Abstract:
The piston ( 1 ) has a ball receptacle ( 14 ) for the rod head ( 4 ) of the piston rod ( 2 ) of a swash plate compressor. For absorbing particularly high pressures the rod head ( 4 ) has a relatively large diameter, so that the ball receptacle ( 14 ) extends close to the circumference of the piston ( 1 ). The piston wall ( 15 ) laterally surrounding the rod head ( 4 ) has a circumferential indentation ( 21 ) which, in the inwards direction, forms an uninterrupted undercut enclosing the spherical rod head ( 4 ).

Description:
This is a divisional of application Ser. No. 08/610,250 filed on Mar. 4,1996, now U.S. Pat. No. 6,098,518. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a piston with a piston rod for high pressures, particularly a swash plate compressor, which has several circumferentially juxtaposed pistons, which are connected to rods, whose two ends are pivotably mounted by spherical rod heads on the one hand on the piston and on the other on the swash plate in ball receptacles. 
     DE-A-4024319 discloses a piston of a hydraulic swash plate mechanism, in which a substantially spherical rod head has a bevelled portion, which allows a fitting into a correspondingly shaped ball receptacle, if the rod is in a position outside its normal pivoting range. 
     The problem of the invention is to find a piston of the aforementioned type, which can absorb particularly high forces and is easy to manufacture. 
     SUMMARY OF THE INVENTION 
     According to the invention this problem is solved by a piston of the aforementioned type, wherein the piston is provided between the front and rear piston area with an uninterrupted, circumferential indentation, which inwardly forms an undercut enclosing the spherical rod head. 
     According to the invention this problem is also solved in that the ball receptacle is hemispherical and in the direction of the rear piston area is bounded by a soft elastic sealing ring sealingly surrounding the spherical piston rod, so that the piston rod is held in the ball receptacle by the gas pressure acting on the rear area of the piston. 
     In additions a method and a die for producing the piston provided with an indentation are proposed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Advantageous embodiments of the invention form the subject matter of the dependent claims and the following description with respect to the attached drawings, wherein show: 
     FIG. 1 A cross-section through a swash plate compressor. 
     FIG. 2 A side view of a piston according to the invention. 
     FIG. 3 A cross-section through the piston of FIG.  2 . 
     FIG. 4 A plan view of the piston of FIG. 2 with three curling tools in deformation engagement. 
     FIG. 5 A cross-section through a die for the hydraulic shaping of a piston according to the invention. 
     FIG. 6 A plan view of the die of FIG.  5 . 
     FIG. 7 A cross-section through a piston with a seal on the rod head circumference. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Pistons  1  with a ball joint-like connection between the piston rod  2  and the piston  1  are required due to the nature of the rod movement e.g. for swash plate compressors. FIG. 1 shows an embodiment of such an application. The basic structure and fundamental operation are known from the literature, e.g. U.S. Pat. No. 5,205,718. 
     The pistons  1  of the swash plate compressor  3 , whereof e.g. seven are juxtaposed in the circumferential direction of the compressor, are operated by rods  2 , whose two ends are pivotably mounted by ball ends  4 ,  5  on the one hand on the piston  1  and on the other on the swash plate  6  in ball receptacles  14 . 
     The swash plate  6  is held by a radial bearing  7  and an axial bearing  8  on a pivotably mounted plate carrier  10  rotating with the drive shaft  9 , so that the rotary movement of the tilting plate carrier  10  brings about a tumbling movement of the swash plate  6  and consequently the pistons  1  connected to the latter perform a lifting movement. The connection between the drive shaft  9  and the plate carrier  10  is provided by a driving joint  12  located at the end of a driving arm  11 . 
     The pivoting movement of the plate carrier  10  and the swash plate  6  mounted thereon about the driving joint  12  results from the difference in the pressures acting on both sides of the piston  1 . Corresponding to the inclination of the swash plate, the bottom dead centre of the piston movement is changed, whereas the top dead centre remains unchanged. The higher the pressure on the bottom of the piston or in the driving space  13  relative to the pressure on the top of the piston or on the suction side of the compressor  3 , the smaller the lift of the piston I and therefor the delivery of the compressor  3 . Since, for modifying the inclination of the swash plate, compressed gas is passed into the driving space, as a function of the application, high pressures can occur there. 
