Igus BoD PlasticBearings

8/13/2019 Igus BoD PlasticBearings

1/4ponsored by igus October 20131

WITH FORTY YEARS OF EXPERIENCE, IGUS HAS ACOMPREHENSIVE DATABASE ON THE TRIBOLOGIC

PROPERTIES OF POLYMERS.

Plain bearings molded of plastic are oftenan economical replacement for needle, ball,and plain metal bearings. o a certain extent,though, plastic bearings face an uphill battlefor respect among the engineering community.

One reason is the erroneous mind-setamong some engineers that plastic is inferiorto metal. Others cling to the notion that high-priced engineered polymers are a must forplastic bearings. Te fact is, low-cost materialswith excellent strength and thermal propertieslet inexpensive plastic bearings outperformtheir metal counterparts in many rotary,

oscillating, and linear-motion applications.For example, self-lubricating plastic bushingsroutinely deliver a longer service life than oil-impregnated sintered bronze bearings, withcost savings up to 40%. And today, plastic

Why plastic bearingsoutperform metalPlastic bearings cost and weigh less than their metal

counterparts. And they often run longer in harsh

environments and under adverse conditions.

MATERIALS-ENGINEERINGADVANCESletplastic bearings match or exceed theperformance of metal bearings in manyhigh-load applications. And plasticbearings weigh less, need no lubrication,and better tolerate dirt, chemicals, andsalt water.

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PLASTICPLAINBEARINGSareavailable in many designs,

sizes, materials, and colorsto meet wide-rangingperformance demands.

2Sponsored by igus October

MORE THAN 3,500 TESTS ARE PERFORMED EACH YEAR ON M

EVERY iglide BEARING HAS BEEN PROV

WITH iglide, SIMPLY INSTALL

oil out and maintain a full film of lubricant. Shaft oscillation,slow speed, and intermittent use can all inhibit this process.If movement stops, the oil on the bearing surface dries up.Tis can lead to higher friction and squeaking. High tem-peratures can also break down the oil.

Also, a lubricated shaft presents problems. One is that the

bearing pushes the oil along the shaft as it moves, eventuallydepleting the oil film unless regularly lubricated. In actualpractice, bearing lubrication is usually haphazard at best, andthe result is shorter bearing life.

Te other problem is that an oil film on the shaft acts asa magnet for dust, dirt, and airborne debris. Tis can clogthe bearing or contaminate a product or process, particu-larly in food or medical settings. Plastic bearings solve theseproblems by first, requiring no lubrication. Ten even underextremely dirty conditions, particles simply embed into thewall of a plastic bearing with little effect on performance.

Plastic bearings offer other advantages as well, includingexcellent chemical compatibility that bronze bearings cannot

match. Most types resist corrosives such as hydrocarbons,alcohols, and alkaline solutions. eflon bearings stand upto virtually all chemicals including etching acids. FDA-approved materials permit contact with food and pharmaceu-ticals.

Plastic bushings are usually underestimated at hightemperatures. Some low-cost bearings operate continuouslyat temperatures approaching 500F and withstand peaks to600F; low-temperature limits are generally to 40F. Engi-neered plastics have an even wider temperature range.

Plastic bearings also run quietly and absorb or damp me-chanical vibrations. Te so-called mechanical loss factor, anindicator of vibration-damping capability, is up to 250 times

plain bearings are available off-the-shelf in manydesigns, sizes, materials, and even colors to meet

wide-ranging performance demands.Engineers with little experience with plastic

bearings might be reluctant to put an expen-sive machine at risk with low-cost componentsthey know nothing about. Many users onlyturned to plastic bearings out of desperationwhen all else failed. But with growing rec-ognition that plastic bearings often outlastmetal versions, need no lubrication, andoffer the potential to cut size and weight,plastic bearings increasingly are the pre-ferred choice in many applications.

Plastics advantagesPlastic bearings typically consist of a thermoplastic alloy

and solid lubricants with a fiber matrix often added forcreep resistance and strength. Te most common low-costmaterials are nylon, ultrahigh-molecular-weight (UHMW)polyethylene, and eflon. High-performance engineeredplastics such as Vespel, orlon, and PEEK are sometimesused for extremely high loads and temperatures, but thesecan be cost prohibitive. Probably the most significantchange in plastic bearings over the last few years is increas-ingly higher load and temperature capabilities. But the pri-mary advantage plastic holds over metal bearings remains

the ability to operate dry without additional lubrication.All bearing constituents the thermoplastic, fibermatrix, and lubricants have excellent antifriction andlow-wear characteristics that produce a self-lubricating ef-fect. Tis is especially critical at initial start-up. A lubricantfilm has not yet formed and the bearing begins operationdry. Tis can accelerate wear in metal bearings, but plasticbearings homogeneously impregnated with solid lubricantrun lubricated from the start.

