Lubricant performance varies greatly by class. Understanding the strengths and weaknesses of each classification can help maximize equipment performance and lifespan.

Perfluoropolyethers (PFPEs) are synthetic lubricants used in demanding applications because of their combination of physical and chemical properties. PFPEs provide excellent thermal and chemical stability, low volatility and unmatched lubricity at extreme temperatures. PFPEs are long-lasting lubricants for extreme environments that require chemical inertness or extreme pressure capabilities.

Mineral Oil lubricants offer excellent hydrolytic stability and good seal material compatibility, lubricating ability and toxicity. However, they have poor fire resistance and only moderate thermal and oxidation stability.

Polyalphaolefins (PAOs) are synthetic lubricants that boast excellent hydrolytic stability and toxicity, along with above average oxidation stability, volatility, seal compatibility and lubricating ability. Yet, PAOs have limited capabilities across temperature extremes, with only moderate thermal stability and poor fire resistance.

Diesters are synthetic lubricants that provide good thermal, oxidation and hydrolytic stability, as well as above average lubricating ability, toxicity and volatility. However, they have only moderate fire resistance and poor seal material compatibility.

Polyol Esters are synthetic lubricants with performance capabilities similar to those of diesters, but with improved hydrolytic stability and slightly stronger thermal stability and overall lubricating ability.

Silicones have good chemical inertness, thermal stability and low volatility, making them a good choice for certain applications. However, their lubricating film-forming capability is low; consequently, silicone lubricants cannot support high loads. Silicones are also known to migrate easily, which can cause serious concerns in some applications.

Petroleum-based hydrocarbon lubricants typically break down quickly and fail at extreme temperatures, under pressure and following chemical exposure, requiring more frequent machine maintenance and reducing component lifespans. Conventional synthetic lubricants may provide only marginally better results in these extreme environments.

Perfluoropolyether lubricants provide the broadest performance capabilities. While these lubricants are more expensive initially, the added expenditure is more than recovered through extended component lifespan, reduced warranty claims and corresponding increase in consumer value.

Challenging Lube Limits

Broader performance capabilities sounds great, but what does that really mean, and how will that impact premature bearing failure? Krytox ran a series of performance tests to determine the relative capabilities of PFPE lubricants compared to the others. For example, In an ASTM D-3336 endurance test to evaluate grease life in ball bearings, PFPEs ran for more than 25,000 hours without failure, compared to less than 1,000 hours for most hydrocarbon lubricants.

Similarly, in an R0F+ Test [Rig with size 0 (small) for Fett (Swedish for Grease)], which measures the useful life of grease at various temperatures and speeds, PFPE greases lasted 21 to 64 times longer than competitive lubricants. The extended performance capabilities enable some components to operate for life without requiring relubrication – considered lubed for life – while others can run for extended periods using significantly less lubricant per application.

PFPE lubricants also outperform other lubricants in high- and low-temperature environments. Petroleum-based lubricants begin to fail as operating temperatures rise above 120 degrees C or fall below 0 degrees C, forcing relubrication and production halts. Conventional synthetic lubricants dont perform much better. PFPE lubricants are effective up to 400 degrees C and down to minus 75 degrees C.

In a high-temperature test, hydrocarbon and PFPE greases were placed in an oven at 232 degrees C for 40 hours. The hydrocarbon grease – specifically advertised for high-temperature applications – lost 40 percent of its weight (losing all effectiveness), and developed tar. The PFPE grease remained unchanged in weight and appearance, and maintained its lubricating ability.

One advantage of PFPE is stability across a wide range of operating conditions. PFPEs are chemically inert, meaning they do not react with chemicals or materials. They are water and oil repellent; solvent resistant; nonflammable; compatible with oxygen, reactive gases and most common elastomers, plastics and metals; and nontoxic. The lubricants are available in several medical and food grades, are environmentally friendly and do not contain hazardous VOCs, and the oils are recyclable.

Finally, PFPE greases provide high load-carrying capability and good lubrication under boundary and mixed-film conditions. PFPE lubricants reach the maximum load in the ASTM D-3233 Pin and Vee Block Test, while hydrocarbon lubricants show signs of extreme wear and often cause catastrophic early failure.

PFPE lubricants also outperform hydrocarbon lubricants in the ASTM D-2596 four-ball test. Petroleum greases typical have a load wear index (LWI) of approximately 50 and synthetic hydrocarbons 100. PFPEs are stable at LWIs more than twice those of hydrocarbons and match or exceed the LWIs of synthetic hydrocarbons.

Total Cost of Ownership

Lubricant selection has a direct impact on the total cost of ownership of any equipment. Even though lubricants make up less than 1 percent of a plants operating cost, they can directly influence more than one-half of plant maintenance costs. Combine that with the corresponding downtime and productivity loss, and lubricant selection can have a significant impact on the total cost of ownership.

Using PFPE-based grease can help cut these costs significantly. For example, a pulp and paper manufacturer used a PFPE grease in a pulp dryer that previously had experienced about 10 bearing failures per year. Replacing the grease in the dryer alone saved the mill an estimated U.S. $1.7 million in downtime each year. If the mill were to use PFPE lubricants in the plants 3,000 electric motors – which run without relubrication, resulting in regular breakdowns – it could potentially save an additional $6 million over six years.

PFPEs stable performance under harsh conditions can help generate significant savings over the equipment lifespan, and specifying the lubricant class upfront can drive significant end user value. According to a separate annual cost of equipment ownership comparison between a PFPE and conventional hydrocarbon lubricant, PFPEs cost 23.5 percent less ($1,772 vs. $2,250 per year).

Thomas Blunt is global technical service engineer for Krytox Performance Lubricants by Chemours, based in Wilmington, Delaware, United States.