Abstract:
A variable flow pump includes: a housing with inlet and outlet chambers interconnected by a main bore and a non-rotating cylinder block positioned in the main bore. The cylinder block includes: a central bore communicating with the inlet chamber; cylinder bores arrayed around the central bore; first feed passages interconnecting the inlet chamber and the cylinder bores, defining a bypass flowpath between the cylinder bores; and at least one discharge valve disposed at the second end which permits fluid flow from the cylinder bores to the discharge chamber but prevents opposite flow; Pistons are disposed in the bores. A shaft is coupled to the pistons so as to cause them to reciprocate through an axial pump stroke when the shaft is rotated. A mechanism is coupled to the cylinder block which modulates the axial position of the cylinder block within the housing, varying the size of the bypass flowpath.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to pumps and more particularly to variable flow rate pumps for hydraulic systems. 
     Aircraft gas turbine engines often incorporate various high pressure hydraulic actuators to operate components such as variable geometry exhaust nozzles, vectoring exhaust nozzles, bypass doors, variable stator vanes, and the like. 
     Depending on which actuators are being used, the flow requirements vary greatly, and it is desirable to match pumping capacity to the demand. Variable displacement high-pressure piston pumps are therefore commonly used in engine and aircraft hydraulic systems. However, prior art variable displacement piston pumps can be complex, heavy, costly and can lack desired reliability. 
     BRIEF SUMMARY OF THE INVENTION 
     These and other shortcomings of the prior art are addressed by the present invention, which provides a high pressure, variable flow rate pump with low weight and high reliability. 
     According to one aspect of the invention, a variable flow pump includes: (a) a housing including an inlet chamber and an outlet chamber interconnected by a main bore; (b) a non-rotating cylinder block with first and second ends disposed in the main bore, the cylinder block including:(i) a central bore disposed in fluid communication with the inlet chamber; (ii) a plurality of cylinder bores arrayed around the central bore; (iii) a plurality of first feed passages interconnecting the inlet chamber and the cylinder bores, the first feed passages defining a bypass flowpath between the cylinder bores; and (iv) at least one check valve disposed at the second end which permits fluid flow from the cylinder bores to the discharge chamber but prevents flow in the opposite direction; (d) a plurality of pistons disposed in the bores; (e) a shaft mechanically coupled to the pistons so as to cause the pistons to reciprocate through an axial pump stroke between predetermined fill and discharge positions, when the shaft is rotated; and (f) a mechanism coupled to the cylinder block which is adapted to selectively axially position the cylinder block within the housing, so as to vary the size of the bypass flowpath. 
     According to another aspect of the invention, a method of operating a variable flow pump includes: (a) receiving fluid into an inlet chamber of a housing of the pump, wherein the pump includes an inlet chamber and an outlet chamber interconnected by a main bore; and (b) using a piston which reciprocates through an axial pump stroke between predetermined fill and discharge positions: (i) drawing fluid from the inlet chamber into a cylinder bore in a non-rotating cylinder block with first and second ends disposed in the main bore; (ii) discharging fluid through the cylinder bore; and (iii) during discharge, selectively bypassing a portion of the fluid from the cylinder bore through a first feed passage into the inlet chamber, the proportion of bypass being controlled by modulating the axial position of the cylinder block within the housing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may be best understood by reference to the following description taken in conjunction with the accompanying drawing figures in which: 
         FIG. 1  is a schematic cross-sectional view of a pump constructed according to an aspect of the present invention; 
         FIG. 2  is another view of the pump of  FIG. 1 ; 
         FIG. 3  is another view of the pump of  FIG. 1 ; 
         FIG. 4  is a view taken along lines  4 - 4  of  FIG. 1 ; and 
         FIG. 5  is a view taken along lines  5 - 5  of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views,  FIG. 1  depicts a variable displacement pump  10 . The major components of the pump  10  are a housing  12 , cylinder block  14 , shaft  16 , wobble plate  18 , pistons  20 , and flow modulating assembly  22 . 
     The housing  12  includes a main bore  24 . An inlet chamber  26  is disposed at one end of the main bore  24  and a discharge chamber  28  is disposed at the opposite end. An inlet  30  connects to the inlet chamber  26 , and an outlet  32  connects to the discharge chamber  28 . 
