Machine Design
PFPE Lubrication Helps End Premature Bearing Failures

PFPE Lubrication Helps End Premature Bearing Failures

Proper lubricant selection significantly reduces failures and increases product efficiency. And PFPEs are often the best choice.

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The average machine has 20 grease points to maintain. That means 20 points of potential failure, all of which can lead to unplanned downtime and maintenance expenses. In up to 50% of all machine failures, the cause can be traced to improper bearing lubrication.

With proper lubrication, however, these failures—along with their associated costs, warranty claims, and downtime—are largely avoidable. In many cases, perfluoropolyethers (PFPEs) are the ideal long-lasting lubricants for extreme environments that require either chemical inertness or extreme pressure and temperature capabilities.

High-performance PFPE lubes, such as Krytox lubricants, offer the broadest performance capabilities. Although these lubricants cost more initially, the added expense is more than recovered through the longer component lifespans, reduced warranty claims, and corresponding increase in consumer value.

Fig. 1

The PFFPE Advantages

Broader performance capabilities sound great, but what does it really mean in terms of lubricants, and how will they affect premature bearing failure?

The ASTM D-3336 endurance test evaluates the life of greases in ball bearings by rotating a bearing at a set speed and temperature and measuring their effective performance life. In this test, PFPEs routinely outperform hydrocarbons. At 10,000 rpm and 350°F (177°C), most lubricants fail in less than 1,000 hr. PFPEs last significantly longer, with one grade (Krytox AUT 2E45 grease) performing for more than 25,000 hr—the equivalent of almost three years—without failing. The test was actually stopped before the lubricant failed.

In ROF tests, which also measure the useful life of greases as a function of various temperatures and speeds, PFPE greases lasted 21 to 64 times longer than competitive lubricants. PFPEs’ extended performance capabilities let some components operate for life without ever being-relubricated; they’re essentially considered “lubed for life.” Other components can get by using significantly less PFPE lubricant per application.

Fig. 2
The results of an ASTM D3336 test of the life of ball bearings as a function of the lubricant used shows that PFPE/PFTE grease does the best. None of the other lubricants could last nearly as long.


PFPE lubricants also out-perform other lubricant classes both in high and low temperatures. As machine temperatures rise above 248°F (120°C) or fall below  32°F (0°C), petroleum-based lubricants begin to fail, forcing relubrication and production interruption . Conventional synthetic lubricants don’t perform much better. PFPE lubricants are effective up to 400+ °C (752+ °F) and down to - 75 °C (-103 °F).

In a side-by-side high temperature test, hydrocarbon and PFPE (Krytox GPL 227) greases were placed in an oven at 232 °C (450 °F) for 40 hours. The hydrocarbon grease, which was specifically advertised for high-temperature applications, lost 40% of its weight and ceased being an effective lubricant. Some of it also degraded into tar. The PFPE remained unchanged in weight and appearance with its lubricating abilities fully intact.

One of PFPEs’ greatest advantages is their stability across a wide variety of operating conditions despite material exposure. Unlike almost all lubricants, PFPEs are chemically inert, meaning they do not react with chemicals or materials. They are nonflammable and nontoxic, repel water and oil, resist solvents, and are compatible with oxygen and reactive gases, as well as most common elastomers, plastics, and metals.

Even with these extreme capabilities, PFPEs are completely safe to handle. According to the Safety Data Sheets, a Krytox PFPE lubricant is safer to handle that sugar. In fact, PFPE lubricants are available in several medical and food grades, are environmentally friendly, and do not contain hazardous VOCs. The PFPE oils are also recyclable.

Fig. 3
The PFPE lubricant outperformed other lubricants when it comes to high and low temperature performance.

Lastly, PFPE greases offer better performance under pressure. PFPE lubricants’ high load-carrying capability and good lubrication characteristics under boundary and mixed friction conditions make them ideal for use in high loading and slow speed conditions.

In a Pin and Vee Block Test (ASTM D-3233)—an evaluation of wear and friction using extreme pressure and line contact sliding motion—PFPE lubricants reach the maximum load in the test, while hydrocarbon lubricants display signs of extreme wear and often cause catastrophic early failures.

PFPE lubricants also outperform hydrocarbon lubricants in a four ball extreme pressure test (ASTM D-2596), which measures a lubricant’s performance under extreme pressure using a point-contact sliding motion. The test steadily increases the load on a rotating steel ball in contact with three fixed balls until they seize and welding occurs. The loads and wear scar diameters leading up to the weld point are used to calculate the load wear index (LWI), an indication of how well the grease prevents wear when operating below the weld point.

Petroleum greases have a LWI of approximately 50, and synthetic hydrocarbons have LWIs close to 100. PFPEs were stable at LWIs more than twice the LWIs of hydrocarbon lubricants, and matching or exceeding the LWIs of synthetic hydrocarbons.

The Performance Gap

Lab data is important, but how does it translate to industrial applications? Here are a few examples of how OEMs and industrial manufacturers used PFPE lubricants to reduce bearing failures and improve overall performance.

