Abstract:
A linear piston pump has a multi-cavity air intake providing improved sound attenuation characteristics. The air intake is formed between a cover mounted in spaced relation to a pump housing so as to define an air intake passage leading around the periphery of the cover to an outer cavity separated from an inner cavity by a partition. Flow orifices are formed in the partition as well as in the inner cavity for air to flow to the intake chamber. Intake air re-expands in the inner cavity to dissipate pulsations in the air flow from the rapidly action of the intake valve, and thereby reduces intake noise. A filter and air baffle filter retainer may also be included to further reduce intake noise.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     Not applicable.  
       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 linear piston pumps.  
         [0004]     Pumps for certain duties, such as oxygen concentration and sewage aeration, generally need to be compact and operate discreetly. It is thus important to properly muffle the working air as well as reduce vibration during operation of the pump without relying on a large, thick-walled housing to attenuate the sound and vibration. Discreet operation of the pump can be obtained by insulating the housing, however, this adds bulk and can cause cooling problems. Mufflers can be added at the output, however, this adds hardware and cost.  
         [0005]     Such compact linear pumps/compressors are often single cylinder devices with a small piston that reciprocates rapidly within a small cylinder to pressurize the air. The rapid movement of the single piston generates considerable vibration. These vibrations are often transferred directly to the pump housing, via a direct rigid mounting connection.  
         [0006]     To facilitate reciprocation of the piston with less vibration, it is known to suspend the drive member, such as the armature of an electromagnetic motor, by springs or like flexible members. Stacks of thin metal leaf springs are well-suited for this. However, when multiple springs are used, it can be difficult to achieve the prescribed spring rate to which the piston drive components of the pump have been tuned. Changes in the angular (about the piston axis) and/or axial (along the piston axis) orientation of the springs relative to one another can effect the spring rate. To ensure that the pump operates efficiently, it is thus important to achieve the intended spring rate and thus ensure the consistent orientation of the suspension springs, which can make pump assembly difficult.  
         [0007]     Another problem is that the intake and exhaust valves of the valve head must open and close rapidly for each stroke of the piston. Typically, thin metal flapper valves are used for this purpose because of their ability to seat and unseat very rapidly. Since the exhaust port opens under the force of the compressed air, a valve stop is used to support the valve and prevent it from being hyper-extended beyond its elastic range. The rapid contact between the intake valve and the valve head or the exhaust valve and the valve stop can generate tapping or clicking sounds. Another problem is that the rapid opening and closing of the intake valve can cause pressure fluctuations or pulsations in the air flow upstream from the valve head. These air pulsations can generate a low-frequency, rumbling noise.  
         [0008]     Another problem confronting the design of compact linear piston pumps is eliminating pulsations in the output air stream. Pulsations in the air downstream from the outlet has been found to alter the resonant frequency of pump when different lengths and/or diameters of output lines are attached to the pump. Changing the operational frequency of the pump causes inefficiencies that can ultimately render the pump unusable for particular applications. It can also exacerbate noise and vibration issues.  
         [0009]     Yet another persistent problem in compact linear pump design is cooling. To decrease noise, or perhaps to make immersible or suitable for outdoor use, the working components of these pumps are often enclosed in a pump housing. With operation of the pump, friction and the current in the electromagnet coil generate heat. As is well understood, heat adversely affects the pump efficiency and life. Many times the need to keep the pump operating efficiently requires the housing to be vented or to have other measures taken which destroy, or at least significantly reduce, the noise retarding features of the housing or other components.  
         [0010]     Accordingly, an improved linear pump is needed that addresses the aforementioned problems.  
       SUMMARY OF THE INVENTION  
       [0011]     This invention is a linear pump with improved sound attenuation characteristics. Lower overall sound is accomplished by introducing the air through a multi-cavity intake in which the intake air is allowed to re-expand before passing further downstream into the pump.  
         [0012]     In particular, the linear pump includes an electro-magnetically driven piston reciprocating within a cylinder in communication with the multi-cavity air intake. The intake has at least two cavities. The first cavity is in communication with ambient air and is in communication with the second cavity through a first orifice. The second cavity has a second orifice allowing the intake air to pass downstream to an intake chamber and ultimately to the cylinder. The second cavity is sized to allow the intake air to re-expand before exiting the second orifice.  
