Wire-Rope Sling D/d Bend Efficiency
Bend efficiency factor and reduced working load limit of a wire-rope sling from the D/d ratio, per the WRTB curve.
Example
You enter
- Catalog WLL, straight pull (lb) 10000
- Bend diameter D (in) - pin / load 3
- Sling diameter d (in) 1
You get
- D/d ratio 3
- Bend efficiency 70%
- Reduced WLL 7000 lb
Checked against published worked examples from: WRTB Wire Rope Users Manual / ASME B30.9.
Details, formula, and sources
ScopeInspect every sling before the lift.
Formula or tableratio = bend_dia / sling_dia; efficiency interpolated from the WRTB 6x19 / 6x37 D/d curve (1->0.50 ... 25->0.93, 40+ ->0.95, plateauing below full strength); reduced_wll = rated_wll x efficiency.
Source and editionWire Rope Technical Board Wire Rope Users Manual (D/d bend efficiency) and ASME B30.9 by name; the curve ships as fixed breakpoints from the WRTB 6x19 / 6x37 chart (75/79/83/86/89/91/93/95% at D/d 4/6/8/10/15/20/25/40).
Free accessThe WRTB D/d efficiency curve is published guidance; ASME B30.9 is a consensus standard.
What governsEstimate. Head rigger and manufacturer working-load-limit charts govern. Inspect every piece of hardware before the show.
API field namesrated_wll_lb, bend_dia_in, sling_dia_in, ratio, efficiency, reduced_wll_lb
- D/d curve fixed [ratio, efficiency] breakpoints for 6x19 / 6x37 wire ropeWRTB Wire Rope Users Manual