Abstract:
Disclosed is a lower track for mounting rubber pads on a track shoe to be attachable and detachable after assembling a steel plate which is adhered with divisionally formed rubber pads as a single unit without additionally perforating coupling through-holes in the track shoe when the rubber pad which forms the lower track and the track shoe which is formed of a metal material are coupled. The lower track of a crawler excavator according to the present invention comprises: first and second rubber pads which are divisionally formed; a first steel plate which supports the bottom surface of the first rubber pad by surrounding the same, and has first hooks formed on the bottom surface thereof and coupling holes formed in a connection stepped-portion.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is the National Phase application of International Application No. PCT/KR2011/007440 filed on Oct. 7, 2011, which is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to a priority control system for a construction machine. More particularly, the present invention relates to a priority control system for a construction machine, in which when a swing apparatus of an excavator and a work apparatus or an attachment such as the arm are simultaneously manipulated, the priority control valve is shifted to a throttle state or a throttle release state depending on the amount of a load occurring in the hydraulic actuator so that an unnecessary pressure loss can be prevented. 
     BACKGROUND OF THE INVENTION 
     A priority control system for a construction machine in accordance with the prior art as shown in  FIG. 1  includes: 
     first and second variable displacement hydraulic pumps (hereinafter, referred to as “first and second hydraulic pumps”)  2  and  3  connected to an engine  1  and a pilot pump  4 ; 
     a boom control valve  7  configured to control the drive of a boom cylinder  6 , a bucket control valve  9  configured to control the drive of a bucket cylinder  8 , and a traveling control valve  11  configured to control the drive of a left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a;    
     a swing control valve  14  configured to control the drive of a swing motor  13 , an arm control valve  16  configured to control the drive of an arm cylinder  15 , and a traveling control valve  18  configured to control the drive of a right traveling motor  17 , wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path  12  of the second hydraulic pump  3  so as to be connected to each other through a parallel flow path  12   a;    
     first and second pressure generation devices  19  and  20  configured to output a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve  23  configured to output a larger pilot signal pressure selected from pilot signal pressures applied to the swing control valve  14  so that the swing motor  13  can be swung in a left or right direction in response to the manipulation of the first pressure generation device (i.e., swing manipulation lever)  19 ; and 
     a priority control valve  21  installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and configured to be shifted to a throttle state or a throttle release state by a pilot signal pressure outputted from the shuttle valve  23  when the swing motor  13  and the arm cylinder  15  are simultaneously manipulated. 
     When the first pressure generation device  19  is not manipulated and thus the pilot signal pressure is not applied to the swing control valve  14 , the priority control valve  21  is maintained in the throttle release state by the elastic force of the valve spring  21   a  (see  FIG. 1 ), so that the opening amount of the spool of the priority control valve  21  is switched maximally. On the other hand, when the pilot signal pressure from the shuttle valve  23  is applied to the priority control valve  21  through the flow path  22  due to the manipulation of the first pressure generation device  19 , an internal spool of the priority control valve  21  is shifted to the top on the drawing sheet to cause the priority control valve to be shifted to the throttle state. 
     When the swing motor generating a high load and the arm cylinder generating a low load to perform an arm-in driving operation are simultaneously manipulated, a load occurring in the arm cylinder  15  is relatively higher than that occurring in the swing motor  13 . Thus, a flow rate of the hydraulic fluid discharged from the second hydraulic pump  3  supplied to the arm cylinder  15  having a relatively low load is higher than a flow rate as supplied to the swing motor  15 . 
     In order to prevent this, when the swing operation and the arm-in driving operation are simultaneously performed, the priority control valve  21  is shifted to the throttle state so that a flow rate of the hydraulic fluid supplied to the arm control valve  16  from the second hydraulic pump  3  is reduced and a flow rate of the hydraulic fluid supplied to the swing control valve  14  is increased as much as the reduced flow rate of the hydraulic fluid. 
     For this reason, when the swing motor  13  generating a high load and the arm cylinder generating a relatively low load to perform an arm-in driving operation are simultaneously manipulated, simultaneous manipulability can be maintained. 
     In the meantime, even when the swing motor  13  generating a high load and the arm cylinder generating a high load to perform an arm-out driving operation are simultaneously manipulated, the priority control valve  21  is maintained in the throttle state by the pilot signal pressure applied thereto due to the manipulation of the first pressure generation device  19 . As a result, there is caused a problem in that the flow path of the priority control valve  21  connected to the arm control valve  16  is reduced, leading to a degradation of the operating speed of the arm cylinder  15  and an unnecessary pressure loss, thereby causing a hydraulic energy loss. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problems 
     Accordingly, the present invention has been made to solve the aforementioned problem occurring in the prior art, and it is an object of the present invention to provide a priority control system for a construction machine, in which when a swing apparatus generating a high operating pressure and an arm generating a high or low operating pressure depending on the driving direction are simultaneously manipulated, the priority control valve is shifted to a throttle state to maintain simultaneous manipulability or is shifted to a throttle release state to prevent an unnecessary pressure loss and secure the operating speed of the hydraulic actuator depending on the amount of a load occurring in the hydraulic actuator so that the distribution of the hydraulic fluid can be controlled optimally. 
