Tire Tread For A Heavy Civil-Engineering Vehicle

Abstract

A radial tire intended to be fitted to a heavy vehicle of construction plant type, and more particularly to the tread thereof. The tread of such a tire is desensitized to attack by indenting bodies that are likely to cause cracks at the cut bottom, in particular in the case of a tread with a high volumetric void ratio and a high degree of surface siping. Such a tire has a degree of surface siping TL of the tread at least equal to 3 m/m.sup.2, a number of cycles to failure N.sub.R of the elastomeric compound of the tread, at least present at the cut bottom, at least equal to 60000 cycles, and a ratio C between the number of cycles to failure N.sub.R of the elastomeric compound and the degree of surface siping TL at least equal to 20000 cycles/(m/m.sup.2).

Claims

1. A tire for a heavy vehicle of construction plant type, comprising: a tread having a radial thickness H.sub.T at least equal to 60 mm and an axial width W.sub.T, the tread comprising cuts having a width W.sub.D and a radial depth H.sub.D, and elements in relief that are separated from one another by the cuts, wherein the cuts, the width W.sub.D of which is at most equal to 20% of their radial depth H.sub.D and the radial depth H.sub.D of which is at least equal to 50% of the radial thickness H.sub.T of the tread, referred to as effective cuts, have a cumulative length L.sub.D measured on a radially outer surface of the tread, the tread having a degree of surface siping TL, expressed in m/m.sup.2, equal to the ratio between the cumulative length L.sub.D of the effective cuts and the area A of the radially outer surface of the tread equal to 2R.sub.E*W.sub.T, where R.sub.E is the external radius of the tire, the tread comprising, at least at the bottoms of the cuts, an elastomeric compound having a crack resistance defined by a number of cycles to failure N.sub.R, wherein the degree of surface siping TL of the tread is at least equal to 3 m/m.sup.2, wherein the number of cycles to failure N.sub.R of the elastomeric compound of the tread is at least equal to 60000 cycles, and wherein the ratio C between the number of cycles to failure N.sub.R of the elastomeric compound of the tread and the degree of surface siping TL of the tread is at least equal to 20000 cycles/(m/m.sup.2).

2. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the ratio C is at least equal to 40000 cycles/(m/m.sup.2).

3. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the degree of siping TL of the tread is at least equal to 3.5 m/m.sup.2.

4. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the degree of siping TL of the tread is at most equal to 9 m/m.sup.2.

5. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the number of cycles to failure N.sub.R of the elastomeric compound of the tread is at least equal to 120000 cycles.

6. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elastomeric compound of the tread has a dynamic shear module G* at least equal to 1.0 MPa.

7. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elastomeric compound of the tread has a dynamic loss tg at most equal to 0.2.

8. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elastomeric compound of the tread comprises an elastomeric matrix consisting of a natural polyisoprene.

9. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elastomeric compound of the tread comprises a reinforcing filler, the content of which is at least equal to 25 phr (part per hundred parts of elastomer) and at most equal to 80 phr.

10. The tire for a heavy vehicle of construction plant type according to claim 9, wherein the reinforcing filler of the elastomeric compound of the tread comprises a carbon black, the content of which is at least equal to 25 phr and at most equal to 60 phr.

11. Tire for a heavy vehicle of construction plant type according to claim 9, wherein the reinforcing filler of the elastomeric compound of the tread comprises an inorganic filler, the content of which is at most equal to 25 phr.

12. The tire for a heavy vehicle of construction plant type according to claim 9, the reinforcing filler of the elastomeric compound of the tread having a dispersion coefficient Z, wherein the dispersion coefficient Z of the reinforcing filler of the elastomeric compound of the tread is at least equal to 65.

13. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the tread is made up of a single elastomeric compound.

14. The tire for a heavy vehicle of construction plant type according to claim 1, the set of cuts having a total volume V.sub.D and the set of elements in relief having a total volume V.sub.R, the tread having a volumetric void ratio TEV, expressed in %, equal to the ratio between the total volume V.sub.D of the cuts and the sum of the total volume V.sub.D of the cuts and the total volume of the elements in relief, wherein the volumetric void ratio TEV of the tread is at least equal to 12%.

15. The tire for a heavy vehicle of construction plant type according to claim 9, wherein the reinforcing filler of the elastomeric compound of the tread comprises a silica, the content of which is at most equal to 25 phr.

16. The tire for a heavy vehicle of construction plant type according to claim 1, the set of cuts having a total volume V.sub.D and the set of elements in relief having a total volume V.sub.R, the tread having a volumetric void ratio TEV, expressed in %, equal to the ratio between the total volume V.sub.D of the cuts and the sum of the total volume V.sub.D of the cuts and the total volume of the elements in relief, wherein the volumetric void ratio TEV of the tread is at least equal to 14%.

Description

[0048] The features of the invention will be better understood with the aid of FIGS. 1 to 3, which are schematic and not to scale:

[0049] FIG. 1 is a half-section, on a meridian plane, of a crown of a tire for a heavy vehicle of construction plant type, according to the invention.

[0050] FIGS. 2A to 2C show embodiment variants of a tread for a tire for a heavy vehicle of construction plant type, according to the invention.

[0051] FIG. 3 shows the range of the number of cycles to failure N.sub.R of the elastomeric compound of the tread as a function of the degree of surface siping TL of the tread for a tire for a heavy vehicle of construction plant type according to the invention.

