Patent Publication Number: US-11655613-B2

Title: Hydraulic system of construction machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a U.S. National Phase of International Application No. PCT/JP2020/029477 filed Jul. 31, 2020, which claims the benefit of Japanese Application No. 2019-152657 filed Aug. 23, 2019. The disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a hydraulic system of a construction machine. 
     BACKGROUND ART 
     In a hydraulic system installed in construction machines such as hydraulic excavators and hydraulic cranes, control valves are interposed between a main pump and hydraulic actuators. Each of the control valves controls supply and discharge of hydraulic oil to and from a corresponding one of the hydraulic actuators. 
     Generally speaking, each control valve includes: a spool disposed in a housing; and a pair of pilot ports for moving the spool. In a case where an operation device that outputs an electrical signal is used as an operation device to move the control valve, solenoid proportional valves are connected to the respective pilot ports of the control valve, and the control valve is driven by the solenoid proportional valves. 
     For example, Patent Literature 1 discloses a configuration for bringing the control valve back to its neutral position when a failure has occurred in the solenoid proportional valves for driving the control valve. In this configuration, a solenoid switching valve is interposed between an auxiliary pump and the solenoid proportional valves for driving the control valve. When a failure has occurred in the solenoid proportional valves for driving the control valve, the solenoid switching valve is switched from an open position to a closed position to stop the supply of the hydraulic oil from the auxiliary pump to the solenoid proportional valves. That is, when a failure has occurred in the solenoid proportional valves for driving the control valve, even if an operator operates the operation device, the control valve is kept in the neutral position and the operation performed on the operation device is invalidated. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Laid-Open Patent Application Publication No. 2017-110672 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the configuration disclosed in Patent Literature 1 requires a solenoid valve that is dedicated for invalidating an operation performed on the operation device. 
     In view of the above, an object of the present invention is to provide a hydraulic system of a construction machine, the hydraulic system making it possible to invalidate operations performed on operation devices without using a solenoid valve that is dedicated for invalidating operations performed on the operation devices. 
     Solution to Problem 
     In order to solve the above-described problems, the inventors of the present invention have paid attention to the fact that, among various hydraulic systems of construction machines, some of them are configured such that the relief pressure of a relief valve for a main pump is changeable by a solenoid proportional valve. Then, the inventors have come up with an idea that it may be possible to use the solenoid proportional for invalidating an operation performed on an operation device. The present invention has been made from such a technological point of view. 
     Specifically, a hydraulic system of a construction machine according to one aspect of the present invention includes: control valves interposed between a main pump and hydraulic actuators, each control valve including pilot ports; first solenoid proportional valves connected to the pilot ports of the control valves; operation devices to move the control valves, each operation device outputting an electrical signal corresponding to an operating amount of the operation device; a controller that controls the first solenoid proportional valves based on the electrical signals outputted from the operation devices; a relief valve for the main pump, the relief valve including a pilot port, the relief valve being a valve whose relief pressure increases when a pilot pressure led to the pilot port becomes higher than a first setting value; a second solenoid proportional valve connected to the pilot port of the relief valve by a secondary pressure line and connected to an auxiliary pump by a primary pressure line; and a switching valve interposed between the auxiliary pump and the first solenoid proportional valves, the switching valve including a pilot port connected to the secondary pressure line by a pilot line, the switching valve switching from a closed position to an open position when a pilot pressure led to the pilot port becomes higher than or equal to a second setting value that is lower than the first setting value. 
     According to the above configuration, whether to switch the switching valve interposed between the auxiliary pump and the first solenoid proportional valves to the closed position or the open position, i.e., whether to invalidate or validate operations performed on the operation devices, can be switched by adjusting the secondary pressure of the second solenoid proportional valve to be lower or higher than the second setting value. Also, while keeping validating operations performed on the operation devices, whether or not to increase the relief pressure can be switched by adjusting the secondary pressure of the second solenoid proportional valve to be lower or higher than the first setting value. This allows the second solenoid proportional valve, which is a single valve, to have two functions. Therefore, a solenoid valve dedicated for invalidating operations performed on the operation devices is unnecessary. 
