Patent ID: 7434655

Claim:
A power steering system comprising: a hydraulic power cylinder for applying a steering assist force to a steering mechanism, the power cylinder having a piston and first and second oil pressure chambers divided by the piston; a reversible pump having a pair of discharge ports; a first oil passage connecting the first oil pressure chamber to one of the discharge ports; a second oil passage connecting the second oil pressure chamber to the other of the discharge ports; an electric motor for driving the reversible pump in forward and reverse directions; a steering assist force detector for detecting a steering assist force to be applied to the steering mechanism; an electric motor control circuit for outputting to the electric motor a drive signal that causes the reversible pump to produce a desired oil pressure on the basis of the steering assist force; a first communication passage interconnecting the first oil passage and the second oil passage; a second communication passage interconnecting the first oil passage and the second oil passage; a third communication passage interconnecting the first communication passage and the second communication passage; an electromagnetic directional control valve disposed in the third communication passage to selectively open and close the third communication passage; a first check valve disposed in the first communication passage at a location between a first connection portion and the first oil passage for allowing working oil to flow in one direction from the first oil passage to the first connection portion only, the first connection portion being a portion at which the first communication passage and the third communication passage are connected; a second check valve disposed in the first communication passage at a location between the first connection portion and the second oil passage for allowing working oil to flow in one direction from second oil passage to the first connection portion only; a third check valve disposed in the second communication passage at a location between a second connection portion and the first oil passage for allowing working oil to flow in one direction from the second connection portion to the first oil passage only, the second connection portion being a portion at which the second communication passage and the third communication passage are connected; and a fourth check valve disposed in the second communication passage at a location between the second connection portion and the second oil passage for allowing working oil to flow in one direction from the second connection portion to the second oil passage only.