Patent Publication Number: US-8523145-B2

Title: Jack assembly with integrated pressure relief assembly

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     This application claims priority to U.S. provisional application No. 61/166,080 filed Apr. 2, 2009, which is hereby incorporated by reference as if fully set forth herein. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to a jack assembly, and more particularly to a jack assembly having an integrated pressure relief assembly. 
     Various types of jack assemblies are used in a wide array of settings; however, in most circumstances, the jack assemblies are configured to perform a generally similar function of providing an axial force. For example, many automotive-style jack assemblies are arranged between the ground and a vehicle to be lifted. Actuating the jack assembly results in the vehicle being raised relative to the ground, and de-actuating the jack assembly results in the vehicle being lowered. 
     Mechanical and material limitations of jack assemblies require that each jack assembly has a maximum load rating, that is, a particular jack assembly is designed and configured to repeatedly provide a certain amount of axial force. Exceeding the maximum load rating of a jack assembly may result in damaging the jack assembly (e.g., degrading internal seals, deforming portions of the jack assembly, and the like). 
     As one example, column-type portable vehicle lifts incorporate a caster jack assembly that urges a caster wheel into engagement with the ground to aid in transporting the lift. However, the maximum load rating of the caster jack assembly is significantly lower than the operating load of the vehicle lift. As a result, if the caster jack assembly is inadvertently left in the extended or engaged position while the vehicle lift is used to support a vehicle, the load transferred to the caster jack assembly may result in damage to the caster jack assembly. Other scenarios commonly arise in which the load placed on the jack assembly exceeds the maximum load rating of the jack assembly, therefore degrading or damaging the components of the jack assembly. 
     To address this potential issue, some jack assemblies incorporate an external pressure relief valve on a hydraulic cylinder that is used to provide the pressurized fluid to extend the jack assembly. If the pressure in the jack assembly exceeds a certain level, the pressure relief valve allows fluid back into the hydraulic cylinder, thus retracting the jack assembly and minimizing potential damage. However, this external tank-valve arrangement is cumbersome and complex. 
     In light of at least the above considerations, a need exists for an improved jack assembly having an integrated pressure relief assembly. 
     SUMMARY OF THE INVENTION 
     In one aspect, a jack assembly comprises a cylinder defining an interior, a piston having a piston head that is slideably engaged with the interior of the cylinder, an extension chamber defined between the piston head and the interior of the cylinder, and a pressure relief assembly integrated proximate the piston head and defining a relief passageway in selective fluid communication with the extension chamber. When a fluid pressure in the extension chamber exceeds a certain level, the pressure relief assembly allows fluid to flow through the relief passageway. 
     In another aspect, a jack assembly comprises a cylinder defining an interior, a piston having a piston head that is slideably engaged with the interior of the cylinder, an extension chamber defined between the piston head and the interior of the cylinder, and a pressure relief assembly integrated proximate the piston head and defining a relief passageway in fluid communication with the extension chamber. The pressure relief assembly is moveable between a closed position, at which fluid is inhibited from flowing along the relief passageway, and an opened position, at which fluid is permitted to flow along the relief passageway. 
     In yet another aspect, a pressure relief assembly comprises a valve body, a relief passageway defined within the valve body, a valve seat formed along the relief passageway, a plug configured to selectively engage the valve seat, an adjustment member moveable along the relief passageway, and a biasing member captured between the plug and the adjustment member to urge the plug toward the valve seat. 
     In another aspect, a jack assembly comprises a cylinder defining an interior, a piston having a piston head including a seal that is slideably engaged with the interior of the cylinder, an extension chamber defined between the piston head and the interior of the cylinder, a valve body coupled to the piston head, a relief passageway defined within the valve body and having an entry port in fluid communication with the extension chamber and an exit port spaced apart from the entry port beyond the seal, a valve seat formed along the relief passageway, a plug configured to selectively engage the valve seat, an adjustment member moveable along the relief passageway, and a biasing member captured between the plug and the adjustment member to urge the plug toward the valve seat. When a fluid pressure in the extension chamber exceeds a certain level, the plug disengages the valve seat such that fluid may flow from the extension chamber, in the entry port, along the relief passageway, and out the exit port. 
