Editorial Feature

An Introduction to the Lubricants Used in the Aerospace Industry

Article updated on 13 May 2020.

Image Credits: superjoseph/Shutterstock.com

In comparison to other industries, the aerospace industry requires stricter demands in the use of lubricants. Selecting the most appropriate lubricant for space mechanisms requires a selection of the most lightweight and most functional lubricant. This is because additional costs are incurred for every pound in total weight added during aerospace operations. Moreover, utilizing heavier lubrication would only cause significant financial consequences on top of possible mechanical or technical concerns.

Research on lubricants used in the aerospace industry would show that solid lubricants were traditionally used for aerospace machinery. However, the advent of technology, globalization, and modernization has brought about significant innovations in the field. New formulations of lubricants with the same, or even higher, quality performance have been developed and are now being utilized widely in many aerospace applications. The success of these new products is evident in the global aerospace market that was reportedly valued at USD760 million in 2014. Global trends examining the industry are expecting a continuous rise in the market value of lubricants.

Considerations in Selecting Aerospace Lubricants

The aerospace industry is extremely selective in choosing lubricants. In order to select the most appropriate lubricant, companies would usually conduct field tests using a variety of lubricants that are available in the market. Lubricants are evaluated based on their viscosity grade, machine operating contexts, volatility, sump cleanliness control, and rate of long-term use. Apart from this, companies would also determine compositional differences among available lubricants in order to determine their individual implications in costing and other operational concerns.

Studies have also found a general selection criterion for aerospace lubricants, as presented below. On top of the previously presented evaluative aspects, the following are the major considerations in selecting the most appropriate lubricant or solution in operations:

  • Operating environment
  • Fluid-film lubrication and viscosity
  • Boundary lubrication performance
  • Compatibility
  • Fire resistance
  • Stability
  • Toxicity and biodegradability
  • Susceptibility to additives

Aerospace Lubricants that are Available in the Market

Presented in the following sections are an overview of the lubricants commonly-used in the aerospace industry.

Perfluoropolyether Lubricants (PFPEs)

The perfluoropolyether lubricant line saw its beginnings in the commercial market in 1965 when it was marketed as a lubricant with low thermal stability and low vapor pressures at low molecular weights. Composed mainly of oxygen, carbon, and fluorine, PFPEs are the least preferred lubrication solution in civil and military aerospace applications due to their complexity and relatively expensive costs. Apart from this, PFPEs are solid lubricants, implying possible weight issues during operations.

Most perfluoropolyether lubricants in the market utilize monomers acquired from crude oil, suggesting a labor-intensive process of developing even its raw materials. Companies that primarily produce PFPEs invest millions, or even billions, in capital in order to meet the necessary regulatory and operational standards required from perfluoropolyether lubricants to function appropriately in aerospace applications. Companies, however, are developing variations in PFPE components in order to make the product more marketable and less complex. For example, a prominent company that markets PFPEs utilize hydrofluoric acid from calcium fluoride to produce new monomers.

Generally, perfluoropolyether lubricants could be used in the following applications:

  • Rocket engines and support systems in ground operations
  • Engine oil and gearbox
  • Couplings (e.g. engine, oxygen system)
  • Bearings, ballscrews, and leadscrews
  • Aerospace instruments, gimbals, and gyroscopes

Research on PFPEs has shown that the material has a poor boundary that would be unable to solubilize newly-formulated additives. In addition, the lubricant was also found to be prone to autocatalytic degradation. These reasons further render PFPEs as an unpopular choice in addressing aerospace industry solutions.

Multiply Alkylated Cyclopentane (MACs) Lubricants

These lubricants have replaced traditional mineral oil-based lubricants due to their low volatile properties and superior candidate additives that are comparable with the performance of lead naphthenate additives and phosphate additives that are currently being utilized in aerospace applications. Moreover, a number of researches suggest that MACs yield better outcomes than PFPEs in room temperature conditions in vacuum environments. This suggests better application outcomes for MACs lubricants. General functions of multiply alkylated cyclopentane lubricants also encompass that of PFPE applications.

Current researches on MACs lubricants and other liquid-based lubricants are examining the use of more exotic products, such as ionic liquid lubricants, in furthering the capability of lubricants in addressing aerospace concerns.

Sources and Further Reading

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