DESCRIPTION Technical field of the invention

The present invention pertains to a tread for an agricultural tire and particularly relates to an improved profile of its lugs.

Background

Traction on the fields is one of the most important performances for agricultural tires besides wear on the roads. These performances have, not necessarily jointly, continuously been improved by providing new shapes of lugs on the tread of agricultural tires.

A lug, also called a bar, of an agriculture tire is defined as that element protruding from the ground of the tread and coming in contact with the pavement surface when the agriculture tire is operated on roads. A lug is also that element of an agriculture tire engaging with the mud or soil when the agriculture tire is operated on a field. In engaging with the mud or soil the lug provides the necessary traction to allow the longitudinal movement of the tire during its rotation. Usually agriculture tires comprise a plurality of lugs.

The shape of the lugs on the tread of the agriculture tire, i.e. their inclination angle or curvature towards the circumferential direction of the tire, is one important feature to tune tire performances. In addition, further improvement can be achieved in providing a particular geometry of the sidewalls of the lugs, especially of the one of the stepping-in side of the lug, i.e. the one first coming in contact with the soil when the tire is rotating. The geometry of this sidewall is mostly responsible for the traction on soft or muddy soil. For example, EP903249A1 discloses an agricultural pneumatic tire comprising a tread provided with lugs extending at a given inclination angle from a central portion of the tread toward both ends of the tread. The lugs are alternately arranged right and left with respect to an equatorial plane of the tire at given intervals in the circumferential direction of the tire.

The same application discloses the sidewall of the stepping-in side of the lug consisting of a first flat surface slanted at a given angle and a second bored or concave surface, which surfaces intersect each other at a given elevation of the total lug height.

WO2015015525A1 discloses an agriculture tire comprising a tread provided with a plurality of ribs, each rib being provided with a front wall, wherein, on the front wall of the rib, a shaped profile is present, wherein the shaped profile has a section that defines at least one flat surface of the shaped profile which is parallel, or inclined at an angle a up to 20°, with respect to the surface of the head of the rib.

An important additional property of an agriculture tire is its ability of self-cleaning, i.e. the capability of removing the soil from between adjacent lugs on the tread without external intervention.

Summary of the invention

The technical problem underlying the present invention is therefore to provide a tread for agricultural tires allowing optimising tire performances with respect to the known art.

Above problem is solved by a tread for an agricultural tire according to claim 1 . Preferred features of the invention are recited in the dependent claims. The tread of the invention comprises at least one lug with a lug profile improving traction on fields, wear on roads and tire self-cleaning performances at the same time. Such improvement is obtained by a variation of the lug profile along its axial length.

In particular, the lug profile improves the resistance to digging-in of the lug into mud or soil, thereby increasing the traction performances of the tire.

Moreover, the lug profile also increases the stiffness of the lug, thereby improving its resistance to wear. The present invention encompasses also embodiments of treads wherein one or more lugs have said improved profiles and others have not. In particular, lugs having said improved profile can be arranged along the tire circumference as well as on the axially left and right side of the tire alternatingly with lugs having a conventional profile known in the art.

Furthermore, the tread according to the invention might comprise lugs having different improved profiles, each according to a preferred embodiment of present invention as disclosed in the following. Other advantages, features and use modes of the present invention will result evident from the following detailed description of some embodiments, presented by way of example and not with limitative purpose.

Brief description of the drawings

Reference will be made to the figures of the annexed drawing, wherein

^■ Figure 1 shows an exemplary front view of an agricultural tire according to a first preferred embodiment of present invention;

^■ Figure 2 shows a sectional view of the tire of Figure 1 , taken along plane A-A of this latter figure; Figure 3 shows an enlarged schematic representation of a lug profile taken along plane B-B of Figure 1 ;

Figure 3b shows an enlarged schematic representation of a variant lug profile with respect to that of Figure 3;

Figure 3c shows an enlarged schematic representation of a variant lug profile with respect to that of Figure 3b;

Figure 4 shows an exemplary, perspective view of a single lug of the tire of Figure 1 ;

Figure 5 shows an exemplary front view of the lug of Figure 4;

Figure 6 shows an exemplary perspective view of a single lug of a tire according to a second preferred embodiment of present invention;

Figure 7 shows an enlarged schematic representation of a lug profile according to the second preferred embodiment of present invention as shown in Figure 6, which lug profile is taken along a plane analogous to plane B-B of

Figure 1 ;

Figure 8 shows an exemplary perspective view of a single lug of a tire according to a third preferred embodiment of present invention;

Figure 9 shows an enlarged schematic view of the lug profile according to the third preferred embodiment of present invention as shown in Figure 8, which lug profile is taken along a plane analogous to plane B-B of Figure 1 .

