Patent Publication Number: US-10309079-B2

Title: Hydraulic control system for work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a hydraulic control system for a work machine. 
     BACKGROUND ART 
     In a hydraulic control system for an excavator or other work machine, a pump delivery amount increases in accordance with the operation amount of an operating device, and at the same time, a spool in a control valve is operated by a pilot pressure based on the operation amount to permit a hydraulic pump to communicate with hydraulic actuators such as a hydraulic cylinder and a hydraulic motor. As the spool in the control valve has an opening formed to vary in accordance with a stroke, the degree of communication between the hydraulic actuators and the hydraulic pump can be changed by the pilot pressure. 
     Consequently, when a combined operation is performed to simultaneously operate a plurality of hydraulic actuators, the pump delivery amount can be divided to operate in combination the hydraulic actuators in accordance with the operation amounts of individual operating devices. 
     A hydraulic control circuit for a construction machine that is described, for instance, in Patent Document 1 controls a first pump and a second pump in order to avoid a decrease in an operating speed when a hydraulic actuator for an attachment and another hydraulic actuator operate in combination with each other. The hydraulic control circuit is capable of supplying hydraulic fluid from the first pump to the hydraulic actuator for the attachment and another hydraulic actuator through an associated spool and from the second pump to the hydraulic actuator for the attachment and another hydraulic actuator through an associated spool. The first pump and the second pump are controlled in such a manner that the flow rate obtained when the hydraulic actuator for the attachment and another hydraulic actuator operate in combination with each other is equal to the sum of the flow rate of the hydraulic actuator for an attachment and the flow rate of the other hydraulic actuator. 
     PRIOR ART LITERATURE 
     Patent Document 
     
