Abstract:
A travel control system is disclosed for preventing off-course travel of equipment due to the overloading of a working piece such as a boom during a complex operation involving vehicle travel and simultaneous driving of the working piece. The travel control system according to the present invention comprises: a left-side travel motor and first working piece coupled to a first hydraulic pump; a plurality of change-over valves for respectively controlling operating fluid supplied from the first hydraulic pump to the left-side travel motor and first working piece; a right-side travel motor and second working piece coupled to a second hydraulic pump; a plurality of change-over valves for controlling operating fluid supplied from the second hydraulic pump to the right-side travel motor and second working piece; a straight-ahead travel valve for supplying the operating fluid of the first hydraulic pump to the left-side and right-side travel motors and supplying the operating fluid of the second hydraulic pump to the first and second working pieces; and a control valve for blocking the supply of operating fluid from the second hydraulic pump, via the straight-ahead travel valve, to the left-side travel motor and right-side travel motor during complex operation involving travel and the working piece.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to traveling control system for a construction machine. More particularly, the present invention relates to a traveling control system for a construction machine, which can prevent a single traveling of the machine due to occurrence of an overload in an attachment (or work apparatus) such as a boom during a combined operation in which a traveling operation and a working operation are performed simultaneously. 
       BACKGROUND OF THE INVENTION 
       [0002]    A traveling control system for a construction machine in accordance with the prior art as shown in  FIGS. 1 and 2  includes: 
         [0003]    first and second variable displacement hydraulic pumps (hereinafter, “first and second hydraulic pumps”)  15  and  18 ; 
         [0004]    a left traveling motor  2  that is connected to the first hydraulic pump  15  and is driven by being supplied with a hydraulic fluid, and a first attachment (not shown) such as an arm; 
         [0005]    a plurality of switching valves  12  and  26  that are installed in a flow path  1  of the first hydraulic pump  15  and are shifted in response to pilot signal pressures a1 and b1 applied thereto to control a hydraulic fluid being supplied to the left traveling motor  2  to the first attachment; 
         [0006]    a right traveling motor  3  that is connected to the second hydraulic pump  18  and is driven by being supplied with the hydraulic fluid, and a second attachment (not shown) such as a boom; 
         [0007]    a plurality of switching valves  11  and  28  that are installed in a flow path  9  of the second hydraulic pump  18  and are shifted in response to pilot signal pressures a2 and b2 applied thereto to control a hydraulic fluid being supplied to the right traveling motor  3  or the second attachment; and 
         [0008]    a straight traveling valve  4  that is installed in the flow path  9  and is shifted in response to a pilot signal pressure a3 to supply the hydraulic fluid discharged from the first hydraulic pump  15  to the left and right traveling motors  2  and  3  and to supply a part of the hydraulic fluid discharged from the second hydraulic pump  18  to the switching valve  26  for the first attachment through a flow path  32  and simultaneously supply a part of the hydraulic fluid discharged from the second hydraulic pump  18  to the switching valve  28  for the second attachment through the flow path  7 , respectively. 
         [0009]    A non-explained reference numeral  10  denotes a main relief valve that drains a hydraulic fluid of an excessive pressure to a hydraulic tank T when an overload exceeding a set pressure in a hydraulic circuit occurs, and a reference symbol s denotes a spool on the switching valve  11  for controlling the hydraulic fluid supplied to the right traveling motor  3 . 
         [0010]    A) The case of performing the traveling operation alone will be described hereinafter. 
         [0011]    When the pilot signal pressure a1 is applied to the switching valve  12  for the left traveling motor, a spool of the switching valve  12  is shifted to the left on the drawing sheet. Thus, a hydraulic fluid discharged from the first hydraulic pump  15  is supplied to the left traveling motor  2  via the flow path  1 , the switching valve  12 , and a traveling line  14  in this order. 
         [0012]    When the pilot signal pressure a2 is applied to the switching valve  11  for the right traveling motor, a spool of the switching valve  11  is shifted to the right on the drawing sheet. Thus, a hydraulic fluid discharged from the second hydraulic pump  18  is supplied to the right traveling motor  3  via the flow path  9 , the switching valve  11 , and a traveling line  20  in this order. In other words, in the case where the left traveling motor  2  or the right traveling motor  3  is driven alone, the hydraulic fluid discharged from the first hydraulic pump  15  is supplied to the left traveling motor  2 , and the hydraulic fluid discharged from the second hydraulic pump  18  is supplied to the right traveling motor  3 . 
