Abstract:
A method for coating a pump component ( 23, 31 ), in particular, a part of an axial piston pump ( 7 ), having the steps of providing a blank of the component ( 23, 31 ), providing at least one recess in the blank, filling a powdery coating material into the associated recess, melting the coating material under a protective gas atmosphere and material-removing processing of the blank to form at least one sliding and/or bearing surface ( 6 ) from the coating.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method for coating a pump component, in particular, a part of an axial piston pump. 
     BACKGROUND OF THE INVENTION 
     Modern designed pumps, as they are used, for example, in hydraulic systems in the form of axial piston pumps, include components having surface areas that are highly stressed during operation. To ensure reliable operation over long operating periods, relevant components, at least on the highly stressed areas, are conventionally provided with a coating. The coating gives the relevant component the desired surface characteristics at the critical points. In particular, coated areas are intended to form sliding points or bearing points, which points enable a relative movement between the relevant component and a fixed pump component that is wear-resistant and low-friction. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to present an improved method that permits the formation of a coating having particularly advantageous surface characteristics. 
     According to the invention, this object is basically achieved by a method comprising a soldering powder method for the coating, in which a powdery coating material is poured into a corresponding molded recess in the carrier material of the blank and melted therein. Melting occurs under a protective gas atmosphere and results in a soldered connection, such that the layer formed is inseparably bonded to the carrier material of the blank. To form a corresponding finished part, the blank is subsequently finished in a material-removing process, for example, machined by turning and/or milling. 
     To form a coating especially suitable for slide bearing points, a coating in the form of a bronze layer may be particularly advantageously formed, by filling the respective recess with powdery tin bronze CuSn6. 
     The blank is preferably roughly turned from steel, for example, carbon steel. 
     Melting may be particularly advantageously carried out without pressure in a vacuum furnace. In this way, the melting process takes place with no temperature differences between the steel and the bronze. Flaws, as these occur during molding, such as bonding flaws, solder cracks and the like, may be eliminated as a result. 
     A tempered state can be achieved through targeted cooling following melting. A tempered steel is preferably used as a blank material for this purpose, wherein tempered states in the range of 750 to 850 N/mm 2  may be achieved. 
     The method according to the invention is particularly suitable for forming a coating on a control plate and/or a guide shoe of an axial piston pump. 
     A control plate of an axial piston pump may be produced by advantageously producing a blank in the form of a circular disk having a central orifice. A recess in the form of an annular space may be formed on the blank between a projecting circumferential edge and an edge in the form of a projecting annular rib surrounding the orifice. 
     A guide shoe of an axial piston pump may be produced by producing a blank in the form of a circular cylindrical body. In the one cylinder surface of the body a recess in the form of an annular surface delimited by a projecting circumferential edge is formed. 
     Other objects, advantages and salient features of the present invention will become apparent from the following detailed description, which, taken in conjunction with the drawings, discloses preferred embodiments of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring to the drawings that form a part of this disclosure: 
         FIG. 1  is a side view in section of an axial piston pump having components, which have a coating produced in accordance with the method according to an exemplary embodiment of the invention; 
         FIG. 2  is a perspective view of the control plate of the axial piston pump of  FIG. 1 , which is provided with a coating produced in accordance with the method according to an exemplary embodiment of the invention; 
         FIG. 3  is a perspective view of a blank, which is prefabricated for the production of the control plate of  FIG. 2 , coated in accordance with the method according an exemplary embodiment of to the invention; 
         FIG. 4  is a side view in section of the blank of  FIG. 3 ; 
         FIG. 5  is a perspective view of the blank of  FIGS. 3 and 4 , having a powdery coating material filled in the recess formed therein; 
