Since the beginning of the Industrial Revolution, lubricating mechanisms was all about petroleum. Alternatives from castor beans to animal fats have found special applications, but even today most oils and greases on the market, including products with special additives, are hydrocarbon-based.
This is good enough for most terrestrial applications, but for advanced devices in demanding service such as aerospace applications, dinosaur juice just doesn’t cut it any more. Advanced non-hydrocarbon fluoropolymer compounds are up to the challenge.
Aerospace is the poster child for tough challenges in lubrication. It’s the perfect storm for lubrication engineers. Operating temperatures range from extreme cold to searing heat, often in the same mechanism over a short period of time. Many applications have little or no lubricant reserve capacity and can’t be easily re-lubricated in the field. Operating conditions often include lubricant contamination from dust, metal particulates and corrosive liquids and gases.
Aerospace platforms have unique requirements, such as exposure to reactive gases like gaseous and liquid oxygen and volatile and potentially corrosive solvents from RP-1 to acid oxidizers. And with the high per hour operating cost of both commercial and military aerospace platforms, durability and lubricant life are important cost considerations as well.
Hydrocarbon formulations with expensive additive packages have been used in these tough applications, but there are challenges with petroleum-based solutions. Hydrocarbons, including alkenes, can be subject to burning, creep, changes in viscosity, and reactivity with other chemicals. In spaceflight, where liquid oxygen is used as an oxidizer and in aerospace breathing oxygen systems, safety is a serious issue. Liquid oxygen is extremely reactive, and not many chemical compounds resist exothermic oxidization in its presence.
How can aerospace engineers deliver while avoiding the limitations of hydrocarbon-based greases and oils?
We spoke with Steve Johnston, Technical and Application Development Manager for Krytox Performance Lubricants at The Chemours Company, about the options for inert lubricants in the aerospace industry.
“When you use anything organic in contact with enriched air, pure gaseous or liquid oxygen, there is a high likelihood that that product will oxidize. This is an exothermic reaction, and it generates heat and things catch fire,” Johnston explained. “Because once it fails, it’s not the fire in the O-ring that causes the problem, it’s what happens when you release the rest of the liquid oxygen, or propellant, or pressurized substance. That’s the problem. It’s not so much the failure of the single component but rather the consequences of that failure.”
In rocketry, for example, moving parts are relatively few. Traditional rolling elements like shafts and gears are bearing supported, but major rotating elements like turbopumps are frequently lubricated by the media flowing through them. In these cases, the role of lubricants in space applications is generally as a sealant, keeping gases and fluids where they need to be.
Actuators and servo systems have similar lubricant needs to their aviation and satellite service counterparts: sustained operating life, generally cold temperatures. Conversely, in booster service, lubricants may have expected lifetime of a few minutes, but may have to endure high temperatures and very high rotational speeds in some assemblies. It’s no place for a lubricant to “shear down” and lose viscosity.
The biggest engineering challenge is oxygen service. Exothermic oxidation is a universal sign of failure in oxygen systems, but charred lube and seals are other ways units can fail. In the past, silicone-based lubricants and sealants have been certified to be used with oxygen systems, and units were designed to accommodate the performance limitations of silicones. Today, these dated formulas have been replaced by fluorinated lubricants, such as Krytox, which is a compound called PFPE (perfluoropolyether).
Without a hydrocarbon base, PFPE lubricants also solve other challenges inherent to aerospace applications. The compounds maintain lubricity at a broad range of temperatures, from -100 °F to 750 °F (-75°C to 400°C). They are also non-flammable, insoluble in water and common solvents, resistant to shock and vibration, and some formulas can be functional under pressures ranging from total vacuum to extreme. PFPE is pretty amazing stuff.
Chemours is of course most famous for Teflon, their brand of PTFE, another synthetic fluoropolymer similar to Krytox PFPE. In fact, Krytox greases often contain PTFE as part of the thickening system of the product. This combination of PFPE and PTFE is responsible for the unique properties of the fluorinated lubricant.
What about the performance of fluorinated lubricants in airplane and jet applications, below 50,000 feet? The applications for specialized lubricants in aircraft are much broader than those for spacecraft. Many complex assemblies are buried deep within the airframe, engine and avionics, and are expensive to service. Regular lubrication of sliding or rolling elements in industry is usually a simple as topping off a gearbox or pumping a Zerk fitting but in aviation, access to components is limited, and design of assemblies to allow access for lubrication adds cost and complexity.
Take fuel systems, for example. Internally sealed wing structures are an extremely lightweight method of carrying fuel, and transfer pumps are normally designed inside that same wing structure. As an immersion pump, access would be nearly impossible for routine service. Even if an access panel could be provided, the need for extremely smooth laminar flow wing services adds more cost and complexity to the access problem. What if internal moving parts could be lubricated for life, or at least for the long services between major inspections? The cost and service uptime advantages are obvious.
Steve Johnston gave an example: “If you think about a modern aircraft system and the general shift away from hydraulics into electronic flight controls, all the control systems are buried deep inside the aircraft. You do not want to be trying to service every three, four years, because they are highly inaccessible. So, there are PFPE applications designed to be sealed for life. These parts and systems are installed into the aircraft permanently and therefore cannot be re-lubricated.”
In the end, performance, reliability and cost are essential attributes, but aerospace safety is paramount. Use of the wrong NLGI grade in a bulldozer’s bronze bushing may cause premature wear, but in aviation, certification is the norm. Unauthorized product substitution is out of the question. PFPE lubricants are one of the critical technologies which allow our aerospace craft to develop and drive higher performance, and products like Krytox are certified and specified across the industry.
In some specifications, United States defense standard (A.K.A “MIL-SPEC”) PFPE greases are called for, but many specifications call for a specific brand due to the variety of specific variants which are available.
In general, PFPE lubricants would not be used on external moving parts such as flap tracks. “It would be complete overkill,” said Johnston. Advanced lubricants are generally only used in very small amounts. After all, good engineers build things that work, but great engineers build things that work efficiently.
For more information about advanced fluorinated lubricants and their interesting applications, visit Chemours here.
This article is sponsored by The Chemours Company. Opinions are mine-- Isaac Maw