Patent Publication Number: US-2010111721-A1

Title: Dual piston pump assembly with anti-rotation guide rails

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119(e) from co-pending U.S. Provisional Patent Application No. 61/194,299, filed Sep. 25, 2008, and further claims priority from co-pending U.S. Provisional Patent Application No. 61/100,225, filed Sep. 25, 2008, both entitled “DUAL PISTON PUMP ASSEMBLY WITH ANTI-ROTATION GUIDE RAILS”, and naming Servin et al. as the inventors, and both of which are incorporated herein by reference in their entirety and for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to dual pump assemblies, and more particularly, relates to dual pump assemblies that accurately deliver fluids at different delivery pressures. 
     BACKGROUND OF THE INVENTION 
     Syringe style pumps commonly consist of a single motor, a syringe barrel and a piston configuration. Typically, these pumps are of the single channel type for pumping a single liquid only. When multiple liquids are required for dispensing, however, such as two or more, additional equipment is required, which entails substantial pump costs. 
     Additionally, the need may arise for pumping liquids at different pressures due to dissimilar downstream restrictions and flow requirements. In these situations, the common solution is to use distinctly separate pumps which also increases costs. 
     Accordingly, there is a need to provide a multi-channel pump device that is capable of pumping two liquids simultaneously at disparate pressures up to 200 psi. There is also a desire to reduce cost through minimizing parts, as well as manufacturing such parts from cost effective techniques such as injection molding, while maintaining structural strength to withstand bending moments resulting from high and different pressures. 
     SUMMARY OF THE INVENTION 
     The present invention provides a dual piston-pump apparatus including a pump chassis assembly having a pair of spaced-apart, elongated piston bores, and a lead screw shaft. This shaft includes a motor driven end, and another portion thereof rotatably mounted to the chassis assembly for rotation about a screw rotational axis. A piston drive member threadably cooperates with the lead screw shaft for selective reciprocating movement longitudinally along the screw rotational axis thereof between a first position and a second position. The drive member includes a pair of spaced-apart piston shafts, each piston shaft of which includes a respective piston head portion slideably received in a respective piston bore of the chassis assembly. As the drive member is driven along the lead screw shaft between the first position and the second position, each piston head is simultaneously reciprocated between a dispensing condition and an aspiration condition, respectively. In accordance with the present invention, an anti-rotation device is cooperatively positioned between the pump chassis assembly and the drive member. This anti-rotation device substantially prevents rotational displacement of the drive member relative to the pump chassis assembly, about the screw rotational axis, during rotational motion of the lead screw shaft. Accordingly, any rotation of the drive member about the shaft rotational axis, resulting from the initial rotation of the lead screw shaft  23  (e.g., from the torque), will be counteracted. 
     In one specific embodiment, the anti-rotation device includes a rail member coupled to the chassis assembly. The rail member includes an elongated contact surface formed and dimensioned for relative sliding contact against a sliding support surface at the bottom of the drive member. The rail member includes an upper rail portion upstanding from a floor portion of the chassis assembly. The upper rail portion includes opposed sidewalls that collectively converge to form an apex portion, defining the elongated contact surface. 
     In yet another specific configuration, the contact surface of the rail member is substantially linear, and is directionally aligned along the chassis floor portion substantially parallel to the rotational axis of the lead screw shaft. The rail member includes a lower rail portion formed and dimensioned to removably mount to the chassis floor portion 
     Another specific embodiment provides a sliding support surface of the drive member that defines an alignment groove configured to slideably receive the upper rail portion therein during the relative reciprocating movement between the first position and the second position. 
     In still another specific embodiment, the chassis assembly includes a first piston barrel defining one piston bore extending therethrough, and a second piston barrel defining the other piston barrel extending therethrough. The first piston barrel is removably disposed on one side of the screw shaft, while the second piston barrel is removably disposed on the opposite side of the screw shaft. Each piston barrel defines a respective distal opening into a respective piston bore, and each the distal opening is sized and dimensioned for reciprocating receipt of the respective piston shaft therein. 
