Patent Publication Number: US-2021188006-A1

Title: Construction vehicle tire and steel cord

Description:
TECHNICAL FIELD 
     The present invention relates to a construction vehicle tire and a steel cord (hereinafter, merely referred to as “cord”), in particular, a steel cord used for reinforcing a rubber product such as a tire. 
     BACKGROUND ART 
     In a pneumatic tire for a heavy load used in a construction vehicle or the like, a belt is generally formed by a plurality of belt layers in order to suppress growth of a diameter when the tire is inflated to a predetermined inner pressure or to improve shear resistance against a projection such as a rock. Further, various methods to improve separation resistance between the belt layers are known. For example, in Patent Literature 1, a belt is formed by four belt layers and an innermost belt layer and a belt layer adjacent to the innermost layer at an outer side in a tire radial direction are mutually inclined in opposite directions against a tire width direction so as to intersect each other. 
     Further, as a reinforcing material for the belt layer, a steel cord formed by twisting a plurality of steel filaments is generally adopted. In particular, a tire used in a large vehicle such as a construction vehicle is used under a heavy load on an uneven ground having large unevenness, and therefore high strength and high durability are required especially to the steel cord served as a reinforcing material. Accordingly, in a large size tire, a steel cord having a so-called multi-twisted structure formed by twisting a plurality of strands, each of the strands being formed by twisting a plurality of the steel filaments, is adopted. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Unexamined Patent Application Publication No. 2004-098850 
     SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
     However, when the tire having the belt layer is damaged and a cut that reaches the belt layer is generated, outside water invaded into the belt layer rusts the steel cord. With this, a contact failure between the steel cord and the rubber is caused and therefore separation between the belt layers might be generated. 
     Accordingly, it is preferable that, in the steel cord, the rubber is filled between the filaments to derive so-called good rubber penetration performance (rust prevention performance) from a viewpoint of preventing rust. However, in the cord having the multi-twisted structure, the number of the filaments is large and therefore the twisted structure is complicated. Consequently, it is difficult to obtain required rust prevention performance. 
     Accordingly, an object of the present invention is, in consideration of the problem described above, to provide a construction vehicle tire and a steel cord including a cord having a multi-twisted structure and being capable of improving rust prevention performance. 
     Means for Solving the Problems 
     The inventors conducted a study to solve the problems described above by adopting the following configuration, so that the present invention was completed. 
     That is, a construction vehicle tire according to the present invention includes a carcass layer, and a belt layer formed by six belts at an outer side of the carcass layer in a tire radial direction. A fourth belt arranged fourth from the carcass layer toward the outer side in the tire radial direction among the belt layer includes steel cords arranged along a predetermined direction. 
     The steel cord includes one core strand having a two-layer-twisted structure in which a plurality of core filaments and a sheath filament forming a sheath are twisted, and a plurality of sheath strands twisted with the core strand, the sheath strand having a layer-twisted structure in which a plurality of steel filaments are twisted. Each of a gap between the core filaments adjacent to each other, a gap between the sheath filaments adjacent to each other, a gap between the core filament and the sheath filament, and a gap between the core stand and the sheath strand is filled with a predetermined rubber material. 
     Further, the steel cord according to the present invention may be used in a belt layer arranged third from a carcass layer toward an outer side in the tire radial direction. Further, the steel cord according to the present invention may be used in a carcass ply of a carcass layer. 
     In the steel cord according to the present invention, it is preferable that a ratio dc/ds of a diameter dc of the sheath filament forming the sheath of the core strand and a diameter ds of an outermost layer sheath filament forming an outermost layer sheath of the sheath strand is set to be more than 1.25 and less than or equal to 1.50. 
