Patent Publication Number: US-7895833-B2

Title: Hydraulic drive apparatus

Description:
TECHNICAL FIELD 
     This invention relates to a hydraulic drive system mounted on a construction machine such as a hydraulic excavator to permit a combined operation of plural hydraulic cylinders. 
     BACKGROUND ART 
     There have conventionally been proposed hydraulic control systems, each of which is mounted on a hydraulic excavator and has a main hydraulic pump and a boom cylinder and arm cylinder as a first hydraulic cylinder and second hydraulic cylinder driven by pressure oil delivered from the main hydraulic pump. This conventional art is provided with a directional control valve for a boom as a first directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the boom cylinder, a directional control valve for an arm as a second directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the arm cylinder, a boom control device as a first control device for selectively controlling the directional control valve for the boom, and an arm control device as a second control device for performing a switching control of the directional control valve for the arm, and is also provided with a communication control means for communicating a rod chamber of the boom cylinder and a bottom chamber of the arm cylinder with each other when a bottom pressure of the arm cylinder has increased to a high pressure equal to or higher than a predetermined pressure. (see, for example, JP-A-2002-339907). 
     DISCLOSURE OF THE INVENTION 
     When the bottom pressure of an arm cylinder has become high as a result of digging work or the like of earth in a combined boom-arm operation performed by feeding pressure oil to both of a bottom chamber of a boom cylinder and a bottom chamber of the arm cylinder, the above-described conventional art can effectively use pressure oil in a rod chamber of the boom cylinder, which was conventionally drained, for an acceleration of the arm cylinder in its extending direction, and can realize an improvement in the efficiency of the work. 
     In some work, however, the bottom pressure of the arm cylinder may not become high upon performing a combined boom-arm operation as in work involving a crowding operation of a bucket in the air. Even in such work, it is desired to realize an acceleration of the arm cylinder, that is, a second hydraulic cylinder. 
     With a view to meeting the above-describe desire, the present invention has as an object the provision of a hydraulic drive system which, in a combined operation to be performed by feeding pressure oil to both of bottom combers of a first hydraulic cylinder and second hydraulic cylinder, can effectively use pressure oil in a rod chamber of the first hydraulic cylinder, which was conventionally drained into a reservoir, irrespective of the level of a bottom pressure in the second hydraulic cylinder. 
     To achieve the above-described object, the present invention is characterized in that, in a hydraulic drive system provided with a main hydraulic pump, a first hydraulic cylinder and second hydraulic cylinder driven by pressure oil delivered from the main hydraulic pump, a first directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the first hydraulic cylinder, a second directional control valve for controlling a flow of pressure oil to be fed from the main hydraulic pump to the second hydraulic cylinder, a first control device for selectively controlling the first directional control valve, and a second control device for selectively controlling the second directional control valve, the hydraulic drive system is provided with a communication control means for communicating a rod chamber of the first hydraulic cylinder and a bottom chamber of the second hydraulic cylinder with each other when a stroke of the second control device has increased to at least a predetermined amount. 
     According to the present invention constructed as described above, when the stroke of the second control device has increased to at least the predetermined amount upon performing a combined operation of these first hydraulic cylinder and second hydraulic cylinder by controlling the first control device and second control device to switch the first directional control valve and second directional control valve and feeding pressure oil from the main hydraulic pump to the bottom chambers of the first hydraulic cylinder and second hydraulic cylinder via the first directional control valve and second directional control valve, respectively, the communication control means is actuated to feed the pressure oil in a rod chamber of the first hydraulic cylinder to the bottom chamber of the second hydraulic cylinder. Described specifically, the pressure oil delivered from the main hydraulic pump and fed via the second directional control valve and the pressure oil fed from the rod chamber of the first hydraulic cylinder are combined and fed to the bottom chamber of the second hydraulic cylinder, and as a result, an acceleration can be achieved in the extending direction of the second hydraulic cylinder irrespective of the level of pressure oil in the bottom chamber of the second hydraulic cylinder. As described above, the pressure oil in the rod chamber of the first hydraulic cylinder, which was conventionally drained into the reservoir, can be effectively used for the selective acceleration of the second hydraulic cylinder. 
