Abstract:
A hydraulic pump, in particular an adjustable axial piston pump, has at least one piston ( 22 ) movable in a reciprocating manner in a longitudinal direction within a pump housing during operation of the hydraulic pump. The piston ( 22 ) has a link head ( 24 ), a piston top ( 54 ) opposite the link head ( 24 ), and at least one hollow chamber ( 60 ) surrounded at least partially by a piston housing ( 62 ) that substantially or completely terminates each hollow chamber ( 60 ) towards the outside. A piston ( 22 ) for such hydraulic pump is also provided.

Description:
FIELD OF THE INVENTION 
     The invention relates to a hydraulic pump, in particular an adjustable axial piston pump, as well as a piston for such a hydraulic pump. 
     BACKGROUND OF THE INVENTION 
     In the operation of a hydraulic, adjustable axial piston pump, mechanical energy is converted to hydrostatic energy. The mechanical energy is supplied, for example, by an electric motor. In the structure of an axial piston pump, hydraulic pumps of this type normally have numerous cylinders, which are attached to a rotatable shaft such that they extend in the axial direction, and are disposed on a partial circle, offset at a radius to the shaft axis. Pistons are displaceably disposed in the cylinders. The pistons, which rotate with the cylinder, are moved, for example, by an adjustable inclined plate. For this movement, in order to minimize the friction, a piston/sliding shoe connection is provided between the inclined plate and the pistons. 
     One problem of conventional axial piston pumps is the mass of the rotating piston, which is normally made of a single piece. Because of this mass, significant energy is needed to accelerate the piston, and to subsequently decelerate it, this energy being associated with significant wear in the region of the piston/sliding shoe connection. In order to reduce the inertia of the mass, a bore hole has been provided previously, in the base of the piston, such that the piston is substantially hollow. A bore hole of this type in the piston has the disadvantage, however, that the dead volume in the cylinder is increased. As a result of the dead volume, there are compression losses and increased volume flow, or pressure, fluctuations, which are a reflection of a lower pumping efficiency. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an improved hydraulic pump and a piston for such hydraulic pump, with which, or by which, respectively, the efficiency of the hydraulic pump is or will be increased. 
     The first part of the object is achieved by a hydraulic pump. The second part of the object is achieved by a piston. 
     According to the invention, the pump, in particular in the form of an adjustable axial piston pump, has at least one piston that can move back and forth in a longitudinal direction inside a pump housing during operation of the hydraulic pump. The piston has a link head and a piston base lying opposite the link head, as well as at least one hollow chamber. The hollow chamber is encompassed, at least partially, by a piston housing. The piston housing closes the respective hollow chamber, substantially or entirely, to the outside. 
     To the extent that reference is made, presently and below, to the hollow chamber being substantially closed off to the outside, this reference means that, starting from the environment, no relevant media discharge, in particular in the form of a fluid, occurs through any existing openings in the piston housing and/or in its end piece, taking the form of a lid, for example, into the interior of the respective hollow chamber. An effective barricade of this type can still be achieved when the hollow chamber is substantially closed, even when, for example, openings are to be formed in the piston housing, such as capillary holes. Substantially closed thus means that even when such openings are formed, passing through the hollow chamber, these openings nevertheless prevent, in a damping manner, or prohibit, in the manner of a capillary hole, accordingly, the flow through, in the manner of a choke or a screen. 
     The hydraulic pump according to the invention is distinguished in that the advantage of having a piston with a lower mass is not obtained at the cost of having a larger dead volume as a result of a hollow chamber that is open to the pump housing. Consequently, the inertia forces are advantageously reduced with a minimized dead volume in the cylinder. Consequently, the efficiency of the hydraulic pump is improved, because less energy must be used in order to convey the same quantity of fluid. Furthermore, a hydraulic pump of this type is distinguished by less wear in the region of the piston/sliding shoe connection. 
     In a particularly advantageous manner, the at least one hollow chamber extends in the direction of the longitudinal axis or parallel to the longitudinal axis of the piston in the piston housing, and passes through the piston, starting from an axial spacing of the link head to the piston base. A bore hole of this type can be readily made in the piston, starting from the piston base. An eccentric configuration of the at least one hollow chamber has the advantage that, in addition to the hollow chamber, a concentric through hole can be provided in the piston, in order to be able to relieve and lubricate the link head during the operation of the hydraulic pump. 
