WO2016169950A1

WO2016169950A1 - Hydraulic circuit and working machine

Info

Publication number: WO2016169950A1
Application number: PCT/EP2016/058696
Authority: WO
Inventors: Kouji Kishida; Mitsuhiro Toyoda; Yoshihiko Hata; Yuya Kanenawa; Shuhei ORIMOTO; Nobuaki Matoba
Original assignee: Caterpillar Sarl
Priority date: 2015-04-21
Filing date: 2016-04-20
Publication date: 2016-10-27
Also published as: JP6493916B2; JP2016205497A

Abstract

Provided are a hydraulic circuit and a work machine being capable of suppressing energy loss and pressure loss during an operation of eliminating excavation load for an excavation. A hydraulic circuit includes an on-board controller which is configured to detect an operation state of a work machine, and to, when detecting the work machine is in an excavation state, control each operation of a solenoid proportional boom control valves (26), (28) and a bleed-off valve (66) in accordance with an operation amount of a lever which operates the work machine at least upward, whereby hydraulic oil fed from pumps (12), (13) to boom cylinders (7c1), (7c2) is reduced, return oil from a rod side of the boom cylinders (7cl), (7c2) is bypassed to a tank (21), and a pump flow rate which is discharged from the pumps (12), (13) is reduced. The hydraulic circuit also includes a making-up circuit (M) which is configured to bypass return oil from a rod side to the pumps (12), (13) by the bleed-off valve (66), and to replenish hydraulic oil to the head side of the boom cylinders (7c1), (7c2) from the tank (21).

Description

[DESCRIPTION]

[Title of Invention]

HYDRAULIC CIRCUIT AND WORKING MACHINE

[Technical Field]

[0001] The present invention relates to a hydraulic circuit that has a hydraulic cylinder for operating a working device vertically, and a working machine equipped with this hydraulic circuit .

[Background Art]

[0002] Hydraulic excavators, for example, have been known as working machines that execute excavation. A hydraulic excavator has, as working devices provided in the machine body thereof, a boom capable of moving up and down, a stick

connected to a tip of the boom, and a bucket connected to a tip of the stick, wherein the boom, the stick and the bucket are operated by hydraulic cylinders respectively.

[0003] In excavation using a hydraulic excavator, hydraulic oil is supplied to each of the hydraulic cylinders. After sticking the bucket in soil and the like, the bucket is closed (bucket-in) while pulling the stick (stick-in) . In this excavation, the operator usually executes the stick-in

operation and the bucket-in operation to the maximum. When the bucket is stuck too deep in the soil or when the soil to be excavated is hard and heavy, the reaction force of the soil applied to the bucket is so large that it generates a load that lifts the boom, disabling the operation of pulling the stick or bucket. In such a case, the operator releases the load by lifting the boom and repeats the excavation.

[0004] There has been known a configuration in which a so- called load reducing operationfor releasing the load on the boom is automatically performed when overload on the bucket or arm is detected (see PTL 1, for example) .

[Citation List]

[Patent Literature]

[0005]

[PTL 1] Japanese Patent Application Publication No. 2011- 252338

[Summary of Invention]

[Technical Problem]

[0006] However, in the excavation operation where the stick-in operation and the bucket-in operation are performed to the maximum, the head pressure of a boom hydraulic cylinder is approximately 0, while the rod pressure of the boom

hydraulic cylinder is high due to the excavation reaction force. For this reason, when executing the load reducing operation by lifting the boom, the pump pressure that becomes high due to the operations of the stick and the bucket merges with the boom cylinder head pressure that is approximately 0, causing high pressure loss.

[0007] When performing control in which the load reducing operationis executed automatically, since it is not easy to execute so-called foundation excavation for excavating below theworking machine, it is desired that the operator be able to execute the load reducing operationon his own will.

[0008] The present invention was contrived in view of these circumstances, and an object thereof is to provide a hydraulic circuit and a working machine capable of reducing energy loss and pressure loss that occur in the load reducing operationin excavation .

[Solution to Problem]

[0009] An invention described in claim 1 is a hydraulic circuit having: a hydraulic cylinder that moves a working device up and down by being operated by a working fluid that is pressurized and supplied by a pump in response to an operation of an operating device; a solenoid proportional control valve for controlling the working fluid supplied from the pump to the hydraulic cylinder; a bypass valve for controlling the amount of communication between a rod of the hydraulic cylinder and a tank; a controller that detects an operating state of the working device, and when detecting an excavation state of the working device, controls an operation of the control valve and an operation of the bypass valve in accordance with an operation amount of the operating device for operating at least the working device upward, thereby reduces the working fluid to be supplied from the pump to the hydraulic cylinder, causes at least some of a return fluid from the rod of the hydraulic cylinder to bypass to the tank, and reduces a pump flow rate of the working fluid discharged from the pump; and a makeup circuit that has a channel connecting a head of the hydraulic cylinder and the tank with a check valve therebetween, uses the bypass valve to cause at least some of a return fluid from the rod to the pump to bypass to the tank, and replenishes the head of the hydraulic cylinder with the working fluid from the tank.

[0010] An invention described in claim 2 is a hydraulic circuit, wherein the hydraulic cylinder of the hydraulic circuit described in claim 1 is pilot-operated, and the controller detects the operating state of the working device based on a pilot pressureof the hydraulic cylinder, a pump pressure, and a head pressure of the hydraulic cylinder.

