Heavy duty pneumatic radial tire

A heavy duty pneumatic radial tire includes a toroidal carcass consisting of plies having cords arranged in parallel with each other, and a plurality of belt layers interposed between the carcass and a tread of the tire. At least one of the plies has turn-up portions folded about a pair of bead cores from inside to outside of the tire. The tire comprises a crown protective layer arranged between the belt layers and the tread. The crown protective layer comprises wave-form cords or wave-form filaments. The tire further comprises a cushion rubber layer arranged radially inward of the protective layer and made of a rubber having a modulus less than 70 kg/cm.sup.2 under 100% elongated condition. A distance between cords of a radially outermost layer of the belt layers and the wave-form cords or the wave-form filaments of the radially innermost layer of the crown protective layer is more than 1.5 mm.

BACKGROUND OF THE INVENTION 
This invention relates to a heavy duty pneumatic radial tire to be used 
under high inner pressure and heavy load. In particular it relates to a 
pneumatic radial tire for an aircraft, which is able to prevent damage of 
the tire resulting from external cuts in a tread of the tire. 
Tread surfaces of pneumatic tires are often cut by stones and metal pieces 
dispersed on roads when the tires run thereon. With pneumatic tires 
adapted to be used for light loads as in passenger cars, even if the tires 
are cut to certain extends, they are scarcely burst due to cuts in treads 
of the tires because the conditions of use are not severe. However, with 
pneumatic tires used under conditions of high inner pressure, heavy load 
and high speed as in aircraft, external cuts in treads immediately result 
in a burst of the tires. 
In general, bias tires are used for aircraft for the following reason. As 
the carcass is bias and includes many ply layers, it is possible to 
arrange on radially inner side of a tread a crown protective layer having 
a plurality of plies including organic fiber cords intersecting with each 
other. Therefore, even if the tread is externally cut by foreign pieces on 
a road surface, any enlargement of cuts in the tread is effectively 
prevented between the crown protective layer and the carcass plies. 
On the other hand, pneumatic radial tires having carcasses of plies 
including radially arranged cords are unsuitable for aircraft for the 
following reason. As the carcass includes only a few plies and belt layers 
include circumferential belt layers, even if a crown protective layer 
having intersecting layers of organic fiber cords is provided, the tire is 
unable to prevent enlargement of external cuts in treads under severe 
condition in an aircraft. 
A radial tire for an aircraft is disclosed in United States Patent 
Specification No. 4,402,356 which discloses a feature of using wave-form 
steel cords for protecting a crown. 
When the wave-form steel cords are used in crown protective layers, it is 
possible to prevent development of external cuts in treads. However, as 
there is a great difference in rigidity between the crown protective layer 
and belt layers, cracks would occur between the crown protective layer and 
belt layers in the event that the tire is subjected to heavy load as in 
landing of an aircraft or going over protrusions on roads. 
SUMMARY OF THE INVENTION 
It is a primary object of the invention to provide an improved heavy duty 
pneumatic radial tire which eliminates all the disadvantages in the prior 
art and prevent cracks apt to occur between a crown protective layer and 
belt layers of a pneumatic radial tire used under conditions of high inner 
pressure and heavy load as for an aircraft, and further facilitates 
recapping of the tire. 
In order to achieve this object, in a heavy duty pneumatic radial tire 
including a toroidal carcass consisting of plies having cords arranged in 
parallel with each other, at least one of the plies having turnup portions 
folded about a pair of bead cores from inside to outside of the tire and, 
a plurality of belt layers interposed between said carcass and a tread of 
the tire, according to the invention the tire comprises a crown protective 
layer arranged between the belt layers and the tread and comprising either 
of wave-form cords and wave-form filaments, and a cushion rubber layer 
arranged radially inward of the crown protective layer and made of a 
rubber having a modulus less than 70 kg/cm.sup.2 under 100% elongated 
condition, and a distance between cords of a radially outermost layer of 
said belt layer and either of wave-form cords and wave-form filaments of a 
radially innermost layer of said crown protective layer being more than 
1.5 mm. 
