Abstract:
There is provided a boom assembly bent at the center in a longitudinal direction so as to form a round shape protruding upward when seen from the side, comprising; a front boom sub assembly ( 47 ), the front boom sub assembly constituting the front of the center in the longitudinal direction of said boom assembly; and a rear boom sub assembly ( 48 ), the rear boom sub assembly constituting the rear of the center in the longitudinal direction of the boom assembly, wherein the front boom sub assembly ( 47 ) and the rear boom sub assembly ( 48 ) are symmetric when seen from the side, and portions from the midway positions in the longitudinal direction of the front boom sub assembly ( 47 ) and the rear boom sub assembly ( 48 ) to the center in the longitudinal direction of the boom assembly ( 12 ) have upper surfaces of arc shape protruding upward when seen from the side, and portions from the middles in the longitudinal direction of the front boom sub assembly ( 47 ) and the rear boom sub assembly ( 48 ) to ends in the longitudinal direction of the boom assembly ( 12 ) have upper surfaces of linear shape when seen from the side.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a boom assembly of an operating machine such as a front loader. 
         [0003]    2. Description of the Related Art 
         [0004]    A boom assembly of a front loader includes a boom assembly formed by joining a front boom sub assembly on the front of the center in a longitudinal direction of the boom assembly, and a rear boom sub assembly on the rear of the center in the longitudinal direction of the boom assembly (for example, see Japanese Patent Application “kokai” No. 11-158907). 
         [0005]    A boom assembly of a front loader also includes a round boom assembly bent at the center in a longitudinal direction so as to form a round shape protruding upward when seen from the side (for example, see Japanese Patent Application “kokai” No. 11-158907 and Japanese Patent Application “kokai” No. 6-313325). 
         [0006]    A boom assembly of a front loader further includes a boom assembly in which a body portion is formed into an angle protruding upward when seen from the side by bending a flat sheet material into a π-shape in section opening downward and then further bending the flat sheet material at the center in a longitudinal direction of the boom assembly (for example, see Japanese Patent Publication No. 7-108410). 
       SUMMARY OF THE INVENTION 
       [0007]    The round boom assembly bent at the center in the longitudinal direction so as to form the round shape protruding upward when seen from the side has good appearance. However, it is difficult to form the round boom assembly from one member from a front end to a rear end. 
         [0008]    Thus, it is considered that the round boom assembly bent at the center in the longitudinal direction so as to form the round shape protruding upward when seen from the side is fabricated to include a front boom sub assembly on the front of the center in a longitudinal direction, and a rear boom sub assembly on the rear of the center in the longitudinal direction. For forming body portions of the front and rear boom sub assembly, for example, a boom forming member cut out of a flat sheet material is bent into a π-shape constituted by right and left side walls and an upper wall connecting upper edges of the right and left side walls and then bent into a round shape protruding upward. 
         [0009]    When a body of each of the front and rear boom sub assemblies is formed into a round shape protruding upward from one end to the other end in the longitudinal direction, and plural types of boom assemblies having different lengths are fabricated, plural types of boom sub assemblies having different lengths can be fabricated by one press die with the bodies of the front and rear boom sub assemblies having the same radius of curvature. However, in a front loader in which a bucket cylinder for swinging a bucket provided swingably at a tip of the boom assembly is placed in an upper front of the boom, expanding and contracting the bucket cylinder for swinging the bucket vertically swings the bucket cylinder, and causes the bucket cylinder to be moved close to or away from an upper surface of the boom. Thus, considering avoiding interference between the bucket cylinder and the boom, it is difficult in design to fabricate plural types of boom sub assemblies in each of which a front boom sub assembly is formed into a round shape protruding upward from one end to the other end in a longitudinal direction, and that have arcs with the same radius of curvature and different lengths. 
         [0010]    Thus, the boom sub assemblies of the round boom assembly are formed to have radii of curvature corresponding to the lengths, thereby solving the problem. In this case, however, press dies corresponding to the boom sub assemblies having different lengths need to be made to increase costs, and a die changing step is required in fabrication of the boom assemblies having different lengths, thereby reducing productivity and increasing labor costs. 
