Excavator and control valve for excavator

An excavator includes a lower travelling body; an upper turning body mounted on the lower travelling body; an engine installed in the upper turning body; a hydraulic pump connected to the engine; a hydraulic actuator driven by hydraulic oil discharged by the hydraulic pump to move a work element; a first control valve configured to control a flow rate of the hydraulic oil flowing from the hydraulic pump to the hydraulic actuator; a second control valve configured to control a flow rate of the hydraulic oil flowing from the hydraulic actuator to a hydraulic oil tank; and a control device configured to control opening and closing of the second control valve.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an excavator having a control valve for adjusting the flow rate of hydraulic oil flowing from a hydraulic cylinder to a hydraulic oil tank, and a control valve for the excavator installed in the excavator.

2. Description of the Related Art

An excavator provided with a control valve for adjusting the flow rate of hydraulic oil flowing from a hydraulic cylinder to a hydraulic oil tank is known in the related art.

The control valve has a switchable valve position including an internal flow path for communicating the hydraulic cylinder and the hydraulic oil tank. In the internal flow path, a first diaphragm is formed, so that the operating speed of the hydraulic cylinder can be suppressed.

Furthermore, the excavator of the related art has a switching valve in the return oil line between the control valve and the hydraulic oil tank. The switching valve can switch between a valve position including the internal flow path having a second diaphragm and a valve position including the internal flow path without the second diaphragm.

With this configuration, in the excavator of the related art, the hydraulic oil can flow from the hydraulic cylinder to the hydraulic oil tank through the flow path including the first diaphragm and the second diaphragm connected in series. As a result, it is possible to set the opening area of the first diaphragm to be larger, and compared to a case without the switching valve, the fluid noise when the hydraulic oil passes through the first diaphragm can be reduced.

SUMMARY OF THE INVENTION

According to an embodiment of the present invention, there is provided an excavator including a lower travelling body; an upper turning body mounted on the lower travelling body; an engine installed in the upper turning body; a hydraulic pump connected to the engine; a hydraulic actuator driven by hydraulic oil discharged by the hydraulic pump to move a work element; a first control valve configured to control a flow rate of the hydraulic oil flowing from the hydraulic pump to the hydraulic actuator; a second control valve configured to control a flow rate of the hydraulic oil flowing from the hydraulic actuator to a hydraulic oil tank; and a control device configured to control opening and closing of the second control valve.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the excavator of the related art, when the hydraulic oil is caused to flow from the hydraulic cylinder to the hydraulic oil tank, the hydraulic oil is passed through the diaphragm, in any case. Therefore, for example, when closing the arm in the air, the closing speed of the arm can be appropriately suppressed; however, in the case of closing the arm for excavation work, unnecessary pressure loss is caused by the diaphragm.

In view of the above, it is desirable to provide an excavator that reduces, when necessary, the pressure loss caused when hydraulic oil is caused to flow from a hydraulic cylinder to a hydraulic oil tank.

First, with reference toFIG. 1, an excavator that is a construction machine according to an embodiment of the present invention will be described.FIG. 1is a side view of the excavator. An upper turning body3is mounted on a lower travelling body1of the excavator illustrated inFIG. 1, via a turning mechanism2. A boom4that is a work element is attached to the upper turning body3. An arm5that is a work element is attached to the tip of the boom4, and a bucket6that is a work element and an end attachment is attached to the tip of the arm5. The boom4, the arm5, and the bucket6are hydraulically driven by a boom cylinder7, an arm cylinder8, and a bucket cylinder9, respectively. A cabin10is provided on the upper turning body3and a power source such as an engine11is mounted on the upper turning body3.

FIG. 2is a block diagram illustrating a configuration example of a driving system of the excavator ofFIG. 1, in which a mechanical power transmission line, a hydraulic oil line, a pilot line, and an electric control line are indicated by a double line, a bold solid line, a broken line, and a dotted line, respectively.

The driving system of the excavator mainly includes the engine11, a regulator13, a main pump14, a pilot pump15, a control valve unit17, an operation device26, a pressure sensor29, a controller30, and a pressure control valve31.

