| 摘要 |
PURPOSE:To prevent a prime mover from overrunning, switch over transmission stage responsively, and prevent a deceleration shock at the time of switch-over. CONSTITUTION:A travel hydraulic motor 4 is driven by an oil delivered from a hydraulic pump 2 driven by a prime mover 1. The output of the travel hydraulic motor 4 is changed by a transmission 5 to obtain a travel drive force. Cross- overload relief valves 9a and 9b for setting pressure in the input and output lines of the hydraulic motor 4 are used as variable relief valves and, when they are switched over from high-speed stage to low-speed stage, the set pressure of the relief valves 9a and 9b is controlled to low pressure and, after that, the set pressure is increased gradually. After changing, much of the return oil from the hydraulic motor 4 is flown into an inlet side line at low pressure, and the flow rate of pressure oil flown into the hydraulic motor 2 is reduced. As a result, rise of speed of the hydraulic pump 2 is prevented to prevent the prime mover 1 from overrunning. Also the return line pressure of the hydraulic motor 4 is reduced to reduce a shock generated when the valves are switched over from a high speed stage to a low speed stage. |