[0011] An invention described in claim 3 is a working machine, having: a machine body; a working device that has a boom provided in such a manner as to be rotatable vertically with respect to the machine body, a stick provided in such a manner as to be rotatable inward and outward with respect to the boom, and a bucket provided in such a manner as to be rotatable inward and outward with respect to the stick; and the hydraulic circuit of claim 1 or 2 that is provided for a hydraulic cylinder that functions at least as a boom cylinder for rotating the boom.

[Advantageous Effects of Invention]

[0012] According to the invention described in claim 1, when the excavation state of the working device is detected, the working fluid to be supplied from the pump to the hydraulic cylinder is reduced in accordance with the operation amount of the operating device for operating at least the working device upward. At least some of the return fluid from the rod of the hydraulic cylinder is caused to bypass to the tank, and the pump flow rate of the working fluid discharged from the pump is reduced, to limit the operation of the solenoid proportional control valve. In addition, the head of the hydraulic cylinder is replenished with the working fluid of the tank, thereby reducing energy loss and pressure loss that occur in the load reducing operation for releasing the load by operating the working device upward in excavation.

[0013] According to the invention described in claim 2, the operating state of the working device is detected by the controller based on the pilot pressure, pump pressure, and head pressure of the hydraulic cylinder. Therefore, the excavation state of the working device can easily be detected with a simple configuration.

[0014] The invention described in claim 3 can reduce the pressure loss that occurs in the load reducing operation for releasing the load by operating the working device upward at the time of excavation performed by the working device of the working machine.

[Brief Description of Drawings]

[0015]

[Fig. 1] Fig. 1 is a circuit diagram showing a way of switching a hydraulic circuit according to an embodiment of the present invention .

[Fig. 2]

Fig. 2 is a circuit diagram showing another way of switching the hydraulic circuit.

[Fig. 3]

Fig. 3 is a circuit diagram showing yet another way of

switching the hydraulic circuit.

[Fig. 4]

Fig. 4(a) is an explanatory diagram schematically showing a control algorithm of one valve of the circuit, and Fig. 4(b) an explanatory diagram schematically showing a control

algorithm of another valve of the circuit.

[Fig. 5]

Fig. 5 is an explanatory diagram schematically showing an outline of a control valve for each block in a controller of the circuit .

[Fig. 6]

Fig. 6 is an explanatory diagram schematically showing control algorithms of a control valve and a bypass valve controlled by the controller.

[Fig. 7]

Fig. 7 is an explanatory diagram schematically showing a control algorithm of a pump flow rate controlled by the controller. [Fig. 8]

Fig. 8 is a side view of a working machine provided with the hydraulic circuit.

[Description of Embodiments]

[0016] The present invention is described hereinafter in detail based on an embodiment shown in Figs. 1 to 8.

[0017] As shown in Fig. 8, a hydraulic excavator HE, which is a working machine, has a machine body 1 that is configured by a lower traveling body 2 and an upper slewing body 3 provided thereon so as to be slewable by a slewing motor 3m, wherein the upper slewing body 3 is equipped with a machine room 4 equipped with the engine, a pump and the like, a cab 5 for protecting an operator, and a working device 6.

[0018] In this working device 6, a base end of a boom 7 that is rotated vertically by two parallel boom cylinders 7cl, 7c2 functioning as hydraulic cylinders is axially supported on the upper slewing body 3, a stick 8 that is rotated back and forth by a stick cylinder 8c functioning as a hydraulic cylinder is axially supported at a tip of the boom 7, and a bucket 9 that is rotated by a bucket cylinder 9c functioning as a hydraulic cylinder is axially supported at a tip of the stick 8. The two boom cylinders 7cl, 7c2 are provided

parallel to the common boom 7 and simultaneously actuate the same operation.

[0019] Fig. 1 to Fig. 3each show an engine power assist system that accumulates position energy of the working device 6 in an accumulator through the boom cylinder 7cl, accumulates kinetic energy of the upper slewing body 3 in the accumulator through the slewing motor 3m, and uses these energies to assist engine power.

[0020] A circuit configuration of this system is described next .

[0021] An assist motor 15 is connected to a main pump shaft 14 of main pumps 12, 13 directly or by a gear, the main pumps 12, 13 being driven by a built-in engine 11 of the machine room 4. The main pumps 12, 13 and the assist motor 15 each have a swash plate capable of variably adjusting the

pump/motor capacity (piston stroke) by the angle thereof. The swash plate angles (tilted angles) are controlled by

regulators 16, 17, 18 and detected by swash plate angle sensors 16φ, 17φ, 18φ. The regulators 16, 17, 18 are

controlled by a solenoid valve. For example, the regulators 16, 17 of the main pumps 12, 13 can be controlled

automatically with a negative flow control pressure (so-called negative control pressure) guided through a negative flow control channel 19nc or with a signal other than the negative control pressure by solenoid switching valves 19a, 19b of a negative flow control valve 19.

[0022] The main pumps 12, 13 discharge, to channels 22, 23, hydraulic oil which is a working fluid drawn up from a tank 21, and have the pump discharge pressures thereof detected by pressure sensors 24, 25. Pilot control valves for controlling the directions and flow rates of the hydraulic oil are

connected to the main pumps 12, 13. The pilot control valves include a boom control valve 26 as a main solenoid

proportional control valve for controlling the boom cylinders 7cl, 7c2 and a boom control valve 28 as a sub-solenoid

proportional control valve. An output channel 27 extending from the boom control valve 26 and an output channel 29 extending from the boom control valve 28 are connected to a boom energy recovery valve 31, which is a composite valve, by a channel 30.

