belt conveyor idler life

T - SU553 IDLER LIFE

Technical Paper T - SU553

BELT CONVEYOR IDLER LIFE;FACTORS AND CONDITIONS

By Bob Domnick, P.E.

CONTENTS

ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2IDLERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2IDLER LIFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3THEORETICAL LIFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4FACTORS THAT AFFECT IDLER LIFE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

Bearing Style . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5Bearing Style - Load . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6Bearing Style - Impact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Bearing Style - Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Bearing Style - Misalignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9Bearing style - Cost . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10

SEAL EFFECTIVENESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .10LUBRICATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13ROLL CONSTRUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14IDLER FRAME . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15MAINTENANCE AND ENVIRONMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16CONCLUSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .16

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ABSTRACTTransporting bulk material from one point to another is an age-old processwith methods that have evolved from primitive and costly to advanced andeconomical. Today, modern belt conveyors can continuously transport materi-al through the toughest terrain, in the most severe environments, and at ratesthat cannot be attained via a batch process. By transferring material from oneconveyor to another, the length of transport is unlimited. As such, the numberof required conveyor components is also unlimited.

Continuous material flow is much like an assembly line. As any manufacturerknows, just one small process on the line can delay the final product. Similarly,the malfunction of just one conveyor component can delay all material pro-duction. The reliability of each conveyor component is vital. The idler (a roll orseries of rolls that support the belt) is one such component, and as idlers arepresent along the entire length of the conveyor, they demand close scrutiny.

How long can an idler supportand protect a belt? That canonly be estimated after a carefulexamination of just what factorsaffect idler life and how thesefactors apply to a producer'sapplication. Pertinent factorsinclude: idler class, bearingstyle, seal type, lubrication, rollconstruction, maintenance andenvironment. A detailed look ateach is required to maximizeidler life and to choose the bestidler type for a given application.

IDLERSAn idler is a roll or series of rollsthat supports and protects theconveyor belt. A troughing idler,which is the most common idleron high capacity belt conveyors,is mounted on the material car-rying side of the conveyor andgenerally consists of three equallength rolls. The two outer rollsare inclined upward while thecenter roll is horizontal Figure 1.The rolls are mounted to aframework that attaches directlyto the conveyor frame. The beltfollows this geometry and aneffective profile for conveyingbulk material is formed. Typicalconveyed materials include butare not limited to sand and grav-

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el, coal, limestone, crushedstone, grain, fertilizer, salt, andwood chips.

There are four classifications ofidlers defined by the ConveyorEquipment Manufacturer'sAssociation (CEMA). They areclasses B, C, D, and E Table 1.The classes are defined byenvelope dimensions, loads, rolldiameters, belt widths, and L10requirements (defined later).

The roll is the heart of an idler.In its most basic form the rollconsists of a cylindrical shellthat rotates concentric to ashaft Figure 2. The shaft is sup-ported on either end by the idlerframe. A housing is attached toeach end of the roll that accom-modates a bearing and sealarrangement. This housing iscalled an enddisc. As the con-veyor belt moves across theidler, the roll and outer bearingrace turn while the shaft and theinner race remain stationary.Typically there is a portion of theseal that rotates and a portionthat remains stationary. It is theroll or its components that usu-ally fail first.

IDLER LIFEIdler life is defined as the length

of time during operation in whichan idler effectively supports andprotects the conveyor belt. It can also be stated as the amount of time operat-ing between idler installation and idler failure. An idler has failed if any one ofthe following conditions has occurred Figure 3:• a roll has stopped turning• a roll has a hole worn through its shell• the enddisc has separated from the shell• the bearings are squeaking at an unacceptable decibel level• any portion of the frame has failed• the deflection of any frame component is so great that it limits the idler's

usefulness.

This paper will describe the conditions and factors that influence idler life.

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THEORETICAL LIFEIdler life is affected by many

variables. However, bearing rat-ing is the only variable for whichlaboratory tests have providedstandard values. ThereforeCEMA uses bearing L10 life asa guide for establishing idler rat-ings. The CEMA idler selectionprocedure does provide chartedguides that can be used to mod-ify the L10 life into a more prac-tical estimate of idler life.