     The drive member further includes a central barrel portion positioned between and adjacent to the pair of piston shafts. The central barrel portion defines a central passage extending longitudinally therethrough which is formed and dimensioned for relative reciprocating receipt of the lead screw shaft therein. A threaded insert is fixedly disposed in the central passage of the central barrel in a manner threadably cooperating with a threaded portion of the lead screw shaft. Accordingly, as the screw shaft is selectively rotated, the threaded insert is moved along the threaded portion, moving the drive member between the first position and the second position. 
     In this embodiment, the anti-rotation device includes a pair of spaced-apart rail members each coupled to the chassis assembly, and disposed on opposite sides of lead screw shaft. Each rail member includes a respective elongated contact surface formed and dimensioned for relative sliding contact against the respective sliding support surface of the drive member during reciprocating movement thereof between the first position and the second position. 
     In another aspect of the present invention, a dual piston-pump apparatus is provided including a pump chassis assembly having a pair of spaced-apart piston barrels, each of which defines a respective piston bore. A lead screw shaft is positioned between the piston barrels, and includes a motor driven end, and mount portion thereof rotatably mounted to the chassis assembly for rotation about a screw rotational axis. A piston drive member is included having a central barrel portion threadably cooperating with the lead screw shaft for selective reciprocating movement longitudinally along the screw rotational axis thereof between a first position and a second position. The drive member includes a pair of spaced-apart piston shafts on opposed sides of the central barrel portion. Each piston shaft includes a respective piston head portion slideably received in a respective piston bore of the chassis assembly between a dispensing condition and an aspiration condition as the drive member is driven along the lead screw shaft between the first position and the second position, respectively. A first sleeve bearing is disposed between the chassis assembly and the lead screw shaft for rotating support thereof, and a second sleeve bearing is disposed between the central barrel portion and the lead screw. This second sleeve bearing not only provides rotational support to the lead screw shaft, but also provides sliding support to the drive member during movement thereof between the first position and the second position. 
     In one specific embodiment, a threaded insert fixedly is disposed in the central passage of the central barrel in a manner threadably cooperating with a threaded portion of the lead screw. The threaded insert is disposed at a proximal portion of the central passage of the drive member, while the second sleeve bearing is disposed at a distal portion of the central passage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The assembly of the present invention has other objects and features of advantage which will be more readily apparent from the following description of the best mode of carrying out the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  is a series of schematic views of a dual pump assembly in operation between an aspiration condition and a dispensing condition. 
         FIG. 2  is a top perspective view of a dual pump assembly constructed in accordance with the present invention, illustrated in a first position. 
         FIG. 3  is a top perspective view of the dual pump assembly of  FIG. 2 , illustrated in a second position. 
         FIG. 4  is another top perspective view of the dual pump assembly of  FIG. 2 , illustrated in the first position. 
         FIG. 5  is a top perspective view of the dual pump assembly of  FIG. 2  with one piston barrel removed, and illustrated in the second position. 
         FIG. 6  is another top perspective view of the dual pump assembly of  FIG. 2  with a piston drive member and a piston barrel removed. 
         FIG. 7  is a front perspective view of the dual pump assembly of  FIG. 2  with the piston drive member removed. 
         FIG. 8  is a fragmentary, enlarged, front perspective view showing the rail member of the anti-rotation device of the dual pump assembly of  FIG. 2 . 
         FIG. 9  is a top perspective view of a chassis apparatus of the dual pump assembly of  FIG. 2 . 
         FIG. 10  is an enlarged top perspective view of a piston barrel of the dual pump assembly of  FIG. 2 . 
         FIG. 11  is an enlarged top perspective view of a piston drive member of the dual pump assembly of  FIG. 2 . 
         FIG. 12  is an enlarged bottom perspective view of a rail member of the dual pump assembly of  FIG. 2 . 
