AU2021212335A1 - Optimized architecture of heavy-duty tires of the agricultural or civil engineering type - Google Patents

Abstract

The present invention relates to a radial tire (1) for a heavy-duty truck of the agricultural or civil engineering type, in which the protection plies (31) contain hyperelastic cords (50) which comprise filamentary elements (54) on a single layer (52). The distance between the center of each filamentary element (54) of the layer (52) and the main axis (A) of the cord is equal to half of the helix diameter Dh and is substantially constant and equal for all the metal filamentary elements (54) of the layer (52), the metal filamentary elements (54) defining an internal arch (58) of the cord, having the diameter Dv. Each metal filamentary element (54) has a diameter Df and a helix radius of curvature Rf, such that: 9 ≤ Rf / Df ≤ 30, and 1.30 ≤ Dv / Df ≤ 4.5, said hyperelastic cords being embedded in a rubber compound.

Description

OPTIMIZED ARCHITECTURE OF A TYRE OF THE HEAVY-DUTY VEHICLE, AGRICULTURAL OR CONSTRUCTION PLANT TYPE

[0001] The subject of the present invention is a radial tyre, intended to equip a heavy vehicle of the construction plant, agricultural or goods transport type, and relates more particularly to the crown reinforcement of such a tyre, and even more particularly to its protective reinforcement.

[0002] Radial tyres intended to equip a heavy vehicle of the construction plant, agricultural or goods transport type are designated within the meaning of the European Tyre and Rim Technical Organization or ETRTO standard.

[0003] For example, a radial tyre for a heavy vehicle of construction plant type, within the meaning of the European Tyre and Rim Technical Organization or ETRTO standard, is intended to be mounted on a rim with a diameter at least equal to 25 inches. Although not limited to this type of application, the invention is described for a radial tyre of large size, which is intended to be mounted on a dumper, in particular on vehicles for transporting materials extracted from quarries or surface mines, by way of a rim with a diameter at least equal to 35 inches, possibly as much as 57 inches, or even 63 inches.

[0004] Since a tyre has a geometry exhibiting symmetry of revolution about an axis of rotation, the geometry of the tyre is generally described in a meridian plane containing the axis of rotation of the tyre. For a given meridian plane, the radial, axial and circumferential directions denote the directions perpendicular to the axis of rotation of the tyre, parallel to the axis of rotation of the tyre and perpendicular to the meridian plane, respectively. The circumferential direction is tangential to the circumference of the tyre.

[0005] In the following text, the expressions "radially inner/radially on the inside" and "radially outer/radially on the outside" mean "closer to" and "further away from" the axis of rotation of the tyre, respectively. "Axially inner/axially on the inside" and "axially outer/axially on the outside" mean "closer to" and "further away from" the equatorial plane of the tyre, respectively, with the equatorial plane of the tyre being the plane that passes through the middle of the tread surface and is perpendicular to the axis of rotation.

[0006] Generally, a tyre comprises a tread intended to come into contact with the ground via a tread surface, the two axial ends of which are connected via two sidewalls to two beads that provide the mechanical connection between the tyre and the rim on which it is intended to be mounted.

[0007] A radial tyre further comprises a reinforcement made up of a crown reinforcement radially on the inside of the tread and of a carcass reinforcement radially on the inside of the crown reinforcement.

[0008] The carcass reinforcement of a radial tyre for a heavy vehicle of the construction plant, agricultural or goods transport type usually comprises at least one carcass layer comprising generally metal reinforcers, coated with a polymeric material of the elastomer or elastomeric type, obtained by mixing and called coating mixture. A carcass layer comprises a main part that joins the two beads together and is generally wound, in each bead, from the inside of the tyre to the outside, around a usually metal circumferential reinforcing element known as a bead wire so as to form a tum-up. The metal reinforcers of a carcass layer are substantially mutually parallel and form an angle of between 85 and 950 with the circumferential direction.

[0009] The crown reinforcement of a radial tyre for a vehicle of the construction plant, agricultural or goods transport type comprises a superposition of crown layers extending circumferentially, radially outside the carcass reinforcement. Each crown layer is made up of generally metal reinforcers that are mutually parallel and are coated in a polymeric material of the elastomer or coating compound type.

[0010] Among the crown layers, a distinction is usually made between the protective layers, constituting the protective reinforcement and radially outermost, and the working layers, constituting the working reinforcement and radially comprised between the protective reinforcement and the carcass reinforcement.

[0011] The protective reinforcement, which comprises at least one protective layer, essentially protects the working layers from mechanical or physicochemical attacks, which are likely to spread through the tread radially towards the inside of the tyre.

[0012] The protective reinforcement often comprises two radially superposed protective layers formed of elastic metal reinforcers that are mutually parallel in each layer and crossed from one layer to the next, forming angles at least equal to 10 with the circumferential direction.

[0013] The working reinforcement, comprising at least two working layers, has the function of belting the tyre and ensuring its stiffness and road-holding. It absorbs both mechanical inflation stresses, which are generated by the tyre inflation pressure and transmitted by the carcass reinforcement, and mechanical stresses caused by running, which are generated as the tyre runs over the ground and are transmitted by the tread. It is also intended to withstand oxidation, impacts and perforation, by virtue of its intrinsic design and that of the protective reinforcement.