     Advantageously, the at least one hollow chamber is closed by the piston housing or by a lid that can be attached to the piston housing. With the lid solution in particular, there is the possibility of providing the hollow chamber in the form of a bore hole in the piston, and to subsequently close the hollow chamber with the lid. The lid can likewise be produced as a rotating part, and be placed in the piston housing with no substantial gaps. As a result, fissures are prevented during the subsequent welding, and it is possible to entirely seal the hollow chamber. 
     The lid and/or the piston can have an equalizing mechanism, which connects the at least one hollow chamber to the environment in a media conducting manner. The development of excess pressure in the hollow chamber can be prevented by the equalizing mechanism, which pressure could lead to process instabilities during the joining. 
     The equalizing mechanism can comprise a spiral groove along an outer circumferential surface of the lid, which is covered when the lid is placed in the housing by the piston housing, except for at a discharge point. The discharge point forms, as part of the equalizing mechanism, the media conducting connection, via the spiral groove, between the environment and the at least one hollow chamber in the piston. An excess pressure can be dissipated during the overall process of inserting the lid in the piston housing by the spiral groove. 
     Particularly advantageously, the spiral groove opens into a bevel in the lid, or borders this bevel. At its other end, the intake point lies opposite the discharge point. The outer diameter of the bevel preferably decreases continuously toward the link head, up to a final diameter. As a result of this bevel, with numerous hollow chambers, they are all ventilated in a uniform manner during the joining of the lid. 
     The piston housing can advantageously have a head part at the piston base. By the head part, the lid can be coupled to the piston base, preferably by welding or brazing. The head part forms a guide and/or attachment possibility for the lid on the piston housing. In this manner, the orientation of the lid is ensured. This structure is particularly advantageous in the case of a subsequent welding of the lid to the piston housing. 
     The lid can be permanently connected to the piston housing by brazing or an electron beam welding process. The piston can preferably be heat treated after the welding. Prior to brazing, the piston housing and the lid are rough-turned, so that the brazing gap between the lid and the piston housing, in particular the walls of the piston housing, is 80 μm to 120 μm, to obtain an optimal capillary effect, such that the brazing gap between the piston and the lid fills with solder. The lid should additionally be provided with a sufficiently large bevel on the side facing the bore holes to prevent oxidation during the brazing in a vacuum. After the turning, the lid is inserted in the piston housing, and the two are non-detachably connected to one another in a material bonded manner in a vacuum, using a metallic filler material, a solder, at 450° C. to 1,200° C. This process is also referred to as joint soldering. The hollow chamber formed by the bore holes is closed in this manner. As a result, the dead volume of the piston cannot increase during the ongoing operation due to a leak in the piston housing/lid connection. 
     The electron beam welding seam advantageously closes at least the discharge point of the spiral groove. In this manner, an entry of fluid into the at least one hollow chamber of the piston is impeded significantly. 
     Preferably numerous hollow chambers are in the piston, preferably having the same diameter, provided on a partial circle, offset to one another. This arrangement enables the volume of the hollow chamber to be maximized, and at the same time, a central bore hole extending in the direction of the longitudinal axis to be provided, which can be used for lubricating the link head during ongoing operation. 
