Patent Publication Number: US-9845814-B2

Title: Hydraulic drive system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2013/075792, filed on Sep. 25, 2013. This U.S. National stage application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application No. 2012-231357, filed in Japan on Oct. 19, 2012, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a hydraulic drive system. 
     Background Information 
     Machinery, such as hydraulic shovels and wheel loaders, is provided with a work implement that is driven by a hydraulic cylinder. The hydraulic cylinder is supplied with the hydraulic fluid discharged from a hydraulic pump. The hydraulic fluid is supplied to the hydraulic cylinder by way of a hydraulic circuit. For instance, Japanese Laid-Open Patent Application Publication No. 2003-21104 proposes a work implement equipped with a closed hydraulic circuit for supplying hydraulic fluid to the hydraulic cylinder. The closed hydraulic circuit regenerates the positional energy of the work implement. Thus, it is possible to reduce the fuel consumption of a prime mover driving the hydraulic pump. 
     The above-mentioned closed hydraulic circuit is also provided with a relief valve. The relief valve opens when hydraulic pressure in the closed hydraulic circuit is at or above a predetermined relief pressure. Thereby, the relief valve regulates the increase in hydraulic pressure in the closed hydraulic circuit. 
     SUMMARY 
     Only the relief valve regulates the hydraulic pressure in the above-described closed hydraulic circuit. Namely, unlike an open hydraulic circuit in which the hydraulic fluid from the hydraulic cylinder is sent to a hydraulic fluid tank, in the closed hydraulic circuit the hydraulic fluid expelled from the hydraulic cylinder circulates through the closed hydraulic circuit and returns to the hydraulic pump. Consequently, the hydraulic pressure in the closed hydraulic circuit tends to increase up to the relief pressure. 
     Therefore, the hydraulic pressure in the closed hydraulic circuit tends to rise rapidly even when an operator operates a control in a manner to slowly lower a work implement. In this case, it can be difficult for the operator to adjust the work implement to a desired height because of the large accelerative force of the work implement. 
     For example, in some cases, a hydraulic shovel is operated such that the upper revolving unit is positioned at approximately 90° relative to the crawler tracks while the bottom of the bucket of the work implement presses against the ground to raise one of the crawler tracks from the ground. With the vehicle set in this orientation, it is possible to remove mud adhered to the crawler track by spraying high-pressure water and rotating the raised crawler track. At this time the operator sets an arm positioned approximately 90° relative to the ground and the bottom of the bucket pushing against the ground. The operator then slowly sets the boom lowered to raise the crawler track from the ground. 
     However, as above described, when the hydraulic pressure in the closed hydraulic circuit rises rapidly, the crawler track rises suddenly from the ground. The operator can find it difficult to adjust the position of the crawler track to a desired height when the crawler track rises suddenly. 
     The present invention aims to provide a hydraulic drive system that facilitates adjusting the position of a work implement to a desired height. 
     A hydraulic drive system according to a first aspect of the present invention includes a hydraulic pump, a drive source, a work implement, a hydraulic cylinder, a hydraulic fluid flowpath, a relief valve, an operating member, a bleed-off flowpath, and a control valve. The hydraulic pump includes a first pump port and a second pump port. The hydraulic pump is switchable between a first state and a second state. In the first state, the hydraulic pump takes in hydraulic fluid from the second pump port and discharges the hydraulic fluid from the first pump port. In the second state, the hydraulic pump takes in the hydraulic fluid from the first pump port and discharges the hydraulic fluid from the second pump port. The drive source drives the hydraulic pump. The hydraulic fluid discharged from the hydraulic pump drives the hydraulic cylinder. The hydraulic cylinder includes a first chamber and a second chamber. The hydraulic cylinder expels the hydraulic fluid from the first chamber, and supplies the hydraulic fluid to the second chamber to lower the work implement. The hydraulic cylinder supplies the hydraulic fluid to the first chamber, and expels the hydraulic fluid to the second chamber to raise the work implement. The hydraulic fluid flowpath includes a first flowpath and a second flowpath. The first flowpath connects the first pump port and the first chamber. The second flowpath connects the second pump port and the second chamber. The hydraulic fluid flowpath forms a closed circuit between the hydraulic pump and the hydraulic cylinder. The relief valve opens when the hydraulic pressure in the hydraulic fluid flowpath is at or above the relief pressure. The operating member is for operating the work implement. The bleed-off flowpath is for bleeding off a portion of the hydraulic fluid from the second flowpath. When an operation amount of the operating member used for lowering the work implement is smaller than a predetermined operation amount, the control valve connects the second flowpath to the bleed-off flowpath via throttle to maintain the hydraulic pressure in the second flowpath at less than the relief pressure. The predetermined operation amount is less than or equal to a maximum operation amount of the operating member for lowering the work implement. 
     A work vehicle according to a second aspect of the present invention is the hydraulic drive system according to the first aspect of the present invention wherein the control valve closes an opening between the second flowpath and the bleed-off flowpath when the operation amount of the operating member is greater than or equal to the predetermined operation amount. 
     A work vehicle according to a third aspect of the present invention is the hydraulic drive system according to the first or the second aspect of the present invention further comprising a pump controller. The pump controller controls the capacity of the hydraulic pump. The hydraulic pump includes a first hydraulic pump and a second hydraulic pump. The pump controller reduces the capacity directed to the second hydraulic pump by a predetermined capacity when the operation amount of the operating member is smaller than a predetermined operation amount. The predetermined capacity is the hydraulic pump capacity corresponding to the flow rate of the hydraulic fluid diverted from the second flowpath to the bleed-off flowpath. 
     A work vehicle according to a fourth aspect of the present invention is the hydraulic drive system according to any one of the first through third aspects of the present inventions wherein the control valve adjusts the opening area between the second flowpath of the bleed-off flowpath such that the hydraulic pressure in the second flowpath increases in accordance with an increase in the operation amount of the operating member when the operation amount of the operating member is smaller than the predetermined operation amount. 
     A work vehicle according to a fifth aspect of the present invention is the hydraulic drive system according to any one of the first through fourth aspects of the present invention further comprising a charge circuit. The charge circuit is a hydraulic circuit that supplements the hydraulic fluid in the hydraulic fluid flowpath. The bleed-off flowpath is connected to the charge circuit. 
     A work vehicle according to a sixth aspect of the present invention is the hydraulic drive system according to any one of the first through fourth aspects of the present invention wherein the bleed-off flowpath is connected to the first flowpath. 
     A work vehicle according to a seventh aspect of the present invention is the hydraulic drive system according to any one of the first through fourth aspects of the present invention further comprising a hydraulic fluid tank. The hydraulic fluid tank stores the hydraulic fluid. The bleed-off flowpath is connected to the hydraulic fluid tank. 
     In the hydraulic drive system according to the first aspect of the present invention, the second flowpath is connected to the bleed-off flowpath via a throttle when the operation amount of the operating member for lowering the work implement is less than a predetermined operation amount. Thus, a portion of the hydraulic fluid in the second flowpath is bled off into the bleed-off flowpath, and the hydraulic pressure in the second flowpath is maintained at less than the relief pressure. Therefore, the acceleration force to lower the work implement can be suppressed. This facilitates the operator&#39;s adjusting the position of the work implement to a desired height. 
     In the hydraulic drive system according to the second aspect of the present invention, the opening between the second flowpath and the bleed-off flowpath is closed when the operation amount of the operation member is greater than or equal to a predetermined operation amount. Therefore, the work implement can be lowered quickly when the operation amount of the operation member is greater than or equal to a predetermined operation amount. Thus, it is possible to improve the work efficiency of the work implement. 
     In the hydraulic drive system according to the third aspect of the present invention, the charge flow rate into the hydraulic fluid flowpath can be reduced. Thus, it is possible to improve the fuel consumption of the drive source. 
