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?

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, because if the lifting sling tightly bends over too much during the lift – the sling will get damaged. This could then lead to unsafe rigging practices. To have a safe lifting operation, continue reading more about D/d Ratio, and the types of slings we carry at US Cargo Control to identify which is right for your lift.

What is D/d Ratio?

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. It is the ratio of the diameter (D) around the object which the sling might be bent, which could be a sheave or another object, then divided by the overall diameter (d) 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.

Polyester Slings vs Nylon Slings: Which Web Sling Type is Better?

Are polyester lifting slings better than nylon lifting slings? This is a question our lifting and rigging product experts commonly get and the answer, as it often is with rigging gear, is that it depends on the job at hand.

Before we dive into the key differences between nylon slings and polyester slings, there are many similar advantages to these two types of synthetic slings that are important to know.

Advantages of Synthetic Webbing

1. Ideal for delicate loads

One of the most common reasons for a rigger to use synthetic slings instead of wire rope slings or chain slings is th fact that they won’t scratch or crush your load. That’s why synthetic slings are extremely popular in the construction industry and with ship haulers.

2. Cost-efficient

Synthetic slings are also an attractive choice due to their lower cost. If you’re wondering whether nylon slings are cheaper than polyester slings, don’t. The prices are more or less the same, and your focus should be on choosing the right sling for the job.

3. Lightweight

Compared to wire rope and chain, synthetic slings are much lighter, making them easier to transport and handle. Again, nylon is typically a little heavier than polyester, but it’s not much of a difference when you consider the weight of chain and wire rope.

4. Strong

It’s true that nylon is stronger on an individual fiber level, but a polyester sling can be made just as strong as a nylon one by adding more threading. And both types can easily lift several thousand pounds. So while it’s important to use a lifting sling that’s rated for the load your lifting, this won’t affect your choice between nylon and polyester.

Physical Differences

Now that you know how they’re similar, let’s look at the differences between the physical characteristics of nylon and polyester to determine which material type is best for your job.

Nylon has more stretch

This is one the most important differences between nylon slings and polyester slings. While nylon slings have more give to them (about 7 to 10 percent stretch when at WLL) that does not mean they are weaker than polyester (typically 2 to 3 percent stretch at WLL). The main reasons you would want more or less stretch when lifting a load include overheight lifting room and the potential for “snapback”.

If you have height limitations, polyester is probably the better choice. If your load could bounce around a lot, the extra stretch of nylon will reduce the danger of the strap snapping back at you or others. This difference in the stretch is why you typically see recovery straps made of nylon and tow straps made of polyester.

Polyester is softer and more flexible

Both nylon and polyester slings are good for delicate loads, but if you’re wondering which one is best for the most delicate loads, it’s polyester. The chemical coating on nylon webbing gives it a slightly more coarse texture.

Polyester is also a bit more flexible than nylon. By flexible, I’m not talking about stretch but rather the ability to wrap tightly around a load and grip the most surface area.

This is an important difference, especially when choosing between nylon endless slings and polyester endless slings. It’s also important to note that there is a difference between flat web polyester slings and round polyester slings.

nylon round endless lifting sling
Nylon Endless Sling
polyester round endless sling
Polyester Endless Sling

Environmental Differences

Another key question to ask when choosing between nylon slings and polyester slings is, what are the environmental conditions? There’s a reason why ship haulers and those in marine environments prefer polyester slings, they absorb less water and are just a bit more resistant to UV rays. But, you also need to consider the differences when it comes to chemical resistance.

Nylon slings can’t resist acids or bleaches

Avoid using nylon slings if you’re operating anywhere near sulfuric, nitric, hydrochloric, or phosphoric acids. Nylon is also unresistant to oxidizing bleach agents such as sodium hypochlorite, hydrogen peroxide, sodium percarbonate, and calcium hypochlorite.

Polyester slings can’t resist ethers or alkalis

On the other hand, polyesters Achilles heel is ethers and alkalis. Among other chemicals, this includes diethyl ether, dimethyl ether, sodium hydroxide, potassium hydroxide, calcium hydroxide, calcium carbonate, and magnesium hydroxide.

Polyester Slings

best eye to eye lifting sling

browse all polyester sling

Conclusion

In short, synthetic lifting slings are a great choice due to their ability to handle delicate loads, lower cost, lightness, and impressive strength. In the polyester sling vs nylon sling matchup, the winner is whichever one meets the demands of your specific job best.

Nylon slings have more stretch but can’t be used near acids or bleaches. Polyester slings are softer and hug to load surfaces better but can’t be used near ethers or alkalis.

If you have any additional questions about nylon slings or polyester slings, give our rigging product experts a call at 800-404-7068.

How to Choose a Lifting Sling

Choosing the right sling for lighting can be confusing, but it’s important to take the environment and conditions where the lifting sling will be used into consideration.

Nylon lifting sling

image of nylon lifting sling from US Cargo Control

Nylon lifting slings are a popular choice because of their multi-purpose uses. A nylon web sling is unaffected by petroleum products like grease and oil. Nylon slings are also resilient to specific chemicals including ethers, strong alkalies and aldehydes. Nylon webbing is not a good choice for uses involving bleaching agents or acids, or for use in temperatures over 194 degrees Fahrenheit (90 degrees Celsius). Nylon also stretches at approximately 8-10%, so it should not be used when elongation is not intended.

 

Polyester lifting sling

Polyester Round Eye & Eye Lifting Sling

Like a nylon lifting straps, polyester round slings also have temperature guidelines, and are not advised for use in temperatures over 194 degrees Fahrenheit (90 degrees Celsius). Unlike nylon, they will stretch only approximately 3% of its rated capacity. A polyester round sling can handle acidic environments because the fabric is not affected by bleaching agents or common acids. Polyester webbing, however, should not be used if it will come in contact with sulfuric acids or alkaline.

 

Chain lifting sling

Image of Double Leg Chain Sling from US Cargo Control

Chain lifting slings are ideal for rugged environments and jobs. They resist abrasions, cutting and can maintain their strength and integrity even in extremely high temperatures up to 400 degrees Fahrenheit (204 degree Celsius). Chain slings are generally constructed in Grade 80 steel, but a Grade 100 chain is usually available on request. Grade 100 chain offers more strength (about 25% more) for lifting chains, yet weighs less than traditional Grade 80 lift chains. Another benefit of a chain sling is its ability to be customized for almost any type of lift. However, a chain hoist can be expensive and the prices can vary due to the market fluctuations.

 

Wire rope lifting sling

image of Wire Rope Lifting Sling from US Cargo Control

Wire rope slings are similar to a chain sling in that it offers excellent durability, strength and resistance to high temperatures. Wire rope, however, is more cost-efficient than chain so it’s a great choice is price is a concern. Because abrasion-resistance and flexibility of the wire rope can change depending on its configuration, a chain sling can be manufactured using a specific type of wire rope. Typically, wire rope is made from either 6×19 or 6×37 classes of rope. A 6×19 is the most widely used because of its ideal combination of flexibility and abrasion resistance. A 6×37 class rope is more flexible, but offers less resistance to abrasion.

 

Other factors will go into determining which lifting sling type is best for your specific job, such as the item(s) being lifted, capacities needed, etc. For help choosing a lift sling, call our product specialists at 800-660-3585. They’ll be happy to any answer questions you may have.