IMPROVEMENTS IN THE BEADS OF PNEUMATIC TIRES FOR VEHICLE WHEELS

Abstract: The metal core used for reinforcing tire beads is consti­ tuted by a plurality of coils of wire (16), (17), (18), axially arranged side-by-side and radially superimposed, in which the wire has a straight section of modular shape with two equal and parallel opposite sides (1), (2), the profiles (15) connecting the corresponding ends of the opposite sides having a distance, from the axis of said pair of sides, whose value varies from one side to the other, said variation comprising at least one inversion of value along the development of the profile.

Description: The present invention concerns pneumatic tires for motor-vehicles and more specifically it relates to their beads, namely to the annular portions of the tire carcass which have the task of anchoring the tire on the corresponding assembly rim. More precisely the invention refers to the reinforcing annular elements inserted in the beads, which are commonly defined as "cores". The various types of assembly rims for tires are so widely known that there is no need of describing them in the present specification; it is sufficient to point out that they are provided at their ends with two more or less conical coaxial surfaces, representing the supporting base for the tire beads; these surfaces terminate with a flange,radially projecting outward, which is intended to support the axially outer surface of the bead and against which the latter abuts by virtue of the tire inflation pressure. The forcement of the bead into its seat is ensured by the conical shape of the supporting base opened outward, in co-operation with the reinforcing metal core, circumferentially inextensible and having therefore a constant diameter, which is contained in the tire bead; this forcement, created by the axial thrust which is exerted on the bead side,from the inside out, on account of the tire inflation pressure, ensures the anchorage of the tire on the rim during use and, in tubeless tires, also the air tightness between tire and rim to prevent a progressive deflation of the tire. It is known that a tubeless tire, namely a tire not provided with inner tube, must be assembled on the so-called grooved rims, i.e. one-piece rims in which the beads supporting bases are considerably conical (15° on the axis of rotation of the rim); also, it can be understood that, to assemble the tire on the rim and to remove it therefrom, the tire bead must step over the rim flange which has a diameter greater than the radially inner diameter of the core. To carry out this operation, the core must be flexible, that is liable to suffer deformation and in particular it must take an elliptical configuration (ovalization) which enables it to step over the rim flange in consequence of the diameter increase which takes place at the greater axis of the ellipse. The solution of this problem proves particularly difficult in respect of large-size single-core tires, owing to the relevant geometrical size of the cores. In fact, it is evident that cores made of rubberized wires, like those at present used for large-size tires, after the tire curing become rigid and extremely compact structures, which are therefore scarcely flexible. For tires of this kind, provision was therefore made of cores composed of bare wires, able to displace circumferentially with respect to one another and consequently to allow the deformation and in particular the ovalization of the core even in the cured tire. These cores are built up in two different versions, namely one having a circular section, commonly known as "cord", made of a single wire of circular section helically wound up in several coils around a central element, and one of quadrangular straight section, obtained by means of a metal strap having quadrangular straight section with at least two opposite sides parallel to each other. However, these cores show some disadvantageous characteristics both as regards their construction and as regards their employment. As to the cores of the "cord" type, their section proves substantially inadequate in view of their use in tubeless tires, since - besides having a certain circumferential elongation - it is little resistant to torsional stresses and consequently is unable to ensure the anchorage of the tire bead on the rim during operation, which is a substantial condition to maintain the regular inflation pressure and to safeguard the structural integrity of the tire. The "metal strap" type cores are made by winding up on an appropriate drum, in several radially superimposed turns, a plurality of straps arranged side-by-side, so that the final core is constituted by a plurality of coils in side-by-side relationship, but not linked to one another. Further, to impart to the base of the finished core the inclination of 15° on the horizontal plane (core axis), which is strictly necessary to use the core in tubeless