Patent ID: 6212886
Filing Date: 2001-04-10
Classification: E02F,F15B

Abstract:
A hydraulic drive system comprising a hydraulic pump, an actuator driven by a hydraulic fluid delivered from said hydraulic pump, a closed center directional control valve connected to said hydraulic pump through a hydraulic fluid supply line and controlling a flow of the hydraulic fluid supplied to said actuator, an unloading circuit interconnecting the hydraulic fluid supply line of said hydraulic pump and a reservoir, and throttle and cutoff means installed in said unloading circuit for throttling and cutting off said unloading circuit depending on an amount by which said directional control valve is operated, wherein:said throttle and cutoff means comprises a logic valve, a pilot circuit including a pilot variable throttle to control a rate of a pilot flow for controlling an opening area of said logic valve depending on the rate of the pilot flow, and operation interlock means for changing an opening of said pilot variable throttle depending on the operation amount of said directional control valve;wherein said logic valve comprises a valve body of the seat valve type arranged to close and open said unloading circuit between an inlet portion connected to said hydraulic fluid supply line and an outlet portion connected to said reservoir, a back pressure chamber formed to face a back side end of said valve body, and a feedback slit formed in said valve body to allow said inlet portion to communicate with said back pressure chamber in such a manner that an opening area of said feedback slit changes depending on an amount by which said valve body is moved, said pilot circuit being connected such that said pilot flow is formed to pass through said inlet portion of the logic valve, said feedback slit and back pressure chamber of the logic valve and said pilot variable throttle to said reservoir; andwherein the hydraulic drive system further comprises a pilot valve for outputting a pilot pressure as an operation signal and driving said directional control valve, and said operation interlock means comprises means for introducing the same pilot pressure output from said pilot valve to said directional control valve and said pilot variable throttle of said throttle and cutoff means.