The thrust bearing's function in marine propulsion is to transmit the axial thrust produced by the propeller (which acts on the water and hence pushes the ship) to the ship's structure, thereby propelling the ship; and to hold the propeller shaft and crankshaft in their correct axial position, preventing the shaft moving fore and aft. Because the propeller thrust is very large (it drives the entire ship), the thrust bearing is critical: without it the propeller thrust would be carried by the main engine bearings and the crankshaft would move axially, causing severe damage and misalignment, so the thrust bearing provides a positive, heavily-loaded, oil-filmed support.

Roles of ahead and astern thrust bearings: A thrust bearing normally has thrust pads on both faces of a thrust collar. The "ahead" pads face in the ahead direction and take the main ahead thrust (when the propeller drives the ship forward). The "astern" pads face the opposite way and take the astern (reversing) thrust when the engine is run astern - they then bear the load in the opposite axial direction. So the two sets of pads allow thrust to be transmitted (and the shaft located) in either direction.

Why both are essential: While ahead thrust is the normal continuous load, the engine must be able to run astern for manoeuvring; without astern thrust pads the shaft would be pushed forward/back against unsupported structure, the shaft/gear/running gear could be pulled out of position, the propeller shaft would move and the engine crank could move axially causing damage. Both bearings hold the shaft/engine axially in both directions, preventing damage in manoeuvring and giving stable location.

Handling directional thrust: The propeller thrust, linear with the propeller's axial force, is transmitted through the propeller shaft flange to the thrust (thrust collar) mounted on the shafting. The thrust collar is a smooth annulus; on each side of it are a set of pivoting thrust pads. When running ahead, the shaft (and collar) is pushed forward; oil is dragged into the wedge between the collar face and the ahead pads, building a hydrodynamic film that carries the load to the bearing housing and ship's structure. When running astern, the collar is pushed astern, and similarly an oil film forms on the astern pads to carry the reverse thrust. The pads are spring/pivot-mounted so the load distributes evenly and each pad tilts to build a wedge.

Monitoring procedures:

1. Temperature monitoring: periodic (or continuous) measurement of each pad temperature (pads have embedded thermocouples), the oil outlet temperature and the bearing housing temperature; abnormal rise indicates overload, oil starvation or misalignment.
2. Oil condition/pressure/level monitoring: check oil pressure at the bearing, oil flow, oil level in the system, filter cleanliness and oil quality; any fall/contamination reduces the film.
3. Axial clearance check: check end float/collar wear periodically; excessive clearance or axial movement indicates pad wear/misalignment.
4. Periodic visual inspection: dismantle and inspect pads for wiping, overheating (discolouration), and pitting; check collar wear and pad surface; verify the ahead and astern clearances and adjust shims if needed.
5. Vibration/alignment checks: monitor vibration of the bearing and check shaft alignment/deflection to ensure loads are shared correctly, especially after long operation.

Keeping both sets of pads, clearances and lubrication in good condition ensures the thrust is supported adequately in both directions.