Experimental study on friction force characteristics between the spool and sleeve of water hydraulic servo valve

Water hydraulic servo valve IS One of the important water hydraulic control components. Because of far lower viscosity and poorer lubrication compared to mineral oil. it is too difficult for water to form fluid film between two contact surfaces. It is desirable to minish and overcome the friction force between the spool and sleeve in water hydraulic servo valve. The friction force characteristic was examined experimentally in this paper and an experiment rig was developed. The results showed that several factors are influential to the friction force between the spool and sleeve of a water hydraulic servo valve, which included the system pressure, iso-pressure grooves of the spool, overlap length between the spool and sleeve. The friction force is definite linear to the system pressure and overlap length; the friction force is less about 10-15% when the spool is with iso-pressure grooves than without iso-pressure grooves; the watershed velocity of boundary lubrication and dry-lubrication is identified as being 0.56 mm/s, and the friction force would increase with the spool velocity increasing under boundary lubrication. The relevant research on the friction force characteristics will be useful to the design of water hydraulic servo valve and other water hydraulic control components.