Australian Patents Act 1990 - Regulation 3.2 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention Title Hydraulic circuit in construction machine The following statement is a full description of this invention, including the best method of performing it known to me/us: P/00/0Il 5 10(2 TECHNICAL FIELD 5 The present invention relates to a hydraulic circuit in a construction machine that allows various attachments to be exchangeably mounted via an adapter at the front end of an articulated work arm. Such attachments may include suspension-type work tools, such as a lifting magnet, a grapple 10 or a fork, which can be operated in a suspended state, or hinged work tools, such as a backhoe bucket, which is operated with a pivoting motion around a fixed hinge. BACKGROUND ART A backhoe bucket (a work tool such as a bucket to be 15 operated with a pivoting action may be referred to hereafter as a hinged work tool) is connected to the front end of an articulated work arm of a construction machine by installing a hydraulic cylinder at a front-end arm of the articulated work arm and linking the piston rod of the hydraulic cylinder to the work tool 20 via a link mechanism which is used to amplify the pivot angle of the work tool. The link mechanism is constituted with a link (hereafter referred to as an arm link) attached to the front end of the articulated work arm and a link (hereafter referred to as a work tool link), one end of which is connected to the arm link and 25 the piston rod of the hydraulic cylinder and the other end of which is connected to the work tool. If a grapple attached to a long front-end arm, such as on a material handling machine, is engaged in operation in a tilted state, the front-end arm is subjected to extra loads due to its length. For this reason, the grapple is usually engaged in work by operating the hydraulic cylinder to ensure that the grapple assumes a vertical orientation. However, the onus on the 5 operator, who must maintain the orientation of the grapple in the upright state, is bound to be considerable, and for this reason, it is desirable to operate the work tool (hereafter any work tool that may be engaged in operation in a suspended state is to be referred to as a suspension-type work tool) by releasing the link 10 between the hydraulic cylinder and the work tool via the link mechanism. However, as the link between the suspension-type work tool and the hydraulic cylinder via the link mechanism is released and the suspension-type work tool is attached to the front-end 15 arm in a suspended state, the end of the work tool link located on the work tool side, which is not linked to the suspension-type work tool, hangs loose. This loose end of the work tool link is bound to swing as the work arm is engaged in operation and the swinging work tool link may collide with and damage the 20 front-end arm. As a means for preventing the swinging movement of the work tool link during the operation of such a suspension-type work tool, Japanese Patent Publication No. 3767399 discloses a work tool link swing prevention device that includes a locking 25 member disposed at the front-end arm, so as to secure via a pin, the end of the work tool link to the locking member. However, the locking member that locks the work tool link onto the front-end arm as described above may become deformed or damaged by a force applied to the locking member via the work tool link as pressure oil escaping from the control valve into the hydraulic cylinder causes the hydraulic cylinder to extend. Accordingly, Japanese Patent Publication No. 3767399 discloses a hydraulic 5 circuit that includes a means for preventing the hydraulic cylinder from extending while a suspension-type work tool is operated with the work tool link locked to the front-end arm by shutting the pipeline connecting the control valve and the hydraulic cylinder, so as to prevent deformation of and damage 10 to the work tool link locking member due to extension of the hydraulic cylinder. As disclosed in Japanese Laid Open Patent Publication No. H9-209391 and Japanese Laid Open Patent Publication No. H11-107307, an adapter that allows a work tool to be 15 attached/detached with ease is often mounted at the front-end arm and any of various types of work tools is exchangeably attached to the adapter by linking the adapter to the hydraulic cylinder via a link mechanism at a construction machine, instead of directly attaching work tools to the front-end arm. The 20 use of such an adapter allows any of various types of work tools, each used in a specific type of work, to be exchangeably mounted easily and quickly. DISCLOSURE OF THE INVENTION 25 It would be possible to exchangeably replace work tools easily and quickly at a construction machine at which a suspension-type work tool such as a clamshell gripper or a hinged work tool such as a bucket, can be exchangeably mounted, as disclosed in Japanese Patent Publication No. 3767399, by exchangeably attaching the work tools to an adapter installed as a permanent fixture at the front-end arm, as disclosed in