What is D/d Ratio & Why is this Critical for Lifting Slings?

What effect does D/d Ratio have on lifting slings and their sling capacities?

Lifting sling with basket

You may wonder what D/d ratio is and what purpose it serves when lifting loads with different types of slings. After all, it can be confusing as there are multiple factors you have to take under consideration such as knowing the weight of the load, the sling’s working load limit (WLL), and the load’s center of gravity.

Identifying the D/d ratio when using any type of sling for a lift is critical. If the lifting sling tightly bends over too much during the lift – the sling will get damaged itself. This then leads to unsafe rigging practices, thus making it difficult to repair and creating high replacement costs.  

To have a safe lifting operation, read about D/d Ratio and the slings we carry at US Cargo Control.

What is D/d Ratio?

In a short sentence, it describes the load size versus the size of the sling placed on the load.

The term D/d ratio works as a simple mathematical equation where you’re learning the diameter or distance of the load you’re picking up. The upper case “D” refers to the ratio of the diameter around the object which the sling is bent, and the lower case “d” represents the diameter of the sling being used.

The Formula for D/d Ratio:

The diameter of the load divided by the diameter of the sling = D/d Ratio

Unirope's definition of D/d ratio
Photo Courtesy of Unirope

For example, you’re picking up an object that’s 12 inches in diameter and you use a 3/8″ chain to pick it up. This is 12″ divided by 3/8″ = 32, which 32 is the D/d Ratio. If the diameter is 10″ and the diameter of the rope is 1/2, the D/d Ratio is 20.

Why is D/d Ratio Important?

The D/d Ratio has a tremendous impact on sling capacity when using slings like wire rope, chain, polyester round slings, and nylon slings. It can determine the sling’s efficiency or capacity reduction, and allow you to make corrections before continuing the lift. If you see a tight bending of the sling, this means there’s a smaller than recommended D/d Ratio.

Using a smaller D/d Ratio that’s not recommended for your lift can aggravate the bending motion. This can result in fatigue, irregular wear, and increased deterioration. Once this occurs, you’d have to perform frequent inspections and go through costly wire rope replacements.

Each sling type has different strength efficiencies, which is why we included a table below that describes the efficiency of various sling constructions with standard D/d Ratios:

Mechanically spliced, single-part slings25 times rope diameter
Hand-Spliced, single-part slings15 times rope diameter
Braided multi-part slings of 6 parts25 times component rope diameter
Braided multi-part slings of 8 parts25 times component rope diameter
Helically laid multi-part slings25 times component rope diameter
Hand-tucked grommets and mechanically joined grommet5 times sling body diameter
Union Rope’s Rated Capacity, Design Factors, and D/d Ratios

What Will Sling Damage Look Like?

wire rope damage

When a sling is tightly bending around another object, there is a loss of sling capacity. As D/d Ratio decreases, this capacity loss becomes greater and the sling will become less efficient. There is a direct correlation between D/d Ratio and the efficiency of the sling (or rated capacity).

Not only is it important to understand the D/d Ratio of the lifting sling, but it’s also equally important that fittings and rigging hardware used in connection points are adequately strong and spatially correct. For instance, if smaller, alloy shackles are used, the webbing edges can get damaged and sling efficiency is lowered due to the bunching of the webbing.

When not used correctly, this could result in bunching, and being crammed into a space will reduce sling efficiency and capacity. Basically, putting too many slings in an undersized shackle or another fitting can result in lost strength.

To understand what slings or fittings should you use for your job, give our team a call at 800-404-7068, and we’ll be glad to assist you.

Different Type of Slings

Now that you have an idea of what D/d Ratio means, continue reading on what lifting slings you should use that’s worth the investment for your lifting situations.

nylon lifting slings

Nylon Slings

Known as synthetic web slings or nylon web slings, our nylon lifting straps perform well for lifting breakable, delicate objects. Its heavy-duty synthetic material has great stretch and flexibility that help the slings mold to the shape of the load. Not only that, the nylon lifting sling’s material is treated to improve abrasion resistance and reduce wear, even in the most rigorous lifting applications.

Chain Slings

If you’re depending on a sling that requires extra strength and durability, the chain sling is your best choice. The chain lifting sling is the strongest and most durable type of sling and is highly popular to use for heavy lifting operations. They will perform better than polyester round slings or nylon slings because they’re more durable, tolerant to hot temperatures, and cut-resistant.

chain lifting sling
Polyester round sling

Polyester Slings

Manufactured in the USA from a continuous loop of polyester yarn that creates exceptional strength, the round slings are versatile, pliable, convenient, and cost-effective. Because of their many benefits, you can use these in vertical, choker, or basket hitches, making them effective to lift a wide variety of cargo!

Wire Rope Slings

Known as steel cable or wire sling, these are more durable than synthetic slings and more cost-effective than chain slings. Wire rope slings are an excellent choice not only for lifting, but also for hoisting, towing, or anchoring loads. Its fabrication offers abrasion-resistance and heat-resistance, as they are made by weaving individual strands or wire around a core.

Wire rope sling

To understand what lifting slings you should use for any lifting operation, read 3 Factors to Consider When Choosing Lifting Slings.

Tackle the Toughest Lifting Jobs with High-Quality Rigging Hardware

We know the importance of quality when it comes to rigging supplies. We carry a variety of rigging hardware, as well as lifting beams and spreader bars that can lift heavy loads safely and efficiently.

Need a custom lifting sling? We can do that! We can customize a lifting sling to meet your specific needs.

Contact our sales team today at 866-444-9990. Our team of product experts is here to answer any questions about rigging hardware, lifting slings, and more.

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