EP0399043B1

EP0399043B1 - Method of operating a working machine for construction vehicles

Info

Publication number: EP0399043B1
Application number: EP88910129A
Authority: EP
Inventors: Noboru Kanayama; Hisashi Fukumoto; Kuniharu Fujii
Original assignee: Komatsu Ltd
Current assignee: Komatsu Ltd
Priority date: 1987-11-26
Filing date: 1988-11-25
Publication date: 1996-01-10
Anticipated expiration: 2008-11-25
Also published as: EP0399043A4; EP0399043A1; DE3854897T2; WO1989004894A1; DE3854897D1

Abstract

A working machine for construction vehicles, which is capable of being folded compactly when it is at rest and being converted into operational condition easily when it is to be operated, without requiring any special apparatus; and a method of operating the same working machine. This working machine is provided with a boom divided into first and second boom members, an arm connected pivotably to the front end portion of the second boom member, a bucket connected pivotably to the front end portion of the arm, and aplurality of hydraulic cylinders for driving these elements. The first boom member (12) is supported pivotably on one side of the base end portion thereof on a predetermined portion (13) of a chassis (10), and at a front end portion (15) thereof on one side of the base end portion of the second boom member (14). The first boom member (12) is adapted to be turned vertically by a first boom cylinder (17) which is supported pivotably at its base end portion on another predetermined portion (18) of the chassis, and at its front end section on a predetermined portion (20) of the front end section of the first boom member, the posture of the second boom member with respect to the first boom member being maintained by a retainer means (A).

Description

The present invention relates to a method for operating a working machine for construction vehicles.
A working machine similar to that one to be operated by the present method is disclosed by the JP-A 56-150235 and the JP-Y2 60-42123.
The working machine according to the JP-Y2 60-42123 comprises

a boom divided into a first and second boom member which are connected to a chassis in the mentioned order, said first boom member being vertically pivotably supported at one side of its proximal end in a pivot point on a predetermined position of said chassis through a bracket and being pivotably connected at its distal end to one side of the proximal end of said second boom member so that said second boom member is pivotable vertically;
an arm which is pivotably connected to the distal end portion of said second boom member;
an arm cylinder which is pivotably connected between the proximal end of said arm and a predetermined position of said second boom member;
a bucket which is pivotably attached to the distal end of said arm;
a bucket cylinder which is pivotably connected between the proximal end of said bucket and a predetermined position of said arm;
a first boom cylinder which is pivotably supported at its proximal end on said chassis and at its distal end on the distal end of said first boom member, to pivot said first boom member vertically; and a retainer means which is connected between said first and second boom members for maintaining said second boom member in a raised position with respect to said first boom member.