     Further, in the steel cord according to the present invention, the number of the sheath strand may be set in a range between 7 and 9. Further, in the steel cord according to the present invention, it is preferable that each of the filaments forming the core strand is formed in the same diameter. Further, in the steel cord according to the present invention, a cord diameter is preferably set to 4.40 mm or more. 
     Further, the steel cord according to the present invention may be used in belt layers arranged first and second from the carcass layer toward the outer side in the tire radial direction. Further, when a tire sectional height SH is defined by a length between a bottom surface of a bead portion and a portion the outer side of the tire in the tire radial direction and a tire sectional height S is defined by a length between the bottom surface of the bead portion and a portion at an outer side of a belt layer arranged sixth toward the outer side in the tire radial direction in a new tire, the tire sectional height SH may be set to be 85% to 95% of the tire sectional height S. 
     Effects of the Invention 
     According to the present invention, the construction vehicle tire and the steel cord capable of improving the rust prevention performance can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating a half part of a tread of a pneumatic tire, which is partially cut, according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view in a tire width direction illustrating one example of a configuration of a steel cord of the present invention. 
         FIG. 3  is a cross-sectional view illustrating a part of the pneumatic tire according to the embodiment of the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The same reference signs are assigned to the same parts in the drawings and the description thereof is therefore omitted. 
     A configuration of a pneumatic tire  1  according to the present embodiment is described with reference to  FIG. 1 . Here, the pneumatic tire  1  is formed as a construction vehicle tire applied to a heavy load vehicle such as a construction vehicle. 
     As shown in  FIG. 1 , the pneumatic tire  1  is provided with a bead portion  2  contacted with a rim (not shown), and a carcass layer  3  that forms a frame of the pneumatic tire  1 . 
     A tread portion  4  is continuously formed along a tire circumferential direction C at an outer side in a tire radial direction of the carcass layer  3 . The tread portion  4  is a portion forming a tread ground contact surface in which an outer circumferential surface is contacted with a road surface. The tread portion  4  is formed by tread rubber. A predetermined tread pattern is formed in the tread portion  4 . 
     A belt portion  5  is formed between the carcass layer  3  and the tread portion  4 . The belt portion  5  is served as a hoop that partially regulates expanding deformation of the carcass layer  3  when the inner pressure is applied to the pneumatic tire  1 . Further, the belt portion  5  is formed to suppress a change of a load applied to the pneumatic tire  1  or to suppress deformation caused by rotation and to receive input from the road surface so as to cushion an impact. The belt portion  5  is embedded in the tread rubber of the tread portion  4  so as to extend in the tire circumferential direction C. 
     In the present embodiment, the belt portion  5  is formed by laminating six belt layers of a first belt layer  1 B, a second belt layer  2 B, a third belt layer  3 B, a fourth belt layer  4 B, a fifth belt layer  5 B, and a sixth belt layer  6 B in this order from an inner side toward an outer side in a tire radial direction R, namely from the carcass layer  3  toward the tread portion  4 . 
     The first belt layer  1 B is arranged at an innermost side, namely arranged first, in the tire radial direction R among a plurality of the belt layers. The second belt layer  2 B is arranged second at an outer side of the first belt layer  1 B in the tire radial direction R. The third belt layer  3 B is arranged third at an outer side of the second belt layer  2 B in the tire radial direction R. The fourth belt layer  4 B is arranged fourth at an outer side of the third belt layer  3 B in the tire radial direction R. The fifth belt layer  5 B is arranged fifth at an outer side of the fourth belt layer  4 B in the tire radial direction R. The sixth belt layer  6 B is arranged sixth at an outer side of the fifth belt layer  5 B in the tire radial direction R. The sixth belt layer  6 B is arranged at an outermost side among a plurality of the belt layers. 
