Patent ID: 8578838

Claim:
A lock valve for a hydraulic steering system comprising: a lock valve body having a spool valve bore therein, the bore having opposite first and second ends and a center, a check valve within a check valve chamber adjacent each end of the bore, each of the check valves having a check valve member adjacent the bore and resiliently biased towards a valve seat near said each end of the bore; a valve spool reciprocatingly received within the bore, the valve spool having first and second lands with an annular space therebetween and the spool being for engaging one of the check valve members when the spool is displaced towards said one check valve member; a pair of helm ports, a first said helm port communicating with the bore near the first end thereof and a second said helm port communicating with the bore near the second end thereof; a pair of steering actuator ports, each said steering actuator port communicating with one of the check valve chambers; a pressure relief port communicating with the bore near the center thereof and disposed between the lands of the valve spool; and a pair of spaced-apart grooves in the lock valve body within the spool valve bore, a first said groove being near the first said end of the bore and a second said groove being near the second said end of the bore, the first said groove being positioned and sized to permit fluid communication past the first said land of the spool valve when said valve spool is displaced towards the first end of the bore by fluid pressure applied to the second end of the bore so as to unseat the check valve member adjacent to the first end of the bore and allow pressurized fluid to pass from the first end of the bore through the first groove between the lock valve body and the first land of the valve spool and into the relief port, and the second said groove being positioned and sized to permit fluid communication past the second land of the spool valve when said valve spool is displaced towards the second end of the bore by fluid pressure applied to the first end of the bore so as to unseat the check valve member adjacent to the second end of the bore and allow pressurized fluid to pass from the second end of the bore through the second groove between the lock valve body and the second land of the valve spool and into the relief port; wherein each of the lands has an inner end facing the annular space and each of the grooves has a first end and a second end, the first end of each of the grooves being closer to the center of the bore than the second ends thereof, and the first ends being formed by a shoulder extending about the bore to provide a substantial release of fluid towards the relief port as soon as said each of the lands moves past the first end of said each groove when moving towards the second end of said each groove, and wherein the bore has a longitudinal axis and said each of the grooves has opposite, parallel sides which extend parallel to the axis, the shoulder extending perpendicular to the sides about a curvature of the bore.