Hydraulic circuit and working machine

Provided are a hydraulic circuit and a work machine capable of improving an initial motion speed of a hydraulic cylinder during a contraction of the hydraulic cylinder and securing a necessary pump flow rate while hydraulic fluid is being accumulated in an accumulator. Hydraulic oil from a head side of a second boom cylinder is regenerated in a first boom cylinder and the second boom cylinder through a second control valve of a regeneration circuit. At the same time, oil from the head side of the first boom cylinder is accumulated to a first accumulator by a pressure accumulating circuit through a first control valve. The hydraulic oil supplied under pressure from a main pump is fed to a rod side of the first boom cylinder by a boom control valve. A bleed-off valve of a bleed-off circuit communicates the first control valve to a tank at the time of initial operation of the first control valve, thereby releasing hydraulic oil from the head end of the first boom cylinder to improve an initial motion speed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national phase application of International Patent Application No. PCT/EP2016/058677 filed Apr. 20, 2016, which claims priority to Japanese Patent Application No. 2015-086579 filed Apr. 21, 2015, both of which are incorporated by reference herein in their entireties for all purposes.

TECHNICAL FIELD

The present invention relates to a hydraulic circuit provided with an accumulator, and a working machine equipped with the hydraulic circuit.

BACKGROUND ART

A working machine is configured to accumulate, in an accumulator, pressure oil that is discharged from a boom hydraulic cylinder when lowering the boom, and to also accumulate, in the accumulator, pressure oil that is relieved from a slewing hydraulic motor when accelerating/decelerating the slewing operation (see, PTL 1, for example).

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

Since the pressure oil discharged from the boom hydraulic cylinder cannot be regenerated to the boom hydraulic cylinder during the accumulation of this pressure oil in the accumulator, a necessary pump flow rate cannot be ensured, slowing down the operating speed of the boom hydraulic cylinder. In addition, the initial speed of the boom hydraulic cylinder needs to be ensured even when the pressure oil discharged from the boom hydraulic cylinder is regenerated to ensure a pump flow rate. Therefore, it is desired that a simpler configuration be employed to regenerate the pressure oil discharged from the boom hydraulic cylinder while ensuring the initial speed of the boom hydraulic cylinder, to ensure a necessary pump flow rate.

The present invention was contrived in view of these circumstances, and an object thereof is to provide a hydraulic circuit and a working machine that are capable of, with a simpler configuration, improving the initial speed of contraction of a hydraulic cylinder and ensuring a necessary pump flow rate even when a working fluid is being accumulated in an accumulator.

Solution to Problem

An invention described in claim1is a hydraulic circuit having: a plurality of hydraulic cylinders that simultaneously actuate the same operation by using a working fluid that is pressurized and supplied by a pump in response to an operation of an operating device; an accumulator in which the working fluid is accumulated; an accumulation circuit that is provided with a first valve for changing the amount of communication between a head of a first hydraulic cylinder of the plurality of hydraulic cylinders and the accumulator in accordance with an operation amount of the operating device, and accumulates a working fluid, which is ejected from the head of the first hydraulic cylinder, in the accumulator through the first valve; a regenerative circuit that is provided with a second valve for blocking communication between heads of the plurality of hydraulic cylinders and enabling communication between the head of a second hydraulic cylinder of the plurality of hydraulic cylinders and rods of the first and second hydraulic cylinders when the accumulation circuit accumulates the working fluid in the accumulator, and regenerates a working fluid, which is ejected from the head of the second hydraulic cylinder, to the first and second hydraulic cylinders through the second valve; a bleed-off circuit that is provided with a third valve for switching between enabling and blocking communication between the first valve and a tank, and returns the working fluid from the first valve to the tank through the third valve at initial operation of the first valve; and a main valve that supplies the working fluid pressurized and supplied by the pump, to the rod of the first hydraulic cylinder while the first valve and the tank communicate with each other by the third valve.

Advantageous Effects of Invention

According to the invention described in claim1, in order to accumulate in the accumulator the working fluid ejected from the head of the first hydraulic cylinder through the first valve with the accumulation circuit and the regenerative circuit being separated from each other, the working fluid is returned to the tank by the bleed-off circuit at initial operation of the first valve, while the working fluid from the pump is supplied to the rod of the first hydraulic cylinder through the main valve. Therefore, the initial speed of contraction of the hydraulic cylinders can be improved. In addition, at the same time with the accumulation of the working fluid in the accumulator, the working fluid ejected from the head of the second hydraulic cylinder is regenerated to the rods of the first and second hydraulic cylinders through the second valve, reducing the regeneration flow rate of the pump at the time of the accumulation of the working fluid in the accumulator, and easily ensuring the necessary pump flow rate with a simple configuration.