         FIG. 13  is a bottom perspective view of the dual pump assembly of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention will be described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications to the present invention can be made to the preferred embodiments by those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims. It will be noted here that for a better understanding, like components are designated by like reference numerals throughout the various figures. 
     Referring now to  FIGS. 1A-7 , a dual piston-pump apparatus, generally designated  20 , is provided including a pump chassis assembly  21  having a pair of spaced-apart, elongated piston bores  22 ,  22 ′, and a lead screw shaft  23  having a motor driven proximal end  25 . Another portion of the lead screw shaft  23  is rotatably mounted to the chassis assembly  21  for rotation thereof about a screw rotational axis  26 . The dual piston-pump apparatus  20  further includes a piston drive member  27  threadably cooperating with the lead screw shaft  23  for selective reciprocating movement longitudinally along the screw rotational axis  26  between a first position ( FIGS. 1A ,  2  and  4 ) and a second position ( FIGS. 1B ,  3  and  5 ). The piston drive member  27  includes a pair of spaced-apart piston shafts  28 ,  28 ′, each having a respective piston head  24 ,  24 ′ slideably received in the respective piston bore  22 ,  22 ′ of the chassis assembly  21 . Collectively, variable volume piston chambers  29 ,  29 ′ are formed between the piston bores  22 ,  22 ′ and the respective piston heads  24 ,  24 ′. As the drive member  27  is driven along the lead screw shaft  23  between the first position and the second position, the piston drive member is moved between a dispensing condition ( FIGS. 1A ,  2  and  4 ) and an aspiration condition ( FIGS. 1B ,  3  and  5 ), respectively. In the aspiration condition, the piston chambers  29 ,  29 ′ aspirate liquids therein as the piston drive member  27  is moved from the first position to the second position, expanding the volume of the piston chambers. In contrast, in the dispensing condition, the piston chambers  29 ,  29 ′ dispense liquids therefrom as the piston drive member  27  is moved from the second position to the first position, contracting the volume of the piston chambers. 
     In accordance with the present invention, the dual piston-pump apparatus  20  includes an anti-rotation device  30  cooperatively disposed between the pump chassis assembly  21  and the drive member  27 . Accordingly, during rotational motion of the lead screw shaft  23 , the piston drive member  27  is substantially prevented from rotational displacement about the screw shaft rotational axis, relative to the pump chassis assembly  21 , when the screw shaft is torqued, thereby resulting in translational motion of the drive member. 
     In another aspect of the present invention, as best viewed in  FIGS. 6-8 , the lead screw shaft  23  further includes a first sleeve bearing  31 , disposed between the chassis assembly  21  and the lead screw shaft  23 , which provides rotating support at a distal end portion of the shaft. A second sleeve bearing  32  is also included that is disposed between the drive member  27  and the lead screw shaft  23 . This second sleeve bearing  32 , however, not only provides relative rotating support for the rotating lead screw shaft, but also provides sliding support to the piston drive member  27  as it reciprocates therealong between the first position ( FIGS. 1A ,  2  and  4 ) and the second position ( FIGS. 1B ,  3  and  5 ). 
     In accordance with this aspect of the present invention, however, this sliding sleeve bearing arrangement between the drive member  27  and the screw shaft  23  significantly accommodates any bending moments exerted upon the screw shaft that result from the disparate dispensing pressures between the two the piston chambers  29 ,  29 ′. Therefore, any associated friction between the piston heads in their respective piston bores that are caused by uneven forces exerted on the piston shafts  28 ,  28 ′, is greatly reduced. As a further consequence, binding of the drive system, and therefore premature failure of the motor, lead screw shaft, pistons, seals and barrels may also be significantly reduced. 