     The above hydraulic system may further include: a first selector that receives a selection of operation lock, which is a selection to invalidate operations performed on the operation devices, or receives a selection of operation lock release, which is a selection to validate operations performed on the operation devices; and a second selector that receives a selection of relief pressure non-increase, which is a selection not to increase the relief pressure of the relief valve, or receives a selection of relief pressure increase, which is a selection to increase the relief pressure of the relief valve While the first selector is receiving the selection of operation lock, the controller may control the second solenoid proportional valve, such that a secondary pressure of the second solenoid proportional valve is lower than the second setting value. While the first selector is receiving the selection of operation lock release, the controller may control the second solenoid proportional valve, such that: during the second selector receiving the selection of relief pressure non-increase, the secondary pressure of the second solenoid proportional valve is higher than the second setting value and lower than the first setting value; and during the second selector receiving the selection of relief pressure increase, the secondary pressure of the second solenoid proportional valve is higher than the first setting value. According to this configuration, when the operator makes the selection of operation lock with the first selector, operations performed on the operation devices are invalidated, whereas when the operator makes the selection of operation lock release with the first selector, operations performed on the operation devices are validated. 
     A hydraulic system of a construction machine according to another aspect of the present invention includes: control valves interposed between a main pump and hydraulic actuators, each control valve including a spool and pilot ports; first solenoid proportional valves connected to the pilot ports of the control valves; operation devices to move the control valves, each operation device outputting an electrical signal corresponding to an operating amount of the operation device; a controller that controls the first solenoid proportional valves based on the electrical signals outputted from the operation devices; a relief valve for the main pump, the relief valve including a pilot port, the relief valve being a valve whose relief pressure increases when a pilot pressure led to the pilot port becomes higher than a first setting value; a second solenoid proportional valve connected to the pilot port of the relief valve by a secondary pressure line and connected to an auxiliary pump by a primary pressure line; and a distribution line that connects between the secondary pressure line and the first solenoid proportional valves. The spool of each control valve moves to a stroke end when a pilot pressure led to each pilot port of the control valve becomes a second setting value, and the first setting value is higher than the second setting value. 
     According to the above configuration, whether to invalidate or validate operations performed on the operation devices can be switched by adjusting the secondary pressure of the second solenoid proportional valve to be zero or to be higher than the second setting value. Also, while keeping validating operations performed on the operation devices, whether or not to increase the relief pressure can be switched by adjusting the secondary pressure of the second solenoid proportional valve to be lower or higher than the first setting value. This allows the second solenoid proportional valve, which is a single valve, to have two functions. Therefore, a solenoid valve dedicated for invalidating operations performed on the operation devices is unnecessary. 
     The above hydraulic system may further include: a first selector that receives a selection of operation lock, which is a selection to invalidate operations performed on the operation devices, or receives a selection of operation lock release, which is a selection to validate operations performed on the operation devices; and a second selector that receives a selection of relief pressure non-increase, which is a selection not to increase the relief pressure of the relief valve, or receives a selection of relief pressure increase, which is a selection to increase the relief pressure of the relief valve. While the first selector is receiving the selection of operation lock, the controller may control the second solenoid proportional valve, such that a secondary pressure of the second solenoid proportional valve is zero. While the first selector is receiving the selection of operation lock release, the controller may control the second solenoid proportional valve, such that: during the second selector receiving the selection of relief pressure non-increase, the secondary pressure of the second solenoid proportional valve is higher than the second setting value and lower than the first setting value; and during the second selector receiving the selection of relief pressure increase, the secondary pressure of the second solenoid proportional valve is higher than the first setting value. According to this configuration, when the operator makes the selection of operation lock with the first selector, operations performed on the operation devices are invalidated, whereas when the operator makes the selection of operation lock release with the first selector, operations performed on the operation devices are validated. 
     Advantageous Effects of Invention 
     The present invention makes it possible to invalidate operations performed on operation devices without using a solenoid valve that is dedicated for invalidating operations performed on the operation devices. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    shows a schematic configuration of a hydraulic system of a construction machine according to Embodiment 1 of the present invention. 
         FIG.  2    is a side view of a hydraulic excavator that is one example of the construction machine. 
         FIG.  3    is a graph showing a relationship between a command current to a second solenoid proportional valve and a secondary pressure of the second solenoid proportional valve in Embodiment 1. 
         FIG.  4    shows a schematic configuration of a hydraulic system of a construction machine according to Embodiment 2 of the present invention. 