The first boom cylinder of this working machine is pivotably supported at its proximal end on the chassis of the working machine through a pivotable bell crank. The bell crank is pivoted by a separate bell crank cylinder to shift the proximal end of the first boom cylinder in such a manner that a dead point situation is avoided when pivoting the first boom by means of the first boom cylinder.
A similar working machine is shown in Figs. 1 and 2 and is described in connection with the method for operating the same as follows:
The working machine is, as shown in Figs. 1 and 2, arranged such that the proximal end portion of a first boom cylinder 3 is pivotably supported on a bell crank 4 that is pivotably supported on a chassis 1 through a bracket 2, and the bell crank 4 is driven by a bell crank cylinder 5, thereby folding the working machine in a compact form. In other words, in order to pivot the proximal end portion of the first boom cylinder 3 in such a manner that no dead point will occur in the link mechanism, it is necessary to provide not only the bell crank 4 and the bell crank cylinder 5 but also a hydraulic circuit means, including a valve and other elements, for controlling the bell crank cylinder 5. Accordingly, the number of parts increases and the structure becomes complicated, and the production cost also rises. In addition, the operation of the working machine also becomes complicated, and a failure is likely to occur. Thus, the prior art involves various problems.
It should be noted that the above-mentioned dead point is a position of the link mechanism in which the pivot point at which the first boom member and the distal end portion of the first boom cylinder are connected pivotably to each other, the pivot point at which the proximal end portion of the first boom member is pivotably attached to the chassis and the pivot point at which the proximal end portion of the first boom cylinder is pivotably attached to the chassis align with each other in a straight line during a pivoting motion of the first boom cylinder. When this position occurs, no hydraulic pressure from the first boom cylinder acts to the outside. In other words, the first boom cylinder cannot function as a driving member. In this application, the term "dead point" is used in the sense of the above.
JP-Y2 61-2200 describes a working machine comprising a one piece boom which is vertically pivotably supported at its proximal end on a chassis, an arm which is pivotably connected at its proximal end to the distal end of the boom, and a bucket which is pivotably attached to the distal end of the arm. The boom is driven by means of a boom cylinder which is arranged under the boom and attached at its distal end to a mid portion of the boom and at its proximal end to the chassis. The arrangement of the boom and the boom cylinder is such that the boom is always directed frontwardly with respect to the chassis and can be raised and lowered without passing a dead point of the boom cylinder. Further, a bucket retainer is provided on the forward end portion of the chassis.
JP-A 53-30105 discloses a working machine which differs from the aforementioned one by the fact that the boom cylinder is not disposed directly under the boom but is displaced laterally so that the boom can pivot between a forwardly directed and rearwardly directed position beyond a dead point of the boom cylinder. To pass this dead point the inertial force of the working machine is used.
It is an object of the present invention to provide a method of operating a working machine for construction vehicle of the type mentioned at the beginning which has a relatively simple arrangement with a reduced number of parts.
This object is achieved by a method of operating a working machine for construction vehicles as defined in claim 1.
According to the invention, a bell crank, a bell crank cylinder and hydraulic circuit means for controlling the bell crank cylinder are avoided, thereby lowering the production costs and making the working machine unlikely to have a failure. Further, the operation of the working machine is remarkably simplified.
To attain the object, there is provided a method of operating the working machine, wherein the first boom cylinder is caused to move beyond the dead point by making use of the hydraulic pressure from each of the cylinders, gravitational force and inertia force, which act on the working machine, and counterforce from a bucket retainer provided at the forward end of the chassis of the construct vehicle.
According to one mode of the present invention, the method of operating the working machine comprises the steps of: bringing the second boom cylinder into a floating state and expanding the first boom cylinder to raise the first boom member to near the dead point of the first boom cylinder; expanding and contracting, after the first boom cylinder has reached the vicinity of the dead point, the bucket cylinder to ensure the retaining of the bucket by the bucket retainer and, at the same time, expanding the arm cylinder with the first and second boom cylinders being brought into a floating state; further raising the first boom cylinder beyond the dead point by making use of the counterforce from the bucket retainer, hydraulic pressures from the bucket cylinder and the arm cylinder, or gravitational force and inertia force, which act on the working machine, thereby bringing the working machine to an operational position; expanding and contracting, after completion of work, the bucket cylinder to retain the bucket on the bucket retainer, and expanding the first boom cylinder and contracting the arm cylinder, with the second boom cylinder being brought into a floating state, to move the first boom member to near the dead point of the first boom cylinder; and gradually bringing, after the first boom member has passed the dead point of the first boom cylinder by the gravitational or inertia force acting on the working machine, the first and second boom cylinders into a floating state, or contracting the first boom cylinder and expanding the second boom cylinder, to fold the first and second boom members by making use of the gravity of the working machine.
According to another mode of the present invention, the method of operating the working machine comprises the steps of: expanding the first boom cylinder to raise the first boom member to near the dead point of the first boom cylinder; expanding and contracting, after the first boom cylinder has reached the vicinity of the dead point, the bucket cylinder to ensure the retaining of the bucket by the bucket retainer, and bringing the first boom cylinder into a floating state and, at the same time, expanding the arm cylinder; further raising the first boom cylinder beyond the dead point and tilting the first boom member toward the forward end of the chassis by making use of the counterforce from the bucket retainer, hydraulic pressures from the cylinders, or gravitational force and inertia force, which act on the working machine; expanding and contracting the arm cylinder to align a pin bore that is formed in the other side of the distal end portion of the first boom member and a pin bore that is formed in the other side of the proximal end portion of the second boom member; inserting, after the two pin bores have been aligned with each other, one or a plurality of fixing pins into the pin bores to maintain an operational position of the working machine; actuating, after completion of work, all the cylinders to retain the bucket on the bucket retainer; finely expanding and contracting, after the bucket has been retained, the first boom cylinder to remove the fixing pins; expanding, after the fixing pins have been removed, the first boom cylinder and the bucket cylinder while retaining the bucket on the bucket retainer by expanding the arm cylinder, to move the first boom member to near the dead point of the first boom cylinder; and gradually bringing, after the first boom member has passed the dead point of the first boom cylinder by the graviational or inertia force acting on the working machine, the first boom cylinder into a floating state to fold the first and second boom members by making use of the gravity of the working machine.
It should be noted that the above-described "floating state" of a cylinder is a state wherein no hydraulic pressure acts on the cylinder, which occurs when each of the ports at the rod and bottom sides of the cylinder is communicated with a tank, for example. In the present application, this term is used in the sense of the above in the following description.
The above and other objects, modes and advantages of the present invention will become apparent to those skilled in the art from the following description of preferred embodiments which are conformable to the principle of the present invention, taken in conjunction with the accompanying drawings.

