Abstract
Tread (2) of a radial tire for a heavy vehicle. The tire alternatingly rolls in laden and unladen states on descent and ascent, respectively. Tread (2) has total width W.sub.T and comprises first median portion (21) having median width W.sub.c, where 0.2W.sub.T≤W.sub.c≤0.5W.sub.T. Tread (2) is axially delimited by second and third lateral portions (22, 23) having respective lateral widths (W.sub.S2, W.sub.S3) at least equal to 25% and at most equal to 40% of total width W.sub.T. Angle A.sub.51 of leading face (51) of every element in relief (31) of first median portion (21) is strictly greater than angle A.sub.61 of trailing face (61) of said element in relief (31). Angle (A.sub.52, A.sub.53) of leading face (52, 53) of every element in relief (32, 33) of each of the second and third lateral portions (22, 23) is strictly less than angle (A.sub.62, A.sub.63) of trailing face (62, 63) of said element in relief (32, 33).
Claims
1. A tire for a heavy vehicle of construction plant type that is adapted for use in quarries, and alternatingly rolls in a laden state on a descending slope and in an unladen state on an ascending slope, wherein the tire comprises: a tread having a total width W.sub.T and comprising a first median portion, axially delimited by a second and a third lateral portion, respectively; the first median portion having a median width W.sub.0 at least equal to 20% and at most equal to 50% of the total width W.sub.T, and comprising elements in relief that are separated from one another by cuts, each said element in relief comprising a leading face, which is adapted to come into contact with the ground first and forms an angle A.sub.51 with a radial plane, and a trailing face, which is adapted to come into contact with the ground last and forms an angle A.sub.61 with the radial plane; each of the second and third lateral portions having a respective lateral width at least equal to 25% and at most equal to 40% of the total width W.sub.T, and respectively comprising elements in relief that are separated from one another by cuts, each said element in relief comprising a leading face, which forms an angle with the radial plane, and a trailing face, which forms an angle with the radial plane, the cuts arranged between each element of each of the second and third lateral portions being configured to have an inclination angle with respect to the tire's equatorial plane having the same sign and magnitude as the inclination angle of the cuts between each element of the first median portion, wherein the angle A.sub.51 of the leading face of every said element in relief of the first median portion is strictly greater than the angle A.sub.61 of the trailing face of said element in relief, and wherein the angle of the leading face of every said element in relief of each of the second and third lateral portions is strictly less than the angle of the trailing face of said element in relief, wherein the angle A.sub.51 of the leading face of every said element in relief of the first median portion is strictly greater than the angle of the leading face of every said element in relief of each of the second and third lateral portions, and wherein the angle A.sub.61 of the trailing face every said element in relief of the first median portion is strictly less than the angle of the trailing face of every said element in relief of each of the second and third lateral portions, wherein differentiation of elastomeric material is provided both across the axial width and in the radial depth of the tread by: the tread having a radial superposition of at least one radially inner first elastomeric material and a radially outermost second elastomeric material different from the at least one radially inner first elastomeric material, and the first median portion having a radially outermost median elastomeric material and each of the second and third lateral portions having a radially outermost lateral elastomeric material different from the radially outermost median elastomeric material, and wherein: under braking torque and in the laden state, the first median portion bears 40% of the total load and the second and third lateral portions bear 60% of the total load; and under engine torque and in the unladen state, the first median portion bears 80% of the total load and the second and third lateral portions bear only 20% of the total load.
2. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the first median portion is symmetric with respect to an equatorial plane passing through the middle of the tread and perpendicular to the axis of rotation of the tire.
3. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the angle A.sub.51 of the leading face of every said element in relief of the first median portion is at least equal to 15° and at most equal to 35°.
4. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the angle A.sub.61 of the trailing face of every said element in relief of the first median portion is at least equal to 6° and at most equal to 12°.
5. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the difference between the angle A.sub.51 of the leading face and the angle A.sub.61 of the trailing face of every said element in relief of the first median portion is at least equal to 5° and at most equal to 30°.
6. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the angle of the leading face of every said element in relief of each of the second and third lateral portions is at least equal to 6° and at most equal to 12°.
7. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the angle of the trailing face of every said element in relief of each of the second and third lateral portions is at least equal to 15° and at most equal to 35°.
