Abstract:
A tank that includes layers of fiber reinforced plastics (FRP) formed by alternately winding hoop and helical bundles of fiber over its outer surface. The winding produces stepped portions, i.e. unevenness, in a helical layer positioned as the innermost layer. Such unevenness affects an outer layer (especially a hoop layer) directly adjacent to the innermost helical layer and lowers fatigue strength of the adjacent outer layer. In order to prevent this fatigue strength decrease, the bundle of fiber used for the innermost helical layer has a smaller sectional area than the bundles used for the outer layers. Consequently, decreasing the sectional area of the innermost helical bundle decreases the stepped portions, which, in turn, decreases the transfer of unevenness to the outer layer (especially a hoop layer) directly adjacent to the helical layer. As a result, fatigue strength of the adjacent outer layer (especially a hoop layer) increases.

Description:
This is a 371 national phase application of PCT/JP2009/057357 filed 10 Apr. 2009, the contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to a tank and a manufacturing method thereof, and more particularly, it relates to the improvement of a structure in a tank filled with a hydrogen gas or the like with a high pressure. 
     BACKGROUND ART 
     As a tank which is utilized for the storage of hydrogen or the like, there has been utilized a tank including a fiber reinforced plastics (FRP) layer formed by alternately laminating hoop layers and helical layers on the outer periphery of a liner (see e.g. Patent Document 1). The hoop layer is a layer formed by hoop-winding a bundle of fibers (e.g. carbon fibers) (a winding way of winding the bundle around a tank trunk part almost vertically to a tank axis), and the helical layer is a layer formed by helically winding a bundle of fibers such as carbon fibers (CF) (a winding way of winding the bundle up to a tank dome part almost in parallel with the tank axis) (see  FIG. 2  of the present application). 
     PRIOR ART DOCUMENT 
     Patent Documents 
     
