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CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is filed as a continuation-in-part of copending patent application Ser. No. 11/775,433, the entire disclosure of which is hereby incorporated by reference herein for all purposes. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The subject matter disclosed herein relates to a flush valve structure for a toilet tank. 
         [0003]    FIG. 4 of copending U.S. patent application Ser. No. 11/775,433 discloses a single volume flush valve structure for a toilet tank. The single volume flush valve structure comprises a valve base that is secured in the outflow hole of the toilet tank. An extension flares upward from the valve base and a first part of a valve seal is attached to, or integrally formed with, the extension at the upper end thereof. A vertical guide is attached to the valve base and extends upward therefrom and a float assembly is fitted slidably over the guide. The float assembly includes a second, complementary, part of the valve seal, which engages the first part of the valve seal when the float assembly is in a lower position relative to the valve base. Because of the flared nature of the extension, the float assembly is typically of somewhat greater diameter than the outlet hole of the toilet tank. A pressure difference across the seal holds the two parts of the seal in sealing relationship. When the float assembly is lifted relative to the valve base, water passes between the two parts of the valve seal and relieves the pressure difference that would otherwise exist across the seal, allowing the float assembly to rise. Water then flows from the tank through the gap between the two parts of the valve. When the level of water in the tank falls approximately to the level of the first part of the seal, the two parts of the valve seal against each other and as water continues to enter the tank the pressure difference across the seal holds the two parts of the seal in position and the seal is thereby restored. U.S. patent application Ser. No. 11/777,433 does not disclose how the float assembly shown in  FIG. 4  of that application may be modified to achieve a dual volume flush, or how to adjust the volume of water that is discharged during a single flush. 
       SUMMARY OF THE INVENTION 
       [0004]    In accordance with a first aspect of the disclosed subject matter there is provided a toilet tank flush valve structure, for use with a toilet tank having a floor formed with an outlet hole, the flush valve structure comprising a valve base for installation in the outlet hole formed in the floor of the toilet tank, the valve base defining a water outflow passage having an upper end and a lower end and including a first valve member at the upper end of the water outflow passage, and a float assembly that is movable relative to the valve base between a sealing position in which a second valve member at a lower end of the float assembly engages the first valve member in sealing relationship and a flushing position in which the second valve member is spaced upwardly from the first valve member, and wherein the float assembly comprises an outer shell that is open downwards and has a lower edge at which the shell is provided with said second valve member, and a closure member within the outer shell and cooperating with the outer shell to define a float chamber, and wherein the closure member is adjustable in position relative to the outer shell, whereby the float chamber is adjustable in volume. 
         [0005]    In accordance with a second aspect of the disclosed subject matter there is provided a toilet tank structure comprising a toilet tank having a floor formed with an outlet hole, a valve base installed in the outlet hole in the floor of the toilet tank, the floor of the toilet tank defining a water outflow passage having an upper end and a lower end, a first valve member at the upper end of the water outflow passage, and a float assembly that is movable relative to the valve base between a sealing position in which a second valve member at a lower end of the float assembly engages the first valve member of the valve base in sealing relationship and a flushing position in which the second valve member is spaced upwardly from the first valve member, and wherein the float assembly comprises an outer shell that is open downwards and has a lower edge at which the shell is provided with said second valve member, and a closure member within the outer shell and cooperating with the outer shell to define a float chamber, and wherein the closure member is adjustable in position relative to the outer shell, whereby the float chamber is adjustable in volume. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0007]      FIG. 1A  is a sectional view of a conventional toilet tank equipped with a first flush valve structure in a first adjustment configuration, 
           [0008]      FIG. 1B  is a view similar to  FIG. 1A  illustrating the flush valve structure in a second adjustment configuration, 
           [0009]      FIGS. 1C and 1D  are views similar to  FIG. 1A  illustrating further modifications of the first flush valve structure, 
           [0010]      FIG. 2  is a sectional view of a conventional toilet tank equipped with a second flush valve structure, 
           [0011]      FIG. 3  is a sectional view of a toilet tank equipped with a third valve structure, 
           [0012]      FIG. 4  is a sectional view of a toilet tank equipped with a fourth valve structure, and 
           [0013]      FIG. 5  is a sectional view of a toilet tank equipped with a fifth valve structure. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    The toilet tank  2  shown in  FIG. 1A  is a conventional low-volume toilet tank having a floor  10  formed with an outlet hole  12  and also having front and rear walls  14 ,  16  and two side walls (not shown). The front and rear walls are typically about 7 inches apart whereas the side walls may be about 14 inches apart. As shown in  FIG. 1A , the hole may be substantially closer to the front wall  14  than the rear wall  16 . 
