Metallic cord for reinforcing elastomeric articles, particularly pneumatic tyres

Abstract: A cord comprising at least a first pair and a second pair of wires (2a, 2b) of different diameter randomly disposed in the transverse section thereof, is obtained by arranging in the nacelle (7) of a double-twisting laying machine, a twister (16) operating upstream of a preformer (15). The twister (16), rotating in a direction opposite to that of the impeller (5) and at a speed which is twice that of the impeller, neutralizes the internal torsional stresses induced in the wires (2a, 2b) by effect of the double twisting carried out upon the action of the impeller itself. Thus, a better control of the preforming operation executed on the wires (2a, 2b) disposed parallelly in respectively coplanar axes is enabled. The obtained cord (1), within each laying pitch and only under a traction condition involving a load not exceeding 5 kg, has at least one right section in which at least one wire (2a, 2b) is spaced apart from at least one of the adjacent wires so as to facilitate the rubberizing step, by enabling access of the blend to the section and penetration of same along the cord (1) axis. The cord is preferably used as a reinforcing element in belt structures for tyres.

Description: The present invention relates to a reinforcing metallic cord to be used in particular in elastomer-matrix composite articles of manufacture, specifically in pneumatic tyres, comprising a plurality of elementary wires twisted together about the longitudinal extension axis of the cord, which cord, under any drawing condition with a tractive load not exceeding 5 kg, in the extension of a laying pitch, has at least one right section provided with at least one inlet port to enable access of the elastomer material to the inside of the cord section. The present invention also concerns a process for the manufacture of said cord, said process comprising the steps of: preforming a plurality of elementary wires submitting them to a permanent bending set along their longitudinal extension; laying the wires together by a double helical twisting about the longitudinal axis of the cord. The invention further relates to an apparatus for the manufacture of said cord, said apparatus comprising: a bearing structure; a flyer in engagement with the bearing structure and drivable in rotation according to a given axis; a stationary cradle oscillatably linked to the bearing structure according to an oscillation axis coincident with the rotation axis of the flyer; feeding means operatively mounted on said stationary cradle to feed a plurality of wires from respective supply reels, said wires being guided onto the flyer along a laying path having end stretches coincident with the rotation axis of the flyer and a central stretch spaced apart from said rotation axis; at least one preformer operatively engaged with the stationary cradle and acting on the wires over a portion of same upstream of the first end stretch of the laying path. The cord in reference is particularly conceived for use in making tyre components for motor-vehicles, such as the carcass and/or belt plies for example, but it can be easily employed for the manufacture of other articles as well, such as pipes for high-pressure fluids, belts, conveyor belts or any other article of elastomer-based composite material. The metallic cords usually employed as the reinforcing structure for articles of manufacture of elastomer material are generally comprised of a plurality of wires helically twisted about an axis coincident with the longitudinal extension of the cord. Usually, the cords of this type are made by a laying machine of the so-called double-twisting type, provided with an flyer operatively in engagement with a bearing structure and operable in rotation by motor means, as well as a so-called stationary cradle oscillatably linked to the bearing structure according to an axis coincident with the rotation axis of the flyer. The stationary cradle carries a plurality of supply reels on which the wires have been previously wound, which wires through appropriate feeding and guide means are picked up and guided onto the flyer along a predetermined laying path. This laying path has a first end stretch coincident with the rotation axis of the flyer, a central stretch stepping over the flyer so as to be spaced apart from the rotation axis, and a second end stretch again coincident with the above specified rotation axis. Rotation of the flyer gives rise during two successive steps and at the end stretches of the laying path, to twisting of the wires and, as a result, formation of the cord, according to an helical winding pitch depending on the relation existing between the rotation speed of the flyer and the pulling speed imposed to the wires upon the action of collecting means usually operating downstream of the laying machine, directly on the obtained cord. Generally, before being submitted to twisting the wires are subjected to a preforming step by their passage over a preforming device imposing a permanent bending set to the wires themselves, in order to promote the subsequent arrangement of the wires in a helical form to ensure maintenance of the structural compactness of the cord. The foregoing being stated, in order to eliminate the risk that the cords may undergo undesired corrosion phenomena once they have been introduced into the tyre or another article of manufacture of elastomeric material, it is of the greatest importance that the wires forming the cords should be completely coated, over the whole surface extension thereof, with the elastomeric material into which the cord itself is incorporated. The above result which is increasingly more difficult to achieve with the increasing of the structural complexity of the cord, cannot be easily attained even when the cords have a low number of wires, which solution, due to the light weight involved, is of particular interest in the production technology of motor-vehicle tyres. Said difficulty originates from the fact that, in order to give the cord the necessary geometric and structural stability, the wires are usually intimately compacted in contact with each other, so as to confine one or more closed cavities extending longitudinally within the cord. These cavities clearly cannot be easily reached by the elastomer material during the usual rubberizing steps of the cord. When for instance, as a result of cuts or punctures caused in the tyre structure or for any other reason, humidity and other external agents can penetrate into said cavities, a quick corrosion process of the wires inevitably occurs, to the detriment of the structural resistance of the cord and the tyre as a whole. In an attempt to overcome this problem the so-called "swollen" cords have been proposed, that is cords in which the wires (generally three to five in number) are maintained always spaced apart from each other during the rubberizing step, carried out by known processes that keep the tractive load applied to the cord to values not exceeding five kilos. An example of these cords is given in the Italian Patent No. 1,099,869 of the same Applicant. The result of a complete rubberizing