Patent Publication Number: US-6708490-B2

Title: Hydraulic system for construction machine

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a hydraulic system for a construction machine. 
     2. Description of the Related Art 
     In a conventional hydraulic circuit of a hydraulic excavator there has been a problem of interference between pressure oil fed to a traveling motor and pressure oil fed to a working actuator in case of performing both travel using the traveling motor and work using the working actuator. In this case, it is difficult to maintain the operation speed of the traveling motor, i.e., the traveling speed of the hydraulic excavator, stably at a desired relatively low speed. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a hydraulic system for a construction machine capable of preventing the occurrence of interference between pressure oil fed to traveling motors and pressure oil fed to working actuators and smoothly performing a work by operation of the working actuators under travel at a stable speed particularly in case of carrying out both travel and work by the working actuators at a time. 
     A hydraulic system for a construction machine according to the present invention, comprising: a first traveling motor and a second traveling motor adapted to actuate a pair of travel devices; actuators adapted to actuate working attachments including a boom and an arm; a first hydraulic pump and a second hydraulic pump adapted to supply pressure oil for actuating said first and second traveling motors and said actuators; a first traveling control valve and a second traveling control valve adapted to control amount of pressure oil to be supplied to said first and second traveling motors in accordance with operation of operating means for the first and second traveling motors; working control valves provided correspondingly to said actuators, said working control valves being classified into a first group including said first traveling control valve and a second group including said second traveling control valve, bleed-off passages in all the control valves belonging to said first group being mutually communicated in series as a first center bypass passage toward an oil tank when all the control valves are in their neutral positions, and bleed-off passages in all the control valves belonging to said second group being mutually communicated in series as a second center bypass passage toward an oil tank when all the control valves are in their neutral positions; a straight-travel valve adapted to switch each flowing direction of pressure oil discharged from said first and second hydraulic pumps, said straight-travel valve supplying pressure oil discharged from said first and second hydraulic pumps to said first and second bypass passages respectively when all of said traveling motors and said actuators are not in operation, while in a simultaneous operation mode in which the traveling motor and the actuator associated with the traveling control valve and the working control valve belonging to one of said first and second groups are operated simultaneously, supplying pressure oil discharged from one of said first and second hydraulic pumps to both said first and second traveling control valves and further supplying pressure oil discharged from the other hydraulic pump to the working control valve; and a cut-off valve and an opening valve provided on a downstream side of each of the bleed-off passages in said traveling control valves, said cut-off valve cutting off the center bypass passage located between the traveling control valve and the working control valve associated with the traveling motor and the actuator which are in operation in said simultaneous operation mode of the traveling motor and the actuator being operated simultaneously, said opening valve causing a downstream side of the bleed-off valve in said traveling control valve to be opened to the oil tank. 
     In this case, the hydraulic system is capable of preventing the occurrence of interference between pressure oil fed to traveling motors and pressure oil fed to working actuators and smoothly performing a work by operation of the working actuators under traveling at a stable speed particularly while carrying out both traveling and work by the working actuators at a time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit configuration diagram of a hydraulic system according to a first embodiment of the present invention; 
     FIG. 2 is a block diagram showing the construction of a control system used in the hydraulic system of FIG. 1; 
     FIG. 3 is a flow chart showing a processing carried out by a controller in the control system of FIG. 2; 
     FIGS. 4A,  4 B are diagrams for explaining the operation of the hydraulic system of FIG. 1; 
     FIGS. 5A,  5 B are diagrams for explaining the operation of the hydraulic system of FIG. 1; 
     FIGS. 6A,  6 B are diagrams for explaining the operation of the hydraulic system of FIG. 1; 
     FIG. 7 is a diagram for explaining the operation of the hydraulic system of FIG. 1; 
     FIG. 8 is a circuit configuration diagram of a hydraulic system according to a second embodiment of the present invention; 
     FIG. 9 is a diagram for explaining the operation of the hydraulic system of FIG. 8; 
     FIG. 10 is a circuit configuration diagram of a hydraulic system according to a third embodiment of the present invention; 
     FIGS. 11A,  11 B are diagrams for explaining the operation of the hydraulic system of FIG. 10; 
     FIG. 12 is a circuit configuration diagram of a hydraulic system according to a fourth embodiment of the present invention; 
     FIG. 13 is a diagram for explaining the operation of the hydraulic system of FIG. 12; and 
     FIGS. 14A,  14 B are diagrams showing other examples of straight-travel valves employable in the embodiments of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A hydraulic system for a construction machine according to the present invention, as a basic construction thereof, comprises a first traveling motor and a second traveling motor for actuating a pair of right and left travel devices respectively in the construction machine; a plurality of working actuators; a first pump and a second pump for supply of pressure oil to actuate the traveling motors and the working actuators; a first traveling control valve and a second traveling control valve provided correspondingly to the traveling motors respectively to control the supply of pressure oil to the traveling motors in accordance with operations of operating levers which are associated with the traveling motors respectively; a plurality of working control valves provided correspondingly to the working actuators respectively to control the supply of pressure oil to the working actuators in accordance with operations of operating levers which are associated with the working actuators respectively, the working control valves being classified into a first group including the first traveling control valve and a second group including the second traveling control valve, a first center bypass passage in which the first traveling control valve and the working control valves included in the first group are disposed successively from an upstream side so that bleed-off passages in the control valves included in the first group are mutually communicated in series toward an oil tank when all the control valves included in the first group are in their neutral positions; and a second center bypass passage in which the second traveling control valve and the working control valves included in the second group are disposed successively from an upstream side so that bleed-off passages in the control valves in the second group are mutually communicated in series toward an oil tank when all the control valves included in the second group are in their neutral positions. 
     The hydraulic system for a construction machine according to the present invention further comprises a straight-travel valve which switches each flow of pressure oil discharged from the first and second pumps so as to supply the pressure oil to the first and second center bypass passages respectively at least when all of the traveling motors and the working actuators are not in operation and to supply pressure oil discharged from one of both pumps to both traveling control valves and further supply pressure oil discharged from the other pump to the working control valves at least in a simultaneous travel/work mode in which the traveling motor and the working actuator corresponding respectively to the traveling control valve and the working control valve belonging to one and same group out of both said groups are operated simultaneously. 
     The present invention provides at least two preferable modes for achieving the foregoing object. In the first mode, the hydraulic system according to the present invention further comprises a cut-off valve for cutting off the center bypass passage between the traveling control valve and the working control valve belonging to the above same group and corresponding respectively to the traveling motor and the working actuator which are in operation, and an opening valve for opening a downstream side of the bleed-off passage in the traveling control valve to the oil tank, at least in the simultaneous travel/work mode, on a downstream side of the bleed-off passage in each of the traveling control valves. 
     According to the first mode of the present invention, at least in the simultaneous travel/work mode, the center bypass passage between the bleed-off passage in the traveling control valve corresponding to the traveling motor in operation and the working control valve corresponding to the working actuator in operation on a downstream side of the traveling control valve, is cut off by the cut-off valve. Consequently, in the simultaneous travel/work mode, pressure oil fed from the one pump through the straight-travel valve to the traveling motor in operation and pressure oil fed from the other pump through the straight-travel valve to the working actuator in operation, do not interfere with each other through the center bypass passage in which are disposed the traveling control valve and the working control valve corresponding to those traveling motor and working actuator. At this time, the bleed-off passage in the traveling control valve corresponding to the traveling motor which is in operation opens to the oil tank through the opening valve, so that bleed-off for the traveling motor is effected appropriately making the most of a characteristic of an opening area of the bleed-off passage in the traveling direction control valve, which characteristic is usually a characteristic of the said opening area becoming smaller with an increase in the amount of operation of the operating lever associated with the traveling direction control valve. 
     Thus, according to the first mode of the present invention, when both travel and work by the working actuators are performed simultaneously, it is possible to prevent interference between pressure oil fed to the traveling motors and pressure oil fed to the working actuators and thereby perform stably the work by operation of the working actuators while allowing the construction machine to travel at a stable speed. 
     It is not always necessary for the opening valve to be fully open in its state of opening. For example, the area of its opening may be changed in accordance with the amount of operation of the operating lever corresponding to the traveling motor which is in operation (for example, the opening area may be made smaller with an increase in the amount of operation of the lever). The opening valve may be provided for each traveling control valve or may be provided as a common opening valve (a single opening valve) for both traveling control valves. 
     According to the first mode of the present invention, in the simultaneous travel/work mode of only one of both traveling motors being operated, it is preferable to control the cut-off valve so that the center bypass passage corresponding to the other traveling motor is cut off by the cut-off valve. That is, in the simultaneous travel/work mode of only one of both traveling motors being operated, pressure oil is fed from the one pump to both traveling control valves through the straight-travel valve. At this time, the traveling control valve corresponding to the other traveling motor (the traveling motor which is not in operation) is in its neutral position and its bleed-off passage is fully open, but the center bypass passage located downstream of the said traveling control valve is cut off. Consequently, the pressure oil from the one pump is prevented from flowing to the traveling control valve corresponding to the other traveling motor and it becomes possible to supply a sufficient amount of pressure oil to the one traveling motor through the traveling control valve corresponding to the one traveling motor which is in operation. In the case where such a cut-off valve as in the foregoing conventional hydraulic system is provided for example on the most downstream side of each center bypass passage, the cut-off valve in the center bypass passage located on the traveling control valve side corresponding to the other traveling motor may be closed, but by allowing the cut-off valve to operate as above it is possible to omit such a conventional cut-off valve. 
     In the first mode of the present invention the cut-off valve and the opening valve may be constituted by separate valves, of course, but both may be constituted by an integrally constructed control valve as unit, whereby it is possible to reduce the number of components of the hydraulic system. 
     Further, in the first mode of the present invention, although the cut-off valve and the opening valve may be operated in the above manner only in the foregoing simultaneous travel/work mode, there preferably is provided means which, when all the working actuators are not in operation and when the first or the second traveling motor is in operation, controls the cut-off valve so as to cut off the center bypass passage between the bleed-off passage in the traveling control valve corresponding to the traveling motor which is in operation and the working control valve located on a downstream side thereof, and controls the opening valve so as to open the downstream side of the bleed-off passage in the said traveling control valve to the oil tank. 
