Hydraulic circuit of construction machine

Abstract

To provide a hydraulic circuit of a construction machine that can reduce swinging of a boom when the boom stops operating.SOLUTION: A hydraulic circuit 16 comprises: a boom switching valve 26 switching a flow direction of hydraulic oil from a hydraulic pump 22 to a boom cylinder 24; a boom operating device 28 outputting a signal to operate the boom switching valve 26; an electromagnetic proportional relief valve 44 installed between a boom cylinder rod line 38 and a hydraulic oil tank 32 and having a set pressure adjusted to a first value; and a controller 70 reducing the set pressure of the electromagnetic proportional relief valve 44 to a second value smaller than the first value when an operation amount of the boom operating device 28 decreases from a predetermined amount or more to less than the predetermined amount. The second value is smaller than a peak pressure generated in the boom cylinder rod line 38 due to swinging of the boom when the operation amount of the boom operating device 28 decreases from a predetermined amount or more to less than the predetermined amount.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a hydraulic circuit for a construction machine. [Background technology]

[0002] A hydraulic excavator, a typical example of a construction machine, comprises a lower chassis, an upper rotating body supported by the lower chassis, and a front working implement attached to the upper rotating body. The front working implement includes a boom connected to the upper rotating body and an arm connected to the boom.

[0003] The hydraulic circuit of the hydraulic excavator also includes a hydraulic pump, a boom cylinder that operates the boom, a boom switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the boom cylinder, a boom operating device that outputs a signal to operate the boom switching valve, an arm cylinder that operates the arm, an arm switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the arm cylinder, and an arm operating device that outputs a signal to operate the arm switching valve.

[0004] An overload relief valve is provided in the boom cylinder rod line connecting the boom cylinder rod-side oil chamber and the boom switching valve to prevent an abnormally high pressure from occurring in the boom cylinder rod line due to an external force.Similarly, an overload relief valve is provided in the boom cylinder head line connecting the boom cylinder head-side oil chamber and the boom switching valve to prevent an abnormally high pressure from occurring in the boom cylinder head line due to an external force (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Microfilm of Utility Model Application No. 63-11195 (Utility Model Application No. 1-119445) Summary of the Invention [Problem to be solved by the invention]

[0006] Hydraulic excavators often suffer from a problem where the boom swings (vibrates) against the operator's will when the boom or arm is stopped. When the boom or arm is stopped, the pressure in the boom cylinder rod line and the boom cylinder head line can fluctuate significantly due to the inertia of the boom or arm, which can cause this problem. The peak pressure in the boom cylinder rod line and the boom cylinder head line when this problem occurs is lower than the set pressure of the overload relief valve. Therefore, hydraulic oil cannot be released from the overload relief valve when the boom or arm is stopped, which can prevent the boom from swinging. Unintended boom swings not only cause discomfort to the operator, but also reduce productivity because it becomes difficult to start the next operation until the front implement stops swinging.

[0007] An object of the present invention is to provide a hydraulic circuit for a construction machine that can reduce the swinging of the boom when the boom or arm stops operating. [Means for solving the problem]

[0008] According to the present invention, there is provided the following hydraulic circuit for a construction machine that solves the above-mentioned problems: "A hydraulic circuit for a construction machine comprising a lower traveling body, an upper rotating body supported on the lower traveling body, and a front working implement attached to the upper rotating body, the front working implement including a boom connected to the upper rotating body and an arm connected to the boom, A hydraulic pump; a boom cylinder that operates the boom; a boom switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the boom cylinder; a boom operating tool that outputs a signal to operate the boom switching valve; a boom cylinder rod line connecting a rod-side oil chamber of the boom cylinder and the boom switching valve; a boom cylinder head line connecting a head side oil chamber of the boom cylinder and the boom switching valve; an electromagnetic proportional relief valve installed between the boom cylinder rod line and a hydraulic oil tank, the set pressure of which is adjusted to a first value; an arm cylinder that operates the arm; an arm switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the arm cylinder; an arm operating device that outputs a signal to operate the arm switching valve; a controller that reduces the set pressure of the electromagnetic proportional relief valve to a second value that is smaller than the first value when the operation amount of the boom operation device or the arm operation device decreases from a predetermined amount or more to less than the predetermined amount, The second value is smaller than the peak pressure generated in the boom cylinder rod line due to the boom swinging when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount.

