Patent Publication Number: US-2007113734-A1

Title: Piston for a two-stroke engine and a method of making the same

Description:
CROSS REFERENCE TO RELATED APPLICATION  
      This application claims priority of German patent application no. 10 2005 055 787.2, filed Nov. 23, 2005, the entire content of which is incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      U.S. Pat. No. 3,460,239 discloses a piston having a piston base and two piston pin bosses. The piston pin bosses each have a bore for the piston pin. A casting process is provided for manufacturing the piston. Thereafter, the piston is machined in a machining method. For this purpose, the piston is clamped at its inner side in two mutually spaced planes. In order to position the piston in the direction of its longitudinal center axis, the piston is pressed with a pressure piece against a stop arranged on the outer side of the piston base. The pressure piece is arranged on the inner side of the piston base. Because of the stop, a machining of the piston base in this clamped state is not possible. The piston must be clamped anew for machining the piston base. The renewed clamping leads to the situation that additional tolerances in machining result which lead to a deteriorated dimensional stability of the piston. The piston must be designed to be stronger in order to prevent a malfunction thereof because all tolerances must be considered. This leads to greater wall thicknesses of the piston and an increased weight. Clamping the piston on the inner side of the piston skirt can lead to a deformation of the piston and therefore to a deteriorated dimensional stability of the machined piston skirt.  
     SUMMARY OF THE INVENTION  
      It is an object of the invention to provide a piston of the kind described above which can be manufactured with reduced tolerances. A further object of the invention is to provide a method for making the piston.  
      The piston of the invention is for a two-stroke engine. The piston includes: a piston body having a base; two piston pin bosses formed on the body; the piston pin bosses having respective bores formed therein for accommodating a piston pin; and, the piston pin bosses each having opposite-lying planar first and second clamping surfaces formed thereon.  
      The piston has two opposite-lying planar clamping surfaces on the piston pin bosses. For this reason, the piston can be positioned on a clamping tool and can be gripped thereby. The clamping tool grips the two clamping surfaces at the piston pin boss. In this way, the piston skirt is not significantly deformed in the clamped state of the piston so that the piston skirt can be machined with high accuracy in the clamped state of the piston.  
      The piston is especially manufactured in a pressure die-casting process. To permit making the clamping surfaces in the pressure die-casting process, the clamping surfaces extend parallel to the longitudinal axis of the bore in the piston pin boss and extend inclined to a transverse plane of the piston. The clamping surfaces are especially inclined at an angle of 1° to 5° to the transverse plane of the piston. The transverse plane includes the longitudinal center axis of the piston and the longitudinal axis of the bore of the piston pin boss. The slight inclination ensures that the piston can be ejected. At the same time, a reliable clamping of the piston and a simple alignment of the piston to the clamping tool are possible because of the slight inclination and the parallel alignment to the longitudinal axis of the bore in the piston pin boss.  
      Preferably, the two clamping surfaces lie at the same spacing to a transverse plane of the piston. The transverse plane contains the longitudinal center axis of the piston and the longitudinal axis of the bore of the piston pin boss. Because of the arrangement of the clamping surfaces at the same distance to the transverse plane, the piston is clamped symmetrically to the transverse plane so that the positioning of the clamping tool to the longitudinal center axis of the piston is possible in a simple manner. The clamping surface, which is arranged on one side of the transverse plane, is narrower in the direction of the longitudinal axis of the bore of the piston pin boss than the clamping surface arranged on the opposite-lying side of the transverse plane. Because of the different widths of the clamping surfaces, a positioning of the piston relative to a clamping tool is possible in a simple manner. For example, a clamping, which is rotated by 180° about the longitudinal center axis of the piston, is prevented by mechanical measures as a consequence of the different geometries of the clamping surfaces. Accordingly, and in a simple manner, a properly-positioned clamping of the piston is ensured.  
