Offset Center of Gravity in a Two-Leg Sling Lift
An 18,000 lb generator skid is being lifted by two single-leg slings. The lifting lugs are 12 ft apart. The center of gravity is 4 ft from the left lug and 8 ft from the right lug. The hook is positioned vertically above the center of gravity, 8 ft above the line between the lugs. That makes the left sling 63.4 degrees from horizontal and the right sling 45 degrees from horizontal. Each sling has a vertical working load limit of 10,000 lb. The rigger says the load is safe because '18,000 divided by two is 9,000 lb per sling, and each sling is good for 10,000 lb.' The load is ready to lift and the crew is waiting. No tag lines are rigged, and the landing area is clear.
1. Calculate the tension in each sling. Show the equilibrium reasoning.
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The rigger's equal-share assumption is wrong because the center of gravity is not centered and the sling angles are not equal. Let T_L be the left sling tension and T_R the right sling tension.
Horizontal equilibrium: T_L cos 63.4° = T_R cos 45° 0.447 T_L = 0.707 T_R T_R = 0.632 T_L
Vertical equilibrium: T_L sin 63.4° + T_R sin 45° = 18,000 0.894 T_L + 0.707 T_R = 18,000 Substitute T_R = 0.632 T_L: 0.894 T_L + 0.707(0.632 T_L) = 18,000 0.894 T_L + 0.447 T_L = 18,000 1.341 T_L = 18,000 T_L = 13,420 lb T_R = 0.632(13,420) = 8,480 lb
The left sling is loaded to about 13,400 lb, which exceeds its 10,000 lb working load limit. The right sling is within its limit.
2. The rigger says each sling carries 9,000 lb. Why is that reasoning unsafe?
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The 9,000 lb figure assumes the load is symmetric and both slings are at the same angle. Neither is true. The center of gravity is 4 ft from the left lug and 8 ft from the right lug, so the left lug carries a larger share of the vertical load. The slings also have different angles: the left sling is steeper, and a steeper sling carries more tension for the same vertical component.
Dividing total weight by the number of legs is only a rough check, and it is not acceptable when the center of gravity is offset. The correct approach is to use statics: sum vertical forces and sum horizontal forces, or take moments about a lug. Here the left sling is overloaded by about 34 percent even though the total load is only 18,000 lb and the two sling ratings add up to 20,000 lb. Adding working load limits is not a valid capacity check.
3. What rigging changes could make the lift acceptable?
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The crew needs a rigging arrangement that accounts for the offset center of gravity, not a simple hook reposition. Options include using a spreader beam sized for the moment, adding a third lifting point near the center of gravity so the load is supported more symmetrically, using a larger sling on the left side and recalculating both tensions, or reducing the load until both slings are within their working load limits.
Moving the hook away from the center of gravity would make the load tilt and would introduce horizontal forces and side loading; that is not an acceptable fix. Any revised arrangement must be recalculated for both sling tension and hardware capacity, including shackles and the hook. Tag lines should be added to control rotation, and the load should be raised slightly to check for tilt before the full lift.