Patent Publication Number: US-8528792-B2

Title: Telescopic piston for pump

Description:
SCOPE OF THE INVENTION 
     This invention relates generally to a piston for a pump and, more particularly, to an arrangement for a disposable plastic pump for dispensing flowable material. 
     BACKGROUND OF THE INVENTION 
     Many dispensers of liquid such as hands soaps, creams, honey, ketchup and mustard and other viscous fluids which dispense fluid from a nozzle leave a drop of liquid at the end of the outlet. This can be a problem that the liquid may harden, as creating an obstruction which reduces the area for fluid flow in future dispensing. The obstruction can result in future dispensing through a small area orifice resulting in spraying in various directions such as onto a wall or user to stain the wall or user or more disadvantageously into the eyes of a user. 
     Many dispensers of material such as creams and for example liquid honey have the problem of stringing in which an elongate string of fluid hangs from fluid in the outlet and dangles from the outlet after dispensing an allotment of fluid. With passage of time the string may form into a droplet and drop from the outlet giving the appearance that the dispenser is leaking. 
     Pump assemblies for fluid dispensers are well known. Such pump dispenser includes those invented by the inventor of this present application including those disclosed in U.S. Pat. No. 5,165,577, issued Nov. 24, 1992; U.S. Pat. No. 5,282,552, issued Feb. 6, 1996; U.S. Pat. No. 5,676,277, issued Oct. 14, 1997, U.S. Pat. No. 5,975,360, issued Nov. 2, 1999, and U.S. Pat. No. 7,267,251, issued Sep. 11, 2007, the disclosures of which are incorporated herein by reference. Of these U.S. Pat. No. 7,267,251 teaches a piston pump in which there is, in a charging stroke of a piston moving in a stepped chamber, drawback of fluid from an outlet through which the fluid is dispensed from the chamber in a dispensing stroke due to the provision of stepped chamber as having two portions of different diameter. Such an arrangement while advantageous has the disadvantage of requiring a stepped chamber. 
     SUMMARY OF THE INVENTION 
     To at least partially overcome these disadvantages of previously known devices the present invention provides a piston pump dispenser in which a piston having a varying length is provided, such that a volume in a compartment defined inside a piston chamber-forming member and between axially spaced discs on the piston varies with movement of the piston in a cycle of operation. 
     The present invention is particularly applicable to fluid dispensers which fluid is to be dispensed out of an outlet with the outlet forming an open end of a tubular member. In many applications, the tubular member has its outlet opening downwardly and fluid passing through the tubular member is drawn downwardly by the forces of gravity. 
     An object of the present invention is to provide a fluid dispenser in which after dispensing fluid out an outlet draws fluid back through the outlet to reduce dripping and/or stringing. 
     An object of the present invention is to provide a simplified piston pump for dispensing fluid and after dispensing draws back fluid from the outlet of a nozzle from which the fluid has been dispensed. 
     Accordingly, in one aspect, the present invention provides a pump for dispensing fluids from a reservoir, comprising: 
     a piston chamber-forming member having an elongate chamber, said chamber having a chamber wall, an outer open end and an inner end in communication with the reservoir; 
     a one-way valve between the reservoir and the chamber permitting fluid flow through the inner end of the chamber, only from the reservoir to the chamber; 
     a piston-forming element slidably received in the chamber extending outwardly from the open end thereof; 
     the piston-forming element having an inner head portion, an outer base portion and a variable length portion intermediate the head portion and the base portion joining the head portion and the base portion, 
     a head disc extending radially outwardly from the head portion, the head disc having an edge portion proximate the chamber wall circumferentially thereabout, the edge portion of the head disc engaging the chamber wall circumferentially thereabout to substantially prevent fluid flow in the chamber past the head disc in an inward direction, the head disc elastically deforming away from the chamber wall to permit fluid flow in the chamber past the head disc in an outward direction, 
     a base disc extending radially outwardly from the stem of the base portion axially outwardly from the head disc, the base disc having an edge portion proximate the chamber wall circumferentially thereabout, the edge portion of the base disc engaging the chamber wall circumferentially thereabout to substantially prevent fluid flow in the chamber past the base disc in an inward direction, 
     the base portion having a central axially extending hollow stem having a central passageway open at an outer end forming an outlet, 
     the passageway extending from the outlet inwardly to an inner end open to the chamber between the head disc and the base disc, 
     the variable length portion having an axial length measured between the head disc and the base disc which is variable between a maximum length and a minimum length, wherein when the variable length portion has the maximum length the variable length portion is in an expanded condition and when the variable length portion has the minimum length the variable length portion is in a compressed condition, 
