Heavy duty radial tire with at least three steel cord belt plies

A heavy duty radial tire has a belt comprising first, second and third plies disposed in this order from the carcass to the radially outside thereof, each belt ply being made of steel cords laid parallel with each other, the inclining direction of the cords with respect to the tire equator of the first ply being the same as that of the second ply, but reverse to the inclining direction of the third ply, the first belt ply cord angle being 35 to 80 degrees, the second belt ply cord angle being 15 to 30 degrees, and the third belt ply cord angle being 15 to 30 degrees, whereby the belt satisfies (A) the total of the second belt ply strength and the third belt ply strength is 3.4 to 10.0 times the first belt ply strength, and/or (B) the second belt ply strength is in the range of from 1.05 to 2.0 times the third belt ply strength, wherein the strength of each belt ply is defined as the total tensile strength of belt cords in a predetermined unit width of the belt ply.

BACKGROUND AND SUMMARY OF THE INVENTION 
The present invention relates to a pneumatic tire, and more particularly to 
a heavy duty radial tire having an improved belt structure by which the 
tire strength (breaking energy) is effectively increased while achieving a 
belt weight reduction. 
In a belted radial tire for heavy duty use such as truck/bus tires, it is 
very important to improve the hoop effect of the belt to withstand a high 
inner pressure and a heavy tire load. Therefore, such a belt layer 
generally comprises at least three plies of steel cords. 
In such heavy duty radial tires, it is also necessary to increase the 
breaking energy (plunger energy) for the belt in order to prevent the 
burst of the tire. A burst of a tire is caused when its belt plies are cut 
by a sharp object on the road surface, such as stones and rocks. Indeed, 
Japanese Industrial Standard (JIS-D4230) in other countries requires the 
breaking energy to score above a regulated value in the tire breaking 
test. 
Hitherto, therefore, the breaking energy is increased by using thicker 
steel cords for the belt and/or increasing the cord count for each of the 
belt plies. 
However, if the cord diameter and cord count are simply increased, the tire 
weight and manufacturing cost are greatly increased, and the dynamic tire 
performance is reduced. Further, the breaking energy sometimes 
unexpectedly decreases. 
Therefore, the present inventor has made various studies and tests, with 
the following discoveries. 
(1) The cords of the second and third plies mainly work as a hoop, and the 
cords of the first ply works to control a cord movement of the second and 
third plies. 
(2) When a tire treads upon a sharp object, the belt plies deflect towards 
the radially inside of the tire and at the same time the angle of the belt 
cords are changed. If the first ply strength is excessively high, the cord 
movements of the second and third plies are excessively controlled and 
their cord angles cannot be changed. As a result, the second and third ply 
cords are liable to be broken. 
(3) When the belt plies deflect toward the radially inside the ply locates 
farther from the sharp object and a larger tensile deformation occurs. 
Accordingly, the second ply is broken easier than the third ply. On the 
other hand, the first ply is difficult to break when the cord angle is 
large and therefore it is soft against bending. 
It is therefore, an object of the present invention to provide a heavy duty 
radial tire, in which the breaking energy is increased without increasing 
the tire weight. 
According to one aspect of the present invention, a heavy duty radial tire 
comprises a carcass extending between a pair of axially spaced bead 
portions of the tire, and a belt disposed radially outward of the carcass 
in a tread portion of the tire, the belt comprising at least three plies 
including first, second and third plies disposed in this order from the 
carcass to the radially outside thereof, each belt ply being made of steel 
cords laid parallel with each other. With respect to the tire equator, the 
inclining direction of the cords of the first belt ply is the same as the 
inclining direction of the cords of the second belt ply, but is reverse to 
the inclining direction of the cords of the third belt ply. The angle of 
the cords of the first belt ply to the tire equator is 35 to 80 degrees, 
the angle of the cords of the second belt ply to the tire equator is 15 to 
30 degrees, and the angle of the cords of the third belt ply to the tire 
equator is 15 to 30 degrees. The total of the ply strength of the second 
belt ply and the ply strength of the third belt ply is 3.4 to 10.0 times 
the ply strength of the first belt ply, wherein the ply strength of each 
belt ply is defined as the total tensile strength of belt cords in a 
predetermined unit width of the belt ply. 