     The present invention can also be characterized in that in the above-described invention, the communication control means may comprise a communication line capable of communicating the rod chamber of the first hydraulic cylinder and the bottom chamber of the second hydraulic cylinder with each other, a check valve arranged on the communication line to prevent a flow of pressure oil from the bottom chamber of the second hydraulic cylinder toward the rod chamber of the first hydraulic cylinder, and a selector valve for feeding pressure oil in the rod chamber of the first hydraulic cylinder to the bottom chamber of the second hydraulic cylinder via the communication line. 
     According to the present invention constructed as described immediately above, when the stroke of the second control device has increased to at least the predetermined amount upon performing a combined operation of these first hydraulic cylinder and second hydraulic cylinder by feeding pressure oil from the main hydraulic pump to the bottom chambers of the first hydraulic cylinder and second hydraulic cylinder, respectively, the selector valve is switched to maintain the communication line in a communicating state, and as a result, the pressure oil in the rod chamber of the first hydraulic cylinder is fed to the bottom chamber of the second hydraulic cylinder via the communication line and check valve. Described specifically, the pressure oil fed to the bottom chamber of the second hydraulic cylinder via the second directional control valve and the pressure oil fed from the rod chamber of the first hydraulic cylinder are combined and fed to the bottom chamber of the second hydraulic cylinder, and as a result, an acceleration can be achieved in the extending direction of the second hydraulic cylinder. 
     When the stroke of the second control device is so small that it does not reach the predetermined amount upon performing a combined operation of the first hydraulic cylinder and second hydraulic cylinder as mentioned above, the selector valve is held to communicate the communication line with the reservoir, and a result, the pressure oil in the rod chamber of the first hydraulic cylinder is drained into the reservoir. In this case, to the bottom chamber of the second hydraulic cylinder, pressure oil is fed only via the second directional control valve so that no acceleration is achieved in the extending direction of the second hydraulic cylinder. 
     The present invention can also be characterized in that in the above-described invention, the selector valve may include a variable restrictor. 
     According to the present invention constructed as described immediately above, the opening of the variable restrictor included in the selector valve changes depending upon the stroke of the second control device. Described specifically, when the stroke of the second control device is relatively small although it is equal to or greater than the predetermined amount, the opening of the variable restrictor in the selector valve becomes smaller so that the flow rate of pressure oil from the rod chamber of the first hydraulic cylinder, which is to be fed to the communication line via the variable restrictor, is reduced. When the stroke of the second control device is equal to or greater than the predetermined amount and is relatively large, on the other hand, the opening of the variable restrictor in the selector valve becomes large so that the flow rate of pressure oil from the rod chamber of the first hydraulic cylinder, which is to be fed to the communication line via the variable restrictor, can be increased. 
     The present invention can also be characterized that the above-described invention may further comprise a branch line connected at an end thereof to a main line, which connects the first directional control valve and the rod chamber of the first hydraulic cylinder with each other, and at an opposite end thereof to the selector valve. 
     According to the present invention constructed as described immediately above, when the stroke of the second control device has increased to at least the predetermined amount upon performing a combined operation of these first hydraulic cylinder and second hydraulic cylinder, the pressure oil in the rod chamber of the first hydraulic cylinder is fed to the bottom chamber of the second hydraulic cylinder from the communication line without going through the first directional control valve. It is, therefore, possible to reduce a pressure loss compared with feeding the pressure oil through the first directional control valve insofar as the diameter of the branch line is set sufficiently large. 
     The present invention can also be characterized in that the above-described invention may further comprise a stroke detector for detecting a stroke of the second control device and outputting an electrical signal, and a controller for outputting, responsive to the signal outputted from the stroke detector, a control signal to selectively control the selector valve. 
     According to the present invention constructed as described immediately above, when it is detected by the stroke detector that the stroke of the second control device has increased to at least the predetermined amount, the electrical signal outputted from the stroke detector is inputted to the controller. As a result, a control signal for switching the selector valve is outputted from the controller so that the selector valve is switched to maintain the communication line in the communicating state. The pressure oil in the rod chamber of the first hydraulic cylinder is, therefore, fed to the bottom chamber of the second hydraulic cylinder via the communication line and check valve. 
     The present invention can also be characterized in that in the above-described invention, the controller may include a function generator for outputting a value which becomes gradually greater as the stroke of the second control device increases. 
