the method and apparatus of the present invention provides a fracture plug seat assembly used in well stimulation for engaging and creating a seal when a plug , such as a ball , is dropped into a wellbore and landed on the fracture plug seat assembly for isolating fracture zones in a well . the fracture plug seat assembly has a fracture plug seat that includes a setting ring , an expandable ring and a lower ring that are capable of locking when a ball that is too large to pass through the setting ring is landed on the fracture plug seat assembly . the setting ring and lower ring collectively form what may be termed an expansion control portion of the overall fracture plug seat assembly . when a ball or fracture plug that is small enough to pass through the setting ring contacts the expandable ring , the expandable ring expands to allow the ball to pass . when the ball designed to plug the seat is launched , it engages the setting ring and actuates the expandable ring into a retracted and locked position in which further expansion is prevented , hence supporting the ball . fig3 illustrates a cross - section of an embodiment of a fracture plug seat assembly 40 according to the present invention . as shown in fig3 , the fracture plug seat assembly 40 includes an expandable ring 42 having an axial opening , a setting ring 44 having an axial opening and a lower ring 46 having an axial opening . according to the embodiment shown in fig3 , the lower ring 46 is also capable of expanding when sufficient force is applied by the expandable ring 42 thereby allowing the expandable ring 42 to move to a locked position . in certain embodiments , the setting ring 44 is integrated with the sleeve 48 . in certain other embodiments , the setting ring 44 may be held axially in the initial position shown in fig3 by means such as shear pins to prevent expandable ring 42 from moving prematurely to a locked position until the ball designed to plug the fracture plug seat assembly 40 is landed on the setting ring 44 . the fracture plug seat assembly 40 shown in fig3 also contains a snap ring 50 which retains the assembly components , namely the expandable ring 42 , the setting ring 44 and the lower ring 46 , within the sleeve 48 . a belleville washer or coned - disc spring 52 keeps pressure on the stack of rings , via an annular spacer 53 bearing on the top side of the setting ring 44 , so that contact between the rings is maintained and so that sand and cement cannot penetrate between the rings . setting ring 44 has an o - ring seal 54 which prevents fluid from passing between the setting ring 44 and the sleeve 48 . expandable ring 42 has a split 58 and a spring 56 which biases the split 58 of the expandable ring 42 to a closed position as shown in fig3 . the expandable ring 42 and the lower ring 46 have respective mating tapered surfaces 60 and 61 which maintain the expandable ring 42 and the lower ring 46 in an axial relationship and initiates expansion of the lower ring 46 when pressure is applied by the expandable ring 42 . the lower ring 46 includes an o - ring 47 for centering purposes . fig4 illustrates the fracture plug seat assembly 40 with a ball 62 passing through the expandable ring 42 . the diameter of the ball 62 is smaller than the diameter of the axial opening of the setting ring 44 and therefore is not large enough to engage and land on the setting ring 44 . the diameter of the ball 62 is larger than the diameter of the axial opening of the expandable ring 42 and exerts sufficient force on the expandable ring to overcome the spring force of spring 56 causing the split 58 to open and allow the ball 62 to pass through the axial opening of the expandable ring 42 . fig5 is an axial view of the fracture plug seat assembly taken along line 5 - 5 of fig4 showing the expandable ring 42 with the spring 56 in tension and the split 58 in the open position . the ball 62 is pressed within the inner diameter of the expandable ring 42 . fig6 illustrates the fracture plug seat assembly 40 with a ball 64 which has been dropped in the direction 66 and is engaged with and landed on the setting ring 44 . significant pressure from the upstream side of the ball 64 forces the setting ring 44 downwardly against the expandable ring 42 . as the setting ring 44 is forced further downward toward the lower ring 46 , force builds on the tapered surface 60 of the expandable ring 42 and the tapered surface 61 of the lower ring 46 causing the lower ring 46 to expand . fig7 illustrates the fracture plug seat assembly 40 with a ball 64 which has been dropped in the direction 66 and is engaged with and landed on the setting ring 44 . pressure from the upstream side of the ball 64 has caused the lower ring 46 to expand to the point at which tapered surface 61 of the lower ring 46 is disengaged from the tapered surface 60 of the expandable ring 42 and the expandable ring 42 is in a concentric relationship with the lower ring 46 . continued pressure from the upstream side of the ball forces the expandable ring 42 downward with respect to the lower ring 46 . fig8 illustrates the fracture plug seat assembly 40 in the condition in which the expandable ring 42 has been forced downward with respect to the lower ring 46 until the tapered surface 60 of the expandable ring 42 engages shoulder 49 of the sleeve 48 . as shown in fig8 , the expandable ring 42 is in a retracted , locked position characterized by a concentric relationship with the lower ring 46 . the ball 64 is now supported by the setting ring 44 and the expandable ring 42 . many prior art fracture plug seat designs only support a ball such as ball 64 with the engagement diameter a . this is because it is the smallest diameter of such designs that is capable of letting the preceding smaller ball 62 pass through . the engagement diameter b which corresponds to the diameter of the axial opening of the expandable ring 42 when it is in the locked position greatly adds to the support of ball 64 helping prevent the cracking or extrusion of the ball 64 . when fracturing is complete , the balls are often purged to the surface . fig9 , 10 and 11 show the fracture plug seat assembly 40 with the larger ball 64 now purged up the well . in fig9 and 10 , the smaller ball 62 has engaged the expandable ring 42 and pressure in the direction 72 is applying an