     the piston-forming element received in the piston chamber-forming member reciprocally coaxially slidable inwardly and outwardly by movement of the base portion in the chamber between a retracted position and an extended position in a cycle of operation to draw fluid from the reservoir and dispense it from the outlet, 
     wherein in movement of the base portion inwardly in the chamber while the length of the variable length portion is greater than the minimum length, resistance to movement of the head disc inwardly in the chamber is sufficient that the length of the variable length portion decreases toward the minimum length before the head disc portion is substantially moved inwardly in the chamber, 
     in movement of the base portion outwardly in the chamber while the length of the variable length portion is less than the maximum length, resistance to movement of the head disc outwardly in the chamber is sufficient that the length of the variable length portion increases toward the maximum length before the head disc portion is substantially moved outwardly in the chamber, and 
     movement of the base portion outwardly in the chamber while the length of the variable length portion increases toward the maximum length draws fluid in the passageway back into the chamber. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further aspects and advantages of the present invention will become apparent from the following description taken together with the accompanying drawings in which: 
         FIG. 1  is a cross-sectional side view of a pump in accordance with a first embodiment of the present invention with a piston in an uncompressed and unexpanded position; 
         FIG. 2  is a pictorial view of the piston of the pump shown in  FIG. 1 : 
         FIG. 3  is a cross-sectional side view of the piston in the same position as in  FIG. 1 ; 
         FIG. 4  is a cross-sectional side view of the piston of  FIG. 3  along section line  4 - 4 ′ in  FIG. 3 ; 
         FIGS. 5 ,  6 ,  7  and  8  are, respectively, cross-sectional views of the pump of  FIG. 1  in an extended and expanded condition, a partially extended and compressed condition, a retracted and compressed condition and a partially retracted and expanded condition; 
         FIGS. 9 ,  10 ,  11  and  12  are each a cross-sectional side view of a pump in accordance with respective second, third, fourth and fifth embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Reference is made first to the pump shown in  FIG. 1  comprising a pump assembly  10  secured to a reservoir or container  26  having a threaded neck  34 . The pump assembly has a body  12 , a one-way valve  14  and a piston  16 . 
     The body  12  provides a cylindrical chamber  18  in which the piston  16  is axially reciprocally slidable in a cycle of operation so as to draw fluid from within the container  26  and dispense it out of an outlet  54 . The chamber  18  has a cylindrical chamber wall  20  disposed coaxially about a chamber axis  22 . 
     The piston  16  has a head portion  47 , a variable length portion  45  and a base portion  49 . The head portion  47  carries a head disc  48 . The head disc  48  is a circular resilient flexible disc located at the inwardmost end of the base portion  49  and extending radially therefrom. The head disc  48  is sized to circumferentially abut the inner chamber wall  20  substantially preventing fluid flow therepast inwardly in the chamber  18 . The head disc  48  is formed as a thin resilient disc having an elastically deformable edge portion to engage the chamber wall  20 . The edge portion extends radially outwardly and in a direction axially outwardly of the chamber  18 . The edge portion is adapted to deflect radially inwardly away from the chamber wall  20  to permit fluid flow outwardly in the chamber  18  therepast. 
     The variable length portion  45  is disposed to bridge between the head portion  47  and the base portion  49  joining them together axially spaced. The variable length portion  45  comprises two elongate members  200 , each having an inner end  202  and an outer end  204 . The inner end  202  of each elongate member  200  is coupled to the head portion  47 . The outer end  204  of each elongate member  200  is coupled to the base portion  49 . Each of the elongate members  200  are coupled to the head portion  47  and the base portion  49  in a manner so as to not interfere with the engagement of the head disc  48  and the base disc  50  with the side wall  20  of the chamber. 
     The base portion  49  is arranged such that the outer ends  204  of the elongate members  200  are coupled to a stem  46  of the base portion  49  radially inwardly from the base disc  50 . The head portion  47  is shown to have a centrally extending stem  43  upon which the head disc  48  is mounted. The inner ends  202  of the elongate members  200  are coupled to the stem  43  radially inwardly from the head disc  48 . Each elongate member  200  includes an inner beam portion  206  and an outer beam portion  208  joined at a juncture  210 . Each inner beam portion  206  thus extends from an inner end  202  to the juncture  210 . Each outer beam portion  208  extends from the juncture  210  outwardly to the outer end  204 . 
