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What is the D/d ratio and how does it affect wire rope strength?

RIG-503 · Rigging
The D/d ratio is the sheave diameter divided by wire rope diameter. Higher ratios reduce bending stress and increase rope life.
The D/d ratio represents the relationship between sheave diameter (D) and wire rope diameter (d). This critical measurement determines how severely the wire rope bends as it passes over sheaves, blocks, and drums. Industry standards typically require minimum D/d ratios of 18:1 for running rope applications, though higher ratios of 20:1 or 25:1 significantly improve performance.
Reference: ASME B30.5 Mobile and Locomotive Cranes standard
Remember: "Big Drum = Long Life" - Larger sheave diameter (D) over rope diameter (d) = less bending stress = longer rope life

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sheave diameter ratio

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