What a Rated Capacity Actually Refers To

Why this matters

The number stamped on a sling tag is not a permission slip for that weight. It is the answer to one narrow question: how much can this sling hold in one specific loading condition, in as-new condition, pulled straight. Every real lift departs from that condition in at least one way, and the departures take capacity away rather than adding it. A rigger who reads the tag as a weight limit and stops there has skipped the entire job. The failure mode is not gradual, and it does not announce itself: the sling holds, holds, holds, and then it does not, with a load in the air and people underneath.

This card is about what the published number covers and what it silently assumes. Its siblings cover the individual corrections. This one owns the two claims the others build on: the rated capacity describes one loading condition, and the design factor is already inside it.

Where nobody stands, before any of the arithmetic

The exclusion zone is not "roughly over there." It is the area under the load plus the area the load can reach if a leg lets go, and on a two-leg bridle a released leg swings the load toward the surviving leg, so the zone is wider on the side you did not expect. Everyone clears that footprint before the load leaves the ground, and the rigger who guides the load does it with a tag line from outside the footprint, not with a hand on the load. Nobody reaches between a sling and a load while there is tension in the sling, because that pinch closes faster than a hand comes out. Nobody stands in line with a taut sling leg, because a failed leg travels along its own axis.

A hoist brake is a stopping device, not a parking device. A load left hanging while people work under it is a load resting on a brake that is not designed for the duty, so the load comes down and gets blocked before anyone goes near.

The one loading condition the number describes

A sling's vertical rated capacity assumes all of the following at once. Every one of them is a condition, and a lift that breaks any of them is no longer described by that number.

• A single sling, pulled straight along its own axis, eye in line with the hook, no angle between the leg and the direction of pull.
• The load hanging free and static. No jerk on pick-up, no snatch, no wind, no sudden stop at the top of the hoist stroke.
• Bearing surfaces at or above the sling manufacturer's stated minimum diameter. The rating is derived over a specified pin or saddle size, not over whatever fits.
• The sling in as-new condition, at ordinary ambient temperature, unrepaired, with a legible identification tag.
• The load's weight known, and the hook over the load's centre of gravity.

Take any one of those away and the number on the tag is still true about the test condition and no longer true about your lift. That is not pedantry. It is the whole reason sling tags carry more than one number.

The design factor is inside the number, not something you add

A sling's rated capacity is derived by dividing its minimum breaking force by a design factor. The factor is a material-and-standard decision, not a per-lift decision: sling manufacturers' ratings and ASME B30.9, in the edition your jurisdiction or your employer's programme has adopted, commonly carry 5 to 1 for wire rope, synthetic web and roundslings, and 4 to 1 for alloy steel chain. The tag governs; if the tag and a remembered figure disagree, the tag wins.

This matters in one direction and one direction only. A correction you apply to a rated capacity is a re-basing of a number that already contains the design factor, not an addition of margin on top of a raw strength. So:

• Applying a second 5 to 1 on top of the tag number is not extra safety, it is a bookkeeping error that sends you looking for a sling four times heavier than the lift needs. Heavier slings are harder to place, harder to keep in the bight, and get chosen for the wrong reasons on the next job.
• The genuinely dangerous version of the same confusion runs the other way. A test certificate, a proof-load figure or a breaking-strength number off a supplier sheet has no design factor in it. Treat one of those as a working number and you have not lost margin, you have lost all of it.

If a number does not come off the sling's own identification tag or the sling manufacturer's rating table for that exact sling, do not use it as a rated capacity. The qualified person your employer has designated owns the call on anything the tag does not answer.

The departures compound, they do not take turns

Angle, hitch, bend severity, side load, shock and an off-centre hook each remove capacity, and they apply to the same lift at the same time. Two riggers arguing about which one matters most have both missed it: they multiply.

The order that keeps the arithmetic honest is share first, then geometry, then the sling body:

1. Share. How much of the load is on this leg, given where the hook is relative to the centre of gravity and how many legs you may legitimately count.
2. Angle. Divide that share by the sine of the leg's angle from horizontal. State the convention every time, because half the trade quotes the angle from the vertical and the two give reciprocal answers.
3. Hitch. Read the column on the tag that matches how the sling is actually rigged. This is not a correction applied to the vertical number; it is a different published number.
4. Bend severity at the bearing point. Re-base to your actual geometry using the sling manufacturer's efficiency table when the bearing diameter is below the minimum the rating assumed.

Shock is not on that list because it is not a factor you multiply in. A design factor computed for a static pull is defeated by a dynamic one, and the multiplier depends on the speed of the stop, which you did not measure. The control is not arithmetic; it is taking up slack slowly, lifting the load clear before travelling, and stopping without slamming.