Detailed description of preferred embodiments of the invention

Fig. 1 shows an agricultural tire 1 comprising a tread 10. The latter is provided with a plurality of lugs, one of which is denoted by way of example with 2. Each lug 2 protrudes from a ground 1 1 of tread 10 in a radially outer direction of the tire.

The lugs are alternately arranged at given intervals in the circumferential direction of the tire on one side and on the other with respect to the equatorial plane of the tire. Consistently with well-established terminology in the field, in the present context the "equatorial plane" of an agriculture tire is defined as the plane orthogonal to the rotation axis of the tire and passing through the axial middle of the tire. More particularly - and with reference also to Fig. 4 - each lug 2 extends from a first end 201 in the axial central part of the tire, i.e. in the area of the equatorial plane, towards a second end 202 in the axial lateral part of the tire, i.e. the shoulder area of the tire. The area close to, and including, the first end 201 of the lug in the central area of the tire defines a so-called nose region, or nose, 22 of the lug. The area close to, and including, the second end 202 of the lug in the shoulder area of the tire is called the shoulder region, or shoulder, 23 of the lug.

The lug extends from the nose 22 to the shoulder 23 at a given inclination angle β with respect to the circumferential direction of the tire, i.e. with respect to said tire equatorial plane. Generally speaking, angle β is defined as the angle between the equatorial plane and a longitudinal axis, or average longitudinal axis, G of lug 2.

Preferably, lug 2 extends from the nose 22 to the shoulder 23 along a curvature. The distance from the first end 201 of the lug to the second end 202 of the lug along the curvature of the lug is called axial length of the lug and is denoted by E.

Preferably - and with reference to Figure 5 - the nose region 22 of a lug 2 is considered the region within about 33%, preferably within 25%, of the axial length E of the lug 2 from the first end 201 along its axial extension, whereas the shoulder region 23 of a lug 2 is considered the region within about 33%, preferably within 25%, of the axial length E of the lug 2 from the second end 202 along its axial extension.

Each lug 2 has a substantially trapezoidal shape when viewed in a sectional view taken, at any point, in a plane orthogonal to the path connecting the nose 22 with the shoulder 23 along the axial length E. A trapezoidal shape is shown also, for example, in Figure 3.

The central part of the lug 2 included between ends 201 and 202 as well as between the nose region 22 and the shoulder region 23 is denoted by 27.

The rotating direction R of the tire is indicated with an arrow in Fig. 1 . During rotation of the tire each lug 2 of the tread 10 sequentially comes in contact with the ground or engages the soil or mud. The first part of each lug 2 that comes in contact with the ground is the one in the central part of the tread 10, i.e. the nose 22 of the lug 2. Upon further rotation of the tire the central part 27 of the lug 2 comes in contact with the ground. Finally, upon further rotation of the tire also the part of the lug on the axially outer side of the tread, i.e. the shoulder 23 of the lug 2, comes in contact with the ground.

The rotation direction R of the tire consequently defines one side of the lugs that first comes in contact with the mud or soil in a field, the so called stepping-in or leading side 21 of the lug 2. The opposite side of the stepping-in side 21 of the lug in circumferential direction is the so called stepping-out, or trailing, side 24 of the lug 2.

Lug 2 also includes a radially outer side 25, defining the lug surface opposite to tread ground 1 1 and arranged substantially orthogonal to the stepping-in side 21 and stepping-out side 24, these latter sides 21 and 24 being connected one to the other by the radially outer side 25.

