Dirty oil spells rapid death for hydraulic machinery and lubricated equipment. Fine tolerance equipment can have clearances between parts of 5 to 10 microns (0.005 - 0.01 mm).  Solid particles larger than the clearance gap will jam into the space. The solid particles will further be broken-up and mangled while ripping out more material from the surfaces. In equipment with larger tolerances the oil film between parts can get as thin as 3 - 5 micron. Solid particles larger than the oil film will be broken up into smaller pieces and produce more solids contamination. Figure No. 1 shows a shaft in a journal bearing lubricated by oil. In the drawing the solid particles are larger than the oil film thickness and when they enter the bearing pressure zone at the bottom of the shaft they will tear into the metal, be broken up and make more particles that cause further wear.

Filters are fitted to hydraulic systems for pump protection as part of the original equipment.  The question most often asked is: "isn't that enough protection?"  The short term answer is "NO" and the long term answer is "A HELP".

• Filters require several passes of the fluid before the particle contamination starts to drop.
• Filter elements that are normally fitted are of the 10 micron range and do not have the capabilities of reducing the high contamination level in the 5 to 15 micron range. This range in NEW OIL may be up to and above 10 million particles of contamination per 1 litre of oil. This is in the range of ISO 4406, 21/18
• Filtering to lower the contamination level prior to use is common practice. Most filters available are of excellent quality, but this does not explain the whole story. Filters will clean the contamination but not the moisture.
• Filter elements dislike moisture. The drier the oil is, the more effective the filtration. Moisture blocks off the filters, giving a false impression the filter is doing its job.
• The lower the micron rating of the filter, the lower the moisture content of the oil needs to be.
• Water interferes with, or coalesces up against the backing membrane face and gives the impression that the filter has blocked off with contamination, and the filter goes into by pass mode.
• The operator then changes the filter element believing the oil in the system is being cleaned. This only adds unnecessary cost to the maintenance program.
• Filtering oil is a workable solution to the contamination problem when the moisture content is low. It is not cost effective when many changes of elements are required due to moisture. There is also the grave risk of emulsifying the oil and setting up another problem to deal with.