Designers working with moving part assemblies have it tough. It’s not enough for the mechanism to work reliably, it has to do so at low initial cost and be maintainable over a life expectancy that might be measured in years, or even decades.
If the designers are lucky, the moving parts they are designing, such as pumps and valves, are lubricated by the media passing through them. For most situations however, sliding and rotating elements need antifriction bearings or bushings to keep friction at bay, usually with some form of lubricant. Low-cost hydrocarbon-based oils and greases are a common off-the-shelf solution along with lithium and silicone products for some applications, particularly where resin part or seal compatibility is an issue.
Most designers default to handbook solutions for lubrication of mechanisms in average service conditions. But what about the non-standard cases? Extreme heat, extreme cold and extreme temperature fluctuations. Corrosion. Reactivity. Long service life. Each of these conditions requires a tailored lubricant for best performance, and each carries an associated cost for that performance.
Service life is a basic design consideration and regular maintenance is scheduled to achieve goals that may be expressed in total hours of runtime, mean-time-between-failures, cycles to end-of-life or simply a manufacturer’s warranty period. Typical strategies involve design for periodic relubrication, but others require a lifetime solution. Inaccessible parts deep within a machine, sealed gearboxes, or remote applications like spacecraft simply can’t be serviced on site, so maintenance-free lubrication is a must.
One interesting option for high-performance lubrication is perfluoropolyether (PFPE). These compounds were originally developed for the space industry, where extremes of chemical exposure and reactivity, outgassing under vacuum and very large temperature fluctuations are combined with high levels of vibration, radiation and isolation. And in space applications, relubrication may not be an option. PFPE compounds were developed to address these challenges and offer the same capability in down-to-Earth applications that can be as demanding as space flight.
Let’s examine a few common applications where PFPE lubricants have solved design challenges.
High-Performance Lubrication Challenges in Aerospace
The aerospace industry has a strong need for maintenance-free lubrication. There are many components which will never be accessible for lubrication after final assembly—especially those going to earth orbit and beyond. In addition, aerospace lubricants have an extreme set of performance requirements.
PFPEs were used on Saturn V rockets. Image source: Wikipedia
In these applications, most lubricants based on hydrocarbons are not sufficient. One alternative is silicone grease which is used in some aerospace applications.
Silicone industrial lubricants have better viscosity control over conventional hydrocarbon lubricants at low temperatures, and good thermal stability at high temperatures. This gives them a wider effective temperature range than petroleum-based lubricants. Some silicone-based lubricant compounds may be filled with polytetrafluoroethylene (PTFE), well known in popular culture as Teflon brand. This is done to confer some of the lubricating properties of PTFE, such as wide temperature range and high moisture resistance, to the product.
PTFE was developed into a solid bearing and bushing material that’s very effective in many dry applications, but most industrial/commercial mechanisms, including aerospace, need lubrication. The first PFPE-based lubricant was invented in the 1965 by DuPont under the brand name Krytox. PFPE takes the PTFE molecule and adds an oxygen backbone. The addition of the oxygen takes the white powder of PTFE and makes it a clear oil. Krytox became known as Liquid Teflon early in its development. It has all the chemical and thermal properties of PTFE but in a more usable lubricant form. The technology behind Krytox has since been used to develop a portfolio of lubricants designed for a wide array of demanding applications. Since the 2015 spin-off of Chemours company from Dupont, Krytox has remained a Chemours brand.
John Keenan, application development engineer at Chemours, explained: “The foremost concern in outer space is the oxygen compatibility. PFPE has been the solution for years. This is crucial when working with highly reactive liquid oxygen, and that was one of the reasons PFPE compounds were used in aerospace originally. Secondly, many spacecraft components must operate at ultra low temperatures, beyond -70F and under deep vacuum. Chemours fluorinated lubricants are tailored for that.”