         
         Patent Document 1 JP 2008-032088 A 
       
    
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     However, a conventional technology described above has a problem that owing to a stepped portion of the surface of helical layer positioned in an inner layer of an FRP layer, especially the stepped portion of the surface of the helical layer positioned in the innermost layer, a fatigue strength of a layer (especially a hoop layer) which is adjacent to the outside of the helical layer lowers. 
     Therefore, an object of the present invention is to provide a tank having a structure which can avoid a phenomenon where owing to an influence of a stepped portion of the surface of a helical layer positioned in an inner layer of an FRP layer, a fatigue strength of a layer (especially a hoop layer) adjacent to the outside of the helical layer lowers, and to provide a manufacturing method of the tank. 
     Means for Solving the Problems 
     To solve such problems, the present inventors have performed various investigations, and have found that when a structural bend of a fiber bundle of the above hoop layer is large, a fatigue strength of the hoop layer lowers. That is, when the hoop layers and helical layers are laminated to form an FRP layer and unevenness occurs in the helical layer itself which is adjacent to the hoop layer, the unevenness is transferred to the adjacent hoop layer. In consequence, a fiber bundle itself of the hoop layer little meanders to generate undulations, and bends (the undulations) due to a structure are generated in the hoop layer. 
     Under such situations, the present inventors have performed investigations on how to decrease the structural bends (undulations) which might be generated in the fiber bundle of the hoop layer, and have obtained a new finding which results in the solution of such a problem. On the basis of such finding, the present invention provides a tank comprising a liner, and an FRP layer including hoop layers and helical layers alternately formed by winding fiber bundles around the outer periphery of the liner, wherein in at least one of a plurality of helical layers positioned in an inner layer of the FRP layer, a sectional area of the fiber bundle constituting the helical layer is smaller than that of the fiber bundle constituting another layer formed outside the helical layer. 
     As means for decreasing the bends (the undulations) of the fiber bundle of the hoop layer due to the structure, it has been considered that the unevenness of the adjacent helical layer is decreased to prevent the unevenness from being transferred. However, as described above, fibers in the helical layer are wound almost in parallel with a tank axis to be folded back in a tank dome part (see  FIG. 2 ), such winding as to eliminate a space between the adjacent fiber bundles is not especially considered, and it is fundamentally difficult to decrease the bends. Usually, the overlap of the fiber bundles with one another, the arrangement of the fiber bundles, or the like in the helical layer is not taken into consideration, and the fibers are wound in a so-called disorder manner. Heretofore, it has been suggested that, for example, a phase of a reinforcing fiber bundle of each helical layer in a cylindrical member peripheral direction is shifted, but there has not been any suggestion or aiming that a helical layer which does not have any unevenness or has only little unevenness is formed. 
     In this respect, according to the present invention, at least one of a plurality of helical layers positioned in the inner layer of the FRP layer, the sectional area of the fiber bundle constituting the helical layer is set to be smaller than that of the fiber bundle constituting another layer formed outside the helical layer, thereby decreasing the unevenness of the helical layer. That is, when the helical layer is formed by using the fiber bundle having such a small sectional area, a thickness of the fiber bundle, accordingly, decreases, which decreases a height of the fiber bundle. Moreover, a space between the fiber bundles also becomes small. Therefore, the unevenness of the helical layer, accordingly, becomes small, and the unevenness can be prevented from being transferred to the other layer (e.g. the hoop layer) formed outside the helical layer. In consequence, it is possible to decrease conventional structural fiber bundle bends due to an uneven helical layer (indicating the helical layer which is not subjected to any treatment for smoothing the surface thereof and has the unevenness occurring on the surface, as denoted with mark  70 B in  FIG. 11  and  FIG. 12 ). Therefore, when the structural bends (undulations) of the fiber bundles of the layer (e.g. the hoop layer) formed outside the helical layer are suppressed, the fatigue strength of the fiber bundle itself can be enhanced. Moreover, when the helical layer and the layer (e.g. the hoop layer) formed outside the helical layer have a decreased thickness and an increased fiber volume content ratio (Vf), a burst strength can be enhanced. 
     In the tank according to the present invention, the helical layer positioned in the inner layer is preferably the helical layer of the innermost layer. Usually, in the fiber bundle layers (the helical layer and the hoop layer), the layer positioned on an inner side (the layer closer to the liner) has a larger degree of contribution to the tank strength. In this respect, when the helical layer of the innermost layer is constituted of the fiber bundle having a small sectional area as in the present invention, the layer (e.g. the hoop layer) which is adjacent to the outside of the innermost helical layer can smoothly be wound. The structural bends (undulations) of the fiber bundle of the layer are suppressed, which can contribute to the enhancement of the tank strength. 
