AU2020347810A1

AU2020347810A1 - Bead wire comprising rubber-coated windings

Info

Publication number: AU2020347810A1
Application number: AU2020347810A
Authority: AU
Inventors: Nicolas Besnard; Guilaine CANEVET; Xavier DEPARIS; Sylvere MAILLARD; Thierry Taillandier
Original assignee: Compagnie Generale des Etablissements Michelin SCA
Current assignee: Compagnie Generale des Etablissements Michelin SCA
Priority date: 2019-09-18
Filing date: 2020-09-15
Publication date: 2022-04-07
Also published as: CN114401851A; CA3148169A1; FR3100746A1; WO2021052922A1; CL2022000648A1; EP4031385B1; CN114401851B; EP4031385A1

Abstract

The subject of the invention is a bead wire for a tyre exhibiting symmetry of revolution about a main axis and comprising N>1 circumferential windings (30) about the main axis, the N circumferential windings (30) being arranged in P>1 radially superposed layers C(i), i ranging from 1 to P, each layer C(i) comprising L(i)>1 axially juxtaposed circumferential windings (30). Each circumferential winding (30) comprises a portion of a metal monofilament (34) of diameter d and a layer (36) of a material having a density less than the density of the metal monofilament (34), the layer (36) covering the portion of metal monofilament (34) such that each circumferential winding (30) is, in a radial section plane (XY) parallel to the main axis, inscribed in a circle of diameter D. The ratio D/d is greater than or equal to 1.15. The bead wire has: - a mean diameter Dm, expressed in mm and defined as the distance radially separating, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, the centres of circles circumscribed (U1, U2) on each half-section of the bead wire in the radial section plane (XY), - a mass M, expressed in g, - each circumferential winding (30) having, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, a section of area S, expressed in mm

Description

Bead wire comprising rubber-coated windings

[001] The present invention relates to a bead wire for a tyre, to a tyre, and to a method for manufacturing this bead wire. A tyre is understood to mean a casing intended to form a cavity by cooperating with a support element, for example a rim, this cavity being able to be pressurized to a pressure higher than atmospheric pressure. A tyre according to the invention has a structure of substantially toroidal shape exhibiting symmetry of revolution about a main axis of the tyre.

[002] A tyre intended to be fitted to a construction plant vehicle and sold under the Michelin brand under the name XDR3 is known from the prior art. Such a tyre comprises a tread, two beads, two sidewalls connecting the beads to the tread, and a crown reinforcement disposed circumferentially between the carcass reinforcement and the tread. Each bead comprises an annular reinforcing element forming a bead wire, this bead wire exhibiting symmetry of revolution about a main axis of the bead wire that is substantially coincident with the main axis of the tyre.

[003] The bead wire of this tyre comprises N=395 circumferential windings about the main axis of the bead wire. These N circumferential windings are arranged in P=23 layers C(i), i ranging from 1 to 23. These layers C(i) are radially superposed on one another. Each layer C(i) comprises L(i)>1 circumferential windings. The circumferential windings of one and the same layer C(i) are axially juxtaposed with one another. Each circumferential winding comprises a portion of a metal monofilament and a layer of a polymer material, in this case a crosslinked elastomer composition, coating the portion of the metal monofilament. The metal monofilament has a substantially circular section of diameter d=3 mm.

[004] In order to manufacture this bead wire, a method is used in which a step of coating the metal monofilament with a layer of polymer material is carried out such that, in a section plane perpendicular to a main axis of the metal monofilament coated with the layer of polymer material, the metal monofilament coated with the layer of polymer material is inscribed in a circle of diameter D. In this instance, the metal monofilament is coated with a sheath of tubular section with a thickness equal to (D d)/2, in this case with a thickness equal to 0.175 mm, such that the metal monofilament coated with the layer of polymer material has an outside diameter D substantially equal to 3.35 mm. Next, a step of assembly the bead wire is carried out, in which the metal monofilament coated with the layer of polymer material is wound so as to form the N circumferential windings.

[005] Such a bead wire is relatively heavy in relation to its section. Thus, on the one hand, if there is a desire to increase the overall section of the bead wire, i.e. the product of its width in the axial direction multiplied by its height in the radial direction, for example in order to improve certain properties of the tyre, the mass increases greatly and requires the use of specific installations just to be able to manipulate the bead wire. On the other hand, if there is a desire to maintain a substantially identical overall section, it is desirable to reduce the weight of the bead wire in order to reduce the costs associated therewith.

[006] The aim of the invention is a bead wire for a tyre that exhibits a better compromise between the mass and the overall section of the bead wire than the prior art.

[007] To this end, the subject of the invention is a bead wire for a tyre, the bead wire exhibiting symmetry of revolution about a main axis of the bead wire and comprising

N>1 circumferential windings about the main axis of the bead wire, the N circumferential windings being arranged in P>1 radially superposed layers C(i), i ranging from 1 to P, each layer C(i) comprising L(i)>1 axially juxtaposed circumferential windings, each circumferential winding comprising a portion of a metal monofilament of diameter d and a layer of a material having a density less than the density of the metal monofilament, the layer coating the portion of metal monofilament such that each circumferential winding is, in a radial section plane parallel to the main axis of the bead wire, inscribed in a circle of diameter D, the ratio D/d being greater than or equal to 1.15.

[008] By virtue of a ratio D/d greater than or equal to 1.15, for one and the same section of the bead wire, the mass of the bead wire is reduced, the material of the layer having a density less than the density of the metal monofilament.

[009] A metal monofilament is understood, by definition, to be a monofilament made entirely (for 100% of its mass) from one or more metal materials. The metal monofilament comprises preferably a core made of steel, more preferably made of perlitic or ferritic-perlitic carbon steel, commonly referred to as carbon steel by those skilled in the art, or of stainless steel (by definition, steel comprising at least 10.5% chromium) and a metal layer coating the steel core, the metal layer being made from a pure metal or from an alloy of pure metals chosen from copper, zinc and tin. In other embodiments, the monofilament is made up of the steel core without the latter being coated with a metal layer.

