Patent Publication Number: US-11644098-B2

Title: Hydraulic system of work machine and work machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional application of the U.S. patent application Ser. No. 16/833,689 filed Mar. 30, 2020, which is a divisional application of the U.S. patent application Ser. No. 15/369,890 filed Dec. 6, 2016, which was issued as U.S. Pat. No. 10,641,388, which claims priority under 35 U. S. C. § 119 to Japanese Patent Application No. 2015-238561, filed Dec. 7, 2015, Japanese Patent Application No. 2016-188002, filed Sep. 27, 2016. 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 of a work machine and to a work machine. 
     Discussion of the Background 
     Conventionally, a hydraulic system that shifts gears by using a variable displacement hydraulic motor in a work machine such as a skid steer loader or a compact track loader is known (see JP 2013-36276 A). 
     The hydraulic system disclosed in JP 2013-36276 A is a system that shifts gears by using a swash-plate variable displacement axial motor (HST motor). The hydraulic system has a speed changing mechanism that can change the speed of the HST motor. The speed changing mechanism includes a direction switching valve, a hydraulic switching valve whose position is switched by the direction switching valve, and a swash-plate switching cylinder that is connected to the hydraulic switching valve and the HST motor. The speed changing mechanism first stretches or contracts the swash-plate switching cylinder by changing the position of the hydraulic switching valve by using the direction switching valve. When the swash-plate switching cylinder stretches or contracts, an angle of the swash plate of the HST motor changes, and thus the HST motor is switched to the first speed or the second speed. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, a hydraulic system of a work machine includes a hydraulic pump, a first hydraulic device, a first oil passage, a second oil passage, a pilot check valve, a third oil passage, a first actuator, a hydraulic actuator, and an operation control valve. The hydraulic pump is to discharge hydraulic oil. The first hydraulic device is to operate in a first operation mode while pressure of the hydraulic oil supplied from the hydraulic oil is equal to or higher than a first pressure threshold. The first oil passage connects the first hydraulic device and the hydraulic pump. The hydraulic oil is to be supplied to the first hydraulic device from the hydraulic pump via the first oil passage. The second oil passage is connected to the first oil passage. The hydraulic oil in the first oil passage is to be discharged via the second oil passage. The pilot check valve is provided in the second oil passage and has a pilot port to receive a pilot pressure of hydraulic oil. the pilot pressure is controlled to be a pressure lower than a fourth pressure threshold when an operation mode of the first hydraulic device is changed to the first operation mode. The pilot check valve is closed to stop discharging the hydraulic oil in the first oil passage through the second oil passage while the pilot pressure is lower than the fourth pressure threshold. The pilot valve is opened such that the hydraulic oil in the first oil passage is discharged through the second oil passage while the pilot pressure is higher than or equal to the fourth pressure threshold. The third oil passage is connected to the pilot port. The first actuator is to control an amount of hydraulic oil flowing to the third oil passage. The operation control valve is to control an operation of the hydraulic actuator. The first hydraulic device is a ride control device to perform a vibration control operation to suppress a pressure fluctuation of the hydraulic actuator in accordance with the pressure of the hydraulic oil. The first actuator is a remote control valve connected to the operation control valve to control the pressure of the hydraulic oil in accordance with an operation of an operating member. 
    
    
     
       BRIEF 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 diagram illustrating a hydraulic system (hydraulic circuit) according to a first embodiment; 
         FIG.  2    is a diagram illustrating a modification of the hydraulic system (hydraulic circuit) according to the first embodiment; 
         FIG.  3    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a second embodiment; 
         FIG.  4    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a third embodiment; 
         FIG.  5    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a fourth embodiment; 
         FIG.  6    is a diagram for explaining anti-stall and illustrates a relationship between primary pressure and engine rotational speed; 
         FIG.  7    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a fifth embodiment; 
         FIG.  8    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a sixth embodiment; 
         FIG.  9    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a seventh embodiment; 
         FIG.  10    is a diagram illustrating a hydraulic system (hydraulic circuit) according to an eighth embodiment; 
         FIG.  11    is a diagram illustrating a hydraulic system (hydraulic circuit) according to a ninth embodiment; 
         FIG.  12    is a side view illustrating a track loader that is an example of a work machine; and 
         FIG.  13    is a side view illustrating part of a track loader in a state where a cabin is raised. 
     
    
    
     DETAILED DESCRIPTION OF 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. 
     Hereinafter, embodiments of a hydraulic system of a work machine and a work machine including the hydraulic system according to the present invention will be described with reference to the drawings. 
     First Embodiment 
     First, an overall configuration of a work machine is described. 
     A work machine  1  according to the embodiments of the present invention includes a frame  2 , a working device  3  attached to the frame  2 , and a travelling device  4  that supports the frame  2 , as illustrated in  FIGS.  12  and  13   . Although a track loader is illustrated as an example of the work machine in  FIGS.  12  and  13   , the work machine according to the embodiments of the present invention is not limited to a track loader and can be, for example, a tractor, a skid steer loader, a compact track loader, or a backhoe. The embodiments of the present invention are described assuming that a front side (left side in  FIG.  12   ) of a driver sitting on a driver&#39;s seat of the work machine is a forward direction, a rear side (right side in  FIG.  12   ) of the driver is a backward direction, a left side (near side in  FIG.  12   ) of the driver is a leftward direction, and a right side (far side in  FIG.  12   ) of the driver is a rightward direction. 
     A cabin  5  is mounted in an upper front part of the frame  2 . A rear part of the cabin  5  is supported by a supporting bracket  11  of the frame  2  so as to be swingable around a supporting shaft  12 . A front part of the cabin  5  can be placed on a front part of the frame  2 . 
     A driver&#39;s seat  13  is provided in the cabin  5 . An operation device for travelling  14  for operating the travelling device  4  is disposed on one side (e.g., left side) of the driver&#39;s seat  13 . 
     The travelling device  4  is realized by a crawler-type travelling device. The travelling devices  4  are provided below the frame  2  on the left and right of the frame  2 . The travelling device  4  has a first travelling part  21 L and a second travelling part  21 R that work by hydraulic driving and is configured to be able to travel by the first travelling part  21 L and the second travelling part  21 R. 
     The working device  3  includes a boom  22 L, a boom  22 R, and a bucket  23  (working tool). The boom  22 L is disposed on the left of the frame  2 . The boom  22 R is disposed on the right of the frame  2 . The boom  22 L and the boom  22 R are linked to each other by a link. The boom  22 L and the boom  22 R are supported by a first lift link  24  and a second lift link  25 . A lift cylinder  26 , which is realized by a double-acting pressure cylinder, is provided between base sides of the boom  22 L and the boom  22 R and a lower rear part of the frame  2 . The boom  22 L and the boom  22 R swing up or down when the lift cylinders  26  stretch or contract concurrently. An attachment bracket  27  is pivotably supported by a front end side of each of the boom  22 L and the boom  22 R so as to be rotatable about a horizontal shaft, and a rear surface side of the bucket  23  is attached to the attachment brackets  27  provided on the left and right. 
     A tilt cylinder  28 , which is realized by a double-acting pressure cylinder, is provided between the attachment brackets  27  and intermediate parts of the boom  22 L and the boom  22 R on front end sides thereof. The bucket  23  swings (scoops or dumps) when the tilt cylinder  28  stretches or contracts. 
     The bucket  23  is detachably attached to the attachment brackets  27 . By detaching the bucket  23  and attaching various attachments (hydraulically-driven working tool having a hydraulic actuator that will be described later) to the attachment brackets  27 , various works other than digging (or a different kind of digging operation) can be performed. A prime mover  29  is provided on a rear side of the frame  2 . The prime mover  29  is an engine. Note that the prime mover  29  may be an electric motor or may be one that has both an engine and an electric motor. Furthermore, a tank (hydraulic oil tank)  31  in which hydraulic oil is stored is provided in the frame  2 . 
