Abstract:
A hydraulic axial piston pump is designed with a secondary swashplate angle for providing improved performance and life. The secondary swashplate angle produces a sideload on the swashplate that is reacted by the pump&#39;s housing. A hydrostatically balanced thrust bearing is used to counteract the sideload produced by the secondary swashplate angle. The bearing design includes a tiltable pad or shoe supplied with pressurized fluid from the pump&#39;s discharge port. The same fluid that supplies the tiltable pad may also supply the pump&#39;s cradle bearings.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates in general to a hydraulic axial piston pump, and more particularly to a pump having a secondary swashplate angle and that provides pressurized fluid to a hydrostatic thrust bearing for reacting the force generated by the secondary swashplate angle. 
       BACKGROUND 
       [0002]    Swashplate-type axial piston pumps are commonly used devices used for generating hydraulic power. These pumps may be configured to provide a variable displacement flow for greater energy efficiency when compared with a non-variable displacement hydraulic pump. The pump component that provides variable displacement flow is a movable, inclined cam plate, typically referred to as a swashplate. The swashplate has a running surface upon which a group of pistons slide or travel. The swashplate ordinarily pivots on either roller or plain bearings. In either bearing case, the bearings are generally designed to provide low friction and long service life. Low friction bearings ensure proper controllability of the outlet flow. The benefits of long service life include lower lifecycle cost and reduced downtime. Other considerations in the design and selection of the swashplate bearings include size, weight, and initial cost among others. The benefits of reduced size, weight, and initial cost may be achieved by using plain swashplate bearings in lieu of roller bearings. The plain bearings are typically referred to as cradle bearings due to their semi-cylindrical shape. For an example of such cradle bearings, refer to U.S. Pat. No. 4,543,876 incorporated herein by reference in its entirety. 
         [0003]    To reduce bearing friction to acceptable levels and to minimize wear, some plain bearing designs incorporate hydrostatic pockets in a semi-circular swashplate backside surface. The hydrostatic pockets may be supplied with pressurized fluid from a discharge port of the pump typically via transfer tubes or passages in the pump&#39;s housing. 
         [0004]    Swashplate pumps are ordinarily designed such that the running surface of the swashplate is inclined solely about a single (primary) axis, which is perpendicular to the pump&#39;s driveshaft. The variable inclination of the swashplate about this axis allows for variable output flow. Some swashplate pumps also incorporate a second swashplate angle that is fixed at typically less than 4 degrees with respect to the variable plane passing through the primary axis. The benefits of incorporating this secondary swashplate angle may include a reduction in the size of the swashplate control mechanism (allowed by reducing the torque exerted on the swashplate by the pumping pistons). The smaller swashplate control mechanism may reduce the corresponding pump&#39;s size and weight. 
         [0005]    Additionally, the pump&#39;s timing may be adjusted by the secondary swashplate angle to reduce fluid-borne noise. 
         [0006]    The disadvantages of incorporating a secondary swashplate angle may include higher manufacturing costs and the need to react the side force exerted by the secondary angle. The thrust load developed by this side force can be approximately 5% of the thrust load reacted by the swashplate bearings. A common method of reacting this side force is via non-pressurized thrust bearings. These bearings may increase manufacturing costs, increase friction that impedes swashplate motion, and generate damaging wear particles due to the nearly continuous motion at the bearing interface. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention uses a hydrostatically balanced thrust bearing to counteract a sideload produced by a secondary swashplate angle of a hydraulic axial piston pump. The design includes a tiltable pad or shoe, supplied with pressurized fluid from the pump&#39;s discharge port, which pivots and/or slides about a flat plate. When supplied with pressurized fluid, the tiltable pad effectively becomes a hydrostatically balanced thrust bearing. Since the force exerted by the secondary swashplate angle is proportional to fixed geometry within the pump and discharge pressure, the tiltable pad is geometrically balanced, independent of varying discharge pressure. The same fluid that supplies the tiltable pad is also used to supply the cradle bearings. 
         [0008]    Hydrostatic support of the sideload generated by the secondary swashplate angle provides the benefits of:
       a) reduced friction, which improves control performance;   b) reduced wear, to improve pump life;   c) reduced contamination and subsequent damage to pump and hydraulic system components due to wear particles; and   d) self-adjusted bearing support, wherein the load and bearing support is directly proportioned to the discharge pressure.       
 
         [0013]    Discharge pressure is supplied to the pump&#39;s hydrostatic bearings via a common flow path for both the cradle bearings and the secondary swashplate angle bearing. 
         [0014]    The embodiments of the pump shown herein reduce the passages in the pump housing, minimize external leak paths, and reduce pump complexity. In summary, the invention reduces overall pump cost, size, and weight, while providing a superior bearing configuration for reacting the secondary swashplate angle force, thereby improving reliability. 