According to the invention described in claim2, some of the working fluid ejected from the head of the first hydraulic cylinder is accumulated in the accumulator through the first valve, while the rest of the working fluid is regenerated to the rod of the first hydraulic cylinder through the fourth valve of the auxiliary regenerative circuit. Therefore, the regeneration flow rate of the pump at the time of the accumulation of the working fluid in the accumulator can further be reduced, and the necessary pump rate can be ensured with a simple configuration.

According to the invention described in claim3, the third valve changes the amount of communication between the first valve and the tank in accordance with the operation amount of the operating device and the accumulator pressure. Such a configuration can effectively return the working fluid, which is ejected from the head of the first hydraulic cylinder, to the tank, adequately improving the initial speed of contraction of the hydraulic cylinders.

The invention described in claim4can improve the initial speed of lowering the working device of the working machine and reduce the regeneration flow rate of the pump at the time of the accumulation in the accumulator when lowering the working device, easily ensuring the necessary pump flow rate.

DESCRIPTION OF EMBODIMENTS

The present invention is described hereinafter in detail based on an embodiment shown inFIGS. 1 to 11.

As shown inFIG. 11, a hydraulic excavator HE, which is a working machine, has a machine body1that is configured by a lower traveling body2and an upper slewing body3provided thereon so as to be slewable by a slewing motor3m, wherein the upper slewing body3is equipped with a machine room4equipped with the engine, a pump and the like, a cab5for protecting an operator, and a working device6.

In this working device6, a base end of a boom7that is rotated vertically by two parallel boom cylinders7c1,7c2functioning as hydraulic cylinders is axially supported on the upper slewing body3, a stick8that is rotated back and forth by a stick cylinder8cis axially supported at a tip of the boom7, and a bucket9that is rotated by a bucket cylinder9cis axially supported at a tip of the stick8. The two boom cylinders7c1,7c2are provided parallel to the common boom7and simultaneously actuate the same operation.

FIG. 1toFIG. 3each show an engine power assist system that accumulates position energy of the working device6in an accumulator through the boom cylinder7c1, accumulates kinetic energy of the upper slewing body3in the accumulator through the slewing motor3m, and uses these energies to assist engine power.

A circuit configuration of this system is described next.

An assist motor15is connected to a main pump shaft14of main pumps12,13directly or by a gear, the main pumps12,13being driven by a built-in engine11of the machine room4. The main pumps12,13and the assist motor15each 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 regulators16,17,18and detected by swash plate angle sensors16ϕ,17ϕ,18ϕ. The regulators16,17,18are controlled by a solenoid valve. For example, the regulators16,17of the main pumps12,13can be controlled automatically with a negative flow control pressure (so-called negative control pressure) guided through a negative flow control channel19ncor with a signal other than the negative control pressure by solenoid switching valves19a,19bof a negative flow control valve19functioning as a flow rate control valve.

The main pumps12,13discharge, to channels22,23, hydraulic oil which is a working fluid drawn up from a tank21, and have the pump discharge pressures thereof detected by pressure sensors24,25. Pilot control valves for controlling the directions and flow rates of the hydraulic oil are connected to the main pumps12,13. The pilot control valves include a boom control valve26as a main valve for controlling the boom cylinders7c1,7c2and a boom control valve28as a sub-valve. An output channel27extending from the boom control valve26and an output channel29extending from the boom control valve28are connected to a boom energy recovery valve31, which is a composite valve, by a channel30.

This boom energy recovery valve31is 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, a regenerative circuit B, a bleed-off circuit C, an auxiliary regenerative circuit D, which are shown inFIG. 1andFIG. 2, and a circuit that guides the hydraulic oil, which is pressurized and supplied by the main pumps12,13in a boom lifting operation shown inFIG. 3, to heads of the two boom cylinders7c1,7c2.

A channel32extending from a head-side end of the boom cylinder7c1is connected to the boom energy recovery valve31by a channel34through a drift reduction valve33, and a channel35extending from a head-side end of the boom cylinder7c2is connected to the boom energy recovery valve31by a channel37through a drift reduction valve36. An output channel38extending from the main boom control valve26is connected to the regenerative circuit B of the boom energy recovery valve31. The rods of the boom cylinders7c1,7c2are connected to the boom energy recovery valve31by channels39,40. The drift reduction valves33,36control the opening/closing and apertures between the ports by controlling the pilot pressure of a spring chamber by means of pilot valves, not shown.

The output channel27extending from the main boom control valve26can communicate with the output channel38by a solenoid switching valve42and a check valve43.

