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CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of and priority to U.S. Provisional Patent Application No. 61/169,621 filed Apr. 15, 2009, the technical disclosures of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to walk-in bathtubs and more specifically to an improved door and hinge system for the side of the tub to allow easier access. 
     BACKGROUND OF THE INVENTION 
     Walk-in bathtubs comprise high tub walls with a high built in seat and a side door, allowing the user to walk into the tub from the side and sit down without having to climb down into a low bath tub.  FIG. 1  shows a typical example of a walk-in bathtub in accordance with the prior art. Walk-in tubs are particularly suited for individuals who have physical limitations that make it difficult or dangerous to climb into and out of a regular, low bathtub or to stand up in a shower for extended periods of time. Such limitations might include physical disabilities or simply the reduced strength, balance and range of motion that typically occur with advancing age. Walk-in tubs are not only easier to enter and exit than conventional bathtubs they also reduce the chances of slips and falls compared to conventional tubs and showers. 
     In addition to safety, the ease of entering the tub via the side walk-in door also provides users with independence, allowing them to bathe without the assistance of another person when getting into and out of the tub. 
     Of central importance for walk-in tubs is the design of side door itself. Specifically, the position and swing path of the door affects the operation of the tub and its ease of use. Prior art designs like the one shown in  FIG. 1 , in which the side door opens to the outside of the tub, have to maintain a proper seal against the outward pressure of the water when the tub is filled. By and large, this task is left up to the locking handle used to close and secure the door. Adding to the difficulty in maintaining a proper seal is the fact that most walk-in tubs do not have a door frame. Instead, the door is merely bolted directly onto the wall of the tub. Depending on the depth of the tub and the materials used in it construction, the perimeter of the door threshold (in the absence of a frame) can buckle and distort under the pressure of the water, leading to potential leaks. 
       FIG. 2  shows an improved prior art design that overcomes many of the problems noted above. In this design, the side door  201  opens to the inside of the tub. Therefore when the door is closed and the bath is filled, the water pushes against the door in the direction of the closed position, thereby enhancing the strength of the water seal instead of working against it. Also present in this design is a door frame  202  onto which the door  201  is mounted. The frame prevents buckling and warping around the perimeter of the door threshold, thereby preventing leaks in the water seal due to buckling from water pressure against the walls of the tub. 
     However, despite the advantages of the improved door design shown in  FIG. 2 , it creates a new problem of its own. As can be seen in the figure, because the door  201  opens inward, it has to swing across the foot well of the tub. If a person is standing or sitting in the tub, the legs have to be moved out of the door&#39;s path during opening and closing. While this may seem like a trivial inconvenience, one must keep in mind that most users of walk-in tubs have some degree of movement impairment. Depending on the size of the foot well of the tub, the size and length of the user&#39;s legs and the degree of impairment, the user may have a great deal of difficulty opening and closing the door while inside the tub. 
     Therefore it would be desirable to have a door design that that opens to the inside of the walk-in tub but does not swing through the foot well of the tub, thereby avoiding interference with the user&#39;s legs. 
     SUMMARY OF THE INVENTION 
     The present invention provides a door assembly for a walk-in bathtub. The invention includes a door that is shaped to fit a door threshold in the side or end of the walk-in tub. The door has a threshold piece that fits within the door threshold and an internal flat panel that faces the interior of the bathtub when the door is closed. The door assembly further comprises at least one hinge assembly that includes a first axis mount coupled to the door threshold and a second axis mount that is coupled to the outer side of the door. A double axis hinge is coupled to both axis mounts, thereby connecting the door to the bathtub. In an alternate embodiment, the door assembly may include multiple hinge assemblies. 
     The double axis hinge opens the door to the inside of the tub by pivoting about the first axis toward the interior of the bathtub as the door in turn counter rotates about the second axis at the distal end of the hinge, in the direction opposite to that of the hinge. As a result of this counter rotation of the door the interior panel of the door faces the interior of the bathtub in both the opened and closed positions. 
