Abstract:
A stepladder provided with a coupling frame in the shape of an equilateral triangle, a front support, a rear right support, a rear left support, and spread stoppers for stopping the front support, the rear right support, and the rear left support from spreading. The front support is rotationally attached to a front shaft of the coupling frame, the rear right fixing member is rotationally attached to a right shaft, and the rear left fixing member is rotationally attached to a left shaft. The rear left support and the rear right support are detachably mounted to the rear left fixing member and the rear right fixing member, respectively. The supports of the stepladder are coupled by the spread stoppers, whereby the three supports are restricted from being displaced forward, to the right rear, and to the left rear.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a stepladder capable of reducing the risk of toppling. 
     Conventionally, stepladders are used in light work at height, for example, pruning of tree branches. A tripod stepladder and a four-legged stepladder are known as such stepladders. For example, a conventional four-legged stepladder  20  shown in  FIG. 11  includes a front leg pair  21  and a rear leg pair  22 , which are shaped as ladders. The front and rear leg pairs  21  and  22  are coupled to each other. The distance between the ground contact points of left and right legs  21 L and  21 R of the front leg pair  21  and the distance between the ground contact points of left and right legs  22 L and  22 R of the rear leg pair  22 , or the widths, are fixed. Further, the distance between the ground contact point of the front left leg  21 L and the ground contact point of the rear left leg  22 L is fixed, and the distance between the ground contact point of the front right leg  21 R and the rear right leg  22 R is fixed. The taller the type of stepladder, the greater the width of the stepladder becomes. 
       FIG. 12  shows a conventional four-legged stepladder  23  including a front left leg  21 L, a front right leg  21 R, a rear left leg  22 L, and a rear right leg  22 R that have adjustable lengths. Thus, even if there is a slight unevenness in the installation location, the height of each leg can be changed in conformance with the unevenness. 
       FIG. 13  shows a conventional tripod stepladder  25  that includes a ladder-shaped front leg pair  23  and a bar-shaped rear leg  24 . The front leg pair  23  includes a front left leg  23 L and a front right leg  23 R that are fixed to form a predetermined width. The taller the type of stepladder, the greater the width of the stepladder becomes. 
     Besides the conventional stepladders shown in  FIGS. 11 to 13 , stepladders described in Patent Documents 1 to 3 are known. 
     However, in the conventional four-legged and tripod stepladders, increase in the distance between the front legs and the rear leg(s) or increase in the width between the left and right legs of the front leg enlarges the footprint of the stepladder. Thus, even when the height of a stepladder is increased in accordance with the purpose and type, increase in the width tends to be restrained. As a result, a higher stepladder has a higher risk of lateral toppling. 
     Moreover, in the conventional four-legged and tripod stepladders, the front and rear legs spread only to the front and rear. Reduction in the angels between the supports of the front and rear legs and the horizontal installation surface does not reduce the risk of lateral toppling. Furthermore, the legs of the ladder that function as the front legs have a large width. This prevents setting of the legs of the ladder in narrow space, such as a place where various items of equipment are already installed and a place with hedges. In addition, since the front and rear legs are fixed, the stepladder cannot be set in a place including facility pipes or tree branches above the ground or in a place having significant slopes or unevenness. 
     PRIOR ART DOCUMENTS 
     Patent Documents 
     
         
         Patent Document 1: Japanese Laid-Open Utility Model Publication No. 7-32198 
         Patent Document 2: Japanese Registered Utility Model No. 3111306 
         Patent Document 3: Registered Utility Model No. 3031542 
       
    
     SUMMARY OF THE INVENTION 
     To solve the problems described above, it is an object of the present invention to provide a stepladder that reduces risk of toppling and restrictions of the installation location. 
     To achieve the foregoing objective and in accordance with a first aspect of the present invention, a stepladder is provided that includes a coupling frame, a front support that is coupled to the coupling frame and includes a ladder, a rear right support coupled to the coupling frame by a rear right fixing member, a rear left support coupled to the coupling frame by a rear left fixing member, and a spread stopper that limits spreading of the front support, the rear right support, and the rear left support. The coupling frame has a shape of a regular triangle. Sides of the coupling frame include a front shaft, a right shaft, and a left shaft. A coupling pipe of the front support is rotationally coupled to the front shaft of the coupling frame. A coupling pipe of the rear right fixing member is rotationally coupled to the right shaft. A pipe of the rear left fixing member is coupled to the left shaft. The rear left support and the rear right support are attached to the rear left fixing member and the rear right fixing member, respectively, in a removal manner. Three supports including the front support, the rear left support, and the rear right support form the stepladder. The spread stopper couples the supports together so that the three supports are arranged to form a regular triangular pyramid and displacement of the three supports in three directions including a front direction, a rear right direction, and a rear left direction is restricted. 
