Load Weight Estimation and Center of Gravity
Estimating Weight
Every lift begins with a known weight. Use shipping papers, nameplates, or manufacturer data first; estimate only when data is unavailable, and mark the estimate clearly. For regular shapes, weight = volume × material density. Steel is about 490 lb/ft³, concrete about 150 lb/ft³, water about 62.4 lb/ft³, and aluminum about 170 lb/ft³. For a rectangular block, volume = length × width × height; convert inches to feet before multiplying.
Finding the Center of Gravity
The center of gravity (CG) is the point where the load balances. For a uniform block, it is at the geometric center. For an irregular load, it shifts toward the heavier side. The hook must be directly above the CG; otherwise the load tilts and sling tensions become unequal. Questions often show a load with an offset heavy component and ask which sling carries more tension - the answer is the one closer to the CG.
Load Distribution and Tension
In a two-point pick, tension is inversely proportional to distance from the CG. If the CG is 3 ft from one sling and 6 ft from the other, the nearer sling carries twice the tension. This is a lever relationship: T1 × d1 = T2 × d2.
Traps
Watch for unit errors (inches vs. feet), forgetting to add rigging weight, and assuming a symmetric shape when a component is offset.
Worked Example
A steel plate 4 ft × 2 ft × 0.5 ft = 4 ft³ × 490 lb/ft³ = 1,960 lb. Add a 200 lb spreader bar: total 2,160 lb. If the CG sits 2 ft from sling A and 4 ft from sling B, A carries 2/3 of the load (1,440 lb) and B carries 1/3 (720 lb).