         [0011]    Therefore, the present invention has an object to provide a boom assembly that solves the problem. 
         [0012]    In order to solve the technical problem, the following technical means are taken. 
         [0013]    Specifically, there is provided a boom assembly bent at the center in a longitudinal direction so as to form a round shape protruding upward when seen from the side, comprising; 
         [0014]    a front boom sub assembly, the front boom sub assembly constituting the front of the center in the longitudinal direction of the boom assembly; and 
         [0015]    a rear boom sub assembly, the rear boom sub assembly constituting the rear of the center in the longitudinal direction of the boom assembly, 
         [0016]    wherein the front boom sub assembly and the rear boom sub assembly are symmetric when seen from the side, and portions from the midway positions in the longitudinal direction of the front boom sub assembly and the rear boom sub assembly to the center in the longitudinal direction of the boom assembly have upper surfaces of arc shape protruding upward when seen from the side, and portions from the middles in the longitudinal direction of the front boom sub assembly and the rear boom sub assembly to ends in the longitudinal direction of the boom assembly have upper surfaces of linear shape when seen from the side. 
         [0017]    Bodies of the front boom sub assembly and the rear boom sub assembly may be formed into a π-shape in section by right and left side walls and an upper wall connecting upper edges of the right and left side walls. 
         [0018]    The boom assembly includes an operating tool swingably at a front end of the boom assembly, and is suitably adopted in a loader in which a hydraulic cylinder that swings the operating tool is placed in an upper front of the boom assembly. 
         [0019]    When a round boom assembly comprising a front boom sub assembly and a rear boom sub assembly, and bent at the center in a longitudinal direction so as to form a round shape protruding upward when seen from the side is adopted in a front loader in which a hydraulic cylinder that swings an operating tool swingably provided at a front end of the boom assembly is placed in an upper front of the boom assembly, the front boom sub assembly that constitutes the front of the boom assembly being formed into an arc shape protruding upward from one end to the other end, it is difficult in design in forming boom assemblies having different lengths to form front boom sub assemblies having the same radius of curvature in view of avoiding interference with the hydraulic cylinder. Thus, the front boom sub assemblies have to be formed into arc shapes with different radii of curvature, which requires a plurality of press dies. 
         [0020]    On the other hand, according to the present invention, the boom assembly is formed so that the portion from the middles in the longitudinal direction to the center in the longitudinal direction of the boom assembly has the upper surface of arc shape protruding upward when seen from the side, and the portions from the middles in the longitudinal direction to the ends in the longitudinal direction of the boom assembly have the upper surfaces of linear shape when seen from the side. Thus, even if the boom sub assemblies are formed to have linear portions with different lengths to form arc portions with the same radius of curvature in fabricating boom assemblies having different lengths, the problem can be easily addressed of avoiding interference between the hydraulic cylinder and the boom assembly in each of the boom assemblies having different lengths. 
         [0021]    Thus, the boom sub assemblies having different lengths can be formed to have arc portions with the same radius of curvature, and the front and rear boom sub assemblies are symmetric in the front and rear when seen from the side, thereby minimizing fabrication of press dies for forming the arc portions to reduce costs, and reduce a die changing time, increase productivity, and reduce labor costs in fabrication of the boom assemblies having different lengths. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a side view of a front loader; 
           [0023]      FIG. 2  is a side view of a tractor to which the front loader is mounted; 
           [0024]      FIG. 3  is an exploded side view of a boom assembly; 
           [0025]      FIG. 4  is a side view of boom assemblies having different lengths; 
           [0026]      FIG. 5  is a side view of boom sub assemblies having different lengths; 
           [0027]      FIG. 6  is a rear sectional view of a pivot connecting portion of a boom cylinder of the boom assembly; 
           [0028]      FIG. 7  is a side view of the pivot connecting portion of the boom cylinder of the boom assembly; and 
           [0029]      FIG. 8  is a side sectional view of the center in the longitudinal direction of the boom assembly. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0030]    Now, an embodiment of the present invention will be described with reference to the drawings. 
