Note: Descriptions are shown in the official language in which they were submitted.
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62-155,397
HIGH INTERNAL PRESSURE TYPE
HEAVY DUTY PNEUMATIC RADIAL TIRES
The present invention relates to high internal
pressure type heavy duty pneumatic radial tires. More
specifically, the invention relates to high internal
pressure type heavy duty pneumatic radial tires which
are to be served under higher internal pressures and
higher loads as compared with conventional heavy duty
pneumatic radial tires for use in trucks, buses, etc.
and which have improved shoulder edge drop wear
resistance and uneven wear resistance.
Conventional heavy duty pneumatic radial tires
used in trucks, buses, etc. are produced by shaping
an outer contour of a tread inside a mold at a single
radius of curvature as viewed in radial planes and
effecting vulcanization there, and are run under
application of a use internal pressure of about
7.25 kg/cm2. These tires have especially no problem in
terms of uneven wear resistance.
However, if conventional heavy duty pneumatic
radial tires of this type for use in trucks, buses, etc.
were employed under higher load, it was revealed that
the tires had troubles in uneven wear resistance.
Under the circumstances, the present inventors
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have tried to develop heavy duty pneumatic radial tires
to be used under loads higher than those in the case of
conventional ones, and proceeded their studies with
respect to the following two points:
(l) An air volume inside a tire is increased.
(2) A use internal pressure of a tire is
increased.
As a result, they found that the above item (l) can be
realized by making a diameter of a use rim smaller,
while with respect to the item (2), a tire can be
designed to withstand such a high use internal pressure
of 9.0 to 12.0 kg/cm2.
When a high internal pressure type heavy
duty pneumatic radial tire having the outer contour of
a tread formed at a single radius of curvature, inside
a mold, as in the case of heavy duty pneumatic radial
tires to be used in trucks, buses, etc., was vulcanized
inside the mold and actually run under application of a
high internal pressure (9.8 kg/cm2) on a test course,
its uneven wear resistance was not satisfactory. Then,
a cause was investigated, and the following fact was
revealed. That is, when a high internal pressure (9.8
kg/cm2 or more) is applied to a tire after vulcaniza-
tion, an edge of a tread of the actual tire is radially
inwardly located by a vertical distance from
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an edge of a tread having an imaginary single radius of
curvature expected at a shoulder of the tread. In this
way, it is seen that application of such a high inter-
nal pressure to the tire makes the outer contour of the
tread shoulder portion to have an unexpected radius of
curvature which is smaller than the radius of curvature
at the central portion of the tread.
Turning to a ground contact shape (foot
print) of the tire to which load is applied, the fol-
lowing fact has been made clear. That is, edge cornersof the ground contact area are round so that the ground
contact length of the edge of the foot print is
shorter. Consequently, the tread is dragged at the
edges of the ground contact area, and thus the edges
are worn at an early stage, which leads to uneven wear-
ing such as shoulder drop wearing.
Further, it was analyzed that when the tire
is inflated at a high use internal pressure, the outer
contour of the tread expands radially outwardly from
that of the tread inside the mold. At that time, since
the use internal pressure is very high, the center por-
tion of the tread between almost 1/4 points, that is,
points which are axially separated from a tread center
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by a distance equal to about 1/2 of a half of the width
of the tread more expands radially outwardly than the
tread shoulder portion. Consequently~ the contour of
the tread shoulder comes to have a second radius o~
curvature due to the application of the internal
pressure to cause a radial drop of the tread edge.
Such a phenomenon is not seen in conventional heavy duty
pneumatic radial tires of this type to be used for
trucks, buses, etc. at low internal pressures.
It is an object of the present invention to
provide high internal pressure type heavy duty pneumatic
radial tire~ which have uneven wear resistance such as
shoulder drop wear re~istance greatly improved, when in
use at an internal pressure 19.0 to 12.0 kg/cm2) higher
than in the case of conventional heavy duty pneumatic
tires ~or use in trucks and buses, by making a dropped
amount of the outer ~ontour of the tread shoulder
smaller than that o~ the tread shoulder having a single
radius oE curvature and thus preventing the ground
contact len~th of edges o~ the tread ~n contact with
ground under load from becoming shorter.
