Water hydraulic proportional control valves using "tap water" as the working fluid are suitable for systems that require high levels of environmental friendliness and safety. Particularly, there are high expectations for their application to mining machinery, wave and wind power generation systems, and ocean development machinery, including underwater drilling machinery. In the previous report, the authors defined the transfer functions of three components of the water hydraulic proportional

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... trol valve, namely the compensation circuit, the solenoid, and the pilot valve, and experimentally and analytically examined the effects of design parameters on valve performance. In water hydraulic proportional control valves that use tap water, with its poor lubricating properties, as the working fluid, the hydrostatic bearings and damping orifices, which comprise the structural characteristics of the valve, function to ensure friction/wear prevention and stable operation of the spool. Moreover, the hydrostatic bearings and damping orifices structurally constitute a meter-in circuit that is effective in improving the response characteristics of the spool and a meter-out circuit that is effective in improving the damping characteristics of the spool; their functions are determined depending on the purpose of the valve. This report focuses on the open loop transfer function represented by the solenoid and the pilot valve and how that function effects the characteristics of the entire valve; more specifically, the effect of the geometric parameters of the hydrostatic bearings and damping orifices is examined; then, the effect of these parameters on the step response characteristics of the entire valve is analytically verified.