Patent Publication Number: US-10781571-B2

Title: Hydraulic system for working machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. P2018-171757, filed Sep. 13, 2018 and to Japanese Patent Application No. P2019-008591, filed Jan. 22, 2019. The contents of these applications are incorporated herein by reference in their entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a hydraulic system for a working machine and to a control valve. 
     Description of Related Art 
     A technique disclosed in Japanese Unexamined Patent Application Publication No. 2010-270527 is known as a hydraulic system for a working machine. The working machine disclosed in Japanese Unexamined Patent Application Publication No. 2010-270527 includes a boom, a bucket, a boom cylinder configured to move the boom, a bucket cylinder configured to move the bucket, an auxiliary actuator configured to operate an auxiliary attachment, a first control valve configured to control the stretching and shortening of the boom cylinder, a second control valve configured to control the stretching and shortening of the bucket cylinder, and a third control valve configured to operate the auxiliary actuator. 
     A technique disclosed in Japanese Unexamined Patent Application Publication No. 2011-208693 is known as a hydraulic system for a working machine. The hydraulic system disclosed in Japanese Unexamined Patent Application Publication No. 2011-208693 includes an unload passage connected to a hydraulic pump, a tank passage connected to a tank, a first directional switching valve connected to the unload passage and configured to control the supplying of pressured fluid from the hydraulic pump to a first actuator, a pair of first supply/discharge passages connecting between the first directional switching valve and the first actuator, a second directional switching valve connected to the unload passage on the downstream side from the first directional switching valve and configured to control the supplying of pressured fluid from the hydraulic pump to the second actuator, and a pair of second supply/discharge passages that connect between the second directional switching valve and the second actuator. In a multiple directional switching valve that returns the pressured fluid from the first supply/discharge passages to the unload passages when the first actuator is actuated, the first directional switching valve has a switching position where the unload passage on the upstream side of the first directional switching valve communicates with one end of the first supply/discharge passage and further where the other end of the first supply/discharge passage communicates with the tank passage and the unload passage on the downstream side of the first directional switching valve. A tank return passage is connected to the first directional switching valve, the tank return passage communicating between the other end of the first supply/discharge passage and the tank passage. 
     SUMMARY OF THE INVENTION 
     A hydraulic system for a working machine, includes: a hydraulic pump; a first hydraulic actuator, a second hydraulic actuator; a first control valve to control a flow rate of operation fluid to be supplied to the first hydraulic actuator; a second control valve to control a flow rate of operation fluid to be supplied to the second hydraulic actuator; a return fluid tube connecting the first control valve and the second control valve and allowing a return fluid to flow in the return fluid tube, the return fluid being operation fluid returning from the first hydraulic actuator to the first control valve; a supply fluid tube connecting the first control valve and the second control valve separately from the return fluid tube and being connected to the return fluid tube at a middle portion of the supply fluid tube; a discharge fluid tube connected to the second control valve; an operation member to operate the second control valve; and a controller to control the second control valve in accordance with operation of the operation member. The second control valve is switched between a primary position, a secondary position, and a tertiary position, the primary position blocking between the supply fluid tube and the discharge fluid tube and allowing the return fluid to be supplied to the second hydraulic actuator, the secondary position communicating between the supply fluid tube and the discharge fluid tube and stopping supplying the return fluid to the second hydraulic actuator, and the tertiary position communicating between the supply fluid tube and the discharge fluid tube and allowing the return fluid to be supplied to the second hydraulic actuator. The controller switches the second control valve to the primary position when the first control valve is under a non-operation state where operation fluid is not supplied to the first hydraulic actuator and when the operation member is at a first operating position, switches the second control valve to the secondary position when the first control valve is under the non-operation state and when the operation member is at a second operating position, and switches the second control valve to the tertiary position when the first control valve is under an operation state where operation fluid is supplied to the first hydraulic actuator and when the operation member is at the first operating position. 
     A hydraulic system for a working machine, includes: a hydraulic pump; a first hydraulic actuator; a second hydraulic actuator; a first control valve to control the first hydraulic actuator; a second control valve arranged on a downstream side of the first control valve, the second control valve being configured to control the second hydraulic actuator; a communication fluid tube in which a return fluid flows to the second control valve, the return fluid being operation fluid discharged from the first hydraulic actuator; a leveling control valve connected to the communication fluid tube, the leveling control valve being configured to perform a leveling operation of the second hydraulic actuator; a return discharge fluid tube to discharge the return fluid flowing toward the second control valve on a side closer to the second control valve than a portion on which the leveling control valve is arranged in the communication fluid tube. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a view illustrating a hydraulic system (a hydraulic circuit) for a working machine according to a first embodiment of the present invention; 
         FIG. 2  is a view illustrating a relation between a pilot pressure and an operation extent (a position) of an operation member according to the first embodiment; 
         FIG. 3  is a view illustrating a hydraulic system (a hydraulic circuit) for a working machine according to a second embodiment of the present invention; 
         FIG. 4A  is a view illustrating a first modified example of the hydraulic system for the working machine according to the second embodiment; 
         FIG. 4B  is a view illustrating a second modified example of the hydraulic system for the working machine according to the second embodiment; 
         FIG. 4C  is a view illustrating a third modified example of the hydraulic system for the working machine according to the second embodiment; 
         FIG. 4D  is a view illustrating a fourth modified example of the hydraulic system for the working machine according to the second embodiment; 
         FIG. 5  is a view illustrating an example of the hydraulic system for the working machine having no operation valve according to the second embodiment; and 
         FIG. 6  is a whole view of a skid steer loader exemplified as the working machine according to the embodiments of the present invention. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     The embodiments will now be described with reference to the accompanying drawings, wherein like reference numerals designate corresponding or identical elements throughout the various drawings. The drawings are to be viewed in an orientation in which the reference numerals are viewed correctly. 
     Hereinafter, an embodiment of the present invention will be described below with reference to the drawings as appropriate. 
     First Embodiment 
     Hereinafter, with reference to the drawings as appropriate, a hydraulic system for a working machine according to a first embodiment of the present invention and a working machine provided with the hydraulic system will be described. 
     First, the working machine will be described.  FIG. 6  shows a side view of the working machine according to the present invention. In  FIG. 6 , a skid steer loader is shown as an example of the working machine. 
     However, the working machine according to the present invention is not limited to the skid steer loader, and may be another type of loader working machine such as a compact truck loader. Moreover, a working machine other than the loader working machine may be employed. 
     The working machine  1  includes a machine body (a vehicle body)  2 , a cabin  3 , a working device  4 , and traveling devices  5 A and  5 B. 
     A cabin  3  is mounted on the machine body  2 . An operator seat  8  is provided at the rear portion of the cabin  3 . 
     In the embodiment of the present invention, the front side (the left side in  FIG. 6 ) of the operator seated on an operator seat  8  of the working machine  1  will be described as the front, the rear side (the right side in  FIG. 6 ) of the operator will be described as the rear, the left side (the front surface side of  FIG. 6 ) of the operator will be described as the left, and the right side (the back surface side of  FIG. 6 ) of the operator will be described as the right. In addition, the horizontal direction, which is a direction orthogonal to the front-rear direction, will be described as the machine width direction. 
     The direction extending from the center portion of the machine body  2  toward the right portion or the left portion will be described as the machine outward direction. In other words, the machine outward direction is the machine width direction and a direction separating away from the machine body  2 . 
     The direction opposite to the machine outward direction will be described as the machine inward direction. In other words, the machine inward direction is the machine width direction and a direction approaching the machine body  2 . 
     The cabin  3  is mounted on the machine body  2 . The working device  4  is a device for the working, and is mounted on the machine body  2 . The traveling device  5 A is a device configured to travel the machine body  2 , and is arranged on the left side of the machine body  2 . The traveling device  5 B is a device configured to travel the machine body  2 , and is arranged on the right side of the machine body  2 . 
     A prime mover  7  is arranged internally at the rear portion of the machine body  2 . The prime mover  7  is a diesel engine (an engine). The prime mover  7  is not limited to the engine, and may be an electric motor or the like. 
     A traveling lever  9 L is arranged on the left side of the operator seat  8 . A traveling lever  9 R is arranged on the right side of the driver seat  8 . The traveling lever  9 L arranged on the left operates the traveling device  5 A arranged on the left, and the traveling lever  9 R arranged on the right operates the traveling device  5 B arranged on the right. 
     The working device  4  includes a boom  10 , a bucket  11 , a lift link  12 , a control link  13 , a boom cylinder  14 , and a bucket cylinder  17 . The boom  10  is arranged on the side of the machine body  2 . The bucket  11  is arranged at the tip end (a front end) of the boom  10 . 
     The lift link  12  and the control link  13  support the base portion (a rear portion) of the boom  10 . The boom cylinder  14  moves the boom  10  up and down. 
     In particular, the lift link  12 , the control link  13 , and the boom cylinder  14  are arranged on the side of the machine body  2 . The upper portion of the lift link  12  is pivotally supported by the upper portion of the base portion of the boom  10 . The lower portion of the lift link  12  is pivotally supported on the side portion of the rear portion of the machine body  2 . 
     The control link  13  is arranged in front of the lift link  12 . One end of the control link  13  is pivotally supported by the lower portion of the base portion of the boom  10 , and the other end of the control link  13  is pivotally supported by the machine body  2 . 
     The boom cylinder  14  is a hydraulic cylinder configured to lift and lower the boom  10 . The upper portion of the boom cylinder  14  is pivotally supported by the front portion of the base portion of the boom  10 . The lower portion of the boom cylinder  14  is pivotally supported by the side portion of the rear portion of the machine body  2 . When the boom cylinder  14  is stretched and shortened, the boom  10  is swung up and down by the lift link  12  and the control link  13 . 
