[RIGGER’S CORNER]
MAXIMIZING THE EFFICIENCY OF SLINGS
As in most industries, precision plays an important role in the performance of equipment—in this case, slings. With so many possible confi gurations and considerations, the process can easily become confusing.
To that end, WRE checked in with Michael Gelskey Sr., CEO of Lift-It Manufacturing Company, Inc., in Pomona, California, to elaborate on the recommended effi ciencies for wire rope slings, roundslings, and web slings. His response addresses and clarifi es the particular formulas that support each specifi c sling scenario:
WIRE ROPE SLINGS AND ROUNDSLINGS D/d is a well-known consideration for wire rope slings, where the uppercase “D” represents the Diameter of the object the sling contacts, and the lowercase “d” signifi es the diameter of the wire rope sling. For example: If a 1-inch diameter wire rope sling is used in a basket hitch around a 25-inch diameter object, a 25/1 D/d results and the wire rope sling has full basket capacity. Conversely, if the same 1-inch diameter wire rope sling is used on an 8-inch Diameter object, resulting in an 8/1 D/d, the wire rope sling loses effi ciency and the basket capacity must be reduced by 16%. T e same effi ciencies apply to roundslings, as there
are prescribed usage diameters recommended by the Web Sling and Tie Down Association and many manufacturers, including Lift-It.
WEB SLINGS T e translation of D/d from wire rope slings ends with roundslings and does not transfer to web slings. To have a D/d, one would need a “d”—and with web slings there is no diameter. If there were a D/d concern with web slings, it would have been realized years ago when some manufacturers were using 3/8-inch steel plate thicknesses for triangle and choker fi ttings. Currently, Lift-It uses 1/2-inch thickness fi ttings, which enable us to justify two ply rated capacities. In this application, one would not have a D/d application,
but rather a T/t—with uppercase “T” representing the thickness of the fi tting, and lowercase “t” indicating the thickness of the web.
80 JULY-AUGUST 2013 WIRE ROPE EXCHANGE
ADDITIONAL CONSIDERATIONS It is important that fi ttings used in connection points are adequately strong and spatially correct. Sometimes, adequately strong fi ttings or connection points are not spatially correct and result in far too much sling being crammed into a space that’s not suffi cient. T is scenario may result in bunching, which reduces sling effi ciency and capacity. It’s imperative to consider hardware and load connection points. In addition, consideration must also be given to the relationship between collection hardware, such as masterlinks or shackles, and the individual slings placed in them to form bridle hitches. Again, too much “stuff ” in an undersized link or shackle can result in loss of assembly strength. T e fi rst set of accompanying fi gures illustrates a 75% rule that Lift-It promotes in their Synthetic Sling and Rigging Training. Depicted are three shackles with overall width, 75% of the overall width, which we will refer to as the eff ective inside width and the corresponding web widths which would be optimal for the three scenarios. If smaller alloy shackles of adequate strength are used, the webbing edges can become damaged, and sling effi ciency will be compromised due to the bunching and edge-loading of the webbing. Simply stated, webbing edges that “ride high in the saddle” take loading fi rst and disproportionally, while the midsection of the webbing “rides low in the saddle” and is not loaded to the same degree. T e second set of pictures depicts a 1-1/2-inch masterlink
that’s strong enough for the four synthetic sling legs, but entirely too small. Also included is the Lift-It replacement bridle, which features a 2-inch masterlink that has an overall width of 8 inches. Applying the 75% rule, 6 inches would be the eff ective inside width of the 2 –inch masterlink ( 8-inch OAL x .75) and is adequate for the four sling eyes, each measuring 1-1/2 inches in width. To ensure the best possible sling assembly: be sure to consider the proper spatial relationships of all applicable components. ❙
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