Abstract:
A method for producing a connecting rod for an internal combustion engine, having a small connecting rod eye for holding a piston pin and having a large connecting rod eye for holding a crank pin, wherein at least one connecting rod eye has at least one geometric deviation from a cylindrical inner contour. It is provided according to the invention that the at least one connecting rod eye is formed by creating a bore with a cylindrical inner contour, and coating the bore with a coating comprising a resin with solid lubricant particles embedded therein. The coating forms the at least one geometric deviation from the cylindrical inner contour of the at least one connecting rod eye.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a divisional of U.S. patent application Ser. No. 12/086,738, filed on Jun. 18, 2008, which is the National Stage of PCT/DE2006/002258 filed on Dec. 15, 2006, which claims priority under 35 U.S.C. §119 of German Application No. 10 2005 061 062.5 filed on Dec. 21, 2005 and German Application No. 10 2006 013 399.4 filed Mar. 23, 2006. The international application under PCT article 21(2) was not published in English. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a connecting rod for an internal combustion engine, having a small connecting rod eye for accommodating a piston pin, and a large connecting rod eye for accommodating a crank journal, whereby at least one connecting rod eye has at least one geometric deviation from a cylindrical inside contour. The present invention furthermore relates to a method for the production of such a connecting rod. 
         [0003]    The connecting rod connects the piston of an internal combustion engine with the crankshaft. In this connection, the small connecting rod eye accommodates the piston pin, while the large connecting rod eye surrounds a crank journal. The small and the large connecting rod eye are provided with slide bearing surfaces, for example in the form of slide bearing layers. The slide bearing surfaces of these components are subject to very great mechanical stress. In particular, the piston pin can bend as a result of the tremendous forces that act on the piston during the oscillating movement of the piston. As a result, the connecting pin eyes are subject to great stress. Therefore, ways are being sought to relieve stress on the connecting rod eyes. This is done, for example, by means of local geometric changes in the usually cylindrical connecting rod eye, which reduce the mechanical stress. Such geometric changes can be, for example, stress relief pockets, oval, conical or spherical bores. This principle is already known for pin boss bores, see, for example, WO 96/07841 A1. Such geometric changes have been produced by means of complicated precision machining of the connecting rod eyes until now. 
         [0004]    Connecting rods having connecting rod eyes with slide bearing surfaces are known from the German patent application 10 2004 059 490.2. The slide bearing surfaces are coated with a self-lubricating coating made of a resin with solid lubricant particles embedded in it. 
       SUMMARY OF THE INVENTION 
       [0005]    It is the task of the present invention to make available a connecting rod in which at least one connecting rod eye can be provided with local geometric deviations in particularly simple manner. 
         [0006]    The solution consists in a connecting rod and a method for producing the connecting rod in which at least one connecting rod eye is formed from a bore having a cylindrical inside contour, that the bore is provided with a coating comprising a resin with solid lubricant particles embedded in it, and that the coating forms the at least one geometric deviation from the cylindrical inside contour of the connecting rod eye. The method according to the invention is characterized in that first, a bore having a cylindrical inside contour is produced, and subsequently, a coating agent comprising a resin with solid lubricant particles embedded in it is applied to the inside surface of this bore, by means of a coating tool, so that the resulting coating forms the at least one geometric deviation from the cylindrical inside contour of the at least one connecting rod eye. 
         [0007]    With the present invention, it is possible to produce connecting rod eyes having at least one geometric deviation from the cylindrical inside contour, and having a self-lubricating coating of their inside surfaces, in one and the same work step. This means a significant saving in time and costs. The complicated and very complex cutting machining of the metallic inside surfaces of the connecting rod eyes for the purpose of introducing the at least one geometric deviation is eliminated. Furthermore, bearing bushings or bearing shells are no longer necessary to achieve sufficient lubrication and an anti-seizure effect of the connecting rod eyes. Coating can take place without prior application of an adhesion layer. The desired dimensional accuracy of the coating rod eyes is reliably achieved. The strength and therefore the useful lifetime of the bearing of piston pin and/or crank journal are significantly improved, as a result of the improved lubrication properties as compared with the previously known coatings made of metal alloys. 