         FIG. 6  is a side view in section of the blank of  FIGS. 3 and 4  having a powdery coating material filled in the recess; 
         FIG. 7  is a side view in section of the finished control plate after final machining of the blank, which control plate is shown in the perspective view in  FIG. 2 ; 
         FIG. 8  is a side view in section of a blank for producing a guide shoe coated in accordance with the method according to an exemplary embodiment of the invention for the axial piston pump of  FIG. 1 ; 
         FIG. 9  is a side view in section of the blank of  FIG. 8 , having a powdery coating material filled in the recess formed; and 
         FIG. 10  is a side view in section of the finished guide shoe after final machining of the blank of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows an axial piston pump of a swash plate design. The pump includes components that have a coating  24  produced in accordance with a method according to an exemplary embodiment of the invention. A rotating driveable cylindrical drum  1  is provided in a pump housing  7  in the manner conventional for axial piston pumps of this type. The cylindrical drum is attached to a swash plate  3 , which may be pivoted for adjusting the output and, therefore, the system pressure producible by the pump, the pivot axis of the swash plate  3  being identified by numeral  37  in  FIG. 1 . The pump housing  7  includes an upper part  9  situated above in the drawing and a lower part  11 . A drive shaft  13  for the cylindrical drum  1  is mounted in the upper part  9  in a tapered roller bearing  16  for rotational movement about the axis  15  and in the lower part  11  via a slide bearing  17 . The cylinder chambers  19  of the cylindrical drum  1  with pistons  21  guided therein (in the section plane of  FIG. 1  only one cylinder chamber  19  is visible) contact a control plate  23 , in the drawing at the lower cylinder end, which control plate abuts the lower housing part  11 . The control plate  23  has control openings  25  and  26  for connections between a connection  29  on the intake side and a connection  27  on the pressure side into the cylinder chambers  19  of the cylindrical drum  1 . The control plate  23 , depicted separately in  FIG. 2  and  FIG. 7 , has a coating  24 , see  FIG. 2 , on the upper side in the drawing facing the cylindrical drum  1 . The coating is produced in accordance with a method according to the invention, and forms the bearing surface on which the slightly concavely curved bottom surface  8  of the cylindrical drum  1  slides as it rotates. In  FIG. 7 , parts  6  of the coating  24  form bearing points between a central orifice  14  and adjacent control openings  25  and  26 . 
     As the cylindrical drum  1  moves, the pistons  21  slide past one guide shoe  31 , each on the sliding surface  33  situated on the underside of the swash plate  3 . A guide shoe  31  is depicted separately in  FIG. 10 . The guide shoes  31  are connected to the upper piston side of the associated piston  21  in a ball-joint manner. The ball joint is formed by a ball-shaped head  34  on the piston  21  and a ball socket  36  in the guide shoe  31 . The ball joint is secured by a crimp  38  on the guide shoe  31 . Oil bores  35  in the ball-shaped head  34  and guide shoe  31  form an access for fluid, such as hydraulic oil, for lubricating the sliding surface  33  and for hydrostatic release of the guide shoe. As in the case of the control plate  23 , the guide shoes  31  also having a coating  24  produced in accordance with a method according to the invention. 
     As previously mentioned, the swash plate  3 , to set the flow volume, is adjustable about the pivot axis  37 , which lies in the plane of the sliding surface  33  of the swash plate  3 . This pivot axis  37  is defined by the swash plate mounting formed between the swash plate  3  and the upper part  9 . The mounting includes a plastic bearing shell  39  on the upper part  9 , on which the swash plate  3  with a concave-shaped sliding surface  41  is guided. In the sliding surface  41 , a passage opening  43  expanding conically upward is formed in the swash plate  3  to allow entry of the drive shaft  13 . Guide rails  45  projecting from the sliding surface  41  are provided on both sides next to the opening  43  as part of the swash plate mounting. For pivotally moving the swash plate  3  about the pivot axis  37 , the side of the swash plate  3  located to the left in  FIG. 1  is screwed to a pivot lever  47 . Pivot lever  47  extends parallel to the axis  15  next to the cylindrical drum  1  and is movable at its lower end  49  in  FIG. 1 , in a direction extending perpendicular to the drawing plane, in order to effect a corresponding pivotal movement of the swash plate  3  about the pivot axis  37 . The pivot lever  47  is screwed to an inner thread situated in a bore  51  on the associated side of the swash plate  3 . 
