Hydraulic drive system for working machine including track device of crawler type

A hydraulic drive system for a working machine including a track device of crawler type, comprising:

• an engine;
• a variable displacement type main pump driven by the engine;
• a plurality of actuators including first and second track hydraulic motors driven by a hydraulic fluid delivered from the main pump;
• a plurality of flow control valves including first and second track flow control valves to control a plurality of flow rates of the hydraulic fluid supplied from the main pump to the actuators;
• a plurality of control devices including first and second track control devices that each have a remote control valve which generates a control pilot pressure for operating the plurality of flow control valves using a hydraulic pressure of a pilot hydraulic fluid source as a source pressure;
• left and right crawlers driven through rotation of the first and second track hydraulic motors, respectively;
• a differential pressure reducing valve that outputs a differential pressure between a delivery pressure of the main pump and a maximum load pressure of the actuators;
• a plurality of pressure compensating valves including first and second track pressure compensating valves which each have a valve opening-side pressure receiving portion to which an output pressure of the differential pressure reducing valve is introduced and each control a respective differential pressure across each of the flow control valves such that the differential pressure across each of the flow control valves equals a target compensating differential pressure that is set based on the output pressure of the differential pressure reducing valve;
• a pump control unit for performing load sensing control of a displacement volume of the main pump such that the delivery pressure of the main pump is higher by a target differential pressure than the maximum load pressure of the actuators;
• and
• a flow rate correction device that is adapted to increase a maximum flow rate output from the lower speed side of the first or the second track flow control valves such that a difference in speed between the first and second track motors does not occur when the control levers of the first and second track control devices are operated all the way,
• wherein the flow rate correction device includes a target compensating differential pressure adjusting device that is disposed on a side on which the valve opening-side pressure receiving portion of the lower speed side of the first or the second track pressure compensating valves is disposed and that is adapted to strengthen a biasing force in a valve opening direction and thereby to increase the target compensating differential pressure.