8. The tire for a heavy vehicle of construction plant type according to claim 1, wherein the difference between the angle of the trailing face and the angle of the leading face of every said element in relief of each of the second and third lateral portions is at least equal to 5° and at most equal to 30°.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Features of embodiments of the invention are illustrated in the schematic figures, which are not to scale and described below:
(2) FIG. 1A: laden outbound downhill cycle of a dumper
(3) FIG. 1B: unladen return uphill cycle of a dumper
(4) FIG. 2A: partial view from above of a tread of a tire according to an embodiment of the invention
(5) FIG. 2B: cross-sectional view of an element in relief of the first median portion
(6) FIG. 2C: cross-sectional view of an element in relief of a second or third lateral portion
(7) FIG. 3A: mechanical operation of an element in relief of the first median portion, under braking torque and in a laden state
(8) FIG. 3B: mechanical operation of an element in relief of a second or third lateral portion, under braking torque and in a laden state
(9) FIG. 4A: mechanical operation of an element in relief of the first median portion, under engine torque and in an unladen state
(10) FIG. 4B: mechanical operation of an element in relief of a second or third lateral portion, under engine torque and in an unladen state
(11) FIG. 5A: canonical curves of wear as a function of the resultant overall force for a tire of the prior art E and for a tire according to the invention I, in a laden state
(12) FIG. 5B: canonical curves of wear as a function of the resultant overall force for a tire of the prior art E and for a tire according to the invention I, in an unladen state.
DETAILED DESCRIPTION OF THE DRAWINGS
(13) FIG. 1A shows a laden outbound downhill cycle of a dumper. The laden dumper is descending a gradient of angle A. Each tire 1, mounted on a driven axle, is subjected to a braking torque T.sub.F and to a load P.sub.C. The reactions of the ground on the tread 2 of the tire are a circumferential braking force R.sub.CX, oriented in the opposite direction to the movement V of the dumper, and a radial force R.sub.CZ, respectively.
(14) FIG. 1B shows an unladen return uphill cycle of a dumper. The unladen dumper is climbing a gradient of angle A. Each tire 1, mounted on a driven axle, is subjected to an engine torque T.sub.M and to a load P.sub.V. The reactions of the ground on the tread 2 of the tire are a circumferential driving force R.sub.VX, oriented in the direction of the movement V of the dumper, and a radial force R.sub.VZ, respectively.
(15) FIG. 2A is partial view from above of a tread 2 of a tire according to the invention. The tread 2 has a total width W.sub.T and comprises a first median portion 21, axially delimited by a second and a third lateral portion 22, 23, respectively. The first median portion 21 has a median width W.sub.c at least equal to 20% and at most equal to 50% of the total width W.sub.T, and comprises elements in relief 31 that are separated from one another by cuts 41, each element in relief 31 comprising a leading face 51, which is intended to come into contact with the ground first, and a trailing face 61, which is intended to come into contact with the ground last. Each of the second and third lateral portions (22, 23) has a respective lateral width (W.sub.S2, W.sub.S3) at least equal to 25% and at most equal to 40% of the total width W.sub.T, and respectively comprises elements in relief (32, 33) that are separated from one another by cuts (42, 43), each element in relief (32, 33) comprising a leading face (52, 53) and a trailing face (62, 63).
(16) FIG. 2B is a cross-sectional view of an element in relief 31 of the first median portion, which is separated from the adjacent elements in relief by a cut 41. Each element in relief 31 comprises a leading face 51, which is intended to come into contact with the ground first and forms an angle A.sub.51 with a radial plane YZ, and a trailing face 61, which is intended to come into contact with the ground last and forms an angle A.sub.61 with a radial plane YZ. The angles A.sub.51 and A.sub.61 are usually known as relief angles. In a local frame of reference XZ defined by a circumferential axis X, tangent to the circumference of the tire and oriented in the direction of rotation of the tire, and by a radial axis Z, perpendicular to the circumference of the tire and oriented towards the axis of rotation of the tire, the angle A.sub.51 of the leading face 51 is an angle oriented positively in the anticlockwise direction. In this same local frame of reference, the angle A.sub.61 of the trailing face 61 is an angle oriented positively in the clockwise direction. According to the invention, the angle A.sub.51 of the leading face 51 of every element in relief 31 of the first median portion 21 is strictly greater than the angle A.sub.61 of the trailing face 61 of said element in relief 31.
