Investigation of Frictional Forces in Hydraulic Spool Valves and Their Effect on the Estimation of Axial Flow Forces

Abstract Model-based design of hydraulic valves, as well as the acquisition of additional variables using soft sensors are rapidly gaining importance. Both, however, require an accurate and comprehensive model of the valve. One of the greatest challenges currently present, is within correctly representing the flow forces acting on the spool, which is why considerable research has been carried out investigating them. Since flow forces are always superimposed by directional frictional, inertial, and, if present, spring forces, they cannot be measured directly. To be able to record the flow forces despite this difficulty, a new measuring method is presented. It enables the measurement of the frictional forces and thereby the calculation of the flow forces. In addition to presenting this new metrology method, the frictional force and factors influencing it, will be further investigated. These factors include the operating point-dependent non-uniform pressure distribution in the annular gap, as well as radial flow forces. The results shown in the paper contribute to a better understanding of frictional forces in hydraulic valves, thus advancing both model-based design, and soft sensor development.