Abstract:
A diaphragm for use in a fluid pump comprising a disc of resilient material having a substantially dished shape. The curvature of the disc is formed from a plurality of steps.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims the benefit of and priority to Great Britain (GB) Application Serial No. 0716294.4, filed on Aug. 21, 2007, the entire content of which is incorporated herein by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a diaphragm for use in a fluid pump, and more particularly, to a diaphragm configured to be coated with a protective layer such that the protective layer would avoid fracturing in use. 
         [0004]    2. Background of Related Art 
         [0005]    Diaphragm-type fluid pumps and, in particular liquid pumps, have a flexible pumping diaphragm which is driven to effect the pumping action of the pump. In such pumps, the diaphragm comprises a flexible circular or oval disc which has its outer peripheral edge clamped and sealed within the body of the pump. The diaphragm may have a central aperture which is secured to a moveable actuator such as a piston, which reciprocates back and forth causing the diaphragm to flex between a concave and a convex configuration. In an alternative pump configuration, however, the diaphragm does not have a central aperture and forms a partition with a driving fluid chamber in which the hydraulic pressure of the driving fluid is repeatedly alternated between high and low pressures, thereby causing the diaphragm to flex between a concave and a convex configuration. In both pump types, the repeated flexing of the pump diaphragm causes fluid displacement in a pumping chamber which results in the pumping action. 
         [0006]    When such pumps are used to pump inert or un-reactive fluids, such as water, the diaphragm can be constructed from plain rubber material, since there is no problem with the fluid reacting with or corroding the diaphragm material. However, if the pump is intended for use with fluids having a more chemically reactive nature, an elastomeric diaphragm, such a natural rubber, alone is unsuitable as it is rapidly corroded, leading quickly to pump failure. 
         [0007]    In order to solve this problem, it is desirable to provide the elastomeric diaphragm with a protective coating to prevent the pump fluid from reacting with or corroding the elastomer. However, the materials which would be most desirable to use for such a protective layer due to their excellent chemical resistance properties, such at PTFE, have plastic material properties, meaning they are unable to stretch and recover their original shape. However, when conventional pump diaphragms are in use, the repeated flexing between convex and concave positions causes the rubber of the diaphragm to repeatedly stretch and deform. Therefore, if a conventional diaphragm is coated with a protective layer, such as PTFE, it results in the protective coating cracking and splitting when the pump is in use, since the protective layer cannot cope with the repeated elastic deformation which the diaphragm experiences. 
         [0008]    It is therefore an object of the present invention to provide a pump diaphragm that substantially alleviates or overcome the problems mentioned above. 
       SUMMARY 
       [0009]    Accordingly, the present invention provides a diaphragm for use in a fluid pump comprising a disc of resilient material having a substantially dished shape, the curvature of the disc being formed from a plurality of steps. 
         [0010]    Preferably, the diaphragm is circular, but may also be oval. 
         [0011]    The plurality of steps are preferably formed from a series of flat annular rings of increasing diameter axially displaced from one another and joined at their adjacent edges by shoulder portions extending in an axial direction. 
         [0012]    In a preferred embodiment, the diaphragm has a substantially inelastic chemically resistant coating on at least one side thereof. The chemically resistant coating may be formed on the concave side of the diaphragm or may be formed on the convex side, or may be formed on both/all sides of the diaphragm. 
         [0013]    Preferably, the chemically resilient material is PTFE. 
         [0014]    The resilient material may be rubber. However, if the diaphragm is for use in a hydraulic pressure type pump in which the diaphragm deflection is achieved by alternating hydraulic pressure of a driving fluid, then the resilient material will need to be compatible with the hydraulic media (e.g. oil). In such cases, the resilient material may be nitrile or a low-temperature resistant rubber material. 
