Choosing a Sling for an Off-Center, Sharp-Edged Load
A crew must lift a 2,400 lb steel weldment from a truck bed to a floor 12 ft away. The weldment has two lifting eyes: one near the center of gravity (CG) and one 5 ft from the CG on a 6 ft-long part, so the load hangs noticeably tilted with a single pick point. The only slings on hand are a 2-leg chain sling rated 4,000 lb per leg at 90 degrees and two 1 in. nylon web slings rated 3,200 lb each in a vertical hitch. The part has freshly flame-cut edges at both eyes and the ambient temperature is 15°F. The supervisor wants to use the chain sling in a 2-leg bridle from a single hook and "just choke it wherever it fits." The floor is clear and the crane capacity is 6 tons.
1. Evaluate the proposed chain bridle. What load-angle and CG issues must be resolved before the lift, and does the chain rating cover it?
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The 4,000 lb per-leg rating applies at a 90-degree included angle (45 degrees from vertical). If the two legs spread wider, each leg's share rises: at 60 degrees from vertical, the tension in each leg is roughly load/(2·cos60°) = 2,400/1.0 = 2,400 lb per leg, still inside 4,000 lb, but the horizontal squeeze on the part grows and the load's tilt from the off-center CG means one leg carries far more than half. With the CG 5 ft from one eye on a 6 ft part, the load's weight distribution is about 5/6 to the near eye and 1/6 to the far eye, so the near leg could see roughly 2,000 lb before any angle penalty. That is within rating, but only if the legs are rigged so the hook sits over the CG; otherwise the load swings and the legs fight each other. The real disqualifier is the geometry: a bridle from one hook cannot place the hook over the CG and keep both legs loaded as designed. The fix is a spreader or two pick points positioned so the hook is plumb over the CG, then recheck each leg's tension at the actual angle. Do not choke the chain on flame-cut edges without edge protection.
2. The nylon slings are offered as an alternative. What two conditions in the setup make them a poor or unsafe choice, and what would you verify first?
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First, nylon loses strength with temperature and is not the right choice at 15°F; many manufacturers rate nylon web for cold service only down to about -40°F but warn of stiffness and reduced elongation, and more importantly nylon absorbs moisture and can freeze stiff, making the hitch unreliable. Second, the flame-cut edges are a cut hazard for web slings; a web sling in a choker or basket on a sharp edge needs edge protection (sleeves, pads, or corner protectors) or it must not be used that way. Before using either sling, verify the actual marked rating on the sling tag (not the crew's memory), confirm the hitch type matches the rating (a choker derates a web sling to about 80 percent of vertical), inspect for cuts, burns, or frozen stiffness, and confirm the load's weight by calculation or scale. The 6-ton crane is not the limiting factor; the sling and the CG are.