     Technical Solution 
     To accomplish the above object, in accordance with a first embodiment of the present invention, there is provided a priority control system for a construction machine, including: 
     an engine; 
     first and second variable displacement hydraulic pumps connected to the engine and a pilot pump; 
     a boom control valve configured to control the drive of a boom cylinder, a bucket control valve configured to control the drive of a bucket cylinder, and a traveling control valve configured to control the drive of a left traveling motor, wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path of the first hydraulic pump so as to be connected to each other through a parallel flow path; 
     a swing control valve configured to control the drive of a swing motor, an arm control valve configured to control the drive of an arm cylinder, and a traveling control valve configured to control the drive of a right traveling motor, wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path of the second hydraulic pump so as to be connected to each other through a parallel flow path; 
     first and second pressure generation devices configured to output a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve configured to output any one selected from pilot signal pressures applied to the swing control valve so that the swing motor can be swung in a left or right direction in response to the manipulation of the first pressure generation device; and 
     a priority control valve installed in a flow path between the parallel flow path on the second hydraulic pump side and an inlet port of the arm control valve, and configured to be switched to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and to be shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously manipulated and an elastic force of a valve spring. 
     In accordance with a second embodiment of the present invention, there is provided a priority control system for a construction machine, including: 
     an engine; 
     first and second variable displacement hydraulic pumps connected to the engine and a pilot pump; 
     a boom control valve configured to control the drive of a boom cylinder, a bucket control valve configured to control the drive of a bucket cylinder, and a traveling control valve configured to control the drive of a left traveling motor, wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path of the first hydraulic pump so as to be connected to each other through a parallel flow path; 
     a swing control valve configured to control the drive of a swing motor, an arm control valve configured to control the drive of an arm cylinder, and a traveling control valve configured to control the drive of a right traveling motor, wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path of the second hydraulic pump so as to be connected to each other through a parallel flow path; 
     first and second pressure generation devices configured to output a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve configured to output any one selected from pilot signal pressures applied to the swing control valve so that the swing motor can be swung in a left or right direction in response to the manipulation of the first pressure generation device; 
     a priority control valve installed in a flow path between the parallel flow path on the second hydraulic pump side and an inlet port of the arm control valve, and configured to be switched to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and to be shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously manipulated; and 
     a signal line shutoff valve installed in a flow path between the shuttle valve and the priority control valve and configured to shut off the flow path only by a pilot signal pressure that is applied to the arm control valve to perform an arm-out driving operation in response to the manipulation of the second pressure generation device. 
     In accordance with a third embodiment of the present invention, there is provided a priority control system for a construction machine, including: 
     an engine; 
     first and second variable displacement hydraulic pumps connected to the engine and a pilot pump; 
     a boom control valve configured to control the drive of a boom cylinder, a bucket control valve configured to control the drive of a bucket cylinder, and a traveling control valve configured to control the drive of a left traveling motor, wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path of the first hydraulic pump so as to be connected to each other through a parallel flow path; 
     a swing control valve configured to control the drive of a swing motor, an arm control valve configured to control the drive of an arm cylinder, and a traveling control valve configured to control the drive of a right traveling motor, wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path of the second hydraulic pump so as to be connected to each other through a parallel flow path; 
     first and second pressure generation devices configured to output a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve configured to output any one selected from pilot signal pressures applied to the swing control valve so that the swing motor can be swung in a left or right direction in response to the manipulation of the first pressure generation device; 
     a priority control valve installed in a flow path between the parallel flow path on the second hydraulic pump side and an inlet port of the arm control valve, and configured to be switched to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and to be shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously manipulated; 
     a signal line shutoff valve installed in a flow path between the shuttle valve and the priority control valve and configured shifted to shut off the flow path in response to an external electric control signal; 
     a first pressure detection means configured to detect the pilot signal pressure that is applied to the arm control valve and output a detection signal to perform an arm-out driving operation in response to the manipulation of the second pressure generation device; and 
     a controller configured to output an electric control signal to the signal line shutoff valve to shift the signal line shutoff valve when the pilot signal pressure for performing an arm-out driving operation reaches a set value in response to the detection signal applied thereto from the first pressure detection means. 