According to another aspect of the present invention, a heavy duty radial 
tire comprises a carcass extending between a pair of axially spaced bead 
portions of the tire, and a belt is disposed radially outward of the 
carcass in a tread portion of the tire, the belt comprising at least three 
plies including first, second and third plies disposed in this order from 
the carcass to the radially outside thereof, each belt ply being made of 
steel cords laid parallel with each other. With respect to the tire 
equator, the inclining direction of the cords of the first belt ply is the 
same as the inclining direction of the cords of the second belt ply, but 
reverse to the inclining direction of the cords of the third belt ply. The 
angle of the cords of the first belt ply to the tire equator is 35 to 80 
degrees; the angle of the cords of the second belt ply to the tire equator 
is 15 to 30 degrees; and the angle of the cords of the third belt ply to 
the tire equator is 15 to 30 degrees. The ply strength of the second belt 
ply is 1.05 to 2.0 times the ply strength of the third belt ply, wherein 
the ply strength of each belt ply is defined as the total tensile strength 
of belt cords in a predetermined unit width of the belt ply. 
In a first aspect, therefore, the movements of the cords of the second and 
third belt plies are appropriately controlled, and the cords are prevented 
from being cut, and the breaking energy for the belt can be increased. 
Similarly, in a second aspect, as the second belt ply is increased in its 
strength S2, it becomes possible to withstand its large tensile stress, 
and the breaking energy can be increased. 
Further, when those two aspects are combined, the breaking energy is 
remarkably increased. 
In either case, as the cord angles of the second and third belt plies is 
relatively small, those plies display a tight hoop effect, and the cords 
of the first belt ply appropriately control the movements of the second 
and third belt ply cords. Further, the cords of the first-third plies form 
a truss structure to provide a desirable rigidity for the belt.

DETAILED DESCRIPTION OF THE INVENTION 
In the figures, the heavy duty radial tire 1 comprises 
a tread portion 2 with tread edges, 
a pair of axially spaced bead portions with a bead core 5 therein, 
a pair of sidewall portions 3 extending radially inwardly from the tread 
edges to the bead portions, 
a toroidal carcass 6 extending between the bead portions 4 through the 
tread portion 2 and sidewall portions 3, and 
a belt 7 disposed on the radially outside of the carcass 6 in the tread 
portion 2. 
The carcass 6 comprises at least one ply of cords extending between the 
bead portions 4 and turned up around the bead cores 5 from the inside to 
outside of the tire to form a pair of turnup portions 6a and a main 
portion 6b therebetween. The turnup portions 6a terminate above the bead 
tore 5 and below the maximum tire width position. The carcass ply cords 
are arranged radially at an angle of 75 to 90 degrees with respect to the 
tire equator C. Preferably and in this embodiment, the carcass 6 comprises 
only one ply of steel cords arranged radially at 90 degrees. However, the 
carcass 6 may be comprised of a plurality of organic fiber cords, e.g., an 
aromatic polyamide, nylon, rayon, polyester and the like. 
In order to increase the lateral stiffness of the tire, each bead portion 4 
is provided with a bead apex 8 between the turnup portion 6a and main 
portion 6b. The bead apex 8 is made of a hard rubber compound extending 
radially outwardly from the bead core 5 so as to reinforce the bead 
portion 4 and lower sidewall portion. 
In this embodiment, the belt 7 consists of four plies; a first ply 11, 
second ply 12, third ply 13 and fourth ply 14, which are disposed in this 
order from the carcass 6 towards the tread face 2A. 
Each of the belt plies 11, 12, 13 and 14 is made of steel cords 20 laid 
parallel to each other. Each belt cord 20 is made of steel filaments 
twisted together. 
The axial width W2 of the second belt ply 12 is larger than the axial width 
W1 of the first belt ply 11. 
The axial width W3 of the third belt ply 13 is substantially the same as, 
but slightly larger than the width W1. 
The second belt ply 12 is widest in the four belt plies, and the width W2 
has a maximum value WM, which is set in the range of from 0.80 to 0.98 
times the tread width TW, whereby a substantially overall width of the 
tread portion 5 is reinforced. 
The fourth belt ply 14 is narrowest, and its width W4 has a minimum value. 
Ply 14 works mainly to protect the inner belt plies 11, 12 and 13 and 
thereby increases the cut resistance of the tread portion. 
In each tread edge portion, cushion rubber layers 15 and 16 are disposed so 
as to cover the edges of the first-third belt plies 11-13 to mitigate 
stress concentration. Since the inner cushion rubber layer 15 has a 
tapered axial inner edge, the space between the carcass and the belt 7 or 
the first belt ply 11 is gradually increased towards the axially outside. 
