Understanding D/d Ratio

Wire Rope Slings

Wire rope offers an economical alternative to chain for many applications and has advantages of its own. It is easier to feed under a load as it can often be pushed through. Note that slings made from galvanised rope and thimbles are more tolerant of marine or similar environments.
The condition of the wire rope (or chain) must always be checked, keeping in mind that it is likely to be worst at those points where it engages most frequently with the rope sheaves or load chain wheel.

The rule of thumb for wire rope is ‘the bigger the better’ when it comes to D/d ratio—and the avoidance of reverse bending, tight sheaves, damaged sheaves, fleet angles, etc. all impact wire rope life. It also of course depends on the construction, lay, and tensile of the wire rope. Many of the newer ropes are more tolerant than the previous 6 by 25 ropes (or six strands of 25 wires) but the same principle of ‘the bigger the better’ still applies.
Note that Standards Australia does specify minimum recommended D/d ratios for various rope constructions, while manufacturer recommendations are referred to with referenced to newer constructions not covered in AS documents.

Synthetic Slings

The term ‘synthetic’ encompasses slings made from fibre rope and common flat webbing slings, endless round slings, and now even synthetic chain. Synthetic slings have come a long way in the last few years with newer generation high-modulus polyethylene (HMPE) fibres providing very high capacity but are physically very small and extremely lightweight.

Synthetic chains are now being manufactured as well to provide further options in the synthetic sling category. Synthetics are soft compared to chain and wire rope and are therefore less likely to damage the surface of a load, but they can easily be cut if loaded over a sharp edge or corner without adequate packing or sling protection (see insert). Synthetic slings were previously limited by lack of length adjustment, but this issue has now been solved with the introduction of synthetic chains.

D/d ratios for common polyester and HMPE sling products should always be considered and should be checked carefully with the manufacturer or supplier. Sling saver shackles are advised when using large capacity HMPE slings as they increase the D/d ratio significantly.

Many loads are vulnerable to local damage and need protecting.

Generally, synthetic slings are less likely to cause local damage than wire rope or chain slings. Round slings flatten under load thus reducing the pressure. A greater spread can be achieved with webbing slings, but particular care is required to ensure they are uniformly loaded across their width. Sling protective sleeves are a good option for synthetic slings to be used at the lift points around shackles etc. These sleeves assist with D/d ratio and prolong the life of the slings as the sleeves are usually a Velcro on option and can be moved along the sling easily so lift points can be shared along the length of a round slings.