     Thus, the stepladder can withstand external force applied from any of the front, rear, right and left directions and maintain uniform balance, thereby reducing the risk of lateral toppling. 
     In accordance with a second aspect of the present invention, a stepladder is provided that includes a coupling frame, a front support that is coupled to the coupling frame and includes a ladder, a rear right support coupled to the coupling frame by a rear right fixing member, a rear left support coupled to the coupling frame by a rear left fixing member, and a spread stopper that limits spreading of the front support, the rear right support, and the rear left support. The coupling frame has a shape of a regular triangle. Sides of the coupling frame include a front shaft, a right shaft, and a left shaft. A coupling pipe of the front support is rotationally coupled to the front shaft of the coupling frame. A coupling pipe of the rear right fixing member is rotationally coupled to the right shaft. A pipe of the rear left fixing member is coupled to the left shaft. The rear left support and the rear right support are attached to the rear left fixing member and the rear right fixing member, respectively, in a removal manner. Three supports including the front support, the rear left support, and the rear right support form the stepladder. The spread stopper couples the supports together so that displacement of the three supports in three directions including a front direction, a rear right direction, and a rear left direction is restricted. 
     Thus, the stepladder can withstand external force applied from any of the front, rear, right and left directions and maintain uniform balance, thereby reducing the risk of lateral toppling. In addition, such an advantage is achieved with a simple structure. 
     The above described stepladder preferably includes the following configuration. That is, the rear left fixing member and the rear right fixing member each include a hollow pipe body that opens at a lower end, and a lower portion of the pipe body of each fixing member includes a cut out groove. A distal end section that extends upward from a body section of each support has a reduced outer diameter over a length that is inserted into the pipe body of a corresponding one of the fixing members. A tip section of each support is tapered. The distal end section of each support includes a projection for positioning. Each support includes a step between the distal end section and the body section. The cut out groove of each fixing member has a width that is greater than an outer diameter of the projection. Each cut out groove includes a first linear section that extends upward from a cut end at an inner side of the lower portion of the pipe body of each fixing member and is parallel with an axis of the fixing member, an inclined section extending outward and diagonally downward from an uppermost position of the first linear section, and a second linear section that extends upward and downward from a lowermost position of the inclined section and is parallel with the axis of the fixing member. When each support is inserted and coupled to the pipe body of a corresponding one of the fixing members, the following actions take place. The projection is aligned with the cut end of the cut out groove after the distal end section of the support is inserted, the projection and the support are moved upward along the first linear section of the cut out groove, the projection and the support are moved downward along the inclined section from a position where the upward movement ends, the projection and the support are moved upward along the second linear section from a position where the downward movement ends, and coupling of the support to the fixing member completes when the upward movement ends. When the coupling of each support completes, the lower portion of the pipe body of the fixing member is in contact with the step of the support. A shape of the cut out groove of the lower portion of the pipe body of the rear right fixing member and a shape of the cut out groove of the rear left fixing member are symmetrical. 
     When the coupling of each support to the corresponding one of the fixing members completes, the lower portion of the pipe body of the fixing member is preferably in contact with the step of the support. 
     A shape of the cut out groove of the lower portion of the pipe body of the rear right fixing member and a shape of the cut out groove of the rear left fixing member are preferably symmetrical. 
     A pipe preferably forms the coupling frame. 
     A chain preferably forms the spread stopper. 
     A top operation platform is preferably arranged between the coupling frame and the front support, and the top operation platform is preferably rotationally coupled to the coupling frame by a pipe and rotationally coupled to the front support by another pipe. 
