Pneumatic radial tire having highly durable bead structure

A pneumatic radial tire having highly durable bead structure is disclosed. One or more carcass ply layers of steel cords are turned up around a bead core from the inside to the outside. A reinforcing strip of steel cords is positioned outwardly adjacent to and without extending radially outwardly beyond the turn-ups of carcass plies. A bead filler of approximately triangular sectional shape is disposed at the region enclosed by the carcass plies and the turn-ups of the carcass plies. The bead filler comprises high hardness rubber stock and medium hardness rubber stock. A cap of organic textile cord fabric covers an upper end of the carcass plies. A fin-shaped rubber buffer having a hardness lower than that of the medium hardness rubber stock by 3.degree. to 20.degree. is disposed radially outwardly of the capped end and axially outwardly adjacent to the medium hardness rubber stock. A reinforcing strip of organic textile cords may be further disposed outwardly adjacent to the reinforcing strip of steel cords.

BACKGROUND OF THE INVENTION 
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. 
SUMMARY OF THE INVENTION 
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.

DETAILED DESCRIPTION OF THE 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. 
EMBODIMENT 1 
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. 
EMBODIMENT 2 
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. 
TABLE 1 
__________________________________________________________________________ 
EMBODI- 
EMBODIMENT 1 MENT 2 
COMISON 
__________________________________________________________________________ 
Reinforcing strip 
Not provided Provided 
Not provided 
of organic textile 
cords 
Test tire No. 
1 2 3 4 5 6 7 8 9 10 11 12 13 
Rubber buffer (A) 
45 50 50 55 55 60 60 62 45 60 62 70 80 
(JIS hardness) 
Medium hardness 
55 65 70 60 65 65 70 65 55 65 62 70 70 
rubber stock (B) 
(JIS hardness) 
Difference in 
10 15 20 5 10 5 10 3 10 5 0 0 -10 
hardness between 
(A) and (B) 
Durability (index) 
160 
250 
220 
160 
220 
140 
170 
120 
220 
180 
100 
90 80 
Damage Bead 
Stop- 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
Bead 
sepa 
ped 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
sepa 
__________________________________________________________________________ 
"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.