         [0013]    B) The case of performing the combined operation of the traveling operation and the working operation will be described hereinafter. 
         [0014]    When the pilot signal pressure a3 is applied to the straight traveling valve  4 , a spool of the straight traveling valve  4  is shifted to the right on the drawing sheet. At the same time, when the pilot signal pressure b1 is applied to the switching valve  26  for the first attachment, a spool of the switching valve  26  is shifted to the left on the drawing sheet. When a signal pressure c1 is applied to a first center bypass valve  22 , a spool of the first center bypass valve  22  is shifted to the left on the drawing sheet to form a pressure in a first center bypass flow path. 
         [0015]    Thus, apart of the hydraulic fluid from the first hydraulic pump  15  is supplied to the left traveling motor  2  via the flow path  1 , the switching valve  12 , and the traveling line  14  in this order. At the same time, a part of the hydraulic fluid from the first hydraulic pump  15  is supplied to the right traveling motor  3  via the flow path  9 , the straight traveling valve  4 , the switching valve  11 , and the traveling line  20  in this order. That is, the hydraulic fluid discharged from the first hydraulic pump  15  is used to drive the left traveling motor  2  and the right traveling motor  3 . 
         [0016]    Meanwhile, the hydraulic fluid from the second hydraulic pump  18  is supplied to the switching valve  26  for the first attachment via the flow path  9 , the straight traveling valve  4 , and the flow path  32  in this order to drive a corresponding attachment (e.g., an arm). That is, the hydraulic fluid discharged from the second hydraulic pump  18  is used to drive a corresponding attachment by being supplied to the switching valve  26  for the first attachment. 
         [0017]    Under the straight traveling condition as described above, when the spool of the switching valve  26  is shifted to a full stroke by gradually increasing a pressure needed to shift the switching valve  26  for the first attachment, the pressure rises up to the set pressure of the main relief valve  10 . In this case, the hydraulic fluid from the second hydraulic pump  18  is not supplied to the switching valve  26  for the first attachment any more. 
         [0018]    In other words, apart of the hydraulic fluid being supplied to the switching valve  26  is supplied to the right traveling motor  3  via the check valve  5  and the orifice  6  after passing through the flow path  32 , the straight traveling valve  4 , the flow path  9 , and the flow path  7 . In addition, a part of the hydraulic fluid being supplied to the switching valve  26  is supplied to the left traveling motor  2  via the flow path  8 . 
         [0019]    In this case, the switching valves  12  and  11  for the traveling motors are shifted in response to the pilot signal pressures a1 and a2 applied thereto. When the combined operation is performed, a pilot signal pressure on the traveling side is maintained at about 10-12K to shift the switching valves  11  and  12 . For this reason, in case of an intermediate shift section, the switching valves  11  and  12  for the traveling motors can be controlled by a P-N notch (i.e., a notch that controls the hydraulic fluid flowing from the hydraulic pump to the hydraulic tank), a P-C notch (i.e., a notch that controls the hydraulic fluid flowing from the hydraulic pump to the hydraulic cylinder), and a C-T notch (i.e., a notch that controls the hydraulic fluid flowing from the hydraulic cylinder to the hydraulic tank). 
         [0020]    In the structure of the conventional hydraulic circuit, in the case where the switching valve  26  and the first center bypass valve  22  are shifted, no hydraulic fluid flows by the P-N notch. For this reason, the switching valves  11  and  12  can be controlled by the P-C notch or the C-T notch. In this case, the spool notches of the switching valves  11  and  12  for the traveling motors have the same structure. On the other hand, it is difficult to maintain the same cross section due to a difference in the stack tolerance for processing the spool and the process conditions. 
         [0021]    In other words, the flow rate of a hydraulic fluid passing through the spool is in proportion to the cross section of the spool. Thus, if there is a difference in cross section of the spool notch, the flow rates of the hydraulic fluids passing through the switching valves  12  and  11  for the traveling motors are different from each other. That is, if the flow rates of the hydraulic fluids passing through the switching valves  12  and  11  for the traveling motors are different from each other, the drive speed of the traveling motor through which a relatively large amount of hydraulic fluid passes is abruptly increased. On the contrary, the drive speed of the traveling motor through which a relatively small amount of hydraulic fluid passes is decreased. 