      The piston is especially provided for a two-stroke engine which operates with a scavenging prestore. For this purpose, it is practical that the piston has at least one piston pocket open toward the piston skirt. The piston pocket connects an air channel of the two-stroke engine with the transfer window of a transfer channel so that scavenging prestored air is stored in advance in the transfer channel. The piston has a center plane which contains the longitudinal center axis of the piston and extends perpendicularly to the longitudinal axis of the bore of the piston pin boss. The piston pocket and the inner wall of the piston are configured to be especially nonsymmetrical to the center plane in the region of the piston pocket. The nonsymmetrical configuration of the piston pocket permits a conduction of air to the transfer channels with slight flow resistance. For approximately the same wall thickness of the piston, there results also a nonsymmetrical configuration of the inner wall of the piston in the region of the piston pocket. The nonsymmetrical configuration of the inner wall of the piston makes possible the correctly positioned clamping of the piston so that the manufacture of the piston is simplified.  
      According to a feature of the invention, a stop surface is configured on the inner side of the piston base. The stop surface is configured to be planar and is arranged perpendicularly to the longitudinal center axis of the piston. The stop surface is configured on the inner side of the piston base and the piston does not have to be pressed against a stop on the outer side of the piston for positioning. For this reason, the piston skirt as well as the piston base can be machined in one clamped state. In this way, the piston skirt and the piston base can be machined to higher accuracy with respect to each other and with respect to the stop surface. Because of the reduced tolerances, the thickness of piston skirt and piston base can be designed comparatively low so that the piston has a reduced weight. Practically, the stop surface has a width of 10% to 25% of the piston diameter and a length of 10% to 25% of the piston diameter. The piston thickness advantageously is 2.5% to 7%, especially 5%, of the piston diameter. The wall thickness of the piston skirt at the elevation of the longitudinal axis of the bore in the piston pin boss amounts advantageously to 1% to 3%, especially 2.5%, of the piston diameter.  
      A clamping tool having a stop is moved up to the stop surface of the piston and the piston base is pressed against a counter holder by the clamping tool for a method for making a piston having a planar stop surface on the inner side of the piston base and at least two opposite-lying clamping surfaces arranged on a piston pin boss. Thereafter, the piston is gripped by the clamping tool at the clamping surfaces and clamped. After clamping the piston at the clamping surfaces, the counter holder is removed and the piston is machined on the piston skirt and the piston base in this clamping state.  
      The method for making the piston provides for a positioning of the piston relative to the clamping tool exclusively at the clamping surfaces and the stop surface on the piston base, that is, exclusively on the inner side of the piston. The counter holder functions only for the purpose to ensure that the stop of the clamping tool lies against the stop surface of the piston. Because the piston is exclusively clamped on its inner side, a machining of the piston skirt and also a machining of the piston base is possible in this clamped state. The machining of the piston skirt as well as the machining of the piston base accordingly takes place with the same tolerances between the stop surface, the clamping surfaces and the clamping tool. In this way, the piston skirt and the piston base can be machined to low tolerances with respect to each other so that the piston can be accurately manufactured and the wall thicknesses can be designed to be thin because of the reduced tolerances.  
      After clamping of the piston, the bore is drilled in the piston pin boss. The bore in the piston pin boss is drilled at reduced clamping force on the clamping surfaces and with the counter holder arranged on the piston base. The bore in the piston pin boss can already be made in advance of removing the counter holder. It can, however, also be practical to first machine the piston skirt and the piston base and thereafter arrange the counter holder anew on the piston base in order to drill the piston pin boss. The reduction of the clamping force at the clamping surfaces ensures that only low stresses are present during the drilling operation in the piston pin boss. In this way, it is ensured that no warping of the bore results when releasing the clamping tool. The bore in the piston pin boss can be made thereby at high accuracy. Because the clamping force is not reduced and the clamping of the piston is, however, not released, the bore in the piston pin boss can be made with low tolerances relative to the piston skirt and the piston base.  
      In the clamped state, an annular slot is cut at the bore in the piston pin boss and a bore is drilled at the annular slot. The annular slot and the bore function for receiving a holding ring for the piston pin. It is practical to cut at least one slot for a piston ring in the clamped state. In this way, it is possible to run through the entire machining operation of the piston in a single clamped state thereof so that the piston can be made with low tolerances. At the same time, the one-time clamping of the piston leads to a simplification of the manufacturing process.  