L10 life for an idler is defined asthe basic rated life (number ofoperating hours at 500 rpm)based on a 90 percent statisticalmodel which is expressed as thetotal number of revolutions 90percent of the bearings in anidentical group of bearings sub-jected to identical operating con-ditions will attain or exceedbefore a defined area of materi-al fatigue occurs on one of itsrings or rolling elements. TheL10 life is also associated with90 percent reliability for a singlebearing under a certain load.Spalling, or material fatigue, isthe chipping or flaking of thecontact surface of either thebearing race or the rolling ele-ments. Fatigue develops gradu-ally over the life of the idler bear-ing as its parts repeatedly exertpressure on each other duringrotation. Eventually small, sub-surface cracks form and grow insize until spalling occurs. This isconsidered the full life of thebearing and is equivalent to"dying of natural causes."

The L10 formula comprises thedynamic load rating of the bear-ing, the radial load, the rpm, andthe bearing style. The allowableload and rpm are specified byCEMA. Bearing style correlates

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to the formula only by essenceof the elements being either ballor roller. The bearing dynamicload rating is the constant radialload a bearing can endure forone million revolutions and ispublished by the bearing manu-facturer. In many cases theseratings were based on tests per-formed many years ago. Bearingadvancements include improve-ments in alloys, material pro-cessing techniques, manufac-turing, design and tribology(interaction between bearingsurface topography with lubri-cants and debris). In responseto these advancements, manymanufacturers have contrivedincreased life factors to be applied to the published dynamic load ratings.

L10 life is most useful in bearing selection for a relative comparison betweendifferent bearings. Actual idler life may have very little correlation to the L10 life.Very seldom does an idler fail due strictly to bearing fatigue. Therefore it is soimportant to look beyond theoretical life and consider the other factors that affectidler life. After all, actual idler life is all that interests the customer.

FACTORS THAT AFFECT IDLER LIFEBearing StyleThere are primarily two styles of bearings used in an idler: tapered roller and

ball bearings. It is not sufficient to state that one is better than the other sincethe considerable difference in design produces advantages or disadvantagesbased on application.

The tapered roller bearing is made up of a cup, a cone, rolling elements, and acage Figure 4. The cone is inserted into the cup in a way that sets the axialclearance, or end play, of the bearing. The extensions of the raceway and rollersconverge at a common point on the axis of rotation resulting in true rollingmotion Figure 5. The angled raceways allow the tapered roller bearing to carryboth radial and thrust loads. The tapered configuration also ensures roller align-ment by generating a seating force Figure 6. The cage provides proper spacingof the rollers.

Deep groove ball bearings are made up of an inner ring, an outer ring, rollingelements, a cage, and seals Figure 7. This bearing is suitable for moderate radi-al loads. It is also suitable for moderate thrust loads because the inner and outerrings are manufactured with a deep groove raceway. The bearing is factoryassembled with inseparable rings that fix the radial clearance. The cage pro-vides proper spacing of the balls.

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It is important to furtherdescribe internal clearance.Bearing internal clearance is thetotal displacement of the innerring to the outer ring under noload conditions. Internal clear-ance permits interference fits onthe bearing rings without caus-ing preload, allows unequalthermal expansion of the rings,and accommodates misalign-ment resulting from shaft deflec-tion. It is called radial clearancewhen the displacement is in theradial direction and axial clear-ance when the displacement isin the axial direction. It is essen-tial that an internal clearanceremain during operation. Aninterference fit between therolling elements and the ringswill produce excessive heat thatwill reduce the life of the bear-ing. Ball bearings are producedwith different classes of radialclearances per bearing size witha guaranteed minimum andmaximum clearance. Axial clear-ance in a tapered roller bearingis set by the idler manufacturerwhen the cup and cone areassembled. The consistency ofthis setting is subject to themanufacturing capabilities of theidler manufacturer.

Bearing Style - LoadThe idler load consists of the

material weight, belt weight, rollweight, impact from lumps ortransfer points, and belt tensiondue to vertical misalignment ofthe idlers Figure 8. The majorityof the load, approximately 70%,is supported by the center roll ofthe troughing idler. This load isused to select the idler class inaddition to calculating the L10life of the bearings.

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The tapered roller bearing hasrolling elements that result inuniform load distribution along aline of contact created by theroller and the raceway Figure 9.Long line contact gives thetapered roller bearing a highload carrying capacity for bothradial and thrust loads. The linecontact also allows the roller tobridge raceway defects.

