May the flow force be with hydraulic systems

In terms of the flow forces in spool valves, the systems engineer can use flow-force-compensated valves to simplify system design and reduce stability issues that might occur when the valve interacts with other parts of the system.

Fluid-power engineers use the term “flow forces” to indicate the axial thrust on metering elements (spool or poppet) generated by momentum changes of oil flow. The resulting force on the spool is obtained through the momentum equation applied to the control volume of fluid, surrounded by permeable and nonpermeable surfaces.

This method works well in the case of most hydraulic valves, where it is possible to define a control volume in which the fluid velocity through the permeable surfaces is mono-dimensional. The shape and the inlet vs. outlet areas of the metering element determine the axial force on the spool. The beauty of this approach, says Parker Hannifin, is that with a simple equation it is possible to evaluate the overall effect of several localized phenomena that would be very difficult to study singularly.