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BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a construction machine such as a hydraulic shovel, and more particularly to a frame constitution of a lower traveling body in a construction machine. 
   2. Description of the Related Art 
   As shown in the side view of  FIG. 8 , a conventional hydraulic shovel  11  comprises a lower traveling body  12  to enable movement, and an upper slewing body  13  which is mounted slewably onto the lower traveling body  12 . 
   In the lower traveling body  12 , a drive shaft  15  and a pivot  16  are attached respectively to the two end portions of a truck frame  14 , and a crawler belt  17   i  is wrapped around the drive shaft  15  and pivot  16 . 
   The hydraulic shovel  11  is driven by a power source engine, the power of which is converted into oil pressure by a hydraulic pump to drive a traction motor. This causes the drive shaft  15  to rotate, which drives the crawler belt  17  to rotate, and as a result, traveling is performed. 
   According to Japanese Patent Application 2002-332379, as shown in  FIG. 9 , in a frame  12 F of the lower traveling body, sheet metal covers  21 ,  22  are welded to the upper face portion and lower face portion of a box-shaped rotary case  20  having an attachment ring  20   r  to which the upper slewing body  13  is attached slewably, and an inverse C-shaped case  23  formed from sheet metal is fixed to the two end portions thereof by welding. An upper face plate  24  is formed on the sheet metal inverse C-shaped case  23 . 
   The truck frame of an axle-equipped vehicle is disclosed in Japanese Unexamined Patent Application Publication H10-236346. 
   Since the hydraulic shovel  11  described above performs operations outside, the crawler belt  17  is rotated to travel over various ground conditions, and hence the crawler belt  17  churns up earth, mud, and so on. 
   As shown in  FIG. 8 , the sheet metal cover  21  and upper face plate  24  disposed in the vicinity of the crawler belt  17  extend in a horizontal direction, and hence the earth, mud, and so on that is churned up by the crawler belt  17  accumulates on the upper face plates  24 ,  24 . 
   Consequently, a worker must remove the accumulated earth, mud, and so on, and if the earth, mud, and so on are left to accumulate, they cause running resistance when the hydraulic shovel  11  is operated. 
   Measures such as opening a hole in the cover  21  may be considered so that the earth falls off the cover  21 . 
   However, the cover  21  and the like invariably have a level part, and it is therefore difficult to solve the problem of earth accumulation. 
   To improve earth removal, a constitution has been considered in which the frame  12 F of the lower traveling body is divided into the central box-shaped rotary case portion and the pair of truck frame portions on the opposite sides, and the truck frames are connected to the rotary case portion via a pair of leg-shaped connecting members. 
   According to this constitution, the rotary case portion and the truck frame portions are connected via a pair of connecting members, and hence an open space is formed between the pair of connecting members through which the earth that is churned up by the crawler belt  17  passes and falls to the ground. Thus earth removal can be precipitated favorably. 
   Conventionally, however, the truck frames  14 , to which excessive loads may be applied, are connected to the rotary case  20  via the plate-form sheet metal covers  21 ,  22  which are single plates as shown in  FIG. 9 . Hence stress generated by a load acting on the truck frames  14  can be transmitted to the single plate covers  21 ,  22  extending in a planar direction, thus eliminating problems relating to strength. 
   In the case of the aforementioned constitution, on the other hand, the truck frame portions on which loads act and the central rotary case portion are connected via the connecting members which are constituted separately, and hence, depending on the connection configuration between the connecting members and the central rotary case portion, the vehicle body may break. 
   Therefore, the connection configuration between the connecting members and the rotary case portion needs to be considered to ensure that stress generated by a load transmitted from the truck frame portions to the connecting members is transferred appropriately from the connecting members to the central rotary case portion. 
   SUMMARY OF THE INVENTION 
   The present invention has been designed in consideration of this situation, and it is an object thereof to provide a construction machine in which a load acting on a vehicle body during an operation is transferred appropriately, thereby ensuring the strength of the vehicle body and improving reliability, and in which welding reliability can also be improved. 
   In a construction machine according to a first invention, a frame of a crawler-type traveling body comprises a box-shaped center frame, a pair of truck frames disposed on an outside of opposite side portions of said center frame, and a hollow connecting member for connecting said center frame to said truck frames, wherein at least one of respective height differences between an upper face or a lower face of said center frame at a joining portion between said center frame and said connecting member and an upper face or a lower face of a center joining portion of said connecting member is set respectively at a dimension that is approximately equal to or less than a thickness of an upper face plate or a lower face plate of said center frame. 
