Takeups ... the Ins-And-Outs

8/3/2019 Takeups ... the Ins-And-Outs

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TTT AAAKKK EEE UUUPPP SSS ttthhheee III nnnsss---aaa nnn ddd ---OOO uuu ttt sss Outside of the drive and belt considerations, no aspect or decision is more important to asuccessful conveyor or elevator operation than that of the takeup. The type of takeup, its weight(or tension inducement ), location, and movement all contribute heavily to a reliable and efficientmaterial handling outcome.

Note: Although various takeup aspectsdiscussed in this document apply to elevators

and package handling conveyors, the primaryfocus of this writing is on bulk material handlingconveyors.

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Takeups are mechanisms that provide apredetermined tension to the conveyor belt at a fixedlocation along the conveyor path. In doing so, thetakeup provides three primary functions vital to theoverall success of any bulk material handlingconveyor:

Accommodate changes in belt length compensates for variations in belt length (ie, bothstretch and shrinkage) resulting from inherent tension fluctuations (reflecting the elongationcharacteristics of the belt itself including permanent, elastic/dynamic, and constructional)

Slip control maintains necessary tension at the slack side of the drive (KT E) to preventthe belt from slipping at the drive pulley (ie, belt slip)

Sag control maintains proper belt tension (T SAG), all along the conveyor path, to preventexcessive belt sag between adjacent idlers (which minimizes unwanted power losses,excessive tension spikes, and possible material spillage issues)

Besides those three main functions, the conveyor takeup can also provide for:

Live storage accommodating additional belt length, for immediate resplicing needsand/or for any anticipated conveyor extensions

Shock absorption minimizing belt stress spikes during startup or braking (in someextreme and demanding applications)


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Fundamentally, there are two basic types of takeups designs:

Fixed or manual These takeups are typically tensioned either through a simplemechanical screw arrangement (as in sketch below), or through winch activation. By far,the most common fixed takeup is the screw variety with the takeup pulley bearings

mounted on guides, and moved manually by simple screw adjustments. In a winch takeup,the takeup pulley is activated orpositioned by a pre-tensionedcable possibly through a ratchetor gear/rack arrangement.Winches tend to accommodatemore takeup travel than screws.

Being fixed in the conveyorprofile, such takeups must bepositioned to sufficiently over -

tension the belt. Doing so willallow for proper conveyance whileaccommodating expected beltelongations both permanent and elastic . Due to inherent mechanical and practical aspects,fixed takeups are typically limited to the shorter-centered and lower tension (and horsepower)conveyor applications often those having no more than 300 centers, or 75 HP motors.Fixed/manual takeups are also found on conveyors with space limitations.

Note: The higher stresses necessary to properly accommodate fixed takeups cancontribute as much as 60% or more belt tension than required with an equivalentautomatic takeup. However, these additional tension influences are seldom of any

consequence (to the belt, splice, or conveyor components) due to the limited tension andhorsepower demands of the smaller conveyors that typically utilize such simple takeups.

Automatic These takeups maintain proper tension, throughout the entire belt (for slip andsag control), by automatically applying apredetermined tensile force to the belt (as from massW in the adjacent sketch) at the designated takeuplocation. By design, these minimum belt tensionlimits are maintained at all times regardless of anyfluctuations in belt length (stretch or shrinkage),operating modes (startup, shutdown, and steady-state),

or changes in load conditions (empty, partial, andfully loaded). As such, automatic takeups are thepreferred takeup design and are typically utilizedwhenever it is possible and practical to do so.

Automatic takeup pulleys , themselves, are most oftenactivated by gravity. Although vertically -actinggravity takeups is a most common design, horizontal gravity takeups are also used.


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Besides the more common passive gravity-type takeup arrangements, there are also a numberof active takeup configurations to choose from. These more complicated arrangements most of which rely on either hydraulic, electric, power-winch, and even pneumatic (forsmaller conveyors) activation can be practical takeup choices when confronting spacelimitations, and frequent extension or relocation possibilities (common in undergroundmining). They can also be an ideal design choice when there is a need to control orprogram differing takeup tensions at various times throughout the conveyor operating cycle.In so doing, overall belt tensions ( and possibly the belt rating) are often reduced.

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With bulk material conveyors (with troughed carry idlers, etc), takeups are simply not practical alongthe carrying strand. Therefore, these takeups must be either located at either one of the terminal regions (load or discharge points) which tend to be convenient choices or somewhere intermediate along the return strand. The more likely options for a takeup location are detailedbelow:

Immediately following the drive With automatic takeups, particularly when the drive isat the head/discharge pulley (as illustrated in the drawing below), this is the preferred locationand clearly is the mostcommon. Havingsuch close proximityto the drive, thetakeup is ideallypositioned to maintainthe required tension inthe belt exiting the

drive fast enough and sufficient enough to properly address all possible drive slip and beltsag concerns.