of the wires is thus achieved, but the cords of this type have some use problems in that the wires keep spaced apart from each other also when the cord is submitted to a strong tensile stress during the tyre manufacture and when the tyre is run, and this condition causes an undesired geometric and structural instability of the cord as a whole, which is substantially prejudicial to the tyre behaviour. Alternatively, cords have been proposed which have still a low number of wires, in which at least one strand is deformed so as to acquire a broken-line course, such as those described in the US Patent No. 5,020,312. In this manner a continuous contact between at least two adjacent wires along the longitudinal extension of the cord is made impossible, thereby maintaining separation areas between said two wires, that is ports for admittance of the rubberizing material, at each zig-zag bending of the strand. A drawback present in this type of cords is a decay in the fatigue resistance values and a consequent decay in the qualitative level of the tyre. Finally the use of the so-called dual-diameter cords has been proposed, that is cords provided with two pairs of wires in which the strand diameter of one pair is suitably differentiated from that of the other pair. In this connection, publication RD 22 404 points out that such a cord, to be obtained by usual laying machines of the above described double-twisting type causes the important central cavity, which is defined internally of the cords provided with four or five wires of same diameter, to be replaced by two opposite cavities of much more reduced sizes, that can be more easily filled with the elastomer material used for rubberizing. In spite of this size reduction, said cavities are at all events closed to the outside. This condition makes it difficult to cause the elastomer material to penetrate into the inner parts of the cord section. Patent EP 0 168 857 discloses a metallic cord for the manufacture of which one pair of wires of same diameter and a second pair of wires of a lower diameter than that of the first pair, are fed to a conventional internal-collection laying machine, after passing through a circular preforming head where the wires of the first and second pairs follow specific paths to be sumbitted to preforming in a suitably differentiated manner with respect to each other. The cord thus achieved has the pair of wires of greater diameter helically twisted together in a mutual contact relationship, whereas the wires of the second pair are each inserted between the two wires of the first pair and extend parallely to the latter, while maintaining suitably spaced apart therefrom. In this manner the presence of closed cavities in the transverse section of the cord is eliminated and, as a result, the complete coating of the wires with the elastomer material employed during the rubberizing step is ensured. However, the wires of smaller diameter keep spaced apart from those of greater diameter also when the cord is subjected to tensile stress under use conditions, which, as in the swollen cords, will cause a certain geometric and structural instability of the cord as a whole, said instability being undesirable. In addition, it is very difficult to give the cord a precise and regular geometric configuration at each point of its longitudinal extension, in that constancy in the mutual positioning of the wires in the cord is ensured by the particular type of preformer used, but distance of the wires of smaller diameter from those of greater diameter tends to vary randomly at the different points of the longitudinal extension of same, both under rest conditions and under use conditions of the cord. In accordance with the present invention, it has been found that by using a roller-type preformer and arranging a twister device upstream of the preformer, which device is adapted to submit the wires to a preliminary step involving successive twisting and untwisting operations, it is possible to obtain a finished cord having the wires arranged in a random order in the transverse section, in which under rest conditions, that is under conditions of weak pulling, for each pitch there is at least one inlet port for the elastomer material, so as to ensure the complete coating of the wires during the rubberizing step, while at the same time eliminating the inner torsional stresses from the wires passing on the preformer, thereby obtaining a cord substantially free of internal stresses so that the following working operations of the semifinished products and/or manufacture of the articles containing said cords are facilitated; when afterwards, during the vulcanization operation of the article of manufacture and the practical use of same, the cord is submitted to high tensile stresses, higher than those used in the rubberizing step of the cords, the wires each lie in contact with at least two of the other wires, so as to give the cord a closed and compact structure with an excellent geometric stability. In particular, the invention relates to a metallic reinforcing cord, to be used especially in elastomer-matrix composite articles of manufacture, characterized in that under any traction condition with a tractive load not exceeding 5 kg in the extension of a laying pitch, has at least one right section with at least one inlet port enabling access of the elastomer material to the inside of the cord section, whereas in use, with tractive loads exceeding 5 kg, at any right section of the cord each strand is intimately in contact with at least two of the other wires, causing elimination of said access ports and a structural compaction of the cord itself. In more detail, the cord in reference comprises a first pair of wires having a given diameter, and a second pair of wires having a lower diameter than the first pair of wires. According to the invention, the cord, at any portion included within a laying pitch, both under rest conditions (traction lower than 5 kg) and under work conditions, has at least one right section in which the wires of the second pair are located on the same side with respect to the direction joining the centres of the first-pair wires, and at least one right section in which the wires of the second pair are located on opposite sides with respect to said direction joining the centres of the first-pair wires. It is also provided that in one and the same right section, by progressively varying the applied tractive load from 0 to 5 kg, the wires of the second pair alternately pass from one situation in which they are both disposed on the same side with respect to a direction joining the centres of the first-pair wires to a situation in which they are disposed on opposite sides respectively, with respect to said direction. Preferably, the wires of the first pair have a diameter included between 0.20 mm and 0.40 mm, whereas the wires of the second pair have a diameter included between 0.12 mm and 0.30 mm, the difference between the minimum and maximum diameters of said wires being in the range of 0.02 to 0.10 mm. Still in a preferential solution, the cord in reference under rest conditions has a maximum diameter included between 1.15 mm and 1.27 mm and a