Patent Publication Number: US-2012031088-A1

Title: Hydraulic drive system for construction machine

Description:
TECHNICAL FIELD 
     The present invention relates to hydraulic excavators and other construction machines in general and particularly to a hydraulic drive system for a construction machine which allows changes in the operational characteristics of a boom directional control valve. 
     BACKGROUND ART 
     A hydraulic excavator, a construction machine, typically comprises the following components: an undercarriage; an upper swing structure mounted swingably atop the undercarriage; a multi-joint front arm structure including a boom, an arm, and a bucket, the arm structure being attached to the upper swing structure in a vertically movable manner; and multiple hydraulic cylinders designed to actuate the boom, the arm, and the bucket. The hydraulic drive system of the excavator includes the following components: a hydraulic pump; multiple operating devices for controlling the operation (operational direction and speed) of the boom and the like; and multiple directional control valves for controlling the flow (flow direction and flow rate) of pressurized oil routed from the hydraulic pump to a hydraulic boom cylinder and the like in response to the operation of the operating devices. An open-center directional control valve includes a center bypass oil passage(s) and meter-in and meter-out oil passages, and the orifice areas of these oil passages determine the operational characteristics of the directional control valve, thereby also determining the operational performance of components to be actuated. 
     Thus far, a method has been proposed in which either of first and second boom directional control valves, both being open center valves but differing in operational characteristics, is selected (see Patent Document 1). The hydraulic drive system of Patent Document 1 includes the following components: a hydraulic pilot operating device; a solenoid switch valve placed on the pilot line of the operating device; and a manual switch for controlling the solenoid switch valve. When the operator turns the manual switch off, the solenoid switch valve is placed in a first switch position, allowing the operating device to output a spool-control pilot pressure to a pressure receiver of a first boom directional control valve. When, on the other hand, the operator turns the manual switch on, the solenoid switch valve is placed in a second switch position, allowing the operating device to output a spool-control pilot pressure to a pressure receiver of a second boom directional control valve. This allows selection of the operational performance suitable for the work at hand. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent Document 1: JP-2005-220544-A 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     By using the technique of Patent Document 1, it would be possible that the orifice area of a center bypass oil passage of the first boom directional control valve is allowed to become larger than that of a meter-in oil passage of the first boom directional control valve when the spool of the first boom directional control valve is in the maximum position of a boom-lowering spool stroke and that the orifice area of a center bypass oil passage of the second boom directional control valve is allowed to become smaller than that of a meter-in oil passage of the second boom directional control valve (or the center bypass oil passage of the second boom directional control valve is allowed to close completely) when the spool of the second boom directional control valve is in the maximum position of a boom-lowering spool stroke. In that case, the operator can be allowed to turn the manual switch off to select the first boom directional control valve while the bucket is in the air without touching the ground at the time of lowering the boom, whereby the amount of oil supplied to the rod side of the hydraulic boom cylinder can be made relatively small. As a result, the own weight of the front arm structure helps to drive the hydraulic boom cylinder, thereby reducing the power required of the hydraulic pump. When, on the other hand, the bucket reaches the ground to start excavation at the time of lowering the boom, the operator can be allowed to turn the manual switch on to select the second boom directional control valve, so that the amount of oil supplied to the rod side of the hydraulic boom cylinder can be made relatively large. As a result, driving pressure (i.e., high hydraulic pressure) is generated on the rod side of the hydraulic boom cylinder, thereby allowing a powerful boom descending motion. 
     However, excavation requires repetitions of boom ascending and descending motions, forcing the bucket to repeatedly move from the ground into the air and vice versa. Thus, every time the boom is lowered, the operator is required to operate the manual switch right after the bucket has touched the ground (in other words, at the timing when the hydraulic boom cylinder requires driving pressure). This is not only bothersome to the operator but could lead to a decrease in labor efficiency. 
     An object of the present invention is thus to provide a hydraulic drive system for a construction machine which allows automatic changes in the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom-lowering operation. 
     Means for Solving the Problem 
     (1) To achieve the above object, the invention provides a hydraulic drive system for a construction machine, the system comprising: a hydraulic pump; a hydraulic boom cylinder for actuating a boom; an operating device for controlling the operation of the boom; and a boom directional control valve for controlling the flow of pressurized oil routed from the hydraulic pump to the hydraulic boom cylinder in response to the operation of the operating device, the boom directional control valve being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the boom directional control valve to become larger than the orifice area of a meter-in oil passage of the boom directional control valve when a spool of the boom directional control valve is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke. The system further comprises: stroke limit varying means for selecting either the middle position or the maximum stroke position as the limit of a boom-lowering spool stroke of the boom directional control valve; pressure judging means for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and control means for controlling the stroke limit varying means such that the limit of the boom-lowering spool stroke of the boom directional control valve is set to the middle position when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the limit of the boom-lowering spool stroke of the boom directional control valve is set to the maximum stroke position when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value. 
     (2) In the above hydraulic drive system (1), the stroke limit varying means preferably includes: a first pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the boom directional control valve without any change to the spool-control pilot pressure; a second pilot oil passage for reducing, with the use of a pressure-reducing valve, a spool-control pilot pressure generated based on a boom-lowering operation by the operating device and then outputting the reduced pressure to the pressure receiver of the boom directional control valve; and pilot-oil-passage selecting means for selecting either the first pilot oil passage or the second pilot oil passage. Preferably, the control means controls the pilot-oil-passage selecting means such that the second pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value. 
     (3) In the above hydraulic drive system (1), the stroke limit varying means preferably includes: a pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the boom directional control valve; and a variable pressure-reducing valve, located on the pilot oil passage, for limiting the maximum value of the spool-control pilot pressure in a variable manner. Preferably, the control means controls a limit value set for the variable pressure-reducing valve such that the limit value becomes a predetermined first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the limit value becomes a predetermined second limit value larger than the first limit value when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value. 
