Axial vibration is the longitudinal (fore-and-aft) oscillation of the crankshaft along its axis. It arises because the crankshaft has axial elasticity and the reciprocating/rotating masses and the propeller thrust produce axial forces that excite the shaft. The crankshaft oscillates axially at its natural axial frequency; if the exciting frequency coincides with the natural frequency (resonance), the axial vibration amplitude becomes large. It can cause damage to the thrust bearing, the coupling, and the engine structure.

[Sketch notes: An axial vibration damper is fitted at the free end of the crankshaft. It consists of a mass (a heavy ring/plate) connected to the crankshaft by a spring/rubber element, arranged so the mass can move axially relative to the shaft. The axial motion of the mass is resisted by the spring/rubber, dissipating the energy of the axial oscillation.]

Axial vibration is minimized by fitting an axial vibration damper at the free end of the crankshaft. The damper consists of a heavy mass (inertia ring) connected to the crankshaft by a spring or rubber element. As the crankshaft oscillates axially, the mass tends to remain stationary (due to its inertia); the relative motion between the mass and the shaft is resisted by the spring/rubber, which dissipates the vibrational energy as heat, damping the axial oscillation. The damper is tuned to the natural axial frequency of the shaft to absorb the critical frequency. This reduces the axial vibration amplitude and the stress on the thrust bearing and the crankshaft.

The thrust bearing is most at risk from axial vibration. Reason: the axial vibration of the crankshaft causes the thrust collar to move axially against the thrust pads, alternately loading and unloading them. This cyclic loading can cause fatigue of the thrust pads, wiping, overheating, and damage to the thrust bearing. The axial vibration also transmits the axial force to the thrust bearing, which is the component that carries the axial load.

Damage to the thrust bearing (e.g. wiping, fatigue, or overheating of the thrust pads) is repaired by:

1. Dismantling the thrust bearing and inspecting the pads and the collar.
2. Replacing the damaged thrust pads with new ones (or re-metalling/re-facing the pads if they are of the re-metallable type).
3. Checking and re-setting the axial clearance (end float) by adjusting the shims behind the pads.
4. Checking the collar face for wear/damage and dressing or replacing it if necessary.
5. Reassembling the bearing, checking the clearances, and testing.

The repair restores the bearing to its correct condition so it can carry the axial load.