Hydraulic drive system for crawler-mounted construction vehicle

A hydraulic drive system for a crawler-mounted construction vehicle comprising:

• a hydraulic pump (1);
• a plurality of hydraulic actuators including first and second traveling hydraulic motors (2, 3) driven by a hydraulic fluid discharged by said hydraulic pump;
• a plurality of flow control valve means including first and second flow control valve means (15, 16;
• 35, 36) for respectively controlling the flow rates of said hydraulic fluid supplied from said hydraulic pump to said first and second traveling hydraulic motors;
• pump controlling means (11) for effecting control during the driving of said first and second traveling motors in such a manner that the discharge pressure of said hydraulic pump becomes higher by a fixed value than the higher one of load pressures of said traveling motors;
• and
• first and second pressure balance valve means (17, 18;
• 37, 38) for respectively controlling the rates of flow through said flow control valve means during the driving of said first and second traveling motors in such a manner that the load pressures of said traveling motors do not affect the rates of flow through said first and second flow control valve means,
• wherein said first and second pressure balance valve means (17, 18:
• 37, 38) are respectively provided with valve control means (62, 63;
• 66, 67) which do not cause the associated pressure balance valve means to be actuated until the differential pressure between the load pressure generated in said first traveling motor (2) and the load pressure generated in said second traveling motor (3) reaches a predetermined value (.DELTA.P.sub.0), and causes said pressure balance valve means to be actuated when said differential pressure exceeds said predetermined value.