Abstract:
The present invention provides a system for driving a working machine, while the system achieves high energy saving, the system improving installability by downsizing hydraulic pumps and motors, and having extensibility enabling an attachment to be easily added. A system for driving a working machine according to the invention includes: a plurality of hydraulic closed circuits that connect hydraulic pumps to hydraulic actuators in a closed circuit manner; hydraulic open circuit that connects a hydraulic pump to hydraulic actuators in an open circuit manner; first assist circuits that connect between the hydraulic closed circuits so as to cause a hydraulic fluid to be mutually supplied between the hydraulic closed circuits; and second assist circuits that connect the hydraulic closed circuits to the hydraulic open circuit so as to cause the hydraulic fluid to be supplied from the hydraulic closed circuits to the hydraulic open circuit.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a system for driving a working machine and more particularly to a system for driving a working machine using a hydraulic closed circuit for causing a hydraulic pump to directly drive a hydraulic actuator. 
         [0003]    2. Description of the Related Art 
         [0004]    In recent years, energy saving has become an important issue for development of construction machines such as hydraulic excavators and wheel loaders. To provide energy-saving construction machines, it is required to save energy consumed by hydraulic systems therefor. The idea under consideration for saving energy consumed by the hydraulic system is the application of a hydraulic closed system in which a hydraulic pump and a hydraulic actuator are connected to each other in a closed circuit manner to cause the hydraulic pump to directly drive the hydraulic actuator. In the case of the hydraulic closed circuit, there is no pressure loss caused by a control valve and no loss of a hydraulic fluid because the hydraulic pump delivers the hydraulic fluid with only a necessary amount. The hydraulic system to which such a hydraulic closed circuit is applied is disclosed in JP-57-54635-A and JP-2004-190845-A. 
         [0005]    JP-57-54635-A discloses a configuration in which a plurality of hydraulic actuators are connected to a plurality of hydraulic pumps through a plurality of solenoid control valves in a closed circuit manner, and the connections between the hydraulic pumps and the hydraulic actuators are switched by controlling the solenoid control valves depending on an operational amount of an operation lever. In this configuration, energy saving is achieved by the closed circuits, and at the same time, the number of hydraulic pumps to be installed is reduced by causing a small number of hydraulic pumps to drive a large number of actuators, allowing the installability to be improved. 
         [0006]    In addition, JP-2004-190845-A discloses a configuration in which hydraulic pumps and motors that drive three actuators of a boom, stick, and bucket of a hydraulic excavator are provided, and an assist circuit that causes a hydraulic fluid to be mutually supplied between hydraulic circuits is arranged. In this configuration, energy saving is achieved by a closed circuit, and at the same time, the hydraulic pumps and the motors can be downsized by reducing demanded delivery rates of the hydraulic pumps, allowing the installability to be improved. 
       SUMMARY OF THE INVENTION 
       [0007]    From the perspective of energy saving, it would be desired that a possible number of actuators be arranged in closed circuits. If all actuators were arranged in closed circuits on a working machine that simultaneously operates multiple actuators, however, it would be necessary to arrange hydraulic pumps and motors by the number of units of the actuators that are simultaneously operated. In addition, a single hydraulic pump needs to support the maximum output of the actuators. Thus, the hydraulic pumps and the motors are large in size, which leads to problems with installability and cost. Furthermore, if an actuator that is frequently operated simultaneously with an existing actuator is to be added, speed control cannot be executed on an individual basis by controlling fluid delivery rates of the pumps, which leads to a problem that extensibility deteriorates. Since, among other things, a hydraulic excavator, needs easy addition of an attachment such as a breaker, the deterioration in extensibility is disadvantageous. 
         [0008]    In a hydraulic circuit described in JP-57-54635-A, since all the actuators are arranged in the closed circuits, high energy saving is achieved. In addition, since the circuit is configured so that a few hydraulic pumps can drive the large number of actuators, the hydraulic pumps and the motors can be downsized, and thus the installability is excellent. A single hydraulic pump, however, cannot individually control the speeds of multiple actuators, and the number of simultaneously operable actuators is limited to that of hydraulic pumps. Thus, the extensibility is deteriorated. 
         [0009]    On the other hand, the assist circuit is arranged in a hydraulic circuit described in JP-2004-190845-A. Thus, the hydraulic pumps and the motors can be each downsized, which leads to excellent installability. In addition, there is no problem with extensibility because an open circuit is arranged. Since only the boom as an actuator is arranged in the closed circuit, however, an effect of energy saving is not sufficient. 
         [0010]    An object of the invention is to provide a system for driving a working machine, while the system achieves high energy saving, the system improving installability by downsizing hydraulic pumps and motors, and having extensibility enabling an attachment to be easily added. 
         [0011]    (1) In order to accomplish the aforementioned object, according to the invention, a system for driving a working machine includes a plurality of hydraulic closed circuits that connect hydraulic pumps to hydraulic actuators in a closed circuit manner; at least one hydraulic open circuit that connects a hydraulic pump to at least one hydraulic actuator through a control valve in an open circuit manner; a plurality of first assist circuits that connect between the plurality of hydraulic closed circuits so as to cause a hydraulic fluid to be mutually supplied between the plurality of hydraulic closed circuits; and at least one second assist circuit that connects at least one of the plurality of hydraulic closed circuits to the hydraulic open circuit so as to cause the hydraulic fluid to be supplied from at least one of the plurality of hydraulic closed circuits to the hydraulic open circuit. 
         [0012]    Since the plurality of hydraulic actuators are driven by the hydraulic closed circuits made up in a closed circuit manner in the configuration described in item (1), there is no pressure loss caused by the control valve and no loss of a delivered hydraulic fluid, the amount of power to be consumed can be suppressed, and energy can be regenerated upon braking. Thus, high energy saving can be achieved. 
         [0013]    In addition, the hydraulic fluid can be mutually supplied between the hydraulic closed circuits and supplied from at least one of the hydraulic closed circuits to the hydraulic open circuit. Thus, the hydraulic pumps can be downsized while ensuring necessary speeds of the actuators, and installability can be improved. 
         [0014]    Furthermore, since the hydraulic open circuit made up in an open circuit manner is arranged, an attachment can be easily added through a control valve, and extensibility necessary for the working machine can be ensured. 