         [0008]    The at least one geometric deviation can be configured as at least one stress-relief pocket and/or ovality (for example as a heightwise or crosswise ovality) and/or as a shaped bore, as it is disclosed in WO 96/07841 A1 for pin bores, for example. 
         [0009]    In advantageous manner, at least one oil collection chamber can be provided in the coating, in order to further improve the lubrication of the bearing. The at least one oil collection chamber can be configured as a channel that runs in the direction of the longitudinal axis, as a channel that runs radially with regard to the longitudinal axis, surrounding it entirely or in part, and/or as a pocket-shaped recess. 
         [0010]    The minimum thickness of the coating depends on the requirements of the individual case and can amount to 5 μm to 15 μm, for example. 
         [0011]    Preferably, the resin contained in the coating is a thermally cured resin, particularly a polyamide resin, which is very temperature-resistant and can withstand the stresses that the bearing is subject to in operation particularly well. 
         [0012]    It has been shown that a proportion of 50 wt.-% to 60 wt.-% of solid lubricant particles in the coating has particularly good lubrication properties. In this connection, the solid lubricant particles can particularly consist of a material that is selected from the materials group that comprises graphite, molybdenum sulfide, tungsten disulfide, hexagonal boron nitride, and PTFE (polytetrafluoroethylene). In this connection, it is advantageous if the solid lubricant particles consist of only one material. In this connection, it is particularly advantageous if all the solid lubricant particles consist of the same material, or if solid lubricant particles that consist of two different materials are mixed, for example solid lubricant particles of graphite with solid lubricant particles of a metal sulfide. For particularly effective lubrication, the solid lubricant particles have a particle size of 1 μm to 3 μm. 
         [0013]    In the case of the method according to the invention, the at least one geometric deviation can be configured by means of varying the amount of the coating agent given off by the coating tool and/or by means of varying the advance of the coating tool in the bore to be coated. 
         [0014]    A possible alternative, of course, consists in applying the coating agent in a uniform thickness and configuring the at least one geometric deviation by means of subsequent working of the resulting coating. Of course, this is significantly more complicated than making the at least one geometric deviation directly during the coating process. However, the result, namely a connecting rod having connecting rod eyes whose coating forms the at least one geometric deviation from the cylindrical inside contour of the connecting rod eyes, is the same. 
         [0015]    It is advantageous if the coating agent is applied to inside surfaces of the bores with a surface roughness of Ra (average roughness value) ≦0.8 μm, in order to achieve particularly good adhesion. In order to further improve the adhesion of the coating agent to the inside surface of the bore, the inside surfaces of the bores can be pre-heated before and/or during application of the coating agent, preferably up to a temperature of 50° C. to 80° C. 
         [0016]    The coating agent can be applied by means of rotation atomization from a rotating nozzle introduced into the bore, for example. In this connection, it has proven to be particularly practical to carry the rotation atomization out at a rotation speed of the nozzle of 14,000 to 18,000 rotations per minute. 
         [0017]    A preferred further development of the method according to the invention consists in using a thermally curing coating agent and subjecting same to heat treatment immediately after completing the application, preferably at a temperature of 200° C. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It is to be understood, however, that the drawings are designed as an illustration only and not as a definition of the limits of the invention. 
           [0019]    In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
           [0020]      FIG. 1  is a front view of an embodiment of a connecting rod according to the invention; 
           [0021]      FIG. 2  is a section along the line II-II in  FIG. 1 ; 
           [0022]      FIG. 3  is a schematic representation of a coating tool and 
           [0023]      FIG. 4  is a section showing an alternative embodiment of the invention with the oil collection chamber in the form of a channel. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]      FIGS. 1 and 2  show an exemplary embodiment of a connecting rod  10  according to the invention. The connecting rod  10  consists, in known manner, of steel, for example. The connecting rod  10  has a connecting rod shaft  11 , a small connecting rod eye  20 , and a large connecting rod eye  30 . The small connecting rod eye  20  accommodates the piston pin of a piston (not shown), while the large connecting rod eye  30  accommodates a crank journal of a crankshaft (not shown). 
