Patent Publication Number: US-10330128-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 
     A known hydraulic control system is intended for a construction machine that is designed to achieve an even more increased speed of a specific actuator that can be driven through merging of hydraulic fluids from two hydraulic pumps. This construction machine includes an engine, variable displacement first and second hydraulic pumps driven by the engine, a specific actuator that can be driven through merging of the hydraulic fluid delivered from each of the first hydraulic pump and the second hydraulic pump, another actuator that is different from the specific actuator, and a third hydraulic pump that is driven by the engine to supply the hydraulic fluid for driving the another actuator. The hydraulic control system includes a merging valve that can merge the hydraulic fluid from the third hydraulic pump with the hydraulic fluid from the first hydraulic pump and the second hydraulic pump to thereby selectively supply the merged hydraulic fluid to the specific actuator and a merging cancellation valve that cancels the merging function of the merging valve (see, for example, Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-2000-337307-A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     A hydraulic control circuit in the known hydraulic control system described above includes the merging cancellation valve that cancels the merging function of the merging valve. When load pressure on an arm cylinder is high, the merging cancellation valve is operated and delivery fluid of the third hydraulic pump is thereby returned from the merging valve to a tank, so that the delivery pressure of the third hydraulic pump is reduced. This reduces the load on the third hydraulic pump to thereby increase a delivery flow rate of other hydraulic pumps. As a result, a flow rate to be supplied to actuators including a bucket cylinder driven by other hydraulic pumps can be obtained, so that favorable combined operability can be achieved. 
     The hydraulic control circuit in the known art described above, however, has the following problem in terms of energy saving. 
     In general, since a leakage flow rate of a hydraulic pump increases with increasing delivery pressure, the leakage flow rate has a greater effect on total loss of the hydraulic pump at higher delivery pressure values. The merging cancellation valve is thus operated to correspond to the load pressure and the delivery pressure of the third hydraulic pump is thereby reduced. The leakage flow rate of all pumps can thereby be reduced. Unfortunately, however, the patent document of the known art does not describe flow rate control of the third hydraulic pump during this time. 
     Application of well-known positive control, for example, causes the third hydraulic pump to deliver a flow rate in accordance with an operation amount of an arm lever. This can increase likelihood that an inoperative flow rate representing the fluid returning to the tank without being supplied to the actuator increases. As a result, a waste of energy occurs. 
     The present invention has been made in view of the foregoing situation and it is an object of the present invention to provide, in a hydraulic control system for a work machine including a specific actuator to which hydraulic fluid can be supplied from a plurality of hydraulic pumps, an energy-saving hydraulic control system for a work machine. 
     Means for Solving the Problem 
     To achieve the foregoing object, an aspect of the present invention provides a hydraulic control system for a work machine including: a first hydraulic actuator; a first hydraulic pump and a second hydraulic pump capable of communicating with the first hydraulic actuator; a first control valve capable of returning a hydraulic fluid delivered by the first hydraulic pump to a tank; and a load detection section that detects a load on the first hydraulic actuator. The hydraulic control system includes: a control valve drive section that takes in a detection signal detected by the load detection section and drives the first control valve such that a communication area between the first hydraulic pump and the tank is enlarged corresponding to an increase in the load on the first hydraulic actuator; and a flow rate control section that, during supply of the hydraulic fluid from the first hydraulic pump and the second hydraulic pump to the first hydraulic actuator, takes in a detection signal detected by the load detection section and controls to reduce a delivery flow rate of the first hydraulic pump corresponding to an increase in the load on the first hydraulic actuator. 
     Effects of the Invention 
     In accordance with an aspect of the present invention, the delivery flow rate of the first hydraulic pump is decreased with an increasing load on the first hydraulic actuator to thereby drive the first control valve so as to enlarge the communication area between the first hydraulic pump and the tank, so that the delivery pressure of the first hydraulic pump can be reduced and a pump total leakage flow rate can be reduced. A void flow rate delivered from the first hydraulic pump can thus be reduced. As a result, an energy-saving hydraulic control system for a work machine can be provided. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a work machine that includes a hydraulic control system for a work machine according to a first embodiment of the present invention. 
         FIG. 2  is a hydraulic control circuit diagram of the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 3  is a conceptual diagram of a configuration of a controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 4  is a characteristic diagram representing an exemplary map used for arithmetic operations performed by a target operation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 5  is a control block diagram representing exemplary arithmetic operations performed by a communication control section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 6  is a conceptual diagram of a configuration of a flow rate control section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 7  is a control block diagram representing exemplary arithmetic operations performed by a boom flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 8  is a control block diagram representing exemplary arithmetic operations performed by an arm target flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 9  is a control block diagram representing exemplary arithmetic operations performed by a pump flow rate command arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 10  is a characteristic diagram representing an exemplary map used for arithmetic operations performed by an arm flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
         FIGS. 11( a ) to 11( e )  are characteristic diagrams illustrating exemplary operations relating to a pump flow rate control section in the hydraulic control system for a work machine according to the first embodiment. 
         FIG. 12  is a hydraulic control circuit diagram of a hydraulic control system for a work machine according to a second embodiment. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     A hydraulic control system for a work machine according to embodiments is described below with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a perspective view of a work machine that includes a hydraulic control system for a work machine according to a first embodiment of the present invention.  FIG. 2  is a hydraulic control circuit diagram of the hydraulic control system for a work machine according to the first embodiment. 
     As shown in  FIG. 1 , a hydraulic excavator that includes the hydraulic control system for a work machine according to the first embodiment includes a lower track structure  1 , an upper swing structure  2  disposed on the lower track structure  1 , a front work implement connected to the upper swing structure  2  rotatably in a vertical direction, and an engine  2 A as a prime mover. The front work implement includes a boom  3 , an arm  4 , and a bucket  5 . Specifically, the boom  3  is installed to the upper swing structure  2 . The arm  4  is installed to a distal end of the boom  3 . The bucket  5  is installed to a distal end of the arm  4 . In addition, the front work implement includes a pair of boom cylinders  6 , an arm cylinder  7 , and a bucket cylinder  8 . Specifically, the boom cylinders  6  drive the boom  3 . The arm cylinder  7  drives the arm  4 . The bucket cylinder  8  drives the bucket  5 . 
     The hydraulic excavator supplies hydraulic fluid delivered by a hydraulic pump unit not shown to the boom cylinders  6 , the arm cylinder  7 , the bucket cylinder  8 , and a swing hydraulic motor  11  via a control valve  10  in accordance with an operation of a first control lever  9   a  or a second control lever  9   b  provided in a cabin of the upper swing structure  1 . A cylinder rod of each of the boom cylinders  6 , the arm cylinder  7 , and the bucket cylinder  8  is extended and contracted by the hydraulic fluid, so that a position and posture of the bucket  5  can be varied. Additionally, the swing hydraulic motor  11  is rotated by the hydraulic fluid, so that the upper swing structure  2  swings with respect to the lower track structure  1 . 
     The control valve  10  includes a track right directional control valve  12   a , a track left directional control valve  12   b , a boom first directional control valve  13   a , a boom second directional control valve  13   b , an arm first directional control valve  14   b , an arm second directional control valve  14   a , an arm third directional control valve  14   c , a bucket directional control valve  15   a , and a swing directional control valve  16   c  to be described later. 
     The engine  2 A includes a speed sensor  2 Ax that detects an engine speed. The boom cylinders  6  each include a pressure sensor A 6  that detects pressure of a bottom-side fluid chamber and a pressure sensor B 6  that detects pressure of a rod-side fluid chamber. The arm cylinder  7  includes a pressure sensor A 7  that detects pressure of a bottom-side fluid chamber as a load acquisition part and a pressure sensor B 7  that detects pressure of a rod-side fluid chamber. Similarly, the bucket cylinder  8  includes a pressure sensor A 8  that detects pressure of a bottom-side fluid chamber and a pressure sensor B 8  that detects pressure of a rod-side fluid chamber. The swing hydraulic motor  11  includes pressure sensors A 11  and B 11  that detect clockwise and counterclockwise swing pressures. Pressure signals detected by the pressure sensors A 6  to A 8 , B 6  to B 8 , A 11 , and B 11  and the engine speed detected by the speed sensor  2 Ax are applied to a controller  100  to be described later. 
     A pump system  20  that constitutes the hydraulic control system for a work machine according to the first embodiment includes, as shown in  FIG. 2 , a first hydraulic pump  20   a , a second hydraulic pump  20   b , and a third hydraulic pump  20   c  that are variable displacement type hydraulic pumps. The first to third hydraulic pumps  20   a  to  20   c  are driven by the engine  2 A. 
     The first hydraulic pump  20   a  includes a regulator  20   d  that is driven by a command signal from the controller  100  to be described later and supplies a first pump line  21   a  with a controlled delivery flow rate of the hydraulic fluid. Similarly, the second hydraulic pump  20   b  includes a regulator  20   e  that is driven by a command signal from the controller  100  to be described later and supplies a second pump line  21   b  with a controlled delivery flow rate of the hydraulic fluid. Additionally, the third hydraulic pump  20   c  includes a regulator  20   f  that is driven by a command signal from the controller  100  to be described later and supplies a third pump line  21   c  with a controlled delivery flow rate of the hydraulic fluid. 