minimum diameter included between 0.48 mm and 0.54 mm. It is also an object of the present invention a process for the manufacture of the above cord, characterized in that before the preforming step the wires are submitted to a torsion action about their own axes, of a quantity substantially equal to that of the double twisting produced on the wires during the laying step. Thus the torsional stresses induced in the wires by effect of said laying step are neutralized, so that said wires can be submitted to the preforming step carried out with said wires disposed parallelly in side by side relation with respect to each other, in the absence of said internal torsional stresses. Advantageously, preforming is executed by making the individual wires, disposed parallelly and in coplanar relation with each other, take respective preforming paths each having a specific radius of curvature. An integral part of the invention is also an apparatus for the manufacture of said cord, characterized in that it comprises at least one twister operatively mounted on said stationary cradle and operating on at least one of said wires at a portion thereof upstream of the preformer to submit the wires to a previous torsional action about their longitudinal axes, aiming at neutralizing internal torsional stresses subsequently induced in the wires by the double twisting produced by said flyer during the laying process. Advantageously, the twister comprises: one fixed frame rigidly in engagement with said stationary cradle; one rotating frame rotatably in engagement with the fixed frame according to a rotation axis substantially coincident with one stretch of the feeding path of the wires to the preformer; and a pair of winding rollers rotatably carried by the rotating frame according to respectively parallel axes, said wires being wound once or more times in succession about the first and second winding rollers in opposite directions; driving means to operate the rotating frame in a rotation direction opposite to the rotation direction of the flyer. Preferably, said driving means kinematically connects the rotating frame with the flyer, so that driving in rotation of the rotating frame is correlated with driving in rotation of said flyer. In more detail, the driving means actuates the twister at a speed which is twice the rotation speed of the flyer. According to another feature of the invention, said preformer has a plurality of preforming seatings, each of them being suitably arranged for operatively engaging a respective wire. In more detail, the preformer consists of an idler roller, said preforming seatings consisting of circumferential races formed in said roller. Each of said circumferential races is substantially as wide as the diameter of the corresponding strand and has a bottom portion of semicircular profile the axis of which is in coplanar relation with that of the bottom portions of the other circumferential races. In a further and different aspect, the invention also relates to a pneumatic tyre containing structural elements reinforced with cords of the above type. Further features and advantages will become more apparent from the detailed description of a preferred embodiment of a metallic reinforcing cord to be employed in particular in elastomer-matrix composite articles of manufacture, and of a process and apparatus adapted to manufacture said cord, in accordance with the present invention. This description will be taken hereinafter with reference to the accompanying drawings, given by way of non-limiting example, in which: Fig. 1 is a diagrammatic side view, partly in section, of an apparatus for the manufacture of cords according to the present invention; Fig. 2 is a top view of the apparatus shown in Fig. 1; Fig. 3 is a side view partly in section and to an enlarged scale with respect to the preceding figures, of a twister device being part of the apparatus of the invention; Fig. 4 is an enlarged view of a roller-type preformer utilized in the apparatus according to the invention; Fig. 5 is a comparison table in which to each horizontal row corresponds a given tractive load value applied to the cord, and to each vertical column corresponds a given cross sectional plane of the cord, the sectional planes corresponding to the five columns are included in one and the same laying pitch. Referring particularly to the drawings, generally identified by 1 is a reinforcing metallic cord, to be used in particular in elastomer-matrix composite articles of manufacture, specifically in pneumatic tyres for motor-vehicles, according to the present invention. In a manner known per se, a tyre for vehicle wheels comprises a carcass of toric form having a crown region, two axially opposite sidewalls terminating at a radially internal position with corresponding beads for anchoring of the tyre to a corresponding mounting rim, said beads being each reinforced with at least one annular metal core, usually referred to as bead core, said carcass comprising at least one rubberized-fabric ply having its ends turned over around said bead cores, and optionally other reinforcing elements such as flippers, strips and bands of rubberized fabric. Said carcass further has a tread band disposed crownwise and moulded with a raised pattern designed to get in contact with a roadway while the tyre is running, and a belt structure, interposed between said tread band and said at least one carcass ply and comprising one or more rubberized-fabric strips reinforced with textile or metallic cords differently inclined in the corresponding strips, relative to the circumferential direction of the tyre. In a manner also known per se, the cord 1 comprises a plurality of wires 2a, 2b, preferably made of steel having a carbon content of 0.65% to 0.95%, helically twisted about the longitudinal extension axis of the cord. More particularly, in a preferential solution of the present invention provision is made for a first pair of wires 2a preferably of a diameter included between 0.20 mm and 0.40 mm and a second pair of wires 2b of a diameter included between 0.12 mm and 0.30 mm, and in any case smaller than that of the wires 2a of the first pair. In each pair the wires could also have different diameter, but preferably they have the same diameter; in a convenient embodiment of the invention the diameters correspond to 0.30 mm and 0.25 mm, respectively. The diameter difference between the wire of larger diameter and that of smaller diameter is included between 0.01 and 0.28 mm, preferably between 0.02 and 0.10 mm and most preferably between 0.03 and 0.05 mm. That being stated, before analysing in detail the peculiarity and construction features of the cord 1 according to the invention, the process and apparatus for the manufacture of same will be described hereinafter. Referring particularly to Figs. 1 and 2, the apparatus for the manufacture of the reinforcing metallic cord 1 is generally denoted by 3. This apparatus, in a manner known per se, comprises a bearing structure 4 with which a so-called flyer 5 is rotatably in engagement, which flyer is drivable in rotation by a motor 6 or equivalent means. Also oscillatably connected to the bearing structure 4, in