Correlation between wear, impurities and particle counting

We permanently filter our hydraulic oil with 5μ filters in the bypass flow. Our particle counting, which we perform on site with a mobile device, shows correspondingly good cleanliness classes.

But how can it be explained that the wear elements such as iron, copper and chromium are increasing in the OELCHECK laboratory reports, although the purity classes are consistently good or even improve from sample to sample?

Is there a correlation between the values for wear, impurities and particle count?

OELCHECK answers:

Despite "good" cleanliness classes, whose values are lower than those required by the plant or machine manufacturers, wear element values can actually increase. This phenomenon usually occurs when such elements are completely in solution in the oil.

If these wear particles are only the size of a molecule, for example, they can no longer be filtered. In such cases, the wear values increase in lubricant analyses, although the particle count is without findings.

There are two possible causes for the coincidence of "low" cleanliness classes despite increasing wear elements:


The wear elements increase when they are detached as corrosive wear from the machine parts as reaction products in molecular layers. In particular, an oil containing water or an oil that has become "acidic" due to aging will preferentially dissolve copper, tin, lead and aluminum from the components of non-ferrous alloys. However, rust particles containing iron, which are due to moisture, can also be extremely minute, e.g. as "film rust". These particles "sucked off" the surfaces remain dissolved in the oil or are neutralized in such a way by additives that they can no longer be filtered even by ultrafine filtration in the main stream with a filter fineness of 1μ. The wear values increase. Despite clean oil, the laboratory analysis provides an indication of an oil change, which becomes necessary because the cause of corrosion-related wear particles often cannot be eliminated even by the best filtration.

Typical metals such as iron, copper, chromium, aluminum and tin, are also used in liquid form in the production of permanently elastic elements as chemical "wetting agents" or "vulcanization accelerators". Therefore, sealing and guide rings, surface seals and hoses may contain these components in their "rubber or plastic" material.

Synthetic oils, VI additives or increased acidity (AN) can dissolve molecules of these metals out of the elastomers. The flushed out elements remain dissolved in the oil and the particle count will not show anything in this case either.


Even a clean oil alone will not solve all the problems that can occur in a hydraulic system. Particle counting performed in the field provides only part of the information essential for oil and component assessment. What good is a clean oil if the oil has aged so much that acidic oil components attack hoses and seals. A meaningful assessment can only be made when all analysis values are considered in context.