ISO 21940-11 Balancing grades

Balancing Grades are a quality characteristic used to compare rotors of different weights and rotating speeds in terms of how well they must be balanced.

The balancing tolerance requirements for rotors with rigid behavior are specified in the ISO 21940-11. Part 11 of the standard includes, besides necessary numbers of correction planes and methods to verify the residual unbalance, a definition of the Balancing Tolerances and Balancing Grades.

This article will look into what Balancing Grades are and how to determine the balancing tolerance for the correction planes of a rotor.

What are Balancing Grades “G”

Based on experience, the ISO Standard recommends certain Balancing Grades for individual rotor types. These recommendations can be used by the manufacturer of components and machines to determine the required balancing tolerance in order to achieve a satisfactory operation of the rotor in service.

The Balance Quality Grade (abbr. G) is expressed in millimeters per second (mm/s) and defines the maximum allowable vibration velocity of a rotating work piece gravity center. It is a product of the

G = eper x Ω

•    eper = permissible eccentricity of the gravity center in mm
•    Ω = Max. Operational Angular Velocity in rad/s (Ω = RPM x π/30)

Example: G 2.5 = max allowable vibration velocity of c.o.g. is 2.5 mm/s

What are balancing tolerances

All Rotors have an initial unbalance that causes vibration while rotating. The complete elimination of unbalance in a rotor is technically and practically impossible. Instead, technical requirements determine how precisely a rotor must be balanced for satisfactory vibration.

Once the Balancing Quality Grade “G” is determined by the application, the permitted residual unbalance Uper can be calculated by multiply the eccentricity eper times the rotor Mass m:

Uper = m x eper = m x G/ω

Instead of calculating the Residual Unbalance the Graph shown later can be used.

How balancing tolerances can save you money

In addition to the technical requirements, economic considerations are also necessary when determining the balancing tolerance. The lower or tighter the rotor tolerance, the more time-consuming and thus cost-intensive it is to achieve. This is why a rotor should only be balanced to the necessary tolerance, rather than balancing as low as possible.

Using Balancing Grades to determine the required balancing tolerance

Three steps are required to use balancing grades to determine the permitted balancing tolerance of a rotor:

1. Set Balancing Grade per Application

Select a balancing grade from the Table, based on the Rotor type and application.

Balance Grade
Machinery Examples
G 4000 Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently unbalanced
G 1600 Crankshaft drives for large, slow marine diesel engines (piston speed below 9 m/s), inherently balanced
G 630 Crankshaft drives, inherently unbalanced, elastically mounted
G 250 Crankshaft drives, inherently unbalanced, rigidly mounted
G 100 Complete reciprocating engines for cars, trucks and locomotives
G 40 Cars: wheels, wheel rims, wheel sets, drive shafts
Crankshaft drives, inherently balanced, elastically mounted
G 16 Agricultural machinery
Crankshaft drives, inherently balanced, rigidly mounted
Crushing machines
Drive shafts (cardan shafts, propeller shafts)
G 6,3 Aircraft gas turbines
Centrifuges (separators, decanters)
Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds up to 950 r/min
Electric motors of shaft heights smaller than 80 mm
Fans Gears
Machinery, general
Machine tools
Paper machines
Process plant machines
Turbo chargers
Water turbine
G 2,5 Compressors
Computer drives
Electric motors and generators (of at least 80 mm shaft height), of maximum rated speeds above 950 r/min
Gas turbines and steam turbines
Machine-tool drives
Textile machines
G 1 Audio and video drives
Grinding machine drive
G 0,4 Gyroscopes
Spindles and drives of high-precision systems

Table for guidance for balance quality grades for rotors with rigid behavior per ISO 21940-11

2. Determine Balancing Tolerance (Permissible Residual Unbalance) for the Rotor

Use the Chart below to determine the permissible residual unbalance eper first and then multiply with the Rotor Mass m to obtain the permissible residual unbalance Uper.

Or as an alternative, you can use the following formula:

Uper = m x eper = m x G/ω

Hofmann Balancing Machines have a balancing tolerance calculator per ISO or API already included in the Software Package. Just enter the required rotor data, then the balancing grade and the balancing tolerance will be automatically calculated and allocated to the correction planes.

3. Allocate the Residual Unbalance of the rotor Uper to the correction planes

The determined permissible residual unbalance Uper applies for the complete Rotor. Depending on Rotor dimensions and configuration the tolerance needs to be split to the correction planes. 

Describing the exact way on how to allocate the total permissible residual unbalance to the correction planes would go beyond the scope of this article. But in general, the following statements can be used for symmetrical Rotors as a rule of thumb:

  • Rotors with a single correction plane:
    → The total Residual Unbalance of the rotor Uper will be considered for this one plane
  • Rotors with two correction planes:
  • →  The total Residual Unbalance of the rotor Uper will be split by 50/50 to each correction plane and only some special cases with outboard center of gravity will require additional considerations

For further questions please contact

Richard Hoer
Product Manager Universal and Aerospace
Phone: +1.434.522.0348
Mail: rhoer2(at)