     In the hydraulic drive system according to the fourth aspect of the present invention, the hydraulic pressure in the second flowpath increases in accordance with the increase in the operation amount of the operation member even when the operation amount of the operation member is smaller than the predetermined operation amount. Thus, it is possible to adjust the operation speed of the work implement using the operation member. 
     In the hydraulic drive system according to the fifth aspect of the present invention, the hydraulic fluid bled off from the second flowpath is returned to the hydraulic pump via the charge circuit. Consequently, the hydraulic fluid bled off can be reused within the hydraulic pump. 
     In the hydraulic drive system according to the sixth aspect of the present invention the hydraulic fluid is sent from the second flowpath through the bleed-off flowpath to the first flowpath. Consequently, the hydraulic fluid bled off from the second flowpath is returned to the hydraulic pump via the first flowpath. 
     In the hydraulic drive system according to the seventh aspect of the present invention the hydraulic fluid is sent from the second flowpath through the bleed-off flowpath to the hydraulic fluid tank. Consequently, the hydraulic fluid bled off from the second flowpath is sent to the hydraulic pump. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  illustrates an outer appearance of a hydraulic shovel containing a hydraulic drive system according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a block diagram of a configuration of a hydraulic drive system according to the first exemplary embodiment; 
         FIG. 3  illustrates boom-lowering opening area information, and bleed-off opening area information; 
         FIG. 4  illustrates a relationship between a boom-lowering operation amount, and the hydraulic pressure in a second pump flowpath; 
         FIG. 5  is a block diagram of a configuration of the hydraulic drive system according to a second exemplary embodiment; 
         FIG. 6  is a block diagram of a configuration of the hydraulic drive system according to a third exemplary embodiment; 
         FIG. 7  is a block diagram of a configuration of the hydraulic drive system according to a fourth exemplary embodiment; 
         FIG. 8  is a flowchart illustrating the process of controlling the capacity directed to the hydraulic pump in the hydraulic drive system according to a fifth exemplary embodiment; 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A hydraulic drive system according to exemplary embodiments of the present invention is described below with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a perspective view of a hydraulic shovel  100  containing the hydraulic drive system according to a first exemplary embodiment of the present invention. The hydraulic shovel  100  includes a vehicle body  1  and a work implement  2 . The vehicle body  1  includes an upper revolving unit  3 , a cab  4 , and an undercarriage  5 . The upper revolving unit  3  is mounted on the undercarriage  5 . The upper revolving unit  3  is provided on the undercarriage  5  to be rotatable with respect to the undercarriage  5 . The upper revolving unit  3  houses apparatus, such as an engine and a hydraulic pump, which are described hereinafter. The cab  4  is located at the front of the upper revolving unit  3 . An operating device, which is described hereinafter, is arranged within the cab  4 . The undercarriage  5  includes crawler tracks  5   a ,  5   b . The hydraulic shovel  100  travels via the rotation of the crawler tracks  5   a ,  5   b.    
     The work implement  2  is attached to the front of the vehicle body  1 . The work implement  2  includes a boom  90 , and arm  91 , and a bucket  92 . The base end of the boom  90  is swingably attached to the upper revolving unit  3  via a boom pin  96 . The base end of the arm  91  is swingably attached to the tip end of the boom  90  via an arm pin  97 . The bucket  92  is swingably attached to the tip end of the arm  91  via a bucket pin  98 . A hydraulic cylinder  14  drives the boom  90 . A hydraulic cylinder  94  drives the arm  91 . A hydraulic cylinder  95  drives the bucket  92 . 
       FIG. 2  is a block diagram illustrating a configuration of the hydraulic drive system. The hydraulic drive system is a system for driving the boom  90 . The hydraulic drive system includes an engine  11 , a main pump  10 , a hydraulic cylinder  14 , a hydraulic fluid flowpath  15 , a control valve  16 , and a pump controller  24 . 
     The engine  11  drives the main pump  10 . The engine  11  is one example of a drive source in the present invention. The engine  11  may be, for example, a diesel engine. The output of the engine  11  is controlled by adjusting the amount of fuel injected from a fuel injection device  21 . An engine controller  22  controls the fuel injection device  21  to thereby control the adjustment of the fuel injection amount. Moreover, the actual rotation speed of the engine  11  may be detected via a rotation speed sensor  23 , and a detection signal from the rotation speed sensor  23  may be input into the engine controller  22  and the pump controller  24  respectively. 
     The main pump  10  includes a first hydraulic pump  12  and a second hydraulic pump  13 . The first hydraulic pump  12  and the second hydraulic pump  13  are driven by the engine  11  and discharge hydraulic fluid. The hydraulic fluid discharged from the main pump  10  is supplied to the hydraulic cylinder  14  via the control valve  16 . 
     The first hydraulic pump  12  is a variable capacity hydraulic pump. The capacity of the first hydraulic pump  12  is controlled by controlling a tilt angle within the first hydraulic pump  12 . The tilt angle within the first hydraulic pump  12  is controlled by a first pump flow control section  25 . The first pump flow control section  25  controls the tilt angle within the first hydraulic pump  12  on the basis of a command signal received from the pump controller  24 . Thereby the first pump flow control section  25  controls the flow rate of the hydraulic fluid discharged from the first hydraulic pump  12 . 
     The first hydraulic pump  12  is a bidirectional discharge type hydraulic pump. More specifically, the first hydraulic pump  12  includes a first pump port  12   a  and a second pump port  12   b . The first hydraulic pump  12  may be switched between a first discharge state and a second discharge state. In the first discharge state, the first hydraulic pump  12  takes in hydraulic fluid from the second pump port  12   b  and discharges hydraulic fluid from the first pump port  12   a . In the second discharge state, the first hydraulic pump  12  takes in hydraulic fluid from the first pump port  12   a  and discharges hydraulic fluid from the second pump port  12   b.    
     The second hydraulic pump  13  is a variable capacity hydraulic pump. The capacity of the second hydraulic pump  13  is controlled by controlling a tilt angle within the second hydraulic pump  13 . The tilt angle within the second hydraulic pump  13  may be controlled by a second pump flow control section  26 . The second pump flow control section  26  controls the tilt angle within the second hydraulic pump  13  on the basis of a command signal received from the pump controller  24 . Thereby, the second pump flow control section  26  controls the flow rate of the hydraulic fluid discharged from the second hydraulic pump  13 . 
     The second hydraulic pump  13  is a bidirectional discharge type hydraulic pump. More specifically, the second hydraulic pump  13  includes a first pump port  13   a  and a second pump port  13   b . The second hydraulic pump  13  may be switched between a first discharge state and a second discharge state, similarly to the first hydraulic pump  12 . In the first discharge state, the second hydraulic pump  13  takes in hydraulic fluid from the second pump port  13   b  and discharges hydraulic fluid from the first pump port  13   a . In the second discharge state, the second hydraulic pump  13  takes in hydraulic fluid from the first pump port  13   a  and discharges hydraulic fluid from the second pump port  13   b.    
     The hydraulic fluid discharged from the first hydraulic pump  12  and the second hydraulic pump  13  drives the hydraulic cylinder  14 . As described above, the hydraulic cylinder  14  drives the boom  90 . When the hydraulic cylinder  14  expands, the tip end of the boom  90  ascends. That is, the work implement  2  ascends. When the hydraulic cylinder  14  contracts, the tip end of the boom  90  descends. That is, the work implement  2  descends. Further, in some attachment condition of the hydraulic cylinder  14 , the extension of the hydraulic cylinder  14  may cause the work implement  2  to descend. In this case, the work implement  2  ascends when the hydraulic cylinder  14  contracts. The hydraulic cylinder  14  includes a cylinder rod  14   a  and a cylinder tube  14   b . The cylinder rod  14   a  divides the inside of the cylinder tube  14   b  into a first chamber  14   c  and a second chamber  14   d.    