In this case, the distance between cords of the radially outermost layer of 
the belt layers and the wave-form cords is the distance h between surfaces 
of the cords as shown in a sectional view of FIG. 2a, wherein h is the 
distance between cords of the radially outermost layer of the belt layers 
and the radially innermost wave-form cords of the crown protective layer. 
With the heavy duty tire according to the invention, the distance h less 
than 4.5 mm is preferable with respect to weight and heating of the tire. 
If the distance h is more than 4.5 mm, a volume of the entire tread of the 
tire is excessively increased so that durability at high speeds is often 
extremely reduced. 
In view of productivity, the modulus of the cushion rubber layer under 100% 
elongated condition is preferably more than 20 kg/cm.sup.2. 
In order that the invention may be more clearly understood, preferred 
embodiments will be described, by way of example, with reference to the 
accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a construction of the tire according to the invention, 
which comprises a carcass 1, an intersecting belt layer 2, a 
circumferential belt layer 3, a tread 4, bead cores 5, a crown protective 
layer 6, and a cushion rubber layer 7. 
In this embodiment, the carcass 1 consists of an up-down laminated layer 
comprising four turn-up plies folded or wound from inside to outside about 
the bead cores 5 and a down ply extending to bead toes along the outsides 
of the turn-up portions of the turn-up plies. 
The intersecting belt layer comprises two plies having organic fiber cords, 
which are folded or doubled at both sides to add two plies on both sides 
of the tread. In this manner, there are two plies at the center of the 
tread and four plies at both the sides of the tread. In this embodiment, 
moreover, folded ends of the outer ply of the intersecting belt layer are 
arranged so as to enclose folded ends of the inner ply in order to prevent 
shearing strains at the folded ends of the inner ply. 
The circumferential belt layer 3 comprises five cord layers whose cords are 
made of the same organic fibers as those of the intersecting belt layer 
and are wound spirally in circumferential directions of the tire. The belt 
layer 3 also has two additional cord layers at the center portion of the 
tread and two additional plies one on each side of the tread. Therefore, 
the circumferential belt layer 3 consists of the seven cord layers at the 
center portion of the tread and six cord layers at both side portions of 
the tread. The circumferential belt layer 3 may be an intersecting belt 
layer. 
The crown protective layer 6 comprises wave-form steel cords 6a arranged in 
parallel with each other between the circumferential belt layer 3 and the 
tread 4. Moreover, between the crown protective layer 6 and the 
circumferential belt layer 3, the cushion rubber layer 7 made of a rubber 
whose modulus under 100% elongated condition is less than 70 kg/cm.sup.2. 
A thickness h of the cushion rubber layer 7 is always more than 1.5 mm. 
The thickness h is the distance between the cords 3a of the 
circumferential belt layer 3 and the steel cords 6a of the crown 
protective layer 6. This distance h is referred to hereinafter "layer 
distance". Moreover, in order to make the tire light-weight, it is 
preferable to make the wave-form cords of organic fibers, for example, 
aromatic polyamide or aliphatic polyamide. 
When the wave-form cords or the wave-form filaments are subjected to 
tensile forces, they are deformed in a manner that amplitudes or distances 
between crests and bottoms of the waves become smaller and wave lengths 
become longer. Upon deforming, the rubber about the cords or filaments is 
deformed following to the deformation of the cords or filaments. 
However, as the belt layers 2 and 3 has very high rigidity, large shearing 
forces occur between the belt layers 2 and 3 and the crown protective 
layer 6 of the wave-form steel cords or wave-form filaments to cause 
separation therebetween. 
Therefore, the cushion rubber layer 7 is provided between the crown 
protective layer 6 and the belt layers 2 and 3 to absorb the shearing 
forces resulting from the particular deformations of the wave-form cords 
or wave-form filaments as deformation of the rubber. 
