Patent Publication Number: US-9903393-B2

Title: Construction machine

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of Japanese Patent Application No. 2014-009842, filed on Jan. 22, 2014, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Technical Field 
     The present invention generally relates to construction machines that include a hydraulic actuator. 
     Description of Related Art 
     A shovel that drives a hydraulic actuator using hydraulic oil discharged by a hydraulic pump is known. 
     Normally, the hydraulic actuator receives hydraulic oil discharged by the hydraulic pump and discharges retained hydraulic oil to a hydraulic oil tank. 
     SUMMARY 
     According to an aspect of the present invention, a construction machine includes a first hydraulic pump, a second hydraulic pump, and a hydraulic circuit. The first hydraulic pump supplies hydraulic oil to a hydraulic actuator of a first system. The second hydraulic pump supplies the hydraulic oil to a hydraulic actuator of a second system. The hydraulic circuit supplies the hydraulic oil flowing out from at least one of the hydraulic actuators of the first and second systems to the intake side or the discharge side of at least one of the first and second hydraulic pumps. At least one of the first and second hydraulic pumps operates as a hydraulic motor so as to assist the other of the first and second hydraulic pumps that operates as a hydraulic pump. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a shovel according to an embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating a configuration of a hydraulic circuit provided in the shovel of  FIG. 1 ; 
         FIG. 3  is a diagram illustrating the correspondence between shovel operation patterns and valve positions of selector valves; 
         FIG. 4  is a diagram illustrating the correspondence between valve positions of selector valves and predetermined pressure conditions; 
         FIG. 5  is a flowchart illustrating an example of a merge point switching operation; 
         FIG. 6  is a schematic diagram illustrating another configuration of the hydraulic circuit provided in the shovel of  FIG. 1 ; 
         FIG. 7  is a diagram illustrating the correspondence between valve positions of selector valves and predetermined pressure conditions; and 
         FIG. 8  is a flowchart illustrating another example of the merge point switching operation. 
     
    
    
     DETAILED DESCRIPTION 
     According to the above-described shovel, however, hydraulic oil flowing out from the hydraulic actuator may be discharged in a high-pressure state to the hydraulic oil tank, so that there is room for improvement in the way hydraulic energy is used. 
     According to an aspect of the present invention, a construction machine that can more efficiently reuse hydraulic oil flowing out from the hydraulic actuator is provided. 
     A description is given, with reference to the accompanying drawings, of an embodiment of the present invention. 
       FIG. 1  is a side view of a shovel that is a construction machine according to the embodiment of the present invention. According to this embodiment, the shovel includes a lower-part traveling (movable) body  1 , a turning mechanism  2  provided on the lower-part traveling body  1 , and an upper-part turning (turnable) body  3  provided on the turning mechanism  2  so as to be turnable relative to the lower-part traveling body  1 . 
     The upper-part turning body  3  includes an excavation attachment provided in its front center part. The excavation attachment includes a boom  4 , an arm  5 , a bucket  6 , a boom cylinder  7  that drives the boom  4 , an arm cylinder  8  that drives the arm  5 , and a bucket cylinder  9  that drives the bucket  6 . The upper-part turning body  3  further includes a cabin  10  into which an operator climbs provided in its front part and an engine  11  serving as a drive source provided in its rear part. In the following description, a left traveling hydraulic motor  1 L, a right traveling hydraulic motor  1 R, the boom cylinder  7 , the arm cylinder  8 , the bucket cylinder  9 , a turning hydraulic motor  21 , etc., are collectively referred to as “hydraulic actuators”. 
       FIG. 2  is a schematic diagram illustrating a configuration of a hydraulic circuit provided in the shovel of  FIG. 1 . In  FIG. 2 , high-pressure oil passages, pilot oil passages, and electrical control lines are indicated by a solid line, a broken line, and a dotted line, respectively. 
     According to this embodiment, the hydraulic circuit circulates hydraulic oil from first and second hydraulic pumps  12 L and  12 R driven by the engine  11  to a hydraulic oil tank via center bypass oil passages  40 L and  40 R, respectively. 
     The first hydraulic pump  12 L is capable of supplying hydraulic oil to each of flow rate control valves  150 ,  151 ,  152  and  153  via a high-pressure oil passage. The second hydraulic pump  12 R is capable of supplying hydraulic oil to each of flow rate control valves  154 ,  155 ,  156  and  157  via a high-pressure oil passage. 
     Specifically, the first and second hydraulic pumps  12 L and  12 R are, for example, swash-plate variable displacement hydraulic pumps. As indicated by double lines in  FIG. 2 , the first and second hydraulic pumps  12 L and  12 R have their respective rotating shafts connected to the drive shaft of the engine  11  so as to be rotated by the engine  11 . According to this embodiment, negative control is employed as a pump control method for controlling the first and second hydraulic pumps  12 L and  12 R. Alternatively, other control methods such as positive control and load sensing control may be employed. 
     Furthermore, the second hydraulic pump  12 R is operable as a hydraulic motor as well. According to this embodiment, when operating as a hydraulic motor, the second hydraulic pump  12 R is rotated by hydraulic oil flowing out from at least one of the hydraulic actuators  7 ,  8 ,  9  and  21  so as to assist the engine  11 . 
