A crankshaft is a rotating shaft which (in conjunction with the connecting rods) converts reciprocating motion of the pistons into rotational motion. Crankshafts are commonly used in internal combustion engines and consist of a series of cranks and crankpins to which the connecting rods are attached.

The crankshaft rotates within the engine block through use of main bearings, and the crankpins rotate within the connecting rods using rod bearings. Crankshafts are usually made from metal, with most modern crankshafts being constructed using forged steel.

The crankshaft is supported by the engine block, with the engine’s main bearings allowing the crankshaft to rotate within the block. The up-down motion of each piston is transferred to the crankshaft via connecting rods. A flywheel is often attached to one end of the crankshaft, in order to store rotational energy and maintain a more consistent rotational speed as the crankshaft received energy from the connecting rods as a series of pulses. This assists in smoothing the power delivery and often in conjunction with a Harmonic damper -attached to the other end of crankshaft- reduces torsional vibration.

A crankshaft is subjected to enormous stresses, in some cases more than 8.6 tonnes (19,000 pounds) per cylinder. Crankshafts for single-cylinder engines are usually a simpler design than for engines with multiple cylinders.

Crankshafts can either be one-piece forgings or pressed together from separate individual crank-webs, shafts and pins, sometimes called a ‘built-up crankshaft’. In most automotive applications (Four-stroke engines) a one-piece forging is used in conjunction with plain/shell bearings that rely on steady supply of pressurized engine oil. This pressurized oil fills the clearance between the bearings and the journals of the crankshaft and creates a thin hydrodynamic layer of oil that ‘floats’ the metal surfaces apart so no metal to metal contact occurs as the parts rotate. This oil film is very low friction and, as long as oil pressure is maintained, can easily handle the forces generated by piston acceleration, deceleration, reversal and swing of conrod big ends.

Two-stroke engines use a sealed crankcase as part of the induction of fresh fuel/air into the cylinder. This precludes the use of oil pressurized plain main and crankpin bearings. Instead rolling element ball and needle roller bearings are used and the cranks shaft is made up of several pieces press-fitted together using one-piece conrods, rather than the 2-piece conrod design ubiquitous in a plain bearing 4-stroke engine. Lubrication of the two-stroke crankshaft is via oil mixed with fuel, either in the fuel tank (pre-mix) or via a separate oiling system that delivers oil to the crankcase to be mixed with the fuel according to rpm and load. While mainly used in two-stroke engines (everything from chainsaws to large multi-cylinder outboard motors) some older 4-stroke engines (mainly motorcycles) use built up roller crankshafts.