         [0015]    One aspect of the invention provides an axial piston pump, including a housing having an outlet port passage and a rotating group disposed in the housing. The rotating group has an axis of rotation and includes a barrel. The barrel includes: a plurality of cylinder bores, a plurality of piston assemblies (with each of the plurality of piston assemblies having a piston slideably disposed within one of the cylinder bores), and a shoe pivotably attached to and extending from each of the pistons. The rotating group is in fluid communication with the outlet port passage of the housing. A swashplate is disposed in the housing and is pivotable about a primary axis that is perpendicular to the axis of rotation of the barrel. The swashplate running surface has a fixed secondary angle with the primary swashplate axis that causes a side force during operation of the pump. The side force is transferred to the pump housing through a hydrostatic thrust bearing. 
         [0016]    According to another aspect of the invention, the axial piston pump is a variable displacement pump. 
         [0017]    According to another aspect of the invention, the hydrostatic thrust bearing includes a tiltable pad and a reaction plate that reacts the side force into the pump housing. 
         [0018]    According to another aspect of the invention, the axial piston pump further includes an arm integral to, or attached to, the swashplate, wherein the side force is transferred through the arm to the hydrostatic thrust bearing. 
         [0019]    According to another aspect of the invention, the tiltable pad includes a fluid passage. 
         [0020]    According to another aspect of the invention, the tiltable pad includes a first side having a socket which forms a semi-spherical joint with the arm, and a second side including a surface which remains flat against the reaction plate during operation of the swashplate. 
         [0021]    According to another aspect of the invention, the arm includes a ball that engages a socket of the tiltable pad. The socket, the ball, and the arm each have a fluid passage therethrough. 
         [0022]    According to another aspect of the invention, the fluid passage through the arm transfers pressurized fluid from the hydrostatic thrust bearing through the swashplate and to a cradle bearing. 
         [0023]    According to another aspect of the invention, the outlet port is in fluid communication with the hydrostatic thrust bearing. 
         [0024]    According to another aspect of the invention, a method is provided for hydrostatically supporting a load generated by a secondary swashplate angle within a housing of an axial piston pump. The method includes: providing a rotating group within the housing (the group having an axis of rotation); providing a swashplate pivotable about a primary axis and oriented at a fixed secondary angle with the primary swashplate axis (the secondary swashplate angle resulting in a side force during operation of the pump); and transferring the side force through a hydrostatic thrust bearing to the pump housing. 
         [0025]    The foregoing and other features of the invention are hereinafter fully described and particularly pointed out in the claims, the following description, and the annexed drawings setting forth in detail one or more illustrative embodiments of the invention, such being indicative, however, of but one or a few of the various ways in which the principles of the invention may be employed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a partial cross-sectional view of an exemplary hydraulic pump in accordance with the invention, taken in a plane parallel to the primary axis of the swashplate. 
           [0027]      FIG. 2  is a cross-sectional view of an exemplary hydraulic pump in accordance with the invention, taken in a plane perpendicular to the primary axis of the swashplate. 
           [0028]      FIG. 3  is an enlarged view of the hydrostatic thrust bearing of  FIG. 1 . 
           [0029]      FIG. 4  is a partial cross-sectional view of a second exemplary hydrostatic thrust bearing and housing configuration in accordance with the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    Referring now to the drawings,  FIG. 1  shows an exemplary pump  10  constructed in accordance with the invention (shown in partial cross-section). The pump  10  includes a housing  12  having an inlet port  13  and an outlet or discharge port  14 . Pressurized fluid is communicated from the discharge port  14  via a passage, passages, or transfer tubes  16  to various hydrostatic bearings (discussed below). 
         [0031]    The pump  10  includes a rotating group  17  disposed in the housing  12 . The rotating group  17  includes a barrel  18  having a plurality of cylinder bores  20 . The barrel  18  further includes a plurality of piston assemblies  22  wherein each of the piston assemblies  22  includes a piston  24  slideably disposed within one of the cylinder bores  20 . The piston assemblies  22  may further include a piston shoe  26  pivotably attached to and extending from the piston  24 . The rotating group  17  is in fluid communication with the inlet port  13  and the outlet port  14 . 
         [0032]    Further, the pump  10  includes a swashplate  28  disposed in the housing  12 . The swashplate  28  is pivotable about a primary axis  30  that is perpendicular to a barrel axis of rotation  32 . The swashplate includes a running surface (shown in  FIG. 1  as plane  33 ) that may have a fixed secondary angle  34  with respect to the primary axis  30 . The piston shoe  26  is slideably disposed on the swashplate running surface. 
         [0033]    Turning to  FIG. 2 , the pump  10  is shown in a cross-section cut perpendicular to the swashplate primary axis. The swashplate  28  acts as a cam and in a variable displacement pump the swashplate  28  is pivotable. For conceptual purposes the swashplate running surface or cam can be represented by the plane  33 , the orientation of which, in combination with the rotation of the barrel  18 , provides the cam action that leads to piston reciprocation and thus pumping. The angle between a vector normal to the cam plane  33  and the barrel axis of rotation  32 , called cam angle  40 , is one variable that determines the displacement of the pump  10  or the amount of fluid pumped per barrel rotation. If the pump  10  is a variable displacement pump, then the cam angle  40  may vary during operation to change the flow rate. 
