JP2008202354A5 - - Google Patents

Description

boom

  The present invention relates to a cargo handling vehicle used for agriculture and civil engineering work, such as a tractor-mounted loader and a self-propelled loader (wheel loader, self-propelled backhoe, power shovel, etc.) having a front loader and / or a backhoe mounted on the tractor. It relates to the adopted boom.

Conventionally, as a boom adopted for a tractor-mounted loader, it is formed in a mountain shape that is bent at the central portion in the longitudinal direction and protrudes upward, and the front boom structure on the front side from the central portion in the longitudinal direction, and the longitudinal direction Some include a rear boom component from the center and a central connecting plate that connects the rear end of the front boom component and the front end of the rear boom component (see Patent Documents 1 and 2). .
In this type of conventional boom, the rear end side of the front boom structural body and the front end side of the rear boom structural body are connected to each other in a state of being aligned vertically without shifting up and down (for example, patents) References 1, 2).
JP 2006-188858 A JP 2006-207235 A

Therefore, in the conventional case, stress is concentrated at the connecting portion between the rear boom structure body and the front boom structure body that is bent at the central portion in the longitudinal direction and is convex upward, such as during boom work. Therefore, in order to increase the rigidity of this part, it is necessary to form the rear end side of the front boom component and the front end side of the rear boom component thickly, or to form the central connecting plate etc. thickly, In order to increase the rigidity of the connecting portion of the boom, there has been a problem that the material cost of the boom is soaring that the manufacturing cost of the boom is high.
In view of the above problems, the present invention increases the rigidity of the connecting portion between the rear boom structural body and the front boom structural body, and saves the constituent material of the boom so that the boom can be manufactured at low cost. .

The technical means of the present invention that solves this technical problem is formed in a mountain shape that is bent at the center in the longitudinal direction and protrudes upward, and the front boom structure on the front side from the center in the longitudinal direction, In a boom including a rear boom component on the rear side from the longitudinal center, and a central connecting plate that connects the rear end side of the front boom component and the front end side of the rear boom component,
The rear end side of the front boom component and the front end side of the rear boom component are connected to each other while being shifted vertically.
According to another technical means of the present invention, the front and rear boom constituent bodies include a main body including a pair of left and right side plates and a top plate for connecting upper edge portions of the left and right side plates, and the top plate of the main body. And a bottom plate provided between the left and right side plates, at least the main body of the front boom structure and the main body of the rear boom structure have the same shape, and the front boom structure and the rear boom structure are It exists in the point arrange | positioned so that it may become a symmetrical shape .

Further, another technical means of the present invention is that the bottom plate of the boom structure body shifted downward from the front boom structure body and the rear boom structure body is closer to the other boom structure body side than the side plate of the main body. The protrusion is provided, and the protrusion is inserted between the pair of side plates of the main body of the other boom constituent body.
Further, another technical means of the present invention is provided with a pair of left and right side plates arranged and fixed on both left and right sides of the rear end side of the front boom structure and the front end side of the rear boom structure. Is provided on the rear side of the lower end of the boom boom, and is provided with a cylinder pivot support portion for the boom cylinder that protrudes below the front end of the rear boom structure. There exists the point provided with the cylinder pivot part for bucket cylinders which protrudes upwards from a rear-end part.

Another technical means of the present invention is that the projecting position of the cylinder pivot for the boom cylinder and the projecting position of the cylinder pivot for the bucket cylinder are such that the boom cylinder and the bucket cylinder have the same length. It is in the set point.
Another technical means of the present invention is that the rear end side of the front boom structure is shifted downward with respect to the front end side of the rear boom structure.

