AU2020382109A1 - Two-layer metal cables having a sheathed inner layer and an improved performance - Google Patents

Abstract

The invention relates to a two-layer cable (50) comprising: - an inner layer (C1) consisting of M>1 inner metal wires (F1); - a cable outer layer (C3) consisting of N outer metal wires (F3) wound around the inner layer (C1) of the cable. The cable (50) is obtained by a method involving a step of manufacturing the sheathed inner layer (CIG) in which the inner layer (CI) is surrounded by an elastomer composition having a thickness G and then by the N outer metal wires to form the outer layer (C3), with N being strictly greater than Nmax, which is the maximum number of outer metal wires (F3) that can be arranged on the theoretical outer layer (C3T) obtained when the inner layer (C1) is in direct contact with the theoretical outer layer (C3T).

Description

TWO-LAYER METAL CORDS HAVING A SHEATHED INNER LAYER AND AN IMPROVED PERFORMANCE

[001] The invention relates to the metal cords that can be used notably for reinforcing tyres, particularly tyres intended to equip vehicles carrying heavy loads and running at sustained speed, such as, for example, lorries, tractors, trailers or buses, aircraft, etc.

[002] Atyre of the heavy-duty type having a radial carcass reinforcement comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread and a belt, or crown reinforcement, arranged circumferentially between the carcass reinforcement and the tread. This crown reinforcement comprises several reinforcements having different functions.

[003] The crown reinforcement generally comprises a working reinforcement comprising two working plies, or crossed plies, comprising filamentary metal working reinforcing elements arranged substantially parallel to one another within each working ply but crossed from one ply to the other, that is to say inclined, symmetrically or asymmetrically, with respect to the median circumferential plane, by an angle generally ranging between 15 and 40°. This working reinforcement makes it possible, amongst other functions, for the transverse loads applied by the ground to be transmitted at least partially to the tyre when the latter is running, so as to provide the tyre with steering capability, namely to give the tyre the ability to allow the vehicle to which it is fitted to corner.

[004] Some current tyres, referred to as "road" tyres, are intended to run at high speed and over increasingly long journeys, as a result of the improvement in the road network and of the growth of the motorway network throughout the world. The combined conditions under which such a tyre is called upon to run undoubtedly enable an increase in the number of kilometres travelled since the wear on the tyre is reduced, but, on the other hand, the endurance of this tyre and in particular of the crown reinforcement is detrimentally affected as a result.

[005] The crown reinforcement of a tyre has to comply, in a known manner, with different, often contradictory, requirements, notably:

- needing to be as rigid as possible for small deformation, because it plays a substantial part in rigidifying the crown of the tyre;

- having the lowest possible hysteresis in order to minimize the heating of the internal region of the crown during running and also to reduce the rolling resistance of the tyre, this going hand in hand with saving fuel;

- and finally needing to have high endurance, particularly with regard to the phenomenon of separation, cracking of the ends of the crossed plies in the shoulder region of the tyre, known by the name of "cleavage", which notably means that the metallic cords that reinforce the working plies need to have high compression fatigue strength, all of this in a fairly corrosive environment.

[006] The third requirement is particularly important to tyre casings for industrial vehicles such as heavy-duty vehicles which are designed to be able to be retreaded one or more times when the treads that they comprise reach a critical level of wear after prolonged running.

[007] In order to reinforce the above-mentioned working plies, use is generally made of so called layered steel cords made up of a central core and of one or more concentric layers of threads disposed around this core. The most commonly used layered cords are essentially cords with an "M+N" or "M+N+P" construction formed from a core of M thread(s) surrounded by at least one layer of N threads, itself possibly surrounded by an external layer of P threads, the M, N or even P threads generally having the same diameter for reasons of simplification and of cost.

[008] For all of the reasons set out hereinabove, the two-layer cords most extensively used at the present time in tyre belts are essentially cords of the 3+N construction made up of a core or internal layer of 3 internal metallic threads and of an external layer of N external metallic threads wound in a helix around the internal layer of the cord (for example, 8 or 9 external metallic threads). The external layer is relatively desaturated because of the high diameter of the internal layer caused by the presence of the three core threads, especially when the diameter of the core threads is chosen to be higher than that of the threads of the external layer.

[009] This type of construction, as is known, encourages the external penetrability of the cord by the skim rubber of the tyre or other rubber item when these are cured, and consequently makes it possible to improve the endurance of the cords in terms of fatigue and fatigue corrosion, particularly in the face of the above-mentioned problem of cleavage.

