AU725389B2 - Crown reinforcement for a tyre - Google Patents

Abstract

The invention concerns a tyre designed for carrying heavy loads, comprising a body reinforcement formed by at least a ply of radial cords (1) and a top reinforcement formed by at least a ply (3) of almost cylindrical reinforcing circumferential elements consisting of main metal and inextensible elements (30), having minimal transverse and radial dimensions at least equal to 0.09 times the square root of the circumferential radius RS of said ply (3), the filling coefficient of which is at least equal to 0.70, and which is topped with a rubber pad (4) with high modulus of elongation, with crescent-shaped transversal cross section.

Description

CROWN REINFORCEMENT FOR A TYRE The present invention relates to a radial tyre, to be fitted on heavy vehicles, construction or agricultural vehicles, such as, for example, lorries, buses, trailers, towing vehicles for highway units, earthmoving equipment or agricultural tractors, and more particularly to the crown reinforcement of such a tyre.

Such a reinforcement is currently formed of several crown plies: generally two so-called triangulation half-plies, arranged on the carcass reinforcement, on either side of the equatorial plane of the tyre, and composed of inextensible metal cables forming an angle which may be between 45' and 900 with the circumferential direction; these triangulation half-plies are surmounted radially by two plies of inextensible metal cables, crossed from one ply to the next and forming angles of between 100 and 30' with the circumferential direction. The two plies of small angle, called working plies, are completed radially on top by at least one ply of extensible, elastic metal cables, generally of the same direction as the subjacent cables of the radially adjacent working ply. The ply or plies of elastic cables are called protective plies.

Such a crown reinforcement architecture, under severe travelling conditions, results in separations between edges of working crown plies, said edges being the favoured location of shear stresses which result in tearing of the layers of vulcanised rubber which separate said edges and in the separation of the working plies, until complete degradation of the crown reinforcement occurs.

Many solutions have been proposed in order to overcome this problem, or at least to alleviate the disadvantages mentioned. It is thus possible to place wedges or pads of rubber between the edges of the working crown plies in order to reduce said inter-ply shear stresses.

According to the hypothesis according to which the expansion of the crown reinforcement of the tyre as a function of the inflation pressure is said to create inter-ply shear pre-stresses which are very detrimental to the strength of the crown reinforcement, it was proposed to arrange radially between the carcass reinforcement and the crown reinforcement at least two plies of practically inextensible metal cables forming a small angle, preferably equal to with the circumferential direction, these plies having the function of surrounding, hooping to the maximum the subjacent radial carcass reinforcement which, under the action of the internal inflation pressure, tends to increase its circumferential development whilst increasing its transverse curvature, which tendency increases with the duration of travel of the tyre. The tyre described, for example, in French application 2 452 390 meets the above definition.

The inextensible cables of the hooping plies above were subsequently replaced by extensible cables, and the small angle became zero or was considered as zero, said plies still being placed between the carcass reinforcement and the crown reinforcement. Such a tyre is described in US Application 4 934 429.

The solutions described above are costly and sensitive to use, more particularly in the case of tyres for heavy vehicles having a form ratio of less than one unit. Many inventors, and in some cases for a long 3 time, have thought about replacing the crown reinforcements formed of plies of crossed cables, or the mixed crown reinforcements formed of plies of crossed cables and plies of circumferential cables, with reinforcements formed of exclusively circumferential reinforcement elements.

French Patent 1 198 141 describes a tyre having an external travelling surface which is approximately cylindrical. Between the carcass reinforcement and the tread there is arranged a hooping reinforcement formed of one or more circumferential reinforcement elements embedded in a plastics material.

French Patent 2 429 678 advocates such a crown reinforcement of circumferential cables for a tyre having a form ratio of between 0.40 and 0.65, the reinforcement being obtained by helically winding a cable with a pitch which is 1 to 4 times the diameter of the cable.