     The hydraulic cylinder  14  expands and contracts due to the supplying or expelling hydraulic fluid to or from the first chamber  14   c  and the second chamber  14   d . More specifically, the hydraulic cylinder  14  expands with the hydraulic fluid supplied to the first chamber  14   c  and the hydraulic fluid expelled from the second chamber  14   d . The hydraulic cylinder  14  contracts with the hydraulic fluid supplied to the second chamber  14   d  and the hydraulic fluid expelled from the first chamber  14   c . The pressure receiving area of the cylinder rod  14   a  in the first chamber  14   c  is larger than the pressure receiving area of the cylinder rod  14   a  in the second chamber  14   d . Accordingly, when the hydraulic cylinder  14  expands, a larger amount of hydraulic fluid is supplied to the first chamber  14   c  than the amount of hydraulic fluid expelled from the second chamber  14   d . Furthermore, when the hydraulic cylinder  14  contracts, a larger amount of hydraulic fluid is expelled from the first chamber  14   c  than the amount of hydraulic fluid supplied to the second chamber  14   d.    
     The hydraulic fluid flowpath  15  connects the first hydraulic pump  12 , the second hydraulic pump  13 , and the hydraulic cylinder  14 . The hydraulic fluid flowpath  15  includes a first flowpath  15   a  and a second flowpath  15   b . The first flowpath  15   a  connects the first pump port  12   a  of the first hydraulic pump  12 , and the first chamber  14   c  in the hydraulic cylinder  14 . The first pump port  13   a  in the second hydraulic pump  13  is connected to the first flowpath  15   a . The second flowpath  15   b  connects the second pump port  12   b  in the first hydraulic pump  12  and the second chamber  14   d  in the hydraulic cylinder  14 . The second pump port  13   b  in the second hydraulic pump  13  is connected to the hydraulic fluid tank  27 . 
     The first flowpath  15   a  includes a first cylinder flowpath  31  and a first pump flowpath  33 . The second flowpath  15   b  includes a second cylinder flowpath  32  and a second pump flowpath  34 . The first cylinder flowpath  31  connects to the first chamber  14   c  in the hydraulic cylinder  14 . The second cylinder flowpath  32  connects to the second chamber  14   d  in the hydraulic cylinder  14 . The first pump flowpath  33  supplies hydraulic fluid to the first chamber  14   c  in the hydraulic cylinder  14  via the first cylinder flowpath  31 . Alternatively, the first pump flowpath  33  collects hydraulic fluid from the first chamber  14   c  in the hydraulic cylinder  14  via the first cylinder flowpath  31 . 
     The first pump flowpath  33  connects to the first pump port  12   a  in the first hydraulic pump  12 . Additionally the first pump flowpath  33  connects to the first pump port  13   a  in the second hydraulic pump  13 . Accordingly, the first pump flowpath  33  is supplied with hydraulic fluid from both the first hydraulic pump  12  and the second hydraulic pump  13 . The second pump flowpath  34  supplies hydraulic fluid to the second chamber  14   d  in the hydraulic cylinder  14  via the second cylinder flowpath  32 . Alternatively the second pump flowpath  34  collects hydraulic fluid from the second chamber  14   d  in the hydraulic cylinder  14  via the second cylinder flowpath  32 . 
     The second pump flowpath  34  connects to the second pump port  12   b  in the first hydraulic pump  12 . The second pump port  13   b  of the second hydraulic pump  13  is connected to the hydraulic fluid tank  27 . Accordingly, the second pump flowpath  34  is supplied with hydraulic fluid from the first hydraulic pump  12 . Thus, as described above, the hydraulic fluid flowpath  15 , including the first flowpath  15   a  and the second flowpath  15   b , configures a closed circuit between the main pump  10  and the hydraulic cylinder  14 . 
     The hydraulic drive system is further provided with a charge pump  28 . The charge pump  28  is a hydraulic pump for supplementing the hydraulic fluid in the first flowpath  15   a  or the second flowpath  15   b . Driven by the engine  11 , the charge pump  28  discharges the hydraulic fluid. The charge pump  28  is a fixed capacity hydraulic pump. The hydraulic fluid flowpath  15  further includes a charge circuit  35 . The charge circuit  35  connects to the first pump flowpath  33  via a check valve  41   a . The check valve  41   a  opens when the hydraulic pressure in the first pump flowpath  33  is less than the hydraulic pressure in the charge circuit  35 . 
     The charge circuit  35  is connected to the second pump flowpath  34  via a check valve  41   b . The check valve  41   b  opens when the hydraulic pressure in the second pump flowpath  34  is less than the hydraulic pressure in the charge circuit  35 . Additionally, the charge circuit  35  is connected to the hydraulic fluid tank  27  via a relief valve  42 . The relief valve  42  maintains the hydraulic pressure in the charge circuit  35  at a predetermined charge pressure. When the hydraulic pressure in the first pump flowpath  33  or the second pump flowpath  34  falls below the hydraulic pressure in the charge circuit  35 , the hydraulic fluid from the charge pump  28  is supplied to the first pump flowpath  33  or the second pump flowpath  34  via the charge circuit  35 . Thus, the hydraulic pressure in the first pump flowpath  33  and the second pump flowpath  34  may be maintained at a predetermined value or greater. 
     The hydraulic fluid flowpath  15  further includes a relief flowpath  36 . The relief flowpath  36  connects to the first pump flowpath  33  via a check valve  41   c . The check valve  41   c  opens when the hydraulic pressure in the first pump flowpath  33  is greater than the hydraulic pressure in the relief flowpath  36 . The relief flowpath  36  is connected to the second pump flowpath  34  via a check valve  41   d . The check valve  41   d  opens when the hydraulic pressure in the second pump flowpath  34  is greater than the hydraulic pressure in the relief flowpath  36 . The relief flowpath  36  is connected to the charge circuit  35  via a relief valve  43 . The relief valve  43  maintains the hydraulic pressure in the relief flowpath  36  at a predetermined relief pressure or less. Therefore, the hydraulic pressure in the first pump flowpath  33  and the second pump flowpath  34  may be maintained at a predetermined relief pressure or less. 
     The hydraulic drive system includes a bleed-off flowpath  37 . The bleed-off flowpath  37  connects to the charge circuit  35 . A portion of the hydraulic fluid in the second flowpath  15   b  flows into the bleed-off flowpath  37  when the work implement  2  is descending. The control of the descent of the work implement  2  is described later in detail. 
     The control valve  16  is an electromagnetic control valve controlled on the basis of a command signal from the pump controller  24 . The control valve  16  controls the flow rate of the hydraulic fluid supplied to the hydraulic cylinder  14  on the basis of a command signal from the pump controller  24 . The control valve  16  is arranged between the main pump  10  and the hydraulic cylinder  14  in the hydraulic fluid flowpath  15 . When the hydraulic cylinder  14  expands, the control valve  16  controls the flow rate of the hydraulic fluid supplied from the first pump flowpath  33  to the hydraulic cylinder  14 , and the flow rate of the hydraulic fluid supplied from the first pump flowpath  33  to the bleed-off flowpath  37 . Additionally, when the hydraulic cylinder  14  contracts, the control valve  16  controls the flow rate of the hydraulic fluid supplied from the second pump flowpath  34  to the hydraulic cylinder  14 , and the flow rate of the hydraulic fluid supplied from the second pump flowpath  34  to the bleed-off flowpath  37 . 
     The control valve  16  includes a first-pump port  16   a , a first-cylinder port  16   b , a first bleed-off port  16   c , and a first bypass port  16   d . The first-pump port  16   a  is connected to the first pump flowpath  33  via a first direction control section  44 . The first direction control section  44  is a check valve that regulates the flow of the hydraulic fluid in one direction. The first-cylinder port  16   b  is connected to the first cylinder flowpath  31 . The first bleed-off port  16   c  is connected to the bleed-off flowpath  37 . The aforementioned first direction control section  44  allows the hydraulic fluid to flow from the first pump flowpath  33  to the first cylinder flowpath  31 , and prevents the hydraulic fluid from flowing from the first cylinder flowpath  31  to the first pump flowpath  33  when the control valve  16  supplies the hydraulic fluid from the first pump flowpath  33  to the first cylinder flowpath  31 . 