At (or near) the tail pulley On conveyors with fixed takeups, or with someregenerative/downhill conveyors, or for suitable structural reasons, or for simply moremaintenance access, this takeup location may be chosen. On a high demand slope belt, atail takeup location will typically result in lower counterweight requirements (a bonus featurewhen securing the takeup mass for splicing purposes).

At (or near) the head/discharge pulley With regenerative/downhill conveyors, and thedrive most likely in the tail/loading region, this takeup location requires the least overall

takeup mass (either running empty or loaded) and will sometimes provide a moreaccessible/maintenance location.

Intermediate Takeups are sometimes located along the return belt strand, somewhereintermediate between the terminal (head/tail) regions. Besides possibly fulfilling someunusual slip or sag requirement, takeups in these locations may also possibly be satisfyingsome accessibility/maintenance or structural concerns.


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The takeup, regardless of its location, must provide enough pounds -weight or mass toaccomodate the calculated belt tensions (pounds- force ) at that location. Although belt tensions arevirtually unaffected, each different takeup location and configuration will likely have a different counterweight mass requirement. Startup and shutdown/braking forces also need to be addressed assuring that the primary slip and sag requirements, along with available takeup movement, is

not in any way compromised throughout these critical operating modes.

FDAs coveted Belt Wizard computer program, in its Take-Up routine, makes thesecounterweight/mass calculations.In the adjacent application example,minimum slip (9431 lbs) and sag(4889 lbs) tensions are noted. BeltWizard then applies the greater of these two values to the takeuplocation (along with an estimatedweight for the takeup itself), resulting

in the counterweight value (19,850 lbsof mass ) as shown.

Note: As with most every calculation in Belt Wizard , takeup counterweight values are based onthe CEMA model. For the automatic takeup calculations, a standard three-pulley, vertical gravitytakeup configuration is assumed. Differing takeup arrangements and locations (eg, horizontal,return strand, etc), and alternative methods of delivering the required counterweight (eg, dead -weight, wire rope and multiple sheave designs, etc) will surely require different mass values.Because of these major modeling assumptions, caution must be used whenever referencing thecwt/mass values in Belt Wizard outside of FDA!

CEMA is a conservative belt calculation model. Having slightly less counterweight mass thanBelt Wizard calculates will probably not cause any operational (slip or sag) issues. Having morecwt/mass than actually calculated is most likely wasteful usually adding unwanted additionaltension and hardship to the belt, the splices, and to all the conveying hardware components. Suchexcess takeup/system tension can also lead to potentially hazardous safety issues!

Bottom Line Any temptation to increase a calculated or existing takeup weight should not bearbitrary one. It should only be done after careful scrutiny, having explored all other availableoptions!!

RR eeccoomm mm eenn dd eedd MM oo vveemm eenn tt / / TTrr aa vveell The recommended takeup movement, which reflects a minimum acceptable value, is based on anumber of criterions. Critical amongst those are:

Average belt operating tension Belt modulus specifically addressing the belt length changes due to inherent elastic stretch

(from dynamic cycling), permanent stretch (non-recoverable), and constructional stretch(from weave or cord design) characteristics


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Acceleration (startup) and deceleration (shutdown) forces Takeup location

Once again When referencing Belt Wizards Take-Up routine, we find that this computerprogram addresses each of the above contributors as it provides an estimate of the Minimum

Recommended Takeup Travel required.

Note : Using the sameapplication example ason the previous page, theminimum recommended Total travel value wascalculated to be 22 ft.

There are also other influences that might contribute to any takeup travel consideration. These, aslisted below, are not directly addressed by Belt Wizard :

Position of the takeup pulley during the final splice or whether there is a run-in periodplanned after initial startup

Thermal expansion/contractioninfluences on the belt carcass which can be considerable withsteel cord belts on long-centeredoverland conveyors (as was truein the adjacent application)

Frequency of loaded belt startupsand shutdowns a highfrequency might add to beltstretch estimates and spliceconcerns

Any live-storage of additionalbelting for emergency,resplicing possibilities

With automatic takeups, and situations where there simply is not enough room to accommodate theminimum travel needed, either of the two following options might provide a possible solution:

1. Double-Reeving the takeup This variation of the standard single -reeved takeup utilizesTWO takeup pulleys rather than one. In so doing, only half as much takeup movement or travel is required. Although belt tensions do not change, the actual takeup weight (or mass)needs to be doubled in this two-takeup pulley configuration!

2. Incorporating a fixed takeup In addition to an automatic takeup, a fixed/manualtakeup could also be utilized. Employing such a fixed takeup, often involving moveable tailor head pulleys, could considerably increase belt storage capacity (eg, for lengthypermanent belt elongation, emergency/splicing needs). Utilizing two separate takeuparrangements has also been a proven solution on long overland conveyors where space-restrictions inhibit adequate takeup accommodation.


