Steel cord and method for producing a steel cord

A first aspect of the present invention relates to a filament (11; 12), especially for reinforcing rubber articles. Said filament (11; 12) features a contact surface (14) and an outer surface (13). In a second aspect, the invention relates to a steel cord (10) comprising two of said filaments (11, 12), the contact surfaces (14) are arranged adjacent to each other. The outer surfaces (13) are configured arcuate shaped and provide a smooth outer contour (15) of the steel cord (10). Due to said construction the largest dimension (d) of the steel cord (10) and the thickness of a rubber coating (18) may be considerably reduced. Additionally the invention relates to a method of producing a steel cord (10) and to a tyre (20) comprising a carcass ply (22) and/or at least one belt (25; 26) including said steel cords (10).

In its first aspect the present invention relates to a filament, especially for reinforcing rubber articles. A second aspect of the present invention relates to a steel cord, especially for reinforcing rubber articles, comprising two filaments combined in parallel in order to form said steel cord.

A third aspect of the present invention relates to a method of producing said steel cord.

In a further aspect, the present invention relates to a pneumatic tyre comprising a tread member, a carcass, two side walls and at least one belt.

A steel cord of the above mentioned kind is known from U.S. Pat. No. 4,606,392 and used for reinforcing vehicle tyres. Said known steel cord comprises several metal wires having each a substantially rectangular cross-section defining opposite broad and narrow sides. The metal wires in the steel cord contact one another along their broad sides to provide a strand of a substantially rectangular cross-section. Other known steel cords comprise two circular filaments with a diameter of around 0.30 mm. Usually, the steel cord is provided with a rubber coating prior to embedding in the material of a tyre. The known steel cord features a rather rough, uneven outer surface. Consequently, the thickness of the rubber coating applied must be at least 0.4 mm higher than the largest dimension of the steel cords. For example a steel cord comprising two filaments with a diameter of 0.30 mm will require a rubber coating with a thickness of 1.10 mm. Accordingly, the known steel cord requires a rather thick coating and is quite heavy.

Therefore it is an object of the invention to provide a filament and a steel cord allowing a reduction in rubber coating thickness. It is another object of the invention to provide a method of producing such a steel cord. A fourth object of the invention is to enhance tyre properties by using relatively small and lightweight steel cords.

Said object is achieved by a filament featuring a contact surface and an arcuate shaped outer surface.

In its second aspect the invention relates to a steel cord as mentioned above which is characterized by the use of filaments in accordance with the invention and in that the contact surfaces of said two filaments are arranged adjacent to each other in order to provide a smooth outer contour of said steel cord.

Additionally the invention relates to a method of producing a steel cord comprising two filaments, each of said filaments featuring a contact surface and an arcuate outer surface, wherein the contact surfaces of said two filaments are arranged adjacent to each other, which is characterized in that said two filaments are pre-shaped and subsequently combined in order to form said steel cord.

The fourth aspect of the invention relates to a pneumatic tyre as set forth above which is characterized in that said carcass and/or said at least one belt includes a steel cord in accordance with the second aspect of the present invention.

The cross-section of the filaments with a contact surface and an arcuate shaped outer surface and the arrangement of the contact surfaces adjacent to each other provide a smooth outer contour of the steel cord. Therefore, the largest dimension of the steel cord may be reduced so that the thickness of the rubber coating may be reduced, too. Reduction of rubber coating thickness leads to a number of advantages. First, the expenses for coating are significantly reduced. Second, the steel cord may be arranged closer to each other, e.g. in the carcass ply or belt of a tyre. Therefore, the weight, stiffness and handling of the tyre may be improved compared to the prior art steel cords.

It is preferred to pre-shape the filaments and combine them subsequently. The filaments may be independently pre-shaped from each other. It is possible to obtain any desired form for the contact surface and the outer surface. Additionally, configuration of both the contact surface and the outer surface may be easily assessed. Combining the pre-shaped filaments is to advantage effected with a double twister as disclosed in EP 396 068 B1 issued to the applicant of the present application. Disclosure of EP 396 068 B1 shall be incorporated by reference.

Advantageous embodiments of the invention read from the dependent claims.

