Abstract:
A method for screwing together two metal parts that are spaced apart from one another close to a screw connection element and elastically deformable relative to one another, and for the remainder lie against one another. In order to obtain a firm and secure screw connection, with little expenditure of torque, one of the metal parts is deformed, before the metal parts are screwed together, in such a manner that regions close to the screw connection element come close to one another, and the metal parts are then screwed together. A region of the one metal parts remains deformed after they have been screwed together, as a result of the screw connection element.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicant claims priority under 35 U.S.C. 119 of German Application No. 10 2007 019 931.9 filed Apr. 27, 2007. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a method for screwing two metal parts together. 
         [0004]    2. The Prior Art 
         [0005]    In the design of a two-part piston consisting of a crown part and a skirt part, which are screwed together, attention is frequently paid to screwing the crown part and the shaft part to one another by way of regions that are elastically resilient and deform when the two parts are screwed together, so that a bias is exerted on the screw connection, which increases the strength of the screw connection. 
         [0006]    German Publication DE 10 2005 060 548 A1 describes a two-part piston that is screwed together. The crown part has a bolt that has an outside thread and is disposed coaxial to the piston axis, on the side facing away from the piston crown. The skirt part is delimited, on the piston crown side, by an elastically resilient ceiling region, which has an opening that lies in the center, into which the bolt is introduced when the crown part and the skirt part are screwed together, before a nut is screwed onto the bolt. The ceiling region deforms, so that a bias is exerted on the screw connection between bolt and nut, which bias improves the strength of the screw connection. 
         [0007]    However, a great torque must be exerted on the nut in order to bring about deformation of the ceiling region of the skirt part. Since the nut rests against the ceiling region, this torque must also serve to overcome the friction resistance that occurs during the relative movement between nut and ceiling region. This friction resistance is dependent on the composition of the contact regions by way of which the nut and the ceiling region lie against one another. If the composition of the contact regions is disadvantageous, the torque to be applied for screwing the piston together is incorrectly estimated at an overly great friction resistance between nut and ceiling region, so that the screw connection can come loose during engine operation, and an engine defect can occur. 
       SUMMARY OF THE INVENTION 
       [0008]    It is an object of the invention to avoid these disadvantages of the state of the art. This object accomplished by a method for screwing together two metal parts that are spaced apart from one another close to a screw connection element and elastically deformable relative to one another, and for the remainder lie against one another. At least one of the metal parts is deformed in such a manner that regions close to the screw connection element come close to one another. The metal parts are subsequently screwed together, and a region of at least one of the metal parts remains deformed after they have been screwed together by the screw connection element. 
         [0009]    The metal parts to be screwed together can be a skirt part and a crown part of a metal piston. The skirt part can have an elastically resilient wall region by way of which the skirt part is screwed to the crown part, and that is deformed before they are screwed together, so that it comes close to the piston crown formed by the crown part. 
         [0010]    Alternatively, the crown part can have an elastically deformable floor region by way of which the skirt part is screwed to the crown part, and that is deformed before they are screwed together, so it comes close to a ceiling region formed by the skirt part. 
         [0011]    As another alternative, the crown part can have an elastically resilient floor region, and the skirt part can have an elastically resilient ceiling region. The crown part and the skirt part are screwed to one another by way of the floor region and the ceiling region, both of which are deformed before the parts are screwed together, in such a manner that the two regions come close to one another. 
         [0012]    Because the metal parts have elastically resilient regions close to the screw connection element, which are deformed by application of force before the parts are screwed together so that they maintain the deformation after the parts are screwed together, activation of the screw connection element with a low expenditure of torque is possible. Furthermore, the deformation of the elastically resilient regions and thus the bias that acts on the screw connection and is supposed to impart strength to the screw connection can be precisely determined in advance, by means of a suitable selection of the force applied. Furthermore, the position of the two parts while they are being screwed together, which is assigned on the bearing side, does not change, since no torque has to be applied. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0013]    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. 
