To better understand the aviation industry’s key concerns and pain points related to grease, ExxonMobil asked MRO Americas attendees to submit their grease-related questions directly to the ExxonMobil team. Having collected more than 40 questions on grease, including color, formulation, compatibility, storage, and handling, ExxonMobil has offered to share the answers in a three-part article series.
In the final part of the grease series, Gary Dudley, PhD., Global Grease Product Technical Advisor for ExxonMobil Research & Engineering Lubricants Technology, tackles questions on evaluating grease color and odor and how this may or may not indicate the status of grease condition.
Q. What causes grease to produce an odor? Does grease odor impact performance?
While many base oils and additives used in greases have a defined odor when fresh, odors can also be the result of in-service degradation of the grease. However, ExxonMobil aviation greases formulated from 100 percent polyalphaolefin (PAO) base oil, such as Mobilgrease 33, will minimize these objectionable odors. Greases containing an ester base stock may be susceptible to hydrolysis and could generate an odor as they degrade in-service.
In general, grease odor does not impact grease performance.
Q. Can you alter grease to make it smell better?
No. Once tested and approved, grease formulations cannot be changed without undergoing a full recertification process.
Q. What could impact a change in the original grease color?
Many factors can cause a color change, but this alone should not be used as the only basis for judging the quality of the grease in use. It’s important to note that many greases in the industry are dyed a specific color and that exposure to light can cause a change in the dye color that has no impact on the grease performance.
Additionally, environmental ingress and contamination could also cause grease color to change. We recommend consulting your lubricant supplier when an unexpected color change occurs.
Q. What happens when you find black grease and now green grease during an inspection?
Two of the most common examples seen are:
• Insufficient grease purging: Grease purging can still leave some residual grease in the component. Instead, it’s generally recommended that the part be removed and cleaned to ensure full removal of the prior grease.
• Incomplete mixing: This could be the result of improper grease distribution throughout the component, leading to what appears to be two different colors as the grease ages or becomes contaminated.
The darker grease color changes in-service can be a result of operating temperatures well above a grease’s temperature rating, or a result of contamination (e.g., dust from carbon brake that causes wheel bearing grease to appear dark).
In some airframe applications, a black grease may signify hydrolysis of an ester-based grease, identified with some BMS3-33 and MIL-PRF-23827 approved greases.
Q. What causes a grease to change colors? Can discoloration affect the grease?
Changes in grease color can be the result of a number of factors, some of them more negative than others. For instance, the change can merely be caused by a change in the chemical dye used in the grease.
However, if the color change is caused by high temperatures, contamination or reactions with other substances, the color change may indicate a change in grease performance. Some of these effects include:
• Chemical change in the grease’s base oils or additives, such as hydrolysis, which may reduce the load-carrying capability or structural stability of the grease.
• Presence of particle contamination that can cause premature wear of bearings or affect grease quality.
• Reaction with unknown material resulting in a change in color.
Q. What does the color of grease mean? Why does aviation have red, green, blue, tan colored greases?
The color of fresh grease is simply a function of dyes added to the formulation, which is often required by aviation industry organizations such as SAE International Aerospace or by an OEM QPL-specific directive. Other greases may be free of dyes.
Different grease colors sometimes help to avoid accidentally mixing greases; however, greases with the same color are not always compatible and may not consist of the same technology. Learn more on aviation grease color here.