     As best seen in  FIG. 2 , each elongate member  200  and its beam portions  206  and  208  have a generally rectangular shape in any cross-section normal the axis  22  with the thickness of each elongate member  200  as seen in  FIG. 1  being less than the width of each elongate member as seen in  FIG. 4 . While not necessary, this rectangular configuration preferably provides some relative rigidity of the elongate members  200  resisting deflection of the elongate members  200  laterally to the left or right as seen in  FIG. 4  as contrasted with an ability of the elongate members  200  to deflect laterally to the left or right as seen in  FIG. 3 . Each of the elongate members  200  has a resiliency by reason of being formed from suitably resilient plastic material. Resiliency of the elongate members  200  is provided in a number of ways as may be appreciated. Firstly, the juncture of each of the inner end  202  with the stem  43  of the head portion  47  may be considered a hinged connection about an inner hinge axis  212  disposed normal to a central axis  201 . Similarly, each of the outer ends  204  may be considered to be joined to the stem  46  of the base portion  49  at a hinged connection about an outer hinge axis  214 . As well, at the junction  210 , each of the inner beam portion  206  and outer beam portion  208  may be considered to be joined at a hinged connection about a mid axis  216 . Each of the inner axis  210 , the mid axis  216  and the outer axis  214  are parallel to each other. Additionally, each of the inner beam portion  206  and the outer beam portion  208  are capable of deflecting due to their inherent resiliency and the nature of the plastic material from which they are made. 
     The variable length portion  45  has an axial length defined as a length measured between the head disc  48  and the base disc  50 . This axial length is measured along the axis  201  between a center  218  on the head portion  47  and a center  220  of the base portion  49 . The axial length is indicated as L on  FIG. 3  and is variable between a maximum length and a minimum length due to the ability of the elongate members  200  to deflect. 
     The piston  16  is shown in each of  FIGS. 1 to 4  in an unbiased inherent condition. 
     The piston  16  is shown in  FIGS. 5 ,  6 ,  7  and  8  in use in a cycle of operation of the pump.  FIGS. 5 and 8  show the piston  16  within the chamber  18  of the body  12  in an “expanded condition” in which the variable length portion  45  is in its maximum length. This maximum length is achieved when each of the inner axis  212 , center axis  216  and outer axis  214  fall within the same flat plane. With movement of the base portion  49  outwardly in the chamber  18 , resistance to movement of the head portion  47  and particularly its head disc  48  within the chamber  18  will give rise to tension forces being applied across each of the elongate members  200 . The response of the elongate members  200  to such tension force will depend upon the nature and resiliency of each elongate members and the amount of the tension force. 
       FIGS. 6 and 7  show the piston  16  received in the chamber  18  of the body  12  with the variable length portion  45  in a “compressed condition”. With movement of the base portion  49  inwardly in the chamber  18 , resistance to inward movement of the head portion  47  and notably resistance to movement of the head disc  48  inwardly in the chamber  18  results in compressive forces being applied to the variable length portion  45  between the base portion  49  and the head portion  47 . Such compressive forces cause the elongate members  200  to deflect to reduce the axial length of the variable length portion  45  to the minimum length compressed condition as seen in  FIGS. 6 and 7 . In this compressed condition, the junctures  210  of the elongate members  200  have been urged radially away from each other, that is, radially outwardly away from each other as seen in  FIG. 3  with the junction portions  210  of each elongate member  200  being restricted in radially outward movement by engagement with the chamber wall  20  of the chamber, however, this is not necessary and the compressed condition could be a condition in which the junction portions  210  are not in contact with the chamber wall  20 . 
     In operation of the pump, the relative tension forces and compression forces which may be applied through the variable length portion  45  between the base portion  49  and the head portion  47  will cause the variable length portion  45  to adopt configurations between its maximum length expanded condition and its minimum length compressed condition. The relative resistance of the head portion  47  to sliding within the chamber  18  is affected by many factors including the friction to movement of the disc portion  47  within the chamber  18 , inwardly and outwardly, the nature of the fluid in the reservoir having regard to, for example, its viscosity, the temperature, the speed with which the base portion  49  is moved and various other features which will be apparent to a person skilled in the art. A person skilled in the art by simple experimentation can determine suitable configurations for the telescopic member  45  so as to provide for the axial length of the variable length portion to vary between a suitable minimum length and a suitable maximum length in cyclical movement of the piston  16  in a cycle of operation. 