[010] The density of the metal monofilament is the ratio of the mass per unit volume of a sample of the metal monofilament to the mass per unit volume of water. The mass per unit volume of the sample of metal monofilament is determined by calculating the ratio of the mass of this sample to the volume of this sample. Similarly, the density of the material of which the layer is made is the ratio of the mass per unit volume of a sample of the material of which the layer is made to the mass per unit volume of water. The mass per unit volume of the sample of the material of which the layer is made is determined by calculating the ratio of the mass of this sample to the volume of this sample. These measurements are taken under conventional metrological conditions known to those skilled in the art.

[011] Each portion of the coated metal monofilament forms a filamentary element comprising a metal core formed by the portion of the metal monofilament and a layer coating the metal core, the layer being formed by the material with a density less than that of the metal monofilament. The set of the portions of the coated metal monofilament together form the N circumferential windings of the bead wire.

[012] The metal monofilament has a substantially circular section of diameter d.

[013] The expression "inscribed in a circle of diameter D" means that the external surface of the layer of each winding is inscribed in the circle of diameter D and is tangent thereto at at least two points.

[014] The invention makes it possible in particular, when the desire is to increase the overall section of a given bead wire, to significantly limit the increase in mass. This increase in the overall section can be achieved by increasing the number of circumferential windings N and/or by increasing the diameter d.

[015] The invention also makes it possible, when the desire is to maintain the overall section of a given bead wire, to reduce its mass. This reduction in mass can be achieved by reducing the number of circumferential windings N while maintaining the diameter d and therefore by reducing the mass of the metal monofilament in relation to the given bead wire.

[016] Thus, in different embodiments, use could be made of a metal monofilament having 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. Thus, in order to lighten the bead wire while maintaining its section, the mechanical strength could be increased, this making it possible to reduce the diameter of the metal monofilament and therefore the metal monofilament mass.

[017] The term "circumferential winding" means the portion of coated metal monofilament that extends all the way around about the main axis of the bead wire. The coated metal monofilament is preferably continuous, such that the bead wire is preferably formed by continuous winding of the coated metal monofilament through N circumferential windings.

[018] The term "axial direction" means the direction substantially parallel to the main axis of the bead wire or the axis of rotation of the tyre, these two axes being coincident once the bead wire is incorporated in the tyre.

[019] The term "circumferential direction" means the direction that is substantially perpendicular both to the axial direction and to a radius of the bead wire or of the tyre (in other words, tangent to a circle centred on the main axis of the bead wire or the axis of rotation of the tyre).

[020] The term "radial direction" means the direction along a radius of the bead wire or tyre, that is to say any direction that intersects the main axis of the bead wire or the axis of rotation of the tyre and is substantially perpendicular to this axis.

[021] A radial section plane parallel to the main axis of the bead wire or the axis of rotation of the tyre is a plane parallel both to the radial direction and to the axial direction. A radial section plane is also perpendicular to the circumferential direction at the intersection point between this radial section plane and the circumferential direction. A view in radial section of the bead wire or of the tyre is a view of the bead wire or of the tyre in a radial section plane parallel to the main axis of the bead wire or the axis of rotation of the tyre.

[022] In a radial section plane parallel to the main axis of the bead wire, the bead wire, on account of its geometry exhibiting symmetry of revolution about the main axis, has two half-sections disposed on either side of the main axis of the bead wire.

[023] Any range of values denoted by the expression "between a and b" represents the range of values from more than a to less than b (i.e. excluding the limits a and b), whereas any range of values denoted by the expression "from a to b" means the range of values from a up to b (i.e. including the strict limits a and b).

[024] In a preferred embodiment, the material of the layer is a polymer material. A polymer material is understood to be a material comprising at least one polymer. A polymer is a macromolecule resulting from the polymerization of one or more monomers. Among the preferred polymers, mention will be made of thermoplastic polymers, thermosetting polymers and co-crosslinkable polymers. Among the thermoplastic polymers, mention will be made for example of polyesters, aromatic polyamides, aliphatic polyamides, polyurethanes, thermoplastic elastomers, preferably diene thermoplastic elastomers, copolymers of these polymers and mixtures of these polymers. Among the thermosetting polymers, mention will be made of aminoplast resins, phenoplasts, polyurethanes, vinyl ester resins and mixtures of these polymers. Among the co-crosslinkable polymers, mention will be made of the polymers comprising at least one unsaturation or double bond, for example, and very preferably, diene elastomers. The polymer material may obviously comprise further constituents, for example a crosslinking system, fillers, resins, or other additives commonly used in the field of tyres.

[025] In addition to reducing the mass of the bead wire, the layer of polymer material makes it possible to create an interface between radially and/or axially adjacent circumferential windings, providing sufficient adhesion to manipulate the bead wire without the structure of the circumferential windings being damaged.

[026] Advantageously, in order to reduce the mass of the bead wire as much as possible, the density of the material of the layer is less than or equal to 3, preferably less than or equal to 2, and more preferably less than or equal to 1.5.

[027] In a preferred embodiment, the ratio D/d is less than or equal to 2.

[028] In an even more preferred embodiment, the ratio D/d ranges from 1.30 to 1.80.

[029] Too high a ratio D/d results in mechanical decohesion of the adjacent windings on account of the thickness of the layer being too great and mechanical functioning in which each winding works independently of the windings adjacent thereto. However, in order to maximize the mechanical properties of the bead wire, in particular the properties of force at break and fatigue strength, the inventors have found that it was preferable for each winding to work simultaneously with the windings adjacent thereto, and therefore it is preferable to have a moderate ratio D/d, which is therefore preferably less than or equal to 2 and more preferably less than or equal to 1.80.

[030] In one advantageous embodiment, the layer forms a sheath of substantially tubular section with a thickness equal to (D-d)/2. The section of the sheath is observed in the radial section plane parallel to the main axis. Such a sheath of tubular section is relatively easy to deposit on the metal monofilament, for example by means of an extruder and a nozzle, as described in W02017/050780.

[031] Other embodiments in which the layer forms a sheath of polygonal section, for example triangular, rectangular, square, hexagonal or octagonal section, may also be envisaged.

[032] In embodiments of bead wires intended for tyres for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles, the thickness (D-d)/2 is greater than or equal to 0.18 mm, preferably ranges from 0.18 mm to 1.00 mm, and more preferably ranges from 0.25 mm to 0.80 mm.