[0014] The working reinforcement usually comprises two radially superposed working layers formed of inextensible metal reinforcers that are mutually parallel in each layer and are crossed from one layer to the next, forming angles at most equal to 60, and preferably at least equal to 15° and at most equal to 45, with the circumferential direction.

[0015] To reduce the mechanical inflation stresses transmitted to the working reinforcement, it is known to arrange, radially outside the carcass reinforcement, a hoop reinforcement. The hoop reinforcement, the function of which is to at least partially absorb the mechanical inflation stresses, improves the endurance of the crown reinforcement by stiffening the crown reinforcement. The hoop reinforcement may be positioned radially on the inside of the working reinforcement, between the two working layers of the working reinforcement, or radially on the outside of the working reinforcement.

[0016] In construction plant type applications, the hoop reinforcement can comprise two hooping layers, radially superimposed, formed of metal reinforcers, parallel to one another in each layer and crossed from one layer to the next, forming, with the circumferential direction, angles at most equal to 10°.

[0017] In heavy-duty vehicle type applications for load transport, the hoop reinforcement usually comprises a hooping layer produced by the circumferential winding of a hooping wire or a continuous hooping strip by forming, with the circumferential direction, angles at most equal to 5°.

[0018] As regards the metal reinforcers, a metal reinforcer is mechanically characterized by a curve representing the tensile force (in N) applied to the metal reinforcer as a function of the relative elongation (in %) thereof, known as the force-elongation curve. Mechanical tensile characteristics of the metal reinforcer, such as the structural elongation As (in %), the total elongation at break At (in %), the force at break Fm (maximum load in N) and the breaking strength Rm (in MPa), are derived from this force-elongation curve, these characteristics being measured in accordance with the standard ASTM D 2969-04 of 2014.

[0019] The total elongation At of the metal reinforcer is, by definition, the sum of its elastic and plastic structural elongations (At = As + Ae + Ap) and particularly at break where each of the elongations is non-zero. The structural elongation As results from the relative positioning of the metal threads making up the metal reinforcer under a low tensile force. The elastic elongation Ae results from the actual elasticity of the metal of the metal threads making up the metal reinforcer, taken individually, the behaviour of the metal following Hooke's law. The plastic elongation Ap results from the plasticity, i.e. the irreversible deformation beyond the yield point, of the metal of these metal threads taken individually. These various elongations and the respective meanings thereof, which are well known to a person skilled in the art, are described, for example, in the documents US5843583, W02005/014925 and W02007/090603.

[0020] Also defined, at each point on the force-elongation curve of a metal reinforcer, is a tensile modulus expressed in GPa, which represents the gradient of the straight line tangential to the force-elongation curve at this point. In particular, the tensile modulus of the elastic linear part of the force-elongation curve is referred to as the tensile elastic modulus or Young's modulus.

[0021] Among the metal reinforcers, a distinction is usually made between elastic metal reinforcers, such as those used in the protective layers, and non-extensible or inextensible metal reinforcers, such as those used in the working layers.

[0022] An elastic metal reinforcer, in its non-rubberized state, is characterized by a structural elongation As at least equal to 1% and a total elongation at break At at least equal to 4%. Moreover, an elastic metal reinforcer has a tensile elastic modulus at most equal to 180 GPa, and usually between 40 GPa and 150 GPa.

[0023] For example, a metal cord comprising a single layer of N=5 metal filamentary elements wound in a helix is known from the prior art. Each metal filamentary element is made up of a steel monofilament and has a diameter equal to 0.38 mm. Each metal filamentary element is wound at a pitch P, here P=6.7 mm, and is individually preformed before the final helical assembly step of the metal filamentary elements. The metal filamentary elements define an internal enclosure of the cord, making it possible to define an enclosure diameter Dv. The preforming and internal enclosure give the cord, once assembled, relatively significant aeration, in other words a relatively large space between each pair of adjacent metal filamentary elements. Such aeration causes structural elongation As of the cord equal to 2.3%. Such a cord is intended in particular to be used in tyres, for example tyres for a heavy duty type vehicle.

[0024] An inextensible metal reinforcer is characterized by a total elongation At, under a tensile force equal to 10% of the force at break Fm, at most equal to 0.2%. Moreover, an inextensible metal reinforcer has a tensile elastic modulus usually between 150 GPa and 200 GPa.

[0025] When the tyre runs over stones or other more or less sharp objects present on tracks on which dumpers travel, or work site approach areas or unsurfaced parking areas on which heavy-duty vehicles frequently travel, the crown of a tyre is frequently subject to cuts which can pass through it radially to the inside and, depending on the size of the object, can perforate the crown and carcass reinforcement assembly, creating a loss of pressure and the failure of the tyre. The use of an elastic metal reinforcer in the protective layers is known to improve the perforation strength of the tyres (W2019/058053); nevertheless, given the cost of these large-size tyres and the frequency of these incidents, it is always useful to improve performance. This improvement is all the more advantageous if it occurs with a reduction in the mass of the crown reinforcement and therefore of the tyre so as to preserve natural resources. Document EP1520069 proposes a solution on the basis of a textile and metal hybrid cord in which an internal layer is a spun yarn on the basis of a multitude of elementary filaments twisted together.

[0026] The inventors have set themselves the objective, for a radial tyre for a vehicle of the construction plant, heavy-duty vehicle or agricultural type, of reducing the risk of perforating the tyre following attacks on the tread when running over sharp stones while reducing the mass of the crown reinforcement.

