The present invention relates to a pneumatic tire, more
particularly to a carcass structure being capable of reducing the tire weight.
In recent years, in order to improve vehicles' fuel consumption,
there is a strong demand for a lightweight tire. Therefore, in order to decrease
the tire weight, an attempt to decrease the carcass weight was made by decreasing
the cord count for the carcass ply. As well known in the art, a pneumatic tire is
vulcanized in a mold while pressurizing the inside of the tire. Therefore, there
is a tendency for the topping rubber of the innermost carcass ply to flow out through
the carcass cords during vulcanizing the tire. Accordingly, if the carcass cord
count is decreased in the innermost carcass ply, the outflow tendency of the topping
rubber increases, and as shown in Fig.5, the thickness of the topping rubber (g)
covering the inside of the carcass cords is greatly decreased from (t') to (t).
As a result, problems such as lowering of airtightness, cord corrosion, cord/rubber
separation and the like arise, and the durability is liable to decrease.
DE-A-28 51 526 discloses a flexible body having textile
cords of different diameters embedded in a rubber-like material. These textile cords
may be used in the belt of a tire for example. The textile cords can provide a tire
with the same stability as steel cords but do not conduct heat and cannot corrode.
US-A-3 783 926 discloses a tire carcass with two plies
merging into one ply. Successive cords of each ply are inserted between adjacent
cords of the other ply, so that the cords of the two plies are located substantially
at the same distance from the inner wall of the tire in regions of the carcass between
the midheights of the two sidewalls.
GB-A-990,392 discloses a tire carcass where textile cords
and metal wires are arranged alternately in the tire circumferential direction.
The textile cords and the metal wires have the same diameter and both reinforce
the tire. These heterogeneous layers comprising cords of different materials help
to decrease the deleterious defects of stresses and to increase the adhesion.
EP-A-555 071 discloses a rubber coated layer for a radial
tire where the reinforcing cords are divided into a plurality of groups and the
distance between mutually adjoining and opposite reinforcing cords not grouped with
each other is wider than a distance between mutually adjoining reinforcing elements
separately arranged at equal spaces.
Attention is also drawn to the disclosure of US-A-4 363
It is therefore, an object of the present invention to
provide a pneumatic tire, in which the tire weight is reduced without deteriorating
the durability. This object is met by a pneumatic tire according to claim 1.
Preferably, the gaps between the carcass cords are set
in a range of from 0.02 to 0.80 mm.
Therefore, the strength necessary for the carcass ply is
provided by the main cords, and the auxiliary cords hinder the outflow of the topping
rubber which causes the topping rubber thickness decrease. Accordingly, the weight
of the auxiliary cords can be minimized as far as they can prevent the outflow during
vulcanizing the tire, and the total weight of the carcass cords and topping rubber
can be minimized without deteriorating the durability.
Embodiments of the present invention will now be described
in detail in conjunction with the accompanying drawings.
- Fig.1 is a cross sectional view of a pneumatic tire according to the present
- Fig.2 is an enlarged schematic cross sectional view of a carcass ply showing
an arrangement of main cords and auxiliary cords.
- Fig.3 is an enlarged schematic cross sectional view of a carcass ply showing
another example of the arrangement of the main cords and auxiliary cords
- Fig.4 is a diagram for explaining the function of the auxiliary cords.
- Fig.5 is an enlarged schematic cross sectional view of a carcass ply used in
reference tires in the undermentioned comparison tests.
In the drawings, pneumatic tire 1 according to the present
invention comprises a tread portion 2, a pair of sidewall portions 3, a pair of
bead portions 4 each with a bead core 5 therein, a carcass 6 extending between the
bead portions 4, and a belt 7 disposed radially outside the carcass 6 in the tread
In the example shown in Fig.1, the tire 1 is a radial tire
for passenger cars.