[0023] The boom energy recovery valve 31 is a composite valve that incorporates a plurality of circuit functions in a single block, the plurality of circuit functions being used for switching an accumulation circuit A and a regenerative circuit B shown in Fig. 2, a circuit that guides the hydraulic oil pressurized and supplied by the main pumps 12, 13, to heads of the two boom cylinders 7cl, 7c2 in a boom lifting operation shown in Fig. 3, a bleed-off circuit C, which is a bypass circuit that causes return oil, which is fed from rods of the boom cylinders 7cl, 7c2, to a tank 21, and a makeup circuit M that replenishes the heads of the boom cylinders 7cl, 7c2 with makeup hydraulic oil from the tank 21.

[0024] A channel 32 extending from a head-side end of the boom cylinder 7cl is connected to the boom energy recovery valve 31 by a channel 34 through a drift reduction valve 33, and a channel 35 extending from a head-side end of the boom cylinder 7c2 is connected to the boom energy recovery valve 31 by a channel 37 through a drift reduction valve 36. An output channel 38 extending from the main boom control valve 26 is connected to the regenerative circuit B of the boom energy recovery valve 31. The rods of the boom cylinders 7cl, 7c2 are connected to the boom energy recovery valve 31 by channels 39, 40. The drift reduction valves 33, 36 control the

opening/closing and apertures between the ports by controlling the pilot pressure of a spring chamber by means of pilot valves, not shown. Note that a pressure sensor 41 for

detecting head pressure acting on the head of the boom

cylinder 7cl is connected to the channel 34.

[0025] The output channel 27 extending from the main boom control valve 26 can communicate with the output channel 38 by a solenoid switching valve 42 and a check valve 43.

[0026] The discharge side of the assist motor 15 is connected to the tank 21 by a discharge channel 44. A tank channel 50 extending from an accumulator channel 47 provided with a plurality of first accumulators 46 is connected to the suction side of the assist motor 15 through a relief valve 48 and a check valve 49, and a suction-side channel 52 extending from the accumulator channel 47 is connected to the same through a solenoid switching valve 51. A pressure sensor 55 for detecting pressure accumulated in the first accumulators 46 is connected to the accumulator channel 47. The tank channel 50 extends through a tank channel 56, a spring check valve 57 which is a back pressure check valve, and an oil cooler 58 or a spring check valve 59 and is connected to the tank 21. In addition, a first makeup channel 60 which is a makeup channel branched off from the tank channel 56 is connected to, for example, the boom energy recovery valve 31 through a check valve 61 and a channel 62 located in the first makeup channel 60. The first accumulators 46, the accumulator channel 47, the relief valve 48, the solenoid switching valve 51, and the pressure sensor 55 are incorporated in the single block to configure an accumulator block 63.

[0027] The boom energy recovery valve 31 has a control valve 64 that is a valve configuring a part of the

accumulation circuit A, a main control valve 65 that is another valve functioning as a boom circuit switching valve to configure a part of the regenerative circuit B, and a bleed- off valve 66 functioning as a bypass valve. Pilot-operated valves are used as the control valve 64, the main control valve 65, and the bleed-off valve 66, the pilot-operated valves being switched when the solenoid switching valves are operated by, for example, the operator in the cab 5 (Fig. 8) or the like operating an operating device such as a lever, not shown, to control the supply and discharge of the pilot pressure. However, for the purpose of clarifying the

explanation, the control valve 64, the main control valve 65, and the bleed-off valve 66 are shown as solenoid proportional direction control valves in the diagrams. [0028] The control valve 64 is a flow rate control valve that allows the hydraulic oil from the boom cylinder 7cl to be accumulated in the first accumulators 46, by switching between enabling and blocking the communication between the channels 68 and 34 connected to the first accumulators 46 (the

accumulator block 63) through a check valve 67. The control valve 64 allows the hydraulic oil to flow in an amount larger than the amount of hydraulic oil returned from the normal cylinders (boom cylinders 7cl, 7c2 and the like) to the tank 21, and prioritizes accumulation of pressure oil in the first ^■accumulators 46. The head of the boom cylinder 7cl is connected to the side of the control valve 64 opposite to the check valve 67 (the upstream side of the control valve 64) by the channel 34.

[0029] The main control valve 65 separates the boom

cylinder 7cl and the boom cylinder 7c2 into an accumulation cylinder and a self-regenerative cylinder by switching the relationship between channels 71 and 72, the relationship between channels 73 and 74, and the relationship between channels 75 and 76. Specifically, the main control valve 65 is configured to block the communication between the heads of the boom cylinders 7cl, 7c2 and enables the communication between the head of the boom cylinder 7c2 and the rods of the boom cylinders 7cl, 7c2 at the time of accumulation in the first accumulators 46 by switching the control valve 64. [0030] The channel 30 is connected to the channel 71 through a check valve 78. The channel 72 is connected to the channel 37 and a channel 79 branching off from the channel 30. The channel 73 branches off from the channel 72. The channel 74 is connected to the channel 40 through a check valve 80. The channel 75 is connected to the output channel 38 and the channel 39. The channel 76 branches off from the channel 40.

[0031] The bleed-off valve 66 is operated when a load reducing operation is executed in an excavation operation described hereinafter. The bleed-off valve 66 is connected to the channels 39, 75 by a channel 82 and to the tank 21 by a channel 83.

