Patent ID: 6192681
Filing Date: 2001-02-27
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 a plurality of actuators, and 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 first setting value .DELTA.PLSref, said pump displacement control means including first setting modifying means for modifying the first setting value .DELTA.PLSref of said pump displacement control means depending on a rotational speed of said engine, wherein said hydraulic drive system further comprises:an unloading valve for controlling the delivery pressure Ps of said hydraulic pump so that the differential pressure .DELTA.PLS between the delivery pressure of said hydraulic pump and the maximum load pressure PLS among said plurality of actuators is maintained at a second setting value .DELTA.Pun higher than said first setting value .DELTA.PLSref, andsecond setting modifying means for modifying the second setting value .DELTA.Pun of said unloading valve depending on the rotational speed of said engine (1) in match with change in the first setting value .DELTA.PLSref modified by said first setting modifying means in such a manner that the second setting value .DELTA.pun does not become smaller than the first setting value .DELTA.PLSref.