Full definition
The process of measuring and correcting the mass distribution of rotating components (rotors, impellers, pulleys, flywheels, fans) so the center of mass coincides with the axis of rotation, minimizing vibration caused by centrifugal forces at operating speed. Balance quality grades per ISO 1940-1: G16 (agricultural machinery, crankshafts), G6.3 (general industrial — electric motors, pumps, fans; the most commonly specified grade), G2.5 (turbochargers, machine-tool drives), G1.0 (grinding machine spindles), and G0.4 (gyroscopes, precision instruments). Permissible residual unbalance: Uper (g·mm) = G × m(kg) / ω(rad/s), where G is the balance grade. Methods: single-plane (static) balancing for narrow rotors (disk-like, width <1/5 diameter — correct by adding or removing mass at one axial location), and two-plane (dynamic) balancing for wide rotors (length/diameter ratio significant — requires correction at two axial planes). Equipment: shop balancing machines (Schenck, IRD, Hofmann — measure both magnitude and phase angle of unbalance) and portable field balancing with vibration analyzers (single- or multi-plane trial-weight method). Per ISO 1940-1 and ISO 21940 series.