In January 2021, I wrote a column on the shelf life of aviation piston engine oils. This brought up several comments from people wondering about the shelf life of aviation greases.
In covering this topic, I realized I had a number of tips for aircraft owners and pilots on how to make sure they are handling and using greases correctly.
First thing to know: One of the functions for grease is anti-rust protection.
Greases are based on roughly the same base oils as piston engine oils but with a thickening agent, so the main concern is a loss of the base stock or incomplete coverage of a bearing.
In a pail or tin, grease should stay usable for a significant amount of time as long as it is kept clean and has no dirt contamination. Some oil separation is OK, especially if the surface is uneven with deep grooves in it.
If you are using grease from a container every week or so, this is not a concern. But if you only use grease from the container once or twice a year, you should take a spatula and smooth the surface after each use.
Also, make sure when parts are stored with grease on them, that they are stored correctly.
A few years back we had a commercial operation that stocked extra wheel bearings for their aircraft. When they had a wheel bearing service, they would remove the bearings from the planes and install different ones from the store room. This reduced down time for the aircraft. The bearings were then cleaned and inspected and repacked.
The problem came when they then wrapped the bearings in a paper shop towel and put them back in the store room. Over time, the paper towel would wick the base oil out of the grease, leaving inadequate lubrication for the bearing when it was put back in use.
If you are going to store a packed bearing, you need to use the same oily paper that is used to wrap new bearings. Do not store them in plastic bags.
Another tip is to clean new bearings well and repack them. Many new bearings are coated with a preservative that is great for coating, but not a very good lubricant.
In the 1990s, one of the commercial airliner models was having trouble with corrosion in the hollow spindles. So, mechanics packed it with cosmoline, a brown wax-like petroleum-based corrosion inhibitors, to prevent rust. The problem was that after a hard landing the cosmoline would melt. Then with a short turnaround time, the airliner would take off with the cosmoline still in a liquid state.
When the landing gear was retracted and the spindal shaft was vertical in the aircraft during flight, the liquid cosmoline would than drain down and hydraulically replace the grease in the outer bearing. After a short time, the poor lubricating characteristics of the cosmoline failed to lubricate the bearing, resulting in failure.
The correct packing of wheel bearings is also important.
When packing a bearing, it is important to thoroughly clean the bearing with mineral spirits to remove all of the old grease or coating. Next carefully inspect the bearing wearing rubber or latex gloves to keep your fingerprints off the bearing surface.
Air dry and thoroughly wash the bearings with an alcohol solvent like Isopropyl Alcohol (IPA). This will remove any accidental fingerprints and mineral spirits left on the bearing and promote proper coating of the bearing surfaces. You can now repack the bearings, again always using gloves.
Last step: Rotate the wheels to ensure the grease is coating all the surfaces.
Another tip is to always exercise all of your flight control surfaces to their maximum degree. This will not only make sure they are working properly, but it will also mix the grease and recoat the different surfaces.
On many of the flight control systems, if they sit in the same spot, the vibration can work the grease out of the bearing contact areas. Movement to the limits can recoat these areas. The same is true for grease-lubricated constant speed props.
The last tip on grease has to do with washing your aircraft. Most pilots take pride in their planes and like to keep them clean. I have seen many of them use power washers to get to the hard-to-reach spots.
The problem is that if the high-pressure stream of water or water and detergent hits the back of the wheel bearing or even other bearings, it can easily force the spray past the seal and into the bearing area. This can increase rust activity and shorten the life of the bearing.