     (4) To achieve the above object, the invention also provides a hydraulic drive system for a construction machine, the system comprising: a hydraulic pump; a hydraulic boom cylinder for actuating a boom; an operating device for controlling the operation of the boom; and a first boom directional control valve for controlling the flow of pressurized oil routed from the hydraulic pump to the hydraulic boom cylinder in response to the operation of the operating device, the first boom directional control valve being an open center valve, the system having characteristics that allow the orifice area of a center bypass oil passage of the first boom directional control valve to become larger than the orifice area of a meter-in oil passage of the first boom directional control valve when a spool of the first boom directional control valve is in the middle position of a boom-lowering spool stroke and that allow the orifice area of the center bypass oil passage to become smaller than the orifice area of the meter-in oil passage or allow the center bypass oil passage to completely close when the spool is in the maximum stroke position of the boom-lowering spool stroke. The system further comprises: a second boom directional control valve, the second boom directional control valve being an open center valve, the orifice area of a center bypass oil passage of the second boom directional control valve being larger than the orifice area of a meter-in oil passage of the second boom directional control valve when a spool of the second boom directional control valve is in the middle position and the maximum stroke position of a boom-lowering spool stroke; directional-control-valve selecting means for selecting either the first boom directional control valve or the second boom directional control valve and actuating the selected boom directional control valve in response to the operation of the operating device; pressure judging means for detecting or receiving an oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom and for judging whether or not the oil-feeding-side pressure is equal to or greater than a predetermined threshold value; and control means for controlling the directional-control-valve selecting means such that the second boom directional control valve is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first boom directional control valve is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value. 
     (5) In the above hydraulic drive system (4), the directional-control-valve selecting means preferably includes: a first pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the first boom directional control valve; a second pilot oil passage for outputting a spool-control pilot pressure generated based on a boom-lowering operation by the operating device to a pressure receiver of the second boom directional control valve; and pilot-oil-passage selecting means for selecting either the first pilot oil passage or the second pilot oil passage. Preferably, the control means controls the pilot-oil-passage selecting means such that the second pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is less than the threshold value and such that the first pilot oil passage is selected when the oil-feeding-side pressure of the hydraulic boom cylinder upon lowering the boom is equal to or greater than the threshold value. 
     Effect of the Invention 
     In accordance with the invention, it is possible to automatically change the operational characteristics of a boom directional control valve by judging whether or not a hydraulic boom cylinder needs driving pressure at the time of a boom-lowering operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a small-sized hydraulic excavator to which the present invention is applied; 
         FIG. 2  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system for a hydraulic excavator according to Embodiment 1 of the invention; 
         FIG. 3  is a graph illustrating the operational characteristics of a boom directional control valve according to Embodiment 1 of the invention; 
         FIG. 4  is a graph related to Embodiment 1, illustrating an example of temporal changes in the rod-side pressure of a hydraulic boom cylinder and in the spool-control pilot pressure input to the boom directional control valve; 
         FIG. 5  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system for a hydraulic excavator according to a modification of the invention; 
         FIG. 6  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system for a hydraulic excavator according to Embodiment 2 of the invention; 
         FIG. 7  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system for a hydraulic excavator according to a modification of the invention; 
         FIG. 8  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system for a hydraulic excavator according to Embodiment 3 of the invention; and 
         FIG. 9  is a graph illustrating the operational characteristics of a second boom directional control valve according to Embodiment 3 of the invention. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. 
       FIG. 1  is a side view of a small-sized hydraulic excavator to which the present invention is applied. Note that the front side, the rear side, the left side, and the right side as viewed from an operator seated on the cab seat of the hydraulic excavator are hereinafter referred to simply as the front side (the left side of  FIG. 1 ), the rear side (the right side of  FIG. 1 ), the left side (the front side of  FIG. 1 ), and the right side (the back side of  FIG. 1 ), respectively. 
     The hydraulic excavator of  FIG. 1  comprises the following components: an undercarriage  2  with right and left trackbelts  1  (crawlers); an upper swing structure  3  mounted swingably atop the undercarriage  2 ; a swing frame  4  that servers as a base structure for the upper swing structure  3 ; a swing post  5  attached to the front of the swing frame  4  in a horizontally movable manner; a multi-joint front arm structure  6  attached to the swing post  5  in a vertically movable manner; a canopy-attached cab  7  located on the left side of the swing frame  4 ; and multiple covers  8  for covering most of the swing frame  4  except the cab  7 . Installed inside the covers  8  of the upper swing structure  3  are devices such as an engine and the like. 
     The undercarriage  2  includes the following components: a substantially H-shaped track frame  9 ; right and left drive wheels  10  attached rotatably to the right and left rear sides of the track frame  9 ; right and left hydraulic travel motors  11  for driving the right and left drive wheels  10 , respectively; and right and left follower wheels  12  (idler wheels) attached rotatably to the right and left front sides of the track frame  9  and driven by the drive force transmitted from the drive wheels  10  via the trackbelts  1 . 
     Attached to the front side of the track frame  9  is a soil-removal blade  13  which is vertically moved by a hydraulic blade cylinder  14 . Between a central portion of the track frame  9  and the swing frame  4  is a rotary wheel, not illustrated. Radially inside this rotary wheel is a hydraulic swing motor  15  which is designed to rotate the swing frame  4  relative to the track frame  9 . 
     The horizontal movement of the swing post  5  relative to the swing frame  4  is achieved by a vertical pin, not illustrated, and by a hydraulic swing cylinder  16 . The horizontal movement of the swing post  5  causes the front arm structure  6  to swing rightward or leftward. 