         
         Patent Document 1: JP-2010-236607-A 
       
    
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     The above-described prior hydraulic control circuit makes it possible to prevent the operating speed of a hydraulic actuator from decreasing due to an insufficient pump flow rate during a combined operation. This circuit not only provides increased, work efficiency, but also avoids an unnecessary increase in a pump flow rate. 
     However, when the load pressure of a hydraulic actuator is different from that of the hydraulic actuator for the attachment when they are operated in combination, a flow division loss occurs in the above-described prior hydraulic control circuit in accordance with the pressure difference and flow rate. Consequently, the flow division loss may increase with an increase in the flow rate of a hydraulic pump. 
     The present invention has been made in view of the above circumstances. An object of the present invention is to provide a hydraulic control system for a work machine that is capable of reducing the loss caused by flow division while reducing a decrease in the speed of a hydraulic actuator due to a combined operation. 
     Means for Solving the Problems 
     In accomplishing the above object, according to a first aspect of the present invention, there is provided a hydraulic control system for a work machine including a first hydraulic actuator, one hydraulic pump, a second hydraulic actuator, another hydraulic pump, and a secondary spool for the first hydraulic actuator. The one hydraulic pump is capable of supplying hydraulic fluid to the first hydraulic actuator through a primary spool for the first hydraulic actuator. The another hydraulic pump is capable of supplying hydraulic fluid to the second hydraulic actuator through a primary spool for the second hydraulic actuator. The secondary spool for the first hydraulic actuator is capable of placing the first hydraulic actuator in communication with the another hydraulic pump. The hydraulic control system further includes operating instruction detection means and pump flow control means. The operating instruction detection means detects that operating instructions are issued to the first hydraulic actuator and the second hydraulic actuator. The pump flow control means is capable of adjusting the delivery flow rate of the one hydraulic pump and the delivery flow rate of the another hydraulic pump on an individual basis in accordance with operation amounts designated by the operating instructions for the first and second hydraulic actuators, which are detected by the operating instruction detection means. When the first and second hydraulic actuators are simultaneously operated, the pump flow control means increases the delivery flow rate of the one hydraulic pump to a higher rate than when the first hydraulic actuator is operated and the second hydraulic actuator is not operated. 
     Advantages of the Invention 
     According to the present invention, the hydraulic control system for a work machine includes the first hydraulic actuator, the one hydraulic pump, the second hydraulic actuator, the another hydraulic pump, and the secondary spool for the first hydraulic actuator. The one hydraulic pump is capable of supplying hydraulic fluid to the first hydraulic actuator through the primary spool for the first hydraulic actuator. The another hydraulic pump is capable of supplying hydraulic fluid to the second hydraulic actuator through the primary spool for the second hydraulic actuator. The secondary spool for the first hydraulic actuator is capable of placing the first hydraulic actuator in communication with the another hydraulic pump. When the first and second hydraulic actuators are simultaneously operated, the delivery flow rate of the one hydraulic pump increases to a higher rate than when the first hydraulic actuator is operated and the second hydraulic actuator is not operated. Therefore, it is possible to reduce a decrease in the speed of the first hydraulic actuator that is caused by the operation of the second hydraulic actuator. Further, in the above instance, the opening for communication between the first hydraulic actuator and the another hydraulic pump is interrupted. Consequently, the amount of divided flow of the delivery from the another hydraulic pump can be decreased to reduce the flow division loss. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating a work machine having an embodiment of a hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 2  is a hydraulic control circuit diagram illustrating an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 3  is a conceptual diagram illustrating a configuration of a controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 4  is a characteristic diagram illustrating an exemplary map of a target operation computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 5  is a control block diagram illustrating an exemplary computation of a communication control section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 6  is a conceptual diagram illustrating a configuration of a flow control section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 7  is a control block diagram illustrating an exemplary computation of a boom flow distribution computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 8  is a control block diagram illustrating an exemplary computation of an arm target flow distribution computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 9  is a control block diagram illustrating an exemplary computation of a pump flow rate command computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 10  is a characteristic diagram illustrating an exemplary operation related to pump flow control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 11  is a characteristic diagram illustrating another exemplary operation related to the pump flow control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 12  is a characteristic diagram illustrating an exemplary operation related to the pump flow control means and communication control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
         FIG. 13  is a characteristic diagram illustrating another exemplary operation related to the pump flow control means and communication control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of a hydraulic control system for a work machine according to the present invention will now be described with reference to the accompanying drawings.  FIG. 1  is a perspective view illustrating a work machine having an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 2  is a hydraulic control circuit diagram illustrating an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
     As illustrated in  FIG. 1 , a hydraulic excavator having an embodiment of the hydraulic control system for a work machine in accordance with the present invention includes a lower travel structure  1 , an upper swing structure  2 , a front work device, and an engine  2 A. The upper swing structure  2  is disposed above the lower travel structure  1 . The front work device is vertically rotatably connected to the upper swing structure  2 . The engine  2 A acts as a prime mover. The front work device includes a boom  3 , an arm  4 , and a bucket  5 . The boom  3  is mounted on the upper swing structure  2 . The arm  4  is mounted on the leading end of the boom  3 . The bucket  5  is mounted on the leading end of the arm  4 . The front work device further includes a pair of boom cylinders  6 , an arm cylinder  7 , and a bucket cylinder  8 . The boom cylinders  6  drive the boom  3 . The arm cylinder  7  drives the arm  4 . The bucket cylinder  8  drives the bucket  5 . 
     In accordance with operations of a first operating lever  9   a  and a second operating lever  9   b , which are disposed in a cab on the upper swing structure  2 , the hydraulic excavator operates in such a manner that hydraulic fluid discharged from a hydraulic pump device not shown is supplied to the boom cylinder  6 , the arm cylinder  7 , the bucket cylinder  8 , and a swing hydraulic motor  11  through a control valve  10 . As cylinder rods of the boom cylinder  6 , arm cylinder  7 , and bucket cylinder  8  are extended and contracted by the hydraulic fluid, the position and orientation of the bucket  5  can be changed. Further, as the swing hydraulic motor  11  is rotated by the hydraulic fluid, the upper swing structure  2  swings with respect to the lower travel structure  1 . 
     The control valve  10  includes various later-described control valves, namely, a travel right directional control valve  12   a , a travel left directional control valve  12   b , a boom first directional control valve  13   a , a boom second directional control valve  13   c , an arm first directional control valve  14   c , an arm second directional control valve  14   b , a bucket directional control valve  15   a , and a swing directional control valve  16   b.    
     The engine  2 A includes a revolving speed sensor  2 Ax, which detects an engine revolving speed. The boom cylinder  6  includes a pressure sensor A 6  and a pressure sensor B 6 . The pressure sensor A 6  detects the pressure in a bottom oil chamber. The pressure sensor B 6  detects the pressure in a rod oil chamber. The arm cylinder  7  includes a pressure sensor A 7  and a pressure sensor B 7 . The pressure sensor A 7  acts as load acquisition means that detects the pressure in a bottom oil chamber. The pressure sensor B 7  detects the pressure in a rod oil chamber. Similarly, the bucket cylinder  8  includes a pressure sensor A 8  and a pressure sensor B 8 . The pressure sensor A 8  detects the pressure in a bottom oil chamber. The pressure sensor B 8  detects the pressure in a rod oil chamber. The swing hydraulic motor  11  includes pressure sensors A 11 , B 11 , which detect left and right swing pressures. Pressure signals detected by the above-mentioned pressure sensors A 6 -A 8 , B 6 -B 8 , A 11 , B 11  and the engine revolving speed detected by the revolving speed sensor  2 Ax are inputted to a later-described controller  100 . 
     As illustrated in  FIG. 2 , a hydraulic pump device  20  included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention supplies a pilot pressure to each directional control valve, which acts as a spool in the later-described control valve  10 , in accordance with operations of the first to fourth operating levers  9   a - 9   d  in order to operate each directional control valve in the control valve  10 . The pump device  20  in the hydraulic control system according to the present embodiment includes a first hydraulic pump  20   a , a second hydraulic pump  20   b , and a third hydraulic pump  20   c , which are variable-displacement hydraulic pumps. The first to third hydraulic pumps  20   a - 20   c  are driven by the engine  2 A. 
     The first hydraulic pump  20   a  includes a regulator  20   d , which is driven by a command signal from the later-described controller  100 , and supplies a controlled delivery amount of hydraulic fluid to a first pump line  21   a . Similarly, the second hydraulic pump  20   b  includes a regulator  20   e , which is driven by a command signal from the later-described controller  100 , and supplies a controlled delivery amount of hydraulic fluid to a second pump line  21   b . Further, the third hydraulic pump  20   c  includes a regulator  20   f , which is driven by a command signal from the later-described controller  100 , and supplies a controlled delivery amount of hydraulic fluid to a third pump line  21   c.    