[033] In a variant that makes it possible to manufacture large bead wires, the thickness (D-d)/2 ranges from 0.40 mm to 0.80 mm and preferably from 0.50 mm to 0.70 mm. In another variant that makes it possible to manufacture smaller bead wires, the thickness (D-d)/2 ranges from 0.25 mm to 0.65 mm, preferably from 0.25 mm to 0.45 mm.

[034] In embodiments of bead wires intended for tyres for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles, d is strictly less than 3.00 mm, preferably ranges from 1.25 mm to 2.90 mm, more preferably from 1.60 to 2.50 mm, and even more preferably from 1.80 mm to 2.40 mm.

[035] In embodiments of bead wires intended for tyres for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles, D is strictly less than 3.35 mm, preferably ranges from 1.75 mm to 3.30 mm, more preferably ranges from 2.30 mm to 3.30 mm, and even more preferably ranges from 2.50 mm to 3.30 mm.

[036] In a variant that makes it possible to manufacture large bead wires, D ranges from 2.80 mm to 3.30 mm. In another variant that makes it possible to manufacture smaller bead wires, D ranges from 2.50 mm to 2.90 mm.

[037] The bead wire has: - a mean diameter Dm, expressed in mm and defined as the distance radially separating, in the radial section plane parallel to the main axis of the bead wire, the centres of the circles circumscribed on each half-section of the bead wire in the radial section plane, - a mass M, expressed in g, - each circumferential winding having, in the radial section plane parallel to the main axis of the bead wire, a section of area S, expressed inmm 2

, and wherein 1000 x M/(N x S x r x Dm) is less than or equal to 2.10, preferably less than or equal to 2.00, more preferably less than or equal to 1.75.

[038] The ratio 1000 x M/(N x S x r x Dm) represents the mean density of the bead wire. The smaller this ratio, the lighter the bead wire will be, independently of its section. The section S of each coated monofilament is determined by the relationship S=r x (D/2) 2 .

[039] Advantageously, 1000 x M/(N x S x r x Dm) is greater than or equal to 1.00, preferably greater than or equal to 1.15.

[040] In a variant that makes it possible to manufacture large bead wires, 1000 x M/(N x S x r x Dm) is less than or equal to 1.50, preferably less than or equal to 1.35. In another variant that makes it possible to manufacture smaller bead wires, 1000 x M/(N x S x r x Dm) is less than or equal to 1.70.

[041] Advantageously, the bead wire has a mass M, expressed in g, and a metal mass Ma equal to the mass of the metal monofilament, expressed in g, such that Ma/M is less than or equal to 0.95, preferably less than or equal to 0.92, and more preferably less than or equal to 0.90. Thus, the mass of metal is significantly reduced in relation to the bead wire of the prior art, that is to say the proportion by mass of metal in the bead wire. The mass of metal may be determined by measuring the total length of the circumferential windings and multiplying said length by the section of the metal monofilament Sa=r x (d/2) 2 and by the density of the metal monofilament. For example, for a monofilament made of carbon steel, the density of the monofilament is substantially equal to 7.8. The mass of metal may also be measured by removing, for example by chemical treatment, the layer of material coating the metal monofilament and by weighing the metal monofilament thus obtained.

[042] Advantageously, Ma/M is greater than or equal to 0.50, preferably greater than or equal to 0.75.

[043] In a variant that makes it possible to manufacture large bead wires, Ma/M is less than or equal to 0.85. In another variant that makes it possible to manufacture smaller bead wires, Ma/M is less than or equal to 0.90.

[044] Advantageously, in the radial section plane parallel to the main axis of the bead wire, with each circumferential winding having a metal monofilament section of area Sa, expressed inmm 2, and each circumferential winding having a section of area S, expressed in mm 2 , the ratio Sa/S is less than or equal to 0.75, preferably less than or equal to 0.65, more preferably less than or equal to 0.60. Thus, for a section of the given metal monofilament, the mass of metal is reduced in relation to the bead wire of the prior prior art. The metal monofilament section Sa of each circumferential winding is determined by the relationship Sa=-rr x (d/2) 2

.

[045] Advantageously, Sa/S is greater than or equal to 0.30, preferably greater than or equal to 0.35.

[046] In a variant that makes it possible to manufacture large bead wires, Sa/S is less than or equal to 0.50, preferably less than or equal to 0.45. In another variant that makes it possible to manufacture smaller bead wires, Sa/S is less than or equal to 0.60.

[047] In embodiments of bead wires intended for tyres for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles, N ranges from 100 to 1000.

[048] In a variant that makes it possible to manufacture large bead wires, N ranges from 400 to 900, preferably ranges from 600 to 850. In another variant that makes it possible to manufacture smaller bead wires, N ranges from 100 to 200, preferably ranges from 100 to 150.

[049] In embodiments of bead wires intended for tyres for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles, P ranges from 10 to 40.

[050] In a variant that makes it possible to manufacture large bead wires, N ranges from 25 to 40, preferably ranges from 25 to 35. In another variant that makes it possible to manufacture smaller bead wires, N ranges from 10 to 25, preferably ranges from 10 to 20.

[051] In an embodiment that promotes the rotation of the products adjacent to the bead wire about the circumferential direction of the bead wire, in particular during the method for manufacturing the green form of the tyre, the bead wire has, in a radial section plane parallel to the main axis of the bead wire, a half-section of substantially hexagonalshape.

[052] The term "substantially hexagonal" means a half-section delimited by six faces, at least 90% of the external circumferential windings of the bead wire delimiting the faces. The term "external circumferential winding" means a circumferential winding that does not comprise a circumferential winding adjacent to at least one of its axial and/or radial sides. These six faces comprise a radially external radial face and a radially internal radial face, the radially external face being substantially parallel to the radially internal face. These six faces also comprise first and second radially external axial faces and first and second radially internal axial faces.

[053] In one embodiment, with the layer C(1) being the radially innermost layer with respect to the main axis of the bead wire and the layer C(P) being the radially outermost layer with respect to the main axis of the bead wire, there is a layer C(k), with k c ]1, P[, arranged radially between the layer C(1) and the layer C(P) such that: - when i ranges from 2 to k-1, L(i-1)<L(i), - L(k)<L(k-1) and L(k)<L(k+1) and L(k-1)=L(k+1), - when i ranges from k+1 to P-1, L(i)>L(i+1), each layer C(i) being arranged radially on the outside of the layer C(i-1).

