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"Is polyurea grease only for bearings or is it also suitable for gears? For example, can I use a multi-purpose EP NLGI #2 polyurea grease in a brushcutter's gearbox? If so, what would happen?"
Polyurea-thickened greases are most often discussed in terms of bearing applications because this encompasses the majority of their usage. Electric motors in particular tend to be exclusively lubricated with polyurea-thickened grease.
It is important to keep in mind that grease is made up of three main components: the base oil, the thickener and additives. Polyurea is the thickener portion of the grease and can affect some of the finished lubricant's functional properties, such as the dropping point, consistency and water-wash resistance. However, simply referring to a grease by its thickener type does not provide enough information as to its true performance characteristics.
The term "multi-purpose" is quite popular for greases on the market and usually relates to some common additives and the viscosity of the base oil. Many of these grease types have extreme-pressure (EP) additives and a base oil viscosity of 220 centistokes (cSt). The "NLGI #2" designation corresponds to the National Lubricating Grease Institute's consistency number for the grease. The scale ranges from #000, which is very thin or almost fluid-like, to #6, which is very hard or a solid block. While grease thickened with polyurea is not disqualified from working in a gearbox, some of these other parameters may prevent it from being the best option.
The majority of gearboxes tend to be highly loaded and as such require a lot from the lubricant to protect the machine's internal surfaces. In this case, you must make certain that the grease's viscosity is in line with what the gearbox needs to be properly lubricated, as you do not want the viscosity to be too low to build a lubricating film. The other issue to be aware of involves the extreme-pressure additives. These additives can be chemically aggressive and lead to chemical corrosion on machine surfaces. This is most prevalent in equipment parts with softer metals.
So, while polyurea greases are commonly used for bearing lubrication, there is nothing preventing them from lubricating gears. The biggest concern when selecting a lubricating grease is ensuring that all the other components of the grease are aligned with the machine's requirements. Once these are balanced, you can rest assured that you have selected the optimum lubricant for the application.