Lifting operations: slinging, spreader attachment, load stability and centre of gravity
This domain tests the mechanics of a safe lift: is the load known, is it attached properly, is the centre of gravity inside the support, and is the lifting gear adequate for the angles involved.
Before the lift
- Confirm the weight independently of the manifest. Verified gross mass information, weighbridge tickets or the terminal system should agree with the crane or spreader capacity.
- Check the container: CSC plate, structural damage, doors secured, no loose contents, corner castings sound.
- Check lifting gear: identification, colour code, current thorough examination certificate, no cuts, broken wires, distortion or heat damage; edge protection where slings contact sharp edges.
Spreader attachment
Twistlocks must be fully engaged at all four corners and the engagement indicators confirmed before hoisting. Partial engagement leads to a corner drop, which twists the container and can unload the other corners. Never rely on the crane's automatic sequence alone; the signaller confirms.
Sling angles and load stability
Tension in a sling leg increases as the angle between the legs widens. For a two-leg sling, leg tension equals load divided by twice the sine of the angle to the horizontal. At 45 degrees each leg carries about 0.71 of the load; at 30 degrees each carries the full load.
Centre of gravity
The centre of gravity must sit within the lifting points. Eccentric loads, poorly packed cargo, an off-centre tank or a shifted load put extra load on one corner and can cause racking. Where the centre of gravity is offset, use a spreader rated for the offset, reduce the lift, or repack.
Control of the lift
One signaller, agreed signals or radio channel, tag lines from outside the fall zone, exclusion zone under the load, no personnel on the landing target, and a slow trial lift to check stability and brake function.
Worked example
A 10-tonne load is lifted by a two-leg sling with legs at 45 degrees to the horizontal. Each leg tension is 10 divided by (2 x sin 45), about 7.1 tonnes. A sling leg rated at 5 tonnes is therefore inadequate even though the load itself is 10 tonnes. At 60 degrees the tension would fall to about 5.8 tonnes each, still above 5 tonnes.
Traps that cost marks
- Using headline sling capacity without applying the angle factor.
- Assuming the manifest weight is correct.
- Ignoring an eccentric centre of gravity because the total weight is within capacity.