[054] Thus, the axial forces applied to the axially outermost circumferential windings are distributed over the two circumferential windings of the layers C(k-1) and C(k+1), thereby reducing the pressure compared with if the axially outermost circumferential winding were only that of the layer C(k). The lifetime of the tyre is thus increased.

[055] Advantageously, with each layer C(i) being arranged radially on the outside of the layer C(i-1), IL(i)-L(i-1)|=1 when i ranges from 2 to P. This promotes the stability of the bead wire.

[056] Preferably, the bead wire has, in the radial section plane parallel to the main axis of the bead wire, a half-section having a radial plane of symmetry perpendicular to the main axis of the bead wire. Such a radial plane of symmetry allows easy rotation of the masses of adjacent compositions, in particular in the case of a method for manufacturing a tyre comprising a step during which these masses of adjacent compositions turn about the bead wire. Moreover, such a radial plane of symmetry allows uniform mechanical functioning of the bead wire on either side of this plane. Lastly, the bead wire is de-lateralized, meaning that it does not have two different sides with respect to the radial plane of symmetry, which would require a specific radial orientation during its use.

[057] Also preferably, the bead wire has, in the radial section plane parallel to the main axis of the bead wire, a half-section having a circumferential plane of symmetry parallel to the main axis of the bead wire. Similarly to the radial plane of symmetry, the circumferential plane of symmetry allows easy rotation of the masses of adjacent compositions and also uniform mechanical functioning of the bead wire on either side of this plane.

[058] In an embodiment that makes it possible to promote co-crosslinking between the material of the layer and the adjacent compositions of the tyre, generally based on an elastomer, preferably a diene elastomer, and a crosslinking system, the material of the layer is based on a crosslinkable elastomer composition.

[059] The term "crosslinkable elastomer composition" means that the composition comprises at least one elastomer, preferably a diene elastomer, a crosslinking system and a filler. The compositions used are conventional compositions for the skim coating of metal filamentary reinforcing elements and comprise a diene elastomer, for example natural rubber, a reinforcing filler, for example carbon black and/or silica, a crosslinking system, for example a vulcanizing system, preferably comprising sulfur, or a molecular crosslinking system, preferably comprising a peroxide.

[060] The expression "based on" should be understood as meaning that the material of the layer comprises the compound and/or the product of the in situ reaction of the various constituents used, some of these constituents being able to react and/or being intended to react with one another, at least partially, during the various phases of manufacture of the crosslinkable elastomer composition; the crosslinkable elastomer composition thus being able to be in the partially crosslinked state or in the non-crosslinked state.

[061] In an advantageous embodiment that makes it possible to easily keep the circumferential windings in contact with one another, the bead wire comprises a sheath covering the radially and axially external circumferential windings.

[062] The covering sheath is preferably based on a crosslinkable elastomer composition, i.e. one comprising at least one elastomer, preferably a diene elastomer, a crosslinking system and a filler. This makes it possible to promote co-crosslinking between the covering sheath and the adjacent compositions of the tyre, generally based on an elastomer, preferably a diene elastomer, and a crosslinking system.

[063] Analogously to the polymer material, the expression "based on" should be understood as meaning that the covering sheath comprises the compound and/or the product of the in situ reaction of the various constituents used, some of these constituents being able to react and/or being intended to react with one another, at least partially, during the various phases of manufacture of the crosslinkable elastomer composition; the crosslinkable elastomer composition thus being able to be in the partially crosslinked state or in the non-crosslinked state.

[064] In a preferred embodiment, the covering sheath is in the form of a strip wound in a helix around the radially and axially external circumferential windings of the bead wire.

[065] The term "radially external circumferential winding" means a circumferential winding that does not comprise a circumferential winding adjacent to at least one of its radial sides. The term "axially external circumferential winding" means a circumferential winding that does not comprise a circumferential winding adjacent to at least one of its axial sides.

[066] A further subject of the invention is a tyre obtained by a method comprising: - a step of assembling a green form of the tyre comprising a step of embedding a bead wire as defined above in a crosslinkable elastomer composition, - a step of heating the green form of the tyre so as to crosslink the crosslinkable elastomer composition in which the bead wire is embedded.

[067] As is known to those skilled in the art, the term "green form" means the assembly of various products comprising crosslinkable elastomer compositions in the non-crosslinked state. A green form of the tyre is distinguished from the final tyre, which, for its part, comprises the assembly of the various products comprising the elastomer compositions in the crosslinked state.

[068] At the end of the heating step, the bead wire is embedded in an elastomer matrix. The term "elastomer matrix" means a matrix exhibiting elastomeric behaviour. The elastomer matrix results from the crosslinking of the crosslinkable elastomer composition and is, to this end, based on the crosslinkable elastomer composition.

[069] The tyres of the invention may be intended for passenger motor vehicles (comprising in particular 4x4 vehicles and SUVs (Sport Utility Vehicles)), but also for two-wheel vehicles such as motorcycles, or for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles. Very preferably, the tyres of the invention are intended for heavy-duty vehicles chosen from construction plant vehicles, trucks and agricultural vehicles.

[070] Advantageously, the tyre comprises a crown comprising a tread and a crown reinforcement, two sidewalls, two beads, each sidewall connecting each bead to the crown, the crown reinforcement extending in the crown in a circumferential direction of the tyre, the tyre comprising a carcass reinforcement that is anchored in each of the beads and extends in the sidewalls and in the crown, the crown reinforcement being radially interposed between the carcass reinforcement and the tread, each bead comprising a bead wire as described above.