tires, the metal strap is appropriately pre-shaped before being wound up on the drum and is then wound up on it with its side facing the drum already inclined by 15°. Clearly, this production system involves a more complicate machinery and increases costs; further, the obtained core shows by itself an insufficient resistance to the stresses exerted onto it during the tire moulding and in operation, so that the necessary compactness of its section, and consequently its stability and torsional strength can be obtained and maintained only if the core body, that is the straps package, is kept closed and compact by a set of metal strips wound up around the package and circumferentially arranged in spaced relationship along the circumferential development of the core in order to prevent any relative displacement between the straps both in axial and in radial direction. Also this expedient involves obviously additional costs and complications. Besides this, in order to achieve a greater compactness of the core to ensure the stability of its shape during the tire moulding and to maintain the integrity of the bead in course of time, the metal core was embedded in a kern of very hard elastomeric material, having a substantially circular section and covered on its turn with a sheet of elastomeric material of still higher hardness so as to create a closed tubular envelope within which is embedded the metal core; in fact, only in this way it was possible to prevent the tears in the elastomer material and the detachments of said material from the core metal surface which - in other cases - take unavoidably place during the tire operation on account of the torsional stresses acting on the bead. A known tyre of this type according to the preamble of claim 1 is shown, e.g. in FR-A-2 220 396. The present invention aims at providing a new core of the metal strap type, intended to be used in particular in large-size tubeless tires, which possesses all the advantageous characteristics of flexibility and elastic deformability of the conventional cores but it devoid of the above indicated inconveniences related thereto. Accordingly, the present invention provides a pneumatic tire for a vehicle wheel comprising a carcass provided with two beads for anchorage to a corresponding assembly rim, each bead being reinforced with a circumferentially inextensible annular metal bead core package, said carcass comprising at least one reinforcement ply, the ends of said ply being turned up around each bead core in a direction from the axial inside to the axial outside of said core, each said bead core package having a cross section generally in the shape of a parallelogram and having generally axially extending bases inclined at a predetermined angle with respect to the axis of rotation of the tire in which said bead core package is positioned, each said bead core package comprising a plurality of radially superimposed coils of straps axially arranged side-by-side, the metal straps forming said bead core package having a cross section including two axially extending rectilinear and parallel opposite sides, the corresponding extremities thereof at one side of such a section being connected by a generally radially extending profile symmetric to the corresponding profile at the other side of the section with respect to a pre-established straight axis perpendicular to said pair of parallel sides, characterized in that the distance from the connecting profile to said axis varies along the profile in such a way as to undergo, along its length, at least one inversion of the trend (increasing/decreasing) of the values of said varying distance, the side-by-side contact among said straps along their corresponding profiles determining their reciprocal radially offset position and the degree of said inclination of the parallelogram base. According to a preferred embodiment of the invention, the distance of said profile from the straight line perpendicular to said pair of opposite and parallel sides varies from a minimum to a maximum value and again to a minimum value by passing from one side to the other and in particular this is obtained by means of a profile specularly symmetrical with respect to a straight line parallel to and equally spaced from said opposite sides, the profile being preferably in the form of a broken line of two sides or of a substantially sinusoidal curvilinear line. In particular, the straight section of the wire is a modular element. The coils axially arranged side-by-side are radially offset from one another by a quantity corresponding to one half the distance between said pair of opposite sides, whilst the radially inner free edges of the side-by-side coils lie on a line inclined by 15° with respect to the core axis; the radially superimposed coils are constituted by a continuous metal strap wound up on itself. The present invention will be better understood from the following description