     According to this construction, when the first or the second traveling motor is in operation (including the case where both traveling motors operate), even if the working actuators are not in operation, the cut-off valve and the opening valve operate in the manner described above, so there is no fear that the cut-off valve and the opening valve may operate immediately upon start-up of operation of the working actuators. Thus, there is no fear of a change in the pressure of pressure oil which is fed to the traveling motor in operation transitionally at the time of operation of the cut-off valve and the opening valve. Consequently, it is possible to keep stable the operating speed of the traveling motor which is in operation and hence possible to carry out the work by the working actuators while maintaining the traveling speed of the construction machine stable. 
     On the other hand, in the second mode of the hydraulic system for a construction machine according to the present invention, each of the traveling control valves is a control valve constructed such that in a neutral position thereof the bleed-off passage thereof opens fully, while in a non-neutral position thereof the said bleed-off passage closes fully, and there are provided an opening valve which, at least when the first or the second traveling motor is in operation, causes an oil passage to open to the oil tank, the said oil passage being located between the traveling control valve associated with the traveling motor which is in operation and the pump for the supply of oil pressure to the said traveling control valve, and means for controlling the area of opening of the opening valve so as to become smaller with an increase in the amount of operation of an operating lever associated with the traveling motor which is in operation. 
     According to this second mode of the present invention, when the first or the second traveling motor is in operation, the bleed-off passage in the traveling control valve corresponding to the traveling motor which is in operation is kept fully closed, so that the center bypass passage corresponding to the said traveling control valve is cut off by the same traveling control valve. 
     Therefore, as in the previous first mode, in the simultaneous travel/work mode, pressure oil fed from the one pump through the straight-travel valve to the traveling motor which is in operation and pressure oil fed from the other pump through the straight-travel valve to the working actuator which is in operation, do not interfere with each other through the center bypass passage in which the traveling control valve and working control valve associated with those traveling motor and working actuator are disposed. At this time, of the pressure oil discharged from the one pump, surplus oil exclusive of the pressure oil fed to the traveling motor which is in operation flows to the oil tank through the opening valve, but the area of opening of the opening valve becomes smaller with an increase in the amount of operation of the operating lever associated with the traveling motor which is in operation. Consequently, bleed-off for the traveling motor is effected appropriately through the opening valve. 
     Thus, according to the second mode of the present invention, as in the first mode, when both travel and work by the working actuators are done simultaneously, it is possible to prevent interference between the pressure oil fed to the associated traveling motor and working actuator respectively and carry out the work by operation of the working actuator smoothly while allowing the construction machine to travel at a stable speed. 
     In the second mode of the present invention, when only one of both traveling motors is operated and when such a cut-off valve as in the foregoing conventional hydraulic system is provided for example on the most downstream side of each center bypass passage, the cut-off valve in the center bypass passage having a traveling control valve corresponding to a traveling motor which is not in operation may be closed. Alternatively, there may be adapted a construction wherein a valve capable of being opened and closed is disposed in each center bypass passage on the downstream side of each traveling control valve and is allowed to operated in the same manner as the aforesaid cut-off valve. 
     Preferably, in the above first and second modes of the present invention, the straight-travel valve is a control valve having a first operating position for providing pressure oils from the first and second pumps independently and respectively to the first and second traveling control valves, a second operating position for providing pressure oil from one of both pumps to only both traveling control valves and providing pressure oil from the other pump to only the plural working control valves, and a third operating position for providing communication through a throttle valve between an oil passage communicating with both traveling control valves in the second operating position and an oil passage communicating with the working control valves in the second operating position, and there is provided means which, at least in the simultaneous travel/work mode, controls the straight-travel valve to the second operating position when the amount of operation of an operating lever associated with the traveling motor which is in operation is not larger than a predetermined amount, while when the amount of operation of the operating lever exceeds the predetermined amount, makes control to switch the position of the straight-travel valve to the third operating position from the second operating position. 
     According to this construction, in the simultaneous travel/work mode, the straight-travel valve is controlled to the second operating position when the amount of operation of the operating lever associated with the traveling motor which is in operation is not larger than the predetermined amount, that is, when the said amount of operation is relatively small, so that the pressure oils from one and the other pumps are fed each independently to the traveling motor and the working actuator which are in operation. Thus, coupled with the foregoing cut-off condition of the associated center bypass passage, the pressure oils from both pumps are sure to be prevented from interference. Consequently, the work by operation of working actuators can be done smoothly while allowing the construction machine to travel stably at a relatively low speed. Besides, since the position of the straight-travel valve is switched to the third operating position when the amount of operation of the operating lever associated with the traveling motor which is in operation exceeds the predetermined amount, pressure oil can be fed to both traveling control valves not only from the one pump but also from the other pump by virtue of the foregoing throttle effect. As a result, it becomes possible to let the traveling motors operate at a sufficiently high speed. 
     Preferably, in the first and second modes of the present invention provided with the straight-travel valve, there is provided means which, when the first or the second traveling motor is in operation with all of the working actuators stopped, controls the position of the straight-travel valve to the second operating position when the amount of operation of the operating lever associated with the traveling motor which is in operation is not larger than the foregoing predetermined amount, while when the amount of operation of the operating lever exceeds the predetermined amount, makes control to switch the position of the straight-travel valve from the second to the first operating position. 
     According to this construction, when the construction machine is traveling with the working actuators stopped and in a state in which the amount of operation of the operating lever associated with the traveling motor which is in operation is not larger than the predetermined amount and is relatively small, the position of the straight-travel valve is controlled to the second operating position. Therefore, even if the working actuators are started to operate in this state, the straight-travel valve is held in the second operating position. Thus, even if the working actuators are started to operate during travel at a relatively low speed, there is no fear that the flow of pressure oil may suddenly change transitionally. As a result, the operating speed of the traveling motor can be kept stable. Moreover, if the amount of operation of the operating lever associated with the traveling motor which is in operation becomes relatively large with the working actuators stopped, the position of the straight-travel valve is switched to the first operating position, thus permitting the supply of pressure oils from both pumps independently to the traveling control valves. Consequently, each traveling motor can be operated at a sufficiently high speed. If the operation of the working actuators is started in this state, the position of the straight-travel valve is switched to the third operating position, so there is no fear of a sudden decrease in the amount of pressure oil fed to the traveling motor which is in operation, whereby a sudden decrease in the traveling speed of the construction machine is prevented. 
     Preferably, there is provided means which holds the straight-travel valve in the second operating position by a predetermined operation at least in the simultaneous travel/work mode. 
     According to this construction, when the driver of the construction machine performs a predetermined operation (e.g., operates a switch or performs a voice input operation), the straight-travel valve is held in the second operating position even if the amount of operation of the operating lever associated with the traveling motor in operation becomes large in excess of the predetermined amount. Thus, for holding the straight-travel valve in the second operating position and for avoiding mutual interference of pressure oil fed to the traveling motor in operation and the working actuators, the amount of operation of the operating lever associated with the traveling motor in operation need not be maintained at a value of not larger than the predetermined amount. That is, by a relatively rough operation of the operating lever, the operating position of the straight-travel valve can be held in the second operating position which permits positive avoidance of the aforesaid interference. As a result, the work by working actuators can be done while the construction machine is allowed to travel easily at a stable speed by operation of the traveling motor. 
     In this case, there preferably are provided means which, at least in the simultaneous travel/work mode, adjusts the discharge rate of the pump for the supply of pressure oil to the traveling motor in operation in accordance with the amount of operation of the operating lever associated with the traveling motor, and means which sets, for the means of adjusting the discharge rate of the pump, a characteristic of a change in the discharge rate based on a change in the amount of operation of the operating lever variably by a predetermined operation. 
     According to this construction, in the simultaneous travel/work mode, the discharge rate in the pump for the supply of pressure oil to the traveling motor in operation, which discharge rate is proportional to the amount of operation of the operating lever associated with the traveling motor, can be adjusted to a flow rate which the driver desires. Consequently, for example, the operating speed of the traveling motor can be limited to a low speed by keeping the discharge rate low. Thus, the work by operation of the working actuators can be done while maintaining the traveling speed of the construction machine by the traveling motor at a low speed stably and easily. 
     Alternatively, in the first mode of the present invention there may be provided means which, at least in the simultaneous travel/work mode, adjusts the area of opening of the opening valve in accordance with the amount of operation of the operating lever associated with the traveling motor in operation, and means which sets, for the means of adjusting the opening area of the opening valve, a characteristic of a change in the opening area in accordance with a change in the amount of operation of the said operating lever variably by a predetermined operation. In the second mode of the present invention there may be provided means which, at least in the simultaneous travel/work mode, sets a characteristic of a change in the opening area in accordance with a change in the amount of operation of the operating lever variably by a predetermined operation for the means of controlling the opening area of the opening valve. 
     According to this construction, in the simultaneous travel/work mode, the flow rate of bleed-off for the traveling motor in operation and proportional to the amount of operation of the operating lever associated with the traveling motor can be adjusted to a flow rate which the driver desires. Therefore, for example it becomes possible to adjust the flow rate of bleed-off to a rather large flow rate and thereby limit the operating speed of the traveling motor to a low speed. Thus, the work by the working actuators can be done while keeping the traveling speed of the construction machine by the traveling motor at a low speed stably and easily. 
     Preferably, according to the first and second modes of the present invention, in the simultaneous travel/work mode, the oil passage for the supply of pressure oil discharged from the other pump to the working control valves through the straight-travel valve is communicated with an inlet side of the bleed-off passage in each working control valve located on an upstream side of in each of the first and second groups and is also communicated with an inlet side of a meter-in passage in each of the working control valves in the first and second groups. 
     According to this construction, in the simultaneous travel/work mode, surplus pressure oil discharged from the other pump as a source of pressure oil supply for the working actuators flows from the straight-travel valve through the oil passage to an inlet side of the bleed-off passage in the working control valve located on the upstream side and flows through the center bypass passage connected to the downstream side of the working control valve. Thus, the operation of each working actuator can be done smoothly while making the most of the opening area characteristic of the bleed-off passage in each working control valve. 
     Concretely, the present invention will be described hereinunder by way of embodiments thereof illustrated in the drawings. It is to be understood that the invention is not limited to those embodiments. 
     A first embodiment of the present invention will be described below with reference to FIGS. 1 to  7 . This embodiment is related to the hydraulic system in a hydraulic excavator. Further, this embodiment is related to the foregoing first mode of the present invention. 