[0009] Preferably, the controller changes the second value in accordance with the weight of the front working implement. The controller may change the second value in accordance with the attitude of the front working implement.

[0010] The controller can reduce the set pressure of the electromagnetic proportional relief valve to the second value for a predetermined time from when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount.

[0011] It is desirable that the controller reduce the set pressure of the electromagnetic proportional relief valve to the second value when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount and when the rate of decrease per unit time of the operation amount of the boom operation device or the arm operation device is greater than a predetermined value.

[0012] When the operation amount of the boom operation tool or the arm operation tool decreases from equal to or greater than the predetermined amount to less than the predetermined amount and the pressure in the boom cylinder head line is greater than a predetermined threshold, the controller preferably reduces the set pressure of the electromagnetic proportional relief valve to the second value. The threshold is preferably greater than the pressure in the boom cylinder head line when the tip of the front working implement is pressed against the ground to tilt the undercarriage and the upper rotating body. [Effects of the Invention]

[0013] In the hydraulic circuit of a construction machine according to the present invention, when the amount of operation of the boom operating device or the arm operating device decreases from a predetermined amount or more to less than the predetermined amount, the set pressure of the electromagnetic proportional relief valve is reduced to a second value smaller than the first value. The second value is smaller than the peak pressure that occurs in the boom cylinder rod line due to the boom swing when the amount of operation of the boom operating device or the arm operating device decreases from a predetermined amount or more to less than the predetermined amount. Therefore, according to the present invention, hydraulic oil can easily escape from the boom cylinder rod line to the hydraulic oil tank when operation of the boom or arm stops, thereby reducing boom swing when operation of the boom or arm stops. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic diagram of a hydraulic excavator equipped with a hydraulic circuit according to the present invention. [Figure 2] FIG. 2 is a hydraulic circuit diagram according to the present invention. [Figure 3] FIG. 2 is a schematic diagram showing the hydraulic excavator shown in FIG. 1 in a state where the tip of the front working implement is pressed against the ground and the lower body and upper rotating body are tilted. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of a hydraulic circuit for a construction machine according to the present invention will now be described with reference to the drawings.

[0016] (Construction machinery: Hydraulic excavator 2) 1 shows a hydraulic excavator 2 on which a hydraulic circuit according to the present invention can be installed. The hydraulic excavator 2 includes a lower traveling unit 4, an upper rotating unit 6 rotatably supported on the lower traveling unit 4, and a front working unit 8 attached to the upper rotating unit 6. The front working unit 8 includes a boom 10 connected to the upper rotating unit 6, an arm 12 connected to the boom 10, and a bucket 14 connected to the arm 12.

[0017] (Hydraulic circuit 16) 2 shows the hydraulic circuit 16 of the hydraulic excavator 2. The hydraulic circuit 16 has a boom circuit 18 for operating the boom 10 and an arm circuit 20 for operating the arm 12. Although not shown, in addition to the boom circuit 18 and the arm circuit 20, the hydraulic circuit 16 also has a bucket circuit for operating the bucket 14, a swing circuit for rotating the upper swing structure 6 relative to the lower running structure 4, a travel circuit for operating the travel motor of the lower running structure 4, and the like.

[0018] (Boom Circuit 18) The boom circuit 18 includes a hydraulic pump 22, a boom cylinder 24, a boom switching valve 26, and a boom operating device 28.

[0019] (Hydraulic pump 22) The hydraulic pump 22 is driven by a drive source 30 such as an engine or an electric motor, and discharges hydraulic oil drawn from a hydraulic oil tank 32 to a pump line 34. A variable displacement pump may be used as the hydraulic pump 22.