     Further, as shown in  FIG. 1 , lengths of the belt layers in a tire width direction W fulfill the following relations. The first belt layer  1 B is longer than the second belt layer  2 B and shorter than the third to sixth belt layers  3 B to  6 B. The second belt layer  2 B is the shortest among the six belt layers. The third belt layer  3 B is longer than the first belt layer  1 B, the second belt layer  2 B, the fourth belt layer  4 B and the sixth belt layer  6 B and shorter than the fifth belt layer  5 B. The fourth belt layer  4 B is longer than the first belt layer  1 B and the second belt layer  2 B and shorter than the third belt layer  3 B and the fifth belt layer  5 B. The fifth belt layer  5 B is the longest among the six belt layers. The sixth belt layer  6 B is longer than the first belt layer  1 B and the second belt layer  2 B and shorter than the third belt layer  3 B and the fifth belt layer  5 B. Here, the ratios of the lengths are not especially limited. 
     Cords  8   a,    8   b,    8   c,    8   d,    8   e,    8   f  are embedded in the belt layers  1 B,  2 B,  3 B,  4 B,  5 B,  6 B, respectively. The belt layers  1 B,  2 B,  3 B,  4 B,  5 B,  6 B having a layer structure are formed by coating the cords  8   a,    8   b,    8   c ,  8   d,    8   e,    8   f  with rubber, respectively. 
     The cords in the belt layers adjacent to each other in the tire radial direction R are inclined in opposite directions to each other against the tire equatorial line CL. That is, the cords in the belt layers adjacent to each other in the tire radial direction R are laminated to intersect each other. Specifically, each group of the first belt layer  1 B and the second layer  2 B, the second belt layer  2 B and the third belt layer  3 B, the third belt layer  3 B and the fourth belt layer  4 B, the fourth belt layer  4 B and the fifth belt layer  5 B, and the fifth belt layer  5 B and the sixth belt layer  6 B is formed as an intersection laminated body in which extending directions of the cords are intersected. With this, even when the pneumatic tire  1  treads on a projection such as a rock in various directions, the break of the pneumatic tire  1  can be prevented, and the rigidity in the tire circumferential direction C can be improved. 
     Next, a steel cord used in the belt portion  5  is described with reference to  FIG. 2 . Here, the steel cord shown in  FIG. 2  is used as a reinforcing cord of the third belt layer  3 B and the fourth belt layer  4 B. 
     As shown in  FIG. 2 , the steel cord is provided with one core strand  11  having a two-layer-twisted structure in which a plurality of steel filaments  10  are twisted, and a plurality of sheath strands  12 , seven sheath strands  12  in an example shown in  FIG. 2 , having a two-layer-twisted structure in which a plurality of the steel filaments  10  are twisted. The steel cord has a multi-twisted structure in which the sheath strands  12  are twisted around the core strand  11 . The core strand  11  has a 3+8 structure in which a sheath is formed by twisting eight sheath filaments  10   b  around a core formed by twisting three core filaments  10   a.  Further, the sheath strand  12  has a 3+8 structure in which a sheath is formed by twisting eight sheath filaments  20   b  around a core formed by twisting three core filaments  20   a.  Further, a spiral filament  30  is wound in a spiral manner on an outer circumference of the sheath strand  12 . 
     Further, as shown in  FIG. 2 , a gap between the core filaments  10   a  adjacent to each other is filled with a predetermined rubber material. Further, a gap between the sheath filaments  10   b  adjacent to each other is also filled with the predetermined rubber material. Similarly, each of a gap between the core filaments  20   a  adjacent to each other and a gap between the sheath filaments  20   b  adjacent to each other is filled with the predetermined rubber material. Further, a gap between the core filament  10   a  and the sheath filament  10   b  is filled with the predetermined rubber material. Further, a gap between the core filament  20   a  and the sheath filament  20   b  is also filled with the predetermined rubber material. Further, a gap between the core strand  11  and the sheath strand  12  is also filled with the predetermined rubber material. Here, the rubber material is not especially limited, and therefore natural rubber or the like may be adopted. 