     These and still other aspects of the invention will be apparent from the description that follows. In the detailed description, preferred example embodiments of the invention will be described with reference to the accompanying drawings. These embodiments do not represent the full scope of the invention; rather, the invention may be employed in other embodiments. Reference should therefore be made to the claims for interpreting the breadth of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partial isometric view of a column-type portable vehicle lift that incorporates a caster jack assembly in accordance with an aspect of the invention; 
         FIG. 2  is an isometric view of an example jack assembly incorporating an aspect of the invention; 
         FIG. 3  is a side view of the example jack assembly shown in the retracted position; 
         FIG. 4  is a side view of the example jack assembly shown in the extended position; 
         FIG. 5  is a partial side view of the example jack assembly along line  5 - 5  of  FIG. 3 ; 
         FIG. 6  is a partial top view of the example jack assembly shown with the handle assembly removed; 
         FIG. 7  is a partial cross-section view along line  7 - 7  of  FIG. 6  showing the example jack assembly in the retracted position; 
         FIG. 8  is a partial cross-section view along line  7 - 7  of  FIG. 6 ; 
         FIG. 9  is an isometric view of an example pressure relief assembly incorporating an aspect of the invention; 
         FIG. 10  is a side view of the example pressure relief assembly; 
         FIG. 11  is a section view along line  12 - 12  of  FIG. 11  showing the example pressure relief assembly in the closed position; 
         FIG. 12  is a partial cross-section view similar to  FIG. 8  showing the example jack assembly in the extended position and the example pressure relief assembly in the closed position; 
         FIG. 13  is a section view similar to  FIG. 12  showing the example pressure relief assembly in the opened position; 
         FIG. 14  is a partial cross-section view similar to  FIG. 7  showing another example jack assembly in the retracted position; 
         FIG. 15  is a partial cross-section view of  FIG. 14 ; 
         FIG. 16  is a partial cut-out view of a portion of  FIG. 14 ; and 
         FIG. 17  is a section view showing another example pressure relief assembly in the closed position. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EXAMPLE EMBODIMENT 
     An example embodiment of the invention will be described in relation to a caster jack assembly of a column-type vehicle lift. However, the present invention is equally applicable to other types and styles of jack assemblies such as lifts, actuators, struts, shocks, dampers, and the like. In addition, while the example embodiment incorporates hydraulic fluid, any other type of fluid may be used depending upon the ultimate application requirements. Furthermore, given the benefit of this disclosure, one skilled in the art will appreciate the various alternative constructions and material compositions that are within the scope of the invention. 
     A simplified example of a column-type portable vehicle lift  10  is shown in  FIG. 1  having an example jack assembly  12  incorporating an aspect of the invention. As shown, the vehicle lift  10  includes center support column  14  and a pair of feet  16  extending from the base  18  of the center support column  14 . A pair of forks  20  is operatively coupled to the center support column  14  and can be hydraulically actuated to various positions along the center support column  14 , thereby selectively raising and lowering a vehicle (not shown) that is carried by the pair of forks  20 . Multiple vehicle lifts  10  can be positioned, for example, at the four wheels of a passenger car, to raise and/or lower the entire vehicle. Control of the vehicle lift  10  may be accomplished mechanically, electromechanically, or by any other technique known to those skilled in the art. 
     As the name implies, the column-type portable vehicle lift  10  is configured to be portable, such as within a vehicle repair shop. To aid the portability of the vehicle lift  10 , spring-biased wheels  22  are integrated into the feet  16  and a caster wheel  24  is carried by the jack assembly  12 . The jack assembly  12  is configured such that when the jack assembly  12  is extended, the caster wheel  24  is urged into engagement with the ground. The caster wheel  24  is intended to engage the ground when the vehicle lift  10  is not carrying or lifting a vehicle, allowing the vehicle lift  10  to be moved about. 
     With additional reference to  FIGS. 2-5 , an embodiment of the jack assembly  12  incorporating an aspect of the invention is shown in greater detail (with the caster wheel  24  and associated components removed). One skilled in the art will appreciate that the jack assembly  12  and components thereof are comprised of application-specific materials. For example, high strength steel may be used for structural components in applications subject to substantial loads (e.g., lifting a vehicle, such as a dump truck). Other components (e.g., seals, handles, springs, etc.) may be comprised of traditional materials (e.g., rubber, plastic, metal, composites, etc.), as will be appreciated by one skilled in the art. 