[0020] (Boom cylinder 24) The boom cylinder 24 operates the boom 10 from the connection point between the upper rotating body 6 and the boom 10. As shown in FIG. 1 , a cylinder tube 24a of the boom cylinder 24 is connected to the upper rotating body 6, and a piston rod 24b of the boom cylinder 24 is connected to the boom 10.

[0021] (Boom switching valve 26) The boom switching valve 26 switches the flow direction of hydraulic oil from the hydraulic pump 22 to the boom cylinder 24. The boom switching valve 26 shown in Figure 2 is an electromagnetic proportional type, but it may also be a hydraulic pilot type. The boom switching valve 26 is connected to the hydraulic pump 22 via a pump line 34, and is connected to the hydraulic oil tank 32 via a return line 36. The boom switching valve 26 is also connected to the rod-side oil chamber 24c of the boom cylinder 24 via a boom cylinder rod line 38, and is connected to the head-side oil chamber 24d of the boom cylinder 24 via a boom cylinder head line 40.

[0022] The boom cylinder rod line 38 is connected to the hydraulic oil tank 32 via a first relief line 42. An electromagnetic proportional relief valve 44 with a variable set pressure is installed in the first relief line 42. The set pressure of the electromagnetic proportional relief valve 44 is adjusted to a first value (for example, approximately 30 MPa). However, when a predetermined condition is satisfied, the set pressure of the electromagnetic proportional relief valve 44 is changed to a second value that is smaller than the first value.

[0023] The boom cylinder head line 40 is connected to the hydraulic oil tank 32 via a second relief line 46. A relief valve 48 with a constant set pressure (for example, about 30 MPa) is installed in the second relief line 46. In addition, a pressure sensor 50 is installed in the boom cylinder head line 40.

[0024] (Boom control device 28) The boom operating device 28 outputs a signal for operating the boom switching valve 26. The boom operating device 28 may be configured to have an input device (for example, a joystick) that outputs a signal with increasing strength as the amount of operation increases. While Fig. 2 shows an example in which the boom operating device 28 outputs an electrical signal, the boom operating device 28 may also be configured to output a hydraulic signal.

[0025] (Arm circuit 20) The arm circuit 20 includes the hydraulic pump 22, an arm cylinder 52, an arm switching valve 54, and an arm operating device 56.

[0026] (Arm cylinder 52) The arm cylinder 52 operates the arm 12 from the connection point between the boom 10 and the arm 12. As shown in FIG. 1, a cylinder tube 52a of the arm cylinder 52 is connected to the boom 10, and a piston rod 52b of the arm cylinder 52 is connected to the arm 12.

[0027] (Arm switching valve 54) The arm switching valve 54 switches the flow direction of hydraulic oil from the hydraulic pump 22 to the arm cylinder 52. The arm switching valve 54 shown in Fig. 2 is of an electromagnetic proportional type, but it may also be of a hydraulic pilot type. The arm switching valve 54 is connected to the hydraulic pump 22 via the pump line 34, and is connected to the hydraulic oil tank 32 via the return line 36. In addition, the arm switching valve 54 is connected to a rod-side oil chamber 52c of the arm cylinder 52 via an arm cylinder rod line 58, and is connected to a head-side oil chamber 52d of the arm cylinder 52 via an arm cylinder head line 60.

[0028] The arm cylinder rod line 58 is connected to the hydraulic oil tank 32 via a third relief line 62. A relief valve 64 with a constant set pressure (for example, about 30 MPa) is installed in the third relief line 62.

[0029] The arm cylinder head line 60 is connected to the hydraulic oil tank 32 via a fourth relief line 66. A relief valve 68 with a constant set pressure (for example, about 30 MPa) is installed in the fourth relief line 66.

[0030] (Arm operating device 56) The arm operating device 56 outputs a signal for operating the arm switching valve 54. The arm operating device 56 may be configured to have an input device (for example, a joystick) that outputs an electric signal whose strength increases as the amount of operation increases. While Fig. 2 shows an example in which the arm operating device 56 outputs an electric signal, the arm operating device 56 may also be configured to output a hydraulic signal.

