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CROSS-REFERENCE TO RELATED APPLICATION 
   This application is based on and claims priority from Korean Patent Application No. 10-2006-0076296, filed on Aug. 11, 2006 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a hydraulic circuit for a construction machine, which can implement an auto idle function by automatically reducing revolution of an engine when a working device of the construction machine is not driven. 
   More particularly, the present invention relates to a hydraulic circuit for a construction machine, which can minimize an energy loss of a hydraulic system by automatically reducing revolution of an engine when a working device such as a boom is not driven. 
   Hereinafter, in the accompanying drawings, only the construction of pilot signal lines related to an auto idle function is illustrated. When corresponding switching valves are switched, the pilot signal lines are intercepted. The switching state of the valves and the connected lines between a main pump and a working device during the switching operation of the corresponding switching valves are not separately illustrated. 
   2. Description of the Prior Art 
   Referring to  FIG. 1 , a conventional hydraulic circuit for a construction machine having an auto idle function includes first, second, and third hydraulic pumps P 1 , P 2 , and P 3 ; a first switching valve A composed of valves installed in a flow path of the first hydraulic pump P 1  and shifted to control hydraulic fluid fed to working devices (right traveling motor, arm, boom, bucket, and so forth); a second switching valve B composed of valves installed in a flow path of the second hydraulic pump P 2  and shifted to control hydraulic fluid fed to working devices (left traveling motor, arm, option device, and so forth); a third switching valve C composed of valves installed in a flow path of the third hydraulic pump P 3  and shifted to control hydraulic fluid fed to a swing device and so on; and a confluence switching valve  8  installed on a downstream side of the flow path of the third hydraulic pump P 3  and shifted to selectively supply the hydraulic fluid from the third hydraulic pump P 3  to the working devices on the first hydraulic pump side P 1  or the working devices on the second hydraulic pump side P 2 , in response to a pilot signal pressure Pi 1  applied thereto. 
   In a general small-sized excavator, the hydraulic fluid fed from the first hydraulic pump P 1  is supplied to the right traveling motor and the hydraulic fluid fed from the second hydraulic pump P 2  is supplied to the left traveling motor to drive the traveling motors. In the case of driving other working devices (arm, boom, bucket, and so forth), the confluence switching valve  8  is used to supply the hydraulic fluid fed from the third hydraulic pump P 3  to the working devices. 
   The confluence switching valve  8  is shifted, in response to the pilot signal pressure Pi 1  applied thereto, to supply the hydraulic fluid fed from the third hydraulic pump P 3  to the working devices (arm, boom, bucket, and so forth) on the first hydraulic pump side P 1  or to the working devices (arm, boom, option device, and so forth) on the second hydraulic pump side P 2 . 
   The pilot signal pressure Pi 1  for shifting the confluence switching valve  8  is supplied from a pilot pump (not illustrated) through a first throttling part  1  installed in a pilot signal line  3 . 
   A signal line  4  includes a signal line  5  passing through the switching valves A and B for the working devices and a signal line  6  passing through a switching valve D for traveling devices. In the case where only either the working devices or the traveling devices are shifted to operate, no signal pressure is formed in the pilot signal line  3 . 
   By contrast, in the case where the working devices and the traveling devices are simultaneously shifted to operate, the pilot signal pressure Pi 1  is formed in the pilot signal line  3 , and the confluence switching valve  8  is shifted in response to the pilot signal pressure Pi 1  formed in the pilot signal line  3 . Accordingly, the hydraulic fluid fed from the third hydraulic pump P 3  is supplied to the working devices (arm, bucket, boom, and so forth) of the first hydraulic pump side P 1  or the working devices (arm, boom, option device, and so forth) of the second hydraulic pump side P 2 . 