     The control valve  16  further includes a second-pump port  16   e , a second-cylinder port  16   f , a second bleed-off port  16   g , and a second bypass port  16   h . The second-pump port  16   e  is connected to the second pump flowpath  34  via a second direction control section  45 . The second direction control section  45  is a check valve that regulates the flow of the hydraulic fluid in one direction. The second-cylinder port  16   f  is connected to the second cylinder flowpath  32 . The second bleed-off port  16   g  is connected to the bleed-off flowpath  37 . 
     The aforementioned second direction control section  45  allows the hydraulic fluid to flow from the second pump flowpath  34  to the second cylinder flowpath  32 , and prevents the hydraulic fluid from flowing from the second cylinder flowpath  32  to the second pump flowpath  34  when the control valve  16  supplies the hydraulic fluid from the second pump flowpath  34  to the second cylinder flowpath  32 . 
     The control valve  16  can be switched between a first position state P 1 , a second position state P 2 , a neutral position state Pn, and a third position state P 3 . In the first position state P 1 , the control valve  16  links the first-pump port  16   a  and the first-cylinder port  16   b , and links the second-cylinder port  16   f  and the second bypass port  16   h . Accordingly, in the first position state P 1 , the control valve  16  connects the first pump flowpath  33  to the first cylinder flowpath  31  via the first direction control section  44 , and connects the second cylinder flowpath  32  to the second pump flowpath  34  bypassing the second direction control section  45 . Moreover, when the control valve  16  is in the first position state P 1  the first bypass port  16   d , the first bleed-off port  16   c , the second-pump port  16   e , and the second bleed-off port  16   g  are insulated from all the other ports. 
     When the hydraulic cylinder  14  expands, the first hydraulic pump  12  and the second hydraulic pump  13  are driven in a first discharge state, and the control valve  16  is set to the first position state P 1 . Thereby the hydraulic fluid discharged from the first pump port  12   a  of the first hydraulic pump  12 , and the first pump port  13   a  of the second hydraulic pump  13  passes through the first pump flowpath  33 , the first direction control section  44 , and the first cylinder flowpath  31 , and is supplied to the first chamber  14   c  of the hydraulic cylinder  14 . Additionally, the hydraulic fluid in the second chamber  14   d  of the hydraulic cylinder  14  passes through the second cylinder flowpath  32  and the second pump flowpath  34  and is collected at the second pump port  12   b  of the first hydraulic pump  12 . As a result, the hydraulic cylinder  14  expands. 
     In the second position state P 2 , the control valve  16  links the second-pump port  16   e  and the second-cylinder port  16   f , and links the first-cylinder port  16   b  and the first bypass port  16   d . Accordingly, in the second position state P 2 , the control valve  16  connects the first cylinder flowpath  31  to the first pump flowpath  33  bypassing the first direction control section  44 , and connects the second pump flowpath  34  to the second cylinder flowpath  32  via the second direction control section  45 . Moreover, when the control valve  16  is in the second position state P 2 , the first-pump port  16   a , the first bleed-off port  16   c , the second bypass port  16   h , and the second bleed-off port  16   g  are insulated from all the other ports. 
     When the hydraulic cylinder  14  contracts, the first hydraulic pump  12  and the second hydraulic pump  13  are driven in a second discharge state, and the control valve  16  is set to the second position state P 2 . Thereby, the hydraulic fluid discharged from the second pump port  12   b  of the first hydraulic pump  12  passes through the second pump flowpath  34 , the second direction control section  45 , and the second cylinder flowpath  32  and is supplied to the second chamber  14   d  of the hydraulic cylinder  14 . Additionally, the hydraulic fluid in the first chamber  14   c  of the hydraulic cylinder  14  passes through the first cylinder flowpath  31  and the first pump flowpath  33  and is collected at the first pump port  12   a  of the first hydraulic pump  12  and the first pump port  13   a  of the second hydraulic pump  13 . As a result, the hydraulic cylinder  14  contracts. 
     In the neutral position state Pn, the control valve  16  links the first bypass port  16   d  and the first bleed-off port  16   c , and links the second bypass port  16   h  and the second bleed-off port  16   g . Accordingly, in the neutral position state Pn, the control valve  16  connects the first pump flowpath  33  to the bleed-off flowpath  37  bypassing the first direction control section  44 , and connects the second pump flowpath  34  to the bleed-off flowpath  37  bypassing the second direction control section  45 . Moreover, when the control valve  16  is in the neutral position state Pn, the first-pump port  16   a , the first-cylinder port  16   b , the second-pump port  16   e , and the second-cylinder port  16   f  are insulated from all the other ports. 
     In the third position state P 3 , the control valve  16  links the second-pump port  16   e  and the second-cylinder port  16   f , and links the first-cylinder port  16   b  and the first bypass port  16   d . Accordingly, in the third position state P 3 , the control valve  16  connects the first cylinder flowpath  31  to the first pump flowpath  33  bypassing the first direction control section  44  and connects the second pump flowpath  34  to the second cylinder flowpath  32  via the second direction control section  45 . Moreover, in the third position state P 3 , the control valve  16  links the second bypass port  16   h  and the second bleed-off port  16   g  via a throttle  17 . Accordingly, in the third position state P 3 , the control valve  16  connects the second pump flowpath  34  to the bleed-off flowpath  37  via the throttle  17 . 
     Consequently, the bleed-off flowpath  37  is connected to the second flowpath  15   b  to branch off from the second flowpath  15   b . When the control valve  16  is in the third position state P 3 , the first-pump port  16   a , and the first bleed-off port  16   c  are insulated from all the other ports. 
     The control valve  16  may be set to any suitable position state between the first position state P 1  and the neutral position state Pn. Consequently, the control valve  16  can control the flow rate of the hydraulic fluid supplied from the first pump flowpath  33  to the first cylinder flowpath  31  via the first direction control section  44 , and the flow rate of the hydraulic fluid supplied from the first pump flowpath  33  to the bleed-off flowpath  37 . That is, the control valve  16  controls the flow rate of the hydraulic fluid supplied from the first hydraulic pump  12  and the second hydraulic pump  13  to the first chamber  14   c  in the hydraulic cylinder  14 , and the flow rate of the hydraulic fluid supplied from the first hydraulic pump  12  and the second hydraulic pump  13  to the bleed-off flowpath  37 . 
     The control valve  16  may be set to any suitable position state between the second position state P 2  and the neutral position state Pn. Consequently, the control valve  16  can control the flow rate of the hydraulic fluid supplied from the second pump flowpath  34  to the second cylinder flowpath  32  via the second direction control section  45 , and the flow rate of the hydraulic fluid supplied from the second pump flowpath  34  to the bleed-off flowpath  37 . That is, the control valve  16  controls the flow rate of the hydraulic fluid supplied from the first hydraulic pump  12  to the second chamber  14   d  in the hydraulic cylinder  14 , and the flow rate of the hydraulic fluid supplied from the first hydraulic pump  12  to the bleed-off flowpath  37 . 
     The control valve  16  may be set to any suitable position state between the second position state P 2  and the third position state P 3 . Therefore, the control valve  16  can control the flow rate of the hydraulic fluid bled off from the second pump flowpath  34  into the bleed-off flowpath  37 . 
     The hydraulic drive system further includes an operating device  46 . The operating device  46  includes an operating member  46   a , and an operation detector  46   b . The operating member  46   a  is a member for operating the hydraulic cylinder  14 . For example, the operating member  46   a  may be a boom operation lever. The operating member  46   a  can be operated from a neutral position in one of the two following directions: a direction causing the hydraulic cylinder  14  to expand, and a direction causing the hydraulic cylinder  14  to contract. 
