Abstract:
A pinch mechanism, which can be used as part of a pump, includes a deformable tube ( 10 ) enclosed within a body ( 11 ) which has a first chamber ( 11   a ). The deformable tube ( 10 ) defines a flow passage. A second chamber ( 19 ) is coupled via passage ( 17 ) to the first chamber ( 11   a ). A piston ( 20 ) is located within the second chamber ( 19 ) and movable between first and second positions. Upon moving to the first position a pressure increase occurs in the first chamber ( 11   a ). Upon moving to the second position a negative pressure is created in the first chamber ( 11   a ). A vent means ( 25 ) is located at a point during movement of the piston ( 20 ) between the first and second positions which enables pressure equalisation within the second chamber ( 19 ) to occur.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to improvements in pinch mechanisms. The invention is particularly suited for elastic rebound pinch mechanisms but is not limited thereto.  
           [0002]    Problems exist when moving liquid with conventional pumping methods in which moving parts are exposed to product flow. For example:— 
           [0003]    Gears, seals, pistons and springs in contact with the product flow can very quickly succumb to corrosion, become blocked and or generally become in operative or faulty in operation  
           [0004]    When used in an hygienic environment, or where one pump is used for a variety of liquids, these parts can be difficult to clean without disassembly  
           [0005]    In some cases, peristaltic pumps have been used to try and address these issues, but poor tube life is often cited as a significant limiting factor.  
           [0006]    Elastic rebound pinch mechanisms are known. The mechanisms can function as a valve or as a pump. Generally the mechanism relies on a flexible tube or conduit having elastic rebound characteristics such that the tube can be pinched to close a flow passage through the tube and then released to enable the elastic rebound to restore the tube to substantially its non-deformed state. An elastic rebound pinch mechanism pump of the type disclosed in WO 99/01687 can overcome many of the above identified problems.  
           [0007]    A problem which can arise with pinch mechanisms is that the rebound characteristics of the tube and/or the material from which it is constructed may not be sufficient to restore the tube to its fully non deformed state. Also the speed of movement of the tube to the non-deformed state can be slow. In a pumping situation failure to rebound fully or quickly can impair or at least limit the desired pump characteristics.  
           [0008]    Furthermore the nature of the fluid material to be pumped or moved through the tube may require the tube to be made of a material (or of such thickness) that the elastic rebound characteristics do not permit the tube to rebound to its non-deformed state as fully or as quickly as desired. Alternately the material to flow through the tube may be of a viscosity or be sticky in nature such that once again the desired elastic rebound characteristics of the tube are impaired.  
         SUMMARY OF THE INVENTION  
         [0009]    It is an object of the present invention to provide an improved pinch mechanism in which the pinch mechanism exhibits favourable restoring characteristics of a deformable tube.  
           [0010]    Broadly in one aspect of the invention there is provided a pinch mechanism including a deformable tube enclosed within a first chamber, the deformable tube defining a flow passage, a second chamber coupled to said first chamber, a piston located within the second chamber, the piston being movable between first and second positions such that upon moving to said first position a pressure increase occurs in said first chamber and upon moving to said second position a negative pressure is established in said first chamber and vent means, which at a point during movement of the piston between the first and second positions enables a pressure equalisation within the second chamber occur.  
           [0011]    According to one form of the invention the deformable tube is resilient and exhibits an inherent rebound characteristic such that the tube tends to revert to a substantially non-deformed state. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 in schematic form illustrates one embodiment of the invention in the form of a rebound pinch mechanism forming part of a pump,  
         [0013]    [0013]FIG. 2 is, in more detailed form, a cross-sectional drawing of a second embodiment of the invention,  
         [0014]    FIGS.  3  to  5  are views of the second embodiment at different operational stages, and  
         [0015]    [0015]FIG. 6 is a more detailed illustration of the valve unit employed in the second embodiment. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0016]    Referring to FIG. 1 there is shown a flexible tube  10  which is subjected to cyclic compression or pinching of the tube into a closed or partially closed position and released to a substantially non-deformed configuration. The flexible tube  10  is typically a silicone tube.  
         [0017]    According to the invention flexible tube  10  is located within a housing  11  which has a cross sectional shape commensurate with that of the exterior wall surface of tube  10 . Thus in one preferred form of the invention tube  10  is of circular cross section as is the housing  11 . A clearance  12  is provided between the inner wall surface  13  and outer wall surface  14  of the respective housing  11  and tube  10 . For the purposes of illustration FIG. 1 exaggerates the extent of clearance  12 .  
         [0018]    The housing  11  is sealed at each end. In the illustrated form the sealing effect is achieved by the positioning at respective ends an inlet valve  15  and an outlet or exhaust valve  16 . In accordance with normal pinch mechanism technology the exhaust valve  16  opens upon the tube  10  being pinched to a closed position. The exhaust valve  16  closes and the inlet valve  15  opens as the tube  10  reverts to its non-deformed state.  