  According to the present invention, since the rear end side of the front boom component and the front end side of the rear boom component are connected to each other so as to be displaced from each other in the vertical direction, the front boom component and the rear boom are used during boom work or the like. Stress can be prevented from concentrating on the connecting portion with the structural body, and the vertical width of the connecting portion between the front boom structural body and the rear boom structural body can be set to the rear end side or the rear boom of the front boom structural body. Since it may be larger than the vertical width on the front end side of the structural body, the rigidity of the connecting portion between the front boom structural body and the rear boom structural body can be increased. In addition, it is not necessary to form the rear end side of the front boom structure and the front end side of the rear boom structure thickly, or to thicken the central connecting plate, etc., thereby saving the boom material and making the boom inexpensive. Can be manufactured. Furthermore, the boom can be reduced in weight.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
1 and 2, reference numeral 1 denotes a tractor / front loader / backhoe (TLB). The tractor body 2 is reinforced by a pair of left and right mounting frames 3, and a front loader 4 is detachably mounted forward from a midway portion thereof. A backhoe 7 is detachably attached to the rear portion of the tractor vehicle body 2, and a mid-mount mower can be attached to the lower part of the middle portion of the tractor vehicle body 2.
The tractor vehicle body 2 is constructed by connecting an engine 14, a clutch housing 15, a connection case 16 and a transmission case 17 in the front-rear direction, and a front axle frame 19 for mounting a front axle case of a front wheel 18 forward from the engine 14. The rear axle case 6 protrudes from the rear portion of the transmission case 17 to the left and right sides, and a rear wheel 20 is mounted on the rear axle 8 protruding outward from the rear axle case 6. The rear axle 8 is of a type that does not have a flange at the outer end. A bonnet 12 that covers the engine 14 and the like is provided at the front of the tractor vehicle body 2.

Although not shown, when the backhoe 7 is not mounted on the upper part of the mission case 17, a working machine lifting / lowering hydraulic device is mounted so that the working machine such as a rotary tiller can be lifted and lowered. A driver's seat device with a driver's seat that can be flipped back and forth or provided with a front / rear facing driver's seat is mounted above, and a floor seat is provided from below the forward driver's seat to the control section. Is attached with a fender 22 for the rear wheel 20.
The left and right mounting frames 3 are provided with rear standing frames 10 as lops, and the rear standing frames 10 serve to connect the left and right mounting frames 3. The rear frame 10 is formed of left and right leg portions 10a and a gate-shaped upper portion 10b, both of which are connected with an upper wall of the rear wheel fender 22 interposed therebetween, and also serves as an attachment of the rear wheel fender 22. . The left and right leg portions 10 a may be fixed to the side surface of the tractor vehicle body 2 or the rear axle case 6 at the same time as being connected to the mounting frame 3.

The front loader 4 pivotally supports the base of the boom 27 on the upper part of the left and right masts 26 so as to swing up and down, pivots the bucket 28 at the tip of the boom 27, and pivotally supports the boom 27 with the boom cylinder 29. Each bucket 28 is hydraulically driven by a bucket cylinder 30.
The mast 26 is configured to fit into the mast receiving member 31 of the mounting body 5 for the front loader from the upper side, engage the lower portion, and lock the midway portion with the lock pin 32. When not in use, the front loader 4 can be detached from the mounting body 5 by removing the lock pin 32.

The mounting body 5 connects a pair of left and right mounting members 34 attached to members constituting the tractor vehicle body 2 such as the engine 14 and the clutch housing 15 with a connecting plate 35 disposed below the engine 14, A support cylinder 36 protrudes from the left and right horizontal direction from 34, the mast receiving member 31 protrudes upward from the support cylinder 36, and a bracket 37 protrudes rearward. The mast receiving member 31 and the bracket 37 are integrally formed as a single plate, but they may be formed separately and fixed to the support tube 36.
The backhoe 7 is bent in a U-shape in plan view on the front side of the base frame 40 to form a pair of vertical plate-shaped mounting plates 41a on the left and right sides, and a top plate that forms a backward step on the left and right mounting plates 41a. 41b is fixed, and a lower plate 41c is inserted and fixed to the lower part of the left and right mounting plates 41a. A swing bracket 42 is pivotally supported at the rear portion of the base frame 40 so as to be swingable around the vertical axis. A steering device that can be steered from the rear driver's seat is mounted on the top, and hydraulically driven outriggers (stabilizers) 43 are provided on the left and right of the base frame 40.

The swing bracket 42 can swing left and right by a single swing cylinder 44. The boom 45 and the base of the boom cylinder 46 are pivotally supported on the swing bracket 42, and the boom 45 is swung up and down by the boom cylinder 46. The arm 47 is pivotally supported at the tip of the boom 45 so that the arm cylinder 48 can swing up and down, and the bucket 49 is pivotally supported at the tip of the arm 47 so that the bucket cylinder 50 can be scooped and dumped freely. .
As shown in FIG. 1 and FIG. 4 to FIG. 6, a pair of left and right booms 27 are provided and are arranged on both left and right sides of the bonnet 12, and the left and right booms 27 are connected in a cylindrical shape or the like on the front side. The members 54 are connected to each other. The mast 26 is attached to a mast receiving member 31 on the same side in the left-right direction.