     The operation detector  46   b  detects an operation amount of the operating member  46   a  (referred to as a “boom operation amount”) and the operation direction. The operation detector  46   b  may be a sensor that detects the position of the operating member  46   a . When the operating member  46   a  is in a neutral position, the boom operation amount is zero. A detection signal indicating the boom operation amount and the operation direction may be input from the operation detector  46   b  to the pump controller  24 . The pump controller  24  computes a target flow rate for the hydraulic fluid supplied of the hydraulic cylinder  14  in accordance with the boom operation amount. 
     The engine controller  22  controls a fuel injection device  21  to control the output of the engine  11 . In the engine controller  22 , a mapping of engine output torque characteristics determined on the basis of set target engine speeds and work modes is stored. The engine output torque characteristics represents a relationship between the output torque and the rotation speed of the engine  11 . The engine controller  22  controls the output of the engine  11  on the basis of the engine output torque characteristics. 
     The pump controller  24  uses the control valve  16  to control the flow rate of the hydraulic fluid supplied to the hydraulic cylinder  14 . Additionally, the pump controller  24  controls the flow rate of the hydraulic fluid supplied to the hydraulic cylinder  14  using the first pump flow control section  25  and a second pump flow control section  26 . The pump controller  24  is one example of the pump control section in the present invention. The more minute flow rate may be controlled using the control valve  16  than that is controlled with the first pump flow control section  25  and the second pump flow control section  26 . 
     For instance, the pump controller  24  may control the flow rate using of the control valve  16  (referred to below as “low-speed control”) when the operation amount of the operating member  46   a  is at a predetermined value or less. The pump controller  24  may control the flow rate using the first pump flow control section  25  and the second pump flow control section  26  (referred to below as “normal control”) when the operation amount of the operating member  46   a  is greater than the predetermined value. 
     During normal control, the pump controller  24  controls the capacity directed to the first hydraulic pump  12  and the second hydraulic pump  13  to thereby control the suction torques in the first hydraulic pump  12  and in the second hydraulic pump  13  on the basis of pump suction torque characteristics. The pump suction torque characteristics represent the relationship between the pump suction torque and the engine rotation speed. The pump suction torque characteristics may be preliminary set on the basis of a work mode and an operation mode, and stored in the pump controller  24 . 
     During low-speed control, the pump controller  24  controls the control valve  16  to maintain the capacity of the first hydraulic pump  12  and the second hydraulic pump  13 , and thereby controls the flow rate of the hydraulic fluid supplied to the hydraulic cylinder  14 . 
     The bleed-off control is described next. The bleed-off control sends a portion of the hydraulic fluid in the second flowpath  15   b  is sent to the bleed-off flowpath  37  when the hydraulic cylinder  14  is contracting, that is, when the work implement  2  is descending. More specifically, the pump controller  24  controls the control valve  16  in accordance with the boom-lowering operation amount on the basis of the bleed-off opening area information L 2  illustrated in  FIG. 3 . The boom-lowering operation amount corresponds to the boom operation amount when the work implement  2  is caused to descend. 
       FIG. 3  illustrates a boom-lowering opening area information L 1 , and bleed-off opening area information L 2 . The boom-lowering opening area information L 1  defines the relationship between the boom-lowering operation amount and the boom-lowering opening area. The boom-lowering opening area is the area of the opening between the second pump flowpath  34  and the second cylinder flowpath  32  in the control valve  16 . In  FIG. 3 , the boom-lowering operation amount is represented as a percentage of the maximum operation amount of the operating member  46   a , where the maximum operation amount is 100%. 
     According to the boom-lowering opening area information L 1 , the boom-lowering opening area increases in accordance with an increase in the boom-lowering operation amount when the boom-lowering operation amount is A1 or greater and less than A2. The aforementioned low-speed control is performed when the boom-lowering operation amount is A1 or greater and less than A2. The aforementioned normal control is carried out when the boom-lowering operation amount is greater than or equal to A2. More specifically, the boom-lowering opening area increases more rapidly in accordance with the increase of the boom-lowering operation amount when the boom-lowering operation amount is A2 or greater and less than A4 than during the low-speed control. Further, when the boom-lowering operation amount is greater than or equal to A4, the boom-lowering opening area is the maximum value Max. That is, the opening area in the control valve  16  between the second pump flowpath  34  and the second cylinder flowpath  32  is maximum. 
     The bleed-off opening area information L 2  defines the relationship between the boom-lowering operation amount and the bleed-off opening area during bleed-off control. The bleed-off opening area is an opening area between the second pump flowpath  34  and the bleed-off flowpath  37  in the control valve  16 . The bleed-off opening area is controlled by setting the control valve  16  between the third position state P 3  and the second position state P 2 . 
     When the boom-lowering operation amount is a predetermined operation amount A2 or greater and less than a predetermined operation amount A3, the bleed-off opening area increases in accordance with an increase in the boom-lowering operation amount. When the boom-lowering operation amount is the predetermined amount A3 or greater and less than a predetermined operation amount A5, the bleed-off opening area remains constant at b2. When the boom-lowering operation amount is the predetermined operation amount A5 or greater and less than a predetermined operation amount A6, the bleed-off opening area decreases in accordance with an increase in the boom-lowering operation amount. When the boom-lowering operation amount is at or greater than the predetermined operation amount A6, the bleed-off opening area is zero. That is, when the boom-lowering operation amount is at or greater than the predetermined operation amount A6, the opening between the second pump flowpath  34  and the bleed-off flowpath  37  is closed. 
     When the boom-lowering operation amount is less than the predetermined operation amount A6, a portion of the hydraulic fluid in the second pump flowpath  34  flows into the bleed-off flowpath  37 . Consequently, an increase in the hydraulic pressure in the second pump flowpath  34  is suppressed.  FIG. 4  illustrates the relationship between the boom-lowering operation amount and the hydraulic pressure in the second pump flowpath  34 . As illustrated in  FIG. 4 , when the boom-lowering operation amount is less than the predetermined operation amount A6, the hydraulic pressure in the second pump flowpath  34  is kept at a lower pressure than the relief pressure Pr of the relief valve  43 . Furthermore, when the boom-lowering operation amount is smaller than a predetermined operation amount A6, the hydraulic pressure in the second pump flowpath  34  increases to within less than the relief pressure Pr in accordance with an increase in the boom-lowering operation amount. 
     Finally, as illustrated in  FIG. 3 , when the boom-lowering operation amount is smaller than the predetermined operation amount A2, the bleed-off opening area remains constant at b1. When the bleed-off opening area remains constant, the control valve  16  is set to a position between the neutral position Pn and the third position state P 3 . 
     Next, one example of the flow of the hydraulic fluid during a bleed off is described based on  FIG. 2 . The ratio of the pressure receiving area of the cylinder rod  14   a  in the first chamber  14   c , and the pressure receiving area of the cylinder rod  14   a  in the second chamber  14   d  is assumed to be 2:1. When the work implement  2  is descending, the hydraulic fluid is supplied to the second chamber  14   d  to cause the hydraulic cylinder  14  to contract. For example, when the inflow rate from the second cylinder flowpath  32  to the second chamber  14   d  is 0.8, the outflow rate from the first chamber  14   c  to the first cylinder flowpath  31  is 1.6. In the following description, the values indicating the flow rate are examples of the division of flows in each flowpath. 
