PURPOSE: To enable the decrease of car speed in turning to be prevented by connecting return circuits of right/left directional switching valves of a hydraulic circuit to an auxiliary relief valve and a tank by means of a switching valve so as to be controlled, operating the switching valve and the right and left directional switching valves while differentiating the meter-in opening areas from each other, and connecting a high- pressure side return circuit to the auxiliary relief valve.
CONSTITUTION: Return circuits 21, 22 of right and left directional switching valves 5, 6 are connected to an auxiliary relief valve 24 and a tank 25 by a switching valve 23. The inlet side of the auxiliary relief valve 24 is connected to upstream side circuits 28, 29 of the right and left directional switching valves 5, 6. The switching valve 23 is held in the drain position B by spring force so as to switch the position to the left position C and the right position D by pressure oil to be supplied to right and left pressure receiving parts 30, 31, and connected to the upstream side circuits 28, 29 by right and left circuits 32, 33 so as to switch the right and left positions C, D by differential pressure of both circuits 28, 29. At this time, the right and left directional switching valves 28, 29 are operated while differentiating the meter-in opening areas from each other.
IKEI KAZUNORI
ISHIHAMA KAZUYOSHI
