Plastic replaces metal in many plain-bearing applications
East Providence, R.I.
Plain bearings molded of plastic are often an economical replacement for needle, ball, and plain metal bearings. To a certain extent, though, plastic bearings face an uphill battle for respect among the engineering community.
One reason is the erroneous mind-set particularly prevalent among older engineers that plastic is inferior to metal. Others cling to the notion that high-priced engineered polymers are a must for plastic bearings. The fact is, low-cost materials with excellent strength and thermal properties let inexpensive plastic bearings outperform their metal counterparts in many rotary, oscillating, and linear-motion applications.
Another impediment to wider acceptance is that only a handful of companies currently stock a broad range of standard products. Many suppliers offer a small complement of standard plastic bearings and resort to custom manufacturing most orders. Metal plain bearings, on the other hand, are available off the shelf from literally thousands of distributors across the country.
Finally, most engineers have little experience with plastic bearings and are thus reluctant to put an expensive machine at risk with low-cost components they know nothing about. Many users only turned to plastic bearings out of desperation when all else failed. But with growing recognition that plastic bearings often outlast metal versions, need no lubrication, and offer the potential to cut size and weight, plastic bearings increasingly are getting a serious look.
Plastic bearings typically consist of a thermoplastic alloy and solid lubricants with a fiber matrix often added for creep resistance and strength. The most common low-cost materials are nylon, ultrahigh-molecular-weight (UHMW) plastic, or Teflon. High-performance engineered plastics such as Vespel, Torlon, and PEEK are sometimes used for extremely high loads and temperatures, but these can be cost prohibitive. Probably the most significant change in plastic bearings over the last few years is higher load and temperature capabilities. But the primary advantage plastic holds over metal bearings remains the ability to operate dry without additional lubrication.
All bearing constituents — the thermoplastic, fiber matrix, and lubricants — have excellent antifriction and low-wear characteristics that produce a self-lubricating effect. This is especially critical at initial start-up. A lubricant film has not yet formed and the bearing begins operation dry. This can accelerate wear in metal bearings, but plastic bearings homogeneously impregnated with solid lubricant run “lubricated” from the start.
As soon as a loaded plastic bearing moves, microscopic bits of solid lubricant and thermoplastic abrade to smooth the shaft surface. The material fills shaft imperfections and provides an optimum surface for continuous low wear and lubrication.
This minimizes slip-stick conditions and wear, and frequently increases operating life compared with plain-metal, ball, and needle bearings. Dimensional changes to the bearing are essentially nonmeasurable, and abrasion decreases rapidly following startup and becomes negligible in continuous operation.
Most plain bearings, on the other hand, are oil-filled, sintered bronze that must rely on a separate lubricating film or coating. A lubricated shaft presents two problems. One is that the bearing pushes the oil along the shaft as it moves, eventually depleting the oil film unless regularly lubricated. In actual practice, bearing lubrication is usually haphazard at best, and the result is shorter bearing life.
The other problem is that an oil film on the shaft acts as a magnet for dust, dirt, and airborne debris. This can clog the bearing or contaminate a product or process, particularly in food or medical settings. Plastic bearings solve these problems by first, requiring no lubrication. Then even under extremely dirty conditions, particles simply embed into the wall of a plastic bearing with little effect on performance.
Plastic bearings offer other advantages as well, including excellent chemical compatibility. Most types resist corrosives such as hydrocarbons, alcohols, and alkaline solutions. Teflon bearings stand up to virtually all chemicals including etching acids. FDA-approved materials permit contact with food and pharmaceuticals.
A few low-cost bearings operate continuously at temperatures approaching 500°F and withstand peaks to 600°F; low-temperature limits are generally to –40°F. Engineered plastics have an even wider temperature range.
Plastic bearings also absorb or damp mechanical vibrations. The so-called mechanical loss factor, an indicator of vibration-damping capability, is up to 250 times higher than that of plain-metal bearings. Consequently, plastic versions typically run quieter, particularly compared with antifriction ball and needle bearings.
While today’s plastic bearings are better than ever, they are not suited for every job. One ongoing problem is that plastic changes dimensions when it absorbs water or changes temperature. This has serious consequences where not properly addressed. Polymer bearings are usually not suited for precision applications unless made from dimensionally stable high-performance plastics or special designs compensate for bearing expansion.
The typical solution for most applications is larger running clearances compared with metal bearings. For example, when specifying 0.001-in. clearance between a metal bearing and shaft, 0.003 in. may be needed with a plastic bearing. Thus, as the latter heats up or absorbs water, there is sufficient room to grow without seizing.
Interestingly enough, the additional play has no effect whatsoever on performance in most applications — a point most engineers find difficult to believe. One reason is that designers often overspecify clearances for metal bearings, particularly in pivot-point and light-duty rotary applications. Another reason is that plastic better absorbs vibration and thus inherently accepts wider clearances. Metal bearings with large clearances often run noisy at high speeds.
Standard plastic bearings are usually built for the same size housings as metal bearings, but with larger shaft clearances. However, wall thicknesses as small as 0.030 in. on some plastic bearings can permit smaller, lighter housings.
Designers also need to be aware that despite better materials, maximum load and speed capabilities do not yet match up with metal bearings. In general, larger running clearances limit rotary speeds to 1,200 to 1,500 rpm, above which the shaft bounces inside the bearing.
Positioning accuracy, not speed, usually limits linear applications. Linear slides, for example, require a little play in all four plastic bearings to compensate for shaft misalignment. Metal bearings provide high accuracy while handling misalignment by holding tight tolerances on one shaft and allowing larger clearances on the other.
Plastic bearings usually have maximum load ratings of 20,000 to 30,000 psi, depending on the application. Higher temperatures reduce load capacity. Metal plain bearings generally carry greater loads and are less sensitive to temperature.
Another drawback is the general inability to accurately predict the life of plastic bearings, although the problem also plagues metal bearings to a certain extent. Researchers have been trying for years without success to establish a universal formula for predicting plastic-bearing life given the material, load, and speed. Most calculations use a K-factor, a measure of wear rate based on a standard laboratory test of metal rubbing against plastic. Unfortunately, there is little correlation with actual bearing performance.
Metal bearings suffer similar shortcomings. Analytical models predict their life with some reliability in a controlled laboratory setting, but results often bear little relation with real-life performance. Changes in operating conditions, lubricants and additives, and even maintenance practices often render predictions useless.
The only sure way to predict life is testing the bearing under applicationlike conditions. Most plastic-bearing manufacturers willingly supply test samples. The good news is that plastic bearings quite often either fail within 24 hr or last for years. But experience plays a major role in selecting the best material and design for a given application, so getting engineering assistance from a bearing manufacturer is almost always prudent. Often there is no one material or style that best suits an application, and selection involves tradeoffs between competing performance characteristics.