   In a construction machine according to a second invention, pertaining to the first invention, at the joining portion between said center frame and said connecting member, the upper face of said center joining portion of said connecting member is disposed slightly upward or slightly downward of the upper face of said center frame, and the lower face of said center frame is disposed at the same height as the lower face of said center joining portion of said connecting member. 
   In a construction machine according to a third invention, pertaining to the first invention, at the joining portion between said center frame and said connecting member, the upper face of said center frame is disposed at the same height as the upper face of said center joining portion of said connecting member, and the lower face of said center joining portion of said connecting member is disposed slightly upward or slightly downward of the lower face of said center frame. 
   In a construction machine according to a fourth invention, pertaining to the first invention, at the joining portion between said center frame and said connecting member, the upper face of said center joining portion of said connecting member is disposed slightly upward or slightly downward of the upper face of said center frame, and the lower face of said center joining portion of said connecting member is disposed slightly upward or slightly downward of the lower face of said center frame. 
   In a construction machine according to a fifth invention, pertaining to the first invention, at the joining portion between said center frame and said connecting member, the upper face of said center frame is disposed at the same height as the upper face of said center joining portion of said connecting member, and the lower face of said center frame is disposed at the same height as the lower face of said center joining portion of said connecting member. 
   According to the constitutions of the first through fifth inventions, the stress of a load that is transmitted via the upper face plate of the connecting member is transferred smoothly and effectively to the upper face plate of the center frame, which has good rigidity, or the stress of a load that is transmitted via the lower face plate of the connecting member is transferred smoothly and effectively to the lower face plate of the center frame. As a result, the load-withstanding strength of the vehicle body is ensured, leading to an improvement in mechanical reliability. 
   When the upper face plate and lower face plate of the center frame are disposed respectively in the vicinity of the upper face plate and lower face plate of the connecting member, stepped portions are formed. Hence the operating position during welding is easy to aim for, and as a result, teaching can be performed accurately, thus facilitating the welding operation and improving the welding reliability. 
   A construction machine according to a sixth invention comprises an upper slewing body and a lower traveling body, a frame of said lower traveling body comprising a hollow center frame on which said upper slewing body is slewably mounted, having a side plate surrounding sides thereof, a pair of truck frames disposed on an outside of opposite side portions of said center frame, and a hollow connecting member for connecting said center frame to said truck frames, a center joining portion of said connecting member being welded to the side plate of said center frame, wherein an upper face of the center joining portion of said connecting member is disposed in the vicinity of and downward of an upper face of said center frame, and a lower face of the center joining portion of said connecting member is disposed in the vicinity of and upward of a lower face of said center frame, whereupon the center joining portion of said connecting member is welded to the side plate of said center frame. 
   According to this constitution, the upper face of the center joining portion on the connecting member is disposed in the vicinity of and downward of the upper face of the center frame, and the lower face of the center joining portion on the connecting member is disposed in the vicinity of and upward of the lower face of the center frame, and as a result, a step is formed between the upper face of the center joining portion on the connecting member and the upper face of the center frame, and a step is formed between the lower face of the center joining portion on the connecting member and the lower face of the center frame. Hence the operating position during welding is easy to aim for, and as a result, teaching can be performed accurately, thus facilitating the welding operation and improving the welding reliability. 
   Further, the upper face plate of the center frame and the upper face plate of the connecting member are brought into proximity and welded, and the lower face plate of the center frame and the lower face plate of the connecting member are brought into proximity and welded, and hence the stress of a load that is transmitted via the upper face plate of the connecting member is transferred smoothly and effectively to the upper face plate of the center frame, which has good rigidity, or the stress of a load that is transmitted via the lower face plate of the connecting member is transferred smoothly and effectively to the lower face plate of the center frame. As a result, the load-withstanding strength of the vehicle body is ensured, leading to an improvement in mechanical reliability. 
   In a construction machine according to a seventh invention, pertaining to the sixth invention, the upper face of the center joining portion of said connecting member is disposed in the vicinity of and downward of the upper face of said center frame so as to form a step having an identical dimension to a height of a weld bead, and the lower face of the center joining portion of said connecting member is disposed in the vicinity of and upward of the lower face of said center frame so as to form a step having an identical dimension to a height of a weld bead, whereupon the center joining portion of said connecting member is welded to the side plate of said center frame. 
