IF YOU CAN'T
USE SIN(),
DON'T RIG
THE ANGLE.

A rigger who cannot use sin() will still set an angle. The sling will still do the math. The hardware will still see the tension. The only person not in the equation is the one who was supposed to check it.

This is the two-leg bridle. Equal legs. Centre of gravity in the middle. Load hanging plumb. If those are not true, stop and get a different method — or an engineer.

WHAT θ IS

θ is the sling angle: the angle between the sling and the horizontal. ASME talks about it that way. This program talks about it that way. It is not the angle between the two legs at the hook. Mix those up and your number is for a different lift than the one in front of you.

When the bridle flattens, θ gets smaller. sin(θ) gets smaller. Tension goes up. The load did not get heavier. The angle did.

The sling is doing sin() whether you are or not.

THE FORMULA

Each leg holds half the weight vertically. That vertical share is T × sin(θ). Solve for tension along the sling:

Same thing written as a factor on the load share: T = (W / 2) × (1 / sin θ). The factor is 1 / sin(θ). At 90° it is 1. At 30° it is 2.

If someone gave you the angle from vertical instead, use cosine of that angle. It is the same triangle: cos(from vertical) = sin(from horizontal). Do not use both and average them.

WORKED LIFT — 4000 kg

On site, kilograms of tension are compared to a WLL in kilograms. The sine does not care. Keep W and T in the same unit.

1. 01

   60° SIN 60° = 0.866

   T = 4000 / (2 × 0.866) = 2309 kg per leg. About 15% more than a straight vertical share. This is a working bridle.

2. 02

   45° SIN 45° = 0.707

   T = 4000 / (2 × 0.707) = 2829 kg per leg. The load is still 4000. Each sling is already carrying 2829.

3. 03

   30° SIN 30° = 0.500

   T = 4000 / (2 × 0.500) = 4000 kg per leg. Each sling sees the whole load. That is what 'it doubled' means in Proven.

If those 4000 kg slings were picked because 'half the load is 2000,' they were already over at 30°. The ticket on the sling did not change. The angle did.

THE TABLE YOU SHOULD KNOW COLD

Below 30° that hitch is not used unless an engineer owns the numbers. sin(20°) is 0.342. The factor is 2.92. You are no longer 'spreading the slings a bit.' You are multiplying the load.

USE IT

Phone calculator. Degree mode. sin(). Then divide. If your calculator is in radians you will get a number that looks official and is wrong. Check: sin(30) must be 0.5. If it is not, you are in the wrong mode.

WHAT THIS FORMULA DOES NOT DO

• It does not split a 3-leg or 4-leg bridle into three or four equal shares. If the load can tilt, design as if two legs are carrying.
• It does not fix an unknown weight. Garbage W in, garbage T out.
• It does not replace a load chart, a sling WLL, or a hitch factor (choker, basket).
• It does not make a flattened bridle acceptable because the crane still has capacity.

If you cannot explain the angle, you cannot use the angle.

That line is already in Proven. This is what it means in numbers. Measure θ from the horizontal. Run sin(). Compare T to the WLL of the sling and the hardware. If it does not fit, change the hitch — do not hope the steel is in a generous mood.

FIND θ FROM HEIGHT AND SLING LENGTH →

06 — RIGGING →

READ PROVEN →