     According to the present invention constructed as described immediately above, a value which becomes gradually greater as the stroke of the second control device increases is determined at the function generator, and a control signal corresponding to the thus-determined value is outputted from the controller to control the amount of switching of the selector valve. It is, therefore, possible to control the speed of the second hydraulic cylinder which is in a state accelerated corresponding to the stroke of the second control device. 
     The present invention can also be characterized in that in the above-described invention, the selector valve may be a pilot-controlled selector valve, and the hydraulic drive system may be provided with an electric-hydraulic converter for outputting a control pressure corresponding to the control signal outputted from the controller and a control line communicating the electric-hydraulic converter and the pilot-controlled selector valve with each other. 
     According to the present invention constructed as described immediately above, when a control signal outputted from the controller is fed to the electric-hydraulic converter, a pilot pressure corresponding to the value of the control signal is applied from the electric-hydraulic converter to the control chamber of the pilot-controlled selector valve via the control line so that the amount of switching of the selector valve is controlled depending upon the level of the pilot pressure. 
     The present invention can also be characterized in that in the above-described invention, the first hydraulic cylinder and second hydraulic cylinder may comprise a boom cylinder and arm cylinder, respectively, the first directional control valve and second directional control valve may comprise a center-bypass-type, directional control valve for a boom and directional control valve for an arm, respectively, and the first control device and second control device may comprise a boom control device and arm control device, respectively. 
     According to the present invention constructed as described immediately above, when the stroke of the arm control device has increased to at least the predetermined amount upon performing a combined operation of the boom cylinder and arm cylinder by controlling the boom control device and arm control device to switch the directional control valve for the boom and the directional control valve for the arm and feeding pressure oil from the main hydraulic pump to the bottom chambers of these boom cylinder and arm cylinder via the directional control valve for the boom and the directional control valve for the arm, that is, upon performing a combined operation of boom raising and arm crowding, the communication control means is actuated such that the pressure oil in the rod chamber of the boom cylinder is fed to the bottom chamber of the arm cylinder. Described specifically, the pressure oil delivered from the main hydraulic pump and fed via the directional control valve for the arm and the pressure oil fed from the rod chamber of the boom cylinder are combined and fed to the bottom chamber of the arm cylinder, and as a result, an acceleration in the extending direction of the arm cylinder, that is, an acceleration in arm crowding can be realized. 
     According to the invention constructed as described immediately above, upon performing a combined operation by feeding oil pressure to the bottom chambers of the first hydraulic cylinder and second hydraulic cylinder, respectively, the pressure oil in the rod chamber of the first hydraulic cylinder, which was conventionally drained into the reservoir, can be effectively used depending upon the stroke of the second control device, which controls the second hydraulic cylinder, irrespective of the level of the bottom pressure of the second hydraulic cylinder, and compared with the conventional art, it is thus possible to perform more work with effective use of pressure oil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a hydraulic circuit diagram showing a first embodiment of the hydraulic drive system according to the present invention. 
         FIG. 2  is a characteristic diagram illustrating a relation between an arm pilot pressure and a flow rate through a communication line, which is available in the first embodiment shown in  FIG. 1 . 
         FIG. 3  is a hydraulic circuit diagram showing a second embodiment of the present invention. 
         FIG. 4  is a hydraulic circuit diagram showing a third embodiment of the present invention. 
         FIG. 5  is a diagram illustrating the construction of an essential part of a controller which the third embodiment shown in  FIG. 4  is provided with. 
     
    
    
     BEST MODES FOR CARRYING OUT THE INVENTION 
     Best modes for carrying out the hydraulic drive system according to the present invention will hereinafter be described based on the drawings. 
       FIG. 1  is a circuit diagram showing the first embodiment of the hydraulic drive system according to the present invention. 
     Not only the first embodiment shown in  FIG. 1  but also the second and third embodiments to be described subsequently herein are arranged on construction machines, for example, on hydraulic excavators, and each comprises a hydraulic drive system of the center bypass type for driving, for example, a boom cylinder  6  as a first hydraulic cylinder and an arm cylinder  7  as a second hydraulic cylinder. The boom cylinder  6  is provided with a bottom chamber  6   a  and a rod chamber  6   b,  and the arm cylinder  7  is likewise provided with a bottom chamber  7   a  and a rod chamber  7   b.    
