Abstract:
A pump uses tubular transfer members for transferring intake and/or exhaust air into the crankcase and/or between valve head chambers. The pump has compact 180° opposed pistons that minimize axial spacing between the pistons on the drive shaft and thereby reduces the shaking couple and noise from reciprocation. Each piston has its own eccentric element press-fit into the connecting rods so as not to occupy space between the pistons. The shaking couple can be further reduced for pistons of different masses by selecting the mass of the cup retainers to compensate for the difference in overall piston masses. The pump includes an improved cylinder sealing arrangement having a circumferential groove in an angled surface at the end of the cylinder. The pump also has a special cover and seal for closing the open neck of the pump crankcase and a multi-lobed valve stop.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     This is a divisional of U.S. patent application Ser. No. 10/338,950 filed Jan. 8, 2003, now issued as U.S. Pat. No. ______. 
     
    
     STATEMENT OF FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not applicable.  
       BACKGROUND OF THE INVENTION  
       [0003]     The present invention relates to pumps and in particular to compact piston pumps.  
         [0004]     Pumps for medical applications, such as used in oxygen concentrators, generally need to be compact and quiet to operate indiscreetly in homes and hospitals. It is thus important to properly muffle the working air as well as to reduce vibration during operation of the pump.  
         [0005]     One problem with conventional pumps is that they can create excessive noise and vibration as the piston(s) are reciprocated, especially if they are improperly balanced. One reason for this in opposed piston pumps is that the pistons may be coupled to the drive shaft by a single retainer or eccentric element between the connecting rods of the piston. Ordinarily, an eccentric element is mounted to the drive shaft and two nibs or bosses extend axially from each side of the eccentric element to mount the pistons to the drive shaft. A moment, or shaking couple, arises as the drive shaft is turn because of the axial spacing between the pistons.  
         [0006]     Another problem with conventional pumps is sealing the crankcase and cylinder(s). Improper sealing of the cylinders to the crankcase or the valve head(s) can cause pressurized air to leak to the outside of the pump, which both reduces pumping efficiency and makes noise. Typical sealing arrangements are either prone to leakage or require costly machining operations on the valve plate or other valve mounting member. Also, many crankcases are made with open necks to allow the pistons to be slid into the crankcase easily during assembly. Typically, the openings in the neck terminate at the cylinders, which have curved exterior surfaces. This makes sealing the crankcase difficult and typically requires separate seals in addition to that sealing the end of the crankcase, thus increasing assembly complexity and creating a potential leak path between the neck seals and the end seal.  
         [0007]     Yet another problem confronting the design of low-noise pumps is properly muffling the working fluid chambers, e.g., the intake and/or exhaust chambers of the valve heads. This can be done by attaching a muffler element to the valve head either directly or via suitable hoses. Another technique is to run the exhaust air into the crankcase on the non-pressure side of the piston head. In this case, if the crankcase is closed and the pistons are in phase, the crankcase will usually be vented through a muffler to avoid generating pulsations in the pump. Even using the later technique, the valve heads are usually exhausted through hoses leading to the crankcase, which is vented through a muffler directly mounted to the crankcase or at the end of a hose.  
         [0008]     Accordingly, an improved pump is needed which addresses the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention provides a pump with one or more transfer tubes that communicate through one or more passageways in the crankcase for passing working fluid from one or more valve head chambers to the crankcase or to another valve head chamber. This provides integrated connections that are resistant to leakage and vibration, and provides opportunities for muffling, communication with the crankcase chamber and communication between chambers of multiple valve heads of the pump.  
         [0010]     In one useful form, a pump of the invention has a crankcase defining a pumping chamber and a transfer opening. A valve mounting member mounted to the crankcase over the pumping chamber has at least one port in communication with the pumping chamber that is opened and closed by a valve mounted to the valve mounting member. At least one working fluid chamber defined at least in part by the valve mounting member on a side of the valve mounting member opposite from the pumping chamber is in communication with the pumping chamber through the port when the valve is open. A transfer port is in communication with the working fluid chamber outside of the pumping chamber. A transfer tube is connected at one end to the transfer port and at the other end to the crankcase transfer opening.  