   According to this constitution, a step having an equal dimension to the height of a weld bead is provided upward of the center joining portion of the connecting member, and a step having an equal dimension to the height of a weld bead is provided downward of the center joining portion. Hence the operating position during robotic welding is easy to aim for, and as a result, teaching can be performed accurately, thus facilitating the welding operation and improving the welding reliability. 
   Further, since the step formed by the upper face of the center joining portion of the connecting member and the upper face of the center frame has an equal dimension to the height of the weld bead, and the step formed by the lower face of the center joining portion and the lower face of the center frame has an equal dimension to the height of the weld bead, the upper face plate of the connecting member and the upper face plate of the center frame can be welded in extremely close proximity, and the lower face plate of the connecting member and the lower face plate of the center frame can be welded in extremely close proximity. 
   Hence the stress of a load that is transmitted via the upper face plate of the connecting member is transferred smoothly and effectively to the upper face plate of the center frame, which has good rigidity, or the stress of a load that is transmitted via the lower face plate of the connecting member is transferred smoothly and effectively to the lower face plate of the center frame. As a result, the load-withstanding strength of the vehicle body is ensured, leading to an improvement in mechanical reliability. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side view showing a hydraulic shovel of an embodiment according to the present invention; 
       FIG. 2  is a perspective view of the frame of a lower traveling body in the hydraulic shovel of an embodiment according to the present invention; 
       FIG. 3  is a top view of the frame of the lower traveling body in the hydraulic shovel of an embodiment according to the present invention; 
       FIGS. 4A ,  4 B, and  4 C are a perspective view showing a center frame of the hydraulic shovel of an embodiment according to the present invention, a perspective view showing a connecting member, and a perspective view showing a connecting member; 
       FIGS. 5A and 5B  are a view seen from the direction of an arrow A and a view seen from the direction of an arrow B of the lower traveling body frame shown in  FIG. 3 ; 
       FIGS. 6A ,  6 B,  6 C, and  6 D are conceptual sectional views (sectional views seen from the A direction in  FIG. 3 ) showing first through fourth modified examples of a method for joining the center frame to a connecting member  8   r  in an embodiment according to the present invention; 
       FIG. 7  is a conceptual sectional view (a sectional view seen from the A direction in  FIG. 3 ) showing a fifth modified example of the method for joining the center frame to the connecting member in an embodiment according to the present invention; 
       FIG. 8  is a side view showing a conventional hydraulic shovel; and 
       FIG. 9  is a perspective view of a lower traveling body frame of the conventional hydraulic shovel. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The present invention will be described in detail below on the basis of the drawings illustrating embodiments thereof. 
   As shown in  FIG. 1 , a hydraulic shovel  1  according to this embodiment comprises a lower traveling body  2  to enable motion, and an upper slewing body  3  which is attached slewably to the top of the lower traveling body  2 , and which is mounted by an operator to perform an operation. 
   An operating seat  3   a  on which the operator sits to perform an operation is provided on the upper slewing body  3 , and a working machine  10  comprising a boom  10   a,  an arm  10   b,  and an excavating bucket  10   c  attached to the tip end of the arm  10   b,  which are hydraulically driven, is axially supported in a vertical direction so as to swing freely. 
   In the lower traveling body  2 , a drive shaft  5  and a pivot  6  are attached respectively to the two end portions of truck frames  4   l,    4   r,  and a crawler belt r is wrapped around the drive shaft  5  and pivot  6 . 
   The hydraulic shovel  1  is driven by a power source engine, the power of which is converted into oil pressure by a hydraulic pump to drive a traction motor. The resultant power is transmitted to the drive shaft  5 , causing the drive shaft  5  to rotate, which drives the crawler belt r to rotate, and as a result, traveling is performed. 
   As shown in the perspective view of  FIG. 2  and the top view of  FIG. 3 , a center frame  7  comprising an attachment ring  7   r,  on which the upper slewing body  3  is mounted, is disposed in the central portion of a frame  2 F of the lower traveling body, and the truck frames  4   r,    4   l  are disposed on the opposite side portions thereof. The truck frame  4   r  is connected to the center frame  7  by two leg-shaped connecting members  8   r,    9   r,  and the truck frame  4   l  is connected to the center frame  7  by two leg-shaped connecting members  8   l,    9   l.    
   These members are joined to each other by welding. 
   Note that as shown in  FIG. 3 , the truck frame  4   r  and the pair of connecting members  8   r,    9   r,  and the truck frame  4   l  and the pair of connecting members  8   l,    9   l  are constituted in plane symmetry about a central face (the center line of the vertical direction of the center frame  7  in  FIG. 3 ) of the center frame  7  in the direction of width (the vertical direction in  FIG. 3 ). 