     The discharge flow rate of the first hydraulic pump  12  and the discharge flow rate of the second hydraulic pump  13  are assumed to be 1.0 respectively. In this case the flow rate in the second pump flowpath  34  is 1.0. The pump controller  24  sets the control valve  16  to a position state between the second position state P 2  and the third position state P 3  so that the size of the bleed-off opening area is proportionate to the boom-lowering operation amount. Consequently, a 0.2 portion of the hydraulic fluid in the second pump flowpath  34  flows into the bleed-off flowpath  37 . The flow rate of the hydraulic fluid sent to the bleed-off flowpath  37  is defined by the bleed-off opening area. The remaining 0.8 portion of the hydraulic fluid passes through the second cylinder flowpath  32  and flows into the second chamber  14   d  of the hydraulic cylinder  14 . 
     When the hydraulic cylinder  14  contracts and the work implement  2  descends, a 1.6 portion of the hydraulic fluid is expelled from the first chamber  14   c  of the hydraulic cylinder  14 . The 1.6 portion of the hydraulic fluid flows through the first cylinder flowpath  31  and into the first pump flowpath  33 . 
     Whereas, the 0.2 portion of the hydraulic fluid from the bleed-off flowpath  37  combines with a 0.2 portion of the hydraulic fluid in the charge circuit  35  coming from the charge pump  28 . A total of a 0.4 portion of the hydraulic fluid flows from the charge circuit  35  into the first pump flowpath  33 . 
     The 1.6 portion of the hydraulic fluid from the first cylinder flowpath  31  combines with the 0.4 portion of the hydraulic fluid from the charge circuit  35  in the first pump flowpath  33 . A 1.0 portion of the hydraulic fluid from the first pump flowpath  33  returns to both the first hydraulic pump  12  and the second hydraulic pump  13  because the first hydraulic pump  12  and the second hydraulic pump  13  are set to have the same capacity. 
     The hydraulic drive system according to this exemplary embodiment has the following features. 
     When the boom-lowering operation amount of the operating member  46   a  is less than the predetermined operation amount A6, the second pump flowpath  34  is connected to the bleed-off flowpath  37  via the throttle  17 . Thus, a portion of the hydraulic fluid in the second pump flowpath  34  is bled off into the bleed-off flowpath  37 , and the hydraulic pressure in the second pump flowpath  34  is kept within the relief pressure. Therefore, the acceleration used to lower the work implement  2  can be controlled. This facilitates the operator&#39;s adjusting the position of the work implement  2  to the desired height. 
     When the boom-lowering operation amount of the operating member  46   a  is at the predetermined operation amount A6 or greater, the opening between the second pump flowpath  34  and the bleed-off flowpath  37  is closed. Accordingly, when the boom-lowering operation amount is at the predetermined operation amount A6 or greater, all the hydraulic fluid in the second pump flowpath  34  is supplied to the second chamber  14   d  in the hydraulic cylinder  14  via the second cylinder flowpath  32 . As a result, the work implement  2  may be lowered rapidly. Consequently, this improves the work efficiency of the work implement  2 . 
     When the boom-lowering operation amount of the operating member  46   a  is less than the predetermined operation amount A6, the bleed-off opening area is adjusted so that the hydraulic pressure in the second pump flowpath  34  increases in accordance with an increase in the boom-lowering operation amount. Thus, the hydraulic pressure in the second pump flowpath  34  increases in accordance with an increase in the boom-lowering operation amount even when the boom-lowering operation amount is less than the predetermined operation amount A6. Therefore, the operator can adjust the operation speed of the work implement  2  using the operating member  46   a.    
     The hydraulic fluid that is bled off returns to the hydraulic pumps  12 ,  13  via the charge circuit  35 . Accordingly, the hydraulic fluid that is bled off may be reused in the hydraulic pumps  12 ,  13 . 
     Second Exemplary Embodiment 
       FIG. 5  illustrates a hydraulic drive system according to a second exemplary embodiment of the present invention. The hydraulic drive system according to the second exemplary embodiment includes a third bleed-off port  16   i  in the control valve  16 . The third bleed-off port  16   i  is connected to the second pump flowpath  34  via a third direction control section  48 . The third direction control section  48  allows the hydraulic fluid to flow from the second pump flowpath  34  to the third bleed-off port  16   i , and prevents the hydraulic fluid from flowing from the third bleed-off port  16   i  into the second pump flowpath  34 . 
     Additionally, in the third position state P 3 , the control valve  16  links the third bleed-off port  16   i  and the first bypass port  16   d  via the throttle  17 . Accordingly, when the control valve  16  is in the third position state P 3  the control valve  16  connects the second pump flowpath  34  to a flowpath  38  via the throttle  17 . The flowpath  38  connects the first bypass port  16   d  and the first pump flowpath  33 . That is, in the second exemplary embodiment the flowpath  38 , which connects the first bypass port  16   d  and the first pump flowpath  33 , corresponds to a bleed-off flowpath. 
     In the third position state P 3 , the control valve  16  links the first-cylinder port  16   f  and the first bypass port  16   d  and links the second-pump port  16   e  and the second-cylinder port  16   b . Accordingly, when the control valve  16  is in the third position state P 3 , a portion of the hydraulic fluid in the second pump flowpath  34  combines with the hydraulic fluid in the first cylinder flowpath  31 , and flows into the first pump flowpath  33 . Other configurations of the hydraulic drive system according to the second exemplary embodiment are the same as the configurations of the hydraulic drive system according to the first exemplary embodiment. 
     Next, an example of the flow of the hydraulic fluid during bleed-off control in the hydraulic drive system according to the second exemplary embodiment is described based on  FIG. 5 . The discharge flow rate of the first hydraulic pump  12  and the discharge flow rate of the second hydraulic pump  13 , are 1.0 respectively. In this case the flow rate in the second pump flowpath  34  is 1.0. The pump controller  24  sets the control valve  16  in a position state between the second position state P 2  and the third position state P 3  so that the bleed-off opening area is proportionate to the boom-lowering operation amount. The bleed-off opening area is an opening area between the third bleed-off port  16   i  and the first bypass port  16   d.    
     When the control valve  16  is set as above mentioned, 0.2 portion of the hydraulic fluid in the second pump flowpath  34  flows into the bleed-off port  16   i . The remaining 0.8 portion of the hydraulic fluid passes through the second cylinder flowpath  32  and flows into the second chamber  14   d  in the hydraulic cylinder  14 . 
     When the hydraulic cylinder  14  contracts and the work implement  2  descends, 1.6 portion of the hydraulic fluid is expelled from the first chamber  14   c  of the hydraulic cylinder  14 . The 1.6 portion of the hydraulic fluid passes through the first cylinder flowpath  31  and flows into the first pump flowpath  33 . In the meantime, the 0.2 portion of the hydraulic fluid from the third bleed-off port  16   i  combines with the 1.6 portion of the hydraulic fluid from the first cylinder flowpath  31 . The total 1.8 portion of the hydraulic fluid passes through the flowpath  38 , and flows into the first pump flowpath  33 . On the other hand, 0.2 portion of the hydraulic fluid from the charge circuit  35  is supplied to the first pump flowpath  33 . 
     The 1.8 portion of the hydraulic fluid from the flowpath  38  combines with the 0.2 portion of the hydraulic fluid from the charge circuit  35  in the first pump flowpath  33 . 1.0 portion of the hydraulic fluid from the first pump flowpath  33  returns to both the first hydraulic pump  12  and the second hydraulic pump  13  because the first hydraulic pump  12  and the second hydraulic pump  13  are set to have the same capacity. 
     As above described, the hydraulic drive system according to the second embodiment exhibits the same effects as the hydraulic drive system according to the first exemplary embodiment. 
     Third Exemplary Embodiment 
       FIG. 6  illustrates a hydraulic drive system according to a third exemplary embodiment of the present invention. The hydraulic drive system according to the third exemplary embodiment excludes the second hydraulic pump  13  in the hydraulic drive system according to the first exemplary embodiment. Accordingly, the main pump  10  is configures by a single hydraulic pump (the first hydraulic pump  12 ). Furthermore, the hydraulic drive system according to the third exemplary embodiment includes a shuttle valve  51 . 
