The crankshaft converts the force generated by the combustion in the engine into rotary motion. The linear upwards and downwards motion of the pistons is converted into a torque by the connecting rod and then transmitted to the fly wheel.
To withstand the heavy loads involved, crankshafts have a hardened surface and a tough core. Therefore, crankshafts are often made of forged steel. The continuous grain course enables a high torsion resistance to be achieved. The bearing positions on the crankshaft are surface hardened, making them more resistant to wear.