Valve apparatus and hydraulic drive system

A valve apparatus for controlling a flow of hydraulic fluid supplied from a hydraulic fluid supply source to a double-acting actuator, comprising a flow control valve having supply passages communicating with said hydraulic fluid supply source, a pair of load passages communicating with said actuator, and a pair of first meter-in variable restrictors disposed between said supply passages and said pair of load passages, respectively, and opened alternatively dependent on the operating direction to an opening dependent on an operation amount thereof;

• a pair of first signal passages located downstream of said pair of first variable restrictors, respectively, and having passage sections for detecting load pressure of said actuator alternatively dependent on the operating direction;
• a pair of tank passages each communicating with a reservoir tank;
• a pair of discharge passages for communicating said pair of first signal passages with said pair of tank passages, respectively;
• and a pair of second variable restrictors provided in said pair of discharge passages, respectively, and having their openings variable dependent on the operation amount of said flow control valve to produce in said pair of first signal passages a control pressure different from the load pressure detected in the corresponding first signal passage, alternatively dependent on the operating direction, the control pressure produced alternatively in said pair of first signal passages being led to said hydraulic fluid supply source through a second signal passage, wherein:
• said valve apparatus further comprises a pair of auxiliary restrictor means disposed in said pair of first signal passages, respectively, for reducing the load pressure detected alternatively in said passage sections of said pair of first signal passages so that a pressure lower than the detected load pressure is produced in the corresponding first signal passage as said control pressure.

A hydraulic drive system comprising a hydraulic fluid supply source, at least one actuator driven by a hydraulic fluid from said hydraulic fluid supply source, and a valve apparatus for controlling a flow of the hydraulic fluid supplied from said hydraulic fluid supply source to said actuator, said valve apparatus comprising a flow control valve having a supply passage communicating with said hydraulic fluid supply source, a load passage communicating with said actuator, and a first meter-in variable restrictor disposed between said supply passage and said load passage and opened dependent on an operation amount thereof;

• a first signal passage located downstream of said first variable restrictor and having a passage section for detecting a load pressure of said actuator;
• a tank passage communicating with a reservoir tank;
• a discharge passage for communicating said first signal passage with said tank passage;
• a second variable restrictor provided in said discharge passage and having its opening variable dependent on the operation amount of said flow control valve to produce in said first signal passage a control pressure different from said load pressure;
• and a second signal passage for leading the control pressure in said first signal passage to said hydraulic fluid supply source, wherein:
• said valve apparatus further comprises auxiliary restrictor means disposed in said first signal passage for reducing the load pressure detected in said passage section of said first signal passage so that a pressure lower than the detected load pressure is produced in said first signal passage as said control pressure.