The ball bearing has compo-nents that result in point contactFigure 10. Because of the pointcontact the load capacity islower than a tapered roller bear-ing of the same size. All otherconditions being equal thisresults in the ball bearing having a lower L10 life than the tapered roller bear-ing.

CEMA idlers meet the minimum L10 life requirement regardless of bearingstyle. Since less than five percent of bearings fail by fatigue it is no surprisethat the benefit of added L10 life for the tapered roller bearing is often disre-garded. Load carrying capacity is significant but must not overshadow theother factors affecting idler life.

Light idler loads can present a problem.The load must be large enough to turnthe rolling elements and overcome seal friction. The combination of a smalldiameter roll, an empty belt, and a roll with high rolling resistance may resultin a roll that will not turn. The point contact of ball bearings results in low fric-tional forces for the load to overcome. A tapered roller bearing with its line con-tact results in high frictional forces. Additional grease retaining seals used in atapered roller bearing idler add to the force needed to turn the roll.

Bearing Style - Impact Because of the line contact tapered roller bearings are more suitable for

impact loading. The point contact of ball bearings has more of a tendency todamage the raceway or the balls when impacted. The geometry of the taperedroller bearing ensures that the impact load is distributed evenly across the faceof the roller or raceway. Therefore, tapered roller bearing idlers are often rec-ommended under crushers and transfer points.

Bearing Style - SpeedConveyor belts move in a speed range of zero to in excess of one thousand

feet per minute. Therefore it is prudent to consider the effect of speed on theidler bearing. There is a limit to the speed at which a bearing may be operat-ed. Generally, it is the operating temperature that can be permitted withrespect to the lubricant being used or to the material of the bearing compo-nents that sets the limit. Friction, which generates heat, is the resistance to

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movement. The total resistanceto rolling is made up of therolling and sliding friction in therolling contacts between rollingelements and cage, as well asthe guiding surfaces for therolling elements or the cage, thefriction in the lubricant and of thesliding friction of rubbing seals ifpresent. The speed at which thelimiting temperature is reacheddepends on this frictional heatand the amount of heat that canbe transferred away from thebearing.

Lubrication type, lubricationamount, bearing style, level ofmaintenance, environmentalconditions, shaft to bearing fit,radial or axial clearance, andstyle of seals all have a signifi-cant impact on the total rollingresistance. There are low tem-perature and high temperaturegreases available. Bearingsmay be over-greased and forcethe rolling elements to pushthrough excess grease leadingto sharp temperature rises.Poor maintenance can lead toover-greasing or insufficientlubrication. Some environmen-tal conditions such as highambient temperatures or pres-ence of water can lead to pre-mature grease breakdown. Aloose fit can cause creep or slipwhich leads to a temperaturerise. A tight fit may cause adamaging radial clearance orpreload. Tight seals can causeunnecessary temperature rises.The allowable bearing speed isdetermined by these factors.

The highest speeds can beachieved by deep groove ballbearings. The point contact ofthe rolling elements, the con-trolled grease fill, and the inde-

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pendence of varying mainte-nance practices aid in makingball bearings a consistent high-speed performer. The taperedroller bearing can also operateat these speeds, but the higheroperating temperature requiresclose attention to maintenance,idler seal design, grease quanti-ty, and grease type.

Bearing Style - MisalignmentAngular misalignment between

the shaft and enddisc occurswhen the shaft deflects underthe operating load Figure 11.Misalignment can also occur ifthe bearing seat of the enddiscis not level. If the enddiscs arenot welded in parallel to eachother, misalignment is intro-duced. Good manufacturingprocesses will reduce this risk.

The shaft slope is typicallydetermined in radians or inchesof deflection divided by inchesof length. There are two vari-ables (other than load) thataffect the shaft slope; the shaftmoment of inertia and themoment arm. The moment ofinertia is a cross sectional prop-erty that quantifies stiffness. Themoment arm is the distancefrom the shaft support to theeffective load center of the bear-ing. For the same bearing loca-tion the moment arm for atapered roller bearing Figure 12is smaller than the moment armfor the ball bearing Figure 13due to the line of action of theforce. This moment arm shouldbe minimized to avoid loss inL10 life. The most successfulmeans to achieve the shortmoment arm is a shallow end-disc with a compact seal config-uration.

