfactory lifespan. Furthermore, the often high loads applied to the tires generate significant heat dissipation, especially in the crown, leading to temperature increases that can cause degradation of the rubber compound in the crown and premature tire failure. Therefore, tread wear resistance and effective crown temperature control are two major considerations for tires intended for heavy construction vehicles.

[0011] Regarding the thermal management of the tread crown, it is known that the tread crown temperature is lower when the tread is well ventilated, meaning the volume of cutouts ensuring ventilation is high. However, a current trend is to develop treads with increased lifespan, thus with a greater volume of material to wear through, i.e., a reduced volume of cutouts. For example, document WO 2017162953A1 proposes a so-called slotted tread, whose cutouts can, however, be susceptible to damage and insufficiently effective for thermal ventilation. Document WO 2020229176A1 proposes a slotted tread with cutouts made more robust against damage thanks to their optimized geometry, but without improved ventilation efficiency from these cutouts.

[0012] The inventors set themselves the objective of improving, for a tire tread for a heavy construction vehicle, comprising at least in part blocks separated from each other by incisions, the compromise between resistance to mechanical aggressions by stony soils and the capacity for thermal ventilation.

[0013] This objective was achieved through a tire tread for a heavy construction vehicle, designed to come into contact with the ground via a tread surface comprising blocks delimited by cutouts, and having a width and a thickness defined as the maximum depth of the cutout. the tread having a median portion, symmetrical with respect to a median longitudinal plane and having a width not exceeding 60% of the tread width, the median portion comprising at least one longitudinal row of blocks, each block being delimited transversely by two longitudinal grooves and longitudinally by two transverse inter-block cutouts, each longitudinal groove and each transverse inter-block cutout having respectively a depth of at least 50% of the tread thickness, and being respectively a stepped cutout, comprising a radially outer portion opening onto the tread surface and a radially inner portion extending from the radially outer portion to a radially inner end (4121, 4221), each radially outer portion having a thickness, measured between the material walls that delimit it,comprising between a minimum and a maximum thickness, each radially inner portion having a thickness, measured between the material walls that delimit it, comprising between a minimum and a maximum thickness, the maximum thickness of each radially outer portion being at most equal to 20% of the depth of the corresponding cut and strictly less than the maximum thickness of the corresponding radially inner portion, and the minimum thickness of each radially outer portion being at least equal to 5% of the depth of the corresponding cut.

[0014] The invention is essentially based on optimizing the respective thicknesses of the radially outer and radially inner portions of any longitudinal or transverse cut delimiting any block belonging to a specific longitudinal row of blocks. This specific longitudinal row of blocks is positioned in a median portion of the tread. The invention aims to ensure a compromise between crack resistance and the thermal ventilation capacity of said cuts.

[0015] In the context of the invention, a median portion of the tread is a portion of the tread symmetrical with respect to a median longitudinal plane passing through the middle of the tread and perpendicular to the tread surface, and having a width not exceeding 60% of the tread width, said tread width being measured in a transverse direction between two lateral edges of the tread surface. The median portion contains one or more longitudinal rows of blocks according to the invention, but may also contain other tread elements. In other words, the median portion does not necessarily contain exclusively longitudinal rows of blocks according to the invention.

[0016] By definition, a longitudinal cut delimiting a block of the middle portion of the tread is called a longitudinal groove, and a transverse cut delimiting a block of the middle portion of the tread is called an inter-block transverse cut.

[0017] Every longitudinal groove, or transverse cut between blocks, is a stepped cut, comprising a radially outer portion opening onto the running surface and a radially inner portion extending from the radially outer portion to the bottom of the cut. Every radially outer portion has a thickness, measured between the material walls that delimit it, which can vary, in the direction perpendicular to the running surface, between the running surface, i.e., the most radially outer point of said portion, and the most radially inner point of said portion: this thickness of the radially outer portion is therefore between a minimum thickness and a maximum thickness.Similarly, any radially inner portion has a thickness, measured between the material walls that delimit it, which can vary, in the direction perpendicular to the tread surface, between the outermost radial point of said portion and the bottom of the cut, that is, the innermost radial point of said portion: this thickness of the radially inner portion is therefore between a minimum and a maximum thickness. The thicknesses described above are usually measured on tread cross-sections, delimited by two transverse planes perpendicular to the tread surface.

[0018] Each longitudinal groove, or each transverse cut between blocks, has a depth measured, along a direction perpendicular to the tread surface, between the tread surface and the bottom of said longitudinal groove, or of said transverse cut between blocks. In the context of the invention, this depth is at least equal to 50% of the maximum cut depth, conventionally called the tread depth, corresponding to the maximum thickness of material to be worn before any cut disappears from the tread surface.

[0019] According to a first essential feature of the invention, the maximum thickness of each radially external portion is at most equal to 20% of the depth of the corresponding cut and strictly less than the maximum thickness of the corresponding radially internal portion.

