Abstract:
A two-piece piston for an internal combustion engine has an upper piece and a lower piece, the lower piece having an upper base part with an opening lying coaxial to the piston axis into which a pin on the upper piece is inserted. The upper piece is connected to the lower piece with a hexagonal nut screwed onto the pin. A resilient sleeve is arranged between the upper piece and the lower piece comprising an inwardly directed collar on the against which the upper face of the hexagonal nut lies and in the region facing away from the piston crown, a contact surface facing the piston crown lying on a surface on the inner side of the opening. An economically produced two-piece piston results with a resilient sleeve between both screwed piston pieces which exerts a pre-tensioning and thus provides security for the screw connection.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of and Applicant claims priority under 35 U.S.C. §§120 and 121 on U.S. application Ser. No. 11/991,238 filed on Jun. 17, 2008, which application is a national stage application under 35 U.S.C. §371 of PCT Application No. PCT/DE2006/001527 filed Aug. 31, 2006, which claims priority under 35 U.S.C. §119 from German Patent Application No. 10 2005 041 409.5 filed Sep. 1, 2005, the disclosures of each of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a two-part piston for an internal combustion engine. 
     A two-part piston for an internal combustion engine, which consists of an upper part and a lower part, is known from the patent application having the file number DE 10 2005 021 427.4. The upper part forms the piston crown and has a cylindrical pin having an outside thread, which pin lies coaxial to the piston axis, on the underside facing away from the piston crown. The lower part has an upper, thin-walled, and elastically resilient crown part, at the central region of which a sleeve pointing in the direction of the piston crown is formed on, having walls that are so thin that this sleeve has the function of an expansion sleeve. An opening that lies coaxial to the piston axis is formed into this expansion sleeve, into which opening the pin is introduced, so that the upper part can be connected with the lower part by means of a nut screwed onto the pin, whereby the elastic resilience of the upper crown part and the expandability of the expansion sleeve puts the screw connection under bias, and thereby imparts great reliability to the latter. 
     It is disadvantageous that the production of the upper crown part of the lower part, with the expansion sleeve formed on, is very complicated. In this connection, time-consuming machining of the central region of the upper crown part and, in particular, of the expansion sleeve, is required, independent of whether the lower part is cast or forged, whereby attention must be paid to the adherence of close tolerances, so that the central region of the upper crown part and the expansion sleeve adhere to the desired elasticity properties. 
     SUMMARY OF THE INVENTION 
     Proceeding from this, the invention is based on the task of making the production of a two-part piston whose upper part and lower part are connected with one another by way of an expansion sleeve, simpler and less expensive. 
     This task is accomplished by a two piece piston for an internal combustion engine, in which a separate expansion sleeve can be produced in a simpler and more cost-advantageous manner than a central region of the upper crown part re-functioned to become an expansion sleeve. The material used for this purpose can be of higher quality than the material of which the upper crown part of the lower part consists. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Some exemplary embodiments of the invention will be described below, using the drawing. This shows: 
         FIG. 1  a two-part piston whose two parts are connected with one another by way of an expansion sleeve, 
         FIG. 2  a section through the piston according to  FIG. 1 , which allows the continuous cooling oil channel to be seen, 
         FIG. 3  a top view of the nut used to screw the two parts of the piston together, 
         FIG. 4  a section through the nut according to  FIG. 3 , 
         FIG. 5  a perspective representation of the nut according to  FIGS. 3 and 4 , 
         FIG. 6  an embodiment of the piston according to the invention, whereby a hexagonal screw is used to screw the two piston parts together, 
         FIG. 7  an enlarged representation of the region A from  FIG. 6 , 
         FIG. 8  an enlarged representation of another embodiment of the screw connection of the two piston parts, using an expansion sleeve, and 
         FIG. 9  a representation in accordance with  FIG. 8 , with the hexagonal nut in section. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a two-part, cooled piston  1  that consists of an upper part  2  and a lower part  3 . The upper part  2  and the lower part  3  can be made from aluminum, steel, or cast iron with spheroidal graphite (GGG cast iron according to DIN 1693). A piston crown  4  delimits the axial upper side of the upper part  2 , whose radially inner region has a combustion bowl  5 . A ring wall  6  is formed into the outer edge of the piston crown  4 , the outer surface of which belt forms a top land  7 , followed on the skirt side by a ring belt  8  having ring grooves for accommodating piston rings not shown in the figure. 