The engine11is a driving source of the excavator. In the present embodiment, the engine11is, for example, a diesel engine that is an internal combustion engine operating to maintain a predetermined rotational speed. An output shaft of the engine11is connected to input shafts of the main pump14and the pilot pump15.

The main pump14supplies hydraulic oil to the control valve unit17via a hydraulic oil line. The main pump14is, for example, a swash plate type variable displacement hydraulic pump.

The regulator13controls the discharge amount of the main pump14. In the present embodiment, the regulator13controls the discharge amount of the main pump14, for example, by adjusting the swash plate tilt angle of the main pump14according to the discharge pressure of the main pump14and control signals from the controller30, etc.

The pilot pump15supplies hydraulic oil to various hydraulic control devices including the operation device26and the pressure control valve31, via the pilot line. The pilot pump15is, for example, a fixed displacement type hydraulic pump.

The control valve unit17is a hydraulic control device for controlling the hydraulic system in the excavator. Specifically, the control valve unit17includes control valves171to176as first control valves (first spool valves) and a control valve177as a second control valve (second spool valves) for controlling the flow of hydraulic oil discharged by the main pump14. The control valve unit17selectively supplies the hydraulic oil discharged by the main pump14to one or more hydraulic actuators through the control valves171to176. The control valves171to176control the flow rate of the hydraulic oil flowing from the main pump14to the hydraulic actuator and the flow rate of the hydraulic oil flowing from the hydraulic actuator to the hydraulic oil tank. The hydraulic actuator includes the boom cylinder7, the arm cylinder8, the bucket cylinder9, a left side traveling hydraulic motor1A, a right side traveling hydraulic motor1B, and a turning hydraulic motor2A. Through the control valve177, the control valve unit17selectively causes the hydraulic oil, which is flowing out from the hydraulic actuator, to flow to the hydraulic oil tank. The control valve177controls the flow rate of the hydraulic oil flowing from the hydraulic actuator to the hydraulic oil tank.

The operation device26is a device used by the operator for operating the hydraulic actuator. In the present embodiment, the operation device26supplies the hydraulic oil discharged by the pilot pump15into the pilot port of the control valve corresponding to each of the hydraulic actuators, via the pilot line. The pressure (pilot pressure) of the hydraulic oil supplied to each of the pilot ports is pressure corresponding to the operation direction and the operation amount of a lever or a pedal (not illustrated) of the operation device26corresponding to each of the hydraulic actuators.

The pressure sensor29detects the operation content of the operator using the operation device26. The pressure sensor29detects, for example, in the form of pressure, the operation direction and the operation amount of a lever or a pedal of the operation device26corresponding to each of the hydraulic actuators, and outputs the detected value to the controller30. The operation content of the operation device26may be detected using a sensor other than the pressure sensor.

The controller30is a control device for controlling the excavator. In the present embodiment, the controller30is formed of a computer including, for example, a CPU, a RAM, and a ROM, etc. The controller30reads programs respectively corresponding to a work content determining unit300and a meter-out control unit301from the ROM, loads the programs into the RAM, and causes the CPU to execute processes corresponding to the programs.

Specifically, the controller30executes processes by the work content determining unit300and the meter-out control unit301based on outputs from various sensors. Subsequently, the controller30appropriately outputs control signals corresponding to the processing results of the work content determining unit300and the meter-out control unit301, to the regulator13and the pressure control valve31, etc.

For example, the work content determining unit300determines whether the closing motion of the arm5is an operation for high load work such as excavation work, or an operation for low load work such as leveling work. In the present embodiment, when the detection value of the arm bottom pressure sensor, which detects the pressure of the bottom side oil chamber of the arm cylinder8, is greater than or equal to a predetermined value, the work content determining unit300determines that the operation is for high load work. Then, when the work content determining unit300determines that the work is high load work, the meter-out control unit301outputs a control instruction to the pressure control valve31.

