Patent Publication Number: US-10760554-B2

Title: Hydrostatic axial piston machine

Description:
This application claims priority under 35 U.S.C. § 119 to patent application no. DE 10 2018 205 010.4, filed on Apr. 4, 2018 in Germany, the disclosure of which is incorporated herein by reference in its entirety. 
     The disclosure concerns a hydrostatic axial piston machine with a cylinder drum which comprises a substantially hollow cylindrical base body in which cylinder bores are arranged around a central axis, and liners which are pressed into the cylinder bores with a fitting outer diameter, an outer end face of which is situated in the region of an opening of the cylinder bores and an inner end face of which is situated deep inside the cylinder bores. In its outer half starting from the outer end face, each liner has an axially delimited, circumferential recess region in its outer casing surface. In particular, hydrostatic axial piston machines in swashplate design are equipped with liners. 
     BACKGROUND 
     Hydrostatic axial piston machines are operated under widely varying operating conditions, wherein the liners must tolerate the loads occurring in the operating states so that the axial piston machine does not fail prematurely. For example, DE 10 2013 208 454 A1 or DE 1 703 403 each describe a hydrostatic axial piston machine in which the liners pressed into the cylinder bores are intact hollow cylinders without any recesses. Such intact liners withstand the loads occurring at high operating pressures and have no tendency to crack. At high operating pressures, the leakage through the gap between the liners and the displacement pistons moving to and fro in the liners is so high that the guide faces between the displacement pistons and the liners are well lubricated and the generated heat is dissipated well. If however the axial piston machine is running with a high rotation speed and at the same time the operating pressures are only low, there is a tendency towards adhesion between the liners and the displacement pistons and hence so-called piston seizure, since the leakage through the gap between the liner and the displacement piston is reduced, so that generated heat is not dissipated so well and the components heat to the point that their expansion is no longer negligible. 
     DE 10 157 248 A1 discloses a hydrostatic piston machine in which the liners pressed into the cylinder bores have an axially delimited, circumferential recess region in their outer half starting from their outer end face. In the known liners, called compensated liners, the recess region is formed as a circumferential groove which has a contour formed as a circle arc in an axial section passing through the axis of the liner. The groove is situated in a region in which the greatest forces occur between the displacement piston and the liner, and in which the liner is accordingly exposed to the greatest heat load. The circumferential groove creates a clearance between the liner and the wall of the cylinder bore, into which the liner can expand. Accordingly, liners with a recess can withstand operating states with high rotation speeds and low pressures. If however a hydrostatic axial piston machine with compensated liners is operated mainly at high operating pressures, there is a possibility that the liners will break prematurely. 
     Therefore, it is already known that a hydrostatic axial piston machine for a specific application in which primarily the one operating state occurs is equipped with intact liners, and a hydrostatic axial piston machine for another application case in which primarily the other operating state occurs is equipped with compensated liners. Such a procedure means additional cost in planning of a plant and additional cost in procurement, stockholding and provision of the different liners, and in assembly of the different variants of an axial piston machine. Also, there are many applications in which the one operation type does not occur significantly more often than the other operation type, but fulfilment of the requirements for both operation types would be of great advantage. 
     SUMMARY 
     The disclosure is based on the object of refining a hydrostatic axial piston machine, with a cylinder drum which comprises a substantially hollow cylindrical base body in which cylinder bores are arranged around a central axis, and liners which are pressed into the cylinder bores with a fitting outer diameter, an outer end face of which is situated in the region of an opening of the cylinder bores and an inner end face of which is situated deep inside the cylinder bores, wherein in its outer half starting from the outer end face, each liner has an axially delimited, circumferential recess region in its outer casing surface so that its possible applications may be extended to application cases with high forces acting between the liners and the displacement pistons. 
     For a hydrostatic axial piston machine with the features given above, this object is achieved in that the recess region of a liner is configured such that in an axial section through the liner enclosing the axis of the liner, the depth of the recess region increases more than once and decreases more than once. 