     For a simplified description, descriptions for, for example, relief valves, return circuits, and load check valves that are not directly connected with the present embodiment are omitted. Additionally, the present embodiment will be described for a case in which the present invention is applied to a well-known open center type hydraulic control system. The application is, however, illustrative only and not limiting. 
     The track right directional control valve  12   a , the bucket directional control valve  15   a , the arm second directional control valve  14   a , and the boom first directional control valve  13   a  are disposed in the first pump line  21   a  that communicates with a delivery port of the first hydraulic pump  20   a . The resultant configuration is a tandem circuit that prioritizes the track right directional control valve  12   a  and a parallel circuit with the remaining bucket directional control valve  15   a , arm second directional control valve  14   a , and boom first directional control valve  13   a.    
     The boom second directional control valve  13   b , the arm first directional control valve  14   b , and the track left directional control valve  12   b  are disposed in the second pump line  21   b  that communicates with a delivery port of the second hydraulic pump  20   b . The resultant configuration is a parallel circuit with the boom second directional control valve  13   b  and the arm first directional control valve  14   b  and a parallel-tandem circuit with the track left directional control valve  12   b . A check valve  17  that permits inflow only from the second hydraulic pump  20   b  side and a restrictor  18  are disposed in the parallel circuit with the track left directional control valve  12   b . Additionally, the track left directional control valve  12   b  can communicate with the first hydraulic pump  20  via a track communication valve  19 . 
     The arm third directional control valve  14   c  and the swing directional control valve  16   c  are disposed in the third pump line  21   c  that communicates with a delivery port of the third hydraulic pump  20   c . The resultant configuration is a tandem circuit that prioritizes the swing directional control valve  16   c.    
     It is noted that an outlet port of the boom first directional control valve  13   a  and an outlet port of the boom second directional control valve  13   b  communicate with the boom cylinders  6  via a merging passage not shown. An outlet port of the arm second directional control valve  14   a , an outlet port of the arm first directional control valve  14   b , and an outlet port of the arm third directional control valve communicate with the arm cylinder  7  via a merging passage not shown. Additionally, an outlet port of the bucket directional control valve  15   a  communicates with the bucket cylinder  5  and an outlet port of the swing directional control valve  16   c  communicates with the swing hydraulic motor  11 . 
     Reference is made to  FIG. 2 . The first control lever  9   a  to a fourth control lever  9   d  are each provided with pilot valves not shown thereinside. The pilot valves generate pilot pressure corresponding to an operation amount in a tilting operation of each control lever. The pilot pressure resulting from each control lever operation is supplied to an operating section of each directional control valve. 
     The pilot lines indicated by broken lines BkC and BkD from the first control lever  9   a  are connected with an operating section of the bucket directional control valve  15   a . A bucket crowding pilot pressure and a bucket dumping pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. Additionally, a pilot line indicated by broken lines BmD and BmU from the first control lever  9   a  are connected with respective operating sections of the boom first directional control valve  13   a  and the boom second directional control valve  13   b . A boom raising pilot pressure and a boom lowering pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. 
     A pressure sensor  105  and a pressure sensor  106  are provided in the pilot lines indicated by the broken lines BkC and BkD. The pressure sensor  105  detects the bucket crowding pilot pressure. The pressure sensor  106  detects the bucket dumping pilot pressure. A pressure sensor  101  and a pressure sensor  102  are provided in the pilot lines indicated by the broken lines BmD and BmU. The pressure sensor  101  detects the boom raising pilot pressure. The pressure sensor  102  detects the boom lowering pilot pressure. The pressure sensors  101 ,  102 ,  105 , and  106  are each an operation instruction detection section. Pressure signals detected by the pressure sensors  101 ,  102 ,  105 , and  106  are applied to the controller  100 . 
     Pilot lines indicated by broken lines AmC and AmD from the second control lever  9   b  are connected with respective operating sections of the arm first directional control valve  14   b , the arm second directional control valve  14   a , and the arm third directional control valve  14   c . An arm crowding pilot pressure and an arm dumping pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. Additionally, Pilot lines indicated by broken lines SwR and SwL from the second control lever  9   b  are connected with an operating section of the swing directional control valve  16   c . A swing right pilot pressure and a swing left pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. 
     A pressure sensor  103  and a pressure sensor  104  are provided in the pilot lines indicated by the broken lines AmC and AmD. The pressure sensor  103  detects the arm crowding pilot pressure. The pressure sensor  104  detects the arm dumping pilot pressure. Additionally, an arm  3  crowding pressure reducing valve  22  is provided in an arm crowding pilot line connected with the operating section of the arm third directional control valve  14   c . The arm  3  crowding pressure reducing valve  22  limits or interrupts an arm crowding pilot hydraulic fluid to be supplied. 
     A pressure sensor  108  and a pressure sensor  107  are provided in the pilot lines indicated by the broken lines SwR and SwL. The pressure sensor  108  detects the swing right pilot pressure. The pressure sensor  107  detects the swing left pilot pressure. The pressure sensors  103 ,  104 ,  107 , and  108  are each an operation instruction detection section. Pressure signals detected by the pressure sensors  103 ,  104 ,  107 , and  108  are applied to the controller  100 . 
     Pilot lines indicated by broken lines TrRF and TrRR from the third control lever  9   c  are connected with an operating section of the track right directional control valve  12   a . A track right forward pilot pressure and a track right reverse pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. 
     Pilot lines indicated by broken lines TrLF and TrLR from the fourth control lever  9   d  are connected with an operating section of the track left directional control valve  12   b . A track left forward pilot pressure and a track left reverse pilot pressure generated corresponding to the operation amount in the tilting operation of the control lever are thus supplied. 
     The hydraulic control system in the present embodiment includes the controller  100 . The controller  100  receives an input of the engine speed from the speed sensor  2 Ax shown in  FIG. 1  and receives inputs of pilot line pilot pressure signals from the respective pressure sensors  101  to  108  described above. Additionally, the controller  100  receives inputs of actuator pressure signals from the respective pressure sensors A 6  to A 8 , B 6  to B 8 , A 11 , and B 11  shown in  FIG. 1 . 
     The controller  100  outputs command signals to the regulator  20   d  of the first hydraulic pump  20   a , the regulator  20   e  of the second hydraulic pump  20   b , and the regulator  20   f  of the third hydraulic pump  20   c , respectively, to thereby control delivery flow rates to the respective hydraulic pumps  20   a  to  20   c . The controller  100  also outputs a command signal to an operating section of the arm  3  crowding pressure reducing valve  22  to thereby control to limit or interrupt pressure of an arm crowding pilot line Amc to be supplied to the operating section of the arm third directional control valve  14   c . An increase in the command signal interrupts the pilot pressure supplied to the operating section of the arm third directional control valve  14   c . As a result, communication between the third hydraulic pump  20   c  and the arm cylinder  7  is interrupted and the hydraulic fluid from the third pump line  21   c  is returned to the tank. 
     The controller that constitutes the hydraulic control system for a work machine according to the first embodiment is described below with reference to relevant drawings.  FIG. 3  is a conceptual diagram of a configuration of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment.  FIG. 4  is a characteristic diagram representing an exemplary map for use by a target operation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment.  FIG. 5  is a control block diagram representing exemplary arithmetic operations performed by a communication control section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
     Reference is made to  FIG. 3 . The controller  100  includes a target operation arithmetic section  110 , a communication control section  120 , and a flow rate control section  130 . Specifically, the target operation arithmetic section  110  calculates a target flow rate using each pilot pressure and each load pressure. The communication control section  120  serves as a communication control section that calculates a command signal for the arm  3  crowding pressure reducing valve  22  for controlling a communication state of the control valve  10 . The flow rate control section  130  serves as a pump flow rate control section that calculates, on the basis of the target flow rates calculated by the target operation arithmetic section  110  and the engine speed from the speed sensor  2 Ax, flow rate command signals of the respective first to third hydraulic pumps  20   a  to  20   c . The flow rate control section  130  outputs command signals to the respective regulators  20   d  to  20   f  of the respective hydraulic pumps to thereby control the delivery flow rates of the respective first to third hydraulic pumps  20   a  to  20   c.    
     The target operation arithmetic section  110  calculates each target flow rate such that the target flow rate increases with an increasing pilot pressure applied thereto and such that the target flow rate decreases with an increasing load pressure applied thereto. During combined operation, each target flow rate is calculated so as to be smaller than during single operation. 
     Exemplary calculations performed by the target operation arithmetic section  110  are described with reference to  FIG. 4  and expressions. The target operation arithmetic section  110  stores, for each actuator, a map used for calculating a reference flow rate from a pilot pressure and shown in  FIG. 4 . For example, a swing target flow rate Qsw is calculated from a swing pilot pressure that represents a value applicable when maximum values of the swing right pilot pressure and the swing left pilot pressure are selected. Similarly, an arm crowding reference flow rate Qamc 0  is calculated from the arm crowding pilot pressure and a dumping reference flow rate Qamd 0  is calculated from the arm dumping pilot pressure. 
     A boom raising reference flow rate Qbmu 0  is calculated from the boom raising pilot pressure. 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 arithmetic section  110  calculates a boom target flow rate Qbm from the swing target flow rate Qsw using an arithmetic expression, Expression 1. 
     Expression 1
 
