crankpin

A crankpin, also known as a crank journal, is the cylindrical bearing surface on a crankshaft that supports a connecting rod. It is offset from the crankshaft’s main bearing axis by the crank throw, and the distance between the centerlines of the main bearing and the crankpin determines the engine stroke.

The big end of the connecting rod attaches to the crankpin and rides on a bearing surface that can be a bush, insert, or a full-floating bearing, depending on design. The crankpin is part of the crankshaft and is machined with a precise diameter and surface finish, with fillet radii at transitions to reduce stress concentrations. Lubrication is essential; oil passages within the crankshaft deliver a film of oil to the crankpin bearing to minimize wear.

Crankpins are arranged according to engine configuration. In multi-cylinder engines, each crankpin or each pair of cylinders may share a crankpin on the crankshaft, and configurations vary between inline, V, and opposed designs. Some designs use split or paired crankpins to balance loads and improve rigidity.

Materials and manufacturing vary, but crankpins are typically formed as an integral part of the crankshaft or as machined bearing surfaces on a forged or cast substrate, followed by hardening and finishing processes to resist wear. The integrity of the crankpin bearing surface is critical for reliability, as excessive wear or scoring can lead to increased clearances, reduced lubrication effectiveness, and potential engine failure.

In operation, the crankpin serves as the pivotal interface through which the reciprocating motion of the piston, via the connecting rod, is converted into the rotational motion of the crankshaft. Regular inspection of bearing clearance and surface condition is part of routine engine maintenance.