     Next, a hydraulic system of the work machine according to the embodiments of the present invention is described below. 
       FIG.  1    is an overall view of a hydraulic system of a travelling system. 
     As illustrated in  FIG.  1   , the hydraulic system (hydraulic circuit) has a first hydraulic pump P 1 . The first hydraulic pump P 1  is a hydraulic pump that is driven by power of the engine  29  to discharge hydraulic oil and is, for example, realized by a fixed displacement gear pump. The first hydraulic pump P 1  is mainly used for supply of a control signal (pilot pressure). For convenience of description, hydraulic oil discharged from the first hydraulic pump P 1  or hydraulic oil for a control signal is hereinafter sometime referred to as pilot oil, and the pressure of pilot oil is hereinafter sometime referred to as pilot pressure. Note that the hydraulic system has a second hydraulic pump (not illustrated). The second hydraulic pump is a pump that mainly supplies hydraulic oil to a hydraulic system of a work system. The lift cylinder  26 , the tilt cylinder  28 , and the like work by the hydraulic oil discharged from the second hydraulic pump. 
     The hydraulic system (hydraulic circuit) includes a first driving circuit  32 A and a second driving circuit  32 B. The first driving circuit  32 A is a circuit that drives the first travelling part  21 L provided on the left, and the second driving circuit  32 B is a circuit that drives the second travelling part  21 R provide on the right. 
     The first driving circuit  32 A and the second driving circuit  32 B each include an HST pump (travelling pump)  50 . The HST pumps  50  are connected to respective travelling motors  51  of the first and second travelling parts  21 L and  21 R by a pair of oil passages for gear shift  71   h  and  71   i . Note that the second driving circuit  32 B has an identical structure to the first driving circuit  32 A and therefore description thereof is omitted. 
     The HST pump  50  is a swash-plate variable displacement axial pump that is driven by power of the engine  29  and is a pilot-type hydraulic pump (swash-plate variable displacement hydraulic pump) in which an angle of a swash plate is changed by pilot pressure. Specifically, the HST pump  50  includes a pressure receiving part (a pilot port) for forward travelling  50   a  and a pressure receiving part (a pilot port) for backward travelling  50   b  on which the pilot pressure acts. 
     The angle of the swash plate is changed by the pilot pressure that acts on the pressure receiving parts  50   a  and  50   b . When the angle of the swash plate changes, a direction and an amount of discharge of hydraulic oil changes, and thus rotation output of the first and second travelling parts  21 L and  21 R changes. When the rotational speed of the HST pump  50  increases, the amount of discharge of the HST pump  50  increases, and the travelling speed increases accordingly. The rotational speed of the HST pump  50 , i.e., the amount of discharge of the HST pump  50  changes depending on output of the engine  29 . 
     The hydraulic system has a first hydraulic device whose operation mode can be changed in a case where the pressure of the hydraulic oil becomes equal to or higher than a predetermined value (a first pressure threshold). In the present embodiment, the first hydraulic device is the travelling motor (hydraulic motor for travelling)  51  provided in each of the first travelling part  21 L and the second travelling part  21 R. The travelling motor  51  is a travelling motor (HST motor) whose speed can be changed by the pressure of the hydraulic oil and works by the hydraulic oil discharged from the HST pump  50 . The travelling motor  51  is, for example, a swash-plate variable displacement axial motor that can be shifted to two speeds, i.e., high speed and low speed. 
     The travelling motor  51  can be operated by a travelling operation device  14 . The travelling operation device  14  has a remote control valve for forward travelling  36 , a remote control valve for backward travelling  37 , a remote control valve for rightward turn  38 , a remote control valve for leftward turn  39 , a travelling lever  40 , which is an operating member, and first through fourth shuttle valves  41 ,  42 ,  43 , and  44 . The remote control valves (first actuators)  36 ,  37 ,  38 , and  39  are operated by a common, i.e., single travelling lever  40 . To the remote control valves  36 ,  37 ,  38 , and  39 , the hydraulic oil discharged from the first hydraulic pump P 1  is supplied. The pressure of the hydraulic oil supplied to the remote control valves  36 ,  37 ,  38 , and  39  changes in accordance with an operation of the travelling lever (operating member)  40 . 
     The travelling lever  40  is tiltable from a neutral position in forward and backward direction, a width direction orthogonal to the forward and backward directions, and oblique directions. By tilting the travelling lever  40 , the remote control valves  36 ,  37 ,  38 , and  39  of the travelling operation device  14  are operated. As a result, pilot pressure that is proportional to the amount of operation of the travelling lever  40  from the neutral position is output from secondary-side ports of the remote control valves  36 ,  37 ,  38 , and  39 . That is, the remote control valves  36 ,  37 ,  38 , and  39  are realized by proportional valves. Note that the remote control valves need not be proportional valves, as long as the pressure of hydraulic oil changes in accordance with an operation of the travelling lever (operating member)  40 . 
     In a case where the travelling lever  40  is tilted forward (in a direction indicated by arrow A 1  in  FIG.  1   ), the remote control valve for forward travelling  36  is operated, and pilot pressure is output from the remote control valve  36 . This pilot pressure acts on the pressure receiving part for forward travelling  50   a  of the first driving circuit  32 A after travelling from the first shuttle valve  41  through a first operation oil passage  75   a  and acts on the pressure receiving part for forward travelling  50   a  of the second driving circuit  32 B after travelling from the second shuttle valve  42  through a second operation oil passage  75   b . As a result, output shafts  51   a  of the first travelling part  21 L and the second travelling part  21 R normally rotate (rotate forwardly) at a speed that is proportional to an amount of tilt of the travelling lever  40 , and thus the track loader  1  travels straight in the forward direction. 
     In a case where the travelling lever  40  is tilted backward (in a direction indicated by arrow A 2  in  FIG.  1   ), the remote control valve for backward travelling  37  is operated, and pilot pressure is output from the remote control valve  37 . This pilot pressure acts on the pressure receiving part for backward travelling  50   b  of the first driving circuit  32 A after travelling from the third shuttle valve  43  through a third operation oil passage  75   c  and acts on the pressure receiving part for backward travelling  50   b  of the second driving circuit  32 B after travelling from the fourth shuttle valve  44  through a fourth operation oil passage  75   d . As a result, the output shafts  51   a  of the first travelling part  21 L and the second travelling part  21 R reversely rotate (rotate backwardly) at a speed that is proportional to an amount of tilt of the travelling lever  40 , and thus the track loader  1  travels straight in the backward direction. 
     In a case where the travelling lever  40  is tilted rightward (in a direction indicated by arrow A 3  in  FIG.  1   ), the remote control valve for rightward turn  38  is operated, and pilot pressure is output from the remote control valve  38 . This pilot pressure acts on the pressure receiving part for forward travelling  50   a  of the first driving circuit  32 A after travelling from the first shuttle valve  41  through the first operation oil passage  75   a  and acts on the pressure receiving part for backward travelling  50   b  of the second driving circuit  32 B after travelling from the fourth shuttle valve  44  through the fourth operation oil passage  75   d . As a result, the output shaft  51   a  of the first travelling part  21 L normally rotates and the output shaft  51   a  of the second travelling part  21 R reversely rotates, and thus the track loader turns rightward. 
     In a case where the travelling lever  40  is tilted leftward (in a direction indicated by arrow A 4  in  FIG.  1   ), the remote control valve for leftward turn  39  is operated, and pilot pressure is output from the remote control valve  39 . This pilot pressure acts on the pressure receiving part for forward travelling  50   a  of the second driving circuit  32 B after travelling from the second shuttle valve  42  through the second operation oil passage  75   b  and acts on the pressure receiving part for backward travelling  50   b  of the first driving circuit  32 A after travelling from the third shuttle valve  43  through the third operation oil passage  75   c . As a result, the output shaft  51   a  of the second travelling part  21 R normally rotates and the output shaft  51   a  of the first travelling part  21 L reversely rotates, and thus the track loader turns leftward. 