 Q   bm =min( Q   bm0   ,Q   bmmax   −k   swbm   ·Q   sw )  (1)
 
     Where, the symbol Qbmmax denotes a boom flow rate upper limit value and is set to correspond with a maximum speed of boom raising. The symbol kswbm denotes a boom flow rate reduction coefficient and the boom target flow rate Qbm decreases with an increasing swing target flow rate Qsw. It is noted that, instead of using the boom flow rate reduction coefficient kswbm, a map that causes the boom flow rate upper limit value Qbmmax to decrease with an increasing swing target flow rate Qsw may be used. 
     The target operation arithmetic section  110  uses arithmetic expressions, Expression 2 and Expression 3, to calculate swing drive power Lsw and boom drive power Lbm. 
     Expression 2
 
 L   sw   =P   sw   ·Q   sw   (2)
 
     Expression 3
 
 L   bm   =P   bmb   ·Q   bm   (3)
 
     Where, the symbol Psw denotes a swing pressure and represents a value of the pressure on a meter-in side selected from among the swing left pressure and the swing right pressure detected by the pressure sensors A 11  and B 11 . The symbol Pbmb denotes a boom bottom pressure and represents the pressure of the bottom-side fluid chamber of the boom cylinder  6  detected by the pressure sensor A 6 . 
     The target operation arithmetic section  110  uses arithmetic expressions, Expression 4 and Expression 5, to calculate a bucket drive power upper limit value Lbkmax and an arm drive power upper limit value Lammax. 
     Expression 4
 
 L   bk max   =k   bk ( L   max   −L   sw   −L   bm )  (4)
 
     Expression 5
 
 L   am max   =k   am ( L   max   −L   sw   −L   bm )  (5)
 
     Where, the symbol Lmax denotes a total drive power upper limit value of the system. The symbol kbk denotes a bucket drive power coefficient and the symbol kam denotes an arm drive power coefficient. The bucket drive power coefficient kbk and the arm drive power coefficient kam are calculated using a bucket crowding pilot pressure BkC, a bucket dumping pilot pressure BkD, an arm crowding pilot pressure AmC, an arm dumping pilot pressure AmD, and an arithmetic expression Expression 6. 
     Expression 6
 
 k   bk   :k   am =max( BkC,BkD ):max( AmC,AmD )  (6)
 
     The target operation arithmetic section  110  uses the bucket crowding reference flow rate Qbkc 0 , the bucket dumping reference flow rate Qbkd 0 , the bucket drive power upper limit Lbkmax, and an arithmetic expression Expression 7 to calculate a bucket target flow rate Qbk. Additionally, the target operation arithmetic section  110  uses the arm crowding reference flow rate Qamc 0 , the arm dumping reference flow rate Qamd 0 , the arm drive power upper limit Lammax, and an arithmetic expression Expression 8 to calculate an arm target flow rate Qam. 
     Expression 7
 
