How to Interpret Oil Analysis Data

Noria Corporation
Tags: oil analysis, hydraulics, particle counting, contamination control, lubricant sampling

"On a recent hydraulic oil sample analysis, the iron count was high, about 30, yet the particle count was fairly clean at 17/14. I would expect the iron count to be low if the system was clean. Can you offer any insight into what we are seeing?" 

In a mission-critical hydraulic system, an X/17/14 cleanliness level may not be all that desirable. When looking at a scale for average hydraulic system cleanliness, a level such as this is borderline between clean and dirty. Generally, a cleanliness level of 16/14/11 or better would be recommended for this type of application.

You mention a cleanliness level of 17/14. This provides the cleanliness at the greater than 6 micron and greater than 14 micron levels (assuming ISO 11171 calibration). If the spectrometer is showing 30 ppm of iron, we know this is measuring particles less than 5 microns due to the inherent limitation of this test. Do you have a cleanliness level for the greater than 4 micron level?

You would also expect the iron count to be low if the system is clean ... provided there are no mechanical conditions that would create iron debris as a leading indicator.

It is possible for a system to be "clean" of environmental and external contamination and still show wear. In fact, this is what we'd prefer to see. Then we can focus our attention on repairing the mechanical problem right away rather than dealing with external contamination control just to find a mechanical problem shortly thereafter.

To help judge the true severity of the iron, it's important to know where you are sampling from. If you are pulling the sample from the drain port of the hydraulic reservoir, you can expect to see a higher concentration of debris, as build-up and settling are likely to occur. If the sample point is located in the middle of the reservoir but away from the return line, you can assume some type of dilution effect, which would suggest a higher concentration of wear is likely.

The best place to pull a sample from a hydraulic system is from the return line prior to passing through a return filter if equipped. From here, it would be ideal if there were secondary sample points after major components within the hydraulic system. This would allow you to pinpoint the source of the debris with a high degree of confidence. In the absence of secondary sample ports, you must rely simply on the knowledge of the hydraulic components within this specific system.

Additional testing will also be required on your sample to help identify the mode of wear that is taking place. Because iron is what is being seen, analytical ferrography would be the test of choice. This will also help to identify the source of the wear. You may want to add ferrous density testing to future samples from this system to help monitor larger particles of iron debris. Check with your lab to see what it offers for ferrous density.

In general, iron in a hydraulic system is likely coming from the pump or cylinders. The precise source and cause must be identified. Some initial ideas to consider include making sure the fluid properties are consistent with system manufacturer recommendations and that they are still within the appropriate tolerances for that fluid. If the system is set up with appropriate and functional contamination control measures and there is a change in fluid properties, abnormal wear can quickly and easily occur.

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