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

1. Thin-shell bearings: The main, big-end (crankpin) and crosshead bearings have changed from thick white-metal-lined bearings to thin-shell (bimetal/trimetal) bearings. Reason: modern engines have much higher MEP and combustion pressures, producing far higher bearing loads. A thin layer of white metal (~0.4-0.5 mm) bonded to a strong steel back has much higher fatigue strength than a thick cast white-metal lining, so it can carry the higher specific loads without cracking/wiping. It is also easier to replace (new shells) and gives accurate clearances.
2. Material development: The bearing surface materials have evolved - from plain white metal to trimetal (a steel back, a copper-lead-bronze or aluminium-tin intermediate layer, and a thin soft overlay) or to aluminium-tin alloys. Reason: to combine high load capacity and fatigue strength (from the strong backing/intermediate) with good conformability and embeddability (from the soft overlay), and to resist the higher temperatures and loads. The crankshaft journals are often hardened (chrome-plated or induction-hardened) to match the harder bearing materials and reduce wear.
3. Geometry: The bearings are designed with the correct clearance, crush, and oil groove/hole arrangement to maintain a hydrodynamic oil film under the high loads. The bearing shells are axially located and prevented from rotating by a tang/groove. Reason: to ensure a reliable oil film, even load distribution, and to prevent edge loading and wiping.
4. Larger bearing area/journal diameter: As engine power per cylinder increased, the bearing journals and bearing areas were enlarged to reduce the specific bearing load and to accommodate the higher peak pressures. Reason: to keep the bearing pressure within the material's capacity and to maintain the oil film.
5. Improved lubrication: Pressure-fed oil with better oil distribution, larger oil grooves, and the use of high-additive crankcase oils. Reason: to maintain the hydrodynamic film and to cool the bearing under the higher loads and speeds.
6. Crosshead bearing design: The crosshead bearing (which has an oscillating motion and cannot build a full hydrodynamic film) has been developed with a special design (e.g. a "floating" or pressure-fed bearing, or a bearing with a special geometry) to cope with the high loads and the oscillating motion. Reason: to prevent wiping and to maintain a film under the severe conditions.

The overall reason for all these changes is the continual increase in engine power output per cylinder (higher MEP) and the need for higher reliability, longer component life, and reduced maintenance, while withstanding the higher loads, temperatures and speeds of modern engines.