The compatibility of lubricating greases is essential in industrial maintenance and helps to prevent failures caused by incompatible mixtures
What happens when different lubricating greases are mixed?
In industrial maintenance, selecting and applying the appropriate lubricating grease is essential to ensure proper operation and equipment durability.
Greases may be substituted for various reasons: performance improvement, changes in operating conditions, or replacement of discontinued products.
However, what happens if the new grease mixes with the previous one? This article analyses the risks of mixing incompatible greases, how their compatibility can be checked, and what measures should be taken during a transition between products.
Negative effects of mixing incompatible greases
Mixing greases with different compositions can cause adverse effects that compromise both the lubricant’s performance and the equipment itself.
The main incompatibility factor is usually the thickener. When greases with incompatible thickeners are mixed, the following may occur:
- excessive softening or, in some cases, hardening outside the specified NLGI range;
- oil separation;
- leaks in seals and gaskets;
- increased operating temperature;
- greater component wear due to loss of lubricating properties.
These effects are not always immediate, which can make early detection difficult. Therefore, it is essential to anticipate and validate compatibility before making any change.
How to determine if two greases are compatible
There is no single universal source for grease compatibility. Compatibility tables published by manufacturers and technical organisations exist, but they do not always agree with each other. This is due to the variety of formulations available in the market, even within the same type of thickener.
For this reason, the best way to determine compatibility between two greases is through specific laboratory tests. The most recognised method is the ASTM D6185 test, which evaluates the binary mixture of two lubricating greases.
What does the ASTM D6185 method involve?
The objective is to simulate the mixture between an existing grease and a new one, and determine whether this combination will negatively affect the lubricant’s performance.
During the test, three mixtures are prepared in different proportions of old grease to new grease:
- 90:10
- 50:50
- 10:90
These proportions represent different situations that may occur, from initial relubrication to complete substitution with remaining residues.
Each mixture is subjected to tests for:
- Thermal stability: analyses how the grease behaves when exposed to elevated temperatures, identifying whether it loses consistency, separates oil, or degrades its structure.
- Shear stability: measures the grease’s resistance to losing consistency under mechanical stress, such as that occurring in bearings or gears.
- Storage stability: evaluates how the mixture’s properties vary after an extended rest period, simulating real storage conditions.
The results are compared with the values of the pure greases. If the parameters remain within acceptable limits compared to the original products, both greases are considered compatible.
What to do if greases are not compatible?
When a grease change involves incompatibility, it is essential to follow certain steps to minimise risks:
Removal of old grease
It is recommended to manually remove at least 90% of the existing grease before introducing the new product.
This significantly reduces the possible negative effects of an incompatible mixture. This proportion is also validated through the 90:10 test result of the ASTM method.
Gradual relubrication
Once the new grease is introduced, it is advisable to apply half the usual volume of grease, but twice as frequently during the first lubrication cycles.
This procedure facilitates the expulsion of residues from the previous grease and ensures a more controlled transition.
Prevention: key to lubricant changes
Changing lubricating grease without verifying its compatibility can cause costly failures and unplanned shutdowns. Therefore, it is recommended to:
- Consult the technical data sheets of both greases.
- Contact a specialised laboratory to perform ASTM D6185 tests.
- Implement cleaning and progressive relubrication protocols if compatibility is not guaranteed.
The importance of lubricant grease compatibility
As we have seen in this article, lubricant grease compatibility is essential for optimal lubrication. For more information on which lubricant is best suited to your application needs, contact our MD Poland experts.