     The front arm structure  6  includes the following components: a boom  17  attached movably to the swing post  5 ; an arm  18  attached movably to the distal end of the boom  17 ; and a bucket  19  attached movably to the distal end of the arm  18 . The boom  17 , the arm  18 , and the bucket  19  are actuated by a hydraulic boom cylinder  20 , a hydraulic arm cylinder  21 , and a hydraulic bucket cylinder  22 , respectively. Note that the bucket  19  can be replaced by an optional attachment (e.g., a crusher). 
     The cab  7  is provided with a cab seat  23  on which the operator is seated. Located in front of the seat  23  are right and left travel levers  24  which are operable with hands or feet and designed to actuate the right and left hydraulic travel motors  11 , respectively, so as to move the hydraulic excavator forward or backward. Located to the left of the left travel lever  24  (at the bottom left section of the cab  7 ) is an attachment control pedal, not illustrated, for controlling a hydraulic attachment actuator. Located to the right of the right travel lever  24  (at the bottom right section of the cab  7 ) is a swing control pedal, not illustrated, for actuating the hydraulic swing cylinder  16  to swing rightward or leftward the swing post  5  (that is, the entire front arm structure  6 ). 
     Located on the left side of the seat  23  are the following components: a crosswise-movable swing/arm control lever  25  for actuating the hydraulic swing motor  15  to swing the upper swing structure  3  right or left when the lever  25  is moved right or left and for actuating the hydraulic arm cylinder  21  to cause the arm  18  to perform a dump or crowd operation when the lever  25  is moved forward or backward; and a lock lever  27 , provided as an anti-false operation lever, for blocking the supply of source pressure from a pilot pump  26  (see  FIG. 2 ). Located on the right side of the seat  23  are the following components: a crosswise-movable bucket/boom control lever  28  (see  FIG. 2 ) for actuating the hydraulic bucket cylinder  22  to crowd or dump the bucket  19  when the lever  28  is moved left or right and for actuating the hydraulic boom cylinder  20  to lower or raise the boom  17  when the lever  28  is moved forward or backward; and a blade control lever, not illustrated, for actuating the hydraulic blade cylinder  14  to raise or lower the blade  13 . 
     The above-mentioned right and left trackbelts  1 , upper swing structure  3 , swing post  5 , blade  13 , boom  17 , arm  18 , and bucket  19  are those components driven by a hydraulic drive system installed in the hydraulic excavator. 
       FIG. 2  is a hydraulic circuit diagram of a hydraulic drive system according to Embodiment 1 of the invention, particularly illustrating essential components related to the operation of the boom  17 . 
     The hydraulic drive system of  FIG. 2  includes the following components: a hydraulic pump  29  and the pilot pump  26  both driven by the engine (not illustrated); a hydraulic pilot operating device  30  with the lever  28  used for controlling the operation (operational direction and speed) of the boom  17  when the lever  28  is moved forward or backward and for controlling the operation of the bucket  19  when the lever  28  is moved right or left; and a boom directional control valve  31  (open center valve) for controlling the flow (direction and flow rate) of the pressurized oil routed from the hydraulic pump  29  to the hydraulic boom cylinder  20  in response to the forward or backward movement of the lever  28 . The hydraulic drive system further includes a swing directional control valve  32  (open center valve) for controlling the flow of the pressurized oil routed from the hydraulic pump  29  to the hydraulic swing motor  15  in response to the rightward or leftward movement of the lever  25 ; and a bucket directional control valve  33  (open center valve) for controlling the flow of the pressurized oil routed from the hydraulic pump  29  to the hydraulic bucket cylinder  22  in response to the rightward or leftward movement of the lever  28 . The three directional control valves, or the swing directional control valve  32 , the boom directional control valve  31 , and the bucket directional control valve  33 , are connected in series in this order. 
     The operating device  30  includes a pair of pressure reducing valves  34   a  and  34   b  for generating a spool-control pilot pressure (a second pilot pressure) by reducing a first pilot pressure supplied from the pilot pump  26  based on how much forward or backward the lever  28  has been moved. When the lever  28  is moved backward (toward the left side of  FIG. 2 ), the pressure reducing valve  34   a  generates a spool-control pilot pressure based on how much the lever  28  has been moved and then outputs the pressure to a pressure receiver  36   a  of the boom directional control valve  31  through a pilot line  35 . This allows the spool of the boom directional control valve  31  to move from its neutral position to the lower side of  FIG. 2  (i.e., in the boom-raising direction) in proportion to how much the lever  28  has been moved. In contrast, when the lever  28  is moved forward (toward the right side of  FIG. 2 ), the pressure reducing valve  34   b  generates a spool-control pilot pressure based on how much the lever  28  has been moved and then outputs the pressure to a pressure receiver  36   b  of the boom directional control valve  31  through a pilot circuit  37  (described later). This allows the spool of the boom directional control valve  31  to move from its neutral position to the upper side of  FIG. 2  (i.e., in the boom-lowering direction) in proportion to how much the lever  28  has been moved. 
