Pneumatic radial tire with zigzag steel cord belt layer

A pneumatic radial tire includes a cylindrical steel cord belt layer having a two-layered structure is formed by winding a tape obtained by burying one or a plurality of steel cords in parallel with one another in a matrix, round the outer circumference of a carcass layer on a tread while bending the tape zigzag.

BACKGROUND OF THE INVENTION 
This invention relates to a pneumatic radial tire having good high speed 
durability and excellent in uniformity, maneuvering stability and driving 
comfort. 
A steel cord belt layer utilizing an excellent strength and a high elastic 
modulus of a steel cord has been used in the past for a belt portion of a 
pneumatic radial tire. This steel cord belt layer has a structure wherein 
steel cords are disposed at a relatively small angle (10.degree. to 
30.degree.) in a tire circumferential direction, and cross one another 
between plies, and a cut fracture exists at both side edges in a 
transverse direction thereof. Therefore, the pneumatic radial tire is not 
free from the drawbacks that a stress concentrates on the cut fracture, 
separation is therefore likely to occur between the steel cords and a coat 
rubber, and high speed durability drops. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a pneumatic 
radial tire which has good high speed durability without using auxiliary 
means such as an organic fiber cover layer, though it uses steel cords for 
a belt layer, and is moreover excellent in uniformity, maneuvering 
stability and driving comfort. 
In a pneumatic radial tire having a belt layer disposed outside a carcass 
layer on a tread, the object of the present invention described above can 
be accomplished by a pneumatic radial tire characterized in that a tape 
formed by burying one or a plurality of steel cords in parallel with one 
another in a matrix, is wound round the outer circumference of the carcass 
layer while being bent zigzag in such a manner as to form a cylindrical 
steel cord belt layer having a two-layered structure, a cord angle .theta. 
of the steel cord belt layer in a tire circumferential direction, a width 
D of the steel cord belt layer, a radius r of the cylinder of the steel 
cord belt layer and the number of zigzag bendings n of the tape per 
circumference of the tire satisfy the relation tan 
.theta.=n.times.D/2.pi.r and 7.degree.&lt;.theta.&lt;20.degree., and the steel 
cord comprises a 0.15 to 0.5 mm-diameter single steel wire shaped into a 
spiral or corrugated shape in the longitudinal direction thereof or has an 
a.times.b twist structure (a=1 to 4 and b=2 to 5) having a blank wire 
diameter of 0.1 to 0.25 mm. 
Here, the term "radius r of the cylinder" means the distance from the axis 
of rotation of the tire to the inner side surface of the cylindrical steel 
cord belt layer having the two-layered structure on the equator of the 
tire. 
Because the steel cord belt layer is formed by winding the tape round the 
outer circumference of the carcass layer while being bent as described 
above, a cut fracture does not exist at both side edges of the steel cord 
belt layer (belt edge portions) in the transverse direction, and the 
possibility of occurrence of separation on the cut fracture is small. 
Accordingly, high speed durability can be improved. 
Because the steel cord belt layer is formed by winding the tape round the 
outer circumference of the carcass layer while being bent, an overlap 
portion (splice portion) does not occur in comparison with the 
conventional pneumatic radial tires wherein both end portions of a steel 
cord rubber-lined sheet for the belt layer are overlapped so as to form 
the belt layer. Therefore, uniformity (UF) can be improved, and radial 
force variation (RFV) representing the change of the reaction which the 
tire receives in the radial direction can be reduced, in particular. 
Moreover, because the steel cord belt layer having a high rigidity is 
disposed at the belt portion without the cords being cut, the steel cord 
belt layer secures transverse rigidity of the tire. For this reason, 
maneuvering stability is not deteriorated. 
Because uniformity (UF) can be improved as described above, driving comfort 
can be improved. However, if the width of the tape is excessively 
increased, bending becomes difficult and moreover, non-uniformity of 
rigidity increases undesirably between the winding start edge (winding end 
edge) of the tape and other portions. Accordingly, the tape width is 
preferably not greater than 15% of the width D of the steel cord belt 
layer (belt bending width).

