GB2106457A - Tyre beads - Google Patents

Abstract

A tyre comprises a carcass 30 turned-up around a bead core 42 and rubber apex strip 44, and an element 46 which is made of rubber having a higher modulus than the rubber of the apex strip 44 and which is placed on the outside of and in direct contact with the carcass turn-up 40 and an outer part of the apex strip 44. The element 46 may have a modulus at 20% extension >/=1.5 MPa and a Shore A hardness >/=75 DEG ,the corresponding properties of the apex strip 44 being approximately 0.65 MPa and 65 DEG . In the Fig. 2 construction the bead has a rubber cover layer 50 but this may be replaced by extension of the element 46. <IMAGE>

Description

SPECIFICATION Improvements in or relating to tyres This invention concerns improvements in or relating to tyres.

More specifically, the invention relates to tyre casings, in particular tyre casings having radial carcass and especially those reinforced with at least one metal ply. The lower part of each side of the tyre casing, called the "bead", includes the end part of the carcass which is folded towards the outside around a bead core. In general, a shaped element made of rubber in one or two parts, called the "apex" is disposed on the bead core, along the side, between the casing fabric and its fold. The apex extends along the side to a certain height, its thickness gradually tapering off.

Such tyre casings are used in particular for heavy goods vehicles.

In the known casings, one or two steel cord reinforcing plies inclined in relation to the bead core, provide reinforcement and increase the rigidity in this area. In addition, the bead has a shaped element made of relatively soft rubber joined to the or each reinforced ply acting as a shock absorber and then a hard rubber cover layer which withstands the friction between the bead and a wheel rim on which the tyre is mounted in use.

Figure 1 of the accompanying drawings shows in partial cross-section an example of such a known tyre casing, specifically showing its bead.

This casing has a radial carcass 10, the top of which has folded breaker layers 12 on the side nearest the tread 1 4. At the wheel rim, marked with reference 1 5 on the drawing, the casing ends in a bead 1 6. More specifically, this bead is formed by the end portion of the carcass which, as indicated by references 1 8 and 20, is folded around a bead core 22 and extends upwards towards the outside forming fold 10. An apex 24 of a relatively hard rubber is disposed between the carcass 10, the bead core 22 and the fold 20, and tapers bff at its external radial part.

In accordance with the known state of the art, reinforcing plies 26 (only one shown in Figure 1) are placed on the outside of fold 20 in order to strengthen and stiffen this area. More specifically, the plies are usually reinforced by metal or textile cords which are parallel with one another and inclined at a slight angle to the longitudinal direction, thus forming a rigid network with fold 20 of the carcass which they cross.

Figure 1 shows that the bead 1 6 also has a shaped element 28 made of a relatively soft rubber at the edge of the strip, and a hard rubber cover layer 30 which extends around the entire bead and withstands the wear caused by friction between the bead and the wheel rim in which the tyre is mounted in use.

Tyre casings of the type indicated above have the disadvantage of needing many precautions and a high level of precision during manufacture.

In fact, the reinforcing plies have to be precisely cut, transported and fitted, using expensive tooiing. The internal movement which inevitably occurs during vulcanisation can lead to displacement of these plies so that the tyre casings produced are sometimes completely unsuitable for use. For this reason, systematic indepth monitoring techniques have to be used so that these defective tyre casings can be eliminated.

The elimination of reinforcing plies from this type of tyre casing has already been proposed.

Thus, French Patent No 1 343 402 described the production of a tyre casing having a bead with an apex of very hard rubber, tapering off at its external radial part into the side, and able to extend towards the internal radial part of the bead, thus forming the rim rubber. These tyres have a certain number of disadvantages. First of all, the assembly of the section on the fold of the carcass, before vulcanisation, causes problems and the union between the edge of the fabric and the rubber is often far from adequate. In addition, during utilisation it has been noticed that tyres with this type of bead do not have the qualities required.

Tyre casings are also known where the outside of the bead is protected with a hard rubber section. This section is placed on the outside of the fold but the bead has no apex. Such a bead is not sufficiently rigid.

According to one aspect of the present invention a tyre casing comprises a carcass extending between a pair of beads wherein at least one bead includes a bead core and a rubber apex strip around which a respective one end portion of the carcass is folded and a rubber element positioned in direct contact with the folded end portion of the carcass and at least part of the apex strip, the element rubber having a modulus which is greater than that of the apex rubber.

