Pneumatic Radial Tire Having Highly Durable Bead Structure

1. Field of the Invention

This invention relates to an improvement of bead structure of a pneumatic radial tire having carcass plies made of steel cords for heavy duty vehicles such as trucks, light trucks, buses and so forth and more particularly to a fundamental improvement of the contact part between a bead region, whose deformation is suppressed by a rim flange, and a lower side region, which undergoes easily load deformation, to eliminate the occurence of separation failure.

2. Prior Art

In a conventional pneumatic radial tire for heavy duty vehicles, carcass plies of a tire are turned up around a bead core and terminate at a lower region of a side wall and turn-ups of the carcass plies are generally covered by a reinforcing strip of rubberized steel cords extending to a side wall. In such a structure, a great difference or dislocation in rigidity occurs between both at the ends of the turn-up of the carcass plies or at the upper end of the reinforcing strip of rubberized steel cords and a surrounding rubber. The dislocation in rigidity generates a shearing stress due to a tire deformation caused by repeated load stress resulted from the rotation of tires. Consequently, separation occurs between the steel cords and a surrounding rubber due to the shearing stress.

Some proposals have been made to overcome the problems. In the Japanese Patent Applications laid open under No. 53-119501 and No. 55-106806, high hardness rubber is so disposed adjacent to both ends of carcass ply turn-ups and of a reinforcing strip of metallic cords as to cover both ends so that the dislocation in rigidity occurred between the metallic cords and the surrounding rubber can be decreased so as to decrease the concentration of strain at the upper ends of the metallic cords and to prevent a separation.

According to the Japanese Utility Model Publication No. 52-48482, the upper end of a reinforcing layer of steel cords is covered with textile cord fabric so as to prevent the free upper end of the steel cords from being in contact with a surrounding rubber and thereby intended to suppress the inducement of separation and to eliminate a separation problem.

The above described proposals for preventing a separation are directed to obtain a separation resistance structure by reinforcing the places liable to cause a separation. But they have not settled the problem substantially because the concentration points of stress are merely transferred to some other points so that a separation is induced in a different manner.

It is therefore an object of the present invention to provide a novel separation resistance structure wherein the shearing stress occurred in the structure can be dispersed and absorbed into a flexible member so as to eliminate a strain in a bead portion and to prevent the occurence of separation at the upper ends of carcass turn-ups and a reinforcing element.

The general feature of the present invention is to dispose a low hardness rubber for a buffer at the contact part between a bead region, whose deformation is suppressed by a rim flange, and a lower side region, which undergoes easily load deformation and more particularly at the part positioned at the lower portion of side wall subjected to the concentration of stress and positioned radially outwardly elongated region of carcass plies. With this arrangement, the stress occurred concentrates into an easily deformable low hardness rubber so that the stress is dispersed and absorbed into a low hardness rubber deformed with the concentration of stress. The incorporation of such a flexible structure at the upper part of the bead region eliminates the strain in a bead portion and prevents the occurence of separation.

More particularly, a pneumatic radial tire according to the present invention comprises one or more carcass plies of steel cords extending from one bead portion to another bead portion across a crown portion and disposed at an angle of 90.degree. relative to the equatorial line of the tire, the opposite ends of the carcass plies being turned up around a respective annular bead core from the inside to the outside and terminating at the bead region, a reinforcing strip of steel cords positioned outwardly adjacent to the turn-ups of the carcass plies and not extending radially outwardly beyond the upper end of the turn-ups of the carcass plies, a bead filler of approximately triangular sectional shape consisting of high hardness rubber stock of approximately triangular sectional shape and medium hardness rubber stock having a radial dimension longer than that of the high hardness rubber stock and positioned outwardly adjacent to the high hardness rubber stock, said bead filler being disposed at the region enclosed by the carcass plies and the turn-ups of the carcass plies with a top portion extending to the side wall, a cap of organic textile cord fabric covering an upper end of the said carcass plies and a fin-shaped rubber buffer having a hardness lower than that of the medium hardness rubber stock by 3.degree. to 20.degree. disposed radially outwardly of the capped end and outwardly adjacent to the medium hardness rubber stock. To further dispose a reinforcing strip of organic textile cords positioned outwardly adjacent to the reinforcing strip of steel cords is also within the range of the present invention.

The terms, employed herein, "outwardly" means "farther from the equatorial plane of a tire in the axial direction" and "radially outwardly" means "farther from the rotating axle of the tire in the radial direction".

As described above, the difference in hardness between the rubber buffer and the medium hardness rubber stock is set in the range of 3.degree.-20.degree. in Shore hardness, but more preferably in the range of 5.degree.-15.degree.. The desirable range of the hardness of the rubber buffer is from 45.degree.-65.degree. in Shore hardness and that of the medium hardness rubber stock is from 55.degree.-70.degree. in Shore hardness. If the difference in the hardness between the rubber buffer and the medium hardness rubber stock is less than 3.degree., the effect for improving the durability of the bead region is small.

