Abstract:
In an axial piston pump, a plurality of reciprocating pistons are at least partially positioned within a barrel. The barrel includes a ring shaped high pressure collection cavity that is positioned between the piston chambers and the outlet of the pump. In order to minimize potential loses due to leakage from the ring shaped collection cavity the barrel is made from a casting that utilizes a ring shaped core supported in a mold. This facilitates the formation of the ring shaped cavity while insuring location and dimensional tolerances, and assisting in the latter machining of the casting to its final form.

Description:
RELATION TO OTHER PATENT APPLICATION 
   This application is a division of Ser. No. 09/995,884, filed Nov. 28, 2001, now U.S. Pat. 6,682,315. 

   TECHNICAL FIELD 
   The present invention relates generally to axial piston pumps, and more particularly to a barrel casting for an axial piston pump having a ring shaped high pressure collection cavity. 
   BACKGROUND 
   Co-owned U.S. Pat. No. 6, 035,828 to Anderson et al. describes a hydraulically actuated system having a variable delivery fixed displacement axial piston pump. This pump is referred to as a fixed displacement because the swash or drive plate has a fixed angle such that each piston reciprocates a fixed distance and displaces a fixed amount of fluid with each rotation of the drive plate. The pump achieves a variable delivery by utilizing sleeves that surround each piston and cover a spill port for at least a portion of each reciprocation of the individual piston. For instance, if the sleeves are positioned at one location, the spill ports in the pistons remain uncovered throughout each reciprocation such that the pump merely circulates fluid between low pressure areas and no high pressure output is produced. When the sleeves are in another position, the spill ports are closed over the entire reciprocation distance of the piston such that the maximum high pressure output of the pump is achieved. The sleeves can be positioned anywhere between these two extremes via an electro-hydraulic control unit so that the effective high pressure delivery of the pump can be varied and controlled at will. 
   While this pump has shown considerable promise, there remains room for improvement. For instance, each of the pistons has one end received in a piston bore of a barrel component. A high pressure collection cavity is located between the barrel and the outlet of the pump. In addition, at least one check valve separates the individual piston chambers from the high pressure collection cavity. In the Anderson et al. pump, these various features are located in body components that are different from the barrel. As such, the effectiveness of the pump has the potential for compromise due to leakage between these components. In addition, insuring the proper location and orientation of these body components relative to one another can add substantial machining costs and assembly complications. 
   The present invention is directed to one or more of the problems set forth above. 
   SUMMARY OF THE INVENTION 
   In one aspect, a barrel assembly for an axial piston pump includes a casting that defines a ring shaped collection cavity that is fluidly isolated from a central bore. 
   In another aspect, a pump includes a barrel assembly mounted in a housing. The barrel assembly includes a casting that defines a ring shaped cavity fluidly isolated from a central bore, and a plurality of parallel piston bores that are open to the ring shaped cavity. A piston is slidably received in each of the piston bores. A drive plate have a slanted drive surface is rotatably mounted in the housing and operably coupled to each of the pistons. 
   In still another aspect, a method of making a barrel assembly for a pump includes a step of casting metal around a ring shaped core. The ring shaped core is then removed from the casting. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partially sectioned isometric view of a pump according to a preferred embodiment of the present invention; 
       FIG. 2  is a sectioned side diagrammatic view of a barrel assembly according to the present invention; 
       FIG. 3  is an isometric view of a ring shaped core according to one aspect of the present invention; 
       FIG. 4  is an isometric view of a base core according to another aspect of the present invention; 
       FIG. 5  is a sectioned side view of a casting mold according to another aspect of the present invention; 
       FIG. 6  is a top view of a casting blank according to one aspect of the present invention; and 
       FIG. 7  is a sectioned side view of the casting blank of  FIG. 5  as viewed along sectioned line  6 — 6 . 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , there is shown an axial piston pump  10  according to the present invention. Pump  10  includes a housing  11  that includes a front flange  12  and an end cap  13 . Housing  11  includes an inlet  14  and an outlet  15 . When pump  10  is installed as part of a hydraulic system, such as a hydraulically actuated fuel injection system, inlet  14  is connected to a source of low pressure fluid, such as engine lubricating oil. Outlet  15  would be fluidly connected to a high pressure reservoir, such as a high pressure common rail that supplies working fluid to hydraulic actuators, such as those associated with hydraulically actuated fuel injectors and/or hydraulically actuated gas exchange valves, and the like. Pump  10  includes a drive shaft  16  having an external end that is coupled to an appropriate rotational power source, such as the crank shaft of an internal combustion engine. Drive shaft  16  preferably has a keyed connection to rotate a drive plate  17 , which preferably has a fixed slant angle. 