     The first embodiment is also provided with an engine  20 , a main hydraulic pump  21  and pilot pump  22  driven by the engine  20 , a first directional control valve for controlling a flow of pressure oil to be fed to the boom cylinder  6 , i.e., a center-bypass-type directional control valve  23  for the boom, a second directional control valve for controlling a flow of pressure oil to be fed to the arm cylinder  7 , i.e., a center-bypass-type directional control valve  24  for the arm. Also provided are a first control device for selectively controlling the directional control valve  23  for the boom, i.e., a boom control device  25  and a second control device for selectively controlling the directional control valve  24  for the arm, i.e., an arm control device  26 . 
     Lines  27 , 28  are connected to a delivery line of the main hydraulic pump  21 , the directional control valve  24  for the arm is arranged on the line  27 , and the directional control valve  23  for the boom is arranged on the line  28 . 
     The directional control valve  23  for the boom and the bottom chamber  6   a  of the boom cylinder  6  are connected via a main line  29   a , while the directional control valve  23  for the boom and the rod chamber  6   b  of the boom cylinder  6  are connected via a main line  29   b . The directional control valve  24  for the arm and the bottom chamber  7   a  of the arm cylinder  7  are connected via a main line  30   a , while the directional control valve  24  for the arm and the rod chamber  7   b  of the arm cylinder  7  are connected via a main line  30   b.    
     The boom control device  25  and arm control device  26  are composed, for example, of pilot control devices which produce pilot pressures, and are connected to the pilot pump  22 . Further, the boom control device  25  is connected to control chambers of the directional control valve  23  for the boom via pilot lines  25   a ,  25   b , respectively, while the arm control device  26  is connected to control chambers of the directional control valve  24  for the arm via pilot lines  26   a ,  26   b , respectively. 
     This first embodiment is provided with a communication control means for communicating the rod chamber  6   b  of the boom cylinder  6 , which makes up the first hydraulic cylinder, and the bottom chamber  7   a  of the arm cylinder  7 , which makes up the second hydraulic cylinder, with each other especially when the stroke of the arm control device as the second control device has increased to a predetermined amount S or greater. 
     As shown by way of example in  FIG. 1 , this communication control means comprises a communication line  40  capable of communicating the rod chamber  6   b  of the boom cylinder  6  and the bottom chamber  7   a  of the arm cylinder  7  with each other, a check valve  41  arranged on the communication line  40  to prevent a flow of pressure oil from the bottom chamber  7   a  of the arm cylinder  7  toward the rod chamber  6   b  of the boom cylinder  6 , and a selector valve  52  for feeding pressure fluid in the rod chamber  6   b  of the boom cylinder  6  to the bottom chamber  7   a  of the arm cylinder  7  via the communication line  40  when the stroke of the arm control device  26  has increased to the predetermined amount S or greater. This selector valve  52  comprises a pilot control device which is switched by an arm pilot pressure guided via a control line  52   a  connected to the pilot line  26   a.    
     Also arranged are a line  46  connected at an end thereof to the part of the communication line  40  located on an upstream side of the check valve  41  and at an opposite end thereof to a reservoir  43 , and a pilot-controlled check valve  47  arranged on the line  46  such that responsive to a predetermined control of the boom control device as the first control device, for example, an operation to feed pressure oil to the pilot line  25   b  to perform boom lowering, the line  46  is opened. The above-described pilot line  25   b  and pilot-controlled check valve  47  are connected together by a control line  48 . 
     In the first embodiment constructed as described above, combined operations of the boom cylinder  6  and the arm cylinder  7  are performed as will be described hereinafter. 
     [Combined Operation of Boom Raising and Arm Crowding] 
     When the boom control device  25  is controlled to feed a pilot pressure to the pilot line  25   a  such that the directional control valve  23  for the boom is switched into the left position as shown in  FIG. 1  and further, the arm control device  26  is controlled to feed a pilot pressure to the pilot line  26   a  such that the directional control valve  24  for the arm is switched into the left position as shown in  FIG. 1 , pressure oil delivered from the main hydraulic pump  21  is fed to the bottom chamber  6   a  of the boom cylinder  6  via the line  28 , the directional control valve  23  for the boom and the main line  29   a , and further, the pressure oil delivered from the main hydraulic pump  21  is also fed to the bottom chamber  7   a  of the arm cylinder  7  via the line  27 , the directional control valve  24  for the arm and the main line  30   a . As a result, the boom cylinder  6  and arm cylinder  7  are both operated in extending directions to perform a combined operation of boom raising and arm crowding. 
