Rigging, Load Attachment, and Load Weight Determination
Know the weight before the rigging
Determine load weight from shipping papers, drawings, or calculation (volume times density). Useful densities: steel about 490 lb per cubic foot, concrete about 150, water 62.4. When the weight is unknown, do not guess - the crane's rated load, sling working load limits, and hook all depend on that number.
Sling angle is the number-one trap
Sling tension is set by the angle measured from horizontal, not from vertical. For a symmetrical two-leg lift, tension per leg equals (load divided by 2) divided by the sine of the angle from horizontal. Memorize the multipliers: 60 degrees = 1.155, 45 degrees = 1.414, 30 degrees = 2.0.
Worked example: two legs at 30 degrees from horizontal carrying 4,000 lb means 2,000 divided by 0.5, or 4,000 lb of tension in each leg - double the load share.
Hitches, hardware, and protection
Vertical, choker, and basket hitches each have their own efficiency, so a sling's rated capacity drops in a choker. Use hooks with functioning latches, rated shackles, and softeners or edge protection at sharp corners. Keep the center of gravity below the hook and centered between legs; an off-center load shifts weight onto one leg and can tilt or rotate.
Below-the-hook devices
Spreader beams, magnets, vacuum lifters, and grabs carry their own ratings, and their weight comes off the crane's usable capacity. Count everything hanging below the hook.
Traps: sling angle measured from the wrong reference, rigging weight ignored, and a choker treated as a vertical hitch.