         [0013]     In preferred forms, the linear pump includes a cover mounted to a pump housing to define the intake air passage, which preferably extends essentially around the entire periphery of the cover. The cover encloses (but for the intake air passage) the two cavities, one being located inward of the other outer cavity. The pump housing at least in part defines both the inner and outer cavities. A partition is mounted beneath the cover to seal off the inner cavity, except for at a pair of orifices in the partition. The outer partition orifices are preferably located near to one side of the intake than are the pair of orifices in the inner cavity. This offset arrangement causes the intake air to turn, approximately 90 degrees, in a serpentine path as it passes from the outer to the inner cavity.  
         [0014]     The intake also preferably include a filter. The filter can be any suitable foam, paper or mesh screen air filter commonly used to screen particulates from the intake air. Preferably, the filter is disposed in a retainer having a plurality of spaced apart riser elements spacing the filter from a base surface of the retainer, which defines the outer orifice, such that intake air can pass between the inner and outer cavities through the filter and around the riser elements.  
         [0015]     The present invention thus provides a quieter linear air pump. The unique intake design attenuates the noise, particularly a low pulsating rumbling noise, that is generated by the air rushing into the intake valve as the valve is opened and closed rapidly. The pulsations in the intake air are diminished in the cavities of the intake, particularly the inner cavity where the intake air is allowed to re-expand, and the associated noise is thus reduced. The offset arrangement of the orifices, which baffles and directs the intake air in a serpentine path, also aids in sound reduction. Similarly, passing the air through an air filter and routing the air around the raised projections of the air filter retainer further reduces noise. Still further, the pump housing enclosure reduces the noise of the pump as does the remote location of the working components and the intake chamber of the pump from the intake.  
         [0016]     These and other advantages of the invention will be apparent from the detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]      FIG. 1  is a perspective view of a linear pump according to the present invention;  
         [0018]      FIG. 2  is a perspective of the pump of  FIG. 1  shown with a housing shroud removed;  
         [0019]      FIG. 3  is an exploded assembly view of the pump;  
         [0020]      FIG. 4  is an exploded assembly view of a drive components of the pump;  
         [0021]      FIG. 5  is a sectional view taken along line  5 - 5  of  FIG. 6 ;  
         [0022]      FIG. 6  is a top view showing the pump with an inlet filter and top cover removed;  
         [0023]      FIG. 7  is a partial sectional view taken along line  7 - 7  of  FIG. 6  albeit with the inlet filter and top cover in place;  
         [0024]      FIG. 8  is a top view of the pump with the housing shroud removed and the drive components shown in phantom;  
         [0025]      FIG. 9  is a top view of a pump base showing the intake and exhaust chambers thereof;  
         [0026]      FIG. 10  is a sectional view take along line  10 - 10  of  FIG. 9 ;  
         [0027]      FIG. 11  is a partial sectional view taken along line  11 - 11  of  FIG. 9 ;  
         [0028]      FIG. 12  is a sectional view taken along line  12 - 12  of  FIG. 1 ;  
         [0029]      FIG. 13  is an enlarged partial sectional view taken along arc  13 - 13  of  FIG. 12 ;  
         [0030]      FIG. 14  is a sectional view taken along line  14 - 14  of  FIG. 2  albeit shown with the housing shroud in place;  
         [0031]      FIG. 15  is a partial sectional view taken along line  15 - 15  of  FIG. 2  albeit with the outer housing in place;  
         [0032]      FIG. 16  is a view of a retainer ring shown in isolation as viewed from line  16 - 16  of  FIG. 12 ; and  
         [0033]      FIG. 17  is a view of a leaf spring shown in isolation as viewed from line  17 - 17  of  FIG. 12 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     The present invention provides an axial or linear piston pump. The term pump used herein includes a device for providing either positive or negative pressure, and thus either acting as a vacuum pump or a compressor. The pump has a compact form factor, with a preferred operating range of 2-30 psi depending upon the application (however, the pump could be designed to operate at other pressures) with low external vibration and noise and less sensitivity to pump attachments (lines, hoses, tubing, etc.) downstream from the outlet.  