     During the above-described combined operation, the pilot line  25   b  of the boom operating system is not fed with the pilot pressure, and remains under the same pressure as the reservoir pressure. Therefore, the control line  48  takes the reservoir pressure so that the pilot-controlled check valve  47  remains in a closed position to prevent communication between the communication line  40  and the reservoir  43  via the line  46 . 
     In a state that the stroke of the arm control device  26  is smaller than the predetermined amount S, the force of an arm pilot pressure corresponding to the is smaller than the spring force of the selector valve  52 , and therefore, this selector valve  52  is held in the right position shown in  FIG. 1 . In this state, the rod chamber  6   b  of the boom cylinder  6  is in communication with the reservoir  43  via the main line  29   b , the directional control valve  23  for the boom, the reservoir line  42 , and the selector valve  52 . During an extending operation of the boom cylinder  6 , the pressure oil in the rod chamber  6   b  of the boom cylinder  6  is, therefore, returned to the reservoir  43 , and the pressure oil in the rod chamber  6   b  is not fed to the communication line  40 . 
     When the stroke of the arm control device  26  increases to the predetermined amount S or greater from such a state as described above, the force produced by an arm pilot pressure guided corresponding to the stroke via the control line  52   a  becomes greater than the spring force of the selector valve  52  so that the selector valve  52  tends to be switched toward the left position in  FIG. 1 . When this state is established, the reservoir line  42  begins to be closed by the selector valve  52  so that a predetermined portion of the pressure oil, which has been guided from the rod chamber  6   b  of the boom cylinder  6  into the main line  29   b , the directional control valve  23  for the boom and the reservoir line  42 , is fed to the communication line  40  via the check valve  41 . As illustrated in  FIG. 2 , the flow rate at which the predetermined portion of the pressure oil is fed at this time increases with the arm pilot pressure which corresponds to the stroke of the arm control device  26 . It is to be noted that in  FIG. 2 , “S” indicates the above-mentioned predetermined stroke and “F” indicates the stroke at the time of a full stroke. The pressure oil fed to the communication line  40  is fed to the bottom chamber  7   a  of the arm cylinder  7  via the main line  30   a . Described specifically, the pressure oil delivered from the main hydraulic pump  21  and fed via the directional control valve  24  for the arm and the pressure oil fed from the rod chamber  6   b  of the boom cylinder  6  are combined and fed to the bottom chamber  7   a  of the arm cylinder  7 . As a result, an acceleration of the arm cylinder  6  in the extending direction can be realized. In other words, the operating speed of arm crowding can be rendered faster. 
     [Combined Operation of Boom Lowering and Arm Crowding] 
     When the boom control device  25  is controlled to feed a pilot pressure to the pilot line  25   b  such that the directional control valve  23  for the boom is switched into the right position shown in  FIG. 1  and further, the arm control device  26  is controlled to feed a pilot pressure to the pilot line  26   a  such that the directional control valve  24  for the arm is switched into the left position, pressure oil delivered from the main hydraulic pump  21  is fed to the rod chamber  6   b  of the boom cylinder  6  via the line  28 , the directional control valve  23  for the boom and the main line  29   b , and as mentioned above, the pressure oil delivered from the main hydraulic pump  21  is also fed to the bottom chamber  7   a  of the arm cylinder  7  via the line  27 , the directional control valve  24  for the arm and the main line  30   a . As a result, the boom cylinder  6  is operated in a retracting direction and the arm cylinder  7  is operated in the extending direction so that a combined operation of boom lowering and arm crowding is performed. 
     As the pilot pressure is being fed to the pilot line  25   b  in the boom operating system during such a combined operation, a control pressure is guided into the control line  48  so that the pilot-controlled check valve  47  is operated to open the line  46 . As a result, the part of the communication line  40  on the upstream side of the selector valve  52  is brought into communication with the reservoir  43 . 
     When the stroke of the second control device  26  increases to the predetermined amount S or greater, the selector valve  52  tends to be switched toward the left position in  FIG. 1  as mentioned above. The part of the communication line  40  is, however, in communication with the reservoir  43  via the pilot-controlled check valve  47  and the line  46  as mentioned above. Consequently, the bottom chamber  6   a  of the boom cylinder  6  is brought into a state communicated with the reservoir  43 . 