European Patent Application 0 093 451 A2, having explained that the exclusive use of circumferential cables for producing a crown reinforcement was abandoned in favour of more complex reinforcements making it possible to obtain sufficient lateral rigidity to respond to the action of the transverse forces to which the tyre is subjected when travelling under drift, nevertheless proposes a crown reinforcement architecture having circumferential cables covered in a layer of rubber having a high elasticity modulus and possessing very specific properties in order to improve and to reduce the rolling resistance of the tyre.

The idea of using a coating having a very high modulus is taken up again in European Patent Application 0 200 055 A2, with the apparent aim of improving the course-holding properties of the tyre described in the European patent application cited above.

US Patent 4 691 752, in the attempt to reduce or even to cancel out the transverse thrust at 0 slip angle, more commonly referred to as "ply-steer", proposes the combination of a crown ply of circumferential cables with a belt formed of rubber reinforced with short fibres and having a rigidity under tension of at least 10 kg/mm 2 Application EP 0,280,674-A2 describes a tyre comprising a crown reinforcement made up of at least three plies of metal reinforcement elements between a radial carcass reinforcement and a tread, at least one of said plies being placed between a radially inner ply and a radially outer ply, and consisting of two lateral parts between which a layer of rubber is provided, the Shore A hardness of which is at least 10 points less than the Shore A hardness of the rubber compound used for the crown plies. The layer of rubber axially between lateral parts can have a lenticular profile.

The solutions proposed above do not make it possible to obtain the satisfactory wear resistance of a tyre intended to bear heavy loads and having a form ratio of less than 1, whilst preserving very good general strength, and such that the mounting of such tyres permits the improvement inthe behaviour of the 20 vehicle thus equipped on the road, in particular when attempts are being made to minimise the effects due to rutting of the roads.

In order to do away with the problems of separation between crown reinforcement plies whilst obtaining good wear resistance, sufficient road-holding and lower rolling resistance, the invention proposes a tyre including firstly a 25 carcass reinforcement formed of at least one ply of radial cables and secondly a crown reinforcement formed of at least one quasi-cylindrical ply of continuous or "interrupted circumferential reinforcement elements, radially surmounted by several layers of rubber including a tread, said tyre characterised in that the ply of circumferential reinforcement elements includes metallic, inextensible main 9.99 elements said main elements having minimum transverse and radial dimensions at least equal to 0.09 times the square root of the circumferential dIus R s of said ply, the fill factor of said ply is at least equal to 0.70, and said ply is surmounted by a lenticular rubber piece of a high modulus of elongation of at least 20 MPa, of crescent-shaped cross-section, having a maximum thickness at least equal to once the maximum radial dimension of the main reinforcement elements, and an axial width Li at most equal to 80% of the axial width L of the ply of reinforcement elements, said width L being at least 0.70 S, S being the maximum axial width of the tyre, mounted on its operating rim and inflated to its operating pressure, and at least equal to 90% of the axial width L 0 of the tread.

"High modulus of elongation" is to be understood to mean a secant modulus, measured at 10% relative elongation, of at least 20 MPa.

The ply of circumferential reinforcement elements is said to be quasicylindrical if its transverse radius of curvature, within the width defined for-the lenticular rubber piece, is at least 3 times its circumferential radius whether this transverse radius be convex or concave.

The axial width of the widest ply of circumferential reinforcement elements is preferable at least 0.70 S, S being the maximum axial width of the tyre, mounted on its operating rim and inflated to its operating pressure, and at least of the axial width of the tread. The road-holding of the tyre, that is to say the ratio of the transverse force exerted by the ground on the tyre to the vertical load supported by said tyre, is thus improved when travelling under a slip angle.

20 A ply of reinforcement elements is a composite assembly formed by the reinforcement elements on one hand and by the coating rubber for said elements on the other hand, said rubber making it possible to fill the spaces between reinforcement elements and to obtain layers of a substantially constant thickness on the upper and lower sides of the elements; said elements may be continuous or interrupted. The fill factor of the ply is then the ratio of the volume occupied in the ply by the reinforcement elements to the total volume. The ply of reinforcement elements may comprise only main reinforcement elements, that is to say elements capable of resisting the tensile, compressive and bending stresses due to inflation, loading and travelling in a straight line and/or with drift of the tyre. Said main elements absorb at least 90% of the inflation stresses, calculated by the formula Tep. N/Rs.p, in which Tep is the tension of the main elements, N is the number of elements per cm of ply, R s is the circumferential radius of said ply, and p the inflation pressure of the tyre in bars. The fill factor may however be advantageously increased by the presence of secondary reinforcement elements, placed axially between the main elements and having shapes such that the so-called coupling surfaces, axially opposite the main and secondary reinforcement elements are larger, which permits an increase in the shearing strength between main elements and, consequently, an increase in the transverse rigidity of the crown reinforcement.

