Explain the difference between static and dynamic balancing of rotating shafts.

Static imbalance occurs when the center of mass is offset from the rotation axis - single plane correction suffices. Dynamic imbalance involves a couple from masses in different axial planes, requiring two-plane correction even if statically balanced. For rigid rotors (operating below 0.7x first critical), balance in two planes using vector calculation: m1.r1 + m2.r2 = 0 (force balance) and m1.r1.l1 + m2.r2.l2 = 0 (moment balance). ISO 1940 specifies balance quality grades G1 to G4000 based on e.omega where e is specific unbalance. Precision machinery requires G2.5 or better.