     Referring to FIG. 1, the hydraulic system of this embodiment is also provided with two variable displacement pumps  20  and  21 , direction control valves (traveling control valves)  22 R and  22 L which are for controlling the supply of pressure oil to right and left traveling motors  2 R,  2 L in a hydraulic excavator, a direction control valve  23  for controlling the supply of pressure oil to a rotating motor  4 , direction control valves  24 ,  25 , and  26  for controlling the supply of pressure oil to a boom cylinder  7 , an arm cylinder  8 , and a bucket cylinder  9 , a center bypass passage  28  in which the direction control valves  22 R,  24 , and  26  belonging to a first group are disposed successively from an upstream side, and a center bypass passage  29  in which the direction control valves  22 L,  23 , and  25  belonging to a second group are disposed successively from the upstream side. The direction control valves  23  to  26  correspond to the working control valves in the present invention. In the following description, the rotating motor  4 , boom cylinder  7 , arm cylinder  8 , and bucket cylinder  9  will sometimes be referred to as working actuators  4  and  7 ˜ 9  generically. 
     Cut-off valves  30  and  31  capable of being opened and closed are disposed downstream of the direction control valves  26  and  25  which are located at most downstream positions in the center bypass passages  28  and  29  respectively. In the same figure, the numeral  35  denotes an arm confluence valve for making pressure oil from both pumps  20  and  21  join together and feeding the joined flow to the arm cylinder  8  where required for actuating an arm in the hydraulic excavator, numeral  36  denotes a boom confluence valve for making pressure oil from both pumps  20  and  21  join together and feeding the joined flow to the boom cylinder  7  where required for actuating a boom, and numerals  20   a  and  21   a  denote regulators for adjusting the discharge rates of the pumps  20  and  21  respectively. 
     On the other hand, as the opening valve and cut-off valve referred to in the first mode of the present invention, the hydraulic system of this embodiment is provided with a pair of traveling bypass cut-off valves  37 R and  37 L possessing the functions of both cut-off valve and opening valve, as well as a straight-travel valve  38 . 
     The traveling bypass cut-off valves  37 R and  37 L are each a three-position change over valve (spool valve) of the same structure having a neutral position A, a position B, and a position C. The traveling bypass cut-off valve  37 R is disposed in the center bypass passage  28  at a position between the direction control valve  22 R for right-hand travel and the direction control valve  24  for boom located downstream of the valve  22 R, while the traveling bypass cut-off valve  37 L is disposed in the center bypass passage  29  at a position between the direction control valve  22 L for left-hand travel and the direction control valve  23  for rotation located downstream of the valve  22 L. 
     When the traveling bypass cut-off valve  37 R, which is located on group G 1  side, is in its neutral position A, it causes a bleed-off port of a bleed-off passage  27  in the direction control valve  22 R for right-hand travel to communicate with an inlet port of a bleed-off passage  27  in the direction control valve  24  for boom located downstream of the valve  22 R. When the traveling bypass cut-off valve  37 R is in its position B, it causes the outlet port of the bleed-off passage  27  in the direction control valve  22 R for right-hand travel to open to an oil tank  32  through an oil passage  37 a formed in the interior of the bypass cut-off valve  37 R and at the same time cuts off the flow of pressure oil from the bleed-off passage  27  in the right-hand traveling direction control valve  22 R located on the upstream side to the bleed-off passage  27  in the direction control valve  24  for boom located on the downstream side (cuts off the center bypass passage  28  between the direction control valves  22 R and  24 ). Further, when the traveling bypass cut-off valve  37 R is in its position C, it cuts off the flow of pressure oil from the bleed-off passage  27  in the right-hand traveling direction control valve  22 R to the bleed-off passage  27  in the direction control valve  24  for boom located on the downstream side and to the oil tank  32  (closes the center bypass passage  28  extending from the valve  22 R to the valve  37 R). The oil passage  37   a  which comes into communication with the oil tank  32  at position B of the traveling bypass cut-off valve  37 R becomes gradually smaller in the area of its opening as the bypass cut-off valve  37 R switches gradually to position C from position B. 
     Like the traveling bypass cut-off valve  37 R, when the traveling bypass cut-off valve  37 L, which is located on second group G 2  side, is in its neutral position A, it causes an output port of a bleed-off passage  27  in the direction control valve  22 L for left-hand travel to communicate with an inlet port of a bleed-off passage in the direction control valve  23  for rotation located downstream of the valve  22 L. When the traveling bypass cut-off valve  37 L is in its position B, it causes the output port of the bleed-off passage  27  in the direction control valve  22 L for left-hand travel to open to the oil tank  32  through an oil passage  37   a  formed in the interior of the bypass cut-off valve  37 L and at the same time cuts off the flow of pressure oil from the bleed-off passage  27  in the left-hand traveling direction control valve  22 L located on the upstream side to the bleed-off passage  27  in the direction control valve  23  for rotation located on the downstream side. Further, at position C of the traveling bypass cut-off valve  37 L, the traveling bypass cut-off valve  37 L cuts off the flow of pressure oil from the bleed-off passage  27  in the right-hand traveling direction control valve  22 L to the bleed-off passage  27  in the direction control valve  23  for rotation located on the downstream side and to the oil tank  32 . 
     Electromagnetic proportional reducing valves  39 R and  39 L are connected respectively to pilot ports of the traveling bypass cut-off valves  37 R and  37 L. The reducing valves  39 R and  39 L, when respective solenoids are energized, produce a pilot pressure of a level proportional to the energizing current from pressure oil of a constant pressure level discharged from a pilot pump (not shown) and provide it to the pilot ports of the traveling bypass cut-off valves  37 R and  37 L. The pilot pressure thus produced becomes larger as the energizing current increases. In the following description the electromagnetic proportional reducing valves  39 R and  39 L will be referred to as the right-hand traveling proportional valve  39 R and the left-hand traveling proportional valve  39 L, respectively. 
     The straight-travel valve  38  is a three-position control valve (spool valve) having a neutral position D (first operating position), a position E (second operating position), and a position F (third operating position). Upstream ends of both center bypass passages  28  and  29  and an upstream end of a working oil passage  40  are connected respectively to three outlet ports of the straight-travel valve  38 , the working oil passage  40  being for the supply of pressure oil to the direction control valves  23 ˜ 26  associated with the working actuators  4  and  7 ˜ 9  without going through both traveling direction control valves  22 R and  22 L. Further, a discharge port of the pump  21  is connected in communication with one of three inlet ports of the straight-travel valve  38  and a discharge port of the pump  20  is connected in communication with the remaining two inlet ports of the straight-travel valve  38 . 
     In this case, the straight-travel valve  38 , in its neutral position D, causes the discharge port of the pump  21  to open to only the center bypass passage  28 , causes the discharge port of the pump  20  to open to only the center bypass passage  29 , and closes the upstream end of the working oil passage  40 . When the straight-travel valve  38  is in its position E, it causes the discharge port of the pump  21  to open to both center bypass passages  28  and  29  and causes the discharge port of the pump  20  to open to only the working oil passage  40 . Further, at the position F of the straight-travel valve  38 , the discharge port of the pump  20  is opened to both center bypass passages  28  and  29  through a throttle passage  38   a  formed in the interior of the straight-travel valve  38 , in addition to opening the discharge port of the pump  21  to both center bypass passages  28  and  29  and opening the discharge port of he pump  20  to the working oil passage  40 . 
     To a pilot port of the straight-travel valve  38  is connected an electromagnetic proportional reducing valve  41  (hereinafter referred to as the “straight-travel proportional valve  41 ) of the same construction as the right- and left-hand traveling proportional valves  39 R,  39 L. 
     The working oil passage  40  is provided with a main passage  40   a  connected to the straight-travel valve  38  and plural branch passages  40   b ˜ 40   g  branched from the main passage  40   a . Of the branch passages  40   b ˜ 40   g , the branch passage  40   b  is connected to the center bypass passage  28  located between the traveling bypass cut-off valve  37 R and the direction control valve  24  for boom on the first group G 1  side and is also connected to an inlet port of a meter-in passage in the direction change-valve  24  for boom. Likewise, the branch passage  40   c  is connected to the center bypass passage  29  located between the traveling bypass cut-off valve  37 L and the direction control valve  23  for rotation on the second group G 2  side and is also connected to an inlet port of a meter-in passage in the direction control valve  23 . The branch passages  40   d  and  40   e  are connected respectively to an inlet port of a meter-in passage in the direction control valve  26  for bucket and an inlet port of a meter-in passage in the direction control valve  25  for arm. Further, the branch passages  40   f  and  40   g  are connected respectively to an inlet port of the arm confluence valve  35  and an inlet port of the boom confluence valve  36 . 
     Referring now to FIG. 2, in this embodiment, for controlling the operation of the hydraulic system described above there are provided an operation quantity detector  44  for detecting operation quantities of operation levers  43  which operate the direction control valves  22 R,  22 L, and  23 ˜ 26  respectively through a pilot operation unit  42 , a controller  45  which controls the switching operations of the traveling bypass cut-off valves  37 R,  37 L and the straight-travel valve  38  through the right- and left-hand traveling proportional valves  39 R,  39 L, and the straight-travel proportional valve  41  and which controls the discharge rates of the pumps  20  and  21  through regulators  20   a  and  21   a , and an operating volume  46  with which the driver of the hydraulic excavator  1  specifies for the operator of the hydraulic excavator a control characteristic for the straight-travel valve  38  by the controller  45  and a flow characteristic of the pumps  20  and  21 . Actually, plural operating levers  43  are provided correspondingly to the direction control valves  22 R,  22 L, and  23 ˜ 26 , but in FIG. 2 there are shown one direction control valve and one operating lever  43  as representative illustrations for convenience&#39; sake. The controller  45  is constituted by an electronic circuit including a microcomputer, etc. (not shown). 
     When the operating levers  43  corresponding respectively to the direction control valves  22 R,  22 L, and  23 ˜ 26  are operated from their neutral positions, the pilot operation unit  42  produces pilot pressures proportional to the amounts of the operations and outputs the pilot pressures to pilot passages  47   a  or  47   b  matching the operated directions of the operating lever  43  out of paired pilot passages  47   a  and  47   b  connected respectively to paired pilot ports of the direction control valves  22 R,  22 L, and  23 ˜ 26 . The operation quantity detector  44  detects the pilot pressures in the pilot passages  47   a  or  47   b  as pressures which represent the amounts of operation of the operating levers  43 , then outputs the detected signals to the controller  45 . The pilot pressures outputted from the pilot operation unit  42  to the pilot passages  47   a  and  47   b  become higher with an increase in the amount of operation of the operating levers  43 . 