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the end portion of a carcass layer 2 is turned up from 
inside to outside of the tire around each of the pair of right and left 
bead cores 1, and a steel cord belt layer 4 is disposed outside the 
carcass layer 2 on a tread 3 in such a manner as to extend throughout the 
circumference of the tire in a tire circumferential direction. In FIG. 1, 
the steel cord belt layer 4 has a two-layered or even-numbered layered 
structure but the number of layers may be four (4) or six (6) so long as 
the layers are even-numbered layers. 
A plurality of grooves 5 extending in the tire circumferential direction 
and a plurality of grooves (not shown in the drawing) in a tire width-wise 
direction are disposed on the surface of the tread 3, that is, on the 
tread surface. 
The steel cord belt layer 4 is formed, as shown in FIG. 2, by moving zigzag 
tapes 6, which are in turn formed by burying one or a plurality of steel 
cords 1, preferably five to ten steel cords, in a matrix in parallel with 
one another, with an even-numbered or odd-numbered number of bendings 
throughout the circumference of the tire of the carcass layer 2 in the 
transverse direction of the tire, and winding the tapes 6 on the outer 
circumference of the carcass layer 2 while bending the tapes at the 
corresponding end portions 7 and 8 of the steel cord belt layer 4 in the 
transverse direction. This winding work is carried out a large number of 
times in the tire circumferential direction while the winding position is 
deviated by the distance corresponding to the width of the tape 6 lest 
gaps are formed between the tapes 6. This winding state will be explained 
with reference to FIG. 3. 
FIG. 3 shows the case where the number of zigzag bending of the tape 6 per 
circumference of the tire is n=2. In FIG. 3, the winding start tape 1 
starts at one of the corresponding end portions 7 in the transverse 
direction, is bent at the other corresponding end portions 8 in the 
transverse direction, returns to one of the corresponding end portions 7 
and then connects with the next tape 2, which is bent at the one of the 
corresponding end portions 7 of the belt layer 4 in the transverse 
direction while the winding position is deviated by the distance 
substantially corresponding to the tape width with respect to the winding 
start tape 1 in the tire circumferential direction. This winding operation 
is sequentially repeated for the tapes 2 to 8. Therefore, since two tapes 
6 are always superposed with each other as a whole, the resulting steel 
cord belt layer 4 has a two-layered (double layer) structure. Though the 
number of the steel cord belt layers 4 may be even, one double layer is 
preferably formed from the aspects of the cost and the weight. 
Incidentally, when the number n of zigzag bendings of the tape 6 per 
circumference of the tire is even, steps occurs concentratedly through the 
tape at the intermediate portion between the winding start edge of the 
winding start tape 1 and the winding end edge of the winding end tape 8 as 
shown in FIGS. 4 and 5. These steps are not preferable for uniformity of 
the tire. 
To further improve uniformity of the tire, it is advisable to set the 
number n of zigzag bendings of the tapes 6 per circumference of the tire 
to an odd number. When the number of zigzag bendings is the odd number, 
the step does not concentratedly occur between the winding starting edge 
of the winding start tape and the winding end edge of the winding end tape 
as represented by asterisk "*" in FIG. 6, and the step occurs between the 
tapes which are under winding, so that the steps are dispersed throughout 
the circumference of the tire. 
Next, the relation between the number n of zigzag bendings of the tape 6 
per circumference of the tire and the cord angle .theta. of the steel cord 
belt layer 4 in the tire circumferential direction is shown in FIGS. 7 to 
12. FIG. 7 shows the case of n=1, FIG. 8 shows the case of n=2, FIG. 9 
shows the case of n=3, FIG. 10 shows the case of n=4, FIG. 11 shows the 
case of n=5 and FIG. 12 shows the case of n=6. As can be clearly 
understood from FIGS. 7 to 12, the cord angle .theta. becomes greater with 
an increasing n value. Therefore, in order to keep the cord angle .theta. 
within the range of .theta.=7 to 20.degree., the present invention sets 
the number of bendings n to two or four in the case of the even number and 
three or five in the case of the odd number. 