         [0035]     Referring to  FIGS. 1-3 , the pump, generally referred to in the drawings by reference number  20 , has a compact housing  22  including a base  24 , a shroud  26  and a top cover  28 . The shroud  26  is bolted to the base  24  (via bolts  23 ), with a gasket  30  therebetween (see  FIG. 15 ), and the cover  28  is bolted (via bolt  69 ) to the shroud  26 . The base has four feet  32  to support the pump  20  and provides an opening for connecting a fitting  34  (see  FIG. 11 ). The shroud  26  has an opening  35  for an electrical socket (or power cord). The cover  28  is spaced off of the shroud  26  slightly to define an air intake passage  36  along the periphery of the cover  28 , as will be described in greater detail below. The top of the shroud  26  and the cover  28  are recessed on opposite sides to provide a hand gripping area  38 .  
         [0036]     Referring to FIGS.  3 ,  5 - 7  and  12 , the pump  20  breathes through an air intake assembly generally designated  40 . The air intake  40  is configured to reduce the low, rumbling noise associated with pulsations in the intake air caused by rapid movement of the intake valve. In particular, the air intake  40  includes an inner cavity  42  defined in part by a recess in the top of the shroud  26 , a seal partition  44 , a filter tray  46 , a filter  48  and the cover  28 , which provides the upper boundary for an outer cavity  50 .  
         [0037]     As mentioned, the recessed top of the shroud  26  defines the inner cavity  42 , in the floor of which are two spaced apart orifices  52  near one end (the right end in the drawings). The inner cavity  42  is bounded at the top by the partition  44  which seals against a peripheral wall  54  of the inner cavity  42 . The partition  44  has a pair of spaced apart orifices  56  located at a (left) end of the inner cavity  42  opposite the orifices  52 . To facilitate assembly the partition  44  includes another set of orifices  56 ′ at the opposite end, however, these are not used when the partition  42  is oriented as shown in the drawings. In any event, intake air flows through only the one set orifices  56  (or  56 ′), which is located opposite the orifices  52  in the floor of the inner cavity  42 . Resting on the partition  44  is the filter tray  46  which holds the filter  48  in the outer cavity  50 . The filter tray  46  has a bottom wall with a pair of openings  58  which align with orifices  56  in the partition  44 . A small alignment feature  59  extends down from the underside of the filter tray  46  and fits into an opening  61  in the shroud  26  to ensure that the filter tray  46  is assembled in the proper orientation. The filter  48  is held spaced off of the bottom of the filter tray  46  by a number of small spaced apart risers  60 , and is retained by a short peripheral wall  62  ringing the filter tray  46 . The wall  62  has a cut-out  64  at one end (opposite the openings  58 ) allowing intake air to flow laterally into the filter  48 . The cover  28  fits onto the shroud  26  over the filter  48  to define the outer cavity  50 . Ribs  66  on the inside of the cover  28  contact the wall  62  of the filter tray  46  to keep the bottom edge of the peripheral wall  68  of the cover  28  spaced slightly from the shroud  26 , and also to funnel the intake air into the outer cavity  50  through the cut-out  64 . The space between the top cover  28  and the shroud  28  defines the intake air passage  36 , which extends around the periphery of the cover  28 . Aligned center openings in the cover  28 , filter  48 , filter tray  46  and partition  44  allow a bolt  69  to screw into a threaded opening  70  in the shroud  26  to secure the assembly.  
         [0038]     Referring now to  FIGS. 5, 7 ,  8  and  12 , the shroud  26  also integrally defines a pair of vent tubes  72  which extend down form the orifices  52  in the floor of the inner cavity  42 . The vent tubes  72  inject the intake air passing from the intake  40  to the inside of the shroud  26  past the drive assembly, described in detail below, through three openings  80  in a base plate  82  (and elongated opening  84  in the gasket  30 ), which is bolted to the base  24 , and into an intake chamber  86  in the base  24  of the housing, as shown in  FIG. 5 . Note that the base plate  82  is bolted to the base  24  by bolts  87  (see  FIGS. 2 and 3 ).  
