The invention relates to a bead reinforcement and particularly to a bead reinforcement of pneumatic tires. The invention also relates to a process of manufacturing a bead reinforcement.

Since a couple of years there has been a general demand for pneumatic tires of a lower weight. The reason behind this demand is saving of fuel consumption of vehicles. This demand has been strengthened by a special tax law on heavy weight truck tires in the U.S.A.

Great efforts have been done in the field of steel cord for the reinforcement of the breaker part and the carcass part of the tire in order to reduce the weight of that steel cord.

The bead area, although being a substantial part of the tire, has remained unchanged because of the following reasons :

There are two main groups of bead reinforcements : cable beads and non-cable beads.

Non-cable bead reinforcements often comprise flat wires. The manufacturing process of these flat wires is very expensive because of the wear of some necessary machine parts. Increasing the strength of these wires with a view to reduce the weight of the tire means a much higher increase of the wear because of the reduction in ductility of the wires with a higher strength.

Cable beads are bead reinforcements which comprise wires which are twisted with each other. During this twisting process part of the theoretical breaking load always gets

lost. (By theoretical breaking load is meant the breaking load of the individual wires multiplied by the number of wire the reinforcement is composed of.) The experience with tire cords, which are also twisted structures, has shown that this loss of theoretical breaking load may increase if the twisted structure is composed of steel wires with an ..increased tensile strength so that the increase of tensile strength of the individual steel filaments does not always result in a structure with an increased breaking load.

Another problem with bead reinforcements, especially with bead reinforcements of the n x m package type are their stability.

A bead reinforcement of the n x m package type comprises n plies lying side by side in axial direction, n being an integer number between two an twelve, preferably between three and nine. Each ply consists of m radially extending layers, m being an integer number between two and twelve, preferably between three and nine.

Each of said plies is formed by at least one steel wire which has at least two parallel flat surfaces and which is wound in m radially extending layers in such a way that the radially inner flat surface of a following layer contacts the radially outer flat surface of a previous layer.

Such a bead reinforcement is preferably used in truck tires.

Such a bead reinforcement and its function are well known in the art and are disclosed e.g. in US-A-3 949800.

It is also well known in the art that, especially in the case of tube!ess tires, the bead seat may form a certain angle α with respect to the axis of the tire. The angle a convenient¬ ly lies between 5 and 20 ^* (degrees) and may be e.g. 12 or

16 ^β . In most cases bead reinforcements have been designed in that way that the inner side is substantially parallel with respect to the bead seat, i.e. that the radially inner side of the bead reinforcement forms an angle β with respect to the axis of the tire, β being almost equal to a.

There are generally two ways to obtain such an "inclined" bead structure. Both of these ways are disclosed in the above-mentioned patent specification US-A-3949800.

In the first way the parallel flat surfaces of the steel wires composing the bead structure remain substantially parallel with respect to the axis of the tire but the subse¬ quent plies are each radially shifted over a fraction of the thickness of the steel wire so as to form a stepped inner (and outer) side of the bead structure with an average incli¬ nation angle of β degrees. A possible embodiment is repre¬ sented in figure 1 of US-A-3 949800. A disadvantage of this embodiment is that two adjacent plies may move with respect to each other and hence form an unstable structure.

In the second way the parallel flat surfaces of the steel wires composing the bead structure form the same angle β as the inner side of the whole bead reinforcement structure forms with respect to the axis of the tire. Examples of such an embodiment are disclosed in figures 5 and 6 of US-A-3 949 800. Such a bead structure is, however, often unstable because each wire tries to regain a position where its flat sides are parallel with respect to the axis of the tire. If each wire remains in its position, then it is sub¬ jected to great internal stresses and its tensile strength is decreased.

In all the examples of US-A-3 949800 the cross-section of the steel wires is quadrangular, i.e. the steel wires have four flat sides.

On the other hand, patent application DE-A-2810847 dis¬ closes bead structures comprising flat wires which extend axially or radially but which do not form a bead reinforce¬ ment of the n x ra package type as described hereabove.

With a flat wire is meant a wire the cross-section of which has two long flat sides which are parallel with respect to each other and which are connected by means of two convex curves so as to form two protruding parts having a thickness smaller than the distance between the two long flat sides. A difference must be made between a flat wire and a rectangu¬ lar wire having a rectangular cross-section the angles of which are rounded but which does not show pronouncedly pro¬ truding parts.

The distance between the two long flat sides is called the "thickness" of the flat wire. The "width" of the flat wire is the distance between the two extreme points of the convex curves. The thickness of steel wires used for bead reinforce¬ ment usually lies between 0.5 and 2.0 mm, and the width between 2.0 and 4.0 mm.

The steel- wires used for bead reinforcement may be coated with zinc or with a bronze or brass alloy in order to ensure adhesion to rubber.

it is an object of the present invention to avoid one or more drawbacks of the prior art.

It is a further object of the present invention to provide a bead reinforcement with an increased tensile strength.

It is also an object of the present invention to lower the weight of tires.

It is another object of the present invention to provide a bead reinforcement of the n x m package type with flat wires whilst still maintaining the torsion stability at a rea¬ sonable level .

It is also an object of the present invention to provide a bead reinforcement structure of the n x m package type where the radial movement of one ply with respect to another ply is prohibited.

According to a first aspect of the present invention there is provided a bead reinforcement comprising flat steel wires made by rolling and having a cross-sectional surface of at least 2.0 mm ² , e.g. at least 2.5 mm ² . The carbon content of these steel wires is at least 0.80 per cent by weight, e.g. at least 0.82 % or at least 0.85 %. The tensile strength of these steel wires is at least 2000 N/mm ² , e.g. at least 2200 N/mrn ² .

Other contents of flat steel wires for bead reinforcements may be a manganese content between 0.40 and 0.70 %, a silicon content between 0.10 and 0.35 % and maximum sulphur and maximum phosphorus contents of 0.04 %, all percentages being percentages by weight. However, the presence of other elements is not excluded.

The flat steel wires are made by rolling since the rolling process is a simple process and is more suitable to manufacture wires with a carbon content of more than 0.80 per cent by weight than the process of manufacturing rectangular wires, by means of Turkish heads. A flat wire made by rolling has a cross-section without sharp angles but with a smooth

border line. As a consequence, stress concentrations due to the high carbon content are absent in contrast with e.g. a rectangular wire where stresses are concentrated in the neighbourhood of the angles.

Another advantage of the bead reinforcement according to the present invention is that the above-mentioned loss of theoretical breaking load is not increased with the use of steel wires having a carbon content above 0.80 %.

Preferably the construction of the bead reinforcement is such that the two long parallel sides of the cross-section of the steel wires are parallel to the axis of the tire when the reinforcement is embedded in the bead of the tire. Long sides which are inclined with respect to the axis of the tire, would have more internal stresses and the use of carbon contents above 0.80 % would be more difficult.

According to a second aspect of the invention there is provided a bead reinforcement of the n x m package type which comprises steel wires which form n plies lying side by side in axial direction. The plies consist of m radially extending layers and each ply is radially shifted over a fraction of the thickness of a steel wire with respect to an adjacent ply. The steel wires are flat wires made by rolling and the axi lly exterior sides of the cross-section of each ply show, due to said flat wires, an undulated curve with maxima and minima. The maxima of one ply fit into the minima of an adjacent ply and vice versa so as to prohibit radial movement of one ply with respect to another ply.

The fitting of maxima into minima is not an exact fitting, i.e. the minima are not an exact negative copy of the maxima.

This means that there are no contacts surface to surface but only line contacts between two adjacent plies.

Preferably the flat surfaces of the steel wires are parallel with respect to the axis of the tire. This provides a more stable construction as the steel wires do not show a tendency to regain another position. Moreover, as explained hereabove, these steel wires have a greater tensile strength.

In a preferable embodiment the radially inner layers form a stepped inner side which has an angle β with respect to the axis of the tire. This angle is substantially equal to the angle α formed by the bead seat with respect to the axis of the tire.

The invention also relates to a process of manufacturing a bead reinforcement of the n x m package type. The n steel wires which are to form the n different plies of the bead reinforcement are led to a cylindrical body. This body has a stepped outer surface which corresponds to the stepped inner side of the ultimate bead reinforcement. The place where the separate wires are first brought into contact with the body is spread over at least a part of the circumference of the body and the order in which the separate wires are first brought into contact with the body is dependent on the ultimate radial position of each ply.