 Q   bk =min( Q   bkc0   ,Q   bkd0   ,L   bk max   /P   bk )  (7)
 
     Expression 8
 
 Q   am =min( Q   amc0   ,Q   amd0   ,L   am max   /P   am )  (8)
 
     Where, the symbol Pbk denotes a value of the pressure on a meter-in side selected from among the bottom-side fluid chamber pressure and the rod-side fluid chamber pressure of the bucket cylinder  8  detected by the pressure sensors A 8  and B 8 . The symbol Pam denotes a value of the pressure on a meter-in side selected from among the bottom-side fluid chamber pressure and the rod-side fluid chamber pressure of the arm cylinder  7  detected by the pressure sensors A 7  and B 7 . 
     Exemplary calculations performed by the communication control section  120  are described below with reference to  FIG. 5 . The communication control section  120  includes a first function generating section  120   a  and a solenoid valve drive command converting section  120   b.    
     As shown in  FIG. 5 , the first function generating section  120   a  receives an input of the bottom-side fluid chamber pressure of the arm cylinder  7  detected by the pressure sensor A 7 . The first function generating section  120   a  stores therein in advance as a map M 1  a table that indicates a limiting characteristic of the arm  3  crowding pilot pressure with respect to the bottom-side fluid chamber pressure of the arm cylinder  7 . The map M 1  exhibits a characteristic that the arm  3  crowding pilot pressure decreases with an increasing bottom-side fluid chamber pressure of the arm cylinder  7 . An arm  3  crowding pilot pressure limiting characteristic signal calculated by the first function generating section  120   a  is output to the solenoid valve drive command converting section  120   b.    
     The solenoid valve drive command converting section  120   b  receives the input of the arm  3  crowding pilot pressure limiting characteristic signal from the first function generating section  120   a  and calculates a command signal for the arm  3  crowding pressure reducing valve  22  corresponding to the limiting characteristic signal. Specifically, an increase in the command signal for the arm  3  crowding pressure reducing valve  22  reduces and interrupts the pilot pressure supplied to the operating section of the arm third directional control valve  14   c , so that the characteristic is such that an output signal increases with an increasing input signal. The command signal calculated by the solenoid valve drive command converting section  120   b  is output to the operating section of the arm  3  crowding pressure reducing valve  22 . 
     Thus, the pilot pressure supplied to the operating section of the arm third directional control valve  14   c  is reduced more with higher bottom-side fluid chamber pressures of the arm cylinder  7 . 
     It is here noted that, in the limiting characteristic of the arm  3  crowding pilot pressure, the value of pressure starting to decrease from a certain value in the bottom-side fluid chamber of the arm cylinder  7  is preferably set to be equal to or higher than a pump delivery pressure at which leakage loss of the hydraulic pump is likely to exceed friction loss of the hydraulic pump and is set on the basis of the loss characteristic of the hydraulic pump. 
     The flow rate control section  130  as a pump flow rate control section is described below with reference to relevant drawings.  FIG. 6  is a conceptual diagram of a configuration of the flow rate control section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment.  FIG. 7  is a control block diagram representing exemplary arithmetic operations performed by a boom flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment.  FIG. 8  is a control block diagram representing exemplary arithmetic operations performed by an arm target flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment.  FIG. 9  is a control block diagram representing exemplary arithmetic operations performed by a pump flow rate command arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. In  FIGS. 6 to 9 , like or corresponding elements are identified by the same reference numerals as those used in  FIGS. 1 to 5  and descriptions for those elements are omitted. 
     Reference is made to  FIG. 6 . The flow rate control section  130  includes a boom flow rate allocation arithmetic section  131 , an arm flow rate allocation arithmetic section  132 , and a pump flow rate command arithmetic section  133 . Specifically, the boom flow rate allocation arithmetic section  131  calculates an allocation of a target flow rate for each of the directional control valves of the boom  3 . The arm flow rate allocation arithmetic section  132  calculates an allocation of a target flow rate for each of the directional control valves of the arm  4 . The pump flow rate command arithmetic section  133  calculates the flow rate of each pump on the basis of the calculated target flow rate allocation and outputs command signals to the respective regulators  20   d  to  20   f  of the respective hydraulic pumps to thereby control the delivery flow rates of the respective first to third hydraulic pumps  20   a  to  20   c.    
     Exemplary calculations performed by the boom flow rate allocation arithmetic section  131  are described below with reference to  FIG. 7 . The boom flow rate allocation arithmetic section  131  includes a first function generating section  131   a , a minimum value selecting section  131   b , a subtractor  131   c , a second function generating section  131   d , a third function generating section  131   e , and a fourth function generating section  131   f.    
     The first function generating section  131   a  receives an input of the boom target flow rate from the target operation arithmetic section  110 . The first function generating section  131   a  stores therein in advance as a map M 3   a  a table that indicates a boom  2  spool target flow rate with respect to the boom target flow rate. The map M 3   a  exhibits a characteristic that the boom  2  spool target flow rate increases with an increasing boom target flow rate. The boom  2  spool target flow rate may be set, for example, to half of the boom target flow rate. In this case, a boom  1  spool target flow rate and the boom  2  spool target flow rate are each half of the boom target flow rate, unless the limiting to be described later is imposed. The calculated boom  2  spool target flow rate signal is output to the minimum value selecting section  131   b.    
     The minimum value selecting section  131   b  receives inputs of the boom  2  spool target flow rate signal from the first function generating section  131   a , a signal from the second function generating section  131   d  to be described later, a limiting signal from the third function generating section  131   e  to be described later, and a limiting signal from the fourth function generating section  131   f  to be described later. The minimum value selecting section  131   b  calculates a minimum value among these signals and outputs the minimum value as the boom  2  spool target flow rate to the subtractor  131   c  and the pump flow rate command arithmetic section  133 . 
     The subtractor  131   c  receives inputs of the boom target flow rate from the target operation arithmetic section  110  and the boom  2  spool target flow rate from the minimum value selecting section  131   b . The subtractor  131   c  then subtracts the boom  2  spool target flow rate from the boom target flow rate to thereby find the boom  1  spool target flow rate. The subtractor  131   c  outputs the calculated boom  1  spool target flow rate signal to the pump flow rate command arithmetic section  133 . 
     The second function generating section  131   d  receives an input of the boom raising pilot pressure detected by the pressure sensor  101  and outputs a limiting signal to the minimum value selecting section  131   b . The second function generating section  131   d  stores therein in advance as a map M 3   b  a table that indicates an upper limit value of the boom  2  spool target flow rate with respect to the boom raising pilot pressure. The map M 3   b  exhibits a trend of substantially proportional to a meter-in opening characteristic of the boom second directional control valve  13   b , increasing with the boom raising pilot pressure. Specifically, the second function generating section  131   d  increases the upper limit value of the boom  2  spool target flow rate corresponding to the opening in the boom second directional control valve  13   c.    
