Arm for construction machine

An arm for a construction machine, including an arm top formed by casting, being easily produced with high dimensional accuracy. The arm includes an arm body and an arm top. The arm top is formed of left and right arm top pieces individually formed by casting and joined together. The left and right arm top pieces include first left and right bosses constituting a first connection section, second left and right bosses constituting a second connection portion, and left and right projecting wall portions constituting a connection wall connecting the first and second connection portions to each other, respectively. The left and right arm top pieces are joined together by welding at respective opposing surfaces of the first left and right bosses, those of the second left and right bosses and those of the left and right projecting wall portions, thereby forming the arm top.

TECHNICAL FIELD

The present invention relates to an arm to constitute a working attachment for a construction machine such as excavator, the arm including an arm top serving as a distal-end member and formed by casting.

BACKGROUND ART

Conventionally, there are known arms for a construction machine, the arm including an arm top formed by casting, as shown in Patent Literatures 1 and 2. The arm top does not include any weld involving high stress concentration, which allows the arm top to have a high strength.

However, the techniques disclosed in Patent Literatures 1 and 2, involving integrally forming the entire arm top by casting, requires a large and complicated mold for the casting which is hard to produce and it is difficult to obtain high production accuracy.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

The object of the present invention aims to provide an arm for a construction machine, the arm being formed by casting and capable of being easily produced with high accuracy.

Provided is an arm to constitute a working attachment for a construction machine, the arm including: an arm body to be attached to a distal end of a boom constituting the working attachment; and an arm top attached to a distal end of the arm body.

The arm top includes: a pair of left and right side walls; a first connection portion disposed at a distal end of the arm top to span the pair of side walls so as to define a first pin hole for allowing a pin for connecting a working tool to the distal end of the arm top to be inserted into the first pin hole; a second connection portion disposed at a position closer to a proximal end of the arm top than the first connection portion to span the pair of side walls so as to define a second pin hole for allowing a pin for connecting a link for actuating the working tool to be inserted in the second pin hole; and connection walls formed on respective inner surfaces of the side walls, the connection walls having respective shapes to connect the pair of side walls to each other and connect the first and second connection portions to each other.

The arm top includes a left arm top piece and a right arm top piece that are individually formed by casting, the left arm top piece having a first left boss constituting the first connection portion, a second left boss constituting the second connection portion, and a left projecting wall portion constituting the connection wall, the right top piece having a first right boss constituting the first connection portion, a second right boss constituting the second connection portion, and a right projecting wall portion constituting the connection wall. The first left boss and the first right boss have respective surfaces opposed to each other in a left-right direction of the arm top, the second left boss and the second right boss have respective surfaces opposed to each other in the left-right direction of the arm top, and the left projecting wall portion and the right projecting wall portion have respective surfaces opposed to each other in the left-right direction.

The left and right arm top pieces are joined together by welding at the opposed surfaces of the first left boss and the first right boss, at the opposed surfaces of the second left boss and the second right boss, and at the opposed surfaces of the left projecting wall portion and the right projecting wall portion, thereby being integrated to form the arm top.

DESCRIPTION OF EMBODIMENTS

There will be described a reference example with reference toFIGS. 12 to 16in advance of describing embodiments of the present invention, the reference example being provided for explanation of the characteristics of an arm according to the embodiments.

FIG. 12shows an excavator including an arm5according to the reference example. The excavator includes a crawler-type lower travelling body1, an upper slewing body2mounted on the lower travelling body1so as to be slewable about an axis normal to the ground, the upper slewing body2including a cabin2a, and a working attachment3attached to the upper slewing body2.

The working attachment3includes a boom4, the arm5, a working tool, a plurality of hydraulic cylinders, a main link10, and an idler link11. The boom4is attached to the upper slewing body2in a raisable and lowerable manner, and the arm5is attached to a distal end of the boom4. The working tool, namely, a bucket6in this reference example, is attached to a distal end of the arm5. The plurality of hydraulic cylinders include a boom cylinder7, an arm cylinder8, and a bucket cylinder9that expand and contract to move the boom4, the arm5, and the bucket6, respectively.

The main link10and the idler link11are disposed in such a way as to transmit an expansion/contraction force of the bucket cylinder9to the bucket6to rotationally move the bucket. Specifically, the main link10includes a first end to be connected to a distal end of the bucket cylinder9through a pin, and a second end to be connected to the bucket6through a pin. Similarly, the idler link11includes a first end to be connected to the distal end of the bucket cylinder9through a pin, and a second end connected to the arm5through a pin.

