Rigging Lab
Forces are not hidden — they are just not printed on the tag. Five stations on real rigging math: drag the sling angle, pick the hitch, fold a line over a block, reeve a second part — then drag the whole lift, hoist to top block, and watch every number recompute.
No account, no signup. Same calculation library as our team Rigging Plan Calculator.
Sling angle
The tag on the sling never mentions the angle. Drag it flatter and watch what each leg really carries.
Shaded wedge: below 30°, where tension takes off. Try 30° with 2 legs.
T = (2,000 ÷ 2) × 1.15 angle factor
Hitch
Same sling, three capacities. The number on the tag is only the vertical one.
The tag as printed — one straight leg, full working load limit.
Typical manufacturer factors — the tag on your sling governs. Basket at 200% assumes both legs vertical; pull them to an angle and Station 01 takes its cut on top.
The block
A top pulley redirects your line — and quietly adds the two pulls together. The anchor feels the sum.
Arrow: the pull the block and its anchor feel. Fold the line straight back — 0° — and it maxes out.
R = 2 × 1,000 × cos(30° ÷ 2). At 120° the block feels exactly your line pull; below that, more — the sling and shackle holding it must be sized for this number, not for the load.
Block load factor 1.93: the block, its sling and shackle see nearly 2x the line tension.
One part or two
Reeve the line back through a traveling block and the hoist pulls half as hard — for twice the rope.
Same tower, same tag line, same geometry — computed by the same lift-system function as our team Rigging Plan Calculator. The only change is the second part of line, and the hoist pull drops to half.
The trade: the hoist reels twice the rope, so the load climbs at half the line speed — and the traveling block itself feels 550 lb, both parts combined. Its sling and shackle get sized for that, not for the load.
Not exactly ×2 on the MA chips? The tag line pulling the load out is in the math too — which is exactly why we compute lifts instead of quoting them.
The whole lift — drag it
The live schematic from our Rigging Plan Calculator. Drag the orange handles — hoist, load, top block, tag anchor — and every force recomputes.
Orange handles move. The dimensions are the lift — not an illustration of it.
Heel blocks and dedicated trolley lines (Types B, E, F), friction, wind, obstructions, equipment checks and the printable plan live in the full Rigging Plan Calculator.
Numbers worth carrying in your head
Load angle factor 1.15 → 1.41 → 2.00. At 30° each leg of a two-leg bridle holds the whole load.
Choker keeps about three quarters of the tag; basket doubles it — while its legs stay vertical.
Fold a line straight back over a block and the block, its sling and its shackle hold both parts.
A traveling block halves the hoist pull and doubles the rope — the load climbs at half the line speed.
Questions crews actually ask
Why does leg tension go up when the sling angle goes down?
Only the vertical part of a leg holds the load, so a flat leg wastes most of its pull squeezing the load sideways. The math is tension = share of load ÷ sin(angle). At 60° that is a 1.15 multiplier, at 45° it is 1.41, and at 30° each leg of a two-leg bridle carries the entire load. Below 30° tension takes off, which is why the lab shades that zone red.
Is this the same math as the BuildRight Rigging Plan Calculator?
Yes — the lab imports the same calculation library the team Rigging Plan Calculator runs, so the numbers cannot drift apart. The plan calculator adds equipment checks, crew sign-off and a printable plan on top; the lab is the physics with sliders attached.
Why does a block or top pulley see double the line pull?
A block feels both parts of the rope that pass over it. When the line folds straight back (0° between the parts) the two pulls add fully: 2 × line tension. As the angle between the parts opens up the resultant drops — at 120° the block feels exactly your line pull, and at 180° the rope runs straight past and the block carries nothing. Size the block, its sling and its shackle for this resultant, not for the load.
Does a two-part load line really cut the hoist pull in half?
Yes — reeving the line back through a traveling block on the load gives a 2:1 mechanical advantage, so each part (and the hoist) carries about half the gross load. The trade is rope and speed: the hoist reels twice the line and the load climbs at half the line speed. And the traveling block itself feels both parts combined, so its sling and shackle are sized for that sum, not for the load alone.
Do choker and basket hitches really change a sling’s capacity?
Yes. The number printed on the tag is the vertical-hitch rating. A choker cinches the sling around the load and typically keeps about 75% of the tag. A basket cradles the load with two parts and rates about 200% — but only while both legs stay vertical; pull them to an angle and the sling angle math cuts that back down. The manufacturer’s tag always governs.
Take it past the sliders
The lab is a teaching tool: frictionless sheaves, symmetric legs, static loads. Real lifts get planned by your competent and qualified persons, from the tags on the actual equipment — the method here is classical statics, consistent with ASME B30.9 sling-use methodology.