Quote:
Originally Posted by BURN-BROS
I believe it is lateral. I imagine that the throws are bending along an axis from the center of one rod journal to it's opposite journal.
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I would tend to agree that the main vibration is not a torsional and as you say the throws are bending.
2nd Order vibrations (2 shakes per rev) are nomal behaviour for V6 engines including 180 degree configurations. These vibrations would be longitudinal and are associated with crankshaft behaviour. I can see them breaking bolts if the resonant frequency coincides with running speed. Softening the crank in the longitudinal direction - increased throws and thinner webs- would depress the critical speed at which resonance occurred.
A 4th order vibration (4 shakes per rev) is much more likely to be a torsional vibration due to gas torques and I am not sure thinning the web by 1 mm has much influence on the torsional resonance of the structure - I do realise that I could be wrong- but it is quite difficult to model accurately.
If the resonance were longitudinal and the flywheel bolts didn't have enough preload to overcome the axial forces caused by the vibration then the bolts could suffer from fatigue failure.
By increasing the preload to a value greater than the axial force you would eliminate the effect of fatigue - same theory as con rod bolts - so it all seems to stack up.
