The crankshaft of an engine (either internal or external combustion) serves to translate the reciprocating motion of the pistons to rotational motion for power output. Where possible, crankshafts are forged from billet steel. Some crankshafts are cast or built of multiple sections bolted together due to the higher cost of forging, however this results in greatly increased weight.

Most crankshafts in 4 cycle or diesel engines share the following design:

The shaft itself spins in the crankcase upon its main bearings. These bearings are pressure lubricated, and one or more of these will feed oil into a series of passages inside the crankshaft for distribution to the connecting rod bearings and / or other main bearings. (A hole in the connecting rod is in turn used to feed lubricating oil to the piston and the wrist pin bearing at the small end of the rod.)

Most crankshafts (especially those which run at high RPM) include a "sludge trap". Due to high RPM rotation the oil passages in the crank act as a centrifuge, meaning that deposits can accumulate in the larger radius sections. This would eventually block flow, therefor upon entry to the crank, the oil passes into a relatively large reservoir (meaning that it resides there longer than elswhere) at the outer radius of revolution. This causes particles which would otherwise clog passages to be caught in the trap. Some engines make the trap too small and thus limit the potential engine life (e.g. '60s BMW motorcycles need to have the sludge trap cleaned about every 150,000 miles).

Most automotive and large stationary engines use journal bearings in both the main and connecting rod bearing locations. The journal is defined as the cylindrical sections which rotate inside of the bearing shell. Many motorcycles and most 2 cycle engines use ball or roller bearings.

Crankshaft design largely determines the vibration characteristics of the engine. This is principally used to set the "balance factor" to obtain the desired behavior, and may accomplished using attached counter weights A balance factor near 1.0 indicates an engine whose primary vibration is in line with piston motion (the engine vibrates up-down), while a value near 0.5 results in an engine which vibrates more in a circle. Additionally, the balance factor is set to minimize vibration at the most common operating RPM ranges. A balance factor not well matched to the engine mounts and frame can result in fatigue failures of the supporting structures.*

Very few crankshaft designs are adequately heavy in themselves to generate a smooth-running engine. For this reason, most engines have an attached flywheel which stores energy, providing smooth operation. Racing engines use very small flywheels, which allows for substantially increased acceleration, but which also may result in an engine which will not idle at a speed lower than say 2-3000 RPM, which would be impractical for daily use.

*Or in some instances of the mechanism. A friend once told me about the failure of a large power generation diesel engine at a US Military installation in Scandanavia. It seems that the engineers skimped on the recommended concrete footings for the engine mounts. After a few months of operation the attachment of a (several ton) flywheel failed. It then fractured its case and the wall of the facility and rolled into the nearby fjord.