     In accordance with a fourth embodiment of the present invention, there is provided a priority control system for a construction machine, including: 
     an engine; 
     first and second variable displacement hydraulic pumps connected to the engine and a pilot pump; 
     a boom control valve configured to control the drive of a boom cylinder, a bucket control valve configured to control the drive of a bucket cylinder, and a traveling control valve configured to control the drive of a left traveling motor, wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path of the first hydraulic pump so as to be connected to each other through a parallel flow path; 
     a swing control valve configured to control the drive of a swing motor, an arm control valve configured to control the drive of an arm cylinder, and a traveling control valve configured to control the drive of a right traveling motor, wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path of the second hydraulic pump so as to be connected to each other through a parallel flow path; 
     first and second pressure generation devices configured to output a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve configured to output any one selected from pilot signal pressures applied to the swing control valve so that the swing motor can be swung in a left or right direction in response to the manipulation of the first pressure generation device; 
     a priority control valve installed in a flow path between the parallel flow path on the second hydraulic pump side and an inlet port of the arm control valve, and configured to be switched to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and to be shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously manipulated; 
     a pressure reduction valve installed in a flow path between the pilot pump  4  and the priority control valve; 
     a first pressure detection means configured to detect the pilot signal pressure that is applied to the arm control valve and output a detection signal to perform an arm-out driving operation in response to the manipulation of the second pressure generation device; 
     a second pressure detection means configured to detect a pilot signal pressure that is outputted from the shuttle valve which outputs any one selected from pilot signal pressures applied to the swing control valve, and output a detection signal so that the swing motor is driven in a left or right direction in response to the manipulation of the first pressure generation device; and 
     a controller configured to output a control signal to the pressure reduction valve to increase a secondary signal pressure that is outputted from the pressure reduction valve when a swing pilot signal pressure is increased by the detection signal applied thereto from the second pressure detection means, and to reduce the secondary signal pressure that is outputted from the pressure reduction valve when the pilot signal pressure for performing the arm-out driving operation is applied to the arm control valve  16  by the detection signal applied thereto from the first pressure detection means. 
     In accordance with a preferred embodiment of the present invention, the priority control valve is shifted to the throttle state by the pilot signal pressure that is outputted from the shuttle valve when the swing operation and the arm-in driving operation are simultaneously performed, and is shifted to the throttle release state by the elastic force of the valve spring of the priority control valve and the pilot signal pressure that is applied to the arm control valve to perform the arm-out driving operation when the swing operation and the arm-out driving operation are simultaneously performed. 
     The first actuator is the swing motor, and the second actuator is the arm cylinder. 
     A solenoid valve that is shifted in response to the electric control signal applied thereto from the controller is used as the signal line shutoff valve. 
     A pressure sensor that detects the pilot signal pressure applied to the arm control valve and transmits the detection signal to the controller is used as the first pressure detection means. 
     A pressure switch, which is turned on/off to generate a signal when the pilot signal pressure applied to the arm control valve reaches the set pressure, is used as the first pressure detection means. 
     An electro proportional control valve for varying the secondary signal pressure outputted therefrom in response to the electric control signal value applied thereto is used as the pressure reduction valve. 
     Pressure sensors for detecting the pilot signal pressure applied to the arm control valve and outputting the detection signal for application to the controller are used as the first and second pressure detection means. 
     Pressure switches, which are turned on/off to generate a signal when the pilot signal pressure applied to the arm control valve reaches the set pressure, are used as the first and second pressure detection means. 
     Advantageous Effect 
     The priority control system for a construction machine in accordance with an embodiment of the present invention as constructed above has the following advantages. 
     When a swing apparatus generating a high operating pressure and an arm generating a high or low operating pressure depending on the driving direction are simultaneously manipulated, the priority control valve is shifted to a throttle state or a throttle release state to maintain simultaneous manipulability or prevent an unnecessary pressure loss and secure the operating speed of the hydraulic actuator depending on the amount of a load occurring in the hydraulic actuator so that workability is improved and the distribution of the hydraulic fluid can be controlled optimally, thereby enhancing the efficiency of the hydraulic system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above objects, other features and advantages of the present invention will become more apparent by describing the preferred embodiments thereof with reference to the accompanying drawings, in which: 
         FIG. 1  is a hydraulic circuit diagram showing a priority control system for a construction machine in accordance with the prior art; 
         FIG. 2  is a hydraulic circuit diagram showing a priority control system for a construction machine in accordance with a first embodiment of the present invention; 
         FIG. 3  is a hydraulic circuit diagram showing a priority control system for a construction machine in accordance with a second embodiment of the present invention; 
         FIG. 4  is a hydraulic circuit diagram showing a priority control system for a construction machine in accordance with a third embodiment of the present invention; 
         FIG. 5  is a hydraulic circuit diagram showing a priority control system for a construction machine in accordance with a fourth embodiment of the present invention; 
     
    
    
     EXPLANATION ON REFERENCE NUMERALS OF MAIN ELEMENTS IN THE DRAWINGS 
     
         
         
           
               1 : engine 
               2 : variable displacement first hydraulic pump 
               3 : variable displacement second hydraulic pump 
               4 : pilot pump 
               5 : first center bypass path 
               6 : boom cylinder 
               7 : boom control valve 
               8 : bucket cylinder 
               9 : bucket control valve 
               10 ,  17 : traveling motor 
               11 ,  18 : traveling control valve 
               12 : second center bypass path 
               13 : swing motor 
               14 : swing control valve 
               15 : arm cylinder 
               16 : arm control valve 
               19 : first pressure generation device 
               20 : second pressure generation device 
               21 : priority control valve 
               22 ,  29 ,  30 : flow path 
               23 : shuttle valve 
               24 : signal line shutoff valve 
               25 : pressure reduction valve 
               26 : first pressure detection means 
               27 : controller 
               28 : second pressure detection means 
           
         
       
    
     Preferred Embodiments of the Invention 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     Now, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The matters defined in the description, such as the detailed construction and elements, are nothing but specific details provided to assist those of ordinary skill in the art in a comprehensive understanding of the invention, and the present invention is not limited to the embodiments disclosed hereinafter. 
