There are two different possible flow patterns in spool valves. One is the flow reattached to the face of the spool piston, and the other is a detached free jet. Valve geometry and Reynold's number determine which pattern exists at any instant. This paper presents the results of an experimental study of these variables using a two dimensional model valve and the numerical solution of Navier Stokes equations in spool valves. It showes that the reattached flow occurs when the chamber width is large, i. e. distance of the sleeve and stem is large, and when the chamber width becomes narrow the jet breakes away from the face of the spool piston. Increasing the chamber width, suddenly the jet is reattached to the spool piston face, and then decreasing the chamber width, the jet breakes away suddenly at some chamber width. But the chamber width when the jet is reattached is larger than that when the flow breakes away. Namely the hysteresis region exists within which the flow pattern may take different formes depending on the direction of the change of increasing or decreasing chamber width. When the flow is reattached, axial force and energy losses in the spool valve are small compared with them on the detached flow. And the reattached flow solution of the Navier Stokes equation is obtained.

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