[0052] FIG. 1 shows a meridian half-section, in a plane YZ, of the crown of a tire 1 for a heavy vehicle of construction plant type, comprising a tread 2 and a crown reinforcement 3 radially on the inside of the tread 2. The tread 2, having a radial thickness H.sub.T at least equal to 60 mm, comprises cuts 21 having a width W.sub.D and a radial depth H.sub.D, and elements in relief 22 separated by the cuts 21. The cuts 21, the width W.sub.D of which is at most equal to 20% of the radial depth H.sub.D thereof, measured between a radially outer surface 23 of the tread 2 and a cut bottom 24, and the radial depth H.sub.D of which is at least equal to 50% of the radial thickness H.sub.T, referred to as effective cuts, have a cumulative length L.sub.D (not shown in the figure) measured on the radially outer surface 23 of the tread 2. The tread 2 has a degree of surface siping TL, expressed in m/m.sup.2, equal to the ratio between the cumulative length L.sub.D of the effective cuts 21 and the area A of the radially outer surface 23 of the tread equal to 2R.sub.E*W.sub.T, where R.sub.E is the external radius of the tire, measured in the equatorial plane XZ, between the axis of revolution YY and the radially outer surface 23 of the tread 2 or tread surface. Radially on the inside of the tread 2, the crown reinforcement 3 comprises, radially from the outside to the inside, a protective reinforcement made up of two protective layers, a working reinforcement made up of two working layers, and a hoop reinforcement made up of two hooping layers.

[0053] FIGS. 2A to 2C show embodiment variants of a tread for a tire for a heavy vehicle of construction plant type, according to the invention. Only one half-tread, in a meridian plane, is shown. FIG. 2A shows a tread 2 made up of a single elastomeric compound 3, which is resistant to cracking within the meaning of the invention, i.e. is characterized by a number of cycles to failure N.sub.R at least equal to 60000 cycles. FIG. 2B shows a tread 2, a radially outer portion of which is made up of an elastomeric compound 3 that is resistant to cracking within the meaning of the invention. Finally, FIG. 2C shows the case in which the elastomeric compound 3 that is resistant to cracking within the meaning of the invention is only located at a cut bottom 24.

[0054] FIG. 3 shows the range of the number of cycles to failure N.sub.R of the elastomeric compound of the tread as a function of the degree of surface siping TL of the tread for a tire for a heavy vehicle of construction plant type according to the invention. According to the invention, the degree of surface siping TL of the tread is at least equal to 3 m/m.sup.2, and the number of cycles to failure N.sub.R of the elastomeric compound of the tread is at least equal to 60000 cycles, with a ratio C between the number of cycles to failure N.sub.R and the degree of surface siping TL at least equal to 20000 cycles/(m/m.sup.2). Consequently, the range of the invention, which is hatched in FIG. 3, is delimited by the straight lines TL=3 m/m.sup.2 and N.sub.R=C*TL=20000*TL cycles/(m/m.sup.2). The graph in FIG. 3 shows an example of the prior art E, outside the range of the invention, characterized by a degree of surface siping TL equal to 1.6 m/m.sup.2, i.e. less than 3 m/m.sup.2, and a number of cycles to failure N.sub.R equal to 80000 cycles. Also shown are two exemplary embodiments of the invention, I1 and I2, for which the degree of surface siping TL is equal to 4.2 m/m.sup.2, and having a number of cycles to failure N.sub.R equal to 120000 cycles and to 140000 cycles, respectively.

[0055] The invention has been studied more particularly in the case of a tire of size 40.00R57. Two examples of tires according to the invention I1 and I2 and a tire of the prior art E, taken as a reference, were compared by the inventors.

[0056] The respective features of siping and of the elastomeric compound of the tread of the tire E and of the tires I1 and I2 are set out in Table 1 below.

TABLE-US-00001 TABLE 1 Tire size I1 (40.00R57) I2 (40.00R57) Degree of 1.6 m/m.sup.2 4.2 m/m.sup.2 4.2 m/m.sup.2 surface siping TL Elastomeric polyisoprene polyisoprene polyisoprene matrix Content of Carbon black: Carbon black: Carbon black: reinforcing 40 phr 42 phr 35 phr filler Silica: 15 phr Silica: 15 phr Silica: 10 phr Dynamic shear 1.4 MPa 1.4 MPa 1.2 MPa modulus G* Dynamic loss 0.14 0.14 0.07 tg Dispersion 55 75 65 coefficient Z of the rein- forcing filler Number of 80000 cycles 120000 cycles 140000 cycles cycles to failure N.sub.R Ratio C = 50000 cycles/ 28571 cycles/ 33333 cycles/ N.sub.R/TL (m/m.sup.2) (m/m.sup.2) (m/m.sup.2)

[0057] According to Table 1, the three tires compared E, I1 and I2 have a tread made up of a single elastomeric compound, the elastomeric matrix of which is a polyisoprene, that is to say a natural rubber, and the reinforcing filler of which comprises both a carbon black and an inorganic filler of the silica type. The tire E of the prior art does not fall within the scope of the invention since it does not meet the criterion of minimum degree of surface siping TL at least equal to 3 m/m.sup.2, that is to say of a tread with a sufficient number of cuts. By contrast, the degree of surface siping TL of the tires I1 and I2 is equal to 4.2 m/m.sup.2 and thus meets this criterion. The elastomeric compound of the tread of the tire I1 is both stiffer and has greater hysteresis than that of the tire I2. Furthermore, the reinforcing filler for the tire I1 is more dispersed than for the tire I2. However, with the number of cycles to failure N.sub.R being lower for the tire I1 than for the tire I2, with the degree of siping TL being the same, the ratio C is lower for the tire I1 than for the tire I2. Therefore, the tire I2 performs better than the tire I1 as regards resistance to attack.