      It is practical that the clamping surfaces of the piston are configured nonsymmetrically to the center plane. The center plane contains the longitudinal center axis of the piston and the longitudinal axis of the bore in the piston pin boss. The clamping tool and the piston are aligned to each other at the nonsymmetry in advance of the clamping of the piston. The piston is made in a pressure die-casting method in advance of the machining operation. The stop surface and the clamping surfaces are made in the pressure die-casting process. The stop surface and the clamping surfaces can be made with sufficiently high accuracy in a pressure die-casting process. In this way, it is possible to completely machine the piston in only one clamping state. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention will now be described with reference to the drawings wherein:  
       FIG. 1  is a schematic longitudinal section taken through a two-stroke engine;  
       FIG. 2  is a section view taken along line II-II in  FIG. 1 ;  
       FIG. 3  is a side elevation view of the piston of  FIGS. 1 and 2 ;  
       FIG. 4  is a longitudinal section taken through the piston of  FIG. 3 ;  
       FIG. 5  is a section view taken along line V-V in  FIG. 4 ;  
       FIG. 6  is a schematic plan view of a clamping device; and,  
       FIG. 7  is a side elevation view of the clamping device of  FIG. 6  viewed in the direction of arrow VII in  FIG. 6 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION  
      The two-stroke engine  1  shown in  FIG. 1  is provided as a drive motor for a portable handheld work apparatus such as a motor-driven chain saw, a cutoff machine, a brushcutter or the like. The two-stroke engine  1  is configured as a single cylinder engine and has a cylinder  2  wherein a combustion chamber  3  is formed. The combustion chamber  3  is delimited by a piston  5  which is journalled for reciprocal movement in the cylinder  2 . The piston  5  drives a crankshaft  7  via a connecting rod  6 . The crankshaft  7  is rotatably journalled in a crankcase  4  and functions to drive the work tool of the work apparatus. The connecting rod  6  is attached to the piston  5  via a piston pin  38 .  
      An intake  9  for an air/fuel mixture opens at the cylinder  2 . The intake  9  is slot controlled by the piston  5  and is connected to the crankcase  4  in the region of top dead center of the piston  5 . A discharge  10  for exhaust gases leads from the combustion chamber  3 . A spark plug  8  projects into the combustion chamber  3 . The piston  5  has two piston rings  21  which seal the combustion chamber  3  to the crankcase  4  during operation. Furthermore, an air channel  15  having an air inlet  16  opens at the cylinder  2  on both sides of the intake  9 . The crankcase  4  is connected to the combustion chamber  3  in the region of bottom dead center shown in  FIG. 1  via transfer channels  11  and  13 . The discharge-near transfer channel  11  opens via a transfer window  12  into the combustion chamber  3  and the intake-near transfer channel  13  opens with a transfer window  14 .  
      As shown in the section view of  FIG. 2 , a first pair of transfer channels  11  and  13  lie opposite a second pair of transfer channels  11  and  13  on the cylinder  2 . The transfer channels  11  and  13  are arranged symmetrically to the center plane  32  of the piston  5 . The center plane  32  partitions the intake  9  and the discharge  10  of the cylinder  2  approximately centrally and contains a cylinder longitudinal axis  17 . The piston  5  is moveably journalled in the cylinder  2  in the direction of the cylinder longitudinal axis  17 . In the section view shown in  FIG. 2 , the piston  5  is shown in the region of top dead center. As shown in  FIG. 2 , an air channel  15  opens at the cylinder  2  on each side of the center plane  32 . The piston  5  has two piston pockets  23  which are configured symmetrically to the center plane  32 . The piston pockets  23  extend from the piston skirt  20  into the cylinder interior. In the region of top dead center of the piston  5 , the two air channels  15  are connected to the transfer windows  12  and  14  of the transfer channels  11  and  13  via the piston pockets  23  so that substantially fuel-free combustion air flows out of the air channels  15  into the transfer channels and displaces the air/fuel mixture from the transfer channels to the crankcase  4 . As  FIG. 2  shows, the piston  5  has two piston pin bosses  24 . The piston pin  38  is journalled in the piston pin bosses  24 .  