           [0014]    In the drawings, wherein similar reference characters denote similar elements throughout the several views: 
           [0015]      FIG. 1  shows two plate-shaped metal parts that are to be screwed to one another, and lie against one another except for a region close to a hex screw, where they are spaced apart from one another, according to one embodiment of the invention; 
           [0016]      FIG. 2  shows the two metal parts after pressure was exerted on one of the metal parts in the region of the hex screw, by a hydraulic device, and this metal part was deformed to such an extent that both metal parts lie against one another even in the region of the hex screw, after which the two metal parts were screwed together with one another; 
           [0017]      FIG. 3  shows a two-part piston consisting of a crown part and a skirt part, whereby the skirt part has an elastically deformable region close to the screw connection element, which region is deformed before the two piston parts are screwed together; 
           [0018]      FIG. 4  shows a two-part piston consisting of a crown part and a skirt part, whereby the crown part has an elastically deformable region close to the screw connection element, which region is deformed before the two piston parts are screwed together; 
           [0019]      FIG. 5  shows a two-part piston consisting of a crown part and a skirt part, whereby the skirt part and the crown part each have an elastically deformable region close to the screw connection element, which region is deformed before the two piston parts are screwed together. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0020]    Referring now in detail to the drawings,  FIG. 1  shows two plate-shaped metal parts  1  and  2  having different wall thicknesses, which rest against one another. Metal part  1 , having the lesser wall thickness, has a bulge  5  in the region of a screw connection element  4  configured as a hex screw  3 , and is elastically deformable in this region. Hex screw  3  is passed through bores  6  and  7  made in the two metal parts  1 ,  2 , in the region of bulge  5 . 
         [0021]    A tubular pressure punch  9  lies against metal part  1  in the region of bulge  5 , surrounding head  8  of hex screw  3 , in such a manner that sufficient room remains between head  8  of hex screw  3  and the inside wall of pressure punch  9 , for a nut driver that fits onto screw head  8 , which nut driver is not shown in the figure. 
         [0022]    Metal part  2  rests against a contact surface  10 , the top of which is connected with a hydraulic device  11  and  12 , on the two sides of bulge  5  of metal part  1 , which devices are connected with a bar element  13  on the side facing away from the contact surface  10 . Bar element  13  is connected with pressure punch  9  and has a bore  14  within the contact region between bar element  13  and pressure punch  9 , the diameter of which bore is equal to the inside diameter of pressure punch  9 . 
         [0023]    If oil is introduced into openings  15  and  16  of hydraulic devices  11  and  12  under pressure, pistons  17 ,  18  disposed on them are put into motion in the direction of arrows  19 ,  20 , which brings about that a force in the direction of arrows  21  and  22  is exerted on pressure punch  9  and thus on metal part  1 , in the region of bulge  5 . 
         [0024]    As shown in  FIG. 2 , metal part  1  thereby makes contact with metal part  2  also in the region of hex screw  3 . In order to screw the two metal parts  1 ,  2  to one another, a nut  24  is screwed onto threaded bolt  23  of hex screw  3  in this deformation state of the metal part  1 . Afterwards, the pressure of the oil situated in hydraulic devices  11 ,  12 , is reduced to such an extent that bar element  13  and thus also pressure punch  9  are lifted off metal parts  1 ,  2 , and metal parts  1 ,  2 , which are now screwed to one another under bias, can be taken off contact surface  10 . 
         [0025]    The advantage of the method according to the invention of screwing metal parts to one another, at least one of which metal parts is elastically deformable and is elastically deformed before they are screwed together, by applying an external load, is that screwing the metal parts together is possible with a lower expenditure of torque than if the elastically deformable metal part has to be deformed by the screw force that occurs when the parts are screwed together. Furthermore, the elastically deformable metal part exerts a bias on the screw connection, which brings about a significant improvement in the strength of the screw connection. 
         [0026]      FIG. 3  shows a piston  27  consisting of a skirt part  25  and a crown part  26  for an internal combustion engine. Piston  27  can consist of steel, gray cast iron, or light metal. Crown part  26  has a threaded bolt  28 , which lies coaxial to piston axis  30  and is affixed in the center, on its side facing away from the piston crown. Crown part  26  and skirt part  25  are connected with one another by a screw connection element  4 ′, configured as a sleeve  29 , which has an inside thread that fits onto the thread of threaded bolt  28 . 
         [0027]    In this connection, piston  27 , viewed in the radial direction, has a cooling chamber  31  within the pin boss support  25 ′, where skirt part  25  and crown part  26  lie against one another. Cooling chamber  31  is delimited, on the one side, by piston crown  32 , and on the shaft side by an elastically resilient wall region  33  of shaft part  25 , which region runs out, radially on the inside, into a tubular molded-on part  34 , which lies coaxial to the piston axis  30  and is oriented in the direction of the piston crown. Molded-on part  34  has a bore  35  that lies in the center, the diameter of which is greater than that of threaded bolt  28 , and the bore has a collar  36  directed radially inward, which is provided, on the skirt side, with a ring-shaped contact surface  37  against which sleeve  29  rests. 
