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Grease Compatibility, Charts and Testing: What is the most reliable method for determining grease compatibility?

We stated in an earlier blog that greases should not be mixed and the general recommendation for compatibility determination is to consult the standard ASTM D6185. Still, grease and equipment manufacturers recognize the practice of mixing greases will occur despite warnings against it. Also, equipment users often don’t have the resources to evaluate grease compatibility and must rely on other sources of information. That being said, it is important users and suppliers understand the compatibility characteristics of any two greases that will inevitably be mixed. The use of compatibility charts alone is not recommended, since contradictions abound. Sometimes overall grease types are presented as compatible, and yet testing of the specific greases could prove incompatible.

Mixing of greases can produce an inferior material to any of its constituent materials. A mixture of incompatible greases often softens, sometimes excessively. Occasionally, it will harden. In extreme cases, the thickener and liquid lubricant will completely separate, causing bleeding that can be so severe the mixed grease will run out of a bearing. Excessive syneresis can occur, forming pools of liquid lubricant separated from the grease. Such events can lead to catastrophic lubrication failures (5.1 ASTM D6185). So how do we approach evaluation of compatibility issues without risk of severe damage?

We first need to understand that compatibility cannot be predicted solely based on grease composition. Many sources state that “generally” greases having the same (or similar) type of thickener can be mixed. Although uncommon, greases with the same thickener can still be incompatible because of a difference in additives, making it important to consider mixing on a case-by-case basis.

For instance, one of the common places mixing of greases occurs is when a lithium complex grease (commonly found in fan and pump bearings) mixes with a polyurea grease that is often used in electric motors for the same application. It’s easy to see how these two greases can be mixed when they are used side by side. Variations in thickener properties of polyurea greases (shear stable vs. non-stable, etc.) further complicates the issue. Also, most grease compatibility charts focus solely on the family of thickener, although there are three components to grease: the base oil, the thickener, and the additives. When dealing with the mixing of oils, the base oil type and the viscosity, which are crucial factors, are almost completely ignored. Thickener compatibility can be the most immediate and obvious consideration, but it is not the only issue. 

Base oil and additive compatibility must also be considered. Combining synthetic vs. mineral oil bases can produce less than desired performance in an application, even if the thickeners are compatible. The same is true with viscosity. Mixing greases with varied base oil viscosities can result in a mixture that is not optimal for the application. It is important to know whether the mixing of greases in a particular application can lead to unwanted consequences. Since compatibility charts are varied, we suggest testing according to the standard (ASTM D6185). 

Three test methods are used, because fewer have been found to be insufficient. If a mixture fails any of these three primary tests, the greases are said to be incompatible. If the mixtures pass the three primary tests, the greases are considered compatible. These tests are 1) dropping point by Test Method ASTM D2265 (or Test Method ASTM D566); 2) shear stability by change in 100,000-stroke worked penetration by Test Methods ASTM D217, and 3) storage stability at elevated temperature by change in 60-stroke penetration by Test Method ASTM D217. To pass a compatibility test, a mixture of two greases must have a result that falls between the results for each grease tested individually. For mixtures that pass these primary tests, there are further secondary (but not mandatory) testing that can be done if the circumstances dictate. 

Most grease suppliers have data on certain grease combinations or are willing to perform required testing for their customers. In addition, you can find details of the testing procedures at www.astm.org.

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