         [0015]    (2) In order to accomplish the aforementioned object, according to the invention, a system for driving a working machine includes a plurality of hydraulic closed circuits that connect hydraulic pumps to hydraulic actuators in a closed circuit manner; at least one fixed pressure source system circuit that includes a hydraulic pump, a common high-pressure line connected to the hydraulic pump and maintaining pressure at a fixed value by receiving the hydraulic fluid delivered from the hydraulic pump, a common low-pressure line connected to a tank, an accumulator connected to the common high-pressure line, and at least one variable displacement hydraulic pump motor connected between the common high-pressure line and the common low-pressure line; a plurality of first assist circuits that connect between the plurality of hydraulic closed circuits so as to cause a hydraulic fluid to be mutually supplied between the plurality of hydraulic closed circuits; and at least one second assist circuit that connects at least one of the plurality of hydraulic closed circuits to the fixed pressure source system circuit so as to cause the hydraulic fluid to be supplied from at least one of the plurality of hydraulic closed circuits to the fixed pressure source system circuit. 
         [0016]    Since the plurality of hydraulic actuators are driven by the hydraulic closed circuits made up in a closed circuit manner in the configuration described in item (2), there is no pressure loss caused by the control valve and no loss of a delivered hydraulic fluid, the amount of power to be consumed can be suppressed, and energy can be regenerated upon braking. Thus, high energy saving can be achieved. In the fixed pressure source system circuit, there is no pressure loss caused by the control valve, compared with the configuration including the hydraulic open circuit, and braking energy can be regenerated upon deceleration of the hydraulic actuators. Thus, significantly high energy saving can be achieved. 
         [0017]    In addition, since the hydraulic fluid can be mutually supplied between the hydraulic closed circuits and supplied from at least one of the hydraulic closed circuits to the fixed pressure source system circuit, the hydraulic pumps can be downsized while ensuring necessary speeds of the actuators, and thus the installability can be improved. 
         [0018]    Since an attachment can be easily added only by adding a variable displacement hydraulic pump motor in the fixed pressure source system circuit, extensibility necessary for the working machine can be ensured. 
         [0019]    (3) In items (1) or (2), the working machine is a hydraulic excavator, and the hydraulic actuators that are connected to the hydraulic pumps in the closed circuit manner in the plurality of hydraulic closed circuits are at least a boom cylinder and an arm cylinder. 
         [0020]    Energy to be consumed by the boom cylinder and the arm cylinder among the actuators of the hydraulic excavator is large. If the boom cylinder and the arm cylinder are arranged in the hydraulic open circuit, energy to be lost due to throttle resistance is large. Thus, high energy saving can be efficiently achieved by causing the hydraulic closed circuits to drive the boom cylinder and the arm cylinder. 
         [0021]    If the boom cylinder is arranged in the hydraulic open circuit, large potential energy is lost upon lowering of a boom. The boom cylinder, however, is driven by the hydraulic closed circuit made up in a closed circuit manner, and whereby potential energy can be regenerated. 
         [0022]    If the arm cylinder is arranged in the hydraulic open circuit, an increase in the speed upon application of a negative load caused by the weight of the arm cylinder is suppressed by a throttle on a meter-out side of a control valve, or by braking effect from a counter balance valve. This causes resistance upon the driving, and whereby energy to be consumed is increased. The arm cylinder, however, is driven by the hydraulic closed circuit made up in such a closed circuit manner, and whereby the hydraulic pumps act as regeneration brakes and throttle resistance is not required. Thus, energy to be consumed for the driving can be significantly reduced. 
         [0023]    (4) In item (2), the working machine is a hydraulic excavator, and the variable displacement hydraulic pump motor that is connected between the common high-pressure line and the common low-pressure line in the fixed pressure source system circuit is a swing hydraulic motor or a travel hydraulic motor. 
         [0024]    If a hydraulic actuator that is driven by the fixed pressure source system circuit is a rotary actuator for swing or traveling, torque of the variable displacement hydraulic pump motor can be used without a change. Thus, it is sufficient if a hydraulic motor that is normally used is replaced with the variable displacement hydraulic pump motor, and the control valve may not be necessary. Thus, installability is excellent. 
         [0025]    According to the invention, the amount of power to be consumed is suppressed by causing the hydraulic closed circuits made up in a closed circuit manner to drive the plurality of actuators, and thus high energy saving can be achieved. In addition, the hydraulic fluid can be mutually supplied between the hydraulic closed circuits and supplied from at least one of the hydraulic closed circuits to the hydraulic open circuit. Thus, the hydraulic pumps can be downsized while ensuring necessary speeds and outputs of the actuators, and the installability can be improved. Furthermore, since the hydraulic open circuit made up in an open circuit manner is arranged, an attachment can be easily added, and extensibility necessary for the working machine can be ensured. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a diagram illustrating an overall configuration of a system for driving a working machine according to a first embodiment. 
           [0027]      FIG. 2  is a diagram illustrating an overall configuration of a system for driving a working machine according to a second embodiment. 
           [0028]      FIG. 3  is a diagram illustrating an overall configuration of a system for driving a working machine according to a third embodiment. 
           [0029]      FIG. 4  is a diagram illustrating an appearance of a hydraulic excavator that is an example of a working machine provided with a drive system according to any of the embodiments of the invention. 
           [0030]      FIG. 5  is a diagram illustrating a table indicating a part of functions of a controller of the system for driving a working machine according to the first embodiment. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0031]    Hereinafter, embodiments of the invention are described with reference to the accompanying drawings. 
       First Embodiment 
       [0032]    First, the first embodiment of the invention is described with reference to  FIGS. 1 ,  4 , and  5 . 
         [0033]    Referring to  FIG. 1 , a system for driving a working machine according to the first embodiment includes hydraulic actuators  7   a,    7   b,    7   c,    10   a,  and  10   b,  hydraulic closed circuits  100  and  101 , a hydraulic open circuit  102 , first assist circuits  200  and  202 , and second assist circuits  201  and  203 . 