In order to attain the above-mentioned object,
according to the present invention, the outer contour of
the tread at a central portion between points
axially separated f rom the tread center by a distance
~alling in a range from 0.64 to 0.85 of a halE of the
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width o~ the tread inside a mold has a posi-tive
radius of curvature which has a curvature center o:l
a side of an axis oE the tire, while the contour o~ the
tread at each o~ the shoulder portions located axially
outside the above points inside the mold have
a negative radius oE curvature which possesses
a curvature center on a side opposite to the tire axis
while the negative radius of curvature has an absolute
value in a range from 1 to 30 times the width of the
tread. As a result, an amount for correcting
a tread shoulder drop (hereinafter referred to as "tread
shoulder edge drop-correcting amount") may be in a range
~rom 0.05 to 0.4 time an amount of a shoulder drop
occurring in the case of a tire with a tread having
a crown of a single radius of curvature inside a mold,
whereby the entire outer contour of the tread expands
radially outwardly from that o~ the tread inside the
mold corresponding thereto through application of a use
internal pre3sure to have a single radius of curvature.
~ ccording to the present invention, despite that
the use internal pressure is high, the foot print is
made almost rectangular by enlarging the ground contact
length of the tread edges. Consequently, uneven wear
resistance can be improved to a large extent.
These and other objects, features, and
advantages of the present invention will be appreciated
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upon reading of the following description of the
invention when taken in conjunction with the attached
drawings, with the understanding that some modiflca-
tions, variations, and changes of the same could be made
by a person having an ordinary knowledge of the art to
which the invention pertains, without departing from the
spirit of the invention or the scope of claims appended
hereto.
For a better understanding of the invention,
reference is made to the attached drawings, wherein:
Fig. 1 is a schematic sectional view
illustrating an outer contour of a tread of a tire
according to the present invention inside a mold;
Fig. 2 is a schematic sectional view of the
outer contour of the tread of the tire according to the
present invention when being inflated at a use internal
pressure;
Fig. 3 is a schematic view showing that the
entire outer contour of the tread of the tire according
to the present invention expands radially outwardly from
that of the tread inside the mold when the tire is
inflated at a use internal pressure;
Fig. 4 is a schematic sectional view in which
outer contours of treads of tires in the present
invention, and Comparative Examples 1 and 2 are shown by
a solid line, a dotted line, and a one dot-chain line,
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respectively, under application of a use internal
pressure;
Fig. S is a foot print showing a ground contact
shape of the tread of the tire according to the present
invention;
Fig. 6 is a schematic sectional view
illustrating an outer contour of a tread of a tire in
Comparative Example 2 inside a mold;
Fig. 7 is a schematic sectional view illustrat-
ing the outer contour of the tread of the tire in
Comparative Example 2 when the tire being inflated at
a use internal pressure;
Fig. 8 is a foot print of the tire in
Comparative Example 2;
Fig. 9 is a schematic sectional view illustrat-
ing the outer contour of a tread of a tire in
Comparative Example 1 inside a mold;
Fig. 10 ia a schematic sectional view of the
outer contour of the tread of the tire in Comparative
Example 1 when the tire is inflated at a use internal
pressure;
Fig. 11 is a front print of the tire in
Comparative Example l;
Fig. 12 is a schematic view showing a maximum
height SH from a bead base to a tread inside a mold; and
Fig. 13 is a schematic sectional view showing
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a location where a tread is worn near a tread edge.
First, numerical limitations of the present
invention will be explained below.
The reason why the boundary "m" between the
tread portions having positive and negative radii of
curvature, respectively, is set in range from
0.64x(TW/2) to 0.85x(TW/2) from the tread center "~" is
as follows:
(1) If the above boundary between the tread
portions having positive and negative radii of
curvature, respectively, is located at less than
0.64x(TW/2) from the tread center, the diameter of the
tread near the 1/4 point increases upon application of
the internal pressure/ so that balance of the crown
portion breaks and the ground contact length of the
shoulder portion of the tread becomes shorter.
Consequently, edge drop wear is likely to occur.
~ 2) On the other hand, if the boundary between
the tread portions having positive and negative radii of
curvature, respectively, is located at more than
0.85x(TW/2) from the tread center, the radius of the
curvature of the latter tread portion remains negative
after the application of the internal pressure. Thus,
the ground contact length of this tread portion becomes
shorter to accelerate wearing there.
The reason why the absolute value of the
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negative radius of curvature is set in a range from 1 TW
to 30 TW is as follows:
(1) If the magnitude of the negative radius of
curvature is less than 1 TW, it remains negative after
the application of the internal pressure. Thus, the
ground contact length thereof becomes shorter to
accelerate wearing there.