[010] Cords of (M>1) + N construction do, however, have the disadvantage of not being able to be penetrated right to the core because of the presence of a channel or capillary at the centre of the N core threads, which remains empty after impregnation with the rubber and therefore, through a kind of "wicking" effect, are susceptible to the propagation of corrosive media such as water. This disadvantage of cords of M+N construction is well known and has been set out for example in patent applications WO 01/00922, WO 01/49926, WO 2005/071157 and WO 2006/013077.

[011] In order to solve the above problem, there has been a proposal to open up the internal layer, by separating its threads, using an individual nucleus thread and to eliminate one thread from the external layer; the cord thus obtained, of 1+3+(P-1) construction, becomes penetrable from the outside all the way to its centre. In comparison with the threads of the internal layer, the nucleus thread needs to be neither too slender, because otherwise it will not produce the intended desaturation effect, nor too fat, because otherwise the thread will not remain at the centre of the cord.

[012] This solution is first of all costly because it requires the addition of a thread which furthermore makes no contribution to the strength of the cord; it also runs into a problem of manufacture: a high level of tension needs to be applied to the nucleus thread in order to keep the thread at the centre of the cord during the twisting, which tension may in certain instances come close to the breaking strength of the thread. Finally, the removal of an external thread has the effect of reducing the strength of the cord per unit of cross-sectional area further still.

[013] All of the above constraints are somewhat detrimental from the industrial viewpoint and conflict with the search for high manufacturing rates.

[014] The object of the invention is a cord of improved performance that solves the above mentioned problems.

[015] CORD ACCORDING TO THE INVENTION

[016] To this end, one subject of the invention is a two-layer cord comprising:

- an internal layer made up of M>1 internal metallic threads,

- an external layer of the cord made up of N external metallic threads wound around the internal layer of the cord, wherein:

the cord is obtained by a method comprising a step of manufacturing the sheathed internal layer in which step the internal layer is surrounded with an elastomer composition having a thickness G and then by the N external metallic threads to form the external layer, where N is strictly greater than Nmax which is the maximum number of external metallic threads that can be laid on the theoretical external layer obtained when the internal layer is directly in contact with the theoretical external layer.

[017] Any range of values denoted by the expression "between a and b" represents the range of values extending from more than a to less than b (namely excluding the end-points a and b), whereas any range of values denoted by the expression "from a to b" means the range of values extending from the end-point "a" as far as the end-point "b", namely including the strict end-points "a" and "b".

[018] In the invention, the cord has two layers of threads, which means to say that it comprises an assembly made up of two layers of threads, neither more nor less, which means to say that the assembly has two layers of threads, not one, not three, but only two.

[019] The internal layer of the cord is surrounded by an elastomer compound having a thickness G and is then surrounded by an external layer.

[020] What is meant by directly in contact with the theoretical external layer is that there is no sheath present between the internal layer and the theoretical external layer. The external layer is thus laid as close as possible to the centre in which the internal layer is circumscribed.

[021] What is meant by an elastomer compound or an elastomeric compound is that the compound contains at least one elastomer or one rubber (the two terms being synonyms) and at least one other component.

[022] The maximum number of external threads of a diameter d3 that can be laid on the theoretical external layer with helix radius Rt and helix angle at obtained when the internal layer is directly in contact with the theoretical external layer, and which is denoted Nmax, is defined by the following formula:

Nmax=INT(/ arctan[(d3/2) 2 /((Rt 2-(d3/2) 2 ) x cos 2 at))]12)

where, by definition, INT is the integer value of the formula between parentheses, the helix radius Rt of the theoretical external layer of the cord is the radius of the theoretical circle passing through the centres of the external threads of the theoretical external layer in a plane perpendicular to the axis of the cord.

[023] The helix angle at is a parameter well known to those skilled in the art and can be determined using the following calculation: at=Arctan [2u x Rt/P] in which formula P is the pitch expressed in millimetres at which each metallic filamentary element is wound, Rt is the helix radius of the theoretical external layer of the cord, expressed in millimetres, and Arctan refers to the Arctangent function.

