The objective of the present paper is an experimental and numerical analysis of the driving forces on a 4/3 hydraulic open center directional control valve. The assembling of the valve has been modified to allow external control of the spool displacement and the flow force measurement. The valve is inserted in a closed hydraulic circuit and is tested with different pump flow rates as well as with different pressure drops. The results obtained show important differences between an open center valve and a closed center one, the latter having been extensively analyzed in the literature. In the first part of the sliding spool movement, a particular trend of the flow forces is observed. Indeed, the flow force acts to open the spool. Finally, a numerical analysis of the flow is performed to confirm and theoretically explain the experimental results. (c) 2005 Elsevier Ltd. All rights reserved.