Figs. 1 and 2 show a prior art, Fig. 1 being a schematic side view of a power shovel which is equipped with a conventional working machine, and Fig. 2 being a view that is employed to explain an operation of the conventional working machine that is shown in Fig. 1;
Fig. 3 is a schematic side view of a construction vehicle which is equipped with a working machine as being a first embodiment of the present invention;
Fig. 4 is a diagram showing a hydraulic control circuit which relates to hydraulic cylinders for driving the working machine according to the present invention;
Figs. 5 to 8 are views which are employed to explain an operation of the working machine that is shown in Fig. 3;
Fig. 9 is a fragmentary schematic view of a modified working machine;
Figs. 10 and 11 are views which are employed to explain the operation of the working machine that is shown in Fig. 3, as a first embodiment of the invention;
Fig. 12 is a schematic side view of a construction vehicle which is equipped with another modified working machine; and
Figs. 13 and 14 are views which are employed to explain an operation of the working machine that is shown in Fig. 12 as a second embodiment of the invention.

The present invention will be described below in more detail in conjunction with Figs. 3 to 14 in the accompanying drawings.
In Fig. 3, which shows a first embodiment of the present invention, reference numeral 10 denotes a chassis, which is provided with a mounting bracket 11. The proximal end portion of a first boom member 12 is vertically pivotably attached to the mounting bracket 11 throuth a pin member 13. The proximal portion of a second boom member 14 is attached vertically pivotably to the distal end portion of the first boom member 12 through a pin member 15. To the chassis 10 are secured brackets 16 which are located at the left and right sides, respectively, of the first boom member 12. The proximal end portion of a first boom cylinder 17 is connected to these brackets 16 through a pin member 18, and a piston rod 19 of the first boom cylinder 17 is connected to the upper portion of the first boom member 12 through a pin member 20.
A bracket 21 is provided at the rear side of the proximal portion of the first boom member 12. The proximal end portion of a second boom cylinder 22, which serves as a retainer means "A" for the second boom member 14, is connected to the bracket 21 through a pin member 23, and a piston rod 24 of the second boom cylinder 22 is connected to the proximal end portion of the second boom member 14 through a pin member 25.
An arm 26 is vertically pivotably attached at the lower side of its proximal portion to the distal end portion of the second boom member 14 through a pin member 27. A bracket 28 is provided on the proximal portion of the second boom member 14. The proximal end portion of an arm cylinder 29 is connected to the bracket 28 through a pin member 30, and a piston rod 31 of the arm cylinder 29 is connected to the proximal end portion of the arm 26 through a pin member 32.
A bucket 33 is pivotably attached to the distal end portion of the arm 26 through a pin member 34. One end portion of a link 36 is connected to a rear bracket 35 of the bucket 33, and one end portion of a link 37 is connected to the distal end portion of the arm 26. A bracket 38 is provided on the proximal portion of the arm 26. The proximal end portion of a bucket cylinder 39 is connected to the bracket 38 through a pin member 40, and a piston rod 41 of the bucket cylinder 39 is connected to the joint of the other end portions of the links 36 and 37 through a pin member 42.
It should be noted that an element which is denoted by reference numeral 43 at the lower side of the forward end portion of the chassis is a bucket retainer for retaining the bucket 33 when the working machine is folded.
Fig. 4 shows a hydraulic control circuit which relates to the hydraulic cylinders for driving the working machine of the present invention. Since this hydraulic control circuit is the same as the one which has heretofore generally been employed to operate this type of working machine, detailed description thereof is omitted.
An operation of the working machine that is shown in Fig. 3 will next be explained in conjunction with Fig. 3 and Figs. 5 to 8, which illustrate in combination an operation of the working machine. This method of operation includes step of but does not completely form the invention which will be further explained below.
Fig. 3 is a side view of the working machine that is folded compactly in such a condition that the first boom cylinder 17 is pivoted to a position below the pivot point 13 at which the proximal end portion of the first boom member 12 is pivotably attached to the chassis 10, and with the second boom cylinder 22 being expanded and the arm cylinder 29 being contracted so that the second boom member 14 and the arm 26 are substantially parallel to each other, the bucket 33 is retained by the bucket retainer 43 on the chassis 10 by the operation of a bucket actuating apparatus.
When the working machine that is in a folded position such as that shown in Fig. 3 is to be unfolded into an operational position, the second boom cylinder 22 is contracted to raise each of the elements of the working machine which are closer to the distal end of the machine than the second boom member 14, as shown in Fig. 5.
Subsequently, the first boom cylinder 17 is expanded. In consequence, the first boom member 12 rises in such a manner as to pivot upwardly, and the center of gravity G of the working machine also moves forwardly. However, since the ratio of the pivoting force applied to the first boom member L12 to the thrust from the first boom cylinder 17 decreases, the hydraulic pressure in the first boom cylinder 12 rises to reach a set relief pressure, so that the pivoting of the first boom member 12 is suspended. At this time, however, the second boom cylinder 22 is actuated so that the center of gravity G of the working machine moves to a position which is forward of the pivot point 13 at which the proximal end portion of the first boom member 12 is pivotably attached to the chassis 10.