     The third function generating section  131   e  receives an input of the arm crowding pilot pressure detected by the pressure sensor  103  and outputs to the minimum value selecting section  131   b  a signal obtained from a map M 3   c  stored in advance as a table. The map M 3   c  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm first directional control valve  14   b  with respect to the arm crowding pilot pressure, reducing the upper limit of the boom  2  spool flow rate corresponding to the arm crowding pilot pressure. 
     The fourth function generating section  131   f  receives an input of the arm dumping pilot pressure detected by the pressure sensor  104  and outputs to the minimum value selecting section  131   b  a signal obtained from a map M 3   d  stored in advance as a table. The map M 3   d  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm first directional control valve  14   b  with respect to the arm dumping pilot pressure, reducing the upper limit value of the boom  2  spool flow rate corresponding to the arm dumping pilot pressure. 
     The boom flow rate allocation arithmetic section  131  limits the boom  2  spool target flow rate using these boom  2  spool flow rate upper limit values and subtracts the boom  2  spool target flow rate from the boom target flow rate to find the boom  1  spool target flow rate. 
     Exemplary calculations performed by the arm flow rate allocation arithmetic section  132  are described below with reference to  FIG. 8 . The arm flow rate allocation arithmetic section  132  includes a first function generating section  132   a , a first minimum value selecting section  132   b , a first subtractor  132   c , a second function generating section  132   d , a third function generating section  132   e , a first maximum value selecting section  132   f , a fourth function generating section  132   g , a second minimum value selecting section  132   h , a second subtractor  132   i , a fifth function generating section  132 J, a sixth function generating section  132   k , a second maximum value selecting section  132 L, a seventh function generating section  132   m , and an eighth function generating section  132   n.    
     The first function generating section  132   a  and the fourth function generating section  132   g  each receive an input of the arm target flow rate from the target operation arithmetic section  110 . The first function generating section  132   a  stores therein in advance as a map M 4   a  a table that indicates an arm  2  spool target flow rate with respect to the arm target flow rate. The fourth function generating section  132   g  stores therein in advance as a map M 4   b  a table that indicates an arm  3  spool target flow rate with respect to the arm target flow rate. The maps M 4   a  and M 4   b  each exhibit a characteristic that the arm  2  spool target flow rate and the arm  3  spool target flow rate increase with an increasing arm target flow rate. Here, for example, each of the arm  2  spool target flow rate and the arm  3  spool target flow rate may be set to ⅓ of the arm target flow rate. In this case, the arm  1  spool target flow rate, the arm  2  spool target flow rate, and the arm  3  spool target flow rate are each ⅓ of the arm target flow rate, unless the limiting to be described later is imposed. The calculated arm  2  spool target flow rate signal is output to the first minimum value selecting section  132   b . The calculated arm  3  spool target flow rate signal is output to the second minimum value selecting section  132   h.    
     The first minimum value selecting section  132   b  receives inputs of the arm  2  spool target flow rate signal from the first function generating section  132   a  and a limiting signal from the first maximum value selecting section  132   f  to be described later. The first minimum value selecting section  132   b  calculates a minimum value of these signals and outputs the minimum value as an arm  2  spool target flow rate signal to the first subtractor  132   c  and the pump flow rate command arithmetic section  133 . 
     The first subtractor  132   c  receives inputs of the arm target flow rate from the target operation arithmetic section  110  and the arm  2  spool target flow rate from the first minimum value selecting section  132   b . The first subtractor  132   c  subtracts the arm  2  spool target flow rate from the arm target flow rate to thereby find an arm  1  spool target flow rate reference signal. The calculated arm  1  spool target flow rate reference signal is output to the second subtractor  132   i.    
     The second function generating section  132   d  receives an input of the arm crowding pilot pressure detected by the pressure sensor  103  and outputs to the first maximum value selecting section  132   f  a signal obtained from a map M 4   c  stored in advance as a table. The map M 4   c  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm second directional control valve  14   a  with respect to the arm crowding pilot pressure, increasing the upper limit value of the arm  2  spool flow rate corresponding to the arm crowding pilot pressure. 
     The third function generating section  132   e  receives an input of the arm dumping pilot pressure detected by the pressure sensor  104  and outputs to the first maximum value selecting section  132   f  a signal obtained from a map M 4   d  stored in advance as a table. The map M 4   d  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm second directional control valve  14   a  with respect to the arm dumping pilot pressure, increasing the upper limit value of the arm  2  spool flow rate corresponding to the arm dumping pilot pressure. 
     The first maximum value selecting section  132   f  receives inputs of an output from the second function generating section  132   d  and an output from the third function generating section  132   e . The first maximum value selecting section  132   f  calculates a maximum value of these outputs and outputs the maximum value to the first minimum value selecting section  132   b.    
     The second minimum value selecting section  132   h  receives inputs of an arm  3  spool target flow rate signal from the fourth function generating section  132   g , a limiting signal from the second maximum value selecting section  132 L to be described later, and limiting signals from the seventh function generating section  132   m  and the eighth function generating section  132   n . The second minimum value selecting section  132   h  calculates a minimum value of these signals and outputs the minimum value as an arm  3  spool target flow rate signal to the second subtractor  132   i  and the pump flow rate command arithmetic section  133 . 
     The second subtractor  132   i  receives inputs of the arm  1  spool target flow rate reference signal calculated by the first subtractor  132   c  and the arm  3  spool target flow rate from the second minimum value selecting section  132   h . The second subtractor  132   i  subtracts the arm  3  spool target flow rate from the arm  1  spool target flow rate reference signal to thereby calculate the arm  1  spool target flow rate reference signal. The calculated arm  1  spool target flow rate signal is output to the pump flow rate command arithmetic section  133 . 
     The fifth function generating section  132 J receives an input of the arm crowding pilot pressure detected by the pressure sensor  103  and outputs to the second maximum value selecting section  132 L a signal obtained from a map M 4   f  stored in advance as a table. The map M 4   f  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm third directional control valve  14   c  with respect to the arm crowding pilot pressure, increasing the upper limit value of the arm  3  spool flow rate corresponding to the arm crowding pilot pressure. It is noted that, as compared with the characteristic of the map M 4   c , the characteristic of the map M 4   f  is such that the output rises with a higher input value (arm crowding pilot pressure). This arrangement results in the following. Specifically, when the operation amount of the second control lever  9   b  that operates the arm  4  is small, the arm  2  spool target flow rate signal is first generated and, after the operation amount of the second control lever  9   b  that operates the arm  4  increases, the arm  3  spool target flow rate signal is generated. 
     The sixth function generating section  132   k  receives an input of the arm dumping pilot pressure detected by the pressure sensor  104  and outputs to the second maximum value selecting section  132 L a signal obtained from a map M 4   g  stored in advance as a table. The map M 4   g  exhibits a trend of substantially proportional to a meter-in opening characteristic of the arm third directional control valve  14   c  with respect to the arm dumping pilot pressure, increasing the upper limit value of the arm  3  spool flow rate corresponding to the arm dumping pilot pressure. It is noted that, as compared with the characteristic of the map M 4   d , the characteristic of the map M 4   g  is such that the output rises with a higher input value (arm dumping pilot pressure). This arrangement results in the following. Specifically, when the operation amount of the second control lever  9   b  that operates the arm  4  is small, the arm  2  spool target flow rate signal is first generated and, after the operation amount of the second control lever  9   b  increases, the arm  3  spool target flow rate signal is generated. 