     Two pin bosses  9 ,  9 ′ each having a pin bore  10 ,  10 ′ are disposed on the underside of the lower part  3 , facing away from the piston crown  4 . The pin bosses  9 ,  9 ′ are connected with one another by way of skirt element  13 . 
     The upper part  2  and the lower part  3  of the piston  1  are connected with one another by way of an inner contact surface  14  and by way of an outer contact surface  15  disposed concentric to the former. The upper part  2  and the lower part  3  each have a dead-end hole  16  and  17 , which lie opposite one another when the upper and lower parts  2 ,  3  are positioned appropriately. A fixation pin  11  introduced into the two dead-end holes  16 ,  17  ensures that the upper part  2  and the lower part  3  always assume the same rotational position relative to one another. 
     An outer cooling channel  22  disposed in the edge region of the piston  1 , on the piston crown side is formed by the upper part  2  and the lower part  3  of the piston  1 , the radially outer limit of which channel is formed by the ring wall  6 , the radially inner limit of which channel is formed partly by a lower part foot  18  disposed on the upper side of the lower part  3  and partly by the piston crown  4 , and the axially lower limit of which is formed by the lower part  3  of the piston  1 . Cooling oil is introduced into the cooling channel  22  by way of at least one oil feed channel  52  ( FIG. 2 ). 
     The piston  1  has another, ring-shaped, inner cooling channel  24  disposed coaxial to the piston axis  12 , which channel has a lesser radial diameter than the outer cooling channel  22 , and which is disposed within the outer cooling channel  22 , seen in the radial direction. Axially on the top, the inner cooling channel  24  is delimited by the piston crown  4 , radially on the outside it is delimited by the lower part foot  18 , axially on the bottom it is delimited by an upper crown part  25  of the lower part  3 , and radially on the inside it is delimited by a pin  31 , whereby the crown part  25  of the lower part  3  has an opening  27  radially on the inside. 
     The pin  31  is configured in cylindrical shape and formed onto the underside of the piston crown  4 , coaxial to the piston axis  12 , whereby the mantle surface of the pin  31  has an end region facing away from the piston crown, having an outside thread  32  ( FIG. 2 ), which corresponds to the inside thread of the hexagonal nut  29 , so that the hexagonal nut  29  can be screwed onto the outside thread  32 . 
     The upper part  2  is screwed to the lower part  3  by means of the hexagonal nut  29 , by way of an expansion sleeve  26  disposed between the upper face surface  23  of the hexagonal nut  29  and the upper crown part  25  of the lower part  3 . For this purpose, the expansion sleeve  26  has a collar  28  directed inward on its end facing the piston crown, and a contact surface  19  that widens conically, radially toward the outside, in the direction facing away from the piston crown, at its end facing away from the piston crown. 
     When the upper part  2  and the lower part  3  are screwed together, the fixation pin  11  is first introduced into the dead-end hole  17  disposed in the upper crown part  25  of the lower part  3 . Subsequently, the upper part  2  and the lower part  3  are oriented coaxial to one another. For this purpose, the edge of the lower part  3 , on the piston crown side, has a support crosspiece  46  that has the shape, in section, of a step directed radially inward and axially in the direction of the piston crown  4 . Furthermore, a cylindrical recess  47  is formed into the inside of the lower face side of the ring wall  6 , the inside shape of which recess is configured to be complementary to the outside shape of the support crosspiece  46 . A coaxial orientation of upper part  2  and lower part  3  can thereby be achieved, in that the support crosspiece  46  is first introduced, at least partway, into the recess  47 . In this connection, however, it is necessary to rotate the upper part  2  about the piston axis  12  until the dead-end hole  16  comes to lie above the fixation pin, after which the support crosspiece  46  can be introduced into the recess  47  in its entirety, and furthermore, the fixation pin  11  can be introduced into the dead-end hole  16 . 