[024] Unlike in the prior art in which the cords have an N=Nmax, the cord according to the invention has N>Nmax threads thus making it possible to increase the force at break of the cord by adding at least one additional thread. The inventors behind the invention are putting forward the hypothesis that the presence of the sheath makes it possible on the one hand to create enough of a vaulted arch around the internal layer that an additional thread can be added and, on the other hand, to relieve the contact pressures through a cushioning effect between the internal layer and the external layer, thereby improving the performance of each of the threads of the cord.

[025] It will be recalled that, as is known, the pitch of a thread represents the length of this thread, measured parallel to the axis of the cord in which it is located, after which the thread that has this pitch has made a complete turn around the said axis of the thread.

[026] What is meant by the direction of winding of a layer of threads is the direction that the threads form with respect to the axis of the cord. The direction of winding is commonly designated by either the letter Z or the letter S.

[027] The pitches, directions of winding, and diameters of the threads are determined in accordance with standard ASTM D2969-04 of 2014.

[028] Advantageously, the cord is made of metal. The term "metal cord" is understood by definition to mean a cord formed of threads made up predominantly (i.e. more than 50% of these threads) or entirely (100% of the threads) of a metallic material. Such a metal cord is preferably implemented with a steel cord, more preferably a cord made of pearlitic (or ferritic pearlitic) carbon steel referred to as "carbon steel" below, or else made of stainless steel (by definition steel comprising at least 11% chromium and at least 50% iron). However, it is of course possible to use other steels or other alloys.

[029] When a carbon steel is advantageously used, its carbon content (% by weight of steel) is preferably between 0.05% and 1.2%, in particular between 0.4% and 1.1%; these contents represent a good compromise between the mechanical properties required for the tyre and the feasibility of the threads.

[030] The metal or the steel used, whether it is in particular a carbon steel or a stainless steel, may itself be coated with a metallic layer which improves for example the workability properties of implementing the metallic cord and/or of its constituent elements, or the use properties of the cord and/or of the tyre themselves, such as the properties of adhesion, corrosion resistance or else resistance to ageing. According to a preferred embodiment, the steel used is covered with a layer of brass (Zn-Cu alloy) or of zinc.

[031] For preference, the threads of the one same (internal or external) layer all have substantially the same diameter. Advantageously, the external threads all have substantially the same diameter. What is meant by "substantially the same diameter" is that the threads have the same diameter to within the industrial tolerances.

[032] Advantageously, the external threads are wound in a helix around the internal thread with a pitch ranging from 10 mm to 30 mm.

[033] For preference, the threads do not undergo pre-shaping.

[034] Advantageously, N= Nmax+1 or Nmax+2 and preferably N=Nmax+1. In order to limit the external diameter of the cord, the person skilled in the art will know how to adapt the thickness G of elastomeric composition required for a good compromise between contact pressure and improved force at break. The external layer comprises a relatively high number of external threads and therefore exhibits a relatively high force at break.

[035] Advantageously, the ratio of the diameter dl of the metallic internal thread or of each metallic internal thread to the diameter d3 of each external metallic thread ranges from 0.9 to 1.2.

[036] Advantageously, the diameter dl of the metallic internal thread or of each internal metallic thread is equal to the diameter d3 of each external metallic thread. Thus, the same diameter is preferably used in the internal metallic thread(s) and for the external metallic threads, thereby limiting the number of different diameters that need to be managed during the manufacture of the cord.

[037] Advantageously, the external layer of the cord is saturated so that the inter-thread distance of the external metallic threads is strictly less than 20 pm.

[038] By definition, a saturated layer of cord is such that the inter-thread distance for the external metallic threads is strictly less than 20 pm. The inter-thread distance of the external layer of external threads is defined, in a section of the cord perpendicular to the main axis of the cord, as being the shortest distance, which, on average, separates two adjacent external metallic threads. Thus, this construction of the cord makes it possible to ensure good architectural stability of the external layer and the saturation of the external layer makes it possible to ensure that the external layer comprises a relatively high number of external metallic threads and therefore exhibits a relatively high force at break.

[039] As a preference, the inter-thread distance of the external metallic threads is less than or equal to 100 pm. By contrast, a desaturated layer of cord is such that the inter-thread distance of the external metallic threads is greater than or equal to 20 pm.

[040] Advantageously, the thickness G of the sheath of elastomer composition is strictly greater than 10 pm, preferably greater than or equal to 12 pm, and more preferably greater than or equal to 15 pm. The greater the thickness G of the elastomer composition, the more metallic threads can be added to the external layer and the more the capillaries between the threads can be filled.