Japanese Laid Open Patent Publication No. 5 H9-209391 and Japanese Laid Open Patent Publication No. H 11-107307. However, the structure disclosed in Japanese Patent Publication No. 3767399, in which the work tool link is locked onto the front-end arm while the suspension-type work tool is 10 engaged in operation cannot be adopted in conjunction with a work tool link disposed as a permanent fixture at the adapter, as disclosed in Japanese Laid Open Patent Publication No. H9-209391 and Japanese Laid Open Patent Publication No. H 11-107307. Thus, a work tool replacement remains 15 labor-intensive and time-consuming. An object of the present invention, having been achieved by addressing the issues discussed above, is to provide a hydraulic circuit in a construction machine that includes an adapter attached to an articulated work arm and rotatably linked via a 20 link mechanism so as to be rotated by a hydraulic cylinder, which allows a suspension-type work tool sustaining a vertical orientation to be mounted at the adapter, successfully addresses the issue of the swinging movement of the work tool link while the suspension-type work tool is engaged in operation and 25 reduces the length of time and labor required for work tool replacement. The present invention provides a hydraulic circuit in a construction machine that includes a work tool mount adapter rotatably attached to a front-end arm of an articulated work arm with a hydraulic cylinder attached to the front-end arm so as to rotate the adapter via a link mechanism and allows a work tool, either a hinged work tool engaged in work with a pivoting action 5 as the hydraulic cylinder operates or a suspension-type work tool that may be used in work in a suspended state hanging from the front-end arm while assuming a vertical orientation regardless of how the tilt angle of the front-end arm may change, to be exchangeably attached to the adapter. The hydraulic circuit 10 comprises variable relief valves via which maximum oil pressures at pipelines connecting a bottom chamber and a rod chamber of the hydraulic cylinder to a control valve for the hydraulic cylinder, are set, check valves, each connected in parallel to one of the variable relief valves and a relief pressure 15 switch-over means capable of selectively switching pressure settings for the variable relief valves to a high-pressure-side relief pressure at which the hinged work tool can be rotated as pressure oil is supplied to the pipelines or a low-pressure side relief pressure at which the suspension-type work tool can rotate 20 to alter its orientation from a tilted orientation to a vertical orientation with its own weight as the suspension-type work tool is attached to the adapter. The hydraulic circuit in the construction machine according to the present invention may comprise an 25 electromagnetic valve via which relief pressure settings at the relief valves are switched and a switch that is installed in an operator's cab of the construction machine and switches the electromagnetic valve.
According to the present invention, the hinged work tool can be engaged in work by selecting the high-pressure-side relief pressure setting for the relief pressures at the variable relief valves and thus rotating the hinged work tool via the hydraulic 5 cylinder. The suspension-type work tool, on the other hand, is used with the relief pressures at the variable relief valves set to the low-pressure side relief pressure. As the articulated work arm moves, the tilt angle of the front-end arm changes relative to the vertical. Thus, the tilt angle of the suspension-type work tool 10 relative to the vertical, too, changes and the dead weight of the suspension-type work tool generates an oil pressure at the bottom chamber (or the rod chamber) of the hydraulic cylinder in correspondence to the tilt angle. The oil pressure thus generated at the bottom chamber (rod chamber), causes reflux of hydraulic 15 operating fluid to the rod chamber (bottom chamber) through the pipeline connected to the bottom chamber (rod chamber), the variable relief valve corresponding to the particular pipeline and the check valve connected to the rod chamber (bottom chamber)-side pipeline, the reflux of the hydraulic operating 20 fluid resets the suspension-type work tool into the vertical orientation and thus, the work tool is allowed to engage in work by sustaining the vertical orientation at all times. As a result, the operator is able to work with better ease and operability without having to perform any complicated operations in order to 25 maintain the suspension-type work tool in the vertical state. In addition, the adapter can be rotated to facilitate work tool replacement by selecting the high-pressure-side relief pressure setting for the relief pressures at the variable relief valves, so as to reduce both the length of time required and the labor required for the work tool replacement. When the suspension-type work tool is used, the work tool link is restrained while still linked to the adapter and thus, the 5 work tool link does not swing