Industrial-grade rigging calculator: leg tension from load, angle and effective leg count with EN 818 derating, hitch factors, G80 chain and round sling WLL databases, 30-degree floor, utilization verdict and full audit trail.
Decision:Run the stated engineering model, review its assumptions and warnings, then make the release decision against the governing standard and verified source data.
SectorCalc SC-031 computes the real tension in every sling leg of a rigged lift and compares it against the working load limit of the selected chain or round sling, with the hitch factor applied. Angle is the silent killer in rigging: at 30° from horizontal each leg of a two-leg sling already carries the full load weight, and below 30° tensions climb toward infinity — the engine blocks those lifts outright. 3- and 4-leg slings are treated per EN 818 practice: unless equal load distribution is proven, only two legs are assumed to carry. This tool checks STATIC equilibrium only — dynamic factors, CG offsets and side loads are the lift planner's responsibility.
Leg tension and the angle multiplier
Tleg = W / (neff · sinθ) θ from horizontal
The 1/sinθ term is the angle multiplier: 90° → 1.00, 60° → 1.15, 45° → 1.41, 30° → 2.00, 15° → 3.86. Halving the angle from 60° to 30° nearly doubles every leg's tension. Always measure the angle against the HORIZONTAL load plane — if your tape measure gives the angle from vertical, use the toggle; the engine converts.
Why 3 and 4 legs count as 2
A rigid load on three or four slings is statically indeterminate: small differences in leg length, hook position or CG location overload one pair while the others run slack. EN 818 rates 3- and 4-leg assemblies at the 2-leg value for exactly this reason. The "verified equal" mode exists only for engineered lifts with measured leg lengths and a known CG — select it consciously, the audit trail records the choice.
Choking bends the leg over itself and crushes the bearing point — 20 % off. A basket doubles capacity per leg ONLY when the load is balanced and the sling cannot slide; an unbalanced basket walk-out is a dropped load. Note the angle rule still applies to each basket leg individually.
The 30° floor and utilization limits
Below 30° from horizontal the lift is blocked: tension exceeds 2× the equal-share value and standard WLL tables no longer apply. 30–45° runs with a warning and 20 % reserve demand. Above 100 % utilization the verdict is fail; 80–100 % is a warning band for wear allowance, because WLL has no extra margin for damaged or aged slings.
Field practice
Check the tag before the math: no tag = no lift, regardless of what the calculator says. The database values are EN-standard single-leg WLLs for new, certified slings; they already contain the standard safety factor (4:1 chain, 7:1 textile). Never re-apply a "safety factor of 4" on top — and never lift with knotted, twisted or chemically degraded textile slings.
Frequently asked questions
My angle is measured with a phone inclinometer on the sling — which reference?
If the phone reads 0° when the sling is horizontal, that is angle-from-horizontal — use it directly. If it reads 0° when the sling hangs plumb, that is from vertical: use the toggle or subtract from 90°.
Can I mix leg lengths to reach an awkward CG?
Only with the "verified equal" mode OFF and engineering judgement: unequal legs shift load toward the shorter (more vertical) legs. For CG-offset loads, size every leg for the worst-case share, or use a spreader beam.
Does this cover edge protection and sling angles at the load?
No. Sharp edges cut textile slings at a fraction of WLL and kink chain links — corner protection is mandatory, and it is a physical check, not a calculation. Choker angle at the basket bite (below 120°) needs additional derating per manufacturer.
What about the dynamic factor when the crane starts?
Crane acceleration, swinging and sudden braking add 10–25 % in normal operation, far more in snatch lifts. The static utilization bands (80 % warning) absorb routine dynamics; high-dynamic lifts (offshore, rescue) need explicit dynamic amplification factors.