     In this embodiment the operating volume  46  is a rotary dial type for example and outputs a signal with a level matching its rotational position to the controller  45 . In this case, the position “OFF” in the figure corresponds to a standard operating position of the operating volume  46 . 
     Next, a description will be given of the operation of the hydraulic system in the hydraulic excavator of this embodiment. First, reference will be made to a basic operation of the hydraulic system. In the description of the basic operation it is assumed that the operating volume  46  is in the “OFF” position. 
     The controller  45  executes a processing for judging an operation mode of the hydraulic system with a predetermined cycle time successively in such a manner as shown in a flowchart of FIG.  3 . 
     First, the controller  45  acquires detection data on the operating levers  43  from the operation quantity detector  44 , that is, acquires detection data on pilot pressures provided to the direction control valves  22 R,  22 L, and  23 ˜ 26 , (STEP 1). Then, the controller  45  compares the level of a pilot pressure Pi (right-hand travel) which represents the amount of operation of the operating lever  43  associated with the right-hand traveling motor  2 R, with a minimum pressure Pis at which a switching operation starts from the neutral position A of the direction control valve  22 R (STEP 2). At this time, if Pi (right-hand travel)≧Pis (with the right-hand traveling motor  2 R ON), the controller  45  sets the value of Flag Fa to “1” (STEP 3), while if Pi (right-hand travel)&lt;Pis (with the right-hand traveling motor  2 R OFF), the controller sets the value of Flag Fa to “0” (STEP 4). 
     The controller  45  further compares the level of a pilot pressure Pi (left-hand travel) which represents the amount of operation of the operating lever  43  associated with the left-hand traveling motor  2 L, with the minimum pressure Pis (STEP 5), and if Pi (left-hand travel)≧Pis (with the left-hand traveling motor  2 L ON), the controller  45  sets the value of Flag Fb to “1” (STEP 6), while if Pi (left-hand travel)&lt;Pis (the left-hand traveling motor  2 L OFF), the controller sets the value of Flag Fb to “0” (STEP 7). 
     Then, the controller  45  compares pilot pressures Pi (work) which represent the amounts of operation of the operating levers  43  associated with the working actuators  4  and  7 ˜ 9 , with the minimum pressure Pis (STEP 8), and if any one of the pilot pressures Pi (work) is Pi (work)≧Pis (when at least one of the working actuators  4  and  7 ˜ 9  is ON), the controller  45  sets the value of Flag Fc to “1” (STEP 9), while if all the pilot pressures Pi (work) are in a relation of Pi (work)&lt;Pis (when all the working actuators  4  and  7 ˜ 9  are OFF), the controller  45  sets the value of Flag Fc to “0” (STEP 10). 
     Then, the controller  45  judges whether the value of Flag Fa or Fb is “1” (including the case of Fa=Fb=1) and whether the value of Fc is “1,” that is, whether the operation of the traveling motor  2 R or  2 L (including simultaneous operation of the two) and the operation of any of the working actuators  4  and  7 ˜ 9  are being done simultaneously (STEP 11). At this time, if Fa=1 or Fb=1 and Fc=1, the controller  45  sets the value of Flag Fd to “1” (STEP 12), while if Fa=Fb=0 or Fc=0, the controller  45  sets the value of Flag Fd to “0” (STEP 13). 
     After thus setting the values of Flags Fa to Fd, if Fa=1 or Fb=1, that is, if the traveling motor  2 R or  2 L is in operation, the controller  45  determines energizing currents for the right- and left-hand traveling proportional valves  39 R,  39 L associated with the traveling bypass cut-off valves  37 R and  37 L respectively, in the following manner. 
     First, with reference to data tables built in advance, as shown in FIGS. 4A and 4B, and in accordance with pilot pressure Pi (right-hand travel) which represents the amount of operation of the operating lever  43  associated with the right-hand traveling motor  2 R, the controller  45  sets energizing currents for the right- and left-hand traveling proportional valves  39 R,  39 L temporarily. 
     In the data table of FIG. 4A, the energizing current for the right-hand traveling proportional valve  39 R becomes a current I 1  which switches the position of the traveling bypass cut-off valve  37 R from the neutral position held by a predetermined lower-limit current Imin to the position B in an instant when the pilot pressure Pi (right-hand travel) becomes the minimum pressure Pi or higher . As the pilot pressure Pi (right-hand travel) increases (the amount of operation of the operating lever  43  for right-hand travel increases), the energizing current in the right-hand traveling proportional valve  39 R increases gradually from the current I 1  up to a predetermined upper-limit current Imax which holds the traveling bypass cut-off valve  37 R at the position C. Pie in the figure represents a pilot pressure corresponding to a nearly maximum operation quantity of an operating lever  43 . 
     In the data table of FIG. 4B, the energizing current in the left-hand traveling proportional valve  39 L increases from the lower-limit current Imin up to a current I 2  (&gt;I 1 ) which switches the traveling bypass cut-off valve  37 L to an intermediate position between the positions B and C in an instant when the pilot pressure Pi (right-hand travel rises to a level above the minimum pressure Pis. As the pilot pressure Pi (right-hand travel) increases (the amount of operation of the operating lever  43  for right-hand travel increases), the energizing current in the left-hand traveling proportional valve  39 L increases gradually from the current I 2  up to the upper-limit current Imax. In an intermediate position between the positions B and C of the traveling bypass cut-off valve  37 L, as added correspondingly to the current I 2  in FIG. 4B, a throttle is formed in the oil passage  37   a  of the traveling bypass cut-off valve  37 L and the opening area of the passage becomes smaller as the energizing current increases. This is also the case with the traveling bypass cut-off valve  37 R. 
     Further, with reference to data tables built in advance, as shown in FIGS. 5A and 5B, and in accordance with pilot pressure Pi (left-hand travel) which represents the amount of operation of the operating lever  43  associated with the left-hand traveling motor  2 L, the controller  45  sets energizing currents for the right- and left-traveling proportional valves  39 R,  39 L temporarily. 
     In the data table of FIG. 5A, the energizing current in the left-hand traveling proportional valve  39 L with respect to the pilot pressure Pi (left-hand travel) possesses the same characteristic as in the data table of FIG.  4 A. Likewise, in the data table of FIG. 5B, the energizing current in the right-hand traveling proportional valve  39 R with respect to the pilot pressure Pi (left-hand travel) possesses the same characteristic as in the data table of FIG.  4 B. 
     In this way energizing currents for the right- and left-hand traveling proportional valves  39 R,  39 L are set temporarily in accordance with pilot pressure Pi (right-hand travel) and energizing currents for the left- and right-hand proportional valves  39 L,  39 R are set temporarily in accordance with pilot pressure Pi (left-hand travel). Thereafter, the controller  45  determines the energizing current of the larger value as the energizing current to be actually fed to the right-hand traveling proportional valve  39 R out of the energizing current which has been determined temporarily with reference to the data table of FIG.  4 A and in accordance with pilot pressure Pi (right-hand travel) and the energizing current which has been set temporarily with reference to the data table of FIG.  5 B and in accordance with pilot pressure Pi (left-hand travel). The controller  45  then supplies the thus-determined energizing current to the right-hand traveling proportional valve  39 R. Likewise, as to the left-hand traveling proportional valve  39 L, the controller  45  determines the energizing current of the larger value as the energizing current to be actually fed to the left-hand traveling proportional valve  39 L out of the energizing current which has been set temporarily with reference to the data table of FIG.  4 B and in accordance with pilot pressure Pi (right-hand travel) and the energizing current which has been set temporarily with reference to the data table of FIG.  5 A and in accordance with pilot pressure Pi (left-hand travel). Then, the controller  45  supplies the thus-determined energizing current to the left-hand traveling proportional valve  39 L. 
     Further, if Fa=1 or Fb=1 (when the traveling motor  2 R or  2 L is in operation), the controller  45  determines an energizing current for the straight-travel proportional valve  41  in the following manner. 
     More specifically, if the value of Flag Fd is “1” (when the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9  are simultaneously in operation), the controller  45  determines an energizing current for the straight-travel proportional valve  41  with reference to a table built in advance, as indicated with a solid line in FIG. 6A, and in accordance with the larger pilot pressure Pi (travel max or max of travel)=max (Pi (right-hand travel), Pi (left-hand travel) out of pilot pressures Pi (right-hand travel) and pilot pressure Pi (left-hand travel). Then, the controller  45  supplies the thus-determined energizing current to the straight-travel proportional valve  41 . 
     In the solid-line data table of FIG. 6A, when the pilot pressure Pi (travel max) becomes the minimum pressure Pis or higher, the energizing current in the straight-travel proportional valve  41  becomes such a current I 1  as switches the straight-travel valve  38  to position E in an instant and holds it in that position. In a state in which the pilot pressure Pi (travel max) is not higher than a predetermined value Pix (Pis&lt;Pix&lt;Pie), that is, in a state in which the pilot pressure Pi (travel max) lies in a range Δ where it is relatively small (when the amount of operation of the operating lever  43  for left-hand travel and that of the operating lever  43  for right-hand travel are both relatively small), the energizing current in the straight-travel proportional valve  41  is maintained in the above current I 1  to hold the straight-travel valve  38  in position E. Further, when the pilot pressure Pi (travel max) exceeds the range Δ (“low-operation range Δ” hereinafter) and becomes the predetermined value Pix or higher, the energizing current in the straight-travel proportional valve  41  increases gradually from the current I 1  up to such a predetermined upper-limit current Imax as holds the straight-travel valve  38  in the position F as the pilot pressure Pi (travel max) increases (as the amount of operation of at least one of the right- and left-hand traveling operating levers  43 ,  43  increases). When the energizing current in the straight-travel proportional valve  41  is of a magnitude between the current I 1  and the upper-limit current Imax, the straight-travel valve  38  assumes a state intermediate between the positions E and F. As to the dot-dash line graph in FIG. 6A, reference will be made later. 
     When the value of Flag Fd is “0” (when either the traveling motor  2 R or  2 L is in operation and all of the working actuators  4  and  7 ˜ 9  are OFF), the controller  45  determines an energizing current for the straight-travel proportional valve  41  with reference to a predetermined data table, as shown in FIG. 6B, and in accordance with pilot pressure Pi (travel max). Then, the controller  45  supplies the thus-determined energizing current to the straight-travel proportional valve  41 . 