         [0039]     As shown in  FIG. 9 , the intake chamber  86  is defined in the base  24  with an exhaust chamber  88  in a yin-yang-like configuration, each with a wide portion and a narrow portion in opposite orientation. The intake air is drawn from the intake chamber  86  up through an intake opening  90  in the base plate  82  (and an associate opening the gasket  30 ) and an intake tube  92  connected to the intake side of the drive assembly, as shown in  FIG. 10 . Exhaust tube  94  runs between an exhaust opening  96  in the base plate  82  (and an associated opening in the gasket  30 ) to the exhaust side of the drive assembly. Importantly, the intake  92  and exhaust  94  tubes are of bellowed construction allowing the tubing to flex in response to vibrations of the drive assembly, without transferring the vibration to the housing base  24 . The intake  92  and exhaust  94  tubes simply snap into the base plate  82  and are clamped (via clamps  98 ) to an associated nipple in the drive assembly.  
         [0040]     Referring now to  FIGS. 2, 4 ,  5 ,  12  and  14 , the drive assembly generally includes a valve head  100 , a cylinder  102 , a piston  104  and an electromagnet motor  106 , all aligned concentrically about a piston axis  108  (see  FIG. 12 ). The entire drive assembly is bolted (via bolts  109 ) to a motor mount  110  and mounted to the base plate  82  via resilient mounts  112 . The four resilient mounts  112  have narrowed necks between enlarged heads and bodies that slideably snap into four open ended slots  114  in the motor mount  110 . Slightly enlarged bottom ends of the resilient mounts  112  can be pushed straight down through openings  116  in the base plate  82 , thereby, the motor mount  110  is captured by the resilient mounts  112  and flexibly mounted to the base plate  82 . The resilient mounts  112  are preferably made of a suitable rubber, such as an EPDM with a durometer of about 60, so as to provide enough stiffness to securely mount the drive assembly while allowing enough flexibility to isolate the vibrations of the drive assembly.  
         [0041]     With primary reference to  FIGS. 2, 4  and  12 , the drive assembly will now be described in detail. The valve head  100  defines an intake side  120  and an exhaust side  122  each having a respective nipple  124  and  126  to which the intake  92  and exhaust  94  tubes connect via clamps  98 . A valve plate  128  mates with the valve head  100 . The valve plate  128  has a groove for a double D-shaped (bisected circle) o-ring  130  sealing against the valve head  100  to isolate the intake side  120  from the exhaust side  122 . The valve plate  128  is generally disk-shaped and defines a pair of intake ports  131  (shown in phantom in  FIG. 4 ) and a pair of exhaust ports  132 . Each pair of ports is covered by respective thin metal flapper valves  134  and  135  (the flapper valves  134  and  135  can be supported by valve stops (not shown)). The intake  131  and exhaust  132  ports are in communication with the associated sides of the valve head  100  and the inside of the cylinder  102 , which fits into another groove in the back side of the valve plate  128  sealed with another o-ring  136  (see  FIG. 13 ). The opposite side of the cylinder  102  fits around a hub of a retainer collar  138 . The retainer collar  138  has four threaded openings, which receive four bolts  140  fit through four ears of each of the valve head  100  and valve plate  128  to clamp the cylinder  102  tightly together with these components.  
         [0042]     The other side of the retainer collar  138  clamps one or more leaf springs  142  with a recessed groove (having alignment features as discussed below) in a spacer ring  144 . The opposite side of the spacer ring  144  receives a stator  146  of the electromagnet motor  106 . The stator  146  is a slotted annular member having a circular base and concentric inner  148  and outer  150  cylindrical walls (with axial slots  169  in outer wall  150 ), which define an annular channel  152  therebetween. A wire coil  154  is disposed in a bobbin  156  within the channel  152 . The bobbin  156  has three posts that extend through openings in the base of the stator  146  and are engaged by retainers  153  to retain the bobbin  156  and coil  154 . A diode (not shown) may be electrically coupled to the coil  154  to rectify the alternating current input signal so that it drives an armature (or shuttle)  158  in only one direction, preferably toward the stator  146 . Conductive tabs  160  for coupling the coil  154  to the power are also included.  
         [0043]     The armature  158  has a series of axial bores therethrough and slides in and out of a side (right in the drawing) of the stator  146  when the coil  154  is energized. The armature  158  has a short hub with an axial bore  162  that receives a bottom end of a connecting rod  164 . The connecting rod  164  is suspended along the piston axis  108  by the leaf springs  142  and passes through the center bore in the stator  146 . The connecting rod  164  is secured to the armature  158  and the piston  104  by a long bolt  166  threaded into the piston  104  and mounting a mass disk  168  under its head.  