Therefore, the radially outer side 25 of the lug delimits the lug in radial outer direction and intersects both the stepping-in side 21 and stepping-out side 24 of the lug. The intersection of the radial outer side 25 with the stepping-in side 21 is called the leading edge L of the lug. The intersection of the radial outer side 25 with the stepping-out side 24 is called the trailing edge T of the lug. In the present embodiment, all lugs of the tire 1 have an improved lug profile according to a first preferred embodiment of present invention on each respective stepping-in side 21 . Such improved profile will now be described with reference to Fig.s 2 to 4.

Fig. 2 shows a sectional view of the agricultural tire of Fig. 1 taken along the plane A-A of the latter figure. Particularly, in the detail Y, the section of lug 2 with its lug profile according to a preferred embodiment of the invention can be seen.

The external profile of the stepping-in side 21 of the lug comprises a first, radially outer, concave surface 21 1 and a second, radially inner, concave surface 212. The first concave surface 21 1 and the second concave surface 212 intersect each other at a transition point, or apex, D. In a preferred embodiment as shown in Fig. 2 the first concave surface 21 1 intersects the radial outer side 25 of the lug at the leading edge L.

The radially inner end of the second concave surface 212 preferably terminates on the ground 1 1 of the tread surface. In order to reduce the stress and strain in the transition area between the second concave surface 212 and the ground 1 1 of the tread 10 a first smoothening radius 213 can be added between the second concave surface 212 and the ground 1 1 of the tread 10.

The external profile of the stepping-out side 24 of the lug shown in Fig. 2 is flat or substantially flat. The transition between the sidewall of the stepping-out side 24 of the lug and the surface of the ground 1 1 of the tread 10 may be rounded with a second smoothening radius 214. The latter may be equal or different from first smoothening radius 213.

As said above, both surfaces 21 1 and 212 are curved, in particular concave, surfaces. Preferably, each surface 21 1 , 212 develops, at least in each circumferential sectional plane B-B or parallel to B-B, according to a spline curve , i.e. the radius of curvature of each surface 21 1 and 212 varies along the radial direction.

In the present embodiment, the spline curve of first surface 21 1 is different from the spline curve of second surface 212.

The greatest radius of curvature of the spline curve of each surface 21 1 and 212 is smaller than 400 mm, preferably smaller than 350 mm and more preferably smaller than 300 mm. Alternatively, each surface 21 1 , 212 develops, at least in each circumferential sectional plane B-B or parallel to B-B, according a constant radius of curvature, being part of a spherical profile.

In such alternative embodiments where the radius of curvature of each surface 21 1 and 212 is constant, the radius of curvature of each surface 21 1 and 212 is smaller than 400 mm, preferably smaller than 350 mm and more preferably smaller than 300mm. The height H of the lug 2 is defined as the distance in radial direction of the tire from the ground 1 1 of the tread 10 to the radially outer side 25.

As better shown in Fig. 3, the transition point D is positioned at radial distance hi from the radial outer side 25 of the lug and is positioned at a radial distance h2 from the ground 1 1 of the tread 10.

In this first preferred embodiment of the invention, the radial distance hi from the transition point D to the radial outer surface 25 of the lug and the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D are constant and do not vary along the whole axial length of the lug 2.

In this first preferred embodiment of the invention the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is the same as the radial distance hi from the transition point D to the radial outer surface 25 of the lug. As said above, in the present embodiment the radial distance h2 between the ground 1 1 and the transition point D is the same as the radial distance hi from the transition point D to the radial outer surface 25 of the lug along the whole axial length E of the lug 2.

As can be seen in Fig. 3, the position on the lug profile of the transition edge D is further defined by the circumferential distance d from a reference plane P, i.e. the distance of edge D from plane P in a direction orthogonal to the lug radial height H. The plane P is the plane connecting the radially outer intersection of the first concave surface 21 1 with the radial outer surface of the lug 25, i.e. the leading edge L, with the radially inner intersection of the second concave surface 212 with the ground 1 1 . In case first smoothening radius 213 is provided, plane P is the plane connecting the radially outer intersection of the first concave surface 21 1 with the radial outer surface 25 of the lug, i.e. the leading edge L, with the radially inner intersection of the imaginary continuation of the second concave surface 212 with the ground 1 1 of tread 10.

