D/d RATIO WIRE ROPE SLINGS
D/d Ratio in Wire Rope Slings: Safe Lifting Practices
D/d RATIO
WIRE ROPE SLINGS
When a wire rope is bent around any sheave or other object there is a loss of strength due to this bending action. As the D/d ratio becomes smaller this loss of strength becomes greater and the rope becomes less efficient.
EYE & EYE SLINGS
Even though the spliced eye of a wire rope sling has nearly DOUBLE the strength of its body, it is still critical that components with the right D/d ratio are placed in the eye. The same D/d ratio that is used on the sling body in a basket hitch should be used in the eye.
In most cases the shackle or hook with the same or similar WLL, over which the sling is placed will have a sufficient D/d ratio. On the other hand, do not place too LARGE an object into the sling eye as this will result in splitting forces affecting the sling splice and sling safety. The object (a shackle, a crane hook, a steel bar, etc.) you place into the sling eye must not be larger than 1/2 of the sling eye length.
When a sling is used in a basket with D/d ratios smaller than listed in the capacity tables, the rated capacities (or WLLs) must be decreased.
For example: The basket capacities listed in all Standards for 6-strand ropes are based on a minimum D/d ratio of 25:1.
An object you place into a 10mm diameter 6-strand wire rope sling using a basket must have a minimum diameter of 250mm If the object is smaller than the listed 25:1 D/d ratio the WLL must be decreased.
The diagrams below and overpage illustrate the percentage of decrease to be expected.
Eye length must NOT be smaller than twice the object (e.g. a hook) diameter.
If the shackle body has AT LEAST 1.5 times the same diameter as the sling (D/d 1:5) the capacity need not be adjusted. Where the D/d is between 1 and 1.5, the sling shall be derated by 25%.
If the object lifted with a 6-strand wire rope sling in a basket hitch is at least 25 x larger than the sling diameter (D/d 25:1) the basket capacity need not to be adjusted.
If the shackle or object has 2 times the diameter of a 6-strand wire rope sling (D/d 2:1) the basket sling capacity must be reduced by 40%.
It is better to use a larger shackle or a Wide Body shackle type. If the shackle or object has at least 5x the sling diameter (D/d 5:1) the basket sling capacity must still be reduced by about 25%.