Abstract:
The invention relates to an axial piston machine, in particular, an air-conditioning compressor for motor vehicles, with at least one piston, an essentially cylindrical piston shaft and an enclosure, which encloses a tilt ring or a tilt disc and a piston slipper sliding on said tilt ring or said tilt disc, whereby the enclosure has spherical recesses for housing the piston slipper.

Description:
The present invention relates to an axial piston machine, in particular, an air-conditioner compressor for motor vehicles, including at least one piston having a substantially cylindrical piston body and a brace that embraces a tilting ring or a tilting plate and piston shoes sliding on said tilting ring or said tilting plate; the brace having spherical cap-shaped depressions for receiving the piston shoes, said depressions being located on the side of the piston body and on the opposite side. 
   BACKGROUND 
   Also, the piston braces of known pistons project radially outward relatively far from the piston axis so as to provide sufficient space for the movement of the tilting plate or tilting ring and the piston shoes, while being sufficiently stiff to prevent the piston shoes from falling out. 
   Moreover, in tilting-ring or tilting-plate type compressors, the lubrication of the radially outward sliding surface of the brace between the piston and the housing is of great importance, especially if, when using CO 2  as the refrigerant, the machine dimensions are smaller than in conventional refrigerant compressors because of the high pressures. As a consequence of the tight spaces in a CO 2  compressor, the spaces between the pistons where lubricant can be distributed, for example, in the drive chamber, become narrower and narrower. The larger the peripheral housing region covered by the piston brace is compared to the exposed peripheral region of the housing, the more difficult is it to supply lubricant to this region. If in tilting plate machines or tilting ring machines of this type, such as for CO 2  applications, the degree of coverage by the piston brace becomes relatively high so that there are only small gaps between the individual piston brace regions for introducing lubricant between the peripheral regions, insufficient lubricant supply and friction damage may occur in this area. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to devise an axial piston machine which will overcome some or all of these disadvantages. 
   It is a feature of an axial piston machine according to the present invention that the cylindrical piston body and the brace are two separate parts from which the piston can be assembled. The advantage of this is that the materials and manufacturing methods for these differently shaped parts can be adapted to the different loads. 
   Also preferred is an axial piston machine whose brace can be made from a strip of sheet metal and, after suitably shaping the metal strip, is connectable to the cylindrical piston body, which can be made as a deep-drawn part of sheet metal. Another preferred axial piston machine is one in which the opening in the brace can be made by punching. Also, the seating of the piston shoes can be produced or largely preformed during the forming process of the brace. Also preferred is an axial piston machine in which the cylindrical piston body and the brace can be made from a steel material. A further preferred axial piston machine is one in which the brace and the cylindrical piston body can be joined together by laser welding or resistance welding. Moreover, the hollow space between the brace and the piston body can be airtight, or nearly airtight. 
   Another embodiment of the axial piston machine according to the present invention is characterized in that, after the brace and the piston body are assembled together, the piston is first provided with an adhesive base coat, for example by phosphating, in a layer thickness of about 2-3 μm, and then provided with a surface coating of PTFE in a layer thickness of about 10 μm. 
   An axial piston machine according to the present invention is characterized in that a first spherical recess is disposed within the bridge of the brace, that is, in the inner radial region of the piston brace. 
   Moreover, the spherical running surfaces of the piston shoes in the brace can seamlessly merge into the first spherical recess in the bridge of the brace, and the spherical running surfaces and the first spherical recess can preferably have equal sphere radii. Also preferred is a first spherical recess which can be processed by and during the machining of the piston shoe bearing surfaces, or fully produced by this machining process. Preferably, the bridge of the brace is adapted, on its inner side, to the contour of the tilting ring or tilting plate by a second spherical recess of larger radius outside the first spherical recess. In accordance with the present invention, the second spherical recess allows the bridge of the brace to be shifted as close as possible to the tilting ring or tilting plate. This reduces the bending load on the brace by shorter lever arms. The first spherical recess only slightly reduces the stiffness of the brace, because the first spherical recess is located very close to the bending line. This is made possible because the second spherical recess shifts the bending line of the brace so close to the tilting plate or tilting ring that the stiffness against bending during the suction movement is only slightly reduced compared to a brace without a first spherical recess. Because of this, less material and installation space are needed, which reduces costs. 