      During the operation of the two-stroke engine  1 , an air/fuel mixture is drawn from the intake  9  into the crankcase  4  in the region of top dead center of the piston  5 . At the same time, substantially fuel-free combustion air flows from the air channels  15  via the piston pockets  23  into the transfer channels  11  and  13  and fills these with air. The air/fuel mixture is compressed in the crankcase with the downward stroke of the piston  5  toward the crankcase  4 . As soon as the transfer windows  12  and  14  open to the combustion chamber  3 , the prestored combustion air flows out of the transfer channels  11  and  13  into the combustion chamber  3  and flushes the still-present exhaust gases from the previous combustion cycle from the combustion chamber  3  through the discharge  10 . Thereafter, the air/fuel mixture flows out of the crankcase  4  via the transfer channels  11  and  13  into the combustion chamber  3 . During the upward stroke of the piston  5 , first the transfer windows  12  and  14  are closed by the piston  5  and then the discharge  10  is closed thereby. The air/fuel mixture is compressed in the combustion chamber  3  and ignited by the spark plug  8  in the region of top dead center of the piston  5 . The combustion of the mixture accelerates the piston  5  again in a direction toward the crankcase  4 . As soon as the discharge  10  opens, the exhaust gases flow out from the combustion chamber  3 . The residual exhaust gases are flushed by the combustion air flowing in from the transfer channels  11  and  13  out of the combustion chamber  3 . Thereafter, the air/fuel mixture for the next combustion cycle passes from the crankcase  4  into the combustion chamber  3 .  
      In  FIG. 2 , the transfer channels  11  and  13  are shown open toward the cylinder outer side. In this way, the cylinder  2  can be made in a simple manner in a pressure die cast process. The transfer channels  11  and  13  are closed by separately manufactured covers not shown in  FIG. 2 .  
      The piston  5  is shown in a side elevation view in  FIG. 3 . The longitudinal center axis  18  of the piston  5  is coincident with the cylinder longitudinal axis  17  in the arrangement of the piston  5  in the cylinder  2  of the two-stroke engine  1 . The piston  5  shown in  FIG. 3  has a piston base  19  which runs planar and perpendicularly to the longitudinal center axis  18  of the piston  5 . The piston base  19  delimits the combustion chamber  3  of the two-stroke engine. The piston  5  is guided in the cylinder  2  at the piston skirt  20 . The piston skirt  20  has two peripherally-extending slots  22  next to the piston base  19  for accommodating the piston rings  21  shown in  FIG. 1 .  
      The piston pocket  23  is next to a bore  27  for the piston pin  38  and is arranged on the side of the bore  27  facing away from the piston base  19 . The bore  27  is arranged in the piston pin boss  24 . The longitudinal axis  39  of the bore  27  runs perpendicularly to the longitudinal center axis  18  and perpendicularly to the center plane  32  of the piston  5  shown in  FIG. 2 . At the bore  27 , the piston  5  has an annular slot  25  for accommodating a holding ring for the piston pin  38 . The annular slot  25  is shown by a broken line in  FIG. 3 . A transverse bore  26  is arranged at the annular slot  25  and is disposed in the region of the periphery of the bore  27  and parallel to the bore  27 .  
      Next to the bore  27 , the piston  5  has a cutout  34  in the piston skirt  20  which functions to reduce weight. The cutout  34  is disposed in the region of the side  31  of the piston  5  facing toward the discharge. The piston  5  is extended toward the crankcase  4  on the opposite-lying side  30  facing toward the intake  9  of the two-stroke engine  1 . The edge  29  of the piston  5  at the crankcase end does not run evenly because the edge  29  controls the intake  9  on the side  30  of the piston  5  facing toward the intake. In the region of the air inlet  16 , the piston pocket  23  must be so configured that the total cross section of the air inlet  16  opens into the piston pocket  23 . At the same time, the edge  29  must be arranged so deep that, at top dead center of the piston  5 , no air can flow from the air inlet  16  directly into the crankcase  4 . At the side  31  of the piston  5  facing toward the discharge, the edge  29  only ensures that the discharge  10  is not connected to the crankcase  4  at top dead center of the piston  5  so that here a shorter height of the piston  5  is necessary. Because of the uneven configuration of the edge  29 , a clamping of the piston  5  on the edge  29  is not easily possible for machining the piston  5 .  