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Takeups provide unique conveyor challenges. Their original design and subsequent maintenancepractices play an important role in the overall efficiency and success of a given bulk conveyingapplication:

Design During the conveyor design/engineering phase, much can be done to the takeup

to make the belt more comfortable as it later cycles around the conveyor profile: Minimize pulley-flexing Takeup arrangements require at least one pulley, andmost likely180 degrees of belt wrap. Beyond that, minimizing both the number of additional pulleys and the degree of overall belt wrap can together enhance both beltlife and splice efficiency.

Size vertical curves accurately If the counterweight tension is of proper design,therell seldom ever be a need to adjust it later (eg, removing tension to reduceconcave curve liftoff, increasing tension to reduce center-buckling in a convex curve,etc). Such counterweight corrections in the field produce unique hardships of theirown, often challenging the belt and/or the conveyor components.

Avoid using a drive snub pulley as a takeup bend pulley In service, this hasproven to be a poor design practice (one that has resulted in belt- flexing and/or pulleymaintenance issues).

Maintenance The takeup pulley,by design, is a mobile conveyorcomponent. As its relative positionwithin the conveyor structurechanges, the takeup pulley may notalways remain square to belt travel.A resulting skewed or cocked pulley often spawns a troubling belt

tracking outcome. Irregularly alignedtakeup pulleys can also bind. Theconsequence here might restrict theavailable slack-side tension at thedrive, possibly resulting in belt slip .

Hence, its important to keep the takeup pulley square to belt travel and, at ALLtimes!! Here are some suggestionsthat might help in that regard:

Properly yoke, or contain,the takeup pulley thereby

minimizing pulley shaftdeflections.

Maintain sturdy and straight pulley guides. Thesebeams, pipes or tracks that thetakeup carriage moves along(and inherently touches) mustnot hinder the takeups

freedom of movement .


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Maintain a clean and material-free environment for both the takeup components andbelt travel through the entire takeup region.

Avoid storing long lengths of belting in the takeup. Excessively long (andsubsequently unsupported) lengths of belting are more exposed and vulnerable to themany potential belt trackingaggravations that may exist (eg,belt camber, pulley mis-alignment, and even thewind).

Pay close attention to anyhorizontal automatic takeup.(See example in the adjacentillustration.) Takeup pulleys inthese arrangements areparticularly troublesome to keepmaintained and properlyaligned.

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FDAs Belt Wizard program (again, based on the CEMA model) is an excellent tension calculationand belt selection tool. It is certainly useful for takeup design and problem analysis on a vastmajority of conveyor applications.

However, to keep the conveyor belt engineering analysis reasonable, numerous simplifyingassumptions have been made in such models and computer programs. Foremost amongst these is thetreatment of the belt as a rigid body during acceleration and deceleration. In reality, a conveyorbelt is NOT rigid! Its an elastic body one that exhibits a non-uniform rate of travel along

the conveyor path when the belt speed varies (typical in start/stop cycles).

Such irregular stretch propagation stemming from both tension andcompression transients producesbuildups (or waves ) within the belt.These waves are eventuallydampened out once belt speed isequalized. For most conveyorapplications, these waves are of littleconsequence to the takeup design or belt

operation. However, with the morechallenging conveyors, such wave

patterns can easily lead to an inadequateor inappropriate takeup design whenfollowing a CEMA-based calculationmodel alone. Component life, systemperformance, and safety concerns arethen most likely compromised andpossibly threatened.


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Dynamic analysis calculations, on the other hand, take into account the elasticity of the belt and itsirregular propagation rate during various speed fluctuations. The focus is on the drive/takeupinterface originally helping with conveyor design, and later troubleshooting any field performanceissues. Such a calculation model is also likely to expand on the popular idler friction assumptions byexploring the idler component energy drains in more depth and thereby generating a morepredictable outcome. (An example of a dynamic analysis output illustrating a conveyor startupsituation is shown on the previous page.)

Possible conveyor candidates for dynamicanalysis evaluations might include any of thefollowing:

Centers greater than 2 miles Multiple drive and/or brake locations High lift, high motor torque conveyors Highly regenerative conveyors with large

brakes High speed (over 800 fpm), high capacity

(in excess of 8000 tph) Existing conveyors with system/dynamic

problems

At present, FDA does not provide dynamicanalysis calculations or evaluations. However,there are several recognized organizations and/orindividuals that do specialize in that expertise. If such a complex conveyor/belt analysis is deemednecessary, we would be more than happy to make an appropriate reference in that regard.

George Big G Frank Belt Engineering & Application Consultant

February 2009

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Takeups ... the Ins-And-Outs