[0031] (Controller 70) The hydraulic circuit 16 includes a controller 70 that executes circuit control based on signals output from the boom manipulator 28 and the arm manipulator 56. The controller 70 is composed of a computer having a processing device and a storage device. When the signals output from the boom manipulator 28 and the arm manipulator 56 are hydraulic signals, the output hydraulic signals are detected by a pressure sensor (not shown), and the detection result of the pressure sensor is input to the controller 70.

[0032] (Attitude detection means 72) The controller 70 of this embodiment receives the detection results of posture detection means 72, which detects the posture of the front work implement 8. The posture detection means 72 may be, for example, a combination of a boom angle detection sensor that detects the angle of the boom 10 relative to the upper rotating body 6, an arm angle detection sensor that detects the angle of the arm 12 relative to the boom 10, and a bucket angle detection sensor that detects the angle of the bucket 14 relative to the arm 12.

[0033] (Operation of hydraulic circuit 16) Next, the operation of the hydraulic circuit 16 as described above will be described.

[0034] When neither the boom operating device 28 nor the arm operating device 56 is being operated (when each operating device 28, 56 is in the neutral position), no signal is output from each operating device 28, 56 to the controller 70. In this case, the controller 70 positions each of the switching valves 26, 54 in the closed position. As a result, no hydraulic oil is supplied to the boom cylinder 24 and the arm cylinder 52, and the boom 10 and the arm 12 do not operate.

[0035] When the boom operating device 28 and the arm operating device 56 are operated by a predetermined amount or more, a signal is output from each operating device 28, 56 in accordance with the amount of operation of each operating device 28, 56. In this case, the controller 70 opens the boom switching valve 26 and adjusts the aperture of the boom switching valve 26 in accordance with the signal output from the boom operating device 28, and similarly opens the arm switching valve 54 and adjusts the aperture of the arm switching valve 54 in accordance with the signal output from the arm operating device 56. As a result, hydraulic oil is supplied from the hydraulic pump 22 to the boom cylinder 24 via the boom switching valve 26, and hydraulic oil is supplied from the hydraulic pump 22 to the arm cylinder 52 via the arm switching valve 54. This causes the boom 10 and the arm 12 to operate.

[0036] The "predetermined amount" related to the amount of operation of the boom operation device 28 is a reference value for determining whether or not the boom 10 will operate. For example, the boom operation device 28 may be configured to output a signal when the amount of operation is equal to or greater than the predetermined amount, and not to output a signal when the amount of operation is less than the predetermined amount. Alternatively, the controller 70 may be configured to open the boom switching valve 26 when the amount of operation of the boom operation device 28 is equal to or greater than the predetermined amount, and not to open the boom switching valve 26 when the amount of operation of the boom operation device 28 is less than the predetermined amount. The same applies to the "predetermined amount" related to the amount of operation of the arm operation device 56. The "predetermined amount" may be set to any value (for example, approximately 5%).

[0037] The operating speed of the boom 10 and the arm 12 changes depending on the amount of operation of the boom manipulator 28 and the arm manipulator 56. That is, as the amount of operation of the boom manipulator 28 increases, the amount of hydraulic oil supplied to the boom cylinder 24 increases, and the operating speed of the boom 10 increases. Similarly, as the amount of operation of the arm manipulator 56 increases, the amount of hydraulic oil supplied to the arm cylinder 52 increases, and the operating speed of the arm 12 increases.

[0038] When the operation amount of the boom operating device 28 and the arm operating device 56 decreases from a predetermined amount or more to less than the predetermined amount, the controller 70 switches the boom switching valve 26 and the arm switching valve 54 to the closed position, thereby stopping the supply of hydraulic oil from the hydraulic pump 22 to the boom cylinder 24 and the arm cylinder 52, and causing the operation of the boom 10 and the arm 12 to stop.