The arm5includes a first connection portion12and a second connection portion13. The first connection portion12is disposed at a distal end of the arm5, and the bucket6is connected to the first connection portion12through a pin. The second connection portion13is disposed at a position closer to a proximal end (i.e., an end to be connected to the boom4) of the arm5than the first connection portion12, the idler link11being connected to the second connection portion13through a pin.

Next will be described the structure of the arm5including the first and second connection portions12and13, with reference toFIGS. 13 to 16. In the description hereinafter, the terms “right” and “left” indicate the respective directions seen from an operator in the cabin2a, and the term “left-right direction” corresponds to a width direction of the arm5.

The arm5includes an arm body14and an arm top15. The arm body14serves as an arm-proximal-end member, including a plurality of plates forming a box structure, namely, a pair of left and right side plates14a, a top plate14band a bottom plate14c. The arm top15serves as a distal-end member to be attached to the distal end of the arm body14.

Each of the top plate14band the bottom plate14cof the arm body14has a length equivalent to the full length of the arm5. Each of the left and right side plates14ais shorter than the top plate14band the bottom plate14c. The arm top15is joined to the distal end of the arm body14by welding in such a way as to make up the dimensional difference of the left and right side plates14afrom the top and bottom plates14b,14c, thereby integrated with the arm body14.

The arm top15includes left and right side walls16,17as shown in the enlarged views ofFIGS. 15 and 16. The first connection portion12and the second connection portion13to which the bucket16and the idler link11are connected, as mentioned above, respectively, are disposed at a distal end of the arm top15and at a position closer to the proximal end of the arm top15than the distal end, respectively, so as to span the side walls16and17.

The first and second connection portions12,13are formed by assembling a plurality of components such as those shown inFIG. 16. The plurality of components include: a pair of ring members18,18and a pair of ring members19,19that are welded to the side walls16,17; a tube member20disposed so as to span the ring members18,18; and a tube member21disposed so as to span the ring members19,19. The plurality of components are assembled to form the first and second connection portions12,13as respective bosses each enclosing an enclosed pin hole.

The arm top15further includes an oil supply inlet22and an oil supply pipe23shown inFIG. 16. The oil supply inlet22is attached to one of the side walls16,17, for example, the left side wall16. The oil supply pipe23is disposed in such a way as to bring the oil supply inlet22into communication with the pin hole enclosed by the tube member19of the first connection portion12.

In the arm top15according to the reference example, stress concentration is likely to occur in respective welds between the side walls16,17and the ring members18,19of the first and second connection portions12,13. The stress concentration hinders the arm top15from having a high strength.

Respective arms according to embodiments of the present invention solve the above-mentioned problem found in the arm5of the reference example. The arms according to the embodiments will be described hereinafter with reference toFIGS. 1 to 11.

The arms according to first to third embodiments shown below share the following common configuration with the arm of the reference example. Specifically, each of the arms according to the first to third embodiments includes an arm body14and an arm top25. The arm body14includes a box structure capable of being attached to a distal end of a boom similar to the boom4of the working attachment3shown inFIG. 12. The arm top25is serves as a distal-end member to be attached to the distal end of the arm body14.

Next will be described respective characteristic features of the arms according to the first to third embodiments, i.e., respective differences of the arm tops25according to the first to third embodiments from the arm top15of the reference example.

FIGS. 1 to 7show the arm top25according to the first embodiment. The arm top25includes a pair of left and right side walls26,27, a first connection portion28, a second connection portion29, and a connection wall30. The side walls26,27are joined to respective distal ends of a pair of left and right side plates14aof the arm body14, as shown inFIGS. 2 and 3. The first connection portion28is disposed so as to span the side walls26,27in order to connect a working tool such as the bucket6to the distal end of the arm top25through a pin. The second connection portion29is disposed so as to span the side walls26,27in order to connect an idler link for actuating the working tool to a part of the arm top25, the part being closer to the proximal end of the arm top25than the first connection portion28through a pin. The connection wall30is disposed so as to connect respective inner surfaces of the left and right side walls26,27to each other and so as to connect the first and second connection portions28,29to each other. The first and second connection portions28,29have respective shapes enclosing first pin hole31and an enclosed second pin hole32. The first pin hole31allows a pin for connecting a working tool such as the bucket6to the distal end of the arm top25to be inserted thereinto, and the second pin hole32allows a pin for connecting the idler link11to a part of the arm top25, the part being closer to the proximal end of the arm top25than the distal end of the arm top25.