     In a case where the travelling lever  40  is tilted in an oblique direction, directions and speeds of rotation of the output shafts  51   a  of the first travelling part  21 L and the second travelling part  21 R are determined by differential pressure between pilot pressure that acts on the pressure receiving part for forward travelling  50   a  and pilot pressure that acts on the pressure receiving part for backward travelling  50   b  of each of the first and second driving circuits  32 A and  32 B, and thus the track loader  1  turns rightward or leftward while travelling in the forward or backward direction. 
     Specifically, in a case where the travelling lever  40  is tilted in a diagonally forward leftward direction, the track loader  1  turns leftward while travelling in the forward direction at a speed corresponding to an angle of tilt of the travelling lever  40 . In a case where the travelling lever  40  is tilted in a diagonally forward rightward direction, the track loader  1  turns rightward while travelling in the forward direction at a speed corresponding to an angle of tilt of the travelling lever  40 . In a case where the travelling lever  40  is tilted in a diagonally backward leftward direction, the track loader  1  turns leftward while travelling in the backward direction at a speed corresponding to an angle of tilt of the travelling lever  40 . In a case where the travelling lever  40  is tilted in a diagonally backward rightward direction, the track loader  1  turns rightward while travelling in the backward direction at a speed corresponding to an angle of tilt of the travelling lever  40 . 
     A circuit (structure) around the first travelling part  21 L is described below. Note that a circuit (structure) of the second travelling part  21 R is similar to that of the first travelling part  21 L. 
     The travelling motor (first hydraulic device)  51  of the first travelling part  21 L includes a swash plate switching cylinder  53 . The swash plate of the travelling motor  51  is linked to the swash plate switching cylinder  53 . The speed of the travelling motor  51  is changed by supplying hydraulic oil to the swash plate switching cylinder  53 , which is realized by a hydraulic cylinder or the like. That is, the angle of the swash plate of the travelling motor  51  is changed by stretching or contracting the swash plate switching cylinder  53 . In this way, the travelling motor  51  is shifted to the first speed or the second speed. 
     A first oil passage (discharge oil passage)  71  is connected to the travelling motor  51 , i.e., the swash plate switching cylinder  53 . The first hydraulic pump P 1  is connected to the first oil passage  71 . Accordingly, hydraulic oil discharged from the first hydraulic pump P 1  can be supplied to the travelling motor  51  (the swash plate switching cylinder  53 ). The reference signs X 1 , X 2 , and X 3  in  FIG.  1    indicates where the oil passage is connected. 
     Specifically, the first oil passage  71  includes a first supply passage  71   a , a second supply passage  71   b , a third supply passage  71   c , and a fourth supply passage  71   d . The first driving circuit  32 A and the second driving circuit  32 B are connected to the first supply passage  71   a , so that pilot oil that is discharged from the first hydraulic pump P 1  can be supplied to the first driving circuit  32 A and the second driving circuit  32 B. The travelling operation device  14  is connected to the second supply passage  71   b , and the pilot oil of the first hydraulic pump P 1  is supplied to the travelling operation device  14  through the second supply passage  71   b . A first actuating part  61  and a second actuating part  62  are connected to the third supply passage  71   c . The pilot oil of the first hydraulic pump P 1  is supplied to the first actuating part  61  and the second actuating part  62 . The fourth supply passage  71   d  connects the first hydraulic pump P 1  and the swash plate switching cylinder  53 . 
     A second oil passage  72  is connected to the first oil passage  71 . The second oil passage  72  is connected to an intermediate part of the fourth supply passage  71   d . In the fourth supply passage  71   d , a restrictor (restricting part)  80  that reduces the amount of hydraulic oil is provided on an upstream side of a connection part where the fourth supply passage  71   d  and the second oil passage  72  are connected. The second oil passage  72  is an oil passage that can discharge the hydraulic oil in the first oil passage  71 . The hydraulic oil tank  31  is connected to the second oil passage  72 . A pilot check valve  65  is provided at an intermediate part of the second oil passage  72 . The pilot check valve  65  has a pressure receiving part (a pilot port)  65   a  that receives the pressure of the hydraulic oil. In a case where the pressure of the hydraulic oil applied to the pressure receiving part  65   a  is equal to or higher than a predetermined value (a second pressure threshold), the pilot check valve  65  closes. That is, the pilot check valve  65  blocks discharge of the hydraulic oil in the second oil passage  72  when the pressure receiving part  65   a  receives the pressure of the hydraulic oil. Meanwhile, in a case where the pressure of the hydraulic oil applied to the pressure receiving part  65   a  is less than the predetermined value, the pilot check valve  65  can be opened. That is, the pilot check valve  65  allows discharge of the hydraulic oil in the second oil passage  72  when the pressure receiving part  65   a  does not receive the pressure of the hydraulic oil. 
     When discharge of the hydraulic oil from the second oil passage  72  is blocked by the pilot check valve  65 , the pressure in the first oil passage  71  (fourth supply passage  71   d ) rises. This makes it possible to stretch the swash plate switching cylinder  53  from the first speed to the second speed. That is, the travelling motor (first hydraulic device)  51  can be shifted from the first speed to the second speed by blocking discharge of the hydraulic oil from the second oil passage  72  by using the pilot check valve  65 . Since the swash plate switching cylinder  53  is stretched or contracted (speed is changed) by using the pilot check valve  65 , it is possible to suppress shock at the time of speed change (at the time of gear shift). It is possible to suppress shock in a case where an operation mode of the travelling motor (first hydraulic device)  51  is changed. Furthermore, it is possible to improve operability at the time of speed change of the travelling motor  51 . 
     The first actuating part  61  causes the hydraulic oil to act on the pressure receiving part  65   a  of the pilot check valve  65 . The pressure receiving part  65   a  of the pilot check valve  65  and the first actuating part  61  are connected by a third oil passage  73 . 
     The first actuating part  61  is a two-position switch-over valve whose position can be switched between a first position  61   a  and a second position  61   b . The first actuating part  61  is switched, for example, by a control device  64 . An operating member  66  is connected to the control device  64 . The operating member  66  is realized by a see-saw type switch that is swingable, a slide-type switch that is slidable, a push-type switch that is capable of being pressed, or a lever. 
     In a case where a command to shift to the first speed is given by the operating member  66 , the control device  64  demagnetizes a solenoid of the first actuating part (two-position switch-over valve)  61 . In a state where the solenoid of the first actuating part  61  is demagnetized, the first actuating part is at the first position  61   a , and therefore the pressure of the hydraulic oil (pilot oil) does not act on the pressure receiving part  65   a  of the pilot check valve  65 . In a case where a command to shift to the second speed is given by the operating member  66 , the control device  64  excites the solenoid of the first actuating part (two-position switch-over valve)  61 . In a state where the solenoid of the first actuating part  61  is excited, the first actuating part  61  is at the second position  61   b , and therefore the pressure of the hydraulic oil (pilot oil) acts on the pressure receiving part  65   a  of the pilot check valve  65 . 
     The hydraulic system has, separately from the travelling motor (first hydraulic device)  51 , a second hydraulic device whose operation mode can be changed in a case where the pressure of hydraulic oil becomes equal to or higher than a predetermined value (a third pressure threshold). In the present embodiment, the second hydraulic device is a braking mechanism  52  for braking the travelling motor  51 . 
     The braking mechanism  52  is switched between a state where the braking mechanism  52  brakes the travelling motor  51  and a state where the braking mechanism  52  releases braking by the pilot oil (hydraulic oil) discharged from the first hydraulic pump P 1 . For example, the braking mechanism  52  has a first disc provided on the output shaft  51   a  of the travelling motor  51 , a second disc that is movable, and a spring that energizes the second disc toward a side on which the second disc contacts the first disc. Furthermore, the braking mechanism  52  includes a containing part (containing case)  52   a  in which the first disc, the second disc, and the spring are contained. A fourth oil passage  74  is connected to a part of the containing part  52   a  in which the second disc is stored. That is, the braking mechanism  52  is connected. A restrictor (restricting part)  81  is provided at an intermediate part of the fourth oil passage  74 . The second actuating part  62  is connected to an upstream side of the restricting part  81  of the fourth oil passage  74 . 