In order to realize the absorption of deformation by means of the cushion 
rubber layer 7, it is essential that the rubber of the cushion rubber 
layer 7 has the modulus under 100% elongated condition is less than 70 
kg/cm.sup.2. If it is more than 70 kg/cm.sup.2, the effect of absorption 
of shearing forces is lost so that separation cannot be prevented. 
Moreover, with the layer distance less than 1.5 mm, separation cannot be 
prevented either. Therefore, the layer distance must be more than 1.5 mm. 
In order to certainly obtain the layer distance more than 1.5 mm, a sheet 
made of rubber or a rubber layer may be provided between the crown 
protective layer and the belt layers. In practice, for example, ply 
coating rubber of belt layer 3 or crown protective layer 6 is made thick 
to some extent to ensure the layer distance h more than 1.5 mm. This 
method is advantageous in cost. 
Further, in tires for aircraft or the like, worn treads are usually 
exchanged with new treads to recover the tires. In this case, because the 
cushion rubber layer keeps the layer distance between the crown protective 
layer and the belt layer, it is possible to peel the tread from the crown 
protective layer to replace with a new tread without damaging the belt 
layers. In this manner the recapping operation is simplified. 
EXAMPLE 
Tires of the size of H 46.times.18.OR 20 having the construction shown in 
FIG. 1 were manufactured with layer distances shown in Table 1. Protruded 
drum tests of these tires were carried out. Results are shown in the Table 
1. Details of the respective layers are as follows. 
Carcass: 
five layers having cords of nylon 66 1,680 d/3 
Intersecting belt layer: 
having cords of nylon 66 1,680 d/4 arranged at angles of 18.degree. 
relative to circumferential directions and intersecting with each other 
Circumferential belt layer: 
having cords of nylon 66 1,680 d/4 extending in circumferential directions 
Crown protective layer: 
construction 1.times.0.2+18.times.0.175, having wave-form steel cords of 6 
mm amplitudes and 20 mm wave lengths 
Cushion rubber layer: 
made of rubber of modulus 35 kg/cm.sup.2 under 100% elongated condition 
TABLE 1 
______________________________________ 
Tire according 
to the invention 
Comparative tire 
______________________________________ 
Layer distance 
1.7 2.2 3.6 0.6 1.0 
h (mm) 
Occurrence of 
none none none occurred 
occurred 
separation in 
protruded drum 
test 
______________________________________ 
Moreover, tires were manufactured which were the same in construction as 
the tires in the Table 1 with exception of crown protective layers having 
3,000 d/3 aromatic polyamide wave-form cords with 6.7 mm amplitudes and 27 
mm wave lengths and cushion rubber layers having thickness of 2.2 mm and 
modulus of 35 kg/cm.sup.2 under 100% elongated condition. 
The same tests were carried out. Any separation did not occur. 
In the extruded drum test, a tire was urged against on a rotating drum 
having hemispherical protrusions attached thereto so as to drive the tire 
intermittently through a predetermined distance (three miles.times.50 
times). Thereafter, the tire was cut to inspect whether any separation 
occurred. Inner pressure and load were 140 PSI and 30,940 Lbs, 
respectively, which were 70% of the normal inner pressure and normal load. 
Speed was 20 miles/hour. 
Further, similar tests were effected by changing the modulus of the cushion 
rubber layers under 100% elongated condition. Results are shown in Table 
2. 
TABLE 2 
______________________________________ 
Tire according 
to the Comparative 
invention tire 
______________________________________ 
Modulus under 50 60 80 
100% elongated 
condition (kg/cm.sup.2) 
Occurrence of none none occurred 
separation 
______________________________________ 
As can be seen from the above explanation, the heavy duty pneumatic tire 
according to the invention is able to prevent external cuts in the tread 
from enlarging so that the durability can be improved. Moreover, recapping 
of the tire is very easy. 
It is further understood by those skilled in the art that the foregoing 
description is that of preferred embodiments of the disclosed tires and 
that various changes and modifications may be made in the invention 
without departing from the spirit and scope thereof.