     The base portion  49  has a stem  46  that carries not only the base disc  50  but also locating webs  66 . The base disc  50  is a circular resilient flexible disc located on the stem  46  spaced axially outwardly from the head disc  48 . The base disc  50  extends radially outwardly from the stem  46  to circumferentially engage the chamber wall  20  substantially preventing fluid flow therebetween outwardly in the chamber  18 . As with the head disc  48 , the base disc  50  is preferably formed as thin resilient disc, in effect, having an elastically deformable edge portion to engage the chamber wall  20 . The stem  46  has a central passageway  52  extending along the axis  201  of the piston  16  from an inner inlet end  58  located on the stem  46  between the head disc  48  and the base disc  50  to the outlet  54  at the outer end of the head portion  49 . The passageway  52  permits fluid communication through the base portion  49  past the base disc  50 , between the inlet  58  and the outlet  54 . Axially extending webs  66  are provided to extend radially from stem  46  of the base portion  49 . These webs  66  engage chamber wall  20  so as to assist in maintaining the base portion  49  axially centered within the chamber  18  when sliding in and out of the chamber  18 . The stem  46  comprises a tubular member and can be seen to have the passageway  52  therethrough between the outlet  54  and an inlet  58  with the inlet  58  open to the chamber  18  between the head disc  48  and the base disc  50 . 
     Each of the base portion  49  and the head portion  47  is circular in any in cross-section in  FIGS. 3 and 4  normal the axis  201  therethrough. Each of the base portion  49  and the head portion  47  is adapted to be slidably received in chamber  18  coaxially within the chamber  18 . 
     An engagement flange  62  is provided on the stem  46  for engagement to move the base portion  49  inwardly and outwardly. The engagement flange  62  also serves the function of a stopping disc to limit axial inward movement of the piston  16  by engagement with the outer end  23  of the body  12 . The stem  46  is shown to extend outwardly from the engagement flange  62  to the discharge outlet  54  as a relatively narrow hollow tube  138  with the passageway  52  coaxially therethrough. 
     The one-way valve  14  comprises a unitary piece of resilient material having a resilient, flexible, annular rim  132  for engagement with the side wall of the chamber  18 . The one-way valve is integrally formed with a shouldering button  134  which is secured in a snap-fit inside an opening  136  in an end wall  320  of the chamber at a central upper end of the chamber  18 . Openings  322  are provided through the end wall  320  for fluid flow from the reservoir  26  to the chamber  18 . 
     As seen in  FIG. 1 , an annular inner compartment  111  is formed inside the chamber  18  between the one-way valve  14  and the head disc  48  and an annular outer compartment  112  is formed inside the chamber  18  between the head disc  48  and the base disc  50 . The volume of the annular outer compartment  112  varies with variance of the length of the variable length portion  45  of the piston  16 . 
     The body  12  carries an outer cylindrical portion  40  carrying threads  130  to cooperate with threads formed on the threaded neck  34  of the container  26 . 
     In use, the pump is preferably orientated such that such that the outlet  54  is directed downwardly, however this is not necessary. 
     The pump operates in a cycle of operation in which the piston  16  is reciprocally moved relative the body  12  inwardly in a retraction stroke and outwardly in a withdrawal stroke. 
     During movement of the head portion  49  inwardly into the chamber, since fluid is prevented from flowing outwardly past the disc  50 , pressure is created in the inner compartment  111  formed in the chamber  18  between the head disc  48  and the one-way valve  14 . This pressure urges rim  132  radially outwardly to a closed position abutting the chamber wall  20 . As a result of this pressure, head disc  48  deflects at its periphery so as to come out of sealing engagement with the chamber walls  20  and permit fluid to flow outwardly past head disc  48  into the annular outer compartment  112  between the head disc  48  and the sealing disc  50  and hence out of chamber  18  via the passageway  52 . 
     During a withdrawal stroke in which the piston  16  is moved outwardly from the chamber  18 , the withdrawal of the piston causes the one-way valve  14  to open with fluid to flow past annular rim  132  which is deflected radially inwardly into the inner compartment  111  in the chamber  18 . In the withdrawal stroke, head disc  48  remains substantially undeflected and assists in creating a vacuum in the inner compartment  111  to deflect rim  132  and draw fluid past rim  132 . 