     The second actuating part  62  causes the braking mechanism  52  to perform a braking action or a braking releasing action. The second actuating part  62  is a two-position switch-over valve whose position can be switched between a first position  62   a  and a second position  62   b . The second actuating part  62  is switched, for example, by the control device  64 . 
     The control device  64  causes the second actuating part  62  to be at the first position  62   a  by demagnetizing a solenoid of the second actuating part (two-position switch-over valve)  62 . In a case where the second actuating part  62  is at the first position  62   a , the hydraulic oil in the fourth oil passage  74  flows to the hydraulic oil tank  31 . This causes the pressure of the pilot oil to be equal to or lower than a predetermined value in the storage part of the containing part  52   a . Accordingly, the second disc moves toward the side on which the second disc contacts the first disc. The travelling motor  51  is braked by the braking mechanism  52 . The control device  64  causes the second actuating part  62  to be at the second position  62   b  by exciting the solenoid of the second actuating part (two-position switch-over valve)  62 . In a case where the second actuating part  62  is at the second position  62   b , hydraulic oil discharged from the first hydraulic pump P 1  flows to the fourth oil passage  74 . This causes the pressure in the storage part of the containing part  52   a  to reach a predetermined value, thereby causing the second disc to move to a side opposite to the case of braking (side opposite to the side toward which the second disc is energized by the spring). Braking of the travelling motor  51  is released by the braking mechanism  52 . 
     In the present embodiment, the third oil passage  73  and the fourth oil passage  74  are connected to each other. The third oil passage  73  includes a first flow passage  73   a , a second flow passage  73   b , and a third flow passage  73   c . The first flow passage  73   a  is an oil passage that is connected to the fourth oil passage  74  so as to be able to discharge the hydraulic oil in the fourth oil passage  74 . A restrictor (restricting part)  82  is provided at an intermediate part of the first flow passage  73   a , and the hydraulic oil tank  31  is connected to the first flow passage  73   a.    
     The second flow passage  73   b  is a flow passage that connects the first flow passage  73   a  and the first actuating part  61 . One end of the second flow passage  73   b  is connected to a section of the first flow passage  73   a  that is located between the restricting part  82  of the first flow passage  73   a  and the hydraulic oil tank  31 . A restrictor (restricting part)  83  is provided at an intermediate part of the second flow passage  73   b , and a non-return valve (check valve)  84  is provided on an upstream side (first actuating part  61  side) of the restricting part  83 . The check valve  84  allows passage of hydraulic oil that flows from the first actuating part  61  to the downstream and blocks passage of hydraulic oil that flows from the downstream to the first actuating part  61 . 
     The third flow passage  73   c  is a flow passage that connects the second flow passage  73   b  and the pressure receiving part  65   a . One end of the third flow passage  73   c  is connected to a section of the second flow passage  73   b  that is located between the restricting part  83  and the check valve  84 . 
     According to the above arrangement, hydraulic oil (pilot oil) can be supplied to the pressure receiving part  65   a  through the first flow passage  73   a  and the third flow passage  73   c  in a state where braking of the travelling motor  51  is released by the braking mechanism  52 . In a case where the first actuating part  61  is switched to the second position  61   b  in the state where braking of the travelling motor  51  is released, the travelling motor  51  can be shifted from the first speed to the second speed. 
     In the embodiment described above, the first oil passage  71  is connected to the travelling motor (first hydraulic device)  51 . Instead, the fourth oil passage  74  may be connected to the travelling motor (first hydraulic device)  51  as illustrated in  FIG.  2   . Specifically, the fourth oil passage  74  has a first flow passage  74   a  that connects the second actuating part  62  and the braking mechanism (second hydraulic device)  52  and a second flow passage  74   b  that connects the second actuating part  62  and the travelling motor (first hydraulic device)  51 . In the second flow passage  74   b , the restricting part  80 , the second oil passage  72 , and the swash plate switching cylinder  53  are provided. Therefore, hydraulic oil can be supplied to the travelling motor (first hydraulic device)  51  in a state where braking is released by the braking mechanism  52  by switching the second actuating part  62  to the second position  62   b . That is, in the hydraulic system of  FIG.  2   , the speed of the travelling motor  51  can be changed in a case where braking is released and travelling is possible. 
     Second Embodiment 
     A second embodiment is a modification of the hydraulic system.  FIG.  3    illustrates modified parts of the hydraulic system of the second embodiment as comparable to the first embodiment. Other parts of the second embodiment that are not shown in  FIG.  3    are similar to those in the first embodiment. In the description of second embodiment set forth below, structures that are different from that of the first embodiment are described. 
     As illustrated in  FIG.  3   , a first hydraulic device is a braking mechanism  52  for braking the travelling motor, and a second hydraulic device is a travelling pump  50 . A first oil passage  71  includes a first supply passage  71   a , a second supply passage  71   b , a fifth supply passage  71   e , and a sixth supply passage  71   f . The fifth supply passage  71   e  connects a first hydraulic pump P 1  and a containing part (containing case)  52   a  of the braking mechanism  52 . The sixth supply passage  71   f  is connected to the fifth supply passage  71   e . The sixth supply passage  71   f  is an oil passage through which hydraulic oil in the fifth supply passage  71   e  can be discharged. A hydraulic oil tank  31  is connected to the sixth supply passage  71   f.    
     A pilot check valve  65  having a pressure receiving part  65   a  is provided at an intermediate part of the sixth supply passage  71   f.    
     A first actuating part is remote control valves (remote control valves  36 ,  37 ,  38 , and  39 ) that change the pressure of hydraulic oil in accordance with an operation of a travelling lever  40 . For convenience of description, the following description takes the remote control valve  36  and the remote control valve  37  as an example. Note that the remote control valves are not limited to the ones illustrated in the embodiment. The first actuating part (remote control valves) and the travelling pump (second hydraulic device)  50  are connected to each other by a fifth oil passage  75 . The fifth oil passage  75  includes a second operation oil passage  75   b  that connects the remote control valve  36  and a pressure receiving part for forward travelling  50   a  of the travelling pump  50  and a fourth operation oil passage  75   d  that connects the remote control valve  37  and a pressure receiving part for backward travelling  50   b  of the travelling pump  50 . 
     The fifth oil passage  75  and a third oil passage  73  are connected to each other. The third oil passage  73  has a fourth flow passage  73   d  that is connected to the second operation oil passage  75   b  and a fifth flow passage  73   e  that is connected to the fourth operation oil passage  75   d . A check valve  85  is connected to an intermediate part of each of the fourth flow passage  73   d  and the fifth flow passage  73   e . The fourth flow passage  73   d  and the fifth flow passage  73   e  merge with each other, and a merged side thereof is connected to the pressure receiving part  65 . Note that a high-pressure selection valve that selects a higher pressure may be used instead of the check valve  85 . A bleed-off circuit (bleed-off oil passage)  88 , i.e., a sixth oil passage  88  through which hydraulic oil is discharged is provided in an oil passage into which the fourth flow passage  73   d  and the fifth flow passage  73   e  merge. 
     According to the above configuration, when the first actuating part (remote control valve) is operated, the travelling pump (second hydraulic device)  50  works in accordance with pressure that is output from the remote control valve. In this case, the pressure of hydraulic oil not only in the fifth oil passage  75 , but also in the third oil passage  73  (the fourth flow passage  73   d  and the fifth flow passage  73   e ) rises. When the pressure of the hydraulic oil in the third oil passage  73  rises, the pilot check valve  65  closes (blocks flow of the hydraulic oil in the sixth supply passage  71   f  to the hydraulic oil tank  31 ). As a result, the pressure of the sixth supply passage  71   f  increases, and the braking mechanism  52  releases braking. That is, according to the second embodiment, it is possible to automatically release braking in a case where a travelling operation is performed by using the travelling lever  40 . 