     The outer side of the door has a channel that extends across the width of the door and accommodates the second axis mount and allows the hinge to recess into the channel when the door is in the opened and closed positions. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objects and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
         FIG. 1  shows a typical example of a walk-in bathtub with an outward swinging side door in accordance with the prior art; 
         FIG. 2  shows a perspective view of a walk-in bathtub with an inward swinging side door in accordance with the prior art; 
         FIGS. 3A-3C  show a perspective view of a lever hinged side door for a walk-in bath tub in accordance with a preferred embodiment of the present invention; 
         FIGS. 4A-4C  show a top plan view of a lever hinged side door for a walk-in bath tub in accordance with a preferred embodiment of the present invention; 
         FIGS. 5A-5C  shows a top plan view of an alternate embodiment of the present invention with the door located at the end of the tub; 
         FIGS. 6A-6C  show another embodiment of the present invention employing multiple double axis hinges; 
         FIGS. 7A-7C  illustrate a locking mechanism used to secure the tub door when the door is in the closed position; and 
         FIGS. 8A-8C  show the door handle of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 3A-3C  show a perspective view of a lever hinged side door for a walk-in bath tub in accordance with a preferred embodiment of the present invention.  FIGS. 3A-3C  illustrate the manner in which the side door opens. Similarly,  FIGS. 4A-4C  show a top plan view of the lever hinged side door. 
     In the example shown, the lever hinged door  300  is mounted to a door frame  310  similar to the one shown in  FIG. 2 . However, it should be mentioned that the present invention can also be used with walk-in tubs that do not utilize a door frame around the threshold. 
     Unlike prior art designs, the door  300  in the present invention does not hinge directly on the door threshold. Instead, the door  300  is connected to the frame  310  by an intermediate double axis hinge  320 . It is this double axis hinge  320  that is connected to the door frame  310  via a first hinge axis mount  311 . The other end of the hinge  320  connects to approximately the middle of the door  300  by means of a second hinge axis mount  321 . 
     The double axis hinge  320  and two axis mounts  311 ,  321  provide the door  300  two degrees of freedom during opening and closing. As shown in  FIGS. 3B and 4B , as the door is opened, the hinge  320  rotates on the frame axis mount  311  and follows the inward arc that would be followed by a conventional door, as denoted by arrow  330  in  FIG. 4B . 
     The door itself  300  counter rotates on the second axis mount  321 , allowing it to pivot in the opposite direction of the hinge  320 , as denoted by arrow  340  in  FIG. 4B . As seen in the figures, this counter rotation of the door  300  on the second axis mount  321  causes the distal end  301  of the door to rotate back toward the first axis mount  311  rather than swinging across the foot well. As a result, the swing path of the door  300  is opposite the user and away from the user&#39;s legs. The door  300  only crosses part of the foot well, specifically the far outside corner of the foot well (relative to the seat) during opening and closing, leaving the leg space in front of the tub seat largely unobstructed. 
     By having the door counter rotate on the second axis mount  321  as the hinge  320  rotates about the first axis mount  311 , the area of the foot well crossed by the door  300  is determined primarily by the length of the double axis hinge rather than the width of the door and will vary according to the width of the door and door frame as well as the width and length of the foot well. Therefore, with the second axis mount  321  positioned approximately in the center of the door  300 , the rotation of the door about the second axis mount reduces the area of the foot well crossed by the door by roughly half compared to the prior art design which hinges at one side of the door, requiring the entire width of the door to swing across the foot well. 
       FIGS. 3C and 4C  show the door in the fully opened position. Like the prior art inward swinging door, the door  300  of the present invention lies along the inner wall of the tub opposite the seat when fully opened, providing easy access into and out of the tub. However, as shown in the figures, because of the pivot around the second hinge axis mount  321 , the inner surface of the door  300  faces inward toward the seat instead of facing the inner wall. 
     As shown in  FIGS. 3A-3C , the door  300  comprises a threshold piece  302  and a flat panel  303 . As the name implies the threshold piece  302  fills the space of the door threshold and forms the outer surface of the door  300 . The flat panel  303  forms the inner surface of the door and is wider than the door threshold, thereby helping to form the water seal by applying pressure to a gasket (not shown) around the perimeter of the door threshold. This gasket may be incorporated into the inner wall of the tub around the perimeter of the threshold or in a preformed frame mounted in the threshold such as frame  310 . In an alternate embodiment, the gasket may be incorporated into the perimeter of the flat panel of the door  300 . 