In aerospace, PFPE-based compounds are often used because no other lubricant can do the job. While high performance requirements require a more expensive lubricant, overall costs can actually be lower. Wing and fuselage designs, for example, are strongest, lightest and lowest cost when they are uninterrupted by openings for service access. A mechanism that can be sealed into a relatively inaccessible part of the airframe and work maintenance-free until a C-check or D-check can allow a simpler structure. The elimination or reduction of lubrication-related service is also a natural cost savings. In the aerospace industry, high upfront costs are not necessarily the lowest lifecycle cost.
However, in the tight-margin world of mass-production manufacturing, such as in automotive or industrial/commercial equipment, use of high-performance materials like PFPEs is subject to rigorous cost-benefit analysis and often an ROI calculation. However, a relatively simple problem in the automobile might look small but when multiplied by millions of units in the field, can have significant negative impact. In these cases the cost benefit analysis must include a holistic view of the impact of the problem.
“Unplanned maintenance or repairs required under warranty can be a significant cost for automakers. So, selecting PFPEs has enabled components to last the entire warranty period without requiring maintenance,” said Keenan.
Maintenance-Free Lubricants for Robotics
Robotic hardware, such as the serial manipulator of an industrial robot, often operates in harsh conditions, including high temperatures, particulates and aerosols such as paints and solvents.
In industrial robots, gears are typically lubricated with lithium grease consisting of synthetic hydrocarbon oil, mineral oil and molybdenum compound. Such greases have extreme pressure properties and good wear resistance, but the oils tend to separate from the mixture and leak out of seals. Heat and other factors contribute to this separation.
Lubrication maintenance including re-greasing typically occurs every 500-700 hours of operation. Using PFPE lubricants in some joints of a robot could prolong this cycle. According to Keenan, PFPE lubrications offer benefits in robotic hardware, and Chemours has developed products specifically for the challenges often encountered in robotics. Keenan said, “Krytox FPG182 was developed as a high vacuum, low out-gassing, anti-corrosion grease for aerospace. Importantly, these application requirements have significant crossover with needs in robotics.”
Specialized lubricants are required in gears, articulations points and end-of-arm tooling that makes robots capable for different tasks. Some robots perform at high temperatures, under long duty cycles, and with environmental factors like solvents, moisture, dust and abrasives. With motors, actuators, valves and electronics on board, it’s important to select the right lubricant when designing a robotic end effector. Plus, like aircraft, downtime can be extremely expensive—so much so that the unit cost of a few ounces of lubricant is a non-factor where a longer mean time between failures (MTBF) can be achieved with existing equipment.
How Much do PFPE Lubricants Cost?
In addition to the industries detailed above, PFPE lubricants have a strong presence in semiconductor manufacturing, chemical processing and petrochemicals, pulp and paper, textiles, energy and metal processing, to name a few. It’s a fact that PFPE lubricants are more expensive than other options.
Part of the reason PFPE products are more expensive is because of the manufacturing process. Krytox, for example, begins with calcium fluoride (Fluorspar), which is converted to hydrofluoric acid. This is then used to make 1-carbon hydrochlorofluorocarbons which are converted to tetrafluoroethylene (TFE) or hexafluoropropylene (HFP). PFPEs are produced from these monomers. At each step in the process, the volume decreases, and additional processing is performed to make different blends.
According to Keenan, PFPEs cost approximately five to ten times more by weight than alternatives. “What PFPEs offer is the cost-benefit of using less and delivering higher performance,” he explained. “We have data that shows that a bearing lubricated with Krytox PFPE can run 25,000 hours versus 500 hours with a bearing lubricated with hydrocarbon and running at the same operating conditions. That's a pretty compelling argument to spend more to deliver maintenance-free operation.”
Add in the cost savings of assembly simplification, lighter weight assemblies, reduced maintenance and a longer MTBF or run time before overhaul and the actual cost of PFPEs can be considerably lower than conventional lubricants.
The first step is for designers and maintainers to investigate the advanced properties of PFPEs and consult experts such as Chemours to determine where a lubricant specification upgrade can deliver a strong ROI. Better lubrication never hurt a moving part, and it also may reduce costs—a rare “win-win” in machine design.
For more information on PFPEs, visit Chemours.
The Chemours Company has sponsored this post. All opinions are mine. --Isaac Maw