Pneumatic tires with cable bead core

Resilient tires and wheels – Tires – resilient – Pneumatic tire or inner tube

Reexamination Certificate

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Details

C245S001500

Reexamination Certificate

active

06302175

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a pneumatic tire comprising a bead core formed by laminating three or more sheath layers on a core wire.
2. Description of Related Art
In pneumatic tires used under conditions of a high speed and a heavy load such as a pneumatic tire for an aircraft and the like, a large stress is usually applied to a bead portion during the running, so that a bead core durable to such a large stress must be embedded in the bead portion.
As the bead core durable to such a large stress, there has been known, for example, a bead core as disclosed in JP-A-53-51804, that is, a bead core
1
called as a cable bead formed by laminating plural sheath layers
4
, each of which layers being constituted by substantially spirally twisting a plurality of fine sheath filaments
3
, on a single thick core wire
2
as shown in FIG.
5
.
In the conventional bead core
1
, the twisting direction of the sheath filament
3
in the sheath layers
4
becomes reverse every the sheath layer. For example, when the sheath filament
3
in a certain sheath layer
4
among the sheath layers
4
is Z-lay, the sheath filaments in both sheath layers adjacent thereto (inside and outside sheath layers) are S-lay. When the twisting direction of the sheath filament
3
is reverse every the sheath layer as mentioned above, the torsional rigidity of the bead core
1
can be increased, whereby the change of deformation bearing ratio in each tire portion can be decreased in correspondence with the change of slip angle to improve steering stability and high-speed straight running performance.
However, when the pneumatic tire provided with the conventional bead core
1
is used over a long period of time, there is caused a problem that the fatigue fracture is often created in the sheath filament
3
of a second sheath layer
4
viewed from the outermost side to degrade the durability of the bead portion.
This is due to the fact as mentioned below. That is, when the pneumatic tire is run under a heavy load, the bead core
1
is repeatedly subjected to torsion in left-handed and right-handed rotations under an action of a large external force. In this case, if the torsion in the right-handed rotation (clockwise rotation) is applied to the bead core
1
, a sheath layer
4
Z in which a twisting direction of a sheath filament
3
is Z-lay is deformed into a direction of opening the sheath filaments
3
Z (or a direction of increasing the size of the sheath layer
4
Z), while a sheath layer
4
S in which a twisting direction of a sheath filament
3
is S-lay is deformed into a direction of closing the sheath filaments
3
S (or a direction of decreasing the size of the sheath layer
4
S). On the other hand, if the torsion in the left-handed rotation (anticlockwise rotation) is applied to the bead core
1
, the sheath layer
4
Z of Z-lay is deformed into the direction of closing the sheath filaments
3
Z (or the size-decreasing direction), while the sheath layer
4
S of S-lay is deformed into the direction of opening the sheath filaments
3
Z (or the size-increasing direction).
As a result, if sheath filaments in a certain sheath layer are Z-lay and sheath filaments in both sheath layers adjacent to the Z-lay sheath layer are S-lay as mentioned above, when the torsion in the right-handed rotation is applied to the bead core, the sheath filaments
3
Z in the Z-lay sheath layer
4
Z deforming into the size-increasing direction directly contact with the sheath filaments
3
S of the outer S-lay sheath layer
4
S deforming into the size-decreasing direction to rub them with each other and also the sheath filaments
3
Z are pushed down by the outer sheath filaments
3
S to locally dent at the contacting position inward in the radial direction. On the other hand, when the torsion in the left-handed rotation is applied to the bead core, the sheath filaments
3
Z in the Z-lay sheath layer
4
Z deforming into the size-decreasing direction directly contact with the sheath filaments
3
S of the inner S-lay sheath layer
4
S deforming into the size-increasing direction to rub them with each other and also the sheath filaments
3
S are pushed down by the outer sheath filaments
3
Z to locally dent at the contacting position inward in the radial direction.
The rubbing force and the dent deformation amount as mentioned above become larger at more outer sheath layer among the sheath layers. However, the sheath filaments in the outermost sheath layer are not rubbed or dented by pushing in the deformation into the size-increasing direction. Therefore, it is frequent that the fatigue fracture is firstly created in the sheath filaments of a second sheath layer viewed from the outermost side of all sheath layers by the rubbing and dent deformation as mentioned above.
SUMMARY OF THE INVENTION
It is, therefore, an object of the invention to provide a pneumatic tire capable of improving the durability of the bead portion by effectively controlling the fatigue fracture in the sheath filaments of a second sheath layer viewed from the outermost side of all sheath layers.
According to the invention, there is the provision of in a pneumatic tire comprising a bead portion provided with a bead core formed by laminating three or more sheath layers, each of which layers being constituted by substantially spirally twisting a plurality of fine sheath filaments, on a single thick core wire and making reverse twisting directions of the sheath filaments in the sheath layers every the sheath layer, an improvement wherein the number of twisting pitches in a sheath filament of a second sheath layer viewed from an outermost side of the sheath layers is made smaller than that in a sheath filament of a sheath layer located inward from the second sheath layer.
When the number of twisting pitches in the sheath filament of the second sheath layer is made smaller than that in the sheath filament of the sheath layer located inward from the second sheath layer as mentioned above, the pitch of the sheath filament in the second sheath layer is made long and the inclination angle of the sheath filament with respect to the circumferential direction of the tire is made small, whereby the cross angle with respect to the sheath filament in the adjoining sheath layer, i.e. outermost sheath layer or a third sheath layer viewed from the outermost side becomes small (more acute angle).
As a result, the contacting length in the contact between the second sheath layer and the sheath layer adjacent thereto becomes long to lower the rubbing stress, whereby the fatigue fracture of the sheath filament in the second sheath layer is effectively controlled. And also, as the pitch of the sheath filament in the second sheath layer becomes long as mentioned above, the radius of curvature in the sheath filament itself becomes large, so that when the sheath filament is pushed and locally dented by the sheath filament in the outer sheath layer, curvature in the dented portion is made small (gentle), whereby the fatigue fracture through repetitive bending deformation of the sheath filament is effectively controlled.
Further, the maximum rubbing force is applied to the sheath filaments in the outermost sheath layer as mentioned above, so that there is a case that the fatigue fracture may be first caused in the sheath filament of the outermost sheath layer. In this case, however, the fatigue fracture in the sheath filament of the outermost sheath layer can effectively be controlled by making the number of twisting pitches in the sheath filament of the outermost sheath layer smaller than that in the sheath filament of the second sheath layer.
Moreover, when the number of the sheath layers is four or more, if the number of twisting pitches in the sheath filament is gradually or stepwise decreased from the innermost sheath layer toward the outermost sheath layer, the fatigue fracture of the sheath filament in a third sheath layer viewed from the outermost side can effectively be controlled.


REFERENCES:
patent: 4887655 (1989-12-01), Imai et al

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