Patent ID: 6105367
Filing Date: 2000-08-22
Classification: E02F,F04B,F15B

Abstract:
A hydraulic drive system comprising an engine, a variable displacement hydraulic pump driven by said engine, a plurality of actuators driven by a hydraulic fluid delivered from said hydraulic pump, a plurality of flow control valves for controlling flow rates of the hydraulic fluid supplied from said hydraulic pump to said plurality of actuators, pump displacement control means for controlling the displacement of said hydraulic pump so that a differential pressure .DELTA.PLS between a delivery pressure Ps of said hydraulic pump and a maximum load pressure PLS among said plurality of actuators is maintained at a setting value .DELTA.PLSref, and setting modifying means for detecting a rotational speed of said engine and modifying the setting value .DELTA.PLSref depending on the detected rotational speed of said engine, wherein said hydraulic drive system further comprises:a plurality of pressure compensating valves for controlling respective differential pressures across said plurality of flow control valves to the same value as said differential pressure .DELTA.PLS,said hydraulic pump and said plurality of flow control vales having flow rate characteristics set in such a relationship that when said rotational speed of the engine is in a region including a rated rotational speed, a total maximum demanded flow rate Qvtotal of said plurality of flow control valves expressed as a function of the respective differential pressure across said plurality of flow control valves controlled by said plurality of flow control valves controlled by said plurality of pressure compensating valves and respective opening areas of said control valves, is larger than a maximum delivery rate Qsmax of said hydraulic pump at the instantaneous engine rotational speed at that time;said setting modifying means being configured such that when the said engine rotational speed is in a region including the lowest rotational speed of said engine, the setting value .DELTA.PLSref of said pump displacement control means is modified so that the total maximum demanded flow rate Qvtotal of said plurality of flow control valves is smaller than the maximum delivery rate Qsmax of said hydraulic pump at the instantaneous engine rotational speed at that time.