The road-holding of the tyre according to the invention can be increased by the presence, radially on the outside of the lenticular rubber piece, of at least one protective ply. Said ply may be formed of cables oriented relative to the circumferential direction at a certain angle a which may be between 30' and 90', which gives rise to a residual transverse thrust when travelling in a straight line which may be beneficial in certain configurations of use. The preferred architecture of the protective ply consists in having two protective half-plies located axially on either ,49 side of the equatorial plane of the tyre and formed of metal cables forming, for one half-ply an angle +a and for the other half-ply an angle a possibly being between 30 and 600, with the circumferential direction. The angles formed with the circumferential direction by the cables of the two half-plies may be different, for example +a for one half-ply and -3 for the other half-ply.

It is possible to replace the two half-plies described above by two axially continuous plies of reinforcement elements crossed from one ply to the next, forming a certain angle with the circumferential direction. Said elements could be elastic cords or cables, forming angles of between 30' and 45' with the circumferential direction. They could also be non-elastic cords or cables forming angles greater than 450 with the circumferential direction. The plies formed of such elements will have a resistance to rupture per centimetre of ply at least three times smaller than the resistance to rupture of the ply of circumferential main elements, and will be constituted such that they only absorb or less of the stresses due to the inflation of the tyre.

The characteristics and advantages of the present invention will be better understood with the aid of the following description, which refers to the drawings illustrating examples of embodiment in non-limitative manner, in which drawings Figure 1 shows a diagram, viewed in meridian section, of a tyre according to the invention, Figures 2A to 2E show a diagram of the portions of crown plies where main reinforcement elements and secondary reinforcement elements run alongside one another, Figures 3A and 3B show a diagram, viewed in meridian section and from above, of a preferred crown reinforcement according to the invention, Figures 4A and 4B show a diagram of other variants of the crown reinforcement according to the invention.

In Figure 1, the heavy-vehicle tyre of dimension 315/50-22.5 has an H/S form ratio substantially equal to 0.50, H and S being the height and the maximum axial width of the tyre mounted on its operating rim J and inflated to its operating pressure, respectively. Said tyre P comprises a carcass reinforcement formed of a single ply of cables of aromatic polyamide, anchored in each bead by winding about a bead wire forming an upturn The carcass ply is hooped by a single crown ply formed of circumferential inextensible reinforcement elements (30) which are none other than inextensible metal cables made of steel, that is to say cables having a relative elongation of at most 0.5% under a force equal to 10% of their rupture force. Circumferential reinforcement elements are elements forming an angle of 0' 2.50 with the circumferential direction. These cables, in the absence of any secondary reinforcement element, are cables having a relatively large cross-section compared with the section of cables which are normally used in the afore-mentioned dimension, to resist the tensile stresses generated by the operating inflation pressure of said tyre, multiplied by a reasonable safety factor.

The reinforcement elements used in the crown reinforcement of the present invention have a circular crosssection of a diameter equal to 2.8 mm, which corresponds to 0.135 times the square root of the circumferential radius R s of the ply and in the case described are normally-constituted cables, that is to say cables formed of a central core on which there are wound in a spiral several strands of elementary cords, the stranding possibly being in several layers.