[0032] As shown in Fig. 1 to Fig. 3, the accumulation circuit A is a circuit where the hydraulic oil flows from the channel 32 extending from the head-side end of the boom cylinder 7cl, passes through the drift reduction valve 33, the channel 34, the control valve 64 and check valve 67 of the boom energy recovery valve 31, and the channel 68, and reaches the first accumulators 46. The accumulation circuit A

functions to accumulate in the first accumulators 46 the oil ejected from the head of the boom cylinder 7cl.

[0033] The regenerative circuit B is a circuit where the hydraulic oil flows from the channel 35 extending from the head-side end of the boom cylinder 7c2, passes through the drift reduction valve 36, the channel 37, the channel 73, main control valve 65, channel 74, check valve 80, and channel 40 of the boom energy recovery valve 31, reaches the rod-side end of the boom cylinder 7c2, flows again from the channel 35, passes through the drift reduction valve 36, the channel 37, the channel 73, main control valve 65, channel 74, check valve 80, channel 76, main control valve 65, channel 75, and channel 39 in the boom energy recovery valve 31, and then reaches the rod-side end of the boom cylinder 7cl. The regenerative circuit B functions to regenerate, to the rods of the boom cylinders 7cl, 7c2, the oil ejected from the head of the boom cylinder 7c2.

[0034] The bleed-off circuit C is a circuit in which the hydraulic oil reaches the tank 21 from the rod of the boom cylinder 7cl through the channel 39, channel 82, bleed-off valve 66, and channel 83, and further reaches the tank 21 from the rod of the boom cylinder 7c2 through the channel 40, channel 76, main control valve 65, channel 75, channel 82, bleed-off valve 66, and channel 83. The bleed-off circuit C functions. to cause at least some of the return oil, which is fed from the rods of the boom cylinders 7cl, 7c2, to the tank 21.

[0035] The makeup circuit M is a circuit in which the hydraulic oil flows from the tank 21 to reach the head of the boom cylinder 7cl through the oil cooler 58, spring check valve 57,_. tank channel 56, first makeup channel 60, check valve 61, channel 62, check valve 78, channel 34, drift reduction valve 33, and channel 32, and further reaches the head of the boom cylinder 7c2 through the check valve 78, channel 71, main control valve 65, channel 72, channel 37, drift reduction valve 36, and channel 35. The makeup circuit M functions to replenish the heads of the boom cylinders 7cl, 7c2 with the hydraulic oil from the tank 21. In other words, the makeup circuit M has a channel that extends from the tank 21 so as to communicate with the heads of the boom cylinders 7cl, 7c2 through the check valves 61, 78.

[0036] Relief valves 94, 95 and check valves 97, 98 that are mutually opposite to each other are provided between channels 92, 93 of a motor drive circuit D that connects a slewing control valve 91 and the slewing motor 3m to each other, the slewing control valve 91 controlling the slewing direction and speed of the slewing motor 3m. A second makeup channel 99, which has a tank channel function for returning · the oil discharged from the motor drive circuit D to the tank 21 and a makeup function capable of replenishing the motor drive circuit D with hydraulic oil, is connected between the relief valves 94, 95 and between the check valves 97, 98. The second makeup channel 99 is connected to a second accumulator 100 that supplies pressure oil. Hydraulic oil is replenished in the channel 92 or 93, whichever is likely to cause a vacuum, from the second makeup channel 99 through the check valves 97, 98 at a pressure that does not exceed the spring biasing pressure of the spring check valve 57. [0037] The channels 92, 93 of the motor drive circuit D are made to communicate with a slewing energy recovery channel 104 by check valves 102, 103. This channel 104 is connected to a channel 106 through a sequence valve 105 where the source pressure at the inlet thereof does not change easily due to the back pressure at the outlet of the same. The channel 106 is connected to the first accumulators 46 and the channel 68.

[0038] In the foregoing circuit configuration, the swash plate angle sensors 16φ, 17φ, 18φ and the pressure sensors 24, 25, 55 input the detected swash plate angle signals and pressure signals to an in-vehicle controller CR (Fig. 8) that functions as a controller, and the valves 42, 51 are switched by an on/off operation using a drive signal output form the in-vehicle controller CR (Fig. 8) or a proportional action in accordance with the drive signal. The boom control valves 26, 28, the slewing control valve 91, and other hydraulic actuator control valves that are not shown (such as a travel motor control valve, a stick cylinder control valve, a bucket cylinder control valve and the like) are pilot-operated by a manually operated valve which is a so-called remote-control valve operated by the operator in the cab 5 (Fig. 8) or the like operating the lever or pedal. The pilot valves of the drift reduction valves 33, 36, which are not shown, are also pilot-operated in conjunction with the foregoing valves.

[0039] The details controlled by the in-vehicle controller CR (Fig. 8) are described functionally hereinafter. [0040] Fig. 2 shows a state of the circuit in which a boom lowering operation for lowering the boom 7 (Fig. 8) is

performed. The hydraulic oil that is ejected from the head of the boom cylinder 7cl due to a load or the like of the working device 6 (Fig. 8) passes through the channel 32 and the drift reduction valve 33, is made to communicate with the channel 68 from the channel 34 through the control valve 64 of the boom energy recovery valve 31 that is switched to the communication position and then through the check valve 67, and is then accumulated in the first accumulators 46 through the channel 68.