         FIG.  5    is a graph showing a relationship between the command current to the second solenoid proportional valve and the secondary pressure of the second solenoid proportional valve in Embodiment 2. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
       FIG.  1    shows a hydraulic system  1 A of a construction machine according to Embodiment 1 of the present invention.  FIG.  2    shows a construction machine  10 , in which the hydraulic system  1 A is installed. Although the construction machine  10  shown in  FIG.  2    is a hydraulic excavator, the present invention is applicable to other construction machines, such as a hydraulic crane. 
     The construction machine  10  shown in  FIG.  2    is a self-propelled construction machine, and includes a traveling unit  11 . The construction machine  10  further includes: a slewing unit  12  slewably supported by the traveling unit  11 ; and a boom that is luffed relative to the slewing unit  12 . An arm is swingably coupled to the distal end of the boom, and a bucket is swingably coupled to the distal end of the arm. The slewing unit  12  is equipped with a cabin  16  including an operator&#39;s seat. The construction machine  10  need not be of a self-propelled type. 
     The hydraulic system  1 A includes, as hydraulic actuators  20 , a boom cylinder  13 , an arm cylinder  14 , and a bucket cylinder  15 , which are shown in  FIG.  2   , an unshown pair of left and right travel motors, and an unshown slewing motor. The boom cylinder  13  luffs the boom. The arm cylinder  14  swings the arm. The bucket cylinder  15  swings the bucket. 
     As shown in  FIG.  1   , the hydraulic system  1 A further includes a main pump  22 , which supplies hydraulic oil to the aforementioned hydraulic actuators  20 . In  FIG.  1   , the hydraulic actuators  20  are not shown for the purpose of simplifying the drawing. 
     The main pump  22  is driven by an engine  21 . Alternatively, the main pump  22  may be driven by an electric motor. The engine  21  also drives an auxiliary pump  23 . The number of main pumps  22  may be more than one. 
     The main pump  22  is a variable displacement pump (a swash plate pump or a bent axis pump) whose tilting angle is changeable. The delivery flow rate of the main pump  22  may be controlled by electrical positive control, or may be controlled by hydraulic negative control. Alternatively, the delivery flow rate of the main pump  22  may be controlled by load-sensing control. 
     Control valves  41  are interposed between the main pump  22  and the hydraulic actuators  20 . In the present embodiment, all the control valves  41  are three-position valves. Alternatively, one or more of the control valves  41  may be two-position valves. 
     All the control valves  4  are connected to the main pump  22  by a supply line  31 , and connected to a tank by a tank line  33 . Each of the control valves  41  is connected to a corresponding one of the hydraulic actuators  20  by a pair of supply/discharge lines. In a case where the number of main pumps  22  is more than one, the same number of groups of the control valves  41  as the number of main pumps  22  are formed. In each group, the control valves  41  are connected to the corresponding main pump  22  by the supply line  31 . 
     For example, the control valves  41  include: a boom control valve that controls supply and discharge of the hydraulic oil to and from the boom cylinder  13 ; an arm control valve that controls supply and discharge of the hydraulic oil to and from the arm cylinder  14 ; and a bucket control valve that controls supply and discharge of the hydraulic oil to and from the bucket cylinder  15 . 
     The supply line  31  includes a main passage and branch passages. The main passage extends from the main pump  22 . The branch passages are branched off from the main passage, and connect to the control valves  41 . In the present embodiment, a center bypass line  32  is branched off from the main passage of the supply line  31 , and the center bypass line  32  extends to the tank. The control valves  41  are disposed on the center bypass line  32 . The center bypass line  32  may be eliminated. 
     A relief line  34  is branched off from the main passage of the supply line  31 , and the relief line  34  is provided with a relief valve  35  for the main pump  22 . The relief line  34  may be branched off from the center bypass line  32  at a position upstream of all the control valves  41 . 
     The relief valve  35  includes a pilot port, and the relief pressure of the relief valve  35  is changeable by a pilot pressure led to the pilot port. To be more specific, when the pilot pressure is lower than or equal to a first setting value α, the relief pressure of the relief valve  35  is kept to the lowest value, and when the pilot pressure becomes higher than the first setting value α, the relief pressure of the relief valve  35  increases. 