The carcass 6 is composed of at least one ply 6A of carcass
cords 10. The carcass cords 10 in the ply 6A are arranged radially at an angle of
from 70 to 90 degrees with respect to the tire equator, and each extend between
the bead portions 4 through the tread portion 2 and sidewall portions 3, and all
is turned up around the bead core 5 in each bead portion 4 from the axially inside
to the axially outside so as to form a pair of carcass ply turnup portions 6b and
a carcass ply main portion 6a therebetween.
The carcass 6 in this example consists of the ply 6A and the cords 10 therein are
arranged radially at 90 degrees with respect to the tire equator. And an inner liner
9 made of a gas-impermeable rubber is disposed along the inside of the carcass 6
to face the tire cavity. However, in order to further decrease the tire weight,
the inner liner 9 can be omitted as explained later.
During building a raw tire, the carcass ply 6A is formed
by applying a sheet of rubberized carcass cords 10 around the tire building drum.
The carcass cords 10 in the sheet are laid parallel with each other. Each side of
the sheet is covered with a topping rubber 12 (thickness t').
According to the present invention, the carcass cords 10
in the sheet include main cords 10M and auxiliary cords 10S which alternate in the
longitudinal direction of the sheet. In the finished tire, accordingly, the main
cords 10M and auxiliary cords 10S in the carcass ply 6A alternate in the circumferential
direction of the tire. Between the adjacent main cords 10M, one to three auxiliary
cords 10S are arranged.
- Fig.2 shows an example in which one auxiliary cord 10S is disposed between the
main cords 10M.
- Fig.3 shows an example in which two auxiliary cords 10S are disposed between
the main cords 10M.
The main cords 10M are for providing support to the tire
inner pressure and tire load. In other words, the main function of the main cords
10M is to reinforce the carcass ply. On the other hand. the auxiliary cords 10S
are for controlling the outflow of the topping rubber 12 during vulcanization.
In order to achieve a tire weight reduction while maintaining
the strength necessary for the carcass ply, a high-strength, high-modulus cord is
used as the main cords 10M and the cord count of the main cords 10M is decreased.
Given that a strength index is the product Tm X Km of the
cord strength Tm (N) of the main cord 10M and the cord count Km of the main cords
10M per 5 cm ply width, usually, the strength index is set in a range of from 4,000
to 20,000 N/5 cm in case of passenger car tires, and in case of heavy duty tires
for trucks and buses, the strength index is set in a range of from 15,000 to 100,000
N/5 cm. It is preferable that the cord strength Tm is not less than 140 (N) in case
of a passenger car tire, and in case of heavy duty tire for trucks and buses, the
cord strength Tm is not less than 800 (N).
Incidentally, the cord strength Tm may be increased by increasing the cord thickness
Dm and/or using a material superior in the rupture strength.
Therefore, it becomes possible to minimize the strength of the auxiliary cord as
far as the auxiliary cords can prevent the outflow of the topping rubber. Thus,
the auxiliary cords may have a less strength and/or less thickness than the main
cords. For example, in case of the passenger car tires in which organic fiber material,
e.g. nylon, polyester, rayon and the like is conventionally used in the carcass
cords, the strength can be increased by 1) increasing the cord thickness Dm, 2)
using high-strength, high-modulus organic fiber material such as high-modulus polyethylene,
high-modulus vinylon, aromatic polyamide, polyolefin ketone and the like, or steel
fiber material, and/or 3) changing the cord structure.
In case of the heavy duty tires in which steel cords are conventionally used as
the carcass cords, the strength can be increased by 1) increasing the cord thickness
Dm and/or 2) changing the cord construction.
As explained above, the main function of the auxiliary
cords 10s is to control the outflow of rubber during vulcanization. Therefore, the
cord strength Ts thereof can be decreased to under the cord strength Tm of the main
cord 10M. Further, in view of the material cost and weight reduction, it is preferable
that the product Ts X Ks of the cord strength Ts and the number Ks (= 1, 2 or 3)
of the auxiliary cord(s) 10s between the main cords 10M is set in a range of not
more than 0.3 times, more preferably not more than 0.2 times, still more preferably
not more than 0.15 times the cord strength Tm.