upward force upon the fracture plug seat assembly 40 . as shown in fig9 and 10 , the sleeve 48 includes a step 74 which prevents the lower ring 46 from moving upwards . thus , as pressure in the direction 72 continues , the expandable ring 42 moves upward with respect to the lower ring 46 and pushes the setting ring 44 ahead of the expandable ring 42 . when the expandable ring 42 and setting ring 44 are moved to their original position as shown in fig3 , the expandable ring 42 is allowed to expand and the ball 62 passes through , as shown in fig1 . tapered surface 76 on the annular spacer 53 prevents the setting ring 44 from moving upward any further and deflects any sand that might have accumulated during fracturing . another embodiment of the present invention is illustrated in fig1 and 13 . fig1 shows a fracture plug seat assembly 80 which includes an expandable ring 82 , a setting ring 84 and a lower ring 86 . according to this embodiment , the expandable ring 82 is a collet with only one end expanding , and with one or more axial slits extending up the length of the expandable ring 82 . a shear tab 88 prevents the expandable ring 82 from sliding down the assembly 80 . in fig1 , a ball 90 is shown passing through expandable ring 82 . as shown in fig1 , when a ball 92 designed to be landed by the fracture plug seat assembly 80 is dropped onto the seat assembly 80 , it engages the setting ring 84 and moves the expandable ring 82 into a nested relationship with the lower ring 86 . in some embodiments , the lower ring 86 is integrated with the sleeve 94 . yet another embodiment of the present invention is illustrated in fig1 in which the lower ring is integrated into the sleeve and in which a shear member is included , both as mentioned above . specifically , fig1 shows a fracture plug seat assembly 100 which includes an expandable ring 102 and a setting ring 104 . according to this embodiment , the expandable ring 102 rests upon a tapered shoulder 107 which is integrated into sleeve 108 . a shear tab 106 is provided on the expandable ring 102 and provides diametrical interference between the expandable ring 102 and the sleeve 108 . a ball 112 has been dropped in the direction 110 and is engaged with and landed on the setting ring 104 . significant pressure from the upstream side of the ball 112 forces the setting ring 104 downward and into the expandable ring 102 . as the setting ring 104 is forced further downward toward the expandable ring 102 , force builds on the expandable ring 102 causing the shear tab 106 to shear and allow the expandable ring 102 to clear the tapered shoulder 107 and move downward with respect to the sleeve 108 until the expandable ring 102 is engaged with the shoulder 114 which is integrated into sleeve 108 . when this occurs , the expandable ring 102 is in a locked position characterized by a concentric relationship with the lower ring sleeve 108 . in a manner similar to that described above with respect to fig9 , 10 and 11 , when fracturing is complete , the balls are often purged to the surface . when a ball smaller than ball 112 engages the expandable ring 102 , pressure in a direction opposite direction 110 applies an upward force upon the fracture plug seat assembly 100 . as pressure in the direction opposite direction 110 continues , the expandable ring 102 moves upward with respect to the sleeve 108 and pushes the setting ring 104 ahead of the expandable ring 102 . when the expandable ring 102 and setting ring 104 are moved to their original position as shown in fig1 , the expandable ring 102 is allowed to expand and the ball smaller than ball 62 passes through , similar to what is shown in fig1 . it is understood that variations may be made in the foregoing without departing from the scope of the disclosure . in several exemplary embodiments , the elements and teachings of the various illustrative exemplary embodiments may be combined in whole or in part in some or all of the illustrative exemplary embodiments . in addition , one or more of the elements and teachings of the various illustrative exemplary embodiments may be omitted , at least in part , and / or combined , at least in part , with one or more of the other elements and teachings of the various illustrative embodiments . any spatial references such as , for example , “ upper ,” “ lower ,” “ above ,” “ below ,” “ between ,” “ bottom ,” “ vertical ,” “ horizontal ,” “ angular ,” “ upwards ,” “ downwards ,” “ side - to - side ,” “ left - to - right ,” “ left ,” “ right ,” “ right - to - left ,” “ top - to - bottom ,” “ bottom - to - top ,” “ top ,” “ bottom ,” “ bottom - up ,” “ top - down ,” etc ., are for the purpose of illustration only and do not limit the specific orientation or location of the structure described above . in several exemplary embodiments , while different steps , processes , and procedures are described as appearing as distinct acts , one or more of the steps , one or more of the processes , and / or one or more of the procedures may also be performed in different orders , simultaneously and / or sequentially . in several exemplary embodiments , the steps , processes and / or procedures may be merged into one or more steps , processes and / or procedures . in several exemplary embodiments , one or more of the operational steps in each embodiment may be omitted . moreover , in some instances , some features of the present disclosure may be employed without a corresponding use of the other features . moreover , one or more of the above - described embodiments and / or variations may be combined in whole or in part with any one or more of the other above - described embodiments and / or variations . although several exemplary embodiments have been described in detail above , the embodiments described are exemplary only and are not limiting , and those skilled in the art will readily appreciate that many other modifications , changes and / or substitutions are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of the present disclosure . accordingly , all such modifications , changes and / or substitutions are intended to be included within the scope of this disclosure as defined in the following claims . in the claims , any means - plus - function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents , but also equivalent structures .