[0041] In this state, the control valve 64 switches the amount of communication between the head of the boom cylinder 7cl and the first accumulators 46, in accordance with the operation amount of the lever, i.e., the pilot pressure set based on this operation amount, and the accumulator pressure of the first accumulators 46 detected by the pressure sensor 55. Specifically, the pilot pressure that is set based on the operation amount of the lever is corrected based on a

predetermined table (converter) Tl, and the accumulator pressure is corrected based on a predetermined table

(converter) T2. Then, the result obtained by integrating these corrected values is obtained as an output for operating the control valve 64. More specifically, in the present embodiment, in the table Tl shown in Fig. 4(a), when the pilot pressure that is set based on the operation amount of the lever is relatively small, the amount of increase in the output pressure thereof becomes relatively greater than the amount of increase in the input pressure of the same.

Therefore, in the region where the pilot pressure that is set based on the operation amount of the lever exceeds a

predetermined threshold TH1, the amount of increase in the output pressure with respect to the amount of increase in the input pressure is reduced more compared to when the pilot pressure is equal to or lower than the threshold TH1.

Furthermore, in the region where the pilot pressure exceeds a predetermined threshold TH2 that is greater than the

predetermined threshold TH1, the output pressure is set constant. Furthermore, according to the table T2, in the region where the accumulator pressure is equal to or lower than a predetermined threshold TH3, a gain increases with respect to the amount of increase in the accumulator pressure and in the region where the accumulator pressure exceeds the predetermined threshold TH3, the gain is set constant (e.g., 1) . In this case, the hydraulic oil is prevented by the chec valve 78 from returning toward the boom control valve 26.

[0042] At the same time, the direction of the hydraulic oi ejected from the head of the boom cylinder 7c2 is controlled to allow the hydraulic oil to flow toward the channel 74 through the channel 35, the drift reduction valve 36, the channel 37, the main control valve 65 of the boom energy recovery valve 31, and the channel 73. The hydraulic oil further passes through the check valve 80 and the channel 40 and is regenerated to the rod of the boom cylinder 7c2. Then, the direction of the hydraulic oil branching off to the channel 76 through the check valve 80 is controlled to allow the hydraulic oil to flow to the channel 75 through the check valve inside the main control valve 65. Consequently, the hydraulic oil passes through the channel 39 and is regenerated to the rod of the boom cylinder 7cl.

[0043] At this moment, the operation amount of the main control valve 65 changes in response to the operation amount of the lever, i.e., the pilot pressure that is set based on this operation amount. Specifically, the pilot pressure that is set based on the operation amount of the lever is corrected based on a predetermined table (converter) T3, and the

resultant pressure is taken as an output for operating the main control valve 65. More specifically, in the present embodiment, the table T3 similar to the table T2 shown in Fig. 4 (a) is used to set the input pressure and the output pressure of the pilot pressure that is set based on the operation amount of the lever, as shown in Fig. 4(b), and basically the main control valve 65 is switched as soon as the boom lowering operation is detected. Note that an excess flow rate of the hydraulic oil ejected from the head of the boom cylinder 7c2 is returned from the boom control valve 26 to the tank 21 after passing through the channel 37, the channel 79, and the channel 30. In addition, for example, in a case where grounding of the working device 6 (Fig. 8) is detected based on the head pressure of the boom cylinders 7cl, 7c2, and thereby it is detected that lowering of the boom results in lifting of the machine body 1 (flag-on for the lifted machine body) , separation of the boom cylinders 7cl, 7c2 into the accumulation cylinder and the self-regenerative cylinder is canceled in accordance with a predetermined set value.

[0044] Using the control valve 64 and the main control valve 65, the boom energy recovery valve 31 accumulates the hydraulic oil in the first accumulators 46 at the time of lowering the boom and at the same time regenerates the

hydraulic oil to the rods of the boom cylinders 7cl, 7c2.

[0045] Some of the hydraulic oil discharged from the main pump 12 at the time of the boom lowering operation is supplied to the rod of the boom cylinder 7cl from the boom control valve 26 through the output channel 38 and the channel 39. At this moment, only when the boom lowering pressure is started, the boom control valve 26 allows the hydraulic oil to be supplied to the rod of the boom cylinder 7cl at the maximum flow rate. And when the boom 7 starts to descend and the main control valve 65 is activated in the regenerative circuit B, the hydraulic oil from the head of the boom . cylinder 7c2 is regenerated to the rods of the boom cylinders 7cl, 7c2, thereby restricting the flow rate.

[0046] Fig. 3 shows a state of the circuit in which the boom lifting operation for lifting the boom 7 (Fig. 8) is performed. In the boom lifting operation, the boom energy recovery valve 31 not only switches the control valve 64 to the blocking position but also switches the main control valve 65 to stop the accumulation of the hydraulic oil in the first accumulators 46 and the regeneration of the same to the rods of the boom cylinders 7cl, 7c2. The boom energy recovery valve 31 also guides the hydraulic oil, which is supplied from the main pumps 12, 13 to the channel 30 through the boom control valves 26, 28, from the channel 79 to the head of the boom cylinder 7c2 through the channel 37, the drift reduction valve 36, and the channel 35, and further guides the hydraulic oil from the check valve 78 to the head of the boom cylinder 7cl through the channel 34, the drift reduction valve 33, and the channel 32. The hydraulic oil ejected from the rod of the boom cylinder 7cl is returned to the tank 21 from the channel 39 and the output channel 38 through the boom control valve 26. The direction of the hydraulic oil ejected from the rod of the boom cylinder 7c2 is controlled to allow the hydraulic oil to flow to the channel 75 through the channel 40, the channel 76, and the main control valve 65, thereby returning the hydraulic oil to the tank 21 from the output channel 38 through the boom control valve 26.

