Engine bearings selection: Which type of bearing is typically used for crankshaft main bearings and connecting-rod big-end bearings in automotive engines?

Introduction / Context:

Crankshaft main journals and connecting-rod big ends operate under high cyclic loads and need compact, quiet, and durable bearings with excellent conformability and damping. The bearing type chosen dictates oil supply, clearances, wear behavior, and rebuild practices. Understanding why engines favor one bearing type over rolling-element alternatives is central to engine design and service.

Given Data / Assumptions:

• Four-stroke automotive engines with pressurized lubrication systems.
• High surface speeds at the journal with periodic load reversals.
• Need for compact package and low noise/vibration.

Concept / Approach:

Engines employ plain (journal) bearings—typically bi-metal or tri-metal shells (steel backing with copper/lead/aluminum-tin overlays). With hydrodynamic lubrication, a wedge of oil supports the journal without metal-to-metal contact at speed, offering high load capacity, good damping, and manufacturability with replaceable shells. Rolling-element bearings would be bulkier for equivalent capacity, noisier, and less tolerant of misalignment and debris in this application.

Step-by-Step Solution:

Verification / Alternative check:

Exploded engine diagrams and overhaul manuals show replaceable half-shell bearings in main saddles and rod caps, confirming the standard practice.

Why Other Options Are Wrong:

• Ball, needle, taper roller: used in some motorcycle/specialty engines but not typical for automotive crank journals due to size, noise, and impact-load sensitivity.
• Hydrostatic thrust: different function (axial load) and supply complexity; not used for mains/rods.

Common Pitfalls:

• Assuming rolling bearings must be ‘‘better’’; in hydrodynamic regimes, plain bearings excel.
• Overlooking the role of oil viscosity, clearance, and alignment in film formation.

Final Answer:

Plain bearings