         [0028]    In order to screw the skirt part and crown part  25 ,  26  of the piston  27  together with one another, the skirt part and crown part  25 ,  26  are placed on top of one another, and, in this connection, threaded bolt  28  is passed through bore  35 . In order to obtain a firm screw connection between skirt part  25  and crown part  26  of piston  27 , crown part  26  is laid onto a contact surface  10 ′ that matches the shape of combustion chamber  38  of crown part  36 , and afterwards, pressure is exerted on wall region  33  of skirt part  25 , in the direction of arrows  40  and  40 ′, using a tubular pressure punch  9 ′, to such an extent that wall region  33  deforms in the direction of piston crown  32 . Using a special screw element  39  that has bolts  39 ′ on its side facing the screw connection element  4 ′, which bolts fit into depressions on the skirt side of sleeve  29  having a complementary shape, sleeve  29  is screwed onto threaded bolt  28  so far until its piston-crown-side face side makes contact with the skirt-side contact surface  37  of collar  36  of molded-on part  34 , without thereby exerting any greater torque on sleeve  29 . Subsequently, screw element  39  is withdrawn from sleeve  29 , and pressure punch  9 ′ is withdrawn from the piston, so that the deformation brought about by pressure punch  9 ′ in wall region  33  is maintained and exerts a bias on sleeve  29  screwed together with threaded bolt  28 , by way of contact surface  37  of the collar  36 , which bias imparts great strength to this screw connection. 
         [0029]    In  FIG. 4 , a piston  43  consisting of a crown part  41  and a skirt part  42 , which are screwed together, is shown, which can consist of steel, gray cast iron, or light metal. Crown part  41  has an elastically deformable floor region  44  close to a screw connection element  4 ″. Crown part  41  and skirt part  42  lie against one another by way of a pin boss support  45 . Viewed in the radial direction, piston  43  has an inner cooling chamber  47  disposed coaxial to piston axis  46 , within pin boss support  45 , which chamber is delimited on the piston crown side by deformable floor region  44  and on the skirt side by piston-crown-side ceiling region  48  of skirt part  42 . 
         [0030]    In the center, ceiling region  48  has a passage bore  49  that lies coaxial to piston axis  46 . On the skirt side, a dead-end bore  50  having an inside thread and disposed coaxial to piston axis  46  is made in floor region  44 . Crown part  41  and skirt part  42  of the piston  43  can be screwed to one another by means of a screw connection element  4 ″ configured as a hex screw  51  that can be passed through passage bore  49  and screwed into dead-end bore  50 . 
         [0031]    In order to screw crown part  41  together with skirt part  42 , crown part  41  and skirt part  42  are joined together as shown in  FIG. 4 , and the two parts are laid onto the contact surface  10  by way of pin boss  70 . Subsequently, pressure is applied to floor region  44  of crown part  41 , by way of a pressure punch  53 , in the direction of arrows  52 ,  52 ′, by means of a bar element  13 ′ that is connected with contact surface  10  by way of hydraulic devices not shown in  FIG. 4 , but as shown in  FIGS. 1 and 2 , to such an extent that in this way, floor region  44  is deformed in the direction of ceiling region  48 . While the pressure is being maintained, hex screw  51  is passed through an opening  54  in contact surface  10  and through passage bore  49  in ceiling region  48  of skirt part  42 , and screwed into dead-end bore  50 , without exerting any overly great torque on hex screw  51  while this is done. Subsequent to this, the pressure being exerted on floor region  44  is reduced, and bar element  13 ′ and pressure punch  53  are lifted off crown part  41 , so that piston  43  can be removed from contact surface  10 . The deformation of floor region  44  of crown part  41  brought about by exerting pressure is maintained when this is done, and exerts a bias on the screw connection between crown part  41  and skirt part  42 , which bias imparts great strength to this screw connection. 
         [0032]      FIG. 5  shows a two-part piston  57  consisting of a crown part  55  and a skirt part  56 , which are screwed together. Piston  57  can consist of steel, gray cast iron, or light metal. Crown part  55  has an elastically resilient floor region  58 , and skirt part  56  has an elastically resilient ceiling region  59 . Viewed in the radial direction, piston  57  has a cooling chamber  61  disposed coaxial to piston axis  62  within pin boss support  60 , which chamber is delimited on the piston crown side by floor region  58  and on the skirt side by ceiling region  59 . 
         [0033]    In the center and coaxial to piston axis  62 , a passage bore  63  having an inside thread is made in ceiling region  59  of skirt part  56 . Floor region  58  of crown part  55  has a passage bore  64  in the center and coaxial to piston axis  62 , which bore is provided, on the skirt side, with a collar  65  directed radially inward, which collar has a ring-shaped contact surface  66  on the piston crown side. Crown part  55  and skirt part  56  can be screwed to one another by means of a screw connection element  4 ′″ configured as a hex socket head screw  67 , whose outside thread fits into inside thread of passage bore  63 , and that has a mushroom-shaped molded-on part  68  on the piston crown side. Molded-on part  68  has a ring-shaped contact surface  69  on the skirt side, which makes contact with contact surface  66  of collar  65  in this connection. 
