Changeover valve unit for power-assisted steering systems

In a changeover valve unit for a power-assisted steering system comprising:

• a housing provided with a fluid inlet and a fluid outlet;
• input and output shafts arranged for relative rotation on aligned axes within said housing;
• a reaction member in the form of a torsion bar interconnecting said input and output shafts to permit relative rotation of said shafts;
• a spool valve element in surrounding relationship with said input shaft and arranged to be displaced in response to relative rotation of said shafts in an axial direction;
• a valve casing in surrounding relationship with said spool valve element within said housing and cooperating with said spool valve element for controlling the flow of fluid between said fluid inlet and said fluid outlet;
• a compression coil spring for biasing said spool valve element in the axial direction;
• and
• means for causing axial displacement of said spool valve element in response to relative rotation of said shafts;
• the improvement wherein an annular retainer member is engaged at an outer periphery thereof with one end of said spool valve element and positioned in place under the biasing force of said compression coil spring to form an oil chamber subdivided from the interior of said spool valve element;
• and orifice means is formed between the inner periphery of said annular retainer member and the outer periphery of said input shaft for throttling the flow of fluid between said oil chamber and the interior of said spool valve element.