Hydraulic driving apparatus for working machine

A hydraulic driving apparatus for a working machine, the hydraulic driving apparatus being designed to move a load in a lowering direction which is the same direction as a self-weight falling direction of the load by means of hydraulic pressure, the hydraulic driving apparatus comprising:

• a hydraulic pump for discharging hydraulic fluid;
• a hydraulic actuator having a first port and a second port, the hydraulic actuator being adapted to be driven so as to move the load in the lowering direction, by receiving a supply of hydraulic fluid discharged from the hydraulic pump through the first port while discharging the hydraulic fluid from the second port;
• a manipulation device adapted to be manipulated to designate an operating speed of the hydraulic actuator;
• a working hydraulic circuit including a meter-in flow passage for introducing hydraulic fluid from the hydraulic pump into the first port of the hydraulic actuator when the hydraulic actuator is driven to move the load in the lowering direction and a meter-out flow passage for introducing hydraulic fluid discharged from the second port of the hydraulic actuator into a tank when the hydraulic actuator is driven to move the load in the lowering direction;
• a control valve for changing a state of the supply of hydraulic fluid from the hydraulic pump to the hydraulic actuator so as to operate the hydraulic actuator at the speed designated by the manipulation device;
• a meter-in flow adjuster for adjusting a meter-in flow rate, which is a flow rate of the hydraulic fluid in the meter-in flow passage, to a flow rate corresponding to the speed designated by the manipulation device;
• a meter-out flow adjuster for adjusting a meter-out flow rate, which is a flow rate of the hydraulic fluid in the meter-out flow passage, to a flow rate corresponding to the speed designated by the manipulation device;
• and
• a relief valve adapted to be opened, when a pressure of the meter-in flow passage becomes equal to or greater than a setting pressure, so as to introduce hydraulic fluid flowing through the meter-in flow passage into the tank to thereby define an upper limit of the pressure of the meter-in flow passage;
• wherein the meter-in flow adjuster and the meter-out flow adjuster have respective flow adjustment characteristics, each of which is a characteristic indicative of a relationship between the speed designated by the manipulation device and a flow rate to be adjusted according to the designated speed, such that the meter-in flow rate adjusted by the meter-in flow adjuster according to any value of the speed designated by the manipulation device is greater than a meter-out flow rate adjusted by the meter-out flow adjuster.