     Each control valve  41  includes: a spool disposed in a housing; and a pair of pilot ports for moving the spool. For example, the housings of all the control valves  41  may be integrated together to form a multi-control valve unit. The pilot ports of each control valve  41  are connected to respective first solenoid proportional valves  43  by respective pilot lines  42 . 
     Each first solenoid proportional valve  43  is a direct proportional valve outputting a secondary pressure that indicates a positive correlation with a command current. Alternatively, each first solenoid proportional valve  43  may be an inverse proportional valve outputting a secondary pressure that indicates a negative correlation with the command current. 
     All the first solenoid proportional valves  43  are connected to a switching valve  52  by a distribution line  53 . The distribution line  53  includes a main passage and branch passages. The main passage extends from the switching valve  52 . The branch passages are branched off from the main passage, and connect to the first solenoid proportional valves  43 . 
     The switching valve  52  is connected to the auxiliary pump  23  by a pump line  51 . A relief line  54  is branched off from the pump line  51 , and the relief line  54  is provided with a relief valve  55  for the auxiliary pump  23 . The relief pressure of the relief valve  55  is set sufficiently high (e.g., 4 MPa) so that the spool of each control valve  41  can move to the stroke end. The relief pressure of the relief valve  55  is higher, to some extent, than the first setting value α of the relief valve  35 . 
     The switching valve  52  interposed between the auxiliary pump  23  and all the first solenoid proportional valves  43  includes a pilot port, and when a pilot pressure led to the pilot port becomes higher than or equal to a second setting value ( 3 , the switching valve  52  switches from a closed position, which is a neutral position, to an open position. When the switching valve  52  is in the closed position, the switching valve  52  blocks the pump line  51 , and brings the distribution line  53  into communication with the tank. When the switching valve  52  is in the open position, the switching valve  52  brings the pump line  51  into communication with the distribution line  53 . In other words, in a state where the switching valve  52  is kept in the closed position, the supply of the hydraulic oil from the auxiliary pump  23  to the first solenoid proportional valves  43  is stopped, and the primary pressure of each first solenoid proportional valve  43  is zero. Accordingly, even when electric currents are fed to the first solenoid proportional valves  43  (even when the first solenoid proportional valves  43  move), the control valves  41  do not move. 
     The second setting value β of the switching valve  52  is set lower than the first setting value α of the relief valve  35 . For example, the first setting value α is 3.0 to 3.9 MPa, and the second setting value β is 0.1 to 1.0 MPa. 
     The auxiliary pump  23  is connected also to a second solenoid proportional valve  62  by a primary pressure line  61 , and the second solenoid proportional valve  62  is connected to the pilot port of the relief valve  35  by a secondary pressure line  63 . The upstream portion of the primary pressure line  61  and the upstream portion of the pump line  51  merge together to form a shared passage. 
     The second solenoid proportional valve  62  is a direct proportional valve outputting a secondary pressure that indicates a positive correlation with a command current. The pilot port of the switching valve  52  is connected to the secondary pressure line  63  by a pilot line  64 . 
     Operation devices  44  to move the control valves  41  are disposed in the aforementioned cabin  16 . Each operation device  44  includes an operating unit (an operating lever or a foot pedal) that receives an operation for moving a corresponding one of the hydraulic actuators  20 , and outputs an electrical signal corresponding to an operating amount of the operating unit (e.g., an inclination angle of the operating lever). 
     For example, the operation devices  44  include: a boom operation device, an arm operation device, and a bucket operation device, each of which includes an operating lever; and a right travel operation device and a left travel operation device, each of which includes a foot pedal. The operating lever of the boom operation device receives a boom raising operation and a boom lowering operation. The operating lever of the arm operation device receives an arm crowding operation and an arm pushing operation. The operating lever of the bucket operation device receives a bucket excavating operation and a bucket dumping operation. Each of the foot pedal of the right travel operation device and the foot pedal of the left travel operation device receives a forward travel operation and a backward travel operation. For example, when the operating lever of the boom operation device is inclined in a boom raising direction, the boom operation device outputs a boom raising electrical signal whose magnitude corresponds to the inclination angle of the operating lever. 
     The electrical signal outputted from each operation device  44  is inputted to a controller  7 . For example, the controller  7  is a computer including memories such as a ROM and RAM, a storage such as a HDD, and a CPU. The CPU executes a program stored in the ROM or HDD. 