As soon as a loaded plastic bearing moves, microscopicbits of solid lubricant and thermoplastic abrade to smooththe shaft surface. Te material fills shaft imperfections andprovides an optimum surface for continuous lubrication.

Tis minimizes slip-stick conditions and wear, andfrequently increases operating life compared with plain-metal, ball, and needle bearings. Dimensional changes tothe bearing are essentially nonmeasurable, and abrasiondecreases rapidly following startup and becomes negligiblein continuous operation. In addition, the fiber-reinforcedmaterials maintain the bushings strength and resistance tohigh forces and edge loads.

Most plain bearings, on the other hand, are oil-filled,sintered bronze that requires a separate lubricating film orcoating. Sintered-bronze bearings rely on capillary actionto create a lubricating oil film. Critically, however, highspeed and rotational motion are both required to draw the
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HEAVY-DUTYQ2 BUSHINGSfrom igus withstand radial

loads exceeding 11,000psi. Tey offer long servicelife in heavy-duty pivotingapplications.

ponsored by igus October 20133

HAN 100 NEW PLASTIC COMPOUNDS.

ILLIONS OF TIMES OVER.

AND FORGET IT.

between competing performance char-acteristics.

Common misconceptionsIn addition to the material, a basic

difference between thin-walled plasticbushings and thick-walled bronze bush-ings is thickness. Te wall thicknesses ofstandard bronze bushings range between0.0625 and 0.156 in. In comparison,plastic bushings have walls typicallyfrom 0.0468 to 0.0625-in. thick.

Tin walls offer some advantages thatlet plastic bushings perform as well as, ifnot better than, thick-walled bushings.For example, thinner walls are betterfor heat dissipation. Heat buildup canaccelerate wear on a bushing, so it iscritical that heat dissipates through theshaft and housing. With thick-walled

bronze bushings, heat buildup is muchmore likely.Better heat dissipation equates to

higher PV values a measure of per-formance capability. Higher PV ratingslet bushings handle higher speeds andloads.

In addition, thin-walled plastic bush-ings can hold tighter tolerances and areless likely to deform, which can be com-

mon with thicker-walled bushings. Finally, thin-walled plas-tic bushings are well suited for applications where weightand fuel economy are an issue. ypical examples include

racing bikes, snowmobiles, automobiles, and motorcycles.Despite the performance advantages, several misconcep-tions may make engineers reluctant to take full advantage ofthe benefits of thin-walled plastic bushings:

1. Durability. One misconception is that thin-walledplastic bushings are not as strong or durable as thick-walledbronze bushings. It is important to remember that the wallthickness of either bushing does not directly correlate to itsstrength. Other more-important factors engineers shouldtake into consideration include the bushings weight, coef-ficient of friction, and wear resistance.

2. Surface pressure. Another mistake is to assume thethin wall of press-fit plastic bushings will affect the surface

higher than that of plain-metal bear-ings. Consequently, plastic versions

typically run quieter, particularlycompared with antifriction ball andneedle bearings.

Predictable lifetimeOne drawback in the past was

the general inability to accuratelypredict the life of plastic bearings.New software programs from someof the major bearing manufacturersnow overcome these concerns. Forinstance, the DryLinand iglideExpert System service-life calculatorsfrom igus Inc., East Providence, R.I.(www.igus.com) are handy tools forpredicting plastic-bearing life undervarious operating conditions.

Users enter data such as theproposed bearing dimensions;maximum loads and exposure toshock or edge loading; whether mo-tion is rotating, linear, pivoting, or acombination of these; speed of mo-tion, and whether its intermittentor constant; operating temperature

range; chemical exposure, the matingsurface; and acceptable limits onbearing wear.

It also asks for details such aswhether lubrication is possible, if thebearing is exposed to dust or othercontaminants, needs to be electrical-ly conductive or insulating, and usedin food processing, outdoors, undervacuum conditions, or exposed toradiation.

Results of the calculations include life in hours and traveldistance for various suitable products. Other results include

wear rate, PV (pressure-velocity) calculations, price/life data,and overall suitability ratings.Calculations are based on an extensive tribological test

database and have been verified with thousands of hours ofactual testing. Results are deemed to be quite accurate.