Figure 3b is showing a variant of this preferred embodiment wherein the intersection at the leading edge L is chamfered or flattened as well as wherein a first smoothening radius 213 is provided in the transition area between the second concave surface 212 and the ground of the tread 1 1 . In this case the plane P is the plane connecting the radially outer intersection of the imaginary continuation of the first concave surface 21 1 with the imaginary continuation of the radial outer surface of the lug 25, i.e. the imaginary leading edge L', with the radially inner intersection of the imaginary continuation of the second concave surface 212 with the ground 1 1 of the tread 10, i.e. point O in Fig. 3b.

With reference to Figure 3c, when the first and the second surface are curved according to a spline, the imaginary continuation of the surfaces can be achieved in drawing a circle Ci passing through the following three point: D; the end point Ff of the respective first or second surface 21 1 or 212; and an intermediate third point Gf defined by the intersection of a plane Mf orthogonal to the line connecting the points D and Ff , such orthogonal plane Mf being positioned at equal distance from D and Ff.

Figure 3c is showing by way of example the circle defined by the point D, the end point Ff of the first surface 21 1 and the intermediate point Gf along the first surface 21 1 .

Therefore, for the definition of the plane P it is irrelevant whether the intersection is between real surfaces or between imaginary continuations of the mentioned surfaces. The first concave surface 21 1 is tangential to the plane P in the leading edge L or in the imaginary leading edge L'.

The second concave surface 212 is tangential to the plane P in the point of radially inner intersection of the second concave surface 212 with the ground 1 1 of the tread 10 or in the point of the radially inner intersection of the imaginary continuation of the second concave surface 212 with the ground 1 1 of the tread 10.

The plane P is inclined with respect to a radial plane passing through the intersection point L or L' and orthogonal to the radial outer surface 25 of the lug at an angle a.

In a preferred embodiment of this invention the angle a is comprised in the range 0 ^° < a < 45 ^° , more preferably in the range 5 ^° < a < 35 ^° and even more preferably in the range 10 ^° < a < 25 ^° According to present invention in order to achieve the best compromise between the tire performances traction, wear and self-cleaning, the profile of the stepping-in side 21 varies along the axial length E of the lug 2 from the nose 22 to the shoulder 23.

According to present invention the distance d is not constant along the axial length E of the lug 2 from the nose 22 to the shoulder 23 of the lug.

In a preferred embodiment of this invention the distance d, along the whole axial length E of the lug 2, is comprised in the range 0 x CLW < d < 0,5 CLW , more preferably in the range 0 x CLW < d < 0,35 CLW and even more preferably in the range 0 x CLW < d < 0,25 CLW, wherein CLW is the circumferential width of the lug 2 measured at the radial outer surface 25 of the lug 2 taken in the same circumferential section as the distance d. An exemplary CLW _e is shown in Figures 3 and 4. In the regions of the nose 22 and the shoulder 23 the distance d is smaller than in the central area 27 of the lug 2 along the axial length of the lug 2 as shown in Figures 4 and 5.

According to a preferred embodiment of the invention the preferred profile distance d in the nose and shoulder region is in the range d < 0,25 d _max wherein d _max is the maximum distance d in the central area 27 of the lug 2.

The nose and shoulder regions of the lugs are the so called self-cleaning areas of the lugs. According to present invention the sidewall of the stepping-in side 21 with a reduced profile width, i.e. a small distance d in the nose region, improves the self-cleaning in such a way that the channelling of the soil or mud in the area between the lugs is improved. The shoulder region of the lug having a reduced profile width with a small distance d improves the self-cleaning by improving the elimination of soil or mud from the tread. Preferably the variation of the distance d along the axial length E of the lug 2 from the nose 22 to the shoulder 23 is parabolic and more preferably being zero at the end of the nose and shoulder region, i.e. respectively at the first end 201 and the second end 202, of the stepping-in side 21 of the lug 2.

It is understood that other variations of the distance d along the axis of the lug are also encompassed by the present invention.

