Grease compatibility articles and charts abound, making the mixing of greases appear to be a convenient method to make decisions. But compatibility charts are not all the same. Add to that the complex interaction of base oils, additives, and thickeners that can change during a machine’s performance, and you have room for a lot of error and potential harm to your equipment.
The compatibility of greases is indeed most often related to the type of thickener. When the base grease is the same, the name of the grease can be similar. However, the two will not be identical. Thickeners can react with each other even if they are made by the same grease manufacturer. So, what is really the best practice? Here is the answer you may not be ready to hear.
Quite simply, to remain consistent with what has been published by NLGI (formerly known as the National Lubricating Grease Institute), “It is best practice to never mix different lubricating greases. Greases should not be mixed because the thickeners, the lubricating fluids, and the additives in different greases may be incompatible. Some grease suppliers publish compatibility charts; those charts are at best a rough guide. If greases must be mixed, as during a product changeover in industrial equipment, compatibility testing, as described in ASTM D6185, should be carried out to determine to what degree the products may be compatible.”
This advice is not to be taken lightly. The mixing of greases can reduce the effectiveness of your machine and lead to problems ranging from running hot to completely breaking down.
If you Have No Other Choice
It can be difficult if not impossible to avoid mixing greases in a plant environment, but steps can be taken to minimize the likelihood and impact of mixing incompatible greases.
Training and Instruction
All personnel involved in applying grease to equipment should be trained and instructed on the correct grease to use on each piece of equipment and their location. All grease containers should be clearly labeled and coded to prevent the wrong product being used.
Involve Purchasing in Grease Specification
Grease mixing is common in new components and rebuilt equipment. Even when all personnel are trained on using the correct products, equipment can enter a plant with a different grease than what will be added while in service. New bearings, gears, motors, etc., often come with a grease that is not compatible with the product being used to relubricate. That is why involving your purchasing department is paramount.
Test Potential Mixtures
As mentioned earlier, when switching greases, for instance during relubrication or component replacement, it is recommended to conduct testing that simulates mixing and dynamic to ensure compatibility. For mixtures of unknown compatibility or that are already proven to be incompatible, then before applying a new grease, all traces of the previous grease should be cleaned from the equipment including housing, supply lines, bearings and gears. Compatibility testing, as described in ASTM D6185, should be carried out to determine to what degree the products may be compatible.
The Last Word
You should not mix greases. If you must, go through the steps needed for testing, as noted in ASTM D6185-11. “In the absence of testing, it is recommended to shorten the relubrication cycle by half for the next 3-4 cycles. This will help to ensure that the undesired grease is removed from the application and also reduce the risk of equipment failure,” says Curt Ellison, Global Account Manager, Chemtool Incorporated.