     Subsequently, the expansion sleeve  26  is pushed through the opening  27 , and the hexagonal nut  29  is screwed onto the outside thread  32  of the pin  31 . In this connection, the hexagonal nut  29  comes to rest against an inside surface  20  of the collar  28 , facing away from the piston crown, by way of its upper face surface  23 , and the contact surface  19  of the expansion sleeve  26  comes to rest against a surface  21  disposed on the radial inside of the opening  27 , which surface opens conically downward. In this connection, the conicity of the surface  21  is at least approximately equal to the conicity of the contact surface  19 . Then the hexagonal nut  29  is screwed tight with such a torque that because of the pressure of the hexagonal nut  29  onto the inside surface  20  of the collar  28  and because of the tensile stress exerted on the expansion sleeve  26  as a result, the expansion sleeve  26  undergoes an expansion that exerts a permanent bias on the screw connection, thereby imparting great reliability to this screw connection. Furthermore, the pin  31  undergoes expansion, in this connection, and the radially inner part of the piston crown  4  undergoes deformation in the manner of a disk spring, in the direction of the pin bosses  9 ,  9 ′. Furthermore, the expansion sleeve  26  is made from a high-quality steel material, thereby contributing to the reliability of the screw connection between upper part  2  and lower part  3  of the piston  1 . 
     In this connection, the contact surface  19  on the side of the expansion sleeve  26  that faces away from the piston crown can also be configured to be spherical or ball-shaped, in an embodiment not shown in the figures, whereby the contact surface  21  of the upper crown part  25  of the lower piston part  3  that stands in contact with it can have a shape complementary to it. 
     As can be clearly seen in  FIG. 2 , the collar  28  delimits a circular opening  30 , the radial diameter of which is greater than the radial diameter of the pin  31 , so that a ring-shaped gap  33  occurs between pin  31  and collar  28 . The hexagonal nut  29  has radially disposed grooves  34  and  34 ′ in its upper face surface  23 , which grooves open into the gap  33  radially on the inside. The radial inside diameter of the expansion sleeve  26  is greater than the radial outside diameter of the hexagonal nut  29 , so that an interstice  35  in the shape of a cylinder and ring-shaped in section occurs between hexagonal nut  29  and expansion sleeve  26 , the end of which interstice, on the side facing away from the piston crown, opens into the piston interior  36 , and the end of which interstice, facing the piston crown, stands in connection with the grooves  34 ,  34 ′. Furthermore, the outer cooling channel is connected with the inner cooling channel  24  by way of radially disposed overflow channels  37 ,  37 ′. 
     As indicated by the line  38 , this results in a continuous oil channel, whereby first of all, cooling oil is introduced into the outer cooling channel  22  by way of the oil feed channel  52 , which oil flows into the inner cooling channel  24  by way of the overflow channels  37 ,  37 ′, and flows back into the piston interior  36  by way of the gap  33 , by way of the grooves  34 ,  34 ′, and by way of the interstice  35 . 
       FIG. 3  shows a top view,  FIG. 4  shows a sectional diagram, and  FIG. 5  shows a perspective representation of the hexagonal nut  29  used to screw the upper part  2  to the lower part  3 . In  FIG. 3 , an exemplary arrangement of the grooves  34  to  34 ′″ is shown, whereby four grooves  34  to  34 ′″ having radial longitudinal axes  48 ,  48 ′,  48 ″, and  48 ′″ are uniformly distributed over the circumference of the face surface  23 , so that the longitudinal axis pairs  48 - 48 ″,  48 ″- 48 ′,  48 ′- 48 ′″, and  48 ′″- 48  each enclose an angle of 90°. 
       FIG. 4  shows a section through the hexagonal nut  29  along the line DD in  FIG. 3 , with a representation of the U-shaped cross-section of the grooves, using the example of the groove  34 ′, and with a top view of the flanks of the grooves  34 ″ and  34 ′″. 
     A perspective representation of the hexagonal nut  29  is shown in  FIG. 5 ; in particular, the grooves  34 ,  34 ′, and  34 ″ can be seen. 
     The embodiment of the present invention according to  FIGS. 6 and 7  differs from the embodiment of the invention shown in  FIGS. 1 to 5  particularly in that the upper part  2 ′ and the lower part  3 ′ of the piston  1 ′ are connected with one another by means of a hexagonal screw  39 , whereby a tube-shaped intermediate piece  41  that is short in the axial direction and has one or more continuous, radially placed bores  42 ,  42 ′ is disposed between the head  40  of the hexagonal screw  39  and the collar  28 ′ of the expansion sleeve  26 ′, by way of which bores the oil can flow from the gap  33 ′ between the collar  28 ′ of the expansion sleeve  26 ′ and the shaft of the screw  39  into the interstice  35 ′ between the expansion sleeve  26 ′ and the screw head  40 . From here, the oil gets into the piston interior  36 ′. 
       FIG. 7  is an enlarged representation of the region A from  FIG. 6 . 
     In  FIG. 8 , an embodiment of the piston  1 ″ is shown in partial section, and in  FIG. 9 , in full section, with an expansion sleeve  26 ″ that has a collar  28  directed radially inward on its end facing the piston crown, and a collar  44  directed radially outward on its end facing away from the piston crown. On the piston crown side, a radially oriented contact surface  19 ′ is disposed on the collar  44 . Furthermore, a cantilever  45  is formed onto the inside wall of the opening  27 ′ in the upper crown part  25 ′ of the lower piston part  3 ″, which cantilever narrows conically radially inward in the direction facing away from the piston crown, and is configured to be elastically resilient, and has a surface  21 ′, also oriented radially, on its side facing away from the piston crown. 
     When the upper piston part  2 ″ is screwed together with the lower piston part  3 ″, the contact surface  19 ′ comes to lie against the surface  21 ′, whereby not only longitudinal expansion of the expansion sleeve  26 ″ and of the pin  31 , and deformation of the center region of the piston crown  4 ′, in the manner of a disk spring, in the direction of the piston skirt, but also deformation of the cantilever  45 , in the manner of a disk spring, in the direction of the piston crown  4 ′, occur. The deformations of the piston elements last mentioned lead to a bias that acts on the screw connection, which imparts great reliability to this connection. 
     REFERENCE SYMBOL LIST 
     
         
           1 ,  1 ′,  1 ″ piston 
           2 ,  2 ′,  2 ″ upper part 
           3 ,  3 ′,  3 ″ lower part 
           4 ,  4 ′,  4 ″ piston crown 
           5  combustion bowl 
           6  ring wall 
           7  top land 
           8  ring belt 
           9 ,  9 ′,  9 ″,  9 ′″ pin boss 
           10 ,  10 ′,  10 ″,  10 ′″ pin bore 
           11  fixation pin 
           12  piston axis 
           13 ,  13 ′ skirt element 
           14  inner contact surface 
           15  outer contact surface 
           16 ,  17  dead-end hole 
           18  lower part foot 
           19 ,  19 ′ contact surface 
           20  inner surface of collar  28   
           21 ,  21 ′,  21 ″ surface 
           22 ,  22 ′ outer cooling channel 
           23  upper face surface of the hexagonal nut 
           24 ,  24 ′ inner cooling channel 
           25 ,  25 ′,  25 ″ upper crown part of the lower part  3   
           26 ,  26 ′,  26 ″ expansion sleeve 
           27 ,  27 ′,  27 ″ opening 
           28 ,  28 ′ collar 
           29  hexagonal nut 
           30  opening of the collar  28   
           31  pin 
           32  outside thread 
           33 ,  33 ′ gap between the collar  28  and the pin  31   
           34 ,  34 ′,  34 ″,  34 ′″ grooves of the hexagonal nut  29   
           35  interstice between the hexagonal nut  29  and the expansion sleeve  26   
           35 ′ third interstice between the head  40  of the hexagonal screw  39  and the expansion sleeve  26 ′ 
           36 ,  36 ′ piston interior 
           37 ,  37 ′,  37 ″ overflow channel 
           38  line 
           39  hexagonal screw 
           40  head of the hexagonal screw  39   
           41  intermediate piece 
           42 ,  42 ′ bore in the intermediate piece  41   
           43  expansion sleeve 
           44  collar 
           45  cantilever 
           46  support crosspiece 
           47  recess 
           48 ,  48 ′,  48 ″,  48 ′″ longitudinal axes of the grooves  34 ,  34 ′,  34 ″,  34 ′″ 
           49  dead-end hole bore 
           50  upper face surface of the intermediate piece  41   
           51  contact surface of the head  40  of the hexagonal screw  39   
           52 ,  52 ′ oil feed channel 
           53  first interstice between the shaft of the hexagonal nut  29  and the intermediate piece  41   
           54  second interstice between the intermediate piece  41  and the expansion sleeve  26 ′