         [0025]    In the exemplary embodiment, the connecting rod eyes  20 ,  30  are shaped bores having a defined inside contour  21 ,  31  that deviates from the cylinder shape, as it is disclosed, for example, in WO 96/07841 A1. This configuration serves to relieve stress on the mechanical system of piston pin, connecting rod  10 , and crank journal, during operation, in order to avoid the risk of cracks around the connecting rod eyes  20 ,  30 . Other configurations of a connecting rod eyes that serve the same purpose are, for example, bores provided with ovality (heightwise and/or crosswise) or with stress relief pockets (not shown). These configurations are already known for pin bores. 
         [0026]    In the exemplary embodiment, the two connecting rod eyes  20 ,  30  are configured, according to the invention, in such a manner that they are composed of a cylindrical bore  22  and  32 , respectively, and a coating  23  and  33 , respectively. In this connection, the surface contour of the coating  23  or  33  is structured in such a manner that the desired inside contour  21  and  31 , respectively, of the connecting rod eye  20  and  30 , respectively, which deviates from the cylinder shape, is obtained. In comparable manner, ovality or a stress relief pocket can also be formed by the surface structure of the coating  23  or  33  (not shown). The coating  23  or  33  essentially consists of a resin with solid lubricant particles embedded in it, and is thus a self-lubricating coating. 
         [0027]    In the exemplary embodiment, the coating  23  or  33  is furthermore provided with oil collection chambers in the form of a channel  24  or  34  that extends in the direction of the longitudinal axis L of the connecting rod eye  20  or  30 , in each instance, from which channel pockets  25  and  35 , respectively, extend radially. These oil collection chambers serve to further improve the lubrication of the bearing of piston pin or crank journal, respectively, in each instance. Of course, they can be configured for any desired application, in any desired shape and size, and can be disposed relative to one another in any desired form. For example,  FIG. 4  shows the oil collection chamber in the form of a channel  34   a  which runs radially relative to the direction of the axis L and surrounds axis L at least in part. 
         [0028]    To produce a connecting rod eye  20 ,  30 , first the cylindrical bore  22  or  32  is made in the connecting rod shaft  11 , and mechanically finished in known manner. The surface roughness Ra (average roughness value) can correspond to the one indicated in DE 41 11 368 A1, whereby in general, the Ra values amount to 0.63 μm or less for bore diameters of less than 30 mm, and Ra values of 0.8 μm or less are achieved for bore diameters between 30 mm and 60 mm. In the exemplary embodiment, the bore diameter is selected, before coating, in such a manner that the piston pin or the crank journal, respectively, has a diametral play of 10 μm to 40 μm in the finished connecting rod eye  20  or  30 , respectively. The cylindrical bore  22  or  32  should be cleaned in such a manner that chips, other particles, machining oils and the like are completely removed. The inside surface of the cylindrical bore  22  or  32  can also be phosphatized. 
         [0029]    If additional oil collection chambers, for example in the form of channels  24 ,  34  or pockets  25 ,  35 , are supposed to be provided, corresponding cover templates are affixed in the cylindrical bore  22  or  32  before coating takes place, in known manner. The cover templates prevent coating of the covered regions of the cylindrical bore  22  or  32 . As an alternative, the finished coating  23  or  33  can subsequently be worked, and can be provided with oil collection chambers in this way. 
         [0030]    The coating agent selected in the exemplary embodiment is formed from a thermally curable resin with solid lubricant particles of one or more of the materials graphite, molybdenum sulfide, tungsten disulfide, hexagonal boron nitride, and PTFE embedded in it. In the exemplary embodiment, the resin is a very temperature-resistant polyamide imide, and the solid lubricant is a mixture of molybdenum sulfide and graphite particles having a particle size of 1 μm to 3 μm. In the exemplary embodiment, the amount of the solid lubricant is selected in such a manner that the finished coating contains about 50 to 60 wt.-% solid lubricant particles. The viscosity of the coating agent is adjusted in such a manner that droplet formation is prevented in the case of sufficient application. 