     When an operation of the first actuating part (remote control valve) is stopped, the hydraulic oil in the third oil passage  73  (the fourth flow passage  73   d  and the fifth flow passage  73   e ) is discharged to the hydraulic oil tank  31  through the bleed-off circuit (bleed-off oil passage)  88 , and the pressure in the third oil passage  73  drops accordingly. The braking mechanism  52  performs a braking action. That is, it is possible to automatically perform a braking action in a case where the travelling operation of the travelling lever  40  is stopped. 
     Although the first actuating part that is capable of changing the amount of flow of hydraulic oil to the third oil passage is the remote control valves (remote control valves  36 ,  37 ,  38 , and  39 ), the remote control valves need not be directly connected to a travelling member (travelling lever  40 ). Furthermore, the hydraulic system may have a structure such that a swash plate of the travelling pump  50  is operated directly by the pressure of hydraulic oil discharged from the remote control valves. 
     Although the first hydraulic device is the braking mechanism  52  and the second hydraulic device is the travelling pump  50  in the above description, the first hydraulic device and the second hydraulic device are not limited to the braking mechanism  52  and the travelling pump  50 . That is, the second hydraulic device can be any hydraulic device that works in accordance with the pressure of hydraulic oil that is output from a remote control valve. This allows the first hydraulic device to work in a case where the operating member (remote control valve) that causes the second hydraulic device to work is operated. That is, the first hydraulic device and the second hydraulic device can be caused to work in conjunction with an operation of the operating member. 
     Third Embodiment 
     A third embodiment is a modification of the hydraulic system.  FIG.  4    illustrates modified parts of the hydraulic system of the third embodiment as comparable to the second embodiment. Other parts of the third embodiment that are not shown in  FIG.  4    are similar to those in the first embodiment or the second embodiment. 
     As illustrated in  FIG.  4   , a first hydraulic device is a braking mechanism  52 , and a second hydraulic device is a travelling pump  50 . Remote control valves (remote control valves  36  and  37 ) are electromagnetic proportional valves whose degrees of opening are changed on the basis of a control signal that is output from a control device  64 . That is, in the second embodiment, an operation of the travelling lever  40  directly acts on the remote control valves, and thus the remote control valves change the pressure of hydraulic oil. Meanwhile, the remote control valves in the third embodiment are valves that electrically act. For convenience of description,  FIG.  4    illustrates the remote control valves  36  and  37 . However, the following description can also be applied to other remote control valves  38  and  39  and hydraulic devices and the like that correspond to the remote control valves  38  and  39 . 
     An operating member  101  is connected to the control device  64 . The operating member  101  is a travelling lever that is swingable. In a case where the travelling lever  101  is operated, the amount of operation and/or a direction of swing are input to the control device  64 . The control device  64  supplies a control signal corresponding to the amount of swing that is output from the travelling lever  101  to a remote control valve. The degree of opening of the remote control valve changes in accordance with the control signal supplied from the control device  64 . Therefore, the travelling pump  50  can be normally rotated or reversely rotated as in the above embodiments by operating the travelling pump  101 . 
     As illustrated in  FIG.  4   , a first switching valve  102  is connected to a bleed-off circuit (sixth oil passage)  88 . The switching valve  102  is a two-position switch-over valve whose position can be switched between a first position  102   a  and a second position  102   b  and is a hydraulic lock valve for hydraulic locking. The hydraulic lock valve  102  is switched, for example, by the control device  64 . A switch  103  is connected to the control device  64 . In a case where the switch  103  is on, the control device  64  demagnetizes a solenoid of the hydraulic lock valve  102 . The hydraulic lock valve  102  is at the first position  102   a  in the state where the solenoid thereof is demagnetized. Accordingly, hydraulic oil in the third oil passage  73  (a fourth flow passage  73   d  and a fifth flow passage  73   e ) is discharged to a hydraulic oil tank  31  through the sixth oil passage  88 . As a result, the pressure in the third oil passage  73  drops, and the braking mechanism  52  performs a braking action. That is, a braking action can be performed in conjunction with hydraulic locking by turning the switch  103  for hydraulic locking on. 
     In a case where the switch  103  is off, the control device  64  excites the solenoid of the hydraulic lock valve  102 . The hydraulic lock valve  102  is at the second position  102   b  in the state where the solenoid thereof is excited. Accordingly, the hydraulic oil in the third oil passage  73  does not flow through the sixth oil passage  88 . As a result, the pressure in the third oil passage  73  rises, and the braking mechanism  52  releases braking. That is, braking can be released in conjunction with hydraulic locking by turning the switch  103  off. 
     Fourth Embodiment 
     A fourth embodiment is a modification of the hydraulic system.  FIG.  5    illustrates modified parts of the hydraulic system of the fourth embodiment as comparable to the second embodiment. Other parts of the fourth embodiment that are not shown in  FIG.  5    are similar to those in the first embodiment or the second embodiment. 
     As illustrated in  FIG.  5   , a first hydraulic device is a braking mechanism  52 , and a second hydraulic device is a travelling pump  50 . A first actuating part is an electromagnetic proportional valve  105  that is provided on an upstream side of remote control valves (remote control valves  36  and  37 ). For convenience of description, the remote control valves  36  and  37  are illustrated in the fourth embodiment, as illustrated in  FIG.  5   . However, the following description can also be applied to other remote control valves  38  and  39  and hydraulic devices and the like that correspond to the remote control valves  38  and  39 . 
     The electromagnetic proportional valve  105  is, for example, a control valve (anti-stall control valve) that prevents engine stall. The degree of opening of the electromagnetic proportional valve  105  is changed by a control device  64 . Anti-stall control performed by the control device  64  and the electromagnetic proportional valve  105  is described below. 
     The control device  64  prevents engine stall by changing the degree of opening of the electromagnetic proportional valve  105  on the basis of a drop amount of an engine, which is a difference between target rotational speed of the engine and actual rotational speed of the engine. Note that the control device  64  is capable of acquiring the actual rotational speed of the engine and the target rotational speed of the engine. 
       FIG.  6    illustrates a relationship among engine rotational speed, travelling primary pressure, and controls lines L 1  and L 2 . 
     The travelling primary pressure is pressure (pilot pressure) of hydraulic oil in a second supply passage  71   b  from the electromagnetic proportional valve  105  to the remote control valves. That is, the travelling primary pressure is primary pressure of hydraulic oil that enters the remote control valves provided in the travelling lever  40  for a travelling operation. The control line L 1  indicates a relationship between the engine rotational speed and the travelling primary pressure in a case where the drop amount is less than a predetermined value. The control line L 2  indicates a relationship between the engine rotational speed and the travelling primary pressure in a case where the drop amount is equal to or larger than the predetermined value. 
     In a case where the drop amount is less than the predetermined value, the control device  64  adjusts the degree of opening of the electromagnetic proportional valve  105  so that the relationship between the actual rotational speed of the engine and the travelling primary pressure coincides with the control line L 1 . In a case where the drop amount is equal to or larger than the predetermined value, the control device  64  adjusts the degree of opening of the electromagnetic proportional valve  105  so that the relationship between the actual rotational speed of the engine and the travelling primary pressure coincides with the control line L 2 . On the control line L 2 , the travelling primary pressure with respect to predetermined engine rotational speed is lower than that on the control line L 1 . That is, the travelling primary pressure of the control line L 2  is lower than that of the control line L 1  with respect to the same engine rotational speed. Accordingly, the pressure (pilot pressure) of the hydraulic oil that enters the remote control valves is kept low by control based on the control line L 2 . As a result, a swash plate of an HST pump  50  is adjusted, and load that acts on an engine  29  decreases. It is therefore possible to prevent stall of the engine  29 . Note that although a single control line L 2  is illustrated in  FIG.  6   , the number of control lines L 2  may be more than one. For example, the control line L 2  may be set for each engine rotational speed. 