   A plurality of pistons  20  are distributed around a centerline of the pump and oriented parallel to one another and to drive shaft  16 . In the illustrated embodiment, there are preferably seven pistons; however, those skilled in the art will appreciate that a pump having any number of pistons could be suitable for use in relation to the present invention. Each individual piston  20  defines a hollow interior  21 , and is aftached via a ball joint to a shoe  29  that is maintained in contact with drive plate  17  via the continuous urging of a return spring  25 . Rotation of drive plate  17  causes the plurality of pistons to serially reciprocate between up and down positions, displacing fluid in a conventional manner. Each of the pistons  20  also includes a hollow interior  21 , which can be thought of as a portion of that pistone pumping chamber, and at least one spill port  26  distributed around the periphery of the piston and opening into hollow interior  21 . One end of each of the pistons is slidably received in a plunger bore  64  defined by a barrel assembly  18 . Together, plunger bore  64  and hollow interior  21  define the pumping chamber for the individual piston. This pumping chamber is separated from a ring shaped high pressure collection cavity  19  in barrel assembly  18  by a check valve  23 . In other words, the plunger bore  64  for each piston is separated from ring shaped collection cavity  19  by a separate check valve  23 . Ring shaped collection cavity  19  is fluidly connected to outlet  15  via a passage that is not shown, but fluidly isolated from a central bore  67 . 
   The output of pump  10  is controlled by an electro-hydraulic control unit  27  that is operable to move a control piece  30  up and down along a line that is parallel to that of the pistons. In particular, electro hydraulic control unit  27  moves a control piece  30 , which is operably coupled to a plurality of sleeves  24  via a connector  22 . An individual sleeve  24  is positioned around each individual piston  20 . The location of sleeves  24  relative to spill ports  26  determines how much of the fluid displaced by piston  21  is pushed into high pressure collection cavity  19  or merely recirculated into low pressure interior  28  of the pump housing  11 . In other words, if sleeve  24  maintains spill port  26  covered during the entire reciprocation distance of a piston  20 , virtually all of the fluid displaced is pushed past check valve  23  into high pressure ring shaped cavity  19 . On the other hand, if sleeves  24  are positioned such that spill ports  26  remain open as piston  20  is moved for its pumping stroke, the fluid displaced by piston  20  merely spills back into low pressure area  28  via spill ports  26  for recirculation. 
   When pistons  20  are undergoing their retracting stroke low pressure fluid is drawn into hollow interior  21  from low pressure pump interior  28  via a center filled inlet  36  in drive plate  17  and an internal fill passage and slot (not shown) that communicates with an opening  31  in shoes  29  at an appropriate rotational position that is out of plane in the sectioned view of  FIG. 1 . In addition to defining the fill passageway, drive plate  17  also defines a plurality of bearing supply passages  37  that communicate fluid from hollow interior  21  to the thrust pads  33  adjacent the underside of drive plate  17  to provide a hydrostatic thrust bearing  34 . A portion of this fluid migrates up the outer radial wall of drive plate  17  to provide a hydrodynamic journal bearing  35 . It should be noted that bearing supply passages  37  are positioned such that they only communicate with openings  31  when the individual piston  20  is undergoing its pumping stroke. When the pistons are undergoing their retracting stroke, they align with a fill slot (not shown) that is fluid communication with center fill inlet  36 . 
   Referring now to  FIG. 2 , the barrel assembly  18  is shown in greater detail. Barrel assembly  18  includes a machined casting  38  and a plurality of attached check valves  23 . Unlike some alternative designed axial piston pumps, barrel assembly  18  of pump  10  remains stationary when the pump is in operation. Those skilled in the art will appreciate that in other axial piston pumps the barrel is rotated by the drive shaft and the drive plate remains stationary. The present invention is compatible with both types of axial piston pumps. As identified earlier, barrel casting  38  includes a ring shaped high pressure collection cavity  19  that is separated from piston bores  64  by a relatively short passage that includes a conical valve seat  65 . Each of the check valves  23  includes external threads that mate to internal threads  69  that are machined in access openings  66  in barrel casting  38 . Thus, check valves  23  are threaded into a position in contact with conical seat  65 . Each check valve  23  includes a check valve member  70  that is biased into a position in contact with a seat  72  via a biasing spring  71 . When in this biased position, a passage  73  is closed to piston bore  64 . When fluid pressure pushing on check valve  70  exceeds the pre-load of biasing spring  71 . check valve member  70  lifts to fluidly connect ring shaped collection cavity  19  to piston bore  64  via passage  73 . Barrel casting  38  also includes a shaft support opening or central bore  67  that extends between first end  63  and second end  68 . Central bore  67  is fluidly isolated from ring shaped cavity  19 . 
