Patent Publication Number: US-9845813-B2

Title: Driving device for work machine and work machine equipped therewith

Description:
TECHNICAL FIELD 
     The present invention relates to a driving device for a work machine such as a hydraulic excavator and a work machine equipped with the driving device. 
     BACKGROUND ART 
     In recent years, development has been underway of a hydraulic circuit (defined as a closed circuit) in a work machine such as a hydraulic excavator, the hydraulic circuit being connected so as to have fewer throttle elements for driving a hydraulic actuator and supply hydraulic fluid from a hydraulic drive source such as a hydraulic pump to the hydraulic actuator before returning the worked hydraulic fluid to the hydraulic drive source without feeding the fluid back to a tank so that the rate of fuel consumption may be lowered. 
     On many work machines, a single rod type cylinder is used as a hydraulic actuator. In the single rod type cylinder, the pressure-receiving area of the internal piston on the head side is different from that on the rod side. It follows that with the cylinder connected to a closed circuit, driving the piston causes excess or shortage of the flow rate of hydraulic fluid within the circuit. There exists a closed hydraulic circuit furnished with a flushing valve to control such excess or shortage of the flow rate of hydraulic fluid (e.g., see Patent Literature 1). 
     There is also provided a driving device for a work machine, the driving device being capable of supplying optimum power in accordance with a load and including: a closed circuit that controls the operating speed of a hydraulic pressure actuator connected to a hydraulic pressure pump through variable displacement control of the hydraulic pressure pump of which the flow rate is controlled by variable displacement device; an open circuit that controls the operating speed of the hydraulic pressure actuator connected to a control valve through variable displacement control of the hydraulic pressure pump of which the flow rate is controlled by variable displacement device different from the above variable displacement device that controls the flow rate of the hydraulic pressure pump in the closed circuit and through flow rate control effected by the control valve for controlling hydraulic fluid supplied from the hydraulic pressure pump and by a bypass valve furnished in parallel with the control valve; and a distribution circuit that distributes the hydraulic fluid from the hydraulic pressure pump in the open circuit to the hydraulic pressure actuator in the closed circuit (e.g., see Patent Literature 2). 
     PRIOR ART LITERATURE 
     Patent Literature 
     Patent Literature 1 
     
         
         JP-58-57559-A
 
Patent Literature 2
 
         JP-2005-76781-A 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     With the closed hydraulic circuit described in the above-cited Patent Literature 1, excess hydraulic fluid is discharged into the tank by use of the flushing valve that operates on a pilot pressure formed by the head-side circuit pressure on the piston in the cylinder and by the rod-side circuit pressure on the piston. This permits control of the flow rate of the hydraulic fluid flowing through flow lines and provides a stable operating speed of the piston rod. 
     However, on the work machine, the load exerted on the cylinder (intra-circuit pressure) varies frequently depending on external force and empty weight. Concomitantly, the flow rate of excess hydraulic fluid discharged into the tank varies with the intra-circuit pressure. In this manner, when the load on the cylinder varies, it is difficult to keep constant the flow rate of the hydraulic fluid flowing into the cylinder. This makes it difficult to maintain the piston rod operating speed as desired by the operator, which reduces the operability of the work machine. 
     The driving device for the work machine described in the above-cited Patent Literature 2 includes an open circuit, a distribution circuit, and a closed circuit furnished with the flushing valve disclosed in Patent Literature 1. The excess hydraulic fluid generated when the piston rod is driven in the contraction direction is discharged into the tank via the flushing valve; the insufficient hydraulic fluid incurred when the piston rod is driven in the extension direction is replenished from the open circuit connected to the head side of the piston in the cylinder. The flow rate of the hydraulic fluid flowing through the flow lines is controlled in this manner, which provides a stable operating speed of the piston rod. 
     However, when the flow rate of the hydraulic fluid passing through a hydraulic pump inside the closed circuit is the same in both the extension and the contraction directions of the piston rod, the operating speed of the piston rod in the contraction direction becomes lower than that in the extension direction. One problem resulting from this is that the operability of the work machine is reduced. 
     The present invention has been made in view of the above circumstances, and an object of this invention is to provide a driving device for use with a work machine having a closed hydraulic circuit system for driving cylinders with hydraulic pumps, and permitting substantially the same operating speed of the piston rod in both the extension and the contraction directions regardless of the load exerted on the cylinder, and a work machine furnished with that driving device. 
     Means for Solving the Problem 
     In order to solve the above problem, the present invention adopts the structures described in the appended claims for example. This application includes a number of means for solving the above problem, exemplarily including: a first hydraulic pump that has flow rate control device for controlling the flow rate and direction of hydraulic fluid to be delivered; a single rod hydraulic cylinder that is driven with the hydraulic fluid to drive one of work members of a work device on the work machine; a closed hydraulic circuit that connects the first hydraulic pump with the single rod hydraulic cylinder to form a closed circuit using flow lines through which the hydraulic fluid flows; a branch line that branches from the flow line between the first hydraulic pump and the single rod hydraulic cylinder; a first flow line of which one end is connected to the branch line; a tank to which the other end of the first flow line is connected; and a hydraulic fluid flow rate control device that is attached to the first flow line to control the flow rate of the hydraulic fluid flowing from the branch line to the tank or from the tank to the branch line. 
     Effect of the Invention 
     The present invention has control device attached to a flow line branched from a closed hydraulic circuit and connected to a tank, the control device controlling the flow rate and direction of hydraulic fluid flowing through the flow line. This allows the operating speed of the piston rod in a cylinder actuated by the closed hydraulic circuit to be substantially the same in both the extension and the contraction directions of the piston rod regardless of the load exerted on the work machine. As a result, excellent operability of the work machine is ensured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a hydraulic excavator furnished with a first embodiment of the present invention made up of a driving device for a work machine and a work machine equipped therewith. 
         FIG. 2  is a hydraulic circuit diagram of the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith. 
         FIG. 3  is a tabular view listing typical operations of solenoid selector valves and hydraulic pumps in different operation modes of the first embodiment and a second embodiment of the present invention each made up of the driving device for a work machine and the work machine equipped therewith. 
         FIG. 4  is a set of characteristic diagrams showing typical relations among the state of a selector valve, the flow rate of a first hydraulic pump, the flow rate of a second hydraulic pump, and the speed of a boom in the first and the second embodiments of the present invention each made up of the driving device for a work machine and the work machine equipped therewith. 
         FIG. 5  is a hydraulic circuit diagram of the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Some embodiments of the present invention each made up of the driving device for a work machine and the work machine equipped therewith are explained below with reference to the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a side view of a hydraulic excavator furnished with the first embodiment of the present invention made up of a driving device for a work machine and a work machine equipped therewith. In  FIG. 1 , a hydraulic excavator  100  includes a track structure  101 , a swing structure  102  mounted swingably on the track structure  101  with a swing device  104  interposed therebetween, a cabin  103  mounted on the swing structure  102 , and an articulated front implement  105  attached to the upper front of the cabin  103  and the swing structure  102  in a vertically rotatable manner. 
     The swing structure  102  is furnished with a driving device including a closed hydraulic circuit and open hydraulic circuits, to be discussed later in detail. 
     The front implement  105  has a boom  2  with its base end attached pivotably to the swing structure  102 , an arm  4  attached pivotably to the tip end of the boom  2 , and a bucket  6  attached pivotably to the top end of the arm  4 . The boom  2 , the arm  4 , and the bucket  6  are actuated by a boom cylinder  1 , an arm cylinder  3 , and a bucket cylinder  5  respectively. 
     The structure of the driving device of this embodiment is explained next with reference to  FIG. 2 .  FIG. 2  is a hydraulic circuit diagram of the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith. For this embodiment, the diagram shows only the driving units of the actuators for driving the boom  2 , the arm  4 , and the bucket  6  making up the hydraulic excavator  100 ; the other driving units of the traveling actuators for the track structure  101  are omitted. In  FIG. 2 , the same reference numerals as those in  FIG. 2  designate the same components, and their detailed explanations are omitted. 
