What is a bearing and what types are used in marine engines?

A bearing's job sounds simple, hold a shaft while letting it turn, but the loads inside a marine engine, the crankshaft carrying the full firing pressure of every cylinder through the connecting rods, make this one of the more demanding pieces of engineering aboard, and bearing failure is one of the fastest routes to serious engine damage.

The plain, or journal, bearing dominates large marine engines because it carries heavy loads over a large surface area. The shaft does not run on bare metal against the bearing shell; a continuous, pressurised film of lubricating oil only a few thousandths of a millimetre thick is maintained between the two by the shaft's own rotation dragging oil into a wedge, hydrodynamic lubrication, so under normal running the surfaces never actually touch. The shell is lined with a soft white metal, traditionally a tin- or lead-based babbitt alloy, chosen because it embeds small hard particles from the oil rather than scoring the shaft, and tolerates brief marginal lubrication at start-up without immediately damaging the journal.

In a large two-stroke engine, main bearings support the crankshaft in the bedplate, crankpin, or bottom-end, bearings connect the connecting rod to the crankshaft, and crosshead bearings and guides carry the crosshead pin unique to that design. The thrust bearing, though carrying axial rather than radial load, uses the same white-metal-on-oil-film principle. Rolling-element bearings, ball and roller types carrying load on hardened steel elements, suit higher speed and lighter load applications: pumps, electric motors, and most auxiliary machines, though turbochargers, given their very high rotational speed, sometimes use plain bearings on an oil film instead.

Bearing health is tracked continuously through temperature, whether by fitted thermocouples on larger bearings or routine feel and crankcase inspection, and periodically by opening bearings during overhaul to measure clearance against maker's limits and inspect the white metal for wiping, pitting or fatigue cracking. Lubricating oil condition, water content, dirt, and wear metals found in routine oil analysis, is often the earliest warning of a bearing degrading, well before temperature or noise becomes noticeable, which is why regular oil sampling is taken as seriously as it is on modern engines.