even as the tilt angle of the front-end arm changes during the work. Consequently, damage to the front-end arm that would otherwise occur as the swinging work tool link collides with the front-end arm, is prevented. The suspension-type work tool, engaged in work, remains 10 connected to the hydraulic cylinder via the link mechanism at all times. Thus, the swinging movement of the suspension-type work tool occurring due to inertia of the work tool as the upper revolving superstructure starts or stops revolving, can be minimized with the resistance imparted from the hydraulic 15 operating fluid flowing in the hydraulic cylinder, eliminating any need for the operator to perform a specific operation to prevent the swinging movement. Thus, the ease of operation is improved, the onus placed on the operator is reduced and better operability is afforded. In particular, by setting the low-pressure side relief 20 pressure to a level slightly higher than the tank pressure for the work performed with the suspension-type work tool, the swinging movement can be inhibited even more effectively. Furthermore, since the swinging movement of the work tool is inhibited, the work tool can be positioned with better ease. 25 Due to specific work requirements, the suspension-type work tool may need to be engaged in work while in a tilted state. Even under such circumstances, the work tool can be engaged in work in a tilted state simply by setting the relief pressures at the relief valves to the high-pressure-side relief pressure, assuring superior adaptability to various requirements that must be met at work sites. In addition, according to the present invention, the switch 5 operated to switch the relief pressure switch-over electromagnetic valve is installed in the operator's cab of the construction machine so as to enable easy and speedy relief pressure switch-over. 10 BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevation presenting an example of a construction machine that may adopt the hydraulic circuit according to the present invention; FIG. 2 shows in an enlargement the structure of the work 15 tool in FIG. 1 and the work tool mounting structure; FIG. 3 is a side elevation presenting an example of an adapter that may be used in conjunction with the present invention; FIGS. 4A - 4D show the procedure through which a work 20 tool is mounted via the adapter in FIG. 3; FIG. 5 is a side elevation showing a bucket representing an example of the hinged work tool used in the construction machine according to the present invention, attached to the arm; FIG. 6 is a side elevation showing a lifting magnet 25 representing an example of the suspension-type work tool used in the construction machine according to the present invention, attached to the arm; FIG. 7 is a hydraulic circuit diagram of the hydraulic circuit in the construction machine, achieved in an embodiment of the present invention; FIG. 8 is a plan view showing the location at which the relief 5 pressure switch-over switch is installed in the embodiment; and FIG. 9 is a hydraulic circuit diagram of the hydraulic circuit in the construction machine, achieved in another embodiment of the present invention. BEST MODE FOR CARRYING OUT THE INVENTION 10 FIG. 1 is a side elevation, presenting an example of a construction machine that may adopt the present invention. Reference numeral 1 indicates a construction machine that is engaged in loading work on a dock 3 near a wall 2 in order to load/unload cargo at a vessel 4. Reference numeral 5 indicates a 15 self-propelled lower traveling superstructure. While the lower traveling superstructure 5 in this example is a crawler-type superstructure, the present invention may be adopted in a construction machine on rubber tires or on metal wheels running on rails. 20 A tower 6 is installed on the frame of the lower traveling superstructure 5 and an upper revolving superstructure 8 is set at the apex of the tower 6 via a revolving device 7. A hydraulic power unit 10, an operator's cab 11, a counterweight 12 and the like are disposed on the upper revolving superstructure 8. From 25 a platform 13 located in the middle of the tower 6, maintenance and inspection of the hydraulic power unit 10 and the like disposed at the upper revolving superstructure 8 above are performed. The platform 13, set at a height at which the devices disposed at the upper revolving superstructure 8 can be inspected through the bottom of the upper revolving superstructure 8, also assumes a width measured along the front/rear direction, which is close to the front/rear width of the 5 upper revolving superstructure 8. Reference numeral 14 indicates a ladder providing access to the platform, through which the operator, workers and the like can reach the platform 13 from the ground. A safety railing 15 is installed around the platform 13. An access ladder 16 is installed at the upper 10 revolving superstructure 8 as well, so as to allow the operator and the like to climb up/down between the upper revolving superstructure 8 and the platform 13. A platform 17, surrounded by a