This guide is for supervisors, site engineers and small contractors who order crane lifts. A trained rigger normally rigs the load, but you need enough knowledge to ask the right questions and spot a bad set-up before the lift. These rigging basics come back to one number, the working load limit.
What is a working load limit (WLL) and where do you find it?
The working load limit is the most that a piece of lifting gear is marked to carry in normal use, for the hitch and angle the marking shows. It is stamped on metal fittings and shown on the tag of a sling.
You may still see SWL (safe working load) on older gear. If the gear is legible and undamaged, ask the supplier or a competent person to confirm it is acceptable; if there is any doubt, do not use it. Never use the breaking strength as a limit. The WLL is set well below it by a safety factor that depends on the type of gear.
The WLL of a rigging set is the WLL of its weakest part. A strong sling on a weak shackle is a weak set, so check the sling, shackle, hook and any lifting beam.
Watch out. If you cannot read the WLL, the gear is not usable.
A simple check for any set:
- Get the real weight of the load, and add the weight of the slings, shackles and beam.
- Note the sling angle and the hitch you plan to use.
- Read the WLL on each tag or stamp, for the angle and hitch where the item has them.
- Compare. The lowest figure in the chain decides what you may lift.
Sling types: wire rope, chain and synthetic
Each sling type suits different work.
| Sling type | Good for | Watch out for |
|---|---|---|
| Wire rope | General lifting, rough conditions | Kinks, broken wires, rust, crushed sections |
| Chain | Rough or abrasive loads. Heat can lower the rating, so use it hot only if the tag or manufacturer says so | Stretched or twisted links, nicks, bent links |
| Synthetic (webbing, round) | Finished or delicate surfaces, easy handling | Cuts, abrasion, heat, chemicals, sun damage |
Use the tag, not the type, to decide what a sling can carry. Colour codes are a hint, not a rating.
How do sling angle and hitch type change what a sling can carry?
Sling angle. Take a two-leg sling with equal legs and a centred load. The tension in each leg is half the load, divided by the sine of the angle between the leg and the horizontal. The flatter the legs, the higher the tension. At 30 degrees from horizontal, each leg carries about the full load by itself.
Note. Worked example only. Take a 10-tonne load on a two-leg sling. At 60 degrees from horizontal, each leg carries about 5.8 tonnes. At 30 degrees, each leg carries 10 tonnes.
This formula is for two equal legs and a centred load. With an off-centre load, unequal legs, or a three- or four-leg set, the legs do not share the load equally, so do not assume they do. Ask the rigger how the load is shared, and use the sling's tag or manufacturer data for multi-leg sets.
Check how the tag is written. If it rates the whole two-leg sling at an angle, compare that figure with the total load. If you are using single slings as legs, compare each sling's straight-pull WLL with the tension in that leg (5.8 tonnes in the example).
Fittings are checked against the force they carry. A shackle or hook at the end of one leg carries that leg's tension (5.8 tonnes in the example). The master link and the crane hook above the slings carry the whole load (10 tonnes). If two sling eyes sit in one shackle, ask the rigger to confirm that shackle is rated for that use.
The fix for a flat angle is simple: use longer legs, or a spreader or lifting beam (a rigid bar that keeps the legs steeper). If the tag shows no angle, or the angle is below the lowest one it shows, do not use it for that rig.
Watch out. Tags may give the angle from the vertical or from the horizontal. Read which one.
Hitch type. The hitch is the way the sling is attached to the load. Vertical hitch: one straight leg from hook to load. Choker hitch: the sling is passed round the load and through its own eye, so it tightens on the load. Basket hitch: the sling passes under the load and both ends go up to the hook.
A sling has different ratings for each hitch, normally shown on its tag or in its certificate. A basket hitch can be rated higher than a single vertical leg, but only at steep leg angles and over a smooth, wide bend. A flatter angle or a sharp edge cuts the rating. A choker hitch is rated lower, and a tight choke angle lowers it further.
Use the figure on the tag for the hitch and angle you will actually use. If the tag does not list them, ask before you lift.
Shackles, hooks and fittings: what to check
Fittings fail quietly. Check these before every lift:
- Rated and marked. The shackle shows a legible WLL. No mark, no use.
- Right pin, fully seated. A screw pin is fully threaded in and tight. A bolt or nail never replaces it.
- No side loading. A shackle should pull in line with its body, not sideways.
- Safety latch. A lifting hook needs a working latch that closes properly.
- Shape. Bent, stretched, opened or cracked fittings are out.
The hook must sit directly above the load's centre of gravity. If the load tilts as it rises, lower it and re-rig.
What should you see before the load lifts?
These are checks a supervisor can make from the ground:
- A test lift. The load rises a few centimetres and stops, so the rigger can check balance, hook position and slack in each leg.
- Rated lifting points. The load's own lugs or lifting points are rated for the lift. Do not hook onto pipes, rails or anything not made for lifting.
- Protected slings. Sharp edges have corner protectors, because a cut sling can fail below its marked WLL. Synthetic slings stay away from heat, weld spatter (hot metal sparks) and chemicals. In strong sun, keep synthetic slings out of direct sunlight when not in use, and check them for UV damage.
- No shock loading. No sudden jerks or drops, which load the gear far beyond the weight of the load.
- Clear ground. No one under the load or in its swing path.
- A tag line where the load can spin.
When should you stop and replace a sling?
Take gear out of service for any of these:
- No legible tag or marking.
- Cut, frayed or broken strands, or broken wires.
- Kinks, crushing or bird-caging (wire strands opened out like a cage) in wire rope.
- Heavy corrosion.
- Burn marks, melt marks or chemical damage.
- Distorted, stretched or cracked fittings.
- Damaged stitching or exposed core on a synthetic sling.
Never knot a sling to shorten it. Do not repair a damaged sling: quarantine it, tag it and replace it.
The person who uses the gear checks it before each use. A competent person also examines it thoroughly at set intervals and keeps a record. The site owner, the insurer and the municipality may each have requirements, so confirm them before you mobilise.
For the daily habit, see daily walk-around inspection and defect reporting.
Who is responsible for rigging on the day?
Agree responsibility before the lift. Ask the supplier and the client three questions:
- Who supplies the slings and shackles, and who checks them?
- Who is the appointed rigger or slinger (the person who attaches the load and signals the operator)?
- Who stops the lift if something looks wrong?
Anyone on site should be allowed to stop a lift. Rigging sits inside the crane lift plan checklist, and rigger and operator must both be competent: see how to check an equipment operator's competence. For what the crane itself can lift, read how to read a crane load chart.
How to do this on Maedah
When you post a request for a crane or hoist, say whether you need a rigger and rigging gear included, and add the load weight, dimensions and site access. When the sealed quotes come back side by side, compare what each includes: rigging gear, rigger, operator and certificates. Use chat to ask for the sling and shackle tags before the date. Browse cranes or hoists and winches to start.