correspondence with the rotation axis of the flyer 5, is a so-called stationary cradle 7 with which a plurality of supply reels 8 is operatively in engagement, on each reel at least one of said wires 2a, 2b being wound up. Combined with the reels 8 is an appropriate unwinding means 9, only partly shown in a diagrammatic form as it is known per se and conventional, which means is operatively mounted on the nacelle 5 to suitably guide the wires coming from reels 8. Still in a manner known per se, on issuing from the stationary cradle 7 the wires 2a, 2b are guided onto the flyer 5 according to a given laying path along which the formation of the cord 1 by effect of the rotation imposed to the flyer 5 by motor 6, takes place, in combination with a dragging action produced on the cord by collecting means not shown as known and not of importance to the ends of the invention. In more detail, the laying path is comprised of a first end stretch 10a coincident with the rotation axis of the flyer 5 and substantially confined between a first stationary intermediate gear 11 integral with the stationary cradle 7 and a second rotating intermediate gear 12 integral with the flyer 5. Along said first end stretch 10a the wires 2 a, 2b undergo a first helical torsion about the rotation axis of the flyer 5, by effect of the first rotating intermediate gear 12 being driven in rotation by said flyer. Downstream of the first rotating gear 12, wires 2a, 2b take a central stretch 10b of the laying path extending on the flyer 5 at a radially spaced apart position with respect to the rotation axis thereof, so that they step over the stationary cradle 7 until they reach a second rotating intermediate gear 13 integrally connected to the flyer itself. The laying path finally has a second end stretch 10c coincident with the rotation axis of the flyer 5 and extending between the first rotating intermediate gear 13 and a second stationary intermediate gear 14. In this second end stretch a second torsion of the wires takes place, by effect of the second rotating intermediate gear 13 being driven in rotation by the flyer 5, the formation of the cord 1 being therefore completed, said cord being gradually drawn away from the second stationary intermediate gear 14 upon the action of said collecting means. The relation existing between the rotation speed of the flyer 5, preferably included between 2000 and 6000 rpm, and the dragging speed of the cord 1, and therefore the wires 2a, 2b, preferably included between 60 and 250 m/min, determines the laying pitch value, that is the pitch according to which the wires 2a, 2b are helically twisted together in the finished cord 1. In a preferential solution of the invention, this laying pitch is maintained to a value included between 3 mm and 50 mm, preferably between 6 mm and 30 mm, and in particular equal to 16 mm. Operatively located along the path taken by the wires 2a, 2b within the stationary cradle 7, and more precisely upstream of the first stationary intermediate gear 11, is a preforming member 15 which essentially comprises an idler roller disposed along an axis perpendicular to the feed direction of the wires 2a, 2b. The wires 2a, 2b, by being wound on the preformer 15 at an angle included between 10° and 180°, preferably of 60°, undergo a permanent bending set aiming at promoting the subsequent laying operations. In accordance with the present invention however, the individual wires 2a, 2b, by effect of the double twisting imposed to them at the end stretches 10a, 10c of the laying path they covered, were each subjected to undergo a torsion about the respective longitudinal axis in the portion thereof extending upstream of the laying path, and more particularly upstream of the first stationary intermediate gear. These torsions, induced by laying in the individual wires (return torsions), did not allow preforming of the wires to be carried out correctly, that is by permanent bending set of the wires exclusively along a generatrix of the side surface of same. In fact, even if bending of the wires on the preformer 15 takes place along a line parallel to the strand axis, the presence of the internal torsional stresses (the so-called return torsions) deformed said wires inducing them to take a helical configuration so that the wires were actually preformed according to a helical bending line. The result is a cord in which the constituent wires keep a stress state hindering the even arrangement of the wires within the predetermined geometric configuration and causing strains in the cord as soon as the corresponding wires are capable of discharging their internal stresses taking a free arrangement in space, which occurs on cutting of the cord, close to the cut end. In particular these strains consist in curling of the cord end portion and fraying out of the cord end and represent a serious inconvenience as regards the whole working process, above all the cutting operations of the rubberized fabrics containing said cords, and a source of serious defects on the finished product. Therefore our objective became that of neutralizing the effect of these return torsions induced in the individual wires: to this end, still in accordance with the present invention, apparatus 3 comprises a twister 16 operatively mounted on the stationary cradle 7 and operating on a portion of the wires 2a, 2b immediately upstream of the preformer 15. More particularly, twister 16 operates between the preformer 15 and a pair of opposite intermediate rollers 17 to which the individual wires 2a, 2b come, being fed by the respective supply reels 8. Twister 16 essentially comprises, as best shown in Fig. 3, a support frame 18 fixedly supported by the stationary cradle 7 and rotatably engaging a rotating frame 19. Engagement between the rotating frame 19 and support frame 18 takes place at end hub-shaped elements 19a coaxially passed through by an inlet channel 20 and an outlet channel 21 through which the wires are caused to run, in such a manner that the rotation axis of the rotating frame 19 is substantially coincident with a stretch of the feeding path of said wires to the preformer, that is the longitudinal extension of the wires within said channels 20, 21. Mounted on the rotating frame 19 is a first and a second freely rotating winding rollers 22a, 22b having parallel axes, preferably slightly inclined to the normal to the rotation axis of the rotating frame. As shown in Fig. 2, the winding rollers 22a, 22b are disposed tangentially on respectively opposite sides in relation to the rotation axis of the rotating frame 19 and, as shown in Fig. 3, they each have at least one groove 23a, 23b formed in their external cylindrical surface. Preferably, said rollers have a plurality of distinct grooves or, alternatively, a single helical groove having several spiral rings: the first solution however is preferred because working of the roller surface in this case is easier. The strand bundle coming from the opposite intermediate rollers 17 runs through the inlet channel 20 to be wound onto the first winding roller 22a, along a corresponding groove 23a, and then onto a second winding roller 22b, along a corresponding groove 23b having an