[071] A further subject of the invention is a method for manufacturing a bead wire for a tyre, the bead wire exhibiting symmetry of revolution about a main axis of the bead wire, comprising: - a step of coating a metal monofilament of diameter d with a layer of a material having a density less than the density of the metal monofilament such that, in a section plane perpendicular to a main axis of the metal monofilament coated with the layer, the metal monofilament coated with the layer is inscribed in a circle of diameter D,

- a step of assembling the bead wire, in which the metal monofilament coated with the layer is wound so as to form N>1 circumferential windings of the metal monofilament coated with the layer about the main axis (A) of the bead wire, each circumferential winding comprising a portion of the metal monofilament and the layer coating the portion of metal monofilament, the N circumferential windings being arranged in P>1 radially superposed layers C(i), i ranging from 1 to P, each layer C(i) comprising L(i)>1 axially juxtaposed circumferential windings, the coating step being carried out such that the ratio D/d is greater than or equal to 1.15 and the bead wire has: - a mean diameter Dm, expressed in mm and defined as the distance radially separating, in the radial section plane parallel to the main axis of the bead wire, the centres of the circles circumscribed on each half-section of the bead wire in the radial section plane, - a mass M, expressed in g, - each circumferential winding having, in the radial section plane parallel to the main axis of the bead wire, a section of area S, expressed in mm2

, and wherein 1000 x M/(N x S x Tr x Dm) is less than or equal to 2.10, preferably less than or equal to 2.00, more preferably less than or equal to 1.75.

[072] The invention will be understood better on reading the following description, which is given purely by way of non-limiting example and with reference to the drawings, in which: - Figure 1 is a view, in a radial section plane parallel to the axis of rotation of the tyre, of a tyre according to the invention comprising a bead wire according to a first embodiment of the invention; - Figure 2 is a view, in a radial section plane parallel to the main axis of the bead wire in Figure 1, of the two half-sections of the bead wire in Figure 1; - Figure 3 is a view, in a radial section plane parallel to the main axis of the bead wire in Figure 1, of a half-section of a bead wire according to the first embodiment before it is incorporated into the tyre in Figure 1; - Figure 4 is a detail view of Figure 3 showing a circumferential winding of the coated metal monofilament; - Figure 5 is a graph illustrating the variation in the mass of a bead wire of the prior art as a function of its section; - Figure 6 is a graph showing the variations in the mass and the section of a bead wire comprising circumferential windings of a coated metal monofilament as a function of the ratio D/d; - Figure 7 is a view similar to the one in Figure 3 of a bead wire according to a second embodiment of the invention before incorporation into a tyre.

[073] Figures 1 to 7 show a reference frame X, Y, Z corresponding to the usual axial (X), radial (Y) and circumferential (Z) directions, respectively, of a tyre exhibiting symmetry of revolution about a main axis of rotation P of the tyre. These directions are analogous for the bead wire according to the invention, which also exhibits the same axial (X), radial (Y) and circumferential (Z) directions of a bead wire exhibiting symmetry of revolution about a main axis A of the bead wire, the axis A being substantially parallel to the axial direction X.

[074] Figure 1 schematically shows a view in radial section of a tyre according to the invention denoted by the general reference 10. The tyre 10 is in this case intended for a heavy-duty vehicle chosen from construction plant vehicles, trucks and agricultural vehicles. In this particular instance, the tyre 10 is intended for a construction plant vehicle.

[075] With reference to Figure 1, the tyre 10 comprises a crown 12 comprising a tread 14 and a crown reinforcement 16, two sidewalls 18, two beads 20, each sidewall 18 connecting each bead 20 to the crown 12. The crown reinforcement 16 extends in the crown 12 in the circumferential direction Z of the tyre 10. The tyre 10 comprises a carcass reinforcement 22 that is anchored in each of the beads 20 and extends in the sidewalls 18 and in the crown 12, the crown reinforcement 16 being radially interposed between the carcass reinforcement 22 and the tread 14. The carcass reinforcement 24 comprises at least one carcass ply 26. Each bead 20 comprises a bead wire 28 about which the carcass reinforcement 24 and in this case the carcass ply 26 is at least partially wound.

[076] With reference to Figures 2, 3 and 4, the bead wire 28 comprises N>1 circumferential windings 30 of a coated metal monofilament 32 about the main axis A of the bead wire 28. N ranges from 100 to 1000 and, in this first embodiment, N ranges from 400 to 900, preferably from 600 to 850, and in this case N=750. The N circumferential windings 30 are arranged in P>1 layers C(i) that are radially superposed on one another. P ranges from 10 to 40 and in this first embodiment, P ranges from 25 to 40, preferably from 25 to 35, and in this case P=31. For each value of i ranging from 1 to 31, each layer C(i) comprises L(i)>1 circumferential windings 30 of the coated metal monofilament 32 that are axially juxtaposed with one another. Each layer C(i) is arranged radially on the outside of the layer C(i-1) and radially on the inside of the layer C(i+1).

[077] The bead wire 28 has, in the radial section plane XY parallel to the main axis A of the bead wire 28 in which the bead wire is shown in Figure 3, a half-section of substantially hexagonal shape. The half-section of the bead wire 28 shown in Figure 3 is delimited by a radially internal radial face F1 and a radially external radial face F2, the face F2 being substantially parallel to the face Fl. The half-section of the bead wire 28 is also delimited by first and second radially external axial faces F3, F4 and first and second radially internal axial faces F5, F6. The bead wire 28 comprises a total of 92 axially and/or radially external circumferential windings, 90 of these axially and/or radially external circumferential windings delimiting the faces F1 to F6 and 2 of these circumferential windings belonging to the layer C(16) and to none of the faces F1 to F6. Thus, 98% of the axially and/or radially external circumferential windings delimit the faces F1 to F6.

[078] As illustrated in Figure 3, in the radial section plane XY parallel to the main axis A of the bead wire 28, the bead wire 28 has a half-section having a radial plane of symmetry R perpendicular to the main axis A of the bead wire 28 and parallel to a circumferential plane YZ. The bead wire 28 also has a half-section having a circumferential plane of symmetry C parallel to the main axis A of the bead wire 28 and perpendicular to the radial direction Y. The bead wire 28 also comprises a sheath 40 covering the radially and axially external circumferential windings delimiting the faces F1 to F6. This sheath 40 is based on a crosslinkable elastomer composition having a relatively high elastic secant modulus in extension at 10% elongation, for example as described in the application W02016/116471.