     Moreover, in the tank according to the present invention, the sectional area of the fiber bundle is preferably set by changing the number of fibers constituting the fiber bundle. When a plurality of fibers are bundled to constitute the fiber bundle and the number of the fibers is decreased, the sectional area of the fiber bundle can, accordingly, be decreased. 
     Furthermore, in the tank according to the present invention, a tensile force onto the fiber bundle constituting the helical layer is preferably increased to decrease a thickness of the fiber bundle in a laminating direction of the FRP layer. When the thickness of the fiber bundle is decreased to flatten the fiber bundle, the unevenness of the helical layer is, accordingly, decreased, and the unevenness can be prevented from being transferred to the other layer (e.g. the hoop layer) formed outside the helical layer. 
     Moreover, as the fiber bundle constituting the other layer formed outside the helical layer, the fiber bundle having a larger diameter than a fiber bundle constituting another layer is preferably used. When the diameter of the fiber bundle of the other layer formed outside the helical layer is increased in this manner, a bend displacement of the fiber bundle is decreased, and an exerted shear stress can be alleviated. 
     A manufacturing method according to the present invention is a manufacturing method of a tank including a liner, and an FRP layer constituted of hoop layers and helical layers alternately formed by winding fiber bundles around the outer periphery of the liner, comprising the steps of: forming at least one of a plurality of helical layers positioned in an inner layer of the FRP layer by winding the fiber bundle having a sectional area which is smaller than that of the fiber bundle constituting another layer formed outside the helical layer. 
     Furthermore, a cylindrical member according to the present invention is a cylindrical member comprising an FRP layer including hoop layers and helical layers alternately formed by winding fiber bundles, wherein in at least one of a plurality of helical layers positioned in an inner layer of the FRP layer, a sectional area of the fiber bundle constituting the helical layer is smaller than that of the fiber bundle constituting another layer formed outside the helical layer. 
     Effect of the Invention 
     According to the present invention, it is possible to avoid a phenomenon where owing to an influence of a stepped portion of the surface of a helical layer positioned in an inner layer of an FRP layer, a fatigue strength of a layer (especially a hoop layer) adjacent to the outside of the helical layer lowers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a sectional view and a partially enlarged view showing a structure of a tank in an embodiment of the present invention; 
         FIG. 2  is a sectional view of the structure of the tank in the embodiment of the present invention; 
         FIG. 3  is a sectional view showing a structure example around a mouthpiece of the tank; 
         FIG. 4  is a partially enlarged view showing a structure example of a helical layer and a hoop layer of an FRP layer; 
         FIG. 5  is a partially enlarged view showing, as a reference, a structure example of a helical layer and a hoop layer of an FRP layer in a conventional tank; 
         FIG. 6  is a partially enlarged view showing, as a reference, a structure example of an innermost helical layer and a hoop layer outside the helical layer in the FRP layer of the conventional tank; 
         FIG. 7  is a diagram showing a sectional shape example of a helical layer in another embodiment of the present invention; 
         FIG. 8  is a diagram showing, as a reference, a sectional shape example of a helical layer in a conventional tank; 
         FIG. 9  is a diagram showing a structure example of a helical layer and a hoop layer in still another embodiment of the present invention; 
         FIG. 10  is a diagram showing, as a reference, a structure example of the helical layer and a hoop layer in the conventional tank; 
         FIG. 11  is a perspective view showing an example of conventional helical winding as a reference; 
         FIG. 12  is a projected diagram along a tank axis direction, showing the example of the conventional helical winding as the reference; 
         FIG. 13  is a diagram showing an example of a filament winding (FW) device; and 
         FIG. 14  is a diagram showing a behavior of winding fiber bundles around the outer periphery of a liner by use of a fiber guide unit of the FW device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a constitution of the present invention will be described in detail with respect to an example of a mode for carrying out the invention shown in drawings. 
       FIG. 1  to  FIG. 4  and the like show an embodiment of a tank according to the present invention and a manufacturing method of the tank. Hereinafter, there will be described illustration of a case where a tank (hereinafter also referred to as the high-pressure tank)  1  according to the present invention is applied to a high-pressure hydrogen tank as a hydrogen fuel supply source. The hydrogen tank can be utilized in a fuel cell system or the like. 