     The boom directional control valve  31  includes the following components: a center bypass oil passage A; meter-in oil passages B 1  and B 2  (oil-feeding passages); and meter-out oil passages C 1  and C 2  (oil-return passages). These oil passages A, B 1 , B 2 , C 1 , and C 2  can change their orifice areas based on the stroke amount of the spool of the boom directional control valve  31 . When the spool is in its neutral position, the center bypass oil passage A opens fully whereas the meter-in oil passages and the meter-out oil passages close completely. In this case, the pressurized oil supplied from the hydraulic pump  29  is not routed to the hydraulic boom cylinder  20  but returned to a tank. When the spool moves in the boom-raising direction, the meter-in oil passage B 1 , designed to supply the pressurized oil from the hydraulic pump  29  to the bottom side of the hydraulic boom cylinder  20 , and the meter-out oil passage C 1 , designed to return the oil from the rod side of the hydraulic boom cylinder  20  to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage A decreases in orifice area; it closes completely at the maximum stroke position. This allows oil the amount of which is proportional to the stroke amount to be supplied to the bottom side of the hydraulic boom cylinder  20 , causing the hydraulic boom cylinder  20  to expand. As a result, the boom  17  is raised. 
     In contrast, when the spool moves in the boom-lowering direction, the meter-in oil passage B 2 , designed to supply the pressurized oil from the hydraulic pump  29  to the rod side of the hydraulic boom cylinder  20 , and the meter-out oil passage C 2 , designed to return the oil from the bottom side of the hydraulic boom cylinder  20  to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage A decreases in orifice area. This allows oil the amount of which is proportional to the stroke amount to be supplied to the rod side of the hydraulic boom cylinder  20 , causing the hydraulic boom cylinder  20  to contract. As a result, the boom  17  is lowered. Note that Embodiment 1 is designed not to completely close the center bypass oil passage A when the spool is placed in the maximum stroke position in the boom-lowering direction but allows it to partially open. This prevents the descending motion of the boom  17  from becoming much faster than the ascending motion of the boom  17  due to the area difference between the rod side and bottom side of the hydraulic boom cylinder  20 . 
       FIG. 3  illustrates the relationship between the spool stroke amount of the boom directional control valve  31  in the boom-lowering direction and the orifice areas of the center bypass oil passage A, the meter-in oil passage B 2 , and the meter-out oil passage C 2 . In the figure, the horizontal axis represents the stroke amount of the spool in the boom-lowering direction while the vertical axis represents the orifice areas of the center bypass oil passage A, the meter-in oil passage B 2 , and the meter-out oil passage C 2 . 
     As illustrated in  FIG. 3 , when the spool is in the middle position L 1  of the boom-lowering stroke, the orifice area of the center bypass oil passage A is approximately ten times as large as that of the meter-in oil passage B 2 . Thus, the meter-in oil passage B 2  is relatively small in flow rate (i.e., the flow rate of oil supplied to the rod side of the hydraulic boom cylinder  20  is small). In contrast, when the spool is in the maximum stroke position L 2  of the boom-lowering stroke, the orifice area of the center bypass oil passage A is approximately one fifth as large as that of the meter-in oil passage B 2 . Thus, the flow rate of the meter-in oil passage B 2  is relatively large. 
     With reference again to  FIG. 2 , the pilot circuit  37  includes the following components: a pilot oil passage  38   a  for routing the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the pressure receiver  36   b  of the boom directional control valve  31  without any change to the pressure; a pilot oil passage  38   b  for reducing, with the use of a pressure reducing valve  39 , the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  and then routing the reduced pressure to the pressure receiver  36   b  of the boom directional control valve  31 ; and a solenoid switch valve  40  for selecting either of the pilot oil passages  38   a  and  38   b.    
     The hydraulic drive system of  FIG. 2  further includes a pressure sensor  41  and a controller  42 . The pressure sensor  41  detects the rod-side pressure of the hydraulic boom cylinder  20  (i.e., the oil-feeding-side pressure at the time of lowering the boom  17 ). The controller  42  receives a pressure signal from the pressure sensor  41  to control the operation of the solenoid switch valve  40  based on that signal. Specifically, the controller  42  examines whether or not the rod-side pressure of the hydraulic boom cylinder  20  detected by the pressure sensor  41  is equal to or greater than a predetermined threshold value, thereby judging whether or not the hydraulic boom cylinder  20  needs driving pressure (the rod-side high hydraulic pressure) upon lowering the boom  17 . The threshold value is slightly lower than the rod-side load pressure resulting from the start of excavation or the like. 
     When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller  42  does not output a drive signal to the solenoid of the solenoid switch valve  40 , placing the solenoid switch valve  40  in the right-side switch position of  FIG. 2 . This allows the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to be routed through the pilot oil passage  38   b  (i.e., through the pressure reducing valve  39 ) to the pressure receiver  36   b  of the boom directional control valve  31 . As a result, the limit of the boom-lowering spool stroke of the boom directional control valve  31  (i.e., the maximum spool stroke position available when moving the lever  28  furthest forward) is set to the middle position L 1  of  FIG. 3 . 
     When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller  42  outputs the drive signal to the solenoid of the solenoid switch valve  40 , placing the solenoid switch valve  40  in the left-side switch position of  FIG. 2 . This allows the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to be routed through the pilot oil passage  38   a  (i.e., not through the pressure reducing valve  39 ) to the pressure receiver  36   b  of the boom directional control valve  31 . As a result, the limit of the boom-lowering spool stroke of the boom directional control valve  31  is set to the maximum stroke position L 2  of  FIG. 3 . 
     The operation of the hydraulic drive system of Embodiment 1 will now be described with reference to  FIG. 4 .  FIG. 4  is a graph illustrating an example of temporal changes in the rod-side pressure of the hydraulic boom cylinder  20  and in the spool-control pilot pressure input to the pressure receiver  36   b  of the boom directional control valve  31 . 
