Why do hydraulic spool valves sometimes stick? In a word: contamination. The radial clearance between the spool and bore in common, garden-variety directional control valves ranges between 3 and 13 microns. And in practice, perfectly round and straight bores are difficult to achieve. This is why servo and servo-proportional valves are fitted with a sleeve between the spool and bore-for high precision. And so the radial clearance typically varies across the length of the spool. This influences spool friction–but so too does the cleanliness of the oil.
If the clearances between the spool and bore become invaded by hard particles (silt) or soft particles (varnish and sludge), more force is required to move the spool.… continue reading »