To make the difference in the hardness between both rubbers more than 20.degree., the hardness of the medium hardness rubber stock should be high and the hardness of the rubber buffer should be low. As a matter of fact, the larger the difference in the hardness between the two rubbers is, the more easily the stress disperses in the rubber buffer. In order to make the hardness of the rubber buffer small, however, the composition ratio of carbon black which is a reinforcing agent of a rubber component should be decreased. As a result, a rubber composition becomes inferior in thermal resistance. Conversely, to make high the hardness of the medium hardness rubber stock results in the decrease in flexural fatigue resistance to deteriorate the durability of the bead portion.

The medium hardness rubber stock of bead filler is preferably divided into two rubbers. A comparatively high hardness rubber is used adjacent to the high hardness rubber stock of bead filler having triangular sectional shape, and a comparatively low hardness rubber is used adjacent to the rubber buffer while keeping the difference range of 3.degree. to 20.degree. higher than the hardness of the rubber buffer. Such arrangement increases the stress dispersion efficiency of the rubber buffer.

The present invention is directed to a pneumatic radial tire which is used with high inflation pressure of more than 3 kg/cm.sup.2 and has one or more strip of carcass plies of steel cords as well as a reinforcing strip of steel cords not extending radially outwardly beyond the upper ends of the turn-ups of the carcass plies.

FIG. 1 is a cross-sectional view of the lower part of a conventional tire.

FIG. 2 is a cross-sectional view of the right half of a tire showing one embodiment in accordance with the present invention.

FIG. 3 is a cross-sectional view of the right half of a tire showing another embodiment in accordance with the present invention.

In FIG. 1, reference numeral 1 designates carcass plies disposed at an angle of 90.degree. with respect to the equatorial line of the tire extending from one bead portion to another bead portion, and turned up around a bead core 2 from the inside to the outside. Reinforcing strip of rubberized steel cords 3 is provided outwardly adjacent to the turn-ups 1a of the said carcass plies 1 to increase the rigidity of the bead portion. A bead filler 4 of high hardness rubber is applied in the area enclosed by the carcass plies 1 and the turn-ups of the carcass plies 1a.

In such a conventional tire, the bead filler 4 of high hardness tends to move in accordance with the movement of the carcass plies, whereas the turn-ups 1a restrained from moving by a rim flange, thereby causing a repeated shearing stress between the bead filler 4 and both upper ends 1b, 3a of the carcass turn-ups and of the reinforcing strip of the steel cords. As a result, both upper ends are liable to be detached from the bead filler and the separation failure is induced.

Referring to FIG. 2 showing an embodiment of the present invention, reference numeral 11 designates carcass plies of steel cords, being disposed at an angle of 90.degree. with respect to the equatorial line of the tire extending from one bead portion B to another bead portion B, the opposite ends of the carcass plies being turned up around the bead core 12 from the inside to the outside to form the turn-ups 11a. Reinforcing strip of steel cords 13 is provided outwardly adjacent to the turn-ups 11a and does not extend radially outwardly beyond the upper end of the turn-ups of the carcass plies 11b.

A bead filler BF is disposed at the area enclosed by the carcass plies 11, the turn-ups 11a and the reinforcing strip of steel cords 13 with a top portion extending to a side wall 18. Said bead filler BF is so formed as to be an approximately triangular shape in cross section consisting of high hardness rubber stock 16 of triangular sectional shape and medium hardness rubber stock 17 positioned outwardly adjacent to said high hardness rubber stock 16.

The upper end 11a of the turn-ups of the carcass plies is covered with a cap 14 made of a rubberized cord fabric of organic textile cords such as nylon cords. Disposed radially outwardly of the capped end and axially outwardly adjacent to the medium hardness rubber stock 17 is a rubber buffer 15.

In the bead structure according to the present invention, the reinforcing strip of steel cords 13 and the high hardness rubber stock 16 enhance the high rigidity of the bead portion B and provide a high driving efficiency. When the lower portion of the side wall 18 undergoes a compressive stress under loaded condition, the rubber buffer 15 is readily deformed depending on the magnitude of the produced stress and absorbs the exerted stress. Accordingly, the stress affecting the vicinity of the upper end 13a of the reinforcing strip of steel cords 13 is diminished so as to enhance the separation resistance and thus a highly durable bead structure is obtained.