     Regulators  13 L and  13 R control the amounts of discharge of the first and second hydraulic pumps  12 L and  12 R, respectively. For example, the regulators  13 L and  13 R control the amounts of discharge per unit time of the first and second hydraulic pumps  12 L and  12 R by adjusting the tilting angles of the swash plates of the first and second hydraulic pumps  12 L and  12 R, respectively. 
     The center bypass oil passage  40 L is a high-pressure oil passage that goes through the flow rate control valves  150  through  153 , and includes a negative control throttle  20 L between the flow rate control valve  153  and the hydraulic oil tank. The center bypass oil passage  40 R is a high-pressure oil passage that goes through the flow rate control valves  154  through  157 , and includes a negative control throttle  20 R between the flow rate control valve  157  and the hydraulic oil tank. 
     The flows of hydraulic oil discharged by the first and second hydraulic pumps  12 L and  12 R are restricted by the negative control throttles  20 L and  20 R. Therefore, the negative control throttles  20 L and  20 R generate control pressures (hereinafter referred to as “negative control pressures”) for controlling the regulators  13 L and  13 R, respectively. 
     Relief valves  19 L and  19 R are safety valves that control the negative control pressures to be lower than a predetermined relief pressure by discharging hydraulic oil to the hydraulic oil tank when the negative control pressures on the upstream side of the negative control throttles  20 L and  20 R become higher than or equal to the predetermined relief pressure. 
     Negative control pressure oil passages  41 L and  41 R are pilot oil passages for transmitting the negative control pressures generated on the upstream side of the negative control throttles  20 L and  20 R to the regulators  13 L and  13 R, respectively. 
     The regulators  13 L and  13 R control the amounts of discharge of the hydraulic pumps  12 L and  12 R by adjusting the tilting angles of the swash plates of the hydraulic pumps  12 L and  12 R in accordance with the negative control pressures. Furthermore, the regulators  13 L and  13 R decrease the amounts of discharge of the hydraulic pumps  12 L and  12 R as the introduced negative control pressures increase, and increase the amounts of discharge of the hydraulic pumps  12 L and  12 R as the introduced negative control pressures decrease. 
     The flow rate control valve  150  is a spool valve for supplying hydraulic oil discharged by the first hydraulic pump  12 L to the left traveling hydraulic motor  1 L and discharging hydraulic oil flowing out from the left traveling hydraulic motor  1 L to the hydraulic oil tank. The flow rate control valve  154  is a spool valve for supplying hydraulic oil discharged by the second hydraulic pump  12 R to the right traveling hydraulic motor  1 R and discharging hydraulic oil flowing out from the right traveling hydraulic motor  1 R to the hydraulic oil tank. 
     The flow rate control valve  151  is a spool valve for supplying hydraulic oil discharged by the first hydraulic pump  12 L to the turning hydraulic motor  21  and discharging hydraulic oil flowing out from the turning hydraulic motor  21  to the hydraulic oil tank. 
     The flow rate control valve  155  is a spool valve for supplying hydraulic oil discharged by the second hydraulic pump  12 R to the bucket cylinder  9  and discharging hydraulic oil flowing out from the bucket cylinder  9  to the hydraulic oil tank. 
     The flow rate control valves  152  and  156  are spool valves for supplying hydraulic oil discharged by the first and second hydraulic pumps  12 L and  12 R to the boom cylinder  7  and discharging hydraulic oil flowing out from the boom cylinder  7  to the hydraulic oil tank. The flow rate control valve  152  is a spool valve that operates every time a boom operation lever (not graphically represented) is operated. The flow rate control valve  156  is a spool valve that operates only when the boom operation lever is operated in a direction to raise the boom  4  with a predetermined amount of lever operation or more. 
     The flow rate control valves  153  and  157  are spool valves for supplying hydraulic oil discharged by the first and second hydraulic pumps  12 L and  12 R to the arm cylinder  8  and discharging hydraulic oil flowing out from the arm cylinder  8  to the hydraulic oil tank. The flow rate control valve  157  is a valve that operates every time an arm operation lever (not graphically represented) is operated. The flow rate control valve  153  is a valve that operates only when the arm operation lever is operated with a predetermined amount of lever operation or more. 
     According to this embodiment, the left traveling hydraulic motor  1 L, the turning hydraulic motor  21 , the boom cylinder  7 , and the arm cylinder  8  that operate using hydraulic oil discharged by the first hydraulic pump  12 L are referred to as “hydraulic actuators of a first system,” and the flow rate control valves  150  through  153  are referred to as “flow rate control valves of the first system.” Furthermore, the right traveling hydraulic motor  1 R, the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  that operate using hydraulic oil discharged by the second hydraulic pump  12 R are referred to as “hydraulic actuators of a second system,” and the flow rate control valves  154  through  157  are referred to as “flow rate control valves of the second system.” 
     A controller  30  is a control unit for controlling the hydraulic circuit. The controller  30  is, for example, a computer that includes a central processing unit (CPU), a random access memory (RAM), and a read-only memory (ROM). According to this embodiment, the controller  30  receives the detection results of various kinds of sensors, performs a predetermined operation based on the received detection results, and controls a first selector valve  51 , a second selector valve  52 , a third selector valve  53 , a fourth selector valve  54 , a fifth selector valve  55 , a sixth selector valve  56 , and a seventh selector valve  57  in accordance with the result of the operation. 
