D/d is the diameter the sling bends around, divided by the diameter of the sling. It sounds like a detail and it is not: the rated load printed on your sling's tag was calculated assuming a gentle bend, and every bend tighter than that assumption takes capacity back. OSHA sets two minimums in the regulation and its rated-load tables tell you what ratio the numbers were based on. Below that, the rated load has to come down.
What D and d are
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, 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 for 8/1.
D/d = D ÷ d. A ½ in rope over a 12 in pin is 24 to 1. The same rope choked hard around a 2 in bar is 4 to 1. Nothing about the sling changed, and its published rating no longer applies. The D/d ratio calculator does the division and checks it against both thresholds below.
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 are forced to slide against each other to make up the difference. The rope is still carrying the same weight, but it is no longer carrying it evenly — and a rope fails at its worst-loaded wire, not at its average.
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 rather than with the diameter alone. It is also why the damage shows up after the lift, not during it: OSHA's own list of conditions that put a wire rope sling out of service immediately includes "kinking, crushing, bird caging or any other damage resulting in distortion of the wire rope structure" and "wear or scraping of one-third the original diameter of outside individual wires" — 1910.184(f)(5)(iii) and (ii). A severe bend manufactures both.
The two minimums that are in the regulation
Figure N-184-4 of 29 CFR 1910.184, "Basic Sling Configurations with Vertical Legs," carries a legend headed Explanation of symbols: minimum diameter of curvature, and it puts a different symbol at each contact surface of each hitch. These are the only D/d figures that are law rather than guidance.
| Where the sling bends | Which hitch puts it there | Minimum D |
|---|---|---|
| The sling body, under the load | Basket hitch | 8 × d |
| The sling body, over the hook or pin | Endless sling or grommet | 8 × d |
| A sling eye on a hook, pin or shackle bolt | Any eye-and-eye sling | 2 × d |
| The load surface inside the choke | Choker hitch | 2 × d |
The logic is consistent: the 8× surfaces are the ones 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.
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 the tables. For ordinary single-part wire rope slings the note reads "rated loads based on minimum D/d ratio of 25/1." That is a long way above the regulatory floor of 8 to 1.
| Sling type | OSHA tables | Rated loads based on |
|---|---|---|
| Wire rope, single-part 6×19 or 6×36 (EIPS, EEIPS) | Tables 7 to 12 | 25/1 |
| Cable-laid wire rope, mechanical splice | Table 13 | 10/1 |
| Braided wire rope, six-part and eight-part | Tables 14 and 15 | 25 × the component rope diameter |
| Synthetic fiber rope (nylon, polyester, polypropylene) | Tables 18 to 20 | the 2× and 8× contact surfaces of Fig. 5 |
| Synthetic web slings and roundslings | Tables 21 to 26 | no numeric ratio published |
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 fine.
Why you will not find a percentage on this page
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 make a number up, and OSHA does not publish one. The reduction values live in ASME B30.9, a paid consensus standard, and in each manufacturer's own chart, and they are not the same 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.
Five ways to get the ratio back
- Increase D. A bigger pin, a bigger shackle bolt, a larger sheave or bail. This is the only fix that improves both sides of the problem: less loss and less damage.
- Use the fitting the eye was designed for. A thimble in the eye, a correctly sized shackle rather than whatever was on the truck. The eye is built to turn on something; bare rope on a bolt of its own diameter is not the design case.
- Put a softener or wear pad where the body bends. It spreads the contact and protects the rope. What it does not do is change the diameter of curvature enough to buy back the rating — treat it as protection, not as capacity.
- Consider a smaller sling, or more of them. Counter-intuitive but real: on a fixed pin, a smaller rope sits at a higher ratio. Two smaller slings can beat one oversized one.
- Change the hitch. A basket puts the rope body in the worst bend on the load. A pair of eye-and-eye legs on lifting lugs puts the bend where the fitting is designed to take it.
D/d is one of four reductions, and they stack
A sling's rated load is reduced by four independent things, and riggers routinely apply one and forget the rest.
- The hitch. Vertical, choker or basket — three different ratings on the same tag. See sling hitch types.
- The leg angle. Two legs at 30° from horizontal each carry the whole weight of the load. That is the sling tension calculator, and the arithmetic is in sling angles explained.
- The angle of choke. A choker rating holds only above 120°; OSHA's Fig. 2 takes it down to 49% below 30°.
- The bend diameter. This article.
They are not alternatives. A hard-choked basket on a small-diameter load at a flat leg angle is all four at once, and no single chart on the wall covers that lift.
Bottom line
- D/d is the bend diameter divided by the sling diameter, and it has no unit.
- The regulation sets 8 × d where the sling body bends and 2 × d for an eye in a fitting or the face inside a choke — 1910.184 Fig. N-184-4.
- Ordinary wire rope sling ratings assume 25/1; cable-laid assumes 10/1; fiber rope assumes 8/1.
- Below the ratio your table assumed, the rated load has to be reduced, and the reduced value comes from the sling manufacturer.
- A bigger sling lowers the ratio. A bigger pin raises it.
- Check the bend before the pick, and inspect the sling after it — a tight bend leaves evidence.