         [0031]    In  FIG. 2 , reference numeral  1  denotes a tractor (running vehicle), and reference numeral  2  denotes a front loader (loader) removably mounted to the front of the tractor  1 . A vehicle body  3  of the tractor  1  mainly includes an engine  4  in the front, a flywheel housing connected to the rear of the engine  4 , and a transmission case  5  connected to the rear of the flywheel housing, and the transmission case  5  includes a front clutch housing  5 A and a rear transmission case  5 B. 
         [0032]    The engine  4  is covered with a hood  6 , and a front axle frame  7  is mounted and secured to a lower portion of the engine  4  so as to protrude forward from the engine  4 . A pair of right and left front wheels  8  are supported by the front axle frame  7 , and a pair of right and left rear wheels  9  are supported in the rear of the transmission case  5 . The vehicle body  3  of the tractor  1  is drivably supported by the pairs of right and left front and rear wheels  8  and  9 . Also as shown in  FIG. 1 , the front loader  2  includes a pair of right and left side frames  11 , a pair of right and left boom assemblies  12 , a pair of right and left boom cylinders  13 , a pair of right and left bucket cylinders  14 , and one bucket  15  (operating tool). 
         [0033]    Each of the right and left side frames  11  mainly includes a pair of right and left side plates  16 , and a connection plate  17  connecting the right and left side plates  16 , and an engagement pin  18  provided across the right and left side plates  16  is provided in a lower end. The right and left boom assemblies  12  are connected at bases (rear ends) to upper portions of the side frames  11  on the same lateral sides pivotably around a lateral shaft by a pivot  19 , and the right and left boom assemblies  12  are connected at the front by a connecting member  20  made of a cylindrical pipe material. 
         [0034]    The boom cylinders  13  include hydraulic cylinders, and the right and left boom cylinders  13  are placed in lower rear portions of the boom assemblies  12  on the same lateral side (below a rear boom sub assembly  48  described later), one ends of the boom cylinders  13  (rear ends, tips of piston rods) are connected to the middles in a vertical direction of the fronts of the side frames  11  on the same lateral side pivotably around a lateral shaft by a pivot  21 , and the other ends of the boom cylinders  13  (front ends, bottom ends of the cylinders) are connected to cylinder pivot portions  60  in lower ends of the rears of the pair of right and left bracket plates  22  provided in the middles in the longitudinal direction of the boom assemblies  12  pivotably around a lateral shaft by a pivot  23 , and the boom cylinders  13  are expanded and contracted (the piston rods are protruded and retracted) to vertically swing the boom assemblies  12  around the pivot  19 . 
         [0035]    The bucket  15  is connected at a lower back portion to tips (front ends) of the right and left boom assemblies  12  pivotably around a lateral shaft by a pivot  24 , one ends of first links  25  are connected to a back side of the bucket  15  pivotably around a lateral shaft by a pivot  27 , and the other ends of the first links  25  are connected to the other ends of second links  26  pivotably around a lateral shaft by a pivot  29 , the second links  26  having one ends connected to the tips of the boom assemblies  12  pivotably around a lateral shaft by a pivot  28 . 
         [0036]    The bucket cylinders  14  include hydraulic cylinders, and placed above the fronts of the boom assemblies  12  on the same lateral side (above a front boom sub assembly  47  described later), one ends of the bucket cylinders  14  (rear ends, bottom ends of the cylinders) are connected to cylinder pivot portions  59  in the fronts of upper ends of the bracket plates  22  pivotably around a lateral shaft by a pivot  31 , the other ends of the bucket cylinders  14  (front ends, tips of piston rods) are connected to connecting portions between the first links  25  and the second links  26  pivotably around a lateral shaft by the pivot  29 , the bucket cylinders  14  are expanded and contracted (the piston rods are protruded and retracted) to vertically swing the bucket  15  (scooping and dumping operation). At this time, the boom cylinders  14  vertically swing around the pivot  31  to be moved close to and away from the boom assemblies  12 . 
