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A Review of Grease Compatibility

December 01, 2012
Nicolas Samman
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To examine this subject, we need to distinguish between Dilution and Compatibility/Incompatibility of greases. Dilution is a physical effect and as a result it is dependent only on concentration. Therefore, the properties of the grease with the larger amount in the mixture predominate. Compatibility/Incompatibility is a chemical effect. As a result, it is both concentration and temperature dependent.

A field example: A gear case located above an open gear system was leaking grease onto the open gears of a mining shovel. The operator experienced operational problems with adhesion of the open gear lubricant to the gear surfaces even though the grease pair were tested for incompatibility and were found to be compatible within the operating temperature range. The explanation was that the leaking grease diluted the open gear lubricant to the extent it changed its character and properties.

Types of Grease Incompatibilities

  1. Incompatibility of base oils or oil blends. For example, mineral oils and polyalkyleneglycols, perfluropolyethers, phenylpolyethers, or silicone oils. This is a fluid immiscibility problem.
  2. Incompatibility of thickener systems, which is the most commonly asked question.
  3. Formulation or additive incompatibility – this is dependent on the robustness of the thickener structure and can occur even when the thickener systems are the same, for example lithium/lithium.
  4. Incompatibility resulting from loss of additive synergy adversely affecting field service performance.

As a general rule, greases should not be mixed. The equipment or component should be cleaned and then re-greased. Grease pair compatibility/incompatibility is generally not predictable. The Grease Compatibility Charts found in company and professional associations publications are to be used only as a guide. It is best to determine the grease mixture compatibility by testing or in-service in the field application wherever possible. 

The now obsolete ASTM D3232 Trident Probe Test was used in the past to determine grease compatibility at various temperatures. The current method in force for testing grease compatibility is ASTM D6185. Many grease laboratories are also now using Rheometers to test grease pair compatibility.

Indicators of grease pair incompatibility include a decrease in the dropping point or heat resistance, softening in product consistency, oil separation, decrease in shear stability (after 10,000 or 100,000 strokes) and a general decrease in additive performance. This can lead to lubricant loss and cause catastrophic lubrication failure if the problem is not addressed.

Summary and Recommendations

  • Grease pair incompatibility is more pronounced at high temperatures, which is critical for high temperature service and long term service life;
  • Grease pair incompatibility is commonly manifested by premature softening, oil separation and accompanied by lowering of the dropping point;
  • For a given grease, the composition is well balanced for that system. Unplanned addition of chemicals occurring on mixing greases can upset that balance and degrade performance;
  • A reminder – grease pair incompatibility is not solely dependent on the thickener system as the compatibility charts may indicate.

As a result:

  • Grease incompatibility is not easy to predict;
  • Mixing greases especially for high temperature applications is not a good strategy or practice;
  • Before converting a major or sensitive system to a new grease, exhaustive compatibility testing is warranted to avoid equipment damage and costly down time;
  • Ideally, the system should be disassembled, cleaned and re-lubricated with the new grease;
  • If this is not possible, then frequent re-lubrication with the new grease is recommended especially if grease softening is observed.  This should be continued until the problem ceases.

References:

  1. ASTM Methods
  2. Manufacture and Application of Lubricating Greases, by Charles J. Boner, 1954
  3. A Brief History of Lubricating Greases, by Arthur T. Polishuk, 1998

Nicolas Samman works at Glide Oil Technology, Inc. You can find his contact information in the membership database.

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