         [0044]     The stator  146  is clamped between the spacer ring  144  and another spacer ring  172 . That spacer ring  172  clamps one or more additional leaf springs  142  against a second retainer collar  174 . Four tie rods  173  extending through ears in the first retainer collar  138  are threaded into openings in ears of the second retainer collar  174  to unite the components of the motor  106 . The retainer collars  138  and  174  also have threaded openings receiving bolts  109  to connect the motor mount  110  and thereby mount the entire drive assembly to the base plate  82  via the resilient mounts  112 , as described above.  
         [0045]     As shown in  FIGS. 16 and 17 , at the side opposite the cylinder  102 , the retainer collar  174  (as well as spacer ring  144 ) has a circular groove  176  about its inner periphery with three alignment pockets  178 , the sides of which taper asymmetrically away from the groove  176 . One or more leaf springs  142  fit into the groove  176  and their asymmetrically tapered alignment tabs  180  fit into the pockets  178 . Due to the asymmetric configuration of the pockets  178  and tabs  180 , the leaf springs  142  can seat properly into the retainer collar  174  (and the spacer ring  144 ) in only one angular and axial orientation (relative to the piston axis  108 ). The alignment tabs  180  are spaced apart about 120 degrees so that the springs can be mounted in one of three angular orientations. If a single angular orientation is desired, the alignment tabs  180  could be spaced asymmetrically. This facilitates and ensures the assembly of multiple leaf springs  142  in the same orientation at both ends of the connecting rod  164 . In particular, the web pattern (and arcuate slots) of the leaf springs  142  are aligned, and any curvature or bowing of the leaf springs  142  out of the plane perpendicular to the piston axis  108  (which can occur from the die cutting process) will be in the same direction for each leaf spring  142 .  
         [0046]     This is important to ensure that the motor  106  has the spring rate for which it was designed. Specifically, during development the pump is tuned to operate at a frequency at or near its natural resonant frequency. In particular, the pump is operated with a load applied and using a calculated spring-mass system (i.e., the combination of spring rate of the springs  142  and mass of the moving components, namely the piston  104 , armature  158 , connecting rod  164  and any mass disk  168 ). The frequency of the input signal to the motor  106  is varied as various parameters are measured. For example, because power consumption goes up as the input frequency strays from the resonant frequency of the spring-mass system, power consumption measurements can be used to adjust the spring-mass system so that its natural frequency will be at or near that of a typical input signal, for example 60 Hz. Operating the pump at the resonant frequency improves efficiency, and reduces vibration, and thereby noise. The spring-mass system can also be adjusted to operate efficiently at different pressures. For example, by increasing mass or spring rate the spring-mass system can be made to operate at or near resonant frequency while the pump is providing increased pressure output. It should be noted that the mass disk  168  is used as a cost effective alternative to increasing or decreasing the mass of the piston, armature and/or connecting rod.  
         [0047]     As is well understood, the piston  104  is driven by movement of the armature  158 , when energy is supplied to the wire coil  154 , to reciprocate within the cylinder  102 . The piston  104  has an enlarged head with a peripheral groove holding a split piston ring  170  that seals against the cylinder  102  when pressure is developed. The stroke length is approximately 8 mm (4 mm in each direction) and is positioned approximately 1 mm from the top of the cylinder when at top dead center.  
         [0048]     Given the single cylinder arrangement of the pump  20 , the reciprocating piston  104 , armature  158  and connecting rod  164  can cause the drive assembly inside the housing to vibrate. The leaf springs  142  absorb much of the energy from these moving components. The number, size and thickness of the leaf springs  142  are selected to achieve a spring rate determined primarily according to the mass of the piston  104  and the input frequency. The leaf springs  142  are selected so that in combination (between the two stacks) they result in a resonant frequency of the piston  104  and springs  142  (i.e., the spring-mass system) approximately equal to the input frequency, which is typically 50 or 60 Hertz. For example, in one preferred embodiment there is a stack of two springs in the second retainer collar  174  and a stack of two springs in the spacer ring  144  near the piston  104 . If the stroke length were to be increased, for example if the pump to be used in an application requiring more air flow, the springs  142  could be of a thinner gauge, in which case the number of springs may be increased to three in each stack to achieve the same spring rate.  
