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Phone (US): 1.706.235.6315 Toll free: 866.54BRUGG (542.7844) Fax: 1.706.235.6035 info: [email protected] www.bruggrope.comTelfono (E.U.): 1.706.235.6315 Llamada gratuita: 866.54BRUGG (542.7844) Fax: 1.706.235.6035 info: [email protected] www.bruggrope.com

TECH TIPS: LOAD MEASUREMEN

WHY MEASUREMENT DEVICES ARE FLAWE

Let us consider a litt le mattercalled constructional st retch.

Over the years industry professionals have sometimeschosen to avoid performing the basic maintenanceroutine of rope load equalization altogether. This happensfor many reasons : the p rohibitive expense of having aprofessional crew correctly perform this tedious, exactingtask; the time it requires to first adjust each rope initially,compoun ded with the amo unt of time it requires tomake sure that all the ropes maintain a certain degreeof equalization with each other; the lack of accuracyinherent in all available tension measurement devices;and a failure to und erstand how other installationcompon ents and related basic factors can impact theresults of the equalization process, and in fact make evenachieving load equalization impossible.

Indeed one of the basic factors that are frequentlyoverlooked in load equalization is constructional stretchof the ropes. Simply put, constructional stretch is thenatural reaction of any helically constructed set of wiresand strands in a hoist rope whenever a load is applied.Loading creates a constrictive reaction that compressesthe rope's core (bringing all elements of the rope closerand reducing the diam eter), which in turn leads to a

lengthening of the hoist rope (elongation).Though a basic range of constructional stretch can be

mathem atically derived (calculated as a percentage ofstretch per amount of rope length), the precise amountof stretch one can expect to encoun ter in a certain rope,and how long it will take to completely stabilize the finalrope length can be d ifficult (if not impossible) to predict.

This can be attributed to the variety of ropes producedand num erous factors including: the type of core used ;rope design; lay length; grade or grades of steel used;and the am ount of preforming present in the rope. Otherfactors affecting constructional stretch include: typeof installation; rope length; acceleration/ deceleration;groove profile; load factors in reference to the MBL;twisting of ropes during installation; and whether the

ropes were equally tensioned upon installation.Critically however, there is no indu stry wide, accepte d

standard that m andates exactly how the rope them selvesare to be hung (though guidelines certainly exist asto safety standards and what constitutes acceptableresults, actual technique is op en to interpretation). Onefinds that th e entire installation/ re-roping process is oftendriven by expediency, convenience and profitability, asmuch as logic and b est practices.

This means that when hoist ropes arrive on-site theyare seldom hung as a group simultaneously. This inequitymeans that all the ropes are never allowed an equal

amount of time and load to stretch naturally as a group,in order to com pensate for constructional stretch.

For instance, if a crew were to hang three rop es andthen install the remaining ropes another day, despiteeven their best efforts in taking up the ropes, so theyall had the sam e length, they would find d ifferences inrope lengthand rope tension in a very short amountof time. And this difference can contribute to a reductionin system p erformance, accelerated rope wear and theeventual degradation of the sheave itself.

Ideally Brugg recommends that an installer suspendcar and ropes, allowing time to reach and equallydistribute constructional stretch additionally, and thenequalize all the rope s accurately with th e provisionthat a maintenance person will accurately equalize theropes again th ree to six month s later. Not only will thisresult in an increase in system performance by reducingconstructional stretch as a factor for consideration inrope tension inequalit y but it will allow each set of ho istropes to attain its ma ximum amount o f life expectancyas well. Addit ionally such mea sures will greatly reducewear and tear on th e sheave, ride quality will improvenoticeably, and on e would see a dramatic reduction inoverall maintenance costs and breakdown incidents which means less system downtime.

Why devices measuring ropetension arent what they arecracked up to be.

Clearly, any tool that wou ld gauge rope ten sioning withaccuracy would be useful to any industry professionals.And the most reliable, simple and efficient way wouldbe to use a tool that measures rope loads (the amountof pressure on a rope) directly. Unfortunately, mosttensioning measurement devices measure loads in arather circuitous fashion.

First one applies a certain am ount of force (orpressure) to defl ect a host rope a certain distance, andthat amoun t of deflection is then measured. Utilizing

both the am ount of pressure applied, and the amountof deflection m easured, the device then calculates theamount of load that is present on the rope. Provided theuser applies the device correctly, the input data derived isaccurate (which sometimes is dependent upon where themeasurements are taken), and the formulas used in theprocess are implemented correctly, one might b e able tocreate a set of readings that one could use as a basis forcomparison. However that is counting on a lot.

