Polymers as power transmission parts. There was a time when people sneered at “plastic gears” – and bearings – figuring them Cracker-Jack box components, not considering the difference between a model-airplane-grade plastic and a high-quality nylon. But disapproval was sometimes justified, brought on by improper selection of materials and misapplication of polymer parts by manufacturers and designers. Today, though, engineering improvements and judicious use have gained these macromolecular materials their due respect in the motion industry.
One of the leading uses of polymers is at bearing surfaces. Many designers have wholeheartedly availed themselves of selflubricating sleeve bearings where wet or chemical environments would chew up steel rolling bearings.
The most common nonmetallic bearing materials are oil-filled UHMW-PE (ultra-high molecular weight polyethylene) and oil-filled nylon, although new polymers and polymer combinations are being developed almost daily. Some recently developed materials can operate at ambient temperatures exceeding 500oF, are unaffected by virtually any chemical, and are FDA compliant – very attractive properties for many applications.
Sometimes it’s easy to tell when polymer bearings will not work, but there may be borderline situations where both metallic and non-metallic varieties have favorable points. A simple PV formula can tell whether a polymer bearing is up to the speed and load requirement. P is the pressure (psi), and V is surface velocity (ft/min). A P vs. V graph is nonlinear. Each material has a maximum pressure and surface velocity, but a plain bearing cannot be exposed to both maximums concurrently; the bearing is rated according to the PV product.
Load is pressure over area; for journal bearings the pressure area is the diameter of the cylinder times its length. The surface velocity is 0.262 x diameter x rpm.
The maximum PV rating for dry (no additional lubrication) UHMW-PE is 2,300 psi-ft/min. With supplemental lubrication it can be increased to 4,000 psift/ min. The generally accepted maximum for oil-filled nylon is 16,000 psi-ft/min.
With any plain bearing, metallic or polymer, frictional heat is a primary concern. Manufacturers often tout polymer bearings’ ability to run without lubrication – below certain speed and load combinations, it is certainly possible – but the capacity of a self-lubricating bearing can be increased substantially by adding lubricant, if only at start-up. Local heat is typically brought on by friction, and any friction- reducing measure will be rewarded with increased load capacity and wear life. Coefficient of thermal expansion is greater than steel for most polymers, so stock polymer bearings are manufactured with the appropriate clearances to allow for thermal effects on size. Maybe your first thought is, “The bore will get larger as it gets hotter,” and that would be correct if the insert was unconstrained, but most polymer bearings are housed or mounted in a way that does not allow for OD expansion, hence the ID constricts as the material swells.
Polymer bearings are ordinarily manufactured to dimensional bearing industry standards. Housing bolt hole configurations and bearing inserts are interchangeable with their steel counterparts. Selfaligning polymer bearings come in most of the standard mounting patterns, including 2, 3, and 4-bolt flange, pillow block, tapped base, cartridge, and take-up. Split bearings, too, are usually available in the 2- and 4-bolt and pillow block arrangements.
Sprockets and gears
There are many cases where polymer sprockets provide an advantage as well. Besides being corrosion-resistant, they produce less noise, and often both sprocket and chain will last longer in situations with limited lubrication. While polymers flex more than steel, that property allows the chain load to be spread out over several teeth. The weak point in a polymer sprocket is the keyway, but cast or machined steel and stainless steel hubs can be integrated for heavy-duty applications.
Most modern polymer sprockets are made from the old standby, nylon (polyamide), and with good reason. Nylon is cost-effective and easy to obtain. It has high strength, excellent wear properties, and is easily machined. FDA-compliant oil-filled nylons are also readily available for food processing applications.
UHMW-PE is also an excellent sprocket material, especially in wet environments or applications involving certain harsh chemicals. Municipal wastewater treatment plants and food processing wire belt conveyors often use UHMW sprockets because of the material’s wear and impact resistance and FDA compliance. UHMW’s mechanical properties actually improve at low temperatures, even those approaching cryogenic levels, so UHMW sprockets are often seen in freezers and other cold environments.
With gearing, surfaces have rolling and sliding contact; a very nice opportunity for polymers to come through. Where plastic gears are applicable, oil-filled nylon (again) usually works about as well as any other formulation. Similar to the sprockets, nylon gears are quiet. Under reasonable circumstances, face surface abrasion is minimal – besides reducing sliding friction, the oil filling is at a microscopic level, and is visually and tactilely subtle, therefore it does not attract dust and dirt. Since nylon gears (and sprockets) have about 20% the weight of comparable steel gears, they add considerably less inertia to the drive. As with sprockets, the keyway is the weakest area, so again, steel or stainless hubs are sometimes in order.
Common nylon formulations are 6, 6/6, 6/10, 6/12, 11, and 12. Each has its pros and cons. They all absorb moisture to some degree, and parts will grow with the absorption. This is generally not a concern unless the components are submerged. If they are, overall growth is easily calculated and parts can be machined or pre-formed to the proper adjusted dimensions.
Oil-filled nylon has higher flexural strength than most other wear-resistant polymers, making it suitable for heavy loads and high-PV applications, and to provide unsupported stiffness.
The most prolific use of polymers in power transmission is with wear strips, and UHMW-PE is easily the most common wear material. Whether it’s a simple PVC conveyor belt, a serpentine chain drive, or a sliding cardboard box, UHMW is the universally recommended choice for polymer guides. It is available in a number of compositions, including virgin, anti-static, UV-inhibiting, oilfilled, and high-wear. Application-specific UHMW types include a non-marking dry-lubricant-filled formula for handling paper and paper containers, and a ceramic- filled formula for wet abrasive environments like bottling lines. With specialty compositions such as these, make certain about vendor claims of “equivalent” alternative brands; often they are not comparable.
UHMW has the highest impact resistance of any polymer, and several formulations are even FDA compliant. Furthermore, its coefficient of friction is surpassed only by PTFE. Virtually any shape can be machined, from long grooved straight sections to intricate curved parts. UHMW comes in sheets up to 5x10 ft, and most suppliers have milling machines that can work pieces that size.
Wear strip capacity is calculated with the same PV formula used for plain bearings. With strips, though, frictional heat is not as localized, therefore they can usually handle higher loads. To account for thermal expansion in these linear applications, firmly fasten the infeed end of the strip, but use slots that allow for growth at the remaining attachment points.
Roger Hirsch is Engineering Services Manager, Power Transmission Div., Poly Hi Solidur, Fort Wayne, Ind.