Queendom Tools

ISO 1940-1 Permissible Residual Unbalance Calculator

Use the highest speed the rotor runs at.
The grade number is e_per × ω in mm/s. Read the grade for your machine type from Table 1 of the standard.
Distance from the shaft axis, per plane.

e_per = 9549 · G / N  (µm),  U_per = e_per × M  (g·mm)

Permissible eccentricity e_per
— µm
Permissible residual unbalance U_per, total
— g·mm
U_per per plane
— g·mm
Residual mass at r, per plane
— g
Residual mass at r, total
— g
Centrifugal force F from the total unbalance at N
— N

For preliminary estimation only, not a substitute for review by a qualified/licensed engineer. Source: ISO 1940-1:2003, Table 1 (balance quality grades); ISO 21940-11:2016 replaces it and keeps these grades.

Permissible residual unbalance of a rigid rotor, from grade G

A balancing shop works to a number, not a feeling: after the last correction the rotor must sit under the permissible residual unbalance, and that limit follows from three facts you already have — the rotor mass, the maximum service speed and the balance quality grade G.

ISO 1940-1 defines eleven grades, from G0.4 to G4000, and the grade number is the permissible eccentricity multiplied by the angular velocity, in mm/s. The calculator turns that into e_per = 9549·G/N in µm (numerically the same as g·mm per kg of rotor), then into U_per = e_per × M in g·mm, per correction plane. It also converts the per-plane limit into the mass you may leave at your correction radius and shows the centrifugal force that the total unbalance produces at speed.

The page does not pick the grade for you: the grade for a machine type is read from the table of the standard, and ISO 21940-11:2016, which replaces ISO 1940-1:2003, keeps the same grades. Use the maximum service speed for N — the limit scales inversely with speed, so a nominal operating speed understates the demand on a rotor that runs faster.

How to use it

  1. Enter the rotor mass M in kilograms.
  2. Enter the maximum service speed N in rpm — the highest speed the rotor runs at, not a nominal operating point.
  3. Pick the balance grade G from the dropdown of the 11 grades, using the machine-type value you read from the standard's table.
  4. Select 1 or 2 correction planes; with 2 planes the permissible unbalance is split equally, which assumes a symmetric rotor.
  5. Optionally enter the correction radius r in mm to convert the per-plane unbalance into the residual mass allowed at that radius.

FAQ

What is the permissible residual unbalance for a 100 kg rotor at 3000 rpm to grade G6.3?

e_per = 9549 × 6.3 / 3000 = 20.05 µm, so U_per = 20.05 × 100 = 2005 g·mm total and 1002.6 g·mm per plane with two planes; at a 50 mm correction radius that is 20.05 g per plane (40.11 g total), and the total unbalance makes 197.9 N of centrifugal force at 3000 rpm.

Which balance grade should I choose — G2.5, G6.3 or G16?

This page does not choose the grade for you: the grade for a given machine type is read from Table 1 of ISO 1940-1:2003, and the grade number is the permissible eccentricity times angular velocity in mm/s.

How does speed affect the permissible residual unbalance?

Inversely — e_per = 9549·G/N, so doubling the maximum service speed halves the permissible eccentricity; the same 100 kg rotor that allows e_per 12732 µm and U_per 1 273 200 g·mm at G4000 allows only 1.273 µm and 127.3 g·mm at G0.4.

Is ISO 1940-1 still the current standard for balance quality grades?

ISO 21940-11:2016 replaces ISO 1940-1:2003 and keeps the same 11 balance quality grades from G0.4 to G4000, so the grade values used here are unchanged.