Rigging, Load Charts, and Load Handling
Rigging
Hitches change capacity
Every sling has a vertical rating; the hitch multiplies or divides it. A vertical hitch uses the full rating. A choker reduces it, commonly to about 0.8 of vertical per the manufacturer's tag. A basket hitch can roughly double capacity only when the legs are truly vertical; as basket legs spread, capacity falls fast. A two-leg bridle never gives twice the single-leg capacity, because each leg carries load divided by (2 cos theta), where theta is the angle measured from horizontal.
Angles, edges, and D/d
At 60 degrees from horizontal each leg carries about 0.58 of the load; at 45 degrees, 0.71; at 30 degrees, a full 1.0 per leg - the point where a two-leg sling has no capacity advantage at all, which is why many sites forbid going below 30 degrees. Sharp edges cut synthetic web and damage wire rope, so use softeners or corner protectors. Wire rope slings need a D/d ratio (sheave or pin diameter to rope diameter) at least equal to the manufacturer's minimum, or capacity is derated.
Hardware and identification
Shackles, hooks, master links, and swivels carry their own working load limits; the assembly is only as strong as its weakest marked link. Never side-load a hook or use a makeshift link. Every sling must have a legible tag showing capacity and type, and rigging hardware must be inspected for cracks, deformation, and worn threads.
Traps
- Splitting the load evenly between legs regardless of angle.
- Using a basket hitch that can slip off the hook.
- Mixing sling types in one bridle without derating.
Worked example
A 4,000 lb load hangs on a two-leg bridle at 45 degrees from horizontal: each leg carries 2,000 / cos 45 = 2,828 lb. Each sling, including its hardware, must be rated at least 2,828 lb.
Load Charts
A capacity is one point on a chart
A load chart gives gross capacity for one combination of radius, boom length, boom angle, quadrant, and configuration (outriggers extended, on rubber, jib stowed or deployed, counterweight). Working radius is measured horizontally from the center of rotation to the hook - not to the boom tip and not along the boom.
Net capacity is what matters
Chart capacity is gross. Deduct everything hanging below the boom tip: hook block, headache ball, slings, shackles, spreader beam, and any stowed jib or auxiliary head the notes require you to subtract. The result is net capacity, which must be at least the load weight plus allowance for dynamic and wind effects. If the actual radius falls between two chart values, use the lower capacity (the longer radius); never interpolate upward and never extrapolate past the chart.
Structural versus stability
At long radii and long booms the crane is usually limited structurally - boom strength or the load moment indicator. At short radii with heavy loads, stability and outrigger ground bearing govern. Knowing which limit your chart line represents tells you whether float area or boom strength is the real risk.
Parts of line
Line pull caps a lift even when the chart allows it: required line pull equals load divided by (number of parts times reeving efficiency). Check the block rating as well.
Traps
- Using boom length where radius belongs.
- Comparing load weight to gross chart capacity.
- Reading the on-rubber chart while the crane is set on outriggers.
Worked example
Chart reads 12,000 lb at 30 ft radius with an 80 ft boom. Deduct hook block 500 lb, slings 300 lb, and deployed jib 800 lb, giving 10,400 lb net. A 9,000 lb load fits; a 10,600 lb load does not.