GB2080845A

GB2080845A - Metal cords for reinforcing elastomeric articles

Info

Publication number: GB2080845A
Application number: GB8122882A
Authority: GB
Original assignee: Dunlop Ltd
Current assignee: Dunlop Ltd
Priority date: 1980-07-29
Filing date: 1981-07-24
Publication date: 1982-02-10
Also published as: LU83518A1; FR2487866B1; AU7347481A; ZA815052B; DE3129963A1; FR2487866A1; JPS5756592A; BE889792A; NL8103579A; IT8123210A0

Abstract

A construction of metal cords 30 having a core 31 and a coil formed of elementary wires 33 coiled in parallel with one another around the core is disclosed which enables better penetration of a coating material to be obtained. In accordance with the invention, the dimension of the core 31, the number and the diameter D of the elementary wires 33 and their relative arrangement are such that the ratio R between the space e between the wires 33 and the sum of this space e and of the diameter D of the elementary wires 33 is at least 0 &cirf& 08 and is preferably between 0 &cirf& 1 and 0 &cirf& 25. The cord is particularly suitable for use in the construction of tyres, especially for heavy goods vehicles. <IMAGE>

Description

SPECIFICATION Metal cords for reinforcing elastomeric articles The present invention concerns a metal cord for reinforcing elastomeric articles for example a tyre.

Tyres of so-called "radial" construction, in particular those which are intended for heavy vehicles, generally have several layers of metal cords, beneath the tread, which together constitute a relatively inextensible reinforcement.

These cords usually have a core surrounded by one or more layers or coils of strands or of elementary wires arranged in such a way that they entirely cover the core. The latter may itself be made of one or more wires, arranged in parallel or twisted together.

Experience has shown that tyres formed with such cords have a serious disadvantage when they meet penetrating objects, such as pins, screws, stones with sharp edges, or following an accidental cut being made in the tread. The damage which results allows the water which is encountered on the road, and which might contain impurities or salts, to penetrate as far as the cords of the outer reinforcement layer, and often even as far as one or more of the underlying layers.

A minute examination of worn tyres containing such cords, particularly at the time of retreading, shows that the water or the moisture has a tendency to infiltrate between the different wires of the cord, by capillary action, and to travel along them, often to the ends.

Surface oxidation of the steel in the cord (and/or of the brass which generally covers the steel in the form of a very thin coating) then takes place; the bond between the rubber mixture coating the cords and the metal is then broken, and local detachment takes place and spreads, which either leads to the tyre being put out of use, or makes it necessary to carry out significant repair work when retreading.

French Patent No 2 001 714 describes a metal cord having a core formed by a strand, and an outer layer formed of strands which are relatively far apart. The space between the strands and the fact that one strand, in cross-section, has a smaller section than that of a full wire of the same outside diameter, allows satisfactory penetration of the rubber when coating the cords.

However, these cords have a disadvantage in that they are expensive. For this reason, the threewire strand has been replaced, for example, by single wires of greater diameter than that of each wire in the strand. Thus, strands having three wires with a diameter of 022mum have been replaced by single wires with a diameter of 038mum.

French Patent No 1 264 878 describes an example of a known cord having a core formed by three elementary wires twisted together, and an outer layer formed by elementary wires having a diameter such that they are not contiguous around the core. British Patent No 1 034 326 describes another cable in which the core is formed by two elementary wires only, twisted together, whilst the outside layer or coil is formed of six elementary wires which are not contiguous.

However, tyres made with such cords, although they are lower in price than tyres made with cords having strands in the outer layer, still have the abovementioned disadvantage of allowing water penetration by capillary action. The parallel elementary wires of the outer layer touch one another along lines or are very close together, whereas strands, even when they are side by side, leave a clearance through which the rubber can pass.

In addition, the cords corresponding to the latter two patents mentioned above usually offer inadequate mechanical characteristics or are too stiff, which makes them unacceptable for.

certain tyres for heavy goods vehicles.

It is an object of the invention to provide a construction of metal cords having an outer layer formed of elementary wires which mitigates the disadvantages of the known cords but nevertheless allow good penetration of the rubber.

Understanding of the invention requires the introduction of the idea of "opening ratio". The opening ratio, applied to metal cords having a layer of elementary wires, is determined in a plane perpendicular to the longitudinal axis of the cord and is defined herein as the ratio R between the distance e separating two elementary wires, and the sum of the diameter D of the wires and this distance e. Therefore we have e e + D According to the present invention a metal cord for reinforcing an elastomeric article comprises a core having a longitudinal axis and a coil formed of elementary wires wound in parallel to each other around the core, the dimensions of the core, the number and the diameter of the elementary wires and their relative arrangement being such that the opening ratio of the elementary wires as herein defined is at least 0-08.

In accordance with the invention, it is established that water or moisture cannot progress along the cords, for example after damage to a tyre, when the coating rubber penetrates perfectly into the cord, and leaves no continuous channel between the wires which form it, this penetration of the rubber being obtained when the cords have an opening ration of at least 0 08 and more preferably an opening ratio of between 0-1 and 0-25.

It is advantageous that the number of elementary wires in the coil should be greater than or equal to 7 since the cords formed then have a high breaking strength with reasonable stiffness and an acceptable cost price.

The core may be formed by a strand or a section, for example being thermoretractable, having a suitable cross-section and containing grooves, for example. The core may also have a multiple structure and may itself have a coil formed of elementary wires around a primary core, the coil than having, to the best advantage, an opening ratio of at least 0-08, in accordance with the invention.

In a first embodiment of the invention, the cords have a structure n1/, + n2/2. The core is formed of n1 elementary wires (n1 being greater than or equal to 1) of the same diameter 1, for example between 0 20 and 0 30mum (to advantage between 0 20 and 028mum), twisted together with a pitch which may for example be between 8 and 1 6mm so that they form a strand.In certain core constructions, a wire of the same diameter or of a smaller diameter is placed at the centre and the wires of the core strand are wound onto this central wire with an opening ratio of at least 0-08. The coil or outer layer of the cord, arranged around the core, is made of n2 elementary wires of the same diameter 2 for example between 0 25 and 0 40mum (to advantage between 0 30 and 0 38mum), wound together parallel to one another around the core with a pitch which may for example be between 1 2 and 22mm.The number and diameter of the elementary wires of the outer layer are such that the opening ratio is at least 0 08 and is preferably between 0 1 and 0-25.

The direction of twisting of the core wires and the direction of winding the wires of the outer layer may be the same, the pitches, however, being sufficiently different to ensure that the wires are only in contact at certain points. The directions may also be opposite, as specifically indicated by the following examples.

In accordance with a second embodiment of the invention, the cords have a structure of the n1/1 + n2/02 type described above but, when carrying out the twisting operation on the wires in the coil or outer layer, the tension is periodically relaxed. In this way, the wires of the outer layer do not tighten round the cable in distant areas along the cord. The rubber penetration is thus improved.

In accordance with a third embodiment of the invention, the core of a cord of one of the above types is replaced by a cord or a section formed of a thermoretractable material such as polyamide 6 or 6-6, known by the term "nylon" and polyester. When the core is made of a section, it is advantageous for this to have a rounded cross-section and to have grooves on the surface which cross the elementary wires of the outer layer. When vulcanising a tyre containing such cords in its reinforcement, at high pressure and temperature, the shrinkage of the cord core and its shortening encourage the penetration of the coating mixture around the wires of the outer layer and specifically at the ends, which always constitute a delicate area of a tyre with radial casing.

In accordance with a fourth embodiment of the invention, a cord has two successive layers of elementary wires, surrounding the core or the preceding layer, and having elementary wires, the number and diameter of which are such that each of the layers has an opening ratio of at least 0-08 and preferably between 0 1 and 025.

In addition, a lapping wire, sometimes called a "lacing", may be arranged around the cords of the different embodiments of the invention The cords in accordance with the invention offer very significant qualities when they are used in the building of tyres. First of all, as indicated above, thanks to their high opening ratio, they allow good penetration of the coating mixture used for calendering the plies, specifically in the case of tyres for heavy goods vehicles. It is known that these mixtures have a high viscosity. In addition, the cords which contain elementary wires in the outer layer are of good compactness.

This characteristic is very interesting because fdr equal mechanical strength properties, the thickness of the layers formed is reduced.

The space between the wires of the cords in accordance with the invention and the penetration of rubber between these wires reduce the stiffness and eliminate practically all breakages of the cords due to friction between the elementary wires. This defect is known in the trade by the term "fretting".

Cords in accordance with the invention also have good axial stability, especially when the direction of twisting of the core and the direction of winding of the wires in the coil or outer layer are opposite. This axial stability is accompanied by good stability of the core when the cord is subjected to deformations, and by good compression and tensile strength properties. In addition, when using layers reinforced with cords in accordance with the invention during the construction of tyres, cutting of the layers is clean and does not lead to deformation.

Cords in accordance with the invention may be used in very large tyres, intended to carry very heavy loads, and specifically in tyres for heavy goods vehicles.

Other characteristics and advantages of the invention will be better deduced from the following description, given by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a cross-section of a known cord having an outer layer formed of strands; Figure 2 is a cross-section of a known cord where the outer layer, formed of elementary wires, does not allow adequate penetration of the rubber; and Figures 3 to 8 are cross-sections of several embodiments of cords in accordance with the present invention.

The cord 10 shown in Fig. 1 having the structure 1 X 4/0-22 + 6 X 4/0-22 + 1 /0-15 has been generally used in the tyre industry for many years and comprises seven strands 11, each having four wires 1 2 of 0-22mm diameter, and it is surrounded by a lapping wire 1 3 with a diameter of 0-1 5mm. The core is formed by one of the four-wire (diameter 0-22mm) strands 11, and the outer layer is formed by the remaining six strands. This cord has very good mechanical properties, but it price is high.

Figure 2 is a cross-section of a cord 20 having a 1 X 3/0-20 + 6/0-38 structure now being used in the tyre industry, and comprises a core formed by a strand 21 made up of three wires 22, each of diameter 0 20mum, and of an outer layer formed of six elementary wires 23 of 0-38mm diameter each. Experience has shown that this cord does not permit good penetration of the rubber and the opening ratio as herein defined is well below the lower limit for the opening ratio of the cord in accordance with the present invention.

Fig. 3 shows a cross-section of a first embodiment of a cord 30 in accordance with the invention. The cord 30 has a 4/0-20 + 7/0-30 structure, i.e. it has a central core strand 31 of four wires 32 of 0 20mum diameter, and seven elementary wires 33 forming a coil and having a diameter of 0-30mm.

Fig. 3 also shows how the opening ratio is evaluated. Space e represents the distance separating two adjacent elementary wires 33 of the coil, these wires each having a diameter D.

The opening ratio R is equal to the ratio between the space e and the sum of the space e and the diameter D. The cord 30 in Fig. 3 has an opening ratio R of 0-177, and experience has shown that it permits good penetration by a mixture of high viscosity, used for calendering the piles for heavy goods vehicle tyres.

Fig. 4 is a cross-section of a second embodiment of a cord 40 in accordance with the invention, derived from the cord 30 shown in Fig. 3. The cord 40 has a 1/0-12 + 4/0-20 + 7/0-30 structure, i.e. it has a wire 41 of diameter 0 12mum forming a primary core, surrounded by four wires 42 of diameter 0 20mum, this assembly forming the core. The opening ratio R of the outer layer comprising the four wires 42 of the core is equal to 0-15.Around this core, seven elementary wires 43 of diameter 0-30mm form a coil, the opening ratio R of which is 0-1 76. The central wire 41 forming the primary core thus ensures spacing between the wires 42 of the strand, compared with the embodiment in Fig. 3, and therefore a greater space between the wires 43 of the outer layer.

Fig. 5 is a cross-section of a third embodiment of a cord 50 in accordance with the invention.

The cord 50 has a (1 + 6)/0-25 + 8/0-35 structure, i.e. it has a core made up of seven wires 51 with a diameter of 0 25mum and an outler layer formed of eight elementary wires 52 of 0-35mm diameter twisted onto the core. The opening ratio R of this cord is 0-169.

Fig. 6 is a cross-section of a fourth embodiment of a cord 60 in accordance with the invention, having a 1 X 3/0-20 + 6/0-38 structure, similar to the one in Fig. 2. However, when producing the cord 60 in accordance with the invention, the elementary wires 61 of the outer layer undergo periodic relaxations so that they are not tight against the core 62 but are separate from the latter. Thus, in Fig. 6 the outer wires 61 are separated from the core 62 by a distance xof 0 1 mum. Therefore, the opening ratio R attains a value of 0-248 and the rubber penetration is excellent.

Fig. 7 is a cross-section of a fifth embodiment of a cord 70 in accordance with another embodiment of the invention. The cord 70 has a core 71, formed of "Nylon" 66, having an outside diameter of 0-6mm and six helicoidal grooves 72 of 0 1 mum depth formed in the peripheral surface. Eight wires 73 of 0 30mum diameter are wound onto the core 71. The opening ratio R of the wires 73 is 0-1 3. The rubber penetration is facilitated by the grooves 72 in the core 71 and by the shrinkage which takes place during vulcanisation of a tyre containing cords having such cores. This shrinkage, which only affects the core 71, causes a separation of the outer wires 73.

Fig. 8 is a cross-section of a sixth embodiment of a cord 80 in accordance with the invention, having a very high mechanical strength. The cord 80 is intended for tyres of very large dimensions, used for example singly on trucks or trailers and has a 4/0-25 + 7/0-35 + 11/0-38 structure. A core formed of a strand 81 made up of four wires 82 of 0.25mm diameter is surrounded by a first layer of seven wires 83 of 0.35mm diameter, having an opening ratio R of 0-1 54 and a second layer of eleven wires 84 of 0-38mm diameter having an opening ratio of 0-20 surrounds the first layer. The intermediate layer has a pitch in the opposite direction to that of the core and tl, outer layer.

