EF7 engine flywheel bolted joint design improvement by theoretical and experimental investigations

Bolted joints are of the most importance in engine connections. One of important engine bolted joints is the flywheel-crankshaft connection which transmits the engine output torque. Any failure or loosening of flywheel bolts may result in a catastrophic engine damage due to the fact that this joint is considered as a safety issue because of its location and high rotational speed. The most known failure mechanism of flywheel joint is the “Bolt Loosening” due to joint slippage which starts with micro slip and propagates to macro slip causing surface fretting and preload loss and ends up with fatigue fracture of the bolts. Many factors may cause or jeopardize flywheel bolts failure e.g. surface roughness, friction coefficient at the interface, flywheel pin and bolts clearance, threads pitch, bolt preload and external loads acting on the joint. In this study, besides defining and validation of the transmitted torque in the joint, using the general bolted joints equations and combining general rules of such joints, at first it was shown that it needs basic changes in the joint to avoid slippage completely. Then, by calculating the critical slip and comparing it with the probable joint slippage, it was proved that the joint slippage can be limited to a value lower than the critical slip in order to prevent the rotational loosening of the bolts by designing the flywheel pin hole to a proper value. At the end, the theory was validated by performing three “Engine Over-Speed” tests.