     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 a preferred embodiment 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 through an axial piston pump according to an exemplary embodiment of the invention; 
         FIG. 2  is a side view in section of the piston of  FIG. 1 ; 
         FIG. 3  is a perspective view of the piston with a lid of  FIG. 1 ; 
         FIG. 4  is a side view in section of the piston of  FIG. 2 , wherein the production of the electron beam welding seam is shown; 
         FIG. 5  is a perspective view of a piston according to the invention, having a lid affixed thereto; and 
         FIGS. 6 to 8  are a top view, a side view, and a perspective view of the lid, respectively. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A detail of a hydraulic pump  10  in the form of an axial piston pump is shown in cross section in  FIG. 1 . Stationary cylinders  14  are disposed on a rotatable shaft  12  as part of the pump housing  16 , such that the cylinders  14  rotate together with the shaft  12 . An odd number of cylinders  14  are provided, which are disposed on a partial circle, circumferentially offset to one another. The longitudinal axes  18  of the cylinders  14  all have the same radius RZ to the longitudinal axis  20  of the shaft  12 . A piston  22  that can be displaced longitudinally is disposed in each cylinder  14 . A spherical surface  26  is provided on a link head  24  of each piston  22 , which is retained in a corresponding receiving part  28  of a sliding shoe  30 . Each sliding shoe  30  is provided with a longitudinal bore  32  for lubricating the receiving part  28 . The sliding shoe  30  is supported on an adjustment mechanism such that it can slide, with an inclined plate  36  disposed therebetween. A fluid, in particular a hydraulic fluid, can be drawn into the cylinder  14  in a first stroke via a fluid inlet  38 . The fluid can be subsequently discharged from the cylinder  14  in a second stroke via a fluid outlet  40 . Thus, during a half rotation of the shaft  12 , a cylinder  14  is filled, and during the other half of the rotation of the shaft  12 , it is again emptied. A high and uniform pump performance can be obtained in this manner. 
     Each piston  22  according to the invention is shown in detail in  FIGS. 2 to 5 . The piston  22  has a link head  24 , which transitions via a neck  42  into a piston body  44 . The link head  24  has the spherical surface  26 . A through hole  46  extends through the piston  22 , which is beveled at one end  48 . A total of five axial bore holes  52  having the same diameter D are provided parallel to the through hole  46 , and thus, to the longitudinal axis L of the piston  22 . These extend from the piston base  54  toward the link head  24  and terminate at an axial spacing A to the link head  24 . An annular recess  56  for a lid  58  is provided on the piston base  54 . The bore holes  52  of the piston  22  form hollow chambers  60 , which are disposed on a partial circle, circumferentially offset to one another. Thus, the hollow chambers  60  are provided in a piston housing  62 . As a result of the annular recess  56 , a protruding wall section  64  of the through bores  46  remains intact. This wall section  64  forms a coupling part  66  for the lid  58  that is to be coupled thereto, and centers this lid when it is inserted. 
     As is shown more clearly in  FIGS. 6 to 8 , the lid  58  is a disk-shaped rotating part having a central bore hole  68  corresponding to the outer diameter of the protruding wall section  64 . A spiral groove  72  is provided, as a part of an equalizing mechanism  74 , on the outer circumferential surface  70  of the lid  58 . Groove  72  connects the hollow chambers  60  to the environment U in a media conducting manner. When the lid  58  has been placed in the piston housing  62 , the spiral groove  72  is covered by the piston housing  62 , except for at a discharge point  76 . The discharge point  76 , as a part of the equalizing mechanism, establishes the media conducting connection between the environment U and the hollow chambers  60  in the piston  22  via the spiral groove  72 . The discharge point  76  is disposed on a back surface  80  of the lid  58 . The spiral groove  72  opens into an intake point  84  in a bevel  86  at the opposite, front surface  82 . The outer diameter of the bevel  86  decreases toward the link head  24 , up to a final diameter AD, when the lid  58  is in the inserted state EZ. 
     As  FIG. 4  shows in greater detail, after the lid  58  has been inserted into the piston housing  62 , the lid  58  is welded, using an electron beam welding process, along the entire circumference of its internal diameter DI and its external diameter DA. As a result, the discharge point  76  is also closed. The electron beam  88  is oriented parallel to the longitudinal axis L of the piston  22  thereby, to enable the most efficient possible welding. For this welding, the lid  58  should be inserted and oriented in the piston housing  62  without gaps to prevent the formation of fissures during the welding. The electron beam welding is carried out in a vacuum, in accordance with DIN 4063. This welding involves an I-axial seam, in which the electron beam  88  is parallel to the longitudinal axis L. As a result of the welding, the hollow chambers  60  are entirely closed off to the environment U to the outside. After the welding, the piston  22  is finished and heat treated. 
     The finished piston  22  is depicted in  FIG. 5  in a perspective sectional view. It is distinguished by a reduced mass, due to the hollow chambers  60 . The lid  58  prevents an increase in the dead volume in the cylinder  14 . In this manner, no additional compression losses can occur through the hollow chambers  60 . As a result, the efficiency of the hydraulic pump  10  is advantageously improved. 
     While one embodiment has 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.