     This embodiment is furnished with a closed hydraulic circuit A that couples the boom cylinder  1  for driving the boom  2  with a first hydraulic pump  9 , an open hydraulic circuit B that couples the arm cylinder  3  for driving the arm  4  with a second hydraulic pump  10 , and an open hydraulic circuit C that couples the bucket cylinder  5  for driving the bucket  6  with a third hydraulic pump  11 . The second and the third hydraulic pumps  10  and  11  making up the open hydraulic circuits B and C are equipped with two-way tilting swash plate mechanisms  10   a  and  11   a  for changing the direction of delivery. The open hydraulic circuits B and C are furnished with solenoid selector valves  25  through  27  and  37  through  39  for changing the delivery direction of hydraulic fluid to any one of the closed hydraulic circuit A and open hydraulic circuits B and C. A controller  57  receives the operation amounts of control levers  56   a  through  56   c  for operating the boom  2 , the arm  4 , and the bucket  6  so as to control the delivery flow rates of the hydraulic pumps  9  through  11 , the opening and closing of the solenoid selector valves  25  through  27  and  37  through  39 , and the operations of proportional selector valves  30  and  42 . 
     As a result, the excess and shortage of hydraulic fluid incurred when the piston rod of the boom cylinder  1  in the closed hydraulic circuit A is extended and contracted can be compensated by the hydraulic pumps  10  and  11  of the open hydraulic circuits B and C constituting a hydraulic fluid flow rate control device. Consequently, it is possible to prevent fluctuations in the piston rod operating speed and equalize the operating speed when the piston rod of the boom cylinder  1  is extended and contracted, thereby improving the operability of the work machine. The operations implementing this functionality will be discussed later in detail. 
     In  FIG. 2 , a power transmission device  8  for distributing the power of an engine  7  is connected to the engine  7  serving as the power source. The power transmission device  8  is furnished with the first hydraulic pump  9  for driving the boom cylinder  1 , the second hydraulic pump  10  for driving the arm cylinder  3 , the third hydraulic pump  11  for driving the bucket cylinder  5 , and a charge pump  12  for replenishing hydraulic fluid to a lower-pressure-side line in the closed hydraulic circuit A, to be discussed later, each pump being attached to the power transmission device  8  with a drive shaft interposed therebetween. 
     The first hydraulic pump  9 , the second hydraulic pump  10 , and the third hydraulic pump  11  are furnished respectively with the two-way tilting swash plate mechanisms each having a pair of inlet and outlet ports, and regulators  9   a ,  10   a  and  11   a  each regulating the tilting angle of the two-way tilting swash plate. The regulators  9   a ,  10   a  and  11   a  are controlled by command signals from the controller  57 . In this manner, the flow rates of suction and delivery and their directions regarding the first through the third hydraulic pumps  9  through  11  are controlled. Also, the first through the third hydraulic pumps  9  through  11  function as hydraulic motors when supplied with hydraulic fluid. 
     The closed hydraulic circuit A is now explained. The boom cylinder  1  making up part of the closed hydraulic circuit A is equipped with a cylinder body, a piston installed movably in the cylinder body, and a piston rod attached to one side of the piston. As such, the boom cylinder  1  constitutes a single rod type hydraulic cylinder furnished with a rod-side oil chamber  1   b  and a head-side oil chamber  1   a.    
     A boom control lever  56   a  is installed in the cabin  103 . An operation amount signal from the boom control lever  56   a  is input to the controller  57 . In turn, the controller  57  controls the hydraulic pumps  9 ,  10  and  11  and the selector valves  25  through  27  and  37  through  39  in a manner attaining the piston rod operating speed corresponding to the operation amount signal. 
     The first hydraulic pump  9  has two hydraulic fluid delivery/suction ports  9   x  and  9   y . One hydraulic fluid delivery/suction port  9   x  is coupled with one end of a first line  13 . The other end of the first line  13  is coupled to the connection port of the head-side oil chamber  1   a  of the boom cylinder  1 . The other hydraulic fluid delivery/suction port  9   y  is coupled with one end of a second line  14 . The other end of the second line  14  is coupled to the connection port of the rod-side oil chamber  1   b  of the boom cylinder  1 . 
     The first line  13  is coupled with the outlet side of a check valve  17   a  permitting suction only, the inlet side of a relief valve  19   a , one inlet port of a flushing valve  20 , and one outlet side of a charge check valve  21  permitting suction only. The inlet side of the check valve  17   a  and the outlet side of the relief valve  19   a  are coupled to the outlet port of the flushing valve  20  and are communicated with a tank  18  via a line  16 . Also, the first line  13  is coupled with one end of a communicating line  15  that permits connection with the second hydraulic pump  10  and the third hydraulic pump  11  via solenoid selector valves, to be discussed later. 
     The second line  14  is coupled with the outlet side of a check valve  17   b  permitting suction only, the inlet side of a relief valve  19   b , the other inlet port of the flushing valve  20 , and the other outlet side of the charge check valve  21  permitting suction only. The inlet side of the check valve  17   b  and the outlet side of the relief valve  19   b  are coupled to the outlet port of the flushing valve  20  and are communicated with the tank  18  via the line  16 . 
     The inlet side of the charge check valve  21  is coupled to the delivery line of the charge pump  12 . The hydraulic fluid delivered by the charge pump  12  is supplied by the charge check valve  21  to the first line  13  or the second line  14 , whichever has the lower pressure. Also, a charge relief valve  22  for limiting the delivery pressure of the charge pump  12  is attached to the delivery line of the charge pump  12 , with the outlet side of the charge relief valve  22  communicated with the tank  18 . Further, the suction port of the charge pump  12  is communicated with the tank  18  via a suction line. 
     The check valves  17   a  and  17   b  attached to the first and the second lines  13  and  14  are designed to supply hydraulic fluid from the tank  18  via the line  16  when the pressure in any one of the lines becomes negative or when the flow rate of hydraulic fluid in the rod-side oil chamber  1   b  or head-side oil chamber  1   a  becomes insufficient upon actuation of the boom cylinder  1 . This prevents the occurrence of cavitation. 
     The relief valves  19   a  and  19   b  attached to the first and the second lines  13  and  14  are designed to discharge hydraulic fluid into the tank  18  via the line  16  when the pressure in any one of the lines has exceeded a predetermined pressure level. This prevents the breakage of pumps or lines. 
     The flushing valve  20  is switched when the difference in pressure between the first line  13  and the second line  14  has exceeded a predetermined pressure level. Thus switched, the flushing valve  20  connects the line having the lower pressure with the line  16 , thereby discharging the excess hydraulic fluid of the lower-pressure-side line into the tank  18 . 
     The open hydraulic circuit B is explained next. As with the boom cylinder  1 , the arm cylinder  3  is a single rod type hydraulic pressure cylinder equipped with a rod-side oil chamber  3   b  and a head-side oil chamber  3   a.    
     An arm control lever  56   b  is installed in the cabin  103 . An operation amount signal from the arm control lever  56   b  is input to the controller  57 . In turn, the controller  57  controls the hydraulic pumps  9 ,  10  and  11 , the solenoid selector valves  25 ,  26  and  27 , and an arm cylinder proportional selector valve  30  in a manner attaining the piston rod operating speed corresponding to the operation amount signal. 
     The second hydraulic pump  10  acting as a hydraulic fluid flow rate control device has two suction/delivery ports  10   x  and  10   y . One suction/delivery port  10   y  is coupled with one end of a line  23 . The other end of the line  23  is coupled to the tank  18 . The other suction/delivery port  10   x  is coupled with one end of a line  24 . The other end of the line  24  branches in three ways, the branches being coupled with the inlet ports of the first through the third solenoid selector valves  25  through  27  respectively. Also, a relief valve  28  for limiting the delivery pressure of the second hydraulic pump  10  is attached to the line  24 , with the outlet side of the relief valve  28  communicated with the tank  18  via the line  23 . 
