Changeover valve unit for power-assisted steering systems

A changeover valve unit for a power-assisted steering system including a driven member for operative connection with the steering road wheels of a wheeled vehicle and arranged to be power-assisted by a hydraulic cylinder, said changeover valve unit comprising:

• a valve housing provided with an inlet port for connection to a source of fluid pressure, outlet ports for connection to said hydraulic cylinder, and a drain port for connection to a fluid reservoir;
• input and output shafts arranged for relative rotation on aligned axes within said valve housing, said output shaft being operatively connected to said driven member;
• a resilient member interconnecting said input and output shafts for permitting relative rotation of said shafts;
• a spool valve element arranged to be moved in response to relative rotation of said shafts in an axial direction;
• a valve casing in surrounding relationship with said spool element within said valve housing and cooperating with said spool valve element for controlling the flow of fluid among said ports;
• and
• means for conducting axial movement of said spool valve element in response to relative rotation of said shafts;
• the improvement wherein a thrust plate is rotatably coupled at its inner periphery with said output shaft to freely permit rotation of said output shaft relative to said valve casing and to permit axial movement of said valve casing in said valve housing caused by axial displacement of said output shaft, said thrust plate being formed with an inner peripheral rim rotatably coupled within an annular groove of said output shaft and with a plurality of radial lugs fixedly engaged with one end of said valve casing.