The discharge side of the assist motor15is connected to the tank21by a discharge channel44. A tank channel50extending from an accumulator channel47provided with a plurality of first accumulators46is connected to the suction side of the assist motor15through a relief valve48and a check valve49, and a suction-side channel52extending from the accumulator channel47is connected to the same through a solenoid switching valve51. A pressure sensor55for detecting pressure accumulated in the first accumulators46is connected to the accumulator channel47. The tank channel50extends through a tank channel56, a spring check valve57, and an oil cooler58or a spring check valve59and is connected to the tank21. The first accumulators46, the accumulator channel47, the relief valve48, the solenoid switching valve51, and the pressure sensor55are incorporated in the single block to configure an accumulator block60.

The boom energy recovery valve31has a control valve61that is a first valve configuring a part of the accumulation circuit A, a main control valve62that is a second valve functioning as a boom circuit switching valve to configure a part of the regenerative circuit B, a bleed-off valve63that is a third valve configuring a part of the bleed-off circuit C, and a regeneration control valve64that is a fourth valve configuring a part of the auxiliary regenerative circuit D. Pilot-operated valves are used as these valves61to64, the pilot-operated valves being switched when the solenoid switching valves are operated by, for example, the operator in the cab5(FIG. 11) 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 valves61to64are shown as solenoid proportional direction control valves in the diagrams.

The control valve61is a flow rate control valve that allows the hydraulic oil from the boom cylinder7c1to be accumulated in the first accumulators46, by switching between enabling and blocking the communication between the channels68and34connected to the first accumulators46(the accumulator block60) through a check valve67. The control valve61allows the hydraulic oil to flow in an amount larger than the amount of hydraulic oil returned from the normal cylinders (boom cylinders7c1,7c2and the like) to the tank21, and prioritizes accumulation of pressure oil in the first accumulators46.

The main control valve62separates the boom cylinder7c1and the boom cylinder7c2into an accumulation cylinder and a self-regenerative cylinder by switching the relationship between channels71and72, the relationship between channels73and74, and the relationship between channels75and76. Specifically, the main control valve62is configured to block the communication between the heads of the boom cylinders7c1,7c2and enables the communication between the head of the boom cylinder7c2and the rods of the boom cylinders7c1,7c2at the time of accumulation in the first accumulators46by switching the control valve61.

The channel30is connected to the channel71through a check valve78. The channel72is connected to the channel37and a channel79branching off from the channel30. The channel73branches off from the channel72. The channel74is connected to the channel40through a check valve80. The channel75is connected to the output channel38and the channel39, and the channel76branches off from the channel40.

The bleed-off valve63is for switching the relationship between a channel82and a channel83, the channel82branching off from the upstream side of the check valve67with respect to the control valve61, i.e., the channel68, and the channel83communicating with the tank21. Operated in conjunction with the control valve61, this bleed-off valve63is configured to enable the communication between the control valve61and the tank21in the initial stage of switching the control valve61, and to block the communication between the control valve61and the tank21during the switching of the control valve61based on a predetermined condition such as after a lapse of a predetermined short time period (e.g., 0.5 seconds) since the initial stage.

The regeneration control valve64is a flow rate control valve that regenerates some (approximately half) of the hydraulic oil, which is discharged from the head of the boom cylinder7c1to the first accumulators46through the control valve61, to the rod of the boom cylinder7c1, by switching between enabling and blocking the communication between a channel84branching off from the upstream side of the check valve67with respect to the control valve61, i.e., the channel68, and a channel86that extends through a check valve85and is connected to the channel39, i.e., the rod of the boom cylinder7c1. Operated in conjunction with the control valve61, this regeneration control valve64enables the communication between the control valve61and the head of the boom cylinder7c1when accumulating the hydraulic oil in the first accumulators46by switching the control valve61, and blocks the communication between the control valve61and the head of the boom cylinder7c1when blocking the communication between the head of the boom cylinder7c1and the first accumulators46by switching the control valve61.

As shown inFIG. 2, the accumulation circuit A is a circuit where the hydraulic oil flows from the channel32extending from the head-side end of the boom cylinder7c1, passes through the drift reduction valve33, the channel34, the control valve61and check valve67of the boom energy recovery valve31, and the channel68, and reaches the first accumulators46. The accumulation circuit A functions to accumulate in the first accumulators46some (approximately half) of the hydraulic oil ejected from the head of the boom cylinder7c1.

The regenerative circuit B is a circuit where the hydraulic oil flows from the channel35extending from the head-side end of the boom cylinder7c2, passes through the drift reduction valve36, the channel37, the channel73, main control valve62, channel74, check valve80, and channel40of the boom energy recovery valve31, reaches the rod-side end of the boom cylinder7c2, flows again from the channel35, passes through the drift reduction valve36, the channel37, the channel73, main control valve62, channel74, check valve80, channel76, main control valve62, channel75, and channel39in the boom energy recovery valve31, and then reaches the rod-side end of the boom cylinder7c1. The regenerative circuit B functions to regenerate, to the rods of the boom cylinders7c1,7c2, the hydraulic oil ejected from the head of the boom cylinder7c2.