     The first through the third solenoid selector valves  25  through  27  are each a two-port two-position type solenoid selector valve of which one end is equipped with a solenoid operation part for receiving a command signal from the controller  57 , the other end of the valve being furnished with a spring part. The presence or absence of the command signal coming from the controller  57  triggers switching of the destination to which to supply the hydraulic fluid fed from the second hydraulic pump  10 . The outlet port of the first solenoid selector valve  25  is coupled via a line to the inlet side of the check valve  29  permitting delivery only. The outlet side of the check valve  29  is connected to the pump port of the arm cylinder proportional selector valve  30  for controlling the flow rate and direction of the hydraulic fluid supplied to the arm cylinder  3 . 
     Also, the outlet port of the second solenoid selector valve  26  is coupled via a check valve  41  to the pump port of a bucket cylinder proportional solenoid valve  42 , to be discussed later. Furthermore, the outlet port of the third solenoid selector valve  27  is coupled via the communicating line  15  to the first line  13  of the closed hydraulic circuit A. 
     The arm cylinder proportional selector valve  30  is a four-port three-position type solenoid proportional selector valve of which one end is equipped with a solenoid operation part for receiving a command signal from the controller  57 , the other end of the valve being furnished with a spring part. A tank port of the arm cylinder proportional selector valve  30  is coupled to the tank  18  via a line  35  communicated with the line  23 . One end of the outlet port of the arm cylinder proportional selector valve  30  is coupled with one end of the first line  31 . The other end of the first line  31  is coupled to the connection port of the head-side oil chamber  3   a  of the arm cylinder  3 . The other end of the outlet port of the arm cylinder proportional selector valve  30  is coupled with one end of the second line  32 . The other end of the second line  32  is coupled to the connection port of the rod-side oil chamber  3   b  of the arm cylinder  3 . 
     In accordance with the command signal from the controller  57 , the arm cylinder proportional selector valve  30  switches the flowing direction of the hydraulic fluid from the check valve  29  to either the first line  31  or to the second line  32  and controls the valve opening, thereby controlling the flow rate of the hydraulic fluid supplied to the arm cylinder  3 . 
     In the first line  31 , a counterbalance valve  33   a  is installed serially so that its inlet side is oriented toward the arm cylinder  3  and its outlet side toward the arm cylinder proportional selector valve  30 . The first line  31  is also coupled with the inlet side of a relief valve  34   a . The outlet side of the relief valve  34   a  is communicated with the tank  18  via a line  35  communicated with the line  23 . 
     In the second line  32 , a counterbalance valve  33   b  is installed serially so that its inlet side is oriented toward the arm cylinder  3  and its outlet side toward the arm cylinder proportional selector valve  30 . The second line  32  is also coupled with the inlet side of a relief valve  34   b . The outlet side of the relief valve  34   a  is communicated with the tank  18  via the line  35  communicated with the line  23 . 
     The counterbalance valves  33   a  and  33   b  installed in the first and the second lines  31  and  32  are designed to prevent the arm cylinder  3  from falling under its empty weight. Likewise, the relief valves  34   a  and  34   b  are designed to discharge the hydraulic fluid into the tank  18  via the line  35  when the pressure in any one of the lines has exceeded a predetermined pressure level, thereby preventing breakage of pumps or lines. 
     The open hydraulic circuit C is explained next. As with the boom cylinder  1 , the bucket cylinder  5  is a single rod type hydraulic cylinder equipped with a rod-side oil chamber  5   b  and a head-side oil chamber  5   a.    
     A bucket control lever  56   c  is installed in the cabin  103 . An operation amount signal from the bucket control lever  56   c  is input to the controller  57 . In turn, the controller  57  controls the hydraulic pumps  9 ,  10  and  11 , the solenoid selector valves  37 ,  38  and  39 , and the bucket cylinder proportional solenoid valve  42  in a manner attaining the piston rod operating speed corresponding to the operation amount signal. 
     The third hydraulic pump  11  acting as a hydraulic fluid flow rate control device has two suction/delivery ports  11   x  and  11   y . One suction/delivery port  11   y  is coupled with one end of a line  47 . The other end of the line  47  is coupled to the tank  18 . The other suction/delivery port  11   x  is coupled with one end of a line  36 . The other end of the line  36  branches in three ways, the branches being coupled with the inlet ports of the first through the third solenoid selector valves  37  through  39  respectively. Also, a relief valve  40  for limiting the delivery pressure of the third hydraulic pump  11  is attached to the line  36 , with the outlet side of the relief valve  40  communicated with the tank  18  via the line  47 . 
     The first through the third solenoid selector valves  37  through  39  are each a two-port two-position type solenoid selector valve of which one end is equipped with a solenoid operation part for receiving a command signal from the controller  57 , the other end of the valve being furnished with a spring part. The presence or absence of the command signal coming from the controller  57  triggers switching of the destination to which to supply the hydraulic fluid coming from the third hydraulic pump  11 . The outlet port of the first solenoid selector valve  37  is coupled via a line to the inlet side of the check valve  41  permitting delivery only. The outlet side of the check valve  41  is connected to the pump port of the bucket cylinder proportional selector valve  42  for controlling the flow rate and direction of the hydraulic fluid supplied to the bucket cylinder  5 . 
     Also, the outlet port of the second solenoid selector valve  38  is coupled via the check valve  29  to the pump port of the arm cylinder proportional solenoid valve  30  of the open hydraulic circuit B. Furthermore, the outlet port of the third solenoid selector valve  39  is coupled via the communicating line  15  to the first line  13  of the closed hydraulic circuit A. 
     The bucket cylinder proportional selector valve  42  is a four-port three-position type solenoid proportional selector valve of which one end is equipped with a solenoid operation part for receiving a command signal from the controller  57 , the other end of the valve being furnished with a spring part. The tank port of the bucket cylinder proportional selector valve  42  is coupled to the tank  18  via a line  48  communicated with the line  47 . One end of the outlet port of the bucket cylinder proportional selector valve  42  is coupled with one end of the first line  43 . The other end of the first line  43  is coupled to the connection port of the head-side oil chamber  5   a  of the bucket cylinder  5 . The other end of the outlet port of the bucket cylinder proportional selector valve  42  is coupled with one end of the second line  44 . The other end of the second line  44  is coupled to the connection port of the rod-side oil chamber  5   b  of the bucket cylinder  5 . 
     In accordance with the command signal from the controller  57 , the bucket cylinder proportional selector valve  42  switches the flowing direction of the hydraulic fluid from the check valve  41  to either the first line  43  or to the second line  44  and controls the valve opening, thereby controlling the flow rate of the hydraulic fluid supplied to the bucket cylinder  5 . 
     In the first line  43 , a counterbalance valve  45   a  is installed serially so that its inlet side is oriented toward the bucket cylinder  5  and its outlet side toward the bucket cylinder proportional selector valve  42 . The first line  43  is also coupled with the inlet side of a relief valve  46   a . The outlet side of the relief valve  46   a  is communicated with the tank  18  via the line  48  communicated with the line  47 . 
     In the second line  44 , a counterbalance valve  45   b  is installed serially so that its inlet side is oriented toward the bucket cylinder  5  and its outlet side toward the bucket cylinder proportional selector valve  42 . The second line  44  is also coupled with the inlet side of a relieve valve  46   b . The outlet side of the relief valve  46   a  is communicated with the tank  18  via the line  48  communicated with the line  47 . 
     The counterbalance valves  45   a  and  45   b  installed in the first and the second lines  43  and  44  are designed to prevent the bucket cylinder  5  from falling under its empty weight. Likewise, the relief valves  46   a  and  46   b  are designed to discharge the hydraulic fluid into the tank  18  via the line  48  when the pressure in any one of the lines has exceeded a predetermined pressure level, thereby preventing breakage of pumps or lines. 