[0020] A maximum thickness limit for the radially outer portion of a cut, set at 20% of the cut's depth, serves two purposes: firstly, to limit the cut's volume, thus reducing the amount of material to be worn, which is beneficial for wear life; and secondly, to stiffen the blocks it delimits by ensuring contact between the walls of the radially outer portion. This, in turn, limits deformation at the bottom of the cut, and therefore reduces the risk of cracking. A radially outer portion with such a thickness limitation is called an incision.

[0021] Furthermore, a maximum thickness limit for the radially outer portion of a groove, strictly less than the maximum thickness of the corresponding radially inner portion, ensures an additional thickness in the radially inner portion of the groove. This limits stress at the groove bottom, thus minimizing the risk of cracking there. Moreover, this additional thickness in the radially inner portion creates an airflow channel at the bottom of the tread, where heat is most difficult to dissipate, thereby contributing to groove ventilation and consequently limiting the tread's temperature.According to a particular embodiment of the radially inner portion, its thickness increases from a minimum value at the interface with the radially outer portion to a maximum value near the bottom of the cut. In this case, the shape of such a radially inner portion is described as a teardrop.

[0022] According to a second essential feature of the invention, the minimum thickness of each radially external portion is at least equal to 5% of the depth of the corresponding cut.

[0023] A lower limit for the minimum thickness of each radially outer portion, set at 5% of the corresponding cut depth, ensures, in the radially outer portion, air circulation contributing to the ventilation of the cut, and consequently to a limitation of the thermal level of the tread.

[0024] Preferably, the minimum thickness of each radially outer portion is at least equal to 10% of the corresponding cut depth. A lower limit for the minimum thickness of each radially outer portion, set at 10% of the corresponding cut depth, further improves airflow in the radially outer portion, contributing to better ventilation of the cut and consequently to even more effective limitation of the tread's thermal level, while maintaining sufficient contact.

[0025] Preferably, the minimum and maximum thicknesses of each radially external portion are equal, such that the thickness at every point of said radially external portion is constant. In this case, the thickness of the radially external portion is constant along its entire height, resulting in an air circulation channel of constant cross-section, ensuring homogeneous and efficient ventilation throughout the entire height of the radially external portion.

[0026] Advantageously, the maximum thickness of each radially inner portion is at least equal to 15% of the corresponding groove depth. A lower limit for the maximum thickness of each radially inner portion, set at 15% of the corresponding groove depth, prevents cracking of the groove bottom during the time required for tread wear to reach the groove bottom. In other words, this lower limit ensures that the wear level is reached at the groove depth without cracking occurring.

[0027] Advantageously, each radially inner portion has a height at least equal to 1.5 times the maximum thickness of the corresponding radially inner portion. In the case of a teardrop-shaped radially inner portion, whose thickness varies from a minimum thickness at the transition with the radially outer portion to a maximum thickness at the bottom of the cutout, the angle of each wall of said radially inner portion, formed with the radial direction, usually called the draft angle, allows, on the one hand, to limit, during manufacturing, the demolding force of the slat intended to mold the cutout, and, on the other hand, to limit the degradation by tearing of the transition zone between the radially outer and radially inner portions respectively.

[0028] Advantageously, each radially internal portion has a circular radially internal end whose diameter is equal to the maximum thickness of the corresponding radially internal portion. A circular arc at the radially internal end of a radially internal portion, i.e., at the bottom of the cut, is the shape that allows for the largest constant radius of curvature, thus optimizing the reduction of stress concentration for a given volume of radially internal portion and reducing the risk of cracking.

[0029] Advantageously, since each block has a longitudinal length, the depth of each transverse cut between blocks is at most equal to the block's longitudinal length, and preferably at most equal to 0.8 times the block's longitudinal length. The ratio between the depth of each transverse cut between blocks and the block's longitudinal length is usually called the block's longitudinal slenderness ratio and characterizes its longitudinal rigidity from a geometric point of view. A longitudinal slenderness ratio limited to 1, preferably 0.8, makes it possible, in particular, to limit the block's deflection under torque, and therefore the irregular wear known as sawtooth wear. A longitudinal slenderness ratio of up to 0.8, or even 1, is made possible by the good ventilation capacity of the cuts delimiting the block, thus mitigating the risk of increased thermal pressure.

[0030] According to a first particular embodiment, the tread comprises external transverse cutouts opening into the longitudinal grooves delimiting the blocks, each having a depth at least equal to 50% of the tread height, and comprising a radially external portion and a radially internal portion, each radially external portion having a thickness between a minimum and a maximum thickness, and each radially internal portion having a thickness between a minimum and a maximum thickness. In this particular embodiment, the maximum thickness of each radially external portion is at most equal to 20% of the depth of the corresponding cutout and strictly less than the maximum thickness of the corresponding radially internal portion.Furthermore, the minimum thickness of each radially external portion is at least equal to 5% of the depth of the corresponding cut. In other words, these external transverse cuts have the same characteristics as the longitudinal grooves and the inter-block transverse cuts, and therefore the same technical advantages of crack resistance and thermal ventilation capacity.