     Whereas in the known hydrostatic axial piston machine, in an axial section through the liner enclosing the axis of the liner, the depth of the groove initially increases to a maximal depth and from there again decreases towards zero, in a hydrostatic axial piston machine according to the disclosure, the recess region of the liners is configured such that in an axial section, the depth increases several times and decreases several times. In this way, inside the recess region, protrusions occur which do not reach as far as the fitting outer diameter of the liner, or several recesses are formed which are separated from each other by faces lying on the fitting outer diameter of the liner. In some cases, the liner is thus permanently supported inside the recess region, or supported after a specific outward expansion, by the cylinder wall so that the risk of cracking is reduced. Secondly, because of the regions of greater depth between a liner and a cylinder wall, there remains sufficient clearance into which the liner can expand so there is no danger of piston seizure. 
     Advantageous embodiments of an axial piston machine according to the disclosure can be found in the claims, description, and drawings. 
     It is particularly advantageous if a recess is present in the recess region and has a protrusion which ends below the fitting outer diameter of the liner. The protrusion creates an increase in wall thickness of the lining inside the recess region, whereby the strength of the lining in the recess region is increased. The liner is first supported by this protrusion when a specific deformation of the liner has already occurred. The protrusion may have a pointed or linear highest point. 
     The base of the recess with the protrusion is curved at least in regions and the curvature there is greater than zero and less than infinity. A curvature greater than zero means that the base has no corner, and a curvature less than infinity means that the base has no straight portions. This reduces the risk of cracking. Advantageously, the entire base of the recess including the protrusion is formed without edges and without straight portions. 
     In a particularly advantageous refinement, the protrusion takes up most of the width, preferably between 70% and 85% of the width of the recess. 
     It is favorable to form the base contour of a recess with circle arcs. Thus, from an edge lying on the fitting outer diameter of a liner, the base of the recess falls away following a first circle arc which is concave towards the fitting outer diameter, i.e. curving inward, and has a small radius, to a lowest point and thereafter is continued. A second circle arc, which is convex towards the fitting outer diameter, i.e. curving outward, and has a substantially larger radius, adjoins the first circle arc with a constant tangent. Preferably, the radius of the first circle arc is approximately 2 mm and the radius of the second circle arc is between 20 mm and 60 mm. The second circle arc forms the protrusion. 
     Advantageously, the protrusion is situated centrally inside the recess. 
     The form of the recess base described above is particularly clear if the data are regarded as data for the course of the base in an axial section through a liner. 
     In a particularly preferred embodiment, the recess runs in the manner of a ring around the liner and has the same contour in each axial section enclosing the axis of the liner. 
     Several individual recesses may be present in the recess region of the liner, which are separated from each other by faces or edges lying on the fitting outer diameter of the liner. Preferably, several individual recesses are arranged successively in the axial direction of the liner, and run in the manner of a ring around the liner and have the same contour in each axial section enclosing the axis of the liner. 
     Here preferably, a first individual recess as a main recess has a first dimension in the axial direction of the liner, and a second individual recess directly following the main recess in the axial direction of the liner, as a secondary recess, has a second dimension in the axial direction which is smaller than the first dimension. 
     Preferably, the main recess has the protrusion which ends below the fitting outer diameter of the liner. The secondary recess has no protrusion, but rather in an axial section enclosing the axis of the liner has a contour following which the depth of the secondary recess increases only once and decreases only once, and in particular is a circle arc. 
     A secondary recess may be situated on either side of the main recess, wherein the secondary recess on the one side of the main recess is formed in the same way as the secondary recess on the other side of the main recess. Thus a symmetrical form of the recess region is possible in a radial plane standing perpendicularly on the axis of the liner and running through the highest point of the protrusion. 
     Two individual recesses arranged successively in the axial direction of the liner may have different maximal depths, i.e. the one individual recess is deeper than the other individual recess. 