[079] The layer C(1) is the radially innermost layer with respect to the axis A of the bead wire 28 and the layer C(31) is the radially outermost layer with respect to the axis A of the bead wire 28. Within the bead wire 28, there is a layer C(k), with k c ]1, P[, in this case k=16, arranged radially between the layer C(1) and the layer (C31) such that, for the one part, when i ranges from 2 to 15, L(i-1)<L(i), and for the other part, L(16)<L(15) and L(16)<L(17) and L(15)=L(17), and finally, when i ranges from 17 to 30, L(i)>L(i+1). It will be noted that IL(i)-L(i-1)|=1 when i ranges from 2 to 31. More specifically, C(1)=17, C(2)=18, C(3)=19, C(4)=20, C(5)=21, C(6)=22, C(7)=23, C(8)=24, C(9)=25, C(10)=26, C(11)=27, C(12)=28, C(13)=29, C(14)=30, C(15)=31, C(16)=30, C(17)=31, C(18)=30, C(19)=29, C(20)=28, C(21)=27, C(22)=26, C(23)=25, C(24)=24, C(25)=23, C(26)=22, C(27)=21, C(28)=20, C(29)=19, C(30)=18, C(31)=17.

[080] Each circumferential winding 30 comprises a circumferential portion of a metal monofilament 34 and a layer 36 of a material coating the portion of the metal monofilament 34. The polymer material of the layer 36 has a density lower than the density of the metal monofilament 34. In this case, the material of which the layer is made has a density less than or equal to 3, preferably less than or equal to 2 and more preferably less than or equal to 1.5, and in this case substantially equal to 1.2. The material of which the layer 36 is made is a polymer material based on a crosslinkable elastomer composition, in this case identical to the crosslinkable elastomer composition of the sheath 40.

[081] Within the tyre 10, with the polymer material of the layer 36 and the crosslinkable composition of the sheath 40 having crept during the method for manufacturing the tyre 10 described below, the geometric characteristics of the layer 36 and of the sheath 40 are variable. Specifically, the tyre 10 is obtained by a manufacturing method comprising a step of assembling a green form of the tyre 10 comprising a step of embedding a bead wire in at least one crosslinkable elastomer composition. In this particular instance, the embedding step is a step of pressurizing the green form, causing creep both of the polymer material of the layer 36 between the circumferential windings 30 and of the crosslinkable elastomer composition of the sheath 40 radially and axially towards the radially and axially external circumferential windings 30 delimiting the faces F1 to F6, thereby embedding the bead wire in the crosslinkable elastomer composition of the sheath 40.

[082] Next, the method comprises a step of heating the green form so as to crosslink the crosslinkable elastomer composition in which the bead wire 28 is embedded. This heating step causes additional creep of the polymer material of the layer 36 between the circumferential windings 30 and of the crosslinkable elastomer composition of the sheath 40 radially and axially towards and between the radially and axially external circumferential windings 30 delimiting the faces F1 to F6.

[083] With reference to Figure 4, before the method for manufacturing the tyre described above, that is to say before any creep of the polymer material of the layer 36 and of the crosslinkable elastomer composition of the sheath 40, each circumferential winding 30 comprises a circumferential portion of a metal monofilament 34 and the layer 36 of the polymer material coating the portion of the metal monofilament 34. In the radial section plane XY parallel to the main axis A of the bead wire 28, the portion of metal monofilament 34 of each circumferential winding 30 has a diameter d and each circumferential winding 30 is inscribed in a circle of diameter D. In this particular instance, the layer 36 of polymer material forms a sheath 38 of substantially tubular section with a thickness equal to (D-d)/2.

[084] The diameter d of the portion of metal monofilament 34 of each circumferential winding 30 is strictly less than 3.00 mm, preferably ranges from 1.25 mm to 2,90 mm, more preferably from 1.60 to 2.50 mm and even more preferably from 1.80 mm to 2.40 mm, and in this case is substantially equal to 2.00 mm. The mechanical strength of the metal monofilament is in this case equal to 2070 MPa.

[085] The diameter D of the sheath 38 is strictly less than 3.35 mm, preferably ranges from 1.75 mm to 3.30 mm, more preferably ranges from 2.30 mm to 3.30 mm and even more preferably ranges from 2.50 mm to 3.30 mm. In this first embodiment, D ranges from 2.80 mm to 3.30 mm and is in this case substantially equal to 3.20 mm.

[086] The thickness (D-d)/2 of the sheath 38 having a substantially tubular section is greater than or equal to 0.18 mm, preferably ranges from 0.18 mm to 1.00 mm and more preferably ranges from 0.25 mm to 0.80 mm. In this first embodiment, the thickness (D-d)/2 ranges from 0.40 mm to 0.80 mm and preferably from 0.50 mm to 0.70 mm and in this case is substantially equal to 0.60 mm.

[087] The bead wire 28 has a mass M in this case equal to 123300 g. The mass of metal Ma equal to the mass of the metal monofilament 34 is in this case equal to 99500 g. The masses M and Ma are such that, expressed in g, Ma/M is greater than or equal to 0.50, preferably greater than or equal to 0.75 and less than or equal to 0.95, preferably less than or equal to 0.92 and more preferably less than or equal to 0.90. In this first embodiment, Ma/M is less than or equal to 0.85 and in this case equal to 0.81.

[088] In the radial section plane XY parallel to the main axis A of the bead wire 28, each circumferential winding 30 has a metal monofilament section of area Sa=r x (d/2) 2 and in this case equal to 3.14mm 2 . In this same plane XY, each circumferential winding 30 has a section of area S=r x (D/2) 2 and in this case equal to 8.04mm 2

. The areas Sa and S are such that, expressed inmm 2, the ratio Sa/S is greater than or equal to 0.30, preferably greater than or equal to 0.35 and less than or equal to 0.75, preferably less than or equal to 0.65, more preferably less than or equal to 0.60. In this first embodiment, Sa/S is less than or equal to 0.50, preferably less than or equal to 0.45 and in this case equal to 0.39.

[089] As illustrated in Figure 2, the bead wire 28 has a mean diameter Dm, an inside diameter Dint and an outside diameter Dext.