     The shuttle valve  51  includes a first input port  51   a , second input port  51   b , a drain port  51   c , a first pressure receiver  51   d , and a second pressure receiver  51   e . The first input port  51   a  is connected to the first flowpath  15   a . The second input port  51   b  is connected to the second flowpath  15   b . More specifically, the first input port  51   a  is connected to the first pump flowpath  33 . The second input port  51   b  is connected to the second pump flowpath  34 . The drain port  51   c  is connected to a drain flowpath  52 . The drain flowpath  52  is connected to the charge circuit  35  via the bleed-off flowpath  37 . The first pressure receiver  51   d  is connected to the first flowpath  15   a  via a first pilot flowpath  53 . Thus, the hydraulic pressure in the first flowpath  15   a  is applied to the first pressure receiver  51   d . A throttle  54  is arranged in the first pilot flowpath  53 . The second pressure receiver  51   e  is connected to the second flowpath  15   b  via a second pilot flowpath  55 . Thus, the hydraulic pressure in the second flowpath  15   b  is applied to the second pressure receiver  51   e . A throttle  56  is arranged in the second pilot flowpath  55 . 
     The shuttle valve  51  can be switched between a first position state Q 1 , a second position state Q 2 , and a neutral position state Qn in accordance with the hydraulic pressure in the first flowpath  15   a  and the hydraulic pressure in the second flowpath  15   b . In the first position state Q 1 , the shuttle valve  51  links the second input port  51   b  and the drain port  51   c . Thus, the second flowpath  15   b  is connected to the drain flowpath  52 . In the second position state Q 2 , the shuttle valve  51  links the first input port  51   a  and the drain port  51   c . Thus, the first flowpath  15   a  is connected to the drain flowpath  52 . In the neutral position state Qn, the shuttle valve  51  blocks the flows among the first input port  51   a , the second input port  51   b , and the drain port  51   c.    
     The shuttle valve  51  includes a spool  57 , a first elastic element  58 , and a second elastic element  59 . The first elastic element  58  pushes the spool  57  from the side of the first pressure receiver  51   d  toward the side of the second pressure receiver  51   e . The second elastic element  59  pushes the spool  57  from the side of the second pressure receiver  51   e  toward the first pressure receiver  51   d . The first elastic element  58  is attached to the spool  57  with the first elastic element  58  contracted from the natural length of the first elastic element  58 . The first elastic element  58  is attached to the spool  57  in such a way that the first elastic element  58  presses the spool  57  with applying a first attachment load to the spool  57  when the spools  57  is in a neutral position. The second elastic element  59  is attached to the spool  57  with the first elastic element  59  contracted from the natural length of the second elastic element  59 . The second elastic element  59  presses the spool  57  with applying a second attachment load to the spool  57  when the spool  57  is in the neutral position. 
     The ratio of the pressure receiving area of the first pressure receiver  51   d  and the pressure receiving area of the second pressure receiver  51   e  is equivalent to the ratio of the pressure receiving area of the first chamber  14   c  and the pressure receiving area of the second chamber  14   d . For instance, when the pressure receiving area the ratio of the pressure receiving area of the first chamber  14   c  and the pressure receiving area of the second chamber  14   d  is 2:1, then the ratio of the pressure receiving area of the first pressure receiver  51   d  and the pressure receiving area of the second pressure receiver  51   e  is 2:1. 
     When the force applied to the first pressure receiver  51   d  due to the hydraulic pressure in the first flowpath  15   a  is greater than the force applied to the second pressure receiver  51   e  due to the hydraulic pressure in the second flowpath  15   b , the shuttle valve  51  switches to the first position state Q 1 . Thus, the second flowpath  15   b  and the drain flowpath  52  are connected. As a result, a portion of the hydraulic fluid in the second flowpath  15   b  flows into the charge circuit  35  via the drain flowpath  52  and the bleed-off flowpath  37 . When the force applied to the second pressure receiver  51   e  due to the hydraulic pressure in the second flowpath  15   b  is greater than the force applied to the first pressure receiver  51   d  due to the hydraulic pressure in the first flowpath  15   a , the shuttle valve  51  switches to the second position state Q 2 . Thus, the first flowpath  15   a  and the drain flowpath  52  are connected. As a result, a portion of the hydraulic fluid in the first flowpath  15   a  flows into the charge circuit  35  via the drain flowpath  52  and the bleed-off flowpath  37 . 
     Other configurations in the hydraulic drive system according to the third exemplary embodiment are the same as the configurations in the hydraulic drive system according to the first exemplary embodiment. Next, an example of the flow of the hydraulic fluid in the hydraulic drive system according to the third exemplary embodiment during bleed-off control is described based on  FIG. 6 . 
     The discharge flow rate of the first hydraulic pump  12  is assumed to be 1.0. In this case, the flow rate in the second pump flowpath  34  is 1.0. The pump controller  24  sets the control valve  16  to a position state between the second position state P 2  and the third position state P 3  so that the size of the bleed-off opening area is proportionate to the boom-lowering operation amount. Consequently, 0.2 portion of the hydraulic fluid from the second pump flowpath  34  flows into the bleed-off flowpath  37 . The remaining 0.8 portion of the hydraulic fluid passes through the second cylinder flowpath  32  and flows into the second chamber  14   d  of the hydraulic cylinder  14 . 
     When the hydraulic cylinder  14  contracts and the work implement  2  descends, a 1.6 portion of the hydraulic fluid is expelled from the first chamber  14   c  of the hydraulic cylinder  14 . The 1.6 portion of the hydraulic fluid passes through the first cylinder flowpath  31  and flows into the first pump flowpath  33 . 
     When the hydraulic cylinder  14  contracts to cause the work implement  2  to descend, the shuttle valve  51  switches to the second position state Q 2 . A 0.6 portion of the hydraulic fluid in the first pump flowpath  33  passes through the shuttle valve  51  and flows into the bleed-off flowpath  37 . The remaining 1.0 portion of the hydraulic fluid returns to the first hydraulic pump  12 . 
     Moreover, a 0.2 portion of the hydraulic fluid from the second pump flowpath  34  is combined with the 0.6 portion of the hydraulic fluid from the shuttle valve  51  in the bleed-off flowpath  37  and flows into the charge circuit  35 . The total 0.8 of the hydraulic fluid flows from the charge circuit  35  into the hydraulic fluid tank  27  through the relief valve  42 . 
     As above described, the hydraulic drive system according to the third exemplary embodiment exhibits the same effects as the hydraulic drive system according to the first exemplary embodiment. 
     Fourth Exemplary Embodiment 
       FIG. 7  illustrates a hydraulic drive system according to a fourth exemplary embodiment of the present invention. Similarly to the hydraulic drive system according to the third exemplary embodiment, the hydraulic drive system according to the fourth exemplary embodiment includes the main pump  10  in the hydraulic drive system according to the second exemplary embodiment configured with a single hydraulic pump (the first hydraulic pump  12 ). Furthermore, similarly to the hydraulic drive system according to the third exemplary embodiment, the hydraulic drive system according to the fourth exemplary embodiment includes a shuttle valve  51 . Other configurations in the fourth exemplary embodiment are the same as in the hydraulic drive system of the second exemplary embodiment. 
     Next, an example of the flow of the hydraulic fluid during bleed-off control in the hydraulic drive system according to the fourth exemplary embodiment is described based on  FIG. 7 . The discharge flow rate of the first hydraulic pump  12  is assumed to be 1.0. In this case the flow rate in the second pump flowpath  34  is 1.0. The pump controller  24  sets the control valve  16  in a position between the second position state P 2  and the third position state P 3  so that the size of the bleed-off opening area is proportionate to the boom-lowering operation amount. 
     Consequently, 0.2 portion of the hydraulic fluid from the second pump flowpath  34  flows into the bleed-off port  16   i . The remaining 0.8 portion of the hydraulic fluid passes through the second cylinder flowpath  32  and flows into the second chamber  14   d  in the hydraulic cylinder  14 . 