     For the sake of brevity of explanation, a relief valve, a return circuit, a load check valve, and other elements not directly associated with the present embodiment are omitted from the description. Although the present embodiment is described with respect to a case where the present invention is applied to a publicly known, open center type hydraulic control system, the present invention is not limited to such a hydraulic control system. 
     The travel right directional control valve  12   a , the bucket directional control valve  15   a , and the boom first directional control valve  13   a  are disposed in the first pump line  21   a  that is in communication with a delivery port of the first hydraulic pump  20   a . A tandem circuit is formed in such a manner as to give priority to the travel right directional control valve  12   a  The remaining bucket directional control valve  15   a  and boom first directional control valve  13   a  are formed as a parallel circuit. 
     The swing directional control valve  16   b , the arm second directional control valve  14   b , and the travel left directional control valve  12   b  are disposed in the second pump line  21   b  that is in communication with a delivery port of the second hydraulic pump  20   b . The swing directional control valve  16   b  and the arm second directional control valve  14   b  are formed as a parallel circuit, and the travel left directional control valve  12   b  is formed as a parallel-tandem circuit. A check valve  17  and a restrictor  18 , which permit only an inflow from the second hydraulic pump  20   b , are disposed in the parallel circuit of the travel left directional control valve  12   b . The travel left directional control valve  12   b  is capable of communicating with the first hydraulic pump  20  through a travel communication valve  19 . 
     An arm  2  flow control valve  23  is disposed in the parallel circuit of the second pump line  21   b  and driven by a command from the controller  100 . 
     The boom second directional control valve  13   c  and the arm first directional control valve  14   c  are disposed in the third pump line  21   c  that is in communication with a delivery port of the third hydraulic pump  20   c . The boom second directional control valve  13   c  and the arm first directional control valve  14   c  are formed as a parallel circuit. An arm  1  flow control valve  22  is disposed in the parallel circuit of the third pump line  21   c  and driven by a command from the controller  100 . 
     An outlet port of the boom first directional control valve  13   a  and an output port of the boom second directional control valve  13   c  are in communication with the boom cylinder  6  through a junction path not shown. An outlet port of the arm first directional control valve  14   c  and an outlet port of the arm second directional control valve  14   b  are in communication with the arm cylinder  7  through a junction path not shown. An outlet port of the bucket directional control valve  15   a  is in communication with the bucket cylinder  5 , and an outlet port of the swing directional control valve  16   b  is in communication with the swing hydraulic motor  11 . 
     Referring to  FIG. 2 , the first to fourth operating levers  9   a - 9   d  each include a pilot valve not shown and generate a pilot pressure in accordance with the amount of tilting operation of each operating lever. The pilot pressure generated by each operating lever is supplied to the operating section of each directional control valve. 
     Pilot lines indicated by broken lines BkC, BkD are connected from the first operating lever  9   a  to the operating section of the bucket directional control valve  15   a  and respectively used to supply a bucket crowding pilot pressure and a bucket dumping pilot pressure. Further, pilot, lines indicated by broken lines BmD, BmU are connected from the first operating lever  9   a  to the operating sections of the boom first directional control valve  13   a  and boom second directional control valve  13   c  and respectively used to supply a boom raising pilot pressure and a boom lowering pilot pressure. 
     A pressure sensor  105  for detecting the bucket crowding pilot pressure and a pressure sensor  106  for detecting the bucket dumping pilot pressure are disposed in the pilot lines indicated by the broken lines BkC, BkD. A pressure sensor  101  for detecting the boom raising pilot pressure and a pressure sensor  102  for detecting the boom lowering pilot pressure are disposed in the pilot lines indicated by the broken lines BmD, BmU. The pressure sensors  101 ,  102 ,  105 ,  106  each act as operating instruction detection means. Pressure signals detected by the pressure sensors  101 ,  102 ,  105 ,  106  are inputted to the controller  100 . 
     Pilot lines indicated by broken lines AmC, AmD are connected from the second operating lever  9   b  to the operating sections of the arm first directional control valve  14   c  and arm second directional control valve  14   b  and respectively used to supply an arm crowding pilot pressure and an arm dumping pilot pressure. Further, pilot lines indicated by broken lines SwR, SwL are connected from the second operating lever  9   b  to the operating section of the swing directional control valve  16   b  and respectively used to supply a swing right pilot pressure and a swing left pilot pressure. 
     A pressure sensor  103  for detecting the arm crowding pilot pressure and a pressure sensor  104  for detecting the arm dumping pilot pressure are disposed in the pilot lines indicated by the broken lines AmC, AmD. A pressure sensor  108  for detecting the swing right pilot pressure and a pressure sensor  107  for detecting the swing left pilot pressure are disposed in the pilot lines indicated by the broken lines SwR, SwL. The pressure sensors  103 ,  104 ,  107 ,  108  act as the operating instruction detection means. Pressure signals detected by the pressure sensors  103 ,  104 ,  107 ,  108  are inputted to the controller  100 . 
     Pilot lines indicated by broken lines TrRF, TrRR are connected from a third lever device  9   c  to the operating section of the travel right directional control valve  12   a  and respectively used to supply a travel right forward pilot pressure and a travel right rearward pilot pressure. 
     Pilot lines indicated by broken lines TrLF, TrLR are connected from a fourth lever device  9   d  to the operating section of the travel left directional control valve  12   b  and respectively used to supply a travel left forward pilot pressure and a travel left rearward pilot pressure. 
     The hydraulic control system according to the present embodiment includes the controller  100 . The controller  100  inputs the engine revolving speed from the revolving speed sensor  2 Ax shown in  FIG. 1  and inputs the pilot pressure signal of each pilot line from the aforementioned pressure sensors  101 - 108 . Further, the controller  100  inputs a pressure signal of each actuator from the pressure sensors A 6 -A 8 , B 6 -B 8 , A 11 , B 11  shown in  FIG. 1 . 
     Moreover, the controller  100  controls the delivery flow rates of the hydraulic pumps  20   a - 20   c  by outputting command signals to the regulator  20   d  of the first hydraulic pump  20   a , to the regulator  20   e  of the second hydraulic pump  20   b , and to the regulator  20   f  of the third hydraulic pump  20   c . Additionally, the controller  100  outputs a command signal to the operating section of the arm  1  flow control valve  22  in order to exercise control to reduce the communication opening between the third hydraulic pump  20   c  and the arm cylinder  7  by increasing the magnitude of the command signal. Similarly, the controller  100  outputs a command signal to the operating section of the arm  2  flow control valve  23  in order to exercise control to reduce the communication opening between the second hydraulic pump  20   b  and the arm cylinder  7  by increasing the magnitude of the command signal. 
     A case where the pressure sensors  101 - 108  are used as the operating instruction detection means has been described. However, an alternative is to employ the operating levers  9   a - 9   d  as electric levers and use signals from the electric levers as the operating instruction detection means. 
     The controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention will now be described with reference to the accompanying drawings.  FIG. 3  is a conceptual diagram illustrating a configuration of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 4  is a characteristic diagram illustrating an exemplary map of a target operation computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 5  is a control block diagram illustrating an exemplary computation of a communication control section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
     As illustrated in  FIG. 3 , the controller  100  includes a target operation computation section  110 , the communication control section  120 , and a flow control section  130 . The target operation computation section  110  computes target flow rates from the pilot pressures and load pressures. The communication control section  120  acts as communication control means that computes a command signal of the arm  1  flow control valve  22 , which controls the communication of the control valve  10 , and a command signal of the arm  2  flow control valve  23 . The flow control section  130  acts as pump now control means that calculates flow rate command signals for the first to third hydraulic pumps  20   a - 20   c  in accordance with the target flow rates calculated by the target operation computation section  110 , the command signal calculated by the communication control section  120 , and the engine revolving speed from the revolving speed sensor  2 Ax. The flow control section  130  outputs command signals to the hydraulic pump regulators  20   d - 20   f  in order to control the delivery flow rates of the first to third hydraulic pumps  20   a - 20   c.    
     The target operation computation section  110  computes the target flow rates in such a manner as to increase the target flow rates in accordance with an increase in each inputted pilot pressure and decrease the target flow rates in accordance with an increase in each inputted load pressure. During a combined operation, the computations are performed such that the target flow rates are lower than those during an independent operation. 
     An example of a computation performed by the target operation computation section  110  will now be described by using  FIG. 4  and equations. The target operation computation section  110  stores a map for each actuator. The map is used to compute a reference flow rate from a pilot pressure shown in  FIG. 4 . For example, a swing target flow rate Qsw is calculated from a swing pilot pressure, which is a value obtained when the maximum values of the swing right pilot pressure and swing left pilot pressure are selected. Similarly, an arm crowding reference flow rate Qamc 0  is calculated from the arm crowding pilot pressure, and an arm dumping reference flow rate Qamd 0  is calculated from the arm dumping pilot pressure. 
     Meanwhile, a boom raising reference flow rate Qbmu 0  is calculated from the boom raising pilot pressure. Further, a bucket crowding reference flow rate Qbkc 0  is calculated from the bucket crowding pilot pressure, and a bucket dumping reference flow rate Qbkd 0  is calculated from the bucket dumping pilot pressure. 
     The target operation computation section  110  uses Equation (1) to calculate a boom target flow rate Qbm from the swing target flow rate Qsw.
 