         [0037]    In the front loader  2 , a stand  33  is provided that supports the boom assemblies  12  with the bucket  15  being grounded when the front loader  2  is removed from the tractor  1 . The stand  33  includes a front end  33   a  connected to lower sides of the boom assemblies  12  pivotably around a lateral shaft, and a grounded portion  33   b  in a rear end removably locked to the lower sides of the boom assemblies  12 , and is changeable in position between a non-use position along the boom assemblies  12  and a use position swung downward from the non-use position. On the other hand, in the tractor  1 , a loader mounting frame  36  for removably mounting the front loader  2  is provided. 
         [0038]    The loader mounting frame  36  includes, as shown in  FIG. 1 , a pair of right and left mounting plates  37  mounted and secured to the vehicle body  3  of the tractor  1 , a pair of right and left support bases  38  provided to protrude laterally outward from the vehicle body  3  of the tractor  1 , and a pair of right and left main frames  39  standing on the support bases  38 , and the mounting plates  37 , the support bases  38 , and the main frames  39  are placed on the right and left of the vehicle body  3  of the tractor  1 . Each mounting plate  37  is formed of one steel sheet, placed in a lower rear portion of the engine  4  and the side in a lower portion of the flywheel housing, the front is secured by bolts to an outer surface of the front axle frame  7 , and upper and lower portions of the rear are secured by bolts to an outer surface of the flywheel housing. 
         [0039]    Each support base  38  is formed of a cylindrical pipe material having a lateral axis, and a lateral inner end thereof is joined by welding to the rear of the mounting plate  37 . Each main frame  39  is formed of one cast steel sheet, and joined by welding at a lower portion to a lateral outer end of the support base  38 . In the middle in a vertical direction of the front of the main frame  39 , a receiving portion  41  is provided into which the engagement pin  18  provided in the lower portion of the side frame  11  is fitted from above to be received. 
         [0040]    An upper portion of the main frame  39  is connected to the side frame  11  by a connection pin  42  inserted through the main frame  39  and the middle in the vertical direction of the rear of the side frame  11 . In the front loader  2  having the above described configuration, the engagement pin  18  of the side frame  11  fits into the receiving portion  41  of the main frame  39  from above with an upper front portion of the main frame  39  being inserted between the right and left side plates  16  of the side frame  11 . The connection pin  42  is inserted through the upper portion of the main frame  39  and the middle in the vertical direction of the rear of the side frame  11  with the engagement pin  18  being received in the receiving portion  41 , thus the side frame  11  is mounted to the main frame  39 , and the front loader  2  is supported by the loader mounting frame  36 . 
         [0041]    For removing the front loader  2  from the tractor  1 , for example, first, the connection pin  42  is removed with the tip of the bottom of the bucket  15  being grounded and the stand  33  being lowered from the non-use position to the use position, then the bucket cylinder  14  is contracted in this state to lower the boom assembly  12  and cause the stand  33  to be grounded. After the stand  33  is grounded, the boom assembly  12  swings around the grounded portion of the stand  33  so as to raise the side frame  11 , and the engagement pin  18  of the side frame  11  is removed upward from the receiving portion  41  of the main frame  39 , and thus the front loader  2  enters a standing state where the boom assembly  12  is supported by the stand  33  with the bottom of the bucket  15  being grounded. 
         [0042]    A bracket  44  extending downward from the lower end of the main frame  39  is integrally formed with the lower end of the main frame  39 , and a front end of a sub frame  45  is secured to the bracket  44  by bolts, the sub frame  45  extends rearward along the vehicle body  3  of the tractor  1 , and the rear end is connected to a member secured to the transmission case  5 B. 
         [0043]    As shown in  FIGS. 1 and 3 , the right and left boom assemblies  12  are bent into a curve at the center in the longitudinal direction so as to form a round shape protruding upward when seen from the side, and have gradually increasing vertical widths from the front and rear ends toward the center in the longitudinal direction. 