(2) If the negative radius of curvature exceeds
30 TW, the diameter of the tread near the 1/4 point
increase to break the balance of the crown shape.
Consequently, the ground contact length of the shoulder
portion becomes shorter by the shortened amount so that
edge drop wear is likely to occur.
In all examples given below, H3/H1 was set at
0.15. However, as discussed later, it is possible to
set the tread shoulder edge drop correcting value H3 in
a range from 0.05 to 0.40 of H1.
H1 is a vertical distance between a center "Q"
of a tread and an edge "O" of the tread which is formed
at a ~ingle radius of curvature, "R1", as viewed in
a tread ~ection inside a mold (see Fig. 1).
H3 is a vertical distance between the tread edge
"O" and an edge, "P", of a tread which is formed at
a negative radius of curvature, "R2", at a shoulder
portion according to the present invention. That is,
H3 is a tread shoulder edge drop-correcting value
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indicating how much the edge "P" of the tread formed by
the above positive radius of curvature is radially
outwardly shifted by the negative radius of curvature,
"R2," according to ~he present invention (see Fig. 1).
As mentioned above, the reason why the tread
shoulder edge drop-correcting value H3 is set in a range
from 0.05 to 0.40 of ~1 is as follows:
(1) If the tread shoulder edge drop-correcting
value H3 is less than 0.05 Hl, the ground contact shape
of the tread shoulder portion dc7 not largely differ from
that of Comparative Example 1 (fc~rmed by a mold having
a crown of a single radiu~ of curvature). Thus,
an effect of the negative radius of curvature, "R2",
upon the tread shoulder portion according to the present
invention is reduoed.
(2) On the other hand, if the tread shoulder
edge drop-correcting value ~3 is more than 0.40 Hl, the
ground contact length o the foot print at the tread
ground contact edge only becomes too great.
Conse~uently, since the length of the tread ground
contact area axially inside the tread edge remains
small, uneven wearing of the tread axially inside ~he
tread ground contact edge is promoted.
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6 (3 0
Fig. 9. shows a schematic sectional view of
a high internal pressure type heavy duty pneumatic
radial tire (Comparative Example 1) having the outer
contour of a tread formed at a single radius of curva-
ture, R6, inside a mold, a~ in the case of heavy duty
pneumatic radial tires to be used in trucks, buses,
etc.
When such a tire was vulcanized inside the
mold and actually run under application of a high
internal pressure (9.8 kg/cm2) on a test course, its
uneven wear resistance was not satisfactory. Then, a
cause was investigated, and the following fact was
revealed. That is, when a hiyh internal pressure (9.8
kg/cm2 or more) is applied to a tire after vulcaniza-
tion, as shown in Fig. 10, an edge "y" of a tread of
the actual tire is radially inwardly located by a ver-
tical distance "S" from an edge "x" of a tread having
an imaginary single radius of curvature (drawn by a
dotted line), R7, expected at a shoulder of the tread.
In this way, it is seen that application of such a high
internal pressure to the tire makes the outer contour
of the tread shoulder portion to have an unexpected
radius of curvature, Rg, which is smaller than the
radius of curvature, R7, at the central portion of the
tread.
Turning to a ground contact shape (foot
print) of the tire to which load is applied, the fol-
lowing fact has been made clear. That is, as shown in
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Fig. 11, edge corners of the ground contact area areround so that the ground contact length, "t", of the
edge of the foot print is shorter. Consequently, the
tread is dragged at the edges of the ground contact
area, and thus the edges are worn at an early stage,
which leads to uneven wearing such as shoulder drop
wearing.
Further, it was analyzed that as shown in
Fig. 9, when the tire in Comparative Example 1 is
inflated at a high use internal pressure, the outer
contour of the tread expands radially outwardly from
that of the tread inside the mold. At that time, since
the use internal pressure is very high, the center por-
tion of the tread between almost 1/4 points, that is,
points which are axially separated from a tread center
by a distance equal to about 1/2 of a half of the width
of the tread expands more radially outwardly than the
tread shoulder portion. Consequently, the contour of
the tread shoulder comes to have a second radius of
curvature, Rg, due to the application of the internal
pressure to cause a radial drop "S" of the tread edge.
Such a phenomenon is not seen in conventional heavy
duty pneumatic radial tires of this type to be used for
trucks, buses, etc. at low internal pressures.
(Examples)
Figs. 1-5 show by way of example embodiments
according to the present invention. Fig. 1 is
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1~ .