opposite rotation direction as compared with that of the preceding winding roller 22a. This path identified as an "eight-shaped" path, can be repeated several times according to several turns about said rollers. It is apparent that in the presence of a helical groove the strand bundle is wound in several turns (corresponding to the number of spiral rings) on both rollers passing only once from the first to the second rollers. The strand bundle leaves the second winding roller 22b through the outlet channel 21 to reach the preformer 15, the wires 2a, 2b being disposed parallelly in side by side relation. Combined with the twister 16 is driving means 24 arranged to drive the rotating frame 19 in rotation, in a discordant rotation direction as compared with that of the flyer 5. In particular, said driving means 24 kinematically connects the twister 16 to the flyer 5 so that rotation of said impeller simultaneously causes driving in rotation of the twister itself, to a speed which is approximately twice that of the flyer. To this end, the driving means 24 contemplates the use of a first driving belt 25 operatively engaged between corresponding pulleys, not shown as known and conventional, fitted on the flyer 5 and a propeller shaft 26 respectively, the shaft being rotatably supported within the stationary cradle 7 at a raised position laterally offset from the stationary cradle rotation axis. A second positive drive belt 27 is operatively engaged between other pulleys fitted on the propeller shaft 26 and an extension 19b of the hub-shaped element 19a carrying the outlet channel 21, respectively. The dimensional ratios between the pulleys associated with the first and second inlet belts 25, 27 respectively are such selected that the rotation speed of the rotating frame 19 is substantially twice, or in any case conveniently correlated with, the rotation speed of the flyer 5. Following rotation of the rotating frame 19, the strand bundle 2a, 2b undergoes a false laying (apparent laying) at the twister inlet, which is eliminated at the twister outlet, so that the bundle wires can become again separated from each other to be guided to the preformer disposed parallelly side by side in a substantially coplanar relationship: however each wire also undergoes a torsion about its own axis the amount of which depends on the twister rotation speed. In accordance with the present invention, it has been found that this torsion is capable of efficiently neutralising the internal torsional stress (return torsion) which is transmitted to the wires 2a, 2b upstream of the first stationary intermediate roller 11 by effect of the double torsion acquired along the laying path on the rotating intermediate rollers, so that the wires pass on the preformer substantially in an untwisted condition and are then bending preformed along a generatrix disposed parallel to the wire axis. In order to ensure accomplishment of a correct preforming on the individual wires 2a, 2b in spite of the difference in diameter that can be found therein, it is originally provided that a plurality of preforming seatings 28, 29 be arranged in the preformer 15, each of which is shaped and sized in conformity with one of the wires. As clearly shown in Fig. 4, these preforming seatings 28, 29 are defined by corresponding circumferential races formed at positions located parallelly in side by side relation on the cylindrical surface of the preformer roller 15 and each showing a depth correlated with the diameter of the corresponding strand 2a, 2b, so that the latter is deviated according to a radius of curvature "r" specifically selected depending on the diameter of the wire itself. In greater detail, as clearly viewed from Fig. 4, the preforming seatings are of a width substantially corresponding to the diameter of the corresponding wires 2a, 2b and have corresponding bottom surfaces of a semicircular profile with respective centres of curvature disposed in a common plane p-p. Then preforming can be controlled by varying either said radius of curvature, or (preferably) tension applied to the wire, that is the traction action exerted by a capstan, the radius of curvature being equal (as already known). The table in fig. 5 shows the structural features of the cord 1 made in accordance with the present invention, and the behaviour of same in operation, in relation to the increasingly growing tractive load applied thereto on passing from one rest condition, to which the complete absence of tensile stresses corresponds, to a use condition to which a tractive load higher than 5 kg corresponds. In this connection, vertical columns "A", "B", "C", "D" and "E" in Fig. 5 refer to respective transverse sections of the cord 1 taken within the same laying pitch, whereas the horizontal rows "a", "b", "c", "d" each represent the configuration taken by said sections at one specific tractive load applied to the cord 1. More particularly, row "a" refers to the cord 1 as such, that is in the absence of stresses, row "b" corresponds to a tractive load of 3 kg, row "c" represents the cord 1 in a tractive load condition, with a load of 5 kg, whereas row "d" represents the cord 1 under any operating condition in which the tractive load exceeds 5 kg. As can be seen by comparing sections "A", "B", "C", "D" and "E" along row "a", the cord 1 in the absence of tractive load has the wires 2a, 2b disposed according to a random configuration and loosely twisted, so that between one strand and the other there is much space left to enable free access to the blend employed during the rubberizing step of the cords, for example when a rubberized fabric for carcass or belt plies of a tyre is to be made. From a comparison between sections "A", "B", "C", "D", "E", reproduced along rows "b" and "c", one can see that as the tractive load applied to the cord 1 increases, in relation to the specific rubberizing process employed, carried out on a calender for example, the wires 2a, 2b tend to gather, but until the applied tractive load does not exceed 5 kg, in any portion of the cord 1 included within a single laying pitch, there is at least one right section having an inlet port, marked by "1", to enable access of the rubberizing material to the inside of the cord. When the load exceeds 5 kg, that is during vulcanization and the use of the product, in particular the tyre, the wires 2a, 2b are each intimately in contact with at least two of the other wires, thereby causing elimination of the access ports "1" and a structural compaction in the cord 1. Since the wires 2a, 2b are completely coated with the elastomeric material which has also penetrated the cord inside during the rubberizing step of the cord and/or the rubberized fabric, the space "s" existing between the wires of the cord 1 under use conditions will be completely filled with this elastomeric material, thereby eliminating any risk of early corrosion of the cord wires as a result of penetration of humidity or other external agents. In addition, a complete rubberizing of the wires 2a, 2b enables the undesired phenomena of mutual rubbing between the wires, which occur in particular in the cords employed in the tyre manufacture, to be efficiently