Patent ID: 7086494

Claim:
A power steering system comprising: a hydraulic power cylinder including a piston for defining first and second hydraulic pressure chambers, hydraulic fluid being supplied to or discharged from each of the first and second hydraulic chambers under action of a hydraulic pressure source to hydraulically operate the piston so as to accomplish steering assisting; first and second hydraulic lines which are respectively connected to first and second hydraulic pressure chambers of the hydraulic power cylinder, the first and second hydraulic lines being connected to the hydraulic pressure source; a bypass passage through which the first and second hydraulic lines are connected with each other; a flow passage change-over device adapted to close the bypass passage during steering assisting under the action of the hydraulic pressure source and to open the bypass passage during non-steering assisting; wherein a supply passage and a return passage are disposed in parallel with each other to form part of each of the first and second hydraulic lines, the supply passage and the return passage being connected to the hydraulic pressure source and to each of the first and second hydraulic pressure chambers of the hydraulic power cylinder, wherein the flow passage change-over device is disposed in each of the first and second hydraulic lines, the flow passage change-over device including a valve member which is adapted to open or close each of the supply passage and the bypass passage in accordance with a pressure differential between pressure at side of the hydraulic pressure source and pressure at side of the hydraulic power cylinder in the hydraulic pressure line in which the flow passage change-over device is disposed, wherein a differential pressure regulating valve is disposed in each of the return passages and adapted to allow hydraulic fluid to flow toward the hydraulic pressure source in a condition where pressure on side of the hydraulic power cylinder becomes higher a set pressure or higher than pressure on side of the hydraulic pressure source.