         [0011]     In a particular embodiment, the pump is a 180 degree opposed piston pump with both pistons located to one or the other side of the motor. The pump has a crankcase defining a chamber, a cylinder and a transfer opening. A valve plate type valve mounting member is mounted to the cylinder. The valve plate has at least one port in communication with the working air inside of the cylinder. The port is opened and closed by a valve mounted to the valve plate. A valve head is mounted to the valve plate to define the chamber. The valve plate further has a transfer port located in the chamber. The transfer tube is connected between the valve plate transfer port and the crankcase transfer opening.  
         [0012]     Multi-cylinder pumps can have multiple transfer tubes connected to one or more transfer ports in the valve plate for each cylinder. For example, the transfer tubes can couple the intake or exhaust chambers to the inside of the crankcase, or they can couple multiple exhaust chambers together and/or multiple intake chambers together or the exhaust chamber of one valve head to the intake chamber of another valve head.  
         [0013]     The crankcase can form integral passageways leading from one or more transfer openings at which the transfer tube(s) are connected. The passageway can open into the crankcase chamber in phase or run between transfer openings to join one or more chambers of one valve head with the chamber(s) of another valve head.  
         [0014]     In preferred forms, the passageways and transfer tubes have opposing flat side walls. The transfer tube can be separate from the valve plate and the crankcase or formed as a unitary part of either the crankcase or the valve plate or both. Resilient seals can be disposed between the ends of the transfer tubes and a transfer opening in the crankcase and/or the intake and exhaust transfer ports in the valve plates as needed. The transfer tube(s) can be made of a resilient material and have stepped ends sized to fit into transfer ports. Preferably, the transfer tube(s) are clamped between the valve plate(s) and the crank case.  
         [0015]     The invention thus provides a compact pump with considerable noise reduction and improved efficiency. These and other advantages of the invention will be apparent from the detailed description and drawings. What follows is a description of the preferred embodiments of the present invention. To assess the full scope of the invention the claims should be looked to as the preferred embodiments are not intended as the only embodiments within the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a perspective view an opposed piston pump of the present invention;  
         [0017]      FIG. 2  is a perspective view of the pump showing its piston assemblies exploded;  
         [0018]      FIG. 3  is another perspective view of the pump showing one of its cylinder and valve head assemblies exploded;  
         [0019]      FIG. 4  is an exploded perspective view showing one valve assembly in isolation;  
         [0020]      FIG. 5  is an enlarged partial cross-sectional view taken along arc  5 - 5  of  FIG. 9  showing a cylinder seal in a circumferential groove in an angled end of the cylinder;  
         [0021]      FIG. 6  is an enlarged partial cross-sectional view taken along line  6 - 6  of  FIG. 9  showing an assembly for sealing the open neck of the pump housing;  
         [0022]      FIG. 7  is a cross-sectional view taken along line  7 - 7  of  FIG. 1  showing the pump (without the intake and exhaust valves) with its pistons 180° out of phase and one piston at top dead center and the other at bottom dead center and with the valve heads coupled;  
         [0023]      FIG. 8  is a cross-sectional view similar to  FIG. 7  albeit with the pistons in a position 180° from that of  FIG. 7 ;  
         [0024]      FIG. 9  is a cross-sectional similar to  FIG. 7  showing the pump with its pistons in phase at bottom dead center and with one valve head exhausted to the crankcase and the other exhausted to the load;  
         [0025]      FIG. 10  is a cross-sectional view similar to  FIG. 9  albeit showing the pistons at top dead center;  
         [0026]      FIG. 11  is a cross-sectional view taken along line  11 - 11  of  FIG. 9 ;  
         [0027]      FIG. 12  is a cross-sectional view taken along line  12 - 12  of  FIG. 9 ;  
         [0028]      FIG. 13  is an enlarged partial cross-sectional view showing one valve assembly;  
         [0029]      FIG. 14  is a cross-sectional view taken along line  14 - 14  of  FIG. 9 ;  
         [0030]      FIG. 15  is a cross-sectional view taken along line  15 - 15  of  FIG. 14  with an exhaust side flapper valve closed;  
         [0031]      FIG. 16  is a view similar to  FIG. 15  albeit with the valve shown open;  
         [0032]      FIG. 17  is a cross-sectional view taken along line  17 - 17  of  FIG. 12 ;  
         [0033]      FIG. 18  is an enlarged partial cross-sectional view taken along arc  18 - 18  of  FIG. 17 ;  
         [0034]      FIGS. 19-21  are enlarged partial cross-sectional view taken along line  19 - 19  of  FIG. 17  showing various alternate constructions of a transfer tube;  
         [0035]      FIG. 22  is a perspective view of an alternate embodiment of the pump of the present invention with different sized cylinders and pistons;  
         [0036]      FIG. 23  is a cross-sectional view taken along line  23 - 23  of  FIG. 22  showing the pump (without the intake and exhaust valves) operating as a pressure-vacuum pump with its pistons in phase at bottom dead center and with the larger valve head exhausted to the crankcase;  
         [0037]      FIG. 24  is a cross-sectional view similar to  FIG. 23  albeit showing the pistons at top dead center; and  
         [0038]      FIG. 25  is a cross-sectional view taken along line  25 - 25  of  FIG. 22 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0039]      FIGS. 1-4  illustrate a pump  30  according to the present invention. Generally, the pump  30  has a motor  32  mounted in an inverted manner in a top opening  34  of a housing or crankcase  36  containing two piston assemblies  38  and  39 . Two cylinders  40  and  41  are mounted to the crankcase  36  in respective side openings  42  and  43 . Valve plates  44  and  45  and valve heads  46  and  47  are mounted to the outer ends of the respective cylinders  40  and  41 . A cover/seal assembly  48  is mounted to the open neck  50  of the crankcase  36  over a bottom end opening  52  so that the interior of the crankcase is completely enclosed when the pump is assembled.  
