In a perfect world, a column on aviation grease would be very short and direct.
I would tell people to consult their airframe manual and only use the mil spec grease called out in the manual, and at the given amount and intervals for each lubrication point.
But, alas, the real world raises its ugly head and problems can — and do —arise.
Most of the problems stem from a belief that all grease is created equal and as long as a bearing has some sort of grease in it, it will be fine.
The most common problem is that pilots and FBOs do not want to have separate inventory and equipment for every application.
When we go back to the basics of grease, we learn that grease is not thick oil, but rather oil base stocks that are thickened.
This is much like making gravy at Thanksgiving. You combine the turkey drippings with, say, chicken stock, which forms a liquid mixture, to which you add flour or corn starch. With heat this changes the liquid to a thick sauce.
In the same way, greases start as oil base stocks and are then thickened by a specific thickening agent.
The most common greases used on general aviation aircraft are Aeroshell Grease 5 and 6. They are similar, but have very different base oils.
The base oil for Grease 5 is similar to that in Aeroshell 120 piston engine oil, compared to Grease 6, which has base oils more like Aeroshell W65.
In an aircraft, Grease 5 is generally used in wheel bearings, while Grease 6 is used for most airframe application points.
One Size Does Not Fit All
If you want to use just one grease for all applications, this will cause problems.
For instance, if you use Grease 6 in wheel bearings, you will not have adequate protection, especially if you make a couple of panic stops and the brakes and wheel bearings get really hot.
Conversely, if you use Grease 5 for everything, your control surfaces will be very stiff and unsafe, especially in cold weather or at altitude.
If you should try Aeroshell Grease 7, it has a pure synthetic base oil and will eat your seals out quickly.
The other significant part of a grease composition is the thickening agent. Some agents are compatible with other chemistry types, but many are not.
For example, say you have a helicopter that requires Aeroshell Grease 14, which has a soap-based thickener. If you use the same greasing equipment with Grease 14 and then switch over and use Grease 5 or 6 on other applications points, the small amount of contamination between the two greases will produce a soupy mess that will probably run out and not properly lubricate that application.
It is best to have dedicated application equipment for each product.
Tips to Get the Most Out of Grease
One of the problem areas with grease becomes evident with the purchase of a used aircraft. The maintenance log for the aircraft should have a record of which grease was used, where, and when.
If it does not, you may want to do some extra work. Look up in your airframe manual which grease to use, when it should be lubed, and how much should be applied.
On plain bearings, it is usually OK to purge the application until good clean product comes out.
Always wipe the excess grease so that it does not collect dirt.
If your manual states do not over grease or limits the amount to be applied, do not try to purge these applications because it can blow out the seals and allow dirt and moisture into the bearing.
On things like wheel bearings, you will need to remove the bearing and re-pack them. When re-packing bearings always use rubber or latex gloves for your safety and to keep your greasy paw prints off the clean bearings.
I usually wash the bearings in mineral spirits and then carefully inspect the bearings and races for pits and scratches. If they are OK, you need to add an additional step, which is to wash the bearing in alcohol like IPA to remove the oil film so that the grease will adhere better to the metal surfaces. Now pack the bearing and install.
Caution: If you are not going to install the bearings right away and want to keep them in inventory, do not wrap them in a paper towel or shop towel to store. If you are going to store them, you will need oil paper like what new bearings are wrapped in, then place them in a paper or cardboard box, not a plastic bag. A plastic bag will collect moisture and the paper towel will wick out the base oil from the bearing.
Finally, there is cleaning your aircraft. Most owners take pride in their aircraft and want to keep it clean. Many of them use a power washer to clean difficult spots, like around the brakes.
The problem here is that if you hit the back of the wheel bearing, it is very easy to force water and detergent past the seal, which will greatly increase corrosion in the bearing and reduce its service life.
Ben.
Thank you for the article. I have a rectified crankshaft, it is in the cardboard box that the rectification workshop sends it to you. Wish is the best grease to protect the crankshaft?
Thank you
I have been waiting years for an article regarding grease. Thank you Ben! Through the years I have overheard hangar arguments ( many times very heated) as to the use of grease in aircraft wheel bearings. There are those who proclaim that automotive bearing grease is perfectly ok for aircraft wheel bearings as the bearings themselves are the same bearings used in cars (manufactured by Timken). Also since the car bearings are in constant use and in an environment of very cold to very hot temperatures, aircraft bearings are used in taxiing, takeoffs and landings—a fraction of the use compared to automotive bearings. As a result, many believe that automotive wheel bearing grease is not only as effective as aircraft bearing grease but also comes at a more economical price. They ignore that automotive grease is technically illegal in certified aircraft and conversely proclaim that homebuilt aircraft with the same bearings can legally use automotive grease and have shown no problem in doing so for years. So how does one respond to the believers in using automotive grease in aircraft wheel bearings? This is interesting and controversial to say the least!
Thank you for bringing this to light.
When I purchased my plane in Florida I flew it to California with a trim wheel that needed a lot of force. The local shop cleaned the old grease from all grease points and regreased properly. it has not been problem since. I wasn’t aware that grease types could be so important, but now I now.
Better yet……stick to the AMM’s (Aircraft Maintenance Manual) specifications on grease use for wheel bearings etc.
Indeed,as Mr. Visser stated in the second sentence,the manual will state what to use.
Many antique/classic aircraft do not have a “maintenance” manual per se,the maintenance instructions are a few paragraphs in the parts manual,but,they always include a lubrication chart.
A few decades ago now I had four separate grease guns hanging on the tool box, plus the small cans the drawer. Now retired from full time aircraft work,two types of grease are sufficient for the aircraft I still maintain.
Ben,
Thank you for this article. Please write more. As someone who enjoys doing the maintenance on their lawn tractor and car, but has little mechanical aptitude and takes five time as long as necessary to change oil ect I am fascinated by the differences in items as mud and but critical as grease. The knowledge needed to work on aircraft is amazing and as a pilot and co owner knowing what is going on is as important as being current.
*Items as small and critical as grease.
Apologies
Thank you for this. It’s good to learn about the differences, why they are there, and about the possible repercussions if we choose the wrong item for the job. Upsides and downsides. Always good to know.