Agricultural tire

Tread (10) for agricultural tire (1) comprising a plurality of lugs protruding from the ground of the tread (11) in radially outer direction extending at a given inclination angle from a central portion of the tread toward both axial ends of the tread and alternately arranged at given intervals in the circumferential direction on one side and on the other with respect to the equatorial plane of the tire. The lugs (2) comprising a stepping-in surface (21) wherein the stepping-in surface (21) of at least one lug (2) comprises on its sidewall a first radially outer, concave surface (211) and a second, radially inner, concave surface (212) intersecting each other in a transition point (D) when viewed in a circumferential section.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No. PCT/EP2016/075180 filed Oct. 20, 2016, claiming priority based on Italian Patent Application No. 102015000066713 filed Oct. 29, 2015.

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 longitudes 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 agriculture pneumatic tire comprising a tread provided with lugs extending at a givers 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 claim1.

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 leg, 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.

FIG. 1shows an agricultural tire1comprising a tread10. The latter is provided with a plurality of lugs, one of which is denoted by way of example with2. Each lug2protrudes from a ground11of tread10in 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 tires and passing through the axial middle of the tire.

More particularly—and with reference also toFIG. 4—each lug2extends from a first end201in the axial central part of the tire, i.e. in the area of the equatorial plane, towards a second end202in the axial lateral part of the tire, i.e. the shoulder area of the tire. The area close to, and including, the first end201of the lug in the central area of the tire defines a so-called nose region, or nose,22of the lug. The area close to, and including, the second end202of the lug in the shoulder area of the tire is called the shoulder region, or shoulder,23of the lug.

The lug extends from the nose22to the shoulder23at a given inclination angle β with respect to the circumferential direction of the tire, i.e. with respect to said tire equator plane. Generally speaking, angle β is defined as the angle between the equatorial plane and a longitudinal axis, or average longitudinal axis, G of lug2.

Preferably, lug2extends from the nose22to the shoulder23along a curvature. The distance from the first end201the lug to the second end202of the lug along the curvature of the lug is called axial length of the lug and is denoted by E.

Preferably—and with reference toFIG. 5—the nose region22of a lug2is considered the region within about 33%, preferably within 25%, of the axial length E of the lug2from the first end201along its axial extension, whereas the shoulder region23of a lug2is considered the region within about 33%, preferably within 25%, of the axial length E of the lug2from the second end202along its axial extension.

Each lug2has a substantially trapezoidal shape when viewed in a sectional view taken, at any point, in a plane orthogonal to the path connecting the nose22with the shoulder23along the axial length E. A trapezoidal shape is shown also, for example, inFIG. 3.

The central part of the lug2included between ends201and202as well as between the nose region22and the shoulder region23is denoted by27.

The rotating direction R of the tire is indicated with an arrow inFIG. 1. During rotation of the tire each lug2of the tread10sequentially comes in contact with the ground or engages the soil or mud. The first part of each lug2that comes in contact with the ground is the one in the central pad of the tread10, i.e. the nose22of the lug2. Upon further rotation of the tire the central part27of the lug2comes 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 shoulder23of the lug2, 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 side21of the lug2. The opposite side of the stepping-in side21of the lug in circumferential direction is the so called stepping-out, or trailing, side24of the lug2.

Lug2also includes a radially outer side25, defining the lug surface opposite to tread ground11and arranged substantially orthogonal to the stepping-in side21and stepping-out side24, these latter sides21and24being connected one to the other by the radially outer side25.

Therefore, the radially outer side25of the lug delimits the lug in radial outer direction and intersects both the stepping in side21and stepping-out side24of the lug. The intersection of the radial cuter side25with the stepping-in side21is called the leading edge L of the lug. The intersection of the radial outer side25with the stepping-out side24is called the trailing edge T of the lug.

In the present embodiment, all lugs of the tire1have an improved lug profile according to a first preferred embodiment of present invention on each respective sleeping-in side21. Such improved profile will flow be described with reference toFIGS. 2 to 4.