     The jack assembly  12  generally includes a cylinder portion  26 , a piston portion  28 , and an actuator assembly  30 . In the example shown, the actuator assembly  30  comprises a manual pump assembly  32 ; however, as one skilled in the art will appreciate given the benefit of this disclosure, the actuator assembly  30  may be any number of constructions and devices capable of actuating the jack assembly  12 . For example, the actuator assembly  30  may comprise a two-way valve in fluid communication with a pressurized fluid supply such that the valve can be manually or automatically operated to extend and/or retract the jack assembly  12  by directing the pressurized fluid accordingly. In the example embodiment, the pressurized fluid comprises hydraulic fluid; however, the pressurized fluid may be water, gas, or any other suitable fluid given the specifics of the particular application. 
     Returning to the example embodiment, the manual pump assembly  32  can be operated to either cause the jack assembly  12  to extend or to allow the jack assembly  12  to retract under load. Specifically, urging the handle  34  from the generally raised position shown in  FIG. 3  to the lowered position shown in  FIG. 4  extends the piston portion  28 . Conversely, urging and maintaining the handle  34  into the extreme position shown in  FIG. 3  allows the piston portion  28  to retract in response to an axial, compressive force on the jack assembly  12 . The handle  34  pivots about a pivot pin  36  that rotatably captures the handle  34  to a mounting arm  38  that extends outwardly from the cylinder portion  26 . A pump pin  40  extends through the handle  34  and a slotted hole  41  (best shown in  FIGS. 7 and 8 ) of a plunger  42  to operationally couple the handle  34  to the plunger  42 , such that movement of the handle  34  about the pivot pin  36  results in generally axial movement of the plunger  42 . 
     Turning to  FIGS. 6-9 , a cross-section of a portion of the jack assembly  12  illustrates the internal operation of the example jack assembly  12 . The cylinder portion  26  includes a pump body  44  that defines a pump chamber  46 . The pump chamber  46  includes a pump valve seat  48  against which a pump valve ball  50  is biased by a pump spring  52  in a lower portion  54  of the pump chamber  46 . An intermediate portion  56  of the pump chamber  46  is sized to slideably engage the plunger  42  during operation of the manual pump assembly  32 . An upper portion  58  of the pump chamber  46  defines a seat  60  into which a plunger collar  62  is secured. The plunger collar  62  captures a lower o-ring  64  proximate the intermediate portion  56  of the pump chamber  46 . In the example embodiment, the plunger collar  62  is threadably engaged with the upper portion  58  of the pump chamber  46 , thereby aiding assembly, disassembly, and repair. An annular groove  66  is formed in an inner wall  68  of a stepped head portion  70  of the plunger collar  62 . An upper o-ring  72  is seated in the annular groove  66  and engages the shaft  74  of the plunger  42  during operation of the manual pump assembly  32 . 
     A release valve assembly  76  is operationally coupled to the pump body  44 . Specifically, the pump body  44  defines a release chamber  78  that includes an extend valve seat  80  against which an extend valve ball  82  is biased by an extend spring  84  in an inner portion  86  of the release chamber  78 . A second end  88  of the extend spring  84  captures a ball guide  90  that rides along an interior passageway  92  of an insert  94 . The insert  94  is secured in an intermediate portion  96  of the release chamber  78  proximate the inner portion  86 . An o-ring  98  is seated in an external annular groove  100  formed in the insert  94 . A release ball  102  is biased against a release valve seat  104  formed at one end  105  of the interior passageway  92  of the insert  94 . Specifically, the release ball  102  is urged toward the release valve seat  104  by the ball guide  90  that is in turn urged by the extend spring  84 . 
     A release plunger  106  is slideably captured by a release plunger collar  108  to the pump body  44  such that the release plunger  106  is moveable generally axially within the intermediate portion  96  of the release chamber  78 . In the example embodiment, the release plunger collar  108  is threadably engaged with the release chamber  78 . An o-ring  110  is captured by a head portion  112  of the release plunger collar  108  proximate an exterior end  114  of the release chamber  78 . A sleeve portion  116  of the release plunger collar  108  extends into the release chamber  78  and engages an end face  118  of the insert  94  thereby capturing the insert  94  within the release chamber  78 . 