     Referring back to  FIGS. 6 ,  8  and  9 , the pump chassis assembly  21  is primarily composed of a thin, injection molded support platform  33  having an interior floor portion  35  that forms a support recess  36 , sized and shaped to accommodate the drive member  27 . To strengthen this support platform  33 , a plurality of strengthening ribs  37  extends downwardly from the bottom surface thereof. These ribs  37  extend longitudinally from a proximal portion to a distal portion of the chassis assembly. 
     The material composition of the chassis material is selected for chemical compatibility and high strength, thereby minimizing the stresses and strains caused by bending moments, as well as achieving high cycle life. Moreover, the chassis composition, along with that of many other of the dual pump apparatus  20  components, such as the piston drive member, are fabricated through injection molding techniques in order to reduce manufacturing costs. Such suitable materials, for example, include PBT or nylon, particularly glass-filled nylon. With respect to other high wear components such as the piston barrels (which define the piston bores, as will be described), injection molding materials, such as polypropylene and polysulfone, are particularly suitable since different material compositions can be applied to the same mold tool as long as the material mold shrink rates are similar. 
     Referring back to  FIGS. 2 and 6 , the lead screw shaft  23  and the reciprocating piston drive member  27  are movably housed in the platform recess  36 . Applying a pillar block mount  40  at a distal portion of the pump chassis assembly  21 , upon which the first sleeve bearing  31  is press-fit into, the distal portion of the lead screw shaft  23  can thus be rotatably mounted. An opposite proximal end of the lead screw shaft  23  is rotatably driven and supported by a conventional stepper type motor  41 . In turn, rigid mounting support to the screw shaft proximal end is provided by the motor  41  which is fixedly mounted to a motor mount wall  42  located at the proximal portion of the chassis assembly  21 . The motor mount wall  42  includes a central through-hole  43  to accommodate passage of the rotating lead screw shaft  23  therethrough. The motor mount wall also defines a plurality of mounting apertures  45  that are aligned with like mounting holes on the motor housing for mounting of the motor  41  thereto, via mounting fasteners (not shown). Accordingly, the lead screw shaft  23  is rotatably supported on the pump chassis assembly  21  by the chassis pillar block  40  on one end, and the mounting of the stepped motor  41  to the chassis, via the motor mount wall  42 , on the other end thereof. 
     Because higher dispensing forces (i.e., piston chamber pressures reaching up to about 200 psi maximum) may be required to drive the drive member piston shafts during liquid dispensing, the stepper motor  41  is designed with a bearing at a distal portion of a stepper motor shaft (not shown), as opposed to a rear end or proximal portion thereof. Contrary to most stepped motor applications in this field, which “push” the piston drive member  27  via the lead screw shaft, the present invention actually “pulls” the drive member back proximally toward the motor during liquid dispensing. Accordingly, the loads generally experienced by the motor shaft of the stepper motor are generally reversed. In this manner, as mentioned, the motor shaft bearing placement is positioned more distally. 
     In accordance with the present invention, the piston bores  22 ,  22 ′ are provided by an opposed pair of syringe-style piston barrels  47 ,  47 ′, as best shown in  FIG. 10 ) that are removably mounted in the recess  36  of the chassis assembly  21 . The syringe-style barrels are also injection molded, providing long straight interior piston bores  22 ,  22 ′ with very low surface roughness. Each respective piston bore extends longitudinally therethrough from a distal opening  50 ,  50 ′ to a proximal end wall  51 ,  51 ′. The distal opening  50 ,  50 ′ of each piston barrel  47 ,  47 ′ is drafted only a short distance to facilitate part and tool separation. Only a specific length is required with no draft and should be sized to allow for an appropriate volume to be dispensed. 
     To provide fluid communication through the proximal end walls  51 ,  51 ′, corresponding communication port  52 ,  52 ′ extends therethrough into each respective piston chamber  29 ,  29 ′. This enables the aspiration of liquid therethrough to fill the piston chamber  29 ,  29 ′, during the aspiration condition, when the piston drive member moves from the first position ( FIGS. 1A ,  2  and  4 ) toward the second position ( FIGS. 1B ,  3  and  5 ), and the dispensing of liquid from the piston chamber  29 ,  29 ′, during the dispensing condition, when the piston drive member moves from the second position back toward the first position. In one specific embodiment, this communication port  52 ,  52 ′ consists of a ¼-28 threaded port configured for receipt of a threaded connector therein. 