     The controller  7  controls the first solenoid proportional valves  43  based on the electrical signals outputted from the operation devices  44 .  FIG.  1    shows only part of signal lines for simplifying the drawing. For example, when a boom raising electrical signal is outputted from the boom operation device, the controller  7  feeds a command current to the first solenoid proportional valve  43  connected to a boom raising pilot port of the boom control valve, and increases the command current in accordance with increase in the boom raising electrical signal. 
     In the cabin  16 , a first selector  81  and a second selector  82  are disposed. With the first selector  81 , an operator selects whether to invalidate or validate operations performed on all the operation devices  44 . With the second selector  82 , the operator selects whether or not to increase the relief pressure of the relief valve  35 . 
     The first selector  81  receives a selection of operation lock, which a selection to invalidate operations performed on the operation devices  44 , or receives a selection of operation lock release, which is a selection to validate operations performed on the operation devices  44 . For example, the first selector  81  may be a micro switch or limit switch including a safety lever, and by shifting or swinging the safety lever, the selection of operation lock or the selection of operation lock release can be made. Alternatively, the first selector  81  may be a push button switch including a button, and by pushing or not pushing the button, the selection of operation lock or the selection of operation lock release can be made. 
     The second selector  82  receives a selection of relief pressure non-increase, which is a selection not to increase the relief pressure of the relief valve  35 , or a selection of relief pressure increase, which is a selection to increase the relief pressure of the relief valve  35 . For example, the second selector  82  may be a slide switch including a knob, and by sliding the knob, the selection of relief pressure non-increase or the selection of relief pressure increase can be made. Alternatively, the second selector  82  may be a push button switch including a button, and by pushing or not pushing the button, the selection of relief pressure non-increase or the selection of relief pressure increase can be made. 
     The controller  7  controls the second solenoid proportional valve  62  in accordance with a selection status of the first selector  81  and a selection status of the second selector  82  as described below. 
     While the first selector  81  is receiving the selection of operation lock, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is lower than the second setting value β as shown in FIG.  3 . As a result, the relief pressure of the relief valve  35  is kept to the lowest value, and the switching valve  52  is kept in the closed position. At the time, the controller  7  may feed no command current to the second solenoid proportional valve  62 , or may feed a command current lower than the electric current value corresponding to the second setting value β to the second solenoid proportional valve  62 . 
     While the first selector  81  is receiving the selection of operation lock release, the control of the second solenoid proportional valve  62  differs depending on the selection status of the second selector  82 . During the second selector  82  receiving the selection of relief pressure non-increase, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is higher than the second setting value β and lower than the first setting value α. As a result, the relief pressure of the relief valve  35  is kept to the lowest value, and the switching valve  52  is switched to the open position. At the time, the value of the command current that the controller  7  feeds to the second solenoid proportional valve  62  may be any value, so long as it is higher than the electric current value corresponding to the second setting value β and lower than the electric current value corresponding to the first setting value α. 
     On the other hand, during the second selector  82  receiving the selection of relief pressure increase, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is higher than the first setting value α. As a result, with the switching valve  52  kept in the open position, the relief pressure of the relief valve  35  is increased to a predetermined value. For example, the controller  7  maximizes the command current to feed to the second solenoid proportional valve  62 . As a result, the secondary pressure of the second solenoid proportional valve  62  is equalized to the primary pressure (the relief pressure of the relief valve  55 ). 
     As described above, in the hydraulic system  1 A of the present embodiment, whether to switch the switching valve  52 , which is interposed between the auxiliary pump  23  and the first solenoid proportional valves  43 , to the closed position or to the open position, i.e., whether to invalidate or validate operations performed on the operation devices  44 , can be switched by adjusting the secondary pressure of the second solenoid proportional valve  62  to be lower or higher than the second setting value ( 3 . Also, while keeping validating operations performed on the operation devices  44 , whether or not to increase the relief pressure of the relief valve  35  can be switched by adjusting the secondary pressure of the second solenoid proportional valve  62  to be lower or higher than the first setting value α. This allows the second solenoid proportional valve  62 , which is a single valve, to have two functions. Therefore, a solenoid valve dedicated for invalidating operations performed on the operation devices  44  is unnecessary. 