The pressure control valve31operates according to a control instruction output from the controller30. In the present embodiment, the pressure control valve31is a solenoid valve that adjusts the control pressure introduced from the pilot pump15into the pilot port of the control valve177in the control valve unit17according to a current instruction output from the controller30. The controller30increases the opening area of the flow path associated with the control valve177by operating the control valve177installed in a pipeline connecting the rod side oil chamber of the arm cylinder8and the hydraulic oil tank, for example. With this configuration, the controller30can reduce the pressure loss caused by the hydraulic oil flowing from the rod side oil chamber of the arm cylinder8to the hydraulic oil tank, when closing the arm5for high load work.

The work content determining unit300may determine whether the operation of lowering the boom4is an operation for high load work or an operation for low load work. In this case, when the detection value of the boom rod pressure sensor that detects the pressure in the rod side oil chamber of the boom cylinder7, is greater than or equal to a predetermined value, the work content determining unit300determines that the operation is for high load work. Then, when the work content determining unit300determines that the operation is for high load work, the meter-out control unit301outputs a control instruction to the pressure control valve31. The pressure control valve31operates the control valve177installed in a pipeline connecting the bottom side oil chamber of the boom cylinder7and the hydraulic oil tank to increase the opening area of the flow path associated with the control valve177. With this configuration, the controller30can reduce the pressure loss caused by the hydraulic oil flowing from the bottom side oil chamber of the boom cylinder7to the hydraulic oil tank when lowering the boom4for high load work.

The work content determining unit300may determine whether regeneration is being performed at the time of lowering the boom. The regeneration at the time of boom lowering is, for example, the control that is implemented to open the arm5by causing the hydraulic oil flowing out from the bottom side oil chamber of the boom cylinder7to flow into the rod side oil chamber of the arm cylinder8. Based on the output of the pressure sensor29, for example, the work content determining unit300determines whether regeneration is being performed at the time of boom lowering. Then, when the work content determining unit300determines that regeneration is being performed at the time of boom lowering, the meter-out control unit301reduces the opening area of the flow path associated with the control valve installed in a pipeline connecting the bottom side oil chamber of the boom cylinder7and the hydraulic oil tank. For example, the meter-out control unit301blocks the flow of hydraulic oil from the bottom side oil chamber of the boom cylinder7to the first control valve (control valve175) by any means. Then, the meter-out control unit301outputs a control instruction to the pressure control valve31to adjust the opening area of the flow path associated with a second control valve (control valve177) installed in the pipeline connecting the bottom side oil chamber of the boom cylinder7and the hydraulic oil tank. Typically, the opening area of the flow path associated with the second control valve is adjusted so as to be smaller than the opening area of the flow path associated with the first control valve, when it is determined that regeneration is not being performed at the time of boom lowering. With this configuration, the controller30can increase the amount (regeneration amount) of hydraulic oil flowing from the bottom side oil chamber of the boom cylinder7to the rod side oil chamber of the arm cylinder8.

Next, with reference toFIG. 3, details of the hydraulic system installed in the excavator will be described.FIG. 3is a schematic diagram illustrating a configuration example of a hydraulic system installed in the excavator ofFIG. 1. InFIG. 3, similar toFIG. 2, the mechanical power transmission line, the hydraulic oil line, the pilot line, and the electric control line are indicated by a double line, a bold solid line, a broken line, and a dotted line, respectively.

InFIG. 3, the hydraulic system circulates hydraulic oil from the main pumps14L,14R driven by the engine11, through center bypass pipelines40L,40R and parallel pipelines42L,42R, to the hydraulic oil tank. The main pumps14L,14R correspond to the main pump14inFIG. 2.

The center bypass pipeline40L is a hydraulic oil line passing through the control valves171,173,175A, and176A disposed in the control valve unit17. The center bypass pipeline40R is a hydraulic oil line passing through the control valves172,174,175B, and176B disposed in the control valve unit17.

The control valve171is a spool valve for switching the flow of the hydraulic oil, in order to supply the hydraulic oil discharged by the main pump14L to the left side traveling hydraulic motor1A, and also to discharge the hydraulic oil discharged by the left side traveling hydraulic motor1A to the hydraulic oil tank.