     
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         [0015]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0016]      FIG. 1  shows a schematic view of a prior art diaphragm pump; 
           [0017]      FIG. 2  shows a series of cross-sectional views of a prior art pump diaphragm as used in the pump of  FIG. 1 ; 
           [0018]      FIG. 3  shows a perspective view of a pump diaphragm according to the present invention in its un-deflected natural state; 
           [0019]      FIG. 4  shows a cross-sectional view along the line X-X shown in  FIG. 3 ; and 
           [0020]      FIG. 5  shows a cross-sectional view of the diaphragm of  FIG. 4  in a deflected position. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0021]    An example of a known diaphragm pump  10  is shown schematically in  FIG. 1  and comprises a chamber  11 , through which fluid being pumped flows, having an inlet  12  and an outlet  13 . The inlet  12  includes a one-way valve  14  which allows fluid to flow into the chamber  11  though the inlet  12  but not out of the chamber  11  therethrough, and the outlet  13  includes a one-way valve  15  which allows fluid to flow out of the chamber  11  through the outlet  13  but not into the chamber  11  therethrough. A flexible diaphragm  16  is mounted in the wall of the chamber  11  separating the chamber  11  from a cavity  17 . The diaphragm  16  is connected at its centre to a piston  18  of a pump driver  19 . The piston  18  is driven backwards and forward in the direction shown by arrow ‘A’ to cause the diaphragm  16  to deform between position I in  FIG. 1  where it extends into the cavity  17 , and position II in  FIG. 1 , where it extends into the chamber  11 . As discussed above, the prior art pump described here is of the type where the diaphragm deflection is actuated by a piston. However, alternative embodiments of prior art pumps use a driving fluid on the side of the diaphragm remote from the fluid being pumped, where the driving fluid pressure is alternated, which causes the diaphragm to flex. In such embodiments, the diaphragm clearly does not have the central aperture. 
         [0022]    The fluid to be transported is caused to flow through the diaphragm pump chamber  11  by repeated reciprocation of the piston  18  between positions I and II. When the diaphragm  16  extends to position I, the volume of the chamber  11  is increased and the fluid pressure therein is reduced. This causes the fluid outside the chamber  11 , which is unable to pass into the chamber  11  through the outlet one-way valve  15 , to be drawn into the chamber  11  through the inlet  12  through the inlet one-way valve  14 . Then, when the diaphragm  16  extends to position II, the volume of the chamber  11  is reduced and the fluid pressure therein is increased. Therefore, the fluid in the chamber  11 , unable to pass through the inlet one-way valve  14 , is forced out of the outlet  13  through the outlet one-way valve  15 . As this cycle is repeated, the resulting repeated fluid displacement causes the fluid to be pumped through the diaphragm pump  10  from the inlet  12 , through the chamber  11  and out of the outlet  13 . 
         [0023]    It will be appreciated that if the fluid being pumped is reactive or corrosive, then the diaphragm  16  will need to be provided with a protective layer interposed between the fluid and the rubber material of the body of the diaphragm  16 , to prevent the diaphragm  16  from being corroded and causing the pump  10  to fail. It is important then, that this protective layer remains intact at all times to protect the rubber diaphragm  16  underneath. In the prior art pump shown, the diaphragm  16  deflects between positions I and II, and in doing so, the surface is stretched and compressed. This can be seen more clearly from  FIG. 2  which shows the prior art diaphragm  16  in more detail in three positions, namely positions I and II at the most concave and convex positions in its range of motion, and also at a third position III, which is intermediate positions I and II where the diaphragm is deflected into a flat shape. The side of the diaphragm exposed to the fluid being pumped is on the concave side in  FIG. 1 . A reference distance between two radially-spaced points a,b on the concave side of the diaphragm in position I is shown as d 1 . As the diaphragm is forced to deflect to position III, the deformation of the diaphragm causes the distance between the same two reference points a,b to reduce to d 2 , as the surface of the diaphragm compresses. Then, as the diaphragm deflects further to position II, the distance between the same two reference points increases to d 3  as the surface of the diaphragm stretches again. In this range of movement, the relationship between the distances is as follows: 
         [0000]      d3&gt;d1&gt;d2 
         [0024]    Therefore, if a protective layer is bonded to the diaphragm which has plastic properties, such as PTFE, the layer cannot cope with the repeated elastic stretching and compressing deformation that the diaphragm undergoes, and so the layer cracks. In use, these cracks would expose the rubber material of the diaphragm to the corrosive fluid being pumped, and so would cause the diaphragm to corrode and the pump to fail. 