     A priority control system for a construction machine in accordance with a first embodiment of the present invention as shown in  FIG. 2  includes: 
     an engine  1 ; 
     first and second variable displacement hydraulic pumps (hereinafter, referred to as “first and second hydraulic pumps”)  2  and  3  that are connected to the engine  1  and a pilot pump  4 ; 
     a boom control valve  7  that controls the drive of a boom cylinder  6 , a bucket control valve  9  that controls the drive of a bucket cylinder  8 , and a traveling control valve  11  that controls the drive of a left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a;    
     a swing control valve  14  that controls the drive of a swing motor  13 , an arm control valve  16  that controls the drive of an arm cylinder  15 , and a traveling control valve  18  controls the drive of a right traveling motor  17 , wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path  12  of the second hydraulic pump  3  so as to be connected to each other through a parallel flow path  12   a;    
     first and second pressure generation devices  19  and  20  that outputs a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve  23  that outputs any one selected from pilot signal pressures applied to the swing control valve  14  so that the swing motor  13  can be swung in a left or right direction in response to the manipulation of the first pressure generation device  19 ; and 
     a priority control valve  21  that is installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and is switched to a throttle state by a pilot signal pressure that is applied thereto when a first actuator (e.g., swing motor) generating a high-load operating pressure and a second actuator (e.g., arm cylinder) generating a low-load operating pressure in accordance with a driving direction (e.g., arm-in driving direction) are simultaneously manipulated, and is shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction (e.g., arm-out driving direction) are simultaneously manipulated and an elastic force of a valve spring  21   a.    
     In this case, the priority control valve  21  is shifted to the throttle state by the pilot signal pressure that is outputted from the shuttle valve  23  when a swing operation and an arm-in driving operation are simultaneously performed, and is shifted to the throttle release state by the elastic force of the valve spring  21   a  of the priority control valve  21  and the pilot signal pressure that is applied to the arm control valve  16  to perform an arm-out driving operation when the swing operation and the arm-out driving operation are simultaneously performed. 
     In this case, the configuration of the priority control system in accordance with the first embodiment of the present invention as shown in  FIG. 2  is the same as that of the conventional priority control system as shown in  FIG. 1 , except the priority control valve  21  that is installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and is shifted to the throttle state to restrict the supply of a hydraulic fluid to the arm control valve  16  from the second hydraulic pump  3  when the swing operation and the arm-in driving operation are simultaneously, and is shifted to the throttle release state to control the supply of the hydraulic fluid to the arm control valve  16  from the second hydraulic pump  3  in response to the manipulation amount of the second pressure generation device  20  when the swing operation and the arm-out driving operation are simultaneously. Thus, the detailed description of the same configuration and operation thereof will be omitted to avoid redundancy, and the same elements are denoted by the same reference numerals. 
     Hereinafter, a use example of the priority control system for a construction machine in accordance with a first embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
     When the first and second pressure generation devices  19  and  20  are simultaneously manipulated to simultaneously perform the swing operation and the arm-in driving operation of the excavator, a pilot signal pressure outputted from the shuttle valve  23  is supplied to a pressure chamber of the priority control valve  21  along a flow path  22  to cause an internal spool of the priority control valve  21  to be shifted to the top on the drawing sheet to switch the priority control valve  21  to the throttle state so that the swing motor  13  can be driven in a left or right direction in response to the manipulation of the first pressure generation device  19 . 
     Thus, the swing control valve  14  is shifted to rotate the swing motor  13  in the left or right direction through the manipulation of the first pressure generation device  19 , so that the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. 
     At the same time, the arm control valve  16  is shifted to the left on the drawing sheet to perform the arm-in driving operation through the manipulation of the second pressure generation device  20 , so that the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to a large chamber of the arm cylinder  15  via the following paths to cause the arm cylinder to be driven in a stretchable manner: the second center bypass path  12 →the parallel flow path  12   a →the priority control valve  21  of the throttle state→the flow path  29 →the arm control valve  16 . 
     In the meantime, since a load value occurring during the swing operation of the upper swing structure relative to the lower traveling structure of the excavator is larger than that occurring during the arm-in driving operation, a flow rate of the hydraulic fluid discharged from the second hydraulic pump  3  supplied to the arm control valve  16  is higher than a flow rate as supplied to the swing control valve  14 . 
     At this time, since the internal spool of the priority control valve  21  is in a state of being shifted to the throttle state, an inlet of the flow path  29  branched off from the parallel flow path  12   a  and connected to the inlet port of the arm control valve  16  is reduced to restrict the supply of the hydraulic fluid to the arm control valve  16  from the second hydraulic pump  3 . 
     Therefore, when the swing operation and the arm-in driving operation are simultaneously performed, the spool of the priority control valve  21  is shifted to the throttle state so that the flow rate of the hydraulic fluid supplied to the arm control valve  16  is restricted and thus simultaneous workability can be maintained. 