The bleed-off circuit C is a circuit branching off from the accumulation circuit A, in which the hydraulic oil reaches the tank21through the control valve61, channel82, bleed-off valve63, and channel83of the boom energy recovery valve31. The bleed-off circuit C functions to return the hydraulic oil, which is ejected from the head of the boom cylinder7c1, to the tank21at initial operation of the control valve61, or in other words in the initial stage of contraction of the boom cylinders7c1,7c2or the initial stage of a boom lowering operation.

As shown inFIG. 1, the auxiliary regenerative circuit D is a circuit branching off from the accumulation circuit A, in which the hydraulic oil flows from the channel32extending from the head-side end of the boom cylinder7c1, passes through the drift reduction valve33, the channel34, the control valve61, channel84, regeneration control valve64, check valve85, and channel86of the boom energy recovery valve31, and reaches the rod-side end of the boom cylinder7c1through the channel39. The auxiliary regenerative circuit D functions to regenerate, to the rod of the boom cylinder7c1, some of the hydraulic oil ejected from the head of the boom cylinder7c1, except for some of which to be accumulated in the first accumulators46.

Relief valves94,95and check valves97,98that are mutually opposite to each other are provided between channels92,93of a motor drive circuit E that connects a slewing control valve91and the slewing motor3mto each other, the slewing control valve91controlling the slewing direction and speed of the slewing motor3m. A makeup channel99, which has a tank channel function for returning the oil discharged from the motor drive circuit E to the tank21and a makeup function capable of replenishing the motor drive circuit E with hydraulic oil, is connected between the relief valves94,95and between the check valves97,98. The makeup channel99is connected to a second accumulator100that supplies pressure oil: Hydraulic oil is replenished in the channel92or93, whichever is likely to cause a vacuum, from the makeup channel99through the check valves97,98at a pressure that does not exceed the spring biasing force of the spring check valve57.

The channels92,93of the motor drive circuit E are made to communicate with a slewing energy recovery channel104by check valves102,103. This channel104is connected to a channel106through a sequence valve105where the source pressure at the inlet thereof does not change easily due to the back pressure at the outlet of the same. The channel106is connected to the first accumulators46and the channel68.

In the foregoing circuit configuration, the swash plate angle sensors16ϕ,17ϕ,18ϕ and the pressure sensors24,25,55input the detected swash plate angle signals and pressure signals to an in-vehicle controller (not shown), and the valves42,51are switched by an on/off operation using a drive signal output form the in-vehicle controller (not shown) or a proportional action in accordance with the drive signal. The boom control valves26,28, the slewing control valve91, 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 cab5(FIG. 11) or the like operating the lever or pedal. The pilot valves of the drift reduction valves33,36, which are not shown, are also pilot-operated in conjunction with the foregoing valves.

The details controlled by the in-vehicle controller are described functionally hereinafter.

FIG. 1andFIG. 2each show a state of the circuit in which the boom lowering operation for lowering the boom7(FIG. 11) is performed. The hydraulic oil that is ejected from the head of the boom cylinder7c1due to a load or the like of the working device6(FIG. 11) passes through the channel32, the drift reduction valve33, and the channel34, and is returned from the control valve61of the boom energy recovery valve31that is switched to the communication position, to the tank21(FIG. 1) by the bleed-off valve63switched to the communication position in the initial stage. The hydraulic oil is further made to communicate with the channel68and channel84from the control valve61through the check valve67when the bleed-off valve63is switched to the blocking position based on a predetermined condition such as a lapse of a predetermined time period. From the channel68, the hydraulic oil is then accumulated in the first accumulators46, passes through the channel84and the regeneration control valve64switched to the communication position, and is regenerated to the rod of the boom cylinder7c1through the check valve85, the channel86, and the channel39(FIG. 2).

In this state, the control valve61switches the amount of communication between the head of the boom cylinder7c1and the first accumulators46, 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 accumulators46detected by the pressure sensor55. Specifically, the pilot pressure that is set based on the operation amount of the lever is corrected based on a predetermined table (converter) T1, 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 valve61. More specifically, in the present embodiment, in the table T1shown inFIG. 4, 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 TH2that 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 check valve78from returning toward the boom control valve26.

