As you might’ve noticed, we refer to D/d Ratio quite a bit in some of our blogs and social media posts. However, there seems to be a bit of confusion regarding what D/d Ratio is and what purpose it serves. Hopefully, we can clear all of that up now.
The D/d Ratio has a tremendous impact and effect on sling capacity when using Wire Rope Slings, and Chain Slings. The D/d is the ratio of the diameter (D) around the object which the sling is bent, then divided by the overall diameter (d) of the sling being used.
When a sling is bent around another object there is a loss of sling capacity due to this bending action. As the D/d ratio decreases, this capacity loss becomes greater and the sling becomes less efficient. There is a direct correlation between the D/d Ratio and the efficiency of the sling, or rated capacity.
Specific to Polyester Round Slings, it varies amongst manufacturers, but usually they recommend minimum hardware diameters. In addition, Round Slings are more negatively affected from users beating the hitch down when in a choker configuration.
All slings lose capacity when they’ve been bent too much. Basket hitch ratings are based on a minimum diameter and the capacity must be reduced when under the minimum D/d Ratio. Keep in mind, a true basket is such that the legs of the sling are at a 90-degree vertical angle.
See the illustration below explaining D/d for Wire Rope Slings, Round Slings and Alloy Chain Slings.
Prior to every lift, the correct D/d Ratio has to be determined and it must be equal or higher to the information listed on the above images. In the rental business we quite often see a number of damaged Rigging Equipment due to the incorrect D/d Ratio being used. Many of the items are damaged beyond repair which often leads to unsafe rigging practices.
Lastly, another common mistake is when an application specifies basket hitches but someone makes a last-minute decision to use choker hitches instead. This is usually done to save headroom, or they do not have other suitable rigging equipment. Without considering any angular forces, the user has just decreased the capacity of each sling from 200% of a single-leg sling capacity, down to only 75%. This often results in overloading of the slings. Read more about which sling should your bring?