Four-Leg Bridle on an Offset Centre of Gravity
You are to lift a steel skid weighing 6,000 kg from a laydown area onto a foundation. The skid is 2.4 m long with a lifting lug at each corner. A gearbox bolted to one end puts the centre of gravity 400 mm toward the A end from the geometric centre. Your rigging store issues a four-leg chain sling, grade 80, each leg marked 2,000 kg working load limit, with a master link and four hooks; the identification tag is faded but readable. You plan to land the master link on the hook with the legs at about 30 degrees from vertical. Two of the skid's lugs are on flame-cut 16 mm plate with sharp, square edges. While checking the gear, you notice one leg has a hook with a missing safety latch and a link with a gouge about 1 mm deep near the hook eye.
1. Calculate the tension in the most heavily loaded leg and determine whether the sling is adequate.
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With the centre of gravity 400 mm toward the A end on a 2.4 m lug spacing, the A-end pair carries:
6,000 x (1.2 + 0.4) / 2.4 = 6,000 x 1.6 / 2.4 = 4,000 kg
The B-end pair carries the remaining 2,000 kg.
Each A-end leg takes half of 4,000 kg, so 2,000 kg vertical share. At 30 degrees from vertical, the leg tension is:
2,000 / cos 30 degrees = 2,000 / 0.866 = 2,309 kg per leg
Against a 2,000 kg leg WLL, that is 115% of the working load limit. Inadequate.
The classic error here is to divide 6,000 kg by four legs, get 1,500 kg, apply the angle factor and conclude the sling is fine at 1,732 kg. That assumes four legs share equally, which requires a rigid load, identical leg lengths, a hook directly above the centre of gravity and lugs symmetrically placed about it. None of those is true here. As a rule, a four-leg sling is rated on two legs only, and an offset centre of gravity concentrates load further still.
2. What rigging changes would make this lift acceptable? Discuss the options with their trade-offs.
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Options, in order of preference:
- Use a sling with adequate WLL at the actual angle. A 13 mm grade 80 chain leg gives around 5.3 t, so four legs at 30 degrees with the same offset sit well inside the limit. Simple, but confirm the hooks and master link are rated to match and fit the lug openings.
- Use a spreader beam. A beam long enough to hang the legs vertically removes the angle factor entirely, and if the beam is set over the centre of gravity the load still hangs level. Heavier to rig, and the beam's own weight becomes part of the gross load.
- Add or reposition lifting points so the pick is balanced about the hook, e.g. two points at the gearbox end instead of one. This changes the load share and must be re-calculated, and the lugs must be designed and certified for their new share, not just welded on.
- Lengthen the legs or lower the hook to reduce the angle. This only helps if the resulting tension falls below the WLL; at 15 degrees from vertical the tension is still 2,070 kg, so longer legs alone do not rescue a 2,000 kg leg.
Whichever is chosen, re-run the calculation with the hook directly above the centre of gravity and confirm the skid will hang level, or plan for the tilt and control it with tag lines.
3. How do you deal with the damaged hook and link and the sharp plate edges, and can you simply derate the sling?
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No. A working load limit is a certified value for the component as manufactured; it is not a variable the operator can adjust in the field. Derating is not a recognised field practice, and there is no calculation that makes a damaged hook acceptable.
The hook with a missing safety latch and a deformed bowl or opening must be taken out of service. A hook is rejected for a missing or broken latch, for an opening increased beyond the manufacturer's limit (commonly 10% of the original), for bending or twisting beyond about 10 degrees, or for cracks and severe wear in the bowl. A 1 mm gouge in a chain link is judged against the chain standard and the manufacturer's wear and damage limits - typically any crack, or wear or damage exceeding a defined percentage of the link diameter, removes the sling. With a faded tag and no reliable history, the correct call is to quarantine the whole assembly, tag it, and send it for competent-person inspection.
For the sharp flame-cut plate, fit softeners or corner protectors, or use a shackle and a protected edge rather than hooking the chain directly over the corner. Cutting or notching of links at an edge is a genuine failure mode, and it is invisible until the lift is in the air.