Crankshaft

The crankshaft, is casually abbreviated to the word crank, is the part of an engine which translates reciprocating linear piston motion into rotation. In order to convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crankpins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach.

Working:

It is connected to a flywheel, to reduce the pulsation characteristic of the four-stroke cycle, and sometimes a torsion or vibration damper at the opposite end. And to reduce the torsion and the vibration often caused along the length of the crankshaft by the cylinders farthest from the output end acting on the torsional elasticity of the metal.

Design:

Heavy engines are mostly multi-cylinder to reduce pulsations from individual firing strokes, with more than one piston attached to a complex crankshaft. Many light (small) engines, such as those found in mopeds or garden machinery, are single cylinder and use only a single piston, simplifying crankshaft design. These engines can also be built with no riveted seam. 

Failure Of Crankshaft:

The shafts are subjected to various forces but generally needs to be analyzed in two positions. At First, the failure may occur at the position of maximum bending; this may be at the centre of the crank or at either end. In such conditions the failure is due to bending and the pressure in the cylinder is maximal. Secondly, the crank has a chance to fail due to twisting, so the crank rod needs to be checked for shear at the position of maximal twisting. This position experiences the maximum pressure.

Production Of Crankshaft:

Crankshafts are forged from steel bars by roll forging or cast in ductile steel. Along the  forged crankshafts, vanadium alloyed steels are mostly used as these steels can be air cooled after reaching high strengths without additional heat treatment, with exception to the surface hardening of the bearing surfaces. The low alloyed mixture makes the material cheaper than high alloy steels.  Iron crankshafts are mostly found in cheaper production engines where the loads are lower.