         [0034]    The hydraulic closed circuit  100  includes a motor  1   a,  a bidirectional delivery type hydraulic pump motor  2   a,  check valves  3   a,    3   b,    3   g,  and  3   h,  relief valves  4   a,    4   b,    4   e,  and  4   f,  and pilot check valves  6   a  and  6   b.  The motor  1   a  is directly connected to the bidirectional delivery type hydraulic pump motor  2   a.  The bidirectional delivery type hydraulic pump motor  2   a  is connected to a boom cylinder  7   a  through closed circuit lines  110   a,    110   b,    111   a,  and  111   b  and a solenoid control valve  5   a  in a closed circuit manner. The motor  1   a  normally and reversely rotates a bidirectional delivery type hydraulic pump  2   a  and thereby causes the bidirectional delivery type hydraulic pump  2   a  to suck and deliver a hydraulic fluid and causes the boom cylinder  7   a  to reciprocate. Specifically, a delivery rate and delivery direction of the hydraulic pump  2   a  are controlled by controlling a speed and direction of rotation of the motor  1   a,  and whereby a driving speed and driving direction of the boom cylinder  7   a  are controlled. When pressure within the circuit is reduced, the check valves  3   a  and  3   b  cause the hydraulic fluid delivered from a charge pump  8   b  to be sucked into the circuit and prevent cavitation in the circuit. When delivery pressure of the hydraulic pump  2   a  is equal to or higher than a set pressure value, the relief valves  4   a  and  4   b  cause the hydraulic fluid to be discharged from the circuit and prevent the pump and the lines from being damaged. The relief valves  4   e  and  4   f  are arranged in order to protect a hydraulic circuit located on the downstream side of the solenoid control valve  5   a.  The pilot check valves  6   a  and  6   b  deliver the hydraulic fluid to a low-pressure line or suck the hydraulic fluid from the low-pressure line in order to eliminate a difference, caused by the reciprocation of the boom cylinder  7   a  (serving as a single rod cylinder), between the amounts of the hydraulic fluids. 
         [0035]    The hydraulic closed circuit  101  includes a motor  1   b,  a bidirectional delivery type hydraulic pump motor  2   b,  check valves  3   c,    3   d,    3   e,  and  3   f,  relief valves  4   c,    4   d,    4   g,  and  4   h,  and pilot check valves  6   c  and  6   d.  The motor  1   b  is directly connected to the bidirectional delivery type hydraulic pump motor  2   b.  The bidirectional delivery type hydraulic pump motor  2   b  is connected to an arm cylinder  7   b  through closed circuit lines  112   a,    112   b,    113   a  and  113   b  and a solenoid control valve  5   e  in a closed circuit manner. The motor  1   b  normally and reversely rotates a bidirectional delivery type hydraulic pump  2   b  and thereby causes the bidirectional delivery type hydraulic pump  2   b  to suck and deliver the hydraulic fluid and causes the arm cylinder  7   b  to reciprocate. Specifically, a delivery rate and delivery direction of the hydraulic pump  2   b  are controlled by controlling a speed and direction of rotation of the motor  1   b,  and whereby a driving speed and driving direction of the arm cylinder  7   b  are controlled. When pressure within the circuit is reduced, the check valves  3   c  and  3   d  cause the hydraulic fluid delivered from the charge pump  8   b  to be sucked into the circuit and prevent cavitation in the circuit. When delivery pressure of the hydraulic pump  2   b  is equal to or higher than a set pressure value, the relief valves  4   c  and  4   d  cause the hydraulic fluid to be discharged from the circuit and prevent the pump and the lines from being damaged. The relief valves  4   g  and  4   h  are arranged in order to protect a hydraulic circuit located on the downstream side of the solenoid control valve  5   e.  The pilot check valves  6   c  and  6   d  deliver the hydraulic fluid to a low-pressure line or suck the hydraulic fluid from the low-pressure line in order to eliminate a difference, caused by the reciprocation of the arm cylinder  7   b  (serving as a single rod cylinder), between the amounts of the hydraulic fluids. 
         [0036]    The hydraulic open circuit  102  includes a motor  1   c,  a hydraulic pump  8   a,  a charge pump  8   b,  a check valve  3   e,  control valves  11   a,    11   b,  and  11   c,  high-pressure relief valves  4   i,    4   j,  and  4   m,  a low-pressure relief valve  4   l,  and a bypass valve  12 . The motor  1   c  is directly connected to the hydraulic pump  8   a  and the charge pump  8   b.  The hydraulic pump  8   a  is connected to a bucket cylinder  7   c,  and right and left travel hydraulic motors  10   a  and  10   b  through a hydraulic fluid supply line  16  and the control valves  11   a  to  11   c.  The hydraulic fluid delivered from the hydraulic pump  8   a  is supplied to the hydraulic actuators  7   c,    10   a,  and  10   b  through the hydraulic fluid supply line  16  and the control valves  11   a  to  11   c.  Returning sides of the control valves  11   a  to  11   c  are connected to a tank  9  through a low-pressure line  17  and the low-pressure relief valve  4   l.  The hydraulic fluid returned from the hydraulic actuators  7   c,    10   a,  and  10   b  is returned to the tank  9  through the control valves  11   a  to  11   c  and the low-pressure line  17 . As described above, the hydraulic open circuit  102  is made up in an open circuit manner that returns the hydraulic fluid returned from the hydraulic actuators  7   c,    10   a,  and  10   b  to the tank  9 . A driving direction and speed of the bucket cylinder  7   c  are controlled by the control valve  11   a.  Driving directions and speeds of the right and left travel hydraulic motors  10   a  and  10   b  are controlled by the control valves  11   b  and  11   c,  respectively. When the pressure within the circuit is reduced, the check valve  3   e  causes the hydraulic fluid delivered from the charge pump  8   b  to be sucked into the circuit and prevents cavitation in the circuit. The high-pressure relief valves  4   i  and  4   j  protects a hydraulic circuit located on the downstream side of the control valve  11   a.  When delivery pressure of the hydraulic pump  8   a  is equal to or higher than a set pressure value, the high-pressure relief valve  4   m  causes the hydraulic fluid to be discharged from the circuit and prevents the pumps and the lines from being damaged. When the solenoid control valves  5   c  and  5   f  are in an ON state and the charge pump  8   b  is directly connected to the tank  9  through the check valves  3   b  and  3   d,  the low-pressure relief valve  4   l  prevents a reduction in charge pressure of the charge pump  8   b  and enables a part of the hydraulic fluid returned from the hydraulic actuators  7   c,    10   a,  and  10   b  of the hydraulic open circuit  102  to return to sucking sides of the hydraulic pumps  2   a  and  2   b.  The bypass valve  12  has a function of causing the hydraulic fluid delivered from the hydraulic pump  8   a  to return to the tank  9  and unloading the delivery pressure when the hydraulic actuators  7   c,    10   a,  and  10   b  are not driven. 
         [0037]    Although the single hydraulic open circuit is arranged in the present embodiment, the number of hydraulic open circuits is not limited to 1 and may be 2 or more. 
         [0038]    The first assist circuit  200  includes hydraulic lines  200   a  and  200   b  and a solenoid control valve  5   b.  The hydraulic lines  200   a  and  200   b  connect the hydraulic closed circuits  100  and  101  to each other. The solenoid control valve  5   b  opens and closes the hydraulic lines  200   a  and  200   b.    