      As shown in the section views of  FIGS. 4 and 5 , clamping surfaces  36  and  37  are provided on both piston pin bosses  24  for clamping the piston  5 . The piston pin bosses  24  each have a strut  28  which extends from the bore  27  to the piston base  19 . The piston  5  can be clamped in the direction of arrows  40  on the clamping surfaces  36  and  37 . Both piston pin bosses  24  have a clamping surface  36  on the side thereof facing toward the side  30  of the piston and a clamping surface  37  on the opposite-lying side facing toward the side  31  of the piston  5 . The clamping surfaces  36  and  37  run parallel to the longitudinal axis  39  of the bore  27  in the piston pin boss  24 . With respect to a transverse plane  33 , the clamping surfaces  36  and  37  are inclined at an angle α which can be from 1° to 5°. In this way, the mutually opposite-lying clamping surfaces have a greater distance from each other in the region of the piston base  19  than at the elevation of the longitudinal axis  39  of the bore  27  in the piston pin bosses  24 . The transverse plane  33  extends perpendicularly to the center axis  32  and contains the longitudinal center axis  18  of the piston and the longitudinal axis  39  of the bore  27  for the piston pin  38 . Perpendicular to the transverse plane, the clamping surfaces  36  and  37  are at the same distance to the transverse plane  33  at each elevation of the piston  5 . The clamping surfaces ( 36 ,  37 ) are symmetrically arranged to the transverse plane  33  aside from their different widths (c, d) as shown in  FIG. 5 .  
      At the elevation of the longitudinal axis  39  of the bore  27 , the piston skirt  20  has a wall thickness L which is 1% to 3% (especially approximately 2.5%) of the piston diameter (e) shown in  FIG. 3 . The piston base  19  has a thickness (k) which is 2.5% to 7% (especially approximately 5%) of the diameter (e) of the piston  5 . A stop surface  35  is formed on the inner side  42  of the piston base  19  and this stop surface runs evenly and perpendicularly to the longitudinal center axis  18  of the piston  5 .  
      In  FIG. 5 , the stop surface  35  is shown in plan view. In the embodiment, the stop surface  35  is rectangular and can be especially configured to be quadratic. The stop surface  35  can, however, also have another form. In the direction of the transverse plane  33 , that is, parallel to the transverse plane  33  and perpendicular to the center plane  32 , the stop surface  35  has a width (a) which is 10% to 25% of the diameter (e) of the piston  5 . The length (b) of the stop surface  35 , which is measured for this purpose in the direction of the center plane  32 , amounts likewise to approximately 10% to 25% of the diameter (e) of the piston  5 . The stop surface  35  is arranged centrally on the piston base  19  in the region of the longitudinal center axis  19 .  
      Referring to  FIG. 5 , the two clamping surfaces  37 , which are on the side of the piston pin boss  24  facing toward the side  31  of the piston  5 , have a width (c) which is measured parallel to the transverse plane  33  and perpendicularly to the center plane  32  and this width (c) is advantageously approximately 5% to 12% of the diameter (e) of the piston  5 . The clamping surfaces  36 , which are arranged on the opposite-lying side of the transverse plane  33 , have a width (d), which is measured in the same direction, and which width (d) is 3% to 10% of the diameter (e) of the piston  5 . The width (d) of the clamping surfaces  36  is less than the width (c) of the clamping surfaces  37 . The clamping surfaces  36  are narrower than the clamping surfaces  37  so that a nonsymmetry of the clamping surfaces ( 36 ,  37 ) to the transverse plane  33  results. The piston pockets  23  are arranged in the region of the clamping surfaces  36  so that the clamping surfaces  36  cannot be configured to be wider. The nonsymmetrical configuration of the piston  5  permits the alignment of a clamping tool at the differently wide clamping surfaces ( 36 ,  37 ) so that, thereby, a position-correct clamping can be ensured. The inner wall  41  of the piston  5  does not run symmetrically to the transverse plane  33  in the region of the piston pockets  23 .  