     In the data table of FIG. 6B, when the pilot pressure Pi (travel max) is in a relation of Pi (travel max)≦Pix, the energizing current in the straight-travel proportional valve  41  is the same as in FIG. 6A (Fd=1). On the other hand, when the pilot pressure Pi (travel max) exceeds the low-operation range Δ and becomes the predetermined value Pix or higher, the energizing current in the straight-travel proportional valve  41  decreases gradually from the current I 1  (the current which holds the straight-travel valve  38  in the position E) down to the lower-limit current Imin which holds the straight-travel valve  38  in the neutral position D. When the energizing current in the straight-travel proportional valve  41  is of a magnitude between the current I 1  and the lower-limit current Imin, the straight-travel valve  38  assumes a state intermediate between the neutral position D and the position E. 
     When the traveling  2 R or  2 L is in operation (Fa=1 or Fb=1), the controller  45  controls the regulator  21   a  for the pump  21  so that the discharge rate of the pump  21  which serves as a pressure oil supply source for both traveling motors  2 R and  2 L is varied in accordance with pilot pressures Pi (right-hand travel) and Pi (left-hand travel) related to the operating levers  43  which are associated with the traveling motors  2 R and  2 L. In this case, according to this embodiment, the regulator  21   a  is controlled in such a manner that, for example as indicated with a solid line in FIG. 7, the discharge rate of the pump  21  is increased gradually from a predetermined minimum flow rate Qmin up to a predetermined maximum flow rate Qmax as the total pilot pressure of pilot pressures Pi (right-hand travel) and Pi (left-hand travel), i.e., Pi (right-hand travel)+Pi (left-hand travel), increases above the minimum pressure Pis. As to the dot-dash line graph in FIG. 7, reference will be made later. 
     Further, when either the traveling motor  2 R or  2 L is in operation (Fa=1 or Fb=1) and with any of the working actuators  4  and  7 ˜ 9  ON (Fd=1), the controller  45  controls the regulator  20   a  for the pump  20  so that the discharge rate of the pump  20  serving as a pressure oil supply source for the working actuators  4  and  7 ˜ 9  is varied in accordance with pilot pressures Pi (work) related to the operating levers  43  which are associated with the working actuators  4  and  7 ˜ 9 . In this case, though not shown, for example as is the case with controlling the regulator  21   a  for the pump  21  described above, the regulator  20   a  for the pump  20  is controlled in accordance with the total sum of pilot pressures (work) corresponding to the working actuators  4  and  7 ˜ 9  in such a manner that the discharge rate of the pump  20  is increased with an increase in the total sum of the said pilot pressures Pi (work). When the traveling motor  2 R or  2 L is in operation and with all of the working actuators  4  and  7 ˜ 9  OFF (Fd=0), and when the pilot pressure Pi (right-hand travel) or the pilot pressure Pi (left-hand travel) is larger than the low-operation range Δ, the controller  45  controls the discharge rate of the pump  20  in accordance with the total sum of both pilot pressures Pi (right-hand travel) and Pi (left-hand travel) for example in the same form as the pump  21  (see FIG.  7 ). 
     When the traveling motor  2 R or  2 L is in operation (Fa=1 or Fb=1) and with any of the working actuators  4  and  7 ˜ 9  ON (Fd=1), and when the boom cylinder  7  and the bucket cylinder  9  in group G 1  are both OFF, the controller  45  makes control so that the cut-off valve  30  located most downstream of the center bypass passage  28  is closed through an electromagnetic proportional reducing valve (not shown). Likewise, in case of Fd=1 and when the rotating motor  4  and the arm cylinder  8  in group G 2  are both OFF, the controller  45  makes control so that the cut-off valve  31  located most downstream of the center bypass passage  29  is closed through an electromagnetic proportional reducing valve (not shown). 
     The above energizing control for the right- and left-hand traveling proportional valves  39 R,  39 L and the straight-travel proportional valve  41 , as well as the above control for the regulators  20   a  and  21   a  associated with the pumps  20  and  21 , are executed successively by the controller  45  with a cycle time synchronized with the cycle time in the processing of FIG. 3 when the value of Flag Fa or Fb is set to “1” in the processing of FIG. 3, that is, when the traveling motor  2 R or  2 L is in operation. 
     By such controls conducted during operation of the traveling motor  2 R or  2 L, the hydraulic system of this embodiment operates in the following manner. 
     When the traveling motor  2 R or  2 L is in operation (including the case where both are ON simultaneously) and when the amount of operation of the operating lever  43  associated with the traveling motor  2 R or  2 L in operation is relatively small (when pilot pressure Pi travel max) lies in the low-operation range Δ), the straight-travel valve  38  is switched from its neutral position D to its position E and is held in the position E constantly irrespective of whether the working actuators  4  and  7 ˜ 9  are ON or OFF. In this state, it is only the pump  21  that serves as a pressure oil supply source for the traveling motors  2 R and  2 L, and at the same time the pump  20  serves as a source for the supply of pressure oil to only the working actuators  4  and  7 ˜ 9  through the working oil passage  40 . 
     In this case, moreover, the traveling bypass cut-off valves  37 R and  37 L are each switched from the neutral position A to a position close to the position B or C, and the downstream sides of the bleed-off passages  27  in the direction control valves  22 R and  22 L for travel communicate with the oil tank  32  through the oil passages  37   a  in the traveling bypass cut-off valves  37 R and  37 L and are disconnected from the direction control valves  23 ˜ 26  for work located downstream of the direction control valves  22 R and  22 L, so that the pressure oil flowing through the bleed-off passages  27  in the direction control valves  22 R and  22 L does not flow through the direction control valves  23 ˜ 26  for work. 
     Therefore, even if any of the working actuators  4  and  7 ˜ 9  is operated simultaneously with operation of the traveling motor  2 R or  2 L, the pressure oil fed from the pump  21  to the traveling motor  2 R or  2 L is not influenced by, for example, a change in pressure of the pressure oil fed from the pump  20  to any of the working actuators  4  and  7 ˜ 9 , nor are conducted switching operations of the straight-travel valve  38  and both bypass cut-off valves  37 R and  37 L in response to the start of operation of the working actuators  4  and  7 ˜ 9  during travel of the hydraulic excavator. As a result, work such as excavation can be done by operation of the working actuators  4  and  7 ˜ 9  while allowing the hydraulic excavator to travel at a relatively low, stable speed under the operation of traveling motors  2 R or  2 L. 
     In this case, in the direction control valve  22 R or  22 L associated with the traveling motor  2 R or  2 L is in operation, surplus oil flows to the oil tank  32  through the bleed-off passage  27  whose opening area varies according to the amount of operation of the associated operating lever  43 . Besides, the discharge rate of the pump  21  serving as a pressure oil supply source for the traveling motor  2 R or  2 L is controlled so as to become smaller as the amount of operation of the operating lever  43  associated with the traveling motor  2 R or  2 L decreases. Consequently, pressure oil can be fed to the energized traveling motor  2 R or  2 L at a flow rate proportional to the amount of operation of the operating lever  43 , making the most of the opening area characteristic of the bleed-off passage in the direction control valve  22 R or  22 L for travel. Thus, the operation for a stable traveling speed can be done smoothly. 
     In the case where the operating lever  43  associated with the traveling motor  2 R or  2 L is operated relatively largely (more specifically, in case of max (Pi (right-hand travel), Pi (left-hand travel)&gt;Pix), the straight-travel valve  38  is switched from the position E to the neutral position D, so that basically pressure oils from the pumps  21  and  20  can be fed to the traveling motors  2 R and  2 L respectively. Therefore, a high traveling speed required for the hydraulic excavator can be ensured to a satisfactory extent. 
     Further, when any of the working actuators  4  and  7 ˜ 9  is operated in such a high-speed traveling state of the hydraulic excavator, the straight-travel valve  38  is switched to the position F side. At this time, the pump  21  serves as a main pressure oil supply source for the traveling motors  2 R and  2 L and the pump  20  serves as a main pressure oil supply source for the working actuators  4  and  7 ˜ 9 , but a portion of the pressure oil from the pump  20  is fed to the traveling motors  2 R and  2 L through the throttle passage  38   a  at position F of the straight-travel valve  38 . Consequently, it is possible to avoid a sudden deceleration of the hydraulic excavator. In the position F of the straight-travel valve  38 , the pressure oil fed to the working actuators  4  and  7 ˜ 9  and the pressure oil fed to the traveling motors  2 R and  2 L somewhat interfere with each other through the throttle passage  38   a  in the straight-travel valve  38 . But this interference will cause no practical trouble because the rate of variation in the traveling speed caused by the interference is smaller in high-speed travel than in low-speed travel of the hydraulic excavator. 
     When only one of the traveling motors  2 R and  2 L is in operation, for example when the traveling motor  2 R is ON, the traveling bypass cut-off valve  37 L on the traveling motor  2 L side which is OFF is switched to a position close to C rather than position B and the oil passage  37   a  in the traveling bypass cut-off valve  37 L, which provides communication of the center bypass passage  29  located upstream of the traveling bypass cut-off valve  37 L with the oil tank  32 , tends to close. Thus, there is no fear that a portion of pressure oil from the pump  21  may flow in a too large amount through the straight-travel valve  38  to the center bypass passage  29  side which is different from the center bypass passage  28  located on the traveling motor  2 R side which is in operation. Consequently, the pressure oil from the pump  21  can be fed sufficiently to the traveling motor  2 R in operation. 
     When the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9  are operated simultaneously, pressure oil is fed from the pump  20  to the working actuators  4  and  7 ˜ 9  through the working oil passage  40 . At this time, a surplus portion of the pressure oil fed through the working oil passage  40  to the working actuator in operation passes through the bleed-off passage  27  in the direction control valve associated with the working actuator in operation and flows to the oil tank  32 . Thus, making the most of the opening area characteristics of the bleed-off passages  27  in the direction control valves  23 ˜ 26  associated with the working actuators  4  and  7 ˜ 9 , pressure oil can be fed to the working actuators  4  and  7 ˜ 9  under operation at flow rates proportional to the amounts of operation of the associated operating levers  43 , whereby the working actuators  4  and  7 ˜ 9  can be operated smoothly. 
     The following description is now provided about the operation performed in response to operation of the operating volume  46  (see FIG. 2) from position “OFF” to position “ON.” In this embodiment, when the operating volume  46  is operated to “ON” position side, a characteristic of the energizing control for the straight-travel proportional valve  41  with Flag Fd=1 (in the simultaneous operation of the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9 ) and a characteristic of control for the discharge rate of the pump  21  are set variably. 