     Meanwhile, in the case the swing operation and the arm-out driving operation of the excavator are simultaneously performed by simultaneously manipulating the first and second pressure generation devices  19  and  20 , since a high load occurs during the swing operation and the arm-out driving operation, the priority control valve  21  is maintained in an initial state in which its throttle state is released (see  FIG. 2 ). 
     Therefore, the swing motor  13  is swung in a left or right direction through the manipulation of the first pressure generation device  19 , so that the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. 
     At the same time, the pilot signal pressure is supplied to the pressure chamber of the arm control valve  16  due to the manipulation of the second pressure generation device  20  to cause an internal spool of the arm control valve  16  to be shifted to the right on the drawing sheet. In addition, apart of the pilot signal pressure supplied to the arm control valve  16  is applied to the valve spring  21   a  so that the priority control valve  21  is maintained in the throttle state. That is, a value obtained by adding the elastic force of the valve spring  21   a  of the priority control valve  21  and the pilot signal pressure applied to the arm control valve  16  during the arm-out driving operation is larger than a value of the pilot signal pressure applied to the pressure chamber of the priority control valve  21 . 
     Thus, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. At the same time, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to a small chamber of the arm cylinder  15  via the following paths to cause the arm cylinder to be driven in a retractable manner: the second center bypass path  12 →the parallel flow path  12   a →the priority control valve  21  of the throttle release state→the flow path  29 →the arm control valve  16 . 
     As described above, according to the priority control system in accordance with the first embodiment of the present invention, when the swing operation and the arm-out driving operation are simultaneously performed, the spool of the priority control valve  21  is shifted to the throttle release state to maximally switch the flow path of the throttle device, thereby preventing an unnecessary pressure loss. 
     A priority control system for a construction machine in accordance with a second embodiment of the present invention as shown in  FIG. 3  includes: 
     an engine  1 ; 
     first and second variable displacement hydraulic pumps  2  and  3  connected to the engine  1  and a pilot pump  4 ; 
     a boom control valve  7  that controls the drive of a boom cylinder  6 , a bucket control valve  9  that controls the drive of a bucket cylinder  8 , and a traveling control valve  11  that controls the drive of a left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a;    
     a swing control valve  14  that controls the drive of a swing motor  13 , an arm control valve  16  that controls the drive of an arm cylinder  15 , and a traveling control valve  18  that controls the drive of a right traveling motor  17 , wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path  12  of the second hydraulic pump  3  so as to be connected to each other through a parallel flow path  12   a;    
     first and second pressure generation devices  19  and  20  that outputs a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve  23  that outputs any one selected from pilot signal pressures applied to the swing control valve  14  so that the swing motor  13  can be swung in a left or right direction in response to the manipulation of the first pressure generation device  19 ; 
     a priority control valve  21  that is installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and is shifted to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and is shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously; and 
     a signal line shutoff valve  24  that is installed in a flow path  22  between the shuttle valve  23  and the priority control valve  21  and shuts off the flow path  22  only by a pilot signal pressure that is applied to the arm control valve  16  to perform an arm-out driving operation in response to the manipulation of the second pressure generation device  20 . 
     In this case, the configuration of the boom control valve  7  that controls the drive of the boom cylinder  6 , the bucket control valve  9  that controls the drive of the bucket cylinder  8 , and the traveling control valve  11  that controls the drive of the left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a  is the same as that of the corresponding elements as shown in  FIG. 2 , and thus redundant illustration of the same configuration thereof is avoided in the accompanying drawings. 
     In the meantime, the configuration of the priority control system in accordance with the second embodiment of the present invention as shown in  FIG. 3  is the same as that of the priority control system as shown in  FIG. 2 , except the signal line shutoff valve  24  that is installed in a flow path  22  between the shuttle valve  23  and the priority control valve  21  and shuts off the flow path  22  only by a pilot signal pressure that is applied to the arm control valve  16  to perform an arm-out driving operation. Thus, the detailed description of the same configuration and operation thereof will be omitted to avoid redundancy, and the same elements are denoted by the same reference numerals. 
     Therefore, in the case the swing operation and the arm-out driving operation of the excavator are simultaneously performed by simultaneously manipulating the first and second pressure generation devices  19  and  20 , since a high load occurs during the swing operation and the arm-out driving operation, the priority control valve  21  is maintained in an initial state in which its throttle state is released (see  FIG. 3 ). 
     The pilot signal pressure is supplied to the pressure chamber of the arm control valve  16  due to the manipulation of the second pressure generation device  20  to cause an internal spool of the arm control valve  16  to be shifted to the right on the drawing sheet. At the same time, a part of the pilot signal pressure supplied to the arm control valve  16  is applied to the signal line shutoff valve  24  to cause an internal spool of the signal line shutoff valve  24  to be shifted to the top on the drawing sheet. For this reason, the supply of the pilot signal pressure to the pressure chamber of the priority control valve  21  is interrupted during the manipulation of the first pressure generation device  19 , so that the priority control valve  21  is maintained in an initial state in which its throttle state is released by the elastic force of the valve spring  21   a.    
