Why Hydraulic Spool Valves Stick

spool1Why 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 »

The Pros and Cons of Wire Reinforced Hydraulic Hose

hydraulic hoseA recent client engaged me to design the hydraulic steering circuit for a luxury motor yacht he was building. During installation, the installer questioned why I specified thermoplastic hose for the sections where stainless steel tube was not able to be used.

For those not familiar, thermoplastic hose (Synflex, Parflex, etc) is entirely reinforced with synthetic fibers. And this is advantageous in a marine application like this one, for a couple of reasons.

The first is corrosion resistance. The steel-wire reinforcement of a conventional hydraulic hose is prone to corrosion–despite the fact it’s covered with a rubber sleeve. This outer sleeve tends to be porous, and is therefore not an impermeable barrier to the saltwater environment.… continue reading »

How to Compare and Select Hydraulic Filter Elements

Filter used on a filter cartOne of our readers wrote me this question:

“We are evaluating filters for a contract and I had the suppliers give me the Beta ratios for the elements they propose to supply. The problem is one company has given me Beta ratios for 2 and 11 microns, and the other company, 6 and 20 microns, i.e.:

Company A’s filter is Beta2 = 2 and Beta11 = 20
Company B’s filter is Beta6 = 2 and Beta20 = 75

So the way I read it:
Company A’s filter is 50% efficient at 2 micron and up and 95% efficient at 11 micron and up.continue reading »