         [0044]    The boom assembly  12  mainly includes a front boom sub assembly  47  on the front of the center in the longitudinal direction of the boom assembly  12 , a rear boom sub assembly  48  on the rear of the center in the longitudinal direction of the boom assembly  12 , and a center connection plate  49  connecting the front and rear boom sub assemblies  47  and  48  at the center in the longitudinal direction of the boom assembly  12 . The front and rear boom sub assemblies  47  and  48  are each constituted by a body  51  and a bottom plate  52 , and the bodies  51  of the front and rear boom sub assemblies  47  and  48  have the same shape (symmetric in the front and rear when seen from the side), thereby achieving sharing of members. 
         [0045]    The body  51  of each of the boom sub assemblies  47  and  48  is formed into a π-shape in section opening downward by a pair of right and left side walls  53  and an upper wall  54  connecting upper edges of the right and left side walls  53 . 
         [0046]    In the body  51  of each of the front and rear boom sub assemblies  47  and  48 , a portion of each of the front and rear boom sub assemblies  47  and  48  from the middle in the longitudinal direction to the end at the center in the longitudinal direction of the boom assembly  12  is a round portion  51   a  having an upper surface of arc shape protruding upward when seen from the side, and a portion of each of the boom sub assemblies  47  and  48  from the middle in the longitudinal direction to the end in the longitudinal direction of the boom assembly  12  is a linear portion  51   b  having an upper surface of linear shape when seen from the side. 
         [0047]    The body  51  of each of the front and rear boom sub assemblies  47  and  48  are formed into a π-shape in section by bending (pressing) one boom forming sheet material cut out of a flat sheet material into a predetermined shape to form the right and left side walls  53  and the upper wall  54 , the walls are bent into a π-shape, then the boom sub assemblies  47  and  48  are bent (pressed) into a curve protruding upward at the centers in the longitudinal direction of the boom assembly  12  to form the round portions  51   a , and the linear portions  51   b  are not bent into a curve. For forming the round portions  51   a  into an arc shape, for example, a press die is provided constituted by a female mold having a concave surface and placed above the boom sub assemblies  47  and  48  so that the concave surface faces upper surfaces of the boom sub assemblies  47  and  48 , and a male mold having a convex surface and placed below the boom sub assemblies  47  and  48  so that the convex surface faces lower edges  53   a  of the side walls of the boom sub assemblies  47  and  48 , and the boom sub assemblies  47  and  48  are press molded between the female mold and the male mold to form the round portions  51   a.    
         [0048]    Thus, in the round portion  51   a , the lower edge  53   a  of the side wall  53  is also formed into an arc shape protruding upward, and in the linear portion  51   b , the lower edge  53   a  of the side wall is also formed into a linear shape when seen from the side. The shape seen from the side of the lower edge  53   a  of the side wall  53  is determined in a cutting-out stage, and thus not limited to the arc shape or the linear shape. The bodies  51  of the front and rear boom sub assemblies  47  and  48  thus formed are abutted and joined by welding at ends of the side walls  53  at the center in the longitudinal direction of the boom assembly  12 . 
         [0049]    For the boom assembly  12  having the above described configuration, in fabrication of boom assemblies  12  having different lengths, the round portions  51   a  of the bodies  51  of the boom sub assemblies  47  and  48  are formed by one press die with the radii of curvature of arcs thereof being the same, and the lengths of linear portions  51   b  are made different to form the boom sub assemblies  47  and  48  having different lengths. The lengths of the boom sub assemblies  47  and  48  are determined in the stage of cutting out the flat sheet material. In the front end of the front boom sub assembly  47 , a front pivot portion  56  is provided made of a cylinder, passing through the right and left side walls  53 , and secured to the side walls  53  by welding, and a bucket  15  is connected to the front pivot portion  56  pivotably around a lateral shaft by the pivot  24 . 