         [0022]    As described above, the spools of the switching valves  12  and  11  for the traveling motors are shifted to an intermediate level to drive the traveling motors  2  and  3 . In this case, the spool of the straight traveling valve  4  is in a state of having been completely shifted. At the same time, a single traveling of the machine is caused due to occurrence of an overload in the attachment during the combined operation in which the traveling operation and the working operation of an attachment such as a boom are performed. 
         [0023]    In addition, when an attachment has a load applied thereto during the traveling of the machine (e.g., a state in which a heavy pipe or the like is lifted), it is not operated. That is, in the case where a boom or the like is operated during the traveling of the machine, when a great load occurs on the boom and a relatively small load occurs on the traveling side, the boom is not operated. 
         [0024]    When the switching valve  26  for the attachment is shifted while driving the switching valves  12  and  11  for the traveling motors, the hydraulic fluid from the first hydraulic pump  15  drives the left traveling motor  2  and the hydraulic fluid from the second hydraulic pump  18  drives the right traveling motor  3 . In this case, the straight traveling valve  4  is not shifted. 
         [0025]    In the case where the switching valve  26  for the attachment is shifted, the straight traveling valve  4  is shifted by the pilot signal pressure a3. In this case, hydraulic fluid from the first hydraulic pump  15  is supplied to the switching valves  12  and  11  via the flow path  1  and the flow path  8 , respectively. In addition, the hydraulic fluid from the second hydraulic pump  18  is supplied to the switching valve  26  via the flow path  9 , and is supplied to the switching valve  11  after passing through the check valve  5  and the orifice  6  via the flow path  7 . 
         [0026]    Meanwhile, in the case where a great load occurs on the hydraulic fluid being supplied to switching valve  26  (e.g., the case in which an orifice is formed), i.e., an orifice smaller than the orifice  6  of the switching valve  11  side is installed, all the hydraulic fluids from the second hydraulic pump  18  are supplied to the switching valve  11 . As a result, there is caused a problem in that the attachment of the switching valve  26  side is not driven. 
         [0027]    In an attempt to address and solve the above-mentioned problem, a gap  16  defined between the outer periphery of a poppet  13  and the inner periphery of a body  17  of a switching valve is machined to have a small size to make the orifice  6  small as shown in  FIG. 2 . A non-explained reference numeral  19  denotes an elastic member (e.g., a compression coil spring) that presses the poppet  13  to elastically biases the poppet  13  to an initial state from the blocked state of the branch flow path  7   a.    
         [0028]    On the other hand, when the gap is machined to have a large size, the poppet  13  and the body  17  of the switching valve come into close contact with each other to cause a noise. For this reason, the gap  16  between the poppet  13  and the body  17  of the switching valve is machined to have a minimum size within a tolerance range in which any noise is not generated from the gap. 
       DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problems 
       [0029]    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 traveling control system for a construction machine, which can prevent a single traveling of the machine due to occurrence of an overload in an attachment such as a boom during a combined operation in which a traveling operation and a working operation are performed simultaneously, and enables the combined operation of a traveling operation and a working operation to be performed even when a load occurs in the attachment 
       Technical Solution 
       [0030]    To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a traveling control system for a construction machine in accordance with an embodiment of the present invention, the system including: 
         [0031]    first and second variable displacement hydraulic pumps; 
         [0032]    a left traveling motor connected to the first hydraulic pump and a first attachment; 
         [0033]    a plurality of switching valves installed in a flow path  1  of the first hydraulic pump and configured to be shifted to control a hydraulic fluid being supplied to the left traveling motor and the first attachment; 
         [0034]    a right traveling motor connected to the second hydraulic pump and a second attachment; 
         [0035]    a plurality of switching valves installed in a flow path of the second hydraulic pump and configured to be shifted to control a hydraulic fluid being supplied to the right traveling motor and the second attachment; 
         [0036]    a straight traveling valve installed in the flow path of the second hydraulic pump and configured to be shifted to supply the hydraulic fluid discharged from the first hydraulic pump to the left and right traveling motors and to supply the hydraulic fluid discharged from the second hydraulic pump to the first attachment and the second attachment, respectively; and 
         [0037]    a control valve installed in a branch flow path having an inlet side that is connected to a flow path branched off from the flow path of the second hydraulic pump and an outlet side that is connected to the flow path of the second hydraulic pump on a downstream side of the straight traveling valve, and configured to serve as a check valve and an orifice so as to interrupt the supply of the hydraulic fluid from the second hydraulic pump to the left traveling motor and the right traveling motor via the straight traveling valve during a combined operation in which a traveling operation and a working operation are performed simultaneously. 
         [0038]    In a preferred embodiment of the present invention, the control valve may include: 
         [0039]    a first poppet configured to open/close the branch flow path that fluidically communicates with an inlet-side flow path of the switching valve for the traveling motor, the first poppet having a first orifice formed thereon; 
         [0040]    a second poppet installed inside the first poppet and having a second orifice formed thereon; 
         [0041]    an elastic member configured to allow the second poppet to be pressed against the first poppet to elastically support the second poppet in a state in which a flow path of the first poppet is closed; and 
         [0042]    a flange securely fixed to a body of the control valve to support the elastic member so as to allow the first and second poppet to be kept at set pressures thereof. 
         [0043]    A control valve that is shifted to an on/off state to open/close the pilot signal line in response to a control signal applied from the outside may be used as a valve installed on a pilot signal line for supplying a pilot signal pressure to the straight traveling valve to shift the straight traveling valve. 
         [0044]    An electronic proportional valve that outputs a secondary pilot signal pressure generated during the driving in proportion to a control signal applied from the outside may be used as a valve installed on a pilot signal line for supplying pilot signal pressure to the straight traveling valve to shift the straight traveling valve. 
         [0045]    The first attachment connected to the first hydraulic pump may be any one selected from a boom, an arm, a bucket, a swing motor, and a winch motor. 
         [0046]    The control valve may include a tapered portion formed on the outer surface of the first poppet that is in close contact with the body of the control valve to serve as a damper when the branch flow path is blocked through the mutual close contact between the first poppet and the body of the control valve. 
         [0047]    The control valve may include a notch portion formed on the outer surface of the first poppet that is in close contact with the body of the control valve to serve as a damper when the branch flow path is blocked through mutual close contact between the first poppet and the body of the control valve. 
         [0048]    The control valve may include a sealing O-ring that prevents the hydraulic fluid from leaking to the outside through a gap of a close contact surface between the body of the control valve and the flange. 
       Advantageous Effect 
       [0049]    The travel control system for a construction machine in accordance with an embodiment of the present invention as constructed above has the following advantages. 
         [0050]    It is possible to prevent a single traveling of the machine due to occurrence of an overload in an attachment such as a boom, and ensure the workability of the attachment, thereby improving the manipulability of the attachment during a combined operation in which a traveling operation and a working operation are performed simultaneously. In addition, when an operation mode is switched to a neutral position, occurrence of a shock can be prevented and the manufacturing cost can be reduced owing to simplicity of the structure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0051]    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: 
           [0052]      FIG. 1  is a hydraulic circuit diagram showing a traveling control system for a construction machine in accordance with the prior art; 
           [0053]      FIG. 2  is an exploded cross-sectional view showing a main element of a switching valve for traveling shown in  FIG. 1 ; and 
           [0054]      FIG. 3  is an exploded cross-sectional view showing a main element of a switching valve for traveling in a control system for a construction machine in accordance with an embodiment of the present invention. 
       
    
    
     EXPLANATION ON REFERENCE NUMERALS OF MAIN ELEMENTS IN THE DRAWINGS 
       [0000]    
       
         
           
               7   a : branch flow path 
               30 : control valve 
               31 : first orifice 
               32 : first poppet 
               33 : second orifice 
               34 : second poppet 
               35 : elastic member 
               36 : fastening member 
               37 : flange 
               38 : O-ring 
           
         
       
     
       PREFERRED EMBODIMENTS OF THE INVENTION 
       [0065]    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. 
         [0066]    As shown in  FIG. 3 , a traveling control system for a construction machine in accordance with an embodiment of the present invention includes: 
         [0067]    first and second variable displacement hydraulic pumps (hereinafter, “first and second hydraulic pumps”)  15  and  18 ; 
         [0068]    a left traveling motor  2  that is connected to the first hydraulic pump  15  and a first attachment (e.g., an arm); 
         [0069]    a plurality of switching valves  12  and  26  that are installed in a flow path  1  of the first hydraulic pump  15  and are shifted to control a hydraulic fluid being supplied to the left traveling motor  2  and the first attachment; 
         [0070]    a right traveling motor  3  that is connected to the second hydraulic pump  18  and a second attachment (e.g., a boom); 
         [0071]    a plurality of switching valves  11  and  28  that are installed in a flow path  9  of the second hydraulic pump  18  and are shifted to control a hydraulic fluid being supplied to the right traveling motor  3  and the second attachment; 
         [0072]    a straight traveling valve  4  that is installed in the flow path  9  of the second hydraulic pump  18  and is shifted to supply the hydraulic fluid discharged from the first hydraulic pump  15  to the left and right traveling motors  2  and  3  and to supply the hydraulic fluid discharged from the second hydraulic pump  18  to the first attachment and the second attachment, respectively; and 
         [0073]    a control valve  30  that is installed in a branch flow path  7   a  having an inlet side that is connected to a flow path  7  branched off from the flow path  9  of the second hydraulic pump  18  and an outlet side that is connected to the flow path  9  of the second hydraulic pump  18  on a downstream side of the straight traveling valve  4 , and serves as a check valve and an orifice so as to interrupt the supply of the hydraulic fluid from the second hydraulic pump  18  to the left traveling motor  2  and the right traveling motor  3  via the straight traveling valve  4  during a combined operation in which a traveling operation and a working operation are performed simultaneously. 
         [0074]    The control valve  30  includes: 
         [0075]    a first poppet  32  that opens/closes the branch flow path  7   a  that fluidically communicates with an inlet-side flow path of the switching valve  11  for the right traveling motor, the first poppet having a first orifice  31  formed thereon; 
         [0076]    a second poppet  34  that is installed inside the first poppet  32  and having a second orifice  33  formed thereon; 
         [0077]    an elastic member (e.g., a compression coil spring)  35  that allows the second poppet  34  to be pressed against the first poppet  32  to elastically support the second poppet  34  in a state in which a flow path  32   a  of the first poppet  32  is closed; and 
         [0078]    a flange  37  that is securely fixed to a body  17  of the control valve by means of a fastening member (e.g., a bolt) to support the elastic member  35  so as to allow the first and second poppet  34  to be kept at set pressures thereof. 
         [0079]    A control valve (not shown) that is shifted to an on/off state to open/close the pilot signal line in response to a control signal applied from the outside may be used as a valve installed on a pilot signal line for supplying a pilot signal pressure to the straight traveling valve  4  to shift the straight traveling valve  4 . 
         [0080]    An electronic proportional valve (not shown) that outputs a secondary pilot signal pressure generated during the driving in proportion to a control signal applied from the outside may be used as a valve installed on a pilot signal line for supplying pilot signal pressure to the straight traveling valve  4  to shift the straight traveling valve  4 . 
         [0081]    The first attachment connected to the first hydraulic pump  15  is any one selected from a boom, an arm, a bucket, a swing motor, and a winch motor, except the traveling motors. 
         [0082]    The control valve  30  includes a tapered portion (not shown) formed on the outer surface of the first poppet  32  that is in close contact with the body  17  of the control valve to serve as a damper when the branch flow path  7   a  is blocked through the mutual close contact between the first poppet  32  and the body  17  of the control valve. 
         [0083]    The control valve  30  includes a notch portion (not shown) formed on the outer surface of the first poppet  32  that is in close contact with the body  17  of the control valve to serve as a damper when the branch flow path  7   a  is blocked through mutual close contact between the first poppet  32  and the body  17  of the control valve. 
         [0084]    The control valve system further includes a sealing O-ring that prevents the hydraulic fluid from leaking to the outside through a gap of a close contact surface between the body  17  of the control valve and the flange  37 . 
         [0085]    In this case, a configuration of a control system for a construction machine in accordance with an embodiment of the present invention is the same as that of the hydraulic system shown in  FIG. 1 , except the control valve  30  that is installed in a branch flow path  7   a  and serves as a check valve and an orifice so as to prevent a single traveling of the machine during a combined operation in which a traveling operation and a working operation are performed 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. 
         [0086]    Hereinafter, a use example of a traveling control system for a construction machine in accordance with an embodiment of the present invention will be described in detail with reference to the accompanying drawings. 
         [0087]    As shown in  FIG. 3 , a case of a combined operation in which an attachment such as an arm is driven during the traveling of the machine will be described hereinafter. 
         [0088]    When a spool inside the straight traveling valve  4  is shifted to the right on the drawing sheet of  FIG. 1  in response to a pilot signal pressure a3 applied to the straight traveling valve  4 , a hydraulic fluid discharge from the first hydraulic pump  15  is supplied to the left traveling motor  2  via the flow path  1 , the switching valve  12 , and a traveling line  14  in this order. In addition, the hydraulic fluid from the first hydraulic pump  15  is supplied to the right traveling motor  3  via the flow path  8 , the straight traveling valve  4 , the switching valve  11 , and a traveling line  20  in this order, so that these elements are driven, respectively. 
         [0089]    At the same time, a hydraulic fluid discharge from the second hydraulic pump  18  is supplied to an attachment such as an arm via the flow path  9 , the straight traveling valve  4 , a flow path  32 , and the switching valve  26  in this order. In addition, the hydraulic fluid from the second hydraulic pump  18  is moved to the flow path  7  via the flow path  32 , the straight traveling valve  4 , and the flow path  9  in this order. The hydraulic fluid moved to the flow path  7  sequentially passes through a check valve  5  and an orifice  6  that are installed in the branch flow path  7   a.    
         [0090]    In other words, the hydraulic fluid from the second hydraulic pump  18  is moved to the branch flow path  7   a  to cause the first poppet  32  to be pushed to the top on the drawing sheet due to a difference in cross section between the second poppet  34  and a pressure-receiving portion, so that the branch flow path  7   a  is opened. At this time, the second poppet  34  is closed by an elastic force of the elastic member  35  to cause the flow path  32   a  of the first poppet  32  to be blocked. 
         [0091]    In this case, a stroke of the first poppet  32  is small, and thus the hydraulic fluid in the branch flow path  7   a  passes through a gap (a) defined between the first poppet  32  and the body  17  of the control valve  30 . That is, the hydraulic fluid in the branch flow path  7   a  passes through the tapered portion or the notch portion formed on the first poppet  32 . The second poppet  34  is pushed to the top on the drawing sheet to open the flow path  32   a  by a pressure introduced into the flow path  32   a  of the first poppet  32  due to an increase in pressure of the branch flow path  7   a.    
         [0092]    Thus, a part of the hydraulic fluid in the branch flow path  7   a  passes through the flow path  32   a  and the first orifice  31  that are formed in the first poppet  32 , and simultaneously passes through the gap (a) defined between the first poppet  32  an the body  17  of the control valve  30 . 
         [0093]    Meanwhile, in the case where the supply of the hydraulic fluid to the branch flow path  7   a  is interrupted, the first poppet  32  returns to an initial position to cause the first poppet  32  an the body  17  of the control valve  30  to come into close contact with each other, so that the branch flow path  7   a  is blocked and then the second poppet  34  returns to an initial position by an elastic restoring force of the elastic member  35  to block the flow path  32   a  of the first poppet  32 . In other words, when the supply of the hydraulic fluid to the branch flow path  7   a  is interrupted, the first poppet  32  and the second poppet  34  are sequentially blocked. Thus, it is possible to prevent a shock (frequently occurring when the operation mode is switched to a neutral position after manipulating a manipulation lever (i.e., RCV lever)) from occurring when the first poppet  32  and the body  17  of the control valve  30  come into close contact with each other. 
         [0094]    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 
       [0095]    As described above, according to the present invention as constructed above, it is possible to prevent a single traveling of the machine due to occurrence of an overload in an attachment such as a boom, and ensure the workability of the attachment, thereby improving the manipulability of the attachment during a combined operation in which a traveling operation and a working operation are performed simultaneously.