How it is calculated
Force in one leg = m / (n × cos α), where m is the load weight, n the number of legs actually carrying the load and α the leg angle from vertical. For a four-leg sling only three legs are counted: on a rigid load one of the four is almost always slack.
The working load limit (WLL) is checked with a mode factor, as in EN 818 and EN 1492: required leg WLL = weight / mode factor. Factors: one leg - 1.0; two legs - 1.4 at 0-45° and 1.0 at 45-60°; four legs - 2.1 at 0-45° and 1.5 at 45-60°.
Example: a 2 t load on two legs at 45°. Force per leg 2000 / (2 × 0.707) ≈ 1414 kg, required leg WLL 2000 / 1.4 ≈ 1.43 t.
An angle over 60° from vertical is prohibited: leg forces grow faster than the load itself.
Force multiplier by angle
At 60° each leg carries twice as much as in a vertical lift.
| Angle from vertical | Multiplier 1/cos α | Force per leg, 1 t on 2 legs |
|---|---|---|
| 0° | 1.00 | 500 kg |
| 15° | 1.04 | 518 kg |
| 30° | 1.15 | 577 kg |
| 45° | 1.41 | 707 kg |
| 60° | 2.00 | 1000 kg |
Common mistakes
- Short slings on a wide load push the angle past 60°. Use longer legs or a spreader beam.
- Assuming four legs each take a quarter is a dangerous illusion; the load hangs on two or three.
- The angle between legs and the angle from vertical are different: 90° between legs is 45° from vertical.
- Choke hitches, sharp edges without protection and knots reduce sling WLL - check the tag.
- Work out the load with the metal weight calculator and the hoist with hoist selection. See lifting equipment in the catalogue.