For prospective riggers and operators, it is important to understand how to properly use slings and to recognise sling weaknesses during pre-use inspections.
Since slings typically handle loads in less than optimal conditions and in combination with equipment of varying ages, strict caution should always be exercised during their use. Adherence to a wide range of general operating practices can ensure secure lifts and reduce the possibility of sling injury.
Overall, the load weight should always be within the sling’s rated capacity, with sling angles of 45 degrees or less and shock load should be avoided. Any sharp corners in contact with the sling should be padded, as failure to do so will allow the edges of the sling to be cut.
Slings must not be shortened or extended by knotting, twisting or wire rope clips unless such action is authorised by the manufacturer. When slings are stored, you should always select an area that is not subject to mechanical damage, corrosive action, dampness or excessive temperatures.
There are three primary types of slings, each with its own advantages.
Wire Rope Slings
Known for their great strength and flexibility, the wire rope slings don’t wear as quickly as any other kind. Core, wire, and strand are the three components of the wire rope sling.
The core serves as a foundation that holds the sling together and is either made up of fibre, wire or an independent wire rope, which is the strongest option. The wires are usually made of carbon steel and laid helically around the core to form a strand. Usually, 16 to 26 wires consist of a line, with six strands rotated in a clockwise direction to form a wire loop.
Each rope throughout its use is subject to fatigue due to bending stress. In order to prevent fatigue, the wires must constantly bend under tension, as when the rope passes over the sheave, without displaying signs of wear or deformation.
A rope made of several wires would have a higher resistance to fatigue than a rope of similar size made of fewer, larger wires because smaller wires bend more quickly when a rope passes over sheaves or drums.
Since wire rope slings gradually lose strength due to natural causes, operators and riggers have to determine the rope’s safe working load before use. It is done by dividing the braking power of the rope by five, which creates a safety margin when new rigging equipment is used.
The daily pre-use inspection of the ropes is of the upmost importance, with the person using the sling being responsible for the visual examination. Such a person should initially look for damaged wires, severe localised abrasions, scraping, and malformed end attachments. Also noteworthy are kinking, corrosion or other damage resulting in permanent distortion of the rope, the rope being removed from service due to such wear.
Synthetic Web Slings
Foldable and capable of moulding around shapes, synthetic web slings are either nylon or polyester. Nylon slings are most widely used and have an elastic section of 6 percent at their indicated capacity. Polyester slings have a 3% stretch, providing a cushion against sudden shock. The size of either synthetic sling shall be determined by the width of the web.
While flexible, these slings are heat sensitive and should never be exposed to temperatures over 180 degrees Fahrenheit. They should also never be used in areas exposed to radiation. Instead, metal mesh slings capable of handling abrasive or hot loads should be used in these situations.
Web slings inspectors should first look for any signs of fabric discolouration resulting from melting, charring, or caustic attack. Any portion of the sling that shows such damage is enough to remove the sling from service/operation.
When on web slings holes, tears, or cuts are identified, it is a judgmental call whether the sling is to be taken out of service. The inspection of the inner threads of the sling will determine what measures should be taken. When signs of wear or degradation are found, the sling must not be used, because the inner threads make up 80% of the strength of the sling. Once the red guard alarm yarn is revealed, the sling must be removed.
Also noteworthy is the appearance of fractured fibres on the webbing surface; this is a sign of excessive wear that is most commonly caused by the load slipping in the sling.
Any sling with knots should not be used, because it reduces the efficiency of the sling. Broken or broken stitching in load bearing splices can also cause a major reduction in sling strength.
Chain Slings
Constructed of alloy steel, chain slings are ideally suited for lifting rough loads and withstand high temperatures; however, they must be closely tested and then used, as the failure of a single chain link can result in an accident.
When checking for defects, test the total length of the chain sling and decide whether it suits the length of the inspection tag. The sling must be taken out of service if the sling is shorter or longer, taking into account acceptable tolerances.
Checking the master link, connecting link and associated chain links for signs of elongation, wear, bending or deformation is also mandatory prior to the use of the chain sling.
Hitches
Hitches are also used in hoisting operations to facilitate slings.
A straight hook uses a single sling to attach the lifting hook to the load. When the hitch is in service in this way, the maximum qualified lifting capability of the attached sling may be used, but not surpassed. The tagline should also be used to avoid the rotation of the load, which could damage the sling.
If two or more slings are attached to the same lifting hook, the total hook becomes a bridle, with the load being divided between the individual slings. In making bridle hitches, the angles of the sling must be carefully calculated to ensure that the legs are not overloaded. In order to disperse the load evenly, it might be appropriate to change the length of the leg with turnbuckles or chain hoists.
The basket hitch is used to divide the load evenly between the two legs of the sling. The efficiency of the sling used in the basket is influenced by the bend or curvature where the sling body is in contact with the load.
When the lift needs the sling to be snug up against the load, the choker brace is used. The choker must be pulled close on the body of the rope, not the choker itself before the lift is made. Operators are recommended to use more than one shock absorber when raising a load which can change or slip during travel.
Since the overall role of slings in raising and moving various loads is unlikely to change going forward, an in-depth understanding of how slings can be properly used and checked remains a necessity for any operator who wishes to perform safe hoisting operations.
About Us
RHT specialises in lifting equipment and is here to help you with all of your lifting conundrums. If you need help or advise of which lifting slings you should buy or how to shorten or lengthen lifting slings you have purchased from us, then please don’t hesitate to get in touch, we will be happy to point you in the right direction or give you the knowledge and tips that we have gained through being an active member of the lifting industry in the UK.