     More specifically, with reference to FIG. 6A, when the operating volume  46  is operated to “ON” position side, the controller  45  makes control, as indicated with dot-dash lines in the same figure, in such a manner that when the pilot pressure Pi (max of travel) is not lower than the predetermined value Pix (when the amount of operation of the operating level  43  associated with the traveling motor  2 R or  2 L is relatively large), the energizing current for the straight-travel valve  41  for the pilot pressure Pi (max of travel) is made smaller than in case of the operating volume  46  being operated to “OFF” position, and that the larger the amount of operation of the operating volume  46  to the “ON” position side, the smaller is made the said energizing current. Particularly, when the operating volume  46  is operated to a maximum degree, as indicated with a dot-dash line “a” in the figure, the energizing current for the straight-travel proportional valve  41  is maintained at current I 1  which holds the straight-travel valve  38  at position E, independently of pilot pressure Pi (travel max), when the pilot pressure Pi (travel max) is not lower than the minimum pressure Pis. 
     Referring to FIG. 7, when the operating volume  46  is operated to “ON” position side, the controller  45  controls the regulator  21   a  for the pump  21  so that the discharge rate of the pump  21  for the pilot pressure Pi (right-hand travel)+Pi (left-hand travel) becomes smaller than in case of the operating volume  46  being operated to “OFF” position. In this case, the controller  45  makes control so that the larger the amount of operation of the operating volume  46 , the smaller the discharge rate of the pump  21 . 
     Since control is thus made according to operations of the operating volume  46 , when the operating volume  46  is operated to “ON” position side and when the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9  are operated simultaneously, the straight-travel valve  38  is controlled to a position closer to position E rather than position F even if the amount of operation of the operating lever  43  associated with the traveling motor  2 R or  2 L which is in operation is made relatively large. Particularly, when the operating volume  46  is operated to a maximum degree, the straight travel valve  38  is held in position E independently of the amount of operation of the operating lever  43  for travel. 
     Consequently, it is no longer necessary to hold the operating lever  43  for travel in the range corresponding to the foregoing low-operation range in order to avoid interference at position F of the straight travel valve  38  between the pressure oil fed to the working actuators  4 ,  7 ˜ 9  and the pressure oil fed to the traveling motors  2 R,  2 L. Thus, the above interference can be avoided under a relatively rough operation of the operating lever  43 . 
     At this time, even if the operating lever  43  for travel is operated to a large extent, the traveling speed of the hydraulic excavator is kept to a low speed because the discharge rate of the pump  21  serving as a pressure oil supply source for the traveling motors  2 R and  2 L is kept to a small value. Consequently, it is possible to easily effect the operation for operating the working actuators  4  and  7 ˜ 9  while ensuring a stable traveling speed of the hydraulic excavator. 
     In this embodiment, when the values of Flags Fa and Fb are both “0” (with both traveling motors  2 R and  2 L OFF), the right- and left-hand traveling proportional valves  39 R,  39 L and the straight-travel proportional valve  41  are subjected to an energizing control so as to respectively hold the traveling bypass cut-off valves  37 R,  37 L and the straight-travel valve  38  at their neutral positions. Therefore, when the working actuators  4  and  7 ˜ 9  are operated with both traveling motors  2 R and  2 L OFF, basically pressure oil is fed from the pump  21  to the working actuators  7  and  9  in group G 1  and pressure oil is fed from the pump  20  to the working actuators  4  and  8  in group G 2 . 
     In this case, for example when the operating lever  43  associated with the boom cylinder  7  is operated in a large amount of operation (an approximately maximum amount of operation), the boom confluence valve  36  is controlled to an open condition by the controller  45  through an electromagnetic proportional reducing valve (not shown) and the cut-off valve  31  is controlled to a closed condition by the controller  45  through an electromagnetic proportional reducing valve (not shown), whereby the pressure oil from both pumps  20  and  21  are joined and fed to the boom cylinder  7 . Likewise, when the operating lever  43  associated with the arm cylinder  8  is operated in a large amount of operation (an approximately maximum amount of operation), the arm confluence valve  35  is controlled to an open condition by the controller  45  through an electromagnetic proportional reducing valve (not shown) and the cut-off valve  30  is controlled to a closed condition by the controller  45  through an electromagnetic proportional reducing valve, whereby the pressure oil from both pumps  20  and  21  are joined and fed to the boom cylinder  7 . 
     A second embodiment of the present invention will be described below with reference to FIGS. 8 and 9. This embodiment is different only partially in construction from the previous first embodiment, so the same constructional portions as in the first embodiment are identified by the same reference numerals as in the first embodiment and explanations thereof will here be omitted. This embodiment is related to the foregoing first mode of the present invention. 
     A hydraulic system of this embodiment is provided with a working oil passage  48  of a different connectional construction from that used in the first embodiment. The working oil passage  48  comprises a main passage  48   a  connected to the straight-travel valve  38  and plural branch passages  48   b ˜ 48   g  branched from the main passage  48   a . Of the branch passages  48   b ˜ 48   g , the branch passages  48   b ,  48   c ,  48   d , and  48   e  are connected respectively to inlet ports of meter-in passages in the direction control valve  24  for boom, direction control valve  23  for rotation, direction control valve  26  for bucket, and direction control valve  25  for arm. The branch passages  48   f  and  48   g  are connected to inlet ports of the arm confluence valve  35  and boom confluence valve  36  respectively. An oil passage  49 R branched from the center bypass passage  28  at a position between the direction control valve  22 R for right-hand travel and the traveling bypass cut-off valve  37 R located downstream of the valve  22 R is connected into communication with upstream portions of the branch passages  48   b ,  48   d , and  48   f  located on the first group G 1  side, while an oil passage  49 L branched from the center bypass passage  29  at a position between the direction control valve  22 L for left-hand travel and the traveling bypass cut-off valve  37 L located downstream of the valve  22 L is connected into communication with upstream portions of the branch passages  38   c ,  48   e , and  48   g  located on the second group G 2  side. 
     The hydraulic system of this system is further provided with an unloading valve  50  for work which can open the main passage  48  of the working oil passage  48  to the oil tank  32  and an electromagnetic proportional reducing valve  51  for actuating the unloading valve  50  for work. The unloading valve  50  for work is a control valve (spool valve) which can open and close and which can adjust the area of its opening. An inlet port of the unloading valve  50  is connected to an oil passage  52  which is branched from the main passage  48   a  on the upstream side of the branch passages  48   b ˜ 48   g  of the working oil passage  48 , and an outlet port thereof is put in communication with an oil tank  32 . The unloading valve  50  for work is closed in a neutral state thereof. The electromagnetic proportional reducing valve  51  (“working proportional valve  51 ” hereinafter) is of the same structure as the proportional valves  39 R,  39 L, and  41  described in the first embodiment and is connected to a pilot port of the unloading valve  50  for work. 
     The cut-off valves  30  and  31  disposed in the center bypass passages  28  and  29  respectively in the first embodiment are not used in this first embodiment The other constructional portions of the hydraulic system of this embodiment are the same as in the first embodiment. Like the first embodiment, the hydraulic system of this embodiment illustrated in FIG. 8, for controlling the operation thereof, is provided with the operation quantity detector  44 , controller  46 , and operating volume  46  which are illustrated in FIG.  2 . But in this embodiment, though not shown, the controller  45  can make an energizing control for the working proportional valve  51  in addition to the proportional valves  39 R,  39 L,  41  and the regulators  20   a ,  21   a  for the pumps  20 ,  21  described in the first embodiment. 
     Reference will now be made to the operation of the hydraulic system of this embodiment. In this embodiment, as in the first embodiment, the controller  45  executes the setting of Flags Fa˜Fd in a successive manner. Then, in accordance with the values of Flags Fa˜Fd the controller  45  makes an energizing control for each of the right- and left-hand traveling proportional valves  39 R,  39 L, the straight-travel proportional valve  38 , and the regulators  20   a  and  21   a  for the pumps  20  and  21  in the same way as in the first embodiment, allowing the traveling bypass cut-off valves  37 R and  37 L and the straight-travel valve  38  to operate and controlling the discharge rate of the pumps  20  and  21  as described in the first embodiment. 
     On the other hand, with Fd=1, namely, in the simultaneous operation of the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9 , the controller  45  determines an energizing current for the working proportional valve  51  with reference to a data table built in advance, as in FIG. 9, and in accordance with a maximum pilot pressure Pi (work max) out of pilot pressures Pi (work) which represent the amounts of operation of the operating levers  43  associated with the working actuators  4  and  7 ˜ 9  respectively. The controller  45  then supplies the thus-determined energizing current to the working proportional valve  51 , causing the unloading valve  50  for work to operate. 
     In the data table of FIG. 9, when the pilot pressure Pi (work max) becomes a predetermined minimum pressure Pis or higher, the energizing current in the working proportional valve  51  increases from a predetermined lower-limit current Imin which holds the unloading valve  50  for work in a closed state to an upper-limit current Imax which switches the unloading valve  50  to a fully open condition in an instant and holds it in that condition. The energizing current in the working proportional valve  51  is held at the upper-limit current Imax until the pilot pressure Pi (work max) rises to a predetermined pressure Piy which is a little higher than the minimum pressure Pis, and thereafter decreases gradually from the upper-limit current Imax to the lower-limit current Imin with an increase of the pilot pressure Pi (work max) (an increase in the amount of operation of the associated operating lever  43 ). In this case, the opening area of the unloading valve  50  for work becomes smaller with a decrease of the energizing current in the working proportional valve  51 . 
     With Fd=0 (when the traveling motor  2 R or  2 L is ON and any of the working actuators  4  and  7 ˜ 9  is OFF), the controller  45  supplies the upper-limit current Imax to the working proportional valve  51  which current holds the unloading valve  50  for work in a fully open condition. When both traveling motors  2 R and  2 L are OFF, the controller  45  supplies the lower-limit current Imin to the working proportional valve  51  which current holds the unloading valve  50  in a closed condition. 
     By operation of the unloading valve  50  for work responsive to such energizing control for the working proportional valve  51 , bleed-off for the working actuators  4  and  7 ˜ 9  in operation is performed through the unloading valve  50  in the simultaneous operation of the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9 . 