     After the operator moves the lever  28  furthest forward (at time t 1 ) to lower the boom  14  for excavation or the like, the solenoid switch valve  40  selects the pilot oil passage  38   b  because the rod-side pressure of the hydraulic boom cylinder  20  stays smaller than the threshold value while the bucket  19  is in the air without touching the ground (from time t 1  to time t 2 ). In other words, a limit is placed on the spool-control pilot pressure so that the limit of the boom-lowering spool stroke of the boom direction control valve  31  can be set to the middle position L 1 . This reduces the amount of oil supplied to the rod side of the hydraulic boom cylinder  20 , keeping the rod-side pressure low. As a result, the own weight of the front arm structure  6  helps to drive the hydraulic boom cylinder  20 , thereby reducing the power required of the hydraulic pump  29 . 
     After the bucket  19  touches the ground to start excavation or the like (after time t 2 ), the rod-side pressure of the hydraulic boom cylinder  20  starts to increase. When the rod-side pressure of the hydraulic boom cylinder  20  reaches the threshold value, the controller  42  outputs the drive signal, allowing the solenoid switch valve  40  to select the pilot oil passage  38   a . In other words, no limit is placed on the spool-control pilot pressure, and the limit of the boom-lowering spool stroke of the boom direction control valve  31  is set to the maximum stroke position L 2 . This increases the amount of oil supplied to the rod side of the hydraulic boom cylinder  20 , increasing the rod-side pressure further. As a result, driving pressure is generated on the rod side of the hydraulic boom cylinder  20 , thereby allowing a powerful boom descending motion. 
     As above, Embodiment 1 of the present invention makes it possible to automatically change the operational characteristics of the boom directional control valve  31  by judging whether or not the hydraulic boom cylinder  20  needs driving pressure at the time of lowering the boom  17 . This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1. 
     As stated above, Embodiment 1 is designed such that the judgment of whether or not the hydraulic boom cylinder  20  needs driving pressure at the time of lowering the boom  17  is made through the examination of whether or not the rod-side pressure of the hydraulic boom cylinder  20  is equal to or greater than the predetermined threshold value. On the other hand, the above judgment may instead be made by, for example, examining whether or not the bottom-side pressure of the hydraulic boom cylinder  20  (i.e., the oil-exhaust-side pressure at the time of lowering the boom  17 ) is less than a predetermined threshold value. This method, however, leaves room for improvement as discussed below. The bottom-side pressure (back pressure) of the hydraulic boom cylinder  20  at the time of lowering the boom  17  increases in proportion to the operational speed of the hydraulic boom cylinder  20  (i.e., the speed of a descending motion of the boom  17 ). Assume now that an excavation is judged to have started when the bottom-side pressure of the hydraulic boom cylinder  20  has become less than the threshold value, and the controller  42  then changes the switch position of the solenoid switch valve  40  to set the limit of the boom-lowering spool stroke of the boom directional control valve  31  to the maximum stroke position L 2  so that a powerful boom descending motion can be achieved. Even so, the bottom-side pressure of the hydraulic boom cylinder  20  will exceed the threshold value when the speed of the descending motion of the boom  17  exceeds a given value during subsequent excavations. Thus, it is likely that the controller  42  may change the switch position of the solenoid switch valve  40  to set the limit of the boom-lowering spool stroke of the boom directional control valve  31  to the middle position L 1  even when the hydraulic boom cylinder  20  does need driving pressure. Consequently, a limit is placed on the speed of the descending motion of the boom  17 . In contrast, Embodiment 1 is designed such that the judgment of whether or not the hydraulic boom cylinder  20  needs driving pressure at the time of lowering the boom  17  is made through the examination of whether or not the rod-side pressure of the hydraulic boom cylinder  20  is equal to or greater than the threshold value. Thus, there is no need to limit the speed of the descending motion of the boom  17 . Accordingly, a powerful boom descending motion can be achieved, irrespective of the operational speed of the boom  17 . 
     As stated above, the hydraulic drive system of Embodiment 1 includes the solenoid switch valve  40  for selecting either of the pilot oil passages  38   a  and  38   b , the pressure sensor  41  for detecting the rod-side pressure of the hydraulic boom cylinder  20 , and the controller  42  for outputting the drive signal to the solenoid of the solenoid switch valve  40  when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For instance, as in the modification of  FIG. 5 , the solenoid switch valve  40  can be replaced by a hydraulic pilot switch valve  43 , and the pressure sensor  41  and the controller  42  by a hydraulic pilot control valve  44  for outputting a hydraulic pressure signal to a pressure receiver of the switch valve  43 . The control valve  44  includes a pressure receiver for receiving the rod-side pressure of the hydraulic boom cylinder  20  and a spring for setting a threshold value for the rod-side pressure. When the rod-side pressure is less than the threshold value, the control valve  44  is placed in the upper-side switch position of  FIG. 5 , allowing the pressure receiver of the switch valve  43  to communicate with the tank (that is, the hydraulic pressure received by the pressure receiver of the switch valve  43  becomes the tank pressure, thus becoming smaller). As a result, the switch valve  43  is placed in the right-side switch position of  FIG. 5  to select the pilot oil passage  38   b . When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the control valve  43  is placed in the lower-side switch position of  FIG. 5 , allowing the pressure receiver of the switch valve  43  to communicate with the pilot pump  26  (that is, the hydraulic pressure received by the pressure receiver of the switch valve  43  becomes the pump pressure, thus becoming larger). As a result, the switch valve  43  is placed in the left-side switch position of  FIG. 5  to select the pilot oil passage  38   a . The above modification also leads to the same advantages of Embodiment 1. 
     As another modification (not illustrated), it is also possible for the hydraulic drive system not to have the control valve  44  and instead route the rod-side pressure of the hydraulic boom cylinder  20  to a pressure receiver of a switch valve  43 A and set a threshold value for the rod-side pressure using the spring of the switch valve  43 A. When the rod-side pressure is less than the threshold value, the switch valve  43 A is placed in a first switch position (same as the right-side switch position of the switch valve  43  of  FIG. 5 ), thereby selecting the pilot oil passage  38   b . When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the switch valve  43 A is placed in a second switch position (same as the left-side switch position of the switch valve  43  of  FIG. 5 ), thereby selecting the pilot oil passage  38   a . This modification also leads to the same advantages of Embodiment 1. 
