1) S.Y. Lee and J.F. Blackburn : Contributions to Hydraulic Control 1. Steady-State Axial Forces on Control-Valve Pistons, Trans. ASME, Vol. 74, 1005/1011 (1952).
2) R.N. Clark, Seattle and Wash. : Compensation of Steady-State Flow Forces in Spool-Type Hydraulic Valves, Trans. ASME, Vol. 79, 1784/1788 (1957)