     The present invention provides a stepladder capable of reducing the risk of toppling and restrictions of the installation location. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall perspective view showing a stepladder of a first embodiment of the present invention; 
         FIG. 2  is an exploded perspective view showing components of the stepladder of the first embodiment; 
         FIG. 3A  is an exploded perspective view showing main components of the stepladder of the first embodiment; 
         FIG. 3B  is a perspective view showing the assembled main components; 
         FIG. 4A  is an exploded perspective view showing main components of a stepladder of another embodiment; 
         FIG. 4B  is a perspective view showing the assembled main components; 
         FIGS. 5A, 5B and 5C  are perspective views showing supports of the stepladder of the first embodiment in assembled and disassembled states; 
         FIG. 6  is a perspective view showing the stepladder of the first embodiment in the first use position; 
         FIG. 7  is a perspective view showing the stepladder of the first embodiment in the second use position; 
         FIG. 8  is a perspective view showing the stepladder of the first embodiment in the third use position; 
         FIG. 9  is a perspective view showing the stepladder of the first embodiment in a folded state; 
         FIG. 10A  is a perspective view showing a use position of a stepladder of a second embodiment of the present invention; 
         FIGS. 10B, 100 and 10D  are perspective views showing a coupling sequence of a spread stopper of the stepladder of the second embodiment; 
         FIG. 11  is a perspective view showing a four-legged stepladder of the first conventional example; 
         FIG. 12  is a perspective view showing a four-legged stepladder of the second conventional example; 
         FIG. 13  is a perspective view showing a garden tripod stepladder of the third conventional example; 
         Fig. 14  is a diagram for comparison of the risks of toppling of the stepladder of the first embodiment according to the present invention, the tripod stepladder of the third conventional example, and the four-legged stepladder of the first conventional example; 
         FIG. 15  is an isometric projection view showing the front, top, and right sides of the stepladder of the first embodiment; 
         FIG. 16  is an isometric projection view showing the rear, bottom, and left sides of the stepladder of the first embodiment; and 
         FIG. 17  is a perspective view showing a use position of a stepladder of further embodiment according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to drawings, embodiments of the present invention will now be described. However, the present invention is not limited to such embodiments. 
       FIG. 1  is an overall perspective view showing a stepladder S of a first embodiment according to the present invention. The stepladder S is a tripod stepladder that includes a coupling frame Tcp, which is a pipe having the shape of a regular triangle, a front support PL including a ladder L, a rear right fixing member Psr for a rear right support, a rear right support Pudr, a rear left fixing member Psl for a rear left support, a rear left support Pudl, and a chain-type spread stopper SOc. 
     The coupling frame Tcp includes a front shaft  31 , a right shaft  33 , and a left shaft  35 , which are arranged in the three sides of the regular triangle and connected to one another. A coupling pipe  32  of the front support PL is attached to the front shaft  31  of the coupling frame Tcp. A coupling pipe  34  of the rear right fixing member Psr is attached to the right shaft  33 . A coupling pipe  36  of the rear left fixing member Psl is attached to the left shaft  35 . Accordingly, the three supports PL, Pudr and Pudl are rotationally coupled to the three shafts  31 ,  33  and  35  of the coupling frame Tcp, respectively. The front shaft  31 , the right shaft  33 , and the left shaft  35  function as three coupling sections of the coupling frame. 
     The left support Pudl and the right support Pudr are attached to the left fixing member Psl and the right fixing member Psr, respectively, in a removable manner. The three supports, including the front support PL, form the stepladder S. Each of the supports Pudl, Pudr and PL has an adjustable length. 
     When the stepladder S is used, the spread stopper SOc connects the front support PL, the rear right support Pudr, and the rear left support Pudl such that each support forms an angle of about 70 degrees with the installation surface. The spread stopper SOc includes two chains. One of the chains is extended from the front support PL, hooked by a hook on the rear right support Pudr, and then hooked by a hook on the rear left support Pudl. The other chain is extended from the front support PL and hooked by a hook on the rear left support Pudl. This fixes the supports. The angle between the three supports and the installation surface is not limited to about 70 degrees. The angle of each support can be set to any angle according to the installation location. 
     The spread stopper SOc and the coupling frame Tcp couple the three supports Pudl, Pudr and PL to have the shape of a regular triangular pyramid so that displacement in three directions, including the front, rear right, and rear left directions, is restricted. This allows the stepladder to withstand external force applied from any of the front, rear, left, and right direction and maintain uniform balance. Thus, the stepladder maintains the most stable position that resists toppling. 
       FIG. 2  is an exploded perspective view showing the components of the stepladder S. The coupling frame Tcp is coupled in advance to the front support PL with ladder. The rear right support Pudr is coupled to the rear right fixing member Psr in a removable manner. The rear left support Pudl is coupled to the rear left fixing member Psl in a removable manner. The spread stopper SOc is coupled to the front support PL in advance. Although the ladder L is shown separated from the front support PL, the front support PL with ladder is an integrated single component. 