     In the present invention, it is important that a ratio dc/ds of a diameter dc of the sheath filament  10   b  forming a sheath of the core strand  11  and a diameter ds of an outermost layer sheath filament  20   b  forming an outermost layer sheath of the sheath strand  12  is set to be more than 1.25 and less than or equal to 1.50. In order to improve the rubber penetration performance, it is effective to enlarge a gap between the filaments, in particular in the core strand  11  located at a center of the cord, as the gap between the sheath filaments  10   b  is larger, the rubber penetration performance becomes superior. On the other hand, in order to form a gap, in a case in which the number of the sheath filaments  10   b  of the core strand  11  is set to be six or less, the strength of the cord is deteriorated, while in a case in which the diameter of the sheath filament  10   b  of the core strand  11  is made small and the number of the sheath filaments  10   b  is set to eleven or more, the shear resistance against the sheath strand  12  is deteriorated. Accordingly, the present invention defines the value of the ratio dc/ds in a case in which a configuration in which the core strand  11  is formed as a two-layer-twisted structure and the number of the sheath filaments  10   b  is set to eight is adopted. 
     It is necessary to set the value of the ratio dc/ds to be more than 1.25 and less than or equal to 1.50, more preferably more than or equal to 1.27 and less than or equal to 1.42. The rubber is hard to penetrate the core strand  11  located at the center of the cord compared to the sheath strand  12  located around the core strand  11 . Accordingly, it is preferable that an average of the gap between the sheath filaments  10   b  is larger than an average of the gap between the sheath filaments  20   b  from a viewpoint of improving the rust prevention performance. By setting the value of the ratio dc/ds to be more than 1.25 and less than or equal to 1.50, the average of the gap between the sheath filaments  10   b  is made larger than the average of the gap between the sheath filaments  20   b.  With this, the rubber penetrates the core strand  11  located at the center of the cord easily. Specifically, by setting the value of the ratio dc/ds to be more than 1.25 and less than or equal to 1.50, a rubber penetration rate is 1.70 to 2.50 times as large as that in a configuration in which the steel cord according to the present invention is not adopted. The rubber penetration rate is preferable as the value thereof is larger. Further, by setting the value of the ratio dc/ds to be more than 1.25 and less than or equal to 1.50, a rust growth rate in a longitudinal direction of the steel cord is approximately 0.50 to 0.60 times as large as that in the configuration in which the steel cord according to the present invention is not adopted. The rust growth rate is preferable as the value thereof is smaller because the growth of the rust is less. 
     Further, by setting the value of the ratio dc/ds to be more than 1.25 and less than or equal to 1.50, shear forces applied to the filaments of the core strand  11  and the sheath strand  12  are reduced, and thereby the core strand  11  having strength that endures fastening force by the sheath strand  12  sufficiently can be obtained. Consequently, the shear resistance of the cord can be improved by improving the shear resistance of the cord itself. Here, Patent Literature (Japanese Unexamined Patent Application Publication No. 2009-299223) discloses that the ratio dc/ds is set to be more than 1.05 and less than 1.25 and in a case in which the ratio dc/ds exceeds 1.25, preceding break of the filaments of the core strand and the sheath strand may be caused. While, the present inventors conducted a study and, as a result, derived knowledge that the shear force is reduced in a case in which the ratio dc/ds is more than 1.25 by forming the core strand  11  as a two-layer-twisted structure and thereby the shear resistance of the cord is improved. In a case in which the ratio dc/ds is less than or equal to 1.25 and the core strand is formed as a two-layer-twisted structure, the improvement effect of the shear resistance of the cord is small, and in a case in which the ratio dc/ds is more than 1.50, the diameter ds is made excessively small against the diameter dc and thereby the preceding break of the outermost layer sheath filament that forms the outermost layer sheath of the sheath strand  12  might be caused easily. 
     In the cord of the present invention, the number of the layers in the layer-twisted structure of the sheath strand  12  is not especially limited, and therefore, for example, the number of the layers may be set to two or three. Accordingly, when a first sheath and a second sheath are defined in this order from an inside of the layer-twisted structure, in a case in which the sheath strand  12  is formed as a two-layer-twisted structure, the outermost layer sheath of the sheath strand  12  corresponds to the first sheath, while in a case in which the sheath strand  12  is formed as a three-layer-twisted structure, the outermost layer sheath of the sheath strand  12  corresponds to the second sheath. 
