Solid lubricants have been around for decades. For example, simple products like PTFE (polytetrafluoroethylene) are used in all kinds of domestic applications. The trade name for PTFE is Teflon, which was originally developed by DuPont in 1938 and is best known for its use providing a non-stick, waterproof, noncorrosive, and nonreactive surface on kitchen equipment.
However, products like this are used in almost every industry, from consumer electronics and semiconductor manufacturing to the oil and gas industry and industrial manufacturing, performing functions including water and corrosion resistance as well as working in high temperature, high stress environments.
These materials can also be used in bearings, especially in dusty or dirty environments such as agricultural machinery, to replace traditional lubrication which would otherwise be contaminated and effectively produce a grinding paste that could over time cause a catastrophic failure.
Many bearing manufacturers have their own versions of these products. For example, TransDev’s bearings with solid lubrication (pictured above, inset) are manufactured under its NTN brand (SolidGrease) and SNR brand (LubSolid). They require a special grease, solidified after injection in the bearings, which is claimed to retain a greater quantity of oil than standard grease, for the lubrication of the bearing during operation.
Bowman’s WMO range of oil-less bearings can be used in projects where lubrication cannot easily be provided or where conventional lubricants may carbonise. It helps to reduce costs of maintenance and oil, as it removes the need for oil-feeding systems.
These have been developed for high-load, low-speed applications and are made of rotationally cast high strength bronze using a specially formed graphite plug as a lubricant. It can withstand temperatures of up to 400°C as well as cryogenic temperatures down to -200°C, depending on which material is selected.
Similarly, Carter Bearings’ Cobra Solid Lubricants are graphite-based solid lubricants injected into clean bearings, either ball or roller, then thermally cured and hardened. The mixture fills almost 100% of the space between inner and outer races, and encapsulates the balls or rollers along with the cage.
The graphite solid lubrication moves in conjunction with the cage and balls or rollers and deposits a thin film of graphite along every surface it comes in contact with. This thin film prevents metal to metal contact, minimising the coefficient of friction within the raceway.
NSK’s Molded-Oil bearings are lubricated with its proprietary oil-impregnated material that consists of lubricating oil and polyolefin resin which has an affinity for oil. The material is thermoset, so is initially a liquid which can be poured into the bearing and then set in an oven.
“The polyolefin polymer in Molded-Oil Bearings is lipophilic, meaning the oil adheres to the polymer when the bearing is at rest,” explains Björn Gudmundsson, applications engineer, NSK UK. “As the bearing starts rotating and heating up, the oil expands thermally quicker than the polymer, and therefore small amounts of the oil are expelled and lubricates the running surfaces of the bearing. Once the bearing stops and cools down, the lipophilic polymer absorbs the oil back, so a minimum of oil is lost in each cycle.”
These bearings can be used in applications in the food and beverage industry where cleanliness is crucial and excess grease is detrimental to the process, as well as where frequent washdowns would flush the grease out of standard bearings.
Gudmundsson adds that they are suitable for “humid areas where bearings suck in air as they cool down. Normally this will cause condensation inside the bearing, but because Molded-Oil Bearings are filled with the polymer, there is no space for humid air to enter, reducing the risk of ‘breathing’.”
One company that has created an entire range of plastic bearings and linear motion solutions is igus. According to UK managing director, Matthew Aldridge, igus blends a minimum of three base components in all its materials that are blended precisely for each product (Iglidur flange bearing pictured above left; sleeve bearing, above right). These are: a solid base polymer, a solid lubricant and reinforcing fibre. Extra components can be added or special technical features.
Aldridge says: “Lifetime of these parts depends on the applications in which they are used and also that bearings are being used on the correct shaft material. With the right pairing (for example a plastic bearing moving on a steel shaft) we can measure wear in microns per kilometre travelled; that’s the order of magnitude that we’re looking at.”
He adds that with the company’s drylin range, igus provides both the bearing and the rail it runs on, both of which have been designed and optimised to work together. In this case a very accurate lifetime for the component can be provided either in time or number of cycles.
“Wear is not the problem for plastic bearings,” Aldridge says. “The biggest enemy of plastic is heat. Once you get to what we would call excessive temperatures then the use of this technology does become more and more difficult.” Generally, these solutions are suitable for use in temperatures between -100°C to +250°C.
Despite the low wear rates of plastic bearings or sealed nature of polymer-filled bearings, there is still a requirement to keep an eye on wear or lubrication loss. “The main causes of bearing damage and wear are ingress of contaminants (water and dust etc) and loss of lubrication,” says Gudmundsson. “As the bearing is filled with the Molded-Oil polymer, there is very little space for contaminants to enter the bearing and the Molded-Oil also supports the sealing lip from the inside, further assisting to keep contaminants out and reducing damage to the bearings. Once fitted, Molded-Oil Bearings are maintenance-free, but they need periodic replacement.”
NSK’s polymer is said to contain up to four times more oil than a normal grease-lubricated bearing, and because almost no oil is lost due to seal leakage the oil store allows a long service life. However, Gudmundsson adds that the oil will run out at some point, and the bearing will then need replacing.
In plastic bearings, clearance will get greater throughout the life of the bearing, and eventually reach a point where it is no longer fit for purpose.
Aldridge says: “We can calculate when that limit will be reached, and we would recommend bearing replacement at that point as a service item. [Then] you can fit a new bearing and you can completely recycle that the old bearing – you can put it in your normal plastic recycling waste.”
A project igus is working on is achieving a closed, circular economy whereby replaced bearings can be sent back and recycled into new engineering applications, something it already does in its chain and cable businesses.
This is something that’s harder to do with polymer-filled metal bearings. Gudmundsson says: “The Molded-Oil is effectively bonded into the bearing, so would be very difficult to extract to be recycled separately. The polyolefin polymers will however burn off when the bearing steel is remelted.”
Addressing the topic of microplastics getting into the environment from industrial use of plastic bearings, Aldridge doesn’t deny the fact that particles do wear off its products. He counters: “We are always developing plastics to wear less, so that effect is reduced as time goes on with more modern materials.”