The whole width of the first belt ply 11 contacts with the second belt ply 
12, but the edge of the third belt ply 13 is spaced therefrom and the 
space is filled with the rubber layer 16. 
As shown in FIG. 2, the belt cords 20A and 20B of the first and second belt 
plies 11 and 12, respectively, are inclined with respect to the tire 
equator C towards a certain direction, for example, towards the right 
side, so as to have different right-side upward inclinations. 
The belt cords 20C and 20D of the third and fourth belt plies 13 and 14, 
respectively, are inclined to the reverse direction to that of the belt 
cords 20A and 20B (in FIG. 2, therefore, towards the left side so as to 
have different left-side-upward inclinations). 
The angle .alpha.1, .alpha.2 and .alpha.3 to the tire equator C, of the 
belt cords 20A, 20B and 20C respectively are: 
EQU 35 degrees.ltoreq..alpha.1.ltoreq.80 degrees (1) 
EQU 15 degrees.ltoreq..alpha.2.ltoreq.30 degrees (2) 
EQU 15 degrees.ltoreq..alpha.3.ltoreq.30 degrees (3) 
whereby, as shown in FIG. 3, the belt cords 20A, 20B and 20C form a stiff 
truss structure, and a desired belt rigidity is provided to maintain the 
steering stability. 
According to one aspect of the present invention, the ply strength S1 of 
the first belt ply 11 is set to be smaller than either of the ply strength 
S2 of the second belt ply 12 and the ply strength S3 of the third belt ply 
13. 
EQU S1&lt;S2 
EQU S1&lt;S3 
The sum total S2+S3 of the ply strengths S2 and S3 is set in the range of 
3.4 to 10.0 times the ply strength S1. 
EQU 3.4 S1.ltoreq.(S2+S3).ltoreq.10.0 S1 (4) 
Here, the ply strength S is the tensile rupture strength of the belt ply 
per unit width. In this embodiment, the ply strength Si is defined as the 
product Ei.times.Ni of the tensile strength Ei of one belt cord 20 in a 
ply and the cord count Ni of the ply per unit width, for example 5 cm 
width, in the direction at a right angle to the belt cords. (i=1, 2 and 3) 
EQU S1=E1.times.N1 
EQU S2=E2.times.N2 
EQU S3=E3.times.N3 
As a result, in the above-mentioned truss structure, the movement of the 
belt cords 20B and 20C is appropriately reduced, but the angles of the 
belt cords 20B, 20C can be changed relatively easily when the tire treads 
upon a sharp object and the tread portion is deflected radially inwardly. 
Therefore, the breaking energy for the belt is increased, and the cord 
breakage in the second and third belt plies 12 and 13 is effectivly 
prevented. 
When the sum total (S2+S3) is less than 3.4 times S1, the movement of the 
belt cords 20B and 20C is excessively controlled, which results in cord 
breakage in the second and third belt plies 12 and 13. When the sum total 
(S2+S3) is more than 10.0 times S1, the truss structure loses its balance, 
and the belt rigidity greatly decreases, which results in uneven wear. 
Further, the cornering force decreases to cause deterioration in the 
steering stability. 
The lower limit for the total (S2+S3) is more preferably 4.0 times S1. The 
upper limit is more preferably 7.0 times S1, still more preferably 4.8 
times S1. 
Further, in the belt 7 in this example, the ply strength S2 of the second 
belt ply 12 is set to be larger than the ply strength S3 of the third belt 
ply 13 to meet the following equation (5). 
EQU 1.05 S3.ltoreq.S2.ltoreq.2.0 S3 (5) 
When the tire treads upon a rock and the like, as the belt plies deflect 
radially inside, the second belt ply 12 is subjected to a tensile 
deformation which Is larger than the third belt ply 13. Therefore, by 
increasing the ply strength S2 of the second belt ply 12 by limiting it 
within the above-mentioned range, the belt cords 20B are prevented form 
being broken. In this embodiment, therefore, this effect and the 
above-mentioned effect from equation (4) are combined to effectively 
increase the breaking energy for the belt 7. 
The cord angle .alpha.1 of the first belt ply 11 is not less than 54 
degrees, whereby the first belt ply 11 is provided with appropriate 
flexibility against the bending deformation and the cords thereof are 
prevented form being broken. 