     To seat and support the piston barrels  47 ,  47 ′ in the chassis platform  33 , barrel seating portions  53 ,  53 ′ of the chassis support recess  36  are provided at proximal portions of the chassis. These seating portions are sized and dimensioned for vertical and lateral seated support of the piston barrels  47 ,  47 ′ longitudinally therein. However, to more rigidly mount, and further align, each piston barrel  47 ,  47 ′, relative to the pump chassis, the barrels include respective alignment flanges  55 ,  55 ′ which are formed to be press-fit into corresponding alignment slots  56 ,  56 ′, further defined by the barrel seating portions  53 ,  53 ′ of the chassis platform. 
     As best shown in  FIGS. 2 and 10 , these annular alignment flanges  55 ,  55 ′ flare radially outward from the proximal portions of the cylindrical shaped exterior of the piston barrels  47 ,  47 ′. These flanges are sized and dimensioned for light press easy friction-fit insertion into the corresponding alignment slots  56 ,  56 ′ which are also positioned at proximally along the barrel seating portions  53 ,  53 ′ of the pump chassis assembly  21 . It will be appreciated, of course, that the annular flanges and corresponding alignment slots can be longitudinally positioned anywhere along the piston barrels  47 .  47 ′. 
     As the respective flanges  55 ,  55 ′ are forcibly inserted vertically into the corresponding alignment slots  56 ,  56 ′, the lower edges of the annular flanges  55 ,  55 ′ seat and align against the respective interior alignment walls  57 ,  57 ′ ( FIGS. 2 and 9 ) defining the bottom portions of the respective alignment slots. Accordingly, once the annular flanges are fully press-fit into the corresponding alignment slots, the piston barrels can be rigidly affixed to the chassis platform. 
     Referring now to  FIGS. 6 and 9 , the lead screw shaft  23  includes a threaded portion  58  extending from the motor driven proximal end  25  to generally a central portion of the shaft. The threaded portion  58  threadably cooperates with the piston drive member  27  for driving movement thereof between the first position and the second position. Extending further distally from the shaft threaded portion  58  is a smooth bearing portion  59  extending all the way to the distal end of the screw shaft that is rotatably supported by the pillar block  40 . 
     Since the lead screw shaft  23  is a load bearing, high life-cycle structure, it is preferably composed of a high strength metallic material, such as  316  Stainless Steel. Depending upon the composition of the screw shaft, the selected diameter thereof is anywhere in the range of about 0.375 inches to about 0.394 inches. 
     As mentioned, the piston drive member  27  is reciprocally mounted to the lead screw shaft  23  for reciprocal movement between the first position ( FIGS. 1A ,  2  and  4 ) and the second position ( FIGS. 1B ,  3  and  5 ). In the preferred form, the drive member  27  is W-shaped having a central barrel portion  60 , flanked by the pair of piston shafts  28 ,  28 ′ on opposed sides of the central barrel portion ( FIG. 11 ). The central barrel portion  60  defines a central bore  61  extending therethrough from a proximal end to a distal end thereof. This central bore  61  is sufficiently sized diametrically to allow press fit insertion of the sleeve bearing  32  which permits reciprocating receipt of the lead screw axially therethrough, along the rotational axis  26  of the lead screw shaft  23 . 
     Each piston shaft  28 ,  28 ′ is integrally mounted to the central barrel portion  60  via support webs  62 ,  62 ′ in a manner such that their respective longitudinal axes are oriented substantially parallel to the central barrel rotational axis  26 . These triangular support webs  62 ,  62 ′ provide reinforced mounting support to the respective piston shafts  28 ,  28 , especially during movement toward the first position when higher piston chamber pressures are exerted on the piston shafts. 