     The first through seventh selector valves  51  through  57  operate in accordance with control instructions from the controller  30 . According to this embodiment, the first through fourth selector valves  51  through  54  are connected to a high-pressure oil passage  42 . Furthermore, the fourth selector valve  54  is a two-port, two-position solenoid selector valve, and the other selector valves are three-port, two-position solenoid selector valves. The first through seventh selector valves  51  through  57  may be hydraulic selector valves. 
     Specifically, the first position of the first selector valve  51  causes the outlet ports of the flow rate control valves  153  and  157  to communicate with the hydraulic oil tank, and the second position of the first selector valve  51  causes the outlet ports of the flow rate control valves  153  and  157  to communicate with the high-pressure oil passage  42 . This configuration makes it possible for the first selector valve  51  to switch discharging hydraulic oil flowing out from the flow rate control valves  153  and  157  directly to the hydraulic oil tank and delivering hydraulic oil flowing out from the flow rate control valves  153  and  157  to the high-pressure oil passage  42 . In  FIG. 2 , parenthesized numbers associated with the first selector valve  51  represent valve positions, and (1) corresponds to the first position and (2) corresponds to the second position. The same applies to the other selector valves  52  through  57 . 
     Furthermore, the first position of the second selector valve  52  causes the outlet ports of the flow rate control valves  152  and  156  to communicate with the hydraulic oil tank, and the second position of the second selector valve  52  causes the outlet ports of the flow rate control valves  152  and  156  to communicate with the high-pressure oil passage  42 . This configuration makes it possible for the second selector valve  52  to switch discharging hydraulic oil flowing out from the flow rate control valves  152  and  156  directly to the hydraulic oil tank and delivering hydraulic oil flowing out from the flow rate control valves  152  and  156  to the high-pressure oil passage  42 . 
     Furthermore, the first position of the third selector valve  53  causes the outlet ports of the flow rate control valves  151  and  155  to communicate with the hydraulic oil tank, and the second position of the third selector valve  53  causes the outlet ports of the flow rate control valves  151  and  155  to communicate with the high-pressure oil passage  42 . This configuration makes it possible for the third selector valve  53  to switch discharging hydraulic oil flowing out from the flow rate control valves  151  and  155  directly to the hydraulic oil tank and delivering hydraulic oil flowing out from the flow rate control valves  151  and  155  to the high-pressure oil passage  42 . 
     Each of the first through third selector valves  51  through  53  may be provided between the associated cylinder and the associated flow rate control valves. In this case, each of the first through third selector valves  51  through  53  is switched between a first position at which hydraulic oil flowing out from the associated cylinder is discharged to the hydraulic oil tank via the associated flow rate control valves and a second position at which hydraulic oil flowing out from the associated cylinder is delivered to the high-pressure oil passage  42  without going through the associated flow rate control valves. 
     Furthermore, the first position of the fourth selector valve  54  disconnects a turning hydraulic circuit and the high-pressure oil passage  42 , and the second position of the fourth selector valve  54  causes the turning hydraulic circuit to communicate with the high-pressure oil passage  42 . The turning hydraulic circuit is a hydraulic circuit including relief valves  22 L and  22 R and a shuttle valve  23 . The relief valve  22 L causes hydraulic oil on a first port  21 L side of the turning hydraulic motor  21  to flow out to the hydraulic oil tank when the pressure of hydraulic oil on the first port  21 L side exceeds a predetermined relief pressure. The relief valve  22 R causes hydraulic oil on a second port  21 R side of the turning hydraulic motor  21  to flow out to the hydraulic oil tank when the pressure of hydraulic oil on the second port  21 R side exceeds a predetermined relief pressure. Furthermore, the shuttle valve  23  causes one of the hydraulic oil on the first port  21 L side and the hydraulic oil on the second port  21 R side that is higher in pressure to flow out to the fourth selector valve  54 . This configuration makes it possible for the fourth selector valve  54  to cause hydraulic oil on the discharge side of the turning hydraulic motor  21  to flow out to the high-pressure oil passage  42  at the time of decelerating the turning mechanism  2 . 
     Furthermore, the first position of the fifth selector valve  55  causes the high-pressure oil passage  42  to communicate with the discharge side (downstream side) of the first hydraulic pump  12 L or the second hydraulic pump  12 R, and the second position of the fifth selector valve  55  causes the high-pressure oil passage  42  to communicate with the intake side (upstream side) of the second hydraulic pump  12 R. This configuration makes it possible for the fifth selector valve  55  to switch merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil discharged from the first hydraulic pump  12 L or the second hydraulic pump  12 R (on its downstream side) and merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil taken into the second hydraulic pump  12 R (on its upstream side). 
     Furthermore, the first position of the sixth selector valve  56  causes the fifth selector valve  55  to communicate with the discharge side (downstream side) of the second hydraulic pump  12 R, and the second position of the sixth selector valve  56  causes the fifth selector valve  55  to communicate with the discharge side (downstream side) of the first hydraulic pump  12 L. This configuration makes it possible for the sixth selector valve  56  to switch merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil discharged from the second hydraulic pump  12 R (on its downstream side) and merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil discharged from the first hydraulic pump  12 L (on its downstream side). 
     Furthermore, the first position of the seventh selector valve  57  causes a discharge port of the second hydraulic pump  12 R to communicate with the center bypass oil passage  40 R, and the second position of the seventh selector valve  57  causes the discharge port of the second hydraulic pump  12 R to communicate with the hydraulic oil tank. This configuration makes it possible for the seventh selector valve  57  to switch delivering hydraulic oil flowing out from the discharge port of the second hydraulic pump  12 R to the center bypass oil passage  40 R and discharging hydraulic oil flowing out from the discharge port of the second hydraulic pump  12 R directly to the hydraulic oil tank. 
