Tire tread for a farm tractor

Abstract

Tire and tread for a vehicle for agricultural use having reduced attack to leading faces of tread bars, at axially outer ends thereof, by stubble remaining after harvest, thereby reducing risk of chunking of axially outer ends of the bars. The tire and tread has a plurality of bars, a bar having, in any axial plane (P.sub.xy) parallel to the axis of rotation, a leading profile and a trailing profile. The angle (A) of the straight line (T) tangential to the leading profile at a point (M) on the leading profile, with respect to an equatorial plane (P), increases continuously, from an axially outermost point (E) of the leading profile, when the axial distance (L) between the point (M) and the point (E) increases, the angle (A) reaches a maximum value (A.sub.m) at a point of inflection (I) of the leading profile, and the radius of curvature (R) at any point (M), positioned axially between the axially outermost point (E) and the point of inflection (I), is at least equal to 0.4 times the height of the bar.

Claims

1. A tire for a vehicle for agricultural use, comprising: a tread comprising a plurality of bars, at least one of said plurality of bars extending radially outwards from a base surface of the tread over a bar height (H), axially towards an inside from an axial end of the tread and circumferentially, in one rotational direction of the tire, between a leading face and a trailing face, at least one of the plurality of bars comprising, in an axial plane (P.sub.xy) parallel to an axis of rotation of the tire, a leading profile, an intersection of the leading face and of the axial plane (P.sub.xy), and a trailing profile, the intersection of the trailing face and of the axial plane (P.sub.xy), wherein an angle (A) of a straight line (T) tangential to the leading profile at a point (M) on the leading profile, with respect to an equatorial plane (P) of the tire passing through a middle of the tread, increases continuously from an axially outermost point (E) of the leading profile to a point of inflection (I) of the leading profile, when an axial distance (L) between the point (M) of the leading profile and the axially outermost point (E) of the leading profile increases, in that the angle (A) reaches a maximum value (A.sub.m) at the point of inflection (I) of the leading profile, and in that a radius of curvature (R), at any point on the leading profile positioned axially between the axially outermost point (E) and the point of inflection (I), is at least equal to 0.4 times the bar height (H).

2. A tire according to claim 1, wherein an axial distance (L1) between the point of inflection (I) of the leading profile and the equatorial plane (P) of the tire is at least equal to 0.8 times an axial distance (L2) between the axially outermost point (E) of the leading profile and the equatorial plane (P) of the tire.

3. A tire according to claim 1, wherein an axial distance (L.sub.1) between the point of inflection (I) of the leading profile and the equatorial plane (P) of the tire is at most equal to 0.93 times the axial distance (L.sub.2) between the axially outermost point (E) of the leading profile and the equatorial plane (P) of the tire.

4. A tire according to claim 1, wherein a circumferential distance (D1) between the axially outermost point (E) of the leading profile and the trailing profile is at most equal to 0.5 times a distance (D2) between the point of inflection (I) of the leading profile and the trailing profile, measured perpendicular to the trailing profile.

5. A tire according to claim 1, wherein the angle (A) of the straight line (T) tangential to the leading profile at the axially outermost point (E) of the leading profile, with respect to the equatorial plane (P) of the tire, is at most equal to 5.

6. A tire according to claim 1, wherein the angle (A) of the straight line (T) tangential to the leading profile at the point of inflection (I) of the leading profile, with respect to the equatorial plane (P) of the tire, is at least equal to 40.

7. A tire according to claim 1, wherein the angle (A) of the straight line (T) tangential to the leading profile at the point of inflection (I) of the leading profile, with respect to the equatorial plane (P) of the tire, is at most equal to 75.

8. A tire according to claim 1, wherein the leading face is connected to the base surface by a connection that is continuous and tangential to the base surface.

9. A tire according to claim 1, wherein the plurality of bars is made up of a first and of a second row of bars which are arranged in a chevron pattern with respect to the equatorial plane (P) of the tire.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will be better understood with the aid of FIGS. 1 to 5 introduced hereinafter:

(2) FIG. 1 shows a perspective view of a tire according to the invention,

(3) FIG. 2 shows a perspective view of an axially outer end of a bar of a reference tire,

(4) FIG. 3 shows a perspective view of an axially outer end of a bar of a tire according to the invention,

(5) FIG. 4 shows a perspective view of a bar of a tire according to the invention,

(6) FIG. 5 shows a section in an axial plane of a bar of a tire according to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

(7) FIG. 1 shows a perspective view of a tire 1 according to the invention. The tread 2 of the tire comprises a plurality of bars (3, 4). In this particular instance, the plurality of bars is distributed in a first row of bars 3 and a second row of bars 4, these being symmetric with respect to the equatorial plane of the tire and arranged in a chevron pattern. A bar (3, 4) extends radially outwards from a base surface 5 of the tread 2, of revolution about the axis of rotation of the tire, over a bar height. The bar 3 extends axially towards the inside from an axial end 6 of the tread 2. The bar 3 extends circumferentially, in a preferred direction of rotation of the tire 1, from a leading face 31 to a trailing face 32. In the tire depicted in FIG. 1, the preferred direction of rotation is indicated by the points of the chevron patterns in which the bars are arranged.