Equation 1
 
 Q   bm =min( Q   bm0   ,Q   bm max   −k   swbm   ·Q   sw )  (1)
 
     Qbmmax is an upper-limit value of a boom flow rate and set in accordance with the maximum boom raising speed. Meanwhile, kswbm is a boom flow rate reduction coefficient. The boom target flow rate Qbm decreases with an increase in the swing target flow rate Qsw. The boom flow rate reduction coefficient kswbm may be substituted by a map that causes the boom flow rate upper-limit value Qbmmax to decrease with an increase in the swing target flow rate Qsw. 
     The target operation computation section  110  uses Equations (2) and (3) to calculate swing power Lsw and boom power Lbm, respectively.
 
Equation 2
 
 L   sw   =P   sw   ·Q   sw   (2)
 
Equation 3
 
 L   bm   =P   bmb   ·Q   bm   (3)
 
     Psw is a swing pressure, which is a value obtained when a meter-in pressure is selected from a swing left pressure and swing right pressure detected by the pressure sensors A 11 , B 11 . Pbmb is a boom bottom pressure, which is the pressure in the bottom oil chamber of the boom cylinder  6  and detected by the pressure sensor A 6 . 
     The target operation computation section  110  uses Equations (4) and (5) to calculate a bucket power upper-limit value Lbkmax and an arm power upper-limit value Lammax, respectively.
 
Equation 4
 
 L   bk max   =k   bk ( L   max   −L   sw   −L   bm )  (4)
 
Equation 5
 
 L   am max   =k   am ( L   max   −L   sw   −L   bm )  (5)
 
     Lmax is a total power upper-limit value of the system, kbk is a bucket power coefficient, and kam is an arm power coefficient. The bucket power coefficient kbk and the arm power coefficient kam are calculated by using the bucket crowding pilot pressure BkC, the bucket dumping pilot pressure BkD, the arm crowding pilot pressure AmC, the arm dumping pilot pressure AmD, and Equation (6).
 
Equation 6
 
 k   bk   :k   am =max( BkC,BkD ):max( AmC,AmD )  (6)
 
     The target operation computation section  110  calculates a bucket target flow rate Qbk by using the bucket crowding reference flow rate Qbkc 0 , the bucket dumping reference flow rate Qbkd 0 , the bucket power upper-limit value Lbkmax, and Equation (7). Further, the target operation computation section  110  calculates an arm target flow rate Qam by using the arm crowding reference flow rate Qamc 0 , the arm dumping reference flow rate Qamd 0 , the arm power upper-limit value Lammax, and Equation (8).
 
Equation 7
 
 Q   bk =min( Q   bkd0   ,Q   bkd0   ,L   bk max   /P   bk )  (7)
 
Equation 8
 
 Q   am =min( Q   amd0   ,Q   amd0   ,L   am max   /P   am )  (8)
 