     As also stated above, the hydraulic drive system of Embodiment 1 includes the pilot oil passages  38   a  and  38   b  and the solenoid switch valve  40  for selecting either of the pilot oil passages  38   a  and  38   b  as stoke limit varying means for setting the limit of the boom-lowering spool stroke of the boom directional control valve  31  to either of the middle position L 1  and the maximum stroke position L 2 . The invention is of course not limited to this configuration but can be modified in various forms without departing from the technical scope of the invention. For instance, when the invention is applied to a hydraulic excavator which includes an operating device having an electrical lever (i.e., an operating device for outputting an electric control signal based on how much its lever is moved), a controller may be provided in order to either limit or not limit the electrical control signal output from the operating device. This modification as well leads to the same advantages of Embodiment 1. 
     Embodiment 2 of the present invention will now be described with reference to  FIG. 6 . In this embodiment, the pilot oil passage is provided with a variable pressure-reducing valve. Note that the same reference numerals as used in Embodiment 1 denote identical components, and such components will not be described again. 
       FIG. 6  is a hydraulic circuit diagram illustrating essential components of a hydraulic drive system according to Embodiment 2. 
     The hydraulic drive system of Embodiment 2 includes the following components: a pilot oil passage  45  for routing the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the pressure receiver  36   b  of the boom directional control valve  31 ; and a solenoid-driven variable pressure-reducing valve  46 , placed on the pilot oil passage  45 , for limiting the maximum value of the spool-control pilot pressure in a variable manner. 
     Similar to Embodiment 1, the hydraulic drive system of Embodiment 2 also includes the pressure sensor  41  and the controller  42 . The pressure sensor  41  detects the rod-side pressure of the hydraulic boom cylinder  20 . The controller  42  examines whether or not the rod-side pressure of the hydraulic boom cylinder  20  detected by the pressure sensor  41  is equal to or greater than the predetermined threshold value, thereby judging whether or not the hydraulic boom cylinder  20  needs driving pressure upon lowering the boom  17 . Based on that judgment, the controller  42  controls the variable pressure-reducing valve  46 . 
     When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller  42  does not output a drive signal to the solenoid of the variable pressure-reducing valve  46 . Thus, a limit value for the variable pressure-reducing valve  46  is set to a predetermined first limit value by the spring. This limits the maximum of the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the first limit value. The limited spool-control pilot pressure is then output to the pressure receiver  36  of the boom directional control valve  31 . As a result, the limit of the boom-lowering spool stroke of the boom directional control valve  31  is set to the middle position L 1  of  FIG. 3 . 
     When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller  42  outputs the drive signal to the solenoid of the variable pressure-reducing valve  46 , thereby setting the limit value for the variable pressure-reducing valve  46  to a predetermined second limit value which is larger than the first limit value. This limits the maximum of the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the second limit value. The limited spool-control pilot pressure is then output to the pressure receiver  36   b  of the boom directional control valve  31  (normally, the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  is output to the pressure receiver  36   b  without any change to the pressure). As a result, the limit of the boom-lowering spool stroke of the boom directional control valve  31  is set to the maximum stroke position L 2  of  FIG. 3 . 
     Similar to Embodiment 1, Embodiment 2 of the invention also makes it possible to automatically change the operational characteristics of the boom directional control valve  31  by judging whether or not the hydraulic boom cylinder  20  needs driving pressure at the time of lowering the boom  17 . This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1. 
     As stated above, the hydraulic drive system of Embodiment 2 includes the solenoid-driven variable pressure-reducing valve  46  placed on the pilot oil passage  45 ; the pressure sensor  41  for detecting the rod-side pressure of the hydraulic boom cylinder  20 ; and the controller  42  for outputting the drive signal to the solenoid of the variable pressure-reducing valve  46  when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For example, as in the modification of  FIG. 7 , the solenoid-driven variable pressure-reducing valve  46  can be replaced by a hydraulic pilot variable pressure-reducing valve  47 , and the pressure sensor  41  and the controller  42  by a hydraulic pilot control valve  44  for outputting a hydraulic pressure signal to a pressure receiver of the variable pressure-reducing valve  47 . The control valve  44  includes a pressure receiver for receiving the rod-side pressure of the hydraulic boom cylinder  20  and a spring for setting a threshold value for the rod-side pressure. When the rod-side pressure is less than the threshold value, the control valve  44  is placed in the upper-side switch position of  FIG. 7 , allowing the pressure receiver of the variable pressure-reducing valve  47  to communicate with the tank (that is, the hydraulic pressure received by the pressure receiver of the variable pressure-reducing valve  47  becomes the tank pressure, thus becoming smaller). As a result, the variable pressure-reducing valve  47  limits the maximum of the spool-control pilot pressure to the first limit value. When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the control valve  43  is placed in the lower-side switch position of  FIG. 7 , allowing the pressure receiver of the variable pressure-reducing valve  47  to communicate with the pilot pump  26  (that is, the hydraulic pressure received by the pressure receiver of the variable pressure-reducing valve  47  becomes the pump pressure, thus becoming larger). As a result, the variable pressure-reducing valve  47  limits the maximum of the spool-control pilot pressure to the second limit value. The above modification also leads to the same advantages of Embodiment 2. 
