To the average pilot ? and even many mechanics ? grease is grease. People feel that as long as they inject a slimy substance into each grease fitting at the proper time interval that all of their bearings should last forever.
To the average pilot ? and even many mechanics ? grease is grease. People feel that as long as they inject a slimy substance into each grease fitting at the proper time interval that all of their bearings should last forever.
It is important to know that grease is not super thick oil. Greases start off as base oils that are thickened by some means. Some greases use a clay or mineral type thickener to change the oil to a thick gooey mixture. Of course there are a lot of other additives in a grease to improve load carrying, reduce water run out, plus a whole host of other properties needed to meet the requirements for each application. There are also long lists of other type thickeners, including soaps, which are used to meet the differing requirements of the numerous applications for which greases are used.
Each thickener has a particular characteristic which makes it desirable for some applications. For example, clay-type thickeners are great for high temperature applications. Most of the wheel bearing greases use clay thickeners so that they stand up to the high temperatures that can occur during a hard brake application. If one of the soap-type greases were used in a wheel bearing, the high temperature would “”coke”” up the grease, resulting in a possible bearing failure. Conversely, a helicopter rotor does not require high temperature grease, but because of the chattering of the rotor, it does have some very unusual load carrying requirements. Therefore most helicopter rotors utilize soap-type grease.
The problem occurs when you try to mix the different greases. For example, if you do not have the proper grease for your propeller, you could ? incorrectly ? use the logic that a helicopter rotor is just a large propeller and substitute a helicopter grease for your prop. If you do this, you will end up with a soup-like mess that will not protect your prop, but will leak out everywhere.
The other major factor about grease is the base oil. The base oil varies by type, mineral vs. synthetic, and viscosity. In general aviation, most wheel bearing greases use a thick mineral oil, while most airframe greases are blended with much thinner mineral base oils. If a person used an airframe grease on his wheel bearings, it probably would not hold up to the high temperatures from severe braking. Conversely, if a person used a heavy wheel bearing grease on some of the control surface bearings, he could have a difficult time moving the stick, especially in cold temperatures. (The technical term for this is a stiff stick, but that does not sound politically correct).
For most commercial or military applications, synthetic-based grease is used. Some of these products will work at temperatures down to -100?F. These are great greases in their proper application, but if you try them in your wheel bearings, the grease would probably ruin the rubber seals and allow dirt ingestion and grease leakage.
Another problem is that synthetics are great lubricants, but poor solvents. If some of these greases are used in an area with high centrifugal loading, like in a prop, the additives and thickeners can centrifuge out, leaving you with a thin base oil that will leak out.
The bottom line is that not all greases are created equal. In all applications ? but especially in aviation ? you should only use the grease recommended for your specific application. So when you are greasing your aircraft, look up the mil spec for each lube point and only use a product that is qualified against that specification.
Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].