[0039] When the operation of the boom 10 and the arm 12 stops, if the tip of the front work implement 8 (the bucket 14 in this embodiment) is off the ground, as shown in FIG. 1 , the inertia of the front work implement 8 may cause the pressure in the boom cylinder rod line 38 and the boom cylinder head line 40 to fluctuate (vibrate) significantly. In this case, the boom 10 may swing (vibrate) against the operator's will. Note that the boom 10 may swing against the operator's will not only when the operation of both the boom 10 and the arm 12 stops, but also when the operation of either the boom 10 or the arm 12 stops.

[0040] Therefore, in order to reduce unintended swinging of the boom 10 by the operator, the controller 70 of this embodiment reduces the set pressure of the electromagnetic proportional relief valve 44 to a second value smaller than the first value when the operation amount of the boom manipulator 28 or the arm manipulator 56 decreases from a predetermined amount or more to less than the predetermined amount. The second value is smaller than the peak pressure that occurs in the boom cylinder rod line 38 due to swinging of the boom 10 when the operation amount of the boom manipulator 28 or the arm manipulator 56 decreases from a predetermined amount or more to less than the predetermined amount. This makes it easier for hydraulic oil to escape from the boom cylinder rod line 38 to the hydraulic oil tank 32 when operation of the boom 10 or the arm 12 stops, thereby reducing swinging of the boom 10 when operation of the boom 10 or the arm 12 stops.

[0041] (Second value: can be changed depending on the weight of the front work implement 8) The second value may be a constant value (for example, about 3 MPa), or may be changed according to the weight of the front working implement 8. In the hydraulic excavator 2, the bucket 14 is replaced with another work tool depending on the work content. This may change the weight of the front working implement 8. Therefore, it is preferable that the controller 70 change the second value according to the weight of the front working implement 8. This makes it possible to quickly reduce the swing of the boom 10 when the operation of the boom 10 or the arm 12 is stopped, regardless of the weight of the front working implement 8.

[0042] The controller 70 can change the second value as appropriate depending on the weight of the front work implement 8. For example, the controller 70 can increase the second value as the weight of the front work implement 8 increases, and decrease the second value as the weight of the front work implement 8 decreases. Conversely, the controller 70 can decrease the second value as the weight of the front work implement 8 increases, and increase the second value as the weight of the front work implement 8 decreases. Note that the weight of the front work implement 8 is input into the controller 70 in advance, but when the bucket 14 is replaced with another work tool, the weight of the front work implement 8 after the replacement is input into the controller 70 by the operator.

[0043] (Second value: can be changed depending on the posture of the front work implement 8) The controller 70 may change the second value in accordance with the attitude of the front work implement 8. The position of the center of gravity of the front work implement 8 differs depending on the attitude of the front work implement 8. Therefore, the peak pressure generated in the boom cylinder rod line 38 and the boom cylinder head line 40 may change depending on the attitude of the front work implement 8 when the operation of the boom 10 or the arm 12 stops. Therefore, it is desirable for the controller 70 to change the second value in accordance with the attitude of the front work implement 8. This makes it possible to quickly reduce the swing of the boom 10 when the operation of the boom 10 or the arm 12 stops, regardless of the attitude of the front work implement 8.

[0044] The controller 70 can change the second value as appropriate depending on the attitude of the front working implement 8. For example, the controller 70 can increase the second value as the distance from an arbitrary reference position, such as the center of rotation of the upper rotating body 6, to the center of gravity of the front working implement 8 increases, and decrease the second value as the distance decreases. Conversely, the controller 70 can decrease the second value as the distance increases, and increase the second value as the distance decreases. Note that, because the controller 70 receives detection results in real time from attitude detection means 72 that detects the attitude of the front working implement 8, it can change the second value based on these detection results.