(17) Analogously, FIG. 2C is a cross-sectional view of an element in relief (32, 33) of a second or third lateral portion, which is separated from the adjacent elements in relief by a cut (42, 43). Each element in relief (32, 33) comprises a leading face (52, 53), which is intended to come into contact with the ground first and forms an angle (A.sub.52, A.sub.53) with a radial plane YZ, and a trailing face (62, 63), which is intended to come into contact with the ground last and forms an angle (A.sub.62, A.sub.63) with a radial plane YZ. The angles (A.sub.52, A.sub.53) and (A.sub.62, A.sub.63) are usually known as relief angles. In a local frame of reference XZ defined by a circumferential axis X, tangent to the circumference of the tire and oriented in the direction of rotation of the tire, and by a radial axis Z, perpendicular to the circumference of the tire and oriented towards the axis of rotation of the tire, the angle (A.sub.52, A.sub.53) of the leading face (52, 53) is an angle oriented positively in the anticlockwise direction. In this same local frame of reference, the angle (A.sub.62, A.sub.63) of the trailing face (62, 63) is an angle oriented positively in the clockwise direction. According to the invention, the angle (A.sub.52, A.sub.53) of the leading face (52, 53) of every element in relief (32, 33) of each of the second and third lateral portions (22, 23) is strictly less than the angle (A.sub.62, A.sub.63) of the trailing face (62, 63) of said element in relief (32, 33).
(18) FIG. 3A schematically shows the mechanical operation of an element in relief 31 of the first median portion, under braking torque T.sub.F and in a laden state, the tire having a direction of rotation R. Given that the angle A.sub.51 of the leading face 51 is strictly greater than the angle A.sub.61 of the trailing face 61, the elementary coupling force C.sub.E applied to the contact face 71, generated by the Poisson effect by the applied pressure p that decreases from the leading edge of the leading face 51 at the large relief angle A.sub.51 to the trailing edge of the trailing face 61 at the small relief angle A.sub.61, is in the same direction as the movement V. Under the action of the braking torque T.sub.F, the elementary slip force G.sub.E applied to the contact face 71, in the opposite direction to the movement V, is added algebraically to the elementary coupling force C.sub.E in order to give the resultant elementary force R.sub.E, in the opposite direction to the movement V.
(19) FIG. 3B schematically shows the mechanical operation of an element in relief 32 of a second (or third) lateral portion, under braking torque T.sub.F and in a laden state, the tire having a direction of rotation R. Given that the angle A.sub.52 of the leading face 52 is strictly less than the angle A.sub.62 of the trailing face 62, the elementary coupling force C.sub.E applied to the contact face 72, generated by the Poisson effect by the applied pressure p that increases from the leading edge of the leading face 22 at the small relief angle A.sub.52 to the trailing edge of the trailing face 62 at the large relief angle A.sub.62, is in the opposite direction to the movement V. Under the action of the braking torque T.sub.F, the elementary slip force G.sub.E applied to the contact face 72, in the opposite direction to the movement V, is added algebraically to the elementary coupling force C.sub.E in order to give the resultant elementary force R.sub.E, in the opposite direction to the movement V.
(20) FIG. 4A schematically shows the mechanical operation of an element in relief 31 of the first median portion, under engine torque T.sub.M and in an unladen state, the tire having a direction of rotation R. Given that the angle A.sub.51 of the leading face 51 is strictly greater than the angle A.sub.61 of the trailing face 61, the elementary coupling force C.sub.E applied to the contact face 71, generated by the Poisson effect by the applied pressure p that decreases from the leading edge of the leading face 51 at the large relief angle A.sub.51 to the trailing edge of the trailing face 61 at the small relief angle A.sub.61, is in the same direction to the movement V. Under the action of the engine torque T.sub.M, the elementary slip force G.sub.E applied to the contact face 71, in the same direction as the movement V, is added algebraically to the elementary coupling force C.sub.E in order to give the resultant elementary force R.sub.E, in the same direction as the movement V.
(21) FIG. 4B schematically shows the mechanical operation of an element in relief 32 of a second (or third) lateral portion, under engine torque T.sub.M and in an unladen state, the tire having a direction of rotation R. Given that the angle A.sub.52 of the leading face 52 is strictly less than the angle A.sub.62 of the trailing face 62, the elementary coupling force C.sub.E applied to the contact face 72, generated by the Poisson effect by the applied pressure p that increases from the leading edge of the leading face 52 at the small relief angle A.sub.52 to the trailing edge of the trailing face 62 at the large relief angle A.sub.62, is in the opposite direction to the movement V. Under the action of the engine torque T.sub.M, the elementary slip force G.sub.E applied to the contact face 71, in the same direction as the movement V, is added algebraically to the elementary coupling force C.sub.E in order to give the resultant elementary force R.sub.E, in the same direction as the movement V.