Nevertheless creating a gauge to measure ropetension is a far different (and simpler) thing than sayingthat such a d evice can b e used to actually tension

Why equalizing hoist rope tensions is so tough to

to perform and why most rope tension measurementdevices are of little practical use at all. 1

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1. Shackle/Wedge Socket

2. Hitch Plate

3. Hoist Rope

4. Car Sheave

5. Counterweight Sheave

6. Shackle/Wedge Socket (CWT side (image obscured by Drive Sheave)

7. Drive Sheave

Image 1: A standard 2:1 elevator presentsmany areas that make accurate loadmeasurement especially challenging.

Brugg Wire Rope, LLCBrugg Wire Rope, LLC


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TECH TIPS: LOAD MEASUREMENT

WHY MEASUREMENT DEVICES ARE FLAWED

the ropes themselves (actually they cant tension

the ropes, the mechanic must tune themhardly asmall distinction). And when m ultiple hoist ropesare involved, any degree of error in the pro cess ofmeasurement is compound ed in the final result.Indeed simply being able to ascertain loads (even ifthe measurements were accurate) is no guaranteethat one could use such m easurements to achieveequalization at allunless one were somehow alsoable to manipulate all the loads on all the ropes in asystem simultaneously and autom atically. And that issimply not what measurement devices are meant todo. In the final analysis, load measurement devicescan be thought o f at best as passive indicators thatreveal rope tensions without actually doing anything atall in regards to equalization.

Until recently every load equalization methodavailable to the professional today used a tensioningmeasurement device in much the sam e way (andwe include our own RPM: Rope PerformanceMeasurement device as wellSee Image 2). Oftencrews would use as many as two men in an involvedback-and-forth manner to adjust shackle nuts, thenmeasure rope tensions (often in more than one p lace,especially in 2:1 arrangement: See Drawing 1) andthen comm unicate their findings, so the nuts could b ereadjusted again. In addition to the length of time theprocedure took, their efforts were not made any easierby the very accuracy of the measurement devicesthemselves. Though many manufacturers like to tout

the accura cy of their devices, ranging anywhere from2 - 8%, this fails to address the very real possibilitythat the workers final actual mea surement couldactually be between 4 -16% off.

Some m anufacturers were inclined to blame theuser for any failure in achieving equalization with t heirtensioning device. Actually there is a degree of truthto this. However we think it unfair to place the m ajorshare of fault on the shoulders of professional crews.Especially since the equalization process, when donecorrectly, is so tedious, labor intensive and complex.Small wonder then that m aintenance professionalshave avoided the process entirely, or due to theinaccuracy of the measuring instrument itself, arewilling to accept something that is less than true rope

load equalization.

Why all tensioning measurementdevices are off .

As explained p reviously, every device available onthe m arket today, no ma tter if it uses springs, wires, ora computer chip, basically works by applying a knownquantity of force to deflect a rope. The amount o ftension is then calculated after basic details such asamount of deflec tion, rope diameter and rope type areinput. Though the form of the device may change and

evolve (from plastic devices to laptop displays) the

essential principle remains the same.Unfortunately this process relies on a fewassumptions. Due to p ractical concerns thesedevices cannot offer the user the degree of sensitivitynecessary to precisely measure rope d iameters.And they don't alert the user if a m easurement wastaken from the peaks or valleys of a rope, creatinga degree o f error in itself. These d evices can onlyprovide measurements that are within a certain range

of precision (or a variable de gree of imprecision if youwill), and all measurements are taken perpendicularto the ropes axis. This means that the measurementsinput are no t in line with the act ual load it selftheyreperpendicular (90) to the direction of the load.

In addition such devices cannot exactly accountfor the many different cross sections of hoist ropesthat are in e xistence. To increase u tility (while at thesame time limiting precision) some devices utilize asimple, arbitrary factor in their calculations to allow

them to quickly create readings that may be ap pliedto fiber or steel core ropes. Measurement devicescannot yet account for the presence of rope wearin their diameter readings. Nor can they accuratelyfactor into their calculations for the impact of ropelubrication. And this is not an insignificant factor asrope lubrication affects the am ount of friction withinthe ropes wires, which impacts the overall accuracy ofthe deflection m easurement itself.

Indeed the deflection m ethod inducing abend in the rope, affects the accuracy of the resultsitself. There is even a degree of variation app arent

depending upon where the actual measurement

is taken in an installation. For instance, due to thefriction imp arted bet ween various elements in asystem, tensions can slightly vary between the car andthe shackles, or the car and the main drive sheave.

Certainly an elevator system is, in theory at least,a balanced arrangement where one force is perfectlycountered by another. But in reality the movementof the ropes themselves is inhibited by the manycomponent s it interacts with. Since most of thedevices used to measure rope tensions state that o neneed only measure tensions in one area of a systemto acquire a satisfactory idea of rope tensions (whichone then uses to tune the rope throughout an entireinstallation), one can see that this indirect methodof measurement is in essence not a true scale foraccurate measurement of tensions but is nothingmore than a samp ling of obtained data tha t one usesto formulate a good guess of tensions.