     Further, in the cord of the present invention, it is preferable that the number of the sheath strands  12  is set in a range between 7 and 9. In a case in which each of the core strand  11  and the sheath strands  12  is formed in the same diameter and the sheath strands are filled without a gap therebetween, six sheath strands can be arranged. However in the present invention, as shown in  FIG. 2 , seven to nine sheath strands  12  can be arranged by setting the diameter of the sheath strand  12  to be smaller than the diameter of the core strand  11 . Accordingly, a resistance of the core strand against fastening force by the sheath strand can be further improved by setting the number of the sheath strands  12  in a range between 7 and 9, and thereby the shear resistance of the cord can be further improved. 
     Further, in the cord of the present invention, as shown in  FIG. 2 , it is preferable that each of the diameters of the filaments  10  that form the core strand  11  is formed in the same diameter. In the core strand  11 , by setting the diameter of the core filament  10   a  to be smaller than the diameter of the sheath strand  10   b,  the shear resistance can be improved. However, in the present invention in which the core strand  11  is formed as a two-layer-twisted structure, it is preferable that the diameter of the core filament  10   a  of the core strand  11  is set to be approximately equal to the diameter of the sheath strand  10   b.  With this, in the core strand  11 , a space for arranging the sheath filaments  10   b  can be secured easily. Accordingly, the rubber easily penetrates the core strand  11  located at the center of the cord. 
     Further, it is preferable that a cord diameter of the cord of the present invention is set to 4.40 mm or more, for example set in a range between 5.00 mm and 6.00 mm. By adopting the cord having a large diameter of 4.40 mm or more, especially both of the strength and the shear resistance required for a large tire used in a large vehicle such as a construction vehicle can be secured. 
     In the cord of the present invention, the wire diameter or the tensile strength of the filament used in the cord, a twisted direction or a twisted pitch of the filament or the strand, or the like is not especially limited, and therefore each of them can be selected appropriately in the usual manner in accordance with the desire thereof. For example, a so-called high tensile strength steel having a carbon content of 0.80 mass % or more may be adopted as the filament. Further, the cord of the present invention may include or may not include a spiral filament. 
     The cord of the present invention is superior in the rust prevention performance and the shear resistance, and therefore the cord of the present invention is suitably used as a reinforcing material of a large tire used in a large vehicle such as a construction vehicle, in particular an extremely large off-road radial tire having a tire size of 40.00R57. 
     Hereinafter, the present invention is described in detail with reference to examples. 
     As shown in tables described below, the steel cords, which are formed by twisting a plurality of the sheath cords having a layer-twisted structure, around one core strand having a layer-twisted structure, according to examples and comparative examples were produced. The steel cords have different ratios dc/ds of the diameter dc of the sheath filament that forms the outermost layer sheath of the core strand and the diameter ds of the sheath filament that forms the outermost layer sheath of the sheath strand derived from different cord structures. Each produced cord was subjected to the evaluation described below, and the result thereof is shown in the tables together with the cord structure. 
     Shear Resistance 
     The shear breakdown resistance of each of the produced cords was evaluated by using a Charpy impact tester. The result is indexed as the result of the cord according to the comparative example 1 is defined as 100. As the value is larger, the shear resistance is superior and preferable. 
     Steel Amount 
     A steel amount per unit length of each of the produced cords was measured. The result is indexed as the result of the steel amount of the cord according to the comparative example 1 is defined as 100. As the value is smaller, the steel amount is less and therefore it is superior in light weight. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
                 Comparative 
               