[0031] Preferably, the minimum thickness of the radially outer portion of each external transverse cut is at least equal to 10% of the corresponding cut depth. A lower limit of 10% of the corresponding cut depth for the minimum thickness of each radially outer portion further improves airflow in the radially outer portion, contributing to better ventilation of the cut and consequently to even more effective limitation of the tread's thermal level, while maintaining sufficient contact.

[0032] Preferably, the minimum and maximum thicknesses of the radially outer portion of each external cross-section are equal. In this case, the thickness of the radially outer portion is constant along its entire height, resulting in an airflow channel with a constant cross-section, ensuring homogeneous and efficient ventilation along the entire height of the radially outer portion.

[0033] Advantageously, the maximum thickness of the radially inner portion of each external transverse groove is at least 15% of the corresponding groove depth. A lower limit for the maximum thickness of each radially inner portion, set at 15% of the corresponding groove depth, prevents cracking of the groove bottom during the time required for tread wear to reach the groove bottom. In other words, this lower limit ensures that the wear level is reached at the groove depth without cracking occurring.

[0034] Advantageously, the radially inner portion of each external transverse cutout has a height at least equal to 1.5 times the maximum thickness of the corresponding radially inner portion. In the case of a teardrop-shaped radially inner portion, whose thickness varies from a minimum thickness at the transition with the radially outer portion to a maximum thickness at the bottom of the cutout, the angle of each wall of said radially inner portion, formed with the radial direction, commonly called the draft angle, allows, on the one hand, for limiting, during manufacturing, the demolding force of the slat intended to mold the cutout, and, on the other hand, for limiting the degradation by tearing of the transition zone between the radially outer and radially inner portions respectively.

[0035] Advantageously, the radially inner portion of each external transverse cut has a circular radially inner end whose diameter is equal to the maximum thickness of the corresponding radially inner portion. A circular arc at the radially inner end of a radially inner portion, i.e., at the bottom of the cut, is the shape that allows for the largest constant radius of curvature, thus optimizing the reduction of stress concentration for a given volume of radially inner portion and reducing the risk of cracking.

[0036] According to a second specific embodiment, each block includes at least one internal cutout having a depth at most equal to the smallest of the respective depths of each longitudinal groove and each transverse cutout between blocks delimiting the block. The presence of at least one cutout inside the blocks results in additional cooling of the tread, which may allow the use of a more hysteresis-resistant tread compound, i.e., one that generates more heat but is more resistant to wear. An internal cutout preferably, but not necessarily, opens into the faces of the block. If it does not open into the faces of the block, it is said to be blind.

[0037] Advantageously, since each internal cutout includes a radially external portion with a thickness between a minimum and a maximum thickness, the maximum thickness of the radially external portion of the internal cutout is at most equal to the smallest of the maximum thicknesses of the radially external portion of each longitudinal groove and each inter-block transverse cutout delimiting the block. An internal cutout having both a shallower depth and a smaller maximum thickness of radially external portion than a longitudinal groove and an inter-block transverse cutout has the same ventilation capacity as a longitudinal groove and an inter-block transverse cutout. Such a limited internal cutout volume prevents excessive reduction in the volume of material subject to wear, and consequently, extends the wear life.

[0038] Furthermore, the maximum thickness of the radially outer portion of the inner cutout is at least 5% and at most 20% of the corresponding inner cutout depth. This range of values ​​for the maximum thickness ensures a satisfactory compromise between the ventilation capacity of the inner cutout and its impact on reducing the volume of material to be worn.

[0039] Advantageously, the minimum thickness of the radially outer portion of the inner cutout is at least 5%, and preferably at least 10%, of the corresponding inner cutout depth. This upper limit of the minimum thickness contributes to optimal ventilation of the inner cutout.

[0040] Preferably, the minimum and maximum thicknesses of the radially outer portion of each internal cutout are equal, such that the thickness of said radially outer portion is constant at every point. In this case, the thickness of the radially outer portion is constant along its entire height, resulting in an airflow channel of constant cross-section, ensuring homogeneous and efficient ventilation along the entire height of the radially outer portion.

[0041] Preferably, for each inner groove comprising a radially inner portion with a thickness between a minimum and a maximum thickness, the maximum thickness of the radially outer portion of the inner groove is strictly less than the maximum thickness of the radially inner portion of the corresponding inner groove. Assuming the tread rubber compound has a slow wear rate, the time required for wear to reach the bottom of an inner groove may be long enough for cracking to have initiated at the bottom of that groove. Therefore, it is advantageous for the groove to include a radially inner portion with an enlarged thickness, such as a teardrop shape, to reduce the risk of cracking.