     In this state, the pressure oil in the bottom chamber  6   a  of the boom cylinder  6  is returned to the reservoir  43  via the main line  29   a  and the directional control valve  23  for the boom. The pressure oil in the bottom chamber  6   a  of the boom cylinder  6  is, therefore, not fed to the bottom chamber  7   a  of the arm cylinder  7  via the communication line  40  so that no acceleration is performed in arm crowding. 
     Upon performing a combined operation including arm dumping in which pressure oil is fed to the rod chamber  7   b  of the arm cylinder  7 , the bottom chamber  7   a  of the arm cylinder  7  is brought into communication with the reservoir  43 . No pressure is, therefore, developed in the communication line  40  so that no acceleration of the arm cylinder  7  is performed. 
     In the first embodiment constructed as described above, during a combined operation of boom raising and arm crowding, the pressure oil in the rod chamber  6   a  of the boom cylinder  6  can be combined to the bottom chamber  7   a  of the arm cylinder  7  as a result of a control of the second control device  26  irrespective of the level of the bottom pressure in the arm cylinder  7 . This makes it possible to effectively use the pressure oil in the rod chamber  6   a  of the boom cylinder  6 , the pressure oil having heretofore been simply drained into the reservoir  43 , for the acceleration of the arm cylinder  7  and hence, to achieve an improvement in the efficiency of the work. It is possible to improve the efficiency of work, for example, not only in digging work of earth that the pressure in the bottom chamber  7   a  of the arm cylinder  7  becomes higher but also in work by a crowding control of a bucket in the air that the pressure in the bottom chamber  7   a  of the arm cylinder  7  becomes lower. As a result, it is possible to accelerate any work that can effectively use the pressure oil in the rod chamber  6   a  of the boom cylinder  6 . 
     Even when the stroke of the arm control device  26  is the predetermined amount S or greater, an acceleration of the arm cylinder  7 , in other words, an acceleration of the operating speed of arm crowding can be reduced by opening the pilot-controlled check valve  47  when boom lowering which requires retraction of the boom cylinder  6  is performed. It is, therefore, possible to continue the desired working performance by combined operations of boom lowering and arm crowding. 
       FIG. 3  is a hydraulic circuit diagram showing a second embodiment of the present invention. 
     This second embodiment is provided with a branch line  56 , which is connected at an end thereof to the main line  29   b  communicating the directional control valve  23  for the boom and the rod chamber  6   b  of the boom cylinder  6  with each other, and at an opposite end thereof to a selector valve  64  which constitutes the communication control means. The selector valve  64  has a variable restrictor  64   a , is arranged on a reservoir line  42 , and is interposed at a point of connection between the branch line  56  and the communication line  40 . 
     The second embodiment is also provided with a bypass line  61 , a pilot-controlled check valve  62  arranged on the bypass line  61 , and a control line  63  connected at an end thereof to the pilot line  25   b  in the boom control system and at an opposite end thereof to the pilot-controlled check valve  62 . The bypass line  61  communicates a part of the reservoir line  42 , said part being located on an upstream side of the selector valve  64 , and another part of the reservoir line  42 , said part being located on a downstream side of the selector valve  64 , with each other. 
     A control chamber, which is arranged opposite a spring case of the selector valve  64 , and the pilot line  26   a  in the arm control system, are connected with each other by a control line  64   b . Further, the control chamber, which is arranged opposite the spring case of the selector valve  64 , and the pilot line  25   a  in the boom control system, are connected with each other by a control line  65 . The remaining construction is similar to that in the above-described first embodiment. 
     In this second embodiment, where the stroke of the boom control device  25  is relatively small when upon performing a combined operation of boom raising and arm crowding, the stroke of the arm control device  26  has increased to the predetermined amount S or greater and the selector valve  64  is about to be switched into the right position, a control pressure to be fed to the control chamber of the selector valve  64  via the pilot line  25   a  and control line  65  as a result of the control of the boom control device  25  is relatively low, and as a result, the amount of switching of the selector valve  64  is small, the opening of the variable restrictor  64   a  included in the selector valve  64  becomes relatively small. Through this reduced opening, the pressure oil in the rod chamber  6   b  of the boom cylinder  6  can be fed at a relatively low flow rate to the bottom chamber  7   a  of the arm cylinder  7  via the branch line  56 , the variable restrictor  64   a  of the selector valve  64 , the check valve  41  and the communication line  40 . As a consequence, the speed of the arm cylinder  7  which is in an accelerated state can be made relatively slow. 