[0027] This objective has been achieved, according to the invention, by a tyre for a vehicle of the heavy-duty vehicle, construction plant or agricultural type, comprising

• a crown reinforcement, radially on the inside of a tread and radially on the outside of a

carcass reinforcement; • the crown reinforcement comprising, radially from the outside towards the inside, a

protective reinforcement and a working reinforcement, * the protective reinforcement comprising at least one protective layer comprising

hyperelastic metal reinforcers, coated in an elastomeric material, parallel to one another and forming, with a circumferential direction (XX') tangential to the circumference of the tyre, an angle at least equal to 10°, each hyperelastic metal reinforcer comprising at least one cord, called hyperelastic cord,

• said hyperelastic cords comprising a single layer consisting of N metal filamentary elements wound in a helix and having an external diameter D, each metal filamentary element of the layer describing, when the cord extends in a substantially rectilinear direction, a trajectory in the form of a helix around a main axis (A) substantially parallel to the substantially rectilinear direction, so that, in a section plane substantially perpendicular to the main axis (A), the distance between the centre of each metal filamentary element of the layer and the main axis (A) is equal to half the helix diameter Dh and is substantially constant and equal for all the metal filamentary elements of the layer, the metal filamentary elements defining an internal enclosure (58) of the cord of diameter Dv, each metal filamentary element having a diameter Df and a helix radius of curvature Rf defined by Rf=P/(R x Sin(2a)) with P the pitch of each metal filamentary element expressed in millimetres and a the helix angle of each metal filamentary element, characterized in that, with Dh, D, Dv, Df and Rf being expressed in millimetres: • 9SRf/DfS30,and • 1.30 < Dv / Df < 4.5, * said hyperelastic cords being embedded in a rubber compound.

[0028] The inventors have surprisingly reduced the risk of perforating the crown without degrading the other performance, by using as metal reinforcer either directly a hyperelastic cord or a multi-strand of hyperelastic cords, said hyperelastic cords having particular geometric characteristics. The enclosure diameter Dv must be large enough compared with the diameter of the metal filamentary elements Df to allow transverse compression deformation improving the resistance to perforation and small enough to define the thickness of the protective layer compatible with the objectives of limiting the mass of the tyres and therefore the material resources necessary for their production. Similarly, the helix radius of curvature Rf must be sufficiently small compared with the diameter of the metal filamentary elements Df to give a significant structural elongation and sufficiently large to obtain an adequate linear rupture strength of the protective layer.

[0029] Such cords have the advantage over conventional hybrid or non-hybrid elastic cords of having not only high elasticity in tension but also in transverse compression. When an indenting tool exerts a pressure on a conventional cord, the latter being less deformable in transverse compression, the cords maintain a substantially cylindrical geometry. The cylindrical shape means that the force of the indenting tool is exerted on a narrow area of the cord and materially on a metal filamentary element. With the cords proposed in the invention, since the central part of the cord situated in the limit of the enclosure diameter Dv is a rubber compound, an elastic material deformable even in comparison with the central textile fibre of the hybrid cords, the cord is deformed until assuming a parallelepipedal squashed shape offering a greater contact area for the indenting tool and thus at the same time a plurality of metal filamentary elements. This increase in the number of filamentary elements in simultaneous contact with the indenting tool reduces the stresses applied to each of the filamentary elements and therefore increases the penetration strength of the ply and hence that of the tyre.

[0030] The values of the characteristics Df, Dv and Rf and of the other characteristics described below are measured on or determined from cords either directly after they have been manufactured, that is to say before any step of embedding in an elastomeric matrix, or once they have been extracted from an elastomeric matrix, for example of a tyre, and have thus undergone a cleaning step during which any elastomeric matrix is removed from the cord, in particular any material present inside the cord. In order to ensure an original state, the adhesive interface between each metal filamentary element and the elastomeric matrix has to be eliminated, for example by way of an electrochemical process in a bath of sodium carbonate. The effects associated with the shaping step of the method for manufacturing the tyre that are described below, in particular the elongation of the cords, are eliminated by the extraction of the ply and of the cord which, during extraction, substantially regain their characteristics from before the shaping step.

[0031] The cord according to the invention comprises a single layer of helically wound metal filamentary elements. In other words, the cord according to the invention comprises one layer, not two or more than two layers, of helically wound metalfilamentary elements. The layer is made up of metal filamentary elements, that is to say a plurality of metal filamentary elements, not just one metal filamentary element. In one embodiment of the cord, for example when the cord has been obtained from its manufacturing process, the cord according to the invention consists of the layer of wound metal filamentary elements.

[0032] The cord according to the invention has a single helix. By definition, a single-helix cord is a cord in which the axis of each metal filamentary element of the layer describes a single helix, in contrast to a double-helix cord, in which the axis of each metalfilamentary element describes a first helix about the axis of the cord and a second helix about a helix described by the axis of the cord. In other words, when the cord extends in a substantially rectilinear direction, the cord comprises a single layer of metalfilamentary elements wound together in a helix, each metal filamentary element of the layer describing a path in the form of a helix around the substantially rectilinear direction so that the distance between the centre of each metal filamentary element of the layer and the axis of the substantially rectilinear direction is substantially constant and equal for all the metal filamentary elements of the layer. By contrast, when a double-helix cord extends in a substantially rectilinear direction, the distance between the centre of each metal filamentary element of the layer and the substantially rectilinear direction is different for all of the metal filamentary elements of the layer.