     The head disc  48 , on one hand, substantially prevents flow inwardly therepast in the withdrawal stroke and, on the other hand, deforms to permit flow outwardly therepast in the retraction stroke. The head disc  48  shown facilitates this by being formed as a thin resilient disc, in effect, having an elastically deformable edge portion near chamber wall  20 . 
     When not deformed, head disc  48  abuts the chamber wall  20  to form a substantially fluid impermeable seal. When deformed, as by its edge portion being bent away from wall  20 , fluid may flow outwardly past the head disc. Head disc  48  is deformed when the pressure differential across it, that is, when the pressure on the upstream side is greater in the inner compartment  111  than the pressure on the downstream side in the outer compartment  112  by an amount greater than the maximum pressure differential which the edge portion of the head disc can withstand without deflecting. When this pressure differential is sufficiently large, the edge portion of the head disc deforms and fluid flows outwardly therepast. When the pressure differential reduces to less than a given pressure differential, the head disc returns to its original inherent shape substantially forming a seal with the wall  20 . 
       FIGS. 5 to 8  which show different conditions the variable length portion  45  assumes in a cycle of operation. In this cycle of operation, the base portion  49  is moved in a retraction stroke from a fully extended position as seen in  FIG. 5  to a fully retracted position as seen in  FIG. 7 . In a withdrawal stroke, the base portion  49  is moved from the fully retracted position of  FIG. 7  to the fully extended position shown in  FIG. 5 . 
       FIG. 5  illustrates the piston  16  with the base portion  49  in the fully retracted condition and the variable length portion  45  in an expanded condition, that is, with the variable length portion  45  at its maximum length. In this extended and expanded condition of  FIG. 5 , the outer compartment  112  formed in the chamber  18  between the head disc  48  and base disc  49  is at a maximum volume. From the extended and expanded in condition of  FIG. 5 , the base portion  49  is moved inwardly in a retraction stroke to assume the condition of  FIG. 6  in which the variable length portion  45  is a compressed condition. On the base portion  49  moving inwardly in the chamber  18  from the position of  FIG. 5 , while the length of the variable length portion  45  is greater than its minimum length, resistance to movement of the head portion  47  and its head disc  48  inwardly in the chamber  18  is sufficient that the length of the variable length portion  45  decreases toward its minimum length as shown in  FIG. 6  before the head portion  47  is moved inwardly in the chamber  18 . Thus, in movement of the base portion  49  inwardly from the position of  FIG. 5 , compressive forces will be applied to the variable length portion  45  which forces will reduce the length of the variable length portion  45  until the compressive transferred by the variable length portion  45  are greater than the resistance to movement of the head portion  47  inwardly in the chamber. The compressive forces may be developed such that the variable length portion substantially decreases to its minimum length before the head portion  47  is substantially moved inwardly. 
     From the position shown in  FIG. 6 , with the variable length portion in the compressed condition, further inward movement of the base portion  49  in the retraction stroke moves the piston  16  with the variable length portion maintained in the compressed condition inwardly to the position of  FIG. 7  in which the base portion  49  is fully retracted and the variable length portion  45  is compressed.  FIG. 7  thus represents a retracted and compressed condition of the piston  16 . 
     From the position of  FIG. 7 , in a withdrawal stroke, the base portion  49  is moved outwardly in the chamber. In movement of the base portion  49  from the position of  FIG. 7  to the position of  FIG. 8 , while the length of the variable length portion  45  is less than the maximum length, resistance to movement of the head portion  47  and therefore its head disc  48  outwardly in the chamber  18  is sufficient that the length of the variable length portion  45  increases toward the maximum length before the head portion is moved outwardly in the chamber  18 . In this regard, in moving from the position of  FIG. 7  to the position of  FIG. 8 , outward movement of the base portion  49  applies tension forces to the variable length portion  45 . These tension forces will act on the variable length portion  45  expanding the variable length portion  45  until such time as the tension forces which are transferred by the variable length portion  49  from the base portion  49  the head portion  47  are greater than the resistance of the head portion for movement outwardly in the chamber. The tension forces may be developed such that the variable length portion substantially increases to its maximum length before the head portion  47  is substantially moved outwardly. 
     From the position of  FIG. 8 , the withdrawal stroke is complete by movement to the position of  FIG. 5 . In moving from the position of  FIG. 8  to the position of  FIG. 5 , the variable length portion  45  is maintained in the expanded condition with the variable length portion  45  at its maximum length and tension forces caused by movement of the base portion  49  are transferred via the variable length portion  45  to the head portion  47 . 