     It is preferable that the control device  64  has data indicative of the control line L 1  and the control line L 2 , control parameters of a function or the like. 
     A third oil passage  73  is connected to a section  71   b   1  of the second supply passage  71   b  that is located between the electromagnetic proportional valve  105  and the remote control valves. With the arrangement, since the pressure in the third oil passage  73  rises while the electromagnetic proportional valve  105  is in operation, braking can be released in conjunction with anti-stall control. Note that although an anti-stall control valve is used as an example of the electromagnetic proportional valve  105  in the present embodiment, the electromagnetic proportional valve  105  need not be an anti-stall control valve. 
     Fifth Embodiment 
     A fifth embodiment is an embodiment in which a pilot check valve  110  is used instead of the pilot check valve  65 , as illustrated in  FIG.  7   . 
     The pilot check valve  110  has a pressure receiving part (a pilot port)  110   a  that receives the pressure of hydraulic oil. In a case where the pressure of the hydraulic oil applied to the pressure receiving part  110   a  is equal to or higher than a predetermined value (a fourth pressure threshold), the pilot check valve  110  opens. That is, the pilot check valve  110  allows discharge of the hydraulic oil in a second oil passage when the pressure receiving part  110   a  receives the pressure of the hydraulic oil. Meanwhile, in a case where the pressure of the hydraulic oil applied to the pressure receiving part  110   a  is less than the predetermined value (the fourth pressure threshold), the pilot check valve  110  closes. That is, the pilot check valve  110  blocks discharge of the hydraulic oil in the second oil passage when the pressure receiving part  110   a  does not receive the pressure of the hydraulic oil. In the present embodiment, the sixth supply passage  71   f  described in the first embodiment is the second oil passage. 
     The pilot check valve  110  is provided at an intermediate part of the second oil passage (sixth supply passage  71   f ), as illustrated in  FIG.  7   . A relief valve  111  is provided on a downstream side of the pilot check valve  110 . A first hydraulic device is a braking mechanism  52 . A first oil passage  71  has a fifth supply passage  71   e  that connects a first hydraulic pump P 1  and a containing part (containing case)  52   a  of the braking mechanism  52  and a second supply passage  71   b  connected to the fifth supply passage  71   e . A braking valve  62  for braking or release of braking of the braking mechanism  52  is provided at an intermediate part of the fifth supply passage  71   e . Note that the braking valve  62  is similar to the second actuating part of the first embodiment. 
     On an upstream side of the braking valve  62 , the fifth supply passage  71   e  is branched, and a switching valve (second switching valve)  113  is connected to a branched oil passage  112 . A third oil passage  73  that is connected to the pilot check valve  110  is connected to the switching valve  113 . The switching valve  113  is a two-position switch-over valve whose position can be switched between a first position and a second position. 
     The switching valve  113  can be switched by a control device  64  or the like. For example, a switch or the like is connected to the control device  64 , and in a case where the switch is turned on, the control device  64  excites a solenoid of the switching valve  113  so that the switching valve  113  is switched to the second position. In a case where the switch is turned off, the control device  64  demagnetizes the solenoid of the switching valve  113  so that the switching valve  113  is switched to the first position. Therefore, the pressure of hydraulic oil that acts on the pilot check valve  110  (a flow amount of hydraulic oil flowing through the third oil passage  73 ) can be changed by operating the switch connected to the control device  64 . As a result, primary pressure of hydraulic oil that acts on remote control valves (remote control valve  36 ,  37 ) of a travelling operation device  14  can be changed. For convenience of description, the remote control valves  36  and  37  are illustrated in the fifth embodiment, as illustrated in  FIG.  7   . However, the above description can also be applied to other remote control valves  38  and  39  and hydraulic devices and the like that correspond to the remote control valves  38  and  39 . 
     Sixth Embodiment 
     A sixth embodiment is a modification of the hydraulic system.  FIG.  8    illustrates a case where a pilot check valve  110  is applied to a hydraulic system of a work system. First, the hydraulic system of the work system is described. Note that the hydraulic system of the work system is not limited to a configuration described below. 
     As illustrated in  FIG.  8   , the hydraulic system of the work system is a system for actuating a boom  22 L, a boom  22 R, a bucket  23  (working tool), and the like and includes a plurality of operation control valves  115  and a hydraulic pump (second hydraulic pump) P 2  of the work system. 
     The second hydraulic pump P 2  is a pump that is provided at a different position from the first hydraulic pump P 1  and is realized by a fixed displacement gear pump. The second hydraulic pump P 2  can discharge hydraulic oil stored in a hydraulic oil tank  22 . In particular, the second hydraulic pump P 2  mainly discharges hydraulic oil for actuating a hydraulic actuator. 
     A main oil passage (oil passage)  116  is provided on a discharge side of the second hydraulic pump P 2 . The plurality of operation control valves  115  are connected to the main oil passage  116 . The operation control valves  115  are valves that can change a direction of flow of hydraulic oil by pilot pressure of pilot oil. 
     The plurality of operation control valves  115  include a first operation control valve  115 A and a second operation control valve  115 B. The first operation control valve  115 A is a valve that controls a lift cylinder  26 . The second operation control valve  115 B is a valve that controls a tilt cylinder  28 . 
     The first operation control valve  115 A and the second operation control valve  115 B are pilot-type direct-acting spool type three-position switch-over valves. The first operation control valve  115 A and the second operation control valve  115 B are switched among a first position, a neutral position, and a second position by the pilot pressure. 
     The lift cylinder  26  is connected to the first operation control valve  115 A via oil passages  151  and  152 , and the tilt cylinder  28  is connected to the second operation control valve  115 B via oil passages. 
     The boom  22 L, the boom  22 R, and the bucket  23  can be operated by a work operation device  119  that is provided around a driver&#39;s seat  13 . The work operation device  119  includes an operating member  120  and a plurality of remote control valves  121 ,  122 ,  123 , and  124 . The operating member  120  is an operation lever that is supported so as to be tiltable from a neutral position in forward, backward, leftward, rightward, and oblique directions. By tilting the operation lever  120 , the plurality of remote control valves  121 ,  122 ,  123 , and  124  that are provided below the operation lever  120  can be operated. The remote control valves  121 ,  122 ,  123 , and  124  and the first hydraulic pump P 1  are connected to each other by a discharge oil passage  71 . 
     The plurality of remote control valves  121 ,  122 ,  123 , and  124  and the plurality of operation control valves  115  are connected to each other by a plurality of oil passages  125   a ,  125   b ,  125   c , and  125   d . Specifically, the remote control valve  121  is connected to the first operation control valve  115 A via the first operation oil passage  125   a . The remote control valve  122  is connected to the first operation control valve  115 A via the second operation oil passage  125   b . The remote control valve  123  is connected to the second operation control valve  115 B via the third operation oil passage  125   c . The remote control valve  124  is connected to the second operation control valve  115 B via the fourth operation oil passage  125   d . Each of the remote control valves  121 ,  122 ,  123 , and  124  can set the pressure of output hydraulic oil in accordance with an operation of the operation lever  120 . 
     Specifically, when the operation lever  120  is tilted forward, the remote control valve for downward movement  121  is operated, and pilot pressure of pilot oil that is output from the remote control valve for downward movement  121  is set. This pilot pressure acts on a pressure receiving part (a pilot port) of the first operation control valve  115 A. As a result, the lift cylinder  26  contracts, and the booms  22 L and  22 R move downward. 
     When the operation lever  120  is tilted backward, the remote control valve for upward movement  122  is operated, and pilot pressure of pilot oil that is output from the remote control valve for upward movement  122  is set. This pilot pressure acts on the pressure receiving part of the first operation control valve  115 A. As a result, the lift cylinder  26  stretches, and the booms  22 L and  22 R move upward. 