It should be noted that, if the center of gravity G of the working machine has already moved to a position which is forward of the pivot point 13 before the hydraulic pressure in the first boom cylinder 17 reaches the set relief pressure, the second boom cylinder 22 need not be actuated.
It is also possible to move the center of gravity G of the working machine to a position which is forward of the pivot point 13 by making use of the inertia force acting on the working machine while the pivoting force applied to the first boom member 12 is sufficiently large relative to the thrust from the first boom cylinder 17.
If, in the above-described state, the first boom cylinder 17 is brought into a floating state, the working machine further pivots forwardly by gravity, as shown in Fig. 6, and consequently the first boom cylinder 17 passes the pivot point 13. Thereafter, the working machine assumes an operational position, as shown in Fig. 7, so that it is possible to perform work by an operation which is similar to that in the prior art.
The following is a description of an operation that is conducted to fold the working machine which is in an operational position. If the first boom cylinder 17 is expanded and the second boom cylinder 22 is contracted when the working machine is in the operational position that is shown in Fig. 7, the section of the working machine that is closer to the distal end than the second boom member 14 rises, and the first boom member 12 pivots rearwardly, so that the center of gravity G of the working machine moves rearwardly and its position approaches that shown in Fig. 6.
However, as the first boom cylinder 17 comes closer to the pivot point 13, the ratio of the pivoting force applied to the first boom member to the thrust from the first boom cylinder 17 decreases. Accordingly, the hydraulic pressure in the first boom cylinder rapidly rises to reach the set relief pressure, thus causing the pivoting of the first boom member 12 to be suspended. At this time, however, the second boom cylinder 22 is contracted so that the center of gravity G of the working machine moves to a position which is rearward of the pivot point 13, as shown in Fig. 8.
It should be noted that, if the center of gravity G of the working machine has already moved to a position which is rearward of the pivot point 13 before the hydraulic pressure in the first boom cylinder 17 reaches the set relief pressure, the second boom cylinder 22 need not be contracted.
It is also possible to move the center of gravity G of the working machine to a position which is rerward of the pivot point 13 by making use of the inertia force acting on the working machine while the pivoting force applied to the first boom member 14 is sufficiently large relative to the thrust from the first boom cylinder 17.
If, in the above-described state, the first boom cylinder 17 is brought into a floating state, the working machine pivots rearwardly by gravity and consequently the first boom cylinder 17 passes the pivot point 13, resulting in the working machine assuming a position such as that shown in Fig. 5.
Thereafter, the working machine can be brought into the folded position that is shown in Fig. 3 by the operation that has been described at the beginning of this embodiment. In Fig. 9, the point 20 at which the piston rod 19 of the first boom cylinder 17 is pivotably attached to the first boom member 12 is on the pin member 15 that connects together the first boom member 12 and the second boom member 14. The other arrangements and operation are the same as those describe above, and the method of operating is also the same as above, Detailed description thereof is therefore omitted.
Figs. 10 and 11 are views which are employed to explain one method of operating the above-described working machine.
When the working machine is changed from a folded position to an operational position, or vice versa, counterforce from the bucket retainer 43 is utilized in addition to the hydraulic pressure from each cylinder and the inertia force resulting from the gravity that acts on the working machine, while ensuring the retaining of the bucket by the bucket retainer 43.
As has been described above, the first boom cylinder 17 has "dead point in the link mechanism", which will be explained below. That is, the pivot point 20 at which the first boom member 12 and the distal end portion of the first boom cylinder 17 are connected pivotably to each other, the pivot point 13 at which one side of the proximal end portion of the first boom member 12 is pivotably attached to the chassis 10 and the pivot point 18 at which the proximal end portion of the first boom cylinder 17 is pivotably attached to the chassis 10 align with each other in a straight line C during a pivoting motion of the first boom cylinder 17. The position B on the straight line C of the pivot point 20 at which the distal end portion of the first boom cylinder 17 is pivotally attached is called the dead point of the first boom cylinder 17.
It should be noted that the border line beyond which the center of gravity G of the working machine moves forwardly of the chassis 10 to a position which is forward of the pivot point 13, at which the proximal end portion of the first boom member 12 is pivotally attached to the chassis 10, is, needless to say, equivalent to the dead point B.
The operating method that is shown in Figs. 10 and 11 is as follows:

(1) From the folded position to near the dead point B:
With the second boom cylinder 22 being brought into a floating state, the first boom cylinder 17 is expanded to raise the first boom member 12 to near the dead point B of the first boom cylinder 17.
(2) From near the dead point B to an operational position:
The distal end portion of the bucket 33 is retained on the bucket retainer 43 by expanding and contracting the bucket cylinder 39, and at the same time as the first and second boom cylinders 17 and 22 are brought into a floating state, the arm cylinder 29 is expanded to cause the first boom member to farther rise beyond the dead point. Thus, the working machine assumes an operational position.
(3) From the operational position to near the dead point B:
The distal end portion of the bucket 33 is retained on the bucket retainer 43 by expanding and contracting the bucket cylinder 39, and with the second boom cylinder 22 being brought into a floating state, the first boom cylinder 17 is expanded and, at the same time, the arm cylinder 29 is contracted to lower the first boom member 12 to near the dead point.
(4) From near the dead point B to the folded position:
After the first boom member 12 has lowered to near the dead point B, all the cylinders are gradually brought into a floating state, or the second boom cylinder 22 is brought into a floating state and the first boom cylinder 17 is contracted, thereby enabling the working machine to return to the folded position by gravity or by a combination of the gravity and the hydraulic pressure from the first boom cylinder 17.