In the steel cord belt layer 4 of the present invention formed in the 
manner described above, the cord angle .theta. is so set as to satisfy the 
relation tan.theta.=n.times.d/2.pi.r and to be within the range of 
7.degree.&lt;.theta.&lt;20.degree. when the cord angle in the tire 
circumferential direction is .theta., the width is D, the radius of the 
cylinder is r and the number of zigzag bendings of the tape 6 per 
circumference of the tire is n. For, the cord angle .theta. is determined 
by the number n, the width D and the radius r of the cylinder. The reason 
why .theta. is set to be within 7.degree.&lt;.theta.&lt;20.degree. is to improve 
rigidity in the tire circumferential direction by reducing as much as 
possible the cord angle .theta. and to improve high speed durability. 
The steel cord used for the tape 6 comprises a 0.15 to 0.5 mm-diameter 
steel single wire shaped into a spiral or corrugated shape in its 
longitudinal direction, or has an a.times.b twist structure (a=1 to 4, b=2 
to 5) of blank wires having a diameter of 0.1 to 0.25 mm. The steel cord 
is constituted in this manner because in order to wind the tape 6 round 
the outer circumference of the carcass layer while bending it zigzag, the 
steel cord must have a small bending rigidity. The steel cord comprising a 
steel single wire is shaped into the spiral or corrugated shape in its 
longitudinal direction because, when the cord angle .theta. is set to a 
small value within the range of 7.degree.&lt;.theta.&lt;20.degree. as described 
above, the steel cord belt layer 4 cannot easily follow up the expansion 
of the tire diameter at the time of vulcanization of the tire unless the 
steel cord is shaped into such a shape. In other words, shaping of the 
steel cord as described above allows the steel cord to smoothly elongate 
in the longitudinal direction and also allows the steel cord belt layer 4 
to easily follow up the expansion of the tire diameter. 
When the steel cord comprises a 0.15 to 0.5 mm-diameter steel single wire 
shaped into the spiral or corrugated shape in the longitudinal direction, 
it satisfies the relation 1.0 (mm).gtoreq.d+.lambda. and 0.30.ltoreq.tan 
.alpha.&gt;0.05 where d is the diameter of the single wire 10 (mm), .lambda. 
is the amplitude of the single wire 10 (mm) as shown in FIG. 13 and 
.alpha. is the angle of inclination of the single wire 10 with respect to 
the axis of the cord longitudinal direction. When d+.lambda. exceeds 1.0 
(mm), the cord diameter becomes so great or the shaped corrugation becomes 
so large and the coat rubber thickness increases so much that the tire 
weight undesirably increases. When tan .alpha. is less than 0.05, the 
bending rigidity of the cord becomes high and when it exceeds 0.30, on the 
other hand, the proportion of the shape of the spiral or corrugated shape 
becomes so great that productivity drops and the elastic modulus drops 
because elongation of the cord becomes great. When the steel cord 
comprises the a.times.b double twist structure (a=1 to 4, b=2 to 5), the 
blank wire diameter is 0.1 to 0.25 mm. When the blank wire diameter is 
less than 0.1 mm, the wire is so thin that the strength becomes low and 
when the blank wire diameter exceeds 0.25 mm, on the other hand, the 
bending rigity becomes excessively high. This structure is typified by 
3.times.3 (0.15), for example. 
The tape 6 is formed by burying one or a plurality of such steel cords in a 
matrix in parallel with one another. The matrix in this case is not 
limited to the rubber, and plastic materials such as a urethane resin can 
be also used. 
EXAMPLE 
(1) High speed durability, uniformity, driving comfort and maneuvering 
stability were evaluated under the following conditions for the Tires Nos. 
1 to 3 of the present invention, the Comparative Tire 1 and Conventional 
Tire 1 listed below. 
Conditions 
air pressure: 1.9 kg/cm.sup.2, 
rim: 14.times.51/2JJ, 
load: 500 kg, 
tire size: 195/70 R14 
Present Tire 1 
The steel cord belt layer had a two-layered structure, and the tape used 
was formed by burying 10 steel cords 3.times.3 (0.15) in parallel in a 
rubber. The tape having a width of 10 mm was wound two times round the 
outer circumference of the carcass layer at n=2 and .theta.=8.degree.. 
tire outer diameter: 655 mm, 
width of belt layer D: 135 mm, 
radius of belt layer r: 315 mm, 
tan.theta.=n.times.D/2.pi.r=n.times.135 mm (2.pi..times.315 mm) 
Present Tire 2 
The same as the Present Tire 1 except that n=4 and .theta.=16.degree.. 