(22) FIG. 5A shows typical canonical curves of wear as a function of the resultant overall force for a tire of the prior art E and for a tire according to the invention I, respectively, in a laden state. On the ordinate axis, the wear indicator U is a loss of mass (for example, expressed in g/km) or a loss of tread pattern height (for example, expressed in mm/km). On the abscissa axis, the resultant overall force R.sub.G (for example, expressed in daN) applied to the tread by the ground is shown. Compared with a tire of the prior art E, the addition of an overall braking coupling force C.sub.GF makes it possible offset a value C.sub.GF of the canonical curve of wear in the direction of the decreasing resultant overall forces R.sub.G. At a given resultant overall braking force R.sub.GF, the wear indicator U decreases from the value U.sub.E for a tire of the prior art to the value U.sub.I for a tire according to the invention, hence a reduction in wear DU.
(23) FIG. 5B shows typical canonical curves of wear as a function of the resultant overall force for a tire of the prior art E and for a tire according to the invention I, respectively, in an unladen state. Compared with a tire of the prior art E, the addition of an overall driving coupling force C.sub.GM makes it possible offset a value C.sub.GM of the canonical curve of wear in the direction of the increasing resultant overall forces R.sub.G. At a given resultant overall driving force R.sub.GM, the wear indicator U decreases from the value U.sub.E for a tire of the prior art to the value U.sub.I for a tire according to the invention, hence a reduction in wear DU.
(24) The invention has been studied more particularly in the case of a tire of size 40.00R57, fitted to a rigid dumper with a total load capacity of 320 tonnes, and in the case of a tire of size 24.00R35, fitted to a rigid dumper with a total load capacity of 100 tonnes.
(25) The following Table 1 presents an example of the distribution of loads and coupling forces, between the first median portion and the second and third lateral portions of a tread of a tire according to the invention, the tire being mounted on a rear axle of a mining dumper carrying out an alternation of laden downhill outbound cycles and unladen uphill return cycles.
(26) TABLE-US-00001 TABLE 1 First median Second lateral Third lateral portion portion portion Overall tread Load applied 0.4 * Z.sub.C 0.3 * Z.sub.C 0.3 * Z.sub.C Z.sub.C Z, in laden state under braking torque Coupling force +X * 0.4 * Z.sub.C −X * 0.3 * Z.sub.C −X * 0.3 * Z.sub.C −X * 0.2 * Z.sub.C C, in laden state under braking torque Load applied 0.8 * Z.sub.V 0.1 * Z.sub.V 0.1 * Z.sub.V Z.sub.V Z, in unladen state under engine torque Coupling force +X * 0.8 * Z.sub.V −X * 0.1 * Z.sub.V −X * 0.1 * Z.sub.V +X * 0.6 * Z.sub.V C, in unladen state under engine torque
(27) In Table 1, the forces Z are the loads applied per portion of tread and generally to the entire tread, and the forces C are the corresponding coupling forces, generated by the Poisson effect. The ratio C/Z=X is, by definition, the level of coupling.
(28) Under braking torque and in a laden state, the first median portion bears 40% of the total load Z.sub.C and the second and third lateral portions bear 60% of the total load Z.sub.C, since the tread is in full contact with the ground across its entire width. With the directions of the coupling forces being reversed between the median portion and the lateral portions, the overall coupling force, equal to −X times 20% of the total load Z.sub.C, is added to the overall braking slip force.
(29) Under engine torque and in an unladen state, the first median portion bears 80% of the total load Z.sub.V and the second and third lateral portions bear only 20% of the total load Z.sub.V, since the tread is in partial contact with the ground in the second and third lateral portions. With the directions of the coupling forces being reversed between the median portion and the lateral portions, the overall coupling force, equal to +X times 60% of the total load Z.sub.V, is added to the overall driving slip force.
(30) In the example described above, the coupling levels are presumed to be identical between the first median portion and the second and third lateral portions. More generally, these respective median and lateral coupling levels can be different.
(31) The scope of protection of the invention is not limited to the examples given hereinabove. The invention is embodied in 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.
