HYDRAULIC WORKING MACHINE

摘要

A hydraulic drive unit of a hydraulic working machine comprising: a hydraulic pump (2) of a variable capacity; at least one actuator (3) driven by pressure oil discharged from said hydraulic pump; a center bypass type direction changeover valve (1) for controlling the flow of the pressure oil supplied from said hydraulic pump to the actuator; a center bypass line (20) for connecting the center bypass path of said direction changeover valve to a tank; a first signal generating means having a fixed throttle (4) provided on the center bypass line, for generating a first control signal for determining a first target capacity of said hydraulic pump by use of pressure generated by said fixed throttle; and a pump regulator (6) for controlling a displacement capacity of said hydraulic pump on the basis of the first control signal, which unit further comprises: a second signal generating means (32 - 26) for generating a second control signal for determining a second target capacity of said hydraulic pump; and a selecting means for selecting one control signal larger in target capacity out of the first and the second control signals as a third control signal to send it to said pump regulator (6). With this arrangement, satisfactory metering characteristics similar to those in the past can be obtained under a light load and satisfactory metering characteristics can be also obtained under a heavy load.