A new class of solid lubricants for bearings
Jeanna Van Rensselar, Senior Feature Writer | TLT Feature Article July 2016
By acting like a sophisticated, oil-soaked sponge, MPLs hold, dispense and reabsorb lubricants during operation.
Photo courtesy of EDT.
KEY CONCEPTS
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An MPL is like a sponge soaked in oil and additives with a microporous structure that holds, dispenses and reabsorbs the lubricant as the bearing operates.
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MPLs are used in a wide range of applications and industries with a strong presence in steel, mining and food processing.
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MPLs cannot be relubricated and rarely need maintenance.
ANDY WARHOL ONCE ADMITTED THAT HE WAS DEEPLY SUPERFICIAL. This joins the list of oxymorons like
Junior Whopper, the only choice, original copy, random order, solid lubricants…clearly confused?
Probably not. Solid coatings that function as lubricants have been around for a long time. But now there is a new class of solid lubricants for bearings: microporous polymeric lubricants or MPLs, which work like a very sophisticated oil-soaked sponge (
1).
Bearing lubricants have three jobs: (1.) separating moving parts in contact with each other (balls, rollers, etc.), (2.) dissipating frictional heat and (3.) providing protection from corrosion, contaminants and moisture—basically the same jobs lubricants perform in other applications. The difference with bearings is that there are a lot of contacting parts and the lubricant is difficult to control; it tends to drip and dissipate faster than in other applications.
Lubricants used in rolling element bearings require:
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Reduced tendency to leak out of the bearing
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A stable viscosity over a wide temperature range
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Film strength that can support loads
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A stable structure for long service life
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Compatibility with contacting components
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Non-corrosivity
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Barrier protection against contaminants and moisture.
Until recently, there were three choices for bearing lubrication: oil, grease and solid coatings (and the highly questionable practice of no lubricant at all in some applications). These lubricants were applied in one of two ways: oil bath, oil recirculation or nozzles; and oil in a thickened form (grease). Now there is solid oil, where the oil is retained in a polymer matrix.
Mitch Wilson, sales manager for Volution Bearing in Statham, Ga., explains, “Solid lubricated bearings are designed to perform when traditional oil or grease lubrication underperforms or cannot be used due to cleanliness, accessibility, bearing orientation or contaminants, or when standard lubrication leaks out and contaminates the area of production.”
SOLID POLYMER BEARING LUBRICANTS
MPL bearings never require relubrication. In fact they
cannot be relubricated, and they rarely need maintenance. All of this is significant when you consider that lubricating a conventional bearing can consume more than a gallon of grease per year, and many bearings are difficult to access.
HOW MPLS ARE FORMULATED AND MANUFACTURED
MPLs contain a microporous polymer with synthetic lubricant and additives. The chemical composition of these components varies depending on the application. MPLs can be formulated for many applications, including high temperature, high speed and food grade. In the case of food grade MPLs in the U.S., the oil will be an FDA/ USDA-approved food-grade lubricant. Viscosity is typically: 140 cST at 40 C (105 F) and 19 cST at 100 C (210 F). Additives can be used to alter the lubricant’s properties and include:
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Corrosion and oxidation inhibitors
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Friction modifiers
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Lubricating solids such as MoS2 and graphite
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EP additives.
The polymer is moulded into the bearing with very narrow gaps around the rolling elements and raceways, which allows the bearing elements to operate freely while the oil seeps into that gap. It is then processed thermally to solidify. Because of this solidification process, there is almost no leakage from the bearing during service. After the mixture is solidified in the bearing, the piece is trimmed and cleaned before shipping.
All of this needs to be done at either the MPL manufacturing facility or the bearing manufacturing facility. It cannot be done once the bearing is placed in machinery. Almost all types of bearings can be lubricated with MPL, which is usually applied in one of two ways:
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Spot-pack. Solid grease is injected into the retainer. This type is standard for deep groove ball bearings, small diameter ball bearings and bearing units.
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Full-pack. All empty space around the rolling elements is filled with solid grease. This type is standard for self-aligning ball bearings, self-aligning roller bearings and needle roller bearings.
MPLs can also be extruded into bars and processed into sheets for specialty applications.
TYPES OF BEARING LUBRICANTS
Oil
Both petroleum-based and synthetic oils are available. Oils are subject to evaporative losses, so the service life in a bearing is less than grease.
Grease
Grease consists of a base oil with a thickener composed of metal soaps or inorganic compounds. Most greases used in rolling element bearings are NLGI Grade 2.
Solid Coatings
These are applied to the frictional surfaces to prevent wear. They are used in harsh environments such as extreme temperatures and radiation. Typical materials are graphite, MoS2, silver, gold and PTFE.
Solid Polymers
These can be loaded into the bearings during production and then solidified, and also installed post-production.