     When the operation lever  120  is tilted rightward, the remote control valve for bucket dumping  123  is operated, and pilot pressure of pilot oil that is output from the remote control valve  123  is set. This pilot pressure acts on a pressure receiving part (a pilot port) of the second operation control valve  115 B. As a result, the tilt cylinder  28  stretches, and the bucket  23  performs a dumping action. 
     When the operation lever  120  is tilted leftward, the remote control valve for bucket scooping  124  is operated, and pilot pressure of pilot oil that is output from the remote control valve  124  is set. This pilot pressure acts on the pressure receiving part of the second operation control valve  115 B. As a result, the tilt cylinder  28  contracts, and the bucket  23  performs a scooping action. 
     A float control device  130  that can bring the lift cylinder  26  into a float state is provided as illustrated in  FIG.  8   . The float control device  130  includes a first float control valve  131 , a second float control valve  132 , and a float switching valve  133 . 
     Each of the first float control valve  131  and the second float control valve  132  is a two-position switch-over valve whose position can be switched between a first position where passage of hydraulic oil is blocked and a second position where passage of hydraulic oil is allowed. The first float control valve  131  and the second float control valve  132  are connected to the float switching valve  133  via an oil passage  141 . The first float control valve  131  is connected to the oil passage (a first additional oil passage)  151  that connects a bottom side (a bottom chamber) of the lift cylinder  26  and the first operation control valve  115 A. Furthermore, the first float control valve  131  is connected to an oil passage  153  that discharges hydraulic oil. In a case where the first float control valve  131  is at the second position, the oil passage  151  and the oil passage  153  are communicated with each other, whereas in a case where the first float control valve  131  is at the first position, communication between the oil passage  151  and the oil passage  153  is blocked. 
     The second float control valve  132  is connected to the oil passage (a second additional oil passage)  152  that connects a rod side (a rod chamber) of the lift cylinder  26  and the first operation control valve  115 A. Furthermore, the second float control valve  132  is connected to the oil passage  153 . In a case where the second float control valve  132  is at the second position, the oil passage  152  and the oil passage  153  are communicated with each other, whereas in a case where the second float control valve  132  is at the first position, communication between the oil passage  152  and the oil passage  153  is blocked. 
     Therefore, by placing the first float control valve  131  and the second float control valve  132  at the second position, the bottom side and the rod side of the lift cylinder  26  are communicated with the oil passage (discharge oil passage)  153 , and therefore the lift cylinder  26  can be brought into a float state. By placing the first float control valve  131  and the second float control valve  132  at the first position, the lift cylinder  26  is brought into a state that is not the float state, and thus the lift cylinder  26  can be moved upward or downward by the first operation control valve  115 A. 
     The first float control valve  131  and the second float control valve  132  can be switched by the float switching valve  133  and the control device  64 . The float switching valve  133  is a two-position switch-over valve whose position can be switched between a first position and a second position. For example, a switch or the like is connected to the control device  64 , and in a case where the switch is turned on, the control device  64  excites a solenoid of the float switching valve  133  so that the float switching valve  133  is switched to the second position. This causes the pressure of hydraulic oil discharged from the first hydraulic pump P 1  to be output to the oil passage  141 . In this way, the first float control valve  131  and the second float control valve  132  can be switched to the second position. Meanwhile, in a case where the switch is turned off, the control device  64  demagnetizes the solenoid of the float switching valve  133  so that the float switching valve  133  is switched to the first position, and thus hydraulic oil in the oil passage  141  is discharged to the hydraulic oil tank  31  or the like. In this way, the first float control valve  131  and the second float control valve  132  can be switched to the first position. 
     As illustrated in  FIG.  8   , a first actuating part that causes hydraulic oil to act on the pressure receiving part  110   a  of the pilot check valve  110  is the float switching valve  133 . A first hydraulic device is the first operation control valve  115 A for actuating the lift cylinder  26 , and a first oil passage includes the discharge oil passage  71  and the first operation oil passage  125   a.    
     A second oil passage  72  is connected to the first operation oil passage  125   a  that connects the remote control valve  121  and the first operation control valve  115 A. That is, the second oil passage  72  is connected to the first operation oil passage (lowering operation oil passage)  125   a  for giving a command to perform an operation of lowering the lift cylinder  26  to the operation control valve  115 A. The pilot check valve  110  is provided in this second oil passage  72 . A third oil passage  73  that is connected to the pressure receiving part  110   a  of the pilot check valve  110  is connected to the oil passage  141 . 
     Therefore, during the float operation, hydraulic oil acts on the pressure receiving part  110   a  of the pilot check valve  110 . That is, in a case where an operation of lowering the booms  22 L and  22 R is performed during the float operation, it is possible to prevent the operation control valve  115 A from working. It is therefore possible to eliminate wasteful use of hydraulic oil. 
     Note that the first hydraulic device may be the second operation control valve  115 B that controls a hydraulic actuator such as the tilt cylinder  28  or may be a different operation control valve. The first oil passage is applicable not only to the first operation oil passage  125   a , but also to any of the second operation oil passage  125   b , the third operation oil passage  125   c , and the fourth operation oil passage  125   d . In this case, the second oil passage  72  is also applicable to any of the second operation oil passage  125   b , the third operation oil passage  125   c , and the fourth operation oil passage  125   d.    
     Seventh Embodiment 
     A seventh embodiment is a modification of the hydraulic system.  FIG.  9    illustrates modified parts of the hydraulic system of the seventh embodiment as comparable to the sixth embodiment. Other parts of the seventh embodiment that are not shown in  FIG.  9    are similar to those in the sixth embodiment. 
     As illustrated in  FIG.  9   , a plurality of second oil passages  72  are connected to a first oil passage (first operation oil passage)  125   a . In the present embodiment, two second oil passages  72  are connected to the first operation oil passage  125   a.    
     Of the two second oil passages  72 , a pilot check valve  110  is connected to one second oil passage  72   a , and a pilot check valve  110  is also connected to the other second oil passage  72   b . A check valve  160  that can change set pressure (differential pressure) is provided in the second oil passage  72   a.    
     For convenience of description, the pilot check valve  110  that is connected to the second oil passage  72   a  is hereinafter referred to as a first pilot check valve  110 A, and the pilot check valve  110  that is connected to the second oil passage  72   b  is hereinafter referred to as a second pilot check valve  110 B. Furthermore, a third oil passage  73  that is connected to the first pilot check valve  110 A is hereinafter referred to as an oil passage  73   a , and a third oil passage  73  that is connected to the second pilot check valve  110 B is hereinafter referred to as an oil passage  73   b.    
     A switching valve  161  is connected to the oil passage  73   a , and a switching valve  162  is connected to the oil passage  73   b . A fourth switching valve (the switching valve  161  and the switching valve  162 ) is connected to a discharge oil passage  71  that is connected to a first hydraulic pump P 1 . Each of the switching valve  161  and the switching valve  162  is a two-position switch-over valve whose position can be switched between a first position and a second position. Therefore, by switching the switching valve  161  and the switching valve  162 , the pressure of hydraulic oil that acts on the first pilot check valve  110 A and the second pilot check valve  110 B (a flow amount of hydraulic oil flowing through the third oil passage  73 ) can be changed. That is, the switching valve  161  and the switching valve  162  are a first actuating part that can change the pressure of the hydraulic oil that acts on the pilot check valves  110 . 
     The first actuating part (the switching valve  161  and the switching valve  162 ) can be switched by a control device  64  or the like. For example, a switch or the like that can be switched among three positions is connected to the control device  64 . In a case where the switch is at a first position, the control device  64  excites a solenoid of the switching valve  161  and demagnetizes a solenoid of the switching valve  162 . This causes the switching valve  161  to be at the second position and causes the switching valve  162  to be at the first position, and therefore part of hydraulic oil in the first operation oil passage  125   a  can be discharged by the first pilot check valve  110 A. In a case where the switch is at a second position, the control device  64  excites the solenoid of the switching valve  162  and demagnetizes the solenoid of the switching valve  161 . This causes the switching valve  161  to be at the first position and causes the switching valve  162  to be at the second position, and therefore part of the hydraulic oil in the first operation oil passage  125   a  can be discharged by the second pilot check valve  110 B. In a case where the switch is at a third position, the switching valve  161  and the switching valve  162  are at the first position. 