     The bucket cylinder  17  is a hydraulic cylinder configured to swing the bucket  11 . The bucket cylinder  17  couples the left portion of the bucket  11  and the boom arranged to the left, and couples the right portion of the bucket  11  and the boom arranged to the right. 
     In addition, instead of the bucket  11 , a auxiliary attachment such as a hydraulic crusher, a hydraulic breaker, an angle broom, an auger, a pallet fork, a sweeper, a mower, a snow blower, or the like, is attached to the tip end (a front portion) of the boom  10 . 
     In the present embodiment, wheel type traveling devices  5 A and  5 B having front wheels  5 F and rear wheels  5 R are employed as the traveling devices  5 A and  5 B. Note that crawler type (including semi-crawler type) traveling devices  5 A and  5 B may be employed as the traveling devices  5 A and  5 B. 
     Next, a working system hydraulic circuit (working system hydraulic system) provided in the skid steer loader  1  will be explained below. 
     The working system hydraulic system is a system for operating the boom  10 , the bucket  11 , the auxiliary attachment, and the like. As shown in  FIG. 1 , the working system hydraulic system includes a plurality of control valves  20  and a hydraulic pump (a first hydraulic pump) P 1  for the working system. In addition, the working system hydraulic system includes a second hydraulic pump P 2  other than the first hydraulic pump P 1 . 
     The first hydraulic pump P 1  is a pump configured to be operated by the power of the prime mover  7 , and is constituted of a fixed capacity gear pump (a constant displacement gear pump). The first hydraulic pump P 1  is configured to output the operation fluid stored in the tank (a operation fluid tank)  15 . 
     The second hydraulic pump P 2  is a pump configured to be operated by the power of the prime mover  7 , and is constituted of a fixed capacity gear pump (a constant displacement gear pump). The second hydraulic pump P 2  is configured to output the operation fluid stored in the tank (the operation fluid tank)  15 . 
     The second hydraulic pump P 2  outputs the operation fluid for signals and the operation fluid for the controlling in the hydraulic system. The operation fluid for signals and the operation fluid for the controlling are called the pilot fluid. 
     The plurality of control valves  20  are valves for controlling various types of the hydraulic actuators provided in the working machine  1 . The hydraulic actuator is a device configured to be operated with the operation fluid, such as a hydraulic cylinder and a hydraulic motor. In this embodiment, the plurality of control valves  20  includes a boom control valve  20 A, a bucket control valve  20 B, and an auxiliary control valve  20 C. 
     The boom control valve  20 A is a valve configured to control the hydraulic actuator (the boom cylinder)  14  for operating the boom  10 . The boom control valve  20 A is a three-position switching valve having a direct acting spool. 
     The boom control valve  20 A is switched between a neutral position  20   a   3 , a first position  20   al  other than the neutral position  20   a   3 , and a second position  20   a   2  other than the neutral position  20   a   3  and the first position  20   al . In the boom control valve  20 A, the switching between the neutral position  20   a   3 , the first position  20   al , and the second position  20   a   2  are performed by movement of the spool through operation of the operation member. 
     The boom control valve  20 A is switched by the spool moved directly through manual operation of the operation member. However, the spool may be moved through the hydraulic operation (the hydraulic operation using a pilot valve, and the hydraulic operation using a proportional valve). In addition, the spool may be moved through the electric operation (the electric operation magnetizing a solenoid). Moreover, the spool may be moved through other methods. 
     The boom control valve  20 A and the first hydraulic pump P 1  are coupled by a output fluid tube  27 . A fluid tube  26  connected to the operation fluid tank  15  is connected to a section of the output fluid tube  27  between the boom control valve  20 A and the first hydraulic pump P 1 . 
     A relief valve (a main relief valve)  25  is arranged in the middle portion of the fluid tube  26 . The operation fluid outputted from the first hydraulic pump P 1  flows through the output fluid tube  27  and is supplied to the boom control valve  20 A. 
     In addition, the boom control valve  20 A and the boom cylinder  14  are connected by a fluid tube  21 . 
     In particular, the boom cylinder  14  includes a cylinder body  14   a , a rod  14   b  arranged movably in the cylinder body  14   a , and a piston  14   c  arranged on the rod  14   b . A first port  14   d  through which the operation fluid is supplied and discharged is arranged at the base end portion of the cylinder body  14   a  (on the side opposite to the rod  14   b  side). A second port  14   e  through which the operation fluid is supplied and discharged is arranged at the tip end portion (on the rod  14   b  side) of the cylinder body  14   a.    
     The fluid tube  21  includes a fluid tube  21   a  that connects the first port  31  of the boom control valve  20 A and the first port  14   d  of the boom cylinder  14 , and a fluid tube  21   b  that connects the second port  32  of the boom control valve  20 A and the second port  14   e  of the boom cylinder  14 . 
     Thus, when the boom control valve  20 A is set to the first position (a lifting position)  20   al , the operation fluid can be supplied from the fluid tube  21   a  to the first port  14   d  of the boom cylinder  14 . In addition, the operation fluid can be discharged from the second port  14   e  of the boom cylinder  14  to the fluid tube  21   b . In this manner, the boom cylinder  14  is stretched and the boom  10  is lifted. 
     When the boom control valve  20 A is set to the second position (a lowering position)  20   a   2 , the operation fluid can be supplied from the fluid tube  21   b  to the second port  14   e  of the boom cylinder  14 . In addition, the operation fluid can be discharged from the first port  14   d  of the boom cylinder  14  to the fluid tube  21   a . In this manner, the boom cylinder  14  is shortened and the boom  10  is lowered. 
     The bucket control valve  20 B is a valve configured to control a hydraulic cylinder (a bucket cylinder)  17  for controlling the bucket  11 . The bucket control valve  20 B is a three-position switching valve having a pilot-type direct acting spool. The bucket control valve  20 B is switched between a neutral position  20   b   3 , a first position  20   b   1  other than the neutral position  20   b   3 , and a second position  20   b   2  other than the neutral position  20   b   3  and the first position  20   b   1 . 
     In the bucket control valve  20 B, the switching between the neutral position  20   b   3 , the first position  20   b   1 , and the second position  20   b   2  is performed by manual movement of the operation member  111  such as a lever. 
     The bucket control valve  20 B is switched by directly moving the spool through the manual operation of the operation member. However, the spool may be moved through the hydraulic operation (the hydraulic operation by a pilot valve, the hydraulic operation by a proportional valve). In addition, the spool may be moved through the electric operation (the electric operation by magnetization of a solenoid). Moreover, the spool may be moved by other methods. 
     The bucket control valve  20 B and the bucket cylinder  17  are connected by a fluid tube  22 . In particular, the bucket cylinder  17  includes a cylinder body  17   a , a rod  17   b  movably arranged on the cylinder body  17   a , and a piston  17   c  arranged on the rod  17   b . A first port  17   d  through which the operation fluid is supplied and discharged is arranged at the base end portion of the cylinder body  17   a  (on the side opposite to the rod  17   b  side). A second port  17   e  through which the operation fluid is supplied and discharged is arranged at the tip end of the cylinder body  17   a  (on the rod  17   b  side). 
     The fluid tube  22  includes a communication fluid tube  22   a  that connects the first port  35  of the bucket control valve  20 B and the second port  17   e  of the bucket cylinder  17 , and a communication fluid tube  22   b  that connects a second port  36  of the bucket control valve  20 B and a first port  17   d  of the bucket cylinder  17 . 
     Thus, when the bucket control valve  20 B is set to the first position (a shoveling position)  20   b   1 , the operation fluid can be supplied from the communication fluid tube  22   a  to the second port  17   e  of the bucket cylinder  17 . In addition, the operation fluid can be discharged from the first port  17   d  of the bucket cylinder  17  to the communication fluid tube  22   b . In this manner, the bucket cylinder  17  is shortened, and the bucket  11  performs the shoveling operation. 
     In addition, when the bucket control valve  20 B is set to the second position (a dumping position)  20   a   2 , the operation fluid can be supplied from the communication fluid tube  22   b  to the first port  17   d  of the bucket cylinder  17 . In addition, the operation fluid can be discharged from the second port  17   e  of the bucket cylinder  17  to the communication fluid tube  22   a . In this manner, the bucket cylinder  17  is stretched, and the bucket  11  performs the dumping operation. 
     The auxiliary control valve  20 C is a valve configured to control an auxiliary attachment, that is, a hydraulic actuator (a hydraulic device such as a hydraulic cylinder and a hydraulic motor)  16  mounted on the working tool. The auxiliary control valve  20 C is a three-position switching valve having a pilot-type direct-acting spool. The auxiliary control valve  20 C has a first input port  44   a , a second input port  44   b , and a third input port  44   c.    
     A return fluid tube  81  to be described below is connected to the first input port  44  and the second input port  44   b , which are first supply ports, and the operation fluid in the return fluid tube  81  is supplied. A center fluid tube (a supply fluid tube)  72 , which will be described below, is connected to the third input port  44   c , which is a second supply port, and the operation fluid in the center fluid tube  72  is supplied. 
     The auxiliary control valve  20 C has a first output port  82   a  and a second output port  82   b . A supply/discharge fluid tube  83   a  and a supply/discharge fluid tube  83   b  are connected to the first output port  82   a  and the second output port  82   b  which are output ports. 
     A connecting member  18  is connected to the supply/discharge fluid tube  83   a  and the supply/discharge fluid tube  83   b , and a fluid tube connected to the hydraulic actuator  16  of the auxiliary attachment is connected to the connecting member  18 . That is, the output ports (the first output port  82   a  and the second output port  82   b ) communicate with the hydraulic actuator  16 . 
     The auxiliary control valve  20 C has a discharge port  84 . The discharge fluid tube  24  is connected to the discharge port  84 . The discharge fluid tube  24  is a fluid tube for discharging the operation fluid to a discharge portion such as the operation fluid tank  15 , and extends toward the operation fluid tank  15 . 