       FIG. 3A  is an exploded perspective view showing the main components of the stepladder S of the first embodiment.  FIG. 3B  is a perspective view showing the assembled main components. The coupling pipe  32  of the front support PL is coupled to the front shaft  31  of the coupling frame Tcp. The coupling pipe  34  of the rear right fixing member Psr is coupled to the right shaft  33 . The coupling pipe  36  of the rear left fixing member Psl is coupled to the left shaft  35 . Thus, the three supports are coupled such that each of the supports rotates about the corresponding one of the three sides of the regular triangular coupling frame Tcp that function as shafts of rotation. 
       FIGS. 5A, 5B and 5C  are perspective views showing disassembled and assembled states of the supports Pudr and Pudl of the stepladder S of the first embodiment. The hooks for the spread stoppers and the feet of the supports Pudr and Pudl need to have predetermined orientations when the supports are coupled. Thus, a mechanism is needed that positions the supports in the same positions in repeated attachment and removal. 
     In addition, a mechanism is needed that prevents separation of the supports Pudr and Pudl from the fixing members Psr and Psl during use and allows the supports Pudr and Pudl to withstand external force applied to the fixing members Psr and Psl. Furthermore, a mechanism is needed that prevents separation of the supports Pudr and Pudl from the fixing members Psr and Psl when carrying the folded stepladder S in an upright position. The required three mechanisms are achieved by the shapes of cut out grooves Hr and Hl of the fixing members Psr and Psl, positioning projections PP of the supports Pudr and Pudl, and steps Gu of the supports. 
       FIG. 5A  is a diagram showing coupling of the supports. The rear left fixing member Psl includes a hollow pipe body having an open lower end. The lower portion of the pipe body of the rear left fixing member Psl includes a cut out groove Hl that resembles letter N. A distal end section  41  extending upward from a body section  40  of the rear left support Pudl has a reduced outer diameter over the length that is inserted into the pipe body of the rear left fixing member Psl. A tip section  42  is tapered to facilitate the insertion. The distal end section  41  of the rear left support Pudl includes a projection PP for positioning. A step Gu is formed at the border between the distal end section  41  and the body section  40  of the rear left support. 
     The cut out groove Hl of the rear left fixing member has a width slightly greater than the outer diameter of the projection PP for positioning. The cut out groove Hl includes a first linear section  45 , an inclined section  46 , and a second linear section  47 . The first linear section  45  extends parallel to the axis of the rear left fixing member Psl from a cut end at the inner side of the lower portion of the pipe body of the rear left fixing member Psl. The inclined section  46  extends from the upper end of the first linear section  45  diagonally downward to the outer side. The second linear section  47  extends both downward and upward from the lower end of the inclined section  46  in parallel with the axis of the fixing member. The shape of the cut out groove Hr of the lower portion of the pipe body of the rear right fixing member Psr and the shape of the cut out groove Hl of the rear left fixing member Psl are symmetrical. 
     As indicated by an arrow, in inserting and coupling the rear left support Pudl to the pipe body of the rear left fixing member Psl, the projection PP is aligned with the cut end of the left cut out groove Hl after the distal end section  41  of the rear left support Pudl is slightly inserted, and the projection PP and the support Pudl are then moved upward along the first linear section  45  of the cut out groove Hl. The projection PP and the support Pudl are then moved downward along the inclined section  46  from the position where the upward movement ends. The projection PP and the support Pudl are then moved upward along the second linear section  47  from the position where the downward movement ends. The coupling of the support Pudl to the rear left fixing member Psl completes when the upward movement ends. 
     In inserting and coupling the rear right support Pudr to the pipe body of the rear right fixing member Psr, the projection PP and the rear right support Pudr are moved along the cut out groove Hr of the rear right fixing member in a symmetrical manner to the movement in which the projection PP of the support Pudl is inserted into the cut out groove Hl of the rear left fixing member. 
       FIG. 5B  is a diagram explaining fixation of the supports. When the projection PP is located at the uppermost position of the second linear section  47  of the cut out groove Hl of the rear left fixing member Psl, the lower portion of the pipe body of the rear left fixing member Psl is in contact with the step Gu of the rear left support Pudl. The rear left support Pudl is not separated from the rear left fixing member Psl. As indicated by arrows, the rear left support Pudl withstands external force applied to the rear left fixing member Psl. Similarly, the rear right support Pudr withstands external force applied to the rear right fixing member Psr. 