inhibited. Advantageously, due to the action exerted by the twister 16, in the finished cord the wires 2a, 2b are substantially free of internal torsional stresses. Thus all problems connected with the presence of these internal stresses are eliminated, above all with reference to the cord cutting operations, in making rubberized fabrics such as carcass plies or belt strips for tyres, or other semifinished products. In this connection, well known to persons of ordinary skill in the art are the problems and difficulties encountered when, due to said internal stresses in the cords, the edges of a cut ply take an undulated appearance or are subjected to undesired strains. Due to the random arrangement of the wires 2a, 2b in the cord section, made possible by the particular structure of the selected preformer 15 in accordance with the present invention, both in the rest and above all in the use conditions of the cord, in any portion of the longitudinal extension included within a laying pitch, there is at least one right section in which the wires 2b of the second pair, of smaller diameter, are located on the same side in relation to a direction Z-Z joining the centres of the first-pair wires, and at least one right section in which the wires of said second pair 2b are located on respectively opposite sides in relation to said direction Z-Z. By comparing the configurations taken by the individual sections in columns "A", "B", "C", "D", "E", it is also possible to ascertain that, in the cord in reference, as the applied tractive load progressively varies within a value range included between 0 and 5 kg, the wires 2b of the second pair alternately pass from a situation in which they are disposed both on the same side in relation to said direction Z-Z, to a situation in which they are disposed respectively on opposite sides relative to said direction. This feature has been found particularly efficient in ensuring a high stability of the cord and an even distribution of the efforts in the individual wires during the use with high loads also of variable intensity, as those applied to a tyre in use. The random arrangement of the wires makes the cord have diameter variations along its longitudinal extension. In a preferential solution, the cord 1 is provided to have, in the absence of tractive load, a maximum diameter included between 1.15 mm and 1.27 mm, preferably equal to 1.21 mm and a minimum diameter included between 0.54 mm and 0.48 mm, preferably of 0.51 mm. Under tractive conditions with a load exceeding 5 kg, since, as said, all the wires are in mutual contact, the maximum and minimum diameters of the cord can be easily determined mathematically, the diameter of the individual wires used being known. In the cord made following the geometric and dimensional parameters specified in the description, the ultimate tensile strength is included between 674 N and 551 N and corresponds to 613 N for example, to which, for steel wires having a carbon content of 0.7, an ultimate elongation included between 2.5% and 3% corresponds: thus it can be proved that the laying process according to the invention has not at all impaired the mechanical strength of the cord as compared with that of the best cords known in the art. The invention achieves the intended purposes. In fact, the action of the twister within the stationary cradle imposes to the wires, in the portion of same coming out of the twister, a torsion in a direction opposite to that induced by the double twisting taking place along the laying path, thereby eliminating the internal torsional stresses and making it possible to carry out a more regular preforming of the wires, disposed in coplanar and parallel relation with respect to each other. The achieved cord can be rubberized in an excellent manner due to the important gaps existing between the wires in a rest condition and, due to the absence of internal torsional stresses, has a better behaviour when fabrics made using such a cord are to be submitted to further workings. At the same time, the cord in reference has a compact structure under use conditions. Such a compact structure is reached when, during the vulcanization step for example, the cord is submitted to a tractive load exceeding 5 kg, and this compact structure is then maintained, following vulcanization of the elastomeric material. The compact configuration thus eliminates all problems of structural instability appearing in known cords having two pairs of wires of different diameters, where the wires of smaller diameter keep a certain distance from the two other wires, even under use conditions. The random arrangement of the wires 2a, 2b also eliminates all problems resulting, in the known art, from the necessity of imposing a well precise and definite geometric positioning to the wires, in the transverse section of the cord, so that the cord in accordance with the invention can be manufactured more easily and its geometrical appearance can be more uniform and constant along its longitudinal extension. As a result of the above, the tyres containing structural elements of rubberized fabric incorporating the cords of the invention as the reinforcing elements have an improved assembling capability, the mutual positioning of the semifinished products takes place more easily and therefore the structure has more stability during the carcass-handling operations preceeding the tyre vulcanization, and the tyre road behaviour in use is ultimately improved. In the finished tyre said cords, in addition, show more resistance to fatigue, less separations from the elastomeric rubberizing material amd more resistance to corrosion, thereby prolonging the structural strength and lifetime of the tyre. In particular said cord is preferably used as a reinforcing element for the belt structure in the tyres. The tyre according to the invention shows the overall structure as previously defined in general, but in particular it has a belt structure comprising two rubberized-fabric strips radially superposed on each other, substantially as large as the tread, mutually staggered at the ends, reinforced with metallic cords disposed parallelly to each other in each strip and symmetrically crossing those of the adjacent strip, relative to the equatorial plane of the tyre. Preferably, in a position radially external to said pair of strips, another layer of textile cords of heat-shrinkage material is provided, which further cords are wound on said pair of strips in a plurality of coils disposed axially side by side and oriented in a direction substantially parallel to said equatorial plane, that is, according to the usual definition, at "0°". Most preferably, as already disclosed, the reinforcing cords of said strips are the metallic cords of the invention, disposed inclined at an angle included between 18° and 26° to the circumferential direction of the tyre, distributed in each strip with a thickness of 80 to 120 cords/decimetre. Obviously, once the invention as above described has been understood, a person skilled in the art will be easily able to make all necessary choices, changes and modifications in the features as claimed, in order to meet the specific technical requirements.

