Hydraulic drive system for construction machine

A hydraulic drive system for a construction machine comprising a hydraulic pump of variable displacement type, a hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, flow control means for controlling a flow rate of the hydraulic fluid supplied to said actuator, and pump control means for controlling a delivery rate of said hydraulic pump such that the pump delivery rate is reduced as a load of said actuator increases, and is increased as the load of said actuator reduces, said hydraulic drive system further comprising:

• first detection means for detecting a magnitude of the load exerted on said actuator;
• and
• flow limiting means for monitoring an abrupt reduction in the load of said actuator based on a signal from said first detection means, and controlling said flow control means to limit a flow increase speed of the hydraulic fluid supplied to said actuator when it is determined that said actuator has reached a predetermined condition related to an abrupt reduction in the load;
• said flow limiting means including means for setting a first flow increase speed adapted for an ordinary work and second flow increase speed smaller than said first flow increase speed, means for selecting said first flow increase speed when it is not determined that said actuator has reached said predetermined condition related to an abrupt reduction in the load, and selecting said second flow increase speed when it is determined that said actuator has reached said predetermined condition related to an abrupt reduction in the load, and means for controlling said flow control means based on the selected flow increase speed.