         [0050]    In the rear end of the rear boom sub assembly  48 , a rear pivot portion  57  is provided made of a cylinder, passing through the right and left side walls  53 , and secured to the side walls  53  by welding, and the side frames  11  are connected to the rear pivot portion  57  rotatably around a lateral shafts by the pivot  19 . The middles of the right and left side walls  53  of the right and left front boom sub assemblies  47  are connected by the connecting member  20 . The bottom plate  52  is formed of a flat sheet, placed below the upper wall  54  and between the right and left side walls  53 , and provided so as to extend from the ends of the front and rear boom sub assemblies  47  and  48  at the center in the longitudinal direction of the boom assembly  12  to the front and rear pivot portions  56  and  57 . 
         [0051]    In the embodiment, the bottom plate  52  of the front and rear boom sub assemblies  47  and  48  is integrally formed of one sheet, and secured to the bodies  51  by welding when (or after) the bodies  51  of the front and rear boom sub assemblies  47  and  48  are joined together. Bottom plates  52  of the front and rear boom sub assemblies  47  and  48  may be formed separately. A rear end of the bottom plate  52  of the rear boom sub assembly  48  is secured to the rear pivot portion  57  by welding. A front end of the bottom plate  52  of the front boom sub assembly  47  may be also secured to the front pivot portion  56  by welding. On the other hand, on a bottom side of one of the right and left boom assemblies  12 , hydraulic pipes for the boom cylinder  13  and the bucket cylinder  14  are provided along the bottom plate  52  from the rear to the connecting member  20 , the hydraulic pipes are provided below and along the connecting member  20  to the other of the right and left boom assemblies  12 , the middles of the hydraulic pipes or the ends thereof on the side of the other boom assembly  12  are connected to the boom cylinder  13  and the bucket cylinder  14  via hydraulic hoses, and rear ends of the hydraulic pipes are connected to a control valve provided in the main frame  39  or the like via the hydraulic hoses. 
         [0052]    For the bottom plate  52  of the front and rear boom sub assemblies  47  and  48 , the bottom plate  52  at the end in the longitudinal direction of the boom assembly  12  is close to the lower edge  53   a  of the side wall  53  of the body  51 , the bottom plate  52  at the center in the longitudinal direction of the boom assembly  12  is positioned in the middle in a vertical width direction at the center in the longitudinal direction of the boom assembly  12  (a middle in the vertical direction of an edge of the side wall  53  at the center in the longitudinal direction of the boom assembly  12 ). 
         [0053]    Thus, a distance from the bottom plate  52  to the lower edge  53   a  of the side wall  53  is long at the center in the longitudinal direction of the boom assembly  12 , and a housing space for the hydraulic pipes is large at the center in the longitudinal direction of the boom assembly  12 . 
         [0054]    The distance from the bottom plate  52  to the lower edge  53   a  of the side wall  53  is long at the center in the longitudinal direction of the boom assembly  12 , and a width between the right and left side walls  53  is narrow, thus in welding the bottom plate  52  to the side walls  53 , a welding torch is hard to be placed between the side walls  53  at the center in the longitudinal direction of the boom assembly  12 , and welding of the bottom plate  52  to the side walls  53  is difficult at the center in the longitudinal direction of the boom assembly  12 . Thus, notches  58  are formed in upper portions of the bodies  51  of the front and rear boom sub assemblies  47  and  48  at the center in the longitudinal direction of the boom assembly  12 , the bottom plate  52  is welded to the side walls  53  from below from the front and rear ends of the boom assembly  12  to the middles in the longitudinal direction of the boom sub assemblies  47  and  48 , and the bottom plate  52  is welded to the side walls  53  from above via the notches  58  at the center in the longitudinal direction of the boom assembly  12 . 
         [0055]    The notches  58  are formed in the upper walls  54  by cutting a predetermined range in the longitudinal direction of the boom from the ends of the upper walls  54  at the center in the longitudinal direction of the boom assembly  12 , and formed so as to extend from the notch portions in the upper walls to the right and left side walls  53 . 