     As above-indicated, a piston head  24 ,  24 ′ is mounted to a proximal end of each respective piston shaft  28 ,  28 ′, and is formed and dimensioned to form for fluid-tight seal with the circumferential walls defining the respective piston bore  22 ,  22 ′. Hence, when the piston head  24 ,  24 ′ is slideably positioned in the corresponding piston bore  22 ,  22 ′, it defines the volume of the corresponding piston chamber  29 ,  29 ′ as the piston shaft  28 ,  28 ′, via the drive member  27 , reciprocates between the aspiration condition ( FIGS. 1B ,  3  and  5 ) and the dispensing condition ( FIGS. 1A ,  2  and  4 ). 
     As best viewed in  FIGS. 1B ,  3  and  5 , in the aspiration condition, the respective piston shaft  28 ,  28 ′ is moved distally or axially out of the respective piston bore  22 ,  22 ′, as the piston drive member  27  is driven from the first position ( FIGS. 1A ,  2  and  4 ) toward the second position ( FIGS. 1B ,  3  and  5 ). When the respective rotary valve  63  is in an aspirate position ( FIG. 1B ), fluidly coupling the pump apparatus  20  to a fluid source  64 ,  64 ′, the vacuum created in the piston chamber  29 ,  29 ′causes fluid flow through the threaded port  52 ,  52  and into the respective chambers of each piston barrel  47 ,  47 ′. 
     In contrast, in the dispensing condition, the respective piston shaft  28 ,  28 ′ is moved proximally or axially into the respective piston bore  22 ,  22 ′, as the piston drive member  27  is driven, via the lead screw shaft  23 , from the second position ( FIGS. 1B ,  3  and  5 ) toward the first position ( FIGS. 1A ,  2  and  4 ). When the respective rotary valve  63  is in a dispense position ( FIG. 1A ), fluidly coupling the pump apparatus  20  to a dispensing nozzle  48 ,  48 ′ (one of which is of a restricted flow  69 , thereby increasing the dispensing pressure), the sample fluid contained in the piston chambers is dispensed therefrom. 
     Preferably, the central barrel, the support webs  62 ,  62 ′ and the respective first and second piston shafts  28 ,  28 ′ of the piston drive member  27  are comprised of a single, injection molded unit. 
     To facilitate movement of the piston drive member  27  along the motor driven lead screw shaft  23 , the drive member  27  incorporates a threaded nut  38  sized and dimensioned for threaded cooperation with the threaded portion  58  of the lead screw. To assure smooth threaded operation between the threaded nut  38  and the threaded portion  58  of the screw shaft  23 , under load, a thread pitch in the range of about 1 mm to about 3 mm, and more preferably about 2 mm is applied. However, other sizes can be used depending on actuation speeds and resulting flow rate requirements. 
     This threaded nut  38  is press-fit into the central bore  61  of the central barrel portion  60  for rigid mounting to the drive member  27 . In one configuration, the threaded nut  38  is preferably disposed near the proximal opening into the central bore  61  of the central barrel portion  60 . This position of the threaded nut  38  is selected to correspond with the lead screw threaded portion  58  such that the respective piston shafts  28 ,  28 ′ is effectively and efficiently reciprocated therein. Thus, upon threaded cooperation of the threaded nut  38  with the threaded portion  58  of the lead screw shaft  23 , the piston drive member  27  can be selectively driven between the retracted and second positions. 
     As indicated above, the second sleeve bearing  32  is included between the central barrel portion  60  and the lead screw shaft  23  to facilitate relative rotating support for the rotating lead screw shaft  23 . Moreover, this second sleeve bearing  32  also provides sliding longitudinal support to the piston drive member  27  as it reciprocates along the smooth bearing portion  59  of the lead screw shaft  23  between the first position and the second position. 