The cord angle .alpha.4 of the fourth belt ply 14 is preferably set at 
substantially the same value as the cord angle .alpha.3, that is, within 
plus or minus 5 degrees, and the ply strength S4 is less than the ply 
strength S3, so that the forth belt ply 14 does not increase the belt 
rigidity and does not hinder the above-explained effect. 
TEST 1: Tires of size 11R22.5 having the construction shown in FIG. 1 and 
specifications given in Table 1 were prepared and tested for the breaking 
energy. 
In the test, the breaking energy for each test tire was measured according 
to the tire strength test specified in Japanese Industrial Standard 
JIS-D4230. The tire inner pressure was 7.00 kgf/sq.cm, and the rim size 
was 7.5.times.22.5. 
The results are indicated in Table 1 by an index based on the fact that the 
reference tire 1 is 100. The larger index is better. 
The belt structures used in the test tires are shown in Table 2. 
TABLE 1 
______________________________________ 
Tire Ex.1 Ex.2 Ref.1 Ref.2 
______________________________________ 
First-belt ply 
Construction B B A A 
Cord angle .alpha.1 (deg) 
67 67 67 67 
Width W1 (mm) 145 145 145 145 
Strength S1 (kgf/5 cm) 
2110 2110 3200 3200 
Cord total weight G1 (g/m) 
65.8 65.8 106.6 106.6 
Second belt ply 
Construction C D C D 
Cord angle .alpha.2 (deg) 
18 18 18 18 
Width W2 (mm) 160 160 160 160 
Strength S2 (kgf/5 cm) 
4160 4980 4160 4980 
Cord total weight G2 (g/m) 
147.2 159.4 147.2 159.4 
Third belt ply 
Construction C D C D 
Cord angle .alpha.3 (deg) 
18 18 18 18 
Width W3 (mm) 140 140 140 140 
Strength S3 (kgf/5 cm) 
4160 4980 4160 4980 
Cord total weight G3 (g/m) 
147.2 159.4 147.2 159.4 
Fourth belt ply 
construction A A A A 
Cord angle .alpha.4 (deg) 
18 18 18 18 
Width W4 (mm) 75 75 75 75 
Strength S4 (kgf/5 cm) 
3200 3200 3200 3200 
Cord total weight G4 (g/m) 
106.6 106.6 106.6 106.6 
Test Results 
(S2 + S3)/S1 4.0 4.7 2.6 3.1 
S2/S3 1.0 1.0 1.0 1.0 
G0(= G1 + G2 + G3) 
360.2 384.6 401.0 425.4 
Breaking energy P (index) 
110 125 100 115 
P/G0 0.305 0.325 0.249 0.270 
______________________________________ 
TABLE 2 
__________________________________________________________________________ 
Belt ply A B C D E 
__________________________________________________________________________ 
Cord structure 3/0.20 + 
2 + 7/0.22 
3/0.20 + 
3 + 8 + 
3 + 9 + 
6/0.35 6/0.35 
13/0.23 
15/0.22 
Breaking strength E (kgf) of a cord 
160 88 160 249 265 
Weight of a cord (g/m) 
5.33 
2.74 5.33 
7.97 
8.50 
Cord count N (/5 cm width) 
20 24 26 20 20 
Ply strength S(= EXN) per 5 cm width 
3200 
2112 4160 
4980 
5300 
Cord total weight G(= gXN) per 5 cm width 
106.6 
65.8 147.2 
159.4 
170 
__________________________________________________________________________ 
As is apparent from Table 1 when comparing Ex.1 with Ref.1 and Ex.2 with 
Ref.2, by decreasing the ply strength S1 of the first belt ply to a value 
in the range expressed by the above-mentioned equation (4), the breaking 
energy could be increased and at the same time the cord weight of the 
first-third belt plies is decreased. 
According to the other aspect of the present invention, the ply strength S2 
of the second belt ply 12 is set in the range of from 1.05 to 2.0 times 
the ply strength S3 of the third belt ply 13. This limitation is the same 
as equation (5). In other words, the limitation by the above-explained 
equation (5) is effective either alone or in combination with the 
limitation of equation (4). 
When the ply strength S2 is less than 1.05 times the ply strength S3, the 
prevention of breaking of the second belt ply cords becomes insufficient. 
When S2 is more than 2.0 times S3, the third belt ply 13 becomes weak and 
causes cord breakage. 
TEST 2: Test tires of size 12.00R24 mounted on a rim of size 8.50V.times.24 
and inflated to an inner pressure of 7.75 kgf/sq.cm were measured for the 
breaking energy in the same way as in the above-mentioned test. 