The arm top25is constituted by a left arm top piece P1and a right arm top piece P2. The arm top pieces P1and P2are individually formed by casting. Specifically, the first connection portion28is divided into first left boss28aand first right boss28b, the second connection portion29is divided into second left boss29aand second right boss29b, and the connection wall30is divided into left projecting wall portion30aand right projecting wall portion30b; the left arm top piece P1includes the first left boss28a, the second left boss29aand the left projecting wall portion30a, while the right arm top piece P2includes the first right boss28b, the second right boss29band the right projecting wall portion30b.

The left and right arm top pieces P1and P2have respective opposed surfaces which are opposed to each other. Specifically, the first left boss28aand the first right boss28bhave respective first opposed surfaces opposed to each other in a left-right direction of the arm top25, the second left boss29aand the second right boss29bhave respective second opposed surfaces opposed to each other in the left-right direction of the arm top25, and the left projecting wall portion30aand the right projecting wall portion30bhave respective third opposed surfaces opposed to each other in the left-right direction of the arm top25. Mutual joining of the first opposed surfaces, mutual joining of the second opposed surfaces and mutual joining of the third opposed surfaces are performed by welding along a weld line Lw extending over the entire circumference. The left and right arm top pieces P1, P2are thereby integrated to form the arm top25. The mutual joinings of the opposing surfaces involve forming the first connection portion28by the connection between the first left boss28aand the first right boss28b, forming the second connection portion29by the connection between the second left boss29aand the second right boss29b, and forming the connection wall30by the connection between the left projecting wall portion30aand the right projecting wall portion30b. The weld line Lw corresponds to the portion indicated by oblique lines extending in different directions inFIG. 4.

In the left and right arm top pieces P1, P2, the left and right projecting wall portions30a,30bare connected to the first left and right bosses28a,28band to the second left and right bosses29a,29bvia respective joining portions having gently curved surfaces CS1, CS2as shown inFIG. 7. Similarly, the left and right projecting wall portions30a,30bare connected to the left and right side walls26,27via joining portions each having a gently curved surface CS3as shown inFIG. 5. The first left and right bosses28a,28band the second left and right bosses29a,29bare connected to the left and right side walls26,27via respective joining portions each having a gently curved surface CS4as shown inFIG. 5.

As shown inFIGS. 4 and 6, the first left and right bosses28a,28bshares a recess28cformed therein. The recesses28care formed in respective forward half peripheries of joint portions of the first left and right bosses28a,28bthat are joined together by welding, the half peripheries being disposed on the distal end side of the arm top pieces P1, P2. The half peripheries are recessed radially inward so as to have a diameter smaller than that of the other parts of the first left and right bosses28a,28bhaving a maximum diameter (the parts on laterally both sides of the recesses28c). The left and right arm top pieces P1, P2are welded together at the recesses28c.

The arm, thus, can be produced by a method including a step of individually forming the left and right arm top pieces P1, P2by casting, a step of joining the left and right arm top pieces P1, P2together by welding at the first opposed surfaces of the first left boss28aand the first right boss28b, at the second opposed surfaces of the second left boss29aand the second right boss29b, and at the third opposed surfaces of the left projecting wall portion30aand the right projecting wall portion30bto thereby integrate the left arm top piece P1and the right arm top piece P2to form the arm top25, and a step of attaching the arm top25to the distal end of the arm body14.

According to the above-described arm, the arm top25is originally divided into the separate left and right arm top pieces P1, P2that are individually formed and then joined to each other by casting to be integrated, in other words, the left and right arm top pieces P1, P2are individually formed as separate casted products; this allows a mold for the casting and the casted products to have a small and simple shape. This makes it possible to easily produce the arm top25with an improved dimensional accuracy, as compared to an arm top which is entirely formed of a single casted product.

Besides, the left and right arm top pieces P1, P2, which are integrated by welding over the entire circumference not at the joints between the first and second connection portions28,29and the side walls26,27where stress concentration is likely to occur but at the widthwise middle of the arm top25with low stress, can have a high strength.

Furthermore, the left and arm top pieces P1, P2include the left and right projecting wall portions30a,30bdisposed on respective inner surfaces of the left and right side walls26,27, the left and right projecting wall portions30a,30bconnecting the left and right side walls26,27to each other and connecting the first and second connection portions28,29to each other, and not only the first left and right bosses28a,28bconstituting the first connection portion28and the second left and right bosses29a,29bconstituting the second connection portion29but also the left and right projecting wall portions30a,30bconstituting the connection wall30are mutually joined by welding; this makes it possible to eliminate a blind spot into which a welding torch cannot make an entry to thereby improve the welding reliability and workability.

