Tire Crown For Heavy Goods Vehicle Of The Civil Engineering Type

Abstract

Crown of a tire for a heavy vehicle of construction plant type is desensitized to attacks. The tire includes a tread (2) 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 cuts (21) and the area A of the radially outer surface (23) of the tread (2), and a protective reinforcement (4) having at least two protective layers (41, 42) that are formed of elastic metallic reinforcers and have a maximum breaking strength R.sub.max, expressed in daN/m, such that the degree of surface siping TL of the tread (2) is at least equal to 3 m/m.sup.2 and a coupling ratio C, equal to the ratio between the maximum breaking strength R.sub.max and the degree of surface siping TL, is at least equal to 30000 daN.

Claims

1. A tire for a heavy vehicle of construction plant type, comprising a tread and a crown reinforcement radially on the inside of the tread: the tread, having a radial thickness H.sub.T at least equal to 60 mm, comprising cuts having a width W.sub.D and a radial depth H.sub.D, and elements in relief separated by the cuts; the cuts, the width W.sub.D of which is at most equal to 20% of the radial depth H.sub.D and the radial depth H.sub.D of which is at least equal to 50% of the radial thickness HT, referred to as effective cuts, having 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 2?R.sub.E*W.sub.T, where R.sub.E is the external radius of the tire; the crown reinforcement comprising a protective reinforcement, a working reinforcement and a hoop reinforcement; the protective reinforcement, which is radially outermost, comprising at least two protective layers, formed of elastic metallic reinforcers, that form an angle of between 15? and 45? with the circumferential direction, each said protective layer having a breaking strength R per unit of layer width, expressed in daN/m, R.sub.max being the maximum value of the breaking strengths R of the protective layers; the working reinforcement comprising at least two working layers, formed of inelastic metallic reinforcers that are crossed from one said working layer to the next and form an angle of between 15? and 45? with a circumferential direction of the tire; the hoop reinforcement comprising at least one hooping layer, formed of metallic reinforcers that form an angle at most equal to 15? with the circumferential direction, wherein the degree of surface siping TL of the tread is at least equal to 3 m/m.sup.2, and wherein a coupling ratio C, equal to the ratio between the maximum value R.sub.max of the breaking strengths R of the protective layers and the degree of surface siping TL of the tread, is at least equal to 30000 daN.

2. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the coupling ratio C is at least equal to 40000 daN.

3. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the coupling ratio C is at most equal to 120000 daN.

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 least equal to 3.5 m/m.sup.2.

5. 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.

6. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the maximum value R.sub.max of the breaking strengths R of the protective layers is at least equal to 150000 daN/m.

7. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the breaking strength R of the radially outermost protective layer is equal to the maximum value R.sub.max of the breaking strengths R of the protective layers.

8. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the breaking strength R of each protective layer is equal to the maximum value R.sub.max of the breaking strengths R of the protective layers.

9. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the minimum value R.sub.min of the breaking strengths R of the protective layers is such that the ratio R.sub.min/TL is at least equal to 30000 daN.

10. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elastic metallic reinforcers of the protective layers are multistrand ropes, made up of a single layer of N strands, N being between 3 and 5, each strand being made up of metal threads.

11. The tire for a heavy vehicle of construction plant type according to claim 10, wherein each strand, of structure (M+P), comprises an internal layer of M metal threads and an external layer of P metal threads wound around the internal layer.

12. The tire for a heavy vehicle of construction plant type according to claim 11, wherein the elastic metallic reinforcers of the protective layers are multistrand ropes, of structure 4*(3+8)*0.35, composed of a single layer of 4 strands, each strand comprising an internal layer of 3 metal threads and an external layer of 8 metal threads wound around the internal layer, and each strand being composed of metal threads with a diameter equal to 0.35 mm.

13. The tire for a heavy vehicle of construction plant type according to claim 11, wherein the elastic metallic reinforcers of the protective layers are multistrand ropes, of structure 4*(4+9)*0.26, composed of a single layer of 4 strands, each strand comprising an internal layer of 4 metal threads and an external layer of 9 metal threads wound around the internal layer, and each strand being composed of metal threads with a diameter equal to 0.26 mm.

14. The tire for a heavy vehicle of construction plant type according to claim 10, wherein each strand, of structure (M+N+P), comprises an intermediate layer of N metal threads wound around the internal layer of M metal threads, the external layer of P metal threads being wound around the intermediate layer of N metal threads.

15. The tire for a heavy vehicle of construction plant type according to claim 11, wherein the external layer of P metal threads is unsaturated.