The control valve172is a spool valve for switching the flow of the hydraulic oil, in order to supply the hydraulic oil discharged by the main pump14R to the right side traveling hydraulic motor1B, and also to discharge the hydraulic oil discharged by the right side traveling hydraulic motor1B to the hydraulic oil tank.

The control valve173is a spool valve for switching the flow of the hydraulic oil, in order to supply the hydraulic oil discharged by the main pump14L to the turning hydraulic motor2A, and to discharge the hydraulic oil discharged by the turning hydraulic motor2A to the hydraulic oil tank.

The control valve174is a spool valve for supplying the hydraulic oil discharged by the main pump14R to the bucket cylinder9and to discharge the hydraulic oil in the bucket cylinder9to the hydraulic oil tank.

The control valves175A,175B are spool valves that are boom-use first control valves for switching the flow of the hydraulic oil, in order to supply the hydraulic oil discharged by the main pumps14L,14R to the boom cylinder7, and to discharge the hydraulic oil in the boom cylinder7to the hydraulic oil tank. In the present embodiment, the control valve175A operates only when the boom4is raised, and does not operate when the boom4is lowered.

The control valves176A,176B are spool valves that are arm-use first control valves for switching the flow of the hydraulic oil, in order to supply the hydraulic oil discharged by the main pumps14L,14R to the arm cylinder8, and to discharge the hydraulic oil in the arm cylinder8to the hydraulic oil tank.

The control valve177A is a spool valve that is an arm-use second control valve that controls the flow rate of the hydraulic oil flowing out from the rod side oil chamber of the arm cylinder8to the hydraulic oil tank. The control valve177B is a spool valve that is a boom-use second control valve that controls the flow rate of hydraulic oil flowing out from the bottom side oil chamber of the boom cylinder7to the hydraulic oil tank. The control valves177A,177B correspond to the control valve177inFIG. 2

The control valves177A,177B have a first valve position with a minimum opening area (opening degree 0%) and a second valve position with a maximum opening area (opening degree 100%). The control valves177A,177B are movable in a stepless manner between the first valve position and the second valve position.

The parallel pipeline42L is a hydraulic oil line parallel to the center bypass pipeline40L. The parallel pipeline42L can supply hydraulic oil to a control valve on a further downstream side, when the flow of the hydraulic oil passing through the center bypass pipeline40L is limited or blocked by any one of the control valves171,173, and175A. The parallel pipeline42R is a hydraulic oil line parallel to the center bypass pipeline40R. The parallel pipeline42R can supply hydraulic oil to a control valve on a further downstream side, when the flow of hydraulic oil passing through the center bypass pipeline40R is limited or blocked by any one of the control valves172,174, and175B.

The regulators13L,13R control the discharge amounts of the main pumps14L,14R, for example, by adjusting the swash plate tilt angles of the main pumps14L,14R according to the discharge pressure of the main pumps14L,14R. The regulators13L,13R correspond to the regulator13inFIG. 2. Specifically, for example, when the discharge pressure of the main pumps14L,14R become greater than or equal to a predetermined value, the regulators13L,13R adjust the swash plate tilt angle of the main pumps14L,14R to decrease the discharge amount. This is done in order to prevent the absorption horsepower of the main pump14, represented by the product of the discharge pressure and the discharge amount, from exceeding the output horsepower of the engine11.

An arm operation lever26A is an example of the operation device26, and is used for operating the arm5. The arm operation lever26A introduces the control pressure corresponding to the lever operation amount into the pilot ports of the control valves176A,176B, by using the hydraulic oil discharged by the pilot pump15. Specifically, when the arm operation lever26A is operated in the arm closing direction, the hydraulic oil is introduced into the right pilot port of the control valve176A, and the hydraulic oil is introduced into the left pilot port of the control valve176B. When the arm operation lever26A is operated in the arm opening direction, the hydraulic oil is introduced into the left pilot port of the control valve176A, and the hydraulic oil is introduced into the right pilot port of the control valve176B.

