About this tool
You know what a leg is carrying. This says the smallest alloy chain that will take it, and adds your rigging’s own weight back on to give the crane its real number.
Grade 80 and grade 100, side by side
Single leg, straight vertical pull. The row this lift lands on is picked out.
| Size | Metric | Rated (lb) | Rated (kg) | This tool uses |
|---|
Where these come from. Single leg, straight vertical pull, at the 4:1 design factor the chain standards use. The pound figures are the North American alloy chain grade ratings (NACM), used with ASME B30.9 for the sling itself; the kilogram figures are the metric series (EN 818-2 / ISO 3076). The two do not quite agree, and the metric one is consistently the more careful of the pair, so the last column — and every size this toolkit picks for you — is the lower of the two. It will never tell you a chain is bigger than it is.
Then go and read the tag. A published table narrows the choice down; it does not rate the chain in your hand. A sling that is unmarked, worn, stretched, nicked, heated, welded or repaired has no rating at all, whatever the table says, and a chain sling has to be inspected and kept to the standard your site works under.
One stage, and that is deliberate
This picks a chain for one set of legs going from the load to the hook. That is the lift most crews do most days, and it is the one a table can honestly answer.
- Stacked rigging is an engineered lift. A spreader hanging under a spreader, blocks under blocks, a four-leg bridle picking two two-leg bridles — every stage changes the angles and the load path of the one above it. That belongs to an engineer with the drawings in front of them, not to a phone.
- The hitch changes the rating. These figures are a straight vertical pull. Choked, a chain is derated; in a basket over a sharp corner it is derated again. That is on the sling’s own tag and chart.
- Softeners and corners. Alloy chain will happily cut into what it is wrapped around, and a sharp edge will happily cut back.
Gross at the hook is not the same number
The chain has to carry the tension in its own leg. The crane has to carry the whole lot — the load, plus every sling, shackle, spreader and block hanging under the hook. Those are two different figures and they get checked against two different charts: the sling’s, and the machine’s.
Capacities are rounded down and the tension is rounded up, so the spare shown is the smallest it could honestly be. Where the two published series disagree this uses the lower one.