     Thus, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. At the same time, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to a small chamber of the arm cylinder  15  via the following paths to cause the arm cylinder to be driven in a retractable manner: the second center bypass path  12 →the parallel flow path  12   a →the priority control valve  21  of the throttle release state→the flow path  29 →the arm control valve  16 . 
     As described above, according to the priority control system in accordance with the second embodiment of the present invention, when the swing operation and the arm-out driving operation are simultaneously performed, the spool of the priority control valve  21  is shifted to the throttle release state by the signal line shutoff valve  24  to maximally switch the flow path of the throttle device, thereby preventing an unnecessary pressure loss. 
     In this case, the configuration of the boom control valve  7  that controls the drive of the boom cylinder  6 , the bucket control valve  9  that controls the drive of the bucket cylinder  8 , and the traveling control valve  11  that controls the drive of the left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a  is the same as that of the corresponding elements as shown in  FIG. 2 , and thus redundant illustration of the same configuration thereof is avoided in the accompanying drawings. 
     A priority control system for a construction machine in accordance with a third embodiment of the present invention as shown in  FIG. 3  includes: 
     an engine  1 ; 
     first and second variable displacement hydraulic pumps  2  and  3  connected to the engine  1  and a pilot pump  4 ; 
     a boom control valve  7  that controls the drive of a boom cylinder  6 , a bucket control valve  9  that controls the drive of a bucket cylinder  8 , and a traveling control valve  11  that controls the drive of a left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a;    
     a swing control valve  14  that controls the drive of a swing motor  13 , an arm control valve  16  that controls the drive of an arm cylinder  15 , and a traveling control valve  18  that controls the drive of a right traveling motor  17 , wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path  12  of the second hydraulic pump  3  so as to be connected to each other through a parallel flow path  12   a;    
     first and second pressure generation devices  19  and  20  that outputs a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve  23  that outputs any one selected from pilot signal pressures applied to the swing control valve  14  so that the swing motor  13  can be swung in a left or right direction in response to the manipulation of the first pressure generation device  19 ; 
     a priority control valve  21  that is installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and is shifted to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and is shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously; 
     a signal line shutoff valve  24  that is installed in a flow path  22  between the shuttle valve  23  and the priority control valve  21  and shuts off the flow path  22  only by a pilot signal pressure that is applied to the arm control valve  16  to perform an arm-out driving operation in response to the manipulation of the second pressure generation device  20 ; 
     a signal line shutoff valve  24  that is installed in a flow path  22  between the shuttle valve  23  and the priority control valve  21  and is shifted to shut off the flow path  22  in response to an external electric control signal; 
     a first pressure detection means  26  that detects the pilot signal pressure that is applied to the arm control valve  16  and outputs a detection signal to perform an arm-out driving operation in response to the manipulation of the second pressure generation device  20 ; and 
     a controller  27  that outputs an electric control signal to the signal line shutoff valve  24  to shift the signal line shutoff valve when the pilot signal pressure for performing the arm-out driving operation reaches a set value in response to the detection signal applied thereto from the first pressure detection means  26 . 
     A solenoid valve that is shifted in response to the electric control signal applied thereto from the controller  27  is used as the signal line shutoff valve  24 . 
     A pressure sensor that detects the pilot signal pressure applied to the arm control valve  16  and transmits the detection signal to the controller  27  is used as the first pressure detection means  26 . 
     A pressure switch, which is turned on/off to generate a signal when the pilot signal pressure applied to the arm control valve  16  reaches the set pressure, is used as the first pressure detection means  26 . 
     In this case, the configuration of the boom control valve  7  that controls the drive of the boom cylinder  6 , the bucket control valve  9  that controls the drive of the bucket cylinder  8 , and the traveling control valve  11  that controls the drive of the left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a  is the same as that of the corresponding elements as shown in  FIG. 2 , and thus redundant illustration of the same configuration thereof is avoided in the accompanying drawings. 
     In the meantime, the configuration of the priority control system in accordance with the third embodiment of the present invention as shown in  FIG. 4  is the same as that of the first priority control system as shown in  FIG. 2 , except the signal line shutoff valve  24  that is installed in a flow path  22  between the shuttle valve  23  and the priority control valve  21  and is shifted to shut off the flow path  22  in response to an external electric control signal, the first pressure detection means  26  that detects the pilot signal pressure that is applied to the arm control valve  16  and outputs a detection signal to perform the arm-out driving operation, and the controller  27  that outputs an electric control signal to the signal line shutoff valve  24  to shift the signal line shutoff valve when the pilot signal pressure for performing the arm-out driving operation reaches the set value. Thus, the detailed description of the same configuration and operation thereof will be omitted to avoid redundancy, and the same elements are denoted by the same reference numerals. 
