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TECHNICAL FIELD 
     The present invention relates to a floating flap gate which is disposed at an opening in a seawall to prevent a rising water from flowing into living spaces or underground spaces at a time of rising water, by raising a door body to block the opening. 
     BACKGROUND ART 
     A floating flap gate exists which is disposed at an opening of a seawall and blocks the opening at the time of a rising water to prevent the rising water from flowing into living spaces or underground spaces, by raising a door body, using a buoyancy of the water which is trying to flow in (e.g., Patent Reference 1). 
     However, the floating flap gate disclosed in Patent Reference 1 has a problem in that if a speed of the inflowing water is high, the rising action of a door body  1  is delayed, resulting in an overflow of water into living spaces or underground spaces (see  FIG. 11 ( a ) .) 
     In addition, when the water level drops, the door body  1  stays at a rising state up to a water level which is about ⅓ the height of the door body  1 , and subsequently exhibits a hazardous behavior such as suddenly falling (see  FIG. 11  ( b ).) 
     In order to prevent the problem of overflow during the initial influx of water, there was proposed a floating flap gate with a rope having a counterweight attached to one end, and with the other end connected to the door body via a pulley (e.g., Patent Reference 2). 
     The floating flap gate disclosed in Patent Reference 2 solves the problem of the delayed rising action of the door body during the initial influx of water by compensating for an insufficient buoyancy of the floating flap gate by using the weight of a counterweight. 
     However, the floating flap gate disclosed in Patent Reference 2 does not readily lower when the water level drops, because weight of the counterweight continually operates in a direction which assists in the operation of raising the door body. 
     Patent Reference 1: Japanese Patent Application Kokai Publication No. 2001-214425 
     Patent Reference 2: Japanese Patent Application Kokai Publication No. 2003-253912 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The problem which the present invention aims to solve is that it becomes difficult to lower the door body when the water level drops, if a device is installed to continually assist in the operation of raising the door body, so as to solve the problem of a floating flap gate in which the operation of raising the door body is delayed when water starts flowing in, resulting an overflow into living spaces or underground spaces. 
     Means for Solving this Problem 
     The present invention was devised with the aim of eliminating the overflow into living spaces or into underground spaces when the raising operation of the door body is delayed when water first starts to flow in, and eliminating difficulty in lowering the door body when the water level drops, and thus eliminating hazardous behavior such as sudden falling of the door body. 
     The floating flap gate according to the present invention is a floating flap gate which is disposed at an opening or at an access way, so as to block the opening or the access way when water flows in, and comprises a door body with a forward end which is able to swing upwards, in a direction in which the water flows in and within a plane in a height direction, around a base end thereof serving as a fulcrum. 
     One end of a rope is attached to the forward end of the door body, and the other end of the rope is attached to a counterweight or a spring via at least a fixed pulley, so that the counterweight is at its lowest point or the spring reaches its natural length when the angle of inclination of the door body with respect to a horizontal plane reaches within a range from 10° to 80° during raising or lowering of the door body. 
     According to the present invention, the door body is assisted in rising by being drawn in an upward direction by the counterweight or by the spring, until the angle of inclination of the door body with respect to a horizontal plane reaches within a range from 10° to 80° during raising of the door body. Further, if the angle of inclination of the door body with respect to a horizontal plane exceeds the range of 10° to 80°, the counterweight or spring causes resistance, thereby reducing the raising speed of the door body. 
     On the other hand, when lowering the door body, until the angle of inclination of the door body with respect to a horizontal plane reaches within a range from 10° to 80°, the door body is assisted in following a downward trajectory which follows the water level as it drops, by being drawn in a downward direction by the counterweight or by the spring. In addition, when the angle of inclination of the door body with respect to a horizontal plane is below the range of 10° to 80°, the counterweight or spring causes resistance, thereby reducing the lowering speed of the door body. 
     Advantageous Effects of the Invention 
     According to the present invention, the predetermined angle of inclination of the door body with respect to a horizontal plane while raising or lowering the door body serves as a point of bifurcation for assisting in raising and lowering the door body and, causing resistance to raising or lowering the door body, thus making it possible to prevent water from overflowing at the time of the initial influx, and also making it possible to prevent the door from suddenly falling before lowering of the door body is completed. In addition, it becomes possible to mitigate the shock which occurs when raising of the door body is completed, and the ability of the door body to follow the water level improves at the initial stage of lowering of the door body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic structural drawing of the floating flap gate according to the present invention, where  FIG. 1 ( a )  is a side view,  FIG. 1 ( b )  is a front view, and  FIG. 1 ( c )  is a planar view. 