         [0019]    Because housing  11  is sealed closed at each end the tube  10  is effectively located within a chamber  11   a.    
         [0020]    According to the present invention a mechanical force contacting the tube is not applied in order to achieve the pinching action. By contrast with known pinch mechanisms the pinching action is preferably achieved pneumatically.  
         [0021]    Thus according to the preferred pneumatic form of the invention a port  17  is formed in the wall of the housing  11 . Port  17  communicates via passage  18  with a chamber  19  (more particularly a cylinder) in which a piston  20  can reciprocate. A piston rod  21  extends from the piston  20 . Rod  21  is coupled to an actuating means such as a motor, linear actuator or the like. Seals  22  associated with piston  20  slidingly engage with the inner wall surface  23  of the chamber housing  24  to provide the required sealing effect.  
         [0022]    Associated with the housing  24  is a transfer port  25 .  
         [0023]    As the piston rod  21  moves in the direction of arrow A (see FIG. 1) the piston  20  moves toward transfer passage  18 . Once the seals  22  have moved beyond the transfer port  25  air located between the piston  20  and inside of housing  11  is gradually compressed. The compressed air acts on the tube  10  to thus cause the tube to collapse inwardly.  
         [0024]    In the preferred form of the invention the tube  10  is confined within the encasement of housing  11  therefore tube  10  is confined in the manner disclosed in our patent specification WO099/01687. Thus tube  10  collapses inwardly in an inverted manner into a sealed closed state as illustrated in FIG. 3 of WO99/01687. However, this inverted collapse of the tube is created not by mechanical means as disclosed in WO99/01687 but via the application of pressurised air.  
         [0025]    It has been found that the tube  10  will inwardly invert in the vicinity of port  17  but not necessarily directly adjacent port  17 . The tube will tend to inwardly invert at the point of least resistance to inversion.  
         [0026]    When the piston  20  retracts the pressure dissipates. As the piston  20  crosses the transfer port  25  pressure in the chamber  19  will be equalized to atmospheric pressure. This occurs because chamber  19  vents via port  25  to atmosphere, the reverse side of piston  20  being exposed to atmosphere.  
         [0027]    As the piston retracts further transfer port  25  will close and a negative pressure will develop within the chamber  19  and hence within chamber  11   a  in the housing  11 . This negative pressure creates a sucking effect on the tube  10  and causes it to revert to its normal state. Then as the piston rod  21  once again moves in the direction of arrow A the negative pressure is dissipated and equalised to atmospheric pressure as the transfer port  25  is once again opened.  
         [0028]    Such negative pressure applied to the tube  10  can actually cause the tube  10  to expand beyond its normal state. Therefore not only does the application of a negative pressure on the tube aid in it reverting to its non-deformed state it can also further assist the efficiency of the pinch mechanism when used in a pump application.  
         [0029]    The throughput of the pinch mechanism when used in a pump configuration can be adjusted by the speed and/or stroke of piston  20 . Tests to date show that a pump according to the present invention can be kept dimensionally compact. Hence the pump can be more compact than a conventional pinch mechanism pump where the pinching action relies on the application of mechanical force to pinch the tube closed and reliance on the rebound characteristics of the tube for the tube to return to its “open” state.  
         [0030]    The invention is open to modification. For example the piston mechanism can be located remote from the housing and coupled by say a tube between transfer passage  18  and port  17 . This may be advantageous when the pump operates as say an immersion pump.  
         [0031]    The embodiment of the invention shown in FIG. 1 demonstrates some excellent attributes such as:— 
         [0032]    No moving parts in contact with the liquid flow.  
         [0033]    A clear unobstructed product flow providing excelling hygiene properties making cleaning simple.  
         [0034]    The pump occupying a small physical space.  
         [0035]    A wide range of motive power possibilities for the pump including small and large electric motors, battery, air, vacuum, water or hand operation.  
         [0036]    Pump sizing being scalable to provide a wide range of volume capabilities.  
         [0037]    Simple or complex electronics being incorporated to control the pump operation including dispense volumes and times.  
         [0038]    However, when seeking pumping solutions for a wider range of applications some limitations can arise. These can be characterised as follows.  
         [0039]    The pump can in some applications display restrictive lift capabilities due to limitations arising from the tube rebound properties and/or the tensions required for springs etc. in the inlet valve.  
         [0040]    Dependency on tube rebound properties can limit potential applications of the pump in terms of viscous fluids and chemical compatibility.  
         [0041]    Siphoning can still possibly occur through the pump where suction (vacuum) on the outlet is greater than the biasing force used to close the valves.  
