Abstract:
A piston for an internal combustion engine has at least two hub bores for holding a piston pin. The hub bores are formed with a cylindrical inner contour and are provided with a coating comprising a resin with solid lubricant particles embedded therein. The coating forms at least one geometric deviation from the cylindrical inner contour of the hub bores. A method for producing a piston of the type, includes producing bores with a cylindrical inner contour and applying a coating medium comprising a resin with solid lubricant particles embedded therein to the inner faces of the bores by a coating tool, such that the resulting coating forms at least one geometric deviation from the cylindrical inner contour of the hub bores.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and Applicants claim priority under 35 U.S.C. §§1.20 and 121 on U.S. application Ser. No. 12/086,797 filed on Jun. 19, 2008, which application is a national stage application under 35 U.S.C. §371 of PCT Application No. PCT/DE2006/002257 filed Dec. 15, 2006, which claims priority under 35 U.S.C. §119 from German Patent Application No. 10 2005 061 063.3 filed Dec. 21, 2005, the disclosures of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a piston for an internal combustion engine and to a method for its production. 
     The piston pin connects the piston with the crankshaft of the internal combustion engine by way of a connecting rod. The piston pin is mounted in pin bores made in pin bosses, and can bend as a result of the tremendous forces that act on the piston during the oscillating movement of the piston. The pin bosses are among the parts of a piston that are subject to the greatest stress. Under great piston stresses, there is the risk of crack formation at the pin bores. Therefore, ways are being sought to relieve stress on the pin bores, particularly in light-metal pistons. This is done, for example, by means of local geometric changes in the usually cylindrical pin bore, which reduce the stress triggered by the deformation of the piston pin. Such geometric changes can be, for example, stress relief pockets, oval pin bores, or conical or spherical bores adapted to the bending line of the piston pin (with regard to the latter see, for example, WO 96/07841 A1). Such geometric changes have been produced by means of complicated precision machining of the pin bore until now. 
     Pistons having pin bores with slide bearing surfaces are known from German patent application 10 2004 059 392.9. 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 
     It is the task of the present invention to make available a piston whose pin bores can be provided with local geometric deviations in particularly simple manner. 
     The solution consists in a piston in which the pin bores are formed from bores having a cylindrical inside contour. The bores are provided with a coating comprising a resin with solid lubricant particles embedded in it, and the coating forms at least one geometric deviation from the cylindrical inside contour of the pin bores. In the method according to the invention, first, bores having a cylindrical inside contour are produced, and subsequently, a coating agent comprising a resin with solid lubricant particles embedded in it is applied to the inside surfaces 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 pin bores. 
     With the present invention, it is possible to produce pin bores 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 pin bores for the purpose of introducing a geometric deviation is eliminated. Furthermore, bearing bushings are no longer necessary to achieve sufficient lubrication and an anti-seizure effect of the pin bores. The desired dimensional accuracy of the pin bores is reliably achieved. The strength and therefore the useful lifetime of the piston pin bearing are significantly improved, as a result of the improved lubrication properties as compared with the previously known coatings made of metal alloys. 
     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 example. 
     In an advantageous manner, at least one oil collection chamber can be provided in the coating, in order to further improve the lubrication of the piston pin bearing. The at least one oil collection chamber can be configured as a channel that runs in the pin boss axis direction, as a channel that runs radially with regard to the pin boss axis direction, surrounding it entirely or in part, and/or as a pocket-shaped recess. 
     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. 
     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 piston pin bearing is subject to in operation particularly well. 
     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. 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. 
     In 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. 
     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 piston having pin bores whose coating forms the at least one geometric deviation from the cylindrical inside contour of the pin bores, is the same. 
     In one embodiment, the coating agent is applied to inside surfaces of the bores with a surface roughness of Ra (average roughness value) ≦0.8 μm. 
     In another embodiment, the coating agent is applied by means of rotation atomization from a rotating nozzle introduced into the bore. 
     In another embodiment, the rotation atomization is carried out at a rotation speed of the nozzle of 14,000 to 18,000 rotations per minute. 
     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. 
     A preferred further development of the method according to the invention consists in using a thermally curing coating agent and subjecting the same to heat treatment immediately after completing the application, preferably at a temperature of 200° C. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the invention will be described in greater detail below, using the attached drawings. These show, in a representation not to scale: 
         FIG. 1  a representation, partly in section, of an embodiment of a piston according to the invention; 
         FIG. 2  a partial representation of the pin bore of the piston according to  FIG. 1 , in section; 
         FIG. 3  a view of the pin bore according to  FIG. 2  in the direction of the arrow A in  FIG. 2 ; 
         FIG. 4  a schematic representation of a coating tool. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows an exemplary embodiment of a piston  10  according to the invention, which is a one-part piston  10  in this case. The piston  10  consists, in known manner, of a light-metal alloy, for example. The piston  10  has a piston head  11  having a combustion bowl  12  and a ring-shaped circumferential side wall having a top land  13  and a ring belt  14  for accommodating piston rings (not shown). The piston  10  has a piston skirt  15  further below the piston head  11 . The piston skirt  15  has two pin boss connections  16  that support themselves on the underside of the piston head  11 , which make a transition into two piston pin bosses  17 . Each piston pin boss is provided with a pin bore  18  equipped with a locking ring groove  19  for a piston ring (not shown). Depending on the construction of the piston (two-part or multi-part), of course, more than two piston pin bosses with corresponding pin bores can be provided. 