FIG. 2shows a sectional view of the agricultural tire ofFIG. 1taken along the plane A-A of the latter figure. Particularly, in the detail Y, the section of lug2with its lug profile according to a preferred embodiment of the invention can be seen.

The external profile of the stepping-in side21of the lug comprises a first, radially outer, concave surface211and a second, radially inner, concave surface212. The first concave surface211and the second concave surface212intersect each other at a transition point or apex, D. In a preferred embodiment as shown inFIG. 2the first concave surface211intersects the radial outer side25of the lug at the leading edge L.

The radially inner end of the second concave surface212preferably terminates on the ground11of tire tread surface, in order to reduce the stress and strain in the transition area between the second concave surface212and the ground11of the tread10a first smoothening radius213can be added between the second concave surface212and the ground11of the tread10.

The external profile of the stepping-out side24of the lug shown inFIG. 2is flat or substantially flat. The transition between the sidewall of the stepping-out side24of the lug and the surface of the ground11of the tread10may be rounded with a second smoothening radius214. The latter may be equal or different from first smoothening radius213.

As said above, both surfaces211and212are curved, in particular concave, surfaces. Preferably, each surface211,212develops, 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 surface211and212varies along the radial direction.

In the present embodiment, the spline curve of first surface211is different from the spline curve of second surface212.

The greatest radius of curvature of the spline curve of each surface211and212is smaller than 400 mm, preferably smaller than 350 mm and more preferably smaller than 300 mm.

Alternatively, each surface211,212develops, 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 surface211and212is constant, the radius of curvature of each surface211and212is smaller than 400 mm, preferably smaller than 350 mm and more preferably smaller than 300 mm.

The height H of the lug2is defined as the distance in radial direction of the tire from the ground11of the tread10to the radially outer25.

As better shownFIG. 3, the transition point D is positioned at radial distance h1from the radial outer side25of the lug and is positioned at a radial distance h2from the ground11of the tread10.

In this first preferred embodiment of the invention, the radial distance h1from the transition point D to the radial outer surface25of the tag and the radial distance h2between the ground11of the tread10and the transition point D are constant and do not vary along the whole axial length of the lug2.

In this first preferred embodiment of the invention the radial distance h2between the ground11of the tread10and the transition point D is the same as the radial distance h1from the transition point D to the radial outer surface25of the lug.

As said above, in the present embodiment the radial distance h2between the ground11and the transition point D is the same as the radial distance h1from the transition point D to the radial outer surface25of the lug along the whole axial length E of the lug2.

As can be seen inFIG. 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 surface211with the radial outer surface of the lug25, i.e. the leading edge L, with the radially inner intersection of the second concave surface212with the ground11. In case first smoothening radius213is provided, plane P is the plane connecting the radially outer intersection of the first concave surface211with the radial outer surface25of the lug, i.e. the leading edge L, with the radially inner intersection of the imaginary continuation of the second concave surface212with the ground11of tread10.

FIG. 3bis 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 radius213is provided in the transition area between the second concave surface212and the ground of the tread11. In this case the plane P is the plane connecting the radially outer intersection of the imaginary continuation of the first concave surface211with the imaginary continuation of the radial outer surface of the lug25, i.e. the imaginary leading edge L′, with the radially inner intersection of the imaginary continuation of the second concave surface212with the ground11of the tread10, i.e. point O inFIG. 3b.

With reference toFIG. 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 surface211or212; 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.

FIG. 3cis showing by way of example the circle defined by the point D, the end point Ff the first surface211and the intermediate point Gf along the first surface211.

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 surface211is tangential to the plane P in the leading edge L or in the Imaginary leading edge L.

The second concave surface212is tangential to the plane P in the point of radially inner intersection of the second concave surface212with the ground11of the tread10or in the point of the radially inner intersection of the imaginary continuation of the second concave surface212with the ground11of the tread10.

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 surface25of the lug at an angle α.

In a preferred embodiment of this invention the angle α is comprised in the range 0°≤α≤45°, more preferably in the range 5°≤α≤35° and even more preferably in the range 10°≤α≤≅°

According to present invention in order to achieve the best compromise between the tire performances traction, wear and sell-cleaning, the profile of the stepping-in side21varies along the axial length E of the lug2from the nose22to the shoulder23.

According to present invention the distance d is not constant along the axial length E of the lug2from the nose22to the shoulder23of the lug.

In a preferred embodiment of this invention the distance d, along the whole axial length E of the lug2, is comprised in the range 0×CLW≤d≤0.5 CLW, more preferably in the range 0×CLW≤d≤0.35 CLW and even more preferably in the range 0×CLW≤d≤0.25 CLW, wherein CLW is the circumferential width of the lug2measured at the radial outer surface25of the lug2taken in the same circumferential section as the distance d. An exemplary CLWeis shown inFIGS. 3 and 4.

In the regions of the nose22and the shoulder23the distance d is smaller than in the central area27of the lug2along the axial length of the lug2as shown inFIGS. 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 dmaxwherein dmaxis the maximum distance d in the central area27of the lug2.

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 side21with a reduced profile width, i.e. a small distance d in the nose region, improves the sell-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 lug2from the nose22to the shoulder23is parabolic and more preferably being zero at the end of the nose and shoulder region i.e. respectively at the first end201and the second end202, of the stepping-in side21of the lug2.

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 side21of lug2comprising the two concave surfaces211and212intersecting at the transition point D can also extend to a front sidewall of the nose22of the lug, as shown inFIG. 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 h2between the ground11of the tread10and the transition point D is not the same as the radial distance h1from the transition point D to the radial outer surface25of the lug2along the whole axial length of the lug2. In those preferred embodiments h1as well as h2vary along the axial length of the lug2.

With reference toFIGS. 6 and 7, in a second preferred embodiment of present invention in the shoulder area of the lug2the radial distance h2between the ground11of the tread10and the transition point D is greater than the radial distance h1from the transition point D to the radial outer surface25of the lug2and in the nose area22of the lug2the radial distance h2between the ground11of the tread10and the transition point D is smaller than the radial distance h1from the transition point D to the radially outer surface25of the lug2.

As can be seen inFIG. 7, in the second preferred embodiment the radial distance h1continuously increases along the axial length E of the lug2from the shoulder23to the nose22whereby the radial distance h2consequently continuously decreases along the axial length E of the lug2from the shoulder23to the nose22of the lug2.

This embodiment is particularly advantageous in field operating conditions where due to the lower inflation pressure of the agriculture tire1there is a higher load distribution in the shoulder areas of the agriculture tire1and consequently in the shoulder areas of the lugs. A reduced radial distance h1on the shoulder, i.e. a transition point D closer to the radial outer surface25of the lug2in 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 h2between the ground of the tread11and the transition point D varies along the axial length of the lug2in a linear way. According to another variant of this preferred embodiment of present invention the radial distance h2between the ground of the tread11and the transition point D varies along the axial length of the lug in a non-linear way, preferably curved or parabolic way.

With reference toFIGS. 8 and 9, in a third preferred embodiment according to present invention, in the shoulder region23the radial distance h2between the ground11of the tread10and the transition point D is smaller than the radial distance h1from the transition point D to the radial eater surface25of the lug2and in the nose region the radial distance h2between the ground11of the tread10and the transition point D is greater than the distance h1from the transition point D to the radially outer surface25of the lug2.

As can be seen in FIG. D, in the third preferred embodiment the radial distance h1continuously decreases along the axial length E of the lug from the shoulder to the nose whereby the radial distance h2consequently 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 tire1there is a higher load distribution in the central area of the agriculture tire1and consequently of the nose region of the lugs. A reduced radial distance h1in the nose area, i.e. a transition point D closer to the radially outer surface25of the lug2in the nose area improves the wear resistance in the central area of the fire.

According to a variant of this preferred embodiment of present invention the racial distance h2between the ground of the tread11and the transition point D varies along the axial length E of the lug2in a linear way. According to another variant of this preferred embodiment of present invention the radial distance h2between the ground11of the tread10and the transition point D varies along the axial length E of the lug2in 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 ofFIG. 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 tire1by alternating them on different lugs. The a trend10for agriculture tire1might 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 tire1might 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 fail within the scope of the following claims.