Hydraulic drive system

A hydraulic drive system having a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from said hydraulic pump, manipulator means manipulated by an operator for commanding operation of said plurality of actuators, a plurality of flow control valves for controlling respective flow rates of the hydraulic fluid supplied to said plurality of actuators, pressure sensor means for detecting a maximum load pressure among said plurality of actuators, an unloading valve opened when a differential pressure between a delivery pressure of said hydraulic pump and said maximum load pressure exceeds a predetermined value, for discharging a part of a flow rate of the hydraulic fluid delivered from said hydraulic pump to a reservoir, resisting means provided downstream of said unloading valve for generating a control pressure corresponding to the flow rate of the hydraulic fluid discharged through said unloading valve, and pump control means for reducing the delivery rate of said hydraulic pump as the control pressure generated by said resisting means is raised, and for increasing the pump delivery rate as the control pressure is lowered, further comprising:

• adjusting valve means connected to said hydraulic pump in parallel to said unloading valve at a position upstream of said resisting means, and
• control means controlling said adjusting valve means such that an opening area of said adjusting valve means is large when an input amount of said manipulator means is small, and the opening area of said adjusting valve means is reduced as the input amount of said manipulator means increases.