     The second maximum value selecting section  132 L receives inputs of an output from the fifth function generating section  132 J and an output from the sixth function generating section  132   k . The second maximum value selecting section  132 L calculates a maximum value of these outputs and outputs the maximum value to the second minimum value selecting section  132   h.    
     The seventh function generating section  132   m  receives an input of the bottom-side fluid chamber pressure of the arm cylinder  7  detected by the pressure sensor A 7  and outputs to the second minimum value selecting section  132   h  a signal obtained from a map M 4   i  stored in advance as a table. The map M 4   i  is set, as is described later, such that the arm  3  spool flow rate upper limit value decreases corresponding to the bottom-side fluid chamber pressure of the arm cylinder  7 . 
     The eighth function generating section  132   b  receives an input of a maximum value out of the swing right pilot pressure and the swing left pilot pressure detected by the pressure sensors  108  and  107 , respectively, and outputs to the second minimum value selecting section  132   h  a signal obtained from a map M 4   h  stored in advance as a table. The map M 4   h  exhibits a trend of substantially proportional to a center bypass opening characteristic of the swing directional control valve  16   c  with respect to the swing pilot pressure, decreasing the upper limit value of the arm  3  spool flow rate corresponding to the swing pilot pressure. 
     The arm flow rate allocation arithmetic section  132  calculates the arm  1  spool target flow rate, the arm  2  spool target flow rate, and the arm  3  spool target flow rate on the basis of, for example, the arm target flow rate, the arm crowding pilot pressure, and the arm dumping pilot pressure calculated by the target operation arithmetic section  110 . As described previously, however, because the rising point of the output with respect to the input is varied between the map M 4   c  of the second function generating section  132   d  and the map M 4   f  of the fifth function generating section  132 J, and between the map M 4   d  of the third function generating section  132   e  and the map M 4   g  of the sixth function generating section  132   k , the arm  1  spool target flow rate, the arm  2  spool target flow rate, and the arm  3  spool target flow rate are generated in sequence as the operation amount of the second control lever  9   b  that operates the arm  4  increases. 
     Thereafter, the arm  1  spool target flow rate and the arm  2  spool target flow rate are generated to correspond to the operation amount of the second control lever  9   b . When the operation amount further increases, the arm  3  spool target flow rate is generated. 
     Exemplary calculations performed by the pump flow rate command arithmetic section  133  are described below with reference to  FIG. 9 . The pump flow rate command arithmetic section  133  includes a first maximum value selecting section  133   a , a first divider  133   b , a first function generating section  133   c , a second maximum value selecting section  133   d , a second divider  133   e , a second function generating section  133   f , a third maximum value selecting section  133   g , a third divider  133   h , and a third function generating section  133   i.    
     The first maximum value selecting section  133   a  receives inputs of a bucket target flow rate signal from the target operation arithmetic section  110 , a boom  1  spool target flow rate signal from the boom flow rate allocation arithmetic section  131 , and an arm  2  spool target flow rate signal from the arm flow rate allocation arithmetic section  132 . The first maximum value selecting section  133   a  then calculates a maximum value of these signals and outputs the maximum value as a first pump target flow rate to the first divider  133   b.    
     The first divider  133   b  receives inputs of the first pump target flow rate from the first maximum value selecting section  133   a  and the engine speed detected by the speed sensor  2 Ax. The first divider  133   b  then divides the first pump target flow rate by the engine speed to find a first pump target command. The calculated first pump target command signal is output to the first function generating section  133   c.    
     The first function generating section  133   c  receives an input of the first pump target command signal calculated by the first divider  133   b . The first function generating section  133   c  outputs as a first pump flow rate command signal a signal obtained from a map M 5   a  stored in advance as a table to the regulator  20   d . The delivery flow rate of the first hydraulic pump  20   a  is thereby controlled. 
     The second maximum value selecting section  133   d  receives inputs of a boom  2  spool target flow rate signal from the boom flow rate allocation arithmetic section  131  and an arm  1  spool target flow rate signal from the arm flow rate allocation arithmetic section  132 . The second maximum value selecting section  133   d  then calculates a maximum value of these signals and outputs the maximum value as a second pump target flow rate to the second divider  133   e.    
     The second divider  133   e  receives inputs of the second pump target flow rate from the second maximum value selecting section  133   d  and the engine speed detected by the speed sensor  2 Ax. The second divider  133   e  then divides the second pump target flow rate by the engine speed to find a second pump target command. The calculated second pump target command signal is output to the second function generating section  133   f.    
     The second function generating section  133   f  receives an input of the second pump target command signal calculated by the second divider  133   e . The second function generating section  133   f  outputs as a second pump flow rate command signal a signal obtained from a map M 5   b  stored in advance as a table to the regulator  20   e . The delivery flow rate of the second hydraulic pump  20   b  is thereby controlled. 
     The third maximum value selecting section  133   g  receives inputs of a swing target flow rate signal from the target operation arithmetic section  110  and an arm  3  spool target flow rate signal from the arm flow rate allocation arithmetic section  132 . The third maximum value selecting section  133   g  then calculates a maximum value of these signals and outputs the maximum value as a third pump target flow rate to the third divider  133   h.    
     The third divider  133   h  receives inputs of the third pump target flow rate from the third maximum value selecting section  133   g  and the engine speed detected by the speed sensor  2 Ax. The third divider  133   h  then divides the third pump target flow rate by the engine speed to find a third pump target command. The calculated third pump target command signal is output to the third function generating section  133   i.    
     The third function generating section  133   i  receives an input of the third pump target command signal calculated by the third divider  133   b . The third function generating section  133   i  outputs as a third pump flow rate command signal a signal obtained from a map M 5   c  stored in advance as a table to the regulator  20   f . The delivery flow rate of the third hydraulic pump  20   c  is thereby controlled. 
     In the pump flow rate command arithmetic section  133 , the arm  2  spool target flow rate is input to the first maximum value selecting section  133   a , the arm  1  spool target flow rate is input to the second maximum value selecting section  133   d , and the arm  3  spool target flow rate is input to the third maximum value selecting section  133   g , and the first pump target flow rate, the second pump target flow rate, and the third pump target flow rate are calculated, respectively. It is here noted that, in the arm flow rate allocation arithmetic section  132 , as described previously, the arm  1  spool target flow rate is first generated, the arm  2  spool target flow rate is next generated, and the arm  3  spool target flow rate is finally generated corresponding to the increase in the operation amount of the second control lever  9   b  that operates the arm  4 . 