         FIG. 2  is a drawing of the operating principle of the floating flap gate according to the present invention, where  FIG. 2 ( a )  is an initial stage of influx of water,  FIG. 2 ( b )  is an intermediate stage of raising or lowering,  FIG. 2 ( c )  is a final stage of raising,  FIG. 2 ( d )  is an initial stage of lowering, and  FIG. 2 ( e )  is a final stage of lowering. 
         FIG. 3  is a drawing illustrating an example of the relationship between the angle of inclination of the door body and the swinging force of the door body in the direction of rising due to the counterweight in the floating flap gate according to the present invention. 
         FIG. 4  is a schematic structural side view of the floating flap gate according to the present invention in a case where the counterweight is in contact with a movable pulley. 
         FIG. 5  is a schematic structural side view of the floating flap gate according to the present invention, using a linear compression coil spring instead of a counterweight. 
         FIGS. 6 ( a )  and  6  ( b ) are drawings illustrating the operating state of the linear compression coil spring with respect to the angle of inclination of the door body.  FIG. 6 ( a )  illustrates a case where the angle of inclination is 45°.  FIG. 6 ( b )  illustrates cases where the angle of inclination is 0° and 90°.  FIG. 6 ( c )  is a graph showing an example of the relationship between the angle of inclination of the door body and the swinging force of the door body in the direction of rising due to the spring of the floating flap gate according to the present invention. 
         FIG. 7  shows drawings illustrating the operating state of linear tension coils with respect to the angle of inclination of the door body.  FIG. 7 ( a )  illustrates a case where the angle of inclination is 45°.  FIG. 7 ( b )  illustrates cases where the angle of inclination is 0° and 90°. 
         FIG. 8  is a drawing illustrating a non-linear combination coil spring. 
         FIG. 9  is a graph illustrating another example of the relationship between the angle of inclination of the door body and the swinging force of the door body in the direction of rising due to the spring of the floating flap gate according to the present invention. 
         FIG. 10  is a drawing illustrating a portion of the floating flap gate according to the present invention corresponding to a portion where the rod is attached to both sides of the door body. 
         FIG. 11  shows drawings illustrating the problems of the floating flap gate according to the prior art.  FIG. 11 ( a )  illustrates the initial stage of influx, and  FIG. 11 ( b )  illustrates a time when the water level has dropped. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to the present invention, the object of preventing an overflow of water, during a beginning of water inflow, into living spaces and underground spaces, preventing difficulties in lower the door body when the water level has dropped, and preventing the door body from suddenly falling, is achieved by assisting in raising and lowering the door body, and by causing resistance to raising or lowering the door body, by having the predetermined angle of inclination of the door body with respect to a horizontal plane serve as a point of bifurcation while raising and lowering the door body. 
     EXAMPLE 
     An example of the present invention is described in detail below using  FIG. 1  to  FIG. 10 . 
       FIG. 1  is a schematic structural drawing of the floating flap gate according to the present invention. 
     In  FIG. 1 , Reference Numeral  11  is a floating flap gate according to the present invention which is disposed on a channel surface rs at an opening in a seawall, for example. When a water w tries to flow from an ocean (or from a river) into a living space or an underground space, the floating flap gate  11  uses the pressure of the water w to swing a forward end  12   b  of a door body  12  upwards around a base end  12   a  as a fulcrum, to block the opening in a water-tight manner. 
     If there is a wide opening to be blocked by the door body  12  of the floating flap gate  11 , then a plurality of door bodies  12  may be linked width-wise at the opening, and the spaces between the various door bodies  12  are joined together with water-tight rubber. In addition, water-tight rubber is provided on the sides corresponding to door bumpers of the door bodies  12  on both sides, which are provided at the opening of the seawall. 
     The floating flap gate  11  shown in  FIG. 1  has, for example, a rod  13  attached across the entire width-wise direction of the forward end of the door body  12 , which functions to support the load resulting from the water pressure, and to attach one end of a wire rope  14 . 
     The other end of the wire rope  14  is attached to a counterweight  20  via a first fixed pulley  16  which is disposed at a door bumper  15  at the top of the forward end of the door body  12  during lowering, and via a second fixed pulley  17  disposed in a position at the same height as the first fixed pulley  16  on the base end side of the door body  12 . Therefore, the weight of the counterweight  20  operates on the door body  12 . 