     The cylinder block  14  is received in the main bore  24 . It is free to move axially, between a maximum flow position (seen in  FIG. 3 ) and a minimum flow position (seen in  FIG. 1 ). The cylinder block  14  is generally cylindrical and has a first end  34  and a second end  36 . A central bore  38  passes down the rotational axis of the cylinder block  14 . It is open at the first end to receive the shaft  16 , and is closed at the second end  36 . A plurality of cylinder bores  40  are arrayed around the central bore  38 . A set of first feed passages  42  (i.e. slots, holes, or the like) are arrayed around the wall  44  separating the central bore  38  and the cylinder bores  40 . A set of second feed passages  46  are located axially downstream of the first feed passages  42 . The second end  36  of the cylinder block  14  carries discharge valves  48  which prevent backflow from the discharge chamber  28  back into the cylinder bores  40 . In this particular example, as seen most clearly in  FIG. 5 , the discharge valves  48  are reed valves which are part of a single valve plate  50  attached to the second end  36  of the cylinder block  14 . Other types of check valves could be substituted for this purpose. Leakage between the housing  12  and the cylinder block  14  is minimized by one or more seals  52 . Preferably the seals  52  are a low-friction type. In the illustrated example, the seals  52  are commercially available “O”-ring energized seals with low-friction caps made from a material such as polytetrafluoroethylene (PTFE), graphite, or the like. 
     The shaft  16  passes through appropriate bearings and seals  54  in the housing  12 . A first end of the shaft  16  extends outside the housing  12  and incorporates one or more mechanical features (not shown) such as a keyway, splines, or a driven gear, allowing the shaft to be connected to a driving element. 
     The opposite end of the shaft  16  is formed into an enlarged plug  55  having a cylindrical outer surface  56  which fits closely in the central bore  38 . A bleed port  57  is provided in the shaft  16  which lets working fluid pass freely between the inlet chamber  26  and the interior of the central bore  38 . This allows the cylinder block  14  to translate axially relative to the shaft  16  without causing excessive loads or hydraulic lock. A rotating port  58  is incorporated near the second end to pass working fluid from the inlet chamber  26  to the second feed passages  46 . As seen in  FIG. 4 , the rotating port  58  may take the form of a groove which extends halfway around the circumference of the plug  55 . The rotating port  58  is positioned or “clocked” such that when a piston  20  is in the “inlet” stroke, (the upper piston  20  in  FIG. 1 ), the rotating port  58  is open to the associated cylinder bore  40 , but when a piston  20  is in the “discharge” stroke, (the lower piston  20  in  FIG. 1 ), the corresponding cylinder bore  40  is closed off. 
     As seen in  FIG. 1 , the wobble plate  18  is mounted to the shaft  16  and is positioned in the inlet chamber  26 . The wobble plate  18  is coupled to the pistons  20  in a manner that permits rotation of the shaft  16  to be converted into reciprocating axial motion of the pistons  20 . In the illustrated example, the wobble plate  18  has a low-friction working face  60 , which may be accomplished through polishing, application of anti-friction coatings, or the like. The working face  60  is disposed at a non-perpendicular angle “A” to the rotational axis of the shaft  16 . Mounted on the working face  60  are annular flanges  62  that define an annular channel  64 . A plurality of slippers  66  are received in the channel  64  and are coupled to connecting rods  68 , for example through the illustrated ball joints  70 . Each of the connecting rods  68  is in turn coupled to one of the generally cylindrical pistons  20 . The pistons  20  can move axially but are restrained from any lateral movement by the cylinder block  14 . As the wobble plate  18  is rotated by the shaft  16 , the individual slippers  66  will be alternately pushed or pulled, in turn pushing or pulling the corresponding connecting rod  68  and piston  20 . At any particular time in the cycle, one of the pistons  20  will be at a fully extended position (to the right in  FIG. 1 ). The diametrically opposite piston  20  will be at a fully retracted position (to the left in  FIG. 1 ), and the remaining pistons  20  will be at intermediate positions. The wobble plate angle A may be selected to provide the desired magnitude of axial piston stroke. The number and size of the pistons  20  as well as the shaft speed may be varied to suit a particular application as well. 
