Bending-Fatigue Failure of a Steel Wire Hoisting Rope for a Stacker Crane
A 13 mm diam 18 x 7 fiber-core improved plow steel nonrotating wire rope, brought into service as a replacement for 6 x 37 improved plow steel ropes, failed after 14 months of service on a stacker crane. The change was reported to have been caused by difficulties twisting of the 6 x 37 rope. The hoist arrangement for this crane was found to consist of one rope with each end attached to a separate drum and the rope was wound around two 30-cm diam sheaves in the block and back up around an equalizer sheave. The rope section that had been in contact with the sheaves was deduced by measurement checks. The presence of broken wire ends, which indicated that the rope failed by fatigue, was revealed by reverse bending of the section of the rope which was normally subjected to this flexing. It was found that minimum sheave diam for a 13-mm 18 x 7 wire rope was 43 cm and hence the currently used smaller sheaves caused excessive bending stresses in the rope. The 18 x 7 rope was replaced by two 6 x 37 side-by-side counter-stranded steel-core ropes as a corrective measure.