"For our gearboxes, we employ oil analysis, which includes testing of viscosity, water concentration, PQ index and 21 elements by inductively coupled plasma (ICP). The wear concentrations generated in worm gearboxes and bronze-bushed sleeve bearings, such as the ones usually found in rotary kilns, are the most difficult to interpret. Wear particles of copper, lead and iron are expected to be found in these applications due to the high-friction bronze parts, but it is difficult to correlate the wear concentration with the actual amount of wear on the gears and sleeves. Are there any guidelines for the absolute or trend limits? Do you have any advice or experience with this?"
In order to reach the proper conclusion about these wear levels, it is important to consider certain details to ensure the reliability of your oil analysis program. A world-class program has two starting points. The first is an appropriate design according to the established objectives. This includes selecting machines (based on their criticality), test slates, limits/targets, sampling locations, machine conditions and procedures for taking a sample, and the tools or devices required to obtain a sample.
The second starting point is field implementation. This is when all hardware is acquired and samples are taken according to best practices and sent for analysis. This sequence should be followed by proper testing procedures in the laboratory, interpretation of the results and taking the appropriate actions when necessary. The illustration below shows some of the factors that help form a chain of success for oil analysis optimization.
To attain an effective diagnosis, verify that all elements mentioned above are implemented and followed correctly. This will help ensure quality and consistency. If your results have significant variations, it may be due to inconsistent procedures.
Also, analyze the laboratory results at three levels: results vs. limits or targets, historical trends and historical statistical information for groups of machines.
To define the limits, you can utilize different sources, including recommendations from the original equipment manufacturer (OEM), the lubricant supplier, a third-party laboratory and statistical analysis of historical results using two and three standard deviations.
Most gearboxes commonly have higher wear debris concentrations than other machines, so seeing iron, copper and other metals as part of the normal operation is to be expected.
A simple evaluation of your procedures and lab results should help you come to the right conclusion and establish a more consistent oil analysis program.