     In the exemplary embodiment, the pin bores  18  are shaped bores having a defined inside contour  23  that deviates from the cylinder shape, as they are disclosed, for example, in WO 96/07841 A1. This configuration serves to relieve stress on the piston pin during operation, in order to avoid the risk of pin boss cracks. Other configurations of a pin bore that serve the same purpose are, for example, pin bores provided with ovality (heightwise and/or crosswise) or with stress relief pockets (not shown). These configurations are known. 
     The pin bores  18  are configured, according to the invention, in such a manner that they are composed of a cylindrical bore  21  and a coating  22 . In this connection, the surface contour of the coating  22  is structured in such a manner that the desired inside contour  23  of the shaped bore, 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  22  (not shown). The coating  22  essentially consists of a resin with solid lubricant particles embedded in it, and is thus a self-lubricating coating. 
     In the exemplary embodiment, the coating is furthermore provided with oil collection chambers in the form of a channel  24  that extends in the direction of the pin boss axis, from which pockets  25  extend radially. These oil collection chambers serve to further improve the lubrication of the piston pin bearing. 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. 
     To produce a pin bore  18 , first the cylindrical bore  21  is made in the piston pin boss  17 , 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 has a diametral play of 10 μm to 40 μm in the finished pin bore  18 . The cylindrical bore  21  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  21  can also be phosphatized. 
     If additional oil collection chambers, for example in the form of channels  24  or pockets  25 , are supposed to be provided, corresponding cover templates are affixed in the cylindrical bore  21  before coating takes place, in known manner. The cover templates prevent coating of the covered region of the cylindrical bore  21 . As an alternative, the finished coating can subsequently be worked, and can be provided with oil collection chambers in this way. 
     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. 
     A device  30  for rotation atomization serves to apply the coating to the inside surface of the cylindrical bore  21 , in the exemplary embodiment. The device  30  has a base body  31  that is connected with a nozzle body  32 . The nozzle body  32  is mounted to rotate on the base body, by means of a bearing  33 . The nozzle body  32  is with a nozzle  34  having an exit opening  35 . The base body  31  possesses feed channels  36 ,  37 , in each instance, which are intended for the liquid coating material and for compressed air, and end in a mixing chamber  38  for mixing and metering. An exit channel  39  extends from the mixing chamber  38 , through the nozzle body  32 , and opens into the exit opening  35 . A baffle plate  41  is disposed perpendicular to the exit opening  35 , so that a ring-shaped gap  42  having a width of 0.5 mm in the exemplary embodiment is formed between the baffle plate  41  and the nozzle body  32 . The coating agent/air mixture exits through the gap  42 , in the form of a spray jet  43 , radially and at a distance from the nozzle body  32 . 
     The nozzle body  32  is put into rotation by means of a drive  44 , 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  35  is accelerated by the centripetal forces that occur at the exit opening  35 , in such a manner that it exits radially as a disk-shaped spray jet  43 . Since the spray jet  43  is configured narrow in the pin axis direction, the inside surface of the cylindrical bore  21  that is to be coated can be sharply delimited, in the pin axis direction, by means of simple feed control of the coating agent/air mixture. In the exemplary embodiment, nozzles  34  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  21  for pistons of all engine types with the device  30 . The diameter of the nozzle  34  is generally selected in such a manner that it approximately corresponds to half the diameter of the cylindrical bore  21 . 
     A centrifuge device S-520 from Sprimag in Kirchheim is also suitable for carrying out the coating method. 
     In the exemplary embodiment, application of the coating agent/air mixture takes place onto the inside surface of the cylindrical bore  21 , which has been pre-heated to 50° C. to 80° C. The nozzle  34  is introduced centrally into the cylindrical bore  21 , 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. 1 , the advance of the nozzle  34  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  35  of the nozzle  34  can be varied. For this purpose, it is practical that the device  30  works with computer control. When the nozzle  34  has reached the end of the cylindrical bore  21 , the device  30  is turned off and retracted. 
     If cover templates are provided in the cylindrical bore  21  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. 
     When the coating agent has been applied, it is thermally hardened, in that the piston, i.e. the piston component that has the coated pin bores  18  is placed in an oven and held at a temperature of 200° C. between 10 and 20 minutes there, in the exemplary embodiment. 
     The finished coating  22  is approximately 5 μm to 20 μm thick at its thinnest point, and the diametral pin play is about 10 μm to 20 μm. This close play is particularly advantageous for avoiding noises caused by pin ticks. The coating  22  furthermore guarantees that despite the close play, no seizing occurs.