The following table compares the characteristics and properties of various known cords (Example 1 and 2) and cords in accordance with the invention (Examples 3 to 10).

In this table, the structure is indicated as described above. The construction indicates the twisting or winding pitch, and its direction (S or Z). The opening ratio is as herein defined. The breaking strength is measured on a dynamometer. The stiffness can be measured with a "Taber" rig, manufactured by Instrument Corporation (U S A), the values representing a torque in (10 4N.m.).

TABLE Structure Construction Opening Breaking Mean Theoretical ratio strength Stiffness outside R N 10-4N.m diameter mm 1. 1 x 4/0,22 + 6 x 4/0,22 9,7 S 18 Z low 2600 126 1,78 PRIOR ART 2. 3/0,20 + 6/0,38 10 Z 16 S 0,063 1776 168 1,20 PRIOR ART 3. 4/0,25 + PRESENT 7/0,30 10 Z 16 S 0,118 1540 85 1,07 INVENTION 4. 3/0,25 + PRESENT 7/0,35 10 Z 16 S 0,093 2158 147 1,25 INVENTION 5. 4/0,25 + PRESENT 7/0,35 10 Z 16 S 0,154 2158 157 1,31 INVENTION 6. 3/0,28 + PRESENT 7/0,38 10 Z 16 S 0,11 2158 219 1,37 INVENTION 7. 7/0,25 + PRESENT 8/0,35 10 Z 16 S 0,169 2865 206 1,47 INVENTION 8. 7/0,25 + PRESENT 9/0,35 12,5 Z 18 S 0,13 2884 226 1,45 INVENTION 9, 3/0,20 + PRESENT 6/0,38 10 Z 18 S 0,248 1776 168 1,40 INVENTION clearance core/outer layer = 0,1mm 10. 4/0,25 + 10 S 16 Z 1 e C 0,154 PRESENT 7/0,35 - 22 S 2 e C 0,20 5101 593 2,064 INVENTION 11/0,38

Claims

1. A metal cord for reinforcing elastomeric articles comprising a core having a longitudinal axis and a coil formed of elementary wires wound in parallel to each other around the core, the dimensions of the core, the number and the diameter of the elementary wires and their relative arrangement being such that the opening ratio of the elementary wires as herein defined is at least 0-08.

2. A cord in accordance with Claim 1, wherein the opening ratio is between 0 1 and 0-25.

3. A cord in accordance with Claim 1 or Claim 2, wherein the coil contains at least seven elementary wires.

4. A cord in accordance with any one of the preceding Claims, wherein the core is formed by a strand.

5. A cord in accordance with any one of Claims 1 to 3, wherein the core contains elementary wires of the same diameter twisted together around a central wire of equal or smaller diameter, the outside elementary wires of the core having an opening ratio as herein defined of at least 0 08.

6. A cord in accordance with any one of Claims 1 to 3, wherein the core is formed by a section of thermoretractable material.

7. A cord in accordance with Claim 6, wherein the core is formed with grooves in the peripheral surface thereof.

8. A cord in accordance with any one of Claim 1 to 5, wherein the wires forming the core have a diameter of between 0-20 and 0-28mm, and a pitch of between 8 and 16mm.

9. A cord in accordance with any one of the preceding Claims, wherein the wires forming the coil have a diameter of between 0 30 and 0 38mum and are coiled with a pitch of between 12 and 22mm.

10. A cord in accordance with any one of Claims 1 to 5, 8 and 9, wherein the direction of twisting of the core wires is opposite to the direction of coiling of the coil wires.

11. A cord in accordance with any one of Claims 1 to 5, and 8 to 10, wherein the elementary wires of the coil are spaced from the core.

12. A cord in accordance with any one of the preceding Claims, wherein the core is surrounded by a first layer of elementary wires wound in parallel to each other, and then by the coil, the number and diameter of the wires in the first layer and in the coil being such that the opening ratio as herein defined of each is greater than 0 08.

1 3. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.

14. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 4 of the accompanying drawings.

1 5. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 5 of the accompanying drawings.

16. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 6 of the accompanying drawings.

1 7. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 7 of the accompanying drawings.

1 8. A metal cord for reinforcing elastomeric articles substantially as hereinbefore described with reference to Fig. 8 of the accompanying drawings.

19. An elastomeric article incorporating a metal cord according to any one of the preceding Claims.

20. A tyre incorporating a metal cord according to any one of Claims 1 to 18.