You can avoid costly repairs by ensuring you aren’t combining incompatible greases on your electric motors.
Keep in mind that grease works differently from oil, even though both act as lubricants. Because of its thickness, grease can stay on surfaces despite gravity and centrifugal force. Another critical aspect of grease, as opposed to oil, is its ability to remain between surfaces even under tremendous pressure. However, not all types of grease are compatible with each other.
What is Grease Made of?
Let’s start off with a quick review of grease for electric motors. For electric motors, and bearings in particular, grease has specific goals: reduce friction, reduce wear, protect against corrosion, and act as a seal to prevent contaminants from making their way into critical rotating components.
To accomplish these goals, grease is made of three key ingredients: base oil, thickener, and additives.
The base oil is the most significant component in grease, making up between 70% and 90%. In addition, the base oil is responsible for the actual lubrication. There are two types of base oil: mineral and synthetic. Mineral oil-based greases serve well in most industrial applications (including electric motors), while synthetic base oils are preferred when extreme temperatures are involved.
The base oil is blended with a thickener. The thickener is primarily responsible for the semisolid texture of grease and makes up between 3% and 30% of its content. There are three types of thickener:
- Simple metal soaps, which are the most common and typically use lithium, clay, aluminum, sodium, calcium, or polyurea
- Complex metal soaps, which have excellent load carrying abilities and are usually lithium-based
- Nonsoap thickeners, which have exceptional load-carrying capabilities in high-temperature environments and are commonly silica aerogel or bentonite based
The grease used on electric motor bearings is often a simple metal soap made of polyurea. Still, other thickeners are sometimes utilized, such as soaps that are lithium or aluminum-based.
According to an article about Grease Basics in Machinery Lubrication magazine, here’s why additives are needed:
- Enhancing already existing desirable properties
- Suppressing already existing undesirable properties
- Imparting new properties
And there are four categories of common additives found in grease:
- Extreme pressure
- Boundary lubricants (for reducing friction and wear without any adverse chemical reactions taking place with the metal surfaces)
- Rust and oxidation inhibitors
For use on electric motor bearings, essential additive packages include antiwear and oxidation inhibitors.
While the base oil may be the same, the thickener and additives may not — and the major issues with mixing different greases lies in the thickeners they use. And keep in mind that two types of grease may have the same color and price or the same base oil and the same performance indicators, the thickeners and additives may still be different.
What Happens When You Use Incompatible Grease?
Mixing incompatible greases for your electric motor bearings can be just as bad as using no grease at all, which is why you need to be careful. In addition, combining incompatible greases can have unpredictable results. For example, if you mix two greases that have incompatible thickeners …
- The grease may harden and no longer release the oil
- Oil dropout may occur, where the grease softens, and all of the oil is released
Mixing incompatible greases mean there is no more lubrication. As a result, moving parts may experience accelerated wear, be left open to corrosion and rust, end up completely unprotected–and catastrophic motor failure can result.
While the main problem lies in the thickener used, there are a few other considerations when two greases are mixed. The base oil should be the same: if the two greases don’t have the same viscosity, then the final result won’t be optimal for your electric motor, and its performance will be compromised.
Another problem area can be the additives: grease additives are crucial to their performance, even though they represent a small fraction of what grease contains. If the additives in two different types of grease are incompatible, they may …
- Reduce the effectiveness of the additive packages
- Cause corrosive conditions to develop in the grease
- Create problematic deposits
How to Tell If Two Greases Are Compatible
For two greases to be compatible, experts look at three characteristics: consistency, mechanical stability, and dropping point. Here’s how that looks:
- The change in viscosity of the mixture following elevated temperature storage must still be within the ranges of the change in consistency for the individual greases (also following high-temperature storage) being mixed
- The mechanical stability of the mixture has to still be within the range of consistency for the individual greases being mixed
- The dropping point of the mixture cannot be significantly lower than that of the individual greases being mixed
There are also very scientific ways to establish grease compatibility, which include testing according to ASTM D6185-11(2017) Standard Practice for Evaluating Compatibility of Binary Mixtures of Lubricating Greases. An alternative approach can be found in EPRI (Electric Power Research Institute) Nuclear Maintenance Applications Center: Effective Grease Practices.
If in Doubt, Change it Out
If you aren’t sure if two greases are compatible and you don’t have enough of the original, then your best bet will be to start with some new grease. This means thoroughly purging the existing grease and, if possible, thoroughly cleaning the parts involved before adding the new grease to your electric motor bearings.
At Hi-Speed Industrial Service, our repair specialists can help you find the right grease for your electric motor bearings or help you resolve any issues you’ve had with mixing incompatible greases. And you can count on our predictive maintenance team to use suitable grease for your motors, never mixing incompatible ones. In fact, we can help you ensure that your bearings have not just the best type of grease but optimum levels as well.