   The present invention furthermore provides an axial piston machine in which the outer side, as a sliding surface, of the brace has at least one opening to the inner radial region of the brace which faces the tilting plate or tilting ring. Preferably, the at least one opening serves to supply lubricant to the sliding surface, because the sliding surface is located in the peripheral region covered by the piston brace and, therefore, can only with difficulty be supplied with the lubricant contained in the refrigerant in the drive mechanism housing. 
   In a further embodiment of the axial piston machine according to the present invention, the peripheral region of the piston brace which is designed as a sliding surface has several and/or differently shaped openings or opening regions. 
   A preferred axial piston machine is one in which the peripheral region of the piston brace which is designed as a sliding surface has formed therein pocket-shaped regions opposite the drive mechanism housing wall which serves as a running surface, said pocket-shaped regions being supplied via at least one lubrication opening. 
   The above-mentioned embodiments allow the peripheral region covered by the piston brace to be supplied with lubricant that is spun off of the rotating tilting plate or tilting ring by centrifugal forces and thus enters the space between the piston and the housing wall through the openings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described with reference to the figures, in which: 
       FIG. 1  is a cross-sectional view of a two-part piston; 
       FIG. 2  shows the same piston in a side view; 
       FIG. 3  shows the same piston in a perspective view; 
       FIG. 4  illustrates the machining of the rear spherical cap; 
       FIG. 5  illustrates the machining of the front spherical cap; 
       FIG. 6  is a top view of a piston; 
       FIG. 7  shows a piston with a first spherical recess; 
       FIG. 8  illustrates the pressures determining the axial forces on the piston; 
       FIG. 9  shows four representations of the first and second spherical recesses; 
       FIG. 10  illustrates the production of the first spherical recess; 
       FIG. 11  shows a piston in a portion of a tilting ring machine; 
       FIG. 12  is a cross-sectional view through a piston brace; 
       FIG. 13  is a top view of a piston; 
       FIG. 14  shows the piston arrangement in the drive mechanism chamber; 
       FIG. 15  shows a rotating tilting plate with a piston according to the present invention; 
       FIG. 16  shows a piston having a lubricant pocket. 
   

   DETAILED DESCRIPTION 
     FIG. 1  is a cross-sectional view of a two-part piston  1 , which is composed of a cylindrical piston body  3  and a U-shaped piston brace  5 . The two parts are joined together in region  7  by laser welding. However, other joining techniques, such as resistance welding, brazing, adhesive bonding, press-fitting, crimping, or form-locking connections, such as circlips, threads, etc., are possible as well. Cylindrical piston body  3  can preferably be made from thin sheet steel using a deep drawing process. The use of steel sheet has the advantage that the piston body can have a thin-walled design in spite of high pressure loads, and that it can advantageously be produced in large quantities by deep drawing. However, the blanks of the parts can also be produced by cold extrusion, hot extrusion, or forging. In some cases, it can be advantageous to manufacture such a piston from aluminum materials. Piston brace  5  can be made from a strip of sheet steel, which is then suitably shaped from a flat metal strip into the U-shaped piston brace in a stamping tool. The use of a two-piece design has the advantage that the two component parts of different basic shapes can be manufactured separately according to their shapes instead of having to be formed from a single piece in a much more complicated way. Thus, piston brace  5  can also be advantageously made from a steel material, which provides significantly greater resistance to the forces occurring during operation. Piston brace  5  has a cylindrical opening  9  at its side opposite the piston body  3 ; center axis  11  of said cylindrical opening coinciding with center axis  13  of cylindrical piston body  3 . On the inner side of the piston brace  5 , opening  9  leads into a spherical cap-shaped region  15 , which serves to receive a spherical cap-shaped piston shoe (not shown here). Likewise, at the side of brace  5  next to cylindrical piston shaft  3 , a spherical cap-shaped region  17  which is capable of receiving a second piston shoe is provided within the brace; the two piston shoes sliding on a tilting plate or tilting ring located therebetween. The portion of brace  5  next to piston body  3  is provided with a smaller opening  19  which provides a connection to the interior of piston body  3 . Piston body  3  is provided at its front end with two grooves  21  which serve to receive piston sealing rings. 