Moreover, in the left and right arm top pieces P1, P2, the joining portions that join the left and right projecting wall portions30a,30bto the first left and right bosses28a,28b, the joining portions that join the left and right projecting wall portions30a,30bto the second left and right bosses29a,29b, the joining portions that join the left and right projecting wall portions30a,30bto the left and right side walls26,27, and the joining portions that join the first left and right bosses28a,28hand the second left and right bosses29a,29bto the left and right side walls26,27are given respective gently curved surfaces CS1, CS2, CS3, CS4to prevent each of the joining portions from have an acute angle; this allows a load to be smooth transmitted through the joining portions to prevent occurrence of stress concentration, thereby enabling the arm top25to have further enhanced strength. Besides, the gently curved surface of each joining portion facilitates the welding to thereby improve the welding reliability and workability.

In addition, forming the recess28calong the forward half periphery of the distal end of each of the left and right top pieces P1, P2and the left and welding right arm top pieces P1, P2together at the recesses28callow the forward half peripheries of the joint portions (weld line Lw) on the distal end side of the arm top pieces P1, P2that are most likely to come into contact with the ground and/or rocks during a work such as excavation to lie at a position radially inner than the other part to be protected from exposure to the outside. This is extremely effective for suppressing wear in the joint portions due to contact with the ground.

FIGS. 8 and 9show an arm top35of the arm according to the second embodiment of the present invention. The arm top35differs from the arm top25of the first embodiment only in the following aspects.

The arm top35according to the second embodiment is provided with an oil supply passage33formed in an inner side of a right arm top piece P2. The oil supply passage33is constituted by a groove extending from a first pin hole31enclosed by a right boss28bto a second pin hole32enclosed by a right boss29bvia a right projecting wall portion30b. In the state that left and right arm top pieces P1, P2are joined together to integrally form the arm top35, the groove and the left arm top piece P1covering the groove defines the oil supply passage33in the form of a tunnel.

The thus formed oil supply passage33can be shared by first and second connection portions28,29. The sharing makes it possible to simplify the oil supply structure and reduce the number of components required for the oil supply structure, and also to improve the efficiency of oil supply work.

The arm top35according to the second embodiment includes an oil supply hole34allowing lubricant oil to be supplied into the oil supply passage33through the oil supply hole34from the outside. The oil supply hole34passes through the right projecting wall portion30bof the right arm top piece P2in the left-right direction, having an outer end opened to the outside and an inner end in communication with the oil supply passage33. The combination of the oil supply hole34and the oil supply passage33allows lubricant oil to be supplied to the first and second connection portions28,29almost simultaneously and evenly.

It is also possible to form the oil supply passage33in the left arm top piece P1. Alternatively, the left and right arm top pieces P1, P2may be formed with respective grooves, which combine to form an oil supply passage having a circular section.

Furthermore, the left and right arm top pieces P1, P2according to the second embodiment includes respective tapered portions28tin place of the recess28c. The tapered portions28t, which serve as means for locating forward half peripheries of respective joint portions of the left and right arm top pieces P1, P2on the distal end side that are joined together at a position radially inner than the other part having a maximum diameter (the parts on laterally both sides of the joint portions), are formed in respective forward half periphery of outer surfaces of the first left and right bosses28a,28bconstituting the first connection portion28on the distal end side of the left and right arm top pieces P1, P2as shown inFIG. 9, each including a portion tapered in such a direction that the boss diameter gradually decreases toward the joint portions lying in the middle (the boundary between the left and right arm top pieces P1, P2). The tapered portions28tenable the same advantageous effect as that by the recess28c, i.e., the effect of preventing the weld from wearing due to contact with the ground or the like to be obtained.

FIGS. 10 and 11show an arm top45of the arm according to the third embodiment of the present invention. The arm top45further has a function of restraining the bottom surface of the distal end of the arm from abrasive wear.

In the arm according to the reference example, as shown inFIGS. 13 and 14, the top plate14band the bottom plate14cconstituting the arm body14extend over the full length of the arm and respective distal ends of the top plate14band the bottom plate14care welded over a top surface and a bottom surface of the arm top15, respectively; therefore, the distal end of the bottom plate14cforms the bottom of the distal end of the arm, i.e., a part most likely to come into contact with the ground or the like during a work such as excavation. The distal end of the bottom plate14cis therefore brought into significant wear and damage.