       FIG. 5C  is a diagram showing a state in which the supports are suspended and carried. When the stepladder S in which the supports are fixed as shown in  FIG. 5B  is folded to be carried in an upright position, the two supports Pudr and Pudl, which are laterally spread, need to be pulled inward toward the front support PL located in the center. To prevent separation of the projections PP of the supports Pudr and Pudl from the respective cut out grooves Hr and Hl, each second linear section  47  is arranged at the outer side of the pipe body of the corresponding fixing member Psr or Psl. This allows the each projection PP to move upward and downward in engagement with the inner wall surface of the second linear section  47  of the corresponding cut out groove Hr or Hl. After the supports Pudr and Pudl are pulled toward the front support PL, each projection PP falls by the weight of the support and is engaged with the lower end of the second linear section  47 . Thus, the supports are suspended from the respective fixing members Psr and Psl. This allows the stepladder to be carried in this position. 
     When the stepladder S is used again after being carried, the left and right supports Pudl and Pudr need to be lifted and spread. To prevent separation of the projection PP of each support from the corresponding one of the cut out grooves Hr and Hl in such movements, the wall surface that resists inward movement of the projection PP, which engages the lower end of the second linear section  47 , needs to extend to the uppermost section associated with the fixed position of the support shown in  FIG. 5B . In addition, each of the cut out grooves Hr and Hl needs to extend to its cut end. Thus, the inclined section  46  extends diagonally upward to the inner side from a middle section of the second linear section  47 . 
       FIG. 6  is a perspective view showing a use position of the stepladder S of the first embodiment. In accordance with the restrictions of the installation location, the legs of the front support PL and the rear left support Pudl are extended, and the leg of the rear right support Pudr is shortened. 
       FIG. 7  is a perspective view showing another use position of the stepladder S of the first embodiment. In accordance with the restrictions of the installation location, the legs of the front support PL and the rear left support Pudl are extended. If the stepladder S cannot be set even after the leg of the rear right support Pudr is shortened, a shorter rear right support sPudr may replace the rear right support Pudr. 
       FIG. 8  is a perspective view showing yet another use position of the stepladder S of the first embodiment. When the installation location is flat and free from restrictions, the legs of the front support PL, the rear left support Pudl, and the rear right support Pudr are shortened. Both legs of the ladder L of the front support PL are on the ground. 
       FIG. 9  is a perspective view showing the folded ladder S of the first embodiment. When the stepladder S is folded to be carried in an upright position, the stepladder S is folded by pulling the rear right support Pudr and the rear left support Pudl, which are laterally spread, inward toward the front support PL having the ladder. This allows for carrying and storing of the stepladder in a compact folded state. 
     Referring to  FIG. 14 , the risks of toppling of the stepladder S of the first embodiment according to the present invention, a tripod stepladder  25  of the third conventional example, and a four-legged stepladder  20  of the first conventional example are compared based on the arrangements of the legs or supports. 
     Under a condition in which the angles between the horizontal installation surface and the legs or supports of the ladders S,  25  and  20  are set at a certain angle, specifically, 75 degrees, and the length of each support is set to 360 cm, the ground contact positions of the legs are indicated by symbols a to j. The stepladder S does not topple when the center of gravity Cc of the operator on the stepladder S is within a triangular installation area SA that is surrounded by the dotted line connecting the ground contact points a, b and c of the stepladder S of the first embodiment. 
     The tripod stepladder  25  does not topple when the center of gravity Cc of the operator on the stepladder  25  is within a triangular installation area  25 A that is surrounded by the dotted line connecting the ground contact points d, e and f of the stepladder  25 . Similarly, the four-legged stepladder  20  does not topple when the center of gravity Cc of the operator who operates on the stepladder  20  is within a rectangular installation area  20 A that is surrounded by the dotted line connecting the ground contact points g, h, i and j of the stepladder  20 . 
     When the angle between the horizontal surface and the stepladders S,  20  and  25  is fixed to 75 degrees and the length of each support is successively changed to 270 cm and 150 cm, the sizes of the installation areas SA,  25 A and  20 A of the stepladders S,  25  and  20  successively decrease as shown in  FIG. 14 . However, each stepladder does not topple when the center of gravity Cc of the operator on the ladder is within the corresponding one of installation locations SA,  25 A and  20 A of reduced size. 
