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A crane’s capacity is not one number — it falls steeply as the load moves out in radius. Add the load, slings, spreader and hook block to get the gross load, find the rated capacity at the actual radius from the load chart, and keep the gross load a safe margin below it. The chart, not the boom length, governs.
- Gross load 10,800 lb; capacity at 30 ft = 14,000 lb.
- % = 10,800 ÷ 14,000 = 77% of chart.
Reference tables
How capacity falls with radius (illustrative)
| Radius (ft) | Rated capacity |
|---|---|
| 10 | 40,000 lb |
| 20 | 22,000 lb |
| 30 | 14,000 lb |
| 40 | 9,500 lb |
| 50 | 6,800 lb |
A number the tables do not give: the same crane that lifts 40,000 lb at 10 ft radius may only rate 9,500 lb at 40 ft — radius, not boom length, governs.
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FAQ
Do I include rigging weight?
Yes — slings, spreaders and the hook block all count as part of the load the crane lifts.
Why does radius matter so much?
Capacity is a moment limit; more radius is more overturning moment, so rated capacity drops sharply as the load moves out.
What margin should I keep?
Practice keeps lifts below a set percentage of chart capacity (often 75–90%); critical or dynamic lifts use larger margins.
Is the chart the final word?
The load chart plus ground bearing, outriggers and crane configuration govern. This is a planning check, not a lift plan.
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