Claims: A reinforcing metallic cord for composite articles of manufacture of elastomer material, comprising at least two pairs of wires (2a, b2) of different diameter, helically twisted together and about the longitudinal extension axis of the cord (1), characterized in that: under a rest condition, corresponding to a traction action with a tractive load not exceeding 5 kg, in the extension of a laying pitch, it has at least one right section in which, between at least two of said wires (2a, 2b), at least one inlet port (1) is defined to enable access of said elastomer material to the inside of the cord (1) section, under any use condition, corresponding to a traction action with a tractive load exceeding 5 kg, at any right section of the cord (1) each strand (2a, 2b) is intimately in contact with at least two of the other wires (2a, 2b), causing elimination of said access ports and a structural compaction of the cord (1). A reinforcing metallic cord according to claim 1, characterized in that it comprises a first pair of wires (2a) having an identical given diameter, and a second pair of wires (2b) of same diameter, having a lower diameter than that of the first pair of the wires (2a). A reinforcing metallic cord according to claim 2, characterized in that within each laying pitch, in the rest condition, it has at least one right section in which the wires (2b) of the second pair are located on the same side with respect to the direction (Z-Z) joining the centres of the first-pair wires (2a), and at least one right section in which the wires (2b) of said second pair are located on opposite sides with respect to said direction (Z-Z) joining the centres of the first-pair wires (2a). A reinforcing metallic cord according to claim 2, characterized in that, within each laying pitch, in the use condition it has at least one right section in which the wires (2b) of the second pair are located on the same side with respect to the direction (Z-Z) joining the centres of the first-pair wires (2a), and at least one right section in which the wires (2b) of said second pair are located on opposite sides with respect to said direction (Z-Z) joining the centres of the first-pair wires (2a). A reinforcing metallic cord according to claim 2, characterized in that in one and the same right section, by progressively varying the applied tractive load from 0 to 5 kg, the wires (2b) of the second pair alternately pass from one situation in which they are both disposed on the same side with respect to the direction (Z-Z) joining the centres of the first-pair wires (2a) to a situation in which they are disposed on opposite sides respectively, with respect to said direction (Z-Z). A reinforcing metallic cord according to claim 2, characterized in that the wires (2a) of the first pair have a diameter included between 0.20 mm and 0.40 mm, whereas the wires (2b) of the second pair have a diameter included between 0.12 mm and 0.30 mm. A reinforcing metallic cord according to claim 6, characterized in that the diameter difference between said wires (2a, 2b) is included between 0.02 and 0.10 mm. A process for the manufacture of a reinforcing metallic cord, in particular for composite articles of manufacture of elastomer material, comprising wires helically twisted together and about the longitudinal extension axis of the cord, said process comprising the steps of: preforming a plurality of wires (2a, 2b) submitting them to a permanent bending set along their longitudinal extension; laying the wires (2a, 2b) together by a double helical twisting about the longitudinal axis of the cord (1), characterized in that before the preforming step the wires (2a, 2b) are submitted to a torsion action about their own axes, in order to neutralize the torsional stresses induced in said wires by effect of said laying step, so that they are submitted to the preforming step in the absence of said internal torsional stresses. A process according to claim 8, characterized in that said torsional actions are of a quantity substantially equal to that of the double twisting produced on the wires (2a, 2b) during the laying step, and of opposite directions. A process according to claim 8, characterized in that preforming is executed by making the individual wires (2a, 2b) disposed parallelly and in coplanar relation with each other take respective preforming paths each having a specific radius of curvature. A process according to claim 10, characterized in that preforming is controlled by varying the traction action exerted on the individual wires (2a, 2b), along the respective preforming paths. An apparatus for the manufacture of reinforcing metallic cords to be used in particular for composite articles of manufacture of elastomer material, comprising: a bearing structure (4); a flyer (5) in engagement with the bearing structure (4) and drivable in rotation according to a given axis; a stationary cradle (7) oscillatably linked to the bearing structure (4) according to an oscillation axis coincident with the rotation axis of the flyer (5); feeding means (9) operatively mounted on said stationary cradle (7) to feed a plurality of wires (2a, 2b) from respective supply reels (8), said wires (2a, 2b) being guided onto the flyer (5) along a laying path having end stretches (10a, 10c) coincident with the rotation axis of the impeller itself and a central stretch (10b) spaced apart from said rotation axis; at least one preformer (15) operatively engaged with the stationary cradle (7) and acting on the wires (2a, 2b) over a portion of same upstream of the first end stretch (10a) of the laying path, characterized in that it further comprises at least one twister (16) mounted on said stationary cradle (7) and operating on