         [0025]      FIGS. 3-5  show a diaphragm  100  of the present invention for use in a pump such as that shown in  FIG. 1 , that does not suffer the drawbacks described above of known prior art pump diaphragms. 
         [0026]    It can be seen that the diaphragm  100  is generally dish-shaped, as are known pump diaphragms, but its dish-shape is formed by a series of tiers or stepped layers  101  in the diaphragm material which are each displaced from one another in an axial direction of the central axis Y-Y of the diaphragm  100 . Each layer  101  is formed as a flat annular ring, the rings increasing in diameter to form the tiers or steps, the inner peripheral edge  101   a  and the outer peripheral edge  101   b  of each ring being connected to the outer/inner peripheral edge  101   b ,  101   a  respectively of the adjacent ring  101  by shoulder portions  102  which extend in an axial direction. 
         [0027]    The embodiment of the invention is shown having a chemically protective layer  103  of PTFE coated on the concave side of the diaphragm, seen more clearly in  FIGS. 3-4 . However, the pump could be configured such that the diaphragm is secured the other way round in the pump, in which case, the chemically protective layer would be coated on the convex side of the diaphragm, so that it is on the side in contact with the fluid being pumped. 
         [0028]    The diaphragm  100  includes a central aperture  104  for a piston of a driver of a fluid pump to be secured thereto in a known manner, as schematically illustrated with the prior art device in  FIG. 1 . 
         [0029]    As explained above in reference to  FIGS. 1 and 2  of a prior art fluid pump and diaphragm, the diaphragm  100  of the present invention is repeatedly moved from its natural shape, shown in  FIG. 4  to a deflected position, as shown in  FIG. 5 . In this repeated movement, the annular rings  101  do not stretch. Instead, they flex such that much of the deflection of the diaphragm  100  is effected at the annular rings  101 . In addition, in the deformation of the diaphragm  100 , the wall or shoulder portions  102  do not themselves bend or flex at all, but remain un-deformed. Therefore, the whole diaphragm  100  is able to repeatedly deflect between concave and convex positions without any part of its surface stretching to any significant degree. Therefore, the coating of PTFE  104  on the concave side (or convex side, in reversed diaphragm embodiments of the pump) of the diaphragm  100  in its natural position is not put under any strain to stretch during the repeated deflections and so the PTFE coating is not at any risk of cracking or fracturing in operation of a fluid pump having such a diaphragm  100  of the present invention. 
         [0030]    The above embodiment is described having a protective coating of PTFE applied thereto. However, the scope of the invention is not limited to the diaphragm having a protective coating and includes an uncoated diaphragm of the configuration to accept such a coating without fracturing in use, as defined in claim  1 . In addition, other protective coatings may be used in conjunction with the diaphragm of the present invention aside from PTFE. 
         [0031]    Although a coating is shown on the concave side of the diaphragm, it may also be provided on both sides thereof to entirely coat the diaphragm, or may be provided on the opposite side thereof. 
         [0032]    Although the embodiment of the diaphragm shown is circular, it may also be an oval disc shape within the scope of the claims. 
         [0033]    The embodiment shown and described is for use in a fluid pump where the diaphragm flexing is actuated by a driving piston. However, the invention is not limited to such a diaphragm, and also covers diaphragms for use in hydraulically actuated fluid pumps, as described above. In such embodiments, the diaphragm would not have a central aperture  104 . 
         [0034]    Although the embodiment shown and described is configured with the diaphragm positioned with the concave side proximate the fluid being pumped, the invention is not limited to this configuration, and the diaphragm may be suitable for use with the convex side proximate the fluid being pumped, in which case, the protective coating would be provided at least on the convex side of the diaphragm.