Hydraulic drive system of construction machine

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

• a turning operation device that outputs a turning operation signal corresponding to an inclination angle of an operating lever;
• a turning motor;
• a variable displacement over-center pump that is connected to the turning motor by a pair of supply/discharge lines in a manner to form a closed loop;
• a regulator that adjusts a tilting angle of the over-center pump;
• a main pump whose rotary shaft is coupled to a rotary shaft of the over-center pump, the main pump sucking hydraulic oil from a tank through a suction line provided with a check valve and supplying the hydraulic oil to a hydraulic actuator different from the turning motor through a delivery line;
• an accumulator that accumulates pressurized oil therein;
• a switching valve that is switched between a pressure accumulation position, in which the switching valve brings the accumulator into communication with the delivery line, a pressure release position, in which the switching valve brings the accumulator into communication with a portion of the suction line downstream of the check valve, and a neutral position, in which the switching valve shuts off the accumulator from the delivery line and the portion of the suction line downstream of the check valve;
• and
• a controller that controls the regulator and the switching valve, wherein
• the controller controls the regulator such that when a turning deceleration operation is performed, the tilting angle of the over-center pump decreases in accordance with decrease in the turning operation signal outputted from the turning operation device, and
• the controller:
• switches the switching valve to the pressure accumulation position when a pressure accumulation condition is satisfied, the pressure accumulation condition being defined to include that the turning deceleration operation is performed alone;
• switches the switching valve to the pressure release position when a pressure release condition is satisfied;
• and
• switches the switching valve to the neutral position when neither the pressure accumulation condition nor the pressure release condition is satisfied.