Often, for one reason or another, it becomes necessary to change the brand of grease used to lubricate a particular machine. If the grease in use may become mixed with a new brand, the question of grease compatibility must be addressed to insure trouble-free changeover.
The NLGI Lubricating Grease Guide, Fourth Edition, 1996 defines grease Incompatibility as follows: “When greases made from different thickeners are mixed the mixture may be poorer in service performance or physical properties than either of the component products. This lessening in performance capability is called incompatibility. It may show up in any of several areas, such as (1) lower heat resistance; (2) change in consistency, usually softening; or (3) decrease in shear stability. Mixtures which show none of these changes are considered compatible.”
Incompatibility is not always caused by the thickener, since each of the greases in the mixture is a complete package – thickener, fluid and additives. Sometimes the thickener of one grease is incompatible with the fluid or the additives present in the second formulation. If the mixture proves to be significantly softer; less shear stable, or less heat resistant than the original grease, the mixture must be deemed incompatible.
“Incompatibility is best determined in service or in service-related tests; it is not predictable. Certain thickener combinations often have been found unsatisfactory and are generally so recognized. These would include lithium and sodium grease and organo-clay and most soap grease. Tests should be run on the specific grease of interest.”
Grease compatibility is a complex subject because of the many variables and changing conditions involved. At one end of the scale, mixing a fresh lubricant with a severely oxidized portion of the same lubricant may produce immediate or progressive changes in the mixture. At the other end of the scale, greases with different thickeners may be mixed resulting in hardening or very soft or low melting mixtures which may not provide adequate lubrication and may lead to early failures. Add to this the diverse operating conditions of time, temperature, and contaminants — and the uncertainties of predicting or measuring compatibility of greases are apparent. There is no practical rule one can apply to all mixtures of different greases to determine compatibility properties.
Grease compatibility is, nevertheless, of more than academic interest, especially when a current or potential customer questions a grease recommendation on the basis of compatibility. Incompatibility alone should not prevent a change in lubricating grease; product performance and overall economics determine lubricant selection. Once the proper lubricant type is thus selected, unwarranted and arbitrary changes should be discouraged.
Ideally, the best procedure to follow when changing grease brands is to completely remove all old grease before new grease is added. This is usually done, for example, when automotive wheel bearings are repacked with grease. In some pieces of equipment such as electric motors, the grease-lubricated bearings are not designed to be relubricated, as the grease lubrication life and the bearing life are about the same. In these cases, the bearing is replaced with a new pregreased bearing and the old one discarded.
In the majority of cases, however, grease lubrication is performed periodically by adding grease to the existing grease in a piece of equipment. During a change from one grease brand to another, when complete removal of the old grease is impractical, much of the old grease may be removed by purging with new grease. This can be done initially and/or progressively by temporarily increasing both the application intervals and the quantity of grease applied. Purging old, hard, contaminated grease with new grease is advisable even if grease brands are not changed to ensure that all parts of the equipment are actually receiving new grease. Sometimes, small openings in the grease-lubricated area become clogged with contaminated or old grease and normal relubrication is not sufficient to distribute fresh lubricant to all parts of the equipment. After thorough purging with the new grease, the equipment should be monitored for signs of possible incompatibility such as grease leakage, abnormally high operating temperatures (if equipment is not overpacked with grease), or noise.