In this research, lubrication characteristics of spool-type valve with groove are studied. This study is carried out for a laminar, incompressible and isothermal flow between sleeve and spool with groove. Spool valves are used a lot in modem hydraulic system to maintain the movement of various actuators. However, the spool-type directional control valve has particular problem called as hydraulic lock. The problem occurs when uneven pressure distribution surrounding the spool in the clearance between spool and sleeve causes the spool to move sideways out of its centered position. And the contact between spool and sleeve causes to increase friction and eventually, the spool is blocked inside the sleeve. To reduce the possibility of the problem, peripheral grooves balancing uneven pressure distribution in the circumferential direction are generally applied to spool lands. In the numerical calculation of spool valve, the Reynolds equation is commonly used to investigate lubrication characteristics of the spool valve. However, the applicability of the Reynolds equation is questionable in spool valve analysis because cavitation often occurs in the grooves and the depth of groove is much higher than the clearance in most cases. Therefore, the validity of the Reynolds equation in spool valve analysis by comparing results obtained from the Reynolds equation and the Navier-Stokes equation is investigated. Results are compared in terms of maximum pressure, lateral force, friction force and volume flow rate (leakage). The cavitation phenomenon is considered by using cavitation model suggested by Singhal in the Navier-Stokes equation and Reynolds cavitation boundary condition in the Reynolds equation. When the cavitation doesn't occur in the flow, the difference of results from the Reynolds equation and the Navier-Stokes equation is nearly small. But in the case of spool with a lot of grooves, there is a difference more than 10% for lateral force. And there is a big difference more than 20% for lateral force and maximum pressure in cases of occurring cavitation. Through the results, when the cavitation occurs and/or many grooves are applied on spool lands, the Navier-Stokes equation should be used to investigate lubrication characteristics of spool valve. Except those conditions, lubrication characteristics of spool valve can be analyzed efficiently by using only the Reynolds equation.
Next, effects of position of groove, number of grooves, tilting angle, eccentricity ratio and groove shape are studied by using the Navier-Stokes equation. The numerical results show that the distance from edge of spool to first groove (l₁), distance between groove and next groove (l₃), number of grooves(n), tilting angle(α), eccentricity ratio(ε), cross sectional shape of groove have a strong influence on the lubrication characteristics of spool valve. When the cross sectional shape of groove is U shape, there is no small eddy current. Therefore, the lateral force and friction force of spool valve with U shape groove are smaller than those of spool valve with other groove shapes. Rectangular grooves are usually used in spool valve but new type grooves (spiral type grooves) are also suggested in the study. And the lubrication characteristics of spiral type grooves are compared with those of typical type grooves in the 4 working conditions. In some conditions, the lateral force and leakage of spool vave with spiral type-1 groove are smaller than those of spool valve with typical type groove.
Through this research, the evaluation of the validity of governing equations (Navier-Stokes equation, Reynolds equation) can be helpful to analyze lubrication characteristics of spool valve with groove more properly. In other words, guidelines about which governing equation is more proper to investigate lubrication characteristics of spool valve according to working conditions are suggested. And it is shown that the different groove shape (U shape) unlike typical groove shape (rectangular) prevents the small eddy current from occurring in the groove. Therefore, lubrication characteristics of spool valve with the groove can be improved. Moreover, spool valve with the new type groove (spiral groove) proposed in the study can be helpful to improve the lubrication characteristics.