In This Issue:

• Today's Tip: Make Oil Sampling Easier

• Book Bits: High-Speed Gear Lubrication

• Lube-Trivia: Up or Down?

• Q & A: Are Oil Analysis Results Repeatable?

Today's Tip

Make Oil Sampling Easier

• When labeling oil sample bottles prior to taking samples, consider identifying the bottle caps as well as the labels. A felt-tip pen works well. This will save a lot of time as you will not have to pick up each bottle to tell which bottle you need next. (Submitted by Craig Palculict, Oil Analyst, Georgia Pacific. Thanks Craig!

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Book Bits

High-Speed Gear Lubrication

From "Practical Lubrication for Industrial Facilities"

The oil furnished to high-speed gears has a dual purpose: lubrication of the teeth and bearings, and cooling. Usually, only 10 to 30 percent of the oil is for lubrication and 70 to 90 percent is for cooling.

A turbine-type oil with rust and oxidation inhibitors is preferred. This oil must be kept clean (filtered to 40 microns maximum, or preferably 25 microns), cooled, and must have the correct viscosity. Synthetic oil should not be used without the manufacturer's approval.

For some reason, the high-speed gear makes all the compromises when oil viscosity for a combined lube oil system is determined. Usually a viscosity preferred for compressor seals or bearings is selected and gear life is probably reduced. The bearings in a gear unit can use the lightest oils available, but gear teeth would like a much heavier oil to increase film thickness between the teeth.

More information about the book "Practical Lubrication for Industrial Facilities"

Lube-Trivia

Up or Down?

Test your knowledge and prepare for ICML lubrication and oil analysis certification with Lube-Trivia.

QUESTION: What effect will thermal and oxidative degradation typically have on the viscosity of mineral oils?

For the answer, visit: http://www.lube-tips.com/trivia/20030827.asp

Q & A

Are Oil Analysis Results Repeatable?

"I received a free oil analysis sample from a lab and decided to try it and compare the results to my current lab. I took two samples from an engine in one of my fleet vehicles and sent one to each lab. The results varied from one lab to the other. For example, the silicon was reported as 20 ppm from one lab and 24 ppm from the other. The aluminum was 13 ppm at one lab and 14 ppm at another. How much discrepancy should I allow between samples, and why is there such a difference between the two?"

When comparing data we need to talk about repeatability and reproducibility. These are simple concepts. Repeatability means if you take the same sample and retest in the same lab using the same instrument, how much variation is normal? For spectometric wear metal analysis (such as silicon and aluminum in your case) this should be <2 percent or < 1 ppm for both sample results you listed. However, what you are concerned with is reproducibility. Reproducibility simply means if I send the same samples to two labs, how will the data compare.

In used oil analysis, there are two factors that affect reproducibility. The most significant is the sample itself. You commented "I took two samples from an engine." Even slight differences in procedure such as how much oil is flushed before sampling can impact data reproducibility.

Secondly, the two labs may be using different instruments. While the two are nominally testing the same thing, the wear metal test has one major limitation, specifically the instrument's sensitivity to particle size.

In general, these instruments cannot see particles greater than three to eight microns. This size limit is instrument-specific, so if the two labs are using difference instruments, there may be differences in the concentrations reported.

In the case of the data you sent, in my opinion, the variation is not significant and is within normal ranges for two separate samples, sent to two different labs. Please be aware however, that in some cases (for example, soot, water, glycol etc.) labs can and do use VERY different instruments and procedures, which can give different levels of precision and accuracy.

The key to oil analysis is consistency - both in sampling procedure and test methodology. Using the same sampling procedure, with the same test methodology should provide consistent data sample-to-sample, allowing accurate trend analysis to be performed.

Mark Barnes, Senior Technical Consultant, Noria Corporation

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