Never Mix Greases

Maintenance Tip article by Paul Murray

In our manuals, as well as many others out there, the section about lubrication states:  “mixing of greases should be kept to a minimum.” After multiple discussions with Shell, Mobil, Texaco, and other large grease manufacturers, the consensus became clear: there should be no mixing of greases - ever.  

To create grease, oil is blended with a base thickener to give it some consistency.  Additives are often blended in as well to improve characteristics, such as preventing rust or improving wear resistance. The compatibility of greases is most often related to the thickener type. When the base grease is the same, the name of the grease can be similar, but the two will not be identical; the thickeners can react with each other even if they are made by the same grease manufacturer.  

David Turner, of Shell, stated it well in the January 2009 issue of Machinery Lubrication: 
In bygone days, when simple soaps and clay were the primary thickener types, compatibility was relatively straightforward. Lithium and calcium soaps were compatible with one another, and neither was particularly good when mixed with a clay-based grease. Today, with not only the aforementioned thickeners – but also complex soaps, polyurea, calcium sulfonate and even more exotic thickeners used in many greases – the issue of compatibility has become much more complicated…To add to the confusion, there are some grease specifications that are based solely on grease performance without regard to grease composition. If greases of different thickener types (both of which meet the performance requirements of the specification) get mixed in service, dire consequences can result.

When two incompatible greases are mixed, one of two things generally happens: the mixture hardens and will not release any of the oil or the mixture softens and releases all of the oil.  In either case, the end result is basically the same; there is effectively no lubrication.

What if you need to start using a new type of grease? How do you avoid the compatibility problem while switching over? Purge as much of the previous grease as possible through the grease-dispensing system and application.

Do not take this step lightly. The mixing of greases can reduce your machine’s effectiveness and lead to problems ranging from running hot to completely breaking down.