         [0039]    The second assist circuit  201  includes hydraulic lines  201   a  and  201   b  and a solenoid control valve  5   c.  The hydraulic lines  201   a  and  201   b  connect the hydraulic closed circuit  100  to the hydraulic open circuit  102 . The solenoid control valve  5   c  opens and closes the hydraulic lines  201   a  and  201   b.    
         [0040]    The first assist circuit  202  includes hydraulic lines  202   a  and  202   b  and a solenoid control valve  5   d.  The hydraulic lines  202   a  and  202   b  connect the hydraulic closed circuits  101  and  100  to each other. The solenoid control valve  5   d  opens and closes the hydraulic lines  202   a  and  202   b.    
         [0041]    The second assist circuit  203  includes hydraulic lines  203   a  and  203   b  and a solenoid control valve  5   f.  The hydraulic lines  203   a  and  203   b  connect the hydraulic closed circuit  101  to the hydraulic open circuit  102 . The solenoid control valve  5   f  opens and closes the hydraulic lines  203   a  and  203   b.    
         [0042]    When the solenoid control valves  5   b  and  5   c  are turned on (or opened), the solenoid control valve  5   a  is turned off (or closed) so as to supply (or assist supply of) a hydraulic fluid from the hydraulic closed circuit  100  to the hydraulic closed circuit  101  and the hydraulic open circuit  102 . Similarly, when the solenoid control valves  5   d  and  5   f  are turned on (or opened), the solenoid control valve  5   e  is turned off (or closed) so as to supply (or assist supply of) the hydraulic fluid from the hydraulic closed circuit  101  to the hydraulic closed circuit  100  and the hydraulic open circuit  102 . 
         [0043]    Although the two second assist circuits are arranged in the present embodiment, the number of second assist circuits is not limited and may be 1. 
         [0044]    The drive system according to the present embodiment has a swing motor  1   d  for turning an upper swing structure of a hydraulic excavator. 
         [0045]    The drive system according to the present embodiment includes an engine  20 , a power generator  21 , inverters  22   a  to  22   d,  a converter  23 , a battery  24 , and a controller  41  as an engine and control system. The power generator  21  is connected to the engine  20 . The inverters  22   a  to  22   d  are connected to the power generator  21 . The converter  23  is connected to the power generator  21 . The battery  24  is connected to the converter  23 . The engine  20  drives the power generator  21 . Power generated by the power generator  21  is supplied to the motors  1   a  to  1   d  through the inverters  22   a  to  22   d,  and part of the power is stored in the battery  24  through the converter  23 . 
         [0046]    The drive system according to the present embodiment includes control lever type operating devices  40   a  and  40   b  and control pedal type operating devices  40   c  and  40   d  as an operation system. The operating devices  40   a  and  40   b  are connected to the controller  41 . An up and down operation of the operating device  40   a  corresponds to an operation of the swing motor  1   d.  A left and right operation of the operating device  40   a  corresponds to an operation of the arm cylinder  7   b.  An up and down operation of the operating device  40   b  corresponds to an operation of the boom cylinder  7   a.  A left and right operation of the operating device  40   b  corresponds to an operation of the bucket cylinder  7   c.  An operation of the operating device  40   c  corresponds to an operation of the right travel hydraulic motor  10   a.  An operation of the operating device  40   d  corresponds to an operation of the left travel hydraulic motor  10   b.  Note that correspondence relationships between operational directions of the operating devices  40   a  and  40   b  and operations of the hydraulic actuators may be based on another scheme. 
         [0047]    The controller  41  executes arithmetic processing on operation signals received from the operating devices  40   a  to  40   d,  outputs control signals after the arithmetic processing to the solenoid control valves  5   a  to  5   f,  the control valves  11   a  to  11   c,  the bypass valve  12 , and the inverters  22   a  to  22   d,  and controls these components. 
         [0048]      FIG. 4  illustrates an appearance of a hydraulic excavator that is an example of a working machine provided with the drive system according to the present embodiment. In  FIG. 4 , parts that are the same as those illustrated in  FIG. 1  are indicated by the same reference symbols. The hydraulic excavator has an upper swing structure  30   d,  a lower travel structure  30   e,  and a front device  30 A. The lower travel structure  30   e  is moved by the right and left travel hydraulic motors  10   a  and  10   b  (only one travel hydraulic motor is illustrated). The upper swing structure  30   d  is swung on the lower travel structure  30   e  by the swing motor  1   d  (refer to  FIG. 1 ). The front device  30 A has a multijoint structure including a boom  30   a,  an arm  30   b,  and a bucket  30   c.  The boom  30 , the arm  30   b,  and the bucket  30   c  are rotationally driven in a vertical plane by the boom cylinder  7   a,  the arm cylinder  7   b,  and the bucket cylinder  7   c,  respectively. 
         [0049]    The driving of the right and left travel hydraulic motors  10   a  and  10   b  (the one travel hydraulic motor is illustrated) is controlled by operating the control valves  11   b  and  11   c  (refer to  FIG. 1 ) on the basis of operational amounts of the operating devices  40   c  and  40   d  (refer to  FIG. 1 ). The driving of the swing structure  30   d  is controlled by operating the inverter  22   d  (refer to  FIG. 1 ) and the swing motor  1   d  (refer to  FIG. 1 ) on the basis of an operational amount of the operating device  40   a  (refer to  FIG. 1 ) in a vertical direction. The driving of the boom cylinder  7   a  is controlled by operating the inverter  22   a  (refer to  FIG. 1 ) and the motor  1   a  (refer to  FIG. 1 ) on the basis of an operational amount of the operating device  40   b  (refer to  FIG. 1 ) in the vertical direction. The driving of the arm cylinder  7   b  is controlled by operating the inverter  22   b  (refer to  FIG. 1 ) and the motor  1   b  (refer to  FIG. 1 ) on the basis of an operational amount of the operating device  40   a  (refer to  FIG. 1 ) in a left-right direction. The driving of the bucket cylinder  7   c  is controlled by operating the control valve  11   a  (refer to  FIG. 1 ) on the basis of an operational amount of the operating device  40   b  (refer to  FIG. 1 ) in the left-right direction. The amount of the hydraulic fluid to be delivered from the hydraulic pump  8   a  (refer to  FIG. 1 ) is controlled by operating the inverter  22   c  (refer to  FIG. 1 ) and the motor  1   c  (refer to  FIG. 1 ) on the basis of an operational amount of the operating device  40   a  (refer to  FIG. 1 ) in the left-right direction and operational amounts of the operating devices  40   c  and  40   d  (refer to  FIG. 1 ). 
         [0050]    Operations of the drive system with the aforementioned configuration are described with reference to  FIG. 5 .  FIG. 5  illustrates a part of functions of the controller  41 . 
