Grease compatibility depends mostly on the compatibility of the thickener or soap base used by different products. Base oil compatibility and additives can also be a consideration. Even grease with similar thickener types can sometimes lead to lubrication problem when mixing different products, resulting in damage of the lubricated parts.
Incompatible grease mixtures will most frequently exhibit a change in consistency. Incompatibility can also exhibit abnormal oil separation, bleeding, at higher operation temperatures. It is the oil suspended in the grease by the thickener that does the actual lubrication. As moving parts exhibit pressure on the grease medium the lubricating oil is squeezed out so that it contacts and lubricates components. When operating pressure decreases the oil is resuspended into the thickener if the grease is performing properly. Mixing incompatible greases can have problematic results due to reactions of the two greases. If excessive oil bleeds out of the thickener then the grease loses its lubricating properties.
Greases can be considered incompatible when adverse situations occur due to chemical or material interaction between the thickener or additive formulation of the different greases. Formulations leading to incompatibility have become more complex because there are many more options today to make high performance greases. Grease suppliers can provide compatibility charts based on different thickener types or soap types. Though these compatibility charts are available, charts from the various manufacturers often disagree with one another on certain thickener-type combinations.
Clay-based greases experience the most compatibility issues due to the de-gelling of the thickener structure by soap based greases. Not all polyurea greases are exactly the same, therefore there’s more variance in their compatibility with other greases. Conventional polyurea greases experience more compatibility problems compared to shear stable polyurea grease. Shear stable polyurea grease is more accommodating than conventional polyurea and can be compatible with a large selection of grease types.
Lithium complex grease, is the most widespread choice in the marketplace. Lithium complex grease works well with most other greases except bentonite clay, barium complex, and conventional polyurea.
Compatibility of grease mixtures is typically categorized as follows:
- The properties of the mixture are similar to those of the individual grease.
- 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. Testing for worked penetration by ASTM D217 can help determine mechanical stability of grease mixtures
- The change in consistency of the mixture following storage at elevated temperature is still within the range of the change in consistency of the individual greases following elevated temperature storage.
- The properties of the mixture may or may not be acceptable, depending on the nature of the application
- The properties of the mixture are significantly different than those of the individual greases.
When changing to a different type of grease in operating equipment it is important to do your research ahead of time. Understand the type of thickener used in the formulation and its expected compatibility with the previous grease formulation in use. When changing to a different grease formation it is difficult to purge all of the previous grease in components. A general practice is to dispense the new grease into the lubrication space, where it will displace as much of the old product as possible. When doing this it is important not to displace seals. In some cases it may be necessary to completely remove a bearing component and clean the bearing of all previous grease prior to changing products. When in doubt if mixing is unavoidable or the most practical solution, testing of the grease mixture for compatibility is recommended. ASTM Standard D6185, “Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases” provides guidelines for testing for determining grease mixing compatibility.
David Doyle, CLS, OMA I, OMA II
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