When one lo oks at all the sources of error that areinherent in a m easurement device based upon simpledeflection one sees its flaws more clearly. Certainly ityields an eye-catching tool. But for load equalizationit offers a solution that is only somewhat better thaninaccuratewhich, in effect, makes the entire idealess than adequate for the job.

Some will argue that achieving perfect equalizationcannot be a chieved. Indeed national and internationalboards are only now working on defining what equaltension actually means. And decisions by such bodies

will largely be confined to how the m atter impactssafety. System performance and rope life issuesare, on the whole, of low priority to the m. And evenif experts can actually find a range of unequal ropetension that they qualify as acceptable, it will be hardto verify when one is within this range due to the veryflaws in the m easurement tool itself.

The advantage t hat RLE of fers

Clearly if the entire concep t of a device to actu allymeasure rope tensions with a high degree of accuracyis impossible, and using an inaccurate device to attainan absolute degree of equalization is untenable, thenclearly another solution had to be found. Rope load

equalization clearly offers too many advantages inride comfort, system performance, benefit s in ropelife expectancy and in a voiding expensive sheave andcomponent wear to simply settle for half measures.

This is why in late 2 00 9, Brugg Wire Rope, LLCreleased Brugg RLE (Rope Load Equa lization:Image 3), a hydraulic device that avoids flawedmeasurements and tedious hand m anipulation of theropes to at tain truly accurate, practically automaticload equalizations. Using a hydraulic fluid pumpedthrough an interconnected system of hoses and

Image 2: Brugg RPM (Rope Performance Measurement)device was an early answer to hoist rope measurement.But as with all other devices suffers from an inherentinaccuracy due to a flawed premise for measurement.


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cylinders, one pressurizes all the cylinders at the sametime, which tensions all the ropes on the shacklessimulta neously. This mean s it is now possible for oneman to achieve perfect rope load equalization quicklyin only minutes.

RLE offers a degree of unmatched accuracy that isso exact that even the most sop histicated measuring

device available on the m arket today cannot matchit in results (especially when that device is part of aneffort to modify rope tensions by hand), nor can those

devices even verify RLE's accuracy due to t he inherentlimits they face in their own precision.

As we stated previously, Brugg RLE is based uponbasic hydraulic principles. Simply put, when a fluidis introduced into a closed system, any pressure orweight when applied to one segment of a system willbe distributed equally to all parts of that system. And

since the cylinders of RLE are atta ched to ro pe endterminations (see area 1 (Car Side Hitchplate) andarea 6 (Counterweight Side Hitchplate) in Image 1),the pull force provided by the cylinders equalizes thetensions of all the ropes and all at the sam e time.This force is exerted directly and in line with th e axisof the load carried. As you review RLE's Stand ardUser Manual you com e to better understand howload equalization is assured and how this method issuperior to other methods that ask you to individuallytune each rope using an inherently inaccuratemeasurement device and direct manual m anipulationof a segment of the ropes (in essence, tuning them asyou would piano strings, but with a wrench).

One note: while RLE makes the equalization

of rope tensions fast and easy to perform,maintaining e qualization requires vigilance, goodmaintenance technique, and above all, equalgroove depths within the drive sheave itself. Evensmall sheave groove variations will inevitably createdifferent rope pitch diameters, which can quicklylead to discrepancies in rope travel distances. Andthis creates unequal rope tensions (which leadsto shortened rope life) once again. A simple wayto accurately measure groove depth within thesheave itself is by using Brugg GDC (Groove DepthComparator). Should groove depth variations be

detected, the only effective way to maintain ropeequalization may be to either to re-machine or replathe sheave (neither option being a cheap p ropositio

The future is here

Naturally there will always be a certain amountof resistance to new ideas. Indeed we expectcompetitors to create newer methods of m easuringtensions and some even attempt to match BruggRLE's idea of u sing cylinders and hydrau lic pressureto equ alize rope load s after all, imitatio n is thesincerest form of flattery.

However if you understand the advantages thatequalization offers to system performance, recognizthe benefits that it provides for increased rope life,and understand how it can save expensive wear andtear on sheaves and surrounding components, thenthe ad vantages of Brugg RLE will be readily apparento you.

For further questions on this or other topicsconcerning hoist ropes review our website atwww.bruggrope.com or contact your BruggLifting representative. This article appearedin Elevator World in 2012.

Image 3 : Brugg RLE (Rope Load Equalizer) is the industry's only patente d hydraulics-based device that allows one man to equally tension all the ropes in a setat t he same t ime in under 3 0 minutes. For more details see review the RLE Manual (in English or Spanish) in our online Pressroom section or our Catalog.

Image 3 : Brugg GDC (Groove Depth Comparator) a llowsprofessionals to quickly measure and compare sheave

groove variations within the sheave itself.

TECH TIPS: LOAD MEASUREMEN

WHY MEASUREMENT DEVICES ARE FLAWE

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