Fluid power steering unit

Claim 1

A fluid power steering unit comprising a gear housing, a supply pipe to said housing, a discharge pipe from said housing, a cross shaft extending within said housing and formed with a gear sector, a manually operable steering shaft extending within said housing, a hollow cylindrical piston slidably fitted within said housing and delineating a pair of opposed fluid chambers therewithin, said piston being provided with rack teeth meshing with said gear sector of said cross shaft, a ball nut housed within said piston coaxially of said steering shaft and mechanically connected thereto, an annular outer valve member rigidly mounted in said piston, said outer valve member being provided with a first annular groove normally fluidically communicating with one of said fluid chambers, a second annular groove normally fluidically communicating with the supply pipe, a third annular groove normally fluidically communicating with the discharge pipe, and a foUrth annular groove normally fluidically communicating with the other fluid chamber, said outer valve member being provided with passage communicating with said annular grooves, an annular inner valve member housed within said piston coaxially of and spaced from said steering shaft and connected to said ball nut and rotatable therewith, said inner valve member being rotatable around said steering shaft by said nut thereby cooperating with said outer valve member for selective communication with said passages by rotation of said steering shaft, and means having a pair of return springs for normally urging said inner valve member to its neutral position in which no hydraulic pressure difference is generated between said fluid chambers.