The bleed-off valve63switches the amount of communication between the control valve61and the tank21, 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 accumulators46detected by the pressure sensor55. Specifically, as shown inFIG. 6, a base pressure, which is set based on a predetermined table (converter) T3in accordance with the pilot pressure that is set based on the operation amount of the lever, a gain, which is set based on a predetermined table (converter) T4for accelerating the lowering of the boom in accordance with a predetermined short time period at the start of the boom lowering operation that is measured by a time counter TC1, such as a lapse of 10 ms, and a gain that is set based on a predetermined table (converter) T5in accordance with the accumulator pressure, are integrated together, and this resultant integrated value is obtained as an output for operating the bleed-off valve63. According to the table T3, in the region where the pilot pressure that is set based on the operation amount of the lever is equal to or lower than a predetermined threshold TH4, the amount of increase in the output pressure becomes relatively greater than the amount of increase in the pilot pressure. In the region where the pilot pressure exceeds the predetermined threshold TH4, 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 TH4. In the region where the pilot pressure exceeds a predetermined threshold TH5greater than the predetermined threshold TH4, the output pressure is set constant. In the table T4, the gain increases as time measured by the time counter TC1passes, and between the time where the pilot pressure exceeds a predetermined threshold TH6and the time where the pilot pressure is equal to or lower than a predetermined threshold TH7greater than the predetermined threshold TH6, the gain is set constant. For a predetermined time period after the predetermined threshold TH7, such as for 0.5 ms, the gain decreases as time passes. In the table T5, the gain is set constant with respect to the amount of increase in the accumulator pressure.

The regeneration control valve64switches the amount of communication between the control valve61and the rod of the boom cylinder7c1, 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 accumulators46detected by the pressure sensor55. Specifically, the pilot pressure that is set based on the operation amount of the lever is corrected based on a predetermined table (converter) T6, and the accumulator pressure is corrected based on a predetermined table (converter) T7. Then, the result obtained by integrating these corrected values is obtained as an output for operating the regeneration control valve64. More specifically, in the present embodiment, in the table T6shown inFIG. 7, when the pilot pressure that is set based on the operation amount of the lever is relatively small, the output pressure thereof increases in proportion to an increase in the input pressure of the same. Therefore, in the region where the pilot pressure set based on the operation amount of the lever exceeds a predetermined threshold TH8, the output pressure is set constant. Furthermore, in the table T7, the gain is set constant with respect to the amount of increase in the accumulator pressure.

At the same time, the direction of the hydraulic oil ejected from the head of the boom cylinder7c2is controlled to allow the hydraulic oil to flow toward the channel74through the channel35, the drift reduction valve36, the channel37, the main control valve62of the boom energy recovery valve31, and the channel73. The hydraulic oil further passes through the check valve80and the channel40and is regenerated to the rod of the boom cylinder7c2. Then, the direction of the hydraulic oil branching off to the channel76through the check valve80is controlled to allow the hydraulic oil to flow to the channel75through the check valve inside the main control valve62. Consequently, the hydraulic oil passes through the channel39and is regenerated to the rod of the boom cylinder7c1. At this moment, the operation amount of the main control valve62changes 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) T8, and the resultant pressure is taken as an output for operating the main control valve62. More specifically, in the present embodiment, the table T8similar to the table T1shown inFIG. 4is 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 inFIG. 5, and basically the main control valve62is 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 cylinder7c2is returned from the boom control valve26to the tank21after passing through the channel37, the channel79, and the channel30. In addition, for example, in a case where grounding of the working device6(FIG. 11) is detected based on the head pressure of the boom cylinders7c1,7c2and thereby it is detected that lowering of the boom results in lifting of the machine body1, separation of the boom cylinders7c1,7c2into the accumulation cylinder and the self-regenerative cylinder is canceled in accordance with a predetermined set value.

Using the control valve61, the regeneration control valve64and the main control valve62, the boom energy recovery valve31accumulates the hydraulic oil in the first accumulators46at the time of lowering the boom and at the same time regenerates the hydraulic oil to the rods of the boom cylinders7c1,7c2.

Some of the hydraulic oil discharged from the main pump12at the time of the boom lowering operation is supplied to the rod of the boom cylinder7c1from the boom control valve26through the output channel38and the channel39. At this moment, only at the start of the boom lowering operation where the bleed-off valve63is in the communication position and thereby the hydraulic oil, which is ejected from the head of the boom cylinder7c1, is returned to the tank21from the bleed-off circuit C through the control valve61, the boom control valve26supplies the hydraulic oil to the rod of the boom cylinder7c1through the output channel38and the channel39at the maximum flow rate, in conjunction with the bleed-off valve63. And when the bleed-off valve63is in the blocking position and thereby the boom7starts to descend, the hydraulic oil from the head of the boom cylinder7c2is regenerated to the rods of the boom cylinders7c1,7c2, thereby restricting the flow rate.