     Means are provided for selectively moving the cylinder block  14  to a desired axial position relative to the housing  12 . Any type of actuator capable of moving the cylinder block  14  (e.g. electrical, hydraulic) may be used. In the illustrated example, the cylinder block  14  is moved by an electrohydraulic servo valve (EHSV)  72  of a known type in which a small pilot valve (not illustrated) is used to port working fluid pressure to either side of a primary cylinder (shown schematically at  74 ). As shown, discharge pressure may be ported to a pressure regulator  76  which in turn feeds regulated fluid pressure to the EHSV  72  through a line  78 . The pressure drop across the EHSV  72  is thus nearly constant over a wide range of pump output pressures, which simplifies control programming. A controller  80  including one or more processors, such as a programmable logic controller (PLC) or computer, is coupled to the EHSV  72 . The controller  80  responds to a flow demand signal and in turn drives the EHSV  72  to an appropriate position. A suitable transducer (not shown), such as a linear variable differential transformer (LVDT), may be used to provide cylinder block axial position feedback information to the controller  80 . 
     The pump  10  operates as follows. Working fluid enters the inlet  30  and floods the inlet chamber  26  volume on the left side of the pump  10 . The fluid is at a relatively low inlet pressure, which may be supplied by a suitable boost pump of a known type (not shown). Meanwhile the shaft  16  is rotating, causing the pistons  20  to reciprocate as described above. When a piston  20  is in the retracted or fill position, (the upper piston  20  in  FIG. 1 ), the associated cylinder bore  40  is flooded with working fluid through the rotating port  58 , and the first and second feed passages  42  and  46 . During the discharge stroke (the lower piston  20  in  FIG. 1 ), the rotating port  58  closes off the second feed passages  46  as described above. As the piston  20  begins its discharge stroke the pumped fluid is initially bypassed back to the inlet chamber  26  through the pressure through the first feed passages  42 . When the piston  20  reaches the end of the first feed passage  42 , the remaining stroke pumps fluid through the discharge valve  48  to the discharge chamber  28  and subsequently through the outlet  32 . 
     Discharge flow is varied by altering the percentage of piston stroke delivering fluid to the discharge chamber  28  versus bypass flow back to the inlet chamber  26 . This is achieved by modulation of the axial position of the cylinder block  14 .  FIG. 1  illustrates a minimum flow position of the cylinder block  14 , where the cylinder block  14  is shifted towards the discharge chamber  28 . This position exposes the first feed passages  42  for the maximum amount of the piston stroke.  FIG. 2  illustrates an intermediate flow position. Relative to  FIG. 1 , the cylinder block  14  is shifted towards the inlet chamber  26 . This causes the first feed passages  42  to be cut off sooner in the piston stroke.  FIG. 3  illustrates a maximum flow position. In this position, the cylinder block  14  is shifted as far towards the inlet chamber  26  as possible. In this position there is no bypass flow through the first feed passages  42 . 
     The pump may also include a balance piston  82 . In operation, discharge pressure is ported to the balance piston  82  through a line  84 . This pressure tends to drive the cylinder block  14  towards the right, in opposition to the force applied by discharge pressure on the second end of the cylinder block  14 . The area of the balance piston  82  may be selected such that the net axial force on the cylinder block  14  is zero or very small, thereby reducing bearing loads. With the balance piston  82 , the EHSV  72  need only have enough capacity to overcome seal friction and allows the EHSV  72  to be much smaller than it would have to be otherwise. 
     If desired, the pump  10  can include a pressure relief valve  86 . If the discharge pressure exceeds the relief valve&#39;s set point, flow is bypassed to the inlet chamber  26 . 
     The foregoing has described a variable flow pump. While specific embodiments of the present invention have been described, it will be apparent to those skilled in the art that various modifications thereto can be made without departing from the spirit and scope of the invention. Accordingly, the foregoing description of the preferred embodiment of the invention and the best mode for practicing the invention are provided for the purpose of illustration only and not for the purpose of limitation.