Elemental analysis is a fundamental tool in used oil analysis. Used appropriately, trends in different wear metals can indicate a problem, allowing corrective action to be taken before catastrophic failures occur. However, the effectiveness of elemental analysis in pinpointing a problem and isolating it to one specific component or group of components can be greatly enhanced by taking some time in advance of the lab reporting “active machine wear” to identify the specific metallurgical composition of all oil-wetted components.
For example, high copper in an oil sample could indicate a host of potential problems. Seen in isolation, the copper is likely due to wear of a purely copper containing component such as a cooler core - baring any copper-containing additives in the oil. However, observed in conjunction with elements such as tin, zinc or aluminum, the copper is more likely to originate from a copper-containing alloy such as brass or bronze.
In order to make the most of elemental spectroscopic data, it is a good practice to spend some time gathering information about the metallurgy of each and every component, and compiling a reference, such as that shown in Table 1 in advance of the lab reporting a problem.
In particular, identifying not just major elements such as iron from steel, but also minor elements such as chromium and nickel, and trace elements such as vanadium and manganese, can help differentiate between different alloys of steel or other components.
It may also be appropriate to record the relative ratios of different elements in the various alloys present, so that these ratios can also be used as a characteristic fingerprint. However, care should be taken not to take the exact ratios too literally, because the ability of both inductively coupled plasma (ICP) and rotating disc electrode (RDE) spectrometers to accurately measure the absolute concentrations of certain elements such as copper and lead is significantly lower than other elements such as iron and chromium, due to difficulties in fully vaporizing and atomizing these elements.
David Doyle of CTC Analytical shares an example of how the identification of major and minor elements allowed a construction company to effectively isolate a specific problem on its cone crusher.
Case Study - Cone Crusher Wear Debris Analysis
Trend analysis of oil samples that were taken routinely on the cone crusher showed a steady increase in the amount of iron wear over successive samples, indicating an active wear problem. Each sample report was carefully reviewed by the original equipment manufacturer (OEM), who agreed with the lab’s assessment that iron levels were excessively high. Because the OEM was involved in the evaluation of the sample data, each wear metal present was carefully considered, as were the metals that did not show up in the analysis.
The interpretation of elemental spectroscopic data can be a haphazard exercise if the origin of different elements is not known. Most labs typically measure anywhere from 15 to 25 elements, ranging from wear metals such as iron, copper, lead and tin, to common contaminants such as silicon (dirt), sodium (coolant or sea water) and oil additives, such as phosphorus, calcium and zinc. Many elements can in fact have multiple sources; therefore, recognizing the relative ratios of major, minor and trace elements helps to successfully diagnose the problem.
Noria’s sourcebook for used oil elements (available online, click here) contains a host of information on how to make the most of spectroscopic data. It includes tables that list the most common sources for each element based on component type, machine metallurgy and working environmental conditions, as well as tips and tricks for interpreting spectroscopic data and performing trend analysis.
Increasing iron in the cone crusher could have been caused by any number of wear problems. However, there was no chromium in the oil analysis, indicating wear from bearing races was not an issue because it was known that the bearings were made from high-alloy steel containing a fairly high degree of chromium. Additionally, normal levels of copper, lead and aluminum indicated that bearing cages were not wearing excessively, also a clue that the wear problem was located somewhere other than the bearings.
The fact that iron was the only wear metal that was high and getting worse, ultimately narrowed the potential problem to a floating plate (similar to a universal joint) or a gear set itself, not bearing wear. Familiarization with the cone crusher and commonly encountered failure modes led the OEM to believe that the floating plate would be the most likely cause of the high iron level.
Based on the OEM’s review of the oil analysis data, the customer was advised to pull the cone head and inspect the unit. The customer was told where to look and what to look for, allowing the initial inspection to be completed in less than five minutes.
Upon inspection of the unit, it was discovered that the floating plate did indeed have extensive wear and was close to catastrophic failure. In other cases where this problem had not been diagnosed soon enough, the floating plate wore so thin that it broke apart, distributing large particles into the bearings, causing additional and more widespread damage. Based on the oil analysis data and visual inspection, immediate replacement of the floating plate was recommended.
Replacing a single $700 part (the floating plate) in one hour of downtime enabled this unit to be quickly put back into service. By contrast, not catching the worn part in time could have cost from $2,000 to $12,000 in additional damage, and an estimated minimum two-day downtime.
In this instance, understanding metallurgy of the key components and the typical failure modes of this system, along with using oil analysis as a predictive maintenance tool, allowed the construction company to prevent a larger problem. Knowing what to look for and having the right parts on hand kept a relatively minor problem from becoming a major one.