   In  FIG. 2 , piston  1  of  FIG. 1  is shown in a side view in which it can be seen that piston brace  5  is provided on its upper side with a beveled step  23  leading to a raised region  25  with which piston  1  bears against a corresponding sliding surface of the housing inside the housing. Moreover, piston body  3  has two bevels  29  and  27  leading to a region  31  which has a larger diameter and acts as a guiding cylinder section within a cylinder liner. Within piston brace  5 , axis  13  of cylindrical piston body  3  crosses an axis  33 , the crossing point defining the center of a spherical shape of the spherical cap-shaped piston shoes and of bearing regions  15  and  17 , respectively. 
   In  FIG. 3 , piston  1  of  FIG. 1  and  FIG. 2  is depicted in a perspective view showing regions  35  in which a suitable coating can be applied to the steel components by means of an adhesive base coat, especially by phosphating the entire piston in a layer thickness of about 2-3 μm, and by subsequently providing marked regions  35  with an anti-friction coating of PTFE in a layer thickness of about 10 μm. However, other coatings, such as WC/C coatings, or heat treatments, such as case hardening, are conceivable as well. The two-piece piston design is especially preferred because the different component shapes can be produced using manufacturing processes that are optimally adapted to the shapes. As has been mentioned earlier, deep-drawing of thin sheet steel is a suitable method for cylindrical piston body  3 , while initial punching of sheet steel and subsequent bending to shape is convenient for piston brace  5 . During the punching process, it is also possible to produce openings  9  and  19  and to preform spherical cap regions  15  and  17  in advance. In some cases, however, it may also be appropriate to select aluminum materials. 
     FIG. 4  is a cross-sectional view of a piston  40 . In this representation, piston  40  is shown solid in cross-section and may be manufactured, for example, from an aluminum material. Piston  40  likewise has a cylindrical piston body  42  and a brace  44 ; the end of brace  44  opposite the piston body  42  being provided with an opening  46  which corresponds to opening  9  of  FIG. 1 . Opening  46  allows a cutting tool  48  to be inserted into the interior of piston brace  44 . Thus, by rotation  52  about piston-cylinder axis  50 , which corresponds to cylinder axis  13  in  FIG. 1 , rear spherical cap  54  can be produced by the machining motion on standard lathes, which is not possible in the case of known forms of braces without such an opening  46 . Moreover, in this machining process, a centering center  56  or a weight-reduction hole (not shown) can be made in piston body  42 , and a second centering center  58  can be produced on the front face of piston body  42 , these openings allowing dimensionally stable chucking during further processing steps on lathes and grinding machines, for example, for turning and grinding a coating. 
     FIG. 5 , finally, shows the machining of the front spherical cap shape  62  in brace  44 . A cutting tool  60  for machining the front spherical cap shape  62  is also inserted through opening  46  in brace  44 , and the spherical cap shape is then produced by suitably moving tool  60  axially and vertically during simultaneous rotation  52  of piston  40  about axis  50 . This means that the piston brace has been altered by opening  46  in piston brace  44  in such a manner that the cutting motion for machining the spherical shape can be produced by rotating piston  40  about axis  50  of piston body  42 , that is, about the cylinder axis. Therefore, neither special machines nor special devices are needed; the machining is not carried out under interrupted cutting conditions, that is, the cutting tool does not move out of and back into the workpiece during machining and, in addition, it is possible to provide the edges of the spherical shape with lubricating wedge chamfers. 
   This results in both considerable cost savings and better quality of manufacture and in operational advantages for a machine having such pistons. Of course, the present invention is not limited in its use to air-conditioner compressors, but may also be used in other axial piston machines, such as axial piston pumps, that use diverse tilting-ring or tilting-plate mechanisms including piston shoes. Moreover, the present invention allows the coating of the piston to be processed on lathes and grinding machines in a very stable chucking position. Therefore, this type of chucking is considerably stiffer and more accurate compared to chucking in a centering center on the left side of the brace. As processing variants to the representations in  FIGS. 4 and 5 , it is also possible to use reversible inserts having a ready-made spherical contour. With these reversible inserts in a tool holder, it is also possible to machine both sides simultaneously. 
