Example: A 1/2" diameter wire rope is bent around a 10" diameter pipe; the D/d Ratio is 10" divided by 1/2" = D/d Ratio of 20:1
This ratio has an effect on the rated capacity of 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. This curve relates the efficiency of a rope diameter to different D/d ratios. This curve is based on static loads and applies to 6-strand class 6x19 and 6x37 wire rope.
|Eye & Eye Slings
The LOOP of an eye & eye sling has nearly DOUBLE the strength of it's body. For this reason the D/d ratio in the LOOP is just half as critical as opposed to when the sling is used in BASKET hitch.
In most cases the shackle or hook 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,....) 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- or CHOKER HITCH with D/d ratios smaller than listed in the capacity tables, the rated capacities (or WLL's) must be decreased.
For example: The BASKET and CHOKER hitch capacities listed (in all Standards and Regulations) for 6-strand ropes are based on a minimum D/d ratio of 25:1
An object you place into a 1" diameter 6-strand wire rope sling using a basket- or choker hitch must have a minimum diameter of 25". If the object is smaller than the listed 25:1 D/d ratio the capacity (or WLL) must be decreased. Table A) illustrates the percentage of decrease to be expected.
Note: The minimum D/d ratio for GATOR-FLEX® and for TRI-FLEX® slings are just 5:1. If you need to lift small objects and don't want your sling to kink or bend permanently use these types.
Endless (or Grommet) slings DO NOT HAVE A LOOP which has double the strength of the sling body. Prior to EVERY lift YOU, the user, has to determine if the D/d ratio is equal or higher than the ones listed in the capacity tables.
For endless 6-strand and Gator-Flex endless type wire rope slings the rated capacities have already been adjusted to be used at a D/d Ratio of 5:1.
See the WLL Tables for details.