   In the case of simultaneously implementing the above-described confluence circuit and the auto idle function, it is required to provide a signal device that can sense the shifting of the switching valves for the working devices and the switching valves for the traveling devices. Since the pressure is not formed in the pilot signal line  3  when either the switching valves for the working devices or the switching valves for the traveling devices are shifted, the pressure in the pilot signal line  3  cannot be used as an auto idle signal pressure. 
   That is, in the case of shifting the switching valves for the working devices or the switching valves for the traveling devices, a separate signal line  7  that can sense the shifting is required. The signal line  7  is connected to the signal line for supplying the pilot signal pressure to the confluence switching valve  8  and is connected to a flow path in which a second throttling part  2  is installed. In addition, the signal line  7  is constructed to pass through all the switching valves A, B, C, and D for the working devices and the traveling devices. 
   Accordingly, in a neutral state of the switching valves A, B, and C connected to the first to third hydraulic pumps P 1 , P 2 , and P 3 , respectively, it is judged that no signal pressure is formed in the signal line  7  and the working devices do not operate, and the engine revolution of the heavy equipment is automatically reduced. In the case of shifting at least one of the switching valves A, B, C, and D, the signal pressure is formed in the signal line  7 , and thus the engine revolution can be accelerated by the formed signal pressure. 
   Referring to  FIG. 2 , another conventional hydraulic circuit for a construction machine having an auto idle function includes a confluence switching valve  8  that is shifted by a pilot signal pressure Pi 1  fed through a third throttling part  11  formed in a pilot signal line  13 ; a signal line  15  which is connected to the pilot signal line  13  and in which a signal pressure is formed when switching valves A and B for working devices are shifted; a signal line  16  which is connected to the pilot signal line  13  and in which a signal pressure is formed when a switching valve D for working devices is shifted; and a signal line  17  which is connected to a pilot signal pressure Pi 2  formed through a fourth throttling part  12  and in which a signal pressure is formed when the switching valves A, B, C, and D for the working devices and the traveling devices connected to first to third hydraulic pumps P 1 , P 2 , and P 3 , respectively, are shifted. 
   The conventional hydraulic circuit of  FIG. 2  further includes the first, second, and third hydraulic pumps P 1 , P 2 , and P 3 ; the first switching valve A composed of valves installed in a flow path of the first hydraulic pump P 1  and shifted to control hydraulic fluid fed to working devices (right traveling motor, arm, and so forth); the second switching valve B composed of valves installed in a flow path of the second hydraulic pump P 2  and shifted to control hydraulic fluid fed to working devices (left traveling motor, boom, and so forth); and the third switching valve C composed of valves installed in a flow path of the third hydraulic pump P 3  and shifted to control hydraulic fluid fed to a swing device and so on. However, since these constituent elements are substantially the same as those of the circuit as illustrated in  FIG. 1 , the detailed description thereof will be omitted. The same drawing reference numerals are used for the same elements across various figures. 
   As illustrated in  FIGS. 1 and 2 , the conventional hydraulic circuits having an auto idle function requires a confluence circuit and separate auto idle signal lines, and this causes the construction of the signal lines to be complicated. In particular, the hydraulic circuit as illustrated in  FIG. 2  has a very complicated signal lines. 
   In addition, since the signal line  7  passes through all the switching valves A, B, C, and D of the working devices and the traveling devices, the hydraulic fluid may leak through joint surfaces of the respective switching valves A, B, C, and D. Particularly, in a high-temperature working environment, the formed auto-idle pressure may become unstable due to the leakage of the hydraulic fluid. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art while advantages achieved by the prior art are maintained intact. 
   One object of the present invention is to provide a hydraulic circuit for a construction machine, which can simplify the construction of signal lines in a hydraulic circuit having a confluence circuit and auto idle signal lines. 
   The hydraulic circuit for a construction machine according to one embodiment of the present invention can stably maintain the formed auto-idle pressure by minimizing the leakage of hydraulic fluid through joint surfaces of switching valves for working devices and traveling devices. 