         [0051]    First, the case where the boom or the arm is independently operated is described below. 
         [0052]    During stop of the boom  30   a  and the arm  30   b,  the operating devices  40   a  and  40   b  are not operated and are in a neutral state. In this case, the solenoid control valves  5   a,    5   b,    5   d,  and  5   e  are in an OFF state (or all closed), the motors  1   a  and  1   b  are not operated, and the hydraulic fluid is not supplied from the hydraulic pumps  2   a  and  2   b  (in operation  1 ). In this case, the boom cylinder  7   a  and arm cylinder  7   b  are prevented from falling due to their own weights. 
         [0053]    To independently drive the boom  30   a  at a low speed, the operating device  40   b  is half operated in a front-back direction, for example. In this case, the solenoid control valve  5   a  is turned on, the hydraulic pump  2   a  is connected to the boom cylinder  7   a,  the motor  1   a  is operated, and whereby the hydraulic fluid is supplied from the hydraulic pump  2   a  to the boom cylinder  7   a  (in operation  2 ). 
         [0054]    To independently drive the arm  30   b  at a low speed, the operating device  40   a  is half operated in the left-right direction, for example. In this case, the solenoid control valve  5   e  is turned on, the hydraulic pump  2   b  is connected to the arm cylinder  7   b,  the motor  1   b  is operated, and whereby the hydraulic fluid is supplied from the hydraulic pump  2   b  to the arm cylinder  7   b  (in operation  3 ). 
         [0055]    To independently drive the boom  30   a  at a high speed, the operating device  40   b  is fully operated in the front-back direction. In this case, the solenoid control valves  5   a  and  5   d  are turned on, the two hydraulic pumps  2   a  and  2   b  are connected to the boom cylinder  7   a,  the motors  1   a  and  1   b  are operated, and whereby the hydraulic fluid is supplied from the two hydraulic pumps  2   a  and  2   b  to the boom cylinder  7   a  (in operation  5 ). 
         [0056]    To independently drive the arm  30   b  at a high speed, the operating device  40   a  is fully operated in the left-right direction. In this case, the solenoid control valves  5   e  and  5   b  are turned on, the two hydraulic pumps  2   a  and  2   b  are connected to the arm cylinder  7   b,  the motors  1   a  and  1   b  are operated, and whereby the hydraulic fluid is supplied from the two hydraulic pumps  2   a  and  2   b  to the arm cylinder  7   b  (in operation  6 ). 
         [0057]    Next, the case where the bucket  30   c  or the left and right travel hydraulic motors  10   a  and  10   b  is or are independently operated is described. 
         [0058]    During stop of the bucket  30   c  and the left and right travel hydraulic motors  10   a  and  10   b,  the operating device  40   b  is not operated and is in the neutral state, and the operating devices  40   c  and  40   d  are not operated. In this case, the bypass valve  12  is in an OFF state (or open), the hydraulic pump  8   a  is unloaded. Specifically, the hydraulic fluid delivered from the hydraulic pump  8   a  is returned to the tank  9  through the bypass valve  12 . In this case, the motor  1   c  rotates at the minimum rotational speed, and power consumed by the motor  1   c  is suppressed to a small value (in operation  1 ). Since the motor  1   c  rotates at the minimum rotational speed and the hydraulic pump  8   a  delivers the fluid with the minimum amount, a response upon start-up is improved. In this case, the motor  1   c  may be stopped, and whereby the power consumed by the motor  1   c  can be further suppressed. 
         [0059]    To independently drive the bucket  30   c  or the left and right travel hydraulic motors  10   a  and  10   b  at a low speed, the operating device  40   b  is half operated in the left-right direction or the operating devices  40   c  and  40   d  are half operated, for example. In this case, the bypass valve  12  is turned on (or closed), the delivery pressure of the hydraulic pump  8   a  is increased, the control valve  11   a  or the control valves  11   b  and  11   c  are switched on the basis of an operational amount of the operating device  40   b  in the left-right direction or operational amounts of the operating devices  40   c  and  40   d,  the rotational speed of the motor  1   c  is increased, the delivery rate of the hydraulic pump  8   a  is increased, and whereby the hydraulic fluid is supplied to the bucket cylinder  7   c  or the right and left travel hydraulic motors  10   a  and  10   b  (in operation  4 ). 
         [0060]    To independently drive the bucket  30   c  or the left and right travel hydraulic motors  10   a  and  10   b  at a high speed, the operating device  40   b  is fully operated in the left-right direction or the operating devices  40   c  and  40   d  are fully operated. In this case, the bypass valve  12  is turned on, and the delivery pressure of the hydraulic pump  8   a  is increased. In addition, at least one of the solenoid control valves  5   c  and  5   f  is turned on (both solenoid control valves  5   c  and  5   f  are turned on in the example illustrated in  FIG. 5 ), and at least one of the hydraulic pumps  2   a  and  2   b  is connected to the hydraulic open circuit  102  (both hydraulic pumps  2   a  and  2   b  are connected to the hydraulic open circuit  102  in the example illustrated in  FIG. 5 ). Furthermore, the control valve  11   a  or the control valves  11   b  and  11   c  are switched on the basis of an operational amount of the operating device  40   b  in the left-right direction or operational amounts of the operating devices  40   c  and  40   d,  and at least one of the motors  1   a  and  1   b  is operated (both motors  1   a  and  1   b  are operated in the example illustrated in  FIG. 5 ). Thus, the hydraulic fluid delivered from the hydraulic pump  8   a  and the hydraulic fluid delivered from at least one of the hydraulic pumps  2   a  and  2   b  join together (hydraulic fluids delivered from up to three hydraulic pumps join together) and are supplied to the bucket cylinder  7   c  or the right and left travel hydraulic motors  10   a  and  10   b  (in operation  7 ). 
         [0061]    Lastly, the case of a combined operation of the boom  30   a,  the arm  30   b  and the bucket  30   c  or traveling is described. 
         [0062]    To simultaneously drive the boom  30   a  and the arm  30   b,  the operating device  40   b  is operated in the front-back direction and the operating device  40   a  is operated in the left-right direction. In this case, the solenoid control valves  5   a  and  5   e  are turned on, the hydraulic pumps  2   a  and  2   b  are connected to the boom cylinder  7   a  and the arm cylinder  7   b,  respectively, the motors  1   a  and  1   b  are operated, and whereby the hydraulic fluid is supplied from the hydraulic pumps  2   a  and  2   b  to the boom cylinder  7   a  and the arm cylinder  7   b,  respectively (in operation  8 ). 