     The shovel illustrated in  FIG. 2  is operated using an operation apparatus (not graphically illustrated). The operation apparatus includes an arm operation lever, a boom operation lever, a bucket operation lever, a turning operation lever, and right and left traveling levers (or traveling pedals). The operation apparatus introduces a pilot pressure corresponding to the amount of lever operation or pedal operation into a right or left pilot port of a corresponding one or more flow rate control valves, using hydraulic oil discharged by a control pump (not graphically illustrated). 
     Specifically, the arm operation lever for operating the arm  5  introduces a pilot pressure corresponding to the amount of lever operation into a right or left pilot port of each of the flow rate control valves  153  and  157 . Furthermore, the boom operation lever for operating the boom  4  introduces a pilot pressure corresponding to the amount of lever operation into a right or left pilot port of each of the flow rate control valves  152  and  156 . Furthermore, the bucket operation lever for operating the bucket  6  introduces a pilot pressure corresponding to the amount of lever operation into a right or left pilot port of the flow rate control valve  155 . Furthermore, the turning operation lever for turning the upper-part turning body  3  introduces a pilot pressure corresponding to the amount of lever operation into a right or left pilot port of the flow rate control valve  151 . The right and left traveling levers (or traveling pedals) for causing the lower-part traveling body  1  to travel introduce a pilot pressure corresponding to the amount of lever operation or pedal operation into a right or left pilot port of the flow rate control valve  154  and a right or left pilot port of the flow rate control valve  150 , respectively. 
     Furthermore, the shovel illustrated in  FIG. 2  detects the amount of operation of the operation apparatus using an operation amount detection part. The operation amount detection part includes an arm pilot pressure sensor, a boom pilot pressure sensor, a bucket pilot pressure sensor, a turning pilot pressure sensor, and a traveling pilot pressure sensor (none of which is graphically illustrated). The operation amount detection part detects the amount of lever operation or the amount of pedal operation as the pressure value of a pilot pressure, and outputs the detected value to the controller  30 . 
     Specifically, the arm pilot pressure sensor detects the amount of lever operation of the arm operation lever as the pressure value of a pilot pressure. Furthermore, the boom pilot pressure sensor detects the amount of lever operation of the boom operation lever as the pressure value of a pilot pressure. Furthermore, the bucket pilot pressure sensor detects the amount of lever operation of the bucket operation lever as the pressure value of a pilot pressure. Furthermore, the turning pilot pressure sensor detects the amount of lever operation of the turning operation lever as the pressure value of a pilot pressure. Furthermore, the traveling pilot pressure sensor detects the amount of lever or pedal operation of each of the right and left traveling levers or pedals as the pressure value of a pilot pressure. 
     Pressure sensors S 1 , S 2  and S 3  detect the pressure of hydraulic oil, and output their respective detected values to the controller  30 . 
     Specifically, the pressure sensor S 1  detects the discharge pressure of the first hydraulic pump  12 L, the pressure sensor S 2  detects the discharge pressure of the second hydraulic pump  12 R, and the pressure sensor S 3  detects the pressure of hydraulic oil inside the high-pressure oil passage  42 . 
     Next, a description is given, with reference to  FIG. 2  and  FIG. 3 , of an operation of the controller  30  controlling the switching of the first through fourth selector valves  51  through  54  in accordance with a detection result of the operation amount detection part in order to collect hydraulic oil having reusable hydraulic energy (hereinafter referred to as “hydraulic oil collecting operation”).  FIG. 3  is a diagram illustrating the correspondence between shovel operation patterns and the valve positions of the first through fourth selector valves  51  through  54 . Furthermore, it is assumed that the valve positions of the first through fourth selector valves  51  through  54  are switched to the respective first positions. 
     In the case where the shovel operation pattern is “arm closing,” that is, when the arm pilot pressure sensor detects the operation of the arm operation lever in a direction to close the arm  5 , the controller  30  switches the valve position of the first selector valve  51  to the second position. 
     This is because hydraulic oil flowing out from the arm cylinder  8  has reusable hydraulic energy in the case of performing “arm closing” using the own weight of the arm  5 . 
     The controller  30  maintains the valve positions of the second through fourth selector valves  52  through  54  in the first positions because the boom cylinder  7 , the bucket cylinder  9 , and the turning hydraulic motor  21  are causing no hydraulic oil having reusable hydraulic energy to flow out. 
     As a result, the hydraulic oil flowing out from the arm cylinder  8  is delivered to the high-pressure oil passage  42  via at least one of the flow rate control valves  153  and  157  and via the first selector valve  51 . 
     Furthermore, in the case where the shovel operation pattern is “boom lowering,” that is, when the boom pilot sensor detects the operation of the boom operation lever in a direction to lower the boom  4 , the controller  30  switches the valve position of the second selector valve  52  to the second position. 
     This is because hydraulic oil flowing out from the boom cylinder  7  has reusable hydraulic energy in the case of performing “boom lowering” using the own weight of the boom  4 . 
