WLL stands for Work Load Limit.It’s the maximum load that should beapplied to a given piece of equip-ment, be it chain, web or wire rope.It’s a portion of the nominal strengthof the chain, web or wire rope, usual-ly expressed as a ratio or percentage.

Som etim es you see productsm arked “capacity” or “m axim umcapacity.” T his is not necessarily theWork Load Limit, and you should getclarification before using the product.

I f you’ve been around awhile, youprobably rem em ber SW L , or SafeWork Load. It was a more descriptiveterm, but had problems. “Safe” is toobroad a term and implies it is safe toapply the stated load, regardless ofcondition.

• Nom inal Str ength, U ltim ateStrength – T he maximum stress amaterial can sustain without fracture.

• Design Factor – T he ratio of theNominal Strength to the Work LoadLimit. For example: I f the NominalStrength is 10,000 lbs. and the WorkLoad Limit is 2,500 lbs., it is a 4 to 1design factor.

• Plastic Defor m ation – T he per-manent distortion of materials underapplied stress.

Now that we know some defini-tions, why do we have work loads?

W hy can’t we use an item to itsNominal Strength? First, some wordsof warning: You should never exceedthe Work Load Limit of a chain, wirerope or web strap. You should neveruse the Nominal Strength of chain,wire rope or web straps when design-ing or specifying a product. All WorkLoad Limits, Nominal Strengths andU ltimate Strengths are determinedusing new m aterials in very con-trolled tests. Now to the good stuff.

As a load is applied to a given itemsuch as a chain, wire rope or webstrap, deformation can occur, gener-ally well below the Nominal Strengthof the item. T his is normal, but dam-aging as well. In chain, the individuallinks elongate. In wire rope, the indi-vidual strands stretch and becomethinner. In web, the individual fiberscan stretch. H ooks and other fittingsbegin to open. While the assembly asa whole is still capable of attaining itsNominal Strength, the process of fail-ure has started. In simple terms, WorkLoad Limits keep the force on theobject below the point where defor-mation occurs.

You can use this analogy: I f you goout and buy a new Corvette and burnrubber at every stop light, red lineevery shift and lock the brakes up atevery stop, som ething is going tobreak sooner rather than later. Eventhough it is a performance car, you

will have exceeded its Work L oadLimits (and probably gotten a fewtickets).

Different Work Loads forDifferent Products

You would think that one work loadratio would be sufficient for everychain, wire rope and web strap you use.Unfortunately it’s not that easy. Workloads can be found in many catalogs,web searches and the like.

If you follow through and search outsome actual work loads, you’ll findsome interesting things. For example,3/8-inch Grade 40 has a higher workload than 5/16 Grade 70, but a lowerbreaking strength. This is because thedesign factor for Grade 40 is lower thanfor Grade 70. Why is the WLL ratio ofone grade different from another? It hasto do with the designed purpose of thechain. Proof Coil and H igh Test aredesigned as light-duty chain, Transportis designed for tie-downs and Alloy foroverhead lifting. These standards areset by the National Association of ChainM anufacturers (N ACM ).

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Wire Rope Wire rope is either 4:1 or 5:1.

Occupational Safety and H ealthAdministration (OSH A) requires thatwire rope used for overhead lifting berated at 5:1. Most, but not all, wirerope manufacturers and wire ropehouses use 5:1. Is a wire rope marked4:1 stronger than a rope marked 5:1?As long as they are of the same con-struction the answer is no, they simplyhave a different design factor. T heWire Rope Technical Board publishesstandards for Nominal Strengths ofeach rope size and type, and WorkL oad L imits are based on thoseNominal Strengths.

Web StrapsWeb straps will be either a 3:1

when used as a tie down, or a 5:1when used as a sling. Check the tag todetermine your strap’s rating, or seem y article “Synthetic Web – T heBasics” in the August 2010 issue ofTow Times.

I hope I haven’t created m orequestions than I ’ve answered. Pleasekeep in m ind that all Work L oadL im its, N om inal Strengths andM inimum Break Strengths are deter-m ined using new com ponents incontrolled conditions. T herefore, theratings are for equipment in wellmaintained, like-new condition. Forfurther reading, see the Wire RopeTechnical Board: http:/ / www.domesticwirerope.org/wrtb/ and the NationalAssociation of Chain M anufacturers:http://www.nacm.info/index.php.

About the author :Fritz Dahlin is vice president of

Operations at B/A Products Companyof Columbia, M aryland. H e has beenwith the company since 1984 and isinvolved in shipping, manufacturing,sales, product development and test-ing. B/A Products Company is a man-ufacturer of custom chain, wire ropeand webbing assemblies for the tow-ing and recovery industry. Contactthem at 800-327-3301 or visi twww.baprod.com.

BY FRITZ DAHLIN

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Y ou s e e the s e le tte rs on c ha in, w ire rope a nd w e ba s s e m blie s . Do y ou k now w ha t it m e a ns , how it ’sde te r m ine d a nd, m os t im por ta ntly, w hy y ou ne e dto k now ? L e t ’s s ta r t w ith s om e de finit ions .

This graph shows the process of deformation in detail. A single side weld link with a minimum breakstrength of 18,800 lbs. was pulled to destruction. The Work Load Limit of the link is 4,700 lbs. The graphshows load in pounds on the left and position (how much the link stretched) on the bottom. From the startto 5,000 lbs., 300 lbs. over the Work Load Limit, the link stretched .075 of an inch. From 5,000 lbs. to17,500 lbs., 1,300 lbs. below the minimum break strength, the link stretched .375 of an inch. Multiply.375-inch out over a chain 100 links long (over three feet), and you can see the damage that is done.

C HAIN TR ADEGR ADE NAME WL L

30 Proof C oil 4:140 High Tes t 3:170 Trans port 4:180 Alloy 4:1

100 Grade 100 Alloy 4:1120 Grade 120 Alloy 4:1

Work L oa ds forS pe c ific I te m s