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:

• a variable displacement type main pump;
• 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 plurality of actuators;
• a plurality of control devices including first and second track control devices that each 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, each of the plurality of pressure compensating valves having a valve opening-side pressure receiving portion to which an output pressure of the differential pressure reducing valve is introduced and a spring that applies a biasing force in a valve opening direction, and which each controls 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;
• an adjusting mechanism-mounted plug that is adapted to increase a maximum flow rate output from a lower speed side track flow control valve which is one of the first and the second track flow control valve and which corresponds to a lower speed side of the one of first and the second track hydraulic motors such that a difference in speed between the first and the second track hydraulic motors will not occur when control levers of the ones of the first and second track control devices are operated all the way;
• and
• wherein the adjusting mechanism-mounted plug has an adjusting pin for strengthening the biasing force of the spring by varying a position of the adjusting plug in an axial direction, thereby increasing the target compensating differential pressure of the lower speed side track flow control valve, and has a lock nut for fixing the position of the adjusting pin in the axial direction, and wherein the adjusting mechanism mounted plug is disposed on the lower speed side track flow control valve such that a differential pressure across the lower speed side track flow control valve, which is the one of the first and the second track flow control valve corresponding to a lower speed side of the one of the first or the second track hydraulic motor equals the target compensating differential pressure that is set based on the output pressure of the differential pressure reducing valve.