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

Abstract

A hydraulic drive system for a track device of crawler type has right and left hydraulic track motors. The hydraulic drive system is capable of correcting for skew occurring in the straight line traveling of the track device. A traveling test is conducted upon shipment from a factory. If skew is noted during the test, a plug disposed on the side of a valve opening-side pressure receiving portion of a pressure compensating valve for the track which is lower in speed is removed and, replaced with an adjusting mechanism-mounted plug having an adjusting pin. The pin is operated so as to strengthen a biasing force of a target compensating differential pressure adjusting spring. An opening in the pressure compensating valve is thereby corrected in an opening direction and a flow rate to one of the left and right track motors is thereby adjusted to be equal to the other motor.

1 . 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 for controlling a flow rate of the hydraulic fluid supplied from the main pump to the actuators; and   left and right crawlers driven through rotation of the first and second track hydraulic motors, respectively, wherein   the hydraulic drive system includes a flow rate correction device for limiting a maximum flow rate output from at least either one of the first and second track flow control valves a predetermined flow rate.   
     
     
         2 . The hydraulic drive system for a working machine including a track device of crawler type according to  claim 1 , further comprising:
 a plurality of pressure compensating valves including first and second track pressure compensating valves, each controlling a differential pressure across each of the flow control valves; and   a pump control unit for performing load sensing control of a displacement volume of the main pump such that a delivery pressure of the main pump is higher by a target differential pressure than a maximum load pressure of the actuators, the pressure compensating valves controlling the differential pressure across each of the flow control valves such that the differential pressure across each of the flow control valves is held at a differential pressure between the delivery pressure of the main pump and the maximum load pressure of the actuators, wherein   the flow rate correction device includes a target compensating differential pressure adjusting device for correcting a target compensating differential pressure of the track pressure compensating valve associated with the track flow control valve of the first and second track pressure compensating valves.   
     
     
         3 . The hydraulic drive system for a working machine including a track device of crawler type according to  claim 2 , wherein
 the target compensating differential pressure adjusting device comprises an adjusting mechanism-mounted plug including an adjusting pin for adjusting a biasing force of a spring that sets the target compensating differential pressure of the track pressure compensating valve.   
     
     
         4 . The hydraulic drive system for a working machine including a track device of crawler type according to  claim 2 , wherein
 the target compensating differential pressure adjusting device comprises a pressure reducing valve unit including a pressure reducing valve for correcting the target compensating differential pressure of the track pressure compensating valve by reducing a pressure of a pilot hydraulic fluid source.   
     
     
         5 . The hydraulic drive system for a working machine including a track device of crawler type according to  claim 1 , further comprising:
 a track operating device including a remote control valve that generates a control pilot pressure for operating the track flow rate control valve, wherein   the flow rate correction device includes a pressure control valve unit including a pressure control valve disposed between the remote control valve of the track operating device and the track flow rate control valve, the pressure control valve for reducing the control pilot pressure of the remote control valve.   
     
     
         6 . The hydraulic drive system for a working machine including a track device of crawler type according to  claim 1 , further comprising:
 a track operating device including a remote control valve that generates a control pilot pressure for operating the track flow rate control valve, wherein   the flow rate correction device includes a pressure reducing valve unit including a pressure reducing valve disposed between the remote control valve of the track operating device and the track flow rate control valve, the pressure reducing valve for reducing the control pilot pressure of the remote control valve.