Heavy duty pneumatic radial tire

Disclosed herein is a heavy duty pneumatic radial tire in which a tread portion of the tire is reinforced by superimposing a belt composed of plural rubberized cord layers upon the outer periphery of a radial structure carcass composed of one or more rubberized cord layers containing cords each arranged in a radial plane of the tire or in a plane deviating at a small angle therefrom; the belt has a substantially lower flexural rigidity at central region than at side regions; and the tread portion has such a tread pattern that has a substantially higher negative ratio at central region than at side regions.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention relates to a heavy duty pneumatic radial tire, and 
more particularly, the invention relates to a heavy duty pneumatic radial 
tire, for instance, a tire for use in the construction vehicles. Such 
tires are advantageously adapted for use in running on rough roads at a 
considerably high speed, wherein durability of belt is improved by 
effectively avoiding belt separation which is caused by shear strain at 
edges of the belt without deteriorating wear resistance and uneven wear 
resistance of such a tire. 
(2) Description of the Prior Art 
In such a radial tire, the tread portion of the tire is generally 
reinforced by superimposing a belt composed of plural rubberized cord 
layers upon the outer periphery of a radial structure carcass composed of 
one or more rubberized cord layers containing cords each arranged in a 
radial plane of the tire or in a plane deviating at a small angle 
therefrom. 
In general, the belt is broadly classified into the following two kinds in 
terms of the structure: 
(i) The cords are arranged at an angle of as small as about 
20.degree.-25.degree. with respect to a plane containing the central 
circumference of the tire, that is, the equatorial plane of the tire, so 
as to restrain the enlargement of the outer diameter caused when the tire 
contacts the ground during running. 
(ii) The cords of the nearest belt layer to the carcass and positioned on 
the innermost side are arranged at an angle of about 65.degree.-70.degree. 
with respect to the equatorial plane, and the cords of the remaining belt 
layers are arranged at an extremely small angle with respect to the 
equatorial plane, so that triangle link structure is formed between the 
carcass cords and the belt cords to give a high rigidity to the crown 
portion of the tire. 
As to the item (i), since the shear strain between the cord layers at the 
edge portion of the belt becomes excessive, which is caused by the tread 
portion contacting the ground, separation problems at that position are 
likely to develop, while as to item (ii), although the occurrence of the 
shear strain when the internal pressure is applied is low due to its high 
rigidity in a direction along the tire equator, the belt is forcedly 
deformed while running on a rough road of an uneven surface on which rocks 
and/or stones are scattered by the deformation of the tread portion when 
it contacts the ground. Hence, the portions near the edges of the belt 
fixed in the triangle link structure exceed the permissible shear strain, 
wherein a separation problem is likely to occur. 
Although there have been various examinations directed to a reduction in 
the strain at the belt edges through making proper the belt width, cord 
angle and cord number per unit length, and changing the quality of the 
cords for the belt and the carcass line to avoid the belt edge separation 
(hereinafter referred to briefly as "end separation") in the heavy duty 
pneumatic radial tire for use in the running road containing the above 
rough road as at least a part thereof, there is naturally a limit on the 
improvement by such a method, since the necessary rigidity of the belt and 
the strain of the belt edges are just in the reverse relation with each 
other, so that when the rigidity of the belt is lowered to decrease the 
strain, a demerit inevitably takes place with respect to the wear 
resistance and the uneven wear resistance of the tread. 
Meanwhile, as to the tread patterns, the so-called lug type pattern is 
commonly used because the conventional tire of this type generally needs 
strong traction, but there has not been reported in the literature any 
studies in which attention was directed to examine the tread pattern as 
the governing factor of the strain behavior at the tread portion, 
particularly, the belt edge portions. 
SUMMARY OF THE PRESENT INVENTION 
It is an object of the present invention to provide a heavy duty pneumatic 
radial tire in which the belt and the tread are functionally divided into 
a central region and the right and left side regions sandwiching the 
central region with respect to the equatorial plane of the tire, and the 
flexural rigidity is appropriately imparted upon the respective regions, 
whereby the end separation can effectively be restrained together with the 
attainment of the wear resistance, and the uneven wear resistance. 
According to the present invention, the heavy duty pneumatic radial tire is 
reinforced by superimposing a belt composed of plural rubberized cord 
layers upon the outer periphery of a radial structure carcass composed of 
one or more rubberized cord layers containing cords each arranged in a 
radial plane of the tire or in a plane deviating at a small angle 
therefrom; the belt has a substantially lower flexural rigidity at central 
region than at side regions; and the tread portion has such a tread 
pattern that has a substantially higher negative ratio at central region 
than at side regions. 
These and other objects, features and advantages of the invention will well 
be appreciated upon reading of the following description of the invention 
when taken in conjunction with the accompanying drawings with 
understanding that some modifications, variations and changes would be 
easily done by the skilled in the art to which the invention pertains 
without departing from the spirit of the invention and the scope of claims 
appended hereto.

DETAILED DESCRIPTION OF THE INVENTION 
The tire construction according to the present invention relates to the 
combination of the belt structure and the tread pattern in the heavy duty 
pneumatic radial tire, and it can be said that the above object is 
attained by appropriately setting the ratio in the flexural rigidity 
between the central region and the both side regions of the belt 1 and the 
tread portion 3. As the substitute measure of the flexural rigidity is 
appropriately adopted the total thickness of the laminated belt as to the 
belt, and the negative ratio as to the tread pattern. 