     This results in the following when the second control lever  9   b  that operates the arm  4  is operated. Specifically, corresponding to the increase in the operation amount, the second pump flow rate command signal is first generated, the first pump flow rate command signal is next generated, and the third pump flow rate command signal is finally generated. 
     It is noted that the present embodiment has been described for a case in which a reduction ratio involved from the engine  2 A to each hydraulic pump is 1. For any reduction ratio other than 1, calculations need to be performed corresponding to the applicable reduction ratio. 
     The setting of the map of the seventh function generating section  132   m  of the arm flow rate allocation arithmetic section  132  is described below with reference to  FIG. 10 .  FIG. 10  is a characteristic diagram representing an exemplary map for use by the arm flow rate allocation arithmetic section of the controller that constitutes the hydraulic control system for a work machine according to the first embodiment. 
     In  FIG. 10 , the abscissa represents pressure of the bottom-side fluid chamber pressure of the arm cylinder  7  and the ordinate represents target flow rate of the arm  3  spool. Additionally, a characteristic line A indicated by the solid line represents the arm  3  crowding pilot pressure limiting characteristic signal of the map M 1  set for the first function generating section  120   a  of the communication control section  120 . A characteristic line B indicated by the broken line represents the map M 4   i  set for the seventh function generating section  132   m , indicating an upper limit limiting characteristic of the arm  3  spool target flow rate with respect to the bottom-side fluid chamber pressure of the arm cylinder  7 . 
     Reference is made to  FIG. 10 . The map M 4   i  (characteristic line B) decreases the arm  3  spool target flow rate upper limit value with an increasing bottom-side fluid chamber pressure of the arm cylinder  7 , so that the map M 4   i  has an operating direction identical to an operating direction of the map M 1  (characteristic line A) that decreases the arm  3  crowding pilot pressure limiting characteristic with an increasing bottom-side fluid chamber pressure of the arm cylinder  7 . The map M 4   i  (characteristic line B) is, however, set to exhibit a characteristic that the reduction in the arm  3  spool target flow rate upper limit starts before the characteristic line A starts decreasing (in a region of small bottom-side fluid chamber pressures of the arm cylinder  7 ). 
     This arrangement results in the following. Specifically, when the bottom-side fluid chamber pressure of the arm cylinder  7  starts increasing, the arm  3  spool flow rate upper limit first decreases and the delivery flow rate of the third hydraulic pump  20   c  decreases; thereafter, the limiting characteristic of the arm  3  crowding pilot pressure causes the arm  3  crowding pressure reducing valve  22  to operate, so that the center bypass opening of the arm third directional control valve  14   c  starts opening. Thus, before the center bypass opening of the arm third directional control valve  14   c  opens, the arm  3  spool flow rate upper limit decreases and the delivery flow rate of the third hydraulic pump  20   c  decreases. As a result, bleed-off loss that is generated in the arm third directional control valve  14   c  can be reduced. Additionally, a small change results in the meter-in flow rate to the arm cylinder  7  at the start of opening of the center bypass opening of the arm third directional control valve  14   c , so that shock at this time can be reduced. 
     Operations of the hydraulic control system for a work machine according to the first embodiment of the present invention are described below with reference to relevant drawings.  FIGS. 11( a ) to 11( e )  are characteristic diagrams illustrating exemplary operations relating to the pump flow rate control section in the hydraulic control system for a work machine according to the first embodiment. 
     In  FIGS. 11( a ) to 11( e ) , the abscissa represents time and the ordinate represents pilot pressure in  FIG. 11( a ) , hydraulic pump delivery pressure in  FIG. 11( b ) , arm third directional control valve  14   c  enter bypass opening in  FIG. 11( c ) , third hydraulic pump delivery flow rate in  FIG. 11( d ) , and fourth hydraulic pump delivery flow rate in  FIG. 11( e ) , respectively. In  FIG. 11( b ) , the solid line represents a delivery pressure characteristic of the second hydraulic pump  20   b  and the broken line represents a delivery pressure characteristic of the third hydraulic pump  20   c . In addition, time T 1  represents time at which an arm crowding operation is started, time T 2  represents time at which the bottom-side fluid chamber pressure of the arm cylinder  7  increases because of, for example, the bucket contacting an excavation surface, and time T 3  represents time at which the bottom-side fluid chamber pressure of the arm cylinder  7  further increases, respectively. It is noted that, for simplification purposes, operations of the first hydraulic pump  20   a  are omitted. 
     When the arm crowding operation is started at time T 1 , the arm crowding pilot pressure rises as shown in  FIG. 11( a ) . The arm first directional control valve  14   b  and the arm third directional control valve  14   c  then operate, the arm cylinder  7  communicates with each hydraulic pump, and the pump delivery pressure shown in  FIG. 11( b )  rises to pressure corresponding to the bottom-side fluid chamber pressure of the arm cylinder  7 . If the bottom-side fluid chamber pressure of the arm cylinder  7  is low at this time, the center bypass opening of the arm third directional control valve  14   c  closes as shown in  FIG. 11( c ) . Additionally, as shown in  FIGS. 11( d ) and 11( e ) , the delivery flow rate of the third hydraulic pump  20   c  and the delivery flow rate of the second hydraulic pump  20   b  increase and the arm  4  operates. 
     When the bottom-side fluid chamber pressure of the arm cylinder  7  increases because of, for example, the bucket  5  contacting an excavation surface at time T 2 , the flow rate control section  130  reduces the delivery flow rate of the third hydraulic pump  20   c  as shown in  FIG. 11( d ) . At this time, the arm flow rate allocation arithmetic section  132  considerably reduces the delivery flow rate of the second hydraulic pump  20   b  to correspond to the bottom-side fluid chamber pressure of the arm cylinder  7 , so that a reduction amount in the delivery flow rate of the second hydraulic pump  20   b  is small as shown in  FIG. 11( e )  and a total arm meter-in flow rate is maintained at the arm target flow rate. 
     When the bottom-side fluid chamber pressure of the arm cylinder  7  further increases thereafter to reach, at time T 3 , a pressure value at which the pressure starts decreasing from a certain value due to the limiting characteristic of the arm  3  crowding pilot pressure in the communication control section  120 , the center bypass opening of the arm third directional control valve  14   c  starts opening as shown in  FIG. 11( c )  and the delivery pressure of the third hydraulic pump  20   c  starts decreasing as shown in  FIG. 11( b ) . It is noted that, preferably, the delivery flow rate of the third hydraulic pump  20   c  after time T 3  shown in  FIG. 11( d )  is a standby flow rate. Operating the third hydraulic pump  20   c  with the standby flow rate improves an energy saving effect. 
     The standby flow rate, as used in the present embodiment, refers to a minimum delivery flow rate of the hydraulic fluid that needs to be delivered in order to protect the hydraulic pump to be operated. 
     In general, the leakage flow rate of the hydraulic pump increases substantially in proportion to the delivery pressure and the leakage flow rate has a greater effect on the loss of the hydraulic pump at higher delivery pressure values. Thus, under high load conditions, driving the arm cylinder  7  using only the second hydraulic pump  20   b  as in the hydraulic control system according to the present embodiment can minimize a total pump loss to thereby achieve energy saving, rather than driving the arm cylinder  7  using both the third hydraulic pump  20   c  and the second hydraulic pump  20   b.    
     Additionally, the delivery flow rate of the third hydraulic pump  20   c  is reduced before the center bypass opening of the arm third directional control valve  14   c  starts opening. This reduces the bleed-off loss generated in the arm third directional control valve  14   c.    
     Additionally, because of a small change in the meter-in flow rate to the arm cylinder  7  at the start of opening of the center bypass opening of the arm third directional control valve  14   c , shock at this time can be reduced. 