Of course, experts always encourage testing a selected bear-ing in the proposed application before releasing a machineto the market. Most plastic-bearing manufacturers willinglysupply test samples. But experience plays a major role in se-lecting the best material and design for a given application, sogetting engineering assistance from a bearing manufacturer isalmost always prudent. Often there is no one material or stylethat best suits an application, and selection involves tradeoffs
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PLASTICBUSHINGSANDBEARINGSare

used in gamingseats to handle roll,pitch, and heavemovements. Te wear-resistant bearingsdampen vibrationsand run silentlywithout lubrication.

4Sponsored by igus October

WITH FORTY YEARS OF EXPERIENCE, IGUS HAS ACOMPREHENSIVE DATABASE ON THE TRIBOLOGIC

PROPERTIES OF POLYMERS.

lubricants attract dust and dirt,which can eventually cause bearings

to seize.For instance, a pasta manufacturer

recently replaced V-grooved, track-guidedrollers on its cartoning machines with plastic

plain bearings. Te machines, which operate 24/7, use ashuttle bucket to carry and unload 1-lb portions of pasta. Tebucket travels 18 in., 240 times a minute, to keep up with themachines load station.

Despite the rapid cycling and extreme acceleration, theplastic bearings last more than three times longer than theprevious roller bearings and have reduced annual repair costsby $7,800. And the lube-free bearings cannot contaminate the

pasta or packaging.Replacement, if necessary, takes less than 2 hr in con-trast to the full day of downtime it takes to rebuild just oneset of rollers. And, as an added benefit, the company reportsvibration issues have been eliminated and the machines runmuch quieter.

In another case, an OEM turned to plastic bearings forequipment that packages flour, sugar, and various types of petfood. Te machines operate around the clock and are expect-ed to last 20 to 30 years. o meet the demanding durabilityrequirements, the companys engineers specified plastic linearbearings on guide rods in the machines trimming and press-ing stations.

Te linear bearings aluminum adapter fits over a plasticliner. Te beefed-up construction lets them carry up to thirty50-lb bags/min on each machine, 43,200 times per day.

Te dry-running bearings are unaffected by flour or sugardust that gets stirred up during packaging. Te bearings willnot contaminate the food or get clogged with flour and sugar,unlike bearings that require constant lubrication.

Plastics bearings have excellent strength, good thermalproperties, and need no external lubrication. And the low-cost, lightweight bearings deliver long life despite exposureto harsh chemicals, dust, dirt, and other contaminants. Withadvances in polymer engineering, plastic bearings now outper-form metal bearings in many applications.

pressure. Actually, the surface pressure of a press-fit bushing,typically rated in pounds per square inch (psi), is deter-

mined by the load divided by the surface area it acts on:Ps = L/(D l)where Ps = surface pressure, psi; L = load, lb; D

= inside diameter, in.; and l = bushing length,in. Whether one is using a thin-walledplastic bushing or a thick-walled bronzebushing, wall thickness has no bearingon surface pressure.

3. Life. A third misconceptionis that thin-walled plastic bush-ings have shorter lives than theirthick-walled bronze counter-parts. It seems reasonable toassume that because a plasticbushing has a thinner wall andless material, it will not last aslong as a thick-walled bronze bush-ing. But its wrong. Te thin wall of aplastic bushing better dissipates heat, whichhelps slow wear.

Bronze bushings are designed with thicker walls tocompensate for wear. But even with the added material,the bushing surface still wears under certain conditions. Inhigh-rotation applications, continually relubricating thebushing helps prevent this wear. But if a bronze bushing

sees other types of motion, such as low-speed rotation orback-and-forth oscillation, it may experience inordinatewear that increases clearance between the shaft and bearing.Tis, in turn, can lead to a number of problems, such as aloss of accuracy, higher friction, excessive noise, and prema-ture failure. It is important to remember that wear dependson the bushing material and not wall thickness.

Successful applicationsPlastic bushings have already replaced sintered bronze

bearings in thousands of applications from a wide range ofdifferent industries, including agricultural machinery, lawnmowers, medical equipment, fitness equipment, pumps,and valves.

Teyre especially important in food and packagingequipment that handles sensitive products and must meethigh standards of cleanliness and hygiene. And, of course,every manufacturer and user demands competitive costs andtrouble-free performance.

o that end, engineers are increasingly turning to plasticbearings for packaging tasks ranging from cartoning andfilling, to palletizing, labeling, and inspection. Plastic bear-ings are inherently corrosion resistant and maintenance-free, making them cost-effective replacements for most ballbearings. Tey are also self-lubricating and operate oil-free a major advantage because FDA regulations prohibitmost lubricants for sanitary reasons, and even approved
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