The invention will be better understood after consideration of the following detailed description together with the attached figures wherein :

FIGURE 1 shows a tire bead provided with a bead reinforcement of the n x m package type according to the invention;

FIGURE 2 shows an embodiment of a bead reinforcement of the n x package type according to the invention ;

FIGURE 3 shows a cross-section of a flat wire made by rolling ;

FIGURE 4 shows a bead reinforcement of the n x m package type according to the invention which has no angle of inclination with respect to the axis of the tire ;

FIGURES 5a and 5b show how a bead reinforcement structure according to the invention is manufactured ; FIGURE 6 shows equipment for measuring the torsion stiff¬ ness of a bead reinforcement ; FIGURES 7a and 7b show curves of the measured torsion stiffnesses of bead reinforcements according to the inven¬ tion and according to the prior art.

^" Figure 1 shows a truck tire bead provided with a bead rein- forcement 1 of the n x m package type according to the inven¬ tion. The structure shown here is a 6 x 5 structure. However, the invention is not limited to a 6 x 5 structure but also applies to other n x m structures, where n and m are integers numbers between two and twelve, preferably between three and nine. Examples of other n x m structures are 7 x 6, 9 x 6, 6 x 6, 8 x 5 and 7 x 5.

The bead seat 2 shows an angle c* with respect to the axis of the tire, α conveniently lies between 5 ^* and 20 ^* and is usual¬ ly about 15 ^* . The radially inner layers of the subsequent plies form a stepped inner side which has an (average) angle β with respect to the axis of the tire, β is preferably equal to α.

Figure 2 shows an enlarged view of the cross-section of a bead reinforcement 1 of the 6 x 5 package type according to the invention. Bead reinforcement 1 has six plies 11, 12, 13, 14, 15 and 16 which ly side by side in axial direction. Each ply is formed by at least one wire and has five radially extending layers. Ill is the cross-section of the first layer

of ply 11, 112 the second layer, 113 the third layer, 114 the fourth layer and 115 the fifth layer. The same applies, mutatis mutandis, to plies 12 to 16. Cross-section 111 of ply 11 contacts cross-section 121 of ply 12. Cross-section 112 contacts cross-sections 121 and 122. Cross-section 113 contacts cross-sections 122 and 123. Cross-section 114 contacts cross-sections 123 and 124 and cross-section 115 contacts cross-section 125. The same applies again, mutatis mutandis, to the other plies 12 to 15. In this way every cross-section contacts two cross-sections of the adjacent left ply except for the cross-sections (111, 121, 131, 141, 151) of the first (radially inner) layer which contact only one cross-section and for the cross-sections (161, 162, 163, 164, 165) of the sixth ply 16 which have no adjacent left ply. Note that the contacts between steel wires of different plies are not contacts surface-to-surface but line contacts. Note also that the fitting of the maxima in to the minima is not exact, i.e. there are still some holes between adjacent plies.

Figure 3 represents an enlarged view of the cross-section (111) of a flat steel wire made by rolling. This cross-section 111 has two long straight parallel sides (1111, 1112) which are parallel to each other and which are con- nected to each other by two short convex curves (1113, 1114). These two curves (1113, 1114) have a radius of curvature R. The two curves (1113, 1114) form two protruded parts 1115 and 1116. These protruded parts have been hatched in figure 3. It is thanks to these protruding parts (1115, 1116), which have a smaller thickness than the thickness T of the cross-section 111 that the axially exterior sides of each ply show an undu¬ lated curve with maxima and minima.

- 10 -

T is the thickness of the wire. W is the width of the wire and is equal to the distance between the two most remote points of the curves 1113, 1114 measured along a line which is parallel to the long sides 1111 and 1112. 5 If, by way of example, the thickness T is 1.5 mm, the width W is 3 mm and the radius of curvature R is 0.99 mm ^" , then an angle & of 15 ^* is obtained in the bead reinforcement , structure according to the invention.