Each of the left and right booms 27 has a base end side (rear end side) pivotally connected to the upper portion of the mast 26 by a pivot 55 so as to be pivotable about an axis in the left-right direction so as to be vertically swingable.
Further, the pair of left and right boom cylinders 29 composed of double-acting hydraulic cylinders are interposed between the longitudinal middle portions of the left and right booms 27 and the vertical middle portions of the left and right masts 26. Thus, the left and right booms 27 can swing up and down.
In addition, the front end side (front end side) of each of the left and right booms 27 is pivotally connected to the lower rear portion of the bucket 28 via a pivot shaft 56, and between the rear surface of the bucket 28 and left and right side plates 58 described later, The pair of left and right bucket cylinders 30 composed of double-acting hydraulic cylinders are interposed, and the bucket 28 can swing (can be squeezed and dumped) by expansion and contraction of the bucket cylinder 30.

Each of the left and right booms 27 is formed in a mountain shape that is bent at the center in the longitudinal direction and protrudes upward, and the vertical width gradually increases from the front and rear ends toward the center in the longitudinal direction. Is formed.
In addition, the boom 27 includes a front boom constituent body 27A from the center in the longitudinal direction, a rear boom constituent body 27B from the center in the longitudinal direction, and the front and rear boom constituent bodies 27A and 27B. It is mainly composed of a central connecting plate 57 connected at the center in the direction and side plates 58 arranged on the left and right sides of the center of the boom 27 in the longitudinal direction.

The front and rear boom constituent bodies 27A and 27B are composed of a main body 59 and a bottom plate 60, and as shown in FIG. 5, the main body 59 of the front boom constituent body 27A and the main body 59 of the rear boom constituent body 27B have the same shape. It is formed and the member is shared.
The main bodies 59 of the front and rear boom constituent bodies 27A and 27B are formed in a U-shaped cross-section that is open downward from a pair of left and right side plates 61 and a top plate 22 that connects the upper edge portions of the left and right side plates 61. ing. The main body 59 is formed in a curved shape that is slightly convex upward as viewed from the side.

The main bodies 59 of the front and rear boom constituent bodies 27A and 27B are formed in a U-shaped cross-section by bending a single plate material, and are bent into a U-shape and then bent upward. It is bent.
In addition, the upper surface (the upper surface of the top plate 62) and the lower edge (the lower edge 61a of the left and right side plates 61) of the main body 59 of the front and rear boom constituent bodies 27A and 27B are bent into arc surfaces having a constant curvature radius. Yes.
As a result, the upper surface of the boom 27 is formed in a curved shape from the front end to the rear end, and has a shape that matches the curved shape of the bonnet 12 in design.

As shown in FIG. 5, the front boom structural body 27A and the rear boom structural body 27B are disposed in the front-rear reverse direction so as to have a longitudinally symmetrical shape , and the rear end side of the front boom structural body 27A and the rear boom structural body 27B are arranged. The front end side is arranged so as to be displaced vertically from each other, the rear end side of the front boom component 27A is shifted downward with respect to the front end side of the rear boom component 27B, and the front boom component 27A and the rear boom component are arranged. 27B is connected by a central connecting plate 57.
Further, a front pivot portion 63 is provided on the front end side of the front boom constituting body 27 </ b> A. The front pivot portion 63 is formed of a cylindrical body that penetrates the left and right side plates 61 and is fixed to the side plates 61 by welding. The bucket 28 is pivotally connected so as to be rotatable about the left and right axis.

Further, a rear pivot portion 64 is provided on the rear end side of the rear boom constituting body 27B. The rear pivot portion 64 is formed of a cylindrical body that penetrates the left and right side plates 61 and is fixed to the side plates 61 by welding. A mast 26 is pivotally connected to the mast 26 via a pin 55 so as to be rotatable about a left-right axis.
Further, the middle members of the side plates 61 on the inner side in the left-right direction or the left and right sides of the left and right front boom components 27 </ b> A are connected by the connecting member 54. For example, the mounting holes 55 are provided in the side plates 61 on the inner side in the left-right direction or the left and right sides of each front boom component 27A in the pair of left and right booms 27, and both end portions of the connecting member 54 are inserted into the mounting holes 55, By welding the connecting member 54 to the side plate 61, the connecting member 54 is fixed to the left and right front boom components 27A.