[0042] Advantageously, the maximum thickness of the radially inner portion of the inner groove is at least equal to 15% of the corresponding inner groove depth. A lower limit for the maximum thickness of each radially inner portion, set at 15% of the corresponding inner groove depth, prevents cracking of the inner groove bottom during the time required for tread wear to reach the bottom of the inner groove. In other words, this lower limit ensures that the wear level is reached at the inner groove depth without cracking occurring.

[0043] Advantageously, the radially inner portion of each inner cutout has a height at least equal to 1.5 times the maximum thickness of the radially inner portion of the corresponding inner cutout. In the case of a teardrop-shaped radially inner portion, whose thickness varies from a minimum thickness at the transition with the radially outer portion to a maximum thickness at the bottom of the cutout, the angle of each wall of said radially inner portion, formed with the radial direction, commonly called the draft angle, allows, on the one hand, for limiting, during manufacturing, the demolding force of the slat intended to mold the cutout, and, on the other hand, for limiting the degradation by tearing of the transition zone between the radially outer and radially inner portions respectively.

[0044] Advantageously, the radially inner portion of each internal cutout has a circular radially inner end whose diameter is equal to the maximum thickness of the radially inner portion of the corresponding internal cutout. A circular arc at the radially inner end of a radially inner portion, i.e., at the bottom of the cutout, is the shape that allows for the largest constant radius of curvature, thus optimizing the reduction of stress concentration for a given volume of radially inner portion and reducing the risk of cracking.

[0045] According to a preferred variant of the second particular embodiment, each inner cut is transverse. A transverse direction of the inner cut is advantageous for tire grip during braking, and therefore for safety. Conversely, the presence of a longitudinal inner cut, close to the median longitudinal plane of the tread, weakens the tread in this area of ​​high contact pressure with the ground.

[0046] In a preferred embodiment, the central portion of the tread comprises a longitudinal row of blocks centered on the median longitudinal plane. For a tire mounted on a heavy construction vehicle and operating under its normal load and pressure conditions, the contact pressure with the ground is maximum at the center of the tread, defined by the median longitudinal plane. The presence of blocks, rather than a longitudinal groove, in this area results in a more robust tread.

[0047] For the purpose of protection against attacks by stones, it may be advantageous to have an extra thickness at the bottom of the cut, especially at the intersections between the various cuts described above: longitudinal grooves, transverse cuts between blocks and external transverse cuts.

[0048] The presence of stone-clearing protuberances, shaped like ribs, can be advantageous, particularly in longitudinal grooves.

[0049] The invention also relates to a tire intended to equip a heavy construction vehicle comprising a tread according to one of the embodiments previously described.

[0050] The features of the invention are illustrated by the figures 1 to 8 The following are schematic and not drawn to scale: Figure 1 Top view of a tread according to the invention Figure 2A : Top view of a central portion of a tread according to the invention Figure 2B Cross-sectional view of a longitudinal groove in the median portion of a tread according to the invention Figure 2C Cross-sectional view of a transverse cut between blocks of the central portion of a tread according to the invention Figure 2DCross-sectional view of an external transverse cut of the median portion of a tread according to the invention Figure 3A : Top view of a central portion of a tread according to a particular embodiment of the invention, comprising internal transverse cutouts Figure 3B Cross-sectional view of a central portion of a tread according to a particular embodiment of the invention, comprising internal transverse cutouts Figure 3C Cross-sectional view of an internal transverse cut comprising a single portion 3D Figure Cross-sectional view of an internal transverse cut comprising a radially external portion and a radially internal portion Figure 4 : First range of variation of the thickness of a radially external portion of a longitudinal groove, an inter-block transverse cut, and an external transverse cut, respectively, as a function of the cut depth Figure 5 : Second range of variation of the thickness of a radially external portion of a longitudinal groove, an inter-block transverse cut and an external transverse cut as a function of the cutting depth Figure 6 : First range of variation of the thickness of the radially outer portion of an internal cutout, as a function of the depth of the internal cutout Figure 7 Second range of variation of the thickness of the radially outer portion of an inner cut, as a function of the depth of the inner cut Figure 8 Third range of variation of the thickness of the radially outer portion of an inner cut, as a function of the depth of the inner cut

[0051] There figure 1is a top view of a tread 1 according to the invention. The tread 1 of a tire for a heavy construction vehicle, intended to come into contact with the ground via a tread surface 2, comprises blocks 3 delimited by cutouts (41, 42), and has a width L, along a transverse direction YY', and a height H (not shown in the figure). figure 1 ), along a direction ZZ', defined as the maximum cutting depth perpendicular to the tread surface 2. The tread has a central portion 11, symmetrical with respect to a median longitudinal plane XZ and having a width L1 at most equal to L1max = 60% of the width L of the tread. In the embodiment shown in the figure 1The central portion 11 consists of a single longitudinal row of blocks 3, each block 3 being delimited transversely, along the transverse direction YY', by two longitudinal grooves 41 and longitudinally, along a longitudinal direction XX', by two transverse inter-block cutouts 42. This single longitudinal row of blocks 3, of width L1, is centered on the median longitudinal plane XZ. More generally, the central portion 11 may comprise several longitudinal rows of blocks according to the invention, or a combination, along the transverse direction, of at least one longitudinal row of blocks according to the invention with other sculptural elements. In the embodiment shown in the figure 1 , the tread 1 also includes external transverse cutouts 43, opening into the longitudinal grooves 41 delimiting the blocks 3.