     The auxiliary control valve  20 C is switched between a neutral position  20   c   3 , a first position  20   c   1  other than the neutral position  20   c   3 , and a second position  20   c   2  other than the neutral position  20   c   3  and the first position  20   c   1 . 
     When the auxiliary control valve  20 C is in the first position  20   c   1 , the first input port  44   a  and the first output port  82   a  are communicated with each other, and the path between the third input port  44   c  and the discharge port  84  is closed. 
     When the auxiliary control valve  20 C is in the second position  20   c   2 , the second input port  44   b  and the second output port  82   b  are communicated with each other, and the path between the third input port  44   c  and the discharge port  84  is closed. 
     That is, in the operation position (the first position  20   c   1 , the second position  20   c   2 ), the auxiliary control valve  20 C communicates the first supply port (the first input port  44   a , the second input port  44   b ) and the output port (the first output port  82   a  and the second output port  82   b ), and block the path between the second supply port (the third input port  44   c ) and the discharge port  84 . 
     In the neutral position (the stop position)  20   c   3 , the auxiliary control valve  20 C blocks the path between the first supply port (the first input port  44   a , the second input port  44   b ) and the output port (the first output port  82   a , the second output port  82   b ), and opens the path between the second supply port (the third input port  44   c ) and the discharge port  84 . 
     The auxiliary control valve  20 C has pressure receiving portions  86   a  and  86   b  configured to receive a pressure of the pilot fluid. The pressure receiving portions  86   a  and  86   b  are respectively connected to pilot fluid tubes  87   a  and  87   b . In addition, the pilot fluid tubes  87   a  and  87   b  are connected to the second hydraulic pump P 2 . 
     The pressure receiving portions  86   a  and  86   b  respectively incorporate proportional valves  88   a  and  88   b , and the pressure of the operation fluid [a pressure of the pilot fluid (a pilot pressure)] applied to the pressure receiving portions  86   a  and  86   b  can be changed by the proportional valves  88   a  and  88   b.    
     In particular, the proportional valves  88   a  and  88   b  are electromagnetic valves whose opening aperture can be changed through magnetization. When the opening apertures of the proportional valves  88   a  and  88   b  are changed, the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  of the auxiliary control valve  20 C changes. In this manner, the spool of the auxiliary control valve  20 C moves in an arbitrary direction. 
     For example, when the proportional valve  88   a  is opened, the pilot fluid is applied to the pressure receiving portion  86   a  of the auxiliary control valve  20 C, and the pilot pressure applied to (acting on) the pressure receiving portion  86   a  is determined by the opening aperture of the proportional valve  88   a . When the pilot pressure applied to the pressure receiving portion  86   a  exceeds a predetermined value, the spool of the auxiliary control valve  20 C moves from the neutral position  20   c   3  to the first position  20   c   1  side. 
     In addition, when the proportional valve  88   b  is opened, the pilot fluid is applied to the pressure receiving portion  86   b  of the auxiliary control valve  20 C, and the pilot pressure applied to (acting on) the pressure receiving portion  86   b  is determined by the opening aperture of the proportional valve  88   b.    
     When the pilot pressure applied to the pressure receiving portion  86   b  becomes equal to or higher than a predetermined value, the spool of the auxiliary control valve  20 C moves from the neutral position  20   c   3  to the second position  20   c   2  side. 
     The magnetization of the proportional valves  88   a  and  88   b  is performed by the control device  90 . The control device  90  is constituted of a CPU and the like. An operation member  93  is connected to the control device  90 . An operation amount (for example, a sliding amount, a swinging amount, or the like) of the operation member  93  is inputted to the control device  90 . 
     The operation member  93  is constituted of, for example, a seesaw type switch that can be swung, a slide type switch that can be slid, or a push type switch that can be pressed. 
     When the operation member  93  is operated in one direction, an operation amount (a first operation amount) in one direction is input to the control device  90 , and the control device  90  changes the opening aperture of the proportional valve  88   a  in accordance with the first operation amount. 
     In addition, when the first operation amount is the maximum, the opening aperture of the proportional valve  88   a  is the maximum. In addition, when the first operation amount is the minimum, the opening aperture of the proportional valve  88   a  is the minimum. That is, the first operation amount is substantially proportional to the opening aperture of the proportional valve  88   a.    
     When the operation member  93  is operated in the other direction, the operation amount (a second operation amount) in the other direction is inputted to the control device  90 , and the control device  90  changes the opening aperture of the proportional valve  88   b  in accordance with the second operation amount. 
     In addition, when the second operation amount is the maximum, the opening aperture of the proportional valve  88   b  is the maximum. In addition, when the second operation amount is the minimum, the opening aperture of the proportional valve  88   b  is the minimum. That is, the second operation amount is substantially proportional to the opening of the second proportional valve  60 B. 
     The operation amount (the first operation amount, the second operation amount) of the operation member  93  can be detected by the operation detector device  110  that detects the swinging amount of the operation member  93  and the like. The operation detector device  110  is constituted of, for example, a potentiometer. 
     As described above, according to the hydraulic system for the working machine, the opening apertures of the proportional valves  88   a  and  88   b  can be set through the operation of the operation member  93 . In addition, when the spool of the auxiliary control valve  20 C is moved, the flow rate of the operation fluid supplied to the auxiliary actuator can be changed. 
     That is, when the auxiliary control valve  20 C is set to the first position  20   c   1 , the operation fluid can be supplied from the supply/discharge fluid tube  83   a  to the hydraulic actuator  16  of the auxiliary attachment. 
     When the auxiliary control valve  20 C is set to the second position  20   c   2 , the operation fluid can be supplied from the supply/discharge fluid tube  83   b  to the hydraulic actuator  16  of the auxiliary attachment. 
     In this manner, the operation fluid is supplied to the hydraulic actuator  16  from the supply/discharge fluid tube  83   a  or the supply/discharge fluid tube  83   b , and thereby the hydraulic actuator  16  (the auxiliary attachment) can be operated. 
     Meanwhile, a series circuit (a series fluid tube) is employed in the hydraulic system. In the series circuit, the operation fluid returned from the hydraulic actuator to the control valve arranged on an upstream side can be supplied to the control valve arranged on a downstream side. 
     For example, focusing on the bucket control valve  20 B and the auxiliary control valve  20 C, the bucket control valve  20 B is a control valve arranged on an upstream side, and the auxiliary control valve  20 C is a control valve arranged on downstream side. 
     Hereinafter, the control valve arranged on an upstream side is referred to as a “first control valve”, and the control valve arranged on a downstream side is referred to as a “second control valve”. A control valve other than the first control valve and the second control valve and arranged on the downstream side of the second control valve is referred to as a “third control valve”. 
     In addition, the hydraulic actuator corresponding to the first control valve is referred to as a “first hydraulic actuator”. The hydraulic actuator corresponding to the second control valve is referred to as a “second hydraulic actuator”. The hydraulic actuator corresponding to the third control valve is referred to as a “third hydraulic actuator”. A fluid tube that supplies the return fluid, which is the operation fluid returning from the first hydraulic actuator to the first control valve, to the second control valve is referred to as a “first fluid tube”. 
     In the embodiment, the bucket control valve  20 B is the “first control valve”. The auxiliary control valve  20 C is the “second control valve”. The boom control valve  20 A is the “third control valve”. In addition, the bucket cylinder  17  is the “first hydraulic actuator”. The hydraulic actuator  16  of the auxiliary attachment is the “second hydraulic actuator”. The boom cylinder  14  is the “third hydraulic actuator”. 
     First, the boom control valve  20 A will be described below. 
     The third control valve  20 A and the output portion of the first hydraulic pump P 1  are connected by an output fluid tube  27 . The output fluid tube  27  is branched at a middle portion  27   a . The branched fluid tube of the output fluid tube  27  is connected to the first input port  46   a  and the second input port  46   b  of the third control valve  20 A. In addition, the output fluid tube  27  is connected to the third input port  46   c  of the third control valve  20 A. 
     In this manner, the operation fluid outputted from the first hydraulic pump P 1  is supplied into the third control valve  20 A through the output fluid tube  27 , the first input port  46   a , the second input port  46   b , and the third input port  46   c.    
     The third control valve  20 A and the first control valve  20 B are coupled by a center fluid tube  51 . The center fluid tube  51  couples the third output port  41   c  of the third control valve  20 A and the third input port  42   c  of the first control valve  20 B. 
     When the third control valve  20 A is set to the neutral position  20   a   3 , by the communication of the center fluid tube  53   c  connecting the third input port  46   c  and the third output port  41   c , the supplied fluid that is the operation fluid supplied from the output fluid tube  27  to the third control valve  20 A is supplied to the center fluid tube  51  through the third control valve  20 A. 
     The third control valve  20 A and the first control valve  20 B are connected by a fluid tube  61  separately from the center fluid tube  51 . The fluid tube  61  is a fluid tube allowing the return fluid that returning from the third hydraulic actuator  14  to the third control valve  20 A to flow through the third control valve  20 A, and the fluid tube supplies the return fluid to the first control valve  20 B. 
     The fluid tube  61  connects the first output port  41   a  of the third control valve  20 A and the first input port  42   a  of the first control valve  20 B, and connects the second output port  41   b  of the third control valve  20 A and the second input port  42   b  of the first control valve  20 B. A middle portion of the fluid tube  61  is connected to the center fluid tube  51 . 
     According to the above configuration, when the third control valve  20 A is set to the second position  20   a   2 , the supply fluid introduced into the second input port  46   b  flows through the second port  32  and the fluid tube  21   b , and enters the second port  14   e  of the third hydraulic actuator  14 . 