         [0063]    To simultaneously drive the boom  30   a,  the arm  30   b,  and the bucket  30   c  or the right and left travel hydraulic motors  10   a  and  10   b,  the operating device  40   b  is operated in the front-back direction, the operating device  40   a  is operated in the left-right direction, and the operating device  40   b  is operated in the left-right direction or the operating devices  40   c  and  40   d  are operated. In this case, the solenoid control valves  5   a  and  5   e  are turned on, the bypass valve  12  is turned on (or closed), the motors  1   a  to  1   c  are operated, and whereby the hydraulic fluid is supplied from the hydraulic pumps  2   a  and  2   b  to the boom cylinder  7   a  and the arm cylinder  7   b,  respectively, and the hydraulic fluid is supplied from the hydraulic pump  8   a  to the bucket cylinder  7   c  or the right and left travel hydraulic motors  10   a  and  10   b  (in operation  9 ). In this case, the independencies of the hydraulic actuators are maintained, and controllability is ensured. 
         [0064]    To simultaneously drive the boom  30   a  and the bucket  30   c,  the operating device  40   b  is operated in the front-back direction, and the operating device  40   b  is operated in the left-right direction. In this case, the solenoid control valve  5   a  is turned on, the bypass valve  12  is turned on, the motors  1   a  and  1   c  are operated, and whereby the hydraulic fluid is supplied from the hydraulic pump  2   a  to the boom cylinder  7   a  and supplied from the hydraulic pump  8   a  to the bucket cylinder  7   c.  In this case, the hydraulic pump  2   b  is operated as follows. 
         [0065]    To drive the boom  30   a  at a high speed and drive the bucket  30   c  simultaneously with the driving of the boom  30   a,  the operating device  40   b  is fully operated in the front-back direction and half operated in the left-right direction, for example. In this case, the solenoid control valves  5   a  and  5   d  are turned on, the bypass valve  12  is turned on, the motors  1   a  and  1   b  are operated, the hydraulic fluids delivered from the hydraulic pumps  2   a  and  2   b  join together and are supplied to the boom cylinder  7   a,  and the hydraulic fluid is supplied from the hydraulic pump  8   a  to the bucket cylinder  7   c  (in operation  10 ). 
         [0066]    To drive the bucket  30   c  at a high speed and drive the boom  30   a  simultaneously with the driving of the bucket  30   c,  the operating device  40   b  is half operated in the front-back direction and fully operated in the left-right direction, for example. In this case, the solenoid control valves  5   a  and  5   f  are turned on, the bypass valve  12  is turned on, the motors  1   a  and  1   b  are operated, the hydraulic fluid is supplied from the hydraulic pump  2   a  to the boom cylinder  7   a,  and the hydraulic fluids delivered from the hydraulic pumps  8   a  and  2   b  join together and are supplied to the bucket cylinder  7   c  (in operation  11 ). 
         [0067]    According to the present embodiment described above, the following effects can be obtained. 
         [0068]    Since the boom  30   a  and the arm  30   b  are driven by the hydraulic closed circuits  100  and  101  made up in a closed circuit manner, respectively, there is no pressure loss caused by the control valves and no loss of the hydraulic fluid, and the amount of power to be consumed can be suppressed. In addition, the bidirectional delivery type hydraulic pump motor  2   a  acts as a motor upon lowering of the boom, and potential energy can be regenerated by driving the motor  1   a  and thereby generating power. Since the bidirectional delivery type hydraulic pump motor  2   a  acts as a regeneration brake upon application of a negative load caused by the weight of the arm  30   b,  energy is not consumed by throttle resistance. Thus, high energy saving can be achieved. 
         [0069]    In addition, since the hydraulic fluid can be mutually supplied between the hydraulic closed circuits  100  and  101  and supplied from the hydraulic closed circuits  100  and  101  to the hydraulic open circuit  102 , the hydraulic pumps and the motors can be downsized while ensuring necessary speeds of the actuators, and whereby installability is improved. 
         [0070]    Furthermore, since the hydraulic open circuit  102  is made up in an open circuit manner, a hydraulic actuator as an attachment can be easily added through a control valve, extensibility that is necessary for the hydraulic excavator can be ensured. 
       Second Embodiment 
       [0071]    Next, the second embodiment of the invention is described with reference to  FIGS. 2 and 4 . The second embodiment describes the case where a motor is not used and the configurations of the hydraulic circuits are nearly the same as the first embodiment. In  FIGS. 2 and 4 , parts that are the same as those illustrated in  FIG. 1  are indicated by the same reference symbols, and a description thereof is omitted. 
         [0072]    Referring to  FIG. 2 , a system for driving a working machine according to the second embodiment includes a swing hydraulic motor  10   c  instead of the swing motor  1   d  (refer to  FIG. 1 ) according to the first embodiment and includes hydraulic closed circuits  100   a  and  101   a  and a hydraulic open circuit  102   a  instead of the hydraulic closed circuits  100  and  101  (refer to  FIG. 1 ) and the hydraulic open circuit  102  (refer to  FIG. 1 ). 
         [0073]    The hydraulic closed circuit  100   a  includes a bidirectional delivery and variable displacement type hydraulic pump motor  13   a  instead of the bidirectional delivery type hydraulic pump motor  2   a  (refer to  FIG. 1 ). The hydraulic closed circuit  101   a  includes a bidirectional delivery and variable displacement type hydraulic pump motor  13   b  instead of the bidirectional delivery type hydraulic pump motor  2   b  (refer to  FIG. 1 ). Hydraulic pumps  13   a  and  13   b  and the hydraulic pump  8   a  of the hydraulic open circuit  102   a  have regulators  14   a,    14   b,  and  14   c,  respectively. The regulators  14   a,    14   b,  and  14   c  control tilting amounts (pump capacity) and tilting directions (delivery directions of the hydraulic fluids) of the hydraulic pumps  13   a,    13   b,  and  8   a  on the basis of operational amounts (demanded fluid amounts) and operation directions of the operating devices  40   a  to  40   d.  The amounts of the hydraulic fluids to be delivered from the hydraulic pumps  13   a  and  13   b  and the directions of the delivery of the hydraulic fluids are controlled by controlling the tilting amounts and tilting directions of the hydraulic pumps  13   a  and  13   b,  and whereby driving speeds and driving directions of the hydraulic actuators  7   a  and  7   b  are controlled. The hydraulic open circuit  102   a  has a control valve  11   d.  The hydraulic pump  8   a  is connected to the swing hydraulic motor  10   c  through the control valve  11   d.  The parts that are related to the control valve  11   d  of the hydraulic open circuit  102   a  are included in a hydraulic open circuit in which the hydraulic fluid is returned from the swing hydraulic motor  10   c  through the control valve  11   d  to the tank  9 . The driving direction and speed of the swing hydraulic motor  10   c  are controlled by the control valve  11   d.    
