Lube-for-life journal bearings

Benjamin A. Shobert
Polygon Co.
Walkerton, Ind.

Edited by Lawrence Kren

Lube-free composite journal bearings from Polygon, Co., Walkerton, Ind., use interwoven PTFE super filaments. The material provides small pockets for contaminants to embed allowing the film transfer (lubrication) process to continue unabated.

The pin (mating surface) before and after PTFE film transfer begins. During break-in, PTFE fills valleys on the shaft. Once the process is completed, the coefficient of friction stabilizes and bearing wear practically flatlines.

A typical sintered journal bearing develops cracks when subjected to repeated impact loading.

Many engineers consider external lubrication systems for journal bearings a necessary evil. The approach adds design complexity and another item to be routinely serviced. Not to mention, liquid lubricants are increasingly regulated. And these environmental issues relate not only to waste grease and oil, but also to potential leaks from equipment. But journal bearings made from self-lubricating materials have the potential to completely eliminate all secondary lubrication in some cases.

Journal bearing materials are broadly classified into five basic types: metallic sintered, metal-backed, woven fabric, filled thermoplastic, and high-load composite. In general, most of these self-lubricate by one of three methods: migration of a semiliquid lubricant from within a sintered structure to the pin/bearing interface; scrubbing using a graphite or MoS₂ dispersion, and film transfer.

Sintered bearings disperse a liquid lubricant such as oil, grease, or silicone fluids trapped in the bearing wall. The lubricant eventually runs dry and must be replenished from an external source. Scrubbing mechanisms also rely on a sintered structure but the lubricant is embedded in the reinforcing matrix. Such bearings have fairly limited life and are typically found in applications that can accommodate significant bearing wear.

Composite bearings from Polygon Co., Walkerton, Ind., combine a high-load continuous-fiberglass-filament backing impregnated with a fatigue-resistant, epoxy-resin matrix. Lubrication comes from a PTFE-fiber liner woven into the fiberglass backing. Relative movement between the bearing I.D. and pin O.D. creates frictional heat, triggering an oxide-build reaction that causes the PTFE fibers to undergo a macroscopic phase change. This phase change smears the material to the pin surface to provide lubrication.

However, lubrication is only one consideration. Equally important is how well a bearing holds up in use. Consider resistance to impact fatigue, for example. Sintered metallic bearings tend to stress crack under repeated impact loading. And impact can shatter thermoplastic bearings. The tendency typically worsens as a bearing is cycled and stressed. Because some metal-backed bearings also use a sintered PTFE resin at the bearing I.D., they can be subject to premature liner failure and cold flow. In contrast, a composite bearing made of a continuous fiberglass/epoxy backing resists impact fatigue and won't relieve or cold flow with repeated use.

Corrosion resistance is another factor. Obviously, all metallic and metal-backed bearings can corrode, especially those not sealed properly against harsh environments. For instance, water can attack flash coatings on such bearings and eventually the bearing itself. Composite bearings (fiberglass/epoxy backing) effectively resist attack from both acidic and caustic compounds.

Yet another factor is embeddability. Embeddability is the capacity of a bearing to continue working properly while ingesting external contaminants. Although greases and oils can purge bearings of contaminants, they also retain and trap them. It is important to note that any lubricated bearing material is rated assuming the lubrication system is fully functional. Contaminants compromising the hydrodynamic film cause two problems. First, bearings can gall and score shafting especially when using a relatively harder (low embeddability) bearing material. And second, bearing load capacity diminishes proportional to declining film integrity.

Next, consider load capacity in both static and dynamic modes. As designs become increasingly optimized, dynamic stresses placed on bearings may also increase, sometimes beyond the capabilities of conventional bearing materials. Composite bearings with a continuous fiberglass backing handle static pressures to 60 to 70,000 psi and dynamic pressures of about half that, or roughly 10 times the capacity of metallic-sintered types.

The bottom line
When specifying a journal bearing, calculate the total cost of ownership. This includes the cost of the bearings themselves, lubrication handling systems (where applicable), and warranty and maintenance expenses. Composite journal bearings made with PTFE monofilaments typically cost more initially than conventional bearings, but can save money in the long haul through reduced maintenance and longer life.