For anyone in the industry, some of your most useful and frequently used tools are the slings that allow you to safely and efficiently complete your projects. Most of us have a basic understanding as to how each type of sling is constructed, but few know the real ins-and-outs of what gives a sling its strength and the amount of work behind its construction. At Lifting Gear, we go through great lengths to understand the driving force behind each piece of equipment in our fleet so we can ensure the safest and most reliable gear makes it onto our customer’s job sites.
I’d like to pass along some insight into what makes these slings, from their construction to the maintenance and inspections that keeps them safe but, at the same time, don’t want to make your eyes bleed. That said, today, we’ll cover wire rope and we’ll cover the other sling types in future installments.
So we all know wire rope derives its strength from the strands of wire that are twisted around a core cable, right? You may ask what else is there to know about their construction beyond that. Well, frankly, a lot.
Our rental fleet has sizes of wire rope ranging from 3/4” up to 4” in diameter for single part slings and 2-1/2″ to 11″ diameter in our 9-part braided slings, but there are larger varieties available for purchase. Each wire rope sling is made up of several strands, which is then twisted in a helix around a central strand that is often the same composition as the outer strands. A wire rope strand is typically made up of anywhere between 19 and 36 wires but can reach as high as 109 wires, which are then twisted around a core wire or strand, depending on the configuration of wire rope.
Knowing the composition of the sling you’re using gives an understanding of what the rope will be able to do or, in some cases, how it will act. Various configurations exist to combat crushing, allow for greater flexibility or reduce the chances of rotation among other things. Ensuring you’ve got the correct wire for the project can be the difference between a successful job or causing damage or complete failure. As a general rule of thumb, improving one aspect will tend to reduce another. For example, a wire rope made up of smaller diameter wires provides increased flexibility, but doesn’t offer great abrasion resistance, whereas a wire rope of larger wires increases abrasion resistance while remaining more rigid.
The most common configuration of wire rope, the 6×19 class, consists of six outer strands comprised of 15-26 wires each that are twisted around a center core of smaller diameter wires in a 6×19 configuration consisting of the same number of wires as the outer strands. This configuration offers greater abrasion and crushing resistance while sacrificing a bit of flexibility and is available in sizes ranging from 3/4″ to 1-1/8” outer diameter. The next variation, for thicker outside diameters, is the 6×36 class comprised of six outer strands made up of 27-49 wires. This configuration offers increased flexibility due to the thinner wire diameters while maintaining crushing resistance but does sacrifice some of the abrasion resistance seen on the 6×19 class. LGH has 6×19 and 6×36 wire ropes from 1-1/4” to 4” outside diameter ranging from 5.6 to 130 tons safe working load (SWL) as well as 9-part braided slings that range from 59 tons to 485 tons SWL.
The next aspect of wire rope to consider is the lay of the wires that make up the strands as well as the way the strands are laid around the core. There are two main classifications of wire rope lay seen with alternating directions within each. These classifications are Regular Lay and Lang Lay.
Regular Lay wire ropes are formed with the wires that make up the strands being twisted in one direction, either left or right, and the completed strands are then laid the opposite direction, which causes the finished product to appear like the wires are running parallel to the axis of the rope. Regular lay rope is more flexible and carries better resistance to crushing forces and is easier to splice than Lang lay rope but has a shorter lifespan. Regular lay rope also tends to spool on a drum more easily and is naturally more rotation resistant.
Lang Lay wire ropes have both the wires forming the strands and the finished strands twisted the same direction, either right or left, and causes the finished product to appear with the wires running diagonally to the axis of the rope. The advantage of Lang lay rope is the increased abrasion resistance leading to a longer lifespan, but that comes at the price of flexibility.
During production, wire rope is heavily lubricated, allowing the lubricant to penetrate throughout the entire rope and into the core, which allows for slight movement of the individual wires within the rope to increase the lifespan of the rope as well as to reduce friction of the ropes as they rub against one another. From this point, maintaining proper lubrication is critical in preserving the life and structure of the wire rope and should be addressed with a combination of penetrating and coating lubricants. Penetrating lubricants reach the core and coat each strand of wire by utilizing petroleum solvent that evaporates once it reaches into the core and leaves the lubricants behind. Coating lubricants would then be used to seal the outside of the cable from moisture and reduce wear and corrosion during use.
The Discover television series “How It’s Made?” came out with an episode in 2011 showing the production of wire rope as well as crane cable and would make for an interesting view if you have five minutes available, providing further insight into the intricacies of wire rope and its composition. You can find that video here.
For more information or to rent wire rope slings and accompanying equipment, reach out to Lifting Gear Hire at www.rentlgh.com or call us at 800-878-7305.