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Permissible residual unbalance is the maximum amount of unbalance allowed to remain in a rotor after balancing, defined by ISO 21940-11 (formerly ISO 1940). It depends on the rotor mass, operating speed, and the required balance quality grade (G).

Exceeding this limit leads to increased vibration, bearing wear, and reduced equipment life.

Select the balance grade from the rotor drawing, manufacturer specification, product standard, or engineering agreement.

G6.3 is common for some fans, pumps and motors, but it is not a universal default. Do not choose a grade solely from this example list when the required grade is unknown.

ISO 21940-11 replaced the older ISO 1940-1 standard. The technical content is essentially the same — the permissible specific unbalance formula and balance grades remain unchanged.

The new number reflects the reorganization of the ISO 21940 series covering all aspects of rotor balancing.

For disc-like rotors with CG approximately centered: allocate 50% to each plane.

For long rotors: use the lever rule — U₁ = U_total × L₂/(L₁+L₂) and U₂ = U_total × L₁/(L₁+L₂), where L₁ and L₂ are distances from CG to each correction plane.

Common units:

• g·mm (gram-millimeters) — most common for industrial rotors
• oz·in (ounce-inches) — used in North America
• kg·m — SI unit (rarely used in practice)

Conversion: 1 oz·in ≈ 720 g·mm