Effects of Variations in Oil Viscosity on Spool–Sleeve Interaction in a Conventional Spool-Type Directional Control Valve for Aircraft Hydraulic Systems

Hydraulic systems in aircraft and other machines operating under extreme environmental conditions are often exposed to mechanical vibration and large temperature variations. Changes in temperature affect the viscosity of the hydraulic oil, and the resulting viscosity changes may alter the friction between the spool and sleeve and influence the dynamic behaviour of the valve. External vibrations can be transmitted to the spool through the valve body and the associated elastic and dissipative connections, causing the spool to oscillate. This paper examines both friction in the spool–sleeve pair of a conventional directional control valve and the contribution of that friction to the transmission of external mechanical vibrations to the spool. Experimental tests were performed at two different hydraulic oil viscosities. The results were used to evaluate how viscosity-dependent friction contributes to the vibrational excitation of the spool. Based on this analysis, a mathematical model of friction in the spool–sleeve pair was developed and incorporated into the equation describing the forced vibration of the spool. The study provides a basis for predicting the behaviour of such directional control valves when subjected to severe mechanical vibration and large temperature variations in aircraft hydraulic systems and other machinery.