     The controller  30  maintains the valve positions of the first, third and fourth selector valves  51 ,  53  and  54  in the first positions because the arm cylinder  8 , the bucket cylinder  9 , and the turning hydraulic motor  21  are causing no hydraulic oil having reusable hydraulic energy to flow out. 
     As a result, the hydraulic oil flowing out from the boom cylinder  7  is delivered to the high-pressure oil passage  42  via at least one of the flow rate control valves  152  and  156  and via the second selector valve  52 . 
     Furthermore, in the case where the shovel operation pattern is “bucket closing,” that is, when the bucket pilot pressure sensor detects the operation of the bucket operation lever in a direction to close the bucket  6 , the controller  30  switches the valve position of the third selector valve  53  to the second position. 
     This is because hydraulic oil flowing out from the bucket cylinder  9  has reusable hydraulic energy in the case of performing “bucket closing” using the own weight of the bucket  6 . 
     The controller  30  maintains the valve positions of the first, second and fourth selector valves  51 ,  52  and  54  in the first positions because the boom cylinder  7 , the arm cylinder  8 , and the turning hydraulic motor  21  are causing no hydraulic oil having reusable hydraulic energy to flow out. 
     As a result, the hydraulic oil flowing out from the bucket cylinder  9  is delivered to the high-pressure oil passage  42  via the flow rate control valve  155  and the third selector valve  53 . 
     Furthermore, in the case where the shove operation pattern is “turning stop,” that is, when the turning pilot pressure sensor detects the operation of the turning operation lever in a direction to stop the turning of the upper-part turning body  3 , the controller  30  switches the valve position of the fourth selector valve  54  to the second position. 
     This is because hydraulic oil on the discharge side of the turning hydraulic motor  21  has reusable hydraulic energy in the case of performing “turning stop” by limiting the amount of hydraulic oil flowing out from the turning hydraulic motor  21 . 
     The controller  30  maintains the valve positions of the first through third selector valves  51  through  53  in the first positions because the boom cylinder  7 , the arm cylinder  8 , and the bucket cylinder  9  are causing no hydraulic oil having reusable hydraulic energy to flow out. 
     As a result, the hydraulic oil on the discharge side of the turning hydraulic motor  21  is delivered to the high-pressure oil passage  42  via the fourth selector valve  54 . 
     In addition, the shove operation pattern may be a combination of two or more of the above-described four operation patterns, namely, “arm closing,” “boom lowering,” “bucket closing,” and “turning stop” as illustrated in  FIG. 3 . The valve positions of the first through fourth selector valves  51  through  54  in the case where the shovel operation pattern is a combination of two or more of the operation patterns are a combination of the valve positions of the individual operation patterns. 
     Next, a description is given, with reference to  FIG. 2 ,  FIG. 4  and  FIG. 5 , an operation of the controller  30  merging reusable hydraulic oil collected in the hydraulic oil collecting operation with a proper point of the hydraulic circuit (hereinafter referred to as “merge point switching operation”). According to this embodiment, the controller  30  controls the switching of the fifth through seventh selector valves  55  through  57  in accordance with detection results of the operation amount detection part and the pressure sensors S 1  through S 3  in the merge point switching operation.  FIG. 4  is a diagram illustrating the correspondence between predetermined pressure conditions and the valve positions of the fifth through seventh selector valves  55  through  57 . Furthermore, a pressure P 1  represents the discharge pressure of the first hydraulic pump  12 L, a pressure P 2  represents the discharge pressure of the second hydraulic pump  12 R, and a pressure P 3  represents the pressure of hydraulic oil of the high-pressure oil passage  42 . Furthermore, in “Second Hydraulic Pump Load State,” “Loaded” means that at least one of the flow rate control valves  154  through  157  of the second system is in operation, that is, at least one of the hydraulic actuators of the second system is in operation, and “No Load” means that none of the flow rate control valves  154  through  157  of the second system is in operation, that is, none of the hydraulic actuators of the second system is in operation. Furthermore, a threshold pressure value Pth is the pressure of hydraulic oil of the high-pressure oil passage  42  that is required to cause the second hydraulic pump  12 R to operate as a hydraulic motor, and is, for example, 10 MPa. Furthermore, “Second Hydraulic Pump Operating State” indicates whether the second hydraulic pump  12 R is operating as a hydraulic pump or a hydraulic motor. It is assumed that the second hydraulic pump  12 R is currently operating as a hydraulic pump. 
       FIG. 5  is a flowchart illustrating an example of the merge point switching operation. The controller  30  repeatedly performs the merge point switching operation at regular control intervals. 
     First, at step ST 1 , the controller  30  determines whether the load state of the second hydraulic pump  12 R is “No Load” and the pressure P 3  of hydraulic oil of the high-pressure oil passage  42  is greater than the threshold pressure value Pth. 
     If the load state of the second hydraulic pump  12 R is “No Load” (that is, the second hydraulic pump  12 R is unloaded) and the pressure P 3  of hydraulic oil of the high-pressure oil passage  42  is greater than the threshold pressure value Pth (YES at step ST 1 ), at step ST 2 , the controller  30  switch the valve position of each of the fifth and seventh selector valves  55  and  57  to the second position so as to cause the second hydraulic pump  12 R to operate as a hydraulic motor. 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  is supplied to the intake side (upstream side) of the second hydraulic pump  12 R. The second hydraulic pump  12 R is rotated as a hydraulic motor by the hydraulic oil flowing out from the high-pressure oil passage  42  so as to assist the first hydraulic pump  12 L operating as a hydraulic pump. As a result, it is possible for the first hydraulic pump  12 L to increase its maximum absorption horsepower determined in accordance with the maximum allowable output of the engine  11 , or it is possible for the second hydraulic pump  12 R as a hydraulic motor to reduce a load on the engine  11  related to the operation of the first hydraulic pump  12 L. 
