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Hong KongRigging and sling angles
As the angle between a sling leg and the horizontal decreases, tension in the leg rises sharply. At 30 degrees from horizontal the tension is double the vertical load.
The geometry that turns an adequate sling into an overloaded one without the load getting any heavier.
The principle
Lift a load with two vertical slings and each carries half the weight. Spread those slings apart and each carries more, because part of the force now pulls sideways rather than up. Geometry alone.
The flatter the angle from horizontal, the worse it gets.
The figure to know
At 30 degrees from horizontal, tension in each leg is double what it would be in a vertical lift.
A 2,000 pound load on two legs at 30 degrees puts 2,000 pounds in each leg rather than 1,000. The slings see the full load each.
That is what makes it dangerous. The rigger has not added weight, lengthened a shackle or changed equipment. They spread the legs to clear an obstruction and doubled the tension by geometry alone.
Practical consequence
Sling capacity tables are rated by angle. A sling adequate at 60 degrees may be overloaded at 30, and the table tells you so if you read the right column.
This is a lookup question in an exam and a fatality in a yard.
Other rigging points
- Slings are inspected before each shift by a competent person
- Damaged or defective slings are removed from service immediately
- Loads are not suspended over workers
- Tag lines control rotation without anybody putting hands on the load
Where to look
The rigging tables in the reference and Subpart CC for the crane requirements. Tab the angle table - it is the one you will want under pressure.
Common questions
How does sling angle affect tension?
As the angle from horizontal decreases, tension rises. At 30 degrees it is double the vertical load.
Why is that dangerous?
Because nothing about the load changed. Spreading the legs to clear an obstruction doubles the tension by geometry alone.
How do sling capacity tables work?
They are rated by angle, so a sling adequate at 60 degrees may be overloaded at 30.
Who inspects slings?
A competent person, before each shift, with damaged slings removed from service immediately.
What is a tag line for?
Controlling rotation of a suspended load without anybody putting hands on it.