Tire crown for a heavy duty civil engineering vehicle

Abstract

Crown of a tire for a heavy vehicle that is desensitized to attacks. The tire (1) comprises tread (2) having a median degree of surface siping TL.sub.C, expressed in m/m.sup.2, equal to the ratio between the cumulative length L.sub.DC of the cuts (21), present in a median portion of tread of axial width W.sub.C, and the median area A.sub.C of the radially outer surface (23) of the tread (2), and protective reinforcement (4) comprising 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 median degree of surface siping TL.sub.C of tread (2) is at least equal to 5 m/m.sup.2 and a coupling ratio C.sub.C, equal to the ratio between the maximum breaking strength R.sub.max and the median degree of surface siping TL.sub.C, is at least equal to 18 000 daN.

Claims

1. A tire for a heavy vehicle of construction plant type, comprising: a tread; and the tread, having a radial thickness H.sub.T at least equal to 60 mm, comprising: cuts having a radial depth H.sub.D and a width W.sub.D, and elements in relief separated by the cuts; at least some of the cuts, referred to as effective cuts, having a radial depth H.sub.D at least equal to 50% of the radial thickness H.sub.T and a width W.sub.D at most equal to 20% of the radial depth a crown reinforcement radially on the inside of the tread, the crown reinforcement comprising: a protective reinforcement: a working reinforcement; and a hoop reinforcement; the protective reinforcement, which is radially outermost, comprising: two protective layers, in contact with one another over a median axial width W.sub.C, wherein a radial spacing between the two protective layers, measured at a same axial position of each of two protective layers, is constant over the median axial width W.sub.C and increases beyond the median axial width W.sub.C, comprising elastic metallic reinforcers, that form an angle of between 15° and 45° with a 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, comprising inextensible 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 of the tire; and the hoop reinforcement comprising at least one hooping layer, comprising metallic reinforcers that form an angle at most equal to 15° with the circumferential direction, wherein the median degree of surface siping TL.sub.C of the tread, defined as the ratio between the cumulative length L.sub.DC of the effective cuts, present on a median portion of the tread having an axial width equal to W.sub.C, and a median area A.sub.C of the radially outer surface of the tread equal to 2ΠR.sub.E*W.sub.C, where R.sub.E is the external radius of the tire, is at least equal to 5 m/m.sup.2 and wherein the coupling ratio C.sub.C, defined as the ratio between the maximum value R.sub.max of the breaking strengths R of the protective layers and the median degree of surface siping TL.sub.C of the tread, is at least equal to 18 000 daN.

2. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the coupling ratio C.sub.C is at least equal to 24 000 daN.

3. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the coupling ratio C.sub.C is at most equal to 72 000 daN.

4. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the median degree of surface siping TL.sub.C is at least equal to 5.8 m/m.sup.2.

5. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the median degree of surface siping TL.sub.C is at most equal to 15 m/m.sup.2.

6. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the circumferential median degree of surface siping TL.sub.CX of the tread, defined as the ratio between the sum of the projections, in the circumferential direction, of the lengths of the effective cuts, present on a median portion of tread having an axial width equal to W.sub.C, and a median area A.sub.C of the radially outer surface of the tread equal to 2ΠR.sub.E*W.sub.C, where R.sub.E is the external radius of the tire, is at least equal to 2.5 m/m.sup.2.

7. The tire for a heavy vehicle of construction plant type according to claim 1, the tire having an axial direction parallel to its axis of rotation, wherein the axial median degree of surface siping TL.sub.CY of the tread, defined as the ratio between the sum of the projections, in the axial direction, of the lengths of the effective cuts, present on a median portion of tread having an axial width equal to W.sub.C, and a median area A.sub.C of the radially outer surface of the tread equal to 2ΠR.sub.E*W.sub.C, where R.sub.E is the external radius of the tire, is at least equal to 3.5 m/m2.

8. 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 150 000 daN/m.

9. 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.

10. 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.

11. 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.sub.C is at least equal to 18 000 daN.

12. 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 K strands, K being between 3 and 5, each strand being made up of metal threads.

13. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the working reinforcement extends beyond the median axial width W.sub.C.

14. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the hoop reinforcement extends less than the median axial width W.sub.C.

15. The tire for a heavy vehicle of construction plant type according to claim 1, wherein an overall volumetric void ratio is a ratio between a total volume of the cuts in the median portion and a sum of the total volume of the cuts in the median portion and a total volume of the elements in relief of the median portion.

16. The tire for a heavy vehicle of construction plant type according to claim 15, wherein an overall volumetric void ratio is at least equal to 12%.

17. The tire for a heavy vehicle of construction plant type according to claim 1, wherein an axial end of the median axial width W.sub.C is beyond a respective axially outermost cut.

18. The tire for a heavy vehicle of construction plant type according to claim 1, wherein at least one of the two protective layers has a convex shape beyond the median axial width W.sub.C so that the two protective layers radially diverge from one another beyond the median axial width W.sub.C, measured at a same axial position of each of two protective layers.

19. The tire for a heavy vehicle of construction plant type according to claim 1, wherein at least one of the two protective layers has a convex shape beyond the median axial width W.sub.C so that the two protective layers radially converge after a maximum radial spacing, measured at a same axial position of each of two protective layers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The features of the invention will be better understood with the aid of FIGS. 1 and 2, which are schematic and not to scale:

(2) 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.

