Radial Flow Force at the Annular Orifice of a Two-Dimensional Hydraulic Servo Valve

A two-dimensional hydraulic servo valve is an innovative servo control element that provides hydraulic systems with a high power-to-weight ratio, great anti-pollution potential, and superior static and dynamic characteristics. The spool of such a valve is subject to two degrees of freedom: rotation around and sliding along the spool axis, to accomplish both pilot control and flow amplifier functions. The structure of the spool at the main stage is similar to that of a traditional slide valve wherein the asymmetrical distribution of the oil paths manufactured in the valve body produces circumferential unevenness in the radial flow force to the spool at the annular orifice, in line with the momentum theory. Three-dimensional computational fluid dynamics analysis of the flow field revealed that the radial flow force at the annular orifice increases sharply with inlet flow velocity and decreases as the orifice opening grows, while changes in outlet pressure do not affect the levels or distribution of this force. Also, net radial force at the annular orifice increases with both inlet velocity and opening size. The paper presents results demonstrating that the net radial force from fluid flow through the orifice could increase friction resistance and cannot be safely ignored, especially under high-flow-rate conditions.