To advantage the contact surface is configured flat or slightly curved. The ideal form of the contact surface is a perfect plane. However, slight deviations from said ideal form are not detrimental.

The outer surface may be configured semi-circular or semi-elliptical. As alternative, it may be configured polygonal. With said aspect the outer surface to advantage comprises more than three sides. In order to minimize the maximum dimension of the steel cord, a semi-circular outer surface is used. However, even with a semi-elliptical or polygonal outer surface of the filament the steel cord in accordance with the invention still features a smooth outer contour and a reduced maximum dimension. Again, rubber coating thickness may be considerably reduced.

According to a further aspect of the invention the transition between the contact surface and the outer surface features a radius. None of the filaments is provided with sharp edges which may damage the rubber coating during use. Accordingly, the life time of the steel cord provided with their rubber coating is increased.

To advantage the steel cord features a gap between the contact surfaces of said two filaments. Said gap may be penetrated by the rubber coating in order to avoid corrosion. Moreover, direct contact between the filaments which is likely to damage a protective coating of the filaments is avoided.

To advantage the two filaments feature the same cross-section. It will not be necessary to manufacture and stock different filaments in order to produce the steel cord in accordance with the invention. Additionally, both filaments exhibit the same physical properties.

In accordance with another embodiment the two filaments may feature different cross-sections. One filament may be configured semi-circular while the other one is configured semi-elliptical. It is possible to create a huge variety of different steel cords which are suited for a number of different applications.

It is preferred if the tyre in accordance with the invention comprises two belts which are arranged between the tread member and the carcass. To advantage, the tyre features are radial-ply concept. The use of two belts provides higher safety and reliability, while the radial-ply concept enhances the properties of the tyre.

FIG. 1shows a cross-section of a prior art steel cord A with two circular filaments B separated by a gap E. The steel cord A is provided with a coating C. The largest dimension of said steel cord A is indicated at D. The thickness of said coating C is determined by said largest dimension D. InFIG. 1, the steel cord A is shown enlarged with a scale of approximately 10:1, the filaments B having a diameter of 0.30 mm (a radius of 0.15 mm) and the coating C having a thickness of 1.10 mm. It is obvious that said coating C is unnecessarily thick at the upper and lower side as shown inFIG. 1.

FIG. 2shows a cross-section of a steel cord10in accordance with the invention. Said steel cord10comprises two filaments11,12which are approximately semi-circular and is provided with a coating18. The filaments11,12are separated by a gap16. The cross-sectional area of said filaments11,12is identical to that of the prior art filaments B. The largest dimension is indicated at d. Again, the steel cord10is shown enlarged with scale of 10:1. A comparision ofFIGS. 1 and 2clearly shows that the steel cord10in accordance with the invention features a reduced largest dimension d. Accordingly, thickness of the coating18may be reduced.

In both the prior art steel cord A and in the steel cord10in accordance with the invention the gap E or16, respectively, is filled with the coating. Direct contact between the filaments and corrosion thereof is avoided.

In the embodiment shown, the filaments11,12are semi-circular. Their respective radius may be calculated as follows:

Cross-sectional area of prior art filaments B (radius r1):
A=r12*π

The calculated cross-sectional areas shall be identical:

Largest dimension D of prior art steel cord A:
D=4*r1+gap

Largest dimension d of steel cord10in accordance with the invention:
d=2*r2+gap=2*√{square root over (2)}*r1+gap≈2.82*r1+gap

The gap E is approximately equal to the gap16. Therefore, the change Δ in the largest dimension d or D, respectively, amounts to:
Δ=D−d≈1.18*r1

The absolute change Δ with r1=0.15 mm equals to 0.177 mm. The relative change Δ/D equals to 0.1609 or a reduction of about 16%. Accordingly, the thickness of the coating may be reduced, too.

The prior art coating thickness T amounts to
T=D+0.4 mm=4*r1+gap+0.4 mm≈1.10 mm
while the coating thickness t in accordance with the invention is
t=d+0.4 mm=2.82*r1+gap+0.4 mm≈0.823 mm

Cross-sectional area A2of coating18in accordance with the invention:

Reduction Δ% of cross-sectional area in percent:
Δ%=(A1−A2)/A1≈44.%

Accordingly, the steel cord10in accordance with the invention allows for a significant reduction in coating thickness.