     Thus, in the case the swing operation and the arm-out driving operation of the excavator are simultaneously performed by simultaneously manipulating the first and second pressure generation devices  19  and  20 , the pilot signal pressure is supplied to the pressure chamber of the arm control valve  16  due to the manipulation of the second pressure generation device  20  to cause an internal spool of the arm control valve  16  to be shifted to the right on the drawing sheet. At this time, the pressure detection means  26  detects the pilot signal pressure that is applied to the arm control valve  16  to perform the arm-out driving operation, and outputs a detection signal for application to the controller  27 . If it is determined that the pilot signal pressure for performing the arm-out driving operation reaches the set value, the controller  27  applies an electric control signal to the signal line shutoff valve  24  to shift an internal spool of the signal line shutoff valve to the top on the drawing sheet to shut off the flow path  22  along which the pilot signal pressure is supplied to the pressure chamber of the priority control valve  21 . Thus, the priority control valve  21  can be maintained in an initial state in which its throttle state is released by the elastic force of the valve spring  21   a  (see  FIG. 4 ). 
     Thus, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. At the same time, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to a small chamber of the arm cylinder  15  via the following paths: the second center bypass path  12 →the parallel flow path  12   a →the priority control valve  21  of the throttle release state→the flow path  29 →the arm control valve  16 . 
     As described above, according to the priority control system in accordance with the third embodiment of the present invention, when the swing operation and the arm-out driving operation are simultaneously performed, the spool of the priority control valve  21  is shifted to the throttle release state by the signal line shutoff valve  24  to maximally switch the flow path of the throttle device, thereby preventing an unnecessary pressure loss. 
     A priority control system for a construction machine in accordance with a fourth embodiment of the present invention as shown in  FIG. 5  includes: 
     an engine  1 ; 
     first and second variable displacement hydraulic pumps  2  and  3  connected to the engine  1  and a pilot pump  4 ; 
     a boom control valve  7  that controls the drive of a boom cylinder  6 , a bucket control valve  9  that controls the drive of a bucket cylinder  8 , and a traveling control valve  11  that controls the drive of a left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a;    
     a swing control valve  14  that controls the drive of a swing motor  13 , an arm control valve  16  that controls the drive of an arm cylinder  15 , and a traveling control valve  18  that controls the drive of a right traveling motor  17 , wherein the swing control valve, the arm control value, and the traveling control valve are installed in a second center bypass path  12  of the second hydraulic pump  3  so as to be connected to each other through a parallel flow path  12   a;    
     first and second pressure generation devices  19  and  20  that outputs a control signal corresponding to a manipulation amount, respectively; 
     a shuttle valve  23  that outputs any one selected from pilot signal pressures applied to the swing control valve  14  so that the swing motor  13  can be swung in a left or right direction in response to the manipulation of the first pressure generation device  19 ; 
     a priority control valve  21  that is installed in a flow path  29  between the parallel flow path  12   a  on the second hydraulic pump  3  side and an inlet port of the arm control valve  16 , and is shifted to a throttle state by a pilot signal pressure that is applied thereto when a first actuator generating a high-load operating pressure and a second actuator generating a low-load operating pressure in accordance with a driving direction are simultaneously manipulated, and is shifted to a throttle release state by a pilot signal pressure that is applied thereto when the first actuator generating a high-load operating pressure and the second actuator generating a high-load operating pressure in accordance with the driving direction are simultaneously; 
     a pressure reduction valve  25  that is installed in a flow path  30  between the pilot pump  4  and the priority control valve  21 ; 
     a first pressure detection means  26  that detects pilot signal pressure that is applied to the arm control valve  16  and outputs a detection signal to perform an arm-out driving operation in response to the manipulation of the second pressure generation device  20 ; 
     a second pressure detection means  28  that detects a pilot signal pressure that is outputted from the shuttle valve  23  which outputs any one selected from pilot signal pressures applied to the swing control valve  14 , and outputs a detection signal so that the swing motor  13  is driven in a left or right direction in response to the manipulation of the first pressure generation device  19 ; and 
     a controller  27  that outputs a control signal to the pressure reduction valve  25  to increase a secondary signal pressure that is outputted from the pressure reduction valve  25  when a swing pilot signal pressure is increased by the detection signal applied thereto from the second pressure detection means  28 , and to reduce the secondary signal pressure that is outputted from the pressure reduction valve  25  when the pilot signal pressure for performing the arm-out driving operation is applied to the arm control valve  16  by the detection signal applied thereto from the first pressure detection means  26 . 
     An electro proportional control valve for varying the secondary signal pressure outputted therefrom in response to the electric control signal value applied thereto is used as the pressure reduction valve  25 . 
     Pressure sensors for detecting the pilot signal pressure applied to the arm control valve  16  and outputting the detection signal for application to the controller  27  are used as the first and second pressure detection means  26  and  28 . 
     Pressure switches, which are turned on/off to generate a signal when the pilot signal pressure applied to the arm control valve  16  reaches the set pressure, are used as the first and second pressure detection means  26  and  28 . 
     In this case, the configuration of the boom control valve  7  that controls the drive of the boom cylinder  6 , the bucket control valve  9  that controls the drive of the bucket cylinder  8 , and the traveling control valve  11  that controls the drive of the left traveling motor  10 , wherein the boom control valve, the bucket control valve, and the traveling control valve are installed in a first center bypass path  5  of the first hydraulic pump  2  so as to be connected to each other through a parallel flow path  5   a  is the same as that of the corresponding elements as shown in  FIG. 2 , and thus redundant illustration of the same configuration thereof is avoided in the accompanying drawings. 
