Gleanings from the Industry
A number of phenomena can limit the useful life of lubricants, but oxidation is the chief scourge, contributing as it does to viscosity increase, formation of sludge and varnish, and acidification.
Now a team of researchers from Imperial College London are proposing to halt those problems by inerting lubricants in air that is pure or nearly pure nitrogen. At the International Colloquium for Tribology in Ostfildern, Germany, in January, a team member of the Imperial group said such environments can be created using portable nitrogen concentrators, allowing lubricant to last longer in a range of applications.
“If my system is closed or semi-closed and we could replace the air in the system that contains oxygen with nitrogen … then we would not need to worry about oxidative degradation any more,” Janet Wong, a reader in the Tribology Group of Imperial’s Department of Mechanical Engineering, said during a keynote talk at the event, which was hosted by Technische Akademie Esslingen.
The effects of oxidation are so ingrained in the use of lubricants that it’s easy to view them as unavoidable — issues that cannot be permanently avoided, only delayed through the use of tools such as antioxidant chemical additives. Wong refuted that idea, saying that eliminating oxygen — or reducing its presence to 2% of the sump atmosphere — can extend lubricant life by an order of magnitude.
It would also impart a number of other benefits. Without oxygen there would be no rust formation nor acid-induced corrosion, Wong said. There would be little or no need for antioxidant additives. Nitrogen flow removes water from the system, yielding benefits such as halting hydrolysis of ester base stocks, which would enable broader use of biolubricants. Absence of oxygen would significantly raise the maximum operational temperature of the lubricant — above 200 degrees C, Wong said — opening the way for use in additional applications. It would also improve efficiency by reducing the amount of cooling needed — a significant advantage when power densities for many types of engines are increasing and sump sizes decreasing.
Inerting lubricants with nitrogen is not a new idea. As Wong noted, in the 1960s supersonic planes were about to significantly raise operating temperatures for engine lubricants — to above 250 degrees — and engineers were puzzling how the lubes could survive. The National Aeronautics and Space Administration researched the possibility of inerting the system by sealing the gearbox and injecting nitrogen.
Wong quoted a 1969 report stating that this allowed a lubricant with a base stock blend of synthetic paraffinic hydrocarbon and perfluoropolymeric fluid to perform satisfactorily for durations of 2 to 10 hours at bearing temperatures of 370 degrees.
However, the gearbox leaked nitrogen so fast that the aircraft would have had to carry more of the gas than was practical, so the idea was dropped.
“The system was not closed, and there was no portable source of nitrogen,” Wong said.
Today, however, such sources do exist. Wong noted that lightweight, portable nitrogen concentrators are now available from a number of sources and that they separate nearly pure streams of nitrogen from air. She displayed photos of a two models easily held in one hand.
“With this kind of component it is now practical to fill closed lubricated components with nitrogen on demand,” she said, “enough to sufficiently eliminate oxygen from contact with the lubricant.”
Designing in concentrators may not be practical in some applications, but the Imperial team believes it might be in some, including aerospace transmissions, electric vehicle transmissions, compressors, industrial gearboxes, hydraulic systems, wind turbines and high-speed grease-lubricated components.
Eager to promote inerting, the Imperial team researched two questions. First, do lubricants still degrade in low-oxygen environments, and if so, how? Second, can existing lubricants function — control friction, wear, scuffing and fatigue — when no or very little oxygen is present?
Concerning the first question, the researchers recognized that lubricants in normal settings can deteriorate both from auto-oxidation, which results from the lubricant contacting and mixing with air that includes oxygen, as well as from tribo-oxidation, where rubbing in the boundary regime breaks the mechano-chemical bond of lubricant molecules to form active free radicals. The team checked for both dynamics by testing base stocks and finished lubricants on a glassware oxidation test and on a high-frequency reciprocating rig. Both tests ran for six hours and were conducted in the presence of dry air, using oxygen-free nitrogen and with 2% oxygen.
Across the board the researchers found that for base oils and commercial lubricants, oxidation, acid formation and viscosity increase were negligible in oxygen-free atmospheres. In 2% oxygen, base oils and lubricants degraded a bit more but at rates 4%-20% slower than in dry air.
To investigate the functioning of lubricants in oxygen-free environments, the researchers ran tribotests that showed two organic friction modifiers provided lower coefficients of friction in oxygen-free nitrogen than in dry air. Molybdenum dithiocarbamate performed similarly in both atmospheres.
Antiwear additives containing both phosphorus and sulfur and some ashless phosphorus returned wear bench test results in nitrogen that were similar to their performance in dry air, but simple alkyl phosphate esters experienced higher wear in nitrogen.
Wong said more investigation is needed, but she concluded that it’s already clear that inerting lubricants enables significant benefits and that it is now very feasible to maintain such operating conditions.
“We believe we are on the threshold of a new era in lubrication technology, where lubricated machines have small, smart nitrogen concentrators to eliminate oxidation and liquid and grease lubricants last indefinitely in most applications.”
Tim Sullivan is Executive Editor of Lubes’n’Greases. Contact him at Tim@LubesnGreases.com