     The release plunger  106  includes an actuation end  120  extending through an opening  122  in the release plunger collar  108  that is selectively engaged by a portion of the handle  34  during operation to allow the jack assembly  12  to retract. A tip  124  is formed on the opposite end  126  and is configured to selectively extend through an opening  128  formed through the insert  94  where it extends into the interior passageway  92  to selectively unseat the release ball  102  from the release valve seat  104 . The release plunger  106  is biased away from the release ball  102  by a spring  130  seated between an end face  132  of the insert  94  and an end face  134  of the release plunger  106 . Specifically, the spring  130  surrounds an inner portion  136  of the release plunger  106 . 
     The cylinder portion  26  also includes a cylinder  138  coupled (e.g., threadably fastened) to the pump body  44  at a first end  140 . The cylinder  138  defines an interior  142  within which the piston portion  28  slideably engages. The piston portion  28  includes a piston  144  defining an exterior surface  146 . The example piston  144  includes a head end  148  to which is coupled a piston head  150  and a support end  152  to which is coupled, for example, the caster wheel  24 . 
     The piston head  150  is sized to slideably fit within the cylinder  138  and includes an annular recess  154  formed in the annular surface  156 . An o-ring  158  is seated in the annular recess  154  and a glide ring  160  is seated within the annular recess  154  radially outward from the o-ring  158  such that the glide ring  160  engages and seals against the interior  142  of the cylinder  138 . 
     The piston head  150  of the example embodiment is captured to the head end  148  by a pressure relief assembly  162 . More specifically, a threaded valve body  164  engages mating threads  166  formed in a post  168  proximate the head end  148 . As one skilled in the art will appreciate, the piston head  150  may alternatively be formed integral with the balance of the piston  144 , be welded to the balance of the piston  144 , and the like. To seal between the piston head  150  and the post  168 , an exterior surface  170  of the post  168  engages an o-ring  172  seated in an annular groove  174  formed along an inner surface  176  of the piston head  150 . 
     The piston  144  extends through an opening  178  formed through an end cap  180  that is secured (e.g., threadably engaged) to a second end  182  of the cylinder  138 . In the example embodiment, the end cap  180  includes internal threads  184  that engage external threads  186  formed proximate the second end  182  of the cylinder  138 . The exterior surface  146  of the piston  144  engages a quad ring  188  seated in an interior annular groove  190  formed in the end cap  180  and a wiper member  192  seated in another interior annular groove  194  formed in the end cap  180 . A reservoir jacket  196  encases the cylinder  138  and is captured between an end face  198  of the pump body  44  and an annular flange  200  formed on the end cap  180 . Specifically, the reservoir jacket  196  fits over a protrusion  202  extending from the pump body  44  and seals against an o-ring  204  seated in an annular groove  206  formed in an exterior surface  208  of the protrusion  202 . Additionally, the end cap  180  includes a collar  210  over which the reservoir jacket  196  extends and seals against another o-ring  212  seated in an annular groove  214  formed in an exterior surface  216  of the collar  210 . In the example embodiment, a mounting flange  218  is coupled (e.g., welded) to the reservoir jacket  196  to allow the jack assembly  12  to be incorporated into another device. The reservoir jacket  196  also defines a reservoir  220  between the cylinder  138  and an interior wall  222  of the reservoir jacket  196 . The reservoir  220  is capable of containing the fluid (e.g., hydraulic fluid) used to operate the jack assembly  12 . 
     With specific reference to  FIGS. 9-11 , an example one-way valve assembly in the form of a pressure relief assembly  162  is described below in greater detail. However, given the benefit of this disclosure, one skilled in the art will appreciate that any type of one-way valve assembly may be coupled and integrated into the piston head  150 . 
     The example pressure relief assembly  162  includes a valve body  224  and a relief passageway  226  defined within the valve body  224 . A valve seat  228  is formed along the relief passageway  226  and selectively engages a plug  230 , shown in the example embodiment as a ball  232  seated in a cradle  234  of an alignment pin  236 . The cradle  234  of the alignment pin  236  selectively engages the ball  232  to align the ball  232  with the valve seat  228  when in the closed position shown in  FIGS. 10-12 . An adjustment member  238  is moveable along the relief passageway  226  and, in the example embodiment, captures a biasing member  240  (e.g., a spring) between the plug  230  and the adjustment member  238 . Specifically, an axial face  242  of the alignment pin  236  is configured to engage a first end  244  of the biasing member  240  and an axial face  246  of the adjustment member  238  is configured to engage a second end  248  of the biasing member  240 . As a result, the biasing member  240  urges the plug  230  toward the valve seat  228 . 
