Oil stiction in hydraulic valves - an experimental investigation

Oil stiction arises whenever two surfaces form a narrow gap filled with oil. For fast switching hydraulic valves with flat anchor solenoids this stiction force may degrade the valve opening response significantly. In spite of a vast literature on oil sticking - about 100 papers from different engineering fields have been studied - an adequate model of the stiction process in hydraulic valves is missing. Some research work has been done in the area of compressor valves where oil stiction often reduces reliability. Most of the literature found concerns basic research addressing adhesion, surface tension, fingering, etc.. The authors’ research intends to combine and extend the existing knowledge to get an appropriate model of the stiction process in hydraulic valves. The calculation of the maximum stiction force and some design rules to avoid the stiction force are the main final objectives. To understand the fundamentals and to check theories a test rig has been built. This test rig can generate the very fast separation of two parallel plates by a servo-valve controlled short stroke hydraulic cylinder and is equipped with sensors for the gap size, the separation speed, and the stiction force. First stiction measurements between parallel plates were done. For small gaps the measured stiction force curves correspond to those computed from the Reynold’s equation, known as Stefan’s force in literature. For larger gaps, however, additional effects, like fluid inertia, become significant and need a refined computational model.