railing 18, is disposed at the upper revolving superstructure 8 as well. 15 Reference numeral 20 indicates an articulated work arm attached to the upper revolving superstructure 8. The articulated work arm 20 includes a first arm 22 mounted at the upper revolving superstructure 8 so that it can be hoisted up/down via a hydraulic cylinder 21 and a second arm (hereafter 20 referred to as a front-end arm) 24 mounted at the front end of the first arm 22 so that it can be rotated freely along the up/down direction via a hydraulic cylinder 23. The articulated work arm 20 may assume an articulated structure that includes three or more arms. 25 A suspension-type work tool is attached to the front end of the front-end arm 24 via an adapter 25. As shown in the figure, the suspension-type work tool in the embodiment is a grapple 26. Reference numeral 28 is a hydraulic cylinder mounted at the front-end arm 24, which causes the work tool to rotate together with the adapter 25 via a link mechanism 29. FIG. 2 shows in an enlargement the mounting structure adopted when mounting the grapple 26 and the structure of the 5 grapple 26 itself. The adapter 25 is rotatably attached to the front end of the front-end arm 24 via a pin 30. As shown in FIG. 1, one end (tube 28a) of the hydraulic cylinder 28 is mounted by linking it via a pin 32 to a bracket 31 disposed at the top surface of the front-end arm 24. Links constituting the link mechanism 29 10 include an arm link 34, one end of which is tightly attached to the front-end arm 24 via a pin 33 and a work tool link 36, one end of which is tightly attached to the adapter 25 via a pin 35. The other end (piston rod 28b) of the hydraulic cylinder 28 is linked to the front ends of the links 34 and 36 via a pin 37. 15 The grapple 26 includes a bracket 42 located at the top thereof, which is mounted at the adapter 25. The bracket 42, constituted with two parallel plates, can be detachably linked to the adapter 25 via pins 40 and 41 with the bottom end of the adapter 25 placed between the two parallel plates. A frame 43 is 20 disposed below the bracket 42 and a plurality of holding tabs 45 are mounted at the frames 43 via individual attachment shafts 44 so as to open/close freely. Hydraulic cylinders 46 via which the tabs are opened/closed are disposed between the frame 43 and the individual holding tabs 45. 25 FIG. 3 is a side elevation showing the structure of the adapter 25. As shown in FIG. 3, a mounting portion (hub) 47 that links with the front-end arm 24 via the pin 30 and a mounting portion (hub) 48 that links with the work tool link 36 via the pin 35 are present at the top of the adapter 25. At an end of a lower frame 50 of the adapter 25, a fixed hook 51, at which a pin 40 linking the grapple 26 is hooked, is disposed. Reference numeral 52 indicates a movable hook that holds the pin 40 in place 5 between the movable hook 52 and the fix took 51 so as to ensure that the pin 40 does not slip out. The movable hook 52 is mounted so as to be allowed to rotate around a shaft 53 disposed at the lower frame 50. Reference numeral 54 indicates a movable arm rotatably 10 mounted so as to rotate around the pin 35 linking the adapter 25 with the work tool link 36. At the movable arm 54, an indented engaging portion 55, at which a pin 41 linking the grapple bucket 26 is engaged and locked. Reference numeral 56 indicates a hydraulic cylinder via which a work tool is attached to/detached 15 from the adapter 25. One end of the hydraulic cylinder 56 is linked via a pin 57 to the movable hook 52, whereas the other end of the hydraulic cylinder 56 is linked via a pin 58 to the movable arm 54. Reference numeral 59 indicates a pin hole formed at the lower frame 50, through which a lock pin 60 (see 20 FIG. 4D) that locks the movable hook 52 is inserted. FIGS. 4A ~ 4D illustrate the procedure through which the grapple 26 placed on the ground his attached to the adapter 25. As shown in FIG. 4A, the hydraulic cylinder 56 is contracted so as to place the movable arm 54 in close proximity to the lower 25 frame 50 and set the movable hook 52 in a released state separated from the fixed hook 51. Then, the first arm 22 in FIG. 1 is lowered so as to lower the adapter 25 and the fixed hook 51 is caused to engage with one of the pins, i.e., the pin 40, among the pins mounted in advance at the grapple 26, as shown in FIG. 4B. Next, as the first arm 22 in FIG. 1 is held up by a slight extent, the hydraulic cylinder 28 attached to the front-end arm 5 24 is set in the extended state so as to rotate the adapter 25 toward the operator's cab 11 and lift up the rear side of the adapter 25, thereby turning the fixed hook 51 to face upward, to prevent the pin 40 from slipping out, as shown in FIG. 4C. At the same time, the engaging portion 55 of the movable arm 54 is set 10 so as to place the engaging portion 55 in contact with the other pin 41 mounted in advance at the adapter 25, by facing opposite the pin 41. Subsequently, the movable arm 54 is caused to rotate by extending the hydraulic cylinder 56 and the engaging portion 