     Pbk is a value obtained when a meter-in pressure is selected from the pressures in the bottom oil chamber and rod oil chamber of the bucket cylinder  8 , which are detected by the pressure sensors A 8 , B 8 . Meanwhile, Pam is a value obtained when a meter-in pressure is selected from the pressures in the bottom oil chamber and rod oil chamber of the arm cylinder  7 , which are detected by the pressure sensors A 7 , B 7 . 
     An exemplary computation performed by the communication control section  120  will now be described with reference to  FIG. 5 . The communication control section  120  includes a first function generator  120   a , a second function generator  120   b , a third function generator  120   c , a minimum value selection section  120   d , and a maximum value selection section  120   e.    
     As illustrated in  FIG. 5 , the first function generator  120   a  and the second function generator  120   b  input a swing pilot pressure that represents the maximum value or the swing right pilot pressure and swing left pilot pressure detected by the pressure sensors  107 ,  108 . The first function generator  120   a  stores beforehand a command pressure for the arm  2  flow control valve  23  with respect to the swing pilot pressure as a map M 1   a  in a table. 
     The map M 1   a  is characterized such that the arm  2  flow control valve command pressure increases with an increase in the swing pilot pressure. Thus, the opening in the arm  2  flow control valve  23  narrows with an increase in the swing pilot pressure, thereby breaking the communication between the second hydraulic pump  20   b  and the arm cylinder  7 . Therefore, when the swing pilot pressure increases, the second hydraulic pump  20   b  drives only the swing hydraulic motor  11 . This makes it possible to avoid a flow division loss that is caused by a load pressure difference between the arm cylinder  7  and the swing hydraulic motor  11 . 
     In the description of the present embodiment, breaking the communication signifies that a passage flow rate is substantially reduced to zero, and that the opening is not necessarily completely closed. 
     The second function generator  120   b  stores beforehand a command pressure for the arm  1  flow control valve  22  with respect to the swing pilot pressure as a map M 1   c  in a table. The map M 1   c  is characterized such that the arm  1  flow control valve command pressure decreases with an increase in the swing pilot pressure. The second function generator  120   b  outputs a calculated arm  1  flow control valve command pressure to the minimum value selection section  120   d.    
     The maximum value selection section  120   e  inputs the bucket crowding pilot pressure and bucket dumping pilot pressure detected by the pressure sensors  105 ,  106 , computes the maximum value of these pressures, and outputs the maximum value to the minimum value selection section  120   d.    
     The minimum value selection section  120   d  inputs the arm  1  flow control valve command pressure from the second function generator  120   b , a signal indicative of the maximum value of the bucket crowding pilot pressure and bucket dumping pilot pressure from the maximum value selection section  120   e , and the boom raising pilot pressure detected by the pressure sensor  101 , and computes the minimum value of these values, and outputs the computed minimum value to the third function generator  120   c.    
     The third function generator  120   c  stores beforehand a command pressure for the arm  1  flow control valve  22  with respect to the minimum value of the maximum value of the bucket crowding pilot pressure and bucket dumping pilot pressure and the boom raising pilot pressure as a map M 1   b  in a table. 
     The map M 1   b  is characterized such that the arm  1  flow control valve command pressure increases with an increase in the minimum value of the maximum value of the bucket crowding pilot pressure and bucket dumping pilot pressure and the boom raising pilot pressure. Thus, the opening in the arm  1  flow control valve  22  narrows with an increase in the minimum value of the maximum value of the bucket crowding pilot pressure and bucket dumping pilot pressure and the boom raising pilot pressure, thereby breaking the communication between the third hydraulic pump  20   c  and the arm cylinder  7 . 
     Consequently, when the bucket  5  does not perform a combined, operation during a combined aerial operation of the arm  4  and boom  3 , the opening in the arm  1  flow control valve  22  is maximized. In this instance, the load pressure of the boom cylinder  6  is higher than that of the arm cylinder  7 . Therefore, the delivery hydraulic fluid from the third hydraulic pump  20   c  is supplied only to the arm cylinder  7 . Thus, the first hydraulic pump  20   a  can drive only the boom cylinder  6 , and the second and third hydraulic pumps  20   b ,  20   c  can drive only the arm cylinder  7 . 
     Meanwhile, when the bucket  5  performs a combined operation during a combined aerial operation of the arm  4  and boom  3 , the load pressure of the boom cylinder  6  is higher than that of the bucket cylinder  8 . Therefore, the delivery hydraulic fluid from the first hydraulic pump  20   a  is supplied only to the bucket cylinder  8 . Thus, the first hydraulic pump  20   a  can drive the bucket cylinder  8 , the second hydraulic pump  20   b  can drive the arm cylinder  7 , and the third hydraulic pump  20   c  can drive the boom cylinder  6 . This makes it possible to avoid a flow division loss that is caused by a load pressure difference. 
     During a swing operation, however, a value to be inputted to the map M 1   b  of the third function generator  120   c  is limited by the map M 1   c  of the second function generator  120   b  to a small value in accordance with the swing pilot pressure. Therefore, an opening command pressure for the arm  1  flow control valve  22  does not increase. This prevents the opening in the arm  1  flow control valve  22  from narrowing. As a result, the delivery from the third hydraulic pump  20   c  is divided and supplied to the boom cylinder  6  and to the arm cylinder  7 . This ensures the operation of the arm cylinder  7 . 
     The flow control section  130 , which acts as the pump flow control means, will now be described with reference to the accompanying drawings.  FIG. 6  is a conceptual diagram illustrating a configuration of the flow control section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 7  is a control block diagram illustrating an exemplary computation of a boom flow distribution computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 8  is a control block diagram illustrating an exemplary computation of an arm target flow distribution computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 9  is a control block diagram illustrating an exemplary computation of a pump flow rate command computation section of the controller included in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. Elements that are shown in  FIGS. 6 to 9  and designated by the same reference numerals as the elements shown in  FIGS. 1 to 5  are identical with the corresponding elements and will not be redundantly described in detail. 
     As illustrated in  FIG. 6 , the flow control section  130  includes the boom flow distribution computation section.  131 , the arm flow distribution computation section  132 , and the pump flow rate command computation section  133 . The boom flow distribution computation section  131  distributively computes a target flow rate for each of a plurality of directional control valves of the boom  3 . The arm flow distribution computation section  132  distributively computes a target flow rate for each of a plurality of directional control valves of the arm  4 . The pump flow rate command computation section  133  calculates the flow rate of each pump in accordance with each of the distributively computed target flow rates and outputs a command signal to the hydraulic pump regulators  20   d - 20   f  in order to control the delivery flow rates of the first to third hydraulic pumps  20   a - 20   c.    
     An exemplary computation performed by the boom flow distribution computation section  131  will now be described with reference to  FIG. 7 . The boom flow distribution computation section  131  includes a variable gain multiplier  131   a , a first maximum value selection section  131   b , a first function generator  131   c , a first minimum value selection section  131   d , a subtractor  131   e , a second function generator  131   f , a third function generator  131   g , a fourth function generator  131   h , a fifth function generator  131   i , a second maximum value selection section  131   j , a second minimum value selection section  131   k , and a sixth function generator  131 L. 
     The variable gain multiplier  131   a  inputs the boom target flow rate from the target operation computation section.  110  and multiplies the boom target flow rate by a gain Kbm 2  outputted from the first function generator  131   c  to compute a boom  2  spool target flow rate. A signal indicative of the calculated boom  2  spool target flow rate is then outputted to the first minimum value selection section  131   d.    
     The first maximum value selection section.  131   b  inputs the bucket crowding pilot pressure and bucket dumping pilot pressure detected by the pressure sensors  105 ,  106 , computes the maximum value of these pressures, and outputs the computed maximum value to the first function generator  131   c.    
     The first function generator  131   c  stores beforehand the gain Kbm 2 , which is based on the maximum value of the bucket crowding pilot pressure and bucket dumping pilot pressure, as a map M 2   a  in a table. For example, if the bucket crowding pilot pressure and the bucket dumping pilot pressure are both minimized, the gain Kbm 2  may be set to 0.5. If, by contrast, either the bucket crowding pilot pressure or the bucket dumping pilot pressure is maximized, the gain Kbm 2  may be set to 1. 
     The first minimum value selection section.  131   d  inputs a boom  2  spool target flow rate signal from the variable gain multiplier  131   a , a limit signal from the second function generator  131   f , and a limit signal from the sixth function generator  131 L, computes the minimum value of these signals as the boom  2  spool target flow rate, and outputs the boom  2  spool target flow rate to the subtractor  131   e  and to the pump flow rate command computation section  133 . 
     The subtractor  131   e  inputs the boom target flow rate from the target operation computation section  110  and the boom  2  spool target flow rate from the first minimum value selection section  131   d  and subtracts the boom  2  spool target flow rate from the boom target flow rate to calculate a boom  1  spool target flow rate. A signal indicative of the calculated boom  1  spool target flow rate is then outputted to the pump flow rate command computation section  133 . 
     The second function generator  131   f  inputs the boom raising pilot pressure detected by the pressure sensor  101  and outputs a limit signal to the first minimum value selection section  131   d . An upper-limit value for the boom  2  spool target flow rate with respect to the boom raising pilot pressure is stored in the second function generator  131   f  as a map M 2   c  in a table beforehand. The map M 2   c  is substantially proportional to the area of the opening in the boom second directional control valve  13   c  and increases in accordance with the boom raising pilot pressure. That is to say, the upper-limit value for the boom  2  spool target flow rate increases in accordance with area of the opening in the boom second directional control valve  13   c.    
     The third function generator  131   g  inputs the arm crowding pilot pressure detected by the pressure sensor  103 , acquires a signal from a map M 2   d  stored in a table, and outputs the acquired signal to the second maximum value selection section  131   j . The map M 2   d  indicates the area of a crowding opening in the arm first directional control valve  14   c  with respect to the arm crowding pilot pressure. 