A copper rod manufacturer believed lubricating its rollers’ bearings—which saw operating temperatures over 400°F (200°C]—with a synthetic hydrocarbon grease every four hours was its best choice. But after switching to a PFPE lubricant, it lowered the re-lubrication interval to a month and went from replacing 186 bearings annually to just four, cutting bearing failures by nearly 98%. Switching to PFPEs also reduced maintenance costs, parts costs, and production downtime. A cost analysis showed a total annual savings of almost $67,000.

Fig. 4
After a heat test, the hydrocarbon-based lubricant was dried out and useless, and some had been converted to tar by the heat. The PTFE lubricant, Krytox GPL 227, was still fresh and able to do its job.

A polyethylene manufacturer was struggling with maintenance costs and frequent shutdowns of its high-capacity, sealed centrifuge because of hazardous material exposure. Specifically, a mineral-oil lithium thickened grease used in the centrifuge bearings leaked and led to premature bearing failures. The leaking solvent also created an additional hazardous exposure threat to workers. The new lubricant needed to be non-reactive and insoluble to hexane, provide enough adhesion to avoid leaks, and remain fluid enough to deliver the required lubrication.

The manufacturer turned to PFPEs. Since doing so, unplanned shutdowns and re-lubrication have both been reduced, with re-lubrication down to twice a year. These dramatic results decreased maintenance costs and increased worker safety.

One chemical manufacturer was plagued by frequent bearings failures in its reformer furnace blower due to the lithium grease decomposing at the 554°F (290°C) operating temperatures. The company sought out a chemically inert lubricant capable of performing alongside ammonia with a once-a-year re-lubrication cycle due to the prosecution loss, maintenance safety risks, and potential fire hazard associated with shutdowns.

A high-temperature PFPE lubricant designed to provide its best performance between 392°F and 572°F (200 °C and 300°C), met the challenge. Because PFPEs are nonflammable and chemically inert, safety improved. As a result, the bearings in the reformer furnace blower do not lose lubricity and the maintenance schedule can remain confidently at 12 month intervals, reducing the company’s operating costs and safety risks.

Quantifying the Total Cost of Ownership

Lubricant selection directly affects the total cost of ownership (TCO) for any piece equipment. According to Power Magazine, lubricants make up less than 1% of a plant’s operating cost but can directly influence more than 50% of a plant’s maintenance costs. Combine that with the corresponding downtime and productivity loss, and lubricant selection significantly impacts end user TCO.

Fig. 5
After a heat test, the hydrocarbon-based lubricant was dried out and useless, and some had been converted to tar by the heat. The PTFE lubricant, Krytox GPL 227, was still fresh and able to do its job.

Here’s an example: A pulp and paper manufacturer used a PFPE grease to prevent unexpected bearing failures in its pulp dryer, which had previously experienced approximately 10 bearing failures per year. The new grease saved the mill an estimated $1.7 million in downtime each year on the pulp dryer alone. If the company had extended the use of PFPE lubricants to the plant’s 3,000 electric motors, which ran without re-lubrication and suffered regular breakdowns, the mill could have saved an additional $6 million.

How is that possible? We know the cost of operating a single 50-hp electric motor without any re-lubrication for six years, which is the average size, state and lifetime of the plant’s electric motors, has an estimated net present value (NPV) of more than $4,800, while the same motor using a PFPE grease has an NPV of approximately $2,700 (a savings of more than 43%). Multiply that savings by the 3,000 electric motors on site and it yields a total saving of more than $6 million over a six-year period.

PFPE’s stable, lasting 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 comparison between a PFPE and a conventional hydrocarbon lubricant, PFPEs cost 23.5% less ($1,772 vs. $2,250).

Thomas Blunt and Carl Walther, Global Technical Service Engineers

Krytox Performance Lubricants

Chemours

The Lubricant Landscape

Fig. 6

A lubricant’s performance varies greatly depending on what type or classification it falls under. Understanding the strengths and weaknesses of each class of lubricant can help in maximizing equipment performance and lifespan. Here’s a quick look at each of them:

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

Mineral oil offers excellent hydrolytic stability and good lubricating ability, low toxicity, and decent compatibility with seal materials. However, mineral lubricants provide poor fire resistance and only moderate levels of thermal and oxidation stability.

Perfluoropolyethers (PFPEs) are synthetic lubricants with excellent thermal and chemical stability, low volatility, and unmatched lubricity at extreme temperatures. PFPEs are well-suited as long-lasting lubricants for extreme environments that require chemical inertness or extreme pressure capabilities.

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

Poly Esters are synthetic lubricants with performance capabilities similar to those of diesters, albeit with better hydrolytic stability and slightly stronger thermal stability and overall lubricating ability.

Silicone lubricants have good chemical inertness, thermal stability, and low volatility, making them a good choice for certain applications. However, their ability to form a lubricating film is low. Consequently, silicone lubricants cannot support high loads. Silicones are also known to move around or migrate easily, which can cause serious concerns in some applications.

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