         [0049]     With reference to  FIGS. 5-7 ,  9 - 11  and  13 , air flow through the pump will now be described in detail for a preferred compressor embodiment of the pump. When the drive assembly is operating, ambient air is drawn into the pump intake through the intake air passage  36  around the periphery of the top cover  28 . As shown in  FIGS. 5-7 , the intake air is drawn through the intake air passage  36  flowing upwardly along the ribs  66  and makes its way into the outer cavity  50  through the cut-out  64  in the peripheral wall  62  of the filter tray  46 . Intake air then moves from that end of the outer cavity  50  through the filter  48  and around the risers  60  in the filter tray  46  to the openings  58  in the filter tray  46  and the partition  44  where it enters the inner cavity  42 . The pressure of the intake air drops as it passes through the small openings into the inner cavity  42 . The inner cavity  42  is effectively larger than the outer cavity  50 , which allows the intake air to expand. The expansion of the intake air in the inner cavity  42  helps to dissipate the pulsations in the intake air arising from the operation of the intake valve. After entering the inner cavity  42 , the intake air turns through a bend of about 90-180 degrees and travels to the other end of the inner cavity  42  to the orifices  52  where it travels down the vent tubes  72  and into the interior of the housing shroud  26 . As shown in  FIG. 5 , the vent tubes  72  are located to direct the intake air into the slots  169  in the stator  146  of the motor  106  so as to convectively cool the coil  154 . After passing through and around the motor  106 , the intake air passes through the openings  80  in the base plate  82  into the wide part of the intake chamber  86 . The air is routed through the narrowed part and up through the intake opening  90  in the base plate  82  and the intake tube  92  to the intake side  120  of the valve head  100 , as shown in  FIG. 10 . Reciprocation of the piston  104  draws air into the cylinder  102  and compresses it. The pressurized air is then passed through the exhaust side  122  of the valve head  100  and down through the exhaust tube  94  in the base plate  82  to the wide part of the exhaust chamber  88  through exhaust opening  96 . As shown in  FIGS. 9 and 11 , the pressurized air flows through the narrow portion of the exhaust chamber  88  and out through the outlet opening where fitting  34  is attached to suitable hose or tubing (not shown).  
         [0050]     Since the pump has only a single cylinder, the pressurized air is pulsed at the rate of the input frequency, for example 60 Hz. The inventors have determined that pulsations in the output air can adversely effect the operation of the pump. In particular, the output lines act as resonant chambers, having their own natural frequency. If the pulsations in the output air are at a different frequency, the air will effectively encounter increased resistance going through the output lines. This creates excessive back pressure on the pump so that the spring-mass system can be made to operate at a different (non-resonant) frequency, thereby decreasing the efficiency of the pump. This makes the pump more sensitive to variations in input frequency which can further decrease efficiency. By reducing the amplitude of the pulsations in the air before leaving the pump, this problem can be avoided.  
         [0051]     To that end, as shown in  FIGS. 3, 9  and  13 , the pulsations in the pressurized output air leaving the pump are dampened by a diaphragm  190  in the exhaust chamber  88 . In particular, the diaphragm  190  is preferably a rubber disk mounted to the top of a cup  192  defined by the housing base  24  in the wide part of the exhaust chamber  88  by a support ring  194  bolted (via bolts  198 ) to the base  24 . The cup  192  defines a trapped air pocket  196  below the diaphragm  190 . As the pressurized air flows through the exhaust chamber  88 , the pulsations act against the diaphragm  190 , tending to make it flex into the cup  192 . The air pocket  196  will compress slightly, but tend to resist inward movement of the diaphragm  190 . This reactive force on the diaphragm  190  will tend to counter, and thus cancel or reduce the amplitude of, the pulsations in the exhaust chamber air. The diaphragm  190  and trapped air pocket  196  act similar to an accumulator, and as a result, allow the pump to output smoother, relatively non-pulsed, air through the output lines. As a result pump inefficiencies are avoided that may otherwise arise from changes in the length or diameter of the output lines or from changes to the input frequency to the motor. As an example of one advantage, the inventors have determined that use of the diaphragm in this way allows a single mass-spring system to be used for both 50 and 60 Hz applications.  
         [0052]     An illustrative embodiment of the present invention has been described above in detail. However, the invention should not be limited to the described embodiment. To ascertain the full scope of the invention, the following claims should be referenced.