Two Slings, One Hook, Unequal Legs
You are asked to lift a 3.2 t machined steel fabrication using a two-leg chain sling in a basket hitch around the load. The legs are unequal: one leg is 2.0 m and the other is 3.0 m, both rated 3.15 t in a single-leg vertical hitch. The centre of gravity of the fabrication is offset toward the short-leg side. The hook has no safety catch and the master link is worn where the legs bear. The landing area is on a slight slope. The crane is a 25 t capacity machine at a radius giving 6 t SWL.
1. Calculate the tension in each leg and determine whether the sling is adequate.
Read the worked answer
With unequal legs and an offset centre of gravity, the load does not share equally. The shorter, steeper leg takes more load.
Approximate share using the leg geometry (treating the two legs as a triangle with the load line through the CoG): the short leg is steeper, so it carries the greater share. A practical conservative estimate: short leg carries roughly 3.0/(2.0+3.0) × 2 = 1.2× its equal share... but the safer engineering approach is to treat each leg independently and check against its rating.
- Equal-share baseline per leg = 3.2 / 2 = 1.6 t
- The offset CoG and the 2:3 leg ratio mean the short leg takes approximately 1.9–2.0 t and the long leg 1.2–1.3 t at the vertical component.
- Apply the angle factor: the long leg at a shallow angle has a larger included angle, raising its tension. Even allowing for this, each leg's tension (≈2.0 t worst case) is below the 3.15 t single-leg rating.
However, the rating assumes a vertical single-leg hitch. In a basket hitch the capacity depends on the included angle and on both parts bearing; with unequal legs and an offset CoG the load will tilt, and the basket may slip. The sling is not automatically adequate - the geometry, not just the numbers, governs.
2. Identify the defects and site conditions that make this lift unacceptable, and state what you would do.
Read the worked answer
Defects and conditions:
- No safety catch on the hook - the sling can jump off under slack or shock loading. Defect; hook must be withdrawn.
- Worn master link - wear reduces strength and indicates the assembly is due for thorough examination; reject.
- Unequal legs with offset CoG - load will tilt; the fabrication may rotate and the basket hitch may slip.
- Basket hitch on machined steel - smooth, hard surfaces give poor grip; edges may damage the chain.
- Sloping landing area - load may slide or topple on landing; bearers/chocks needed.
Action: Stop. Do not lift. Tag out the sling and hook. Report to the appointed person. Re-plan using a matched-leg sling or a spreader beam so legs are equal and vertical, position the hook over the CoG, use edge protectors, fit a safety catch, and prepare a level, chocked landing area. Only resume with a revised lift plan and verified equipment.