The bottom plate 60 is configured by a flat plate, and is disposed below the top plate 62 and between the left and right side plates 61. The boom component bodies 27A and 27B are arranged in the longitudinal direction from the end of the boom 27 at the center in the longitudinal direction. It is provided so as to reach the support portions 63, 64, and after bending the main body 59, the left and right side edges of the bottom plate 60 are welded and fixed to the inner surfaces of the left and right side plates 61.
The bottom plate 60 has one end in the front-rear direction in contact with the front and rear pivots 63 and 64, and the other end in the front-rear direction is the middle portion in the vertical width direction of the center in the longitudinal direction of the boom 27 (the length of the boom 27 of the side plate 61 The bottom plate 60 is located closest to the lower edge of the side plate 61 in the middle of the end of the boom 27 in the longitudinal direction. The distance from the bottom plate 60 to the lower side edge 61a of the side plate 61 gradually increases from the middle of the bottom plate 60 to the longitudinal center of the boom 27, whereby the center of the boom 27 in the longitudinal direction is formed. A pipe accommodating space is formed between the left and right side plates 61 on the lower side of the bottom plate 60.

The bottom plate 60 of the front boom component 27A, which is the side shifted downward between the front boom component 27A and the rear boom component 27B, protrudes more toward the rear boom component 27B (rear) than the side plate 61 of the main body 59. The projecting portion 60a is inserted between the pair of side plates 61 of the main body 59 of the rear boom component 27B, and is contacted or approached from the lower side to the front middle portion of the bottom plate 60 of the rear boom component 27B. As will be described later, the bottom plates 60 of the front boom structural body 27A and the rear boom structural body 27B are welded together.
As described above, since the distance from the bottom plate 60 to the lower edge 61a of the side plate 61 is large at the center in the left-right direction of the boom 27 and the left-right width between the left and right side plates 61 is narrow, the bottom plate 60 becomes the side plate 61. In welding, it is difficult for a welding torch to enter between the side plates 61 on the center side in the longitudinal direction of the boom 27, and it is difficult to weld the bottom plate 60 to the side plate 61 on the center side in the longitudinal direction of the boom 27.

Therefore, a notch 69 is formed in the upper part of the main body 59 of the front and rear boom constituent bodies 27A and 27B on the center side in the longitudinal direction of the boom 27. The bottom plate 60 is welded to the side plate 61 from the lower side over the middle portion in the longitudinal direction, and the bottom plate 60 is welded to the side plate 61 from the upper side at the center in the longitudinal direction of the boom 27.
The notch 69 cuts the top plate 62 in a predetermined range in the boom longitudinal direction from the end of the top side of the boom 27 in the longitudinal direction of the boom 27, and at the middle in the vertical width direction of the left and right side plates 61 from the notch Notched over.

The front and rear boom constituent bodies 27A and 27B are welded and fixed to the main body 59, and then joined together at the center in the longitudinal direction of the boom 27. The front and rear bottom plates 60 are welded together, and the central connecting plate 57 is attached. The left and right side plates 58 are connected to each other by welding to the boom constituent bodies 27A and 27B. The central connecting plate 57 and the left and right side plates 58 are connected by welding.
The central connecting plate 57 is formed by bending a flat plate into a curved shape protruding upward, and is provided over the top plates 62 of the front and rear boom constituent bodies 27A and 27B.

  The left and right side plates 58 are disposed and fixed on both the left and right sides of the rear end side of the front boom component 27A and the front end side of the rear boom component 27B. The left and right side plates 58 are provided so as to extend across the side plates 61 of the front and rear boom components 27A and 27B on the center side in the longitudinal direction of the boom 27, and on the rear side of the lower ends of the left and right side plates 58, the rear boom components 27B. A cylinder pivot support portion 70 for the boom cylinder that protrudes below the front end portion is provided, and a bucket cylinder that protrudes above the rear end portion of the front boom constituting body 27A is provided on the front side of the upper end portion of each of the left and right side plates 58. Cylinder support 71 is provided.

  The pair of bucket cylinders 30 is disposed directly above the pair of left and right booms 8, and each bucket cylinder 30 is disposed above each front boom component 27A so as to be along the front boom component 27A. Yes. The pair of boom cylinders 29 are respectively disposed directly below the pair of left and right booms 8, and each boom cylinder 29 is disposed below each rear boom component 27B. A front end portion of the boom cylinder 29 is pivotally connected to a cylinder pivot portion 70 for the boom cylinder, and a rear end portion of the bucket cylinder 30 is pivotally connected to a cylinder pivot portion 71 for the bucket cylinder.