A boom operation lever26B is an example of the operation device26and is used for operating the boom4. The boom operation lever26B introduces the control pressure corresponding to the lever operation amount into the pilot ports of the control valves175A,175B, by using the hydraulic oil discharged by the pilot pump15. Specifically, when the boom operation lever26B is operated in the boom raising direction, the hydraulic oil is introduced into the right pilot port of the control valve175A, and the hydraulic oil is introduced into the left pilot port of the control valve175B. On the other hand, when the boom operation lever26B is operated in the boom lowering direction, hydraulic oil is introduced only into the right pilot port of the control valve175B, without introducing hydraulic oil into the left pilot port of the control valve175A.

The pressure sensors29A,29B are examples of the pressure sensor29, and detect, in the form of pressure, the operation contents by the operator with respect to the arm operation lever26A and the boom operation lever26B, and output the detected values to the controller30. The operation content is, for example, a lever operation direction and a lever operation amount (lever operation angle), etc.

Left and right traveling levers (or pedals), a bucket operation lever, and a turning operation lever (none are illustrated), are operation devices that respectively operate the traveling of the lower travelling body1, the opening and closing of the bucket6, and the turning of the upper turning body3. Similar to the case of the arm operation lever26A, these operation devices introduce the control pressure corresponding to the lever operation amount (or the pedal operation amount) to the left or right pilot port of the control valve corresponding to each of the hydraulic actuators, by using the hydraulic oil discharged by the pilot pump15. Similar to the case of the pressure sensor29A, the operation contents by the operator for each of these operation devices are detected in the form of pressure by the corresponding pressure sensors, and the detection values are output to the controller30.

The controller30receives the output of the pressure sensor29A, etc., outputs a control signal to the regulators13L,13R as necessary, and changes the discharge amount of the main pumps14L,14R.

The pressure control valves31A,31B adjust the control pressure introduced from the pilot pump15into the pilot ports of the control valves177A,177B, according to a current instruction output from the controller30. The pressure control valves31A,31B correspond to the pressure control valve31inFIG. 2.

The pressure control valve31A is capable of adjusting the control pressure so that the control valve177A can be stopped at any position between the first valve position and the second valve position. The pressure control valve31B is capable of adjusting the control pressure so that the control valve177B can be stopped at any position between the first valve position and the second valve position.

Here, negative control adopted in the hydraulic system ofFIG. 3will be described.

The center bypass pipelines40L,40R are provided with negative control diaphragms18L,18R between the respective control valves176A,176B located at the most downstream side and the hydraulic oil tank. The flow of the hydraulic oil discharged by the main pumps14L,14R is limited by the negative control diaphragms18L,18R. Then, the negative control diaphragms18L,18R generate control pressure (hereinafter referred to as “negative control pressure”) for controlling the regulators13L,13R.

Negative control pressure pipelines41L,41R indicated by broken lines are pilot lines for transmitting the negative control pressure generated upstream of the negative control diaphragms18L,18R to the regulators13L,13R.

The regulators13L,13R control the discharge amounts of the main pumps14L,14R by adjusting the swash plate tilt angle of the main pumps14L,14R according to the negative control pressure. In the present embodiment, the regulators13L,13R decrease the discharge amounts of the main pumps14L,14R as the introduced negative control pressure increases, and increase the discharge amounts of the main pumps14L,14R as the introduced negative control pressure decreases.

Specifically, as illustrated inFIG. 3, when none of the hydraulic actuators in the excavator are operated (hereinafter referred to as a “standby mode”), the hydraulic oil discharged by the main pumps14L,14R passes through the center bypass pipelines40L,40R and reaches the negative control diaphragms18L,18R. Then, the flow of the hydraulic oil discharged by the main pumps14L,14R increases the negative control pressure generated upstream of the negative control diaphragms18L,18R. As a result, the regulators13L,13R decrease the discharge amounts of the main pumps14L,14R to the allowable minimum discharge amount, and suppress the pressure loss (pumping loss) when the discharged hydraulic oil passes through the center bypass pipelines40L,40R.

