PUMP CONTROL DEVICE FOR HYDRAULIC DRIVE MACHINE

PROBLEM TO BE SOLVED: To constitute a hydraulic system with a simple structure at a low cost and also cope with softly for the addition of a service valve and improve a control accuracy by carrying out a negative control without providing a bleed opening on the spool of a flow rate control valve and without arranging a bypass line. SOLUTION: A variable bleed valve 9a for discharging the pressurized oil on the pressurized oil supply route 14 between a hydraulic pump 1 and flow rate control valves 5a, 5b in response to the bigness of an opening amount is provided. The opening amount of the varianble bleed valve 9a is changed so that the opening amount of the variable bleed valve 9a becomes smaller, as the operation amount of operation levers 7a, 7b becmes larger and the glow rate of the pressurized oil discharged through the variable bleed valve 9a whose opening amount is changed or the pressure difference between the pressure of the flow in side and that of flow out side of a throttle valve 23a provided between the variable bleed valve 9a and a tank 8a is detected and the hydraulic pump 1 controlled so that the delivery flow rate of the hydraulic pump 1 (push aside volume) becoms smaller, as the flow rate or pressure difference of the detected pressurized oil becomes bigger.