What You Should Know about the D/d Ratio
If you’ve followed our blog for a while, chances are you’ve heard the term D/d Ratio quite a few times. That’s because identifying the D/d Ratio when using any type of sling is critical to safe rigging. Still, there can be confusion about what exactly D/d Ratio is and what purpose it serves. This blog aims to clear that up.
What is D/d Ratio?
D/d Ratio is the ratio of the diameter (D) around the object which a sling is bent, divided by the sling’s overall diameter (d). More simply stated, it’s the diameter of a load or rigging hardware divided by the diameter of the sling.
Why is D/d Ratio Important?
There will always be a loss of sling capacity whenever a sling bends around another object. The D/d Ratio aids in determining the sling’s capacity reduction, allowing you to make corrections before continuing a lift.
As the ratio decreases, capacity loss becomes more significant, and the sling becomes less efficient.
If you see a tight bending of the sling, this means there’s a smaller than recommended D/d ratio, which can intensify the bending motion. This may result in fatigue, irregular wear, and increased deterioration of your slings. Therefore, before every lift, it’s critical to determine the correct ratio and account for any reductions.
Each sling type has different strength efficiencies. The illustrations below explain the Ratio for wire rope and alloy chain slings.
- Specific to polyester round slings, it varies among manufacturers, but usually, they recommend minimum hardware diameters to protect the inner core yarns. The illustration above provides an example of minimum shackle size requirements when using round slings. Bear in mind that round slings are more negatively affected by users bunching up the sling and not allowing the yarns to spread out properly and disperse the load when in a choker configuration.
- Using choker hitches may save headroom; however, even following manufacturer-specified hardware diameter, the sling’s rated capacity reduces to 75% of the listed vertical rating. Failing to understand this relationship often results in overloading the sling.
- While all slings lose capacity when they are bent beyond a certain point, basket hitch ratings are based on a minimum diameter and the capacity must be reduced when the sling’s D/d falls below the minimum D/d ratio. Keep in mind, a true basket is one in which the legs of the sling remain at a 90-degree vertical angle.
In the rental business, we often see rigging equipment damaged due to sling capacity reduction being ignored or miscalculated. Often, the equipment is damaged beyond repair, leading to unsafe rigging practices and additional repair or replacement costs for our customers. This is why it’s imperative to understand and abide by D/d Ratio specifications.
For more related to this topic, read Which Sling Should You Bring?
If you have additional questions or need high-quality, inspected, tested, and certified lifting equipment rentals, LGH has you covered. Click to chat with a live rental support specialist or call 800-878-7305 now.