   In order to accomplish these objects, there is provided a hydraulic circuit for a construction machine, according to one aspect of the present invention, which includes first, second, and third hydraulic pumps; a first switching valve composed of valves installed in a flow path of the first hydraulic pump and shifted to control hydraulic fluid fed to a right traveling device and working devices; a second switching valve composed of valves installed in a flow path of the second hydraulic pump and shifted to control hydraulic fluid fed to a left traveling device and working devices; a third switching valve composed of valves installed in a flow path of the third hydraulic pump and shifted to control hydraulic fluid fed to working devices; a confluence switching valve installed on a downstream side of the flow path of the third hydraulic pump and shifted to selectively supply the hydraulic fluid from the third hydraulic pump to the working devices on the first hydraulic pump side or the working devices on the second hydraulic pump side; a first shuttle valve selecting any one of a pressure of a first signal line in which a signal pressure is formed when the third switching valve for the working devices connected to the third hydraulic pump is shifted and a pressure of a second signal line in which a signal pressure is formed when a switching valve for the traveling devices is shifted; and a second shuttle valve selecting any one of the pressure selected by the first shuttle valve and a pressure of a third signal line in which a signal pressure is formed when switching valves for the working devices connected to the first and second hydraulic pumps are shifted. 
   The hydraulic circuit according to one aspect of the present invention further includes a valve having an inlet that is connected to a flow path connecting the second shuttle valve and the third signal line and an outlet that is connected to a pilot signal line for supplying a pilot signal pressure to the confluence switching valve. 
   In another aspect of the present invention, there is provided there is provided a hydraulic circuit for a construction machine, which includes first, second, and third hydraulic pumps; a first switching valve composed of valves installed in a flow path of the first hydraulic pump and shifted to control hydraulic fluid fed to a right traveling device and working devices; a second switching valve composed of valves installed in a flow path of the second hydraulic pump and shifted to control hydraulic fluid fed to a left traveling device and working devices; a third switching valve composed of valves installed in a flow path of the third hydraulic pump and shifted to control hydraulic fluid fed to working devices; a confluence switching valve installed on a downstream side of the flow path of the third hydraulic pump and shifted to selectively supply the hydraulic fluid from the third hydraulic pump to the working devices on the first hydraulic pump side or the working devices on the second hydraulic pump side; a first shuttle valve selecting any one of a pressure of a first signal line in which a signal pressure is formed when the third switching valve for the working devices connected to the third hydraulic pump is shifted and a pressure of a third signal line in which a signal pressure is formed when switching valves for the working devices connected to the first and second hydraulic pumps are shifted; and a second shuttle valve selecting any one of the pressure selected by the first shuttle valve and a pressure of a second signal line in which a signal pressure is formed when a switching valve for the traveling devices is shifted. 
   The hydraulic circuit according to another aspect of the present invention further includes a valve having an inlet that is connected to a flow path connecting the second shuttle valve and the second signal line and an outlet that is connected to a pilot signal line for supplying a pilot signal pressure to the confluence switching valve. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a circuit diagram of a conventional hydraulic circuit having an auto idle function; 
       FIG. 2  is a circuit diagram of another conventional hydraulic circuit having an auto idle function; 
       FIG. 3  is a circuit diagram of a hydraulic circuit for a construction machine having an auto idle function according to an embodiment of the present invention; and 
       FIG. 4  is a circuit diagram of a hydraulic circuit for a construction machine having an auto idle function according to another embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, preferred embodiments of the present invention will be described 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 thus the present invention is not limited thereto. 
     FIG. 3  is a circuit diagram of a hydraulic circuit for a construction machine having an auto idle function according to an embodiment of the present invention. 
