Analysis of fracture and cracks of oldham’s couplings used in anchor hoisting. Case study

The usual failure of an anchor hoisting assembly was investigated. The anchor ensures stability in cases of ship mooring in a harbor, anchorage or bay and provides adequate safety combined with convenient and easy handling. For the anchor hoisting a hydraulic motor is used consisting of a main two-speed crankshaft, the oldham’s coupling and a valve spool. The oldham’s coupling of a specific oil-tanker frequently fails during the anchor descent and blocks the whole hoisting system. Three pieces used to the same anchor were examined. Two of them exhibited total fracture while the third one had been plastically deformed. The study involved chemical analysis, hardness measurements and in-depth examination of microstructures and fracture morphologies of specimens derived from the failed parts. The paper presents conclusions related to the fracture mechanism and the type of fractures respectively. It was found that poor design and wrong material selection were the main causes of failure.