How to use the sling safe working load calculator
The same wire rope sling lifts very different weights depending on whether it hangs vertically, chokes the load, or spreads into a two-leg bridle. This calculator starts from the single-leg vertical safe working load, which is the minimum breaking strength divided by the design factor, then applies the hitch and angle factors from ASME B30.9 practice.
| Hitch | Applied factor |
|---|---|
| Single vertical | 1.00 |
| Single choker | 0.75 |
| Basket (two parts) | 2 × sinθ |
| Two-leg bridle | 2 × sinθ |
| Three-leg bridle | 3 × sinθ |
| Four-leg bridle | 3 × sinθ (three legs effective) |
The horizontal angle is measured between the load surface and the sling leg. The further it drops below 90 degrees, the more tension each leg carries and the lower the rated capacity becomes. A four-leg bridle is rated on three legs because a rigid load and an uneven surface keep the fourth leg from taking its share.
Enter the breaking strength from the manufacturer catalog or the sling certificate whenever possible; leaving it blank falls back to a conservative estimate from diameter and grade. Before the lift, check the sling tag capacity table, the ratings of shackles and hooks, and the crane load chart, and have a qualified rigger inspect the assembly.
Frequently asked questions
On a rigid load, fabrication tolerance and an uneven surface keep all four legs from sharing equally. Standard practice is to rate a four-leg bridle on three legs, and this calculator follows that conservative approach.
A choker forces a sharp bend where the rope grips the load, concentrating stress at that point. Rigging practice accounts for the loss by rating a choker hitch at about 75 percent of the vertical rating.
At a 60 degree horizontal angle each leg carries about 1.15 times the vertical share, at 45 degrees about 1.41 times, and at 30 degrees exactly twice. Angles below 30 degrees are generally prohibited, so this calculator stops there.