[0033] The cord according to the invention does not have a central metal core. This is also referred to as a cord of structure lxN, in which N is the number of metalfilamentary elements, or as an open cord.

[0034] The enclosure of the cord according to the invention is delimited by the metal filamentary elements and corresponds to the volume delimited by a theoretical circle that is, on the one hand, radially on the inside of each metal filamentary element and, on the other hand, tangent to each metal filamentary element.

[0035] A filamentary element means an element extending longitudinally along a main axis and having a section perpendicular to the main axis, the largest dimension G of which is relatively small compared with the dimension L along the main axis. The expression "relatively small" means that L/G is greater than or equal to 100, preferably greater than or equal to 1000. This definition covers both filamentary elements with a circular section and filamentary elements with a non-circular section, for example a polygonal or oblong section. Very preferably, each metal filamentary element has a circular section.

[0036] By definition, the term metal means a filamentary element made up mostly (i.e. more than 50% of its weight) or entirely (100% of its weight) of a metal material. Each metal filamentary element is preferably made of steel, more preferably pearlitic or ferritic-pearlitic carbon steel, commonly referred to as carbon steel by a person skilled in the art, or made of stainless steel (by definition steel comprising at least 10.5% chromium).

[0037] The helix angle a is a parameter that is well known to a person skilled in the art and can be determined using the following iterative calculation comprising three iterations and wherein the index i indicates the number of the iteration 1, 2 or 3. Knowing the structural elongation As expressed in %, the helix angle a(i) is such that a(i)=Arcos [ (100/(100+As) x Cos [ Arctan ( (R x Df) / (P x Cos(a(i-1)) x Sin(R/N)) ] ], in which formula P is the pitch expressed in millimetres at which each metal filamentary element is wound, N is the number of metal filamentary elements in the layer, Df is the diameter of each metal filamentary element expressed in millimetres, Arcos, Cos and Arctan and Sin denote the arccosine, cosine, arctangent and sine functions, respectively. For the first iteration, that is to say for the calculation of a(1), a()=. At the third iteration, a(3)=a is obtained with at least one significant digit after the decimal point when a is expressed in degrees.

[0038] The helix diameter Dh, expressed in millimetres, is calculated using the relationship Dh=P x Tan(a) / u, in which P is the pitch expressed in millimetres at which each metal filamentary element is wound, a is the helix angle of each metalfilamentary element determined above, and Tan is the tangent function. The helix diameter Dh corresponds to the diameter of the theoretical circle passing through the centres of the metalfilamentary elements of the layer in a plane perpendicular to the axis of the cord.

[0039] The enclosure diameter Dv, expressed in millimetres, is calculated using the relationship Dv=Dh-Df, in which Df is the diameter of each metal filamentary element and Dh is the helix diameter, both expressed in millimetres.

[0040] The radius of curvature Rf, expressed in millimetres, is calculated according to the relationship Rf=P/(R x Sin(2a)) in which P is the pitch expressed in millimetres, a is the helix angle of each metal filamentary element and Sin is the sine function.

[0041] It will be recalled that the pitch at which each metal filamentary element is wound is the length covered by this filamentary element, measured parallel to the axis of the cord in which it is located, after which the filamentary element that has this pitch has made a complete turn about said axis of the cord.

[0042] The optional features described below could be combined with one another in so far as such combinations are technically compatible.

[0043] In an advantageous embodiment, all the metal filamentary elements have the same diameter Df.

[0044] The orientation of an angle means the direction, clockwise or anticlockwise, in which it is necessary to rotate from a reference straight line, in this instance the circumferential direction of the tyre, defining the angle in order to reach the other straight line defining the angle.

[0045] In preferred embodiments, 9 5 Rf / Df < 25. In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 9 < Rf/Df < 15.

[0046] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 1.70 < Dv/Df < 2.50.

[0047] Advantageously, the helix radius of curvature Rf is such that 2 mm < Rf < 7 mm. In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 4 mm < Rf < 6 mm and preferably 4 mm < Rf < 5 mm.

[0048] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 4 mm < Rf < 7 mm and preferably 4.5 mm < Rf < 6.5 mm.

[0049] Advantageously, the helix diameter Dh of each metal filamentary element is such that 0.40 mm < Dh < 1.60 mm.

[0050] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 0.85 mm < Dh < 1.60 mm and preferably 0.90 mm < Dh < 1.6 mm.

[0051] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 0.95 mm < Dh < 1.40 mm and preferably 1.00 mm < Dh < 1.35 mm.

[0052] Advantageously, Df is such that 0.10 mm < Df < 0.50 mm.

[0053] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 0.22 mm < Df < 0.50 mm and preferably 0.25 mm < Df < 0.45 mm.

[0054] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 0.32 mm <Df < 0.50 mm and preferably 0.35 mm < Df < 0.50 mm.

[0055] Advantageously, Dv is such that Dv > 0.46 mm, and more preferably 0.50 mm < Dv < 1.20 mm.

[0056] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 0.65 mm < Dv < 0.80 mm.

[0057] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 0.55 mm < Dv < 1.00 mm.

[0058] Advantageously, each metal filamentary element is wound at a pitch P such that 3 mm < P < 15 mm.

[0059] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 7 mm < P < 15 mm and preferably 7.5 mm < P < 11 mm.

[0060] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 9 mm < P < 15 mm.

