1. Field of the Invention
The present invention relates to a steel cord for radial tire, and more particularly relates to a steel cord to be used in a carcass of a radial tire for truck and bus.
2. Description of the Prior Art
As the steel cord to be used in the carcass portion of a radial tire for truck and bus, there had previously been predominantly used a strand construction steel cord 7.times.4+1 illustrated in FIG. 1, but has recently been predominantly used a layer construction steel cord 3+9+15+1 illustrated in FIG. 2A. The reason is as follows. In the strand construction steel cord 7.times.4+1, filaments constituting the steel cord are brought into point contact with each other in their length direction, and are rubbed with each other in many portions, and hence the cord is not satisfactorily high in fatigue resistance and wear resistance. In order to obviate these drawbacks, a layer construction steel cord 3+9+15+1 has recently been used. In the layer construction steel cord 3+9+15+1, filaments constituting the cord are brought into line contact with each other, and are very slight in the mutual rubbing, and hence the cord is excellent in fatigue resistance and wear resistance, and has a long durable life.
The layer construction steel cord predominantly used at present is formed of 28 filaments twisted in a layer construction. The steel cord is indicated by the numeral 1 in FIG. 2A, and consists of 3 layers A, B and C, each of which consists of filaments 2, and a wrapping wire 3. In the layer construction cord illustrated in FIG. 2A, filaments constituting the outer layer C are compactly twisted such that substantially no clearance is formed between adjacent filaments, and the cord periphery of the layer construction cord illustrated in FIG. 2A is more smooth than the cord periphery of the strand construction cord 5 illustrated in FIG. 1, which has almost the same number of filaments as that of the cord illustrated in FIG. 2A. Further, when a cylindrical calendered material consisting of cords and unvulcanized rubber is inflated and is toroidally formed to build a tire having a given outer diameter as illustrated in FIG. 7, if a conventional compact cord is used, the amount of rubber to be penetrated between filaments is small, and the unvulcanized rubber is easily separated from the cord, and a vacant space is formed around the cord. That is, when a portion having a high stickiness between a cord and unvulcanized rubber, and a portion having a low stickiness between a cord and unvulcanized rubber are concurrently present in an intermingled state, a large vacant space and a small vacant space are formed around the cords, and the resulting carcass 6 has an uneven surface formed of non-periodically and irregularly arranged valleys a and mountains b as illustrated in FIGS. 8A and 8B, and the green tire 7 has an irregular outward appearance. When a reinforcing belt layer is superposed on the carcass having such irregular outward appearance in the next building step, the carcass portion and the belt portion are ununiformly stuck to each other to disturb the adhesion of rubber of the tire after vulcanization and to deteriorate the durable life of the tire.
Radial tires are required to have an accuracy remarkably higher in the dimension of parts materials and in the building of the tire than the accuracy required in bias tires due to the difference of the tire construction. It is generally known that the above described unevenness in the carcass surface of a green tire deteriorates noticeably the durable life of a vulcanized tire.