(3) FIG. 2 shows the range of the maximum breaking strengths R.sub.max as a function of the median degree of surface siping TL.sub.C of the tread for a tire for a heavy vehicle of construction plant type, according to the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(4) 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 tread 2 comprises a median portion, positioned in line with a portion of the protective reinforcement 4, having an axial width W.sub.C across which the two protective layers (41, 42) are in contact with one another. 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, present on the median portion of tread 2 of width W.sub.C, have a cumulative length L.sub.D (not shown in the figure) measured on a radially outer surface 23 of the tread 2. This cumulative length L.sub.DC makes it possible to define a median degree of surface siping TL.sub.C, expressed in m/m.sup.2, equal to the ratio between the cumulative length L.sub.DC and the median area A.sub.C of the radially outer surface 23 of the tread equal to 2ΠR.sub.E*W.sub.C, 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 inextensible 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′.

(5) FIG. 2 shows the range of the maximum breaking strengths R.sub.max as a function of the median degree of surface siping TL.sub.C of the tread of a tire for a heavy vehicle of construction plant type according to the invention. According to the invention, the median degree of surface siping TL.sub.C of the tread is at least equal to 5 m/m.sup.2, and a coupling ratio C.sub.C, equal to the ratio between the maximum value R.sub.max of the breaking strengths R of the protective layers and the median degree of surface siping TL.sub.C of the tread 2, is at least equal to 18 000 daN. Consequently, the range of the invention is defined by the maximum breaking strengths R.sub.max, at least equal to 18 000*TL.sub.C, where TL.sub.C is at least equal to 5 m/m.sup.2. Shown on the abscissa axis of the graph in FIG. 2 is the minimum value of the median degree of surface siping TL.sub.C of the tread equal to 5 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 90 000 daN/m, corresponding to the minimum coupling rate C.sub.C equal to 18 000 daN. Also shown in the graph are a first exemplary embodiment of the invention I1, in which the median degree of surface siping TL.sub.C is equal to 7 m/m.sup.2 and the maximum value R.sub.max of the breaking strengths R of the protective layers is equal to 160 000 daN/m, the protective layers comprising elastic multistrand ropes of structure 4*(4+9)*0.26, and a second exemplary embodiment of the invention 12, in which the median degree of surface siping TL.sub.C is also equal to 7 m/m.sup.2 and the maximum value R.sub.max of the breaking strengths R of the protective layers is equal to 200 000 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 median degree of surface siping TL.sub.C equal to 2.7 m/m.sup.2 and a maximum value R.sub.max, of the breaking strengths R of the protective layers equal to 102 000 daN/m, i.e. outside the range of the invention.

(6) 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.

(7) 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 inextensible metallic reinforcers, and a hoop reinforcement made up of two hooping layers with inextensible 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 inextensible 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 inextensible 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.

(8) 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 an overall volumetric void ratio TEV at least equal to 12%.

(9) For the example 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:

(10) TABLE-US-00001 TABLE 1 E I1 I2 Tire size (40.00R57) (40.00R57) (40.00R57) Axial width W.sub.T of the tread (m) 0.98 m 0.98 m 0.98 m Median axial width W.sub.C (=0.6 W.sub.T) of 0.59 m 0.59 m 0.59 m the tread (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 2IIR.sub.E * W.sub.T (m.sup.2) Median area of the tread surface A.sub.C = 6.6 m.sup.2 6.6 m.sup.2 6.6 m.sup.2 2IIR.sub.E * W.sub.C (m.sup.2) Cumulative length L.sub.D of the 17.3 m 45.8 m 45.8 m effective cuts, across the total axial width W.sub.T (m) Cumulative length L.sub.DC of the 17.3 m 45.8 m 45.8 m effective cuts, present in the median part of axial width W.sub.C (m) Cumulative length L.sub.DCX of the 11 m 28 m 28 m effective cuts, present in the median part, in the circumferential direction (XX′) (m) Cumulative length L.sub.DCY of the 9.5 m 33 m 33 m effective cuts, present in the median part, in the axial direction (YY′) (m) Overall degree of surface siping TL = 1.6 m/m.sup.2 4.2 m/m.sup.2 4.2 m/m.sup.2 L.sub.D/A (m/m.sup.2) Overall volumetric void ratio TEV 18.1% 14.2% 14.2% (%) Median degree of surface siping TL.sub.C = 2.7 m/m.sup.2 7 m/m.sup.2 7 m/m.sup.2 L.sub.DC/A.sub.C Circumferential median degree of 1.7 m/m.sup.2 4.2 m/m.sup.2 4.2 m/m.sup.2 surface siping TL.sub.CX = L.sub.DCX/A.sub.C Axial median degree of surface 1.4 m/m.sup.2 5.0 m/m.sup.2 5.0 m/m.sup.2 siping TL.sub.CY = L.sub.DCY/A.sub.C Median volumetric void ratio TEV.sub.C 16.9% 12.0% 12.0% (%) 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 102 000 daN/m 160 000 daN/m 200 000 daN/m strengths R of the protective layers (daN/m) Coupling ratio C.sub.C = R.sub.max/TL.sub.C (daN) 37 778 daN 22 857 daN 28 571 daN

(11) 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.

(12) 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.

(13) 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.

(14) 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:

(15) TABLE-US-00002 TABLE 2 E I1 I2 Tire size (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″)
According to Table 2, 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.

(16) The scope of the protection of the invention is not limited to the examples given hereinabove. The invention is embodied each novel characteristic and each combination of characteristics, which includes every combination of any features which are stated in the claims, even if this feature or combination of features is not explicitly stated in the examples.