A Guide to Grease Compatibility

Industrial plants thrive on consistency because it allows for the smooth operation of equipment. However, sometimes various factors can affect the working of the machinery and may demand change in lubricants.

It is often possible to change the lubricating oils completely. If not, the compatibility of the fluids is relatively easy to determine.

On the other hand, in grease-lubricated applications, it is usually impossible to remove old grease entirely when changing products. Grease suppliers advise purging as much of the grease as possible through the grease-dispensing system and application.

However, this may not be necessary if the two greases in question are compatible with one another.

Note: This chart is a general guide to grease compatibility. Specific properties of greases can dictate suitability for use. Please ensure that you test the compatibility before using.

Unlike the compatibility of oils, which is most often related to the interaction between the additives or sometimes the nature of the base fluids, the compatibility of grease is most often related to the thickener, although base fluid compatibility is also important.

Grease compatibility can be confusing even if most grease manufacturers produce compatibility charts. This can be because the charts from the various manufacturers often disagree with one another on certain combinations.

Earlier, when simpler combinations were in use, Lithium and Calcium soaps were compatible with one another, and neither was particularly good when mixed with a clay-based grease. Today, with many new complex soaps, polyurea, calcium sulfonate and even more exotic thickeners being used in many greases, the issue of compatibility has become much more complicated.

A typical grease compatibility chart is shown in Figure 1.

Compatibility of grease mixtures is typically categorized as follows:

• Compatible – The properties of the mixture are similar to those of the individual grease.
• Incompatible – The properties of the mixture are significantly different than those of the individual greases.
• Borderline – The properties of the mixture may or may not be acceptable, depending on the nature of the application.

However, some grease specifications are based solely on grease performance and not on grease composition. Using incompatible greases can result in heavy damage.

For example:

The compatibility of polyurea greases with soap-thickened greases is probably the most debated area of grease compatibility today. Greases based on simple lithium soaps (lithium stearate or lithium 12-hydroxy stearate) and lithium complex soaps may or may not be compatible with polyurea greases. The reason being, some polyurea thickeners are completely compatible with lithium and lithium complex thickeners, while others are incompatible.

It is necessary to perform compatibility testing. Most grease suppliers have data on certain grease combinations or are willing to perform the required testing for their customers, but the responsibility lies on the user.

However, for ease of understanding, ASTM International (formerly the American Society for Testing and Materials) Committee D02.G developed ASTM D6185 Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases in 1997. It details the procedures for evaluating the basic compatibility of greases by measuring:

1. The dropping point
2. The mechanical stability
3. The change in consistency of the mixture upon heating

Once two greases are deemed compatible in the areas mentioned above, further testing is done to determine the impact on other performance parameters. Any test designed for measuring grease performance may be used on a mixture of greases to determine the effect on that parameter.