The crankshaft converts the up and down (reciprocating) motion of the
pistons into a turning (rotary) motion. It provides the turning
motion for the wheels. It works much like the pedals of a bicycle,
converting up-down motion into rotational motion.
The crankshaft is usually either alloy steel or cast iron. The
crankshaft is connected to the pistons by the connecting-rods.
Some parts of the shaft do not move up and down; they rotate in the
stationary main bearings. These parts are known as journals. There
are usually three journals in a four cylinder engine.