   Referring to  FIG. 3 , the hydraulic circuit for a construction machine having an auto idle function according to an embodiment of the present invention includes first, second, and third hydraulic pumps P 1 , P 2 , and P 3 ; a first switching valve A composed of valves installed in a flow path of the first hydraulic pump P 1  and shifted to control hydraulic fluid fed to a right traveling device and working devices (arm, boom, bucket, and so forth); a second switching valve B composed of valves installed in a flow path of the second hydraulic pump P 2  and shifted to control hydraulic fluid fed to a left traveling device and working devices (arm, boom, option device, and so forth); a third switching valve C composed of valves installed in a flow path of the third hydraulic pump P 3  and shifted to control hydraulic fluid fed to working devices (swing device and so on); a confluence switching valve  8  installed on a downstream side of the flow path of the third hydraulic pump P 3  and shifted to selectively supply the hydraulic fluid from the third hydraulic pump P 3  to the working devices on the first hydraulic pump side P 1  or the working devices on the second hydraulic pump side P 2 , in response to a pilot signal pressure Pi 1  applied through a pilot signal line  31 ; a first shuttle valve  41  selecting any one of a pressure of a first signal line  34  in which a signal pressure is formed when the third switching valve C for the working devices connected to the third hydraulic pump P 3  is shifted and a pressure of a second signal line  33  in which a signal pressure is formed when a switching valve D for the traveling devices is shifted; and a second shuttle valve  42  selecting any one of the pressure selected by the first shuttle valve  41  and a pressure of a third signal line  32  in which a signal pressure is formed when switching valves A and B for the working devices connected to the first and second hydraulic pumps P 1  and P 2  are shifted. 
   The hydraulic circuit according to an embodiment of the present invention further includes a valve  100  having an inlet that is connected to a flow path  35  connecting the second shuttle valve  42  and the third signal line  32  and an outlet that is connected to a pilot signal line  31  for supplying a pilot signal pressure Pi 1  to the confluence switching valve  8 . 
   The pilot signal line  31 , in which first and second throttling part  21  and  22  are installed, is connected to a flow path for supplying the pilot signal pressure Pi 1 . 
   The second signal line  33  is installed to pass through the first throttling part  21  of the pilot signal line  31  and then through the switching valve D for the traveling devices, and is connected to a right end of the valve  100  along with the pilot signal line  31 . 
   The third signal line  32  is installed to pass through a third throttling part  23  and then through the switching valves A and B for the working devices, and is connected to a left end of the valve  100  through the flow path  35 . 
   Hereinafter, the operation of the hydraulic circuit for a construction machine according to an embodiment of the present invention will be described with reference to the accompanying drawings. 
   As illustrated in  FIG. 3 , the hydraulic fluid fed from the first hydraulic pump P 1  is supplied to the right traveling motor and the hydraulic fluid fed from the second hydraulic pump P 2  is supplied to the left traveling motor to drive the traveling motors. In the case of driving other working devices (arm, boom, bucket, and so forth), the confluence switching valve  8  is used to supply the hydraulic fluid fed from the third hydraulic pump P 3  to the working devices. 
   The confluence switching valve  8  is shifted, in response to the pilot signal pressure Pi 1  applied thereto through the first and second throttling parts  21  and  22  installed in the pilot signal line  31 , to supply the hydraulic fluid fed from the third hydraulic pump P 3  to the working devices (arm, boom, bucket, and so forth) on the first hydraulic pump side P 1  or to the working devices (arm, boom, option device, and so forth) on the second hydraulic pump side P 2 . 
   The pilot signal pressure Pi 1  for shifting the confluence switching valve  8  is supplied from a pilot pump (not illustrated) through a first throttling part  1  installed in a pilot signal line  3 . 
   In the case of shifting only the switching valves A and B for the working devices connected to the first and second hydraulic pumps P 1  and P 2 , a signal pressure is formed in the third signal line  32 , but no signal pressure is formed in the second signal line  33  connected to the switching valve D for the traveling devices and in the pilot signal line  31  for supplying the pilot signal pressure Pi 1  to the confluence switching valve  8 . Accordingly, the confluence switching valve  8  is not shifted. 