In a variant preferred embodiment of the invention the same profile disclosed for the stepping-in side 21 of lug 2 comprising the two concave surfaces 21 1 and 212 intersecting at the transition point D can also extend to a front sidewall of the nose 22 of the lug, as shown in Figure 4.

A second and third preferred embodiment according to present invention comprise all the features as described in the previous first preferred embodiment with the exception of the radial position of the transition point D.

In a second and third preferred embodiment according to present invention the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is not the same as the radial distance hi from the transition point D to the radial outer surface 25 of the lug 2 along the whole axial length of the lug 2. In these preferred embodiments hi as well as h2 vary along the axial length of the lug 2. With reference to Fig.s 6 and 7, in a second preferred embodiment of present invention in the shoulder area of the lug 2 the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is greater than the radial distance hi from the transition point D to the radial outer surface 25 of the lug 2 and in the nose area 22 of the lug 2 the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is smaller than the radial distance hi from the transition point D to the radially outer surface 25 of the lug 2.

As can be seen in Figure 7, in the second preferred embodiment the radial distance hi continuously increases along the axial length E of the lug 2 from the shoulder 23 to the nose 22 whereby the radial distance h2 consequently continuously decreases along the axial length E of the lug 2 from the shoulder 23 to the nose 22 of the lug 2.

This embodiment is particularly advantageous in field operating conditions where due to the lower inflation pressure of the agriculture tire 1 there is a higher load distribution in the shoulder areas of the agriculture tire 1 and consequently in the shoulder areas of the lugs. A reduced radial distance hi on the shoulder, i.e. a transition point D closer to the radial outer surface 25 of the lug 2 in the shoulder area, improves the wear resistance in the shoulder area.

According to a variant of this preferred embodiment of present invention the radial distance h2 between the ground of the tread 1 1 and the transition point D varies along the axial length of the lug 2 in a linear way. According to another variant of this preferred embodiment of present invention the radial distance h2 between the ground of the tread 1 1 and the transition point D varies along the axial length of the lug in a non-linear way, preferably curved or parabolic way. With reference to Fig. s 8 and 9, in a third preferred embodiment according to present invention, in the shoulder region 23 the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is smaller than the radial distance hi from the transition point D to the radial outer surface 25 of the lug 2 and in the nose region the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D is greater than the radial distance hi from the transition point D to the radially outer surface 25 of the lug 2.

As can be seen in Figure 9, in the third preferred embodiment the radial distance hi continuously decreases along the axial length E of the lug from the shoulder to the nose whereby the radial distance h2 consequently continuously increases along the axial length E of the lug from the shoulder to the nose area.

This embodiment is particularly advantageous in road operating conditions where due to the higher inflation pressure of the agriculture tire 1 there is a higher load distribution in the central area of the agriculture tire 1 and consequently of the nose region of the lugs. A reduced radial distance hi in the nose area, i.e. a transition point D closer to the radially outer surface 25 of the lug 2 in the nose area improves the wear resistance in the central area of the tire.

According to a variant of this preferred embodiment of present invention the radial distance h2 between the ground of the tread 1 1 and the transition point D varies along the axial length E of the lug 2 in a linear way. According to another variant of this preferred embodiment of present invention the radial distance h2 between the ground 1 1 of the tread 10 and the transition point D varies along the axial length E of the lug 2 in a non-linear way, preferably curved or parabolic way.

Transition point D can also be implemented as a thin region instead that as a single point. In particular, a junction profile can be applied between the first and the second concave surface. For example, a circular junction may be present, having, in a sectional view like that of Fig. 2, a radius of curvature comprised in a range of about 0.5-1 .0 mm. Also in this case, reference may be made to a transition point, meant to indicate also a thin transition region as just defined. It is understood that the single embodiments can simultaneously be implemented on a single agriculture tire 1 by alternating them on different lugs. This means that a tread 10 for agriculture tire 1 might comprise a plurality of lugs wherein the plurality of lugs might comprise different groups of plurality of lugs each group according to different embodiments of present invention alternately arranged on the tread. The same agricultural tire 1 might further comprise also standard lugs known in the art. The present invention has been described so far with reference to preferred embodiments. It is intended that there may be other embodiments which refer to the same inventive concept and fall within the scope of the following claims.