Present Tire 3 
The tape was produced by aligning in parallel five individual wires shaped 
into the corrugated shape (d=0.30 mm, d+.lambda.=0.60 mm, tan.alpha.=0.18) 
and burying them in a rubber, and had a tape width of 5 mm. The tape was 
wound two times round the outer circumference of the carcass layer at n=2 
and .theta.=8.degree.. The rest were the same as those of the Present Tire 
1. 
Comparative Tire 1 
The same as the Present Tire 1 except that n=6 and .theta.=23.degree.. 
Conventional Tire 1 
Two steel cord belt layers; cord angle in the tire circumferential 
direction =20.degree.; cord crossed mutually between plies. 
inner steel cord belt layer: 
steel cord 2+2 (0.25), 40 ends/50 mm, 
width =130 mm 
outer steel cord belt layer: 
steel cord 2+2 (0.25), 40 ends/50 mm, 
width=120 mm 
belt reinforcing layer: 
one, formed by winding spirally and continuously a sheet of two-twist cord 
of nylon fiber of 840 D at 55 ends/50 mm, round the outer circumference of 
outer steel cord belt layer at width of 10 mm at a very small angle in 
tire circumferential direction; width =140 mm. 
Uniformity: 
The test was carried out in accordance with "Uniformity Testing Method of 
Automobile Tires", JASO C607. The result was indicated by an index using 
the value of the Conventional Tire 1 as 100. The greater this value, the 
better uniformity. 
High Speed Durability: 
After a JATMA high speed durability test using a drum diameter of 1,707 mm 
was completed, the test was continued at an acceleration rate of 10 km/hr 
until each test tire was broken. The result was indicated by an index 
using the value of the Conventional Tire 1 as 100. The greater this value, 
the higher became the durability. 
It could be understood that the smaller the cord angle .theta. (the smaller 
the n value), the higher became the high speed durability. 
Maneuvering Stability & Driving Comfort 
Maneuvering stability was evaluated by a feeling test of an actual car. In 
this case, the test tires were fitted to a Japanese-make car of a 2.5 l 
class, and three test panelists evaluated maneuvering feeling by moving 
thrice the car in the transverse direction. In Table 1, circle 
".smallcircle." represents "very good", double circle ".circleincircle." 
represents "excellent", triangle ".DELTA." represents "good" and "X" 
represents "poor". 
Driving comfort was evaluated also by the feeling test. The test tires were 
fitted to a Japanese-make car of a 2.5 l class; and three test panelists 
evaluated driving comfort by feeling by driving the car on an irregular 
road surface. In Table 1, circle ".smallcircle." represents "very good", 
double circle ".circleincircle." represents "excellent", triangle 
".DELTA." represents "good" and "X" represents "poor". 
TABLE 1 
__________________________________________________________________________ 
No. of high speed 
bendings cord durability 
driving 
maneuvering 
n angle .theta. 
uniformity 
(index) 
comfort 
stability 
__________________________________________________________________________ 
Conventional 
-- 20.degree. 
100 100 .DELTA. 
.DELTA. 
Tire 1 
Comparative 
6 23.degree. 
105 95 .circleincircle. 
X 
Tire 1 
Present 
2 8.degree. 
100 125 .largecircle. 
.circleincircle. 
Tire 1 
Present 
4 16.degree. 
105 115 .circleincircle. 
.largecircle. 
Tire 2 
Present 
2 8.degree. 
100 120 .largecircle. 
.circleincircle. 
Tire 3 
__________________________________________________________________________ 
As can be understood clearly from Table 1, the Present Tires Nos. 1 to 3 
were excellent in all the aspects of high speed durability, uniformity, 
driving comfort and maneuvering stability. 
