Rotary valve for power steering gear

Abstract

A low noise rotary valve for a hydraulic power steering gear that offers flexibility in the design of its boost characteristic has inlet ports to receive hydraulic fluid from a pump, return ports to return hydraulic fluid to the pump, and cylinder ports to communicate the hydraulic fluid to left and right-hand cylinder chambers of the hydraulic power steering gear. The valve has an input-shaft with a plurality of axially extending grooves separated by lands. A sleeve that has in its bore an array of axially extending slots that circumferentially align with the lands on the input-shaft is journalled on the input shaft. The interfaces between the coacting input-shaft grooves and sleeve slots define axially extending orifices which open and close when relative rotation occurs between the input shaft and the sleeve. These orifices are ported as a network such that they form primary and secondary hydraulic Wheatstone bridges each having right and left-hand inlet orifices and return orifices. When the input-shaft is rotated in relation to the sleeve, the flow of oil through the Wheatstone bridges creates a differential pressure between the right-hand and left-hand cylinder ports. The sides of the input-shaft grooves are contoured to provide relatively steep sloping metering edge contours which are cost efficient to manufacture and allow flexibility in the design of a rotary valve having a linear boost characteristic with a fast turn around. The return paths of each secondary bridge provide a restriction in hydraulic flow to the upstream inlet orifice that causes a back pressure to be applied to the upstream inlet orifice. This back pressure significantly suppresses the generation of cavitation noise in the inlet orifices.