An Analytical Investigation of the Crankshaft-Flywheel Bending Vibrations for a V6 Engine

High vibration levels at the rear bearing cap and oil pump were observed in dyno tests for a particular design of a V6 engine at a rated speed of 4800 r/min. It was found experimentally that the crankshaft-flywheel assembly had a bending resonance at 240 Hz which was excited at around 4800 r/min by 3rd order forces on the crankshaft.

A newly developed crankshaft system model (CRANKSYM) was used to analytically verify the above finding and propose possible solutions to the problem. CRANKSYM can perform a coupled analysis among the crankshaft structural dynamics, main bearing hydrodynamics and engine block flexibility. It considers the flywheel dynamics (including the gyroscopic effect), belt loads, crankshaft “bent” and block misboring, and the anisotropy of the block flexibility as seen from a rotating crankshaft. It can also calculate the dynamic stresses on the crankshaft throughout the whole engine cycle. A brief description of CRANKSYM is given in the paper.

It was found using CRANKSYM, that internal balancing of the crankshaft and reduced flywheel inertia reduced the flywheel vibration and the dynamic load on the rear main bearing considerably. On the contrary, stiffening of the rear main bearing cap did not decrease the flywheel vibration or the rear main bearing dynamic load.