Whenever I give talks I usually put in a few slides about grease. In discussions on this topic, questions mainly fall into three areas.
The first is why is it so critical to use only the grease that is approved for a certain application. A lot of people think grease is grease, so they should be able to use whatever is available.
An important point to remember is that grease is not really thick oil. It is base oil that has a thickening agent mixed in, much like my mother would mix corn starch into thicken the gravy.
The thickening agent gives the grease certain properties. For example, Aeroshell Grease 5 is a Mil-G-3545C qualified, clay-based grease that has a very high drop point. That means when you come in for a hot landing and the brakes heat up, it will stay in the wheel bearings and protect them. If you used a soap-type thickened grease with a lower drop point, all of your grease could melt and you could ruin a bearing — or worse.
Conversely, if a non-approved grease is used in the flight control system, it may not have the low temperature characteristics to allow proper operation of your aircraft at very low temperatures.
The primary reason to use only an approved product is to ensure that the grease will meet the performance requirements of your application. Another critical consideration is compatibility.
For example, if you mix Grease 5 with Aeroshell Grease 14 (a Mil-G-25537C qualified soap-based grease) you will end up with a soup-like mixture that will run out of almost any bearing.
In order to get a military qualification for a grease, a company must run compatibility tests with all other products approved to that specification. Where this can become critical is when you buy a different aircraft.
Always check the log books to see what grease was used in each application. If it does not say which grease was used, or if an unapproved product was used, you probably should wash out the bearing and repack with the correct grease. This will ensure that the lubricant in the bearing meets the requirements of your application, and should not cause a performance problem.
The second area of questions concerns the proper procedure for repacking bearings. Most people get the washing with mineral spirits or solvent and then inspecting the bearing for pits and other problems. But many forget the next step, which is to wash the bearings in Isopropyl Alcohol or the equivalent to remove the mineral spirits and your finger print residue from the bearings. After you air dry, this will give the bearing parts a clean surface for the grease to adhere to.
Also remember to always wear rubber gloves to protect your hands and keep your grimy paw prints off the bearing when repacking.
If you are going to store the bearing for an extended period, always use an approved paper that will not wick out the base oil during storage.
The third area of questions concerns washing your airplane. Most pilots take pride in their aircraft and like to keep it clean at all times. I have seen people wash their aircraft with a high pressure washer. They work well and can remove the oil and grime off the belly in very little time. The problem is if they accidently hit a bearing, the high pressure water can easily be forced past the grease seal. This will greatly increase corrosion in the bearing and, worst case scenario, could freeze up at altitude and cause a problem.
So remember, all greases are not created equal. Always follow the manufacturer’s recommendation and procedures.