     In the present example of the invention, because the angle of inclination θ of the door body  12  is 90° when raising of the door body is completed, the position at which the first fixed pulley  16  is set is such that the counterweight  20  reaches its lowest point when the angle of inclination θ is 45° with respect to a horizontal plane when the door body  12  swings upwards (see  FIG. 2 ( b ) ), for example. As a result of investigations conducted by the inventors, there was found to be no problem if the angle of inclination θ ranges from 10° to 80°. 
     The floating flap gate  11  of the present invention which has the above-described configuration exhibits the functions described below when raising and lowering the door body  12 . 
     When Raising the Door Body  12   
     When water first starts flowing in, the counterweight  20  drops, pulling the door body  12  upwards, thereby assisting the door body  12  to rise (see  FIG. 2  ( a )). When the angle of inclination θ of the door body  12  with respect to a horizontal plane reaches 45°, the door body  12  and the wire rope  14  line up (see  FIG. 2 ( b ) ) and the counterweight  20  reaches the position of the lowermost end. When the angle of inclination θ of the door body  12  with respect to a horizontal plane exceeds 45°, the counterweight  20  rises due to the upwardly swinging action of the door body  12 , so that the counterweight  20  causes resistance, thereby decelerating the rising of the door body  12 , thus mitigating the shock when raising of the door body  12  is completed (see  FIG. 2 ( c ) ). 
     When Lowering the Door Body  12   
     At the initial stage of lowering the door body  12 , the counterweight  20  drops, pulling the door body  12  downward so that it follows the water level as it drops (see  FIG. 2 ( d ) ). Further, when the angle of inclination θ of the door body  12  with respect to a horizontal plane reaches 45°, the door body  12  and the wire rope  14  line up (see  FIG. 2 ( b ) ) and the counterweight  20  reaches the position of the lowermost end. When the angle of inclination θ of the door body  12  with respect to a horizontal plane is less than 45°, the counterweight  20  rises due to the lowering of the door body  12 , so that the counterweight  20  causes resistance, thereby decelerating the lowering of the door body  12 , thus mitigating the shock when lowering of the door body  12  is completed (see  FIG. 2 ( e ) ). 
       FIG. 3  shows the relationship between the angle of inclination θ of the door body  12  and the swinging force of the door body  12  in the direction of rising due to the counterweight  20  in the floating flap gate  11  according to the present invention. 
     The floating flap gate  11  according to the present invention makes it possible to implement a variety of functions such as assisting in raising the door body  12 , mitigating shock, and imparting a trajectory that follows the water level, as described above, by utilizing a raising/lowering mechanism which has the counterweight  20 . 
     As shown in  FIG. 4 , the floating flap gate  11  according to the present invention may have a movable pulley  18  arranged behind the second fixed pulley  17 , and while the counterweight  20  is attached to the movable pulley  18 , the other end of the wire rope  14  may be secured to a securing member  19  disposed at the door bumper  15  via the movable pulley  18 . 
     As shown in  FIG. 5 , the floating flap gate  11  according to the present invention may have a compression pressure coil spring  22  attached, instead of the counterweight  20 . A tension coil spring may be attached, although it is not shown in the drawing. Reference Numeral  23  in  FIG. 5  is a third fixed pulley which is disposed between the movable pulley  18  and the securing member  19 . The compression coil spring  22  or the tension coil spring may come in direct contact with the other end of the wire rope  14  as shown in  FIG. 1  and  FIG. 2 , instead of having the movable pulley  18  disposed between them, as shown in  FIG. 5 . 
     If the compression coil spring  22  is used, in the case of a flap gate having a door body  12  with an angle of inclination θ of 90° when raising of the door body  12  is completed, the spring reaches its natural length as shown in  FIG. 6  ( a ) when the angle of inclination θ of the door body  12  is 45°, and, as shown in  FIG. 6 ( c ) , the swinging force of the door body in the direction of rising is set to reach a minimum by means of the spring. Further, when the angle of inclination θ of the door body  12  is 0° and 90°, the spring is compressed as shown in  FIG. 6 ( b ) , and the swinging force of the door body in the direction of rising is set to reach a maximum by means of the spring, as shown in  FIG. 6 ( c ) . 
