Sling selection: sling types, choke and basket hitches, and the D/d ratio

A sling’s marked working load limit (WLL) applies to a straight, vertical pull with undamaged end fittings. Every other way of using it, whether choked, in a basket, at an angle or round a small pin, changes the load it can safely carry.

1. Choosing the sling type

Textile slings lose strength at relatively low temperatures. Follow the maker’s limits, typically around 80–100 °C for polyester, and keep them away from hot work. Alloy chain keeps its rating up to a temperature stated by the maker, then needs a reduction or must be withdrawn after heat exposure.

2. Choke hitch

In a choke hitch the sling bends sharply at the point of choke, and the eye or fitting rubs on the sling body. Capacity falls. EN slings carry a mode factor of 0.8 for choked use. For wire rope, the choker rating is usually about 75–80% of the vertical rating at a normal choke angle of 120° or more. If the choke angle is smaller, the rating drops further:

A freely hanging load chokes naturally at about 135°. Smaller angles occur when something else supports the load at the same time: turning a load over, tailing a column, or two chokers trying to rotate the load in opposite directions. Never force the choke down towards the load; let it settle.

3. Basket hitch and D/d

In a basket the sling passes under the load (or through a lug) and both legs go to the hook, so in principle it carries twice the vertical WLL. That is only true if the sling is not bent too sharply. The D/d ratio compares the diameter the sling bends round (D) with the sling’s own diameter (d). Bending a wire rope round a small D overstresses its outer wires and reduces its strength.

Basket capacity ≈ 2 × vertical WLL × D/d efficiency × cos β, where β is the angle of the legs from vertical. Use the sling maker’s values where they are published.

Example. A 32 mm wire rope sling with a vertical WLL of 10 t is used in a basket round a 160 mm trunnion, with the legs 30° from vertical. D/d = 160 ÷ 32 = 5, efficiency about 77%. Capacity ≈ 2 × 10 × 0.77 × cos 30° = 13.3 t, not the 20 t a quick look suggests.

The same principle applies to shackle pins and hook saddles. A sling eye over a small shackle pin is a bend with a low D/d. Many makers require a minimum pin diameter or a thimble for this reason.

4. Edges and protection

• A sharp edge cuts textile slings in seconds under load and crushes wire rope. A corner counts as sharp if its radius is smaller than the sling’s thickness or diameter.
• Use purpose-made edge protectors, corner pads or sleeves, rated for the sling type. Rags and cardboard are not protection.
• Keep labels and tags readable. A sling with no legible WLL must not be used.

5. Selection checklist

1. Calculate the tension in each leg (Chapters 2 and 3).
2. Apply the hitch factor (choke, basket) and the D/d efficiency.
3. Choose a sling whose reduced capacity is at least the leg tension.
4. Check the sling against the environment: edges, temperature and chemicals.
5. Confirm it is in date for thorough examination and passes its pre-use check.

6. Worked example: choosing a two-leg choked sling

A 12 t bundle is to be lifted with two slings, each in a choke hitch round the bundle, gathered on the hook. The legs make 40° to the vertical. The choke angle is over 120°, and the manufacturer gives a choke factor of 0.8.

• Load per leg (even split, CoG central): T = 12 ÷ (2 × cos 40°) = 12 ÷ 1.532 = 7.83 t.
• The choke reduces each sling’s capacity to 0.8 × its vertical WLL, so the vertical WLL needed is 7.83 ÷ 0.8 = 9.79 t.
• Select slings with a vertical WLL of at least 10 t, and check that the hook, shackles and any edge protection suit that size.

Had the 12 t been divided by two and the slings chosen at 6 t, each choked leg would have been loaded to 7.83 ÷ (0.8 × 6) = 163% of its choked rating.

The choke angle

The choke factor applies only when the choke angle (the angle the sling makes as it leaves the choke point) is about 120° or more. If the choke is pulled tighter, the capacity falls further. Typical reductions for wire rope used in published handbooks are:

Never force the choke down onto the load with a bar or by hammering the eye. Let it bed in under load. Never choke a sling round its own body against a shackle pin, and put the shackle bow, not the pin, against the sling running through it.

7. Baskets: inward force and crushing

A basket hitch carries the load on both legs, but it also squeezes the load. Each inclined leg pulls inwards with a horizontal force H = T × sin β: a leg carrying 10 t at 30° from vertical pulls inwards with 5 t. For a double basket under a long load, that pull acts across the load at each basket position.

Where the sling wraps round a round object, it presses on it all round the arc of contact. Treating the sling like a rope over a sheave, the average contact pressure for vertical legs is:

p = W ÷ (B × D)   W = load carried by that basket, B = width of the sling in contact, D = diameter of the object.

Example. A coated 8 t pipe spool, 508 mm outside diameter, is lifted in two baskets of 100 mm wide webbing, each carrying 4 t (39.2 kN). p = 39.2 ÷ (0.100 × 0.508) = 772 kPa (0.77 MPa). That is enough to mark soft coatings and, on large thin-walled pipe or ducting, to dent or ovalise the wall. Use wider slings, wide flat saddles or cradles, or spread each basket over a timber or plastic sleeve.

8. Inspection and removal from service

Every sling is inspected by the user before each use and thoroughly examined by a competent person at the intervals set by law (in the UK, at least every 6 months for accessories under LOLER). Remove the sling from service if you find any of the following. The detailed limits are in ASME B30.9 and the relevant EN standards; the manufacturer’s limits apply where stricter.

Temperature and chemicals

• Wire rope: fibre core about −40 °C to 82 °C (180 °F); steel core (IWRC) up to about 204 °C (400 °F) without reduction, above which the maker’s data is needed.
• Alloy chain: full rating to about 200 °C for most grade 80 and 100 chain; above that use the maker’s reduction table. After exposure to high heat, the chain must be examined before reuse.
• Polyester and nylon (polyamide): −40 °C to about 90–100 °C depending on the standard. Polypropylene: up to 80 °C.
• Chemicals: polyester resists acids but is attacked by strong alkalis; nylon resists alkalis but is attacked by acids. If a sling has been in contact with an unknown chemical, take it out of service and ask the manufacturer.

9. Common mistakes

• Dividing the load by the number of legs and ignoring the leg angle and hitch factor.
• Using the basket rating of a wire rope sling round a small pin or a tight corner, where the D/d ratio cuts the capacity far below the rating.
• Choking round a sharp edge without protection, or relying on a sleeve that the edge will cut straight through.
• Shortening a chain leg by knotting it or wrapping it round the hook, instead of using a proper shortening clutch.
• Leaving a webbing sling in sunlight for months and still trusting its label.
• Mixing slings of different stretch in one lift.

10. Self-check questions

General guidance only. It does not replace the manufacturer's data, the applicable standards or the judgement of a competent person.

References

Technical references consulted for this chapter. The text and worked examples are original to LiftPlan Pro.

• J. Keith Anderson, Rigging Engineering Calculations (Industrial Training International).
• Ronald G. Garby, IPT’s Crane and Rigging Training Manual: Mobile, EOT and Tower Cranes (IPT Publishing and Training).
• Rigging Handbook, 2nd edition (Bechtel Equipment Operations, 2002).
• ASME B30.9 Slings; ASME B30.26 Rigging Hardware; ASME B30.10 Hooks.
• BS EN 13414-1 (wire rope slings), BS EN 818-4 (chain slings), BS EN 1492-1/-2 (textile slings).

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