     The high-pressure tank  1  includes a cylindrical tank main body  10  having both ends, for example, with a substantially semispherical shape, and a mouthpiece  11  attached to an end of the tank main body  10  in a longitudinal direction. It is to be noted that in the present description, a substantially semispherical part is referred to as a dome part, a cylindrical trunk part is referred to as a straight part, and the parts are denoted with marks  1   d  and  1   s , respectively (see  FIG. 1 ,  FIG. 2 , etc.). Moreover, the high-pressure tank  1  according to the present embodiment has the mouthpieces  11  at both ends, but for the convenience of the description, the tank is described with reference to  FIG. 3  showing a main section of the high-pressure tank  1  in which a positive direction of an X-axis (a direction shown by an arrow) is a tip side and a negative direction thereof is a base side. The positive direction (the direction shown by an arrow) of a Y-axis which is vertical to the X-axis indicates a tank outer peripheral side. 
     The tank main body  10  includes a wall layer having, for example, a two-layer structure, and includes a liner  20  as an inner wall layer and, for example, an FRP layer  21  as a resin fiber layer (a reinforcing layer) which is an outer wall layer outside the liner. The FRP layer  21  is formed by, for example, an only carbon fiber reinforced plastics (CFRP) layer  21   c , or the CFRP layer  21   c  and a glass fiber reinforced plastics (GFRP) layer  21   g  (see  FIG. 1 ). 
     The liner  20  is formed into almost the same shape as the tank main body  10 . The liner  20  is made of, for example, a polyethylene resin, a polypropylene resin, another hard resin or the like. Alternatively, the liner  20  may be a metal liner made of aluminum or the like. 
     On the tip side of the liner  20  provided with the mouthpiece  11 , an inwardly bent folded part  30  is formed. The folded part  30  is folded back toward the inside of the tank main body  10  so that the folded part is disposed away from the outside FRP layer  21 . The folded part  30  has a tapered portion  30   a  having a diameter which gradually decreases closer to a folding tip, and a cylindrical portion  30   b  connected to the tip of the tapered portion  30   a  and having a constant diameter. The cylindrical portion  30   b  forms an opening of the liner  20 . 
     The mouthpiece  11  has a schematically cylindrical shape, and is fitted into the opening of the liner  20 . The mouthpiece  11  is made of, for example, aluminum or an aluminum alloy, and manufactured into a predetermined shape by, for example, a die cast process or the like. The mouthpiece  11  is fitted into a divided liner formed by injection. Moreover, the mouthpiece  11  may be attached to the liner  20  by, for example, insert forming. 
     Moreover, in the mouthpiece  11 , for example, a valve fastening flange  11   a  is formed on the tip side (outside the high-pressure tank  1  in an axial direction), and an annular recess portion  11   b  is formed behind the valve fastening flange  11   a  (inside the high-pressure tank  1  in the axial direction) with respect to the axis of the high-pressure tank  1 . The recess portion  11   b  is curved to project on an axis side and has an R-shape. The recess portion  11   b  comes in contact with the vicinity of the tip of the FRP layer  21  having the same R-shape in an airtight manner. 
     For example, the surface of the recess portion  11   b  which comes in contact with the FRP layer  21  is provided with solid lubrication coating C such as a fluoric resin. This decreases a friction coefficient between the FRP layer  21  and the recess portion  11   b.    
     Further behind the recess portion  11   b  of the mouthpiece  11 , there is formed a collar portion  11   c  adapted to, for example, the shape of the folded part  30  of the liner  20  and having, for example, a large diameter continuously with the recess portion  11   b , and behind the collar portion  11   c , there is formed a mouthpiece cylindrical portion  11   d  having a predetermined diameter. The tapered portion  30   a  of the folded part  30  of the liner  20  comes in contact closely with the surface of the collar portion  11   c , and the cylindrical portion  30   b  comes in contact closely with the surface of the mouthpiece cylindrical portion  11   d . Between the cylindrical portion  30   b  and the mouthpiece cylindrical portion  11   d , seal members  40  and  41  are interposed. 
     A valve assembly  50  controls supply/discharge of a fuel gas between an external gas supply line (a supply path  22 ) and the inside of the high-pressure tank  1 . Seal members  60  and  61  are interposed between the outer peripheral surface of the valve assembly  50  and the inner peripheral surface of the mouthpiece  11 . 
     The FRP layer  21  is formed, for example, by winding a fiber bundle (a reinforcing fiber bundle)  70  impregnated with a resin around the outer peripheral surface of the liner  20  and the recess portion  11   b  of the mouthpiece  11  through filament winding forming (FW forming) and hardening the resin. As the resin of the FRP layer  21 , for example, an epoxy resin, a modified epoxy resin, an unsaturated polyester resin or the like is used. Moreover, as the fiber bundle  70 , a bundle of carbon fibers (CF), metals fiber or the like is used. During the FW forming, while rotating the liner  20  around a tank axis, a guide of the fiber bundle  70  is moved along the tank axial direction, whereby the fiber bundle  70  can be wound around the outer peripheral surface of the liner  20 . 
     Next, there will be described a fiber winding structure for decreasing structural bends of the fiber bundles (e.g. the bundles of carbon fibers CF)  70  in the high-pressure tank  1  (see  FIG. 2 , etc.). 
