Understanding Aviation Grease: A Comprehensive Guide

Aviation grease is a crucial component in aircraft maintenance, providing essential lubrication and protection for various aircraft components. Below, we address common questions about aviation grease to help you better understand its importance and usage.

Q. What is a grease?

A. Oils that have been thickened to provide protection, seal, cushion and extend service life for components. Greases are typically referred to based on the thickening agent used. (E.g. Calcium, Clay, Polyurea, Lithium, etc…)

Q. Why does my grease have oil separation?

A. Grease is oil that has been mixed into a soap in order to hold the lubricating oil in place. Some degree of oil separation from the soap is necessary for the oil to provide lubrication to the intended application. Oil separation varies with storage, time and temperature. Some separation is normal. As long as the oil is not leaking from any seals in the airplane, the grease will perform its intended task.

Q. Does free oil on the top of grease indicate the grease is not good?

A. No, some degree of oil separation is tolerable and needed for the oil to provide lubrication to the intended application. If you find small traces of oil on the surface, you should be able to mix the oil appearing on the surface back into the grease. If however, you find free oil covering the total surface, we suggest contacting your A S Harrison & Co representative for guidance.

Q. Why do we see more oil separation with Mobilgrease™ 33 than with Mobilgrease™ 28?

A. Oil does separate from grease -both in use and in storage. And the lower the viscosity of the base oil used in the grease the more separation you are likely to see. For example, the base oil viscosity of Mobilgrease 33 is lower than the base oil viscosity of Mobilgrease 28 and the rate of oil separation is correspondingly higher.

Q. What is considered normal oil separation?

A. If oil is collecting around the dome or in the crevices at the top of a pail of grease, this is considered normal, and the oil can be remixed into the grease. If, however, the surface of the
pail is completely covered with a layer of separated oil, then operators should contact your A S Harrison & Co representative for guidance.

Q. What do we do when we see normal oil separation?

A. If the separation appears normal, the oil can be mixed back into the grease. Refer to the aviation shelf life bulletin which addresses the process for remixing the oil and soap in a container in which you see separation. Basically, it says to remix the separated oil into the top 2.5 cm of grease.

Q. Is there anything we can do to minimise oil separation?

A. You can help minimise oil separation by storing in a climate control environment and releasing pressure on the pumping devices used to apply the grease when not in use.

Q. What do we do when we think oil separation is abnormal?

A. Please contact your A S Harrison & Co representative for guidance.

Q. What is oil bleed and how does it impact the grease?

A. Oil bleed is a term used to explain the separation of oil or seepage of oil during normal grease operating conditions. Oil bleed is easily identified by the presence of oily sections of greased components and/or the formation of small pools of oil around componentry. Excessive oil bleed will cause the grease to harden and render it unsuitable for use.

Q. What is the maximum use temperature for a grease?

A. Grease high-temperature claims are based on different standards that can vary widely. ExxonMobil has chosen to base continuous-operation recommendations upon the use bearing tests, at the same time recognising that operation at temperatures exceeding this recommendation can be tolerated for short periods with appropriate adjustments. It is recommended you review the ExxonMobil PDS for the specific product being considered.

Q. What is the best way to pack a bearing with grease?

A. We recommend using a bearing packer or to pack the grease by hand and to make sure to only apply the grease from one direction to avoid trapping air.

Q. How do you know a grease needs to
be replaced?

A. Look for noticeable changes in color, odor or consistency. Grease should be replaced if/when it becomes contaminated.

Q. How do the properties of thickening
agents vary?

A. Clay-based greases (bentonite) are sometimes used in high temperature greases. Market by some suppliers as Microgels. Lithium thickeners are also commonly used and have high melting points (“drop out”) and adequate water resistance.

Q. What is flash point and why is it important to grease?

A. Flash point is the temperature at which a grease gives off sufficient vapor to ignite in air. The flash point of a grease is often governed by the flash point of the oil used in the grease.

Q. How much grease should I use?

A. For specific advice, we recommend consulting the aircraft maintenance manual
(AMM) or aircraft service bulletin (ASB) for
recommendations on what types of grease to
use and the appropriate method of application.

Q. What is the role of base oils in greases?

A. The base oil in a grease is used to provide
adequate lubrication to reduce friction and to
prevent harmful wear of components.

Q. How do the base oil properties vary and
what is the impact on the grease?

A. Base oils can consist of a wide range of fluids from low viscosity mineral oils, to heavy cylinder stocks, to specialise synthetic lubricants. The inherent features seen the in the base oil are generally also seen in the grease such as good high temperature performance or low temperature performance. Base oil selection is generally made based on the desired performance features of the grease.

Q. What is the difference between mineral based greases and synthetic based greases?

A. Greases made with mineral oils generally provide what is seen as satisfactory performance in most applications. Synthetic greases are typically used in those applications where temperature may very over a wide range, greases made with synthetic base oils are generally used to expand operating temperature ranges and increase grease life.

Q. What makes greases compatible?

A. A grease may be deemed compatible if they
show no impact consistency or performance features. Typically the industry used ASTM D 6185 to assess grease compatibility. A compatible rating only indicates that there may be a low risk of incompatibility issues. Any mixing of greases should be closely monitored.

Q. Why should you not mix greases?

A. Mixing of greases have the potential to cause unexpected interactions between the two greases such as change in consistency, shear stability, oil separation, and/or oxidation stability. It is always best to clean and fully re-grease the component or thoroughly purge out the old grease with the new grease.

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