On the other hand, when any of the hydraulic actuators is operated, the hydraulic oil discharged by the main pumps14L,14R flows into the operated hydraulic actuator via the control valve corresponding to the operated hydraulic actuator. Then, the flow of the hydraulic oil discharged by the main pumps14L,14R reduces or eliminates the amount reaching the negative control diaphragms18L,18R, and lowers the negative control pressure generated upstream of the negative control diaphragms18L,18R. As a result, the regulators13L,13R receiving the reduced negative control pressure increase the discharge amounts of the main pumps14L,14R, and circulate a sufficient amount of hydraulic oil to the operated hydraulic actuator, to reliably drive the operated hydraulic actuator.

With the above configuration, in the hydraulic system ofFIG. 3, it is possible to suppress wasteful energy consumption in the main pumps14L,14R in the standby mode. Wasteful energy consumption includes pumping loss in the center bypass pipelines40L,40R caused by the hydraulic oil discharged by the main pumps14L,14R.

In the hydraulic system ofFIG. 3, when operating the hydraulic actuator, it is possible to reliably supply a necessary and sufficient amount of hydraulic oil from the main pumps14L,14R to the operated hydraulic actuator.

Next, with reference toFIGS. 4 to 6, the configuration of the control valve177A and the control valve177B (invisible inFIG. 4) will be described.FIG. 4is a partial cross-sectional of the control valve unit17.FIG. 5is a partial cross-sectional view of the control valve177A and the control valve177B as viewed from the −X side of a plane including a line segment L1indicated by a one-dot chain line inFIG. 4.FIG. 6is a partial cross-sectional view of the control valve176A as viewed from the −X side of a plane including a line segment L2indicated by a two-dot chain line inFIG. 4.FIG. 4corresponds to a partial cross-sectional view as viewed from the +Z side of a plane including a line segment L3indicated by a one-dot chain line inFIG. 5and a line segment L4indicated by a one-dot chain line inFIG. 6. The bold solid arrows inFIG. 4indicate the flow of hydraulic oil in the center bypass pipeline40L.

In the present embodiment, the control valve175A, the control valve176A, the control valve177A, and the control valve177B are formed in a valve block17B of the control valve unit17. The control valve177A and the control valve177B are disposed between the control valve175A and the control valve176A. That is, the control valve177A and the control valve177B are disposed on the +X side of the control valve175A and on the −X side of the control valve176A.

As illustrated inFIG. 4, the center bypass pipeline40L branches into two right and left pipelines on the downstream side of the spool of the control valve175A, and then joins together as one pipeline. Then, the center bypass pipeline40L leads to the next control valve176A in the state of one pipeline. When the arm operation lever26A and the boom operation lever26B are both in a neutral state, the hydraulic oil flowing through the center bypass pipeline40L crosses the spool of each control valve and flows to the downstream side of the spool of each control valve, as indicated by the thick solid lines inFIG. 4.

As illustrated inFIG. 5, the control valve177B is disposed on the +Z side of the control valve177A.FIG. 5illustrates that the control valve177A is at the first valve position with an opening degree of 0%, and the control valve177B is at the second valve position with an opening degree of 100%. The control valve177A blocks the communication between a meter-out pipeline45and a return oil pipeline49at the first valve position. Then, when a spring177As contracts in accordance with the rise in the control pressure generated by the pressure control valve31A, the control valve177A moves to the −Y side to increase the opening area of the flow path connecting the meter-out pipeline45and the return oil pipeline49. The meter-out pipeline45is a pipeline connecting the rod-side oil chamber of the arm cylinder8and the control valve177A. Similarly, the control valve177B blocks the communication between a meter-out pipeline46and the return oil pipeline49at the first valve position. When a spring177Bs contracts according to the rise of the control pressure generated by the pressure control valve31B, the control valve177B moves to the −Y side to increase the opening area of the flow path connecting the meter-out pipeline46and the return oil pipeline49. The meter-out pipeline46is a pipeline connecting the bottom-side oil chamber of the boom cylinder7and the control valve177B.