     Hydraulic oil flowing out from the second hydraulic pump  12 R rotated as a hydraulic motor is discharged to the hydraulic oil tank through the second position of the seventh selector valve  57 . 
     In this case, the sixth selector valve  56  may be in either the first position or the second position because no hydraulic oil of the high-pressure oil passage  42  arrives at the sixth selector valve  56  through the fifth selector valve  55 . In  FIG. 4 , “-” in the column of “Sixth Selector Valve” indicates that the valve position of the sixth selector valve  56  is either the first position or the second position. The same applies to “-” in  FIG. 5 . 
     On the other hand, in response to determining that the second hydraulic pump  12 R is “Loaded” or the pressure P 3  is less than or equal to the threshold pressure value Pth (NO at step ST 1 ), at step ST 3 , the controller  30  determines whether the pressure P 3  is greater than the discharge pressure P 2  of the second hydraulic pump  12 R. 
     In response to determining that the pressure P 3  is greater than the discharge pressure P 2  (YES at step ST 3 ), at step ST 4 , the controller  30  maintains the state as is. Specifically, the controller  30  maintains the fifth through seventh selector valves  55  through  57  in their respective first positions, and causes the second hydraulic pump  12 R to continue to operate as a hydraulic pump. 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  arrives at the downstream side of the seventh selector valve  57  through the fifth and sixth selector valves  55  and  56  so as to merge with hydraulic oil discharged by the second hydraulic pump  12 R. As a result, it is possible for the second hydraulic pump  12 R to reduce the amount of discharge for causing the hydraulic actuators of the second system to operate. 
     Furthermore, in response to determining that the pressure P 3  is less than or equal to the discharge pressure P 2  (NO at step ST 3 ), at step ST 5 , the controller  30  determines whether the pressure P 3  is greater than the discharge pressure P 1  of the first hydraulic pump  12 L. 
     In response to determining that the pressure P 3  is greater than the discharge pressure P 1  (YES at step ST 5 ), at step ST 6 , the controller  30  switches the valve position of the sixth selector valve  56  to the second position. Specifically, the controller  30  switches the valve position of the sixth selector valve  56  to the second position while maintaining the fifth and seventh selector valves  55  and  57  in the first positions and causing the second hydraulic pump  12 R to continue to operate as a hydraulic pump. 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  arrives at the discharge side (downstream side) of the first hydraulic pump  12 L through the fifth and sixth selector valves  55  and  56  so as to merge with hydraulic oil discharged by the first hydraulic pump  12 L. As a result, it is possible for the first hydraulic pump  12 L to reduce the amount of discharge for causing the hydraulic actuators of the first system to operate. 
     In response to determining that the pressure P 3  is less than or equal to the discharge pressure P 1  (NO at step ST 5 ), at step ST 7 , the controller  30  switches the valve position of the fifth selector valve  55  to the second position. Specifically, the controller  30  switches the valve position of the fifth selector valve  55  to the second position while maintaining the seventh selector valve  57  in the first position and causing the second hydraulic pump  12 R to continue to operate as a hydraulic pump. In this case, the sixth selector valve  56  may be in either the first position or the second position because no hydraulic oil of the high-pressure oil passage  42  arrives at the sixth selector valve  56  through the fifth selector valve  55 . 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  is supplied to the intake side (upstream side) of the second hydraulic pump  12 R. The second hydraulic pump  12 R operates as a hydraulic pump while taking in hydraulic oil flowing out from the high-pressure oil passage  42 . As a result, it is possible for the second hydraulic pump  12 R to take in and discharge to the downstream side hydraulic oil having higher hydraulic pressure than hydraulic oil taken in from the hydraulic oil tank and to reduce a loan on the engine  11  related to the operation of the second hydraulic pump  12 R. 
     In the above-described embodiment, of the two hydraulic pumps  12 L and  12 R, only the second hydraulic pump  12 R can operate as a hydraulic motor. Alternatively, of the two hydraulic pumps  12 L and  12 R, only the first hydraulic pump  12 L may operate as a hydraulic motor. In this case, the fifth selector valve  55  is configured to switch merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil discharged from the first hydraulic pump  12 L or the second hydraulic pump  12 R (on its downstream side) and merging hydraulic oil flowing out from the high-pressure oil passage  42  with hydraulic oil taken into the first hydraulic pump  12 L (on its upstream side). Furthermore, the seventh selector valve  57  is configured to switch delivering hydraulic oil flowing out from a discharge port of the first hydraulic pump  12 L to the center bypass oil passage  40 L and discharging hydraulic oil flowing out from the discharge port of the first hydraulic pump  12 L directly to the hydraulic oil tank. 
     Next, a description is given, with reference to  FIG. 6 ,  FIG. 7  and  FIG. 8 , of an operation of another hydraulic circuit provided in the shovel according to the embodiment of the present invention.  FIG. 6  is a schematic diagram illustrating a configuration of another hydraulic circuit provided in the shovel of  FIG. 1 . The hydraulic circuit of  FIG. 6  is the same as the hydraulic circuit of  FIG. 2  except that the first hydraulic pump  12 L can operate as a hydraulic motor and that an eighth selector valve  58  and a ninth selector valve  59  are further provided. Therefore, a description of configurations common to the hydraulic circuits of  FIG. 2  and  FIG. 6  is omitted. 
