Two-Leg Bridle on an Off-Center Load
An 8,000 lb machine skid has two lifting lugs 10 ft apart. The center of gravity is 3 ft from the left lug and 7 ft from the right lug. The hook is placed directly above the center of gravity, 6 ft above the line between the lugs, so the left leg runs 3 ft horizontally and 6 ft vertically, and the right leg runs 7 ft horizontally and 6 ft vertically. Each leg is a single-leg sling rated 5,000 lb vertical. The rigger plans a two-leg bridle with no spreader beam. The floor around the skid is clear and the crane is otherwise in good condition.
1. Calculate the tension in each leg and determine whether either sling is overloaded.
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Step 1 - vertical share by moment. With the hook over the CG, the vertical load splits in inverse proportion to horizontal distance from each lug:
- Left leg vertical share = 8,000 x 7/10 = 5,600 lb
- Right leg vertical share = 8,000 x 3/10 = 2,400 lb
Step 2 - convert to leg tension. Leg length left = sqrt(3^2 + 6^2) = 6.71 ft; sin of the angle from horizontal = 6/6.71 = 0.894. Tension = 5,600 / 0.894 = 6,260 lb. Right leg length = sqrt(7^2 + 6^2) = 9.22 ft; sin = 6/9.22 = 0.651. Tension = 2,400 / 0.651 = 3,690 lb.
Step 3 - compare to rating. The left sling is at 6,260 lb against a 5,000 lb vertical rating, so it is overloaded by about 25%. The right sling is fine. The bridle cannot be used as planned.
2. How does the off-center center of gravity affect the lift, and what would happen with vertical legs?
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The CG being 3 ft from the left lug means the left side carries 70% of the weight. That is the reason the left leg is the problem, not the total weight: 8,000 lb split evenly would be 4,000 lb per side, but the actual split is 5,600/2,400.
With the hook over the CG, the skid should hang level, but it will hang with the left sling steep and the right sling shallow. If the hook were not over the CG, the skid would tilt until the CG came under the hook, and the leg tensions would change again.
If vertical legs were used under a spreader, the angle penalty would disappear, but the share would not: the left leg would still see 5,600 lb, which is still over the 5,000 lb rating. So a spreader alone does not solve this; the left leg must be upgraded or the pick points relocated.
3. Propose a rigging change that makes the lift acceptable.
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Options, in order of preference:
- Upgrade the left leg and control the geometry. Use slings rated at least 7,000 lb on the left (with a matching or larger right leg), and keep the hook over the CG so the share stays 5,600/2,400. Verify the lugs and the skid frame for the higher local load.
- Use a spreader beam with vertical legs and upgraded left sling. The spreader removes the angle penalty but the left vertical load is still 5,600 lb, so the left sling still needs a rating above that.
- Relocate or add pick points so the CG is closer to the midpoint between pick points; this reduces the 70/30 split.
In every option, re-check that the hook is rated for the total load, that the lugs are loaded in the intended direction, and that tag lines are used. Do not simply 'lift slower' or 'keep the load low' to compensate for an overloaded sling.