crankweb

A crankweb is the solid arm of a crankshaft that connects the main journal (the rotating axis passing through the engine block) to the crankpin, where the connecting rod attaches. In a four-cylinder engine, there are typically four crankwebs and four crankpins arranged around the main shaft. The web carries the full side load generated by combustion and inertia forces as the piston moves, so it must be thick and rigid enough to resist bending and fatigue.

Crankwebs are integral to the crankshaft forging; they are not bolted on. During forging, the metal is worked to align grain flow along the stress paths, which runs from the crankpin through the web and into the main journal. This grain alignment is crucial because it raises fatigue strength without adding mass. A typical automotive crankshaft has crankwebs made from ductile iron or alloy steel, often in the range of 30 to 50 mm thick at the root where the web joins the main journal.

The crankweb is not symmetrical. On one side of the crankpin, material extends outward; on the opposite side, a counterweight is often cast or forged as part of the web itself. This counterweight reduces vibration by offsetting the mass of the crankpin and connecting rod. The angle between adjacent crankwebs follows the engine's firing order; in a four-stroke four-cylinder engine arranged at 90-degree intervals, the webs alternate front-to-back to optimize balance and reduce torsional vibration.

Cracks in crankwebs typically initiate at the fillet (the radius where the web meets the main journal) under cyclic loading. This is why modern crankshafts are often shot-peened at these stress concentrations to introduce compressive residual stress and extend service life. The fillet radius itself is a design trade-off: too sharp, and fatigue cracks start early; too large, and packaging becomes difficult.

In large diesel engines, crankwebs may be separate from the crankpin and journal, pinned or shrink-fitted in place, to allow for replacement or repair. In automotive engines, the single-piece design is standard because cost and packaging efficiency outweigh the advantage of replaceability. The crankweb's strength directly limits engine power output and maximum rpm, making it one of the most analyzed components in engine design.