   More specifically, in  FIG. 3 , the connecting member  8   r  and connecting member  8   l  take a symmetrical form about the central face of the center frame  7  in the direction of width, and the connecting member  9   r  and connecting member  9   l  take a symmetrical form about the central face of the center frame  7  in the direction of width. 
   The center frame  7  is manufactured as a cast using cast steel, and as shown in  FIG. 4A , is constituted in a hollow box form. 
   The attachment ring  7   r,  on which the upper slewing body  3  is mounted slewably, is formed on an upper face  7   u  of the center frame  7  with an opening in the central portion thereof, and a side plate  7   b  to which the connecting members  8   r,    8   l,    9   r,    9   l  are welded so as to jut out radially from the attachment ring  7   r  is formed on the side face portion of the center frame  7 . 
   Note that the side plate  7   b  is formed in a plate form having a vertical plane which is rectilinear when seen from above (see  FIG. 3 ). 
   The connecting member  8   r  is manufactured as a cast using cast steel, and as shown in  FIG. 4B  (which is a perspective view of the connecting member  8   r  alone seen from the side of the center frame  7  shown in  FIG. 3 ), is constituted in a hollow form comprising a core portion  8   r   1  formed with a ridge in the longitudinal direction of the upper face which has a substantially pentagonal cross section with an angled portion at the upper portion thereof, a center joining portion  8   r   2  having a quadrilateral cross section and formed with an opening  8   r   0 , and a truck joining portion  8   r   3  formed with an opening. 
   In other words, the connecting member  8   r  is constituted as a pipe-form hollow member extending in a longitudinal direction having a closed transverse section. 
   The center joining portion  8   r   2  of the connecting member  8   r  comprises a joining face  8   rb  which is joined to the side plate  7   b  of the center frame  7 , and as shown in  FIG. 3 , this joining face  8   rb  is formed in planar form to form a straight line when seen from above, and is thus non-perpendicular to the axis (the straight line P in  FIG. 3 ) which runs along the direction in which the connecting member  8   r  extends. 
   Further, as shown in  FIG. 4B , a height dimension hr 1  between the upper face  8   ru  and a lower face  8   rs  of the center joining portion  8   r   2  having the joining face  8   rb  is set to be shorter than a height dimension hc 1  (see  FIG. 4 ) between an upper face  7   u  and a lower face  7   s  of the center frame  7  by identical dimensions b 1 , b 2  to the height dimensions of the weld beads (see  FIG. 5A , which is a view seen from the direction of the arrow A in  FIG. 3 ). 
   In this manner, a welding configuration is employed in which slight steps b 1 , b 2  are formed respectively between the upper and lower faces  7   u,    7   s  of the center frame  7  and the upper and lower faces  8   ru,    8   rs  of the center joining portion  8   r   2  on the connecting member  8   r  such that the upper face plate and lower face plate of the center frame  7  and the upper face plate and lower face plate of the connecting member  8   r  are connected in alignment with each other, and hence stress generated by a load transmitted via the upper face plate and lower face plate of the connecting member  8   r  is transferred smoothly to the upper face plate and lower face plate of the center frame  7 . 
   As shown in  FIGS. 3 and 5A , a welding location y 1  between the joining face  8   rb  of the connecting member  8   r  and the side plate  7   b  of the center frame  7  is formed to be capable of taking a rectilinear form. 
   A truck joining portion  8   r   3  of the connecting member  8   r  is formed so as to match the joining face  4   b  (to be described below) of the truck frame  4  to enable welding to the joining face  4   b  of the truck frame  4 . 
   The connecting member  8   l  shown in  FIGS. 2 ,  3  is manufactured as a cast using cast steel, and as described above, is constituted in plane symmetry with the connecting member  8   r  about the central face of the center frame  7  in the direction of width. Similarly to the connecting member  8   r,  the connecting member  8   l  is formed as a hollow pipe-form member extending in a longitudinal direction having a closed transverse cross section. 
   As shown in  FIG. 3 , the connecting member  8   l  is welded to the center frame  7  in a similar configuration to the connecting member  8   r,  and hence detailed description thereof has been omitted. 