     Therefore, the pressure of the hydraulic oil that acts on the first operation oil passage  125   a  (a flow amount of hydraulic oil flowing through the third oil passage  73 ) can be changed by the first pilot check valve  110 A, the second pilot check valve  110 B, and the check valve  160 . Note that the number of pilot check valves  110 , the number of check valves  160 , and the number of switching valves  161  and  162  are not limited to the ones described in the embodiment. Although the plurality of second oil passages  72  are provided in the first operation oil passage  125   a , the second oil passages  72  may be provided in a different oil passage. 
     Eighth Embodiment 
     An eighth embodiment is a modification of the hydraulic system.  FIG.  10    illustrates modified parts of the hydraulic system of the eight embodiment as comparable to the sixth embodiment. Other parts of the eight embodiment that are not shown in  FIG.  10    are similar to those in the sixth embodiment. 
     As illustrated in  FIG.  10   , a base oil passage  70  connects a first hydraulic pump P 1  and remote control valves  121  and  122 . A first oil passage  71  branches off from the base oil passage  70  and includes a seventh supply passage  71   g  that connects a ride control device  170  and the first hydraulic pump P 1 . The ride control device  170  is a device that performs a vibration control operation for suppressing a pressure fluctuation of a hydraulic actuator. In the present embodiment, the ride control device  170  is a device that controls vibration of a lift cylinder  26  (a boom  22 L and a boom  22 R). The ride control device  170  includes a vibration control switching valve  171  and a vibration control part  172  that performs a vibration control operation. The vibration control switching valve  171  is a two-position switch-over valve whose position can be switched between a first position and a second position. The vibration control switching valve  171  can be switched by a control device  64  or the like. For example, a switch or the like is connected to the control device  64 , and in a case where the switch is turned on, the control device  64  excites a solenoid of the vibration control switching valve  171  so that the vibration control switching valve  171  is switched to the second position. In a case where the switch is turned off, the control device  64  demagnetizes the solenoid of the vibration control switching valve  171  so that the vibration control switching valve  171  is switched to the first position. In a case where the vibration control switching valve  171  is at the second position, hydraulic oil acts on the vibration control part  172 , thereby causing the vibration control part  172  to control vibration of the lift cylinder  26 . In a case where the vibration control switching valve  171  is at the first position, hydraulic oil does not act on the vibration control part  172 , and therefore vibration control of the lift cylinder  26  by the vibration control part  172  stops. 
     A second oil passage  72  is connected to the seventh supply passage  71   g . A pilot check valve  110  is connected to an intermediate part of the second oil passage  72 . A third oil passage  73  is connected to the pilot check valve  110 . The third oil passage  73  has a sixth flow passage  73   f  that is connected to a first operation oil passage  125   a  and a seventh flow passage  73   g  that is connected to a second operation oil passage  125   b . A check valve  173  is connected to intermediate parts of the sixth flow passage  73   f  and the seventh flow passage  73   g . The sixth flow passage  73   f  and the seventh flow passage  73   g  merge with each other, and a merged side thereof is connected to a pressure receiving part  110   a . Note that a high-pressure selection valve that selects a higher pressure may be used instead of the check valve  173 . 
     In the eighth embodiment, a first hydraulic device is the ride control device  170 , and a first operating valve is remote control valves  121  and  122 . Therefore, an operation of controlling vibration of the lift cylinder  26  can be performed by switching the vibration control switching valve  171 , and the vibration control operation can be released in a case where an operation lever  120  is operated. 
     Ninth Embodiment 
     A ninth embodiment is a modification of the hydraulic system.  FIG.  11    illustrates modified parts of the hydraulic system of the ninth embodiment as comparable to the ninth embodiment. Other parts of the ninth embodiment that are not shown in  FIG.  11    are similar to those in the above embodiments and the like. 
     As illustrated in  FIG.  11   , the hydraulic system includes an operation device  181  that can perform both a travelling operation and a work operation by a single operating member  180 . The operation device  181  has a remote control valve for forward travelling  36 , a remote control valve for backward travelling  37 , a remote control valve for downward movement  121 , and a remote control valve for upward movement  122 . The remote control valve  36  and the remote control valve  37  are connected to an HST pump  50 , and the remote control valve  121  and the remote control valve  122  are connected to a first operation control valve  115 . 
     Specifically, the remote control valve  36  is connected to the HST pump  50  via a first operation oil passage  75   a . The remote control valve  37  is connected to the HST pump  50  via a third operation oil passage  75   c . The remote control valve  121  is connected to a first operation control valve  115 A via a first operation oil passage  125   a . The remote control valve  122  is connected to the first operation control valve  115 A via a second operation oil passage  125   b . For convenience of description, operation oil passages of a travelling system (the first operation oil passage  75   a  and the third operation oil passage  75   c ) are hereinafter referred to as a first travelling oil passage  75   a  and a second travelling oil passage  75   c.    
     A second oil passage  72  is connected to the first travelling oil passage  75   a  and the second travelling oil passage  75   c . The second oil passage  72  has an oil passage  72   c  that is connected to the first travelling oil passage  75   a , an oil passage  72   d  that is connected to the second travelling oil passage  75   c , and an oil passage  72   e  into which the oil passage  72   c  and the oil passage  72   d  merge. A check valve  183  is provided in the oil passage  72   c  and the oil passage  72   d.    
     A pilot check valve  110  is connected to the oil passage  72   e . A switching valve (third switching valve)  182  is connected to a third oil passage  73  that is connected to the pilot check valve  110 . The switching valve  182  is a two-position switch-over valve whose position can be switched between a first position and a second position. Therefore, the pressure of the hydraulic oil that acts on the pilot check valve  110  can be changed by switching the switching valve  182 . The switching valve  182  is a first actuating part. 
     The switching valve  182  can be switched by a control device  64  or the like. For example, a switch or the like is connected to the control device  64 . In a case where the switch is turned on, the control device  64  excites a solenoid of the switching valve  182 . This causes the switching valve  182  to be at the second position, and therefore part of hydraulic oil in the operation oil passages of the travelling system (the first operation oil passage  75   a  and the third operation oil passage  75   c ) can be discharged by the first pilot check valve  110 . In a case where the switch is turned off, the control device  64  demagnetizes the solenoid of the switching valve  182 . In this way, the HST pump  50  can be operated as usual by an operation of the operating member  180 . 
     The embodiments disclosed herein are given only for illustration and should not be construed as being restrictive. The scope of the present invention is indicated not by the above description but by the claims, and it is intended that meanings equivalent to the scope of the claims and all changes within the scope are encompassed within the embodiments of the present invention. In the above embodiments, the first actuating part  61  and the second actuating part  62  are two-position switch-over valves. Instead, the first actuating part  61  and the second actuating part  62  may be proportional valves. In the above embodiments, the travelling motor  51  and the braking mechanism  52  are described as hydraulic devices. However, the hydraulic devices may be any devices that work by the pressure of hydraulic oil. In the above embodiments, a travelling motor is a motor that is switched between the first speed and the second speed (motor whose travelling state is changed). However, the stages between which the travelling motor is switched is not limited to the first speed and the second speed. In the above embodiments, hydraulic oil is discharged to a hydraulic oil tank. However, hydraulic oil may be discharged to a different place. That is, an oil passage for discharging hydraulic oil may be connected to a place other than a hydraulic oil tank. For example, an oil passage for discharging hydraulic oil may be connected to a sucking part of a hydraulic pump (part that sucks in hydraulic oil) or may be connected to a different part. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.