         [0074]    The drive system according to the second embodiment includes a controller  41   a  and a power transfer device  15  that is connected to the engine  20  and distributes power of the engine  20  to the hydraulic pumps  13   a,    13   b,  and  8   a  and the charge pump  8   b  as an engine and control system. 
         [0075]    The controller  41   a  executes arithmetic processing on operation signals received from the operating devices  40   a  to  40   d,  outputs control signals after the arithmetic processing to the solenoid control valves  5   a  to  5   f,  the control valves  11   a  to  11   d,  the bypass valve  12 , and the regulators  14   a  to  14   c  of the hydraulic pumps  13   a,    13   b,  and  8   a,  and controls these components. 
         [0076]    According to the second embodiment described above, high energy saving, installability, and high extensibility, which are the same as or close to the first embodiment, can be obtained without using a motor. 
         [0077]    In the second embodiment, the swing hydraulic motor  10   c  is driven by the hydraulic open circuit made up in an open circuit manner. Another bidirectional delivery and variable displacement type hydraulic pump motor may be added and driven by the hydraulic closed circuit made up in a closed circuit manner. In this case, large braking energy can be regenerated upon deceleration of the swing hydraulic motor  10   c,  and whereby higher energy saving can be obtained. Specifically, since load torque is reduced for the engine  20  upon the regeneration of the braking energy, the amount of a fuel to be injected to maintain the revolution of the engine  20  can be reduced, and the amount of the fuel to be consumed can be reduced. 
       Third Embodiment 
       [0078]    The third embodiment of the invention is described with reference to  FIGS. 3 and 4 . In the third embodiment, the hydraulic open circuit according to the second embodiment is replaced with a fixed pressure source system circuit (secondary control system circuit), and a hydraulic closed circuit for the bucket cylinder is added. In  FIGS. 3 and 4 , parts that are the same as those illustrated in FIGS.  1  and  2  are indicated by the same reference numerals and symbols, and a description thereof is omitted. 
         [0079]    Referring to  FIG. 3 , a system for driving a working machine according to the third embodiment includes a variable displacement type right travel hydraulic pump motor  13   d,  a variable displacement type left travel hydraulic pump motor  13   e,  and a variable displacement type swing hydraulic pump motor  13   f  instead of the right and left travel hydraulic motors  10   a  and  10   b  (refer to  FIG. 2 ) and the swing hydraulic motor  10   c  (refer to  FIG. 2 ) and includes a hydraulic closed circuit  103  and a fixed pressure source system circuit  104  instead of the hydraulic open circuit  102   a  (refer to  FIG. 2 ). The system for driving a working machine according to the third embodiment includes a first assist circuit  201 A and a second assist circuit  203 A instead of the second assist circuits  201  and  203  (refer to  FIG. 2 ) and further includes a first assist circuit  204  and a second assist circuit  205 . 
         [0080]    The hydraulic closed circuit  103  includes a bidirectional delivery and variable displacement type hydraulic pump motor  13   c,  the check valves  3   e  and  3   f,  the relief valves  4   i,    4   j,    4   n  and  4   o,  and pilot check valves  6   e  and  6   f.  The bidirectional delivery and variable displacement type hydraulic pump motor  13   c  includes a regulator  14   d  that controls a tilting amount (pump capacity) and tilting direction (delivery directions of the hydraulic fluids) of a hydraulic pump  13   c.  The bidirectional delivery and variable displacement type hydraulic pump motor  13   c  is connected to the bucket cylinder  7   c  through closed circuit lines  114   a,    114   b,    115   a,  and  115   b  and a solenoid control valve  5   h  in a closed circuit manner. The amount and direction of the hydraulic fluid to be delivered from the hydraulic pump  13   c  are controlled by controlling the tilting amount and tilting direction of the hydraulic pump  13   c,  and whereby the driving speed and driving direction of the bucket cylinder  7   c  are controlled. 
         [0081]    The fixed pressure source system circuit  104  includes the hydraulic pump  8   a  and the charge pump  8   b  as hydraulic sources. The engine  20  drives the variable displacement hydraulic pump motors  13   a,    13   b,  and  13   c,  the hydraulic pump  8   a,  and the charge pump  8   b  through a power transfer device  15   a.    
         [0082]    The first assist circuit  201 A includes hydraulic lines  201 Aa and  201 Ab and a solenoid control valve  5   c.  The hydraulic lines  201 Aa and  201 Ab connect the hydraulic closed circuits  100   a  and  103  to each other. The solenoid control valve  5   c  opens and closes the hydraulic lines  201 Aa and  201 Ab. 
         [0083]    The second assist circuit  203 A includes hydraulic lines  203 Aa and  203 Ab and a solenoid control valve  5   f.  The hydraulic lines  203 Aa and  203 Ab connect the hydraulic closed circuit  101   a  to the fixed pressure source system circuit  104 . The solenoid control valve  5   f  opens and closes the hydraulic lines  203 Aa and  203 Ab. 
         [0084]    The first assist circuit  204  includes hydraulic lines  204   a  and  204   b  and a solenoid control valve  5   g.  The hydraulic lines  204   a  and  204   b  connect the hydraulic closed circuits  103  and  100   a  to each other. The solenoid control valve  5   g  opens and closes the hydraulic lines  204   a  and  204   b.    
         [0085]    The second assist circuit  205  includes hydraulic lines  205   a  and  205   b  and a solenoid control valve  5   i.  The hydraulic lines  205   a  and  205   b  connect the hydraulic closed circuit  103  to the fixed pressure source system circuit  104 . The solenoid control valve  5   i  opens and closes the hydraulic lines  205   a  and  205   b.    
         [0086]    When the solenoid control valves  5   g  and  5   i  are turned on (or opened), the solenoid control valve  5   h  is turned off (or closed) so as to supply (or assist of supply of) the hydraulic fluid from the hydraulic closed circuit  103  to the hydraulic closed circuit  100   a  and the fixed pressure source system circuit  104 . 
         [0087]    When the solenoid control valves  5   a,    5   d,  and  5   g  are turned on, the boom cylinder  7   a  is connected to the three variable displacement hydraulic pump motors  13   a,    13   b,  and  13   c  and can be driven at a higher speed when necessary. Similarly, when the solenoid control valves  5   c  and  5   h  are turned on, the bucket cylinder  7   c  is connected to the two variable displacement hydraulic pump motors  13   a  and  13   c  and can be driven at a high speed when necessary. 
