Rigging, slinging and load calculations
Slinging fundamentals
Rigging and slinging determine whether the load arrives intact and whether the crane stays within capacity. Know the common sling types - chain, wire rope, synthetic webbing and round slings - and their rated capacities, markings and inspection criteria. Understand the effect of sling angle: as the angle between the sling legs and the horizontal decreases, the tension in each leg increases. For a two-leg sling, tension per leg = (load ÷ 2) ÷ sin(angle to horizontal). A 30° angle doubles the tension compared with a vertical lift.
Know the centre of gravity (CoG): the load must hang level and stable. If the CoG is off-centre, the load tilts and the load distribution between slings changes. Use taglines to control rotation. Protect slings from sharp edges with padding, and never exceed the sling's working load limit (WLL).
Traps in exam questions
- Using the load's mass as the sling tension without accounting for angle.
- Assuming a 4-leg sling shares load equally - in practice, two legs often carry the load; use the appropriate factor.
- Ignoring the CoG and the resulting tilt.
- Using damaged slings or slings without legible tags.
- Forgetting that the total rigging mass counts against the crane's rated capacity.
Worked example
A 4.0 t load is lifted with a two-leg sling at 45° to the horizontal. Tension per leg = (4.0 ÷ 2) ÷ sin 45° = 2.0 ÷ 0.707 = 2.83 t. Each leg must have a WLL of at least 2.83 t.
Exam tip
Draw the triangle: vertical load, horizontal spread, sling length. The sling tension is always the hypotenuse.