[041] Advantageously, the thickness G of the sheath of elastomer composition is less than or equal to 300 pm, preferably less than or equal to 250 pm, and more preferably less than or equal to 230 pm. This thickness makes it possible to optimize the relatively high number of external metallic threads and therefore to have a force at break that is relatively high while at the same time limiting the external diameter of the cord.

[042] Advantageously, the elastomer compound comprises an elastomer selected from the group consisting of polybutadienes, natural rubber, synthetic polyisoprenes, butadiene copolymers, isoprene copolymers and the mixtures of these elastomers.

[043] Preferably, the elastomer compound comprises an elastomer selected from the group consisting of natural rubber, synthetic polyisoprenes, isoprene copolymers, and mixtures of these elastomers.

[044] For preference, the elastomer compound also contains a vulcanization system and a filler. More preferentially, the elastomer is a diene elastomer.

[045] As a preference, the elastomer compound contains carbon black as reinforcing filler.

[046] Advantageously, M=2, 3 or 4, and preferably M=3 or 4. The most severe transverse loadings applied to the cord when the latter is tensioned are the transverse loadings applied between the internal metallic threads.

[047] Cords exhibiting an architecture in which M>1 and comprising a number of external metallic threads such that the external layer of the cord is saturated so as to maximize the breaking strength by adding a maximum number of external metallic threads are known from the prior art. In this instance, the cord according to the invention having an architecture in which M>1, thanks to the creation of a cushion of elastomer composition that at least partially absorbs the transverse loadings applied between the internal metallic threads, the cord exhibits a markedly improved force at break.

[048] Advantageously, N= 9,10, 11 or 12, and preferably N=10 or 11.

[049] Advantageously, each metallic thread has a respective diameter dl, d3 ranging from 0.22 mm to 0.60 mm, preferably from 0.22 mm to 0.50 mm.

[050] In a first variant, M=2 and N=9 or 10.

[051] In a second variant, M=3, and N=10.

[052] In a third variant, M=4, and N=11.

[053] In these three variants of cords according to the invention, the transverse loadings applied between the internal metallic threads are absorbed by the sheath and the cord exhibits an improved force at break owing to the presence of an additional external metallic thread, while at the same time having a limited external diameter.

[054] REINFORCED PRODUCT ACCORDING TO THE INVENTION

[055] Another subject of the invention is a reinforced product comprising an elastomer matrix and at least one cord as defined above.

[056] Advantageously, the reinforced product comprises one or several cords according to the invention embedded in the elastomer matrix and, in the case of several cords, the cords are arranged side-by-side in a main direction.

[057] TYRE ACCORDING TO THE INVENTION

[058] Another subject of the invention is a tyre comprising at least one cord or a reinforced product as defined above.

[059] For preference, the tyre has a carcass reinforcement anchored in two beads and surmounted radially by a crown reinforcement which is itself surmounted by a tread, the crown reinforcement being joined to the said beads by two sidewalls, and comprising at least one cord as defined above.

[060] In one preferred embodiment, the crown reinforcement comprises a protective reinforcement and a working reinforcement, the working reinforcement comprising at least one cord as defined hereinabove, the protective reinforcement being interposed radially between the tread and the working reinforcement.

[061] The cord is most particularly intended for industrial vehicles selected from heavy vehicles such as "heavy-duty vehicles" - i.e. underground trains, buses, road haulage vehicles (lorries, tractors, trailers), off-road vehicles, agricultural vehicles or construction plant vehicles, or other transport or handling vehicles.

[062] As a preference, the tyre is for a vehicle of the heavy-duty type.

[063] A better understanding of the invention will be obtained on reading the examples which will follow, given solely by way of non-limiting examples and made with reference to the drawings, in which:

- Figure 1 is a view in cross section perpendicular to the circumferential direction of a tyre according to the invention;

- Figure 2 is a view in cross section of a reinforced product according to the invention;

- Figure 3 is a schematic view in cross section perpendicular to the axis of the cord (which is assumed to be straight and at rest) of a cord (50) according to a first embodiment of the invention;

- Figure 4 is a view similar to that of Figure 3 of a cord (50') according to a second embodiment of the invention;

- Figure 5 is a view similar to that of Figure 3 of a cord (50") according to a third embodiment of the invention.