     The fourth function generator  131   h  inputs the arm dumping pilot pressure detected by the pressure sensor  104 , acquires a signal from a map M 2   e  stored in a table, and outputs the acquired signal to the second maximum value selection section  131   j . The map M 2   e  indicates the area of a dumping opening in the arm first directional control valve  14   c  with respect to the arm dumping pilot pressure. 
     The second maximum value selection section  131   j  inputs the output of the third function generator  131   g  and the output of the fourth function generator  131   h , computes the maximum value of these outputs, and outputs the computed maximum value to the second minimum value selection section.  131   k.    
     The fifth function generator  131   i  inputs an arm  1  flow control valve command pressure signal from the communication control section  120 , acquires a signal from a map M 2   f  stored in a table, and outputs the acquired signal to the second minimum value selection section  131   k . The map M 2   f  indicates the area of the opening in the arm  1  flow control valve  22  with respect to the arm  1  flow control valve command pressure. 
     The second minimum value selection section  131   k  inputs a signal indicative of the maximum value of the output of the third function generator  131   g  and the output of the fourth function generator  131   h , which are obtained from the second maximum value selection section  131   j , and an output signal of the fifth function generator  131   i , computes the minimum value of these signals, and outputs the computed minimum value to the sixth function generator  131 L. 
     The sixth function generator  131 L inputs a signal from the second minimum value selection section  131   k  and outputs a limit signal to the first minimum value selection section  131   d . A limit value for the boom  2  spool target flow rate with respect to the minimum value of the maximum value of values computed from the arm crowding pilot pressure and arm dumping pilot pressure by using the maps M 2   d , M 2   e  and a value computed from the arm  1  flow control valve command pressure by using the map M 2   f  is stored in the sixth function generator  131 L as a map M 2   g  in a table. 
     That is to say, the boom  2  spool target flow rate is limited to a small value in accordance with a value computed by using the map M 2   g . This limits the boom  2  spool target flow rate in accordance with the degree of communication between the third hydraulic pump  20   c  and the arm cylinder  7 . 
     An exemplary computation performed by the arm flow distribution computation section  132  will now be described with reference to  FIG. 8 . The arm flow distribution computation section  132  includes a variable gain multiplier  132   a , a first function generator  132   b , a minimum value selection section  132   c , a subtractor  132   d , a second function generator  132   e , a third function generator  132   f , a maximum value selection section  132   g , and a fourth function generator  132   h.    
     The variable gain multiplier  132   a  inputs the arm target flow rate from the target operation computation section.  110  and multiplies the arm target flow rate by a gain Kam 2  outputted from the first function generator  132   b  to compute an arm  2  spool target flow rate. A signal indicative of the calculated arm  2  spool target flow rate is then outputted to the minimum value selection section  132   c.    
     The first function generator  132   b  inputs an arm  1  flow control valve command pressure signal from the communication control section  120 , handles a signal obtained from a map M 3   a  stored in a table as a gain Kam 2 , and outputs the gain Kam 2  to the variable gain multiplier  132   a . For example, if the arm  1  flow control valve command pressure signal indicates the minimum pressure, the gain Kam 2  may be set to 0.5. If, by contrast, the arm  1  flow control valve command pressure signal indicates the maximum pressure, the gain Kam 2  may be set to 1. 
     The minimum value selection section  132   c  inputs an arm  2  spool target flow rate signal from the variable gain multiplier  132   a , a limit signal from the later-described maximum value selection section  132   g , and a limit signal from the fourth function generator  132   h , computes the minimum value of these signals, and outputs the computed minimum value, as the arm  2  spool target flow rate, to the subtractor  132   d  and to the pump flow rate command computation section  133 . 
     The subtractor  132   d  inputs the arm target flow rate from the target operation computation section.  110  and the arm  2  spool target flow rate from the minimum value selection section  132   c , and subtracts the arm  2  spool target flow rate from the arm target flow rate to calculate an arm  1  spool target flow rate. A signal indicative of the calculated arm  1  spool target flow rate is then outputted to the pump flow rate command computation section  133 . 
     The second function generator  132   e  inputs the arm crowding pilot pressure detected by the pressure sensor  103 , acquires a signal from a map M 3   b  stored in a table, and outputs the acquired signal to the maximum value selection section  132   g . The map M 3   b  is substantially proportional to the area of a crowding opening in the arm second directional control valve  14   b  with respect to the arm crowding pilot pressure. 
     The third function generator  132   f  inputs the arm dumping pilot pressure detected by the pressure sensor  104 , acquires a signal from a map M 3   c  stored in a table, and outputs the acquired signal to the maximum value selection section  132   g . The map M 3   c  is substantially proportional to the area of a dumping opening in the arm second directional control valve  14   b  with respect to the arm dumping pilot pressure. 
     The maximum value selection section  132   g  inputs the output of the second function generator  132   e  and the output of the third function generator  132   f , computes the maximum value of these outputs, and outputs the computed maximum value to the minimum value selection section  132   c.    
     The fourth function generator  132   h  inputs an arm  2  flow control valve command pressure signal from the communication control section  120 , acquires a signal from a map M 3   d  stored in a table, and outputs the acquired signal to the minimum value selection section  132   c . The map M 3   d  is substantially proportional to the area of the opening in the arm  2  flow control valve  23  with respect to the arm  2  flow control valve command pressure. 
     That is to say, the arm  2  spool target flow rate is limited in accordance with the maximum value of values computed from the arm crowding pilot pressure and arm dumping pilot pressure by respectively using the maps M 3   b , M 3   c , and with a value computed from the arm  2  flow control valve command pressure by using the map M 3   d . This increases the upper-limit value for the arm  2  spool target flow rate in accordance with the degree of communication between the second hydraulic pump  20   b  and the arm cylinder  7 . 
     An exemplary computation performed by the pump flow rate command computation section  133  will now be described with reference to  FIG. 9 . The pump flow rate command computation section  133  includes a first maximum value selection section  133   a , a first divider  133   b , a first function generator  133   c , a second maximum value selection section  133   d , a second divider  133   e , a second function generator  133   f , a subtractor  133   g , a third divider  133   h , and a third function generator  133   i.    
     The first maximum value selection section  133   a  inputs a bucket target flow rate signal from the target operation computation section  110  and a boom  1  spool target flow rate signal from the boom flow distribution computation section.  131 , computes the maximum value of these signals, and outputs the computed maximum value, as a first pump target flow rate, to the first divider  133   b.    
     The first divider  133   b  inputs the first pump target flow rate from the first maximum value selection section  133   a  and the engine revolving speed detected by the revolving speed sensor  2 Ax, and divides the first pump target flow rate by the engine revolving speed to calculate a first pump target command. A signal indicative of the calculated first pump target command is then outputted to the first function generator  133   c.    
     The first function generator  133   c  inputs the first pump target command signal calculated by the first divider  133   b , acquires a signal from a map M 4   a  stored in a table, and outputs the acquired signal to the regulator  20   d  as a first pump flow rate command signal. This controls the delivery flow rate of the first hydraulic pump  20   a.    
     The second maximum value selection section  133   d  inputs a swing target flow rate signal from the target operation computation section  110  and an arm  2  spool target flow rate signal from the arm flow distribution computation section  132 , computes the maximum value of these signals, and outputs the computed maximum value, as a second pump target flow rate, to the second divider  133   e.    
     The second divider  133   e  inputs the second pump target flow rate from the second maximum value selection section.  133   d  and the engine revolving speed detected by the revolving speed sensor  2 Ax, and divides the second pump target flow rate by the engine revolving speed to calculate a second pump target command. A signal indicative of the calculated second pump target command is then outputted to the second function generator  133   f.    
     The second function generator  133   f  inputs the second pump target command signal calculated by the second divider  133   e , acquires a signal from a map M 4   b  stored in a table, and outputs the acquired signal to the regulator  20   e  as a second pump flow rate command signal. This controls the delivery flow rate of the second hydraulic pump  20   b.    
     The subtractor  133   g  inputs the boom  2  spool target flow rate signal from the boom flow distribution computation section  131  and an arm  1  spool target flow rate signal from the arm flow distribution computation section  132 , and adds the boom  2  spool target flow rate signal to the arm  1  spool target flow rate signal to calculate a third pump target flow rate. A signal indicative of the calculated third pump target flow rate is then outputted to the third divider  133   h.    
     The third divider  133   h  inputs the third pump target flow rate from the subtractor  133   g  and the engine revolving speed detected by the revolving speed sensor  2 Ax, and divides the third pump target flow rate by the engine revolving speed to calculate a third pump target command. A signal indicative of the calculated third pump target command is then outputted to the third function generator  133   i.    
     The third function generator  133   i  inputs the third pump target command signal calculated by the third divider  133   b , acquires a signal from a map M 4   c  stored in a table, and outputs the acquired signal to the regulator  20   f  as a third pump flow rate command signal. This controls the delivery flow rate of the third hydraulic pump  20   c.    
     The present embodiment is described on the assumption that the reduction ratio between the engine  2 A and each hydraulic pump is 1. If the reduction ratio is other than  1 , it is necessary to perform computations in accordance with the reduction ratio. 
     Operations of an embodiment of the hydraulic control system for a work machine will now be described in accordance with the present invention.  FIG. 10  is a characteristic diagram illustrating an exemplary operation related to the pump flow control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 11  is a characteristic diagram illustrating another exemplary operation related to the pump flow control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 12  is a characteristic diagram illustrating an exemplary operation related to the pump flow control means and communication control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention.  FIG. 13  is a characteristic diagram illustrating another exemplary operation related to the pump flow control means and communication control means in an embodiment of the hydraulic control system for a work machine in accordance with the present invention. 