     When the supply fluid is supplied to the second port  14   e , the third hydraulic actuator  14  is shortened, for example. When the third hydraulic actuator  14  is shortened, the return fluid discharged from the first port  14   d  of the third hydraulic actuator  14  flows to the third control valve  20 A through the fluid tube  21   a . The return fluid in the third control valve  20 A flows toward the first control valve  20 B through the fluid tube  61 . 
     Thus, the return fluid in the third hydraulic actuator  14  can be supplied to the first control valve  20 B. 
     Next, the relation between the first control valve  20 B and the second control valve  20 C will be described in detail. 
     The first control valve  20 B and the second control valve  20 C are connected by a center fluid tube (a supply fluid tube)  72 . The supply fluid tube  72  connects the third output port  43   c  of the first control valve  20 B and the third input port (the second supply port)  44   c  of the second control valve  20 C. 
     Thus, when the first control valve  20 B is set to the neutral position  20   b   3 , the supply fluid that is the operation fluid supplied to the first control valve  20 B flows through the center fluid tube  73   c  that connects the third input port  42   c  and the third output port  43   c , and is supplied to the supply fluid tube  72  connected to the third output port  43   c.    
     The first control valve  20 B and the second control valve  20 C are connected by a return fluid tube  81  separately from the supply fluid tube  72 . The return fluid tube  81  is a fluid tube that supplies, to the second control valve  20 C, the return fluid returning from the first hydraulic actuator  17  to the first control valve  20 B. 
     In particular, one end of the fluid tube  81  is connected to the first input port  44   a  of the second control valve  20 C and the second input port  44   b . In addition, the other end of the return fluid  81  is connected to the first output port  43   a  of the first control valve  20 B and the second output port  43   b.    
     The supply fluid tube  72  is confluent with the middle portion of the return fluid tube  81 , and thereby the return fluid tube  81  and the supply fluid tube  72  communicate with each other. 
     According to the above configuration, when the first control valve  20 B is set to the second position  20   b   2 , the supply fluid introduced to the second input port  42   b  flows through the second port  36  and the communication fluid tube  22   b , and enters the first port  17   d  of the first hydraulic actuator  17 . When the supply fluid is supplied to the first port  17   d , the first hydraulic actuator  17  is stretched, for example. 
     When the first hydraulic actuator  17  is stretched, the return fluid discharged from the second port  17   e  of the first hydraulic actuator  17  flows to the return fluid tube  81   a  through the communication fluid tube  22   a , and the return fluid in the return fluid tube  81   a  flows toward the second control valve  20 C. 
     Thus, the return fluid in the first hydraulic actuator  17  can be supplied to the second control valve  20 C. 
     When the second control valve  20 C is in the first position  20   c   1 , the second control valve  20 C supplies the return fluid in the first hydraulic actuator  17  to the second hydraulic actuator  16  to communicate the path between the first input port  44   a  and the first output port  82   a.    
     Also when the second control valve  20 C is in the second position  20   c   2 , the second control valve  20 C supplies the return fluid in the first hydraulic actuator  17  to the second hydraulic actuator  16  to communicate the path between the second input port  44   b  and the second output port  82   b.    
     That is, when the second control valve  20 C is in the operation position (the first position  20   c   1 , the second position  20   c   2 ), the return fluid can be supplied to the second hydraulic actuator  16 . 
     In addition, the second control valve  20 C blocks the path between the second supply port (the third input port  44   c ) and the discharge port  84  in both the first position  20   c   1  and the second position  20   c   2 . Thus, the center fluid tube (the supply fluid tube)  72  is blocked from the discharge fluid tube  24 . 
     When the auxiliary control valve  20 C is in the stop position  20   c   3 , the auxiliary control valve  20 C closes off the path between the first input port  44   a  and the first output port  82   a  and the path between the second input port  44   b  and the second output port  82   b . Thus, the return fluid is not supplied to the second hydraulic actuator  16 . 
     In addition, when the auxiliary control valve  20 C is in the stop position  20   c   3 , the auxiliary control valve  20 C opens the path between the second supply port (the third input port  44   c ) and the discharge port  84 . Thus, the central fluid tube (the supply fluid path)  72  and the discharge fluid tube  24  communicate with each other. 
     In the hydraulic system for the working machine, the operation of the second control valve  20 C is changed in accordance with the operation of the first control valve  20 B. 
       FIG. 2  shows the relation between the operation amount (a position) of the operation member  93  and the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  of the second control valve  20 C. 
     The pilot pressure applied to the pressure receiving portions  86   a  and  86   b  of the second control valve  20 C can be replaced with the opening apertures of the proportional valves  88   a  and  88   b . That is, the pilot pressure may be replaced with the electric current outputted from the control device  90  to the solenoids of the proportional valves  88   a  and  88   b.    
     Thus, for convenience of the explanation, the explanation may be made with use of the opening apertures (the current values) of the proportional valves  88   a  and  88   b  instead of the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  of the second control valve  20 C. 
     As shown in  FIG. 2 , when the operation member  93  is gradually operated in one direction from the neutral state, the pilot pressure (the opening aperture, the current value) gradually increases in accordance with the operation amount of the operation member  93 . 
     In addition, when the operation member  93  is gradually operated in the other direction from the neutral state, the pilot pressure (the opening aperture, the current value) gradually increases in accordance with the operation amount of the operation member  93 . That is, the operation amount of the operation member  93  is proportional to the pilot pressure (the opening aperture, the current value). 
     Referring to  FIG. 1  and  FIG. 2 , the operation of the second control valve  20 C in a non-operation state in which the first control valve  20 B stops supplying the operation fluid to the first hydraulic actuator  17  will be described below. 
     The non-operation state is a state where the operation fluid for stretching and shortening the first hydraulic actuator  17  is not substantially outputted from the first control valve  20 B when the first control valve  20 B is in the neutral position  30   b   3 , in other words, when the first control valve  20 B is not switched to one of the first position  20   b   1  and the second position  20   b   2 . 
     The state whether or not the first control valve  20 B is in the non-operation state can be detected by the state detector device  112  connected to the control device  90 . 
     The state detector device  112  is, for example, a potentiometer that detects the position of the operation member  111  for operating the first control valve  20 B, and detects the non-operation state when the position of the operation member  111  is a position that corresponds to the neutral position  30   b   3 . 
     The state detector device  112  is an example, and the state whether or not the first control valve  20 B is in the non-operation state may be detected through movement of the spool of the first control valve  20 B. In addition, the state may be detected based on the pilot pressure applied to the pressure receiving portion when the first control valve  20 B is configured to be switched by the pilot pressure applied to the pressure receiving portion. In addition, when the first control valve  20 B is switched by an electric operation, the state may be detected through an electric signal outputted to the first control valve  20 B. 
     As shown in  FIG. 2 , it is assumed that the operating member  93  is operated in one direction or the other direction in a state where the first control valve  20 B is in the non-operation state. Here, when the first operation amount of the operation member  93  detected by the operation detector device  110  is the maximum value, the control device  90  maximizes the opening aperture of the proportional valve  88   a , that is, the second control valve  20 C is set to the first position  20   c   1  by setting the pressure applied to the pressure receiving portion  86   a  to the first pressure PS 1  that is the highest pressure. 
     In addition, when the second operation amount of the operation member  93  detected by the operation detector device  110  is the maximum value, the control device  90  maximizes the opening aperture of the proportional valve  88   b . That is, the control device  90  sets the second control valve  20 C to the second position  20   c   2  by setting the pressure applied to the pressure receiving portion  86   b  to the first pressure PS 1  that is the highest pressure. 
     That is, the control device  90  detects that either the first operation amount or the second operation amount of the operation member  93  is the maximum with the operation detector device  110  in a state where the first control valve  20 B is in the non-operation state. Thus, when it is determined that the operation member  93  is at the first operating position (the position where the first operation amount reaches the maximum, the position where the second operation amount reaches the maximum) at which the operation member  93  has been fully strokes, the second control valve  20 C is set to the operation position (the first position  20   c   1 , the second position  20   c   2 ). 
     When the operation amount of the operation member  93  detected by the operation detector device  110  is the minimum value under the state where the first control valve  20 B is in the non-operation state, the control device  90  minimizes the opening aperture of the proportional valve  88   a . That is, the control device  90  sets the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  to the second pressure PS 2  that is the lowest pressure, thereby setting the second control valve  20 C to the stop position  20   c   3 . 
     That is, when the operation detector device  110  detects that the operation amount of the operation member  93  is the minimum in the state where the first control valve  20 B is in the non-operation state, the control device  90  sets the second control valve  20 C to the stop position  20   c   3  when the control device  90  determines that the operation member  93  is in the neutral position (the second operating position). 
     Accordingly, when the first control valve  20 B is in the non-operation state, the second control valve  20 C is set to the operation position (the first position  20   c   1 , the second position  20   c   2 ) by fully stroking the operating member  93 . 
     Next, the operation of the second control valve  20 C in the operation state where the first control valve  20 B supplies the operation fluid to the first hydraulic actuator  17  (in the state where the first control valve  20 B is in either the first position  20   b   1  or the second position  20   b   2 ) will be described. The state detector device  112  is capable of detecting whether or not the first control valve  20 B is in the operation state. 
     The case where the state detector device  112  detects the non-operation state corresponds to the case where it is detected that the first control valve  20 B is not in the operation state. The case where the operation state is not detected corresponds to the case where the first control valve  20 B detects the operation state. 
     That is, the state detector device  112  is configured to detect whether in the non-operation state or the operation state. 
     In the case where the first control valve  20 B is in the operation state, the control device  90  sets the opening aperture of the proportional valve  88   a  to be smaller than the maximum opening aperture when the first operation amount of the operation member  93  detected by the operation detector device  110  is the maximum value. That is, the control device  90  sets the pilot pressure applied to the pressure receiving portion  86   a  to the third pressure PS 3  that is lower than the first pressure PS 1  and higher than the second pressure PS 2 . 