   By contrast, in the case of shifting only the switching valve D for the traveling devices connected to the first and second hydraulic pumps P 1  and P 2 , a signal pressure is formed in the second signal line  33  and in the pilot signal line  31 , but no signal pressure is formed in the third signal line  32  connected to the switching valves A and B for the working devices. 
   Accordingly, as a piston inside the valve  100  is moved to the left as illustrated in  FIG. 3  and the pilot signal line  31  is connected to a hydraulic tank, no signal pressure is formed in the pilot signal line  31 , and thus the confluence switching valve  8  is not shifted. 
   On the other hand, in the case of simultaneously shifting the switching valve D for the traveling devices and the switching valves A and B for the working devices, the signal pressure is formed in the pilot signal line  31 , the third signal line  32 , and the second signal line  33 , and thus the confluence switching valve  8  is shifted. 
   Accordingly, the hydraulic fluid fed from the third hydraulic pump P 3  is supplied to the working devices (arm, boom, bucket, and so forth) of the first hydraulic pump side P 1  or the working devices (arm, boom, option device, and so forth) of the second hydraulic pump side P 2  to drive the working devices. 
   Specifically, the first shuttle valve  41  compares the pressure of the first signal line  34  in which the signal pressure is formed when the third switching valve C for the working devices connected to the third hydraulic pump P 3  is shifted with the pressure of the second signal line  33  in which the signal pressure is formed when the switching valve D for the traveling devices is shifted, and selects one of the pressures. 
   The second shuttle valve  42  compares the pressure selected by the first shuttle valve  41  with the pressure of the third signal line  32  in which the signal pressure is formed when the switching valves A and B for the working devices connected to the first and second hydraulic pumps P 1  and P 2  are shifted. 
   Accordingly, the signal pressure is formed in the signal lines  31 ,  32 ,  33 , and  34  when the switching valves A, B, C, and D connected to the first, second, and third hydraulic pumps P 1 , P 2 , and P 3 , respectively, and the signal pressure is used as an auto idle pressure. 
   As described above, in the case of forming the confluence circuit and the auto idle signal lines in the hydraulic circuit for a construction machine according to one embodiment of the present invention, a signal line  34  for passing through only the switching valve C of the third hydraulic pump side P 3  is separately formed to implement the auto idle function. 
   The hydraulic circuit as constructed above according to the present invention can minimize the leakage of the hydraulic fluid through the joint surfaces of the respective switching valves in comparison to the conventional hydraulic circuit in which the auto idle signal line passes through all the working devices. Also, the hydraulic circuit according to the present invention can stably maintain the auto idle pressure. 
     FIG. 4  is a circuit diagram of a hydraulic circuit for a construction machine having an auto idle function according to another embodiment of the present invention. 
   Referring to  FIG. 4 , the hydraulic circuit for a construction machine having an auto idle function according to another embodiment of the present invention includes first, second, and third hydraulic pumps P 1 , P 2 , and P 3 ; a first switching valve A composed of valves installed in a flow path of the first hydraulic pump P 1  and shifted to control hydraulic fluid fed to a right traveling device and working devices (arm, boom, bucket, and so forth); a second switching valve B composed of valves installed in a flow path of the second hydraulic pump P 2  and shifted to control hydraulic fluid fed to a left traveling device and working devices (arm, boom, option device, and so forth); a third switching valve C composed of valves installed in a flow path of the third hydraulic pump P 3  and shifted to control hydraulic fluid fed to working devices (swing device and so on); a confluence switching valve  8  installed on a downstream side of the flow path of the third hydraulic pump P 3  and shifted to selectively supply the hydraulic fluid from the third hydraulic pump P 3  to the working devices on the first hydraulic pump side P 1  or the working devices on the second hydraulic pump side P 2 , in response to a pilot signal pressure Pi 1  applied through a pilot signal line  31 ; a first shuttle valve  41  selecting any one of a pressure of a first signal line  34  in which a signal pressure is formed when the third switching valve C for the working devices connected to the third hydraulic pump P 3  is shifted and a pressure of a third signal line  32  in which a signal pressure is formed when switching valves A and B for the working devices connected to the first and second hydraulic pumps P 1  and P 2  are shifted; and a second shuttle valve  42  selecting any one of the pressure selected by the first shuttle valve  41  and a pressure of a second signal line  33  in which a signal pressure is formed when a switching valve D for the traveling devices is shifted. 