[064] EXAMPLE OF A TYRE ACCORDING TO THE INVENTION

[065] A frame of reference X, Y, Z corresponding to the usual respectively axial (X), radial (Y) and circumferential (Z) orientations of a tyre has been depicted in Figure 1.

[066] The "median circumferential plane" M of the tyre is the plane that is normal to the axis of rotation of the tyre and that is located equidistantly from the annular reinforcement structures of each bead.

[067] Figure 1 depicts a tyre according to the invention and denoted by the general reference 10.

[068] The tyre 10 is for a vehicle of the heavy-duty type. Thus, the tyre 10 has a dimension of the type 315/80 R22.5.

[069] This tyre 10 comprises a crown 2 reinforced by a crown reinforcement or belt 6, two sidewalls 3 and two beads 4, each of these beads 4 being reinforced with a bead wire 5. The crown 2 is surmounted by a tread, not represented in this diagrammatic figure. A carcass reinforcement 7 is wound around the two bead wires 5 in each bead 4, the turn-up 8 of this reinforcement 7 being, for example, positioned towards the outside of the tyre 10, which is represented here fitted onto its wheel rim 9. The carcass reinforcement 7 is, in a way known per se, formed of at least one ply reinforced by "radial" cords, that is to say that these cords are positioned virtually parallel to one another and extend from one bead to the other so as to form an angle of between 80 and 90 with the median circumferential plane (plane perpendicular to the axis of rotation of the tyre which is located midway between the two beads 4 and passes through the middle of the crown reinforcement 6).

[070] The tyre according to the invention is characterized in that its belt 6 comprises at least, by way of reinforcements of at least one of the belt plies, a two-layer metal cord according to the invention. In this belt 6, it will be understood that the cords of the invention may for example reinforce all or part of the belt plies referred to as working plies. Of course, this tyre 10 additionally comprises, in a known manner, an inner rubber or elastomer layer (commonly referred to as "inner liner") which definesthe radially inner face of thetyre and which is intended to protect the carcass reinforcement from the diffusion of air originating from the space inside the tyre.

[071] EXAMPLE OF A REINFORCED PRODUCT ACCORDING TO THE INVENTION

[072] Figure 2 depicts a reinforced product according to the invention and denoted by the general reference 100. The reinforced product 100 comprises at least one cord 50, in this instance several cords 50, embedded in the elastomer matrix 102.

[073] Figure 2 depicts the elastomer matrix 102, the cords 50 in a frame of reference X, Y, Z, in which the direction Y is the radial direction and the directions X and Z are the axial and circumferential directions. In Figure 3, the reinforced product 100 comprises several cords 50 arranged side-by-side in the main direction X and extending parallel to one another within the reinforced product 100 and collectively embedded in the elastomer matrix 102.

[074] CORD ACCORDING TO A FIRST EMBODIMENT OF THE INVENTION

[075] Figure 3 depicts the cord 50 according to a first embodiment of the invention.

[076] The cord 50 is metal and has two layers. Thus, it will be understood that there are two layers, not more, not less, of metallic threads of which the cord 50 is made.

[077] The cord 50 comprises an internal layer C1 of the cord, made up of M>1 internal metallic threads Fl. In this instance, M=2, 3 or 4, and preferably M=3 or 4 and here M=2. The internal layer C1 is surrounded by an elastomer compound of a thickness G thus forming the sheathed internal layer CIG. The external layer C3 is made up of N>Nmax external metallic threads F3 wound around the sheathed internal layer CIG of the cord.

Nmax=INT(/ arctan[(d3/2) 2 /((Rt 2-(d3/2) 2 ) x cos 2 at))]112)=INT(/ arctan[(0.30/2)2/((0.472_ (0.30/2)2) x cos2 (3.14x10.9/180)))]1 2 )= INT(9.58)=9. In this instance, N=9, 10, 11 or 12, and preferably N=10 or 11 and N=Nmax +1=9+1=10.

[078] Each internal metallic thread F1 and each external metallic thread F3 respectively has a diameter dl and d3. The diameter dl of each internal metallic thread F1 is preferably equal to the diameter d3 of each external metallic thread F3. Here d1=d3=0.30 mm.

[079] The external layer C3 of the cord is saturated. The inter-thread distance of the external threads is strictly less than 20 pm and is here equal to 0 pm.

