A hydraulic drive apparatus installed in a traveling type working machine, which includes a machine body and a working arm, the machine body including a first traveling body and a second traveling body that are arranged left and right, each being capable of performing a traveling motion of traveling on a traveling surface forward and backward, the working arm being supported by the machine body and capable of performing a working motion including a traveling assist motion of moving the machine body forward while a tip of the working arm is stuck into the ground, to hydraulically cause the traveling motions of the first traveling body and the second traveling body and the working motion of the working device, the hydraulic drive apparatus comprising:
- a plurality of working actuators that are supplied with hydraulic fluid to thereby actuate the working arm, the working actuators including a specific working actuator that makes the working arm perform the traveling assist motion;
- a first traveling motor that is supplied with hydraulic fluid to thereby actuate the first traveling body;
- a second traveling motor that is supplied with hydraulic fluid to thereby actuate the second traveling body;
- a first hydraulic pump and a second hydraulic pump that discharge hydraulic fluid to be supplied to a plurality of hydraulic actuators including the plurality of working actuators, the first traveling motor and the second traveling motor;
- a traveling selector valve selectively switchable to a neutral position and a straight traveling position as a position for forming a flow passage to lead hydraulic fluid discharged from the first hydraulic pump and the second hydraulic pump to the plurality of hydraulic actuators, the traveling selector valve forming, in the neutral position, a fluid passage that allows hydraulic fluid discharged from the first hydraulic pump to be supplied to the first traveling motor and allows hydraulic fluid discharged from the second hydraulic pump to be supplied to the second traveling motor and the specific working actuator and forming, in the straight traveling position, a working fluid passage that allows hydraulic fluid discharged from the first hydraulic pump to be supplied to the specific working actuator while preventing hydraulic fluid discharged from the first hydraulic pump from being supplied to the first traveling motor and the second traveling motor, and a traveling fluid passage that allows hydraulic fluid discharged from the second hydraulic pump to be supplied to the first traveling motor and the second traveling motor;
- a communication throttle portion provided in a communication passage providing communication between the working fluid passage and the traveling fluid passage so as to allow hydraulic fluid to flow from the working fluid passage to the traveling fluid passage, the communication throttle portion having an opening degree variable to increase and decrease a flow rate of the hydraulic fluid in the communication passage;
- a first pump pressure detector that detects a first pump pressure which is a pressure of hydraulic fluid that the first hydraulic pump discharges;
- a second pump pressure detector that detects a second pump pressure which is a pressure of hydraulic fluid that the second hydraulic pump discharges;
- and
- a switching control part configured to switch the traveling selector valve to the neutral position when a single operation action of making only one of a traveling operation for making the first traveling body and the second traveling body travel and a specific working operation for moving the specific working actuator is performed and configured to switch the traveling selector valve to the straight traveling position when a specific combined operation action of simultaneously making a forward traveling operation for making the lower traveling body travel forward and the specific working operation is performed, wherein the switching control part is configured to perform a communication control of adjusting the opening degree of the communication throttle part so as to reduce the flow rate of the hydraulic fluid in the communication passage with an increase in a pump pressure difference, which is a difference between the first pump pressure and the second pump pressure, in the case where the pump pressure difference is positive when the specific combined operation action is performed.