         [0031]    A device  40  for rotation atomization serves to apply the coating to the inside surface of the cylindrical bore  22  or  32 , in the exemplary embodiment. The device  40  has a base body  41  that is connected with a nozzle body  42 . The nozzle body  42  is mounted to rotate on the base body, by means of a bearing  43 . The nozzle body  42  has a nozzle  44  having an exit opening  45 . The base body  41  possesses feed channels  46 ,  47 , in each instance, which are intended for the liquid coating material and for compressed air, and end in a mixing chamber  48  for mixing and metering. An exit channel  49  extends from the mixing chamber  48 , through the nozzle body  42 , and opens into the exit opening  45 . A baffle plate  51  is disposed perpendicular to the exit opening  45 , so that a ring-shaped gap  52  having a width of 0.5 mm in the exemplary embodiment is formed between the baffle plate  51  and the nozzle body  42 . The coating agent/air mixture exits through the gap  52 , in the form of a spray jet  53 , radially and at a distance from the nozzle body  42 . 
         [0032]    The nozzle body  42  is put into rotation by means of a drive  54 , and rotates in the speed of rotation range from 14,000 to 18,000 rotations per minute in the exemplary embodiment. The coating agent/air mixture that exits from the exit opening  45  is accelerated by the centripetal forces that occur at the exit opening  45 , in such a manner that it exits radially as a disk-shaped spray jet  53 . Since the spray jet  53  is configured narrow in the direction of the longitudinal axis L, the inside surface of the cylindrical bore  22  or  32  that is to be coated can be sharply delimited, in the direction of the longitudinal axis L, by means of simple feed control of the coating agent/air mixture. In the exemplary embodiment, nozzles  44  having a diameter in the range between 5 and 25 mm and having depths up to 50 mm are available, so that it is possible to coat cylindrical bores  22 ,  32  for connecting rods of all engine types with the device  40 . The diameter of the nozzle  44  is generally selected in such a manner that it approximately corresponds to half the diameter of the cylindrical bore  22  or  32 . 
         [0033]    A centrifuge device S- 520  from Sprimag in Kirchheim is also suitable for carrying out the coating method. 
         [0034]    In the exemplary embodiment, application of the coating agent/air mixture takes place onto the inside surface of the cylindrical bore  22  and/or  32 , which has been pre-heated to 50° C. to 80° C. The nozzle  44  is introduced centrally into the cylindrical bore  22  or  32 , from the outside to the inside. To configure the geometric deviation from the cylindrical inside contour, for example of the shaped bore shown in  FIG. 2 , the advance of the nozzle  44  is varied in a range of 10 to 20 mm/s, for example. In addition or as an alternative, the amount of the coating agent/air mixture exiting from the exit opening  45  of the nozzle  44  can be varied. For this purpose, it is practical that the device  40  works with computer control. When the nozzle  44  has reached the end of the cylindrical bore  22  or  32 , the device  40  is turned off and retracted. 
         [0035]    If cover templates are provided in the cylindrical bore  22  or  32  to produce oil collection chambers, the feed of the coating agent/air mixture is shut off when such a template is reached, so that residues of the spray jet are sprayed onto the cover template. When the end of the cover template has been reached, the feed of the coating agent/air mixture is achieved again. 
         [0036]    When the coating agent has been applied, it is thermally hardened, in that the connecting rod, i.e. the connecting rod component that has the coated connecting rod eyes  20  and/or  30 , is placed in an oven and held at a temperature of 200° C. between 10 and 20 minutes there, in the exemplary embodiment. 
         [0037]    The finished coating  23  or  33  is approximately 5 μm to 20 μm thick at its thinnest point, and the diametral pin play of the piston pin or crank journal amounts to about 10 μm to 20 μm. This close play is particularly advantageous for avoiding noise development. The coating  23  or  33  furthermore guarantees that despite the close play, no seizing occurs. 
         [0038]    Accordingly, while only a few embodiments of the present invention have been shown and described, it is obvious that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention.