Abstract:
The invention relates to an assembled piston ( 1 ) for an internal combustion engine, said piston consisting of an upper part ( 4 ) and a lower part ( 5 ) that are interconnected by means of an internal hexagon screw ( 6 ) consisting of an upper half( 37 ) and a lower half ( 38 ). The upper half ( 37 ) of the internal hexagon screw ( 6 ) comprises an external thread ( 39 ) which is parallel to the external thread ( 40 ) of the lower screw half ( 38 ) and has a larger lead angle than the same ( 40 ). The lead angle resulting from the difference between the lead angles of the two external threads ( 39, 40 ) is very small and contributes to a high strength of the screw connection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Applicants claim priority under 35 U.S.C. §119 of German Application No. 10 2004 029 877.7 filed Jun. 19, 2004.Applicants also claim priority under 35 U.S.C. §365 of PCT/DE2005/001092 filed Jun. 17, 2005. The international application under PCT article 21(2) was not published in English. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a composite piston for an internal combustion engine, in accordance with the preamble of claim  1 . 
   A composite piston that consists of an upper part and a lower part is known from the German Auslegeschrift [version of patent application published for public scrutiny before issuance] 22 12 922. The two piston parts are connected with one another by means of a hexagonal socket screw, which consists of a pin head and a pin shaft, whereby the pin head has an outside thread that runs opposite to the pin shaft. Since the strength of a screw connection is generally all the stronger the lower the pitch angle of the screw thread, and the pitch angle, which is the deciding factor in this regard, results from the sum of the pitch angles of the two outside threads that run in opposite directions, this screw connection has a very low strength. 
   SUMMARY OF INVENTION 
   It is the task of the present invention to avoid this disadvantage of the state of the art, i.e. to create a permanent, strong screw connection between the upper part and the lower part of a composite piston. 
   This task is accomplished with the characteristics contained in the characterizing part of the main claim, whereby a hexagonal socket screw having a thread consisting of two halves is used, which are configured to run opposite one another and have different pitch angles, so that the pitch angle that is the decisive factor for the strength of the screw connection here can be calculated from the difference of the pitch angles of the two thread halves, which is very slight, so that this results in a relatively great strength of the screw connection. 
   A practical embodiment of the invention is the object of the dependent claim, whereby a region around a passage bore, having a thread, is configured to have such a thin wall that it is elastic in the manner of a disk spring, and deforms when the hexagonal socket screw is screwed into the passage bore, to such an extent that a bias that increases the strength of the screw connection is exerted on the latter in this way. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     An exemplary embodiment of the invention will be described below, using a drawing. This shows a composite piston consisting of an upper part and a lower part, in a sectional diagram consisting of two halves, which shows two longitudinal sections of the piston, offset by 90°. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The FIGURE shows a composite piston  1  in a sectional diagram, the left half of which shows a section through the piston  1  along a longitudinal axis  2  of a pin bore  3 , and the right half of which shows a section through the piston  1  offset by 90° relative to the former. The piston  1  consists of an upper part  4  and a lower part  5 , which are connected with one another by means of a hexagonal socket screw  6  disposed in the center, in such a manner that the hexagonal socket  42  of the hexagonal socket screw  6  comes to lie on the piston inside. 
   The upper part  4  and the lower part  5  of the piston  1  are preferably produced from forged steel. It is also possible, however, to produce the upper part  4  from steel and the lower part  5  from aluminum, or to produce the upper part  4  from forged aluminum and the lower part from cast aluminum. 
   The cylindrically shaped upper part  4  forms the piston crown  7 , into which a combustion bowl  8  having rotation symmetry is worked. The mantle surface of the upper part  4 , which lies radially on the outside, is configured as a ring belt  9  that has three ring grooves  10 ,  11 , and  12  for accommodating piston rings, not shown in the FIGURE. The underside of the upper part  4 , facing away from the piston crown  7 , has a circumferential recess  13  radially on the outside, which forms a ring-shaped, outer cooling channel  15 , together with a corresponding recess  14  of the lower part  5 , on the piston crown side, which channel is delimited radially on the outside by a ring wall  29  formed onto the piston crown  7 . 
   Radially on the inside, the outer cooling channel  15  is delimited partly by a ring flange  16  disposed on the underside of the upper part  4 , and partly by a ring rib  17  disposed on the top of the lower part  5 , whereby the upper part  4  and the lower part  5  of the piston  1  rest on one another by way of the ring flange  16  and the ring rib  17 . In this connection, the ring flange  16  has a first contact surface  18  and the ring rib  17  has a second contact surface  19 , by way of which contact surfaces  18 ,  19  of the ring flange  16  and the ring rib  17  stand in contact with one another. 
   Radially within the ring flange  16 , the upper part  4  is provided, on its underside, with another circumferential recess  20 , which forms an inner, ring-shaped cooling channel  22  together with a corresponding other recess  21  formed into the top of the lower part  5 . In this connection, the outer cooling channel  15  is connected with the piston interior  24  by way of an oil run-in opening  23 , and with the inner cooling channel  22  by way of an oil channel  25 . The inner cooling channel  22  is connected with the piston interior  24  by way of an oil run-off opening  26 . To cool the piston  1 , oil is injected into the outer cooling channel  15  by way of an oil run-in opening not shown in the FIGURE; after some time, it gets into the inner cooling channel  22  by way of the oil channel  25 , and runs back into the piston interior  24  by way of the oil run-off openings  23  and  26 . 
   The lower part  5  of the piston  1  consists of two pin bosses  27 ,  27 ′ that lie opposite one another and are trapezoid in section, each having a pin bore  3 ,  3 ′, respectively, which are disposed at such a distance from one another that the upper part of a piston rod (not shown in the FIGURE) finds room between them. Furthermore, the lower part  5  has skirt elements  41 ,  41 ′ that connect the pin bosses  27 ,  27 ′ with one another. On the piston crown side, the lower part  5  has a circumferential collar  28 , rectangular in section, radially on the outside, which collar fits into a recess  30  disposed radially on the inside in a face of the ring wall  29  that faces away from the piston crown, so that during assembly of the piston  1 , the lower part  5  is guided by way of the collar  28  and the recess  30 , and centered relative to the upper part  4 . 
   On the surface of the lower part  5  on the piston crown side, the recess  14  follows the collar  28 , radially on the inside, which recess forms the outer cooling channel  15  together with the recess  13  of the upper part  4 , followed by the ring rib  17  and the other recess  21 , which forms the inner cooling channel  22  together with the other recess  20  of the upper part  4 . In this connection, the recess  21  is worked so far into the lower piston part  5  that a thin-walled region  43  is formed between piston interior  24  and recess  21 , which region is configured elastically, in the manner of a disk spring. 
   Coaxial to the axis  31  of the piston  1 , a dead-end bore  32  with inside thread  33  is worked into the underside of the piston crown  7 , and a passage bore  34  with a thread  35  that runs in the same direction as the inside thread  33  is worked into the top of the lower part  5 , i.e. into the elastically resilient region  43 , whereby the pitch angle of the inside thread  33  is greater than the pitch angle of the thread  35 . Furthermore, the inside diameter of the dead-end bore  32  is ½ mm to 1 mm smaller than the inside diameter of the passage bore  34 . 
   Using the hexagonal socket screw  6  consisting of an upper half  37  and a lower half  38 , the upper part  4  is screwed together with the lower part  5 , whereby the upper screw half  37  has such a diameter and such an outside thread  39  that it can be screwed into the dead-end bore  32 , and the lower screw half  38  has such a diameter and such an outside thread that it can be screwed into the passage bore  34 . Accordingly, the diameter of the upper screw half  37  is ½ mm to 1 mm smaller than that of the lower screw half  38 , and the outside thread  39  of the upper screw half  37  has a greater pitch angle than that of the lower screw half  38 . 
   When the upper part  4  is screwed together with the lower part  5  of the piston  1 , the hexagonal socket screw  6  is first screwed into the passage bore  34 , by way of the outside thread  40  of its lower half  38 , from below, to such an extent that the upper half  37  of the hexagonal socket screw  6  projects out of the passage bore  34 . Subsequently, the upper part  4  is set onto the lower part  5  in such a manner that the upper and lower part of the piston  1  are disposed in a certain position relative to one another, and that the outside thread  39  of the upper half  37  of the hexagonal socket screw  6  comes into contact with and touches the dead-end bore  32  of the upper part  4 . A further rotation of the hexagonal socket screw  6  has the result, because of the greater pitch angle of the outside thread  39  of the upper half  37  of the screw  6 , as compared with the pitch angle of the outside thread  40  of the lower half  38  of the screw  6 , that the first contact surface  19  of the upper part  4  makes contact with the second contact surface  18  of the lower part  5  sooner than the outside thread  40  of the lower half  38  of the screw  6  is completely screwed into the thread  35  of the passage bore  34 . 
   In this connection, the upper and lower part of the piston  1  are configured in such a manner, in the region close to the bores  32  and  34 , that a gap occurs here between upper and lower part, without any screw forcing. If the torque exerted on the screw  6  while the upper and lower part are being screwed together is increased after the surfaces  18  and  19  have entered into contact with one another, this results in a deformation of the region  43  in the manner of a disk spring, leading to a reduction in size of the gap between piston upper part and piston lower part, and to a bias exerted on the screw connection by the deformed region  43 , which results in an increase in strength of the screw connection according to the invention. 
   A further increase in the strength of the screw connection results from the fact that the self-locking of a screw that prevents loosening of a screw connection is all the greater the lower the pitch angle of the screw thread. The pitch angle that is the deciding factor in the case of the screw connection according to the invention can be calculated from the difference of the pitch angles between the inside thread  33  of the dead-end bore  32  and the thread  35  of the passage bore  34 , or between the outside thread  39  of the upper half  37  and the outside thread  40  of the lower half  38  of the hexagonal socket screw  6 , respectively. This difference is very slight, which brings about the further improvement in the strength of the screw connection according to the invention. 
   REFERENCE SYMBOL LIST 
   
       
         1  piston 
         2  longitudinal axis 
         3 ,  3 ′ pin bore 
         4  upper part 
         5  lower part 
         6  hexagonal socket screw 
         7  piston crown 
         8  combustion bowl 
         9  ring belt 
         10 ,  11 ,  12  ring groove 
         13 ,  14  recess 
         15  outer cooling channel 
         16  ring flange 
         17  ring rib 
         18  first contact surface 
         19  second contact surface 
         20 ,  21  recess 
         22  inner cooling channel 
         23  oil run-off opening 
         24  piston interior 
         25  oil channel 
         26  oil run-off opening 
         27 ,  27 ′ pin boss 
         28  collar 
         29  ring wall 
         30  recess 
         31  longitudinal axis of the piston  1   
         32  dead-end bore 
         33  inside thread of the dead-end bore  32   
         34  passage bore 
         35  thread of the passage bore  34   
         37  upper half of the hexagonal socket screw  6   
         38  lower half of the hexagonal socket screw  6   
         39  outside thread of the upper half  37  of the hexagonal socket screw  6   
         40  outside thread of the lower half  38  of the hexagonal socket screw  6   
         41 ,  41 ′ skirt element 
         42  hexagonal socket 
         43  region in the manner of a disk spring between the other recess  21  and the piston interior  24