[080] Each thread has a breaking strength, denoted Rm, such that 2500 5 Rm 5 3100 MPa. The steel for these threads is said to be of SHT ("Super High Tensile") grade. Other threads may be used, for example threads of an inferior grade, for example of NT ("Normal Tensile") or HT ("High Tensile") grade, just as threads of a superior grade, for example of UT ("Ultra Tensile") or MT ("Mega Tensile") grade.

[081] METHOD FOR MANUFACTURING THE CORD ACCORDING TO THE INVENTION

[082] The cord according to the invention is manufactured using a method comprising steps well known to those skilled in the art.

[083] The aforementioned cord is manufactured according to known methods involving the following steps, preferably performed in line and continuously:

- first of all, a first step of assembling, by twisting, the M>1 internal threads F1 of the internal layer C1 at the pitch p1 and in the S-direction to form the internal layer C1 at a first assembling point;

- followed by a second step in the manufacture of the sheathed internal layer CIG, where the internal layer C1 is surrounded with an elastomer composition having a thickness G and then the N external metallic threads F3 are assembled by twisting around the internal layer CIG at the pitch p3 and in the S-direction to form the assembly of the layers CIG and C3, with N strictly greater than Nmax which is the maximum number of external metallic threads F3 that can be laid on the theoretical external layer C3T obtained when the internal layer C1 is directly in contact with the theoretical external layer C3T;

- preferably a final twist-balancing step.

[084] What is meant here by "twist balancing" is, as is well known to those skilled in the art, the cancellation of the residual torque pairs (or the elastic return of the twist) applied to each thread, in the intermediate layer as in the external layer.

[085] After this final twist-balancing step, the manufacture of the cord is complete.

[086] In this instance N>Nmax=9, here N=10.

[087] The thickness G of the sheath of elastomeric composition is strictly greater than 10 pm, preferably greater than or equal to 12 pm and more preferably greater than or equal to 15 pm, and the thickness G is less than or equal to 300 pm, preferably less than or equal to 250 pm, and more preferably less than or equal to 230 pm. Here, G=71 pm.

[088] The elastomer composition contains a vulcanization system, a filler and a diene elastomer.

[089] The elastomer composition used is a diene elastomer composition conventionally used in tyres, based on natural (peptized) rubber and carbon black N330 (65 phr), also containing the following usual additives: sulfur (7 phr), sulfenamide accelerator (1 phr), ZnO (8 phr), stearic acid (0.7 phr), antioxidant (1.5 phr), cobalt naphthenate (1.5 phr) (phr meaning parts by weight per hundred parts of elastomer); the E10 modulus of the coating elastomer composition is around 10 MPa.

[090] Possibly, in a last assembly step, the wrapper F is wound, at the pitch pf and in the Z direction, around the assembly previously obtained.

[091] The cord is then incorporated by skimming into composite fabrics formed from a known composition based on natural rubber and carbon black as reinforcing filler, conventionally used for manufacturing crown reinforcements of radial tyres. This compound essentially contains, in addition to the elastomer and the reinforcing filler (carbon black), an antioxidant, stearic acid, an extender oil, cobalt naphthenate as adhesion promoter, and finally a vulcanization system (sulfur, accelerator and ZnO).

[092] The composite fabrics reinforced by these cords have an elastomer compound matrix formed from two thin layers of elastomer compound which are superposed on either side of the cords and which have a thickness ranging between 0.6 and 1.5 mm, respectively. The skim pitch (spacing at which the cords are laid in the elastomer compound fabric) ranges from 1 mm to 4 mm.

[093] These composite fabrics are then used as working ply in the crown reinforcement during the method of manufacturing the tyre, the steps of which are otherwise known to a person skilled in the art.

[094] CORD ACCORDING TO A SECOND EMBODIMENT OF THE INVENTION

[095] Figure 4 depicts a cord 50' according to a second embodiment of the invention. Elements similar to those of the first embodiment are denoted by identical references.

[096] Unlike in the first embodiment described hereinabove, the cord 50' according to the second embodiment is such that M=3 and L=10.

[097] CORD ACCORDING TO A THIRD EMBODIMENT OF THE INVENTION

[098] Figure 5 depicts a cord 50" according to a third embodiment of the invention. Elements similar to those of the first embodiment are denoted by identical references.

[099] Unlike in the first embodiment of the cord 50 described hereinabove, the cord 50" according to the third embodiment is such that M=4 and L=11.