HOW MPLS FUNCTION
The finished product is like a sponge soaked in oil and additives with the polymer functioning as a microporous structure that holds, dispenses and reabsorbs the lubricant as the bearing operates. The polymer pores have to be small enough to retain the oil by surface tension. As the bearing temperature increases, more oil is released by the MPL, and as the temperature decreases, oil is reabsorbed.
Carl Klinge, Engineering Manager, EDT Corp., Vancouver, Wash. explains, “The polymer structure is comprised of millions of small pores that hold and release oil. As the bearing rotates and the ball and roller track warm, the oil is released. When the bearing is idle, the oil is trapped within the pores.”
When MPLs are used in wet environments, most bearing and MPL manufacturers recommend including contact seals. The combination of MPLs and contact seals improve sealing performance since the MPL prevents the seals from opening under pressure.
“During normal operation the solid polymer matrix will not break down,” Wilson says. “The polymer matrix is completely inert and includes antifriction additives that increase lubricity and lubricate independently.”
He continues, “Since solid lubrication is just another lubrication option, the failure modes of the bearing remain the same, but solid lubrication likely will change the mode. For example, if the bearing is failing from dust contamination, solid lubrication may significantly extend the life of the bearing by resisting the contamination for a longer period.”
John Cox, aftermarket-senior technical support for NSK Corp. in Tokyo adds, “The failure modes tend to be similar to greased bearings with the typical variation being the amount of time before that failure occurred. Solid lubricants are proven to be more durable than standard greases.”
An assortment of EDT Corporation’s solid lubricated bearings. (Photo courtesy of EDT.)
THE PROS OF MPLs
MPLs are an excellent solution in many but not all applications. They provide continuous, reliable lubricant to the bearing and act as an effective seal, but there are speed and temperature limitations.
SKF TEMPERATURE LIMITS (2)
The following SKF temperature limits apply to both open and sealed solid-lubricated bearings. In general, sealed bearings have higher operating temperatures.
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Maximum continuous operating temperature 85 C (185 F)
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Maximum intermittent operating temperature 95 C (205 F)
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Minimum start-up temperature (standard oil) –40 C (–40 F)
“Solid lubricated bearings do not require regreasing,” Wilson says. “The oil and polymer matrix keeps the lubricant inside the bearing and delivers the right amount of lubrication to the raceways and rolling elements. Solid lubricated bearings eliminate the cost of grease and labor to regrease bearings.”
MPLs save money in many ways. In addition to eliminating the product and labor expense of relubrication, they extend bearing life by:
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Reducing the effects of contaminants and chemicals that can lead to early bearing failure.
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Ensuring a constant and consistent supply of oil to the bearing. Because the base oil is retained in a solid mixture, it is less likely to leak out of the bearing. And because spotpack bearings are not subject to shear resistance, they have a lower running torque than bearings with standard lubricants (
3). Centrifugal force keeps the lubricant in the pack or on the bearing and cannot be channeled out of the raceway groove.
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Creating a seal. Because solid oil fills the bearing cavity completely, it is difficult for contaminants to intrude. A bearing with solid oil contains between two and four times more oil than a grease-lubricated bearing. Seals are not needed to retain the lubricant in the bearing, even on vertical shafts. But for applications that need reliable sealing performance (i.e., wet or highly contaminated environments), manufacturers recommend seals.
Other advantages include:
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The ability to survive harsh applications and wash downs (
4) (and reduced wash down time)
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Dramatically improving equipment cleanliness
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The ability to withstand high g-forces
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Eliminating the need for grease purging and regreasing
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Keeps grease out of the waste stream
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May be USDA/FDA and HACCP friendly (
5)
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Can be difficult to wash out of the bearing
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Eliminating lubricant churning
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Can provide lubrication at difficult-to-reach locations
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Releasing oil to bearing surfaces on demand
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Improving plant housekeeping and safety conditions.
MPLs are also environmentally friendly. “Solid lubrication protects the environment in two ways,” Wilson says. “First, it keeps the oil trapped within the polymer matrix so it doesn’t leak out and contaminate the surrounding area. Second, since no regreasing is necessary, it replaces the traditional model where grease is pumped into the bearing and purged. The purged grease is usually washed away and flows into the waste stream to further contaminate the environment.”
THE CONS OF MPLs
MPLs don’t do well at high operating speeds. “Limiting speeds are greatly reduced compared to typical greased bearings,” Cox says. “Every application should be investigated on an individual basis to determine if the impact on limiting speeds is detrimental to targeted performance.”
MPLs also have maximum recommended operating temperatures since they do not dissipate heat rapidly. If the high-temperature limit is exceeded, the polymer softens and can eject from the bearing.
While MPLs generally resist contamination better than greased bearings, they are not waterproof and will not prevent bearing corrosion. Also, although they generally resist chemical damage, direct contact with strong detergents and acids is not recommended. They also are not ideal in environments that involve high shock or excessive vibration.