     FIG. 6  is a top view of a piston  1  according to the present invention. Here, the reference numerals used correspond to those in  FIGS. 1 and 2  again. In the top view of  FIG. 6 , it can be seen, in particular, that bevel  23 , which is shown in a side view in  FIG. 2 , leads to a raised region  25  on piston brace  5 , said raised region serving as a suitable contact and sliding surface with respect to the compressor housing wall. This sliding surface  25  exists both on the right and on the left side, that is, here, both at the top and bottom of  FIG. 6 , and serves both as a sliding surface and to prevent the piston from rotating or tilting sideways. 
     FIG. 7  is a perspective view of a piston  1  having a brace  5  and a first spherical recess  80  in the of brace  5 . The components described hereinbefore are provided with the same reference numerals as, for example, in  FIG. 1 , and will not be described again in order to avoid repetitions. Additionally shown here is first spherical recess  80 , which can be produced simultaneously with the bearing surfaces  62  and, not visible here,  54  for the piston shoes during machining by rotation about cylinder axis  50 . 
     FIG. 8  shows the pressures and forces acting on piston  1  and piston brace  44 ,  5  during the suction stroke. During the suction stroke of the piston, tilting ring  82  or the tilting plate pulls piston  1  out of the cylinder block by means of the piston shoes (not shown here). In this process, the movement of tilting ring  82  results in forces PA acting within piston brace  44 ,  5 , said forces PA being transferred to brace  44  or  5  by tilting ring  82  and the piston shoe and tending to bend the brace  44 ,  5  open. In addition, inside the drive chamber, drive chamber pressure PC acts on the piston cylinder surface in region  62  of piston brace  44 ,  5 , said drive chamber pressure acting against suction pressure PS on the front face of cylindrical piston body  42  or  3 , respectively. Thus, during operation, brace  44  or  5  of piston  1  is primarily loaded by bending during the suction movement. In order to achieve maximum possible stiffness during this bending, the back of the brace is shifted radially inward as close as possible to tilting ring  82  or to the tilting plate, respectively; so that, in comparison with a brace that projects radially further outward, recess  80  is located so close to the bending line of the brace that the stiffness against bending during the suction movement is only slightly reduced compared to a brace that does not have a spherical recess  80  and which is located radially further outward and therefore has longer lever arms for bending. To this end, the back of brace  44  or  5 , respectively, is adapted, on its inner side, to the cylindrical contour of the tilting ring or tilting plate and their moving positions by a second spherical recess  81 , which can be seen in  FIG. 9 . This results in a space-saving geometry, thus reducing the cost of the compressor. 
   In  FIG. 9 , spherical recesses  80  and  81  in the piston brace are shown in four views.  FIG. 9   a  is a view of the inner side of the brace  44  or  5 , respectively, showing the first spherical cap-shaped depression, that is, spherical recess  80 , in the bridge of brace  44 ,  5  and a second spherical contour  81 , which can occupy the entire inner side of the brace.  FIG. 9   b  shows section B-B of  FIG. 9   a . Front bearing surface  62  for the front piston shoe can be seen within the cut brace  44  or  5 . In cut region  88  of the piston brace, both the raised sliding regions  25  of  FIG. 6 , which serve as a contact surface with housing contour  86 , and first spherical recess  80  can be seen. It can also be clearly seen that second spherical recess  81  provides sufficient clearance from tilting ring contour  84  and the envelope generated by its pivotal movement, and that section B-B follows the contour of tilting ring  84  and housing  86 . 
     FIG. 9   c  shows that bearing surface  62  and/or opposite bearing surface  54  can seamlessly merge into first spherical recess  80  and form a spherical shape. Similarly to  FIG. 9   b , the section shows second spherical recess  81 , which is of considerably larger diameter than first spherical recess  80 , and thus is adapted to the radius of the envelope of the tilting plate or of tilting ring  84  of  FIG. 9   b.    
   In  FIG. 9   d , spherical recess  81  of the inner surface of the brace can be seen particularly well from the side because of the perspective view. It also becomes clear that by producing bearing surface  62  for the piston shoe, the first spherical recess is simultaneously produced as well. 