     Explained next with reference to  FIGS. 3 and 4  are the operations of the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith.  FIG. 3  is a tabular view listing typical operations of solenoid selector valves and hydraulic pumps in different operation modes of the first and the second embodiment of the present invention each made up of the driving device for a work machine and the work machine equipped therewith.  FIG. 4  is a set of characteristic diagrams showing typical relations among the state of a selector valve, the flow rate of a first hydraulic pump, the flow rate of a second hydraulic pump, and the speed of a boom in the first and the second embodiments of the present invention each made up of the driving device for a work machine and the work machine equipped therewith. In  FIGS. 3 and 4 , the same reference symbols as those in  FIGS. 1 and 2  designate the same components, and their detailed explanations are omitted. 
       FIG. 3  lists typical operations of the solenoid valves, proportional selector valves, and hydraulic pumps in different operation modes under control of the controller  57  in this embodiment. First, the non-operating state (stopped state) indicated in  FIG. 3  refers to a state in which none of the boom control lever  56   a , the arm control lever  56   b , and the bucket control lever  56   c  is operated and in which none of the signals from these control levers is input to the controller  57 . In this case, the controller  57  outputs a minimum tilting angle control command signal to the regulators  9   a ,  10   a  and  11   a  of the first through the third hydraulic pumps  9 ,  10  and  11  shown in  FIG. 2 . At the same time, the controller  57  outputs a cut-off close command signal to the first through the third solenoid selector valves  25  through  27  of the open hydraulic circuit B and to the first through the third solenoid selector valves  37  through  39  of the open hydraulic circuit C. Also, the controller  57  outputs a cut-off command signal to the arm cylinder proportional selector valve  30  and bucket cylinder proportional selector valve  42 . As a result, the boom cylinder  1 , the arm cylinder  3 , and the bucket cylinder  5  are held in the non-operating state. Also in  FIG. 3 , a pump “OFF” refers to a minimum tilting angle state, and a pump “ON” refers to a state larger than the minimum tilting angle state. 
     The individual operation of the boom  2  is explained next. In  FIG. 4 , the horizontal axis denotes time. On the vertical axis from the top down, reference character (a) stands for the operation amount Lb of the boom lever, (b) for the state Cs of the selector valve  27 , (c) for the flow rate Qcp of the first hydraulic pump, (d) for the flow rate Qop of the second hydraulic pump, and (e) for the piston rod speed Vb of the boom cylinder  1 . The period from time t1 to time t3 indicates the characteristics in effect when the piston rod of the boom cylinder  1  is extended (to raise the boom); the period from time t4 to time t6 depicts the characteristics in effect when the piston rod of the boom cylinder  1  is contracted (to lower the boom). 
     The raising operation of the boom  2  is explained first. Returning to  FIG. 2 , when the operator starts operating the boom control lever  56   a  in the direction of piston rod extension, the controller  57  outputs a command signal to the regulator  9   a  of the first hydraulic pump  9  causing the tilting angle of the swash plate to be raised. Here, if the operation amount of the boom control lever  56   a  is as small as X1 as indicated at time t1 in  FIG. 4 , the delivery flow rate of the first hydraulic pump  9  reaches Qcp1 so that the piston rod of the boom cylinder  1  is extended at speed V1 (low speed). 
     At this point, in  FIG. 2 , the hydraulic fluid from the first hydraulic pump  9  is supplied to the head-side oil chamber  1   a  of the boom cylinder  1  via one hydraulic fluid delivery/suction port  9   x  of the first hydraulic pump  9  and the first line  13 . On the other hand, the hydraulic fluid in the rod-side oil chamber  1   b  of the boom cylinder  1  is returned to the other hydraulic fluid delivery/suction port  9   y  of the first hydraulic pump  9  via the second line  14 . At this point, the flow rate of the hydraulic fluid returning from the rod-side oil chamber  1   b  of the boom cylinder  1  to the first hydraulic pump  9  is lower than the flow rate of the hydraulic fluid supplied from the first hydraulic pump  9  to the head-side oil chamber  1   a  of the boom cylinder  1 . The insufficient flow rate of the hydraulic fluid is compensated by the charge pump  12  supplying the hydraulic fluid to the other hydraulic fluid delivery/suction port  9   y  of the first hydraulic pump  9  via the charge check valve  21  and the second line  14 . 
     When the operator increases the operation amount of the boom control lever  56   a  to further increase the speed at which to extend the piston rod of the boom cylinder  1 , the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  causing the tilting angle of the swash plate to be raised. At the same time, the controller  57  outputs a communication command signal to the third solenoid selector valve  27  of the open hydraulic circuit B. This causes the head-side oil chamber  1   a  of the boom cylinder  1  to be replenished with the hydraulic fluid coming from the second hydraulic pump  10  via third solenoid selector valve  27 . Here, if the operation amount of the boom control lever  56   a  has exceeded X1 to reach X2 as indicated at time t2 in  FIG. 4 , the third solenoid selector valve  27  is placed in the communicating state, and the delivery flow rates of the second and the first hydraulic pumps  10  and  9  reach Qop1 and Qcp2 respectively. As a result, the hydraulic fluid flows into the head-side oil chamber  1   a  of the boom cylinder  1  at a flow rate of Qop1+Qcp2 so that the piston rod is extended at speed V2 (high speed). 
     When the above-described lever manipulation is performed to increase the speed at which to extend the piston rod of the boom cylinder  1 , the controller  57  may output a command signal to the third hydraulic pump  11  and to the third solenoid selector valve  39  of the open hydraulic circuit C, instead of issuing the command signal to the second hydraulic pump  10  and to the third solenoid selector valve  27  of the open hydraulic circuit B, thereby attaining the high-speed operation. 
     The lowering operation of the boom  2  is explained next. Returning to  FIG. 2 , when the operator starts operating the boom control lever  56   a  in the direction of piston rod contraction, the controller  57  outputs a command signal to the regulator  9   a  of the first hydraulic pump  9  causing the tilting angle of the swash plate to be lowered. Here, if the operation amount of the boom control lever  56   a  is as small as −X1 as indicated at time t4 in  FIG. 4 , the delivery flow rate of the first hydraulic pump  9  reaches −Qcp1 causing the piston rod of the boom cylinder  1  to contract at speed −V1 (low speed). 
     At this point, in  FIG. 2 , the hydraulic fluid from the first hydraulic pump  9  is supplied to the rod-side oil chamber  1   b  of the boom cylinder  1  via the other hydraulic fluid delivery/suction port  9   y  of the first hydraulic pump  9  and the second line  14 . On the other hand, the hydraulic fluid in the head-side oil chamber  1   a  of the boom cylinder  1  is returned to one hydraulic fluid delivery/suction port  9   x  of the first hydraulic pump  9  via the first line  13 . At this point, the flow rate of the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the first hydraulic pump  9  is higher than the flow rate of the hydraulic fluid supplied from the first hydraulic pump  9  to the rod-side oil chamber  1   b  of the boom cylinder  1 . The excess hydraulic fluid is returned from the first line  13  to the tank  18  via the flushing valve  20  and the line  16 . 
     At this point, the pressure of the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the first hydraulic pump  9  is boosted under the empty weight of the front implement  105 . When supplied with the pressurized hydraulic fluid, the first hydraulic pump  9  is driven as a hydraulic motor. The power of the first hydraulic pump  9  generated by the pressurized hydraulic fluid is transmitted to and absorbed by the engine  7  and other hydraulic pumps via the power transmission device  8 . Although not shown, the power transmission device  8  may be coupled with a motor generator and an electrical storage device to store the power that has overflowed and cannot be absorbed so that the power can be recycled. 
     When the operator raises the operation amount of the boom control lever  56   a  to further increase the speed at which to contract the piston rod of the boom cylinder  1 , the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  causing the tilting angle of the swash plate to be lowered. At the same time, the controller  57  outputs a communication command signal to the third solenoid selector valve  27  of the open hydraulic circuit B. This causes the second hydraulic pump  10  to act in a manner sucking the hydraulic fluid from the other suction/delivery port  10   x . As a result, the discharge of the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1  into the tank  18  is promoted through the communicating line  15  and the third solenoid selector valve  27 . 