     As described above, the high-pressure tank  1  is formed by winding the fiber bundle (e.g. the carbon fiber bundle)  70  around the outer periphery of the liner  20  and hardening the resin. Here, the winding of the fiber bundle  70  includes hoop winding and helical winding, a hoop layer (denoted with mark  70 P in  FIG. 4 ,  FIG. 5 , etc.) is formed by hoop-wound layers of the resin, and a helical layer (denoted with mark  70 H in  FIG. 4 ,  FIG. 4 , etc.) is formed by helically wound layers, respectively. In the former hoop winding, the fiber bundle  70  is wound around the straight part (the tank trunk part) of the high-pressure tank  1  in the form of a coil spring to tightly wind the corresponding part, and a force for countering a force directed in the Y-axis positive direction (a force to expand to the outside in a diametric direction) is exerted on the liner  20  by a gas pressure. On the other hand, the latter helical winding is a winding way for a main purpose of tightly winding the dome part in a tight winding direction (inwardly in the tank axial direction), and the fiber bundle  70  is wound around the whole high-pressure tank  1  so that the fiber bundle is caught by the dome part, thereby mainly contributing to the enhancement of a strength of the dome part. It is to be noted that an angle (an acute angle) formed by a helix (a screw thread line in a screw) of the fiber bundle  70  wound in the form of the coil spring and the center line (a tank axis  12 ) of the tank  1  is “the winding angle with respect to the tank axis ( 12 )” of the fibers  70  mentioned in the present description and denoted with mark α in  FIG. 2  (see  FIG. 2 ). 
     In the hoop winding among these various winding ways, the fiber bundle  70  is wound around the straight part almost vertically to the tank axis, and in this case, a specific winding angle is, for example, from 80 to 90° (see  FIG. 2 ). The helical winding (or impress winding) is a winding way of also winding the fiber bundle  70  around the dome part, and the winding angle with respect to the tank axis is smaller than in the hoop winding (see  FIG. 2 ). The helical winding is roughly divided into two types of high-angle helical winding and low-angle helical winding, and in the high-angle helical winding in the two types, the winding angle with respect to the tank axis is comparatively large, and a specific example of the winding angle is from 70 to 80°. On the other hand, in the low-angle helical winding, the winding angle with respect to the tank axis is comparatively small, and a specific example of the winding angle is from 5 to 30°. It is to be noted that in the present description, helical winding with a winding angle of 30 to 70° which is an angle between the above angles is referred to as medium-angle helical winding sometimes. Furthermore, helical layers formed by the high-angle helical winding, the medium-angle helical winding and the low-angle helical winding are referred to as a high helical layer, a medium helical layer (denoted with mark  70 MH) and a low helical layer (denoted with mark  70 LH), respectively. Moreover, a folded portion of the dome part  1   d  of the high-angle helical winding in the tank axial direction is referred to as a folded portion (see  FIG. 2 ). 
     Usually, in the hoop winding itself, the fiber bundles  70  are spirally wound while the fiber bundles are disposed adjacent to each other, and in the winding way, the fiber bundles can be wound so that the fiber bundles  70  are prevented from being overlapped, to prevent unevenness from being generated. On the other hand, the helical winding is usually performed for the main purpose of tightly winding the dome part, and in the winding way, it is difficult to decrease the overlap of the fiber bundles  70  with one another and the unevenness, or the decreasing of these defects is not sufficiently taken into consideration. The hoop winding and the helical winding are appropriately combined in accordance with specifications such as an axial length, a diameter and the like of the high-pressure tank  1 , and the hoop layer  70 P and the helical layer  70 H are laminated around the liner  20  (see  FIG. 1 , etc.). At this time, when the helical layer  70 H is disposed adjacent to the hoop layer  70 P, the unevenness of the helical layer  70 H is transferred to the hoop layer  70 P, and bends (undulations) are generated in the fiber bundles  70  of the hoop layer  70 P sometimes. 
     In this respect, the present embodiment relates to at least one helical layer  70 H among a plurality of helical layers  70 H positioned in an inner layer of the FRP layer  21 , in which a sectional area of the fiber bundle  70  constituting the helical layer  70 H is set to be smaller than that of the fiber bundle  70  constituting another layer formed outside the helical layer  70 H (see  FIG. 4 ). In this case, any of the plurality of helical layers  70 H positioned in the inner layer can be set as an object, but among these objects, the innermost helical layer (the helical layer closest to the liner  20 )  70 H is preferably set as the object (see  FIG. 6 ). 