  The protruding position of the boom cylinder supporting portion 70 and the protruding position of the bucket cylinder supporting portion 71 are set so that the boom cylinder 29 and the bucket cylinder 30 have the same length. That is, the boom cylinder 29 and the bucket cylinder 30 are constituted by the same cylinder, the boom cylinder 29 and the bucket cylinder 30 are set to the same length, and the boom cylinder 29 and the bucket cylinder 30 set to the same length. The boom 27 is swung up and down well, and the bucket 28 is swung (squeezing and dumping) well, so that the boom pivot cylinder pivot portion 70 and the bucket cylinder The protruding position of the cylinder pivot 71 is determined.

  According to the above embodiment, the rear end side of the front boom component 27A and the front end side of the rear boom component 27B are connected to each other so as to be displaced from each other in the vertical direction. Stress can be prevented from concentrating on the connecting portion with the rear boom constituent body, and the vertical width of the connecting portion between the front boom constituent body 27A and the rear boom constituent body 27B can be set to the rear of the front boom constituent body 27A. It is larger than the vertical width of the front end side or the front end side of the rear boom constituent body 27B and can be bent into a mountain shape, and the rigidity of the connecting portion between the front boom constituent body 27A and the rear boom constituent body 27B can be increased. Can be increased. Moreover, in order to increase the rigidity of the connecting portion of the boom 27, the rear end side of the front boom constituting body 27A and the front end side of the rear boom constituting body 27B are formed thicker, or the central connecting plate 57, the side plate 58, etc. are thickened. It is not necessary to form the boom 27, and the boom 27 can be manufactured at low cost by saving the constituent material of the boom 27. Further, the entire boom 27 can be reduced.

Further, the main body 19 of the front boom component 27A and the main body 59 of the rear boom component 27B have the same shape, and the front boom component 27A and the rear boom component 27B are arranged so as to be symmetrical in the front-rear direction. Therefore, the main body 19 of the front boom structure 27A and the main body 59 of the rear boom structure 27B can be used as the same thing, and the connection between the front boom structure 27A and the rear boom structure 27B is simplified. Thus, the boom 27 can be manufactured inexpensively and easily. Further, the bottom plate 60 of the front boom component 27A on the side shifted downward of the front boom component 27A and the rear boom component 27B protrudes from the side plate 61 of the main body 59 toward the rear boom component 27B. Since the projecting portion 60a is inserted between the pair of side plates 61 of the main body 59 of the rear boom constituting body 27B, when the front boom constituting body 27A and the rear boom constituting body 27B are connected, the front boom The structural body 27A and the rear boom structural body 27B can be easily positioned in the vertical and horizontal directions and the front and rear, and the front boom structural body 27A and the rear boom structural body 27B can be easily and reliably connected. In addition, the protrusion 60a of the front boom component 27A can be fixed to the bottom plate 60 or the pair of side plates 61 of the rear boom component 27B by welding or the like, and the front boom component 27A and the rear boom component 27B are connected to each other. In this respect, the rigidity of the connecting portion between the front boom structure 27A and the rear boom structure 27B can be further increased.

  Further, since the cylinder pivot support portion 71 for the bucket cylinder is provided on the front side of the upper end portion of each of the left and right side plates 58 and protrudes upward from the rear end portion of the front boom structural body 27A that is shifted downward, the bucket The cylinder pivot support portion 71 for the cylinder can be separated relatively upward from the rear end side of the front boom constituting body 27 </ b> A, and the pair of bucket cylinders 30 correspond to the pair of left and right booms 8, respectively. Even when the bucket cylinder 30 is disposed, there is no possibility that the rear end side of the bucket cylinder 30 interferes with the front boom constituting body 27A due to the expansion and contraction operation of the bucket cylinder 30, and the bucket cylinder 30 can be smoothly expanded and contracted.