The tires had the construction shown in FIG. 1 and specifications given in 
Table 3. The belt structures used in the test tires are also given in 
Table 2. 
The results are indicated in Table 3 by an index based on the fact that the 
reference tire 3 is 100. The larger index is better. 
TABLE 3 
__________________________________________________________________________ 
Tire Ex.3 
Ex.4 
Ex.5 
Ref.3 
Ref.4 
Ref.5 
Ref.6 
Ref.7 
Ref.8 
__________________________________________________________________________ 
First belt ply 
Construction 
A A A A A A A A A 
Cord angle .alpha.1 (deg) 
67 67 67 67 67 67 67 67 67 
Width W1 (mm) 
165 
165 
165 
165 
165 
165 
165 
165 
165 
Strength S1 (kgf/5 cm) 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
Cord total weight G1 (g/m) 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
Second belt ply 
Construction 
D E E C C C D D E 
Cord angle .alpha.2 (deg) 
21 21 21 21 21 21 21 21 21 
Width W2 (mm) 
190 
190 
190 
190 
190 
190 
190 
190 
190 
Strength S2 (kgf/5 cm) 
4980 
5300 
5300 
4160 
4160 
4160 
4980 
4980 
5300 
Cord total weight G2 (g/m) 
159.4 
170 
170 
147.2 
147.2 
147.2 
159.4 
159.4 
1702 
Third belt ply 
Construction 
C C D C D E D E E 
Cord angle .alpha.3 (deg) 
21 21 21 21 21 21 21 21 21 
Width W3 (mm) 
170 
170 
170 
170 
170 
170 
170 
170 
170 
Strength S3 (kgf/5 cm) 
4160 
4160 
4980 
4160 
4980 
5300 
4980 
5300 
5300 
Cord total weight G3 (g/m) 
147.2 
147.2 
159.4 
147.2 
159.4 
170 
159.4 
170 
170 
Fourth belt ply 
Construction 
A A A A A A A A A 
Cord angle .alpha.4 (deg) 
18 18 18 18 18 18 18 18 18 
Width W4 (mm) 
130 
130 
130 
130 
130 
130 
130 
130 
130 
Strength S4 (kgf/5 cm) 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
3200 
Cord total weight G4 (g/m) 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
106.6 
Test Results 
(S2 + S3)/S1 
2.86 
2.96 
3.21 
2.6 
2.86 
2.96 
3.11 
3.21 
3.31 
S2/S3 1.20 
1.27 
1.06 
1.00 
0.83 
0.78 
1.00 
0.94 
1.00 
G0(= G1 + G2 + G3) 
413.2 
423.8 
436.0 
401.0 
413.2 
423.8 
425.4 
436 
446.6 
Breaking energy P (index) 
127 
153 
160 
100 
107 
107 
133 
133 
160 
P/G0 0.307 
0.361 
0.367 
0.249 
0.259 
0.252 
0.313 
0.305 
0.358 
__________________________________________________________________________ 
As is apparent form Table 3 when comparing Ex.3 with Ref.4, Ex.4 with Ref.8 
and Ex.5 with Ref. 7, by increasing the ply strength S2 than the ply 
strength S3 to meet the equation (5), the breaking energy could be 
increased without increasing the cord weight of the first-third belt 
plies. 
Further, as apparent when comparing Ref.4 with Ref.5, Ref.6 with Ref.7, and 
Ex.5 with Ref.8, by specifically decreasing the ply strength S3 than the 
ply strength S2, the cord weight could be decreased without decreasing the 
breaking energy. 
Incidentally, the ply strength S is substantially in proportion to the 
total weight of the belt cords embedded in a unit width of the belt ply. 
Accordingly, by adjusting the ply strength S1, S2 and S3 to meet the 
above-mentioned equation (4) and/or equation (5), the breaking energy for 
the belt plies can be increased without increasing the tire weight. 
As explained above, in the heavy duty radial tires according to the present 
invention, as the cord directions, cord angles and ply-strength of the 
first-third belt plies are set at specific ranges, the tire strength 
(breaking energy) can be increased without increasing the belt weight. 
The invention being thus described, it will be obvious that the same may be 
varied in many ways. Such variations are not to be regarded as a departure 
from the spirit and scope of the invention, and all such modifications as 
would be obvious to one skilled in the art are intended to be included 
within the scope of the following claims.