Engineered lifts utilizing synthetics often require grommets for a variety of reasons, and in these scenarios there are inherent reductions in efficiencies associated with the D/d ratios at the bearing surfaces; this is no different than wire sings in concept, but the actual efficiencies do vary. In our experience we have found that a conservative way of calculating efficiencies of synthetic ropes in a grommet or basket via the DNV method for calculating bend efficiency of wire rope, however this is outside of the intended use of the calculator and may not hold true for all rope manufacturers and rope constructions.
Rigorous testing done by the provider of the ropes being specified is the only way to truly know the efficiency of your rope when using D/d ratios below 3:1. Most manufacturers will agree that a 1.6 multiplier can be taken on the minimum breaking strength of a rope in eye and eye configuration when converting it into a grommet, or basketing it, if the D/d exceeds 3:1, e.g. 100Te rope becomes a 160Te grommet. However, it is known that (i) D/d ratios below 3 are the norm and (ii) there are greater efficiency losses when you get below that threshold.
Test data is critical to determining efficiencies below 3:1 and if your lift requires rope slings to be used in grommet form or basket just do a quick check on your spec:
Does the minimum breaking strength of your spec’d sling equal 1.6x the minimum breaking strength of the rope being used to build it?
If so, there is a good chance no reduction was applied and you’re eating into your safety factor.
If not, and it appears there is a reduction taken, ask the provider of the data to support it with test data, or more importantly third party certification (not verification).
Precision Tension and Lankhorst Ropes not only calculate sling efficiencies based on large datasets of real world tensile tests, we are able to provide DNV Certification for the slings being proposed.
Check out Rui’s Pedro Faria’s recent article: