High levels of wear metals with normal purity classes?

Year of publication: 2013


How can there be high levels of wear metals with normal purity classes?

For years we have been using your analyses to monitor the hydraulic fluids in our excavators. Poor oil purity, contamination or excessive water content have been found repeatedly. However, we recently received a laboratory report from you which shows high wear values but completely normal purity classes, to our considerable surprise. Shouldn't the purity of the fluid be significantly worse with a relatively large increase in wear metal?


OELCHECK answers:

This conclusion appears to be correct. In theory, all of the values should increase. However, this depends on whether the detected metals are really particles which originate from abrasive wear. In the case of your hydraulic excavators, this does not appear to be the case. The wear values which we measured, with higher levels of copper, chromium and iron, are clear. However, these metals do not result from the wear of bearings, pumps or valves. For example, a high level of iron or copper does not always mean that there is something

wrong with the pump. Wear in a pump, when it does occur, is fostered by factors such as dust (silicon) or other wear particles. In such case, the purity class clearly indicates that the filtration is deficient. However, in a hydraulic system wear can also result from chemical processes. Lubricants and hydraulic fluids can attack elastomers, especially if the fluids contain synthetic components or detergent additives. Sealing rings and guide rings, hoses or other synthetic materials are composed of more than just plastic. In addition to the raw polymer, they contain up to 25 other components such as carbon black and oil, fillers and swelling agents, plasticisers, wetting agents, pigments or glide enhancers. There are good reasons why seal compatibility tests are specified in the standards for hydraulic fluids. If the „chemistry“ between the seals and the lubricant is not optimal, metallic elements may be dissolved out of the seal material. For example, O-rings may contain iron oxide, hoses may contain zinc oxide, or guide rings may contain chromium or copper compounds with particles in the nanoscale range. If these are leached out of the elastomer, they remain in solution in

the oil. In the ICP tests used to determine approximately 30 elements with particle sizes down to 3 µm, they appear in the laboratory report as elevated levels of iron, zinc, chromium or copper. Elastomer particles can be so extremely small that even in elaborate tests, such as those which we perform with scanning electron microscopy, we are unable to filter them out even with an especially fine filter with a pore size of only 0.25 µm. This also explains why they cannot be seen at all in the particle count. For the determination of the purity classes according to ISO 4406, particles are only counted if they are larger than 4 μm. If the levels of wear metals are outside the usual range but the purity classes indicate that better filtration is not required, we must rely strongly on our expertise and instinct. Our diagnostic engineers have extensive knowledge of where the elements or metal oxides dissolved in the oil may come from. Thanks to their experience and supported by the information in the large OELCHECK database, they can provide a clear indication of the origin of the elements, and if necessary they can point out the harmless wear of seals and guide rings without sounding an alarm about pump wear.