     When the hydraulic cylinder  14  contracts and the work implement  2  descends, a 1.6 portion of the hydraulic fluid is expelled from the first chamber  14   c  of the hydraulic cylinder  14 . The 1.6 portion of the hydraulic fluid passes through the first cylinder flowpath  31  and flows into the first pump flowpath  33 . At this point, the 0.2 portion of the hydraulic fluid from the third bleed-off port  16   i  combines with the 1.6 portion of the hydraulic fluid from the first cylinder flowpath  31 . The total 1.8 portion of the hydraulic fluid passes through the flowpath  38 , and flows into the first pump flowpath  33 . 
     When the hydraulic cylinder  14  contracts to cause the work implement  2  to descend, the shuttle valve  51  switches to the second position state Q 2 . 0.8 portion of the hydraulic fluid in the first pump flowpath  33  passes through the shuttle valve  51  and flows into the bleed-off flowpath  37 . The remaining 1.0 portion of the hydraulic fluid returns to the first hydraulic pump  12 . 
     Moreover, 0.8 portion of the hydraulic fluid from the shuttle valve  51  passes through the bleed-off flowpath  37  and flows into the charge circuit  35 . The total 0.8 of the hydraulic fluid flows from the charge circuit  35  into the hydraulic fluid tank  27  through the relief valve  42 . 
     As above described, the hydraulic drive system according to the fourth exemplary embodiment exhibits the same effects as the hydraulic drive system according to the first exemplary embodiment. 
     Fifth Exemplary Embodiment 
     During normal control, the pump controller  24  controls the capacity directed to the first hydraulic pump  12  and the second hydraulic pump  13  to control the suction torque of the first hydraulic pump  12  and the suction torque of the second hydraulic pump  13  on the basis of the pump suction torque characteristics. However, when the boom-lowering operation amount is smaller than a predetermined operation amount A6, the pump controller  24  may subtract a capacity corresponding to the flow rate of the hydraulic fluid bled off from the second pump flowpath  34  from the capacity directed to the second hydraulic pump  13 .  FIG. 8  is a flowchart illustrating the process of controlling the capacity directed to the second hydraulic pump  13  in the hydraulic drive system according to the fifth exemplary embodiment. 
     In step S 1 , the pump controller  24  detects the boom-lowering operation amount. The pump controller  24  detects the boom-lowering operation amount using a detection signal from the operation detector  46   b.    
     In step S 2 , the pump controller  24  computes a bleed-off opening area (A). The pump controller  24  computes the bleed-off opening area (A) from the boom-lowering operation amount on the basis of the bleed-off opening area information L 2 . 
     In step S 3 , the pump controller  24  detects a pump pressure (P 2 ), and a charge pressure (Pc). The pump pressure (P 2 ) is the hydraulic pressure in the second pump flowpath  34 . The charge pressure (Pc) is a hydraulic pressure in the charge circuit  35 . The pump controller  24  detects the pump pressure (P 2 ) and the charge pressure (Pc) using a pressure sensor provided in the hydraulic circuit, for example. 
     In step S 4 , the pump controller  24  calculates the bleed-off flow rate (Qb). The bleed-off flow rate (Qb) is the rate of the hydraulic fluid bled off from the second pump flowpath  34 . The pump controller  24  computes the bleed-off flow rate (Qb) from the following Formula 1.
 
 Qb=CA √{square root over ( P 2 −Pc )}  Formula 1:
 
     Where C is a predetermined constant; A is the bleed-off opening area computed in step S 2 ; P 2  is the pump pressure detected in step S 3 ; and Pc is the charge pressure detected in step S 3 . 
     In step S 5 , the pump controller  24  computes the pump rotation speed (N). The pump rotation speed (N) is the rotation speed of both the hydraulic pumps  12 ,  13 . For example, the pump controller  24  computes the pump rotation speed (N) from the rotation speed of the engine  11  as detected by the rotation speed sensor  23 . 
     In step S 6 , the pump controller  24  computes a reduced capacity (ΔD) of the second hydraulic pump  13 . The pump controller  24  computes the reduced capacity (ΔD) of the second hydraulic pump  13  from the following Formula 2.
 
Δ D=Qb/N   Formula 2:
 
     Where Qb is the bleed-off flow rate computed in step S 4 ; and N is the pump rotation speed detected in step S 5 . 
     In step S 7 , the pump controller  24  reduces the capacity directed to the second hydraulic pump  13  by the reduced capacity (ΔD). The pump controller  24  sends a command signal corresponding to the capacity subtracting the reduced capacity (ΔD) from the directed capacity to the second hydraulic pump  13 . 
     In the hydraulic drive system according to the fifth exemplary embodiment the charge flow rate, which is augmented from the charge pump  28 , can be reduced. Thus, the fuel consumption of the drive source may be further improved. For instance, in the first exemplary embodiment when the flow rate into the bleed-off flowpath  37  is 0.2 as illustrated in  FIG. 2 , the flow of 0.2 does not travel through the hydraulic cylinder  14 . Accordingly, the outflow rate from the hydraulic cylinder  14  does not increase from 0.2 to 0.4. Thus, the difference of 0.2 in the flow rate may be provided from the charge pump  28 . In contrast, in the hydraulic drive system according to the fifth exemplary embodiment, the capacity of the second hydraulic pump is reduced by a capacity of 0.2. Therefore, there is no need to supplement the hydraulic fluid flowing in the first pump flowpath  33  from the charge pump  28 . Thus, it is possible to reduce the flow rate of the charge pump  28 . 
     Here ends the description of exemplary embodiments of the present invention. The present invention is not limited to these descriptions but may be modified in various ways and so far as the modifications do not deviate from the spirit of the present invention. 
     The hydraulic drive system is not limited to driving the boom of a hydraulic shovel, and may be used to drive other work implements in other types of work vehicles. For example, the hydraulic drive system may be used to drive the lift arm in a wheel loader. Alternatively the hydraulic drive system may be used to drive the blade of a bulldozer. 
     The drive source is not limited to an engine and may be an electric motor. 
     The control valve  16  may be a hydraulic control valve controlled using a pilot pressure. In this case, an electromagnetic proportional pressure-reducing valve may be arranged between the pump controller  24  and the hydraulic control valve. The pump controller  24  uses a command signal to control the electromagnetic proportional pressure-reducing valve. The electromagnetic proportional pressure-reducing valve supplies the hydraulic control valve with a pilot pressure in accordance with the command signal. The hydraulic control valve may switch controls in accordance with the pilot pressure. The electromagnetic proportional pressure-reducing valve reduces the pressure of the hydraulic fluid discharged from the pilot pump and generates a pilot pressure. The hydraulic fluid discharged from the charge pump  28  may be used instead of the hydraulic fluid discharged from the pilot pump. 
     In the above-mentioned embodiments the bleed-off flowpath  37  is connected to the charge circuit  35 ; however, the bleed-off flowpath  37  may be connected to another element in the circuit such as the hydraulic fluid tank  27 . However, when the bleed-off flowpath  37  is connected to the hydraulic fluid tank  27 , the hydraulic fluid from the bleed-off flowpath  37  cannot be reused in the hydraulic pumps  12 ,  13 . Therefore, the charge pump  28  needs to be enlarged. Consequently, it is preferable that the bleed-off flowpath  37  is connected to the charge circuit  35 . 
     In the above-mentioned exemplary embodiments, the pump controller  24  carries out normal control and low-speed control. However, these controls may be omitted. For instance, the low-speed control may be omitted. 
     In the above-mentioned exemplary embodiments, the predetermined operation amount A6 is a value less than 100%. However, the predetermined operation amount may be 100%. 
     The present invention provides a hydraulic drive system that facilitates adjusting the position of a work implement to a desired height.