A sling that lifted a steel beam last month, failed yesterday, with a lighter load. No one expected this. The load limit tag said 2 tonnes. The steel beam weighed 1,800kg. The angle seemed correct. Except it wasn’t right.
Mistakes like this happen too often. Not due to negligence. Sling selection appears simple. The sheer number of mistakes comes from a lack of understanding that the load limit tag says what a sling can lift with no angle, no load, and in the perfect conditions. Any change to the conditions, and the limits change too.
The angle that nobody thinks of
Most of the mistakes come from a miscalculation of what is referred to at the job site as the angle of the load, or tension, as opposed to the angle of the sling.
Think about a load that is being lifted by two slings from the same point and at a 30-degree angle. This does not give you a lift limit of 4 tonnes. Instead it gives you the same lift limit of 2 tonnes, as one sling that’s doing the lift straight up. The variation is amplified the greater the angle is.
Choker hitches lose capacity
Using a choker hitch, where the sling passes through its own eye after wrapping around the load, already cuts lifting capacity by 25%, before considering the angle.
The reason for this reduction is because the sling is forced around a corner, which applies pressure unevenly on the wire fibres and strands. If the load pinches the sling, expect a potentially massive reduction up to 50%.
Choker hitches cleanly grip the load and help prevent load rotation. However, the choker hitch tied using a 2 tonne sling, can provide a capacity as low as 1.5 tonne and potentially lower.
Choker hitches also can lead to sling and load failure, and potential injury, if a hammer is used to tighten the choker hitch.
The hitch makes the difference
The three main configurations for slings are the choker, the basket, and the vertical.
For a vertical hitch, the capacity is whatever the tag says, and there is no capacity loss due to the hitch.
The basket hitch however may provide the potential doubling of the capacity, although this is strongly dependent on the angle and if the load is balanced. An angle exceeding 45 degrees, the advantages of the basket hitch quickly diminish.
Choker hitch will only knock off 25% straight away, and you still have to account for additional angle reductions.
This isn’t any guesswork; the formulas have been established. It’s mainly just human negligence, or people presupposing that the years they’ve accumulated in the field justifies not practicing these things. It may be some of the time, and some of the time, it may not be.
Sharp edges and missing tags
Synthetic slings are great for protecting surfaces and are generally easier to handle. Unfortunately, they’re generally easier to break as well. A sharp edge of a steel beam or a concrete block will literally cut the webbing while under load.
Wire rope slings can generally take edges better, but they’re not exactly invincible. Individually breaking wires of a ragged edge will still weaken the sling, as a few strands breaking will break the integrity of the sling.
This can be ‘fixed’ by using proper edge protection rated for the job, or simply moving the sling away form the edge it is supporting. Padding the edge rated for the job may or may not do the trick, but that’s a monster’s respawn pad at the end of the level.
Every sling has to be identified with a tag which shows the manufacturer, inspection date, and the working load limit. If the tag showing the working load limit is missing or is illegible in any way, that sling is out of service. If you don’t know the capacity, you’re guessing. Or, you can look at the tag, and that’s your answer.
Quick reference for foreman
Vertical hitch: Full capacity from the tag.
Basket hitch: At 90 degrees, capacity doubles. At 60 degrees, about 1.7x. At 45 degrees, puts it at about 1.4x. At 30 degrees, back to just the single-leg capacity
Choker hitch: Capacity decreases to 75% vertical rating. Choking tighter reduces capacity further.
Sling angle is important. The angle between the sling leg and the vertical should be more than 45 degrees. If it is lower than 30 degrees, a different method should be found to execute the lift.
Slings should be protected against sharp edges to ensure longevity.
Sling conditions should be checked. Damaged slings, slings missing tags, etc. result in no usage.
Choosing the Right Sling Size: Overestimating the capacity of the sling is more advantageous than underestimating it.
Importance of the statement
A poor choice of a sling results in a failed execution of a drop. It can result in a whip back. This can result in injuries, damages to the equipment or further failures of the rigging system. The HSE gets involved, the site is closed, and in result there is a loss of life.
It is important to choose the right sling. It takes 30 seconds of your time but the tag takes a second and the hitch type, the angle, the load shape and the sling condition change the value.
The expression is only valid until the time it is not. It results in an expiration of an HSE inspector and it does not end well.
Looking for lifting gear training? ACT Construction Training is your source for the provision of LOLER Compliant lifting gear training to your nearest and approved providers. Please contact via the email: info@act-constructiontraining.co.uk.