55 15 is caused to engage at the pin 41 mounted at the grapple 26, as shown in FIG. 4D. At the same time, the movable hook 52 is engaged at the pin 40 so as to hold the pin 40 between the movable hook 52 and the fixed hook 51. Next, the lock pin 60 is inserted through the hole 59 formed at the lower frame 50 so as 20 to lock the movable hook 52 by restraining any movement of the movable hook 52. The grapple 26 is disengaged from the adapter 25 by placing the grapple 26 on the ground, pulling the lock pin 60 out of the lower frame 50, contracting the hydraulic cylinder 56 so as 25 to move the movable hook 52 away from the pin 40, rotating the movable arm 54 so as to disengage the engaging portion 55 from the pin 41, contracting the hydraulic cylinder 28 and lifting the first arm 22 up by a slight extent.
AL I FIG. 5 shows a backhoe bucket 61 representing an example of the hinged work tool mounted at the front-end arm 24 via the adapter 25. FIG. 6 shows a lifting magnet 62 representing an example of the suspension-type work tool mounted at the 5 front-end arm 24 via the adapter 25. It is to be noted that the grapple 26 or the lifting magnet 62 having been described as a suspension-type work tool may be used as a hinged work tool as well. In addition, a fork, for instance, may be used either as a hinged work tool or a suspension-type work tool. 10 FIG. 7 is a hydraulic circuit diagram of the hydraulic circuit according to the present invention as achieved in an embodiment. Reference numeral 63 indicates a main pump included in the hydraulic power unit 10, whereas reference numerals 64 and 65 respectively indicate a pilot pump and an oil 15 tank in the hydraulic power unit 10. Reference numeral 66 indicates a control valve installed in the hydraulic circuit, between the main pump 63 and the hydraulic cylinder 28. Reference numeral 67 indicates a pilot valve for the control valve 66. 20 As an operation lever 68 is operated along the direction indicated by an arrow 69 at the pilot valve 67, pilot pressure oil departing the pilot pump 64 is supplied into an operation chamber 66a of the control valve 66 via a pipeline 70, a valve 71 and a pipeline 72, thereby switching the control valve 66 to the 25 upper position. As a result, pressure oil from the main pump 63 flows into a bottom chamber 28c at the hydraulic cylinder 28 via ports 66c and 66d at the control valve 66 and a pipeline 75, whereas the hydraulic operating fluid in a rod chamber 28d flows back to the oil tank 65 through a pipeline 76, ports 66e and 66f at the control valve 66 and a pipeline 77, thereby causing the hydraulic cylinder 28 to extend. In contrast, as the operation lever 68 is operated along the 5 direction indicated by an arrow 78, pilot pressure oil departing the pilot pump 64 is supplied into an operation chamber 66b of the control valve 66 via the pipeline 70, a valve 79 and a pipeline 80, thereby switching the control valve 66 to the lower position. As a result, pressure oil from the main pump 63 flows into the 10 rod chamber 28d at the hydraulic cylinder 28 via the ports 66c and 66e at the control valve 66 and the pipeline 76, whereas the hydraulic operating fluid in the bottom chamber 28c flows back to the oil tank 65 through the pipeline 75, the ports 66d and 66f at the control valve 66 and the pipeline 77, thereby causing the 15 hydraulic cylinder 28 to contract. Reference numerals 82 and 83 respectively indicate a variable relief valve inserted between the pipeline 75 connecting the control valve 66 to the bottom chamber 28c and the oil tank 25 and a variable relief valve inserted between the pipeline 76 20 connecting the control valve 66 to the rod chamber 28d and the oil tank 65. Reference numerals 93 and 94 indicate check valves respectively disposed in parallel to the variable relief valves 82 and 83. The variable relief valves 82 and 83 respectively include operation chambers 82a and 83a, and when the pilot pressure oil 25 is supplied into the operation chambers 82a and 83a, the relief pressures at the variable relief valves 82 and 83 are both set to a high level (e.g., 17.6 MPa). The high-pressure-side relief pressure setting must be set when work is conducted by rotating a hinged work tool such as the backhoe bucket 61 shown in FIG. 5. However, when the pilot pressure oil is not supplied into the operation chambers 82a and 83a, the relief pressures at the variable relief valves 82 and 83 are set to a low level (e.g., 3.9 5 MPa). At the low-pressure side relief pressure setting, a suspension-type work tool such as the grapple 26 shown in FIGS. 1 and 2 can be rotated to assume a vertical orientation under its own weight. Reference numeral 84 indicates an electromagnetic valve 10 installed to enable relief pressure switch-over at the variable relief valves 82 and 83. Reference numeral 85 indicates a power source mounted at the upper revolving superstructure 8 and reference numeral 86 (see FIG. 8) indicates a switch installed in the operator's cab 11, which is operated to switch the 15 electromagnetic valve 84. The switch 86 is turned off (set in the open state) when a hinged work tool such as the backhoe bucket 61 is used in operation. In this situation, no power is supplied to a solenoid 84a at the electromagnetic valve 84 and thus, the electromagnetic valve 84 assumes the lower position in the figure. 