Ball bearings are praised fortheir ability to tolerate up to 16minutes or .005 in/in of mis-alignment while maintainingnormal contact Figure 14.Misalignment causes edgecontact in tapered roller bear-ings severely reducing bearinglife Figure 15. Early designs oftapered roller bearings couldonly tolerate 3 minutes or .0009in/in. The flat tapered roller andflat raceways gave way to mod-ern designs utilizing edgecrowning. If the cup and coneare misaligned on a flat taperedroller the stress is concentratedon one end of the roller Figure16. When the tapered roller and

raceway are crowned the stress will be evenly distributed along the length ofthe roller Figure 17. Edge crowning allows up to 7 minutes or .002 in/in of mis-alignment. Misalignment beyond the aforementioned values derates the L10life similar to the curve shown Table 2. Each bearing has a unique curve, butthe relationship between tapered roller bearings and ball bearings is consis-tent.

Bearing Style - CostPower consumption is an important cost consideration when selecting idlers.

The friction caused by rolling resistance increases the amount of powerrequired to move the conveyor belt. The CEMA Kx factor designates the over-all drag force and is comprised of the sliding friction between the belt and theidler rolls and the frictional rolling resistance of the roll. The CEMA Ai value isthe force required to overcome frictional resistance and rotate idlers. Ballbearing idlers have a significantly lower Ai value than tapered roller bearingidlers. On long horizontal conveyors this can represent noteworthy savings inpower consumption as well as increased life of associated drive components.

The bearing price tag is an obvious cost consideration. Typically a ball bear-ing can be purchased at a lower cost than a tapered roller bearing. It is easyto assemble requiring less labor than a tapered roller bearing. A sealed for lifeball bearing can provide an excellent cost saving by eliminating maintenancelabor, grease expenditure, and grease piping for single point lubrication.

Tapered roller bearings are usually more cost effective in terms of dollars perL10 hour. This measure becomes less meaningful considering less than fivepercent of bearings reach theoretical life. However, designers often specifyincreased idler spacing for tapered roller bearing idlers, thereby reducing cap-ital costs. The idler can withstand the increased load and still have an L10 lifeequal to a ball bearing idler. Therefore the entire system must be explored todetermine which idler will provide the greatest cost advantage.

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SEAL EFFECTIVENESSIdler roll seals are responsible

for retaining grease in the bear-ing cavity, excluding contami-nants, maintaining low rollresistance, and maximizingwear life. Ball bearing idlers relyon the seal attached to the bear-ing itself as well as the seal con-figuration provided by the idlermanufacturer. Tapered rollerbearing idlers rely only on theidler manufacturer's seal.

It is necessary to understandthe importance of excludingcontaminants. Despiteadvances in sealing and lubrica-tion methods, enough contami-nants still enter the bearing cav-ity to cause fifty percent of allbearing failures typically at afraction of the calculated L10life. If dust and dirt reaches thebearing, it will mix with the lubri-cant to form an abrasive com-pound that causes wear on thebearing. If water contacts themetal parts of the bearing, aniron oxide is produced thatforms a grinding compound thatalso causes wear on the bear-ing. Contaminants can be largeenough to make dents in thecontact surfaces of the race-ways and rolling elements. Thissignificantly reduces the life ofthe bearing.

Idler manufacturers have devel-oped an assortment of labyrinthseals to aid in excluding con-taminants. The labyrinth com-bines a tortuous path with cen-trifugal forces that trap andremove dirt and water. The cen-trifugal force creates a flingingeffect between the rotating andstationary surfaces that helpsremove particles. In verticallabyrinths the path extends radi-

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ally and effectively utilizes cen-trifugal forces to exclude con-taminants by making it difficultfor the particles to move axiallyFigure 18. In horizontallabyrinths the path extends axi-ally and utilizes centrifugalforces to exclude contaminantsby making it difficult for the par-ticles to move radially Figure19. Horizontal labyrinths canbe easily assembled and usual-ly require fewer componentsthan a vertical labyrinth. Aneffective vertical labyrinth canbe created in less distancethereby minimizing the momentarm. Double, triple, or multiplelabyrinths simply describe thenumber of paths present in thelabyrinth (Figure 19). Alabyrinth seal is a non-contactseal. Consequently it does notlimit the speed of the bearing.

A lip seal is a contact seal thatis often incorporated into theseal configuration of an idlerFigure 20. Lip seals are madeout of a variety of materials andhelp to minimize the dirt orother air-borne abrasives thatcan contaminate the bearinglubricant. The seal can bearranged to exert a force radial-ly or axially against a movingsurface usually depending onthe labyrinth style. A lip seal isvery effective at excluding con-taminants but has some draw-backs that must be considered.First of all, lip seals typicallyhave a short service life. Overtime the lips will wear and losecontact and allow contami-nates, humid air, and moistureinto the bearing cavity. Next,the lip exerts a spring forceradially or axially on the adja-cent seal component. Thisresults in added power require-

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ment just to turn the idler roll. This can be substantial on long conveyors.Lastly, there is a limit to the lip sliding speed. The lip seal is generally veryclose to the bearing cavity so the heat caused by the friction of the lip seal canincrease the running bearing temperature. High bearing temperatures breakdown lubricant and ultimately reduce bearing life. These drawbacks shouldnot minimize the value of excluding contaminants. There are applications inwhich the use of lip seals is imperative. Also, modern lip seals have beendeveloped that work longer and more effectively with narrow contact bandsand minimal radial loads that are uniform over time.

Ball bearings are often manufactured with seals, called closures, integrateddirectly into the bearing. Closures can be simple trash shields or rubber lippedcontact seals. Elaborate closures incorporate multiple lip seals and a greasefilled cavity that protects the inner lip seal Figure 21. The idler manufacturer'slabyrinth seal configuration in combination with the closures creates a formi-dable barrier against contaminants.

LUBRICATIONAll bearings require lubricants. The use of grease lubrication is prevalent in

the idler industry. Grease is easily retained in its cavity and is effective in seal-ing against moisture and particles. Lubricants have four roles:• to prevent the direct metallic contact between the rolling elements, race

ways and cages• to prevent bearings from corrosion and wear• to prevent ingress of contamination• to cool the bearing

Up to thirty percent of all bearing failures are a result of improper lubrication.Over-greasing forces the rolling elements to push through excess greaseleading to temperature rises. Under-greasing will allow the direct contact ofmetallic surfaces. Chemical attacks or thermal conditions can decompose orbreak down the lubricant. Low temperature greases or the mixing of incom-patible greases may not provide adequate viscosity.

If a bearing cannot operate without grease, then grease retention becomesan important consideration. The pumping action caused by the wide surfaceand line contact of the roller on a tapered roller bearing forces the greaseaway from the rolling elements Figure 22. The only way to replace it is toregrease the idler. The grease is retained by the idler manufacturer's seal.This seal is usually too far away to hold the grease close to the tapered rollers.Ball bearings travel a narrow path with only point contact, so very little greaseis displaced Figure 23. The bearing manufacturer's seal is effective in hold-ing the grease in close contact with the rolling elements.

For many years, the industry demanded a CEMA C, D, and E series idler thatcould be relubricated. However, the use of factory sealed non-relubricated ballbearings in idlers has grown in popularity. Even when tapered roller bearingidlers are required there remains the desire for a sealed-for-life idler. Thisdemand has prompted the use of factory sealed tapered roller bearing idlerswhere effective grease retention is critical, yet difficult to attain.Idlers that can be relubricated have non-purgeable and purgeable seal con-

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figurations. A purgeable seal-ing system allows the user topurge contaminants out of thebearing cavity. However, thegrease mixed with the air pock-ets causes the labyrinths to"breathe" allowing contami-nants to be drawn into the bear-ing cavity. Proper maintenancebecomes critical with thisdesign. Non-purgeable sealingsystems are designed to lubri-cate the bearing, not to act as aseal against contaminants.They also restrict the exit ofgrease from the cavity. Singlepoint lubrication is effective fornon-purgeable designs, but it isimpossible to purge all six bear-ing cavities and labyrinth sealsfrom one point on a purgeabledesign. Purgeable sealsrequire lubrication approxi-mately every 2000 hours whilenon-purgeable seals can oftenoperate at 10,000 hour greas-ing intervals.

Grease selection is important.A wide temperature range, min-eral oil-based grease isrequired for a standard idlerapplication. Some applicationsrequire special grease. High orlow ambient temperatures, thepresence of chemicals, and therequired reliability may necessi-tate special grease.