and from the attached drawings, both given only by way of example and therefore not restrictive, where : FIGURE 1 - illustrates the generic straight section of the wire according to the invention; FIGURE 2 - illustrates a profile specularly symmetrical with respect to the median line of the wire section; FIGURE 3 - shows a straight section of modular type; FIGURE 4 - shows a preferred embodiment of a wire according to the invention; FIGURE 5 - shows a second preferred embodiment of the invention; FIGURE 6 - illustrates the straight section of the bead of a tubeless tire provided with the core according to the invention, obtained with the strap of straight section as in figure 5; FIGURE 7 - represents in perspective view a finished core provided with check elements to maintain the compactness of the package of metal coils. FIGURE 1 shows the straight section of a metal wire of the type defined as "metal strap", used to build up - in accordance with the invention - the reinforcing elements of the tire bead, i.e. the so-called "cores", in particular those to be used in large-size tubeless tires comprising only one core. The section of said wire, generically in accordance with the invention, shows two opposite and parallel sides (1) and (2) and an end profile (3) which connects the corresponding ends of said pair of sides. According to the invention, taking as a reference a straight line "r" perpendicular to said pair of sides (1) and (2), the distance "d" of the profile from said straight line varies along the profile development and said variation between the two end points of the profile shows at least one inversion of value; in other words said distance at first increases and then decreases, or vice versa, at least once. FIGURE 2 illustrates a less generic embodiment of the invention according to which the opposite and parallel sides (1) and (2) are also equal to each other, so that axis a-a of said pair of sides (1) and (2) can now be taken as reference straight line "r", whilst the connection profile (4) is specularly symmetrical with respect to the median line m-m of the section. FIGURE 3 illustrates a still more advantageous embodiment of the invention, in which profile (5) connecting the opposite sides (1) and (2), besides being specularly symmetrical with respect to the median line m-m (not shown), is the same for both sides of the strap and is of modular type; in other words several straps (6),(7),(8), all having the same straight section, can be exactly coupled together so as to give rise to a unit constituted by many equal and distinct elements (modular elements) in which the straight section is compact, i.e. does not comprise hollow spaces or interferences and has an area corresponding to the sum of the section areas of the components, namely of the individual straps forming the core. The strap illustrated in FIGURES 1, 2 and 3 is shown to explain the invention in detail but has not an optimum section, since profiles (5) connecting the ends of the parallel sides are difficult and troublesome to obtain and, for instance those of convex shape represented in FIGURES 2 and 3 require complicate systems to wind up the strap on the drum, being it impossible - owing to interferences in radial direction which might take place between the strap edges - to obtain coils axially arranged side-by-side and radially superimposed by carrying out the winding operating only by means of the rotation of the building drum. FIGURE 4 illustrates instead a more convenient embodiment of the strap, obtained according to the principle of modularity, in which profile (9) connecting the opposite and parallel sides is substantially sinusoidal and consequently facilitates the fitting of the straps arranged side-by-side (10)-(14) and at the same time allows an expedite performance. From said Figure it is evident that, by arranging conveniently the straps in side-by-side relationship, a flat-base core can be easily obtained. Another advantageous embodiment of the strap according to the invention is shown in FIGURE 5, where profile (15) connecting the opposite and parallel sides (1) and (2) is represented by a broken line of two sides, projecting axially outward with respect to said pair of sides, so that the strap has a substantially hexagonal section, which can be easily and economically carried out by means of the present technology regarding wire processing and drawing. Also this section is modular and facilitates the mutual fitting of a plurality of straps, axially arranged side-by-side. It is clear that in this case said straps will be radially offset with respect to one another by a portion equal to one half the strap thickness. FIGURE 6 illustrates the bead B of a large-size tubeless tire, provided with a core C built up with the strap of hexagonal section shown in Figure 5, and assembled on a grooved rim R with the bead supporting surface inclined, with respect to the line parallel