Hydraulic drive traveling system

A hydraulic drive traveling system for construction vehicles comprising a hydraulic pump, at least one hydraulic motor of variable displacement type driven by a hydraulic fluid delivered from said hydraulic pump, a pressure compensated flow control valve connected between said hydraulic pump and said hydraulic motor for controlling a flow rate of the hydraulic fluid supplied to said hydraulic motor dependent on an operation amount of control means, pump control means for controlling a delivery rate of said hydraulic pump such that the delivery rate of said hydraulic pump is increased with an increase in the flow rate demanded by said flow control valve, and motor control means for shifting said hydraulic motor based on a shift decision value between a first capacity and a second capacity smaller than the first capacity, wherein:

• said motor control means includes first detection means for detecting the flow rate demanded by said flow control valve, second detection means for detecting a delivery pressure of said hydraulic pump, computing means in which a function of said flow rate demanded by the flow control valve and said shift decision value in which the shift decision value increases with an increase in the demanded flow rate is set in advance for determining a corresponding decision value based on the demanded flow rate detected by said first detection means, and means for comparing the delivery pressure detected by said second detection means with the shift decision value determined by said computing means and controlling said hydraulic motor to have said first capacity when said delivery pressure is larger than the shift decision value and to have said second capacity when the delivery pressure is smaller than the shift decision value.

A hydraulic drive traveling system for construction vehicles comprising a hydraulic pump, at least one hydraulic motor of variable displacement type driven by a hydraulic fluid delivered from said hydraulic pump, a pressure compensated flow control valve connected between said hydraulic pump and said hydraulic motor for controlling a flow rate of the hydraulic fluid supplied to said hydraulic motor dependent on an operation amount of control means, pump control means for controlling a delivery rate of said hydraulic pump such that the delivery rate of said hydraulic pump is increased with an increase in the flow rate demanded by said flow control valve, and motor control means for shifting said hydraulic motor based on a shift decision value between a first capacity and a second capacity smaller than the first capacity, wherein:

• said motor control means includes first detection means for detecting the flow rate demanded by said flow control valve, a second detection means for detecting a delivery pressure of said hydraulic pump, computing means in which a function of said delivery pressure of the hydraulic pump and said shift decision value in which the shift decision value increases with an increase in the pump delivery pressure is set in advance for determining a corresponding decision value based on the pump delivery pressure detected by said second detection means, and means for comparing the flow rate demanded by said flow control valve detected by said first detection means with the shift decision value determined by said computing means and controlling said hydraulic motor to have said first capacity when said demanded flow rate is smaller than the shift decision value and to have said second capacity when the demanded flow rate is larger than the shift decision value.