[0045] (Time to decrease to second value) The controller 70 can reduce the set pressure of the electromagnetic proportional relief valve 44 to the second value for a predetermined time from the time when the operation amount of the boom manipulator 28 or the arm manipulator 56 decreases from a predetermined amount or more to less than the predetermined amount. The above-mentioned "predetermined time" may be any time that is sufficient to suppress unintended swing of the boom 10, and may be, for example, about three seconds. This prevents the set pressure of the electromagnetic proportional relief valve 44 from being reduced to the second value when it does not contribute to reducing unintended swing of the boom 10.

[0046] (Additional condition: reduction rate of operation amount) As described above, the controller 70 reduces the set pressure of the electromagnetic proportional relief valve 44 to the second value smaller than the first value when the operation amount of the boom manipulator 28 or the arm manipulator 56 decreases from a predetermined amount or more to less than the predetermined amount. In addition to this condition, the controller 70 may also be configured to reduce the set pressure of the electromagnetic proportional relief valve 44 to the second value when the rate of decrease per unit time of the operation amount of the boom manipulator 28 or the arm manipulator 56 is greater than a predetermined value.

[0047] The greater the rate of decrease per unit time of the operation amount of the boom manipulator 28 or the arm manipulator 56, the greater the pressure fluctuations in the boom cylinder rod line 38 and the boom cylinder head line 40. In other words, the greater the speed at which the boom manipulator 28 or the arm manipulator 56 is returned to the neutral position, the more likely it is that the boom 10 will swing.

[0048] On the other hand, the smaller the rate of decrease per unit time of the operation amount of the boom manipulator 28 or the arm manipulator 56, the smaller the pressure fluctuations in the boom cylinder rod line 38 and the boom cylinder head line 40. In other words, the slower the speed at which the boom manipulator 28 or the arm manipulator 56 is returned to the neutral position, the less likely the boom 10 to swing.

[0049] Therefore, it is desirable that the controller 70 reduce the set pressure of the electromagnetic proportional relief valve 44 to the second value when the operation amount of the boom manipulator 28 or the arm manipulator 56 decreases from a predetermined amount or more to less than the predetermined amount and when the rate of decrease per unit time of the operation amount of the boom manipulator 28 or the arm manipulator 56 is greater than a predetermined value. This prevents the set pressure of the electromagnetic proportional relief valve 44 from being reduced to the second value when this does not contribute to reducing the swing of the boom 10, which is against the operator's intention.

[0050] (Additional condition: Pressure of boom cylinder head line 40) Incidentally, in the hydraulic excavator 2, as shown in Fig. 3, an operation may be performed in which the bucket 14 is pressed against the ground to tilt the lower traveling structure 4 and the upper rotating structure 6, and such an operation may be called a "machine lift." When a machine lift is performed, the rod-side oil chamber 24c of the boom cylinder 24 becomes high pressure, and the head-side oil chamber 24d becomes low pressure.

[0051] Even when the set pressure of the electromagnetic proportional relief valve 44 is reduced to the second value when the operation amount of the boom operating device 28 or the arm operating device 56 is reduced to below the predetermined amount after the machine lift is performed to tilt the lower traveling unit 4 and the upper rotating unit 6, high-pressure hydraulic oil is discharged from the boom cylinder rod line 38 to the hydraulic oil tank 32 via the electromagnetic proportional relief valve 44. As a result, the boom cylinder 24 extends against the operator's will, and the tilted attitude of the lower traveling unit 4 and the upper rotating unit 6 cannot be maintained.

[0052] Therefore, it is preferable that the controller 70 lowers the set pressure of the electromagnetic proportional relief valve 44 to the second value when the operation amount of the boom operating device 28 or the arm operating device 56 decreases from a predetermined amount or more to less than the predetermined amount and when the pressure in the boom cylinder head line 40 is greater than a predetermined threshold. This prevents the set pressure of the electromagnetic proportional relief valve 44 from being lowered to the second value when a machine lift is performed. Therefore, the attitudes of the undercarriage 4 and the upper rotating body 6 that have been tilted when a machine lift is performed can be maintained. The above-mentioned "threshold value" is a value greater than the pressure in the boom cylinder head line 40 when a machine lift is performed, and may be, for example, approximately 5 MPa.