     Advantageously, the recess region comprises a central main recess running in the manner of a ring around the liner, and on each side of the main recess a secondary recess running in the manner of a ring around the liner, wherein the secondary recesses are separated by the ring faces of the main recess lying on the fitting outer diameter of the liner. Here, the main recess is substantially wider than the two secondary recesses, preferably five to seven times wider than the two secondary recesses, and in its middle comprises the protrusion which ends below the fitting outer diameter of the liner, while the depth of the secondary recesses increases and decreases only once. The secondary recesses therefore have no protrusion. 
     In a further advantageous embodiment, the main recess between the protrusion and a ring face lying on the fitting outer diameter of the liner has a maximal depth which is greater than the maximal depth of the secondary recesses. The minimal depth of the main recess, namely the depth at the highest point of the protrusion, is smaller than the maximal depth of the secondary recesses. 
     Even if the main recess between the protrusion and a ring face lying on the fitting outer diameter of the liner has a maximal depth which is greater than the maximal depth of the secondary recesses, the main recess and the secondary recesses may fall away from a ring face lying on the fitting outer diameter of the liner following a contour which can be described by the same mathematical formula, in particular a circle arc, first steeply and then flatly to the respective maximal depth. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Two exemplary embodiments of a hydrostatic axial piston machine according to the disclosure are shown in the drawings, wherein only an extract of a liner of the second exemplary embodiment is shown. The disclosure is now explained in more detail with reference to the drawings. 
       The drawings show: 
         FIG. 1  a longitudinal section through a hydrostatic axial piston machine of adjustable displacement volume according to the first exemplary embodiment, 
         FIG. 2  an extract from  FIG. 1  in enlarged scale, wherein however a liner is shown in a front view, 
         FIG. 3  in a further enlarged scale, an axial section through part of the liner fitted in the hydrostatic axial piston machine from  FIGS. 1 and 2 , with a first embodiment of a circumferential recess region, and 
         FIG. 4  a section similar to that of  FIG. 3  through a second liner which has a different configuration of a circumferential recess region. 
     
    
    
     DETAILED DESCRIPTION 
     The hydrostatic axial piston machine in  FIGS. 1 and 2  is a variable displacement pump in swashplate design for hydrostatic drives in an open hydraulic circuit. The volume flow of the variable displacement pump is proportional to the drive rotation speed and to the displacement volume, i.e. to the quantity of pressure medium conveyed per revolution. The variable displacement pump comprises a pot-like housing  10 , a connecting plate  11  closing the open end of the housing  10 , a drive shaft  12 , a cylinder drum  13 , a control plate  14  situated between the cylinder drum  13  and the connecting plate  11  and fixed relative to the connecting plate, and a swashplate  15  which is adjustable in its tilt relative to the axis of the drive shaft and is also called a pivot cradle because of its pivotability. The pivot cradle  15  may be pivoted from a position in which it stands almost perpendicular to the axis of the drive shaft  12 , in a direction towards a maximum pivot angle. 
     The pivot angle may be not reduced fully to zero in order to always have a certain quantity of pressure fluid for cooling, for supplying the adjustment, for compensating for leakage fluid and for lubrication of all moving parts. 
     The drive shaft  12  is mounted rotatably in the base of the housing  10  and in the connecting plate  11  via roller bearings  16  and  17 , and extends centrally through the cylinder drum  13 . The latter is connected rotationally fixedly but axially movably to the drive shaft  12  and therefore may lie on the control plate  13  without play. 
     The cylinder drum  13  has a substantially circular cylindrical base body  21  with a central axis  22 . The base body  21  has a central cavity  23  extending in the direction of the central axis, through which the drive shaft  12  passes. The base body  21  contains, evenly divided over the circumference, a plurality of for example nine cylinder bores  24  lying on the same pitch circle and, in this exemplary embodiment, set slightly obliquely relative to the central axis  22  which coincides with the central axis of the drive shaft  12 . The diameter of the cylinder bores  24 , in a front portion starting in an outer end face towards the swashplate and extending over around 60% of the total length of the cylinder bore, is slightly larger than in a rear portion. The two portions of a cylinder bore  24  transform into each other at a radial step. 