     On the other hand, if a tension coil spring  24  is used, the spring reaches its natural length as shown in  FIG. 7 ( a )  when the angle of inclination θ is 45°, and when the angle of inclination θ is 0° and 90°, the spring becomes extended, as shown in  FIG. 7 ( b ) . 
     The compression coil spring  22  or the tension coil spring  24  is not limited to the linear coil spring as shown in  FIG. 6  or  FIG. 7 . A spring which has non-linear properties such as a taper coil spring, a conical coil spring, a cylindrical coil spring, a barrel-shaped spring, or an irregular pitch coil spring may be used. 
     As shown in  FIG. 8 , a combination coil spring  25  formed from a large-diameter and short-length first compression spring  25   a , a medium-diameter and medium-length second compression spring  25   b , and a small-diameter and long-length third compression spring  25   c  arranged sequentially from the central axis, may be used to produce a tensile force which is non-linear. 
     When the combination coil spring  25  shown in  FIG. 8  is used, and the angle of inclination θ of the door body  12  is 0°, the three compression springs  25   a - 25   c  are all in a state of compression. Further, from the initial stage of rising to the early stage of rising of the door body  12 , when the angle of inclination of the door body  12  is about 10-30°, the three compression springs  25   a - 25   c  each start to expand, and when the early stage of rising is reached, the first compression spring  25   a  returns to its original state. 
     Next, from the early stage of rising to the intermediate stage of rising when the angle of inclination θ of the door body  12  is 45°, the second compression spring  25   b  and the third compression spring  25   c  successively return to their original state, and the three compression springs  25   a - 25   c  all return to their natural length. 
     From the intermediate stage of rising to the completion of rising when the angle of inclination θ of the door body  12  is 90°, the third compression spring  25   c , the second compression spring  25   b , and the first compression spring  25   a  successively contract, and when raising of the door body  12  is completed, the three compression springs  25   a - 25   c  are all in a compressed state. 
     When lowering the door body, the state is the reverse of that for when raising the door body. 
       FIG. 9  shows the relationship between the angle of inclination θ of the door body  12  and the swinging force of the door body in the direction of rising due to the non-linear combination coil spring  25 . 
       FIG. 1 ,  FIG. 2 ,  FIG. 4 , and  FIG. 5  show examples in which a single rod  13  is attached across the entire width of the door body  12 , but in  FIG. 10 , the rod  13  may be attached only to both sides of the door body  12 . 
     The present invention is not limited to the above-described example, and the preferred embodiment may, of course, be advantageously modified within the scope of the technical ideas recited in the claims. 
     For example, the wire rope  14  was used in the above example, but a polyamide-series, polyester-series, polyethylene-series, polypropylene-series, aramid-series, polyarylate-series, or ultra-high density synthetic fiber rope may be used. 
       FIG. 1 ,  FIG. 2 ,  FIG. 4 , and  FIG. 5  show a floating flap gate in which the door body  12  is a single floating body, but floating body connection-type flap gate may be used, in which a plurality of floating bodies are connected in a direction of height. 
     The movable pulley  18 , the counterweight  20 , and the springs  22 ,  24 , and  25  may be attached at a position on the outside of the door bumper  15 , but the weight of the counterweight  20 , the number of fixed pulleys  16 ,  17  and movable pulleys  18 , and the characteristics of the springs  22 ,  24 ,  25  may, of course, be set at the most appropriate values depending on the size of the door body  12 . 
     Moreover, instead of attaching the rod  13 , a hanging piece may be attached to the forward end of the door body  12 , and one end of the wire rope  14  may be attached to the hanging piece. 
     EXPLANATION OF THE REFERENCE NUMERALS 
       11  Floating flap gate 
       12  Door body 
       13  Rod 
       14  Wire rope 
       16  First fixed pulley 
       17  Second fixed pulley 
       18  Movable pulley 
       20  Counterweight 
       22  Compression coil spring 
       24  Tension coil spring 
       25  Combined coil spring

Summary:
A floating flap gate to prevent water from overflowing into underground spaces, with ease in raising or lowering the door body when water level changes. The floating flap gate comprises a door body with a forward end which swings upwards or downwards in a same direction following a rise or a drop in water level, wherein an opposite end serves as a fulcrum. A rod is attached to the forward end of the door body. One end of a wire rope is attached to the rod, and the other end is attached to a counterweight via fixed pulleys. The fixed pulleys are arranged so that the counterweight is at its lowest point when the angle of inclination of the door body with respect to a horizontal plane reaches within a range from 10° to 80° during raising or lowering of the door body.