Shafting axial vibrations
Shafting axial vibrations are characterized by shafting segments oscillating in a fore-and-aft direction around some neutral position. For simplified description purposes only, this motion may be compared to the movement of an accordion during play.
Shafting axial vibrations are mainly excited by the propeller's thrust variations, as well as by forces generated in the engine's crank mechanism. Namely, the excitation forces coming from the gas pressure and from the inertia of alternating masses are converted into the equivalent crank throw opening-closing forces, acting along the longitudinal, axial direction. In some cases, due to torsional-axial coupling, excessive axial vibrations may be excited by shafting torsional vibrations.
Although shafting axial vibrations alone are rarely the cause of severe shafting damages, they are usually the cause of a vessel's hull vibration, excited by the variable force acting on the engine's trust block.
To minimize unfavorable side effects of the shafting axial vibrations, most enginebuilders of the low-speed diesel engines integrate an axial vibration damper into the engine casing. That way, the axial vibration damper becomes a standard building block of modern low-speed diesel engines.