(2) High speed durability, uniformity, driving comfort (bump get-over test) 
and maneuvering stability were similarly evaluated for the Present Tires 
Nos. 4 to 6, Comparative Tire 2 and Conventional Tire 2 under the 
following conditions. The results were tabulated in Table 2. 
Conditions 
air pressure: 1.9 kg/cm.sup.2, 
rim: 14.times.51/2JJ, 
load: 500 kg, 
tire size: 195/70 R14 
Present Tire 4 
The steel cord belt layer had a two-layered structure, and the tape used 
was produced by burying ten steel cords 3.times.3 (0.15) in parallel in a 
rubber, and had a width of 10 mm. The tape was wound in two layers around 
the outer circumference of the carcass layer at n=3 and 
.theta.=12.degree.. 
tire outer shape: 655 mm, 
width of belt layer D=135 mm, 
radius of belt layer: r=315 mm, 
tan.theta.=n.times.D/2.pi.r=nx135 mm/(2.pi..times.315 mm) 
Present Tire 5 
The same as the Present Tire 4 except that n=5 and .theta.=19.degree.. 
Present Tire 6 
The tape was produced by aligning in parallel five individual wires shaped 
into the corrugated shape (d=0.30 mm, d+.lambda.=0.60 mm, 
tan.alpha.(=0.18) and burying them in a rubber, and had a tape width of 5 
mm. The tape was wound two times round the outer circumference of the 
carcass layer at n=3 and .theta.=12.degree.. The rest were the same as 
those of the Present Tire 4. 
Comparative Tire 2 
The same as the Present Tire 4 except that n=7 and .theta..degree.=26. 
Conventional Tire 2 
Same as Conventional Tire 1. 
TABLE 2 
__________________________________________________________________________ 
No. of high speed 
bendings cord durability 
driving 
maneuvering 
n angle .theta. 
uniformity 
(index) 
comfort 
stability 
__________________________________________________________________________ 
Conventional 
-- 20.degree. 
100 100 .DELTA. 
.DELTA. 
Tire 2 
Comparative 
7 26.degree. 
110 90 .circleincircle. 
X 
Tire 2 
Present 
3 12.degree. 
105 125 .largecircle. 
.circleincircle. 
Tire 4 
Present 
5 19.degree. 
110 105 .circleincircle. 
.largecircle. 
Tire 5 
Present 
3 12.degree. 
105 120 .largecircle. 
.circleincircle. 
Tire 6 
__________________________________________________________________________ 
As can be clearly understood from Table 2, the Present Tires 4 to 6 were 
excellent in all the aspects of high speed durability, uniformity, driving 
comfort and maneuvering stability. 
As explained above, in the pneumatic radial tire according to the present 
invention, the tape formed by burying one or a plurality of steel cords in 
parallel with one another in the matrix is wound round the outer 
circumference of the carcass layer while the tape is bent zigzag in such a 
manner as to form the cylindrical steel cord belt layer having the 
two-layered structure. When the cord angle of the steel cord belt layer in 
the tire circumferential direction is .theta., the width of the steel cord 
belt layer is D, the radius of the cylinder of the steel cord belt layer 
is r and the number of zigzag bendings of the tape per circumference of 
the tire is n, these parameters satisfy the relation tan 
.theta.=n.times.D/2.pi.r and 7.degree. &lt;.theta.20.degree., and the steel 
cord comprises a 0.15 to 0.5 mm-diameter single steel wire shaped into the 
spiral or corrugated shape in the longitudinal direction thereof or has 
the a.times.b twist structure (a=1 to 4 and b=2 to 5) having a blank wire 
diameter of 0.1 to 0.25 mm. Accordingly, the pneumatic radial tire of the 
present invention has excellent high speed durability and moreover, can 
improve uniformity, maneuvering stability and driving comfort. 
Further, in the pneumatic radial fire according to the present invention, 
the steel cord belt layer is formed by winding the tape outside the 
carcass layer while being bent throughout one circumference of the tire. 
Therefore, this steel cord belt layer can be produced by the existing belt 
forming drum that has been used in the past. Since new equipment need not 
be installed additionally, the pneumatic radial tire can be produced 
economically.