The ply has an axial width L of 260 mm, or 0.91 times the width of the tread L 0 or alternatively 0.82 times the maximum axial width S. The ply is radially adjacent to the carcass ply over a large width, whilst the edges thereof are separated from said ply by rubber wedges The ply is surmounted radially by a crescent-shaped lenticular piece or pad of vulcanised rubber having a maximum thickness at the level of the equatorial plane XX' of the tyre, which thickness is equal, for the dimension in question, to 6 mm, that is to say 2.14 times the diameter of the cables of the ply and an axial width L 1 equal to 150 mm, or 61% of the width L of the ply The rubber of the lenticular piece has a secant modulus of rigidity under tension at relative elongation of 40 MPa. The lenticular piece is itself covered with a layer of rubber referred to as lower tread layer, the rigidity modulus of which, measured under the same conditions as previously, is at most 6 MPa, that is to say very much less than the modulus of the same name of the lenticular piece A so-called upper tread (6) completes the crown of the tyre, said tread being joined to the beads of the tyre by outer rubber sidewall pieces and inner rubber pieces Figure 2A shows a very partial view of a ply of a tyre according to the invention. The reinforcement elements (30) of this ply have a cross-section of a shape known per se, since it is elliptical, but are arranged in the ply such that the major axes of said elements are parallel to one another and parallel to the equatorial plane XX'. The above preferred orientation permits better coupling between adjacent elements owing to the increase in the effective coupling surface, that is to say the total surface of adhesion of the adjacent elements to the portion of coating rubber which separates them. There is thus generated a greater overall inter-element shear force, without the pinpoint stresses at the level of the join between elements and rubber being increased. Said elliptical elements or cables may be formed of a core (32) of any shape, and constituted either of plastics material or of metal, said core being surrounded either by one or several plastic and/or metal cords or cables or by a layer (33) of plastics material, or by a layer (33) of elastomeric material of a very high modulus.

The effective coupling surface can be considerably enlarged by the presence between two main reinforcement elements which in this case are metal cables, of secondary reinforcement elements as shown in Figures 2B to 2D. In Figure 2B, the secondary element (31) is a strip of brass-coated metal of low thickness e, equal in the example described to 0.2 mm, said strip being curved circularly at its lower and upper ends and wound, in order to produce the ply, like the main reinforcement elements. This strip of rectangular cross-section may stick to the coating rubber of the main elements and may be interrupted along the circumference of the ply Preferably, the strip (31) has radial notches on its circumference, which alternate between the radially upper edge and the radially lower edge of the strip.

Figure 2C shows a secondary reinforcement element (31) of optimised transverse shape: its lateral walls are in fact parallel to the transverse contour of the main elements in this case cables of circular section, which makes it possible to make the coupling surfaces maximal and to leave between the lateral walls S respectively of the elements (31) and of the cables the nominal thickness of rubber, that is to say the thickness strictly necessary for sufficient adhesion between reinforcement elements and rubber, for the strength of the tyre. The structure illustrated in Figure 2D is the simplest to use industrially, the secondary reinforcement elements (31) being simple cables of metal or of plastics material of smaller diameter than that of the main cables and each gap between two main elements being provided with two secondary elements on either side of the median line of the ply The above three cases relate to the addition of secondary reinforcement elements separated from the main elements by rubber. It is also possible to have secondary reinforcement elements integral with the main elements such as shown in Figure 2E: the cables (30) are then covered in sheaths which may be plastic, metallic or elastomeric sheaths with a high modulus and make it possible to impart to the assembly a transverse shape more favourable for producing shearing strength.

Figures 3A and 3B show a preferred variant of a tyre according to the invention. The improvement made consists in arranging two half-plies of socalled elastic metal cables radially on the outside of the lenticular rubber piece A cable is said to be elastic if it has a relative elongation of at least for a force equal to 10% of the rupture force.