     Internal threads  250  formed along at least a portion of the relief passageway  226  are configured to engage mating external threads  252  defined by an exterior surface  254  of the adjustment member  238 . As a result, the adjustment member  238  is moveable along a longitudinal axis of the valve body  224 . In the example embodiment, the adjustment member  238  defines a hexagonal opening  256  into which a mating tool (e.g., a hex key) may be inserted to adjust the position of the adjustment member  238  and ultimately alter the biasing force provided by the biasing member  240  urging the plug  230  toward the valve seat  228 . 
     Returning briefly to  FIG. 8 , the pressure relief assembly  162  of the example embodiment is shown threadably engaged with a threaded opening  258  formed in the piston head  150  along a longitudinal axis of the piston portion  28 . Specifically, the valve body  224  includes engagement threads  260  formed in an external surface  262 . An o-ring  264  is seated in an annular groove  266  formed proximate the interface between a head portion  268  and a shaft portion  270  of the valve body  224 . In the example embodiment, the valve body  224  is generally in the form of a bolt that has been modified to define the relief passageway  226 , valve seat  228 , and internal threads  250 . One skilled in the art, given the benefit of this disclosure, will appreciate that the valve body  224  may be integral with the piston head  150  or formed of multiple components. 
     In the example embodiment, the relief passageway  226  is formed generally along a longitudinal axis of the valve body  224  from an entry port  286 , through the adjustment member  238 , and to an exit port  288 . However, the relief passageway  226  need not extend the entire length of the valve body  224  nor through the adjustment member  238 . For example, the relief passageway  226  may extend at least partially radially along the valve body  224  such that the exit port  288  is formed along the valve body  224 . In addition, the relief passageway  226  may be offset radially from the longitudinal axis of the valve body  224 , may be skewed relative to the longitudinal axis of the valve body  224 , may include multiple entry ports  286 , and may include multiple exit ports  288 . The size and contour of the relief passageway  226  need not be generally uniform as illustrated in  FIG. 11  and may comprise a variety of contours to further influence the flow of fluid along the relief passageway  226 . 
     Turning to  FIGS. 8 ,  12 , and  13 , the operation of the example jack assembly  12  and the example pressure relief assembly  162  are illustrated and described. In operation, the jack assembly  12  is urged toward the extended position by pumping the manual pump assembly  32 . Specifically, as one skilled in the art will appreciate, axial movement of the plunger  42  draws fluid (e.g., hydraulic fluid) through a pickup tube  272  located in the reservoir  220  and past the pump valve ball  50 . Returning the plunger  42  to the position shown in  FIG. 8  seats the pump valve ball  50  against the pump valve seat  48  and urges the fluid past the extend valve ball  82  and into an extension chamber  274  defined between the piston head  150  and the interior  142  of the cylinder  138 . 
     Urging the handle  34  into engagement with the release valve assembly  76  allows pressurized fluid to flow from the extension chamber  274 . Specifically, axially moving the release plunger  106  results in the tip  124  unseating the release ball  102  from the release valve seat  104  such that fluid flows from the extension chamber  274 , through a release port  282  and through a release passageway  284  into the reservoir  220 . 
     One skilled in the art will appreciate the variety of structures and configurations available to extend and retract the jack assembly  12 . Therefore, the example jack assembly  12  construction described is not limiting. 
     In the example embodiment, the pressure relief assembly  162  is substantially integrated proximate the piston head  150  such that the relief passageway  226  provides selective fluid communication between the extension chamber  274  and the reservoir  220  (i.e., a generally lower pressure volume). When a fluid pressure in the extension chamber  274  exceeds a certain level (i.e., any acceptable level for the particular application), the pressure relief assembly  162  allows fluid to flow out of the extension chamber  274  and through the relief passageway  226 . Specifically, in the example embodiment, the plug  230  allows the fluid to flow past toward a relief cavity  276  defined between the interior  142  of the cylinder  138  and the piston  144 . That is, the relief cavity  276  is in fluid communication with the relief passageway  226 . 