   The connecting member  9   r  is manufactured as a cast using cast steel, and as shown in  FIG. 4C  (which is a perspective view of the connecting member  9   r  alone seen from the center frame side in  FIG. 3 ), is constituted in a hollow form comprising a tapered core portion  9   r   1  formed with a ridge in the longitudinal direction of the upper face which has a substantially pentagonal cross section with an angled portion at the upper portion thereof, a center joining portion  9   r   2  having a quadrilateral cross section and formed with an opening  9   r   0 , and a truck joining portion  9   r   3  formed with an opening. 
   In other words, the connecting member  9   r  is constituted as a pipe-form, tapered hollow member extending in a longitudinal direction having a closed transverse section. 
   The center joining portion  9   r   2  of the connecting member  9   r  comprises a joining face  9   rb  which is joined to the side plate  7   b  of the center frame  7 , and as shown in  FIG. 3 , this joining face  9   rb  is formed in planar form to form a straight line when seen from above, and is thus non-perpendicular to the axis (the straight line Q in  FIG. 3 ) which runs along the direction in which the connecting member  9   r  extends. 
   Further, as shown in  FIG. 4C , a height dimension hr 2  between an upper face  9   ru  and a lower face  9   rs  of the center joining portion  9   r   2  having the joining face  9   rb  is set to be shorter than a height dimension hc 1  (see  FIG. 4A ) between the upper face  7   u  and lower face  7   s  of the center frame  7  by identical dimensions b 3 , b 4  to the height dimensions of the weld beads (see  FIG. 5B , which is a view seen from the direction of the arrow B in  FIG. 3 ). 
   In this manner, a welding configuration is employed in which slight steps b 3 , b 4  are formed respectively between the upper and lower faces  7   u,    7   s  of the center frame  7  and the upper and lower faces  9   ru,    9   rs  of the center joining portion  9   r   2  on the connecting member  9   r  such that the upper face plate and lower face plate of the center frame  7  and the upper face plate and lower face plate of the connecting member  9   r  are connected in alignment with each other, and hence stress generated by a load transmitted via the upper face plate and lower face plate of the connecting member  9   r  is transferred smoothly to the upper face plate and lower face plate of the center frame  7 . 
   As shown in  FIGS. 3 and 5B , a welding location y 2  between the joining face  9   rb  of the connecting member  9   r  and the side plate  7   b  of the enter frame  7  is formed to be capable of taking a rectilinear form. 
   A truck joining portion  9   r   3  of the connecting member  9   r  is formed so as to match the joining face  4   b  (to be described below) of the truck frame  4  to enable welding to the joining face  4   b  of the truck frame  4 . 
   The connecting member  9   l  shown in  FIGS. 2 ,  3  is manufactured as a cast using cast steel, and as described above, is constituted in plane symmetry with the connecting member  9   r  about the central face of the center frame  7  in the direction of width. Similarly to the connecting member  9   r,  the connecting member  9   l  is formed as a tapered, hollow, pipe-form member extending in a longitudinal direction having a closed transverse cross section. 
   As shown in  FIG. 3 , the connecting member  9   l  is welded to the center frame  7  in a similar configuration to the connecting member  9   r,  and hence detailed description thereof has been omitted. 
   As described above, the connecting members  8   r,    8   l,    9   r,    9   l  are constituted in a form having a ridge with an angled portion on the upper face thereof, and hence earth that is churned up by the crawler belt  9  during traveling slides down the inclined surface of the upper face of the connecting members  8  and falls to the ground, thus being prevented from accumulating on top of the connecting members  8 . 
   The truck frames  4   r,    4   l  extend in a front-rear direction (the left/right direction in  FIGS. 1 ,  2 , and  3 ) and have a quadrilateral cross section. As shown in  FIG. 2 , the upper face portion thereof is constituted with an inclined face plate  4   a  forming a singly tapered face which inclines downward toward the outside of the vehicle body. An earth removing hole  4   ah  is pierced through the inclined face plate  4   a  of the truck frames  4  near the center thereof. 
   Next, a method of joining the center frame  7  to the connecting members  8   r,    8   l,    9   r,    9   l  will be described. 
   As shown in  FIGS. 3 and 5A , when the center frame  7  is joined to the connecting member  8   r,  the center joining portion  8   r   2  of the connecting member  8   r  is disposed on the side plate  7   b  of the center frame  7  such that the upper face  8   ru  of the center joining portion  8   r   2  is positioned downward of the upper face  7   u  of the center frame  7  by the dimension b 1 , thus forming a step, and such that the lower face  8   rs  of the center joining portion  8   r   2  is positioned upward of the lower face  7   s  of the center frame  7  by the dimension b 2 , thus forming a step, whereupon robotic welding is performed at the welding location y 1  between the side plate  7   b  of the center frame  7  and the center joining portion  8   r   2  of the connecting member  8   r.    