In the arm top45according to the third embodiment, each of left and right projecting wall portions30a,30bconstituting a connection wall30includes a bottom surface30scapable of solving the above-mentioned problem. Each bottom surface30sis in the form of a flat surface substantially flush with a bottom surface of the bottom plate14cof the arm body14. These bottom surfaces30s, which constitute a bottom surface of the arm top45, namely, a bottom surface defining the bottom of the distal end of the arm, and are in the form of flat surfaces substantially flush with the bottom surface of the bottom plate14cof the arm body14, can be effectively restrained from being worn and damaged.

In this case, preferred is that the distal end of the bottom plate14cof the arm body14are welded, as shown inFIG. 11, to respective parts of the bottom surfaces30sof the left and right projecting wall portions30a,30b, the parts being rearward of the second connection portion29, for example, between the first left and right bosses28a,28band the second left and right bosses29a,29b, so as to avoid the second left and right bosses29a,29bconstituting the second connection portion29on which a great load acts.

The present invention is not limited to those arms described above that constitute a working attachment for an excavator. The present invention can be similarly applied to those arms provided for construction machines other than excavators such as dismounting machines and crushing machines, the arms having a distal end for allowing a working tool other than a bucket, such as a breaker or a grinder, to be attached thereto.

As described above, there is provided an arm to constitute a working attachment for a construction machine, the arm being formed by casting and capable of being easily produced with high accuracy. The arm includes: an arm body to be attached to a distal end of a boom constituting the working attachment; and an arm top attached to a distal end of the arm body. The arm top includes: a pair of left and right side walls; a first connection portion disposed at a distal end of the arm top to span the pair of side walls so as to define a first pin hole for allowing a pin for connecting a working tool to the distal end of the arm top to be inserted into the first pin hole; a second connection portion disposed at a position closer to a proximal end of the arm top than the first connection portion to span the pair of side walls so as to define a second pin hole for allowing a pin for connecting a link for actuating the working tool to be inserted in the second pin hole; and connection walls formed on respective inner surfaces of the side walls, the connection walls having respective shapes to connect the pair of side walls to each other and connect the first and second connection portions to each other. The arm top includes a left arm top piece and a right arm top piece that are individually formed by casting, the left arm top piece having a first left boss constituting the first connection portion, a second left boss constituting the second connection portion, and a left projecting wall portion constituting the connection wall, the right top piece having a first right boss constituting the first connection portion, a second right boss constituting the second connection portion, and a right projecting wall portion constituting the connection wall. The first left boss and the first right boss have respective surfaces opposed to each other in a left-right direction of the arm top, the second left boss and the second right boss have respective surfaces opposed to each other in the left-right direction of the arm top, and the left projecting wall portion and the right projecting wall portion have respective surfaces opposed to each other in the left-right direction. The left and right arm top pieces are joined together by welding at the opposed surfaces of the first left boss and the first right boss, at the opposed surfaces of the second left boss and the second right boss, and at the opposed surfaces of the left projecting wall portion and the right projecting wall portion, thereby being integrated to form the arm top.

In the above-described arm, the arm top is originally divided into the separate left and right arm top pieces that are individually formed by casting and joined to each other to be integrated to thereby form the arm top, in other words, the piece obtained by casting for forming the arm top are divided into the left and right arm top pieces, allowing a mold for the casting and the casted products to have a small and simple shape. This makes it possible to easily produce the arm top with an improved dimensional accuracy, as compared to an arm top entirely formed of a single casted product.

In addition, the left and right arm top pieces, which arc welded not at the joints between the first and second connection portions and the left and right side walls where stress concentration is likely to occur but at the widthwise middle of the arm top with low stress, can have a high strength.

If the left and right arm top pieces are integrated by welding only at the opposed surfaces of the first left and right bosses and the oppose surfaces of the second left and right bosses, many parts would be out of reach of a welding tool such as a welding torch, which would reduce the welding reliability and workability. In contrast, providing the left and right arm top pieces with respective left and right projecting wall portions having respective shapes connecting the left and right side wall portions to each other and the first and second connection portions to each other and integrating the left and right arm top not only by welding of the first left and right bosses and the second left and right bosses but also by welding of the left and right wall portions make it possible to improve the welding reliability and workability.

