Full definition
A condition where the center of mass of a rotating component (rotor, impeller, pulley, flywheel) does not coincide with its axis of rotation, generating a centrifugal force proportional to the mass eccentricity and the square of rotational speed (F = m·e·ω²). Imbalance is the #1 most common rotating machinery vibration problem and the easiest to correct. Vibration signature: dominant at exactly 1× RPM (once per revolution), predominantly radial, sinusoidal waveform. Per ISO 1940-1, balance quality grades: G6.3 (standard industrial machinery — electric motors, pumps, fans), G2.5 (precision machinery — turbochargers, machine-tool drives), G1.0 (high precision — grinding spindles), and G16 (general industrial — crankshafts, flywheels). Permissible residual imbalance: Uper = (G × m) / ω grams·mm. Correction: single-plane (static) balancing for narrow rotors (width < 1/5 diameter), and two-plane (dynamic) balancing for wide rotors. Methods: field balancing with vibration analyzer (portable, in-situ), or shop balancing on a balancing machine (more precise). Causes: manufacturing tolerance, deposit buildup (fans, impellers), material loss (erosion, corrosion), and eccentric mounting. Per ISO 1940-1 and ISO 21940 series.