         [0040]     Referring to  FIGS. 1, 3  and  5 , more specifically, to improve the seal between the cylinders  40  and  41  and valve plates  44  and  45 , the outer rims of each cylinder are tapered radially inward to define an angled surface  54  (one shown in  FIG. 5 ) with a circumferential groove  56  therein sized to a retain seal  58 , preferably a resilient o-ring. Each of the valve plates  44  and  45  have an underside with a circular angled surface  60  against which the seal  58  can seat when the pump is assembled. The cylinders  40  and  41  are clamped to the crankcase  36  by fasteners  63  connecting the valve heads  46  and  47  to the crankcase  36  which compresses the seals between the grooves and the respective seats of the valve plates. This assembly provides a good seal as well as promotes serviceability in that the angled surfaces reduce the occurrence of the o-ring sticking to the valve plate over time and locking the valve plate to the cylinder. Also, the inwardly angled seat can be formed during casting of the valve plate without the need for additional machining.  
         [0041]     Referring to  FIGS. 2 and 6 , the cover/seal assembly  48  improves the seal at the bottom opening  52  and open neck  50  of the crankcase  36 . The unique cover/seal assembly  48  includes a resilient seal  64  and a rigid backing plate  66 . In particular, the seal  64  is a generally ring shaped structure defining a central opening  68  and sized to fit onto the open end  52  of the crankcase  36 . The seal  64  defines two axially extending neck plugs  70  and  71  at opposite locations on the ring, for example at the 12 and 6 o&#39;clock positions. The neck plugs  70  and  71  are sized and shaped to fit into the openings  72  and  73  in the neck  50  of the crankcase  36 . The neck plugs  70  and  71  define concave sealing surfaces  74  and  75  shaped to fit against the convex contour of the outside of the cylinders  40  and  41 . The sealing surfaces  74  and  75  have pointed ends that fit snugly against the intersecting surfaces of the neck  50  and the cylinders  40  and  41  (see  FIG. 6 ). The seal  64  also defines two channel plugs  76  and  77  extending radially outward from the ring at the 3 and 9 o&#39;clock positions. These channel plugs  76  and  77  fit into the end of channels  78  and  79  formed in the crankcase  36  (as discussed below). The seal  64  is retained by the backing plate  66 , which is generally a circular plate with four openings  80  through which four fasteners  82  are disposed to fasten the cover/seal assembly  48  to the crankcase  36 . The backing plate  66  has axially extending plug supports  84  and  85  aligned with the neck plugs  70  and  71  with curved edges  86  and  87  contacting ledges  88  and  89  defined by the neck plugs  70  and  71 . The backing plate  66  also has two tabs  57  and  59  located and sized to support respective channel plugs  76  and  77  of the seal  68 .  
         [0042]     The plug supports  84  and  85  help maintain the seal of the neck plugs  70  and  71 . However, the pointed corners of the neck plugs  70  and  71  can flex away from the crankcase and cylinders somewhat to allow a leak path to relieve transient high pressure situations. The seal is designed primarily for low pressure applications to seal off air leaks for noise reductions. The corners of the neck plugs will unseat slightly when the internal pressure reaches about  15  psi as a pressure relief. The assembly could, of course, be used in higher pressure applications by using a more rigid elastomer or modifying the backing plate to prevent the seal from unseating.  
