The thrust bearing transfers the axial thrust of the propeller (driving the ship ahead or astern) to the ship's structure. It consists of a thrust collar (a large collar mounted on or integral with the crankshaft) running against a set of thrust pads (segments). Each pad rests on a pivot/edge or a spherical seat so it can tilt slightly. As the collar rotates, oil is dragged into the wedge-shaped space between the collar face and the tilted pad, building up a hydrodynamic pressure film which carries the load. The pad pivots to generate a converging oil film wedge. The thrust is transmitted through the pads to the bearing ring and then to the engine bedplate/chocks. Ahead pads carry the forward (ahead) thrust; astern pads (on the opposite face of the collar) carry the engine astern thrust and hold the shaft in place.

The critical clearance is the axial (end) float/clearance - the total movement of the shaft collar between the ahead and astern pads (the "end clearance"/thrust bearing axial clearance). This must be within maker's limits because:

• if too small, the pads may bind/overheat and the collar may not build a proper oil film; thermal growth of the shaft could wipe the pads;
• if too large, the shaft can move excessively, causing propeller thrust collar hammering, rapid pad and collar wear, misalignment of the crankshaft (affecting main bearing deflections), the crankshaft web deflections going out of limits, and vibration.

The radial/pad clearances and the oil film thickness to the pads are also critical for load capacity. The complete total clearance is typically of the order of tenths of a millimetre.

The axial clearance is adjusted by fitting/adjusting shims or liners behind the thrust pads (ahead and astern), or by moving the complete thrust bearing housing fore/aft in the chock location. Removing shims reduces clearance; adding shims increases it. In some designs, thin adjusting liners are placed between the support ring and the bearing housing. The clearance is measured using a dial gauge made up on the shaft collar or by feeler gauges, with the method of checking the total end float by levering the shaft. Adjustment is done by dismantling access covers, adding/removing equal shims on each pad to keep pads parallel, and re-checking the clearance after tightening.

• Oil starvation: insufficient oil supply (low pressure/flow, blocked oil passage, oil cooler fouled) so no proper hydrodynamic film.
• Misdistribution of load among pads or misalignment of the thrust collar relative to the pads (e.g. from crankshaft/hull deflection), causing one pad to take excessive load.
• Excessive thrust due to overload, propeller damage, fouled hull/propeller, or wrong astern operation.
• Oil viscosity too low (hot oil, wrong oil) so the film cannot support the load.
• Contamination of oil with abrasive particles causing metal-to-metal wear.
• Incorrect running-in: new or reconditioned pads not bedded in.
• The collar thrust face distorted/out-of-flat, or edge loading due to pad geometry errors.

The oil film is maintained by ensuring a continuous, adequate, filtered supply of oil at the correct pressure and temperature to each bearing pad's leading edge. The pad shape, keel/pivot position and the collar rotation create the wedge; oil is dragged into the convergent gap by viscosity generating the hydrodynamic film. The bearing must be checked for correct oil flow, the oil free of contaminants, correct viscosity (by cooling), proper pad pivot action, and relief of excess thrust. Keeping the thrust loading reasonable and the pads/collar smooth and parallel also helps maintain the film.