16. The tire for a heavy vehicle of construction plant type according to claim 10, wherein the diameter of the constituent threads of each strand is at least equal to 0.22 mm.

17. The tire for a heavy vehicle of construction plant type according to claim 10, wherein the elastic metallic reinforcers of the protective layers have, in the air permeability test, a mean air flow rate of less than 30 cm.sup.3/min.

18. The tire for a heavy vehicle of construction plant type according to claim 10, wherein the elastic metallic reinforcers of the protective layers are distributed at a mean spacing of between 3.5 mm and 5 mm.

19. 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%.

Description

[0068] 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.

[0069] FIG. 2 shows the range of the maximum breaking strengths R.sub.max as a function of the degree of siping TL of the tread for a tire for a heavy vehicle of construction plant type, according to the invention.

[0070] 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 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 a 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 2?R.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 a protective reinforcement 4, a working reinforcement 5 and a hoop reinforcement 6. The protective reinforcement 4, which is radially outermost, comprises two protective layers (41, 42), formed of elastic metallic reinforcers, that form an angle of between 15? and 45? with the circumferential direction XX. Each protective layer (41, 42) has a breaking strength R per unit of layer width, expressed in daN/m, R.sub.max being the maximum value of the breaking strengths R of the protective layers (41, 42). The working reinforcement 5 comprises two working layers (51, 52), formed of inelastic metallic reinforcers that are crossed from one working layer to the next and form an angle of between 15? and 45? with the circumferential direction XX. The hoop reinforcement 6 comprises two hooping layers (61, 62), formed of metallic reinforcers that form an angle at most equal to 15? with the circumferential direction XX. According to the invention, the degree of surface siping TL of the tread 2 is at least equal to 3 m/m.sup.2, and a coupling ratio C, equal to the ratio between the maximum value R.sub.max of the breaking strengths R of the protective layers (41, 42) and the degree of surface siping TL of the tread 2, is at least equal to 30000 daN.

[0071] FIG. 2 shows the range of the maximum breaking strengths R.sub.max as a function of the degree of 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 a coupling ratio C, equal to the ratio between the maximum value R.sub.max of the breaking strengths R of the protective layers and the degree of surface siping TL of the tread 2, is at least equal to 30000 daN. Consequently, the range of the invention is defined by the maximum breaking strengths R.sub.max at least equal to 30000*TL, where TL is the degree of surface siping, with TL being at least equal to 3 m/m.sup.2. Shown on the abscissa axis of the graph in FIG. 2 is the minimum value of the degree of surface siping TL of the tread equal to 3 m/m.sup.2. Shown on the ordinate axis of the graph in FIG. 2 is the minimum value of the maximum strength R.sub.max of the breaking strengths R of the protective layers equal to 90000 daN/m, corresponding to the minimum coupling rate C equal to 30000 daN. Also shown in the graph are a first exemplary embodiment of the invention I1 in which the degree of surface siping TL is equal to 4.2 m/m.sup.2 and the maximum value R.sub.max of the breaking strengths R of the protective layers is equal to 160000 daN/m, the protective layers comprising elastic multistrand ropes of structure 4*(4+9)*0.26, and a second exemplary embodiment of the invention I2, in which the degree of surface siping TL is equal to 4.2 m/m.sup.2 and the maximum value R.sub.max of the breaking strengths R of the protective layers is equal to 200000 daN/m, the protective layers comprising elastic multistrand ropes of structure 4*(3+8)*0.35. Also shown in FIG. 2 is an example of the prior art E that is characterized by a degree of surface siping TL equal to 1.6 m/m.sup.2 and a maximum value R.sub.max of the breaking strengths R of the protective layers equal to 102000 daN/m, i.e. outside the range of the invention.

[0072] 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.

[0073] In the case studied, the tires of the prior art E and according to the invention I1 and I2, respectively, have a crown reinforcement comprising, radially from the outside to the inside, a protective reinforcement made up of two protective layers with elastic metallic reinforcers, a working reinforcement made up of two working layers with inelastic metallic reinforcers, and a hoop reinforcement made up of two hooping layers with inelastic metallic reinforcers. As regards the protective reinforcement, the elastic metallic reinforcers of the two protective layers, which are crossed from one layer to the next, form, with the circumferential direction XX, an angle equal to 24? for the tire of the prior art E, and an angle equal to 33? for the tires according to the invention I1 and I2. As regards the working reinforcement, the inelastic metallic reinforcers of the two working layers, which are crossed from one layer to the next, form, with the circumferential direction XX, angles equal to 33? and 19?, respectively, for the tire of the prior art E, and angles equal to 33? and 24?, respectively, for the tires according to the invention I1 and I2. As regards the hoop reinforcement, the inelastic metallic reinforcers of the two hooping layers, which are crossed from one layer to the next, form, with the circumferential direction XX, an angle of between 6? and 8? for the tire of the prior art E and for the tires according to the invention I1 and I2.