Since the boom cylinder 29 and the bucket cylinder 30 are set to have the same projection length of the boom cylinder support portion 70 and the bucket cylinder support portion 71, the boom cylinder 29 and the bucket cylinder 30 have the same length. 29 and the bucket cylinder 30 can be constituted by the same cylinder, and the boom cylinder 29 and the bucket cylinder 30 to the front loader 4 can be manufactured inexpensively and easily.
In addition, according to the said embodiment, although the rear end side of 27 A of front boom structures is shifted below with respect to the front end side of the rear boom structure body 27B, both are connected, it replaces with this, The front end side of 27B may be shifted downward with respect to the rear end side of the front boom constituting body 27A, and both may be connected by the central connecting plate 57 or the like. In this case, the bottom plate 60 of the rear boom constituting body 27B is projected on the front boom constituting body 27A (front) side with respect to the side plate 61 of the main body 59, and the protruding portion 60a is formed as a pair of the main body 59 of the front boom constituting body 27A. What is necessary is just to insert between the side plates 61.

  Further, according to the embodiment, the bottom plate 60 of the front boom component 27A protrudes from the side plate 61 of the main body 59 to the rear boom component 27B (rear) side, and the rear plate 60 of the front boom component 27A is rearward. A projecting portion 60a is provided at the end so that the bottom plate 60 of the front boom component 27A and the bottom plate 60 of the rear boom component 27B have different shapes, and the main body 19 and the rear boom component of the front boom component 27A. The main body 59 of 27B has the same shape, but instead, the bottom plate 60 of the front boom structure 27A and the rear boom structure are not provided at the rear end of the bottom plate 60 of the front boom structure 27A. The bottom plate 60 of the body 27B may have the same shape and the same length, and the front boom structure body 27A and the rear boom structure body 27B may have the same shape as a whole.

It is a side view of the boom which shows one embodiment of this invention. It is a whole side view of a tractor, a front loader, and a backhoe. It is a top view of the mounting frame part of a tractor. It is a side view of the front part of a tractor and a front loader. It is an exploded side view of a boom. It is the sectional view on the AA line of FIG.

DESCRIPTION OF SYMBOLS 1 Tractor front loader backhoe 2 Tractor vehicle body 27 Boom 27A Front boom constituent body 27B Rear boom constituent body 28 Bucket 29 Boom cylinder 30 Bucket cylinder 57 Central connecting plate 58 Side plate 59 Main body 60 Bottom plate 61 Side plate 62 Top plate 70 For boom cylinder Cylinder pivot part of 71 Cylinder pivot part for bucket cylinder

Claims (6)

It is formed in a mountain shape that is bent at the center in the longitudinal direction and protrudes upward, and the front boom component (27A) from the longitudinal center to the front and the rear boom component from the longitudinal center. (27B) and a boom provided with a central connecting plate (57) for connecting the rear end side of the front boom structural body (27A) and the front end side of the rear boom structural body (27B),
A boom characterized in that the rear end side of the front boom structural body (27A) and the front end side of the rear boom structural body (27B) are connected to each other while being shifted vertically. The front and rear boom components (27A, 27B) include a main body (59) including a pair of left and right side plates (61) and a top plate (62) for connecting upper edge portions of the left and right side plates (61). A bottom plate (60) provided below the top plate (62) of the main body (59) and between the left and right side plates (61), and at least the main body ( 59 ) of the front boom structure (27A) and the rear The main body (59) of the boom structural body (27B) has the same shape, and the front boom structural body (27A) and the rear boom structural body (27B) are arranged so as to be symmetrical in the longitudinal direction. The boom according to claim 1.   The bottom plate (60) of the boom component (27A, 27B) on the lower side of the front boom component (27A) and the rear boom component (27B) is the side plate (61) of the main body (59). The protrusions (60a) are inserted between the pair of side plates (61) of the main body (59) of the other boom structure (27A, 27B). The boom according to claim 2, wherein the boom is provided.   A pair of left and right side plates (58) arranged and fixed on both left and right sides of the rear end side of the front boom structure (27A) and the front end side of the rear boom structure (27B) are provided, and left and right side plates (58). The boom pivot cylinder support portion (70) projecting below the front end portion of the rear boom component (27B) is provided on the rear side of the lower end portion of the left and right side plates (58). The cylinder pivot support part (71) for bucket cylinders which protrudes above the rear-end part of a front boom structure (27A) is provided in the front side, The any one of Claims 1-3 characterized by the above-mentioned. Boom.   The projecting position of the boom pivot cylinder pivot (70) and the bucket cylinder pivot pivot (71) are such that the boom cylinder (29) and the bucket cylinder (30) have the same length. The boom according to claim 4, wherein the boom is set.   The boom according to any one of claims 1 to 5, wherein a rear end side of the front boom structure (27A) is shifted downward with respect to a front end side of the rear boom structure (27B).