It is preferred that the left and right arm top pieces include respective joining portions that join the left projecting wall portion to each of the first and second left bosses, a joining portion that joins the left projecting wall portion to the left side wall, respective joining portions that join the first and second left bosses to the left side wall, respective joining portions that join the right projecting wall portion to the first and second right bosses, a joining portion that joins the right projecting wall portion to the right side wall, and respective joining portions that join the first and second right bosses to the right side wall, each of the joining portions having a curved surface. This makes it possible to suppress stress concentration by allowing a load to be smooth transmitted through the joining portion to thereby further improve the strength of the arm top, differently from the case where each joining portion forms an acute angle. Besides, providing each joining portion with the curved surface facilitates the welding, thereby allowing the welding reliability and workability to be improved.

It is preferred that at least one of the left and right arm top pieces is provided with an oil supply passage extending from the first pin hole defined by the first connection portion to the second pin hole defined by the second connection portion through the connection wall. The oil supply passage can be shared by the first and second connection portions, and the sharing makes it possible to simplify the oil supply structure and reduce the number of components required for the oil supply structure, and also to improve the efficiency of oil supply work.

In this case, it is preferred that at least one of the left and right arm top pieces is provided with an oil supply hole for supplying lubricant oil into the oil supply passage from the outside of the arm top piece, the oil supply hole passing through at least one of the left and right projecting wall portions and having an outer end opened to the outside and an inner end in communication with the oil supply passage. The oil supply hole allows the lubricant oil to be supplied to the first and second connection portions almost simultaneously and evenly.

It is preferred that the left and right first bosses constituting the first connection portion includes respective joint portions joined to each other by welding, at least a forward half periphery of each of the joint portions on the distal end side of the arm top pieces lying at a position radially inner than a forward periphery of an outer surface of the first connection portion that has a maximum diameter. Thus locating the forward half peripheries of the joint portions of the first left and right bosses on the distal end side of the left and right arm top pieces, i.e., the parts that are most likely to come into contact with the ground and/or rocks during a work such as excavation, at a position radially inner than the part having a maximum diameter makes it possible to effectively protect the forward half peripheries of the joint portions from wear due to contact with the ground and/or rocks.

Specifically, it is possible either that at least the forward half periphery of the joint portion of each of the first left and right bosses on the distal end side of the arm top pieces is recessed radially inward so as to have a smaller diameter than that of the other part or that at least a forward half periphery of each of the first left and right bosses on the distal end side of the arm top pieces is tapered in such a way that the diameter of the forward half periphery decreases toward the joint portions in a width direction of the arm top.

The arm body, for example, appropriately includes left and right side plates, a top plate, and a bottom plate that form a box structure. In this case, it is preferred that the connection wall has a flat bottom surface substantially flush with a bottom surface of the bottom plate of the arm body. The bottom surface can be restrained from being worn and damaged, though being the bottom of the distal end of the arm that is most likely to come into contact with the ground or the like during a work such as excavation.

The present invention also provides a method for producing an arm to constitute a working attachment for a construction machine, the arm including an arm body to be attached to a distal end of a boom constituting the working attachment, and an arm top serving as an arm distal-end member to be attached to a distal end of the arm body, the arm top including: a pair of left and right side walls; a first connection portion disposed at a distal end of the arm top and spanning the side walls so as to define a first pin hole for allowing a pin for connecting a working tool to the distal end of the arm top to be inserted into the first pin hole; a second connection portion disposed at a position between the first connection portion and a proximal end of the arm top and spanning the side walls so as to define a second pin hole for allowing a pin for connecting a link for actuating the working tool to be inserted into the second pin hole; and a connection wall formed on respective inner surfaces of the side walls and having a shape connecting the side walls to each other and connecting the first and second connection portions to each other. The method includes: individually forming a left arm top piece and a right arm top piece by casting, the left arm top piece having a first left boss constituting the first connection portion, a second left boss constituting the second connection portion, and a left projecting wall portion constituting the connection wall, and the right top piece having a first right boss constituting the first connection portion, a second right boss constituting the second connection portion, and a right projecting wall portion constituting the connection wall, the first left boss and the first right boss having respective surfaces opposed to each other in a left-right direction of the arm top, the second left boss and the second right boss having respective surfaces opposed to each other in the left-right direction of the arm top, and the left projecting wall portion and the right projecting wall portion having respective surfaces opposed to each other in the left-right direction; joining the left arm top piece and the right arm top piece by welding at the opposed surfaces of the first left boss and the first right boss, at the opposed surfaces of the second left boss and the second right boss, and at the opposed surfaces of the left projecting wall portion and the right projecting wall portion, thereby integrating the left arm top piece and the right arm top piece to form the arm top; and attaching the arm top to the distal end of the arm body.