As indicated by the bidirectional arrow inFIG. 6, the spool of the control valve176A moves to the −Y side when the arm operation lever26A is operated in the closing direction, and moves to the +Y side when the arm operation lever26A is operated in the opening direction. When the arm operation lever26A is operated, the hydraulic oil in the center bypass pipeline40L is blocked by the spool of the control valve176A, and does not flow to the downstream side thereof. The control valve176A is structured such that the parallel pipeline42L can selectively communicate with either an arm bottom pipeline47B or an arm rod pipeline47R via the bridge pipeline44L. Specifically, when the spool moves in the −Y direction, the center bypass pipeline40L is blocked. Then, the bridge pipeline44L and the arm bottom pipeline47B communicate with each other, and the arm rod pipeline47R and the return oil pipeline49communicate with each other, by grooves formed in the spool. Then, the hydraulic oil flowing through the parallel pipeline42L flows into the bottom side oil chamber of the arm cylinder8through a connection pipeline42La, the bridge pipeline44L, and the arm bottom pipeline47B. Furthermore, the hydraulic oil flowing out from the rod side oil chamber of the arm cylinder8is discharged to the hydraulic oil tank through the arm rod pipeline47R and the return oil pipeline49. As a result, the arm cylinder8expands and the arm5is closed. Alternatively, when the spool moves in the +Y direction, the center bypass pipeline40L is blocked. Then, the bridge pipeline44L and the arm rod pipeline47R communicate with each other, and the arm bottom pipeline47B and the return oil pipeline49communicate with each other, by grooves formed in the spool. The hydraulic oil flowing through the parallel pipeline42L flows into the rod side oil chamber of the arm cylinder8through the connection pipeline42La, the bridge pipeline44L, and the arm rod pipeline47R. The hydraulic oil flowing out from the bottom side oil chamber of the arm cylinder8is discharged to the hydraulic oil tank through the arm bottom pipeline47B and the return oil pipeline49. As a result, the arm cylinder8is contracted and the arm5is opened.

Next, with reference toFIGS. 7 to 9, a process (hereinafter referred to as “meter-out process”) in which the controller30controls the opening and the closing of the control valve177A will be described.FIG. 7is a flowchart illustrating the flow of a meter-out process. During the arm closing operation, the controller30repeats this meter-out process in a predetermined control cycle.FIGS. 8 and 9correspond toFIG. 4and illustrate the state of the control valve unit17when the arm operation lever26A is operated.FIG. 8illustrates a state when high load work is performed, andFIG. 9illustrates a state when low load work is performed.

When the arm operation lever26A is operated in the arm closing direction, the control valve176A moves in the −Y direction as indicated by the arrow AR1inFIGS. 8 and 9to block the center bypass pipeline40L. Furthermore, the bridge pipeline44L and the arm bottom pipeline47B communicate with each other, and the arm rod pipeline47R and the return oil pipeline49communicate with each other, by grooves formed in the spool of the control valve176A. Then, the hydraulic oil flowing through the parallel pipeline42L flows into the bottom side oil chamber of the arm cylinder8through the connection pipeline42La, the bridge pipeline44L, and the arm bottom pipeline47B. Furthermore, the hydraulic oil flowing out from the rod side oil chamber of the arm cylinder8is discharged to the hydraulic oil tank through the arm rod pipeline47R and the return oil pipeline49. As a result, the arm cylinder8expands and the arm5is closed. InFIGS. 8 and 9, the hydraulic oil flowing through the parallel pipeline42L and the bridge pipeline44L is indicated by thick dotted arrows. Also, the hydraulic oil flowing from the bridge pipeline44L to the arm bottom pipeline47B and the hydraulic oil flowing from the arm rod pipeline47R to the return oil pipeline49are indicated by thick solid arrows.

In the meter-out process, as illustrated inFIG. 7, the work content determining unit300of the controller30determines whether high load work by closing the arm is being performed (step S1). For example, when the detection value of the arm bottom pressure sensor is greater than or equal to a predetermined value, it is determined that high load work by arm closing is being performed.