     In accordance with the present invention, the second sleeve bearing  32  is preferably disposed at a distal portion of the central bore  61 . At this location along the central bore  61  of the central barrel portion  60 , the second sleeve bearing  32  is positioned closest to the first sleeve bearing  31 . In this manner, the two sleeve bearings  31 ,  32  cooperate to counteract any bending moment acting upon the screw shaft  23  caused by the disparate forces applied to the piston shafts of the drive member, which is ultimately caused by the pressure differential between each piston chamber. 
     As mentioned, such dissimilar forces exerted upon the spaced piston shafts will transmit a bending moment about screw shaft which could cause binding of the drive system, high friction between pistons heads  24 ,  24 ′ and piston barrels  47 ,  47 ′ and therefore premature failure of the motor, screw drive, pistons, seals and other moving components. By positioning the second sleeve bearing  32  and the first sleeve bearing  31  in this association, bending moments can be minimized. 
     In accordance with another aspect of the present invention, as previously indicated, an anti-rotation device  30  is incorporated to prevent any rotation of the drive member  27  relative to the chassis assembly  21 . This anti-rotation device  30  counteracts any rotation of the drive member  27  about the shaft rotational axis  26 , caused by the torque of the lead screw shaft  23 . Thus, by positioning the anti-rotation device between the chassis assembly  21  and the piston drive member  27 , any such rotation will be counteracted. 
     To counteract rotation of the drive member  27 , regardless of the rotational direction of the lead screw shaft  23 , anti-rotation structure is provided along the platform on both sides of the lead screw shaft  23 . Such structure preferably includes a pair of elongated rail members  65 ,  65 ′, as best shown in  FIGS. 2 ,  6  and  8 , upstanding from the floor portion  35  of the chassis assembly  21  and disposed on opposed sides of the lead screw shaft  23 . Through abutting contact between the rail members  65 ,  65 ′ and the bottom side surface of the piston drive member  27 , rotation thereof relative to the chassis assembly  21  is substantially eliminated. 
     Each rail member  65 ,  65 ′ is also preferably oriented longitudinally in the direction of reciprocation of the piston drive member  27 . Accordingly, as the drive member  27  reciprocates along the lead screw between the first position and the second position, low friction sliding contact between the opposed rail member  65 ,  65 ′ and the bottom side surface of the piston drive member  27  provides vertical sliding support of the piston drive member  27  relative to the chassis assembly  21 . 
     Referring now to  FIGS. 7 and 12 , it will be appreciated that only one rail member  65  will be described in detailed. Each rail member  65  includes a lower rail portion  66 , configured to removably mount to the floor portion  35  of the chassis assembly  21 , and an opposed upper rail portion  67 , configured for sliding support against a sliding support surface  68  at the bottom of the drive member  27 . 
     The lower rail portion  66  has a generally rectangular-shaped bottom footprint (i.e., bottom base surface  70 ), and a transverse cross-section dimension that includes a pair of upwardly facing shoulder walls  71 ,  72  converging toward one another. Collectively, this lower rail portion  66  is sized for sliding insertion into an elongated receiving channel  73  defined by a rail support structure  75  upstanding from the floor portion  35  of the chassis assembly  21 . The transverse cross-sectional dimension of the elongated channel  73 , thus, is sized and dimensioned for friction-fit sliding receipt therein. In this manner, the respective rail member  65  is retained in the support structure  75  as the piston drive member  27  reciprocates in a sliding motion thereatop. 
     As best illustrated in  FIG. 7 , the elongated receiving channel  73  is defined by an alternating series of spaced bottom support surfaces  76  and spaced upper opposed beveled edges  77 . Due in part to the alternating disposition of these bottom support surfaces  76  and opposed beveled edges  77 , alternating openings  78  are formed between spaced bottom support surfaces, as well as those between the spaced beveled edges. 