     Thus, in the simultaneous operation (Fd=1) in this embodiment, pressure oil does not flow through the bleed-off passages  27  in the direction control valves  23 ˜ 26  corresponding to the working actuators  4  and  7 ˜ 9 , but a surplus portion of pressure oil fed from the pump  20  serving as a pressure oil supply source for the working actuators  4  and  7 ˜ 9  to the working oil passage  48  through the straight-travel valve  38  flows from the main passage  48   a  of the working oil passage  48  to the oil tank  32  through the oil passage  52  and the unloading valve  50  for work. At this time, the area of opening of the unloading valve  50  becomes smaller as the amount of operation of the operating lever  43  associated with the working actuator which is in operation increases (as the pilot pressure Pi (work max) increases), whereby bleed-off for the working actuators  4  and  7 ˜ 9  is effected appropriately in the above simultaneous operation (Fd=1) and the working actuators  4  and  7 ˜ 9  can be operated smoothly. Other operations (including the operation performed upon operation of the operating volume  46 ) and functions and effects are the same as in the first embodiment. 
     In this embodiment, for example when the operating lever  43  associated with the boom cylinder  7  is operated in a large amount of operation, with the traveling motors  2 R and  2 L OFF, and when pressure oils from both pumps  20  and  21  are joined and fed to the boom cylinder  7  (“boom joining operation” hereinafter) as described in the first embodiment, the controller  45  causes the boom confluence valve  36  to open as in the first embodiment and makes an energizing control for the left-hand traveling proportional valve  39 L so as to hold the traveling bypass cut-off valve  37 L in position C. Likewise, when the operating lever  43  associated with the arm cylinder  8  is operated in a large amount of operation and pressure oils from both pumps  20  and  21  are joined and fed to the arm cylinder  8  (“arm joining operation” hereinafter), the controller  45  causes the arm confluence valve  35  to open as in the first embodiment and holds the traveling bypass cut-off valve  37 R in position C. Thus, the cut-off valves  30  and  31  used in the first embodiment are not necessary in this second embodiment. 
     A third embodiment of the present invention will now be described with reference to FIGS. 10 and 11. This second embodiment is different only partially in construction from the previous second embodiment, so the same constructional portions as in the second embodiment are identified by the same reference numerals as in the second embodiment and explanations thereof will here be omitted. This embodiment is related to the foregoing first mode of the present invention. 
     In this embodiment, instead of the traveling bypass cut-off valves  37 R and  37 L used in the second embodiment, traveling bypass cut-off valves  53 R and  53 L which can merely open and close are disposed in the center bypass passages  28  and  29  respectively. The traveling bypass cut-off valves correspond to the cut-off valve in the foregoing first mode of the present invention and are open in their neutral state. A right-hand traveling proportional valve  54 R and a left-hand proportional valve  54 L, which are constituted by electromagnetic proportional reducing valves of the same structures as the right- and left-hand traveling proportional valves  39 R and  39 L, are connected respectively to pilot ports of the traveling bypass cut-off valves  53 R and  53 L. 
     In this embodiment, the center bypass passage  28  between the right-hand traveling direction control valve  22 R and the traveling bypass cut-off valve  53 R located downstream of the valve  22 R and the center bypass passage  29  between the left-hand traveling direction control valve  22 L and the traveling bypass cut-off valve  53 L located downstream of the valve  22 L are connected into communication with each other through an oil passage  55 . The hydraulic system of this embodiment is further provided with an unloading valve  56  for travel which can open the oil passage  55  to an oil tank  32  and an electromagnetic proportional reducing valve  57  for actuating the unloading valve  56 . 
     The unloading valve  56  for travel is a control valve (spool valve) which-can open and close and which can adjust the area of its opening. An inlet port of the unloading valve  56  is connected into communication with the oil passage  55  through an oil passage  58  and an outlet port thereof is put in communication with the oil tank  32 . The unloading valve  56  for travel, which is closed in its neutral state, corresponds to the opening valve in the foregoing first mode of the present invention. The electromagnetic proportional reducing valve  57  (“traveling proportional valve  57 ” hereinafter) is of the same structure as the straight-travel proportional valve  41  and is connected to a pilot port of the unloading valve  56  for travel. 
     The other constructional portions than above of the hydraulic system of this embodiment are the same as in the second embodiment. Further, in this embodiment, for controlling the hydraulic system illustrated in FIG. 10, there are provided such operation quantity detector  44 , controller  46  and operating volume  46  as are illustrated in FIG. 2, like the first and second embodiments. But in this embodiment, though not shown, the controller  45  can make an energizing control for the straight-travel proportional valve  41 , right- and left-hand proportional valves  53 R,  54 L, traveling proportional valve  56 , working proportional valve  51 , and regulators  20   a  and  21   a  for the pumps  20  and  21 . 
     The following description is now provided about the operation of the hydraulic system of this embodiment. In this embodiment, as in the second embodiment, the controller  45  executes the setting of Flags Fa˜Fd in a successive manner. In accordance with the values of Flags Fa˜Fd the controller  45  makes an energizing control for each of the straight-travel proportional valve  38 , the working proportional valve  51 , and the regulators  20   a  and  21   a  for the pumps  20  and  21  in the same manner as in the second embodiment, allowing the straight-travel valve  38  and the unloading valve  50  for work to operate and controlling the discharge rate of the pumps  20  and  21  as described in the first embodiment. 
     On the other hand, in case of Fa=1 or Fb=1, namely, the traveling motor  2 R or  2 L is in operation, the controller  45  makes control to supply an energizing current (upper-limit current) which holds both traveling bypass cut-off valves  53 R and  53 L in a closed condition to the right- and left-hand traveling proportional valves  54 R,  54 L independently of pilot pressures Pi (right-hand travel) (&gt;Pis) and Pi (left-hand travel) (&gt;Pis) which are related to the amount of operation of the operating lever  43  for travel. 
     Further, with reference to a data table built in advance, as indicated with a solid line in FIG. 11A or  11 B, the controller  45  determines an energizing current for the traveling proportional valve  57  in accordance with the higher pilot pressure Pi (travel max)=max (Pi(right-hand travel), Pi(left-hand travel)) out of pilot pressures Pi(right-hand travel) (&gt;Pis) and Pi(left-hand travel) (&gt;Pis). The controller  45  then supplies the thus-determined energizing current to the traveling proportional valve  57 , causing the unloading valve  56  for travel to operate. The data table indicated with a solid line in FIG. 11A is to be used when both traveling motors  2 R and  2 L are in operation (Fa=Fb=1), while the data table indicated with a solid line in FIG. 11B is to be used when only one of the traveling motors  2 R and  2 L is in operation (Fa=1 and Fb=0, or Fa=0 and Fb=1). 
     The dot-dash line graphs in FIGS. 11A and 11B are concerned with the case where the operating volume  46  is operated to “ON” position. On this regard, a description will be given later. Here it is assumed that the operating volume  46  is operated to “OFF” position. 
     In the data table indicated with a solid line in FIG. 11A, when the pilot pressure Pi (travel max) becomes a predetermined minimum pressure Pis or higher, the energizing current in the traveling proportional valve  57  increases from a predetermined lower-limit current Imin which holds the unloading valve  56  for travel in a closed condition to an upper-limit current Imax which switches the unloading valve  56  to a fully closed condition in an instant and holds it in that condition. The energizing current in the traveling proportional valve  57  is held in the upper-limit current Imax until the pilot pressure Pi (travel max) rises to a predetermined pressure Piz which is higher than the minimum Pis, then decreases gradually from the upper-limit current Imax to the lower-limit current Imin with an increase of the pilot pressure Pi (work max) (an increase in the amount of operation of the operating lever  43  for travel). In this case, the area of opening of the unloading valve  56  for travel becomes smaller as the energizing current in the traveling proportional valve  57  decreases. 
     In the data table of FIG. 11B, when the pilot pressure Pi (travel max) becomes a predetermined minimum pressure Pis or higher, the energizing current in the traveling proportional valve  57  increases from a predetermined lower limit Imin which holds the unloading valve  56  for travel in a closed condition to an upper-limit current Imax which switches the unloading valve  56  to a fully open condition in an instant. Thereafter, as the pilot pressure Pi (work max) increases (as the amount of operation of the operating lever  43  for travel increases), the energizing current in the traveling proportional valve  57  decreases gradually from the upper-limit current Imax to the lower-limit current Imin. Consequently, with an increase of pilot pressure Pi (work max), the area of opening of the unloading valve  56  for travel becomes smaller more rapidly than in case of using the data table of FIG. 11A (Fa=Fb=1). This for preventing the operating pressure of the traveling motor  2 R from becoming higher in the operation of only one of the traveling motors  2 R and  2 L, e.g., only  2 R, than in the operation of both traveling motors  2 R and  2 L and for preventing the resultant deepening (increase in the amount of operation) of the operation lever  43  associated with the traveling motor  2 R which is in operation. 
     By such operations of the bypass cut-off valves  53 R,  53 L and the unloading valve  56  for travel responsive to the energizing control for the right-traveling proportional valves  54 R,  54 L and the unloading valve  56  for travel, in the operation of the traveling motor  2 R or  2 L, the downstream sides of bleed-off passages  27  in the traveling direction control valves  22 R and  22 L come into communication with the oil tank  32  through the unloading valve  56  and are disconnected from the working direction control valves  23 ˜ 26  located downstream of the direction control valves  22 R and  22 L by means of the traveling bypass cut-off valves  53 R and  53 L which are closed, so that the pressure oil flowing through the bleed-off passages  27  in the direction control valves  22 R and  22 L does not flow through the working direction control valves  23 ˜ 26 . Thus, the traveling bypass cut-off valves  53 R,  53 L and the unloading valve  56  for travel used in this embodiment fulfill the same function as that of the traveling bypass cut-off valves  37 R and  37 L used in the first and second embodiments. In the hydraulic system of this embodiment, the other constructions and operations than those of the traveling bypass cut-off valves  53 R,  53 L and the unloading valve  56  for travel are the same as in the second embodiment. Therefore, also in this embodiment there can be attained the same functions and effects as in the second embodiment. 
     In this embodiment, when the operating volume  46  is operated from position “OFF” to position “ON,” the controller  45  supplies the traveling proportional valve  57  with such an energizing current as keeps the opening area of the unloading valve  56  for travel at a constant opening area in a relatively high pilot pressure Pi (travel max), as indicated with dot-dash lines in FIGS. 11A and 11B. In this case, the larger the amount of operation of the operating volume  46 , the larger the energizing current in the traveling proportional valve  57 . 