         [0015]    The flush valve structure that is shown in  FIG. 1A  comprises a valve base  100  that is installed in the hole  12  in the floor  10  of the toilet tank and is held in position by a nut  108  that is in threaded engagement with the base  100 . The valve base  100  includes a flange  110 , and an extension  112  extends upward from the flange. The extension  112  may be integrally formed with the valve base  100 , e.g. by concurrent injection molding of the base and the extension, or may be formed as a separate component that is attached to the valve base. The extension defines a passage that is circular in horizontal section at all points over its height, but the horizontal cross-sectional area of the passage is substantially greater at the top of the extension than the bottom. For example, the diameter of the passage at the bottom of the extension may be less than 2 inches, in order to match a valve base sized to fit in a standard 2 inch outlet hole, whereas the diameter of the passage at the top of the extension may be almost 5 inches. Thus, as shown in  FIG. 1A , the extension  112  flares upward from the flange  110 . Moreover, the extension has a skewed configuration, in that the locus of the center of the circular horizontal cross section of the passage is inclined to vertical, so that the center of the circular cross section of the passage at the top of the extension is located approximately midway between the front and rear walls of the tank even though the center of the outlet hole is closer to the front wall than the rear wall. A sealing member  116  is attached to the extension  112  at the top. In this case, the sealing member  116  is a flexible gasket. 
         [0016]    A guide  120  extends vertically upward from the valve base. The lower end of the guide  120  is a relatively slender rod that is in threaded engagement with the flange  110  of the base. Above its lower end, the guide has a cross-like configuration. At its upper end, the guide is provided with a removable stop member  122 . 
         [0017]    The flush valve structure shown in  FIG. 1A  also includes a float assembly  128 . The float assembly  128  comprises an overflow tube  132  through which the guide  120  extends with a clearance that is sufficient to permit movement of the float assembly lengthwise of the guide without binding yet is small enough to ensure that the float assembly remains properly aligned relative to the sealing member  116 . 
         [0018]    The float assembly further comprises an outer shell  130  that is attached to the overflow tube. The outer shell has a cylindrical outer wall  134  provided at its lower end with a sealing member  136  that cooperates with the sealing member  116  of the extension  112 , a first annular disc-wall  138  extending inward from the outer wall  134  at the upper end thereof, and an inner boss  140  that extends upwardly from the first annular disc-wall  138  and through which the overflow tube  132  extends with a small clearance. The inner boss  140  is threaded at its upper end and lock nut  142  having an internal flange is in threaded engagement with the threads of the inner boss. When the lock nut  142  is tightened onto the inner boss  140 , an O-ring  144  is clamped between the internal flange of the lock nut and the upper end of the boss. By clamping the O-ring in this manner, the O-ring grips the overflow tube, thus holding the outer shell in position relative to the overflow tube, and seals the clearance between the inner boss and the overflow tube. 
         [0019]    A second or lower annular disc-wall  146  is attached to the lower end of the overflow tube, e.g. by screw threads, and a narrow clearance is provided between the outer periphery of the second annular disc-wall  146  and the interior surface of the outer cylindrical wall  134 , allowing vertical movement of the second annular disc-wall  146  relative to the cylindrical outer wall  134 . The outer shell  130 , the overflow tube  132  and the second annular disc-wall  146  together define a float chamber. By releasing the lock nut  142 , moving the outer shell  130  lengthwise of the overflow tube  132 , and re-tightening the locknut, the position of the second annular disc-wall  146  relative to the first annular wall  138 , and hence the volume of the float chamber, is adjustable.  FIG. 1A  shows the float assembly with the second annular disc-wall  146  positioned to maximize the volume and resultant buoyancy of the float chamber, whereas  FIG. 1B  shows the float assembly with the wall  146  positioned to provide a float chamber of substantially smaller volume and resultant lesser buoyancy. 
         [0020]    Because the O-ring not only grips the overflow tube but also seals the float chamber at the upper end, air cannot be displaced from the float chamber by water entering the chamber through the clearance between the lower annular disc-wall  146  and the cylindrical outer wall  134 . Otherwise, water entering the float chamber through the clearance could lead eventually to the float chamber becoming waterlogged. It may nevertheless be desirable to provide drain holes in the lower annular disc-wall to ensure that any water that enters the float chamber is able to drain from the float chamber. 
         [0021]    The float assembly  128  is provided with an attachment ring  148  that can be connected to a flush lever (not shown) for lifting the float assembly from the seal. 