     In the present invention, the threshold piece  302  includes a channel across its width, shown most clearly in  FIG. 3B . This channel accommodates the hinge  320  and second axis mount  321  and allows the hinge to recess into the door  300  when the door is in the fully closed and opened positions, as shown in  FIGS. 3A and 3C .  FIG. 3A  shows how the U-shaped geometry of hinge ends allow the second axis mount  321  to fit with the hinge  320  in the closed position. Conversely,  FIG. 4C  shows how the goose neck geometry of the second axis mount  321  creates a space to allow a hinge cover (shown in  FIGS. 5 and 6 ) mounted on the hinge  320  to nest within the hinge components when the door is fully open. 
     By allowing the double axis hinge  320  to recess into the threshold piece  302 , the channel enables the door  300  of the present invention to occupy the same space as prior art doors, thereby allowing the present invention to be implemented with current tub and door frame designs and be retrofitted to current tubs. 
     The point of attachment of the second axis mount  321  on the door may vary according to the design of the tub. In the example shown in  FIGS. 3A-3C  and  4 A- 4 C the hinge  320  and corresponding second axis mount  321  are coupled at approximately the middle of the door  300 . However, the location of the hinge axis on the door can vary depending on the dimensions of the door and tub and the desired swing path of the door. 
       FIGS. 5A-5C  show an alternate embodiment of the present invention.  FIGS. 5A-5C  show both perspective and top plan views of the tub  500  as the door moves between the closed and open positions. In this embodiment, the door  510  is located at the far end of the tub  500 , opposite the seat  520 . As shown in the figures, the door  510  opens along the side of the tub  500 . The opening and closing motion of the door  510  is the same as that of door  300  described above. Like the previous embodiment, this one allows the door  510  to swing away from the user&#39;s legs without having to cross over the foot well. 
     The location of the door on the end as shown in  FIG. 5  is not practical for conventional door designs. A conventional door placed at the end of the tub would hit the knees of the user as it swings inward toward the user and across the foot well. The only way in which a conventional door could avoid this problem is if the length of the tub (especially the foot well) is increased, which would increase the distance the user would have to move to get into and out of the seat, as well as the amount of reach necessary to open and close the door from the seated position. Again, one must always remember that the primary user population will have some degree of movement impairment. Therefore, these seemingly trivial inconveniences due to increased tub length can pose significant problems for many users. By allowing the door  510  to swing to the side and away from the user&#39;s knees, the door system of the present invention permits an inward opening door to be placed at the end of tubs that are not very long (e.g., 36×36 in.) without interfering with the user&#39;s legs. 
       FIGS. 6A-6C  show another embodiment of the present invention employing multiple double axis hinges. Multiple hinges provide additional structural stability to the door during opening and closing depending on the size and weight of the door. In the example shown in  FIGS. 6A-6C , two double axis-hinges  620 ,  630  are connected to the door  610 . However, more than two hinges may be used. The structure and operation of the hinges  620 ,  630  is the same as hinge  320 . 
     It should be understood that the multiple double-axis hinge door shown in  FIGS. 6A-6C  can also be applied to the tub configuration shown in  FIGS. 5A-5C  with the door on the end opposite the seat rather than in the side wall. 
       FIGS. 7A-7C  illustrate a locking mechanism used to secure the tub door when the door is in the closed position.  FIG. 7A  shows the locking mechanism in the open position, and  FIG. 7B  shows the locking mechanism in the closed position.  FIG. 7C  shows a closer view of the locking mechanism, door and door frame in isolation from the tub. 
     The locking mechanism is internal to the door and is illustrated here with the outer door panel removed. The mechanism comprises two locking levers  711 ,  712  that are pivotally coupled to locking pins  721 ,  722  at one end and a central rotating mount  731  at the other. The locking pins are held within respective pins guides  723 ,  724  that keep the pins moving in a straight line as they are pushed and pulled by the pivotally coupled locking levers  711 ,  712 , similar to cylinders for pistons driven by a crank shaft. The rotating mount  730  is operated by a handle  800  (shown in  FIG. 8A ) on the inside of the door. 