Worked example: turning a known weight into a required tag reading

A skid the fabricator's shipping document lists at 4,000 lb, picked with a two-leg bridle, hook centred over the centre of gravity, both legs equal length, both at 45 degrees from horizontal. Each leg is a straight pull from its eye to the hook, so each leg is a vertical hitch. The legs land on a crane hook whose saddle diameter is below the sling manufacturer's stated minimum for that sling body.

Every qualifier from the section above gets its own line, so the correction is visible rather than assumed:

• Load weight, from the shipping document: 4,000 lb. Not estimated, not judged by eye.
• Legs counted: 2, valid here because the hook is over the centre of gravity and the legs are equal.
• Vertical share per leg: 4,000 / 2 = 2,000 lb.
• Angle factor, from horizontal: sine 45 degrees = 0.707. Leg tension = 2,000 / 0.707 = 2,829 lb.
• Hitch: vertical, so read the tag's vertical column. Factor applied: 1.00, and printing a factor of one is a decision, not an absence of one.
• Bend severity at the hook saddle: say the sling manufacturer's efficiency table gives 0.85 at that ratio. That figure is illustrative and belongs to the table for your sling, not to this article. Required rated capacity = 2,829 / 0.85 = 3,328 lb per leg.
• Design factor: not applied. The 3,328 lb is already a rated-capacity figure, so it is compared directly against the tag's vertical rating.

Select a sling whose tag vertical rating is at least 3,328 lb, rounding up to the next size the manufacturer publishes. Rounding up is the conservative direction because rounding a required capacity down, or a rated capacity up, is the arithmetic that drops loads.

Notice what the untreated version looks like. A rigger who reads 4,000 lb, halves it, and picks a sling rated 2,000 lb has under-selected by a factor of 3,328 / 2,000 = 1.66. Against the 5 to 1 design factor inside a wire rope, web or roundsling rating, that sling is now working at an effective 5 / 1.66 = 3.0 to 1. On an alloy steel chain sling, whose rating commonly carries 4 to 1, the same under-selection lands at 4 / 1.66 = 2.4 to 1. It will very probably hold. It will hold every time until the day something else on the list, a slightly flatter angle or a shock on pick-up, uses up what is left.

What would flip this

• The weight is not known. If no nameplate, drawing or shipping document gives it, that is the finding. Say so and stop. There is no arithmetic in this card that survives a guessed weight, and adding margin to a guess produces a confident wrong answer rather than a safe one.
• The hook is not over the centre of gravity. The share stops being weight divided by legs, and the article on computing bridle leg tension owns that correction.
• The lift is a crane lift rather than a hoist pick. The crane's load chart, not the sling, becomes the governing document, and the chart figure already contains the machine's own weight and often a rigging allowance, so read what is inside it before you subtract anything. Crane work in construction sits under 29 CFR 1926 Subpart CC.
• The sling is not in as-new condition. A rating describes a sling that has not been damaged. Damage is not a percentage you deduct; it is a removal decision.

How to verify you got this right

Read the tag with the sling in your hand and the load on the ground, and confirm three things: the tag is legible and belongs to that sling, the column you read matches the hitch you actually rigged, and the rated capacity you wrote down is greater than the tension you computed rather than greater than the load weight. Those are different numbers, and confusing them is the single most common way a correct calculation ends in an under-rated sling.

Then check that your arithmetic went in the conservative direction at every rounding. Required capacity rounds up. Angle from horizontal rounds down if you are estimating it. An efficiency factor rounds down. If any of your roundings made the lift look easier, redo it.

Finally, before the load leaves the ground, take the strain and hold it just clear, with everyone outside the footprint described above, and look at the slings under tension. A sling that is going to slide, a leg that is longer than you thought, or a load that wants to tip shows itself in that first inch and nowhere else.

References

• 29 CFR 1910.184, slings, for general industry work, and 29 CFR 1926.251, rigging equipment for material handling, for construction work. Name which Part applies to the job you are on before quoting either.
• 29 CFR 1926 Subpart CC for cranes and derricks in construction, where a load chart rather than a sling tag governs.
• ASME B30.9, slings, in the edition your authority having jurisdiction, your contract or your employer's programme has adopted. A consensus standard binds through one of those, never on its own.
• The sling manufacturer's rating table and identification tag for the specific sling in your hand, which owns every efficiency figure and every temperature limit this card routes to.
• See related: Why Sling Angle Costs More Capacity Than Anyone Expects; What the Three Basic Hitches Each Do to Capacity; What the D Over d Ratio Does to a Sling.