“Most solid lubrication formulas have speed and temperature limitations,” Wilson says. “However, Volution continues to develop various products with different operating temperatures and speeds. Additionally, if the application is subject to wash down, integral seals are recommended to protect the solid polymer from high pressure spray that can wash out the lubricating oil from the polymer matrix.”
Cox summarizes, “The key advantages that solid lubricants bring to all applications are reduced maintenance, reduced leakage and improved lubrication. Individual formulas then address specific concerns such as wash down, heavy contamination, high heat, etc. The main disadvantage to solid lubricants is the significant reduction in obtainable speeds.”
NTN’S TESTS (8)
NTN subjected their solid lubricants for bearings to the following three tests.
Lubricant Leakage Test. Non-shielded/sealed test bearings (spot-pack) were subjected to a centrifugal acceleration of 3,000 G (5,000 min-1) for a period of four hours. Lubricant leakage from the NTN bearings with solid grease was approximately 2% by weight for the horizontally mounted condition and approximately 5% by weight for the vertically mounted condition. Standard grease-filled bearings using contact and non-contact seals were also subjected to this test. Within 10 minutes of starting the test, centrifugal force caused the seals to become displaced allowing the grease to expel.
Rotating Torque Test. When tested, the required rotating torque level for NTN bearings with solid grease utilizing the spot-pack configuration was found to be less than bearings using lithium-diester grease (an acknowledged low-torque grease). However, the full-pack configuration exhibited running torque levels greater than those of standard bearings.
Saltwater Test. A saltwater endurance test was performed to compare the performance of bearings with solid grease to that of bearings using a conventional lubricant (lithium-mineral oil based grease). NTN bearings with solid grease were found to outperform standard bearings although some surface deterioration had been detected.
APPLICATIONS
“Solid lubricants are used in a wide range of applications and industries with a strong presence in steel, mining and food processing,” Cox says. In most applications, traditional lubricants provide adequate bearing lubrication, but MPLs shine in such demanding conditions as:
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Highly contaminated environments
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Wet environments
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Environments with low operating temperatures
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Plants that require high levels of sanitation and are subject to USDA-inspection
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Bearings that are subjected to frequent wash down and/or chemical cleaning
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Bearings that are difficult to reach or maintain
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Bearings are vertical shaft-mounted.
Successful MicroPoly/Phymet Applications (9)
Klinge explains, “Customers that typically would use solid lube bearings have applications in areas that are hard to reach or maintain in some way and industries that require high levels of sanitation where exposed grease is a contaminant. Other applications are where a bearing that is designed to not require relubrication presents the lowest cost of ownership over a specified time and so becomes the most economical.”
MPLs don’t make sense for every application, but in places where they do such as in food processing facilities and difficult-to-access areas, they are lifesavers, increasing the service life over traditionally lubricated bearings by as much as 300%, according to Cox.
Typical applications include: (10)
Right now MPL formulators and bearing manufacturers are investigating specialty products and ways to overcome temperature and speed limitations. Klinge says that EDT is currently working on a bacterial inhibitor additive to enhance the cleanliness and HACCP compliance of food grade solid lube.
REFERENCES
1.
The terms microporous polymeric lubricants and MPL were coined by PhyMet/MicroPoly, but are used here generally as a reference to solid oils self-contained in bearings. Other terms include: SmartLube (BDI) Molded-Oil (NSK), EPL (EDT), and V-PolyLube (Volution).
2.
From: SKF Bearings with Solid Oil, may be available
here.
3.
Running torque is the amount of torque required to keep the bearing rotating at a constant speed once it starts rotating.
4.
Wash down is the process of high-pressure cleaning with water and/or chemicals in industries such as food and beverage and pharmaceuticals.
5.
HACCP (Hazard Analysis Critical Control Point) is an FDA management system under which food safety is handled through the analysis and control of biological, chemical, and physical hazards from raw material production, procurement and handling; to manufacturing, distribution and consumption of the product.
6.
From: SKF Bearings with Solid Oil, may be available
here.
7.
A = n ¥ dm where A = speed factor (mm/min), n = rotational speed (r/min), dm = bearing mean diameter = 0.5 (d + D), d = bearing bore diameter (mm), D = bearing outside diameter (mm)
8.
From: NTN: Bearings with Solid Grease: CAT. NO. 3022-*/E : may be available
here.
9.
From MicroPoly/PhyMet PowerPoint, The Solid Idea for Lubrication.
10.
From: NTN: Bearings with Solid Grease: CAT. NO. 3022-*/E : may be available
here.
Jeanna Van Rensselar heads her own communication/public relations firm, Smart PR Communications, in Naperville, Ill. You can reach her at jeanna@smartprcommunications.com.