         [0056]    The center connection plate  49  is formed by bending a flat sheet into a curve protruding upward, and provided across the upper walls  54  of the front and rear boom sub assemblies  47  and  48  so as to close the notches  58  from above. The bracket plates  22  are placed on the right and left of the boom assembly  12  at the center in the longitudinal direction of the boom assembly  12 , provided across the side walls  53  of the front and rear boom sub assemblies  47  and  48 , and placed on outer surfaces of the side walls  53  of the front and rear boom sub assemblies  47  and  48  and secured by welding. Thus, the bracket plates  22  also serve as reinforcing plates. 
         [0057]      FIG. 4  shows three types of round boom assemblies  12  of the embodiment having different lengths, and  FIG. 5  shows boom sub assemblies  47  and  48  of the three types of boom assemblies  12  in  FIG. 4 . The boom sub assemblies  47  and  48  of the three types of boom assemblies  12  having different lengths are formed so that radii of curvature of arcs of upper walls  54  at round portions  51   a  are the same, lengths A in a longitudinal direction of the upper walls  54  are the same, and radii of curvature of lower edges  53   a  of side walls  53  at the round portions  51   a  are the same, and the round portions  51   a  of the boom sub assemblies  47  and  48  of the three types of boom assemblies  12  are formed by the same press die. 
         [0058]    Arc portions of the boom sub assemblies  47  and  48  of plural types of boom assemblies  12  having different length are formed by the same press die, and the front and rear boom sub assemblies  47  and  48  are made symmetric in the front and rear, thereby minimizing press dies for forming the round portions  51   a  of the boom sub assemblies  47  and  48  to reduce costs, and eliminating a press die changing step in production of plural types of boom assemblies  12  having different lengths to increase productivity and reduce labor costs. In the boom sub assemblies  47  and  48  of the three types of boom assemblies  12  having different lengths, lengths B in the longitudinal direction of the upper walls  54  at the linear portions  51   b  are relatively significantly different, and the lengths of the linear portions  51   b  are made different to form the three types of boom assemblies  12  having different lengths. 
         [0059]    Forming portions of the notches  58  at the round portions  51   a  of the three types of boom sub assemblies  47  and  48  in  FIGS. 4 and 5  have slightly different lengths, but if the arc portions have the same radius of curvature with different lengths, the round portions  51   a  can be formed by the same press die. Thus, the lengths A of the arc portions of the upper walls  54  may be slightly different. 
         [0060]    Two types or four or more types of boom assemblies  12  having different lengths may be formed in the same manner. 
         [0061]    As shown in  FIGS. 6 to 8 , the cylinder pivot portion  60  of each of the right and left bracket plates  22  to which the end of the boom cylinder  13  is pivotably connected is provided with a cylindrical boss  61  that supports the pivot  23  for supporting the boom cylinder  13 , and formed with a boss insertion hole  62  through which the boss  61  is inserted. 
         [0062]    One end in an axial direction of the boss  61  (a lateral inner end) is inserted through the boss insertion hole  62  from a lateral outer surface of the bracket plate  22 , and the boss  61  is joined to the cylinder pivot portion  60  by fillet welding between an outer peripheral surface of the boss  61  and the outer surface of the bracket plate  22 . The front end of the boom cylinder  13  is placed between the right and left cylinder pivot portions  60 , and the front end of the boom cylinder  13  is pivotably connected to the right and left cylinder pivot portions  60  by the pivot  23  inserted through the right and left bosses  61  and passing through the front end of the boom cylinder  13 . According to the above described configuration, the inner end of the boss  61  is inserted from the outer surface of the bracket plate  22  through the boss insertion hole  62  formed in the cylinder pivot portion  60  of the bracket plate  22  to join the outer surface of the cylinder pivot portion  60  and the outer peripheral surface of the boss  61  by welding. Thus, an external force acting on the boss  61  can be received by a welding bead  63  around the boss  61  and the bracket plate  22  (the inner surface of the boss insertion hole  62 ), thereby reducing load on the welding bead  63  around the boss  61 . 