         [0043]     Referring to  FIG. 2 , the piston assemblies  38  and  39  each include pistons  90  and  91  and with heads  92  and  93 , forming pan sections having pistons seals  94  and  95  mounted by retainers  96  and  97  (shown in phantom), and connecting rods  98  and  99  defining circular openings  100  and  101 , respectively. Bearings  102  and  103  (having inner races  104  and  105  rotatable with respect to outer races  106  and  107 , respectively) press-fit into the respective openings  100  and  101  to fix the outer races to the connecting rods  98  and  99 . Circular eccentric elements  108  and  109  are then press-fit into respective openings  110  and  111  of the bearings to fix them to the respective inner races  104  and  105 . The eccentric elements  108  and  109  have through bores  112  and  113  radially offset from their centers.  
         [0044]     Referring to  FIGS. 7, 8 ,  11  and  12 , the piston assemblies  38  and  39  are press-fit onto a drive shaft  114  of the motor  32  one at a time in the through bores  112  and  113  of the eccentric elements  108  and  109 , respectively. The drive shaft  114  is journalled to the crankcase  36  by bearing  116 . The crankcase openings  42  and  43  and cylinders  40  and  41  are offset somewhat to account for the different axial locations of each piston assembly  38  and  39  so that piston  90  reciprocates along the centerline of cylinder  40  and piston  91  reciprocates along the centerline of cylinder  41  allowing the piston seals  94  and  95  of each assembly creating a sliding seal with the inner surfaces of the cylinders.  
         [0045]     Importantly, the connecting rods  98  and  99  of the pistons  90  and  91  are mounted on the drive shaft  114  so that the connecting rods  98  and  99  are substantially adjacent to one another, that is within ⅛ inches (preferably less than {fraction (1/16)}″) or as close as possible. Preferably, the pistons are mounted on the drive shaft as close as possible with only air space between the connecting rods. This is to reduce the moment or shaking couple about the drive shaft  114  caused by the axial displacement of the piston assemblies  38  and  39 . While some moment remains, this arrangement provides a significant improvement over the prior art in that there is no other element (eccentric or otherwise) on the shaft between the pistons so that their axial displacement is minimized.  
         [0046]     As shown in  FIGS. 7 and 8 , the pump  30  can operate as a parallel pressure or parallel vacuum pump in which the pistons reciprocate 180 degrees out of phase.  FIG. 5  shows piston  90  at top dead center while piston  91  is at bottom dead center.  FIG. 6  shows the pistons when the drive shaft is rotated 180 degrees so that piston  90  is at bottom dead center when piston  91  is at top dead center. This configuration of the pump results from the eccentric elements  108  and  109  being mounted to the drive shaft  114  so that the through bores  112  and  113  in positions opposite 180 degrees with respect to their pistons. For example, the through bore  112  would be at a 12 o&#39;clock position (toward the piston head) and the through bore  113  would be at a 6 o&#39;clock position.  
         [0047]      FIGS. 9 and 10  show an alternate configuration in which the pump operates as a pressure-vacuum pump with the pistons reciprocating in phase (i.e., moving in and out of the cylinders in unison). In this case, the eccentric elements would be mounted to the drive shaft when both are in the same orientation with respect to their piston, for example, both through bores being at 12 o&#39;clock. This version of the pump can be otherwise identical to that shown in  FIGS. 1-4 .  
         [0048]     Air flow through the cylinders is controlled by the valving on the valve plates  44  and  45 . Referring to  FIGS. 3, 4 , and  13 - 16 , the valve plate  44  includes pairs of intake ports  120  and exhaust ports  122 . The pairs of intake  120  and exhaust  122  ports are separated by a partition  124  of the valve head  46  defining two intake  126  and exhaust  128  chambers. A specially shaped head seal  130  lies between the valve plate  44  and the valve head  46  to seal and isolate the two chambers  126  and  128 .  