20 As a result, the pilot pressure oil from the pilot pump 64 is supplied into the operation chambers 82a and 83a at the variable relief valves 82 and 83 via a pipeline 87, the electromagnetic valve 84, a pipeline 88 and pipelines 89 and 90, thereby setting the relief pressures at the variable relief valves 82 25 and 83 to the high level. Thus, when the hydraulic cylinder 28 is engaged in extending/contracting operation with the control valve 66 switched to the upper position or the lower position, the oil pressure at the pipelines 75 and 76 is set equal to or higher L.I than the high-pressure-side relief pressure, enabling rotation of the hinged work tool. When engaging a suspension-type work tool in work in a suspended state, hanging from the front-end arm 24, the switch 5 86 is turned on (set in the closed state). As a result, power from the power source 85 is supplied to the solenoid 84a at the electromagnetic valve 84, switching the electromagnetic valve 84 to the upper position. This, in turn, causes the secondary pipeline 88 of the electromagnetic valve 84 to communicate via 10 the electromagnetic valve 84 with a pipeline 91 connected to the oil tank 65. Consequently, the oil pressure in the operation chambers 82a and 83a become equal to the tank pressure, thereby setting the relief pressures at the variable relief valves 82 and 83 to the low level. 15 If the gravitational center G of the grapple 26, mounted via the adapter 25, as shown in FIG. 2, assumes a position further frontward relative to the pin 30 (toward the opposite side from the upper revolving superstructure 8) while the relief pressures at the variable relief valves 82 and 83 are set to the low pressure 20 side, as described above, the weight F of the grapple 26 itself generates a rotational force centered on the pin 30 and imparted along the direction indicated by the arrow 92. Under these circumstances, an oil pressure attributable to the weight F of the grapple is generated in the rod chamber 28d 25 at the hydraulic cylinder 28. Provided that the relief pressure is set equal to or higher than 3.9 MPa, i.e., the low-pressure side relief pressure setting, when the tilt angle of the line connecting the gravitational center G and the pin 30, formed relative to the vertical, is equal to or greater than the angle that can be substantially regarded to represent verticality, the grapple 26 is caused to rotate under its own weight even though the control valve 66 assumes the neutral position in the figure. In other 5 words, the hydraulic operating fluid in the rod chamber 28d flows into the bottom chamber 28c through the pipeline 76, the variable relief valve 83, the check valve 93 and the pipeline 75. The difference between the sectional area of the rod chamber 28d and the sectional area of the bottom chamber 28c causes the 10 bottom chamber 28c to be filled with the hydraulic operating fluid flowing therein from the oil tank 65 through the pipeline 77, the check valve 93 and the pipeline 75. If the gravitational center G is located further rearward relative to the pin 30, the oil pressure attributable to the weight 15 of the work tool is generated in the bottom chamber 28c. Provided that this oil pressure is equal to or greater than the low-pressure side relief pressure setting, the hydraulic operating fluid in the bottom chamber 28c flows into the rod chamber 28d through the pipeline 75, the variable relief valve 82, the check 20 valve 94 and the pipeline 76, even with the control valve 66 in the neutral position. The difference between the sectional areas of the rod chamber 28d and the bottom chamber 28c causes part of the hydraulic operating fluid leaving the bottom chamber 28c to flow out into the oil tank 65 via the pipeline 77. 25 Compared to the oil pressure generated when the work tool is tilted forward, the oil pressure generated in the bottom chamber 28c attributable to the weight of the work tool itself, as described above, i.e., the oil pressure generated when the work tool is tilted rearward, is lower due to the difference between the sectional areas of the rod chamber 28d and the bottom chamber 28c. As long as matching low-pressure side relief pressures are assumed for the variable relief valves 82 and 83, the smallest tilt 5 angle at which rotation attributable to the weight is enabled is slightly greater when the work tool is tilted rearward. This rotation-enabling smallest tilt angle can be set smaller by lowering the low-pressure side relief pressure setting, which, in turn, can be adjusted in correspondence to the weight of the 10 work tool, the type of the work tool and the like. It is desirable, from the viewpoint of work efficiency, to set the smallest tilt angle that can be substantially regarded to represent the vertical orientation, to 10* or less. In