     Where the stroke of the boom control device  25  is relatively large, the control pressure to be fed to the control chamber of the selector valve  64  via the control line  65  as a result of the control of the boom control device  25  becomes higher, and as a result, the opening of the variable restrictor  64   a  in the selector valve  64  becomes large. Through this enlarged opening, the pressure oil in the rod chamber  6   b  of the boom cylinder  6  can be fed at a high flow rate to the bottom chamber  7   a  of the arm cylinder  7 . As a consequence, the speed of the arm cylinder  7  which is in an accelerated state can be made still faster. 
     When upon performing a combined operation of boom lowering and arm crowding, the stroke of the arm control device  26  has increased to the predetermined amount S or greater and the selector valve  64  becomes prone to be switched into the right position in  FIG. 3  and further, the boom control device  25  is controlled and a control pressure is applied to the pilot-controlled variable restrictor  62  via the pilot line  25   b  and control line  63 , the pilot-controlled variable restrictor  62  is opened, the pressure oil in the bottom chamber  6   a  of the boom cylinder  6  is returned to the reservoir  43  via the main line  29   a , the directional control valve  23  for the boom, the reservoir line  42 , the line  61  and the pilot-controlled check valve  62 . It, therefore, becomes possible to perform the desired retracting operation of the boom cylinder  6 , namely, the boom lowering operation. 
     Even when during such a combined operation of boom lowering and arm crowding, the stroke of the arm control device  26  has increased to the predetermined amount S or greater and the selector valve  64  tends to be switched into the right position in  FIG. 3 , the bypass line  25   a  in the boom control system is brought to the reservoir pressure, the control line  65  is also brought to the reservoir pressure, and therefore, the variable restrictor  64   a  in the selector valve  64  is closed. As a consequence, the pressure oil in the rod chamber  6   b  of the boom cylinder  6  is not combined into the bottom chamber  7   a  of the arm cylinder  7 . 
     According to the second embodiment constructed as described above, the pressure oil in the rod chamber  6   a  of the boom cylinder  6 , as in the above-described first embodiment, can be combined into the bottom chamber  7   a  of the arm cylinder  7  irrespective of the level of the bottom pressure of the arm cylinder  7  as a result of a control of the second control device  26  upon performing a combined operation of boom raising and arm crowding. In particular, it is also possible to control the flow rate through the communication line, that is, the acceleration of the arm cylinder  7  by relying upon the stroke of the boom control device  25  which controls the boom cylinder  6 . 
     When the stroke of the arm control device  26  has increased to the predetermined amount S or greater in the combined operation of the boom raising and arm crowding, the pressure oil in the rod chamber  6   b  of the boom cylinder  6  is fed from the communication line  40  to the bottom chamber  7   a  of the arm cylinder  7  via the branch line  56 , that is, without going through the directional control valve  23  for the boom. Compared with the feeding of the pressure oil through the directional control valve  23  for the boom, it is, therefore, possible to reduce the pressure loss and hence, the energy loss provided that the diameter of the branch line  56  is set sufficiently large. 
       FIG. 4  is a hydraulic circuit showing a third embodiment of the present invention, and  FIG. 5  is a diagram illustrating the construction of an essential part of a controller which the third embodiment shown in  FIG. 4  is provided with. 
     The third embodiment shown in these  FIGS. 4 and 5  is constructed that a communication control means for communicating the rod chamber  6   b  of the boom cylinder as the first hydraulic cylinder and the bottom chamber  7   a  of the arm cylinder  7  with each other when the stroke of the arm control device  26  as the second control device has increased the predetermined amount S or greater is arranged on the pilot line  26   a , and that the third embodiment includes a stroke detector, i.e., an arm pilot pressure detector  67  for detecting an arm pilot pressure, which corresponds to the stroke of the arm control device  26 , and outputting an electrical signal, a controller  68  for outputting a control signal to selectively control a selector valve  44  responsive to the signal outputted from the arm pilot pressure detector  67 , an electric-hydraulic converter  69  for outputting a control pressure corresponding to the value of the control signal outputted from the controller  68 , and a control line  57   a  communicating the electric-hydraulic converter  69  and the control chamber of the selector valve  44  with each other. As illustrated in  FIG. 5 , the controller  68  includes a function generator  68   a  for outputting a value which becomes gradually greater as the arm pilot pressure corresponding to the stroke of the arm control device  26  increases. The remaining elements of the construction are similar to the corresponding elements in the above-described first embodiment shown in  FIG. 1 . 