[0061] Advantageously, the cord has a diameter D such that D < 2.10 mm.

[0062] The diameter or visible diameter, denoted D, is measured by means of a thickness gauge, the diameter of the contacts of which is at least equal to 1.5 times the winding pitch P of the filamentary elements (the model JD50 from Kaefer may be mentioned for example, which makes it possible to achieve a precision of 1/100 of a millimetre, is equipped with a type a contact, and has a contact pressure of around 0.6 N). The measurement protocol consists of three repetitions of a set of three measurements (carried out perpendicularly to the axis of the cord and under zero tension), wherein the second and third of these measurements are carried out in a direction offset angularly from the previous measurement by one third of a turn, by rotation of the measurement direction about the axis of the cord.

[0063] In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 1.15 mm < D < 1.55 mm.

[0064] In one embodiment of a cord intended to reinforce a tyre for off-road vehicles, for example agricultural or construction plant vehicles, 1.5 mm < D < 2 mm.

[0065] In one embodiment, each metal filamentary element comprises a single metal monofilament. Here, each metal filamentary element is advantageously made up of a metal monofilament. In a variant of this embodiment, the metal monofilament is directly coated with a layer of a metal coating comprising copper, zinc, tin, cobalt or an alloy of these metals, for example brass or bronze. In this variant, each metal filamentary element is then made up of the metal monofilament, made for example of steel, forming a core, which is directly coated with the metal coating layer.

[0066] In this embodiment, each metal elementary monofilament is, as described above, preferably made of steel, and has a mechanical strength ranging from 1000 MPa to 5000 MPa. Such mechanical strengths correspond to the steel grades commonly encountered in the field of tyres, namely the NT (Normal Tensile), HT (High Tensile), ST (Super Tensile), SHT (Super High Tensile), UT (Ultra Tensile), UHT (Ultra High Tensile) and MT (Mega Tensile) grades, the use of high mechanical strengths potentially allowing improved reinforcement of the matrix in which the cord is intended to be embedded and lightening of the matrix reinforced in this way.

[0067] Advantageously, with the layer being made up of N helically wound metalfilamentary elements, N ranges from 3 to 18, preferably from 5 to 12, more preferably from 6 to 9.

[0068] Advantageously, the ratio K of the pitch P to the diameter Df of each metal filamentary element of said hyperelastic cords, P and Df being expressed in millimetres, is such that 19 < K < 44, preferably 20 < K < 40 and more preferably 23 < K < 39, P and Df being expressed in millimetres. In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 19 < K < 35 and preferably 23 < K < 30.

[0069] Advantageously, the helix angle a of each metal filamentary element is such that 130 < a < 30°, preferably 17° < a < 26°. In one embodiment of a cord intended to reinforce a tyre for industrial vehicles chosen from vans, heavy-duty vehicles, for example light rail vehicles, buses, heavy road transport vehicles (lorries, tractors, trailers), 18.5° < a < 30° and preferably 18.5° < a < 260.

[0070] In the case of values of the ratio K that are too high or in the case of values of the helix angle that are too low, the longitudinal compressibility of the cord is reduced. In the case of values of the ratio K that are too low or in the case of values of the helix angle that are too high, the longitudinal stiffness of the cord and thus its reinforcement capacity are reduced.

[0071] Advantageously, the cord has a structural elongation As such that As > 1%, preferably

As > 2.5%, more preferably As > 3%.

[0072] Lastly, the relative radial clearance Jr is representative of the distance separating each pair of adjacent metal filamentary elements brought to within the length available for positioning the metal filamentary elements on the layer. More precisely, Jr =N/(R*(D-Df)) x (Dh x Sin(R/N) - (Df / Cos(a x /180))), a being the helix angle, expressed in degrees, of each metal filamentary element (54) and Dv=Dh-Df. Thus, the greater Jr, the greater the space separating two adjacent metal filamentary elements relative to the maximum number of metal filamentary elements that the layer could accommodate. Conversely, the smaller Jr, the smaller the space separating two adjacent metal filamentary elements relative to the maximum number of metal filamentary elements that the layer could accommodate. Within the range according to the invention, Jr makes it possible to maximize the number of metal filamentary elements present on the layer and therefore the reinforcing capacity of the cord but not at the expense of the capacity to adapt to longitudinal compressive deformations.

[0073] Advantageously, the relative radial clearance between two adjacent filamentary elements, Jr of said hyperelastic cords included in the elastic metal reinforcers of the protective layer (311) is such that 0.10 < Jr < 0.6, preferably 0.30 < Jr < 0.60. The relative radial clearance between two adjacent filamentary elements makes it possible to adjust the ventilation of the cord and the balance in this type of application between its, in particular transverse, stiffness and the number of single threads possibly in contact with the indenting tool. A cord that is too poorly ventilated will behave similarly to conventional cords and will not provide the same level of performance gain. A cord that is too ventilated, beyond a relative clearance of 0.6, will deform easily but will not put up sufficient resistance to the indenting tools.

[0074] Depending on the type of tyre designed, depending on the need for tensile strength for each specific use, i.e. vans, heavy-duty vehicles or construction plant vehicles, whose dimensions vary from 16 inches to 63 inches, each metal reinforcer of the single protective layer or of the protective layers consists of a single hyperelastic cord as defined above or is composed of a plurality of said cords, assembled together, namely is a strand of hyperelastic cords. Specifically, the assembly of a plurality of hyperelastic cords retains the transverse squashing properties of the hyperelastic cord.