     The comparison among the risks of toppling of the three stepladders is as follows. In the conventional tripod stepladder  25  and the four-legged stepladder  20 , even if the height of the stepladder is increased, the increase in the width between the front left leg and the front right leg is restrained. The widths of the ladders are the distance between the ground contact points c and b, the distance between the ground contact points d and e, and the distance between the ground contact points g and h. 
     In a conventional stepladder, increase in the width is restrained. Thus, increase in the height increases the risk of lateral toppling. In contrast, in the stepladder S of the first embodiment, the distance between adjacent two ground contact points can be increased according to the height. Thus, increase in the height reduces the risk of lateral toppling. 
     Further, in the tripod stepladder  25  and the four-legged stepladder  20 , the front legs and rear legs spread only to front and rear. Thus, even if the angles between the horizontal installation surface and the front and rear leg supports are reduced, the distance between the ground contact points is increased only in the front and rear direction. This does not reduce the risk of lateral toppling. In contrast, in the stepladder S of the first embodiment, reduction in the angles between the legs of the stepladder and the horizontal surface increases the size of the installation area SA shown in  FIG. 14 . This reduces the risk of lateral toppling. 
     As is evident in the foregoing descriptions, the present invention has the following advantages. 
     (1) As shown in  FIGS. 1, 15 and 16 , the coupling pipe  32  of the front support PL is coupled to the front shaft  31  of the regular triangular coupling frame Tcp. The coupling pipe  34  of the rear right fixing member Psr is coupled to the right shaft  33 . The coupling pipe  36  of the rear left fixing member Psl is coupled to the left shaft  35 . Thus, each support is coupled to the coupling frame Tcp to be rotational about the corresponding one of the three shafts of rotation  31 ,  33  and  35 . Accordingly, the three supports are arranged at regulated positions to form a regular triangular pyramid. This allows the stepladder to withstand external force applied from any of the front, rear, right and left directions and maintain uniform balance. This reduces the risk of lateral toppling. In particular, regarding the conventional problem that an increase in the height of the stepladder increases the risk of lateral toppling, the legs of the three supports, which are extended to increase the height of the stepladder, can be easily spread in three directions from the coupling frame with limited restrictions of the installation location. This reduces the risk of toppling. 
     In addition, each of the three supports is coupled to be rotational about the corresponding one of the three rotation shafts of the coupling frame Tcp. Thus, each of the three supports can individually form any angle with the horizontal installation surface. Reduction in the angles enlarges the installation area. Thus, unlike the conventional stepladders, the risk of toppling can be reduced. 
     (2) Although including a ladder attached to one of the three supports, the present embodiment is stable. This allows the front support PL with the ladder to be set in small space such as a place where equipment is already installed and a place with hedges. 
     (3) The rear right support Pudr is attached to the rear right fixing member Psr. The rear left support Pudl is attached to the rear left fixing member Psl. The three supports including the front support with ladder form the stepladder. The right and left supports are removal. Thus, when the stepladder cannot be set with the supports coupled in a place where a facility pipe or a tree branch is located above the ground, the support that prevents the setting is removed and then coupled again at a position beyond the facility pipe or the tree branch. This allows for setting of the stepladder. 
     (4) The legs of the three supports can extend and contract. This allows the stepladder to be set in various places with steps. Further, the stepladder can be set in places with significant slopes and unevenness by replacing the right and left supports with other shorter or longer supports. 
     (5) Although including a ladder attached to the support PL of the three supports, the present embodiment is stable. Thus, the front leg of the ladder does not have to include two legs. In addition, the width of the ladder can be reduced, and the rear leg can be free from steps. Thus, compared to a conventional four-legged or tripod stepladder, the overall size and the weight of the stepladder are reduced. 
       FIGS. 4A and 4B  are perspective views showing a coupling frame of another embodiment. This coupling frame differs from the first coupling frame Tcp of the first embodiment in the following points. A second coupling frame Tcb has the shape of a regular triangle and includes coupling pieces  51 ,  52  and  53  that function as coupling sections. The coupling pieces  51 ,  52  and  53  project from the three vertices located at equal angular intervals. 