addition, the low-pressure side relief pressure for the variable relief valve 82 located toward the 15 bottom chamber 28c with the greater sectional area can be set lower than the low-pressure side relief pressure for the variable relief valve 83 located toward the rod chamber 28d with the smaller sectional area at the hydraulic cylinder 28, so as to equalize the front-side smallest tilt angle and the rear-side 20 smallest tilt angle of the grapple 26. In the embodiment described above, equipped with the variable relief valves 82 and 83 via which the oil pressures in the pipelines 75 and 76 connected to the hydraulic cylinder 28 are set, a suspension-type work tool such as the grapple 26 is 25 engaged in work by setting the relief pressures at the variable relief valves 82 and 83 to the low-pressure side relief pressure setting. Thus, regardless of the angle of the front-end arm 24, the suspension-type work tool such as the grapple 26 is caused to rotate to assume the vertical orientation under its own weight and is engaged in work in the suspended state. Since the operator does not have to perform any complicated operation of the hydraulic cylinder 28 in order to keep the suspended work 5 tool in the vertical orientation, ease of operation is afforded and, at the same time, the operability improves. In addition, work tools can be exchanged with ease by setting the relief pressures at the variable relief valves 82 and 83 to the high-pressure-side relief pressure setting and thus allowing the adapter 25 to rotate 10 to facilitate the replacement procedure. Consequently, regardless of whether a hinged work tool or a suspension-type work tool is attached to the front-end arm 24 via the adapter 25, the work tool can be engaged in work in a desirable rotation mode or suspension mode and the work tool 15 can be replaced with ease. When a suspension-type work tool is attached, the work tool link 36 is restrained while linked to the adapter 25 and thus, the work tool link 36 does not swing even as the articulated work arm 20 moves. Thus, damage to the front-end arm 24, which would otherwise occur as the swinging 20 work tool link 36 collides with the front-end arm, is prevented. In addition, a suspension-type work tool such as the grapple 26 or the lifting magnet 62 engaged in work remains connected to the hydraulic cylinder 28 via the link mechanism 29 at all times. Thus, any swinging movement attributable to the 25 inertia of the work tool 26 or 62, which may occur as the upper revolving superstructure 8 starts or stops revolving, can be inhibited with the resistance imparted from the hydraulic operating fluid flowing between the bottom chamber 28c and the rod chamber 28d at the hydraulic cylinder 28 and flowing from the chambers 28c and 28d to the oil tank 65. As a result, there is no need for the operator to perform any operation specifically to prevent such a swinging movement, the onus placed on the 5 operator can be reduced and the operability can be improved in this regard as well. Furthermore, since the swinging movement of the work tool is inhibited, it can be positioned with better ease. It is to be noted that while the low-pressure side relief pressure may be set equal to the tank pressure, the swinging movement 10 can be inhibited even more effectively by setting the low-pressure side relief pressure for the suspension-type work tool to a level slightly higher than the tank pressure, as has been described in reference to the embodiment. Moreover, if the grappling target object for the 15 suspension-type work tool such as the grapple 26 is present under a certain structure, the relief pressures at the variable relief valves 82 and 83 may be set to the high-pressure-side relief pressure so as to grasp the target object with the grapple 26 in a lateral orientation. In other words, the suspension-type work tool 20 can be engaged in operation in diverse work modes. In the embodiment, the switch 86 operated to switch the relief pressure setting at the switchover circuit for the variable relief valves 82 and 83 is installed in the operator's cab 11 of the construction machine, as shown in the plan view presented in 25 FIG. 8, and thus, the relief pressure switchover can be achieved quickly and easily in the embodiment. While the relief pressures at the variable relief valves 82 and 83 are switched by switching the electromagnetic valve 84 via the switch 86 in the embodiment, the relief pressure switchover may instead be achieved via manual operating means 82b and 83b such as those shown in FIG. 9. The present invention is not limited to the embodiments 5 described above and it may be adopted in any of various other construction machines at which a work tool that may be engaged in operation in a suspended state, is attached to an articulated work arm. Throughout this specification and the claims which follow, 10 unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. 15 The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the 20 common general knowledge in the field of endeavour to which this specification relates. 25