"Our organization is looking to make a change in the grease we use, but we are concerned with the compatibility of the new grease with the current grease in use. Considering all the different grease formulas on the market, is there a field test for compatibility?"
Currently, there is not a field test for grease compatibility. However, if you were to make any assumptions about the compatibility of two greases, you should assume that they are not compatible. Not only is there the concern about the compatibility of the base oils, but there is also the extra dimension of the compatibility of the grease thickeners. For instance, with polyurea-based greases, many times these greases are not compatible with other greases, including other types of polyurea grease.
Too often people presume that oil is oil and grease is grease. This leads to an oversight of the compatibility issues. Of course, all oils or greases are not equal. You would not want to use a passenger car motor oil in a turbine any more than you would use a turbine oil in your car or truck. These oils are formulated for specific applications. It is the same with grease. While there are hundreds of options to choose from, each type is designed to work better in a specific application. Keep in mind that the various types of grease thickeners have different strengths and weaknesses. There are also different base oils and grease consistency grades to consider.
One point of concern is the use of general-purpose grease in couplings. Typically, general-purpose grease is not formulated to withstand the centrifugal force exerted by the rotation of the coupling or the motion of the grid inside the covers. This can cause grease to be flung out of the coupling. It is easy to determine if this has occurred by simply looking inside the coupling guard. If there is grease inside the cover, then you have the wrong grease in the coupling.
Mixing greases usually results in one of two conditions: the thickener either solidifies to a consistency similar to gypsum in drywall or thins to the point where it will run out of the housing. In either case, the bearings are not receiving the lubrication necessary, and a failure is eminent.
When changing greases, the best practice is to completely clean out the bearings and housings prior to applying the new grease.