Q & A

"We change many of our lubricants according to a schedule, ranging from six months to two years, depending on the lubricant and application. Because we use a scheduled oil change criteria (instead of condition-based oil change) we are uncertain as to whether the time interval we have chosen is optimum. How do we know if we should extend or shorten the scheduled interval?"

In many cases the original equipment builder can suggest helpful guidelines in this area. There may be useful technical support information available from your lubricant supplier as well. However, because a lubricant's service life has a lot to do with the stressing environment it is exposed to, the best way to tweak your oil change interval is to make actual assessments of its condition and remaining useful life. Unlike routine oil analysis, the type of tests you might want to select could be more similar to what you see on your lubricant's spec sheet.

Begin by talking to your lubricant supplier about which performance properties would need to be evaluated. These could include oxidation stability, rust/corrosion protection, air release and foam stability, demulsibility, antiwear protection, VI (Viscosity Index), silt particle concentration and other essential performance properties. Expect the cost of these tests to run several hundred dollars. However, you only have to run tests on samples from a few representative machines.

Obtain the sample just prior to the currently scheduled oil change. Send this sample along with a sample of the new lubricant (for baselining purposes) to a lab that can perform ASTM performance tests. You may need to obtain several hundred milliliters of fluid of each sample.

Once the tests are complete and an assessment made on remaining useful life a decision can then be made on whether the oil change interval can be shortened or lengthened. A reasonable safety margin needs to be included in the decision. Once changes in the drain interval are made monitor the oil carefully.

Today's Tip

• Viscosity is not the only issue associated with high pressure drop across filters on start-up. It might also have to do with the fact that the oil does not have a high enough cloud point. For indoor equipment where filter elements are involved, this can be much more important than the pour point.

  The cloud point is the point when you can first see wax coming out of solution. If you can see the wax then you know that the filter will also see it. In most cases the wax goes back into solution as the oil warms up. There is generally not a good correlation between cloud point and pour point because pour point depressants (an additive to lower the pour point) enter into the picture.

  (Tip submitted by Ken Brown, Utility Service Associates)

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Book Bits

From the book "Machinery Failure Analysis and Troubleshooting":

A rather large number of factors influences lubricating oil degradation and, consequently, pump bearing life. If your centrifugal pumps are equipped with rolling element bearings, there is little doubt that medium viscosity turbine oils (ISO Grade 68) will perform better than the lighter oils originally specified by many pump manufacturers. But, by far, the most frequent cause of lube-oil-related failure incidents is water and dirt contamination. With only 20 ppm water in pure mineral oil, bearing surface and rolling element fatigue life is reduced by an incredible 48 percent. Although the fatigue life reduction is less pronounced with inhibited lubricants, there are always compelling reasons to exclude dirt and water from pump bearing housings. Lip seals are a poor choice for centrifugal pump installations demanding high reliability. Face seals represent superior, "hermetic" sealing and should be given serious consideration.

For more information about the book "Machinery Failure Analysis and Troubleshooting" go to:

http://www.noria.com/secure/product_detail.asp?catalogid=20

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Lube-Tips is published by Noria Corporation. Oil Analysis and Lubrication Experts