A component within an internal combustion engine which converts the reciprocating motion of the prisons to rotary motion which can be transferred to the transmission.
This engine piece provides a means by which to turn the reciprocating motion of the pistons into rotational motion, then provides this power out to the drivetrain.
Here is a representation of a 4-cylinder crankshaft:
Inside the engine, the pistons are connected to connecting rods via wrist pins. The connecting rods are then connected to the crankshaft. There are bearings which the connecting rods holds, which ride on journals on the crankshaft. The journals are very smooth and round portions of the crankshaft. The bearings go around the crankshaft as it rotates. There is a thin film of oil which is pumped into the space between the bearing journals and the bearings themselves, which provides a way where the bearing never even touches the surface of the crankshaft. The oil is fed through the block via oil gallery passages, the out through holes in the main journals. There are a second set of holes in the main journal which directs oil to the rod journals (labelled crankpin journals in the picture).
A crankshaft can be internally or externally balanced. The balance offsets the massive rotational weight which occurs as the crankshaft rotates. This allows for smoother engine operation. Without it, the engine would tear itself apart.
When a crankshaft has power applied to it via when a cylinder fires, massive twisting occurs where torque is loaded onto the crankshaft, then unloaded as the crankshaft twists back to right itself. The vibrations which occur due to this loading and unloading are controlled partially by the vibration/torsional damper which is attached to the front of the engine. This damper can also be used to help balance the front end of the engine.
The flywheel mounting flange is at the back of the crankshaft. It is the main mounting point for either a flywheel on a manual shift transmission or a flexplate on an automatic. If flexplate, a torque converter is attached to it to couple the power output of the engine to the transmission. If flywheel, there is a clutch plate and friction disk combo.