Tire Tread for a Heavy-Duty Vehicle of Construction Plant Type

Abstract

A tire tread for a heavy-duty vehicle of construction plant type and aims to reduce the risk of cracking at the sipe bottom, without significantly reducing the volume of material of the tread so as not to shorten the lifetime of the tire in respect of wear. The tread (I) has transverse sipes (5) having a depth H and a width E, each transverse sipe (5) having a radially inner end formed by a bulge (53) having an end radius R, the transverse sipes (5) being distributed longitudinally at a longitudinal spacing B, the depth H, the width E of each transverse sipe (5), the end radius R of the bulge (53) and the longitudinal spacing B satisfying the relationship (R*B)/(E*H)>=1.8.

Claims

1. A tire tread for a heavy-duty vehicle of construction plant type, said tread being intended to come into contact with the ground via a tread surface, having a radial thickness H.sub.T at least equal to 60 mm and comprising cuts delimiting elements in relief, the cuts being, in a longitudinal direction (XX′), longitudinal furrows, or, in a transverse direction (YY′), transverse grooves or transverse sipes, each transverse sipe being delimited by two walls intended to come into contact with one another when the tread surface comes into contact with the ground as the tire runs along, each transverse sipe having a depth H, measured radially towards the inside from the tread surface, at most equal to the radial thickness H.sub.T, and a width E, measured perpendicularly to the two walls delimiting said transverse sipe and corresponding to the minimum distance between the two walls, each transverse sipe having a radially inner end made up of a bulge having an end radius R, the transverse sipes being distributed longitudinally at a longitudinal spacing B, wherein the depth H, the width E of each transverse sipe, the end radius R of the bulge and the longitudinal spacing B satisfy the relationship (R*B)/(E*H)>=1.8.

2. The tread according to claim 1, wherein the width E of each transverse sipe is at least equal to 1 mm and at most equal to 20% of the radial thickness H.sub.T.

3. The tread according to claim 1, wherein the end radius R of the bulge is at least equal to 2 times the width E of the transverse sipe.

4. The tread according to claim 1, wherein each bulge is a cylinder of radius R.

5. The tread according to claim 1, wherein the transverse sipes are distributed longitudinally at a spacing B such that their depth H is at least equal to 0.5 times the spacing B.

6. The tread according to claim 1, wherein the depths H of two consecutive transverse sipes are different.

7. The tread according to claim 6, wherein the depth H alternately assumes two different values H1 and H2 for two consecutive transverse sipes.

8. The tread according to claim 1, wherein each transverse sipe has an undulating mean surface in a radial direction (ZZ′) and/or in the transverse direction (YY′).

9. The tread according to claim 1, wherein all of the cuts have a total volume V.sub.c and all of the bulges of the transverse sipes have a total volume V.sub.I, and wherein the total volume V.sub.I of the bulges of the transverse sipes is at most equal to 10% of the total volume V.sub.c of the cuts.

10. The tread according to claim 1, wherein the tread comprises at least two longitudinal furrows.

11. A tread for a heavy-duty vehicle of construction plant type, comprising a tread according to claim 1.

12. The tread according to claim 5, wherein the transverse sipes have their depth H at least equal to the spacing B.

Description

[0039] The features of the invention are illustrated in the schematic FIGS. 1 to 8, which are not shown to scale: [0040] FIG. 1: Top view of a tread according to the invention, [0041] FIG. 2: View in cross section, on a longitudinal plane, of a tread according to the invention, [0042] FIG. 3: Detail view in cross section, on a longitudinal plane, of a transverse sipe with a bulge at the sipe bottom, [0043] FIG. 4: Detail view in cross section, on a longitudinal plane, of a transverse sipe having a bulge at the sipe bottom and surface widening, [0044] FIG. 5: View in cross section, on a longitudinal plane, of a tread according to the invention comprising a succession of transverse sipes with alternating depths, [0045] FIG. 6: Detail view in cross section, on a longitudinal plane, of an undulating transverse sipe, [0046] FIG. 7: Mechanical operating principle of a transverse sipe, under the action of shear loading, [0047] FIG. 8: Curves representing the maximum shear (or stress slip) T.sub.max at the sipe bottom as a function of the tensile load F.sub.x to be transmitted by the tire.

[0048] FIG. 1 is a top view of a tread according to the invention. The tread 1, which is intended to come into contact with the ground via a tread surface 2, comprises cuts (3, 4, 5) delimiting elements in relief 6. The cuts (3, 4, 5) are longitudinal furrows 3 in a longitudinal direction XX′, or transverse grooves 4 or transverse sipes 5 in a transverse direction YY′.

[0049] FIG. 2 is a view in cross section, on a longitudinal plane XZ, of a tread according to the invention. In the tread, which has a radial thickness H.sub.T at least equal to 60 mm, each transverse sipe 5 has a depth H, measured radially towards the inside from the tread surface 2, at most equal to the radial thickness H.sub.T, and comprises a radially inner end made up of a bulge 53. The transverse sipes 5 are distributed longitudinally at a longitudinal spacing B. In the example shown, the tread also comprises, periodically, a transverse groove 4 positioned longitudinally between two transverse sipes 5, thereby locally creating a greater spacing between the two transverse sipes in question.