       FIG. 10  is a characteristic diagram illustrating an exemplary operation that is performed when arm crowding is conducted during a boom raising operation. 
     In  FIG. 10 , the horizontal axis represents time, and the vertical axis represents (a) a pilot pressure, (b) the delivery flow rate of a hydraulic pump, (c) an actuator speed, and (d) an actuator pressure. In (a), the solid line indicates boom raising pilot pressure characteristics, and the broken line indicates the arm crowding pilot, pressure characteristics. In (b), the solid line indicates the delivery flow rate characteristics of the first hydraulic pump  20   a , and the broken line indicates the delivery flow rate characteristics of the third hydraulic pump  20   c . In (c), the solid line indicates the actuator speed characteristics of the boom cylinder  6 , and the broken line indicates the actuator speed characteristics of the arm cylinder  7 . In (d), the solid line indicates the bottom oil chamber pressure characteristics of the boom cylinder  6 , and the broken line indicates the bottom oil chamber pressure characteristics of the arm cylinder  7 . Time T 1  is the time at which a boom raising operation is started. Time T 2  is the time at which an arm crowding operation is started. 
     First of all, when a boom raising operation starts at time T 1 , the boom raising pilot pressure rises as indicated in (a). The first hydraulic pump  20   a  and the third hydraulic pump  20   c  then communicate with the bottom oil chamber of the boom cylinder  6  such that the delivery flow rates of the first and third hydraulic pumps  20   a ,  20   c  increase in accordance with the boom raising pilot pressure as indicated in (b). This causes the boom  3  to operate. As a result, the actuator speed of the boom cylinder  6  increases as indicated in (c), and the bottom oil chamber pressure of the boom cylinder  6  increases as indicated in (d). 
     Next, when an arm crowding operation starts at time T 2 , the arm crowding pilot pressure rises as indicated in (a). The second hydraulic pump  20   b  and the third hydraulic pump  20   c  then communicate with the bottom oil chamber of the arm cylinder  7 . During an aerial operation, the delivery hydraulic fluid from the third hydraulic pump  20   c  is supplied to the arm cylinder  7  without being significantly divided because the bottom oil chamber pressure of the boom cylinder  6  is higher than that of the arm cylinder  7  as indicated in (d). 
     In the above instance, as indicated in  FIG. 10 , the flow control section  130  of the hydraulic control system according to the present embodiment decreases the boom  2  spool target flow rate in accordance with the arm crowding pilot pressure and increases the boom  1  spool target flow rate. As a result, the delivery flow rate of the first hydraulic pump  20   a  becomes higher as compared to a period before time T 2  as indicated in (b). Therefore, a decrease in the boom raising speed can be reduced as indicated in (c) without dividing the delivery hydraulic fluid from the third hydraulic pump  20   c . In this instance, the bottom oil chamber pressure of the arm cylinder  7  increases as indicated in (d). 
     If, in a situation where two hydraulic actuators (boom cylinder  6  and arm cylinder  7 ) operate in a combined manner, the boom cylinder  6  is regarded as the first hydraulic actuator, a hydraulic pump communicating with the first and second hydraulic actuators through different spools is defined as the other hydraulic pump. In the above-described operation, the third hydraulic pump  20   c  corresponds to the other hydraulic pump. 
     Further, a hydraulic pump communicating with the first hydraulic actuator (boom cylinder  6 ) through a primary spool for the first hydraulic actuator (boom first directional control valve)  13   a  is defined as the one hydraulic pump. In the above-described operation, the first hydraulic pump  20   a  corresponds to the one hydraulic pump. 
     Furthermore, the arm cylinder  7 , which is a hydraulic actuator communicating only with the other hydraulic pump  20   c  without communicating with the one hydraulic pump  20   a , is defined as the second hydraulic actuator. 
     That is to say, the first hydraulic actuator is either one of two simultaneously operated hydraulic actuators that communicates with the one hydraulic pump  20   a  through the first hydraulic actuator primary spool (boom first directional control valve)  13   a  and communicates with the other hydraulic pump  20   c  through a first hydraulic actuator secondary spool (boom second directional control valve)  13   c.    
     When the above definition is formulated, the pump flow control means (flow control section  130 ) of the controller according to the present embodiment exercises control to increase the delivery flow rate of the one hydraulic pump (first hydraulic pump  20   a ) to a higher rate when the first hydraulic actuator (boom cylinder  6 ) and the second hydraulic actuator (arm cylinder  7 ) are simultaneously operated than when the first hydraulic actuator (boom cylinder  6 ) is operated and the second hydraulic actuator (arm cylinder  7 ) is not operated. 
     An operation performed when bucket dumping is conducted during a boom raising operation will now be described with reference to  FIG. 11 . 
     In  FIG. 11 , the horizontal axis represents time, and the vertical axis represents (a) a pilot pressure, (h) the delivery flow rate of a hydraulic pump, (c) an actuator speed, and (d) an actuator pressure. In (a), the solid line indicates the boom raising pressure characteristics, and the broken line indicates bucket dumping pilot pressure characteristics. In (b), the solid line indicates the delivery flow rate characteristics of the third hydraulic pump  20   c , and the broken line indicates the delivery flow rate characteristics of the first hydraulic pump  20   a . In (c), the solid line indicates the actuator speed characteristics of the boom cylinder  6 , and the broken line indicates the actuator speed characteristics of the bucket cylinder  8 . In (d), the solid line indicates the bottom oil chamber pressure characteristics of the boom cylinder  6 , and the broken line indicates the rod oil chamber pressure characteristics of the bucket cylinder  8 . Time T 1  is the time at which a boom raising operation is started. Time T 0  is the time at which a bucket dumping operation is started. Operations that are indicated in  FIG. 11  and performed before time T 0  are the same as those described with reference to  FIG. 10  and will not be redundantly described. 
     When a bucket dumping operation starts at time T 2 , the bucket dumping pilot pressure rises as indicated in (a). The first hydraulic pump  20   a  then communicates with the rod oil chamber of the bucket cylinder  8 . During an aerial operation, the delivery hydraulic fluid from the first hydraulic pump  20   a  is supplied to the bucket cylinder  8  without being significantly diverged because the bottom oil chamber pressure of the boom cylinder  6  is higher than the rod oil chamber pressure of the bucket cylinder  8  as indicated in (d). 
     In the above instance, as indicated in  FIG. 7 , the flow control section  130  of the hydraulic control system according to the present embodiment increases the boom  2  spool target flow rate in accordance with the bucket dumping pilot pressure and decreases the boom  1  spool target flow rate. As a result, the delivery flow rate of the third hydraulic pump  20   c  becomes higher as compared to a period before time T 2  as indicated in (b). Therefore, a decrease in the boom raising speed can be reduced as indicated in (c) without dividing the delivery hydraulic fluid from the first hydraulic pump  20   a . In this instance, the rod oil chamber pressure of the bucket cylinder  8  increases as indicated in (d). 
     If, in a situation where two hydraulic actuators (boom cylinder  6  and bucket cylinder  8 ) operate in a combined manner, the boom cylinder  6  is regarded as the first hydraulic actuator, a hydraulic pump communicating with the first and second hydraulic actuators through different spools is defined as the other hydraulic pump. In the above-described operation, the first hydraulic pump  20   a  corresponds to the other hydraulic pump. 
     Further, a hydraulic pump communicating with the first hydraulic actuator (boom cylinder  6 ) through a primary spool for the first hydraulic actuator (boom second directional control valve)  13   c  is defined as the one hydraulic pump. In the above-described operation, the third hydraulic pump  20   c  corresponds to the one hydraulic pump. 
     Furthermore, the bucket cylinder  8 , which is a hydraulic actuator communicating only with the other hydraulic pump  20   a  without communicating with the one hydraulic pump  20   c , is defined as the second hydraulic actuator. 
     That is to say, the first hydraulic actuator is either one of two simultaneously operated hydraulic actuators that communicates with the one hydraulic pump  20   c  through the first hydraulic actuator primary spool (boom first directional control valve)  13   a  and communicates with the other hydraulic pump  20   a  through the first hydraulic actuator secondary spool (boom second directional control valve)  13   c.    
     When the above definition is formulated, the pump flow control means (flow control section  130 ) of the controller according to the present embodiment exercises control to increase the delivery flow rate of the one hydraulic pump (third hydraulic pump  20   c ) to a higher rate when the first hydraulic actuator (boom cylinder  6 ) and the second hydraulic actuator (bucket cylinder  8 ) are simultaneously operated than when the first hydraulic actuator (boom cylinder  6 ) is operated and the second hydraulic actuator (bucket cylinder  8 ) is not operated. 
     An operation performed when a swing is conducted during an arm dumping operation will now be described with reference to  FIG. 12 . 
     In  FIG. 12 , the horizontal axis represents time, and the vertical axis represents (a) a pilot pressure, (b) the area of an opening, (c) the delivery flow rate of a hydraulic pump, (d) an actuator speed, and (e) an actuator pressure. In (a), the solid line indicates arm dumping pilot pressure characteristics, and the broken line indicates swing pilot pressure characteristics. In (b), the solid line indicates the opening area characteristics of the arm  2  flow control valve. In (c), the solid line indicates the delivery flow rate characteristics of the third hydraulic pump  20   c , and the broken line indicates the delivery flow rate characteristics of the second hydraulic pump  20   b . In (d), the solid line indicates the actuator speed characteristics of the arm cylinder  7 , and the broken line indicates the actuator speed characteristics of the swing hydraulic motor  11 . In (e), the solid line indicates the rod oil chamber pressure characteristics of the arm cylinder  7 , and the broken line indicates the supply pressure characteristics of the swing hydraulic motor. Time T 1  is the time at which an arm dumping operation is started. Time  12  is the time at which a swing operation is started. 
     First of ail, when an arm dumping operation starts at time T 1 , the arm dumping pilot pressure rises as indicated in (a). The third hydraulic pump  20   c  and the second hydraulic pump  20   b  then communicate with the rod oil chamber of the arm cylinder  7  such that the delivery flow rates of the second and third hydraulic pumps  20   b ,  20   c  increase in accordance with the arm dumping pilot pressure as indicated in (c). This causes the arm  4  to operate. As a result, the actuator speed of the arm cylinder  7  increases as indicated in (d), and the rod oil chamber pressure of the arm cylinder  7  increases as indicated in (e). 
     Next, when a swing operation starts at time  12 , the swing pilot pressure rises as indicated in W. The second hydraulic pump  20   b  then communicates with the swing hydraulic motor  11 . 
     In the above instance, the communication control section  120  of the hydraulic control system according to the present embodiment increases the arm  2  flow control valve command pressure in accordance with the swing pilot pressure as indicated in  FIG. 5 , and interrupts the opening in the arm  2  flow control valve  23  as indicated in (b) of  FIG. 12 . This causes the delivery hydraulic fluid from the second hydraulic pump  20   b  to be supplied to the swing hydraulic motor  11  without being significantly divided. 
