why do we calculate natural frequency of crankshafts?

Question

modal analysis of crankshafts using ANSYS,free vibrtation analysis

Answer 1

Because the natural vibration frequencies
of anything called a shaft, from crankshafts
to propellershafts, are going to determine
two critical parameters of the problem.

How long is the shaft, and how many
monkeys does it take to turn the shaft.

Answer 2

You would like to know if a vibrational frequency matches the power pulses in the operational range wouldn't you.
(Might cause shaft failure.)