[0050] FIG. 3 is a detail view in cross section, on a longitudinal plane, of a transverse sipe having a bulge at the sipe bottom. The transverse sipe 5 has a depth H, measured radially towards the inside from the tread surface 2, at most equal to the radial thickness H.sub.T, and a width E, measured perpendicularly to the two walls (51, 52) delimiting said transverse sipe 5 and corresponding to the minimum distance between the two walls. Moreover, it has a radially inner end made up of a bulge 53 having an end radius R.

[0051] FIG. 4 is a detail view in cross section, on a longitudinal plane XZ, of a transverse sipe having a bulge at the sipe bottom and a surface widening. It differs from FIG. 3 by the presence of a widening 54 of the sipe at the tread surface 2, over a limited radial height. The thickness E of the sipe is not measured at the widening, at the radially outer end, or at the bulge, at the radially inner end: it is measured in the main portion of the sipe.

[0052] FIG. 5 is a view in cross section, on a longitudinal plane XZ, of a tread according to the invention comprising a succession of transverse sipes with alternating depths. In the embodiment shown, the depths H of two consecutive transverse sipes 5 are different. More specifically, the depth H alternately assumes two different values H1 and H2 for two consecutive transverse sipes 5. Moreover, each transverse sipe 5 has an undulating mean surface in a radial direction ZZ′. FIG. 6 is a detail view in cross section, on a longitudinal plane XZ, of an undulating transverse sipe.

[0053] FIG. 7 illustrates the mechanical operating principle of a transverse sipe, under the action of a shear load. When running, the ground exerts a longitudinal shear load F.sub.x on the tread at the tread surface, which generates, at the bottom of the radially inner bulge of the sipe, a stress concentration characterized by a maximum shear T.sub.max.

[0054] FIG. 8 illustrates three curves representing the maximum shear (or shear slip) T.sub.max, at the sipe bottom, as a function of the tensile load Fx to be transmitted by the tire, for two prior art treads R1 and R2 and for a tread I according to the invention. These three curves are discussed below in the description of the examples tested by the inventors.

[0055] The inventors studied this invention more particularly for a dumper tire of the size 40.00 R 57. A tread I according to the invention was compared with two prior art treads R1 and R2, these three treads all comprising transverse sipes.

[0056] The transverse sipes of each of these treads are characterized by their radial height H, their width E and by their end radius R. The treads R1 and R2 do not have a bulge at their radially inner end, the radius R characterizes the rounded portion of their radially inner end and is, consequently, equal to half the width E of the sipe. Since the tread I according to the invention has a bulge at its radially inner end, the radius R is that of said bulge, the diameter of which is, by definition, greater than the width E of the sipe. Moreover, these transverse sipes are distributed longitudinally at a longitudinal spacing B.

[0057] Table 1 below presents the characteristics H, E, R and B, and the resultant characteristic ratio (R*B)/(E*H):

TABLE-US-00001 TABLE 1 Characteristics H E R B (R*B)/(E*H) R1  80 mm 10 mm 5 mm 240 mm 1.5 R2 100 mm  2 mm 1 mm  60 mm 0.3 I  80 mm  2 mm 5 mm  70 mm 2.19

[0058] The transverse sipes of the treads R1 and R2 have a characteristic ratio (R*B)/(E*H) equal to 1.5 and to 0.3, respectively, i.e. less than 1.8. By contrast, the transverse sipes of the tread I have a characteristic ratio equal to 2.19, i.e. greater than 1.8, in accordance with the invention.

[0059] It should also be noted that, for the tread I, the total volume V.sub.I of bulges of the transverse sipes is equal to 6%, i.e. less than 10%, of the total volume Vc of the cuts: this implies that the presence of these sipe bottom bulges does not significantly reduce the volume of rubber to be worn away and therefore the lifetime in terms of wear.

[0060] The invention was tested for a dumper tire of the size 40.00 R 57, intended to carry a load equal to 588 600 N, for an inflation pressure equal to 6.5 bar, according to the ETRTO standard. A dumper usually comprises two tires on its front steered axle and four tires on its rear driven axle: the vehicle as a whole can therefore carry 6*588 600 N=3 531 600 N. Assuming there is a mean gradient equal to 10%, when used in a mine, the tensile load to be transmitted by the rear driven axle equipped with 4 wheels is equal to 0.1*3 531 600 N=353 160 N, or 353 160 N/4=88 290 N for each driven wheel.

[0061] Following simulations involving finite-element calculations, the inventors estimated that the risk of cracking at the transverse sipe bottom becomes high above a maximum shear T.sub.max equal to 2 N/mm.sup.2. The curves in FIG. 8 show that, for the two prior art treads R1 and R2, the maximum shear T.sub.max increases rapidly and significantly above a tensile load value F.sub.x substantially equal to 20 000 N and 60 000 N, respectively, and reaches a value equal to 3.5 N/mm.sup.2 when the tensile load F.sub.x reaches the value, estimated above, equal to 88 290 N. Consequently, for the prior art treads R1 and R2, at the level of the tensile load in question, the risk of cracking is high since the maximum shear is greater than 2 N/mm.sup.2. By contrast, in the case of the tread I, for a tensile load equal to 88 290 N, the maximum shear T.sub.max at the sipe bulge bottom is equal to 1.5 N/mm.sup.2, and therefore remains below 2 N/mm.sup.2.