The present invention relates to a rubber reinforcing steel cord, which is excellent in the rubber permeability and the fatigue resistance and also has a high productivity, and to a heavy duty pneumatic radial tire using the steel cord as the cord for the carcass layer.
Hitherto, in a heavy duty pneumatic radial tire used for large vehicles such as trucks, buses, etc., in order to make endurable heavy load, steel cords are frequently used as cords for the carcass layers forming the tire skeleton.
As the steel cord, as shown in FIG. 3, a layer-twisted structure of 3+9+15+W obtained by intertwining nine (9) steel filaments 2 around a core portion formed by intertwining three (3) steel filaments 1, intertwining 15 steel filaments 3 around the 9 steel filaments 2, and further binding them by wrapping a wrapping filament W around the outside of them; and a bunched structure (bundle-twisted structure) of 1xc3x9712+W obtained by intertwining 12 steel filaments and binding them by wrapping a wrap filament around the outside of them, or of 1xc3x9727+W obtained by intertwining 27 steel filaments and binding them by wrapping a wrap filament around the outside of them are generally used. In addition, the wrap filament is for binding the intertwined steel filaments so that the cord is not dispersed.
However, in these steel cords of the layer-twisted structure or the bunched structure, there was a problem that because of the existence of the wrap filament, fretting occurred between the wrap filament and the steel filaments just under the wrap filament during the use to lower the strength of the steel filament, which resulted in lowering the fatigue resistance of the steel cord.
Also, in the steel cord of the layer-twisted structure, there was a problem that because of the closest structure having no gaps amount the steel filaments, a rubber permeability into the inside of the cord was bad, whereby when water penetrated during the use or storage of the tire reached the cord, the cord was corroded to lower the corrosion resistance and the fatigue resistance. On the other hand, in the steel cord of the bunched structure, the productivity can be improved because the number of the production steps is less as compared with the steel cord of the layer-twisted structure but there was a problem that because the structure was also the closest structure as the layer-twisted structure, the rubber permeability into the inside of the cord was bad.
Thus, recently, for improving lowering of the fatigue resistance by fretting, a steel cord having no rap filament is proposed (Japanese patent application Kokai publication No. 8-176977), wherein the cord is a three-layer structure as shown in FIG. 4 but the number of steel filaments of out outermost layer is reduced by one or two than the closest number (the maximum number capable of being disposed) of steel filaments to increase the rubber permeability of the outermost layer (3+9+14 layer-twisted structure in FIG. 4) and the cord is bound by the permeated rubber. However, in the steel cord, because other inside layers than the outermost layer are the closest structure, the rubber permeability is insufficient.
Also, a steel cord of the layer-twisted structure or the bunched structure using at least one wave-like shaped steel filament at the core portion for increasing the rubber permeability is proposed (Japanese patent application Kokai publication No. 9-31875). However, in the steel cord, the rubber permeability is improved but the form of the cross section of the cord does not become a homogeneous round (becomes distorted), when the steel cord is used as a cord of the carcass layer and a largely bending input force is applied to the cord, an ununiform strain occurs in the cord, whereby the fatigue resistance of the cord is lowered. An embodiment of the cord is shown in FIG. 5. The embodiment of FIG. 5 is a structure that one of three steel filaments of the core portion is wave-like shaped and in the embodiment, a steel cord of a layer-twisted structure of 3Cr+9+14 is constructed by intertwining 9 steel filaments 2 around the core portion formed by intertwining 3 steel filaments including the one above-described steel filament and intertwining 14 steel filaments 3 around the 9 steel filaments 2 (wherein, Cr means a wave-like shaped form). However, in the steel cord of FIG. 5, the cord is produced by three times twisting, which is disadvantageous in the production cost.
As other steel cords, for example, the steel cords shown in FIG. 6 and FIG. 7 are proposed. In the embodiment of FIG. 6, a steel cord of the bunched structure of 3Cr/24 obtained by disposing total 24 steel filaments 2 and 3 around three(3) steel filaments 1 wave-like shaped in the lengthwise direction is constructed. In the steel cord of FIG. 6, the productivity is high because the production step is one-time twisting only but because the cross-sectional form of the cord is nearly a triangle (straw bag form) and thus a strain becomes ununiform, the fatigue resistance is inferior and also because the steel cord has almost the closest structure, the rubber permeability is insufficient. In the embodiment shown in FIG. 7, a steel cord of the layer- twisted structure of 3Cr+9+14 obtained by intertwining 9 steel filaments 2 around a core portion formed by arranging 3 steel filaments 1 wave-like shaped in the lengthwise direction without intertwining, and intertwining 14 steel filaments 3 around the 9 steel filaments 2 is constructed. In the steel cord of FIG. 7, because 3Cr of the core portion are straight, when a cord tension is applied, the tension burden is largely applied to the core portion (the core portion is stretched), the partial charge of the tension becomes ununiform and the fatigue resistance is no good.
The present invention provides a rubber reinforcing steel cord being excellent in the rubber permeability and the fatigue resistance and also having a high productivity and also the invention provides a heavy duty pneumatic radial tire using the above-described steel cord as the cord of the carcass layer.
That is, the rubber reinforcing steel cord of the invention is a steel cord of a bundle-twisted+layer-twisted structure obtained by arranging 9 steel filaments around three steel filaments 1 each being two-dimensionally wave-like shaped in the lengthwise direction, intertwining all of these steel filaments in one operation to form a bundle-twisted cord, and further arranging 14 steel filaments around the bundle-twisted cord followed by intertwining, wherein when the diameter of the two-dimensionally wave-like shaped steel filament is d, the wave-form height thereof is h, and the wave form pitch thereof is P, the parameter F shown by (hxe2x88x92d)/P is in the relation of 0.001xe2x89xa6Fxe2x89xa60.03 and further, the ratio b/a of the long diameter axe2x80x2 and the short diameter b of the circumscribed circle of the cross section of the bundle-twisted cord is 0.94xe2x89xa6b/axe2x89xa61.00, and the ratio bxe2x80x2/axe2x80x2 of the long diameter axe2x80x2 and the short diameter bxe2x80x2 of the circumscribed circle of the cross section of the steel cord is 0.96xe2x89xa6bxe2x80x2/axe2x80x2xe2x89xa61.00.
As described above, in the invention, because the 3 steel filaments of the core portion are wave-like shaped in the lengthwise direction, the existence of gaps in the core portion is ensured and thus the rubber permeability of the cord becomes good. Also, because the steel cord of the invention has the bundle-twisted+layer-twisted structure having the above-described core portion, the cross sectional form of the steel cord can be made almost homogeneous and round and thus, when a large bending input force is applied to the steel cord, an ununiform strain does not occur in the steel cord, whereby the fatigue resistance of the steel cord is not lowered. Furthermore, because the steel cord of the invention has the bundle-twisted+layer-twisted structure, the number of the production steps can be reduced as compared with the relates art layer-twisted structure, whereby the productivity can be increased.
Also, in the heavy duty pneumatic radial tire of the invention using the above-described rubber reinforcing steel cord excellent in the rubber permeability and the fatigue resistance as described above as the cord of the carcass layer, the durability can be increased.