               
                   
                 example 1 
                 example 2 
                 example 3 
                 example 4 
                 example 5 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Cord structure 
                 1 + 7 
                 1 + 7 
                 1 + 7 
                 1 + 6 
                 1 + 7 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Core 
                 Structure 
                 3 + 8 + 10 
                 3 + 8 + 10 
                 3 + 8 + 10 
                 3 + 7 + 13 
                 3 + 8 
               
               
                 strand 
                 Diameter 
                 0.320 
                 0.310 
                 0.330 
                 0.240 
                 0.565 
               
               
                   
                 of core 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 0.320 
                 0.310 
                 0.330 
                 0.340 
                 0.565 
               
               
                   
                 of first 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 0.455 
                 0.455 
                 0.475 
                 0.340 
                 — 
               
               
                   
                 of second 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                 Sheath 
                 Structure 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 9 + 12 
                 3 + 8 
               
               
                 strand 
                 Diameter 
                 0.385 
                 0.390 
                 0.385 
                 0.240 
                 0.375 
               
               
                   
                 of core 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 0.385 
                 0.390 
                 0.385 
                 0.240 
                 0.375 
               
               
                   
                 of first 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 — 
                 — 
                 — 
                 0.340 
                 — 
               
               
                   
                 of second 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                 Cord 
                 Diameter 
                 5.44 
                 5.44 
                 5.52 
                 5.23 
                 5.46 
               
               
                   
                 of cord 
               
               
                   
                 (mm) 
               
            
           
           
               
               
               
               
               
               
            
               
                 Ratio dc/ds 
                 1.18 
                 1.17 
                 1.23 
                 1.00 
                 1.51 
               
               
                 Shear resistance 
                 100 
                 102 
                 99 
                 96 
                 99 
               
               
                 (index number) 
               
               
                 Steel amount 
                 100 
                 102 
                 100 
                 102 
                 99 
               
               
                 (index number) 
               
               
                 Ratio of Shear 
                 100 
                 100 
                 99 
                 94 
                 100 
               
               
                 resistance/ 
               
               
                 Steel amount 
               
               
                 (index number) 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
                 Example 
               
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Applied belt 
                 4B only 
                 3B + 4B 
                 3B + 4B 
                 3B + 4B 
                 3B + 4B 
                 3B + 4B 
                 3B + 4B 
               
               
                 Cord structure 
                 1 + 7 
                 1 + 7 
                 1 + 7 
                 1 + 7 
                 1 + 7 
                 1 + 7 
                 1 + 7 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Core 
                 Structure 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
               
               
                 strand 
                 Diameter 
                 0.510 
                 0.510 
                 0.520 
                 0.540 
                 0.540 
                 0.540 
                 0.540 
               
               
                   
                 of core 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 0.510 
                 0.510 
                 0.520 
                 0.540 
                 0.540 
                 0.540 
                 0.540 
               
               
                   
                 of first 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 of second 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                 Sheath 
                 Structure 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 3 + 8 
                 1 + 6 
               
               
                 strand 
                 Diameter 
                 0.380 
                 0.380 
                 0.395 
                 0.385 
                 0.290 
                 0.300 
                 0.385 
               
               
                   
                 of core 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 0.400 
                 0.400 
                 0.395 
                 0.385 
                 0.380 
                 0.370 
                 0.385 
               
               
                   
                 of first 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                   
                 Diameter 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
                 — 
               
               
                   
                 of second 
               
               
                   
                 sheath 
               
               
                   
                 filament 
               
               
                   
                 (mm) 
               
               
                 Cord 
                 Diameter 
                 5.36 
                 5.36 
                 5.44 
                 5.44 
                 5.01 
                 5.02 
                 4.55 
               
               
                   
                 of cord 
               
               
                   
                 (mm) 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Ratio dc/ds 
                 1.28 
                 1.28 
                 1.32 
                 1.40 
                 1.42 
                 1.46 
                 1.40 
               
               
                 Shear resistance 
                 109 
                 109 
                 112 
                 108 
                 104 
                 100 
                 87 
               
               
                 (index number) 
               
               
                 Steel amount 
                 101 
                 101 
                 102 
                 100 
                 98 
                 96 
                 85 
               