         [0022]    When the flush lever is pressed sufficiently to lift the float assembly, the seal between the sealing members  116  and  136  is broken. Because of the large volume of the float chamber, the buoyancy of the float assembly overcomes the force due to the pressure of water on the top of the annular wall and any downward force acting on the float assembly due to flow of water into the passage defined by the extension. Accordingly, even though the user may immediately release pressure on the flush lever, the float assembly immediately rises until the upper end of the overflow tube  132  engages the stop member  122 . When the upper end of the overflow tube engages the stop member, the float assembly initially remains in this position as the water level in the toilet tank falls due to outflow of water through the valve base, while water flow continues into the tank under control of the conventional fill valve. After this initial phase, the float assembly falls as the water level in the tank continues to fall until the float assembly is close enough to the sealing member  116  for the force acting on the float assembly due to flow of water into the outflow passage to overcome the buoyancy of the float assembly and pull the sealing member  136  into contact with the sealing member  116 . The water level in the tank then rises, as is conventional, until the fill valve shuts off supply of water. 
         [0023]    When the toilet bowl contains only liquid waste, the user activates the flush valve by pressing and immediately releasing the flush lever. Depending on the adjustment position of the second annular disc-wall  146  (as discussed below), the quantity of water that is discharged from the tank in response to a press and immediate release activation of the flush lever is sufficient to remove liquid waste and most solid waste from the toilet bowl. In the event that the user observes that solid waste remains in the bowl after pressing the flush lever, the user may maintain pressure on the flush lever, in which event the float assembly will remain elevated relative to the seal member at the upper rim of the extension and water will continue to flow through the valve and into the toilet bowl. The user releases the flush lever upon observing that the waste has been removed from the toilet bowl. 
         [0024]    The quantity of water discharged from the toilet tank in response to a press and immediate release activation of the flush lever depends on the extent of the upward travel of the float assembly from the lower position, in which the sealing member  136  in contact with the sealing member  116 , to the upper position, in which further upward movement is limited by the stop member  122 . The greater the distance, the longer is the time between breaking of the seal and restoration of the seal, and the greater is the quantity of water discharged. In the case of  FIG. 1A , the extent of the upward travel of the float assembly  128  is equal to the distance between the top of the overflow tube  132  and the stop member  122  when the float assembly is in its lower position. In order to limit upward travel of the float assembly, and avoid waste of water, an overflow tube extension piece  133  may be attached to the overflow tube  132 , as shown in  FIG. 1C . The overflow tube extension piece  133  may be attached to the overflow tube  132  by, for example, use of screw threads, a taper fit, a ridge and groove snap lock, a worm drive hose clamp, or other means. The overflow tube extension piece  133  reduces the upward travel of the float assembly in response to a press and immediate release activation of the flush lever and thereby reduces the quantity of water discharged from the toilet tank. 
         [0025]      FIG. 1D  illustrates further mechanisms for selectively limiting the extent of upward travel of the float assembly  128  in response to a press and immediate release activation of the flush lever.  FIG. 1D  shows that the upper end region  121  of the guide  120 , above the section having a cross-like configuration, is threaded over its entire length. A nut  123  is in threaded engagement with the upper end region  121  and can be positioned at any desired height above the upper end of the overflow tube  132 . Similarly to the overflow tube extension piece  133  shown in  FIG. 1C , the nut  123  reduces the upward travel of the float assembly in response to a press and immediate release activation of the flush lever and thereby reduces the quantity of water discharged from the toilet tank. Alternatively, as also shown in  FIG. 1D , the stop member  122 , which is the same as that shown in  FIG. 1A  and is in threaded engagement with the upper end of the guide  120 , may be provided with a collar or sleeve  124  that is attached to the stop member. When the flush valve is activated by pressing the flush lever, and the seal between the upper and lower seal members is broken, the float assembly immediately rises and the upper end of the float assembly engages the collar  124 , which is used instead of the nut  123 . In this case, the upward travel of the float assembly depends on the axial length of the collar  124 . 
         [0026]    Different models of toilet assemblies (toilet bowl and toilet tank) have different flushing characteristics. The different flushing characteristics of different models of toilet assemblies result in a greater or lesser quantity of water being needed to both remove liquid waste from the toilet bowl and restore its trapway water level. It has been found that by adjusting the height of the lower annular disc-wall relative to the upper annular disc it is possible to adjust the quantity of water that is discharged from the toilet tank in response to a press and immediate release activation of the flush lever. In particular, as the distance between the lower annular disc-wall and upper annular wall decreases, and correspondingly the height of the upper end of the overflow tube when the float assembly is in its sealing position, increases, the volume of water discharged from the toilet tank in response to a press and immediate release activation decreases. Accordingly, adjustability of the outer shell relative to the overflow tube, and corresponding adjustability of the volume of the float chamber, allows a given model of flush valve structure to be used with multiple models of toilet assemblies, each of which may require its own unique combination of flush volumes for satisfactory liquid and solid waste removal. This is useful because it allows a single model of flush valve structure to be used as a replacement part in multiple models of toilet assemblies. 