In order to appropriately distribute the flexural rigidity onto the belt, 
the thickness of the laminated belt may be adjusted in such a way that the 
rubber gauge between the belt layers is made thicker at both side or the 
belt is designed as a folded belt structure, or to the contrary, the belt 
is designed in a central portion-lacking belt structure. 
The central region of the belt is symmetrical on the right and left sides 
with respect to the equatorial plane of the tire and is defined as an 
A-region in FIG. 1 which corresponds to 40% of the largest belt width BW, 
and the remaining B-regions are taken as the side regions. Taking the 
largest thicknesses of the A-region and the B-regions as t.sub.A and 
t.sub.B respectively, the ratio of the t.sub.B /t.sub.A is preferably set 
in a range of 1.25-1.55, more preferably 1.30-1.45. 
On the other hand, as to the tread pattern, the central region of the tread 
is defined as a C-region in FIG. 1 which is symmetrical on the right and 
left sides sandwiching the equatorial plane of the tire and corresponds to 
50% of the tread width T.sub.W, and the remaining D-region is taken as the 
both side regions. The ratio of the negative ratio N.sub.C at the C-region 
to the negative ratio N.sub.D at the D-region is preferably set at 1.15 to 
1.50, more preferably 1.25-1.45. The negative ratio used herein means an 
area ratio of the grooves to the landing portion in the tread 
ground-contacting surface when the normal internal pressure and the normal 
load is applied to the tire. 
The ratio N.sub.C /N.sub.D of 1.15-1.50 can advantageously be attained by 
employing the tread pattern of the center block/shoulder lug (transversely 
long block) type as shown in FIG. 2, and other tread patterns different 
from that shown in FIG. 2 may be employed. For reference purposes, the 
conventional common lug type tread pattern is shown in FIG. 3. 
The thicknesses t.sub.A and t.sub.B of the laminated belt are measured as 
including a high elasticity rubber of the elasticity modulus of 0.35-0.55 
kg/mm.sup.2 which is used as a covering rubber or squeegee rubber of the 
cord layers of the belt. If the ratio of t.sub.B /t.sub.A is less than 
1.25, any special superiority can not be obtained as compared with the 
conventional belt, and it is not suitable for the object of the invention. 
On the contrary, if the ratio exceeds 1.55, the increase in the heat 
generation at the B-region begins to have an adverse affect, and the 
separation is liable to occur around the cords due to the excessive 
movement at the A-region which becomes relatively too thin. 
Needless to explain, the belt 3 composed of cord layers of steel cords of 
an elasticity modulus of 5,000-20,000 kg/mm.sup.2 at a number of cords per 
unit length of around 13 cords/25 mm according to the conventional manner 
may be used in combination with the carcass 2 composed of one layer of 
steel cords or Kevlar cords or the carcass 2 composed of at least two 
layers of the polyester cords. 
As to the tread pattern, if the ratio of N.sub.C /N.sub.D is less than 
1.15, end separation is not actually prevented, while if it exceeds 1.50, 
movement of the rubber at the central region is too large, so that it is 
disadvantageous in terms of the wear life, which is also adversely 
affected by a relatively small amount of rubber, and there is a danger of 
vigorous cut damage particularly on the rough roads. 
The invention will be explained more in detail with reference to the 
following specific Examples together with Comparative Example, but 
Examples are merely illustrative of the invention and never interpreted to 
limit the scope thereof. 
Test tires A, B and C of a tire size of OR 18.00 R 25 were prepared 
according to FIGS. 1 and 2 together with a conventional similar tire D 
having the lug type tread pattern as shown in FIG. 3. They were subjected 
to a durability test while being actually mounted on a car. After 50,000 
Km running, the occurrence state of the end separation (length of the 
cracks) and the produced state of the wear are compared in the following 
Table. 
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Tire kind 
A B C D 
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Belt t.sub.A (mm) 
12 12.5 13 13 
t.sub.B (mm) 
17 16 15 13 
t.sub.B /t.sub.A 
1.42 1.28 1.15 1.00 
Tread N.sub.C 0.40 0.43 0.43 0.32 
pattern 
N.sub.D 0.28 0.34 0.34 0.41 
N.sub.C /N.sub.D 
1.43 1.26 1.26 0.78 
End separation 
10 45 70 100 
(Crack length) 
(Index) 
Uneven wear 
Not Not Observed 
Slightly 
observed observed observed 
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In the above tests, the belt composed of steel cords of a cord elasticity 
modulus of 13,000 kg/mm.sup.2 at a number of cords per unit length of 13 
cords/25 mm was applied to each of the tires. 
According to the test results, end separation could be avoided without 
being accompanied by the wear resistance deterioration with respect to the 
tires A and B in which both the flexural strength of both the belt and the 
tread portion were simultaneously taken into account, whereas there 
remained a defect in end separation and local wear resistance in the case 
of the tire C in which no due attention has been paid on the belt and in 
the case of the tire D in which both the belt and the tread didn't meet 
the requirements. 
As mentioned above, according to the present invention, the end separation 
resistance, the wear resistance and the uneven wear resistance can all be 
attained.