[090] The mean diameter Dm is defined as the radial distance radially separating, in the radial section plane XY parallel to the main axis A of the bead wire 28, the centres of the circles U1, U2 circumscribed on each half-section of the bead wire 28 in the radial section plane XY. In this case, Dm=5208.6 mm. The ratio 1000 x M/(N x S x rr x Dm) is greater than or equal to 1.00, preferably greater than or equal to 1.15 and less than or equal to 2.10, preferably less than or equal to 2.00, more preferably less than or equal to 1.75. In this first embodiment, 1000 x M/(N x S x r x Dm) is less than or equal to 1.50, preferably less than or equal to 1.35 and in this case equal to 1.25.

[091] The inside diameter Dint is defined as the distance radially separating the radially innermost circumferential windings of the bead wire. In other words, Dint=Dm Ha where Ha is the height in the radial direction of each half-section of the bead wire 28 defined as the distance radially separating the radially innermost circumferential windings from the radially outermost circumferential windings.

[092] The outside diameter Dext is defined as the distance radially separating the radially innermost circumferential windings of the bead wire. In other words, Dext=Dm+Ha.

[093] The width La in the axial direction X of a half-section of the bead wire 28 is in this case equal to 99.2 mm. The height Ha in the radial direction Y of this half-section is in this case equal to 79.6 mm.

[094] According to the invention, the ratio D/d is greater than or equal to 1.15 and less than or equal to 2. Preferably, D/d ranges from 1.30 to 1.80 and in this case is equal to 1.6.

[095] Figure 5 shows the variations, in base 100, of the mass as a function of the overall section of the bead wire of the prior art, in which D/d=1.11. The overall section is defined as the product of the width La in the axial direction of a half-section multiplied by the height Ha in the radial direction of this half-section. It will be noted that the increase in the mass is linear with the increase in the section. Thus, in order to have a bead wire having a section that is twice as large (section equal to 200), the mass of the bead wire will have doubled (mass equal to 200).

[096] By virtue of the invention, and as can be seen in Figure 6, it is now possible to obtain a bead wire having a section that is twice as large by increasing the mass only by about 30% by virtue of a ratio D/d substantially equal to 1.6. More generally, this Figure 6 demonstrates that the increase in the section of the bead wire does not necessarily bring about an increase of the same magnitude in the mass, this being all the more so the higher the ratio D/d is.

[097] A method according to the invention for manufacturing the above-described bead wire will now be described.

[098] The method comprises a step of coating the metal monofilament 34 with the layer 36 such that, in a section plane perpendicular to a main axis of the metal monofilament 34 coated with the layer 36, the metal monofilament 34 coated with the layer 36 is inscribed in a circle of diameter D equal to the diameter of the sheath 38. In this particular instance, the polymer material of the layer 36 is extruded around the metal monofilament 34.

[099] Next, the method comprises a step of assembling the bead wire 28 in which the metal monofilament 34 coated with the layer 36 is wound so as to form the N circumferential windings of the metal monofilament 34 coated with the layer 36 about the main axis A of the bead wire 28. According to the invention, it is ensured that the coating step is carried out such that the ratio D/d is greater than or equal to 1.15.

[0100] Lastly, the method comprises a step of winding in a helix a strip based on the crosslinkable elastomer composition of the sheath 40 around the radially and axially external circumferential windings of the bead wire 28 so as to form the sheath 40.

[0101] A bead wire 28'according to a second embodiment will now be described with reference to Figure 7. Elements similar to those of the first embodiment are denoted by identical references.

[0102] Unlike in the first embodiment, the bead wire 28' according to the second embodiment is such that: - N ranges preferably from 100 to 200 and more preferably from 100 to 150, and in this case N=125, - P ranges preferably from 10 to 25 and more preferably from 10 to 20, and in this case P=13, - C(1)=7, C(2)=8, C(3)=9, C(4)=10, C(5)=11, C(6)=12, C(7)=11, C(8)=12, C(9)=11, C(10)=10, C(11)=9, C(12)=8, C(13)=7, - D ranges preferably from 2.50 mm to 2.90 mm, and in this case D=2.70 mm, - the thickness (D-d)/2 ranges preferably from 0.25 mm to 0.65 mm and very preferably from 0.25 mm to 0.45 mm, and in this case is equal to 0.35 mm, - the ratio Ma/M is preferably less than or equal to 0.90 and in this case equal to 0.89, M=7410 g and Ma=6580 g,

- the ratio Sa/S is preferably less than or equal to 0.60 and in this case equal to 0.55, S=5.72 m 2 and Sa=3.14 mm 2

, - Dm=2076.40 mm, S=5.72 mm2 and the ratio 1000 x M/(N x S x Tr x Dm) is preferably less than or equal to 1.70 and in this case equal to 1.59.

[0103] The characteristics of the bead wires 28 and 28' are collated in Table 1 below, as are the characteristics of the respectively corresponding bead wires EDT1, EDT2 of the prior art. Table 1

EDT1 Bead wire 28 EDT2 Bead wire 28' N 395 750 179 125 P 23 31 15 13 Ha (mm) 65.2 79.6 27.3 32.4 La (mm) 71.9 99.2 35.3 28.9 Dint (mm) 5086 5129 2041 2044 Dm (mm) 5151.2 5208.6 2068.3 2076.4 Dext (mm) 5216.4 5288.2 2095.6 2108.8 d (mm) 3 2 2 2 D (mm) 3.35 3.20 2.25 2.70 (D-d)/2 (mm) 0.175 0.60 0.125 0.35 D/d 1.11 1.60 1.13 1.35 Ma (g) 115400 99500 9400 6580 M (g) 119800 123300 9780 7410 Ma/M 0.96 0.81 0.96 0.89 Sa (mm 2 ) 7.07 3.14 3.14 3.14 2 S (mm ) 8.81 8.04 3.97 5.72 Sa/S 0.80 0.39 0.79 0.55 1000*M/(N*S*Dm) 2.13 1.25 2.12 1.59 FR(daN) 369325 369750 88247 61625

[0104] On comparing the bead wire EDT1 and the bead wire 28, it will be noted that, while increasing the overall section of the bead wire by 68%, the mass remains substantially constant (increase limited to 3%), by virtue of the use of a layer such that D/d is greater than or equal to 1.15, in this case equal to 1.60.

[0105] On comparing the bead wire EDT2 and the bead wire 28', it will be noted that, while maintaining more or less the same overall section (decrease of 3%), the mass is reduced by 24%, again by virtue of the use of a layer such that D/d is greater than or equal to 1.15, in this case equal to 1.35.

Claims

1. Bead wire (28, 28') for a tyre (10), the bead wire exhibiting symmetry of revolution about a main axis (A) of the bead wire and comprising N>1 circumferential windings (30) about the main axis (A) of the bead wire (28, 28'), the N circumferential windings (30) being arranged in P>1 radially superposed layers C(i), i ranging from 1 to P, each layer C(i) comprising L(i)>1 axially juxtaposed circumferential windings (30), each circumferential winding (30) comprising a portion of a metal monofilament (34) of diameter d and a layer (36) of a material having a density less than the density of the metal monofilament (34), the layer (36) coating the portion of metal monofilament (34) such that each circumferential winding (30) is, in a radial section plane (XY) parallel to the main axis (A) of the bead wire, inscribed in a circle of diameter D having a ratio D/d greater than or equal to 1.15, characterized in that the ratio D/d is greater than or equal to 1.15 and the bead wire has: - a mean diameter Dm, expressed in mm and defined as the distance radially separating, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, the centres of the circles (U1, U2) circumscribed on each half-section of the bead wire in the radial section plane (XY), - a mass M, expressed in g, - each circumferential winding (30) having, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, a section of area S, expressed inmm 2

, and wherein 1000 x M/(N x S x r x Dm) is less than or equal to 2.10.

2. Bead wire (28, 28') according to the preceding claim, wherein 1000 x M/(N x S x r x Dm) is less than or equal to 2.00 and preferably less than or equal to 1.75.

3. Bead wire (28, 28') according to either one of the preceding claims, wherein the density of the material of the layer (36) is less than or equal to 3, preferably less than or equal to 2, and more preferably less than or equal to 1.5.

4. Bead wire (28, 28') according to any one of the preceding claims, wherein the ratio D/d is less than or equal to 2, preferably ranging from 1.30 to 1.80.

5. Bead wire (28, 28') according to any one of the preceding claims, wherein the layer (36) forms a sheath (38) of substantially tubular section with a thickness equal to (D d)/2.

6. Bead wire (28, 28') according to any one of the preceding claims, which has a mass M, expressed in g, and a metal mass Ma equal to the mass of the metal monofilament (34), expressed in g, such that Ma/M is less than or equal to 0.95, preferably less than or equal to 0.92, and more preferably less than or equal to 0.90.

7. Bead wire (28, 28') according to any one of the preceding claims, wherein, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, with each circumferential winding (30) having a metal monofilament (34) section of area Sa, expressed in mm 2 , and each circumferential winding (30) having a section of area S, expressed in mm 2 ,the ratio Sa/S is less than or equal to 0.75, preferably less than or equal to 0.65, more preferably less than or equal to 0.60.

8. Bead wire (28, 28') according to any one of the preceding claims, wherein N ranges from 100 to 1000.

9. Bead wire (28, 28') according to any one of the preceding claims, wherein P ranges from 10 to 40.

10. Bead wire (28, 28') according to any one of the preceding claims, which has, in a radial section plane (XY) parallel to the main axis (A) of the bead wire, a half-section of substantially hexagonal shape.

11. Bead wire (28, 28') according to any one of the preceding claims, wherein, with the layer C(1) being the radially innermost layer with respect to the main axis (A) of the bead wire and the layer C(P) being the radially outermost layer with respect to the main axis (A) of the bead wire, there is a layer C(k), with k c ]1, P[, arranged radially between the layer C(1) and the layer C(P) such that: - when i ranges from 2 to k-1, L(i-1)<L(i), - L(k)<L(k-1) and L(k)<L(k+1) and L(k-1)=L(k+1), - when i ranges from k+1 to P-1, L(i)>L(i+1), each layer C(i) being arranged radially on the outside of the layer C(i-1).

12. Bead wire (28, 28') according to any one of the preceding claims, wherein, with each layer C(i) being arranged radially on the outside of the layer C(i-1), IL(i)-L(i-1)|=1 when i ranges from 2 to P.

13. Bead wire (28, 28') according to any one of the preceding claims, wherein the material of the layer (36) is based on a crosslinkable elastomer composition.

14. Tyre (10), characterized in that it is obtained by way of a method comprising: - a step of assembling a green form of the tyre comprising a step of embedding a bead wire (28, 28') according to any one of the preceding claims in a crosslinkable elastomer composition, - a step of heating the green form of the tyre so as to crosslink the crosslinkable elastomer composition in which the bead wire (28, 28') is embedded.

15. Method for manufacturing a bead wire (28, 28') for a tyre, the bead wire (28, 28') exhibiting symmetry of revolution about a main axis (A) of the bead wire, comprising: - a step of coating a metal monofilament (34) of diameter d with a layer (36) of a material having a density less than the density of the metal monofilament (34) such that, in a section plane (XY) perpendicular to a main axis of the metal monofilament (34) coated with the layer (36), the metal monofilament (34) coated with the layer (36) is inscribed in a circle of diameter D, - a step of assembling the bead wire, in which the metal monofilament (34) coated with the layer (36) is wound so as to form N>1 circumferential windings (30) of the metal monofilament (34) coated with the layer (36) about the main axis (A) of the bead wire, each circumferential winding (30) comprising a portion of the metal monofilament (34) and the layer (36) coating the portion of metal monofilament (34), the N circumferential windings (30) being arranged in P>1 radially superposed layers C(i), i ranging from 1 to P, each layer C(i) comprising L(i)>1 axially juxtaposed circumferential windings (30), characterized in that the coating step is carried out such that the ratio D/d is greater than or equal to 1.15 and such that the bead wire has: - a mean diameter Dm, expressed in mm and defined as the distance radially separating, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, the centres of the circles (U1, U2) circumscribed on each half-section of the bead wire in the radial section plane (XY), - a mass M, expressed in g,

- each circumferential winding (30) having, in the radial section plane (XY) parallel to the main axis (A) of the bead wire, a section of area S, expressed in mm2

, and wherein 1000 x M/(N x S x Tr x Dm) is less than or equal to 2.10.