   Referring now to  FIGS. 3 ,  4 , and  5 , the various core pieces and mold assembly are illustrated to show how the barrel is casted to include its ring shaped collection cavity  19  ( FIGS. 1 and 2 ). The barrel is preferably cast in a sand mold using discardable core pieces that are preferably premanufactured using a sand epoxy mixture in a conventional manner. In particular, ring shaped core  40  includes a ring shaped portion  41  and a plurality of pillars  42  that correspond to the number of pistons in the pump. A base core  45  is-likewise preferably manufactured from a suitable sand and epoxy mixture to include a central bore core  46  centrally located atop a base portion  48  that defines a plurality of pillar holes  47 . An additional central bore core  49  ( FIG. 5 ) can also be used in molding the barrel casting. When placed in mold  50 , the pillars  42  of ring shaped core  40  are received in respective pillar bores  47  in base core  45 . This arrangement insures that ring portion  41  will remain at its desired location when the molten metal is poured into mold  50 . In other words, this mating arrangement between ring shaped core  40  and base core  45  insures that the pillars  48  are properly located in the cast component and that ring portion  41  is precisely located within mold  50  and remains at that location throughout the molding process. 
   Referring now in addition to  FIGS. 6 and 7 , a casting blank  60  is illustrated as would be produced using the mold  50  as illustrated in  FIG. 5 . Casting blank  60  includes a central bore  61 , a plurality of pillar openings  62  and ring shaped high pressure collection cavity  19 . This casting blank is then machined using conventional techniques to arrive at the barrel casting  38  shown in  FIG. 2 . During the machining process, the individual pillar openings  62  are enlarged to produce access openings  66 , internal threads  69  and conical seats  65 . 
   INDUSTRIAL APPLICABILITY 
   The present invention finds potential application in any case where there is a desirability to cast a cavity into a casting, especially when it is important to maintain a certain geometry for the cavity and precisely locate the same with regard to the other surface features of the component. In the present case, the casting technique of the present invention allows for the formation of a high pressure ring shaped cavity that is virtually free of potential leakage concerns that could become associated with pumps that utilize one or more joined components to form their high pressure cavity(s). The present invention also exploits that fact that the core for the ring shaped cavity can be located and supported using other attached core components that are located at or near where openings are intended to be located in the finished component. This allows the casting technique to exploit the anticipated location of access openings  66  ( FIG. 2 ) in order to help facilitate the formation of internal ring shaped cavity  19 . Furthermore, by combining this technique with the particular structure and attachment strategy of check valves  23  allows the individual check valves to provide the check valve functionality while sealing ring shaped collection cavity  19  from any leakage to the outside in a cost effective and efficient manner. 
   Returning to  FIGS. 1 and 2 , when in operation, pump  10  can preferably produce between zero and its maximum output depending upon the positioning of electro hydraulic control unit  27  and hence sleeves  24 . As drive shaft  16  rotates, drive plate  17  rotates to cause each of the pistons  20  to reciprocate. Those undergoing their retracting stroke drawl fresh low pressure fluid from low pressure interior  28  through central fill inlet  36  and on to opening  31  via a passage in drive plate  17  not visible in  FIG. 1 . The pistons undergoing their pumping stroke push fluid out of piston bore  64  and hollow interior  21  past check valve  23  into high pressure collection cavity  19  for whatever portion of the piston stroke that sleeve  24  covers spill ports  26 . For that portion of the pumping stroke in which spill ports  26  are open, the fluid is merely displaced back into low pressure interior  28 . However, when spill ports  26  are closed, a portion of the fluid displaced by piston  20  is pushed down through bearing supply passages  37  to produce the hydrostatic fluid bearing that separates drive plate  17  from thrust bearing pads  34 . 
   The above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. For example, the casting technique of the present invention could permit for the formation of more than one ring shaped cavity and possibly permit the usage of a single check valve as opposed to an individual check valve for each of the reciprocating pistons. Thus, those skilled in the art will appreciate the other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims