Hydraulic circuit system for civil engineering and construction machines

A hydraulic circuit system for civil engineering and construction machines comprising first and second hydraulic pumps;

• a plurality of hydraulic actuators driven by hydraulic fluid delivered from said first and second hydraulic pumps;
• a first group of valves connected to a delivery line of said first hydraulic pump for controlling flow rates of hydraulic fluid supplied to the plurality of hydraulic actuators;
• and a second group of valves connected to a delivery line of said second hydraulic pump for controlling flow rates of hydraulic fluid supplied to the plurality of hydraulic actuators;
• said plurality of hydraulic actuators including first and second traveling motors for driving a pair of traveling devices, respectively, and a plurality of working actuators for driving a plurality of working elements, respectively, said first group of valves including a first traveling directional control valve for controlling a flow rate of hydraulic fluid supplied to said first traveling motor and a plurality of first directional control valves for controlling flow rates of hydraulic fluid supplied to at least part of said plurality of working actuators, said plurality of first directional control valves being connected to said first traveling directional control valve so as to supply a hydraulic fluid from a hydraulic pump to said plurality of working actuators with a priority over said first traveling motor, said second group of valves including a second traveling directional control valve for controlling a flow rate of hydraulic fluid supplied to said second traveling motor and a plurality of second directional control valves for controlling flow rates of hydraulic fluid supplied to at least part of said plurality of working actuators, said second directional control valve being connected to said plurality of second directional control valves so as to supply hydraulic fluid from said second hydraulic pump to said second traveling motor with a priority over said plurality of working actuators, said first and second traveling directional control valves having first and second variable restrictors for controlling the flow rate of said hydraulic fluid by changing an open area in accordance with an input amount of first and second operation means, respectively, and further comprising a communication circuit for communicating a hydraulic fluid supply circuit of said second traveling directional control valve with a hydraulic fluid supply circuit of said first traveling directional control valve when at least one of said plurality of working actuators is operated, wherein said hydraulic circuit system further comprises:
• (a) first pressure adjusting means arranged between said first variable restrictors and said first traveling motor for controlling a pressure downstream of said first variable restrictors to a value corresponding to a first signal pressure;
• (b) second pressure adjusting means arranged between said second variable restrictors and said second traveling motor for controlling a pressure downstream of said second variable restrictors to a value corresponding to a second signal pressure;
• (c) pressure selection means for detecting a higher one of load pressures of said first traveling motor and said second traveling motor as a maximum load pressure;
• and
• (d) signal selection means for supplying said maximum load pressure to said first and second pressure adjusting means as said first and second signal pressures when a combined operation is performed in which said first and second traveling motors and at least one of said plurality of working actuators are driven simultaneously.