Every rigger learns that sling angle matters. The numbers explain why. As the angle between a sling leg and the horizontal gets smaller, the tension in each leg rises, even though the load itself hasn't changed.
The formula
For a symmetrical lift with n legs sharing the load equally:
Leg tension = W ÷ (n × sin θ)
where W is the load and θ is the angle between each leg and the horizontal.
| Angle to horizontal | Load factor (1 ÷ sin θ) | Tension per leg, 10 t load on 2 legs |
|---|---|---|
| 90° (vertical) | 1.00 | 5.0 t |
| 60° | 1.15 | 5.8 t |
| 45° | 1.41 | 7.1 t |
| 30° | 2.00 | 10.0 t |
At 30°, each leg of a two-leg sling carries the full weight of the load. Below 30°, tension climbs steeply, and most rigging practice treats this as a limit that needs specific engineering approval.
Things the simple formula doesn't capture
- Unequal legs or an off-centre centre of gravity put more load on one leg. Many standards assume only two legs of a four-leg sling carry the load, unless the rigging is designed to share it.
- Horizontal compression at the lifting points, which can bend lugs or the load itself
- Dynamic effects from sudden starts and stops
- The rated capacity of the whole assembly: shackles, hooks, lugs and slings, each derated for the angle
The regulatory framework
In South Africa, lifting machinery and lifting tackle fall under the Driven Machinery Regulations, 2015, issued under the OHS Act. These cover the appointment of competent persons, inspection and load-testing intervals, and record-keeping. Lifting machinery is inspected and load-tested by a registered Lifting Machinery Inspector (LMI).
Try the calculator
Our free sling load calculator gives leg tension and the load factor for any sling angle and number of legs.
Lifting at Redsafe
Redsafe has an in-house ECSA-registered Lifting Machinery Inspector, and lifting plans form part of its mechanical installation and structural steel scopes.
For planning and education only. Every lift must be planned and approved by a competent person using the actual rigging configuration and rated capacities.
T-03 · Free toolSling load calculatorTension in each sling leg for any load, number of legs and sling angle, with the load factor and warnings.