         [0034]    Crown part  55  and skirt part  56  of piston  57  are screwed together by joining crown part  55  and skirt part  56  together as shown in  FIG. 5 , and laying them onto contact surface  10  in such a manner that piston crown  71  makes contact with it. Subsequently, a tubular pressure punch  9 ″ is introduced into the side of skirt part  56  that faces away from the piston crown, and is brought into contact with elastically resilient ceiling region  59  of skirt part  56  on the side facing away from the piston crown. An upper bar element  72  is connected with pressure punch  9 ″. Subsequently, a tubular pressure punch  9 ′″ connected with a lower bar element  73  is passed through opening  54  of contact surface  10  in such a manner that upper face side of pressure punch  9 ′″ makes contact with piston crown  71  in the region of combustion chamber  79 , and thus with elastically resilient floor region  58 . 
         [0035]    In accordance with  FIGS. 1 and 2 , upper and lower bar elements  72  and  73  are connected by way of hydraulic devices, not shown in  FIG. 5 , which bring about the result that pressure is exerted on the upper and the lower bar elements  72  and  73  in the direction of arrows  74 ,  74 ′ and  75 ,  75 ′. This pressure is passed on to the floor region  58  and to ceiling region  59  by way of pressure punches  9 ″ and  9 ′″, so that these two regions  58 ,  59  deform and move towards one another as this happens. Hex socket head screw  67  is passed through passage bore  64  in floor region  58 , by way of a central opening  76  in lower bar element  73 , and screwed tightly into passage bore  63  having an inside thread, by a hex driver  78  that is introduced into hex socket opening  77 , without exerting any overly great torque on hex socket head screw  67  while this is done. Subsequent to this, the pressure exerted on bar elements  72  and  73  is reduced, hex driver  78  is withdrawn from hex socket opening  77  and from pressure punch  9 ″, pressure punches  9 ″ and  9 ′″ are removed from piston  57 , and piston  57  is removed from contact surface  10 . In this connection, the deformations produced in floor region  58  and in ceiling region  59 , brought about by hex socket head screw  67 , are maintained, thereby causing a bias to be exerted on the screw connection between crown part  55  and skirt part  56 , which bias imparts great strength to this screw connection. 
         [0036]    Accordingly, while only 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. 
       REFERENCE SYMBOL LIST 
       [0000]    
       
           1 ,  2  metal part 
           2  region 
           3  hex screw 
           4 ,  4 ′,  4 ″,  4 ′″ screw connection element 
           5  bulge, region 
           6 ,  7  bore 
           8  head of the hex screw  3 , screw head 
           9 ,  9 ′,  9 ″,  9 ′″ pressure punch 
           10 ,  10 ′ contact surface 
           11 ,  12  hydraulic device 
           13 ,  13 ′ bar element 
           14  bore 
           15  opening in the hydraulic element  11   
           16  opening in the hydraulic element  12   
           17  piston of the hydraulic element  11   
           18  piston of the hydraulic element  12   
           19 ,  20 ,  21 ,  22  arrow 
           23  threaded bolt 
           24  nut 
           25  skirt part, metal part 
           25 ′ pin boss support 
           26  crown part, metal part 
           27  piston 
           28  threaded bolt 
           29  sleeve 
           30  piston axis 
           31  cooling oil chamber 
           32  piston crown, region 
           33  wall region, region 
           34  molded-on part 
           35  bore 
           36  collar 
           37  contact surface 
           38  combustion chamber 
           39  screw element 
           39 ′ bolt 
           40 ,  40 ′ arrow 
           41  crown part, metal part 
           42  skirt part, metal part 
           43  piston 
           44  floor region, region 
           45  pin boss support 
           46  piston axis 
           47  cooling chamber 
           48  ceiling region, region 
           49  passage bore 
           50  dead-end bore 
           51  hex screw 
           52 ,  52 ′ arrow 
           53  pressure punch 
           54  opening 
           55  crown part, metal part 
           56  skirt part, metal part 
           57  piston 
           58  floor region, region 
           59  ceiling region, region 
           60  pin boss support 
           61  cooling chamber 
           62  piston axis 
           63 ,  64  passage bore 
           65  collar 
           66  contact surface 
           67  hex socket head screw 
           68  molded-on part 
           69  contact surface 
           70  pin boss 
           71  piston crown 
           72  upper bar element 
           73  lower bar element 
           74 ,  74 ′,  75 ,  75 ′ arrow 
           76  opening 
           77  hex socket opening 
           78  hex driver 
           79  combustion chamber