Four-Leg Choker on an Off-Center Skid
A mechanical contractor is lifting a skid-mounted pump weighing 9,600 lb with a 4-leg chain sling: 5/16 in. Grade 80, each leg rated 4,500 lb in a single vertical hitch, fitted with 3/4 in. screw-pin shackles and a rated master link. Each leg is choked on a steel lug welded to the skid, and with the legs at 45 degrees from horizontal. The skid's center of gravity sits 8 in. off the center of a 5 ft x 5 ft lug pattern. During the pre-lift inspection the crew finds one leg with a deep gouge and nick at a link, a hook whose latch is broken off, and one leg with no identification tag. The crew has a 6 ft spreader beam and a set of 3/8 in. Grade 80 chain legs available in the gang box.
1. Does the 4-leg sling as rigged have adequate capacity? Show the numbers.
Read the worked answer
Do not use the sling as rigged.
- Choker hitch derating: a Grade 80 chain leg rated 4,500 lb vertical is typically rated about 80% of that in a choker, so effective WLL is about 3,600 lb per leg.
- A 4-leg sling is never assumed to share load equally. Standard practice is to assume only 3 legs carry the load (the fourth is treated as a stabilizer). With legs at 45 degrees: T = W / (n x sin 45) = 9,600 / (3 x 0.7071) = 4,525 lb per leg - already over the 3,600 lb choked WLL.
- The off-center center of gravity makes it worse. With the CG shifted toward two lugs, the practical assumption becomes a 2-leg share: T = 9,600 / (2 x 0.7071) = 6,788 lb per leg - nearly double the choked rating.
Both the geometry and the CG offset independently disqualify this rigging. Adding legs or upsizing hardware is not optional.
2. Evaluate the inspection findings. Which items require removal from service, and which require a judgment call?
Read the worked answer
Remove from service immediately (no judgment call):
- The hook with the broken latch. A hook that cannot retain the load is defective; a missing or inoperative latch is a removal criterion for the assembly.
- The leg with no identification tag. An unmarked sling has no known capacity and cannot be used.
- The gouged/nicked link. Deep gouges, nicks, cracks, bent or elongated links, heat damage, or weld splatter are removal criteria for chain. A deep gouge at a link is a stress riser and a rejection item.
Points to document:
- The reason for the defects (dragging the sling over the load, contact with the weld at the lugs, catching on sharp edges) because the same cause will damage the replacement.
- Any sling that was in service when the defects occurred should prompt a review of the inspection interval and the storage/handling practice, not just a swap.
Do not attempt to compute broken-wire or wear percentages on chain, and do not downgrade the sling - tag it, remove it from the job, and destroy or return it to the supplier.
3. Using the available equipment, describe a rigging plan that will work, and explain why it works.
Read the worked answer
Plan: spreader beam, vertical legs, corrected hook position.
- Use the 6 ft spreader beam. The lugs are on a 5 ft pattern; a 6 ft spreader puts the legs very close to vertical, which changes the trigonometry in your favor: at near-vertical, leg tension approaches W/n instead of W/(n x sin theta).
- Upsize to the 3/8 in. Grade 80 legs (roughly 7,100 lb vertical each). With the spreader and near-vertical legs, still assuming only 3 legs share: 9,600 / 3 = 3,200 lb per leg, well under rating. Even the conservative 2-leg assumption (9,600 / 2 = 4,800 lb) stays inside the 7,100 lb rating.
- Avoid chokers if possible. If the lugs allow a shackle or a basket, use it; if a choker is unavoidable, apply the 0.8 derate and re-check that the 2-leg case still passes.
- Deal with the CG explicitly. Position the hook directly over the actual center of gravity, not the geometric center of the lug pattern; if that is not possible, use two different leg lengths or a below-the-hook adjuster so the skid hangs level, and verify level with a spirit level before the load leaves the ground.
- Add softeners or corner protection at the lugs so the new sling is not damaged the same way as the old one.
- Test lift a few inches, check level and hardware, then proceed.