Figure 1: Section view of flexible coupling arrangement left

Investigation

A geisslinger damper is attached to the forward end of the engine for reducing torsional vibration effects. Deflection of the radial springs in the damper as a result of fluctuations in rotational inertia due to torsional vibration causes flow of viscous oil into respective chambers. This has an overall damping effect on vibration. The damper was found to be in good working order with minimal engine vibrations. 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.

Figure 2: Linkage arm coupling

Results

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. 

 
Figure 3: Linkage arm showing failure on its right side mounting end

Figure 4: Fracture surface of linkage arm from engine side of coupling

Some of the arms have failed completely through via fatigue. This would have resulted in rapid crack propagation prior to final failure of the coupling. For these elements to fail by fatigue, the loading stress would have to exceed the endurance limit. In addition, the edges of the washer mounting face on certain linkage arms show evidence of localised yielding. The modified endurance limit for the material in this region is approximately 300 MPa. A coupling axial deflection of approximately 4 mm is necessary for bending stresses in these areas to be greater than the yield strength of the material. With additional tensile loading from vertical motion of the engine, the combined stress state is brought beyond the endurance limit of the component material. 

Conclusions

• 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
• The source of axial and vertical loading should be identified by means of engine vibration and monitoring tests.