     By so doing, not only there are performed such discharge rate control for the pump  21  and operation control for the straight-travel valve  38  responsive to operations of the operating volume  46  as described in the first embodiment, but also the operating speed of the traveling motors  2 R and  2 L can be kept to a low speed effectively even if the associated operating lever  43  for travel is operated relatively largely. As a result, operations for operating the working actuators  4  and  7 ˜ 9  can be done easily while ensuring a stable speed of the hydraulic excavator. 
     In this embodiment, when the traveling motors  2 R and  2 L are OFF, the unloading valve  56  for travel is held in its closed state (neutral state). Then, in the foregoing boom joining operation, the controller  45  causes the boom confluence valve  36  to open in the same manner as in the first embodiment and makes an energizing control for the left-hand traveling proportional valve  54 L so as to keep the traveling bypass cut-off valve  53 L closed. Likewise, in the foregoing arm joining operation, the controller  45  causes the arm confluence valve  35  to open in the same manner as in the first embodiment and holds the traveling bypass cut-off valve  53 R in a closed condition. Thus, also in this embodiment, like the second embodiment, the cut-off valves  30  and  31  used in the first embodiment are not necessary. 
     Although in this embodiment the unloading valve  56  for travel is used in common to both traveling motors  2 R and  2 L, separate unloading valves for travel may be connected to the downstream sides of the bleed-off passages  27  of the traveling direction control valves  22 R and  22 L (upstream sides of the traveling bypass cut-off valves  53 R and  53 L). In this case, when both traveling motors  2 R and  2 L are in operation, the separate unloading valves may be operated according to pilot pressures Pi (right-hand travel) and Pi (left-hand travel) corresponding respectively to the traveling motors  2 R and  2 L for example with such a characteristic as shown in FIG.  11 A. When only one of the traveling motors  2 R and  2 L is in operation, for example when the traveling motor  2 R is in operation, the unloading valve for travel associated with the traveling motor  2 R which is in operation is operated according to pilot pressure Pi (right-hand travel) with such a characteristic as shown in FIG. 11A, while the unloading valve for travel associated with the traveling motor  2 L which is OFF is held in a closed condition. 
     Next, a fourth embodiment of the present invention will be described with reference to FIGS. 12 and 13. This embodiment is different only partially in construction from the previous third embodiment, so the same constructional portions as in the third embodiment are identified by the same reference numerals as in the third embodiment and explanations thereof will here be omitted. This embodiment is related to the foregoing second mode of the present invention. 
     In this embodiment, the spool shape of traveling direction control valves  22 RR and  22 LL and an elastic force characteristic of a return spring (a spring for urging to a neutral position) are set beforehand so that the bleed-off passages  27  in the direction control passages  22 RR and  22 LL vary in the area of opening in accordance with pilot pressures Pi (right-hand travel) and Pi (left-hand travel) which are applied to pilot ports of the valves  22 RR and  22 LL. More specifically, when the pilot pressures Pi (right-hand travel) and Pi (left-hand travel) proportional to operations of the associated operating levers  43  become a minimum pressure Pis at which the direction control valves  22 RR and  22 LL for travel are switched into operation, the bleed-off passages  27  in the direction control valves  22 RR and  22 LL assume a fully closed state immediately from a fully open state and are thereafter held in the fully closed state independently of an increase of pilot pressures Pi (right-hand travel) and Pi (left-hand travel). Immediately after the bleed-off passages  27  were put in the fully closed state, meter-in passages in the direction control valves  22 RR and  22 LL become larger in their opening area with an increase of pilot pressures Pi (right-hand travel) and Pi (left-hand travel). 
     In this embodiment, an unloading valve  56  for travel, which corresponds to the opening valve in the foregoing second mode of the present invention, is connected to an oil passage  59  extending from the pump  21  to the straight-travel valve  38 , through an oil passage  60  branched from the oil passage  59 . The other constructional portions than above are just the same as in the previous third embodiment. 
     Next, the operation of the hydraulic system of this embodiment will be described. In this embodiment, as in the third embodiment, the controller  45  executes the setting of Flags Fa˜Fd in a successive manner, then in accordance with the values of Flags Fa˜Fd the controller  45  makes an energizing control for the straight-travel valve  38 , the working proportional valve  51 , and the regulators  20   a  and  21   a  for the pumps  20  and  21 , causing the straight-travel valve  38  and the unloading valve  50  for work to operate, and controls the discharge rate of the pumps  20  and  21 . 
     On the other hand, in case of Fa=1 or Fb=1, namely, when either the traveling motor  2 R or  2 L is in operation, the controller  45  supplies the left-hand traveling proportional valve  54 L with an energizing current (upper-limit current) which holds the traveling bypass cut-off valve  53 L associated with the left-hand traveling motor  2 L in a closed state when only the traveling motor  2 R is in operation (Fa=1 and Fb=0), while when only the traveling motor  2 L is in operation (Fa=0 and Fb=1), the controller  45  supplies the right-hand traveling proportional valve  54 R with an energizing current (upper-limit current) which holds the traveling bypass cut-off valve  53 R associated with the right-hand traveling motor  2 R in a closed state. Thus, the traveling bypass cut-off valve  53 R or  53 L associated with the traveling motor  2 R or  2 L which is OFF is closed when only one of the traveling motors  2 R and  2 L is ON, whereby the pressure oil from the pump  21  flows through the center bypass passage  28  or  29  associated with the traveling motor  2 R or  2 L which is OFF and what is called pressure relief is prevented thereby. 
     The bleed-off passage  27  in the direction control valve  22 R or  22 L associated with the traveling motor  2 R or  2 L which is in operation is fully closed, therefore, the state of the traveling bypass cut-off valve  53 R in case of Fa=1 and Fb=0, the state of the traveling bypass cut-off valve  53 L in case of Fa=0 and Fb=1, and the state of both traveling bypass cut-off valves  53 R and  3 L in case of Fa=Fb=1 (both traveling motors  2 R and  2 L are ON), are not specially limited. In this embodiment, they are closed states for example. As in the third embodiment, both traveling bypass cut-off valves  53 R and  53 L may be kept closed in case of Fa=1 or Fb=1. 
     With Fa=1 or Fb=1, the controller  45  determines the foregoing energizing current for the traveling proportional valve  57  with reference to, for example, the data table of FIG. 11A described in the third embodiment and in accordance with pilot pressure Pi (travel max)=max (Pi(right-hand travel), Pi(left-hand travel)) independently of whether only one of the traveling motors  2 R and  2 L is in operation or both are in operation. Then, the controller  45  supplies the traveling proportional valve  57  with the thus-determined energizing current and causes the unloading valve  56  for travel to operate. 
     In such a hydraulic system of this embodiment, when the traveling motors  2 R and  2 L are in operation, the bleed-off passages  27  in the direction control valves  22 RR and  22 LL associated with the energized traveling motors  2 R and  2 L are fully closed constantly, so that the center bypass passages  28  and  29  are cut off at the positions of the direction control valves  22 RR and  22 LL. Therefore, even if any of the working actuators  4  and  7 ˜ 9  associated with any of the working direction control valves  23 ˜ 26  located downstream of the direction control valves  22 RR and  22 LL which are associated with the energized traveling motors  2 R and  2 L is operated, the occurrence of pressure interference between the pressure oil fed to the traveling motors  2 R,  2 L and the pressure oil fed to the working actuators  4  and  7 ˜ 9  is prevented. Then, by operating the unloading valve  56  for travel in the manner described above, there is made an appropriate bleed-off for the traveling motors  2 R and  2 L. Consequently, there can be attained the same functions and effects as in the third embodiment. 
     The other operations (including operation of the operating volume  46  and operation of the traveling bypass cut-off valves  53 R and  53 L in the boom and arm joining operations) than the above are the same as in the first embodiment. 
     In this embodiment, the traveling bypass cut-off valves  53 R and  53 L may be disposed at the positions of the cut-off valves  35  and  36  used in the first embodiment and illustrated in FIG. 1, or the cut-off valves  35  and  36  illustrated in FIG. 1 may be used as the traveling bypass cut-off valves  53 R and  53 L in this embodiment. 
     Although the straight-travel valve  38  of such a construction as shown in FIGS. 1,  8 ,  10 , and  12  is used in the first to fourth embodiments, the straight-travel valve used in the present invention is not limited thereto. For example, there may be used a straight-travel valve of such a construction as shown in FIG. 14A or  14 B. In FIGS. 14A and 14B, the same functional portions as in the previous embodiments are identified by the same reference numerals as in the previous embodiments. The straight-travel valves shown in both figures exhibit the same function as that of the straight-travel valve  38  used in the previous embodiments, and how to operate and control them may also be the same as in the previous embodiments. 
     In the above embodiments, when the traveling motor  2 R or  2 L and any of the working actuators  4  and  7 ˜ 9  are operated simultaneously, a control characteristic (see FIG. 6A) for the pilot pressure Pi (travel max) in the straight-travel valve  38  is changed stepwise according to the amount of operation of the operating volume  46 , but there may be adapted a modification in which when the operating volume  46  lies in its “ON” position for example and during operation of the traveling motor  2 R or  2 L, the straight-travel valve  38  is controlled constantly with such a characteristic as indicated by a dot-dash line “a” in FIG.  6 A and is thereby held in its position E. 
     Moreover, although in the above embodiments the operating volume  46  is used for making the control characteristic of the straight-travel valve  38 , etc. variable, the control characteristic of the straight-travel valve  38 , etc. may be rendered variable by operating a two-stage control switch having only two operating positions corresponding to “OFF” and “ON” positions of the operating volume  46  or by driver&#39;s voice indication or the like. 
     Further, although in the third and fourth embodiments the working oil passage  48  is constructed in the same manner as in the second embodiment, there may be adapted such a working oil passage  40  as in the first embodiment. For example, in the fourth embodiment, in case of adopting the working oil passage  40  used in the first embodiment in place of the working oil passage  48 , the unloading valve  50  for work, the working proportional valve  51  and the oil passage  52  used in the fourth embodiment are removed and the unloading valve  56  and the traveling bypass cut-off valves  53 R and  53 L are controlled in the manner described in the fourth embodiment. In case of performing the foregoing boom joining operation and arm joining operation, such cut-off valves  30  and  31  as those used in the first embodiment are disposed in the most downstream portions of the center bypass passages  28  and  29  and may be operated as described in the first embodiment. 
     Although embodiments of the present invention have been described above, the scope of protection of the invention is not limited thereto.