BIZOMAT MAKEOS · Mechanics
Sling Leg Tension
What tension does each sling leg carry at this angle, and is it within the rating?
Tension in each leg of a multi-leg sling from the load mass, the number of legs assumed to share the load and the leg angle from vertical.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load mass | kg | Load mass including lifting beams and attachments below the hook. |
| Legs assumed to share the load | legs | Number of legs that carry load; many codes count only two legs of a three- or four-leg sling unless load sharing is assured. |
| Leg angle from vertical | ° | Angle of each leg from vertical (half the included angle). |
| Working load limit per leg | kg | Working load limit of one leg as marked on the sling tag (in a straight vertical pull). |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Load mass
- What it is
- The total mass hanging below the hook: the load plus lifting beams, shackles and other attachments.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The tension in each leg rises in proportion to the mass.
- Get it right
- Use a confirmed mass, for example from a weighbridge or the maker’s data, not an estimate.
What this means: Legs assumed to share the load
- What it is
- The number of sling legs that are assumed to share the load equally.
- Units and format
- Enter a number in legs: legs. The form accepts 1 legs to 8 legs. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 legs as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- More sharing legs lower the tension per leg.
- Get it right
- Many lifting codes count only two legs of a three- or four-leg sling unless load sharing is assured; follow your code.
What this means: Leg angle from vertical
- What it is
- The angle of each sling leg measured from vertical, which is half the included angle between two legs.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 89°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 30° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Leg tension rises steeply as the angle from vertical increases.
- Get it right
- Do not enter the included angle between legs or the angle from horizontal.
What this means: Working load limit per leg
- What it is
- The working load limit of one sling leg in a straight vertical pull, as marked on the sling tag. The tool compares each leg’s tension with it.
- Units and format
- Enter a number in kg: kilograms. The form accepts 0.000001 kg to 10000000 kg. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1500 kg as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the single-leg vertical rating from the tag, not the rating of the whole sling assembly at an angle.
Method and formulas
- Tension per leg = m g ÷ (n cos β), where n is the number of legs sharing the load and β the angle from vertical.
- Angle factor = 1 ÷ cos β; the leg tension in kilograms-force = mass ÷ (n cos β).
- Example: 2000 kg on 2 legs at 30° from vertical puts 1,154.7 kg in each leg.
Limitations
- Symmetrical load with legs at equal angles; an off-centre centre of gravity loads legs unequally.
- This is a calculation aid, not a lift plan; follow your lifting code and a competent person’s assessment.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check slings and shackles before a lift
- Plan sling length to keep angles small
- Explain why wide sling angles overload legs
Who it is for
- Riggers and supervisors
- Maintenance planners
- Lifting engineers
What you get
- Tension per leg
- Angle factor
- Leg tension against the WLL
Interpreting the result
- Shorten the angle (longer legs) or use a spreader beam when the tension is near the rating.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2613–2650. Topic reference; no source tables or recommended settings are reproduced.
- AS 1418 and AS 3775 series (cranes and chain slings) or your national lifting code — consult the current editions
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