Greases are made up of three components: base oil, thickener and additives. While thickener differences are the most common cause of incompatibility, base oil differences (for example, mineral versus synthetic) and adverse additive interactions can also cause grease incompatibility. Not only do the compatibility charts ignore the impact of base oils and additives, they also generalize about each thickener type.

Lets look at some examples. Mobil Industrial and Machinery Lubrication both provide publicly available compatibility charts, which clearly demonstrate a disagreement about the compatibility of lithium complex and aluminum complex greases. Mobil Industrials chart indicates that lithium complex and aluminum complex thickener types are compatible, while Machinery Lubrications chart labels them as incompatible.

A review of 21 grease compatibility charts found online shows major inconsistencies. In fact, 12 charts indicate that aluminum complex and lithium complex are compatible, four indicate they are borderline compatible and five indicate they are incompatible. It is obvious that trusting these compatibility charts is unreasonable and potentially a danger to grease-lubricated machinery.

What is grease compatibility?

According to the National Lubricating Grease Institutes definition, Two lubricating greases are incompatible when a mixture of the products has physical or performance properties that are inferior to those of the individual greases. Physical or performance properties inferior to one of the products and superior to the other may be due to simple mixing and would not be considered as evidence of incompatibility.

So, what can happen when incompatible greases are mixed?

Richard N. Wurzbach and Gretchen Kowalik of York, Pennsylvania-based MRG Labs explained in an article in Machinery Lubrication that when greases are incompatible, many mixtures will initially soften, often to the point of migration through seals or away from lubricated surfaces. Some mixtures will cause the thickener to release the oil, and the separated phase will run freely from the bearing, gear or housing. Other mixtures harden initially and cause component load issues and poor grease motility.

Other properties that may be influenced by mixing different greases are dropping point, shear stability, pumpability and oxidation stability, according to Lubrication Engineers.

An ExxonMobil white paper states that, although less probable but not unknown, the greases performance additives may act antagonistically, adversely affecting the lubrication performance such as protection against friction, wear, rust or corrosion.

That being said, it is obvious that end users must carefully consider compatibility when changing greases in their equipment.

Experimenting with Compatibility

The grease industry is being challenged by the recent increase in price and competition among users of lithium hydroxide, driven by the growing demand for lithium used in batteries for mobile electronics and electric vehicles. This has contributed to a decrease in simple lithium grease production. While lithium complex grease production continues to grow, it is uncertain how much the tightness of supply and increased costs of lithium hydroxide may dampen future lithium complex grease production.

This potential shortage may necessitate switches to different grease thickener types, and since changing from one type to another must be carefully managed, Grease Technology Solutions performed a study of the compatibility of different commercial greases. Christopher Horvath and Teresa Makuvek of Bethlehem, Pennsylvania-based FedChem conducted the laboratory work.

A test program was conducted on six commercial greases, three of lithium complex and three of aluminum complex thickener types. The testing was performed using a modification of the ASTM D6185 method (Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases) Option 2, using the ASTM D1831 Roll Stability test in place of the ASTM D217 100,000-stroke penetration test. Dropping point testing (ASTM D2265) and storage stability testing (FTM 3467.1) were included, as specified in ASTM D6185.

The data were analyzed using both the ASTM D6185 definitions and Grease Technology Solutions definitions for compatibility, borderline compatibility and incompatibility. The ASTM acceptability limits are based on test method repeatability, whereas the GTS acceptability limits are based on practical experience and are more generous than the ASTM limits (see sidebar on Page 32). For illustration, Figures 1 and 2 show the compatibility test data of the worst and the best pairs.

In general, greases are considered to be compatible if the following conditions are met:

The dropping point of the mixture is not significantly lower than that of the individual greases;

The mechanical stability of the mixture is within the range of consistency of the individual greases;

The change in consistency of the mixture following elevated temperature storage is within the range of the change in consistency of the individual greases following elevated temperature storage.

By the modified ASTM D6185 method, none of the mixtures were found to be compatible, and none were compatible by dropping point acceptance limits. Four of the mixtures would be judged compatible or borderline compatible based solely on the roll stability test, and one mixture would be judged compatible based solely on the storage stability test.

By GTS acceptance limits, two of the mixtures would be judged compatible. Based solely on the roll stability test, all mixtures would be judged compatible or borderline compatible. Based on only the dropping point, seven of the mixtures would be judged compatible or borderline compatible. Only two of the mixtures passed the storage stability test.

The storage stability test is most severe, with only two of the mixtures passing using the GTS acceptance limits and none passing by ASTM limits. The dropping point test is the next most severe, with all mixtures failing by ASTM acceptance limits and two failing by GTS acceptance limits. In the roll stability test (proxy for the 100,000-stroke penetration test), four mixtures passed using ASTM limits, and all mixtures passed using GTS acceptance limits.

There was a wide compatibility performance differential among the lithium complex greases, although their performance ranking is not the same by ASTM and GTS acceptance limits. However, the same lithium complex grease (L3) was ranked best by both ASTM and GTS acceptance limits. The aluminum complex greases performed similarly to one another. This tells us that it makes a real difference which lithium complex grease is being replaced, but it seems to be less important which aluminum complex grease is selected to replace it.

The relative overall performance (high and low temperature, load carrying, mechanical stability, etc.) of the three aluminum complex greases should be compared in a future study.

Conclusions

The results of the grease compatibility testing led to several conclusions. First, for a given pair of greases, different tests yield different results. For example, a pair may be compatible by shear stability testing but incompatible by storage stability or dropping point testing.

Another conclusion was that the compatibility rating by the ASTM D6185 criteria is both severe and rigid. Because of this, the compatibility test methods chosen should reflect application demands. For instance, in a low- to moderate-temperature application, dropping point reduction of 40 degrees Celsius may be reasonable.

Furthermore, rating compatibility by dropping point change assumes the test is predictive of service life or high-temperature operating limits, which is a poor assumption. Rating compatibility by storage stability penetration change under the test conditions of 1,400 hours at 75 C is also rather severe for applications at low to moderate temperatures. (It is important to note that this is a static test, and static tests are not expected to predict performance in a dynamic application.)

Rating compatibility by shear stability, using either ASTM D1831 roll stability or ASTM D217 100,000-stroke penetration testing, is probably a better way to predict the most common incompatibility, which is excessive softening.

Finally, ASTM D6185 has proven to be impractical and should be significantly revised or replaced with a more realistic and predictive compatibility test method. Compatibility testing by ASTM D6185 is so severe that it doesnt allow for any differentiation in compatibility level among the tested greases. Meanwhile, compatibility testing using GTS acceptance limits does allow for differentiation of the level of compatibility among the nine grease pairs tested.

Overall, this study showed that the thickener used in greases can contribute to considerable variation in their level of compatibility. It clearly demonstrates the danger of relying on published compatibility charts and shows the importance of actually testing the greases involved in any potential change-over. z

Chuck Coe is the president and principal consultant for Grease Technology Solutions LLC. He holds a bachelor of science in chemical engineering from Pennsylvania State University as well as NLGI Certified Lubricating Grease Specialist and STLE Certified Lubrication Specialist certifications. He worked for Mobil and ExxonMobil for 32 years, including 6 years as ExxonMobils Grease Technology Manager. Contact him at chuckcoe@grease-tech.com.

ASTM Grease Compatibility Criteria and Proposed Alternative

Dropping Point

ASTM Definitions:

Compatible: The dropping point of the mixture is equal to or greater than that of either constituent grease.

Borderline: The dropping point of the mixture is less than the lower of the constituent greases by an amount equal to or less than the repeatability of the test method. (ASTM D2265 dropping point repeatability = 7 degrees Celsius)

Incompatible: The dropping point of the mixture is less than the lower of the constituent greases by an amount greater than the repeatability of the test method.

GTS Definitions:

Compatible: The dropping point of the mixture is less than 40 C lower than the predicted dropping point.

Borderline: The dropping point of the mixture is less than the predicted value by 40-60 C.

Incompatible: The dropping point of the mixture is less than the predicted value by more than 60 C.

Shear Stability

ASTM Definitions:

Compatible: The penetration of the mixture is equal to or between the constituent greases.

Borderline: The penetration of the mixture is less than the lower of the constituent greases or greater than the higher of the constituent greases by an amount equal to or less than the repeatability of the test method. (ASTM D1831 roll stability test repeatability = 11 decimillimeters)

Incompatible: The penetration of the mixture is less than the lower of the constituent greases or greater than the higher of the constituent greases by an amount greater than the repeatability of the test.

GTS Definitions:

Compatible: The penetration of the mixture is 30 points or less different from the predicted penetration.

Borderline: The penetration of the mixture is 31-45 points different from the predicted penetration.

Incompatible: The penetration of the mixture is more than 45 points different from the predicted penetration.
Storage Stability

ASTM Definitions:

Compatible: The 60-stroke penetration change of the mixture is equal to or between those of the constituent greases.

Borderline: The 60-stroke penetration change of the mixture is less than that of the lower of the constituent greases or greater than that of the higher of the constituent greases by an amount equal to or less than the repeatability of the test method. (ASTM D1403 (half scale) 60-stroke penetration = 10 dmm)

Incompatible: The 60-stroke penetration change of the mixture is less than that of the lower of the constituent greases or greater than that of the higher of the constituent greases by an amount greater than the repeatability of the test.

GTS Definitions:

Compatible: The 60-stroke penetration of the mixture is 30 points or less different from the predicted penetration.

Borderline: The 60-stroke penetration of the mixture is 35-45 points different from the predicted penetration.

Incompatible: The 60-stroke penetration of the mixture is more than 45 points different from the predicted penetration.