     If the operation amount of the boom control lever  56   a  has exceeded −X1 to reach −X2 as indicated at time t5 in  FIG. 4 , the third solenoid selector valve  27  is placed in the communicating state. At the same time, the delivery flow rates of the second and the first hydraulic pumps  10  and  9  become −Qop1 and −Qcp2 respectively. As a result, the hydraulic fluid flows from the head-side oil chamber  1   a  of the boom cylinder  1  at a flow rate of −(Qop1+Qcp2), so that the piston rod is contracted at speed −V2 (high speed). At this point, the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the second hydraulic pump  10  is highly pressurized. When supplied with the pressurized hydraulic fluid, the second hydraulic pump  10  is driven as a hydraulic motor. The power of the second hydraulic pump  10  generated by the pressurized hydraulic fluid is transmitted to and absorbed by the engine  7  and other hydraulic pumps via the power transmission device  8 . 
     When the above-described lever manipulation is performed to increase the speed at which to contract the piston rod of the boom cylinder  1 , the controller  57  may output an operation command signal to the third hydraulic pump  11  and to the third solenoid selector valve  39  of the open hydraulic circuit C, instead of issuing the operation command signal to the second hydraulic pump  10  and to the third solenoid selector valve  27  of the open hydraulic circuit B, thereby attaining the high-speed operation. 
     In this embodiment, when the lever manipulation is performed to increase the speed at which to contract the piston rod of the boom cylinder  1 , the second hydraulic pump  10  and the first hydraulic pump  9  are used together to admit the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1 . In this manner, the operating speed of the piston rod of the boom cylinder  1  is boosted. 
     The individual operation of the arm  4  is explained next. In  FIG. 2 , when the operator starts operating the arm control lever  56   b  in the direction of piston rod extension, the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  causing the tilting angle of the swash plate to be raised. At the same time, the controller  57  outputs a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a forward opening command signal to the arm cylinder proportional selector valve  30 . This causes the tilting angle of the swash plate to be raised in the second hydraulic pump  10  and opens the arm cylinder proportional selector valve  30  in the direction coupling the check valve  29  with the first line  31 . 
     As a result, the hydraulic fluid from the second hydraulic pump  10  is supplied to the head-side oil chamber  3   a  of the arm cylinder  3  via the other suction/delivery port  10   x  of the pump  10 , the line  24 , and the first line  31 . Meanwhile, the hydraulic fluid in the rod-side oil chamber  3   b  of the arm cylinder  3  is returned to the tank  18  via the second line  32 , the arm cylinder proportional selector valve  30 , and the line  35 . Consequently, the piston rod of the arm cylinder  3  is extended. 
     An arm damping operation is explained next. When the operator starts operating the arm control lever  56   b  in the direction of piston rod contraction, the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  causing the tilting angle of the swash plate to be raised. At the same time, the controller  57  outputs a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a reverse opening command signal to the arm cylinder proportional selector valve  30 . This causes the tilting angle of the swash plate to be raised in the second hydraulic pump  10  and opens the arm cylinder proportional selector valve  30  in the direction coupling the check valve  29  with the second line  32 . 
     The hydraulic fluid from the second hydraulic pump  10  is supplied to the rod-side oil chamber  3   b  of the arm cylinder  3  via the other suction/delivery port  10   x  of the pump  10 , the line  24 , and the second line  32 . Meanwhile, the hydraulic fluid in the head-side oil chamber  3   a  of the arm cylinder  3  is returned to the tank  18  via the first line  31 , the arm cylinder proportional selector valve  30 , and the line  35 . Consequently, the piston rod of the arm cylinder  3  is contracted. 
     The individual operation of the bucket  6  is performed in the same manner as that of the arm  4  and thus will not be discussed further. 
     A combined operation of the actuators is explained next with reference to  FIGS. 2 and 3 . As shown in  FIG. 3 , it is assumed that the boom  2 , the arm  4 , and the bucket  6  are operated in a combined manner. In that case, if the boom  2  is to be operated at low speed, the boom cylinder  1 , arm cylinder  3 , and the bucket cylinder  5  are supplied with the hydraulic fluid respectively from the first hydraulic pump  9 , the second hydraulic pump  10 , and the third hydraulic pump  11  driving the respective piston rods. Specifically, the controller  57  outputs a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B, an opening command signal to the arm cylinder proportional selector valve  30 , a communication command signal to the first solenoid selector valve  37  of the open hydraulic circuit C, and an opening command signal to the bucket cylinder proportional selector valve  42 . 
     On the other hand, if the boom  2  is to be operated at high speed, e.g., if the piston rod of the boom cylinder  1  is to be extended at a speed exceeding a predetermined threshold value, the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  causing the tilting angle of the swash plate to reflect the operation amount of the boom control lever  56   a . At the same time, the controller  57  outputs a cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a communication command signal to the third solenoid selector valve  27 . 
     As a result, the head-side oil chamber  1   a  of the boom cylinder  1  is replenished with the hydraulic fluid from the second hydraulic pump  10 , so that the piston rod of the boom cylinder  1  is extended at a speed corresponding to the operation amount of the boom control lever  56   a.    
     Meanwhile, the controller  57  outputs a command signal to the regulator  11   a  of the third hydraulic pump  11  causing the tilting angle of the swash plate to reflect the operation amount of the arm control lever  56   b , and also outputs a communication command signal to the second solenoid selector valve  38  of the open hydraulic circuit C. This causes the arm cylinder  3  to be supplied with the hydraulic fluid from the third hydraulic pump  11  via the arm cylinder proportional selector valve  30 , whereby the piston rod of the arm cylinder  3  is drive-controlled. 
     When the above operation is carried out, the controller  57  may control the swash plate of the third hydraulic pump  11  instead of the second hydraulic pump  10  and may output a cut-off command signal to the first solenoid selector valve  37  of the open hydraulic circuit C and a communication command signal to the third solenoid selector valve  39  instead of the cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and the communication command signal to the third solenoid selector valve  27 , thereby replenishing the head-side oil chamber  1   a  of the boom cylinder  1  with the hydraulic fluid from the third hydraulic pump  11 . 
     Where the boom  2 , the arm  4 , and the bucket  6  are operated in combined fashion and where the piston rod of the boom cylinder  1  is contracted at low speed, the first hydraulic motor  9  is driven as a hydraulic motor as described above. For this reason, the power of the first hydraulic pump  9  generated by the pressurized hydraulic fluid is transmitted to and absorbed by the engine  7  and other hydraulic pumps via the power transmission device  8 . 
     Meanwhile, if the piston rod of the boom cylinder  1  is to be contracted at a speed exceeding a predetermined threshold value, the controller  57  outputs a command signal to the regulator  10   a  of the second hydraulic pump  10  reflecting the operation amount of the boom control lever  56   a  in the opposite direction of the above-mentioned high-speed extension. At the same time, the controller  57  outputs a cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a communication command signal to the third solenoid selector valve  27 . 
     As a result, the second hydraulic pump  10  acts to suck the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1 , so that the piston rod of the boom cylinder  1  is controlled to be contracted at a speed corresponding to the operation amount of the boom control lever  56   a . At this point, the hydraulic fluid returning to the second hydraulic pump  10  is highly pressurized. When supplied with the pressurized hydraulic fluid, the second hydraulic pump  10  is driven as a hydraulic motor. The power of the second hydraulic pump  10  generated by the pressurized hydraulic fluid is transmitted to and absorbed by the engine  7  and other hydraulic pumps via the power transmission device  8 . 
     Meanwhile, the controller  57  outputs a command signal to the regulator  11   a  of the third hydraulic pump  11  causing the tilting angle of the swash plate to reflect the operation amount of the arm control lever  56   b , and also outputs a communication command signal to the second solenoid selector valve  38  of the open hydraulic circuit C. This causes the arm cylinder  3  to be supplied with the hydraulic fluid from the third hydraulic pump  11  via the arm cylinder proportional selector valve  30 , whereby the piston rod of the arm cylinder  3  is drive-controlled. 