     In the portion of each cylinder bore  24  of larger diameter, a liner  25  is inserted which with its one outer end face  26  lies approximately flush with the opening of the cylinder bore  24 . The fitting outer diameter D of the lining  25  and the inner diameter of the cylinder bore  24  are adapted to each other such that a press fit exists between the liner and the cylinder drum. A displacement piston  28  is guided axially movably in each liner  25 . The inner diameter of a liner  25  is slightly smaller than the diameter of the rear portion of the cylinder bore  24 , so that in this rear portion a clear ring gap exists between a displacement piston  28  and the wall of the cylinder bore  24 . 
     At the end facing the pivot cradle  15 , the displacement pistons  28  each have a ball-shaped head  29  which dips into a corresponding recess of a sliding shoe  30 , so that a ball joint is formed between the displacement piston and the sliding shoe. By means of the sliding shoe  30 , the displacement pistons rest on the pivot cradle  15  so that the displacement pistons  28  execute a reciprocating motion in the liners and in the cylinder bores during operation. The size of the stroke is determined by the tilt of the pivotable pivot cradle  15 . To adjust the tilt of the pivot cradle  15 , an adjustment device  31  is provided. 
     On their side facing away from the cylinder drum  13 , the control openings of the control plate  14  are open to a first fluid channel  34  and to a second fluid channel  35  which are formed in the connecting plate  11 , wherein fluid channel  34  leads to a pressure port (not shown in  FIG. 1 ) and fluid channel  35  leads to a suction port  36  on the connecting plate  11 . The cylinder bores  24  are open via passages to the end face of the cylinder drum  13  facing the control plate  14 . On rotation of the cylinder drum  13 , the passages sweep over the control openings of the control plate  14  and during a revolution are successively connected to the fluid channel  34  and the fluid channel  35  of the connecting plate  11 . 
     In their outer half, in their outer casing surface  39 , the liners  25  have a circumferential recess region  40  which is configured such that the risk of piston seizure at a high rotation speed and low operating pressure, and the risk of breakage of the liner at a high operating pressure, are reduced in comparison with known hydrostatic axial piston machines with compensated liners. 
     In the liners  25  from  FIGS. 1 to 3 , the circumferential recess region  40  consists of a single circumferential recess or groove  41  within which the outer diameter of a liner  25  is overall smaller than the fitting outer diameter with which the liner  25  is pressed into a cylinder bore  24 . The width of the recess  41  in the axial direction of the liner  25  is around 25 to 30% of the total length of the liner. In an axial section through the liner  25  as shown in  FIG. 3 , and in which by definition the axis  32  of the liner lies, the contour starting from each peripheral side edge  42  of the recess  41  first falls in an inwardly curved first arc  43 , which is convex viewed from the fitting outer diameter D of the liner and has a small radius of for example 2 mm, within a short axial extent amounting to around 10% of the width of the recess  41 , to a lowest point  44  which lies for example 0.5 mm below the fitting outer diameter of the liner. The first circle arc extends upwardly for a short distance beyond the lowest point in order then to transform with constant tangent, i.e. without an edge, into an outwardly curved second circle arc  45 , the radius of which is for example 50 mm. In this way, in the recess  41 , a protrusion  46  is created which extends over around 80% of the width of the recess  41  and the highest point  47  of which is situated in the middle of the recess  41  and lies for example 0.2 mm below the fitting outer diameter of the liner. Viewed three-dimensionally, the protrusion  46 , like the recess  41  with the cross-section evident from  FIG. 3 , runs on the outside around the liner  25 . 
     In a hydrostatic axial piston machine which is equipped with liners  25  as shown in  FIGS. 1 to 3 , because of the recess  41 , on severe heating of the particularly heavily loaded portion, a clearance is present into which the material of the liner may expand. Secondly, because of the protrusion  46 , the strength of the liner  25  in the region of the recess  41  is improved in comparison with the known solution, so that the liner there deforms less. Also, on very severe load from high forces and an associated deformation, the liner is supported by the wall of the cylinder bore at the highest point of the protrusion  46 . As a whole therefore, the risk of breakage of the liner is very low. 
