Rotor Balance (ISO 1940)

The grade is defined so that G = e × ω is constant, which means the permissible unbalance halves every time you double the speed. A rotor balanced perfectly acceptably at 500 rpm can be hopeless at 5000 — and because unbalance force goes as ω², the bearing load still doubles even when you meet the tighter grade.

The per-plane figure assumes a symmetric two-plane correction; an overhung or asymmetric rotor splits differently, and ISO 21940-11 gives the rules. A rotor is only "rigid" below about 70 % of its first critical speed — above that it flexes and single-speed balancing does not hold, so check Critical Speed first. ISO 1940-1 was superseded by ISO 21940-11 in 2016; the grades are unchanged.