[0047] In the boom lowering operation and the boom lifting operation, engine power assist can be performed in which the assist motor 15 with a motor function, which is coupled to the main pump shaft 14 directly or by a gear, is caused to function as a hydraulic motor as shown in Fig. 3, to reduce the engine load. For example, in the boom lowering operation, the engine power assist is performed when the pressure sensor 55 detects that the accumulator pressure of the first

accumulators 46 that is accumulated through the control valve 64 is egual to or greater than a predetermined first

threshold. Other than the boom lowering operation, such as in the boom lifting operation or the like, the engine power assist is performed when the pressure sensor 55 detects that the accumulator pressure of the first accumulators 46 is equal to or greater than a predetermined second threshold different from the predetermined first threshold. In this engine power assist, the solenoid switching valve 51 is switched to the communication position in response to the flag, and the assist motor 15 is rotated by the energy accumulated in the first accumulators 46, to assist the hydraulic outputs of the main pumps 12, 13 and reduce the engine load. When the machine body 1 is lifted, the engine power assist is not performed using the assist motor 15.

[0048] Therefore, by rotating the assist motor 15 by means of the energy from the head of the boom cylinder 7cl that is accumulated in the first accumulators 46, the engine power assist function reduces, by using the assist motor 15, the load of the built-in engine 11 that is coupled thereto by the main pump shaft 14. [0049] In the excavation operation by the working device 6, such as the one shown in Fig. 8, the boom lowering operation and the boom lifting operation described above are each executed in random combination with a stick-in/out operation and a bucket-in/out operation. In a case where, for example, the bucket 9 is stuck too deep in the soil or the soil to be excavated is too hard and heavy, reaction force of the soil that acts on the bucket 9 is so large that it generates a load that lifts the boom 7, disabling the operation of pulling the stick 8 or bucket 9. In such a case, the operator often executes a so-called load reducing operation (shown by the imaginary line) in which the load is released by turning the working device 6 (boom 7) upward. Therefore, when the load reducing operation is executed, the pump pressure that becomes high due to the load flows into the heads of the boom

cylinders 7cl, 7c2, resulting in energy loss.

[0050] As shown in Fig. 5. the in-vehicle controller CR described above is provided with, generally, an operation detector 107 for detecting the operating state of the working device 6 (Fig. 8), a valve controller 108 as an opening area controller, and a pump flow rate controller 109. When the operation detector 107 detects an excavation state of the working device 6, the valve controller 108 sets an output for operating the boom control valves 26, 28 and an output for operating the bleed-off valve 66, and the pump flow rate controller 109 sets a pump necessary flow rate. When the operation detector 107 does not detect the excavation state of the working device 6, that is, when the operating state of the working device 6 detected by the operation detector 107 is not the excavation state, the output for operating the boom control valves 26, 28, the output for operating the bleed-off valve 66, and the pump necessary flow rate are set at 0, thereby reducing the pressure loss.

[0051] Specifically, the operation detector 107 detects the operating state of the working device 6 (Fig. 8) based on the stick-in operation amount obtained by operating the lever, i.e., the pilot pressure set based on this operation amount, the bucket-in operation amount, i.e., the pilot pressure set based on this operation amount, the pump pressure of the main pumps 12, 13, the head pressure of the boom cylinders 7cl, 7c2, and the boom lifting operation amount, i.e., the pilot pressure set based on this operation amount. More

specifically, when the pilot pressure set based on the stick- in operation amount is greater than a predetermined pressure (e.g., 1.8 MPa) ; the pilot pressure set based on the bucket-in operation amount is greater than a predetermined pressure (e.g., 1.3 MPa); the pump pressure is greater than a

predetermined pressure (e.g., 15 MPa); the head pressure of the boom cylinders 7cl, 7c2 is greater than a predetermined pressure (e.g., 5 MPa); and the pilot pressure set based on the boom lifting operation amount is smaller than a

predetermined pressure (e.g., 1.5 MPa), the operation detector 107 determines that the working device 6 (Fig. 8) is executing the excavation operation, and in other cases the operation detector 107 determines that the excavation operation is not executed .

[0052] The valve controller 108 sets an output for

operating the boom control valves 26, 28 and the bleed-off valve 66, based on the boom lifting operation obtained by operating the lever, i.e., the pilot pressure set based on this operation amount, the pump pressure of the main pumps 12, 13, and the rod pressure of the boom cylinders 7cl, 7c2. More specifically, the valve controller 108 integrates a gain that is set based on a predetermined table (converter) T4 in accordance with the pump pressure, with a gain that is set based on a predetermined table (converter) T5 in accordance with the rod pressure of the boom cylinders 7cl, 7c2 detected by a pressure sensor (not shown) , and integrates the resultant integrated value with an opening area that is set based on a predetermined table (converter) T6 in accordance with the pilot pressure set based on the operation amount of the lever in the boom lifting operation. The resultant integrated value or the opening area, whichever is smaller, is subjected to area pressure conversion, and the resultant value is set as the output for operating the bleed-off valve 66. The valve controller 108 further performs area pressure conversion on the difference between the opening area that is set based on the table T6 in accordance with the pilot pressure set based on the operation amount of the lever in the boom lifting operation and the smaller opening area described above, and sets the resultant value as an output for restricting the operations of the boom control valves 26, 28. According to the table T4, in the region where the pump pressure is equal to or lower than a predetermined threshold TH4, the gain is set constant. In the region where the pump pressure exceeds the predetermined threshold TH4 but is equal to or lower than a predetermined threshold TH4 greater than the predetermined threshold TH5, the gain decreases in proportion to the