The pump flow rate from the main pump12controlled by the boom control valve26to the boom cylinder7c1is set by the solenoid switching valve19aof the negative flow control valve19in accordance with the operation amount of the lever, i.e., the pilot pressure that is set based on this operation amount, and the accumulator pressure of the first accumulators46. Specifically, in the present embodiment, as shown inFIG. 8, the base flow rate of this pump flow rate is set as follows. In other words, the minimum value of a flow rate that is set based on a predetermined table (converter) T9in accordance with the pilot pressure set based on the operation amount of the lever is obtained, as well as the minimum value of a flow rate that is set based on a predetermined table (converter) T10in accordance with a predetermined short time period at the start of the boom lowering operation that is measured by a time counter TC2, such as a lapse of 10 ms. Then, an accelerated flow rate that is set based on a predetermined table (converter) T11in accordance with a predetermined short time period at the start of the boom lowering operation that is measured by the time counter TC2, such as a lapse of 10 ms, is integrated with a gain that is set based on a predetermined table (converter) T12in accordance with the pilot pressure that is set based on the operation amount of the lever. The foregoing minimum values or the resultant integrated value, whichever is bigger, is set as the base flow rate. In the table T9, the flow rate is set constant in the region where the pilot pressure that is set based on the operation amount of the lever is equal to or lower than a predetermined threshold TH9. However, in the region where the pilot pressure exceeds the predetermined threshold TH9but is equal to or lower than a predetermined threshold TH10that is greater than the predetermined threshold TH9, the flow rate decreases in proportion to an increase in the pilot pressure. Thus, the flow rate is set constant in the region where the pilot pressure exceeds the predetermined threshold TH10. According to the table T10, the flow rate increases as time measured by the time counter TC2passes, and the flow rate is set constant from the time where the pilot pressure exceeds a predetermined threshold TH11. According to the table T11, the flow rate increases as time measured by the time counter TC2passes, and then the flow rate is set constant between the time where the pilot pressure exceeds a predetermined threshold TH12and the time where the pilot pressure is equal to or lower than a predetermined threshold TH13that is greater than the predetermined threshold TH12. From the time where the pilot pressure exceeds the predetermined threshold TH13, the flow rate decreases as time passes. According to the table T12, when the pilot pressure that is set based on the operation amount of the lever is relatively small, the gain increases in proportion to an increase in the pilot pressure, and the gain is set constant (e.g.,1) in the region where the pilot pressure exceeds a predetermined threshold TH14.

As shown inFIG. 9, a flow rate that is obtained by integrating the base flow rate described above with a gain that is set based on the predetermined table (converter) T13in accordance with the accumulator pressure, is set as the foregoing pump flow rate for the boom lowering operation alone. When a lever operation such as a stick-in operation, a stick-out operation, a bucket-in operation, or a bucket-out operation is performed simultaneously with the boom lowering operation, flow rates that are set based on predetermined tables (converters) T14to T17in accordance with the pilot pressures set based on these operations are added up. In the table T13, the gain is set constant (e.g.,1) when the accumulator pressure is equal to or lower than a predetermined threshold TH15. In the region where the accumulator pressure exceeds the predetermined threshold TH15, when the accumulator pressure is relatively small, the amount of increase in the gain is relatively greater than the amount of increase in the accumulator pressure. In the region where the accumulator pressure exceeds the predetermined threshold TH15but is equal to or lower than a predetermined threshold TH16that is greater than the predetermined threshold TH15, the amount of increase in the gain with respect to the amount of increase in the accumulator pressure is reduced more compared to when the accumulator pressure is equal to or lower than the threshold TH15. Furthermore, in the region where the accumulator pressure exceeds a predetermined threshold TH17that is greater than the predetermined threshold TH16, the gain is set constant (greater than 1). In each of the tables T14to T17, in the region where the pilot pressure set by the operation amount of the lever is equal to or lower than a predetermined threshold TH18, the amount of increase in the flow rate is relatively greater than the amount of increase in the pilot pressure, and in the region where the pilot pressure exceeds the predetermined threshold TH18but is equal to or lower than a predetermined threshold TH19that is greater than the predetermined threshold TH18, the amount of increase in the flow rate with respect to the amount of increase in the pilot pressure is reduced more compared to when the pilot pressure is equal to or lower than the threshold TH18. Furthermore, in the region where the pilot pressure exceeds the predetermined threshold TH19, the flow rate is set constant. These tables T14to T17may be identical or have the values of the thresholds TH18and TH19different from each other.