       FIGS. 6 and 7  show a clamping tool  45  for clamping the piston  5 . The clamping tool  45  has a first jaw  47  which clamps the piston at the side  30  facing toward the intake and a second jaw  48  which clamps the piston  5  at the opposite-lying side  31  facing toward the discharge. The two jaws  47  and  48  are moveably journalled on a base  52 . A guide  46  extends between the two jaws  47  and  48  and a stop  49  is arranged on the guide  46 . The stop  49  is configured slightly smaller than the stop surface  35  of the piston  5 . As also shown in  FIG. 7 , the mutually opposite-lying grip surfaces  50  and  51  of the jaws  47  and  48  extend inclined by an angle β relative to the longitudinal axis  54  of the clamping tool  45 . The angle β corresponds to the inclination angle α of the clamping surfaces  36  and  37  of the piston  5 . The stop  49  runs perpendicularly to the longitudinal axis  54  and is configured to be planar. As shown in  FIG. 7 , the guide  46  has a bore  53  which is arranged in the region of the bore  27  in the piston pin boss  24  when the piston is clamped. The bore  53  permits a drilling of the bore  27  through both piston pin bosses  24  when the piston  5  is clamped.  
      As shown in  FIG. 6 , the stop surface  49  has a width (h) and a length (i). The width (h) is somewhat less than the width (a) of the stop surface  35  of the piston  5  and the length (i) is somewhat less than the length (b) of the stop surface  35 . The width (f) of the grip surface  51  is slightly less than the width (c) of the clamping surface  37  and the width (g) of the grip surface  50  is somewhat less than the width (d) of the clamping surface  36 . In this way, it is ensured that the grip surfaces  50  and  51  lie planar against the clamping surfaces  36  and  37 . At the same time, the largest possible stop surface is ensured so that the clamping forces can be well introduced into the piston pin bosses  24 .  
      For making the piston  5 , the piston  5  is first made in a pressure die cast process. Here, the stop surface  35  and the clamping surfaces  36  and  37  are produced with a sufficiently high accuracy. For the machining operation, the piston  5  is clamped on the clamping tool  45 . For this purpose, the clamping tool  45  is configured as a gripper and is moved into the piston  5 . Only a position-correct introduction into the piston  5  is possible because of the nonsymmetrical configuration of the piston  5  and the clamping tool  45 . The second jaw  48  of the clamping tool  45  is too wide in order to be introduced into the piston  5  in the region of the clamping surfaces  36 . The clamping tool  45  moves into the piston  5  in the direction of the longitudinal axis  54  of the tool until the stop  49  lies against the stop surface  35 . The clamping tool  45  presses the piston base  19  of the piston  5  shown in phantom outline in  FIG. 7  against a counter holder  44  to ensure that the stop  49  lies firmly against the stop surface  35 . Thereafter, the jaws  47  and  48  move together and clamp the piston  5  at the clamping surfaces  36  and  37 .  
      The jaws  47  and  48  first clamp the piston  5  with reduced clamping force and the bore  27  is drilled into the piston pin bosses  24  while the counter holder  44  still lies against the piston base  19 . Thereafter, the clamping force is increased so that the piston  5  is held tightly against the clamping tool  45  and the counter holder  44  is removed. In that the clamping tool  45  only grips at the piston pin bosses  24 , there results only minimal deformation at the piston skirt  20 . Thereafter, the piston  5  is machined at the piston skirt  20  and the piston base  19  until the desired roughness of the surfaces is achieved. In the same clamping state, at least one slot for a piston ring is cut in. It can also be advantageous to make the slot  22  for the piston ring  21  in advance of machining the piston skirt  20 . An annular slot  25  is cut into the piston pin boss  24  and the bore  26  is drilled. This machining too can take place at reduced clamping force on the piston pin bosses  24 . No high accuracy in the radial direction toward the longitudinal axis  39  of the piston pin  38  is needed for the annular slot  25 . For this reason, the annular slot  25  can, however, also be introduced at increased clamping forces on the clamping tool  45 . A bore  26  is drilled for the bosses on the holding ring at the annular slot  25 . In this way, all machining operations of the piston  5  can take place in one clamping state.  
      The piston  5  can be made with high accuracy so that reduced wall thicknesses at the piston skirt  20  and at the piston base  19  can be realized. Because of the nonsymmetry of the piston  5  to the center axis  32 , the clamping tool  45  and the piston  5  can be aligned to each other in advance of clamping the piston  5  so that a position-correct clamping of the piston  5  is ensured.  
      It is understood that the foregoing description is that of the preferred embodiments of the invention and that various changes and modifications may be made thereto without departing from the spirit and scope of the invention as defined in the appended claims.