     In a cycle of operation in moving from the position of  FIG. 5  to the position of  FIG. 6 , the volume of the outer compartment  112  reduces and hence fluid is discharged from the outer compartment  112  through the passageway  52  out the outlet  54  by reason of fluid within the outer compartment  112  being prevented from passing inwardly past the head disc  48  and being prevented from passing outwardly past the base disc  50 . In moving from the position of  FIG. 6  to the position of  FIG. 7 , pressure is created within the inner compartment  111  which closes the one-way valve  14 . Fluid within the inner compartment  111  becomes compressed by movement of the head disc  48  inwardly. Such pressure causes the deformable edge portion of the head disc  48  to deflect away from the chamber wall  18  thus permitting flow of fluid from the inner compartment  111  into the outer compartment  112 . Since the volume of the outer compartment  112  remains the same in the compressed condition, any fluid which is passed outwardly past the head disc  48  causes fluid within the outer compartment  112  to be dispensed through the passageway  52  out from the outlet  54 . 
     In movement from the position of  FIG. 7  to the position of  FIG. 8 , the volume of the outer compartment  112  increases. This increase in volume of the outer compartment  112  causes a drawback of fluid in the passageway  52  from the outlet  54  back into the outer compartment  112 . This drawback may not only be a drawback of fluid in the passageway but also possibly of any air existing in the passageway. 
     To facilitate drawback of fluid, the relative nature of the head disc  48  and the base disc  50  and the engagement of each with the chamber wall  20  are preferably selected such that vacuum created within the outer compartment  112  will drawback fluid from the passageway  54  rather than deflect the head disc  48  to draw liquid from the inner compartment  111  past the head disc  48  into the outer compartment  112 , or, deflect the base disc  50  to draw atmospheric air between the base disc  50  and the chamber wall  20 . 
     In movement from the position of  FIG. 8  to the position of  FIG. 7 , the volume in the outer compartment  112  is maintained constant with the variable length portion  45  in the expanded condition, however, movement of the head disc  48  outwardly increases the volume in the inner compartment  111  thus drawing fluid from the reservoir inwardly past the one-way valve  14  into the inner compartment  111 . 
     The drawback pump in accordance with the present invention may be used in manually operated dispensers such as those in which, for example, the piston  16  is moved manually as by a user engaging an actuator such as a lever which urges the piston  16  either outwardly or inwardly. The drawback pump can also be used in automated systems in which a user will activate an automated mechanism to move the piston in a cycle of operation. 
     A preferred arrangement for operation of the drawback pump in accordance with the present invention is for the pump to assume a position between the condition shown in  FIG. 8  and the condition shown in  FIG. 5  as a rest position between cycles of operation. For example, in the context of a manual dispenser, the dispenser may be arranged such that the base portion  49  is biased to assume as a rest position between cycles of operation, the extended position seen in  FIG. 5 . A person would manually operate a lever to move the dispenser from the position of  FIG. 5  to the position of  FIG. 7 . On release of the lever, a spring will return the lever and base portion  48  to the position of  FIG. 5 . In such a cycle of operation, on movement from the position of  FIG. 5  to the position of  FIG. 7 , fluid is dispensed from the outlet  54 . In a return stroke, for example, due to the bias of the spring, fluid in the passageway  54  is withdrawn in movement from the position of  FIG. 7  to the position of  FIG. 8  and the inner compartment  111  is filled in movement of the piston to the rest position of  FIG. 5 . In automated operation, a rest position between cycles may be at some point in between the position of  FIG. 8  and the position of  FIG. 5 . 
     The preferred embodiment illustrates the piston as being formed from a unitary piece of plastic preferably by injection molding. It is to be appreciated that a similar structure could be formed from a plurality of elements, for example, with the variable length portion formed together with at least one of the head portion and the disc portion as a unitary piece of plastic. 
     In the context of the embodiment of  FIGS. 1 to 8 , the piston  16  and its variable length portion  45  could have an inherent condition when molded as seen in  FIGS. 5 and 8 , which is also the condition in which the length L is a maximum. In such an alternate embodiment,  FIGS. 1 to 4  represent a partially compressed condition. The members  200  would be molded so as to deflect radially away from each other when the variable length member is compressed, and adopt the bent profile as seen in  FIGS. 1 to 4  and  FIGS. 6 and 7 . 
