3.3 D/d ratio: what a tight bend costs

1 D and d

D is the diameter of curvature the sling has to follow: the round load it baskets under, the pin it is seated on, the bail of the hook, the bolt of a shackle. d is the diameter of the sling itself — the nominal rope size on the tag, and on a cable-laid or braided sling the diameter of one component rope rather than of the finished body.

Both are measured in the same unit, so the ratio has none. OSHA states it in exactly this form in Fig. 4 of its Guidance on Safe Sling Use: "when D is 25 times the component rope diameter (d), the D/d ratio is expressed as 25/1." The companion figure for synthetic fiber rope, Fig. 6, does the same at 8/1.

2 Why a bend costs strength

A wire rope in a straight pull shares the load across every wire in it. Bend that rope over something and the geometry stops being fair: the wires on the outside of the curve have farther to travel and are stretched, the wires on the inside are compressed, and the strands slide against each other to make up the difference. The rope is carrying the same weight, but no longer carrying it evenly — and a rope fails at its worst-loaded wire, not at its average one.

The tighter the curve, the bigger the difference between the outside and the inside of the rope, which is why the effect scales with the ratio and not with the diameter alone. It is also why the damage often shows up after the lift rather than during it: 1910.184(f)(5)(iii) puts a wire rope sling out of service for "kinking, crushing, bird caging or any other damage resulting in distortion of the wire rope structure," and (f)(5)(ii) for "wear or scraping of one-third the original diameter of outside individual wires." A severe bend manufactures both.

3 The two minimums that are in the regulation

Figure N-184-4 of 29 CFR 1910.184 carries a legend headed Explanation of symbols: minimum diameter of curvature, and it places a different symbol at each contact surface of each hitch. These are the only D/d figures that are law rather than guidance.

The logic is consistent: the 8× surfaces are where the rope body does the bending, and the 2× surfaces are an eye seated in a fitting, which is built to turn, plus the load face a choker grips. OSHA repeats the same two contact symbols for synthetic fiber rope slings in Fig. 5 of its sling guidance.

4 The ratio your rated load was calculated from

Every published rated load has a D/d ratio baked into it, and OSHA prints it in the general notes under its tables.

The same tables carry a second note that gets even less attention: "rated load based on pin diameter no larger than natural eye width or less than the nominal sling diameter." A pin that is too wide spreads the eye open; a pin that is too narrow pinches it. Either one puts you outside the table even when the ratio looks healthy.

5 Why there is no percentage in this course

The obvious next question is how much capacity a sling loses at 10 to 1, or at 4 to 1. We are not going to invent a number, and OSHA does not publish one. The reduction values live in ASME B30.9, a paid consensus standard, and in each manufacturer's chart, and they differ from one rope construction to the next — so a single generic percentage would be wrong on most of the slings it was applied to.

OSHA's guidance says what to do instead, twice, on its wire rope sling page: "when D/d ratios are smaller than those listed in the tables, consult the sling manufacturer," and "when D/d ratios smaller than those cited in the tables are necessary, ensure that the rated load of the sling is decreased. Consult the sling manufacturer for specific data or refer to the WRTB (Wire Rope Technical Board) Wire Rope Sling Users Manual."

That is the whole procedure: know your ratio, know what your table assumed, and get the reduced value from the people who tested the sling.

6 Five ways to get the ratio back

1. 1
   Increase D. A bigger pin, a bigger shackle bolt, a larger bail. The only fix that improves both the loss and the damage.
2. 2
   Use the fitting the eye was designed for — a thimble, a correctly sized shackle. Bare rope on a bolt of its own diameter is not the design case.
3. 3
   Add a softener where the body bends. It protects the sling and spreads the contact; treat it as protection, not as recovered capacity.
4. 4
   Consider a smaller sling, or two of them. Counter-intuitive but real: on a fixed pin, a smaller rope sits at a higher ratio.
5. 5
   Change the hitch. A basket puts the rope body in the worst bend on the load; eye-and-eye legs on lifting lugs put the bend where the fitting is designed to take it.