[0100] Table 1 below summarizes the characteristics of the various cords 50, 50' and 50".

[0101] Table 1

Cords 50 50' 50"

M/N 2/10 3/10 4/11

d1/d3 0.30/0.30 0.30/0.30 0.30/0.30

direction for Cl/pitch p1 S/7.7 S/7.7 S/7.7 (mm)

direction for C3/pitch p3 S/15.4 S/15.4 S/15.4 (mm)

13 (pm) 0 0 0

Sheath 71 48 57 G (pm)

at (degrees) 10.4 10.9 11.8

Rt (mm) 0.45 0.48 0.51

Nmax 9 9 10

D (mm) 1.3 1.3 1.4

[0102] COMPARATIVE TESTS

[0103] Variation in NMAX as a function of thread diameter

[0104] Tables 2, 3 and 4 below summarize the characteristics for variations in diameter of the various cords 50, 50'and 50", and Nmax has been calculated.

[0105] Table 2

Cords 50-1 50-2 50-3 50-4

M/N 2/10 2/10 2/10 2/10

d1/d3 0.15/0.15 0.23/0.23 0.38/0.38 0.60/0.60 direction for S/7.7 S/7.7 S/7.7 S/7.7 Cl/pitch p1 (mm) direction for S/15.4 S/15.4 S/15.4 S/15.4 C3/pitch p3 (mm)

13 (pm) 0 0 0 0

Sheath 32 50 97 223 G (pm)

at (degrees) 5.7 8.6 14.5 20.8

Rt (mm) 0.24 0.37 0.63 0.93

Nmax 9 9 9 8

D (mm) 0.63 0.96 1.64 2.70

[0106] Table 3

Cords 50'-1 50'-2 50'-3 50'-4

M/N 3/10 3/10 3/10 3/10

d1/d3 0.15/0.15 0.23/0.23 0.38/0.38 0.45/0.45

direction for S/7.7 S/7.7 S/7.7 S/7.7 Cl/pitch p1 (mm)

direction for S/15.4 S/15.4 S/15.4 S/15.4 C3/pitch p3 (mm)

13 (pm) 0 0 0 0

Sheath 20 32 66 89 G (pm)

at (degrees) 5.7 8.6 14.5 17.2

Rt (mm) 0.24 0.37 0.63 0.76

Nmax 9 9 9 9

D (mm) 0.63 0.96 1.65 2.00

[0107] Table 4

Cords 50"-1 50"-2 50"-3 50"-4

M/N 4/11 4/11 4/11 4/11

d1/d3 0.15/0.15 0.23/0.23 0.38/0.38 0.60/0.60

direction for S/7.7 S/7.7 S/7.7 S/7.7 Cl/pitch p1 (mm)

direction for S/15.4 S/15.4 S/15.4 S/15.4 C3/pitch p3 (mm)

13 (pm) 0 0 0 0

Sheath 25 40 81 190 G (pm)

at (degrees) 6.2 9.4 15.9 25.5

Rt (mm) 0.27 0.40 0.70 1.17

Nmax 10 10 10 9

D (mm) 0.69 1.04 1.78 2.94

[0108] BREAKING STRENGTH TESTS

[0109] This test makes it possible to determine the breaking strength of the cords tested by measuring the force at break, denoted Fm (maximum load in N), under tension in accordance with standard ISO 6892-1, of October 2009.

[0110] The characteristics of the control cord T1 and of comparative cords C1 and C2 not in accordance with the invention are summarized in Table 5.

[0111] Table 5

Cords T1 C1 C2

M/N 3/8 3/9 3/9

d1/d3 0.30/0.30 0.30/0.30 0.35/0.30

direction for S/7.7 S/7.7 S/7.7 Cl/pitch p1 (mm)

direction for S/15.4 S/15.4 S/15.4 C3/pitch p3 (mm)

13(pm)/S13(mm) 58/0.47 19/0.17 55/0.50

Sheath 0 0 0 G (pm)

Nmax 9 9 10

D (mm) 1.25 1.25 1.36

[0112] Table 6 below collates the results of the breaking strength test for the control cord T1 and the comparative cords C1 and C2 and for the cord 50' according to the invention. The results of these tests are indicated in base 100. Thus the result of 100 for any one of these tests indicates that the cord tested exhibits a higher breaking strength than the control cord. The force at break with respect to the diameter is also compared in that same table.

