An important part of an Internal Combustion Engine
. It serves the biggest role of an engine, converting Linear motion to Rotary motion. It takes the piston's up-and-down motion, pushing the connecting rod therefore turning the crankshaft. It contains counter-weights to balance the crankshaft so to reduce noise and vibration. It is also connected to the camshaft for timing purposes via a timing belt.
The crankshaft must turn without friction, therefore it has bearings on the case and bearings around the connecting rod. Oil must be supplied, in fact the crankshaft sits in the oil pan. There must be little play, but lateral play occurs, see Crankwalk
On Manual Transmissions, the wheel with teeth at the end of the crankshaft is called the Flywheel. On Automatic Transmissions, it is called a Flex Plate.