     A flexible tube  5  forming a component of a feed and pressure device is, as shown in  FIG. 1 , arranged laterally next to the cylindrical drum  1  in a direction extending parallel to the axis  15 . The flexible tube  5  is seated at its lower end in  FIG. 1  in a mount  53  in a connector block  55  on the lower housing portion  11 . The mount  53  allows for an axial displacement of the flexible tube  5 . The block  55  includes a connection channel to the pressure side  29 , not visible in  FIG. 1 , which opens into the mount  53  of the flexible tube  5 . The upper end of the flexible tube  5  is flexibly connected to the swash plate  3  via a connecting piece  58  arranged laterally outside of the sliding surface  33  on the underside of the swash plate  3 . The flexible connection is realized by a type of ball joint and includes a ball-shaped head  59  at the upper end of the flexible tube  5 , which is accommodated in a ball socket  61  of the connecting piece  58 . The flexible tube  5  is braced via the connecting piece  58  against the swash plate  3 . For this purpose, a disk spring set  63  is arranged between the lower end of the flexible tube  5  and the bottom of the mount  53 . A fluid orifice  67  in the connecting piece  58  continues the fluid connection to the pressure side  29  beyond the tube opening on the ball-shaped head  59  to the swash plate  3 . Adjoining the orifice  67  of the connecting piece  58  are lubrication channels  73 ,  75  formed within the swash plate  3 , of which only some are visible in  FIG. 1  and of which the vertical ducts  75  lead to the locations of the sliding surface  41  relevant for supplying lubricant for the swash plate mounting. 
       FIGS. 3 through 7  illustrate the steps of a method according to the invention for forming a coating  24  on the control plate  23  of the axial piston pump depicted in  FIG. 1 .  FIG. 3  shows the form of a blank  81  produced from tempered steel as a rotating part in the form of a circular cylindrical disk having a circular recess  83  between a projecting circumferential edge  85  and a projecting annular rib  87  at the edge of the central orifice  14 . As shown in  FIG. 4 , the recess  83  is consistently recessed in the radial direction (i.e., recess  83  continuously deepens in the radially outward direction).  FIGS. 5 and 6  illustrate the further method step, in which the recess  83  is filled with the powdery coating material  4 . To form the coating  24  in the form of a bearing bronze, a powdery tin bronze CuSn6 is filled in as the filler material. The coating material  4  is then melted under a protective gas atmosphere without pressure in a vacuum furnace. The bronze layer is inseparably bonded to the carrier part as a result of the soldered connection produced. A specific tempered state can be achieved through targeted cooling following the soldering process.  FIG. 7  illustrates the finished state of the control plate  23 , once the semi-finished product shown in  FIGS. 5 and 6  is rendered in the form shown in  FIG. 2  and  FIG. 7  by turning. As is apparent, slide bearing regions  6  are formed from the coating  24  between the central orifice  14  and the control opening  25  on the intake side, and the control opening  26  on the pressure side, as well as between a flange-like circumferential region  89  and the control openings  25 ,  26 , as they are identified in  FIG. 7 . The bearing region  6  formed from the coating  24  is convexly curved, corresponding to the slightly concavely curved bottom surface  8  of the cylindrical drum  1 . The region  6  of the coating  24  is lapped together with the bottom surface  8  of the cylindrical drum  1 . 
       FIGS. 8 through 10  illustrate the coating of a guide shoe  31  of the axial piston pump of  FIG. 1  in accordance with a coating method according to the invention. Again, a steel blank  82  is roughly turned so as to form a recess  84  in the form of an annular surface bordered by a projecting circumferential edge  88 , which is filled with the powdery coating material  4 , again tin bronze. Melting then takes place as described in the example of the control plate  23 . The semi-finished product formed after melting is depicted in  FIG. 9 . The guide shoe  31  is brought to final form as shown in  FIG. 10  by subsequent final machining by turning and milling (some of the machining lines are indicated by dashed lines  91  and  93  in  FIG. 9 ). As is apparent here, the upper side of the guide shoe  31 , provided for interacting with the sliding surface  33  of the axial piston pump, is machined in such a way that bearing regions  6  in the form of adjoining circular rings are formed from the coating  24  on the circumferential region of the upper side  95 . 
     While various embodiments have been chosen to illustrate the invention, it will be understood by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined in the claims.