Preferably the modulus of the element rubber is at least double that of the apex rubber. More specifically, it is advantageous for the stabilised modulus at 20% of the element rubber to be at least equal to 1.5 MPa, the Shore A hardness of the rubber being at least equal to 750, whilst the stabilised modulus at 20% of the apex rubber is only of the order of 0.65 MPa, its Shore A hardness being of the order of 650.

In one particular embodiment, the element is extended around part of the bead core and forms the cover layer which contacts the rim on which a tyre incorporating the tyre casing is mounted in use.

It is advantageous for the radial dimension of the element, measured in a plane perpendicular to the axis of rotation of the casing, to be between 20 and 60% of the radial dimension of the casing, also measured in a plane perpendicular to the axis of rotation. It is advantageous for the thickness of the element to be between 5 and 30% of its radial dimension.

The design indicated above has several advantages. First of all, the fatigue life of the shaped casings is very high. Then, the manufacture of the tyre casings does not pose any particular problems because the internal movements of the elements during vulcanisation are smaller than those of reinforcing plies and in any event have fewer disadvantages. Thus the production accuracy can be increased.

Furthermore when the shaped element also forms the cover layer, the number of elements incorporated in the tyre casing is reduced so that the cost of the latter can be decreased.

Preferably the carcass is of radial construction comprising one or more plies each comprising a plurality of parallel reinforcement members e.g.

cords of flexible material such as nylon, Kevlar, or non-textile material such as steel wire. Preferably the reinforcement members have a high modulus, for example a modulus greater than 2500 Kg/cm2.

The bead core may be of any known construction e.g. cable, creel or close-packed wires and have any desired cross-section e.g.

circular, hexagonal, rectangular. Preferably the bead core is substantially inextensible.

The invention will now be described in more detail, by way of example only, with reference to Figures 2 and 3 of the accompanying drawings, (Figure 1 already having been described), in which: Figure 2 is a diagrammatic cross-section of part of a tyre casing in accordance with one embodiment of the invention; and Figure 3 is a cross-section similar to that of Figure 2, in accordance with a second embodiment of the invention.

Referring to Figure 2 the casing shown comprises a radial carcass 30 comprising one or more reinforcement plies the end portion of which, indicated by reference numerals 39 and 40, is folded towards the outside around a bead comprising a bead core 42 and an apex strip 44 of rubber. The apex strip 44 is tapered towards its radially outer extremity and projects beyond the end of the carcass fold 40. An element 46 of rubber having a higher modulus than the apex rubber is positioned next axially outwards of and in direct contact with the carcass fold 40 and part of the apex strip 44. Element 46 extends from the bead heel to a point below the radially outer extremity of the apex strip 44. A cover layer 50 of hard rubber is positioned next axially outwards of the element 46 and extends below the bead base and around the bead toe.Layer 50 contacts the bead seat of a rim 35 on which the tyre is mounted in use.

Assuming the radial dimension of the casing, measured in a plane perpendicular to the axis of rotation, is H as shown in Figure 1, the element 46 has a radial dimension h which, advantageously, is between 20 and 60% of the total radial dimension H, depending on the particular dimensions and the aspect ratio of the casing. It is advantageous for the thickness of the element 46 to be between 5 and 30% of its radial dimension h.

In the particular example shown, which is that of a tyre casing of standard dimensions 8.5-17.5, the element 46 is made of a rubber, or rather of a mixture of rubbers, where the stabilised modulus at 20% is greater than or equal to 1.5 MPa, its Shore A hardness being at least equal to 7 50. As a comparison, the apex 44 is made of a rubber or rather a mixture of rubbers where the stabilised modulus at 20% is of the order of 0.65 MPa, its Shore A hardness being of the order of 650.

The modulus indicated above is measured at just 20% extension. In fact, this extension is very representative of the conditions of use of the rubber in the apex and in the shaped element. This modulus is called "stabilised" because each value is measured several times in succession and it is noted that the measurements stabilise, generally after the fifth one. It is this stabilised value which is indicated in the present specification.

It will be apparent from the foregoing description that the element 46 made of hard rubber replaces the reinforcing plies 26 and the element 28 of soft rubber in the casing shown in Figure 1.

Figure 3 shows another embodiment, intended for a tyre casing of similar dimension to that shown in Figure 2. Those elements in Figure 3 which are similar to those in the embodiment in Figure 2 have been given identical reference numbers. Thus it will be seen that the only difference between the embodiments in Figures 2 and 3 is that the element 46 of hard rubber and the cover layer 50 in the embodiment in Figure 2 are replaced by a single component 52 made of a rubber having properties corresponding to those of the rubber indicated for element 46.

The advantage of the embodiment in Figure 3 over the one in Figure 2 is that it provides a reduction in the number of elements used to make up the tyre casing. It therefore allows a certain reduction in the cost of the casing to be made.

