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As part of my job, I regularly present oil analysis training seminars. One of the tests we discuss is ferrous density. I find that this test is seldom well understood, and, probably as a result, is seldom used.
Explanation of the Test
Ferrous density is the density of ferrous, or more correctly, of ferromagnetic particles distributed in the oil. The other ferromagnetic elements that one might encounter in lubricated components are nickel and cobalt, but in practice, these elements are likely to be present only in comparatively small quantities. Therefore, we are effectively measuring the concentration of magnetic steel alloys.
There is no single way to conduct a ferrous density analysis. There are many ways of performing this rather essential test. Perhaps the two most common are the direct-reading ferrography, a technique which uses light blockage analysis of material deposited on a glass slide; and electromagnetic induction, which measures the voltage induced in a current-carrying coil by the presence of ferromagnetic material.
The test reports the concentration of ferrous debris in a sample. While no units are reported (the numbers generated are indices), you may conceptualize the results as mass of ferrous debris per mass of oil - something like grams of steel per kilogram of oil. The concentration index does not tell us anything about the size distribution of the particles; but practically, the test is biased toward larger particles.
Ferrous density is suited to small-sump systems in which the major wearing components are ferrous-based, and in which there is no filtration or at least relatively course filtration. Good candidates include engines and gear-heavy components, like most automotive and industrial gearboxes and transmissions. I would strongly recommend making ferrous density analysis routine on such systems.
Systems which have large oil volumes and experience relatively fine filtration, like turbine systems and hydraulic systems, are not good candidates for this test, at least not by using the oil sample. It is useful to perform a ferrous density analysis of debris which has been washed out of a section of the filter element.
In the case of the electromagnetic induction technology, it is also possible to perform ferrous density on a filtergram produced either from the oil or the filter debris. On such lubrication systems, I would recommend performing a ferrous density analysis on filter debris either suspended in solvent or deposited on a patch. As always, make sure the test procedure is reproduced correctly each time to ensure trendable results.
Ferrous density is not useful on systems which have comparatively small ferrous composition, like worm gearboxes which use a cupric alloy-based bull gear.
A failing ferrous density analysis should be followed up with a microscopy-based test, such as analytical ferrography or patch microscopy.
Comparison with Elemental Iron Analysis
All ferrous density testers produce at least a single index of contamination proportional to density of the ferrous particles in the oil. I will use this index in the following discussion. There are also many ways of determining the elemental iron (Fe) content, but I will limit this discussion to inductive-coupled plasma (ICP) atomic emission spectroscopy, because this is the most commonly performed method of elemental analysis.
To highlight the differences between ferrous density and elemental analysis, two theoretical situations are presented. The first is illustrated in Figure 1. A sample has been prepared by taking a ball out of a ball bearing and putting it into some oil.
If you were to analyze this sample for both Fe and ferrous density, the ferrous density would, not surprisingly, come out as high, but the Fe would be zero. This relates to the limitation of the sizes of particles analyzed by an ICP spectrometer. Approximately five microns is typically quoted as the upper limit.
If we took the same ball bearing and ground it to a fine powder (Figure 2), the ferrous density would be the same as in Figure 1 (the concentration of the ferrous material has not changed). However, the Fe reading would now be high, as particles have been presented to the spectrometer which it can detect.
Figures 3, 4 and 5 represent real-life situations. Figure 3 illustrates a normal wearing situation, predominantly rubbing wear particles. There are a few small particles suspended in the oil. Predictably, both the Fe and the ferrous density are both low.
Figure 4 illustrates corrosion. During corrosion reactions, the particles generated are extremely small, usually submicron in size and often completely dissolved. The concentration of ferrous debris is actually quite low, but because virtually all the particles can be detected by the ICP spectrometer, its reading will be high.
Figure 5 represents a sample taken from a system in the latter stages of fatigue failure - such wear debris distributions are characterized by relatively few small particles and many large spalls. The ferrous density reading would be high, but the Fe would be small due to the relatively few particles the spectrometer could accurately process.
Ferrous density analysis presents a rapid and powerful picture of the overall wear situation of a lubricated component. Comparing ferrous debris concentration indices and elemental iron gives you even more information about the type of wear occurring and the distribution of wear particle sizes.