How to read the compatibility chart of lubricating greases

Read time: 2 min
Published: 16 March 2023

Mixing different greases can lead to ineffective lubrication. By consulting the lubricating grease compatibility table, it is possible to ensure optimal performance.

How is lubricating grease obtained?

Lubricating greases are essentially oils to which thickening components, called soaps, are added to make them semi-solid and prevent leakage or system leaks (e.g. calcium, lithium, aluminium soaps, etc...even in complex versions).

Several characteristics depend on the soaps. The main ones include the consistency, NLGI (National Lubricating Grease Institute), and the drop point, the temperature at which the soap melts.

What is the problem with mixing different greases?

During working activities, one sometimes finds oneself having to mix greases of different nature because the product already in use is not available or, indeed, the type of grease already in the system is not even known. It is therefore necessary to assess what interference might occur between the components of the products that come into contact. Mixing different greases can lead to ineffective lubrication, resulting in damage to the lubricated components. Performance additives are normally focused for all greases on AW (anti-wear) and EP (extreme pressure) performance levels, and therefore chemical interaction is generally negligible. Leaving aside incompatibilities related to the chemical nature of the base (e.g. silicone grease with hydrocarbon grease), the main aspect to consider remains the interaction between the soaps of the greases being examined.

How to analyse grease compatibility?

The compatibility standard can be assessed using the ASTM D6185 (American Society for Testing and Materials) method, which determines the characteristics of the greases in the mixture: two greases are defined as incompatible if the chemical-physical characteristics of the mixture differ from the intermediate characteristics of the two starting greases. The principle of the ASTM D6185 test is to mix the two greases in different proportions under controlled and identical conditions and determine, after a short resting time at room temperature, any changes in structural stability from the original stability of the individual new greases. The compatibility of greases is also affected by operating conditions such as temperature, shear stress, the condition of the previously used and replaced grease, and the volume ratio of the greases in the mixture.

For this reason, a compatibility chart of different greases formulated with different soaps has been compiled, which provides a guideline on possible permissible combinations.

Compatible means that there is a LOW risk of structural instability; Incompatible means there is a high probability that structural instability will occur; 

But what happens in practice if I mix incompatible lubricating greases?

In reality, since these are semi-solid products, it is very difficult in normal applications to have conditions in which the greases mix homogeneously as in laboratory tests, and typically the contact between the products is rather limited, therefore reducing the real negative effects. Furthermore, lubricated systems tend to be designed to remove the grease present precisely to facilitate lubricant replacement.

When choosing a grease, the main recommendation is to follow the manufacturer's instructions. If forced to combine greases containing soaps classed as incompatible, the first step should be a very generous initial grease application in order to remove as much residue of the previous grease as possible, and if possible reduce the time to the next relubrication.

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