     For compressing to high pressures, which can e.g. be  120  bar for a supercritically operating CO 2  vapour compression refrigeration process, the piston shape and the size of the spherical rod ends  4 ,  5  must be so adapted due to the high forces acting on the piston  1  that the size of the diameter of the rod head  4  significantly approaches, e.g. to more than 70% the diameter of the piston  1 . The invention inter alia deals with the fact that the piston  1  or the connection between the latter and the piston rod  2 , in the case of low manufacturing costs, can so adapt to extreme conditions of this type that it is particularly reliable and stable even in the case of low surface pressures on the ball receptacles  14 . 
     In the embodiment of a piston according to the invention shown in FIGS. 2 and 3, the rod head  4  has a diameter of approximately 70% of the largest piston diameter. However, compared with the piston diameter in the vicinity of the location of the rod head  4 , where the piston  1  is inwardly offset, the rod head diameter is more than 80%, so that the remaining thickness of the lateral piston wall  15  enclosing the rod head  4  is reduced to a minimum with respect to the strength requirements. 
     In its front area  16  and its rear area, the piston  1  has a larger diameter in order on the one hand to provide a groove  18  for receiving a not shown piston ring and on the other a guide surface on the rear piston end. 
     In the inwardly offset, central area  20  of the piston  1 , as a result of a material deformation starting from a cylindrical wall, there is a circumferential indentation  21 , so that it so engages below the spherical rod head  4  that it is positively securely held in the ball receptacle  14  of the piston  1 . 
     FIG. 4 diagrammatically shows an example for the arrangement of rotatably mounted, disk-like deforming tools  22  on the circumference of the piston  1  during the manufacture of the circumferential indentation  21 , which is intended to provide the permanent connection between the piston  1  and the piston rod  2 . On their circumference the deforming tools  2  are rounded, so that during the rotation thereof relative to the piston  1  and radial pressure against the piston wall  15 , production is possible of the cross-sectionally rounded, circumferential indentation  21 . 
     Instead of by mechanical material deformation by means of the curling tools  22 , there can be a deformation of the piston wall  15  closely adapted to the shape of the spherical rod head by the pressure of a hydraulic medium. FIGS. 5 and 6 show a suitable forming die  24 . In said die are provided several cylindrical reception spaces  25  for in each case one piston  1  in which, starting from a central channel  26 , several branch lines  27  issue into an area on which is located the wall area  15  of the piston  1 . The e.g. seven reception spaces  25  and their arrangement in the die  24  correspond to those of the cylinder spaces of a swash plate compressor  3 , so that the die  24  is simultaneously an assembly tool. 
     The cylindrical inner wall of the reception spaces  25 , at least on the marginal area thereof, is adapted with small tolerance to the external diameter of the piston  1  in its rear area, so that a correspondingly small sealing gap there permits an adequate hydraulic pressure to curve outwards the previously cylindrical piston wall  15 , so that the spherical rod head  4  is positively enclosed in the piston  1 . An adjustment of the hydraulic pressure ensures that the rod head  4  is not jammed in the cavity of the piston  1 . For securing the piston-rod units  1 ,  2  during the hydraulic deformation in the die  24 , the latter is covered by a cover plate  29 . The latter can be turned backwards and forwards between two positions by guidance on four arcuate elongated holes  30  and threaded bolts  31  located therein, so that the openings  32  can be positioned above the reception spaces  25  when the piston-rod units  1 ,  2  are inserted in the die  24  and in order to move them into the closure position shown in FIG. 6, in which only the piston rod  2  can extend outwards through a slot  33  following onto the opening  32 . 
     Instead of having a circumferential indentation  21  of the piston wall  15 , the spherical rod head  4  can also be held by a sealing ring  35  inserted inwards on said piston wall  15 . As a result of a U-shaped cross-section of this sealing ring  35 , it has a soft elastic sealing lip  36  engaging on the rod head  4  and directed towards the piston rod  2  and is located adjacent to the hemispherical inner face  37  of the ball receptacle  14 . The sealing lip  36  engages on the area  38  of the rod head  4  tapering from the largest ball diameter towards the piston rod  2 . The sealing ring  35  is also so shaped that the inner face  37  of the ball receptacle  14  is substantially continuously adjacent to the sealing ring  35 . This avoids a clearance, so that the gap space  39  of the ball receptacle  14  bounded by the sealing ring  35  and rod head  4  cannot form a gas pressure, which could lead to a reduction of the force resulting from the gas pressure in the driving space  13  of the swash plate compressor  3  and which presses the rod head  4  against the inner face  37  of the ball receptacle.