to the rim axis, by an angle "w" of 15°. As shown in FIG. 6 the carcass ply reinforcing cords 22 are turned up around the bead core in a direction to run from the axial inside to the axial outside thereof. It is now evident that, if the values of L, 1 and h of the straight section of the strap (FIGURE 5) are appropriately selected, the radially inner edges of the straps axially arranged side-by-side which are free, i.e. are not in contact with other straps, are all aligned on a straight line "s", also inclined by 15° with respect to the axis of the core, as shown in Figure 5. The advantages afforded by the core forming the object of the invention are many and relevant. First of all, like the conventional ones, the core is built up by winding up a plurality of straps (16)-(19) in Figure 5, each in several superimposed coils, on an appropriate drum to which they are fed in mutual side-by-side arrangement. However, as the side-by-side arranged coils remain blocked with respect to the mutual radial displacements (owing to the particular profile which connects the opposite and parallel sides of the strap) and are therefore able to provide a core base having a permanent inclination of 15°, the pre-shaping of the wire becomes quite useless, so that also the surface supporting the building drum is simpler in processing, being it represented by a series of steps having a cylindrical, and not conical, radially outer surface. Being not subjected to pre-shaping operations, the core is devoid of internal tensions and therefore is not affected by forces which tend to disarrange the geometry of its section; moreover, the core section, provided with straps mutually fitting together, is naturally stable and extremely resistant to torsional stresses, so that the section compactness can be easily maintained and ensured even when the core is to be assembled in the tire carcass and during the tire moulding and curing only by means of a reduced number of suitable check elements, as the strips (20), illustrated in Figure 7, which are wound up around the kern, i.e. the package of straps, and are situated circumferentially along the peripheral development of the core at a few points, for instance only at two points in diametrically opposite position. The strips (20), which are not to bear excessive stresses owing to the peculiar stability of the section, can be made of any suitable material, for instance also of a material not resistant to compression. Further, it is evident that, in spite of the axial interconnection between the sections of the straps arranged side-by-side, each strap is given the possibility of mutual circumferential movements with respect to the other straps, so that the core maintains in the largest extent the characteristics of flexibility and deformability (ovalization) typical of the best conventional cores, affording in this way a considerable technical and economical improvement in the present tire technology. As regards the bond with the elastomeric material in which the core is embedded, the value of said bond is extremely high and stable in course of time. In fact the core according to the invention, by virtue of the particular straight section of the strap, has two main advantages. First of all the core surface is well articulated; the core profile, considered on the straight section, shows projections (vertices of the hexagons) in axial position and steps in radial position. The whole formed by projections and steps is such as to create a series of joints between the surface of the metal core and the elastomeric material embedding it; the result is a very high resistance to the torsional stresses applied to the bead, which tend to detach the elastomeric material from the metal. Moreover, the projections and the steps increase in a considerable way the area of the core surface, without substantially modifying the area of the resistant section (straight section) of the metal in comparison with that of the conventional cores of quadrangular section; since the area of the resistant section is dimensioned according to the entity of the stresses (tension and torsion) applied to the core, it can be understood that - under an equal stress - a greater surface of contact with the elastomeric material means a lesser specific strain per unit of surface (Kg/mm²) acting on the rubber/metal interface. The combination of these favourable characteristics eliminates substantially the risk of tears in the elastomeric material and of detachments in the rubber/metal bond, so that it is no longer necessary to embed the annular core in the kern covered by specific elastomeric materials of high hardness; on the contrary, the core in accordance with the invention can be embedded in the tire bead by the usual methods of assembly used for the conventional rubberized cores. At last, it will be understood that the present specification is solely given by way of non-limiting example.