     The eighth selector valve  598  and the ninth selector valve  59  operate in accordance with control instructions from the controller  30 . According to this embodiment, the eighth and ninth selector valves  58  and  59  are three-port, two-position solenoid selector valves. The eighth and ninth selector valves  58  and  59  may alternatively be hydraulic selector valves. 
     Specifically, the first position of the eighth selector valve  58  causes the fifth selector valve  55  to communicate with the intake side (upstream side) of the second hydraulic pump  12 R. Furthermore, the second position of the eighth selector valve  58  causes the fifth selector valve  55  to communicate with the intake side (upstream side) of the first hydraulic pump  12 L. This configuration makes it possible for the eighth selector valve  58  to switch merging hydraulic oil flowing out from the high-pressure oil passage  42  through the fifth selector valve  55  with hydraulic oil taken into the first hydraulic pump  12 L (on its upstream side) and merging hydraulic oil flowing out from the high-pressure oil passage  42  through the fifth selector valve  55  with hydraulic oil taken into the second hydraulic pump  12 R (on its upstream side). 
     Furthermore, the first position of the ninth selector valve  59  causes the discharge port of the first hydraulic pump  12 L to communicate with the center bypass oil passage  40 L, and the second position of the ninth selector valve  59  causes the discharge port of the first hydraulic pump  12 L to communicate with the hydraulic oil tank. This configuration makes it possible for the ninth selector valve  59  to switch delivering hydraulic oil flowing out from the discharge port of the first hydraulic pump  12 L to the center bypass oil passage  40 L and discharging hydraulic oil flowing out from the discharge port of the first hydraulic pump  12 L directly to the hydraulic oil tank. 
       FIG. 7  is a diagram illustrating the correspondence between predetermined pressure conditions and the valve positions of the fifth through ninth selector valves  55  through  59 , and corresponds to  FIG. 4 .  FIG. 8  is a flowchart illustrating another example of the merge point switching operation, and corresponds to  FIG. 5 . Specifically, determinations at steps ST 15  and ST 17  in  FIG. 8  are equal to those at steps ST 3  and ST 5 , respectively, of  FIG. 5 . Furthermore, the valve positions of the fifth through seventh selector valves  55  through  57  and the operating state of the second hydraulic pump  12 R at steps ST 14 , ST 16 , ST 18  and ST 19  are equal to those at steps ST 2 , ST 4 , ST 6  and ST 7 , respectively, of  FIG. 5 . Therefore, a description is given of determinations at steps ST 11  and ST 13  and settings at step S 12 . It is assumed that both the first and second hydraulic pumps  12 L and  12 R are operating as hydraulic pumps. 
     First, at step ST 11 , the controller  30  determines whether the load state of the first hydraulic pump  12 L is “No Load,” the load state of the second hydraulic pump  12 R is “Loaded” and the pressure P 3  of hydraulic oil of the high-pressure oil passage  42  is greater than the threshold pressure value Pth. 
     In response to determining that the load state of the first hydraulic pump  12 L is “No Load,” the load state of the second hydraulic pump  12 R is “Loaded” and the pressure P 3  is greater than the threshold pressure value Pth (YES at step ST 11 ), at step ST 12 , the controller  30  switches the valve positions of the fifth, eighth and ninth selector valves  55 ,  58  and  59  to their respective second positions so as to cause the first hydraulic pump  12 L to operate as a hydraulic motor. 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  is supplied to the intake side (upstream side) of the first hydraulic pump  12 L. The first hydraulic pump  12 L is rotated as a hydraulic motor by the hydraulic oil flowing out from the high-pressure oil passage  42  so as to assist the second hydraulic pump  12 R operating as a hydraulic pump. As a result, it is possible for the second hydraulic pump  12 R to increase its maximum absorption horsepower deter mined in accordance with the maximum allowable output of the engine  11 , or it is possible for the first hydraulic pump  12 L as a hydraulic motor to reduce a load on the engine  11  related to the operation of the second hydraulic pump  12 R. 
     Hydraulic oil flowing out from the first hydraulic pump  12 L rotated as a hydraulic motor is discharged to the hydraulic oil tank through the second position of the ninth selector valve  59 . 
     In this case, the sixth selector valve  56  may be in either the first position or the second position because no hydraulic oil of the high-pressure oil passage  42  arrives at the sixth selector valve  56  through the fifth selector valve  55 . 
     On the other hand, in response to determining that the load state of the first hydraulic pump  12 L is “Loaded”, the load state of the second hydraulic pump  12 R is “No Load,” or the pressure P 3  is less than or equal to the threshold pressure value Pth (NO at step ST 11 ), at step ST 13 , the controller  30  determines whether the load state of the first hydraulic pump  12 L is “Loaded,” the load state of the second hydraulic pump  12 R is “No Load,” and the pressure P 3  of hydraulic oil of the high-pressure oil passage  42  is greater than the threshold pressure value Pth. 
