filtering PS fluid(looking for info on a thread a few years back)

[JanDebL at aol.com]

There are several methods used for filtering hydraulic fluid.  Each have 
their advantages & disadvantages.
Filters in the high pressure system are expensive, place a high restriction 
(and added stress) on the system and are seldom used.  The filters need to be 
substantial to withstand 2-5K PSI pressure spikes when the oil is cold and 
viscous. They are usually made of a porous metal or sintered metal 
construction. Their advantage is that little if any contamination can reach 
down stream components.  They are normally used in combination with other 
filter systems.
Filters on the return section of the system are cheaper but starve the pump 
(and ruin it) when they become plugged and restrict the flow of oil.  For 
that reason, the mesh size is usually fairly large (and ineffective for 
filtering smaller contaminants).
Another type which I prefer is a parallel loop system.  This uses a small 
auxiliary pump and filtering system which cleans the oil (on the pressure 
side of the aux. pump at pressures compatible with the filter) similar to the 
oil filtration system on your engine.  Added cost is again the drawback.
Maybe one of us could invent a dialysis type of machine where you 
periodically connect it to your car to machine and clean the hydraulic system.
Until then, I believe Blau's tech tip, which promotes regular fluid changes 
is the most effective approach to long service life
Toilette paper filter aren't used too much any more but I bet the military is 
probably still using them and paying  ___ X the list price for them. The 
"Luberfiner" filters (the large canisters you see hanging on the side of semi 
tractors and off road equipment) often use toilet paper, wood chips or other 
questionable media.  Even after a massive engine failure, I have never seen 
one of these filters catch even a small amount of the contaminants. 
Happy filtering, Jan Lahtonen