Is Your New Grease Compatible With Your Old Grease?

Did you even know there is a compatibility concern when mixing different greases? Even greases from the same manufacturer? Well, there could if you are not careful. WikiPedia defines grease at THIS URL as “Grease is a solid or semisolid lubricant formed as a dispersion of thickening agents in a liquid lubricant. Grease generally consists of a soap emulsified with mineral or vegetable oil“. Using a differently formulated grease than what is already in the joint can cause unintended–and in some cases unwanted–interactions between the different thickening agents in the mixed grease resulting in a grease mixture with unknown characteristics.

One of the more widely used thickeners is a Lithium complex soap, I found this by downloading the descriptions of the Shell Rotella Greases, the MP, SD, ET, and HD. Even within the 4 listed Shell Rotella greases, there are 2 different thickeners. The HD, SD, and ET greases all use a Lithium Comples. The MP grease uses a Lithium hydroxystearate soap. Fortunately, ins a grease compatibility chart I found, all lithium based thickeners are listed as compatible with each other, but this exercise does server to make my point about knowing what the joint had been previously greased with, and knowing what your new grease uses as a thickener. If you have unknowns, my sources I researched–multiple sources made this same recommendation–recommended to clean the joint, and purge to old grease to avoid the possibility of unintended interactions between incompatible grease thickeners.

When purging a grease joint, all the grease must be washed out of the joint. This can be accomplished using a grease joint rejuvinator and penetrating oil. The rejuvinator forces the penetrating oil into the joint at high pressure and it will clean the grease out. Once you believe you have all the grease out, you must now fill the joint with your new grease, and keep notes on the make and model of the grease you are using so you can avoid this action in any future changes of grease manufacturer. Ideally, the base oil in your new grease should match the base oil in your old grease, In my Shell Rotella example, the base oil is mineral oil on all the listed greases. There are some greases made with vegetable oil. All these factors go into determining grease compatibility. 

UPDATE!  Researching the differences between Polyurea(conventional) and Polyurea(Shear Stable) and their compatibilities with Lithium greases, we came upon the following useful paper that may help explain the ongoing confusion (Click For Paper).  One of the consistent findings we came across is the Polyurea greases are very prevalent in the electric motor industry.  From our research we are also finding that industrial equipment manufacturers are turning increasingly to a Polyurea based grease for longer grease life and extreme pressure applications.  As part of our research we were unable to find any definitive reference that clearly defines the differences between a conventional Polyurea and a shear stable Polyurea.  Indeed what we turned up is that there is a range of Polyurea thickeners that are used in greases, and in the above referenced paper, 2 different types of Polyurea greases were intentionally mixed and tested with the final analysis being that the mix that was tested resulted in unacceptable lowering of critical performance characteristics. 

Our suggestion to KNOW what type of grease is in ANY joint you are planning to lubricate still stands as as your BEST protection against accidentally mixing grease types that ultimately prove to be incompatible. Our second recommendation related to mixing greases is that unless there is some compelling reason, and you are willing to do the appropriate testing to find the performance differences such as what are found in the referenced paper, you should not mix grease types.

When changing grease types(thickeners) and the chart below shows not compatible, you must also clean you grease gun.  Leftover grease will contaminate and possibly interact with the new grease resulting in a compound that has unpredictable characteristics!


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