     In the hydraulic control system for a work machine according to the first embodiment of the present invention described above, the delivery flow rate of the first hydraulic pump (third hydraulic pump  20   c ) decreases with an increasing load on the first hydraulic actuator (arm cylinder  7 ) and the first control valve (arm third directional control valve  14   c ) is driven to enlarge a communication area between the first hydraulic pump and the tank, so that the delivery pressure of the first hydraulic pump (third hydraulic pump  20   c ) can be reduced and the pump total leakage flow rate can be reduced. A void flow rate delivered from the first hydraulic pump (third hydraulic pump  20   c ) can thus be reduced. An energy-saving hydraulic control system for a work machine can thus be provided. 
     Additionally, in the hydraulic control system for a work machine according to the first embodiment of the present invention described above, the delivery flow rate of the first hydraulic pump (third hydraulic pump  20   c ) is reduced before the communication area between the first hydraulic pump (third hydraulic pump  20   c ) and the tank is enlarged corresponding to the load on the first hydraulic actuator (arm cylinder  7 ). This reduces the bleed-off loss generated in first control valve (arm third directional control valve  14   c ). Additionally, because of a small change in the meter-in flow rate to the first hydraulic actuator (arm cylinder  7 ) when the first control valve (arm third directional control valve  14   c ) is opened or closed, shock at this time can be reduced. 
     Second Embodiment 
     A hydraulic control system for a work machine according to a second embodiment of the present invention is described below with reference to a relevant drawing.  FIG. 12  is a hydraulic control circuit diagram of the hydraulic control system for a work machine according to the second embodiment. In  FIG. 12 , like or corresponding elements are identified by the same reference numerals as those used in  FIGS. 1 and 11 ( a ) to  11 ( e ) and descriptions for those elements will be omitted. 
     The hydraulic control system for a work machine according to the second embodiment of the present invention has a general system configuration substantially identical to a general system configuration of the hydraulic control system for a work machine according to the first embodiment. The hydraulic control system for a work machine according to the second embodiment differs from the hydraulic control system for a work machine according to the first embodiment in that the hydraulic control system in the second embodiment is configured to incorporate only a hydraulic circuit without the controller  100 . 
     Specifically, as shown in  FIG. 12 , a regulator  20   f  of a third hydraulic pump  20   c  is operated by a sub-regulator  20   g  that is driven by pilot hydraulic pressure. A pilot hydraulic fluid is supplied to the sub-regulator  20   g  via a first selecting section valve  23  from a pilot hydraulic fluid source  25 . To correspond to the supply of the hydraulic fluid to the sub-regulator  20   g , the regulator  20   f  controls the delivery flow rate of the third hydraulic pump  20   c  in a decreasing direction. 
     The first selecting section valve  23  is a three-port two-position selecting section valve having a spring disposed on one side and receives a hydraulic fluid of a bottom-side fluid chamber of an arm cylinder  7  introduced to an operating section thereof. The first selecting section valve  23  has an inlet port connected with a hydraulic line from the pilot hydraulic fluid source  25  and an outlet port connected with a hydraulic line to the sub-regulator  20   g . The first selecting section valve  23  has a drain port connected with a hydraulic line to a tank. 
     An arm  3  crowding pressure reducing valve  22   b  is provided in an arm crowding pilot line that is connected with an operating section of an arm third directional control valve  14   c . The arm  3  crowding pressure reducing valve  22   b  limits or interrupts the arm crowding pilot hydraulic fluid to be supplied. The arm  3  crowding pressure reducing valve  22   b  is driven by the pilot hydraulic pressure. The pilot hydraulic fluid is supplied to the arm  3  crowding pressure reducing valve  22   b  via a second selecting section valve  24  from the pilot hydraulic fluid source  25 . The arm  3  crowding pressure reducing valve  22   b  enlarges a communication area between the third hydraulic pump  20   c  and the tank so as to correspond to the supply of the hydraulic fluid to the arm  3  crowding pressure reducing valve  22   b.    
     The second selecting section valve  24  is a three-port two-position selecting section valve having a spring disposed on one side and receives the hydraulic fluid of the bottom-side fluid chamber of the arm cylinder  7  introduced to an operating section thereof. The second selecting section valve  24  has an inlet port connected with a hydraulic line from the pilot hydraulic fluid source  25  and an outlet port connected with a hydraulic line to an operating section of the arm  3  crowding pressure reducing valve  22   b . The second selecting section valve  24  has a drain port connected with a hydraulic line to the tank. 
     It is noted that, preferably, characteristics of the first selecting section valve  23  and the second selecting section valve  24  are adjusted such that the first selecting section valve  23  performs a changeover operation before the second selecting section valve  24  does so as to correspond to an increase in pressure of the hydraulic fluid of the bottom-side fluid chamber of the arm cylinder  7  introduced to the respective operating sections. 
     Additionally, in the present embodiment, a maximum value of control pilot pressures that drive directional control valves disposed in respective pump lines  21   a ,  21   b , and  21   c  may be detected and the regulators  20   d ,  20   e , and  20   f  may be driven on the basis of the detected value. 
     The hydraulic control system for a work machine according to the second embodiment of the present invention described above can achieve effects similar to those achieved by the hydraulic control system for a work machine according to the first embodiment. 
     It should be noted that the present invention is not limited to the above-described first and second embodiments and may include various modifications. The entire detailed configuration of the embodiments described above for ease of understanding of the present invention is not always necessary to embody the present invention. Part of the configuration of one embodiment may be replaced with the configuration of another embodiment, or the configuration of one embodiment may be added to the configuration of another embodiment. The configuration of each embodiment may additionally include another configuration, or part of the configuration may be deleted or replaced with another. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           1 : Lower track structure 
           2 : Upper swing structure 
           2 A: Engine 
           3 : Boom 
           4 : Arm 
           5 : Bucket 
           6 : Boom cylinder 
           7 : Arm cylinder (first hydraulic actuator) 
           8 : Bucket cylinder 
           9 : Control lever (operating section) 
           10 : Control valve 
           11 : Swing hydraulic motor 
           13   a : Boom first directional control valve 
           13   b : Boom second directional control valve 
           14   a : Arm second directional control valve 
           14   b : Arm first directional control valve 
           14   c : Arm third directional control valve (first control valve) 
           15   a : Bucket directional control valve 
           16   c : Swing directional control valve 
           20 : Hydraulic pump system 
           20   a : First hydraulic pump 
           20   b : Second hydraulic pump (second hydraulic pump) 
           20   c : Third hydraulic pump (first 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  3  crowding pressure reducing valve (first control valve) 
           22   b : Arm  3  crowding pressure reducing valve (first control valve) 
           23 : First selecting section valve 
           24 : Second selecting section valve 
           100 : Controller 
           101  to  108 : Pilot pressure sensor 
           110 : Target operation arithmetic section 
           120 : Communication control section (control valve drive section) 
           130 : Flow rate control section (flow rate control section) 
         A 7 : Boom cylinder bottom-side fluid chamber pressure sensor (load detection section)