     When the above operation is carried out, the controller  57  may control the swash plate of the third hydraulic pump  11  instead of the second hydraulic pump  10  and may output a cut-off command signal to the first solenoid selector valve  37  of the open hydraulic circuit C and a communication command signal to the third solenoid selector valve  39  instead of the cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and the communication command signal to the third solenoid selector valve  27 , thereby supplying the third hydraulic pump  11  with the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1 . 
     According to the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, the second hydraulic pump  10  and the third hydraulic pump  11  are attached to the communicating line  15  branched from the closed hydraulic circuit and connected to the tank  18 , the pumps  10  and  11  serving as the device for controlling the flow rate and direction of the hydraulic fluid (i.e., operating oil) flowing through the communicating line  15 . With this structure, the operating speed of the piston rod of the boom cylinder  1  actuated by the closed hydraulic circuit is made substantially the same in both the extension and the contracting directions regardless of the load exerted on the work machine. As a result, excellent operability of the work machine is ensured. 
     Also according to the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, a two-way tilting swash plate mechanism pump is used as the second hydraulic pump  10  capable of controlling the direction of delivery. Thus the second hydraulic pump  10  makes the flow rate of the hydraulic fluid replenishing the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is extended at high speed, substantially the same as the flow rate of the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is contracted at high speed. As a result, the operating speed of the piston rod of the boom cylinder  1  is made substantially the same in both the extension and the contracting directions, so that excellent operability of the work machine is provided. 
     Further, according to the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, when the piston rod of the boom cylinder  1  is operated at low speed, the charge pump  12  and the flushing valve  20  combine to compensate the excess or shortage of the hydraulic fluid in the flow rate balance caused by the difference in volume between the head-side oil chamber  1   a  and the rod-side oil chamber  1   b  of the boom cylinder  1 ; when the piston rod of the boom cylinder  1  is operated at high speed, the second hydraulic pump  10  compensates the above-mentioned excess or shortage of the hydraulic fluid in the flow rate balance of the boom cylinder  1 . In this manner, in keeping with the operating speed of the piston rod of the boom cylinder  1 , the use or nonuse of the second hydraulic pump  10  is selected in the closed hydraulic circuit A, which makes it possible to downsize the charge bump  12 . Also, when there occur fluctuations of the pressure inside the lines during high-speed operation, the second hydraulic pump  10  provides flow rate control, thereby ensuring a stable operation state. 
     Also according to the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is contracted at high speed is guided to the first hydraulic pump  9  and the second hydraulic pump  10 . This allows the displacement of the first hydraulic pump  9  to be smaller than that of its counterpart in the past. 
     Furthermore, according to the first embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, the second hydraulic pump  10  and the third hydraulic pump  11  are provided as the hydraulic pumps of the open hydraulic circuits. With this structure, if the second hydraulic pump  10  is used to drive the piston rod of the boom cylinder  1  for example, the third hydraulic pump  11  may be used to drive the piston rod of the arm cylinder  3  as well as the piston rod of the bucket cylinder  5 . 
     Second Embodiment 
     Explained below with reference to the relevant accompanying drawings is the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith.  FIG. 5  is a hydraulic circuit diagram of the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith. In  FIG. 5 , the same reference numerals as those used in  FIGS. 1 through 4  designate the same components, and their detailed explanations are omitted. 
     The second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith as shown in  FIG. 5  is configured with approximately the same components as those of the first embodiment except for the following structures: In the first embodiment, the first through the third hydraulic pumps  9  through  11  and the charge pump  12  are driven by the power transmission device  8  distributing the power of the engine  7  by way of the drive shafts of these pumps. In the second embodiment, by contrast, a first through a third hydraulic pumps  60  through  62  and a charge pump  61  are driven by a first through a third motor generators  50  through  52  and a charge motor generator  53  that are coupled with these pumps by way of their drive shafts. And in the first embodiment, the first through the third hydraulic pumps  9  through  11  are each a two-way tilting swash plate mechanism hydraulic pump having a pair of inlet and outlet ports. In the second embodiment, by contrast, the first through the third hydraulic pumps  60  through  62  are each a hydraulic pump capable of forward and reverse rotations. 
     In  FIG. 5 , a power unit  54  acting as a power supply is connected electrically to the first motor generator  50  that drives the first hydraulic pump  60  for supplying the hydraulic fluid to the boom cylinder  1 , the second motor generator  51  that drives the second hydraulic pump  61  for supplying the hydraulic fluid to the arm cylinder  3 , the third motor generator  52  that drives the third hydraulic pump  62  for supplying the hydraulic fluid to the bucket cylinder  5 , and the charge motor generator  53  that drives a charge pump  63  for supplying the hydraulic fluid to the lower-pressure line of the closed hydraulic circuit A, the power unit  54  being connected thereto via power control units  50   a  through  53   a  for controlling these motor generators  50  through  53  and via electric wiring. Electric power is exchanged between the power unit  54  on the one hand and the power control units  50   a  through  53   a  on the other hand. The power unit  54  may store the electric power coming from the power control units  50   a  through  53   a.    
     The revolution speeds of the first through the third motor generators  50  through  52  and the charge motor generator  53  are controlled with the outputs from the power control units  50   a  through  53   a  responding to command signals from the controller  57 . In this manner, the flow rate and the direction of suction and delivery of the hydraulic fluid by each of the first through the third hydraulic pumps  60  through  62  are controlled. When supplied with the hydraulic fluid, the first through the third hydraulic pumps  60  through  62  also function as a hydraulic motor each. 
     The lines coupled with the first hydraulic pump  60 , second hydraulic pump  61 , the third hydraulic pump  62 , and the charge pump  63 , and the like components are the same as those used in the first embodiment and thus will not be discussed further. 
     Explained below with reference to  FIGS. 3 through 5  is the operation of the second embodiment of this embodiment made up of the driving device for a work machine and the work machine equipped therewith. First of all, where none of the boom control lever  56   a , the arm control lever  56   b , and the bucket control lever  56   c  in the non-operating state (stopped state) as shown in  FIG. 3  is operated, the controller  57  outputs a stop control command signal to the power control units  50   a ,  51   a ,  52   a  and  53   a  of the first motor generator  50  that drives the first hydraulic pump  60 , the second motor generator  51  that drives the second hydraulic pump  61 , the third motor generator  52  that drives the third hydraulic pump  62 , and the charge motor generator  53  that drives the charge pump  63 , all shown in  FIG. 5 . At the same time, the controller  57  outputs a cut-off close command signal to the first through the third solenoid selector valves  25  through  27  of the open hydraulic circuit B and to the first through the third solenoid selector valves  37  through  39  of the open hydraulic circuit C. The controller  57  further outputs a cut-off command signal to the arm cylinder proportional selector valve  30  and the bucket cylinder proportional selector valve  42 . As a result, the boom cylinder  1 , the arm cylinder  3 , and the bucket cylinder  5  are held in the non-operating state. 
     The individual operation of the boom  2  is explained next. The raising action of the boom  2  is first explained. In  FIG. 5 , when the operator starts operating the boom control lever  56   a  in the direction of piston rod extension, the controller  57  outputs a forward rotation torque increase command signal to the power control unit  50   a  of the first motor generator  50  and a torque increase command signal to the power control unit  53   a  of the charge motor generator  53 . As a result, the first hydraulic pump  60  and the charge pump  63  are driven. Here, if the operation amount of the boom control lever  56   a  is as small as X1 as indicated at time t1 in  FIG. 4 , the delivery flow rate of the first hydraulic pump  60  reaches Qcp1 so that the piston rod of the boom cylinder  1  is extended at speed V1 (low speed). 