FIGS. 3 to 11show cross-sections of nine different embodiments of a steel cord10in accordance with the invention. As already shown inFIG. 2, the steel cord10comprises two filaments11,12, each featuring an outer surface13and a contact surface14. The contact surfaces14are arranged adjacent to each other so that the outer surfaces13provide a smooth outer contour15of the steel cord10. The contact surfaces14are separated by the gap16.

FIGS. 3 and 4show a steel cord10comprising two substantially semi-circular filaments11,12. In the embodiment shown inFIG. 3, the contact surfaces14are configured flat.FIG. 4shows filaments11,12with contact surfaces14which are slightly curved. It should be noted that curvature of the contact surfaces14is exaggerated in the figures for better understanding.

FIGS. 5 and 6show a steel cord comprising filaments11,12which are configured semi-elliptical. Again, the contact surfaces14may be flat or slightly curved.

FIGS. 3 to 6show different embodiments of a steel cord10the filaments11,12of which feature the same cross-sectional area and cross-section. It is, however, possible to combine filaments11,12of different cross-sections and cross-sectional areas as shown inFIGS. 7 and 8.FIG. 7depicts schematically a semi-circular filament11together with a semi-elliptical filament12.FIG. 8shows a filament11configured slightly bigger than a semi-circle. Corresponding filament12, accordingly, is configured smaller than a semi-circle. The gap16is formed by curved contact surfaces14arranged in parallel.

In all embodiments shown the outer surface13is configured arcuate, especially semi-circular or semi-elliptical. It is possible to use a polygonal outer surface13, too. It is, however, important that the outer contour15formed by the two outer surfaces13be smooth.

FIGS. 9 to 11show steel cords10comprising one or two filaments11,12with a polygonal outer surface13. Said steel cords10do still feature a smooth outer contour15.FIG. 9shows an embodiment with two filaments11,12featuring the same cross-section. The outer contour comprises six sides19.

FIG. 10shows a combination of a semi-elliptical filament11and a polygonal filament12. The outer contour13of filament12comprises seven sides.FIG. 11shows two filaments11,12which are basically semi-elliptical, but provided with a polygonal outer surface13. Both filaments11,12feature seven sides.

It is important that the outer surface13of filaments11,12features at least three sides. Otherwise, the steel cord10would feature a nearly rectangular outer contour15which would not allow the desired reduction in rubber coating thickness.

FIG. 12shows an enlarged view of detail X inFIG. 7. The transition between the contact surface14and the outer surface13features a radius17. Said radius17ensures that no sharp edges are present which might damage the rubber coating during use.

Preferably, the above described filaments are wound together according to a winding pitch comprised between 5 mm and 30 mm.

FIG. 13shows a partial cross-section of a tyre20comprising a tread member21, at least a carcass ply22and two side walls23provided with beads24to which the extremities of the carcass ply are associated. Each bead portion24comprises a reinforcing bead core50provided, in a radially external position, with a filler60. Arranged between the tread member21and the carcass ply22are at least two belts25,26. Said belts25,26comprise steel cords10as described above, parallel to each other in each belt and crossed with the cords of the adjacent belt.

In the embodiment shown, the tread member21comprises a base27and a cap28, the base27being provided with shoulders29extending up to the tread member surface. Arranged between the right-hand shoulder29and the cap28is a groove30.

Profile blocks31,32of the base27and the cap28are obtained by grooves33.

Due to the reduction of the largest dimension of the steel cords10the thickness of their rubber coating18may be reduced. Therefore, the steel cords10may be arranged closer to each other in the belts25,26. At the same time, the radial dimension of belts25,26may be reduced, so that the distance between the tread member21and the carcass ply22may be reduced, too. As alternative additional safety means may be inserted.

The steel cord10in accordance with the invention allows a significant reduction of the largest dimension. At the same time, a smooth outer contour15of the steel cord10is provided. Consequently, the thickness of the coating18may be considerably reduced, leading to significant cost savings. Additionally, weight, stiffness and handling of a tyre20provided with a steel cord10in accordance with the invention may be improved.