at least one of said wires (2a, 2b) at a portion thereof upstream of the preformer (15) to submit each strand to a torsional action about its own longitudinal axis, aiming at neutralizing internal torsional stresses induced in the wires themselves by effect of the twisting action produced by said flyer (5). An apparatus according to claim 12, characterized in that said twister (16) comprises: one supporting frame (18) rigidly in engagement with said stationary cradle (7); one rotating frame (19) rotatably in engagement with the fixed frame according to a rotation axis substantially coincident with one stretch of the feeding path of said wires (2a, 2b) upstream of the preformer; and a pair of winding rollers rotatably carried by the rotating frame according to respectively parallel axes, said wires being successively wound about the first and second winding rollers (22a, 22b) in respectively opposite directions; driving means (24) to operate the rotating frame (19) in a rotation direction discordant from the rotation direction of the flyer (5). An apparatus according to claim 13, characterized in that said driving means (24) kinematically connects the rotating frame (19) with the flyer (5), so that driving in rotation of the rotating frame (19) is correlated with driving in rotation of said impeller. An apparatus according to claim 13, characterized in that said driving means (24) actuates the twister (16) at a speed which is twice the rotation speed of the flyer (5). An apparatus according to claim 12, characterized in that said preformer (15) has a plurality of preforming seatings (28, 29), each of them being suitably arranged for operatively engaging a respective wire (2a, 2b). An apparatus according to claim 16, characterized in that said preformer (15) consists of an idler roller, said preforming seatings (28, 29) consisting of circumferential races formed in said roller. An apparatus according to claim 17, characterized in that each of said circumferential races (28, 29) is substantially as wide as the diameter of the corresponding wire(2a, 2b) and has a bottom portion of semicircular profile the axis of which is in coplanar relation with that of the bottom portions of the other circumferential races. A pneumatic tyre for vehicle wheels comprising a carcass of toric form having a crown region, two axially opposite sidewalls terminating at a radially internal position with corresponding beads for anchoring of the tyre to a corresponding mounting rim, said beads being each reinforced with at least one annular metal core, usually referred to as bead core, said carcass comprising at least one rubberized-fabric ply having its ends turned over around said bead cores, and optionally other reinforcing elements, said tyre further having a tread band disposed crownwise and moulded with a raised pattern designed to get in contact with a roadway while the tyre is running, and a belt structure, interposed between said tread band and said at least one carcass ply and comprising one or more rubberized-fabric strips reinforced with textile or metallic cords differently inclined in the corresponding strips, relative to the circumferential direction of the tyre, characterized in that it comprises at least one reinforcing structural element of rubberized fabric incorporating metallic cords according to any one of claims 1 to 7. A pneumatic tyre for vehicle wheels comprising a carcass of toric form having a crown region, two axially opposite sidewalls terminating at a radially internal position with corresponding beads for anchoring of the tyre to a corresponding mounting rim, said beads being each reinforced with at least one annular metal core, usually referred to as bead core, said carcass comprising at least one rubberized-fabric ply having its ends turned over around said bead cores, and optionally other reinforcing elements, said tyre further having a tread band disposed crownwise and moulded with a raised pattern designed to get in contact with a roadway while the tyre is running, and a belt structure, interposed between said tread band and said at least one carcass ply and comprising two rubberized-fabric strips radially superposed on each other, substantially as large as the tread, mutually staggered at the ends, reinforced with metallic cords disposed parallelly to each other in each strip and symmetrically crossing those of the adjacent strip, relative to the equatorial plane of the tyre, characterized in that said reinforcing cords comprise at least two pairs of wires (2a, b2) of different diameter, helically twisted together and about the longitudinal extension axis of the cord, under a rest condition, corresponding to a traction action with a tractive load not exceeding 5 kg, in the extension of a laying pitch, have at least one right section in which, between at least two of said wires (2a, 2b), at least one inlet port (1) is defined to enable access of the rubberizing material to the inside of the cord (1) section, under any use condition, corresponding to a traction action with a tractive load exceeding 5 kg, at any right section of the cord (1), have each wire (2a, 2b) intimately in contact with at least two of the other wires (2a, 2b), causing elimination of said access ports and a structural compaction of the cord (1), are arranged inclined at an angle included between 18° and 26° relative to the circumferential direction of the tyre, are distributed in each strip with a thickness of a value included between 80 and 120 cords/decimetre.

Inventors: CIPPARRONE MARCO, NOFERI OMERO, CIPPARRONE, MARCO, NOFERI, OMERO
Applicants: PIRELLI, PIRELLI PNEUMATICI SOCIETA PER AZIONI
Application year: 1996
Main classification: D07B1
IPC codes: B60C9, D07B1, D07B7, D07B3