Q & A

"We are looking at changing lube suppliers soon and I have been told that mixing products from the two suppliers can present compatibility issues. What advice can you offer?"

In general, it is never a good idea to mix different oils because there is always a possibility that the two will be incompatible, particularly the additives in each oil. Of course, changing lube suppliers is a fact of life and you cannot be expected to drain every last drop of oil from the old supplier from your equipment and replace it with the new oil!

The first step is to discuss with the new supplier any incompatibilities that are known between corresponding products from each vendor that will be used in the same application. Usually, they will provide a cross-reference guide of equivalent products that are compatible and have the same minimum performance specifications. If any doubt exists, compatibility testing should be conducted as follows:

Take three different mixes of oil A and oil B, one at 50:50, one at 90:10 and the third at 10:90 and test for filterability, sediment (possibly caused by additive drop-out), color and clarity, RPVOT, storage stability, demulsibility, rust and corrosion inhibition, air release and foaming tendency. Provided all mixtures pass these performance tests, there is a good possibility that the two oils are compatible. More care should be taken with grease compatibility issues. With greases, the biggest problem is compatibility of soap thickeners. Incompatible greases can either thicken or thin when mixed and extreme care must be taken when switching from one product to another. Without any evidence to the contrary, all greases should be treated as incompatible and equipment be thoroughly cleaned and purged before introducing the new grease.

Today's Tip

• Suction line leaks can cause air entrainment and cause problems such as excessive aeration, air lock, pump cavitation, poor lubrication and premature oil degradation. A pinhole-sized suction line leak can be hard to find, however, using a small amount of shaving foam sprayed over the suspected leak area can indicate the source of the problem as the foam is drawn into the line. This method should not be used for large leaks and care should be taken not to use too much soap since this can cause contamination of the lube, resulting in foaming and aeration problems and poor demulsibility.

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Book Bits

From the book "Machinery Failure Analysis and Troubleshooting":

Any load-carrying contact, even when rolling, requires the presence of lubricants for reliable operations. The curvature of the contact areas between rolling element and raceway in normal operation results in minute amounts of sliding motion in addition to the rolling. Also, the cage must be carried on either of the rolling elements or on some surface of the bearing rings, or on a combination of these. In most types of roller bearings, there are roller end faces which slide against a flange or a cage. For these reasons, adequate lubrication is even more important at all times. The term "lubrication failure" is too often taken to imply that there was no oil or grease in the bearing. While this happens occasionally, the failure analysis is normally not that simple. In many cases, a study of the bearing leaves no doubt in the examiner's mind that lubrication failed, but why the lubricant failed to prevent damage to the bearing is not obvious.

When searching for the reason why the lubricant did not perform, one must consider first its properties; second, the quantity applied to the bearing; and third, the operating conditions. These three concepts comprise the adequacy of lubrication. If any one concept does not meet requirements, the bearing can be said to have failed from inadequate lubrication.

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