Lock-Step Trending

Tags: oil analysis

When analyzing data from elemental spectroscopy, it is important to consider not just single elements in isolation, but rather to look at multiple elements simultaneously. By doing so, certain inferences can be drawn with respect to wear metals, contaminants and additive elements.

A good case in point is the determination of the location of a known wear problem within a complex piece of equipment, which may contain many different components. For example, a gearbox may contain steel gears and steel rolling element bearings. Simply looking at the iron concentration in ppm as the sole indicator of a wear problem will not help to isolate the problem as either gear wear or bearing wear. However, if the rolling element bearing is made of a high carbon steel alloy, the presence of chromium as a minor trace element, moving in lock-step with the iron may help to differentiate between gear wear and bearing wear.
(Figure 1)

Similar lock-step trending can be used to look for signs of additive depletion or the addition of wrong make-up oil. For example, a small increase in zinc may result from wear of a galvanized or zinc-plated component, an alloy of brass or the accidental addition of a small quantity of an oil containing a zinc-based additive such as ZDDP. In this case, an increase in both zinc and phosphorus may indicate that the wrong oil had indeed been added, while an increase in copper and zinc in lock-step may indicate that the problem is related to brass wear.

When it comes to contaminants, silicon is of prime interest because its presence often signifies dirt ingression due to the mineral silica found in most dust and dirt. However, other sources of silicon can and do occur in oil, including silicon-based antifoam agents, leaching of silicone sealants and residual sand from casting of new components. So how can one differentiate an increase in silicon due to dirt ingression from these other sources?

The most common way is to look at silicon and aluminum simultaneously. With dirt ingression, aluminum often shows up at the same time as silicon because dirt contains not only silica, but also alumina. The relative ratios of these elements will vary depending on the type of dirt, and specifically the geology of the area in which the equipment is operating. However, in many cases, the ratio of silicon to aluminum is typically 3 to 4:1. To determine the ratio of these two elements in your working environment (or any other contaminant for that matter), it’s as simple as taking a fresh sample of clean oil, and leaving it in a strategically dirty area of the plant or site for a day or two with the sample bottle lid removed. At the end of the day, cap the sample and submit it to the lab for elemental analysis. It is a safe bet that after a day or two exposed to the ambient environment, the bottle will be laced with dirt, allowing the ratio of silicon to aluminum to be estimated.

We recently learned of a rather unique situation where this ratio was found to vary seasonally. This story comes from a Caterpillar SlOlSTM lab in India, which analyzes thousands of samples annually from mining equipment. The lab noticed that the ratio of silicon to aluminum varies depending on whether the sample is taken during the monsoon season in June and July, or in a dry time of year. During monsoon, the rains cause vast quantities of clay-based soil to be washed away. Because clay is comprised of silica and alumina in the approximate ratio 3.3:1.0, high levels of both elements are seen when dirt ingress occurs during the monsoon season. However, the lab also noticed that under drier conditions, the source of dirt and the ability of dirt particles to become airborne vary such that small amounts of aluminum are seen, but significant quantities of silicon are observed whenever a dirt ingress problem occurs. By recognizing this seasonal variation, the lab is able to allow for these differences in the relative ratios of silicon and aluminum, and is able to make an accurate diagnosis based on historical trends.

POA would like to thank Dr. D. Mukherjee of TIL Ltd., West Bengal India for bringing this interesting “Field Fact” to our attention.

For more information on lock-step trending and using minor and major trace elements for root cause analysis, refer to “The Sourcebook for Used Oil Elements” available online at www.oilanalysis.com.

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