2.4 Bridles: two, three and four legs

1 What a bridle is, and what it is not

1910.184(b) defines a bridle wire rope sling as a sling composed of multiple wire rope legs with the top ends gathered in a fitting that goes over the lifting hook, and a master link or gathering ring as the forged or welded steel link used to support all the legs. Chain bridles work the same way through a master link.

A bridle is not a fourth hitch. Each leg is rigged as a vertical or a choker at the load end, and everything you learned about hitches still applies to it. What the bridle adds is a question the single hitches do not raise: how much of the load does each leg actually carry?

2 Two legs: the honest case

Two matched legs on a symmetrical load, with the hook above the center of gravity, share the weight. Each carries half of it, multiplied by the angle factor for the sling angle. That is the case OSHA's own rigging workbook states its formula for: sling leg force equals the applied load divided by two, times the angle factor, for symmetrically loaded basket hitches and two-leg bridle hitches.

Take those words seriously. Symmetrically loaded is a condition, not a description. Move the center of gravity toward one leg and the split stops being even, and nothing about the rigging looks different from the ground.

3 Three and four legs: design on two

Here is the part that surprises people. On a three- or four-leg bridle, the legs share equally only if the load is rigid and the legs are genuinely the same length under tension. Real slings differ by an inch; real lugs are not perfectly placed; real loads flex. The shortest leg takes the load first, and it keeps taking it until it stretches enough for the next one to pick up.

The consequence is standard rigging practice and it is also the default of this site's tension calculator: design a three- or four-leg bridle as if two legs carry the load, unless the load is rigid and the leg lengths are matched. Notice that OSHA's own workbook gives the sling force formula for two-leg bridles and symmetric baskets, and stops there.

Exactly twice, and the difference decides which sling you pick. Assuming four when it is really two is not a small optimism — it halves the sling you think you need.

4 Nonsymmetrical loads go to a qualified person

OSHA's Guidance on Safe Sling Use is unusually direct about this: for multiple-leg slings used with nonsymmetrical loads, an analysis by a qualified person is to be performed to prevent overloading of any leg; multiple-leg slings are selected according to the tables at the specific angles given in them, with other angles limited to the rated load of the next lower angle or calculated by a qualified person; and when using a multiple-leg sling, the rating shown for the single-leg sling must not be exceeded in any leg.

That last clause is the useful one on a tailgate. Whatever the assembly is rated for, no individual leg may be asked for more than a single leg of that sling is rated for. It converts a hard sharing question into a check you can actually perform: work out the worst-case leg tension, and compare it with the single-leg figure on the tag.

"Qualified person" is defined in 1926.1401 for crane work: someone who, by a recognized degree or professional standing, or by extensive knowledge, training and experience, has successfully demonstrated the ability to solve or resolve problems relating to the subject matter, the work or the project. It is a determination the employer makes about a named individual, not a document you buy.

5 Practical ways to make legs share

1. 1
   Use matched slings from the same assembly. A bridle made up on the spot from four singles off the rack is four different lengths.
2. 2
   Put the hook over the center of gravity before worrying about anything else. An off-center hook guarantees uneven legs whatever the lengths are.
3. 3
   Use adjusters deliberately, not to hide a problem. Chain shorteners and turnbuckles are for setting geometry on an unequal load; they are not a way of making a mismatched set of slings behave.
4. 4
   Consider a spreader or lifting beam. It converts angled legs into vertical ones, which removes both the angle multiplication and the inward crushing force. Its own weight and rating join the assembly.
5. 5
   Do the trial lift and look at the legs. Slack in one leg of a four-leg bridle is the whole lesson of this chapter, visible from twenty feet away.

6 The master link and the gathering point

One component in a bridle carries everything: the master link. Every leg passes through it, so it sees the full weight on the hook regardless of how the legs share below it, and it is rated accordingly by the sling manufacturer as part of the assembly.

Two rules apply to it directly. 1910.184(e)(2)(i) requires attachments — hooks, rings, oblong links, pear shaped links, welded or mechanical coupling links — to have a rated capacity at least equal to the chain they are used with, or the sling is used at the rating of the weakest component. And (e)(2)(ii), with 1926.251(b)(3), prohibits makeshift links or fasteners formed from bolts or rods.

There is also a geometry trap at the top: a master link that is too small for the crane hook, or too large, does not seat properly. OSHA's guidance says it plainly for wire rope slings — do not use a fitting unless it is of the proper shape and size to ensure that it seats properly in the hook or lifting device. A link riding on the tip of a hook instead of its saddle is a hardware failure waiting for the first swing.