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, the meter-in flow adjuster having a first flow adjustment characteristic which is a characteristic indicative of a relationship between the speed designated by the manipulation device and a meter-in flow rate to be adjusted according to the designated speed, the meter-in flow adjuster being configured to adjust the meter-in flow rate to a flow rate corresponding to the speed designated by the manipulation device according to the first flow adjustment characteristic;
• the meter-in flow adjuster including a meter-in orifice having a flow passage area variable according to the manipulation of the manipulation device and a meter-in flow adjustment valve which receives an input of an upstream-side pressure on an upstream side of the meter-in orifice and an input of a downstream-side pressure on an downstream side of the meter-in orifice and varies the meter-in flow rate so as to bring a pressure difference between the upstream-side pressure and the downstream-side pressure of the meter-in orifice into agreement with a predetermined value;
• 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, the meter-out flow adjuster having a second flow adjustment characteristic which is a characteristic indicative of a relationship between the speed designated by the manipulation device and a meter-out flow rate to be adjusted according to the designated speed, the meter-out flow adjuster being configured to adjust the meter-out flow rate to a flow rate corresponding to the speed designated by the manipulation device according to the second flow adjustment characteristic, the meter-out flow adjuster including a meter-out orifice having a flow passage area variable according to the manipulation of the manipulation device and a meter-out flow adjustment valve which receives an input of an upstream-side pressure on an upstream side of the meter-out orifice and an input of a downstream-side pressure on an downstream side of the meter-out orifice and varies the meter-out flow rate so as to bring a pressure difference between the upstream-side pressure and the downstream-side pressure of the meter-out orifice into agreement with a predetermined value;
• 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 first and second flow adjustment characteristics are set so as to make the meter-in flow rate adjusted by the meter-in flow adjuster based on the first flow adjustment characteristic be greater than the meter-out flow rate adjusted by the meter-out flow adjuster based on the second flow adjustment characteristic, with respect to any value of the speed designated by the manipulation device.