               
                 (index number) 
               
               
                 Ratio of Shear 
                 108 
                 108 
                 110 
                 108 
                 106 
                 104 
                 102 
               
               
                 resistance/Steel 
               
               
                 amount (index 
               
               
                 number) 
               
               
                   
               
            
           
         
       
     
     As described above, in the cord formed in which a plurality of the sheath strands  12  having a layer-twisted structure is twisted around one core strand  11  having a two-layer-twisted structure and the value of the ratio dc/ds is set in the predetermined range, a gap between the sheath filaments  10   b  is made large, and thereby the rubber penetration rate is improved and the rust prevention performance is improved. That is, in a configuration in which the steel cord according to the present invention is applied to the fourth belt layer  4 B, the fourth belt layer  4 B served as a rust prevention cord derives a sufficient effect of preventing water from invading. Further, in a configuration in which the steel cord according to the present invention is applied to the third belt layer  3 B in addition to the fourth belt layer  4 B, the rust prevention performance is further improved. 
     Further, by adopting the steel cord according to the present invention, an increase of the steel amount is suppressed and the shear resistance is improved. With this, a lifetime of each of the third belt layer  3 B and the fourth belt layer  4 B against the cut separation is extended. Specifically, in a configuration in which the steel cord according to the present invention is applied to the third belt layer  3 B and the fourth belt layer  4 B, a lifetime against the cut separation is approximately 1.27 times as long as a lifetime of a configuration in which the steel cord according to the present invention is not adopted. With this, a lifetime of the pneumatic tire  1  can be extended. 
     In the present invention, the steel cord shown in  FIG. 2  is applied to the third belt layer  3 B and the fourth belt layer  4 B, however it is not limited to this. The steel cord may be applied to the first belt layer  1 B and the second belt layer  2 B in addition to the third belt layer  3 B and the fourth belt layer  4 B. With this, the rust prevention performance of the first belt layer  1 B and the second belt layer  2 B is improved, and therefore the pneumatic tire  1  having excellent rust prevention performance can be provided. 
     Further, the tread portion  4  shown in  FIG. 1  includes a shear resistant function, however since the shear resistance of the third belt layer  3 B and the fourth belt layer  4 B is improved, the shear resistant function of the tread portion can be reduced. That is, a gauge thickness of the tread portion  4  can be made thin. With this, a cost can be reduced, and a failure of the pneumatic tire  1  caused by heat generated by a contact between the tread portion  4  and a road surface can be suppressed. Specifically in the gauge thickness, as shown in  FIG. 3 , when a tire sectional height SH is defined by a length between a bottom surface of the bead portion  2  and a portion at an outer side of the tire in the tire radial direction and a tire sectional height S is defined by a length between the bottom surface of the bead portion  2  and a portion at an outer side of the sixth belt layer  6 B in the tire radial direction, the tire sectional height SH may be set to be smaller by 3% to 5% of the tire sectional height S. 
     Further, even in a case in which the steel cord according to the present invention is adopted in the third belt layer  3 B and the fourth belt layer  4 B, a failure except the cut separation is not affected. It is similarly applied to a configuration in which the steel cord according to the present invention is adopted in the first belt layer  1 B and the second belt layer  2 B. 
     Further, the steel cord according to the present invention may be applied to a carcass ply of the carcass layer  3 . With this, the rust prevention performance against the water invasion due to a side cut is improved. 
     The entire contents of Japanese Patent Application No. 2016-082120 (filed on Apr. 15, 2016) are incorporated in the present specification by reference. 
     REFERENCE SIGNS LIST 
     
         
           1 : pneumatic tire 
           2 : bead portion 
           3 : carcass layer 
           4 : tread portion 
           5 : belt portion 
           10 ,  20 : steel filament 
           10   a,    20   a:  core filament 
           10   b,    20   b:  sheath filament 
           11 : core strand 
           12 : sheath strand 
           30 : spiral filament