         [0042]    One or more of these limitations can be overcome by the pump arrangement which incorporates the invention and is shown in a second embodiment in FIG. 2.  
         [0043]    As with the first embodiment the pump shown in FIG. 2 includes a length of silicone rubber (or equivalent) tube  10  an inlet valve  15  and an exhaust valve  16 . Once again valves  15  and  16  are contained in a pressure tight fit with housing  11 . In accordance with the first embodiment the operative mechanism is a small air cylinder that can generate positive and negative pressures. The cylinder  24  is connected to the housing  11  via port  17 .  
         [0044]    As will hereinafter be described, the inlet valve  15  is positioned within the tube  10  with one or more apertures which is/are actually closed by the tube. The exhaust valve  16  is in the preferred form of the invention identical to the inlet valve  15  except that the aperture(s) is/are located external of the housing  11 .  
         [0045]    [0045]FIGS. 2 and 3 show the pump in the “at rest” state. It will be observed that the piston  20  is located at the transfer port  25 . The chamber  19  and the chamber  11   a  in housing  11  are thus both at atmospheric pressure.  
         [0046]    [0046]FIGS. 2 and 6 show a valve body B which with the tube  10  forms each of inlet valve  15  and exhaust valve  16 . The valve body B comprises a tubular body  26  with a bore  26   a . The tubular body  26  is closed at one end by a wall  27  preferably formed integrally with body  26 . A peripheral outwardly projecting rib  28  extends from the body  26 .  
         [0047]    The tubular body  26  is inserted into the tube  10 . In the case of the inlet valve  15  the body  26  is inserted in the tube so that wall  27  thereof is inboard of the end of the tube  10 . The exhaust valve  16  is formed by body  26  inserted so that the end wall  27  is located outside the chamber ila formed in housing  11 .  
         [0048]    As show in FIG. 2 an external screw thread  29  is applied to each end of the housing II. An end cap  30  is coupled to each end of the housing  11 . The end cap  30  has an annular wall  31  with an internal screw thread  32  to facilitate this coupling.  
         [0049]    A concentric opening  33  is formed in the end cap  30 . Extending through this opening  33  is a fitting  34 . This fitting has a peripheral rim  35  at one end so that it engages not only with the underside of the top  36  of the cap  30  but also the tube  10  where the tube extends over the peripheral rib  28 . Thus by screwing on the end cap  30  not only is the fitting  34  attached but also the valve body  26  is located firmly in position so that it cannot move axially relative to the tube  10 . Equally the tube  10  is also anchored into position so that it is held in a correct position within the housing  11 .  
         [0050]    In use appropriate conduits will be coupled to the pump via fittings  34 .  
         [0051]    Located adjacent end wall  27  of each valve body  26  is a plurality of radial ports  37 .  
         [0052]    The tube  10  where it fits over valve body  26  thus actually forms a part of the valve mechanism. Hence an extremely simple yet effective valve is formed. In the “at rest” state of the pump the tube  10  forms a seal over the ports  37  of the inlet valve  15 . This is shown in FIGS. 2 and 3.  
         [0053]    [0053]FIG. 4, shows that when a negative pressure is applied to the chamber  11   a  in housing  11  tube  10  is caused to expand and this expansion lifts the portion  10   a  of tube  10  off the outer wall surface of the body  26  adjacent the end wall  27  thereby opening the port(s)  37 . This allows liquid from an input conduit (not shown) fixed to the inlet fitting  34  to flow into and fill the tube  10 . Outflow from the tube  10  is prevented due to the sealing effect of portion  10   b  of the tube  10  over the outlet port(s)  37  of the valve body  26  of exhaust valve  16 .  
         [0054]    When the air cylinder pressure increases by movement of the piston  20  toward the transfer passageway  18 , the tube  10  is forced to collapse inward (see FIG. 5) thereby increasing the pressure of liquid in the tube which forces the portion  10   b  of tube  10  to move off the port(s)  37  of the body B of exhaust valve  16 . Fluid thus flows through the exhaust valve  16  and into an outlet conduit (not shown) attached to the outlet fitting  34 . This pressure increase in the chamber  11   a  in housing  11  on the other hand causes the tube portion  10   a  where it fits over body  26  of inlet valve  15  in the vicinity of port(s)  37  to maintain a good seal over the port(s)  37  of the inlet valve  15 .  
         [0055]    [0055]FIG. 3 shows a chamber or clearance  38  formed in the wall of housing  11  by a counterboring within housing  11  adjacent inlet valve  15 . This provides a clearance for the tube  10  to be lifted by the negative pressure build up from the port end of the inlet valve body  26  such that liquid can flow through the ports  37  and into the main body of the tube  10 . Chamber  38  is shown occupied by the lifted wall portion  10   a  of tube  10  in FIG. 4.  