The half-plies in the case described, are arranged symmetrically relative to the equatorial plane of the tyre both from the point of view of widths and of angles, the elements of the half-ply being oriented with an angle +c of and the elements of the half-ply being oriented with an angle -a of they could be oriented asymmetrically. The axial width of each of these half-plies is at most equal to 48% of the axial width of the hooping ply such that the axially outer end of each is axially on the inside of the corresponding end of the ply and that the axially inner end is separated from the equatorial plane by a distance equal to at least 3% of the width L of the ply Figure 4A shows the variant of crown reinforcement in which the lenticular rubber piece is radially surmounted by two plies (10) of cables of aromatic polyamide crossed from one ply to the next and forming an angle of 45 with the circumferential direction, this latter arrangement making it possible firstly to improve very substantially the road behaviour of the heavy vehicle equipped with tyres having such crown reinforcements, and secondly to provide sufficient protection for the main crown reinforcement in the case of construction and transportation tyres. The crown reinforcement variant illustrated in Figure 4B differs from that of Figure 4A by the widths of the two plies said width being less, whilst retaining the same overall axial width for both plies, but only permitting the crossing of the reinforcement elements over a reduced width of at least 50% of the overall width i.

Claims (9)

1. A tyre including firstly a carcass reinforcement formed of at least one ply of radial cables and secondly a crown reinforcement formed of at least one quasi- cylindrical ply of continuous or interrupted circumferential reinforcement elements, radially surmounted by several layers of rubber including a tread, said tyre characterised in that the ply of circumferential reinforcement elements includes metallic, inextensible main elements, said main elements having minimum transverse and radial dimensions at least equal to 0.09 times the square root of the circumferential radius R S of said ply, the fill factor of said ply is at least equal to 0.70, and said ply is surmounted by a lenticular rubber piece of a high modulus of elongation of at least 20 MPa, of crescent-shaped cross-section, having a maximum thickness at least equal to once the maximum radial dimension of the main reinforcement elements, and an axial width L 1 at most equal to 80% of axial width L of said ply, said width L being at least 0.70 S, S being the maximum axial width of the tyre, mounted on its operating rim and inflated to its operating pressure, and at least equal to 90% of the axial width L 0 of the tread.

2. A tyre according to claim 1, characterised in that said fill factor is increased Sby the presence of secondary reinforcement elements, placed axially between the main elements.

3. A tyre according to claim 2, characterised in that the secondary elements are strips of brass-coated metal of low thickness e, each strip being curved circularly at its lower and upper ends and being interrupted along the circumference of the ply of reinforcement elements, or having radial notches on its circumference which alternate between the radially upper edge and the radially lower edge thereof. 14

4. A tyre according to claim 2, characterised in that the secondary reinforcement elements have an optimised transverse shape, their lateral walls being parallel to the transverse contours of the main elements of circular section, which makes it possible to leave between the lateral walls respectively of the secondary elements and the main cables the thickness of rubber strictly necessary in order to have sufficient adhesion between reinforcement elements and rubber.

A tyre according to claim 2, characterised in that the secondary reinforcement elements are cords or cables, of metal or of textile material or of plastics material, of smaller diameter than that of the main cables.

6. A tyre according to claim 2, characterised in that the secondary reinforcement elements are integral with the main elements, said main elements then being covered in sheaths which are plastic, or elastomeric with high hardness, or metallic, making it possible to impart to the assembly a transverse shape more favourable for producing shearing strength.

7. A tyre according to claim 1 wherein said lenticular rubber piece is covered radially on the outside by at least one protective ply, formed of cords or cables oriented relative to the circumferential direction at an angle o of between 300 and 900

8. A tyre according to claim 7, characterised by the presence of two *protective plies of elastic or non-elastic cords or cables, crossed from one ply to the next, forming with the circumferentialdirection an angle of 450 V

9. A tyre according to claim 7 characterised in that the protective ply is formed by two half-plies of elastic metal cables oriented relative to the circumferential direction by an angle a, which plies are arranged symmetrically relative to the equatorial plane XX' of the tyre, and each having an axial width of at most 48% of the axial width L of the ply of reinforcement elements, their axially inner ends being separated from said equatorial plane by a distance equal to at least 3% of the width L of the ply of reinforcement elements. DATED this 31"s day of July 2000 COMPAGNIE GENERALE DES ETABLISSEMENTS MICHELIN -MICHELIN CIE WATERMARK PATENT AND TRADE MARK ATTORNEYS 290 BURWOOD ROAD HAWTHORN VICTORIA 3122 AUSTRALIA SKP:DHS:VRH P10327AU00.DOC S o 5 9 **o G