Changeover valve unit for power-assisted steering systems

摘要

A changeover valve unit for a power-assisted steering system includes input and output shafts arranged within a valve housing for relative rotation on aligned axes, a spool valve element concentrically assembled with the input shaft to be axially moved in response to relative rotation of the shafts for controlling the supply of fluid under pressure to a hydraulic cylinder associated with the steering system. An annular seal member of elastic material is secured at its inner periphery to the input shaft and is integrally formed with an annular lip facing the inner periphery of the spool valve element with a predetermined annular clearance. The annular lip is expanded outwardly by back pressure acting on the spool valve element at a higher level than a predetermined value and is brought into engagement with the inner periphery of the spool valve element.