Modeling Spool-Valve Flow Forces

Abstract

This paper has been written to reconsider the important topic of transient flow forces that act on spool-type hydraulic control valves. Though this subject has been addressed and somewhat laid to rest many years ago, the high-speed applications of our present day require a fresh look at this topic for gaining deeper insight into the significant contributions that determine the stability and frequency response of the hydraulic control valve. Over thirty years ago, it became an accepted practice to neglect the pressure transient flow-forces acting on the spool valve as they were deemed to be minor compared to the velocity effects of the spool itself. In this paper, flow force models with and without the pressure transient term are compared and it is shown that the historic practice of neglecting pressure transients may no longer be valid. As a result, this paper provides justification for considering the pressure transients in the analysis and may provide an explanation for some of the discrepancies that have existed between theoretical expectations and laboratory results.