The honest answer is that it depends, and the most important variable is the one most operators get wrong. It is not the sling, it is the angle.
A polyester webbing sling or round sling stamped with a working load limit gives you that capacity in a straight vertical pull. The moment you put two legs on a load at an angle, the working load limit changes. Get the angle wrong, and a sling rated for 4 tonnes can be working well over its safe capacity without anyone noticing, until something fails.
This guide covers how to read the colour-coded working load limits on polyester slings, how the lifting angle affects what you can safely lift, and the inspection and operating practice that keeps everything in spec.
The Colour Code Chart
Polyester webbing slings and round slings are colour-coded so the working load limit can be identified at a glance, without reading the label. The colour code is the same across both sling types, which means a yellow webbing sling and a yellow round sling have the same straight-pull working load limit. Once you know the colours, you know the capacities.
The leftmost columns show the sling specification (webbing width, or round sling reference). The colour column shows the visual identifier. The right-hand columns show the working load limit in tonnes for different lifting configurations, from a single straight-pull leg through to multi-leg arrangements at varying angles.
The colour is a quick visual check. The label on the sling is the legal record and the source of truth. If the label is missing or illegible, the sling must come out of service.
Why The Angle Changes The Answer
When you lift with two or more sling legs, the load is shared between them, but not in the way most people think. The wider the angle between the legs, the harder each leg has to work to hold the same load.
A two-leg sling lifting with both legs vertical gives you the full combined capacity. Spread the legs apart, and the strain on each one climbs. At 30 degrees from vertical, each leg is working at about 87% of its straight-pull rating. Open it to 45 degrees, and that drops to around 71%. By the time the legs are 60 degrees from vertical, each leg is doing twice the work it would in a straight pull, and any wider than that, the loads climb steeply.
This is why the chart shows different working load limits for different configurations. The numbers are not arbitrary, they are the safe capacity at the lifting angles shown. Lifting at a wider angle than the chart allows is overloading the sling, even if the total load weighs less than the straight-pull rating.
The practical rule is simple. Plan the lift, check the angle, and read the working load limit for that configuration, not the straight-pull figure stamped on the label.
Selecting The Right Sling
The colour tells you the capacity. Selecting the right sling for the job means thinking about the load itself.
Consider how the load will be lifted. A balanced load with smooth surfaces is straightforward. A load with sharp edges, friction points, or an uneven centre of gravity needs more thought and may require lifting protection or abrasion sleeves to prevent the sling from being damaged in service.
Consider the working environment. Polyester slings are suitable for use across a temperature range of -40°C to +100°C and have selective resistance to chemicals. If the sling will be exposed to chemicals, solvents, or temperatures outside that range, get advice on the right specification before you commit to a sling type.
Consider how the sling sits on the load:
- The load needs to be uniform across the sling’s width
- The load needs to be balanced and unable to tilt or fall once tensioned
- The load needs to be free to be lifted, which is what a trial lift confirms
Pre-Use Inspection
A sling should be examined before every use. The check is quick once you know what to look for.
Lay the sling out on a flat, well-lit surface. The first thing is the identification label. If it is missing, damaged, or illegible, the sling cannot stay in service. The label is what proves the sling is what it claims to be, including its working load limit, batch, and manufacturing date.
Examine the sling along its full length, looking for:
- Surface chafe or abrasion damage
- Cuts across or along the weave
- Damage to the eyes
- Broken stitching at the load-bearing seams
- Damage to coupling components or steel terminal fittings
The contrasting-colour thread on the load-bearing seams is there to make this faster, so any broken or pulled stitches stand out.
If anything looks questionable, the sling does not go back into service until it has been examined by a competent person. When in doubt, ask. A second opinion costs nothing. A failed lift costs everything.
Safe Operating Practice
Once the sling is in service, the operating rules are short, but every one of them matters.
Plan The Lift
Know the load weight, lifting points, angle, destination, and route. You will want to make sure the area below the load is clear and stays clear throughout the lift.
Tension And Lift Carefully
When tensioning the sling and landing the load, keep fingers, hands, and feet clear. Take a trial lift, lower it back down, confirm the load is balanced, and the sling has settled correctly, then make the lift properly. When lowering, take care not to trap the sling under the load, and never try to pull a sling out from beneath a load that is resting on it.
Protect The Sling
Avoid dragging the load in the sling or dragging the sling itself across rough ground. Polyester is tough, but abrasive surfaces will damage it. Avoid snatch or shock loading, which can spike the load on the sling well beyond its working capacity, even briefly.
What Never To Do
- Never overload a sling
- Never knot or twist a sling
- Never use a damaged sling
- Never repair a damaged sling (damaged means out of service)
Storage And Care
When not in use, slings should be stored clean, dry, and well ventilated, at ambient temperatures, on a rack rather than on the floor, and away from contact with chemicals. Polyester is a durable material, but ultraviolet light, chemical contamination, and prolonged dampness will all shorten its working life.
A sling that has been used in an unusual environment, exposed to chemicals, soaked, or subjected to a shock load should be examined by a competent person before it goes back into service, even if it looks fine.
When To Remove A Sling From Service
A sling comes out of service if:
- The label is illegible or missing
- There is visible damage to the webbing, eyes, or seams
- There is damage to the coupling components or the steel terminals
- It has been subjected to a shock load or any incident that may have exceeded its rated capacity
Under LOLER 1998, lifting accessories, including webbing slings and round slings, must be thoroughly examined at least every six months by a competent person. That examination is the legal check that the sling remains safe to use, and it is a separate, more rigorous process than the pre-use inspection an operator carries out before every lift.
Need Slings?
SWL manufactures polyester webbing slings and round slings in our own UK workshops, to BS EN1492-1 and BS EN1492-2, respectively. Standard sizes off the shelf, custom lengths and configurations made to order, and full certification supplied with every sling.