     In other words, when the first control valve  20 B is in the operation state and the first operation amount is the maximum value, the control device  90  lowers the current value to be outputted to the proportional valve  88   a  to be lower than the current value in the operation position (the first position  20   c   1 ). More specifically, the control device  90  sets the maximum value (an upper limit value) of the current value to be outputted to the proportional valve  88   a  to be smaller than the maximum value in the non-operation state. 
     In this manner, the control device  90  switches the second control valve  20 C to the middle position (a first middle position) between the first position  20   c   1  and the stop position  20   c   3 . When the second control valve  20 C is in the first middle position, the first input port  44   a  and the first output port  82   a  communicate with each other, and the second supply port (the third input port  44   c ) and the discharge port  84  also communicate with each other. 
     That is, when the second control valve  20 C is in the first middle position, the supply fluid tube  72  and the discharge fluid tube  24  communicate with each other, and the return fluid in the first control valve  20 B is supplied to the second hydraulic actuator  16  through the supply/discharge fluid tube  83   a.    
     In the case where the first control valve  20 B is in the operation state, the control device  90  sets the opening aperture of the proportional valve  88   b  to be smaller than the maximum opening aperture when the second operation amount of the operation member  93  detected by the operation detector device  110  is the maximum value. That is, the control device  90  sets the pilot pressure applied to the pressure receiving portion  86   b  to the third pressure PS 3  that is lower than the first pressure PS 1  and higher than the second pressure PS 2 . 
     In other words, when the first control valve  20 B is in the operation state and the second operation amount is the maximum value, the control device  90  lowers the current value to be outputted to the proportional valve  88   b  to be lower than the current value in the operation position (the second position  20   c   2 ). More specifically, the control device  90  sets the maximum value (an upper limit value) of the current value to be outputted to the proportional valve  88   b  to be smaller than the maximum value in the non-operation state. 
     In this manner, the control device  90  switches the second control valve  20 C to the middle position (a second middle position) between the second position  20   c   2  and the stop position  20   c   3 . When the second control valve  20 C is in the second middle position, the second input port  44   b  and the second output port  82   b  communicate with each other, and the second supply port (the third input port  44   c ) and the discharge port  84  also communicate with each other. 
     That is, when the second control valve  20 C is in the second middle position, the supply fluid tube  72  and the discharge fluid tube  24  communicate with each other, and the return fluid in the first control valve  20 B is supplied to the second hydraulic actuator  16  through the supply/discharge fluid tube  83   b.    
     The hydraulic system for the working machine, includes: the hydraulic pump P 1  to output the operation fluid; the first hydraulic actuator  17 ; the second hydraulic actuator  16 ; the first control valve  20 B to control the flow rate of the operation fluid to be supplied to the first hydraulic actuator  17 ; the second control valve  20 C to control the flow rate of the operation fluid to be supplied to the second hydraulic actuator  16 ; the return fluid tube  81  connecting the first control valve  20 B and the second control valve  20 C and allowing the return fluid to flow in the return fluid tube  81 , the return fluid being operation fluid returning from the first hydraulic actuator  17  to the first control valve  20 B; the supply fluid tube  72  connecting the first control valve  20 B and the second control valve  20 C separately from the return fluid tube and being connected to the return fluid tube  81  at the middle portion of the supply fluid tube  72 ; the discharge fluid tube  24  connected to the second control valve  20 C; the operation member  93  to operate the second control valve  20 C; and the control device (controller)  90  to control the second control valve  20 C in accordance with operation of the operation member  93 . 
     In addition, the second control valve  20 C is switched between the operation position (a primary position) (the first position  20   c   1 , the second position  20   c   2 ), the stop position (a secondary position)  20   c   3 , and the middle position (a tertiary position). The operation position blocks between the supply fluid tube  72  and the discharge fluid tube  24  and allows the return fluid to be supplied to the second hydraulic actuator  16 . The stop position  20   c   3  communicates between the supply fluid tube  72  and the discharge fluid tube  24  and stops supplying the return fluid to the second hydraulic actuator  16 . And, the middle position communicates between the supply fluid tube  72  and the discharge fluid tube  24  and allows the return fluid to be supplied to the second hydraulic actuator  16 . 
     The control device (a controller)  90  switches the second control valve  20 C to the operation position (the first position  20   c   1 , the second position  20   c   2 ) when the first control valve  20 B is under the non-operation state where the operation fluid is not supplied to the first hydraulic actuator  17  and when the operation member  93  is at the first operating position. The control device  90  switches the second control valve  20 C to the stop position  20   c   3  when the first control valve  20 B is under the non-operation state and when the operation member  93  is at the second operating position. And, the control device  90  switches the second control valve  20 C to the middle position when the first control valve  20 B is under the operation state where the operation fluid is supplied to the first hydraulic actuator  17  and when the operation member  93  is at the first operating position. 
     In this manner, when the operating member  93  is operated to the first operating position in the state where the first hydraulic actuator  17  is not operated by the first control valve  20 B (in the non-operation state), the second hydraulic actuator  16  can be operated by the return fluid supplied from the first control valve  20 B to the second control valve  20 C. In addition, when the operation member  93  is in the second operating position, the second hydraulic actuator  16  can be stopped. 
     On the other hand, when the operation member  93  is operated to the first operating position in the state (the operation state) in which the first hydraulic actuator  17  is operated by the first control valve  20 B, the second hydraulic actuator  16  can be operated by the return fluid supplied from the first control valve  20 B to the second control valve  20 C as in the non-operation state. In addition, when the second control valve  20 C stops the operation for some reason, the return fluid flowing from the first control valve  20 B to the second control valve  20 C is discharged from the supply fluid tube  72  to the discharge fluid tube  24 . 
     That is, when the first hydraulic actuator  17  and the second hydraulic actuator  16  are operated in combination, the return fluid flowing from the first hydraulic actuator  17  to the second control valve  20 C is allowed to flow from the supply fluid tube  72  to the discharge fluid tube  24 . In this manner, the first hydraulic actuator  17  arranged on the upstream side can be easily operated. 
     The second control valve  20 C includes a first supply port (a first input port  44   a , a second input port  44   b ) to which the return fluid tube  81  is connected, a second supply port (a third input port  44   c ) to which the supply fluid tube  72  is connected, an output port (a first output port  82   a , a second output port  82   b ) communicating with the second hydraulic actuator  16 , and a discharge port  84  to which the discharge fluid tube  24  is connected. 
     In addition, the second control valve  20 C communicates (connects) the first supply port (the first input port  44   a , the second input port  44   b ) and the output port (the first output port  82   a , the second position  20   c   2 ) at the operation position (the first position  20   c   1 , the second position  20   c   2 ). The second control valve  20 C closes the path between the second supply port (the third input port  44   c ) and the discharge port  84  at the operation position. The second control valve  20 C closes the path between the first supply port (the first input port  44   a , the second input port  44   b ) and the output port (first output port  82   a , second output port  82   b ) at the stop position  20   c   3 . The second control valve  20 C opens the path between the second supply port (the third input port  44   c ) and the discharge port  84  at the stop position  20   c   3 . The second control valve  20 C communicates (connects) the first supply port (the first input port  44   a , the second input port  44   b ) and the output port (the first output port  82   a , the second output port  82   b ) at the middle position. The second control valve  20 C communicates (connects) the second supply port (the third input port  44   c ) and the discharge port  84  at the middle position. 
     According to that configuration, the communicating (the connecting) between the first supply port (the first input port  44   a , the second input port  44   b ), the second supply port (the third input port  44   c ), the output port (the first output port  82   a , the second output port  82   b ), and the discharge port  84  is easily switched in accordance with the switching between the operation position (the first position  20   c   1 , the second position  20   c   2 ), the stop position  20   c   3 , and the middle position. 
     The second control valve  20 C is a switching valve having the operation position (the first position  20   c   1 , the second position  20   c   2 ), the stop position  20   c   3 , and the middle position, and is configured to be switched between the operation position, the stop position  20   c   3 , and the middle position in accordance with the pilot pressure that is the pressure of the pilot fluid supplied to the pressure receiving portions  86   a  and  86   b  of the second control valve  20 C. The control device  90  sets the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  to a first pressure corresponding to the operation position (the first position  20   c   1 , the second position  20   c   2 ) when the first control valve  20 B is in the non-operation state and the operating member  93  is in the first operating position. The control device  90  sets the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  to a second pressure corresponding to the stop position  20   c   3  when the first control valve  20 B is in the non-operation state and the operating member  93  is in the second operating position. When the first control valve  20 B is in the operation state and the operation member  93  is in the first operating position, the control device  90  sets the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  to a third pressure corresponding to the middle position. 
     In this manner, since the pilot pressure (the first pressure, the second pressure, the third pressure) can be easily set in accordance with the position (the first operating position, the second operating position) of the operation member  93 , the second control valve  20  can be switched smoothly in accordance with the position of the operation member  93 . 
     The hydraulic system for the working machine includes the proportional valves  88   a  and  88   b  configured to set the pilot pressure applied to the pressure receiving portions  86   a  and  86   b  of the second control valve  20 C, the operation detector device  110  configured to detect either the first operating position and the second operating position of the operation member  93 , and the state detector device  112  configured to detect either the non-operation state or the operation state of the first control valve  20 B. The control device  90  sets the opening apertures of the proportional valves  88   a  and  88   b  based on either one of the first operating position and the second operating position detected by the operation detector device  110  and on one of the non-operation state and the operation state detected by the state detector device  112 , and thereby setting the first pressure, the second pressure, and the third pressure. 
     According to that configuration, the opening apertures of the proportional valves  88   a  and  88   b  can be easily set in accordance with the position (the first operating position, the second operating position) detected by the operation detector device  110  and with the state (the non-operation state, the operation state) detected by the state detector device  112 . 