     Embodiment 3 of the present invention will now be described with reference to  FIGS. 8 and 9 . The hydraulic drive system of Embodiment 3 includes first and second boom directional control valves which differ in operational characteristics and is designed to select either of the two directional control valves. Note that the same reference numerals as used in Embodiments 1 and 2 denote identical components, and such components will not be described again. 
       FIG. 8  is a hydraulic circuit diagram illustrating essential components of the hydraulic drive system of Embodiment 3. 
     The hydraulic drive system of Embodiment 3 includes the boom directional control valve  31  (open center valve) and a boom directional control valve  48  (open center valve) that differs from the boom directional control valve  31  in operational characteristics. The swing directional control valve  32 , the boom directional control valves  31  and  48 , and the bucket directional control valve  33  are connected in series in this order. 
     The boom directional control valve  48  includes the following components: a center bypass oil passage D; meter-in oil passages E 1  and E 2  (oil-feeding passages); and meter-out oil passages F 1  and F 2  (oil-return passages). These oil passages D, E 1 , E 2 , F 1 , and F 2  can change their orifice areas based on the stroke amount of the spool of the boom directional control valve  48 . When the spool is in its neutral position, the center bypass oil passage D opens fully whereas the meter-in oil passages and the meter-out oil passages close completely. When the spool moves in the downward direction of  FIG. 8  (in the boom-raising direction), the meter-in oil passage E 1 , designed to supply the pressurized oil from the hydraulic pump  29  to the bottom side of the hydraulic boom cylinder  20 , and the meter-out oil passage F 1 , designed to return the oil from the rod side of the hydraulic boom cylinder  20  to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage D decreases in orifice area; it closes completely at the maximum stroke position. 
     In contrast, when the spool moves in the upward direction of  FIG. 8  (in the boom-lowering direction), the meter-in oil passage E 2 , designed to supply the pressurized oil from the hydraulic pump  29  to the rod side of the hydraulic boom cylinder  20 , and the meter-out oil passage F 2 , designed to return the oil from the bottom side of the hydraulic boom cylinder  20  to the tank, increase in orifice area in response to the stroke amount of the spool. At the same time, the center bypass oil passage A decreases in orifice area. In this case, the orifice area of the center bypass oil passage D 1  is, as illustrated in  FIG. 9 , approximately ten times as large as that of the meter-in oil passage E 2  when the spool is in the middle position L 3  of the boom-lowering spool stroke and also when it is in the maximum stroke position L 4 . Thus, the meter-in oil passage E 2  is relatively small in flow rate. 
     When the lever  28  is moved backward (toward the left side of  FIG. 8 ), the pressure reducing valve  34   a  generates a spool-control pilot pressure based on how much the lever  28  has been moved and then outputs the pressure to a pressure receiver  49   a  of the boom directional control valve  48  through the pilot line  35 . This allows the spool of the boom directional control valve  48  to move from its neutral position to the lower side of  FIG. 8  (i.e., in the boom-raising direction) in proportion to how much the lever  28  has been moved. In contrast, when the lever  28  is moved forward (toward the right side of  FIG. 8 ), the pressure reducing valve  34   b  generates a spool-control pilot pressure based on how much the lever  28  has been moved and then outputs the pressure to a pilot circuit  50 . 
     The pilot circuit  50  includes the following components: a pilot oil passage  51   a  for routing the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the pressure receiver  36   b  of the boom directional control valve  31 ; a pilot oil passage  51   b  for routing the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to the pressure receiver  49   b  of the boom directional control valve  48 ; and a solenoid switch valve  52  for selecting either of the pilot oil passages  51   a  and  51   b.    
     As in Embodiments 1 and 2, the hydraulic drive system of Embodiment 3 also includes the pressure sensor  41  and the controller  42 . The pressure sensor  41  detects the rod-side pressure of the hydraulic boom cylinder  20 . The controller  42  examines whether or not the rod-side pressure of the hydraulic boom cylinder  20  detected by the pressure sensor  41  is equal to or greater than the predetermined threshold value, thereby judging whether or not the hydraulic boom cylinder  20  needs driving pressure upon lowering the boom  17 . Based on that judgment, the controller  42  controls the switch valve  52 . 
     When the rod-side pressure is less than the threshold value (i.e., when driving pressure is not necessary), the controller  42  does not output a drive signal to the solenoid of the solenoid switch valve  52 , placing the solenoid switch valve  52  in the right-side switch position of  FIG. 8 . This allows the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to be routed through the pilot oil passage  51   b  to the pressure receiver  49   b  of the boom directional control valve  48 . As a result, the spool of the boom directional control valve  48  moves from its neutral position to the upper-side position of  FIG. 8  (in the boom-lowering direction) in proportion to how much the lever  28  has been moved. Even if, in this case, the limit of the boom-lowering spool stroke of the boom directional control valve  48  is set to the maximum stroke position L 4  by the operator moving the lever  28  furthest forward, the amount of oil supplied to the rod side of the hydraulic boom cylinder  20  becomes relatively small, keeping the rod-side pressure low. Accordingly, the own weight of the front arm structure  6  helps to drive the hydraulic boom cylinder  20 , thereby reducing the power required of the hydraulic pump  29 . 
     When, on the other hand, the rod-side pressure is equal to or greater than the threshold value (i.e., when driving pressure is necessary), the controller  42  outputs the drive signal to the solenoid of the solenoid switch valve  52 , placing the solenoid switch valve  52  in the left-side switch position of  FIG. 8 . This allows the spool-control pilot pressure generated by the pressure reducing valve  34   b  of the operating device  30  to be routed through the pilot oil passage  51   a  to the pressure receiver  36   b  of the boom directional control valve  31 . As a result, the spool of the boom directional control valve  31  moves from its neutral position to the upper-side position of  FIG. 8  (in the boom-lowering direction) in proportion to how much the lever  28  has been moved. When, in this case, the limit of the boom-lowering spool stroke of the boom directional control valve  31  is set to the maximum stroke position L 2  by the operator moving the lever  28  furthest forward, the amount of oil supplied to the rod side of the hydraulic boom cylinder  20  becomes relatively large, thus increasing the rod-side pressure. Accordingly, driving pressure is generated on the rod side of the hydraulic boom cylinder  20 , thereby allowing a powerful boom descending motion. 