     The variable length portion in the preferred embodiments shown in  FIGS. 1 to 8  which is injection molded from plastic typically will have an inherent tendency to assume an unbiased condition being the condition of the elongate members  200  forming the variable length portion when they are injection molded. In the embodiment illustrated in  FIG. 2 , the unbiased condition is a condition shown in  FIG. 2  which is intermediate the compressed condition shown in  FIG. 5  and the expanded condition shown in  FIG. 6 . However, it is to be appreciated that the unbiased condition may be any position in between the compressed condition and the expanded condition. A preferred arrangement is for the variable length portion to be inherently biased to assume the expanded condition or at least a condition proximate the expanded condition. This will have, amongst other things, the advantage that the inherent bias of the variable length portion will assist in expanding the volume of the outer compartment  112  to assist in providing drawback and in a rest position of  FIG. 5  assist in maintaining the volume of the outer compartment  112  at a maximum. 
     The embodiment shown in  FIG. 2  shows two opposing elongate members  200 . While merely two such elongate members  200  are provided, three or more such elongate members could be provided spaced circumferentially about the piston. However, in the context of the piston comprising a unitary element to be injection molded, providing elongate members merely at two oppositely directed sides of the piston can facilitate manufacture by injection molding. 
     The particular variable length portion may be selected so as to provide the head portion and its head disc maintained coaxially arranged within the chamber. Alternatively, the head disc may be permitted to, at least some extent, tilt or pivot so as to not be coaxially disposed within the chamber and thus provide additional advantages to the invention similar to those provided in pumps with pivoting pivot heads as disclosed in the applicant&#39;s U.S. Pat. No. 6,557,736, issued May 6, 2003, the disclosure of which is incorporated herein by reference. 
     In the embodiment illustrated in  FIGS. 1 to 8 , when the telescopic section  45  is compressed and the head disc  48  is moved inwardly as, for example, in moving from the position of  FIG. 6  to the position of  FIG. 7 , tilting of the head disc  48  can reduce the resistance to fluid flow past the head disc  48  outwardly. The tilting of the head disc  48  may preferably be sufficient that the edge portion of the head disc  48  becomes displaced from the side wall  20  of the chamber  18  over at least one segment about the circumference of the head disc. In any event, whether or not the tilting is so substantial that the edge portions of the head disc  48  are disposed by tilting alone radially inwardly from the chamber wall  20 , to the extent that due to tilting at least some segment the edge portions of the head disc are moved radially inwardly away from the chamber wall  20 , the extent to which deflection of the edge portion is required to permit fluid flow outwardly past the head disc  48  is reduced. Tilting of the head disc  48  can assist in pumping fluids containing particulate matter including solid particles such as pumice, sands and other solid particulate matter mixed with liquids to provide a slurry-like composition which is fluid. Tilting may also be of assistance with extremely viscous fluids. Tilting of the head disc  48  may be considered in the first embodiment as an arrangement in which an axis coaxially through the head portion  49  comes to be disposed at an angle relative the chamber axis  22 . In the embodiment illustrated in  FIG. 2 , tilting can occur merely by increased deflection of one of the elongate members  200  compared to the other elongate member  200  as may occur by one of the elongate members  200  having a tendency to deflect under lesser compressive forces than the other elongate member  200 . For example, one elongate member  200  could have a reduced cross-sectional area compared to the other elongate member over its length or at one of the living hinges. Such a reduced cross-sectional area could permit one elongate member  200  to compress to a greater extent than the other elongate member yet would not affect, in tension, the elongate members having effectively the same length and thus when in tension providing the head portion  47  and the head disc  48  to be coaxially located in the chamber  18  not tilted to ensure a good seal is formed to prevent movement of fluid inwardly therepast. 
     While it may be advantageous to have the head disc tilt in some applications on movement of the piston  16  inwardly, on movement of the base portion  49  and the head portion  47  outwardly, it is desired that the head disc be coaxially untilted. In the embodiments illustrated with the head disc  48  extending radially outwardly and axially outwardly on movement of the head disc outwardly, it will tend to assume an untilted configuration. 
     Reference is made to  FIG. 9  which illustrates in a second embodiment a modified form of a piston  16  as shown in  FIGS. 1 to 8  with a left-hand elongate member  200  having an increased thickness compared to the right hand elongate member  200  such that when the piston  16  moves from the position of  FIG. 6  to the position of  FIG. 7 , the head disc  48  assumes a tilted position as shown. In  FIG. 9 , the stronger left-hand elongate member  200  is shown to not bend so far as to engage the chamber wall  20 . Of course, in the embodiment of  FIGS. 1 to 8 , both elongate members  200  could be provided such that in a compressed condition, the members  200  do not engage the chamber wall  20 . 
