Developments in the design of bearings of slow-speed marine diesel engines, focusing on the reasons:

1. Main (big-end, crankpin) and crosshead bearings have evolved from white-metal (Babbitt) lined plain bearings to thin-shell (trimetal/bimetal) bearings. The reason: higher MEP and higher combustion pressures in modern engines produce far higher bearing loads. A thick white-metal lining cracks and wipes under these loads; a thin layer of white metal (~0.4-0.5 mm) bonded to a strong steel back is a fatigue-resistant surface that can carry much higher specific loads and is easier to replace when worn by simply fitting new shells. So the geometry (thin lining on a steel backing rather than thick cast) and the material (special trimetal overlays, or a bronze-lead-bronze surface) evolved to give high strength, good friction and wearing-in, accurate clearances, and to cope with high peak bearing loads and temperature.
2. The bearings are made as two half-shells (top and bottom) that are axially-located and prevented from rotating by a tang/groove and clamped in the housing. Thin shells are precision matched to the fine clearance required and to allow the correct oil film.
3. Larger journal diameter/bearing area per cylinder for the higher MEP and to reduce specific load; the bearing is designed with an oil groove and pressure oil feed to maintain a hydrodynamic film, hence the geometry is optimised for the peak load.
4. Material: development of high-strength, fatigue-resistant bearing materials, including chrome plating of the shaft/crankpin surfaces, and use of materials with good load capacity, wear resistance, and resistance to wiping, and matching to the harder (chromed) crankshaft surface. A "platform" of copper-lead-bronze (trimetal) on steel or of Pb-bronze overlay on steel.
5. Oil lubrication improvements: pressure feed to the bearings with improved oil distribution, larger oil grooves and oil holes, and the use of high-additive (crankcase) oils to withstand the severe boundary conditions at reversals, plus the improvement in side clearance/axial location to ensure the whole bearing length is lubricated.
6. Development in bearing geometry (e.g. circular/semi-circular section, the correct clearance and crush) so the shells conform to the journal perfectly and distribute load evenly, reducing edge loading and extending life.
7. The design supports higher peak cylinder pressures, higher speeds and cooling of the bearing by circulating oil, so the bearing can sustain continuous operation under the higher power-to-weight ratios of modern engines. The reason for all these changes is the continual increase in engine power output per cylinder (higher MEP), and the need to improve reliability, component life, and reduce maintenance and the risk of bearing failure.