Claims: A pneumatic tire for a vehicle wheel comprising a carcass provided with two beads for anchorage to a corresponding assembly rim, each bead being reinforced with a circumferentially inextensible annular metal bead core package (C), said carcass comprising at least one reinforcement ply, the ends of said ply being turned up around each bead core in a direction from the axial inside to the axial outside of said core, each said bead core package (C) having a cross section generally in the shape of a parallelogram and having generally axially extending bases inclined at a predetermined angle (w) with respect to the axis of rotation of the tire in which said bead core package is positioned, each said bead core package (C) comprising a plurality of radially superimposed coils of straps (6-8, 10-14, 16-19) axially arranged side-by-side, the metal straps (6-8, 10-14, 16-19) forming said bead core package having a cross section including two axially extending rectilinear and parallel opposite sides, the corresponding extremities thereof at one side of such a section being connected by a generally radially extending profile (5, 9, 15) symmetric to the corresponding profile at the other side of the section with respect to a pre-established straight axis perpendicular to said pair of parallel sides, characterized in that the distance from the connecting profile to said axis varies along the profile (5, 9, 15) in such a way as to undergo, along its length, at least one inversion of the trend of the values of said varying distance, the side-by-side contact among said straps (6-8, 10-14, 16-19) along their corresponding profiles (5, 9, 15) determining their reciprocal radially offset position and the degree of said inclination of the parallelogram base. Pneumatic tyre of claim 1 characterized by the fact that said profiles (5, 9, 15) are so shaped that when two straps are radially stacked, their combined profiles form an outline that is complementary to the profile of an axially adjacent similar strap that can then interfit therewith, the arrangement being such that only a portion of the profile of one strap contacts only a portion of the profile of the axially adjacent strap Pneumatic tyre of claim 2 characterized by the fact that said profiles (9, 15) are shaped to form an axially extending projection on each profile, said profiles (9, 15) being so shaped that when two straps are radially stacked, their axial extending projections form an indentation therebetween, said indentation being shaped to receive an axial projection from an axially adjacent similar strap that can then interfit therewith. Pneumatic tyre of claim 3, characterized by the fact that each radially extending profile (15) comprises two rectilinear lines lying at an angle to each other. Pneumatic tyre of claim 1 characterized by the fact that said predetermined angle (w) is about 15° with respect to the axis of rotation of the tire in which the bead core package (C) is mounted. Pneumatic tyre of any one of claims 1 to 5 characterized by the fact that said bead core package (C) comprises a plurality of check elements (20) spaced around its circumference for holding together said plurality of coils of straps (6-8, 10, 16-19), each check element (20) formed as a closed ring surrounding the coils of said bead core package. Metal strap (16, 19) having a generally hexagonal cross section including two axially extending rectilinear and parallel opposite base-sides, the outer extremities at one side of said cross section being connected by a generally radially extending profile (15) projecting axially outward with respect to said base-sides, characterized in that the maximum axial length L of the cross section, the length 1 of the base-sides and the section height h are appropriately selected in such a way as to form, upon side-by-side arrangement of a plurality of straps (16, 19), a bead core package (C) as in any one of the preceding claims. Metal strap of claim 7 characterized in that a straight line passing through one extremity of one base-side and said axial projection of the profile at the outer extremity of the same base-side is inclined at about 15° with respect to said base-side. Circumferentially inextensible annular metal bead core package for reinforcing the beads of a pneumatic tyre, said bead core package (C) having a cross section generally in the shape of a parallelogram and having generally axially extending bases inclined at a predetermined angle (w) with respect to the axis of rotation of the tire in which said bead core package is positioned, each said bead core package (C) comprising a plurality of radially superimposed coils of straps (6-8, 10-14, 16-19) axially arranged side-by-side, the metal straps (6-8, 10-14, 16-19) forming said bead core package having a cross section including two axially extending rectilinear and parallel opposite sides, the corresponding extremities thereof at one side of such a section being connected by a generally radially extending profile (5, 9, 15) symmetric to the corresponding profile at the other side of the section with respect to a pre-established straight axis perpendicular to said pair of parallel sides, characterized in that the distance from the connecting profile to said axis varies along the profile (5, 9, 15) in such a way as to undergo, along its length, at least one inversion of the trend of the values of said varying distance, the side-by-side contact among said straps (6-8, 10-14, 16-19) along their corresponding profiles (5, 9, 15) determining their reciprocal radially offset position and the degree of said inclination of the parallelogram base. Circumferentially inextensible annular metal bead core package of claim 9 characterized by comprising said metal straps according to any one of claims 7 and 8.

Inventors: MAIOCCHI, LUIGI
Applicants: PIRELLI COORDINAMENTO PNEUMATICI SOCIETA PER AZIONI
Application year: 1988
Main classification: B60C15
IPC codes: B60C15