     At this point, in  FIG. 5 , the hydraulic fluid from the first hydraulic pump  60  is supplied to the head-side oil chamber  1   a  of the boom cylinder  1  via the first line  13 . On the other hand, the hydraulic fluid in the rod-side oil chamber  1   b  of the boom cylinder  1  is returned to the first hydraulic pump  60  via the second line  14 . At this point, the flow rate of the hydraulic fluid returning from the rod-side oil chamber  1   b  of the boom cylinder  1  to the first hydraulic pump  60  is lower than the flow rate of the hydraulic fluid supplied from the first hydraulic pump  60  to the head-side oil chamber  1   a  of the boom cylinder  1 . The insufficient flow rate of the hydraulic fluid is compensated by the charge pump  63  supplying the hydraulic fluid to the first hydraulic pump  60  via the charge check valve  21  and the second line  14 . 
     When the operator increases the operation amount of the boom control lever  56   a  to further increase the speed at which to extend the piston rod of the boom cylinder  1 , the controller  57  outputs a forward rotation torque increase command signal to the power control unit  51   a  of the second motor generator  51  and a communication command signal to the third solenoid selector valve  27  of the open hydraulic circuit B. This causes the head-side oil chamber  1   a  of the boom cylinder  1  to be replenished with the hydraulic fluid sucked from the tank  18  and forwarded by the second hydraulic pump  61 . Here, if the operation amount of the boom control lever  56   a  has exceeded X1 to reach X2 as indicated at time t2 in  FIG. 4 , the third solenoid selector valve  27  is placed in the communicating state, and the delivery flow rates of the second and the first hydraulic pumps  61  and  60  reach Qop1 and Qcp2 respectively. As a result, the hydraulic fluid flows into the head-side oil chamber  1   a  of the boom cylinder  1  at a flow rate of Qop1 Qcp2 so that the piston rod is extended at speed V2 (high speed). 
     When the above-described lever manipulation is performed to increase the speed at which to extend the piston rod of the boom cylinder  1 , the controller  57  may output a command signal to the power control unit  52   a  of the third motor generator  52  for driving the third hydraulic pump  62  and to the third solenoid selector valve  39  of the open hydraulic circuit C, instead of issuing the command signal to the power control unit  51   a  of the second motor generator  51  for driving the second hydraulic pump  61  and to the third solenoid selector valve  27  of the open hydraulic circuit B, thereby attaining the high-speed operation. 
     The lowering operation of the boom  2  is explained next. Returning to  FIG. 5 , when the operator starts operating the boom control lever  56   a  in the direction of piston rod contraction, the controller  57  outputs a reverse rotation torque increase command signal to the power control unit  50   a  of the first motor generator  50 . Here, if the operation amount of the boom control lever  56   a  is as small as −X1 as indicated at time t4 in  FIG. 4 , the delivery flow rate of the first hydraulic pump  60  reaches −Qcp1 causing the piston rod of the boom cylinder  1  to contract at speed −V1 (low speed). 
     At this point, in  FIG. 5 , the flow rate of the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the first hydraulic pump  60  is higher than the flow rate of the hydraulic fluid supplied from the first hydraulic pump  60  to the rod-side oil chamber  1   b  of the boom cylinder  1 . The excess hydraulic fluid is returned from the first line  13  to the tank  18  via the flushing valve  20  and the line  16 . 
     Also at this point, the pressure of the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the first hydraulic pump  60  is boosted under the empty weight of the front implement  105 . When supplied with the pressurized hydraulic fluid, the first hydraulic pump  60  acts as a hydraulic motor to drive the first motor generator  50 . The power generated by the first motor generator  50  in this manner is stored into the power unit  54  via the power control unit  50   a.    
     When the operator raises the operation amount of the boom control lever  56   a  to further increase the speed at which to contract the piston rod of the boom cylinder  1 , the controller  57  outputs a reverse rotation torque increase command signal to the power control unit  51   a  of the second motor generator  51  and a communication command signal to the third solenoid selector valve  27  of the open hydraulic circuit B. This causes the second hydraulic pump  61  to act in a manner sucking the hydraulic fluid. As a result, the discharge of the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1  into the tank  18  is promoted through the communicating line  15  and the third solenoid selector valve  27 . 
     At this point, if the operation amount of the boom control lever  56   a  has exceeded −X1 to reach −X2 as indicated at time t5 in  FIG. 4 , the third solenoid selector valve  27  is placed in the communicating state. At the same time, the delivery flow rates of the second and the first hydraulic pumps  61  and  60  become −Qop1 and −Qcp2 respectively. As a result, the hydraulic fluid flows from the head-side oil chamber  1   a  of the boom cylinder  1  at a flow rate of −(Qop1+Qcp2), so that the piston rod is contracted at speed −V2 (high speed). At this point, the hydraulic fluid returning from the head-side oil chamber  1   a  of the boom cylinder  1  to the second hydraulic pump  61  is highly pressurized. When supplied with the pressurized hydraulic fluid, the second hydraulic pump  61  acts as a hydraulic motor to drive the second motor generator  51 . The power generated by the second motor generator  51  in this manner is stored into the power unit  54  via the power control unit  51   a.    
     When the above-described lever manipulation is performed to increase the speed at which to contract the piston rod of the boom cylinder  1 , the controller  57  may output an operation command signal to the power control unit  52   a  of the third motor generator  52  and to the third solenoid selector valve  39  of the open hydraulic circuit C, instead of issuing the operation command signal to the power control unit  51   a  of the second motor generator  51  and to the third solenoid selector valve  27  of the open hydraulic circuit B, thereby attaining the high-speed operation. 
     With this embodiment, when the lever manipulation is performed to increase the speed at which to contract the piston rod of the boom cylinder  1 , the second hydraulic pump  61  and the first hydraulic pump  60  are used together to receive the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1 , so that the operating speed of the piston rod of the boom cylinder  1  is increased. 
     The individual operation of the arm  4  is explained next. In  FIG. 5 , when the operator starts operating the arm control lever  56   b  in the direction of piston rod extension, the controller  57  outputs a forward rotation torque increase command signal to the power control unit  51   a  of the second motor generator  51 , a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B, and a forward opening command signal to the arm cylinder proportional selector valve  30 . As a result, the second hydraulic pump  61  delivers the hydraulic fluid sucked from the tank  18 , and the arm cylinder proportional selector valve  30  opens in the direction coupling the check valve  29  with the first line  31 . 
     The hydraulic fluid from the second hydraulic pump  61  is supplied to the head-side oil chamber  3   a  of the arm cylinder  3  via the line  24  and the first line  31 . On the other hand, the hydraulic fluid in the rod-side oil chamber  3   b  of the arm cylinder  3  is returned to the tank  18  via the second line  32 , the arm cylinder proportional selector valve  30 , and the line  35 . As a result, the piston rod of the arm cylinder  3  is extended. 
     The arm damping operation is explained next. When the operator starts operating the arm control lever  56   b  in the direction of piston rod contraction, the controller  57  outputs a forward rotation torque increase command signal to the power control unit  51   a  of the second motor generator  51 , a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B, and a reverse opening command signal to the arm cylinder proportional selector valve  30 . As a result, the second hydraulic pump  61  delivers the hydraulic fluid sucked from the tank  18 , and the arm cylinder proportional selector valve  30  opens in the direction coupling the check valve  29  with the second line  32 . 
     The hydraulic fluid from the second hydraulic pump  61  is supplied to the rod-side oil chamber  3   b  of the arm cylinder  3  via the line  24  and the second line  32 . On the other hand, the hydraulic fluid in the head-side oil chamber  3   a  of the arm cylinder  3  is returned to the tank  18  via the first line  31 , the arm cylinder proportional selector valve  30 , and the line  35 . As a result, the piston rod of the arm cylinder  3  is contracted. 
     The individual operation of the bucket  6  is performed in the same manner as that of the arm  4  and thus will not be discussed further. 
