Forces on Hydraulic Valve Spool

The analysis and the evaluation of the steady-state flow forces acting on the spool of hydraulic valves is a keyaspect in performance assessment of many working conditions. These flow forces are generated by the different pressures on the spool faces caused by the high gradients of velocity near the metering orifice edge. In some working conditions these forces can cause significant instabilities in valve operation and a better insight is needed, with respect to classical approximations, in order to acquire informations on its causes and to guide design changes. An experimental and numerical analysis has been carried out in order to analyse the overall force needed to operate a valve spool in different working conditions in a controlled environment. From the experimental side, test results were used in order to identify critical operating conditions, and C.F.D. fl ow analysis (computational fluid dynamics based on the finite element method) was checked against the measured value of the operating force in order to tune a numerical model for a parametric investigation of valve flow characteristic. The paper focuses on the numerical aspect of the study, highlighting possibilities offered by C.F.D. in hydraulics, its expected degree of approximation, and problems in comparing numerical results with experimental evidence. Some detail considerations are reserved to a sensitivity analysis on aspects usually unknown in setting up a numerical C.F.D. analysis, such as turbulence model parameters and effect of geometric simplification.