[0053] As described above, in the hydraulic circuit 16 of this embodiment, when the operation amount of the boom operation device 28 or the arm operation device 56 decreases from a predetermined amount or more to less than the predetermined amount, the set pressure of the electromagnetic proportional relief valve 44 is reduced to a second value smaller than the first value. The second value is smaller than the peak pressure that occurs in the boom cylinder rod line 38 due to the swinging of the boom 10 when the operation amount of the boom operation device 28 or the arm operation device 56 decreases from a predetermined amount or more to less than the predetermined amount. Therefore, according to this embodiment, when the operation of the boom 10 or the arm 12 stops, hydraulic oil is more likely to escape from the boom cylinder rod line 38 to the hydraulic oil tank 32, thereby reducing the swinging of the boom 10 when the operation of the boom 10 or the arm 12 stops. [Explanation of symbols]

[0054] 2: Hydraulic excavator (construction machinery) 4: Lower running body 6: Upper rotating body 8: Front work equipment 10: Boom 12: Arm 16: Hydraulic circuit 22: Hydraulic pump 24: Boom cylinder 26: Boom switching valve 28: Boom control device 32: Hydraulic oil tank 38: Boom cylinder rod line 40: Boom cylinder head line 44: Electromagnetic proportional relief valve 50: Pressure sensor 52: Arm cylinder 54: Arm switching valve 56: Arm operating tool 70: Controller

Claims

1. A hydraulic circuit for a construction machine comprising: a lower traveling body; an upper rotating body supported on the lower traveling body; and a front working implement attached to the upper rotating body, the front working implement including a boom connected to the upper rotating body; and an arm connected to the boom, A hydraulic pump; a boom cylinder that operates the boom; a boom switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the boom cylinder; a boom operating tool that outputs a signal to operate the boom switching valve; a boom cylinder rod line connecting a rod-side oil chamber of the boom cylinder and the boom switching valve; a boom cylinder head line connecting a head side oil chamber of the boom cylinder and the boom switching valve; an electromagnetic proportional relief valve installed between the boom cylinder rod line and a hydraulic oil tank, the set pressure of which is adjusted to a first value; an arm cylinder that operates the arm; an arm switching valve that switches the flow direction of hydraulic oil from the hydraulic pump to the arm cylinder; an arm operating device that outputs a signal to operate the arm switching valve; a controller that reduces the set pressure of the electromagnetic proportional relief valve to a second value that is smaller than the first value when the operation amount of the boom operation device or the arm operation device decreases from a predetermined amount or more to less than the predetermined amount, a hydraulic circuit of a construction machine, wherein the second value is smaller than a peak pressure generated in the boom cylinder rod line due to the boom swinging when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount.

2. The hydraulic circuit for a construction machine according to claim 1 , wherein the controller changes the second value in accordance with the weight of the front working implement.

3. The hydraulic circuit for a construction machine according to claim 1 , wherein the controller changes the second value in accordance with the attitude of the front working implement.

4. 2. The hydraulic circuit for a construction machine according to claim 1, wherein the controller reduces the set pressure of the electromagnetic proportional relief valve to the second value for a predetermined time from when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount.

5. 2. The hydraulic circuit for a construction machine according to claim 1, wherein the controller reduces the set pressure of the electromagnetic proportional relief valve to the second value when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount and when a rate of decrease per unit time of the operation amount of the boom operation device or the arm operation device is greater than a predetermined value.

6. 2. The hydraulic circuit for a construction machine according to claim 1, wherein the controller reduces the set pressure of the electromagnetic proportional relief valve to the second value when the operation amount of the boom operation device or the arm operation device decreases from equal to or greater than the predetermined amount to less than the predetermined amount and when the pressure in the boom cylinder head line is greater than a predetermined threshold value.

7. 7. The hydraulic circuit for a construction machine according to claim 6, wherein the threshold value is greater than the pressure in the boom cylinder head line when the tip of the front working implement is pressed against the ground to tilt the undercarriage and the upper rotating body.

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