A modified working machine is shown in Fig. 12. Unlike the first embodiment in which the second boom cylinder 22 is employed as a position retaining means A for the second boom member 14, the third embodiment employs as a position retaining means A for the second boom member 14 one or a plurality of fixing pins 45 which are inserted into a pin bore 25a that is bored in the other side of the distal end portion of the first boom member 12 and a pin bore 44 that is bored in the other side of the proximal end portion of the second boom member 14a, when these pin bores 25a and 44 align with each other.
A method of operating this embodiment will be explained below in conjunction with Figs. 12 to 14.

(1) From the folded position to near the dead point B: The first boom cylinder 17 is expanded to raise the first boom member 12 to near the dead point.
(2) From the dead point B to an operational position: The bucket cylinder 39 is expanded and contracted to retain the distal end portion of the bucket 33 on the bucket retainer 43, and while doing so, the first boom cylinder 17 is brought into a floating state and, at the same time, the arm cylinder 29 is expanded to cause the first boom member 12 to further rise beyond the dead point B.
(3) Retaining of the position of the second boom member 14:
The pin bore 25a that is bored in the second side of the distal end portion of the first boom member 12 and the pin bore 44 that is provided in the second side of the proximal end portion of the second boom member 14 are aligned with each other by finely actuating the first boom cylinder 17, and the fixing pin(s) 45 is inserted into the two pin bores aligned with each other by a manual operation. Thus, the position of the second boom member 14 is maintained by the first boom member 12 that is held by the first boom cylinder 17.
(4) Cancellation of the retaining of the position of the second boom member 14:
After the distal end portion of the bucket 33 has been retained on the bucket retainer 43 by expanding and contracting all the hydraulic cylinders, the fixing pin(2) 45 is loosened by finely actuating the first boom cylinder 17 and then removed by a manual operation. Thus, the first boom member 17 and the second boom member 14 become pivotable relative to each other.
(5) From the operational position to near the dead point B:
The first boom cylinder 17 and the bucket cylindor 39 are expanded and the arm cylinder 29 is also expanded to retain the distal end portion of the bucket 33 on the bucket retainer 43 and pivot the first boom member 12 to near the dead point B.
(6) From near the dead point B to the folded position:
After the first boom member 12 has pivoted (lowered) to near the dead point B, all the cylinders are gradually brought into a floating state, or the first boom cylinder 17 is contracted, thereby enabling the working machine to return to the folded position by gravity or a combination of the gravity and the hydraulic pressure from the first boom cylinder.

Claims

A method of operating a working machine for construction vehicles, comprising:
- a boom divided into a first and second boom member (12,14) which are connected to a chassis (10) in the mentioned order, said first boom member (12) being vertically pivotably supported at one side of its proximal end in a pivot point (13) on a predetermined position of said chassis (10) through a bracket (11) and being pivotably connected at its distal end to one side of the proximal end of said second boom member (14) so that said second boom member (14) is pivotable vertically;

- an arm (26) which is pivotably connected to the distal end portion of said second boom member (14);

- an arm cylinder (29) which is pivotably connected between the proximal end of said arm (26) and a predetermined position of said second boom member (14);

- a bucket (33) which is pivotably attached to the distal end of said arm (26);

- a bucket cylinder (39) which is pivotably connected between the proximal end of said bucket (33) and a predetermined position of said arm (26);

- a bucket retainer (43) provided on the forward end portion of said chassis (10) is adapted to retain the bucket (33) when the arm (26) is moved relative to the second boom member (14) to exert a counterforce on said second boom member (14);

- a first boom cylinder (17) which is pivotably supported at its proximal end on said chassis (10) and at its distal end on the distal end of said first boom member (12), to pivot said first boom member (12) vertically;

- said first boom cylinder (17) is directly connected at its proximal end on another predetermined position of said chassis (10) through another bracket (16);

- a retainer means (22,45) which is connected between said first and second boom members (12,14) for maintaining said second boom member (14) in a raised position with respect to said first boom member (12);