     When the first pressure is referred to as PS 1 , the second pressure is referred to as PS 2 , and the third pressure is referred to as PS 3 , the equation, PS 1 &gt;PS 3 &gt;PS 2 , is satisfied. 
     According to that configuration, the supply fluid tube  72  and the discharge fluid tube  24  can be easily connected while the second hydraulic actuator  16  is operated by the second control valve  20 C by lowering the second pressure applied to the pressure receiving portion of the second control valve  20 C when the first control valve  20 B is in the operation state in comparison with the first pressure applied to the pressure receiving portion of the second control valve  20 C when the first control valve  20 B is the non-operation state. 
     In the hydraulic system for the working machine, the first hydraulic actuator  17  is constituted of a hydraulic cylinder configured to be stretched and shortened. The first control valve  20 B is a switching valve configured to stretch and shorten the hydraulic cylinder. The control device  90  switches the second control valve  20 C to the middle position in the case where the first control valve  20 B supplies the operation fluid to the hydraulic cylinder to shorten the hydraulic cylinder in the operating state. 
     According to that configuration, even in the case where the return fluid to the second control valve  20 C cannot be supplied, the first hydraulic actuator  17  is capable of being shortened. 
     In the above description, the embodiment of the present invention has been explained. However, all the features of the embodiment disclosed in this application should be considered just as examples, and the embodiment does not restrict the present invention accordingly. A scope of the present invention is shown not in the above-described embodiment but in claims, and is intended to include all modifications within and equivalent to a scope of the claims. The first control valve and the second control valve are not limited to the embodiment mentioned above, and may be any types of control valves provided in the working machine. 
     In the embodiment described above, the operation fluid is discharged to the operation fluid tank, but may be discharged to other places. That is, the fluid tube for discharging the operation fluid may be connected to a portion other than the operation fluid tank. For example, the fluid tube for discharging the operation fluid may be connected to a suction portion (portion for sucking the operation fluid) of the hydraulic pump, or may be connected to other portions. 
     In the embodiment described above, the control valve is constituted of a three-position switching valve, but the number of the switching positions is not limited. The control valve may be constituted of a two-position switching valve, a four-position switching valve, or another switching valve. 
     In the embodiment described above, the hydraulic pump is constituted of a constant displacement pump (a fixed displacement pump). However, the hydraulic pump may be constituted of, for example, a variable displacement pump whose output rate is changed by changing the swash plate, or constituted of another type of hydraulic pump. 
     In addition, the first hydraulic actuator, the second hydraulic actuator, the third hydraulic actuator, the first control valve, the second control valve, and the third control valve are not limited to those of the above-described embodiment, and may be other types of components configured to be provided in the working machine  1 . 
     Second Embodiment 
     Hereinafter, a hydraulic system for a working machine according to a second embodiment of the present invention and the working machine including the hydraulic system will be described with reference to the drawings as appropriate. 
     The hydraulic system according to the present embodiment is provided in the working machine  1  described in the first embodiment. The description of the configuration of the working machine  1  is omitted. 
     The working system hydraulic system according to the present embodiment is a system for operating the boom  10 , the bucket  11 , the auxiliary attachment, and the like. As shown in  FIG. 3 , the working system hydraulic system includes a plurality of control valves  20  and a working system hydraulic pump (a first hydraulic pump) P 1 . In addition, the working system hydraulic system is provided with a second hydraulic pump P 2  other than the first hydraulic pump P 1 . 
     The first hydraulic pump P 1  is a pump configured to be operated by the power of the prime mover  7 . The first hydraulic pump P 1  is constituted of a constant displacement gear pump (a fixed displacement gear pump). The first hydraulic pump P 1  is configured to output the operation fluid stored in the tank (the operation fluid tank)  15 . 
     The second hydraulic pump P 2  is a pump configured to be operated by the power of the prime mover  7 . The second hydraulic pump P 2  is constituted of a constant displacement gear pump (a fixed displacement gear pump). The second hydraulic pump P 2  is configured to output the operation fluid stored in the tank (the operation fluid tank)  15 . 
     The second hydraulic pump P 2  outputs the operation fluid for signal and the operation fluid for control in the hydraulic system. The operation fluid for signal and the operation fluid for control are referred to as a pilot fluid. 
     The plurality of control valves  20  are valves configured to control various types of hydraulic actuators provided in the working machine  1 . The hydraulic actuator is a device configured to be operated by the operation fluid, such as a hydraulic cylinder and a hydraulic motor. 
     In this embodiment, the plurality of control valves  20  are a boom control valve  20 A, a bucket control valve  20 B, and an auxiliary control valve  20 C. 
     The boom control valve  20 A is a valve for controlling a hydraulic actuator (a boom cylinder)  14  configured to operate the boom  10 . The boom control valve  20 A is a three-position switching valve having a direct acting spool. 
     The boom control valve  20 A is switched between to a neutral position  20   a   3 , a first position  20   al  other than the neutral position  20   a   3 , and a second position  20   a   2  other than the neutral position  20   a   3  and the first position  20   al . The switching between the neutral position  20   a   3 , the first position  20   al , and the second position  20   a   2  in the boom control valve  20 A is performed by moving the spool through the operation of the operation member. 
     The boom control valve  20 A is switched by moving the spool directly through the manual operation of the operation member. However, the spool may be moved through the hydraulic operation (the hydraulic operation by a pilot valve, the hydraulic operation by a proportional valve). In particular, the spool may be moved through the electric operation (the electric operation by magnetizing a solenoid). Further, the spool may be moved in other methods. 
     The boom control valve  20 A and the first hydraulic pump P 1  are connected by an output fluid tube  27 . A discharge fluid tube  24   a  connected to the operation fluid tank  15  is connected to a section between the boom control valve  20 A and the first hydraulic pump P 1  in the output fluid tube  27 . 
     A relief valve (a main relief valve)  25  is arranged in the middle portion of the discharge fluid tube  24   a . The operation fluid outputted from the first hydraulic pump P 1  flows through the output fluid tube  27 , and is supplied to the boom control valve  20 A. 
     In addition, the boom control valve  20 A and the boom cylinder  14  are connected by a fluid tube  21 . 
     In particular, the boom cylinder  14  includes a cylinder body  14   a , a rod  14   b  arranged movably in the cylinder body  14   a , and a piston  14   c  arranged on the rod  14   b . A first port  14   d  through which the operation fluid is supplied and discharged is arranged at the base end portion of the cylinder body  14   a  (on the side opposite to the rod  14   b  side). A second port  14   e  through which the operation fluid is supplied and discharged is arranged at the tip end (on the rod  14   b  side) of the cylinder body  14   a.    
     The fluid tube  21  includes a first connection fluid tube  21   a  that connects the first port  31  of the boom control valve  20 A and the first port  14   d  of the boom cylinder  14 , and a second connection fluid tube  21   b  that connects the second port  32  of the boom control valve  20 A and the second port  14   e  of the boom cylinder  14 . 
     Thus, when the boom control valve  20 A is set to the first position  20   al , the operation fluid can be supplied from the first connection fluid tube  21   a  to the first port  14   d  of the boom cylinder  14 . In addition, the operation fluid can be discharged from the second port  14   e  of the boom cylinder  14  to the second connection fluid tube  21   b . In this manner, the boom cylinder  14  is stretched, and the boom  10  is lifted. 
     When the boom control valve  20 A is set to the second position  20   a   2 , the operation fluid can be supplied from the second connection fluid tube  21   b  to the second port  14   e  of the boom cylinder  14 . In addition, the operation fluid can be discharged from the first port  14   d  of the boom cylinder  14  to the first connection fluid tube  21   a . In this manner, the boom cylinder  14  is shortened, and the boom  10  is lowered. 
     The bucket control valve  20 B is a valve for controlling a hydraulic cylinder (a bucket cylinder)  17  configured to control the bucket  11 . The bucket control valve  20 B is constituted of a pilot-type three-position switching valve having a direct acting spool. 
     The bucket control valve  20 B is switched between a neutral position  20   b   3 , a first position  20   b   1  other than the neutral position  20   b   3 , and a second position  20   b   2  other than the neutral position  20   b   3  and the first position  20   b   1 . In the bucket control valve  20 B, the switching between the neutral position  20   b   3 , the first position  20   b   1 , and the second position  20   b   2  is performed by moving the spool through the operation of the operation member. 
     In addition, the bucket control valve  20 B is switched by moving the spool directly through the manual operation of the operation member. However, the spool may be moved through the hydraulic operation (the hydraulic operation by the pilot valve, the hydraulic operation by the proportional valve). In addition, the spool may be moved through the electric operation (the electric operation by magnetizing a solenoid). In addition, the spool may be moved in other methods. 
     The bucket control valve  20 B and bucket cylinder  17  are connected by a fluid tube  22 . In particular, the bucket cylinder  17  includes a cylinder body  17   a , a rod  17   b  movably arranged on the cylinder body  17   a , and a piston  17   c  arranged on the rod  17   b.    
     A first port  17   d  through which the operation fluid is supplied and discharged is arranged at the base end portion of the cylinder body  17   a  (on the side opposite to the rod  17   b  side). A second port  17   e  through which the operation fluid is supplied and discharged is arranged at the tip portion (on the rod  17   b  side) of the cylinder body  17   a.    
     The fluid tube  22  includes a first connection fluid tube  22   a  that connects the first port  35  of the bucket control valve  20 B and the second port  17   e  of the bucket cylinder  17 , and includes a second connection fluid tube  22   b  that connects a second port  36  of the bucket control valve  20 B and the first port  17   d  of the bucket cylinder  17 . 
     Thus, when the bucket control valve  20 B is set to the first position  20   b   1 , the operation fluid can be supplied from the first connection fluid tube  22   a  to the second port  17   e  of the bucket cylinder  17 . In addition, the operation fluid can be discharged from the first port  17   d  of the bucket cylinder  17  to the second connection fluid tube  22   b.    
