Abstract:
A variable delivery fixed displacement pump is provided which has a rotating drive plate with a fixed angle. The drive plate has a fill passage extending between its radial outer surface and its drive surface, which allows fluid to be supplied to a fluid journal bearing. The drive plate also has a series of bearing supply passages which fluidly connect the drive surface of the drive plate with a base surface, allowing fluid to be supplied to a fluid thrust bearing. A method of pumping fluid is also provided which is comprised of the steps of reciprocating a plurality of pistons at least in part by rotating the drive plate, and by fluidly connecting a pumping chamber of the pistons to an annular groove that is part of the drive plate fill passage.

Description:
TECHNICAL FIELD 
     The present invention relates generally to axial piston pumps, and more particularly to a variable delivery axial piston pump with outer diameter inlet filling via a fixed angle drive plate. 
     BACKGROUND 
     The invention described in U.S. Pat. No. 6,035,828 to Anderson et al. shows a variable delivery fixed displacement pump. Anderson also discloses a fixed angle drive plate and an electronic control unit which can alter the effective fluid displacement achieved by each pumping stroke. This design has met with tremendous success and represents a substantial improvement over earlier systems, however, there remains room for improvement. 
     For instance, the drive plate in Anderson is mounted on frustoconical roller bearings to ensure smooth rotation. While this design achieves its intended purpose, a significant amount of engine torque is wasted in overcoming the roller bearings&#39; friction. In addition, frustoconical bearings are relatively expensive and subject to failure like any other moveable metallic component. It would thus be desirable to reduce the cost and the friction between the drive plate and the pump housing. In addition, refilling of the hollow piston interiors takes place by drawing fluid from the pump&#39;s low pressure interior via an opening in the outer radius of the drive plate. Consequently, engine power used to supply the pump with hydraulic fluid is less than fully exploited, resulting in a reduction in efficiency. It would thus be desirable to employ a design which takes advantage of the hydraulic fluid inlet pressure. 
     The present invention is directed to overcoming one or more of the problems or disadvantages set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect, a drive plate for an axial piston pump is provided which comprises a metallic component having a centerline and a drive surface oriented at a drive angle that is different from 90 degrees relative to the centerline. The metallic component further includes a radial outer surface surrounding the centerline, and defines a fill passage that extends between the radial outer surface and the drive surface. The fill passage includes an annular groove that is defined by the radial outer surface. 
     In another aspect, a pump is provided which comprises a housing defining an inlet. A plurality of pistons are provided, each defining a hollow interior, and are arranged around a centerline. A rotatable drive plate is also provided and defines a fill passage extending between a radial outer surface and a drive surface. The hollow interiors of the plurality of pistons are in fluid communication with the inlet via an annular groove defined by at least one of the housing and the drive plate. 
     In still another aspect, a method of pumping fluid is provided which comprises the step of reciprocating a plurality of pistons at least in part by rotating a drive plate. The method also includes the step of fluidly connecting a pumping chamber of a portion of the pistons to an inlet via an annular groove that is a portion of a fill passage extending between a radial outer surface and a drive surface of the drive plate. The method also includes the step of fluidly connecting a pumping chamber of a different portion of the pistons to an outlet. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a partial sectioned diagrammatic isometric view of a pump according to the present invention; 
     FIG. 2 is a top view of the drive plate included in the present invention; 
     FIG. 3 is a sectioned side view of the drive plate as viewed along section line  3 — 3  of FIG. 2; 
     FIG. 4 is a sectioned bottom view of the drive plate as viewed along section line  4 — 4  of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, there is shown an axial piston pump  1  according to the present invention. Pump  1  includes a housing  3  and an electro-hydraulic control unit  32 . A front flange  5  and an end cap  7  are provided, and are attached to housing  3  at opposite ends. An inlet  8  which is defined by housing  3  allows hydraulic fluid to be supplied to pump  1  from an exterior source (not shown). A barrel assembly  18  is provided which includes a barrel  19  positioned at least partially within housing  3  that is preferably adjacent one end of a plurality of pistons  20 . A drive plate  12 , which is preferably metallic, is positioned adjacent the opposite end. A rotatable drive shaft  9  is attached to drive plate  12 , and is supported by a bearing collar  10 . Drive shaft  9  is preferably coupled directly to the output of an engine (not shown), such that the rotation rate of shaft  9  and drive plate  12  is directly proportional to the rotation rate of the engine drive shaft. 