When the work content determining unit300determines that the high load work by arm closing is performed (YES in step S1), the meter-out control unit301of the controller30increases the opening area of the flow path connecting the meter-out pipeline45and the return oil pipeline49(step S2). In the present embodiment, the meter-out control unit301raises the control pressure generated by the pressure control valve31A by outputting a current instruction to the pressure control valve31A. As indicated by an arrow AR2inFIG. 8, the control valve177A moves to the −Y side in accordance with the rise of the control pressure and increases the opening area of the flow path connecting the meter-out pipeline45and the return oil pipeline49. As a result, most of the hydraulic oil flowing out from the rod-side oil chamber of the arm cylinder8passes through the meter-out pipeline45and the return oil pipeline49and is discharged to the hydraulic oil tank. InFIG. 8, the hydraulic oil flowing from the arm rod pipeline47R through the meter-out pipeline45to the return oil pipeline49is indicated by thick broken line arrows. With this configuration, the controller30can reduce the pressure loss that is caused when the hydraulic oil flows out from the rod-side oil chamber of the arm cylinder8to the hydraulic oil tank, and it is possible to prevent the hydraulic energy from being wastefully consumed in the high load work.

When the work content determining unit300determines that low load work of the arm closing is performed (NO in step S1), the meter-out control unit301does not increase the opening area of the flow path connecting the meter-out pipeline45and the return oil pipeline49. The control valve177A remains stationary as illustrated inFIG. 9and does not allow the communication of the flow path connecting the meter-out pipeline45and the return oil pipeline49. As a result, the hydraulic oil flowing out from the rod-side oil chamber of the arm cylinder8flows through the flow path connecting the arm rod pipeline47R and the return oil pipeline49, which are communicated by a groove formed in the spool of the control valve176A, and is discharged to the hydraulic oil tank. With this configuration, the controller30can appropriately limit the flow rate of the hydraulic oil flowing out from the rod-side oil chamber of the arm cylinder8to the hydraulic oil tank, so that the movement of the arm5is prevented from becoming excessively fast at the time of the low load work.

In the embodiment described above, the controller30controls the control valve177A to increase the opening area when it is determined that high load work of the arm closing is being performed, to reduce the pressure loss that is caused when hydraulic oil flows from the rod side oil chamber of the arm cylinder8to the hydraulic oil tank. This process is also executed when it is determined that high load work including boom lowering is being performed. Specifically, when the controller30determines that high load work including boom lowering is performed, the controller30controls the control valve177B to increase the opening area, to reduce the pressure loss that is caused when hydraulic oil flows from the bottom side oil chamber of the boom cylinder7to the hydraulic oil tank.

Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications and substitutions may be made to the above-described embodiments without departing from the scope of the present invention.

For example, in the above-described embodiment, the control valve177is incorporated in the valve block17B of the control valve unit17. Therefore, it is unnecessary to attach the control valve177to the outside of the valve block17B, and it is possible to realize a low-cost and compact hydraulic system including the control valve177. However, a configuration in which the control valve177is attached to the outside of the valve block17B is not excluded. That is, the control valve177may be disposed outside the valve block17B.

Furthermore, in the above-described embodiment, a configuration is adopted in which the first spool valve corresponding to each hydraulic actuator individually executes the bleed-off control; but it is also possible to adopt a configuration in which the bleed-off control for a plurality of hydraulic actuators is executed in a unified manner, by using a unified bleed off valve provided between the center bypass pipeline and the hydraulic oil tank. In this case, even when each first spool valve moves from the neutral position, the flow path area of the center bypass pipeline is prevented from decreasing, that is, each first spool valve does not block the center bypass pipeline. Even when this unified bleed-off valve is used, in the application of the present invention, a parallel pipeline is formed separately from the center bypass pipeline.

According to an embodiment of the present invention, an excavator that reduces, when necessary, the pressure loss caused when hydraulic oil is caused to flow from a hydraulic cylinder to a hydraulic oil tank, can be provided.