     Since the present embodiment includes the first selector  81 , when the operator makes the selection of operation lock with the first selector  81 , operations performed on the operation devices  44  are invalidated, whereas when the operator makes the selection of operation lock release with the first selector  81 , operations performed on the operation devices  44  are validated. 
     Instead of including the second selector  82 , the controller  7  may detect a particular operation, and in response thereto, the controller  7  may automatically control the second solenoid proportional valve  62 , such that the relief pressure of the relief valve  35  is increased. For example, while traveling, the controller  7  may automatically control the second solenoid proportional valve  62 , such that the relief pressure of the relief valve  35  is increased. In this case, when the foot pedal of the right travel operation device or the foot pedal of the left travel operation device is operated, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  becomes higher than the first setting value α. 
     Embodiment 2 
       FIG.  4    shows a hydraulic system  1 B according to Embodiment 2 of the present invention. In the present embodiment, the same components as those described in Embodiment 1 are denoted by the same reference signs as those used in Embodiment 1, and repeating the same descriptions is avoided. 
     In the present embodiment, the switching valve  52  shown in  FIG.  1    is eliminated, and instead, the upstream end of the distribution line  53  is connected to the secondary pressure line  63 . That is, the distribution line  53  connects between the secondary pressure line  63  and all the first solenoid proportional valves  43 . 
     Further, in the present embodiment, the spool of each control valve  41  moves to the stroke end when a pilot pressure led to each pilot port of the control valve  41  becomes a second setting value γ. The first setting value α of the relief valve  35  is higher than the second setting value γ. For example, the second setting value γ is 2.0 to 3.4 MPa, and the first setting value α is 3.5 to 3.9 MPa. 
     Next, the control of the second solenoid proportional valve  62  by the controller  7  is described with reference to  FIG.  5   . 
     While the first selector  81  is receiving the selection of operation lock, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is zero. That is, the controller  7  feeds no command current to the second solenoid proportional valve  62 . As a result, the relief pressure of the relief valve  35  is kept to the lowest value, and the primary pressure of each first solenoid proportional valve  43  is zero (even if the first solenoid proportional valves  43  are fed with electric currents, the control valves  41  do not move). 
     While the first selector  81  is receiving the selection of operation lock release, the control of the second solenoid proportional valve  62  differs depending on the selection status of the second selector  82 . During the second selector  82  receiving the selection of relief pressure non-increase, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is higher than the second setting value γ and lower than the first setting value α. As a result, the relief pressure of the relief valve  35  is kept to the lowest value, and the primary pressure of each first solenoid proportional valve  43  is higher than the second setting value γ (the spool of each control valve  41  can move to the stroke end). At the time, the value of the command current that the controller  7  feeds to the second solenoid proportional valve  62  may be any value, so long as it is higher than the electric current value corresponding to the second setting value γ and lower than the electric current value corresponding to the first setting value α. 
     On the other hand, during the second selector  82  receiving the selection of relief pressure increase, the controller  7  controls the second solenoid proportional valve  62 , such that the secondary pressure of the second solenoid proportional valve  62  is higher than the first setting value α. As a result, with the primary pressure of each first solenoid proportional valve  43  kept higher than the second setting value γ, the relief pressure of the relief valve  35  is increased to a predetermined value. For example, the controller  7  maximizes the command current to feed to the second solenoid proportional valve  62 . As a result, the secondary pressure of the second solenoid proportional valve  62  is equalized to the primary pressure (the relief pressure of the relief valve  55 ). 
     As described above, in the hydraulic system  1 B of the present embodiment, whether to invalidate or validate operations performed on the operation devices  44  can be switched by adjusting the secondary pressure of the second solenoid proportional valve  62  to be zero or to be higher than the second setting value γ. Also, while keeping validating operations performed on the operation devices  44 , whether or not to increase the relief pressure of the relief valve  35  can be switched by adjusting the secondary pressure of the second solenoid proportional valve  62  to be lower or higher than the first setting value α. This allows the second solenoid proportional valve  62 , which is a single valve, to have two functions. Therefore, a solenoid valve dedicated for invalidating operations performed on the operation devices  44  is unnecessary. 
     Other Embodiments 
     The present invention is not limited to the above-described embodiments. Various modifications can be made without departing from the scope of the present invention.