     As shown in the figures, the locking levers  711 ,  712  are pivotally coupled to opposite ends of the rotating mount  730  across from each other. Each lever pivots on a respective axis  713 ,  714  (shown more clearly in  FIG. 7C ). In the present example, both levers  711 ,  712  are coupled to the outer face of the rotating member  730 , but they can be coupled to the inner face or on opposite faces of the mount as well. 
     Because they point in opposite directions, the levers  711 ,  712  move in opposite directions as the central rotating mount  730  turns. In the example shown, when the rotating mount  730  move counterclockwise, the locking levers  711 ,  712  are pushed outward, causing the pivotally coupled locking pins  721 ,  722  to slide into respective locking recesses  741 ,  742  in the door frame, thereby locking the door in the closed position (pictured in  FIG. 7B ). Turning the rotating mount  730  in the clockwise direction pulls the locking pins  721 ,  722  out of the locking recesses  741 ,  742 , thereby unlocking the door (pictured in  FIG. 7A ). 
     It should be noted that if the rotating mount  730  were to continue rotating counterclockwise past the locked position shown in  FIG. 7B  it would pull the locking pins  721 ,  722  back out. However, as explained in more detail below, the handle that operates the rotating mount  730  prevents it from rotating past the locked position. 
     In the example shown in  FIGS. 7A-7C  the door includes two locking mechanisms. To enable them to operating in unison, a drive chain  750  links the two rotating members  730 ,  731  so that both can be operated by the door handle, similar to a bicycle chain. 
     The number of locking mechanisms incorporated into the door will depend on the size of the door and depth of the tub. One locking mechanism might be sufficient for smaller doors, whereas three or more might be needed for larger ones. In addition, depending on the size and width of the door, the locking mechanism might only include one locking lever and locking pin on one side of the door, rather than two levers moving in opposite directions to secure both sides of the door. 
       FIGS. 8A-8C  show the door handle of the present invention.  FIG. 8A  shows the handle  800  mounted on the internal panel of the tub door  850 .  FIG. 8B  is an exploded view showing how the handle assembly is coupled to the door panel  850 .  FIG. 8C  shows exploded and assembled views of how the handle assembly is coupled to the rotating mount  730  of the locking mechanism shown in  FIGS. 7A-7C . 
     The handle  800  turns the rotating mount  730  by means of a shaft  820  that passes through the inner door panel  850 . The shaft  820  is secured to the rotating mount  730  by means of a screw  840  that passes through a hole  821  in the shaft  820  when the shaft is inserted into the rotating mount, as shown in  FIG. 8C . The other end of the shaft  820  is coupled to a fitted insert  820  that engages a recess  801  in the base of the handle  800 . 
     The rotation of the handle  800  and rotating mount  730  is controlled by a flanged handle mount  830  secured to the inner door panel  850 . The flange  831  on the door mount  830  is semicircular, which can be seen most clearly in  FIG. 8B . The fitted insert  820  in the base of the handle  800  accommodates the flange  810  of the door mount  830 , and has a turn stop protrusion  811  (shown in  FIG. 8C ) that buts into the flange  831 , thereby restricting the rotation of the handle  800  to approximately 180°. 
     The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated. It will be understood by one of ordinary skill in the art that numerous variations will be possible to the disclosed embodiments without going outside the scope of the invention as disclosed in the claims.

Summary:
The disclosure concerns a door assembly for a walk-in bathtub that includes a door that fits within a door threshold in the wall of the walk-in tub. The door assembly further comprises a hinge assembly that includes a first axis mount coupled to the door threshold and a second axis mount coupled to the outer side of the door. A double axis hinge is coupled to both axis mounts, to thereby connect the door to the bathtub. The double axis hinge is able to open the door towards the inside of the tub by pivoting about the first axis toward the inside of the bathtub as the door in turn counter rotates about the second axis at the distal end of the hinge, in the direction opposite to that of the hinge.