Patent ID: 7128182

Claim:
A power steering system, comprising: a torque sensor that is configured to sense a steering torque; first and second passages, each including a pump-side portion and a cylinder-side portion; a hydraulic cylinder with first and second chambers, the first and second chambers being connected to the first and second passages, respectively; a hydraulic pump which is configured to supply and discharge a hydraulic pressure to and from the first and second chambers of the hydraulic cylinder in accordance with the steering torque sensed by the torque sensor; a drain passage connected to at least one of the first and second passages; a switching valve arranged at a connection of the drain passage and the at least one passage, wherein when the hydraulic pump operates to discharge the hydraulic pressure to the at least one passage, the switching valve provides a fluid communication between the pump-side and cylinder-side portions of the at least one passage and closes-off the drain passage, and wherein when the hydraulic pump stops, the switching valve provides a fluid communication between the cylinder-side portion of the at least one passage and the drain passage, wherein the switching valve comprises a valve body formed with a plurality of passage holes communicating with the pump-side and cylinder-side portions of the at least one passage and the drain passage, wherein a spool valve element is slidably arranged in the valve body and comprises a first valve portion for opening and closing the passage holes corresponding to the pump-side and cylinder-side portions and a second valve portion for opening and closing the passage hole corresponding to the drain passage, and wherein a resilient member is configured to bias the spool valve element in the direction of closing-off a fluid communication between the pump-side and cylinder-side portions by the first valve portion and is configured to provide a fluid communication between the cylinder-side portion and the drain passage by the second valve portion.