     In this manner, the bucket cylinder  17  is shortened, and the bucket  11  performs the shoveling operation. 
     When the bucket control valve  20 B is set to the second position  20   a   2 , the operation fluid can be supplied from the second connection fluid tube  22   b  to the first port  17   d  of the bucket cylinder  17 . In addition, the operation fluid can be discharged from the second port  17   e  of the bucket cylinder  17  to the first connection fluid tube  22   a.    
     In this manner, the bucket cylinder  17  is stretched, and performs the dumping operation. 
     Meanwhile, a drain fluid tube  24   c  is connected to the first connection fluid tube  21   a  and the second connection fluid tube  21   b . In addition, the discharge fluid tube  24   c  is connected to the first discharge port  33   a  and the second discharge port  33   b  of the first control valve  20 A, and can discharge the operation fluid to the discharge portion. 
     The discharge portion is the operation fluid tank and the suction portion of the hydraulic pump (a portion for sucking the operation fluid). The discharge portion is a portion from which the operation fluid is discharged. In addition, the discharge portion may be other than the operation fluid tank and the suction portion of the hydraulic pump, and is not limited thereto. 
     In addition, a relief valve  38  is arranged in the drain fluid tube  24   c . The set pressure of the relief valve  38  is set to be higher than the set pressure of the main relief valve  25 , for example. 
     It should be noted that the hydraulic actuator arranged on the upstream side may be easily operated by setting the set pressure of the relief valve  38  to be lower than the set pressure of the main relief valve  25 . 
     The auxiliary control valve  20 C is a valve for controlling the hydraulic actuator (the hydraulic cylinder, the hydraulic motor, or the like)  16  attached to the auxiliary attachment. The auxiliary control valve  20 C is constituted of a three-position switching valve having a pilot-type direct acting spool. The auxiliary control valve  20 C is switched between a neutral position  20   c   3 , a first position  20   c   1  other than the neutral position  20   c   3 , and a second position  20   c   2  other than the neutral position  20   c   3  and the first position  20   c   1 . 
     In the auxiliary control valve  20 C, the switching between the neutral position  20   c   3 , the first position  20   c   1 , and the second position  20   c   2  is performed by moving the spool through the pilot fluid pressure. A connecting member  18  is connected to the auxiliary control valve  20 C via the supply/discharge fluid tubes  83   a  and  83   b.    
     The connecting member  18  is connected to a fluid tube connected to the hydraulic actuator (the auxiliary hydraulic actuator)  16  for the auxiliary attachment. 
     Thus, when the auxiliary control valve  20 C is set to the first position  20   c   1 , the operation fluid can be supplied from the supply/discharge fluid tube  83   a  to the hydraulic actuator  16  of the auxiliary attachment. When the auxiliary control valve  20 C is set to the second position  20   c   2 , the operation fluid can be supplied from the supply/discharge fluid tube  83   b  to the hydraulic actuator  16  of the auxiliary attachment. 
     In this manner, when the operation fluid is supplied from the supply/discharge fluid tube  83   a  or the supply/discharge fluid tube  83   b  to the hydraulic actuator  16 , the hydraulic actuator  16  (the auxiliary attachment) can be operated. 
     Now, a series circuit (a series fluid tube) is employed in the hydraulic system. In the series circuit, the operation fluid returned from the hydraulic actuator to the control valve arranged on the upstream side can be supplied to the control valve arranged on the downstream side. For example, focusing on the boom control valve  20 A and the bucket control valve  20 B, the boom control valve  20 A is the control valve arranged on the upstream side, and the bucket control valve  20 B is the control valve arranged on the downstream side. 
     Hereinafter, the control valve arranged on the upstream side is referred to as a “first control valve”, and the control valve arranged on the downstream side is referred to as a “second control valve”. In addition, the hydraulic actuator corresponding to the first control valve is referred to as a “first hydraulic actuator”. The hydraulic actuator corresponding to the second control valve is referred to as a “second hydraulic actuator”. The operation fluid returning from the first hydraulic actuator to the first control valve is referred to as the return fluid. 
     In this embodiment, the boom control valve  20 A is the “first control valve”, and the bucket control valve  20 B is the “second control valve”. In addition, the boom cylinder  14  is the “first hydraulic actuator”, and the bucket cylinder  17  is the “second hydraulic actuator”. 
     The working system hydraulic system includes the communication fluid tube  69 . The communication fluid tube  69  is a fluid tube allowing the return fluid discharged from the first hydraulic actuator  14  to be supplied to the second control valve  20 B through the first control valve  20 A. The communication fluid tube  69  includes a first return fluid tube and a second return fluid tube  61 . 
     The first return fluid tube is a fluid tube through which the return fluid discharged from the first hydraulic actuator  14  flows to the first control valve  20 A. In this embodiment, the first return fluid tube is the second connection fluid tube  21   b . Hereinafter, the second connection fluid tube  21   b  is referred to as a “first return fluid tube”. 
     The first return fluid tube  21   b  is a fluid tube allowing the return fluid discharged from the first hydraulic actuator (the boom cylinder)  14  to flow to the first control valve (the boom control valve)  20 A when the first hydraulic actuator (the boom cylinder)  14  is stretched with the first control valve (the boom control valve)  20 A in the first position  20   al , that is, when the boom is lifted. 
     The second return fluid tube  61  is a fluid tube through which the return fluid having flowed through the first control valve  20 A flows to the second control valve  20 B. The second return fluid tube  61  connects the first output port  41   a  of the first control valve  20 A and the first input port  42   a  of the second control valve  20 B, and connects the second output port  41   b  of the first control valve  20 A and the second input port  42   b  of the second control valve  20 B. A middle portion of the second return fluid tube  61  is connected to the center fluid tube  51 . 
     The first control valve  20 A is connected to the output portion of the first hydraulic pump P 1  by an output fluid tube  27 . The output fluid tube  27  is branched off at the connecting portion  27   a . The branched fluid tube of the output fluid tube  27  is connected to the first input port  46   a  and the second input port  46   b  of the first control valve  20 A. In addition, the output fluid tube  27  is connected to the third input port  46   c  of the first control valve  20 A. 
     Thus, the operation fluid outputted from the first hydraulic pump P 1  is supplied into the first control valve  20 A through the output fluid tube  27 , the first input port  46   a , the second input port  46   b , and the third input port  46   c.    
     The first control valve  20 A and the second control valve  20 B are connected by the center fluid tube  51 . The center fluid tube  51  couples the third output port  41   c  of the first control valve  20 A and the third input port  42   c  of the second control valve  20 B. 
     The communication fluid tube  69  is provided with a bypass fluid tube  82  that branches from the communication fluid tube  69 . The bypass fluid tube  82  is a fluid tube branched from a middle portion of the first return fluid tube  21   b , and is a fluid tube to which the leveling control valve  81  is connected in the middle portion. 
     At least two or more branched portions  40   a  and  40   b  are arranged on the middle portion of the first return fluid tube  21   b  of the communication fluid tube  69 . One end of the bypass fluid tube  82  is connected to the branched portion  40   a . The other end of the bypass fluid tube  82  is connected to the branched portion  40   b.    
     In the first return fluid tube  21   b , an operation valve  43  is arranged in a section between the branched portion  40   a  and the branched portion  40   b . In this embodiment, the operation valve  43  is provided, but the operation valve  43  is not a necessarily-required component. 
     As shown in  FIG. 5 , when the operation valve  43  is not provided, the second check valve  85  is arranged on a side closer to the first control valve  20 A with respect to the branched portion  40   a  in the first return fluid tube  21   b . For example, the second check valve  85  is arranged between the branched portion  40   a  and the branched portion  40   b  at a position corresponding to the operation valve  43 , that is, in the first return fluid tube  21   b.    
     The second check valve  85  allows the operation fluid to flow from the first control valve  20 A to the first hydraulic actuator  14 , and prevents the operation fluid from flowing from the first hydraulic actuator  14  to the first control valve  20 A. 
     The leveling control valve  81  is a valve for performing a leveling operation of the second hydraulic actuator (the bucket cylinder)  17 . The operation valve  43  is a valve configured to be switched between the first position  43   a  and the second position  43   b.    
     When the operation valve  43  is in the first position  43   a , the operation valve  43  communicates between the ports  71  and  72 , and supplies the return fluid having returned from the first hydraulic actuator (the boom cylinder)  14  to flow to the first control valve  20 A side. That is, when the operating valve  43  is in the first position  43   a , the operating valve  43  is in a state of stopping supplying the return fluid to the bypass fluid tube  82  and the leveling control valve  81 . 
     When the operating valve  43  is in the second position  43   b , the communication between the ports  71  and  72  is blocked. That is, when the operating valve  43  is in the second position  43   b , the operating valve  43  is in a state of supplying the return fluid having returned from the first hydraulic actuator (the boom cylinder)  14  to the bypass fluid tube  82  and the leveling control valve  81 . 
     In other words, the operation valve  43  is a valve configured to stop supplying the return fluid to the leveling control valve  81  to stop the leveling operation and to supply the return fluid to the leveling control valve  81  to operate the leveling operation. 
     In addition, the operation valve  43  is an electromagnetic switching valve that is switched to the first position  43   a  by a spring and switched to the second position  43   b  through magnetization of the solenoid  43   c.    
     The operation valve  43  may be a switching valve configured to be switched between the first position  43   a  and the second position  43   b  through the manual operation or the like. 
     In addition, the operation valve  43  may be other than the switching valve, and may be a proportional valve or another type of valve. 
     In addition, the switching of the operation valve  43  (the switching between the first position  43   a  and the second position  43   b ) can be performed by a switch or the like arranged in the vicinity of the operator seat  8  or the like. 