     When the sectional area of the fiber bundle  70  constituting the helical layer  70 H which is at least one (preferably the innermost helical layer) of the plurality of helical layers  70 H positioned in the inner layer of the FRP layer  21  is decreased in this manner, the following function and effect can be obtained. That is, when the helical layer  70 H is formed by using the fiber bundle  70  having the small sectional area, a thickness t of the fiber bundle  70  itself becomes smaller than a conventional thickness t′ (see  FIG. 4  and  FIG. 5 ). Moreover, the wound fiber bundle  70  has a flattened sectional shape owing to a tensile force, a pressure from another layer, or the like during the winding. Furthermore, since the sectional area is small, a space between the fiber bundles  70  is made smaller than before (see  FIG. 4 ). Therefore, unevenness in the surface of the helical layer  70 H constituted of the fiber bundle  70  becomes small, and hence it is possible to prevent the unevenness from being transferred to another layer (in the present embodiment, the hoop layer  70 P) formed outside the helical layer  70 H (see  FIG. 4 ). In consequence, it is possible to decrease conventional structural bends of the fiber bundle due to an uneven helical layer (indicating the helical layer which is not subjected to any treatment for smoothing the surface thereof and has the unevenness occurring on the surface, as denoted with mark  70 B in  FIG. 11  and  FIG. 12 ). Therefore, the structural bends (undulations) of the fiber bundle  70  of the layer (in the present embodiment, the hoop layer  70 P) formed outside the helical layer  70 H are suppressed, and a fatigue strength of the fiber bundle  70  itself can be enhanced. Specifically, in the conventional helical layer  70 P, the fiber bundles  70  having a comparatively large sectional area overlap with one another, so that the surface of the helical layer  70 P is not smooth sometimes (see  FIG. 6 ), but in the present embodiment, the surface of the helical layer  70 H can be more smooth (see  FIG. 4 ). Additionally, in the present embodiment, the structural bends (undulations) of the hoop layer  70 P formed outside the helical layer  70 H are apparently smaller than before (see  FIG. 4  and  FIG. 5 ). Furthermore, the helical layer  70 H itself and the layer formed outside the helical layer  70 H (in the present embodiment, the hoop layer  70 P) have a decreased thickness and an increased density to obtain a high Vf, whereby a burst strength can be enhanced. It is to be noted that Vf indicates a fiber volume content ratio. When the value (Vf value) increases, the content ratio of the fibers becomes high, and the content ratio of the resin becomes small. If the value of Vf is excessively high, fatigue durability deteriorates. If the value is excessively lowered, the outer shape of the tank becomes large. 
     Additionally, in the present embodiment, the innermost layer (the innermost helical layer  70 H) of a plurality of helical layers  70 H positioned in the inner layer is set as the object, and hence the tank strength can more noticeably be enhanced. That is, usually, in the hoop layer  70 P and the helical layer  70 H formed by winding the fiber bundles  70  (especially, the hoop layer  70 P), the layer positioned on an inner side (i.e., the layer closer to the liner  20 ) has a larger degree of contribution to the tank strength. Therefore, when the innermost helical layer  70 H is constituted of the fiber bundle  70  having a small sectional area as described above, the layer adjacent to the outside of the innermost helical layer  70 H (in the present embodiment, the hoop layer  70 P) can more smoothly be wound. In consequence, the structural bends (undulations) of the fiber bundle  70  constituting the hoop layer  70 P are suppressed, which can further contribute to the enhancement of the tank strength. 
     It is to be noted that “the inner layer” mentioned in the present description is the layer closer to the liner  20  in a case where the FRP layer  21  is divided into a plurality of layers. For example, in the present embodiment, when the FRP layer  21  is divided into three layers having substantially the same thickness 1/3, the layer on the inner side is referred to as the inner layer. Moreover, the layer on an outer side is referred to as the outer layer, and an intermediate layer is referred to as the middle layer. In Table 1 and Table 2 described later, the layers are described as “outer”, “middle” and “inner”. 
     Moreover, when the sectional area of the fiber bundle  70  is decreased as described above, the sectional area can be changed, for example, by changing the number of the fibers constituting the fiber bundle  70 . Specifically, in the helical layer  70 H positioned in the inner layer of the FRP layer  21 , the fiber bundle (a small tow)  70  having a smaller number of the fibers is used, and in the other layers (the helical layers  70 H of the middle layer and the outer layer and the hoop layers  70 P of all the layers), a fiber bundle (a large tow) having a larger number of the fibers is used, whereby the surface of the helical layer  70 H as the object can further be smoothed. A configuration of the arrangement of the fiber bundles  70  will be illustrated and described as follows (see Table 1 and Table 2). 
     In Table 1 described hereinafter, the fiber bundle  70  constituted of 6 k (6,000) fibers is used in the helical layer  70 H positioned in the inner layer of the FRP layer  21 , and the fiber bundle  70  constituted of 12 k (12,000) fibers is used in the other layers (the helical layers  70 H of the middle layer and the outer layer and the hoop layers  70 P of all the layers). In this case, the sectional area of the fiber bundle  70  may be decreased in all the helical layers  70 H positioned in the inner layer, or the sectional area of the fiber bundle  70  may be decreased in part of the helical layers. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Arrangement Example 1 of Fiber Bundles 
               