     In response to determining that the load state of the first hydraulic pump  12 L is “Loaded,” the load state of the second hydraulic pump  12 R is “No Load,” and the pressure P 3  is greater than the threshold pressure value Pth (YES at step ST 13 ), at step ST 14 , the controller  30  switches the valve positions of the fifth and seventh selector valves  55  and  57  to the second positions so as to cause the second hydraulic pump  12 R to operate as a hydraulic motor. 
     As a result of this setting, hydraulic oil flowing out from the high-pressure oil passage  42  is supplied to the intake side (upstream side) of the second hydraulic pump  12 R. The second hydraulic pump  12 R is rotated as a hydraulic motor by the hydraulic oil flowing out from the high-pressure oil passage  42  so as to assist the first hydraulic pump  12 L operating as a hydraulic pump. As a result, it is possible for the first hydraulic pump  12 L to increase its maximum absorption horsepower determined in accordance with the maximum allowable output of the engine  11 , or it is possible for the second hydraulic pump  12 R as a hydraulic motor to reduce a load on the engine  11  related to the operation of the first hydraulic pump  12 L. 
     Hydraulic oil flowing out from the second hydraulic pump  12 R rotated as a hydraulic motor is discharged to the hydraulic oil tank through the second position of the seventh selector valve  57 . 
     Furthermore, at steps ST 16 , ST 18  and ST 19 , the controller  30  maintains the ninth selector valve  59  in the first position and causes the first hydraulic pump  12 L to continue to operate as a hydraulic pump. Furthermore, at steps ST 16  and ST 18 , the eighth selector valve  58  may be in either the first position or the second position because no hydraulic oil of the high-pressure oil passage  42  arrives at the eighth selector valve  58  through the fifth selector valve  55 . In  FIG. 7 , “-” in the column of “Eighth Selector Valve” indicates that the valve position of the eighth selector valve  58  is either the first position or the second position. The same applies to “-” in  FIG. 8 . Furthermore, at step ST 19 , the eighth selector valve  58  may be in either the first position or the second position because hydraulic oil from the high-pressure oil passage  42  may be merged with hydraulic oil taken in by either the first hydraulic pump  12 L or the second hydraulic pump  12 R. 
     According to the above-described configuration, it is possible for the shovel according to the embodiment of the present invention to merge hydraulic oil flowing out from a hydraulic actuator with hydraulic oil on the intake side (upstream side) or the discharge side (downstream side) of a hydraulic pump in accordance with the pressure of the hydraulic oil flowing out from a hydraulic actuator. Therefore, it is possible to efficiently reuse hydraulic oil flowing out from a hydraulic actuator and to save energy. 
     Furthermore, it is possible for the shovel according to the embodiment of the present invention to cause the second hydraulic pump  12 R to operate as a hydraulic motor in the case of merging hydraulic oil flowing out from a hydraulic actuator with hydraulic oil on the intake side (upstream side) of the second hydraulic pump  12 R. Accordingly, it is possible to cause the first hydraulic pump  12 L to operate as a hydraulic pump, using the driving force of the engine  11  and the driving force of the second hydraulic pump  12 R operating as a hydraulic motor. As a result, it is possible to increase the maximum absorption horsepower of the first hydraulic pump  12 L or reduce a load on the engine  11  related to the operation of the first hydraulic pump  12 L. 
     Furthermore, the shovel according to the embodiment of the present invention merges hydraulic oil flowing out from a hydraulic actuator with hydraulic oil on the discharge side (downstream side) of a hydraulic pump when the pressure of the hydraulic oil flowing out from a hydraulic actuator is higher than the discharge pressure of the hydraulic pump, and merges hydraulic oil flowing out from a hydraulic actuator with hydraulic oil on the intake side (upstream side) of a hydraulic pump when the pressure of the hydraulic oil flowing out from a hydraulic actuator is lower than the discharge pressure of the hydraulic pump. Therefore, even when the pressure of hydraulic oil flowing out from a hydraulic actuator is lower than the discharge pressure of a hydraulic pump, it is possible to reuse the hydraulic oil to reduce a load on the hydraulic pump. 
     In the above-described embodiment, the controller  30  compares the pressure P 3  of hydraulic oil of the high-pressure oil passage  42  and the discharge pressure P 1  of the first hydraulic pump  12 L after comparing the pressure P 3  and the discharge pressure P 2  of the second hydraulic pump  12 R. Alternatively, the controller  30  may compare the pressure P 3  and the discharge pressure P 2  after comparing the pressure P 3  and the discharge pressure P 1 . As yet another alternative, the controller  30  may compare the pressure P 3  with the lower of the discharge pressure P 1  and the discharge pressure P 2  after comparing the pressure P 3  with the higher of the discharge pressure P 1  and the discharge pressure P 2 . 
     All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority or inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention. 
     For example, in the above-described embodiment, the operation amount detection part detects the amount of lever operation of an operation lever as the pressure value of a pilot pressure. Alternatively, the amount of operation may be detected as other physical quantity (such as a voltage, electric current or angle) using other sensors such as a potentiometer. 
     Furthermore, in the above-described embodiment, the flow rate control valves  150  through  157  are spool valves that operate in accordance with a pilot pressure. Alternatively, the flow rate control valves  150  through  157  may be solenoid spool valves that operate in accordance with a control instruction from the controller  30 . 
     Furthermore, the shovel may be provided with a turning electric motor in place of a turning hydraulic motor. 
     Furthermore, the construction machine according to the embodiment of the present invention may also be a lifting magnet, a crane, a high reach demolition machine or the like.