         [0027]    In the case of the flush valve structure shown in  FIGS. 1A and 1B , the central axis of the float assembly is offset horizontally from the central axis of the flange of the valve base by virtue of the skewed configuration of the flared extension  112 .  FIG. 2  illustrates a modification in which the flared extension  212  is not skewed but the central axis of the float assembly is nevertheless offset from the central axis of the flange of the valve base by virtue of a skewed lower extension  214 . Thus, in the case of  FIG. 2  the extension comprises a lower, skewed portion and an upper, flared portion. The structure of the float is the same in the case of  FIG. 2  as in  FIG. 1A . 
         [0028]      FIG. 3  illustrates a further modification of the flush valve structure shown in  FIG. 1A . As shown in  FIG. 3 , the extension of the valve base comprises a lower portion in the form of an S-shaped tube  770  and a flared upper portion  212 . The S-shaped tube allows the flush valve to be positioned near one of the side walls of the toilet tank. 
         [0029]      FIG. 4  illustrates a further modification of the flush valve structure. In this case, a valve base  514  provided with a seal  516  is fitted in a circular recess in the floor of the tank and the circular recess is surrounded by an annular plateau  518 . The seat  530  is provided at the top of the annular plateau, which has a frusto-conical interior surface tapering downward towards the entry to the outflow tube  526 . In the case of  FIG. 4 , therefore, the flared extension is built into the toilet tank. 
         [0030]      FIG. 5  illustrates a further modification of the flush valve structure shown in  FIG. 1A . The outlet hole  12  of the toilet tank is centered between the front and rear walls  14 ,  16  of the toilet tank and accordingly it is not necessary that the extension  512  be skewed. Thus, the circular openings at the top and bottom respectively of the extension  512  are coaxial. In addition, the outlet hole  12  is of greater diameter than the outlet hole in the case of  FIG. 1A . For example, in the case of  FIG. 5  the outlet hole may be 3.5 inches in diameter. 
         [0031]    In each of the illustrated embodiments of the claimed subject matter, the float assembly has a substantially greater cross-sectional area than the water outflow passage defined by the valve base. By employing a float assembly with a large cross-sectional area, the buoyancy of the float assembly is sufficient to ensure that immediately upon breaking the seal between the valve members, the float assembly will rise to the extent permitted by the stop member  122 . 
         [0032]    As noted previously, the stop member  122  is removable from the guide  120 . This allows the height of the stop member to be adjusted by attaching the stop member to the guide using one or more adapters, for example as disclosed in U.S. Pat. No. 6,637,042. Adjusting the height of the stop member is an alternative means for adjusting the upward travel of the float assembly when the seal between the valve members is broken. Another means of adjusting the upward travel of the float assembly when the seal between the valve members is broken may employ interchangeable overflow tubes of different lengths, or an adjustable mechanism similar to the lock nut  142  may be used for attaching the second annular disc-wall  146  to the overflow tube  132 , such that the overflow tube is adjustable in vertical position relative to the disc-wall  146 . Another possibility would be to provide a single overflow tube with multiple circumferential lines of weakness spaced apart therealong over an upper end region so that, if needed, one or more short segments of the overflow tube may be easily broken off to provide a shorter tube, having a longer upward travel. 
         [0033]    It will be appreciated that the invention is not restricted to the particular embodiment or embodiments that has or have been described, and that variations may be made therein without departing from the scope of the invention as defined in the appended claims, as interpreted in accordance with principles of prevailing law, including the doctrine of equivalents or any other principle that enlarges the enforceable scope of the claims beyond the literal scope. Unless the context indicates otherwise, a reference in a claim to the number of instances of an element, be it a reference to one instance or more than one instance, requires at least the stated number of instances of the element but is not intended to exclude from the scope of the claim a structure or method having more instances of that element than stated.

Summary:
A toilet tank flush valve structure includes a valve base for installation in the outlet hole formed in the floor of a toilet tank and defining a water outflow passage and including a first valve member at the upper end of the water outflow passage. A float assembly is movable relative to the valve base between a sealing position in which a second valve member at a lower end of the float assembly engages the first valve member in sealing relationship and a flushing position in which the second valve member is spaced upwardly from the first valve member. The float assembly includes an outer shell that is open downwards and has a lower edge at which the shell is provided with the second valve member, and a closure member within the outer shell and cooperating with the outer shell to define a float chamber. The closure member is adjustable in position relative to the outer shell, whereby the float chamber is adjustable in volume.