             
          
           
               
                   
                 Helical 
                 Hoop 
               
               
                   
                   
               
             
          
           
               
                   
                 Outer ⅓ layer 
                 12k 
                 12k 
               
               
                   
                 Middle ⅓ layer 
                 12k 
                 12k 
               
               
                   
                 Inner ⅓ layer 
                  6k 
                 12k 
               
               
                   
                   
               
             
          
         
       
     
     Moreover, in Table 2 described hereinafter, the fiber bundle  70  constituted of 12 k (12,000) fibers is used in the helical layer  70 H positioned in the inner layer of the FRP layer  21 , and the fiber bundle  70  constituted of 24 k (24,000) fibers is used in the other layers (the helical layers  70 H of the middle layer and the outer layer and the hoop layers  70 P of all the layers). Also in this case, the sectional area of the fiber bundle  70  may be decreased in all the helical layers  70 H positioned in the inner layer, or the sectional area of the fiber bundle  70  may be decreased in part of the helical layers. The sectional area can appropriately be changed in this manner. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Arrangement Example 2 of Fiber Bundles 
               
             
          
           
               
                   
                 Helical 
                 Hoop 
               
               
                   
                   
               
             
          
           
               
                   
                 Outer ⅓ layer 
                 24k 
                 24k 
               
               
                   
                 Middle ⅓ layer 
                 24k 
                 24k 
               
               
                   
                 Inner ⅓ layer 
                 12k 
                 24k 
               
               
                   
                   
               
             
          
         
       