         [0049]     The intake  120  and exhaust  122  ports are controlled by respective flapper valves  130  and  132 . The flapper valves  130  and  132  are identically shaped thin, metal valves. The valves  130  and  132  each have a middle section  134  defining an opening  136  and an alignment tab  139  as well as two identical paddles  140  extending from the middle section  130  in opposite directions approximately 30 degrees from vertical. The paddles  140  have narrow necks  142  and relative large flat heads  144 . The heads are sized slightly larger than the intake and exhaust ports and the necks are narrow to let the valves flex more easily under the force of the pressurized air, and thus reduce power consumption. Each flapper valve  130  and  132  is mounted to the valve plate  44  by a fastener  146  inserted through the opening  136  in the middle section  134  of the valve and threaded into bores in the valve plate. The intake valve  130  is mounted at the inside of the cylinder  40  and the exhaust valve  132  is mounted in the exhaust chamber  128 .  
         [0050]     Referring to  FIGS. 4 and 13 - 16 , because the exhaust valve  132  opens under the force of the compressed air in the cylinder, it is backed by a valve stop  138  preferably made of a rigid plastic. No valve stop is used (besides the piston) for the intake valve which opens during the expansion stroke. In particular, the valve stop  138  has a middle body  148  with an alignment tab  149  and an opening therethrough for the fastener  146 . Two arms  150  extend out from the body  148  at the same angles as the valve paddles  140 . Two hands  152  have fingers or lobes  154 , preferably three, extending outward and spaced apart at equal angles. The underside of the arms  150  and hands  152  tapers away from the valve plate, preferably with a slight convex curve, so that the lobes  154  are spaced away from the valve plate  44  enough to allow the valve paddles  140  to move sufficiently to open the ports. As shown in  FIG. 16 , the paddles follow the contour of the underside of the arms and lobes when opened and are supported along their entire length (except at the tips). The arms  150  are approximately the width of the valve paddle necks  142  and the lobes  154  are sized to support the entire paddle heads  144  to prevent them from hyper-extending at the narrow necks. Collectively, the underside of the lobes  154  are of less surface area than the paddle heads  144  and end inside of the boundaries of the heads. This design limits the surface contact between the paddles and thus reduces or eliminates valve chatter. This valve stop design has two main advantages: first, it reduces the surface attracting forces or “stiction” between these elements which could cause the valves to stick to the stop and remain open, and second, it reduces noise/vibration in the valves that would otherwise be present were the valve tips to contact the stops. It should also be noted that the valves are mounted to the valve plates with their middle sections disposed over recesses  156  shaped like the middle sections only larger. This allows the valves to be assembled and aligned by a fixture having pins that extend below the underside of the valves and into the recesses  156 . The alignment tabs  139  and  149  ensure that the valve and stop are in the proper orientation.  
         [0051]     Another feature of the pump  30  is the use of transfer tubes  158  with air passageways formed in the body of the crankcase  36  (outside of the internal chamber) to either couple an intake or exhaust chamber to the inside of the crankcase or to couple the valve heads together (in parallel between exhaust chambers and/or between intake chambers or in series with the exhaust chamber of one valve head connected to the intake chamber of the other valve head) without the need for hoses. Referring now to  FIGS. 11, 12  and  17 - 21 , the pump  30  includes small tubular members  158 , preferably having two opposite flat sides, extending from intake  160  and exhaust  162  transfer ports through the valve plates outside of the cylinders. In one preferred form, these transfer tubes  158  are formed as a unitary part of the valve plates (see  FIGS. 17 and 19 ). The free ends of the transfer tubes  158  are coupled to two sets of transfer openings  164  and  165  in the crankcase  26  preferably with a special resilient seal  166  therebetween having a flange  168  that fits inside the transfer openings  164  and  165  in the crankcase. It should be noted that the transfer tubes need not be integral with the valve plates but instead could be as shown in  FIGS. 20 and 21  in which they are entirely separate elements. In  FIG. 20 , each transfer tube  158 A is a separate rigid member with (or without) stepped ends mounting resilient seals  166 A. Or, as shown in  FIG. 21 , each transfer tube  158 B could be made of a entirely of a resilient material so that no separate seals are needed. Preferably, it would have stepped ends that fit inside the corresponding openings in the crankcase and valve plate.  
