Plane Bearings That Grease Themselves

July 10, 2008
Self-lubricating resins can make plane bearings quiet, durable, and put them on the FDAapproved list.

Dave Biering
Technical Director, Plastics
TriStar Plastics Corp.
Shrewsbury, Mass.

Edited by Jessica Shapiro

Plane bearings are simple: Two surfaces moving against each other without the aid of a rolling element. A shaft turning in a bushing, a slide pad damping vibration, a bridge joint expanding, and a chain running through a channel are all examples of plane bearing. Sleeve or flange bushings, thrust washers, linear-slide plates, friction bearings, and wear pads all fall into the plane-bearing category. If there are no balls, needles, retainers, or races, it’s a plane bearing.

Plane Advantages
In addition to their simplicity and low up-front cost, plane bearings can be made of resin with added lubricants. During operation, the lubricants transfer onto the metal mating surfaces.

Polytetrafluoroethylene (PTFE), graphite, molybdenum disulfide (moly), and silicone are commonly added to the resin for lubrication. Each of these interacts differently with the microscopic structure of the mating surface, but they all extend wear life by lowering friction. Less friction means less heat and ultimately leads to longer maintenance- free periods.

Plane bearings stand up to most industrial and outdoor environments. Bearing grade plastics can be chosen for their resistance to fresh water, saltwater, deionized water, slurries, acids, or bases. Because they have few moving parts, most plane bearings are not adversely affected by particulates like coal dust, quartz debris, sand, and road ballast. Plane bearings are safe for clean rooms, too, because they produce minimal debris and don’t attract dirt. Many plastic bearing materials also meet FDA, USDA, 3A, or NSF standards.

Pressure Performance
When selecting a plastic bearing, consider all aspects of its application. Some design considerations apply to any kind of plane bearing. However, nonmetallic bearings need some special design attention.

All bearings are rated for maximum loads or pressures (P), expressed as psi, maximum speeds (V) in surface feet-per-minute (sfpm), and the limiting product of the two, PV, expressed in psi sfpm. Selecting appropriate bearings based on an accurate P, V, and PV assessment will permit longer maintenance-free operation.

Heat sensitivity
Pay close attention to the temperature of operation. All plastic materials have a maximum continuous service temperature. This temperature, somewhere below the melting point, is the highest at which a material will retain enough physical integrity to continue operating as designed. Material properties can suffer at elevated temperatures, so it’s important to know the maximum transient temperature as well as the normal operating temperature.

Thermal expansion and contraction coefficients of the insulative plastic material are other material properties to be aware of. Plane bear ings count on press fits and running clearances. If thermal expansion isn’t considered, the press fit could be lost or the bearing could close in on the shaft and seize up. Every plastic has a different rate of thermal expansion, so make sure the bearing can maintain its dimensions across the full range of operating temperatures.

Environmental awareness
In addition to temperature, other aspects of the operating environment can affect bearing life. Will the bearing be exposed to dry, abrasive debris? Are acids or alkalis present? Is the environment very moist or wet? Every material has environments where it shines and conditions it cannot withstand, so be sure the material you choose matches the surroundings in which it works.

There can be subtleties to the hardware in which the plastic plane bearing will be used. Plastic plane bearings work best when the dynamic mating surface finish is 12 to 16 RMS. Surface finishes that are too rough saw away at the plastic. On the flip side, very smooth surfaces can actually boost friction and prevent dissipation of frictional heat.

The ability of the metal surrounding the bearing to dissipate heat is also important. Heat is a natural killer of any plastic bearing, so thermal management is a critical design factor.

Material matters
Self-lubricating plane bearings are classified by their base material. The largest subset contains bearings made from filled PTFE materials. Other bearings use common engineering plastics like nylons, acetals, and polyesters with added lubricants. For high-performance applications, a designer may choose to use more-expensive engineering thermosets like polyamide- imide, polyetheretherke tone, polyimide, and polybenzimidazole. Some widely used plastic bearing materials are listed in the accompanying table.

The newest generation of selflubricating plane bearings uses composite materials. These bearings’ self-lubricating resins are reinforced by synthetic fibers. The reinforcement makes these bearings well suited for high-load, slow rotary, and linear applications.

Sweet success
One such application is food production. An ice-cream producer was able to cut bearing change-outs from three times annually to once a year after switching to composite plane bearings for conveyor belt rollers, dashers (the blade that fills the ice cream molds), and other production-line components. The bearings’ inert nature makes them compatible with both the stainless-steel and aluminum shafts on the line.

The bearings retain their dimensions and low friction in the frosty confines of ice-cream production, even down to cryogenic temperatures. Their high strength and wear resistance, in addition to their self-lubricating properties, help them beat the industry lifespan average while eliminating the need for expensive food-grade grease. For this application, doing away with the food-grade grease saved $18,000 annually on a single machine.

As in any food production, hygiene and sanitation are of chief concern. The composite plane bearings absorb minimal moisture despite the daily high-pressure steaming needed to clean and sterilize production-line machinery.

Lights, Camera, Action
Composite bearings have also made their way into the entertainment industry. A manufacturer of camera and sound equipment for TV and movies uses composite bearings to ensure smooth movement.

Boom-mounted cameras often attach to large, heavy dollies that run on tracks to follow the action. To get a steady shot, the pivot points on the camera boom arm must move smoothly while supporting an extremely cantilevered load. Microphone booms are subject to similar stresses, with the added requirement that all movement be virtually silent.

The composite plane bearings in the booms can carry up to 15,000 psi and handle speeds up to 400 sfpm without lubrication. These capabilities translate to lowmaintenance, long-life equipment for this demanding application.

Make Contact
TriStar Plastics Corp.

Composite bearings like Tri-Star’s Ultracomp line pair selflubricating resin with reinforcing synthetic fibers. The bearings operate well with low speeds and can tolerate heavy loads.

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