         [0063]    An upper portion of an end surface  61   a  at the lateral inner end of the boss  61  laterally overlaps the lower end of the side wall  53  of the rear boom sub assembly  48 , the upper portion of the end surface  61   a  at the lateral inner end of the boss  61  abuts against the lower end of the side wall  53  of the rear boom sub assembly  48 , and the end surface  61   a  at the lateral inner end of the boss  61  and the lower edge  53   a  of the side wall  53  of the rear boom sub assembly  48  are joined by fillet welding so as not to close an inner hole  61   b  of the boss  61 . The lateral inner surface of the bracket plate  22  and the lower edge  53   a  of the side wall  53  of each of the front and rear boom sub assemblies  47  and  48  are joined by fillet welding from the front end to the rear end of the bracket plate  22 . A welding bead  64  that joins the lateral inner surface of the bracket plate  22  and the lower edge  53   a  of the side wall  53  of each of the front and rear boom sub assemblies  47  and  48 , and a welding bead  65  that joins the end surface  61   a  at the lateral inner end of the boss  61  and the lower edge  53   a  of the side wall  53  of the rear boom sub assembly  48  are continuous. 
         [0064]    A portion of the lower edge  53   a  of the side wall  53  of the rear boom sub assembly  48 , welded to the end surface  61   a  at the lateral inner end of the boss  61  is formed with an arc-shaped notch  66  along the inner hole  61   b  of the boss  61 , and the side wall edge  53   a  of the boom assembly  12  of the portion against which the end surface  61   a  at the lateral inner end of the boss  61  abuts is formed along the inner hole  61   b  of the boss  61 . The inner end of the boss  61  is configured to be inserted through the boss insertion hole  62  from the outer surface of the bracket plate  22 , and thus the end surface  61   a  at the inner end of the boss  61  and the side wall edge  53   a  of the boom assembly  12  can be joined by welding, and the end surface  61   a  at the inner end of the boss  61  and the side wall edge  53   a  of the boom assembly  12  are joined by welding to significantly reduce stress around the boss  61 . 
         [0065]    As described above, the boss insertion hole  62  is formed through the cylinder pivot portion  60  of the bracket plate  22 , one end in the axial direction of the cylindrical boss  61  is inserted through the boss insertion hole  62  from the outer surface of the bracket plate  22 , the outer surface of the cylinder pivot portion  60  and the outer peripheral surface of the boss  61  are joined by welding, and the end surface  61   a  of one end in the axial direction of the boss and the side wall edge  53   a  of the boom assembly  12  are joined by welding so as not to close the inner hole of the boss  61 . 
         [0066]    The end surface  61   a  of one end in the axial direction of the boss  61  of the portion welded to the side wall edge  53   a  of the boom assembly  12  abuts against the outer surface of the side wall  53  of the boom assembly  12 , and the side wall edge  53   a  of the boom assembly  12  of the portion against which the end surface  61   a  of one end in the axial direction of the boss  61  abuts is formed along the inner hole  61   b  of the boss  61 . 
         [0067]    According to the present invention, one end in the axial direction of the boss  61  is inserted through the boss insertion hole  62  formed in the cylinder pivot portion  60  of the bracket plate  22  from the outer surface of the bracket plate  22  to join the outer surface of the cylinder pivot portion  60  and the outer peripheral surface of the boss  61  by welding, and thus an external force acting on the boss can be received by the welding bead around the boss and the bracket plate. This reduces load on the welding bead around the boss  61 . One end in the axial direction of the boss  61  is inserted through the boss insertion hole  62  from the outer surface of the bracket plate  22 , and thus the end surface  61   a  of one end in the axial direction of the boss  61  and the side wall edge  53   a  of the boom assembly  12  can be joined by welding. Then, the end surface  61   a  of one end in the axial direction of the boss  61  and the side wall edge  53   a  of the boom assembly  12  are joined by welding to significantly reduce stress around the boss  61 . This ensures strength against the external force acting on the boss  61 , eliminates the need for increasing an outer diameter of the boss  61  or providing a reinforcing plate for ensuring the strength of the boss  61 , reduces costs, and improves design.