The purpose of this component is to change the reciprocating motion(up and down movement) of the piston into rotary motion. Crankshafts are usually alloy steel forgings with their journals and crankpins hardened to resist wear. The crankpins and journals are usually hollow, to reduce weight, these spaces being interconnected by drillings in the crank webs to provide passages for lubricating oil.
A shaft is classified according to the number of ‘throws’ or cranks, for instance a ’six throw’ shaft has six crankpins. The crankwebs are sometimes extended, the extra metal providing a means of balancing the assembly or provide provisions for attachment of damping weights. Suitable drives at each end of the crankshaft transmit the torque to the reduction gear and the accessory drives and In direct drive engines the crankshaft is connected to the propeller with or without the propeller governor.
The simplest crankshaft is the single-throw or 360° type. This type is used in a single-row radial engine. It can be constructed in one or two pieces. Two main bearings (one on each end) are provided when this type of crankshaft is used.
The double-throw or 180° crankshaft is used on double-row or 180° crankshaft is used on double-row radial engines. In the radial-type engine, one throw is provided for each row of cylinders.