Influence of Spool Impact on Preloaded Threaded Plugs in Hydraulic Valves

Preloaded threaded plugs that limit axial movement of spools in hydraulic valves are repeatedly loaded by spool impacts, which affects their fatigue strength. For proper dimensioning of these plugs, their loadings must be known. The aims of the study are to: (a) determine the spool impact force (Fimpact) on the plug head, (b) determine its influence on the force in the plug neck (FIPN) and (c) give design recommendations and practical considerations for plug–valve connections. Modified versions of the plugs were produced and equipped with strain gauges. The plugs were tightened into valves and loaded with spool impacts at three different pilot pressures. Meanwhile, Fimpact, FIPN, pilot pressure, and spool displacement were measured. As the FIPN was measured in the modified plug–valve connections, which had different stiffnesses from the original connections, the FIPN in the original plug–valve connections was calculated by considering the stiffness ratios of the original connections obtained from finite element analyses. The results show that the spool impact force and the resulting additional FIPN increase linearly with pilot pressure. To achieve smaller amplitudes of the FIPN and, consequently, higher fatigue strength, the tensioned parts of the plugs should have lower stiffness and the clamping parts higher stiffness.