Synthetic Roundslings – What you need to know
Thursday, 20 September 2018
Roundslings are amongst the most common types of rigging equipment used for a wide variety of lifting applications. They range from small and inexpensive slings which are one of the few genuinely ‘throwaway’ types of rigging available right up to very large high capacity slings which have successfully displaced big wire rope grommets for major heavy lifts.
The chief benefits of roundslings are that they are generally inexpensive and light weight. However, their use is controversial in some sectors as there are now a number of well publicised incidents where they have failed under load, to the extent where some people have even called for them to be banned.
Good care of roundslings and a thorough understanding of their safe use is critical to avoiding failure.
What are Synthetic roundslings?
Roundslings are simple. The fundamental component of any round sling is its core, which bears the load. The core consists of a continuous monofilament synthetic fibre yarn. This is wound around and around until there is enough strength to form a useful sling. The new Australian Standard AS4497:2018 requires that there are at least 11 turns of yarn.
Sling manufacturers may use as many turns as they need and may use as many pieces as they like by using a simple join, often made with a basic loop and/or adhesive tape. AS4497 requires that any joins be at least 4 turns apart from one another.
This construction is not useful on its own and can easily fall apart into a tangled mess. That is why every round sling is encased in a protective woven cover. This is basically a tube like a very long sock. The cover is joined by overlapping this ‘sock’ and stitching it together. Then a tag is attached to identify the sling type. material and working load limit.
Sharp corners and the myth of the indestructible sling
Roundslings are not indestructible. Roundslings can and do fail if not used correctly. One of the easiest ways to destroy a roundsling is to load it around a sharp edge. However, this risk is often not treated with the respect it deserves.
For an edge to be sharp enough to damage the roundsling, the edge need not be sharp enough to cut it. To be sharp enough to degrade, damage and ultimately break a roundsling under load an edge needs only to be ‘relatively sharp’, that is – to have a relatively sharp corner radius.
How sharp? The new Australian Standard defines this as the ratio of Corner Radius to Sling Thickness:
- Corner radius / sling thickness ≥ 1 at an end fitting, and
- Corner radius / sling thickness ≥ 3 anywhere else.
Firstly, slings can vary greatly in thickness. Higher capacity slings are thicker than lower capacity ones, and different brands of sling and materials used result in variations in the thickness. In addition, the thickness we are interested in here is the ‘compressed thickness’. The measurement method is given in the new standard.
Secondly, what appears to be a ‘blunt’ edge can actually be sharp, especially with high capacity round slings. A 10 tonne roundsling might be about 20mm thick, but a corner with a 10mm radius won’t cut your hand so how can it hope to cut the tough woven cover of a sling?
The key is to understand that sharp corners hurt slings in a variety of ways. These include:
- Cutting. If a corner is so sharp that it can cut anything, with the forces involved in lifting it will cut a sling. This is a proper sharp edge and you might already be wearing gloves to protect yourself from this.
- Pressure. When a corner has too tight a radius, the area of contact to the sling becomes small. Once lifting forces are applied this equates to a high pressure and compressive stress upon the sling material. Being synthetic, this resistance to pressure is limited – if there is any movement of the sling involved such as where a sling is choked around a load for example then pressure plus movement also creates heat due to friction which can rapidly destroy synthetic materials.
- Bending. Although they are very flexible, roundslings are still subject to some of the same physical effects as any other material is when bend around a corner. Just as the outer fibres of a rope experience more tension going around a bend so too do the roundsling core fibres on the outside of a corner on a load. If the corner is too tight relative to the thickness of the sling, the forces on the outer fibres inside the core can become too high with inevitable risk that they will break.
Good practice roundsling use
Once you’ve checked the thickness of your sling, you must also check the corners on your load. These must be greater than three times the sling thickness. Often they are not – in which case protection must be used. Protection can be provided against wear, abrasion and to provide cut resistance. If the corners of the load are less than three times the thickness the nett radius presented to the sling is also important. Guidance is provided in the new standard (AS4497-2018)
Within an end fitting, and in any position about an interface, it is important that roundslings are able to form into their natural shape under load. Where a roundsling is fitted into a tight space so that the edges are bunched up eventually the sling jams into place and is damaged. Before that happens however, the bunching up can act to reduce the strength of the sling, disrupting the normal behaviour and performance of the sling’s load bearing core. The new standard sets rules describing the width of the compressed sling and requiring that the space within the fitting be at least as wide.
Although it is permissible to use a contact radius that is only one times the sling thickness within an end fitting, using a smaller radius is not allowed. With smaller contact radii the loss of strength within a roundsling can be severe to the point where slings can rupture when lifting payloads which are within the working load limit (WLL) of the sling.
It is vital that all end users of roundslings are aware of the dangers of sharp corners and are aware that as slings get thicker what constitutes a sharp edge and therefore a hazard can be unexpected. Some examples of unexpected sharp edges that can occur are:
- In the belly of a DIN standard crane hook
- Where inappropriate corner protection has been used and this fails to provide cut protection
- Where inappropriate corner protection has been used and this crushes under load so that the corner radius against the sling is still too tight
- Where there are side loads against a sling and the sling rubs or bears across an obstruction, or at the sides of the sling interface
- Where an end fitting or interface is damaged and has dents, burrs and chips that are sharper than intended.
Inspecting synthetic roundslings
With the risk of abuse and vulnerability to ‘blunt’ sharp edges it is only natural that roundslings should have a stringent inspection regime and discard criteria.
As with all slings, roundslings must be inspected before each use. In addition, the Australian Standard AS 4497 mandates that and all roundslings must have a periodic and documented visual inspection with an inspection frequency of at least every 3 months. These periodic inspections must be performed by a person who meets the criteria of a competent person as defined within the standard.
This is important, since in order to inspect a roundsling thoroughly, the inspector must be able to sufficiently perceive internal and external wear, tear and damage. A competent inspector must be able to objectively interpret any features that may present themselves as visual and tactile indications upon closely examining the cover and feeling the core along the entirety of each sling.
The inspector should remove any synthetic roundsling from service if they see any of the following:
- Any cuts or holes in the cover which expose the internal yarns of the round sling
- If any identification tags have been removed/damaged or are not legible
- If a round sling has been tied into one or more knots
- Any melting, charring or weld spatter of any part of the sling
- Acid or alkali burns
- Broken or worn stitching in the cover which exposes the core fibres
- Any other conditions which cause doubt as to the strength and integrity of the round sling.
- Any evidence of chemical damage whatsoever including discolouration.
Nobles are acknowledged experts in the field of sling inspection and have are able to manage and schedule competent inspections of roundslings that are in use. This competence is important because natural wear and environmental exposure detracts from the pristine ‘new’ appearance of slings but it is important to be able to distinguish between superficial and significant deterioration which may arise from not only mechanical damage but also chemical and environmental exposure.
For more information on the latest Australian Standard on roundslings see the Quicklinks. To organise a professional Nobles inspection of your roundslings or other lifting & rigging gear call 1300 711 559 or email us at email@example.com.
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