FIG. 3shows a state of the circuit in which the boom lifting operation for raising the boom7(FIG. 11) is performed. In the boom lifting operation, the boom energy recovery valve31not only switches the control valve61and the regeneration control valve64to the blocking position but also switches the main control valve62to stop the accumulation of the hydraulic oil in the first accumulators46and the regeneration of the same to the rods of the boom cylinders7c1,7c2. The boom energy recovery valve31also guides the hydraulic oil, which is supplied from the main pumps12,13to the channel30through the boom control valves26,28, from the channel79to the head of the boom cylinder7c2through the channel37, the drift reduction valve36, and the channel35, and further guides the hydraulic oil from the check valve78to the head of the boom cylinder7c1through the channel34, the drift reduction valve33, and the channel32. The hydraulic oil ejected from the rod of the boom cylinder7c1is returned to the tank21from the channel39and the output channel38through the boom control valve26. The direction of the hydraulic oil ejected from the rod of the boom cylinder7c2is controlled to allow the hydraulic oil to flow to the channel75through the channel40, the channel76, and the main control valve62, thereby returning the hydraulic oil to the tank21from the output channel38through the boom control valve26.

In the boom lowering operation and the boom lifting operation, engine power assist can be performed in which the assist motor15with a motor function, which is coupled to the main pump shaft14directly or by a gear, is caused to function as a hydraulic motor as shown inFIG. 3, to reduce the engine load. For example, in the boom lowering operation, the engine power assist is performed when the pressure sensor55detects that the accumulator pressure of the first accumulators46that is accumulated through the control valve61is equal 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 sensor55detects that the accumulator pressure of the first accumulators46is equal to or greater than a predetermined second threshold different from the predetermined first threshold. In this engine power assist, the solenoid switching valve51is switched to the communication position, and the assist motor15is rotated by the energy accumulated in the first accumulators46, to assist the hydraulic outputs, of the main pumps12,13and reduce the engine load. When the machine body1is lifted, the engine power assist is not performed using the assist motor15.

Specifically, as shown inFIG. 10, a logical sum of a logical product of flags that are set at 0 and 1 for the boom lowering operation (only when lowering the boom) and an operation other than the boom lowering operation (a state other than when lowering the boom) and a flag that is set according to the accumulator pressure based on a predetermined table (converter) T18corresponding to the operation other than the boom lowering operation, and a flag that is set according to the accumulator pressure based on a predetermined table (converter) T19corresponding to the boom lowering operation, is output. When this flag is ON or in other words 1, the solenoid switching valve51is switched to the communication position. When the assist flag is OFF or in other words 0, the solenoid switching valve51is switched to the blocking position. In the table T18in which a predetermined threshold TH20and a predetermined threshold TH21greater than the predetermined threshold TH20are set, the flag is switched from 0 to 1 when the accumulator pressure increases to become equal to or greater than the threshold TH21, and the flag is switched from 1 to 0 when the accumulator pressure decreases to become equal to or lower than the threshold TH20. In the table T19in which a predetermined threshold TH22greater than the predetermined threshold TH21and a predetermined threshold TH23greater than the predetermined threshold TH22are set, the flag is switched from 0 to 1 when the accumulator pressure increases to become equal to or greater than the threshold TH23, and the flag is switched from 1 to 0 when the accumulator pressure decreases to become equal to or lower than the threshold TH22. These tables T18and T19, therefore, each have so-called hysteresis in which the thresholds vary depending on the increase and decrease of the accumulator pressure.

Therefore, by rotating the assist motor15by means of the energy from the head of the boom cylinder7c1that is accumulated in the first accumulators46, the engine power assist function reduces, by using the assist motor15, the load of the built-in engine11that is coupled thereto by the main pump shaft14.

As a result, when, for example, the boom lowering operation is executed, four sequences are established: a first sequence in which the control valve61is switched to the communication position and the main control valve62is switched to the position for blocking the communication between the heads of the boom cylinders7c1,7c2and enabling the communication between the head of the boom cylinder7c2and the rods of the boom cylinders7c1,7c2, to form the accumulation circuit A and the regenerative circuit B; a second sequence (FIG. 1) following the first sequence, which is a short period of time in which the bleed-off valve63is switched to the communication position to return the hydraulic oil to the tank21through the bleed-off circuit C, and the hydraulic oil is supplied from the main pump12to the rod of the boom cylinder7c1by the boom control valve26to accelerate the contraction of the boom cylinders7c1,7c2, i.e., the lowering of the boom; a third sequence (FIG. 2) following the second sequence, in which the bleed-off valve63is switched to the blocking position to accumulate the hydraulic oil from the control valve61in the first accumulators46, and some of the hydraulic oil to be accumulated in the first accumulators46is regenerated to the rod of the boom cylinder7c1by switching the regeneration control valve64to the communication position, and the supply of hydraulic oil to the rod of the boom cylinder7c1is minimized using the boom control valve26; and a fourth sequence following the third sequence, in which, while accumulating the hydraulic oil in the first accumulators46, the assist motor15is rotated using the accumulated excess energy.