[0052] There figure 2Ais a top view of a central portion of a tread according to the invention. As seen on the figure 1 The central portion of the tread has a width L1 at most equal to 60% of the tread width L. Every block 3 of the central portion has a longitudinal length B, which may vary from one block to another. The section plane AA defines a longitudinal groove cross-section 41, as represented by the figure 2B The CC cutting plane defines a cross-section of inter-block cross-section 42, as represented by the figure 2C The cutting plane DD defines a transverse section with an external cross-sectional cut 43, as defined by the figure 2D .

[0053] There figure 2B is a cross-sectional view of a longitudinal groove 41 of the central portion of the tread according to the invention. figure 2BFigure AA represents, on the one hand, the cross-section of the longitudinal groove 41 as a whole, and on the other hand, a detailed view A1 of the radially inner portion 412 of said longitudinal groove 41. The longitudinal groove 41 has a depth P1 at least equal to 50% of the tread height H, and comprises, respectively, a radially outer portion 411 and a radially inner portion 412. In the general case, the radially outer portion 411 has a thickness, measured between the material walls that delimit it, between a minimum thickness E11min and a maximum thickness E11max. In the case shown, the minimum thickness E11min and the maximum thickness E11max of the radially outer portion 411 are equal, such that the thickness at every point of said radially outer portion 411 is constant.The radially inner portion 412 has a thickness, measured between the material walls that delimit it, between a minimum thickness E12min and a maximum thickness E12max, which, in the general case, are not equal. According to the invention, the maximum thickness E11max of the radially outer portion 411 is at most equal to 20% of the depth P1 of the corresponding cutout 41 and strictly less than the maximum thickness E12max of the corresponding radially inner portion 412, and the minimum thickness E11min of each radially outer portion 411 is at least equal to 5% of the depth P1 of the corresponding cutout 41. In the detail view A1, according to a particular embodiment, the radially internal portion 412 has a height H12 at least equal to 1.5 times its maximum thickness E12max, and a circular radially internal end 4121 whose diameter D1 is equal to its maximum thickness E12max.

[0054] There figure 2C is a cross-sectional view of a transverse inter-block cut 42 of the central portion of the tread according to the invention. figure 2CFigure 42 represents, on the one hand, section CC of the inter-block crosscut 42 as a whole, and on the other hand, a detail view C1 of the radially inner portion 422 of said inter-block crosscut 42. The inter-block crosscut 42 has a depth P2 at least equal to 50% of the tread height H, and comprises, respectively, a radially outer portion 421 and a radially inner portion 422. In the general case, the radially outer portion 421 has a thickness between a minimum thickness E21min and a maximum thickness E21max. In the case shown, the minimum thickness E21min and the maximum thickness E21max of the radially outer portion 421 are equal, such that the thickness at every point of said radially outer portion 421 is constant.The radially inner portion 422 has a thickness between a minimum thickness E22min and a maximum thickness E22max, which, in the general case, are not equal. According to the invention, the maximum thickness E21max of the radially outer portion 421 is at most equal to 20% of the depth P2 of the corresponding cutout 42 and strictly less than the maximum thickness E22max of the corresponding radially inner portion 422, and the minimum thickness E21min of each radially outer portion 421 is at least equal to 5% of the depth P2 of the corresponding cutout 42. In the detailed view C1, according to a particular embodiment, the radially inner portion 422 has a height H22 at least equal to 1.5 times its maximum thickness E22max, and a circular radially inner end 4221 whose diameter D2 is equal to its maximum thickness E22max.