In FIG. 3, a reinforcing strip of organic textile cords 19 is further provided outwardly adjacent to the reinforcing strip of steel cords 13 so as to extend radially outwardly to the upper end of the turn-ups 11b of carcass plies. The provision of the reinforcing strip of organic textile cords suppresses the movement of the upper end of the carcass plies and reduces the dislocation in rigidity so that the concentration of stress occurred in the structure can be restrained. Thus, a tire having this structure is very effective in preventing the occurence of separation described above.

In accordance with the present invention, the tires of 10.00 R 20 in size are prepared. The detailed structure of the tires are described hereunder. A single layer of carcass ply of steel cords is employed. The opposite ends of the carcass ply 11 is turned up around the bead core 12 from the inside to the outside with the turn-up 11a having radial dimension within 25% of the tire sectional height, that is, a vertical dimension from a bead heel to the crown outer surface. A reinforcing strip of steel cords 13 is provided outwardly adjacent to the turn-up 11a of the carcass ply with steel cords inclined at an angle of 45.degree. with respect to the radial direction of the tire. The reinforcing strip of steel cords 13 extends from the radially inward position of the bead core 12 up to 20% of the tire sectional height. The upper end of the carcass plies is covered with a cap 14 made of 45.degree. bias cut nylon cord fabric rubberized with a rubber compound having a good adhesive property to steel cords. A rubber buffer 15 having a hardness shown in Table 1 is disposed on the cap. High hardness rubber stock 16 of the hardness 80.degree. and medium hardness rubber stock 17 having a hardness shown in Table 1 are disposed at the area enclosed by the carcass plies 11 and the turn-ups 11a of the carcass plies with the top extending to the side wall 18 to form an approximately triangular shaped bead filler BF.

Drum tests for durability were conducted upon the tires 10.00 R 20 constructed as above. The test results are shown in Table 1. The drum tests for durability were conducted by comparing the running distance of each test tire until the separation occurs at the bead portion under such high loading conditions as an inflation pressure of 9 kg/cm.sup.2, a load of 5,400 kg and a running speed of 40 km/hr. The running distances are represented for comparison by means of an index by setting 100 for a distance of test tire No. 11 wherein the difference of hardness is zero.

As shown in FIG. 3, a reinforcing strip of organic textile cords 19 comprising two nylon cord plies is further provided outwardly adjacent to the reinforcing strip of steel cords and extends from the radially inner side of the carcass plies 11 to a radially outward position of the turn-ups of the carcass plies 11a. The hardness of the medium hardness rubber stock 17 and that of the rubber buffer 14 and the test results are also shown in Table 1.

"Bead sepa" indicated in the damage column in Table 1 means the occurence of the separation at the bead portion. "Stopped" in the same column means that the tests were discontinued because no damage occurred at the bead region when the running distance of the tires was 2.5 times as long as that of the standard test tire No. 11.

The drum tests show that the running distance of the embodiment tires No. 1-No. 10 is longer than that of the comparison tires with conventional structure until the separation damage occurs at the bead region. Especially, the test tires No. 2-No. 5 whose hardness of the rubber buffer is 50.degree.-55.degree. and having a hardness difference of 5.degree.-15.degree. between the rubber buffer and the medium hardness rubber stock is so durable in the bead regions that no damage occurs in the running distance 1.6 times as long as that of the test tire No. 11. The tests also reveal that, as shown in Table 1, the tires having the hardness difference of 3.degree. to 20.degree. between the rubber buffer and the medium hardness rubber stock shows considerable effect for enhancing the durabilities in the bead regions.

The running distance of the embodiment tires Nos. 9 and 10 in which a reinforcing strip of nylon cord plies is provided outwardly adjacent to the reinforcing strip of steel cords is longer than that of the tires in which the same hardness of rubbers are employed as for the rubber buffer and the medium hardness rubber stock and no reinforcing strip of nylon cord plies is provided, until the separation damage occurs at the bead regions. Hence, it is preferable to provide a reinforcing strip of nylon cord plies in a tire. The tests also indicate that, as shown in the comparison tire No. 12, the running distance of a tire in which the hardness of rubber buffer is higher than that of medium hardness rubber stock is shorter than that of the test tire No. 11.

Also recognized from the results in Table 1, the preferable ranges of the hardness of the rubber buffer and that of the medium hardness rubber stock is 45.degree. to 65.degree. and 55.degree. to 70.degree. respectively. If the hardness of the medium hardness rubber stock is more than 70.degree., the running distance of tires would become rather short.

While certain representative embodiments and details have been shown for the purpose of illustrating the invention, it will be apparent to those skilled in this art that various changes and modifications may be made therein without departing from the spirit or scope of the present invention.

Summing up the tests results, the tire having the bead structure described before in accordance with the present invention wherein the rubber buffer has a hardness lower than that of the medium hardness rubber stock by 3.degree. to 20.degree. can attain the remarkable improvement in durability more than expected.