     Collectively, the spaced bottom support surfaces  76  and opposed beveled edges  77  alternatively cooperate to form the elongated channel  73  that define a transverse cross-sectional dimension substantially similar to that of the corresponding rail member. These channels accommodate sliding assembly of the respective guide rails in a press-fit manner. 
     The plurality of generally rectangular spaced support surfaces  76  function to provide vertical support to the rail member  65 , while the opposed beveled edges  77  cooperate with one another and the support surfaces  76  to retain the rail member there against. Thus, the opposed beveled edges must be sized and dimensioned to provide sufficient structural integrity to retain the rail member  65  against the support surfaces  76 , while simultaneously permit sliding passage of the upper rail portion  67  therebetween during insertion into the elongated channel  73 . Accordingly, alternating these two support structure features along the length of the chassis mold creates an elongated channel through which the guide rails pass and are held in place. 
     To further prevent longitudinal or axial dislodgement of the rail member  65  in the corresponding elongated channel  73 , the bottom base surface  70  includes a plurality of friction stops in the form spaced, domed protrusions  80  ( FIGS. 12 and 13 ). These protrusions  80  are strategically spaced and positioned such that when the rail member  65  is properly mounted in the support structure elongated channel  73 , the downwardly extending protrusions extend into the openings  78  spaced between the bottom support surfaces  76 . Accordingly, as the domed protrusions  80  are slid past the bottom support surfaces  76 , during assembly, into the openings  78 , the resiliency of the rail member retains the same in the channel. 
     Referring now to  FIGS. 7 and 12 , the upper rail portion  67  incorporates a thin or narrow elongated contact surface  81  formed to provide reduced friction and vertical sliding support to the piston drive member  27  as it reciprocates between the first position and the second position. Preferably, the elongated upper rail portion  67  upstands from the lower rail portion  66 , is substantially linear, and includes opposed sidewalls  82 ,  83  that collectively converge in an inverted U-shaped manner to define the upper elongated contact surface  81 . Accordingly, by significantly reducing contact between an apex portion of the elongated contact surface  81  and the corresponding sliding support surface  68  (which defines alignment grooves  85 ,  85 ′, as will be described below) of the drive member  27 , the collective friction is also significantly reduced. 
     In one specific embodiment, the upper elongated contact surface  81  of the upper rail portion  67  is preferably positioned such that its longitudinal axis is oriented substantially parallel to the reciprocating movement of the piston drive member  27 , and thus also parallel to that of the lead screw shaft  23 . Again, in this orientation, the overall frictional contact area against the bottom support surface  68  of the drive member  27  is significantly reduced. Essentially, such contact against the bottom support surface  68  would be confined to a very thin rectangular region, if not nearly just a line. 
     To further facilitate aligned movement of the piston drive member  27  along the rail members  65 ,  65 ′, the sliding support surfaces  68 ,  68 ′ define corresponding alignment grooves  85 ,  85 ′ that are formed and dimensioned for sliding receipt of the opposed rail members  65 ,  65 ′ therein. As best illustrated in  FIGS. 2 ,  3 ,  8  and  11 , thus, as the drive member  27  reciprocates between the first position and the second position, the stationary rail members  65 ,  65 ′ are slideably received in the corresponding alignment grooves  85 ,  85 ′. 
     Accordingly, depending upon the rotational direction of the lead screw shaft  23 , at least one of the opposed rail members  65 ,  65 ′, in sliding contact in the alignment groove  85 ,  85 ′, function to oppose rotation of the drive member  27 . Simultaneously, friction between rail members and the drive member is reduced during sliding reciprocation between the first position and the second position. Such sliding contact therebetween further provides such anti-rotation of the W-shaft when torqued. 
     Although the present invention has been described in connection with the preferred form of practicing it and modifications thereto, those of ordinary skill in the art will understand that many other modifications can be made thereto within the scope of the claims that follow. Accordingly, it is not intended that the scope of the invention in any way be limited by the above description, but instead be determined entirely by reference to the claims that follow.