[0055] There figure 2Dis a cross-sectional view of an external transverse cut 43 of the central portion of a tread according to a particular embodiment of the invention. figure 2DFigure 1 represents, on the one hand, the cross-section DD of the outer transverse cut 43 as a whole, and on the other hand, a detail view D1 of the radially inner portion 432 of said outer transverse cut 43. The outer transverse cut 43 has a depth P3 at least equal to 50% of the tread height H, and comprises, respectively, an outer radial portion 431 and an inner radial portion 432. In the general case, the outer radial portion 431 has a thickness between a minimum thickness E31min and a maximum thickness E31max. In the case shown, the minimum thickness E31min and the maximum thickness E31max of the outer radial portion 431 are equal, such that the thickness at every point of said outer radial portion 431 is constant.The radially inner portion 432 has a thickness between a minimum thickness E32min and a maximum thickness E32max, which, in the general case, are not equal. According to a particular embodiment of the invention, the maximum thickness E31max of the radially outer portion 431 is at most equal to 20% of the depth P3 of the corresponding cutout 43 and strictly less than the maximum thickness E32max of the corresponding radially inner portion 432, and the minimum thickness E31min of each radially outer portion 431 is at least equal to 5% of the depth P3 of the corresponding cutout 43. In the detailed view D1, according to a particular embodiment, the radially inner portion 432 has a height H32 at least equal to 1.5 times its maximum thickness E32max, and a circular radially inner end 4321 whose diameter D3 is equal to its maximum thickness E32max.

[0056] There figure 3Ais a top view of a central portion of a tread according to a particular embodiment of the invention, comprising internal transverse cutouts 44. In this particular embodiment, each block 3 comprises an internal transverse cutout 44, substantially parallel to the inter-block transverse cutouts 42 delimiting it, and opening into the two longitudinal grooves 41 delimiting it. The section plane EE, along the median longitudinal plane XZ, defines a cross-section of the alternating internal transverse cutouts 44 and inter-block transverse cutouts 42, as shown in the figure 3B In addition, two types of internal cutouts 44 are represented by the detail view E1 respectively of the figures 3C and 3D .

[0057] There figure 3Bis a cross-sectional view of a median portion of tread according to a particular embodiment of the invention, comprising internal transverse cutouts 44. Along the longitudinal direction X, an internal transverse cutout 44, having a depth P4, alternates with an inter-block transverse cutout 42, having a depth P2.

[0058] There figure 3C is a cross-sectional view of an internal transverse cut 44 comprising a single portion, corresponding to detail E1 of the figure 3B . In this particular case, the radially inner and radially outer portions respectively are merged into a single portion 441. Furthermore, in the embodiment shown, the minimum thickness E41min and the maximum thickness E41max of the portion 441 are equal to each other, such that the thickness at every point of said portion 441 is constant.

[0059] There 3D figureis a cross-sectional view of an internal transverse cut 44 comprising respectively a radially external portion 441 and a radially internal portion 442, corresponding to detail E1 of the figure 3BIn the general case, the radially external portion 441 has a thickness between a minimum thickness E41min and a maximum thickness E41max. In the case shown, the minimum thickness E41min and the maximum thickness E41max of the radially external portion 441 are equal, such that the thickness at every point of said radially external portion 441 is constant. The radially internal portion 442 has a thickness between a minimum thickness E42min and a maximum thickness E42max, which, in the general case, are not equal. In the detail view E1, according to a particular embodiment, the radially internal portion 432 has a height H42 at least equal to 1.5 times its maximum thickness E42max, and a circular radially internal end 4421 whose diameter D4 is equal to its maximum thickness E42max.

[0060] There figure 4is a first range of variation of a thickness Ei1 of the radially external portion 4i1 of a cut 4i, as a function of the depth Pi of said cut 4i, the index i being able to take the values ​​1, 2 and 3. An index i equal to 1 characterizes a longitudinal groove 41, an index i equal to 2 characterizes an inter-block transverse cut 42 and an index i equal to 3 characterizes an external transverse cut 43. The thickness Ei1 is the thickness measured at any point of the radially external portion 4i1. For a depth Pi between 50% of H and H, the thickness Ei1 of said radially external portion, at any point of the cut 4i, is between 5% of Pi and 20% of Pi.

[0061] There figure 5is a second domain of variation of a thickness Ei1 of radially outer portion 4i1 of a cut 4i, as a function of the depth Pi of said cut 4i, the index i being able to take the values ​​1, 2 and 3. The thickness Ei1 is the thickness measured at any point of the radially outer portion 4i1. For a depth Pi between 50% of H and H, the thickness Ei1 of said radially outer portion, at any point of the cut 4i, is between 10% of Pi and 20% of Pi: it is therefore a more restricted domain than the first domain.

[0062] There figure 6is a first domain of variation of a thickness E41 of radially external portion 441 of an internal cut 44, as a function of its depth P4. The thickness E41 is the thickness measured at any point of the radially external portion 441. In this first range of variation, an internal cut 44, formed in a block 3, has a depth P4 at most equal to min (P1, P2), that is to say the smallest of the respective depths (P1, P2) of each longitudinal groove 41 and of each transverse inter-block cut 42 delimiting the block 3 and a thickness E41, measured at any point of the radially external portion 441, therefore a maximum thickness E41max at most equal to min (E11max, E21max), that is to say the smallest of the maximum thicknesses (E11max, E21max) of the radially external portion (411, 421) of each longitudinal groove 41 and of each transverse inter-block cut 42 delimiting the block 3.