     FIG. 10  illustrates the production of first spherical recess  80  together with the production of piston shoe bearing surfaces  62  and  54 . While a tool is rotated about an axis of rotation  90  within brace  44 , the piston is rotated about its cylinder axis  50  to produce the spherical shape in the brace; the cutting edge of tool  92  producing the contours of bearing surfaces  62  and  54  for the piston shoes as well as spherical recess  80 . Thus, by, as it were, shifting the brace  44 ,  5  closer to the outer contour of the tilting plate or tilting ring, spherical recess  80  is formed in the portion of brace  44  parallel to the cylinder axis during the machining of the spherical shape in brace  44  when using a rotation of piston  1  about its cylinder axis  50 . This allows a cost-effective manufacturing process to be combined with a cost- and space-saving geometry of brace  44 . 
   Spherical recess  80  is also formed when rotating the piston about an axis extending perpendicular between tool rotation axis  90  and cylinder axis  50  and running through their intersection point (center of the sphere), while a non-rotating tool cuts the spherical or nearly spherical contour. 
     FIG. 11  shows a piston  101  having a cylindrical part  102  which is capable of reciprocating in the opening of a cylinder block  103  and whose cylindrical outer surface therefore forms the first sliding surface with respect to cylinder block bore  113 . Piston  101  merges into a second part  104 , which serves as brace for tilting plate  106  and piston shoes  105 . When tilting plate  106  rotates, piston  101  is caused to reciprocate by means of piston shoes  105 , during which tilting plate  106  slides between the flat sides of piston shoes  105 , while the piston shoes  105  themselves perform a kind of a wobbling motion within the piston brace. Piston brace  104 , in turn, slides in drive mechanism housing  107 , which is only partially shown, along inner wall  108 , thus forming a second sliding surface  109 . 
     FIG. 12  is a cross-sectional view through the piston brace, such as is described in the present invention and shown in a top view in  FIG. 13 . In  FIG. 13 , second sliding surface  109  is pierced by an opening  111  via which lubricant from the interior, especially that thrown off of rotating tilting plate  106  ( FIG. 11 ) by centrifugal forces, is conveyed through the piston brace to the upper side, that is, to sliding surface  109 . A sliding surface  115  for the front piston shoe can be seen on piston brace front surface  114  below the cut piston brace surface  112 ; a piston shoe  105  of  FIG. 11  performing a wobbling motion in said sliding surface. Opening  111  can be frustoconical in shape so as to catch the lubricant over a wider area. 
     FIG. 13  is a top view of a piston according to the present invention. Cylindrical piston part  101 , the diameter of which is smaller than that of the curvature of brace surface  109 , is adjoined by the second portion, piston brace  104 . Located in brace part  104  is the opening  111  provided for lubricant supply, which here is, for example, oval in cross-section, and is surrounded by a pocket-shaped recess  116  for receiving the lubricant. This pocket-shaped opening  116  is shown in cross-section in  FIG. 16 . Also indicated in  FIG. 13  is an adjacent piston brace  104 ′, which shows that, in a machine according to the present invention, there is only a very small gap  117  left between the piston braces, which may not be sufficient for lubricant supply to brace sliding surfaces  109 . 
     FIG. 14  shows, by way of example, six piston braces in cross-section in one machine. It can be seen that there are only very narrow gaps  117  between the six piston braces  104  with their sliding surfaces  109 . This means that lubricant that is spun off of a rotating slant or tilting plate within the drive chamber may possibly not be able to make its way from gaps  117  to the center of sliding surfaces  109 . 
   Therefore, in accordance with the present invention, and as shown in  FIG. 15 , lubricant supply is provided through opening  111  in that lubricant  118  is passed, under the action of the centrifugal forces, from the rotating slant or tilting plate or tilting ring  106  through the opening to surface  109 , where it can lubricate second sliding surface  109  between the drive chamber housing wall and the radial outer surface of piston brace  104 . 
     FIG. 16  also shows a cross-section of a piston brace  104  according to the present invention; the surface  109  of said piston brace being provided with a lubricant pocket  116  in addition to lubricant opening  111 ; it being possible for the lubricant pocket to be made in different shapes, as required. The purpose of this lubricant pocket is to collect the lubricant that has passed through opening  111  above the piston brace, and to supply it to sliding surface  109  in sufficient quantities.