Tire Tread For Heavy Vehicle Of Construction Plant Tire

Abstract

Tire (1) for a heavy vehicle of construction plant type with tread (2) comprising raised elements (3), separated by grooves (4), extending radially outwards from a bottom surface (5) as far as a contact face (6) over a height (H) and comprising base compound (9) and coating compound (10). Coating compound (10) has a constant maximum thickness (e.sub.max), at the contact face (6) of each raised element (3), at most equal to 0.15 H, the coating compound (10) has a constant minimum thickness (e.sub.min), at each groove bottom (8), at least equal to 0.04 H, and the elongation at break at 23 C. A.sub.R2 of the coating compound (10) is at least equal to 1.05 times the elongation at break of 23 C. A.sub.R1 of the base compound (9).

Claims

1. A tire for a heavy vehicle of construction plant type, comprising: a tread comprising raised elements separated by grooves; each raised element extending radially outwards from a bottom surface as far as a contact face over a height H and comprising lateral faces; each said groove being a cut in the tread which cut is delimited laterally by lateral faces of raised elements that face one another and is delimited radially towards the inside by a groove bottom resting on the bottom surface; the tread comprising at least one first elastomeric compound, referred to as base compound, and at least one second elastomeric compound, referred to as painting compound, fully covering the base compound; the base compound and coating compound respectively having elongations at break at 23 C. A.sub.R1 and A.sub.R2; wherein the coating compound has a constant maximum thickness e.sub.max, at the contact face of each raised element, at most equal to 0.15 times the height H of the raised element, wherein the coating compound has a constant minimum thickness e.sub.min, at each groove bottom, at least equal to 0.04 times the height H of the raised element, and wherein the elongation at break at 23 C. A.sub.R2 of the coating compound is at least equal to 1.05 times the elongation at break at 23 C. A.sub.R1 of the base compound.

2. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the minimum thickness e.sub.min of the coating compound at each groove bottom is at least equal to 1 mm.

3. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the maximum thickness e.sub.max of the coating compound at the contact face of each raised element is at most equal to 10 mm.

4. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the elongation at break at 23 C. A.sub.R2 of the coating compound is at least equal to 1.08 times the elongation at break at 23 C. A.sub.R1 of the base compound.

5. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the base compound contains a diene elastomer of natural rubber (NR) type.

6. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the coating compound comprises a blend of diene elastomers of NR/SBR or NR/BR/SBR type, NR denoting a natural rubber, BR a polybutadiene and SBR a stirene-butadiene copolymer.

Description

[0034] The features of the invention will be better understood with the aid of the description of the attached FIGS. 1 to 3 which have not been drawn to scale:

[0035] FIG. 1 is a view from above of a portion of tread for a tire for a heavy vehicle of construction plant type, according to the invention,

[0036] FIG. 2 is a view in section, on a plane perpendicular to the axis of rotation of the tire, of a portion of tire tread for a heavy vehicle of construction plant type, according to the invention, and

[0037] FIG. 3 is a graph depicting how the thickness of the coating compound evolves from the middle of a contact face to the middle of a groove bottom.

[0038] FIG. 1 depicts a plan view of a portion of tread 2 of a tire 1 for a heavy goods vehicle of construction plant type, according to the invention. The tread 2 comprises raised elements 3 separated by grooves 4. Each raised element 3 extends radially outwards from a bottom surface 5 as far as a contact face 6 over a height Hnot depicted because it is perpendicular to the plane of the figureand comprises lateral faces 7. Each groove 4 is a cut in the tread 2 which is delimited laterally by lateral faces 7 of raised elements 3 facing each other and delimited radially towards the inside by a groove bottom 8 resting on the bottom surface 5. Circumferential planes of section AA, parallel to the equatorial plane of the tire are also depicted, the equatorial plane being the plane perpendicular to the axis of rotation of the tire and passing through the middle of the tread.

[0039] FIG. 2 depicts a view in section, on a plane of section AA as depicted in FIG. 1, of a portion of tread of a tire for a heavy vehicle of construction plant type, according to the invention. In addition to showing the elements described in the case of FIG. 1, FIG. 2 shows that the tread 2 comprises a first elastomeric compound, referred to as base compound 9, which is the predominant component of the tread 2, and a second elastomeric compound, referred to as coating compound 10, fully covering the base compound 9, and which is a surface or skin component. FIG. 2 shows, in section, a raised element 3 comprising a contact face 6 and two lateral faces 7, the raised element 3 extending radially outwards from the bottom surface 5 (in dotted line) as far as the contact face 6. It also shows a groove 4, delimited axially by the respective lateral faces 7 of two raised elements 3 facing each other, and delimited radially on the inside by a groove bottom 8 positioned at the bottom surface 5. FIG. 2 in particular shows the variation in thickness e of the coating compound 10 according to the zone of tread. At the contact face 6, which has a middle O and an axial end A, the thickness of the coating compound 10 is constant and has a maximum value e.sub.max, at most equal to 0.15 times the height H of the raised element 3. The maximum thickness e.sub.max is measured at the point O, the middle of the contact face 6. The thickness e of the coating compound 10 then decreases from the axial end A of the contact face 6, which coincides with the radially outer end of the lateral face 7, as far as the radially inner end B of the lateral face 7. Then, in the groove bottom 8, the thickness of the coating compound 10 is once against substantially constant and equal to a minimum value e.sub.min at least equal to 0.04 times the height H of the raised element 3. The minimum thickness e.sub.min is measured at the point C but is the middle of the groove bottom 8.

[0040] FIG. 3 is a graph showing how the thickness e of the coating compound 10 evolves from the middle O of a contact face 6 as far as the middle C of a groove bottom 8. As was seen earlier, the thickness e is constant and equal to the maximum value e.sub.max between the middle O and the axial end A of the contact face 6 then decreases from the axial end A of the contact face 6 as far as the radially inner end B of the lateral face 7 where it reaches a minimum value e.sub.min, and remains constant and equal to this minimum value e.sub.min as far as the middle C of the groove bottom 8.

[0041] The invention has been studied in particular in the case of a 29.5R25 tire for an articulated dumper.

[0042] Thickness measurements were taken in two circumferential planes of section, positioned axially in a lateral portion of the tread near the edge of the tread, and in a central portion of the tread, respectively. The results obtained are collated in table A below:

TABLE-US-00001 TABLE A H = 40 mm e.sub.max (mm) e.sub.min (mm) e.sub.max/H e.sub.min/H Plane of section 4 2.5 0.10 0.06 lateral portion Plane of section 4.5 2 0.11 0.05 central portion

[0043] Elongation at break measurements were taken on samples taken from the tire, from the coating compound and from the base compound respectively, in the form of test specimens measuring 18 mm long, 1.1 mm wide and 0 3 mm thick, subjected to uniaxial tension until the point of breaking, at a rate of 50 mm/min and at a temperature of 23 C. For each of the elastomeric compounds, the elongations at break and the stress at break are the averages of results obtained over at least two test specimens.

[0044] The results obtained are given in table B below:

TABLE-US-00002 TABLE B Coating Base compound compound Ratio Mean elongation at 814 752 1.08 break at 23 C. (%) Mean stress at 21.7 24.6 0.88 break at 23 C. (MPa)