[0063] There figure 7 is a second range of variation of a thickness E41 of the radially outer portion 441 of an internal cut 44, as a function of its depth P4. The thickness E41 is the thickness measured at any point of the radially outer portion 441. In this second range of variation, an internal cut 44, formed in a block 3, has a depth P4 at most equal to min(P1, P2), that is, the smallest of the respective depths (P1, P2) of each longitudinal groove 41 and each transverse inter-block cut 42 delimiting the block 3, and a thickness E41, measured at any point of the radially outer portion 441, between 5% of P4 and 20% of P4. This range of variation is more restricted than the first range described previously.

[0064] There figure 8is a third range of variation of a thickness E41 of the radially outer portion 441 of an internal cut 44, as a function of its depth P4. The thickness E41 is the thickness measured at any point of the radially outer portion 441. In this third range of variation, an internal cut 44, formed in a block 3, has a depth P4 at most equal to min(P1, P2), that is, the smallest of the respective depths (P1, P2) of each longitudinal groove 41 and each transverse inter-block cut 42 delimiting the block 3, and a thickness E41, measured at any point of the radially outer portion 441, between 10% of P4 and 20% of P4. This range of variation is more restricted than the second range described previously.

[0065] A tread pattern according to the invention, as described in the preceding figures, has been studied by the inventors in tire size 53 / 80 R 63, for a tire intended to equip a heavy construction vehicle, more particularly a rigid dump truck. Such a tire is designed to carry a load of 82,500 kg, at an inflation pressure of 6 bar.

[0066] Table 1 below presents the characteristics of the tested tread: [Table 1] Features Values Tread width L 1188 mm Tread height (H) 110 mm Width L1 of the central portion of the tread 235 mm Longitudinal length B of a block 3 of median portion 260 mm Constant thickness (E11max=E11min) of a radially outer portion 411 of a longitudinal groove 41 10 mm Minimum thickness E12min of a radially inner portion 412 of a longitudinal groove 41 10 mm Maximum thickness E12max of a radially internal portion 412 of a longitudinal groove 41 16 mm Height H12 of a radially internal portion 412 of a longitudinal groove 41 25 mm Depth P1 of a longitudinal groove 41 96 mm Constant thickness (E21max=E21min) of a radially external portion 421 of a transverse inter-block cut 42 10 mm Minimum thickness E22min of a radially internal portion 421 of a transverse inter-block cut 42 10 mm Maximum thickness E22max of a radially internal portion 421 of a transverse inter-block cut 42 16 mm Height H22 of a radially internal portion 421 of a transverse inter-block cut 42 33 mm Depth P2 of a transverse inter-block cut 96 mm 42 Constant thickness (E31max=E31min) of a radially external portion 431 of an external transverse cut 43 10 mm Minimum thickness E32min of a radially internal portion 431 of an external transverse cut 43 10 mm Maximum thickness E32max of a radially internal portion 431 of an external transverse cut 43 16 mm Height H32 of a radially internal portion 431 of an external transverse cut 43 30 mm Depth P3 of an external transverse cut 43 96 mm Constant thickness (E41max=E41min) of a radially external portion 441 of an internal transverse cut 44 7 mm Minimum thickness E42min of a radially internal portion 441 of an internal transverse cut 44 7 mm Maximum thickness E42max of a radially internal portion 441 of an internal transverse cut 44 7 mm Height H42 of a radially internal portion 441 of an internal transverse cut 44 0 mm Depth P4 of an internal transverse cutout 44 36 mm

[0067] Rolling tests in mines were carried out on tires according to the invention and showed significantly improved resistance to mechanical stress compared to a reference tire whose tread pattern includes simple cuts, particularly transverse cuts, without an inner radial portion. Specifically, there was an absence of cracks at the base of the inner radial portion of the cuts, and an absence of tread block detachment. In other words, a tread pattern according to the invention reaches complete wear without cracking, whereas the reference tread pattern suffers cracking and / or block detachment during wear.

[0068] Regarding the improvement of the thermal ventilation capacity of the stepped cutouts, numerical simulations have shown a decrease in internal temperature of the blocks of 8°C when they include in particular an internal transverse cutout.

