Four-Leg Bridle Lift of a Pump Skid
You are asked to approve a lift of an 8,000 lb skid-mounted pump using a four-leg bridle sling with the legs at 45° from horizontal. The four slings in the crib are identical, each with a vertical working load limit of 3,000 lb. The skid has four lifting lugs, each rated 2 tons. The overhead crane is a 5 ton hoist with a 500 lb block and rigging weight. The pump end of the skid is visibly heavier than the motor end, and the center of gravity is well off the geometric center. A rigger proposes to hook all four legs, take up slack, and lift; he has a tag line available but says it is "not really needed for a short pick." The floor area below is barricaded.
1. Calculate the sling tension and determine whether the four-leg bridle is acceptable. Explain the assumption that governs a four-leg lift and what your options are if the tension is too high.
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Governing assumption: with a four-leg bridle you cannot assume the load splits evenly four ways. Because of small differences in leg length and lug position, the standard conservative assumption is that only two legs share the entire load. That is the case you must design for.
Tension per leg, two legs sharing, 45° from horizontal:
- T = W / (2 × sin 45°) = 8,000 / (2 × 0.7071) = 8,000 / 1.4142 ≈ 5,660 lb per leg
That is nearly double the 3,000 lb vertical WLL of each sling. Reject the lift as proposed.
Note the trap: dividing by four legs gives 8,000 / (4 × 0.7071) ≈ 2,830 lb, which looks acceptable. It is not, because the load will not distribute evenly - and with the center of gravity off-center, it will distribute even less evenly than usual.
Options:
- Use slings with a higher WLL (confirmed by tag, not by guessing).
- Improve the angle (see next task) within the limits of the hardware and hook height.
- Use a spreader bar so the legs hang closer to vertical and the bending load is carried by the bar instead of the slings.
- Reduce the load (separate the pump and motor, remove the base fill, drain fluids).
- Verify the lugs as well - 5,660 lb per leg is 2.83 tons on a 2 ton lug, which is also an overstress even if the slings were upgraded.
2. The angle seems to be the problem, so the rigger proposes lowering the hook to "make the legs flatter." Evaluate that idea, and describe how the off-center center of gravity affects the pick even if the slings were strong enough.
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Flatter is worse, not better. Tension rises as the angle from horizontal decreases:
- At 45° (90° included): 8,000 / (2 × sin 45°) ≈ 5,660 lb per leg
- At 60° from horizontal (60° included): 8,000 / (2 × sin 60°) ≈ 4,620 lb per leg
- At 30° from horizontal (120° included): 8,000 / (2 × sin 30°) = 8,000 lb per leg - double the 45° case
Lowering the hook flattens the legs toward horizontal and drives tension up, exactly the wrong direction. Steep legs (closer to vertical) reduce tension but require more hook height. The usual guidance is to keep the included angle between opposite legs at or below 90° (legs at 45° or steeper from horizontal) unless the slings and hardware are rated for it.
Off-center center of gravity:
- The skid will hang tilted. Load shifts to the two legs nearer the CG, so the "two legs share the load" assumption can become "two legs carry more than half the load," and the lightly loaded legs may go slack and allow the load to shift when it swings.
- A tilted load swings, rotates, and can slip out of hooks. It also makes setting it down accurately much harder.
- The lift points near the heavy end will see well above the average share - check them against the 2 ton lug rating with the actual distribution, not the average.
Fix it before lifting: find the CG (calculate from component weights if unknown, or make a short trial lift just off the floor and adjust), then either move the hook point over the CG using an adjustable bridle or a four-leg with two legs shortened, or add a spreader/lifting beam that lets you position the pick point. Use the tag line to control rotation - for an off-center load a tag line is not optional.
3. List the pre-lift checks you would require before this pick, including crane capacity, and describe the trial lift and what you would do with the information from it.
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Crane and hardware:
- Verify the hoist's capacity at the actual configuration (capacity may be reduced at long radius or with the trolley at the end of the bridge). 8,000 lb load plus 500 lb block is 8,500 lb against a 10,000 lb rated hoist - about 85%, which is acceptable on paper but leaves little room, so verify the rating at the actual radius rather than assuming the nominal capacity applies everywhere.
- Inspect slings (tags, WLL, cuts, broken wires, damaged eyes, hooks with functioning latches), shackles, and the lugs and their welds.
- Confirm the lifting lugs are rated for the calculated tension, not the average.
Site:
- Barricade and exclusion zone under and around the load - already done; keep it during the lift and the travel path.
- Confirm the travel path is clear, the landing area is prepared and level, and there are no overhead obstructions or power lines.
- Tag lines on the load, and a designated signal person with a clear stop signal.
- Softeners or pads anywhere the sling contacts an edge.
- Rigging plan reviewed with the crew; everyone knows who is giving signals.
Trial lift:
- Lift a few inches, stop, and hold. Check the load for tilt and the slings for slack legs or signs of slipping, check that the CG is where you thought, look for lug or weld distress, and confirm the hoist brake holds.
- If the load tilts, lower it, adjust the pick point or leg lengths, and repeat - do not chase a tilt by hand or with a cheater on the hook.
- Only after a clean trial lift do you proceed with the full lift and travel, at a controlled speed, with the load as low as practical.