     Three sets of a bolt, washer, and nut couple the front support PL, the rear right fixing member Psr, and the rear left fixing member Psl to the coupling pieces  51 ,  52  and  53  of the second coupling frame Tcb, respectively. The front support PL is coupled to the second coupling frame Tcb to be rotational about the corresponding bolt, which functions as the shaft of rotation. The fixing members Psr and Psl couples the respective rear right support and the rear left support to the second coupling frame Tcb such that the rear right support and the rear left support rotate about the respective bolts, which function as the shafts of rotation. 
     The stepladder including the second coupling frame Tcb has the same advantages as the stepladder of the first embodiment. 
       FIG. 10A  is a perspective view showing a use position of a stepladder of a second embodiment according to the present invention. This embodiment is used only in a place that has a flat surface and is free from installation restrictions. The present embodiment reduces the risk of toppling compared to conventional four-legged or tripod stepladders. 
     The coupling pipe  32  of a front ladder Ln is coupled to the front shaft  31  of the first coupling frame Tcp. The coupling pipe  34  of a rear right support Pnr is coupled to the right shaft  33 . The coupling pipe  36  of the rear left support Pnl is coupled to the left shaft  35 . Thus, the three supports are coupled to the first coupling frame Tcp to be rotational about the three sides of the first coupling frame Tcp that function as the shafts of rotation. A slip stopper  50  is attached by a universal joint to each of the legs of the front ladder Ln, the leg of the rear right support Pnr, and leg of the rear left support Pnl. 
     A spread stopper SOp includes one fixed piece  151  and two movable pieces  152 . The spread stopper SOp is suspended from the front ladder Ln with the fixed piece  151  of the spread stopper SOp fixed to the front ladder Ln. The two movable pieces  152  are rotationally coupled to the fixed piece  151 . Each movable piece  152  of the spread stopper SOp includes a cut out hole that engages with the hook of the rear right support Pnr or the hook of the rear left support Pnl to limit spreading of the supports. The angle between the horizontal surface and each of the front ladder Ln, the rear right support Pnr, and the rear left support Pnl is maintained at 70 degrees. 
       FIGS. 10B, 10C and 10D  show the assembly sequence of the spread stopper SOp. The spread stopper SOp may include three cut out holes so that the angle between the horizontal installation surface and each support is selectively set to 75 degrees, 70 degrees, and 65 degrees. 
       FIG. 17  is a perspective view showing a use position of a stepladder of further embodiment according to the present invention. This embodiment differs from the first embodiment shown in  FIG. 1  in that a top operation platform TFL is arranged between the first coupling frame Tcp and the front support PL. The top operation platform TFL has a large area that allows an operator to stand on the top operation platform TFL. The top operation platform TFL is rotationally coupled to the coupling frame Tcp by the coupling pipe  32  in the same manner as the first embodiment. In addition, another coupling pipe  153  rotationally couples the top operation platform TFL to the front support PL. Thus, the coupling pipe  153 , the top operation platfonn TFL and the coupling pipe  32  rotationally couple the front support PL to the first coupling frame Tcp. The pipes  34  and  36  rotationally couple the rear left support Pudl and the rear right support Pudr, respectively, to the first coupling frame Tcp. 
     In the stepladder of the further embodiment shown in  FIG. 17 , the top operation platform TFL is arranged between the first coupling frame Tcp and the front support PL. This allows an operator to stand on the stable top operation platform TFL having a large area. In addition, the stepladder of the present embodiment has the same advantages as the first embodiment shown in  FIG. 1 . 
     DESCRIPTION OF THE REFERENCE NUMERALS 
     S: stepladder, Tcp: first coupling frame, Tcb: second coupling frame, Psr: rear right fixing member, Psl: rear left fixing member, Pl: front support, L: ladder, Pudr: rear right support, Pudl: rear left support, Hr: right cut out groove, Hl: left cut out groove, PP: projection for positioning, Gu: step of support, SOc: first spread stopper, SOp: second spread stopper, Ln: front ladder, Pnr: rear right support, Pnl: rear left support, a: ground contact point of front leg of stepladder S, b: ground contact point of rear right leg of stepladder S, c: ground contact point of rear left leg of stepladder S, d: ground contact point of front left leg of stepladder  25 , e: ground contact point of front right leg of stepladder  25 , f: ground contact point of rear leg of stepladder  25 , g: ground contact point of front left leg of four-legged stepladder  20 , h: ground contact point of front right leg of four-legged stepladder  20 , i: ground contact point of rear right leg of four-legged stepladder  20 , j: ground contact point of rear left leg of four-legged stepladder  20 , Cc: center of gravity of operator, TFL: top operation platform