[0113] Table 6

Cords T1 C1 C2 50'

M/N 3/8 3/9 3/9 3/10

Force at break (base 100_Ti) in N 100 109 117 120

Force at break with respect to the diameter 100 109 108 116 (base 100 TI) in N/mm

Penetrability + - + ++

[0114] It may be noted that the cord 50' according to the invention exhibits a force at break of the same order as that of the comparative cord C2 and better in comparison with the control cord T1 and the comparative cord C1.

[0115] It will be noted that the force at break with respect to the diameter of the cord is significantly higher than that of the control cord T1 and than those of the comparative cords C1 and C2, with better penetrability. The cord according to the invention has a better arrangement of threads for the same overall size. This test clearly demonstrates that the presence, here, of the sheath of elastomer composition enabling two additional threads to be added to the external layer according to the invention makes it possible to obtain a vaulted arch effect and thus more effective contribution of each thread to the breaking strength of the cord by comparison with the cord T1, thus overcoming the problems set out in the preamble.

[0116] Of course, the invention is not restricted to the exemplary embodiments described above.

[0117] For reasons of industrial feasibility, of cost and of overall performance, it is preferable to implement the invention with linear threads, that is to say straight threads, having a conventional circular cross section.

[0118] It will also be possible to combine the characteristics of the various embodiments described or envisaged above, with the proviso that these characteristics are compatible with one another.

Claims (14)

1. Two-layer metal cord (50), characterized in that it comprises:

- an internal layer (Cl) made up of M>1 internal metallic threads (Fl),

- an external layer (C3) of the cord, made up of N external metallic threads (F3) wound around the internal layer (Cl), of the cord, wherein:

the cord (50) is obtained by a method comprising a step of manufacturing the sheathed internal layer (CIG) in which step the internal layer (CI) is surrounded with an elastomer composition having a thickness G and then by the N external metallic threads to form the external layer (C3), where N is strictly greater than Nmax which is the maximum number of external metallic threads (F3) that can be laid on the theoretical external layer (C3T) obtained when the internal layer (Cl) is directly in contact with the theoretical external layer (C3T).

2. Cord (50) according to the preceding claim, wherein N= Nmax+1 or Nmax+2 and preferably N=Nmax+1.

3. Cord (50) according to either one of the preceding claims, wherein the diameter dl of each internal metallic thread (Fl) is equal to the diameter d3 of each external metallic thread (F3).

4. Cord (50) according to any one of the preceding claims, wherein the external layer (C3) of the cord is saturated so that the inter-thread distance of the external metallic threads (F3) is strictly less than 20 pm.

5. Cord (50) according to any one of the preceding claims, wherein the thickness G of the sheath of elastomer composition is strictly greater than 10 pm, preferably greater than or equal to 12 pm, and more preferably greater than or equal to 15 pm.

6. Cord (50) according to any one of the preceding claims, wherein the thickness G of the sheath of elastomer composition is less than or equal to 300 pm, preferably less than or equal to 250 pm, and more preferably less than or equal to 230 pm.

7. Cord (50) according to any one of the preceding claims, wherein the sheath of elastomer composition comprises an elastomer selected from the group consisting of polybutadienes, natural rubber, synthetic polyisoprenes, butadiene copolymers, isoprene copolymers, and mixtures of these elastomers.

8. Cord (50) according to the preceding claim, wherein the sheath of elastomer composition comprises an elastomer selected from the group consisting of natural rubber, synthetic polyisoprenes, isoprene copolymers, and mixtures of these elastomers.

9. Cord (50) according to any one of the preceding claims, wherein the sheath of elastomer composition contains carbon black by way of reinforcing filler.

10. Cord (50) according to any one of the preceding claims, wherein M= 2, 3 or 4 and preferably M=3 or 4.

11. Cord (50) according to any one of the preceding claims, wherein N= 9,10, 11 or 12, and preferably N=10 or 11.

12. Cord (50) according to any one of the preceding claims, wherein each metallic thread (Fl, F3) has a respective diameter dl, d3 ranging from 0.22 mm to 0.60 mm and preferentially from 0.22 mm to 0.50 mm.

13. Reinforced product (100), characterized in that it comprises an elastomer matrix (102) and at least one cord (50) according to any one of Claims 1 to 12.

14. Tyre (10), characterized in that it comprises at least one cord (50) according to any one of Claims 1 to 12 or a reinforced product according to Claim 13.