increase of the pump pressure. In the region where the pump pressure exceeds the predetermined threshold TH5, the gain is set constant. According to the table T5, in the region where the head pressure is equal to or lower than a predetermined threshold TH9, the amount of increase in the gain is

relatively greater than the amount of increase in the head pressure. In the region where the head pressure exceeds a predetermined threshold TH6 but is equal to or lower than a predetermined threshold TH7 greater than the predetermined threshold TH6, the amount of increase in the gain with respect to the amount of increase in the head pressure is reduced more compared to when the head pressure is equal to or lower than the threshold TH6. In the region where the head pressure exceeds the predetermined threshold TH7 but is equal to or lower than a predetermined threshold TH8 greater than the predetermined threshold TH7, the amount of increase in the gain with respect to the amount of increase in the head pressure is reduced more compared to when the head pressure is equal to or lower than the threshold TH7. In the region where the head pressure exceeds the predetermined threshold TH8, the gain is set constant. According to the table T6, when the pilot pressure that is set based on the operation amount of the lever is relatively as small as a predetermined threshold TH9 or lower, the opening area increases in proportion to the increase in the pilot pressure. In the region where the pilot pressure exceeds the predetermined threshold TH9 but is equal to or lower than a predetermined threshold TH10 greater than the predetermined threshold TH9, the amount of increase in the opening area with respect to the amount of increase in the pilot pressure is increased more compared to when the pilot pressure is equal to or lower than the predetermined threshold TH9. In the region where the pilot pressure exceeds the predetermined threshold TH10, the opening area is set

constant .

[0053] Returning to Fig. 5, the pump flow rate controller 109 sets the pump necessary flow rate based on the boom lifting operation amount obtained by operating the lever, i.e., the pilot pressure set based on this operation amount, the output for operating the boom control valves 26, 28 that is output from the valve controller 108, and a predetermined flow rate distribution. More specifically, as shown in Fig. 7, the pump flow rate controller 109 divides an opening area that is set based on a predetermined table (converter) T7 in accordance with the pilot operation set based on the operation amount of the lever in the boom lifting operation, by the output for restricting the operations of the boom control valves 26, 28 that is output from the valve controller 108, integrates the resultant value with the difference between 1 and a predetermined base flow rate coefficient, and adds the foregoing base flow rate coefficient to the resultant

integrated value. The pump flow rate controller 109 then integrate this resultant value with a value obtained by integrating the predetermined flow rate distribution with the maximum flow rate, and outputs the resultant value as the pump necessary flow rate. In the table T7, as with the table T6, when the pilot pressure that is set based on the operation amount of the lever is relatively as small as the

predetermined threshold TH9 or lower, the opening area

increases in proportion to the increase in the pilot pressure. In the region where the pilot pressure exceeds the

predetermined threshold TH9 but is equal to or lower than a predetermined threshold TH10 greater than the predetermined threshold TH9, the amount of increase in the opening area with respect to the amount of increase in the pilot pressure is increased more compared to when the pilot pressure is equal to or lower than the predetermined threshold TH9. In the region where the pilot pressure exceeds the predetermined threshold TH10, the opening area is set constant. [0054] In the load reducing operation, therefore, as shown in Fig. 1, the hydraulic oil from the rods of the boom

cylinders 7cl, 7c2 (at least some of the hydraulic oil) is returned to the tank 21 through the bleed-off valve 66 of the bleed-off circuit C, and the supply of the hydraulic oil from the main pumps 12, 13 to the rods of the boom cylinders 7cl, 7c2 is blocked (reduced) by the boom control valves 26, 28, whereas for the heads of the boom cylinders 7cl, 7c2 where a vacuum may occur, the heads of the boom cylinders 7cl, 7c2 are replenished with the hydraulic oil of the tank 21 through the makeup circuit M, at a pressure that does not exceed the spring biasing pressure of the spring check valve 57, because the back pressure that is set based on the spring force of the spring of the spring check valve 57 is generated in the tank channel 56 of the makeup circuit M. As a result, the

occurrence of a vacuum is inhibited. The main pumps 12, 13 do not need to supply the hydraulic oil to the heads of the boom cylinders 7cl, 7c2 or supply less hydraulic oil thereto, reducing the pump flow rate thereof.

[0055] In this manner, the hydraulic oil to be supplied from the main pumps 12, 13 to the boom cylinders 7cl, 7c2 is reduced (blocked) in accordance with the operation amount of the lever in the boom lifting operation where at least the working device 6 is operated upward when the excavation state of the working device 6 is detected. Furthermore, the high- pressure oil (at least some of the return oil) from the rods of the boom cylinders 7cl, 7c2 is caused to bypass to the tank 21, the pump flow rate of the oil discharged from the main pumps 12, 13 is reduced to restrict the operations of the solenoid proportional boom control valves 26, 28. In

addition, replenishing the heads of the boom cylinders 7cl, 7c2 with the hydraulic oil of the tank 21 can reduce the energy loss and pressure loss that occur in the load reducing operation in which the load is released by operating the working device 6 upward at the time of excavation.

[0056] Moreover, instead of performing the load reducing operation on the working device 6 automatically, the foregoing configuration can allow the operator to perform the load reducing operation by himself if desired. Therefore,

excavation such as foundation excavation that cannot be done easily by the automatic load reducing operation can be

realized more efficiently.

[0057] In addition, because the in-vehicle controller CR

(the operation detector 107) detects the operating state of the working device 6 based on the pilot pressure, pump

pressure and head pressure of the boom cylinders 7cl, 7c2, the excavation state of the working device 6 can easily be

detected with a simple configuration.