- the center of gravity (G) of the working machine being shiftable between a first position in front of said pivot point (13) and a second position behind said pivot point (13) by accordingly moving the second boom member (14) and/or the arm (26) relative to the first boom member (12), to move said first boom cylinder (17) beyond its dead point (B);

- said first boom cylinder (17) being moved beyond its dead point (B) by making use of the hydraulic pressure from each of said cylinders (17,22, 29,39), gravitational force which acts said working machine and counterforce from said bucket retainer (43).
A method according to claim 1, characterized by the steps of:
- providing a second boom cylinder (22) as retainer means connecting the first and second boom members (12,14);

- bringing said second boom cylinder (22) into a floating state and expanding said first boom cylinder (17) to raise said first boom member (12) to near said dead point (B);

- expanding and contracting, after said first boom cylinder (17) has reached the vicinity of its dead point (B), said bucket cylinder (39) to ensure the retaining of said bucket (33) by said bucket retainer (43) and, at the same time, expanding said arm cylinder (29) with said first and second boom cylinders (17,22) being brought into a floating state, to raise said first boom cylinder (17) beyond its dead point (B) by making use of the counterforce from said bucket retainer (43) and the hydraulic pressures from said bucket cylinder (39) and said arm cylinder (29), thereby bringing said working machine to an operational position;

- expanding and contracting, after completion of work, said bucket cylinder (39) to retain said bucket (33) on said bucket retainer (43), and expanding said first boom cylinder (17) and contracting said arm cylinder (29), with said second boom cylinder (22) being brought into a floating state, to move said first boom member (12) to near said dead point (B); and

- gradually bringing, after said first boom member (12) has passed said dead point (B) by the gravitational or inertia force acting on said working machine, said first and second boom cylinders (17,22) into a floating state, or bringing said second boom cylinder (22) into a floating state and contracting said first boom cylinder (17), to fold said first and second boom members (12,14) by making use of the gravitational force acting on said working machine.
A method according to to claim 1, characterized by the steps of:
- providing one or a plurality of pin members (45) as retaining means which are inserted into a first pin bore (25a) formed in the other side of the distal end portion of said first boom member (12) and into a second pin bore (44) formed in the other side of the proximal end portion of said second boom member (14), when these two pin bores (25a,44) are aligned with each other;

- expanding said first boom cylinder (17) to raise said first boom member (12) to near said dead point (B);

- expanding and contracting, after said first boom cylinder (17) has reached the vicinity of its dead point (B), said bucket cylinder (39) to ensure the retaining of said bucket (33) by said bucket retainer (43), and bringing said first boom cylinder (17) into a floating state and, at the same time, expanding said arm cylinder (33);

- further raising said first boom cylinder (17) beyond its dead point (B) and tilting said first boom member (12) toward the forward end of said chassis (10) by making use of the counterforce from said bucket retainer (43) and the hydraulic pressures from said bucket cylinder (39) and said boom cylinders (17,22);

- expanding and contracting said arm cylinder (29) to align said first and second pin bores (25a,44);

- inserting, after said two pin bores (25a,44) have been aligned with each other, said one or plurality of fixing pins (45) into said pin bores (25a,44) to maintain an operational position of said working machine;

- actuating, after completion of work, all said cylinders (17,29,39) to retain said bucket (33) on said bucket retainer (43);

- finally expanding and contracting, after said bucket (33) has been retained, said first boom cylinder (17) to remove said fixing pins (45);

- expanding, after said fixing pins (45) have been removed, said first boom cylinder (17) and said bucket cylinder (39) while retaining said bucket (33) on said bucket retainer (43) by expanding said arm cylinder (33), to move said first boom member (12) to near said dead point (B); and

- gradually bringing, after said first boom member (12) has passed said dead point (B) by the gravitational or the inertia force acting on said working machine, said first boom cylinder (17) into a floating state or contracting the same, to fold said first and second boom members (12,14) by making use of the gravitational force acting on said working machine.
A method according to one of the preceding claims, characterized in that the distal end portion of said first boom cylinder (17) is pivotably connected to a joint (15) of the distal end portion of said first boom member (12) and said one side of the proximal end portion of said second boom member (14).