| 摘要 |
PROBLEM TO BE SOLVED: To reduce a workload of a hydraulic pump as much as possible at neutral time of steering operation, and prevent the occurrence of a shortage of a flow rate of an hydraulic fluid even when sudden steering operation is performed. SOLUTION: A first branch passage 3 and a second branch passage 4 branching off from an hydraulic passage 2 are provided. A first control valve 7 arranged arranged so that four throttle valves whose throttle quantity changes according to steering torque constitute a four-side bridge type circuit, is arranged between the first branch passage 3 and a drain passage 5. A second control valve 9 arranged so that four restriction valves whose restriction changes according to steering torque constitute a four-side bridge type circuit, is arranged between the second branch passage 4 and the drain passage 5. A power cylinder 10 is attached to the second control valve 9. A flow control valve 15 to increase and decrease an operating fluid quantity according to an increase- decrease of load pressure is arranged in the hydraulic passage 2. An orifice 31 is arranged in the second branch passage 4. After the throttle valves of the first control valve 7 are fully closed, the throttle valves of the second control valve 9 are fully closed. |