     The combined operation of the actuators is explained next with reference to  FIGS. 3 and 5 . As shown in  FIG. 3 , it is assumed that the boom  2 , the arm  4 , and the bucket  6  are operated in a combined manner. In that case, if the boom  2  is to be operated at low speed, the boom cylinder  1 , the arm cylinder  3 , and the bucket cylinder  5  are supplied with the hydraulic fluid respectively from the first hydraulic pump  60 , the second hydraulic pump  61 , and the third hydraulic pump  62  driving the respective piston rods. Specifically, the controller  57  outputs a communication command signal to the first solenoid selector valve  25  of the open hydraulic circuit B, an opening command signal to the arm cylinder proportional selector valve  30 , a communication command signal to the first solenoid selector valve  37  of the open hydraulic circuit C, and an opening command signal to the bucket cylinder proportional selector valve  42 . 
     On the other hand, if the boom  2  is to be operated at high speed, e.g., if the piston rod of the boom cylinder  1  is to be extended at a speed exceeding a predetermined threshold value, the controller  57  outputs to the power control unit  51   a  of the second motor generator  51  a forward rotation torque increase command signal corresponding to the operation amount of the boom control lever  56   a . At the same time, controller  57  outputs a cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a communication command signal to the third solenoid selector valve  27 . 
     As a result, the head-side oil chamber  1   a  of the boom cylinder  1  is replenished with the hydraulic fluid from the second hydraulic pump  61 , so that the piston rod of the boom cylinder  1  is extended at a speed corresponding to the operation amount of the boom control lever  56   a.    
     Meanwhile, the controller  57  outputs to the power control unit  52   a  of the third motor generator  52  a forward rotation torque increase command signal corresponding to the operation amount of the arm control lever  56   b . The controller  57  also outputs a communication command signal to the second solenoid selector valve  38  of the open hydraulic circuit C. This causes the arm cylinder  3  to be supplied with the hydraulic fluid from the third hydraulic pump  62  via the arm cylinder proportional selector valve  30 , whereby the piston rod of the arm cylinder  3  is drive-controlled. 
     When the above operation is carried out, the controller  57  may control the output of the power control unit  52   a  of the third motor generator  52  instead of the output of the power control unit  51   a  of the second motor generator  51 , and may output a cut-off command signal to the first solenoid selector valve  37  of the open hydraulic circuit C and a communication command signal to the third solenoid selector valve  39  instead of the cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and the communication command signal to the third solenoid selector valve  27 , thereby replenishing the head-side oil chamber  1   a  of the boom cylinder  1  with the hydraulic fluid from the third hydraulic pump  62 . 
     Where the boom  2 , the arm  4 , and the bucket  6  are operated in combined fashion and where the piston rod of the boom cylinder  1  is contracted at low speed, the first hydraulic motor  60  acts as a hydraulic motor to drive the first motor generator  50  as described above. The power generated by the first motor generator  50  in this manner is stored into the power unit  54  via the power control unit  50   a.    
     Meanwhile, if the piston rod of the boom cylinder  1  is to be contracted at a speed exceeding a predetermined threshold value, the controller  57  outputs to the power control unit  51   a  of the second motor generator  51  a reverse rotation torque increase command signal corresponding to the operation amount of the boom control lever  56   a . At the same time, the controller  57  outputs a cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and a communication command signal to the third solenoid selector valve  27 . 
     As a result, the second hydraulic pump  61  acts to suck the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1 , so that the piston rod of the boom cylinder  1  is controlled to be contracted at a speed corresponding to the operation amount of the boom control lever  56   a . At this point, the hydraulic fluid returning to the second hydraulic pump  61  is highly pressurized. When supplied with the pressurized hydraulic fluid, the second hydraulic pump  61  acts as a hydraulic motor to drive the second motor generator  51 . The power generated by the second motor generator  51  in this manner is stored into the power unit  54  via the power control unit  51   a.    
     Meanwhile, the controller  57  outputs to the power control unit  52   a  of the third motor generator  52  a forward rotation torque increase command signal corresponding to the operation amount of the boom control lever  56   b . At the same time, the controller  57  outputs a communication command signal to the second solenoid selector valve  38  of the open hydraulic circuit C. This causes the arm cylinder  3  to be supplied with the hydraulic fluid from the third hydraulic pump  62  via the arm cylinder proportional selector valve  30 , whereby the piston rod of the arm cylinder  3  is drive-controlled. 
     When the above operation is carried out, the controller  57  may control the output of the power control unit  52   a  of the third motor generator  52  instead of the output of the power control unit  51   a  of the second motor generator  51 , and may output a cut-off command signal to the first solenoid selector valve  37  of the open hydraulic circuit C and a communication command signal to the third solenoid selector valve  39  instead of the cut-off command signal to the first solenoid selector valve  25  of the open hydraulic circuit B and the communication command signal to the third solenoid selector valve  27 , thereby supplying the third hydraulic pump  62  with the hydraulic fluid from the head-side oil chamber  1   a  of the boom cylinder  1 . 
     The above-described second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith provides the same effects as the first embodiment discussed earlier. 
     Also according to the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, a hydraulic pump capable of forward and reverse rotations is used as the second hydraulic pump  61 . As such, the second hydraulic pump  61  can make the flow rate of the hydraulic fluid replenishing the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is extended at high speed, substantially the same as the flow rate of the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is contracted at high speed. As a result, the speed at which to extend and contract the piston rod of the boom cylinder  1  is made substantially the same, so that excellent operability the work machine is obtained as in the case of the first embodiment. 
     Further, according to the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, when the piston rod of the boom cylinder  1  is operated at low speed, the charge pump  63  and the flushing valve  20  compensate the excess or shortage of the hydraulic fluid in the flow rate balance caused by the difference in volume between the head-side oil chamber  1   a  and the rod-side oil chamber  1   b  of the boom cylinder  1 . When the piston rod of the boom cylinder  1  is operated at high speed, the second hydraulic pump  61  compensates the excess or shortage of the hydraulic fluid in the flow rate balance of the boom cylinder  1  mentioned above. In this manner, in keeping with the operating speed of the piston rod of the boom cylinder  1 , the use or nonuse of the second hydraulic pump  61  is selected in the closed hydraulic circuit A, which makes it possible to downsize the charge bump  63 . Also, when there occur fluctuations of the pressure inside the lines during high-speed operation, the second hydraulic pump  61  provides flow rate control such as to ensure a stable operation state. 
     Also according to the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, the hydraulic fluid flowing from the head-side oil chamber  1   a  of the boom cylinder  1  when the piston rod of the boom cylinder  1  is contracted at high speed is guided to the first hydraulic pump  60  and the second hydraulic pump  61 . This allows the displacement of the first hydraulic pump  60  to be smaller than that of its counterpart in the past. 
     Furthermore, according to the second embodiment of the present invention made up of the driving device for a work machine and the work machine equipped therewith, the motor generators for driving the hydraulic pumps are directly coupled thereto. As a result, the transmission losses incurred when the hydraulic pumps are driven or serve to regenerate power are smaller than in the case of the first embodiment. 
     The present invention is not limited to the embodiments discussed above and may also be implemented in diverse variations. The embodiments above have been explained as detailed examples helping this invention to be better understood. The present invention, when embodied, is not necessarily limited to any embodiment that includes all the structures described above. 
     DESCRIPTION OF REFERENCE SYMBOLS 
     
         
           1  Boom cylinder 
           1   a  Head-side oil chamber 
           1   b  Rod-side oil chamber 
           2  Boom 
           3  Arm cylinder 
           4  Arm 
           5  Bucket cylinder 
           6  Bucket 
           7  Engine 
           8  Power transmission device 
           9  First hydraulic pump 
           10  Second hydraulic pump 
           11  Third hydraulic pump 
           12  Charge pump 
           13  First line 
           14  Second line 
           15  Communicating line 
           18  Tank 
           20  Flushing valve 
           21  Charge check valve 
           25  First solenoid selector valve 
           26  Second solenoid selector valve 
           27  Third solenoid selector valve 
           30  Arm cylinder proportional selector valve 
           42  Bucket cylinder proportional selector valve 
           56   a  Boom control lever 
           56   b  Arm control lever 
           56   c  Bucket control lever 
           57  Controller 
         A Closed hydraulic circuit 
         B Open hydraulic circuit 
         C Open hydraulic circuit