In order to lower the working device6of the hydraulic excavator HE with the accumulation circuit A and the regenerative circuit B being separated from each other as described above, when some (approximately half) of the hydraulic oil ejected from the head of the boom cylinder7c1is accumulated in the first accumulators46through the control valve61, the hydraulic oil is returned to the tank21through the bleed-off valve63of the bleed-off circuit C at initial operation of this control valve61, and the hydraulic oil from the main pump12is supplied to the rod of the boom cylinder7c1through the boom control valve26, improving the initial speed of contraction of the boom cylinders7c1,7c2. In other words, because connecting the head of the boom cylinder7c1to the first accumulators46by means of the accumulation circuit A leads to an increase of the back pressure, the contraction of the boom cylinders7c1,7c2can easily be accelerated at initial operation by releasing the back pressure instantaneously through the bleed-off valve63of the bleed-off circuit C for a certain period of time.

Furthermore, because the hydraulic oil ejected from the head of the boom cylinder7c2is regenerated to the rods of the boom cylinders7c1,7c2through the main control valve62at the same time as when the hydraulic oil is accumulated in the first accumulators46, the regeneration flow rates of the main pumps12,13at the time of the accumulation in the first accumulators46can be reduced, 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. Moreover, the size of the main pumps12,13can be reduced.

In addition, because some of the hydraulic oil ejected from the head of the boom cylinder7c1is accumulated in the first accumulators46through the control valve61and the rest of the hydraulic oil is regenerated to the rod of the boom cylinder7c1through the regeneration control valve64of the auxiliary regenerative circuit D, the regeneration flow rate of the main pump12at the time of the accumulation in the first accumulators46can be further reduced, easily ensuring the necessary pump flow rate with a simple configuration.

Moreover, even in an simultaneous operation where the boom cylinders7c1,7c2are operated in conjunction with the other hydraulic actuators (the slewing motor3m, the stick cylinder8c, the bucket cylinder9c, and the like), some of the hydraulic oil ejected from the head of the boom cylinder7c1is regenerated to the rod of this boom cylinder7c1by the auxiliary regenerative circuit D, while the hydraulic oil ejected from the head of the boom cylinder7c2is regenerated to the rods of the boom cylinders7c1,7c2by the same. Therefore, the amount of oil to be regenerated can be sent from the main pump12,13to the other hydraulic actuators, preventing a reduction of the speed of the simultaneous operation and improving the operability of the simultaneous operation. In addition, such a configuration can effectively prevent a sudden descent of the boom7which is caused when the regeneration flow rates to the rods of the boom cylinders7c1,7c2increases drastically at the time of stroke end of the other actuators.

In addition, because some of the oil from the head of the boom cylinder7c1is accumulated in the first accumulators46, the load of the working device6is concentrated on the single boom cylinder7c1instead of being dispersed to the two boom cylinders7c1,7c2. As a result, the energy density can be increased, and the pressure generated from the boom cylinder7c1can be increased, resulting in an increase in the energy to be accumulated in the first accumulators46. In other words, the sizes of the components such as the first accumulators46and the assist motor15can be reduced, resulting in a cost reduction and a simple layout of the circuit.

By causing the bleed-off valve63to change the amount of communication between the control valve61and the tank21in accordance with the operation amount of the lever and the accumulator pressure, the hydraulic oil ejected from the head of the boom cylinder7c1can be returned to the tank21effectively, improving the initial speed of contraction of the boom cylinders7c1,7c2adequately.

Furthermore, the control valve61changes the amount of communication between the head of the boom cylinder7c1and the first accumulators46in accordance with the operation amount of the lever and the accumulator pressure of the first accumulators46, and the regeneration control valve64changes the amount of communication between the control valve61and the rod of the boom cylinder7c1in accordance with the operation amount of the lever and the accumulator pressure. Thus, not only is it possible to accumulate the hydraulic oil in the first accumulators46more adequately without compromising the operability of the boom lowering operation, but also the operability and energy accumulation can be satisfied at the same time. In addition, the flow rate of the hydraulic oil discharged from the main pumps12,13to the rod of the boom cylinder7c1can be reduced by effectively regenerating the hydraulic oil to the rod of the boom cylinder7c1, ensuring the necessary pump flow rate more easily.

With the boom energy recovery valve31configured 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.

In addition, concentrating a load on the boom cylinder7c1alone can increase the energy to be accumulated in the first accumulators46. Therefore, substantial assist can be performed with a small accumulator, resulting in a cost reduction and a compact machine body layout.

INDUSTRIAL APPLICABILITY

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