   In so doing, a step having the dimension b 1  is provided above the center joining portion  8   r   2  of the connecting member  8   r,  or in other words the welding location y 1 , and a step having the dimension b 2  is provided below the center joining portion  8   r   2 . Hence the operating position during robotic welding is easy to aim for, and as a result, teaching can be performed accurately. 
   As shown in  FIGS. 2 ,  3 , welding of the center frame  7  to the connecting member  8   l  is performed similarly to the welding described above between the center frame  7  and the connecting member  8   r.    
   As shown in  FIGS. 3 and 5B , when the center frame  7  is joined to the connecting member  9   r,  the center joining portion  9   r   2  of the connecting member  9   r  is disposed on the side plate  7   b  of the center frame  7  such that the upper face  9   ru  of the center joining portion  9   r   2  is positioned downward of the upper face  7   u  of the center frame  7  by the dimension b 3 , thus forming a step, and such that the lower face  9   rs  of the center joining portion  9   r   2  is positioned upward of the lower face  7   s  of the center frame  7  by the dimension b 4 , thus forming a step, whereupon welding is performed at the welding location y 2  between the side plate  7   b  of the center frame  7  and the center joining portion  9   r   2  of the connecting member  9   r.    
   In so doing, a step having the dimension b 3  is provided above the center joining portion  9   r   2  of the connecting member  9   r,  or in other words the welding location y 2 , and a step having the dimension b 4  is provided below the center joining portion  9   r   2 . Hence the operating position during robotic welding is easy to aim for, and as a result, teaching can be performed accurately. 
   As shown in  FIGS. 2 ,  3 , welding of the center frame  7  to the connecting member  9   l  is performed similarly to the welding described above between the center frame  7  and the connecting member  9   r  by means of robotic welding at the welding location y 2  around the joining face  9   lb  of the connecting member  9   l.    
   Note that in this embodiment, an example was described in which the center frame  7  and connecting members  8   r,    8   l,    9   r,    9   l  are manufactured as casts, but the center frame  7  and connecting members  8   r,    8   l,    9   r,    9   l  are not limited to casts, and may be manufactured from sheet metal. 
   Also in this embodiment, the step dimensions b 1 , b 2 , b 3 , b 4  are set to be equal to the dimensions of the weld beads, but may be set to different dimensions. 
   According to the constitution described above, the hydraulic shovel  1  travels over various ground surfaces, causing various loads to act on the truck frames  4   r,    4   l  via the crawler belt r, and hence stress generated by these loads is transmitted to the connecting members  8   r,    8   l,    9   r,    9   l  from the truck frames  4   r,    4   l.  However, the upper and lower faces of the center joining portions  8   r   2 ,  8   l   2 ,  9   r   2 ,  9   l   2  of the connecting members  8   r,    8   l,    9   r,    9   l  are connected to the upper and lower faces  7   u,    7   s  of the center frame  7  with slight steps (b 1 , b 2 , b 3 , b 4 ) therebetween, and hence the load stress that is transmitted via the upper face plate and lower face plate of the connecting members  8   r,    8   l,    9   r,    9   l  is transferred smoothly and effectively to the upper face plate and lower face plate of the center frame  7 , which extend in a horizontal direction and have good rigidity. As a result, the load-withstanding strength of the vehicle body is ensured, leading to an improvement in reliability. 
   Since steps are provided between the upper and lower faces  7   u,    7   s  of the center frame  7  and the upper and lower faces of the center joining portions  8   r   2 ,  8   l   2 ,  9   r   2 ,  9   l   2  on the connecting members  8   r,    8   l,    9   r,    9   l,  the operating positions during robotic welding are easy to aim for, enabling teaching to be performed easily and accurately, and thus leading to an improvement in welding reliability. 
   Next, first through fourth modified examples of the method for joining the center frame  7  to the connecting members  8   r,    8   l,    9   r,    9   l  according to the above embodiment will be described. 
   Note that since the methods of joining the center frame  7  and the connecting members  8   l,    9   r,    9   l  are identical to the method of joining the center frame  7  and the connecting member  8   r,  only the method of joining the center frame  7  and the connecting member  8   r  will be described, omitting description of the other methods. 
     FIGS. 6A ,  6 B,  6 C, and  6 D are conceptual sectional views (sectional views seen from the A direction of  FIG. 3 ) showing various joining states between a center frame  7   i  ( 71 ,  72 ,  73 ,  74 ) and the connecting member  8   r  ( 8   ra,    8   rb,    8   rc,    8   rd ). 
