WLL, SWL, MBL and design factor: four numbers people mix up

Minimum breaking load (MBL). The lowest force at which the accessory is expected to fail, established by testing and by the manufacturer's design. It is a strength, in kilonewtons or tonnes. It is not a load anyone is allowed to apply, ever, to anything.

Design factor. The number the breaking load is divided by to get a rating. Every accessory family has one fixed by the standard it is made to, and they are not the same: wire rope slings sit around 5, alloy chain around 4, synthetic round slings around 7, shackles around 6. It is sometimes called the safety factor, which is a worse name, because it is not a margin against your uncertainty. It is a margin against the material's, the manufacture's, and the wear the accessory will accumulate before it is condemned.

Working load limit (WLL). The breaking load divided by the design factor. It is the maximum force the accessory is rated to carry, in a stated configuration, marked on the accessory itself. This is the number you compare your calculated leg force against.

Proof load. A test load, typically a stated multiple of the working load limit, applied by the manufacturer or during thorough examination to confirm the item was made or remains sound. It proves the item, and it does not rate it. An accessory that has passed a proof test at twice its working load limit is still rated at its working load limit.

Read that table twice. Five accessories with the same 12 t rating have breaking loads spanning 48 t to 84 t. The rating is the number that means something across the whole set; the breaking load is not comparable between families, because each family's factor answers a different set of concerns.

The design factors above are the values these families are conventionally built to. Yours come from the marking on the item and from the standard the manufacturer declares, not from this article and not from memory.

The relationship is one line:

Every engineer can do that division, and doing it is almost never the task. Two things follow from it.

You cannot recover a bigger rating by choosing a smaller factor. The factor belongs to the standard the accessory is made to and to the wear it is allowed to accumulate before it is condemned. A wire rope sling at 5 to 1 has that factor partly because a rope with broken wires within the removal criteria still has to be safe. Reducing the factor because "this one is new" reprices a risk that is not yours to reprice.

The rating is for a stated configuration. A sling's rating assumes a particular hitch and a particular angle; a shackle's assumes in-line loading through the bow, on the pin, with the load in the plane of the bow. Depart from that and the rating reduces, and the reduction is applied by you, not by the accessory.

The common reductions, all of which are yours to apply:

• Angle. A sling leg at an angle carries more than its share of the vertical load. See the sling angle factor for the whole of that.
• Choke. A choked hitch reduces the rating, typically to around three quarters of the straight-pull value, because the rope is bent sharply against itself.
• Side load. A shackle loaded out of the plane of its bow, or an eyebolt loaded at an angle to its axis, is derated steeply. An eyebolt at 45 degrees off axis can be down to about a quarter of its in-line rating.
• Edge contact. A synthetic sling over a sharp edge can be cut through at a small fraction of its rating, and the loss is not a published percentage because it depends on the radius and the sling. It is a reason to fit protection, not a factor to apply.
• Temperature. Both synthetic and alloy accessories derate outside their stated range.

Safe working load is the older term, and in current practice it has three separate lives.

As a synonym for working load limit. Common in speech and on older equipment. Harmless when everyone means the same thing.

As the reduced, configuration-specific limit. In some traditions the working load limit is the general rating and the safe working load is the limit for a particular use: this sling, on this hitch, at this angle, on this job. Under that reading a sling can have one working load limit and many safe working loads.

As whatever the competent person signed off. In some organisations the safe working load is a value on a document rather than a property of the equipment.

New equipment is marked with a working load limit. That is the term the current standards use, and it is unambiguous.

The practical rule: on a drawing or a lift plan, never write a bare capacity. Write "WLL 12 t (straight pull)" for the accessory, and write "design load 78 kN per lift point" for what your calculation carries. Two different quantities with two different names, and neither of them is "SWL 12 t".

An accessory is rated. A sling, a shackle, a hook, an eyebolt, a chain: somebody built it, tested it, divided by a factor and stamped a number on it. Your job is to work out the force in it and compare that force with the stamped number. The design factor is inside the stamp. You do not apply it again.

A structure is designed. A padeye, a trunnion, a spreader beam, a lifting frame, a foundation: nobody stamped it, because you are the one making it. Your job is to compute a demand, compute a resistance from a design code, and show that one is smaller than the other. The margins live in the code's own factors and in the allowable stresses you declare.

The mistake is to bring the accessory's design factor across into the structural check. The reasoning behind it is sound as far as it goes: somebody knows rigging carries 5 to 1, sees a padeye in the rigging, and applies 5 to 1 to it.

That final number, 78.2 kN, is the design load. It goes two places, and it means something different in each:

• Into the accessory comparison, where it is checked against the shackle's and sling's working load limits. Those already contain their factors.
• Into the structural calculation, where it is the demand the padeye's checks are performed against, with resistances from the design code you have chosen.

It does not get multiplied by five on the way into either.

The lift is 12 t, with the ladder above giving 78.2 kN at the worst lift point, arriving 15 degrees off the padeye's axis. The padeye is 25 mm plate in S355, 200 mm wide, 45 mm hole on a 42 mm pin, with two 8 mm fillet welds 180 mm long.

Now apply a 5 to 1 design factor to the same demand, as though the padeye were an accessory being rated.

The cost is not only steel. A padeye sized this way is heavier, its weld is larger and slower to lay, its inspection takes longer, and the plate it is welded to now needs a local check it would not otherwise have needed. The arithmetic in it is correct throughout, which is what makes it hard to catch: the margin it appears to demonstrate came from applying a number outside the context that gave it meaning.

A design that is conservative for the wrong reason is not conservative. It is unexplained.

The gap is deliberate and it is where the engineering sits. The accessory standards rate accessories. The design codes give resistances for structures. The regulations require both to be respected. Nobody writes down the demand for your particular lift, so the demand is yours, and the two comparisons it feeds are different comparisons.

1. The design factor applied twice. The worked example. A factor that is already inside an accessory's rating applied again to a structure.

2. A breaking load used as a capacity. Rare, catastrophic, and it happens when a specification sheet quotes MBL and a spreadsheet reads it as WLL.

3. Design factors swapped between families. Sizing a chain sling with a wire rope sling's factor, or a round sling with a shackle's. They differ by nearly two to one between the extremes in the table above.

4. A proof load read as a capacity. A proof test at twice the working load limit does not create a rating at twice the working load limit.

5. The rated configuration ignored. A shackle side-loaded, an eyebolt pulled at an angle, a sling choked or over an edge. Every one of those derates the accessory, and every one of those reductions is the user's to apply.

6. SWL written without saying which SWL. The general rating of the item, or the limit for this particular use. On a lift plan those can differ by a factor of two.

7. The accessory checked and the structure forgotten. The shackle is fine and the padeye it is pinned through was never calculated. The accessory has a number stamped on it, which makes it the easy half to check, which is exactly why it is the half that gets checked.