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.
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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The effectiveness of the film transfer process and the ease of embeddability are functions of the wear fiber orientation to the primary bearing axis. But the coefficient of friction is also affected by fiber position with respect to the bearing surface. Friction is lowest when the surface contact is on the ends of the fibers. Furthermore, end contact ensures a larger amount of PTFE near the surface to act as a dry lubricant. Although some designs lay the fibers in a concentric or parallel orientation, better performance results when the PTFE wear fibers are arranged so they’re radial or perpendicular to the sliding surface. The wear fibers should also be transversely locked so they don’t separate under very high loading.
A PTFE-rich surface helps prevent scoring and excessive wear seen in sliding fluid-lubricated metallic bearings that can seize or rub under initial load and cycling (when the lubricant is not fully active). A self-lubricating surface mated to a hard pin will generally begin the film transfer process at startup, given loads high enough to pose a threat of binding or cause significant abrasion.
PV...nothing more than PV
The PV rating relates the bearing pressure and surface velocity capabilities. From this value, engineers can derive the amount of heat generated as the bearing wear surface interacts with the shaft. Going deeper, the PV characteristic tells if and when the film transfer process will occur. As a general guideline, the PTFEfiber wear surface in composite bearings will have a PV limit of 20,000. Running this type of bearing at 15,000 PV for 10 million cycles with a ±25° oscillation movement caused frictional wear of only 0.002 in.
Out of whack
Shaft misalignment can cause a poor load distribution across the bearing surface, placing excessive stress at the edge. Premature failure, including crushed ends, may result. A linear pressure region usually indicates a small or moderate misalignment, while a parabolic pressure area is a symptom of gross misalignment.
With continuously reinforced composites, the reinforcing filament’s wind angle can be modified to reflect such adverse conditions. Altering the fiber orientation allows the structural properties – including both flexural and compressive characteristics – to be redesigned accordingly.
Benjamin Shobert is Vice President of Polygon Company, Walkerton, Ind.