         [0034]    As the pistons  24  engage the swashplate  28  via the piston shoes  26 , they apply a force to the swashplate  28 . The force applied to the swashplate  28  is ordinarily reacted by a single cradle bearing or pair of cradle bearings  42 . 
         [0035]    Turning back to  FIG. 1 , the force applied by the pistons  24  to the swashplate  28  is no longer purely normal to the primary swashplate axis  30  due to the secondary angle  34  of the swashplate  28 . Therefore, in addition to the normal forces reacted by the cradle bearings  42 , a side force F is applied to the swashplate  28  that is reacted by a hydrostatic thrust bearing  44  (shown in a dashed circle and in  FIG. 3 ). 
         [0036]    The hydrostatic thrust bearing  44  includes a tiltable pad  46  and a reaction plate  48 . The reaction plate  48  provides a rigid, smooth, flat surface  50  against which the tiltable pad  46  rides. The reaction plate  48  is secured to the housing  12  and reacts the side force F generated by the secondary swashplate angle  34  into the housing  12 . Additionally, the reaction plate  48 , via passages  52 , communicates pressurized fluid from the passages  16  in the housing  12  to the tiltable pad  46 . 
         [0037]    Turning to  FIG. 3 , the tiltable pad  46  may include on one side a machined recess  54  that, by its pre-determined geometry, provides a hydrostatic pressure balance with the reaction plate  48 . This hydrostatic balance may be set such that a small resultant force exists to prevent separation between the tiltable pad  46  and the reaction plate  48 . Such a separation may result in leakage and possible loss of supply fluid to the cradle bearings  42  shown in  FIGS. 1 and 2 . 
         [0038]    Additionally, a pair of bearing surfaces  50  and  58  between the reaction plate  48  and the tiltable pad  46  should be sufficiently flat, smooth, and rigid to prevent uneven bearing loads that may contribute to leakage and wear at this interface. The reaction plate  48  may be constructed from a strong, hard material such as heat-treated steel to provide a good bearing surface and for adequate support against structural and pressure loads. 
         [0039]    The tiltable pad  46 , on its opposite side, may include a semi-spherical socket  60  that mates with a semi-spherical ball  62  that is integral to an arm  64 . The tiltable pad  46  contains a passage  65  through which pressurized fluid is communicated from the passage  52  to the semi-spherical ball  62 . The pressurized fluid provides lubrication for this interface and is further provided through a passage  66  in the semi-spherical ball  62 , and through the arm  64 , to the cradle bearings  42 . The resulting semi-spherical joint provides the freedom to allow the tiltable pad  46  to remain flat against the reaction plate  48  despite any misalignments caused by machining tolerances and structural deflections as the swashplate  28  tilts about the primary pivot axis. Due to the bearing loads on both of its sides, the tiltable pad  46  may be constructed of a good bearing material such as bronze. 
         [0040]    Turning back to  FIG. 1 , the arm  64 , which may be either integral with or affixed to the swashplate  28  by a bolted connection or other securement, acts as a conduit for the transfer of pressurized fluid from the tiltable pad  46  through passages  67  in the the swashplate  28  and into hydrostatic bearing pockets  68  at the cradle bearing  42  interface. The force F generated by the secondary swashplate angle  34  is transmitted through the arm  64  to the tiltable pad  46 . The arm  64  should be of sufficient strength to withstand the applied bending loads and may be constructed of a strong material such as steel. 
         [0041]    There are numerous optional designs for communicating fluid from the discharge port  14  to the tiltable pad  46 . The design selected for use in  FIG. 1  is merely exemplary and the invention is not intended to be limited to this configuration. 
         [0042]    Turning now to  FIG. 4 , a second thrust bearing and housing configuration constructed in accordance with the invention is shown in partial cross-section. The housing  12 ′ includes a second configuration of passage, passages, or transfer tubes  16 ′ for communicating pressurized fluid to the various hydrostatic bearings discussed above. 
         [0043]    In the configuration of  FIG. 4 , a hydrostatic thrust bearing  44 ′ includes a tiltable pad  46 ′ and a reaction plate  48 ′. The reaction plate  48 ′ again provides a rigid, smooth, flat surface against which the tiltable pad  46 ′ rides. The reaction plate  48 ′ is secured to the housing  12 ′ and reacts the side force generated by the secondary swashplate angle into the housing  12 ′. Additionally, the reaction plate  48 ′ communicates pressurized fluid from the passages  16 ′ in the housing  12 ′ to the tiltable pad  46 ′ and through an arm  64 ′ to supply fluid to the cradle bearings (not shown). 
         [0044]    The configuration of the housing  12 ′ and the securement of reaction plate  48 ′ in  FIG. 4  differs from that of  FIG. 1  in that it reduces external leakpaths and thus the need for external seals. 
         [0045]    Although the invention has been shown and described with respect to a certain preferred embodiment or embodiments, it is obvious that equivalent alterations and modifications will occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including any reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several illustrated embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application.