The above schematic illustrates the pin (mating surface) before and after the film transfer process begins. The high load continuous fiberglass filament backing is impregnated with a fatigue-resistant epoxy resin matrix. The backing material is complemented by a woven PTFE fiber liner that provides self-lubrication during normal cycling conditions. The resulting high load capacity bearing can operate successfully in environments that typically assault traditional journal bearings.
Select figure to enlarge.

Lubrication media have for some time been seen as a necessary evil. At the very least, the need to re-lube has added a level of complexity to system design. Allowances must be made for lubricant transport; plus, improper maintenance of lubricated joints often leads to warranty claims.

The potential to completely eliminate all need for secondary lubrication through true self-lubricating bearing materials is indeed intriguing. It’s also now possible.

Incorporating PTFE super-filaments into composite journal bearing materials is the key, eliminating the need for secondary lubrication without compromising other performance issues, such as innate strength, resilience, and self-lubrication properties, that typically plague bearing designers.

Self-sufficient bearings

Most journal bearing materials offer some level of self-lubrication. Lubrication types can typically be characterized into three general transport phenomena: migration of semi-liquid lubricants from within their sintered structure to the pin/bearing interface; a “scrubbing” mechanism using a graphite or MoS₂ dispersion, and third, an actual film transfer of the PTFE from the bearing ID to the OD of the mating surface.

The migration of semiliquid lubricants, that is, traditional oil or grease as well as silicone fluids trapped in a sintered metal or thermoplastic resin structure, has limitations. Primarily, this lubrication method has a finite life span. These types of journal bearings, especially sintered metal systems, are almost always externally lubricated. As the assemblies are purged and re-lubricated, the transfer of lubricant media from the sintered structure must start completely over. This cycle repeats itself and limits the total life possible with a sintered structure bearing. Also, as these bearings release lubricant they can undergo a fairly typical stress relief resulting in diminished crush strength and depleted fatigue resistance.

Scrubbing mechanisms, typically molly or graphite based, also rely on a sintered resin structure with the material embedded in the reinforcing matrix. As opposed to the generation of a hydrodynamic film with semiliquid lubricants, these bearings rely on a scrubbing phenomenon that removes the peaks of the mating surface (pin material) and smoothes the shaft during cycling. Bearings with this type of lubrication system typically have limited life cycles and are configured for wear all the way through the wall.

The most efficient self-lubrication mode is film transfer. This process typically has a bearing ID composed of high tenacity PTFE filaments. As the pin is subjected to rotational movement, the mating components between the bearing ID and pin OD begin to rub creating frictional heat. This heat triggers an oxide-build reaction that results in the PTFE filaments undergoing a macroscopic phase change. This phase change allows the PTFE to actually smear from the bearing ID to the mating surface of the pin. As the bearing continues to dissipate friction, this ongoing film transfer reaches an equilibrium point where sufficient PTFE has been transferred to the shaft and all valleys are filled with the PTFE film. Once this break-in period is over, the coefficient of friction stabilizes and wear practically flat-lines.

Performance drivers

True self-lubrication presents an enormous design opportunity when properly understood and applied. However, ultimate design freedom comes with an understanding of how other primary design drivers can be fully optimized with the appropriate composite bearing material. The first such property is impact fatigue.

Select figure to enlarge.

With any sintered structure material, impact fatigue presents a very real and defined performance limitation. For a metallic sintered structure bearing, repeated impact quickly results in crack propagation and structural failure.

With sintered thermoplastic bearing materials, impact fatigue, especially when taking place in very cold environments, can cause the bearing to shatter. This condition becomes aggravated as the bearing is cycled and stress release occurs. Because many metalbacked bearings also use a sintered PTFE resin structure at the bearing ID, which means they are also subject to cold flow.

In contrast to thermoplastic and metal bearings, a composite bearing is composed of a continuous fiberglass/epoxy backing which is extremely resistant to impact fatigue and will simply not relieve or cold flow during repeated stress or strain.

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