Selecting Slings for an Off-Center Load
A 6,000 lb steel tank must be lifted with a two-leg bridle. The tank's center of gravity is 2 ft from the left lifting lug and 4 ft from the right lifting lug, so the load is not centered. The rigger has 1/2-inch wire rope slings rated at 9,800 lb vertical capacity each, and 1-inch polyester roundslings rated at 12,000 lb vertical each. The included angle between the two legs at the hook is 90 degrees. The tank has no lifting points on its sides, only the two top lugs. The rigger plans to use the wire rope slings because they are stiffer and easier to attach.
1. How does the off-center center of gravity change the tension in each sling leg?
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The load's weight does not divide equally between the legs. The tension in each leg is proportional to its horizontal distance from the center of gravity. Using the moment principle about the CG, the left leg carries the share proportional to the right distance (4 ft) and the right leg carries the share proportional to the left distance (2 ft). Total span is 6 ft. Left leg share = 4/6 of 6,000 lb = 4,000 lb; right leg share = 2/6 of 6,000 lb = 2,000 lb. These are vertical shares before applying the sling angle factor. With a 90-degree included angle, each leg's angle from vertical is 45 degrees, so the tension multiplier is 1/cos(45) = 1.414. Left leg tension = 4,000 x 1.414 = 5,657 lb; right leg tension = 2,000 x 1.414 = 2,829 lb. The left leg is the critical one.
2. Are the proposed wire rope slings adequate, and what should the rigger do?
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The left wire rope sling sees about 5,657 lb. Its vertical rating is 9,800 lb, but that rating applies to a vertical lift; for a 90-degree included angle the working load limit must be reduced by the same 1.414 factor, giving about 6,930 lb capacity per leg. The 5,657 lb tension is below that, so the wire rope sling is adequate on capacity. However, the rigger should verify the sling's actual WLL tag and configuration, because a choker or basket configuration changes the rating. The rigger should also check that the tank's lugs can take the load and that the load will hang level; an off-center load will tilt, so the rigger may need to adjust the hook point or use a spreader to keep the tank level. The rigger should not choose slings based on stiffness alone; capacity, angle, and load control govern.
3. What happens if the rigger ignores the off-center CG and assumes equal sharing?
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If the rigger assumes each leg carries 3,000 lb, the left leg is actually carrying about 5,657 lb, nearly double the assumed value. That error could lead to selecting a sling that is undersized for the left leg, or to assuming the load will hang level when it will tilt toward the heavier side. The tilt changes the geometry and can increase tension further, and it can cause the tank to slide or rotate. The rigger must calculate each leg separately and control the load's orientation before the lift.