     Reference is made to  FIG. 10  which shows a third embodiment of a piston element in accordance with the present invention. The embodiment in  FIG. 10  differs from the embodiment shown in  FIG. 2  in that the head portion  47  includes locating means in the form of a plurality of circumferentially spaced axially extending webs  166  similar to webs  66  to assist in maintaining the head portion coaxially received within the chamber  18  and the two elongate members  200  are replaced by a single string-like tension member  200  which will substantially totally collapse upon itself and not transfer any compressive forces from the base portion  49  to the head portion  47  yet will, when at its maximum length, have adequate strength to transfer tension forces. The stem  46  on the base portion  49  is shown to have an inner axial extension that will come to engage the stem  46  on the head portion  47  in a totally compressed condition. 
     Reference is made to  FIG. 11  which shows a fourth embodiment of a piston  16  in accordance with the present invention. In the embodiment of  FIG. 11 , the stem of the head portion  47  extends axially outwardly with the stem of the head portion  47  telescopically coaxially received in sliding engagement within a cylindrical guide bore  152  in the stem on the base portion  49 . The stem  43  on the head portion  47  has slots  153  at diametrically opposed portions of its side wall through which a diametrically extending pin  154  may be provided with the pin fixed at each of its ends in the opposed wall of the stem  46  of the base portion  49  so as to limit the axial extent of relative sliding of the head portion  47  relative the base portion  49  and thus set the maximum length and minimum length of the variable length portion  45 . As well, a radial passageway  226  is shown through the stem  43  of the head portion  47  to permit fluid to flow into the passageway  52 . 
     In the embodiments illustrated in  FIG. 11 , an optional, biasing spring member in the form of a helical coil spring  227  is provided between the head portion  47  and the base portion  49  to bias the head portion  47  and the base portion  49  apart to the expanded condition. The strength of the spring  227  needs to be selected such that it compresses under forces less than the forces required to slide the head portion  47  inwardly. 
     The preferred embodiment in  FIGS. 1 to 8  illustrates a three-piece pump having as the three pieces, the body  12 , the one-way valve  14  and the piston  16 , and in which the chamber  18  in the body  12  has a constant diameter. The invention of the present application is also adaptable for use with two piece pumps having a stepped chamber. Such pumps have been disclosed in U.S. Pat. No. 5,676,277 to Ophardt, issued Oct. 14, 1997, the disclosure of which is incorporated herein by reference. 
     Reference is made to  FIG. 12  which shows as a fifth embodiment of the present invention a two piece pump  10  which is substantially the same as the pump of the first embodiment of  FIGS. 1 to 8  with the exception that the one way valve  14  of  FIG. 1  has been replaced by the provision on the body  12  of a stepped chamber with an inner chamber portion  304  coaxially inward of an outer chamber portion  18  and the provision on the head disc portion  47  of the piston  16  of an innermost disc  216 . The inner chamber portion  304  is of a different, smaller diameter than the diameter of the outer chamber portion  18 . An outer end of the inner chamber portion opens coaxially into the inner end of the outer chamber portion  18 . The innermost disc  216  is formed as a thin resilient disc having an elastically deformable edge portion to engage a chamber wall  220  of the inner chamber portion  304  so as to prevent fluid flow inwardly there past. This deformable edge portion is adapted to deflect radically inwardly away from the chamber wall  220  to permit fluid flow outwardly in the inner chamber portion  304  there past. The arrangement of the stepped cylindrical chamber portions  304  and  18 , the innermost disc  216  and the head disc  48  forms a one way valve arrangement in  FIG. 12  functionally similar to the one way arrangement in the embodiment of  FIGS. 1 to 8 . On moving the head disc portion  47  inwardly, fluid in the stepped annular space between innermost disc  216  and head disc  48  is forced outwardly past head disc  48 . On moving the head disc portion  47  outwardly fluid is drawn from the container past innermost disc  216  into the stepped annular space between the innermost disc  216  and the head disc  48 . The operation of the head disc  48  and the base disc  50  is the same as in the first embodiment of  FIGS. 1 to 8 . 
     While the invention has been described with reference to preferred embodiments, many modifications and variations will now occur to persons skilled in the art. For a definition of the invention, reference is made to the following claims.