     In the preferred embodiment, the plurality of pistons  20  are arranged in a parallel orientation around centerline  11 . Bach individual piston  20  defines e hollow interior  21 , and is attached via a ball joint  36  to a shoe  34  that is positioned in contact with drive plate  12 . Hollow interior  21  is a portion of the pumping chamber for the piston. Return springs  25  continuously urge each piston  20  toward drive plate  12  in a conventional manner such that the piston shoes  34  remain in continuous contact with drive plate  12 . Drive plate  12  has a fixed angle, β (see FIG.  3 ), and its rotation causes the plurality of pistons  20  to serially reciprocate between an up and a down position, displacing fluid in a conventional manner. Because each piston shoe  34  is maintained in contact with the drive plate, the pistons&#39; hollow interiors  21  can allow fluid supplied via drive plate  12  (described below) to flow from an opening  37  in each shoe  34  to the opposite end of the piston  20 . From this point, the fluid can be forced past a check valve  26  into a collector ring  28 , and from there to an outlet via an outlet passage  29 . 
     A sleeve  24  is movably mounted around each of the plurality of pistons  20 . The sleeves&#39;  24  position determines the proportion of displaced fluid flowing to collector ring  28 , and the proportion which flows to the low pressure interior  52  of pump  1 . Each sleeve  24  is attached to a connector  22  which surrounds drive shaft  9 . Connector  22  is movable between an up and a down position by electro-hydraulic control unit  32  in a conventional manner, allowing simultaneous movement of all the sleeves  24 . When the sleeves  24  are in their down position, a plurality of spill ports  30  can fluidly connect the hollow piston interiors  21  to low pressure interior  52  when the pistons  20  travel upward during a pumping stroke. In their up position, sleeves  24  cover the spill ports  30  and allow pressure to build in the piston interiors  21 , resulting in a relatively greater proportion of fluid being forced past check valve  26  and into collector ring  28  by the pistons&#39;  20  pumping action. Because electro-hydraulic control unit  32  can be used to control the vertical position of each sleeve  24  on its respective piston  20 , the relative discharge of pump  1  can be controlled by selectively allowing sleeves  24  to cover or uncover the spill ports  30  during different portions of a piston pumping stroke. Electro-hydraulic control unit  32  defaults when un-energized via spring  69  to bias the piston sleeves  24  in their down position, at which the pump produces no high pressure output. 
     Referring in addition to FIGS. 2-4, there is shown the metallic drive plate  12  of the present invention. Drive plate  12  has a centerline  11 , and a radial inner surface  61  and a radial outer surface  62  which surround the centerline  11 . A drive surface  63  extends between outer surface  62  and inner surface  61 , and is oriented at a drive angle β which should be different from 90 degrees relative to the centerline  11 . Drive plate  12  defines a fill passage  60  which extends between radial outer surface  62  and drive surface  63 . Fill passage  60  includes an annular groove  71  which is preferably machined around radial outer surface  62 , and a fill slot  65  which opens to drive surface  63 . It should be appreciated that the present invention might be designed such that groove  71  was at least partially defined by housing  3  rather than drive plate  12  itself. The cross-sectional area of groove  71  should have sufficient flow area to accommodate the fluid pumping and bearing demands of the pump. The portion of fill passage  60  which connects groove  71  and fill slot  65  can be designed in any suitable manner, so long as adequate flow area is provided. The present description shows, for instance, a plurality of spoke-like bores. However, it should be appreciated that some other design might be employed such as a continuous slot through radial outer surface  62 . In the preferred embodiment, till slot  65  is arcuate shaped, and follows a path that has a substantially constant radius, circle  66 , relative to centerline  11 , preferably sweeping out an angle δ which is less than 180 degrees. As drive plate  12  rotates, the hollow interior  21  of at least one of the plurality of pistons  20  is in fluid communication with inlet  8  via fill passage  60  and annular groove  71 . 