Incidentally, the cord strength Ts of the auxiliary cord 10S can be decreased by
making the cord thickness Ds smaller than the main cords 10M and/or employing a
material having a smaller rupture strength.
When the cord strength Ts is decreased by only decreasing
the cord thickness Ds, it is preferable that the cord thickness Ds is set in a range
of from 0.1 to 0.67 times, more preferably 0.1 to 0.4 times the cord thickness Dm
of the main cords 10M. If the thickness Ds is more than 0.67 times the thickness
Dm, the tire weight increases contrary to the purpose of the present invention.
If the thickness Ds is less than 0.1 times the thickness Dm, it becomes difficult
to control the outflow of the topping rubber.
Further, in order to effectively prevent the outflow of
the topping rubber, the cord count of the carcass cords 10 (inclusive of 10M and
10S) is determined such that the gaps L therebetween are within a range of from
0.02 to 0.80 mm and substantially constant.
If the gaps L are more than 0.80 mm, the outflow is liable to occur. If the gaps
L are less than 0.02 mm, fretting wear is liable to be caused between the adjacent
cords and separation from the toping rubber tends to occur.
The above-mentioned inner liner 9 is made of air-impermeable
rubber and extends over the tire inner surface facing the tire cavity with a substantially
constant thickness. As the air-impermeable rubber, used is a butyl rubber compound
including not less than 20 weight % of butyl rubber and/or halogenated butyl rubber
as its base rubber material. A diene rubber can be used as the remaining part of
the base rubber material, if any. In the air-impermeable rubber compound, a halogenide
of isobutylene-paramethyl styrene copolymer can be also used instead of the butyl
rubber and/or halogenated butyl rubber.
As mentioned before, in order to further reduce the tire
weight, it is preferable that the air-impermeable rubber is used as the topping
rubber 12 for the carcass ply 6A and the inner liner rubber 9 is omitted. In this
case, it is possible that the air-impermeable rubber is used only in an inside part
of the topping rubber facing the tire cavity, and in the outside part, a different
rubber compound, e.g. diene rubber and the like is used.
The above-mentioned belt includes a breaker 7 and optional
band disposed radially outside the breaker 7. The breaker 7 comprises at least two
cross plies 7A and 7B of cords laid parallel with each other at an angle of from
10 to 35 degrees with respect to the tire equator. For the breaker cords, steel
cords and high performance organic fiber cords such as aromatic polyamide and the
like can be suitably used. In this example which is a passenger car radial tire,
the belt 7 is composed of two cross breaker plies ply 7A and 7B. In case of heavy
duty tire, the breaker 7 composed of three or four plies is usually used.
Radial tires having a tire size of 195/65R14 for passenger
cars and a tire size of 11R22.5 for trucks and buses were made and tested as follows.
The test tires were disassembled, and the thickness of
the carcass topping rubber covering the carcass cords was measured as the thickness
t after the tire vulcanization and the thickness (t) is shown in Table 1 together
with the thickness (t') before vulcanization.
Further, undulation of the carcass topping rubber due to
the outflow during vulcanization was visually observed. According to the position
of the boundary between the carcass topping rubber and inner liner rubber, the degree
of undulation was ranked as follows.
- A: The undulation was not observed.
- B: The boundary did not run out beyond the inner ends of the cords as shown
- C: The boundary ran out to a position between the inner ends of the cords and
the cord center line H.
- D: The boundary ran out beyond the cord center line H as shown in Fig. 5.
The test results and the specifications of the carcass
ply are shown in Table 1. The cord thickness and cord strength were measured according
to Japanese Industrial Standard (JIS) L-1017 in case of organic fiber cords and
JIS G-3510 in case of steel cords.
Tm X Km
(Ts X Ks)/Tm