In composite bearings, a typical wear surface consists of PTFE fibers woven into a material. Proper fiber orientation optimizes friction resistance, boundary lubrication, and contaminant placement.
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The mention of composite self-lubricating bearings may bring to mind bushings made essentially of plastic – expensive devices that can only be used in limited plain bearing applications. Actually, there is great contrast between composite bearing families. Fabricating methods and material types differ, and therein lies the ability to create strong, resilient components that can cost-effectively replace rolling and sliding greased bearing assemblies in many applications.

Backing it up

Among the different varieties of selflubricating composite materials used in the backing of the bearing, one has continuous- fiber reinforcement placed in a thermoset resin, as opposed to injectionmolded thermoplastic composites containing short, broken reinforcing fibers.

The continuous-reinforcement version can handle static compressive loads of 60,000 psi and dynamic compressive loads of 30,000 psi without the need for hydrodynamic lubrication. Where applicable, these composites put an end to problems such as shaft scoring and galling, bearing assembly corrosion, and lubricant and fittings maintenance. They have high shock load capability, being that a resin matrix combined with continuous fiberglass strands lends to elasticity and resiliency. Traditional thermoplastic bearings, besides having lower load capacities, are susceptible to contraction and expansion from changing temperatures and can also absorb moisture and swell.

The thermoset resin family of bearings is made with a filament-winding process. This involves wrapping a bundle of resin-impregnated filaments around the bearing liner, which is set onto a mandrel. The winding goes across the mandrel’s entire length, and is repeated until the stock tube is built up approximately to the desired final outer diameter. Then, everything goes into an oven while the resin cures, and afterward the hardened composite is cut and finished to the specified diameter and length.

The unbroken strands of resin-soaked fiberglass provide an extremely strong backing. And, the ability to control their placement and positioning lets the properties of the finished composite backing be optimized, even customized, to allow for abnormal load patterns.

Down to the surface

Composite bearing performance is heavily reliant on the self-lubricating wear surface. With the form of PTFE (polytetrafluorethylene, commonly known as Teflon) used in the lining, lubrication occurs through a film transfer process, also called boundary lubrication. With this kind of lubrication, surfaces remain in contact during operation, and a surface film of lubricant alleviates friction and wear.

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The nature of the self-lubricating materials enables good bearing performance under cyclic and oscillatory load conditions. PTFE molecules are easily sheared and compacted into the surface of the mating pin, actually filling surface voids. At a melting point of 612°F, the PTFE causes an oxide-building reaction, which lets the material smear around the shaft as it undergoes a phase change. This temperature point is high enough to ensure that wear and smearing do not occur unless significant shaft rotation and loading are present.

The high-performance composite backing and the process it’s made by complement the specially woven wear lining. During manufacture, the resin works into the coarse liner back and, after hardening, the backing and liner become inseparable.

Such a wear surface can be further improved to allow embeddability. A well-designed lining is semi-porous, with the PTFE wear fibers forming tiny pockets that can take in contaminants. Dirt and dust particles become embedded into the pockets, thus their damaging effects on the shaft are minimized and the self-lubricating process can continue unimpeded. The small pockets also provide a disposal for resin debris during the break-in period so the film transfer process can be initiated quickly.

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