Hydraulic drive system of construction machine

A hydraulic drive system of a construction machine, the hydraulic drive system comprising:

• an operation device that receives a first operation for moving an actuator in a first direction and receives a second operation for moving the actuator in a second direction, in which a load on the actuator is lower than the load on the actuator moved in the first direction;
• a variable displacement pump that supplies hydraulic oil to the actuator and that is driven by an engine;
• a solenoid proportional valve that outputs a secondary pressure corresponding to a command current;
• a regulator that adjusts a tilting angle of the pump in accordance with the secondary pressure outputted from the solenoid proportional valve;
• an engine controller that controls a fuel injector of the engine;
• a rotation speed selector that receives a selection of a reference rotation speed of the engine;
• and
• a pump controller that outputs a command rotation speed to the engine controller and feeds the command current to the solenoid proportional valve, wherein
• the pump controller:
• when the operation device receives neither the first operation nor the second operation, outputs a standby rotation speed as the command rotation speed, the standby rotation speed being lower than the selected reference rotation speed;
• when the operation device receives the first operation, changes the command rotation speed from the standby rotation speed to a first target rotation speed lower than or equal to the selected reference rotation speed in such a manner that as an amount of the first operation increases, an increasing rate of the command rotation speed decreases gradually;
• when the operation device receives the second operation, changes the command rotation speed from the standby rotation speed to a second target rotation speed lower than or equal to the selected reference rotation speed in such a manner that as an amount of the second operation increases, the increasing rate of the command rotation speed increases gradually;
• and
• feeds the command current to the solenoid proportional valve, such that a discharge flow rate of the pump is proportional to the amount of the first operation and the amount of the second operation.