[0075] For tyres for heavy vehicles of the construction plant type, according to a preferred embodiment of the protective layers, the elastic metal reinforcers of the protective layers form, with the circumferential direction, an angle at least equal to 150 and at most equal to 350.

[0076] For tyres for heavy vehicles of the construction plant type, a preferred embodiment comprises two protective layers comprising hyperelastic metal reinforcers. More advantageously, the respective metal reinforcers of the two protective layers are crossed from one protective layer to the next. More advantageously for productivity gains, the metal reinforcers of the radially innermost protective layer form with the circumferential direction (XX') an angle equal in absolute value to the angle formed by the metal reinforcers of the radially outermost protective layer with the circumferential direction (XX'). And more advantageously, to have homogeneous behaviour in the distribution of the forces taken up by each of the layers of the crown reinforcement, the absolute value of the angles formed by the hyperelastic metal reinforcers of the protective layers with the circumferential direction is substantially equal to the average of the absolute values of the angles formed by the metal reinforcers of the working layers with the circumferential direction (XX').

[0077] According to a preferred embodiment of the radially outermost protective layer, the hyperelastic metal reinforcers of the radially outermost protective layer form, with the circumferential direction, an angle equal to that formed by the inextensible metal reinforcers of the radially outermost working layer. These angles are oriented in the same direction with respect to the equatorial plane of the tyre and are therefore equal in terms of their algebraic value. In other words, the reinforcers of said protective layer are parallel to those of said working layer.

[0078] According to a preferred embodiment of the crown reinforcement, to improve the hammering performance, namely the cracking of the rubber materials, the radially innermost protective layer has an axial width LP1 at least equal to 1.05 times and at most equal to 1.25 times the maximum axial width LTmax of the working layer having the greatest axial width. Below 1.05 times the axial width LTmax, the radially innermost protective layer does not project sufficiently beyond the working layer of greatest axial width to be able to provide effective protection against hammering. Above 1.25 times the axial width LTmax, the axial end of the radially innermost protective layer is very close to the axial end of the tread, thereby increasing the risk of cracking between the axial end of said protective layer and the axial end of the tread.

[0079] According to a preferred embodiment of the crown reinforcement of a construction plant tyre, the crown reinforcement comprises a hoop reinforcement comprising two hooping layers, the respective metal reinforcers of which, coated in an elastomeric material, parallel to one another and forming, with the circumferential direction, an angle at most equal to 10, are crossed from one hooping layer to the next. A distinction is usually made between angled hooping layers, with reinforcers that form angles at least equal to 5° and at most equal to 8°, and circumferential hooping layers, with reinforcers that are substantially circumferential and form angles close to 0° and at most equal to 5°. The metal reinforcers of the hooping layer may be either elastic or inextensible. The hoop reinforcement can be positioned radially inside the working reinforcement, between the two working layers of the working reinforcement, or radially outside the working reinforcement.

[0080] The features of the invention are illustrated in the schematic Figures 1 and 2, which are not to scale, with reference to a tyre of size 53/8R63: • Figure 1: meridian cross section of a tyre crown according to the invention.

• Figure 2: cross section of a hyperelastic cord making up all or part of the metal reinforcers of the protective reinforcement according to the invention.

[0081] Figure 1 shows a meridian cross section through a tyre 1 for a heavy vehicle of construction plant type comprising a crown reinforcement 3 radially on the inside of a tread 2 and radially on the outside of a carcass reinforcement 4. The crown reinforcement 3 comprises, radially from the outside to the inside, a protective reinforcement 31, a working reinforcement 32 and a hoop reinforcement 33. The protective reinforcement 31 comprises two protective layers (311, 312) comprising hyperelastic metal reinforcers that are coated in an elastomeric material, are mutually parallel and form an angle equal to 240 with a circumferential direction XX' tangential to the circumference of the tyre, the respective metal reinforcers of each protective layer being crossed from one protective layer to the next. The working reinforcement 32 comprises two working layers 321, 322 whose respective inextensible metal reinforcers that are coated in an elastomeric material, are mutually parallel and form, with the circumferential direction XX', angles respectively equal to 33, for the radially innermost working layer 321, and 19°, for the radially outermost working layer 322, are crossed from one working layer to the next. The radially innermost protective layer 311 projects axially beyond the working layer of greatest axial width, here the radially innermost working layer 321. In the case represented, the axial width LP1 is equal to 1.2 times the axial width LTmax. The hoop reinforcement 33 comprises two hooping layers 331, 332, the respective metal reinforcers of which, which are coated in an elastomeric material, are mutually parallel and form an angle of between 50 and 100 with the circumferential direction XX', are crossed from one hooping layer to the next.

[0082] Figure 2 illustrates the cords involved in the production of the reinforcing elements of the protective layers either constituting the reinforcing elements, or being one of the cords of the strand which the reinforcing element of the protective layer constitutes. The cord 50 according to the invention comprises a single layer 52 of metal filamentary elements 54 wound helically. In this instance, the cord 50 is made up of the single layer 52, in other words the cord 50 does not comprise any other metal filamentary element than those of the layer 52. The layer 52 consists of 9 metal filamentary elements wound in a helix and being embedded in the rubber compound matrix of the protective layer, the internal enclosure 58 itself being filled with said rubber compound.