10 Another embodiment of the bead reinforcement structure according to the invention is shown in figure 4. It has a stepped inner and outer side and no global inclination with respect to the axis of the tire (β = 0 ^* )

15 Figures 5a and 5b show how a bead reinforcement structure according to the invention is manufactured. The different steel wires (11 - 17) which form the different plies of the bead structure are drawn from bobbins 20 and led to a cylin¬ der 22 which is placed between two flanges 21. The cylinder

20 22 has a stepped outer surface 23 which corresponds to the stepped inner side of the ultimate bead structure. The steel wire 11 which forms the most radially inner ply is first brought into contact with the cylinder, followed by the steel wire 12 which forms the second most radially inner ply, etc.

25 .For bringing all the steel wires in contact with the cylinder 22 at the same place is impossible because of the ultimate intermeshed structure.

Table 1 hereunder compares the tensile strength and the 30 breaking load of a flat wire with a carbon content of 0.65 % and of a flat wire with a carbon content of 0.80 ...

TABLE 1

| 3.0 x 1.5 mm | 3.0 x 1.5 mm | 1 0.65 % C 1 0.80 % C 1

breaking load | 8263 | 9362 |

(N) 1 1 i

tensile strength | 1800 | 2119 | (N/mm ² ) j j j

Table 2 hereunder shows that the loss of theoretical breaking load is not increased when steel wires with a carbon content of 0.80 % are used instead of steel wires with a carbon content of 0.65 %.

TABLE 2

I Loss of theoretical breaking load |

| S = 3.0 x 1.5 Dim |

Bead construction | 0.65 % C | 0.80 % C | n x m

6 5 | 16.4 % | 16.7 % |

6 x 6 | 12.7 % j 15.5 % j

7 x 6 | 18 % | 15.8 % j

According to the invention, using a bead reinforcement with high carbon wires (>= 0.80 % C) may result in a reinforcement which weighs ten to twelve per cent less than a steel core with only low carbon wires (<= 0.65 % C) and which provides the same reinforcing effect as these low carbon wires.

A test has been developed in order to compare the torsion stability of an in rubber embedded bead structure according to the invention with the torsion stability of prior art bead structures. Figure 6 shows equipment used for this test. The bead reinforcement structure 1, embedded in rubber, is fixed at the two positions 32. Between these positions 32 the bead reinforcement structure is clamped between two metal beams 31. The upper of said metal beams 31 is fixed to a bar 33, at one end of which a weight 34 may be hung in order to exercise a torsion to the bead reinforcement structure and at the other end of which a needle 35 is fixed. The deflection -j of the needle 35 (expressed in degrees - ^* ) may be read at the scale 36 and is a measure for the torsion stability of the bead reinforcement structure. In this way couple - deflection angle curves may be recorded. Figure 7a show such curves for an embedded 6 x 6 bead struc¬ ture having a width of 3.0 mm and a thickness of 1.5 mm. The couple is expressed in Newton-centimeter (Ncm), the angle -j in degrees ( ^* ). Curve 41 is the curve for an embedded bead reinforcement structure according to the invention with flat wires made by rolling. Curve 42 is the curve for a bead rein¬ forcement structure comprising steel wires with a rectangular cross-section. Note that the angle of inclination curve 42 is somewhat higher than the angle of inclination of curve 41, which means that the prior art bead structure is somewhat stiffer than the bead structure according to the invention as far as torsion goes. This may be explained by the fact that in the prior art structure with rectangular wires the con¬ tacts between wires of different plies are surface-to-surface contacts whereas with flat wires as in the invention this is not necessarily the case.

The same may be seen on figure 7b. Curve 43 relates to a 7 x 6 bead reinforcement structure according to the invention comprising flat wires made by rolling. Curve 44 relates to a

7 x 6 bead structure comprising steel wires with a rectan¬ gular section. The width is here also 3.0 mm, and the thick¬ ness 1.5 mm.

As a conclusion the invention provides a bead reinforcement structure comprising flat wires which, in comparison with prior art structures and according to one aspect, has a higher tensile strength, and according to a second aspect, prohibits adjacent plies to move with respect to each other and has a torsion stiffness which is only slightly smaller despite the fact that the invention has not necessarily surface contacts between the different plies.