(8) FIG. 2 shows a perspective view of an axially outer end of a bar of a conventional reference tire. The bar 3 comprises a leading face 31, a trailing face 32 and an axially outer face at the axial end 6 of the tread. The bar 3 extends radially outwards from a base surface 5 of the tread. The leading face 31 is connected to the base surface 5 by a connecting surface 7 comprising a fillet radius. In the reference tire, the leading face 31 is connected to the axial end 6 by a sharp edge corner, characterized by a very small blend radius, for example of the order of 1 to 3 mm. The leading face, shown in FIG. 2, has a concave curved shape, i.e. the centre of curvature of the leading profile, which is the intersection of the leading face with an axial plane, at a given point on the leading profile, is forward of the leading face 31, on the opposite side to the trailing face 32. In this configuration, it is difficult for the end of some stubble to be removed towards the outside of the tread.

(9) FIG. 3 shows a perspective view of an axially outer end of the bar of a tire according to the invention. From the axially outer end 6 of the bar, the leading face 31 is convex, with a centre of curvature positioned behind the leading face 31, on the trailing face 32 side, and then, following a reversal of curvature, becomes at least partially concave, with a centre of curvature positioned forward of the leading face 31, on the opposite side to the trailing face 32. The connection between the leading face 31 and the axial end 6 occurs without sharp edges. Likewise, the connection between the leading face 31 and the base surface 5 is via a connecting surface 7 that has no sharp edges. The absence of any sharp edges at the axially outer end of the bar prevents any stubble ends from becoming trapped in this zone.

(10) FIG. 4 shows a perspective view of a bar 3 of a tire according to the invention, with its leading 31 and trailing 32 faces. An axial cross section through the bar 3 on an axial plane P.sub.xy parallel to the axis of rotation of the tire is shown in dotted line. The leading profile 311 is defined by the intersection of the leading face 31 with the axial plane P.sub.xy. The trailing profile 321 is defined by the intersection of the trailing face 32 with the axial plane P.sub.xy. The points E and I are respectively the axially outermost point and the point of inflection of the leading profile 311.

(11) FIG. 5 shows a cross section in an axial plane P.sub.xy of a bar of a tire according to the invention. In the circumferential direction X, the bar 3 extends between the leading profile 311 and the trailing profile 321. In the axial direction Y, the leading profile 311 comprises, near the axially outer end of the bar, an axially outermost point E positioned at an axial distance L.sub.2 away from the equatorial plane P of the tire, and a point of inflection I positioned at an axial distance L.sub.1 away from the equatorial plane P of the tire. At any point M positioned axially between the point E and I, the tangent T to the leading profile 311 makes an angle A with respect to the equatorial plane P. The angle A of the tangent T increases continuously when the axial distance L from the point M to the axially outermost point E increases. The curvature at any point M positioned axially between the points E and I is defined by the centre of curvature C and the radius of curvature R. According to the invention, the curvature at any point M is convex, i.e. is such that the centre of curvature is positioned circumferentially behind the leading profile 311, on the trailing profile 321 side. At the point of inflection I, the curvature at the leading profile 311 cancels and changes direction at any point axially inside the point of inflection I, the centre of curvature then being positioned circumferentially forward of the leading profile 311, on the opposite side to the trailing profile 321.

(12) The invention has been developed more particularly for an agricultural tire of size 520/85 R 42. For this under sized study, in a given axial plane, the axial distance of the point of inflection of a leading profile with respect to the equatorial plane of the tire is comprises between 0.8 times and 0.95 times the axial distance of the axially outermost point with respect to the equatorial plane of the tire. The radius of curvature at a point positioned axially between the axially outermost point and the point of inflection is at least equal to 24 mm, and therefore to 0.4 times the height of the bar, this height being equal to 60 mm.

(13) In order to quantify the technical effectiveness of the invention, the inventors have compared the mean number of impacts, caused by the ends of stubble in the convex axially outer portion of the leading face of a bar, on several bars, between a reference tire and a tire according to the invention. They observed a 30% improvement in terms of the mean number of impacts for the tire according to the invention as compared with the reference tire.

(14) The principle of a reversal of curvature in the vicinity of the axially outer end of the bar can be extended to the trailing face of a bar.

(15) The present invention can be extended to treads comprising more than two rows of bars.

(16) The invention may also prove to be a technical solution to any problem of a bar being attacked by any indenter behaving in a similar way to stubble, i.e. an indenter having mobility in rotation about a fixed end anchored in the ground, a sharp and aggressive free end, and sufficient axial rigidity to be able to perforate the elastomeric compound of the bar.

(17) The invention can finally be generalized to any tire the tread of which comprises raised elements and which is liable to run on ground comprising aggressive indenters, such as a tire for a construction plant vehicle.