[0083] The invention has been tested on tyres of size 24.00R35. The tyres according to the invention are compared with reference tyres of the same size for each of the tests.

[0084] With regard to the crown perforation strength performance, two types of tests are carried out, namely dynamic or quasi-static tests. The two tests use an indenting tool whose geometry approximates to the obstacles causing the most crown rupture in construction plant tyres according to certain users. The indenting tool is a steel prism with a triangular base. The height of the prism is 55 mm. The tests are carried out in such a way that the tyre comes into contact with the prism on one of its lateral edges, termed contact edge.

[0085] In the dynamic test, the prism is fastened to the road by the lateral face opposite to the contact edge. The angle of the prism opposite to the lateral face for fastening to the road is 45°. The tyre is tested by mounting it on a vehicle, with the recommended pressure and load. The vehicle is guided in such a way that the indenting tool stresses the tyre on the same axial position of the crown. The tyre is adjusted in rotation such that the impact does not occur twice at the same azimuth of the tyre. The vehicle runs at a speed of 5 km/h. A mechanism makes it possible to increase the distance from the contact edge to the running surface. The test is stopped upon perforation of the crown. The result of the test is the distance from the edge to the running surface. The greater the height, the better the tyre resists running over obstacles.

[0086] The quasi-static test uses the same type of indenting tool inserted at a rate of50 mm/min. The tyre is squashed on a planar surface with a force equal to the recommended load, the tyre being inflated to the recommended pressure. The polar is inserted at the centre of the contact area. The result of the test is the penetration distance required to rupture the crown reinforcement.

[0087] The reference tyres and those according to the invention are identical except for the protective layers. They have the same tread pattern and the same reinforcers for the carcass layer and the same rubber compounds for the different parts of the tyre. With regard to the crown reinforcement, they are made up, from the radially outermost element to the radially innermost element, of a protective reinforcement, of a working reinforcement and of a hoop reinforcement.

[0088] For all the tyres tested, the hoop reinforcement consists of a first hooping layer formed of inextensible metal reinforcing elements forming an angle with the circumferential direction equal to 80 and of a second hooping layer radially outside the first, formed of inextensible metal reinforcing elements forming an angle with the circumferential direction equal to 80 and crossed with the cords of the preceding layer.

[0089] For all the tyres tested, the working reinforcement consists of a first working layer formed of inextensible metal reinforcing elements forming an angle with the circumferential direction equal to 330 oriented on the same side with respect to the circumferential direction as the cords of the radially innermost layer of the hoop reinforcement and of a second working layer radially outside the first, formed of inextensible metal cords forming an angle with the circumferential direction equal to 19 and crossed with the metal reinforcing elements of the radially innermost layer of the working reinforcement.

[0090] The inextensible metal reinforcers of the hoop and crown reinforcements are 26.30 cords, namely cords of 26 threads of 30 hundredths of a mm in diameter, arranged in three layers, the central layer comprising 3 threads, the second comprising 9 threads and the outer layer comprising 14 threads. Said cords are arranged at a pitch of 3.4 mm.

[0091] The reference tyre comprises a protective reinforcement composed of a first protective layer formed of elastic metal reinforcing elements forming an angle with the circumferential direction equal to 24° and of a second layer, radially outside the first, formed of elastic metal reinforcing elements forming an angle with the circumferential direction equal to 24° and crossed with the cords of the preceding layer. The angles of the different layers are such that the reinforcing elements of the radially innermost protective layer are crossed with the reinforcing elements of the radially outermost working layer.

[0092] The elastic metal reinforcers of the protective reinforcement of the reference tyre are 24.26 strands, namely strands of 4 cords 6 threads of 26 hundredths of a mm in diameter. Said strands are arranged at a pitch of 2.5 mm.

[0093] The first tyre P1 according to the invention comprises a protective reinforcement composed of a first protective layer formed of hyperelastic metal reinforcing elements forming an angle with the circumferential direction equal to 240 and of a second layer, radially outside the first, formed of hyperelastic metal reinforcing elements forming an angle with the circumferential direction equal to 24° and crossed with the cords of the preceding layer. The angles of the different layers are such that the reinforcing elements of the radially innermost protective layer are crossed with the reinforcing elements of the radially outermost working layer.

[0094] The hyperelastic metal reinforcers of the protective reinforcement of the tyre P1 according to the invention are 5.45 cords according to the invention, namely a cord comprising 5 threads with a diameter Df of 45 hundredths of a mm. Said cords are arranged at a pitch of 2 mm. The enclosure diameter Dv of said threads is equal to 1.11 mm. The helix radius of curvature Rf is equal to 4.2 mm. Jr, the relative radial clearance between two adjacent filamentary elements, is equal to 0.43.

[0095] The second tyre P2 according to the invention comprises a protective reinforcement composed of a single protective layer formed of hyperelastic metal reinforcing elements forming an angle with the circumferential direction equal to 24°. The angles of the different layers are such that the reinforcing elements of the protective layer are crossed with the reinforcing elements of the radially outermost working layer in a first version and are not crossed in a version P2'.

[0096] The hyperelastic metal reinforcers of the protective reinforcement of the tyre P2 according to the invention, whichever the version, are 18.45 strands, namely strands according to the invention composed of 3 cords comprising 6 threads with a diameter Df of 45 hundredths of a mm. Said strands are arranged at a pitch of 4.4 mm. The enclosure diameter Dv of said threads is equal to 1.11 mm. The helix radius of curvature Rf is equal to 4.2 mm. Jr, the relative radial clearance between two adjacent filamentary elements, is equal to 0.35.