     A base surface  64  is located opposite drive surface  63  and separates radial inner surface  61  from radial outer surface  62 . Base surface  64  preferably lies in a plane that is substantially perpendicular to centerline  11 , and is separated from housing  3  by a fluid thrust bearing  43 . A thrust bearing plate  40  which provides a plurality of thrust pads  42  is positioned beneath fluid thrust bearing  43  (FIG. 1) and drive plate  12 . Drive plate  12  defines a plurality of bearing supply passages  67  which extend from base surface  64  through drive surface  63 , and provide the fluid for thrust bearing  43 . The bearing supply passages  67  are preferably distributed on a circle  66  that is centered on centerline  11  and includes the arc swept out by fill slot  65 . In the preferred embodiment, a majority of the radial outer surface  62  is a portion of a regular cylinder and is separated from housing  3  by a fluid journal bearing  44 . Hydraulic fluid is pushed into the area between radial outer surface  62  and housing  3  to provide the journal bearing  44 . Although preferred, it is not necessary that the present invention include both fluid thrust and fluid journal bearings. A conventional roller bearing might be substituted for either of the fluid bearings provided by the present invention. 
     INDUSTRIAL APPLICABILITY 
     Returning now to FIG. 1, the rotation of drive plate  12  causes pistons  20  to reciprocate up and down by elevating and de-elevating the shoes  34  of each piston  20  as the plate passes underneath. The axial lodes produced by piston reciprocation can be balanced by the plurality of thrust pads  42 . As drive plate  12  passes underneath one of the pistons  20 , drive surface  63  can act on the piston shoe  34  to drive the piston  20  up for a pumping stroke. Each shoe  34  is connected to its respective piston  20  by a ball joint  36  which allows the shoe  34  to remain in continuous contact with drive surface  63 . The amount of fluid displaced by the piston  20  into high pressure collector ring  28  depends on the position of its respective sleeve  24 . When relatively greater fluid displacement is desired, electro-hydraulic control unit  32  can be used to move sleeves  24  up. The sleeves  24  then cover spill ports  30  and a maximum amount of fluid can be displaced by each piston&#39;s  20  pumping stroke to flow past check valve  26  into collector ring  28 . By varying the time that the sleeves  24  are held in their up position, a broad spectrum of fluid displacement quantities can thus be obtained. 
     When drive plate  12  has moved piston  20  its maximum displacement, it begins to move down, its shoe  34  remaining in continuous contact with drive surface  63 . Shortly after the piston  20  begins to retract, the rotation of drive plate  12  brings fill slot  65  under the opening  37  in piston shoe  34 . Because fluid is continuously supplied via inlet  8  to fill passage  60 , the retracting movement of piston  20  acts to draw fluid from fill slot  65  into its hollow interior  21 . Because fill passage  60  is supplied with hydraulic fluid directly from inlet  8  rather than the pump&#39;s  1  low pressure interior  52 , fluid is drawn into the pistons&#39; hollow interior  21  more readily than in prior art pumps. Low pressure interior  52  is preferably fluidly connected to inlet  8  via a pressure balancing passage which is not shown. Shortly before the piston  20  reaches its fully retracted position, the rotation of drive plate  12  moves fill slot  65  out of fluid communication with the opening  38  in piston shoe  34 . 
     As drive plate  12  rotates, fluid which is supplied via inlet  8  is pushed into the area between drive plate  12 &#39;s radial outer surface  62  and housing  3 , resulting in a relatively low friction fluid journal bearing  44 . The bearing supply passages  67  which fluidly connect drive surface  63  with base surface  64  allow a continuous supply of fluid to be provided to the area between drive plate  12  and thrust bearing plate  40 , constituting the invention&#39;s fluid thrust bearing  43 . In other words, a portion of the fluid pumped by pistons  20  is pushed through bearing supply passages  67  to produce a fluid thrust bearing  43  that separates drive plate  12  from contact with thrust pads  42 . The substitution of conventional roller bearings for the fluid journal  44  and thrust bearings  43  allows the present invention to be manufactured for lower cost and to operate under a significantly decreased frictional load. The present invention represents a further improvement over earlier designs by taking advantage of the fluid supply pressure at the inlet  8  to assist in replenishing the hydraulic fluid in the pistons  20  rather than relying only upon the reciprocating action of the pistons  20  to draw fluid back into their interiors  21 . 
     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 fluid bearing design utilized in the present invention might be modified to use a combination of fluid and roller bearings. Additionally, the drive plate-fill passage design might be employed as a means of reducing plumbing in a pump with space constraints. 
     Thus, those skilled in the art will appreciate that other aspects, objects and advantages of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.