ROLL CONSTRUCTIONThe only rotating element

affecting idler life yet to consid-er is the roll itself. A roll with ahole worn through its shell orthe separation of the enddiscfrom the shell constitutes anidler failure to the same extentas a seized bearing. There areno CEMA standards for rollconstruction other than rolldiameter.

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The diameter of a roll and itsshell thickness can drasticallyaffect idler life. Selecting a larg-er diameter roll reduces the rpmof the bearing as well asincreases the wearable surfacearea of the roll. For abrasivematerials a thicker shell will pro-vide longer life at only slightlyhigher costs.

Roll concentricity is important inproviding smooth running andan even wear pattern for rollsand belt. A roll that does notrotate concentric with the shaftwill introduce vibration within thebearing that will decrease bear-ing life. Extreme cases canaggravate belt training which in turn can subject more load and higher belttensions to one roll.

A very controversial issue in the industry is the method of attachment of theenddisc to the shell. Some idler manufacturers butt the enddisc up against theroll end and weld a fillet weld around the roll Figure 24. Some manufacturerscounterbore the end of the roll, inset the enddisc inside the roll, and weld a fil-let weld around the inside of the roll Figure 25. Companies that manufacturethe inset enddisc (Figure 25) purport that the belt will abrade the exposed fil-let weld (Figure 24) and cause the enddisc to separate from the shell prema-turely. They also claim that the exposed fillet weld will wear the bottom coverof the conveyor belt. These allegations have initiated much marketing hypethat has needlessly confused idler consumers. Very seldom will the belt trav-el to the edge of an outer roll. It is common to find paint still adhering to theroll on the edge of the outer roll where the exposed weld is located Figure 26.The radius of the belt at the junction of the center roll and the outer roll is usu-ally sufficient to bridge the two exposed welds if the idler roll gap is minimalFigure 27. The drawback to the inset weld (Figure 17) is that some wear thick-ness is removed in the counterbore process. If the shell did wear in this areait would reach the enddisc more swiftly than the exposed fillet weld roll.Ultimately, either design is effective since the wear at the enddisc attachmentis minimal.

The roll material can also affect idler life. Steel rolls are regularly construct-ed using electric resistance welded tubing. It provides excellent wear for mostapplications. For tough applications of corrosion, abrasion, and severe mate-rial build-up there are several roll materials available: urethane, ceramic, poly-ethylene, and rubber discs.

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IDLER FRAMEThe idler frame has less effect on idler life than the rotating components. The

frame must adequately support the rolls without excessive deflection. Therolls must be retained in the frame at all times. The frame must be built with-in CEMA dimensional tolerances such that an adjacent idler is not overloadeddue to idler misalignment. The roll shafts must be affixed in the frame in sucha way that shaft rotation is resisted. The idler must be adequately secured tothe conveyor preferably with the ability to double bolt mount the idler on larg-er sizes. Smaller idlers only require single bolt mounting which offers savingsthrough fewer holes in the idler, fewer bolts, and fewer holes in the conveyor.

MAINTENANCE AND ENVIRONMENTPoor conveyor maintenance can lead to idler failures. If idlers are not lubri-

cated at correct intervals it can accelerate bearing failure. If a failed roll is notreplaced the adjacent idler may be overloaded. If material spillage has notbeen cleared away from the idler it may suffer additional wear. Idlers that areout-of-square are subjected to more friction and additional wear. Idler instal-lation resulting in idler misalignment can cause premature bearing failure.

Some environments are detrimental to idler life. Dusty applications presentmore frequent contamination problems. Wet applications can severely reducebearing life since most seals cannot exclude water. Acidic material can cor-rode and rust the frame, roll, and bearings. High ambient temperatures cansignificantly increase bearing operating temperature and reduce bearing lifeand lubrication viscosity. It is important to know the environment of the appli-cation in order to select the proper idler options that maximize idler life.

CONCLUSIONIn conclusion, the length of time an idler will support and protect a belt can

only be estimated after careful consideration of the factors affecting idler lifeand how they apply to the application. The perfect idler would operate for thefull L10 life of the bearing and then all the components would fail at the sametime. Realistically, our estimating tools only allow us to select the idler com-ponents that will maximize idler life for a given application. According to theapplication, consider idler class, bearing style, seal type, lubrication, roll con-struction, maintenance and environmental conditions. Knowledge of howthese criteria relate to idler life is crucial if the perfect idler is ever going to beobtained.

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NOTES:

belt conveyor idler life

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