Equations of motion of spool valves have been established under very ideal conditions. In this paper, some important factors are brought into considerations. One of them is the eccentric clearance existing between the valve spool and the sleeve due to fabrication errors. Another is the viscosity of a Newtonian fluid assumed to flow through the valve, which generally results in shear forces acting on the valve spool. Under these conditions, equations of motion of the spool valves are derived. Concepts of Poiseuille flow and Couette flow are used to obtain axial forces acting on the valve spool. The resulting equations of motion of spool valves have the simple form of a second order linear homogeneous ordinary differential equation under the imposed idealizations. The influence of these factors on the motion characteristics of spool valves are in turn discussed.