Crankshaft

The crankshaft is the engine's principal rotating member, the component that collects the separate pushes from every piston and merges them into a single, continuous turning output. It is the final mechanical link before power leaves the engine through the flywheel or flexplate, and from there to the clutch or torque converter and ultimately the wheels. In essence, it is the part that lets a series of small explosions become smooth, usable rotation.

It achieves this through its distinctive cranked shape. The main journals run along the shaft's centreline and spin in main bearings bolted into the engine block, defining the axis of rotation. Between them sit the crankpins, offset from that centreline by a fixed distance; it is to these offset pins that the big ends of the connecting rods attach. When a piston is driven down its bore, its rod presses on the crankpin and, because the pin is offset, applies a turning moment to the shaft, much as a foot on a bicycle pedal turns the chainwheel. The offset distance is the crank throw, and twice that throw equals the piston's stroke, one of the two dimensions that, together with the bore, fixes the engine's displacement.

Managing the order and spacing of these impulses is central to refinement. The crankpins are arranged at carefully chosen angles so that the cylinders fire in a sequence that spreads the power strokes evenly and balances forces. Heavy counterweights are forged or cast opposite the crankpins to offset the inertia of the rotating rod ends and to damp the vibrations inherent in reciprocating machinery; without them an engine would shake heavily. A harmonic balancer or damper on the front pulley further tames torsional vibration that would otherwise fatigue the shaft.

Crankshafts are typically forged from steel for strength in demanding or high-output engines, or cast from nodular iron where cost and lower stress allow. Their journals are precision-ground and surface-hardened, and they ride on thin plain bearings supplied with pressurised oil through drillings inside the shaft. The same nose of the crankshaft usually drives the timing belt or chain that keeps the camshafts and valves synchronised with piston position.

In service the crankshaft is extremely durable, but it depends utterly on clean, pressurised oil; a loss of oil pressure quickly wipes the main and big-end bearings and can score or seize the journals. Excessive end float, a worn thrust bearing, or a fatigue crack are serious faults, and regrinding the journals undersize with matching bearing shells is the traditional remedy when wear sets in.