   As shown in  FIG. 6A , the first modified example is a case in which an upper face  8   rau  of a center joining portion  8   ra   2  on the connecting member  8   ra  is disposed upward of an upper face  71   u  of the center frame  71  by a dimension h 1  (&lt;≈n 1 ) that is substantially identical to or less than the thickness n 1  of the upper face plate of the center frame  71 , and a lower face  8   ras  of the center joining portion  8   ra   2  on the connecting member  8   ra  is disposed downward of a lower face  71   s  of the center frame  71  by a dimension h 2  (&lt;≈n 2 ) that is substantially identical to or less than the thickness n 2  of the lower face plate of the center frame  71 , whereupon the center frame  71  and connecting member  8   ra  are joined by welding via a weld bead yb. 
   Note that the upper face  8   rau  of the center joining portion  8   ra   2  on the connecting member  8   ra  may be disposed downward of the upper face  71   u  of the center frame  71  by the dimension h 1  (&lt;≈n 1 ) that is substantially identical to or less than the thickness n 1  of the upper face plate of the center frame  71 , and the lower face  8   ras  of the center joining portion  8   ra   2  on the connecting member  8   ra  may be disposed upward of the lower face  71   s  of the center frame  71  by a dimension h 2  (&lt;≈n 2 ) that is substantially identical to or less than the thickness n 2  of the lower face plate of the center frame  71 . 
   As shown in  FIG. 6B , the second modified example is a case in which an upper face  8   rbu  of a center joining portion  8   rb   2  on the connecting member  8   rb  is disposed at the same height as an upper face  72   u  of the center frame  72 , and a lower face  8   rbs  of the center joining portion  8   rb   2  on the connecting member  8   rb  is disposed at the same height as a lower face  72   s  of the center frame  72 , whereupon the center frame  72  and connecting member  8   rb  are joined by welding via the weld bead yb. 
   As shown in  FIG. 6C , the third modified example is a case in which an upper face  8   rcu  of a center joining portion  8   rc   2  on the connecting member  8   rc  is disposed upward of an upper face  73   u  of the center frame  73  by a dimension h 3  (&lt;≈n 3 ) that is substantially identical to or less than the thickness n 3  of the upper face plate of the center frame  73 , and a lower face  8   rcs  of the center joining portion  8   rc   2  on the connecting member  8   rc  is disposed at the same height as a lower face  73   s  of the center frame  73 , whereupon the center frame  73  and connecting member  8   rc  are joined by welding via the weld bead yb. 
   Note that as a variation of the third modified example, the upper face  8   rcu  of the center joining portion  8   rc   2  on the connecting member  8   rc  may be disposed and joined downward of the upper face  73   u  of the center frame  73  by the dimension h 3  (&lt;≈n 3 ) that is substantially identical to or less than the thickness n 3  of the upper face plate of the center frame  73 . 
   As shown in  FIG. 6D , the fourth modified example is a case in which an upper face  8   rdu  of a center joining portion  8   rd   2  on the connecting member  8   rd  is disposed at the same height as an upper face  74   u  of the center frame  74 , and a lower face  8   rds  of the center joining portion  8   rd   2  on the connecting member  8   rd  is disposed downward of a lower face  74   s  of the center frame  74  by a dimension h 4  (&lt;≈n 4 ) that is substantially identical to or less than the thickness n4 of the lower face plate of the center frame  74 , whereupon the center frame  74  and connecting member  8   rd  are joined by welding via the weld bead yb. 
   Note that as a variation of the fourth modified example, the lower face  8   rds  of the center joining portion  8   rd   2  on the connecting member  8   rd  may be disposed and joined upward of the lower face  74   s  of the center frame  74  by the dimension h 4  (&lt;≈n 4 ) that is substantially identical to or less than the thickness n 4  of the lower face plate of the center frame  74 . 
   According to the constitutions of the first through fourth modified examples, the upper face of the center joining portion on the connecting member is disposed near to the upper face of the center frame by a dimension that is approximately equal to or less than the thickness of the upper face plate of the center frame, or at the same height as the upper face of the center frame, and the lower face of the center joining portion on the connecting member is disposed near to the lower face of the center frame by a dimension that is approximately equal to or less than the thickness of the lower face plate of the center frame, or at the same height as the lower face of the center frame, whereupon the connecting member and center frame are joined. In so doing, stress that is transmitted to the center frame from the connecting members as the hydraulic shovel  1  travels is transferred smoothly to the upper face plate of the center frame from the upper face plate of the connecting member and to the lower face plate of the center frame from the lower face plate of the connecting member. 