     In the meantime, the configuration of the priority control system in accordance with the fourth embodiment of the present invention as shown in  FIG. 5  is the same as that of the priority control system as shown in  FIG. 2 , except the pressure reduction valve  25  that is installed in a flow path  30  between the pilot pump  4  and the priority control valve  21 , the first pressure detection means  26  that detects pilot signal pressure that is applied to the arm control valve  16  to perform an arm-out driving operation, the second pressure detection means  28  that detects a pilot signal pressure that is outputted from the shuttle valve  23  which outputs any one selected from pilot signal pressures applied to the swing control valve  14 , and the controller  27  that outputs a control signal to the pressure reduction valve  25  to increase a secondary signal pressure that is outputted from the pressure reduction valve  25  when a swing pilot signal pressure is increased, and to reduce the secondary signal pressure that is outputted from the pressure reduction valve  25  when the pilot signal pressure for performing the arm-out driving operation is applied thereto. Thus, the detailed description of the same configuration and operation thereof will be omitted to avoid redundancy, and the same elements are denoted by the same reference numerals. 
     Thus, in the case the swing operation and the arm-out driving operation of the excavator are simultaneously performed by simultaneously manipulating the first and second pressure generation devices  19  and  20 , the pilot signal pressure is supplied to the pressure chamber of the arm control valve  16  due to the manipulation of the second pressure generation device  20  to cause an internal spool of the arm control valve  16  to be shifted to the right on the drawing sheet. At this time, the pressure detection means  26  detects the pilot signal pressure that is applied to the arm control valve  16  to perform the arm-out driving operation, and outputs a detection signal for application to the controller  27 . 
     In addition, the pilot signal pressure is applied to the swing control valve  14  so that the swing motor  13  can be driven in a left or right direction in response to the manipulation of the first pressure generation device  19 . At this time, the second pressure detection means  28  detects the pilot signal pressure outputted from the shuttle valve  23  and outputs a detection signal for application to the controller  27 . 
     The controller  27  outputs a control signal to the pressure reduction valve  25  to increase a secondary signal pressure that is outputted from the pressure reduction valve  25  when a swing pilot signal pressure is increased by the detection signal applied thereto from the second pressure detection means  28 . Further, the controller  27  outputs a control signal to the pressure reduction valve  25  to reduce the secondary signal pressure that is outputted from the pressure reduction valve  25  when the pilot signal pressure for performing the arm-out driving operation is applied to the arm control valve  16  by the detection signal applied thereto from the first pressure detection means  26 . 
     Thus, when the swing pilot signal pressure is increased, the controller  27  controls the secondary signal pressure outputted from the pressure reduction valve  25  to be increased so that the priority control valve  21  is shifted to a throttle state to restrict the supply of the hydraulic fluid to the arm control valve  16  from the second hydraulic pump  3 . On the other hand, when the pilot signal pressure for performing the arm-out driving operation is applied to the arm control valve  16 , the controller  27  controls the secondary signal pressure that outputted from the pressure reduction valve  25  to be reduced so that the priority control valve  21  is shifted to a throttle release state to maximally switch the flow path  29  along which the hydraulic fluid from the second hydraulic pump  3  is supplied to the arm control valve  16 . 
     Thus, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to the swing motor  13  via the swing control valve  14  along the second center bypass path  12  to cause the swing motor to be driven. At the same time, a part of the hydraulic fluid discharged from the second hydraulic pump  3  is supplied to a small chamber of the arm cylinder  15  via the following paths: the second center bypass path  12 →the parallel flow path  12   a →the priority control valve  21  of the throttle release state→the flow path  29 →the arm control valve  16 . 
     As described above, according to the priority control system in accordance with the fourth embodiment of the present invention, when the swing operation and the arm-out driving operation are simultaneously performed, the spool of the priority control valve  21  can be shifted to the throttle release state or the throttle state by pressure reduction valve  25 . 
     While the present invention has been described in connection with the specific embodiments illustrated in the drawings, they are merely illustrative, and the invention is not limited to these embodiments. It is to be understood that various equivalent modifications and variations of the embodiments can be made by a person having an ordinary skill in the art without departing from the spirit and scope of the present invention. Therefore, the true technical scope of the present invention should not be defined by the above-mentioned embodiments but should be defined by the appended claims and equivalents thereof. 
     INDUSTRIAL APPLICABILITY 
     As described above, according to the priority control system for a construction machine in accordance with the first to fourth embodiments of the present invention, when a swing apparatus of an excavator and a work apparatus or an attachment such as the arm are simultaneously manipulated, the priority control valve is shifted to a throttle state or a throttle release state depending on the amount of a load occurring in the hydraulic actuator so that simultaneous manipulability is maintained or an unnecessary pressure loss is prevented. In addition, the operating speed of the actuator can be secured to optimally control the distribution of the hydraulic fluid.