The Load That Wasn't on the Chart
You are asked to lift a prefabricated skid-mounted pump unit from a truck tray onto a plinth. The unit's nameplate says 3.2 t. The rigger proposes a two-leg sling with a 60° included angle at the hook, using two 2 t rated synthetic roundslings in a basket hitch around the skid frame. The lift radius is 8 m; your chart allows 6.8 t there. The truck is parked on a slight cross-slope, and the skid has an off-centre motor that the rigger says 'won't matter'. You notice the skid frame has two lifting lugs on one side only, and the rigger intends to basket the frame rather than use the lugs.
1. Check the rigging: are the slings adequate, and what does the 60° included angle do to the leg tension?
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At a 60° included angle between the legs, each leg is 30° from vertical, and leg tension = (load/2) / cos 30° = (3.2/2)/0.866 = 1.85 t per leg. Two 2 t roundslings in basket hitch have a rated capacity of 2 × 2 t × 0.8 (basket efficiency, typically 0.8 for a basket with the load bearing on the sling) ≈ 3.2 t total, or about 1.6 t per leg if you split it - which is below the 1.85 t required. So the slings are marginal to inadequate. Even using the full 2 t per leg without a basket derate, the margin is thin. The rigger must use higher-rated slings, reduce the included angle (which increases leg tension further, so that is the wrong direction), or use a four-leg or spreader arrangement. The 60° angle is the problem: a wider angle means more tension, not less.
2. What about the off-centre motor and the one-sided lifting lugs?
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The off-centre motor shifts the unit's centre of gravity away from the geometric centre, so the skid will hang tilted and the load will not be shared equally between the two legs - one leg takes more than the calculated 1.85 t. That destroys the equal-share assumption in the calculation. The one-sided lugs are worse: using them alone would put the lifting point off the centre of gravity and cause the load to rotate or hang badly. Basketing the frame instead of using the lugs is acceptable only if the frame is strong enough for the sling bearing pressure and the basket is positioned so the sling passes through or under the centre of gravity. The correct approach is to determine the true centre of gravity (the manufacturer's data or a trial lift just clear of the tray), then rig so the hook is over it - using the lugs if they are rated and positioned for it, or a spreader beam with slings positioned to balance the offset.
3. The truck is on a cross-slope. Does that change your answer?
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Yes. A cross-slope means the load will not lift vertically - as it comes clear of the tray it will swing to find vertical under the hook, which introduces side loading on the boom and a shock load on the slings. It also means the crane itself may not be level, which invalidates the load chart (charts assume a level crane). Before lifting, the crane must be set up level on its outriggers, and the truck should be repositioned to level ground or the load lifted with the crane positioned so the hook is directly over the load's centre of gravity at the moment of lift. If the truck cannot be moved, the lift should be re-planned - a swinging load on a cross-slope is a rigging and stability problem, not a convenience.