         [0056]    As the piston  20  retreats along cylinder  24  the tube  10  reverts to its non-deformed state thereby causing the area  10   a  of tube  10  to once again seal over the port(s)  37  of outlet valve  16 .  
         [0057]    With the pinch mechanism according to the present invention, no rigid pushers or rollers make contact with and pinch the tube. Therefore, significantly longer tube life is achieved.  
         [0058]    Also efficient operation is achievable because the mechanism operates with very little friction, consequently motor power efficiency can be extremely high. Indeed, in applications where it may be desirable a battery power source could be used.  
         [0059]    It is believed that the gradual build up of pressure acting on the tube  10  and the gradual development of the negative pressure in the chamber also results in less wear and tear on the tube  10 . Furthermore the gradual pressure changes (rather than a sudden change of the type typical with known mechanisms of this type) improves flow characteristics within the tube  10  can be achieved.  
         [0060]    With known pumps of this type the means of driving the tube in the chamber can involve a compressed pressure source and a vacuum source. Consequently a complex arrangement of valves, control gear and compressors/vacuum pumps is required. Not only does this represent a capital cost in plant but also higher running costs. The present invention thus represents a radical departure by using the piston  20  in cylinder  19  with transfer port  25  to generate the required positive and negative pressures to operate the tube. The overall result is an effective and economic means of driving the pump with reduced maintenance and running costs.  
         [0061]    Previous proposals to reduce the capital costs and running costs mentioned above with prior pumps of this type have included a piston in cylinder arrangement charged with a hydraulic fluid. However, such arrangements are prone to leakage thereby resulting in the need to routinely recharge the cylinder. Also leakage into the chamber is possible and if this leakage of hydraulic fluid takes place into the tube then a serious problem exists, especially if the pump is being used in a food or medical situation.  
         [0062]    To overcome this latter problem it has been proposed to charge the cylinder with air or other gaseous medium. However, once again leakage can result in a drop off of performance and thus a need to routinely recharge the cylinder.  
         [0063]    With the present invention there is no air consumption during operation. Any leakage which does occur (say due to a worn piston seal) is automatically replenished when the piston passes through the zero pressure point i.e. passes the transfer port  25 .  
         [0064]    From a commercial point of view, the low number of parts making up the pump provides benefits not only at the initial costs but also ongoing costs. Because of the construction and its operation it is believed that maintenance costs can be kept low.  
         [0065]    A further factor which contributes to the favourable maintenance characteristics of the pump is in the area of the seal(s)  22  on piston  20 . Because the pressure within cylinder  24  is essentially at atmospheric pressure when the seal(s)  22  pass over the ends of transfer port  25  there is little or no tendency for the seal(s) to be pushed into the port. Thus seal  22  is not subjected to damaging contact with the port  25  and hence a long seal life is achieved.  
         [0066]    The pump exhibits good characteristics of dry and wet priming. With the second embodiment the effectiveness of the valves will ensure that no siphoning occurs.  
         [0067]    With the present invention there is no requirement that tube  10  have rebound characteristics. Indeed tube  10  can be of thin wall construction (e.g. in the nature of a membrane) which exhibits no rebound characteristics. For the food and medical industries the thin wall tube can be made of a suitable grade polyurethane.  
         [0068]    Because of its design the pinch mechanism when in a pump configuration develops good suction aided by the negative pressure on the inlet strike. The level of suction can be altered by design. Output pressure can be preset by adjustment to the air cylinder. The output pressure is also limited by the drive pressure ensuring the pump, and the equipment that may be attached, will not overload. A pressure relief switch is therefore not required. Furthermore without heat generation or abrasion dry running can occur without damage.  
         [0069]    A problem which often arises with pumps of this type occurs at the inlet valve. The operation of the inlet valve generally relies on the negative pressure in the tube to lift valve element from the valve seat. This requires a pressure differential to occur at the valve and consequently a pressure drop will take place which can have an adverse impact on the flow into the tube.  
         [0070]    With the present invention, however, the operation of the inlet valve is actively driven by the negative pressure build up in the chamber. Consequently there is no or little pressure differential across the inlet valve. This active control of the inlet valve also occurs at closure of the valve due to the build up of greater than atmospheric pressure in the chamber.  
         [0071]    In the modified form of the invention the port  17  can be located immediately adjacent the inlet valve  15 . Consequently the pressure change in the chamber commences in the vicinity of the valve which results in even better active control of the lifting from or sealing on of the tube  10  with the port(s)  37 .  
         [0072]    It is envisaged that a series of housings and tubes could be located adjacent one another and operated simultaneously from one source of positive pressure followed by the application from the same source or a separate source of a negative pressure. Therefore one driving arrangement could be used to operate a series of tubes  10  within housing II.  
         [0073]    Other modifications, common uses and different arrangements will be apparent to those skilled in the art within the context of the present invention.