         [0088]    Although the two second assist circuits are arranged in the present embodiment, the number of second assist circuits is not limited to 2 and may be 1. 
         [0089]    The fixed pressure source system circuit  104  includes a common high-pressure line  25 , a common low-pressure line  26 , the low-pressure relief valve  4   l,  a high-pressure relief valve  4   m,  an accumulator  18 , a pressure sensor  19 , and a check valve  3   g.    
         [0090]    The common high-pressure line  25  is connected to the hydraulic pump  8   a.  The hydraulic fluid is supplied from the hydraulic pump  8   a  to the common high-pressure line  25 , and pressure of the common high-pressure line  25  is maintained at a fixed level. The structure of a fixed pressure source system circuit that maintains the pressure of the common high-pressure line  25  at the fixed level is well known. As an example, in the present embodiment, a regulator  14   c  is arranged on the hydraulic pump  8   a,  the pressure sensor  19  is arranged on the common high-pressure line  25 , and a detection signal of the pressure sensor  19  is input to a controller  41   b.  The controller  41   b  compares a pressure value detected by the pressure sensor  19  with a target pressure value. If the detected pressure value is lower than the target pressure value, the regulator  14   c  is controlled so as to increase the tilting amount (pump capacity) of the hydraulic pump  8   a.  If the detected pressure value is higher than the target pressure value, the regulator  14   c  is controlled so as to reduce the tilting amount (pump capacity) of the hydraulic pump  8   a.    
         [0091]    The common high-pressure line  25  has the relief valve  4   m  and the accumulator  18  connected thereto. The common low-pressure line  26  has the low-pressure relief valve  4   l  and the check valve  3   g  connected thereto. The check valve  3   g  is connected to the common low-pressure line  26  in parallel with the low-pressure relief valve  4   l  so as to allow the hydraulic fluid to flow from the tank  9  to the common low-pressure line  26 . 
         [0092]    The variable displacement type right and left travel hydraulic pump motors  13   d  and  13   e  and the variable displacement type swing hydraulic pump motor  13   f  are connected between the common high-pressure line  25  and the common low-pressure line  26 . The variable displacement type hydraulic pump motors  13   d,    13   e,  and  13   f  respectively include regulators  14   e,    14   f,  and  14   g  that control tilting directions and tilting amounts. 
         [0093]    Rotation torque of the hydraulic pump motors  13   d,    13   e,  and  13   f  is represented by products of the tilting amounts (motor capacity) and the driving pressure (pressure of the common high-pressure line  25 ). Since the pressure of the common high-pressure line  25  is a fixed value, the rotation torque of the hydraulic pump motors  13   d,    13   e,  and  13   f  can be changed by changing the tilting amounts of the hydraulic pump motors  13   d,    13   e,  and  13   f.  The rotational speeds of the hydraulic pump motors  13   d,    13   e,  and  13   f  can be changed by changing the rotation torque of the hydraulic pump motors  13   d,    13   e,  and  13   f.  In the fixed pressure source system circuit  104 , the rotational directions and rotational speeds of the hydraulic pump motors  13   d,    13   e,  and  13   f  can be controlled by controlling the tilting directions and tilting amounts of the hydraulic pump motors  13   d,    13   e,  and  13   f  without using a control valve. 
         [0094]    The variable displacement hydraulic pump motors  13   d,    13   e,  and  13   f  act as motors upon the driving of the loads and act as pumps upon braking. Upon the braking, the variable displacement hydraulic pump motors  13   d,    13   e,  and  13   f  suck the hydraulic fluid from the tank  9  through the check valve  3   g  and deliver the hydraulic fluid to the common high-pressure line  25 . Hydraulic energy (pressure) generated in this case is collected by the accumulator  18  and reused for acceleration of the hydraulic pump motors. Note that the accumulator  18  also has an effect of absorbing pulsation of pressure within the circuit. 
         [0095]    Although the single fixed pressure source system circuit is arranged in the present embodiment, the number of fixed pressure source system circuits is not limited to 1 and may be 2 or more. 
         [0096]    The controller  41   b  executes arithmetic processing on operation signals received from the operating devices  40   a  to  40   d,  outputs control signals after the arithmetic processing to the solenoid control valves  5   a  to  5   i  and the regulators  14   a,    14   b,  and  14   d  to  14   g  of the hydraulic pump motors  13   a  to  13   f  of the variable displacement type, and controls these components. In addition, to maintain the pressure of the common high-pressure line  25  at the fixed level, the detection signal of the pressure sensor  19  is monitored and the regulator  14   c  is controlled so that the delivery pressure of the hydraulic pump  8   a  is fixed. 
         [0097]    According to the present embodiment described above, the same effects of high energy saving and installability as the second embodiment and the following effects can be obtained. 
         [0098]    In the present embodiment, since the bucket is driven by the hydraulic closed circuit  103  made up in a closed circuit manner, the energy saving is high. In addition, energy can be significantly saved by causing the fixed pressure source system circuit  104  capable of regenerating braking energy to drive the right and left travel hydraulic pump motors  13   d  and  13   e  and the swing hydraulic pump motor  13   f  without pressure loss caused by the control valves. 
         [0099]    The hydraulic fluid can be mutually supplied among the three hydraulic closed circuits  100   a,    101   a,  and  103  made up in a closed circuit manner and can be supplied from the hydraulic closed circuits  101   a  and  103  to the fixed pressure source system circuit  104 . Thus, the hydraulic pumps can be downsized while necessary speeds of the actuators are ensured, and the installability can be improved. 
         [0100]    If the hydraulic actuators that are driven by the fixed pressure source system circuit  104  are rotary actuators such as swing actuators or travel actuators, the rotation torque of the variable displacement hydraulic pump motors can be used without conversion, hydraulic motors that are normally used are simply replaced with the variable displacement hydraulic pump motors, and a control valve is not required. Thus, the installability is excellent. 
         [0101]    An actuator can be easily added by arranging an additional variable displacement hydraulic pump motor between the common high-pressure line  25  and the common low-pressure line  26 , and thus extensibility can be ensured. 
         [0102]    In the present embodiment, the fixed pressure source system circuit drives the rotary actuators. However, when a hydraulic transformer in which a fixed displacement hydraulic pump motor is directly connected to a rotary shaft of a variable displacement hydraulic pump motor is used, the fixed pressure source system circuit can drive a linear actuator by causing the hydraulic fluid to be supplied from the fixed displacement hydraulic pump motor to a cap side of a hydraulic cylinder.