     Similar to Embodiments 1 and 2, Embodiment 3 of the invention also makes it possible to automatically change the operational characteristics of the boom directional control valves by judging whether or not the hydraulic boom cylinder  20  needs driving pressure at the time of lowering the boom  17 . This is not bothersome to the operator and leads to high labor efficiency, compared with when the operator has to do the above with the use of a manual switch as in Patent Document 1. 
     As stated above, the hydraulic drive system of Embodiment 3 includes the solenoid switch valve  52  for selecting either of the pilot oil passages  51   a  and  51   b , the pressure sensor  41  for detecting the rod-side pressure of the hydraulic boom cylinder  20 , and the controller  42  for outputting the drive signal to the solenoid of the solenoid switch valve  52  when the rod-side pressure is equal to or greater than the threshold value. Note, however, that the invention is not limited to such an electrical configuration. For instance, the solenoid switch valve  52  can be replaced by a hydraulic pilot switch valve (not illustrated), and the pressure sensor  41  and the controller  42  by a hydraulic pilot control valve (not illustrated) for outputting a hydraulic pressure signal to a pressure receiver of that switch valve. The control valve can include a pressure receiver for receiving the rod-side pressure of the hydraulic boom cylinder  20  and a spring for setting a threshold value for the rod-side pressure. When the rod-side pressure is less than the threshold value, the control valve is placed in a first switch position, allowing the pressure receiver of the switch valve to communicate with the tank (that is, the hydraulic pressure received by the pressure receiver of the switch valve becomes the tank pressure, thus becoming smaller). As a result, the switch valve is placed in a first switch position to select the pilot oil passage  51   b . When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the control valve is placed in a second switch position, allowing the pressure receiver of the switch valve to communicate with the pilot pump  26  (that is, the hydraulic pressure received by the pressure receiver of the switch valve becomes the pump pressure, thus becoming larger). As a result, the switch valve is placed in a second switch position to select the pilot oil passage  51   a . The above modification also leads to the same advantages of Embodiment 3. 
     As another modification (not illustrated), it is also possible for the hydraulic drive system not to have the control valve and instead route the rod-side pressure of the hydraulic boom cylinder  20  to a pressure receiver of a switch valve and set a threshold value for the rod-side pressure using the spring of the switch valve. When the rod-side pressure is less than the threshold value, the switch valve is placed in a first switch position, thereby selecting the pilot oil passage  51   b . When, on the other hand, the rod-side pressure is equal to or greater than the threshold value, the switch valve is placed in a second switch position, thereby selecting the pilot oil passage  51   a . This modification also leads to the same advantages of Embodiment 3. 
     As also stated above, the hydraulic drive system of Embodiment 3 includes the pilot oil passages  51   a  and  51   b  and the solenoid switch valve  52  for selecting either of the pilot oil passages  51   a  and  51   b  as directional-control-valve selecting means for selecting either of the boom directional control valves  31  and  48 . The invention is of course not limited to this configuration but can be modified in various forms without departing from the technical scope of the invention. For instance, when the invention is applied to a hydraulic excavator which includes an operating device having an electrical lever, a controller may be provided in order to select the destinations of the electrical control signal. This modification as well leads to the same advantages of Embodiment 3. 
     We have also stated that, in all the foregoing embodiments 1 to 3 and modifications, the center bypass oil passage of the boom directional control valve  31  is allowed to completely close when its spool is in the maximum position of a boom-raising stroke and to partially open when the spool is in the maximum position of a boom-lowering stroke. The invention is not limited to the above, however. The center bypass oil passage may instead close completely also when the spool is in the maximum position of a boom-lowering stroke. This also leads to the same advantages of the invention. 
     It should also be noted that the invention is not limited to the above-described examples in which the invention is applied to a small-sized hydraulic excavator. 
     The invention is of course applicable to medium- or large-sized hydraulic excavators and to other construction machines as well. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           17 : Boom 
           20 : Hydraulic boom cylinder 
           28 : Hydraulic pump 
           30 : Operating device 
           31 : Boom directional control valve 
           38   a : Pilot oil passage (stroke limit varying means) 
           38   b : Pilot oil passage (stroke limit varying means) 
           39 : Pressure reducing valve (stroke limit varying means) 
           40 : Solenoid switch valve (pilot-oil-passage selecting means, stroke limit varying means) 
           41 : Pressure sensor (pressure judging means) 
           42 : Controller (pressure judging means, control means) 
           43 : Hydraulic pilot switch valve (pilot-oil-passage selecting means, stroke limit varying means) 
           43 A: Hydraulic pilot switch valve (pilot-oil-passage selecting means, stroke limit varying means, pressure judging means, control means) 
           44 : Control valve (pressure judging means, control means) 
           45 : Pilot oil passage (stroke limit varying means) 
           46 : Solenoid-driven variable pressure-reducing valve (stroke limit varying means) 
           47 : Hydraulic pilot variable pressure-reducing valve (stroke limit varying means) 
           48 : Boom directional control valve 
           51   a : Pilot oil passage (directional-control-valve selecting means) 
           51   b : Pilot oil passage (directional-control-valve selecting means) 
           52 : Solenoid switch valve (pilot-oil-passage selecting means, directional-control-valve selecting means)