     According to the third embodiment constructed as described above, especially when upon performing a combined operation of boom raising and arm crowding, the boom control device  25  is controlled to feed a pilot pressure to the pilot line  25   a  and to switch the directional control valve  23  into the left position and the arm control device  26  is controlled to feed a pilot pressure to the pilot line  26   a  and to switch the directional control valve  24  for the arm into the left position, as illustrated in  FIG. 4 , the pressure oil delivered from the main hydraulic pump  1  is fed to the bottom chamber  6   a  of the boom cylinder  6  and the bottom chamber  7   a  of the arm cylinder  7 . As a result, the boom cylinder  6  and arm cylinder  7  both operate in their extending directions so that the combined operation of boom raising and arm crowding is performed. 
     During this combined operation, the pilot pressure is not fed to the pilot line  25   b  in the boom control system so that the pilot line  25   b  is brought to the reservoir pressure. Accordingly, the control line  48  is brought to the reservoir pressure, the pilot-controlled check valve  47  is maintained in a closed state, and the communication of the communication line  40  with the reservoir  43  via the line  46  is prevented. 
     When the stroke of the arm control device  26  is smaller than the predetermined amount S, the signal value detected by the arm pilot pressure detector  67  is small so that the signal value outputted from the function generator  68   a  of the controller  68  shown in  FIG. 5  becomes smaller. The control signal of the small value is outputted from the controller  68  to the electric-hydraulic converter  69 . The electric-hydraulic converter  69  outputs a relatively low control pressure to the control line  57   a . In this state, the force by the control pressure applied to the control chamber of the selector value  44  is smaller than the spring force so that the selector valve  44  is held in the right position depicted in  FIG. 4 . Accordingly, the pressure oil in the rod chamber  6   b  of the boom cylinder  6  is not fed to the communication line  40  during the extending operation of the boom cylinder  6 . 
     When the stroke of the arm control device  26  has increased to the predetermined amount S or greater in the above state, the signal value detected by the arm pilot pressure detector  67  becomes large so that the signal value outputted from the function generator  68   a  of the controller  68  depicted in  FIG. 5  becomes greater. The control signal of this large value is outputted from the controller  68  to the electric-hydraulic converter  69 . Responsive to the control signal, the electric-hydraulic converter  69  outputs a high control pressure to the control line  57   a . As a result, the force by the control pressure applied to the control chamber of the selector valve  44  becomes greater than the spring force so that the selector valve  44  tends to be switched into the left position in  FIG. 4 . When this state has been achieved, the reservoir line  42  is cut off by the selector valve  44  so that the pressure oil, which has been guided from the rod chamber  6   b  of the boom cylinder  6  to the main line  29   a , the directional control valve  23  for the boom and the reservoir line  42 , is fed to the communication line  40  via the check valve  41 . The pressure oil fed from the communication line  40  is fed to the bottom chamber  7   a  of the arm cylinder  7  via the main line  30   a . Described specifically, the pressure oil fed via the directional control valve  24  for the arm and the pressure oil fed from the rod chamber  6   b  of the boom cylinder  6  are combined and fed to the bottom chamber  7   a  of the arm cylinder  7 . As a result, an acceleration can be realized in the extending direction of the arm cylinder  6 , and therefore, the operating speed of arm crowding can be made faster. 
     In the third embodiment constructed as described above, as in the above-described first embodiment shown in  FIG. 1 , the pressure oil in the rod chamber  6   a  of the boom cylinder  6 , which was conventionally drained into the reservoir  43 , can also be effectively used for the acceleration of the arm cylinder  7  irrespective of the level of the bottom pressure of the arm cylinder  7 , and therefore, an improvement can be realized in the efficiency of work. 
     Corresponding to the stroke of the arm control device  26 , this third embodiment can also achieve an acceleration of the arm cylinder  7  based on the function relation in the function generator  68   a  of the controller  68  so that in conformity with the operator&#39;s control sensation, the arm cylinder  7  can be smoothly accelerated to perform an arm crowding operation.