     The leveling control valve  81  has a first switching valve  44  and a second switching valve  45 . The first switching valve  44  is constituted of a pilot-switching two-position switching valve configured to be switched between the first position  44   a  and the second position  44   b . The first switching valve  44  is connected to the bypass fluid tube  82 . The pressure of the operation fluid in the bypass fluid tube  82  (the pressure of the return fluid) is applied to the pressure receiving portion  44   c  of the first switching valve  44 . 
     The second switching valve  45  is constituted of a pilot-switching three-position switching valve configured to be switched between a first position  45   a , a second position  45   b , and a third position  45   c . The first switching valve  44  and the second switching valve  45  are coupled each other by fluid tubes  47   a  and  47   b.    
     A fluid tube  47   c  is connected to the fluid tube  47   a , and a fluid tube  47   d  is connected to the fluid tube  47   b . The fluid tube  47   c  and the fluid tube  47   d  are coupled each other by a fluid tube  47   e , and the pressure receiving portion  45   d  of the second switching valve  45  is connected to the fluid tube  47   e.    
     In addition, the second switching valve  45  and the first connection fluid tube  22   a  are coupled by a fluid tube  48 . Thus, when the leveling control valve  81  operates, the pressure of the operation fluid in the fluid tubes  47   a  and  47   b  is applied to the pressure receiving portion  45   d  of the second switching valve  45 . 
     The bypass fluid tube  82  includes a fluid tube  49  coupling the branched portion  40   a  and the first switching valve  44 , a fluid tube  47   a  couples the first switching valve  44  and the second switching valve  45 , the fluid tube  47   c  connected to the fluid tube  47   a , and the fluid tube  84  coupling the fluid tube  47   c  and the branched portion  40   b.    
     Thus, in the case of the first position  43   a  (when the leveling operation is off), the first hydraulic actuator (the boom cylinder)  14  can be stretched and shortened by the switching of the first control valve  20 A. In addition, the second hydraulic actuator (the bucket cylinder)  17  can be stretched and shortened by the switching of the second control valve  20 B. 
     In the case of the second position  43   b  (when the leveling operation is on), when the first hydraulic actuator (the boom cylinder)  14  is stretched, that is, when the boom  10  is lifted, the return fluid from the first hydraulic actuator (the boom cylinder)  14  (referred to as the boom return fluid) is shut off by the operation valve  43 . 
     The boom return fluid enters the fluid tube  49  from the branched portion  40   a , is applied to the pressure receiving portion  44   c  of the first switching valve  44 , and is applied to the pressure receiving portion  45   d  of the second switching valve  45 . Then, the boom return fluid is applied to the first connection fluid tube  22   a  through the fluid tube  48  by the switching of the first switching valve  44  and the second switching valve  45 . 
     As the result, the second hydraulic actuator (the bucket cylinder)  17  dumps, that is, performs the leveling operation with use of the boom return fluid. 
     In addition, the return fluid having flowed through the fluid tube  47   a  of the first switching valve  44  flows through the fluid tube  47   c  and the fluid tube  84 , flows to the first return fluid tube  21   b , and then flows toward the first control valve  20 A. 
     The working system hydraulic system includes a return discharge fluid tube  100 . The return discharge fluid tube  100  is a fluid tube configured to discharge, to the discharge portion, at least a part of the return fluid flowing through the communication fluid tube  69 . The return discharge fluid tube  100  is a fluid tube connected, in the communication fluid tube  69 , to a side closer to the second control valve  20 B with respect to a portion where the leveling control valve  81  is arranged. 
     In other words, paying attention to the communication fluid tube  69 , the leveling control valve  81  is connected to the upstream side (the first hydraulic actuator  14  side) through which the return fluid flows. In addition, the return drain fluid tube  100  is connected to the downstream side of the leveling control valve  81  (the second control valve  20 B side). 
     In this embodiment, the return discharge fluid tube  100  is connected to the second return fluid tube  61  of the communication fluid tube  69 . In particular, one end of the return discharge fluid tube  100  is connected to the second return fluid tube  61 , and the other end is connected to the discharge fluid tube  24   c.    
     A throttle  101  is connected to the return discharge fluid tube  100 . The throttle portion  101  is configured, for example, by making a part of the return discharge fluid tube  100  thinner than the other parts. 
     In other words, the throttle portion  101  is configured by making the cross-sectional area of the portion where the operation fluid flows smaller than the other portions in the return discharge fluid tube  100 . In addition, the structure of the return discharge fluid tube  100  is not limited to the example mentioned above. 
     A first check valve  102  is connected to the second return fluid tube  61 . The first check valve  102  is a valve configured to allow the return fluid to flow from the first control valve  20 A to the second control valve  20 B, and to prevent the operation fluid from flowing from the second control valve  20 B to the return discharge fluid tube  100 . 
     In particular, the first check valve  102  is arranged, in the second return fluid tube  61 , on a section between the first connecting portion  110  and the second connecting portion (the first input port  42   a  and the second input port  42   b ), the first connecting portion  110  coupling the second return fluid tube  61  and the return discharge fluid tube  100 , the second connecting portion coupling the second return fluid tube  61  and the second control valve  20 B. 
     Thus, when the first control valve  20 A is set to the second position  20   a   2  under the state where the operation valve  43  is in the first position  43   a  (the state where the leveling operation is stopped), the first return fluid tube  21   b  and the second return fluid tube  61  communicate with each other. Thus, the return fluid from the first hydraulic actuator  14  can flow to the second return fluid tube  61  through the first return fluid tube  21   b.    
     As described above, since the return discharge fluid tube  100  is connected to the second return fluid tube  61 , a part of the return fluid can be discharged to the return discharge fluid tube  100 . 
     On the other hand, when the first control valve  20 A is set to the second position  20   a   2  under the state where the operation valve  43  is in the second position  43   b  (the state where the leveling operation is activated), the return fluid from the first hydraulic actuator  14  can be supplied to the leveling control valve  81  through the bypass fluid tube  82 . 
     That is, the return fluid from the first hydraulic actuator  14  flows to the leveling control valve  81  through at least the bypass fluid tube  82  under the state where the leveling operation is activated, and thus the operation of the leveling control valve  81  can be stabilized. 
     In addition, the second return fluid tube  61  is provided with a first check valve  102 . In this manner, the operation fluid in the second return fluid tube  61  is prevented from reversely flowing to the first control valve  20 A in accordance with the relation between the pressure of the operation fluid in the center fluid tube  51  and the pressure of the operation fluid flowing from the second return fluid tube  61  toward the second control valve  20 B. 
       FIG. 4A  to  FIG. 4D  show modified examples of the working system hydraulic system. In  FIG. 4A  to  FIG. 4D , the leveling control valve  81  is omitted. However, the leveling control valve  81  is connected to the branched fluid tube  46  as in  FIG. 3 , and further the leveling control valve  81  and the like are provided as in  FIG. 3 . Thus, the explanations thereof will be omitted. 
     In the modified example shown in  FIG. 4A , the second return fluid tube  61  includes a fluid tube  61   a  connected to the first output port  41   a , a fluid tube  61   b  connected to the second output port  41   b , and a fluid tube  61   c  connected to the fluid tube  61   a  and the fluid tube  61   b  and connected to the second control valve  20 B. In addition, one end of the return discharge fluid tube  100  is connected to the fluid tube  61   b , and the other end of the return discharge fluid tube  100  is connected to the discharge fluid tube  24   c . In addition, the fluid tube  61   b  is provided with a first check valve  102 . 
     In the modified example shown in  FIG. 4B , the working system hydraulic system has at least two (a plurality of) return discharge fluid tubes  100 . One of the return discharge fluid tubes  100  is connected to the fluid tube  61   a  and the drain fluid tube  24   c . The other one of the return discharge fluid tubes  100  is connected to the fluid tube  61   b  and the drain fluid tube  24   c . The first check valve  102  is arranged in one of the return discharge fluid tubes  100  and the other one of the return discharge fluid tubes  100 . 
     In the modified example shown in  FIG. 4C , the return discharge fluid tube  100  is arranged inside the first control valve  20 A. In particular, the first control valve  20 A includes a fluid tube (an internal fluid tube)  62   a  that communicates between the first port  31  and the first output port  41   a  when the first control valve  20 A is in the second position  20   a   2 . One end of the return discharge fluid tube  100  is connected to the internal fluid tube  62   a , and the other end is connected to the first discharge port  33   a.    
     The first control valve  20 A includes a fluid tube (an internal fluid tube)  62   b  that communicates between the second port  32  and the second output port  41   b  when the first control valve  20 A is in the first position  20   a   1 . The return discharge fluid tube  100  may be arranged in the internal fluid tube  62   b.    
       FIG. 4D  is a modified example of the configuration shown in  FIG. 4C , and the first check valve  102  is arranged in the fluid tube  61   a  and the fluid tube  61   b . In any one of the modified examples shown in  FIG. 4A  to  FIG. 4D , at least a part of the return fluid from the first hydraulic actuator  14  can be discharged from the return discharge fluid tube  100  to the discharge fluid tube  24   c . In addition, the remaining return fluid can be supplied to the second control valve  20 B. 
     In the above description, the embodiment of the present invention has been explained. However, all the features of the embodiment disclosed in this application should be considered just as examples, and the embodiment does not restrict the present invention accordingly. A scope of the present invention is shown not in the above-described embodiment but in claims, and is intended to include all modifications within and equivalent to a scope of the claims. 
     A first control valve and a second control valve are not limited to those of the embodiments described above, and any types of control valves arranged on the working machine may be employed as the first control valve and the second control valve. In the embodiment described above, the hydraulic pump is a fixed displacement pump. However, for example, the hydraulic pump may be a variable displacement pump configured to change the output flow late by changing an angle of the swash plate, or may be another type of hydraulic pump.