Sling angle factor: how to work out the tension in each leg

Two slings rated 8,000 lb each can still be overloaded by a 10,000 lb load. Whether they hold depends on the sling angle. Here is how the angle changes the tension in each leg, and how the Red Seal millwright exam tests it.

Video released 7 November 2026 · 14 min · Watch on YouTube · AI narration

Why a rated sling can still be overloaded

In a vertical hitch the sling hangs straight, and the tension in it equals the weight of the load. A 5,000 lb load on a sling rated 8,000 lb uses 62.5 % of the rating. That rating, printed on the tag, is for a straight vertical pull.

Put a 10,000 lb load on a two-leg bridle. If both legs could hang straight down, each would carry half, 5,000 lb. Spread the legs and the tension climbs. At 60° from horizontal each leg carries 5,775 lb. At 45° it is 7,070 lb, about 88 % of an 8,000 lb sling. At 30° each leg carries the full 10,000 lb, so 8,000 lb slings are overloaded by 25 %, on a load lighter than the two slings combined. Go flatter, to 25°, and the factor jumps to 2.366. Every degree lower costs more than the last one.

Where the extra tension comes from

Look at the forces on one leg. The upward part of the pull has to hold up half the load no matter what the angle is. An angled sling also pulls sideways, toward the other leg, so the two legs pull against each other. As the angle drops, the upward part stays the same but the sideways part keeps growing, and the sling carries both. That sideways pull lifts nothing. It is pure extra tension, and it also pushes in on the load and the lifting lugs.

Load angle factors to memorise

The load angle factor (LAF) is 1 ÷ sin θ, where θ is the sling angle measured from horizontal. Four values cover most exam questions:

Below 30°: not without a qualified person's approval and an engineered lift plan

At 30° the factor is 2, so each leg carries the whole weight of the load. That is why 30° from horizontal is the practical minimum.

Convert the angle first

The formula wants the angle from horizontal, but a question may give it another way. The angle from vertical is measured at the hook; subtract it from 90°. The included angle is the full angle between the two legs; halve it, then subtract from 90°. So 30° from vertical is 60° from horizontal, and a 120° included angle is only 30° from horizontal. At 45° from horizontal all three agree: 45° from vertical and 90° included.

The L ÷ H shortcut

On the shop floor you don't need trigonometry. Measure the sling length, L, and the height, H, straight up from the pick point to the hook. L ÷ H is the load angle factor. A 10 ft sling with the hook 8.66 ft above the pick point gives 1.155, the same as 1 ÷ sin 60°. It works at every angle because it is the same triangle. If H ÷ L gives you a number under 1, you have it upside down.

This is also why shorter slings make things worse. Pick points 10 ft apart on 10 ft slings give 60°. Swap in 7 ft slings because the hook won't go high enough, and the angle drops to about 44°, the factor rises to 1.429 and a 10,000 lb load puts 7,145 lb on each leg. If headroom is short, the answer is a spreader beam or a different lift plan, not shorter slings.

Chokers, baskets and four-leg bridles

A choker hitch is rated lower than a vertical hitch because the sling bends sharply at the choke: about 75 % of the vertical rating for wire rope and about 80 % for chain and synthetics. Check the tag. If the angle of choke is under 120°, derate again from the maker's table. An 8,000 lb wire rope sling is good for 6,000 lb as a choker; pull the choke down to 100° and it drops to 5,220 lb; below 90° it is 4,440 lb.

A basket with both sides vertical is rated at twice the vertical rating, but open the legs to 60° and the angle factor applies just as it does on a bridle: 16,000 lb becomes 13,853 lb. The load must also be big enough around for the sling (the D/d ratio).

On paper, four legs share the load. In practice the legs are never exactly equal and the centre of gravity is never exactly centred, so two legs can end up carrying it all. The conservative practice taught in Canadian rigging courses is to plan three- and four-leg bridles as if only two legs carry the load.

The formula

Tension in each leg equals the load weight, divided by the number of legs that actually carry it, times the load angle factor. Get three things right: the weight, the number of legs carrying, and the angle measured from horizontal. Most wrong answers come from one of those three, not from the arithmetic.

Worked exam questions

These questions are from the video. Try each one before you read the solution.

Common mistakes

• Angle given from vertical, not horizontal
• Included angle used as the sling angle
• Dividing by 4 on a four-leg bridle
• Forgetting the choker rating or the choke reduction
• H ÷ L instead of L ÷ H

Read the question twice, convert the angle first, and remember the load angle factor is always 1 or more.

Practise it

Sources

• Red Seal Occupational Standard (RSOS), Industrial Mechanic (Millwright): Task B-9, Plans rigging, hoisting/lifting and moving
• IHSA (Ontario), Hoisting and Rigging Safety Manual: sling angles, load angle factors, multi-leg bridle practice
• ASME B30.9, Slings: horizontal sling angles below 30° (as summarised in Lift-All and Mazzella sling user guides)
• Wire Rope Technical Board, Wire Rope Sling Users Manual: angle-of-choke reduction table (100 / 87 / 74 / 62 / 49 %)
• Kennedy Wire Rope & Sling, sling catalogue (Dec 2025): same choke table; note bracket edge worded 'over 120°' (some charts say 120–180°)
• Mazzella Companies, Wire Rope Slings: General Information: free-hanging choke ≈ 135°
• Every figure is computed by the CrewForge simulator and checked by hand against the sources above.

Exam preparation, not a procedure. On the job, check the tag, the manufacturer's data and your lift or work plan.