In each of the embodiments described with reference to Figures 2 and 3 the apex strip 44 and element 46 or 52 of high modulus, harder rubber provide a rigidity in the bead which varies progressively in a transverse direction enabling the bead to fully play its part as a connection between the rim and a tyre incorporating the casing.

Tests performed on tyre casings in accordance with the invention, compared with tyre casings of the known type, show the significant advantages of the invention.

First of all, a study of the distribution of deformation stresses in loaded tyre casings shows that the deformation stresses are better distributed in casings made in accordance with the invention. This better distribution of the deformation stresses allows an improvement in the fatigue life.

More specifically, the fatigue life of the tyre casings in accordance with the invention is determined during rolling trials on a rolling machine, with the application of very high loads, the maximum loads reaching 200% of the nominal load standardised by the ETRTO organisation.

More specifically, tyre casings in accordance with the invention, and traditional casings, of the standard 11 R 22.5 SC type, have been subjected to rolling tests on overload. The casings of the traditional type all split in the bead region before 250 hours of rolling. Tyres in accordance with the invention withstood this test for a longer period, one of them reading 51 8 hours of rolling under the same conditions.

Moreover, the tests have been carried out on the type casings in accordance with the invention under real conditions of use. These tyres of standard dimensions, 8.5 R 17.5 SC, were fitted to a vehicle with a total load equal to 6250 kg. It was confirmed that the beads of the casings were intact after an initial utilisation until the tread was worn, and also after a second period of utilisation until complete wear permitted by retreading the crown.

Therefore, the previous tests show that the invention provides tyre casings having a better fatigue life than traditional tyre casings, and yet allows these casings to be produced at reduced cost, thanks to the new structure used.

Claims (24)

1. A tyre casing comprising a carcass extending between a pair of beads wherein at least one bead includes a bead core and a rubber apex strip around which a respective one end portion of the carcass is folded and a rubber element positioned in direct contact with the folded end portion of the carcass and at least part of the apex strip, the element rubber having a modulus which is greater than that of the apex rubber.

2. A casing according to claim 1 wherein the element rubber has a modulus at least double that of the apex rubber.

3. A casing according to claim 1 or 2 wherein the element rubber has a stabilised modulus at 20% extension at least equal to 1.5 MPa and the apex rubber has a stabilised modulus at 20% extension of the order of 0.65 MPa.

4. A casing according to any one of the preceding claims wherein the Shore A hardness of the element rubber exceeds that of the apex rubber.

5. A casing according to claim 4 wherein the Shore A hardness of the element rubber is at least equal to 750 and that of the apex rubber is of the order of 650.

6. A casing according to any one of the preceding claims wherein the element has a radial dimension of between 20 and 60% the radial dimension of the casing, both measurements being taken in a plane normal to the axis of rotation of the casing.

7. A casing according to claim 6 wherein the element extends radially outwards from the bead heel and the bead further includes a rubber cover layer on at least that part of the bead which contacts a wheel rim on which a tyre incorporating the casing is mounted in use.

8. A casing according to claim 6 wherein the element extends around at least that part of the bead which contacts a wheel rim on which a tyre incorporating the casing is mounted in use.

9. A casing according to claim 6 wherein the thickness of the element is between 5 and 30% of its radial dimension.

10. A casing according to any one of the preceding claims wherein the carcass is of radial construction.

11. A casing according to any one of the preceding claims wherein the carcass comprises one or more plies.

12. A casing according to claim 11 wherein the or each ply comprises a plurality of parallel reinforcement members.

13. A casing according to claim 12 wherein the reinforcement members comprise metal cord.

14. A casing according to claim 12 or claim 13 wherein the reinforcement members have a high modulus.

15. A casing according to any one of the preceding claims wherein the bead core is substantially inextensible.

1 6. A tyre casing substantially as hereinbefore described.

17. A tyre casing substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

1 8. A tyre casing substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

1 9. A tyre provided with a tyre casing according to any one of the preceding claims.

20. A tyre bead comprising a bead core and a rubber apex strip around which one end of a tyre carcass is folded and a rubber element positioned in direct contact with the folded end portion of the tyre carcass and at least part of the apex strip, the element rubber having a modulus which is greater than that of the apex rubber.

21. A tyre bead substantially as hereinbefore described.

22. A tyre bead substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.

23. A tyre bead substantially as hereinbefore described with reference to Figure 3 of the accompanying drawings.

24. A tyre having at least one bead according to any one of claims 20 to 23.