TechArchive

Original article date: November 1999

Grease incompatibility has serious implications for the performance of rolling bearings. We asked Tony Synnott of NSK-RHP to explain how problems can be avoided

Imagine these scenarios: a laboratory technician tests grease from a problem bearing and finds that, despite meeting all of the specifications, it is not performing as it should; a hot strip steel mill changes over to a grease that is highly rated by other mills, but complaints are soon pouring in; during a rush production run, a critical motor fails even though it had been properly lubricated. What has happened?

In every case, the problems occurred after one type of grease, which met the specifications, was changed to another type with the exact same formula. The cause was grease incompatibility; some greases cannot be mixed with others, even when both types meet specifications. Unless this incompatibility is understood and accounted for, a switch to a different grease can have serious consequences.

Incompatibility occurs when a mixture of two greases shows properties or performances significantly inferior to those of either grease before mixing. Some grease bases are intrinsically incompatible. Different fatty acids and/or additive packages also affect compatibility. To make it even more confusing, sometimes a grease is manufactured as a mixed-base grease even though the two bases are incompatible when mixed in operation.

Usually, problems are not obvious until the bearing is in use. By that time, major problems can develop. It’s best to know in advance which types of greases can be used together and which should not.

Testing Greases

Results from compatibility tests on ten different types of grease are given in the table on the left. For the series of tests, two greases were tested separately and then blended at three different ratios at room temperature. The blended greases were tested at room temperature and again after storage at 250degF.

Every grease proved to be incompatible with at least one other grease. Most difficult to use in mixtures were aluminium complex, calcium complex, clay and polyurea thickened greases. Usually, substantial softening occurred when greases were incompatible. This is especially dangerous since softened grease can run out of a bearing, particularly in vertical applications. However, lithium grease hardened in some mixtures, as did some clay mixes. Barium grease blends looked like grease on the bottom and oil on the top; mixing may have caused the second grease to liquefy while the barium remained intact.

It is a mystery why engineers don’t foresee more problems when greases are mixed either deliberately or accidentally. The answer is that even a soupy mix of incompatible greases may work for a while when the bearings are in good shape, if shock loading isn’t too severe, the seals are adequate, and demands are not extreme. And most instances of incompatibility are brief – a little old grease gets mixed in with a different grease for a short time.

But what if changing the grease is necessary? There are ways to ensure a safe changeover. Incompatible greases don’t have to be eliminated completely; a little common sense by the user can prevent problems:

1. Ask the supplier about compatibility. They should have the information or be willing to run tests.
2. Use up as much of the old grease as possible before bringing in the new grease. It is ideal, if not always practical, to completely drain and clean the system before changing over.
3. Once the new grease is added, grease consumption should be increased temporarily. This will move the interface (the area of grease mixing) through the system and out as quickly as possible. The increased grease flow also assures good lubrication and proper sealing while overly soft grease may be in the bearings.
4. When in doubt, expect incompatibility and watch for problems.

•  NSK-RHP

November 1999