     
     The above-mentioned configuration of the arrangement of the fiber bundles  70  is merely illustration, and the number of the fibers may appropriately be changed. However, for smoothing the surface of the helical layer  70 H which is the object to sufficiently develop the above function and effect, the sectional area of the fiber bundle  70  of the helical layer  70 H as the object is preferably ⅔ or less of the sectional area of the fiber bundle  70  in the other layers. In other words, the number of the fibers of the fiber bundle  70  in the layers other than the object is preferably selected so that the number is 1.5 or more times the number of the fibers of the fiber bundle  70  of the helical layer  70 H which is the object, to arrange the fiber bundles  70 . 
     It has been described above that the sectional area of the fiber bundle  70  in part of the helical layers  70 H is decreased, but the surface of the helical layer  70  can be smoothed by another means. For example, when a tensile force onto the fiber bundle constituting the helical layer  70 H is increased, the thickness of the fiber bundle  70  in a laminating direction of the FRP layer  21  can be decreased. In such a case, the fiber bundle  70  spreads in a width direction to have a flatter state, a stepped portion, accordingly, becomes small, and the surface of the helical layer  70 H becomes smoother. Heretofore, owing to an influence of unevenness of the surface of the helical layer  70 H, a shear stress has been generated by the bend of the fiber bundle  70  of the layer (e.g. the hoop layer) outside the helical layer  70 H (see a part surrounded with an imaginary line in  FIG. 10 ), breakdown has proceeded, and the fatigue strength might lower. According to the present embodiment, however, such an influence is suppressed, so that the deterioration of the fatigue strength can be avoided. 
     For example, in another embodiment of the present invention shown in  FIG. 7  and the like, a tensile force onto the fiber bundle  70  is about 20 N in the present situation (see  FIG. 8 ), but the tensile force onto the fiber bundle  70  of the innermost helical layer  70 H is increased to about 40 to 50 N (see  FIG. 7 ). In such a case, the fiber bundle  70  more tightly wound with the increase of the tensile force has a further flattened sectional shape to decrease a thickness thereof (the thickness t′→t). In consequence, the surface of the helical layer  70 H has a smaller stepped portion, and become smoother (see  FIG. 7 ). For example, when the tensile force is increased from 20N to about 40 to 50 N as described above, the thickness of the fiber bundle  70  and the thickness of the helical layer  70 H are decreased as much as about 20%, and the unevenness is decreased, whereby the surface can further be smoothed. 
     Moreover, when the tensile force is increased as described above to smooth the helical layer  70 H, a linear diameter of the fiber bundle  70  of the layer formed outside the helical layer  70 H is preferably increased. When the fiber bundle diameter of the other layer (e.g. the hoop layer  70 P) formed outside the helical layer H is increased as much as, for example, from 20 to 30% in this manner, bend displacement in the fiber bundle is decreased, so that the shear stress can be alleviated (see  FIG. 9 ). An example will be described. Heretofore, for example, a bend angle from 6 to 7° has been generated in the hoop layer  70 P outside the helical layer  70 H owing to the influence of the unevenness of the helical layer  70 H (see  FIG. 10 ), but the bend angle can be decreased to about 4.2 to 5.5° only by increasing the linear diameter of the fiber bundle  70  of the hoop layer  70 P (see  FIG. 9 ). That is, when the linear diameter of the fiber bundle  70  of the hoop layer  70 P is increased in this manner, rigidity per fiber lowers. Therefore, even when the same force is applied, a bending angle becomes small, and the bend angle decreases. 
     It is to be noted that hereinafter, an example of a filament winding (FW) device for winding the fiber bundle  70  will briefly be described. In an FW device  80  shown in  FIG. 13  and  FIG. 14 , while rotating the liner  20  around the tank axis, a guide unit (referred to as “a dagger” or the like)  81  of the fiber bundle  70  is reciprocated along the tank axial direction to wind the fiber bundle  70  around the outer periphery of the liner  20 . When a relative speed of movement of the guide unit  81  with respect to a rotation number of the liner  20  is changed, a winding angle of the fiber bundle  70  can be changed. The guide unit  81  is supported operably by, for example, a jig. 
     As described above, in the present embodiment, the unevenness of the surface of the helical layer  70 H is decreased, whereby the unevenness or structural bend which might be transferred to the layer (e.g., the hoop layer  70 P) adjacent to the outside of the helical layer  70 H is decreased, so that the deterioration of the fatigue strength can be avoided. In this case, the above-mentioned constitution of the helical layer  70 H using the fiber bundle  70  having a smaller number of the fibers and a comparatively small sectional area, the increasing of the tensile force onto the fiber bundle  70  to decrease the thickness t of the fiber bundle  70 , or the like is effective as means for decreasing the unevenness of the surface of the helical layer  70 H. Moreover, a winding amount of the fibers per unit time is increased, whereby productivity of the high-pressure tank  1  can be enhanced. 
     Moreover, needless to say, the smooth helical layer  70 H itself, and the hoop layer  70 P adjacent to this layer have a high fiber volume content ratio (Vf), and the resin, accordingly, accumulates less, whereby an amount of the resin for use in the FRP layer  21  decreases. When the amount of the resin decreases in this manner, the weight saving of the high-pressure tank  1  can be achieved as much as the decrease. 
     Furthermore, when the FRP layer  21  has a high Vf and the amount of the resin decreases as described above, an amount of hardening heat to be generated (the generation of the heat due to reaction heat during the thermal hardening of the resin) decreases. Usually, when peak temperature during the thermal hardening is high, a problem such as bonding (after the FW forming and the resin hardening, the liner  20  and the FRP layer  21  are partially or entirely bonded) or liner material deterioration might occur. However, these problems can be suppressed in the high-pressure tank  1  of the present embodiment which can decrease the amount of the hardening heat to be generated in this manner. 
     In addition, the present embodiment also has an advantage that the tank strength can noticeably be enhanced. That is, the decreasing of the structural bends of the fiber bundle  70  of the hoop layer  70 P preferably contributes to the enhancement of the fatigue strength of the straight part  1   s  of the high-pressure tank  1 . Moreover, the decreasing of the structural bends of the fiber bundle  70  of the helical layer  70 H preferably contributes to the enhancement of the fatigue strength of the dome part  1   d  of the high-pressure tank  1 . Usually, in both the helical layer  70 H and the hoop layer  70 P, the layer positioned on the inner side (the layer closer to the liner  20 ) has a larger degree of contribution to the tank strength, and the innermost hoop layer  70 P has a large function especially in that the straight part  1   s  is tightly wound to exert a sufficient pressure resistance. In this respect, according to the above embodiment, when at least the surface of the innermost helical layer  70 H is formed to be smooth, the layer (e.g. the hoop layer  70 P) adjacent to the outside of the smooth helical layer  70 H can smoothly be formed, and the layer (e.g. the hoop layer  70 P) can noticeably contribute to the enhancement of the tank strength. 
     It is to be noted that the above embodiments are an example of the preferable embodiment of the present invention, but the present invention is not limited to this example, and can variously be modified without departing from the scope of the present invention. For example, in the above embodiments, there has been described the case where the innermost helical layer  70 H is formed as the smooth helical layer (the innermost smooth helical layer), but in view of various functions and effects in the case where the smooth helical layer  70 H is formed, a treatment to form the surface of the other helical layer  70 H into a smooth surface is preferably performed. Moreover, when the helical layer  70 H in the middle layer or the outer layer is smoothed, the above function and effect can similarly be realized also in the helical layer  70 H. 
     Moreover, in the above embodiment, the case where the layer outside the helical layer is the hoop layer  70 P has mainly be described (see  FIG. 4 , etc.), but the present invention can be applied to a case where the layer outside the helical layer  70 H is the helical layer  70 H. 
     Furthermore, in the above embodiments, there has been described illustration of a case where the present invention is applied to a hydrogen tank which can be utilized in a fuel cell system or the like, but needless to say, the present invention can be applied to a tank to be filled with a fluid other than a hydrogen gas. 
     Additionally, the present invention can be applied to a member other than the tank (the pressure container), for example, a cylindrical member (including a cylindrical portion) such as a long member or structure including the FRP layer. An example will be described. When the FRP layer  21  including the helical layers  70 H or the hoop layers  70 P is formed by winding the fiber bundles  70  around the outer side of an axle (e.g. a mandrel or the like) or a mold through the helical winding or the hoop winding, the smooth helical layer  70 H is formed. In this case, the structural bends of the fiber bundles  70  are decreased, the fatigue strength is enhanced, and the thickness of each layer is decreased. In this manner, a function and an effect can be realized in the same manner as in the above embodiments. 
     Moreover, when the present invention is applied to a cylindrical member  1 ′ in this manner, the layer adjacent to the hoop layer is formed as another hoop layer  70 P or the smooth helical layer  70 H in one of preferable configurations. Alternatively, the layer adjacent to the uneven helical layer  70 H is formed as the smooth helical layer in another preferable configuration. It is to be noted that specific examples of the cylindrical member  1 ′ include a sporting equipment such as a shaft of a golf club or a carbon bat, leisure goods such as a fishing rod, an engineering product such as a plant facility and a structure such as a construction material. 
     INDUSTRIAL APPLICABILITY 
     The present invention is preferably applied to a tank having an FRP layer, and further to a cylindrical member such as a long member or a structural member. 
     DESCRIPTION OF REFERENCE NUMERALS 
       1  . . . high-pressure tank (tank),  1 ′ . . . cylindrical member,  20  . . . liner,  21  . . . FRP layer,  70  . . . fiber bundle,  70 H . . . helical layer,  70 P . . . hoop layer, and t and t′ . . . thickness of (helical layer  70 H).