[0058] In order to lower the working device 6 of the hydraulic excavator HE with the accumulation circuit A and the regenerative circuit B being separated from each other, the hydraulic oil ejected from the head of the boom cylinder 7cl is accumulated in the first accumulators 46 through the control valve 64, and at the same time the hydraulic oil ejected from the head of the boom cylinder 7c2 is regenerated to the rods of the boom cylinders 7cl, 7c2 by the main control valve 65. Therefore, the pump flow rate corresponding to the regeneration flow rate can be reduced even at the time of the accumulation in the first accumulators 46, and the necessary pump flow rate including the main pump flow rates required by the other hydraulic actuators can easily be ensured with a simple configuration using the control valves 64, 65.

Moreover, the size of the main pumps 12, 13 can be reduced.

[0059] In addition, because the oil from the head of the boom cylinder 7cl is accumulated in the first accumulators 46, the load of the working device 6 is concentrated on the single boom cylinder 7cl instead of being dispersed to the two boom cylinders 7cl, 7c2. As a result, the energy density can be increased, and the pressure generated from the boom cylinder 7cl can be increased, resulting in an increase in the energy to be accumulated in the first accumulators 46. In other words, the sizes of the components such as the first

accumulators 46 and the assist motor 15 can be reduced, resulting in a cost reduction and a simple layout of the circuit .

[0060] Moreover, the control valve 64 changes the amount of communication between the head of the boom cylinder 7cl and the first accumulators 46 in accordance with the operation amount of the lever, and the accumulator pressure of the first accumulators 46. Therefore, the hydraulic oil can be

accumulated in the first accumulators 46 more adequately without compromising the operability of the boom lowering operation, and the operability and energy accumulation can be satisfied at the same time.

[0061] Moreover, in an simultaneous operation where the boom cylinders 7cl, 7c2 are operated in conjunction with the other hydraulic actuators (the slewing motor 3m, the stick cylinder 8c, the bucket cylinder 9c, and the like) , the hydraulic oil ejected from the head of the boom cylinder 7c2 is regenerated to the rods of the boom cylinders 7cl, 7c2.

Therefore, the oil to be regenerated can be fed from the main pump 12, 13 to the other hydraulic actuators, preventing a reduction of the speed of the simultaneous operation and improving the operability of the simultaneous operation.

[0062] Furthermore, with the boom energy recovery valve 31 configured by integrating the plurality of circuit functions into a single block, not only is it possible to obtain a simple layout, but also a cost reduction can be achieved by reducing the number of assembly steps.

[0063] In addition, concentrating a load on the boom cylinder 7cl alone can increase the energy to be accumulated in the first accumulators 46. Therefore, substantial assist can be performed with a small accumulator, resulting in a cost reduction and a compact machine body layout . [0064] By supplying the accumulated hydraulic oil from the first accumulators 46 to the assist motor 15 to assist with the operation of the pumps 12, 13, the load of the built-in engine 11 for operating the pumps 12, 13 can be reduced through an effective use of the accumulated hydraulic oil. [Industrial Applicability]

[0065] The present invention is industrially applicable to all businesses that are concerned in manufacturing and sales of hydraulic circuits or working machines.

[Reference Signs List]

[0066]

CR In-vehicle controller as controller

HE Hydraulic excavator as working machine

Makeup circuit

1 Machine body

6 Working device

7 Boom

7cl, 7c2 Boom cylinder as hydraulic cylinder

8 Stick

9 Bucket

12, 13 Main pump as pump

21 Tank

26, 28 Boom control valve as control valve

61, 78 Check valve

66 Bleed-off valve as bypass valve

Claims

[CLAIMS]

[Claim 1] A hydraulic circuit, comprising:

a hydraulic cylinder that moves a working device up and down by being operated by a working fluid that is pressurized and supplied by a pump in response to an operation of an operating device;

a solenoid proportional control valve for controlling the working fluid supplied from the pump to the hydraulic

cylinder;

a bypass valve for controlling the amount of

communication between a rod of the hydraulic cylinder and a tank;

a controller that detects an operating state of the working device, and when detecting an excavation state of the working device, controls an operation of the control valve and an operation of the bypass valve in accordance with an

operation amount of the operating device for operating at least the working device upward, thereby reduces the working fluid to be supplied from the pump to the hydraulic cylinder, causes at least some of a return fluid from the rod of the hydraulic cylinder to bypass to the tank, and reduces a pump flow rate of the working fluid discharged from the pump; and a makeup circuit that has a channel connecting a head of the hydraulic cylinder and the tank with a check valve

therebetween, uses the bypass valve to cause at least some of a return fluid from the rod to the pump to bypass to the tank, and replenishes the head of the hydraulic cylinder with the working fluid from the tank.

[Claim 2] The hydraulic circuit according to claim 1, wherein the hydraulic cylinder is pilot-operated, and

the controller detects the operating state of the working device based on a pilot pressureof the hydraulic cylinder, a pump pressure, and a head pressure of the hydraulic cylinder.

[Claim 3] A working machine, comprising:

amachine body;

a working device that has a boom provided in such a manner as to be rotatable vertically with respect to the machine body, a. stick provided in such a manner as to be rotatable inward and outward with respect to the boom, and a bucket provided in such a manner as to be rotatable inward and outward with respect to the stick; and

the hydraulic circuit according to claim 1 or 2 that is provided for a hydraulic cylinder that functions at least as a boom cylinder for rotating the boom.