Failure Investigation of a gearbox coupling on a fishing vessel


A fishing vessel is powered with a 9 cylinder diesel engine. Power transfer from the engine to the gearbox is enabled through a flexible coupling arrangement. This coupling is made up of a rubber component attached to a finger coupling, shown in Figure 1 with an array of linkage arms. Failure of the linkage arms in this assembly occurred on repeated occasions. After the initial failure a 2mm vertical misalignment of the rubber component was found and fixed. The coupling and the new linkage arms failed after a further 400 hours.


A damper is attached to the forward end of the engine for reducing torsional vibration effects Torque is transferred from the engine via the rubber elements in shear, then through tensile and compressive tangential force transfer through the linkage elements to the gearbox. The linkage elements provide a degree of axial displacement from the engine side. The rubber components were offset in the circumferential direction passed their recommended permanent deflection marking. SONY DSC Linkage arm coupling parts from engine side         SONY DSC Rubber elements        

What We Found


Compositional analysis and hardness tests show the material to be quench and tempered AISI 5140 H steel. These properties were used to calculate the fatigue properties of the linkage arms. Scanning electron microscopy images of the linkage arm fracture surfaces are indicative of a fatigue fracture i.e. crack propagation due to cyclic loading.  Additionally, some of the couplings had failed by fast fracture.  With additional tensile loading from vertical motion of the engine, the combined stress state is brought beyond the endurance limit of the component material. SONY DSC

Failed linkage arm


  • The linkage arms failed by fatigue fracture and once a number of arms had failed in this way, additional loading on the remaining arms caused rapid fast fracture.
  • Failure was due to axial loading on the coupling, however vertical loading would also create sufficient stress

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