, . . .
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a schematic sectional view illustrating an outer contour
of a tire according to the present invention inside
a mold. Fig. 2 is a schematic sectional view
illustrating the outer contour of the tire according to
the present invention which is inflated at a use
internal pressure. Fig. 3 is a schematic sectional view
showing that the entire outer contour of the tread of
the tire according to the present invention radially
outwardly expands from the tread outer contour inside
the mold. Fig. 4 is a schematic sectional view
illustrating outer contours of treads of the tires
according to the present invention, and Comparative
Examples l and 2 in a solid line, a dotted line, and
a single dotted chain line, respectively. Fig. 5 is
a foot print showing the shape of a ground contact area
of the tread according to the present invention.
In the drawings, "E" denotes a high internal
pressure type heavy duty pneumatic radial tire as
an embodiment according to the present invention in
which an outer contour of a center portion of a tread
between the tread center "Q" and a point "m" separated
from the tread center line CL by a distance equal to
from 0.64 to 0.85 of a half of the tread width, (TW/2)
inside a mold has a positive radius of curvature, Rl,
having a center of the curvature on a side of an axis of
the tire, while the outer contour of the tread shoulder
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portion axially outwardly located from the above
point "m" inside the mold has a negative radius of
curvature, R2, which is in a range from 1 to 30 of the
tread width (TW) and which has a curvature center on the
side opposite to the tire. ~ tread shoulder edge drop-
correcting value H3 is in a range from 0.05 to 0.40 of
a drop amount Hl in the case that a tread crown is
formed at a single radius of curvature. When the tire
is inflated at a use internal pressure, the entire outer
contour of the tread expands radially outwardly from
that of the tread inside the mold to make the outer
contour of the tread at a single radius of curvature so
that a ground contact shape of the tread under load
becomes almost rectangular.
Now, explanation will be made in more detail.
In the specific examples, H3/Hl was all set at
0.15, but as mentioned later, it is possible to set H3/H
in a range from 0.05 to 0.40 Hl.
The tire size in the examples was
TBRE 13.50/85 R 16, and a rim used was 9.00 V x 16.
The other dimensions are given in the following Table 1.
In Fig. 3, outer contours of the tread exhibited
when the tire is assembled to a rim and inflated at
an internal pressure of 0.5 kg/cm2 and a use internal
pressure of 9.8 kg/cm2 are drawn by a solid line and
a dotted line, respectively.
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Steel cords reinforcing a carcass (not shown)
are arranged at 90 with respect to a tire
circumferential direction. A belt B is constituted by
steel cords. Cord angles of lower, intermediate and
upper belt layers Bl, ~2, and B3 are 23 rightwardly
inclined, 72 rightwardly inclined, and 72 leftwardly
inclined with respect to the tire circumferential
direction, respectively A main belt portion is
constituted by intersecting belt cords of the
intermediate belt layer B2 and those of the upper belt
layer B3.
The shape of a buttress leading to a side wall
of the tire from the tread edge influences an edge drop-
wearing which is likely to occur near the tread edge.
As shown in Fig. 1, the buttress is formed from the
tread edge "P" at a curvature having its center on
a straight line passing through the tread edge and in
parallel to the tire rotary axis or in a range X which
falls within 5% of the sectional height SH (see Fig. 12)
of the tire radially and axially outwardly from this
straight line, and it is preferable to set the radius of
the curvature, "r", in a range from 5% to 25% of the
tread width (=2 x Ll).
Further, according to this tire, the radius of
curvature, "r", of the buttress is set at 20 mm (about
9.3% of the tread width). As shown in Fig. 1, the
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131960Q
center of the curvature of the buttress is located near
the line passing through the tread edge "P" and in
parallel with the tire rotary axis.
By designing the buttress like this, rigidity of
the tread edge portion can be optimized and occurrence
of edge drop wearing can be delayed.
(Comparative Examples)
Comparative Examples have the same fundamental
constituent features as in Examples other than the
dimensions given in Table 1.
In Comparative Example 2, the sectional shape of
the shoulder portion of the tread inside a mold was
straight. As shown in Fig. 6, the shoulder portion was
formed by a straight line continuing to the curve of the
radius of curvature, Rl. Thus, the outer contour of the
tread shoulder radially outwardly expands as compared
with the tire in which the tread is formed at a single
positive radius of curvature, Rl (the shoulder portion
is depicted by a dotted line).
That is, the outer contour of the tread is
formed by the straight line such that the tread edge may
be located at "P" radially outside the edge "O" of the
tread having the imaginary single positive radius of
curvature R1.
The tangential point between the curve of the
radius of curvature, Rl, and the straight line is
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located at L2=68 mm from the tread center. The tread
shoulder edge drop-correcting value H3 is 1.1, and H3/H
is 0.15.
Fig. 7 shows the tire in Comparative Example 2
which was vulcanized in the mold and inflated at a use
internal pressure of 9.8 kg/cm2. In the tire of
Comparative Example 2, the outer contour of the tread is
formed at radii of curvature, R4 and R5 (smaller than R4)
at the center portion and the shoulder portion of the
tread, respectively (the tread shoulder portion is
depicted by a dotted line). Consequently, the tread
edge "Y" drops radially inside the tire by "S" from
an edge "X" of a tread which would be formed at
an imaginary radius of curvature, R4.
As shown in Fig. 8, ground contact corners of
a foot print of the tire in Comparative Example 2 are
rounded 80 that edge drop wearing at an early stage
cannot be restrained due to variation in the ground
circumferential length of the edge portion of the ground
contact area.
Test Method:
Tires were tested at an internal pressure of
9.8 kg/cm2 under a load of 4 ton/tire in the Bridgestone
te~t course as mounted on a truck.
In Table 2, Tl and T2 denote edge drop wearing
at a tread edge and wearing axially inside the tread
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edge, respectively.
Since the negative radius of curvature of the
crown portion in Comparative Example 3 is too small and
H3/Hl in Comparative Example 5 is too large, the outer
contour of the tread when the tires were inflated at
a use internal pressure remained at a reverse radius of
curvature at the tread edges. Consequently, the
circumferential ground contact length of the tread in
the foot print at a zone near the tread edge becomes
smaller so that T2 as a core of uneven wearing, which
causes a portion of the tread having smaller ground
contact length to be more worn, occurs at an early stage
to deteriorate uneven wearing resistance (Fig. 13).
Fig. 8 shows a foot print of the tire in
Comparative Example 2. Ground contact corners are
rounded to make the circumferential ground contact
length at the edge portions ununiform. Thus, edge drop
wearing at the early stage cannot be prevented.
To the contrary, as shown in Fig. 5, the foot
print of Examples according to the present invention
exhibit a substantially rectangular shape to suppressed
edge drop wearing.
In Table 2, with respect to each of Tl and T2,
results are shown by index taking that of Example 1
as 100. The greater the index of each of Tl and T2, the
more excellent the wearing resistance.
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i i i ~ _ 1
D, ~ ~ n = n R n ~ ~ o o
u ~ ~ n s r~ s n ~n _l o o
11 o ~ n n n o R v~l ~ _i u a
Cr=~= ~ O~ _ l ~D l = l ~n l
,1 R N ~ ~ _ o _ _
E~ R ~ L L ~ R N 11~ _I L O
K N o N o s o m _i o o
n _ L ~ . ~` ~ ~
R ~ i r R R = .~ o
~ ~ ~ O,~ r C
~ ~ ~3 ~ _ ~ _ ~ ~ ~ ~ 3
3 ~ N N ~! ~ ~! ~ tl~ _ )~ ~2:
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~ ~ U~
U
,
~ ~ aJ
~ ~ o
U~-a
~'V ~ CO o
U~
~,,o o
E V 13 ~ o
U~W
~-=o
~ o o
.0~ i~ cn _
_l o o
~ o ~o
o ~
~ o ,ol
Pq
.~c ~ ~ -- ~a --
tl- C ~ ~ S tl- tJ~ C
~ ~d'C ~ D. ~ '~
~ ~ 3 ~ o 1- c: JJ 3
c: 0 ~ O ~ ~ ~ ul o ~ n~
~ 3 1~ 3
13196~0
As mentioned above, the edge drop wearing (Tl)
and wearing (T2) occurring axially inside the tread edge
at the early stage as cores of uneven wearing can be
suppressed as seen in Table 2. Thus, high internal
pressure type heavy duty pneumatic radial tires having
excellent wear resistance can be obtained.
As shown in Fig. 4, the present invention is to
provide high internal pressure type heavy duty pneumatic
radial tires having the tread of a single radius of
curvature and more excellent wear resistance with
a substantially rectangular foot print when being
inflated at a high use internal pressure as compared
with Comparative Examples.
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