     In the example embodiment, the relief passageway  226  includes the entry port  286  proximate the piston head  150  and the exit port  288  positioned beyond the seal (e.g., the glide ring  160  and the o-ring  158 ) such that the relief passageway  226  selectively permits fluid to flow from the extension chamber  274  and along the relief passageway  226 , thereby reducing excessive pressure within the extension chamber  274 . The example relief passageway  226  includes a radial portion  227  formed in the piston  144  and extending toward the relief cavity  276 . One skilled in the art will appreciate, given the benefit of this disclosures, that the exit port  288  may include a variety of configurations and orientations. 
     Two relief ports  278  are formed through the cylinder  138  to allow fluid communication between the relief cavity  276  and the reservoir  220 . In the example embodiment, two relief ports  278  are shown and each has a stepped construction with a smaller opening  280  proximate the relief cavity  276 . Any number, location, and contour of relief ports  278  may be incorporated depending upon the application requirements. 
     The pressure required to alter the state of the pressure relief assembly  162  from closed to opened may be adjusted. When the fluid pressure exceeds the certain level, as illustrated in the example embodiment, the ball  232  is unseated from the valve seat  228  and the alignment pin  236  compresses the biasing member  240  against the adjustment member  238 . Conversely, when the fluid pressure drops below the certain level, the biasing member urges the plug  230  from the opened to the closed position, thereby allowing fluid pressure to increase within the extension chamber  274 . Adjusting the location of the adjustment member  238  allows a spring force of the biasing member to be altered, hence increasing or decreasing the fluid pressure in the extension chamber  274  required to open the pressure relief assembly  162  and permit the flow of fluid along the relief passageway  226 . 
     As one skilled in the art will appreciate, given the benefit of this disclosure, if the example jack assembly  12  is extended when the vehicle lift  10  is attempting to lift and/or carry a vehicle, the pressure relief assembly  162  may be configured such that an excessive pressure within the extension chamber  274  will not occur. As one result, damage to the jack assembly  12  may be substantially prevented. 
     Another example embodiment of a jack assembly  512  incorporating an aspect of the invention is shown generally in  FIGS. 14-17 . Again, the jack assembly  512  generally includes a cylinder portion  526 , a piston portion  528 , and an actuator assembly  530 . In a preferred form, an end cap  580  secured to a second end  582  of a cylinder  538  is elongated, as compared to the end cap  180  shown best in  FIG. 8 , and is generally cylindrical in shape. This end cap  580  provides additional bearing surface for the piston portion  528  as it moves axially, enhancing side-loading (e.g., radial loading) capabilities. A bearing sleeve  600  is also included to aid operation of the piston portion  528 . One skilled in the art will appreciate the various application-specific modifications that may be made to provide an end cap  580  suited for a particular application. 
     In another preferred form shown in  FIGS. 14-16 , a pickup tube  572  is secured at both ends by barbed couplers  602  to a pump body  544  at one end and a screen  604  at the opposite end. The barbed couplers  602  are preferably made of plastic and provide a sufficient seal to enhance proper flow of fluid through the pickup tube  572 . As best shown in  FIG. 16 , the pickup tube  572  is preferably secured relative to the cylinder  538  by a cable tie  606 . The cable tie  606  inhibits the pickup tube  572  from “curling” or otherwise being displaced from a location allowing fluid to flow through the pickup tube  572  while in use. For instance, when used in a vertical orientation (such as shown in  FIG. 1 ), the screen  604  of the pickup tube  572  is preferably located sufficiently below a fill line  608  such that the screen  604  will remain within the fluid during normal operation. 
     Turning briefly to  FIG. 17 , an alternative example pressure relief assembly  662  is illustrated. The notable distinction between the example described with reference to  FIGS. 9-11  is the alteration of a biasing member  638 , that is, the example spring shown in  FIG. 17  includes more turns of a higher gauge wire. 
     Given the benefit of this disclosure, one skilled in the art will appreciate that various one-way valve assemblies may be integrated with the piston to provide an integrated pressure relief assembly, each of which is within the scope of the present invention. 
     While there has been shown and described what is at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made, given the benefit of this disclosure, without departing from the scope of the invention defined by the following claims.