The Calculation, Explained
Resolving the forces
Each leg of a symmetrical bridle carries a tension T at angle θ to the horizontal. Only the vertical components lift; they must sum to the weight:
The horizontal components cancel between legs — but they squeeze the load and pull the hooks inward, which is why shallow angles also crush and slip loads. Watch the angle factor 1/sinθ: at 90° it is 1.00, at 60° it is 1.15, at 45° 1.41, at 30° a full 2.00 — and it runs away entirely below that. Nothing about the load changed; the geometry taxed it.
Assumptions and applicability
- The two-leg rule for four-leg bridles. Unless the load is rigid and the legs are length-matched or equalised, a four-leg bridle is assumed — in LOLER/ASME practice — to hang on two of its legs, the others merely steadying. This calculator defaults to that conservative assumption; select equal sharing only when the rigging genuinely guarantees it.
- Symmetrical load with the centre of gravity under the hook. An offset CG loads the legs unequally in proportion to their geometry — a separate, deliberate calculation, not this one.
- Static, vertical lift. Snatch, slewing, wind and travelling with the load all add dynamic factors the lift plan must cover.
- The tension computed is what each leg's sling, shackle and lug must be rated for — compare against WLL at the configuration used (choke hitches derate a sling to about 80%, basket arrangements differ again), never against breaking strength.
Worked example (the defaults above)
An 8-tonne machine on a four-leg bridle, legs at 60° to horizontal, two legs assumed carrying: T = 8 / (2 × 0.866) = 4.62 tonnes per leg (45.3 kN). Re-rig the same lift flatter, at 30°, and each leg sees 8.00 tonnes — the leg tension doubled while the load did nothing at all. That single comparison has prevented more rigging failures than any poster on the workshop wall.
References
ASME B30.9 — Slings; LOLER 1998 and LEEA guidance on bridle load-sharing and angle factors.
Related reading: Inside a 60,000-Hour Overhaul.
Frequently Asked Questions
How is sling tension calculated?
For a symmetrical bridle, tension per leg T = W divided by (n times sin of the sling angle to horizontal), where W is the load weight and n is the number of legs assumed to be carrying. Only the vertical components of the leg tensions lift the load.
Why are only two legs assumed to carry a four-leg bridle?
Because four legs are never exactly equal in length and the load is never perfectly rigid, two legs typically end up carrying most of the weight while the others steady it. LOLER and ASME practice therefore rates four-leg bridles on two effective legs unless equalising gear guarantees sharing.
What is the minimum recommended sling angle?
45 degrees to the horizontal is the preferred minimum and 30 degrees is the absolute floor. At 30 degrees the tension in each leg has already doubled compared with a vertical lift, and below it tension climbs uncontrollably and the load can slip.
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