   The hydraulic circuit according to another embodiment of the present invention further includes a valve  100  having an inlet that is connected to a flow path connecting the second shuttle valve  42  and the third signal line  32  and an outlet that is connected to a pilot signal line  31  for supplying a pilot signal pressure Pi 1  to the confluence switching valve  8 . 
   Since the constituent elements, such as the first, second, and third hydraulic pumps P 1 , P 2 , and P 3 , the first switching valve A composed of the valves installed in the flow path of the first hydraulic pump P 1  and shifted to control the hydraulic fluid fed to the right traveling device and the working devices (arm, boom, and so forth), the second switching valve B composed of the valves installed in the flow path of the second hydraulic pump P 2  and shifted to control the hydraulic fluid fed to the left traveling device and the working devices (boom, option device, and so forth), the third switching valve C composed of the valves installed in the flow path of the third hydraulic pump P 3  and shifted to control the hydraulic fluid fed to the working devices (swing device and so on), are substantially the same as those of the circuit as illustrated in  FIG. 3 , the detailed description thereof will be omitted. The same drawing reference numerals are used for the same elements across various figures. 
   The first shuttle valve  41  compares the pressure of the first signal line  34  in which the signal pressure is formed when the third switching valve C for the working devices connected to the third hydraulic pump P 3  is shifted with the pressure of the third signal line  32  in which the signal pressure is formed when the switching valves A and B for the working devices connected to the first and second hydraulic pumps P 1  and P 2  are shifted, and selects one of the pressures. 
   The second shuttle valve  42  compares the pressure selected by the first shuttle valve  41  with the pressure of the second signal line  33  in which the signal pressure is formed when the switching valve D for the traveling devices is shifted, and selects one of the pressures. 
   Accordingly, the signal pressure is formed in the signal lines  31 ,  32 ,  33 , and  34  when the switching valves A, B, C, and D connected to the first, second, and third hydraulic pumps P 1 , P 2 , and P 3 , respectively, and the signal pressure is used as the auto idle pressure. 
   As described above, the hydraulic circuit for a construction machine according to the present invention has the following advantages. 
   The construction of the signal lines in the hydraulic circuit having the confluence circuit and the auto idle signal lines can be simplified and thus the manufacturing cost can be reduced. 
   The leakage of the hydraulic fluid through the joint surfaces of the respective switching valves for the working devices and the traveling devices can be minimized, and thus the formed auto idle pressure can be stabilized to heighten the reliability of the hydraulic circuit. 
   Although preferred embodiment of the present invention has been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Summary:
A hydraulic circuit for a construction machine is disclosed, which can prevent an energy loss of a hydraulic system by automatically reducing revolution of an engine when a working device such as a boom is not driven. The hydraulic circuit includes first to third hydraulic pumps, a first switching valve, a second switching valve, a third switching valve, a confluence switching valve, a first shuttle valve selecting any one of a pressure of a first signal line in which a signal pressure is formed when the third switching valve for working devices connected to the third hydraulic pump is shifted and a pressure of a second signal line in which a signal pressure is formed when a switching valve for traveling devices is shifted, and a second shuttle valve selecting any one of the pressure selected by the first shuttle valve and a pressure of a third signal line in which a signal pressure is formed when switching valves for the working devices connected to the first and second hydraulic pumps are shifted.