   Note that when steps exist respectively between the upper face or the lower face of the center frame and the upper face or the lower face of the center joining portion of the connecting member, the weld bead yb connects the steps smoothly, and hence the aforementioned stress is transferred smoothly from the upper face plate or the lower face plate of the center joining portion on the connecting member via the weld bead yb to the upper face plate or the lower face plate of the center frame. 
   As a result, the load-withstanding strength of the vehicle body to loads transmitted from the connecting members is improved. 
   Further, when steps exist respectively between the upper face or the lower face of the center frame and the upper face or the lower face of the center joining portion of the connecting member, the operating position during robotic welding is easy to aim for, and as a result, teaching can be performed easily and accurately, leading to an improvement in welding reliability. 
   In the constitutions of the first through fourth modified examples, the aforementioned fixed effects are obtained when either of the relationship between the lower face of the center frame and the lower face of the center joining portion on the connecting member and the relationship between the upper face of the center frame and the upper face of the center joining portion on the connecting member is constituted as described in the first through fourth modified examples. 
   For example, as shown in  FIG. 7 , a lower face  8   res  of a center joining portion  8   re   2  on a connecting member  8   re  may be disposed at the same height as a lower face  75   s  of a center frame  75 , and an upper face  8   reu  of the center joining portion  8   re   2  on the connecting member  8   re  may be disposed downward of an upper face  75   u  of the center frame  75  by a dimension h 5  (&gt;≈n 5 ) that is substantially identical to or greater than the thickness n 5  of the upper face plate of the center frame  75 . 
   For example, the thickness n 5  of the upper face plate of the center frame  75  may be set between 10 and 14 mm, and the step h 5  between the upper face  75   u  of the center frame  75  and the upper face  8   reu  of the center joining portion  8   re   2  on the connecting member  8   re  may be set at 15 mm. 
   In this case, stress transmitted to the center frame  75  from the connecting member  8   re  is transferred smoothly from the lower face plate of the connecting member  8   re  to the lower face plate of the center frame  75 , and moreover, irregularities in the vertical width dimension of the center frame  75  and the vertical width dimension of the connecting member  8   re  occurring during manufacture can be absorbed by the step h 5  between the upper face  75   u  of the center frame  75  and the upper face  8   reu  of the center joining portion  8   re   2  on the connecting member  8   re.    
   As opposed to the case shown in  FIG. 7 , the upper face  8   reu  of the center joining portion  8   re   2  on the connecting member  8   re  may be disposed at the same height as the upper face  75   u  of the center frame  75 , and the lower face  8   res  of the center joining portion  8   re   2  on the connecting member  8   re  may be disposed upward of the lower face  75   s  of the center frame  75  by a dimension that is substantially identical to or greater than the thickness of the lower face plate of the center frame  75 . 
   In this case, stress transmitted to the center frame  75  from the connecting member  8   re  is transferred smoothly from the upper face plate of the connecting member  8   re  to the upper face plate of the center frame  75 , and moreover, irregularities in the vertical width dimension of the center frame  75  and the vertical width dimension of the connecting member  8   re  occurring during manufacture can be absorbed by the step between the lower face  75   s  of the center frame  75  and the lower face  8   res  of the center joining portion  8   re   2  on the connecting member  8   re.    
   Note that in the embodiment and the first through fourth modified examples described above, an example was provided in which the center frame and connecting members are joined by welding, but a method other than welding, for example using bolts or the like, may be used to join the center frame and connecting members. 
   Note that in the embodiment described above, an example was provided in which a hydraulic shovel is used as the construction machine, but the construction machine according to the present invention may of course be applied effectively to a construction machine other than a hydraulic shovel having a similar constitution. 
   For example, the present invention may be applied effectively to a machine other than a hydraulic shovel which has a base carrier constitution, such as a crawler dump, a bulldozer, or an agricultural machine.

Summary:
A construction machine where a load acting on a vehicle body during an operation is transferred to ensure the strength of the vehicle body, thereby improving reliability including welding reliability. In the construction machine, at least one of the height differences between an upper face or a lower face of the center frame at a joining portion between the center frame and the connecting member and an upper face or a lower face of a center joining portion of the connecting member is set respectively at a dimension that is approximately equal to or less than the thickness of an upper face plate or a lower face plate of the center frame.