Claims

1. Tyre tread (1) for a heavy civil engineering vehicle, designed to come into contact with the ground via a running surface (2), comprising blocks (3) delimited by cut-outs (41, 42), and having a width L and a height H, defined as the maximum cut-out depth, - the tread having a median portion (11), symmetrical about a median longitudinal plane (XZ) and having a width L1 of not more than 60% of the width L of the tread, - the median portion (11) comprising at least one longitudinal row of blocks (3), wherein each block (3) is delimited transversely by two longitudinal channels (41) and longitudinally by two inter-block transverse cut-outs (42), - each longitudinal channel (41) and each inter-block transverse cut-out (42), respectively, having a depth (P1, P2) equal to at least 50% of the tread height H, and being, respectively, a staged cut-out comprising a radially outer portion (411, 421) opening on to the running surface (2) and a radially inner portion (412, 422) extending from the radially outer portion (411, 421) to a radially inner end (4121, 4221), - each radially outer portion (411, 421) having a thickness, measured between the walls of material delimiting it, within the range from a minimum thickness (E11min, E21min) to a maximum thickness (E11max, E21max), - each radially inner portion (412, 422) having a thickness, measured between the walls of material delimiting it, within the range from a minimum thickness (E12min, E22min) to a maximum thickness (E12max, E22max), characterized in that the maximum thickness (E11max, E21max) of each radially outer portion (411, 421) is equal to not more than 20% of the depth (P1, P2) of the corresponding cut-out (41, 42) and strictly less than the maximum thickness (E12max, E22max) of the corresponding radially inner portion (412, 422) and in that the minimum thickness (E11min, E21min) of each radially outer portion (411, 421) is equal to at least 5% of the depth (P1, P2) of the corresponding cut-out (41, 42).

2. Tread (1) according to Claim 1, wherein the minimum thickness (E11min, E21min) of each radially outer portion (411, 421) is equal to at least 10% of the depth (P1, P2) of the corresponding cut-out (41, 42).

3. Tread (1) according to either of Claims 1 and 2, wherein the minimum thickness (E11min, E21min) and the maximum thickness (E11max, E21max) of each radially outer portion (411, 421) are equal to one another, so that the thickness at all points of said radially outer portion (411, 421) is constant.

4. Tread (1) according to any of Claims 1 to 3, wherein the maximum thickness (E12max, E22max) of each radially inner portion (412, 422) is equal to at least 15% of the depth (P1, P2) of the corresponding cut-out (41, 42).

5. Tread (1) according to any of Claims 1 to 4, wherein each radially inner portion (412, 422) has a height (H12, H22) equal to at least 1.5 times the maximum thickness (E12max, E22max) of the corresponding radially inner portion (412, 422).

6. Tread (1) according to any of Claims 1 to 5, wherein each radially inner portion (412, 422) has a circular radially inner end (4121, 4221) whose diameter (D1, D2) is equal to the maximum thickness (E12max, E22max) of the corresponding radially inner portion (412, 422).

7. Tread (1) according to any of Claims 1 to 6, each block (3) having a longitudinal length B, wherein the depth P2 of each inter-block transverse cut-out (42) is equal to not more than the longitudinal length B of the block (3), or preferably not more than 0.8 times the longitudinal length B of the block (3).

8. Tread (1) according to any of Claims 1 to 7, comprising outer transverse cut-outs (43), opening into the longitudinal channels (41) delimiting the blocks (3), each having a depth P3 equal to at least 50% of the tread height H, and comprising a radially outer portion (431) and a radially inner portion (432), each radially inner portion (431) having a thickness within the range from a minimum thickness E31min to a maximum thickness E31max, each radially inner portion (432) having a thickness in the range from a minimum thickness E32min to a maximum thickness E32max, wherein the maximum thickness E31max of each radially outer portion (431) is equal to not more than 20% of the depth P3 of the corresponding cut-out (43) and strictly less than the maximum thickness E32max of the corresponding radially inner portion (412, 422, 432), and wherein the minimum thickness E31min of each radially outer portion (431) is equal to at least 5% of the depth P3 of the corresponding cut-out (43).

9. Tread (1) according to any of Claims 1 to 8, wherein each block (3) comprises at least one internal cut-out (44) having a depth P4 not greater than the smallest of the respective depths (P1, P2) of each longitudinal channel (41) and of each inter-block transverse cut-out (42) delimiting the block (3).

10. Tread (1) according to Claim 9, each internal cut-out (44) comprising a radially outer portion (441) having a thickness in the range from a minimum thickness E41min to a maximum thickness E41max, wherein the maximum thickness E41max of the radially outer portion (441) of the internal cut-out (44) is not greater than the smallest of the maximum thicknesses (E11max, E21max) of the radially outer portion (411, 421) of each longitudinal channel (41) and of each inter-block transverse cut-out (42) delimiting the block (3).

11. Tread (1) according to Claim 10, each internal cut-out (44) comprising a radially inner portion (442) having a thickness in the range from a minimum thickness E42min to a maximum thickness E42max, wherein the maximum thickness E41max of the radially outer portion (441) of the internal cut-out (44) is strictly less than the maximum thickness E42max of the radially inner portion (442) of the corresponding internal cut-out (44).

12. Tread (1) according to any of Claims 9 to 11, wherein each internal cut-out (44) is transverse.

13. Tread (1) according to any of Claims 1 to 12, wherein the median portion of the tread (1) comprises a longitudinal row of blocks (3) centred on the median longitudinal plane (XZ).

14. Tyre to be fitted to a heavy civil engineering vehicle, comprising a tread according to any of Claims 1 to 13.