Improving bend-over-sheave fatigue in fiber ropes

One of the limitations of synthetic fiber ropes in industrial uses has been the premature wear of these materials when subjected to continuous bend-over-sheave fatigue. While one-way passage over sheaves is not generally damaging, repeated back-and-forth movement over one part of the rope, as in heave compensation units, can lead to damaging heat buildup and unexpected failure modes. This report details the results of bend-over-sheave fatigue testing on 18 mm diameter fiber ropes conducted at Cortland Cable Company. Cycles-to-failure data is presented for the fiber materials, coatings, and constructions tested to date. Fatigue life was significantly improved by fiber blending and with specialty coatings, resulting in a new braided rope design specifically optimized for bending fatigue (BOB). The BOB design was found to have a significant CTF advantage over aramid constructions and steel wire currently in use today. The importance of test parameters such as sheave design and cycle rate is highlighted.