A
crankshaft is more than just a chunk of
metal. It is one of the most
painstakingly and
precisely manufactured
devices in an automobile
engine. It must be very
hard so that it does not flex or bend as the pistons (via the rods) pound on it to transfer torque to the
drivetrain. It must also be hardy, so that it does not wear out from heat fatigue and other mechanical stresses, or break from the induced torque.
A crankshaft is typically connected to the pistons via connecting rods at several key locations (journals). These journals are positioned so that the reciprocating linear motion of the pistons during each combustion cycle act to further the rotation of the shaft. Each connecting rod is bound to the shaft at the journal with a rod bearing; the journals must be machined to be ultra-smooth and round so that there is negligible chance for friction to occur between it and the bearing. Most crankshafts have criss-crossing oil passages machined inside them with openings at each journal. This allows the bearing to continuously ride on a film of pressurized oil so that no metal-to-metal contact occurs during crankshaft rotation.
A crankshaft typically has an elaborate system of counterweights attached to it, which are machined at the factory so that the shaft is nearly perfectly rotationally balanced. The shaft is mounted into the bottom of the engine block, and rides at its ends on main bearings. In most automobiles, one end of the crankshaft is connected to a pulley which drives other accessory parts, such as the air conditioner, the alternator, and the camshaft(s). The other end of the shaft is directly connected to the drivetrain through either a clutch or a torque converter and then the transmission.