D. Borchers
E. Engelberth

20th International Conference on Hydraulics and Pneumatics
Sep 29 - Oct 1, 2008 - Prague

Two undesired phenomena occur in some hydraulic directional control valves: A rotation of the spool inside the valve and the so called „hydraulic lock”. The rotation is induced by a tangential net force which may be the result of an asymmetric incoming flow. The hydraulic lock is the result of a radial net force, acting on the spool. Based on this force occurs a high friction in the metal-metal contact between spool and housing. Reasons for the radial net force are unbalanced pressure fields around the piston or flow forces. The disadvantage of the rotation is the friction between the rotating spool and stationary parts of the valve. This may cause e. g. a deformation or even a welding of the actuator pin and the spool at its end face. The disadvantage of the hydraulic lock is a less sensitive moving of the spool, which makes proper proportional control impossible and requires higher actuating power in case of switching valves. One of these undesired phenomena can be suppressed by the other. They cannot appear at the same time. This paper presents proposals for suppressing the spool rotation. The investigations are realized by CFD simulated design modifications of the housing. The solutions are based on a symmetrically piped flow to the spool for equalizing the tangential flow components. The problem of the most of these solutions is an increase of the radial flow force which may cause the hydraulic lock. In not modified housings the hydraulic lock has been analyzed for special flow and spool combinations on a test bench.