Flow-Force Compensation in a Hydraulic Valve

Flow-reaction forces acting in hydraulic valves have been studied for many decades. Despite this, they are difficult to account for due to the complexities of the jet flow. This paper focuses only on the reduction, also referred to as compensation, of the flow force as applied to a valve spool featuring a profile of a turbine bucket. Fluid power textbooks explain the compensation taking place on such a profile by applying Newton laws of motion to the profile and deliver an equation for the magnitude and the direction of the flow force. This paper shows that both the magnitude and the direction of the compensating flow force are incorrect if calculated from the textbook equation. A corrected analysis of the dynamic forces is presented that are in agreement with earlier experiments by this author. It follows that the compensating flow force should be calculated from the static-pressure imbalance on the spool profile. That is, not Newton but Pascal law should be applied to calculate the compensating flow force.