     Further, as indicated in  FIG. 8 , the flow control section  130  of the hydraulic control system according to the present embodiment decreases the arm  2  spool target flow rate in accordance with the arm  2  flow control valve command pressure and increases the arm  1  spool target flow rate. As a result, the delivery flow rate of the third hydraulic pump  20   c  becomes higher as compared to a period before time T 2  as indicated in (c). Therefore, a decrease in the arm dumping speed can be reduced as indicated in (d) without dividing the delivery hydraulic fluid from the second hydraulic pump  20   b . In this instance, the pressure of the swing hydraulic motor  11  increases as indicated in (e). 
     If, in a situation where two hydraulic actuators (arm cylinder  7  and swing hydraulic motor  11 ) operate in a combined manner, the arm cylinder  7  is regarded as the first hydraulic actuator, a hydraulic pump communicating with the first and second hydraulic actuators through different spools is defined as the other hydraulic pump. In the above-described operation, the second hydraulic pump  20   b  corresponds to the other hydraulic pump. 
     Further, a hydraulic pump communicating with the first hydraulic actuator (arm cylinder  7 ) through a primary spool for the first hydraulic actuator (arm first directional control valve)  14   c  is defined as the one hydraulic pump. In the above-described operation, the third hydraulic pump  20   c  corresponds to the one hydraulic pump. 
     Furthermore, the swing hydraulic motor  11 , which is a hydraulic actuator communicating only with the other hydraulic pump  20   b  without communicating with the one hydraulic pump  20   c , is defined as the second hydraulic actuator. 
     That is to say, the first hydraulic actuator is either one of two simultaneously operated hydraulic actuators that communicates with the one hydraulic pump  20   c  through the first hydraulic actuator primary spool (arm first directional control valve)  14   c  and communicates with the other hydraulic pump  20   b  through the first hydraulic actuator secondary spool (arm second directional control valve)  14   b.    
     When the above definition is formulated, the pump flow control means (flow control section  130 ) of the controller according to the present embodiment exercises control to increase the delivery flow rate of the one hydraulic pump (third hydraulic pump  20   c ) to a higher rate when the first hydraulic actuator (arm cylinder  7 ) and the second hydraulic actuator (swing hydraulic motor  11 ) are simultaneously operated than when the first hydraulic actuator (arm cylinder  7 ) is operated and the second hydraulic actuator (swing hydraulic motor  11 ) is not operated. 
     An operation performed when boom raising is conducted during a combined operation of arm crowding and bucket crowding will now be described with reference to  FIG. 13 . 
     In  FIG. 13 , the horizontal axis represents time, and the vertical axis represents (a) a pilot pressure, (b) the area of an opening, (c) the delivery flow rate of a hydraulic pump, (d) an actuator speed, and (e) an actuator pressure. In (a), the solid line indicates arm crowding pilot pressure characteristics and bucket dumping pilot pressure characteristics, and the broken line indicates boom raising pilot pressure characteristics. In (b), the solid line indicates the opening area characteristics of the arm  1  flow control valve  22 . In (c), the solid line indicates the delivery flow rate characteristics of the second hydraulic pump  20   b , and the broken line indicates the delivery flow rate characteristics of the third hydraulic pump  20   c . For brevity of explanation, the delivery flow rate characteristics of the first hydraulic pump  20   a  are omitted. In (d), the solid line indicates the actuator speed characteristics of the arm cylinder  7 , and the broken line indicates the actuator speed characteristics of the boom cylinder  6 . In (e), the solid line indicates the bottom oil chamber pressure characteristics of the arm cylinder  7 , and the broken line indicates the bottom oil chamber pressure characteristics of the boom cylinder  6 . Time T 1  is the time at which a combined operation of arm crowding and bucket crowding is started. Time T 2  is the time at which a boom raising operation is started. 
     First of all, when a combined operation of arm crowding and bucket crowding starts at time T 1 , the arm crowding pilot pressure and the bucket crowding pilot pressure rise as indicated in (a). Then, the first hydraulic pump  20   a  communicates with the bottom oil chamber of the bucket cylinder  8 , and the third hydraulic pump  20   c  and the second hydraulic pump  20   b  communicate with the bottom oil chamber of the arm cylinder  7 . Thus, the delivery flow rates of the second and third hydraulic pumps  20   b ,  20   c  increase in accordance with the arm crowding pilot pressure and the bucket crowding pilot pressure as indicated in (c). This causes the arm  4  and the bucket  5  to operate. As a result, the actuator speed of the arm cylinder  7  increases as indicated in (d), and the bottom oil chamber pressure of the arm cylinder  7  increases as indicated in (e). 
     Next, when a boom raising operation starts at time T 2 , the boom raising pilot pressure rises as indicated in (a). The first and third hydraulic pumps  20   a ,  20   c  then communicate with the bottom oil chamber of the boom cylinder  6 . When the bottom oil chamber pressure of the bucket cylinder  8  is low, the delivery hydraulic fluid from the first hydraulic pump  20   a  is supplied to the bucket cylinder  8  without being significantly divided. 
     In the above instance, the communication control section  120  of the hydraulic control system according to the present embodiment increases the arm  1  flow control valve command pressure in accordance with the boom raising pilot pressure as indicated in  FIG. 5 , and interrupts the opening in the arm  1  flow control valve  22  as indicated in (b) of  FIG. 13 . This causes the delivery hydraulic fluid from the third hydraulic pump  20   c  to be supplied to the boom cylinder  6  without being significantly divided. 
     Further, as indicated in  FIG. 8 , the flow control section  130  of the hydraulic control system according to the present embodiment increases the arm  2  spool target flow rate in accordance with the arm  1  flow control valve command pressure and decreases the arm  1  spool target flow rate. As a result, the delivery flow rate of the second hydraulic pump  20   b  becomes higher as compared to a period before time T 2  as indicated in (c). Therefore, a decrease in the arm crowding speed can be reduced as indicated in (d) without dividing the delivery hydraulic fluid from each hydraulic pump. In this instance, the bottom oil chamber pressure of the boom cylinder  6  increases as indicated in (e). 
     If, in a situation where two hydraulic actuators (arm cylinder  7  and boom cylinder  6 ) operate in a combined manner, the arm cylinder  7  is regarded as the first hydraulic actuator, a hydraulic pump communicating with the first and second hydraulic actuators through different spools is defined as the other hydraulic pump. In the above-described operation, the third hydraulic pump  20   c  corresponds to the other hydraulic pump. 
     Further, a hydraulic pump communicating with the first hydraulic actuator (arm cylinder  7 ) through a primary spool for the first hydraulic actuator (arm second directional control valve)  14   b  is defined as the one hydraulic pump. In the above-described operation, the second hydraulic pump  20   b  corresponds to the one hydraulic pump. 
     Furthermore, the boom cylinder  6 , which is a hydraulic actuator communicating only with the other hydraulic pump  20   c  without communicating with the one hydraulic pump  20   b , is defined as the second hydraulic actuator. 
     That is to say, the first hydraulic actuator is either one of two simultaneously operated hydraulic actuators that communicates with the one hydraulic pump  20   b  through the first hydraulic actuator primary spool (arm second directional control valve)  14   b  and communicates with the other hydraulic pump  20   c  through the first hydraulic actuator secondary spool (arm first directional control valve)  14   c.    
     When the above definition is formulated, the pump flow control means (flow control section  130 ) of the controller according to the present embodiment exercises control to increase the delivery flow rate of the one hydraulic pump (second hydraulic pump  20   b ) to a higher rate when the first hydraulic actuator (arm cylinder  7 ) and the second hydraulic actuator (boom cylinder  6 ) are simultaneously operated than when the first hydraulic actuator (arm cylinder  7 ) is operated and the second hydraulic actuator (boom cylinder  6 ) is not operated. 
     According to an embodiment of the present invention, the hydraulic control system for a work machine includes the first hydraulic actuator, the one hydraulic pump, the second hydraulic actuator, the other hydraulic pump, and the secondary spool for the first hydraulic actuator. The one hydraulic pump is capable of supplying hydraulic fluid to the first hydraulic actuator through the primary spool for the first hydraulic actuator. The other hydraulic pump is capable of supplying hydraulic fluid to the second hydraulic actuator through the primary spool for the second hydraulic actuator. The secondary spool for the first hydraulic actuator is capable of placing the first hydraulic actuator in communication with the other hydraulic pump. When the first and second hydraulic actuators are simultaneously operated, the delivery flow rate of the one hydraulic pump increases to a higher rate than when the first hydraulic actuator is operated and the second hydraulic actuator is not operated. Therefore, it is possible to reduce a decrease in the speed of the first hydraulic actuator that is caused by the operation of the second hydraulic actuator. Further, in the above instance, the opening for communication between the first hydraulic actuator and the second hydraulic pump is interrupted. Consequently, the amount of divided flow of the delivery hydraulic fluid from the second hydraulic pump can be decreased to reduce the flow division loss. 
     The present invention is not limited to the above-described exemplary embodiments, but extends to various modifications that nevertheless fall within the scope of the present invention. The foregoing embodiments have been described in detail to facilitate the understanding of the present invention. The present invention is not necessarily limited to a configuration having all the above-described elements. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : Lower travel structure 
           2 : Upper swing structure 
           2 A: Engine 
           3 : Boom. 
           4 : Arm 
           5 : Bucket 
           6 : Boom cylinder 
           7 : Arm cylinder 
           8 : Bucket cylinder 
           9 : Operating lever (operating device) 
           10 : Control valve 
           11 : Swing hydraulic motor 
           13   a : Boom first directional control valve (spool) 
           13   c : Boom second directional control valve (spool) 
           14   b : Arm second directional control valve (spool) 
           14   c : Arm first directional control valve (spool) 
           15   a : Bucket directional control valve (spool) 
           16   b : Swing directional control valve (spool) 
           20 : Hydraulic pump device 
           20   a : First hydraulic pump 
           20   b : Second hydraulic pump 
           20   c : Third hydraulic pump 
           20   d : First hydraulic pump regulator 
           20   e : Second hydraulic pump regulator 
           20   f : Third hydraulic pump regulator 
           21   a : First pump line 
           21   b : Second pump line 
           21   c : Third pump line 
           22 : Arm  1  flow control valve 
           23 : Arm  2  flow control valve 
           100 : Controller 
           101 - 108 : Pilot pressure sensor (operating instruction detection means) 
           110 : Target operation computation section 
           120 : Communication control section (communication control means) 
           130 : Flow control section (pump flow control means)