         [0052]     As mentioned, the crankcase  36  has two sets of interior passageways  170  and  171  in the walls of the crankcase opening at the transfer openings  164  and  165 . Depending on the desired operation of the pump, there can be only one of these passageways  170  and  171  or one set of these passageways in one side of the crankcase. One or both of these passageways may also open to the channels  78  and  79 , which open to the interior of the crankcase. This can be done by boring through section  174  or by casting the crankcase to block off or connect passageways as needed. In the parallel pressure embodiment of the pump shown in  FIGS. 11, 17  and  18 , preferably the passageways  170  and  171  couple the exhaust chambers of each valve head and the intake chambers of each valve head. In this way, the load can be connected at a hose barb or socket of either of the intake chambers (to pull a vacuum) or either of the exhaust chambers (to provide pressure) or both, without connecting to both of the intake chambers and/or exhaust chambers. A suitable muffler (not shown) can be connected to either the intake or exhaust side if not otherwise connected to a load.  
         [0053]      FIGS. 22-25  show another preferred pressure-vacuum embodiment of the pump  30 C such as can be used in a medical application, such as an oxygen concentrating apparatus. This embodiment of the invention is identical to that described above, with the following exceptions. Here, cylinder  40 C, valve plate  44 C, valve head  46 C and the head of piston assembly  38 C are of a lesser size (diameter) than cylinder  41 C, valve plate  45 C, valve head  47 C and the head of piston assembly  39 C, respectively. Preferably, the smaller side is the pressure side and the cylinder  40 C has a 1.5 inch diameter and the larger side is the vacuum side with the cylinder  41 C having a 2 inch diameter. Preferably, in this embodiment, the piston assemblies  38 C and  39 C are in phase as shown in  FIGS. 23 and 24  (although they could be out of phase as well), the pressure side providing roughly 5 to 10 psi of pressure and the vacuum side drawing a vacuum of about −10 to −5 psi, which is preferred for oxygen concentrator devices.  
         [0054]     Since the pistons are of different sizes, they have different masses. The difference in masses will make the pistons out of balance and thus effect unequal moments on the drive shaft, which would cause vibration, noise and lower pump efficiency. Preferably, the retainers  96 C and  97 C are selected to have different masses, substantially equal to the difference in the masses of the other parts of the pistons (such as the connecting rods and the heads/pans). This can be accomplished by making the retainers  96 C and  97 C from disparate materials or of different thicknesses. For example, the retainer  96 C could be made of a suitable zinc composition so that it has a greater mass (despite its smaller diameter) than retainer  97 C, which could be made of an aluminum. Thus, the heavier retainer  96 C would make up the difference in mass of the smaller piston  90 C. The result is equally balanced piston assemblies and improved operation of the pump when the application requires different flow volumes in the cylinders.  
         [0055]     The pump also differs from that described above in that it has only one transfer tube  158 C connecting the exhaust side of valve head  47 C to passageway  171 C (through a transfer opening) in the crankcase  36 C. Passageway  171 C intersects with channel  78 C (as shown in  FIG. 25 ). The crankcase  36 C has no other internal passageways as did the previously described embodiment.  
         [0056]     This embodiment of the pump is thus constructed so that air can be drawn from the load (through a hose (not shown) connected to barb  200 ) and into the intake chamber of valve head  47 C. Surrounding air can also be brought in through barb  202  (to which preferably a muffler (not shown)) is mounted. Air from the higher pressure side valve head  46 C exhaust chamber will be exhausted through barb  204  to the load (after passing through hoses and valves as needed). The exhaust chamber of the vacuum side valve head  47 C will exhaust through the transfer tube  158 C and the crankcase passageway  171 C to the non-pressure side of the inside of the crankcase  36 C, which is vented through barb  206  and another muffler (not shown). Passing the exhaust through the crankcase prior to the muffler provides further (two-stage) sound attenuation beneficial in low-noise applications, such as when used with medical devices.  
         [0057]     It should be appreciated that preferred embodiments of the invention have been described above. However, many modifications and variations to these preferred embodiments will be apparent to those skilled in the art, which will be within the spirit and scope of the invention. For example, while only two-cylinder embodiments were shown, the principles of the invention could apply to a single-cylinder pump or to three or four cylinder pumps, such pumps having a double shafted motor and additional crankcases, cylinders, pistons and valve heads. For multi-cylinder pumps, the valve heads of all of the cylinders could be coupled in series or parallel through the transfer tubes and integral crankcase passageways, like those described above. Shared valve heads for multiple cylinders could also be incorporated into such a pump. The pump of the present invention could also include transfer tubes which connect directly to the valve heads/plates to join air chambers without connected to passageways in the crankcase.  
         [0058]     Therefore, the invention should not be limited to the described embodiments. To ascertain the full scope of the invention, the following claims should be referenced.