“Don’t go breaking my heart” —Elton John & Kiki Dee
Whether it’s a shackle, a hoist, a crane, or a beefy fly frame, never forget that everything breaks. The only question is under what load and under what conditions.
To address this issue, the rigging industry has adopted the term “SWL,” which stands for Safe Working Load. You might also see “WLL,” which stands for Working Load Limit. When you purchase a shackle, a pairing ring, or a length of wire rope, they’ll all carry a load rating - unless they are cheap copies of the real thing (more on this later).
Let’s start with the common anchor shackle. An anchor shackle is the bread and butter of the entertainment rigging industry.
It can be obtained in many sizes, and made from several types of material. It’s close cousin is the chain shackle, which normally carries the same ratings for a given size, but is not as commonly used (Figure 1).
The stated size of a shackle relates to the nominal diameter of the threads that secure the pin into the body of the shackle. The unthreaded section of the pin will always be larger in diameter.
This is important to know if you’re building a steel frame or fixture that the shackle will attach to. A 1/2-inch shackle will need a hole diameter of at least 5/8 of an inch or larger, to fit without jamming.
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Loudspeaker rigging mainly utilizes shackle sizes of 1/2-inch, 5/8-inch, and 3/4-inch It’s rare to encounter any that are larger, though smaller ones are sometimes seen for light loads.
Typically, a 1/2-inch shackle will be rated at two-and-a-half tons or 5,000 pounds. A 5/8-inch shackle is usually rated at three-and-one-quarter tons (6,500 pounds). The rating is usually displayed on the shackle like this: “3¼T”.
But what does this mean precisely? Can you load them up to their stated rating? Absolutely not!
Most ratings, especially of older products, are based on a 4:1 safety margin. This was the safety standard that was used in the crane and rigging industry for many decades. Some newer parts may be rated at 5:1, or an even higher ratio, but you can’t count on it unless it’s clearly notated on the part itself, or the spool it’s wound on (for wire rope).
Nonetheless, in our industry we have to go further. We don’t use shackles on construction sites with trained crane operators and ground-personnel - all working with a constant awareness of the possibility of a shackle or a cable failing. No.
Instead, we suspend loads over people’s heads who are paying attention to anything but the loads we suspend in the air. They’re looking at the stage, they’re dancing, they’re socializing.
Entertainment rigging has largely adopted a 7:1 safety margin in the U.S., while in the European Union, the safety margin is mandated at 10:1, usually followed in most countries.
At a 10:1 ratio, if the failure point (also called “ultimate strength”) of a shackle or other device is 10,000 pounds, then you should never load the device to more than 1,000 pounds.
And even that rating, gentle rigger, is only valid under a static load. A dynamic load, which occurs when a chain motor starts and stops - or a gust of wind hits – can put a far greater force on the suspension equipment than that of a static load.
And then there’s material fatigue. As the sidebar explains, the shackles and the wire rope slings you routinely grab from your rigging box are all not necessarily created equally.
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Direction Of Load
Shackles are designed to be loaded in a straight line; they should never be turned sideways (Figure 2).
While this isn’t breaking news to anyone, it’s not so well known that bridling two wire rope slings, or two Spansets in the eye of a shackle, is NOT what a shackle is intended for.
Yes, you can do this and usually get away with it, but it can weaken the shackle’s ultimate strength by a considerable margin.
The weakening is a function of the angle of the bridle: the more acute the angle, the more the shackle is being pulled in multiple directions and therefore weakened. The proper method is to use a pairing ring.
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In Figure 3, the vertical arrows represent the vector force within the shackle, while the angular arrows represent the actual force when a shackle has been subjected to a semi lateral line-of-force.
Anchor shackles may look like they’re designed to take angular loads, because of the way they conveniently flare out in the top part of their eye, but they are not! They are intended for straight line-of-force loads only, or at best, very shallow bridle angles.
Some shackles, usually those of very high quality, will be imprinted with marks that represent the maximum permissible vector loading. In such cases, it may be O.K. to utilize a shackle as a pairing ring, but only when a shallow bridle angle is presented.