[0074] In the case studied, the tires of the prior art E and according to the invention I1 and I2, respectively, have treads that comprise at least three circumferential cuts or furrows, the cuts having a width W.sub.D at least equal to 8 mm The corresponding treads have a volumetric void ratio at least equal to 12%.

[0075] For the case studied of 40.00R57, the characteristics of the crown for the tire of the prior art E taken as a reference and for the tires according to the invention I1 and I2 are presented in Table 1 below:

TABLE-US-00001 TABLE 1 Tire size E (40.00R57) I1 (40.00R57) I2 (40.00R57) Axial width W.sub.T of the tread (m) 0.98 m 0.98 m 0.98 m Radial thickness H.sub.T of the tread (m) 0.098 m 0.098 m 0.098 m External radius R.sub.E of the tire (m) 1.79 m 1.79 m 1.79 m Area of the tread surface A = 11 m.sup.2 11 m.sup.2 11 m.sup.2 2?R.sub.E*W.sub.T (m.sup.2) Cumulative length L.sub.D of the 17.3 m 45.8 m 45.8 m effective cuts (m) Degree of siping TL = L.sub.D/A (m/m.sup.2) 1.6 m/m.sup.2 4.2 m/m.sup.2 4.2 m/m.sup.2 Volumetric void ratio TE.sub.V (%) 18.1% 14.2% 14.2% Type of metallic reinforcers of the 4*(1 + 5)*0.26 4*(4 + 9)*0.26 4*(3 + 8)*0.35 protective layers Spacing of the metallic reinforcers of 2.5 mm 3.7 mm 4.8 mm the protective layers (mm) Maximum value R.sub.max of the breaking 102000 daN/m 160000 daN/m 200000 daN/m strengths R of the protective layers (daN/m) Coupling ratio C = R.sub.max/TL (daN) 63750 daN 38095 daN 47620 daN

[0076] The tires of the prior art and according to the invention were subjected to measurements and tests, in particular to evaluate the heat level of the crown, when the tire is subjected to recommended pressure, loading and speed conditions, and to quantify the breaking strength of the crown, when the tire is subjected to attacks by indenting bodies.

[0077] As far as the heat level is concerned, the temperature of the crown is measured close to the axial ends of the crown reinforcement, which are generally the hot points of the crown, with the aid of a temperature sensor. The results of these thermal measurements, in terms of temperatures at the axial ends of the crown reinforcement, are presented in Table 2 below, in relative values with respect to the tire of the prior art taken as a reference.

[0078] In order to characterize the breaking strength of a tire crown reinforcement subjected to impacts, a person skilled in the art is familiar with carrying out tests that consist in causing a tire, inflated to a recommended pressure and subjected to a recommended load, to run over a cylindrical indenting body, referred to as a polar, with a diameter of between 1 inch, or 25.4 mm, and 2.2 inches, or 55 9 mm, depending on the size of the tire, and with a given height. The breaking strength is characterized by the critical height of the polar, i.e. the maximum height of the polar that results in complete breakage of the crown reinforcement, i.e. in the breakage of all the crown layers. The results of these attack tests, in terms of maximum heights of a cylindrical polar with a diameter equal to 2 inches, are presented in Table 2 below, with respect to the tire of the prior art taken as a reference as base 100.

[0079] Table 2 below presents the results of thermal performance and performance with regard to resistance to attacks for the studied tires of the prior art E and according to the invention I1 and I2:

TABLE-US-00002 TABLE 2 Tire size E I1 I2 (40.00R57) (40.00R57) (40.00R57) Thermal performance reference ?10? C. ?9? C. (temperatures at the axial ends of the crown reinforcement) Performance with regard 100 140 180 to resistance to attacks (maximum height of the cylindrical polar with a diameter of 2)

[0080] The heat level of the tires according to the invention I1 and I2 is lower by 10? and 9?, respectively, compared with that of the tire of the prior art E. The performance with regard to resistance to attacks of the crown of the tires according to the invention I1 and I2 is increased by 40% and 80%, respectively, compared with that of the tire of the prior art E.