[0097] The tyres P1, P2, P2' according to the invention lead, whichever the version, to a gain at least equal to 7% as far as the dynamic test is concerned and of at least 18% according to the static test compared with the reference tyre. The crown reinforcement is about 8% lighter for the versions of the tyre according to the invention with two protective layers and 17% lighter for the versions of the tyre according to the invention with a single protective layer. The invention as proposed therefore makes it possible to improve the crown perforation resistance while reducing the mass of the crown reinforcement and therefore the mass of the tyre.

Claims (9)

1. Tyre (1) for a vehicle of the heavy-duty, construction plant or agricultural type, comprising: - a crown reinforcement (3), radially on the inside of a tread (2) and radially on the outside of a carcass reinforcement (4), - the crown reinforcement (3) comprising, radially from the outside towards the inside, a protective reinforcement (31) and a working reinforcement (32), - the protective reinforcement (31) comprising at least one protective layer (311, 312) comprising hyperelastic metal reinforcers, coated in an elastomeric material, parallel to one another and forming, with a circumferential direction (XX') tangential to the circumference of the tyre, an angle at least equal to 10, each hyperelastic metal reinforcer comprising at least one cord, called hyperelastic cord (50): characterized in that said hyperelastic cords (50) comprise a single layer (52) consisting of N metal filamentary elements (54) wound in a helix and having an external diameter D, each metal filamentary element (54) of the layer (52) describing, when the cord (50) extends in a substantially rectilinear direction, a trajectory in the form of a helix around a main axis (A) substantially parallel to the substantially rectilinear direction, so that, in a section plane substantially perpendicular to the main axis (A), the distance between the centre of each metal filamentary element (54) of the layer (52) and the main axis (A) is equal to half the helix diameter Dh and is substantially constant and equal for all the metal filamentary elements (54) of the layer (52), the metal filamentary elements (54) defining an internal enclosure (58) of the cord of diameter Dv, each metal filamentary element (54) having a diameter Df and a helix radius of curvature Rf defined by Rf=P/(R x Sin(2a)) with P the pitch of each metal filamentary element expressed in millimetres and a the helix angle of each metalfilamentary element (54), characterized in that, with Dh, D, Dv, Df and Rf being expressed in millimetres: 9<Rf/Df<30, and 1.30 < Dv / Df < 4.5, - said hyperelastic cords (50) being embedded in a rubber compound.

2. Tyre (1) according to Claim 1, in which the ratio K of the pitch P to the diameter Df of each metal filamentary element (54) of said hyperelastic cords (50) is such that 19 5 K 5 44, preferably 20 < K < 40 and more preferably 23 < K < 39, P and Df being expressed in millimetres.

3. Tyre (1) according to one of Claims 1 or 2, in which there is a relative radial clearance, Jr, between two adjacent filamentary elements of said hyperelastic cords (50), Jr =N/(7*(D-Df)) x (Dh x Sin(7u/N) - (Df / Cos(a x u/180))), a being the helix angle,

expressed in degrees, of each metal filamentary element (54) and Dv=Dh-Df, and Jr of the hyperelastic metal reinforcers of the protective layer (311) being such that 0.10 < Jr < 0.6.

4. Tyre (1) according to any one of the preceding claims, in which each hyperelastic metal reinforcer of said protective layer (311, 312) is a hyperelastic cord (50).

5. Tyre (1) according to any one of Claims I to 3, in which each hyperelastic metal reinforcer of said protective layer (311, 312) is a strand of hyperelastic cords (50).

6. Tyre (1) for a heavy vehicle of the construction plant type according to any one of the preceding claims, in which the protective reinforcement (31) comprises two protective layers (311, 312), the respective metal reinforcers of which are hyperelastic and crossed from one protective layer to the next.

7. Tyre (1) for a heavy vehicle of the construction plant type according to any one of the preceding claims, the protective reinforcement (31) of which comprises two protective layers (311, 312), the respective metal reinforcers of which are hyperelastic and crossed from one protective layer to the next, and the working reinforcement (32) of which comprises at least two radially superposed working layers (321, 322) formed of inextensible metal reinforcers, in which the metal reinforcers of the radially innermost protective layer (311) form with the circumferential direction (XX') an angle equal in absolute value to the angle formed by the metal reinforcers of the radially outermost protective layer (312) with the circumferential direction (XX'), the absolute value of these two angles being substantially equal to the average of the absolute values of the angles formed by the metal reinforcers of the working layers (321, 322) with the circumferential direction (XX').

8. Tyre (1) for a heavy vehicle of the construction plant type according to any one of the preceding claims, the working reinforcement (32) of which comprises at least two radially superposed working layers (321, 322) formed of inextensible metal reinforcers, in which the radially innermost protective layer (311) has an axial width LP1 at least equal to 1.05 times and at most equal to 1.25 times the maximum axial width LTmax of the working layer with the greatest axial width.

9. Tyre (1) for a heavy vehicle of the construction plant type according to any one of the preceding claims, in which the crown reinforcement (3) comprises a hoop reinforcement (33) comprising two hooping layers (331, 332), including the respective metal reinforcers, coated in an elastomeric material, parallel to one another and forming, with the circumferential direction (XX'), an angle at most equal to 100.