     In the liner  25  of which an extract is shown in axial section in  FIG. 4 , the recess region  50  consists of three circumferential recesses  51 ,  52  and  53 . These circumferential recesses are separated from each other by the circumferential ring faces  55  and  56  on the fitting outer diameter D of the liner. The middle recess  51  is narrower than the recess  41  in  FIG. 3 . Its width is around only 83% of the width of the recess  41 . In its axial contour, the recess  51  however resembles the recess  41  from  FIG. 3 . Starting from each circumferential side edge of the recess  51 , the contour first falls in a first circle arc, which is convex or inwardly curved viewed from the fitting outer diameter D and has a small radius of for example 2 mm, within a short axial extent to a lowest point which lies for example 0.5 mm below the fitting outer diameter D of the liner. The first circle arc extends upwardly for a short distance beyond the lowest point in order then to transform with constant tangent, i.e. without an edge, into a second outwardly curved circle arc with a radius of for example 30 mm. In this way, a protrusion  57  is created in the recess  51  which extends however only over around 75% of the width of the recess  51 . The highest point of the protrusion  57  is again in the middle of the recess  51  and for example lies 0.2 mm below the fitting outer diameter of the liner. 
     The two side recesses  52  and  53  of identical form have a contour in the axial direction which is formed by a single circle arc  54  with a radius of 2 mm. This radius is selected such that at a maximal depth of 0.3 mm of a recess  52  and  53 , a desired width of each recess  52  and  53  results of around 11% of the total width of the recess region  50  from  FIG. 4 . Each of the ring faces  55  and  56  extends over around 5%, and the middle recess  51  extends over around 68% of the total width of the recess region  50 . The middle recess  51  could therefore be regarded as the main recess, and the two side recesses  52  and  53  as secondary recesses. The convex first circle arc of the main recess  51  and the circle arcs  54  of the secondary recesses  52  and  53  have the same radius, so that the main recess  51  and the secondary recesses  52  and  53  fall from a ring face  55  or  56  lying on the fitting outer diameter D of the liner  25 , following the circle arcs  54  with the same radii, first steeply and then flatly to the respective maximal depth. 
     It has been found that, because of the additional support from the ring faces  55  and  56  inside the recess region  50 , while retaining the same quality with regard to the avoidance of piston seizure, a liner is even more able to resist breakage than a liner with the recess region  40  from  FIG. 3 . However, the production of the recess region  50  is more complex in comparison with the production of the recess region  40 . 
     LIST OF REFERENCE SIGNS 
     
         
           10  Pot-like housing 
           11  Connecting plate 
           12  Drive shaft 
           13  Cylinder drum 
           14  Control plate 
           15  Pivot cradle 
           16  Roller bearing 
           17  Roller bearing 
           21  Base body of  13   
           22  Centre axis of  12  and  21   
           23  Central cavity in  21   
           24  Cylinder bore 
           25  Liner 
           26  Outer end face of  25   
           27  Inner end face of  25   
           28  Displacement piston 
           29  Ball-shaped head of  28   
           30  Sliding shoe 
           31  Adjustment device 
           32  Axis of  25   
           34  First fluid channel 
           35  Second fluid channel 
           36  Suction port 
           39  Outer casing surface 
           40  Recess region  40  in  25   
           41  Recess 
           42  Side edge  41   
           43  First circle arc 
           44  Lowest point of  43   
           45  Second circle arc 
           46  Protrusion 
           47  Highest point 
           50  Recess region 
           51  Circumferential recess 
           52  Circumferential recess 
           53  Circumferential recess 
           54  Circle arc in  51 ,  52 ,  53   
           55  Circumferential ring face 
           56  Circumferential ring face 
           57  Protrusion 
         D Fitting outer diameter of  25